Noble Metal-Metal Oxide Hybrid Nanoparticles

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NOBLE METAL-METAL OXIDE HYBRID NANOPARTICLES

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Micro and Nano Technologies

NOBLE METAL-METAL OXIDE HYBRID NANOPARTICLES Fundamentals and Applications Edited by

SATYABRATA MOHAPATRA University School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, Dwarka, Delhi, India

TUAN ANH NGUYEN Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

PHUONG NGUYEN-TRI Department of Chemistry, University of Montre´al, Montreal, Quebec, Canada De´partement de ge´nie de la construction, E´cole de Technologie Supe´rieure, Montreal, Quebec, Canada

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright r 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-814134-2 (print) ISBN: 978-0-12-814135-9 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

3.2 Chemical Synthesis Methods 52 3.3 Physical Fabrications of Hybrid Nanoparticles 57 3.4 Summary and Future Trend 59 References 60

List of Contributors xi

PART I FUNDAMENTALS

4. Nanoscale Characterization

1. Hybrid Nanoparticles: An Introduction

SRABANTI GHOSH AND RAJENDRA N. BASU

4.1 Introduction 65 4.2 Morphological Characterization 67 4.3 Quantification of Metal Content in Nanohybrids 73 4.4 Crystal Phase Characterization Through X-Ray Techniques 75 4.5 Surface Characterization 77 4.6 Spectroscopic Characterization 79 4.7 Electrochemical Characterization 85 4.8 Other Techniques 86 4.9 Conclusion 89 References 89

DONGLING MA

1.1 Introduction References 5

3

2. Theoretical Aspects of Synthesis for Controlled Morphological Nanostructures SOUGATA SARKAR AND TARASANKAR PAL

2.1 Nucleation and Growth of Nanoparticles: Theoretical Perspectives 7 2.2 Mechanism of Growth and LifshitzSlyozovWagner (LSW) Theory 9 2.3 Stabilization of Nanoparticles: The Role of Ligands 12 2.4 Interactions Between Ligands and Surface of Noble Metal Nanoparticles 14 2.5 Interactions Between Ligands and Surface of Metal Oxide Nanoparticles 23 2.6 Synthesis of Noble Metal and Metal Oxide Nanomaterials: A Brief Discussion 34 2.7 Conclusions 45 References 45

5. Physics, Electrochemistry, Photochemistry, and Photoelectrochemistry of Hybrid Nanoparticles PHUONG NGUYEN TRI, SAMI RTIMI, TUAN ANH NGUYEN AND MINH THANH VU

5.1 Overview 95 5.2 Physical Properties 96 5.3 Effect of Noble Metal NPs on the Specific Capacitance of Noble MetalMetal Oxide Based Supercapacitors 109 5.4 Photoelectrochemical (PEC) Properties 110 5.5 Photochemical Properties 111 5.6 Summary and Future Trend 114 References 114

3. Methods for Synthesis of Hybrid Nanoparticles PHUONG NGUYEN TRI, CLAUDIANE OUELLETPLAMONDON, SAMI RTIMI, AYMEN AMINE ASSADI AND TUAN ANH NGUYEN

3.1 Introduction

51

v

vi

CONTENTS

6. Electronic Transport in Hybrid Nanoparticles ANTOINE KHATER

6.1 Introduction 125 6.2 Electronic Transport in Nanoparticle Assemblies 127 6.3 Electronic Transport by Excitons in Hybrid NMMO NP Systems 130 6.4 Summary and Perspectives 137 Acknowledgments 138 References 138

7. Antibacterial Behavior of Hybrid Nanoparticles PHUONG NGUYEN TRI, TUAN ANH NGUYEN, THE HUU NGUYEN AND PASCAL CARRIERE

7.1 Overview 141 7.2 Effect of Metal Oxide Nanoparticles on the Antibacterial Behavior of Noble Metals in Their Nanohybrids 145 7.3 Effect of Noble Metal Nanoparticles on the Antibacterial Behavior of Metal Oxides in Their Nanohybrids 145 7.4 Challenges and Perspective 149 References 150

8. Exciton 2 Plasmon Interactions in Noble MetalSemiconductor Oxide Hybrid Nanostructures WEIHUA LIN AND MENGTAO SUN

8.1 Introduction 8.2 Mechanisms 8.3 Femtosecond 8.4 Applications References 173

157 159 Absorption 168

185

10. Sonochemical Synthesis of PalladiumMetal Oxide Hybrid Nanoparticles S. SIVASANKARAN AND M.J. KISHOR KUMAR

10.1 Introduction 189 10.2 Synthesis of PdCuO Hybrid Nanoparticles 190 10.3 The Chemical Reactions Involved in the Synthesis of Hybrid Nanoparticles 191 10.4 Concluding Remarks 193 References 194

11. Laser-Induced Heating Synthesis of Hybrid Nanoparticles RINA SINGH AND R.K. SONI

11.1 Introduction 195 11.2 Experimental Methodologies 205 11.3 Hybrid Nanoparticles Synthesized by Laser Heating 208 11.4 Hybrid Nanoparticles Synthesized by Two-Step Laser Ablation 218 11.5 Trimetallic Hybrid Nanoparticles 225 11.6 Summary 233 References 234

PART II 163

9. Chemical Methods for Synthesis of Hybrid Nanoparticles BALAKRISHNAN KARTHIKEYAN, R. GOVINDHAN AND M. AMUTHEESAN

9.1 9.2 9.3 9.4 9.5 9.6

9.7 Wet-Chemical Synthesis 185 9.8 Hydrothermal/Solvothermal Method 9.9 Concluding Remarks 186 References 187

Introduction 179 Seed Growth Method 180 Coprecipitation Method 181 Sonochemical Synthesis 181 SolGel Method 182 Photochemical Method 184

APPLICATIONS 12. Hyperthermia Treatments GEETA NIJHAWAN, SIDDHARTH SAGAR NIJHAWAN AND MINU SETHI

12.1 Overview 241 12.2 Physical Fundamentals 243 12.3 Magnetic-Induced Thermal Cancer Therapy 254 12.4 Photo-Induced Thermal Cancer Therapy 257 12.5 Application of Noble MetalFe3O4 Hybrid Nanoparticles for Dual Magnetic Photothermal Cancer Therapy 258

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CONTENTS

15.2 Role of Fe3O4 in Antibacterial Action of AgFe3O4 Nanoparticles and Antibacterial Agents Based on Magnetic AgFe3O4 Nanoparticles 305 15.3 Role of MnO2 in Antibacterial Action of AgMnO2 Nanoparticles and Antibacterial Agents Based on AgMnO2 309 15.4 Concluding Remarks 310 References 311

12.6 Challenging Problems 259 12.7 Conclusions 260 References 260

13. Optical Absorption Modeling of Plasmonic Organic Solar Cells Embedding AgSiO2 CoreShell Nanoparticles KEKELI N’KONOU AND PHILIPPE TORCHIO

16. Noble MetalManganese Oxide Hybrid Nanocatalysts

13.1 Introduction 265 13.2 Mechanism of the Optical Absorption Enhancement in Plasmonic Organic Solar Cells 266 13.3 Why Coat Metal Nanoparticles (MNPs) with a Dielectric Shell? 269 13.4 Numerical Model 269 13.5 Results and Discussion 272 13.6 Case of the Dye-Sensitized Solar Cells (DSSCs) and Perovskite Solar Cells (PSCs) 278 13.7 Conclusion 279 Acknowledgment 279 References 279 Further Reading 282

SUJIT KUMAR GHOSH AND HASIMUR RAHAMAN

16.1 Introduction 313 16.2 The Chemistry of Manganese Oxides: Different Oxidation States 314 16.3 Noble MetalManganese Oxide Hybrids 315 16.4 Applications in Catalysis 316 16.5 Concluding Remarks and Future Outlook 331 References 333

17. Smart Coatings

14. Noble MetalsMetal Oxide Mesoporous Nanohybrids in Humidity and Gas Sensing Applications VIJAY K. TOMER, RITU MALIK, VANDNA CHAUDHARY, ARABINDA BARUAH AND LORENZ KIENLE

14.1 Introduction 283 14.2 Materials for Humidity and Gas Sensors 14.3 AgSnO2/SBA-15 Nanohybrids-Based Humidity Sensors 286 14.4 Mesoporous AgTiO2/SnO2 Nanohybrids-Based Gas Sensors 292 14.5 Conclusion 298 14.6 Future Outlook 299 References 299 Further Reading 302

284

15. Role of Oxides (Fe3O4, MnO2) in the Antibacterial Action of AgMetal Oxide Hybrid Nanoparticles R.K. KUNKALEKAR

15.1 Introduction

303

SARAH B. ULAETO, JERIN K. PANCRECIOUS, T.P.D. RAJAN AND B.C. PAI

17.1 Introduction 341 17.2 Classification of Smart Coatings 342 17.3 Applications and Commercial Viability of Smart Coatings 363 17.4 Conclusion 363 17.5 Sources of Further Information 365 Acknowledgments 365 References 366

18. Photocatalytic Application of Ag/TiO2 Hybrid Nanoparticles FRANCESCA PETRONELLA, ALESSANDRA TRUPPI, MARINELLA STRICCOLI, M. LUCIA CURRI AND ROBERTO COMPARELLI

18.1 Introduction 373 18.2 Ag/TiO2 Hybrid Nanoparticles for Environmental Application 376 18.3 Energy Production Mediated by Ag/TiO2 Hybrid Nanoparticles 382 18.4 Multifunctional Ag/TiO2 Hybrid Nanoparticles for Quality Life Improvement 386

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CONTENTS

18.5 Conclusions References 390

389

19. Noble MetalTransition Metal Oxides/Hydroxides: Desired Materials for Pseudocapacitor RAMKRISHNA SAHOO, ANJALI PAL AND TARASANKAR PAL

19.1 Introduction 395 19.2 Fundamentals of TMOs and TMHs Pseudocapacitor 396 19.3 Single Transition Metal Oxides or Hydroxides (TMOs) 397 19.4 Mixed Transition Metal Oxides (MTMOs) and Mixed Transition Metal Hydroxides (MTMHs) 411 19.5 Hybrid Materials 413 19.6 Noble MetalTransition Metal Oxide/Hydroxide Hybrid Based Materials 419 19.7 Conclusion 425 References 426

20. Applications of Hybrid Nanoparticles in Biosensors: Simulation Studies YUANKAI TANG, XIANTONG YU, JIANHUA XU, BENJAMIN AUDIT AND SANJUN ZHANG

20.1 Introduction 431 20.2 Fundamental Theory of Hybrid Nanoparticles 432 20.3 Simulation Methods 437 20.4 Applications 443 20.5 Summary and Outlook 451 Acknowledgments 451 References 451

21. SERS Application of Noble MetalMetal Oxide Hybrid Nanoparticles VIPUL SHARMA, RAMACHANDRAN BALAJI, NISHA KUMARI AND VENKATA KRISHNAN

21.1 Introduction 457 21.2 Noble Metal Nanoparticle Based SERS Platforms 459

21.3 Metal Oxide Nanostructures in SERS 461 21.4 Noble MetalMetal Oxide NanohybridsBased SERS Substrates 462 21.5 Summary and Outlook 480 Acknowledgment 482 References 482

22. Plasmonic Perovskite Solar Cells Utilizing Noble MetalMetal Oxide Hybrid Nanoparticles NILESH KUMAR PATHAK, P. SENTHIL KUMAR AND R.P. SHARMA

22.1 Introduction 487 22.2 Theoretical Analysis 489 22.3 Results and Discussion 492 22.4 Conclusion 496 Acknowledgment 496 References 496

23. Hydrogen Gas-Sensing Application of Au@In2O3 CoreShell Hybrid Nanoparticles RAMA KRISHNA CHAVA

23.1 Introduction 499 23.2 Synthesis and Characterizations of Au@In2O3 CoreShell Hybrid Nanoparticles 501 23.3 Hydrogen Gas-Sensing Application 23.4 Conclusions 513 References 513

507

24. Development of CeO2and TiO2-Based Au Nanocatalysts for Catalytic Applications RAJASHREE BORTAMULY, ABU TALEB MIAH AND PRANJAL SAIKIA

24.1 Introduction 517 24.2 Synthesis of CeO2- and TiO2-Based Au Nanocatalysts 518 24.3 Catalytic Applications 519 24.4 Conclusions 529 References 529

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CONTENTS

25. Radiolabeled Theranostics: Magnetic and Gold Hybrid Nanoparticles AYUOB AGHANEJAD AND YADOLLAH OMIDI

25.1 25.2 25.3 25.4 25.5

Introduction 535 Imaging Modalities 536 Radiolabeled Hybrid AuNPs 537 Radiolabeled Hybrid MNPs 541 Radiolabeled AuFe3O4 Hybrid Nanoparticles 543 25.6 Conclusion 544 Acknowledgment 544 References 545

26. Noble MetalManganese Oxide Nanohybrids Based Supercapacitors THUY T.B. HOANG

26.1 Introduction 549 26.2 AgMnO2 Nanohybrids-Based Supercapacitors 550 26.3 AuMnO2 Nanohybrids-Based Supercapacitors 556 26.4 Concluding Remarks 558 References 561

27. Palladium-Based Hybrid Nanocatalysts: Application Toward Reduction Reactions BIRAJ JYOTI BORAH, MANOJ MONDAL AND PANKAJ BHARALI

27.1 Introduction 565 27.2 Oxygen Reduction Reaction (ORR) 566 27.3 Reduction of Organic Substrates 572 27.4 Conclusions 576 Acknowledgments 577 References 577

28. Photoelectrochemical Water Splitting PRABHAKARN ARUNACHALAM AND ABDULLAH M. AL MAYOUF

28.1 Introduction 585 28.2 Principles of PEC Water Splitting Process 587 28.3 Photoanode Materials 589 28.4 Noble MetalMetal Oxide Nanohybrids-Based Photoanode 597 28.5 Conclusion 597 References 599

29. Theranostic Application of Fe3O4Au Hybrid Nanoparticles S. RAJKUMAR AND M. PRABAHARAN

29.1 Introduction 607 29.2 Design and Synthesis of Fe3O4Au Hybrid NPs 608 29.3 Theranostic Application of Fe3O4Au Hybrid NPs 612 29.4 Concluding Remarks 620 Acknowledgments 620 References 620

30. Synthesis and Application of AuFe3O4 Dumbbell-Like Nanoparticles XUEPING ZHANG AND SHAOJUN DONG

30.1 Introduction 625 30.2 Synthesis of AuFe3O4 Dumbbell-Like Nanoparticles 626 30.3 Optical and Magnetic Properties 630 30.4 Potential Applications 633 30.5 Conclusions 641 References 641

Index 645

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List of Contributors Ayuob Aghanejad Research Center for Pharmaceutical Nanotechnology, Tabriz University of Medical Sciences, Tabriz, Iran Abdullah M. Al Mayouf Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia M. Amutheesan Department of Aeronautical Engineering, Hindustan Institute of Technology & Science, Chennai, Tamil Nadu, India Prabhakarn Arunachalam Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia Aymen Amine Assadi E´cole Nationale Supe´rieure de Chimie de Rennes, Rennes, France Benjamin Audit Univ Lyon, Ens de Lyon, Univ Claude Bernard Lyon 1, CNRS, Laboratoire de Physique, Lyon, France Ramachandran Balaji School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh, India Arabinda Baruah Indian Institute of Science Education and Research, Mohali, Punjab, India Rajendra N. Basu Fuel Cell & Battery Division CSIR-Central Glass and Ceramics Research Institute, Kolkata, West Bengal, India Pankaj Bharali Department of Chemical Sciences, Tezpur University, Napaam, Assam, India Biraj Jyoti Borah Department of Chemical Sciences, Tezpur University, Napaam, Assam, India Rajashree Bortamuly Department of Applied Sciences (Chemical Science Division), Gauhati University, Guwahati, Assam, India Pascal Carriere Laboratoire MAPIEM (EA 4323), Materiaux Polymeres Interfaces Environnement Marin, Universite de Toulon, Toulon, France Vandna Chaudhary Center of Excellence for Energy and Environment Studies, D.C.R. University of Science & Technology, Murthal (Sonepat), Haryana, India Rama Krishna Chava Department of Chemistry, College of Natural Sciences, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea Roberto Comparelli CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico Fisici, S.S. Bari, Italy M. Lucia Curri CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico Fisici, S.S. Bari, Italy Shaojun Dong State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, P.R. China; University of Chinese Academy of Sciences, Beijing, P.R. China

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LIST OF CONTRIBUTORS

Srabanti Ghosh Fuel Cell & Battery Division CSIR-Central Glass and Ceramics Research Institute, Kolkata, West Bengal, India Sujit Kumar Ghosh Department of Chemistry, Assam University, Silchar, India R. Govindhan Department of Chemistry, Annamalai University, Chidambaram, Tamil Nadu, India Thuy T.B. Hoang School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam Balakrishnan Karthikeyan Department of Chemistry, Annamalai University, Chidambaram, Tamil Nadu, India Antoine Khater Institute of Physics, Jan Dlugosz University, Czestochowa, Poland; Department of Physics, Universite´ du Maine, Le Mans, France Lorenz Kienle Synthesis & Real Structure Group, Technical Faculty, Institute for Materials Science, Kiel University, Kiel, Germany M.J. Kishor Kumar Department of Chemical Engineering, National Institute of Technology, Surathkal, Karnataka, India Venkata Krishnan School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh, India Nisha Kumari School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh, India R.K. Kunkalekar Parvatibai Chowgule College of Arts and Science (autonomous), Margao, Goa, India Weihua Lin Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, P.R. China Dongling Ma Institut National de la Recherche Scientifique (INRS), Center of Energy, Materials and Telecommunications, Varennes, QC, Canada Ritu Malik Synthesis & Real Structure Group, Technical Faculty, Institute for Materials Science, Kiel University, Kiel, Germany Abu Taleb Miah Department of Applied Sciences (Chemical Science Division), Gauhati University, Guwahati, Assam, India Manoj Mondal Department of Chemical Sciences, Tezpur University, Napaam, Assam, India The Huu Nguyen Faculty of Chemical Technology, Hanoi University of Industry, Bac Tu Liem, Hanoi, Vietnam Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam Phuong Nguyen Tri Department of Chemistry, University of Montre´al, Montre´al, QC, Canada; Ecole de Technologie Superieure, University of Quebec, Quebec City, QC, Canada; De´partement de ge´nie de la construction, E´cole de Technologie Supe´rieure, Montre´al, QC, Canada Geeta Nijhawan Manav Rachna International University, Faridabad, Haryana, India Siddharth Sagar Nijhawan Netaji Subhas Institute of Technology, Delhi University, New Delhi, India

LIST OF CONTRIBUTORS

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Kekeli N’Konou Aix-Marseille Universite´, Institut Mate´riaux Microe´lectronique Nanosciences de Provence  IM2NP, CNRS-UMR 7334, Domaine Universitaire de Saint-Je´roˆme, Marseille Cedex, France Yadollah Omidi Research Center for Pharmaceutical Nanotechnology, Tabriz University of Medical Sciences, Tabriz, Iran; Department of Pharmaceutics, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran Claudiane Ouellet-Plamondon De´partement de ge´nie de la construction, E´cole de Technologie Supe´rieure, Montre´al, QC, Canada B.C. Pai CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala, India Anjali Pal Department of Civil Engineering, Indian Institute of Technology, Kharagpur, West Bengal, India Tarasankar Pal Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India Jerin K. Pancrecious CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala, India Nilesh Kumar Pathak Department of Physics & Astrophysics, University of Delhi, New Delhi, India; Plasma and Plasmonic Simulation Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India Francesca Petronella CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico Fisici, S.S. Bari, Italy M. Prabaharan Department of Chemistry, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu, India Hasimur Rahaman Department of Science and Humanities, Contai Polytechnic, Purba Medinipur, West Bengal, India T.P.D. Rajan CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala, India S. Rajkumar Department of Chemistry, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu, India Sami Rtimi Swiss Federal Institute of Technology School of Engineering (STI), Powder Technology Laboratory (LTP), EPFL-STI-IMX-LTP, Lausanne, Switzerland Ramkrishna Sahoo Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India Pranjal Saikia Department of Applied Sciences (Chemical Science Division), Gauhati University, Guwahati, Assam, India Sougata Sarkar Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Kolkata, West Bengal, India P. Senthil Kumar Department of Physics & Astrophysics, University of Delhi, New Delhi, India Minu Sethi Manav Rachna International University, Faridabad, Haryana, India R.P. Sharma Plasma and Plasmonic Simulation Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India Vipul Sharma School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh, India

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LIST OF CONTRIBUTORS

Rina Singh Environment Science Division, CSIR-Central Road Research Institute, New Delhi, India S. Sivasankaran Manipal Institute of Technology, Manipal Academy of Higher Education (MAHE), Manipal, India R.K. Soni Physics Department, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India Marinella Striccoli CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico Fisici, S.S. Bari, Italy Mengtao Sun Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, P.R. China Yuankai Tang State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, P.R. China Vijay K. Tomer Berkeley Sensor & Actuator Center, University of California, Berkeley, CA, United States; Synthesis & Real Structure Group, Technical Faculty, Institute for Materials Science, Kiel University, Kiel, Germany Philippe Torchio Aix-Marseille Universite´, Institut Mate´riaux Microe´lectronique Nanosciences de Provence  IM2NP, CNRS-UMR 7334, Domaine Universitaire de Saint-Je´roˆme, Marseille Cedex, France Alessandra Truppi CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico Fisici, S.S. Bari, Italy Sarah B. Ulaeto CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala, India Minh Thanh Vu Institute of Chemistry and Materials, Hanoi, Vietnam Jianhua Xu State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, P.R. China Xiantong Yu State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, P.R. China Sanjun Zhang State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, P.R. China; Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, P.R. China; NYU-ECNU Institute of Physics at NYU Shanghai, Shanghai, P.R. China Xueping Zhang State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, P.R. China; University of Chinese Academy of Sciences, Beijing, P.R. China

P A R T

I

FUNDAMENTALS

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C H A P T E R

1 Hybrid Nanoparticles: An Introduction Dongling Ma Institut National de la Recherche Scientifique (INRS), Center of Energy, Materials and Telecommunications, Varennes, QC, Canada

1.1 INTRODUCTION During the past three decades, nanomaterials have attracted tremendous research interest because of their unique properties, mainly arising from the quantum confinement effect and extremely large surface-to-volume ratios [1,2], which offer new routes to address challenging and pressing issues facing humanity, such as the growing demand of green energy, the shortage of sufficient clean water for everyone, the lack of ultrasensitive early-stage cancer diagnostic tools and effective personalized therapeutics, and the everdeteriorating global environment, just to name a few. The most prominent example of nanomaterials is perhaps semiconductor nanocrystals, also known as quantum dots, which show size-tunable bandgap and thereby size-tunable optical properties [3,4]. In particular, they possess marked advantages of broader excitation, narrower emission, brighter photoluminescence, and higher stability as compared to conventional imaging probes and thus hold high potential for bioimaging and cancer detection [5]. These remarkable properties also make them excellent candidate materials for energy- and environment-related applications, such as solar cells, luminescent solar concentrators, photocatalytic degradation of pollutants, solar water splitting, and lightemitting devices [69]. Plasmonic metal nanoparticles, such as Au and Ag, represent another type of highly interesting nanomaterial. With unique surface plasmon resonance, originating from the resonant oscillation of free electrons at the metal/dielectric interface, they distinguish themselves by intense absorption and/or scattering at tunable resonance wavelengths, as well as largely amplified local electric fields, especially at sharp tips, corners, and coupling space between two adjacent plasmonic nanoparticles [10,11]. These

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00001-2

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© 2019 Elsevier Inc. All rights reserved.

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1. HYBRID NANOPARTICLES: AN INTRODUCTION

outstanding attributes endow them with the capability of acting as powerful antennas, which largely stimulated the development of the area of surface-enhanced Raman scattering that can find many important applications in biosensors and environmental sensors, as well as promises a broad range of other applications related to photonics and optoelectronics [12]. Another striking characteristic, unique to nanomaterials, is superparamagnetism of magnetic nanoparticles that are of single-magnetic-domain size [13]. Each individual nanoparticle behaves like a giant paramagnetic atom with a fast response to applied magnetic fields with negligible remanence and coercivity, while simultaneously having a large magnetic moment. The invention of superparamagnetic nanoparticles initiated new avenues in medical diagnostics, targeted drug delivery, and hyperthermia treatment [14]. The properties of nanomaterials highly rely on their size, but also their shape and other structure details (e.g., surface chemistry and porosity). As such, immense efforts were initially mainly devoted to achieving precise structural control of single-component nanomaterials, uncovering the growth mechanism of specific structures and elucidating their properties [15,16]. Great progress was soon made in the synthesis of nanoparticles in a controlled fashion in the 1990s, such as the invention of the organometallic approach for the reliable synthesis of high-quality quantum dots land-marked by Murray et al. [16]. It now already becomes routine to controllably synthesize a wide variety of singlecomponent nanoparticles, including, but not limited to, CdSe and PbS quantum dots, Au nanoparticles, and Fe superparamagnetic nanoparticles, in many research laboratories. The paradigm of research focus was subsequently shifted toward the development of hybrid nanostructures, which are constructed from at least two different materials, in order to overcome the limits of single components, to improve properties, to achieve new properties not possible for single-component nanoparticles, and/or to achieve multiple functionalities for single nano-architectures. Diverse hybrid nanostructures, such as coreshell, yolkhell, heterodimer, Janus, dot-in-nanotube, dot-on-nanorod, nanobranches, etc., have been designed and synthesized, and some are illustrated in Fig. 1.1

FIGURE 1.1 Different types of hybrid nanostructures: (A) single-core/single-shell nanoparticles; (B) multiple-core/single-shell nanoparticles; (C) single-core/multiple-shell nanoparticles; (D) yolkshell nanoparticles; (E) heterodimer nanoparticles; (F) Janus nanoparticles; and (G) nanoparticle decorated one-dimensional nanostructures. Source: (AD) Reused with permission from M.R. Kim, Z. Xu, G. Chen, D. Ma, Semiconductor and metallic coreshell nanostructures: synthesis and applications in solar cells and catalysis, Chem. Eur. J. 20 (2014) 1125611275. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

I. FUNDAMENTALS

REFERENCES

5

[1721]. As for materials, a great number of combinations, such as magnetic/luminescent, plasmonic/catalytic, magnetic/catalytic, have been attempted and realized, hugely expanding the library of functional materials and even leading to new research areas. For instance, the integration of plasmonic metal nanoparticles with semiconductor materials directly prompted the emergence of two highly promising fields, plasmon-enhanced photovoltaics [22] and plasmon-enhanced photocatalysis [23]. Plasmonic nanostructures are anticipated to contribute to solar cell and photocatalyst performance mainly by enhanced electronhole pair generation rates due to the near field effect and elongation of the average photon path length in semiconductors via the scattering effect [2426]. Similarly important, surface plasmon resonance-induced hot electron transfer from plasmonic nanostructures to semiconductor catalysts also plays a prominent role in boosting the photocatalytic activity in many plasmonicsemiconductor photocatalytic systems [2729]. The exact underlying enhancement mechanism depends on the structure and optical property of each individual nanocomponent as well as the way they are assembled. Therefore, the rational design, controlled synthesis, advanced characterizations, and in depth understanding of structureproperty relationships are all highly important in pushing forward the development of highly functional hybrid nanomaterials and their useful applications in the real world. The combined experimental/theoretical approach is highly demanded to rapidly advance this research area that promises to revolutionize many aspects of our life and society from health, to clean energy, and to the environment.

References [1] A.P. Alivisatos, Semiconductor clusters, nanocrystals, and quantum dots, Science 271 (1996) 933937. [2] G.C. Papaefthymiou, Nanoparticle magnetism, Nano Today 4 (2009) 438447. [3] X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, et al., Quantum dots for live cells, in vivo imaging, and diagnostics, Science 307 (2005) 538544. [4] W.W. Yu, X. Peng, Formation of High-Quality CdS and Other IIVI Semiconductor Nanocrystals in Noncoordinating Solvents: Tunable Reactivity of Monomers, Angew. Chem. Int. Ed. 41 (2002) 23682371. [5] W.C.W. Chan, S. Nie, Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection, Science 281 (1998) 20162018. [6] M.R. Kim, D. Ma, Quantum-Dot-Based Solar Cells: Recent Advances, Strategies, and Challenges, J. Phys. Chem. Lett. 6 (2015) 8599. [7] Y. Zhou, H. Zhao, D. Ma, F. Rosei, Harnessing the Properties of Colloidal Quantum Dots in Luminescent Solar Concentrators, Chem. Soc. Rev. (2018). Invited. [8] M. Gratzel, Photoelectrochemical Cells, Nature 414 (2001) 338344. [9] X. Yang, F. Ren, Y. Wang, T. Ding, H. Sun, D. Ma, et al., Iodide capped PbS/CdS core-shell quantum dots for efficient long-wavelength near-infrared light-emitting diodes, Sci. Rep. 7 (2017) 14741. [10] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment, J. Phys. Chem. B 107 (2003) 668677. [11] P.K. Jain, W. Huang, M.A. El-Sayed, On the Universal Scaling Behavior of the Distance Decay of Plasmon Coupling in Metal Nanoparticle Pairs: A Plasmon Ruler Equation, Nano Lett. 7 (2007) 20802088. [12] Y.C. Cao, R. Jin, C.A. Mirkin, Nanoparticles with Raman Spectroscopic Fingerprints for DNA and RNA Detection, Science 297 (2002) 15361540. [13] J. Frenkel, J. Dorfman, Spontaneous and Induced Magnetisation in Ferromagnetic Bodies, Nature 126 (1930) 274275. [14] J.H. Lee, Y.M. Huh, Y.W. Jun, J.W. Seo, J.T. Jang, H.T. Song, et al., Artificially engineered magnetic nanoparticles for ultra-sensitive molecular imaging, Nat. Med. 13 (2007) 9599.

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1. HYBRID NANOPARTICLES: AN INTRODUCTION

[15] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Shape-Controlled Synthesis of Colloidal Platinum Nanoparticles, Science 272 (1996) 19241925. [16] C.B. Murray, D.J. Norris, M.G. Bawendi, Synthesis and characterization of nearly monodisperse CdE (E 5 sulfur, selenium, tellurium) semiconductor nanocrystallites, J. Am. Chem. Soc. 115 (1993) 87068715. [17] F. Yang, A. Skripka, A. Benayas, X. Dong, S.H. Hong, F. Ren, et al., An Integrated Multifunctional Nanoplatform for Deep-Tissue Dual-Mode Imaging, Adv. Funct. Mater. 28 (2018) 1706235. [18] M.R. Kim, Z. Xu, G. Chen, D. Ma, Semiconductor and metallic coreshell nanostructures: synthesis and applications in solar cells and catalysis, Chem. Eur. J. 20 (2014) 1125611275. Available from: https://doi. org/10.1002/chem.201402277. [19] M. Lattuada, T.A. Hatton, Synthesis, properties and applications of Janus nanoparticles, Nano Today 6 (2011) 286308. [20] A. Benayas, F. Ren, E. Carrasco, V. Marzal, B. del Rosal, B. Gonfa, et al., PbS/CdS/ZnS Quantum Dots: A Multifunctional Platform for In Vivo Near-Infrared Low-Dose Fluorescence Imaging, Adv. Funct. Mater. 25 (2015) 66506659. [21] L. Weng, H. Zhang, A.O. Govorov, M. Ouyang, Hierarchical synthesis of non-centrosymmetric hybrid nanostructures and enabled plasmon-driven photocatalysis, Nat. Commun. 5 (2014) 4792. [22] H.A. Atwater, A. Polman, Plasmonics for improved photovoltaic devices, Nat. Mater. 9 (2010) 205213. [23] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911921. [24] B.A. Gonfa, M.R. Kim, P. Zheng, S. Cushing, Q. Qiao, N. Wu, et al., Investigation of the plasmonic effect in air-processed PbS/CdS coreshell quantum dot based solar cells, J. Mater. Chem. A 4 (2016) 1307113080. [25] S.T. Kochuveedu, Y.H. Jang, D.H. Kim, A study on the mechanism for the interaction of light with noble metal-metal oxide semiconductor nanostructures for various photophysical applications, Chem. Soc. Rev. 42 (2013) 84678493. [26] S.K. Cushing, J. Li, F. Meng, T.R. Senty, S. Suri, M. Zhi, et al., Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor, J. Am. Chem. Soc. 134 (2012) 1503315041. [27] Z. Xu, M. Quintanilla, F. Vetrone, A.O. Govorov, M. Chaker, D. Ma, Harvesting Lost Photons: Plasmon and Upconversion Enhanced Broadband Photocatalytic Activity in Core@Shell Microspheres Based on Lanthanide-Doped NaYF4, TiO2, and Au, Adv. Funct. Mater. 25 (2015) 29502960. [28] Q. Zhang, J. Deng, Z. Xu, M. Chaker, D. Ma, High-Efficiency Broadband C3N4 Photocatalysts: Synergistic Effects from Upconversion and Plasmons, ACS Catal. 7 (2017) 62256234. [29] A.O. Govorov, H. Zhang, H.V. Demir, Y.K. Gun’ko, Photogeneration of hot plasmonic electrons with metal nanocrystals: Quantum description and potential applications, Nano Today 9 (2014) 85101.

I. FUNDAMENTALS

C H A P T E R

2 Theoretical Aspects of Synthesis for Controlled Morphological Nanostructures Sougata Sarkar1 and Tarasankar Pal2 1

Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Kolkata, West Bengal, India 2Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India

2.1 NUCLEATION AND GROWTH OF NANOPARTICLES: THEORETICAL PERSPECTIVES Different existing methods of fabrication towards the growth of nanoparticles, nanocrystals, and quantum dots are worth mentioning, where each method is more ideal for the generation of one single category of nanoparticle having a different size and shape. Little changes in the experimental parameters can considerably affect the properties of the nanomaterials. And hence, the growth mechanisms of them are often difficult to understand in detail and therefore offer a challenge to the scientific community to find out the same. This understanding of the mechanism leading to the growth of the particles in the nanoregime is important for both scientific and for technological considerations. For many years, there has been a common acceptance that the nucleation and growth of nanoparticles can be analyzed by LaMer burst nucleation and a diffusion limited Ostwald ripening process. So here we will highlight the theoretical aspects of nucleation and growth of nanoparticles following the anticipated theoretical frameworks as described below.

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00002-4

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© 2019 Elsevier Inc. All rights reserved.

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

2.1.1 LaMer Theory The first mechanism was the LaMer mechanism, which has the conceptual separation of the nucleation and growth into two stages. Using this concept LaMer developed an understanding of the mechanism for the formation of colloids or nanocrystals from a homogeneous, supersaturated medium. They studied the synthesis of sulfur sols from the decomposition of sodium thiosulfate, consisting of two steps: the first to form free sulfur from the thiosulfate and the second form sulfur sols in solution. The mechanism suggested by LaMer encompasses the following routes: (a) synthesis of the colloid should be premeditated in such a way that the concentration of the free monomers increases rapidly, rising above the saturation concentration for a brief period; (b) the monomers undergo a short burst of nucleation with the formation of a large number of nuclei in a short space of time which lowers the concentration of the monomers below the nucleation level; (c) following nucleation of these particles, the growth occurs rapidly through diffusion of the particles [1]. LaMer’s theory can be nicely addressed with Fig. 2.1. Here the concentration of the monomers has been plotted against the function of time. The plot actually manifests the above key points. As understood from this diagram for the monodispersed growth of the particles, the burst of the monomer should occur in a short period leading to the formation of nuclei followed by initial fast rate of growth of these nuclei to reduce the concentration below the nucleation concentration and finally a slow rate of growth for a long period compared to the nucleation period. This theory was then applied with success for the derivation of the rate of growth. The method was also claimed to be important for the estimation of diffusion coefficients for any colloidal system having small particle size distribution at all stages of growth. Sugimoto et al. have reported the validation of this theory during the spontaneous nucleation of a monodisperse AgCl system containing a silvergelatin complex as the source of Ag(I) ion, chloride ion, and dimethyl sulfate. Still this theory of nucleation does not turn out to be a generalized approach for a wide variety of systems. Besides the LaMer’s nucleation theory, it has been later observed that the specific growth process also plays an important role in the synthesis of monodispersed particles.

FIGURE 2.1 Schematic presentation of LaMer diagram. Source: Reprinted with permission from T. Sugimoto, J. Colloid Interface Sci. 309 (2007) 106118. Copyright 2007 Elsevier.

I. FUNDAMENTALS

2.2 MECHANISM OF GROWTH AND LIFSHITZSLYOZOVWAGNER (LSW) THEORY

9

2.2 MECHANISM OF GROWTH AND LIFSHITZSLYOZOVWAGNER (LSW) THEORY From the above discussed LaMer theory we have gathered a general concept regarding the nucleation process where it has been observed that nucleation occurs over some time with constant monomer concentration. Then surface growth of the nucleated clusters start and the monomer concentration begins to fall, and eventually it falls below the critical supersaturation level and then the process comes to an end. Therefore the mechanism of the growth process then becomes an intriguing parameter for the synthesis of the nanoparticles in solution and hence needs to be understood. It is well known that the surface area to volume ratio is reasonably high for smaller particles and therefore the surface excess energy ought to be a key factor for tiny particles. During the growth of the nanocrystal from a solution, which is not in the thermodynamic equilibrium stage initially, the monomers present in the solution diffuse towards each other through their surface in order to lower their surface energy and thus larger particles (or nanocrystal) are formed at the cost of the smaller particles (monomers). The growth continues through the reaction of the monomers on the surface of these preformed nanocrystals. This coarsening effect of monomers attachment during the growth process is majorly controlled by either the diffusion or by mass transport and is often termed as the Ostwald ripening process [2]. The process was first introduced by Wilhelm Ostwald in 1896 in a book published in German for which the first English representation appeared in 1900. And this is an observed phenomenon in solid solutions or liquid sols which actually describes the change of an inhomogeneous structure over time, i.e., small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles. This thermodynamically-driven spontaneous process occurs because larger particles are more energetically stable than smaller particles (as their internal pressure is reversely proportional to the radius of the particles). This indicates that the molecules on the surface of a particle, which are coordinatively unsaturated, are energetically less stable than the ones already well ordered and packed in the interior (which are coordinatively saturated). Large particles, with their lower surface to volume ratio, result in a lower energy state (and have a lower surface energy). As the system tries to lower its overall energy, molecules on the surface of a small (energetically unfavorable) particle will tend to detach and diffuse through solution and then attach to the surface of a larger particle. Therefore, the number of smaller particles continues to shrink, while larger particles continue to grow. Ostwald ripening occurs mostly in colloids, but also in emulsions, alloys, and other fluid systems where two phases are separating. A quantitative treatment of the Ostwald ripening process was first developed in April 1961 when Lifshitz and Slyozov from the USSR published their theory of precipitation [3]. But in the same year in September, Wagner from the Max Planck Institute submitted a theory of Ostwald ripening which cited the Lifshitz and Slyozov paper and provided the exact same quantitative conclusions [4]. However, Wagner used a very different mathematical treatment to solve the kinetics of the ripening process, and he also extended the treatment to examine the reaction limited ripening, which Lifshitz and Slyozov neglected. Wagner’s treatment is more approachable, while Lifshitz and Slyozov’s treatment is more mathematically rigorous, though both of these treatments provide identical quantitative

I. FUNDAMENTALS

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

size distribution functions and growth laws. The unified theory of the trio on the precipitation and Ostwald ripening is more commonly known as “LSW theory” [5].

2.2.1 GibbsThomson Effect Now before going into the details of this LSW theory, we will briefly discuss another theoretical perspective, the GibbsThomson effect which deciphers the thermodynamic basis for particles grown from a solution [6]. The diffusion process of the particles is dominated by their surface energy. The interfacial energy is the energy associated with an interface due to differences between the chemical potential of atoms in an interfacial region and atoms in neighboring bulk phases (or condensed phases). When this interface is flat, then addition or removal of materials only cause a change in the volume of the bulk phase whereas for a curved interface, adding new material to a finite cluster causes a change in both the volume as well as in the surface of the cluster. Hence the chemical potential of material in the cluster will depend on its size. If the free energy of a spherical cluster contains a volume term and a surface term as below: G5

4πR3 gcond 1 4πR2 γ 3v

(2.1)

then the standard chemical potential of the solute in such a particle is given by the following equation:     @G @G v 2γv 0 (2.2) 5 5 gcond 1 μR 5 2 @N T;P @R T;P 4πR R From this equation we can predict the solubility near the curved surface with radius of curvature R:   gcond 2γv CR 5 exp 1 (2.3) 5 CN eð2γv=RkTÞ RkT kT This above equation is referred as the GibbsThomson equation. From this equation it could be understood that the high surface-to-volume ratio of the smallest clusters (or a curved surface) makes them exponentially more amenable to dissolving than a larger cluster (or a flat surface). As we have previously stated, for any crystal, a molecule on the surface always coordinatively remains unsaturated due to coordination through fewer neighbor molecules than a molecule in the interior phase which remains mostly coordinatively saturated, i.e., the molecules in the bulk, on a face, at an edge, and at a vertex all have different coordination numbers. A bulk molecule has the most neighbors (i.e., 6 in a cubic crystal or 12 in an f.c.c. crystal), a vertex molecule has the fewest (i.e., 3 in cubic or 5 in f.c.c.), and molecules on a face and at an edge have a coordination number in between. Principally, the molecules in the vertex are the weakest-bound molecules and are thus most likely to desorb. Now with the lowering of the radius for smaller nanocrystals or particles with curved surface, the fraction of edge- or vertex- molecules increases, and simultaneously the probability of the surface molecules desorbing from the crystal increases accordingly.

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2.2 MECHANISM OF GROWTH AND LIFSHITZSLYOZOVWAGNER (LSW) THEORY

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2.2.2 LifshitzSlyozovWagner (LSW) Theory The theory has been nicely interpreted by different research groups [7]. We have discussed the theory here based on the interpretations. This theory was a quantitative approach to the Ostwald’s ripening process and majorly considers a diffusion limited growth of the nanocrystallites. There are some basic assumptions to be considered. For example: The coarsening second phase is spherical having radius R. The particles are fixed in space. It considers the growth of the particles in a supersaturated solution. Particles are assumed to grow or shrink only in relation to the mean field concentration set at infinity. 5. The total mass of the solute is conserved. 6. The interparticle distance between the particles are infinitely large compared with the particle radius and hence there would be no interaction among the particles. 7. The solute atoms diffuse to the particles spherical particles under steady state conditions. 1. 2. 3. 4.

If f is considered as the number of particles per unit volume at time t in a size class R to R 1 dR then the radius distribution of the spherical particles are given by f(R,t) and we have in hand the following equation: ðN fn 5 Rn f ðR; tÞdR (2.4) 0

Thus the flux of particles within the size class R and R 1 dR is f. dR/dt. Therefore the time rate of change of f is given by the equation: _ @f @ðf∙RÞ 1 5J @t @R

(2.5)

Now in LSW theory, J is set to zero so that there would be no interaction among the particles. Finally considering the assumptions, this theory reaches an asymptotic solution and makes the following solutions concerning the behavior of the two-phase mixtures experiencing the Ostwald ripening process:   4 1=3 3 R ðtÞ5 R ð0Þ1 t 9   4 2ð1=3Þ θm ðtÞ5 R3 ð0Þ1 t 9  N ðtÞ5ψ

R3 ð0Þ1

4 t 9

21

I. FUNDAMENTALS

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

t is defined as the beginning of the coarsening in the long-time regime. The overbar denotes an average and 4/9 is the dimensionless prefactor and denotes the coarsening rate, where ψ can be given as below: θ0 ψ 5 Ð 3=2 a 0 ρ3 gðρÞdρ Therefore, in summary, we have studied the general phenomenon particles nucleation and growth like burst nucleation, GibbsThomson effect, LSW theory, etc. to give a brief idea regarding the formation of nanoparticles or nanocrystals in solution.

2.3 STABILIZATION OF NANOPARTICLES: THE ROLE OF LIGANDS Colloidal systems are dispersed phases finely subdivided in a dispersion medium. This subdivision results in a very high interfacial surface area which determines largely the physical properties of the system. Colloidal particles in a dispersion medium always show Brownian motion and hence collide with each other frequently. The stability of colloids is thus determined by the interaction between the particles during such a collision. There are two basic interactions: one being attractive and the other repulsive. When attraction dominates, the particles will adhere with each other and finally the entire dispersion may coalesce but when repulsion dominates, the system will be stable and remain in a dispersed state. Van der Waals forces are the primary contribution for attraction between nanocolloidal particles. The term includes: permanent dipolepermanent dipole (Keesom) forces, the permanent dipoleinduced dipole (Debije) interactions, and transitory dipoletransitory dipole (London) forces. The first two are very short range interactions, but the London forces are longer range attractions. Therefore, a colloidal dispersion is said to be stable only when a sufficiently strong repulsive force counteracts the van der Waals attraction. A brief outline on stabilization of nanoparticles has been well documented in a report by Shi et al. [8]. Since there are always strong, long-range attractive forces between similar colloidal particles, it is necessary to provide a long-range repulsion between the particles to impart stability. This repulsion should be at least as strong as the attractive force and comparable in range of the attractive interaction. Stability can be obtained by surrounding colloidal particles: • With an electrical double layer (electrostatic or charge stabilization). • With adsorbed or chemically attached polymeric molecules (steric stabilization). • With free polymer in the dispersion medium (depletion stabilization). Combination of the first two stabilization mechanisms leads to electrosteric stabilization. The latter two types of stabilization are often realized by the addition of polymers to stabilize dispersions and are known as polymeric stabilization.

2.3.1 Electrostatic Stabilization As we have already stated, van der WaalsLondon (VDWL) attraction is decisive in determining strategies for stabilizing colloid particles. An effective way to counterbalance I. FUNDAMENTALS

2.3 STABILIZATION OF NANOPARTICLES: THE ROLE OF LIGANDS

13

this VDWL attraction between colloidal particles in polar liquids is to provide the particles with Coulombic repulsion. In liquid dispersion media, ionic groups can adsorb to the surface of a colloidal particle through different mechanisms to form a charged layer. To maintain electroneutrality, an equal number of counterions with the opposite charge will surround the colloidal particles and give rise to overall charge-neutral double layers. In charge stabilization, it is the mutual repulsion of these double layers surrounding particles that provides stability. It can be shown that most charge-stabilized dispersions coagulate when increasing the ionic strength of the dispersion medium. Hence, one great disadvantage of charge stabilization of particles is its great sensitivity to the ionic strength of the dispersion medium. In addition it only works in polar liquids which can dissolve electrolytes [9].

2.3.2 Polymeric Stabilization For polymers with molecular weights .10,000 Da, the chain dimensions are comparable to, or in excess of, the range of the VDWL attraction. Hence, as long as they can generate repulsion, these polymer molecules can be used to impart colloid stability. There are two different mechanisms accepted for such stabilization: steric stabilization and depletion stabilization. Steric stabilization of colloidal particles is achieved by fastening (grafting or chemisorption) macromolecules to the surfaces of the particles. The stabilization due to the adsorbed layers on the dispersed particle is generally called steric stabilization, whereas the depletion stabilization of colloidal particles is imparted by macromolecules that are free in solution [10].

2.3.3 Electrosteric Stabilization This kind of stabilization is generally achieved with either ionic surfactants or from polyoxoanions such as the couple tetrabutyl ammonium (Bu4N1)/polyoxoanion (P2W15Nb3O6292). The significant steric repulsion of the associated bulky Bu4N1 countercations associated with the highly charged polyoxoanions provide an efficient electrosterical stability toward agglomeration in solution of the resultant nanoclusters. Thus the origin may be a net charge on the particle surface and/or charges associated with the polymer attached to the surface (i.e., through an attached polyelectrolyte) [11]. Finally, the steric and electrostatic stabilization could be pictorially represented as shown in Fig. 2.2. Beyond the abovementioned stabilization methodologies, coordinating molecules have also been observed to be responsible for nanoparticle stabilization. Phosphines, Thiols, amines, acids, etc. belong to this category (Fig. 2.3). Therefore the above discussion gives us a brief idea regarding different types of interactions between the surface of the nanoparticles and the stabilizing ligands.

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.2 Schematic presentation of steric stabilization and electrostatic stabilization.

FIGURE 2.3 Schematic presentation of steric interaction of different ligands with the surface of a metallic nanoparticle.

2.4 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF NOBLE METAL NANOPARTICLES Noble metal nanoparticles are chemically described as the metallic nanoparticles which are mostly resistant to corrosion and aerial oxidation and these primarily include ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), and gold (Au). Though in atomic physics, only copper, silver, and gold are considered to be the noble metal nanoparticles having a completely filled d-subshell. Here the role of “Hard and Soft Acids and Bases (HSAB)” theory as described by Ralph G. Pearson in 1963 [12] may be considered to describe the interactions of the ligands with the metal nanomaterials surface. In this theory metal centers are considered as Lewis acids (LA) and the ligands may be considered as Lewis bases (LB) and they have been categorized as typically hard or soft and borderline hard or soft acids and bases. The theory predicts the propensity of product formation in a reaction or the feasibility of a reaction based on the softsoft LALB interactions and hardhard LALB interactions. The softhard interactions are said to be less preferred over softsoft and hardhard interactions.

I. FUNDAMENTALS

2.4 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF NOBLE METAL NANOPARTICLES

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Here we will consider the following category of ligands majorly employed for the stabilization of noble metal nanoparticles. A. Ligands with thiol functional group Thiol (RaSH; an organosulfur compound containing a carbon-bonded sulfhydryl group where R represents an alkyl or other organic substituent) has been recognized as one of the most popular ligands in stabilization of noble metal nanoparticles. As described already, these ligands bear the soft donor “S” site and hence the stabilization is principally driven by softsoft interactions. Way back to the BrustSchiffrin method (BSM), highly stable small size (22.5 nm) gold nanoparticles have been synthesized employing alkanethiol as the stabilizer [13]. In brief, first chloroauric acid (Au31) is phase transferred into toluene from an aqueous phase using a phase transfer catalyst such as tetraoctylammonium bromide (TOAB). Then the organic phase is separated and then the addition of a desired amount of dodecanethiol to the organic phase results in the reduction of the Au31 ion to Au1. This is indicated by the disappearance of the yellow color of the organic phase which transforms into colorless. Then an aqueous solution of sodium borohydride, the principal reducing agent, is injected into the organic phase. The particle formation is indicated by the change of color of the organic phase to pink/blue. The methodology can be schematically presented as shown in Fig. 2.4 [14]. Here it is believed that after phase transfer, the reduction of Au31 ion by the thiol results in the formation of polymeric units of Au1 coordinated through the S end of the thiol, i.e., (AuSR)n and simultaneously its oxidized product RSSR. In a recent article [14], Kumar et al. have reported a detailed survey related to the mechanistic approach of the BSM route. They have also used a thorough population balanced mathematical model to explain the formation mechanism in a newer approach, i.e., continuous nucleation, growth, and capping of particles throughout the synthesis process. Their proposed mechanism also differs from the already established mechanism in the literature like the classical LaMer mechanism, sequential nucleationgrowth-capping, and thermodynamic mechanism, etc. and is able to successfully explain key features of BSM, including size tuning by varying the amount of capping agent instead of the widely used approach of varying the amount of reducing agent. In a different report [15], Lennox has thoroughly investigated and confirmed that the

FIGURE 2.4 Synthesis of thiol capped gold nanoparticles in organic medium as described by Brust et al. Source: Reprinted with permission from S.R.K. Perala, S. Kumar, Langmuir 29 (2013) 98639873. Copyright 2013 American Chemical Society.

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Au(I)-thiolate species is not the intermediate in the two-phase synthesis; rather Au(I)and Au(III)-tetraalkylammonium complexes are the relevant Au species in solution prior to reduction with NaBH4. However the thiolate species is proved to be the intermediate during the synthesis in a polar (such as water) medium. And it has also been shown that in water, thiol is the active surface stabilizer and produces small clusters, whereas in the organic solvent medium, disulfide is more successful. More interestingly, following this two-phase thiol-mediated method, silver and copper nanoparticles were also synthesized. Finally, in lieu of using the tetraalkylammonium salts (i.e., TOAB), analogues phosphonium salts (TOP1) are also used and have been observed to solve the purpose successfully. Along with alkylthiols, other sulfur-containing ligands, including xanthates [16], disulfides [17], dithiols [18], trithiols [19], and resorcinarene tetrathiol [20], have also been examined and reported in literature for the stabilization of gold nanoparticles. For example, Wei et al. have shown the spontaneous assembly of dithiocarbamate (DTC) ligands on gold surface [21]. A series of the DTC ligands were synthesized using CS2 and the corresponding secondary amines and were also observed to assemble on gold nanoparticles surface affecting their dispersion properties. For example, aqueous suspensions of gold nanoparticles (40 nm), when treated with CS2 and tetra(N-methyl) aminomethyl resorcinarene (TMAR) at millimolar concentrations, could be extracted from the aqueous phase into CH2Cl2 as shown in Fig. 2.5. No extractions occurred in the absence of CS2 or TMAR, demonstrating that phase transfer was successful only when the nanoparticles were encapsulated by the DTC-resorcinarene surfactant layer.

FIGURE 2.5 Dithiocarbamate ligands (left panel) and extraction of gold nanoparticles from aqueous phase to organic (CH2Cl2) phase using DTC 5 with the respective TEM image of the gold nanoparticles. Source: Reprinted with permission from Y. Zhao, W. Pe´rez-Segarra, Q. Shi, A. Wei, J. Am. Chem. Soc. 127 (2005) 73287329. Copyright 2005 American Chemical Society.

I. FUNDAMENTALS

2.4 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF NOBLE METAL NANOPARTICLES

HN

NH

S – S

+

CS2

H2N

N

[Ru]Cl2

– S

S – S

S

N CS2

N

N

N

– S S

N

S S

N

+

[Ru]

FIGURE 2.6 Anchoring + [Ru] of Ru comples on Au nanoS particles employing DTC method. Source: Reprinted with permission from E.R. Knight, A.R. Cowley, G. + S Hogarth, J.D.E.T. Wilton-Ely, [Ru] Dalton Trans. (2009) S 607609. Copyright 2009 Royal Society of Chemistry. S

+

H2N NEt3

17

HAuCl4 NaBH4

N S S

+

[Ru]

[Ru]Cl2 = cis-RuCl2(dppm)2

Similarly, this DTC-based strategy has been extended for the surface functionalization of gold nanoparticles with heterometallic (e.g., Ru, Pd, etc.)DTC complexes and was shown to be an efficient strategy for the anchoring of transition metal on the gold nanoparticles surface (Fig. 2.6) [22]. Again, thiol-functionalized ionic liquids (TFILs) have been introduced [23] for the synthesis and stabilization of gold and platinum crystalline nanoparticles. Thiols have also been used for the tailor-made assembly of gold nanoparticles, nanorods, etc. leading to the fabrication of gold nanochain, etc. For example, Thomas and coworkers [24] have reported the use of different α,ω-alkane thiols for the end-to-end oligomerization of gold nanorods along with mechanistic interpretation (Fig. 2.7). Similarly tripodal thiol-functionalized porphyrin molecules have also been attempted for fabrication of self-assembled monolayer (SAM) on gold nanoparticles [25]. In the last decade, emphasis has been given to the design and fabrication of thiolated gold nanoclusters owing to their promising applications in biomedical fields, electronic devices, etc. [26]. An ensemble of such nanoclusters of composition Aux(SR)y {x 5 10, 15, 18, 23, 24, 25, 29, 33, 36, 38, 39, 40, 68, 130, 187, 279, etc.; y 5 10, 13, 14, 16, 17, 18, 20, 22, 24, 40} are reported in literature [27]. Attempts have already been made to give an idea of the structure of these thiolated clusters. For example, Jin’s research group of University of California at Berkeley has successfully reported the single crystal structure of Au25 nanoclusters (1.27 nm diameter, surface-tosurface distance) protected by 18 phenylethanethiol ligands [28]. In the same way, the group has also addressed the synthesis and structural evolution of another cluster, Au38(SC2H4Ph)24. Pradeep and Udayabhaskararao have reported series of fluorescent gold nanoclusters surface passivated with thiol ligands (Fig. 2.8) [29]; for example, synthesis of Au18(SG)14 has been made using a biomolecule, glutathione, as the corresponding thiol. On the other hand, reports on thiolated silver nanoclusters are much less and this is due to the lack of suitable synthetic methods to produce the latter and also due to higher susceptibility of silver towards oxidation. This converts the Agn core to AgnOx, resulting in the loss of characteristic optical features. However, the group of Professor Pradeep has illustrated a benchmark reaction for the fabrication of

I. FUNDAMENTALS

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.7 Schematic presentation of thiolmediated assembly of gold nanorods. Source: Reprinted with permission from S.T.S. Joseph, B.I. Ipe, P. Pramod, K.G. Thomas, J. Phys. Chem. B110 (2006) 150157. Copyright 2006 American Chemical Society.

FIGURE 2.8 Synthesis of thiol-protected gold nanoclusters. Source: Reprinted with permission from T. Udayabhaskararao, T. Pradeep, J. Phys. Chem. Lett. 4 (2013) 15531564. Copyright 2013 American Chemical Society.

I. FUNDAMENTALS

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19

thiol-protected silver nanoclusters, which involves solid state grinding protocol of solid Ag(I) salt as the metal precursor, mercaptosuccinic acid/glutathione/ phenylethanethiol as the thiol and NaBH4 as the suitable reducing agent. And in this way Ag7, Ag8, Ag9, Ag32, and even Ag152 clusters were successfully synthesized and characterized. In a recent review they have described the synthesis and other different aspects of noble metal nanoclusters in a detailed way [30]. B. Ligands with amine functional group Molecules with amine functional groups have also been chosen as one of the surface modifying ligands, like the thiols, for noble metal nanoparticles. Though it should be clearly remembered here that gold surface has an inherent fascination towards thiol functionality as described in the SHAB principle and hence the reports with amino ligands are comparatively less. The first attempt was made in the formation of SAMs on gold films with alkylamine [31]. Xu et al. has shown the deposition and successive formation of ordered monolayer of octadecylamine (ODA) over a gold surface. Here it is worth noting that the deposition was carried out instrumentally, however when it was tried in solution phase the monolayer was incomplete as both the gold surface and the solvent medium remain in competition to access the amine molecules. The ODA capped gold nanoparticles were easily dispersible in different organic solvent media and the dispersion was stable. The UVvis studies also demonstrated that the alkylamine protected gold nanoparticles can be easily stored as a dry powder and can be again dispersed in a suitable organic solvent like toluene, chloroform, etc. with little indication of aggregation of the particles. Again, like the thiols, amine also provides a protective layer on nanoparticles surface that helps provide it with a resistance to oxidation. Similarly another amine, oleyl amine (OLA) has been introduced for onestep fabrication of water dispersible gold nanoparticles through complexation of Au (III) salts, with subsequent reduction followed by stabilization of the nanoparticles [32]. Regarding the mechanism, it was considered that gold in its 11 oxidation state (i.e., AuCl22 ion) has a tendency to form complexes with different ligands like alkyl isocyanate, cyclohexylamine, 3-bromopyridine, alkyl amines, in the form AuCl(NH2R) and finally reduction occurs to the formation of gold nanoparticles. Similarly, in another report, Williams and coworkers have studied the molecular and electronic structure of alkylamine SAMs on gold nanoparticles surface [33]. From XPS results they have concluded that the self-assembled amine molecules remain tilted B30 with respect to the surface normal and having a molecular surface density of approximately ˚ 2/molecule (Fig. 2.9; left panel). Again from UPS study it was shown that the 25A assembly of the amines results in a lowering in the work function value of the metal which was due to the change in the surface potential caused by the modification of the surface layer dipole. The study also discovered that the amine SAMs have a dipole perpendicular to the surface, with the positive charges at the monolayer/vacuum interface and negative charges at the metal/monolayer interface (Fig. 2.9; right panel). Therefore the study provides a comprehensive physical insight into the molecular and electronic structures of amine-based SAMs on gold surface. Similarly, quaternary ammonium salts like cetyl trimethyl ammonium bromide (CTAB) or the corresponding chloride (CTAC), tetraoctyl ammonium bromide (TOAB), cetyl pyridinium chloride (CPC), etc. are being used as growth controlling agents for

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.9 Assembly of amine molecules on gold nanoparticles surface in a tilted fashion (right panel) and the related energy level diagram (left panel). Source: Reprinted with permission from E. De la Llave, R. Clarenc, D.J. Schiffrin, F.J. Williams, J. Phys. Chem. C 118 (2014) 468475. Copyright 2014 American Chemical Society.

synthesis of nanoparticles. For example, in one of our previous reports, we have shown the formation of gold nanocubes under UV photoactivation, using 2-napthol as the reducing agent and in the presence of CTAB as stabilizer [34]. Apart from gold substrates, amine ligands have also been explored as the surface stabilizer for other noble metal nanomaterials like silver, platinum, etc. For example, monodispersed colloidal silver nanoparticles were synthesized using dimethylformamide (DMF) as a reducing agent as well as a stabilizer [35]. Similarly, silver myristate, an insoluble silver salt has been used as a single source precursor for the synthesis of monodispersed silver nanoparticles through reduction with a tertiary amine (triethylamine) [36]. It was claimed that the silver myristate forms a 1:2 adduct with triethylamine which on thermal decomposition results in the formation of monodispersed silver nanoparticles. Attempts have also been taken for the transfer of silver nanoparticles from aqueous phase to organic medium (silver organosol) using a long chain ODA where these amine ligands offer surface hydrphobicity to the silver nanoparticles and in turn make them easily dispersible in organic medium [37]. Other long-chain amines, like dodecylamine, have been used in the same way for transfer of platinum nanoparticles synthesized in aqueous medium to organic solvent through the surface modification of the platinum nanoparticles with the DDA molecules [38]. Tsukamoto and coworkers have reported the fabrication of octylaminestabilized nanosized copper surface with enhanced electrical conductivity [39]. Amine ligands have also been used to stabilize uncommon/nonconventional nanoparticles like well-dispersed ruthenium nanoparticles of size 23 nm [40]. C. Ligands with phosphate/phosphine functional group Though thiol and amine ligands have been explored a lot in the synthesis, stabilization, or surface functionaliztion of noble metal nanoparticles, still other ligands have also been attempted and ligands with phosphate functional groups are worth mentioning in this category. Bawendi and coworkers [41] in a report has shown the synthesis of water-soluble nanoparticles (Au, Pd, Fe2O3, etc.) assisted with a phosphine oxidepolyethylene glycol appended polymer (Fig. 2.10). Similarly, Pd and Ru nanoparticles have been synthesized using their organometallic polymers in the presence of borane-protected phosphine ligands which act as the stabilizers through surface capping of the nanoparticles [42]. Again, an important member in the series of

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21

FIGURE 2.10 Water-soluble nanoparticles (left panel) stabilized with phosphine oxidepolyethylene glycol appended polymer (right panel). Source: Reprinted with permission from S. Kim, S. Kim, J.B. Tracy, A. Jasanoff, M.G. Bawendi, J. Am. Chem. Soc. 127 (2005) 45564557. Copyright 2005 American Chemical Society.

these ligands is secondary phosphine oxide (SPO) which has been shown to stabilize nanoparticles apart from their common use in coordination chemistry. Their oxidative stability, easy synthesis strategy, and strong electron donation ability have made these SPOs interesting in the nanopartices chemistry. They generally exist in an equilibrium between the pentavalent phosphorus oxide and the trivalent phosphinous acid. This equilibrium depends on solvents, substituents, and metal coordination [43]. Leeuwen and coworkers have shown [44] the application of SPO-stabilized Au nanoparticles in chemoselective hydrogenation of substituted aldehydes and have highlighted the promising role of the ligands in the catalytic activity. Similarly, in another report by Hyeon and coworkers [45], trioctyl phosphine (TOP)-stabilized monodispersed Pd nanoparticles have been synthesized and then ligand exchange reactions were carried out to replace the TOP ligands with other phosphine ligands. The presence and the surface coordination behavior of the ligands were corroborated with 31P NMR study. In this regard, a series of different monodentate as well as bidentate ligands were examined for the ligand exchange reactions (Fig. 2.11). On the other hand, different silver precursors, RAg(PPh3)n (R 5 Cl, Br, or NO3, and n 5 1 or 3), have been exploited for the synthesis of 2.57.1 nm Ag nanoparticles using tert-butylamine borane (TBAB) in the presence of dodecanethiols (C12) at a temperature between 100 and 160 C [46]. However here it was shown that the thiol can replace the phosphine ligands during the growth of the nanoparticles. It was also shown that the rate of the PPh3thiols exchange depend on the nature of the silver precursor and influences the final NPs size. D. Ligands with silanol functional group A silanol is a compound having SiOH functional groups and different compounds containing this moiety have been used as typical surface capping/ modifying ligands in metal nanoparticles chemistry. For example, 3aminopropyltrimethoxysilane (3-APTMS) and 3-glycidoxypropyltrimethoxy silanes (3-GPTMS) have been used for both the synthesis and stabilization of gold nanoparticles. 3-APTMS and 3-GPTMS are hydrophilic and hydrophobic in nature, respectively. Thus with such ligands Au nanoparticles could be prepared which are spontaneously dispersible in water as well as in organic media [47]. The silanol chemistry is also well known in making glass surface coatings with Au, Ag, Pt nanoparticles. In this method, first clean glass slides are functionalized with

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.11 Different phosphine ligands used as a stabilizing ligands. Source: Reprinted with permission from S.U. Son, Y. Jang, K.Y. Yoon, E. Kang, T. Hyeon, Nano Lett. 4 (2004) 11471151. Copyright 2004 American Chemical Society.

silanol groups through their incubation with 3-(mercaptopropyl)-trimethoxysilane (MPTMS), 3-(aminopropyl)-triethoxysilane (APTES), etc., and then the freely appended amino groups are exposed to colloidal solution of Au or Ag or Pt, etc. nanoparticles where these nanoparticles are bound. Similarly, Jana et al. have shown the synthesis of water soluble nanoparticles through surface modification with silica shell [48]. They have carried out a silica coating strategy. The method involves the condensation of alkoxysilanes on the nanoparticles surface. The silane includes have trimethoxysilyl or silanol groups at one terminal and an amino or phosphonate group at the other terminal. Finally hydrolysis is carried out in a basic/acidic medium to render the formation of a silica shell. They have synthesized gold nanoparticles in toluene solvent in the presence of a thiol containing silane, 3-(mercaptopropyl)-trimethoxysilane (MPTMS) and then 2-aminoethyl-aminopropyltrimethoxysilane (AEAPS) was used further which undergoes a cross-linking with the previously anchored silane and results in a water-soluble gold nanoparticles which was shown to be stable for a couple of months. A similar strategy was adopted to make silver nanoparticles also. Thus the above discussion gives us a precise idea regarding the stabilization as well as the surface modification of plasmonic nanoparticles.

I. FUNDAMENTALS

2.5 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF METAL OXIDE NANOPARTICLES

23

2.5 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF METAL OXIDE NANOPARTICLES Here it is worth mentioning that oxide nanoparticles can also be made stabilized/surface-modified with molecules having suitable functionalities. Here, for the sake of a brief idea we have chosen three mostly studied oxide nanoparticles: iron oxide, zinc oxide, and titanium dioxide. A. Ligands with amine functional group Stabilizer with amine functional group has been interestingly employed in the oxide nanomaterials chemistry for their surface capping or functionalization. For example, Dravid and coworkers [49] have shown the synthesis of Fe3O4 nanoparticles through parallel reduction and surface capping of a Fe(II) precursor with dodecylamine (DDA). This capping turns the nanoparticles air-stable with regards to surface oxidation as well as making them dispersible both in aqueous and organic solvents. It was proposed that the reaction mechanism is simple. It involves a reduction reaction assisted with the amine which first makes complex with the Fe(III) followed by hydrolysis of these complex. This renders the formation of the metal hydroxide or metal oxide where the amine molecules remain surface bound. The higher pH required for the hydrolysis reaction is provided by the amines. Similarly, there are also reports on the synthesis of iron oxide nanoparticles from suitable iron precursors like Fe(III) acetylacetonate [50], Fe(II) cuppferonate [51], Fe(CO)5 [52], etc. For example, thermal decomposition of Fe(acac)3 (acac 5 acetylacetone) in the presence of pyrrolidone results in water-soluble Fe3O4 nanoparticles [50]. Here pyrrolidone provides three functions: (i) strong polarity; (ii) high boiling point; and (iii) coordination ability with transition metal ion, i.e., Fe(III) here, and hence they also remain ligated over the nanoparticles surface. The role of amine has also been nicely presented in the article on the fabrication of hierarchical nanostructures of zinc oxide (ZnO) [53]. Here the synthesis was performed hydrothermally with zinc nitrate and amines like hexamethylenetetramine (hmt) and 1,3-diaminopropane (dap). In brief, first ZnO nanorods were synthesized on a glass surface using hmt and this nanorod was employed as the template for the secondary growth of other ZnO crystalline nanostructures with the other amine. It was shown that these primarily grown nanorods have well-defined hexagonal crystallographic planes and their growth direction is in the ,1 1 0. direction (perpendicular to the surface of the glass slide). When DAP was introduced during the secondary growth, new crystals grew on the columnar facets of the primary rods. It was nicely shown that DAP concentration highly affected the growth of the secondary morphologies (Fig. 2.12). At low concentration level of DAP, tapered nanostructures were formed, whereas at higher concentration long, needle-like growth was observed on the nanorods’ surface indicating a key role in the growth process. With increasing concentration of DAP, the pH of the medium gradually increases and it helps in the formation of more ZnO crystallites which renders the growth of the different secondary nanostructures over the nanorod template. With a considerably high concentration of the amine, the pH of the medium is soon increased so that it again

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.12

DAP concentration dependent hierarchical growth of ZnO nanostructures. Source: Reprinted with permission from T. Zhang, W. Dong, M. Keeter-Brewer, S. Konar, R.N. Njabon, Z.R. Tian, J. Am. Chem. Soc. 128 (2006) 1096010968. Copyright 2006 American Chemical Society.

promotes dissolution of the amphoteric crystallites and hence finally we have the tapered morphology composed of stacked nanoplates grown along the ,0 0 1. axis. Titanium dioxide (TiO2) nanocrystals have been well acknowledged for the last few decades in the area of oxide nanomaterials. They can also be synthesized using different Ti(IV) precursors as well as with varieties of amine molecules. For example, titanic sulfate Ti(SO4)2 and other titanium precursor salts can be hydrolyzed with hydrazine under hydrothermal conditions and different morphologies including spindle-like, wire-like nanostructures are obtained [54]. Titaniumtriethanolamine complex has also been employed for the fabrication of TiO2 nanoparticles [55]. It was observed that the complex, when dissolved in ammonia and aged for 3 days at 140 C, results in the formation of anatase TiO2 phase having spherical/spindle-like morphology. It was proposed that here NH3 acts as the shape-directing agent, causing elongation of the particle by adsorption onto the crystal planes parallel to the C-axis of the particles. The role of ammonia in controlling the morphology of the oxide/ hydroxide nanostructures has also been reported by us in one of our research findings on the fabrication of β-Ni(OH)2 nanostructures [56]. It was shown that a variety of nanostructures from nanoflowers to stacked nanoplates/nanocolumn (Fig. 2.13) could

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25

FIGURE 2.13 NH3 concentration dependent hierarchical growth (flower to columnar) of Ni(OH)2 nanostructures. Source: Reprinted with permission from S. Sarkar, M. Pradhan, A.K. Sinha, M. Basu, Y. Negishi, T. Pal, Inorg. Chem. 49 (2010) 88138827. Copyright 2010 American Chemical Society.

FIGURE 2.14 Triethylamine concentration dependent hierarchical growth (hexagonal, elongated hexagonal, stacked hexagonal, columnar, stacked octagonal, octagonal) of uranyl oxy hydroxide nanostructures. Source: Reprinted with permission from M. Pradhan, S. Sarkar, A.K. Sinha, M. Basu, T. Pal, Cryst. Eng. Comm. 13 (2011) 28782889. Copyright 2011 Royal Society of Chemistry.

be synthesized by tuning the NH3 concentration. The surface of the materials was observed to be anchored with the NH3 molecules as indicated from the FTIR studies. We also noticed the ammonia-mediated shape transformation of the nanoplates from triangular to truncated triangular to finally hexagonal shape. On the other hand, we have also observed the formation of monoclinic CuO nanoflowers through simple hydrolysis of a cation exchange resin bound Cu(II) complex, R2[Cu(1,10-phen)2]21 [57]. Similarly, ethanolamine has also been addressed in the fabrication of a SnO2polyaniline nanocomposite [58]. Similarly a simple tertiary amine (Et3N) has been chosen by our group for the fabrication of diverse and hierarchical nanostructures of uranyl oxy hydroxide. The growth of the different shape was explained from their PXRD analysis [59] (Fig. 2.14).

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

Thus, in brief, molecules with amine functionality provide a wide scope in the fabrication as well as surface functionalization of oxide nanomaterials. B. Ligands with carboxylate functional group Different carboxylates have been employed again for the fabrication/stabilization of oxide nanomaterials. In a recent report it has been shown that superparamagnetic iron oxide nanoparticles (SPIONs) can be surface functionalized with different acid functionalized capping agents, such as ascorbic acid, hexanoic acid, salicylic acid, and amino acids like L-arginine and L-cysteine, etc. (Fig. 2.15) [60]. Such surface modification makes the nanoparticles into a promising avenue towards biomedical applications like drug delivery and others. The magnetic nanoparticles prepared by the coprecipitation O O OH HO OH O Fe3O4

O HO HO OH O O

C4H9 O O

OH O – O O



HO OH OH O – O O HO OH

O C4H9 O

OH HO – O O HO

(A)

O C H 4 9 O

Fe3O4

O O C4H9

OH

C4H9

O

OH

(B) OH OH

HO HO NH2

HO

SH H2N

O

HO

SH

H2N

O O

O Fe3O4

OH

O HO HO

OH OH

O O HS

Fe3O4

O

OH O O

H2N O N NH HO NH2 H

NH2 SH

OH O O

NH2

(E)

H2N

H N –

HN

NH2 O O

Fe3O4

O O NH2

H2N O O

N H –

HN



NH2

NH2

NH

(C)

NH2 N H –

SH

NH2 O O

O O HO

O O HO

O O H N

Fe3O4

NH

(D)

FIGURE 2.15 Schematic representation of esterification of iron oxide nanoparticles with different coating agents: (A) ascorbic acid; (B) hexanoic acid; (C) salicylic acid; (D) L-arginine; and (E) L-cysteine. Source: Reprinted with permission from D. Rehana, A.K. Haleel, A.K. Rahiman, J. Chem. Sci. 127 (2015) 11551166. Copyright 2015 Royal Society of Chemistry.

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27

method are enriched with the presence of surface hydroxyl groups which in turn help the anchoring/attachment of the different acid molecules through hydrogen bonding. Similarly, benzene carboxylic acids like terephthalic acid, aminoterephthalic acid, trimesic acid, pyromellitic acid, etc. have also been employed for surface coatings of SPIONs [61]. Surface modification with such ligands leads to the improvement of the magnetic property, water dispersibility through hydrophilic functional groups, good cytocompatibility, etc. The presence of these acid/amino acid functional moieties was ascertained from the FTIR studies. Similarly folic acid [62] has been highly appreciated in this category of surface functionalizing agents for Fe3O4 nanoparticles. These nanoparticles are then used for magnetic resonance (MR) imaging. For example, polyethyleneimine-mediated synthesized Fe3O4 nanoparticles could be surface modified with pegylated folic acid or hyaluronic acid for targeted MR imaging of tumors overexpressing FA receptors (FAR) or CD44 receptors, respectively [63]. Again Fe2O3 nanoparticles have also been surface grafted with folic acid and the bioconjugated water dispersible nanoparticles have been used for photodynamic therapy against human colorectal carcinoma cell lines (HCT 16) through the generation of intracellular ROS [64] which helps in significant nuclear DNA damage. For the biological activity of these nanoparticles, their water dispersibility is a key issue which has also been achieved chemically. For example, oleic acid capped Fe3O4 nanoparticles are stable in organic medium which on oxidative cleavage of the ethyleneic bond becomes transferred to the aqueous phase. Here such oxidation converts the oleic acid molecules to ligands having surface appended free carboxylic groups and thus makes the nanoparticles water dispersible [65]. Hence such surface functionalization of iron oxide nanoparticles have turned them into advanced materials for applications in medicinal fields like MRI contrast agents, etc. Similarly, oxalate ligand has been applied in the synthesis of TiO2 nanostructures having anatase and rutile phase [66]. The synthesis was oxalate concentration dependent. At lower range it results in the formation of a phase mixrure of anatase and rutile whereas at higher concentration it allows the formation of rutile phase. This difference was ascribed to oxalate ligand which can either act as a ligand in the formation of a hydrated species, Ti2O3(H2O)2(C2O4) 3H2O (R 5 1), at its lower concentration or as a chelating ligand with the stabilization of the rutile phase TiO2. The Ti2O3(H2O)2(C2O4) 3H2O was thermolyzed for its conversion to TiO2. TiO2 nanostructures with a range of morphological variety (like solid smooth TiO2 particles, urchin-like solid spheres, yolkshell TiO2 nanostructures, and TiO2 hollow spheres) have been synthesized hydrothermally where disodium salt of ethylenediaminetetraacetic acid (Na2EDTA) has been chosen as the structural regulating ligand [67]. Similarly amino acid (para aminobenzoic acid or PABA) grafted TiO2 nanoparticles have been synthesized starting from titanium tetraisopropoxide (TTIP) [68]. We have already discussed the interesting roles of hmt and DAP in the growth of different ZnO nanostructures [53]. In the work, the authors have also sighted the introduction of citrate (a hydroxyl tricarboxylate) ions as an additional shape/growth regulating agent instead of the secondary growth controlling agent DAP or along with DAP. It was observed that when it is used in place of DAP, nanoplates of ZnO grew up on the columnar facets of the primary ZnO rods (Fig. 2.16), where the thickness of



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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

the plates is controlled by citrate concentration. On the other hand, when it was used on the DAP-assisted grown up secondary nanostructures in the auxiliary role, it was noticed that ZnO nanoplates (thicknesses from 100 to 250 nm) covered the columnar facets of the primary rods and secondary branches and therefore resulted in tertiary growth to form big branched columnar nanostructures (Fig. 2.17). FIGURE 2.16 Citrate concentration-dependent hierarchical growth of ZnO nanostructures grown on primarily synthesized nanorods. Source: Reprinted with permission from ref [53]. Copyright 2006 American Chemical Society.

FIGURE 2.17 Citrate concentration-dependent hierarchical growth of ZnO nanostructures secondarily grown on the amineassisted grown columnar nanostructures. Source: Reprinted with permission from T. Zhang, W. Dong, M. Keeter-Brewer, S. Konar, R. N. Njabon, Z.R. Tian, J. Am. Chem. Soc. 128 (2006) 1096010968. Copyright 2006 American Chemical Society.

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29

FIGURE 2.18 Hierarchical ZnO nanostructures synthesized through: ZnO nanorods by hmt assisted growth; citrate as the secondary growth controlling agent; and DAP as the tertiary controlling agent. Source: Reprinted with permission from T. Zhang, W. Dong, M. Keeter-Brewer, S. Konar, R.N. Njabon, Z.R. Tian, J. Am. Chem. Soc. 128 (2006) 1096010968. Copyright 2006 American Chemical Society.

In the above tertiary growth by citrate the primary growth of the ZnO nanorods was carried out with hmt and secondary growth of branched nanostructures was assisted with DAP. Here it is truly interesting to note that if the secondary growth controlling agent was citrate and the tertiary agent was DAP sequentially, then, another set of new hierarchical ZnO nanostructure is obtained (Fig. 2.18). Hence this article clearly presents the intriguing role of both the amine and carboxylate in the morphological variation of the nanostructures. Na2EDTA has also been exploited in the fabrication of hierarchical ZnO nanostructures [69]. For example, ZnO nanostructures with morphologies like short/ long nanorods, nanoflowers etc. have been achieved with zinc acetate and sodium hydroxide hydrothermally (Fig. 2.19) where we can clearly understand the function of EDTA in controlling the morphology. Like the instances of ZnO, in one of our reports we have also shown the introduction of citrate in the secondary columnar growth of uranyl oxy hydroxide nanostructures starting from hexagonal nanoplates [59]. Hence both amine and carboxylate, either in some cases alone or in some cases jointly can nicely and precisely regulate the growth of different metal oxide nanostructures.

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES Zn-EDTA Complex

NUCLEI

HYDROTHERMAL GROWTH

RESULTANT MORPHOLOGY OF ZnO

EDTA 3 mM 2+

Zn

+ OH– High

+

Agglomerated Nanofiber Branch

EDTA 5 mM

Medium

Discrete Hexagonal Long Nanorods

EDTA 10 mM

Low

Branch of Tapered Flower Petals

RESULTANT MORPHOLOGY OF ZnO

FIGURE 2.19 Schematic presentation of nucleation and growth of different ZnO nanostructures from ZnEDTA complex. Source: Reprinted with permission from S.D. Gopal Ram, G. Ravi, M.R. Manikandan, T. Mahalingam, M. Anbu Kulandainathan, Superlattices Microstruct. 50 (2011) 296302. Copyright 2011 Elsevier.

C. Ligands with silanol functional group As stated earlier, silanol ligands can anchor on the nanoparticles surface through their hydroxyl moiety and they have been therefore used for surface functionalization of different oxide nanostructures. For example, Bronstein and coworkers [70] have reported the surface modification of different iron oxide nanoparticles synthesized via different methods like thermal decomposition of Fe(CO)5, Feoleate, etc. with silane. And for that purpose they have used two silanes: (i) N-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS) where only one end of the molecule reacts with the surface hydroxyl groups, i.e., with FeOH; and (ii) 3-(triethoxysilyl) propylsuccinic anhydride (SSA) where both ends react with the FeOH moities. AHAPS can attach to the surface with its primary amine group. On the other hand SSA ligand anchors onto the surface through a chelation mode and the freely appended silane group could be further linked to another SSA ligand and this way a thick shell of silane can be designed on the iron oxide surface. Such tailor-made functionalization prevents the nanoparticles from aggregation and make them increasingly dispersible in aqueous medium (Fig. 2.20). In the same way, the surface of TiO2 nanoparticles can also be modified with silane groups. Dutschk and coworkers [71] has employed a simple solution chemistry approach for the surface modification of commercial TiO2 nanoparticles using silane compounds 3-aminopropyltrimethoxysilane (APTMS) and 3isocyanatopropyltrimethoxysilane (IPTMS), respectively. The recipe of modification can be presented as follows (Fig. 2.21). As we can observe from the mechanism, the trimethoxy silane functionality first hydrolyzes to the corresponding silanol moieties.

I. FUNDAMENTALS

2.5 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF METAL OXIDE NANOPARTICLES

O OH

OC2H5 Si OC2H5

O

OC2H5

O

OC2H5

O

Si OC2H5

O

OH

O O O

O

O

Si

C2H5O

O

OC2H5

OC2H5

O

Si OC2H5

O O OH

31

+ SSA (1)

OC2H5

OH

O

OC2H5 Si OC2H5

O

OH -EtOH

O

+ SSA (2)

O

O O

OC2H5 Si OC2H5 OC2H5 Si OC2H5

O O

O

O OH

OC2H5

O

FIGURE 2.20 Growth of silane shell on iron oxide nanoparticles surface. Source: Reprinted with permission from X. Huang, A. Schmucker, J. Dyke, S.M. Hall, J. Retrum, B. Stein, et al., J. Mater. Chem. 19 (2009) 42314239. Copyright 2009 Royal Society of Chemistry.

The individual silanol compound undergoes self condensation and the resulting hydroxylated silane makes condensation with the oxide surface bound OH groups and is grafted on the surface through TiaOaSi bond formation. It was also noticed that the grafting efficiency was higher for IPTMS than APTMS as for IPTMS, the isocyanate group hydrolyzes and finally results in cross-linking. The anchoring of the silane moieties was ascertained from FTIR and other microanalytical techniques like thermogravimetry, zeta potential measurement, etc. The silanization of the oxide surface may also have an effect on their physical property. For example, ZnO nanowires [72] when silanized using tetraethyl ortho silicate (TEOS) show an enhancement in their impedance property due to the presence of the bulky silane moieties covering the surface which offer insulation of the surface, thereby decreasing the capacitance and increasing the resistance to charge transfer. Here the surface silanization was confirmed from FTIR studies through the appearance of a vibrational band at B1000/cm indicating the ZnaOaSi bond. In a recent report [73], surface of ZnO quantum dots (QDs) has been modified by a bi-silanization technique employing first silanization with a hydrophobic silane followed by second

I. FUNDAMENTALS

32

2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.21 Grafting of organosilane onto titanium dioxide nanoparticles surface. Source: Reprinted with permission from J. Zhao, M. Milanova, M.M.C.G. Warmoeskerken, V. Dutschk, Colloids Surf. A: Physicochem. Eng. Aspects 413 (2012) 273279. Copyright 2012 Elsevier.

silanization with a hydrophilic silane. Such modification renders the QDs dispersible both in aqueous and organic media as well as effectively enhances their fluorescence property. It was also noticed that this bi-silanization was more profitable towards their enhanced fluorescence or aqueous dispersibility than the single silanization either by the two silanes or when compared to the bare ZnO QDs.

I. FUNDAMENTALS

2.5 INTERACTIONS BETWEEN LIGANDS AND SURFACE OF METAL OXIDE NANOPARTICLES

33

Hence such silanization of oxide nanoparticles surface makes them highly beneficial for emerging applications such as in biosensors and bioelectronics. D. Ligands with phosphate/phosphine functional group Phosphates or phosphonic acid salts have been also used to stabilize metal oxide nanoparticles as well as for surface modification of the particles. For example, alkyl phosphates and phosphonates have been used to cap magnetite nanoparticles [74]. These ligands bound to the surface in a quasi-bilayer fashion where the primary layer remains strongly bound to the surface. The affinity of such phosphonate ligands indicates their use as a suitable alternative to fatty acids like oleic acid. Recently hexadecylphosphonic acid (HDPA) has been similarly introduced to make surface-coated Fe3O4 nanoparticles of average diameter B12 nm [75]. In both the cases this protects the nanoparticles from aerial oxidation. These HDPA functionalized magnetic nanoparticles were used in the efficient and exclusive oxidation of benzyl C 2 H bonds to carbonyls in a series of compounds only by molecular oxygen under mild conditions. And it was noticed that HDPA plays a typical role in the catalytic process. Too dense packing of the ligands blocks the approach of the reactants to the catalyst surface and lowers the catalytic activity. Again, too low concentration of them fails to build up an emulsion system which is the required environment for the catalytic process to occur [76]. The catalytic activity is presented in Fig. 2.22.

FIGURE 2.22 HDPA functionalized magnetic nanoparticles and their application in catalysis. Source: Reprinted with permission from L. Li, J. Lv, Y. Shen, X. Guo, L. Peng, Z. Xie, et al., ACS Catal. 4 (2014) 27462752. Copyright 2014 American Chemical Society.

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In a recent report, phosphonic acid coating has been chosen to transfer TiO2 nanoparticles from aqueous phase to organic phase (ionic liquid) [77]. It was observed that when this exchange was carried out with phosphonic acid ligands having a cationic moiety, like in methylimidazolium dodecylphosphonic acid (MImDPA), where a methylimidazolium group is at the end of the long chain, then this phase transfer occurs efficiently, whereas with simple octylphosphonic acid, this phase transfer does not occur. Such surface grafting helps TiO2 nanoparticles to be well dispersed in the ionic liquid medium. Such adsorption of phosphonic acid ligands may lead to their different configuration or geometry of the ligands over the surface, which has been recently studied theoretically with different adsorption models.

2.6 SYNTHESIS OF NOBLE METAL AND METAL OXIDE NANOMATERIALS: A BRIEF DISCUSSION In the previous sections we have assembled a brief idea regarding the theory of nucleation of nanoparticles growth and the nature of different stabilizing ligands frequently applied for surface capping and modification of plasmonic and oxide nanoparticles. Here we will focus on a brief idea regarding the different soft chemistry-based synthetic protocols for the synthesis of plasmonic and oxide nanoparticles. In association with this discussion we will also shed light on the different hybrid nanostructures formations, for example, coreshell, Janus-like, dumbbell-like, raspberry-like nanostructures, out of these plasmonic-oxide nanoduets.

2.6.1 Wet-chemical synthesis of noble metal nanoparticles A. Citrate reduction method This method was first invented by J. Turkevich et al. in 1951 and then refined by G. Frens in the 1970s. This reaction helps to produce almost monodispersed nanoparticles. It involves the spontaneous reduction of chloroauric acid with trisodium citrate under heating condition. Here citrate serves both as reducing agent and capping agent to control the growth of the nanoparticles. It is interesting to note that extensive networks of gold nanowires are formed as a transient intermediate. It has been proposed that under boiling condition with citrate and at a relatively low pH (3.76.5), [AuCl3(OH)]2 is first formed which is reduced to gold nuclei (nucleation step). After this nucleation, the nuclei form nanowires through fast random attachment and intraparticle ripening. Such formation of nanowires was confirmed by TEM and UVvis spectroscopy. These chains then undergo fragmentation and then Ostwald ripening to result in the formation of spherical gold nanoparticles [78] (Fig. 2.23). This nanochain intermediate is also formed when the reduction is carried out with sodium borohydride in lieu of trisodium citrate. These gold nanowires are responsible for the dark blue or blueblack appearance of the reaction solution before it finally turns ruby-red. The nanocolloid exhibits excellent stability and a relatively narrow size distribution for particles between 10 and 20 nm in diameter. The particle size is

I. FUNDAMENTALS

2.6 SYNTHESIS OF NOBLE METAL AND METAL OXIDE NANOMATERIALS: A BRIEF DISCUSSION

[AuCl3(OH)]– nucleation + H2Ct–/HCt2– < 10 sec Na3Ct

(1)fast random attachment (2)intra-particle ripening

35

intra-particle ripening ctd...

pH ≈ 3.7–6.5

HAuCl4 boiling water

[AuCl2(OH)]2– nucleation /[AuCl(OH)]3– pH ≈ 6.5–7.7 + HCt2–/Ct3– ~ 60 sec Na3Ct

slow growth

FIGURE 2.23 Schematic presentation of reaction pathways for the synthesis of gold nanoparticles by citrate reduction. Source: Reprinted with permission from N.T.K. Thanh, N. Maclean, S. Mahiddine, Chem. Rev. 114 (2014) 76107630. Copyright 2014 American Chemical Society.

controlled by the ratio of citrate to AuCl42 ions; a larger amount of citrate leads to smaller particle size. The reduction in the amount of sodium citrate will reduce the amount of the citrate ions available for stabilizing the particles, and this will cause the small particles to aggregate into bigger ones (until the total surface area of all particles becomes small enough to be covered by the existing citrate ions). When citrate reduction of AuCl42 was carried out under boiling conditions, the color of the solution changed rapidly in the following sequence: pale yellow, colorless, very dark blue, purple, and finally ruby-red. The dark blue intermediate was observed to be an extensive network of Au nanowires of approximately 5 nm diameter. Shortly after the addition of sodium citrate, the solution contained spherical Au nanoclusters of 35 nm in diameter and some irregularly shaped chain-like segments. The short chain-like segments apparently increased in length as the solution darkened, forming an extensive network of Au nanowires, whose diameter was approximately 5 nm. As the reaction progressed, the nanowires increased in diameter to about 8 nm, and at the same time the network fractured into smaller segments. As the color of the reaction solution started to lighten to purple, spherical particles between 10 and 13 nm in diameter began to “cleave” off from the nanowires. Finally, when the solution had turned to a ruby-red color, well-defined spherical particles of diameter 1315 nm were formed. Here it was proposed that the citrate ions undergo oxidation, with the formation of acetone dicarboxylate ions, carbon dioxide, etc., and give up two electrons. These electrons are received by the chloroaurate ions, i.e., AuCl42 ions and subsequently the Au(III) ions are reduced to Au(I) ions (i.e., to AuCl). These AuCl, formed as the intermediate, are stabilized by the acetone dicarboxylate ions. Finally, the AuCl disproportionates to form Au(0) and Au(III). The Au(III) ions again react in the previous way. Therefore in this way the formed gold nanoparticles are capped by the citrate ions and are stabilized in the colloidal state. Silver nanoparticles have also been synthesized using this citrate reduction method (LeeMeisel method) under boiling condition [79]. However this method results in silver nanoparticles with a variety of size distribution, and thus different modifications like variation in pH of the reaction medium, addition of additives, surfactants, seed mediated synthesis, etc. have been carried out to increase the homogeneity of the particles size and shape.

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B. Polyol method Polyols have been interestingly applied for the controlled synthesis of metal and metal oxide nanostructures [80]. Polyols are the high boiling, multivalent alcohols and have the advantages of the following factors: (i) metal salts are easily soluble in such polyhydroxylated solvent like in water; (ii) the solvents are coordinating in nature and hence they can solubilize the metal salts through metal ion coordination; this coordination property also helps them to anchor on the surface of the nanoparticles thereby polyols can stabilize the nanoparticles; and (iii) reducing property. In a recent review, Feldmann and coworkers [80] has nicely described the reports on polyol methods of synthesis of metal, metal oxide, and other chalcogenide nanoparticles. The range of different metal nanoparticles synthesized by this method is summarized in Table 2.1. Xia et al. have applied this synthetic approach largely for synthesis of hierarchical noble metal nanostructures. For example, silver nanowires having pentagonal cross-section were synthesized by this method in the presence of a capping agent, polyvinyl pyrrolidone (PVP) [81]. PVP plays a key role in controlling the nanostructures. It is proposed that the nanowires growth occurs from initially formed multiply twinned particles (MTPs) with a decahedral shape. The MTPs have fivefold symmetry with the surface bounded by 10 {1 1 1} facets with a set of five twinned boundaries. These boundaries are the sites of highest energy on the surface and hence they provide the sites of attraction of silver ions. This growth TABLE 2.1 Synthesis of Different Metal Nanoparticles by Polyol synthesis Method Metal

Particle Size (nm)

Shape

Polyol

Ref.

Fe

10150

Spheres

EG

[22]

FeCo

30

Spheres

EG

[24]

Co

20 3 50275

Rods

BD

[25]

Ni

15

Spheres

Castor oil

[52]

11 3 75

Rods

Natural polyol

Ru

26

Spheres

EG, PDO, BD

[26]

Ru

1.47.4

Spheres

EG, DEG, TrEG (triethylene glycol)

[56]

Rh

250

Various

EG, DEG, TrEG, TEG (tetraethylene glycol)

[55]

Rh

11

Tripods

EG

[27]

Rh

6.5 (edge length)

Polyhedra

EG

[28]

6.5 (edge length)

Cubes

Pd

515 (edge length)

Bipyramids

EG

[58]

Pd

2 (diameter)

Wires

Various

[58]

Pd

16

Icosahedra

DEG

[29] (Continued)

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2.6 SYNTHESIS OF NOBLE METAL AND METAL OXIDE NANOMATERIALS: A BRIEF DISCUSSION

37

TABLE 2.1 (Continued) Metal

Particle Size (nm)

Shape

Polyol

Ref.

PdNi

911 (edge length)

Cubes

PD

[30]

PdCo

47

Spheres

EG

[58]

PdRh

3

Dendrites

EG

[31]

PdCu

3.55.8

Spheres

EG

[32]

PdAg

5.5

Spheres

EG

[33]

PdBi

100 (edge length)

Crystals

EG

[58]

Pt

10-15 (diameter)

Branches

PDO

[34]

Pt

2030

Flowers

EG

[35]

PtFe3

3.3

Spheres

EG

[36]

Pt3Co

2.3

Spheres

EG

[37]

PtNi

45

Spheres

EG

[38]

PtRh

611

Spheres

BD

[39]

PtRu

15

Spheres

EG

[40]

PtZn

35

Spheres

EG

[51]

PtSn

25

Spheres

EG

[51]

PtPdBi

35 (diameter)

Wires

EG

[41]

Cu

2

Spheres

EG

[42]

Ag

1832

Cubes

EG, DEG

[54]

Ag

5150

Spheres

EG, PDO, BD, PD

[57]

Ag

60 (diameter)

Wire

EG

[58]

AuCu

Various

Coreshell

EG

[43]

AuAg

30 3 80

Rods

EG

[44]

AuAg

4354

Coreshell

DEG

[45]

Sn

510

Spheres

DEG

[46]

Sb

50

Spheres

PD

[47]

30 (diameter)

Wires

Bi

3050

Spheres

PDO

[48]

Bi3lr

4060

Spheres

EG

[23]

Bi2lr

50

Spheres

EG

[23]

BiRh

60 3 20

Plates

EG

[23]

Reprinted with permission from H. Dong, Y.-C. Chen, C. Feldmann, Green. Chem. 17 (2015) 41074132. Copyright 2012 Royal Society of Chemistry.

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process finally leads to the elongated nanostructure. Here it is also proposed that PVP preferentially interacts with {1 0 0} facets (side-wall of the nanorods) compared to the {1 1 1} facets (two ends of the nanorods) and thus the growing occurs continuously along the two ends because of their high chemical potential and reactivity, and finally the nanorods grow into nanowires. The side surfaces of the nanowires thus remain passivated by PVP. The corresponding SEM images of the nanowires clearly depict the pentagonal cross-section and thus support the proposed mechanism (Fig. 2.24). Controlling the PVP/AgNO3 molar ratio, other nanostructures, like nanocubes, could also be synthesized. Xia and Sun have described the synthesis of uniform silver nanocubes using ethylene glycol as solvent and reducing agent in presence of PVP [82]. The group has also studied the interference of additives such as metal salts in the growth process. For example, they have studied the morphological growth of Pt nanoparticles in the presence of NaNO3. H2PtCl6 when was reduced with ethylene glycol in the presence of PVP and different concentrations of NaNO3, it was observed that higher concentration of NaNO3 results in branched nanostructure [83].

FIGURE 2.24 Schematic illustration of the growth of silver nanowires with pentagonal cross-sections. (i) Evolution of a nanorod from a multiply twinned nanoparticle (MTP) of silver. The ends of this nanorod are terminated by {1 1 1} facets, and the side surfaces are bounded by {1 0 0} facets. The strong interaction between PVP and the {1 0 0} facets is indicated with a dark-gray color, and the weak interaction with the {1 1 1} facets is marked by a light-blue color. The red lines on the end surfaces represent the twin boundaries that can serve as active sites for the addition of silver atoms. The plane marked in red shows one of the five twin planes that can serve as the internal confinement for the evolution of nanorods from MTP. (ii) Schematic illustration of the diffusion of silver atoms towards the two ends of a nanorod, with the side surfaces completely passivated by PVP. Source: Reprinted with permission from Y. Sun, B. Mayers, T. Herricks, Y. Xia, Nano Lett. 3 (2003) 955960. Copyright 2003 American Chemical Society.

I. FUNDAMENTALS

2.6 SYNTHESIS OF NOBLE METAL AND METAL OXIDE NANOMATERIALS: A BRIEF DISCUSSION

39

C. Seed-mediated method This method involves use of tiny preformed nanoparticles (seeds; size 35 nm) in a reaction medium from where growth of nanoparticles is being considered. There are a couple of reports on the synthesis of gold nanorods using seed-mediated methods. For example, Jana et al. have shown that gold nanorods can be synthesized in the presence of gold seeds through the reduction of chloroauric acid with ascorbic acid, CTAB (acts as a micellar template), and a little AgNO3 [84]. The seed solution was prepared by borohydride reduction of HAuCl4 using citrate as a stabilizer at room temperature. Here Ag1 ion was observed to be necessarily present as this favors the nanorods formation. Its absence results in the ultimate formation of spherical gold nanoparticles. It is supposed, that AgBr formed in situ (by the reaction of AgNO3 with CTAB) adsorbs on the surface of the spherical as well as spheroid nanoparticles and controls the growth of the spheroidal nanorods. El-Sayed and Nikoobakht [85] have developed a similar strategy, except for the use of CTAB also during the preparation of seed solution. The seed plays a role of template in the growth process, where after addition to the growth solution, the Au atoms diffuse to the template surface and grow into nanorods. Sau and Murphy [86] have reported the seed-mediated high-yield synthesis of multiple shaped gold nanoparticles. They have shown the careful control of the reaction parameters, i.e., concentration of (i) seed solution, (ii) CTAB, (iii) Au31 ion, (iv) ascorbic acid, and (v) Ag1 ion could bring a large variation in the morphology of the gold nanostructures. Palladium nanocubes have also been synthesized using preformed Pd seed [87]. Xia and coworkers [88] have reported seed-mediated synthesis of Ag nano-octahedral where the seed solution was prepared using polyol reduction method followed by their final growth to nano-octahedra through citrate reduction of Ag1 ions of the growth solution. D. Redox transmetallation method An increasingly deserving approach employing galvanic replacement reactions/ transmetallation reactions has become a key and novel means towards syntheses of a broad spectrum of hollow/porous metal nanoframeworks, metal alloy nanostructures, etc. This single-step methodology is based on the sacrificial oxidative dissolution of a reactive (less noble) metal template by a suitable metal salt and the process is solely driven by their distinctly different reduction potential values, and finally comes up with reductive deposition of the nobler element from the precursor salt. A group of materials’ research community has exploited the technique both in aqueous and organic media to design engineered nanomaterials with intriguing properties but to date, a major part of the area is confined with the decoration of monometallic/hybrid nanocomposites, but made up largely with Au, Ag, Pd, and Pt. For example, Ag nanocubes were employed as sacrificial template for the formation of gold nanocages [89]. It was observed that the sharp cornered nanocubes react with AuCl42 ions and preferential dissolution of the cube corners results in a pinhole where further dissolution of Ag atoms occur; epitaxial growth of gold occurs and it results in the formation of hollow nanobox. Finally gold atoms diffuse to the corners and porous nanocages are formed (Fig. 2.25). It was surprisingly noticed that use of nanocubes with rounded corners however results in morphologically different

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.25 Stepwise fabrication of porous gold nanostructures from silver nanocubes (sharp corners). Source: Reprinted with permission from S.E. Skrabalak, J. Chen, Y. Sun, X. Lu, L. Au C.M. Cobley, et al., Acc. Chem. Res. 41 (2008) 15851595. Copyright 2008 American Chemical Society.

nanostructure (Fig. 2.26). Again, the porous nanocubes of Au/Ag have been further treated with ferric nitrate to remove the excess silver present in the nanostructures and ultimately results in the formation of porous gold nanoframe. This transmetallation pathway thus happens to be a suitable platform for easy fabrication of a choice of hierarchical noble metal nanostructures. And of course, the skill has also been assessed with certain 3d block transition metal elements or nanoparticles. As for illustration, commercially available nulvalent Mn has very recently been exploited for large-scale synthesis of nickel nanoscrolls from aqueous nickel(II) chloride solution [90]. We have also applied this strategy for fabrication of different noble metal nanostructures. For example, treatment of prickly nickel nanowires with HAuCl4 finally results in the formation of dendritic gold nanostructures [91]. Again, gold nanoflowers were synthesized using dissolution of nano Cu template [92] or dendritic silver nanostructures were synthesized using dissolution of nano Fe template [93]. This method has also been employed for synthesis of alloy nanostructures. For example, Pradeep et al. have reported the synthesis of platinum telluride nanoparticles (Pt3Te4) from tellurium nanowires template using PtCl622 [94]. E. Ion-exchange resin method Both cation and anion exchange resin matrix have been widely applied by our group to promote the synthesis of different nanostructures including metal nanostructures. For example, anion exchange resin matrix (chloride form) has been employed for the large-scale synthesis of gold nanowires using amines like EDTA,

I. FUNDAMENTALS

2.6 SYNTHESIS OF NOBLE METAL AND METAL OXIDE NANOMATERIALS: A BRIEF DISCUSSION

41

FIGURE 2.26

Stepwise fabrication of porous gold nanostructures from silver nanocubes (different from above; curved corners). Source: Reprinted with permission from S.E. Skrabalak, J. Chen, Y. Sun, X. Lu, L. Au C.M. Cobley, et al., Acc. Chem. Res. 41 (2008) 15851595. Copyright 2008 American Chemical Society.

triethylamine as reducing agents. It was proposed that the resin surface promotes an electrostatic field force directed growth of the nanowires in the absence of which only irregular shaped particles were observed to be formed (Fig. 2.27) [95]. Similarly, cation exchange resin matrix was applied for synthesis of Ni, Cu, etc. nanoparticles [92,96]. F. Interfacial growth method This methodology majorly involves a liquidliquid interface for the growth of plasmonic nanoparticles. For example, we have shown the formation of gold nanoleaves in an organicaqueous interface through the reduction of HAuCl4 (present in aqueous medium) by a suitable reducing agent, 1,4-dihydropyridine ester (DHPE) (present in organic medium) under UV light irradiation (Fig. 2.28) [97]. We obtained a porous morphology of the nanoleaves. This methodology was observed to be equally efficient for the fabrication of mesoporous palladium nanoleaves. Here we have briefly discussed a couple of strategies for the synthesis of different plasmonic nanostructures. There are also other solution chemistry-based methods for the fabrication of such nanostructures.

2.6.2 Wet-chemical synthesis of metal oxide nanoparticles There are also several synthetic strategies for the fabrication of hierarchical oxide nanostructures. Patzke et al. [98] have nicely described the different techniques majorly adopted for the synthesis of the nanomaterials. The methodologies primarily involve (i) hydrothermal/solvothermal method, (ii) microwave irradiation technique, (iii) ionic-liquid assisted

I. FUNDAMENTALS

42

2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

R+AuCl4–

RAu

Au nanowire Resin

Reaction R+Cl– + HAuCl4 R+AuCl4–

Immobilization Triethylamine (0.01M)

R+AuCl4– RAu

MHT 200W

FIGURE 2.27 Growth of gold nanowires employing ion-exchange resin support. Source: Reprinted with permission from A.K. Sinha, M. Basu, S. Sarkar, M. Pradhan, T. Pal, Langmuir 26 (2010) 1741917426. Copyright 2010 American Chemical Society.

FIGURE 2.28 Growth of gold nanowires employing ion-exchange resin support. Source: Reprinted with permission from S. Dutta, S. Sarkar, C. Ray, A. Ray, R. Sahoo, T. Pal, ACS Appl. Mater. Interfaces 6 (2014) 91349143. Copyright 2014 American Chemical Society.

synthesis, (iv) sonochemical methods, and (v) combined synthetic methods which involve two of the above methods subsequently. Following the literature, we can observe that the metal oxides can be synthesized using the hydrolysis or solvolysis of the corresponding metal salts following the above methodologies. Controlled hydrolysis can produce interesting morphologies. Different amines and other bases have been widely used for the synthesis of the metal hydroxides/oxides. For example, we have fabricated a variety of β-Ni(OH)2 nanostructures in an aminolytic approach employing a hydrothermal strategy where Ni(CH3COO)2 was hydrolyzed in the presence of NH3. We have already discussed the growth of different ZnO nanostructures using hydrothermal process in the presence of amines (hmt and DAP) [53]. In most cases it is considered that the metal salts undergo a solvolysis reaction to form the hydroxide/ oxide monomeric nanocrystallites which on further condensation (olation) produce the hydroxide/oxide nanostructures (Fig. 2.29). Here it is worth noting that, we have

I. FUNDAMENTALS

2.6 SYNTHESIS OF NOBLE METAL AND METAL OXIDE NANOMATERIALS: A BRIEF DISCUSSION

43

FIGURE 2.29

Olation mechanism towards the polycondensation mechanism of metal hydroxides which finally results in the metal oxide products. Source: Reprinted with permission from N.T.K. Thanh, N. Maclean, S. Mahiddine, Chem. Rev. 114 (2014), 76107630. Copyright 2014 American Chemical Society.

employed the ion-exchange resin matrix as a suitable platform for the fabrication of different oxide nanostructures, for example, box-like Cu2O [99], rod-like β-MnO2[100], waferlike Fe3O4 [101], coccun-like ZnO [102], etc., where the metal precursor salts are first bound to the resin matrix through ion-exchange method and then solvothermal reactions are performed to get the nanostructures.

2.6.3 Wet-chemical synthesis of noble metal-metal oxide hybrid nanoparticles In this section we have described the solution chemistry-based synthesis of hybrid nanostructures composed of metal oxide-plasmonic nanoparticles. A. Coreshell nanoparticles Metal oxide coreplasmoinc nanoparticles shell hybrid nanostructures are well reported. For example, Sun et al. have applied Fe3O4 nanoparticles as the core on which a shell of gold nanoparticles was preformed through olylamine reduction of HAuCl4 in chloroform solvent. These gold-coated Fe3O4 nanoparticles were then transferred to the aqueous medium with the aid of CTAB and citrate and then this was used as a seed matrix for further growth of Au or Ag nanoparticles over them. Therefore the simple wet-chemical strategy directs the synthesis of magneticplasmonic coreshell Fe3O4Au and Fe3O4Au or Ag nanoparticles [103] having tunable surface plasmon behaviors. Such a seed-based recipe has also been employed in other cases for the hierarchical synthesis. For example, polyethyleneimine (PEI)-stabilized Fe3O4 nanoparticles were first synthesized and gold nanoparticles were then anchored on their surface. This hybrid was then applied for the further growth of gold nanoshell over them and this ultimately resulted in the formation of different nanostructured morphologies, such as nanostars, nanopopcorns, etc. [104]. The reverse nanostructure, i.e., gold nanoparticle in the core and Fe3O4 nanoparticle in the shell was also synthesized by Alivisatos et al. [105]. Here, firstly iron nanoparticles were grown on the gold core through thermal decomposition of Fe(CO)5 in an oleic acid/ olylamine medium which aerially oxidized to result the iron oxide shell. Similarly, Kamat et al. [106] have described the synthesis of Ag coreTiO2 shell nanoparticles through a simultaneous hydrolysis and reduction method employing titanium-(triethanolaminato) isopropoxide complex as the titanium precursor and AgNO3 as the silver precursor. DMF acts as the reducing agent. Au coreTiO2 shell nanoparticles with branched shell can also be prepared hydrothermally through hydrolysis of TiF4 over the Au nanoparticles (Fig. 2.30A) [107]. Li et al. have

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2. THEORETICAL ASPECTS OF SYNTHESIS FOR CONTROLLED MORPHOLOGICAL NANOSTRUCTURES

FIGURE 2.30 (A) Schematic presentation of formation of Agcore@TiO2shell nanoparticles. (B) Dumbbell like Au@Fe3O4 nanoparticles. (C) Janus-like AuTiO2 nanoparticles. Source: Reprinted with permission from T. Hirakawa, P.V. Kamat, J. Am. Chem. Soc. 127 (2005) 39283934. Copyright 2005 American Chemical Society. (B) Reprinted with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Nano Lett. 5 (2005) 379382. Copyright 2005 American Chemical Society. (C) Reprinted with permission from Z.W. Seh , S. Liu , M. Low , S.-Y. Zhang , Z. Liu , A. Mlayah, et al., Adv. Mater. 24 (2012) 23102314. Copyright 2012 Wiley online library.

synthesized large-scale AuZnO pyramidal nanohybrids with a similar strategy [108]. Again, Cu2O shell was also fabricated over Ag core using reduction of a Cu(II) salt over preformed Ag nanoparticles [109]. These nanohybrids with plasmonic components show modified surface plasmonic behaviors and thus become spectroscopically significant for different studies like catalysis. B. Dumbbell-shaped nanoparticles Sun et al. have reported different strategies for the synthesis of dumbbell-shaped nanostructures. For instance, PtPdFe3O4 nanoparticles were synthesized [110] through solution phase synthesis of PtxPd100x nanoparticles in the first step, followed by controlled nucleation of Fe0 (obtained by thermal decomposition of Fe(CO)5) over the nanoparticles and finally the Fe0 is aerially oxidized to Fe3O4 and makes the magnetic shell over the PtPd nanoparticle in such a way that it forms a dumbbell-like nanostructure. Similarly AuFe3O4 nanoparticles were synthesized through first making Fe over preformed Au, followed by the oxidation of Fe to Fe3O4 in octadecene I. FUNDAMENTALS

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45

solvent under reflux condition (Fig. 2.30B) [111]. Here the dumbbell structures are formed through epitaxial growth of iron oxide on the Au seeds, and the growth can be affected by the polarity of the solvent, as more polar solvent leads to flower-like AuFe3O4 composite particles. Similarly, epitaxial growth of the iron oxide on Pt nanoparticles results in PtFe3O4 dumbbell nanostructure [112]. C. Janus-like nanoparticles AuTiO2 hybrid Janus-like nanoparticles (where one side of the spherical TiO2 nanoparticle is partially covered with Au nano and the other surface remain exposed to the solvent medium; Fig. 2.30C) have been fabricated through the hydrolysis of titanium diisopropoxide bis(acetylacetonate) in an aqueous-alcoholic solution of Au nanoparticles at RT [113]. Here the growth of the Janus-like morphology was achieved through the onetime addition of the titanium salt to the gold sol solution. ZnOAu Janus-like nanohybrid has similarly been reported [114], where ZnO quantum dots have been anchored on Au nanoparticles surface through 3-mercaptopropanoic acid, whose sulfur end is chemically bonded to the Au surface and the carboxylate moiety binds to the ZnO QDs. Thus the above discussion gives us a brief idea regarding the different types of hybrid nanostructures obtained from the combination of noble metal and metal oxides like Fe3O4, ZnO, TiO2, etc. These nanostructures have newborn or modified physicochemical aspects and thus they become an interesting platform for a set of emerging applications like in electrochemical cells, drug delivery, fluorimetric sensing, catalysis, photocatalysis, etc.

2.7 CONCLUSIONS Here we have made an attempt to give a brief overview of the synthesis of noble metal and metal oxide nanostructures based on the nucleation and growth mechanism and the related theories like LaMer theory, LSW theory, etc. and other aspects. Controlled growth of nanostructured materials are majorly arrested using a capping agent. These agents generally prohibit the particles growth through their adsorption on the surface of the nanoparticles and stabilize the nanoparticles either electrostatically or sterically. There are plenty of such stabilizers. In this chapter we have presented different categories of ligands which are commonly applied for the stabilization of metallic and metal oxide nanostructures. These include thiols, amines, phosphates, silanols, carboxylates, etc. We have also discussed the surface functionalization or ligand exchange reactions with the above-said ligands/stabilizers. Apart from this we have also described the different solution chemistry-based protocols for the synthesis of metal (noble metals) and metal oxide nanoparticles. Along with this, we have also discussed the hybrid nanostructures based on “noble metalmetal oxide” in a nut shell. Hopefully, the discussion will be advantageous to the materials scientific community and will help lead to a better understanding of the ideas.

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C H A P T E R

3 Methods for Synthesis of Hybrid Nanoparticles Phuong Nguyen Tri1,2, Claudiane Ouellet-Plamondon2, Sami Rtimi3, Aymen Amine Assadi4 and Tuan Anh Nguyen5 1

Department of Chemistry, University of Montre´al, Montre´al, QC, Canada 2De´partement de ge´nie de la construction, E´cole de Technologie Supe´rieure, Montre´al, QC, Canada 3 Swiss Federal Institute of Technology School of Engineering (STI), Powder Technology Laboratory (LTP), EPFL-STI-IMX-LTP, Lausanne, Switzerland 4E´cole Nationale Supe´rieure de Chimie de Rennes, Rennes, France 5Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

3.1 INTRODUCTION Hybrid nanoparticles are expected to have larger applications in biomedicine, antibacterial, energy storage, electronics, and catalysis than single nanoparticles. In general, the synthetic methods could be classified into two main categories: physical and chemical methods. As compared to the chemical methods, the process for the preparation of hybrid nanoparticles by physical methods are simple but the control of the size of hybrid nanoparticles seems to be complex. Regarding the chemical synthesis, especially the controllable nanostructures, they are expected to provide a better performance than that obtained from physical methods. In the physical preparation methods, the individual nanoparticles remain sometimes separate and distinct within the final nanostructure. However, a direct and stronger hybridization between nanoparticles is often obtained by this pathway while the nanoparticles prepared by chemical methods can exhibit unwanted properties due to the presence of chemical species, such as the stabilizing ligands, polymer shells.

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00003-6

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3.2 CHEMICAL SYNTHESIS METHODS Chemical synthesis can be classified either by the reduction of metal precursor or by the processing routes. Based on the reduction of metal precursors, there are the chemical reduction (CR) and photoreduction (PR) methods. In addition, various processing routes were used for fabricating the hybrid nanoparticles, such as hydrothermal, thermal decomposition, solgel, coprecipitation, electrodeposition, sonochemical, and seeding growth.

3.2.1 Chemical Reduction (CR) and Photoreduction (PR) Methods These methods are widely used for the deposition of noble metal (NM) nanoparticles on the surface of oxide metal (OM) nanoparticles, forming the NM-decorated OM nanoparticles. In these cases, the OMs, such as TiO2 and ZnO (commercial or customized products), were often dispersed into a solution containing NM precursors. During the synthesis process, NM ions firstly were adsorbed on the surface of OMs, then were reduced by chemical reducing agents (in CR method) or by photoirradiation (in PR method). The ultrasonication is sometimes used to assist the synthesis. In CR method, some reducing agents, such as sodium borohydride [1] and ascorbic acid [2], were used for the synthesis of Au/TiO2 hybrid nanoparticles. Fig. 3.1 presents the main steps for the synthesis of Au/TiO2 hybrid nanoparticles by reduction methods according to three main steps [2]. Similarly, the AuNPs decorated ZnO nanohybrids were also synthesized by using sodium borohydride as the reducing agent [3]. The drawback of these methods relates to the mixture of both pure NM nanoparticles and hybrid TiO2-Au1

TiO2 AuCl –

4(aq), PVP

ascorbic acid, 90ºC

1st reduction step

2nd reduction step

– AuCl4(aq)

AuCl–4(aq)

3rd reduction step

TiO2-Au3

TiO2-Au2

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FIGURE 3.1 Synthesis steps of chemical reduction for AuNPs decorated TiO2 nanoparticles. Source: Reprinted with permission from T.C. Damato, C.C. de Oliveira, R.A. Ando, P.H. Camargo, A facile approach to TiO2 colloidal spheres decorated with Au nanoparticles displaying well-defined sizes and uniform dispersion, Langmuir 29 (2013) 16421649. Copyright 2013 American Chemical Society.

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nanoparticles. Three approaches to resolve this drawback are: (i) by using local immobilization of chemical reducing agent on the surface of OM nanoparticles [4]; (ii) by using a redox reaction of metal hydroxide and noble metal ion (without using the reducer [5]); and (iii) by using a low weight ratio of NM sprecursors and OMs (e.g., NMs: OMs , 1:30). In a recent unpublished work, we fabricated the AgNPs decorated nano-TiO2 particles by using sodium borohydride as the reducing agent, with the weight ratio of Ag precursors:TiO2 5 1:30. Fig. 3.2 shows an electron microscopy image of the original nano-TiO2 particles (Fig. 3.2A) and then after the decoration with Ag nanoparticles (Fig. 3.2B). As can be seen in this figure, Ag nanoparticles were well dispersed on the surface of nano-TiO2 particles. The AgNPs exhibit quasi-spherical form with an average size 510 nm (Fig. 3.2). The Ag nanoparticles size and concentration can be tuned by controlling the synthetic FIGURE 3.2 (A) SEM image of nanoTiO2, (B) TEM image of AgNPs decorated TiO2 nanoparticles.

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process. The photocatalytical properties of these hybrid nanoparticles to blue methyl are much higher than the single nanoparticles. The UVVis spectra show some new peak assigned to the conjunction state of these two metals. Other chemical reducing agents have been found for the synthesis of hybrid nanoparticles, such as sodium citrate [6], N,Ndimethylformamide [79], oleylamine [10,11], diethanolamine [12,13], hydrazine [14,15], arcobic acid [16], formaldehyde [14], and glucose [17]. For PR methods, photoelectrons from OMs (e.g., TiO2 or ZnO) play an important role during the reducing process upon light irradiation. Fig. 3.3 shows an example of the synthesis of metal nanoparticles by photoreduction; under light irradiation, the free electron from transition metal can act with metal ions on the surface to form nanoparticles on the hosting nanoparticles surface [18]. Large number of light sources (λ . 300 nm) can be used for this purpose including high-pressure mercury arc (Au/TiO2 hybrid [19]), low pressure mercury lamp (Ag/TiO2 hybrid [20]), or sunlight (Pd/TiO2 hybrid [21]).

3.2.2 SolGel Method In the solgel method, the size of hybrid nanoparticles is precisely controllable, but the bonding/hybridization between hybrid nanoparticles is relatively weak, as compared to other methods. In the solgel method, the ligand or surfactant was added into the solution containing the NM and/or OM precursors. Depending on the ligand/surfactant/stabilizers, various nanostructures of hybrids can be obtained, such as (i) AuTiO2 coreshell nanoparticles [22], with polyvinylpyrrolidone as surfactant; (ii) Ag/TiO2 coreshell nanoparticles [23], with cetyltrimethylammonium bromide as protective agent; and (iii) Ag/TiO2 hybrids [24,25] with diethanolamine/triethanolamine as stabilizer/chelating agents. Olteanu et al. [26] have fabricated the Au@SiO2 and SiO2@Au coreshell nanoparticles using a microemulsion-assisted solgel method coupled with photoreduction reaction. In their study, the sodium 3-sulfonatemercaptopropane was used as the stabilizer.

3.2.3 Hydrothermal and Thermal Decomposition Processes The advantage of these methods is the ease in controlling the size and shape of nanoparticles. Hydrothermal processes usually require the high reaction temperature and high pressure, whereas the thermal decomposition processes might involve multiple steps. In general, the ligands, stabilizers, or surfactants could be present or not in the hydrothermal synthesis of hybrid nanoparticles. In case of nano-TiO2-based hybrids, by using the Mn+ n+ M n+ M Mn+ Reduce hv TiO2

M NPs FIGURE 3.3 Synthetic mechanism for NMs decorated TiO2 nanoparticles. Source: Reprinted with permission from S.F. Chen, J.P. Li, K. Qian, W.P. Xu, Y. Lu, W.X. Huangand, et al., Large scale photochemical synthesis of M@TiO2 nanocomposites (M 5 Ag, Pd, Au, Pt) and their optical properties, CO oxidation performance, and antibacterial effect, Nano Res. 3 (2010) 244255. Copyright 2010 Springer. M NPs@TiO2

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hydrothermal process with redox procedure, Yu et al. have synthesized the Au/TiO2 flower-shaped hybrids [27] and Pt/TiO2 coreshell NPs [28], without ligands or surfactants. In the other hand, Huang et al. [29] have prepared the branched AgZnO nanohybrids using the hydrothermal route in the presence of diethanolamine as both the stabilizer and reducing agent. It has been reported that the hydrothermal reaction at high temperatures was made more effective by using microwave irradiation with reduced reaction time. Song et al. [30] have produced the Au@TiO2 coreshell nanoparticles by using the microwave-assisted hydrothermal method. Regarding the thermal decomposition route for synthesis of nano-Fe3O4 based hybrids, Brollo et al. [10,31] have fabricated the brick-like Ag@Fe3O4 coreshell nanoparticles, by using thermal decomposition process with the mixture of oleylamine and oleic acid (as surfactants). Similarly, Lopes et al. [32] have fabricated the AgFe3O4 dimer nanoparticles by using these surfactants. With hexamine as the stabilizer, Padmanaban et al. [33] have fabricated the AgTiO2 nanohybrids through the thermal decomposition. The rose-like PdFe3O4 nanohybrids are also fabricated by thermal decomposition of Fe(CO)5 and reduction of Pd(OAc)2 in oleylamine and 1-octadecene [34].

3.2.4 Coprecipitation Method It has been reported that the Au-metal oxide hybrids, such as AuFe2O3, AuNiO, and AuCo3O4, could be synthesized by the coprecipitation method [6,35] using sodium carbonate, HAuCl4, and metal nitrate. In this direction, Donkova et al. [36] have synthesized the AuZnO nanoparticles using Na2CO3, HAuCl4, and Zn(NO3)2. Similarly, AuZnO hybrid nanoparticles were also fabricated by the same procedure [37]. Ma et al. [38] have successfully synthesized the AgFe3O4 core 2 shell nanowires by coprecipitation method using FeCl3, FeCl2, and polyvinylpyrrolidone (PVP). This method is simple and efficient, but the coprecipitation protocol might affect the controllability of nanoparticle shape, thus their size had a broad distribution with certain aggregations. The purity and stoichiometric phase of nanohybrids were difficult to obtain by this chemical coprecipitation.

3.2.5 Sonochemical Synthesis Sivasankaran et al. [39] reported a novel sonochemical synthesis of Pdmetal oxide hybrid nanoparticles. In the sonochemical reactor batch, they have successfully fabricated the PdCuO nanohybrids from copper salt in the presence of palladium and water. They signaled that, by aid of ultrasound energy (20 KHz and 32 KHz), the transition metal salts could be converted into their oxides in the presence of palladium and water. In this study, either the palladium source was pure metallic palladium Pd(0) or palladium salts (palladium acetate, palladium nitrate). Before sonication, Pd(0) should be synthesized from palladium salts by using ethyl alcohol as the reducing agent. Ziylan-Yavas et al. [40] have synthesized the PdTiO2 and AuTiO2 nanohybrids by using both high-frequency ultrasound (35 KHz) and UV-irradiation (254 nm). In this work, metal salts (Na2PdCl4 3H2O, Na(AuCl4) 2H2O), commercial TiO2 powder, and





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polyethyleneglycol monostearate were used. In this direction, AgNP decorated TiO2 nanoparticles were also fabricated through the sonochemical route in alkaline solution in the presence of ethylene glycol [41]. Similarly, Pol et al. [42] produced the AgNPs decorated SiO2 by using the ultrasound irradiation (20 KHz) in an aqueous solution containing the silica slurry, silver nitrate, and ammonia. The sonochemical synthesis was found to be faster, safer, less complicated, and is an eco-friendly method, as compared to other traditional preparative methods of hybrid nanoparticles.

3.2.6 Seeding Growth Method To synthesize the hybrid nanoparticles, the lattice mismatch between the two nanoparticles is an important issue. For this problem, the seeding growth approach is a very effective method, especially in the case of dumbbell-like hybrid nanoparticles. Fan et al. [43] argued that the lattice mismatch between the two noble nanoparticles was below 5% when using the epitaxial growth. By using the epitaxial seeding growth, Zhang et al. [44] have synthesized the AgFe3O4 heterodimeric hybrid nanoparticles, by using the preformed Fe3O4 nanoparticles as the seeds in oleylamine and toluene. In addition, the seed-mediated growth method has been effectively used for synthesizing the high-quality noble metal nanoparticles [45] due to its high controllability. In this method, the typical shape-directing reagents were cetyltrimethylammonium bromide and cetylpyridinium chloride [46]. Recently, we reported the synthesis of 816 nm AgFe3O4 dumbbell-like nanoparticles (Fig. 3.4) using nano-Fe3O4 (8 nm, as the seeds, Fig. 3.5) and oleylamine as both the reducing and capping agents [47]. Liu et al. [48] fabricated the dumbbell-like 514 nm AuFe3O4 nanoparticles, using 5 nm AuNPs as the seeds in the presence of oleic acid and oleylamine. Similarly, the dumbbell-like 310 nm PtFe3O4 NPs were synthesized by using 3 nm PtNPs as seeding agent in the presence of oleylamine [11]. FIGURE 3.4 TEM image of Fe3O4 NP seeds dispersed in DCB [47].

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FIGURE 3.5 TEM image of Fe3O4Ag hybrid nanoparticles dispersed in DCB [47]. The larger ones being AgNP nanoparticles (d  16 nm) and the smaller ones being Fe3O4 NPs (d  8 nm).

When using AuNPs or PtNPs as the seeds, AuZnO hybrid nanopyramids [49] or PtZnO nanoflowers [50] have been synthesized, respectively.

3.3 PHYSICAL FABRICATIONS OF HYBRID NANOPARTICLES 3.3.1 Laser-Induced Heating Process The pulsed laser ablation in liquid (PLAL) method has been mostly used to synthesize noble metal nanoparticles from their bulk metal targets. The PLAL synthesized nanoparticles were charged and free of surfactants, thus their colloids are very stable and pure [51]. By using the laser-induced heating (532-nm laser), Singh and Soni [52] have fabricated the rattle-type Ag@Al2O3 nanohybrids. AgNPs and AlNPs were first synthesized by laser ablation of sliver and aluminum metallic substrates in an aqueous solution of PVP. The colloidal solution of as-prepared AgAl nanoparticles was then post-irradiated by the unfocused laser beam in the aqueous solution. Fig. 3.6 presents the schematic route for the synthesis of rattle-type AgAl2O3 coreshell nanoparticles in water. In this figure, TEM photos observed at different post-irradiation stages are included. Zhang et al. [53] fabricated the gold nanoparticles (5 nm) decorated ceria nanotubes by using PLAL method (248-nm laser). Similarly, Siuzdak et al. [54] produced the platinum nanoparticles (320 nm) decorated TiO2 nanoparticles using the 1064-nm laser.

3.3.2 Atom Beam Cosputtering Method The nanocomposite thin films, which are composed of noble metal nanoparticles (Ag, Au) and oxide matrices (SiO2, ZnO, Al2O3, GeO2), have been successfully fabricated by using the atom beam cosputtering method [5560]. Fig. 3.7 shows the basic setup of this atom beam cosputtering technique. In this direction, Mishra et al. [60] have synthesized AuZnO nanohybrids by using the subsequent annealing at 600oC after atom beam cosputtering. I. FUNDAMENTALS

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FIGURE 3.6 Evolution procedure from sintered structure to coreshell and finally to rattle structure by pulsed laser ablation process. Source: Reprinted with permission from R. Singh, R.K. Soni, Synthesis of rattle-type Ag@Al2O3 nanostructure by laser-induced heating of Ag and Al nanoparticles, Appl. Phys. A 121 (2015) 261. Copyright 2015 Springer.

FIGURE 3.7 Schematic of diagram of atom beam cosputtering setup. Metal: Ag; matrix: SiO2, atom source: 1.5 keV neutral Ar atoms. Source: Reprinted with permission from Y.K. Mishra, S. Mohapatra, D. Kabiraj, B. Mohanta, N.P. Lalla, J.C. Pivin, et al., Synthesis and characterization of Ag nanoparticles in silica matrix by atom beam sputtering, Scr. Mater. 56 (2007) 629632.

3.3.3 Ion Implantation Method The ion implantation has been widely applied for optical waveguides and semiconductor chips. In this technique, ionized atoms (guest) were accelerated and oriented into the target (host) substrate. The ion energies used ranged from several hundred to several

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million electron volts. In the host lattice, these guest atoms collide with the host atoms, thus losing their energy, and finally embed within the solid matrix. The implant amount depends on the ion dose. In the case of silver nanoparticles, the ion doses are in the 1015 to 1017 ions/cm2 range [61]. Fig. 3.8 shows the basic physical stages of nanoparticle synthesis by ion implantation with different ranges of ion dose [62]. By using high-energy ion implantation, Wang et al. [63] have fabricated Au nanoparticles in TiO2 single crystal. Similarly, Chang et al. [64] have produced the Ag-implanted TiO2 nanoparticles.

3.4 SUMMARY AND FUTURE TREND The synthesis of hybrid nanoparticles is a promising field and could bring various new discoveries and novelties in the next decades. The hybridization of two or more metals (noble or nonnobles) can provide new and unexpected properties, which are not present in their counterparts. In this chapter, we have traced the most recent chemical and physical methods for the synthesis of hybrid nanoparticles, issued from the literature and the authors’ own works. Each preparative method exhibits both advantages and drawbacks. By controlling the synthesis processes (treatment temperature, seeding agents, chemical composition, surfactants, reducing agents, and preparative methods), various morphologies of hybrid nanoparticles can be obtained, such as coreshell-like, rattle-type, bricklike, flowers-like, and dumbbell-like nanoparticles. The current tendency for the synthesis of hybrid nanoparticles relates to simple, costeffective, and eco-friendly methods with multifunctional properties. Regarding the application of hybrid nanoparticles, there are promising applications of these materials in various application fields, such as biomedicine, antibacterial, energy storage, electronics, and catalysis. The development of future synthetic methods of nanostructural hybrid particles will be focused on the desired final properties, linked to end-use applications. The use of the recent advanced nanoscale characterization techniques and computable molecular modeling will be expected to provide more useful information for the design of the complex hybrid systems and thus help lead to a larger pathway for the development of hybrid nanostructural materials.

FIGURE 3.8 Basic physical stages of nanoparticle synthesis by ion implantation vs ion dose. Source: Reprinted with permission from A.L. Stepanov, Optical properties of metal nanoparticles synthesized in a polymer by ion implantation: a review, Tech. Phys. 49 (2) (2004) 143153. Copyright 2004 Springer.

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[39] S. Sivasankaran, S. Sankaranarayanan, S. Ramakrishnan, A novel sonochemical synthesis of metal oxides based bhasmas, Mater. Sci. Forum 754 (2013) 8997. Available from: https://doi.org/10.4028/www.scientific.net/MSF.754.89. [40] A. Ziylan-Yavas, Y. Mizukoshi, Y. Maeda, N.H. Ince, Supporting of pristine TiO2 with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis, Appl. Catal. B: Environ. 172 (2015) 717. [41] Y.-Y. Jhuang, W.-T. Cheng, Fabrication and characterization of silver/titanium dioxide composite nanoparticles in ethylene glycol with alkaline solution through sonochemical process, Ultrason. Sonochem. 28 (2016) 327333. [42] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, et al., Sonochemical deposition of silver nanoparticles on silica spheres, Langmuir 18 (2002) 33523357. [43] F.-R. Fan, D.-Y. Liu, Y.-F. Wu, S. Duan, Z.-X. Xie, Z.-Y. Jiang, et al., Epitaxial growth of heterogeneous metal nanocrystals: from gold nano-octahedra to palladium and silver nanocubes, J. Am. Chem. Soc. 130 (22) (2008) 69496951. Available from: https://doi.org/10.1021/ja801566d. [44] L. Zhang, Y.H. Dou, H.C. Gu, Synthesis of AgFe3O4 heterodimeric nanoparticles, J Colloid Interface Sci. 297 (2) (2006) 660664. May. [45] N. WenXin, Z. Ling, X. Guo Bao, Seed-mediated growth method for high-quality noble metal nanocrystals, Sci. China Chem. 55 (11) (2012) 23112317. Available from: https://doi.org/10.1007/s11426-012-4681-z. [46] J.X. Gao, C.M. Bender, C.J. Murphy, Dependence of the gold nanorod aspect ratio on the nature of the directing surfactant in aqueous solution, Langmuir 19 (2003) 90659070. [47] P. Nguyen Tri, T.A. Nguyen, C. Ouellet-Plamondon, T. Le Xuan, S. Rtimi, Antibacterial mechanism and nanoscale structure of dumbbell-like Fe3O4Ag hybrid/PE nanocomposites, J. Hazard. Mater., under review paper, Hazmat D-17-05675. [48] S. Liu, S. Guo, S. Sun, X.-Z. You, Dumbbell-like AuFe3O4 nanoparticles: a new nanostructure for supercapacitors, Nanoscale 7 (2015) 48904893. Available from: https://doi.org/10.1039/C5NR00135H. [49] P. Li, Z. Wei, T. Wu, Q. Peng, Y. Li, Au 2 ZnO hybrid nanopyramids and their photocatalytic properties, J. Am. Chem. Soc. 133 (15) (2011) 56605663. Available from: https://doi.org/10.1021/ja111102u. [50] J. Yuan, et al., Synthesis of ZnOPt nanoflowers and their photocatalytic applications, Nanotechnology 21 (2010) 185606. [51] S. Barcikowski, G. Compagnini, Advanced nanoparticle generation and excitation by lasers in liquids, Phys. Chem. Chem. Phys. 15 (2013) 3022. Available from: https://doi.org/10.1039/c2cp90132c. [52] R. Singh, R.K. Soni, Synthesis of rattle-type Ag@Al2O3 nanostructure by laser-induced heating of Ag and Al nanoparticles, Appl. Phys. A 121 (2015) 261. Available from: https://doi.org/10.1007/s00339-015-9429-0. [53] J. Zhang, G. Chen, M. Chaker, F. Rosei, D. Ma, Gold nanoparticle decorated ceria nanotubes with significantly high catalyticactivity for the reduction of nitrophenol and mechanism study, Appl. Catal. B: Environ. 132133 (2013) 107115. [54] K. Siuzdak, M. Sawczak, M. Klein, G. Nowaczyk, S. Jurgac, A. Ceniana, Preparation of platinum modified titanium dioxide nanoparticles with the use of laser ablation in water, Phys. Chem. Chem. Phys. 16 (2014) 1519915206. Available from: https://doi.org/10.1039/C4CP01923G. [55] S. Mohapatra, Plasmonic properties of Ag nanoparticles embedded in GeO2SiO2 matrix by atom beam sputtering, Phys. Chem. Chem. Phys. 18 (2016) 38783883. [56] J. Singh, K. Sahu, A. Pandey, M. Kumar, T. Ghosh, B. Satpati, et al., Atom beam sputtered AgTiO2 plasmonic nanocomposite thin films for photocatalytic applications, Appl. Surf. Sci. 411 (2017) 354. Available from: https://doi.org/10.1016/j.apsusc.2017.03.152. [57] D.K. Avasthi, Y.K. Mishra, R. Singhal, D. Kabiraj, S. Mohapatra, B. Mohanta, et al., Synthesis of plasmonic nanocomposites for diverse applications, J. Nanosci. Nanotechnol. 10 (2010) 27052712. [58] Y.K. Mishra, S. Mohapatra, D. Kabiraj, B. Mohanta, N.P. Lalla, J.C. Pivin, et al., Synthesis and characterization of Ag nanoparticles in silica matrix by atom beam sputtering, Scr. Mater. 56 (2007) 629632. [59] M. Tiwary, D.C. Agarwal, S. Mohapatra, J.C. Pivin, D.K. Avasthi, S. Annapoorni, Synthesis and characterizations of Aualumina nanocomposites prepared by atom beam co-sputtering, Phys. Status Solidi A 209 (2012) 24992504. [60] Y.K. Mishra, S. Mohapatra, R. Singhal, D.K. Avasthi, D.C. Agarwal, S.B. Ogale, AuZnO: a tunable localized surface plasmonic nanocomposite, Appl. Phys. Lett. 92 (2008) 43107.

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[61] A.L. Stepanov, Ion-synthesis of Metal Nanoparticles and their Optical Properties, NovaSci. Publ., New York, 2011. [62] A.L. Stepanov, Optical properties of metal nanoparticles synthesized in a polymer by ion implantation: a review, Tech. Phys 49 (2) (2004) 143153. Available from: https://doi.org/10.1134/1.1648948. [63] C.M. Wang, Y. Zhang, V. Shutthanandan, S. Thevuthasan, G. Duscher, Microstructure of precipitated Au nanoclusters in TiO2, J. Appl. Phys. 95 (2004) 8185. Available from: https://doi.org/10.1063/1.1748859. [64] Y.-Y. Chang, Y.-N. Shieh, H.-Y. Kao, Optical properties of TiO2 thin films after Ag ion implantation, Thin Solid Films 519 (20) (2011) 69356939.

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C H A P T E R

4 Nanoscale Characterization Srabanti Ghosh and Rajendra N. Basu Fuel Cell & Battery Division CSIR-Central Glass and Ceramics Research Institute, Kolkata, West Bengal, India

4.1 INTRODUCTION Noble metalmetal oxide hybrid nanoparticles (HNPs) represent an important class of nanomaterial for the tuning of the optical, electrical, magnetic, and catalytic properties of nanocrystals [14]. In HNPs, two different functional materials (i.e., metal/magnetic oxides or metal/semiconductor oxides) are combined through surface reconstruction around the junction [5], lattice mismatch-induced crystal strain [6], and electron interaction/ transfer across the interface [7], etc. These multifunctional nanomaterials can exhibit novel physical and chemical properties, which are essential for future technological applications. For example, coreshell [8,9], yolkshell [10,11], and heterodimers [12,13], which can enable electronic [14,15] and magnetic [16] coupling between the constituent domains and therefore allow for multifunctionalities that are not possible in single-component systems. Broad range of applications of HNPs originated from strong interaction between metal and metal oxides, which facilitate the dispersion of small particles hereby promoting utilization of expensive noble metal and also improved the catalytic properties of metal NPs [17,18]. Control of the dimension of each component of HNPs permits the widespread engineering of electronic energy state configuration within the nanoscale architecture, which makes them promising materials for a wide range of applications in biomedical imaging [19], cancer treatment [20], solar-energy harvesting [21,22], heterogeneous catalysis [23,24], photonics [25], and optoelectronics [26]. Moreover, a controlling mechanism for improved physicochemical properties can be achieved by precisely manipulating the charge transfer, charge carrier dynamics, and electronhole separation within the HNPs. Up to now, the synthesis of HNPs has been accomplished by various approaches, most of which have been generally based on seeded growth techniques which allowing magnetic, metallic, and fluorescent spherical components to be combined into binary (such as of Fe3O4Ag [27],

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00004-8

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AuFe3O4, [28], NiFe2O3 [29], PdCu2O [30], Fe3O4Pt [31,32], AuTiO2 [33], AgTiO2 [34], Pt, Pd, Ag, AuCu2O [9]) and ternary HNPs (such as ZnO/Pt/Cd12xZnxS [2], AuFe3O4Au [35], Fe3O4AuPbSe [36] PtFe3O4MnOx [37], AgPtFe3O4 [38,39]). Further developments have demonstrated the possibility to grow foreign material sections onto selected locations of anisotropically shaped NCs. Examples include ZnO decorated with Ag [40], AuZnO hybrid nanopyramids [41], etc. Over the past decades, a number of excellent reviews describing the various aspects of metalmetal oxides HNPs have been published [4244]. In particular, methods for characterization of hybrid nanomaterials, such as preparations, final compositions, or large-scale products, generally are in the stage of development. This chapter provides an overview of recent progress in the characterization of metal oxidenoble metal nanohybrids, particularly highlighting the general characterization techniques to understand the morphology controlled formation of metalmetal oxides HNPs that contain noble metal and magnetic or semiconductor nanoparticles, and illustrating the interesting optical and magnetic properties found in these hybrid particles determined through various advanced characterization tools. For example, integration of noble metals (e.g., Au, Ag, Pt, Pd, etc.) and metal oxides (e.g., Fe3O4, TiO2, ZnO, CeO2, ZrO2, etc.) into the coreshell or yolkshell single nanostructures can be studied by measuring the metal oxide shell of a certain thickness via transmission electron microscopy. Electron and atomic force microscopy, optical spectroscopy, and radiation scattering techniques are widely used. Applying these techniques to measure nanoparticle size, structure, and composition can help to understand the underlying synthesis mechanism. High resolution transmission electron microscopy (HRTEM) coupled with energy dispersive X-ray spectroscopy (EDS) are essential tools for the structural characterization of the HNPs providing lattice parameters and revealing the crystal structure [45]. Aberration-free high-angle annular dark field (HAADF) [46] and scanning TEM (Z-STEM) [47] are also attractive tools to analyze the chemical composition of the HNPs as they allow different elements to be imaged separately and it is possible to have a clear understanding of the structural basis of the hybrid structures, especially the core/multishell structures. Z-STEM is highly sensitive to the atomic number and can thus be exploited to achieve atomic resolution elemental mapping of the hybrid structures. All of these measurements give valuable information regarding the nanoparticle’s physical behavior, but it is important to realize under which conditions the measurements have been performed. Moreover, combining metal and a semiconductor nanoparticle has been found to be interesting as the metal can provide an anchor point for electrical and chemical connections to the functional semiconductor part. Significant changes of the optical properties of the semiconductor material upon coupling with the metal nanoparticles have also been observed. The optical properties of these HNPs are determined by the complex interaction between the enhancement of the local excitation field and the modification of radiative and nonradiative exciton decay rates which includes a shift in the plasmon resonance of noble metal nanocrystals or changes in the photoluminescence intensity of semiconductor nanocrystals [48]. These structures are found to be photocatalytically active because of their appropriate band alignment for water photolysis. Hence, it is deemed important to characterize a final HPNs product to obtain better insight into the design and application of well-defined nanohybrids in both the energy and environmental fields (Fig. 4.1).

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FIGURE 4.1 Schematic representation of metalmetal oxides hybrid nanoparticles.

4.2 MORPHOLOGICAL CHARACTERIZATION 4.2.1 Transmission Electron Microscopy The most widely used technique for characterizing nanoparticles is transmission electron microscopy (TEM) or high resolution TEM (HR-TEM), which provide direct visual information on the size, dispersity, structure, and morphology of nanoclusters [4951]. TEM has been used for materials characterization for a long time but its need has increased after the realization of the vast possible scope of property tailoring with the decreasing length scale of materials in various dimensions like thin-films, nanotubes/nanorods/quantum wires, nanoparticles/quantum dots, etc. Manna and coworkers [52] provided a study on the formation of AuFexOy heterostructures in which spinel ferrite (FexOy) was grown on a spherical gold (Au) from the TEM images. Fig. 4.2A demonstrates a typical sample of AuFexOy heterostructures, the FexOy nanorod section grown on the Au seeds in the presence of dodecyl dimethyl ammonium bromide (DDAB) has diameters in the 56 nm range and is 6080 nm long and in each nanoparticle the Au seed could be located at any position along the rod section. HRTEM analysis of these nanostructures give clear evidence that the rods always grew along the [2 2 0] direction of spinel ferrite (Fig. 4.2B). In addition, a full preferential orientation relationship is determined between the gold and the spinel ferrite nanocrystals, i.e., Au(0 0 2)//spinel ferrite(1 1 1) and Au[2 2 0]//spinel ferrite[2 2 0].

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FIGURE 4.2 (A) TEM image of AuFexOy HSs. (B) HRTEM image of AuFexOy HSs with the epitaxial relationship between Au NCs and FexOy HSs, the lower inset sketching the growth direction of the FexOy nanorod on the Au seed. (C) Atomic models sketching the atomic periodicity along one couple of gold and FexOy facets that are in contact in the AuFexOy HSs. For iron oxide only the Fe tetrahedral atomic sub-lattice is shown. Source: Reproduced from C. George, A. Genovese, F. Qiao, K. Korobchevskaya, A. Comin, A. Falqui, et al., Optical and electrical properties of colloidal (spherical Au)(spinel ferrite nanorod) heterostructures, Nanoscale 3 (2011) 46474654 with permission from The Royal Society of Chemistry, 2011.

This symmetry relation can be rationalized by considering that eight surface unit cells of the (0 0 2) gold facets match with one surface unit cell of the (1 1 1) spinel ferrite facet (Fig. 4.2C). Such periodicity leads to a commensurate epitaxy with low lattice mismatch values (m) along two directions lying in the interfacial plane, the former parallel to the rod elongation direction (m 5 1.03%) and the latter perpendicular to it (m 5 1.73%). Sun et al. [53] developed a general method for coating oxides having NPs, nanowires (NWs), and nanotubes of different compositions combined with noble metal to create a

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69 FIGURE 4.3 TEM images of the Au@oxide NPs (dAu 5 40 nm) with different kinds of oxide shells: (A) Au@TiO2, (B) Au@Fe3O4, (C) Au@MnO, (D) Au@Eu2O3. Insets show magnified views of typical NPs. Source: Reproduced from H. Sun, J.T. He, J.Y. Wang, S.-Y. Zhang, C.C. Liu, T. Sritharan, et al., Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation, J. Am. Chem. Soc. 135 (2013) 9099 9110 with permission from American Chemical Society, 2013.

large variety of coreshell nanostructures. Fig. 4.3 shows TEM images of four distinct nanohybrids that are generated by growing Au nanoparticles using different metal oxides. Notably, a common problem in generating these oxide shells is that their rates of reactions can be quite different. In contrast to pure oxide NPs, a large amount of oxide is obtained in the product. For oxide shells, the reaction needed to be slowed to reduce homogeneous oxide nucleation; in fact the most effective way is to use less reactive reactants, and also change the solvent, lower the reactant concentration, or increase the seed concentration. The sample shown in Fig. 4.3A represents a typical Au@TiO2, which is formed using ethanol as the solvent, which reduced the hydrolysis rate of TiF4 and led to successful TiO2 encapsulation. Fig. 4.3B shows a different sample of AuFe3O4 coreshell synthesized using FeCl2 which reacted slower, allowing the formation of uniform oxides shells, that can be suitably oxidized to give Fe3O4, and while FeCl3 can be used to generate iron oxides, it leads to fast reaction, causing the homogeneous nucleation of pure α-Fe2O3 spindles and therefore the presence of FeCl2 was a significant criteria for the synthesis of the AuFe3O4 coreshell structures. Further, Mn(CH3COO)2 is used as the Mn source for generating MnO shells on Au in place of using MnCl2 which leads to fast hydrolysis (Fig. 4.3C). Fig. 4.4D shows Au@Eu2O3 coreshell structures synthesized from a rare earth oxide of Eu2O3 nanoparticle seeds. Sun et al. [53] further extended this study to developed a general method for coating of ZnO on different seeds. Citrate-stabilized Au, Ag, and Pt nanospheres can be easily coated with ZnO shells using 4-mercaptobenzoic acid as ligand (Fig. 4.4AC). Moreover, Pd nanospheres, Ag nanocubes, and Ag nanowires that were stabilized by polyvinylpyrrolidone (PVP) can be directly coated with ZnO without addition

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FIGURE 4.4 TEM images and photographs of metal@ZnO NPs that were synthesized from different noble metal cores: citrate-stabilized NPs, including (A) Au nanospheres (dAu 5 15 nm); (B) Ag nanospheres (dAg 5 60 nm); and (C) Pt nanospheres (dPt 5 40 nm); and PVP-stabilized NPs, including (D) Pd nanospheres (dPd 5 20 nm); (E) Ag nanocubes (dAg 5 150 nm); and (F) Ag NWs (dAgNW 5 120 nm, lAgNW 5 35 μm). Insets show magnified views of typical NPs. Scale bar: 200 nm. Source: Reproduced from H. Sun, J.T. He, J.Y. Wang, S.-Y. Zhang, C.C. Liu, T. Sritharan, et al., Investigating the multiple roles of polyvinylpyrrolidone for a general methodology of oxide encapsulation, J. Am. Chem. Soc. 135 (2013) 90999110 with permission from American Chemical Society, 2013.

of a ligand as shown in TEM images (Fig. 4.4DF). Particularly, for the Ag nanocubes and nanowires, it can be observed that the uniform ZnO shells conformed to the shape of the seeds.

4.2.2 Atomic Number Contrast Scanning Transmission Electron Microscopy Atomic number contrast scanning transmission electron microscopy (Z-STEM) provides an exceptional ability to achieve structural and chemical information from individual nanostructures at the atomic level [54]. Principally, Z-STEM uses a HAADF detector to collect an incoherent image, a direct image of the object’s structure [5557]. In contrast to traditional HRTEM that uses phase-contrast imaging to gain insight into the crystalline nature of the particles, the intensity seen in the Z-STEM images depends on the scattering power of the atom being imaged, yielding chemical information simultaneously with structural position, which makes Z-STEM an ideal tool for studying coreshell structures at the atomic level. The intensity difference between the core and shell or heterodimer or heterotrimer seen in Z-STEM images allows precise characterization of shell shape, coverage, chemical composition, and the presence of any extended defects. For example, ZSTEM tomography was used to probe the chemoselective addition of Ag to PtFe3O4 heterodimer seeds to form AgPtFe3O4 heterotrimers [58]. Fig. 4.5AC displays HAADFSTEM images of AgPtFe3O4 samples at different time interval (A) 15 min aliquot, (B) 60 min aliquot, and (C) the final product after the 14-h reaction, respectively. Fig. 4.5A and B demonstrates small particles decorating both the Pt and Fe3O4 surfaces

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FIGURE 4.5 Representative HAADFSTEM images of aliquot samples taken from the AgPtFe3O4 synthesis at (A) 15 min and (B) 60 min into the reaction, as well as (C) the final AgPtFe3O4 heterotrimer product isolated after 14 h. The corresponding EDS elemental maps are shown in panels DG, HK, and LO, respectively, and indicate that Ag indiscriminately nucleates on the PtFe3O4 heterodimers to form various Ag (PtFe3O4) intermediates, followed by coalescence onto the Pt domain to form AgPtFe3O4. Source: Reproduced from J.M. Hodges, J.R. Morse, M.E. Williams, R.E. Schaak, Microscopic investigation of chemoselectivity in AgPtFe3O4 heterotrimer formation: mechanistic insights and implications for controlling high-order hybrid nanoparticle morphology, J. Am. Chem. Soc. 137 (2015)1549315500 with permission from American Chemical Society, 2015.

of the PtFe3O4 heterodimer seeds while, after 14 h, the expected final nanoparticle product has AgPtFe3O4 heterotrimer architecture as in Fig. 4.5C. Further STEMEDS is used to create elemental maps of each sample. The STEMEDS maps for the aliquot taken after 15 min (Fig. 4.5DG), which show small Ag nanoparticles attached to both the Pt and Fe3O4 surfaces of the PtFe3O4 heterodimer seeds (marked as Ag (PtFe3O4)), as well as PtFe3O4 heterodimers having no detectable Ag. After 60 min, the number and size of the Ag NPs growing on the PtFe3O4 seeds has increased, and the formation of AgPtFe3O4 heterotrimers has initiated, as indicated by the STEMEDS maps shown in Fig. 4.5HK. The STEMEDS maps illustrated in Fig. 4.5LO confirm that after 14 h the final nanoparticle product is AgPtFe3O4 heterotrimer configuration.

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The formation of the AgPtFe3O4 heterotrimers initiates with indiscriminate Ag nucleation onto both the Pt and Fe3O4 surfaces of PtFe3O4, followed by surface diffusion and coalescence of Ag onto the Pt surface to form the AgPtFe3O4 product. These results provide exceptional microscopic insights into the pathway by which AgPtFe3O4 heterotrimer NPs form and controlling nucleation and growth therefore permits the development of high-order HNPs with precisely targeted morphologies and properties. Ortalan and coworkers [59] identified a strong metalsupport bonding in nanoengineered AuFe3O4 dumbbell-like NPs by in situ TEM where the average diameters of the Au NPs and the Fe3O4 NPs are 5.0 and 10.4 nm, respectively. Drastic morphological changes of Au NP from a spherical NP to Au thin films (i.e., complete wetting) on Fe3O4 during the vacuum annealing directly indicates the presence of strong bonds between Au and Fe3O4 during the in situ annealing experiment. Further, in situ STEM-coupled with electron energy loss spectroscopy (EELS) as well as in situ electron diffraction results indicates that the core part of the final state still has a form of unreduced Fe3O4 and this suggests that the drastic morphology change of the dumbbell NPs is due to the interaction between the Au thin film and the iron oxides. Very recently, Lord et al. [60] tested electrical contacts with multiprobe electrical transport measurements and correlated this behavior directly to aberration-corrected scanning transmission electron microscopy (ac-STEM) for the Au catalystZnO nanowire system which is the only known material that exhibits quantummechanical edge tunneling in such a way that the effect can be used to modulate the transport properties from Schottky to ohmic. The ac-STEM analysis shows atomic- and nanoscale modifications to the interface edge can entirely alter the transport properties rather than the less-influential central zone of the circular AuZnO interfaces.

4.2.3 Scanning Tunneling Microscopy Scanning tunneling microscopy is the most suitable technique to obtain a single-dot image at room temperature (TE 300K) as well as low temperatures (T 5 4.2100K) [61]. The atomic-resolution of STM is attributed to the imaging mechanism based on the quantum tunneling phenomena and STM measures the tunneling current I that is generated by a bias voltage V between the atomically sharp STM tip and the material surface [61,62]. ˚ reduction in disThe tunneling current increases by an order of the current for every 1 A tance. The distance in the xyz-directions is also controlled by a piezoelectric scanner, which provides angstrom-order changes in the distance, and a feedback loop, which controls the z-direction, is installed to keep the tunneling current constant. By observing the tunneling current as the tip scans a surface, the morphology of the surface can be precisely detected. Measurements can be carried out at room temperature in solution as well as at low temperature under ultrahigh vacuum (UHV) conditions. For example, Rieboldt et al. [63] utilized STM measurement to study the nucleation and growth of Pt NPs on TiO2 (1 1 0) surfaces of different oxidation state such as with O on-top atoms (oxidized TiO2 represented as o-TiO2), surface O vacancies (represented as r-TiO2), and H adatoms, respectively (reduced TiO2 represented as h-TiO2). At room temperature, Pt is found to be trapped at O on-top atoms and surface O vacancies, leading to rather small Pt NPs. In contrast, on surfaces with H adatoms the mobility of Pt is much larger and large Pt NPs are found at room temperature on TiO2 (1 1 0) surfaces with H adatoms. Fig. 4.6 illustrates

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FIGURE 4.6 STM images of the r-TiO2 (A and D), h-TiO2 (B and E), and o-TiO2 (C and F) surfaces after evaporation of Pt at RT. The STM images were acquired at RT. Source: Reproduced from F. Rieboldt, L.B. Vilhelmsen, S. Koust, J.V. Lauritsen, S. Helveg, L. Lammich, et al., Nucleation and growth of Pt nanoparticles on reduced and oxidized rutile TiO2 (1 1 0), J. Chem. Phys. 141 (2014) 214702 with permission from American Institute of physics, 2014.

STM images acquired following the evaporation of B2.5% ML Pt at room temperature (RT). In case of the r-TiO2 (Fig. 4.6A and D) and o-TiO2 (Fig. 4.6C and F) surfaces, high densities (0.07 6 0.01 nm22) of small Pt NPs with homogeneous NP distributions (15% 20% of Pt NPs at step edges) are obtained. In contrast, larger Pt NPs are found on the h-TiO2 surface and their density was at 0.030 6 0.005 nm22 rather low (Fig. 4.6B and E). Hence, the STM results indicate that Pt NPs or Pt atoms on the h-TiO2 surface have a higher mobility at RT than Pt on r- and o-TiO2 surfaces and thus larger NPs are formed on h-TiO2.

4.3 QUANTIFICATION OF METAL CONTENT IN NANOHYBRIDS The mass percentages of metal and metal oxides in HNPs are measured by inductively coupled plasma optical emission spectra (ICP-OES) analysis or inductively coupled plasma mass spectrometry (ICP-MS) [64]. ICP-MS has become the technique of choice for detection and characterization of nanoparticles in solution. Compared with other techniques, ICPMS is unique in its ability to provide information on elemental composition, with high sensitivity, multielement capability, wide linear dynamic range, high sample throughput, and ability to discriminate between isotopes [65,66]. ICP-MS is capable of scanning

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mass-to-charge (m/z) range 5240 amu with a minimum resolution of 0.9 amu at 10% peak height. In principle, ICP-MS can be used to directly detect and quantify the signal generated from the atomization and ionization of a single particle introduced to the plasma, known as “single particle mode.” If the masses and densities of the elemental constituents of the particle are known along with the elemental response factor based on an ionic calibration standard, then the theoretical size of the particle, calculated as a sphere, can be determined. If the transport efficiency from the nebulizer to the plasma is also known, then the particle number concentration can be determined. For example, the mass percentages of Pd and ZnO in four Pd/ZnO composites are determined by ICP-OES analysis. The measured mass percentages of Pd in each composite (2.67%3.5%) were generally larger or close to the theoretical mass percent of Pd (B2.9%) based on the experimental dosages of Pd(acac)2 and Zn(acac)2, indicating that the addition of Pd(acac)2 to the precursor solution is almost completely transferred into Pd NPs and incorporated into the metal oxide support reported by Bao et al. [67]. Recently, single-particle or particle-mode ICPMS (spICP-MS) has been considered as a novel nanoparticle characterization technique, which is used in the time-resolved mode for the measurement of dilute NP dispersions (particle concentrations less than 105/mL are adequate) [68,69]. After the statistical evaluation of the signal time profile and assuming a spherical NP geometry, information can be obtained about not only the elemental (isotopic) composition of the NPs, but also their characteristic size and distribution, as well as the particle concentration. For metallic NPs, size detection limits ranging from 10 to 30 nm and upper detectable size limits around or above a few hundred nm are typically reported [70]. The spICP-MS technique has been used to determine the concentration of Pt in a Pt/silica nanocomposite by Sa´pi et al. [71]. Fig. 4.7A shows that the lognormal Pt NP peak in the signal histogram is well resolved

FIGURE 4.7

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Pt spICP-MS signal histogram recorded for the Pt/SiO2 nanocomposite. Please note that the lognormal NP signal peak is well separated from the solution background signal. The characteristic intensity of the NP peak in the histogram corresponds to an equivalent NP size of 20.4 nm in the spICP-MS size calibration curve. Source: Reproduced from A. Sa´pi, A. Ke´ri, I. Ka´lomista, D.G. Dobo´, A. Szamosvo¨lgyi, K.L. Juha´sz, et al., Determination of the platinum concentration of a Pt/silica nanocomposite decorated with ultra-small Pt nanoparticles using single particle inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 32 (2017) 9961003 with permission from The Royal Society of Chemistry, 2017.

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from the background peak. The mode of the particle peak was found to be equivalent to the signal from a 20.4 nm diameter spherical Pt particle. Considering the size and density of the support and the load particles, the Pt wt.% concentration is calculated to be 0.0966 wt.%, with a standard deviation of 0.0025 based on three repeated measurements.

4.4 CRYSTAL PHASE CHARACTERIZATION THROUGH X-RAY TECHNIQUES XRD is powerful technique that has long been used to address all issues related to the crystal structure of solids, including lattice constant, geometry, identification of unknown materials, orientation of single crystal, preferred orientation of polycrystals, defect, etc., and can also be used to determine the interfacial strain by comparing and calculating the shift from the spectra of the individual component [72,73]. The diffraction pattern is used to identify the crystalline phases and measure its structural properties. The broadening of the XRD peaks reflects either crystallinity or the size of the nanocrystal. Assuming that the crystallinity of nanoparticles is not too different, the broadening of the XRD peaks reflects the size of nanocrystals only: smaller nanocrystals have a wider reflection peak. However, the nanoparticles often form twinned structures; therefore, the Scherer’s formula may produce results different from the true particle size. In addition, X-ray diffraction only provides the collective information of the particle sizes and usually requires a sizable amount of powder. Compared to electron diffraction, the low intensity of diffracted X-rays is obtained particularly for low Z-materials, i.e., XRD is more sensitive to high Z-materials. Bao et al. [67] developed a one-pot synthetic methodology of noble metal/zinc oxide composites with controllable morphology and high catalytic performance controllable morphology including tube-like, flower-like, star-like, and skin needling-like. The crystal structures of the Pd/ZnO composites are examined by the XRD patterns (Fig. 4.8). All XRD diffraction peaks of the four Pd/ZnO composites could be indexed as a combination of the typical wurtzite structure of ZnO (JCPDS 36-1451) and the face centered-cubic structure of Pd (JCPDS 46-1043). FIGURE 4.8 XRD patterns of the tube-like (A), flower-like (B), star-like (C), and skin needlinglike (D) Pd/ZnO composites. The red color belongs to the wurtzite structure of ZnO and the blue color belongs to the face-centered-cubic structure of Pd, respectively. Source: Reproduced from Z. Bao, Y. Yuan, C. Leng, L. Li, K. Zhao, Z. Sun, One-pot synthesis of noble metal/zinc oxide composites with controllable morphology and high catalytic performance, ACS Appl. Mater. Interfaces 9 (2017) 1641716425 with permission from American Chemical Society, 2017.

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However, it is difficult to observe the signal from Pd NPs in the composites due to the small size of the Pd NPs in the four composites compared to ZnO particles. Tremel and coworkers [74] used Rietveld refinements to the powder XRD data to characterize directly the epitaxial growth of γ-Fe2O3 nanorods to the Pd nanotetrahedra (Pdnth@Fe2O3) and nanoplates (Pdhnp@Fe2O3). Crystalline phases were identified according to the PDF-2 database using Bruker diffractometer. Full profile fits (Le Bail/Pawley/ Rietveld) were performed with TOPAS Academic version 4.1 by applying the fundamental parameter approach [75,76]. Fig. 4.9A shows the X-ray diffractogram of the Pdnth@γFe2O3 superparticles. The reflections can be assigned to Pd as well as γ-Fe2O3. The γ-Fe2O3 nanorods show a (1 1 1) orientation on the Pd(1 1 1) surface. The intensity of the Pd reflections is much lower compared to corresponding reflections for the Pd nanotetrahedra due to the absorption by the surrounding γ-Fe2O3 nanorods (Fig. 4.9A). Hence, the maghemite [Fe0.67(1)O] content with a crystallite size of 13(1) nm could be refined. For Pdhnp@Fe2O3 superparticles (Fig. 4.9B), a FIGURE 4.9 Rietveld refinements to the powder XRD data of (A) Pdnth@Fe2O3 superparticles and (B) Pdhnp@Fe2O3 superparticles. Red dots mark the experimental data; the black line corresponds to the calculated pattern, and the red line shows the difference between the experimental and calculated data. Black ticks mark reflections of Pd. Q 5 [4π sin(Θ)]/λ is the scattering vector. Source: Reproduced from M. Kluenker, M.N. Tahir, R. Ragg, K. Korschelt, P. Simon, T.E. Gorelik, et al., Pd@Fe2O3 superparticles with enhanced peroxidase activity by solution phase epitaxial growth, Chem. Mater. 29 (2017) 11341146 with permission from American Chemical Society, 2017.



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Pd content of 11(1) wt.% could be extracted from the Rietveld refinement. Hence, the presence of a Pd core, selectively overgrown with maghemite nanorods could be established for nanohybrids.

4.5 SURFACE CHARACTERIZATION The surface characterization of the HNPs involve common techniques like X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES), and they have penetration depths comparable to the typical dimensions of the HNPs [77,78]. The electronic structure of the constituent elements in the HNPs can be well-defined by XPS, giving further composition information of these NPs. The depth of the photoemitted electrons escaping from the very top surface of samples by the Al Kα X-ray source (1486.6 eV photons) is usually in the 0.53 nm range, comparable to half of the diameters of a NPs (for example, B5 nm). Thereby, XPS can help us to obtain the electronic structures of elements in cores and coatings, or whether they are metallic or oxidized. For example, XPS has been used to analyze surface composition of Au/ZnO nanowire hetero-nanoarrays, in which C 1s (284.8 eV) is used to calibrate the binding energies [79]. The full XPS spectra confirm the existence of Zn, O, Au, and C elements and also reveal that the cross-linked Au/ZnO nanowire arrays are successfully formed without any existing impurity (Fig. 4.10A). The high resolution XPS spectrum for Zn element (Fig. 4.10B) in Au/ZnO nanowire arrays shows two broad peaks centered at 1021.8 eV and 1044.8 eV, which can be indexed as the signals from Zn 2p3/2 and Zn 2p1/2, respectively the binding energy of Zn oxides. Remarkably, there is a distinct peak located at about 530.6 eV which is associated with the lattice oxygen of ZnO, whereas the weaker shoulder peak at about 531.9 eV can be attributed to chemisorbed oxygen caused by the surface hydroxyl groups, as shown in Fig. 4.10C. The high energy resolution XPS spectrum for Au element (Fig. 4.10D) shows noticeable peaks centered at 83.6 eV and 87.3 eV that are attributed to Au 4f7/2 and 4f5/2, which exhibit a negative shift of 0.2 eV in comparison to 83.8 eV of the bulk Au. This minor shift is caused by electron transfer from plasmonic Au thin nanowires to ZnO nanowire arrays due to the strong electronic interaction between the Au and oxide support [80,81]. XANES spectroscopy is a well-established technique providing information on the electronic and structural properties of materials. X-ray absorption occurs in the region of approximately 40 eV above the edge and is sensitive to the treatment of interactions between the photoelectron and the core hole. In XANES, a photon is absorbed and electron is excited from a core state to an empty state and photon energy has to be equal or higher than the binding energy of this core level. Hence, the energy of an absorption edge corresponds to the core-level energy, which is characteristic for each element, making XANES an element selective technique. The evolution of the phases involved during the annealing can also be followed through the semiquantitative analysis of XANES spectra. The thermal evolution of Pt-Rich FePt/Fe3O4 heterodimers during the annealing under an inert atmosphere is followed by in situ time-resolved Pt L3 and Fe K edges XANES spectroscopy experiments [82]. Fig. 4.11A shows the Fe K edge and Pt L3 XANES spectra of Pt-Rich FePt/Fe3O4 heterodimers. XANES features at the Fe Kedge similar to the magnetite (Fe3O4). The main component of the pre-edge peaks of Fe3O4 arises from tetrahedrally

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FIGURE 4.10 (A) Wide XPS spectrum and high resolution XPS spectra of (B) Zn 2p, (c) O 1s, and (D) Au 4f of cross-linked Au/ZnO nanowire arrays. Source: Reproduced from T. Wang, B. Jin, Z. Jiao, G. Lu, J. Ye, Y. Bi, Photodirected growth of Au nanowires on ZnO arrays for enhancing photoelectrochemical performances, J. Mater. Chem. A 2 (2014) 1555315559 with permission from The Royal Society of Chemistry, 2014.

coordinated Fe3þ, and the shoulder corresponds to the octahedrally coordinated Fe2þFe3þ ions. The average oxidation state of iron from these XANES data is 2.5(1)þ, which is slightly less than expected for Fe3O4 (2.67þ), indicating the major iron phase is magnetite, and also contains a small percentage of a metallic phase (γ-Fe2O3). Fe K edge EXAFS are suitably fitted to a cubic spinel structure and the Fe-O distances are close to ˚ as expected for the tetrahedral and octahedral sites in Fe3O4, the values of 1.89 and 2.06 A respectively. A reduction of the amplitude of the second main peak is observed in comparison to Fourier transform of bulk Fe3O4, which is associated to the local structural disorder due to the higher percentage of atoms at the particle surface layer in the NPs. Further detailed analysis can be obtained from the Fourier transforms χ(R) of the EXAFS spectra at the Pt L3 edge shown in Fig. 4.11C. ˚ , which is shorter than The nearest neighbor distance PtPt (or PtFe) is close to 2.74 A ˚ the value expected for pure platinum (2.77 A). This indicates the incorporation of low quantity of iron into platinum forming a FePt alloy. The inhomogeneity in the Pt

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FIGURE 4.11 XANES spectra at the (A) Pt L3 and (B) Fe K edges of as-made Pt-rich FePt/Fe3O NPs. Spectra of Fe and Pt reference foils are also shown for comparison. (C) Fourier transforms of k3χ(k) EXAFS at Pt L3 edge and at Fe K edge of the as-made Pt-rich FePt/Fe3O4 NPs. Solid lines correspond to the fitting results. XANES of bulk Fe3O4 is also shown for comparison. Source: Reproduced from M. Ahmad, S. Yingying, A. Nisar, H. Sun, W. Shen, M. Wei, et al., Synthesis of hierarchical flower-like ZnO nanostructures and their functionalization by Au nanoparticles for improved photocatalytic and high performance Li-ion battery anodes, J. Mater. Chem. 21 (2011) 77237729 with permission from American Chemical Society, 2011.

concentration lies in the different nucleation velocity, Pt atoms nucleate faster than the Fe ones, leading to a Pt concentration that decreases from inside to outside of the NP.

4.6 SPECTROSCOPIC CHARACTERIZATION 4.6.1 UVVis and Photoluminescence Spectroscopy To unravel the photophysical properties of the HNCs, a combination of spectroscopic techniques are needed. Absorption, photoluminescence (PL), and PL excitation (PLE) provide the basic information about the exciton energy level structure. PLE spectra are

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particularly better suited than the absorption spectra for the identification and assignment of absorption transitions since only the emitting states contribute to this spectra. Timeresolved PL spectroscopy serves as a quantitative tool for the analysis of photoexcitation dynamics in HNCs yielding information about both radiative and nonradiative exciton recombination channels. To probe the dynamics of other ultrafast processes, such as intraband relaxation, multiexciton generation, and exciton spatial separation, several advanced spectroscopic techniques like transient absorption, femtosecond fluorescence upconversion, and THz time domain spectroscopy can be used which provide complimentary information regarding the fast relaxation of electrons and holes. Optical spectroscopic technique can be generally categorized into two groups • Absorption and emission spectroscopy—determines the electronic structure of atoms or molecules or crystals through exciting electrons from the ground to excited states, i.e., absorption, and relaxing from the excited to ground states, i.e., emission. • Vibrational spectroscopy—involving the interaction of photons with the species that results in energy transfer via vibrational excitation or de-excitation and provides useful information about molecular structure. UVvisible absorption spectroscopy is the easiest tool available to characterize nanocrystals [83]. In addition one can get an estimate of the size distribution and concentration from the sharpness of the absorption peak. In the case of the combination of the metal and semiconductor, hybrid structures exhibit significant changes in the absorption spectrum which are not typical linear additions of the absorbance of the individual components. Amalgamation of the electronic states of metal and semiconductor results in modified density of states which in turn affects the absorption spectrum. Typically, upon the growth of the metal domains onto semiconductor substrate, the excitonic peak and the fine structure of the semiconductor component becomes less pronounced with the increase and shifting of the plasmonic band of the metals. Fig. 4.12 demonstrates the UVvis diffuse reflectance spectra of the pure porous TiO2 and the Aux/TiO2 nanohybrids with different Au loading [84]. The pure porous TiO2 shows strong absorption in the UV region, whereas Aux/TiO2 nanohybrids reveals a broad absorption feature at B600650 nm, which is assigned to the localized surface plasmon resonance (LSPR) of Au NPs supported on TiO2. The intensity of Au LSPR increases as the Au loading increased from 2 to 10 wt.%. The wavelength and intensity of the Au LSPR signal both depend on the Au particle size and shape, as well as the surrounding medium. The broadening of LSPR peaks of Au/TiO2 hybrid materials has been observed due to the broad size distribution Au NPs or located at different positions in the porous TiO2 support. Fluorescence spectroscopy is one of the most widely used spectroscopic techniques in the fields of material chemistry [85]. Although fluorescence measurements do not provide detailed structural information, the technique has become quite popular because of its acute sensitivity to changes in the structural and dynamic properties of nanomaterials. The fluorescence spectroscopic studies can be carried out at many levels, ranging from simple measurement of steady-state emission intensity to quite sophisticated timeresolved studies. The fluorescence spectrum which owes its origin to the semiconductor component, in the presence of metal domains leads to interplay between fluorescence quenching and enhancement effects [86,87]. Fluorescence quenching may arise from

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FIGURE 4.12 UVvis diffuse reflection spectra (DRS) of (i) pure porous TiO2, (ii) Au2/TiO2, (iii) Au5/TiO2, and (iv) Au10/TiO2. Source: Reproduced from B. Li, Y. Hao, X. Shao, H. Tang, T. Wang, J. Zhu, et al., Synthesis of hierarchically porous metal oxides and Au/TiO2 nanohybrids for photodegradation of organic dye and catalytic reduction of 4-nitrophenol, J. Catal. 329 (2015) 368378 with permission from Elsevier, 2015.

energy transfer from the exciton in the semiconductor to the metal. Alternatively, enhancement has been observed by chemically linking gold nanocrystals to the surface of semiconductor nanowires. Such an enhancement is often seen in cases in which the semiconducting and metallic domains are separated by a small distance and a large potential barrier is present between the two domains. Kostopoulou et al. [88] reported the synthesis of anisotropic HNPs that are individually comprised of a single rod-shaped ZnO section, ubiquitously decorated with multiple nearly spherical Fe@FexOy domains. Fig. 4.13A and B shows TEM images of two representative examples of HNPs which are distinguished by a relative high and low surface coverage of the relevant ZnO NR cores, respectively (referred to as HNC-1 and HNC-2). The variable-dimension of ZnO NRs are studied by PL spectroscopy as shown in Fig. 4.13C and D. The nanorods exhibit a pronounced near band-edge (NBE) UV emission located at 3.2 eV for the seeds of the HNC-1 sample (Fig. 4.13A, upper curve) and at 3.25 eV for ZnO seeds used in the growth of the HNC-2 sample (Fig. 4.13B, upper curve). While the difference in the NBE between the two batches is less as the ZnO NR dimensions are much larger than the exciton Bohr radius (B2.34 nm) due to quantum confinement effects. Moreover, a broader and much weaker band emission was observed in the visible spectral region (B2.4 meV) for all samples, related to such deep level defects. It is important to note that the NBE emission is shifted to the blue spectral region in comparison to the parent NR seeds (Fig. 4.13). The NBE spectral shift upon coverage is ΔNBE 5 175 meV for the HNC-1 sample, while it is moved by ΔNBE 5 128 meV for the HNC-2 sample. The modifications in the optical properties of the ZnO should be a consequence of the coverage of its surface by the Fe@FexOy nanodomains. Time-resolved photoluminescence spectroscopy was applied to study the distinct differences between magneticplasmonic heterodimers, Au@MnO and Au@Fe3O4 altered by the variation of the electronic structure of the metal oxides by Tremel and coworkers [89].

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FIGURE 4.13 PL at 300K of the HNC-1 (A) and HNC-2 (B) samples compared to the corresponding ZnO NRs seeds (top spectra). The vertical dash line shows the blue shift of the NBE emission due to the Fe@FexOy coverage attained over ZnO. Each spectrum has been normalized to the associated NBE UV emission max intensity. Source: Reproduced from A. Kostopoulou, F. The´tiot, I. Tsiaoussis, M. Androulidaki, P.D. Cozzoli, A. Lappas, Colloidal anisotropic ZnOFe@FexOy nanoarchitectures with interface-mediated exchange-bias and bandedge ultraviolet fluorescence, Chem. Mater. 24 (2012) 27222732 with permission from American Chemical Society, 2012.

Fig. 4.14 displays the fluorescence spectra and decay dynamics of the pristine Au NPs in comparison to Au@Fe3O4 and Au@MnO heterodimers. Thiol-functionalized Au NPs and Au@Fe3O4 heterodimers show a maximum of photoluminescence at 481 and 475 nm, respectively, and it is shifted to 463 nm for Au@MnO with an additional peak at 632 nm as shown in the time-integrated fluorescence spectra (Fig. 4.14A). The emission spectrum of Au NPs is almost corresponding with the one of Au@Fe3O4 heterodimers, which suggests the Au domains to be the origin of the fluorescence.

4.6.2 Fourier Transforms Infrared Spectroscopy Infrared spectrometry is a vibrational technique that involves coupling of highfrequency infrared (IR) electromagnetic radiation ranging from 1012 to 1014 Hz (3300 μm wavelength), with vibration of chemical bond [90]. In the infrared spectroscopy, the intensity of a beam of infrared radiation is measured before and after it interacts with the sample as a function of light frequency. A plot of relative intensity versus frequency is the “infrared spectrum.” As the interferogram is measured, all frequencies are being measured simultaneously. Thus, the use of the interferometer results in extremely fast measurements. Then the intensitytime output of the interferometer is subjected to a well-known mathematical technique called the Fourier transformation to convert it to the familiar infrared spectrum. This transformation is performed by the computer, which then presents

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FIGURE 4.14 (A) Time-integrated emission spectra of pure Au nanoparticles, Au@Fe3O4 and Au@MnO heterodimers. The samples were excited at 400 nm by a 100-fs laser pulse. (B) Photoluminescence dynamics monitored at the emission peak wavelength and stretched-exponential fits using the parameter inverse decay rates τ and stretching exponent β. Source: Reproduced from I. Schick, D. Gehrig, M. Montigny, B. Balke, M. Pantho¨fer, A. Henkel, et al., Effect of charge transfer in magneticplasmonic Au@MOx (M 5 Mn, Fe) heterodimers on the kinetics of nanocrystal formation, Chem. Mater. 27 (2015) 48774884 with permission from American Chemical Society, 2015.

FIGURE 4.15 PSDDRIFT spectra collected at 498K during a MES experiment for both Au0.75Pd0.25/Al2O3 and Au0.80Pd0.20FexOy/Al2O3; inset: magnification of the peak, indicating the evolution of the band components at increasing ϕdelay (from the weakest green spectrum to the weakest orange spectrum). Source: Reproduced from C. George, A. Genovese, A. Casu, M. Prato, M. Povia, L. Manna, et al., CO oxidation on colloidal Au0.80Pd0.20FexOy dumbbell nanocrystals, Nano Lett. 13 (2013) 752757 with permission from American Chemical Society, 2013.

the user with the desired spectral information for analysis. The surface active sites can be identified by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [91]. The enhanced catalytic activity of CO oxidation by the dumbbell (Au0.80Pd0.20FexOy) nanocrystalline catalyst in comparison to Au0.75Pd0.25 NPs can be determined by using time-resolved diffuse reflectance infrared Fourier transform spectroscopy coupled with modulation excitation spectroscopy (MES) (Fig. 4.15). The kinetic

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information provided by phase sensitive detection (PSD) processed spectra (Fig. 4.15A and B) shows that the formation of CO surface species, in the spectral range 21001900 cm21, and the formation of gaseous CO2, in the spectral range 24002300 cm21, are evidently faster in the case of the dumbbell catalyst than for the metal “only” catalyst (phase delay, ϕdelay 5 260 , Fig. 4.15B, vs ϕdelay 5 280 , Fig. 4.15A, for CO adsorption; ϕdelay 5 290 , Fig. 4.15B, vs ϕdelay 5 2120 , Fig. 4.15A, for CO2 formation). The surface species are detected on both catalysts (Au0.75Pd0.25/Al2O3 and Au0.80Pd0.20FexOy/Al2O3) by IR spectra. This could be correlated to the presence of the epitaxial connection between the FexOy and the Au0.80Pd0.20 domains, resulting in an electron flow from the FexOy domain to the Au0.80Pd0.20 domain and influence favorably the nature and composition of the catalytically active surface sites of the dumbbells. In fact, when the metal alloy domain is attached to the metal oxide domain, surface Pd species are more active compared to the noble metal Au0.75Pd0.25 domain and also Auδ2 sites are formed that are not present on the initial Au0.75Pd0.25 NCs.

4.6.3 Nuclear Magnetic Resonance Spectroscopy Nuclear magnetic resonance (NMR) serves as a powerful tool for assigning MRI commercial tomographs dedicated to clinical use and laboratory research [92]. In modern diagnosis where highly accurate information is desired, single-mode contrast agents are not always sufficient. Dual-mode T1T2 contrast agents, combining the advantages of positive and negative contrasts, may allow for improved diagnosis by sharpening anatomical details in the MR image [92,93]. However, development of dual-mode agents with strong T1T2 contrast effects is very challenging. 1H NMR relaxometry characterizations have been used to measure the longitudinal and the transverse nuclear relaxation times, T1 and T2, respectively, in the 5212 MHz frequency range, which corresponds to an external magnetic field spanning from 0.15 to 5 T. The superparamagnetic metal oxides NPs have the ability to enhance the image contrast in MRI techniques by modifying the proton relaxation rates in different tissues. The NPs induce magnetic field inhomogeneities in the surrounding medium that significantly decrease the transverse relaxation time (T2) of the protons and. the shortening in T2 leads to a signal loss and, in turn, to negatively contrast images. Figuerola et al. explored bimagnetic hybrid nanocrystals, comprising size-tuned FePt and inverse spinel iron oxide domains epitaxially arranged in a heterodimer configuration as MRI contrast agents [94]. The NMR dispersion (NMRD) profile allows measuring the frequency dependence of the longitudinal R1 and transverse R2 nuclear relaxivities. Fig. 4.16 displays the longitudinal R1 and transverse R2 relaxivities (panels A and B, respectively) as a function of the frequency for four FePtiron oxide HNP samples with different dimensional features. The Endorem contrasting agent (commercial material) is used as the reference. The HNPs mainly behaved as T2 relaxing MRI contrast agents and the effects of HNP dimensions and geometry on their longitudinal relaxivity (R1) are not significant, as observed in Fig. 4.16A. While, a variation in the proton transverse relaxivity as a function of the HNC dimensions is noticeably observed. The R2 values for each sample remained constant over the whole frequency range investigated, as shown in Fig. 4.15B. The preliminary studies on the proton nuclear relaxation in the presence of the

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FIGURE 4.16 Plots of roomtemperature 1H NMRD relaxivities R1 (A) and R2 (B) vs frequency for the various HNC samples dissolved in water. The FePt and iron oxide domain dimensions were, respectively: (a) 10.0 and 16.0 nm, (b) 4.0 and 11.1 nm, (c) 6.2 and 15.4 nm, (d) 8.9 and 12.0 nm. For a comparison, the relaxivity values for Endorem contrasting agent are also shown. Source: Reproduced from A. Figuerola, A. Fiore, R.D. Corato, A. Falqui, C. Giannini, E. Micotti, et al., One-pot synthesis and characterization of size-controlled bimagnetic FePtiron oxide heterodimer nanocrystals, J. Am. Chem. Soc. 130 (2008)14771487 with permission from American Chemical Society, 2008.

FePtiron oxide HNCs have proven the possibility to reach relaxivity values comparable or even higher compared to the commercial Endorem contrasting agent. According to the NMRD profiles, the transverse relaxation becomes progressively faster with comparatively larger heterodimers, whereas, such effect cannot be obtained for FePt seeds. The enhanced performances of the HNPs may associate with the iron oxide component in the heterostructures. The experimental result suggests that the proton relaxation rate scales up with the overall dimensions of HNPs, so that an improvement with respect to Endorem contrasting agent can be achieved for heterodimers larger than B20 nm.

4.7 ELECTROCHEMICAL CHARACTERIZATION Photoelectrochemical cell is a photocurrent-generated device composed of an electrolyte, a photoactive semiconductor electrode [95]. Under irradiation of the interface electrolytesemiconductor with an energy level greater than the band gap of the

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semiconductor, electronhole pairs are generated. The charge in an oxides-based semiconductor is distributed creating a space charge region that enables the separation of the electronhole pairs. Photoelectrochemical (PEC) characterizations can be conducted in a single compartment cell with a pyrex window, using a three-electrode configuration system where the prepared samples are the working electrodes, a Pt wire is the counter electrode, and Ag/AgCl (saturated KCl) is a reference electrode in the presence of aqueous electrolyte (NaOH, KCl, or Na2SO4 aqueous solution). A low power UV-LED (365 nm) can be used as a light source. Linear scan voltammetry (LSV) has been carried out using a Potentiostat. It is generally known that transient photocurrent always reflects the transfer and separation of photoinduced charge carriers under intermittent illumination. As the light is turned on, the photocurrent values increase while the photocurrent values decrease rapidly as the light is turned off. Photocurrent measurements of an anatase/rutile mixedphase titanium dioxide (TiO2) hierarchical network deposited with Au nanoparticles (Au/ TiO2 ARHN) are investigated in Fig. 4.17 [96]. The measured photocurrent was normalized to the sample area to obtain the photocurrent density for comparison. Fig. 4.17A shows the LSV curves of the different samples in the dark and under light irradiation. Remarkably, a 4.5-fold enhancement of the photocurrent for Au/TiO2 ARHN was observed as compared to that for TiO2 under AM 1.5G solar illumination, suggesting its potential application in PEC cells. The photocurrents of the Au/TiO2 ARHN samples improved compared with that of bare TiO2, suggesting that the ARHN revealed a stronger ability to separate photogenerated electronhole pairs. The low photocurrent density is observed due to the limit of the wide bandgap characteristics of TiO2 (3.2 eV for anatase and 3.0 eV for rutile), which allows only UV light absorption. The generation of photocurrent for the samples is observed via many onoff cycles which indicate that the electrodes are stable and the photocurrent is quite reversible. In Fig. 4.17B all electrodes show a good reproducibility and stability as the illumination was turned on and off.

4.8 OTHER TECHNIQUES Elam and coworkers [97] examined the atomic layer deposition of Pd and Pt films onto a variety of metal oxide surfaces including Al2O3, ZrO2, and TiO2 using in situ quartz crystal microbalance (QCM) and quadrupole mass spectrometry (QMS) to explore the nucleation and growth of the Pd and Pt on the different metal oxide surfaces. QCM measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator. QMS provides intensity of a specific fragment where particular ions of interest are being studied, as it can stay tuned on a single ion for extended periods of time. Fig. 4.18A displays QCM measurements of Pd atomic layer deposition (ALD) on an Al2O3 surface. The Pd deposition can be divided into two stages: nucleation (below B100 cycles) during which the Pd film thickness changes very slowly, and growth (above B100 cycles) during which the Pd film thickness increases linearly with the number of cycles. This transition occurs at a Pd film thickness of B1 Pd monolayer as indicated in Fig. 4.18A. Both HCOH and hydrogen gas (H2) were used as the reducing agent for Pd ALD. It is not possible to

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FIGURE 4.17 (A) Linear sweep voltammograms and (B) amperometric It curves of TiO2 NW, TiO2 ARHN and Au/TiO2 ARHN photoelectrode. Source: Reproduced from Y.-C. Yen, J.-A. Chen, S. Ou, Y.-S. Chen, K.-J. Lin, Plasmon-enhanced photocurrent using gold nanoparticles on a three-dimensional TiO2 nanowire-web electrode, Sci. Rep. 7 (2017) 42524 with permission from Nature Publishers, 2017.

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FIGURE 4.18 Pd nucleation and growth on Al2O3 at 200 C examined by (A) QCM and (B, C) QMS. Source: Reproduced from J.W. Elam, A.V. Zinovev, M.J. Pellin, D.J. Comstock, M.C. Hersam, Nucleation and growth of noble metals on oxide surfaces using atomic layer deposition, ECS Trans. 3 (2007) 271278 with permission from The Electrochemical Society, 2007.

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nucleate the Pd ALD on Al2O3 surfaces using H2, however, once a film has been nucleated using HCOH, then continued deposition of the Pd is possible using H2. Further, QMS has been used to monitor the HCOH (m 5 30) and H2 (m 5 2) signals during the HCOH exposures for Pd deposition on Al2O3 (Fig. 4.18B and C). The HCOH signal decreases while the H2 signal increases during the Pd nucleation, and both of these signals remain constant during the Pd growth which may be explained by the decomposition of HCOH to form H2 that occurs on Pd. The rates of H2 production and HCOH consumption are low initially because the Pd coverage is low. Both of these rates increase and then level off as the Pd nucleates and grows to cover the entire Al2O3 surface.

4.9 CONCLUSION In this chapter, we describe the recent progress made in the study of metalmetal oxides nanohybrids characterized by microscopy, X-ray techniques, spectroscopy, and electrochemical measurements. Basic principles of such measurements for HNPs are summarized. We highlight the results of optical and photoelectrochemical properties of HNPs studied by UVvis and photoluminescence spectroscopy and LSV and chronoamperometric measurements. Photocurrent from HNPs is also introduced, based on PEC measurements. 1H NMR relaxometry characterization are presented to measure the longitudinal and the transverse nuclear relaxation times for MR images. This chapter provides an overview of HNPs characterization methods, which include the detailed understanding of formation and the optoelectronic properties of HNPs. A deep insight into the synthesis and mechanism of formation of HNPs enable the tuning of the physicochemical properties or imparting of multiple functionalities to HNPs for a broad range of applications. However, theoretical calculation and prediction for hybrid nanostructures and their mutual interaction have received little attention, which limits the rapid growth process of the field.

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C H A P T E R

5 Physics, Electrochemistry, Photochemistry, and Photoelectrochemistry of Hybrid Nanoparticles Phuong Nguyen Tri1, Sami Rtimi2, Tuan Anh Nguyen3 and Minh Thanh Vu4 1

Department of Chemistry, University of Montre´al, Montre´al, QC, Canada 2Swiss Federal Institute of Technology, School of Enginnering (STI), Powder Technology Laboratory (LTP), EPFL-STI-IMX-LTP, Lausanne, Switzerland 3Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam 4Institute of Chemistry and Materials, Hanoi, Vietnam

5.1 OVERVIEW Hybrid nanoparticles (HNPs) are expected to have more applications in biomedicine, antibacterial, energy storage, electronics, and catalysis than single nanoparticles. The chemical syntheses, especially the controllable nanostructures, are expected to provide better properties than the physical methods. In the physical preparation methods, the individual nanoparticle might remain separate and distinct within the finished nanostructure. Morphology and structure also significantly affect the properties of hybrid particles. Depending on end-used application fields, the type of HNPs, such as core/shell, dumbbell-like, Janus-like, and raspberry nanoparticles, could be selected appropriately. Among these hybrid nanostructures, dumbbell-like nanoparticles are expected to provide the best hybridization between single nanoparticles. This hybridization would be obtained

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00005-X

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by the heterostructural nanoparticles with two joined nanoparticles sharing a common interface. Regarding the localized surface plasmon (LRP) of noble metal NPs, the hybridizations with NPs oxide (with high refractive index) are expected to modify the local dielectric function around the noble metal NPs. Thus, the hybrid-NPs exhibit a shift of plasmon resonance (SPR) band. In the case of semiconducting oxides, the excitonplasmon interactions can be strong, weak, or neutral, depending on the degree of hybridization between the noble metal NPs and oxide NPs. In this direction, the coreshell structure is expected to have strong excitonplasmon interaction, whereas the excitonplasmon interactions in nanocomposites are expected to be weak or neutral. For the photoelectrochemical properties, depending on the wavelength of excited light, the electron transfers were taking place from semiconducting oxide NPs (UV radiation) to noble metal NPs and vice versa (visible light). In addition, the formation of Schottky barriers at the interface of noble metal NPs and semiconducting oxide NPs further improves the separation of photogenerated electrons and holes, thus reducing significantly their recombination rate.

5.2 PHYSICAL PROPERTIES 5.2.1 Effect of Metal Oxide NPs on the Localized Surface Plasmons of Noble Metal NPs Localized surface plasmons are excitations of the conductive free electrons of metallic nanostructures induced by the electromagnetic field [1]. In an oscillating electromagnetic field, the curved surface of the nanoparticles also exerts an effective restoring force on the driven electrons, so that a resonance can arise [1]. This resonance is called the localized surface plasmon resonance (LSPR). Another consequence of the curved surface of nanoparticles is that the plasmon resonances can be excited by direct light illumination. For gold and silver nanoparticles, the LSP can be observed under illumination with visible light [29]. The resonance frequency for the surface plasmon can be tuned by changing various parameters concerned with the type of metal [10], the nanoparticles size [11], the nanoparticle morphology [12,13], and surface charge [14], as well as dispersion concentration [15] and dispersion medium [16]. Fig. 5.1 shows an example of the extinction spectra of silver (Ag) nanoparticles, which can be turned with size and form of the nanoparticles. This figure indicates that the LSPR-induced an electric field enhancement around a single isolated 60-nm Ag nanoprism at 700 nm. 5.2.1.1 AuNPs-Based Hybrids It is well known that SPR is sensitive to dielectric environment [18]. Nano-Fe3O4 particles are reported to be most used for hybridization with noble metals due to its high refractive index [19] and thus its effect on the LSPR of noble nanoparticles is large. The presence of nano-Fe3O4 particles in the AuNPs-based hybrids exhibit LSPR similar to AuNPs, but the peak position is shifted as reported in the literature [20]. This

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0

FIGURE 5.1 Effect of metal natures (A) and its shape (B) and size (C) on extinction spectra of sub-100-nm Ag, Au, and Cu nanoparticles; (D) simulated spatial distribution of the LSPR-induced electric field enhancement around a single isolated 60-nm Ag nanoprism at 700 nm. Color corresponds to electric field enhancement. The large red arrow shows the point of maximum electric field enhancement. E is the direction of polarization; (E) spatial electric field enhancement (left) of two Ag nanoprisms arranged point-to-point at a separation of 2 nm with a three-dimensional contour plot (right) to emphasize the four orders of magnitude field enhancement in the “hot spot” between the particles. Source: Reproduced from S. Linic, U. Aslam, C. Boerigter, M. Morabito, Photochemical transformations on plasmonic metal nanoparticles, Nat. Mater. 14 (2015) 567576 with permission from Nature Publishing Group [17].

phenomenon was obtained in both the nanostructure of coreshell and dumbbell-like HNPs [20]. These two types of HNPs could be chemically synthesized by choosing the right solvent polarity. Unpolar solvent leads to dumbbell-like nanostructures, whereas more polar solvent leads to coreshell nanoparticles [21,22]. Absorption spectrum of Au nanoparticles with the average size 10 nm shows a LSPR peak at 520 nm [20,23]. However, coating of Au nanoparticles with nano-Fe3O4 shell causes a red shift of the Au-LSPR. Increasing the shell thickness leads to an increase of the red shift of the absorption peak (peak positions for incomplete shell, 2 nm shell, and 3 nm shell are 546, 559, and 573 nm, respectively). In the case of the dumbbell-like AuFe3O4 NPs, the slight red shift of LSPR peak was also observed [20], increasing from 520 nm to 528.5 nm. Wei et al. [24] also reported that the growth of nano-Fe3O4 particles around the gold core was accompanied by the red shift and broadening of the Au-SPR peak. Based on the classical Mie theory [25,26], Wei et al. explained that, the red shift is due to nano-Fe3O4, which increases the effective local dielectric function around gold nanoparticles. Broadening and decreasing

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intensity of the LSPR band are the results of interfacial charge transfer. The role of the surfactants was found to be important in the formation of well-defined and monodispersed AuFe3O4 heterostructured nanocrystals (HNCs) [27]. Nonnoble metal/noble metal ratio, temperature, and the nature of solvent were reported to influence the size and morphology of the final hybrid nanoparticles [27]. HNCs show broader plasmonic peak than the corresponding Au seeds and it was found to be shifted toward higher wavelengths (557 nm) with respect to the starting Au NCs (519 nm) (Fig. 5.2) due to the presence of iron oxide in direct contact with the Au domain [27]. In the case of AuTiO2 nanocomposites, the red shift of LPR peak position was also observed from 535 nm for virgin Au to 548 nm for hybrid nanoparticles [28]. A possible explanation is related to the densely attached nano-TiO2 particles, having a high refractive index with respect to bare AuNPs. Table 5.1 presents the values of SPR wavelengths, λ (nm), of AuNPs in various hybrid NPs. Table 5.1 shows that hybridization of AuNPs with oxide nanoparticles leads to the red shift of LPR peak position and hybrid of Au nanoparticles with nano-SiO2 shows the lowest shift due to its low refractive index (B1.46) compared to other nanoparticles such as nano-ZnO (B2), nano-ZrO2 (2.16), nano-Fe3O4 (B2,4), and nano-TiO2 (B2.6). The higher the refractive index of the oxide nanoparticles, the stronger red shift of the LPR peak position is observed. (A) 1.2

0.6

1.0

0.4

0.8

0.2

0.6

0.0

A (a.u.)

(B)

400

500

600

700

0.4 0.2 0.0 300

HNCs_acac Au NCs_B

400

(C) 1.2

700

800

(D) 0.7

1.0

0.6 0.5

0.8 A (a.u.)

500 600 λ (nm)

0.4

0.7

400

500

600

0.5 0.3 0.2 0.0 300

HNCs_CO Au NCs_A

400

500 600 λ (nm)

700

800

FIGURE 5.2 UVvis spectra (A, C) and HRTEM (B, D) of AuFe3O4 hybrid heterostructured nanocrystals (HNCs) modified by different surfactants. Source: Adapted with permission from E. Fantechi, et al. Seeded growth synthesis of AuFe3O4 heterostructured nanocrystals: rational design and mechanistic insights. Chem. Mater. 29 (2017) 40224035. Copyright (2017) American Chemical Society.

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TABLE 5.1 Values of Surface Plasmon Resonance (SPR) wavelength, λ (nm), of AuNPs in Various Hybrid NPs [20,23,2933] Nanocomposite

AuNPs

AuFe3O4 Dumbell Like

Au@SiO2 (coreshell)

Au@Fe3O4 (coreshell)

Au@ZrO2

AuTiO2

AuZnO

520 nm

528.5 nm

525 nm

546573 nm



580650 nm

560 nm

515 nm

520 nm





550 nm

550 nm



535 nm







548 nm



500 nm









583 nm, 505615 nm



5.2.1.2 AgNPs-Based Hybrids The red shift of the LSPR peak position was also seen in the AgNPs-based hybrids. The synthesis of dumbbell-like AgFe3O4 HNPs by seeding growth method was recently reported [34]. The TEM images of these AgFe3O4 hybrid nanoparticles (Fig. 5.3) showed large Ag-NPs (d  16 nm) and small Fe3O4 NPs (d  8 nm). The UVvis spectra of nanoFe3O4 NPs presents a broad absorption band in the region of 300600 nm (Fig. 5.4), which can be attributed to the absorption and scattering of UV radiation by magnetic NPs [35]. The absorption band at B360 nm indicates the formation of nanosized particles [36]. Fig. 5.4 shows that the AgNPs (16 nm of diameter) have a broad band around 398 nm, which is the characteristic of the SPR peak of AgNPs [37]. Fig. 5.4 shows that the hybridization of AgNPs and nano-Fe3O4 leads to red shift in SPR and also to the significant broadening of the SPR peak. It was reported that the SPR wavelength of AgNPs can be tuned by tailoring the size, shape, interparticle spacing and the surrounding medium (coating) [34]. For the single AgNPs, the characteristic SPR peak of AgNPs is located at 398 nm, whereas it is 415 nm for the Fe3O4Ag hybrid nanoparticles. The contribution of Fe3O4 NPs into the HNPs could be observed by the presence of the absorption band at B360 nm. The new band at 670 nm could be attributed to the hybridization of AgNPs and nano-Fe3O4. In a recent unpublished work, we also fabricated the AgNPs-decorated nano-TiO2 particles by using the chemical reduction of AgNO3 with the presence of nano-TiO2 dispersion (the weight ratio of Ag/TiO2 was 3 wt.%). Fig. 5.5 presents the electron microscopy images of nano-TiO2 particles before and after the decoration. As can be seen in this figure, Ag nanoparticles were dispersed on the surface of nano-TiO2 particles. For the majority of the nanoparticles, the AgNPs is quasi-spherical with an average size ,20 nm (Fig. 5.5B). The UVvisible absorption spectra of nano-TiO2 and AgNPs-doped nano-TiO2 particles (dispersed in water) shows that the hybridization of nano-TiO2and AgNPs leads to broadening of the AgNPs’ SPR peak, as compared with the SPR peak of single AgNPs (Fig. 5.5C). Table 5.2 collects the values of SPR wavelength, λ (nm), of AgNPs in various hybrid NPs [34,3747]. As can be seen in this table, hybridization of AgNPs with oxide nanoparticles exhibits the red shift of LPR peak position. In addition, the core/shell-like

I. FUNDAMENTALS

(A)

50 nm

(B)

20 nm

(C) 20 nm

Fe 3O4

Ag Fe 3O4

Ag

Ag

Fe 3O4

Dumbbell-like AgFe3O4 hybrid nanoparticles

FIGURE 5.3 TEM image of Fe3O4Ag hybrid nanoparticles: dumbbell-like nanoparticles with the larger ones are AgNP nanoparticles (d  16 nm) and the smaller ones are Fe3O4 NP (d  8 nm).

2.0

OL/OLA coated AgNPs OL/OLA coated Fe3O4NPs

360.8 nm 415.5 nm

1.5 Absorption (a.u.)

OL/OLA coated FeAgNPs

617.8 nm

398.5 nm 1.0

0.5

0.0 300

400

700 500 600 Wavelength (nm)

800

900

FIGURE 5.4 UVvisible absorption spectra of the AgNp, Fe3O4 NPs, and AgFe3O4 HNPs.

101

5.2 PHYSICAL PROPERTIES

(A)

FIGURE 5.5 TEM images of AgNPs decorated nano-TiO2 (A), pure nanoTiO2 (B), and their UVvis spectra (C). The different colors reflect solar light spectrum.

(B)

20 μm

50 μm

1.2 Nano-TiO2

AgNPs decorated nano-TiO2

1

Absorption (a.u.)

0.8 UVC UVB

UVA

Visible light

IRA (NIR)

0.6

0.4

0.2

0 250

350

450

550 650 Wavelength (nm)

750

850

nanostructure leads to the broadband absorption in UVvis spectra, whereas the dumbbell-like nanostructure brought a new peak in UVvis spectra for the HNPs.

5.2.2 SERS Effect of Hybrids Nanoparticles Surface-enhanced Raman spectroscopy or surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces or by nanostructures [48]. The enhancement factor can be as high as 1010 to 1011 that could detect single molecules [4952]. As compared to the traditional Raman scattering signals for single molecules, it can produce the random formation of localized plasmons or “hot spots” at the junctions, giving SERS enhancements up to 1014 as described in the literature [5355]. The mechanism of the enhancement effect of SERS is still an open question in the literature [56]. There are two main mechanisms that could explain this enhancement: (i) the chemical theory used for a substance having chemical bond with the metallic surface; or (ii) the electromagnetic theory used for a substance that was physically absorbed to the surface (Fig. 5.6).

I. FUNDAMENTALS

TABLE 5.2 Values of Surface Plasmon Resonance (SPR) Wavelength, λ (nm), of AgNPs in Various Hybrid NPs [34,3747] Coreshell nanostructure AgNPs Ag@TiO2

TiO2@Ag

Ag@ZnO ZrO2@Ag

Nanocomposite Ag@ZrO2 AgTiO2

AgZnO

AgZrO2

AgFe3O4 AgFe3O4 dumbell-like

Ref.

416 nm 416445 nm

[28]

407 nm 454 nm

[29]

410 nm 425430 nm

[30]

413 nm 474499 (super broadband)

[31]

408 nm 425 nm

[32]

410 nm 380 nm

Super broadband

[33]

410430 nm broadband

[33] 440 nm

[34] 420 nm broadband

[35] 425 nm

[35] 480530 nm

[21]

450 nm

490 nm

[35]

400 nm

480500 nm

[36] 434487 nm

435 nm 398 nm

[37] 453 nm

[38] 418 nmNew band 620 nm

[39]

103

Absorption (a.u.)

5.2 PHYSICAL PROPERTIES

(A)

(C)

Yeast SERS response

523 nm laser excitation

Ag NP’s

250 450 650 Wavelength (nm)

Yeast cell (D) 400 nm (B)

Cell wall Cell membrane

Exchange transporters

Ag NP’s 5 μm

Plasmonic field Chemical components to be detected

FIGURE 5.6 (A) Transmission electron microscope image of silver nanoparticles synthesized. Inset: ultraviolet visible absorbance spectra of the silver nanoparticle suspension. (B) Environmental scanning electron microscope image of yeast cells coated with silver nanoparticles. (C) Schematic of silver bonding to yeast cell wall, the exposure to the light source and the consequent generation of SERS signals. (D) Inset schematic of the silvers attachment to the cell wall showing the chemical component exchanges in and out of cell. The chemicals pass near the silver nanoparticles plasmonic SERS enhancement area. Source: Reproduced with permission A.F. Chrimes, et al. In situ SERS probing of nano-silver coated individual yeast cells, Biosens. Bioelectron. 49 (2013) 536541 [57].

The electromagnetic theory proposes the excitation of LSPs, whereas the chemical theory believes that there is a formation of charge-transfer complexes. However, it has been reported recently that SERS enhancement can be obtained even when the molecule is located relatively far from the nanostructural metallic surface [55]. Visible and near-infrared radiations (NIR) are generally used to excite Raman modes. Thus, regarding the electromagnetic theory, the silver and gold nanostructures are typically subtracted from SERS experiments because their plasmon resonance frequencies fall within these wavelength ranges, providing maximal enhancement under the visible and NIR light [58]. Platinum and palladium nanostructures also display plasmon resonance within visible and NIR frequencies [59]. SERS substrates were primarily restricted to noble metal structures for many years. However, it has been reported recently that various semiconducting oxides, such as ZnO [60], TiO2 [61], Cu2O [62], and CuO [63] can also generate weak SERS activity with the enhancement factors ranging from 101 to 103. Therefore, hybridizations between semiconductors (ZnO and TiO2) and noble metals (Au and Ag)

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have attracted a great deal of attention. These hybridizations are expected to have better SERS enhancement by the contributions from both the electromagnetic enhancement (excited by the LSPR of noble metals) and the semiconductor supporting chemical enhancement (caused by the charge transfer between the noble metal and the adjacent semiconductor) [6467]. For the magnetic nanobeads decorated with silver nanoparticles (AgMNBs), Di Corato et al. [68] reported that their size and adsorption peaks at 420 nm are observed to be increased with increasing AgNO3 concentration. Whereas, the progressive red-shift of these AgMNBs absorption band (from 420 to 440 nm) is observed when reducing the NaBH4 concentration. Tang et al. [69] reported that the AgNPs-decorated ZnO nanorods manifested a high SERS sensitivity to rhodamine and polychlorinated biphenyls at the detection limits as low as 10212 and 10211 mol, respectively. Chen et al. [70] also reported that the ZnO/Au nanoarrays had an enhancement factor to 1.2 3 107 with rhodamine (at concentration of 1 3 1027 mol). In SERS application, fabrication of noble metal-coated insulating oxide NPs is a costeffective method to reduce the expensive noble metals. By this direction, Pal et al. [71] reported that the SiO2@Au coreshell nanoparticles provided much more sensitive SERS detection of molecules with less gold matter than pure gold nanoparticles. The SiO2@Au NPs could detect the bis(4-pyridyl) ethylene (BPE) molecules at its concentration as low as 10211 mol. Moreover, by adjusting the ratio of the core (nanosilica) and shell (Au) radii, the plasmon resonance could be tuned to any wavelength of interest [72]. This SiO2@Au coreshell nanostructure was very stable and highly reproducible [7375]. Sun et al. [73] reported also a new route for synthesis of silicagold coreshell (SiO2@Au) nanospheres by thermally induced morphological self-reorganization and characteristic surface plasmon (SP) absorption of the metal shell. When the coverage of the nanoshell increases, the SP band in UVvisNIR spectra increases in intensity and shifts toward longer wavelengths from 554 to 915 nm. Noble metals SERS is inherently a double-edged sword: on the one hand, the intense electromagnetic field concentrated at localized hot spots enables ultrasensitive detection. On the other hand, the same effect can trigger photothermal and/or photochemical reactions as a result of heat dissipation and/or plasmon-driven surface reactions. In turn, this strong perturbation of chemical species in the close proximity of hot spots offers the opportunity to investigate chemical reactions triggered by metallic nanojunctions. In the past few years, several attempts have been made to maintain plasmonic SERS without inducing spurious side reactions. Coating plasmonic NPs or nanostructures with a thin dielectric layer is a straightforward option. The coating layer serves as a spacer between the plasmonic particles and the analyte. The concept of shell-isolated plasmonic NPs was extended by Li et al. [76]. to the use of silica coatings to keep metal NPs separated from each other and from the adsorbates. Both experimental evidence and theoretical simulations indicate that maximum SERS enhancement occurs when analyte molecules are located near the surface or in the nanogap regions of metal nanoparticles [77,78]. Accoding to the electromagnetic theory, SERS effect is mainly attributed to the SPR at very close to

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105

the surface of noble metal substrate. SPR is often observed at the gaps between microstructures, resulting in the so-called hot spots. When the incident laser light strikes the noble metallic surface, LSPs are excited. In order for scattering to occur, the plasmon oscillations must be perpendicular to the surface; if they are in-plane with the surface, no scattering will occur. It is because of this requirement that roughened surfaces or arrangements of noble metal nanoparticles are typically employed in SERS experiments. The use of magnetic oxides/noble metal hybrid NPs makes it possible to assemble noble metal NPs in a relatively orderly aggregated pattern with the assistance of an external magnetic field and has shown higher enhancement of SERS signals [79,80]. Bao et al. [81] fabricated AgFe3O4 NPs and maximized SERS sensitivity through changing the size and density of Ag particles. The optimized NPs could be used to detect rhodamine at a concentration as low as 10212. Wheeler et al. [80] demonstrated that magnetically induced aggregation of the Fe3O4Au coreshell NPs enhanced SERS activity by B7 times higher compared to nonmagnetically aggregated Fe3O4Au NPs. By preparing under an external magnetic field, Yu et al. [82] also reported that Fe3O4/Ag hybrid NPs could detect furazolidone at the levels as low as 40 ng/mL. For detection of arsenic contamination, Du et al. [83] reported a higher SERS sensitivity for Fe3O4@Ag coreshell magnetic NPs, as compared to the AgNPs. Fe3O4@Ag coreshell magnetic NPs had the detection limit of 10 μg/L, whereas AgNPs-based substrate was susceptible to matrix effects that could not even detect 10 mg/L As(V) in a groundwater sample.

5.2.3 Effect of Noble Metal Nanoparticles on the (Optical) Band Gap Energy of Semiconducting Oxide NPs In solid-state physics, for the insulators and semiconductors, a band gap energy generally refers to the difference of energy between the top of the valence band and the bottom of the conduction band. It is closely related to the HOMO/LUMO gap in chemistry. Band gap energy (Eg) is a major factor determining the electrical conductivity of material. Some metal oxides are isolators with a large band gap, such as Al2O3 (EgB8.3 eV), and many oxides have a wide band gap, such as ZrO2 (Eg B5.7 eV) [82], Ga2O3 (Eg B4.8 eV), In2O3 (Eg B3.6 eV), SnO2 (Eg B3.6 eV), ZnO (Eg B3.3 eV), CuAlO2 (Eg B2.22 eV) [83], and TiO2 (Eg B3.2 eV) [84]. Among these oxides, several semiconducting oxides with direct wide band gap, such as ZnO and TiO2 have the greatest practical impact for electronic and photovoltaic industry [8593]. However, the band gap of these semiconducting oxides is about B3.2 eV, which is only excited by UV light, which largely limits their practical applications in normal light condition (UV light makes up only B5% of solar energy) [94]. To extend the absorption band edge of these semiconducting oxides from UV to visible light region, many approaches have been developed. The intensification of the transverse plasmon resonance by coating TiO2 onto Au nanorods is also reported. The transverse plasmon mode of the resultant Au@TiO2 nanorods with a sufficiently thick shell can be comparable to or even stronger than the longitudinal one in intensity. The shell thickness

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FIGURE 5.7 FDTD-simulated scattering spectra of the uncoated Au nanorod and Au@TiO2 nanorods. (A, B) Simulated scattering spectra of the Au nanorod and Au@TiO2 nanorods under the transverse and longitudinal excitations, respectively. (C, D) Scattering efficiency spectra of the Au nanorod and Au@TiO2 nanorods under the transverse and longitudinal excitations, respectively. Under the transverse and longitudinal excitations, the electric field of the incident electromagnetic wave is perpendicular and parallel to the length axis of the nanorod, respectively, as shown in the insets of (c) and (D). For clarity, some spectra in (A) and (C) have been multiplied by the indicated factors. (E) Longitudinal and transverse LSPR wavelengths of the nanorods as functions of the shell thickness. (F) Peak scattering cross-sections for the longitudinal and transverse LSPR modes of the nanorods as functions of the shell thickness. (G) Ratio of the peak scattering cross-sections between the transverse and longitudinal modes of the nanorods as a function of the shell thickness. Source: Reproduced with permission Q. Ruan, et al. Highly enhanced transverse plasmon resonance and tunable double Fano resonances in gold@titania nanorods, Nanoscale 8 (2016) 65146526.

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107

has an affect not only on the plasmon peak position but also on its intensity for Au@TiO2 nanorods [95] (Fig. 5.7). Some of these works were based on doping various transition metals or rare earth ions into the oxides crystal lattice. For example, Umebayashi et al. [96] reported that S doping caused the absorption edge of TiO2 to be shifted into the lower energy region. Similar studies were also carried out by other researchers. Sakthivel et al. [97] compared the photonic efficiency of Pt-, Au-, and Pd-doped TiO2 with undoped TiO2. They revealed that absorption of light in the visible region by TiO2 increased with metal content. Recently, the coupling of semiconducting oxides with noble metals (Au, Ag, Pd, Pt) has turned out to be the most promising strategy to defeat a larger band gap [98]. The energy level alignment between the nanoparticles is combined at the heterojunction. In the hybrid nanoparticles, the noble metal nanoparticles (gold and silver nanoparticles) exhibit LSPR absorption in visible light which can have significant impact at the heterointerfaces. However, the SPR of platinum nanoparticles is found to be ranged in the UV range (215264 nm), unlike the other noble metal nanoparticles which display SPR in the visible range [96]. Since the absorption spectrum of Pt nanoparticles is shorter than the band gap of ZnO NP, i.e., B380 nm, it may thus produce a blue-shift phenomenon by the energy transfer of plasmonic effect. This slight blue shifting absorption peak has been observed in [96,99,100,97] for the PtZnO nanohybrids. The phenomenon can be attributed to the LSPR coupling, and the absorption band wavelength of Pt NPs is shorter than the band gap of ZnO, it may generated a blue shift phenomenon by LSPR coupling [101]. From the absorption spectrum of hybrid NPs, an estimated optical band gap can be derived using the following equation: αEp 5 KðEp 2Eg Þ1=2 where α stands for the absorption coefficient, K is a constant, Ep is the discrete photo energy, and Eg is the band gap energy. Then, the classical Tauc approach is further employed to estimate the Eg value of hybrid nanoparticles [102,103]. In the similar way, the band gap of a material can be estimated from its UV absorption spectrum [104,105]. Eg 5 h

C λg

where h 5 Planck’s constant 5 6.626 3 10234 J s, C 5 speed of light 5 3.0 3 108 m/s, λg 5 wavelength of absorption edge (m). Or the band gap energies (Eg) can be calculated simply according to the below equation (λg in nm): [106] Eg 5

1240 λg

Table 5.3 demonstrates the Eg (eV) values for various nano-TiO2-based hybrid NPs. As can be seen in this table, the hybridization of noble metals and nano-TiO2 reduced the optical band gap of nano-TiO2 particles, with AgNPs being the highest reduction. Similar results are obtained for nano-ZnO-based hybrid NPs (Table 5.4) with the presence of AgNPs, AuNPs, and PdNPs. However, a slight increase of optical band gap was reported for PtZnO nanohybrids as explained above [97,101].

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TABLE 5.3 Eg (eV) Values of Various Nano-TiO2-Based Hybrid NPs [107118] Hybrid nanoparticles Single nano-TiO2

Ag NPs

Au NPs

3.26 eV

2.64, 2.46 and 2.34 eV

Pt NPs

Pd NPs

2.84 eV 3.26 eV

2.93 eV

2.93 eV

2.84 eV

2.9 eV

2.8 eV

3.21 eV 3.3 eV

3.11 eV 3.0 eV

3.05 eV

2.192.9 eV

3.4 eV

2.22.3 eV

.3.1 eV

2.482.76 eV

TABLE 5.4 Eg (eV) Values of Various Nano-ZnO Based Hybrid NP [45,119124] Hybrid Nanoparticles Single nano-ZnO

Ag NPs

Au NPs

Pd NPs

Pt NPs

3.36 eV 3.25 eV

3.18 eV

3.3 eV

3.1- 3.2 eV

3.3 eV

3.05 eV

3.36 eV

3.28 eV

3.41 eV

3.38 eV

3.27 eV

3.26 eV

3.05 eV

2.9 eV

3.38 eV

3.4 eV

3.21 eV

3.23 eV

5.2.4 Effect of Noble Metal NPs on the Magnetic Behavior of Noble MetalMagnetic Oxide HNPs The magnetic property of magnetic oxide nanoparticles was sensitive to the particle sizes, crystallinity, and the presence of other molecules on their surface [125]. By

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109

considering them as nonmagnetic elements, the incorporation of noble metals into the lattice of magnetic oxides is expected to reduce their inherent saturation magnetization. Desai and Athawale [126] had successfully synthesized the Ag-doped lanthanum ferrite (LaFeO3), by using the microwave combustion route. The authors reported that Ag doping into the LaFeO3 lattice decreased the saturation magnetization (Ms), but increased the magnetic coercivity (Hci). For their LaFe0.25Ag0.75O3 nanoparticles, silver substitution increased the coercive force, from 82 Gauss to 361 Gauss. This observed data was attributed to the structural changes in the lattice by having different orbital configuration of Fe ions. Recently, Kowlgi et al. [127] have successfully synthesized magnetic noble metal nanoparticles and (nano)particle clusters by using the magnetic field during the chemical synthesis. In their study, the external magnetic field was produced by either magnetic stirrer plate (producing 20 mT at the point of contact) or permanent magnets (0.51.8 T). They found that the stronger magnetic fields yielded more ferromagnetic particles, whereas the absence of a magnetic field yielded no magnetic particles. At room temperature, 300 K, the as-prepared PtNPs had the highest saturation magnetization (4.22 emu/g), followed by AgNPs (0.88 emu/g) and AuNPs (0.18 emu/g). The authors also concluded that the magnetism of these noble metals nanoparticles was due to the surface anisotropy induced by the external magnetic field during the growth of particles. From these interesting findings, the combination of noble metals nanoparticles and magnetic oxides nanoparticles is expected to enhance their magnetic behavior. Bertolucci et al. [128] have successfully synthesized the noble metalsFe3O4 hybrid nanoparticles by using the microwave-hydrothermal method. The authors indicated that the presence of noble metals significantly reduced the magnetic hysteresis of nano-Fe3O4 particles. In addition, regarding the saturation magnetization, PdNPs and RuNPs significantly enhanced the saturation of the magnetization of nano-Fe3O4 particles (37 emu/g), reaching values of 63 emu/g and 40 emu/g, respectively. However, a slight reduction of saturation of the magnetization was observed for Pt@Fe3O4 NPs, at the value of 35 emu/g. These above interesting findings gave the potential applications for the hybridization between the noble metals and magnetic oxides, especially for application under the effect of magnetic field, such as MRI and hyperthermia therapy [129132].

5.3 EFFECT OF NOBLE METAL NPs ON THE SPECIFIC CAPACITANCE OF NOBLE METALMETAL OXIDE BASED SUPERCAPACITORS The common metal oxide materials for supercapacitor are transition-metal oxides, such as such as manganese oxides, nickel oxides, copper oxides, silver oxides, cobalt oxides, due to their high specific capacitance. However, these metal oxides have low electrical conductivity that restricts their specific capacitance and cycling stability. Since the noble metals have high conductivity and good electrochemical stability, their hybridization with these metal oxides is expected to facilitate the transport of electrons arising from the oxidationreduction of pseudo-capacitors to the current collectors. Thus noble metals might improve their specific capacitance and cycling stability.

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5. PHYSICS, ELECTROCHEMISTRY, PHOTOCHEMISTRY

For the AuNPs based supercapacitors, Dai et al. [133] have successfully fabricated the MnO2 nanorodAu nanoparticle hybrids that exhibited superior specific supercapacitance and long-term durability. Their nanohybrids had the specific capacitance of 406.8 F/g at a scan rate of 50 mV/s, which was five times higher than that of the pure MnO2 nanorods. A similar result was obtained by Khandare and Terdale [134] for gold nanoparticlesdecorated MnO2 nanowires. The specific capacitance of their nanohybrids had the high value of 249 and 164 F/g at scan rate of 1 and 5 mV/s, respectively. These values were much higher than that of MnO2 nanowires (50 and 70 F/g at scan rate of 1 and 5 mV/s, respectively). Qu et al. [135] also successfully prepared the Au nanoparticles decorated on NiO nanostructures. The authors reported that AuNiO nanohybrids exhibited highly improved rate performance as pseudo-capacitors, and a much higher specific capacitance value of 619 F/g (at a high rate of 20 A/g), than that of pure NiO electrodes (216 F/g). The same interesting results are reported for the 714 nm AuFe3O4 dumbbell-like nanoparticles by Liu et al. [136]. These dumbbell-like NPs have the best specific capacitance of 464 F/g at 1 A/g, which was much higher than that of the pure Fe3O4 NPs (160 F/g). Regarding the PtNPs based supercapacitors, Xia et al. [138] successfully designed the hierarchical Co3O4@Pt@MnO2 core/shell/shell-like structure. Their nanohybrids provided the higher specific capacitance of 539 F/g (at the current density of 1 A/g), than that of the MnO2 thin-film electrode (171 F/g). The high specific capacitance was also reported for the core/shell-like Pt/MnO2 nanotubes (810 F/g at scan rate of 5 mV/s). For the AgNPs-based supercapacitors, Zhang et al. [140] have successfully fabricated the nanohybrids of Ag nanoparticle/MnO2 nanosheets. Their as-prepared nanohybrids showed a high specific capacitance value of 272 F/g (at a scan rate of 10 mV/s), which was much higher than that of the MnO2 nanosheet materials (90 F/g). Similar results were reported for the nanohybrids of Ag nanoparticles-decorated MnO2 nanowires [141]. These nanohybrids delivered a specific capacitance of 293 F/g (at a scan rate of 10 mV/s), which was twofold higher than that of the neat MnO2 material (B130 F/g). Sawangphruk et al. [142] also reported the high value of specific capacitance for Ag/MnO2 nanocomposite (557 F/g, at a scan rate of 5 mV/s), which was about 2.7-fold higher than that of the pure MnO2 material. In the case of Ag/MnO2 nanotubes, their specific capacitances were 170 F/g (at a scan rate of 1 mV/s) and 150 F/g (at the current density of 1 A/g) [143]. The hybrization between noble metal nanoparticles and metal oxide nanostructures improves the specific capacitance. In the nanohybrids, Au nanoparticles were not only improving the electrical conductivity but also enhancing the structural stability. As reported in the literature [133143], the hybrization between noble metal nanoparticles and metal oxide nanostructures improves the specific capacitance. In the nanohybrids, noble nanoparticles not only improve the electrical conductivity but also enhance the structural stability.

5.4 PHOTOELECTROCHEMICAL (PEC) PROPERTIES Semiconducting oxides, such as TiO2, ZnO, and Fe2O3, have been widely used as photoelectrode materials [144150]. Hybridization of these oxides as host materials loaded with noble metals is expected to provide the enhancement of photoelectrochemical

I. FUNDAMENTALS

5.5 PHOTOCHEMICAL PROPERTIES

111

FIGURE 5.8 Plasmon-enhanced photoPhotocurrent

catalytic activity of iron oxide on gold nanopillars. Source: Adapted with permission from H. Gao, C. Liu, H.E. Jeong, P. Yang, Plasmonenhanced photocatalytic activity of iron oxide on gold nanopillars. ACS Nano 6 (2012) 234240. Copyright (2012) American Chemical Society.

1.0

1.2 1.4 Voltage (V vs RHE)

1.6

performance. By acting as an antenna that localizes the optical energy by SPR, plasmonic AuNPs have been suggested to sensitize TiO2 to light with energy below the bandgap, generating additional charge carriers for water oxidation. Such plasmon-enhanced photoactivity has been reported in some AuTiO2 nanocomposites for high efficient solar water splitting [144150]. In this direction, Liu et al. [151] reported that the performance of AuNP-deposited TiO2 films under visible light illumination for photoelectrochemical water splitting was enhanced 66-fold. This enhancement was explained by the fact that the SPR induced electric field amplification near the TiO2 surface, which increased the photon absorption rate of TiO2 and therefore improved the photoelectrochemical performance. In addition, Zhang et al. [152] also showed that Au nanocrystals assembled with TiO2-based substrate, achieved a high photocurrent density of B150 μA/cm2 under visible-light illumination. They argued that there was a matching of Au SPR wavelength with the photonic band gap of TiO2, which significantly enhanced the SPR intensity to boost hot electron injection and therefore increased the photoelectrochemical performance. The photocatalysis is found to be increased with the hybridization of iron oxide on gold nanopillars (Fig. 5.8). A summary of the hybrid strategies used to enhance the photocurrent density of nanoZnO and nano-TiO2 based photoanode is presented in Table 5.5. As can be seen in this table, the hybridization of noble metals and oxides NPs enhanced significantly the photocurrent density of oxide photoanode.

5.5 PHOTOCHEMICAL PROPERTIES Photochemical property is an important property of nanomaterials; this concerns the chemical effect of light induced by the absorption of UV radiation (200400 nm) or visible light (400800 nm). Many kinds of hybrid nanomaterials have been successfully synthesized in order to be used in photosynthesis and solar energy conversion. There has recently become a very attractive field relating to the use of hybrid nanoparticles on the hydrogen production and the reduction of CO2 and the synthesis of several vitamins. Song [163] reported in his review paper the synthesis methods of metal hybrid nanoparticles to conduct catalytic organic and photochemical reactions, dealing with four groups of hybrid nanoparticles: (i) yolk/shell-like nanoparticles; (ii) dumbbell- and tipped rod-like nanoparticles; (iii) core/shell and relative nanoparticles; and (iv) double shell hollow-like nanoparticles and compared their catalytic capacity. The yolk/shell-like structure

I. FUNDAMENTALS

TABLE 5.5 Summary of Hybridization Strategies to Enhance the Photocurrent Density of Semiconducting Nano-Oxides [152162] Photocurrent Density (µA/cm2)

Counter Electrode

Reference Electrolyte Electrode

Applied Potential (V)

Control Electrode

Hybrid Electrode

Photoanode Electrode

Preparation Method

AgNPZnO nanorods hybrids

Hydrothermal 1 electron beam evaporation

Pt

0.5 M Na2SO4

Ag/AgCl

0.28 and 0.34 (Voc) short-circuit

89

616

[153]

AgZnO nanocomposite

Spray pyrolysis technique

Graphite

0.5 M Na2SO4

SCE

0.22 and 0.5 (Voc) short-circuit

20

249

[154]

59

303

AuNPZnO nanopencil arrays hybrids

Hydrothermal 1 photoreduction Pt

0.5 M Na2SO4

Ag/AgCl

1

700

1500

AuNPZnO nanocomposites

Solgel spin coating 1 electrochemical deposition

Pt

0.1 M NaOH

SCE

0.5

1500

21002600 [156]

AuNPZnO nanorod hybrids

Hydrothermal and photoreduction

Pt

0.1 M Na2SO4

Ag/AgCl

1

330

9110

0.5

200

350

AuNPZnO nanowire hybrids

Hydrothermal and thermalreduction

Pt

0.5 M Na2SO4

Ag/AgCl

1/RHE

700

1450

[158]

AgTiO2 nanocomposite

Photoreduction-thermal treatment

Pt

0.1 M KNO3

Non

Non

0.005

0.015

[159]

AuTiO2 nanocomposite

Anodization and electrophoretic deposition

Pt

0.05 M NaOH

Non

Non

120

280

[160]

AuNPTiO2 nanocomposite

Adsorption of AuNPs

Pt

0.05 M NaOH

SCE

0.75

0.04

0.15

[161]

Au NPTiO2 Nanowires

Hydrothermal, seed-growth and thiol-modification

Pt

1M NaOH,

Ag/AgCl

0

820

1490

[162]

AuNPTiO2 nanotube hybrids

Photocatalytic reduction

Pt

1 M KOH

Ag/AgCl

1.23/RHE

3

150

[152]

AuNPTiO2 nanorod hybrids

Photoreduction method

Pt

0.5 M Na2SO4

Ag/AgCl

1.23/RHE

8001000

2500

[113]

Ref.

[155]

[157]

5.5 PHOTOCHEMICAL PROPERTIES

113

exhibited a high thermal and chemical durability due to the protection of the SiO2 or metal oxide layer. Metal@metal oxide coreshell are mainly composed of a metal core covered by thin silica (or metal shell). It was reported that this structure has mutual interactions between the core/shell structures leading to higher activity and could be an effective catalyst for various organic and gas-phase reactions, including hydrogen transfer, Suzuki coupling, and steam methane reforming. In this work, Metalsemiconductor hybrid nanostructures were found to be effective visible photocatalysts for hydrogen evolution and CO oxidation reactions. Dumbbell- or tipped rod-like NPs have an asymmetric arrangement of different domains and can express a large variety of photochemical properties upon irradiation of light depending on their nanostructure; these structures offer a higher surface area and high local concentration of the reactants. The double-shell hollow nanoparticle presents the highest photochemical properties compared to the other nanoparticles, especially for the monitoring of photocatalytic hydrogen. Recently, a large number publications have been reported on the synthesis and application of the high effective activity of noble metalmetal oxide hybrid nanoparticles for photocatalysis dealing with AgZnO [164], silver niobate [165], AuAg metal oxide [166], PtCdSe [163], PtphotosystemsPSI [167169], PtAu biconjugates [168], PtAuC3N4 [170], AuBiSbS3 [171], and MoS2TiO2Au hybrids [172]. Trandafilovic et al. [164]. reported a procedure for the preparation of ZnO nanocubes and ZnO/Ag heterostructures in an alginate biopolymer. These structures exhibit different optical properties compared to the starting alginateZnO system and the photocatalytic activity of the ZnO/Ag heterostructure was faster than that of the pure ZnO and further improved with increasing the concentration of silver. Gold and platinum nanoparticles are also used for the photocatalytic production of hydrogen gas by reforming on the surface of catalysts. The author argued that the photosystem can generate a light induced hydrogen due to the covalent attachment of cofactors of photosystem (PSI) on Au and Pt nanoparticle surface (Fig. 5.9).

FIGURE 5.9 Schematic representation of the cofactors of photosystem (PSI). Source: Reprinted with permission from L.M. Utschig, S.C. Silver, K.L. Mulfort, D.M. Tiede, Nature-driven photochemistry for catalytic solar hydrogen production: a photosystem I-transition metal catalyst hybrid, J. Am. Chem. Soc. 133 (2011) 1633416337. Copyright (2011) American Chemical Society.

I. FUNDAMENTALS

114

5. PHYSICS, ELECTROCHEMISTRY, PHOTOCHEMISTRY

5.6 SUMMARY AND FUTURE TREND We have reviewed, in this chapter, several main publications in the literature, in combination with our works on the design, synthesis, preparation, and characterization of hybrid nanoparticles in term of photochemical properties, especially for silver (Ag), gold (Au), and platinum (Pt) metals in conjunction with various materials. It demonstrates that the photochemical properties can be tuned by controlling the nanoparticle morphology, the chemical composition, and the synthesis methods depending on the end-use application fields, such as biomedicine, antibacterial, energy storage, electronics, and catalysis. Morphology and structure also significantly affect the properties of hybrid particles. Depending on the end-use application fields, a large range of nanostructural morphology can be obtained and selected, including: (i) yolk/shell-like nanoparticles; (ii) dumbbelland tipped rod-like nanoparticles; (iii) core/shell and relative nanoparticles; (iv) double shell hollow-like nanoparticles; (v) janus-like; and (vi) raspberry-like nanoparticles. Among these hybrid nanostructures, dumbbell-like nanoparticles are expected to provide the best hybridization between single nanoparticles. This hybridization would be obtained by the heterostructural nanoparticles with two joined nanoparticles sharing a common interface. In many cases, the hybrid-NPs exhibit a shift of SPR band due to the excitonplasmon interactions. The intensity of these interactions depends on the degree of hybridization between the noble metal NPs and oxide NPs. In this direction, the coreshell structure is expected to have the strongest excitonplasmon interactions, whereas the excitonplasmon interactions in nanocomposite are expected to be weak or neutral. Regarding the potential application of hybrid nanoparticles, the coverage of this domain will be expanded in the next decades with the explosion of conjoined hybrid systems providing special properties which are not present in single systems. The future trend of the hybrid nanoparticles is expected to be related to the development of modern photocatalytic systems, which can be used in larger applications domain, especially for solar energy conversion, increasing productivity of hydrogen production, efficient reduction of carbon dioxide, contaminant self-cleaning, and intelligent artificial devices. More studies need to be done to further understand the relationship between the nanoscale structure and catalytic properties and also the extreme activity, thermal stability, and recyclability of hybrid nanomaterials.

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[150] E. Thimsen, F. Le Formal, M. Gratzel, S.C. Warren, Influence of plasmonic Au nanoparticles on the photoactivity of Fe2O3 electrodes for water splitting, Nano Lett. 11 (2011) 3543. Available from: https://doi.org/ 10.1021/nl1022354. [151] Z. Liu, W. Hou, P. Pavaskar, M. Aykol, S.B. Cronin, Plasmon resonant enhancement of photocatalytic water splitting under visible illumination, Nano Lett. 11 (2011) 11111116. Available from: https://doi.org/ 10.1021/nl104005n. [152] Z. Zhang, L. Zhang, M.N. Hedhili, H. Zhang, P. Wang, Plasmonic gold nanocrystals coupled with photonic crystal seamlessly on TiO2 nanotube photoelectrodes for efficient visible light photoelectrochemical water splitting, Nano Lett. 13 (2013) 1420. Available from: https://doi.org/10.1021/nl3029202. [153] Y. Wei, et al., Enhanced photoelectrochemical water-splitting effect with a bent ZnO nanorod photo anode decorated with Ag nanoparticles, Nanotechnology 23 (2012) 235401. Available from: https://doi.org/ 10.1088/0957-4484/23/23/235401. [154] N.L. Tarwal, P.S. Patil, Enhanced photoelectrochemical performance of AgZnO thin films synthesized by spray pyrolysis technique, Electrochim. Acta 56 (2011) 65106516. Available from: https://doi.org/ 10.1016/j.electacta.2011.05.001. [155] T. Wang, R. Lv, P. Zhang, C. Li, J. Gong, Au nanoparticle sensitized ZnO nanopencil arrays for photoelectrochemical water splitting, Nanoscale 7 (2015) 7781. Available from: https://doi.org/10.1039/c4nr03735a. [156] B. Kumari, S. Sharma, V.R. Satsangi, S. Dass, R. Shrivastav, Surface deposition of Ag and Au nano-isles on ZnO thin films yields enhanced photoelectrochemical splitting of water, J. Appl. Electrochem. 45 (2015) 299312. Available from: https://doi.org/10.1007/s10800-015-0790-7. [157] M. Wu, et al., In situ growth of matchlike ZnO/Au plasmonic heterostructure for enhanced photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 6 (2014) 1505215060. Available from: https://doi. org/10.1021/am503044f. [158] X. Zhang, Y. Liu, Z. Kang, 3D branched ZnO nanowire arrays decorated with plasmonic au nanoparticles for high-performance photoelectrochemical water splitting, ACS Appl. Mater. Interfaces 6 (2014) 44804489. Available from: https://doi.org/10.1021/am500234v. [159] H. Zhang, G. Wang, D. Chen, X. Lv, J. Li, Tuning photoelectrochemical performances of AgTiO2 nanocomposites via reduction/oxidation of Ag, Chem. Mater. 20 (2008) 65436549. Available from: https://doi.org/ 10.1021/cm801796q. [160] V. Subramanian, E. Wolf, P.V. Kamat, Semiconductormetal composite nanostructures. To what extent do metal nanoparticles improve the photocatalytic activity of TiO2 films? J. Phys. Chem. B 105 (2001) 1143911446. Available from: https://doi.org/10.1021/jp011118k. [161] N. Chandrasekharan, P.V. Kamat, Improving the photoelectrochemical performance of nanostructured TiO2 films by adsorption of gold nanoparticles, J. Phys. Chem. B 104 (2000) 1085110857. Available from: https://doi.org/10.1021/jp0010029. [162] Y.C. Pu, et al., Au nanostructure-decorated TiO2 nanowires exhibiting photoactivity across entire UVvisible region for photoelectrochemical water splitting, Nano Lett. 13 (2013) 38173823. Available from: https://doi.org/10.1021/nl4018385. [163] H. Song, Metal hybrid nanoparticles for catalytic organic and photochemical transformations, Acc. Chem. Res. 48 (2015) 491499. Available from: https://doi.org/10.1021/ar500411s. [164] L.V. Trandafilovi´c, et al., ZnO/Ag hybrid nanocubes in alginate biopolymer: synthesis and properties, Chem. Eng. J. 253 (2014) 341349. Available from: https://doi.org/10.1016/j.cej.2014.05.056. [165] X. Liu, C. Qin, Y. Huang, L. Qin, H.J. Seo, A new silver niobate photocatalyst AgNb13O33: synthesis, structure and photochemical properties, J. Taiwan Inst. Chem. Eng. 78 (2017) 530538. Available from: https:// doi.org/10.1016/j.jtice.2017.06.034. [166] S.-I. Naya, et al., Solid-phase photochemical growth of composition-variable AuAg alloy nanoparticles in AgBr crystal, J. Phys. Chem. C (2017). Available from: https://doi.org/10.1021/acs.jpcc.7b04531. [167] L.M. Utschig, S.C. Silver, K.L. Mulfort, D.M. Tiede, Nature-driven photochemistry for catalytic solar hydrogen production: a photosystem I-transition metal catalyst hybrid, J. Am. Chem. Soc. 133 (2011) 1633416337. Available from: https://doi.org/10.1021/ja206012r. [168] R.A. Grimme, C.E. Lubner, D.A. Bryant, J.H. Golbeck, Photosystem I/molecular wire/metal nanoparticle bioconjugates for the photocatalytic production of H2, J. Am. Chem. Soc. 130 (2008) 63086309. Available from: https://doi.org/10.1021/ja800923y.

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C H A P T E R

6 Electronic Transport in Hybrid Nanoparticles Antoine Khater1,2 1

Institute of Physics, Jan Dlugosz University, Czestochowa, Poland 2Department of Physics, Universite´ du Maine, Le Mans, France

6.1 INTRODUCTION The myriad of nano objects assembled in remarkable configurations in 1D, 2D, and 3D nanostructures, and their potential applications, have motivated an unprecedented quantity of research work in recent years. The preparation and study of diverse forms of nanoparticles (NPs), in different domains, represent a major component of this research work. Nanoparticles are highlighted by their promising potential for a wide range of physical, chemical, biological, and other applications, as for example in medical applications [1], nanofluidics [2], photovoltaic nanoelectronics [3], mechanical structural reinforcement [4], and others. Amongst the different types of NPs, bimetallic ones have attracted a great deal of attention due to the rich variety of their physical and chemical properties (see for reviews [510]). In other chapters of the present book are discussed, in great depth, a wide variety of topics on the bimetallic noble metalmetal oxide hybrid nanoparticles (NMMO NPs). These constitute a special class of NPs that are gaining interest, since the metal oxide component carries intrinsic semiconductor properties [1115], alongside the metallic properties of the noble metal component. The hybridization of the two components under different morphological forms [1622] can lead to remarkable properties for nanomaterial applications. In the present chapter, we focus on the electronic characteristics of individual conducting NPs, and on the electronic conductivity of NP assemblies, notably the NMMO NP

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hybrid assemblies, under DC bias. For many of the potential applications of these conducting assemblies, from photovoltaics to nanoelectronics, it is essential to have a detailed understanding of the mechanisms behind their electronic transport on the local and assembly scales. Nanoparticles have sizes between 1 and 100 nm by scientific tradition. At the lower end of this range [B1 to B6 nm], the number of NP surface boundary atoms exceeds the number of atoms in the NP core. The number of NP surface atoms remains significant in comparison with the number of NP core atoms up to NP sizes B20 nm. Owing to these features, the physical and chemical properties of NPs are significantly influenced by their surfaces (in contact with other nano objects). The properties of a single NP are hence markedly different to those of the corresponding macroscopic bulk material. It is useful to point out at this point that although the NPs are often modeled as spherical objects for expediency, their real forms do not correspond in general to a sphere. There are, however, intensive investigations in the literature on the NP morphological forms, a topic outside the scope of this chapter. Further, the surface boundaries of NPs can have a significant thickness with an outer shell of atoms surrounding the NP core, and their surface topographies are disordered at the atomic scale, no matter how careful is the process by which they are prepared. Fig. 6.1 here illustrates this issue. Despite recent advances to control the NP morphology for bimetallic NPs in their synthesis, towards a singular form, and to narrow the size and composition distributions of NP assemblies, for example [5,2326], the size histograms and disordered NP surface topographies remain recurrent features in NP assemblies. Nanoparticles are considered as the primary building blocks of NP assemblies and their nanostructures, and these are intensively studied in the search for advanced nanomaterial functionalities since the early work of Refs. [2729]. The disordered and random features of individual NPs contribute to the complexity of electronic transport in NP assemblies, such as the hybrid NMMO NPs. However, the electronic properties of NP assemblies are determined not only by the properties of the individual constitutive NPs, but also by the NP networks inside the NP assemblies. FIGURE 6.1 Electro conducting powders of metal nanoparticles for additive technologies. Source: From website: http:// www.solid.nsc.ru/developments/img/materials/ poroshkinanochastits/1.png.

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The basic types of NP assemblies of recurrent interest are notably: (i) the dense NP aggregates in colloidal suspensions; and (ii) the consolidated granular NP assemblies in thin 2D layers and 3D structures. An important example of (ii) are the layered assemblies of conducting NPs as a component of organic photovoltaic (OPV) devices. The chapter is divided into sections. Section 6.2 contains a review of the literature on the principal mechanisms that are employed to interpret the experimental measurements of electronic transport in NP assemblies. In Section 6.3, we present a review of the dynamics of excitons and their important role in electronic transport due to hopping mechanisms. Further, a theoretical model is developed for the DC electronic transport by excitons in 2D hybrid NMMO NP granular assemblies, which yields novel theoretical relations for the thermally activated electronic mobility and conductivity. Our model elucidates the role of excitons in electronic transport, and corrects the phenomenological features of the nearest neighbour hopping mechanism widely used in the literature to interpret experimental results for DC electronic transport in NP assemblies. The topical summary and perspectives are given in Section 6.4.

6.2 ELECTRONIC TRANSPORT IN NANOPARTICLE ASSEMBLIES A brief review is presented in this section on the electronic transport in NP assemblies, under DC bias, in particular for systems which have attracted attention in recent years; see for example the reviews of Refs. [25,3035].

6.2.1 Electronic Characteristics of a Singular Nanoparticle Consider, in a hypothetical experiment, the system of an individual (metal or semiconductor) NP of spherical form, connected to outside electrodes in a closed circuit. Suppose that the NP is initially electrically neutral with equal negative (atomic electrons) and positive (nuclear protons) charges. Consider the possibility of introducing an extra electron into the NP from the outside electrodes. If successful, this experiment will charge the solid sphere with the electronic charge e, engendering a charged sphere with capacitance Cnp 5 4πκε0 d, electrical potential Vnp 5 e=4πκε0 d, and potential energy Unp 5 e2 =4πκε0 d. The variables are d, the diameter of the spherical NP, and κ, the dielectric constant of the milieu surrounding the sphere. ε0 is the permittivity of vacuum. It can be shown for a metal or semiconductor NP, which size is at the lower end of the nanometric range, with a diameter dB 10 nm for example, that its charged potential energy Unp is greater than the electron thermal energy kTB 25 meV at room temperature; k is the Boltzmann constant. This implies that if the thermal electron attempts to transport its charge into the NP, it is blocked by its self-fulfilling potential energy Unp on the NP. This effect is called the Coulomb blockade. Numerous experimental techniques are available to investigate the electronic structure in metal or semiconductor NPs, such as optical spectroscopy, UV photoemission, and STM/STS. It is hence well known that the electronic structure for metal NPs presents a hierarchy of discrete electronic levels [36], in contrast with the electronic band structure in

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bulk materials [37,38]. The transition from bulk to NP properties is gauged by the manifestation of the discrete levels, for which the mean level spacing δ increases with decreasing NP size. δ is inversely proportional to the NP volume Vd , such that δ 5 ðDF Vd Þ21 , where DF is the density of electronic states at the Fermi level. Typically δ is B of a few meV in metal NPs with diameters dB few nm. For semiconductor NPs, the electronic states present a comparable configuration to that of metal NPs though slightly different; the NP characteristic discrete electronic levels appear near the edges of the energy band gap, and this effectively widens the bulk material band gap in NPs [37,38]. Moreover, the optical and electronic properties of semiconductor NPs are dependent in large measure on the excitonic elementary excitations.

6.2.2 Electronic Transport Across NP Assemblies Faced with the Coulomb blockade, the electronic transport through consolidated metal and semiconductor NP assemblies in 2D and 3D can be envisaged only with applied electric voltages V above threshold voltages Vth characteristic of the systems under study. For any given NP system, its assembly contains NPs which may mechanically connect via their surface boundary contacts, generating the NP networks (percolation paths; or forcechains, as in granular materials; see [39,40], for example). Nevertheless, even if the mechanical contact is missing between neighboring conducting NPs, they may still electronically connect by quantum tunneling transitions (hops) across the van der Waal spatial gaps; see [30,32,34,35,41] and references therein. The multiscale complexity due to the combined NP surface boundary contacts and percolating NP networks [42], and to van der Waal gaps, imposes the need to apply a voltage across the DC circuit, equal or greater than the threshold voltage, V $ Vth , to achieve a DC current I through the considered NP assembly. The current I which is generated by probabilistic electronic hopping events through the NPs, may be expressed [43,44] as   ξ V IðVÞ 5 I0 (6.1) 21 Vth The agreement between this relation and experimental results, obtained by conventional DC measurements, confirms that electrons can tunnel through the NPs, overriding the Coulomb blockade for V $ Vth . The power index ξ depends on the dimensionality of the NP assembly (see for example [45] for a study of dimensionality effects), and can be experimentally large. Furthermore, the conductance of the NP system may be expressed as   dI (6.2) g5 dV V For small circuit voltages, the conductance g 5 dI=dV through NP assemblies has been intensively investigated as a function of temperature, because this yields a great deal of information about the electronic transport mechanisms. The experimental results for a large variety of different NP systems show that the temperature variation of the conductance g differs from one temperature range to another. There are two distinct ranges, corresponding to low and intermediate temperatures T. The

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experimental measurements of the electronic transport in consolidated NP assemblies are presented consequently either in terms of the electronic conductance g or the electronic conductivity σ. These are related by a multiplying prefactor which depends on the geometry of the system under study. The experimental DC conductivity σ measurements for NP assemblies are hence generally understood by invoking two electronic transport mechanisms for two T-ranges, the low range, [Tl sTsTi ], for which the low Tl is not typically zero Kelvin, and the intermediate range [Ti sTsTh ], where the relatively high Th and intermediate Ti vary from one NP system to another. For the low T-range, the experimental conductivity measurements for electronic transport at small bias are interpreted according to the so-called variable range hopping mechanism (VRH) which involves distant NP neighbors and possibly tunneling. Commonly known as the Efros and Shklovskii ES-VRH model for electronic transport [46,47], it expresses the electronic conductivity as "   # T ð1=2Þ σ ~ exp 2 (6.3) T The ES-VRH σ of Eq. (6.3) has been generally employed to interpret the experimental electronic conductivity measurements for different NP systems (see the reviews in Refs. [30,34,35,48]), and also for other disordered systems such as semiconductor nanocrystal solids [44] and colloidal crystals [43], with phenomenological fitting of the numerical results. T is a characteristic temperature predicted by the ES-VRH model, but which may also serve as a needed adjustable parameter. Note that Ti marks the crossover temperature between the ES-VRH mechanism over to the so-called nearest neighbor hopping (NNH) mechanism, and correspondingly from the low T-range to the intermediate T-range. The idea for NNH hopping electronic transitions was first developed by Miller and Abraham [49] for electrons which can make quantum transitions between localized electronic states by thermally activated mechanisms. In their paper these authors compute effectively the conductivity of n-type semiconductors with site disorder at low impurity concentrations. Their model introduces phonon-assisted electron quantum transitions between donor sites where a fraction of the sites is vacant due to compensation, and leads to an Arrhenius activation process for these electrons. This model, developed initially for bulk semiconductors, has been adopted as a mechanism for the hopping conductivity in disordered NP assemblies in the intermediate T-range, and is expressed as   2 EA σ ~ exp (6.4) kT EA is the Arrhenius activation energy, predicted by the original semiconductor model, but which serves also as an adjustable parameter for NP systems. This expression for σ is employed to interpret also the experimental conductivity results in different disordered systems of other types of nano objects. It is a remarkable achievement that we are able to interpret the multitude of relevant experimental measurements of the DC electronic transport in NP systems, and in I. FUNDAMENTALS

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disordered systems of other types of nano objects, by the two cited mechanisms (and sometimes by slight variants of the ES-VRH mechanism). Nevertheless, the theoretical research on the electronic transport for these systems remains tributary to phenomenological fittings as pointed out by Nenashev et al. [50], with model attachments which often make little fundamental sense.

6.3 ELECTRONIC TRANSPORT BY EXCITONS IN HYBRID NMMO NP SYSTEMS The synthesis of hybrid NPs from noble metals (NM) as Au, Ag, Pt, Pd, etc., and from metal oxides (MO) as ZnO, TiO2, NiO, Cu2O, CeO2, etc., have received considerable attention for applications, as described in recent reviews [1622]. In particular, Ref. [18] gives a review of the principal morphological types of hybridized NMMO NPs, including NM decorated MO NPs, NM/MO core/shell NPs, NM/MO yolk/shell NPs, and Janus NMMO NPs. It is interesting to note that none of these forms are alloy NPs, the noble metal and the metal oxide seemingly insoluble to one another. The experimental and theoretical studies of electronic transport in consolidated assemblies of metalsemiconductor hybrid nanoparticles, including NMMO NPs are still rare [25]. The increased complexity of the morphology and structure of a single NP, and of the staggered porosity of the disordered percolating networks of NP assemblies in 2D and 3D, are challenging problems to investigate. In this section we present a theoretical model for the electronic transport in the plane of two-dimensional (2D) nanometrically thick mono-layered assemblies of NMMO NPs, employing an exciton hopping mechanism for the electronic transport across the structural and stoichiometric disorder in these NP assemblies. The intermediate states for the mobile electrons are considered as 3D excitons of which holes are trapped in the MO component. These WannierMott excitons (typical of inorganic semiconductors), present 3D model characteristics inside the nanometric thickness of the hybridized NMMO NP 2D monolayered arrays, because the diameter of the hopping excitons is for our purpose much smaller than the size of the considered NPs. The developed model will shed new light on the exciton hopping mechanism in NP systems, and yields an expression for the temperature dependence of the thermally activated electronic mobility for NMMO NP assemblies. It integrates the relevant physical characteristics of the NP aggregate assemblies, and enables us to calculate the system conductivity without resorting to adjustable parameters. This approach is essential for a deeper understanding of electronic transport in NP systems. The presented model for the electronic transport in NMMO NP assemblies is based on a theoretical approach developed previously by Khater et al. [51], for the DC electronic transport in disordered layered semiconductors, where the interactions normal to the layers are weak van der Waals forces due to the stacking disorder. The theoretical results of this approach were applied numerically to compute the absolute value of the electronic mobility for the layered InSe semiconductor, and are in agreement with the experimental measurements for this compound as shown in Fig. 6.2.

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FIGURE 6.2 From [51]: The line is the theoretical plot of the absolute value of the electronic mobility normal to the stacking faults of atomic layers in InSe, as a function of temperature, with no adjustable parameters. The black dots are the corresponding experimental measurements from [52].

6.3.1 Electronic Transport by Excitons in NMMO NP Systems In hybrid NMMO NP assemblies, the hole conduction is effectively hampered by localized oxygen 2p dangling bonds, and by the deep valence band maxima (VBM) levels where holes would be trapped owing to the structural and stoichiometric disorder. Note that the energy band gaps in metal oxides are relatively wider than in the traditional semiconductors such as GaAs, even at the nano-scale of NP sizes. Monolayer hybrid NMMO NP assemblies are promising nanomaterials [15,53,54] for applications ranging from photovoltaics to nano electronics. The NMMO NP assemblies of particular interest in this chapter are those for which the noble metal and metal oxide components constitute Janus NP morphologies with an interface between the two components. The other morphologies with sizeable MO cores and NM satellites, though geometrically different, present electronic attributes reducible to those of the Janus type. The metalsemiconductor interface in these NPs gives rise to a Schottky barrier. In a recent review Tung [55] emphasizes that the Schottky barrier at the MS interface depends on its atomic structure, which is inevitably disordered at the atomic scale (regardless of the preparation technique), and that there are no simple equations to describe the Schottky barrier for all MS interfaces, despite recent quantum mechanical contributions for its study. Note that this NM/MO interface may support plasmonexciton interactions. In view of the limited space accorded per chapter in the present book, we will not discuss the subject of the plasmonexciton interactions which can arise in NMMO NP assemblies via AC oscillations and optical effects (see [5658], and references therein). The possibility of multiple exciton generation under plasmon influence is, however, of current interest. In materials with a high dielectric constant as is the case for MO semiconductors, the strength of Coulomb interactions is reduced by their large dielectric constant. As a consequence, the MO excitons have a hydrogenic character with a Bohr radius axB2 nm, greater than that in the hyrdogen atom. If the size of a MO NP is equal or smaller than ax , the excitons will experience quantum size effects, which will modify their properties [37,38,59,60].

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To illustrate the development of our theoretical model, consider as a reference the 2D monolayer assembly of hybrid AuZnO NPs as in Fig. 6.3 [61]. The ZnO component per NP is in the form of a nanopyramid of average size B40 nm, and the attached Au Janus satellite is much smaller. In these cases the MO exciton ax is much smaller than the NP size. It is pertinent to point out here that the formation of the Schottky barriers in ZnO/Au nanocomposites has been investigated experimentally by different groups and techniques [62,63]. They show that the Schottky barrier at the AuZnO interface permits excited Au electrons to inject into the ZnO, but restricts the reverse transfer of electrons from the conduction band of ZnO to Au. This implies for our 2D monolayer hybrid NMMO NP assembly that the NM component acts effectively as a reservoir for the transfer of mobile electrons to the MO component. The exciton properties in a nanoparticle are determined by two potentials: the habitual holeelectron hydrogen-like potential, and the confinement potential due to the quantum size effects in very small NPs. For the relatively large model MO (referenced by ZnO B40 nm in the hybrid AuZnO NP assembly), we can neglect the confinement potential. The Schro¨dinger equation for the localized exciton is calculated then with the exciton potential Vex ðrÞ 5 e2 =4πκε0 r, where r is the holeelectron distance. Note the high dielectric constant κ which reduces the strength of the potential. Furthermore, the exciton in the effective mass theory (EMT) has a reduced mass meh , given by 1=meh 5 1=me 1 1=mh , where me and mh are the effective masses of the electron and the hole in the semiconductor, respectively. Under these conditions, the wave-function of the excitonic electron is considered as the hybrid product of a characteristic localized hydrogenic wave-function and those of electrons in the lowest-energy conduction band (CB) Bloch eigenstates, so that X ψi 5 cp φp ðrÞϕ1s;i ðrÞ (6.5a) p

FIGURE 6.3 From [61] (referenced in [18]): The AuZnO hybrid NPs present metal oxide nano-pyramid ZnO component B40 nm in size, and smaller noble metal component Au NP B6 nm in size. The NPs have well defined interfaces between the ZnO and Au components.

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ϕ1s;i ðrÞ 5 Be2r=ax corresponds to a localized 1s state of the hydrogenic exciton, and φp ðrÞ to the Bloch eigenstates; the sum is taken over a sufficient number n of representative eigenstates. An adequate choice is n 5 14 [64]. Moreover, it can be shown that the Bohr radius ax for the isolated exciton is ax 5

κh ¯2 e2 meh

(6.5b)

To appreciate the model scales, the referenced ZnO semiconductor, for example, with an average NP component size dB40 nm, dielectric constant κD7, and reduced mass meh 5 0:19m0 ðm0 is the free electron mass), has excitons of Bohr radius ax B18:6 A˚ 5 1.86 nm, with a binding energy in the excitonic 1 s state ground B60 meV [65,66]. Excitons submit to two conditions in a nanoparticle, which are related to their diameters. The first is as follows. If the exciton diameter is smaller than the size of the NP, we say that the excitons are in weak confinement. If it is bigger the excitons are in strong confinement; for this latter case the excitons cease to exist as an elementary excitation, and break up into independent holes and electrons. The second condition is related to the quantum size effects which arise if the exciton diameter is comparable or greater than the MO size. If this is the case we need to solve the Schro¨dinger equation for the exciton under the combined confinement and hydrogen-like potentials (see for example [59,67]). This implies hybrid eigenstates which are different from those expressed by Eq. (6.5b). Since our model for the 2D monolayer hybrid NMMO NP assembly is referenced by the AuZnO hybrid NP, then we deal with excitons in weak confinement which are treated as described by Eqs. (6.5a) and (6.5b).

6.3.2 Theoretical Modeling of Electronic Transport in 2D NMMO NP Assemblies Consider two NN trapped exciton holes i and j in the MO components of the NMMO NP assembly, which are separated by a random distance rij . The holes are trapped in the deep states engendered by the structural and stoichiometric disorder, and are strongly localized. The electronic hopping involves then the following process. The electron phonon-assisted quantum mechanical hop from i to j causes the effective annihilation of an exciton at i and the creation of another at j. The hop is either within the same NP, or from one NP to another across the interfacial boundaries or the van der Waal gaps which separate the nanoparticles at the atomic scale. The hopping transitions which occur across the stacked structural and stoichiometric disorder of the NP assembly are reversible under zero bias. However, under bias, the hopping is directional leading to a current in the direction of the externally applied electric fields. The electronic conduction, due to the hopping of exciton electrons in the NMMO NP system under bias, is formulated in terms of the normalized directional probability for the net electronic transfer between a pair of sites i and j. As the NP assembly is disordered, the electronic mobility is calculated as a configuration average over stochastic distributions for site separations and energy splittings.

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For the electronphonon interactions and the consequent phonon-assisted hopping, we use the wave functions ψi of Eq. (6.5a). The transition rate for an electron phonon-assisted hop can then be given, following [51], in the form  3=2    rij Uij 5 A e22rij =ax ½ΔEij =ζ=½exp βΔEij 2 1 (6.6) ax where ax is the Bohr radius of the exciton in the ground state, and β 5 ðkTÞ21 . The energy splitting and its statistical variance are (6.7a) ΔEij 5 Ei 2 Ej D

E

2 ζ 2  ΔEij

(6.7b) Ei are the zeroth-order energy levels of the i and j excitons, respectively. where

Ei and

rij 5 r i 2 r j is the distance between the two localized holes. ΔEij of Eqs. (6.6) and (6.7a) is a random quantity, and ζ 2 of Eq. (6.7b) is the variance of the distribution of the energy splittings over the stacked structural and stoichiometric defects of the disordered NP assembly. We calculate the factor A in Eq. (6.6), applying the deformation potential to the longitudinal optical phonon-assisted hops inside the MO and at its surfaces. These phonons are known for their active interactions with localized excitons on defect sites in polar semiconductors. Further, the free MO surface in the NP presents a structural asymmetry at the atomic scale: on one side there is the metal oxide, and on the either the van der Waal gaps between NPs in their aggregated assembly or the interfacial NP contacts. The structural defects and the surface asymmetry can activate the phonon-assisted exciton hop. The factor A can hence be expressed as follows  2 1 2 a2x 1 1 2e2 ζε 2 A5 (6.8) 2π a0 ρv5 h ¯ 4 n 3ð4πκE0 Þax The units of the transition rate A are in s21. Also ¯h is Planck’s constant, ρ the density of the MO, v the speed of the LO phonons (sound), ε the deformation potential in energy units, and a0 the lattice constant of the metal oxide (see [68,69] for deformation potentials). The normalized directional probability for the net electronic charge transfer from i to j sites, across the 2D NP assembly, using Eq. (6.6), is   Uij 2 Uji βΔEij 5 tanh Pij 5 (6.9) Uij 1 Uji 2 The random distribution of the structural defects and van der Waal gaps throughout the NP assembly create the need for configuration averages for the random variables rij and ΔEij . These averages are then calculated with the use of two distributions. The first , . . . . r depicts the probability distribution function (PDF) for the rij spatial separations between two hole sites i and j, and the second , . . . . ΔE that for the energy splittings in the hopping processes between sites. The two PDFs are not correlated. This assumption is realistic because of the absence of long range order in the NP assemblies, and because the NPs have no structural correlations one to another given the individual and randomized manner in which they are habitually synthesized. The first PDF is taken in the following normalized form I. FUNDAMENTALS

6.3 ELECTRONIC TRANSPORT BY EXCITONS IN HYBRID NMMO NP SYSTEMS

 ð 4 rij eð22rij =r Þ . . . drij , ... .r 5 2 r

135 (6.10)

This is a modified Poisson PDF which, with no loss of generality, assigns the dominant probability to the mean separation , . . . . r 5 r between nearest neighbor defects. As regards the second PDF there is no detailed information available. We know, however, from photoluminescence measurements in bulk semiconductors [70,71], that the envelope of the density of states is a Gaussian spectrum. The second PDF is hence taken as the normalized Gaussian "  2 # ð 2 , ΔEij ðminÞ . ðN ΔEij 1 , . . . . ΔE 5 ... 1 . . . of pffiffiffiffiffiffi exp 2 (6.11) dΔEij ζ 2π 2ζ 2 2N , ΔEij ðminÞ . This choice is appropriate since the corresponding symmetry yields a net zero current for zero bias. Further, the limits on the integral 6 , ΔEij ðminÞ . are finite to depict a significant probability only for nonzero energy splitting over the stacking faults. Note that the lower integration limit , ΔEij ðminÞ . is related to ζ [51,72,73], and can be evaluated numerically. The total configuration average is then , . . . . 5 , , . . . . ΔE . r 5 , , . . . . r . ΔE

(6.12)

where we have assumed that ΔEij and rij are not explicitly correlated over the disordered assembly of NPs. However, there have been efforts to model certain disordered systems assuming that these two variables are correlated (see for example [50,74], and references therein). It follows from the above, that the drift velocity ve for the electronic hopping in the plane of a quasi-2D monolayer NP assembly is given, using Eq. (6.6) and Eqs. (6.8)(6.12), by the configuration average ve 5 , rij Pij Uij .

(6.13)

Under an externally applied electric field E, the energy splitting between the excitons for the hop from i to j, is modified owing to the work done on the electronic charge during its hop, so that ΔEij ðE 6¼ 0Þ 5 ΔEij ðE 5 0Þ 1 erij E

(6.14)

For electric fields E, where Eth , E&10 V=cm, the term erij E in Eq. (6.14) is ,, than the term , jΔEij ðE 5 0Þj . ; for example , jΔEij ðE 5 0Þj . is B73 meV in ZnO (see [65,66]), for example. Eth corresponds to the threshold electric field. Substituting Eq. (6.14) into Eq. (6.9), we obtain the configuration average for the net electronic transport of the hopping transition i to j, to the first order of the applied field E in the form h i , rij Pij ðE 6¼ 0Þ . 5 12 1 2 P2ij ðE 5 0Þ βer 2 E (6.15) 2

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This is a key result of the model. Substituting Eq. (6.15) in Eq. (6.13), and using Eqs. (6.6) and (6.8), and the well known relation ve 5 μE for semiconductors, the configuration average of the electronic mobility μ of the 2D NMMO NP hybrid assemblies becomes *  ( )  + rij 3=2 22rij =ax ΔEij =ζ 2 βΔEij 2 2

1 2 tanh μ 5 A12r e e β (6.16) ax 2 expðβΔEij Þ 2 1 Eqs. (6.9) and (6.14) can lead to other forms of Pij ðE 6¼ 0Þ when developed to higher orders of the applied field E. Let N 5 ðNe Nh Þ1=2 denote the 3D spatial concentration of mobile excitons in the nanometrically thick NMMO NP monolayer system, where Ne and Nh are the electron and hole concentrations. Though some variables, such as the concentration of mobile excitons N, and the lattice constant a0 in A of Eq. (6.8), may vary with temperature, we may neglect this at present. Using Eq. (6.16), the configuration average of the electrical conductivity for the disordered monolayer NMMO NP assembly becomes *  3=2 5 N 2=3 A12r2 e3

rij ax

8 σ 5 N eμ 9 2 0 13+ < = ΔE =ζ βΔE ij ij A5  

β 41 2 tanh2 @ e22rij =ax : exp βΔEij 2 1 ; 2 2=3

(6.17)

Putting , ΔEij ðminÞ . 5 kT  , the configuration averages of Eq. (6.17) are computed as follows *   + rij 3=2 22rij exp  fr ðax ; r Þ ax ax r )  *(  + ΔEij =ζ βΔEij expð 2 2T =TÞ   β 12tanh2  fΔE ðζ; T; T Þ kT 2 exp βΔEij 21 ΔE

Substituting the functionals fr ðax ; r Þ and fΔE ðζ; T; T  Þ in Eq. (6.17), we obtain the theoretical expression for the configuration average of the electronic conductivity in disordered 2D monolayer NMMO NP assemblies as σ 5 N 2=3 A12r 2 e2 fr ðax ; r ÞfΔE ðζ; T; T Þ

expð2 2T =TÞ kT

(6.18)

Eq. (6.18) is the central theoretical result of the model. The derived functionals fr and fΔE dress, with the factor A, the contributions to the magnitude of the thermally activated electronic conductivity in the intermediate T-range, by the MO nanomaterial characteristics. The derived fΔE varies slowly with temperature, as compared to the expð2 2T =TÞ and 1=kT terms. Equally, the derived fr is independent of temperature, and characterizes the NMMO NP nanomaterial. Comparing σ in Eq. (6.18) with Eq. (6.4), it is clear that the former equation models the fundamental physics of the hopping process avoiding the phenomenological inadequacies. 2kT  in Eq. (6.18) corresponds to the habitual activation energy, as EA of Eq. (6.4), in the intermediate T-range. Our theoretical results incorporate a novel 1=kT term which is absent from the phenomenological form in Eq. (6.4); however,

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6.4 SUMMARY AND PERSPECTIVES

137

this term has been evoked recently by Wang et al. [45] for the experimental results of the electronic conductance in 2D and 3D NP arrays. Comparing Eq. (6.18) to experimental measurements should yield, without resorting to adjustable parameters, valuable information as regards the unknown nanomaterial characteristics of the NP assembly, notably ζ the variance, N the concentration of mobile excitons in 3D NP assemblies, and r which is related to N.

6.4 SUMMARY AND PERSPECTIVES A brief review is presented in this chapter of the literature on the principal mechanisms that are habitually employed to interpret the experimental measurements of DC electronic transport in conducting granular nanoparticle assemblies. In particular, we discuss the two primary mechanisms, ES-VRH and NNH, which have been extensively employed to interpret the wide range of experimental conductivity measurements as a function of temperature. The implications of the choice of these mechanisms for conducting NP assemblies are discussed. In particular, the NNH mechanism, adopted to analyze and interpret DC electronic transport through NP assemblies, is presented in the literature as an offshoot of the hopping mechanism for electronic transport in bulk semiconductors. However, in bulk systems, the hopping conductivity is intimately related to the dynamics of WannierMott excitons, weakly localized on bulk impurities and stacking faults. Given the phenomenological manner by which the NNH mechanism is applied, it was necessary to detail the role of exciton dynamics towards the hopping mechanism of electronic transport in disordered granular NP assemblies, distinguishing explicitly between the 2D layer properties of the nanomaterial NP assembly and the 3D properties of the excitons in the NP assembly. Further, we develop a theoretical model, seemingly missing in the literature, for the electronic transport by exciton hopping through the 2D assemblies of hybrid noble metalmetal oxide nanoparticles (NMMO NP), referenced by 2D hybrid AuZnO NP assemblies. The model yields full theoretical relations for the electronic mobility and conductivity for these systems, without adjustable parameters. Our model results show a thermally activated electronic conductivity σ, and correct the phenomenological features of

the nearest neighbor hopping mechanism, σ ~ exp 2 EA =kT , adopted heuristically from former bulk material models to interpret experimental results for the electronic transport in NP assemblies. This novel theoretical approach is essential for a deeper understanding of the electronic transport in disordered hybrid NMMO NP assemblies, and can provide valuable information on the intrinsic contribution of the semiconductor nanomaterial characteristics in the hybrid NPs to the magnitude of their mobility and conductivity. The theoretical results are general, and can be applied to different types of hybrid NMMO NP granular assemblies which present physical attributes comparable to those of the referenced 2D hybrid AuZnO NP assembly. In the topical domain of this chapter, there is a need to continue to develop the research work. Indeed, little is known at present about a number of important issues behind the hopping mechanisms for electronic transport. For example, we know very little about the impact on σ of the porosity of a granular NP system, and their associated percolating NP networks. The dependence of the electronic transport on the dimensionality of the NP I. FUNDAMENTALS

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assemblies is still not fully understood, with open questions as regards the transition from 2D to 3D transport, whether for a NP monolayer or for systems of different stacked NP layers. The question of the impact of quantum size effects on excitons in NPs of very small size, and of how AC plasmonexciton interactions can augment σ in NMMO NP assemblies, are important and open. There are other questions which concern the quantum ballistic electronic transport for Fermi electrons through disordered NP systems. The answers to these questions, and to others, are very important for a wide range of potential applications involving the hybrid NMMO NP assemblies.

Acknowledgments I am indebted to Dominik Szczesniak (Czestochowa) and Doried Ghader (Beirut) for useful discussions, and would like to thank Maha Khater (Le Mans), Zygmunt Bak (Czestochowa), Philippe Daniel (Le Mans), and Benoit Piro (Paris), for their support. I am grateful to Tuan Anh Nguyen (Hanoi) for his kindness and editorial efforts.

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C H A P T E R

7 Antibacterial Behavior of Hybrid Nanoparticles Phuong Nguyen Tri1,5, Tuan Anh Nguyen2, The Huu Nguyen3 and Pascal Carriere4 1

Department of Chemistry, University of Montre´al, Montre´al, QC, Canada 2Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam 3Faculty of Chemical Technology, Hanoi University of Industry, Bac Tu Liem, Hanoi, Vietnam 4 Laboratoire MAPIEM (EA 4323), Materiaux Polymeres Interfaces Environnement Marin, Universite de Toulon, Toulon, France 5Ecole de Technologie Superieure, University of Quebec, Quebec City, QC, Canada

7.1 OVERVIEW The number of bacteria on Earth was estimated at around 5 3 1030/1 mL of fresh water and 1 g of soil contains about 140 million bacterial cells, respectively [13]. Bacteria have an important role in our environment and ecosystem. They are essential not only to humans and animals, but also to plants on our planet. For the human body, by the defense mechanisms of the immune system, the majority of bacteria are harmless. However, some types of bacteria are pathogenic. These harmful bacteria cause infectious diseases, such as cholera, syphilis, anthrax, leprosy, and bubonic plague [1]. Then, antibiotics are developed to treat the bacterial infections. The antibiotics are bactericidal or bacteriostatic, depending on whether they kill bacteria or they prevent bacterial growth, respectively [1]. As the osmotic barrier, the cell membrane protects bacterial cells from their surroundings. The bacteria cell membrane, composed primarily of phospholipids, has a net negative

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00007-3

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charge [4]. Bacteria cell walls, located outside the cell membrane, are made of peptidoglycan. Based on the thickness of the peptidoglycan layer, bacteria can be classified into two classes: (i) Gram-positive bacterium (thick layer of peptidoglycan B2080 nm), or (ii) Gram-negative bacterium (thin peptidoglycan layer B8 nm) [5]. Regarding the size of bacteria (110 μm) and molecular antibiotics (B1 nm for penicilline) [1], the molecular antibiotic can permeate easily through bacteria pore channels. In addition the chemical and physical properties of molecular antibiotics are important, such as hydrophobicity, stoichiometry, and charge [2]. Similarly, bacteria are around 100 times bigger than nanoparticles, but the surface area of nanoparticles are important. In this direction, the smaller nanoparticles have better antimicrobial activity [6]. The antimicrobial mechanism of nanoparticles is still under discussion [7,8]. Six main mechanisms have been proposed [7,920]: (i) direct contact between nanoparticles and bacterial cell (degrading the cell wall and peptidoglycan layer); (ii) release of toxic ions; (iii) interaction of nanoparticles with bacterial efflux pumps; (iv) bacterial membrane rupture by formation of reactive oxygen species (ROS); (v) ROS degrading DNA, RNA, and proteins; and (vi) depletion of intracellular Adenosine Triphosphate (ATP) production [1]. In general, most of the current molecular antibiotics affect bacteria via one of the three bacterial targets: cell wall, translational machinery, and DNA replication [21]. Whereas, nanoparticles could react simultaneously through various processes, such as (i) production of ROS, (ii) electrostatic interaction with the cell membrane, (iii) ion release, (iv) internalization, etc. Thus, nanoparticles would have the superior antibacterial activity over the traditional molecular antibiotics, especially for the antibiotic-resistant bacteria. Fig. 7.1 presents the summary of mechanisms of toxicity of nanoparticles (NPs) against bacteria [22] By the distinct physicochemical properties and high surface areas to volume ratios, noble metal nanoparticles have been reported as an effective antibacterial activity against pathogenic bacteria. Among the noble metal nanoparticles, silver (AgNps) and gold (AuNPs) have been mostly used for antibacterial applications. Silver nanoparticles (AgNPs) have been reported as effective antibiotics against both Gram-negative and Gram-positive bacteria [23,24]. They are extensively used as antimicrobial agents in various applications, such as medical devices, potable water filters textile, food storage, refrigerators, and environmental [25]. AgNPs are unique nanoparticles, which can attack bacteria through all six of the antimicrobial mechanisms mentioned above [2628], thus they could be used as the sole antimicrobial agent. Their antimicrobial mechanism includes the photocatalytic production of ROS in solution [29]. Fig. 7.2 shows the possible antimicrobial interactions of Ag nanoparticles in cells [30]. In the case of AuNPs, they can form weaker hydrophobic interactions with lipopolysaccharide (LPS) in cell walls of Gram-negative bacteria [31]. AuNPs also can interact with many biologically active compounds [32]. The metal oxide nanoparticles-based antibiotics, such as NiO, Co3O4, ZnO, Fe2O3, Fe3O4, MgO, CuO, TiO2, SiO2, have been studied extensively due to their high and

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7.1 OVERVIEW

Cell membrane Protein

Nucleus

Cell membrane disruption

DNA damage

Oxidized cellular component Ag+ Zn2+

Release of heavy metal ions

Reactive oxygen species (ROS)

e– ROS production

Mitochondria damage

e– Interrupted transmembrane electron transport

FIGURE 7.1 Mechanisms of toxicity of nanoparticles (NPs) against bacteria. NPs and their ions (e.g., silver and zinc) can produce free radicals, resulting in induction of oxidative stress (i.e., reactive oxygen species; ROS). The produced ROS can irreversibly damage bacteria (e.g., their membrane, DNA, and mitochondria), resulting in bacterial death. Source: Reprinted with permission from M.J. Hajipour, K.M. Fromm, A.A. Ashkarran, D. Jimenez de Aberasturi, I. Ruiz de Larramendi, T. Rojo, et al., Antibacterial properties of nanoparticles, Trends Biotechnol. 30 (10) (2012) 499511. Copyright 2012 Elsevier.

stable microbial sensitivity [3339]. ROS is the predominant antibacterial mechanism for these nanoparticles, especially for nano-ZnO and nano-TiO2 [36,4042]. In the case of nano-CuO, the release of copper ions was reported [43,44], whereas Fe2O3 NPs generate Fe21 ions, which then react with oxygen to produce hydrogen peroxide [45]. Fig. 7.3 presents the effect of nanosized Cu species on the Escherichia coli cells. Nowadays, the rise of antibiotic-resistant bacteria is becoming more dangerous to human health and food security in the developing world. In the United States, the costs of additional health care due to antibiotic-resistant infections could reach US$20 billion [46,47]. Thus, the combination and hybridization of above nanoparticles would be the new strategic approach for fighting these dangerous bacteria. In the case of noble metals, such as silver nanoparticles, their hybridization with nano-Fe3O4 (or nanoMnO2) could accelerate their ionization. On the other hand, for semiconductor oxides (ZnO or TiO2), the generation of ROS was also enhanced by their hybridization with noble metals (Ag or Au).

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FIGURE 7.2 Possible antimicrobial interactions of Ag nanoparticles in cells. Source: Figures reproduced with permission from Royal Society of Chemistry, Z. Wang, T. Xia, S. Liu, Mechanisms of nanosilver-induced toxicological effects: more attention should be paid to its sublethal effects, Nanoscale 7 (2015) 74707481.

FIGURE 7.3 TEM images of Escherichia coli cells treated with Cu species. Red arrows indicate Cu particles; green arrows indicate membrane damage. Source: Reprinted with permission from C. Kaweeteerawat, C.H. Chang, K.R. Roy, R. Liu, R. Li, D. Toso, et al., Cu nanoparticles have different impacts in Escherichia coli and Lactobacillus brevis than their microsized and ionic analogues, ACS Nano 9 (2015) 72157225. Copyright (2015) American Chemical Society. Nanoparticles (n-Cu, n-CuO, n-Cu(OH)2), microparticles (m-Cu, m-CuO) and the ionic Cu species (CuCl2, CuSO4).

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7.2 EFFECT OF METAL OXIDE NANOPARTICLES ON THE ANTIBACTERIAL BEHAVIOR OF NOBLE METALS IN THEIR NANOHYBRIDS Although silver is an effective antibiotic against broad-spectrum bacteria, its free ionic form is rather toxic to human cells. In its antibacterial application, it has been reported that doping of silver in metal oxide could minimize the toxicity of free silver to human cells [48]. Today, one of the most promising strategies for increasing antibacterial activity of AgNPs relates to the combination of AgNPs with other oxides, such as silver ferrite nanocomposites [49], Fe3O4Ag coreshell nanoparticles [50,51], AgNPs containing silica microbeads [52], and Fe3O4Ag hybrid nanoparticles [53]. For Fe3O4Ag coreshell nanoparticles, Chudasama et al. [50] observed that their antibacterial effect against the Gram-negative bacteria (including E. coli) was higher than that of AgNPs. Similarly, for silver ferrite nanocomposites, Kondala et al. [49] reported that their antibacterial activity was better than those of AgNPs and other antibiotic drugs. The authors concluded that their findings could be attributed to the faster Ag1 release rate from the silver ferrite nanocomposite. Recently, we reported that the Fe3O4Ag dumbell-like hybrid nanoparticles had a higher antibacterial effect against E. coli than the lone AgNPs [54]. We proposed two similar reasons: (i) the faster Ag1 release rate from the Fe3O4Ag hybrid nanoparticles due to the electron transfer from AgNP to Fe3O4 nanoparticle; and (ii) the ionization of AgNPs in the nanohybrids might be accelerated by Fe31 ions. Other authors also reported that the cointensification of Ag and Fe31 greatly enhanced the bioleaching efficiency of As-bearing gold ore [55]. Similarity, Takahashi et al. found electron transfer from Ag core to the FeCo shell in their 15 nm hybrid nanoparticles [56]. Their XPS data indicated the relative proportions of oxidation states of iron were Fe0/Fe21/Fe31 5 15:56:29. In this direction, for manganese oxide nanohybrids, Kunkalekar et al. reported that the AgMnO2 nanohybrids were found to be more effective than the AgMn2O3 nanohybrids against six test bacteria, which were three Gram-positive bacterial cultures (Staphylococcus aureus ATCC 6538, Streptococcus epidermis ATCC 12228, Bacillus subtilis ATCC 6633) and three Gram-negative cultures (Escherichia coli ATCC 8739, Salmonella abony NCTC 6017, Klebsiella pneumoniae ATCC1003) [57]. Chen et al. [58] observed that hybridization of AgNPs (220 nm) with nano-TiO2 (200400 nm) enhanced the antibacterial effect of AgNPs against E. coli, with very low concentration of AgNPs (10 μg/mL). A similar result was also obtained for Ag/TiO2 hybrid nanoparticles (2656 nm) [59], which had higher antibacterial effect in the dark against E.coli, than that of AgNPs, with low concentration (25 μg/cm2). In their test, pure nano-TiO2 nanoparticles had no antibacterial effect against E. coli.

7.3 EFFECT OF NOBLE METAL NANOPARTICLES ON THE ANTIBACTERIAL BEHAVIOR OF METAL OXIDES IN THEIR NANOHYBRIDS In general, metal oxide semiconductor nanoparticles, such as ZnO and TiO2, can destroy the pathogenic bacteria by ROS mechanism under UV light radiation. In this case, I. FUNDAMENTALS

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when a photon of higher energy than their optical band gap energy (Eg B3.23.4 eV) is absorbed by these nanoparticles, the electronhole pairs were created and then generated ROS. Since these nanoparticles had a wide band gap, such as B3.2 eV (nano-TiO2 [60]) and 3.37 eV (nano-ZnO [61]), only a light source in the UV radiation range (B5% of the solar energy) could be used for their activation. In addition, these nanoparticles had the low photoenergy conversion efficiency [62] (with low charge separation efficiency and fast recombination of photogenerated charge carriers), which limited their practical applications [63,64]. Many scientists have tried to improve the photocatalytic ability of these materials through two main pathways: (i) reduction of the recombination of photogenerated electronhole pairs; or (ii) improvement of the visible light sensitivity (the visible light contains about 45% of the solar energy [60]). For the first approach, the design of heterostructures (heterojunctions) was a promising pathway, such as (i) deposition of noble metals (Ag, Au or Pt) on the these nanoparticles surface, and (ii) coupling other semiconductor (CdSe, Ag2O, CdS) with these oxide nanoparicles [6569]. In the case of noble metals, the formation of the Schottky barriers at the interface of noble metals and these semiconductor oxide nanoparticles enhanced the segregation of charges and reduced the charge recombination [70,71]. As result, Ubonchonlakate et al. reported that Ag-doped TiO2 had higher antibacterial efficiency (100% in 10 min) against P. aeruginosa bacteria, than that of pure TiO2 (57% in 15 min), under UV irradiation [72]. In the case of the second approach, doping various transition metals or rare earth ions into these oxide crystal lattices could reduce their optical band gap. For example, the absorption edge of TiO2 was shifted into the lower energy region with S doping [73]. Similarly, the absorption of light in the visible region of TiO2 increased with the noble metals’ (Pt, Au, and Pd) doping content [74]. Recently, the hybridization of semiconducting oxides with noble metals (Au, Ag, Pd, Pt) has turned out to be the most promising strategy to defeat larger band gap [75]. The energy level alignment between the nanoparticles is combined at the heterojunction. In the hybrid nanoparticles, the noble metal nanoparticles (gold and silver) exhibit localized surface plasmon resonance (LSPR) absorption in visible light which can have significant impact at the heterointerfaces. However, the SPR of platinum nanoparticles is found to be located in the UV range (215 nm/264 nm), unlike the other noble metal nanoparticles which display SPR in the visible range [76]. Since the absorption spectrum of Pt nanoparticles is shorter than the band gap of ZnO NP, such as B380 nm, it may thus produce a blue-shift phenomenon by the energy transfer of plasmonic effect. This slight blue-shifting absorption peak has been observed for the PtZnO nano-hybrids [74,7678]. The phenomenon can be attributed to the LSPR coupling, and the absorption band wavelength of PtNPs is shorter than the band gap of ZnO, it may generate a blue-shift phenomenon by LSPR coupling [79]. Table 7.1 demonstrates the Eg (eV) values for various nano-TiO2-based hybrid NPs. As can be seen in this table, the hybridization of noble metals and nano-TiO2 reduced the optical band gap of nano-TiO2 particles, with AgNPs giving the highest reduction. Similar results are observed for nano-ZnO-based hybrid NPs (Table 7.2) with the presence of AgNPs, AuNPs, and PdNPs. However, a slight increase of optical band gap was reported for PtZnO nanohybrids as explained above [74,79].

I. FUNDAMENTALS

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7.3 EFFECT OF NOBLE METAL NANOPARTICLES ON THE ANTIBACTERIAL BEHAVIOR

TABLE 7.1 Optical Band Gap Energy Eg (eV) Values of Various Nano-TiO2-Based Hybrid NPs [8091] Hybrid Nanoparticles Single Nano-TiO2 (eV)

AgNPs (eV)

AuNPs (eV)

3.26

2.64, 2.46 and 2.34

PtNPs (eV)

PdNPs (eV)

2.84 3.26

2.93

2.93

2.84

2.9

2.8

3.21 3.3

3.11 3.0

3.05

2.192.9

3.4

2.22.3

.3.1

2.482.76

TABLE 7.2 Optical Band Gap Energy Eg (eV) Values of various Nano-ZnO-Based Hybrid NP [9298] Hybrid Nanoparticles Single Nano-ZnO (eV)

AgNPs (eV)

AuNPs (eV)

PdNPs (eV)

PtNPs (eV)

3.36 3.25

3.18

3.3

3.13.2

3.3

3.05

3.36

3.28

3.41

3.38

3.27

3.26

3.05

2.9

3.38

3.4

3.21

3.23

In a recent unpublished work, we fabricated the AgNPs-decorated nano-TiO2 particles by using the chemical reduction of AgNO3 with the presence of nano-TiO2 dispersion (the weight ratio of Ag/TiO2 was 3 wt.%). Fig. 7.4A and B presents the SEM images of nanoTiO2 particles before and after the decoration. Ag nanoparticles were well dispersed on

I. FUNDAMENTALS

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7. ANTIBACTERIAL BEHAVIOR OF HYBRID NANOPARTICLES

FIGURE 7.4 TEM images of AgNPs decorated nano-TiO2 (A), SEM images of pure nano-TiO2 (B) and their UVvis spectra (C). The different colors reflect solar light spectrum.

the surface of nano-TiO2 particles. The UVvisible absorption spectra of nano-TiO2 and AgNPs-doped nano-TiO2 particles (dispersed in water) are illustrated in Fig. 7.4C, showing that the hybridization of nano-TiO2 and AgNPs leads to shifting the absorption edge into the lower energy region (visible region), as compared with the pure nano-TiO2 (UV region). Similar results were reported for AgTiO2 nanocomposites [99,100]. The authors signaled that visible light absorption by surface plasmon resonance of AgNPs induced electron transfer to TiO2, resulting in charge separation and therefore activated by visible light. Yue et al. [101] reported the antibacterial properties against E. coli of the AgTiO2 coreshell hybrid nanoparticles without the presence of UV light. They observed the obvious zone of inhibition around the hybrid nanoparticles, whereas there was no inhibition detected around the pure TiO2 nanoparticles. Similar results also were observed by Dhanalekshmi et al. [102] for AgTiO2 coreshell hybrid nanoparticles against E. coli and S. aureus bacteria.

I. FUNDAMENTALS

7.4 CHALLENGES AND PERSPECTIVE

149

In the case of AgZnO nanohybrids, Wu and Kao [103] reported that they exhibited highly antibacterial activities against E. coli, under visible-light illumination and dark conditions. Zhang et al. [104] reported that the antibacterial activity against E. coli of AuTiO2 nanohybrids was more than five time higher than that of pure TiO2. This antibacterial activity of these nanohybrids was obtained under visible light illumination or in the darkness. Chen et al. [58] also observed that AuNPs (5 nm)-decorated nano-TiO2 particles (200400 nm) had higher antibacterial activity against E. coli, than pure nano-TiO2 particles, with low concentration of nano-TiO2 particles (15 μg/mL). Regarding the AuZnO nanohybrids, it was reported that the deposition of AuNPs onto ZnONPs dramatically increased the light-induced generation of hydroxyl radical, superoxide and singlet oxygen, holes and electrons [105]. The authors also indicated that hybridization with AuNPs enhanced the photocatalytic and antibacterial activity of ZnONP.

7.4 CHALLENGES AND PERSPECTIVE Today there is an increasing resistance of bacteria to a large number antibiotics, which are used to treat a wide range of infectious diseases, leading to a warning about the security and safety for patients. The comeback of forgotten natural antibiotics like silvers or its hybrizidation with other metal brings new pathway for overcoming the problems relating to bacteria and bacterial infection treatments. Research being undertaken on the combination of noble metals with other metals have shown that this combination has a positive effect on the biological activity of the nanoparticles. However, there is not a common mechanism to explain the biological enhancement in the hybrid systems. Moreover, although it is believed that antibiotic behavior of nanoparticles on bacteria occurs through one of three targets: (1) cell wall; (2) translational machinery; and (3) DNA replication and nanoparticles could react simultaneously through various processes, such as (i) production of ROS; (ii) electrostatic interaction with the cell membrane, (iii) ion release, (iv) internalization, etc. There is a lack of an international standard for the clear determination of the interaction between nanoparticles and bacteria and thus it is impossible to compare the biological activity of each nanoparticle. However, the bacteria cell has a complex structure and the real interaction between these cells with nanoparticles remains a big question. Regarding the synthesis of hybrid nanoparticles, it is evident that will be a promising research field and numerous studies can be found on this subject using physical, mechanical, and chemical methods. There is no doubt that with the continued progress of the nanotechnology used in the characterization of the nanoparticles, we will continue to shed more light on many aspects of hybrid nanoparticles, especially on electron transfer and mutual ionization mechanisms. The studies in toxicity of hybrid nanoparticles must be considered because they will be essential for this promising field and to avoid the future harmful impact on human health.

I. FUNDAMENTALS

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C H A P T E R

8 Exciton 2 Plasmon Interactions in Noble MetalSemiconductor Oxide Hybrid Nanostructures Weihua Lin and Mengtao Sun Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, P.R. China

8.1 INTRODUCTION Surface plasmons (SPs) are the coherent collective electrons oscillating along the interface where the signs of the real part of the dielectric function are different on the two sides [1]. Moreover, the localized surface plasmon (LSPs) have been broadly applied in the fields of surface-enhanced Raman scattering (SERS) [2], surface plasmon resonance sensors [3,4], tip-enhanced Raman scattering (TERS), etc. Nowadays, the novel applications of SPs on chemical reactions have been extensively investigated [518], such as photocatalytic reactions. To use solar energy in the photocatalytic field more efficiently, it is of paramount importance to reveal the internal mechanism of SPs. In most cases, with a proper designed nanostructure which is usually efficient light-trapping [1921], localized surface plasmon resonance (LSPR) can occur, where the confined free electrons oscillate with the same frequency as the incident radiation, leading to intense and highly localized electromagnetic fields. Based on that, SERS has been broadly studied, where the Raman signals can be enhanced over a large frequency range. After light absorption and LSPR excitation, the accumulated energy is transferred to electrons in the conduction band. Highly energetic electrons generated from plasmon decay

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00008-5

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© 2019 Elsevier Inc. All rights reserved.

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are named as “hot electrons” as a critical part in driving the surface catalytic reactions, that can not only produce the energy to overcome the reaction barrier but also provide electrons for reduction reactions. Besides, by using propagating surface plasmon polaritons (PSPPs), the damage caused by a direct incident laser can also be avoided. Hence, plasmon-driven surface catalytic reactions have several outstanding merits, e.g., the extremely high surface sensitivity and promoted catalytic efficiency [22]. However, according to the reaction dynamics investigated by ultrafast transient absorption spectroscopy, the lifetime of “hot electrons” is relatively short, which is the major challenge for surface plasmon-induced hot electron transfer catalytic reactions. Semiconductor and metal oxide emerged as the potential candidates for solving the problems. For example, monolayer MoS2, two-dimensional material with a direct band gap of 1.8 eV [23,24], has a wide range of electronic, mechanical, thermal, optical, and chemical properties to attract a great deal of attention [25]. Monolayer MoS2 has high transparency ( . 92%) in visible light region, large surface-to-bulk ratio, quantum confinement effects, and good potential in promoting catalytic reactions [23]. Moreover, the monolayer MoS2 can protect plasmonic metals (usually Ag) from rapid oxidation. However, the large band gaps and low yield of hot electrons limit the developments of metal oxide for application in catalytic field. TiO2 also has been attracting much interest in the photocatalytic field as an outstanding electron-accepting metal oxide. The conduction band of TiO2 has a high density of states. According to its merits, TiO2 has the ability to permit fast electron injection. Many studies have investigated the hybrid systems consisting of Au or Ag NPs with TiO2 [2632]. According to transient absorption spectroscopy [3335], TiO2 has many outstanding optical properties compared with other dyesensitized semiconductors, such as ZnO [36,37], SnO2, and In2O3. Many investigations were applied to improve the photocatalytic efficiency [3840], and the reaction dynamics of photocatalytic reactivity on TiO2 have been investigated extensively [4145]. Moreover, the thermal stability, photostability, low cost, and harmlessness make TiO2 a more robust competitor. However, TiO2 has some drawbacks including the large band gap of 3.3 eV, which limits photoabsorption to the UV region of the solar spectrum [4649]. Another crucial obstacle is the chargecharge recombination in metal oxide, whose rate should be reduced to improve the efficiency of chemical reaction [47,50]. The recombination results in an overall loss of the charge carriers before reaching the surface, and can be addressed by maximizing photon absorption with the help of plasmonic nanostructure [51]. Although the connection between plasmonic nanostructures and semiconductor forms a metalsemiconductor Schottky junction [20], combining the merits of noble metal and metal oxide is a promising way to optimize photocatalytic devices with several limitations addressed, and further promote surface catalytic reaction efficiency. The lifetime of hot electrons is obviously prolonged from femtosecond to picosecond, which is vital in driving the surface catalytic reaction, as confirmed by Ding and coworkers with the fabricated grapheneAg NPs system [52]. On the other hand, the band gap of metal oxide is decreased and the density of states (DOS) is adjusted for improving the efficiency in hybrid structures. The localized SPR

I. FUNDAMENTALS

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159

effect induced by plasmonic nanostructure can increase photon absorption due to the confined field enhancement. Understanding the internal mechanism and tenability of these hybrid systems, still experimentally and theoretically challenging subjects, are important for further investigations towards dynamics and various applications. When excitons of metal oxide strongly couple to the localized SPs, the novel hybridized energy states formed as plexcitons (also excitonpolaritons) are formed as a type of polariton. Several studies have reported the optical advanced properties of plexcitonic nanostructures [5357]. Importantly, the hybrid systems are already applied in many fields such as chemical sensors, pH meters [58], light harvesting [59], and optically active devices [60,61]. Hence, the mechanism of plasmonexciton coupling interactions with semiclassical theory and quantum theory will be introduced to reveal a deeper understanding of the unique properties of noble metalmetal oxides hybrid system. And the merits of hybrid system will be confirmed experimentally and theoretically, consisting of ultrafast transient absorption and plasmonexciton codriven catalytic reactions.

8.2 MECHANISMS Cavity quantum electrodynamics (QED) is the proper mechanical description to study the quantum interactions between light and matter inside a microcavity. Taking an excited isolated atom for example, there is no mechanism leading to electron decay due to the two orthogonal eigenstates (excited and ground levels). However, Purcell [62] discovered that the spontaneous emission not only relates to the emitter, but also depends on the environment in 1946, which can be confirmed by the case where an atom is inside of a cavity with perfectly reflecting walls. Based on this theory, we can only consider the emitter and its environment as a whole system.

8.2.1 Free Space Spontaneous Emission To illustrate the basic physical mechanism underlying spontaneous emission, we first consider the electric dipole interaction between the single two-level system and the single mode of the electromagnetic field. The two-level system, which is formally analogous to a spin-1/2 system with two possible states, can be conveniently described by the Hamiltonian



Ha 5 ¯hωe jeihej 1 ¯hωg g g

(8.1) where the ¯hωe and ¯hωg are the energies of the excited The wave

 and the ground level. 

g , where Ce and function of the two-level atom can be described as ψðtÞ 5 Ce ðtÞjei 1 Cg ðtÞ

 Cg are the probability amplitudes of finding the atom in states jei and g which represent the upper and lower level states of the atom, respectively. In the absence of interaction, an atom initially in its excited state jei will remain there for all times. Transitions between the eigenstates of the atom result from the coupling of

I. FUNDAMENTALS

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the atom to some other system. In the case of dipole coupling to a single electromagnetic field mode of frequency ωc , the total atom-field system can be described by the JaynesCummings Hamiltonian Hs 5 Ha 1 Hf 1 Ha2f y

(8.2) y

where Hf 5 ¯hωc a a and the annihilation and creation operators a and a obey the boson commutation relation ½a; ay  5 1. In the dipole and rotating wave approximation, the interaction Hamiltonian Ha2f between this field mode and the two-level atom can be described as



b E b Ha2f 5 2 d

(8.3)

b is the dipole moment operator of the transition, and E b is the electric field operator where d related with the polarization of the field mode. Hence the total atom-field system in Eq. (8.2) can be reexpressed as H 5 E0 1 ¯hω0 σz 1 ¯hωc ay a 1 ¯hðgðRÞay σ2 1 h:c:Þ, where ω0  ωe 2 ωg is the atomic transition frequency, and g is half the so-called vacuum Rabi frequency. The vacuum Rabi frequency 2gðRÞ normally depends on the location R of the atom. The interaction is physically transparent: the atom can either absorb a photon and undergo a transition towards the excited state, or emit a photon when undergoing a transition from the excited to the ground state. Within the framework based on the JaynesCummings model, the excitations number in the atom-field system is conserved. On the resonance ðωc 5 ω0 Þ, when the frequency of single electromagnetic field mode equals the frequency of atom transition, the timedependent states of atom-field system are





ψð0Þ 5 cosðgtÞje; 0i 2 isinðgtÞ g; 1 (8.4)

 So that probability Pg ðtÞ for the atom to be in its ground electronic state g is



 2

Pg ðtÞ 5 g; 1 ψðtÞ 5 sin2 ðgtÞ (8.5) The equation can simply demonstrate the form of spontaneous emission: the quantized field (fluctuation) in the vacuum induces transitions between the two states. Besides, the puzzling oscillatory behavior of Pg ðtÞ at the “vacuum Rabi frequency,” 2g can also be presented by the simple model, which results from the periodic exchange between the atom and cavity. Based on the Fermi’s golden rule, the transition rate of corresponding radiation from an excited state towards the lower energy level is able to be calculated:



 2 2π X X

γ 5 2

f

μ E

i

ρðωÞ (8.6) ¯h



In the equation, the γ represents transition decay rate from the initial (excited) state jii

 X X to the final state f . μ and E are the electric dipole and vacuum-field operators, respectively. ρðωÞ represents the final photonic density of states.

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8.2 MECHANISMS

And the decay rate of the excited atomic state population in the free space can be described as: γ free 5





X X 2



based on f μ E

i

5 ð1=3Þμ2if E2vac , pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Evac 5 ¯hω=2ε0 V [6366].



1 τ free

5

μif 2 ω3

(8.7)

3πε0¯hc3

where

τ free

is

the

radiative

lifetime

and

8.2.2 Spontaneous Emission in Cavities The Hamiltonian of a system that consists of an atom coupling to a single-mode field can be described as Hs 5 Ha 1 Hf 1 Ha2f . The atom-cavity mode system has two dissipative processes: to the free space electromagnetic field background or to the outside world due to the mirror losses and diffraction. And the master equation of the atom-cavity mode can be described by i ih 0 y (8.8) ρ_ s 5 2 Heff ρ 2 ρHeff 1 κaρs ay 1 γ σ2 ρs σ1 ¯h ¯ ðγ0 =2Þσ1 σ2 2 ih ¯ ðκ=2Þay a. where Heff 5 Hs 1 Hloss and Hloss 5 2 ih According to the master Eq. (8.8), the evolution of the state can be described as:



 d ψðtÞ 5 Heff ψðtÞ ih ¯ (8.9) dt

  where the state can be introduced as ψðtÞ 5 Ce ðtÞeð1=2Þδt je; 0i 1 Cg ðtÞeð1=2Þδt g; 1 . The coupling between LSPs and excitons results from the coherent dipoledipole interaction [67]. Based on the brief introduction of basic concepts, we move forward to distinguishing the different regimes. The interactions could be divided into two different kinds (in the weak or strong coupling regime) by the lifetime of the LSPs and excitons. When the system is in the weak coupling regime, the lifetime of plasmonic resonance is relatively short compared with the spontaneous decay rate of the isolated emitter. On the opposite, when the lifetime of LSPR is very much longer, then it is in the limit of strong coupling. In another way, regimes are also able to be determined by comparing the three typical constants. 1. The dipole coupling constant between the atom and the cavity mode g; 2. The decay rate of the mode κ; 3. The rate of spontaneous emission into electromagnetic field modes γ 0 . When the κ and γ 0 overwhelm the Hamiltonian dipole interaction represented by g (g{κ; γ 0 ), the system is in the weak coupling regime. In this limit regime, the role of the vacuum field can be considered as a perturbation to the emitter, where the cavity and emitter can be separately treated. In contrast, the system is in the strong coupling regime if gcκ; γ0 . In this limit regime, the emitter and cavity can only be treated as a system, and can only be descripted with quantum electrodynamics treatment.

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a. Weak coupling In the weak coupling regime, the interaction between matter and light is not stronger than outside the system, for example, the fast relaxation of LSPs [67,68]. LSPs can modify the absorption cross-section due to plasmonic nanostructures. Final density of states, for instance, has its maximum at the plasmon resonance wavelength when the emitter is near metal NPs. When the cavity only has one mode of frequency ωc , ρðωÞ can be described as: ρðωÞ 5

2 Δω π 4ðω2ωc Þ2 1 Δω2

(8.10)

which represents the density of states and can be influenced by the quality factor of cavity Q 5 ωc =Δω. And according to Eq. (8.10), it is obvious to find the local density of states (LDOS) maximum at ω 5 ωc [69]. And the decay rate of an emitter which is placed within a plasmonic cavity can be calculated: γ cav 5 β 2

2μ2if Q ε0¯h V

(8.11)

Using the Purcell factor FP [6971], the way that the decay rate of the emitter is modified by the effect of the cavity can be described. If FP . 1, the spontaneous decay rate is enhanced; otherwise the cavity inhibits the emission.  3  τ free γ cav 3 λ FP 5 5 5 2Q 3 (8.12) 4π γ free τ cav n V Moreover, the overall enhancement in a quantum yield (η 5 γ rad =ðγ rad 1 γ nonrad Þ) can be further determined by the competition between radiative and nonradiative rates, and the maximum can be achieved by optimizing the radiative rate enhancements and minimizing the nonradioactive losses [67,72]. b. Strong coupling In a strong coupling regime, the new quasiparticle (plexciton) is formed with distinct properties possessed by neither original particle. The coherent coupling interaction between the atom and the cavity mode is so strong that the photon emitted into the cavity is similar to be reabsorbed before escape (the light and matter exchange energy periodically), and results in two new mixed states separated energetically (named as Rabi splitting) [68,73,74]. The perturbative analysis of the coupling between the atom and the cavity mode ceases to be justified. The general solution for arbitrary g, γ 0 , and κ is of the form Ce ðtÞ 5 Ce1 eα1 t 1 Ce2 eα2t

(8.13)

where " #1=2   2 1 γ0 κ 1 γ0 κ 2 α1;2 5 2 1 1 iδ 6 1 1iδ 24g 2 2 2 2 2 2

I. FUNDAMENTALS

(8.14)

8.3 FEMTOSECOND ABSORPTION

163

and the constants Ce1 , Ce2 are determined from the initial condition Ce ð0Þ 5 1, Cg ð0Þ 5 0. In strong coupling regime where gcκ; γ0 and δ, it can be adjusted to   1 γ0 κ (8.15) 1 1 iδ 6 ig α1;2 5 2 2 2 2 According to Eq. (8.15), the evolution of the upper state population can be described, as well as the split at the vacuum Rabi frequency. Based on the JaynesCummings Hamiltonian, the system only has two states je; ni

and g; n 1 1 . When the atom-field interaction is considered, the eigenenergies can be described as   1 E2n 5 ¯h n 1 (8.16) ωc 2 ¯hRn 2   1 E1n 5 ¯h n 1 (8.17) ωc 1 ¯hRn 2 Here Rn is the n-photon generalized Rabi frequency qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 δ2 1 4g2 ðn 1 1Þ Rn 5 (8.18) 2

 The energies of the state je; ni and state g; n 1 1 cross at δ 5 0, but the atom-field interaction removes this degeneracy, causing the dressed states j1; ni and j2; ni to repel each other, or anticross. In other word, the anticrossing corresponds to the resonance condition ω0 5 ωc (a central peak at ωc and two sidebands at ωc 6 Rn ) (Fig. 8.1).

8.3 FEMTOSECOND ABSORPTION Hot electrons generated from plasmon decay in plasmonic nanostructures, which can be treated as an efficient light-trapping components, can significantly improve the photocatalytic efficiency of traditional semiconductor devices. The hybrid system of noble metal and metal oxide was once considered as two separated components, where the electron transfer was considered as impossible, because the individual electrons were not able to get enough energy to overcome the Schottky barrier [76]. However, the electron transfer from gold into TiO2 electrode had been proved by the generated photocurrent under excitation of plasmon band [77,78], which is meaningful because the wide band gap of TiO2 limits the generation of photocurrents. The generated hot electrons will go through three steps as generation, injection, and regeneration. The reverse electron transfer from TiO2 to gold, as the inverse process, is also proved in coreshell AgTiO2 NPs [79]. Based on the investigations, the energy that is needed to overcome the reaction barrier in hybrid system is relatively smaller than the band gap of the TiO2. Hence, putting the plasmonic nanostructures in contact with a semiconductor is a promising way to develop photocatalytic devices.

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8. EXCITON 2 PLASMON INTERACTIONS IN NOBLE METAL

FIGURE 8.1 The mechanism of strong coupling between light and matter revealed by reflectance spectra [75].

The electromagnetic decay in plasmonic nanostructures takes places on a femtosecond timescale, either radiatively through reemitted photons [80] or nonradiatively by transferring the energy to hot electrons [8184]. To further reveal the internal mechanism of the prolonged lifetime of hot electrons in plasmonexciton coupling interaction, the ultrafast pumpprobe transient absorption spectroscopy is usually applied to investigate the timescale of the hot electron transfer process. Among the hot electrons transfer processes, the fast-electron injection into the metal oxide before recombination is crucial for improving the conversion efficiency. Direct evidence of electron transfer from Au NPs to TiO2 can be obtained. Furube and coworkers [15,85,86] revealed the internal mechanism of ultrafast plasmon-induced electron transfer from 10 nm Au NPs to TiO2 NPs with femtosecond transient absorption spectroscopy, and revealed that the hot electron generation and injection were completed within 50 fs [42,45,8797]. When the plasmons of gold NPs are excited, the electrons with a nonFermi distribution relaxation through the reemission of photons or carrier multiplication within 100 fs is due to the electronelectron interaction [52,98,99], electronphonon interaction at the timescale of 110 ps, and phononphonon interactions around 100 ps [100,101]. Hence the researchers concluded that the hot electron injection resulted from electronelectron interaction.

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10

FIGURE 8.2 Transient absorption of N3/TiO2, Au/TiO2 and Au/ZrO2 at 3500 nm [15].

Silicon N3/TiO2

Absorbance (10–3)

8

Au/TiO2 6

Au/ZrO2

4 2 0

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The N3/TiO2 system, whose carrier injection efficiency is nearly 100%, is used for the comparison of the electron transfer yield [102]. According to the transient absorption intensity ratio between Au/TiO2 and N3/TiO2 (B100% injection efficiency), the electron injection yield in Au/TiO2 was evaluated to be about 40% under 550 nm excitation. In addition, the Au/ZrO2 system is set for monitoring the response of excited Au nanodots as a control group, where no electron is able to transfer from Au nanodots to ZrO2, since the conduction band edge of ZrO2 is located 0.9 V above that of TiO2 [102]. The observed transient absorption of Au/TiO2 can be attributed to the electrons transfer into TiO2, because there is no transient absorption for Au/ZrO2 with a probe laser of 3500 nm. And the electron transfer was completed within 240 fs, as shown in Fig. 8.2. The timescale of the regeneration process was also investigated by Tian and coworkers [103,104]. Similarly, Ag NPsgraphene hybrid have also been investigated widely as a proper candidate. With the unique properties of graphene, the grapheneAg nanostructure hybrid systems have been widely applied in investigating plasmonexciton codriven surface catalytic reactions, whose mechanism has not been clearly elucidated. Ding and coworkers fabricated the grapheneAg nanowire hybrid system (Fig. 8.3A) to reveal the dynamic process of plasmonexciton coupling interaction with ultrafast transient absorption spectroscopy, as shown in Fig. 8.3BE. The fitted curve in Fig. 8.3C indicates that the lifetime of plasmonic hot electrons is about 3.2 6 0.8 ps, which is obviously prolonged compared with the situation of isolated Ag NW (150 fs). Furthermore, the graphene can not only prolong the lifetime of hot electrons dramatically, but also results in a significant accumulation of hot electrons. Moreover, the mechanism of plasmonexciton coupling interaction can also be investigated based on the MoS2Ag NPs hybrid system, according to the transmission spectra (Fig. 8.9) and ultrafast absorption spectroscopy (Fig. 8.4) with a pump laser of 400 nm. It is revealed that the enhancement factors of excitonic states of MoS2 are different, where the A excitonic state (637 nm) is enhanced significantly with plasmonexciton coupling

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FIGURE 8.3 (A) The SEM image of Ag NWgraphene hybrid system. (B) The corresponding ultrafast pumpprobe transient absorption spectroscopy excited by 400 nm, and (C) fitted at 532 nm. (D) The corresponding ultrafast pumpprobe transient absorption spectroscopy excited in NIR region, and (E) fitted at 1103 nm [52].

(A)

200 nm (B)

(C) 05

0.0000 ΔA (OD)

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interaction, instead of B excitonic state (595 nm). Comparing the lifetimes of the two excitonic states of MoS2 in Fig. 8.4, we can find out that the plasmonexciton coupling interaction has a strong impact on the lifetime of excitonic states. According to the fitted transient absorption spectra of MoS2Ag NP (size of 21 nm) in Fig. 8.4EH, the lifetime of electronelectron interaction in the hybrid system is enhanced about eight times over that for MoS2 alone. As for the electronphoton interaction, the lifetime is also increased significantly for excitonic state A and B, respectively. Hence, it is confirmed that the plasmonexciton coupling interaction is able to improve the probability and efficiency of surface catalytic reactions due to the enlarged lifetime of carriers.

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8.3 FEMTOSECOND ABSORPTION

(A)

(B) 100

100

0.006800

80

80 0.003900

Time (ps)

Time (ps)

0.003660 0.002100

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3.000E-04 –0.001500

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60

5.200E-04 –0.002620

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FIGURE 8.4 The ultrafast transient absorption spectra of (A) monolayer MoS2, and (BD) monolayer MoS2Ag NPs hybrid system where sizes of Ag NPs are 6.1, 14.5, and 21 nm, respectively. The transient absorption spectra of (EF) monolayer MoS2 fitted at excitonic state A and B, respectively, and (GH) monolayer MoS2Ag NPs hybrid fitted at excitonic state A and B, respectively, where the size of Ag NPs is 21 nm [105]. I. FUNDAMENTALS

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8.4 APPLICATIONS Since 2010, plasmonic hot electrons have been found to be critical in the field of surface catalytic reactions monitored by the SERS [12,106] and TERS [107]. However, based on the investigations, the efficiency of surface plasmon-to-hot electron conversion is considered as lower than 1% [108]. Hence, several studies have been done to achieve the goal of increasing the efficiency of plasmon-driven surface catalytic reactions. Before depicting the specific application of hybrid system, we can experimentally corroborate that the plasmonexciton coupling degree can be well manipulated, for example, by changing the size of Ag NPs in the Ag NPsTiO2 film hybrids. Ding firstly synthesized the nano-sized TiO2 film on the quartz, with the thickness of 208 nm approximately and the absorption peak is centered at 524 nm. Above the TiO2 film, Ag NPs with different sized are synthesized, as shown in Fig. 8.5. The optimal parameters of components can be studied by UVvisible absorption spectroscopy, and the ultrafast transfer process of plasmon-induced hot electron from Ag NPs into TiO2 film can be investigated by ultrafast transient absorption spectroscopy. When the Ag NPs are generated under UV irradiation, accompanied by the enlarging size, the absorption intensity gradually increases and the strong absorbance peak is red

FIGURE 8.5 The SEM images of Ag NPs synthesized on TiO2 film under different UV irradiation for (A) 2, (B) 5, (C) 15, (D) 30, and (E) 60 min. (F) The corresponding in situ real-time UVvis absorbance spectra of (AE), respectively [109].

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169

shifted. The strong plasmonexciton coupling interaction is formed only when the SPR peak of Ag NPs has an overlap with the absorption peak of TiO2 film (524 nm). According to the in situ real-time UVvisible absorbance spectra of hybrid systems (Fig. 8.5), it can be found that the absorption intensity increases gradually when the UV irradiation time increases from 2 min to 15 min, and the SPR peak of Ag NPs at 15 min is around 532 nm, while the growth is halted at 30 min and decreased when the time is up to 60 min. According to the absorption peaks of Ag NPsTiO2 film where the Ag NPs were fabricated within 2 min (Fig. 8.6A), it is clear that there are two ultrafast absorption peaks around 475 nm and 532 nm. Focusing on the case of 532 nm, the electronelectron interaction is 2 ps while the electronphonon interaction can approach 71 ps, as shown in

FIGURE 8.6 (A) The 3D ultrafast transient absorption spectrum of AgNPsTiO2 film which is synthesized within 2 min, and (B) fitted 532.7 nm. (C) The ultrafast transient absorption spectra at 532.7 nm for AgNPsTiO2 film hybrids with different synthesized time [109].

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Fig. 8.6B. According to the Ag NPs size-dependent ultrafast dynamics of plasmonexciton coupling of Ag NPsTiO2 film hybrids in Fig. 8.6C, when the size of Ag NPs increases, the intensity of the absorption spectrum becomes stronger while the lifetime decreases. The shorter lifetime represents the stronger plasmonexciton coupling in Ag NPsTiO2 film hybrids. We can conclude that the degree of plasmonexciton coupling interaction can be well manipulated. To further reveal the relationship between plasmonexciton coupling interaction and the efficiency of surface catalytic reactions, the typical oxidation reaction of PATP dimerized to DMAB can be investigated on Ag NPsTiO2 film hybrids synthesized with different UV irradiation time. In this case, hot electrons generated from plasmon decay transfer from Ag NPs to TiO2 film, and the left hot holes on the Ag NPs play a dominated role in driving oxidation reactions. Fig. 8.7A proves again that the superposition between the absorption peak of Ag NPs and TiO2 film can monitor the degree of coupling interaction, due to the strongest SERS intensity of reactions on 15 min Ag NPsTiO2 film. The efficiency of oxidation reaction can be monitored by the ratio of intensity at 1437 and 1071/cm, where the former is attributed to Ag mode of DMAB and the latter to A1 mode of PATP. The excitation wavelength-dependent oxidation reactions illustrate that, based on the match between excitation laser wavelength and SPR peak of Ag NPs, we can obtain the highest yield of product excited on 532 nm due to the plasmonexciton coupling interaction. And the oxidation reaction is efficient and stable on the Ag NPsTiO2 film hybrids under different environments, including atmospheric, aqueous, and icy environments [110]. In a word, based on UVvisible absorption spectroscopy, ultrafast transient absorption spectroscopy and SERS, the plasmonexciton coupling interaction in Ag NPsTiO2 film can be investigated in detail. To obtain maximum catalytic activity and oxidation reactions, the degree of coupling interaction can be adjusted by changing the size of Ag NPs, monitoring by the superposition between SPR peak of Ag NPs and the absorption peak of TiO2 film.

FIGURE 8.7 (A) The SERS spectra of plasmon-driven oxidation reactions on AgNPsTiO2 film with different UV irradiation time. (B) Laser wavelength-dependent SERS spectra on 15 min AgNPsTiO2 film hybrid. (C) Relative ratio between intensities at 1437/cm and 1071/cm, with different excitation wavelengths [109].

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8.4 APPLICATIONS

Yang synthesized monolayer MoS2/Ag NPs hybrids where the size of Ag NPs was monitored for shifting LSPR peak to match the exciton energy of monolayer MoS2, and hence, to further monitor the degree of plasmonexciton coupling interaction. As shown in Fig. 8.8, when the size increases, the LSPR peak of Ag NPs is red-shifted, as well as the absorption peak of hybrid system attributed to the plasmonexciton interaction. When they are coupled, LSPR can significantly enhance the excitation rate of the monolayer MoS2 exciton through EM, and the generated collective states result in stronger optical absorption than the individual components. The changes in the degree of plasmonexciton coupling interaction are also demonstrated by the photoluminescence (PL) spectra. When the plasmonexciton coupling interaction is increased by changing the thickness of Ag NPs, the PL intensity of MoS2 is strongly enhanced by LSPR up to 52 times, due to the Purcell effect. Yang and coworkers investigated the photocatalytic reactions of 4-nitrobenzenethiol (4NBT) on monolayer MoS2Ag NPs hybrid system, compared with that on MoS2 or Ag NPs, respectively.

90

80 Transmittance (%)

(B) 100

Transmittance (%)

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Transmittance (%)

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FIGURE 8.8 (A) The transmission spectra of Ag NPs, monolayer MoS2 and MoS2Ag NPs hybrids; (B) the transmission spectra of Ag NPs with different diameters; (C) the transmission spectra of MoS2Ag NPs hybrids with different sizes of Ag NPs; (D) the absorbances for hybrids with different sizes Ag NPs at 532 nm [111].

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d)

e)

f)

0.75 mW

0.75 mW

0.375 mW

0.15 mW

0.15 mW

0.15 mW

0.015 mW

0.015 mW

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x2

Raman Intensity (a.u.)

Raman Intensity (a.u.)

x2

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0.0015 mW

x2

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1600

300 400

1200 1400 Wavenumber (cm–1)

1600

300 400

1200 1400 Wavenumber (cm–1)

1600

FIGURE 8.9 (AC) The SEM images of MoS2Ag NPs with different sizes of Ag NPs, and (DF) laser power-dependent SERS spectra of the MoS2Ag NPs hybrids that correspond to (AC) [111]. 4NBT+Ag NPs 4NBT+MoS2+Ag NPs

(B)

Raman Intensity (a.u.)

(A)

0.15 mw

0.015 mw

x5

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x5

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1200

1350

1500

1650

Wavenumber (cm–1)

FIGURE 8.10 (A) The SEM of the Ag substrate, where the right regime is covered by MoS2, and the corresponding surface catalytic reactions in both regimes [111].

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We all know that in the reduction reaction, plasmonic hot electrons not only provide electrons, but also kinetic energy for overcoming the reaction barrier. And the existence of plasmonexciton coupling interaction can prolong the lifetime of hot electrons, and increase the density of states (DOS), and further promote the reduction reaction. On the monolayer MoS2 substrate, there is no obvious phenomenon about occurring reactions. With a low-intensity laser, the probability of the reduction reaction performed on the Ag NPs substrate is much lower than on the MoS2Ag NPs hybrid system. And the advantages of plasmonexciton coupling interaction can be confirmed by investigating the Ag NPs size-dependent plasmonexciton codriven surface catalytic reactions in Fig. 8.9. With the help of strongest plasmonexciton coupling interaction near 532 nm, the highest probability and efficiency of reduction reaction can be achieved. In Fig. 8.10, comparing the ratio between the Raman intensities of reactant (1338/cm) and product (1432/cm), we can conclude that the probability of the reduction reaction for 4NBT adsorbed on the Ag NPs directly is much weaker than on MoS2Ag NPs hybrid substrate, which supports the aforementioned conclusion. Combining with theoretically investigation, although the MoS2 layer weakens the electric field by approximately 30%, the efficiency of the plasmonexciton codriven surface catalytic reaction at low laser intensities increases by 3.6 times due to the plasmonexciton coupling interaction.

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C H A P T E R

9 Chemical Methods for Synthesis of Hybrid Nanoparticles Balakrishnan Karthikeyan1, R. Govindhan1 and M. Amutheesan2 1

2

Department of Chemistry, Annamalai University, Chidambaram, Tamil Nadu, India Department of Aeronautical Engineering, Hindustan Institute of Technology & Science, Chennai, Tamil Nadu, India

9.1 INTRODUCTION Nanomaterial is described exactly as material that is having one or more dimension(s) in the nanoscale range (,100 nm). Atoms/molecules are combined in a controllable way by bottom-up procedures to prepare (chemical synthesis) nanostructural materials. Larger surface to volume ratio is the master key for nanomaterials which make them work with more reactive surfaces and different functionality than bulk materials. As compared to the single nanoparticles of noble metals or metal oxides, their hybrid nanoparticles may have different optical, magnetic, electronic, and structural properties. The properties of these nanohybrids are based not only on the structural arrangement of each individual nanocomponent, but also on the concentration of the mixed single-component nanoparticles. Bottom-up chemical synthesis is the ideal methodology for the preparation of nanohybrids. It uses a chemical reducing agent that reduces the metal ion and integrates on another metal surface at the nano-level. Bottom-up procedures have been followed by the scientists to combine the atomic molecules in the nano-scale in the controllable way. In this method, the reaction time, heat treatment during the preparation process, and the choice of reducing agent affect the size, morphology, and the structure of the nanocomposite. Purity of the material and the synthetic route affect the nanomaterial preparation in a greater way. But in this method, the desired applications of the nanomaterial are affected

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00016-4

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due to the aggregation of the nanoparticles during sorption processes. Various chemical methods are discussed as follows.

9.2 SEED GROWTH METHOD A novel method of microwave (MW) 2 polyol method used in this study; reported in [1,2]. The synthesis of Au@Ag coreshell nanocrystals was prepared by two steps. The first method is synthesis of Au core, and the next step is the preparation of Ag shell. Initially, 2.4 mM of HAuCl4 4H2O is added into the solution in 20 mL of ethylene glycol solution. After that, 1 M of PVP in terms of monomeric units (molecular weight 40,000) is slowly added to the above solution. The mixture is heated by MW irradiation in a CW mode (Shikoku Keisoku, 400 W) for 2 min. After the solution is heated, it is then cooled to room temperature, and then AgNO3 is added. The [AgNO3]/[HAuCl4] molar ratio is varied in the range of 1:10. The mixture solution is heated again by MW irradiation for 2 min. Au coreAu/Ag alloy shell particles, denoted as Au@Au/Ag, are prepared by the addition of 2.4 mM of HAuCl4 4H2O to Au@Ag nanocrystals obtained at an [AgNO3]/ [HAuCl4] molar ratio of 1:1. Product particles after the first and second MW irradiations were characterized by using TEM (JEOL JEM-2010 and JEM 3000F). The images are shown in Scheme 9.1. It was reported that noble metal (Au, Ag, Pt)Fe3O4 hybrid nanoparticles could be synthesized by using seeding growth methods [3,4]. In these studies, the seeds were either the preformed Fe3O4 nanoparticles [3] or noble metal nanoparticles (AuNPs [4]).





SCHEME 9.1 Crystal structures and seed growth mechanism of Au@Ag coreshell nanohybrids prepared by the microwavepolyol synthetic routes. Source: Reprinted with permission from M. Tsuji, N. Miyamae, S. Lim, K. Kimura, X. Zhang, S. Hikino, et al., Crystal structures and growth mechanisms of Au@Ag coreshell nanoparticles prepared by the microwavepolyol method, Cryst. Growth Des. 6 (8) (2006) 18011807. Copyright 2006 American Chemical Society.

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9.3 COPRECIPITATION METHOD ZnOMgO nanocomposites are prepared by coprecipitation method by dissolving 0.58 g of ZnSO4 7H2O, 0.49 g of MgSO4 7H2O, and 0.75 g of NaOH in DI water (60 mL) under constant blending in a 100 mL glass beaker. Final solution is refluxed for 24 h in a 100 mL glass conical flask at 90 C. The precipitate mixture is centrifuged at 8000 rpm at room temperature, continuously and repeatedly washed with DI water. The obtained colorless semisolid samples are allowed to dry for 12 h at 80 C. Likewise, ZnO and MgO nanoparticles are prepared by using the homogeneous precipitation method [5], where 0.58 g of ZnSO4 7H2O (or 0.49 g of ZnSO4 7H2O) and 0.75 g of NaOH are homogeneously added in 60 mL of DI water. Colorless powder nanoparticles are formed in the final condition. Pure ZnO and MgO nanoparticles are formed via reaction (9.1) and (9.2), and (9.3) and (9.4), respectively. ZnOMgO nanocomposites are formed via reaction (9.5) and (9.6).











ZnSO4 7H2 O 1 NaOH-ZnðOHÞ2 1 NaSO4 1 6H2 O

(9.1)

ZnðOHÞ2 -ZnO 1 6H2 O

(9.2)

MgSO4 7H2 O 1 NaOH-MgðOHÞ2 1 NaSO4 1 6H2 O

(9.3)

MgðOHÞ2 -MgO 1 6H2 O

(9.4)









ZnSO4 7H2 O 1 MgSO4 7H2 O 1 NaOH-ZnðOHÞ2 MgðOHÞ2 1 NaSO4 1 6H2 O



ZnðOHÞ2 MgðOHÞ-ZnO 2 MgO 1 2H2 O

(9.5) (9.6)

Aumetal oxide hybrids, such as AuFe2O3, AuNiO and AuCo3O4, are prepared by coprecipitation method [6] using the sodium carbonate, HAuCl4, and metal nitrate. Similarity, AuZnO hybrid nanoparticles have been synthesized by using Na2CO3, HAuCl4, and Zn(NO3)2 [7,8]. AgFe3O4 coreshell nanowires have been also successfully synthesized by coprecipitation method using FeCl3, FeCl2, and polyvinylpyrrolidone (PVP) [9].

9.4 SONOCHEMICAL SYNTHESIS 9.4.1 Synthesis of (Pd, Co)@Pt Nanohybrids Ultraviolet photoelectron (UPS) reactions are used to prepare Pd, Co@Pt nanohybrid materials, through Pd(acac)2, Pt(acac)2, Co(acac)2, and the carbon support. These are added to a three-necked flask containing ethylene glycol (30 mL), through which pure argon gas was bubbled for 45 min before the addition. Three reactions with different mM ratios of (Pd:Co:Pt as 0.05:0.025:0.05, 0.025:0.025:0.05, and 0.0125:0.025:0.05 mM) and weighed amounts of carbon (about 30 mg) were added. Amplitude of 30% ultrasound from a 500 kW ultrasound generator (Sonic and Materials, VC-500, 20 kHz with a 13 mm solid probe) was applied for 3 h under Ar gas at room temperature. The final solution of blackish slurry is filtered, washed with ethanol, and then dried under vacuum for 12 h at room temperature. In the end, all samples are heated at 350 C under the flow of mixed gas (4% H2 and 94% N2) for 4 h to remove the residual organics [10]. I. FUNDAMENTALS

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9.4.2 Synthesis of PdMetal Oxide Hybrid Nanoparticles PdCuO nanohybrids have been successfully fabricated by the sonochemical synthesis using copper salt in the presence of palladium and water [11]. The authors reported that in the presence of palladium and water, transition metal salts could be converted into their oxides by aid of ultrasound energy. In their work, the palladium source was either pure metallic palladium Pd(0) or the palladium salts (palladium acetate, palladium nitrate). Ziylan-Yavas et al. [12] have synthesized PdTiO2 nanohybrids by using both highfrequency ultrasound (35 KHz) and UV-irradiation (254 nm). In their study, palladium salt (Na2PdCl4 3H2O), commercial TiO2 powder, and polyethyleneglycol monostearate were used.



9.5 SOLGEL METHOD 9.5.1 Synthesis of Trimetallic Nanoparticles Au/Ag/Pt Based on the known method [13], Au nanoparticles are first prepared by the reducing agent trisodium citrate. 10 mL of aqueous 0.1 % of metal salt (HAuCl4 3H2O) is heated to boiling, and 2 mL of 1 % trisodium citrate is added with continuous stirring. The reaction mixture is allowed to heat for 4 min and cooled to room temperature. The change in color indicates the formation of Au nanoparticles. Then, 10 mL of 0.1% metal salt (H2PtCl6 6H2O) is added to the Au nanoparticles, accompanied by the inclusion of 2 mL of 1% trisodium citrate with constant stirring. Finally 10 mL of 0.1% of metal salt (AgNO3) is added into the Au/Pt nanoparticles. The heating operation is carried out in a (microwave) MW oven for 7 min. The synthesized colloidal sol is sonicated with 30 min with a “fast-clean” ultrasonic cleaner. The preparation method of trimetallic Au/Pt/Ag nanocomposites is shown in Scheme 9.2.





SCHEME 9.2 Modified microwave irradiation method for the synthesis of Au/Pt/Ag trimetallic nanocomposites. Source: Reprinted with permission from S. Sivasankaran, S. Sankaranarayanan, S. Ramakrishnan, A novel sonochemical synthesis of metal oxides based Bhasmas, Mater. Sci. Forum 754 (2013) 8997. Copyright 2013 Elsevier.

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183

9.5.2 Synthesis of Amine-Functionalized Silica Nanopowder (SiO2 Nanopowder) In order to achieve the faster solgel transition, tetraethoxysilane (TEOS) is chosen as a silica source. During the preparation procedure, 3 mL of TEOS and 1 mL of amino propyl tetra ethoxy silane (APTES) are added to 3 mL of distilled ethanol. This final mixture is transferred into a beaker which is carefully shielded to reduce the unwanted evaporation. The mixture is stirred for 30 min at room temperature for the formation of solgel product. This product is placed in a hot-air oven for 12 h at 100 C for the total hydrolysis of TEOS/APTES to form SiO2 nanopowder. Scheme 9.3 shows the synthetic route of the SiO2 nanopowder [14].

9.5.3 Synthesis of Trimetallic Au/Pt/Ag Nanocomposites-Doped Amine-Functionalized Silica Nanopowder (Au/Pt/Ag@SiO2) Solgel technique is used to prepare Au/Pt/Ag@SiO2 nanopowder. Scheme 9.2C represents the preparation route of the Au/Pt/Ag@SiO2 nanopowder. Briefly, TEOS (3 mL) and APTES (1 mL) are dissolved in distilled ethanol (3 mL) in a beaker and stirred for 30 min at room temperature. In parallel, as-prepared trimetallic Au/Pt/Ag nanocomposites sol (3 mL) is added into the obtained mixture. The final mixture is stirred for another 30 min at room temperature to form Au/Pt/Ag@TEOS/APTES solgel mixture [13,14]. This solgel mixture is kept in a hot-air oven and dried for 12 h at 100 C for aging, drying, and shrinking. At the end, the solgel matrix is well ground to form Au/Pt/Ag@SiO2 nanopowder (Scheme 9.4).

SCHEME 9.3 Solgel chemical route for the synthesis of amine-functionalized SiO2 nanocomposite. Source: Reprinted with permission from A. Ziylan-Yavas, Y. Mizukoshi, Y. Maeda, N.H. Ince, Supporting of pristine TiO2 with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis, Appl. Catal. B: Environ. 172 (2015) 717. Copyright 2015 Springer.

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SCHEME 9.4 Solgel chemical route for the synthesis of Au/Pt/Ag TNC-doped SiO2 nanocomposite (Au/Pt/ nanocomposite). Ag@SiO2 Source: Reprinted with permission from A. Ziylan-Yavas, Y. Mizukoshi, Y. Maeda, N.H. Ince, Supporting of pristine TiO2 with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis, Appl. Catal. B: Environ. 172 (2015) 717. Copyright 2015 Springer.

SCHEME 9.5 Schematic illustrations for the synthesis of Ag/ZnO hybrid nanocomposites via photochemical environments. Source: Reprinted with permission from B. Karthikeyan, B. Loganathan, A close look of Au/Pt/Ag nanocomposites using SERS assisted with optical, electrochemical, spectral and theoretical methods, Phys. E: Low Dimens. Syst. Nanostruct. 49 (2013) 105110. Copyright 2012 American Chemical Society.

9.6 PHOTOCHEMICAL METHOD Ag/ZnO nanohybrid is prepared using the modified photochemical method [15]. Reactant solution, containing colloidal zinc oxide (5 3 1024 M), silver nitrate (1 3 1024 M), is prepared from stock solutions in 2-propanol. The irradiation of solution is executed using filter-cut 310390 nm light segment of 1000 W high-pressure mercury lamp or 500 W incandescent lamp in glass 1.0 cm cuvettes. A 5-cm water filter has been placed between the light source and the work cuvette to reduce the heating of reacting mixtures. Before the experiments, oxygen has been removed from the cuvettes via continuous argon atmosphere. Finally, the formation of Ag/ZnO colloidal nanocomposites is obtained (Scheme 9.5).

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9.8 HYDROTHERMAL/SOLVOTHERMAL METHOD

9.7 WET-CHEMICAL SYNTHESIS Synthesis of open-ended, cylindrical AuAg alloy nanostructures on a Si/SiOx surface is reported [16]. In a typical experiment, a 45-nm-thick Ag film (Scheme 9.1) from the thermally evaporated Ag film with the relatively large Ag grains of size 40140 nm is achieved. Ag is oxidized by HAuCl4, and the reduced Au is deposited onto the surface of the Ag. Wet-chemical process results in the formation of the open-ended cylindrical structures that are different from what has been observed for the previously studied etched colloidal nanoparticles in three ways: (i) the Ag dots are attached to a substrate surface rather than being dispersed in solution; (ii) the Ag nanodots are anisotropically functionalized with 16-mercaptohexadecanoic acid (MHA) group; and (iii) the Ag dot is in a polycrystalline structure, consisting of multiple grains. These differences are critical and account for the unusual open cylindrical shape of the resulting nanostructures. All data are consistent with the MHA acting as a resist layer forcing the electroless deposition of gold to occur on the sidewalls of the MHA-capped nanostructures (Scheme 9.6). Gold gets initially plated on the outside of the nanostructure and continues to grow vertically as the Ag core is dissolved and supplies electrons for further electroless Au plating. Dramatically, the growth in the vertical direction exceeds in the plane parallel to the substrate. This is a result of the relative surface area of the gold on the sidewalls as compared with that on the top rim. The sidewalls always have significantly greater surface area than the top rim of the open cylinder. Therefore, more gold is required to increase the diameter as compared with the height. This is in contrast to the etching process of spherical nanoparticles or nanocubes, where the particle surfaces are more homogeneously passivated with ligands and isotropic etching is observed, leading to closed hollow structures rather than the open-ended structures.

9.8 HYDROTHERMAL/SOLVOTHERMAL METHOD Usually, in the hydrothermal synthesis of nanohybrids, the ligands, stabilizers, or surfactants can be involved or not. SCHEME 9.6 Synthetic procedure for the open-ended cylindrical AuAg alloy nanostructures on Si/SiOX surfaces. Source: Reprinted with permission from B. Loganathan, V.L. Chandraboss, M. Murugavelu, S. Senthilvelan, B. Karthikeyan, J. SolGel Sci. Technol. 74 (2015) 114. Copyright 2004 American Chemical Society.

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In case of nano-TiO2-based hybrids, without ligands or surfactants, Au/TiO2 nanohybrids [17] and Pt/TiO2 core/shell NPs [18] have been synthesized, by using the hydrothermal process with redox procedure. Whereas, with hexamine as the stabilizer, AgTiO2 nanohybrids have been fabricated [19]. In case of nano-Fe3O4 based hybrids, under thermal decomposition process, Ag@Fe3O4 coreshell nanoparticles have been synthesized by using the mixture of oleylamine and oleic acid (as surfactants) [20]. Similarly, by using these surfactants, AgFe3O4 dimer nanoparticles were also fabricated [21]. The PdFe3O4 nanohybrids were successfully synthesized by the thermal decomposition of Fe(CO)5 and the reduction of Pd(OAc)2 in oleylamine and 1-octadecene [22]. It was reported that in the presence of diethanolamine, acting as stabilizer and reducing agent, AgZnO nanohybrids could be prepared under the hydrothermal method [23].

9.9 CONCLUDING REMARKS Among these methods discussed, chemical synthesis has some advantages for creating new generation hybrid nanocomposites. In current societal needs, miniaturization of electronic components are determining the generation of the new materials. The advantage and the desired applications among the hybrid nanomaterial are making researchers think more on the novel and simple nanohybrid preparation techniques. Among the various chemical methods of nanohybrid preparation discussed in this chapter, bottom-up approaches like seed growth mechanism, coprecipitation method, ultrasonochemical synthesis, solgel approach, hydrothermal routes, and photochemical synthetic procedures are well studied. Sonochemical is the best and the simplest methodology for the preparation of metal oxides-related nanohybrids with controlled size and morphology for special application like photovoltaic cell or photoelectrochemical application. Coprecipitation method provides the advantage of being a low cost, simple, waterbased reaction, with flexibility, mild reaction conditions, and size control. It is highly acceptable for the synthesis of nanohybrids for mechanical applications: tribological, light weight structures, thermal defensive structures, etc., and highly applicable for biomedical treatment due to the heat producing capability of (Fe2O3) IOAu coreshell nanohybrids during the cancer cell treatments. Seed growth mechanism is the best method for converting the less observed metal oxides to highly visible light observing nanohybrid structures by arranging the shell over the core of the metal oxides for solar cell applications. Solgel method is highly captivating for the preparation of nanostructures having more than one component, since a good structural resultant product is formed due to slow reaction kinetics. It is a fine way to prepare superhydrophobic nanomaterial and to coat nanohybrids over metal surfaces for various technological applications, as well as to determine the water contacting angle and the surface modification, self-cleaning activities of the bulk materials. Hydrothermal method is the superior way to make nanohybrids for novel applications like cosmetology, optoelectronic, etc. by the simple and precise preparation technique.

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References [1] M. Tsuji, N. Miyamae, K. Matsumoto, S. Hikino, T. Tsuji, Rapid formation of novel Au coreAg shell nanostructures by a microwave-polyol method, Chem. Lett. 34 (2005) 15181519. Available from: https://doi.org/ 10.1246/cl.2005.1518. [2] M. Tsuji, N. Miyamae, S. Lim, K. Kimura, X. Zhang, S. Hikino, et al., Crystal structures and growth mechanisms of Au@Ag coreshell nanoparticles prepared by the microwavepolyol method, Cryst. Growth Des. 6 (8) (2006) 18011807. Available from: https://doi.org/10.1021/cg060103e. [3] L. Zhang, Y.-H. Dou, H.-C. Gu, Synthesis of AgFe3O4 heterodimeric nanoparticles, J. Colloid Interface Sci. 297 (2006) 660664. Available from: https://doi.org/10.1016/J.JCIS.2005.11.009. [4] S. Liu, S. Guo, S. Sun, X.-Z. You, Dumbbell-like AuFe3O4 nanoparticles: a new nanostructure for supercapacitors, Nanoscale 7 (2015) 48904893. Available from: https://doi.org/10.1039/C5NR00135H. [5] S. Das, V.C. Srivasatava, Synthesis and characterization of ZnOMgO nanocomposite by co-precipitation method, Smart Sci. 4 (4) (2016). Available from: https://doi.org/10.1080/23080477.2016.1260425. [6] Y. Wang, H. Arandiyan, J. Scott, A. Bagheri, H. Dai, R. Amala, Recent advances in ordered meso/macroporousmetal oxides for heterogeneous catalysis: a review, J. Mater. Chem. A 5 (2017) 8825. Available from: https://doi.org/10.1039/c6ta10896b. [7] B. Donkova, P. Vasileva, D. Nihtianova, N. Velichkova, P. Stefanov, D. Mehandjiev, Synthesis, characterization, and catalytic application of Au/ZnO nanocomposites prepared by coprecipitation, J. Mater. Sci. 46 (2011) 71347143. Available from: https://doi.org/10.1007/s10853-011-5503-y. [8] K.-J. Kim, P.B. Kreider, C.-H. Chang, C.-M. Park, H.-G. Ahn, Visible-light-sensitive nanoscale AuZnO photocatalysts, J. Nanopart. Res. 15 (2013) 1606. Available from: https://doi.org/10.1007/s11051-013-1606-5. [9] J. Ma, K. Wang, M. Zhan, Growth mechanism and electrical and magnetic properties of AgFe3O4 coreshell nanowires, ACS Appl. Mater. Interfaces 7 (2015) 1602716039. Available from: https://doi.org/10.1021/ acsami.5b04342. [10] J. Li, Q. Wu, J. Wu, Synthesis of nanoparticles via solvothermal and hydrothermal methods, Handbook of Nanoparticles, Springer International Publishing, Cham, 2015, pp. 128. Available from: https://doi.org/ 10.1007/978-3-319-13188-7_17-1. [11] S. Sivasankaran, S. Sankaranarayanan, S. Ramakrishnan, A novel sonochemical synthesis of metal oxides based Bhasmas, Mater. Sci. Forum 754 (2013) 8997. Available from: https://doi.org/10.4028/www.scientific.net/MSF.754.89. [12] A. Ziylan-Yavas, Y. Mizukoshi, Y. Maeda, N.H. Ince, Supporting of pristine TiO2 with noble metals to enhance the oxidation and mineralization of paracetamol by sonolysis and sonophotolysis, Appl. Catal. B Environ. 172173 (2015) 717. Available from: https://doi.org/10.1016/J.APCATB.2015.02.012. [13] B. Karthikeyan, B. Loganathan, A close look of Au/Pt/Ag nanocomposites using SERS assisted with optical, electrochemical, spectral and theoretical methods, Phys. E Low-Dimensional Syst. Nanostructures 49 (2013) 105110. Available from: https://doi.org/10.1016/J.PHYSE.2013.02.008. [14] B. Loganathan, V.L. Chandraboss, M. Murugavelu, S. Senthilvelan, B. Karthikeyan, Synthesis and characterization of multimetallic-core and siliceous-shell Au/Pt/Ag@SiO2 solgel derived nanocomposites, J. Sol-Gel Sci. Technol. 74 (2015) 114. Available from: https://doi.org/10.1007/s10971-014-3564-5. [15] Q. Deng, X. Duan, D.H.L. Ng, H. Tang, Y. Yang, M. Kong, et al., Ag nanoparticle decorated nanoporous ZnO microrods and their enhanced photocatalytic activities, ACS Appl. Mater. Interfaces 4 (2012) 60306037. Available from: https://doi.org/10.1021/am301682g. [16] H. Zhang, R. Jin and C.A. Mirkin, Synthesis of open-ended, cylindrical Au 2 Ag alloy nanostructures on a Si/SiOx surface, 4(8) (2004) 1493uˆ1495. https://doi.org/10.1021/NL0492281. [17] X.-F. Wu, Y.-F. Chen, J.-M. Yoon, Y.-T. Yu, Fabrication and properties of flower-shaped Pt@TiO2 coreshell nanoparticles, Mater. Lett. 64 (2010) 22082210. Available from: https://doi.org/10.1016/J. MATLET.2010.07.027. [18] X.-F. Wu, H.-Y. Song, J.-M. Yoon, Y.-T. Yu, Y.-F. Chen, Synthesis of core 2 shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology and their photocatalytic properties, Langmuir 25 (2009) 64386447. Available from: https://doi.org/10.1021/la900035a. [19] A. Padmanaban, T. Dhanasekaran, S. Praveen Kumar, G. Gnanamoorthy, S. Munusamy, et al., Visible light photocatalytic property of Ag/TiO2 composite, Mech. Mater. Sci. Eng. J. Magnolithe 9 (1) (2017). Available from: https://doi.org/10.2412/mmse.97.67.748.

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[20] M.E.F. Brollo, R. Lo´pez-Ruiz, D. Muraca, S.J.A. Figueroa, K.R. Pirota, M. Knobel, Compact Ag@Fe3O4 coreshell nanoparticles by means of single-step thermal decomposition reaction, Sci. Rep. 4 (2014). Available from: https://doi.org/10.1038/srep06839. Article number: 6839. [21] G. Lopes, J.M. Vargas, S.K. Sharma, F. Be´ron, K.R. Pirota, M. Knobel, et al., AgFe3O4 dimer colloidal nanoparticles: synthesis and enhancement of magnetic properties, J. Phys. Chem. C 114 (22) (2010) 1014810152. Available from: https://doi.org/10.1021/jp102311u. [22] H. Woo, J.C. Park, S. Park, K.H. Park, Rose-like PdFe3O4 hybrid nanocomposite-supported Au nanocatalysts for tandem synthesis of 2-phenylindoles, Nanoscale 7 (2015) 83568360. Available from: https://doi. org/10.1039/C5NR01441G. [23] Q. Huang, Q. Zhang, S. Yuan, Y. Zhang, M. Zhang, One-pot facile synthesis of branched AgZnO heterojunction nanostructure as highly efficient photocatalytic catalyst, Appl. Surf. Sci. 353 (2015) 949957. Available from: http://dx.doi.org/10.1016/j.apsusc.2015.06.197.

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10 Sonochemical Synthesis of PalladiumMetal Oxide Hybrid Nanoparticles S. Sivasankaran1 and M.J. Kishor Kumar2 1

Manipal Institute of Technology, Manipal Academy of Higher Education (MAHE), Manipal, India 2Department of Chemical Engineering, National Institute of Technology, Surathkal, Karnataka, India

10.1 INTRODUCTION Hybrid nanoparticles of palladiummetal oxides could be used for wide range of applications in the fields of chemical engineering, such as the synthesis of value-added chemicals, hydrogen, hydrogen peroxide [1], and mineralization of toxic chemicals. Also, they find extensive use in medical diagnosis, electrical and electronic instruments, energy production and storage. These hybrid nanoparticles could be used either alone or in combination with alternative energies, such as solar and ultrasound energy. Thus, there is a need for a singlestep process for the synthesis of hybrid nanoparticles. The use of alternative energy such as ultrasound energy is an emerging method for the synthesis of these hybrid nanoparticles [2]. A novel green, facile, faster, and inherently safer single-step ultrasound-assisted process of making noble metalnonnoble metal oxide hybrid nanoparticles has been developed and patented [3]. The process involved the irradiation with ultrasound energy of an aqueous solution of metal salt precursors of a noble metal, nonnoble metal, and alcohol leading to the simultaneous formation of noble metal and nonnoble metal oxide nanoparticles in a single step in the slurry form. From the slurry, a wet solid could be separated by centrifugation or filtration and then by drying. Further calcination may be done if required. Palladiumcobalt oxide, palladiumiron oxide, palladiumcopper oxide, palladiummanganese oxide, and multimetallic oxides such as palladiumcopper oxideiron oxide could be synthesized. Synthesis of palladiumcopper oxide has been presented to Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00015-2

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describe the general synthesis procedure. The product synthesized was analyzed for confirming the chemical composition by using XRD. The size distribution of the particles was determined by DLS method. TEM-EDX determined the shape and elemental composition of the synthesized particles. This novel sonochemical process is versatile and has many advantages such as the single-step synthesis of noble metalnonnoble metal oxide and noble metalmultimetal oxide hybrid nanoparticles. Also supported catalysts of noble metal nanoparticles on metal oxides and carbon materials could be readily synthesized.

10.2 SYNTHESIS OF PdCuO HYBRID NANOPARTICLES A 100 mL glass vessel was used for conducting the reaction which may be called a batch sonochemical reactor Fig. 10.1. 1 mmol of palladium acetate and 1 mmol of copper acetate were taken in the vessel and mixed with 70 mL water and 20 mL ethanol. The solution was subjected to sonication for 1 h by using a 20 kHz frequency 1/2-in. probe operating at 45% amplitude. The black color slurry of the product obtained was centrifuged at 7000 rpm for 15 min [4]. Two different methods used for the synthesis and chemical reaction involved during the synthesis of PdCuO hybrid nanoparticles are shown in Fig. 10.2. Samples were

FIGURE 10.1

Experimental setup for the synthesis of PdCuO.

FIGURE 10.2 Flow chart of two different methods used for the synthesis of PdCuO. I. FUNDAMENTALS

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analyzed for particle size distribution and also chemical composition. The XRD analysis of the dried product confirmed the presence PdCuO phases. The particle size analysis by using Zeta sizer confirmed that the slurry contained nanoparticles. The histogram obtained showed a nano-size distribution of the particles.

10.3 THE CHEMICAL REACTIONS INVOLVED IN THE SYNTHESIS OF HYBRID NANOPARTICLES Palladium salt was reduced to palladium by ethyl alcohol. Alternately palladium metal could be used directly. Palladium-catalyzed the formation of “sono” H2O2 from the “sono” hydrogen produced during the sonolysis of water Eqs. (1) and (2). These “sono” H2O2 converts the metal salts into their corresponding metal oxides Eq. (3). Thus palladium, water, and ultrasound energy were used to convert transition metal salts into their oxides Eq. (4) in Fig. 10.3. The sonolysis may further include sparging the mixture with gas during sonolysis to enhance cavitation. Air may be used for the sparging, but oxygen or inert gases such as nitrogen or argon may also be used. The gas may be introduced into the mixture using, e.g., plastic tubing and a simple aquarium pump. The different steps involved during the synthesis have been given in Fig. 10.4. The X-ray diffraction patterns of the as-prepared PdCuO hybrid nanoparticles are shown in Fig. 10.5. The obtained peaks were indexed using Joint Committee on Powder Diffraction Standards (JCPDS) files (now renamed the International Center for Diffraction Data (ICDD)). It can be seen from Fig. 10.5 that the sharp diffraction peaks at 40 , 45.8 , and 67.3 are attributed to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively. These peaks represent the face-centered cubic structure of the palladium (Pd) (JCPDS no. 46-1043). We did not find any peaks corresponds to palladium oxide (PdO). Meanwhile, the peaks of CuO at 31.8 , 35 , 37.5 , and 48.7 show the monoclinic system of CuO (JCPDS no. 05-0661). Transmission electron microscopy (TEM) analysis is a most suitable method to know the size and shape of the material. TEM images were captured at 20 nm and 50 nm scale Fig. 10.6. We could observe the cluster of particles and most of the particles are spherical in shape. The TEM image is one of the evidences that the synthesized sample contains particle size less than 50 nm. EDX elemental analysis confirmed the presence of PdCuO in the synthesized sample. The sample contains the mass% of Pd (34.28%), Cu (55.33%), and O (10.039%) (Fig. 10.7). This result shows the purity of the material. Zeta size analysis FIGURE 10.3 The proposed chemical reactions involved in the synthesis of PdCuO.

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FIGURE 10.4

The different steps involved in the synthesis of PdCuO.

FIGURE 10.5 XRD pattern of PdCuO.

FIGURE 10.6 TEM images of PdCuO taken at different scale bar.

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FIGURE 10.7 Elemental composition of PdCuO hybrid nanoparticles.

FIGURE 10.8 Size distribution histogram of PdCuO hybrid nanoparticles.

shows the histogram of the PdCuO hybrid particles (Fig. 10.8). The size of the particles is in the range of 8100 nm. Most particles lie within the range 2070 nm.

10.4 CONCLUDING REMARKS The process presented here is based on the original applied research work disclosed in the publications [1,3] for a novel ultrasound-assisted method for the green, faster, facile synthesis of bulk Pdmetal oxide hybrid nanoparticles. This method could be easily extended to the synthesis of bulk Pdmultimetallic oxide nanohybrids and also supported Pdmetal oxide nanohybrids on carbon and metallic materials including nanomaterials. Also, this method could be used for the synthesis of Rumetal oxide and Aumetal oxide nanohybrids. The nanoparticles of noble metalmetal oxides could be explored for their medicinal properties as Bhasmas [5]. The proof of concept process developed by using a batch sonochemical reactor could be used with large-scale continuous sonochemical reactors for the industrial scale production of these hybrid nanoparticles. Also, hybrid reactors such as solar-sono reactors could be developed for making the process energy efficient.

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References [1] S. Sivasankaran, An ultrasound assisted process of water splitting and catalytic hydrogenation for the synthesis of hydrogen peroxide, IN 201641034424. [2] S. Sivasankaran, M.J. Kishor Kumar, A novel sonochemical synthesis of nano-size silicon nitride and titanium carbide, Ceram. Int. 41 (2015) 1130111305. [3] Sivasankaran S., Synthesis of palladium-based metal oxides by sonication, JP5841661 (2015), US20130004412 (2013), PCT/IB2011/053412, CN103608293A, IN2042/CHE/2011. [4] S. Sivasankaran, M.J. Kishor Kumar, A Novel Single Step Sonochemical Synthesis of Micro-Nano Size Palladium-Metal Oxides, Material, Energy and Environment Engineering, Springer Selected Proceedings of ICACE, 2015, pp. 6974. [5] S. Sivasankaran, S. Sankaranarayanan, S. Ramakrishnan, A Novel Sonochemical Synthesis of Metal Oxides Based Bhasmas, Materials Science Forum, vol. 754, Trans Tech Publications, Switzerland, 2013, pp. 8997.

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11 Laser-Induced Heating Synthesis of Hybrid Nanoparticles Rina Singh1 and R.K. Soni2 1 2

Environment Science Division, CSIR-Central Road Research Institute, New Delhi, India Physics Department, Indian Institute of Technology Delhi, Hauz Khas, New Delhi, India

11.1 INTRODUCTION Hybrid nanoparticles (HNs) are composites commonly used to describe a multicomponent nanoparticle where at least one of the constituent is at the nanometer or molecular level. There are many different possible types or morphologies of hybrid nanoparticles like: metal@dielectric coreshell, dielectric@metal coreshell, metal@metal coreshell, metal@dielectric@metal multilayer coreshell, dielectric@metal@metal multilayer coreshell, alloys, hetero dimers, etc. [14]. HNs play a very important role in nanoscience and nanotechnology and therefore have become a topic of intense research in recent years. This is due to the special physical and chemical properties of these materials that open new avenues in advanced applications [46]. The development of new materials and devices leading to ultrasensitive detection of molecules has gained an exponential growth because of their exciting features and their potential application in important fields like environmental monitoring of toxic pollutants in the concentration range between picomolar and attomolar using surface-enhanced Raman spectroscopy, photovoltaic devices, optics, outstanding catalysis effect, magnetic and photonic properties, etc. [58]. Among these fields, improvements of sensors at the nanoscale are providing new solutions in physical, chemical, and environmental areas. Air and water pollutants create significant health hazards around the globe. Indeed, over the last decade, environmental pollution remediation became a global priority. HNs especially in the form of coreshell, multilayer coreshell, and alloy give a promising avenue for the development of rapid, cost-effective and highly sensitive sensor platforms for the on-site in situ detection of various environmental pollutants [911].

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HNs, especially coreshell, alloys, trimetallic nanostructures are of considerable commercial interest as heterogeneous catalysts [1214]. Multimetallic structures provide many active intermetallic interfaces where electronic structure is changed. Bimetallic compounds like alloy and coreshell HNs have gained much interest due to the additional new properties arising from the combination of different compositions of two different metals at the nanoscale which allows tuning of the localized surface plasmon resonance (LSPR) and their catalytic activity via composition ratios [15,16]. Coreshell HNs with thinner metal shell layer produce LSPR towards lower frequency and a larger shift toward the red region of spectrum and we can have wide LSPR tunability covering a broad region from visible to infrared (IR) [17]. Therefore even spherical HNs in alloy and coreshell form lead to wide tunability and almost covering a broad region from visible to IR since nonspherical geometries are much more difficult to synthesize. As a comparison, trimetallic compounds provide far more opportunities for the tailoring of unique shapes and thus multiparameter optimization for more fine property tuning [4,6,18,19]. In particular, trimetallic interfaces are more active due to the presence of crystal defects and fast electron interchange. In the field of catalytic applications bi- and trimetallic HNs are highly efficient than monometal nanoparticles (NPs). The improved catalytic efficiency stems from their modulated surface electronic properties by addition of one or more metal [20]. Having so many advantages, there is still no general method for reliable synthesis of multimetallic coreshell or alloy HNs. Concentric nanoparticles consisting of a dielectric core and a metallic shell, also called multilayered nanoshells/nanomatryushkas, are another important class of HNs with multiple SPR bands at different wavelength, exhibiting attractive features due to the hybridization of the plasmon’s supported by the nanoscale sphere and a cavity in the surrounding medium and therefore widely used for multiplex sensing of various toxic pollutants [21,22]. Therefore, continuous efforts have been made towards the development of such materials in order to achieve the desired properties and activities. Many studies used spherical HNs as the plasmonic unit because of their easy chemical synthesis or their commercial availability. These materials because of LSPR generate a significantly enhanced electromagnetic field close to the nanoparticles surface making it possible to detect even a single molecule and therefore LSPR provides the basis for surface-enhanced Raman spectroscopy (SERS)/surface-enhanced fluorescence (SEF)-based detection [2325]. Using this material any water/air pollutants, bacteria, viruses can be detected and differentiated based upon their fingerprint spectra when they enter SERSactive hot spots. Gold (Au) and silver (Ag) are the most frequently studied materials in the context of the optical properties of metallic NPs. Similarly another very important plasmonic material aluminum (Al) is also capable of sustaining a strong surface plasmon resonance (SPR) in the UV region but is far less studied because of its chemical reactivity and, hence, difficulties in preparation and application. There is a strong need for the development of new HNs based on plasmonic materials, aluminum (Al), silver (Ag), and gold (Au), that enables the physical trapping of a wide variety of analyte molecules and therefore combining these three elements we can have wide tunability of LSPR from UV to IR range and hence wide applications. Recently, the growing ability to fabricate complex multicomponent HNs has opened new opportunities for designing multifunctional materials. Porous alumina-based HN

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have been used in catalysis because of the high surface area of alumina which acts as a support for the dispersion of the catalysts [2628], and multilayered structures in SERS probes for enhancing the Raman signals [4]. The coreshell arrangement is particularly attractive as the metal core and the shell interaction creates more boundary sites for additional functionality and allows encapsulation of active molecules extending their application to areas such as drug delivery, energy storage, enhanced optical devices, and high efficiency catalysis because of the synergistic interactions between the core and shell. Double shell hybrid particles have been shown to enhance photocatalytic activity and multiplex sensing, further broadening their use for different applications [1,3,2934]. There are many different possible types or morphologies of HNs; however we will be focusing mainly on alloy and coreshell HNs because of their potential application in sensing and catalysis. The activity of nanoparticles can be further enhanced by making porous structures. Porous structure in the form of yolkshell or rattle-type structures with empty space between the encapsulated NP (yolk) and the hollow shell, have been developed recently for SERS sensing, pollutants adsorbent, and nanosensors [3538]. The rattlestructure particles synthesized by Kirkendall hollowing effect [39,40] have gained attention because of their novel structural properties and potential applications [4143]. In a spherical material system where the fast diffusion species is enclosed by the slower one, the Kirkendall effect occurs and transforms the system by forming hollow structures of a compound shell [26,44,45]. The major methods to prepare rattle-type structures are hard/ soft-templating methods, Kirkendall, Ostwald ripening effect, and selective etching [4648]. Various types of rattle nanostructures have been synthesized, however these methods are often complicated because of tedious procedures and poor reproducibility. Thus, there is a need to develop simple, controllable, and environmentally friendly methods for the synthesis of the rattle-type HNs. Moreover, the trimetallic porous structured HNs are yet to be explored. Trimetallic HNs consisting of Al, Ag, and Au in a solid or porous form have not yet been explored. Thus, the present chapter highlights the development of bimetallic and trimetallic HNs. Among the physical synthesis routes, pulsed laser ablation in liquids (PLAL) is one of the most promising methods for generation of varieties of multimetallic HNs and may efficiently overcome some of the problems arising during chemical synthesis [4956]. This method allows the generation of surfactant-free nanoparticle colloids without the use of chemical precursors or toxic preservatives, which could be beneficial for potential catalytic and SERS applications [57,58]. PLAL is capable of producing particles from wide classes of materials and therefore the interest in exploiting PLAL is growing. However, there is still a considerable lack of understanding of most of the fundamental processes like nucleation and growth during this procedure. There is still no general method for reliable synthesis of multimetallic coreshell or alloy NPs. The generation of nanoalloys/rattle-type/coreshell HNs of immiscible metals are still a challenge using conventional methods. However, because these materials are currently attracting much attention, alternative methods are needed. In this chapter, we demonstrate a simple and effective method for the generation of a new multicomponent structure of immiscible metals by the pulsed laser ablation/irradiation of colloidal nanoparticles. This new mechanism of PLAL technique opens new opportunities to develop a large variety of novel hybrid materials with controllable morphology and hence properties.

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In this chapter a very effective and a versatile one-step method to fabricate multicomponent HNs of various types: AgAl rattle-type HNs with hollow cavities, bimetallic (Al2O3@Ag, Al2O3@Au), and trimetallic Al2O3@AgAu HNs using PLAL and post-laser heating is presented. Also, synthesis mechanism, morphology, and optical properties are discussed. To the best of our knowledge, it is the first time that an all in-one system using Al, Ag, and Au has been designed, built, and characterized.

11.1.1 Pulsed Laser Ablation in Liquid Laser ablation is the ejection of macroscopic amounts of materials from the surface of a solid induced by the interaction of short (B10213 to 1028 s), intense (B106 to 1014 W/cm2) laser pulses with the surface [59]. This can occur in any medium like vacuum, gas, and liquid, providing that the gas or liquid does not strongly attenuate laser energy and the light intensity (fluence) on the solid surface is still enough to ablate the material. The laser ablation of solid in liquid is mainly used for the generation of NPs of various sizes and shapes [56,57]. In PLAL, liquid confines the movement of the plasma plume and therefore, the various formation processes like generation, transformation, and condensation of the plasma plume resulting from laser ablation of solids in liquid environments take place under the condition of the liquid confinement [60]. This confinement process from liquid greatly influences the thermodynamic and kinetic properties of the plasma plume, and hence causes the different environments of the condensing phase formation from that of laser ablation of solids in vacuum or diluted gas. Therefore, the understanding of fundamental aspects of the evolution of the plasma plume from laser ablation of solids in liquids is required for their potential applications in nanotechnology. In ablation the interaction of laser beam with a material is mainly determined by the temporal width of the laser pulse, which is related to the electronphonon coupling time constant of the material. Laser ablation is the irradiation of material with a large number of pulses Np with pulse duration τ p and the repetition rate Re with pulse fluence F exceeding the melting threshold fluence FM. The melt duration at the surface after each pulse is τ m such that (Re21 . τ m . τ p) and the duration of elevated surface temperature state after the solidification is τ TBτ m [61]. The overall duration of molten phase is tm 5 τ m  NP (no. of pulses) and the overall duration of the heated solid phase existing in the subsequent pulses is tT 5 τ T NP [61]. The nature of the ablation phenomena is determined mainly by the parameters of lasers like pulse duration, wavelength, fluence, and also the thermal and optical properties of solid material. In this process the incoming photons are absorbed by the target material forming gas of hot carriers which eventually transfer their energy to the ions through emissions of phonons. The ion and electron gas after some time reach equilibrium state, i.e, the temperature of electron gas equals that of the lattice, which occurs on a picosecond timescale τ EB10212 to 10211 s. This timescale decides the boundary between the thermal and nonthermal routes of ablation as well as long and short pulses. If τ p . τ E, equilibrium between electrons and phonons prevails throughout the heating stage and phase changes can be regarded as slow thermal processes involving quasi-equilibrium dynamics. However in contrast, for ultrashort pulses (femto and picosecond) the material is driven into highly nonequilibrium state and Te . T. In this case the timescale with which

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11.1 INTRODUCTION

the structural change takes place, τM determines whether the thermal mechanisms are involved (τ M . τ E) or not (τ M , τ E) [61]. There are three kinds of thermal processes in ablation mechanism: (i) vaporization, (ii) normal boiling, and (iii) explosive boiling, which are determined by thermodynamic and kinetic limits. Vaporization is the emission of atoms or molecules from the laser-irradiated surface by local perturbations. Normal boiling mainly occurs by heterogeneous nucleation, in which a vapor bubble nucleates at the interface between superheated liquid with another phase, such as impurities and involved solids. By contrast, explosive boiling occurs by homogenous nucleation completely within a superheated liquid [62,63].

11.1.2 Fundamentals of PLAL As mentioned above, the laser ablation of a target starts with the absorption of incoming photons, which can produce the heating and photoionization of the irradiated area. The phase and the amount of ablated material depend on the absorbed energy Qabs. Amendola observed the following trends for the ablation depth, Ld, the duration of the ablation process, τ a, and the electronic temperature during the ablation process τ E [64]: 2=3

1=2

1=3

τ a . . τ p ; Ld ~ Qabs ; τ a ~ Qabs ; τ E ~ Qabs

(11.1)

where τ p is the laser pulse duration. The various mechanisms of plasma formation like boiling, vaporization, and explosive processes are observed. The profile of absorbed energy is nonconstant in time and nonuniform on the whole target area, which results in large size distributions of metallic NPs. The charging of the irradiated area due to photoionization results in expulsion of material from the target. Due to the coexistence of all these different processes, the ablation mechanism of PLAL is also called explosive boiling or explosive ablation mechanism. One of the main characteristics of laser-induced plasma inside liquids is fast quenching due to a dense liquid medium which results in formation of NPs. The interaction of nanosecond pulsed laser of sufficient threshold energy with the interface between liquid and the metal target results in the formation of a microplasma layer. Once the laser beam strikes the sample, mass leaves the surface in the form of electrons, ions, atoms, molecules, vapors, liquid drops, clusters, and particles, where each of the processes like fragmentation, sublimation, and atomization are separated in time and space. Mechanisms like multiphoton absorption and cascade ionization are associated with this materials removal process. The multiphoton absorption mechanism is observed in the case of femtosecond pulses, whereas for nanosecond pulse duration, cascade ionization becomes dominant due to the wavelength-dependent inverse Bremsstrahlung (IB) process. The IB absorption coefficient, σIB, which is proportional to the cube or square of the wavelength for ionelectron and atomelectron, is given by [65]:   e2 8kB Te σIB ðλÞatom2electron 5 n n σ (11.2) e 0 col πme cν 2 πme

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σIB ðλÞion2electron 5

3:7x1018 1=2

Te ν 3





  hν Z exp 2 1 ne ni k B Te 2

(11.3)

where ν, Te, me, Z, h, ne, ni, and n0 denote the laser irradiation frequency, the electronic temperature, the electron mass, the ionic charge, Planck’s constant, the number densities of the electrons, ions, and neutral atoms, respectively, and σcol is the cross-section for electronneutral atom collisions. During this process the free electrons which gain sufficient energy release the bound electrons of metal atoms by frequent collisions with them leaving more ions which results in cascade ionization through IB absorption. This process leads to the multiplication of free electrons to form plasma plume which is accompanied by the generation of shock waves, followed by cavitation bubbles. The disturbances in the path of liquid because of shock waves create cavitation bubbles and the frequent expansion and collapse of this bubble results in the formation of secondary shock waves. The detachment of the shock wavefront from the plasma takes place immediately after the plasma formation because its velocity is much larger than the particle velocity behind the front. So the plasma plume once formed starts absorbing a significant portion of the remaining part of the laser pulse through IB effect to increase plasma temperature. Plasma expansion takes place in thermal adiabatic conditions and finally as the plasma plume cools down, the solid particles nucleate and grow subsequently to form NPs (Fig. 11.1). The heatingmeltingevaporation mechanism is responsible for the size change. If a nanoparticle absorbs sufficient laser beam energy, it will melt or evaporate. Thus, the change of shape and size quantitatively relates to the temperature, which is directly related to laser fluence. Metal NPs interact with visible light due to their distinct surface plasmon resonance (SPR) and interband transition. Therefore intense pulsed laser light can induce size reduction, enlargement, and morphological changes of metal NPs whose SPR lies in the region of the wavelength of laser light used.

FIGURE 11.1

Mechanism of various stages of ns-laser ablation in liquid and time sequence. Source: Reprinted by permission from V. Amendola, S. Scaramuzza, F. Carraro E. Cattaruzza, J. Colloid Interface Sci. 489 (2017) 1827 [56]. Copyright 2017 Elsevier.

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201

11.1.3 Localized Surface Plasmon Resonance (LSPR) Plasmonic metal nanostructures can be differentiated on the basis of plasmonic modes they support, i.e., localized surface plasmons (LSPs) or propagating surface plasmons (PSPs). In LSPs, the time-varying electric field associated with the light (E0) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (ω), this oscillation in resonance with the incident light results in a strong collective oscillation of the surface electrons, known as a localized surface plasmon resonance (LSPR). The propagation length of PSPs depends on the losses in the metal and typically ranges from less than a micrometer to a few micrometers. In a metallic particle smaller than the propagation length, surface plasmons are confined to the particle geometry. This effect is extremely pronounced in metallic nanoparticles (NPs) which act as cavity resonators for the surface plasmons. For this reason surface plasmon resonances of NPs are called LSPRs and depend strongly on the exact size, shape, interparticle spacing, and dielectric properties of the nanoparticle [17]. For plasmonic metals like Al, Ag, Au, and Cu, the resonant frequencies lie in the UV to IR region of the electromagnetic spectrum, and therefore surface plasmons interact strongly with light. As a result, metallic NPs that support LSPR are promising materials for highly sensitive optical nanosensors, photonic components, and surface-enhanced spectroscopies. Theoretical modeling represents another important approach of understanding the plasmonic properties of metal nanostructures. Theoretical models create greater understanding of the effect of experimental parameters on plasmonic properties, and have been used to describe plasmon coupling leading to hybridization, the substrate effect on plasmons, and structures composed of different kinds of NPs.

11.1.4 Spherical Nanoparticles: Quasi-Static Mie Theory Mie theory, describes the optical response of a spherical particle. Mie theory requires the dielectric constants of the particle and the surroundings as input parameters. The dielectric constants of metals are strongly frequency dependent and contain both real and imaginary components. The real part determines the resonance wavelength of the LSPR or the polarizability, and the imaginary part determines the dephasing or loss in the material which is absorption or extinction (absorption scattering). When the particles are much smaller than the wavelength λ of light (the quasistatic limit), only the dipole contributions are important, and the extinction is dominated by absorption. In this limit the absorption cross-section is given by [66]:  2  m 21 σabs 5 4πR2 xIm (11.4) m2 1 2 Using m2 5 ε/εm, where ε is the complex dielectric constants of the particle, εm is the dielectric constant of the medium, R is the radius of the particle, x 5 2πRnm/λ, nm is the

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refractive index of the medium, and m 5 n/nm is the refractive index of the particle versus refractive index of medium, Eq. (11.4) can be rewritten in the form [66]: 3=2

24π2 R3 εm ε2

σext 5 λ ðε1 12εm Þ2 1 ε22

(11.5)

where σext is the extinction cross-section. This equation shows that the interaction between a metal nanoparticle and light depends strongly on its dielectric properties (ε1 and ε2). When ε1 5 22εm, σext becomes extremely large and the optical absorption and scattering at this particular frequency would also be exceedingly strong. This is known as a resonance condition. Ag and Au have the largest quality factor across most of the spectrum from 300 to 1200 nm. In contrast, Al has the highest quality factor in the UV region making it suitable for plasmonic applications in the UV region. Small spherical Al NPs with a radius of 10 nm show an extremely strong and narrow SPR peak with a full width at half-maximum (FWHM) of about 20 nm [67]. According to the Drude model, the resonance frequency ωsp for free-electron metals in a small metallic sphere is given by [66,67]: ωp ωsp 5 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 2εm  2 1=2 nc e ωp 5 me ε0

(11.6) (11.7)

Here, nc, e, and me are the conduction electron concentration, conduction electron electric charge, and effective mass, respectively, and εm is the dielectric constant of the medium. The bulk plasmon frequency for metallic Al (12.70 eV) is very large as compared to metallic gold (5.25 eV) [67]. The reason for such a large difference is the number of free electrons in their valence shell. Aluminum has three conduction electrons for each Al atom, however it is one for each Au atom, making nc value of Al larger than that of Au and thus ωp is much higher for Al, which results in much shorter resonance wavelength of Al. As compared to gold NPs, the LSPR of aluminum shows large sensitivity to size, shape, and dielectric constant of surrounding medium. When particle size increases, the electromagnetic wave incident on the particle can no longer be considered to have a negligible phase difference over the extent of the particle, and the dipole resonance peak starts to shift toward longer wavelengths due to phase retardation effect. For spherical Al NPs, multipole resonances appear even for relatively small particle sizes, which is not the case for other metal nanospheres (such as Au, Ag, or Cu). For larger NPs ( . 25 nm) the extinction coefficient explicitly depends on the nanoparticle size as the higher-order terms also contribute, which are functions of R. For these large particles the plasmon bandwidth increases with increasing size as the wavelength λ of the interacting light becomes comparable to the dimension of the nanoparticle. This leads to an inhomogeneous polarization of the nanoparticle by the electromagnetic field. The broadening of the plasmon band is then usually ascribed to retardation effects. For very small

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particles, R ,, λ, the scattering cross-section is negligible and the extinction cross-section, mainly dominated by absorption is given as [68]: 3=2

σext 5

24π2 R3 εm ε2 λ½ðε1 1ð21ð12=5Þx2 Þεm Þ2 1 ε22 

(11.8)

For the dipole resonance, the electrons respond collectively to the electromagnetic radiation and give rise to an excess and depletion of charge at opposite sides of the nanoparticle.

11.1.5 Alloy Hybrid Nanoparticles For an alloy system the absorption and dispersion of light in NPs depend on the nature of the metals, along with their chemical composition, morphology, and sizes. In the case of spherical NPs separated by long distance, with no substance adsorbed on their surfaces, their absorbance (Aalloy) can be calculated as (for alloy of Ag and Au NPs) [69]: Aalloy 5 xl Cext

CAux Agx21 fa R3

(11.9)

where R is the particle radius, Cext is the extinction cross-section for a single particle in nm2, xl is the optical path length in nm, CAux Agx21 is the concentration of the alloy AuxAgx21 in g/cm3, and fa 5 4πρa =3 is a constant which depends on the average bulk density ρa of the alloy. Considering this size dependence and based on Drude’s model, size-dependent dielectric functions such as the damping frequency of the particle ωR can be calculated using the relation [69]: ωR 5 ωd 1 A

vf R

(11.10)

where ωd is the bulk alloy damping constant and A is a theory-dependent factor, close to one. The bulk dielectric function ε(ω) 5 ε1(ω) 1 iε2(ω) for bimetallic systems AuxAg(12x) can be defined by considering the weighted average for each component [70]: εav ðAuxAu Agð12xAu Þ Þ 5 xAu εðAuÞ 1 ð1 2 xAu ÞεðAgÞ

(11.11)

where xAu is the volume fraction of Au in the alloy. The size-dependent dielectric functions of the alloy nanoclusters can be expressed considering Drude’s model as [69]: ε1 ðω; RÞ 5 ε1bulk 1 ε2 ðω; RÞ 5 ε2bulk 1

ω2p ðω2 1 ω2d Þ iω2p ωR

ωðω2 1 ω2R Þ

2 2

ω2p ðω2 1 ω2R Þ iω2p ωd ωðω2 1 ω2d Þ

(11.12)

(11.13)

where ω, ωp, and ωd are the light frequency, plasmon frequency of the bimetallic AuAg alloy, and bulk alloy damping constant, respectively. A linear dependence of the plasmon absorption maximum on the composition of the NPs is observed for alloy particles [70].

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11.1.6 CoreShell Hybrid Nanoparticles Tapia et al. reported a new method to calculate the local refractive index, resonance condition, maximum spectral shift, plasma wavelength, and the sensitivity of the wavelength maximum to variations in the refractive index of the environment for metallic coredielectric shell HPs [71]. This method is based on the fact that a layered core dispersed in a dielectric environment (coreshell model) can be considered as an uncoated sphere dispersed in a medium with a local refractive index. The optical response of the coreshell HNs to changes in the local refractive index is a function of the thickness of the shell; such response can be easily measured by UVvis spectroscopy. On the other hand, to understand the main optical features of coreshell HNs, simulations of their UVvis spectra are carried out using Mie theory for spheres covered with a shell layer. Fig. 11.2 shows the diagram of coreshell nanostructures in an electric field E0. The extended Mie theory can be used to calculate the optical absorption of coreshell nanostructures embedded in a nonabsorbing medium of dielectric constant εm. The absorption coefficient, σcoreshell, for core spherical particles covered with a concentric spherical shell is given by [71]: )   pffiffiffiffiffiffi ( ðεs 2 εm Þðεc 2 2εs Þ 1 1 2 g ðεc 2 εs Þðεm 1 2εs Þ 6πϕ εm   Im σcore2shell 5 ; (11.14) λ ðεs 1 2εm Þðεc 1 2εs Þ 1 1 2 g ð2εs 2 2εm Þðεc 2 εs Þ where ϕ and g are the volume fractions of the core and the shell layers, respectively. λ is the wavelength of light, and εs and εc are the complex dielectric functions of the shell and the core, respectively. The volume fraction, g, in general, is determined by the ratio between the volume of the shell, Vshell, and the total volume of the particles, Vtotal, as is expressed in Eq. (11.14). The volume fraction g of the shell layer determines the position of the surface plasmon peak of the coreshell nanostructure and can be expressed in terms of geometrical size parameters [71]:  3   Vshell R1 R1 3 g5 512 512 (11.15) Vtotal R2 R2 1t

FIGURE 11.2

Schematic configuration of a metal nanoparticle or nanoshell in an electric field.

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205

where R2 is the total particle radius (R2 5 R1 1 t) (Fig. 11.2). In particular, if t .. R1 (thick shell) then g-1, and if tB0 (thin shell) then g ,, 1. The plasmon resonance in small metallic NPs covered with a shell layer occurs when [71]:   εs g 1 εm 3 2 g  

εc 5 2 2εs (11.16) εs 3 2 2g 1 2εm g For g 5 1, the resonance condition is given by εc 5 22εs (the core “perceives” the shell as its surrounding medium), whereas for g 5 0 (uncoated particle case) the resonance condition becomes εc 5 22εm. For a thin shell (g ,, 1), the resonance condition is given by dielectric function εc for the coupled coreshell nanoparticle, the shift in peak maximum is: Δλmax 5 2

λ2p ðεc 1 2εm Þ 2λm

(11.17)

where λm is the unshifted wavelength of the maximum for g 5 0, λp is the wavelength of volume oscillations of electron plasma, and εc is given as in Eq. (11.16). Combining the above, the surface plasmon peak position, λpeak, is determined by [72,73]. λ2peak λ2p λ2peak λ2p

5 εN 1 2εm ðuncoatedÞ

5 εN 1 2εm 1

2gðεs 2 εm Þ ðsurface protectedÞ 3

(11.18)

(11.19)

Eqs. (11.18) and (11.19) clearly show the differences between coated and uncoated particle optical properties. The equations first predict that, since g increases with chain length, longer-chain ligands should shift the plasmon band position, λpeak, to longer wavelengths. Second, if the refractive index of the monolayer, ns 5 εs1/2, increases, the band wavelength should increase.

11.2 EXPERIMENTAL METHODOLOGIES Two different approaches were adopted for the synthesis of aluminum-, silver-, and gold-based multicomponent HNs: laser heating and two-step/multistep ablation method.

11.2.1 Laser Heating Selective laser-induced heating is one of the most important techniques in which hybrid nanoparticles can be formed by selectively melting successive layers of metal nanoparticles using a laser beam comparable to the resonant wavelength of the metal used. The unique interaction of laser light with a material can lead to permanent changes in the material’s properties not easily achievable through other means [74]. Laser irradiation has been shown to induce changes to the local chemistry, the local crystal structure, and the local

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morphology, all of which affect how the material behaves in a given application [75]. This method relies on melting behavior of metal NPs and their resonant absorption at the LSPR peak wavelength. The Au nanoparticles when irradiated with 532 nm get heated to liquid form and therefore act as nanosolders to join the material for composite formation, this phenomena is known as nanosoldering in laser ablation, if the resonance of metal matches with the wavelength of laser light [7678]. Fluences above the threshold of melting can lead to the formation of transient pools of molten material on the surface. The molten material will support much higher atomic mobilities and solubilities than in the solid phase, resulting in rapid material homogenization [79]. High self-quenching rates can be achieved by rapid dissipation of heat into the cooler surrounding bulk material. Such rapid quenching can freeze in defects and supersaturated solutes as well as form metastable material phases [80]. At temperatures far above the melting temperature, hydrodynamic motion can reshape and redistribute material which is easily achievable using resonant laser wavelength. For the synthesis of AlAg HNs, silver and aluminum NPs were prepared individually by ablating sliver and aluminum metal plates (99.99% pure) for 20 min in deionized water and polyvinyl pyrrolidone (PVP) polymer using second-harmonics (532 nm) wavelength of Nd: YAG laser (Quanta Systems, Model SYL 202) having 5-ns pulse duration and 10-Hz pulse repetition rate. The laser beam was focused by a convex lens of focal length 150 mm to obtain a laser spot size of 0.5 mm on the target immersed in liquid. The laser fluence used for silver and aluminum ablation was 14.3 and 4.5 J/cm2, respectively; the lower fluence for aluminum ablation was necessary because of the lower melting point of aluminum compared to silver; further aluminum oxidizes rapidly at higher laser fluence. The metal plate (Ag or Al) was placed at the bottom of a glass vessel filled with 5 mL of deionized water or 0.001 mM concentration of aqueous solution of PVP (24 kDa, Central Drug House). The freshly prepared nanoparticle solutions were then mixed in 1:1 volumetric ratios to obtain 3 mL as-mixed AgAl solution. The as-mixed solution was post-irradiated with an unfocussed beam of the same laser at fluence 0.6 J/cm2 for different irradiation time (20, 40, and 60 min). The irradiated nanoparticle solutions were immediately characterized by UVvis absorption spectroscopy using a dual-beam spectrophotometer (PerkinElmer Lambda35) having spectral resolution of 0.5 nm. The extinction spectra of the samples were periodically monitored at regular intervals for their chemical stability and nanoparticle aggregation. TEM analysis was performed at 100 kV with a Philips Model CM 12 Microscope. HRTEM analysis was performed at an accelerating voltage of 200 kV with Technai G20-stwin HRTEM microscope. The HRTEM was equipped with energy-dispersive X-ray analysis (EDX) for elemental studies. For TEM and HRTEM, the nanoparticle suspensions were dropped onto a carbon-coated Cu grid and then dried at room temperature

11.2.2 Two-Step Laser Ablation Method Conventional syntheses of binary HNs are based on the coprecipitation of metal salts, reverse micelle methods, hard/soft-templating methods, Kirkendall, Ostwald ripening effect, selective etching [4648]. Various nanostructures have been synthesized, however

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these methods are often complicated because of tedious procedures and poor reproducibility. To prepare multimetallic HNs with a core/shell structure, the most effective method is to deposit one metal on an existing colloid or cluster of another metal. This is similar to the growth process following the seed formation for preparation of growth of large particles in colloid solution, ignoring the difference in two metals. If the metal seeds are chemically inert to the metal designed to be used for depositing a shell on the preformed metal core, i.e., the metal has a lower redox potential than that of the seed metal, and if the deposited metal atoms can stably stand on the surface of the metal core without migrating into the phase of the core under the preparation condition, one can easily grow a metal shell. The seed metal should not be protected by ligands which have strong coordination ability to the metal core otherwise the core will migrate to the shell position. With the increasing complexity of synthetic reactions, an understanding of the formation mechanisms of the nanoparticles is needed to enable a systematic synthetic approach. For the synthesis of bimetallic Al2O3@Ag, Ag@Al2O3, Al2O3@Au, and Au@Al2O3 twostep laser ablation steps were used. Laser ablation of metal target (M1—Al) was carried out to form colloidal solution of metal M1. Another metal target (M2—Ag/Au) was then ablated in the (M1) colloidal solution to form (M1@M2) bimetallic HNs (Fig. 11.3). This method was adopted in order to invert the geometry of coreshell structure. All the parameters and materials used are similar to laser heating methodology. The laser fluence of 17.6 J/cm2 was used for ablating gold target. This approach may be considered as a seedmediated growth for synthesis of coreshell nanostructures where seed existing in the solution acts as a nucleation center for other particles to grow. Depending upon seed one can therefore change the morphology of nanoparticles. Particle size analysis was performed using UTHSCSA image processing program by measuring the diameter of 100200 particles from TEM images and the mean size was calculated as the number averaged mean size.

Nd: YAG Laser

M2

M2

Lens 532 nm

532 nm M1

Glass

Two-step laser ablation

M1 colloid

M2

Metal target

M1 Post irradiation M1@M2 Core-shell

FIGURE 11.3 Scheme showing generation of HNs of metal M1 (Al) and M2 (Au/Ag) using two-step laser ablation method where the metal target M2 was ablated in M1 colloid to form coreshell structure of M1@M2 (the morphology could be interchanged as M2@M1 by ablating M1 target in M2 colloid).

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11.3 HYBRID NANOPARTICLES SYNTHESIZED BY LASER HEATING 11.3.1 AgAl Hybrid Nanoparticles Combining silver and aluminum metal to form HNs is nontrivial, as the phase diagrams of silver and aluminum are complicated and include a miscibility gap [81]. In addition, aluminum’s tendency to oxidize makes the controlled formation of AgAl HNs even more challenging. The coexistence of aluminum and silver in the same nanostructure, for instance, is interesting for plasmonic solar cell [82], catalysis [30,83], SERS sensing [84], and nanophotonics [85]. Here the synthesis of AlAg HNs is described by passing thermodynamics constraints by the PLAL approach. The synthetic approach is of general applicability and virtually permits the preparation in one step of a large variety of multielement HNs. Using laser ablation one can attempt out of equilibrium synthetic approaches to place immiscible metal inside the same nanostructure, since it operates in the conditions where the kinetics of NPs formation is very fast and often leads to metastable/intermediate product which is not the thermodynamically favored one [51,52]. Therefore we exploited the laser irradiation using selective heating to obtain in a single step plasmonic AgAl HNs. The UVvisible spectra of Al, Ag, and as-mixed Ag and Al NPs solution in a 1:1 ratio in water before and after the laser irradiation are shown in Fig. 11.4. The two distinct LSPR peaks at 404 nm [86] and 264 nm [8789], correspond to Ag and AlAl2O3 NPs. Aluminum in water initially shows a weak localized LSPR at around 225 nm (Fig. 11.4, curve 1). However, after 23 h of residency time a new LSPR peak at 264 nm appears due to the formation of natural aluminum oxide layers around aluminum surface (Fig. 11.4, curve 3). The silver NPs show an intense and broad LSPR peak at 404 nm in the absorption spectrum (Fig. 11.4, curve 2). The LSPR peak of aluminum NPs lie in deep UV region 200220 nm [89,90]. Therefore the shift in plasmonic peak from 200 nm to 264 nm in water is due to the formation of thin aluminum oxide shell over the highly reactive aluminum core to form coreshell structure of Al@Al2O3. So the plasmonic peak at 264 nm is because of the coreshell structure Al@Al2O3 and the large red shift of about 44 nm is because of Deionized Water

0.30

Absorbance

2

3

0.25

6

0.20

FIGURE 11.4

Absorption spectra of Al, Ag NPs and as-mixed AgAl HNs post irradiated for different times in deionized water (circle in the figure shows broadening in the peak indicating shape anisotropy). Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 121 (2015) 261271. Copyright 2015 Springer.

1) Al in water 2) Ag in water 3) Al-Ag as mixed 4) 20min post irrad 5) 40min post irrad 6) 60 min post irrad

4

5

0.15 0.10 0.05

1

0.00 250

300

350

400

450

500

550

600

Wavelength(nm)

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higher refractive index of alumina as compared to water. Merely mixing the colloidal solution of silver and Al NPs does not shift the plasmonic peak and therefore both the plasmonic peaks can be clearly seen without any shift (Fig. 11.4, curve 3). The time of laser heating was varied from 20 to 60 min in order to get the maximum heating effect. After 20 min of post-laser irradiation there was marginal shift in plasmonic peak (B1 to 2 nm) because of the weak interaction between adsorbed silver particles and alumina in the sintered structure (Fig. 11.4, curve 4). The broadening of LSPR peak suggests the formation of hybrid structure in water and the damping of surface plasmon of silver core in the presence of alumina over its surface. Broad plasmon absorption indicates the anisotropic particle growth along one preferential direction, i.e., by unidirectional coalescence and soldering of Ag NPs with alumina NPs (evident from TEM image, Fig. 11.5A). Unidirectional nanocrystals assembly arises from random collisions of NPs diffusing in the liquid medium. The plasmonic peak of AlAl2O3 coreshell structure red-shifted by 5 nm (from 264 nm to 269 nm) after 40 min of laser heating which indicated the formation of HNs or decrease in electron density of aluminum, resulting from electron transfer to silver or electron scattering to some defect states at the interface [31]. Increasing the effective refractive index of porous alumina shell with thickness may also cause gradual red shift of the LSPR peak due to enhance diffusion of aluminum through oxide shell. The higher the refractive index of the embedding medium, the greater will be the polarization, which will weaken the restoring forces inside the polarized particles. This gives rise to the excitation of another oscillating mode resulting in shifting of resonance towards the higher wavelength side. After further irradiating the mixed solution for 20 min, a total 60 min of laser heating, the LSPR peak of AlAl2O3 blue-shifted by 2 nm (Fig. 11.4, curve 6) suggesting a decrease in the refractive index around the Al core due to formation of a hollow or void between the core and shell which is also clearly visible in the TEM image (Fig. 11.5B). The formation of hollow cavity at the metal and metal oxide interface is due to the faster outward diffusion of metal ions compared to the inward diffusion of oxygen ions in the process of the formation of a surface oxide layer [44,91]. Longer irradiation time induces the Kirkendall effect [26,45] which consequently lowers the effective refractive index by forming hollow particles and therefore blue shifts the plasmonic peak. Also, the peak absorption decreases rapidly and broadens for thick oxide layers due to diminishing plasmonic response of the core electrons undergoing retardation effect at the AlAl2O3 interface [1]. FIGURE 11.5 TEM images of AlAg HNs post-irradiated with 532 nm laser in deionized water, for (A) 40 min and (B) 60 min. Inset in (B) shows complete coreshell structure with Ag (dark) as core and Al2O3 (gray color) as shell. Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 121 (2015) 261271. Copyright 2015 Springer.

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The silver LSPR peak was not shifted but intensity was drastically reduced with increase in post-irradiation time. The peak broadening suggests the formation of coreshell structure Ag@Al2O3 in deionized water and the influence of alumina over the silver surface which reduces the intensity of silver peak [1]. The morphological changes of AgAl HNs in deionized water with changing postirradiation are evident from their TEM image (Fig. 11.5A and B). At 40 min of post-laser irradiation time as shown in Fig. 11.5A the AgAl HNs gradually cluster into sintered/ network type of structure. Due to laser-induced electric dipole moment, the HNs tend to align one-dimensionally to minimize electrostatic potential of the dipoledipole interaction leading to network-type structure [92]. The major mechanisms of sintering involve both Ostwald ripening and coalescence [93]. Ostwald ripening occurs by the diffusion of Ag atoms in the oxide layers from one Ag NP, which then transfer to another. Because the coordination of a surface atom for a small Ag NP is lower than that for a large one, the surface atoms for the small particle will be more easily removed. Accordingly, when the Ag in the oxide layers is supersaturated, the free Ag atoms will condense on the surfaces of the larger AgNPs. Therefore, the smaller AgNPs shrink, while the larger AgNPs grow. This is also because of the coalescence when two AgNPs collide and merge to form a large particle. Because the large nanoparticles always have relatively high stability and more energy is required to eliminate the interfaces among them, coalescence is more difficult for large particles. However, the small nanoparticles are less stable due to their higher specific surface areas and thus tend to agglomerate during sintering. After further post-irradiation for 20 min, gradual morphological transformation to stable nearly spherical rattle-type coreshell structure is seen due to enhanced material diffusion and interaction at the coreshell interface (Fig. 11.5B) [31]. The TEM image clearly shows the rattle-type of coreshell morphology of AgAl HNs after 60 min of post-irradiation. The TEM analysis reveals narrow size dispersion and isolated particles of mean size 25 nm and shell thickness 46 nm. The image shows that the particles are porous (alumina on the surface of silver makes the structure porous) and there is a hollow formation inbetween core and shell [94,95]. The aluminum shell gets highly oxidized in water forming porous γ-alumina along with a hollow region or variable gap between the core and shell, similar to the rattle-type structure formed in other synthesis routes [96]. Actually long-period laser ablation (60 min) of AlAg HNs in water triggers a secondstage growth of sintered particles. The assembly of sintered particles into large hollow rattle-type of particles may be because of high concentration of sintered particles well above the Ccrit (concentration above which nucleation takes place) in which case a second stage growth is switched on with the sintered particles as building blocks for the synthesis of rattle-type HNs [49,93,97,98]. There is a large amount of evidence that nanomaterials obtained by PLAL form by coalescence of smaller crystalline nuclei with nanometric size. In the present case, however, the lack of miscibility between Ag and alumina is the reason for the formation of new interfaces between alumina and Ag inside the same NP (clearly seen marked by the arrow in Fig. 11.5A). In addition, the diffusivity inside NPs is hampered by the presence of multiple dislocations and boundaries between grains with different crystalline structure, such as FCC silver and γ-alumina. The final result is the formation of rattle-like HNs with an intermediate shape between the sphere and the hollow particles obtained by diffusion

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limited aggregation of laser-generated nanometric nuclei. In particular, the rattle-like morphology of AlAg HNs suggests that nuclei temperature is close to the melting point when their coalescence takes place, and that the process is fast enough to run out of thermodynamic equilibrium, where the formation of rattle structure would be favored. Controlled coreshell morphology of porous structure using pulsed laser ablation method has not been achieved before. In Fig. 11.5B, the alumina occupies shell position due to its lower surface energy than silver. Porosity results from differential solid-state diffusion rates of the two mixing components in an alloying or oxidation reaction. This diffusion difference generates vacancies in the lower-melting component side of the diffusion couple near the interface and hence low melting material appears as a porous-like structure. The nanoscale pores can develop inside nanocrystals with a mechanism analogous to void formation in the Kirkendall effect, in which the mutual diffusion rates of two components in a diffusion couple differ by a considerable amount. Within the nanocrystals of transforming structure the supersaturated vacancy will coalesce into a single void to give the rattle form of morphology. In the physical mixture of Ag and Al colloidal NPs, absorbance for Ag is larger than Al NPs at 532 nm wavelength [99,100], and the dielectric at the surface of alumina melts faster than the aluminum metal. The melted form of alumina (light gray color) depositing over melted silver particles (black) can be clearly seen in TEM image Fig. 11.5A. The laser produced plasma (LPP) behavior of metal (Al) and dielectric (Al2O3) under the influence of intense laser beam was studied by Rothenberg and Koren [101] and their result proved that the threshold fluence for producing LPP in dielectric was much lower than that of pure metal. This unexpected outcome was justified by taking into account the differences in the absorptivity, thermal diffusivity, and volatility of the two materials used for the study [101]. For forming laser-induced plasma from a solid target a minimum threshold value of absorbed energy density (U) at the surface is required. The threshold value Uth is lower for more volatile materials, with smaller heat capacity, but is more difficult to reach in materials with higher thermal diffusivity, which results in higher Uth value [31,101], U 5 Fð1  Rref Þαabs ;

(20)

where, F is the incident fluence, Rref is the reflectivity of material, and αabs is the absorption coefficient. Because of the higher volatility of aluminum as compared to alumina, the Uth value required for plasma formation in aluminum is lower than that of alumina, however, at the laser fluence sufficient for plasma formation in alumina, LPP in Al would not be obtained because of its greater thermal diffusivity which would not allow U to attain Uth. Such effects in alumina could lead to a significant enhancement in the vaporization of surface atoms and thus explain the melting of alumina for the formation of rattle-type NCs [31]. So when the colloidal mixtures are irradiated with 532 nm, both alumina and Ag NPs can be effectively excited to high temperature. The post-laser irradiation induces fragmentation of the parent NPs with some sintered structures (evident from Fig. 11.5A) as NPs prepared in deionized water are mostly in physical contact, therefore revealing interconnected structures. The melting depression in NPs can be used to thermally control their shape. Because of the lower melting point of aluminum (660 C) as compared to silver (961 C) and also because of size-dependent melting depression, the aluminum nanoparticles in a colloidal solution will reach their melting point before Ag NPs and therefore

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significant laser-induced heating of both the particles in a form of sintered or chain-type hybrid structure can be achieved [102104]. Finally, post-irradiating the mixture for 60 min results in complete melting of the sintered particles and by lowering the interface energy, which is done by lowering the surface tension and surface energy of Al2O3, the whole system then transforms from sintered HNs to spherical Ag@Al2O3 rattle-type HNs [105]. In fact, the formation of spherical shapes is the way the interface energy is minimized during the synthesis of metal NPs. Fig. 11.6AC) shows the HRTEM, SAED, and EDX pattern, respectively, of AgAl2O3 rattle-type HNs in deionized water. The high resolution image reveals lattice space value

FIGURE 11.6

(A) HRTEM image of AlAg HNs in deionized-water subjected to post-ablation nanosecond pulse laser heating for 60 min. Inset image shows complete coreshell morphology. (B) Selected area electron diffraction pattern of AgAl HNs subjected to post-ablation nanosecond pulse laser heating for 60 min. (C) EDX of AlAg in water showing Al and Ag as main constituent elements. Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 121 (2015) 261271. Copyright 2015 Springer.

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of 0.779 nm corresponding to γ-phase of alumina [106]. The scattering effect differentiates the color of two materials in a single image and therefore the metallic silver, because of large electron density, scatters more and will appear dark as compared to γ alumina (dielectric material). The SAED pattern shows d spacing corresponding to both alumina and silver phase. The d spacing values of 0.23, 0.20, and 0.88 nm corresponds to (2 2 2), (4 0 0), and (8 4 0) planes of γ alumina, and the d values of 0.12, 0.10, and 0.83 nm correspond to (3 1 1), (4 0 0), and (4 2 2) planes of silver phase. The EDX spectrum shows the presence of aluminum, oxygen, and silver besides copper arising from the carbon-coated grid. The large peak intensity of aluminum and oxygen compared to silver in the spectrum corroborates the formation of aluminum oxide on the surface of the rattle particle encapsulating the silver core. Fig. 11.7 shows the pictorial scheme of AlAg HNs formation in water using laser pulses at 532 nm. It shows that initially fragmentation and fusion of NPs occurs depending upon the absorbance in material at 532 nm, and then melting of fragmented particles and finally solidification in a rattle-type form of coreshell structure following Kirkendall voiding with experimental TEM pictures observed at different post-irradiation time. During laser heating, the formation of the HNs takes place in the confined plasma composed of different species evaporated from the target by laser materials interaction. The free electrons of the particle absorb the laser radiation and within 35 ps electron excitation is transferred to the NP lattice. If the absorbed energy is high enough, the particle is molten, and which favors the reaction of the molten NP with the vapors arising from the surrounding liquid or other NPs in the same solution [107]. To form HNs, these NPs must collide with each other, and at least one of the colliding particles should be in molten phase and such a condition only prevails during the nanosecond laser pulse, as the particles solidify via heat dissipation to the surrounding liquid.

FIGURE 11.7 Schematic diagram of morphological transformation from sintered to coreshell HNs by postlaser heating of mixed AgAl colloidal solution in water.

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11.3.2 Polymer Effect on the Synthesis of AgAl Hybrid Nanoparticles The rattle particles prepared in water aggregated after a few hours of residency time, therefore they require stabilizer, such as PVP polymer, to prevent their aggregation. In this section, we aim to combine HNs synthesis encapsulated with polymers to form HNs. The polymer chain confined to the network acts as capping agent and therefore prevents the particle from agglomeration. The UVvisible spectra of Al, Ag NPs and 1:1 ratio of as-mixed Ag and Al NPs solution in PVP solution is shown in Fig. 11.8. The optical absorption spectrum of the aluminum NPs in PVP solution (Fig. 11.8, curve 1) shows a featureless spectra extending toward the UVvis wavelength range. The spectra also shows complete absence of LSPR peak at 264 nm assigned to AlA2O3 coreshell structure observed in water even after 34 days of exposure time. This is due to the formation of pure aluminum NPs of smaller size and the role of PVP in reducing oxidation and agglomeration of aluminum NPs [89]. Actually PVP modifies the surface of aluminum nanoparticles by strongly coordinating with the nanoparticles surface and therefore damps the SPR peak of Al NPs [89]. In PVP solution the ablation efficiency gets enhanced because of its higher viscosity than water. The overall effect of PVP is to increase the oxidation stability of the formed HNs and to increase the formation efficiency of HN because of its confinement effect [108,109]. The absorbance spectra clearly show increased intensity of LSPR peak because of the PVP addition. This is explained by the fact that ablation products when emitted from the metal plate like Ag and Al are confined by the solvent, and this confinement becomes stronger for high-density and viscous solvent. The generated plasma already confined near solids is at high pressure and temperature, and therefore can etch the surface and again generate NPs by so-called secondary ablation [108]. With increasing post-irradiation time, the intensity of the plasmonic peak of silver NPs (404 nm) reduces along with a small blue shift in peak wavelength, and simultaneously, the appearance of broadband plasmon absorption near 264 nm increases very slowly in comparison with sample prepared in water. The UVvis spectra clearly show smaller LSPR broadening resulting from NP surface protection by polymers. In PVP, Al generally forms pure Al NPs because of its excellent

Absorbance

1 5

0.4 4

FIGURE 11.8 Absorption spectra of Al, Ag NPs and as-mixed AgAl HNs post-irradiateded for different times in 0.001 mM PVP solution (circle in the figure shows broadening in the peak indicating shape anisotropy). Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 121 (2015) 261271. Copyright 2015 Springer.

1) Ag in PVP 2) Al in PVP 3) Al-Ag as mixed 4) 20min post irrad 5) 40min post irrad 6) 60min post irrad

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steric stability, however for longer post-irradiation the steric stability is lost and oxidation is increased. The polymer had the function of coating the particle surface before and after the photofragmentation, to prevent coalescence at longer times and preserving the shape of Ag nanocrystals just after the interaction with laser pulses. Fig. 11.9 shows the TEM image of 1:1 ratio of AgAl in 0.001 mM PVP solution at different post-irradiation time. Post-irradiation of 40 min in PVP solution reveals weakly aggregated AlAl2O3 as well as dispersed small spherical silver particles in the TEM image (Fig. 11.9A and B). The morphology of the HN shows strong dependence on concentration of the polymer used and the particle characteristics. The figure shows distinguishable Ag NPs (dark) adsorbed on Al/Al2O3 aggregates (gray) [31]. The spherical morphology in PVP is mainly due to steric stability supported by polymer because of limited diffusion or encounters of the two fusing particles. The TEM image shows that the alumina particles are mainly agglomerated and larger in size and smaller silver NPs are deposited on the surface. As Al2O3 is a porous material with very high surface area, it can support wide dispersion of deposited silver NPs which could be useful for various spectroscopy and catalytic applications. Since larger silver particles absorb 532 nm wavelengths more efficiently than aluminum NPs the photofragmentation of silver NPs is possible for generation of smaller size particles. Longer post-irradiation for 60 min changes the morphology to stable rattle-type HNs, Fig. 11.9B. The laser ablation of aluminum in PVP solution usually produces oxide free chemically pure Al NPs because of excellent steric stability of the polymer. However, concomitant loss of steric stability and enhancement in oxidation rate at longer post-irradiation time results in Al2O3-coated aluminum particles. The aggregation of Al NPs after each laser pulse depends on the coverage of the photoproducts by stabilizer molecules, and on the probability of encounters of heated Al NPs with photofragments of silver. It has been reported that photofragmentation of NPs in the presence of strong ligands leads to the formation of tiny NPs rather than large sintered structures [99,107,110]. In addition to the protecting ligand, the concentration of the parent

FIGURE 11.9 TEM images of AgAl HNs post-irradiated with 532 nm laser wavelength in 0.001 mM PVP polymer: (A) 40 min (gray color aluminum oxide is in fused form and over that small silver NPs (dark color) are deposited); (B) 60 min (inset showing complete rattle nanostructure). Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 121 (2015) 261271. Copyright 2015 Springer.

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NPs is also an important aspect because it determines the concentration of the photo fragments. When concentration of parent NPs is low, the released photo fragments also have low concentrations, so that the possibility of collisions between them decreases. These conditions are not advantageous for the formation of sintered structures but are suitable to produce spherical NPs. This recombination process between released atoms and clusters during photofragmentation of the parent NPs is the basis for the HNs formation. Fig. 11.10AC shows the HRTEM, SAED, and EDX patterns, respectively, of AgAl HN in 0.001 mM PVP solution. The HRTEM image shows both dark and bright contrast,

FIG. 11.10 (A) HRTEM image of AlAg HNs in 0.001 mM PVP subjected to post-ablation nanosecond pulse laser heating for 60 min. Inset shows the lattice fringes 0.23 nm corresponding to γ-alumina. (B) Selected area electron diffraction pattern of AgAl HNs subjected to post-ablation nanosecond pulse laser heating for 60 min. (C) EDX of AlAg in polymer showing Al and Ag as constituent elements. Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 121 (2015) 261271. Copyright 2015 Springer.

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suggesting the formation of composite structure. The measured lattice spacing of 0.28 nm in HRTEM image corresponds to (2 2 0) plane of alumina. The SAED pattern of AlAg HNs exhibits ring patterns: the diffraction spots are assigned to (2 2 2), (4 0 0), (4 4 0), (4 4 4), (8 0 0), and (8 4 0) planes of alumina crystals and confirm polycrystalline nature alumina. The EDX spectrum shows the presence of aluminum, oxygen, and silver in the AgAl2O3 rattle-type HNs. The EDX spectrum shows the presence of aluminum, oxygen, and silver besides copper arising from the carbon-coated grid. Fig. 11.11 shows a scheme of AlAg HNs formation in PVP solution using 532 nm laser wavelengths with experimental TEM image observed at various stage of growth of nanocomposites. Here the polymer protects the NPs from diffusing and therefore depositedtype of nanostructure is formed. Post-irradiation for a long time breaks the surface barrier ˚ ) and Al (4.0495 A ˚ ), the leading to rattle-type HNs. The lattice constant of Ag (4.0853 A great similarity in their crystalline structures (FCC cubic crystal), and the relatively small ˚ for AlAg makes it possible to grow aluminum layers on the lattice mismatch 0.0358 A surface of Ag NPs or vice versa with minimal lattice stress under appropriate conditions [111,112]. The cohesive energy also decides the stability and morphology of HNs since the structure with higher cohesive energy tends to be more stable [15,105]. The experimental cohesive energy of bulk Al (3.39 eV per atom) [113] and Ag (2.95 eV per atom) [105] indicates that Al is superior to Ag in stability. Further the surface energy of AlAl2O3 would be reduced in comparison to pure Al and Ag. However in the case of Al and Ag as per their cohesive energy values Al@Ag should be a more stable structure than that of Ag@Al. But Al2O3 on the surface of Al lowers the surface energy and therefore Ag@Al2O3 would be the stable structure. In addition, the analyses of diffusion behavior and atomic distribution suggest that the minimization of surface energy tends to form Al surface segregation because of its lower surface energy and faster diffusivity than silver [27].

FIGURE 11.11 Schematic diagram of morphological transformation from sintered to coreshell HNs by post laser heating of mixed AgAl colloidal solution in polymer matrix.

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11.4 HYBRID NANOPARTICLES SYNTHESIZED BY TWO-STEP LASER ABLATION 11.4.1 AgAl Hybrid Nanoparticles Bimetallic HNs Ag@Al and Al@Ag were synthesized by ablating aluminum plate in silver colloid and silver plate in aluminum colloid, respectively, with laser pulses at 532 nm. The motivation for using this method was to interchange the core and shell material. Unlike laser heating method (post-irradiation) the thermodynamics and surface energy allows aluminum oxide to occupy shell position because of its lower surface energy, however in two-step laser ablation method it is possible to invert the structure of Ag@Al2O3 as Al2O3@Ag by simply providing alumina as a seed in existing colloidal solution. Heterogeneous nucleation takes place in a heterogeneous reaction medium that contains the nucleation seeds. In the synthesis of multicomponent HNs, heterogeneous nucleation is of great importance. In heterogeneous nucleation, there is a strong tendency to minimize the interface energy of the nucleus and the seed. If the nucleus and the seed form a heteroepitaxial interface, the interface energy increases with their lattice mismatch. As a result, their lattices are mechanically strained to reduce the mismatch at the interface and therefore form anisotropic structure (Fig. 11.12). In liquid phase, heterogeneous nucleation occurs much easier, since a stable nucleating surface is already present. When nanoparticles get dispersed in the colloidal solution, during cooling they spontaneously assemble at the interface between the already existing nucleated particles and the liquid matrix, resulting in immobilization or thin nanoparticle coating over the nucleated particles to significantly retard, if not completely block, the diffusional transport to significantly limit their growth. By inducing the formation of a secondary phase at the surface of the seed nanoparticles, this process enables the synthesis of a surprisingly wide variety of multicomponent HNs. (A)

(B) Ag in water Al in Ag(10 min) 20 min 30 min 40 min

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FIGURE 11.12 (A) UVvisible spectra and (B) TEM image of AgAl HNs synthesized by ablating aluminum in silver colloids.

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Fig. 11.12 shows the UVvisible spectra and TEM image of Ag@Al in deionized water. Here at first silver collidal solution was prepared by ablating silver target in deionized water for 20 min. After the colloidal solution of silver was formed the target was removed and in that solution aluminum target was ablated for different ablation time. The absorbance of silver NPs was drastically reduced and LSPR peak red-shifted (8 nm) after aluminum was ablated in the solution for 10 min. The morphology of AgAl HNs was mainly chain-type structure. The UVvis spectra also shows the broadening in the region 450500 nm this is because of shape anisotropy of the synthesized HNs (Fig. 11.12B). The morphology completely shows jointed type of morphology with spherical silver NPs. Fig. 11.13 presents the UVvisible spectra of Al2O3@Ag by ablating silver in aluminum colloid. Here, we form the core as Al@Al2O3 and silver as deposited material (Fig. 11.13B). The morphology is not a complete coreshell, rather it showed deposited type of nanostructures. The γ-alumina acts as a support or substrate to uniformly disperse silver NPs over it. By increasing the time of ablation we can change the concentration of deposited particles and hence core to shell ratio can be changed. Oriented attachment is a well-recognized process for crystal growth from primary nanoparticles [97]. If two nanoparticles are attached to each other but their crystallographic orientations are not perfectly aligned parallel to each other, defects such as grain boundaries, twinning, or misfit dislocations are formed at the interface [97]. For the particlemediated growth process, in order to reduce the overall energy to a minimum during the coalescence of two nanoparticles, the nanoparticles prefer to attach through the facets with the highest surface energy or highest surface area to decrease the overall surface energy of the two nanoparticle system. The particle-mediated growth not only involves the attachment of nanoparticles, but also includes the movement of atoms between two attached nanoparticles. After the nanoparticles become attached, the atoms on the surface diffuse toward each other to fuse the nanoparticles together into a single crystal; on the other (A)

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FIGURE 11.13 (A) UVvisible spectra and (B) TEM image of AlAg HNs synthesized by ablating Ag in Al colloid for different ablation time [30]. The circles in TEM image clearly show deposited silver particles on alumina surface. Alumina helps in dispersion of silver NPs on its surface [30].

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hand, the atoms at positions with high energy will be rearranged to reduce the total energy of the system. This process is denoted as intraparticle coarsening or ripening which refers to the diffusion of atoms along the surface of the nanocrystals, thereby changing the morphology of the nanoparticle with time [114,115]. In the present case, however, the lack of miscibility between Ag and alumina is the reason for the deposited type of structure. In addition, the diffusivity inside NPs is hampered by the presence of multiple dislocations and boundaries between grains with different crystalline structure, such as FCC silver and γ-alumina. Fig. 11.14 shows the size distribution of both the type of HNs with an average size 34 6 13 nm and 21 6 10 nm for Ag@Al2O3 and Al2O3@Ag. For Al2O3@Ag the size calculation is done for silver NPs which are deposited over the surface of alumina. Fig. 11.14B clearly shows that the alumina NPs are larger in size; the average size for alumina is around 50 nm. The larger size of Ag@Al2O3 HNs is because of alumina at the outer layer surface of silver. Theoretical calculation using Mie theory (Eq. (11.14)) for 15 nm core radius of silver of varying oxide shell is shown in Fig. 11.15A. Comparison between the experimental and theoretical peak shows that an oxide layer of 3 nm over the silver surface will give a 8 nm shift. Therefore theory predicts an oxide layer of around 3 nm for 10 min of laser ablation. Fig. 11.15B shows the calculated absorption spectra for Al2O3@Ag coreshell with varying shell thickness of silver. For thin metal shells, the damping due to electronsurface collisions broadens the SPR peak. The radius of alumina core is fixed at 25 nm. When the thickness of Ag shell is in the region of 12 nm, the SPR band is significantly affected by the Ag shell. When the thickness is more than 10 nm, the SPR band of Al2O3@Ag coreshell further shifts to the red and becomes strong in intensity with increase in the thickness of Au shell. As can be seen in Fig. 11.13A, with increasing time of ablation, the peak absorbance of silver increases. Initially pure aluminum NPs are formed which are then transformed to AlAl2O3 coreshell structure and the plasmonic peak shifts to 264 nm (Fig. 11.13A, curve 1), however, when silver is ablated in that colloidal solution the plasmonic peak of Al NP gets broadened. After 40 min of ablation time silver masks the plasmonic peak of (A)

(B) 80 Mean Size = 34 nm S.D(σ) = 13 nm

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Size distribution of (A) Ag@Al2O3 HNs, and (B) Al2O3@Ag HNs prepared in deionized water.

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FIGURE 11.15 Calculated variation of LSPR spectrum with radius of core and shell: (A) Ag@Al2O3 for varying shell thickness of Al2O3 (core radius 5 15 nm); (B) Al2O3@Ag for varying thickness of Ag (core radius 5 25 nm).

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FIGURE 11.16 (A) UVvisible spectra and (B) TEM image of AuAl HN synthesized by ablating aluminum in gold colloids (arrows indicate coreshell morphology of the nanostructure).

aluminum and therefore peak of aluminum disappears and only the plasmonic peak of silver appears. Depending on ablation time we can monitor the thickness of the oxide shell. In comparison to pure silver NPs in water the HNs of silver in alumina display a broad plasmon peak which is due to the presence of alumina.

11.4.2 AuAl Hybrid Nanoparticles Figs. 11.16 and 11.17 show UVvisible spectra and TEM images of Au@Al2O3 and Al2O3@Au HNs in deionized water prepared by two-step laser ablation method and it can

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2 1

300

400

500

600

700

20nm

Wavelength(nm)

FIGURE 11.17 (A) UVvisible spectra and (B) TEM image of AlAu HN synthesized by ablating Au in Al colloid for different ablation time. The TEM image clearly shows that the gold particles are deposited on alumina surface.

be seen that for Au@Al2O3, the Au core (black) is completely covered by a shell of alumina (gray), as clearly seen in the TEM image, Fig. 11.16B. The average particle size is 23 6 15 nm for Au@Al2O3 HN and 14 6 6 nm for Au in Al2O3@Au HN, the average particle size of alumina is 50 nm, which is clearly visible in Fig. 11.17B. The UVvisible spectra show a concomitant decrease in peak intensity with large broadening in peak. With ablation for 40 min the peak shift was 15 nm. The large red shift and peak broadening suggests that the Au core is surrounded by alumina layer and therefore damps the LSPR peak of gold. Fig. 11.17B shows that the alumina is partially covered with gold NPs. When gold was ablated in Al colloidal solution for 1040 min the intensity of gold LSPR peak was enhanced and that of aluminum LSPR peak was broadened because of an increase in concentration of gold with an increase in ablation time which would then mask the plasmonic peak of Al. The presence of plasmonic peak of aluminum also suggests the insufficient coverage of AlAl2O3 with gold and therefore both the peaks appear. This spectral behavior reasonably indicates the coreshell structure. It is also known that the absorption intensity of metal nanoshell particles increases when the shell growth is complete. The intense absorptions observed here suggest complete coverage of the alumina surfaces. Fig. 11.18A shows the calculated LSPR spectra of gold NPs in the presence of alumina of varying shell thickness. For an Au core size of 7 nm, the shell thickness (alumina) of 7 nm will shift the peak to 15 nm (from 520 to 535 nm), the theoretical calculation fits well with the experimental value. Similarly for 25 nm alumina core radius, 4 nm Au shell will shift the peak to 535 nm; however experimentally we observed red shift until 528 nm in the presence of alumina which corresponds to 3 nm Au thick shells. The UVvis spectra clearly show a LSPR red shift with ablation time correlated with increasing shell thickness, Fig. 11.16A. Fig. 11.18A shows the behavior of LSPR peak shift and shell thickness with

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11.4 HYBRID NANOPARTICLES SYNTHESIZED BY TWO-STEP LASER ABLATION

ablation time. To experimentally estimate the gold NP LSPR red shift caused by Al2O3 shell, we have drawn a fitting curve of SPR peak position to the shell thickness with a good regression correlation (R2 5 0.99), Fig. 11.20B (Fig. 11.19). Since for Al2O3@Au the morphology was not complete coreshell structure the sample was post-irradiated with 532 nm laser wavelength for coreshell formation. After postirradiating the sample, Al2O3@Au, the sample prepared by ablating Au in Alumina for 40 min, as shown in Fig. 11.17B for 15 min with a laser fluence 0.6 J/cm2 the morphology showed complete coreshell structure with gold as shell and it can be clearly seen from the TEM picture, Fig. 11.20A, that there is no alumina structure left and all the gold NPs have deposited over it. The large size of NPs also confirms the formation of Al2O3@Au (B)

(A) R R

Absorption

R

λmax = 535 nm

R R R R R R

300

400

500 600 700 Wavelength(nm)

Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3

λmax = 533 nm

= 9 nm

RAu = 1 nm RAu = 2 nm

λmax = 528 nm

= 8 nm = 7 nm

RAu = 3 nm

= 6 nm

RAu = 4 nm

= 5 nm

RAu = 5 nm

Absorption

R

λmax = 532 nm

= 4 nm = 3 nm = 2 nm = 1 nm

RAu = 6 nm RAu = 7 nm RAu = 8 nm RAu = 9 nm RAu = 10 nm

= 0 nm

800

300

900

400

500 600 700 Wavelength(nm)

800

900

FIGURE 11.18 Variation of LSPR spectrum with changing radius of core and shell: (A) Au@Al2O3 for varying shell thickness of Al2O3 (core radius 5 7 nm); and (B) Al2O3@Au for varying thickness of Au (core radius 5 25 nm). (A)

(B) 540

530

5

525

4 3

520

2 515

1

510

0 0

10

20

30

40

50

LSPR Peak 2 Linear fit (R = 0.99)

534 531 λmax (nm)

6

Al2O3 (thickness)

λ max (nm)

535

537

7

LSPR Peak Al2O3 shell thickness

528 525 522 519

60

Ablation Time(min)

0

1

2

3

4

5

6

7

Shell thickness(nm)

FIGURE 11.19 Variation of LSPR maximum with changing radius of core and shell: (A) Au@Al2O3 for varying shell thickness of Al2O3 (core radius 5 7 nm); and (B) Al2O3@Au for varying thickness of Au (core radius 5 25 nm).

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11. LASER-INDUCED HEATING SYNTHESIS OF HYBRID NANOPARTICLES

coreshell because of larger size of alumina in Fig. 11.17B (the average size of alumina is 50 nm in diameter). ED patterns confirm that the particles are polycrystalline in nature. Figs. 11.20 and 11.21 present the HRTEM image of Al2O3@Au. As can be seen clearly in these figures, the proper coating of gold as shell has been fabricated on alumina shell. The HRTEM shows lattice fringes of 0.234 nm which matches with the FCC phase of gold. EDAX spectra (Fig. 11.22) also prove that Au is in shell position and therefore the percentage count of gold is more than that of aluminum, though some percentage of aluminum is also there, this is because we still have some free alumina NP in pure water and therefore the EDAX spectrum will show some peak of aluminum with oxygen. Fig. 11.23 shows a scheme of AlAu HNs formation in de-ionized water using 532 nm laser wavelengths with experimental TEM image observed at various stages of growth of nanocomposites. The mechanisms of interaction of laser radiation with NPs and the corresponding changes are still not well understood. Therefore, an important condition for optimizing the laser-induced modification is to establish the contributions of different physical parameters (heating, melting, evaporation, laser-induced charging of particles, and particle photofragmentation and aggregation). Pulsed laser-induced morphological changes of metal NPs like gold are caused by photothermal process. So, when a colloidal solution is laser-irradiated, the radiation is absorbed by NPs (the liquid solvent is generally transparent for laser radiation). As a result, the particles can be heated to the melting and even boiling temperature, and their morphology may change. The possibility of reaching such temperatures depends on the laser fluence, pulse duration, and irradiation time. If the laser radiation density is low and the NP temperature does not reach the melting point, FIGURE 11.20 (A) TEM image of AlAu HNs synthesized by ablating Au in Al colloid for different ablation time. The sample was post-irradiated with 532 nm laser for 15 min with a laser fluence 0.6 J/cm2. The TEM image clearly shows that the gold particles are deposited on alumina surface. (B) SAED pattern of AuAl HNs.

FIGURE 11.21 (A) HRTEM image of AlAu HN synthesized by ablating Au in Al colloid for different ablation time (the sample post-irradiated for 15 min).

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11.5 TRIMETALLIC HYBRID NANOPARTICLES

FIGURE 11.22 EDAX spectrum of AlAu HNs prepared by two-step laser ablation method.

FIGURE 11.23 Schematic diagram of morphological transformation from deposited strcuture to complete coreshell Al2O3@Au HNs by two-step laser ablation method of mixed AgAl colloidal in water.

the particles do not change in shape and size. However, if the NP temperature is above the melting temperature but below the boiling point, it is only the particle shape that changes. The particles change in both shape and size only when heated to the boiling temperature [116,117].

11.5 TRIMETALLIC HYBRID NANOPARTICLES The same methodology has been applied for the formation of trimetallic HNs of AlAgAu. For formation of trimetallic HNs, a third metal M3 was ablated in colloidal solution of bimetal M1 1 M2. When M3 (Au) was ablated in M1@M2 (Al@Ag) colloidal solution, Al oxidizes in water to form Al2O3 and therefore this could either result in Al2O3@AgAu alloy or coreshell structure of Al2O3@Au@Ag depending upon the laser energy and ablation time.

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11. LASER-INDUCED HEATING SYNTHESIS OF HYBRID NANOPARTICLES

11.5.1 Al2O3@AgAu Alloy Hybrid Nanoparticles Chemical synthesis of homogenous solid solution of alloy nanoparticles of noble metals requires very high temperature and often leads to phase separation. PLAL allows synthesis of complete homogenous structure of multicomponent system at room temperature without addition of any surfactants, reducing agents, or stabilizing ligands [51,57]. However, to date, the dominant alloy formation process during PLAL is not fully understood. We have successfully prepared HNs of Au/Ag in the form of alloy in aqueous solution of alumina through PLAL. Alumina in the solution acts as a nucleation center and enhances the interaction between the two particles in forming alloy. The alloy formation was monitored by the optical absorption measurement which exhibited a single broad LSPR band characteristic of alloy structure rather than two-band characteristic of coreshell structures [32]. Fig. 11.24A shows experimental and theoretical spectra of AgAu alloy HNs in the presence of a third metal Al. As discussed in the previous section, aluminum in water forms an AlAl2O3 coreshell structure, therefore aluminum target was ablated in water for 40 min. Next, in the prepared colloidal solution of Al@Al2O3, silver was ablated for 40 min to form Al2O3@Ag coreshell HN or intermediate structure like deposited silver over the surface of alumina (Fig. 11.13B). Subsequently, the silver target was removed and a gold target was ablated for various ablation times which resulted in alloy formation in the form of Al2O3@AgAu which was also evident from the UVvisible spectra (linear shift of single LSPR peak with ablation time/alloy composition). The experimental results show linear LSPR shift from 403 nm to 500 nm corresponding to silver and gold NPs with increasing ablation time. Unlike a coreshell structure where the plasmon resonance of the core nanosphere is rapidly masked or attenuated by that of the growing shell, the optical absorption spectra exhibit one SPR band for various ablation times suggesting the formation of an alloy of AgAu on alumina surface, indicating the Ag and Au elements are (B)

Absorption

1 5 2 3 4

8

1) Ag in Al(40 min) 2) Au in above(10 min) 3) 20 min 4) 30 min 5) 40 min 6) 50 min 7) 60 min 8) 70 min 9) 80 min 10) 90 min 11) 100 min 12) 110 min

11

9

6

7

1

9 2

3

4

5

6

7

8

10

= 0.14

3) x

= 0.20

4) x

= 0.31

Au Au

5) x

Au

= 0.55

Au

7) x

= 0.64

8) x

= 0.78

Au

Au

9) x

Au

600

= 0.45

6) x

12

400 500 Wavelength(nm)

= 0.03

2) x

Au

10

300

1) x

Au

Absorption

(A)

10) x

= 0.81

Au

400

450

500

= 0.83

550

Wavelength(nm)

FIGURE 11.24 UVvisible spectra of AlAgAu trimetallic HNs: (A) experimental, and (B) calculated for 9 nm size alloy NPs. Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 116 (2014) 955967. Copyright 2014 Springer.

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11.5 TRIMETALLIC HYBRID NANOPARTICLES

homogeneously distributed without compositional interfaces over the whole nanoparticle. We therefore concluded that alumina helps in alloying silver and gold and uniform dispersion over its surface during ablation. The results were further confirmed by comparing the experimental value with a theoretical value. Eq. (11.9) was used for calculating frequency-dependent LSPR shift and the dielectric constant was taken considering the weighted average of each component, Eqs. (11.11), (11.12), (11.13). Fig. 11.24B shows the calculated absorption spectra of AuAg alloy and their composition-dependent LSPR peak. With increasing ablation time the concentration of gold atoms increases and therefore consistent collision between gold and silver resulted in alloy HNs [118]. The theoretical plasmon maximum is plotted against the gold mole fraction xAu and a linear relationship is found, Fig. 11.25B. The LSPR shift observed is due to the perturbation of the d band energy levels because of their difference in high frequency dielectric constant. Both the metals have identical bulk plasma frequencies, so an LSPR shift due to changing electron density is not expected. This results in a steady increase in the effective value of εN for the alloy and consequently a shift in the position of the LSPR peak. During ablation, the ablated Ag and Au atoms gets trapped inside a laser-induced plasma plume, which is followed by the formation of a cavitation bubble and because the plasma plume temperature reaches up to 7000K and is subjected to strong spatial confinement by the liquid, the Au and Ag atoms are homogeneously mixed, and phase separation is minimized. The alloying is because of enhanced self-diffusion that results from depression of the melting point with size or by surface melting of the NPs [32]. The formation of alloy over alumina, also indicates that defects, such as vacancies, at the bimetallic interface enhance the radial migration (as well as displacement around the interface) of one metal into the other [119]. Table 11.1 shows the calculated composition of alloy for various LSPR peak shifts; the calculated value exactly matches with the results obtained by Link et al. [70]. Table 11.2 shows the physical parameters used for calculation. In Fig. 11.25A, the observed FWHM of LSPR peak wavelength of Al2O3@AgAu HNs with ablation time is shown. As the ablation time increases, both λmax and FWHM (A)

(B)

500

LSPR FWHM

500 λmax(nm)

150

450

FWHM(nm)

λmax(nm)

200

450

100 400 400 0

20

40

60

80

100

50 120

0.0

0.2

0.4

0.6

0.8

Mole fraction(xAu)

Ablation time(min)

FIGURE 11.25 (A) Variation of FWHM and LSPR peak wavelength with ablation time for AlAgAu HNs. (B) Plot of calculated LSPR peak wavelength with gold mole fraction in AlAgAu HNs. Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 116 (2014) 955967. Copyright 2014 Springer.

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11. LASER-INDUCED HEATING SYNTHESIS OF HYBRID NANOPARTICLES

TABLE 11.1 Calculated Composition of AuAg with Ablation Time [32] Ablation time (min)

LSPR shift (nm)

Composition of alloy (xAu)

10

406

0.03

20

417

0.14

30

429

0.20

40

442

0.31

50

458

0.45

60

470

0.55

70

481

0.64

80

495

0.78

90

496

0.79

100

498

0.81

110

499

0.82

TABLE 11.2 Physical Parameters of Ag, Au, and Al [32] Bulk properties

Symbol

Silver (Ag)

Gold (Au)

Aluminum (Al)

Fermi speed

vF

1.39 3 10 m/s

1.40 3 10 m/s

2.03 3 106 m/s

Plasma frequency

ωp

1.38 3 1016 rad/s

1.36 3 1016 rad/s

2.20 3 1016 rad/s

Relaxation time

τ

4.01 3 10214 s

2.94 3 10214 s

10.2 3 10215 s

Damping constant

Γ

3.23 3 1014/s

1.08 3 1014/s

7.6 3 1014/s

Dielectric constant

εN

3.7

9.5

0.7

Ionization energy

Ei

7.57 eV

6

6

9.22 eV 2

5.99 eV 2

Thermal diffusivity

D

174 mm /s

127 mm /s

84.2 mm2/s

Cohesive energy

U

2.95 eV

3.81 eV

3.39 eV

increases rapidly. The FWHM of the absorption peak for Au nanoshells increases with increasing the collision frequency and is maximum (B250 nm) at 50 min ablation time where the alloy has nearly 50:50 Ag and Au composition. With increasing shell thickness the total size of particles gets increased which then enhances the phase retardation effect and increases FWHM of the LSPR peak. Other factors like shape anisotropy, aggregation effect, broad size distribution arising from laser ablated product also contribute to broadening of LSPR band. Intense pulsed laser light can induce size reduction, enlargement, and morphological changes of metal NPs whose SPR lies in the region of wavelength of

I. FUNDAMENTALS

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11.5 TRIMETALLIC HYBRID NANOPARTICLES

laser light used and therefore gold nanoparticles in the solution on irradiation with 532 nm laser wavelength gets heated to its boiling point in a few picoseconds via electronphonon interactions. Elevated temperature of the parent Au NPs by heat transfer simultaneously melts Ag particles in the close vicinity of the Au NPs to form alloy HNs. Gold nanoparticles therefore act as nucleation sites for the deposition of silver particles as well as heat sources for formation of alloy structure. Fig. 11.26 shows the UVvis spectra, TEM image, and size distribution of trimetallic Al2O3@AgAu HNs prepared in deionized water at higher laser energy (36.7 J/cm2). The SAED pattern, in Fig. 11.26B, shows polycrystalline nature of the synthesized particles. Fig. 11.26C displays the size distribution of trimetallic Al2O3@AgAu HNs with an average (A)

(B)

0.6

Al2O3@Ag

0.5

40 min 60 min

Absorption

Au in Al2O3@Ag(20min)

0.4 0.3 0.2 0.1 200

300

400

500

600

700 100 nm

Wavelength(nm)

(C) Mean Size = 15 nm S.D = 4 nm

No of Counts

100 80 60 40 20 0 5

10 15 20 25 30 Particles Diameter (nm)

35

FIGURE 11.26 (A) UVvisible spectra, (B) TEM image, and (C) size distribution of Al2O3@AgAu HNs in deionized water. Inset in (B) shows SAED pattern of trimetallic HNs. Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 116 (2014) 955967. Copyright 2014 Springer.

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11. LASER-INDUCED HEATING SYNTHESIS OF HYBRID NANOPARTICLES

size of 15 6 4 nm. The morphology clearly confirms the formation of alloy HNs over alumina, which is also confirmed by UVvis spectra, the linear shift of single LSPR peak from 403 nm (silver) to 504 nm (alloy composition of xAu 5 0.87) with increasing ablation time. High laser fluence allows the faster transformation of mixed colloids to alloy particles; the whole process of alloying took almost 60 min to complete. Melting behavior of nanoparticles and the laser wavelength used for irradiation governs the formation of coreshell or alloy morphology of the resulting particles, since 532 nm radiation is more effective in reducing the particle size due to the strong plasmon absorption by gold which results in both melting and vaporization and therefore strong absorption of the 532 nm light with the Au NPs leads to very rapid heating, melting, and evaporation of the Au NPs. This indicates that the 532 nm laser wavelength is very effective in alloying dispersed Au and Ag NPs in the presence of alumina.

11.5.2 Al2O3@Au@Ag CoreShell Hybrid Nanoparticles Since ablating gold with 532 nm wavelength in the presence of Al2O3@Ag colloidal solution resulted in an alloy form of HNs: Al2O3@AgAu, we tried to see the effect of ablating silver with 532 nm wavelength (laser wavelength not comparable with resonance frequency of the metal used) in the presence of Al2O3@Au colloidal solution and the results showed different morphology for this combination of laser and metal. Figs. 11.27 and 11.28 show the UVvisible spectra and TEM image of Al2O3@Au@Ag HNs. The visible spectra show two LSPR peaks (nonlinear LSPR peak shift) confirming coreshell, rather than alloy structure. Fig. 11.27A (curve 1) describes the absorption spectrum of Al2O3@Au coreshell structure obtained by ablating Au plate in aluminum colloid for 40 min, which gets confirmed by the presence of plasmon peak of Au and AlAl2O3 at 532 and 260 nm,

(A)

(B) 1) Au in Al (40 min) 2) Ag in Au in Al (10 min) 3) 30 min 4) 45 min 5) 55 min 6) 65 min 7) 90 min

Absorption

7 2 5

1) rAg = 1 nm 2) rAg = 3 nm 3) rAg = 5 nm 4) rAg = 7 nm 5) rAg = 9 nm

Absorption

Al-Al2O3 NP LSPR

4 3

10 9 87 65 4 3 2 1

6) rAg = 11 nm 7) rAg = 13 nm 8) rAg = 15 nm 9) rAg = 17 nm 10) rAg = 19 nm

1

200

300

400 500 Wavelength(nm)

600

700

300

400

500 600 Wavelength(nm)

700

FIGURE 11.27 UVvisible spectra of AlAuAg trimetallic HNs: (A) experimental, and (B) calculated for 10 nm Au core radius with Ag shell thickness (119 nm). Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 116 (2014) 955967. Copyright 2014 Springer.

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11.5 TRIMETALLIC HYBRID NANOPARTICLES

(A)

(B) 70

No of Counts

60 Mean Size = 22 nm S.D = 10 nm

50 40 30 20 10 0

100nm

10

20 30 40 50 Particles Diameter (nm)

60

FIGURE 11.28 (A) TEM image and (B) size distribution of Al2O3@AuAg HNs in de-ionized water (inset shows complete coreshell structure of Al2O3@Au@Ag and arrow shows that NPs deposition over alumina surface). Source: Reprinted by permission from R. Singh, R.K. Soni, Appl. Phys. A 116 (2014) 955967. Copyright 2014 Springer.

respectively. There is a red shift of 12 nm in LSPR peak of gold because of the presence of high refractive alumina surface. Simultaneously, Al@Al2O3 peak blue shifts by 4 nm (from 264 to 260 nm), therefore the red shift of gold and blue shift of alumina LSPR confirms the interfacial communication at the defect sites of alumina by electron transfer. Consistent ablation of silver and gold in alumina broadens its LSPR peak as its plasmonic behavior is completely masked by the presence of silver and gold over it (Fig 11.27A, as marked with red circle). After ablating silver in the Al2O3@Au solution, the blue shift of Au LSPR peak with increasing concentration of silver has been observed due to damping of Au core plasma resonance (Fig. 11.27A, curve 27). At the initial ablation time of 1045 min a blue shift of the LSPR wavelength from 532 to 525 to 516 nm was observed (Fig. 11.27A, curve 24). High electron density and chemical potential of silver allows a flow of electrons from silver to gold and therefore the shift. The LSPR shift was subsequently followed by the growth of a second LSPR peak at B409 nm with a core peak at 500 nm (broad shoulder at around 500 nm). A major characteristic of these peaks is the significant shoulder observed at B500 nm, along with a second LSPR centered at B409 nm. Further, the red shift of LSPR at 409 nm to 416 nm (Fig. 11.27A, curve 57) with ablation time indicates the formation of Ag-rich nanostructure as shell. Further increasing the ablation time increases the shell thickness of silver shielding the gold LSPR at 500 nm. Further ablating silver resulted in single LSPR peak at B416 nm corresponding to silver nanoparticles only. At thicker shells only a single LSPR is observed because plasmon resonance of the core nanosphere is rapidly masked by that of the growing shell and after passing a stage where two plasmon resonances are present, the shell resonance dominates in an absorbance spectra.

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11. LASER-INDUCED HEATING SYNTHESIS OF HYBRID NANOPARTICLES

The absorption spectra show gradual damping of LSPR peak of both Au and Al@Al2O3 coreshell NPs with increasing thickness of silver shell. The LSPR peak at 260 nm which corresponds to Al@Al2O3 coreshell NPs gets broadened (Fig 11.27A, as marked with red circle) after ablating silver and gold in the existing solution (after deposition of silver and gold over alumina surface the plasmonic behavior is almost lost and LSPR peak is completely broadened). In the coreshell structure having two metals we can only shift the plasmonic peak between the characteristic peak of two metals but not beyond them. The absorption spectra of the coreshell can be easily distinguishable from the alloy structure because of the nonlinear shift of LSPR peak with concentration/ablation time. When the shell thickness is large, the LSPR peak of shell is seen, however for thin shell, hybridization effect starts playing a role, where surface plasmon of the sphere interacts with surface plasmon of the cavity to give two or more LSPR peaks with almost equal intensity (intensity of peak depends on the strength of interaction). In the colloidal mixture of Au and Ag NPs, absorbance for Au and Ag nanoparticles at 532 nm wavelength is 0.81 and 0.23, respectively [120]. Au NPs is more efficiently excited than Ag NPs while laser heating Au and Ag NPs of the same size with 532 nm wavelength. Thus, when silver is ablated by 532 nm wavelength, the silver NPs does not reach the melting temperature because of the difference in absorption of silver with the laser wavelength used and therefore the structure results in coreshell of Au@Ag over the surface of alumina rather than alloy. Fig. 11.28 shows the TEM image and size distribution of Al2O3@Au@Ag HNs prepared in water and the morphology mainly shows spherical coreshell HNs with an average size of 22 nm (inset in the TEM picture shows clear coreshell morphology of the nanoparticles). TEM picture also confirms the presence of deposited Ag and Au particles over the alumina surface (marked with arrow). However, the TEM image does not reveal a clear coreshell structure partly due to difficulty in image contrast at interface for similar metals. It may be pointed out that either or when shell grows in a different crystal orientation to the core, the interface can be distinguished in an HRTEM image [121]. The core shell structure provides an outer bright and a central dark contrast in the TEM image [32]. To understand the changes of the absorption spectra with the ablation time, we simulated the absorption spectrum of Au/Ag core/shell NP using Mie theory (Eq. (11.14)). Fig. 11.27B shows the calculated surface plasmon absorption spectrum of the Au/Ag core/shell NPs with the Au core diameter 10 nm and Ag shell thickness from 1 to 19 nm. There are two peaks: one is located at B510 nm, the other is located at B340 nm. The peak at 340 nm can be well interpreted using hybridization theory which is discussed by Singh and Soni [32]. As the Ag shell thickness increases, the long wavelength peak blue shifts from 510 to 490 nm, which is consistent with experimental results. The blue shift of the SPR absorption spectrum confirms that the Au/Ag core/ shell HNs are formed during Ag ablation. By comparing the measured blue shift of the long wavelength peak with results from the Mie calculations, we can obtain the thickness of the Ag shell as a function of the ablation time. Fig. 11.29 presents the HRTEM image of Al2O3@AuAg HNs in which some particles reveal coreshell morphology. Light silver clusters (shell) are clearly distinguished from dark gold particles (core) due to the different scattering power of the two metals as a consequence of the different surface electronic density [122].

I. FUNDAMENTALS

11.6 SUMMARY

233

FIGURE 11.29 HRTEM image of Al2O3@AuAg HNs prepared in de-ionized water, inset shows lattice fringes corresponding to FCC phase of Ag NPs (arrows indicate coreshell morphology). Source: Reprinted with permission from R. Singh, R.K. Soni, Appl. Phys. A 116 (2014) 955967. Copyright 2014 Springer.

11.6 SUMMARY We reported the synthesis of aluminum or alumina-based bimetallic and trimetallic hybrid nanoparticles Al2O3@Au, Au@Al2O3, Al2O3@Ag, Ag@Al2O3, and trimetallic Al2O3@AuAg and Al2O3@AgAu by nanosecond laser ablation in water. Different methodologies of PLAL like laser heating and two-step laser ablation method were adopted to synthesize these hybrid nanoparticles. Using laser heating method sintered and rattle-type Ag@Al2O3 HNs were synthesized. The results show that morphology changed with increasing post-irradiation time. With increasing post-irradiation time the morphology changed from sintered-type structure to spherical rattle-type of hybrid structure. The porous rattle formation is due to the Kirkendall voiding effect. The lower surface energy of alumina than silver resulted in Ag as core and alumina as shell. Unlike the laser heating method (post-irradiation) the thermodynamics and surface energy always allow aluminum oxide to take the shell position and silver occupies the core position because of their lower surface energy, therefore a two-step laser ablation method was adopted in order to invert the geometry of HNs. Using a two-step method the morphology was mainly deposition of nanoparticles (shell) over the surface of existing alumina (core) in the colloidal solution. The wavelength dependence of laser heating method resulted in successful preparation of Au@Al2O3 HNs because of the strong plasmonic absorption in gold at 532 nm. The role of laser wavelength and time of postirradiation on controlled morphological synthesis of HNs provided a novel method for synthesis of sintered structure, coreshell, rattle-type, and trimetallic HNs. We expect that our approach will be applicable to a wide range of inorganic solids, yielding new metastable solids.

I. FUNDAMENTALS

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C H A P T E R

12 Hyperthermia Treatments Geeta Nijhawan1, Siddharth Sagar Nijhawan2 and Minu Sethi1 1

Manav Rachna International University, Faridabad, Haryana, India 2Netaji Subhas Institute of Technology, Delhi University, New Delhi, India

12.1 OVERVIEW Cancer rates are increasing at an alarming rate worldwide and it is a major cause of concern for the medical fraternity. Based on a World Health Organization (WHO) study, it is expected that there would be about 70% increase in cancer cases in the next two decades [1]. At present, the various treatment options for cancer patients are chemotherapy, radiation, and removal of tumor through surgery. But, these treatments suffer from several drawbacks. Chemotherapy is quite effective in destroying cancerous cells but it causes serious side effects. It lowers the immunity and hence increases the risk of infection. It also affects the proper functioning of the digestive and nervous system. In radiotherapy, the tumor is destroyed by radiation energy. The radiation cannot eliminate the entire tumor effectively as it is difficult to target the whole tumor. There is also the risk of organs losing their functionality if they are located in the vicinity of the targeted radiation region. Surgery may be an option only if the exact tumor location is known. Surgery may increase the chances of getting infection and may be very painful sometimes. It is not suitable for deep-seated tumors. Clinicians are trying to find alternative therapies to treat the cancer in a noninvasive manner. Thermotherapy can prove to be a promising treatment method as heat can cause cancer cells death. Hyperthermia and thermal ablation are the two types of heating which are used in thermotherapy. In hyperthermia, the normal body temperature of 37 C of either whole body or some part of the body is increased to a temperature range of 4145 C. Hyperthermia is always used with either radiation and/or chemotherapy. The cancerous cells are more sensitive to heat and hence it can selectively kill cancerous cells

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00012-7

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without causing much damage to surrounding healthy tissues [2]. Thermal ablation is a technique to create intense local heat resulting in high temperature, i.e., above 45 C, which results in significant cell destruction. In addition to destroying tumorous cells it may have adverse effects on healthy tissues; therefore it requires to be dealt with great care. During the course of hyperthermia, the tumor cells are exposed to a temperature of about 42 C for a period of one hour or even longer duration [3]. The rate at which cell death occurs in hyperthermia is proportional to the temperature and duration of heat. The bimolecular system of cells gets changed due to applied heat and cell death occurs because of either apoptosis or necrosis. The biological rationale in accepting hyperthermia is the fact that the sensitivity of tumor cells to heat is greater than compared to normal cells. Hence, when the temperature of tumor is increased to 4145 C, the tumor cells get killed without causing damage to healthy cells in the surrounding area. The high glycolytic activities inside cancer cells make them highly acidic and as a result they become more sensitive to heat. The excess heat is removed by increasing the blood flow. This is called the thermoregulation system of the body. In response to the rise in temperature, the flow rate of blood in normal tissues increases more rapidly than in tumorous cells. Normally, it increases to about 20 times its preheating rate in healthy tissues, whereas it could just double in tumorous cells [4]. Thus, heat has a more damaging effect on tumorous cells than normal cells. Three kinds of hyperthermia are usually employed: whole-body, regional, and local hyperthermia. The treatment method depends on the organ affected by cancer, the stage or extent to which cancer has developed, and the mode by which energy can be delivered to the patient. When the metastatic stage has developed in a patient and the cancer has spread throughout the body, then the patient has to undergo whole-body hyperthermia. There are several techniques which are used to raise the body temperature to the 4145 C, which include hot water blankets and thermal chambers. Regional hyperthermia is recommended for tumors which are large, deeply seated, and cannot be operated easily. Cancers of cervical and bladder are treated by this technique. In regional hyperthermia some part of the body is treated. It may be an organ, limb, or a cavity (a hollow space) within the body. The available techniques for regional hyperthermia are regional perfusion technique and continuous hyperthermic peritoneal perfusion (CHPP) technique. In regional perfusion technique, the blood from the patient’s body is taken out, heated, and again pumped back into the effected organ. The cancers within the peritoneal cavity (the space within the abdomen that contains the intestines, stomach, and liver) and also primary peritoneal mesothelioma and stomach cancer are treated by CHPP technique [5]. Both whole-body and regional hyperthermia techniques result in poor tumor specificity. The whole-body hyperthermia may cause gastrointestinal symptoms, such as diarrhea, nausea, and vomiting. Sometimes, it may result in serious side effects, including cardiac and related vascular disorders. Regional hyperthermia is invasive and the setup required is somewhat challenging. Local hyperthermia is used for treatment of primary malignant tumors before the metastases stage. RF, microwave, or ultrasound sources are usually used to deliver energy to heat the tumor cells. The following approaches are used for local hyperthermia: • External/superficial: Usually used for the treatment of tumor below the skin. Energy is delivered with the help of external applicator.

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• Intraluminal: Tumors which lie within or near body cavities (e.g., rectum and esophagus) are treated. Radioactive probes are placed inside the body cavity. • Interstitial: It can treat tumors deep within the body (e.g., brain tumors). Probes or needles are placed into the tumor to deliver energy with the use of anesthesia. In summary, despite the fact that hyperthermia has become an important treatment modality in the treatment of cancer, it is often invasive and lacks specificity. This has inspired researchers to explore nanoparticles for generating hyperthermia. Tumors can be loaded with systematically administered nanoparticles to maximize the energy deposited in the tumor. The remaining sections provide an insight to some of the significant concepts involved in the technical development of hyperthermia.

12.2 PHYSICAL FUNDAMENTALS For the past many years heat therapy has been used to treat many diseases, including cancers. In modern oncology, hyperthermia is given in conjunction with radiotherapy and chemotherapy. The implementation of available hyperthermia techniques is difficult as some of them may require surgery. Also, they are not universally effective as they are cancer-specific and depend on tumor position and stage of development. Nanoparticles promise to be a novel approach to treat cancer. In 1957, Gilchrist et al. proved that by inductively heating the magnetic particles administered into the lymph nodes, the lymphatic metastases can be destroyed [6]. In his experiments, 5 mg of Fe2O3 particles having 20100 nm diameters were delivered to lymph nodes. The temperature increased by 14 C, when an alternating magnetic field (AMF) of strength in the range 200240 Oe (1619.2 kA/m) at 1.2 MHz was applied. But it was expected that the temperature rise occurred due to the induced electric field because of the high frequency used and not because of heat produced by nanoparticle. Thereafter, various techniques were tried to deliver heat for treatment of cancer [7,8] but these modalities could not achieve many encouraging results. In the mid-1990s, the use of nanoparticles for hyperthermia treatment again gained momentum [9]. Magnetic nanoparticles were directly injected into tumors and then these nanoparticles were excited with external magnetic fields to produce heat. Significant advancements have been made in the past few years and nanoparticles can be directed to tumors loaded with targeting agents such as antibodies [10,11]. Currently, the nanoparticle heating techniques available for diagnostic and therapeutic purposes are: (1) optical heating by lasers; (2) heating of small bubbles using ultrasound; and (3) magnetic heating through AMF [12,13]. The optical method has proved to be very effective in heating particles, but it has a disadvantage in that the laser light gets severely limited by the attenuation by tissue. In the case of ultrasound heating, the energy can be easily focused to a specific location but speed of sound varies in tissues and also the applicator’s limited aperture may pose problems. Magnetic particle heating can be achieved for treatment of tumors which are located anywhere and at any depth in the human body. These particles not only provide heat but also can be used for medical imaging.

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Nanoparticle hyperthermia would prove to be an effective technique if (1) the concentration of particles is adequately high in the tumor and also relatively higher than in adjacent healthy tissue, and (2) the specific absorption rate (SAR, in W/g) of particles is high enough. Mathematically, SAR is given by Eq. (12.1), SAR 5 c 

ΔT ; Δt

(12.1)

where c is called the specific heat capacity, and Δt is the time interval during which the temperature rises by ΔT to provide significant amount of heat within the tumor with magnetic field strengths that are within the tolerable limits of the normal tissues. It is also called specific loss power (SLP, or heat dissipation per unit mass of magnetic material). The important factors that determine the extent of damage caused by heat are tissue sensitivity, temperature, and exposure time. Thermotolerance also affects the rate of cell killing. It has been proved that the rate of cell death increases exponentially with a rise in temperature over a limited temperature range (4055 C) [14]. Although sensitivity to heat depends upon species and varies across different tissues and organs, around 43 C, a breakpoint in the rate of cell death was detected in cell culture and it has been generalized as a part of the calculation of thermal dose. Cumulative Equivalent Minutes (CEMs) is a technique that determines and normalizes the total thermal dose (thermal history) of a heated tissue. The CEM is defined by Eq. (12.2), CEM43 C 5 t  Rð432TÞ ; 

(12.2) 

where CEM43 C is the heat dose normalized to number of minutes at 43 C, T represents the average temperature during the time interval, t (in minutes). R represents time in minutes that is required for a 1 C change in temperature with reference to 43 C. R is taken as 0.25 for temperatures below 43 C and 0.45 when temperatures are above 43 C.The resulting CEM43 C value represents the affect of the entire history of heat exposure on cell death. Electromagnetic radiation, such as RF, microwaves, and lasers, are used for various forms of thermal therapies as they can strongly interact with tissues. But, they get highly attenuated in surface layers and are not advisable for treatment of deeply located tumors. It has been established that magnetic fields having frequency below 10 MHz do not get attenuated in tissues, and hence are capable of uniformly penetrating deep in tissue areas. Dielectric properties are very important for the study of the microwave hyperthermia heating process in the biological tissues. The penetration of energy in the human tissues is determined by the dielectric properties of the tissue and the frequency of the radiation. The interaction of the high frequency fields with the biological tissues is determined to a large extent by the characteristics of the tissue: permittivity (ε), permeability (μ), and conductivity (σ). Table 12.1 lists the electrical properties for biological tissues at 915 MHz. It gives an idea of the range of conductivity values. The permittivity of biological tissues is determined by water and electrolyte contents. Thus, tissues such as blood and muscle, which have higher water content than tissues such as fat, have higher dielectric constants and conductivities.

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TABLE 12.1 Electrical Properties of Human Tissues Human Tissue

Conductivity (S/m)

Relative Permittivity

Muscle

0.94809

54.997

Bone cancello

0.34353

20.756

Bone marrow

0.040588

Skin

0.87169

41.329

Blood

1.5445

61.314

Heart

1.2378

59.796

Fat

0.051402

Nerve

0.57759

5.5014

5.4596 32.486

12.2.1 Physical Fundamentals of Magnetic-Induced Heating The use of nanotechnology for cancer treatment has advanced at a great pace in the last two decades. Magnetic nanoparticles are preferred due to following three reasons: - Human body has no magnetic materials. - Human body is transparent to the magnetic field. - When a magnetic field is applied to the magnetic materials, they release heat at a particular amplitude and frequency conditions. The magnetic nanoparticles are used for different applications in the biomedical sector [1518]. In the year 1957, Gilchrist et al. studied the use of magnetic fluid hyperthermia (MFH) as a cancer therapy on animal model [6]. It was used for the treatment of lymph nodes. The use of MFH generates enough power so that it can completely eliminate a tumor [1921]. It can successfully solve the hotspots and targeting issues that may occur in other hyperthermia techniques. Using nanoparticles, the energy deposition in the tumors can be better controlled by heating only the targeted particles [22]. The surface to volume ratio of magnetic nanoparticles is high which allows particle surface to be functionalized with cancer-targeting molecules. The performance of nanoparticles can be improved by coating them with suitable materials [23]. The magnetic particle imaging can be used to guide the targeting process [24,25]. The tumors which are inside the vital organs and hence for which surgery is not possible can be treated by this method. Different approaches used to deliver magnetic nanoparticles to the tumor site are the arterial injection or direct injection into the tumor region, the in situ implant formation with the help of entrapped nanoparticle gel, and the active targeting of nanoparticles by coating with the targeting ligands [26]. The magnetic nanoparticles can be inductively heated by an alternating magnetic field. The cancerous cells show more sensitivity to heat and hence the generated heat by AMF results in the death of cancerous cells. The principle of the energy conversion process is based on hysteresis loss or the Ne´elian and Brownian

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relaxation (explained later). The main criterion for design of nanoparticle is that the power deposition in a tumor which produces localized hyperthermia is to be maximized. The heating of nanoparticles using an AMF results in direct cytotoxicity, or sensitizes the tumor to radiation treatment or chemotherapy. It is required to maximize the SAR of magnetic nanoparticles in order to optimize the heating. The following five factors determine the magnetic properties of nanoparticles: Physical dimensions of the nanoparticles; Intraparticle magnetic interactions; Magnetic interactions between two particles also called interparticle interactions; Magnetic interactions that takes place between the nanoparticles and the matrix material; - Particle and applied magnetic field interactions. -

The other factors which affect treatment outcome of MFH are described in detail in the next paragraphs [27]. These factors need to be optimized so as to get the optimum heating for the treatment [28]. 12.2.1.1 Type of Material The heating capability is not the same for different types of material. Iron oxide (maghemite or magnetite) are immuno-evasive materials. The surrounding healthy tissue may absorb some of the nanoparticles. Hence, in order to avoid unwanted immune response it is required that particles have immuno-evasive behavior. Also, iron oxide has high SAR to induce MFH. Therefore, it is the most frequently used nanoparticle in MFH [29]. Researchers have explored several other materials with better heating properties. The ironcobalt nanoparticles have a high saturation magnetization and therefore exhibit significantly high specific loss power as compared to ferrite coreshell nanoparticles. Hence, ironcobalt is a better heating nanoparticle. This reduces the concentration levels of nanoparticles required to be injected due to higher power dissipation. However, cobalt requires coating using biomaterials to reduce its toxicity. The comparative study of magnetite, maghemite, ironcobalt, ironplatinum, ferrites of barium and cobalt [30] for magnetic hyperthermia has been carried out in various literatures. Bariumferrite and cobaltferrite change with temperature slowly whereas ironcobalt causes a large rate of temperature change. Hence, these nanoparticles are not suitable for hyperthermia. On the other hand, the heating powers of magnetite, ironplatinum, and maghemite are quite high and hence they are preferred for MFH. Also, ironplatinum nanoparticles can act as high-performance nanoheaters because they have high saturation magnetization and are chemically stable. The high heating rate reduces the amount of ironplatinum nanoparticles required to generate the same quantity of heat as produced by iron oxide nanoparticles. Also, high heating rate can be obtained by using smaller diameter of ironplatinum nanoparticles as compared to the magnetite. The smaller the diameter the greater will be the surface to volume ratio. This increases the selectivity of ironplatinum nanoparticles in targeting the tumor and functionalization can be done on the particle surface. But the toxicity of ironplatinum materials has to be investigated before they can be safely used for injection in the human body.

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12.2.1.2 Particle Size and Concentration The amount of heat generated by nanoparticles is dependent on the size and concentration of the nanoparticles. The particle size influences SAR outcome significantly [31]. A lot of work has been done to find the most suitable size of nanoparticles so as to get the desired heating effect [32,33]. The frequency and strength of the magnetic field determines the optimum particle size used for the treatment. Particle size is also dependent on specific loss power and also on other parameters such as anisotropy [34]. The concentration of nanoparticles directly affects the quantity of heat generated in MFH. The increase in concentration has no affect on SAR generated by each nanoparticle. The rise in temperature is because the number of heat sources increases due to increase in concentration. It was found that in an experiment using the collagen, as the concentration of particles is doubled, the rise in temperature also doubled and the toxicity effect on cancer cells also increases. 12.2.1.3 Anisotropy The heat generation process is dependent on the magnetic property of the material. The heat generated increases as the anisotropy of nanoparticles or the applied magnetic field strength used for treatment is increased. Anisotropy depends on crystal structure and shape. The size at which the nanoparticle generates maximum heat decreases as the anisotropy constant increases [35]. The way nanoparticles are synthesized, determines the anisotropy constant of the nanoparticles. Coating the nanoparticles also changes their anisotropy constant. 12.2.1.4 Viscosity Viscosity of the medium has a direct impact on the amount of heat generated. The heating rate also depends on carrier viscosity. When the specific loss power of monodispersed particles of maghemite and cobalt ferrite were studied in three different solvents, namely water, collagen, and glycerol, it was found that for the same concentration and magnetic field conditions, the maximum rise in temperature occurs in water. The temperature rise was moderate in glycerol and lowest in collagen as its high viscosity reduces the Brownian relaxation effect. 12.2.1.5 Magnetic Field Strength and Frequency The changes in the strength of magnetic field and its frequency affect the amount of heat generated by superparamagnetic nanomaterials. The magnetic field less than 15 kA/m is usually used for MFH. The increase in strength of magnetic field raises the temperature of the tumor cells which in turn reduces the treatment duration. SAR varies linearly with magnetic field strength and frequency [36]. Usually magnetic field frequency lies in the range of 0.051.2 MHz in magnetic hyperthermia. Although a large amount of heat can be produced if the magnetic field strength and frequency is increased, it may have adverse effects on human body. Hence, a tradeoff between applied magnetic field and heat generated is required. More extensive research is required to be carried out to determine the maximum allowable limits that can be applied to different parts of the human body.

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12.2.2 Physical Fundamentals of Photo-Induced Heating Light was used in medicine for the first time in the 19th century. This was possible because of the better understanding of both the physical nature of light and fundamental lightmatter interactions. The advancements in laser technology have provided many new treatment options and many therapeutic and diagnostic devices are frequently used [37]. The process of destroying cancer cells by utilizing heat generated by nanoparticles induced by infrared light has shown great promise as a treatment option, along with surgery, radiation therapy, and chemotherapy. The processes include photodynamic therapy and photothermal therapy. Photoinduced therapies are less invasive and highly localized. This destroys cells at tumor sites without affecting the adjacent cells [38]. Unlike the case with magnetically induced hyperthermia, prior knowledge of biodistribution of nanoparticles is required in the photothermal technique so that the lasers can be properly focused on the nanoparticles. Photons interact with biological matter in tissues by means of different processes. These are basically classified as absorption and scattering. The scattering causes change in the propagation path due to nonuniformities in the medium. It also changes polarization and spectrum of scattered light. The analysis of scattered light can be used for diagnostic and imaging purposes. The absorbed energy is converted to electronic or vibrational energy in photons. The reemission of this energy from the tissue may occur through luminescence, inelastic scattering, or acoustomechanical waves. The reemitted wave provides the information about its microstructure and molecular contents. It also helps in optical diagnostics and imaging. On the contrary, the excitation of intrinsic molecules by light or extrinsic light-sensitive agents upon introduction in the body gives rise to various effects on the tissue and cells within it through the generation of heat (photothermal), chemical reactions (photochemical), and biological processes. These effects are used in a controlled manner in optical therapy and laser surgery. The absorption properties due to the phenomenon of surface plasmon resonance (SPR) associated with metal nanoparticles is being explored. This mainly happens in gold nanoparticles. They can convert laser light into heat. Two major types are nanoshells and nanorods. They are very efficient in generating heat upon illumination and exhibit inertness, stability, and get readily associated with biomolecules for specific targeting of tumors. At a particular frequency of light, the electrons on the gold nanoparticle surface collectively oscillate, which results in surface plasmon resonance. Both absorption and scattering of light occurs simultaneously. The frequency of light at which this takes place is dependent on the physical dimensions, surface, and buildup of gold nanoparticles. However, for most cancers, it is required that the light passes through a tissue whose thickness may lie between a few centimeters. The nanoparticles are designed so that absorption occurs in the near-infrared, i.e., in the range 650900 nm. At this wavelength, certain tissues are fully transparent over a few centimeters. It is possible to shift the SPR absorption band into this range by changing the size, shape, and chemical composition of the particles. Optical parameters, which include wavelength λ, exposure time τ, beam area A, energy E, power P 5 E/τ, intensity P/A, and fluence E/A, are to be optimally selected for different diagnostic and therapeutic applications of light.

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Scattering of Light and Penetration Depths The scattering of light occurs when an incident light ray which is electromagnetic (EM) in nature changes its path when it encounters an obstacle in the scattering material. Scattering of light can be elastic or inelastic. The reason for elastic light scattering is electric polarization density is inhomogeneous in tissues, whereas inelastic light scattering occurs due to time-varying scatters, hypersonic waves, which are produced thermodynamically, and molecular vibrations. Rayleigh scattering and Mie scattering are forms of elastic light scattering. Inelastic scattering includes Brillouin scattering, Raman scattering, and inelastic X-ray scattering. The magnitude of Rayleigh scattering from atoms and molecules varies inversely with λ, and Mie-type scattering occurs if the scatter’s dimensions are larger than the wavelength of the incident electromagnetic radiation. It is almost independent of λ. Spontaneous inelastic scattering has the scattering coefficients several times lower than that for elastic scattering in magnitude. Scattering and absorption determines the depth of penetration of light into tissue. The effective penetration depth in skin tissues at which the incident optical energy decreases to 1/e, i.e., 37%, is normally in the range of 50100 μm for UV and blue light (λ 5 400450 nm). The main limiting factor is light absorption by water. The penetration depth is a few hundreds of micrometers for green light (λ 5 500550 nm), because of high light absorption by melanin and hemoglobin. The penetration is maximum for red and near-infrared light (λ 5 6001350 nm). It is typically 13 mm. Light absorption by tissue increases its local temperature. During hyperthermia the temperature rises to 42.543.0 C, and tissue ablation occurs at higher temperatures (100 C). The factors which determine the amount of energy required are the desired temperature, penetration depth of light, and target tissue volume. Without heat dissipation, water has to absorb 4.18 J of energy (heat capacity) to raise the temperature of 1 cm3 of tissue by 1 C, and approx. 2.7 kJ of heat (latent heat of water) is needed for vaporization of water in the tissue. Normally, in order to avoid heat diffusion and to reduce adjacent tissue damage pulsed irradiation is used. Pulse widths of 1100 ms are adequate for ablation of large tissue volumes; micro- and nanosecond pulses are needed for high-precision ablation. A molecule gets excited when it absorbs a photon. It interacts with adjacent molecules to produce photochemical effects. These include creation of reactive radicals (ROS) and singlet oxygen. Several photobiological effects, such as the destruction of enzymes in cellular signaling pathways, the opening of ion channels, and the promotion of specific gene expression also occurs. The number of photons needed is very large for this process and it is given by A/σa, where σa is the absorption cross-section of molecule and A is the beam area. For photoexcitation, the optical energy required is greater in tissues as compared to in vitro, so as to counteract for optical loss. The efficiency of a photochemical reaction is usually very less. Hence, to enhance the photochemical effect, it is required to excite a molecule multiple times. The optimal dose for biomedical applications is usually more than 1 J/cm2. Low optical powers are necessary for safety in diagnostic applications. Maximum permissible exposure gives the requirement for optical radiation. It is defined as one tenth of the damage threshold resulting from photothermal and photochemical effects. Light gives photon energy in the range of 0.53 eV in the near-infrared (NIR) region. These energies

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do not cause any harm to the human body. At higher energies, bond dissociation and ionization can occur. Similarly, at lower energies, water absorption dominates and thereby prevents any specific targeting of molecules. [37]

12.2.3 Physical Models of Magnetic-Induced Heating In magnetic hyperthermia, electromagnetic energy is converted into heat when AMF is applied to magnetic nanoparticles. The three mechanisms which cause heating in magnetic particles are: (1) eddy currents losses in materials which have high electrical conductivity; (2) frictional heating in an anisotropic magnetic particle; and (3) hysteresis effect. Eddy currents losses are the most common heating mechanism which occurs in bulk materials. A magnetic nanoparticle produces in addition to a magnetic field, an electric field. Due to this eddy currents are induced, which generate heat in normal tissues. The strength of the eddy current is directly proportional to the strength of AMF and the size of the tissue. In magnetic nanoparticles, the heat produced by eddy currents is not appreciable because the electrical conductivity of nanoparticles is poor. The effect of frictional forces is still not fully established. A magnetically anisotropic particle shows automatic rotation when an AMF is applied in the case of low viscosity liquid [39], which causes frictional heating of the surrounding medium. This is referred to as Brownian losses. For an immobilized particle which is bound to a cell, heating will not occur. For micro- or nanoscale particles, hysteresis loss heating is quite significant. Hysteresis heating is dependent on the physical dimensions and coercivity of the particles. The strength and frequency of the magnetic field also affects hysteresis heating. 12.2.3.1 Hysteresis Heating Different magnetic materials behave differently in magnetic fields and hence some magnetic nanoparticles heat better than others. Three basic magnetisms are: • Diamagnetism: It exists only when an external field is applied. It is very weak and nonpermanent. • Paramagnetism: Becomes slightly magnetized when a magnetic field is applied, and loses the magnetic property in the absence of field. • Ferromagnetism: Two types are antiferromagnetism and ferrimagnetism. Fig. 12.1 shows the response of these materials to a magnetic field by a plot showing their magnetizations against applied magnetic fields. When the applied magnetic field (H) is absent, the remnant magnetization (M) in paramagnetic materials is zero. When H is increased, the dipoles within the material start aligning with the field and the magnetization increases. M attains its maximum value when all the dipoles have completely aligned with the applied magnetic field and it is termed the saturation magnetization (MS). M does not increase beyond MS with increasing H. Ferromagnetic materials have magnetic “memory.” Above Curie temperature (TC 5 770 C for iron), the dipoles become randomly aligned within the material and ferromagnetic materials behave as paramagnets. Below TC, the dipoles get moderately aligned and the net magnetization of the material becomes zero. The magnitude of H decreases

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B Ms Superparamagnet

MR Ferromagnet

Paramagnet

H HC

FIGURE 12.1 Hysteresis curves.

when the direction of the magnetic field is reversed. At H 5 0, the value of M within the ferromagnet does not become zero and it is known as remnant magnetization (MR). On increasing H in negative direction, the magnitude of M decreases. The absolute value of this H at which M becomes zero is called the coercivity (HC) or the residual magnetism of the material. The area under the hysteresis curve is defined by MS, HC, and MR. The area gives the quantity of heat dissipated when H varies from H (MS) to H (2MS) and back. Hence, the total energy lost as heat is equal to the frequency multiplied by the integral of the BH curve over a closed loop. It has been established that when a varying magnetic field is applied to a ferromagnetic material, energy will be dissipated as heat and the SAR will be high if the coercivity of the nanoparticles is high [40]. 12.2.3.2 Heat Generation Model Based on Ne´elian and Brownian Relaxation Supermagnetism occurs in ferromagnetic or ferrimagnetic nanoparticles. When a magnetic moment is placed into a magnetic field, the time for which it relaxes into its equilibrium state is termed as magnetic relaxation. In small nanoparticles, the direction of magnetization gets changed randomly due to thermal fluctuations. There is no remnant magnetization when there is no applied field. Hence, a ferro- or ferrimagnetic material exhibits magnetism only in the presence of an applied field. This happens because below a critical volume, the anisotropic energy barrier (KuVm) of the magnetic material is so much reduced that it can be overcome by the energy generated by random thermal motion

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(KBT). By assuming a spherical geometry (Vm 5 ð4πr3c =3Þ) for superparamagnetic behavior the approximate critical radius (rc) can be determined. The equation which gives the probability of thermal relaxation can be modified as:       Ʈm Ku Vm 23 Ʈ KB T 1=3 ln 0 5 exp -rc 5 4π Ʈ0 KB T Ʈm K u

(12.3)

where Ʈm is the average time taken by the nanoparticle’s magnetization to randomly change its direction. Ʈ0 is the attempt time (generally taken to be 10210 s), Vm represents the volume of magnetic material, KB is Boltzmann’s constant, and T denotes the absolute temperature. Superparamagnetic particles do not have remnant magnetization and hence hysteresis behavior is not present. The energy dissipation occurs due to moment relaxation mechanisms. The physical mechanism of relaxation which leads to energy dissipation in superparamagnetic iron oxide nanoparticles was studied by Rosensweig [41]. The losses can be broadly classified as Brownian and Ne´elian. The Brownian motion represents the rotational motion in a given medium. The complete particle tries to oscillate in the direction of the field; the rotational motion is opposed by the suspending medium, thereby generating heat. In Ne´elian motion, the individual magnetic moments move towards the alternating field whereas the particle remains fixed (Fig. 12.2). When an alternating magnetic field is applied, the magnetic moment moves in the direction opposite to the crystal axis so as to reduce its potential energy. The remaining energy is dissipated as heat. In Brownian relaxation, the actual physical rotation of the particle itself is within the carrier media it is suspended in via an external AMF. The Brownian relaxation time

FIGURE 12.2

(A) Ne´el rotation, (B) Brownian rotation.

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constant (τ b ) is defined as the time taken by a magnetic nanoparticle to align itself with the applied magnetic field and is given by: τb 5

3ηVH Kb T

(12.4)

where η represents viscosity and the hydrodynamic volume of the particle including coatings is given by VH. Neel relaxation mechanism occurs due to the rapid changes in direction of magnetic moments within the particles thus leading to rotation within the particle itself leading to heat loss. The relaxation time is given by: τn 5

pffiffiffi eΓ Ku Vm πτ 0 pffiffiffiffi ; Γ 5 Kb T Γ

(12.5)

where Γ is the ratio of anisotropy energy to thermal energy. Since, Neelian and Brownian processes occur in parallel, the effective relaxation time (τ) gives the complete description of the process: τbτn τ5 (12.6) τb 1 τn This equation is the same as that of equivalent resistance of electronic resistors connected in parallel and that the overall behavior is dominated by a shorter time constant. A high SAR can be achieved if we can distinguish between the quantities of heat produced by each mechanism. Ne´elian relaxation time varies with particle volume and magnetic anisotropy. On the contrary, Brownian relaxation time is dependent on properties of the solvent, i.e., its density and viscosity. 12.2.3.3 Bioheat Transfer Model for Heat Distribution The Pennes’ model can be used to describe bioheat transfer in a tissue. It takes into account the convective flow effects of blood perfusion and conduction of heat between two adjoining nodes. The bioheat transfer equation can be solved, if we have the complete knowledge of anatomy of all the vascular components in the region, i.e., diameter, lengths, and positions of all the arteries, arterioles, capillaries, veins, and venules, and the velocity field within each such vessel. This is a cumbersome task involving huge computations. The assumption that blood flow is a scalar and not a vector field helps in simplification of the modeling process. This was described by Pennes in the year 1948 for the first time [42]. He performed experiments on human forearm. Pennes combined the complete perfusion information to make a single blood perfusion term. The correctness of this assumption was checked when experimentally recorded temperature was compared with the predicted values in the human forearm. The perfusion term is determined by the difference between the arterial temperature “Ta” (in  C) and the temperature measured at a given point. The assumption made was that “Ta” is constant throughout the tissue while the vein temperature is equal to the temperature of tissue denoted by “T” at a particular location. Hence, a term Wb Cb (TTa) which represented the blood perfusion was added. The blood flow is a pulsatile flow of two-phase blood in flexible tubes. This situation is

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similar to turbulent flow problems in engineering for which complete velocity profile is not known and various approximations are used. Blood enters through arteries and flows to the tissue cells through blood capillaries. Pennes stated that exchange of energy takes place between blood vessels and the nearby tissues through the wall of capillaries. The velocity or blood flow rate is inversely proportional to the total cross-sectional area of the blood vessels. Hence, the blood flow velocity decreases if the effective cross-sectional area of the vessels increases. The blood velocity is very low in capillaries. Pennes’ bioheat transfer equation (PBHTE) does not take into account the effect of large blood vessels. The 1D bioheat transfer equation gives satisfactory results when the direction of heat propagation is in a perpendicular direction to the skin surface and is described by Eq. (12.7): ρCp

δT 5 rðKrTÞ 2 Wb Cb ðT 2 Ta Þ 1 qp 1 qm δt

(12.7)

where Cb and Cp represents the specific heat of blood and tissue in J/kg/C, respectively, blood perfusion rate is denoted by Wb (kg/m3/s), K (W/m/C) represents the thermal conductivity of tissue, ρ (kg/m3) indicates the tissue density, qp is the rate at which energy deposition takes place, and qm is the metabolism rate in (W/m3). qm is almost negligible as compared to the term qp. When considering heating processes, the term WbCb(T 2 Ta), represents the dominant form of energy removal. Hence, the blood enters the control volume at an arterial temperature Ta, and attains equilibrium state at the tissue temperature, T. The blood carries away the energy when it leaves the control volume. Hence, blood acts as an energy source in hyperthermia treatment. Pennes’ equation is based on approximations and it does not have a theoretical basis but still it helps in predicting temperature fields correctly in many applications. However, the equation has some limitations. Several physical effects are not handled by PBHTE. Also, it considers blood flow as a scalar component and thereby ignores the direction of blood flow, hence convective heat transfer mechanisms are not described by it. Many modifications have been suggested by various researchers to overcome the shortcomings. The blood flow direction was taken into account by Wulff [42a] and Klinger (1974) [42d] by considering the local blood mass flux. Further modifications were suggested by Chen and Holmes [42b] and how the blood temperature is effected by thermal equilibration length was examined and the dispersion and microcirculatory perfusion terms were added to the Klinger equation [42c].

12.3 MAGNETIC-INDUCED THERMAL CANCER THERAPY The magnetic nanoparticles can be activated with the help of an alternating magnetic field (AMF) and this technique is being used for targeted therapeutic and diagnostic heating of tumors. Various types of magnetic nanoparticles with different coatings and targeting agents can be used for hyperthermia treatment. They allow localized treatment to cancer cells. Magnetic nanoparticle hyperthermia has been effective in treating cancer in addition to conventional chemotherapy and radiation therapy [43]. Different approaches have been used to apply hyperthermia in tumor regions [44], but most of them may cause harmful side effects in the healthy tissues. The techniques using laser, ionizing radiation, and microwaves can successfully increase the temperature

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resulting in the cellular death, but they can cause ionization of the genetic material. The lack of selectivity in radiation and microwave techniques may affect the surrounding healthy tissues. These drawbacks are addressed by magnetic hyperthermia through which the temperature of damaged areas can be increased without affecting the healthy tissues. The basic principle of magnetic hyperthermia is that it allows remotely induced local heat generation by means of the energy losses in magnetic nanoparticles when an oscillating magnetic field is applied. Some magnetic nanoparticles can change the electromagnetic energy into heat, resulting in temperature increase in tumor regions in the human body. The activation of these nanoparticles as nanoheaters can be controlled externally by means of an alternating magnetic field. The electromagnetic radiation is in the range of the radiofrequency extending from KHz and MHz. It is completely safe and penetration depth is quite good so that inner organs or tissues in the body can be accessed. The tumoral cells are more sensitive than normal tissues to temperature increases above 42 C. At such high temperatures the natural enzymatic processes that keep the cells alive are destroyed, thus resulting in killing of tumoral cells. A superparamagnetic behavior of magnetic nanoparticles is preferred as they lose magnetization in the absence of the applied magnetic field. Since there are no coercive forces or remaining field, the interparticle magnetic dipolar interactions can be prevented. Their amassment could result in serious adverse effects by forming clots in the blood circulation system. It is required that the saturation magnetization should be high for the efficient heating of the nanoparticles. It is related to the particle size and distribution of the nanoparticles. As the particle size increases the saturation magnetization values also increases. But magnetic nanoparticles become ferromagnetic (superparamagnetic limit) if the size exceeds a critical size. Hence, a trade off is required between the nanoparticles size, distribution, and their magnetic properties. It is observed that very small nanoparticles cannot show a hyperthermia effect, whereas large nanoparticles are not capable of crossing the endothelial barrier through the continuous capillaries. Hence, only the external tumor cells would be affected by the generated heat whereas the inner cells would experience less heat which is not enough to cause cell death. On the contrary, small magnetic nanoparticles would cross the blood barrier easily and could penetrate and distribute more homogeneously inside the tumor. This may result in the death of large number of neighboring tumor cells due to heat diffusion. Magnetic nanomaterials of different types ranging from iron oxide-based nanomaterials to nanoparticles of metals, such as manganese, iron, cobalt, zinc, nickel, and their oxides are being explored for their use in hypothermia treatment [45]. Magnetite (Fe3O4) and maghemite nanoparticles (γ-Fe2O3) [46] stabilized by ligands such as dextran, cationic liposomes [47], polyvinyl alcohol, hydro-gel, and lauric acid are used as hypothermic agents. Ferrites of cobalt (CoFe2O4), manganese (MnFe2O4), nickel (NiFe2O4), lithium iron (Li0.5Fe2.5O4), and mixed ferrites such as cobaltnickle and nickelzinccopper are commonly used [48]. Ferromagnetic nanoparticles which include Fe-doped Au, ZnMn- and MnZnGd-doped iron oxide composites have also been investigated for hyperthermia. FeCo metallic nanoparticles are capable of providing very high heating performance in the range of 13001600 W/g [49]. Superparamagnetic iron oxide nanoparticles (SPIONs) have also been used for hyperthermia applications. They heat up rapidly when an alternating magnetic field is applied. Rate of heating is dependent on the frequency and magnetic field strength, concentration of particles, and the depth of the tumor within the body. Iron

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oxide-based magnetic nanoparticles are preferred because they lack toxicity and are biocompatible. It has been observed in preclinical tests that as compared to cobalt nanoparticles, magnetite has higher Curie temperature, saturation magnetization (MS) (9098 emu/g, or B450500 emu/cm3) and lower toxicity. Deger et al. tested hyperthermia in conjunction with 3D conformal radiotherapy for prostate cancer. Using CoPd thermo seeds the intraprostatic temperatures of about 4246 C were achieved with little side effects [50]. The coating of nanoparticles with suitable materials enhances their properties and makes them biocompatible, lowers toxicity, and helps in easy evasion from the renal filtration. Selective heating of nanoparticles is required through focused AMF so that nanoparticles that collect in somewhat sensitive organs like the liver and kidney remain unaffected. It is desirable that nanoparticles absorb enough power to attain cytolytic tumor temperatures (CEM43 C of greater than 60), without causing much heat in the healthy tissues. The real-time monitoring of temperature rise in the tumor is required during the hyperthermia treatment to take care of blood flow changes and physiological effects to heating. The efficacy of treatment is to be properly analyzed to optimize the factors such as dose and administration technique of nanoparticles, temperature and time of treatment. More specific drug targeting and delivery is required for the treatment of nonlocalized tumors (metastases). Surface functionalization of the nanoparticles is another important parameter which needs to be addressed. Surface functionalization is done to avoid deterioration of physicochemical properties of nanoparticles in the medium. Nanoparticles are required to be attached to ligands which increase cellular binding. Such increased adherence helps in increasing the effectiveness of the drug given in nanoparticles. If certain proteins such as antibodies are attached to the nanoparticle surface, it helps in a more specific targeting of the particles. Therefore, the success of using magnetic nanoparticles in cancer therapy depends mainly on how effectively they can be delivered to the target of interest. There are many techniques of surface functionalization using different coating agents and biomolecules like polymers, viruses, antibodies, aptamers, etc. Magnetic nanoparticles can be administered through intravenous injection and they could be guided to the target area by the strong magnetic forces generated by a permanent magnet. In order to preserve the chemicalphysical properties of the nanoparticles, a suitable particle surface modification or coating can be done to improve their biocompatibility and decrease as much as possible their toxicity. For hyperthermia therapy applications, it is required that nanoparticles reach their target and stay in place long enough to allow a continued treatment. Research is being done on several multifunctional nanostructures for biomedical applications. Coreshell nanostructures have been suggested. The shell protects the chemical and physical properties of the core and also a more favorable surface is provided to the nanoparticle which can be further modified with organic and inorganic functional molecules and compounds. Coreshell nanomaterials can offer combinations of different properties because they can have compositions of different materials in a single particle. They offer multifunctionality. A shell can provide a protective covering for sensitive core material, for example, magnetic nanomaterials like cobalt can be protected from air sensitivity. By the shell’s dimensions and its composition, the properties of the core can be changed and vice versa. A toxic core material, for example, cobalt NPs can be made less toxic by having

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a biocompatible shell around it. For catalytic applications, expensive nanomaterials are formed as shells around inexpensive core nanomaterials. Hence, a single coreshell particle can provide multiple treatment options. Dimers (two-linked constituents) or labeled entities (i.e., virus with nanoparticles attached around it) are other complex structures which have been tested for biomedical applications. The coating agent also increases the specific absorption rate of nanoparticles. High specific absorption rates means the nanoparticles will stay for a shorter duration in the human body and also lower dosages are required to be given to the patient. Particle functionalization is also very helpful; modification of nanoparticle surface improves their targeting specificity. More selective killing of target cancer cells will be achieved if the specificity is higher.

12.4 PHOTO-INDUCED THERMAL CANCER THERAPY Metal nanoparticles, when illuminated at their plasmonic resonance, become very effective nanosources of heat due to strong absorption of light [51]. In photothermal therapy, the plasmon resonances are tuned to the near-infrared (NIR) wavelengths and are used for generating heat for thermal ablation of cancer cells. The major ones include various types of gold nanoparticles while carbon nanotubes (CNTs) are also being used recently. In the near-infrared region, absorption is minimal and hence light can penetrate deep within human tissues [52]. The metallic nanoparticles such as gold and silver have unique bright colors due to strong optical resonances. Metal nanoparticles upon illumination by light exhibit coherent oscillations of their valence electrons. These are called surface plasmons. The absorption or scattering increases sharply at the plasmon resonant wavelength of the nanoparticle. The wavelength at which resonance occurs depends primarily on the type of metal, dimensions of the metal nanoparticle, and also on its local environment. The smaller nanoparticles absorb light more and the nanoparticle scattering cross-section increases on increasing its size. The strong absorption properties of metallic nanoparticles at resonance help in highly target-specific photothermal heating. This produces enough heat to cause cancer cell death. The most commonly used gold nanostructures are nanoshells and nanorods. Nanoshells are spherical nanoparticles and have a dielectric (silica) core and a shell layer of metal. The size of the inner and outer shell layer can be altered to adjust the plasmon resonance of nanoshell to the desired wavelength in the range of the visible and infrared range. In the near-infrared region having wavelength in the range of 7001100 nm, absorption by water is less and blood and tissue are highly transparent [53]. Nanoshells having optical resonances in this range cause strong absorption or scattering of near-infrared light. Gold nanoshells are biocompatible and have infrared plasmon resonance wavelength, which makes them useful for in vivo biomedical applications. Gold nanorods or nanocages also exhibit near-IR optical resonances which are shape-dependent. They are also being used for cancer treatment [54,55]. CNTs are 1D nanomaterials made up of a single sheet of graphene which are in the shape of a tube. The optical properties can be tuned by changing the aspect ratio and the direction of rolling at the time of synthesis. Single-walled CNTs are strong absorbers of

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electromagnetic waves which can be converted into heat. Unlike gold nanoparticles, plasmon resonance does not occur in CNTs. The absorption of light by CNTs causes transition of electrons inside the nanomaterial. The relaxation results in amplification of vibrations within the carbon structure. Absorption in CNTs covers a wide range of frequencies, which include visible, near-infrared, and radiofrequency light. The absorption has an extinction coefficient which is greater than that of tissues such as melanins, hemoglobin, and water [56]. Arrays of metal nanostructures have been explored for hyperthermia applications. These have the benefit of studying nanoscale processes over a large number of identical systems. This causes an increase in the signal-to-noise ratio. Govorov, Richardson, et al. have proved experimentally that a large quantity of gold nanoparticles dispersed in solution upon illumination produces an increase in the temperature in the system which may not be limited around each nanoparticle due to some thermal collective effects [57,58]. The temperature is uniform within the tumor target. This is not suitable for providing localized heat for nanoscale applications.

12.5 APPLICATION OF NOBLE METALFe3O4 HYBRID NANOPARTICLES FOR DUAL MAGNETIC PHOTOTHERMAL CANCER THERAPY Magnetic hyperthermia and photothermal therapy have shown encouraging results in treating cancer in conjunction with already existing techniques such as radiation and chemotherapy. These techniques deliver nanoscale particles into targeted tumor tissues, which can release enough heat either upon alternating magnetic field or laser exposure to cause death of cancer cells. In photothermal therapy, noble metal nanoparticles (gold, silver) are activated by near-infrared light, as the absorption by tissues is less. This technique has been successfully used in ablation of tumor cells both in vitro and in vivo in animal models. Magnetic iron oxide nanoparticles were tested with direct injection of nanoparticles into solid tumors for treatment of brain tumor and prostate cancer in humans. Although magnetic hyperthermia and photothermal therapy have individually proved to be quite successful, the idea of combining both the techniques is yet in the nascent stage. Both magnetic and photothermal techniques have certain disadvantages. It is suspected that high doses of laser irradiation can damage adjoining tissues. Also, gold nanoparticles used for treatment may not be biodegradable and may be potentially toxic. On the contrary, magnetic nanoparticles such as iron oxide are found to be safe for human use as magnetic resonance imaging (MRI) contrast agents. They are easily biodegradable in vivo, and the iron ions released upon dissolution are assimilated by the body through a fully regulated physiological process. But in magnetic hyperthermia applications the main disadvantage is that high nanoparticles concentrations are required to get the desired results (Fe required is approx. 12 M, which is many times higher than that used for MRI). Hence, it is required that the heating efficiency of iron oxide nanoparticles be optimized. In the case where both magnetic hyperthermia and photothermia are used simultaneously, it could result in an increase in the amount of heating and would decrease the dose of iron oxide nanoparticles needed for magnetic hyperthermia. This should be also

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considered as a strategy to supplement the efforts devoted to optimizing nanoparticles heating efficiency. Elsherbini et al. [59] applied the laser and radiofrequency-induced hyperthermia treatment for subcutaneous Ehrlich carcinoma cells, by using the gold-coated magnetic nanoparticles. Their Fe3O4Au coreshell hybrid nanoparticles had spherical shapes, which contained the core of Fe3O4 nanoparticles (2255 nm diameter). In their study, the Magneto-Optical Resonance Hyperthermia (MORH) method was performed under MRI guidance, by using the green, near-infrared diode laser, and magnetic field. Their findings indicated that the coreshell hybrid nanoparticles had higher therapeutic efficacy as compared with gold nanospheres (15 6 1.5 nm diameter). In their study, the high therapeutic efficacy of hybrid nanoparticles might be attributed to both the homogenous heat distribution across the surface area of tumor (by applying green diode laser) and the interstitial heating (by near infrared and radiofrequency), which had a higher penetration depth than that of gold nanospheres. In other work, Mohammad et al. [60] studied the effect of gold nanoshell (0.4 2 0.5 nm thickness) on hyperthermia of Fe3O4 nanoparticles. By applying low frequency oscillating magnetic fields (44430 Hz), the authors observed the four- to fivefold increase in the amount of heat released with hybrid nanoparticles (6.3 nm size) in comparison to Fe3O4 nanoparticles (5.4 nm size). Recently, Das et al. [61] reported that the heating efficiency of AgFe3O4 coreshell nanoflowers has been highly enhanced by combined magnetic hyperthermia and photothermia. By using low magnetic field (200 Oe), the SAR of nanohybrids was enhanced by an order of magnitude, under simultaneous laser irradiation. The authors also indicated that the same SAR value could be obtained by using lower magnetic fields and lower laser power densities, than those required for the individual technique. In summary, the noble metalFe3O4 hybrid nanoparticles exhibited higher efficacy of inducing hyperthermia than individual nanoparticles. The combination of both magnetic hyperthermia and photothermia was considered as a new strategic approach for clinical practice.

12.6 CHALLENGING PROBLEMS Hyperthermia is a novel method of cancer treatment, but it is still in its nascent stage. This technique has some potential risks and challenges. Researchers are still working to find ways to treat deep-seated tumors for which hyperthermia treatment is presently not available. The conventional chemotherapy suffers from lack of specificity and it can be overcome with targeted nanoparticles. It is very difficult to achieve uniform temperature in the tumor region. This is because the penetration of nanoparticles is not uniform into the poorly vascularized tumor core. It is required to explore ways through which temperature can be raised uniformly in the core. Biocompatibility is another problem in the clinical acceptance of nanoparticles. The nanoparticles may get accumulated at the tumor site. There are problems of immediate toxicity and also retained nanoparticles may be hazardous. It is possible to resolve direct toxicity issues by doing standardized testing on animal models but the more difficult

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challenge is the long-term effect of the nanoparticles left in the body. Drug resistance may be developed by some cancer cell types over the duration of treatment, thus making the drugs which are released from the nanoparticles ineffective. Combined therapies using targeted nanoparticles can be used to deliver both chemotherapeutics and gene therapeutics to prevent this drug resistance and to inhibit the growth of tumor. The SAR is dependent on factors like magnetization, strength and frequency of the magnetic field. The size and distribution of particles also affect SAR. However, all these parameters are to be considered together to maximize SAR. It is required that maximum SAR is obtained at low dosages of nanoparticles. Proper selection of appropriate materials along with optimal selection of physical dimensions and distribution is important to design nanoparticles which are target specific. Other factors like sedimentation, drug encapsulation efficacy, desired drug release profiles, circulation, and cost need to be addressed to get optimum results. Particle size plays an important role because the very small nanoparticles have high clearance rate and chances are that these might get settled in liver and kidney. This reduces the effectiveness of targeted nanoparticles. In contrast, large nanoparticles might not be able to cross small capillaries making the drug delivery process difficult. Thus, the selection of the right material and particle size is crucial for proper delivery and targeting of nanoparticles for cancer therapy. It is very difficult to keep track of the location and distribution of nanoparticles after they have been administered in the body. This is the main reason for the slow development of nanoparticles cancer therapy. It is also not possible to monitor nanoparticles in real time in vivo, and precise biodistribution cannot be ascertained. Hence, it becomes difficult to know their therapeutic effects. Therefore, real-time monitoring of treatment effects on cancer cells is the biggest challenge which needs to be overcome to develop efficient targeted nanoparticles.

12.7 CONCLUSIONS Cancer continues to pose a major threat to human life and researchers are trying to find treatments for this dreadful disease. Chemotherapy is still the most commonly used cancer therapy, but drug-resistance and biotoxicity are its important limitations. Nanotechnology has proved to be a novel technique of generating hyperthermia and it has many advantages over traditional methods. In addition, it was reported that hybridization of noble metal (Au, Ag) and magnetic iron oxide nanoparticles enhanced the effectiveness of hyperthermia, in comparison with the individual nanoparticles. Hyperthermia however is still not a fully developed technique; there are some issues which need to be addressed for its clinical acceptance, and there is scope for further technological improvements.

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[46] N. Kawai, M. Futakuchi, T. Yoshida, A. Ito, S. Sato, T. Naiki, et al., Effect of heat therapy using magnetic nanoparticles conjugated with cationic liposomes on prostate tumor in bone, The Prostate 68 (2008) 78479244. a) S. Bae, S.W. Lee, Y. Takemura, E. Yamashita, J. Kunisaki, S. Zurn, et al., Dependence of frequency and magnetic field on self-heating characteristics of NiFe2O4 nanoparticles for hyperthermia, IEEE Trans. Magn. 42 (2006) 35663568. [47] K. Park, G. Liang, X. Ji, Z.-P. Luo, C. Li, M.C. Croft, et al., Structural and magnetic properties of gold and silica doubly coated γ-Fe2O3 nanoparticles, J. Phys. Chem. C 111 (2007) 1851218519. [48] D.-H. Kim, S.-H. Lee, K.-N. Kim, K.-M. Kim, Y.-K. Lee, Temperature change of various ferrite particles with alternating magnetic field for hyperthermic application 293 (1) (May 2005) 320327. [49] H. Nojima, S. Ge, Y. Katayama, S. Ueno, K. Iramina, Effect of the stimulus frequency and pulse number of repetitive transcranial magnetic stimulation on the inter-reversal time of perceptual reversal on the right superior parietal lobule, J. Appl. Phys. 107 (2010). 09B32009B320-3. [50] S. Deger, D. Boehmer, I. Tu¨rk, J. Roigas, V. Budach, S.A. Loening, Interstitial hyperthermia using selfregulating thermoseeds combined with conformal radiation therapy, Eur. Urol. 42 (2002) 147153. Department of Radiology, Case Western Reserve University, 10900 Euclid Avenue Cleveland, Ohio 44106. [51] P. Biagioni, J.S. Huang, B. Hecht, Nanoantennas for visible and infrared radiation, Rep. Prog. Phys. 75 (2012) 024402. [52] L.R. Hirsch, A.M. Gobin, A.R. Lowery, et al., Metal nanoshells, Ann. Biomed. Eng. 34 (1) (2006) 1522. [53] S. Lal, S.E. Clare, N.J. Halas, Nanoshell-enabled photothermal cancer therapy: impending clinical, Acc. Chem. Res. 41 (12) (2008) 18421851. [54] G. Baffou, P. Berto, E.B. Urena, R. Quidant, S. Monneret, J. Polleux, et al., Photoinduced heating of nanoparticle arrays, ACS Nano 7 (8) (2013) 64786488. Available from: www.acsnano.org. [55] M. Zhu, G. Baffou, N. Meyerbro¨ker, J. Polleux, Micropatterning thermoplasmonics gold nanoarrays to manipulate cell adhesion, ACS Nano 6 (2012) 72277233. [56] N. Huang, H. Wang, J. Zhao, H. Lui, M. Korbelik, H. Zeng, Single-wall carbon nanotubes assisted photothermal cancer therapy: animal study with a murine model of squamous cell carcinoma, Lasers Surg Med. 42 (9) (2010) 798808. [57] A.O. Govorov, W. Zhang, T. Skeini, H. Richardson, J. Lee, N.A. Kotov, Gold nanoparticle ensembles as heaters and actuators: melting and collective plasmon resonances, Nanoscale Res. Lett. 1 (2006) 84. [58] H.H. Richardson, M.T. Carlson, P.J. Tandler, P. Hernandez, A.O. Govorov, Experimental and theoretical studies of light-to-heat conversion and collective heating effects in metal nanoparticle solutions, Nano Lett. 9 (2009) 1139. [59] A.A.M. Elsherbini, M. Saber, M. Aggag, A. El-Shahawy, H.A.A. Shokier, Laser and radiofrequency-induced hyperthermia treatment via gold-coated magnetic nanocomposites, Int. J. Nanomed. 6 (2011) 21552165. [60] F. Mohammad, G. Balaji, A. Weber, R. Uppu, C. Kumar, Influence of gold nanoshell on hyperthermia of superparamagnetic iron oxide nanoparticles, Phys. Chem. C 114 (2010) 1919419201. [61] R. Das, N. Rinaldi-Montes, J. Alonso, Z. Amghouz, E. Garaio, J.A. Garcı´a, et al., Boosted hyperthermia therapy by combined ac magnetic and photothermal exposures in Ag/Fe3O4 nanoflowers, ACS Appl. Mater. Interfaces 8 (2016) 2516225169.

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C H A P T E R

13 Optical Absorption Modeling of Plasmonic Organic Solar Cells Embedding AgSiO2 CoreShell Nanoparticles Kekeli N’Konou and Philippe Torchio Aix-Marseille Universite´, Institut Mate´riaux Microe´lectronique Nanosciences de Provence  IM2NP, CNRS-UMR 7334, Domaine Universitaire de Saint-Je´roˆme, Marseille Cedex, France

13.1 INTRODUCTION A plasmonic organic solar cell is a thin-film solar cell using conductive organic polymers or small organic molecules for light absorption and charge transport that converts sunlight into electricity with the assistance of plasmons. In recent years, organic solar cells (OSCs) have been revealed as a promising solution for photovoltaic devices [13] owing to low-production cost, high absorption coefficient, easy processability, mechanical flexibility, large-area manufacturing, and light weight. The power conversion efficiency (PCE) of OSCs is steadily progressing forward and the highest PCEs over 13% [4] have been obtained recently using the new materials. However, this PCE is still relatively low, compared to inorganic solar cells, which limits their further application. This is due to the short exciton diffusion length and low charge carrier mobilities of most of the organic semiconductor (on the order of 1024 cm2/V s). As a consequence, the photoactive layers thicknesses in OSCs are compelled to be thin (100 nm or less) to achieve efficient photogenerated carrier collection [5,6]. However such configuration reduces the photons absorption efficiency, and thereby the PCE. Therefore, the key challenge for absorption enhancement in OSCs is to find a compromise/trade-off between an efficient photogenerated carriers collection, with thin active layers, and an optimal light

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absorption, which requires a thick active layer. Noble metallic nanoparticles (MNPs) (Ag, Au, Pt, etc.) present an important optical extinction (absorption 1 scattering) cross-section thanks to the collective oscillation of free electrons called localized surface plasmons resonance (LSPR). In this way, the incorporation of MNPs inside or near the active layer is proposed as one of the efficient light-trapping techniques for enhancing light absorption, and consequently improved short-circuit current density (Jsc) via LSPR, light scattering effects, or a synergy of both effects can be engineered to enhance absorption and/or optical path length [710]. The nature of metal, shape, size, and surrounding medium of MNPs are also reported to affect their resonance wavelengths. Although the MNPs generate a beneficial amplified local electromagnetic field, the contact of bare MNPs with the active layer in OSCs can increase the exciton quenching or charge recombination leading to electrical losses [11,12], offsetting the optical enhancement [13]. Moreover, the possible migration or segregation of the MNPs in the undesired areas of the solar cell can lead to shunt currents which are also detrimental to electrical properties of the devices. To overcome these issues, some research groups [12,1420] have suggested coating the MNPs with a dielectric or polymeric shell, physically introducing these coreshell structures into or near the active layer. The coreshell nanoparticle (CSNP) is commonly defined as comprising a core (inner material) and a shell (outer coated material). MNPs are commonly made of silver, gold, or aluminum due to their strong interactions with the sunlight; gold leads to surface plasmons resonance in the visible spectrum while aluminum and silver lead to surface plasmons resonance in the ultraviolet spectrum [21]. Silver is usually preferred for its intrinsic features such as its low cost, low absorption, and LSPR wavelength matching the spectra of organic materials. In this chapter, silver will be preferably chosen as metal core, silica (SiO2) is often chosen as a dielectric shell due to its high transparency. A threedimensional finite difference time domain method (3D-FDTD) numerical study of the optical behavior of OSCs containing dielectric coated silver nanoparticles is explored. The chapter presents the influence on calculated intrinsic absorption enhancement of geometrical parameters such as the periodicity of the array, the Ag core diameter, the P3HT:PCBM active layer thickness, the shell thickness, and the shell nature.

13.2 MECHANISM OF THE OPTICAL ABSORPTION ENHANCEMENT IN PLASMONIC ORGANIC SOLAR CELLS The theoretical mechanism involving the light absorption enhancement by the MNPs embedded inside the PV devices, has been described by Atwater and Polman [22]. The integration of MNPs is able to enhance the absorption by two main light-trapping processes, such as the electromagnetic near-field resonance and far-field scattering (Fig. 13.1).

13.2.1 Electromagnetic Near-Field Resonance A strong interaction can appear between the electromagnetic exaltation (radiation) and the free conduction electrons in the metal when the MNPs size are smaller than the

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267

FIGURE 13.1 Two light trapping mechanisms: (A) light scattering from large diameter ( . 50 nm) MNPs into high angles within the photoactive layer, causing elongated optical path lengths; (B) light concentration induced by LSPR from MNPs.

FIGURE 13.2 Schematics of the surface plasmon resonance where the free conduction electrons of the MNP are driven into oscillation due to a strong coupling with incident light. Source: Reprinted with permission from K.A. Willets, R.P. Van Duyne, Localized surface plasmon resonance spectroscopy and sensing, Annu. Rev. Phys. Chem. 58 (2007) 267297 [23].

wavelength (λ) of the light. This interaction can be defined by a simplified formalism or quasi-static approximation. The electric field (E) of the light wave is nearly constant across the entire particle. This electric field applies a force on the conduction electrons of the MNP, which leads them to collectively migrate at the MNP surface (as shown in Fig. 13.2). At the same time, a restoring force originating from the Coulomb attraction between these displaced electrons and the ionized metal lattice occurs. These two forces simultaneously produce a collective oscillation of the electrons with a maximal amplitude at the so-called localized surface plasmon resonance frequency. The Ep plasmon energy in a free electron model can be determined as follows [24]: sffiffiffiffiffiffiffiffi ne2 Ep 5 ħ 5 ħωp (13.1) mε0 where m is the electron mass, ε0 the permittivity of free space, e the elementary charge, n the density of free electrons, ħ the Planck constant, and ωp the bulk plasmon frequency. The εr dielectric function can be defined by the Drude model, for metals with low interband absorption, which describes the response of damped, free electrons to an applied electromagnetic field with a frequency of ω [24]: εr ðωÞ 5 1 2

ωp ω2 1 iγω

(13.2)

where τ 5 1/γ is the relaxation time and ω the angular frequency of an applied electromagnetic field.

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FIGURE 13.3 Illustration of the integration of a metaldielectric CSNPs in a surrounding medium.

By considering an MNP with a radius r, integrated in an infinitely large active-layer medium, the LSPR is most important when the wavelength of light is higher than the MNP dimension: r ,, λ. Knowing that the applied field generates a dipole moment inside the MNP, according to the quasi-static approximation [25], the polarizability α can be defined as: In the case of a metaldielectric CSNP incorporated in an infinitely large medium, for example, active layer (Fig. 13.3), the polarizability α will be redefined as follows: α 5 4πr32

ðεdielectric 2 εactive layer Þ 1 ðεNP 1 2εdielectric Þ 1 fðεNP 2 εdielectric Þðεactive layer 1 2εdielectric Þ ðεdielectric 1 2εactive layer ÞðεNP 2 εdielectric Þ 1 fð2εdielectric 2 2εactive layer ÞðεNP 2 εdielectric Þ (13.3)

where r1 is the inner radius, r2 is the outer radius, εdielectric is the relative permittivity of the dielectric medium, and f 5 (r1/r2)3 is the fraction of the total particle volume occupied by the inner sphere [26]. The cross-section scattering (Cscat) and absorption (Cabs) are defined as:   1 2π 4 2 2π Im½α (13.4) Cscat 5 jαj Cabs 5 6π λ λ The interaction of the free electrons of a MNP with the electromagnetic wave leads to a local enhancement of the electromagnetic fields, defined as “near-fields,” in the proximity of the MNP. By incorporating MNPs inside or near the OSC active layers, the electromagnetic field can be enhanced in their vicinity to increase absorption, which is directly proportional to the electric field. The MNPs in OSCs can also act as optical antenna for the incident light to concentrate the electric field intensity and increase absorption (Fig 13.1B).

13.2.2 Far-Field Scattering Embedded MNPs with a large diameter (above 50 nm) can scatter the incident light in the OSC’s active layer, improving the effective optical path length and enhancing absorption. This absorption enhancement is attributed to the optical scattering (far-field) process of the MNPs as the near-field is reduced to only a few tens of nanometers. Depending on the diameter, the light scattering can be predominant in comparison with the local absorption [27]. By increasing the diameter of the MNPs, scattering can be favored to absorption (CScat(D/2)6 and Cabs(D/2)3) [8]. It has been reported that the scattering cross-section of a nanoparticle is much larger than its geometric cross-section [28] due to the LSPR. The light

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can be scattered in both forward and reverse directions when an MNP incorporated in a homogeneous medium [28,29]. Furthermore, light will scatter more intensely into the dielectric medium with the higher permittivity [30] when the MNP is closely situated to the interface of two dielectrics. Thus, the optical path length can be increased by the angular spread of the light or by scattering in different directions within the device [31]. Finally, the metal contact which reflects the light back to the outer surface induces multiple passes of the light through the OSCs, when MNPs are placed, as shown in Fig. 13.1A.

13.3 WHY COAT METAL NANOPARTICLES (MNPs) WITH A DIELECTRIC SHELL? Regardless of the notable progress achieved by mixing directly bare MNPs in the active layers of OSCs, some challenges always remain: (i) difficulty of uniformly dispersing the MNPs in the active layer without disturbing the film morphologies; (ii) electrical losses due to the undesired exciton quenching and charge-trapping which can offset the optical enhancement generated by the MNPs; (iii) shunt currents can be occurred in the thin film photovoltaic devices because of the possible migration of NPs; and (iv) some MNPs could be easily oxidized under ambient conditions, specifically silver nanoparticles (Ag NPs). To overcome these issues, it has been suggested to insulate MNPs with a thin dielectric shell. Such so-called metal@dielectric CSNPs have already drawn many attention, due to their interesting properties and wide range of applications in biomedical [32], catalysis [33], electronics [34], etc. The CSNPs have numerous advantages over bare NPs, for example, reduction in reactivity, reduction in consumption of precious material, surface modification, stability, etc. [35]. The role of the dielectric shell around the metallic core is then to avoid the aforementioned problems, such as oxidation, recombination process, or exciton quenching, by separating the bare metal cores and the active layer surrounding the NPs. The dielectric shell of CSNP can also prevent NPs aggregation and active layer morphology disturbance. Chen et al. reported that the metal core induces the LSP effect while the dielectric shell provides a good dispersion of the NPs [17]. Kim et al. [36] indicated that the use of a chemically stable polymeric shell on the CSNP can sufficiently prevent the excitons to be quenched at the surface of the metal core. Lee and Peumans revealed that the incorporation silica-coated silver CSNPs in organic thin films enhance absorption. However, the challenge to find appropriated MNPs with protective shell depending on the PV devices design remains to investigate.

13.4 NUMERICAL MODEL The classical Mie scattering theory (analytical method) and several numerical models such as finite element method (FEM), discrete dipole approximation (DDA), rigorous coupled wave analysis (RCWA), finite difference time domain (FDTD) method, etc. have been carried out to study the optical properties behavior of plasmonic NPs or CSNPs. Nowadays, some researchers perform computational modeling and employ simulation tools to predict laboratory experiments and analyze various properties of the plasmonic

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structures. The FDTD method can efficiently model the optical behavior of plasmonic nanoparticles in 3D by solving Maxwell’s equations [37], and was chosen for the present works.

13.4.1 Finite Difference Time-Domain Method Yee introduced in 1966 the FDTD method to evaluate magnetic (H) and electric (E) field components based on a discrete mesh, so-called Yee cells. The H and E vector fields represent the light in a three-dimensional space grid. The H-components are centered on the faces and the E-components are centered on the edges of Yee’s cube. Fig. 13.4 presents Yee’s cube with the electric and magnetic components. FDTD method is carried out to computationally model electromagnetic wave and light interactions with materials based on Yee’s algorithm for various applications such as radar [39], photonic crystal fibers [40], microscopy [41], etc. FDTD method requires less running time and does not demand more of the computer memory. It also is capable of modeling the surfaces and interiors of any dielectric and inhomogeneous materials. Hence, owing to the fact that the arbitrary electrical parameters can be attributed to each cell of the spatial grid of Yee cells [42], FDTD method can be used to simulate and model the wave interaction with any type of matter [43]. Several open source tools and commercial software packages are available for FDTD modeling. In this work Lumerical FDTD solutions (commercial software package) will be used to model the optical properties of the plasmonic OSCs. FIGURE 13.4 Electric and magnetic field components of Yee’s cube. Source: Reprinted with permission from M. Hamidi, Mode´lisation par la me´thode FDTD des plasmons de surface localise´s, The`se de doctorat, Universite´ Mouloud Mammeri—Tizi Ouzou, 2012 [38].

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13.4.2 Simulation Parameters Fig. 13.5A presents the schematic structure of the plasmonic solar cell, where a periodic square array of NSs is placed on top of ITO. The simulation mesh size is fixed at 1 nm in the region encircling the NSs. To eliminate parasitic reflection in the stack direction, PML boundary conditions are used in the glass substrate and in aluminum. The periodic boundary condition in x and y direction is applied for considering the multiple scattering and cross-coupling between the neighboring NPs as shown in Fig. 13.5B. The incident light is a plane wave propagating at the normal incidence along the z direction with a wavelength varying between 350 and 700 nm. Our OSCs design is composed of five thin films, namely a glass substrate, a 180-nm thick transparent conducting indium tin oxide (ITO) layer, a 20-nm thick poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) anode layer, a variable nm-thick active layer of P3HT:PCBM with 1:1 weight ratio, and a 100-nm thick aluminum (Al) cathode. The optical constants (refractive index n and extinction coefficient k) of ITO, PEDOT:PSS, P3HT:PCBM, Al, and Ag are taken from literature [45,46]. The refractive index of glass is 1.5. The Ag@SiO2 NSs are placed on top of ITO, as shown in Fig. 13.5A. The intrinsic absorption inside the only active layer (i.e., without Ag material absorption) and the optical absorption enhancement are calculated in order to evaluate the absorption efficiency of our multilayer solar cells. For that, the dissipated power (or loss of power) L(λ) into the volume (x, y, z) of a material can be calculated from the Poynting vector L(x, y, z, λ) according to the formula (13.5) [29]: ðxðyðz 2 divfΠðx; y; z; λÞgdx dy dz (13.5) LðλÞ 5

FIGURE 13.5 (A) Schematic structure of a plasmonic organic solar cell (Glass/ITO/Ag@SiO2/PEDOT:PSS/ P3HT:PCBM/ Al); (B) side view of the unit cell of our simulation model in the xz plane, including perfectly matched layers (PML) and periodic boundary conditions. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silica-coated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44].

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Then, the intrinsic absorption A(λ) in each layer is determined by the ratio of the loss of power L(λ) in each volume to the total power of the incident light source Psource(λ) using the relation: AðλÞ 5

LðλÞ Psource ðλÞ

(13.6)

The optical absorption enhancement (AE) in the 350700 nm wavelength range is calculated using the following Eq. (13.7): Ð λ2 ðλ=hcÞAðλÞIðλÞdλ AE 5 Ð λ2 λ1 (13.7) λ1 ðλ=hcÞAwithout NSs ðλÞIðλÞdλ In this expression, c is the speed of light in vacuum and h is the Planck’s constant, while I(λ) represents the AM 1.5G solar spectral irradiance. A(λ) and Awithout NSs(λ) are the intrinsic optical absorption (inside the active blend heterojunction) with and without nanospheres, respectively. The dissipated power (DP) through the active layer is then deduced over the 350700 nm wavelength range by integrating the intrinsic absorption weighted by the AM 1.5G solar spectrum (IAM1.5) using Eq. (13.8): ð λ2 DP 5 AðλÞIAM1:5 dλ (13.8) λ2

13.5 RESULTS AND DISCUSSION 13.5.1 Effect of Bare Ag NSs Size Versus Period on Absorption Enhancement The effect of the period (P) and the diameter (D) of the NSs on the absorption enhancement AE are simultaneously studied, in this section. Fig. 13.6 presents the calculation of AE inside the 50-nm thick P3HT:PCBM active layer by varying the period P (spacing between NSs) for several Ag core diameters D (ranging between 30 and 70 nm). It can be observed that the absorption is very low AE , 1 when particles are very close to each other. This is due to the close packed array which can act as a mirror for incident light, and avoid light to sufficiently enter the active region. For each value of D, an optimum value of AE is obtained. When P increases beyond the optimum peak, absorption enhancement slightly decreases and absorption tends to be equivalent to that obtained without NSs (i.e., AE 5 1) which is in accordance with the fact that a large increase of P corresponds to a decrease in NP concentration. The maximum value of absorption enhancement is summarized in Table 13.1 for each Ag core diameter with the corresponding P optimal value. Small Ag NSs (D 5 30 nm) incorporated in a 20-nm thick PEDOT:PSS buffer layer drive to a negligible maximum AE in the active layer (AE 5 1.037). However, AE becomes significant when D is increased (taller than the 20-nm thick PEDOT:PSS buffer layer with D . 30 nm). The AE value increases with the diameter of NPs. This can be explained by considering that, as the NSs become larger, they penetrate more into the active layer.

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FIGURE 13.6 Intrinsic absorption enhancement (AE) of P3HT:PCBM as function of the period of the NSs array for various Ag core diameters (D) in Glass/ITO/Ag@SiO2/PEDOT:PSS/P3HT:PCBM/Al plasmonic organic solar cell. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silica-coated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44]. TABLE 13.1 Maximum Absorption Enhancement (AE) Obtained for Each Ag Core Diameter D and Corresponding Optimal Period P Ag NSs Diameter D (nm)

Absorption Enhancement AE

Optimal Period P (nm)

30

1.037

75

40

1.124

110

50

1.162

160

60

1.166

190

70

1.116

210

Thus, the spatial concentration of energy associated with the near-field plasmonic resonance of NSs provides more photons to be absorbed within the active layer. As previously described in Section 13.2.2, the far-field scattering effect also contributes to absorption enhancement since NPs diameter is beyond 40 nm. For Ag NSs with D 5 70 nm, AE drops from 1.166 to 1.116. This matches the case where D equals the sum of the active and buffer layer thickness (50-nm thick P3HT:PCBM 1 20nm thick PEDOT:PSS). At this point, the beneficial optical effect of the NSs cannot compensate for the loss of active material, which limits absorption. A maximum value of AE 5 1.166 is calculated for D 5 60 nm and P 5 190 nm, which indicates that the absorption is increased by 16.60% compared to the cell without any NSs.

13.5.2 Effect of the Shell Thickness on Absorption Enhancement The influence of the shell thickness on optical absorption enhancement is investigated and results are presented in Fig. 13.7. The active layer thickness is fixed at 50 nm, the

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FIGURE

13.7 Intrinsic absorption enhancement (AE) of P3HT:PCBM as a function of the silica shell thicknesses of core Ag (50 nm) NSs in Glass/ITO/Ag@SiO2/ PEDOT:PSS/P3HT:PCBM/Al plasmonic organic solar cell. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silica-coated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44].

FIGURE 13.8 Intrinsic absorption spectra of P3HT:PCBM inside the 50-nm thick active layer for plasmonic organic solar cells without NSs and with NSs for various shell thicknesses. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silica-coated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44].

diameter (D) of the Ag core at 50 nm, and the period at 160 nm, according to the previous results described in Section 13.3.1, while the shell thickness (Tshell) is varied from 0 to 10 nm. The optical absorption enhancement decreases as Tshell increases. For Ag@SiO2 NSs with 5 nm silica shell, only a slight enhancement is calculated (AE 5 4.8%). The intrinsic absorption spectra inside P3HT:PCBM when Ag NSs with and without silica shell are incorporated in a 50-nm thick P3HT:PCBM active layer, and the reference spectrum (without NSs) are presented in Fig. 13.8. A strong decrease in absorption is observed upon incorporation of NSs below λ 5 450 nm. It is interesting to note that the addition of a thin silica shell (1 nm) reduces these losses, even if absorption remains lower than for the reference (without NSs). Beyond λ 5 450 nm, a broadband absorption enhancement is observed for coreshell NSs with Tshell 5 1 nm and Tshell 5 5 nm. This enhancement is ascribed to two major phenomena. First, the particles can act as optical antennas for incident light. When the nanoparticles are positioned near the active

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(A)

E2/E02

(B)

2

E2/E0

10

–260

10

–260

–290

6 4

–320

2 –350 –50

0 –20 10 x (nm)

40

8 z (nm)

z (nm)

8 –290

6 4

–320

2 –350 –50

0 –20

10 x (nm)

40

FIGURE 13.9 Normalized electromagnetic field intensity distributions in the xz plane around (A) the bare Ag NS and (B) the Ag@SiO2 NS with a 5-nm thick silica shell at the wavelength of λ 5 600 nm. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silica-coated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44].

region, the electromagnetic field intensity concentrated near the NSs as a result of LSPR can improve absorption. Second, the NSs can act as scattering centers for incident light, which provides a longer path length for photons and increases their probability of being absorbed in the active layer. The presence of the silica shell limits absorption enhancement in the 450700 nm range, and for a 5-nm thick shell, AE falls from 1.22 to 1.04 (Fig. 13.7). The electromagnetic field intensity around NSs in the OSC is mapped at the wavelength of λ 5 600 nm for an easier understanding of this phenomenon. The field is concentrated around the metallic part of the nanoparticle both in the presence of dielectric shell and without, as shown in Fig. 13.9. However, the electromagnetic field is much more intense within the silica shell (Fig. 13.9B), which limits the beneficial near-field effect. This explains the reduction of the absorption enhancement in the active layer. Some reports have demonstrated that the performance of OSCs can also be enhanced by embedding Ag nanoprisms coated with ultrathin (12 nm) dielectric shell (TiO2 or SiO2). Their results showed that ultrathin oxide shells can efficiently avoid exciton recombination [47]. Zhang et al. obtained photocurrent enhancement (from 16.5 mA cm22 to 21.2 mA cm22) and PCE (from 7.52% to 9.55%) enhancement by embedding Au nanorods coated with ultrathin shells (23 nm) in the active layer or at the interface of active layer/hole extraction of an OSC. [48]

13.5.3 Effect of the Active Layer Thickness on Absorption Enhancement A relatively thick active layer is generally used to fabricate efficient OSCs devices in order to absorb the maximum of the incident light. However, several numerical studies in plasmonic OSCs considered thin active layers to demonstrate enhancement of both the optical and electrical properties of OSCs [4951].

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FIGURE 13.10 Intrinsic absorption enhancement (AE) of P3HT:PCBM as a function of the active layer thickness for P 5 160 nm and D 5 50 nm in Glass/ITO/ Ag@SiO2/PEDOT:PSS/P3HT:PCBM/Al plasmonic OSC. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silica-coated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44].

In this section, the Ag NSs core diameter and the period P are fixed at 50 nm and 160 nm, respectively. The optical absorption enhancement is studied as a function of the active layer thickness as presented in Fig. 13.10. An absorption enhancement is observed when the P3HT:PCBM active layers are thinner than 70 nm. This can be explained by the fact that P3HT:PCBM possesses a very high absorption coefficient (B5 3 106/m), and about 100 nm of material is needed to absorb most of the light. In contrast, a decrease of absorption enhancement (AE , 1) is achieved by using an active layer over 70 nm thick, the beneficial effect of NSs on absorption being not compensated with the loss of active material. This result confirms that it is necessary to use a thin active layer to make plasmonically enhanced solar cells. These numerical results match with other previous works [5255], while experimental studies reported that performances of plasmonic solar cell can be improved using an ultrathin active layer [56,57].

13.5.4 Effect of the Nature of the Dielectric Shell Material on Absorption Enhancement Titanium oxide (TiO2), zinc oxide (ZnO), aluminum oxide (Al2O3), and silica (SiO2) are chosen as dielectric shell materials with a corresponding refractive index of 2.5, 2, 1.77, and 1.5, respectively. Fig. 13.7 presents the results of the influence of the refractive index and thickness of the shell materials on optical absorption. Considering the optimal results previously demonstrated, the Ag core diameter (D) and the active layer thickness are both fixed at 50 nm while the shell thickness (Tshell) is varied from 0 to 5 nm. Among the four different coreshell structures, for shell thicknesses varying from 1 to 2 nm, Ag@ZnO NSs present the highest AE (AE 5 1.27 for 1-nm thick ZnO shell) compared to the other dielectric shells which exhibit lower values as shown in Fig. 13.11. The refractive index of shell material slightly affects the optical absorption enhancement, except for the case of Ag NSs coated with a TiO2 shell which gives rise to a significant reduction of AE, as the shell thickness increases from 3 to 5 nm. As a result, it is pointed out that the refractive index of the shell materials can influence the optical absorption. It is worth noting that we have considered all dielectrics as entirely transparent. In fact, the imaginary part of the refractive index for these materials is not really equal to zero in the blue part of the visible spectrum, and should be taken into account. II. APPLICATIONS

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13.5 RESULTS AND DISCUSSION

FIGURE 13.11 Intrinsic absorption enhancement (AE) inside P3HT: PCBM as a function of the refractive index and thickness of the shell materials in Glass/ITO/Ag@dielectric oxide/PEDOT:PSS/P3HT:PCBM/Al plasmonic organic solar cell. Source: Reprinted with permission from K. N’Konou, L. Peres, Ph. Torchio, Optical absorption modeling of plasmonic organic solar cells embedding silicacoated silver nanospheres, Plasmonics 13 (1) (2018) 297303 [44].

FIGURE 13.12

Absorption enhancement (AE) of different active layers as a function of the ZnO optical spacer layer thickness in Glass/ITO/PEDOT:PSS/P3HT:PCBM/ZnO/Al, and Glass/ITO/PEDOT:PSS/PTB7:PCBM/ ZnO/Al organic solar cells. Source: Reprinted with permission from K. N’Konou, P. Torchio, Optical absorption enhancement by inserting ZnO optical spacer in plasmonic organic solar cells, J. Nanophotonics 12 (2017) 012502 [58].

13.5.5 Influence of ZnO Optical Spacer Layer and Active Layer Material on Absorption Enhancement In this section, we simultaneously study the influence of ZnO optical spacer layer thickness inserted between the active layer and Al and those of the active layer material nature on optical AE as presented in Fig. 13.12. P3HT:PCBM and PTB7:PCBM are alternatively employed as photoactive layer materials of OSCs. The AE is calculated here as the ratio between the DP inside the only active layer of the OSC including ZnO spacer and the DP in the same active layer of the OSC without ZnO layer. The AE value is calculated inside a 50-nm thick P3HT:PCBM or PTB7:PCBM active layer by varying the ZnO optical spacer thickness (ranging between 0 and 40 nm). The

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numerical results of AE from Fig. 13.2 demonstrate that the effect of the ZnO optical spacer is more pronounced in PTB7:PCBM than in P3HT:PCBM layer. An optimum value of 20 nm for the thickness of the ZnO optical spacer is found for both materials. Optimizing the ZnO thickness allows enlarging the electromagnetic field distribution inside the active layer to further enhance light absorption. As a result, the optimal 20-nm thick ZnO optical spacer inserted between the metal cathode and the active layer is able to enhance the spatial distribution of light within the multilayer stack of the OSCs device. The insertion of a ZnO optical spacer layer changes the optical interferential system (DescartesSnell’s laws) and the distribution of the electromagnetic field in the multilayers. But this acts differently with PTB7:PCBM and P3HT:PCBM active layers due to their different optical constants (refractive index n(λ) and extinction coefficient k(λ)). Consequently, by varying the ZnO layer thickness, the optical properties (reflectance, transmittance, and absorption) of the multilayers are modified, so that the electromagnetic field distribution, which affects the AE value and differs versus the considered active material. Such variations can also be correlated to the bandgap energy value of each active material, which is lower for PTB7:PCBM than for P3HT:PCBM. Some researchers [59] recently confirmed this absorption gain and reported that the optical properties of ZnO used as the optical spacer can beneficially redistribute the optical electric field in the OSC structure. Furthermore, a thin layer of ZnO around 2030 nm thick used as an optical spacer in the OSC is also proved to have a high conductivity [60]. Due to the highest AE values obtained with PTB7:PCBM active layer, this low bandgap PTB7 polymer with extended light absorption spectrum and enhanced hole mobility can be preferentially chosen as active layer [61,62].

13.6 CASE OF THE DYE-SENSITIZED SOLAR CELLS (DSSCs) AND PEROVSKITE SOLAR CELLS (PSCs) In the past few years, the role of metaldielectric CSNPs on optical properties of the other solar solar cells of the third-generation namely dye-sensitized solar cells (DSSCs) and perovskite solar cells has also been investigated using both experiments and numerical modeling. Several studies stated that the incorporation of metaldielectric CSNPs with different shapes inside the active layer of DSSCs can enhance the light absorption. This enhancement effect is attributed to the enhanced excitation rate of dye sensitizers owing to LSPR [6366]. In particular, using titanium oxide (TiO2) as dielectric shell can generate two additional mechanisms related to the efficiency enhancement. First, the metal cores can incur charge equilibrium with the surrounding semiconductor and change the Fermi level of TiO2, causing an improved cell potential [6769]. Second, the hot electrons produced within the resonant plasmonic cores can contribute directly to the photocurrent enhancement. Moreover, it has been demonstrated that the use of TiO2 shell can reduce the internal impedance of the cell [70] and take advantage of the compatible affinity to dye sensitizers [71]. Embedding of metaldielectric CSNPs in dye-sensitized molecules layer leads to plasmon improved charge injection from the dyes into the TiO2, which was attributed to a combination of near-field and far-field effects.

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Recently, metaldielectric oxide CSNPs were used as light harvesting enhancers in perovskite solar cells. Wu et al. [72] combined both numerical modeling and experimental approaches to demonstrate that the integration of silica-coated gold (Au@SiO2) nanorods at the interface between the PEDOT:PSS hole transport layer and the CH3NH3PbI3 active layer can significantly improve light absorption as well as transport and collection of charge carriers. The origin of the photocurrent enhancement is attributed to a previously unobserved and unexpected mechanism of reduced exciton binding energy with the incorporation of the Au@SiO2 NPs, rather than enhanced light absorption, as reported by Zhang et al. [73]. These findings make metaldielectric oxide CSNPs a promising functional material for facile tuning of the transport and collection of the charge carriers that can be utilized to fabricate high-efficiency and low-cost perovskite solar cells. In a similar way, other authors have demonstrated that embedding Au@SiO2 coreshell nanorods in perovskite solar cell enhance not only the light absorption but also the external quantum efficiency across a broad range of wavelengths, which can contribute to the enhanced cross-sectional scattering and spectrally absorbing energy density [74].

13.7 CONCLUSION The basic physical mechanisms and FDTD numerical model for exploitation of plasmon effect in OSCs are reviewed. Our results prove that the optical light absorption can be efficiently enhanced by embedding metaldielectric oxide CSNPs. Our predictive numerical analysis demonstrates that the right choice of architectural (optical spacer) and geometrical (metal core size, period of the NPs array, dielectric shell thickness, active layer thickness, nature of materials, etc.) parameters of metaldielectric oxide CSNPs can lead to optical absorption enhancement inside the organic photovoltaic device. Such photonic concept can pave the way towards new highly efficient plasmonic organic solar cells.

Acknowledgment The authors thank the IDB Merit Scholarship Programme for High Technology (MSP) for financial support.

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Further Reading J.Y. Lee and P. Peumans, Energy and Charge Transport on the Nanoscale in Organic and Organic Metallic Composite Solar Cells, NEI Workshop, Nice, September 2426, 2006.

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14 Noble MetalsMetal Oxide Mesoporous Nanohybrids in Humidity and Gas Sensing Applications Vijay K. Tomer1,2, Ritu Malik2, Vandna Chaudhary3, Arabinda Baruah4 and Lorenz Kienle2 1

Berkeley Sensor & Actuator Center, University of California, Berkeley, CA, United States 2 Synthesis & Real Structure Group, Technical Faculty, Institute for Materials Science, Kiel University, Kiel, Germany 3Center of Excellence for Energy and Environment Studies, D.C.R. University of Science & Technology, Murthal (Sonepat), Haryana, India 4 Indian Institute of Science Education and Research, Mohali, Punjab, India

14.1 INTRODUCTION The necessity for controlling the indoor environment for balancing the microclimate conditions in museums, agriculture, paper industry, defense, drugs, electronics, packaging, drugs, research laboratories, medical industry, and metrology has led to the development of new gas/humidity sensing materials [19]. Therefore, a variety of novel, functional nanomaterials based on polymer, electrolytes, metal oxide, silica, organicinorganic hybrid nanocomposites, and carbon materials have been utilized for the fabrication of advanced gas sensors in recent years [1018]. The advancement in the sensing technology is expected to create sensors with good linearity, high accuracy, sensitivity, wide range of operating temperatures, fast response and recovery time, and stability [1924]. Depending upon their physical and chemical properties these materials can be used under different environmental condition, but still it is hard for a single material to satisfy all the requirements of being the best sensing material. Low cost and easy production of polymer materials has made them good for sensors, but they have relative low working range of operating

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temperatures and also poor antijamming property. Similarly, ceramic materials suffer from poor recovery and so need to be heated to reuse them again. More recently, the invention of highly ordered mesoporous silica materials (MSM) has stimulated intensive studies in sensors, nanoelectronic/optical devices, drug delivery, and catalysis [2531]. MSM possess high mechanical strength and thermal stability along with uniform channels, highly porous, tunable pore size, and large pore volume with abundant SiOH active bonds on the pore walls [3235]. The pores of silica increase the adsorption efficiency of water vapors and thus provide an easy path for free traveling of analyte molecules, and so, the inorganic silica due to its high intrinsic impedance, relative inertness, and mechanical strength has become an attractive material for humidity/gas sensing applications. Such type of porous material is synthesized using surfactants, ionic and cationic block copolymers, and modified with nanometallic particles and metal oxides to improve its sensing performances [3641]. The mesoporous semiconducting metal oxide (SMOx) exhibits high surface area, high surface to volume ratio, and large pore diameter, which is essential for development of highly sensitive gas/humidity sensors and other applications [4247]. These pores help analyte molecules to enter and transfer all through the material’s layer and support the movement of charge carriers across the material surface. The traditional approach of using surfactants for developing mesoporous materials results in the materials possessing semicrystalline and amorphous pore walls which often collapse at high operating temperatures and thus restrict their use in various applications. The MSM, besides being itself used as a porous matrix in the design of sensing materials, has also been extensively utilized as “hard templates” in the synthesis of mesoporous semiconducting metal oxide (SMOx) in order to enhance surface area, large pore diameter, and high mechanical/thermal stability [4850]. This hard template method has been proved as an efficient approach for the synthesis of SMOx because of the stable support provided by the hard templates which results in thermal/mechanical stable, crystalline mesoporous materials. In this regard, a variety of porous materials have been successfully prepared by using the nanocasting method [5153].

14.2 MATERIALS FOR HUMIDITY AND GAS SENSORS 14.2.1 Mesoporous Silica The possibility of synthesizing various types of mesostructures with multiple pore architecture enhances its versatility for various applications. The Santa Barbara Amorphous (SBA-15) is a type of mesoporous silica with hexagonal-ordered tunable uniform mesopores (4 2 30 nm) with pore walls (0.3 2 4 nm), depending upon synthesis conditions. SBA-15 was prepared by using triblock copolymer (EOnPOmEOn) with polypropyleneoxide (PPO) blocks [54]. The micropores in the walls of the SBA-15 generate from polyethylene-oxide (PEO) blocks whereas the PPO is more hydrophobic and gives rise to the internal structure of the mesopore. The changes in the pore wall thickness and different amounts of micropores could be obtained by changing the length of polyethylene oxide blocks and also, altering the length of PPO blocks will result in different diameter. The synthesis parameters like pH, temperature, and some additives, such as salts,

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FIGURE 14.1 Schematic of process for synthesis of SBA-15. Source: Adapted with permission from V.K. Tomer, S. Devi, R. Malik,S. Duhan, Mesoporous materials and their nanocomposites, Nanomaterials and Nanocomposites, Wiley-VCH Verlag, Germany, 2016, pp. 223254. Copyright 2016 Wiley-VCH.

swelling, cosurfactants, etc. also govern the morphologies of SBA-15. The SBA-15 materials with straight and short channels can also be synthesized by decreasing the stirring rate/time or by using salts during the synthesis. The curvature and shape of the pore is important for the diffusion of molecules and ultimate adsorption capacity. The schematic process of synthesis of SBA-15 using micelles has been shown in Fig. 14.1.

14.2.2 Semiconductor Metal Oxide (MOx) The electrical resistance of semiconductor depends upon the presence of impurities at the surface. This effect was confirmed for germanium in 1953. It was studied that the conductivity of ZnO thin films after heating becomes more sensitive towards the reactive gases in the air and the same was reported for SnO2. Over the years, the SMOx-based gas sensors have emerged as the most investigated materials for the gas sensors [5559]. They have gained much attention in the field of hand-held sensing systems under ambient conditions due to the flexibility in their design, the simplicity of their use, their low cost, their compact size, and flexibility in production. Many metal oxides such as NiO, SrO, WO3, V2O3, MoO3, La2O3, CeO2, In2O3, Ta2O5, and Co3O4 are suitable for detecting humidity and oxidizing/reducing/combusting gases by conductive measurements. The target gas interacts with the surface of the metal oxide films which give rise to a change in charge carrier concentration of the materials and alter the conductivity of the materials. Typically, in an n-type semiconductor, the majority of charge carriers are electrons and upon interaction with a reducing and oxidizing gas, a respective increase and decrease in the conductivity of the materials is observed [60]. Whereas, in a p-type semiconductor, the holes are in the majority and so the opposite effects are observed. With reducing gas, an increase in resistance is observed and the negative charge induced in the materials reduces the hole charge carrier concentration, therefore a decrease in the conductivity is observed. Recently, hard templating (nanocasting) method has been used to synthesize various kinds of mesoporous SMOx (Fig. 14.2). The desired materials are inserted inside the pores of silica template, which afterwards was removed selectively. During the synthesis of mesoporous SMOx typically metal precursor salts are filled into the silica matrix [21]. The precursor is then converted into the metal oxide by using thermal decomposition

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FIGURE 14.2 Schematic of process for synthesis of mesoporous Ag/(TiO2SnO2) nanocomposite. Source: Adapted with permission from V.K. Tomer, S. Duhan, Ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite based highly sensitive and selective VOC sensors, J. Mater. Chem. 4 (2016) 10331043. Copyright 2016 The Royal Society of Chemistry.

sometimes preceded by a pH-induced conversion. By chemical etching with hydrofluoric acid (HF) or sodium hydroxide (NaOH) solution, the parent silica matrix is removed in order to get the final mesoporous SMOx.

14.3 AgSnO2/SBA-15 NANOHYBRIDS-BASED HUMIDITY SENSORS Recently, Tomer et al. have reported the synthesis of highly ordered mesoporous AgSnO2/SBA-15 nanohybrids for humidity sensing applications [61]. To improve the sensitivity, response-recovery characteristic, selectivity, and stability of the sensor, the SnO2 was in situ loaded in the pristine SBA-15 by using a simple hydrothermal method. To the compound of SnO2/SBA-15, the highly conducting Ag nanoparticles were introduced by using wet-impregnation process. The mesoporous SBA-15 matrix offers multifunctionality by encouraging the large-scale molecular diffusion and transportation of charge carriers across the material’s surface. In AgSnO2/SBA-15, the SnO2 fills the double need of a scattering agent for controlling the aggregation of Ag nanoparticles and also

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being an n-type semiconductor, playing an important role in enhancing the sensing performance of the nanocomposite. By combining both in situ and wet-impregnation methods, the resulting nanohybrid had exhibited excellent sensitivity towards detection of relative humidity [22]. The LAXRD patterns of SBA-15, SnO2/SBA-15, and AgSnO2/SBA-15(X) nanocomposites are shown in Fig. 14.3(A). The strongest reflection was observed at 2θ 5 0.8 , which relates to the presence of cylindrical hexagonally arranged mesopores [62]. The AgSnO2/SBA-15(X) nanocomposite present similar pattern as that of pure SBA15, with diffraction plane indexed to the (1 0 0), (1 1 0), and (2 0 0) reflections of twodimensional hexagonal mesostructure [63], however, their d-spacing shifts slightly to a higher angle with increasing Ag concentration due to the filling of pore walls of SBA-15 (Table 14.1). Also, a decrease in the intensity of [1 0 0] reflection was observed with the increase in Ag content which could be due to an increase in the number of Ag

FIGURE 14.3 (A) Low-angle XRD (LAXRD); (B) wide-angle XRD (WXRD) spectra; (C) N2 adsorptiondesorption isotherms curves; and (D) pore size distributions curves for SBA-15, SnO2/SBA-15, and AgSnO2/SBA-15(X) nanohybrids, where X 5 1, 2, 3. Source: Adapted with permission from V.K. Tomer, S. Devi, R. Malik, S.P. Nehra, S. Duhan, Fast response with high performance humidity sensing of AgSnO2/SBA-15 nanohybrid sensors, Microporous Mesopor. Mater. 219 (2016), 240248. Copyright 2016 Elsevier.

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TABLE 14.1 Structural and Textural Properties of Mesoporous SBA-15, SnO2/SBA-15, and AgSnO2/SBA15(X) Nanocomposites as Determined From XRD, N2 AdsorptionDesorption, and EDX Analysis Sample

d100-spacing (nm)a

a0 (nm)b

DP (nm)c

DW (nm)e

VP (cm3/g)d

SBET (m2/g)f

Effective Ag (wt.%)g

SBA-15

10.54

12.17

8.98

3.19

1.25

878



3.69 3 102

SnO2/ SBA-15

10.24

11.82

7.11

4.71

1.02

661



2.1 3 105

X51

9.83

11.35

6.68

4.75

0.94

607

0.62 (62%)

4.7 3 105

X52

9.15

10.57

5.77

4.8

0.85

558

1.12 (56%)

12.3 3 105

X53

8.37

9.66

4.82

4.84

0.79

506

1.52 (51%)

2.5 3 105

Sensitivity (S)h

a

d100: d-spacing. a0: unit cell parameter [a0 5 2 3 d100/O3]. c DP: pore size. d VP: pore volume. e DW: pore wall thickness [DW 5 a0DP]. f SBET: total surface area. g Calculated from EDX studies. h Sensitivity, S 5 Im11%/Im98%. Adapted with permission from V.K. Tomer, S. Devi, R. Malik, S.P. Nehra, S. Duhan, Fast response with high performance humidity sensing of AgSnO2/SBA-15 nanohybrid sensors, Microporous Mesopor. Mater. 219 (2016), 240248. Copyright 2016 Elsevier. b

nanoparticles attached to the inner pore walls of the SnO2/SBA-15. The result also indicates that loading of Ag in SnO2/SBA-15 does not destroy the mesoporous structure of nanocomposite. The WXRD patterns of SBA-15, SnO2/SBA-15, and AgSnO2/SBA-15(X) in Fig 14.3(B) demonstrates well resolved diffraction peaks of (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (1 1 2), and (3 0 1) that can be indexed to SnO2 with JCPDS no. 03-1116 [64]. Small diffraction peaks of Ag0 reflection at 2θ 5 44.2 and 64.3 which correspond to (2 0 0) and (2 2 0) planes were observed for 3 wt.% of Ag (JCPDS no. 04-0783). For the samples containing 12 wt.% Ag, no reflection for Ag was observed. The growing pattern of Ag reflection peaks confirms the loading of Ag in SnO2/SBA-15 nanocomposite. The N2 adsorptiondesorption isotherms curves and pore size distributions curves of SBA-15, SnO2/SBA-15, and AgSnO2/SBA-15(X) materials are shown in Fig. 14.3(C) and (D), respectively. A sharp inflection in P/P0 . 0.6 associated with capillary condensation was observed for all the samples which denote a steep rise in the N2 adsorption volume, thus confirming the mesoporous structures and narrow pore size distributions. The summarized results in Table 14.1 suggest that the textural properties (specific surface area, pore diameter, and pore volume) of hybrid nanocomposites systematically decrease with the increased amount of Ag nanoparticles but this decreasing pattern is not directly proportional to the amount of Ag nanoparticles. Also, the decrease in pore diameter indicates that the pore wall becomes thicker, showing that some SnO2 and Ag nanoparticles have entered and stuck to the mesopores of SBA-15. The HRTEM image of the SBA-15 and SnO2/SBA-15 in Fig. 14.4(A) and (B) shows a well-ordered 2D p6mm regular hexagonal mesostructure which are in the characteristic

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FIGURE 14.4 HRTEM image showing uniform channels with long range order of (A) SBA-15, (B) SnO2/SBA15, and (C) Ag-SnO2/SBA-15(2) nanocomposite, (DF) corresponding EDX spectra of SBA-15, SnO2/SBA-15, and Ag-SnO2/SBA-15(2) nanocomposites. Source: Adapted with permission from V.K. Tomer, S. Devi, R. Malik, S.P. Nehra, S. Duhan, Fast response with high performance humidity sensing of AgSnO2/SBA-15 nanohybrid sensors, Microporous Mesopor. Mater. 219 (2016), 240248. Copyright 2016 Elsevier.

(0 0 1) direction of the one-dimensional channels templated [65]. The SnO2 nanoparticles were located uniformly in the mesoporous channels of SBA-15. As discussed, the Ag0 nanoparticles were loaded in SnO2/SBA-15 nanocomposite in their metallic state. The loading process initiated during the process of calcination in wet impregnation method and the pore channels of SBA-15 remain intact even after loading of Ag nanoparticles. Fig. 14.4(D)(F) shows the EDX spectra of SBA-15, SnO2/SBA-15, and AgSnO2/SBA-15(2),

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respectively. The EDX results reveal that Ag and SnO2 nanoparticles have been loaded in the mesoporous SBA-15 silica material. However, the efficiency of Ag loading in SnO2/SBA-15 decreases with loading of Ag nanoparticles in the SBA-15 matrix as shown in Table 14.1. This could be due to constant pore blockage of SBA-15 leading to the reduction of effective Si 2 OH bonding sites for Ag nanoparticles. The dependence of sensor impedance with respect to a change in humidity is shown in Fig. 14.5(A). The pure SBA-15 shows a poor response during the 1198 %RH range, however, a steep drop in impedance was observed when SnO2 and Ag nanoparticles were

FIGURE 14.5 (A) Humidity sensing curves showing decrease in impedance with increase in %RH for SBA-15, SnO2/SBA-15, and AgSnO2/SBA-15(X) nanocomposites; (B) hysteresis curve showing adsorptiondesorption responses measured in the 1198 %RH range for AgSnO2/SBA-15(2) nanocomposite; (C) response/recovery time of the AgSnO2/SBA-15(2) sensor for humidity levels between 11 %RH and 98 %RH and repeated measurement of impedance in four RH loop to determine the stability of the sensor. Source: Adapted with permission from V.K. Tomer, S. Devi, R. Malik, S.P. Nehra, S. Duhan, Fast response with high performance humidity sensing of AgSnO2/SBA-15 nanohybrid sensors, Microporous Mesopor. Mater. 219 (2016), 240248. Copyright 2016 Elsevier.

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loaded in the SBA-15 matrix. A change of around 5.5 orders was observed for SnO2 loaded in SBA-15, but when the Ag nanoparticles were loaded in the SnO2/SBA-15, a change of six orders was observed in the 1198 %RH range. Therefore the hybridization of SnO2 and Ag nanoparticles improved the response of SBA-15 based sensor. Besides, the response to %RH increases from 1 wt.% and 2 wt.% Ag loaded in SnO2/SBA-15, where after, the sensitivity decreases due to blockage of pore channels of SBA-15 with Ag nanoparticle. The blockage causes the obstacle in charge carrier movements across the pore channels. To determine stability of the sensor, the hysteresis error was calculated by measuring the time slack in absorption/desorption processes in 1198 %RH range. The hysteresis error ðγHÞ was calculated by using the expression, γH 5 6 ðΔHmax =2FFS Þ, where, ΔHmax is the difference in output of adsorption and desorption processes and FFS is the full scale output [66]. As can be seen in Fig 14.5(B), the adsorption and desorption curves closely coincide with each other, which illustrates the excellent reversible characteristics of the sensor, however, the hysteresis error, was calculated to be 0.9%, which indicates a good reliability of the sensor. The response and recovery times are the vital parameters to measure the performance of humidity sensing materials and can be defined as the time taken by the sensor to achieve 90% of impedance change in the case of the process of adsorption and desorption, respectively [67,68]. As can be seen in Fig 14.5(C), the sensor demonstrates a response time of 5 s, and recovery time of 8 s. To determine the dynamic response, the AgSnO2/SBA15(2) nanocomposite-based sensor was switched between closed chambers in four loops (11 %RH-98 %RH-11 %RH-98 %RH-11 %RH). The standard deviation in the process of exchanging of sensor reaction was calculated to be 0.8% which also determines an excellent stability of the sensor. The %RH sensing mechanism for AgSnO2/SBA-15(2) can be illustrated as follows. Due to its high surface area, uniform pore channels, and high pore volume, the mesochannels encourage the easier absorption of water molecules and help in charge transportation on the internal and external surface. The water molecules are primarily physisorbed onto the available active sites of the AgSnO2/SBA-15(2) surface through double hydrogen bonding, which is also known as first-layer physisorption of water. However, the water molecules are unable to move freely because of the constraints from double hydrogen bonding. The hopping of protons between adjacent hydroxyl groups in the first-layer physical adsorption is very difficult and so the sensor exhibits strong impedance under low %RH conditions. The multilayer physical adsorption of water molecules takes place with an increase in %RH. Further physisorption of water molecules proceeds through single hydrogen bonding on hydroxyl groups and these physisorbed water molecules can be ionized by applying an electrostatic field to produce a large number of hydronium ions (H3O1) as charge carriers. The physisorbed water layers gradually show liquid-like behavior (Fig. 14.6) with further increase in %RH, and the process of proton hopping becomes easier between the adjacent water molecules which causes a dramatic increase in conductivity of the AgSnO2/SBA-15(2) sensor [69]. The conductivity of the sensor depends upon the water absorbed by surface of complex nanocomposite AgSnO2/SBA-15(2). The presence of water vapor is not much at low % RH conditions. However, with the increase in %RH, successive layers of water molecules

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FIGURE 14.6 A schematic of adsorption of water layers on the AgSnO2/SBA15 surface. Source: Adapted with permission from V.K. Tomer, S. Devi, R. Malik, S.P. Nehra, S. Duhan, Fast response with high performance humidity sensing of AgSnO2/SBA15 nanohybrid sensors, Microporous Mesopor. Mater. 219 (2016), 240248. Copyright 2016 Elsevier.

are absorbed on the sensor surface and resembles a bulk liquid phase of water. The protons are released from the hydration of H3O1 (H3O1-H2O 1 H1) at these layers and become the main source of charge carriers. The oxygen molecules of water atoms are adsorbed on the Ag nanoparticles by giving their lone-pair of electrons, which in turn increases the number of e2 in the conduction sp band of Ag. The process of ionization increases from 11 %RH to 98 %RH and a significant increase in charge carriers leads to a remarkable approximate six orders change in impedance.

14.4 MESOPOROUS AgTiO2/SnO2 NANOHYBRIDS-BASED GAS SENSORS The detection of VOCs commonly present in our indoor climate is an important aspect in order to ensure a healthy and clean environment. In this regard, Tomer et al. have reported the selective detection of important VOCs (ethanol, methanol, acetone, isopropanol, benzyl alcohol, and ethyl acetate) using highly ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite [42]. The mesoporous silica, SBA-15 was utilized to prepare ordered mesoporous Ag(TiO2/SnO2) nanocomposite (Fig. 14.2). In a typical synthesis procedure, the mesopore channels of SBA-15 were sequentially filled with SnO2, TiO2, and AgNO3 precursor salt and the mesoporous negative replica was recovered later by removing the SBA-15 by using NaOH as etchant. Herein, the materials SBA-15, TiO2/SnO2 (soft template), TiO2/SnO2 (hard template), and AgTiO2/SnO2 (hard template) have been denoted as S-15, TS-s, TS-h, and ATS-h, respectively. The low-angle XRD patterns of S-15, TS-s, TS-h, and ATS-h is shown in Fig. 14.7(A). For sample S-15, a broad characteristic peak of amorphous materials was observed at 2θ 5 0.8 along with two other peaks in the 2θ region of 1 2 2 which indicates the hexagonal mesoporous structure of the SBA-15 material. For the material TS-s prepared by using soft templates, the peak at 2θ 5 0.8 indicated the disordered mesoporous structure. The nanocomposites prepared by using hard templates, TS-h and ATS-h, shows a similar

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FIGURE 14.7

(A) LAXRD spectra (2θ range 0.53.0 ); (B) XRD spectra in (2θ range 1570 ); (C) N2 adsorptiondesorption isotherms curves; and (D) pore size distributions curves for S-15, TS-s, TS-h and ATS-h materials. Source: Adapted with permission from V.K. Tomer, S. Duhan, Ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite based highly sensitive and selective VOC sensors, J. Mater. Chem. A, 4 (2016), 10331043. Copyright 2016 The Royal Society of Chemistry.

strong reflection between 2θ 5 0.8 2 0.9 . The presence of this (1 0 0) peak represents the long range mesoporous structure of the nanocasted samples [70]. For the sample ATS-h, a little rightward shift in reflection at 2θ 5 0.8 towards higher angles was observed as compared to TS-h, which can be related to the reduction in X-ray scattering contrast between pore channels and the Ag nanoparticles. The information obtained from the LAXRD results is listed in Table 14.2. Fig. 14.7(B) shows the XRD pattern for S-15, TS-s, TS-h, and ATS-h materials. For pure mesoporous silica, S-15, a broad peak centered at 2θ 5 22 was observed which corresponds to the fingerprint reflection for the pristine material. For TS-s, TS-h, and ATS-h samples, well resolved reflections of tetragonal rutile SnO2 were observed at 2θ 5 26.9 , 34.2 , 38.2 , 52.3 , 54.9 , 57.8 , 62.2 , 64.6 , and 66.2 which, respectively, correspond to (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (1 1 2), and (3 0 1) planes (JCPDS no. 03-1116). No obvious peaks are observed for the impurities or silica contents. The TiO2 characteristic peaks were also observed at 2θ 5 25.3 , 37.6 , 48.4 , 53.8 , and 68.7 . No characteristic diffraction peaks of Ag, Ag2O, or Ag2O3 in the sample ATS-h was observed, which could be due to the low amounts of Ag.

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TABLE 14.2

Physiochemical Properties of S-15, TS-s, TS-h, and ATS-h Materials

Sample

SBET (m2/g)a

DP (nm)b

VP (cm3/g)c

S-15

872

8.98

1.31

TS-s

21

5.01

0.19

TS-h

78

6.32

0.33

ATS-h

49

5.74

0.26

a

SBET: total surface area. DP: pore size. VP: pore volume. Adapted with permission from V.K. Tomer, S. Duhan, Ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite based highly sensitive and selective VOC sensors, J. Mater. Chem. A, 4 (2016), 10331043. Copyright 2016 The Royal Society of Chemistry. b c

The adsorptiondesorption isotherms for SBA-15 template in Fig. 14.7(C) display type IV isotherms with H1-type hysteresis loops which represent the characteristic of capillary condensation within uniform pores [24]. A steep increment in the adsorption curves at P/P0 . 0.6 denotes sudden adsorption of nitrogen and confirms the coexistence of a mesoporous structure. A linear increment in the slope from P/P0 5 0.60.9 was observed which corresponds to the capillary condensation of N2 in the mesoporous structure and is related to the substantial interparticle porosity and irregular pores of gas bubbles. The information related to the surface area, pore size (Fig 14.7(D)), and pore volume information are summarized in Table 14.2. The structures and morphologies of the S-15, TS-s, TS-h, and ATS-h were characterized by HRTEM analysis. Fig. 14.8(A) 2 (C) clearly show the presence of long range of ordered pore channels in S-15 template. A similar pattern of pore channels was observed for TS-h and ATS-h in Fig. 14.8(D) 2 (I), respectively, showing that nanocasted sample retains the inverse replica of the mesoporous S-15 template. Highly distributed Ag nanoparticles can also be located within the pore channels of TiO2/SnO2 nanocomposite in Fig 14.8(I)(K) shows the HRTEM of TiO2/SnO2 nanocomposites synthesized using conventional surfactant and FESEM images in Fig. 14.8(L) and (M) show the beautiful hierarchical flower-like morphology for sample TS-s. The temperature-dependent sensing response of the as-prepared materials towards 50 ppm ethanol was tested in the temperature range of 150400 C (Fig 14.9(A)). The pure S-15 template shows no response towards ethanol gas. The gas sensor based on TiO2/SnO2 nanocomposite prepared using soft template shows a maximum response (R 5 Ra/Rg) of 14 at 275 C, however, the TiO2/SnO2 sample synthesized using hard templating of SBA-15 shows a good response of 38 at 275 C which could be due to its high intrinsic surface area and ordered mesoporous pore channels. For Ag-doped mesoporous TiO2/SnO2, the response rises monotonically in the range of 200275 C and reaches the maximum response of 53 at 275 C then decreases with a further increase of the operating temperature. The response of the ATS-h sensor towards ethanol gas was B1.4 and 3.7 times higher than TS-h and TS-s sensor, respectively, at the same operating temperature.

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FIGURE 14.8 HRTEM image showing uniform channels with long range order of (AC) S-15 material; (DF) TS-h material; (GI) ATS-h material; (J, K) TS-s material; and (L, M) FESEM images of TS-s material. Source: Adapted with permission from V.K. Tomer, S. Duhan, Ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite based highly sensitive and selective VOC sensors, J. Mater. Chem. A, 4 (2016), 10331043. Copyright 2016 The Royal Society of Chemistry.

Fig. 14.9(B) illustrates the response of the as-prepared sensors against different concentrations of ethanol gas (1, 5, 10, 30, 50, 75, 100, 200, 300, 400, and 500 ppm) at fixed working temperature of 275 C. It was observed that the sensor ATS-h shows highly linear response in the concentrations range from 1 to 500 ppm. An enlarged view of all the materials in 150 ppm ethanol concentration range in the inset of Fig. 14.9(B) also shows the excellent linear behavior of the ATS-h sensor. A sensor with swift response and rapid recovery time usually made it an excellent real-time detector and found excellent usage in practical applications. Response and recovery times for gas sensors are usually defined as the time required while reaching 90% of the final and initial resistance in the case of the process of adsorption and desorption, respectively. The single cycle response and recovery behavior of the ATS-h sensor for 50 ppm ethanol operating at 275 C in Fig. 14.9(C) shows a fast response time of 3.5 s and rapid recovery time of 7 s. The long-term stability tests of the ATS-h toward 50 ppm ethanol at 275 C were carried out on alternate days. Fig. 14.9(D)

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FIGURE 14.9

(A) Responses of S-15, TS-h, ATS-h, and TS-s materials to 50 ppm ethanol at different operating temperatures (150400 C); (B) responses of the sensors based on S-15, TS-h, ATS-h, and TS-s materials to ethanol in the range from 1 to 500 ppm at 275 C (the inset shows the calibration curve in the range of 150 ppm); (C) the response and recovery of mesoporous ATS-h to 50 ppm ethanol at 275 C; (D) the long-term stability of mesoporous ATS-h to 50 ppm ethanol at 275 C; (E) the response of ATS-h versus varying concentration of test gases; and (F) the response and recovery times of mesoporous ATS-h to 50 ppm test gases at 275 C. Source: Adapted with permission from V.K. Tomer, S. Duhan, Ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite based highly sensitive and selective VOC sensors, J. Mater. Chem. A, 4 (2016), 10331043. Copyright 2016 The Royal Society of Chemistry.

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shows that the sensors exhibit nearly constant response signals without any notable increase/decrease in response even after 30 days. A standard deviation of 0.8% was recorded which further confirmed that the ATS-h sensor possesses excellent stability. The response of the ATS-h sensor versus varying concentrations (1500 ppm) of test gases at 275 C is shown in Fig. 14.9(E). Among all the test gases, the sensor responded excellently towards ethanol gas, which might be owing to the easier oxidization of the hydroxyl group at the particular temperature. Fig. 14.9(F) shows the response and recovery times of ATS-h sensor towards detection of 50 ppm test gases at 275 C operating temperature. As can be seen, the response/recovery time (3.5/7 s) for the ATS-h sensor towards detection of ethanol gas is significantly less than other gases, illustrating that the sensor ATS-h responds quickly to ethanol gas under the same test conditions. The process of ATS-h sensing towards ethanol proceeds from the interaction of gas molecules with the surface of materials which leads to a change in its electrical conductivity. When the sensor is exposed to the hot air at 275 C, the oxygen molecules in air trap free electron from the conductance band of n-type sensing layer of TiO2 and SnO2, thereby leading to the formation of O2, O22, O22, and O222. This trapping of the electrons causes an electron depletion layer and a space charge region is formed on the surface of the sensor and increases its resistance [71,72]. The ethanol gas when introduced into the gas chamber interacts with the ATS-h sensor, its surface gets oxidized by reactive oxygen species and in the process, electrons are released, which results in the increase in conductivity of the sensor. Fig. 14.10 shows the schematic of gas sensing mechanism for ATS-h material. Any sensor performance towards a target gas primarily depends upon the factors including surface area of the sensor, formation of heterojunction, and nature of the target gas. The presence of mixed oxide heterostructures causes the electron transport to precede through different potential barriers namely TiO2 and TiO2, TiO2 and SnO2, and SnO2 and SnO2. The electrons are transferred from TiO2 to SnO2 by band bending due to higher Fermi level of TiO2 thus creating a potential barrier between the two materials. These potential differences cause a barrier in the electron transportation through the nanostructures and the electrons present on the sensor surface are absorbed by the oxygen species which significantly increases the sensor response [73]. The hybridization of Ag nanoparticles and TiO2/SnO2 nanocomposite also enhanced the gas sensing response by the process of catalytic oxidation, which results in increasing the quantities of active oxygen species on the surface of the TiO2/SnO2 nanocomposite. Ag nanoparticles facilitate the faster adsorption and desorption of the oxygen molecules (O2) over the sensor surface. This process increases the quantity of adsorbed O2 molecules and enhances the moleculeion conversion rate. The catalytic nature of Ag nanoparticles promotes the disintegration of ethanol into active radicals and increases the reaction between adsorbed surface oxygen ions and ethanol gas. Due to the lower work function of Ag, the electrons move on to the TiO2/SnO2 heterojunction. This process facilitates the dissociative adsorption of oxygen on the TiO2/SnO2 surface by enhancing the formation of electron charge transfer dynamics [74,75]. Hence, the mesoporous nanocomposite-based sensor exhibited a high response, good reproducibility, quick response/recovery characteristics, and good selectivity to ppm-level ethanol, which provide a promising opportunity to exploit the use of mesoporous materials in designing futuristic commercial sensors.

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FIGURE 14.10 A schematic of an ethanol sensing mechanism by Ag(TiO2/SnO2) material. Source: Adapted with permission from V.K. Tomer, S. Duhan, Ordered mesoporous Ag-doped TiO2/SnO2 nanocomposite based highly sensitive and selective VOC sensors, J. Mater. Chem. A, 4 (2016), 10331043. Copyright 2016 The Royal Society of Chemistry.

14.5 CONCLUSION This work illustrates the tremendous success story of mesoporous materials as humidity/gas sensing materials. The presence of Agmetal oxide nanohybrids in mesoporous structures of these sensors enhanced their sensitivity and stability. The unique properties of mesoporous metal oxide materials, like high density of surface sites, high surface area, large surface to volume ratio, pore volume, availability of micropores, unique physicochemical properties, and interconnected with tunable pore channels, increases the chemical reactivity and physical adsorption of analyte molecules across their surfaces and results in enhancing the conductivity of the sensor on exposure to humid/ gas conditions. The mesoporous structure of the sensor improves its adsorption efficiency and facilitates the propagation of charge carriers across its surface. Thus, the optimized sensor exhibited a high sensitivity, rapid response/recovery time, excellent linearity, minimal hysteresis, and high stability over the period of time. Overall, the highly efficient sensor using the highly ordered mesoporous hybrid metal oxide presented a strategy to develop highly sensitive humidity/gas sensors with good stability, short response/recovery times, and selectivity for utilization in biomedical and environmental applications both in terms of cost-effectiveness and performance.

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14.6 FUTURE OUTLOOK The indoor environment and the need to combat its associated negative effects constitute one of the key global challenges of the modern age. There are many advantages of porous materials in gas/humidity sensing applications. The mesoporous nanorods of semiconducting metal oxides are known for their piezoelectric effects which can be used in the development of self-powered sensors. Such kind of sensors harvest energy from the environment (light, wind, thermal energy, mechanical vibration, and body movement). This approach can bring a suitable change in the commercial application of the gas sensors for wearable devices. The synthesis of novel functional materials which are flexible and can operate at room temperature can be utilized to design wearable electronic devices equipped with sensors for detecting pollutant, toxic, and combustible gases.

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[60] V.K. Tomer, S. Duhan, A facile nanocasting synthesis of mesoporous Ag-doped SnO2 nanostructures with enhanced humidity sensing performance, Sens. Actuators B: Chem. 223 (2016) 750760. [61] V.K. Tomer, S. Devi, R. Malik, S.P. Nehra, S. Duhan, Fast response with high performance humidity sensing of AgSnO2/SBA-15 nanohybrid sensors, Microporous Mesopor. Mater. 219 (2016) 240248. [62] V.K. Tomer, S. Jangra, R. Malik, S. Duhan, Effect of in-situ loading of nanoTitania particles on structural ordering of mesoporous SBA-15 framework, Colloids Surf. A: Physicochem. Eng. Asp. 466 (2015) 160165. [63] V.K. Tomer, S. Duhan, A.S. Sharma, R. Malik, S.P. Nehra, S. Devi, One pot synthesis of mesoporous ZnOSiO2 nanocomposite for room temperature relative humidity sensor, Colloids Surf. A: Physicochem. Eng. Asp. 483 (2015) 121128. [64] R. Malik, V.K. Tomer, P.S. Rana, S.P. Nehra, S. Duhan, Effect of annealing temperature on the photocatalytic performance of SnO2 nano-flowers towards degradation of Rhodamine B, Adv. Sci. Eng. Med. 7 (2015) 448456. [65] V.K. Tomer, R. Malik, S. Jangra, S.P. Nehra, S. Duhan, One pot direct synthesis of mesoporous SnO2/SBA15 nano-composite by the hydrothermal method, Mater. Lett. 132 (2014) 228230. [66] R. Malik, V.K. Tomer, M.S. Dahiya, V. Chaudhary, S.P. Nehra, P.S. Rana, et al., Ordered mesoporous In (TiO2/WO3) nanohybrid: an ultrasensitive n-butanol sensor, Sens. Actuators B: Chem. 239 (2017) 364373. [67] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, et al., Reporting physiosorption data for gas/solid interface with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (1985) 603. [68] S. Duhan, V.K. Tomer, Mesoporous silica: making “sense” of sensors, Advanced Sensor and Detection Materials, Wiley-Scrivener, U.S.A, 20149781118773482, pp. 149192 (Chapter 6). [69] Y. Chen, L. Yu, D. Feng, M. Zhuo, M. Zhang, E. Zhang, et al., Superior ethanol-sensing properties based on Ni-doped SnO2 pn heterojunction hollow spheres, Sens. Actuators B 166 (2012) 61. [70] S. Duhan, V.K. Tomer, Advance electronics: looking beyond silicon, Advanced Energy Materials, WileyScrivener, U.S.A, 20149781118686294, pp. 295326 (Chapter 7). [71] V.K. Tomer, S. Devi, R. Malik, S. Duhan, Mesoporous materials and their nanocomposites, Nanomaterials and Nanocomposites, Wiley-VCH Verlag, Germany, 20169783527337804, pp. 23254 (Chapter 7). [72] S. Duhan, V.K. Tomer, A.K. Sharma, B.S. Dehiya, Development and properties study of micro-structure silver-doped silica nanocomposites by chemical process, J. Alloys Compd. 583 (2014) 550553. [73] R. Malik, V.K. Tomer, V. Chaudhary, M.S. Dahiya, A. Sharma, S.P. Nehra, et al., Excellent humidity sensor based on InSnO2 loaded mesoporous graphitic carbon nitride, J. Mater. Chem. A 5 (2017) 14134. [74] V.K. Tomer, R. Malik, K. Kailasam, Near room temperature ethanol detection using Ag-loaded mesoporous carbon nitrides, ACS Omega 2 (2017) 36583668. [75] R. Malik, V. Chaudhary, V.K. Tomer, S.P. Nehra, Nanocasted synthesis of Ag/WO3 nanocomposite with enhanced sensing and photocatalysis applications, Energy Environ. Focus 6 (2017) 4348.

Further Reading R. Malik, V.K. Tomer, P.S. Rana, S.P. Nehra, S. Duhan, Facile preparation of TiO2/SnO2 catalysts using TiO2 as an auxiliary for photocatalytic and gas sensing applications, MRS Adv. 46 (2016) 31573162.

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C H A P T E R

15 Role of Oxides (Fe3O4, MnO2) in the Antibacterial Action of AgMetal Oxide Hybrid Nanoparticles R.K. Kunkalekar Parvatibai Chowgule College of Arts and Science (autonomous), Margao, Goa, India

15.1 INTRODUCTION In the olden days various antibacterial drugs such as penicillin and streptomycin were used extensively in the treatment and prevention of bacterial infections. These antibacterial drugs were highly toxic to pathogenic bacteria and were useful in controlling their growth. However, constant use of these antibacterial drugs generates resistance in pathogenic bacterial strains. Thus, these antibacterial drugs are futile against drug-resistant bacterial pathogens and fail to control different bacterial diseases. Researchers from different institutes are engaged in producing new types of antibacterial agents which are highly toxic to multiple drug-resistant bacteria and which can sustain their activity for a longer period of time. These new antibacterial agents must be effective in controlling the growth of multiple drug resistant bacterial cells and must be nontoxic towards human cells and other living being [15]. Extensive research has been focused on synthesizing metal-based antibacterial agents. Various metals, such as Cu, Au, Zn, Ag, Hg, Pb, and Te, are dreadfully harmful to bacterial cells and are effective in eradicating bacterial infections and diseases [615]. Very low concentrations of metals are required to show antibacterial effect. Different processes occur when these metals, in the form of different metal-based compounds, interact with bacterial cells. Metals inhibit the normal protein functions of bacterial cells, various enzyme-catalyzed reactions are repressed, which also leads to generation of

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reactive oxygen species, metals affect the bacterial cell membrane leading to impaired membrane function and cell destruction, and metals also affect the DNA membrane leading to DNA damage [68]. Among these various metals, silver is found to be most efficient for antibacterial application. Silver is well known for its antimicrobial effect and was used for water purification and food packing in olden days. Silver is currently used as an antibacterial agent in different applications, such as hospitals, wound and burns, dental work, fisheries, food preservation, etc. It is well known that Ag metal and Ag-based compounds are highly active in killing the bacterial cells including both Gram-positive and Gram-negative bacteria. Further, synthesis of Ag nanoparticles (Ag NPs) has attracted considerable attention of researchers and/or medical microbiologists all over the world due to their exceptional properties. Silver nanoparticles have wide applications in different fields such as catalysis, optoelectronics, biomedicine, wound dressing, biomedical devices, food preservation, etc. [1621]. Various synthetic roots are utilized for the synthesis of Ag NPs with different shapes, size, and morphological features, such as Ag nanorods, nanowires, nanoflowers, and nanospheres, which exhibit excellent catalytic and antibiotic applications against pathogenic bacteria [1722]. Ag NPs have large surface area (high surface to volume ratio) with different morphological features, which is a key factor in their application. Ag NPs can easily penetrate into the bacterial cell membrane and act as a catalyst to destroy the bacterial cell organelles through various different processes such as disruption of cell membrane, DNA damage, inhibiting functions of proteins and enzymes, etc. [68]. However, it is found that excessive use of silver nanoparticles as antibacterial agent in various fields results in potential risk to humans and the environment. Ag in its free form is toxic to human cells, it causes argyrosis and argyria. Besides this, it also affects the liver and kidney, causes irritation of the skin, eyes, respiratory, and intestinal tract. Thus, it becomes essential to maintain the concentration of free Ag nanoparticles below the permissible limits given by WHO, in order to control the detrimental effects of Ag nanoparticles on humans and the environment [6,7]. The major challenge here is to eliminate the Ag NPs from the solution after the disinfection process, as the particle size is too small in nano range, it is difficult to separate Ag NPs based on physical or chemical properties of Ag. Thus complete extraction of Ag nanoparticles is improbable, and this leads to the toxicity of Ag NPs for humans and other living systems. When Ag NPs are used as antibiotic agents alone, they tend to agglomerate which reduces their antibacterial activity due to lower numbers of Ag NPs being exposed to bacterial cells. Due to agglomeration, their stability under natural conditions decreases and they cannot be used for long-term periods, also a large amount of Ag NPs are required to exhibit better antibacterial activity. In order to overcome the different drawbacks a wide range of inorganic as well as organic materials, such as TiO2, ZnO, SiO2, Al2O3, iron oxides, manganese oxides, zeolites, activated carbon fibers, hydroxyapatite, polyvinyl alcohol, polyvinylpyrrolidone and graphine, can be employed as a supporting material for Ag NPs [615]. Here, the ultrafine Ag NPs can be highly dispersed over the surface of supporting materials, thus agglomeration is prevented and uniform dispersion leads to more exposure of Ag NPs towards bacterial cells which results in better activity. Ag NPs are immobilized over the surface of the support material by strong interactions and thus remain attached along with support material. This gives the advantage of extracting the Ag NPs along with support material

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from the solution, reducing the toxicity of free Ag NPs, which otherwise is difficult to remove. Due to greater dispersion, a lower amount of Ag NPs are required to show superior activity when combined with other materials compared to the pristine Ag NPs [1115,2326]. Among different types of materials, MnO2 nanopaticles and magnetic Fe3O4 nanoparticles with different structural and morphological features are found to be good material as a support for Ag NPs for antibacterial applications. MnO2 and Fe3O4 NPs are very stable and important inorganic materials, they have wide applications in catalysis and various other fields [2734]. Both manganese and iron are essential elements for biological system, and hence nonhazardous to human cells and are widely used in antimicrobial applications. Nano-size MnO2 and Fe3O4 with different sizes and shape can be easily synthesized by using eco-friendly synthesis methods which exhibit exceptional properties. Antibacterial agents based on Ag NPs supported MnO2 and based on Ag NPs supported on Fe3O4 can be used in different fields for controlling growth of various types of Grampositive bacterial cultures such as Staphylococcus aureus, Streptococcus epidermis, Bacillus subtilis, and Gram-negative bacterial cultures, such as Escherichia coli, Salmonella abony, and Klebsiella pneumoniae [3538].

15.2 ROLE OF Fe3O4 IN ANTIBACTERIAL ACTION OF AgFe3O4 NANOPARTICLES AND ANTIBACTERIAL AGENTS BASED ON MAGNETIC AgFe3O4 NANOPARTICLES The nano-size Fe3O4 (magnetite) is an attractive material due to its unique characteristic properties such as superparamagnetism, low toxicity, high surface to volume ratio, easy synthesis, and biocompatibility. Fe3O4 NPs with different morphological features have been synthesized using various synthetic approaches which have potential application in different fields. Fe3O4 NPs are widely used in magnetic applications such as magnetic storage devices, superparamagnetic relexometry, catalysis, gas sensing, etc. Due to their unique properties, they are extensively applied in biomedical applications, such as targeted drug delivery, high sensitivity biomolecular magnetic resonance imaging, to diagnose and treat cancer using magnetic hyperthermia, immobilization of biomaterials, environmental treatment, and biological separation [37]. The important features of Fe3O4 NPs are their nontoxicity to human cells, high stability, good potency, and easy magnetic separation from the solution which is usually a challenging task. Fe3O4 NPs can also promote faster release of Ag1 due to transfer of electron form Ag to Fe3O4 NPs. Certain chemicals produced, such as reactive oxygen species, in the presence of magnetic Fe3O4 NPs are fatal for the bacterial cells [38,39]. Thus, it can act as a good support material for Ag NPs and can be employed for antibacterial application. Ag-immobilized Fe3O4 NPs have been synthesized and employed as catalysts and antibacterial agents. Impregnation of Ag NPs on magnetic Fe3O4 NPs exhibits excellent superparamagnetic and antibacterial properties and increases functionality of both materials. Ag NPs are highly dispersed over the surface of Fe3O4, which results in higher antibacterial activity. The AgFe3O4 NPs are always found to show higher antibacterial activity with lower concentration of Ag as compared to pure Ag NPs. After treatment, extraction

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of AgFe3O4 NPs becomes easy due to magnetic separation, thus reducing the toxicity of free Ag and furthermore extracted AgFe3O4 NPs can be recycled and reused [4042]. AgFe3O4 nanocomposites also exhibit good bactericidal property. An interesting one-pot method of synthesis was used to synthesize these AgFe3O4 nanocomposites, which are used for antibacterial application [4346]. Fe3O4Ag coreshell nanostructures also have been reported to exhibit bactericidal properties. The silver coating was created by directly reducing AgNO3 with oleylamine onto the surface of the Fe3O4 nanoparticles. The effect of these coreshell nanostructures was more pronounced on Gram-negative bacterial strains, which suggests the additive action of the iron in the case of Gram-negative bacteria. This enhanced antibacterial activity of the nanostructures is due to their stability as a colloid in the medium, which results in modulation of the phosphotyrosine profile of the bacterial proteins and the arrest of bacterial growth [47]. The nickel-modified Fe3O4@Ag@Ni nanoparticles can inhibit both Gram-positive and Gram-negative bacteria, this material also has sensor properties [48]. A new type of Ag core/magnetic Fe3O4 shell nanoparticle with strong magnetic responsiveness and tunable plasmonic properties has been developed as a multifunctional platform for bacterial disinfection. These as-prepared magnetic photothermal Ag core/Fe3O4 NPs exhibit both a wide range of bacterial capture ability and excellent photothermal antibacterial activity under near-infrared irradiation due to a combination of magnetic and photothermal properties. High magnetic recyclability and low cytotoxicity are the other promising features of the magnetic photothermal antimicrobial agent [49]. Another approach is to use multifunctional hybrid nanomaterials with magnetic Fe3O4 NPs as a core and biocompatible polymer as the shell over which Ag NPs are immobilized. Different methods have been exploited to get the soluble and biocompatible Fe3O4 nanoparticles. The surface coating of magnetic Fe3O4 NPs with a biopolymer shell is an effective way to improve their biocompatibility, and improves greater stability and water dispersity. Hybrid nanocomposites NiFe2O4, NiFe2O4@PAMA (poly(acrylonitrile-co-maleic anhydride)), NiFe2O4@Ag, NiFe2O4@PAMA@Ag, and pristine Ag NPs were applied for bactericidal property against Gram-positive and Gram-negative bacterial cultures, including Staphylococcus aureus (ATCC29213), Bacillus cereus (ATCC14579), Escherichia coli (ATCC35218), and Salmonella typhimurium (ATCC14028). From Fig. 15.1 it is clear that NiFe2O4@PAMA@Ag exhibits much higher antibacterial activity as compared to NiFe2O4@Ag and Ag NPs, whereas NiFe2O4 and NiFe2O4@PAMA are inefficient materials [50]. The multifunctional polyethylene glycol (PEG) encapsulated Fe3O4Ag hybrid NPs (Fe3O4@AgPEG), provides highly efficient antibacterial activity and fluorescence, in addition, they are magnetically stable against environment and temperature. Small Ag nanocrystals well-encapsulated in the PEG shell do not exhibit any cytotoxicity [51]. The antibacterial activities of Fe3O4 nanoparticles with different organic parts, including humic acid, nicotinic acid and histidine against different bacteria were studied. These composite materials were found to be inactive, however when they are immobilized with Ag NPs, they were found to show high antibacterial activity [52]. A new type of hybrid nanocomposites NR/PE blends containing Ag NPs and NR/PE blends containing Fe3O4Ag NPs were prepared using natural rubber (NR) and polyethylene (PE). Antibacterial results show higher activity of NR/PE blends containing Fe3O4Ag NPs. Two similar reasons were proposed: (i) the faster Ag1 release rate from the Fe3O4Ag hybrid due to the

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FIGURE 15.1 Zone of inhibition of (1) NiFe2O4, (2) NiFe2O4@Ag, (3) NiFe2O4@PAMA, (4) NiFe2O4@PAMA@Ag, (5) blank disks, and (6) Ag NPs against tested bacteria.

electron transfer from Ag NPs to Fe3O4 NPs; and (ii) the fact that the ionization of Ag NPs in hybrid nanostructure might be accelerated by Fe31 ions. This hybrid nanomaterial appears to be very promising for clean water and food preservation [38]. Novel modified halloysiteFe3O4Ag/polyurea nanocomposites (mHNTFe3O4Ag/PUA) were synthesized and possess excellent magnetic properties and exhibit good antibacterial activities, better thermal stability, and mechanical properties [53]. Chitosan-functionalized endcapped Ag NPs composited with Fe3O4 NPs were also prepared which show high antibacterial activity [54]. Similarly, Ag@Fe3O4@cellulose nanocomposites exhibit good antibacterial activity and have promising application in biomedical field and public health area. The higher magnetic behavior enhanced their dye degradation capacity and increased their potential for applications in wastewater treatment [55]. Ag/Fe3O4 nanocomposites were effectively synthesized by using starch. Starch acts both as a biocompatible capping agent for Fe3O4 NPs as well as a reducing agent for the reduction of Ag ions. The schematic illustration is shown in Fig. 15.2 for stepwise synthesis of starch intervene Ag/Fe3O4 nanocomposites. These nanocomposites can be used as a targeted antibacterial therapy on bacterial infected sites under an external magnetic field [56]. The Ag immobilized Fe3O4@SiO2 coreshell nanostructures have been extensively used as a novel antibacterial agent. It is seen that reported preparation methods of Fe3O4Ag magnetic composites are inconvenient, and toxic reagents are required. Furthermore, the amount of Ag NPs combined with Fe3O4 is insufficient and the Fe3O4 core tends to be oxidized or dissolved in acidic conditions during the treatment procedure. Thus, a covering of SiO2 onto Fe3O4 NPs core has been used as a supporting matrix for immobilization of Ag NPs, it will increase the stability of Fe3O4 core, and also prevents the aggregation of the Fe3O4 Nps. Fig. 15.3 shows the TEM images for spherical Fe3O4 NPs, Fe3O4@SiO2 coreshell nanostructures and Ag NPs decorated over magnetic Fe3O4@SiO2 coreshell

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FIGURE 15.2

Schematic representation of the Ag/Fe3O4 nanocomposites synthesized in the presence of starch.

FIGURE 15.3 TEM micrographs of (A) Fe3O4, (B) Fe3O4@SiO2, (C) Fe3O4@SiO2@Ag, and (D) size distributions of Ag Nps (inset shows diffraction pattern of core shell). Insets of (AC) show their respective high magnification images.

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nanospheres, along with size distribution of Ag NPs [57]. Thus, Fe3O4@SiO2 coreshell@Ag nanocomposites combine the advantages of magnetic separation and stability and are efficient material for elimination of organic contaminants and inhibition of bacterial pathogens [5759]. A vancomycin-modified Fe3O4@SiO2@Ag microflower is a new class of hybrid nanomaterial, wherein magnetic nanomaterial combines with antibiotics, showing strong bactericidal ability, a wide antimicrobial spectrum, and good recyclability. They are efficient in rapid and effective killing of pathogenic bacteria at low concentration and exhibit higher antibacterial activity than Fe3O4@SiO2@Ag core [60]. Similarly, the antibacterial activity of Fe3O4Au@Ag magnetic nanocomposite was enhanced after combination with streptomycin, which indicates the presence of synergistic effect. Thus, Fe3O4Au@Agstreptomycin showed great potential as an excellent antibacterial agent in water environment [61]. Graphene oxide (GO)-based magnetic nanocomposites incorporated Ag NPs attract considerable research interest due to their outstanding properties, including thermal and chemical stability, high mechanical strength, large specific surface area, good electron conductivity, and water solubility. This nanocomposite is a two-dimensional monolayer of carbon nanostructure with abundant epoxy, hydroxyl, and carboxyl groups [62,63]. Ag NPs can bind with GO nanostructure by physisorption, electrostatic binding, or chargetransfer interactions. GOAg nanocomposite can be separated from solution when magnetic Fe3O4 NPs are incorporated in to GO. Therefore, AgFe3O4GO nanocomposite exhibits good stability, is easily separable, and has enhanced antibacterial property due to the synergistic effect. Various GO-based nanocomposites, such as Ag/Fe3O4@GO, AgNPs/ Fe3O4/GO, GO-Fe3O4@N-alkylated poly-4-vinylpyridine (NPVP)Ag, and Ag@GOFe3O4polyethylenimine (PEI), have been employed for antibacterial application and show better activity and can be easily recovered and reused [6265]. The fly ash-based nanocomposite (AgFe3O4/fly ash) was synthesized by simultaneous deposition of Ag and Fe3O4 NPs on fly ash particles to produce magnetically separable composites. These materials have high metal absorption efficiency and antibacterial activity due to the synergetic effect of Fe3O4 and Ag NPs [66]. Also, carbon-based magnetic Ag(Fe3O4@carbon) nanocomposites showed efficient antibacterial activities, high catalytic activity, strong adsorption ability to organic molecules, and efficient separability under magnetic field [67]. Magnetic photocatalyst nanocomposites of AgTiO2/Fe3O4 have efficient antimicrobial properties through a mechanism including photocatalytic production of reactive oxygen species which will damage the cell. Due to AgFe3O4 synergistic effect, TiO2 shows enhanced photocatalytic activity by separation of electronhole pairs [68,69].

15.3 ROLE OF MnO2 IN ANTIBACTERIAL ACTION OF AgMnO2 NANOPARTICLES AND ANTIBACTERIAL AGENTS BASED ON AgMnO2 Mn can exist in different oxidation states and forms different oxides, such as MnO, MnO2, Mn2O3, Mn3O4, and Mn2O7 depending on preparation conditions. Among different Mn oxides, manganese dioxide (MnO2) NPs are of special interest because of their unique

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properties, such as high surface area, high stability, environmental friendly, nontoxic to human cells, and easy preparation using eco-friendly methods. MnO2 NPs with different crystallographic forms and morphological features have useful technological applications in catalysis, electrochemistry, molecular adsorption, ion exchange processes, and magnetic materials [2730,35,36,7073]. The properties of MnO2 are considerably influenced by its structure, morphology, and preparative methods. The basic building block of MnO2 is MnO6 octahedra which are connected in different ways by shearing their edges and corners which results in variety of polymorphs of MnO2 having infinite channels and different dimensions [74,75]. Several reports are available which demonstrate the use of different manganese oxide NPs for antimicrobial application. Bioactive curcuminaniline functionalized manganese oxide NPs show better bactericidal activity by resisting bacterial growth when compared with nonfunctionalized manganese oxide NPs [76]. Comparative studies have been done with respect to bactericidal property of ZnO and MnO2 NPs, and results show better activity by ZnO NPs [77]. Other nanocomposites like Mn- and Fedoped ZnO showed maximum antimicrobial and photodegradation activity due to the generation of reactive oxygen species due to the synergistic effects of Mn and Fe loading [78]. Similarly, citric acid-modified manganese ferrite nanocomposites were able to inhibit fungal pathogens, but were not capable of inhibiting Gram-negative bacteria and Grampositive bacteria [79]. MnO2 NPs synthesized by a biomineralization approach were found to show very poor activity in controlling the growth of microbial pathogen (Shewanella oneidensis) [80]. Similarly, MnO2 NPs synthesized by simple coprecipitation method and by green chemistry reduction approach do not exhibit any antibacterial activity against either Gram-negative bacteria or Gram-positive bacteria [35,36,81]. Thus, it is found that MnO2 and other Mn oxide NPs have poor activity to inhibit the growth of bacterial pathogens. However, when MnO2 NPs combine with some other base metals and noble metals, they tend to show exceptional properties. As it is known that, the Ag NPs get agglomerated easily, this reduces the surface-tovolume ratio, and they are required in large amounts to obliterate harmful bacterial cells. Thus, incorporation of Ag in metal oxides will be advantageous and useful in antibacterial applications. In this regards, Ag incorporated into MnO2 NPs will be of great importance as MnO2 NPs are very stable, nontoxic, and environmental friendly. Ag-doped MnO2 nanomaterial has been applied for antibacterial applications and exhibits superior activity [35,36,81,82]. These materials have high surface area and silver is uniformly dispersed over the MnO2 matrix. Due to higher dispersion, Ag can easily interact with bacterial cells, furthermore a very much lower concentration of Ag is required to show higher antibacterial activity as compared to pristine Ag NPs which are usually required in higher amounts. Ag has strong interactions with MnO2 lattice, and due to these strong interactions Ag will be retained within the MnO2 lattice and consequently the toxicity of free silver to the human cells and the environment will be reduced.

15.4 CONCLUDING REMARKS Silver nanoparticles are good antibacterial agents for controlling the growth of various multidrug resistant Gram-positive and Gram-negative bacterial cultures. However,

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pristine Ag NPs have several disadvantages, such as toxicity to human cells, low stability, high agglomeration rate, due to which larger amounts of Ag NPs are required for better activity (as surface-to-volume ratio decreases), and there is difficulty in separation after treatment. To overcome all these drawbacks, it is advantageous to use metal oxide NPs such as Fe3O4 and MnO2 over which the Ag NPs are immobilized. Both Fe3O4 and MnO2 NPs are very stable, nontoxic materials over which Ag NPs are finely dispersed and show much better activity due to synergistic effect. Fe3O4 NPs also promote the antibacterial property of Ag NPs. Due to strong interactions of Ag NPs with Fe3O4 and MnO2 NPs, the toxicity of free silver is greatly reduced. Separation of these nanomaterials becomes easy, and furthermore they can be recycled and reused. These nanomaterials are very stable and can be coalesced with effective antibiotics to treat bacterial infections.

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C H A P T E R

16 Noble MetalManganese Oxide Hybrid Nanocatalysts Sujit Kumar Ghosh1 and Hasimur Rahaman2 1

Department of Chemistry, Assam University, Silchar, India 2Department of Science and Humanities, Contai Polytechnic, Purba Medinipur, West Bengal, India

16.1 INTRODUCTION During the last three decades, with the advent of the art of nanomaterials synthesis, the design of hybrid assemblies by controlled miniaturization of two or more disparate materials containing two or more different functionalities represents a potential approach for achieving advanced materials that have become more and more indispensable to fulfill both scientific and technological perspectives [16]. Synthetic approaches offering high degree of control over composition and morphology with a wide range of material combinations have become an effective strategy for inducing new physicochemical properties and realizing multifunctionality governed by the synergistic effect among the individual components [7]. These hybrid assemblies exhibit new optical, electrical, magnetic, mechanical, chemical, thermodynamic, and catalytic properties that can be tuned by controlling their composition, size, shape, and organization at the nanoscale. The tunable properties, along with the chemical and biological accessibility, open up new opportunities to spread their interest in a diverse range of niche applications, such as ferrofluids, medical imaging, drug targeting and delivery, cancer therapy, separations, and catalysis [8]. An exponential growth of research activities has been seen in nanoscience and nanotechnology since the 1990s. When the size of the materials becomes smaller and smaller and shrink to the nanometer length scale, new physicochemical properties emerge inside the nanostructures. Manganese oxides are materials of considerable importance due to their remarkable diversity of atomic architectures that inspire newer technological applications [9]. When manganese oxides are allowed to interact with noble metal nanostructures

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to form metal oxide hybrid assemblies, new functionalities emerge which are very promising for a wide range of applications. There are many excellent reviews on this topic that report on the preparation, properties, and application of these nanoscale materials [1015]. In this chapter, we have reviewed the recent developments in the catalytic applications of structure-diversified manganese oxide nanostructures and their composites with noble metals along with mesoporous, hollow, or multilayered structures. Since the increasing energy demand and environmental pollution seek for clean and sustainable energy solutions, special emphasis has been given to elucidate their representative catalytic applications, including catalytic conversion and sensing of CO, NOx, SOx, and other volatile organic compounds (VOCs), decomposition of ozone and hydrogen peroxide, organic reduction and oxidation, removal of bacterial pathogens, epoxidation of olefins, coupling reaction, photo-/electrochromics, and water splitting reaction. The aim of this chapter is not only to inspect the current progress and provide a comprehensive insight towards the application in catalysis, but the accumulated knowledge establishes the foundation of the mechanism of the several applications. Emerging research directions and arenas are envisioned at the end to solicit more exploitation of the versatile catalytic activities of manganese oxides and metal oxide hybrids at the smaller dimensions.

16.2 THE CHEMISTRY OF MANGANESE OXIDES: DIFFERENT OXIDATION STATES Manganese (Mn) is the 10th most abundant element in the Earth’s crust and geochemically, behaves like Mg, Fe, Ni, and Co and tends to partition into minerals that form in the early stages of magmatic crystallization [16]. Mn is readily depleted from igneous and metamorphic rocks by interactions with surface water and groundwater and is highly mobile, as Mn(II), in acidic aqueous systems. Near the Earth’s surface, Mn is easily oxidized, giving rise to more than 30 different naturally occurring crystal forms of oxides/ hydroxides (referred to generally as manganese oxide, MnOx, where x ranges from 1.0 to 2.0) of manganese minerals [17] that can adopt a vast number of structural geometries [18]. These oxides participate in a variety of natural chemical reactions that govern the composition of natural waters, the arability of soils, and the potability of both subsurface and surface waters [16]. Thus, these oxides play a major role in determining the mineralogy and geochemistry of Mn in the upper crust and are the major source of manganese in the industry. There are several reasons that account for the complexity of chemistry, physics, biology, and materials science [1921] of manganese oxide minerals. First, manganese is available in nature in various oxidation states (II, III, IV), giving rise to a range of multivalent phases. Second, the oxides of manganese display a remarkable diversity of atomic architectures, many of which easily accommodate a wide variety of other metal cations. Finally, manganese is abundant in most of the geological environments and forms minerals under a wide range of chemical and temperature conditions, and through biological interactions. Manganese forms the stable oxides MnO (manganosite), α-Mn2O3 (bixbyite), α-Mn3O4 (hausmannite), and β-MnO2 (pyrolusite) as well as the metastable Mn5O8 [22]. Moreover, MnO2 exists in many polymorphic forms (such as, α, β, γ, and δ), which are different in

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FIGURE 16.1 Structural representation of nine crystalline MnOx materials, viz., α-MnO2, β-MnO2, R-MnO2, γ-MnO2, λ-MnO2, δ-MnO2, LiMn2O4, Mn2O3, and Mn3O4. Source: Reprinted with permission from R. Pokhrel, M.K. Goetz, S.E. Shaner, X. Wu, S.S. Stahl, J. Am. Chem. Soc. 137 (2015) 83848387. Copyright (2015) American Chemical Society.

the linkage of the basic octahedral unit [MnO6]. They generally show different physicochemical properties for various applications. Furthermore, manganese oxyhydroxide, MnOOH, is used as an effective precursor to synthesize intercalation compounds and lithium manganese oxides for rechargeable lithium-ion batteries [23]. A structural representation of nine crystalline MnOx materials [24] is shown in Fig. 16.1.

16.3 NOBLE METALMANGANESE OXIDE HYBRIDS Considering the earth abundance and biogeneticity, manganese oxides have received a great deal of attention as the model compound due to their unique physical, chemical, structural, and thermodynamic properties [25]. The synthesis and study of manganese oxides particles with well-controlled morphology and a narrow size distribution is desirable for achieving practical applications, such as catalysis. The catalytic activities of manganese oxides are dependent on their chemical compositions and crystallographic structures, as well as morphologies and pore structures [2629]. While the catalytic activity of these materials at the nanoscale dimension depends strongly on their specific surface area and surface-to-volume ratio, the reactivity and selectivity of the specific nanostructures can be tuned through controlling the morphology due to their limited size and distinct crystallographic planes governed by their shape [30]. On the other hand, precious metals are alluring and magical because of their inertness toward chemical reactions; they are extremely stable such that they are known as “noble metals.” However, by careful manipulation of their chemical inertness, the chemistry of precious metals has emerged as one of the exciting frontiers for advanced understanding and applications, particularly, in catalysis. Therefore, noble metal nanoparticles (Ag, Au, Pd, and Pt) have, often, been doped or decorated into/on the manganese oxides. The resultant nanocomposites have enhanced thermal stability and sintering resistance exhibiting two or more different functionalities that have garnered attention for enhanced catalytic properties [31]. While the synergistic properties of the metalmanganese oxide conjugates have shown promising catalytic activity towards various chemical reactions, the mechanistic understanding has been obvious and far more sophisticated with our increased understanding and control of the atomic world [32].

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16.4 APPLICATIONS IN CATALYSIS Although, the efficacy of earth-abundant transition metal catalysts was introduced at the very beginning of the 20th century for synthesis of ammonia in the HaberBosch process, the earth-abundant transition metal catalysts with dimension at the nanoscale have only been extensively investigated during the past couple of decades [20,33]. The earth abundance, low cost, environmentally benign nature, high thermal and mechanical stability make the family of manganese oxides promising candidates for outstanding catalytic applications [34,35]. Nanostructured noble metal particles have, typically, been dispersed on inorganic metal oxides, providing mankind with the largest class of technologically useful catalysts. The chemical interactions of the metal oxides with the noble metal nanoclusters have been explored through the development of atomic-scale microscopes in the landscape of scientific discoveries. Fundamental understanding of underlying mechanisms for catalytic processes has been an obvious requirement to meet the clean energy demand for the sustainable survival of the society [3641].

16.4.1 CO Oxidation During the past several decades, the oxidation of carbon monoxide (CO) formed during the combustion of fossil-derived fuels has become environmentally and industrially important for air/gas purification, life-support respirations, removal of exhaustive automobiles/fractions, and protection of Pt electrode in low temperature fuel-cell system [42]. Since, most applications involving CO oxidation occur in moist environments with rapid flow rates under ambient condition, manufacturing of highly moisture tolerant and operative at low temperature catalysts is necessary. Manganese oxides have shown very high activity towards the catalytic oxidation of CO at low temperatures [43]. Moreover, manganese oxides in combination with noble metals can serve as highly active thermally stable catalysts that can operate at temperatures significantly lower than the respective noble metal-based catalysts [44]. When combined with gold nanoparticles, significantly enhanced activity has been achieved over the Au/MnOx system for low-temperature CO oxidation in the absence or presence of H2 for fuel cell applications. Gardner’s group reported that Au/MnOx catalysts show excellent catalytic performance at low-temperature (,100 C) in stoichiometric mixtures of carbon monoxide and oxygen containing no carbon dioxide in the feed gas [45]. Haruta and coworkers [46] observed 100% CO conversion on Au/MnOx catalysts in H2 stream, which can be applicable for selective CO oxidation in H2 streams of polymer electrolyte fuel cells. Hoflund et al. [47] suggested that the ratio of Au to Mn in the precursor solution must be properly chosen to obtain high CO conversion. This observation is consistent with Haruta’s assertion that the total surface area of exposed Au metal increases initially with Au content, but may decline when the gold loading increases further due to the coagulation of Au particles [48]. In fact, gold nanoparticles supported on manganese oxides have been found to be the most active gold catalysts to convert CO to CO2 at low temperatures [49,50]. Gold nanoparticles loaded onto MnOx/C (Au/MnOx/C) shows higher catalytic activity in CO oxidation compared with the

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corresponding Au/C [51]. It is observed that the Au/MnOx/C catalyst system combines the high activity and temporal stability of the AuMnOx interface. Metal-loaded octahedral molecular sieves are also highly active for CO oxidation at low temperatures and show a correlation among average oxidation number of Mn and the position and nature of the doped cation [52]. Catalytic oxidation of CO over coppermanganese oxide catalysts with various Cu content is shown in Fig. 16.2. The improved catalytic activity in CO oxidation observed for the CuMn catalysts could be attributed to a greater amount of adsorbed oxygen species and high lattice oxygen mobility due to the formation of a Cu1.5Mn1.5O4 spinel active phase (Cux 21 2 Mnx 31 2 ½Oðy2zÞ ʘz ) species [53]. A schematic illustration of the proposed mechanism for NO reduction by CO over CuMn mixed oxides is presented in Scheme 16.1. The deposition of gold to a mixed coppermanganese oxide (Hopcalite) surface results in enhanced activity of the Hopcalite for ambient temperature CO oxidation [54]. It is expected that the introduction of gold will introduce new active sites to the Hopcalite that are associated with the gold nanoparticles. The enhanced activity of metal oxide catalysts has imbued growing interest in understanding the mechanistic aspects of CO oxidation at low temperatures [55]. For goldcoupled binary catalysts, it is now well-established that the catalytic activity of gold depends not only on the size of the particles, but also the nature of the support material, the preparation method, and the activation procedure [56,57]. It has been proposed that the reducible metal oxide support supplies oxygen to form active gold sites [58]. The unique catalytic nature of supported gold can be explained by assuming that the goldmetal oxide perimeter interface acts as a site for activating at least one of the reactants, for example, oxygen [59,60]. Other explanations focus on the charge transfer between the support,

FIGURE 16.2 Catalytic oxidation of CO (averages of three measurements) over CuMn catalysts with various Cu contents. Source: Reprinted with permission from T. Liu, Y. Yao, L. Wei, Z. Shi, L. Han, H. Yuan, et al. J. Phys. Chem. C 121 (2017) 1275712770. Copyright (2017) American Chemical Society.

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SCHEME 16.1 Schematic illustration of the proposed mechanism for NO reduction by CO over CuMn mixed oxides (blue boxes, surface oxygen vacancies; red boxes, surface synergetic oxygen vacancies). Source: Reprinted with permission from T. Liu, Y. Yao, L. Wei, Z. Shi, L. Han, H. Yuan, et al. J. Phys. Chem. C 121 (2017) 1275712770. Copyright (2017) American Chemical Society.

particularly, negatively charged defects (F centers), and the gold particles [61]. In addition, it has also been suggested that the effects of low coordinated sites and surface roughness may play an active role during the reaction [62].

16.4.2 Selective Reduction and Decomposition of NOx and SOx One of main challenges in the power and chemical industries is to remove generated toxic or environmentally harmful gases before atmospheric emission [63]. Nitrogen oxides (NOx) are major pollutants in the atmosphere, being a precursor to acid rain, photochemical smog, and ozone accumulation, and are corrosive and hazardous to health. On the other hand, SOx emitted from the combustion of fossil fuel is directly harmful to human health and even can be fatal. Manganese oxides have been exploited for the reduction of nitric oxide [64], and NO and NO2 decomposition [37,65]. In the studies of decomposition of NO2 over Mn2O3 and Mn3O4 catalysts, it has been suggested that this reaction requires temperatures of .773K for significant rates to occur [37]. Again, Mn2O3 is more active than Mn3O4 for NO

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decomposition, perhaps because Mn2O3 releases more lattice oxygen than Mn3O4 during the reaction [66]. The reaction pathway follows the LangmuirHinshelwood model involving a surface reaction between two NO molecules [37]. Manganese oxides deposited with metallic nanoparticles have been found to participate in selective catalytic reduction of NOx with NH3 [67,68] and to remove NOx and SO2 from air as well as to catalyze their decomposition [69,70].

16.4.3 H2O2 Decomposition Manganese oxides have been found to be very active catalysts for the decomposition of H2O2 into hydroxyl radicals (aOH) in the catalytic peroxidation of organic effluents, especially at low concentrations (,10 mM) of organic compounds [71]. The excellent activity could be attributed to the high surface area as well as the large number of oxygen vacant sites [72]. From an environmental viewpoint, the experiment could be useful for H2O2 removal applications, such as water treatment or peroxide bleaching.

16.4.4 Decompositon of Ozone Catalytic ozone decomposition is of practical significance because ozone is a toxic substance, which is commonly found or generated in human environments (aircraft cabins, offices with photocopiers, laser printers, sterilizers). Manganese oxides [73] and supported manganese oxides [74] have revealed exceptionally good ability for the decomposition of ozone at ambient temperature and this efficiency increases with increasing temperature. The high activity of these materials could be correlated to their redox properties and to the facile formation of radical species on their surface. The mechanism of ozone decomposition on a MnO2/γ-Al2O3/cordierite catalyst involves the formation of an ionic intermediate with partial superoxide (O22) or peroxide (O222) character on the p-type oxide, MnO2 species [75]. The derivatized cryptomelane-type manganese oxide, OMS-2-Ac serves as a promising catalyst for purifying waste gases containing ozone under high-humidity conditions [76]. It is supposed that the greater surface area and higher amount of Mn31 are the main factors that contribute to the excellent performance of OMS-2-Ac. These studies have improved our understanding and serve as a guide for ozone removal over the manganese oxide catalysts.

16.4.5 Oxidation of VOCs Volatile organic compounds are widely used and produced by industrial processes, transportation, and domestic activities [77] and continue to be the major source of direct and indirect air pollution. These pollutants cause atmospheric pollution (photochemical smog and destruction of ozone layer) and have raised severe concern owing to probable short- and long-term adverse health effects (carcinogenic, mutagenic, or teratogenic) [78]. Catalytic oxidation using manganese oxide materials has been demonstrated to be a promising technology for both partial oxidation of petrochemical materials to make products of economical value and complete oxidation of gas-phase VOCs to CO2 and H2O, minimizing

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formation of undesirable byproducts [79]. Manganese oxide nanostructures showed excellent oxidative decomposition of cyclohexane [80], ethylbenzene [81], formaldehyde [82], etc. at low temperatures. Supported metal nanocatalysts hold great promise for heterogeneous catalysis; however, their practical application is, sometimes, hampered by poor thermodynamic stability. Numerous supported metal catalysts have been employed for the efficient elimination of VOCs [70]. These materials exhibit an excellent catalytic activity due to the induced lattice defects [83] and exceptional stability as a result of strong metalsupport interactions [84]. The catalysts, cryptomelane-type octahedral molecular sieve (OMS-2) manganese oxide, amorphous manganese oxide (AMO), and mixed copper manganese oxide (CuO/ Mn2O3) nanomaterials, along with commercial MnO2, have been investigated for gasphase total oxidation of toluene, benzene, ethylbenzene, p-xylene, m-xylene, and o-xylene under air atmosphere [85]. Moreover, dispersed single adatoms on nanostructured hollandite manganese oxide (Ag1/HMO) surface show high catalytic activity in the complete conversion of benzene [86]. It has been realized that lattice oxygen is involved in the VOC oxidation, suggesting that the reaction could proceed via the Mars 2 van Krevelen mechanism [87].

16.4.6 Sensing of VOCs Volatile organic compounds are carbon-based chemicals of specific chemical reactivities that can affect the regional as well as global environment and may cause adverse health hazards, including the potential cause of cancer [88]. Therefore, on-site and realtime detection of VOCs in the gas phase has received global attention to overcome the challenges for environmental management, process control, and medical diagnosis of human health [89]. Trimanganese tetraoxide (Mn3O4) 2 silver (Ag) nanocomposites containing 2, 5, 10, 15, and 20 nm silver particles (designated as sets 1, 2, 3, 4, and 5, respectively) have been demonstrated towards the sensing of various hazardous VOCs, exhibiting high sensitivity to ethanol compared to other components [90]. A histogram showing the sensitivity of Mn3O4Ag@1 nanocomposite sensors upon exposure to different VOCs at an operating temperature of 350 C is shown in Fig. 16.3. It is seen that the all the sets of nanocomposites exhibit more dramatic improvement in sensitivity toward VOCs than that of pure metal oxide nanoparticles which indicates that conjugation of silver adds functionality to the Mn3O4 matrix [91]. The direct band gap energy (Eg) of pure Mn3O4 NPs appears as 3.08 eV, the band gap values have been calculated as 2.27, 2.50, 2.78, 2.88, and 2.97 eV for Mn3O4Ag nanocomposites containing 2, 5, 10, 15, and 20 nm silver particles, respectively [90]. Moreover, in semiconductormetal composites, the photoinduced charge carriers are trapped by the noble metal particles and become able to promote the interfacial charge-transfer processes [92]. A schematic presentation of the mechanism of improved sensory activity in the presence of Mn3O4Ag nanocomposites [90] is presented in Scheme 16.2. Thus, the semiconducting metal oxide nanostructures impregnated with plasmonic metal nanoparticles offer functional heterogeneous catalysis, owing to the synergetic effects of Ag and Mn3O4 in the nanocomposites.

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321 FIGURE 16.3 Histogram showing the sensitivity of Mn3O4Ag@1 nanocomposite sensors upon exposure to different volatile organic compounds at an operating temperature of 350 C. Source: Reprinted with permission from H. Rahaman, S. Kumdu, S.K. Ghosh, ChemistrySelect 2 (2017) 69916999. Copyright (2017) Wiley-VCH.

SCHEME 16.2 Schematic presentation of the mechanism of improved sensory activity in the presence of Mn3O4Ag nanocomposites. Source: Reprinted with permission from H. Rahaman, S. Kumdu, S.K. Ghosh, ChemistrySelect 2 (2017) 69916999. Copyright (2017) Wiley-VCH.

16.4.7 Removal of Bacterial Pathogens Due to evolution of multiple drug resistance in bacterial pathogens in the human body, there is growing demand for novel bacterial pathogens. Nanotechnology can be employed to fight and prevent infections caused by multidrug-resistant bacterial pathogens. It has been found that manganese oxides at the nanoscale dimension exhibit reactivity similar to

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laccase, a phenol oxidase that exhibits promising applications [93]. The laccase-like reactivity of manganese oxide nanostructures of different crystallinity, including, α-, β-, γ-, δ-, and ε-MnO2, and Mn3O4, with 2,20 -azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 17β-estradiol (E2) has been examined with regard to substrate oxidation and oxygen reduction. It is observed that the reactivity of manganese oxide nanostructures correlate well with their laccase-like catalytic activities amongst which γ-MnO2 exhibits the best performance [94]. It has also been investigated that silver-doped MnO2 nanoparticles have antibacterial activity against multidrug-resistant Staphylococcus aureus and lead-resistant Pseudomonas aeruginosa strain 4EA at low levels [95]. Therefore, based on these observations, the principle of these experiments could be employed for pollutant oxidation in wastewater treatment.

16.4.8 Hydrocarbon Oxidation Aerobic oxidation of sp3CH bonds at low temperature is a profound challenge in organic synthesis and petroleum industry [96]. The activation of CH bonds generally requires high temperature (for example, B600 C for propane dehydrogenation) and excessive energy input, often resulting in uncontrolled product selectivity and undesirable cokes [97]. Featuring superior ability to activate and supply oxygen, manganese oxides have proved their ability as stoichiometric oxidants for oxidation of hydrocarbons in organic synthesis [98,99] and have also been extensively investigated as an excellent support or promoter for various metal or metal oxide catalysts [100102].

16.4.9 Alcohol Oxidation Manganese oxides have, successfully, been innovated to be efficient catalysts not only for the aerobic oxidation of various alcohols to aldehydes or ketones [103,104], but also for the selective aerobic oxidation of mixed alcohols [105]; and in certain cases, overoxidation of aldehydes occurs to esters or carboxylic acids [106]. Radical intermediates have been proposed in such reactions [107]. The catalytic activity is found to be enhanced over the octahedral molecular sieves [108,109] and supported metal catalysts [110].

16.4.10 Coupling Reactions A coupling reaction in organic chemistry indicates a reaction where two hydrocarbon fragments are coupled with the aid of a metal catalyst. A series of manganese oxide catalysts have been probed for oxidative coupling of methane [111], abiotic transformation of 2,4,6-trinitrotoluene reduction products [112], and synthesis of azobenzenes from anilines [113]. The coupling of alkynes has been tested over mesoporous copper/manganese oxide catalysts [114]. Model systems on the Cu/MnOx surface capable of oxidative coupling of alkynes are shown in Fig. 16.4. It is noted that the labile lattice oxygen of the meso Cu/MnOx plays a vital role in the deprotonation of the alkyne proton. The materials also possess superior catalytic activity in the aerobic oxidative coupling of terminal alkynes. The Ullmann-type cross-coupling reactions for the formation of a new CC bond between

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FIGURE 16.4 Model systems on the Cu/MnOx surface capable of oxidative coupling of alkynes: (A) Mn atom, squeezed between two active Cu atoms; (B) Cu sites adjacent to each other; (C) O2 as the bridging ligand between Cu atoms. Source: Reprinted with permission from S. Biswas, K. Mullick, S.-Y. Chen, D.A. Kriz, M.D. Shakil, C.-H. Kuo, et al. ACS Catal. 6 (2016) 50695080. Copyright (2016) American Chemical Society.

two aryls have also been performed by heterogeneous mesoporous copper manganese oxide (meso Cu/MnOx) materials [115]. Experimental observations and DFT calculations suggest an oxidative addition of the phenol derivatives at the Cu center with a charge delocalization over the oxide surface.

16.4.11 Epoxidation of Olefines The catalytic epoxidation of olefins in the liquid phase is an important reaction in the chemical industry since epoxides are versatile and important intermediates to manufacture a wide range of fine chemicals and pharmaceuticals [116]. An efficient epoxidation of a broad range of olefins with hydrogen peroxide as a clean oxidant over manganese catalysts has been extensively studied over the last three decades, not only for their activity [117119] but also their catalytic selectivity [120122]. Moreover, manganese oxide octahedral molecular sieves having tunnel structures have been exploited for studying styrene oxidation [123]. These studies corroborate that high porosity plays a crucial role in these catalytic reactions and moreover, transition metal-doped octahedral molecular sieves show better selectivity of styrene oxide when compared to their undoped catalysts. Moreover, films of polyions and octahedral layered manganese oxide nanoparticles on carbon electrodes made by layer-by-layer alternate electrostatic adsorption were active for electrochemical catalysis of styrene epoxidation in solution in the presence of hydrogen peroxide and oxygen [117]. Furthermore, the activated layered manganese oxides with deposited nanosized gold or silver exhibit higher catalytic activity of the manganese oxides toward the epoxidation of olefins in the presence of H2O2 and NaHCO3 [124].

16.4.12 Oxidative Dehydrogenation Catalytic activity of manganese complexes towards hydrogenation of carbon dioxide to methanol [125], one-dimensional tunnel structured manganese oxide catalysts in the oxidative dehydrogenation of ethylbenzene to styrene [126], and transition metals (Ni21, Cd21,

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Fe31, Co21, Mg31) doped into the framework of octahedral molecular sieve materials for oxidative dehydrogenation of ethanol to acetaldehyde [108] have been examined. Porous manganese oxide (OMS-2) and platinum supported on OMS-2 catalysts have been shown to facilitate the hydrogenation of the nitro group in chloronitrobenzene to give chloroaniline with no dehalogenation [127]. The hydrogenation of nitrobenzene has been found to form nitrosobenzene, azoxybenzene, and azobenzene/hydrazobenzene intermediates before complete conversion to aniline. These results indicate that for Pt/OMS-2 the hydrogenation proceeds predominantly over the support with the metal acting to dissociate hydrogen. The experimental results show that both the conversions and selectivities of this reaction over each individual OMS material are highly dependent on the nature of molecular sieve materials and the transition metal dopants in their framework structures. The liquid-phase hydrogenation of crotonaldehyde over silvermanganese oxide catalysts exhibits the selectivity to 2-buten-1-ol [128]. A detailed investigation of the mechanism of CO hydrogenation over cobalt/manganese oxide catalyst proposes a reaction mechanism for carboncarbon bond formation in the FischerTropsch reaction involving α-hydroxylated metal alkyl as an important intermediate, the formation of which involves the coupling of a number of electrophilic and nucleophilic C1 surface intermediates [129].

16.4.13 Photocatalysis The removal of the nonbiodegradable organic chemicals is a global ecological problem. Dyes are an important class of synthetic organic compounds which are commonly used in textile industries and therefore, are common industrial pollutants. Photocatalysis, where photons are used for catalytically activating chemical reactions on the surface of photosensitized catalysts, remains one of the leading hubs of research for harvesting the solar light [92]. The efficiency of a photocatalytic process, mostly depends on the nature of the photosensitized catalyst, a suitable photon source for excitation, a substrate which can rapidly accept the photogenerated charge carriers, and the spatial distance between the catalyst and substrate [130]. Manganese is an essential component in photosynthetic systems; reactions that mimic biological systems in order to utilize solar energy have often focused on manganese catalysts [131]. Unfortunately, there are relatively few manganese complexes that have longterm photochemical stability. This has provided impetus for the preparation of various manganese compounds that are stable and exhibit photoredox functions that might be used to catalyze photochemical reactions [132]. Although, manganese oxides have, successfully, been used as catalysts for thermally catalyzed reactions, dandelion-shaped Mn3O4 microstructures have been screened by studying the photocatalytic degradation of alizarin red under visible light illumination [133]. Manganese oxides, in the form of dispersed powders, have been tested as potential catalysts for the oxygen evolution at the oxidation of water under photochemical conditions [134]. Moreover, a variety of amorphous manganese oxide materials has been examined for the photocatalytic oxidation of 2-propanol to acetone [135], degradation of organic pollutants into benign products, such as, CO2 and H2O [136], and decomposition of halogenated hydrocarbons, such as methyl bromide, to yield CO2, H2O, and Br2. The amorphous manganese oxide materials have high photoactivity, mixed valent manganese oxidation states, high surface areas

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(200 m2/g), microporous structures, and the ability to lose lattice oxygen and create lattice vacancies that may be important in photooxidation reactions. Again, synthetic tunnel structures of manganese oxides have also been explored as photocatalysts in various reactions [137]. Being inspired by the structural diversity of manganese oxides that occur naturally as minerals in at least 30 different crystal structures, Dismukes and colleagues have chosen to, systematically, compare eight synthetic oxide structures containing Mn(III) and Mn(IV) with cubic phases. Measurement of water oxidation rate by oxygen evolution in aqueous solution has been conducted with dispersed nanoparticulate manganese oxides and a standard ruthenium dye photooxidant system. It has been concluded that electronically degenerate Mn(III) imparts lattice distortions due to the JahnTeller effect that are hypothesized to contribute to structural flexibility important for catalytic turnover in water oxidation at the surface [138]. Water oxidation in all oxygenic photosynthetic organisms is catalyzed by the Mn4CaO4 cluster of Photosystem II that has inspired the development of synthetic manganese catalysts for solar energy production [139]. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis [139]. An in situ X-ray absorption spectroscopy reveals that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. A Mn K-edge XANES of a Nafion-coated [Mn4O4L6]1-loaded glassy carbon electrode measured in different “states” of photochemical cycling is shown in Fig. 16.5. A schematic

FIGURE 16.5 Mn K-edge XANES of a Nafion-coated [Mn4O4L6]1-loaded glassy carbon electrode measured in

different ‘states’ of photochemical cycling. State 1 5 initial load; State 2 5 State 1 1 1.0 V (versus Ag/AgCl) applied potential in electrolyte; State 3 5 State 2 1 40 min of light excitation in electrolyte; State 4 5 State 3 1 1.0 V applied potential in electrolyte; State 5 5 State 4 1 20 min of light excitation in electrolyte. Two clean isosbestic points can be observed indicating that repeated cycling between an oxidized birnessite-like state and a reduced Mn(II) state can be achieved. Source: Reprinted with permission from R.K. Hocking, R. Brimblecombe, L.-Y. Chang, A. Singh, M.H. Cheah, C. Glover, et al., Nat. Chem. 3 (2011) 461a466. Copyright (2011) Macmillan Publishers Limited.

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illustration of the change in the oxidation state of Mn catalytic centers and mechanism of photochemical water oxidation with [Ru(bpy)3]21S2O822 system on MnOx/AuNPs catalysts is presented in Scheme 16.3. The proposed electron transfer pathways describe two possibilities involving oxidation of water (loss of an electron) on AuNPs or on MnO2 [140].

16.4.14 Electrocatalysis 16.4.14.1 Water Oxidation Increasing energy consumption throughout the globe demands new energy resources and coupled storage techniques to substitute for the fossil fuels—coal, natural gas, and petroleum [141]. Therefore, there is a strong need to seek viable, clean, and highly efficient alternative energy resources that could replace finite fossil fuels and lead to surviving on the planet in a sustainable way; this poses a great challenge to humanity in the 21st century. Hydrogen generation and carbon dioxide reduction driven by solar energy have become the ultimate goal for realizing fuel production through the use of electrons and protons as reducing agents [142]. Water upon oxidation can provide abundant protons and electrons acting as an excellent reducing agent and thus, becomes the key in solar fuel production to meet our exponentially increasing rates of energy consumption [143]. The generation of hydrogen from water may be the best solution not only for the problem of the depletion of fossil fuels but also for global warming [144]. There is a growing need for electrocatalytic water oxidation to give dioxygen for the conversion of electrical energy to stored chemical energy in the form of fuels. Plant life depends upon complex SCHEME 16.3 (A) The changes in the oxidation state of Mn catalytic centers photochemical water oxidation. (B) Schematic illustration of the mechanism for photochemical water oxidation with the [Ru(bpy)3]21S2O822 system on MnOx/AuNPs catalysts. The proposed electron transfer pathways describe two possibilities involving oxidation of water (loss of an electron) on AuNPs (left) or on MnO2 (right). Source: Reprinted with permission from C.-H. Kuo, W. Li, L. Pahalgedara, A.M. El-Sawy, D. Kriz, N. Genz, et al., Angew. Chem. Int. Ed. 54 (2015) 23452350. Copyright (2015) Wiley-VCH.

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catalytic systems for the disintegration of water into its elements; in natural and many protocols designed for artificial photosynthesis, water oxidation, 2H2O-4H1 1 O2 1 4e2, is a significant step and our understanding of these systems could pioneer the design of effective catalysts. However, water oxidation is thermodynamically unfavorable and requires high energy input [145]. The salient features of the technical impediment to such a reaction is the need for efficient and inexpensive electrocatalysts capable of oxidizing water to drive this energetically highly unfavorable reaction (ΔH 5 572 kJ/mol). Therefore, to facilitate water oxidation, growing efforts have been made to develop efficient, inexpensive, and robust electrocatalysts [146]. Manganese oxides are a highly promising class of water-oxidation catalysts (WOCs). Several reports have been published in the literature using manganese oxides and complexes as electrocatalysts for water oxidation. Dismukes et al. have described the thermodynamic and mechanistic aspects that nature appears to use to catalyze in vitro water oxidation by photosynthesis and have designed bioinspired and photoactive Mn4O4cubane clusters [147]. Mixed calcium manganese (III) oxides with elemental compositions and structures mimicking the active site of photosystem II were found to be highly active catalysts for the oxidation of water to molecular oxygen [148150]. Drawing inspiration from these cubane-like CaMn(4)O(x), the biological catalyst found in the oxygen evolving center in photosystem II, Gorlin and Jaramillo have investigated the electrocatalytic activity of nanostructured manganese oxide surfaces that exhibited similar oxygen electrode activity to the best known precious metal nanoparticle catalysts, viz., platinum, ruthenium, and iridium [146]. Jiao and Frei have probed the nanostructured cobalt and manganese oxide clusters in mesoporous silica scaffolds that afford efficient WOCs with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites [151]. Zaharieva and coworkers have shown that a binuclear manganese molecular complex [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)]31, which is the most prominent structural and functional model of the water-oxidizing manganese complexes operating in plants and cyanobacteria, could be supported on montmorillonite clay and, using Ce(IV) as a chemical oxidant, the complex could act as one of the best manganese-based molecular catalysts towards water oxidation [152]. Wiechen et al. have reported the syntheses of layered manganese oxides where the interlayer cations, viz., K-, Ca-, Sr-, and Mg-containing birnessites, was varied and observed that oxygen-evolving complexes require the presence of calcium in their structures to reach maximum catalytic activity [153]. Spiccia and coworkers have shown that for the nanoparticulates of manganese oxides, formed in Nafion polymer, the catalytic activity towards the water oxidation is dependent on the dispersity of the nanoparticles. They have also reported on the synthetic methodologies for the preparation of highly active mixed valent MnOx catalysts by partial oxidation of crystalline MnIIO nanoparticles and analyzed the catalytic activity in water splitting devices [154]. Stahl and colleagues have reported a systematic study of nine crystalline MnOx materials as WOCs and showed that the identity of the “best” catalyst changes, depending on the oxidation method used to probe the catalytic activity [24]. Jaramillo and colleagues have also demonstrated that addition of Au to MnOx produces an order of magnitude higher turnover frequency (TOF) than that of the best pure MnOx catalysts and a local rather than bulk interaction between Au and MnOx leads to the

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observed enhancement in the activity of the reaction [155]. Ghosh and coworkers [156] have investigated the electrocatalytic activity of goldmanganese oxide nanocomposites towards water oxidation and oxygen reduction at low overpotential (370 mV) and under neutral pH condition. The water-oxidizing activities of the composites comprising of gold nanorods deposited on nanolayered Mn oxide have been studied in the presence of cerium(IV) ammonium nitrate as a non-oxo transfer oxidant and these results show that gold has no significant effect on the water oxidizing activity of the Mn oxide phase at least in the presence of Ce(IV) [157]. The Najafpour group have reported self-assembled layered hybrid [Ru(bpy)3]21/manganese(III,IV) [158] and manganese oxide-coated montmorillonite as water oxidizing systems [159] that suggest the mechanism of water oxidation to be multielectron reactions. Suib and coworkers have compared the catalytic activity of mixed valent porous amorphous manganese oxides, cryptomelane-type tunnel manganese oxides, and layered birnessite as WOCs and observed that amorphous manganese oxides exhibit significantly higher turnovers compared to tunnel and layered structures [160]. Infrared spectral measurements prove that the AMO catalysts maintain the structural stability and reusability after regeneration. O-18 labeling studies proved that water is the source of dioxygen. The AMO is related to hexagonal birnessites, such as layered biogenic manganese oxides or H1-birnessites, that have cation vacancies in the MnO2 sheets rather than completely filled Mn31/Mn41 sheets, and this is influential in catalytic activity. Nakamura and coworkers have exploited layered manganese oxide nanoparticles δ-MnO2(K0.17[Mn410.90 Mn310.07&0.03]O2 0.53H2O) deposited on fluorine-doped tin oxide electrodes to catalyze water oxidation at pH from 4 to 13 [161]. It has been suggested that the control of disproportionation or comproportionation efficiencies of Mn31 is essential for the development of Mn catalysts that afford water oxidation with a small overpotential at neutral pH.



16.4.14.2 Oxygen Reduction Reaction The large overpotential associated with the oxygen reduction reaction (ORR) is one of the major challenges to be overcome for the development of a high performance cathode catalyst [162]. The ORR is a reaction of indispensable importance electrochemical energyconversion devices, such as in metalair batteries, alkaline fuel cells, as well as in oxygen sensors. Although, platinum and its alloys are superior to many other electrocatalysts [163,164], the slow oxygen reduction kinetics and scarce availability of costly Pt limits their application in the energy conversion efficiency of the state-of-the-art low-temperature polymer electrolyte membrane fuel cells (PEMFCs). Therefore, another viable alternative is needed for the development of inexpensive yet efficient electrocatalysts with a comparable or even better electrocatalytic activity compared to that of platinum. Manganese oxides of various crystalline phases (α-, β-, and δ-MnO2 and amorphous) [165], morphologies [166], and porous structures [167,168] have, systematically, been examined for catalyzing ORR in alkaline media. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened MnaO bonds along ˚ , which provides an optimum requirement for the with a MnaMn distance of 2.842 A adsorbed oxygen in a bridge mode favoring the direct four electron reduction [166]. In accordance with the first principles-based density functional theory (DFT), the enhancement in ORR activity is due to the lower activation energy needed for the reaction by the

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(2 1 1) surface than all other surfaces [166]. Moreover, bifunctional electrocatalysts composed of, viz., Au nanoparticlesMnOx nanoparticles [169], AuMn3O4 nanocomposites [156,170], Ag nanocrystals anchored on α-MnO2 nanorods [171], AgMnO2 catalysts supported onto single-walled carbon nanotubes [172], Ni nanoparticles deposited on α-MnO2 nanowires [28], doping the MnOx/C nanoparticles with nickel or magnesium divalent cations [173], silver manganese oxide octahedral molecular sieve (AgOMS-2) nanofibers with cryptomelane structure [174], have been probed towards the oxygen reduction in aqueous alkaline medium. This is based on the fact that manganese oxide has an excellent catalytic activity towards the chemical disproportionation of hydrogen peroxide into water and molecular oxygen. The superior catalytic activity is reflected either by an increase in ORR currents and/or a positive shift in the half-wave potential. This bifunctional mechanism leads ultimately to an apparent four-electron reduction of oxygen [175,176]. Cyclic voltammograms of water oxidation oxygen reduction using AuMn3O4 nanocomposites as catalysts [156] in PBS at pH B7.5 are shown in Fig. 16.6. The enhanced catalytic activity of the AuMn3O4 catalyst was attributed to the beneficial presence of a higher amount of oxidizable gold species and surface oxygen vacancies resulting from the strong interaction between Au and the reactive surface of Mn3O4 nanoparticles [177]. An increase in 5d vacancy of Au increases the interaction of O2 and Au, thereby enhancing the catalytic activity of Au in the composites [178]. It is also likely that oxygen can dissociate on the Au surface and spill over from Au to the oxygen vacancies in the oxide, which synergistically promotes the adsorption and dissociation of O2. The AuMn3O4 particles, after evacuation and exposure to oxygen, form radical species on the surface of this catalyst. The ability to form such radical species in the presence of oxygen [179] leads to enhanced performance of the AuMn3O4 composites in the ORR. Therefore, the different

FIGURE 16.6 (A) Cyclic voltammograms of water oxidation in the presence of 4-ATP/gold (red), Au NPs/4ATP/gold (green), Mn3O4 NPs/4-ATP/gold (blue), and AuMn3O4 NCs/4-ATP/gold (black) electrodes in PBS at pH B7.5; (B) cyclic voltammograms for oxygen reduction in the presence of 4-ATP/gold (blue), Au NPs/4ATP/gold (green), Mn3O4 NPs/4-ATP/gold (red), and AuMn3O4 NCs/ATP/gold (black) electrodes in PBS at pH B7.5. Source: Reprinted with permission from H. Rahaman, K. Barman, S.K. Jasimuddin, S.K. Ghosh, RSC Adv. 4 (2014) 4197641981. Copyright (2014) Royal Society of Chemistry.

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surface structural features clearly determine the strength of metalsupport interaction and thus the catalytic activity. 16.4.14.3 Oxygen Evolution Reaction Oxygen evolution reaction (OER) is a limiting reaction in the process of generating molecular oxygen through chemical reaction, such as the oxidation of water during oxygenic photosynthesis, electrolysis of water into oxygen and hydrogen, and electrocatalytic oxygen evolution from oxides and oxoacids. Developing improved catalysts for the OER is the key to the advancement of a number of renewable energy technologies, involving solar fuels production and metalair batteries [180]. Although precious metal oxides, such as ruthenium and iridium oxides, show the best OER activities, their scarcity and high costs limit scalable applications [146]. First-row metals have been a target for the development of OER catalysts because they compromise noncritical elements. Owing to their stability under oxygen evolution conditions, manganese oxides have emerged as the material of choice to improve the efficiency of gas evolution [181]. Electrochemical characterization of manganese oxides (MnOx) over a wide pH range shows that MnOx is a functionally stable OER catalyst owing to self-healing derived from MnOx redeposition that offsets the catalyst dissolution during turnover [182]. It has also been noted that electrodeposited manganese oxide films (MnOx) are promising stable catalysts for the OER in acidic solutions but only with modest activity [183]. Investigating interactions of MnOx films deposited on Au nanoparticles suggests that the enhancement in OER activity arises from interfacial sites between Au and MnOx [184]. Jaramillo and coworkers [185] have examined a catalytic system comprising of nanoparticulate Au, and nanoparticulate MnOx as promising OER catalyst. Cyclic voltammetry of MnOGC and bare GC support in the presence and absence of trace amount of Au (0.1 mM HAu(III)Cl4) demonstrating the increasing OER activity with increasing deposition of Au is presented in Fig. 16.7. It has been demonstrated that adding Au to MnOx significantly enhances OER activity, producing an order of magnitude higher TOF, and a local rather than bulk interaction between Au and MnOx that leads to the observed enhancement in the OER activity [185].

16.4.15 Photoelectrocatalysis Mimicking photosynthesis, water splitting has been adopted as the key step for solar energy conversion. Ghosh and coworkers have explored the electrocatalytic activity of Mn3O4NiO nanocomposites towards water oxidation reaction in the presence of sunlight and ultraviolet light at low overpotential and under neutral pH condition [186]. Results show that these nanocomposites could catalyze water oxidation in neutral phosphate buffer saline (pH B7.0) with a current density of 2.2 mA cm22 and TOF of 4.3 s21 and in the presence of UV light, with a current density of 4.5 mA cm22 and TOF of 5.1 s21 at an applied overpotential of just 280 mV. Typical controlled potential electrolysis (CPE) curves, in the dark, the presence of sunlight, and UV light, are presented in Fig. 16.8. The profile shows the constancy of current density with time which indicates high stability of the Mn3O4NiO modified gold electrode during the electrolysis and also better catalytic activity in the presence of

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FIGURE 16.7 (A) Cyclic voltammetry of MnOGC and bare GC support in the presence and absence of trace amount of Au (0.1 mM HAu(III)Cl4) demonstrating the increasing OER activity with increasing deposition of Au; (B) 10-fold magnification of panel a focusing on the OER region; and (C) 100-fold magnification of panel a focusing on the reductive region. Source: Reprinted with permission from Y. Gorlin, C.-J. Chung, J.D. Benck, D. Nordlund, L. Seitz, T.-C. Weng, et al., J. Am. Chem. Soc. 136 (2014) 49204926. Copyright (2014) American Chemical Society.

sunlight and UV light compared to the dark condition. This is, due to the fact that in the presence of comparatively higher energy UV light, the electron transfer for water oxidation reaction is more feasible in Mn3O4NiO NCs (band gap 2.78 eV, which is narrower than 3.0 eV, the typical band edge potential suitable for overall water splitting) [187].

16.5 CONCLUDING REMARKS AND FUTURE OUTLOOK In conclusion, we have summarized a rich database for the catalytic applications of manganese oxides and metal oxide nanoscale hybrids over the decades. The continuing

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FIGURE 16.8 Controlled potential electrolysis with Mn3O4NiO modified electrode in 0.1 M PBS (pH B7.0) at 11.5 V versus RHE in the dark (black), presence of sunlight (blue), and UV light (red). Source: Reprinted with permission from H. Rahaman, K. Barman, S.K. Jasimuddin, S.K. Ghosh, RSC Adv. 6 (2016) 113694a113702. Copyright (2016) Royal Society of Chemistry.

breakthroughs in the art of nanomaterials synthesis and advent of characterization techniques have enabled to unravel their unique characteristics and to exploit their plausible applications in diverse catalytic reactions with a tremendous potential for technological applications. The unique features of multivariance, crystalline phases, band structures, magnetism, thermodynamic stability, and biogenicity have revealed the chemistry of manganese for diverse prospective technological applications. It seems that many successful catalysts have resulted from the fortuitous combination of a metal, a ligand, a support, and reaction conditions. The ability to successfully model and predict the structurefunction relationship of manganese oxides and their metal oxide composites depends to a large degree on the detailed understanding of their crystal structures. Theoretical models have been developed and experimentally verified that predict the distribution of the metal precursor across the nanostructured surfaces for both high and low precursor loadings. At a conceptual level, a better understanding of the subtle links among size, shape, and structure is necessary for a rational design of the catalytic systems and the avenues that these properties affect in the electronic and chemical behavior of the metalmanganese oxide hybrid nanostructures. The catalytic activities are expanded from oxidation and reduction reactions to ozone and H2O2 decomposition, epoxidation, abatement and sensing of VOCs, removal of bacterial pathogens, photocatalysis, electrocatalysis, and photoelectrocatalysis. This steady progress demonstrates that manganese oxides are playing and will continue to play an important role in the search for renewable and clean energy technologies and in the protection of the environment. Lastly, the simple and facile approaches build a good platform towards fabricating metalmanganese oxide nanohybrids over a wide range of material combinations with morphological anisotropy and functional diversities. From these simple examples have emerged excellent strategies, clean reactions, and often high yields which inspire their scale up to industrial applications. Increased application of combinatorial techniques may occur in the future, and therefore, contribute to faster developments, with the condition that the creativity of the human mind can keep pace with the speed of experimentation.

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17 Smart Coatings Sarah B. Ulaeto, Jerin K. Pancrecious, T.P.D. Rajan and B.C. Pai CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala, India

17.1 INTRODUCTION Smart coatings are innovative coatings that provide a spontaneous response to changes in the microenvironment such as heat, light irradiation, mechanical induction, wettability, temperature, pressure, ion exchange, pH variation, aggressive corrosive ions, etc. Smart coatings are also referred to as “stimuli-responsive”, “intelligent” or “environmentally sensitive” coatings [14]. Smart coatings are designed to maintain their passive feature, provide superior performance, and recover the functional performance of coatings by exhibiting stimuli-responsive behavior when the need arises. The stimuli-responsive characteristic of smart coatings enhances the efficiency of the coated system. These categories of coatings with phenomenal innovations are unique for its accelerating developments. The driving force in developing smart coating technologies is the continuous demand for higher performance, extending product lifetimes and significantly reducing maintenance cost. Enhancing energy efficiency for both cost and environmental reasons, as well as coping with the change in substrates towards new and lightweight materials, like composites, and lightweight metals, such as aluminum and its alloys, etc. Examples of smart coatings include but are not limited to anti-inflammatory, antimicrobial, and antifouling coatings; corrosion, degradation, and defect sensing coatings; self-healing and pressure-sensing paints; reversible thermochromic; piezoelectric paint; hydrophilic/hydrophobic switching; self-cleaning, pH-responsive, light sensing coatings; self-erasing inks; smart window coatings; photochromic and electrochromiccoatings; radio frequency identification coatings; antibacterial, anti-icing, intumescent coatings, etc. [37]. Shifting demand towards nano-based coatings instead of conventional polymer coatings and other microparticle-based coatings due to superior properties and low Volatile

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00017-6

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Organic Contents (VOC) emissions are expected to be the significant mainspring for the nanocoatings market. But with the growing preference for advanced materials in sectors such as medical and automotive industry, there is a surge in nanotechnology research and development, expected to boost market growth. With nanotechnology development, coatings nowadays become smarter, stronger, and more durable. The nanocoatings market embraces nanotechnology-based coatings due to its superior properties such as abrasion resistance, ductility, hardness, lubricity, and transparency to mention a few as compared to other conventional coatings. A boost in market demand is expected from the application of antimicrobial and self-cleaning coatings in mechanical and aerospace applications for extending the durability of parts. Nanostructured coatings, however, provide essential functions such as protection from ice, pollutant, UV, fire, heat, bacteria, marine fouling, touch, and corrosion. Properties such as antimicrobial action, product longevity, thermal insulation, antigraffiti, self-cleaning, moisture absorbing, gloss retention, dirt and water repellency, hardness, corrosion resistance, flame retardancy, ultraviolet radiation stability, improved energy efficiency, chemical and mechanical properties to mention a few are significantly enhanced with the use of nanostructured materials [8]. Smart coatings are increasingly used in a variety of applications in the medical, textile, transport, construction, military, electronics, aviation, and several other industries for providing different functionalities alongside protecting against corrosion. They are considered pragmatic candidates without manual intervention. The goal of smart coatings is directed at improving a system’s efficiency by reducing inspection times, significantly reducing the maintenance costs and equipment downtime in many industrial applications. It is a class of coatings capable of impacting on the society in a significant way [1,35,9]. The synergy between modern engineering science and nanotechnology has resulted in rapid developments of high-performance multifunctional corrosion-resistant coatings to cater for a broader range of hostile environments. These innovative coating systems are expected to accelerate gainful transformation in the corrosion world [10]. In the case of organic coatings, the incorporation of nanoparticles might enhance their barrier property and decrease the trend for coatings to blister or delaminate. Whereas, by introducing the hard nanocrystalline phases within a metallic matrix, the high hardness could be obtained for metallic coatings [11]. In the following reviews [1,6] studies are presented on corrosion protection with smart coatings. In the present contribution, an update on smart anticorrosive coatings with relevance in corrosion sensing, self-healing, antifouling and self-cleaning actions is considered with a few notable studies. With this brief introduction, this chapter aims to provide a concise perspective on smart coatings and its developments as their utilization cuts across various aspects of science. It is a vibrant topic, quite expansive, and cannot be entirely covered in a chapter. An attempt to highlight some selected smart coatings based on their function is taken into consideration including the update on smart anticorrosive coatings. Applications and commercial viability of smart coatings are explored. A brief conclusion and sources of further information are provided.

17.2 CLASSIFICATION OF SMART COATINGS Smart coatings may be classified based on application, function, responsiveness, material types, level of complexity [12], functional constituent and fabrication methods, etc. [13].

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The functional component within these intelligent coatings can be the resin or from its variety of additives such as pigments, bioactive species, enzymes, microencapsulated agents, nanomaterials, inhibitors, radiofrequency identification devices, micro-electromechanical devices, etc. [14]. Triggers for the required responses come from both internal and external stimuli. When applied to the surfaces of the desired substrates these smart coatings contribute to decoration in their functionality. These chemically active coatings exhibit various functionalities either on filmsubstrate interfaces, airfilm interfaces or in the bulk of the film. In the different sectors of application, multifunctionality of smart coatings is an advantage [1,3,4,6,9]. To streamline the discussion, selected smart coatings discussed herein are categorized based on the functions exhibited by the coatings. Self-healing coatings can also be referred to as smart-repair coatings. Active sensing coatings include corrosion-sensing and pressure-sensing coatings. Flame-retardant coatings are intumescent and nonintumescent coatings. Antifouling and antibacterial coatings are referred to as bioactive coatings. Easy-to-clean coatings include self-cleaning and antigraffiti coatings. Smart window coatings are optically active coatings. Others are antifingerprint, antireflective, anti-icing, and antifogging coatings.

17.2.1 Self-Healing Coatings The concept of self-healing is a significant property of biological materials and has drawn considerable research interest in the medical sector and pharmaceutical industries and is now very relevant in the development of corrosion-resistant coatings [15]. The self-healing effect is modeled after the natural wound-healing process, and the functional nature of the self-healing coating depends on its chemical composition and structure [1]. The self-healing concept was first proposed in 1979 by Jud and Kausch through molecular interdiffusion across crack interfaces and later advanced by White and coworkers in 2001 by embedding microcapsules containing healing liquid and catalyst particles into the matrix material [16,17]. Self-healing materials are known to provide some advantages over the conventional coating. These include (i) automatic repair process upon damage; (ii) autopreservation of esthetics of surface appearance of coatings, plastic, and films; and (iii) restoration of the mechanical integrity of load-bearing materials as in composites, etc. [15]. Self-healing materials are classified into nonautonomous and autonomous systems. Nonautonomous self-healing (or stimuli-assisted) can be induced and controlled by heat, light, mechanical forces, chemical reactions, pH, etc. while autonomous self-healing does not require any external intervention, the damage itself triggers repair processes. Autonomous systems, therefore, behave as smart, adaptive materials and the autonomous self-healing process can either be intrinsic or extrinsic. Intrinsic involves mending mechanical failures since it is based on the formation of either covalent or noncovalent chemical bonds between cracked interfaces. Although engineering intrinsic processes is quite hard for most of the materials. Extrinsic requires the presence of some externally loaded healing agents, responding when triggered by mechanical damage [18]. Amongst the different approaches for self-healing coatings, the use of nanocontainers has been the most widely accepted and employed due to the versatility of nanocontainer fabrication and the variety

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of possible applicable healing agents [19]. Nanocontainers, such as halloysite clay, mesoporous silica [2,20], cyclodextrins [21], layered double hydroxide, polyelectrolyte multilayers, and mesoporous zirconia, exist and are being utilized [2,6]. Various self-healing mechanisms in polymeric materials are described in Fig. 17.1 [22]. The illustration explains self-healing based on the actions of healing agents in enclosed micro- or nanocapsule embedded in polymer matrices. The self-healing ability of coatings has become a treasured and desirable effect of protective coatings, due to the considerable delay in corrosion when they are in use and a subsequent reduction upon actual corrosion occurrence. Smart self-healing coatings provide a protective film build up via adsorption on the exposed area of the metal or alloy. Physisorption and strong covalent bonds are involved in the attachment of the nanoparticles to the polymer molecules as well as with other functional molecules of micron sizes [1]. It is best to design and establish such a stimuli-responsive release system (self-healing system) that entraps corrosion inhibitors in containers with a high uptake capacity and releases it only in response to specific stimuli. Hence, nanocontainer-based coatings serve as the solution for the challenge of self-healing for corrosion inhibition processes. The nanoparticles, also, occupy small defects and holes formed during curing of the coating and also act as a bridge interconnecting more molecules. The incorporation of

FIGURE 17.1 (AE) Self-healing of different enclosed micro- or nanocapsule healing agents in polymeric matrices. Source: Reproduced with permission Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (2013) 74467467. Copyright 2013, Royal Society of Chemistry.

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nanocontainers loaded with inhibitors into the coating, suppresses further corrosion occurrence by the long-term self-healing effect derived from the high inhibitor uptake and release capacity and response to local changes resulting from corrosion [19,20,23]. Recent developments in self-healing coatings are discussed with emphasis on corrosion protection. Qian et al. synthesized a novel superhydrophobic coating that exhibited good self-healing properties in their anticorrosion studies of Q235 carbon steel in 3.5 wt.% NaCl. The fabricated coating contained benzotriazole (BTA) as the corrosion inhibitor. The selfhealing induced by the shape memory effect of the damaged epoxy coating was investigated under two conditions: (i) heating in an oven at 60 C for 20 min; and (ii) exposure to sunlight for different durations. Healing through thermally triggered shape memory effect of the epoxy polymers is an entropically driven process. The inspiring practicality presented for use in actual outdoor environments was the self-healing achieved under sunlight (Fig. 17.2), revealing a potential for long-term corrosion protection from external

FIGURE 17.2 (A) Confocal laser scanning microscopy (CLSM) images of the scratched coating surface containing BTA-5% after (a1) 0 h, (a2) 1 h, and (a3) 7 days of exposure to sunlight under an outdoor environment. (B) Temperature evolution of the superhydrophobic coating surfaces with time during the outdoor exposure. (C) Bode plots of the healed BTA-5% coatings after 0 h, 1 h, and 7 days of exposure to sunlight in the outdoor environment. Source: Reproduced with permission H. Qian, D. Xu, C. Du, D. Zhang, X. Li, L. Huang, et al. Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (5) (2017) 23552364. Copyright 2017, Royal Society of Chemistry.

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damage. The 45 straight line in the Bode plot of the impedance investigation indicated that the anticorrosion performance was already fully repaired after 1 day and the recovered surface maintained after 7 days of exposure [24]. Garnweitner et al. fabricated the thermoresponsive polymer/silica nanocomposite thin film [25]. The authors signaled that the larger the temperature range changes, the higher the magnitude of the swelling/deswelling response is. In another significant study, light-responsive and self-healing anticorrosive coatings were investigated by Chen et al. The incorporation of hollow nanocontainers loaded with benzotriazole having smart molecular switches (photoresponsive azobenzene molecular switches) were used in the fabrication of the intelligent coating to protect the aluminum alloy AA2024. Exposure of the modified water-borne alkyd coating to visible light illumination had a reverse effect. The cis-isomer of the azobenzene molecules grafted in the mesopores of hollow nanocontainers transformed into the trans-isomer closing the pores (Fig. 17.3A). The residual corrosion inhibitors were not sequentially released/wasted but remained encapsulated within the nanocontainers to prevent the next corrosion occurrence. Thus, the reversible light-responsive release system achieved in the self-healing coating was aimed at providing flexibility and avoiding the excess release of the corrosion inhibitors after repairing the initial corrosion affected area. The nanocontainers possessed a high loading capacity and could also control the entrance and release of trapped active molecules by the dynamic motion of azobenzene molecules. With the scanning vibration electrode technique (SVET), local current density measurements around the artificial scratches were performed to verify the active anticorrosion performance of the lightsensitive nanocontainers in 0.1 M NaCl aqueous solution represented in Fig. 17.3B [26].

17.2.2 Active Sensing Coatings 17.2.2.1 Corrosion-Sensing Coatings Corrosion sensing coatings are used to monitor the corrosion risk of the material at the early stages of deterioration. It is well-known that when the corrosion happens, the anodic region possesses an acidic pH and the cathodic region has an alkaline pH. The pHtriggered release of corrosion indicators and healing agents from the microcapsules or nanocapsules is one of the active research areas. The corrosion sensing indicators can be of color dyes or fluorescent compounds. The size and pH sensitivity of the microcapsules or nanocapsules can be controlled by adjusting the preparation method and time. A corrosion-sensing fluorescent coating made of phenylfluorone (PF) into the acrylic paint was prepared for monitoring corrosion of aluminum alloys. This system was sensitive to underlying corrosion processes by reacting with the Al31 produced by the anodic reaction. The corroded areas under the fluorescence quenching spots were identified using an optical microscope. The anodic reaction sensitivity associated with corrosion was determined by applying constant charge current and measuring the charge, where the fluorescence quenching was detected in the coating [27]. A carbon nanotube (CNT) polyelectrolyte composite multilayer thin film has been made for measuring both strain and corrosion processes. The layer-by-layer fabrication exhibited changes in the electrical properties to strain and pH [28]. The corrosion-sensing compounds with color-change or

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FIGURE 17.3 (A) Schematic of reversible release system by utilizing trans-cisphotoisomerization of azobenzene molecules grafted in the mesopores of HMSs. UV irradiation at 365 nm converts azobenzene to the cis form, resulting in pore opening. The cis isomer of the azobenzene molecules transforms into the trans isomer under visible light irradiation (450 nm), leading to pore closing. (B) SVET current density maps of aluminum coated with the alkyd coating without (a) and with (b) the BTA@Azo-HMSs nanocontainers obtained after 1 h (left column) and 10 h (middle column) immersion in 0.1 M NaCl. The passive protection performance of the pure coating and the active self-healing performance of the coating containing BTA@Azo-HMSs was compared when the scratches were exposed to the UV light (right column). Source: Reproduced with permission from T. Chen, R. Chen, Z. Jin, J. Liu, Engineering hollow mesoporous silica nanocontainers with molecular switches for continuous self-healing anticorrosion coating, J. Mater. Chem. A 3 (18) (2015) 95109516. Copyright 2015, Royal Society of Chemistry.

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fluorescing to the pH increase associated with the cathodic reaction were also used by applying constant cathodic current and measuring the charge at which color change or fluorescence occurred. The color change of modified acrylic coating systems was controlled by the sensitivity of the coating to pH increase [29]. 17.2.2.2 Pressure-Sensing Coatings Pressure-sensing coatings have been used on wind tunnel models and flight vehicles to measure the surface pressures for aerodynamic and acoustic investigations. It is an optical technique for determining surface pressure distributions by measuring changes in the intensity of emitted light. Mainly, luminescent coatings are painted on the surface which will be excited by light of appropriate wavelength and imaged with digital cameras [30]. Porous pressure-sensitive paint (PSP) containing porous material as the binder with the large surface area can hold luminophores directly. Hence, the response time of porous PSPs is of the order of 1 μs owing to the feasibility of oxygen molecules in a test gas instantly quenching luminescence without having to permeate into a binder layer, as shown in Fig. 17.4. Anodized aluminum, anodized titanium, polymer/ceramic, thin-layer chromatography (TLC) plate, etc. have been used as porous materials [32]. A pressure-sensitive luminescent coating containing tris(4,7-diphenylphenanthroline) ruthenium(II) ([Ru(dpp)3]21) on porous anodized aluminum was made to measure nonperiodic unsteady pressure distribution. The pressure distribution on a delta wing at a high angle of attack in transonic flow is unstable due to the interaction between shock waves and leading-edge vortices [33]. Fast pressure-sensitive paint is an extension applicable to unsteady flow and acoustics. Most fast PSP formulations are associated with porous binders that allow rapid oxygen diffusion and interaction with the chemical sensor. Calibrations of the dynamic response of them show a flat frequency response to at least 6 kHz, with some paint formulations exceeding a response of 1 MHz [32]. Besides the luminescent coatings, functional graphene sensors, such as Raman-based strain sensors, are also prepared by different methods. The strain in graphene/poly

FIGURE 17.4 Schematic illustration of porous pressure-sensitive paint (PSP). Source: Reproduced with permission H. Sakaue, T. Tabei, M. Kameda, Hydrophobic monolayer coating on anodized aluminum pressure-sensitive paint, Sens. Actuators B: Chem. 119 (2) (2006) 504511 [31]. Copyright 2006, Elsevier.

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(methyl methacrylate) sensors made of mechanically-exfoliated single-crystal graphene flake, as well as scalable chemical vapor deposition (CVD) graphene film, was investigated. The former was more sensitive to strain than the CVD graphene due to exfoliated graphene being a large single-crystal domain with high stiffness. CVD graphene films had an absolute accuracy of  6 0.01% and resolution of  27 με [34]. A highly sensitive, mechanically flexible pressure sensor was made by sandwiching ultrathin gold nanowireimpregnated tissue paper between two thin polydimethylsiloxane sheets for wearable optoelectronic devices. The sensors could be operated at a battery voltage of 1.5 V with low energy consumption. They were able to detect very low pressure with fast response time, high sensitivity, and high stability. This type of material with mechanical flexibility and robustness enabled real-time monitoring of blood pulses as well as detection of small vibration forces from music [35].

17.2.3 Optically-Active Coatings Smart window coatings are thin films with spectrally selective properties on the surface of the glass. They are characterized by their ability to regulate the amount of radiant energy between broad limits. These coatings are commonly referred to as Energy Efficient Window Coatings [3638]. Smart window coatings enable blocking of inbound infrared radiation from the sun during warm weather conditions and provide heat retention inside a room in the cold seasons. Since the active materials in these coatings block both near infrared (NIR) light and visible light, this allows customization of the window settings to maximize energy efficiency. Thus, energy consuming processes are significantly eliminated. Smart window coatings can be fabricated from any of the following: photochromic coatings, thermochromic coatings, suspended particle devices (SPD), polymer dispersed liquid crystal devices (PDLC), electrochromic coatings, etc. [36,39,40]. SPD or electrochromic windows seem encouraging for dynamic daylight and solar energy applications in buildings based on the achieved transmittance modulation ranges. The electrochromic windows appear to be the most promising in comparison to the suspended-particle type because, with the suspended particle windows, electric field should be maintained whenever the transparent mode of the glass is required, resulting in higher energy consumption [40]. A notable study by Li et al. described an approach where a single nanoparticle structure (VO2@TiO2 coreshell nanorods) with both thermochromic and photocatalytic properties offered significant potential for creating a multifunctional energy-saving smart coating. With the VO2 nanorod core, a remarkable modulation ability for solar infrared light was achieved, while the anatase TiO2 shell displayed significant photocatalytic degradation of organic dye [37]. Studies by Liu et al. on the combined effect of KxWO3 and F-TiO2, to achieve FTKWO nanocomposite films for windows exhibited strong near-infrared, ultraviolet light-shielding ability, well visible light transmittance, high photocatalytic activity, and excellent hydrophilic capacity. The proposed multifunctional NIR shielding-photocatalytic nanocomposite window film is meant to solve the energy crisis and deteriorating environmental issues. The working model is described in Fig. 17.5 [41].

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FIGURE 17.5 Schematic of the FTKWO and F-TiO2-coated window applied to dissimilar conditions. Source: Reproduced from T. Liu, B. Liu, J. Wang, L. Yang, X. Ma, H. Li, et al. Smart window coating based on F-TiO2KxWO3 nanocomposites with heat shielding, ultraviolet isolating, hydrophilic and photocatalytic performance., Sci. Rep. 6 (2016). licensed under a Creative Commons Attribution 4.0 International License.

17.2.4 Easy-to-Clean Coatings 17.2.4.1 Self-Cleaning Coatings Self-cleaning coatings inspired by nature and centered on surface contact angles ensure an independent action of dirt removed from a surface by either a hydrophilic or hydrophobic technique. Self-cleaned surfaces can be elucidated using two distinct approaches. The first approach involves the application of a photocatalytic coating to the substrate surface, where the effect of the sun’s ultraviolet rays catalytically breaks down organic dirt. Simultaneously, the surface becomes superhydrophilic spreading the water evenly over the surface, and less drying traces are formed by dripping. In the other approach, a selfcleaning surface is achieved in congruence with the lotus effect phenomenon in which the surface becomes superhydrophobic. The extent of hydrophobicity is determined by the contact angle of water. Contact angles greater than 90o are regarded as hydrophobic while the surface with contact angles of 150o, or higher are% superhydrophobic and repel water % droplets completely [1,8]. Superhydrophobic surfaces are obtained from roughened hydrophobic surfaces. The effects of both the contact and the sliding angle are essential in defining a superhydrophobic surface with self-cleaning effect. This is because, for self-cleaning in addition to the high contact angle, the water drop should slide off the surface [42]. Fig. 17.6 illustrates self-cleaning related actions such as (i) natural self-cleaning surfaces and corresponding SEM micrographs; (ii) the hydrophilic action of self-cleaning coating via photocatalysis reaction of titanium dioxide coating on a pre-painted aluminum surface; (iii) lotus effect (a) self-cleaning and (b) wetting based on contact angle differences; and (iv) hydrophobic/superhydrophobic action of self-cleaning coatings made of organosilanecoated alumina particles deposited via electrospraying. Anticorrosive self-cleaning coatings with superhydrophobic surfaces are particularly attractive candidates for achieving enhanced anticorrosive performances. In the study led

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FIGURE 17.6 (A) Natural self-cleaning surfaces and corresponding SEM micrographs. (B) Hydrophilic action of a self-cleaning coating. Schematic shows photocatalysis reaction of titanium dioxide coating on a pre-painted aluminum surface. (a) UV light energizes the electrons in TiO2 which transfer energy to oxygen and water in the air forming free radicals like .OH. (b) The free radicals attack organic matter by oxidation as hydroxyls accumulate on the surface. (c) The hydroxyls collapse water molecules on the surface in the cleansing action [1]. (C) Lotus effect (i) self-cleaning and (ii) wetting based on contact angle differences. (D) Hydrophobic/superhydrophobic action of self-cleaning coatings made of organosilane-coated alumina particles deposited via electrospraying. Source: (A and C) Reproduced with permission J. Zhang, X. Suo, J. Zhang, B. Han, P. Li, Y. Xue, et al. One-pot synthesis of Au/TiO2 heteronanostructure composites with SPR effect and its antibacterial activity, Mater. Lett. 162 (2016) 235237 [43]. Copyright 2016, Elsevier; (D) Reproduced with permission H. Yoon, H. Kim, S.S. Latthe, M. Kim, S. AlDeyab, S.S. Yoon, A highly transparent self-cleaning superhydrophobic surface by organosilane-coated alumina particles deposited via electrospraying., J. Mater. Chem. A 3 (21) (2015) 1140311410 [44]. Copyright 2015, Royal Society of Chemistry.

by Li et al. a hierarchical structured superhydrophobic coating with nanoflakes was formulated via electrodeposition and solution-immersion processes. The water contact angle was about 157 and sliding angle around 3 after fluorination modification. The prepared self-cleaning superhydrophobic coating was investigated for anticorrosive properties. The fabricated coating had higher Ecorr (20.346 V) and lower Icorr (4.128 3 1026 A/cm2) values compared to the bare steel substrate (Ecorr 5 20.684 V, Icorr 5 7.270 3 1026 A/cm2). The superhydrophobic coating was also investigated for the antiscaling property, and this was confirmed when CaCO3 crystals on the superhydrophobic coating had needle-like features compared to the rhombohedral CaCO3 crystals on the surface of the bare steel substrate.

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FIGURE 17.7 (A) Photos of water droplets on the superhydrophobic coating surface; (B) the superhydrophobic coating can sustain the water droplet due to intrusion pressure (Δp) . 0; (C) sliding process of the water droplet on the superhydrophobic coating; (D) Tafel plots of the bare pipeline steel and the superhydrophobic coated pipeline steel; (E) schematic mechanism of the anticorrosion process; (F and G) SEM micrographs of the superhydrophobic coating before and after the potentiodynamic polarization test. Source: Reproduced with permission H. Li, S. Yu, X. Han, Y. Zhao, A stable hierarchical superhydrophobic coating on pipeline steel surface with self-cleaning, anticorrosion, and anti-scaling properties, Colloids Surf. A: Physicochem. Eng. Asp. 503 (2016) 4352. Copyright 2016, Elsevier.

The superhydrophobic coating maintained good long-term stability in air, mechanical and thermal stability under certain environment. After the polarization test, the aggressive NaCl solution did not destroy the robust superhydrophobic coating which had dandelionlike hierarchical structures. An illustration of the coating properties is presented in Fig. 17.7 [45]. A self-cleaning superhydrophobic coating with a contact angle of 155.2 6 0.5 and a sliding angle of 3.5 6 1.3 was fabricated by Zheng et al. using a facile and low-cost method and investigated for anticorrosive protection on an aluminum surface. Different sample surfaces were examined during the study. Myristic acid modified samples were denoted as MA-x, and the reference substrate was anodized Al samples without surface modification denoted as AAO-x, where x represents the value of anodization voltage. In Fig. 17.8 samples A, B, C representing hydrophilic aluminum (Al), superhydrophilic (AAO-20), and

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FIGURE 17.8

The self-cleaning process of (A) Al; (B) AAO-20; (C) MA-20 pristine samples before the test; (D) Al; (E) AAO-20; (F) MA-20 samples after immersion in a dirty solution for 1 min and (G) Al; (H) AAO-20; (I) MA20 cleaned samples after water spray and drying; (J) potentiodynamic polarization curves of Al, MA-0, and MA20. Source: Reproduced with permission S. Zheng, C. Li, Q. Fu, W. Hu, T. Xiang, Q. Wang, et al. Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and anti-icing applications, Mater. Des. 93 (2016) 261270. Copyright 2016, Elsevier.

superhydrophobic coating (MA-20) were immersed in a dirty solution and dried. Dirt accumulated on Al (D) and AAO-20 (E) surfaces, but a little dirt was observed on MA-20 coating (F). After spraying water on the studied surfaces, the MA-20 coating (I) was as clean as before, in comparison to both Al (G) and AAO-20 (H) surfaces which were still covered with a significant amount of dirt. The findings revealed that it is difficult for dirt to attach to a superhydrophobic surface and if any, can be easily cleaned by spraying water. This is due to the joint action of high capillary forces induced by water droplets and weak adhesion of dirt to the superhydrophobic surface. The anticorrosive performance of the coatings was investigated using potentiodynamic polarization technique in comparison with the uncoated Al substrate (Fig. 17.8J). The Ecorr of the superhydrophobic MA-20 coating was 59 mV more positive than the bare Al substrate and 39 mV higher than the hydrophobic MA-0 coating. The corresponding Jcorr (1.527 3 1029 A/cm2) was reduced by two orders of magnitude compared to the bare Al substrate. Meanwhile, the MA-20 superhydrophobic coating showed higher polarization resistance (Rp), 408 times that of the bare Al substrate and 147 times that of the hydrophobic MA-0 coating. In comparison with other previously reported superhydrophobic anticorrosive coatings, the results showed a remarkable corrosion resistance on Al and its alloy [46]. A recent study by Li et al. on superhydrophobic multiwalled carbon nanotubes dispersed in a thermoplastic elastomer (MWCNT/TPE) smart coating revealed the development of a high-performance coating with sensing ability toward stretching, bending, and torsion. The intelligent coating can be easily fabricated under ambient conditions with no special requirements for cleaning or activation of the substrate. The superhydrophobicity of the coating is maintained irrespective of the substrate (glass, plastic, cloth, and metals) for self-cleaning, drag reduction, or other related applications. The coating showed

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excellent stability to UV radiation, acid/alkali stress, repetitive bending and kneading, and extreme repellency to acidic/alkaline droplets. The smart coating can be exploited as a flexible, high-performance, and multifunctional wearable sensor in human healthcare and humanmachine interface applications, and being superhydrophobic can be used under wet and corrosive conditions [47]. Regarding the polymer/oxide nanocomposite coatings, Ding et al. fabricated the superhydrophobic self-cleaning fluorinated polysiloxane/TiO2 nanocomposite coatings with longterm durability [48]. Manca et al. synthesized the self-cleaning and antiglare coating on glass plates using modified nanosilica particles. Their nanocoatings exhibited an enormous water repellency (contact angle 5 168 ) and stable self-cleaning property during 2000 hours of outdoor exposure [49]. Bayer’s group developed several superhydrophobic coatings for various applications, by using nano-silica particles and flourinated polymers [5052]. 17.2.4.2 Antigraffiti Coatings Graffiti is a significant challenge for architectural heritage materials. Graffiti affects a wide variety of surfaces, and the cleaning is costly and quite often, the penetration into the pores contained in the substrate material induces an irreversible effect onto the painted surface. Antigraffiti coatings have been developed by functionalizing nanoparticles and polymers to form a coating repellent to both water and oil simultaneously. As a result, the coated surface can be a non-stick surface, an easy to clean surface, and also able to withstand repeated graffiti attacks [8,53]. To enable hassle-free cleaning of the substrates fluorinating agents have been used to lower surface energy of the coating by migrating onto the coating surface. This offers good hydrophobicity reducing the adherence of staining agent to substrates. Also, outstanding thermal and oxidative stability, low coefficient of friction, and good chemical resistance are achieved with its use [54]. Fluoropolymers exhibit a great deal of oleophobicity since the extent of electronegativity and low polarizability of fluorine atoms in the backbone or side chains induce hydrophobic and oleophobic properties in the coatings synthesized from them [55]. Furthermore, most commercially available antigraffiti paints are siloxane/silicone-based formulations. They repel most of the waterbased paints and markers [56].

17.2.5 Bioactive Coatings 17.2.5.1 Antifouling Coatings Biofouling occurs when microorganisms in water adhere to wet surfaces, multiply, and gradually cover the surface with biofilm. The effects of biofouling costs marine, shipping, and other global industries billions of dollars every year [57]. Fouling occurs in two categories: micro- and macrofouling. Antifouling coatings are meant to reduce biofouling and its attendant effects and are categorized into biocide-free coatings and biocide-containing coatings [1]. Specifically, antifouling coatings can be further classified into chemically active self-polishing coatings (SPC) with booster biocides, or silicone- and fluorine-based fouling release coatings (FRC), based on physicochemical and mechanical effects. The efficacy of SPC is connected to biocidal effects, while for the biocide-free FRC, the adhesion between foulant and the surface is minimized due to the low surface energy and elastic

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FIGURE 17.9 (AD) Marine antifouling coating approaches. Source: Reproduced from A.G. Nurioglu, A.C.C. Esteves, G. de With, Non-toxic, non-biocide-release antifouling coatings based on molecular structure design for marine applications, J. Mater. Chem. B 3 (32) (2015) 65476570 [61]. Licensed under a Creative Commons Attribution 3.0 Unported Licence.

modulus. This ensures that biofouling can be removed by hydrodynamic stress during movement or mechanical cleaning. FRC fails to prevent colonization of biofilm but inhibits the adhesion of most macrofouling under dynamic conditions. The biofilms predominantly consist of diatoms which adhere tenaciously to hydrophobic surfaces and are not released from FRC, even on vessels operating at high speeds such as .30 knots [5860]. Fig. 17.9 illustrates the marine antifouling coating approaches. Marine biofouling is of great concern to the maritime sector due to its related substantial economic losses since substrates are rapidly colonized by both micro- and macroorganisms when immersed in seawater. This process is accountable for important economic and ecological effects related to shipping hulls, which increase fuel consumption and drydocking operations. This further leads to significant increases in pollution and financial burden [58,62]. Characteristics of an effective antifouling coating should include: antifouling properties, durability, good adhesion, corrosion inhibition, smoothness, easy to apply, fast drying, economical, and readily available, etc. [63]. Over time, tributyltin (TBT) has been the most widely used chemical antifoulant compound. To avoid the known adverse effects of TBT and its derivatives on marine environments, rigorous studies have been ongoing towards obtaining eco-friendly antifouling coatings able to withstand the rigours of practical applications such as shipping. As a significant advancement towards the production of eco-friendly antifouling paint, natural, biodegradable compounds have been utilized, such as bacteria isolated from living surfaces in the marine environment to produce chemicals that are potential antifoulants [62].

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Self-cleaning foul release coatings have been developed by Selim et al. in a recent study and the anticorrosive performance was investigated through salt spray test. Low-surface free energy, low microroughness, and ultrasmooth topology of self-cleaning surfaces prevent fouling settlements. Amongst the concentrations studied, the addition of welldispersed 0.5% ZnOSiO2 nanospheres minimized the surface tension inside the polymer matrix, leading to enhanced surface inertness against fouling adhesion alongside excellent mechanical and anticorrosive properties in aqueous salt fog environments. Foulant investigated for biofilm formation were Gram-positive (Micrococcus sp.) and Gram-negative (Pseudomonas putida) strains of bacteria and the fungi (Aspergillusniger), which are widely used in the evaluation of the antifouling performance of marine paints. In a nutshell, the nanocomposite coating demonstrated inert and superhydrophobic properties with a contact angle of 165 6 2 . Superior physical characteristics, lotus effect, thermal stability, long-term durability under UV radiation, and resistance against a wide range of pH solutions was observed making them promising as efficient, eco-friendly fouling release selfcleaning coatings for ship hulls without biocides [64]. Biodegradability measurements of the studied nanocomposites against the different foulant are presented in Fig. 17.10A and a comparison of the prepared nanocomposite smart coating with some commercial fouling release coatings is shown in Fig. 17.10B. Furthermore, antifouling coatings developed by incorporating butenolide derived from marine bacteria into biodegradable poly(ε-caprolactone)-based polyurethane were investigated by Ma et al. Mass loss measurements revealed that the polymer degraded in seawater and the degradation rate increased in the presence of marine organisms or enzymes. Butenolide was released from the biodegradable polymer for at least 3 months, and the release rate dependent on both the concentration of butenolide and temperature. With the incorporation of a naturally occurring resin (rosin) into the biodegradable polymer, the self-renewal rate increased, and afterwards, the release rate of butenolide improved. A field test indicated that the system had excellent antifouling properties [65]. 17.2.5.2 Antibacterial Coatings Several factors dominate bacterial interactions and one such factor is the surface charge. The bacterial surface charge varies depending on the species, the containing medium, the age of the bacteria, and the rod or round surface structure [66]. Bacteria survive by attaching to solid substrates via biofilms, where they can persist for extended periods, acting as a reservoir of pathogens and multiplying their pathways of transmission. In the biofilms, they are drastically more resistant to antibiotics and external forces and can withstand host immune responses. In-dwelling devices and implants, as well as surfaces in the nearpatient environment, play a significant role in the spread of hospital-acquired (nosocomial) infections. These nosocomial infections can be attributed to Gram-negative bacterial pathogens, for which there is an inadequate supply of antibiotics. The need for antibacterial coatings is strongly stimulated by the increasing urgency of identifying alternatives to the traditional administration of antibiotics. Hence, the development of controlled release strategies is vital to optimize therapeutic effects [67]. A study by Su et al. revealed that a shaking condition applied to the self-polymerization of dopamine in alkaline solution could facilitate the formation of roughened polydopamine (rPDA) coatings which remarkably enhanced antibacterial activities against Gram-positive Staphylococcus aureus, and

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FIGURE 17.10 (A) Represents biodegradability measurements of the virgin polydimethylsiloxane (PDMS) and filled ZnOSiO2/PDMS nanocomposites against different foulant. (B) A comparison of the prepared PDMS/ ZnOSiO2 nanocomposites with some commercial fouling release coatings. Source: Reproduced with permission M.S. Selim, M.A. Shenashen, A. Elmarakbi, N.A. Fatthallah, S. Hasegawa, S.A. El- Safty, Synthesis of ultrahydrophobic and thermally stable inorganicorganic nanocomposites for self-cleaning foul release coatings, Chem. Eng. J. 320 (2017) 653666. Copyright 2017, Elsevier.

Gram-negative Escherichia coli and Pseudomonas aeruginosa. The cell membranes of the bacteria incubated with the rPDA coatings were damaged by the contact-kill mechanism [68]. The gradual release of antibacterial agents, contact-killing, and antiadhesion/bacteriarepelling approaches are the three primary strategies for designing antibacterial coatings [67,69], as demonstrated in Fig. 17.11AC. As can be used with a low dose, recently nanoparticles have been reported as an alternative to common antibiotics. They could affect bacteria simultaneously through various processes, such as: (i) production of reactive oxygen species (ROS); (ii) electrostatic interaction with the cell membrane; (iii) ion release; (iv) internalization [70]. Many studies have

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FIGURE 17.11 Schematic of bacterial action in contact with (A) non-antibacterial coating; (B) antibacterial coating featuring contact-killing mechanism; (C) antibacterial coating featuring antiadhesive mechanism.

focused on the antibacterial coatings, which incorporated these antibiotic nanoparticles. In the case of noble metal nanoparticles, Kuma et al. reported that silver-nanoparticle (AgNPs) paints exhibited the high antimicrobial activity against both Gram-positive human pathogens (S. aureus) and Gram-negative bacteria (E. coli) [71]. Similarly, Ploux et al. reported the antibacterial action against E. coli of AgNPs-loaded plasma polymer coatings [72]. Regarding the metal oxide nanoparticles, nano-TiO2 and nano-ZnO particles are mostly used in the antibacterial polymer nanocomposite coatings [73,74]. To enhance the antibacterial activity of AgNPs, the hybridization of AgNPs and other oxide nanoparticles is a promising approach. Chudasama et al. reported that Fe3O4Ag coreshell nanoparticles have a better antibacterial activity against the Gram-negative bacteria (including E. coli) than that of AgNPs [75]. In the case of silver ferrite nanocomposites, Kondala et al. indicated that their antibacterial activity was higher than that of AgNPs and other antibacterial drugs [76]. Ngo et al. also reported that Fe3O4Ag dumbbell-like hybrid nanoparticles had a higher antibacterial action against E. coli than the lone AgNPs [77]. On the other hand, for nano-TiO2 and nano-ZnO particles, their production of ROS could be enhanced by their hybridization with noble metal nanoparticles. In the case of AuTiO2 nanohybrids, Zhang et al. reported that the antibacterial action against E. coli was more than five times higher than that of pure TiO2. Chen et al. also indicated that AuNPs (5 nm) decorated nano-TiO2 particles had higher antibacterial activity against E. coli, than that of pure nano-TiO2 particles. Regarding the Au/ZnO nanohybrids, He et al. observed that the deposition of AuNPs onto nano-ZnO particles significantly increased the light-induced generation of hydroxyl radical, superoxide and singlet oxygen, holes, and electrons [7880].

17.2.6 Fire-Retardants Coatings 17.2.6.1 Intumescent Coatings Intumescent coatings are used as a protective material for fire resistance by decreasing heat transfer to the substrate structure. An intumescent coating swells with heat exposure

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leading to increasing volume and decrease in density. They can be applied to structural members as a fireproofing paint. Intumescent paint contains many ingredients such as acid source, a charring agent, blowing agent, binder, etc. The binder is a crucial part of an intumescent coating. Its primary task is to bind all compounds, but it also acts as a carbon source and influences the foaming process. The role of different binders and their chemical reactivity have been investigated. It was found that the thermal insulation much improved when using a mixture of a linear copolymer presenting a good reactivity with the acid source and a cross-linked copolymer as the binder in the intumescent paint [81]. A series of intumescent coatings with different binders of vinyl ester/ethylene, vinyl acetate/ethylene, vinyl acetate/acryl ester, and vinyl acetate/vinyl ester of versatic acid from two different manufacturers were investigated regarding insulation, foaming, mechanical impact resistance, and residue morphology. The insulation performance had moderate influence with different binders, whereas the foaming dynamics, thickness achieved, and the inner structure of the residues showed a rich variety [82]. The selection of suitable flameretardant fillers influences the physical and chemical properties of the coatings. Intumescent coatings made of an acrylic binder and flame-retardant ingredients on steel substrates showed better fire protection as well as mechanical properties. It was found that the combination of aluminum hydroxide (Al(OH)3) and titanium dioxide (TiO2) had significantly improved the fire protection, thermal stability, and water resistance [84]. Intumescent coatings made of epoxy resin as a binder with variable concentrations of coconut fiber (CCN), wood waste (MDP), and peach stones (PEA) biomasses were studied. The optimum dry mass percentage of CCN and MDP was 9%, while 6% was ideal for PEA. The total expansion of the coatings after fire resistance testing was more than 600% for CCN9, 1500% for blank, 1300% for MDP9, and 1600% for PEA6. The biomass-based intumescent coatings showed better thermal insulation with the maximum temperatures on the back of substrate at 120 C, whereas it was at 474 C for uncoated steel [85]. The effect of fly ash cenospheres on the heat shielding performance was studied. Less feasibility of the heat transfer to the substrate was observed due to the large quantity of residual char (39.30 wt.%) containing titanium phosphate and boron phosphate [86]. Influence of kaolin clay on the expansion of coating and heat shielding was investigated. The coating with 5 wt.% of kaolin clay was enhanced up to 49% residual weight than that of the kaolin-free counterpart [87]. Intumescent flame-retardant coatings that incorporate chicken eggshell (CES) waste as the filler along with three flame-retardant additives, namely, ammonium polyphosphate phase II, pentaerythritol, and melamine in the acrylic binder has proven to be efficient in the protection of plywood against fire [83]. Epoxy resin coatings prepared with ginger powder, coffee husk, and other vegetable compounds as the carbon source showed better flame resistance [88]. 17.2.6.2 Nonintumescent Coatings Active nonintumescent coatings release fire-retarding species including gas phase radicals from the formulations of water-soluble salts containing phosphate, nitrate, halides sulfate, sulfamate, boric acid, and borax [89]. Cold plasma polymerized fluorinated acrylate on polyamide-6 was made by Errifai et al. for fire resistance. The deposit reduced both peak heat release rate (HRR) and time to ignition (tig) by 50%. This reduction was due to the dilution of combustible gases caused by the reaction of CFx radicals with the

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degraded polymer fragments [90]. Methacrylated phenolic melamine (MAPM) incorporated epoxy acrylate gave better flame retardancy. It was found that decomposition of melamine released nonflammable nitrogen volatiles like ammonia with no increase in char formation [91]. C ¸ akmakc¸i et al. prepared flame-retardant epoxy acrylate coatings with the incorporation of phosphate containing allyldiphenylphosphine oxide (ADPPO) [92]. Phosphoruscontaining UV-cured hyperbranched polyurethane acrylate showed a limiting oxygen index of 27.0. Synergistic effect of phosphorus and nitrogen at the phosphorus content around 0.7 wt.% gave the best value. The breaking of POC bonds formed the POP bonds. The presence of phosphorus in coating supported the char formation, which reduced the flame attack of underlying polymeric materials [93].

17.2.7 Other Smart Coatings 17.2.7.1 Antifingerprint Coatings These smart coatings provide a fingerprint-hiding effect. Antifingerprint coatings are widely used for touch screens in consumer electronic devices. With the release of surfactants or enzymes in response to surface contact, the deposited fingerprints become faded [5]. Furthermore, antifingerprint surfaces should have low energy, for the formation of an oleophobic weak boundary layer. Low energy surfaces are known to reduce attractive intermolecular forces [94]. The fingerprint problem on a touch screen surface is a pressing issue requiring antifingerprint coatings. The development of protective coating materials with amphiphobic (water and oil repellent) properties can resolve this with amphiphobic (water and oil repellent) properties. The amphiphobic property can be achieved by the construction of the morphological structure with re-entrant curvature in combination with the chemical composition and roughness on surfaces. Derived from a nano-scaled concave structure consisting of cavities, in which the capillary force produced at the liquidair interface inside the re-entrant can repel liquid (water or oil) from entering the void. In a study by Siriviriyanun and Imae, an attempt to make antifingerprint properties from nonfluorinated organosiloxane self-assembled monolayer-coated glass surfaces was achieved. The substrate with nonfluorinated (methyl terminated) organosiloxane (TMSglass) provided both amphiphobic and antifingerprint features on the surfaces. Also, the hybrid with both hydrophobic and nonfluorinated moieties (octadecyltrimethoxysilane, ODS, and trimethoxymethylsilane, TMS) imparted the same properties evaluated by the contact angle of oleic acid. It was concluded that antifingerprint is related to amphiphobicity [95]. Furthermore, the use of fluoride films on glass substrates of electronic panel devices providing antifingerprint effects experiences adhesion problems. A thin SiO2 layer should be inserted between the fluoride film and the glass to overcome the poor adhesion. An attempt to realize an enhanced mechanical durability of an antifingerprint coating by further incorporation of silver nanoparticles has been reported. For strong adhesion, the SiO2 layer was inserted between the silver nanoparticles and the glass substrate and the fluoride/SiO2 films deposited on that composition. An efficient antifingerprint property based on a wetting angle of about 116 was achieved [96]. Recently, both antifingerprinting and antibacterial effects from 30 nm thick ZnO thin films without an additional protective layer

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for smartphone panel application have been achieved. As a result of a simple annealing treatment of the film, high transmittance (B91.3%) comparable to that of a glass substrate at a wavelength of 550 nm was realized [97]. 17.2.7.2 Antireflective Coatings The idea of antireflective coatings in physical sciences dates back to Lord Rayleigh (John Strutt) in the 19th century when he observed the tarnishing on a glass increasing its transmittance rather than reducing it. However, Fraunhofer in 1817 produced antireflective coatings when he noticed that reflection was decreased due to etching a surface in an atmosphere of sulfur and nitric acid vapors. The quest for ways to maximize light collection efficiency has encouraged countless investigations for antireflective coatings to cater for the growing demand of optical and optoelectronic equipment in diverse areas of application [98]. With antireflection coatings, there is an increased performance of optical components fabricated from glass-based optical materials and a reduction in reflective losses at interfaces. Thus, antireflective coatings on flat-panel displays in electronics eliminate the effects of spurious images or veiled glares originating from stray and multiple reflections from optical surfaces. High-efficiency antireflection coatings fabricated from phaseseparated polyelectrolyte multilayer films that undergo a reversible pH-induced swelling transition have been realized [99]. Similarly, a double-layer coating based on λ/4λ/4 index-gradient design wavelength system has been reported. Prepared with dense and porous silica films as high and low index layers separately. When coated on a glass substrate, a satisfactory broadband antireflective performance was realized for the demand of the amplifier blast-shields used in high power laser systems. With a subsequent NH3-heat treatment and trimethylchlorosilane (TMCS) post-treatment, weak mechanical property and optical instability of the coating were overcome [100]. Using modified nanosilica particles, Manca et al. fabricated the antireflective surfaces by a double-layer coating. This antireflection effect was attributed to the nanotextured topology producing by the nanoparticles-based top layer [49]. 17.2.7.3 Anti-icing Coatings In the modern society, icing is a source of a variety of problems. The icing on the wings and surfaces of aircraft could cause crash accidents. During flights, aircraft intercept supercooled water droplets through clouds or encounter freezing rain. The impacting water freezes rapidly to form deposits of ice. The ice accretion results in drag increase and sometimes may lead to dangerous loss of lift force, which may cause tragic aircraft crash accidents. Similarly, frost and ice accumulation in heat exchangers and refrigerators often results in a decrease of heat transfer efficiency up to 50%75% due to the frost formation [101]. Icing can result in major electrical outages, interrupt offshore oil and gas production, and decrease the efficiency of wind power generation [102]. The need for anti-icing strategies and coatings cannot be overemphasized. Anti-icing coatings are coatings with properties that can prevent or delay freezing of the impacting and condensed water as well as decrease the ice adhesion on its surface. On such surfaces, condensed water droplets can spontaneously jump away before freezing due to its superhydrophobicity. In another scenario, the coating is impregnated with antifreeze lubricants [101,102]. Anti-icing properties of coatings may depend on the following (i) the state of the solid surface if colder than the

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air/vapor; (ii) how large the temperature gradient is; and (iii) if a thin film of water tends to form on the solid surface due to capillary effects, disjoining pressure, etc. [103]. An ideal superhydrophobic surface does not allow frost formation due to the coalescence induced self-removal of condensed water. But in reality, most of the superhydrophobic surfaces have been susceptible to the frosting and the initiation of any defect on the coating surface further encourages frost formation [101,102]. To improve the anti-icing performance of aircraft, the superhydrophobic anti-icing coatings should be effectively deposited on the aluminum and copper substrates. In a contribution to overcoming the shortcomings of antiicing coatings, projects such as ICECOAT project have developed new types of coating matrices, new nanoparticles for nanocomposites, and a new method for coating surface modification to produce anti-icing coating with superior hardness and erosion resistance [103]. Similarly, Zheng et al. have fabricated anti-icing coatings with low ice adhesion strength and higher contact angles for aluminum substrates in contribution to overcoming ice accumulation on aluminum surfaces at low temperatures [46]. Furthermore, focus on bio-inspired strategies for anti-icing are ongoing. A hybrid method of icing-prevention by antifreeze dispensing and by repelling impinging drops has been developed. The fabricated bio-inspired anti-icing coating was aimed at reducing the need for antifreeze liquid by dispensing antifreeze only when required and directed towards having significant economic and environmental benefits. The semipassive coating consisted of a porous superhydrophobic epidermis and wick-like underlying dermis that was infused with antifreeze liquid. The outer layer served as a barrier between the antifreeze and environment. The coating was architectured to respond remarkably when eventually iced over by releasing stored antifreeze liquid amongst other factors [102]. To reduce ice deposition, Li et al. have fabricated the hydrophobic polydimethylsiloxane/nano-silica hybrid coating. The authors indicated that their hybrid coating exhibited super hydrophobicity, both in terms of multiscale and low surface energy [104]. 17.2.7.4 Antifogging Coatings Fogging involves the formation of water droplets on transparent solid surfaces which scatters light and reduces optical transmission. Fogging is a challenge for the everyday use of eyeglasses, goggles, and windshields. Fogging also reduces the efficiency of solar energy panels, medical/analytical instruments, as well as other industrial equipment. In antifogging research, superhydrophilic surfaces with very low θS (,5 ) are well known to prevent water droplets by promoting the formation of a continuous thin film of water condensed from the air, and this has gained significant attention. However, these rely on external stimulation by UV-light. Highly transparent superhydrophilic antifogging coatings were prepared and coated on various substrates, such as glass slides, silicon, copper and poly(methyl methacrylate) (PMMA). The antifogging coatings fabricated with polyvinylpyrrolidone (PVP) and aminopropyl-functionalized, nanoscale clay platelets exhibited 90% transmission of visible light. The chemical/physical properties of the coated surface remained almost unchanged after several antifogging tests and exposure to humid air for 30 days [105]. Similarly, the improvement of water repellency by designing a micro- or nanostructure onto a low surface-energy material surface, with a water contact angle (CA) greater than 150 and a low sliding-off angle of less than 10 , enhances antifogging abilities. Antifogging coatings are therefore relevant for surfaces in cold and humid

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environments. Antifogging coatings with icing-delay properties have been investigated using poly(vinylidenedifluoride) (PVDF) polymer together with zinc oxide (ZnO) which produced ZnO-on-PVDF micro/nanostructure (ZP-MN) composite. On the ZP-MN surface, condensed water droplets were easily shed at 5 C for B1600 s via a slight wind or tilting. Interestingly, the droplets did not completely freeze on the ZP-MN surface at 10 C until B7360 s [106].

17.3 APPLICATIONS AND COMMERCIAL VIABILITY OF SMART COATINGS End-user applications of smart coatings are in the following areas: medical and healthcare, military, aerospace, automotive, marine, energy, consumer electronics, construction, oil and gas, packaging, textiles and apparel industries, etc. Smart coatings can be customized to meet the specific consumers’ requirements [107,3]. Self-cleaning thin nanofilms are applicable on household surfaces. They are safe for skin contact and protect the surfaces from dust or grease accumulation, scale deposition, and corrosion attack. From computer equipment to kitchen surfaces to shower screens these selfcleaning surfaces will minimize the use of detergents and time spent on cleaning [108]. Antifingerprint coatings have found application in optics, kitchens, cafes, restaurants, electronic display screens, textiles, floor sealants, automotive and HVAC (heating, ventilation, and air conditioning) systems, etc. [8]. Self-healing coatings for the aviation industry have been proposed by a team of researchers from the Center for Research in Ceramics and Composite Materials (CICECO), University of Aveiro. When applied to the fuselage of the aircraft, it repairs, as a skin, small discontinuities resulting from mechanical and environmental impacts suffered by appliances during flight by releasing its embedded healing molecules when required [109]. Likewise, an innovative nanopaint technology called Ultra-Ever Dry, a contribution to development in the world’s first selfcleaning car, has been tested by Nissan. It repels mud, rain, oil, sleet, dirt, and everyday road spray minimizing the car wash trips. Although there are no current plans to apply the self-cleaning paint to Nissan’s vehicle lineup as a standard feature, the technology is considered as a potential aftermarket option [110]. A few commercialized products are presented in Table 17.1.

17.4 CONCLUSION The purpose of this chapter is not to provide a complete summary of the literature on the present topic but rather to highlight some of the functions exhibited by these coatings with conceivable industrial relevance. As presented above, many of the smart coatings contain metal and metal oxide nanoparticles, which provide various functional properties and enhanced performance. Their hybrid nanoparticles are expected to produce the new multifunctional coatings. The area of smart or intelligent coatings is, in reality, a hot topic with a wide area of application due to the achievable multifunctionality. The list of coatings exhibiting

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TABLE 17.1 Selected Commercially Available Smart Coatings Products

Characteristics

Applications

Producers

Deletum 5000 & 3000

Antigraffiti paints

Coatings repellant to water and oil.

Victor Castan˜o

2C Marine Sealant PRO

Antifouling coating

Intended for gel coat marine surfaces with no or only minimal sign of wear.

Nanosafeguard

Burgundy Cukote

Antifouling coating

Cuprous oxide self-polishing coating

Sea Hawk

Smart solution

Copper free antifouling

Eco-friendly metal free bottom paint

Sea Hawk

Intercept 8500 LPP

Antifouling coating

Biocidal antifouling for marine vessels

AkzoNobel

ECONTROL

Intelligent solar control glass

Provides shading in the glass

EControl-Glas GmbH & Co. KG

NanoChar

Fire protection coating, VOC-free epoxy intumescent coatings based on nanotechnology

Protects steel amongst other nonmetallic substrates

Intumescent Associates Group

SPD-SmartGlass

Smart window coating

Changes the tint of any window, sunroof, or skylight

Research Frontiers

NANOMYTE

Waterborne, solventborne, superhydrophobic, self-healing, anticorrosion, acid resistant, etc.

Pretreatments, primers, and topcoats

NEI Corporation

Ultra-Ever Dry

Self-cleaning, antiicing, antiwetting, anticorrosion, etc.

Superhydrophobic and oleophobic coating that will repel most water-based and some oil-based liquids. For industrial use only

UltraTech International, Inc.

Victor Castano: http://www.nanotechproject.org; Nanosafeguard: http://www.nanotechproject.org; Intumescents Associates Group: http://www.advancedepoxycoatings.com; NEI Corporation: http://www.neicorporation.com/products/coatings/ anticorrosion-paints-coatings/; Sea Hawk: www.seahawkpaints.com; UltraTech International, Inc.: http://www.spillcontainment. com/; AkzoNobel: https://www.akzonobel.com; Research Frontiers: http://www.smartglass.com; EControl-Glas GmbH & Co. KG: www.econtrol-glas.de/en/company.

intelligent characteristics is gradually becoming inexhaustive, especially when dual to multiple functionalities can now be achieved in most categories due to coating formulation maximizing its potential. With recent developments focused mainly on smart coatings for anticorrosive applications, one of the essential approaches for self-healing coatings is apparently the use of nanocontainers capable of being loaded with active agents and having shells possessing controlled permeability specific to several triggers. Sustainability/durability is related to a continuous healing action. Likewise, leading approaches for self-cleaning coatings involves high superhydrophobicity with extreme repellency to varying corrosive droplets which are maintained irrespective of the metal substrate and also UV stability. Essential strategies for the active corrosion-sensing coatings involve the inclusion of fluorescent or color indicators responsive to varying environmental triggers. These could be any of the

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following: pH, ion, or redox indicators. Approaches for enhancing antifouling coatings are concerned with self-cleaning, water-resistant and superhydrophobic surfaces with lowsurface free energy, low microroughness, and ultrasmooth topology to prevent fouling settlements when biocides are not utilized. Otherwise eco-friendly biocides are encouraged. Besides the self-cleaning and water-resistant properties, superhydrophobic surfaces support antifouling characteristics alongside anti-icing, antiscratching, and antibacterial properties which contribute immensely to anticorrosion [111]. Smart materials and coatings can also be categorized into high-performance materials, property-changing materials, and energy-exchanging materials. Processed as a combination of several constituents to exploit the best properties of the individual constituents in the passive and active modes [112]. Smart coatings containing nanoparticles are versatile in formulation and functionalities. Nanocontainers of various types of materials coupled or loaded to obtain different active centers gain more attention for the innovations and application in various smart systems from drug delivery through bioactive surfaces to corrosion protection and beyond. Presently, efforts are devoted to the up-scaling of nanocontainer production and evaluation [113]. The challenge of translating smart coatings concepts in fundamental academic research into practical coating systems with commercially viable industrial applications is still of great concern. This obviously requires a lot more dialog and collaboration between academics and industrialists to bridge the gap between the technological progress and the market demand. Within the industry, there is continued product development, and the focus is on higher performance, not only to fulfill customer needs but also to comply with regulatory requirements. Whereas, the results from the academia aided by computer simulations may contribute to providing some leverage to addressing certain technical difficulties in the coatings industries. Furthermore, durability is a key challenge to advancing current smart coating technologies for external and internal applications in the different sectors where it is being utilized. Similarly, adding functionality to coatings often increases cost which is another challenge. But, effective multifunctional smart coatings in real-time service will boost market demands far beyond predictions [5,114].

17.5 SOURCES OF FURTHER INFORMATION The following articles and patents provide additional insights and advances in antibacterial coatings [115,116], antifouling coatings [117,118], antigraffiti coatings [119], antireflective coatings [120,121], anti-icing coatings [122,123], and antifogging coatings [121,124]. Also, some website sources include [125] (www.coatingsworld.com), [126] (www.european-coatings.com), and [127] (www.paint.org).

Acknowledgments Sarah B. Ulaeto acknowledges Council of Scientific and Industrial Research (CSIR) and The World Academy of Science for Developing Countries (TWAS) for the award of CSIRTWAS Postgraduate Fellowship. The authors appreciate Knowledge Resource Center of CSIRNIIST for access to some valuable articles utilized in the preparation of this manuscript.

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[80] W. He, H.K. Kim, W.G. Wamer, D. Melka, J.H. Callahan, J.J. Yin, Photogenerated charge carriers and reactive oxygen species in ZnO/Au hybrid nanostructures with enhanced photocatalytic and antibacterial activity, J. Am. Chem. Soc. 136 (2) (2014) 750757. Available from: https://doi.org/10.1021/ja410800y. [81] S. Duquesne, S. Magnet, C. Jama, R. Delobel, Intumescent paints: fire protective coatings for metallic substrates, Surf. Coat. Technol. 180181 (2004) 302307. Available from: https://doi.org/10.1016/j. surfcoat.2003.10.075. [82] M. Morys, B. Illerhaus, H. Sturm, B. Schartel, Size is not all that matters: residue thickness and protection performance of intumescent coatings made from different binders, J. Fire Sci. 35 (4) (2017) 284302. Available from: https://doi.org/10.1177/0734904117709479. [83] M.C. Yew, N.H. Ramli Sulong, M.K. Yew, M.A. Amalina, M.R. Johan, Eggshells: a novel bio-filler for intumescent flame-retardant coatings, Prog. Org. Coat. 81 (2015) 116124. Available from: https://doi.org/ 10.1016/j.porgcoat.2015.01.003. [84] M.C. Yew, N.H. Ramli Sulong, M.K. Yew, M.A. Amalina, M.R. Johan, Influences of flame-retardant fillers on fire protection and mechanical properties of intumescent coatings, Prog. Org. Coat. 78 (2015) 5966. Available from: https://doi.org/10.1016/j.porgcoat.2014.10.006. [85] M.M. de Souza, S.C. de Sa´, A.V. Zmozinski, R.S. Peres, C.A. Ferreira, Biomass as the carbon source in intumescent coatings for steel protection against fire, Ind. Eng. Chem. Res. 55 (46) (2016) 1196111969. Available from: https://doi.org/10.1021/acs.iecr.6b03537. [86] R.G. Puri, A.S. Khanna, Effect of cenospheres on the char formation and fire protective performance of water-based intumescent coatings on structural steel, Prog. Org. Coat. 92 (2016) 815. Available from: https://doi.org/10.1016/j.porgcoat.2015.11.016. [87] S. Ullah, F. Ahmad, A.M. Shariff, M.A. Bustam, Synergistic effects of kaolin clay on intumescent fire retardant coating composition for fire protection of structural steel substrate, Polym. Degrad. Stab. 110 (2014) 91103. Available from: https://doi.org/10.1016/j.polymdegradstab.2014.08.017. [88] S.C. de Sa´, M.M. de Souza, R.S. Peres, A.V. Zmozinski, R.M. Braga, D.M. de Arau´jo Melo, et al., Environmentally friendly intumescent coatings formulated with vegetable compounds, Prog. Org. Coat. 113 (2017) 4759. Available from: https://doi.org/10.1016/j.porgcoat.2017.08.007. [89] S. Liang, N.M. Neisius, S. Gaan, Recent developments in flame retardant polymeric coatings, Prog. Org. Coat. 76 (11) (2013) 16421665. Available from: https://doi.org/10.1016/j.porgcoat.2013.07.014. [90] I. Errifai, C. Jama, M. Le Bras, R. Delobel, L. Gengembre, A. Mazzah, et al., Elaboration of a fire retardant coating for polyamide-6 using cold plasma polymerization of a fluorinated acrylate, Surf. Coat. Technol. 180181 (2004) 297301. Available from: https://doi.org/10.1016/j.surfcoat.2003.10.074. [91] H. Liang, A. Asif, W. Shi, Thermal degradation and flame retardancy of a novel methacrylated phenolic melamine used for UV curable flame retardant coatings, Polym. Degrad. Stab. 87 (3) (2005) 495501. Available from: https://doi.org/10.1016/j.polymdegradstab.2004.10.006. [92] E. C ¸ akmakc¸i, Y. Mu¨lazim, M.V. Kahraman, N.K. Apohan, Flame retardant thiol-ene photocured coatings, React. Funct. Polym. 71 (1) (2011) 3641. Available from: https://doi.org/10.1016/j.reactfunctpolym.2010.11.011. [93] S.W. Zhu, W.F. Shi, Flame retardant mechanism of hyperbranched polyurethane acrylates used for UV curable flame retardant coatings, Polym. Degrad. Stab. 75 (3) (2002) 543547. Available from: https://doi.org/ 10.1016/S0141-3910(01)00257-9. [94] G. Wang, H. Wang, Z. Guo, A robust transparent and anti-fingerprint superhydrophobic film, Chem. Commun. 49 (66) (2013) 7310. Available from: https://doi.org/10.1039/c3cc43677b. [95] A. Siriviriyanun, T. Imae, Anti-fingerprint properties of non-fluorinated organosiloxane self-assembled monolayer-coated glass surfaces, Chem. Eng. J. 246 (2014) 254259. Available from: https://doi.org/ 10.1016/j.cej.2014.02.066. [96] S.Y. Heo, B.J. Park, J.R. Jeong, S.G. Yoon, Enhanced transmittance, mechanical durability, and antifingerprinting qualities of silver nanoparticles deposited onto glass substrates, J. Alloys Compd 602 (2014) 255260. Available from: https://doi.org/10.1016/j.jallcom.2014.03.019. [97] H.J. Choi, B.J. Park, J.H. Eom, M.J. Choi, S.G. Yoon, Achieving antifingerprinting and antibacterial effects in smart-phone panel applications using ZnO thin films without a protective layer, ACS Appl. Mater. Interfaces 8 (1) (2016) 9971003. Available from: https://doi.org/10.1021/acsami.5b11024. [98] H.K. Raut, V.A. Ganesh, A.S. Nair, S. Ramakrishna, Anti-reflective coatings: a critical, in-depth review, Energy Environ. Sci. 4 (10) (2011) 3779. Available from: https://doi.org/10.1039/c1ee01297e.

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C H A P T E R

18 Photocatalytic Application of Ag/TiO2 Hybrid Nanoparticles Francesca Petronella, Alessandra Truppi, Marinella Striccoli, M. Lucia Curri and Roberto Comparelli CNR-IPCF, Consiglio Nazionale delle Ricerche, Istituto per i Processi Chimico Fisici, S.S. Bari, Italy

18.1 INTRODUCTION In recent years, photocatalytic processes have been recognized as a powerful tool to take action on critical environmental concerns, mainly related to an effective pollution control and to a sustainable energy production [1]. The process of photocatalysis takes place upon the photoexcitation of a semiconductor. After absorption, electromagnetic radiation of a proper wavelength causes the generation of electron and hole pairs (e2/h1). Holes (h1) in the valence band (VB) and electrons (e2) in the conduction band (CB) can undertake several paths including migration and trapping at the semiconductor surface, or recombination both at the bulk and at the surface of the semiconductor [2]. The competition between migration/trapping and recombination affects the overall efficiency of the photocatalytic process [3]. Indeed, when e2 and h1 are trapped at the semiconductor surface, they can be collected in an external circuit in order to generate electricity [4], produce H2 by water splitting [5], or they can react with H2O molecules or O2 molecules. In this case, e2 and h1 can trigger the formation of reactive oxygen species (ROS), namely chemical compounds containing oxygen, with a high reactivity, as hydroxyl radical •OH, hydrogen peroxide H2O2, and superoxide anion radical •O2. ROS are able to activate consecutive oxidation (or reduction) reactions that can potentially lead to the mineralization of a target pollutant [1]. Among semiconductors, TiO2 is the most appealing photocatalyst, because of its great stability, large commercial availability, low toxicity, and high photocatalytic activity [6].

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00018-8

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Recently, the nano-sized TiO2 has been extensively investigated and it is utilized in relevant applications, such as the photocatalytic cements used for the facades of the church “Dives in Misericordia” in Rome, designed by Richard Mayer, and in antipollution asphalts because of its well-established photocatalytic properties [7,8]. Indeed, from a practical point of view, nano-sized TiO2 is extremely convenient not only because of its small size, which implies a high surface area and a high number of surface active sites, but also thanks to the great achievements obtained in the field of nanoparticle synthesis [6]. Preparation protocols allow to customize chemicalphysical and optoelectronic properties of TiO2 nanoparticles (NPs) according to the specific application, because they enable careful control of the morphology, the crystalline phase, the crystalline facets, as well as the surface chemistry [9]. However, despite its high photoactivity, because of its energy gap of 3.2 eV nanosized TiO2 has the intrinsic limit to absorb light only at wavelengths lower than 384 nm, which corresponds to the ability to harvest only 4% of the solar spectrum [10]. Such a drawback can be circumvented by synthesizing hybrid NPs combining TiO2 with other semiconductors, redox couples, organic sensitizers, or metal NPs, thus obtaining multifunctional photocatalysts that can potentially work under solar light irradiation [11]. In this framework, coupling TiO2 with noble metal NPs is an extensively exploited strategy. A great deal of work has been devoted to the investigation of Ag/TiO2-based heterostructures due to the affordable synthetic protocols for Ag NPs [12], their reduced cost, their excellent performances in accumulating photogenerated electrons from TiO2 CB under UV light, and their plasmonic properties that are observed and exploited under visible light irradiation. It has been extensively demonstrated that the modification of TiO2 NPs with Ag NPs provides a significant enhancement of the photocatalytic efficiency under UV light irradiation [1318]. Upon TiO2 photoexcitation, photogenerated e2 can flow from the conduction band (CB) of TiO2 to the metal. Such a charge transfer occurs, under UV light irradiation, because the work function of metal NPs is more negative than that of TiO2, therefore Ag NPs collect and accumulate photogenerated e2 from TiO2 CB, behaving like an electron sink [19,20]. Upon this interfacial charge transfer, the pseudo Fermi level of the composite shifts to more negative potentials [21]. Further, interfacial e2 transfer determines the formation of a Schottky barrier at the metal semiconductor interface [22]. The ensemble of these phenomena determines an efficient e2 and h1 separation, limits their detrimental recombination events, promotes a more efficient production of ROS, resulting in an increase of the overall photocatalytic efficiency [13,23]. It is worth noting that in Ag/TiO2 heterostructures, the charge transfer efficiency from the TiO2 CB to the metal is influenced by several parameters, including metal NP size [20] and shape, TiO2 morphology, crystal facets, and surface chemistry, and nature of the interaction between Ag and TiO2 [24]. A boost to the field of photocatalytic application of metal NPs has been triggered by the investigation of the benefits deriving from the plasmonic properties of metal NPs. When metal NPs, such as Ag, Au, and Cu, are irradiated with light of suitable wavelength, electrons oscillate in phase with the incoming electromagnetic radiation. This phenomenon is called surface plasmon resonance (SPR) and can be detected by absorption spectroscopy. Indeed, metal NPs show, in their absorption spectrum, intense plasmonic bands that in the case of Au, Ag, and Cu are located in the visible range. The SPR phenomenon, as well as absorption spectrum features, are strongly affected by size, shape, surface chemistry,

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and dielectric constant of the metal and the surrounding medium [25,26]. A further crucial consequence of the SPR is the occurrence of an intense electric field in the proximity of metal NPs [27]. As the SPR phenomenon occurs under visible light irradiation, SPR can be exploited to collect and concentrate visible light, opening up a plethora of opportunities in photocatalysis-related applications. Indeed, in semiconductor/metal hybrid NPs, such as Ag/TiO2, the presence of plasmonic NPs enable the absorption of visible light. As reported in Fig. 18.1B, Ag NPs can extend in the visible light range the optical response of TiO2. In particular, semiconductor/metal hybrid NPs as Ag/TiO2 can be photoactivated by visible light by means of three possible mechanisms occurring under SPR conditions: (i) the near-field effect; (ii) the hot electron generation; or (iii) the far-field effect. The nearfield effect occurs when a thin organic layer that acts as dielectric separates the metal and the semiconductor. Under this condition the generation of e2 and h1 on TiO2 CB and valence band (VB), respectively, occur thanks to the intense electric field, generated in the

FIGURE 18.1 (A) EDS spectrum of TiO2 NRs/Ag NPs nanocomposite showing the presence of Ti and Ag species. (B) Absorbance spectra of as-prepared TiO2 NRs and TiO2 NRs/Ag NPs chloroform solution. (C) Rietveld refinement of the XRD pattern of TiO2 NRs/Ag NPs nanocomposite. Black dots: experimental data; red line: calculated powder pattern; vertical bars: expected Bragg reflections positions for the different identified phases. Source: Reprinted with the permission of F. Petronella, S. Diomede, E. Fanizza, G. Mascolo, T. Sibillano, A. Agostiano, et al., Photodegradation of nalidixic acid assisted by TiO2 nanorods/Ag nanoparticles based catalyst, Chemosphere 91 (2013) 941947.

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proximity of the metal NPs [28]. The generation of “hot electrons” is associated with the plasmon decay that can produce an energy transfer to electrons of the metal. As a consequence, these electrons gain a high energy, therefore they are identified as “hot electrons.” Such “hot electrons” can be captured by the semiconductor in close contact with the metal NP, provided that the “hot electrons” have enough energy to overcome the Schottky barrier between the metal and the semiconductor [27]. Finally, the far-field effect occurs when metal NPs are larger than 50 nm in size, and, acting as “nanomirror,” they can increase the electron mean optical free path, intensifying the rate of e2/h 1 pairs formation in the semiconductor [28]. Therefore, Ag/TiO2 hybrid NPs clearly show a great potential for several photocatalytic applications both under UV and visible light irradiation. From a practical point of view, with respect to other metal/semiconductor hybrid NPs, Ag/TiO2 systems show the advantages of affordability, efficient CB electron release to the solvent [29], high range of resonance wavelength (from 300 to 1200 nm, according to their size, shape and dielectric environment) [12], and antimicrobial properties. Further, the outstanding progress in the field of synthesis and characterization techniques allow the tailoring of the properties of Ag/TiO2 hybrid NPs and maximizing of specific features according to the specific application field. This chapter aims at providing an overview of the possible application of Ag/TiO2 nanosized heterostructures. In particular Section 18.2 is devoted to relevant environmental applications as water and air depollution. In Section 18.3, the production of green fuels, as H2, by the photocatalytic water splitting, and solar energy conversion through the dyesensitized solar cells are described. Section 18.4 illustrates how Ag/TiO2 hybrid NPs can be integrated in smart surfaces in order to convey self-cleaning, photocatalytic, and/or antibacterial properties. Finally, the application of Ag/TiO2-based NPs in photocatalytic filters, designed in order to improve air quality in factories, aircrafts, hospitals, stations, but also domestic environments, such as kitchens, car interiors, pet litters, is summarized. Several TiO2/Ag-based architectures are examined in each application class and recent reported experimental results are described as a function of the peculiar properties of the photocatalyst.

18.2 Ag/TiO2 HYBRID NANOPARTICLES FOR ENVIRONMENTAL APPLICATION 18.2.1 Photoactive Ag/TiO2 Hybrid Nanoparticles for Water Treatment Nowadays, the global water pollution is notoriously one of the major concerns in developing and industrialized countries, due to worldwide population growth, to the heavy industrialization, and to the changing of both climate and lifestyles of people [30,31]. Chemical pollutants can be dispersed into the aquatic environment in several ways, directly from industrial effluents or urban wastewater treatment plants (WWTP), or indirectly through the use of biocides and fertilizers in agriculture [30]. The WWTP are also considered as hotspots of antibiotics promoting the growth of pathogens in water [31,32].

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Indeed, the presence of antibiotic-resistant bacteria is another important problem in urban effluents. Several strategies have been reported in order to achieve a progressive reduction of water pollutants. Among the different water treatment technologies, adsorption or coagulation methods present limitations, because they are able to concentrate and transfer pollutants to other phases, without accomplishing their complete destruction. Further, conventional water cleaning methods involve more expensive processes and could release toxic secondary pollutants into the ecosystem [33]. Currently, a great deal of research activity is focused on the field of advanced oxidation processes (AOPs) that aim at eliminating hazardous water pollutants that are resistant to conventional purification methods [11]. The AOPs show several advantage including: (i) fast reaction rates; (ii) nonselective oxidation (allowing the treatment of multiple contaminants at the same time); and (iii) reduction of the contaminant toxicity [32,34]. The photocatalytic degradation of organic pollutants, assisted by nanosized TiO2-based photocatalysts, is one of the most investigated AOPs processes, developed in order to remove organic pollutants and bacteria from water. In this framework, plasmonic TiO2-based nanostructures are emerging as photocatalysts for potentially sustainable water treatment [35]. In addition, these hybrid materials are also applied in drinking and underground water supplies cleaning, to remove biological contaminants. Compared with other materials, Ag and Ag-based oxides are the most suitable for practical applications thanks to their high efficiency and to their low-cost [36]. In particular, nanosized TiO2-based photocatalysts, modified with Ag NPs, such as for example Ag/TiO2 [3741], Ag2O/TiO2-F [42], and Ag/TiO2/CuO [35], are attracting a lot of attention in the field of photocatalytic degradation of pollutants in water, because due to their peculiar plasmonic properties, they can potentially promote the photocatalytic degradation of pollutants under visible light irradiation. Further, the presence of Ag, enables to achieve a synergistic disinfection effect thanks to the intrinsic antimicrobial properties of Ag NPs. Indeed, Kowalska et al. reported the synthesis of Ag-modified TiO2 photocatalysts, with a coreshell-configuration, (Ag@TiO2). The Ag@TiO2 was prepared by photodeposition, and was applied for the photocatalytic decomposition of chemical and biological contaminants, such as methanol, acetic acid, 2-propanol, and Escherichia coli (E. coli) [43]. In detail, the Ag@TiO2 showed enhanced activity for 2-propanol and acetic acid oxidation under visible light and UV irradiation, respectively. Moreover, the hybrid photocatalyst presented improved disinfection properties under visible light, that were ascribed to the interplay between the antimicrobial properties of Ag in the dark and the plasmonic properties of nanomaterials [43]. Ag/TiO2-based ternary photocatalysts showed interesting photocatalytic properties for water purification. A promising candidate for practical use is the Ag/AgCl/TiO2 proposed by Guo and coworkers [44]. The Ag/AgCl/TiO2 was synthesized by a novel one-pot process that is based on three steps, namely (i) deposition, (ii) precipitation, and (iii) photoreduction. The Ag/AgCl/TiO2 system exhibited high photocatalytic activity for the degradation of 4-chlorophenol and photoreduction of Cr(VI) ion under visible light irradiation. Such a result was attributed to the SPR effect of highly dispersed Ag NPs along with the coexistence of Ag and AgCl at the surface of material. However, the photocatalytic activity was found affected by the material synthetic parameters, such as photoreduction time [44].

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Unfortunately, photocatalytic water treatment, assisted by nanosized nanoparticles, show drawbacks associated with the separation and the recovery of nanophotocatalysts from wastewater, which are technologically difficult and uneconomical due to the small particle size [45]. In order to address this issue, various methods, were comprehensively investigated. A proposed possible strategy is based on the immobilization of TiO2 particles on substrates such as fibers and natural or synthetic polymers. A plethora of studies reported the use of polymer materials, which exhibit a high level of resistance against UV irradiation and improved corrosion resistance [45]. Natural polymers, such as polysaccharides, lignin, cellulose, hemicellulose, chitin, chitosan, starch, or xylan, are more attractive, with respect to their synthetic counterparts. Hybrid photocatalysts, supported on natural polymers, present several advantages, such as low-cost, recyclability, and are environmentally friendly. Yu et al. reported on an Ag/TiO2 nanosponge material, prepared by solgel process using biological-template cellulose fibers (Fig. 18.2) [15]. In particular, Ag/TiO25.1% nanosponge materials showed the highest photocatalytic activity for the degradation of Rhodamine B under UV light. Such a high efficiency can be accounted for (i) by the large BET surface areas and mesoporosity of the TiO2 nanosponge, (ii) by the high dispersion of metallic Ag particles, and (iii) by a strong interaction between Ag and TiO2 in the hybrid structure [15]. Chitosan (CS) is the second most abundant natural polymer and is characterized by relevant properties, including its environmental safety, and can undergo microbial degradation. CS-supported TiO2, exhibits well-known multifunctional performances, because it provides an increase of heavy metals adsorption and enhances the photocatalytic degradation of organic pollutants. In a recent study by Zhao and coworkers, Ag2OTiO2 coated FIGURE 18.2 Schematic illustration of the synthetic process for TiO2/Ag nanosponge materials. Source: Reprinted with the permission of D.H. Yu, X. Yu, C. Wang, X. C. Liu, Y. Xing, Synthesis of natural cellulose-templated TiO2/Ag nanosponge composites and photocatalytic properties, ACS Appl. Mater. Interfaces, 4 (2012) 27812787.

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on CS-based polypropylene fibers (ATCPF) was reported as a novel real application for wastewater treatment [36]. Specifically, a multilayer composite with graft-like structure was obtained, showing high photocatalytic efficiency for the degradation of two different target molecules, ampicillin (AMP) and methyl orange (MO) under sunlight. Furthermore, the ATCPF system displayed good stability and excellent recycling properties (Fig. 18.3). Another natural support, proposed as host materials of NPs, is wood, as it preserves the unique properties of photocatalysts. Gao et al. reported the synthesis of superhydrophobic antibacterial AgTiO2 grown on wood substrates as a green application for the degradation of phenol. Such a composite photocatalyst has the additional advantage to be removed easily from the polluted water after use. Moreover, it showed antibacterial actions against both Gram-negative (E. coli) and Gram-positive bacteria (Staphylococcus aureus (S. aureus)) due to the bactericidal effects of Ag NPs [46]. Currently, novel floating photocatalytic systems have successfully addressed the issue of the separation and recycling of nanosized photocatalyst. Li et al. reported Ag NPsdecorated self-floating porous black TiO2 foams (Ag-FBTFs) synthesized by wet-impregnation and high-temperature hydrogen-reduction strategy [47]. They used self-floating porous black TiO2 foams, inspired by the formation of volcanic rocks, able to float on the surface of water due to the huge amount of closed pores in their frameworks [48]. In particular, experimental data showed that Ag-FBTFs exhibited excellent photocatalytic performance under solarlight for the removal of some highly toxic organic contaminants, such as thiobencarb, atrazine, phenol, and octane. This enhancement was attributed to the plasmonic FIGURE 18.3 (A) Schematic structure of the ATCPF. The degradation of MO (B) and AMP (C) during simulated solar irradiation in the presence of ATCPF, TCPF, CPF, Ag2O/TiO2 nanocomposites (AT), and blank polypropylene film. Source: Reprinted with the permission of Y. Zhao, C. Tao, G. Xiao, H. Su, Controlled synthesis and wastewater treatment of Ag2O/TiO2 modified chitosan-based photocatalytic film, RSC Adv. 7 (2017) 1121111221.

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properties of Ag NPs that improve the visible light absorption and efficiently retard the photogenerated e/h1 pair recombination. Furthermore, hybrid nanophotocatalysts combined with magnetic moieties were proposed as an alternative strategy for a safe recovery and reuse of NPs, since they enable the magnetic recovery of the system. For instance, Petronella et al. reported an innovative approach for the preparation of heterostructures merging a photoactive moiety (TiO2/Ag) and magnetic properties (FexOy) that, in principle, allows the recovery and recycling of the nanosized photocatalyst by applying an external magnetic field. The synthesis was carried by preparing first the TiO2NRs/FexOy-based heterostructure and then growing Ag NPs, by a photochemical approach that allows the control of the metal NP size. Specifically, the photocatalytic properties of the obtained multifunction TiO2NRs/FexOy/Ag heterostructures were tested under visible light irradiation for the degradation of the nalidixic acid, demonstrating a 1.9-fold enhancement of photocatalytic activity with respect to TiO2 P25. Such an enhancement was attributed to the size-dependent plasmonic properties of Ag NPs [49].

18.2.2 Atmospheric Pollution Abatement by Means of Photocatalytic Ag/TiO2 Hybrid Nanoparticles Air pollution is a great issue with harmful consequences for the environment and for the human health throughout the world [11]. The increasing emissions of NOx and VOCs, is associated mainly to anthropogenic activities, such as combustion processes, fossil fuels, car exhausts, paints and coatings, cleaning products, refrigerants, and furnishings. Photocatalytic oxidations of hazardous air pollutants assisted by semiconductors can be a promising approach to face air pollution [11,50]. In this framework, recently Ag/TiO2based photocatalysts have received enormous attention for pollution control and outdoor/ indoor air purification. For this purpose, Sofianou et al. investigated Ag-decorated TiO2 nanoplates [51] for the photocatalytic oxidation of NO gas to NO32. All the TiO2 nanoplates decorated with Ag of different size, and especially those with 9 nm Ag NPs, exhibited enhanced photocatalytic activity in comparison to the bare TiO2 nanoplates. However, the increase of the Ag NPs size may cause the covering of active sites on the TiO2 surface, thus negatively affecting the electron storage capacity and, consequently, the photocatalytic activity. Xu et al. synthesized and characterized anatase Ag/TiO2 photocatalyst prepared by one-step solvothermal method [52]. Such a photocatalyst was tested for the NOx conversion into N2 under UV and visible light irradiation, in a gas phase photocatalytic reactor. It was found that the selectivity of photodecomposition process of anatase TiO2 was increased by Ag modification, provided that the 1% of Ag loading is used. The improved selectivity can be associated to the coexistence of Ag0 and Ag1 on the surface of TiO2 [52]. Volatile organic compounds (VOCs), such as trichloroethylene, ethylbenzene, xylene, toluene, formaldehyde, acetone, 1-butanol, butylaldehyde, and 1,3-butadiene [53] deriving from a variety of sources including cigarette smoke, household products, furnishings materials, building materials, vehicle exhaust, or vapors from stored fuel are regarded as a hazardous for human health, because they cause the well-known Sick Building Syndrome

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(SBS) [11]. These compounds are also involved in the production of tropospheric ozone and secondary organic aerosol [54]. Photocatalytic processes assisted by Ag/TiO2-based photocatalyst are under investigation also for the VOCs degradation, due to their potential ability to convert VOCs into benign and odorless compounds, such as water vapor (H2O) and carbon dioxide (CO2) [55]. Recently, Tongon et al. proposed the Ag/TiO2/MCM-41 nanocomposite as a smart film for indoor air treatment, where MCM-41 stands for porous silica and/or mesoporous silica [56]. As a result, the photodegradation of benzene assisted by Ag/TiO2/MCM-41 was 2.7 times higher than the bare TiO2 film, due to the synergic effect of Ag plasmonic properties and TiO2 photoactivity and due to the presence of MCM-41, which promotes the adsorption of benzene on the film and contributes to the production of highly reactive hydroxyl radicals. ´ Zielinska-Jurek et al. proposed Ag/Pt-modified TiO2 nanocomposites as a promising material for air-cleaning treatment [53]. They investigated the photocatalytic degradation of toluene using light-emitting diodes (LEDs) irradiation emitting at λ 5 415 nm and evaluated the effect of Ag and Pt amount and the respective ratio. It was observed that the photocatalytic activity of Ag/PtTiO2 was affected by the size of Ag and Pt NPs deposited on TiO2 surface. More specifically, Ag/PtTiO2 sample with 0.5 mol% of Pt and 2.5 mol% of Ag was the best performing photocatalyst among the investigated materials. Remarkably, in the most photoactive Ag/PtTiO2 sample, Ag NPs have an average size ranging from 15 to 30 nm while the size of Pt NPs varies from 0.8 to 2.0 nm. The photodegradation activity of Ag/PtTiO2 sample increased about 1.5 times compared with pure TiO2, which underwent a drop of performance during subsequent cycles, probably due to adsorption of intermediates on the TiO2 surface. However, the photoactivity of the sample was reestablished by heating it at 350 C for 2 h. Formaldehyde, whose presence in the air is due to high dispersion from combustion and building materials, is another irritant and carcinogen pollutant and can be considered one of the main causes of SBS. The photodegradation of formaldehyde assisted by Ag/ TiO2 photocatalyst was successfully explored by Shie et al., who proposed their Ag/TiO2 photocatalyst, immobilized on glass plates, for photocatalytic indoor air purification [57]. In this work, Ag/TiO2 was prepared by the wet impregnation method, and ultraviolet light-emitting diode (UVLED) was employed as light source. In the work of Ma et al., Ag/TiO2 photocatalyst was investigated for the degradation of gaseous isopropanol, one the most common pollutants in industrial air streams [58]. Thanks to the ability of Ag NPs to improve the e2/h1 separation, Ag/TiO2 photocatalyst showed a reaction rate constant 2.0 times higher than bare TiO2. However, the authors pointed out that a 0.04% Ag loading gave rise to the most interesting results because the higher Ag amount, by acting as recombination centers, decreased the photocatalytic efficiency, as often reported. From a practical point of view, Ag/TiO2-based photocatalysts can also be exploited to functionalize commonly used fabrics, with the goal to exploit the high photoactivity of Ag/TiO2 to reduce the accumulation of VOCs inside the fibers. Recently, viscose and cotton were modified by Ag nanowiresTiO2 NPs (AgNWs/TiO2) and the resulting smart fabrics were investigated in the photodegradation of nicotine under UV and visible

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light [59]. It was demonstrated that the photocatalytic activity was affected by the type of cellulose fabrics. In particular, the decomposition rate of nicotine assisted by AgNWs/TiO2 was three and four times (cotton fabric) and 1.8 and 1.5 (viscose fabric) faster than unmodified fabrics, under UV and visible light, respectively. This substantial difference in terms of photocatalytic performance was probably due to several properties of viscose under the treatment conditions, as the high ability of water sorption and subsequently swelling caused the cracking of the TiO2 coating and the loss of contact between Ag NWs, reducing their interplay in the separation/transport of photogenerated electrons.

18.3 ENERGY PRODUCTION MEDIATED BY Ag/TiO2 HYBRID NANOPARTICLES Hydrogen (H2) is a sustainable and clean alternative to fossil fuels, having a high energy-capacity and allowing the limiting of CO2 emissions. The production of H2 by the photocatalytic water spitting is a real sustainable earth-friendly process, because it merges several advantages as: (i) it prevents the release of harmful by-products; (ii) it makes use of a renewable energy source, sunlight, to activate the process; (iii) it exploits water as H2 source which is nontoxic for human health and environment; and (iv) it uses solid state photocatalysts that can be, in principle, recovered and reused. The water splitting consists of the following reaction: H2 O-O2 1 H2

(18.1)

Such a reaction is endergonic, because it requires a standard Gibbs energy change ΔG0 of 1.23 eV and further the electron transfer needs an activation energy higher than that necessary for carrying on the water splitting. The well-known process of heterogeneous photocatalysis is considered extremely effective to achieve water splitting. Indeed, in the presence of a suitable semiconductor, such as TiO2, the water splitting can be simply triggered by electromagnetic radiation. As demonstrated by the pioneering work of Fujishima and Honda, in a photoelectrochemical cell with the anode of TiO2 and a black Pt electrode as a cathode, the irradiation of TiO2 promotes the generation of e2/h1 pairs that can migrate to the surface of photocatalysts. As a result, holes can oxidize H2O producing O2, while electrons reach the cathode giving rise to H2 evolution. The amount of produced H2 is proportional to the amount of photogenerated electrons (Fig. 18.4). Due to its high photoactivity, low recombination of photogenerated e2/h1 pairs, and optimal match between energetic levels of CB and VB and standard redox potentials of H1/H2 and O2/H2O, respectively, TiO2 is one of the most investigated photocatalysts for water splitting [60]. Coupling TiO2 nanocrystals with metallic Ag NPs is a powerful tool to improve the photocatalytic activity of TiO2 both under UV and solar light irradiation. The presence of Ag in combination with TiO2 NPs, under UV light irradiation, has been found to promote an increase of the amount of electrons available for the H2O reduction, with a consequent increase of H2 production. Anisotropic TiO2 nanocrystals are extensively investigated for solar energy conversion, because their peculiar morphology enables a facile charge transport along the longitudinal dimension and decreases the e2/h 1 recombination rate [62,63].

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FIGURE 18.4 Photoelectrochemical water splitting with a TiO2 photoanode. Source: Reprinted with the permission of K. Maeda, Photocatalytic water splitting using semiconductor particles: history and recent developments, J. Photochem. Photobiol. C: Photochem. Rev. 12 (2011) 237268 [61].

The effectiveness of the system is enhanced when TiO2/Ag anisotropic heterostructures are conveniently organized. For instance, TiO2NPs can be assembled in an array of nanotubes that can be functionalized in a second step by Ag NPs. In the report of Wu et al. [62] an array of TiO2 nanotubes, fabricated by anodic oxidation, was decorated with Ag NPs, introduced by microwave-assisted chemical reduction. This technique allows to improve the morphological and structural features, providing control over Ag size by varying the amount of Ag precursor. Following this approach, Ag NPs can be integrated both at the surface and into the nanotube inner cavity. The resulting heterostructures resulted in a H2 evolution rate up to twice as high as that obtained for bare TiO2 nanotubes. Remarkably, the improvement of water splitting rate was found to be affected by the size of Ag NPs. In particular, the optimal diameter of Ag NPs was 20 nm [64]. The relation between photoactivity efficiency improvement under UV light irradiation, and noble metal NP size can be explained by considering that smaller Ag NPs possess a higher capability of storing electrons due to their higher volume ratio and therefore, to their more negative redox potential [65]. The specific geometry of the TiO2/Ag heterostructures can govern the efficiency of the photocatalytic water splitting. Indeed, Ag phase can also be integrated in a coreshell heterostructure, where the Ag phase is the core of TiO2 nanowire, resulting in coaxial TiO2/Ag nanowire. Interestingly, the coreshell configuration was demonstrated to be more effective in photocatalytic H2 production by water splitting, than Ag NPs at the surface of TiO2 nanotube. Such a difference was rationalized considering that the coreshell structure can accommodate a higher Ag amount and, of note, a higher surface contact between TiO2 and Ag phase, that could promote a more efficient accumulation of photogenerated electrons on the Ag core, thus reducing the e2/h1 recombination events [66]. In the most recent years, plasmonic properties of noble metal NPs were deeply investigated and exploited also in solar light conversion. Indeed the SPR phenomena increases the steady state concentration of chemically effective charge carriers, namely e2 and h1 trapped at the photocatalyst surfaces and able to take part to photocatalytic transformations [28]. The most extensively studied configuration of TiO2/Ag-based plasmonic photocatalyst designed for photocatalytic water splitting consists of anisotropic TiO2NPs decorated with

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Ag spheres in direct contact with the titania support. As previously mentioned, the TiO2 anisotropy is expected to enable an improved transport of photogenerated carriers, while the close contact with the plasmonic NPs, i.e., Ag, should improve the charge transfer from the metal to the semiconductor surface, under visible light irradiation. As in the case of UV light source, the efficiency in H2 production rate, assisted by plasmonic heterostructures under visible light irradiation, is influenced by the material synthetic parameters, as they dictate the morphological and structural properties of the resulting plasmonic NPs. The ratio between TiO2 and Ag precursor can affect the water reduction efficiency. Indeed the H2 production can increase from 38.2 μmol/h to 72.1 μmol/h when the Ti:Ag ratio increases from 10:1 to 20:1 and decreases to 36.8 μmol/h for a Ti amount 40 time higher than Ag [67]. The size of Ag NPs decorating the TiO2 is a fundamental parameter that can dramatically affect the performance of the heterostructures in H2 production rate. 8 nm Ag NPs decorating a vertically aligned array of TiO2 nanotubes can enable an H2 production rate up to 15 higher than that obtained when the water spitting is catalyzed by bare TiO2 nanotubes. The same heterostructures can undergo a drop of performance when the Ag NPs are 60 nm in size. Such a difference is associated with the shorter e2/h1 lifetime and the lower e2/h1 separation that is often detected for Ag NPs larger than 20 nm in size [68]. Further, the size of Ag NPs not only affects the redox potential and the ability to store electrons but also governs the Fermi energy level of Ag NPs and therefore, the ability to transfer electrons to TiO2 CB. Indeed, under visible light irradiation, when SPR excitation occurs, the nonradiative decay of plasmons through the “hot electrons” generation is recognized to be more efficient for smaller NPs [27,69]. Photoactive nanosized heterostructures are extensively investigated also as lightharvesting components in photovoltaic devices designed to convert solar light in electricity. An outstanding example of devices designed for visible light conversion in electric energy are the dye-sensitized solar cells (DSSCs). The facile fabrication along with the good performances under visible light irradiation established, in the most recent years, their worldwide success as a possible alternative to conventional solid-state pn junction photovoltaics. A DSSC consists of a conductive transparent substrate of ITO or FTO that supports a nanocrystalline mesoporous TiO2 film, with a monolayer of dye. The film is placed in contact to a redox electrolyte, typically I2/I32. Visible light irradiation produces the dye photoexcitation, therefore electrons are injected from the LUMO level of the dye to the CB of TiO2 and subsequently run in the outer circuit, toward the counterelectrode and produce the electric power. Nanosized TiO2-based film are particularly effective for use as electrodes in DSSCs because of their high surface area that maximizes dye chemisorption and because of their reduced dimensions that enable an efficient charge transport from the adsorbed dye to the electrode [4]. Recently, the integration of plasmonic nanostructures in photovoltaic cells has gathered enormous attention, because, thanks to the SPR phenomenon, plasmonic NPs are able to trap, guide, and concentrate visible light at the nanoscale. In particular, Ag NPs in combination with TiO2, bring a positive contribution to the DSSCs efficiency, because they have high stability and a reduced cost compared to other metal NPs, they can improve the interfacial charge transfer, and they can enhance electrical conductivity and charge separation [70]. Furthermore, Ag NPs have higher extinction coefficient compared to Au [71] providing a more intense light absorption [72]. II. APPLICATIONS

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A great deal of possible configurations for Ag/TiO2-based heterostructures designed for the integration in DSSCs has been reported, following two main criteria; (i) maximizing light harvesting; and (ii) improving charge collection efficiency. For this purpose, a typical configuration consists of anisotropic TiO2 NPs decorated with Ag nanospheres. The use of anisotropic TiO2 NPs is very common because, as mentioned for photocatalytic water splitting, it improves charge collection efficiency. An example is provided by Li et al. who presented TiO2 nanorods with the surface decorated by Ag NPs, directly grown by a photochemical method in order to enable a very precise dimensional control of Ag NPs at the TiO2 nanorod surface. Alternatively, Ag NPs can grow on mesoporous TiO2 film organized in a close array, prepared by nanoimprinting lithography [73]. Coreshell configuration is also extensively used, because it has the advantages of: (i) combining the properties of different materials; (ii) enabling the light collection by LSPR action; and (iii) protecting the Ag phase from the high temperature processing and from the eventual corrosion due to the interaction with I2/I32 electrolyte. Some examples of coreshell TiO2/Ag heterostructures, designed for DSSCs, include Ag@SiO2 incorporated on spherical TiO2NPs film [74], or Ag NPs protected by an anisotropic layer of TiO2. Moreover, Ag NPs can behave as an active component of a multiple phase heterostructure as in the case presented by Yun et al., who prepared hollow TiO2 particles, decorated by coreshell Au@Ag nanospheres. TiO2 hollow NPs were selected in order to improve an efficient interaction with the redox electrolyte, while the Au@Ag nanospheres, thanks to the interaction between their plasmon bands, resulted in an intense and broad plasmon band (Fig. 18.5B). It is worth mentioning that the Au@Ag nanospheres were designed so that the Ag phase is the outer shell because Ag NPs have a larger extinction coefficient than Au NPs [71,75].

FIGURE 18.5 (A) Schematic illustration of a dye sensitized solar cell (DSSC) device. (B) TEM micrograph of Au@Ag/TiO2 composite for visible light harvesting in DSSCs. Source: (A) Reprinted with the permission of J. Gong, K. Sumathy, Q. Qiao, Z. Zhou, Review on dye-sensitized solar cells (DSSCs): advanced techniques and research trends, Renew. Sust. Energy Rev. 68 (2017) 234246 [77]. (B) Reprinted with the permission of J. Yun, S.H. Hwang, J. Jang, Fabrication of Au@Ag core/shell nanoparticles decorated TiO2 hollow structure for efficient light-harvesting in dyesensitized solar cells, ACS Appl. Mater. Interfaces 7 (2015) 20552063.

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However, irrespective of the architecture adopted to design TiO2/Ag heterostructures, the common issue for application in DSSCs is the dependence of the conversion efficiency on the Ag NP size. Indeed, generally for each specific heterostructure the optimal PCE was measured for a defined Ag NP size or Ag concentration; therefore, it is crucial to master the control of the size and shape of Ag NPs to be integrated in DSSC devices. Size and shape of metal NPs govern the position, the bandwidth, and the intensity of Ag plasmon band [76], and therefore their light harvesting ability. Further, the electron transfer to the TiO2 phase is related to optical properties of metal NPs [28]. Finally, it was proposed that Ag NPs, in a specific size range, can also be detrimental, acting as recombination centers in plasmonic DSSCs. As a consequence, it confirmed how essential is the control on size and shape of metal NPs in TiO2/Ag heterostructures.

18.4 MULTIFUNCTIONAL Ag/TiO2 HYBRID NANOPARTICLES FOR QUALITY LIFE IMPROVEMENT 18.4.1 Bactericidal Coatings Currently, there is a special interest in developing bactericidal coatings that show a high effectiveness against microorganisms with low-cost systems for industrial application. Several research studies are devoted to synthesizing a variety of materials, based on inorganic and natural compounds, such as Ag, Cu, chitosan, tea extracts, etc., as possible platforms for antibacterial surfaces [78]. In the literature, Ag/TiO2-based nanomaterials have received much attention for their nontoxicity, biocompatibility, low cost, and ability to destroy several bacteria [79,80]. Remarkably, the antibacterial ability of Ag/TiO2-based nanocoatings can be potentially extended to several application fields, such as sterilizing products for hospital, medical applications, as well as biocide products for monuments and building materials, etc. [81,82]. Mesoporous Ag/TiO2 composite nanofilms were prepared on silicon wafer via solgel method by spin-coating technique. Their antimicrobial effect was tested against Gramnegative bacteria, such as E. coli under UV illumination, as a function of different Ag loadings [83]. The obtained results showed enhanced bactericidal activities with the increase of the Ag content. Moreover, the improvement of antibacterial properties of Ag/TiO2 coating compared to TiO2 nanofilm both in the dark and under UV illumination, respectively, was attributed to the synergistic antibacterial effects of TiO2 and Ag. The interplay between the TiO2 photocatalyst and the surface-bound Ag NPs was demonstrated to be effective also under hospital lighting conditions [84]. In particular, an Ag/TiO2 film was prepared by a two-step solgel method that consisted of (i) coating glass slides with the anatase TiO2 and successively (ii) depositing Ag NPs on the surface by UV photoassisted reduction of AgNO3. Specifically, the experimental data showed an excellent antibacterial activity against Gram-negative bacterium E. coli and Gram-positive bacterium S. aureus. Notably, the synergistic effect of TiO2 and Ag allows to deactivate many different types of bacteria in a short time through a multidirectional action mechanism which does not involve exclusively the photocatalytic process and the intrinsic Ag NPs antimicrobial effect.

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In recent years, textiles functionalization with Ag and metal oxides has been regarded as a promising strategy against airborne bacteria and skin diseases [85,86]. Plenty of works are devoted to cotton functionalization because cotton itself is a nutrient for bacteria and fungi, due to its ability to adsorb a large amount of moisture [85,87]. Miloˇsevic et al. reported on the functionalization of cotton and cotton/PET fabrics by Ag/TiO2 nanotubes (Ag/TNT) deposited in situ through the photoreduction of Ag1 adsorbed on TiO2 NPs [88]. The experimental results demonstrated the two-fold advantage of Ag/TNT in that it inhibits the growth of microorganisms, especially E. coli and S. aureus, and protects the fabric from UV radiation. Additionally, the properties of functionalized fabrics were preserved after five washing cycles. Cotton fabrics were also functionalized with flower-like hierarchical structure of Ag/TiO2 that exhibited UV-protective property and high antibacterial activity against S. aureus and E. coli bacteria [80]. The antibacterial activity is associated with Ag NPs that attack the bacterial cell membrane causing the denaturation of both the sulfur-containing proteins and DNA after membrane penetration, thus slowing down bacterial growth, ultimately leading to cell death. In addition, Ag ions may be released, resulting in a further enhancement of the antibacterial activity of coating. Ag/N-TiO2 was also employed to functionalize sheepskin leather coating [89]. The sensitivity of this coating was evaluated at direct contact with Candida albicans (C. albicans) ATCC 26790 and 1760, Epidermophyton floccosum (E. floccosum), E. coli ATCC 25922, and S. aureus ATCC 2592. The surface covered with Ag/N-TiO2 exhibited good bacterial sensitivity and mechanical resistance, attributed to its photocatalytic-induced hydrophilic properties. Moreover, the improvement of antimicrobial and fungitoxic properties plays a crucial role for designing new durable and useable materials with multifunctional functionalities. The bactericidal properties of Ag/TiO2-based coatings showed also encouraging results in the orthodontic field as micro-implants. Mai et al. developed Ag/TiO2 films on titanium plates, prepared by solgel method [90]. In this study, the antimicrobial properties were evaluated as a function of the annealing temperature using E. coli as a reference microorganism. Experimental results indicated that the bactericidal ability enhances as Ag NP size decreases, due to the increase of light absorption strength that improves the visible light photoresponse of TiO2. Besides, they confirmed that the bactericidal properties of Ag are enhanced by the positive antibacterial contribution of TiO2. Ag/TiO2-based coatings were exploited to address concerns related to the biological deterioration of buildings and monuments of historical and architectural interest. It is well known that microbial contamination is one of the main factors leading to deterioration of stone surfaces due to the acidolytic and oxidoreductive corrosive process of stone minerals [91]. The photocatalytic and antibacterial properties of Ag/TiO2-based coatings can be exploited to prevent biological colonization of microbes on building surfaces, thus providing a reliable alternative to the use of harmful chemical biocides. For example, Aflori et al. designed and proposed Ag/TiO2 silsesquioxane-based coatings (POSS-Ag/Ti) with antimicrobial/antifungal properties for the protection of cultural heritage [91]. In particular, they selected the Repedea limestone, a porous bioclastic oolitic limestone, as monumental stone. POSS-Ag/Ti materials displayed a higher antibacterial/ antifungal activity against E. coli and C. albicans with respect to POSS-Ag, reported as a

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reference material. In addition, the coating application on stone evidenced only a slightly change in the stone mineral composition. Indeed, Repedea stone is affected by the polymer addition, due to the acidic environment typical of polymer solutions. However, the composition of stone is not significantly modified by POSS-AgTi coating. Also, Ag/TiO2 film were successfully employed in order to convey antibacterial properties to glazed surfaces of ceramic tiles, that are extensively used in the hospital environment and in any household, but are extremely sensitive to bacteria colonization, especially in the presence of moisture [92]. The Ag/TiO2 film was obtained by liquid phase deposition method (LPD) and it was tested against E. coli and S. aureus using the so-called antibacterial drop-test, without UV irradiation. The experimental results showed that Ag/TiO2 thin films are more effective antibacterial materials than TiO2-based films, and produce an antibacterial rate of 99% and 91%, in the case of E. coli and S. aureus, respectively, after 24 h. Recently, an innovative treatment for biofouling prevention on building materials was reported by Graziani et al. [93]. They applied Ag/TiO2 nanocoatings on brick substrates and tested their inhibitory efficiency toward algae and cyanobacteria. The obtained results showed that these materials are able to limit algae adhesion and their growth, so encouraging their application on modern building facades, reducing the maintenance costs.

18.4.2 Photoactive Ag/TiO2 Hybrid Nanoparticles for Odor-Control Filters “Odor gases” are substances that negatively affect the sense of smell [94]. They not only cause people discomfort but decrease air quality [95] and, in conjunction with airborne bacteria and fungi, can induce respiratory illness [96,97]. “Odor gases” can be produced in raw garbage, WWTP, factories, smog over the cities, but can also be generated indoors. Conventional odor control methods consist of covering the odor with pleasant smells or in physical methods that can temporally remove the odor gas and harmful substances by trapping them (HEPA filters) [98]. Currently, photocatalytic methods are being proposed as a powerful alternative for air purification from odor gases and bacteria. Indeed photocatalytic components can be integrated into devices designed for air quality control, with the aim to destroy smelling substances and bacteria and transforming them into nonharmful and odorless compounds [96]. One of the most striking examples of photocatalytic filters is the AiroCide technology patented by NASA and KES Science & Technology Inc. This is an air sanitation device that basically consists of a TiO2-based photocatalytic filter. It was revealed to be effective in removing the Bacillus thurengiensis, a microorganism used to simulate the Anthrax spores, airborne contaminants, and ethylene. The AiroCide technology was also tested in the Space Shuttle Columbia mission STS-73 in 1995 [99]. In the last 15 years, Ag/TiO2-based photoactive coatings gained attention also in the field of air filtration devices equipped with photocatalytic components. In such context, the performances of Ag/TiO2-based heterostructures were investigated both for the removal of odors and harmful organic compounds as well as for the control of pathogenic bacteria strains in the air. The photoactive component of a photocatalytic air filter is usually composed of a proper substrate, such as ceramic, alumina, or glass fiber, that supports the photocatalytic material. Therefore, Ag/TiO2-based heterostructures can be directly

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deposited on the substrate, generally by impregnation, or alternatively Ag nanoparticles can be immobilized in a second step on a previously fixed layer of TiO2, for example by photodeposition methods. Ag/TiO2-based photocatalytic filters were revealed to be successful for the removal of gaseous sulfur compounds (ethyl mercaptan C2H6S, hydrogen sulfide H2S, methyl mercaptan CH4S, propyl mercaptan C3H8S) which are identified as the main compounds responsible of odors, from a variety of sources including WWTP, composting, rendering plants, factories, and raw garbage [100]. In particular, an Ag/TiO2-based photocatalytic filter was tested for the photocatalytic oxidation of H2S and CH3SH, which have an extremely low odor threshold (20 ppb and 0.5 ppb, respectively). The presence of Ag gives rise to an increase up to 7 and 14-times higher degradation rate for H2S and CH3SH, respectively, compared to conventional filters. Such a result, was considered consistent not only with the photochemical behavior of Ag/TiO2 nanocomposite under UV light, but also to the affinity of SH groups toward the surface of Ag NPs that can enhance the interaction with the target pollutant [96]. Further, Ag/TiO2-based photocatalytic filter was also applied in the removal of acetic acid, isopropyl alcohol, and dimethylsulfide that are regarded as a source of odor problems in optoelectronic industries. As a result, Ag/TiO2based photocatalytic filters can produce an improvement of up to 15% higher with respect to the commercial functional filter from Ishihra Sangyo, Japan [101]. Photocatalytic filters displayed their potential also for the protection of public health and environment from harmful microorganisms. There are few reports on integration of Ag NPs into photocatalytic air sanitation filters for airborne bacteria removal. However, the introduction of Ag NPs can be advantageous not only for their contribution in enhancing the photocatalytic activity but also because of their intrinsic antimicrobial effect. Indeed, a TiO2 P25/Ag-based nanocomposite was used in order to functionalize a glass fiber-based photocatalytic filter that was tested against the Mycobacterium tuberculosis (the nonhuman pathogenic strain), obtaining an enhancement in the removal of 20% higher than that obtained for bare TiO2 P25 [98]. Further TiO2/Ag-based nanocomposites were also integrated in a photocatalytic filter designed for a bacteria-restraining unit in the heating, ventilation, and air-conditioning of buildings. The equipment was installed and tested in a medical institution in Taiwan, and it showed promise for improving air quality, being able to remove up to 71% of airborne bacteria [97].

18.5 CONCLUSIONS The recent advances in material chemistry offer powerful tools to develop new multifunctional nanomaterials with outstanding properties that can be finely tuned as a function of the target application. In particular, photocatalytic applications have attracted enormous efforts from the material chemistry community because they represent a potential trustworthy solution to urgent environmental concerns as the pollution control and the energy production. This chapter is dedicated to the photocatalytic applications of a peculiar nanosized material: the Ag/TiO2. Several Ag/TiO2-based nanoarchitectures have been examined in three application areas, namely pollution control, energy production, and

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improving quality of life. As a result, the chapter highlights the versatility and the effectiveness of Ag/TiO2-based photocatalysts in such appealing application areas, due to their ability to improve the photoactivation mechanism of TiO2 and due to the dominant plasmonic properties of Ag NPs as well as due to its peculiar antimicrobial properties.

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[68] M.-Z. Ge, C.-Y. Cao, S.-H. Li, Y.-X. Tang, L.-N. Wang, N. Qi, et al., In situ plasmonic Ag nanoparticle anchored TiO2 nanotube arrays as visible-light-driven photocatalysts for enhanced water splitting, Nanoscale 8 (2016) 52265234. [69] J. Yu, G. Dai, B. Huang, Fabrication and characterization of visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2 nanotube arrays, J. Phys. Chem. C 113 (2009) 1639416401. [70] Q. Lu, Z. Lu, Y. Lu, L. Lv, Y. Ning, H. Yu, et al., Photocatalytic synthesis and photovoltaic application of AgTiO2 nanorod composites, Nano Lett. 13 (2013) 56985702. [71] J. Yun, S.H. Hwang, J. Jang, Fabrication of Au@Ag core/shell nanoparticles decorated TiO2 hollow structure for efficient light-harvesting in dye-sensitized solar cells, ACS Appl. Mater. Interfaces 7 (2015) 20552063. [72] P. Nbelayim, G. Kawamura, W. Kian Tan, H. Muto, A. Matsuda, Systematic characterization of the effect of Ag@TiO2 nanoparticles on the performance of plasmonic dye-sensitized solar cells, Sci. Rep. 7 (2017) 15690. [73] W. Jiang, H. Liu, L. Yin, Y. Ding, Fabrication of well-arrayed plasmonic mesoporous TiO2/Ag films for dyesensitized solar cells by multiple-step nanoimprint lithography, J. Mater. Chem. A 1 (2013) 64336440. [74] M.A. Hossain, J. Park, D. Yoo, Y.-K. Baek, Y. Kim, S.H. Kim, et al., Surface plasmonic effects on dyesensitized solar cells by SiO2-encapsulated Ag nanoparticles, Curr. Appl. Phys. 16 (2016) 397403. [75] D.B. Ingram, S. Linic, Water splitting on composite plasmonic-metal/semiconductor photoelectrodes: evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface, J. Am. Chemi. Soc. 133 (2011) 52025205. [76] A. Moores, F. Goettmann, The plasmon band in noble metal nanoparticles: an introduction to theory and applications, New J. Chem. 30 (2006) 11211132. [77] J. Gong, K. Sumathy, Q. Qiao, Z. Zhou, Review on dye-sensitized solar cells (DSSCs): advanced techniques and research trends, Renew. Sust. Energy Rev. 68 (2017) 234246. [78] Y. Liu, X. Wang, F. Yang, X. Yang, Excellent antimicrobial properties of mesoporous anatase TiO2 and Ag/TiO2 composite films, Micropor. Mesopor. Mater. 114 (2008) 431439. [79] K. Zawadzka, K. Kadziola, A. Felczak, N. Wronska, I. Piwonski, A. Kisielewska, et al., Surface area or diameter—which factor really determines the antibacterial activity of silver nanoparticles grown on TiO2 coatings?, New J. Chem. 38 (2014) 32753281. [80] S. Li, T. Zhu, J. Huang, Q. Guo, G. Chen, Y. Lai, Durable antibacterial and UV-protective Ag/TiO2@fabrics for sustainable biomedical application, Int. J. Nanomed. 12 (2017) 25932606. [81] P. Munafo`, G.B. Goffredo, E. Quagliarini, TiO2-based nanocoatings for preserving architectural stone surfaces: an overview, Const. Build. Mater. 84 (2015) 201218. [82] M.F. La Russa, A. Macchia, S.A. Ruffolo, F. De Leo, M. Barberio, P. Barone, et al., Testing the antibacterial activity of doped TiO2 for preventing biodeterioration of cultural heritage building materials, Int. Biodeterior. Biodegrad. 96 (2014) 8796. [83] Y. Binyu, L. Kar Man, G. Qiuquan, L. Woon Ming, Y. Jun, Synthesis of AgTiO2 composite nano thin film for antimicrobial application, Nanotechnology 22 (2011) 115603. [84] C.W. Dunnill, K. Page, Z.A. Aiken, S. Noimark, G. Hyett, A. Kafizas, et al., Nanoparticulate silver coatedtitania thin films—photo-oxidative destruction of stearic acid under different light sources and antimicrobial effects under hospital lighting conditions, J. Photochem. Photobiol. A: Chem. 220 (2011) 113123. [85] J. Kiwi, C. Pulgarin, Innovative self-cleaning and bactericide textiles, Catal. Today 151 (2010) 27. [86] Y. Gao, R. Cranston, Recent advances in antimicrobial treatments of textiles, Text. Res. J. 78 (2008) 6072. [87] Y. Zhang, Q. Xu, F. Fu, X. Liu, Durable antimicrobial cotton textiles modified with inorganic nanoparticles, Cellulose 23 (2016) 27912808. ˇ [88] M. Miloˇsevi´c, Z. Saponji´ c, T. Nunney, C. Deeks, M. Radoiˇci´c, M. Mitri´c, et al., In situ photoreduction of Ag1-ions on the surface of titania nanotubes deposited on cotton and cotton/PET fabrics, Cellulose 24 (2017) 15971610. [89] C. Gaidau, A. Petica, M. Ignat, O. Iordache, L.-M. Ditu, M. Ionescu, Enhanced photocatalysts based on AgTiO2 and AgNTiO2 nanoparticles for multifunctional leather surface coating, Open Chem. 14 (2016) 383. [90] L. Mai, D. Wang, S. Zhang, Y. Xie, C. Huang, Z. Zhang, Synthesis and bactericidal ability of Ag/TiO2 composite films deposited on titanium plate, Appl. Surf. Sci. 257 (2010) 974978. [91] M. Aflori, B. Simionescu, I.-E. Bordianu, L. Sacarescu, C.-D. Varganici, F. Doroftei, et al., Silsesquioxanebased hybrid nanocomposites with methacrylate units containing titania and/or silver nanoparticles as antibacterial/antifungal coatings for monumental stones, Mater. Sci. Eng. B 178 (2013) 13391346.

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C H A P T E R

19 Noble MetalTransition Metal Oxides/Hydroxides: Desired Materials for Pseudocapacitor Ramkrishna Sahoo1, Anjali Pal2 and Tarasankar Pal1 1

Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal, India 2 Department of Civil Engineering, Indian Institute of Technology, Kharagpur, West Bengal, India

19.1 INTRODUCTION Supercapacitor and rechargeable battery are the two primary areas of research for the materials scientists. Both are excellent energy storage devices, but based on the activity and practical application, the rechargeable battery is well ahead compared to the electrochemical supercapacitor (ES) in terms of commercial point of view. In 1957, the first patent was filed on ES [1]. After that, no such improvement was observed on ES technology until the 1990s. Apparently, the electrochemical supercapacitors have been used mainly for the betterment of the battery and fuel cell in a hybrid electric vehicle to supply essential power for acceleration [2]. Since then ES technology has been improved. It plays a crucial role in complementing battery and fuel cell. During the energy storage function, ES is utilized to provide backup power while powering disruption. Thus, ES has been designated as an important device for future energy storage application [3]. In recent years, enormous development has occurred on ES in the theoretical as well as practical research. But, some serious disadvantages still exist in ES, such as low energy density, low specific capacitance, and high production cost [4]. Due to the high capacity and large potential window, rechargeable battery exhibits high energy density, but its power density is very low. However, a supercapacitor exhibits high power density compared with the excellent cyclic stability and rate capability. Thus, the supercapacitor has become a hot field. There

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00019-X

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are mainly two types of supercapacitor: electrical double layer capacitor (EDLC) and pseudocapacitor (PC) [58]. Mainly carbonaceous materials have been utilized as the double layer capacitor. Recently, scientists are working on a hybrid supercapacitor where both battery-like material and supercapacitor are coupled together to make a device. In every case, electrochemical activity of the device depends on the material which is used as the electrode [57]. Thus, choice of electrode material is an important parameter for the fabrication of an advanced energy storage device. There are several redox active materials which have been used as the pseudocapacitor, but among them, transition metal oxides (TMOs) and hydroxides (TMHs) are the groups of materials which have been utilized most as the pseudocapacitor electrode. Here, the chapter is mainly based on the comprehensive discussion of the pseudocapacitive activities of different TMOs and TMHs. Over the last few years, nanomaterials have played a major role in the fabrication of advanced energy storage devices. TMOs and TMHs as pseudocapacitor energy storage electrodes have drawn an immense interest of the researchers due to their fast electron transfer reactions and high theoretical specific capacitance values [57]. RuO2 nanomaterials were first to be used in pseudocapacitor electrode. Enormous work has been done on this particular nanomaterial. But, due to the high cost of RuO2, scientists have started working on MnO2. MnO2 is the most used nanomaterials pseudocapacitor [59]. In the last few years, researchers became interested in the Ni- or Co-based oxides or hydroxides due to their high specific capacitance values [59]. Recently, other metal oxides/hydroxides have also been used as the pseudocapacitor. Electrodeposition, a solgel method, hydrothermal methods, etc. are the few typical synthetic techniques which have been used to synthesize different TMOs and TMHs [9]. Among them the hydrothermal method has mostly been used by the researchers.

19.2 FUNDAMENTALS OF TMOs AND TMHs PSEUDOCAPACITOR Nanostructured materials with suitable morphology play a vital role in energy storage application. Among the nanostructured materials, TMOs and TMHs are the most commonly used pseudocapacitor electrode material [59]. Nanomaterials with fast redox activity in the aqueous or organic electrolyte are used as the pseudocapacitor electrode. Where the activity is concerned, utilization of the surface of the active material is very important in the case of a pseudocapacitor. Thus, nanostructured materials are of utmost importance as they have the high surface to volume ratio. Sometimes aggregation on nanomaterial during electrochemical reaction makes it less efficient. Thus, morphology is also important for a nanomaterial to behave as an advanced pseudocapacitor electrode. Mainly, 0D nanoparticles aggregate during the electrochemical reaction. Hence, scientists generally avoid using 0D nanoparticles as electrode materials. Mostly one-dimensional (1D) and two-dimensional (2D) nanostructured materials are used as the pseudocapacitor electrode due to their exceptional chemical and physical properties [5]. In recent days, three-dimensional (3D) porous nanomaterials are also used as the advanced pseudocapacitor electrode [5]. The high specific surface area, pore diameter, and large pore volume make the nanomaterials wettable in the electrolyte which in turn makes the charge transportation facile. Thus, TMOs and TMHs of different morphologies have a different role in pseudocapacitance activities.

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19.3 SINGLE TRANSITION METAL OXIDES OR HYDROXIDES (TMOs)

397

19.3 SINGLE TRANSITION METAL OXIDES OR HYDROXIDES (TMOs) There are several TMOs or TMHs which have been used as the pseudocapacitor electrode material, such as RuO2, MnO2, CoO/Co(OH)2/Co3O4, NiO/Ni(OH)2, V2O5, MoO3, etc. Above them, RuO2 was used as the pseudocapacitor electrode for the first time. The hydrous form of RuO2 has been designated as the model system of a pseudocapacitor. The pseudocapacitance activity of RuO2 has been discovered by Trassati in the 1970s [10,11]. Scientists used RuO2 instead of carbonaceous material in supercapacitors to get an elevated specific capacitance value. In 1995 Zheng et al. reported the use of hydrated ruthenium dioxide, RuO2 nH2O which exhibited very high specific capacitance value [12,13]. They prepared the material via the solgel process. The energy storage mechanism of RuO2 nH2O is based on the faradic redox reaction on the surface of the metal oxide. In an acidic medium, ruthenium oxide varies from Ru(II) to Ru(IV). The electron transfer process is associated with adsorptiondesorption of protons. The typical redox reaction occurring in the case of RuO2 nH2O in acidic medium is:







RuO2 1 δH1 1 δe2 -RuO22δ ðOHÞδ 0 # δ # 2 Change in δ is proportional to the insertion or de-insertion of protons within the potential window of 1.2 V, considering the evolution of hydrogen and oxygen [14]. Till now, several research groups have tried to prepare different RuO2 to get better results [1219]. Susanti et al. prepared a 1D vertical nanorod using the chemical vapor deposition (CVD) which was further used as the electrochemical capacitor [15]. They got a total capacitance of 520 F/g (870 mF/cm2). In 2009, Kurtani et al. reported a porous RuO2 for pseudocapacitor electrode [16]. It exhibited a specific capacitance of 200 F/g. To get a better electrochemically active pseudocapacitor Hyun et al. prepared a highly conductive RuO2 fiber via the electrospinning method (Fig. 19.1) [17]. Due to the excellent electron transporting pathway provided by the RuO2 hydrous nanofiber, it exhibited a specific capacitance value of 886 F/g (Fig. 19.2). Sanchez and his group fabricated a RuO2-based mesoporous thin film [18]. The morphological and structural advantages made the as-prepared RuO2 an excellent pseudocapacitor exhibiting a specific capacitance of 1000 F/g at 10 mV/s. Very recently, Ryu et al. reported supercapacitive properties of coaxial RuO2 nanoparticles which were electrodeposited on ITO nanopillar substrate [19]. Generally, scientists use 2D substrates for the fabrication of electrodes. Here, the unconventional structure of the current collector made the nanostructure more accessible towards electrolyte ions which facilitate the ion transport. The ITO-RuO2 nanopillars exhibited a specific capacitance of 1235 F/g at 50 mV/s. From this discussion, it is evident that RuO2, based on its fabrication and morphology could be an excellent pseudocapacitor. But the limitation regarding RuO2 is that it is very expensive and not environmentally friendly. Keeping these limitations of RuO2 in mind, scientists started to work on different metal oxide and hydroxide nanostructures. MnO2 nanomaterial is the best replacement of RuO2 nanomaterials and has been considered to be the best TMO for energy storage application due to its facile redox property, good reversibility, high theoretical capacitance value (1100 F/g), high energy density, environmental friendliness, and, most importantly, low

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FIGURE 19.1 FESEM images of RuO2 nanoparticles electrodeposited on the ITO nanopillars. The number of electrodeposition cycles are (A) 10, (B) 15, (C) 20, (D) 30, (E) 50, and (F) 100. Source: Reproduced with the permission from the publisher of I. Ryu, M. Yang, H. Kwon, H.K. Park, Y.R. Do, S.B. Lee et al., Langmuir 30 (2014) 1704. Copyright r 2014, American Chemical Society.

cost [2022]. Here, pseudocapacitance of MnO2 arises due to the fast transition of Mn41/Mn31 which actually occurs on the solid surface of the metal oxides. During the redox transition, adsorption/desorption (at surface) and intercalation/deintercalation (at subsurface) of protons and other cations occur: Mnðx1yÞ III Mn12ðx1yÞ IV OOCx Hy -MnIV O2 1 xC1 1 yH1 1 ðx 1 yÞe2 ðC1 5 Li1 ; Na1 ; K1 Þ The pseudocapacitance property of MnO2 was first reported by Lee and Goodenough in 1999 [23]. They prepared hydrated MnO2 using KMnO4 and Mn(CH3COO)2 as precursor reagents. They examined the electrochemical activity of the MnO2 sample in KCl, NaCl, and LiCl electrolytes and got ideal supercapacitor behavior of MnO2. After that, several scientists studied different types of MnO2 to get better electrochemical results. Belanger and his group reported a crystal structure-dependent pseudocapacitance of MnO2 [24]. According to their report, amorphous and 2D birnessite MnO2 exhibited better pseudocapacitance activity. Chen et al. published a paper on the pseudocapacitance activity of MnO2 (Fig. 19.3) [25]. They prepared MnO2 with different morphologies (such as needles, rods, and spindles) and different crystal forms (α and γ). They investigated the electrochemical activities of all the samples and according to the experimental results, the needle-like MnO2 sample exhibited best electrochemical activity as a pseudocapacitor

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FIGURE 19.2 (A)Representative cyclic voltammograms of the RuO2 2 ITO coaxial nanopillars measured in 0.5 M aqueous H2SO4 electrolyte. RuO2 layers were grown at various number of electrodeposition cycles. (B) Specific capacitance (’) and RuO2 thickness (▲) of the coaxial nanopillars as a function of the number of RuO2 electrodeposition cycles. (C) Transmittanc spectra of the ITO film and ITO nanopillar before and after deposition of RuO2 for 30 cycle, respectively. Inset is photos of ITO film, RuO2 deposited-ITO film, ITO nanopillars, and RuO2 deposited-ITO nanopillars, which are arranged left to right. Source: Reproduced with the permission from the publisher of I. Ryu, M. Yang, H. Kwon, H.K. Park, Y.R. Do, S.B. Lee et al., Langmuir 30 (2014) 1704. Copyright r 2014, American Chemical Society.

FIGURE 19.3

TEM and HRTEM images of as-obtained needle- (A, B, C), rod- (D, E, F), and spindle-like (G, H) products; the HRTEM images of the tip of the needle- and rod-like products are shown in the inset of (A) and (D). (I) The formation mechanism for 1D MnO2 nanostructures. Source: Reproduced with the permission from the publisher of S. Chen, J. Zhu, Q. Han, Z. Zheng, Y. Yang and X. Wang, Cryst. Growth Des. 9 (2009) 4356. Copyright r 2009, American Chemical Society.

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electrode. Devaraj and Munichandraiah also studied the crystallographic structuredependent pseudocapacitance property of MnO2 [26]. According to their investigation, α and δ crystallographic structures offer better electrochemical activities over the others. Rusi et al. prepared a flower-like MnO2 nanomaterial for pseudocapacitor application [27]. They synthesized the material using the electrodeposition method. The as-synthesized material exhibited a specific capacitance of 238 F/g in 0.5 M Na2SO4 electrolyte solution. Anandan and his group sonochemically synthesized MnO2 nanoparticle which was used as the supercapacitor electrode [28]. They used 1 M Ca(NO3)2 as the electrolyte and got the maximum specific capacitance value of 282 F/g at 0.5 mA/cm2. Zhang et al. reported a MnO2-based asymmetric supercapacitor in the ionic liquid electrolyte. Due to the use of ionic liquid, MnO2 exhibited a large potential window of 2.1 V and a specific capacitance of 523.3 F/g at 3 A/g [29]. Among all the TMOs, Co3O4 has been considered to be the most promising candidate as a pseudocapacitor due to its low cost, environmental friendliness, excellent redox activity, and very high theoretical specific capacitance value (3560 F/g) [30]. Until now, several scientists have tried with different Co3O4 nanomaterials, but the observed specific capacitance value did not match with the theoretical specific capacitance value. Pseudocapacitance activity of Co3O4 has been generated from the fast redox reactions between Co(II), Co(III), and Co(IV). The signature reactions are as follows: Co21 1 3OH2 2CoOOH 1 H2 O 1 e2 CoOOH 1 OH2 2CoO2 1 H2 O 1 e2 Numerous works have been done on the pseudocapacitance activity of Co3O4 and Co (OH)2. Yang et al. prepared a heterostructure of hierarchical Co3O4 nanosheet@nanowire to improve the pseudocapacitance activity of Co3O4 [31]. They synthesized the material via hydrothermal method followed by annealing (Fig. 19.4). The novel structured nanomaterial exhibited specific capacitance value of 715 F/g with rate capability of 69% after increasing the current six times (Fig. 19.5). Xiong et al. reported a paper where they had studied the supercapacitive activity of Co3O4 of tunable morphologies [32]. They synthesized the material via solvothermal strategies. The volume ratio of water (H2O) and ethanolamine (EA) played the crucial role in the morphology of the materials. Electrochemical results demonstrate that 2D nanosheets exhibit better pseudocapacitance activity over the 3D structure. Zhao et al. studied the morphology-dependent pseudocapacitance activity [30]. They prepared 1D nanochain and 2D nanosheet using the hydrothermal method. The experimental studies revealed that 1D nanochain exhibited better pseudocapacitance activity over 3D nanosheet. Xia et al reported a hollow 1D Co3O4 nanowire for pseudocapacitance activity [33]. The self-supported hollow Co3O4 nanowire was synthesized via the hydrothermal method and used as the positive electrode for supercapacitor application. It exhibited specific capacitance values of 599 F/g and 439 F/g at specific currents of 2 A/g and 40 A/g, respectively. The excellent rate capability of the as-prepared nanowire was due to the fast ion and electron transport through the porous structure of the nanomaterials. Recently, we have prepared a 2D ultrathin Co3O4 nanosheet via hydrothermal method using ammonia as hydrolyzing agent [5]. Here, ammonia plays a vital role in the fabrication of 2D sheet structure. Due to the ultrathin nature, it has exhibited very high specific

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FIGURE 19.4 SEM images of the products at various reaction stages by setting the reaction time to (A) 6 h, (B) 7 h, (C) 8 h, (D) 9 h. The insets are the corresponding magnified SEM images with scale bars of 200 nm. (E) Scheme of the possible formation process of the Co3O4 hierarchically structure. Source: Reproduced with the permission from the publisher of Q. Yang, Z. Lu, Z. Chang, W. Zhu, J. Sun, J. Liu, et al., RSC Adv. 2 (2012) 1663. Copyright r 2011, American Chemical Society.

FIGURE 19.5 Electrochemical characterization of the Co3O4 NSWAs: (A) CV curves of the Co3O4 NSWAs at different scan rates; (B) galvanostatic discharge curves of the Co3O4 NSWAs at various discharge current densities; (C) average specific capacitance versus cycle number of the Co3O4 NSWAs at a galvanostatic charge and discharge current density of 20 mA/cm2; (D) specific capacitance versus current densities of NSWAs, NSAs, and NWAs; (E) schematic image of the electron transmission in hierarchical structure. Source: Reproduced with the permission from the publisher of Q. Yang, Z. Lu, Z. Chang, W. Zhu, J. Sun, J. Liu, et al., RSC Adv. 2 (2012) 1663. Copyright r 2011, American Chemical Society.

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FIGURE 19.6 (A) Photograph of CF paper coated with cobalt hydroxide nanoflakes. (B) Schematic diagram illustrating the loading procedure of cobalt hydroxide on CF. (C) SEM image of bare CF. (D) SEM image of cobalt hydroxide nanoflakes coated on CF. Inset: magnified SEM image of the nanoflakes. Source: Reproduced with the permission from the publisher of A.D. Jagadale, G. Guan, X. Du, X. Hao, X. Li and A. Abudulaa, RSC Adv. 5 (2015) 56942. Copyright r 2015, Royal Society of Chemistry.

capacitance value (1256 F/g at 1 A/g) with excellent rate capability and stability. Co(OH)2 is also an excellent TMH which has widely been used as the pseudocapacitor electrode. Gao et al. prepared a layered α-Co(OH)2 nanomaterial and studied their electrochemical activity for a supercapacitor [34]. The Co(OH)2 aggregates were exfoliated in formamide solution and then used as a supercapacitor electrode. Due to the ultrathin nature of the assynthesized material, it exhibited a high specific capacitance value of 952 F/g at current density of 5 mA/cm2, but the rate was not good. Lokhande and his group fabricated a Co (OH)2 thin film electrode for supercapacitor [35]. Jagadale et al. fabricated a flexible supercapacitor using Co(OH)2 as electrode material (Fig. 19.6) [36]. The as-prepared electrode with mass loading of 2.5 mg/cm2 exhibited specific capacitance value of 386.5 F/g at current density of 1 mA/cm2. The as-fabricated electrode exhibited stability of 92% up to 2000 chargedischarge cycles (Fig. 19.7). Wang et al. reported an interesting work on the pseudocapacitance activity of Co(OH)2 depending on their crystallographic structure [37]. They prepared single layer α-Co(OH)2 and transformed it to β-Co(OH)2. It is very difficult to prepare β-Co(OH)2 directly. They studied the pseudocapacitance activity for both the samples and β-Co(OH)2 was proved to be a better pseudocapacitor. NiO and Ni(OH)2 have also been treated as an admirable candidate for pseudocapacitor application due to their high theoretical specific capacitance value and fast redox behavior

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(B) 0.012

Specific capacitance (F/g)

Current density (A/cm2)

(A) CF20 CF30 CF40 CF50 CF60

0.008 0.004 0.000 –0.004 –0.008 –0.4

–0.2

0.0

0.2

0.4

350 300 250 200 150 100

0.6

CF20

CF30

Potential (V vs. Ag/AgCl)

CF50

CF60

(D)

0.12

Specific capacitance (F/g)

Current density (A/cm2)

(C)

CF40

Electrode

5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s

0.09 0.06 0.03 0.00 –0.03 –0.06 –0.09

320 280 240 200 160

80 40 0

–0.4

–0.2

0.0

0.2

0.4

56.2%

120

0.6

0

20

Potential (V vs. Ag/AgCl)

40

60

80

100

Scan rate (mV/s)

FIGURE 19.7

(A) CV curves of CF20, CF30, CF40, CF50 and CF60 electrodes at the scan rate of 5 mV/s; (b) variation of specific capacitance with mass loading of each electrode; (c) CV curves of CF40 electrode at scan rates of 5, 10, 20, 50 and 100 mV/s; (d) specific capacitances of CF40 electrode at scan rates of 5, 10, 20, 50, and 100 mV/s. Source: Reproduced with the permission from the publisher of A.D. Jagadale, G. Guan, X. Du, X. Hao, X. Li and A. Abudulaa, RSC Adv. 5 (2015) 56942. Copyright r 2015, Royal Society of Chemistry.

[3841]. The pseudocapacitance-behavior of NiO and Ni(OH)2 arises due to the faradic redox reaction similar to the Co-based oxides and hydroxides [3841]. The reactions are as follows: NiO 1 OH2 2NiOOH 1 e2 NiðOHÞ2 1 OH2 2NiOOH 1 H2 O 1 e2 Ni(OH)2 has two crystallographic structure, α-Ni(OH)2 and β-Ni(OH)2. α-Ni(OH)2 is composed of exchangeable anions and water molecules intercalated into the interlayer whereas β phase is composed of a well-oriented Ni(OH)2 layer. Due to the larger interlayer space of α phase, it exhibits better electrochemical activity over the β phase. Another interesting factor is that in γ-NiOOH (the oxidized form of α-Ni(OH)2 during the charging), Ni possesses an oxidation state of about 3.33.7 [42]. On the other hand, in β-NiOOH which is the oxidized form of β-Ni(OH)2 the oxidation state of Ni is about 2.9. This also justifies

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the better electrochemical activity of α-Ni(OH)2 than β-Ni(OH)2. Several research groups have worked on these Ni-based oxides and hydroxides to fabricate an advanced supercapacitor device. Xiong et al. prepared an ultrathin Ni(OH)2 on Ni foam. The binder-free electrode behaved as an excellent supercapacitor which exhibited specific capacitance values of 2384 F/g and 1288 F/g at specific currents of 1 A/g and 5 A/g, respectively. The excellent electrochemical activity came from the fast ion transport through the ultrathin porous Ni(OH)2 nanosheet [38]. Sun et al. also prepared Ni(OH)2 nanosheets for supercapacitor application. Due to the high specific surface area and thin nature, the electrode material exhibited a specific capacitance value of 2064 F/g at 2 A/g specific currents with excellent rate capability [43]. Du et al. prepared a flower-like α-Ni(OH)2 which was used as the supercapacitor electrode (Fig. 19.8) [39]. It is very difficult to prepare α-Ni(OH)2 as it is very unstable in nature and easily transformed to its β phase. The prepared material exhibited a specific capacitance of 1788.9 F/g at 0.5 A/g with excellent rate capability and cyclic durability (Fig. 19.9). Jiang and his group developed a method to prepare a 3D NiO with network morphology which was proved to be an efficient supercapacitor (Fig. 19.10) [40]. 3D nanochannels of that NiO architecture offered maximum pore volume. Due to the high specific surface area and large pore volume, electrode and electrolyte came in contact FIGURE 19.8 SEM images of α-Ni (OH)2 products obtained at (A) 2, (B) 4, (C) 6, (D) 8, (E) 12, and (F) 16 h. Source: Reproduced with the permission from the publisher of H. Du, L. Jiao, K. Cao, Y. Wang, and H. Yuan, ACS Appl. Mater. Interfaces 5 (2013) 6643. Copyright r 2013, American Chemical Society.

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Potential (V vs. Hg/HgO)

(B) Potential (V vs. Hg/HgO)

(A) 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0.6

0.4 0.3 0.2 0.1 0.0 –0.1

0

10, 5, 2, 1, 0.5 Ag–1

0.5

3000 6000 9000 12000 15000 18000 21000

0

Time (s) (C)

1000

1500

(D)

1.8 1.6

0.04

Ni foam

1.4

Z⬘ (ohm)

0.06

Ni(OH)2

0.02 0.00 –0.02

1.2 1.0

Cdl

0.8

Rs

0.6 0.4

–0.04

CL

0.2

–0.06 0.0

0.1

0.2

0.3

0.4

2000

Time (s)

0.08

Current (A)

500

0.5

0.6

Rct Ws

0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Z⬘ (ohm)

Potential (V vs. Hg/HgO) (E)

100

2000

90

1600 1400

80

1200

70

1000 60

800 600

50

400

Capacitance retention (%)

Specific capacitance (F g–1)

1800

40

200 0 0

100

200

300

400

500

600

30 700

Cycle number (n)

FIGURE 19.9 Electrochemical performances of α-Ni(OH)2 prepared in the TEG/H2O mixed solvent. (A) Typical galvanostatic charge/discharge curve. The measurements were carried out in the 2 M KOH aqueous solution with a constant current of 0.5 A/g. (B) Galvanostatic discharge curve at various discharge current densities. (C) CV curves of Ni(OH)2 sample and pure Ni foam. (D) Nyquist plot. Inset is the proposed equivalent circuit for the EIS spectrum. (E) Cyclic performance of α-Ni(OH)2 prepared in the mixed solvent of TEG/H2O at the current density of 0.5 A/g. Source: Reproduced with the permission from the publisher of H. Du, L. Jiao, K. Cao, Y. Wang, and H. Yuan, ACS Appl. Mater. Interfaces 5 (2013) 6643. Copyright r 2013, American Chemical Society.

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FIGURE 19.10 Schematic diagram showing the formation process of (A) flower-, (B) slice-, and (C) particle-like Ni(OH)2 under different pH conditions; corresponding SEM images of asprepared samples are in the right panel. Source: Reproduced with the permission from the publisher of S. Kim, J. Lee, H. Ahn, H. Song, J. Jang, ACS Appl. Mater. Interfaces 5 (2013) 1596. Copyright r 2013, American Chemical Society.

very easily facilitating the charge diffusion and ion transportation through the electrode material. It exhibited specific capacitance value of 480 F/g at 0.5 A/g with high rate and stability (Fig. 19.11). G.R. Rao and his group prepared a microwave-assisted NiO nanostructure [41]. The mesoporous NiO offered very high specific surface area and pore volume. The measured specific capacitance value obtained from the as-fabricated NiO was 370 F/g at 2 A/g. Cheng et al. prepared ultrathin NiO nanosheet which was grown on 3D Ni foam. In the actual case, NiO is not a good electrical conductor (1022 to 1023 S/cm). Due to the in situ growth of NiO over Ni foam, electrical conductivity increases. On the other hand, the mesoporous and ultrathin nature of the prepared NiO increases the charge diffusion and charge transportation. Due to these structural and morphological advantages, the hybrid nanostructured material exhibited very high specific capacitance value of 2504 F/g. The most interesting factor is that it kept its stability up to 45,000 chargedischarge cycles [44]. Kolathodi et al. fabricated an asymmetric supercapacitor using NiO as a cathode material [45]. They prepared nanofiber using cost-effective solgel-based electrospinning method. The as-fabricated aqueous asymmetric supercapacitor exhibited a specific capacitance of 141 F/g with a specific energy of 43.75 Wh/kg. Kim and his group reported on an asymmetric supercapacitor where they used NiO ultrathin 2D NiO nanoflake as the positive electrode [46]. They grew the ultrathin 2D NiO nanosheet on 3D Ni foam using the solvothermal method. The unique porous structure and ultrathin 2D nature made charge diffusion very fast enhancing the electrochemical activity of the electrode material. In the three-electrode system, it exhibited a specific capacitance of 2013.7 F/g and 1465.6 F/g at specific currents of 1 A/g and 20 A/g, respectively. It also exhibited 100% capacitance retention up to 5000 cycles. Authors had fabricated an asymmetric supercapacitor using rGO as a negative electrode and 2D NiO as a positive

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(A)

407

(B) 5 4

1.00 0.75

2

Current (A/g)

Current (A/g)

3

1.25

5 mV/s 10 mV/s 20 mV/s 50 mV/s

1 0

5 mV/s 10 mV/s 20 mV/s 50 mV/s

0.50 0.25 0.00

–1 –0.25 –2 –3 –0.1

–0.50 0.0

0.2 0.3 0.1 Potential (V vs Hg/HgO)

0.4

(C)

–0.1

0.0

0.1 0.2 0.3 Potential (V vs Hg/HgO)

0.4

(D) 5

4

Current (A/g)

2

4

Nanoflower Nanoslice Nanoparticle

3 Current (A/g)

3

5 mV/s 10 mV/s 20 mV/s 50 mV/s

1 0

2 1 0 –1

–1 –2 –2 –3 –0.1

–3 0.0

0.1 0.2 0.3 Potential (V vs Hg/HgO)

0.4

–4 –0.1

0.0

0.1 0.2 0.3 Potential (V vs Hg/HgO)

0.4

FIGURE 19.11

Cyclic voltammogram curve of the capacitor at different scan rates of NiO nanostructure: (A) nanoflower, (B) nanoslice, and (C) nanoparticle. (D) Comparison of CV curves of three different shaped NiO nanostructures at a scan rate of 50 mV/s. Source: Reproduced with the permission from the publisher of S. Kim, J. Lee, H. Ahn, H. Song, J. Jang, ACS Appl. Mater. Interfaces 5 (2013) 1596. Copyright r 2013, American Chemical Society.

electrode which exhibited a specific capacitance of 145 F/g at 1 A/g with a specific energy of 45.3 Wh/kg. Apart from the above TMOs and TMHs, some other TMOs and TMHs are used as the pseudocapacitor electrode. These are MoO3, MoO2, V2O5, Fe3O4, Fe2O3, and FeOOH. Molybdenum exhibits a wide range of oxidation states from 12 to 17. Among them, MoO3 and MoO2 exhibit pseudocapacitance activity. α-MoO3 exhibits excellent ion intercalation. α-MoO3 is of the layered orthorhombic structure. Inside the structure, each layer is attached by van der Waals forces along the (0 1 0) direction [47]. Nanostructured MoO3 and MoO2 exhibit their excellent supercapacitor application in a neutral electrolyte (Na2SO4), acidic electrolyte (H2SO4), and basic electrolyte (KOH) [47]. In the last few years,

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numerous works have been done on the pseudocapacitance activity of Mo-oxides. Sanchez et al. published a work on nanostructured α-MoO3 thin film for supercapacitor application [47]. Liu and his group prepared α-MoO3 of several morphologies [48]. They experimentally proved that due to the morphological advantages, MoO3 nanobelts exhibited better supercapacitive activity over nanoplate, nanowire, and nanorods morphologies of MoO3. Tang et al. prepared MoO3 nanoplate for pseudocapacitor application (Fig. 19.12) [49]. They compared the electrochemical activity of the MoO3 nanoplate with the bulk MoO3 material in 0.5 M Li2SO4 electrolyte. The nanoplate exhibited a specific current of 45 Wh/ kg at a specific power of 450 W/kg. The material has been proved to be an alternative anode material for supercapacitor other than carbon material (Fig. 19.13). V2O5 is also an important TMO which can be used as the negative electrode for supercapacitor application [50,51]. Lee et al. prepared an amorphous V2O5 nH2O [50]. The as-prepared material exhibited ideal supercapacitor behavior in KCl electrolyte solution. G. Murlidharan and his group prepared an interconnected V2O5 nanoporous network structure material (Fig. 19.14) [51]. They prepared the material via a coprecipitation method. The as-prepared material exhibited a maximum specific capacitance of 316 F/g (Fig. 19.15). The interconnected porous network created nanochannel facilitating the ion diffusion and charge transport which actually enhanced the electrochemical activity of the material.



FIGURE 19.12 SEM micrographs of (A) the MoO3nanoplates and (B) the bulk MoO3, and (C) their X-ray diffraction patterns. Source: Reproduced with the permission from the publisher of W. Tang, L. Liu, S. Tian, L. Li, Y. Yue, Y. Wu, et al., Chem. Commun. 47 (2011) 10058. Copyright r 2011, Royal Society of Chemistry.

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FIGURE 19.13 (A) CV curves and (B) change of the capacitance with the scan rate for the nanoplates and bulk MoO3 in 0.5 M Li2SO4 aqueous electrolytes. Source: Reproduced with the permission from the publisher of W. Tang, L. Liu, S. Tian, L. Li, Y. Yue, Y. Wu, et al., Chem. Commun. 47 (2011) 10058. Copyright r 2011, Royal Society of Chemistry.

(A) SEM image of the V2O5 nanoporous network (VNN); (B) VNN annealed at 300 C; (C) VNN annealed at 400 C; (D) VNN annealed at 500 C; (E, F) TEM images of VNN. Source: Reproduced with the permission from the publisher of B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, ACS Appl. Mater. Interfaces 4 (2012) 4484. Copyright r 2012, American Chemical Society.

FIGURE 19.14

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(B)

1

Potential (V)

100

a) VNN b) VNN3 c) VNN4 d) VNN5

0.8 0.6

Retention of capacity (%)

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19. NOBLE METALTRANSITION METAL OXIDES/HYDROXIDES: DESIRED MATERIALS FOR PSEUDOCAPACITOR

0.4 0.2 c

0

d

b

80

100%

60 76% 40 20

a 0

–0.2 0

0

1000 2000 3000 4000 5000 6000 7000

100

200

300

400

500

600

Cycle number Time (s) 10

2

8

1.5

6 4 Current (A/g)

Current (A/g)

1 0.5 0 VNN –0.5

VNN3

0 50 mVs–1 25 mVs–1 15 mVs–1 10 mVs–1 5 mVs–1 2 mVs–1

–2 –4 –6

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FIGURE 19.15 (A) Charge/discharge curves for VNN, VNN3, VNN4, and VNN5 measured at 100 mA/g. (B) Cycling behavior of VNN measured at 1000 mA/g. Source: Reproduced with the permission from the publisher of B. Saravanakumar, K.K. Purushothaman, G. Muralidharan, ACS Appl. Mater. Interfaces 4 (2012) 4484. Copyright r 2012, American Chemical Society.

Another group of materials which can be used as the negative electrode for supercapacitor application are Fe-based oxides and oxy-hydroxides. Fe-based oxides have better electrical conductivity compared to the other TMOs and TMHs. Several works have been done on these materials. Wu et al. prepared a nanostructured iron oxide thin film [52]. It exhibited a specific capacitance of 146 F/g at 0.05 mA/cm2 current density in 1 M Na2SO4 electrolyte. Huang and his group fabricated a highly ordered iron oxide nanotube array [53]. They used it as the electrode material for electrochemical storage, a pseudocapacitor. It exhibited a specific capacitance of 138 F/g at 1.3 A/g specific current. It kept 89% of its specific capacitance values after 500 chargedischarge cycles.

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19.4 MIXED TRANSITION METAL OXIDES (MTMOs) AND MIXED TRANSITION METAL HYDROXIDES (MTMHs)

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The above discussion is completely based on the pseudocapacitance activity of the single TMOs and TMHs. From the above discussion, it is clear that single TMOs and TMHs exhibit good electrochemical activities as a pseudocapacitor electrode. But they also display some limitations like low specific capacitance value, low rate capability, and most importantly lower cyclic stability. To overcome these limitations, scientists are working on mixed transition metal oxides (MTMOs) or mixed transition metal hydroxides (MTMHs), hybrids of TMOs/TMHs/MTMOs/MTMHs with different carbonaceous materials. Here, we will discuss the advantages of using MTMOs or MTMHs and hybrid materials as an electrode material for supercapacitor application.

19.4 MIXED TRANSITION METAL OXIDES (MTMOs) AND MIXED TRANSITION METAL HYDROXIDES (MTMHs) Among the mixed metal oxides, binary metal oxides of spinel structures have drawn most of the attention of the researchers. Among the spinel compounds, NiCo2O4, CoFe2O4, NiFe2O4, ZnMnO4, and ZnCo2O4 have been used as the electrode material for pseudocapacitor application [5461]. NiCo2O4 is an interesting compound and it was mostly used by the researchers as an energy storage electrode due to the high theoretical specific capacitance value and interconnected arrangements [6]. Several research groups prepared different NiCo-based oxides or hydroxides of different morphologies for that purpose. Zhou et al. prepared a multiple hierarchical NiCo2O4 composed of 1D nanowires and 2D nanosheets (Fig. 19.16) [62]. They adopted the hydrothermal technique for the synthesis of this material. Due to the high specific surface area and facile electroactive sites, the material exhibited excellent electrochemical activity. It produced a maximum specific capacitance value of 2623.3 F/g at 1 A/g specific current with good rate capability and stability at higher specific current (Fig. 19.17). Wang et al. prepared 2D NiCo2O4 nanosheet using a FIGURE 19.16 (A 2 C) FESEM images at different magnifications of NiCo2O4 MHSs (NCO2) on Ni foam; (D 2 H) TEM and HRTEM images of the nanosheet and nanowire exfoliated after intense ultrasonication; (i) corresponding SAED pattern of the nanosheet and nanowire. Source: Reproduced with the permission from the publisher of Q. Zhou, J. Xing, Y. Gao, X. Lv, Y. He, Z. Guo, et al. ACS Appl. Mater. Interfaces 6 (2014) 11394. Copyright r 2014, American Chemical Society.

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FIGURE 19.17

Electrochemical performances of the as-prepared samples: (A) CV curves (after 10 cycles) of the three samples at a scan rate of 10 mV/s; (b) specific capacitance derived from the CV measurements at scan rates of 1, 2, 5, 10, 20, 30, and 40 mV/s; (c) CD curves (after 10 cycles) of the three samples at a current density of 10 A/g; (d) specific capacitance derived from the discharge curves at current densities of 1, 2, 5, 10, 20, 30, and 40 A/g; (e) Ragone plot of the estimated energy density and power density at various charge 2 discharge rates; and (f) cycling performance of the three samples measured at a current density of 10 A/g. Source: Reproduced with the permission from the publisher of Q. Zhou, J. Xing, Y. Gao, X. Lv, Y. He, Z. Guo, et al. ACS Appl. Mater. Interfaces 6 (2014) 11394. Copyright r 2014, American Chemical Society.

nickelcobalt citrate complex [63]. It exhibited a maximum specific capacitance of 1846 F/g with excellent rate capability. Most importantly, when it was coupled with polyaniline-activated carbon to fabricate aqueous asymmetric supercapacitor (AAS), it exhibited a maximum specific energy of 71.7 Wh/kg and a maximum power density of 16 kW/kg. Apart from these spinel structures, we reported a few other mixed metal oxides and hydroxides for pseudocapacitor application [5,6]. G. Shen and his group prepared ZnCo2O4 on Ni foam (Fig. 19.18) [59]. It exhibited a very high specific capacitance value of 1400 F/g at 1 A/g specific current with very high rate capability, 72% capacitance retention at 20-fold increase in specific current (Fig. 19.19). We prepared an ultrathin 2D

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19.5 HYBRID MATERIALS

413 FIGURE 19.18 (A) Schematic representation of the ZnCo2O4 nanorods/nickel foam synthesis. (B, C) photographs of the obtained products without and with bending, (D) SEM image of the pristine nickel foam, and (E, F) the ZnCo2O4 nanorods grown on it. Source: Reproduced with the permission from the publisher of K. Karthikeyan, D. Kalpana, N. Renganathan, Ionics 15 (2009) 107. Copyright r 2013, American Chemical Society.

Co3V2O8 nanostructure for pseudocapacitor application [5]. We used ammonia as the hydrolyzing agent and it played a major role in the fabrication of nanosheet. Due to the mixed oxide structure and ultrathin nature, Co3V2O8 exhibited a maximum specific capacitance of 4194 F/g at 1 A/g specific current with excellent rate and stability. When an AAS was fabricated using Co3V2O8 as positive electrode and AC as a negative electrode, it exhibited a maximum specific energy of 107 Wh/kg. In another report, we prepared CoSn (OH)6 nanostructure of two different morphologies, cubic nanostructure (CNS) and hierarchical nanostructure (HNS) (Fig. 19.20) [6]. Any Sn-based compound generally exhibits cubic morphology. In this case also ammonia played a crucial role for the fabrication of uncommon 2D CoSn(OH)6 nanomaterials which were assembled together to form an HNS sample. Due to the morphological advantages, the HNS sample exhibited better electrochemical activity over the CNS sample. The HNS sample exhibited a maximum specific capacitance value of 2545 F/g at 2.5 A/g specific current. As an AAS it exhibited a maximum specific energy of 63.5 Wh/kg with 92% stability after 10,000 chargedischarge cycles (Fig. 19.21). Liu et al. also prepared two metal vanadate (M3V2O8, M 5 Ni, Co) samples for pseudocapacitor application [64].

19.5 HYBRID MATERIALS Scientists have prepared several hybrid materials of different TMOs and TMHs with noble metals, other TMOs or TMHs, or with carbonaceous materials, like conductive

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FIGURE 19.19 (A) Schematic of the fabrication process for the three-electrode measurement system. (B) CV curves of the ZnCo2O4 nanorods/nickel foam at the different scan rates. (C) Discharging curves at different current densities. (D) The specific capacitance as a function of the current densities of the ZnCo2O4 nanorods. (E) Cycling performance of the ZnCo2O4 nanorods/nickel foam electrodes at various current densities of 2 and 6 A/g. Source: Reproduced with the permission from the publisher of K. Karthikeyan, D. Kalpana, N. Renganathan, Ionics 15 (2009) 107. Copyright r 2013, American Chemical Society.

polymers, graphene, carbon nanotube, porous carbon, etc. to fabricate an advanced supercapacitor. Hybrid material is prepared mainly to increase the electrical conductivity. A hybrid structure of gold nanoparticles and amorphous MnO2 was reported where the hybrid structure exhibited a specific capacitance value of 1145 F/g [65]. The Au nanoparticles not only increased the electronic conductivity but also facilitated the ion diffusion between the TMO and electrolyte which actually increases the cyclic durability and rate capability of the electrode material. There are a few reports where researchers have doped metal atoms, such as V, Fe, Co, etc., to increase the electrochemical activity of MnO2 [66]. A 3D Co3O4MnO2 hybrid hierarchical structure was synthesized using the stepwise hydrothermal technique for advanced energy storage application [67]. When it was used as supercapacitor electrode, it exhibited a specific capacitance value of 1632.5 F/g at 1 A/g

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19.5 HYBRID MATERIALS

415

FIGURE 19.20

TEM images of (A, B) HNS sample and (C) CNS sample. Source: Reproduced with the permission from the publisher of R. Sahoo, A.K. Sasmal, C. Ray, S. Dutta, A. Pal, T. Pal, ACS Appl. Mater. Interfaces 8 (2016) 17987. Copyright r 2016, American Chemical Society.

specific current with very high rate capability and stability. The important factor is that the hybrid structure exhibited superior electrochemical activity over the individual Co3O4 1D nanoneedles and 2D MnO2 nanosheets. A MnO2-based ternary hybrid sandwich structured material was reported by Li et al. [68]. In the MnO2/Mn/MnO2 hybrid nanotubes, Mn layer serves as an excellent electronic conductor and the sandwich structure allows the electrolyte to access the redox active sites more actively. These dual factors in the hybrid structure actually increased the electrochemical activity when it was used as the supercapacitor electrode. It exhibited a specific capacitance of 955 F/g at a specific current of 1.5 A/g with excellent rate capability. It also exhibited a specific energy of 45 Wh/kg without sacrificing the specific power, 23 kW/kg. S.B. Lee and his group prepared an interesting hybrid material, TiN-MnO2 [69]. They used deposition technique to synthesize the hybrid nanotube. Interestingly, they deposited redox active MnO2 on both sides of TiN nanotubes. Due to the high electronic conductivity of TiN and highly accessible redox active sites (here MnO2) of the electrode, the hybrid material exhibited a very high specific capacitance value, 662 F/g at 45 A/g specific current. Lu et al. reported an interesting compound for flexible supercapacitor application, WO3 x@Au@MnO2 [70]. The composite exhibited specific capacitance value of 1195 F/g at a specific current of 0.75 A/g. As a flexible supercapacitor, it exhibited very high specific power of 30.6 kW/kg with the excellent specific energy of 78.1 W/kg. Several reports are of scientists using carbonaceous materialbased MnO2 hybrid structure, such as MnO2/graphene, MnO2/CNT, MnO2/PANI, etc., to improve the capacitive activity of MnO2 [7173]. A graphene supported needle-like MnO2 was fabricated for superior pseudocapacitance activity [71]. Experimental data revealed that due to the synergistic effect between GO and MnO2, the composite exhibited better electrochemical activities over individual MnO2 and GO. Rate capability was an issue for MnO2 when it was used as pseudocapacitor. A rGO/CNT/MnO2 ternary composite was synthesized to solve this problem [72]. The maximum specific capacitance value exhibited by the composite was 319 F/g at 1 A/g specific current, which is not so high compared to the other reported MnO2-based composite materials, but an interesting factor is that it

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FIGURE 19.21 (A) CV curve of AC and (red line) and HNS CoSn(OH)6 (black line) at 50 mV/s in three-electrode system, (B) CV curve at different voltage window, (C) CV curves at different scan rate, and (D) charge 2 discharge curve at different specific currents of HNS CoSn(OH)6//AC asymmetric cell. Source: Reproduced with the permission from the publisher of R. Sahoo, A.K. Sasmal, C. Ray, S. Dutta, A. Pal, T. Pal, ACS Appl. Mater. Interfaces 8 (2016) 17987. Copyright r 2016, American Chemical Society.

exhibited very high rate capability (222 F/g at 60 A/g specific current). Another MnO2based ternary composite was reported for advanced supercapacitor application (Fig. 19.22) [73]. The high synergistic property and high specific surface area of core double shell CNT@PPy@MnO2 hybrid structure exhibited excellent rate capability as well as excellent cyclic stability (Fig. 19.23). Muniraj et al. synthesized a hybrid composite of RuO2 nH2O nanoparticles and carbon nano-onions [74]. They used this composite to prepare flexible solid-state supercapacitor using poly(vinyl alcohol)/H2SO4 gel electrolyte. As a solid-state supercapacitor, it exhibited a maximum specific energy of 10.62 Wh/kg and maximum specific power of 4.456 kW/kg. Kim and his group prepared a graphene/Co(OH)2 nanosheet composite [75]. Interestingly, they did not use GO for the preparation of graphene. They synthesized graphene directly from graphite using SDS surfactant. As a result they nullified the probability of restacking of the graphene sheet. Here, due to the



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417

FIGURE 19.22 Schematic for the fabrication process from CNT sponge to CNT@PPy@MnO2 core-double-shell sponge. A uniform PPy layer was firstly coated on the surface of individual CNTs by electropolymerization to produce an intermediate CNT@PPy coreshell sponge. Then, through a hydrothermal process, a MnO2 shell was attached outside the PPy shell, leading to the formation of the core-double-shell CNT@PPy@MnO2 sponge. Source: Reproduced with the permission from the publisher of P. Li, Y. Yang, E. Shi, Q. Shen, Y. Shang, S. Wu, et al., ACS Appl. Mater. Interfaces, 6 (2014) 5228. Copyright r 2014, American Chemical Society.

presence of ultrathin 2D Co(OH)2 nanosheet and monolayer graphene in the composite, the electrolyte ions could easily diffuse across the electrode material and the ions transportation become facile. Due to the morphological advantages, the hybrid structure exhibited a very high specific capacitance value of 960 F/g at a specific current of 10 A/g. It exhibited high cyclic stability as well as rate capability. Fu et al. fabricated an excellent hybrid structure of mixed TMOs and TMHs, ZnCo2O4@NixCo2x(OH)6x coreshell nanostructure (Fig. 19.24) [76]. They used an electrodeposition method to deposit 2D NixCo2x(OH)6x nanosheets on hydrothermally prepared 1D ZnCo2O4. Due to the morphological advantages and synergistic effect between two mixed oxides, the composite exhibited extraordinary electrochemical activity. It displayed areal capacity of 419 μAh/cm2 with high cyclic stability. When the hybrid structure was coupled with AC for the fabrication of asymmetric supercapacitor, it exhibited a specific energy of 26.2 Wh/kg at specific power of 511.8 W/kg (Fig. 19.25). Zhai et al. has also fabricated a hybrid structure of two mixed TMOs, NiCo2O4@MnCo2O4 [77]. 3D hierarchical morphology of the composite structure enhanced the electrical conductivity and diminished the probability of volume change during the chargedischarge cycles which actually enhanced the electrochemical activity of the composite. A solid-state supercapacitor was reported using NiCo2O4/porous graphene as the positive electrode [78]. No binder was used to fabricate the electrode. The hierarchical NiCo2O4 nanostructure was in situ grown on the carbon cloth which was used as the electrode. It exhibited a maximum specific capacitance of 1768 F/g. As an asymmetric electrode it exhibited very high specific energy value of 60.9 Wh/kg with

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FIGURE 19.23 Supercapacitor application of CNT@PPy@MnO2 core-double-shell sponges. (A) CV curves of the CNT, CNT@PPy, CNT@MnO2, and CNT@PPy@MnO2 sponges at a scan rate of 100 mV/s. (B) Calculated specific capacitances of the four type sponges at different scan rates. (C) Galvanostatic charge/discharge curves of the four sponges at a current of 0.5 A/g. (D) Nyquist plots of the EIS for the four sponges. (E) Cycling tests showing a better stability of CNT@PPy@MnO2 sponge than CNT@PPy and CNT@MnO2 sponges after 1000 charging and discharging cycles at 100 mV/s. (F) CV curves of cycle 1st, 50th, 100th, 500th, and 1000th for the CNT@PPy@MnO2 sponge. Source: Reproduced with the permission from the publisher of P. Li, Y. Yang, E. Shi, Q. Shen, Y. Shang, S. Wu, et al., ACS Appl. Mater. Interfaces, 6 (2014) 5228. Copyright r 2014, American Chemical Society.

maximum specific power of 11.36 kW/kg. Remarkably, the asymmetric cell kept 96.8% of its original specific capacitance value after 5000 chargedischarge cycles. Liang et al. prepared a hierarchical NiCo2O4 nanosheet@halloysite nanotube composite using a coprecipitation method followed by thermal annealing treatment [79]. Morphologies of the individual materials in the composite offer the hybrid structure a high specific surface area which facilitates the ions transportation. Thus, morphology and synergistic effect inside the composite make it an advanced electrode for supercapacitor application. It exhibited maximum specific capacitance value of 1728 F/g at 1 A/g specific current. Due to the robust nature and morphological advantages it also exhibited high specific rate and excellent cyclic chargedischarge stability. It retained B95% of its initial specific capacitance value after 9000 chargedischarge cycles at 10 A/g. Recently, we have fabricated an aqueous asymmetric supercapacitor using two-hybrid materials, Ni3V2O8@MWCNT (as positive electrode) and FeOOH@rGO (as negative electrode) (Figs. 19.26 and 19.27) [7]. We documented that for both the materials, pseudocapacitance depends on the mass loading of NPs over the carbonaceous materials and at a particular mass loading, the composites exhibited extraordinary electrochemical activity. It was observed that when Ni3V2O8

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FIGURE 19.24 TEM image (A), HRTEM image (B) and SAED pattern (C) of the ZnCo2O4 nanowire. TEM images (D and E) and EDX spectrum (F) of the ZnCo2O4@NixCo2x(OH)6x NWAs. Source: Reproduced with the permission from the publisher of W. Fu, Y. Wang, W. Han, Z. Zhang, H. Zha, E. Xie, J. Mater. Chem. A 4 (2016) 173. Copyright r 2015, Royal Society of Chemistry.

loading on MWCNT was 30%, it exhibited a maximum specific capacitance value of 2920 F/g at 2 A/g specific current. Due to synergistic effect between Ni3V2O8 and MWCNT, it exhibited high rate capability and excellent cyclic stability (95% after 10,000 chargedischarge cycles). For the FeOOH@rGO composite, when FeOOH loading was 12% it exhibited the best electrochemical activity (maximum specific capacitance value of 1306 F/g with cyclic durability of 92% after 10000 chargedischarge cycles). As an asymmetric cell, Ni3V2O8@MWCNT//FeOOH@rGO, it exhibited very high specific capacitance value of 86.7 Wh/kg (Fig. 19.28). Nagaraju et al. reported a composite of 2D V2O5 and rGO as a supercapacitor electrode [80]. They demonstrated that the 2D V2O5/rGO composite exhibited superior electrochemical activity over 2D V2O5. At 1 A/g specific current, V2O5/rGO composite and V2O5 exhibited a maximum specific capacitance value of 635 F/g and 253 F/g, respectively. The composite also exhibited higher specific energy value (79.5 Wh/kg) compared to the pure V2O5 (39 Wh/kg).

19.6 NOBLE METALTRANSITION METAL OXIDE/HYDROXIDE HYBRID BASED MATERIALS The low electrical conductivity of TMOs and TMHs is always an issue for these materials during the electrochemical performance. Use of noble metals as an additive with

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19. NOBLE METALTRANSITION METAL OXIDES/HYDROXIDES: DESIRED MATERIALS FOR PSEUDOCAPACITOR

(B) 60

Current density (mA cm–2)

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FIGURE 19.25

(A) A comparison of CV curves of the as-prepared ZnCo2O4@NixCo2x(OH)6x, NixCo2x(OH)6x and ZnCo2O4 electrodes at a scan rate of 10 mV/s. (B) CV curves of the ZnCo2O4@NixCo2x(OH)6x electrode at different scan rates. (C) GCD curves of the ZnCo2O4@NixCo2x(OH)6x electrode at various current densities from 5 to 50 mA/cm2. (D) The areal capacity as a function of current density for the ZnCo2O4@NixCo(OH)6x, NixCo2x(OH)6x and ZnCo2O4 electrodes. Source: Reproduced with the permission from the publisher of W. Fu, Y. Wang, W. Han, Z. Zhang, H. Zha, E. Xie, J. Mater. Chem. A 4 (2016) 173. Copyright r 2015, American Chemical Society.

TMOs or TMHs can be an excellent alternative for pseudocapacitor electrodes. It can not only enhance the electrical conductivity of the electrode but also it can enhance the specific capacitance values, cycling stability etc. To date, several researchers have worked on these materials and got some excellent results [81101]. Researchers have adopted different synthetic strategies (solvothermal/hydrothermal, polymerization, electrodeposition method, etc.) to prepare noble metal-TMOs/noble metal-TMHs hybrids. Among the noble metals, Ag has the highest electrical conductivity. Thus there are several reports on AgTMOs/ TMHs composites in the literature which have been used as the pseudocapacitor electrode. Wang et al. reported an Ag-doped MnO2 film which was prepared using a KMnO4 aqueous solution containing AgNO3 as a precursor by cathodic electrolytic deposition [81]. The obtained specific capacitance was 770 F/g at a scan rate of 2 mV/s when it was used as a pseudocapacitor. Another group synthesized a 3D nanoporous silver (NPS)/ MnO2 composites Ag45Mg35Ca20 metallic glass via the electrochemical de-alloying and electrodes plating of nanocrystalline MnO2 [82]. The as-prepared composite as pseudocapacitor electrode resulted in a specific capacitance of 1088 F/g at 1 A/g due to the fast ionic conduction and excellent electronproton transport. Zhang et al. had successfully synthesized the

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FIGURE 19.26 (AC) FESEM and (DF) TEM images of the different Ni3V2O8@MWCNT composites (A and D, CNV1; B and E, CNV2; C and F, CNV3). Source: Reproduced with the permission from the publisher of R. Sahoo, A. Pal, T. Pal, J. Mater. Chem. A 4 (2016) 17440. Copyright r 2016, Royal Society of Chemistry.

nanohybrids of AgNPs/MnO2 nanosheets [83]. Their nanohybrids provided the specific capacitance value of 272 F/g (at a scan rate of 10 mV/s), which was much higher than that of the pure MnO2 nanosheets (90 F/g). Similar results were obtained for the AgNPs decorated MnO2 nanowires [84]. These nanohybrids showed the specific capacitance of 293 F/g (at a scan rate of 10 mV/s), which was twofold higher than that of the pure MnO2 materials (B130 F/g). For an Ag/MnO2 nanocomposite, its specific capacitance was 557 F/g (at a scan rate of 5 mV/s), which was about 2.7-fold higher than that of the MnO2 electrodes [85]. Not only MnO2, but other TMOs, such as NiO, Ni(OH)2, CuO, Co3O4, etc., have also been used to prepare AgTMOs/TMHs hybrids. Wu et al. reported the AgNiO hybrid, which exhibited excellent rate capability [86]. The composite exhibited specific capacitance values of 330 F/g and 281 F/g at 2 A/g and 40 A/g currents, respectively. Huang et al. synthesized the Ag-doped CuO by using a template-free growth method and Agmirror reactions [87]. The doped CuO exhibited a sharp increase in the specific capacitance value of 689 F/g at 1 A/g, whereas the undoped CuO only exhibited the specific capacitance value of 418 at the same current density. Wang et al. reported the porous Ag decorated Co3O4 electrode for pseudocapacitive activity, which exhibited high specific capacitance value as well as high rate (specific capacitance values of 1276 F/g and 986 F/g at 1 A/g and still 10 A/g, respectively) [88]. Not only TMOs, but there are also some reports of

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FIGURE 19.27 FESEM and TEM images of different β-FeOOH@rGO composites (A and D, GF1; B and E, GF2; and C and F, GF3). Source: Reproduced with the permission from the publisher of R. Sahoo, A. Pal, T. Pal, J. Mater. Chem. A 4 (2016) 17440. Copyright r 2016, Royal Society of Chemistry.

AgTMHs in the literature for pseudocapacitor application. Ghosh et al. prepared a Agdeposited Ni(OH)2/graphene composite electrode which showed specific capacitance of 496 F/g at 1 A/g current density. The composite exhibited 93% specific capacitance retention after 500 chargedischarge cycles [89]. In another report, Lan et al. synthesized a ternary composite of Ag NPs/Ni(OH)2 nanosheet/3D graphene [90]. The composite exhibited very high specific capacitance value of 2167 F/g at 10 A/g with 98% cyclic stability after 1000 cycles at 25 A/g current. Regarding the AuNPs hybrids-based supercapacitors, Dai et al. successfully synthesized the AuNPs/MnO2 nanorod hybrids that exhibited superior specific capacitance and longterm durability [91]. The specific capacitance of these nanohybrids was 406.8 F/g at a scan rate of 50 mV/s, which was five times higher than that of pure MnO2 nanorods. The similar result was also reported by Khandare and Terdale for the AuNPs-decorated MnO2 nanowires [92]. These nanohybrids had the specific capacitances of 249 and 164 F/g, at a scan rate of 1 and 5 mV/s, respectively. These values were much higher than that of pure MnO2 nanowires (50 and 70 F/g at a scan rate of 1 and 5 mV/s, respectively). Chen et al. fabricated an Au-doped MnO2 film, which possessed an ultrahigh specific capacitance of 626 F/g at 5 mV/s [93]. The cyclic stability of the materials was very high. It increased to 7% stability after 15,000 cycles. Qu et al. also successfully synthesized the AuNPs-decorated NiO nanostructures [94]. Their AuNiO nanohybrids provided the much higher value of specific

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FIGURE 19.28 Pictorial representation of the mechanism of the action inside the asymmetric supercapacitor where CNV is the positive electrode and GF is the negative electrode. (A) Cyclic voltammetry curves of the β-FeOOH@rGO and Ni3V2O8@MWCNT composite at 50 mV/s scan rate, (B) CV curves of different voltage windows, (C) cyclic voltammetry curves at different scan rates and (D) chargedischarge curves at different specific currents of Ni3V2O8@MWCNT// β-FeOOH@rGO asymmetric supercapacitor. Source: Reproduced with the permission from the publisher of R. Sahoo, A. Pal, T. Pal, J. Mater. Chem. A 4 (2016) 17440. Copyright r 2016, Royal Socity of Chemistry.

capacitance (619 F/g at a high rate of 20 A/g), compared to that of pure NiO nanomaterials (216 F/g). Liu et al. also obtained the high value of specific capacitance for the 714 nm AuFe3O4 dumbbell-like nanoparticles [95]. The specific capacitance of their dumbbell-like NPs was 464 F/g at 1 A/g, which was much higher than that of pure Fe3O4 NPs (160 F/g). Kim et al. adopted a simple Au deposition on Ni(OH)2 to enhance the electrochemical activity of Ni(OH)2. This composite exhibited the specific capacitance value of 1927 F/g at 1 A/g [97]. Compared to the pristine Ni(OH)2, the composite exhibited almost 41% capacitance increment. The composite maintained 48% capacitance retention after 5000 cycles at 20 A/g, whereas the pristine Ni(OH)2 kept only 30% of its initial capacitance value. In case of PtNPs hybrids based supercapacitors, Xia et al. designed the hierarchical Co3O4@Pt@MnO2 core/shell/shell-like structure [98]. These nanohybrids exhibited the higher specific capacitance of 539 F/g (at the current density of 1 A/g), compared to that

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of pure MnO2 thin film electrode (171 F/g). Lee et al. synthesized Pt/MnO2/CNFs composite, which exhibited the 252 F/g specific capacitance at 10 mV/s scan rate. This composite showed high energy density as well [99]. Wen et al. prepared the Pt/MnO2 coreshell nanotube through an atomic layer deposition process [100]. It showed a high gravimetric capacitance (810 F/g at 5 mV/s) and areal capacitance value (75 mF/cm2 at 5 mV/s). This composite exhibited excellent rate capability of 68% after 50-fold increase in current density. The cyclic stability was also very good. Table 19.1 presents the summary of hybridization strategies to enhance the specific capacity of noble metals-based supercapacitors in the literature. As can be seen in this table, the hybridization between noble metal nanoparticles and metal oxide nanostructures improved the specific capacitance. TABLE 19.1 Summary of Hybridization Strategies to Enhance the Specific Capacity of Noble Metals-Based Supercapacitors in the Literature Materials

Current Collector

Potential Electrolyte Window (V)

Capacity (F/g)

Reference

AUNPS-BASED HYBRID MATERIALS MnO2 nanorod/AuNPs

Nickel foam 1 M Na2SO4

01.0 (Ag/AgCl)

406.8 F/g (at 50 mV/s)

[91]

Gold nanoparticles-decorated MnO2 nanowires

platinum metal

(-0.2 10.8) (Ag/AgCl)

249 and 164 F/g (at 1 and 5 mV/s)

[92]

619 F/g (at 20 A/g)

[94]

1M Na2SO4

Au-decorated hierarchical NiO nanostructures 714 nm AuFe3O4 dumbbell

Pt mesh

6 M KOH

-1.0 10.4 (Hg/HgO)

464 F/g (at 1 A/g)

[95]

Nanosized MnO2 spines on Au stem

Au nanowires

1M Na2SO4

00.8 (Ag/AgCl)

1130 F/g (at 2 mV/s)

[96]

PTNPS-BASED HYBRID MATERIALS Co3O4@Pt@MnO2 nanowire arrays

Ti foil

1M Na2SO4

01.0 (Ag/AgCl)

539 F/g (at 1 A/g)

[98]

Core/shell Pt/MnO2 nanotubes

Aluminum foil

1M Na2SO4

00.9 (Ag/AgCl)

810 F/g (at 5 mV/s)

[100]

AGNPS-BASED HYBRID MATERIALS Ag-nanoparticle-loaded MnO2 nanosheets

Pt foil

1M Na2SO4

-0.05 10.95 V (SCE)

272 F/g (at 10 mV/s)

[83]

Ag nanoparticles-decorated MnO2 nanowires

Pt foil

1M Na2SO4

01.0 (Ag/AgCl)

293 F/g (at 10 mV/s)

[84]

Ag/MnO2 nanocomposite

Platinum mesh

0.5 M Na2SO4

01.0 (Ag/AgCl)

551557 F/g, (at 5 mV/s)

[85]

Ag/MnO2 nanotubes

Platinum foil

1M Na2SO4

01.0 (Ag/AgCl)

170 F/g (at 1 mV/s)

[101]

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150 F/g (at 1 A/g)

19.7 CONCLUSION

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19.7 CONCLUSION A desired energy storage device must have high energy density value as well as power density value. It is known that a supercapacitor provides low energy density and a battery provides low power density. Pseudocapacitor somewhat exhibits excellent electrochemical activity with high specific capacitance value, high power density value, moderately high energy density with good cyclic durability. Thus, in all cases, the choice of electrode materials matters significantly in the electrochemical activity. Among the pseudocapacitive electrode materials, nanostructured TMOs and TMHs are mostly used. Nanostructured materials are more important due to their high surface to volume ratio, a higher percentage of active material etc. Then above discussion reveals that based on their structure, morphology, hybridization, composition, TMOs and TNHs exhibit different pseudocapacitance activity. Single TMOs and TMHs exhibit better electrochemical activity, such as high specific capacitance value, high energy density etc., compared to the carbonaceous materials. TMOs and TMHs possess low electrical conductivity and due to the faradic redox reaction on the surface, they exhibit instability. As a matter of fact, single TMOs and TMHs suffer from low rate capability and low cyclic stability. When these single TMOs or TMHs are coupled with the other TMOs/TMHs or hybridized with the carbonaceous materials or other mixed TMOs or TMHs, they exhibit better electrochemical activity. In mixed oxide condition, robustness and electrical conductivity increase which in turn increases the electrochemical activity of the pseudocapacitor. Again, carbonaceous materials are 2D in nature with high electrical conductivity which favors the TMOs or TMHs for better electrical conductivity and high specific surface area. In the case of carbonaceous composite it has been observed that electrochemical activity depends on the proportion of composition of the composite. At a particular mass loading of NPs on the carbonaceous material, the composite exhibits the best electrochemical activity. Electrochemical activity also depends on the morphology of the TMOs and TMHs. Metal oxides with porous structure offer high specific surface area and large pore volume which diminish the diffusive resistance of the electrode and facilitate the ion transportation through the electrode, enhancing the rate as well as the cyclic stability of the electrode material. Among the different morphologies, 2D nanostructures of TMOs and TMHs are very important for pseudocapacitor application. 2D nanostructures participate in the electrochemical activity through their whole body participation. On the other hand, the ultrathin wall of the material facilitates the fast ion transportation. These two factors combined increase the specific capacitance value as well as cyclic durability of the electrode material. Since the noble metals have high conductivity and good electrochemical stability, their hybridization with TMOs/TMHs could facilitate the transport of electrons arising from the oxidation-reduction of pseudo-capacitors to the current collectors, thus enhancing the specific capacitance values and cycling stability. From the above discussion it can be concluded that TMOs, TMHs, and their hybrids with noble metals have proven to be exceptional pseudocapacitive materials.

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[75] G.H. Jeong, H.M. Lee, H. Lee, C.K. Kim, Y. Piao, J.H. Lee, et al., One-pot synthesis of thin Co(OH)2 nanosheets on graphene and their high activity as a capacitor electrode, RSC Adv. 4 (2014) 51619. [76] W. Fu, Y. Wang, W. Han, Z. Zhang, H. Zha, E. Xie, Construction of hierarchical ZnCo2O4@NixCo2x(OH)6x core/shell nanowire arrays for high-performance supercapacitors, J. Mater. Chem. A 4 (2016) 173. [77] Y. Zhai, H. Mao, P. Liu, X. Ren, L. Xu, Y. Qian, Facile fabrication of hierarchical porous rose-like NiCo2O4 nanoflake/MnCo2O4 nanoparticle composites with enhanced electrochemical performance for energy storage, J. Mater. Chem. A 3 (2015) 16142. [78] Z. Gaoa, W. Yangb, J. Wang, N. Song, X. Li, Flexible all-solid-state hierarchical NiCo2O4/porous grapheme paper asymmetric supercapacitors with an exceptional combination of electrochemical properties, Nano Energy 13 (2015) 306. [79] J. Liang, Z. Fan, S. Chen, S. Ding, G. Yang, Hierarchical NiCo2O4 nanosheets@halloysite nanotubes with ultrahigh capacitance and long cycle stability as electrochemical pseudocapacitor materials, Chem. Mater. 26 (2014) 4354. [80] D.H. Nagaraju, Q. Wang, P. Beaujuge, H.N. Alshareef, Two-dimensional heterostructures of V2O5 and reduced graphene oxide as electrodes for high energy density asymmetric supercapacitors, J. Mater. Chem. A 2 (2014) 17146. [81] Y. Wang, I. Zhitomirsky, Cathodic electrodeposition of Ag-doped manganese dioxide films for electrodes of electrochemical supercapacitors, Mater. Lett. 65 (2011) 1759. [82] R. Li, X. Liu, H. Wang, Y. Wu, Z.P. Lu, Development of electrochemical supercapacitors with uniform nanoporous silver network, Electrochim. Acta 182 (2015) 224. [83] G. Zhang, L. Zheng, M. Zhang, S. Guo, Z. Liu, Z. Yang, et al., Preparation of Ag-nanoparticle-loaded MnO2 nanosheets and their capacitance behavior, Energy Fuels 26 (2012) 618. [84] H. Xia, C. Hong, X. Shi, B. Li, G. Yuan, Q. Yao, et al., Hierarchical heterostructures of Ag nanoparticles decorated MnO2 nanowires as promising electrodes for supercapacitors, J. Mater. Chem. A 3 (2015) 1216. [85] M. Sawangphruk, S. Pinitsoontorn, J. Limtrakul, Surfactant-assisted electro deposition and improved electrochemical capacitance of silver-doped manganese oxide pseudocapacitor electrodes, J. Solid State Electrochem. 16 (2012) 2623. [86] J.B. Wu, Z.G. Li, Y. Lin, Porous NiO/Ag composite film for electrochemical capacitor application, Electrochim. Acta 56 (2011) 2116. [87] J. Huang, H. Wu, D. Cao, G. Wang, Influence of Ag doped CuO nanosheet arrays on electrochemical behaviors for supercapacitors, Electrochim. Acta 75 (2012) 208. [88] X. Wang, P. Zhang, S. Vongehr, S. Tang, Y. Wang, X. Meng, Large-scale fabrication of porous bulk silver thin sheets with tunable porosity for high-performance binder-free supercapacitor electrodes, RSC Adv. 5 (2015) 45194. [89] D. Ghosh, S. Giri, A. Mandal, C.K. Das, Graphene decorated with Ni(OH)2 and Ag deposited Ni (OH)2stacked nanoplate for supercapacitor application, Chem. Phys. Lett. 573 (2013) 41. [90] W. Lan, Y. Sun, Y. Chen, J. Wang, G. Tang, W. Dou, et al., Ultralight and flexible supercapacitor electrodes made from Ni(OH)2 nanosheets doped with Ag nanoparticle/3D graphene composite, RSC Adv. 5 (2015) 20878. [91] D. Yuming, T. Shaochun, W. Xiaoyu, H. Xiang, Z. Chao, H. Zusheng, et al., MnO2Au composite electrodes for supercapacitors, Chem. Lett. 43 (2014) 122. [92] L. Khandare, S. Terdale, Gold nanoparticles decorated MnO2 nanowires for high performance supercapacitor, Appl. Surf. Sci. 418 (2017) 22. [93] J. Kang, A. Hirata, L. Kang, X. Zhang, Y. Hou, L. Chen, et al., Enhanced supercapacitor performance of MnO2 by atomic doping, Angew. Chem., Int. Ed. 52 (2013) 1664. [94] B. Qu, L. Hu, Y. Chen, C. Li, Q. Li, Y. Wang, W. Wei, L. Chen, T. Wang, Rational design of AuNiO hierarchical structures with enhanced rate performance for supercapacitors, J. Mater. Chem. A 1 (2013) 7023. [95] S. Liu, S. Guo, S. Sun, X.Z. You, Dumbbell-like Au-Fe3O4 nanoparticles: a new nanostructure for supercapacitors, Nanoscale 7 (2015) 4890. [96] Y.-L. Chen, P.-C. Chen, T.-L. Chen, C.-Y. Lee, H.-T. Chiu, Nanosized MnO2 spines on Au stems for high-performance flexible supercapacitor electrodes, J. Mater. Chem. A 1 (2013) 13301. [97] S. Kim, P. Thiyagarajan, J. Jang, Great improvement in pseudocapacitor properties of nickel hydroxide via simple gold deposition, Nanoscale 6 (2014) 11646.

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[98] H. Xia, D. Zhu, Z. Luo, Y. Yu, X. Shi, G. Yuan, et al., Hierarchically structured Co3O4@Pt@MnO2 nanowire arrays for high-performance supercapacitors, Sci. Rep. 3 (2013) 2978. [99] Y. Lee, G. An, H. Ahn, Comparison of electrodes for high-performance electrochemical capacitors: multilayer MnO2/Pt and composite MnO2/Pt on carbon nanofibres, J. Nanosci. Nanotechnol. 15 (2015) 8931. [100] L. Wen, Y. Mi, C. Wang, Y. Fang, F. Grote, H. Zhao, et al., Cost-effective atomic layer deposition synthesis of Pt nanotube arrays: application for high performance supercapacitor, Small 10 (2014) 3162. [101] Y. Li, H. Fu, Y. Zhang, Z. Wang, X. Li, Kirkendall effect induced one-step fabrication of tubular Ag/MnOx nanocomposites for supercapacitor application, J. Phys. Chem. C 118 (2014) 6604.

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20 Applications of Hybrid Nanoparticles in Biosensors: Simulation Studies Yuankai Tang1, Xiantong Yu1, Jianhua Xu1, Benjamin Audit2 and Sanjun Zhang1,3,4 1

State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, P.R. China 2Univ Lyon, Ens de Lyon, Univ Claude Bernard Lyon 1, CNRS, Laboratoire de Physique, Lyon, France 3Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, P.R. China 4NYU-ECNU Institute of Physics at NYU Shanghai, Shanghai, P.R. China

20.1 INTRODUCTION The development of nanotechnologies including observation, synthesis, and simulation methods is allowing the production and investigation of more and more nanoparticles with innovative and interesting properties. Hybrid nanoparticles are one category of these original nanoparticles, constituted of the combination of diverse materials, such as metal@metal, metal@MOs (metal oxides), metal@organics, and metal@inorganics particles [1]. They are discrete multicomponent structures of different kinds, such as coreshell, heterodimer, and heterowire particles [1]. Differences in compositions and morphologies result in unique optical and chemical properties, which explain the increasing interest of many researchers in hybrid nanoparticles. These nanoparticles have specific properties that can be useful in numerous applications [1]. Localized surface plasmon resonance (LSPR) [2], a phenomenon of collective electrons resonance, is the most significant of these properties. Many biosensors are based on LSPR [3], e.g., surface-enhanced Raman scattering (SERS) [410], metal-enhanced fluorescence [1115], and modified spontaneous emission [16], due to its sensitivity to the surrounding environment and electromagnetic field [3]. Furthermore, LSPR properties of hybrid nanoparticles are different from common nanoparticles and can result in novel phenomena, such

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00020-6

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as Fano resonance [17] and strong coupling [18]. Biosensors design can take advantage of these new properties. Numerical simulations are necessary to the understanding of experimental phenomena, providing direction for further investigations. They are essential for the study of hybrid nanoparticles characteristics and the design of new applications. The unified physical model of surface plasmonics was established in the 1960s, although LSPR phenomena have been observed thousands of years ago and the mathematical formulation of these phenomena was proposed more than one hundred years ago [2]. The classical analytical and numerical descriptions of LSPR are based on Maxwell’s equations [2]. They include Mie theory, FDTD method, discrete dipole approximation (DDA) method, and finite element method (FEM), which are focused on exploring optical and electromagnetic properties of nanoparticles. Semiclassical and quantum descriptions have also been developed to model these phenomena to account for quantum effects in materials’ properties. With the rapid advances in computing capacities and precision of numerical methods, the usefulness of simulation studies in the investigations of hybrid nanoparticle properties is continually increasing with time. In this chapter, we will first introduce the background of classical hybrid-nanoparticle modeling theory. Then, several numerical simulation methods will be presented, including the fundamental details of their implementation. Finally, we will present a selection of applications of these simulation methods to investigate the optical and electromagnetic properties of biosensors based on hybrid nanoparticle technology.

20.2 FUNDAMENTAL THEORY OF HYBRID NANOPARTICLES Structures and materials are the two main control parameters of the optical properties of hybrid nanoparticles. Thanks to the development of nanoparticle synthesis methods, there are a lot of achievable structures for hybrid nanoparticles, such as coreshells, rods, and prisms. In the same manner, the possible choices of materials for multicomponent systems is wide, including intermetallic (alloyed) nanoparticles, metalmetal hybrids (such as Ag@Au and Cu@Au) and metaldielectric hybrids (such as Au@SiO2 and Ag@TiO2) [1]. In general, hybrid nanoparticles’ optical properties depend on their unique LSPR characteristics, which are caused by collective electrons resonance driven by incident electromagnetic waves. Therefore, investigation of hybrid nanoparticles’ LSPR is a key point to understand why different structures and materials modify the optical properties of hybrid nanoparticles.

20.2.1 Maxwell’s Equations in Matter and Dielectric Constants The classical description of hybrid nanoparticles’ LSPR is based on Maxwell’s equations for classical electromagnetism, which describe the interaction between light and matter. In this description, electromagnetic waves are used to represent light, and the dielectric constants (the refractive indexes) account for the material properties. Maxwell’s equations in matter can be written as:



r D 5 ρf

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(20.1)

20.2 FUNDAMENTAL THEORY OF HYBRID NANOPARTICLES

r3E52



@B @t

r B50 r 3 H 5 jf 1

433 (20.2) (20.3)

@D @t

(20.4)

where E, D, H, and B are the electric field, the dielectric displacement, the magnetic induction, and the magnetic field, respectively. In addition, ρf is the free charge density and jf are the free current densities. This formalization is very common in electrodynamics textbooks, so we do not discuss it further here. The main difference between the Maxwell’s equations in matter and in vacuum is the presence of the D and H fields, which represent the interaction between light and matter by: D 5 εε0 E H5

(20.5)

1 B: μμ0

(20.6)

ε0 and μ0 are the electric permittivity and magnetic permeability of vacuum. ε is the dielectric constant, and μ is the relative permeability of the material which is equal to 1 in nonmagnetic media. These two parameters describe the response of matter to light and are the key points to study the LSPR of hybrid nanoparticles. Because LSPR depends on the electric field of light, we focus on the dielectric constant ε in the following. The dielectric constant ε describes the response of a material to the electric field. This interaction between light and the material is due to the electrons’ response to the electric field. In general, εðωÞ 5 εr ðωÞ 1 iεi ðωÞ

(20.7)

where εr ðωÞ and εi ðωÞ are the real and the imaginary part of the dielectric constant for electromagnetic field at angular pffiffiffi frequency ω. The link between the dielectric constant and the refractive index is n 5 ε. Different materials have different properties, so there are a lot of models for the dielectric constant. Drude model is a familiar and simple method to describe the dielectric constant of ideal free-electron metal, which treats electrons in metal as a free electron gas. In a realistic metal, however, a dielectric constant term εN should be added to represent the interaction between electrons and background atomic cores at high frequency [2,19]. The resulting equation for the dielectric constant can be written as: εðωÞ 5 εN 2

ω2p ω2 1 iγω

(20.8)

where ωp is the plasmon frequency and γ is the characteristic collision frequency. γ 5 1=τ, where τ is the relaxation time of the free electron gas. For instance, we use this model to fit the dielectric constant of gold and silver, experimentally obtained by Yakubovsky et al. [20] and McPeak et al. [21], respectively (Fig. 20.1). For gold, ωp , γ, and εN are 9.141 eV, 0.1278 eV, and 8.279, respectively. For the silver, ωp , γ, and εN are 9.419 eV, 0.0537 eV, and 4.819, respectively. The resulting model dielectric constant curves match the experimental curves well (Fig. 20.1).

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FIGURE 20.1 (A) Experimental and fitted dielectric functions of gold. (B) Experimental and fitted dielectric functions of silver.

FIGURE 20.2

The dispersion curves for gold and silver.

From the model (Eq. (20.8)), we can obtain the dispersion relation of the metallic materials easily: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi 2 2 ω εðωÞ ω εN 2 ðωp =ω 2 iγωÞ (20.9) 5 k5 c c where k is the wave vector, c is the velocity of light in vacuum. Eq. (20.9) describes the relation between the wave vector and the frequency of the electromagnetic wave in metallic materials whose dielectric constant can be described by Eq. (20.8), such as gold and silver. It implies that if ωcγ the electromagnetic wave can only propagate when pffiffiffiffiffiffiffi pffiffiffiffiffiffiffi ωp =ω , εN [2,19]. When ωcωp , the dispersion (Eq. (20.9)) reduces to k 5 ω εN =c. The dispersion curves for gold and silver are shown in Fig. 20.2. A better description of metal dielectric constants in wider range of wavelength can be obtained adding Lorentz oscillator model to Eq. (20.8) [2,19]. Lorentz oscillator model can be written as: 1 εðωÞ 5 A 2 (20.10) 2 ω0 2 ω 2 iγ 0 ω where A is a constant related to intrinsic properties of materials, ω0 and γ0 are the resonance frequency and linewidth of the Lorentz oscillator. Lorentz oscillator model can also describe some exciton materials, such as dye molecule and organic semiconductor [2223]. II. APPLICATIONS

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In brief, dielectric constants characterize the optical response of materials to an incoming electromagnetic wave. Although, as we have just illustrated, there are many physical models to describe dielectric constants behavior as a function of the wavelength of the electromagnetic field, these models cannot satisfactorily capture the full diversity of materials’ responses. Hence, several empirical methods have been developed in order to numerically fit and represent dielectric constant curves, including polynomial fitting and interpolation of experimentally measured data points.

20.2.2 Fundamental Theory of LSPR and Mie Theory Due to their subwavelength size, nanoparticles are unable to support propagative surface plasmon resonance. The collective electrons oscillation driven by and coupled to electromagnetic field are thus LSPR [2] (Fig. 20.3). Near-field enhancement can be caused by LSPR resulting in high sensitivity to surrounding environment showing up as an extinction peak at the plasmon resonance frequency. When the nanoparticle structure has a spherical symmetry, LSPR properties can be fully accounted for by Mie theory [24]. Gustav Mie established Mie theory in 1908 based on Maxwell’s equations. It describes light scattering by spherical particles very well, although the physical concept of plasmonics was not clear at that time [2,2425]. According to Mie theory, the optical cross-sections— the scattering, extinction, and absorption cross-sections—of homogeneous spherical nanoparticles are Csca 5

N 2π X ð2n 1 1Þðjan j2 1 jbn j2 Þ; k2 n51

(20.11)

Cext 5

N 2π X ð2n 1 1ÞReðan 1 bn Þ k2 n51

(20.12)

Cabs 5 Cext 2 Csca

FIGURE 20.3 Sketch of localized surface plasmon resonance.

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(20.13)

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respectively, where an and bn are related to the decomposition of the scattered electromagnetic field on the spherical vector harmonics of order n, and k is the wave vector in the surrounding of the nanoparticles. an and bn can be written as [24]:   0     0  mψn my ψn y 2 ψn my ψn y        an 5 (20.14) 0  mψn my χ0n y 2 ψn my χn y and

  0     0  ψn my ψn y 2 mψn my ψn y    bn 5   0   (20.15) 0  ψn my χn y 2 mψn my χn y pffiffiffiffiffiffi pffiffiffiffi where ψn ; χn are the RiccatiBessel functions, m 5 εm = εs , where εm and εs are dielectric constants of nanoparticles and surrounding respectively, and y 5 km r, where km is the wave vector in media of nanoparticles and r is radius of nanoparticles. In addition, the prime denotes derivation of the function. In these expansions, n 5 1 corresponds to the dipole term, n 5 2 the quadrupole, n 5 3 the octupole, and so on. In the simple case where y{1, only order y3 has to be considered and Eqs. (20.14) and (20.15) can be simplified to [24]: a1 

i2ð1 2 εm =εs Þy3 3ð2 1 εm =εs Þ b1  0:

(20.16) (20.17)

We take Au and Ag spherical nanoparticles as illustrative examples of using the Mie theory. A gold nanoparticle with 30 nm radius in water has the optical cross-sections shown in Fig. 20.4A. The peaks of absorption, scattering, and extinction cross-sections are all near 530 nm, which means its LSPR frequency is near 2.34 eV (B530 nm). In contrast, the optical cross-sections’ peaks of a silver nanoparticle of the same size in waterare at 415 nm corresponding to a LSPR frequency of 2.99 eV (Fig. 20.4B). In addition, the ratio

FIGURE 20.4 Absorption, scattering, and extinction cross-sections in water of (A) Au spherical nanoparticles with 30 nm radius; (B) Ag spherical nanoparticles with 30 nm radius. Optical cross-sections were calculated with the Mie theory using the dielectric constant models for gold and silver determined above (Fig. 20.1). Note that these optical cross-sections match experimental spectroscopy measurement very well.

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between scattering and absorption at the resonance frequency, and the amplitudes of cross-sections are different for the two metals. Furthermore, it is also apparent in Fig. 20.4 that the quadrupole of silver nanoparticles is distinct from the gold quadrupole, which has a resonance near 379 nm. The cause of the differences between the LSPR of these two nanoparticles of the same size is their dissimilar dielectric constant (Fig. 20.1). It illustrates that dielectric constants of materials have a major effect on their LSPR properties.

20.3 SIMULATION METHODS 20.3.1 Generalized Mie Theory Although Mie theory is good to simulate the optical properties of spherical nanoparticles, it cannot deal with a multilayer coreshell sphere, which is a very common structure of hybrid nanoparticles. There are several methods to solve this problem. One of them is the generalized Mie theory, which was initially developed by Bhandari et al. in the 1980s. Subsequently, Sinzig and Quiten [26] simplified this generalized Mie theory and transferred the method from a matrix formalism to a recursive formalism that is more convenient for numerical computation. Therefore, we present here the latter one. According to the conclusion of Sinzig and Quiten [26], for the coresphere with r 2 1 shells as shown in Fig. 20.5, the coefficients of the transverse magnetic (TM) modes an can be expressed as   0         0  0  mr ψn mr yr ψn yr 1 Tnr21 χn yr 2 ψn mr yr ψn yr 1 Tnr21 χn yr   0    

      an 5 2 (20.18) 0 mr ψn mr yr ψn yr 1 Tnr21 χ0n yr 2 ψn mr yr ψn yr 1 Tnr21 χn yr and the coefficients of the transverse electric (TE) modes bn can be expressed as   0         0  0  2 mr ψn mr yr ψn yr 1 Sr21 ψn mr yr ψn yr 1 Sr21 n χn yr n χn yr  0          bn 5 2  0  0 2 mr ξn mr yr ψn yr 1 Sr21 ξn mr yr ψn yr 1 Sr21 n χn yr n χn yr

(20.19)

where n is the order of the Bessel functions, r 2 1 is the number of shells, ψn ; χn andξ n are RiccatiBessel functions ψn ðzÞ 5 zjn ðzÞ; χn 5 zyn ðzÞ, ξn 5 zhð1Þ n ðzÞ, where jn and yn are the first and second kind of spherical Bessel functions, respectively, and hð1Þ n are the first kind of spherical Hankel functions, and the prime denotes derivation of the function. Finally, coefficients Tnr21 and Sr21 can be computed recursively using the following expressions [26]: n   0     0  m1 ψn m1 y1 ψn y1 2 ψn m1 y1 ψn y1 1   0      (20.20) Tn 5 2 m1 χn m1 y1 ψn y1 2 χ0n m1 y1 ψn y1   0         0  0  ms ψn ms ys ψn ys 1 Tns21 χn ys 2 ψn ms ys ψn ys 1 Tns21 χn ys s   0           (20.21) Tn 5 2 ms χn ms ys ψn ys 1 Tns21 χ0n ys 2 χ0n ms ys ψn ys 1 Tns21 χn ys   0     0  ψn m1 y1 ψn y1 2 m1 ψn m1 y1 ψn y1 1  0      (20.22) Sn 5 2  χn m1 y1 ψn y1 2 m1 χ0n m1 y1 ψn y1

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Ssn

  0         0  0  ψn ms ys ψn ys 1 Ss21 2 ms ψn ms ys ψn ys 1 Ss21 n χn ys n χn ys  0           52  0 χn ms ys ψn ys 1 Ss21 2 ms χ0n ms ys ψn ys 1 Ss21 n χn ys n χn ys

(20.23)

where ns11 ns

(20.24)

ys 5 ks Rs

(20.25)

ms 5

s 5 2; 3; ?; r is the layer index, ns is the refractive index of the sth layer, when s 5 r 1 1, ns is the refractive index of surrounding, ks is the wave vector of the sth layer and Rs is the radius of the sth layer. If the incident light is unpolarized, the cross-sections of scattering, extinction, and absorption are obtained similarly to Mie theory using Eqs. (20.11)(20.13). The crosssections are the sum of infinite series from Bessel function. However, n cannot be calculated to infinity due to the finite computing power. Therefore a suitable maximal order n should be chosen that ensures calculation convergence [17]. Higher eigenmodes become more and more important with increasing size of nanoparticle. For example, when the radius of Ag sphere is less than 20 nm, the quadrupole has almost no contribution to the extinction cross-section of the sphere. However, when the radius is more than 20 nm, the quadrupole has to be taken into account to compute the extinction spectrum, and when radius is larger than 60 nm, the octupole becomes significant. In general, the larger the nanoparticles, the larger the number of meaningful terms in Eqs. (20.11)(20.13). This generalized Mie theory is a suitable and general way to calculate optical crosssections of hybrid nanoparticle with coreshell structure [2735], including the specific phenomena based on LSPR, such as strong coupling [3638] and Spasers [3940]. In addition to this generalized Mie theory, there are many other analytical methods extending Mie theories [41], which are able to deal with spheroids [4244], cylinders [4546], and aggregate of spheres [4750]. Note that Mie theory can also address far-field scattering problems dependent on scattering direction.

FIGURE 20.5

Schematic of coreshell structure.

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20.3.2 Finite-Difference Time-Domain Method Analytical methods such as the ones discussed above only apply to regular and simple geometries, but are not suitable for arbitrary complex structures. Nevertheless, the study of optical properties of irregular nanostructures can be achieved using (fully) numerical methods. FDTD method is one of the most popular such numerical methods used in plasmonic numerical studies. It is very powerful as it allows simulating most of the important optical properties such as the optical cross-sections, the transmission and reflection coefficients, the near-field properties, the field maps in the frequency domain, and the time-resolved spectroscopy spectra [5152]. There are many mature and widely used commercial products implementing FDTD, such as Lumerical FDTD Solution and OptiFDTD. FDTD method was established by Yee in 1966 [53]. The elementary unit of FDTD is Yee’s cell as shown in Fig. 20.6, which describes the lattice for discretization of the electromagnetic field in space. Maxwell’s equations are then solved in time on the lattice. It results in the numerical simulation of electromagnetic wave propagation on discretized space and time. Optical response in the frequency domain can then be obtained using Fourier transform. In short, Faraday’s law (Eq. (20.2)) and Ampere’s law (Eq. (20.4)) are solved using Taylor expansions for the derivatives, for the components of the electromagnetic field shown in Fig. 20.6 [54]. In the following, we further describe the implementation of the 2D FDTD in order to convey the principles of the methodology. In this case, electric field has two components, Ex and Ey, and magnetic field has one component Bz, it corresponds to using an horizontal cut of the 3D Yee’s cell in Fig. 20.6 (Fig. 20.7). According to Maxwell’s equations, the evolution of the electromagnetic field on Yee’s cells for time steps of size τ indexed by n, can be written as [54]:   τ @Hzn n11=2 n21=2 n 2 Jx 5 Ex 1 Ex (20.26) ε @y

FIGURE 20.6

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Schema of Yee’s cell in 3D FDTD.

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FIGURE 20.7

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Schema of Yee’s cell in 2D FDTD.

  τ @Hzn 2 2 Jxn ε @y " # n11=2 n11=2 τ @Ez @Ex n11 n 2 Hz 5 Hz 2 ε @x @y Eyn11=2 5 Eyn21=2 1

(20.27) (20.28)

From Figs. 20.6 and 20.7, we can see that each electric and magnetic field components are surrounded by the component they depend on through the curl operator [19]: the electric and magnetic field component sublattices are shifted by half the mesh size of Yee’s cell. The electric components are calculated at integer times (nτ) alternately with the magnetic components at intermediate times (ðn 1 1=2Þτ). From Eqs. (20.7) and (20.8), we can see that electric component and magnetic components interaction is described by the dielectric constant ε following the principles of Maxwell’s equations. Furthermore, the Yee’s cell will affect the adjacent Yee’s cells in the calculation, which is the key point of FDTD. Yee’s cells provide the mean to link the electromagnetic field in discretized space and discretized time. When the electromagnetic field components are calculated for the last Yee’s cell, the distribution of electromagnetic field in time and space can be analyzed. The mesh size and time step are dependent on each other. It means that when one of them is fixed, the range of another is limited, which should meet the steady condition [51]:  2 21 c c2 c2 2 1 1 (20.29) τ , Δx2 Δy2 Δz2 The mesh size should be fine enough to ensure that the discretized structure represents well all the important details of the actual continuous one; it is usually chosen smaller than 5 nm [51]. In FDTD calculation, the structures, dielectric constant of materials, field sources, mesh size, and boundary conditions need to be considered and inputted into the numerical software. There are many kinds of field sources, such as plane wave, Gaussian wave-packet, dipole sources, and sum of Bloch’s waves. The choice of field sources is dependent on the objectives of the simulation. Furthermore, the parameters of boundary conditions are very important in FDTD simulations. The simulation space size cannot be infinite, due to

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441

limited computing resource. However, many problems need to be simulated in open space, such as scattering spectra of nanoparticles. Therefore, special boundary conditions are required to solve these problems, namely absorbing boundary conditions (ABCs) [55]. The perfectly matched layer (PML) is one of the most robust ABCs that was introduced by Berenguer [56]. PML is a special medium layer covering the simulation space with designed parameters to create a wave-impedance matching condition, which is independent of the angles and frequencies of the wave incident on this boundary [55]. FDTD is a very useful method to simulate electromagnetic field propagation and its interactions with matter. FDTD has been successfully used to deal with a lot of problems and applications, such as scattering, microwave circuits, waveguides, fiber optics, antennas, medical applications, shielding, coupling, and electromagnetic compatibility [55]. In addition, FDTD has a lot of benefits: it can solve Maxwell’s equations for structures made of different materials and for diverse light sources; it is easy to derive the optical properties of a system from FDTD results; it is computationally effective so that it requires very reasonable computer resources [51,55,57].

20.3.3 Discrete Dipole Approximation DDA is another very popular and flexible numerical method to simulate LSPR of nanoparticles including nonspherical nanoparticles. It was introduced by Purcell and Pennypacker [58] in 1973. DDA focuses on the computations of scattering and absorption of electromagnetic plane waves by irregular particles and periodic arrangements. Compared with FDTD, DDA has the advantages to calculate optical properties and energy transferring processes. There are many DDA open source codes, such as DDSCAT [5961], ADDA [6263], and OpenDDA [64]. In this method, a particle (irregular structure) is divided into N smaller particles, small enough to be treated as a radiative dipole [41,54,59]. It is the elemental idea of DDA. For example, a material sphere can be divided into 365 or 33059 discrete cubic volume elements (Fig. 20.8) [65]. For the small volume to be treated as a dipole, it is required that the following condition be met: 2πna {2π λ

(20.30)

4πχa {1 λ

(20.31)

where n is the refractive index, a is the characteristic dimension of the element, and χ is the extinction index of the material. Each dipole is described by its polarizability αi : [41,54,59]: Pi 5 αi Et;i Et;i 5 Einc;i 2

N X



Aij Pj ;

i 5 1; 2; 3; . . .; N

j51;j6¼i

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FIGURE 20.8 Sphere divided into (A) 365 and (B) 33059 discrete cubic volume elements, respectively. Source: Reprinted with permission from F. Enguehard, Mie theory and the discrete dipole approximation. Calculating radiative properties of particulate media, with application to nanostructured materials, Therm. Nanosyst. Nanomater. (2009) 151212. Copyright 2009 Springer Nature.

where the electric field Et;i is the sum of the incident electric field Einc;i and the retarded contribution from the N 2 1 other dipoles. Aij is the element in the dipole interaction matrix:



Aij Pj 5 k2 eikrij



½r2ij Pj 2 3rij ðrij Pj Þ rij 3 ðrij 3 Pj Þ ikrij 1 e ð1 2 ikr Þ ; ij r3ij r5ij

i 5 1; 2; 3; . . .; N; j 5

1; 2; 3; . . .; N; and i 6¼ j (20.33) where k 5 2π=λ and rij is the vector from dipole i to dipole j. Once Et;i and Pj are solved, the extinction and absorption cross-sections can be calculated from Poynting’s law [41,54,59]: N 4πk X ImðEinc;j Pj Þ jEinc j2 j51

(20.34)

N 4πk X 2 3 2  ImðPj ðα21 j Þ Pj Þ 2 k jPj j : 2 3 jEinc j j51

(20.35)

Cext 5

Cabs 5







The scatter cross-section is Csca 5 Cext 2 Cabs , which is the same as in Mie theory. Before calculating DDA, expressions of polarizability need to be determined. There are several formulations used to model polarizability, such as ClausiusMossotti formulation, Draine formulation, and Doyle formulation [65]. Simultaneously, the size of element volume, the incident wavelength, and dielectric constant of materials need to be inputted into the DDA calculating program. Discrete dipole approximation is a very useful and powerful method to deal with the electromagnetic wave absorption and scattering by particles up to the micron scale [41].

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The particles need to be divided into enough elementary volumes, depending on both incident wavelength and material properties. If the size of particle is too large, the required number of elementary volumes results in computational costs exceeding the available computing resources. DDA only includes dipoledipole coupling and is thus unable to deal with multipolar resonances. Hence, DDA is suitable for small nanoparticles whose main resonance is dipolar and multipole contributions can be neglected. It results that in practice, its computational accuracy is generally lower than other simulation methods, such as Mie theory and FDTD.

20.3.4 Finite Element Methods FEMs are another category of methods to solve problems that can be expressed with partial differential equations with boundary conditions. They are very suitable methods to resolve the electromagnetic response of nanoparticles. Applications of FEM to hybrid nanoparticles focus on LSPR properties by solving the inhomogeneous vector wave equation [54]:   1 r 3 E 2 k20 εr E 5 0: (20.36) r3 μr FEMs allow computing the solution of such problems over a set of discrete locations after decomposition of the geometry into elementary volumes such as tetrahedrons. Thanks to the flexibility in the choice of these volume elements, the discretization can be adapted to complex geometries by covering the domain of analysis with more elements of smaller volumes in regions having finer structural details. This results in the ability of FEM to deal with electromagnetic properties of more complex geometry than FDTD and DDA [54,6667].

20.4 APPLICATIONS In the above section, we introduced four simulation methods of nanoparticles, Mie theory, FDTD, DDA, and FEM. These simulation methods are widely used in investigations of nanoparticles to support experimental results, to explain experimental phenomena, and to understand physical processes. They can also be used in theoretical research to predict novel phenomena and to design new experimental setups to test them. Here we review several papers on nanoparticles including simulations of plasmonics as examples to illustrate the use of the above methods to simulate the optical properties of nanoparticles.

20.4.1 Mie Theory Mie theory is used widely in electromagnetic simulations of spherical nanostructures [6876]. Here we take several detailed examples. In Tang et al. [77], the properties of nanoparticle’s LSPR are simulated by generalized Mie theory [26], which was coded in Matlab. Based on the optical properties of plasmonexciton nanoparticles, they designed the principle of novel ratiometric sensor. First, they compared optical cross-sections of

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silver sphere and SiO2@Ag by simulation. It shows that the full width at half maximum (FWHM) of SiO2@Ag nanoparticle is narrower, the intensity of LSPR of SiO2@Ag nanoparticle is stronger, and SiO2@Ag nanoparticle is able to suppress the appearance of the quadrupole. Subsequently, a J-aggregated cyanine dye TDBC is used as active material to form an exciton because of its narrow absorption band and strong enough oscillator strength. Lorentz mode is used to describe the permittivity of TDBC in generalized Mie theory, and the imaginary permittivity shows that the absorption frequency of TDBC is 590 nm, which corresponds to a quantum two-level system. The strong coupling based on TDBC-core and silver-shell nanoparticles in water is realized in simulation with obvious Rabbi splitting, near 110 meV. The simulation shows that the intensities of two splitting peaks change inversely with the change of size of nanoparticle due to the interaction between LSPR and emitter. The frequencies of the two splitting peaks depend on both frequency of LSPR and the absorption frequency of TDBC [78], which are expressed by qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 ω 6 5 ðωpl 1ω0 Þ 6 g2 1 δ4 =4 (20.38) 2 where ω1 is the high frequency of splitting peak, ω2 is the low one, ωpl is frequency of LSPR, ω0 is the frequency of TDBC exciton, g is the coupling rate, and δ is the detuning. The article assumes that the local refractive index near the nanoparticle will change when molecules approach the nanoparticle or due to molecular interactions near the nanoparticle surface. Therefore strong coupling of the nanoparticle is able to detect approaching molecules or the nearby molecular interactions. These molecules are treated as a 2-nm-thickness shell around the nanoparticle, and generalized Mie theory discussed above is used to calculate their optical cross-sections. As the insert in Fig. 20.9A shows, a TDBC(r 5 20.5 nm)@Ag (r 5 25.5 nm) nanoparticle is used as a sensor here because the splitting peaks are of similar amplitude in water, which is very sensitive to the change of local refractive index. An external layer of refractive index ns models changes in the local environment. The extinction cross-sections of TDBC(r 5 20.5 nm)@Ag(r 5 25.5 nm) nanoparticle with the local refractive index ns 5 1:033; 1:333; 1:633 are shown in Fig. 20.9A. Their frequencies of splitting peaks have red shifts with the increasing local refractive index as shown in Fig. 20.9C. As the intensity of splitting peaks is a function of ns , shown in Fig. 20.9B, the intensity of ω2 is decreasing with increasing ns . Conversely, the intensity of ω1 is increasing with increasing ns . When the local refractive index changes from 1.033 to 2.033, the intensity of ω1 decreases to half and the intensity of ω2 increases more than twice. The ratio of the intensity of ω2 and ω1 provides a feasible way to quantify the local refractive index, as shown in Fig. 20.9D, which is not affected by the absolute intensity of single peak. In addition to optical cross-sections, Mie theory can be used in calculations of angular radiation properties. The optical properties of goldsilicagold multilayer nanoshells, including radiation patterns, were studied by Mie theory [8081] in water by Hu et al. [79]. In this article, incident wave is plane, the particles are rigidly spherical, and the layers are concentric. Gold’s dielectric constant is obtained from Cristy and Johnson [82], and the dielectric constant for silica was set to 2.04. The corrections of size are ignored. The optical properties of goldsilicagold multilayer nanoshells (MNS) and goldsilica conventional nanoshells (CNS) are compared. First, the tunable optical properties of MNS

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FIGURE 20.9 TDBC@Ag nanoparticle in water: (A) the extinction cross-sections of TDBC(r 5 20.5 nm)@Ag (r 5 25.5 nm) with different local refractive index; (B) the intensity of splitting peaks as a function of ns ; (C) the wavelength of splitting peaks as a function of ns ; (D) the ratio of intensity of ω2 and ω1 as a function of ns . ; (E) the schematic of measurement. Source: Reprinted with permission from Y. Tang, X. Yu, H. Pan, J. Chen, B. Audit, F. Argoul, et al., Numerical study of novel ratiometric sensors based on plasmonexciton coupling, Appl. Spectrosc. 2017. Copyright 2017 SAGE Publications.

are displayed by varying the size of gold core with the fixed size of the particle. A red shift shows in the spectra with a thinner silica layer by Mie theory. Subsequently, the refractive index of the surroundings changes to affect the LSPR response of MNS. Fig. 20.10C shows that this nanoparticle is sensitive to the surrounding media and could be used as a sensor. Finally, the angular radiation properties of the MNS and CNS are calculated by Mie theory, which shows the obvious difference between MNS and CNS in Fig. 20.10D. MNS angular radiation patterns are more complex than those of CNS due to the interactions of different resonance modes. CNS is mainly forward-scatter with incident wavelengths shorter than

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FIGURE 20.10

Sketch of (A) CNS; (B) MNS; (C) calculated extinction spectra of MNS immersed in various media with distinct refractive indices; (D) angular radiation pattern of R90/125/140 nm MNS (blue) and R125/ 140 nm CNS (green) at 550 nm, 755 nm, 1145 nm, and 1270 nm. Source: Reprinted with permission from Y. Hu, R.C. Fleming, R.A. Drezek, Optical properties of goldsilicagold multilayer nanoshells, Opt. Express 16 (24) (2008) 1957919591. Copyright 2008 Optical Society of America.

the LSPR wavelength, however MNS radiates more in the back and side directions with these incident waves. These theoretical researches are beneficial for future realistic applications.

20.4.2 Finite-Difference Time-Domain Method Many researchers use FDTD to simulate optical properties of nanostructures [10,8495] including irregular structures. The following shows two detailed examples. LSPR of Albased nanostructures was studied in both experiments and FDTD simulations by Knight et al. [83]. They showed the high-quality LSPR of Al and explained the discrepancies between the previous two simulations and experiments on LSPR of aluminum. Its LSPR sensitively depends on the presence of oxide within the bulk metal, which will change the surrounding refractive index dramatically. For Al nanodisks, Knight et al. observed that the wavelength of the LSPR is determined by the percentage of oxide present within the aluminum. FDTD simulations corroborated this understanding. A modified Drude model and Bruggeman model are used to represent dielectric functions of aluminum and composite Al/Al2O3, respectively, which match the data from experiments very well (Fig. 20.11D and E). First, the scattering spectra of Al nanodisks with different sizes from both experiential dark-field spectra and theoretical FDTD calculations are similar, as shown in Fig. 20.11A and B. Subsequently, the scattering spectra of

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FIGURE 20.11 (A) Experimental dark-field spectra of individual nanodisks with D 5 70, 80, 100, 120, 130, 150, 180 nm. (B) FDTD simulations of the nanodisk spectra, assuming a 3-nm surface oxide and a SiO2 substrate. (C) Scattering spectra of 100-nm diameter nanodisks with varying metal oxide fractions. The calculated spectra (solid lines) assume a 3-nm pure surface oxide and a SiO2 substrate. The experimental dark-field spectra (dotted lines, scaled for clarity) correspond to evaporations performed under exposure to varying trace levels of oxygen, producing 9% (green), 19% (blue), and 27% (orange) metal oxide content. (D) Ellipsometrically measured dielectric functions for the three deposited Al purities. (E) Bruggeman dielectric functions for Al oxide fractions of 0% (black), 9% (green), 19% (blue), 27% (orange), 40% (gray), and 50% (light gray). Source: Reprinted with permission from M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum for plasmonics, ACS Nano 8 (1) (2013) 834840. Copyright 2013 American Chemical Society.

nanoparticles with varying trace levels of Al2O3 were obtained from experimental darkfield spectra. The FDTD simulations reproduced the scattering spectra very well, as shown in Fig. 20.11C. It gave a reasonable explanation for the previous difference between the data from experiments and simulations, due to the neglect of oxide. In addition to optical cross-sections, FDTD is able to calculate electromagnetic field distribution, which is an important property in sensor research, especially in surfaceenhanced Raman scattering (SERS). Prinz et al. [9] created DNA origami nanostructure novel SERS substrates which have single-molecule SERS sensitivity. In this article, FDTD is used to calculate electromagnetic field of nanoparticle dimers. It shows that the enhancement factors of dimers are much higher than single nanoparticles, as shown in Fig. 20.12. In this work, Lumerical FDTD Solutions 8.6.3, which is a commercial software, was used to perform FDTD calculations. The parameters of light source in the simulations are set as follows. The wavelength of incident was assumed as 532 nm in all simulations. The nanoparticle dimer are illuminated with polarization along the axis of the dimer.

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FIGURE 20.12 FDTD calculations showing the distribution of the electromagnetic field enhancement that can be expected for a single 60-nm AuNP (top) as well as for a dimer consisting of two 60-nm AuNPs with a 2.5-nm silver shell (below). Source: Reprinted with permission from J. Prinz, C. Heck, L. Ellerik, V. Merk, I. Bald, DNA origami based AuAg-coreshell nanoparticle dimers with single-molecule SERS sensitivity, Nanoscale 8 (10) (2016) 56125620. Copyright 2016 The Royal Society of Chemistry.

20.4.3 Discrete Dipole Approximation DDA is similar to FDTD and able to calculate various electromagnetic properties of complexed nanostructures [7,97100]. The sensing efficiency and factor of gold nanoframes were studied by experiments and DDA in Mahmoud and El-Sayed [96]. The nanoframes with different size and thickness show different sensitivity factors due to different plasmon field strength. It shows that the sensitivity factors increase linearly with the aspect ratio (wall length/wall thickness) of the nanoframes, which are especially sensitive to a decrease in the wall thickness. Compared with other nanoparticles, it is found that nanoframes have sensitivity factors that are several times higher than other gold nanoparticles with other structures, such as gold nanospheres, gold nanocubes, and gold nanorods. In this article, DDA was used to calculate LPSR spectra and field enhancement, which matched previous experiment’s data well. First, sensitivity factors of synthesized nanoframes with different aspect ratios were detected in experiments. Second, the plasmonic fields of several nanoframes were calculated by DDA. Extinction, absorption, and scattering spectra of nanoframes of different size (Fig. 20.13A) show that a red shift in the spectrum occur with the wall thickness decrease, along with the increasing ratio of the scattered to the absorption intensity. In Fig. 20.13B, field enhancement contour maps show that the wavelength of nanoframes decreases with the intraframe distance at the same aspect ratio. Subsequently, sensitivity factors of the nanoframes were calculated by DDA. SPR spectra of nanoframes were obtained for different dielectric constants to

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FIGURE 20.13 (A) Left: Results of the DDA calculation of the extinction (black), absorption (red), and scattering (green) spectra of 50 nm AuNF with wall thicknesses of 9, 10, 11, and 12 nm. Right: Decrease of the SPR maximum peak position with increasing wall thickness for each studied wall length. (B) Field enhancement contour maps of AuNFs of the same aspect ratios with different wall lengths and wall thicknesses. (C) Relationship between the SPR peak position of AuNFs as calculated from the DDA method and the refractive indices of different surrounding solvents. Source: Reprinted with permission from M.A. Mahmoud, M.A. El-Sayed, Gold nanoframes: very high surface plasmon fields and excellent near-infrared sensors, J. Am. Chem. Soc. 132 (36) (2010) 1270412710. Copyright 2010 American Chemical Society.

determine their sensitivity factors. As shown in Fig. 20.13C, nanoframes of different size and different wall thickness have different sensitivity factors. It can be concluded that higher aspect ratio and thinner thickness make the sensor more sensitive. Finally, the DDA calculations were compared with experimental results, which agree with each other very well.

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20.4.4 Finite Element Methods FEM is used widely in simulations of complex nanostructures as well [8,102108]. Plasmonexciton coupling based on hybrid metal 2 organic nanoparticles was studied in Balci et al. [101]. The Rabi splitting based on plasmonexciton coupling can be changed from weak coupling to the ultrastrong coupling regime by a method. Its phenomena were observed in both experiments and FEM simulations. Furthermore, plasmonexciton coupling was studied by steady and time-resolved spectra, which shows the nature of the coupling. FEM was used to calculate the extinction spectra and local electric field enhancement of plexcitonic nanoparticles to investigate the properties of plasmonexciton coupling, as shown in Fig. 20.14. First, a silver nanoprism with an edge length of 54 nm and a thickness of 10 nm was put in air. The dielectric constants of the J-aggregate film were taken from previous studies [109110]. Extinction spectra of plexcitonic nanoparticles with different J-aggregate film thickness are shown in Fig. 20.14A. The Rabi splitting increases with the thickness of J-aggregate film, as shown in Fig. 20.14B. Subsequently, the local electric field of plexcitonic nanoparticles with different sizes were calculated. These simulation results agree with the experimental data very well.

FIGURE 20.14

Electrodynamics simulations of tunable plexcitonic nanoparticles. (A) Extinction spectra of a single Ag NP with varying J-aggregate film thickness computed using COMSOL. The J-aggregate film thickness ranges from 0.2 to 1.0 nm. The Rabi splitting energy, the separation between the polaritonic branches, increases with the dye film thickness. (B) Calculated Rabi splitting energy values from the extinction spectra in (A). (C) Distribution of the calculated electric field on the single plexcitonic nanoparticle for three different incident wavelengths: (1) 579 nm, (2) 595 nm, and (3) 635 nm. Source: Reprinted with permission from S. Balci, B. Kucukoz, O. Balci, A. Karatay, C. Kocabas, G. Yaglioglu, Tunable plexcitonic nanoparticles: a model system for studying plasmonexciton interaction from the weak to the ultrastrong coupling regime, ACS Photonics 3 (11) (2016) 20102016. Copyright 2016 American Chemical Society.

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20.5 SUMMARY AND OUTLOOK In this chapter, we review some simulation methods of hybrid nanoparticles biosensors, especially for the simulations of LSPR. First, the fundamental theories of plasmonics and hybrid nanoparticles are introduced, including the LSPR and description of dielectric constant. Second, generalized Mie theory, FDTD, DAA, and FEM, the four methods of LSPR simulation are presented briefly, including theories and concepts. Finally, we take several applications in previous research as examples of these simulating methods. With the development of research in hybrid nanoparticles biosensors, the simulations of properties of biosensors are becoming more and more important. Simulations are able to reliably predict the characteristics of biosensors and offer directions of biosensor designs. The power of computers is improving fast, which will make simulations play a more significant role in the investigations of hybrid nanoparticles biosensors.

Acknowledgments This work was partly supported by the National Science Foundation of China (11774096 and 11674101), the Science and Technology Commission of Shanghai Municipality (15520711500), and the 111 project (B12024). BA and SZ (corresponding authors) acknowledge support from Joint Research Institute for Science and Society (JoRISS).

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C H A P T E R

21 SERS Application of Noble MetalMetal Oxide Hybrid Nanoparticles Vipul Sharma, Ramachandran Balaji, Nisha Kumari and Venkata Krishnan School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Kamand, Mandi, Himachal Pradesh, India

21.1 INTRODUCTION 21.1.1 Background of Raman Spectroscopy The phenomenon of Raman scattering was first discovered by C.V. Raman in 1928, where he discovered that there is a new radiation from the vibrations of molecules [1]. The same phenomenon was also reported by Landsberg and Mandelstam in crystals in the same decade [2]. This phenomenon explains the interaction between light and the molecular vibrations in solids. Raman scattering can be used as a versatile and powerful analytical tool, which has applications in biomedical [3], biotechnology [4], forensics [5], chemical industry [6], and defense technology [7]. The phenomenon of Raman scattering can be explained by the electromagnetic theory in the simple terms. The vibrational polarizability of the analyte molecules in a given time harmonic oscillation electric field results in a dipole moment vibration which can be written as [8]: P 5 α0 E0 Cosð2πωL tÞ 1

@α0 1 E0 Q0 ½Cos 2πðωL 2ωL Þt 1 Cos 2πðωL 1ωL Þt 2 @Q

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00021-8

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In this equation, E0 is amplitude of electric field from incident light, ωL is the frequency of light wave which is incident over the molecule, α0 is the polarizability of the molecule which vibrates with a certain frequency ωM, and Q denotes the nuclear displacement of the molecule. The change in the frequency of scattered light is usually observed, which depends on the composition of the molecule. This phenomenon is called Stokes scattering if it changes to a longer wavelength ðωL 2ωL Þ, where a photon having a lower energy is emitted [9]. Similarly, this phenomenon is called anti-Stokes scattering if the frequency shifts to a shorter wavelength ðωL 1ωL Þ, where a photon with a larger energy is emitted [9]. When no frequency change in the scattered light is present, it is called Rayleigh scattering which corresponds to α0 E0 Cosð2πωL tÞ. A fingerprint signature in the Raman spectra is generated due to the vibrations in the chemical bonds present in the molecules which can be due to the bending or the stretching of these bonds when the incident light interacts with these bonds. The only drawback of the Raman scattering which limits its use in a variety of specialized applications is the very small scattering cross-section (several fold smaller than fluorescence). Due to this reason, a very weak signal is produced, which is sometimes very hard to detect and interpret. Surface-enhanced Raman scattering (SERS) spectroscopy is a powerful analytical technique that is capable of enhancing the weak signals generated during the Raman scattering [10]. This enables us to detect the analytes molecules up to single molecule level and a direct molecule specific information can be retrieved. The first discovery of the SERS was inspired by the reports of Fleischmann and Hill [11] in 1974, where they published the Raman spectra of the pyridine that was adsorbed on a roughened Ag surface. The high intensity of the pyridine spectra in their studies inspired Van Duyne and Jeanmaire to explore more about the origin of the high-intensity SERS spectra obtained in that study [12]. Subsequently, after more detailed investigation, they proposed that such drastic enhancement of the signals was caused by the electrochemical interfacial field gradient and later on they proposed the “electromagnetic theory” of the SERS effect [13].

21.1.2 Mechanism of Surface Enhanced Raman Scattering There was a major debate for decades on the mechanism of the SERS enhancement and now it has been accepted that the major contributor of the SERS phenomenon is the electromagnetic enhancement mechanism [14]. The enhancement of the signals is due to the amplification of the light energy by the excitations of the localized surface plasmon resonances (LSPR). It has been reported in several studies that SERS originates from the intensity anomaly [15]. This takes place when the sum of the dipole induced in the adsorbed molecule is added to its image in the metal and in the limit of zero separation between the two. Later on, Moskovits proposed that a localization and amplification of the incident light by LSPR of noble metals are responsible for the drastic amplification of the Raman signals [16]. One major thing to be noted is that this signal amplification preferentially occurs in the gaps, cavities, or the rough/sharp edges of the plasmonic materials. Mostly these plasmonic materials are coinage metals or the noble metals such as Ag, Au, and Cu which have nanoscalar features [17]. Theoretically calculated electromagnetic enhancement for SERS can reach factors more than B1011 which depends on the structure of the plasmonic material on to which the analyte is adsorbed [18].

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Another mechanism which is reported for the signal enhancement is chemical enhancement [19]. In this mechanism, the charge transfer mechanisms occur, where the excitation wavelength interacts with the charge transfer electronic states within the analyte and metal. Theoretically reported chemical enhancement factors go up to 103 which were calculated using time-dependent density functional theory for para and meta substituted analytes interacting with metal clusters. It has been observed that the magnitudes of enhancement through charge transfer transitions are highly specific to the analyte molecules [20]. The overall accepted SERS enhancement factor is dependent on the sum of electromagnetic and chemical enhancement mechanisms. Resonance Raman effects are also believed to play a large role in SERS-based experimentation because in many applications, the dye molecules with extremely large resonance Raman cross-sections are employed [21].

21.2 NOBLE METAL NANOPARTICLE BASED SERS PLATFORMS Numerous SERS platforms have been developed since the discovery of the SERS, many of which are heavily focused on the plasmonic and noble metal nanoparticles [22,23]. Nanoparticles synthesized by top-down and bottom-up approaches have been widely used as the SERS substrates [2426]. Primarily the nanoparticles synthesized by these methods are used as these materials display large enhancement factors, facile synthesis routes, and possibility to tailor the geometries [27]. There are many studies in the literature which reported different geometries of the nanoparticles which include spherical, nanorods, nanocubes, nanostars, nanoprisms, octahedra, octopods, porous nanoparticles, etc. [2832]. The only drawback of using nanoparticles as the SERS substrates is the difficulty in scale-up which limits their use in the large-scale industries. Also, the surfactants used in the synthesis of the nanoparticles, such as cetyltrimethylammonium bromide (CTAB), trisodium citrate, polyvinyl alcohol (PVA), etc., are hard to remove from their surface, which provides the complications in practical applications, such as SERS-based sensing [33]. The removal of these surfactants from the surface of the nanoparticles disturbs their stability and can lead to Ostwald ripening [34] and aggregation [35], which effects the overall reproducibility of the SERS signals. There are some interesting approaches to synthesize the surfactant-free nanoparticles, which can be utilized for SERS applications [36,37]. The main mechanism of the SERS strongly depends on the interaction of the adsorbed analyte molecules and the surfaces of the plasmonic nanoparticles, and the majority of the classic SERS substrates are based on the Au, Ag, and Cu nanoparticles [38,39]. Mostly Au and Ag are the first choice metals for the fabrication of SERS substrates as these are airstable metals and Cu is more reactive and is more prone to oxide formation [40]. Also, most of the Raman measurements occur between the visible and near-infrared wavelength range where the LSPR of these metals fall, which makes these metal nanoparticles convenient to use, as seen in Fig. 21.1. Recently, efforts have been made to identify new plasmonic materials so that the wavelength range can be tuned to achieve the selectivity [41,42]. There are many other reports as well, where different engineered metals, which include Na, K, Li, Rb, Cs, Al, Ga, Pt, Rh, etc. and metal alloy nanoparticles have been used as the

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FIGURE 21.1 Wavelength range of metal nanoparticles for SERS applications.

plasmonic substrates for SERS-based applications [38,4346]. Among these metals, Al was found to be more suitable for UV SERS [47,48]. Most of these metals are prone to oxidation and are very reactive, limiting their use in SERS applications. Some newly discovered materials in the past decade such as graphene [49,50], oxides of semiconductors, such as TiO2 [51,52], and quantum dots [53,54] also show a major SERS enhancement, although the definitions provided by the traditional mechanisms do not agree with the observed enhancement. There are various aspects which cannot be ignored while choosing the material, which are reliable and provide a platform for the high enhancement of SERS signals. As the plasmonic and noble metals are proved to support the electromagnetic enhancement, an SPR spectra which is separate from the charge transfer and absorption spectra must be obtained. Also, by the conventional definitions, excitation through the chemical enhancement mechanism of charge transfer alone does not result in the enhancement of the Raman signals. To claim that a nanoparticle is plasmonic, the negative real component of the dielectric constant and a positive imaginary component of the dielectric constant should exist [55]. As the noble metal nanoparticle-based SERS research is getting saturated these days, the researchers have begun to explore the nanoparticles based on the metal oxides [56], semiconductor oxides [5759], and carbon materials [6062] as substrates for SERS for applications in the characterization of novel materials, sensors, and other devices. The reported results from these materials show that the mechanisms of the enhancements are purely chemical enhancements which can be attributed to the charge transfer bands, interband transitions, surface resonances, and the interactions based on ππ stacking. Electromagnetic enhancement was not evident in these cases and hence very high enhancement factors were not achieved. Progress is being made in establishing metal oxide nanomaterials as a plasmonic material in ranging from the visible region to the infrared, leading to the future possibility of SERS on these metal oxides-based materials. Further exploration of such novel substrates will open up new avenues of research for SERS-based applications.

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21.3 METAL OXIDE NANOSTRUCTURES IN SERS It has been established that several metal oxides such as TiO2, SiO2, ZnO, Fe2O3, CuO, etc. also show SERS signal enhancement [63]. But it has been observed that the overall enhancement factor obtained is much weaker than that observed for the noble metals and other plasmonic metal-based substrates. In the metal oxide-based systems, the charge transfer happens at the interface of the metal oxide and the analyte molecules [64]. The main advantage of using these metal oxides-based materials is that the three-dimensional (3D) structures of these metal oxide particles increase the overall surface area and subsequently provide larger adsorbing area for analyte molecules [65,66]. In addition, a greater number of hot spots can be generated by engineering the active sites of the metal oxides, which can lead to the higher enhancements. In a chemical enhancement mechanism, the analyte molecule is adsorbed on the surface of the substrate and the chemical interaction occurs between the analyte and the surface [67]. This type of interaction depends on the electron density distribution of the analyte molecule which exists in correlation with the vibrational energy states. This variation in the electron density distribution directly enhances the Raman scattering cross-section, which depends on the specific molecular vibrational modes. The photon-induced charge transfer is a widely accepted mechanism explaining the SERS enhancement on metal oxide nanoparticles [57,63,68]. In this mechanism, the photoactivated charge transfer occurs from the metal oxide to the analyte. For this process to happen, a thermodynamically favored condition under light excitation should exist. This means that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the analyte molecule that is adsorbed on to the substrate has to energetically match the valence band and the conduction band of the metal oxides. This mechanism mainly consists of three electronic transitions as shown in Fig. 21.2. The photon-induced charge transfer from the metal oxide surface to the analyte adsorbed on the surface is summarized in Fig. 21.2A. In the first step, an electron in the VB of the metal oxide is excited by the incident light to the CB. In the next step, the photogenerated electron is rapidly transferred by resonant tunneling from the CB of metal oxide to an equal energetic level above the LUMO of the analyte molecule involving a vibrational level. In the final step, the electron returns back to the VB of metal oxide via an

FIGURE 21.2 Mechanism of SERS enhancement in metal oxide-based SERS substrates.

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electronhole recombination. On the other hand, the photon-induced charge transfer from the analyte to the metal oxide involves a slightly different mechanism, which is shown in Fig. 21.2(B). In the first step, an electron in the HOMO of the adsorbed analyte molecule gets photoexcited to an energy level above the LUMO. In the next step, the photogenerated electron is rapidly transferred by resonant tunneling to a matching energy level in the metal oxide. Subsequently, in the final step, the electron returns to the HOMO of molecule, and a vibrational quantum of energy is involved which gives a Stokes Raman scattering process.

21.4 NOBLE METALMETAL OXIDE NANOHYBRIDS-BASED SERS SUBSTRATES In the previous sections, it has been highlighted that the overall SERS enhancement factor is dependent on the sum of electromagnetic and chemical enhancement mechanisms. The overall enhancement in the SERS signals can be obtained by using the hybrids or the so-called nanocomposites of noble metals and metal oxides. This strategy integrates the mechanism of the electromagnetic enhancement coming from the noble metal nanoparticles and the chemical enhancement arising from both metal oxide nanostructures and noble metal nanoparticles, and charge transfer processes happening between the noble metal nanoparticles and the adsorbed analyte molecules, as depicted schematically in Fig. 21.3. Due to their nanostructured morphology, the surface areas of the metal oxide nanocomposites are very high which allows the increased loading of the noble metal particles. This provides the possibilities for the creation of 3D plasmonic hot spots, thus providing high enhancement in the SERS signals. The 3D noble metalmetal oxide nanocomposites also provide high surface area for the adsorption of the analyte molecules as compared to the other SERS substrates. Among the noble metals used in these types of hybrids, Au offers greater chemical stability, biocompatibility, and easier surface FIGURE 21.3 Mechanism of the noble metalmetal oxide-based nanocomposites for SERS enhancement.

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chemistry. Ag is more prone to oxidation, but still is often used as it provides the highest efficiency in enhancing the Raman scattering signals due to its optical absorption properties.

21.4.1 TiO2Noble Metal Nanohybrids for SERS Recent advances in SERS spectroscopy has seen tremendous interest in semiconductor materials hosting plasmonic particles. Among the noble metalmetal oxide based assemblies, the hybrids based on noble metalTiO2 are the most explored and considered as the best. TiO2 is reported to be chemically more stable in comparison with other oxides typically used for noble metal deposition. In a report, Yang et al. synthesized nanocomposites based on AgTiO2 for SERS applications [69]. They prepared the anatase phase TiO2 particles by hydrothermal procedure and synthesized the nanocomposite via photoreduction of silver nitrate on TiO2 under UV light illumination. The surface-treated Ag on TiO2 efficiently transferred the electrons to the analyte molecules, 4-mercaptobenzoic acid (4-MBA) adsorbed on the surface of TiO2 through the conduction band of TiO2 particles because of SPR absorption of Ag under incident visible laser in addition to the intrinsic TiO2-to-molecule charge transfer as schematically illustrated in Fig. 21.4 [69]. In another report, interesting self-cleaning AgTiO2 nanocomposites comprising of coreshell assembly were utilized for SERS by Zou et al. [70]. They synthesized the AgTiO2 coreshell nanocomposite through in situ hydrolysis and condensation of Sn21 grafted titanium glycolate microsphere in the presence of Ag1 ions. In this study, the SERS enhancement of the AgTiO2 hybrids could be increased due to its porosity, which produced a large contact area between the analyte molecule, 4-mercaptopyridine (4-Mpy), and the TiO2 surfaces, wherein the authors reported an enhancement factor of 6.5 3 105. The overall SERS enhancement could be credited to the chemical interaction between the 4-Mpy molecules and the Ag surfaces (chemical enhancement) and also the contacts between Ag nanoparticles, which led to the formation of hot spots (electromagnetic enhancement). The hybrids based on coreshell AgTiO2 assembly also exhibited selfcleaning properties under UV light illumination [70]. In another report, electrospinning technique was used to synthesize AgTiO2 hybrids where TiO2 nanofelt was decorated with Ag nanoparticles, which were claimed as highly sensitive, cost-effective, and recyclable SERS substrate and are shown in Fig. 21.5. Ag FIGURE 21.4 Proposed mechanism that contributed to SERS in the AgTiO2 nanocomposites. Source: Reproduced with permission from L. Yang, X. Jiang, W. Ruan, J. Yang, B. Zhao, W. Xu, et al., Charge-transfer-induced surface-enhanced Raman scattering on AgTiO2 nanocomposites, J. Phys. Chem. C 113 (2009) 1622616231 (Copyright 2009, American Chemical Society).

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FIGURE 21.5 Electrospun TiO2 nanofelt surface-decorated with Ag nanoparticles. Source: Reproduced with permission from Y. Zhao, L. Sun, M. Xi, Q. Feng, C. Jiang, H. Fong, Electrospun TiO2 nanofelt surface-decorated with Ag nanoparticles as sensitive and UV-cleanable substrate for surface enhanced Raman scattering, ACS Appl. Mater. Interfaces, 6 (2014) 57595767 (Copyright 2014, American Chemical Society).

nanoparticle aggregates were separated with nanosized interparticle gaps which led to the generation of electromagnetic hot spots, which efficiently enhanced the local electromagnetic fields leading to strong SERS activity. In this work, 4-MBA was used as a model analyte and an enhancement factor of 5.62 3 106 was achieved. The UV-cleanable property of the nanofelt TiO2/Ag using R6G and 4-ATP as an analyte was also reported [71]. In another report by Dai et al., an ultrasensitive 3D-SERS substrate was fabricated based on an unique angular, cuboid-shaped TiO2 nanowire (NW) arrays decorated with Ag nanoparticles [72]. The rutile cuboid TiO2 nanoparticles were prepared by the hydrothermal route and Ag nanoparticles were deposited on to TiO2 through the ultrahigh vacuum magnetron sputtering technique, as shown in Fig. 21.6. The high SERS activity could be attributed to the concentrated electromagnetic effect and the chemical effect caused by the AgTiO2 which was demonstrated by using Rhodamine 6G (R6G) as a model analyte for the SERS activity where a very high enhancement factor of 1 3 1012 was achieved [72]. In another interesting strategy of coupling Ag with titania, an alluring hydrogenated black TiO2 decorated with Ag nanoparticles (Ag/H-TiO2) was reported as a sensitive and reusable SERS substrate by Shan et al. [73]. The synthesis was done by using a hydrothermal procedure and the SERS active substrate material scaffold was hydrogenated by treating with sodium borohydride under the vacuum condition followed by calcination at 300 C for 3 h. It was interesting to observe that the hot spots arising from Ag nanoparticles grown on TiO2 nanowires were inefficient for making the AgTiO2 assembly as a sensitive SERS substrate. So, the hydrogenation led to the activation of hot spots by addition of more photogenerated electrons to the Fermi level of Ag nanoparticles and therefore, enabling the powerful chemical enhancement by promoting more electron-to-analyte molecule locomotion. The SERS enhancement was studied using R6G as a model analyte and the reported enhancement factor was calculated to be 1 3 108 [73]. The assemblies of TiO2 in combination with Au nanoparticles have also been widely explored as SERS substrates. Stroyuk et al. prepared nanocrystalline AuTiO2 films for the efficient SERS activity, where mesoporous titania films on ITO substrate (ITO/TiO2) were fabricated by using dip-coating procedure with titanium(IV) tetraisopropoxide as precursor. In the next step, ITO/Au/TiO2 films were fabricated by treating the ITO/TiO2 with NaAuCl4 (sodium tetrachloroaurate) aqueous solution under high-pressure mercury

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FIGURE 21.6 Schematic diagram of the preparation of arrayed angular, cuboid-shaped TiO2-NW decorated with Ag-nanoparticles on FTO glass. Source: Reproduced with permission from Z. Dai, G. Wang, X. Xiao, W. Wu, W. Li, J. Ying, et al., Obviously angular, cuboid-shaped TiO2 nanowire arrays decorated with Ag nanoparticle as ultrasensitive 3D surface-enhanced Raman scattering substrates, J. Phys. Chem. C 118 (2014) 2271122718 (Copyright r 2014, American Chemical Society).

1000 W lamp (λ 5 310390 nm) using photodeposition technique [74]. The SERS effect was investigated by comparing the ITO/TiO2 with ITO/Au/TiO2 films where the basic anatase phonon mode of TiO2 at 150/cm was investigated. In this case, the overall SERS enhancement is due to the photo-assisted transformations occurring at the interface between TiO2 and Au nanoparticles which forms a more intimate contact between the metal oxide and the noble metal leading to a high enhancement of the Raman signal from the titania at the expense of excitation of surface plasmon resonance in the adjacent gold nanoparticles [74]. Ji and Bang prepared SERS active hydrophobic TiO2 nanorod arrays decorated with Au nanoparticles by combination of hydrothermal and photochemical reactions where the reusability was highlighted and credited to its high photocatalytic properties [75]. A significant SERS enhancement was attained with noble metal nanoparticles deposited on a 1D nanorod array due to the existence of two types of hotspots: Au nanoparticles on individual rods and the interparticle distance between adjacent TiO2 rods decorated with Au nanoparticles [75]. In a similar report by Jiang et al., AuTiO2 nanocomposites were prepared by the photoreduction method of HAuCl4 on TiO2 nanoparticles under UV radiation [76]. The efficient SERS activity was demonstrated using 4-MBA as analyte molecule which due to the charge-transfer induced an enhancement effect with an intimate interaction of Au with TiO2. The intrinsic TiO2-to-analyte charge transfer and the additional charge transfer assisted by localized SPR of Au were also highlighted to be the main factors for the SERS enhancement of analytes on these substrates [76]. Another unique composition of Au/TiO2/Au nanosheets acts as highly sensitive SERS substrates and has been reported by Jiang et al. [77]. The SERS substrate were prepared by sputtering of Au layer onto the TiO2 nanosheets, as seen in Fig. 21.7, where the heterogeneous nanosheets provide high-density hot spots as well as high surface to volume area for analyte molecules. In order to investigate the SERS activity, as-prepared Au/TiO2/Au nanosheets were tested by 4-MBA as analyte. The authors reported the strong enhancement factor of 107 [77].

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FIGURE 21.7 SERS substrate based on Au/ TiO2/Au nanosheets. Source: Reproduced with permission from L. Jiang, X. Liang, T. You, P. Yin, H. Wang, L. Guo, et al., A sensitive SERS substrate based on Au/TiO2/Au nanosheets, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 142 (2015) 5054 (Copyright 2015, Elsevier B.V.).

21.4.2 ZnONoble Metal Nanohybrids for SERS As a metal oxide, ZnO is a practically nontoxic and biosafe material, and thus its combination with the noble metals is also of special interest for biomolecular applications, such as SERS [78]. In a common strategy to form noble metalZnO-based nanocomposites, ZnO microspheres are usually prepared by low temperature hydrothermal method and then photochemical deposition is done to prepare the AgZnO hybrids. In one of the studies reported by Liu et al. [79], porous ZnOAg microspheres were prepared using such strategy and employed for SERS applications, as shown in Fig. 21.8. In such assembly, the porous structured microspheres enhanced the light trapping for SERS activity and the higher surface-to-volume ratio was responsible for the immobilization of analyte molecules. Moreover, ZnO nanoparticles helped the Ag nanoparticles to adsorb and immobilize the organic molecules/analytes around them leading to the high enhancement of the SERS signals. Secondly, a strong absorption of sputtered Ag nanoparticles was observed in the range between 400 nm and 550 nm. Under an excitation wavelength of 514.5 nm laser (photochemical deposition), the Ag nanoparticles excited resonantly which led to the formation of localized electromagnetic oscillations [79]. The heterogeneous nanostructure arrays of AgZnO have also been reported for the SERS applications. Hu et al. reported AgZnO composite nanoarrays synthesized via photocatalytic method and proposed two plausible mechanisms for the SERS activity which was demonstrated using crystal violet and sudan dyes as model analytes [80]. The electromagnetic effect corresponds with the large local fields due to resonances occurring between the microstructures on the metal surface leading to high orders of enhancement. The second is the chemical effect which involves the scattering process associated with chemical interaction between the analyte molecule and the metal surface [80]. Shan et al. have constructed these SERS active and photocatalytic substrates, AgZnO nanorod arrays using the hydrothermal procedures. The proposed SERS mechanism for the high SERS activity which uses 4-aminothiophenol (4-ATP) as model analyte, demonstrates the formation of interfacial electric field between ZnO nanorods and Ag nanoparticles [81]. In another interesting assembly, a ZnO nanodome (ND) array decorated with the Ag nanoclusters (NC) as a hybrid SERS active substrate is reported [82]. The Ag nanoclusters were deposited on the focused ion beam fabricated ZnO nanodome array to form a hybrid noble metalmetal oxide SERS active nanosystem, as displayed in Fig. 21.9. In this case, improved SERS activity could be attributed to the generation of strong local electromagnetic fields induced by the nanoclusters on nanodome and intrananocluster interactions.

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FIGURE 21.8 Porous ZnOAg microspheres as recyclable and ultrasensitive SERS substrates. Source: Reproduced with permission from Y. Liu, C. Xu, J. Lu, Z. Zhu, Q. Zhu, A.G. Manohari, et al., Template-free synthesis of porous ZnO/Ag microspheres as recyclable and ultra-sensitive SERS substrates, Appl. Surf. Sci. 427 (2018) 830836 (Copyright 2018, Elsevier B.V.).

Fabrication of Ag NCs on ZnO ND arrays ZnO ND by varying Dt-t

Ag NCs on ZnO ND ND_2 ND_1

Dt-t

ZnO nanodome Ag

Si (100)

FIGURE 21.9 Schematic illustration of ZnO ND and Ag NCs on ZnO ND. Source: Reproduced with permission from K. Sivashanmugan, J.-D. Liao, B.H. Liu, C.-K. Yao, S.-C. Luo, Ag nanoclusters on ZnO nanodome array as hybrid SERS-active substrate for trace detection of malachite green, Sens. Actuators B: Chem. 207 (2015) 430436 (Copyright 2014, Elsevier B.V.).

In this study, malachite green was utilized as the analyte molecule, which showed an enhancement factor of B106 [82]. A multifunctional AgZnO sea urchin-like hybrid has been presented for the SERS applications by playing with the surface morphology. Li et al. reported AgZnO sea urchin-like structures synthesized by using photochemical technique, wherein these hybrid nanostructures consist of many needle-shaped branches, which can form multiple horn-like structures. These structures uninterruptedly amplified the surface plasmon excitations and under these conditions, the electromagnetic field of the light was greatly enhanced. It was concluded that this novel “echo effect” can further increase the SERS signals. In this study, R6G was taken as a model analyte for the SERS activity investigations and enhancement factors of 3 3 106 has been reported [83]. In another report an interesting morphology comprising of hydrothermally grown ZnO nanoflowers decorated with sputter-deposited Ag nanoparticles was presented. These nanohybrids were subjected to the detection of Raman-inactive trinitrotoluene (TNT) explosive

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material via SERS where an enhancement factor of 4.12 3 106 was achieved. It was proposed in this study that TNT initiated the high Raman scattering of nonresonated 4-ATP through the formation of a p-donorp-acceptor interaction between the p-acceptor, TNT, and the p-donor, the 4-ATPAgZnO complex, on the flower-like hybrids. In this work, this p-donorp-acceptor interaction was utilized to effectively induce the hot spots for the enhancement in the Raman signals while controlling the energy [84]. The assembly of noble metals with ZnO-based metal oxides have also been achieved using thin layer deposition techniques [85]. In a recent report, recyclable 3D-SERS active substrate based on ZnO nanosheets decorated with Au nanoparticles were fabricated [86]. AuZnO nanosheets were synthesized through ultrathin liquid layer electrodeposition and galvanic reduction techniques. The synthesized Zn/ZnO micro/nanostructure acted as a unique template as well as the reducing agent for the decorated Au nanoparticles on the surface of ZnO nanosheets. When the concentration of Au31 aqueous solution was varied, two kinds of gaps contributed to the 3D SERS sensitivity and creation of hot spots, the gaps between small Au nanoparticles growing on the same ZnO nanosheets and gaps between the Au nanoparticles growing on the two neighboring ZnO nanosheets. R6G was utilized as a model analyte for the SERS investigation in this study and an enhancement factor of 1.5 3 106 was achieved [86]. Similarly, Chen et al. prepared AuZnO composite nanoarrays for SERS applications. The well-aligned ZnO nanoneedle were grown on the Al-doped ZnO buffer layer by chemical vapor deposition method and the composite of AuZnO was prepared by the hydrothermal method. The melamine detection in the egg white was demonstrated using the composite to prove its high SERS activity. The SERS enhancement factor in this case was mainly controlled by Au coverage area and surface morphologies of the AuZnO nanoarrays composites. In another work, Chan et al. reported on the chemical deposition method of preparation of a highly ordered flower-like Au/ZnO/nanoporous Si pillar array (NSPA) structure for SERS application. Combining the morphological and structural analysis of Au/ZnO/ NSPA, the high SERS sensitivity and high enhancement factor performance in these composites were also directly related to the local strong electromagnetic effect and the reasons were as follows: 1. The periodically ordered ZnO/NSPA nanostructures with high-density ZnO branches provided a much larger surface area for loading Au nanoparticles in comparison to that of ZnO grown onto planar Si substrates. 2. The three-dimensional periodic nanostructure array also enhanced the excitation light trapping in the Au/ZnO/NSPA system, consequently increasing the light interaction with Au nanoparticles. At the interface of an individual Au nanoparticleZnO nanowire, the plentiful ZnO nanowires enhanced the light scattering from Au nanoparticles, and both the scattered and incident lights would contribute to the total field enhancement at the interface. 3. The close inter distances between adjacent Au nanoparticles contributed to the formation of SERS hotspots, which gave rise to large electromagnetic enhancement [87]. In yet another report where deposition method was used for fabrication, the ZnO layers were deposited on to silica substrates by the atomic layer deposition technique followed by gold deposition over the substrates by the means of sputtering.

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Electromagnetic mechanism and charge transfer mechanism has been proposed for the high SERS activity of the materials towards the analyte, p-mercaptobenzoic acid (pMBA), where a high enhancement factor of B107 could be achieved [88]. In another report, an alluring nanocable array based on the Au and ZnO for the SERS has been reported by Zhou et al. [89]. They prepared nanocable like morphology for SERS where Au nanoparticles were deposited over the ZnO nanocables using the electrochemical deposition technique. It is well established fact that SERS is a local phenomenon happening on the surface plasmon near-field, and large SERS enhancement can be achieved on a substrate with particle gaps in the tens of nanometers region. Since the AuZnO nanocable arrays used as SERS substrates were vertically aligned, hotspots originated from junctions between the crossing nanocables had to be minimized. In this case, the morphology of Au coatings was the main factor for the enhancement ability of SERS as seen in Fig. 21.10. The Au existed as separated single crystalline nanoparticles, which further evolved to continuous layers in thicker coatings. The multiple gaps between adjacent

FIGURE 21.10 (A) Low magnification TEM image and (B) high resolution image of single ZnO/Au core/shell nanocable with 10 s Au deposition time. (C) Low magnification TEM image of single ZnO/Au core/shell nanocable with 30 s Au deposition time. (D) EDX spectrum. Source: Reproduced with permission from M. Zhou, K. Diao, J. Zhang, W. Wu, Controllable synthesis of plasmonic ZnO/Au core/shell nanocable arrays on ITO glass, Phys. E: LowDimensional Syst. Nanostruct. 56 (2014) 5963 (Copyright 2014, Elsevier B.V.).

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nanoparticles yielded a strong enhancement of the electromagnetic field, giving rise to well pronounced Raman signals from the analyte (R6G in this case) which provided a high enhancement factor of 9.2 3 106 [89].

21.4.3 Fe3O4Noble Metal Nanohybrids for SERS Another metal oxide utilized with the noble metal nanoparticles for SERS applications is based on magnetite, a conducting mixed-valence iron oxide, which can act as a SERS substrate capable of electromagnetically enhancing the Raman signal for diverse surfaceadsorbed organic molecules. The main difficulty in preparing such hybrid particles lies in the weak interactions between iron oxide (e.g., α-Fe2O3 and Fe3O4) and noble metal nanoparticles [90]. The key to forming such nanoparticles is to suppress the homogeneous nucleation and to promote the heterogeneous growth of magnetite-based nanoparticles on Au seeds where Fe3O4 is the most preferred. The discovery that magnetite is an effective SERS substrate in combination with the noble metal nanoparticles considerably extends the scope for the study of interfacial adsorption reactions at metal oxide interfaces by relaxing the requirements for specific molecular structure and binding mechanism. In a report, Hu et al. reported Ag-coated Fe3O4@SiO2 three-ply composite microspheres with ferromagnetic property, using a wet-chemical method for SERS application, which has been illustrated in Fig. 21.11. In this case, two major mechanisms contributed to the SERS enhancement effect, one is the electromagnetic effect associated with large local fields due to resonances happening in the microstructures on the metal surface, and other is the chemical effect which involves a scattering process associated with chemical interaction between the analyte molecule (R6G in this case) and the metal surface. It was concluded in this study that the smaller metallic particles show higher enhancement, but when the particle size was less than 15 nm, the enhancement was saturated because the separation between metal nanoparticles was equal to their diameter. Here, the average size of the Ag nanoparticles coated on Fe3O4@SiO2 microspheres was in close proximity to

FIGURE 21.11 Synthesis route of Ag-coated Fe3O4@SiO2 composite microspheres. Source: Reproduced with permission from H. Hu, Z. Wang, L. Pan, S. Zhao, S. Zhu, Ag-coated Fe3O4@SiO2 three-ply composite microspheres: synthesis, characterization, and application in detecting melamine with their surface-enhanced Raman scattering, J. Phys. Chem. C 114 (2010) 77387742 (Copyright 2010, American Chemical Society).

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this size, which was responsible for the ultrasensitivity of the Ag-coated Fe3O4@SiO2 SERS substrate [91]. In another report, a hydrothermal synthesis route was employed for the fabrication of the hybrid nanoparticles of AgFe3O4 by Yu et al. In this case, the substrates assembled under magnetic field displayed stronger SERS signals and gave about fourfold increase in SERS enhancement compared to those without magnetic field. Hybrid nanoparticles assembled in a more orderly manner by the magnetic force led to the formation of more relatively homogeneous SERS hot spots and therefore led to better enhancement factors. The SERS investigation using composite based on the AgFe3O4 was utilized for the analysis of furazolidone in fish feeds [92]. In another interesting report, the portable SERS sensor based on the AgFe3O4 combination was employed for the in situ detection of arsenic species as described in Fig. 21.12 by Du and team. AgFe3O4 magnetic SERS substrate is an active arsenic accumulator and a strong surface Raman enhancer with high sensitivity. With a detection limit of 10 mg/L and a linear SERS response for arsenic concentrations up to 1000 mg/L, this in situ SERS platform allowed easy and reliable detection and identification of arsenic species in multiple matrices as shown in Figs. 21.13 and 21.14 [93]. The authors claim both electromagnetic mechanism and charge transfer mechanism simultaneously play a vital role in the SERS enhancement of the target molecules. By the utilization of thermal reduction and decomposition, the hybrid nanoparticles based on AuFe3O4 can also be prepared as shown in Fig. 21.15. Mezni et al. investigated the light-induced phase transition from magnetite to hematite (α-Fe2O3) [94]. Owing to the strong SERS effect, they were able to detect the formation of diironoxo bonds during the phase transition. These bonds are attributed to the presence of a mixed magnetite/maghemite phase. Hence, they proposed a new scheme where the phase transition is triggered by the iron hydroxide surface layer, as displayed in Fig. 21.16. This transition was studied for the first time in AuFe3O4 hybrid nanoparticles where the gold core acted as plasmonic nanoheaters responsible for the thermally induced phase transition [94]. A new version of multifunctional plasmonic nanoparticles were prepared based on the cobalt-doped Fe2O3@polydopamineAu (AuCoFe2O3@PDA) by coating polydopamine (PDA) through self-polymerization onto CoFe2O3 and later loading gold nanoparticles by in situ reduction onto the surface of PDA shell as shown in Fig. 21.17 [95]. The surface plasmon resonance (SPR) effect by Au nanoparticles present in the assembly enhanced the Raman signal of the molecular adsorbates (R6G in this case). The adsorption of R6G on

FIGURE 21.12 Photographic representation of the procedure for As field SERS detection. Source: Reproduced with permission from J. Du, J. Cui, C. Jing, Rapid in situ identification of arsenic species using a portable Fe3O4@Ag SERS sensor, Chem. Commun. 50 (2014) 347349 (Copyright 2013, Royal Society of Chemistry).

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FIGURE 21.13

SERS spectra of As(III) (A) and As(V) (B) at different concentrations of 10, 50, 100, 500, and 1000 mg/L. The inset pictures show calibration curves for As(III) and As(V), respectively. Source: Reproduced with permission from J. Du, J. Cui, C. Jing, Rapid in situ identification of arsenic species using a portable Fe3O4@Ag SERS sensor, Chem. Commun. 50 (2014) 347349 (Copyright 2013, Royal Society of Chemistry).

FIGURE 21.14 Raman (a) and SERS (b) spectra of As-contaminated water treatment sludge (A), As-spiked juice (B), and As-spiked wine (C). Source: Reproduced with permission from J. Du, J. Cui, C. Jing, Rapid in situ identification of arsenic species using a portable Fe3O4@Ag SERS sensor, Chem. Commun. 50 (2014) 347349 (Copyright 2013, Royal Society of Chemistry).

AuCoFe2O3@PDA is due to the strong adsorption activity of PDA as well as the weak electrostatic adsorption by Au nanoparticles, which played a vital role in the SERS activity and photocatalysis as shown in Fig. 21.18 [95]. Gan et al. reported that the reason for the enhanced SERS sensitivity can be generalized. Accordingly, the number of the analyte molecules adsorbed on the magnetic-induced Fe3O4noble metal aggregates is far more than that adsorbed on the naturally dispersed noble metal nanoparticles in the experimental cross-sectioned Raman laser range [96]. Besides, the Fe2O3noble metal aggregates, especially AgFe2O3 aggregates create more hot spots that can result in huge electromagnetic field enhancements [96]. Fe3O4noble

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FIGURE 21.15 Schematic illustration of Fe3O4-decorated Au nanoparticles synthesis. Formation of Au nanoparticles in the polyol solvent (A), decomposition of the iron(III) precursor at the surface of the preformed Au nanoparticles and formation of an intermediate iron alkoxy salt complex (B), nucleation of Fe3O4 seeds (C), and growth of Fe3O4 at the Au nanoparticles surface leading to Fe3O4-decorated Au nanoparticles (D). Source: Reproduced with permission from A. Mezni, I. Balti, A. Mlayah, N. Jouini, L.S. Smiri, Hybrid AuFe3O4 nanoparticles: plasmonic, surface enhanced Raman scattering, and phase transition properties, J. Phys. Chem. C 117 (2013) 1616616174 (Copyright 2013, American Chemical Society).

FIGURE 21.16 Schematic illustration of the transition of the iron oxide shell from the magnetite (Fe3O4) phase to the hematite (α-Fe2O3) phase under laser irradiation. Source: Reproduced with permission from A. Mezni, I. Balti, A. Mlayah, N. Jouini, L.S. Smiri, Hybrid AuFe3O4 nanoparticles: plasmonic, surface enhanced Raman scattering, and phase transition properties, J. Phys. Chem. C 117 (2013) 1616616174 (Copyright 2013, American Chemical Society).

metal composites were obtained by combining Au, Ag nanoparticles with 3aminopropyltrimethoxysilane-functionalized Fe3O4 nanoparticles for the superior SERS performance. The nanocomposite was prepared by using the solvothermal procedures as shown in Fig. 21.19.

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FIGURE 21.17 (A) Synthesis scheme of AuCoFe2O3@PDA. Red, gray, and yellow colors represent CoFe2O3, PDA, and Au, respectively. SEM images of (B, E) CoFe2O3, (C, F) CoFe2O3@PDA, and (D, G) AuCoFe2O3@PDA. Source: Reproduced with permission from W. Xiong, Q. Zhao, X. Li, L. Wang, Multifunctional plasmonic codoped Fe2O3@polydopamineAu for adsorption, photocatalysis, and SERS-based sensing, Part. Part. Syst. Charact. 33 (2016) 602609 (Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).

21.4.4 Other OxidesNoble Metal Nanohybrids for SERS There are many other hybrids based on the noble metals and oxides other than TiO2, ZnO, and Fe3O4, which have attracted a lot of attention in recent years as these can also form versatile nanophotonic systems as SERS substrates because the plasmon resonance can be tuned to any wavelength of interest by adjusting the various parameters, which can be used to control their morphology. They are reported to be very stable and highly reproducible [97]. In one such assembly, MnO2-protected Ag nanoparticles were reported as highly active SERS substrates by Abdulrahman et al., prepared by the wet chemical synthesis method [98]. Under visible light, when plasmonic metal nanoparticles were deposited on the Au surface, they observed an electromagnetic coupling between the localized surface plasmons in plasmonic nanoparticles and the surface plasmons in the underlying Au substrate. This coupling led to the appearance of strong electromagnetic field enhancement in the slits between deposited nanoparticles and the gold substrate, and hence, molecules present in these gaps gave rise to very strong Raman signals. This means that the enhancement efficiency of the Raman scattering observed for these systems was not only induced by the Au@MnO2 or Ag@MnO2 nanoresonators themselves, but was also due to the coupling of surface plasmons in the underlying Au substrate and deposited Au@MnO2 or Ag@MnO2 nanoparticles [98]. In another report based on MnO2, a high performance nanowall film based on the AuMnO2 was fabricated using hydrothermal method for the

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FIGURE 21.18 (A) Adsorption behaviors of various dyes on 20 mg of AuCoFe2O3@PDA. (B) Photocatalytic degradation of R6G by CoFe2O3, PDA, CoFe2O3@PDA, and AuCoFe2O3@PDA under the simulated solar light. (C) Raman spectra of R6G of various concentrations with AuCoFe2O3@PDA as the substrates. (D) Schematic of reversible adsorption, photocatalysis, and SERS behaviors of the multifunctional AuCoFe2O3@PDA. Source: Reproduced with permission from W. Xiong, Q. Zhao, X. Li, L. Wang, Multifunctional plasmonic codoped Fe2O3@polydopamineAu for adsorption, photocatalysis, and SERS-based sensing, Part. Part. Syst. Charact. 33 (2016) 602609 (Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).

SERS applications. The SERS enhancement demonstration in this case was done using the crystal violet as a model analyte and the overall enhancement factor obtained was B107. According to finite-difference time-domain (FDTD) simulations as shown in Fig. 21.20, the high enhancement of Raman signal was observed with increased gold coating in AuMnO2 hybrid nanowall system, which could be ascribed to the couplings of two gaps, i.e., one gap between the nano-islands and another gap between the nanowalls. Initially, the gap between nano islands plays an important role in the contribution of the high enhancement; due to the change in structure with increase in the deposition time, the coupling enhancement for the gap between two nanowalls plays a vital role instead of that between two gold nano islands [99]. The main barrier so far in using the metal oxide-based materials is their instability and these can be easily decomposed. Silica is such a material which is very abundant and is

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OEt

Synthesis Process H2N

NH

OEt

OEt

OH OH OH

Si

2

Si O O O

O Si

NH2 O

Si NH

2

Fe3O4 NPs

Noble metal NPs

Fe3O4-noble metal composites

Detection Process

S

R6G

Analyte

La

se

r

N

Monitor tank

FIGURE 21.19 Schematic depiction of the synthetic route to Fe3O4noble metal composites and their applications in SERS. Source: Reproduced with permission from Z. Gan, A. Zhao, M. Zhang, D. Wang, H. Guo, W. Tao, et al., Fabrication and magnetic-induced aggregation of Fe3O4noble metal composites for superior sers performances, J. Nanopart. Res. 15 (2013) 1954-1-12 (Copyright 2013, Springer Nature).

FIGURE 21.20 FDTD simulations for AuMnO2 hybrid nanowall film; (A) the model of AuMnO2 hybrid nanowall film for simulation. (BE) FDTD simulations of the electric field distribution in cross-section view of AuMnO2 hybrid nanowall film with varied gold thickness. Source: Reproduced with permission from M. Zhou, X. Liu, B. Yu, J. Cai, C. Liao, Z. Ni, et al., MnO2/Au hybrid nanowall film for high-performance surface-enhanced Raman scattering substrate, Appl. Surf. Sci. 333 (2015) 7885 (Copyright 2015, Elsevier B.V.).

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FIGURE 21.21 TEM images of SiO2 nanoparticles (A) and Au@SiO2 nanoshells prepared with different growth cycles: (B) one growth cycle, (C) two growth cycles, and (D) three growth cycles. Source: Reproduced with permission from Y. Xie, T. Chen, Y. Cheng, H. Wang, H. Qian, W. Yao, SiO2@Au nanoshells-based SERS method for detection of sunset yellow and chrysoidine, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 132 (2014) 355360 (Copyright 2014, Elsevier B.V.).

also very stable in the harsh environments [100]. A novel SERS substrate made up of AuSiO2 nanoshells has been developed by Xie et al., as shown in Fig. 21.21, wherein the researchers used a facile chemical synthesis process and demonstrated the SERS activity using sunset yellow and chrysoidine as a model analytes [101]. As the composites based on SiO2 are very stable, pH of the environment of the colloidal solution had a significant effect on the SERS intensity. The SERS spectra were recorded using environments with a range of pH by adding hydrochloric acid or sodium hydroxide. Use of hydrochloric acid increased the enhancement effect for sunset yellow at pH 4.11 but in contrast it weakened the enhancement effect for chrysoidine at pH 4.32. Meanwhile, poor enhancement was produced by sodium hydroxide for both sunset yellow and chrysoidine at pH 8.74 and 9.45, respectively. The positive charge on the NH21 groups in the chrysoidine molecules made them attracted to the reduced nanoparticles. On the other hand, the OH in the sodium hydroxide molecules also gets attracted to the nanoparticles, thereby the addition of either HCl or NaOH has an influence on the SERS activity. Due to the weak SERS signals resulting from the addition of NaOH solution, HCl solution was chosen for enhancement of the SERS spectra of sunset yellow at pH 4.11. The chrysoidine was detected without HCl or NaOH at pH 7.81. An impressive EF of 8.6 3 105 was obtained by the unique pH-dependent mechanism of SERS detection [101]. Multifunctional Fe3O4@Ag/SiO2/Au core 2 shell microspheres were reported for the high SERS performance by Shen et al. [102]. A high SERS enhancement was obtained from Fe3O4@Ag/SiO2/Au nanostructures where the electromagnetic enhancement was due to

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FIGURE 21.22 Schematic illustration of in situ SERS detection of RhB absorbed on Fe3O4@Ag/SiO2/Au microspheres. Enlarged part A is the schematic illustration of the RhB molecules on Fe3O4@Ag/SiO2/Au microspheres in the vial and the Raman experiments. Source: Reproduced with permission from J. Shen, Y. Zhu, X. Yang, J. Zong, C. Li, Multifunctional Fe3O4@Ag/SiO2/Au coreshell microspheres as a novel SERS-activity label via long-range plasmon coupling, Langmuir 29 (2012) 690695 (Copyright 2013, American Chemical Society).

the interaction of the excitation wavelength with plasmon excitations in the nanostructures, as well as chemical enhancement of the RhB (Rhodamine B) molecules adsorbed on the gold-coated nanomaterials. Due to the long-range interparticle plasmon coupling, the plasmon resonance wavelength of the Fe3O4@Ag/SiO2/Au nanostructures shift to the range of near-IR, and the nanostructures are on resonance using near-IR laser excitation, such as 785 nm. The SERS intensity of Rhodamine B (RhB) on Fe3O4@Ag/SiO2/Au nanostructures had a very high enhancement which was due to the long-range interparticle plasmon coupling as shown in Fig. 21.22 [102].

21.4.5 Noble MetalMetal Oxide Hybrid Nanoparticles in Controlling the Selectivity of Photocatalytic Reactions Monitored by SERS In the field of catalysis, plasmon-driven surface catalytic reactions are of high importance and these reactions can be monitored by in situ SERS technique [44,103]. In these types of reactions, the use of noble metal nanostructures allows the photon energy from light to be used to perform chemical transformations. Plasmon-driven catalytic reaction was first discovered on metal surface, taking advantage of the distinctive SPR property of the applied metal nanostructures. The noble metal nanoparticles cannot provide photocatalytic performance like conventional metal oxide-based photocatalysts. Although, the reactions on the noble metal nanoparticles can be efficiently monitored using SERS, the enhancement on the metal oxide based photocatalysts is usually low. Therefore, to address this issue, metal and visible-light active metal oxides can be combined to form nanocomposites which bring novel SPR features for the plasmon-driven surface catalytic reactions

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while being monitored efficiently using SERS spectroscopy. The SPR features of the noble metalmetal oxide nanocomposites have been utilized to assist the visible light driven catalytic oxidation reactions such as NH3 oxidation and CO oxidation with low intensity illumination [104106]. However, these reactions cannot be monitored by SERS technique because it is very difficult to detect these molecules in the gaseous phase. By exploiting the photochemistry mechanism of the metal oxide nanostructures, plasmon-driven photocatalysis of para-aminothiophenol (PATP) to para-nitrothiophenol (PNTP) occurring on the noble metalmetal oxide nanoparticles shows very distinctive characteristics than on the surfaces of noble metals. As these catalytic reactions are of high importance, the selectivity control of these reactions, which can be monitored using SERS technique, are desired. This can be achieved using a noble metal and a composite of noble metalsmetal oxide nanoparticles. Recently, a work on the selectivity control was demonstrated using Au nanoparticles and AuTiO2 nanoparticles by employing different light sources [107]. When Au nanoparticles were used as the catalysts, PATP gets converted to p,p-dimercaptobenzene (DMAB). This oxidation reaction did not proceed in the case of AuTiO2 because the hot electrons induced by SPR from the Au nanoparticles migrated to TiO2. However, when the AuTiO2 were used as catalysts under UV illumination and SPR excitation, PNTP was formed directly in a single step from photooxidation of PATP instead of the formation of DMAB. The PNTP produced in this process further can be converted to the DMAB when UV illumination is removed. In this process, hot electrons produced at Au nanoparticles surface gets transferred to PNTP molecules to facilitate this reduction reaction. This showed that the processes of charge transfer are responsible for the control of product formation and selectivity in photocatalytic plasmon-enhanced reactions. This whole process was efficiently monitored by SERS as the different product formation was evident by the respective SERS spectra of the products formed and can be seen in Fig. 21.23. The special interest in this work lies in the fact that apart from the control of reaction atmosphere and light sources, selectivity control of the plasmon-driven catalytic reactions can be realized with the assistance of proper semiconductors which can act as efficient SERS substrates as well. In a related study, Sharma et al. also utilized the Autitania assembly to fabricate highly sensitive biomimetic SERS substrates comprising of 3D plasmonic surface [51]. The fabricated SERS substrates were applied in the SPR-driven photooxidation of paminothiophenol, wherein the selectivity of the product formation was controlled by choice of the photocatalyst deposited on to the substrate and by the modulation of charge transfer between the constituting homo/hetero nanojunctions. When Au nanoparticles were used as photocatalysts, the SPR-driven photooxidation of PATP led to the formation of DMAB, but when Autitania nanocomposite was used as the photocatalyst, PNTP was formed in addition to DMAB. The results showed that the selectivity of the charge transfer processes in the SPR-driven catalytic reactions can be manipulated by choosing appropriate photocatalysts (noble metal nanoparticles or noble metalmetal oxide hybrids) and by controlling the interactions with the substrate as presented in Fig. 21.24. In the case of the SERS substrates with biomimetic 3D plasmonic surface and Au nanoparticles, as shown in Fig. 21.24A, laser (visible light) falls on the surface and leads to the formation of hot electrons. These hot electrons get transferred to the O2 molecules that are adsorbed on to the

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FIGURE 21.23 SERS spectra recorded for TiO2Au NPs that had been functionalized with PATP: before UV illumination (bottom trace), under UV illumination (middle trace), and after the UV illumination was turned off (top trace). Before UV excitation, only peaks assigned to PATP were detected (DMAB peaks displayed very low intensities). Under UV exposure for 5 min, the formation of PNTP was detected. PNTP could be further reduced to DMAB as the UV illumination was removed. Source: Reproduced with permission from J. Wang, R.A. Ando, P.H. Camargo, Controlling the selectivity of the surface plasmon resonance mediated oxidation of p-aminothiophenol on au nanoparticles by charge transfer from UV-excited TiO2, Angew. Chem. 127 (2015) 70137016 (Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).

surface of Au nanoparticles which activates the oxygen to photooxidize PATP to DMAB. In the case of SERS substrates having Autitania as photocatalysts, as shown in Fig. 21.24B, the synergistic effect of SPR excitation of 3D plasmonic surface and the Autitania nanocomposite led to the photooxidation of PATP to PNTP, in addition to DMAB. In this case, the photooxidation process occurs on the hetero nano-junction comprising of Au nanoparticlestitaniaAu thin film from the 3D plasmonic surface aided by the migration of hot electrons (excited by visible light) to the conduction band of titania via Schottky barrier. This leads to accumulation of an excess of electrons on the conduction band of titania and facilitates the direct oxidation of PATP to PNTP in a single step on SERS substrate with Autitania photocatalysts. Thus, by choosing suitable substrate having noble metal and noble metametal oxide nanocomposites, the selectivity of the PATP photooxidation can be maneuvered.

21.5 SUMMARY AND OUTLOOK In this chapter, we have summarized the noble metalmetal oxide hybrid nanoparticles having a diverse range of morphologies synthesized using different synthesis protocols, and have explained their plasmonic and SERS related properties. The nanoparticles based on these noble metalmetal oxide hybrids exhibit a strong plasmonic resonance that produces an intense SERS signal. By combining SERS experiments and numerical simulations

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FIGURE 21.24 Proposed mechanism for the SPR-driven photooxidation of PATP on substrates (A) SERS substrates with Au nanoparticles and (B) SERS substrates with Autitania nanocomposites. Source: Reproduced with permission from V. Sharma, R. Balaji, A. Kumar, N. Kumari, V. Krishnan, Bioinspired 3D surface-enhanced Raman spectroscopy substrates for surface plasmon driven photoxidation reactions: role of catalyst and substrate in controlling the selectivity of product formation, ChemCatChem 10 (2018) 975979 (Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).

of the plasmonic near-fields, detailed investigation can be performed using the morphologies of the metal oxides present either as a shell material or as a conjugate material shell to understand the interplay of different mechanisms that have an important role in the phase transition properties of the hybrid nanoparticles.

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The discovery of noble metalmetal oxide hybrid nanoparticles as effective SERS substrates considerably extends the scope for the study of interfacial adsorption reactions at noble metalmetal oxide interfaces by relaxing the requirements for specific molecular structure and binding mechanism. To understand the mechanisms of SERS enhancement using noble metalmetal oxide hybrid nanoparticles in depth, theoretical studies that will permit more detailed interpretation of the mechanism of the SERS are required. Also, the main barrier so far in using the metal oxide-based materials is their instability and these can be easily oxidized or decomposed further by laser irradiation or when these come in contact with the corrosive substances. There is likely to be a large range of other materials, which can be used along with the noble metals, that need to be considered for SERS analysis, including other conductive minerals, such as pyrite or pyrolusite. In addition, the combination of noble metals with doped metal oxide nanoparticles with strong plasmonic behavior may also provide SERS-active oxide substrates which have a diversity of interfacial structures and properties.

Acknowledgment This work is supported by Himachal Pradesh Council for Science, Technology and Environment (HIMCOSTE), India.

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[64] H. Yamada, Y. Yamamoto, N. Tani, Surface-enhanced raman scattering (SERS) of adsorbed molecules on smooth surfaces of metals and a metal oxide, Chem. Phys. Lett. 86 (1982) 397400. [65] H. Tang, G. Meng, Q. Huang, Z. Zhang, Z. Huang, C. Zhu, Arrays of cone-shaped ZnO nanorods decorated with Ag nanoparticles as 3D surface-enhanced raman scattering substrates for rapid detection of trace polychlorinated biphenyls, Adv. Funct. Mater. 22 (2012) 218224. [66] V. Hadjiev, M. Iliev, I. Vergilov, The Raman spectra of Co3O4, J. Phys. C: Solid State Phys. 21 (1988) L199. [67] L.A. Dick, A.D. McFarland, C.L. Haynes, R.P. Van Duyne, Metal film over nanosphere (MFON) electrodes for surface-enhanced Raman spectroscopy (SERS): improvements in surface nanostructure stability and suppression of irreversible loss, J. Phys. Chem. B. 106 (2002) 853860. [68] A. Otto, The ‘chemical’ (electronic) contribution to surface-enhanced Raman scattering, J. Raman Spectrosc. 36 (2005) 497509. [69] L. Yang, X. Jiang, W. Ruan, J. Yang, B. Zhao, W. Xu, et al., Charge-transfer-induced surface-enhanced Raman scattering on Ag 2 TiO2 nanocomposites, J. Phys. Chem. C 113 (2009) 1622616231. [70] X. Zou, R. Silva, X. Huang, J.F. Al-Sharab, T. Asefa, A self-cleaning porous TiO2Ag coreshell nanocomposite material for surface-enhanced Raman scattering, Chem. Commun. 49 (2013) 382384. [71] Y. Zhao, L. Sun, M. Xi, Q. Feng, C. Jiang, H. Fong, Electrospun TiO2 nanofelt surface-decorated with Ag nanoparticles as sensitive and UV-cleanable substrate for surface enhanced Raman scattering, ACS Appl. Mater. Interfaces 6 (2014) 57595767. [72] Z. Dai, G. Wang, X. Xiao, W. Wu, W. Li, J. Ying, et al., Obviously angular, cuboid-shaped TiO2 nanowire arrays decorated with Ag nanoparticle as ultrasensitive 3D surface-enhanced Raman scattering substrates, J. Phys. Chem. C 118 (2014) 2271122718. [73] Y. Shan, Y. Yang, Y. Cao, H. Yin, N.V. Long, Z. Huang, Hydrogenated black TiO2 nanowires decorated with Ag nanoparticles as sensitive and reusable surface-enhanced Raman scattering substrates, RSC Adv. 5 (2015) 3473734743. [74] O.L. Stroyuk, V.M. Dzhagan, A.V. Kozytskiy, A.Y. Breslavskiy, S.Y. Kuchmiy, A. Villabona, et al., Nanocrystalline TiO2/Au films: photocatalytic deposition of gold nanocrystals and plasmonic enhancement of Raman scattering from titania, Mater. Sci. Semicond. Process. 37 (2015) 38. [75] I.A. Ji, J.H. Bang, Synthesis of gold-coated TiO2 nanorod array and its application as a Raman substrate, Mater. Lett. 97 (2013) 158161. [76] X. Jiang, X. Sun, D. Yin, X. Li, M. Yang, X. Han, et al., Recyclable AuTiO2 nanocomposite SERS-active substrates contributed by synergistic charge-transfer effect, Phys. Chem. Chem. Phys. 19 (2017) 1121211219. [77] L. Jiang, X. Liang, T. You, P. Yin, H. Wang, L. Guo, et al., A sensitive SERS substrate based on Au/TiO2/Au nanosheets, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 142 (2015) 5054. [78] H. Yang, S.-Q. Ni, X. Jiang, W. Jiang, J. Zhan, In situ fabrication of single-crystalline porous ZnO nanoplates on zinc foil to support silver nanoparticles as a stable SERS substrate, CrystEngComm 14 (2012) 60236028. [79] Y. Liu, C. Xu, J. Lu, Z. Zhu, Q. Zhu, A.G. Manohari, et al., Template-free synthesis of porous ZnO/Ag microspheres as recyclable and ultra-sensitive SERS substrates, Appl. Surf. Sci. 427 (2018) 830836. [80] H. Hu, Z. Wang, S. Wang, F. Zhang, S. Zhao, S. Zhu, ZnO/Ag heterogeneous structure nanoarrays: photocatalytic synthesis and used as substrate for surface-enhanced Raman scattering detection, J. Alloys Compd. 509 (2011) 20162020. [81] G. Shan, S. Zheng, S. Chen, Y. Chen, Y. Liu, Multifunctional ZnO/Ag nanorod array as highly sensitive substrate for surface enhanced Raman detection, Colloids Surf. B: Biointerfaces 94 (2012) 157162. [82] K. Sivashanmugan, J.-D. Liao, B.H. Liu, C.-K. Yao, S.-C. Luo, Ag nanoclusters on ZnO nanodome array as hybrid SERS-active substrate for trace detection of malachite green, Sens. Actuators B: Chem. 207 (2015) 430436. [83] R. Li, C. Han, Q.-W. Chen, A facile synthesis of multifunctional ZnO/Ag sea urchin-like hybrids as highly sensitive substrates for surface-enhanced Raman detection, RSC Adv. 3 (2013) 1171511722. [84] X. He, H. Wang, Z. Li, D. Chen, Q. Zhang, ZnOAg hybrids for ultrasensitive detection of trinitrotoluene by surface-enhanced Raman spectroscopy, Phys. Chem. Chem. Phys. 16 (2014) 1470614712. [85] S.-Y. Pung, K.-L. Choy, X. Hou, C. Shan, Preferential growth of ZnO thin films by the atomic layer deposition technique, Nanotechnology 19 (2008) 435609. [86] C.-H. Xiao, B.-X. Xiao, J. Zhang, S.-M. Wang, P. Wang, T.-Y. Yang, et al., Synthesis of ZnO nanosheets decorated with Au nanoparticles and its application in recyclable 3D surface-enhanced Raman scattering substrates, RSC Adv. 5 (2015) 1794517952.

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[87] Y.F. Chan, H.J. Xu, L. Cao, Y. Tang, D.Y. Li, X.M. Sun, ZnO/Si arrays decorated by Au nanoparticles for surface-enhanced Raman scattering study, J. Appl. Phys. 111 (2012) 033104. ´ ´ [88] A. Kaminska, A.A. Kowalska, D. Snigurenko, E. Guziewicz, J. Lewinski, J. Waluk, ZnO oxide films for ultrasensitive, rapid, and label-free detection of neopterin by surface-enhanced Raman spectroscopy, Analyst 140 (2015) 50905098. [89] M. Zhou, K. Diao, J. Zhang, W. Wu, Controllable synthesis of plasmonic ZnO/Au core/shell nanocable arrays on ITO glass, Phys. E: Low-Dimensional Syst. Nanostruct. 56 (2014) 5963. [90] Y. Zhai, J. Zhai, Y. Wang, S. Guo, W. Ren, S. Dong, Fabrication of iron oxide core/gold shell submicrometer spheres with nanoscale surface roughness for efficient surface-enhanced Raman scattering, J. Phys. Chem. C 113 (2009) 70097014. [91] H. Hu, Z. Wang, L. Pan, S. Zhao, S. Zhu, Ag-coated Fe3O4@SiO2 three-ply composite microspheres: synthesis, characterization, and application in detecting melamine with their surface-enhanced Raman scattering, J. Phys. Chem. C 114 (2010) 77387742. [92] W. Yu, Y. Huang, L. Pei, Y. Fan, X. Wang, K. Lai, Magnetic Fe3O4/Ag hybrid nanoparticles as surfaceenhanced raman scattering substrate for trace analysis of furazolidone in fish feeds, J. Nanomater. 2014 (2014) 103. [93] J. Du, J. Cui, C. Jing, Rapid in situ identification of arsenic species using a portable Fe3O4@Ag SERS sensor, Chem. Commun. 50 (2014) 347349. [94] A. Mezni, I. Balti, A. Mlayah, N. Jouini, L.S. Smiri, Hybrid AuFe3O4 nanoparticles: plasmonic, surface enhanced Raman scattering, and phase transition properties, J. Phys. Chem. C 117 (2013) 1616616174. [95] W. Xiong, Q. Zhao, X. Li, L. Wang, Multifunctional plasmonic codoped Fe2O3@polydopamineAu for adsorption, photocatalysis, and SERS-based sensing, Part. Part. Syst. Charact. 33 (2016) 602609. [96] Z. Gan, A. Zhao, M. Zhang, D. Wang, H. Guo, W. Tao, et al., Fabrication and magnetic-induced aggregation of Fe3O4noble metal composites for superior sers performances, J. Nanopart. Res. 15 (2013). 1954-1-12. [97] X. Wang, W. Shi, G. She, L. Mu, Surface-enhanced Raman scattering (SERS) on transition metal and semiconductor nanostructures, Phys. Chem. Chem. Phys. 14 (2012) 58915901. [98] H.B. Abdulrahman, K. Koła˛taj, P. Lenczewski, J. Krajczewski, A. Kudelski, MnO2-protected silver nanoparticles: new electromagnetic nanoresonators for Raman analysis of surfaces in basis environment, Appl. Surf. Sci. 388 (2016) 704709. [99] M. Zhou, X. Liu, B. Yu, J. Cai, C. Liao, Z. Ni, et al., MnO2/Au hybrid nanowall film for high-performance surface-enhanced Raman scattering substrate, Appl. Surf. Sci. 333 (2015) 7885. [100] K.B. Krauskopf, The geochemistry of silica in sedimentary environments. In: Silica in sediments, Society of Econ. Paleontologists and Mineralogists. Spec. publ. 7 (1959) 419. [101] Y. Xie, T. Chen, Y. Cheng, H. Wang, H. Qian, W. Yao, SiO2@Au nanoshells-based SERS method for detection of sunset yellow and chrysoidine, Spectrochim. Acta A: Mol. Biomol. Spectrosc. 132 (2014) 355360. [102] J. Shen, Y. Zhu, X. Yang, J. Zong, C. Li, Multifunctional Fe3O4@Ag/SiO2/Au coreshell microspheres as a novel SERS-activity label via long-range plasmon coupling, Langmuir 29 (2012) 690695. [103] B.-B. Xu, R. Zhang, X.-Q. Liu, H. Wang, Y.-L. Zhang, H.-B. Jiang, et al., On-chip fabrication of silver microflower arrays as a catalytic microreactor for allowing in situ SERS monitoring, Chem. Commun. 48 (2012) 16801682. [104] S. Linic, U. Aslam, C. Boerigter, M. Morabito, Photochemical transformations on plasmonic metal nanoparticles, Nat. Mater. 14 (2015) 567576. [105] S. Linic, P. Christopher, D.B. Ingram, Plasmonicmetal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911921. [106] P. Christopher, H. Xin, S. Linic, Visible-light-enhanced catalytic oxidation reactions on plasmonic silver nanostructures, Nat. Chem. 3 (2011) 467472. [107] J. Wang, R.A. Ando, P.H. Camargo, Controlling the selectivity of the surface plasmon resonance mediated oxidation of p-aminothiophenol on au nanoparticles by charge transfer from UV-excited TiO2, Angew. Chem. 127 (2015) 70137016.

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C H A P T E R

22 Plasmonic Perovskite Solar Cells Utilizing Noble MetalMetal Oxide Hybrid Nanoparticles Nilesh Kumar Pathak1,2, P. Senthil Kumar1 and R.P. Sharma2 1

Department of Physics & Astrophysics, University of Delhi, New Delhi, India 2 Plasma and Plasmonic Simulation Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India

22.1 INTRODUCTION Plasmonic properties of a noble metal nanoparticle have tremendous applications due to the inherent optical features. Its optical properties can be explored when it is irradiated by an electromagnetic field. After irradiating with an electromagnetic field two fundamental excitations are observed. The first fundamental excitation is surface plasmon polaritons (SPPs), which is propagating, and the second excitation is surface plasmon resonance (SPR), which is localized or nonpropagating. These two excitations have a broader range of applications, such as in sensing, detection of weak Raman signal, and also in photovoltaics to harvest sunlight and convert it into electricity [16]. The main concept in plasmonics is to confine or manipulate light in a metal dielectric interface or on the surface of nanostructures. The optical signature of the nanostructure is highly dependent on the parameters like size, shape, composition, and surrounding environments. Several studies have been already explored in the literatures based on the plasmonic geometries which exhibit SPRs in different regimen of electromagnetic spectrum [711]. The materials which are frequently used as a plasmonic elements are metals like silver, gold, copper, and aluminum and few others [1115]. The optical signature of these metallic nanogeometries is observed in terms of SPRs. These SPRs in metal nanostructures are observed when incident light frequency matches with the collective oscillation of metal’s electron frequency.

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00022-X

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The SPR physics can be utilized to enhance the sensing capability, thin film photovoltaics performances, and several other purposes [1620]. As we know, the traditional photovoltaic absorbers are thick which are very costly for the photovoltaic society. Therefore, we have to make the cheaper devices which can be easily sent from the laboratory to the roof. Plasmonics is an emerging technology which is being used to increase the performance of thin-film devices via light trapping mechanism. It can be used to increase the optical thickness with reduced physical thickness of thinfilm materials. There are several ways to analyze the plasmonic interaction to the thin-film materials [7,2024]. The first way is the plasmonic scattering mechanism wherein the plasmonic geometries protruded onto the surface of thin film which facilitates to increase the optical pathlength of light inside the active material. This increased pathlength could enhance the photon absorption capability inside thin-film materials. The scattering of light from the metal nanogeometries which are embedded in an homogeneous medium are uniformly distributed in all directions. This is the general condition of scattering of light which will change when the nanoparticle is placed inside or at a certain distance from the active layer. In such a situation, light will preferentially be scattered more in the high refractive index medium. The trapping of light mechanism is most effective at its SPRs under which maximum absorption and scattering of light are observed. The second approach is to use the localized resonance concept in which metal nanogeometries are embedded in a medium having either constant or wavelength-dependent refractive index. The nanogeometries which are embedded in the medium could increase the exciton or electron and whole generation rate under resonance condition due to localized hot spot field effects [2528]. The work presented in this chapter focuses on the study of SPRs of noble metalmetal oxide nanogeometries surrounded in perovskite environment and how the spectral width and peak position of SPRs can be tuned in different regimen of solar spectrum with the parameters like metal radii and metal oxide layer thickness. The surrounding material is perovskite (CH3NH3PbI3), which is highly anisotropic, whose dielectric constant depends on the wavelength. This material has good photovoltaic properties like good absorption coefficient, tunable bandgap, and is frequently used as a thin-film material. The dielectric constants used in the study are taken from the literatures [29]. The analysis of optical properties of metalmetal oxide coreshell nanogeometries has been done using the first principle approach and its schematics shown in Fig. 22.1, wherein two different situations like noble metalmetal oxide hybrid and single metal nanosphere are taken in which an incident electric field E0 is interacted with them separately.

FIGURE 22.1 Interaction of electric field to isolated coated and noncoated metal nanosphere.

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22.2 THEORETICAL ANALYSIS 22.2.1 Polarizability of Noncoated and Coated Nanosphere The optical properties of metalmetal oxide coreshell nanoparticles are analyzed using the first principle approach known as the electrostatic approach, wherein the particle dimension is smaller than the wavelength of incident light. For such analysis, we have solved the Laplace equation for different situations to find out the potential, field, and polarizability expression. These parameters consist of size and shape of nanostructures, type of materials used, and media which are very sensitive to tune the plasmonic resonances. The Laplace equation can be expressed as: r2 V 5 0

(22.1)

The general solution of the Laplace equation in spherical polar coordinate ðr; θ; φÞ which gives the electrostatic potential profile is [3032] X X VðrÞ 5 Al;m r2l21 Ym Bl;m rl Ym (22.2) l ðθ; φÞ 1 l ðθ; φÞ l;m

l;m

where l; m are the running index in which l 5 0 to N and m 5 2 l to 1 l. Using the proper boundary condition we solved the potential inside and outside the sphere of radius r. The first term in Eq. (22.1) represents the potential due to sphere of radius smaller than r while the second term is the potential due to sphere of radius greater than r. the term Al;m gives the amplitude of field which is decayed by a factor of r2l21 and it also represents the amplitude of the multipolar field. This multipolar field amplitude may also be represented by incident field amplitude as: X Alm 5 αlm;l0 m0 Bl0 m0 ; for l 6¼ 0 (22.3) l0 m0

0

The prime indices used as super index of the summation indicates that l 6¼ 0. Note that 0 αlm;l0 m0 is not symmetric for the interchange of lm and l m0 in the polarizable dipole model 0 one takes all amplitudes with l and l 6¼ 1 equal to zero. Because of the symmetry in the sphere the polarizability matrix becomes diagonal and depends only nonprime number l. αlm;l0 m0 5 αl δll0 δmm0 Let us focus on one particular geometry to study its optical properties in electrostatic approximation. Consider a sphere of radius r whose dielectric constant ε is surrounded by medium having dielectric constant εm and corresponding multipolar polarizability expressed as αl 5

lðε 2 εm Þ r2l11 lε 1 ðl 1 1Þεm

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(22.4)

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This is the multipolar polarizability expression of spherical geometry which contains dipole for l 5 1, quadrupole for l 5 2, and octapole for l 5 3, etc. As the work is based on the metalmetal oxide hybrid nanoparticle, therefore we have to develop the model based on the same. In the same electrostatic approximation, we have to solve the Laplace equation for a sphere coated with a different layer of material surrounded by a medium and the corresponding polarizability expression is αl 5

l½ðεc 2 εm Þðlε 1 ðl 1 1Þεc Þr2l11 1 ðε 2 εc Þðlεm 1 ðl 1 1Þεc Þr2l11  c rc 2l11 ðlεc 1 ðl 1 1Þεm Þðlε 1 ðl 1 1Þεc Þr2l11 1 lðl 1 1Þðε 2 εc Þðεc 2 εm Þr2l11 c

(22.5)

where, ε and εc , are the complex dielectric constants of the core and the coating layer and εm is the dielectric constant of surrounding medium. Furthermore, r is the radius of the core and rc , the radius of the core plus coating layer. For the different values of l we have found the different expression of polarizability. For l 5 1, we have found the dipolar polarizability of isolated nanosphere as ε 2 εm αd 5 4πεm r3 (22.6) ε 1 2εm And for l 5 2, quadrupolar polarizability is αq 5

4π ε 2 εm εm r5 3 2ε 1 3εm

(22.7)

The physics of resonances are brought into the picture by putting the magnitude of the denominator part of polarizability expression equal to zero. This whole phenomenon is known as Frolich condition jε 1 2εm j 5 0 for an isolated sphere for dipolar resonance. While, for quadrupole resonance this Frolich condition becomes jε 1 ð2=3Þεm j 5 0 [8,33]. These are the simplest pictures of resonance conditions corresponding to quadrupole moments. With these Frolich conditions, which are the main case of plasmonic resonances, one can easily establish the relations between metal plasmon frequency and incident lightffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi frequency. These relations are ω 5 ωp = 1 1 2εm for dipolar case and ω 5 ωp = 1 1 0:67εm for quadrupolar case. The polarizability expression has also been obtained in the case of a sphere coated with different material using Eq. (22.5) for two different situations, dipolar and quadrupolar. To obtain the expression of dipolar polarizability substituting l 5 1 in Eq. (22.5), we have   ðεc 2 εm Þðε 1 2εc Þr3c 1 ðε 2 εc Þðεm 1 2εc Þr3 αd 5 4πεm rc 3 (22.8) ðεc 1 2εm Þðε 1 2εc Þr3c 1 2ðε 2 εc Þðεc 2 εm Þr3 For quadrupolar polarizability substituting l 5 2 in Eq. (22.5), we have   4 ðεc 2 εm Þð2ε 1 3εc Þr5c 1 ðε 2 εc Þð2εm 1 3εc Þr5 αq 5 πεm rc 5 3 ð2εc 1 3εm Þð2ε 1 3εc Þr5c 1 6ðε 2 εc Þðεc 2 εm Þr5

(22.9)

This semianalytical approach gives the physical insights to understand the optical properties of very small size as well as medium size nanoparticle, which is one step ahead from the dipolar approximation. The dipolar approximation is only limited to the small size nanoparticle. Therefore, by introducing the quadrupolar concepts one can at least

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discuss the optical signature of a medium size nanoparticle. In the medium size nanoparticle, the average size of the nanoparticle is greater than 40 nm but less than 70 nm.

22.2.2 Dielectric Constant of Metal The optical properties of metal depends on the dielectric constant which can be explained by the plasma model, wherein the free electron movements are considered due to light weight in comparison to a positive ion. In this model we have considered that the free electron density oscillates under the influence of applied electromagnetic field and the motion of these electrons are damped due to the mutual collision of electrons. The simple expression of dielectric constant of metal can be obtained by solving the equation of motion of electron in external applied field [34]. my¨ 1 mγ y_ 5 2 eE

(22.10)

We have taken the time-dependent profile of electric field EðtÞ 5 E0 e2iωt and solved it to find out the complex amplitude is yðtÞ 5

e E mðω2 1 iγωÞ

(22.11)

The polarization which is the dipole moment per unit volume is given by P 5 2 ney 5 The displacement vector

ne2 E mðω2 1 iγωÞ

 D 5 ε0 E 1 P 5 ε0 1 2

 ωp 2 E mðω2 1 iγωÞ

The dielectric function of free electron gas is εðωÞ 5 1 2

ωp 2 1 iγωÞ

(22.12)

mðω2

where ωp 2 5 ne2 =mε0 is plasmon frequency, γ 5 1=τ is the collision frequency of the order of 100 THz at room temperature, and ω is the incident light frequency. Eq. (22.12) is the dielectric constant of metal which contains both real and imaginary parts. In Eq. (22.12), we have only to consider the free electron contribution via the Drude model which defines the optical signature of metals without considering the interband effects. In the noble metals like silver and gold, the interband effects start to occur for energy greater than 1 eV [34]. The interband effects are taken into account by considering the role of bound electron in the equation of motion whose resonance frequency is ω0 , and then calculating the final expression of the dielectric constant which contains size effect εðωÞ 5 εbulk 1

ω2p ðω2 1 jτ 21 bulk ωÞ

2

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ω2p ðω2 2 jτ 21 ωÞ

(22.13)

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This Eq. (22.13) is the modified dielectric constant of a metal nanoparticle considering the size of particle smaller than the bulk mean free path of electron. The plasmon frequency ωp depends on the choice of metal and it is different for different metals due to the different value of electron density. In the study of optical properties of coated and noncoated metal nanogeometry several parameters are taken into account. These parameters are type of material, size, shape of the geometry, thickness of coating materials, and embedding media. The optical properties in terms of optical cross-section can be expressed in dipolar approximation [30].



k4 8π 4 6

ε2εm

2 2 hCscat i 5 k r

jαd j 5 (22.14) 3 6π ε12εm

  ε 2 εm 3 hCabs i 5 kImfαd g 5 4πkr Im (22.15) ε 1 2εm Cext 5 Cscat 1 Cabs

(22.16)

While these optical properties can also be expressed by considering quadrupolar effects which appear in Eqs. (22.7) and (22.9) corresponding to noncoated and coated nanosphere, respectively. The extinction cross-section of metal nanogeometry under dipolar and quadrupolar approximation can be expressed as  (22.17) hCext i 5 kIm αd;q   π2 8π3 r5 hCext i 5 kIm αd 1 2 αq 1 ð ε 2 1Þ (22.18) λ 15λ2 The extinction cross-section is the sum of the scattering and absorption cross-section and it is normalized by the geometrical cross-section known as Qext which signifies how much incident light interacted with the chosen nanogeometry. The extinction of coated and noncoated spheres in dipolar and (dipolar 1 quadrupolar) approximations are derived as can be seen from Eq. (22.14) to Eq. (22.18). These equations can used to simulate optical properties of metal nanospheres having different size and thickness of coating materials.

22.3 RESULTS AND DISCUSSION When a small spherical metallic nanoparticle is irradiated by light, metal gets polarized, that is the separation of positive and negative charges are observed. Since positive charge is sluggish due to heavy weight as compared to the electron, therefore, the effects of electrons are considered in the optical properties discussion. Further, a discussion through light is how the oscillation builds up in the metallic nanostructures under the influence of electric field. The applied electric pushes the negative charge in the opposite direction of the incident field and the positive charge is trying to bring it back to its original position. In this way, a natural frequency of oscillation will be produced, known as the plasmon frequency. When this plasmon frequency matches with the incident light frequency the

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resulting resonance is known as SPR. The study of SPRs and its tuning in different regimen of the solar spectrum is the main interest of the present chapter. We have chosen the different types of metals, like silver and aluminum, to analyze their optical signatures, and their dielectric constants are taken from literature [35]. Fig. 22.2 shows the wavelength dependent extinction cross-section of isolated aluminum metal embedded in perovskite environment. For all the three radii r 5 10, 11, 12 nm, we have found the different magnitude of extinction cross-section. The SPR for all the radii almost coincide at one wavelength around 380 nm. This is due to the anisotropic behavior of the perovskite material which has a wavelength-dependent dielectric constant. In Fig. 22.3, we have discussed the metalmetal oxide coreshell hybrid nanoparticle surrounded by perovskite environment. The influence of this hybrid nanostructure on the optical properties is analyzed in terms of SPRs and extinction cross-section. To do this, we have used aluminum metal as a core and aluminum oxide (Al2O3) as a coating layer. The different shell thicknesses are considered to study the optical cross-section of the hybrid nanostructure. The core 1 shell radius (rc) is 15 nm which is fixed throughout the discussion while the core radii (r) are variable. The results are consistent with the results observed in Fig. 22.2 with some additional features, including the appearance of extra SPR peaks around wavelength 500 nm. These additional peaks, corresponding to different shell thicknesses, are observed due to the metal oxide coating effect. Hence, it was concluded that the advantage of coated geometry shows extra features in terms of SPR peaks over the noncoating geometry. These dual SPR peaks can be utilized to excite excitons, which are bound states of electrons and holes paired, in two different regimen of the solar spectrum at a time.

6

FIGURE 22.2 Wavelength-dependent extinction cross-section of aluminum metal surrounded by perovskite environment.

x 104 r = 10 nm

Al

11 nm

5

12 nm

Cextn (nm2)

4

3

2

1

0 300

400

500

600

700

800

Wavelength (nm)

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Now we discuss the optical properties of isolated metal silver metal surrounded by perovskite environment, as shown in Fig. 22.4. Three different radii are taken to plot the extinction cross-section of silver metal whose SPR wavelength were observed around

5

x 105 r = 10 nm

rc = 15 nm Al–Al2O3

4.5 4

11 nm 12 nm

FIGURE 22.3 Wavelength-dependent extinction cross-section of aluminumaluminum oxide core shell hybrid nanoparticles surrounded by perovskite environment.

Cextn (nm2)

3.5 3 2.5 2 1.5 1 0.5 0 300

400

500

600

700

800

Wavelength (nm)

5

x 104

4.5

r = 10 nm 11 nm 12 nm

Ag

4

FIGURE 22.4 Wavelength-dependent extinction cross-section of silver metal surrounded by perovskite environment.

Cextn (nm2)

3.5 3 2.5 2 1.5 1 0.5 0 300

400

500

600

700

800

Wavelength (nm)

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wavelength 584 nm. We know that the smaller size silver metals surrounded by air exhibit their SPRs from 350 to 360 nm wavelength range, whereas due to the surrounding medium effects its SPR was observed at a higher wavelength range. Further, we have discussed the extinction cross-section of silveraluminum oxide coreshell hybrid nanoparticles surrounded by perovskite environment as shown in Fig. 22.5. When the metal is coated with the metal oxide, the extinction cross-section and SPRs are just opposite to the noncoated situation. The coating of the aluminum oxide over the silver metal shows dual SPR peaks, one at around 400 nm and other at 500 nm. These dual SPR peaks were observed due to separate excitation of SPR at two interfaces after the interaction between incident electromagnetic waves and the coated geometry. The extinction spectra are plotted by using Eq. (22.16) for dipolar case. The above discussion involves the study of extinction spectra for small size coated and noncoated nanospheres under dipolar approximation. For the estimation of optical properties of larger size metal nanoparticles quadrupolar approximation must be taken into account. Fig. 22.6 represents the extinction spectra of AlAl2O3 coreshell hybrid nanoparticles surrounded by perovskite environment. The extinction cross-section has been plotted by using Eq. (22.17), which contains both dipolar and quadrupolar polarizability term. The aluminum metal shows dipolar and quadrupolar peaks in two different regimen of the solar spectrum. The dipolar peaks are observed at wavelength 380 nm while the quadrupolar peaks are at 270 nm, which suggest lightmetal interaction in two different regimen of the solar spectrum. We have also observed that the magnitude of extinction cross-section increases with the increment of radius while its SPR peaks position are coincident at one particular wavelength. Therefore, after considering the quadrupolar effects over the dipolar effects one can target the smaller as well as larger size nanoparticles.

2

FIGURE 22.5 Wavelength-dependent extinction cross-section of silver metal coated with aluminum oxide nanoparticles surrounded by perovskite environment.

x 105 r = 10 nm 11 nm 12 nm

rc = 15 nm

1.8

Ag–Al2O3

1.6

Cextn (nm2)

1.4 1.2 1 0.8 0.6 0.4 0.2 0 300

400

500

600

700

800

Wavelength (nm)

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10

x 105 Al

9

r = 50 nm

FIGURE 22.6 Dipolarquadrupolar response of aluminum metal nanoparticles surrounded by perovskite environment.

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22.4 CONCLUSION The optical properties of noble metal-oxide coreshell nanoparticle are studied in terms of optical cross-section and SPRs. The causes of resonances appearing in different situations are explored by the Frolich resonance conditions, which are entirely different for isolated and hybrid cases. The aluminum and silver metals are chosen as a core material which have different resonance positions due to their different dielectric constants. These different resonance peak positions could increase the exciton generation rate only in one particular wavelength that is SPR wavelength. Moreover, the spectra of the resonance indicate the excitation of an electronhole pair inside an active medium in a wavelength range. The work presented in the chapter brought new opportunities to compare the optical properties of medium and smaller size nanoparticle having coated and noncoated geometries. It also suggested to visualize the dual SPR peaks either in terms of size effects or coating effects, which opens new avenues for the experimentalist to fabricate both types of nanostructure for thin-film application.

Acknowledgment This work is financially supported by the Science and Engineering Research Board (SERB) with File No PDF/ 2016/000161, Government of India.

References [1] K.R. Catchpole, A. Polman, Plasmonic solar cell, Opt. Express 6 (2008) 2179321800. [2] H.A. Atwater, A. Polman, Plasmonics for improved photovoltaic devices, Nat. Mater. 9 (2010) 205213.

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[3] N.K. Pathak, N. Chander, V.K. Komarala, R.P. Sharma, Plasmonic perovskite solar cells utilizing Au@SiO2 coreshell nanoparticles, Plasmonics 12 (2017) 237244. [4] W. Runsheng, Y. Bingchu, Z. Chujun, H. Yulan, C. Yanxia, L. Peng, et al., Prominent efficiency enhancement in perovskite solar cells employing silica-coated gold nanorods, J. Phys. Chem. C 120 (2016) 69967004. [5] G.K. Pandey, N.K. Pathak, R. Uma, R.P. Sharma, Electromagnetic study of surface enhanced Raman scattering of plasmonicbiomolecule: an interaction between nanodimer and single biomolecule, Solid Stat. Commum. 255 (2017) 4753. [6] M. Verma, A. Kedia, M. Boazbou Newmaia, P. Senthil Kumar, Differential role of PVP on the synthesis of plasmonic gold nanostructures and their catalytic and SERS properties, RSC Adv. 6 (2016) 8034280353. [7] N.K. Pathak, A. Ji, R.P. Sharma, Tunable properties of surface plasmon resonances: the influence of coreshell thickness and dielectric environment, Plasmonics 9 (2014) 651657. [8] S.A. Maier, Plasmonics: Fundamentals and Applications, Springer, Berlin, 2007. [9] E. Prodan, C. Radloff, N.J. Halas, P. Nordlander, A hybridization model for the plasmon response of complex nanostructures, Science 302 (2003) 419422. [10] H. Wang, X. Wang, C. Yan, H. Zhao, J. Zhang, C. Santschi, et al., Full color generation using silver tandem nanodisks, ACS Nano 11 (2017) 44194427. [11] P.S. Chandrasekhar, N. Chander, O. Anjaneyulu, V.K. Komaral, Plasmonic effect of Ag@TiO2 coreshell nanocubes on dye-sensitized solar cell performance based on reduced grapheme oxideTiO2 nanotube composite, Thin Sol. Films 594 (2015) 4555. [12] K. Thyagarajan, C. Santschi, P. Langlet, O.J.F. Martin, Highly improved fabrication of Ag and Al nanostructures for UV and nonlinear plasmonics, Adv. Opt. Mater. 4 (2016) 871876. [13] L. Dong, X. Yang, C. Zhang, B. Cerjan, L. Zhou, M.L. Tseng, et al., Nanogapped Au antennas for ultrasensitive surface-enhanced infrared absorption spectroscopy, Nano Lett. 17 (2017) 57685774. [14] A. Ji, R. Sharma, H. Pathak, N.K. Pathak, R.P. Sharma, Numerical simulation of plasmonic light trapping in thin-film Si solar cells: surface coverage effect, J. Phys. D: Appl. Phys. 48 (2015) 275101275107. [15] E.P. Bellido, Y. Zhang, A. Manjavacas, P. Nordlander, G.A. Botton, Plasmonic coupling of multipolar edge modes and the formation of gap modes, ACS Photonics 4 (2017) 15581565. [16] N.K. Pathak, A. Ji, R.P. Sharma, Study of efficiency enhancement in layered geometry of excitonicplasmonic solar cell, Appl. Phys. A 115 (2014) 14451450. [17] D. Zhang, O.J.F. Martin, A universal law for plasmon resonance shift in biosensing, ACS Photonics 2 (2015) 144150. [18] T. Ye, S. Ma, X. Jiang, L. Wei, C. Vijila, S. Ramakrishna, Performance enhancement of tri-cation and dualanion mixed perovskite solar cells by Au@SiO2 nanoparticles, Adv. Functional Mater. 27 (13) (2017) 1606545. [19] H. Robatjazi, H. Zhao, D.F. Swearer, N.J. Hogan, L. Zhou, A. Alabastri, et al., Plasmon-induced selective carbon dioxide conversion on earth-abundant aluminumcuprous oxide antenna-reactor nanoparticles, Nat. Commun. 8 (2017) 27. [20] J. Butet, O.J.F. Martin, Surface-enhanced hyper-Raman scattering: a new road to the observation of low energy molecular vibrations, Phys. Chem. C 119 (2015) 1554715556. [21] S.S. Mali, S.S. Chang, K. Hyungjin, S.P. Pramod, K.H. Chang, In situ processed gold nanoparticle-embedded TiO2 nanofibers enabling plasmonic perovskite solar cells to exceed 14% conversion efficiency, Nanoscale 8 (2016) 26642677. [22] P.K. Parashar, R.P. Sharma, V.K. Komarala, Double-layer antireflection from silver nanoparticle integrated SiO2 layer on silicon wafer: effect of nanoparticle morphology and SiO2 film thickness, J. Phys. D: Appl. Phys. 50 (2017) 035105. [23] N.K. Pathak, R.P. Sharma, Study of broadband tunable properties of surface plasmon resonances of noble metal nanoparticles using Mie scattering theory: plasmonic perovskite interaction, Plasmonics 11 (2016) 713719. [24] F. Rundong, W. Ligang, C. Yihua, Z. Guanhaojie, L. Li, Z. Lia, et al., Tailored Au@TiO2 nanostructures for the plasmonic effect in planar perovskite solar cells, J. Mater. Chem. A 5 (2017) 1203412042. [25] Carretero-Palacios, A. Jime´nez-Solano, H. Mı´guez, Plasmonic nanoparticles as light-harvesting enhancers in perovskite solar cells: a user’s guide, ACS Energy Lett. 1 (2016) 323331. [26] R.A. Pala, J. White, E. Barnard, J. Liu, M.L. Brongersma, Design of plasmonic thin-film solar cells with broadband absorption enhancements, Adv. Mater. 21 (2009) 35043509.

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[27] V.E. Ferry, L.A. Sweatlock, D. Pacifici, H.A. Atwater, Plasmonic nanostructure design for efficient light coupling into solar cells, Nano Lett. 8 (2008) 43914397. [28] Z. Li, Y. Xiao, Y. Gong, Z. Wang, Y. Kang, S. Zu, et al., Active light control of the MoS2 monolayer exciton binding energy, ACS Nano 9 (2015) 1015810164. [29] W. Zhang, M. Saliba, D.S. Samuel, S. Yao, S. Xian, W. Ulrich, et al., Enhancement of perovskite-based solar cells employing coreshell metal nanoparticles, Nano Lett. 13 (2013) 45054510. [30] Bohren C.F., Huffman D.R., Absorption and Scattering of Light by Small Particles, 1983, John Wiley, New York. [31] J.D. Jackson, Classical Electrodynamics, John Wiley, New York, 1962. [32] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light, North Holland Publ. Co., Amsterdam, 1989. [33] C. Noguez, Surface plasmons on metal nanoparticles: the influence of shape and physical environment, J. Phys. Chem. C. 111 (2007) 38063819. [34] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [35] E.D. Palik, Handbook of Optical Constants of Solids, Academic, Orlando, 1985.

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23 Hydrogen Gas-Sensing Application of Au@In2O3 CoreShell Hybrid Nanoparticles Rama Krishna Chava Department of Chemistry, College of Natural Sciences, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea

23.1 INTRODUCTION Gas sensors play a significant role among the various technologies in our daily life. Typical uses of gas sensors comprise the detection of toxic and explosive gases for the purpose of domestic and public safety, industrial applications, and monitoring of environmental pollution and then air quality. At present, the important application fields are, industrial and automotive sector (to detect NH3, NOx, SO2, O2, O3, CO2, hydrocarbons), the food industries (gas sensors are used here for control of fermentation processes), in domestic sector (for the detection of humidity, CO2, and combustible gases), in medical sector (in patient monitoring and diagnostics), and security fields (to detect the traces of explosives) [15]. Among the different types of gas sensors, chemiresistors or resistive gas-sensing elements are the most attractive due to the simple fabrication methods, ease of operation, and low production costs. In chemiresistors, metal oxides are typically used as gas-sensing materials, which change their electrical resistance when reducing or oxidizing gases are applied [2]. At first, Brattein and Bardeen observed this effect for Ge [6], and later by Heiland for ZnO [7] materials. The pioneering works of Seiyama et al. [8] and Taguchi [9] in the early 1960s led to the fabrication of the first gas-sensing elements. Since that time, several technological efforts have been explored to enhance the sensor

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selectivity, sensitivity, stability, as well as response and recovery time. The research and developments in gas sensors field have attained significant progress driven by the novel fabrication strategies based on nanoscience and nanotechnology, mainly from bottom-up fabrication resulting in the low-cost fabrication of sensor elements. Nanostructured materials with tailored structures and in small size and dimension have been established a great potential for the use as the sensing materials. Due to the high specific surface area, large surface-to-volume ratio, abundant surface active sites, and effect of crystal facets with high surface reactivity benefits, the nanostructured materials are being used as active materials in the gas sensors [1013]. Metal oxide semiconductors with interesting properties have been regarded as promising materials for gas-sensing applications. The success of this type of device is due to several advantages over chemical analysis methods, including portability, high sensitivity, simplicity in device structure, small dimension, ease of use, rapid response time, stable repeatability for reuse in the same process, simplicity in fabrication, compact size, real-time detection, low detection limits, low cost, low power consumption, and flexibility to a wide variety of oxidizing and reducing gases [14]. The most studied metal oxides for gas sensors are fabricated from n-type semiconductors such as ZnO, SnO2, In2O3, WO3, TiO2. This is because, compared to n-type oxides, p-type oxides are relatively unstable, having the tendency to exchange the lattice oxygen easily with air. However, some p-type oxide semiconductors, such as Co3O4, NiO, CuO, and Mn2O3, can also be used to fabricate new gas sensors capable of contributing a high selectivity and functionality due to their ability to catalyze the oxidation of various volatile organic compounds [14]. The sensing mechanism in metal oxide semiconductors is based on the surface redox reactions between the testing gas molecules and surface adsorbed oxygen species [15]. Among the studied metal oxide semiconductors, indium oxide (In2O3) is an important n-type semiconductor and has been identified as the gas-sensing material with the most potential due to its high electric conductance. Until now several authors reported the different In2O3 nanostructures and their gas-sensing properties [1623]. The gas-sensing properties of In2O3 nanostructures can be improved by making hybrid systems such as alloying and/or doping with the other metals or metal oxides. Some researchers performed experiments on noble metals, Au, Ag, Pd, and Pt modified In2O3 nanostructures and their gas-sensing properties were determined [2430]. The noble metals act as a catalyst, which can enhance the reactions between the sensing materials and target gases [31]. The resultant metal@In2O3 hybrid nanoparticles (HNPs) can be considered as an assembly of multicomponent heterostructures that exhibit new synergistic properties obtained from the nanoscale interaction of two different components [32]. The loaded metal NPs have the larger surface to volume ratio which in turn produces higher activity. Even though the noble metal NPs are used as catalysts to improve the performance of metal oxide semiconductor gas sensors, these noble metals, specifically Au NPs with small sizes have high surface energy and thus tend to undergo undesired aggregation during the high temperature treatment and surface reactions which results in a loss of catalytic activity [3336]. Therefore in order to avoid the sintering and retaining the number active sites on metal NPs, a protective shell is necessary and termed as coreshell structures. Coreshell structures provide a variety of benefits, such as the metal NP can be protected from sintering and aggregation, improved stability, and the enhanced catalytic performance [37,38].

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Hydrogen (H2) is one of the best clean energy carriers and as an alternative to the fossil fuel energy sources to overcome the problems of energy supply security and global warming. Hydrogen has a high heat of combustion (142 kJ/g), low minimum ignition energy (0.017 mJ), and wide flammable range (4%75%), as well as high burning velocity. The combustion product of hydrogen is water, which is free from impurity and can be converted into hydrogen and oxygen again for periodic duty. Hydrogen also acts as a strong reducing agent for many elements and is also useful in the fields of semiconductor processing, nuclear power stations, metal smelting, glassmaking, petroleum extraction, and the daily chemical industry. In addition, hydrogen can also be applied in biomedical, environmental protection, and seismic surveillance areas, such as for indicating a certain type of bacterial infection, detection of environmental pollution, etc.[39,40]. Currently H2 is being widely investigated as sustainable and renewable fuel that has the potential to meet the energy needs of tomorrow [41]. However, its safe storage and transportation are major concerns and need to be adequately addressed before it can replace present day fossil fuels in automotive and power industries [4244]. Hydrogen is a tasteless, colorless, and odorless flammable gas so it cannot be detected by human senses and therefore rapid and accurate detection is highly necessary during the production, storage, and in usage. For this, great effort has been made in developing the materials for highly sensitive, selective hydrogen sensors, efficient and low-cost devices capable of real-time fast detection of even very small amounts of hydrogen gas [44]. Based on the abovementioned facts, the current chapter deals with the synthesis and H2 gas-sensing properties of Au@In2O3 coreshell hybrid NPs [45]. The introduction of Au NPs into Au@In2O3 coreshell HNPs as a core significantly improved the H2 gas-sensing behavior. The proposed coreshell structure not only protects the Au NPs from corrosion/aggregation but also introduces the metal support interaction through the threedimensional contacts between the Au metal core and In2O3 shell. When Au surface is exposed with H2 molecules, the Au metal dissociates into H atoms and thereby transfers to the surface of In2O3 shell [46]. The enhanced H2 gas-sensing property of Au@In2O3 coreshell HNPs was presented by comparing with the performance of bare In2O3NPs.

23.2 SYNTHESIS AND CHARACTERIZATIONS OF AU@IN2O3 CORESHELL HYBRID NANOPARTICLES 23.2.1 Synthesis To prepare the hybrid nanoparticles, Au NPs (seeds) having a particle diameter of B20 nm were presynthesized by sodium citrate reduction method [47,48]. Initially, 500 mL DI water containing 1 mM HAuCl4 solution was heated until boiling with mild stirring. After boiling point (above 95oC) was reached, Na3cit solution (25 mL, 34 mM) was added with rigorous stirring. The resulting solution was stirred for 15 min at boiling temperature and then cooled down. The obtained deep red wine colored Au NPs solution was stored for further use. For the synthesis of Au@In2O3 coreshell hybrid NPs [45], 5 mL of the above prepared Au NPs are kept stirring for 5 min. The solution containing 2 mM of InCl3 and 2.4 mM of

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Na3cit was added to the Au NPs solution under stirring. The solution was stirred at room temperature for 30 min and then 2 mM of urea was added and stirred for another 30 min. After that, the resultant solution was changed to 50 mL Teflon-lined stainless steel autoclave and was heated at 140oC for 24 h in an electric oven. After cooled down to room temperature, the purple colored precipitate was washed repeatedly, centrifuged, and dried to obtain the Au@InOOH coreshell hybrid NPs. Finally, Au@In2O3 coreshell NPs were obtained after calcining in air at 350oC for 2 h. The heating rate was maintained at 2oC/min. For comparison, bare In2O3NPs were also synthesized in similar manner without adding Au NPs.

23.2.2 Characterizations The formation of In2O3 shell on the surface of Au NPs was achieved by a single-step hydrothermal method with trisodium citrate as an organic ligand and urea as a complexing agent and then subsequent annealing. Fig. 23.1 describes the formation mechanism of Au@In2O3 coreshell HNPs. At first, indium oxy hydroxide shell was formed onto Au NPs. After calcination at 350oC for 2 h, the preformed indium oxy hydroxide shell was transformed to indium oxide shell. When InCl3 is dissolved into the deionized water, it produces In31 and it is easily attached to sodium citrate molecules. At moderate temperatures, urea molecular releases OH2 and then In31 will be gradually converted into InOOH

FIGURE 23.1

Schematic representation of the synthesis of Au@In2O3 coreshell NPs.

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during the chemical reaction. These formed nanocrystallites began to aggregate and selfassemble on to the surface of Au NPs surface. After calcination at the required temperature, the obtained Au@InOOH coreshell will be converted to Au@In2O3 coreshell HNPs. Fig. 23.2 depicts the morphology of as-prepared NPs. Fig. 23.2A shows the TEM images of citrate stabilized Au NPs and having a diameter of around 15 nm. After being coated with InOOH shell (Fig. 23.2B), the size of Au NPs was increased up to 100 nm. Due to the aggregation of InOOH NPs, the obtained coreshell NPs are found to be rough. Next, Fig. 23.2C represents the final Au@In2O3 coreshell HNPs. For the sake of comparison, bare In2O3 NPs were also prepared and their corresponding TEM image is given in Fig. 23.2D. High-resolution transmission electron microscopy (HRTEM) measurements were also performed to check the interface between the core and shell NPs. As per HRTEM images (Fig. 23.3AC), the core material is Au NP and the shell is composed of only In2O3 NPs with an average 20 nm size. In order to find the crystalline features and sharp interfaces of Au@In2O3 coreshell NPs, the lattice fringes for both Au and In2O3 were recorded and are given in Fig. 23.3B and C, respectively. The interplanar spacing of 0.274 nm corresponds to the (1 1 0) plane of cubic In2O3 phase, whereas the lattice spacing of 0.235 nm corresponds to the (1 1 1) plane of cubic Au NPs. From these observations, it is confirmed that the proposed synthesis method successfully yields the Au@In2O3 coreshell HNPs without any other impurities. Later selected area electron diffraction (SAED) patterns of In2O3 shell were also recorded and are given in Fig. 23.3D which displays the circular rings corresponding to (0 1 2), (1 1 0), (0 2 4), (1 1 6), (0 1 8), (2 1 4), and (2 2 0) diffraction planes revealing their crystalline nature. Further, the prepared Au@In2O3 coreshell HNPs were confirmed by high-angle annular dark-field scanning

FIGURE 23.2 TEM images of Au NPs (A), Au@InOOH coreshell HNPs (B), Au@In2O3 coreshell HNPs (C), and In2O3 NPs (D). Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

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FIGURE 23.3 (AC) HR-TEM images, (D) selected area electron diffraction patterns, and (E, F) HAADFSTEM images of Au@In2O3 coreshell NPs. STEM-EDX line scanning profiles (GJ) and STEM-EDX elemental mapping profiles (KN) of Au@In2O3 coreshell NPs. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

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transmission electron microscopy (HAADF-STEM) imaging (Fig. 23.3E and F), energy dispersive X-ray (EDX) line scanning, and elemental mapping. From EDX line scanning profiles (Fig. 23.3GJ), the coreshell HNPs consist of only Au, In, and O elements with their corresponding valleys. Fig. 23.3KN displays the EDX elemental mapping and revealed that the core particle is Au NP and shell material is composed of In and O. These features recommend that the formed coreshell HNPs are of only Au@In2O3 and there were no other phases or impurities present in the sample. The morphology of Au@In2O3 coreshell HNPs displayed in Fig. 23.3A represents the coreshell NPs with a flower-shaped structure when observed using TEM/HRTEM images. Such a flower-like morphology is beneficial for gas-sensing applications since the larger area is available for the adsorption of gas molecules. The formation of such flower-like structure can be explained as follows. When the reaction time is considerably prolonged, the formed NPs keep crystallizing and the ions in the solution are repetitively adsorbed on the surface of the particles to support further growth. Owing to the coalescence and assembly, together with the crystallization and growth of In2O3 NPs, the larger-sized spherical particles are formed. As a result, the nanoflakes gradually appear, arising from Ostwald ripening of the nanoparticles, and the coreshell NPs are elongated and larger [49,50]. The crystallinity, purity, and structural features of Au@In2O3 coreshell HNPs were determined by using powder X-ray diffraction (XRD) studies. The powder XRD patterns of as-prepared Au@InOOH and Au@In2O3 coreshell nanoparticles were given in Fig. 23.4A. The observed peaks were assigned to either InOOH/In2O3 or Au nanoparticles. As seen from Fig. 23.4A, the powder XRD pattern of Au@InOOHshell HNPs exhibited the diffraction peaks related to cubic Au phase (marked with *) with JCPDS no: 04-0784 and the other peaks (marked with Δ) correspond to the orthorhombic phase of InOOH (JCPDS no.71-2284). Likewise, Au@In2O3 coreshell HNPs also displayed the diffraction peaks at 2θ 5 38.2 , 64.5 , and 77.50 corresponding to (1 1 1), (2 2 0), and (3 1 1) lattice planes of AuNPs. The remaining diffraction peaks (marked with ▲) were indexed to the lattice planes of In2O3 with cubic structure (JCPDS no: 22-0336). No other significant peaks related to impurities were observed, verifying the high quality of the products. It is also obeserved that the crystallinity of In2O3 in the coreshell HNPs is increased after calcination. The diffraction peaks of the In2O3 are sharp and intense, whereas the Au NPs are broad and weak, suggesting the small size of the Au crystallites. The formation of Au@In2O3 coreshell HNPs can also be monitored by recording the plasmonic curves of Au-based NP seeds in the reaction. The UVvis absorption spectrum of coreshell HNPs were given in Fig. 23.4B together with Au NPs and bare In2O3. The citrate-stabilized Au NPs displayed a localized surface plasmon resonance (LSPR) peak at 520 nm. After In2O3 shell coating, the plasmon peak of Au NPs showed a significant broadening and red shifted (550 nm) due to the increase in the refractive index of the neighboring medium [5154], which is the unique characteristic of this coreshell nanocomposite materials [55,56]. This red shift is due to the formation of In2O3 shell around Au NPs and also due to the increase in dielectric constant surrounding the Au NPs. In addition to this, Au@In2O3 also exhibited an absorption peak at 305 nm which is the characteristic of In2O3 NPs. Generally in coreshell NPs, the shell layer provides thermal stability to the metal NPs, and results not only in the red shift of LSPR peak, but also shields the electron oscillation on the metal, causing plasmon dampening [57,58]. Therefore a decrease in SPR peak intensity for Au NPs was observed. II. APPLICATIONS

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FIGURE 23.4 Powder X-ray diffraction patterns (A) and UVvis absorption spectra (B) of Au@In2O3 coreshell HNPs. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

The chemical states, binding energy, and valence chemistry of constituent elements in the Au@In2O3 coreshell HNPs were studied by X-ray photoelectron spectroscopy and the corresponding curves are shown in Fig. 23.5. As seen from Fig. 23.5A, the survey spectrum shows the presence of gold, indium (In3d3/2, In3d5/2), and oxygen (O1s) elements. Fig. 23.5B shows the characteristic binding energy values of Au 4f electrons. The peaks observed at 84.1 eV and 87.8 eV are assigned to Au 4f7/2 and Au 4f5/2 core levels, respectively [59]. There were no peaks observed at 85.5 and 86.3 eV suggesting that the Au in the Au@In2O3 coreshell structure is in a metallic state [60,61]. Fig. 23.5C exhibits the characteristic spin-orbit splittings of In3d3/2 and In3d5/2 signals, whose positions are observed at 452.4 and 444.8 eV, respectively. The observed peaks with symmetric shape indicate the oxidation states rather than the metallic states [62] and also confirm that the indium element in the prepared sample exists in the oxide form only [63,64] and the valence of

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FIGURE 23.5 X-ray photoelectron spectra of Au@In2O3 coreshell HNPs. (A) Survey graph, (B) Au 4f, (C) In 3d, and (D) O 1s regions; black circles are the experimental data, which are deconvoluted into their respective peaks. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

indium is mainly 13 in the product [65]. The XPS spectrum of O-1s could be resolved using the XPS peak fitting program. From Fig. 23.5D, the O-1s spectrum showed a peak at 530.3 eV corresponding to lattice oxygen in In2O3 and the other peak observed at 532.2 eV was characterized by adsorbed oxygen species [66,67].

23.3 HYDROGEN GAS-SENSING APPLICATION 23.3.1 Gas Sensor Device Fabrication and Measurements The gas-sensing measurements were carried out on thick Pt-interdigitated electrodes (1 3 1 cm2) made onto an alumina substrate (1.5 3 1.5 cm2). For the fabrication of Au@In2O3 coreshell and In2O3 NPs-based gas sensors, a paste made by grinding the asprepared NPs with a few μL of α-terpinol (as a binder) was deposited onto the substrates. The sensor devices were sintered at a temperature of 300 C in a heating furnace to remove the organic binders. The sensing device was connected to an external electrical circuit to measure the change in electrical resistance at a fixed voltage. Changes in the electrical

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resistance during the adsorption and desorption of testing gas molecules were calculated by measuring the electrical resistance in presence of air (Ra) and in gas (Rg). The gas flow of air, N2, and testing gases (H2, CO, C2H5OH, CH3CHO) was controlled by UNIT Instruments Inc. made high performance mass flow controller (MFC). The change in resistance of the device in the presence of testing gases was measured using a high resistance Agilent 34970A Data Acquisition Unit. The prepared gas-sensing devices were tested at different temperatures from 150 to 300oC with gas concentrations of 2100 ppm. The sensor response (S) was calculated from the formula, S 5 Rg/Ra for oxidizing gases and/or S 5 Ra/Rg for reducing gases. Next, the response time and the recovery time were calculated as the time required for the variation in the resistance to reach 90% of the equilibrium value after a test gas was injected, and the time necessary for the sensor to return to 10% above the original resistance in air after releasing the test gas, respectively.

23.3.2 Hydrogen Gas-Sensing Studies To test the ability of Au@In2O3 coreshell HNPs to reflect in a practical application, the H2 gas-sensing tests were performed in a dry ambient air. For convenience, the sensing properties of bare In2O3NPs were also studied. The time-dependent resistance behavior of Au@In2O3 coreshell HNPs and bare In2O3 NPs-based sensor devices was checked with various concentrations of hydrogen at different temperatures. The characteristic curves are shown in Fig. 23.6A and B. When the gas flow is turned from hydrogen to air, the resistance of the sample attained its original high resistance value. Several tests were conducted on the sensors by using high and low concentrations (2100 ppm) of hydrogen gas and the similar activities were monitored. From the measurements, it is observed that Au@In2O3 coreshell HNPs exhibited a good response to H2 gas, whereas the bare In2O3 NPs samples showed a little response. In the case of gas-sensing devices, it is important to identify the optimal operating temperature (highest response to the test gas) since the response of every sensor is strongly dependent on the operating temperature. This operating temperature significantly influences the surface states of sensing materials, as well as the interactions during the gas-sensing reactions. Therefore, the response of both Au@In2O3 coreshell HNPs and In2O3 NPs were measured as a function of temperature from 150 to 300oC. Fig. 23.6C shows the response of both sensor devices at 100 ppm of H2 gas with increasing temperature. Apparently, the response of both sensors changed with the temperature and reached a maximum at 300oC, after that the response of both sensors decreased. Hence this temperature was chosen as the optimal operating temperature and showed higher responses of 34.4 and 9.3 for Au@In2O3 coreshell HNPs and In2O3 NPs, respectively. Further increase in temperature leads to the decrement of trapped metastable electrons from the conduction band due to desorption of oxygen species and hydroxyl groups, consequently the sensor response decreases [68]. The response of Au@In2O3 coreshell HNPs is almost four times that of bare In2O3 NPs and moreover coreshell HNPs showed higher response than that of the In2O3 sensor at all testing temperatures, which is probably due to the sensitization effect of Au nanoparticles. For practical applications, the stability of gas sensor is of great interest, therefore the stability of Au@In2O3 coreshell HNPs was studied. This can be defined as the

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FIGURE 23.6 Resistance changes of Au@In2O3 coreshell HNPs and In2O3 NPs at various H2 gas concentrations (A) at 300oC and (B) at 250 C. (C) Dependence of the sensor response of Au@In2O3 coreshell and In2O3 NPs at different temperatures to 100 ppm H2 gas; (D) stability test of Au@In2O3 coreshell HNPs sensor device with 100 ppm hydrogen gas exposure at 300 C. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

reproducibility of sensor characteristics during a certain period of time at working conditions in the presence of testing gas. Here, the sensor stability of the Au@In2O3 coreshell HNPs has been investigated towards 100 ppm of H2 gas and is displayed in Fig. 23.6D. From Fig. 23.6D, the response is quite stable and reversible even after the H2 gas was switched several times relative to air, confirming its good stability. In other words, we can say that there were no changes that occurred during the exposure of hydrogen gas repeatedly at higher temperatures. However, the lower desorption of H2 gas molecules was observed initially for a few cycles and later attained its original baseline resistance. Besides good stability, a quick response and recovery time was also observed for the Au@In2O3 coreshell HNPs sensor device. The typical dynamic response curves for both Au@In2O3 and In2O3 sensor devices towards 100 ppm H2 gas at an operating temperature of 300 C is given in Fig. 23.7A. As seen from Fig. 23.7A, the resistance of the sensor decreased abruptly on the injection of H2, and then increased rapidly and recovered to its initial value after the test gas was released. The response and recovery times for Au@In2O3 coreshell HNPs was 31 s and 10 min, whereas for bare In2O3 NPs it was 1 and

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(A) Response and recovery curves at a temperature of 300 C towards 100 ppm H2 gas; (B) H2 gas concentration versus response curves of Au@In2O3 coreshell and In2O3NPs at operating temperature of 300 C. H2 gas concentration versus response curves of Au@In2O3 coreshell HNPs (C) and In2O3 NPs (D) sensors at different temperatures. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

FIGURE 23.7

30 min, respectively. Fig. 23.7B shows the response data of both sensors with H2 gas concentrations from 2 to 100 ppm at an operating temperature of 300 C. The response values for Au@In2O3 coreshell HNPs were calculated as 34.38, 15.90, 7.30, 4.58, 2.88, and 1.73 at 100, 50, 20, 10, 5, and 2 ppm of H2 gas, respectively, whereas for bare In2O3 NPs they were measured as 9.26, 6.80, 4.00, 3.06, 2.22, and 1.57, respectively. At all ppm levels of H2 gas exposures, Au@In2O3 coreshell HNPs showed higher response than that of bare In2O3 NPs. Similarly, H2 gas-sensing activity of both Au@In2O3 coreshell and In2O3 NPs at different temperatures with various concentration of H2 gas levels are shown in Fig. 23.7C and D. It is observed that as temperature increases the sensor response was also increased in both sensors. In practical applications, selectivity is an important factor, i.e., to detect a specified gas in the presence of several gas molecules, especially with similar physicochemical properties. In the current work, the selectivity towards H2 gas was examined by exposing the sensor devices to a variety of gases such as NO2, CO, CH3CHO, and C2H5OH at an operating temperature of 300oC towards 100 ppm gas levels. As given in Fig. 23.8, the responses towards NO2, CO, CH3CHO, and C2H5OH are lower than that of

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FIGURE 23.8 Responses of Au@In2O3 and In2O3 sensor devices to different gases at an operating temperature of 300oC to 100 ppm gas levels. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

H2 gas, highlighting the selectivity toward H2 gas. It is also seen that the selectivity of Au@In2O3 coreshell HNPs towards H2 gas was greatly increased by combining the Au and In2O3 in a coreshell hybrid structures. In this coreshell hybrid structure, Au shows a sensitization effect which causes a dominant mechanism for increasing the sensitivity, as well as selectivity. The electronic sensitization effect arises due to the change in Schottky barrier at the interface between the two components Au and In2O3 as a consequence of hybrid structures [69].

23.3.3 Role of Au Metal NPs in Improved Hydrogen Gas-Sensing of the Hybrid Nanoparticles In most of the metal oxide semiconductor gas sensors, the sensing mechanism depends on the redox reactions happening between gaseous molecules and active ions, such as O22, O22, O2, adsorbed on the surface of the metal oxide depending on its temperature [70] and reported by several authors [21,7177]. Such red-ox reactions modulate the size of space charge layer formed within the metal oxide. This work also adopted the same phenomenon to discuss the enhanced hydrogen gas-sensing mechanism of Au@In2O3 coreshell HNPs over In2O3 NPs. In2O3 nanostructures are n-type semiconductors and when exposed to air atmosphere, the oxygen molecules can be adsorbed on the surface and extract the electrons from the conduction band of In2O3 to form oxygen ions. Hence there will be a chance for the formation of an electron depletion region near the surface which can significantly increase the resistance due to the decrease of charge carrier density. Immediately when the sensor is exposed to hydrogen gas, the hydrogen molecules will react with the adsorbed oxygen species. The redox reaction is exothermic and results in the fast desorption of produced H2O molecules from the surface. The released electrons will lessen the thickness of the depletion layer and as a consequence the resistance of the semiconductors decreases. Next, when the sensor is exposed to air again, the depletion region will be rebuilt by adsorbed oxygen species. The resistance will regain the initial level before hydrogen response [78].

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FIGURE 23.9 Schematic illustration for the H2 gas-sensing mechanism of Au@In2O3 coreshell HNPs sensor device in air and H2 gas environments. Source: Reproduced from R.K. Chava, S.-Y. Oh, Y.-T. Yu, CrystEngComm 18 (2016) 36553666 with permission from the Royal Society of Chemistry.

Fig. 23.9 describes the reaction mechanisms of Au@In2O3 or In2O3 NPs sensor devices during the air and hydrogen gas environments. The improved H2 gas-sensing performance of Au@In2O3 coreshell NPs includes two key mechanisms: (i) the electronic sensitization induced by interface between In2O3 and Au nanoparticles, and (ii) the chemical sensitization resulting from the catalytic Au metal NPs [24,7174,76,7884]. As per electronic sensitization effect, the work function of metal Au (5.1 eV) is greater than that of In2O3 (4.8 eV) and hence a few electrons from conduction band of In2O3 must move to metal NP to balance the Fermi levels. The transferred electrons will cause a greater band bending at the interface of Au@In2O3, which creates the Schottky barrier, preventing the transportation of electrons. During the air exposure, the Schottky barrier height (SBH) is expanded by the adsorbed oxygen species, and thus the electrical resistance increases. In contrast, when the Au@In2O3 coreshell NPs sensor was exposed to H2 gas, a significant decrease in the SBH is caused and consequently lower resistance will be obtained [81,85,86]. In addition to this, chemical sensitization also played a role in improving the H2 gas-sensing properties of Au@In2O3 coreshell HNPs. It is well known fact that the metal Au nanoparticles effectively serves as adsorption sites to bind and dissociate the oxygen molecules as well as the dissociation and ionization of H2 gas molecules into H atoms [87,88]. This phenomenon will increase the amount of adsorbed oxygen species and thereby increases the degree of electron extractions from the conduction band of In2O3. Moreover, these Au NPs also quicken the reactions between H2 atoms and adsorbed oxygen species due to the spillover effect of Au NPs [24,82,83,89]. Hence Au@In2O3 coreshell HNPs-based sensors showed superior sensing performance over bare In2O3.

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23.4 CONCLUSIONS Owing to its coreshell structure, Au@In2O3 shows many advantages, such as high performance, high stability, and high recyclability in H2 gas-sensing devices. The designed Au@In2O3 coreshell HNPs displayed a greater response and selective towards H2 gas with a highest response of 34 at an operating temperature of 300 C, which is four times higher than that of bare In2O3 NPs. The enhanced H2 gas-sensing performance of Au@In2O3 coreshell HNPs was attributed to the electronic sensitization effects, as well as the chemical sensitization effect of metal Au NPs. The present chapter not only underscores the benefit of coreshell structure in the performance of Au@In2O3 sensor devices but also can serve as a lead for the fabrication of future noble metal@metal oxide coreshell hybrid nanoparticles for the gas sensor devices.

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24 Development of CeO2- and TiO2-Based Au Nanocatalysts for Catalytic Applications Rajashree Bortamuly, Abu Taleb Miah and Pranjal Saikia Department of Applied Sciences (Chemical Science Division), Gauhati University, Guwahati, Assam, India

24.1 INTRODUCTION Gold has always been regarded as a precious material throughout the ages. The introduction of gold (Au) into the world of catalysis was brought about by Prof. M. Haruta in a laboratory of Japan in the year 1987 [1]. In contrast to the catalytic inertness of bulk gold, highly dispersed nanosized gold particles reveal tremendous activity towards different chemical reactions. The metalsupport interaction (MSI) plays a vital role in the catalysis by gold. The beneficial perspectives in exploiting solid materials as supports for metal nanoparticles (MNPs) are manifold. So far, various solid materials have been used in this regard. The oxide support contributes to the catalytic activity of Au through several mechanisms: (i) charge transfer to/from the oxide support from/to the gold particle; (ii) supply of adsorption sites for reactants, in particular, oxidants, that may migrate to the Au particle; and (iii) formation of a reactive gold oxide interface and the particle perimeter. Against the abovementioned background, in this chapter, we are trying to summarize some recent advances pertaining to the development of ceria (CeO2) and titania (TiO2)based Au nanocatalysts and their catalytic applications in some important reactions. In the subsequent sections of this chapter, these two types of catalysts will be represented by Au/CeO2 and Au/TiO2 nanohybrids for convenience.

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00024-3

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24.2 SYNTHESIS OF CeO2- AND TiO2-BASED Au NANOCATALYSTS The routes pertaining to the synthesis of metal oxide supported (e.g., CeO2, TiO2, ZnO, Al2O3, etc.) Au nanocatalysts are an important aspect. Generally, the formation of Au NPs is accomplished by loading a gold precursor (usually a gold salt or complex) onto solid supports followed by reduction or calcination [2]. The reduction or calcination process leads to reduction of the cationic gold species into Au atoms that undergo aggregation to form Au NPs. The degree of agglomeration during this stage relies on different factors, such as the ambient temperature and duration of the process as well as the nature of solid supports [3]. Literature reveals that impregnation (IMP) is the first technique for synthesizing supported Au nanocatalysts [4]. This method involves the addition of a corresponding metal precursor to a preformed solid scaffold followed by a subsequent reduction reaction for impregnating the thus formed metal nanoparticles (MNPs). Though this technique is preferably used over the others owing to its simplicity, it is invariably associated with some disadvantages that restrict its utility. As a prominent example, this method with the most common gold precursor HAuCl4, often produces large Au particles after thermal treatment and hence yields catalytically less active nanostructures [5]. Further modifications to this method could however alleviate the critical issues to a great extent. Post impregnation washing with aqueous NH3 solution is revealed to be an efficient strategy for making highly dispersed, stable, and active catalysts with sub-4-nm Au particles [4]. This approach is intriguingly applicable to any type of oxide supports regardless of their nature. Moreover, leaching of Au during the washing treatment is very limited, and a large range of Au loadings could be realized [4,5]. In conventional IMP method, the formation of large and catalytically inactive Au particles is attributed to the high amount of Cl2 ligands generating from the gold precursor HAuCl4, which promote sintering of Au particles during thermal activation [5]. The inclusion of an extra washing step with aqueous NH3 in this method removes the Cl2 ligands and assists the formation of small Au particles [4]. Apart from this approach, gaseous NH3 treatment method followed by washing with H2O is also recognized to favor the removal of the chlorine residues after impregnation for obtaining small Au particles on TiO2 [6]. Depositionprecipitation (DP) has so far been known to be the most effective and preferred method for synthesizing CeO2- and TiO2-based Au nanocatalysts. The key reason to this assumption is that this method could convene the most enviable aspect for realizing tremendous activity (i.e., very small Au particle size; especially # 5 nm) of the Au nanocatalysts in heterogeneous catalysis [7,8]. In this method, a basic environment (pH 8) is required to be maintained for the suspension containing the gold precursor and the support material via the addition of a precipitant, i.e., base (e.g., NaOH, KOH, NH2CONH2, NH4OH) which plays a prominent role in the morphology of the nanosized Au particles and thereby on the catalytic activity of the final catalysts. Another method, namely colloidal deposition (CD), is quite popular for synthesizing CeO2- and TiO2-based oxide supported Au nanocatalysts [9]. The method of colloidal deposition includes the synthesis of colloidal gold nanoparticles followed by deposition of the nanoparticles on a support material. It excludes many factors which are difficult to

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control in coprecipitation or depositionprecipitation synthetic protocols. The method avails the separation of the gold nanoparticle formation from the supporting process which is highly important for the identification of the support influence. Recently, photodeposition has been explored for preparing supported Au NPs. Photodeposition is sometimes a method preferred over others in the context that the requirement of elevated temperature or applied voltage is replaced by mere illumination to deposit metal nanoparticles on support suface [10]. The sonication-assisted reductionprecipitation method has gained interest to prepare the heterogeneous mono- and bi-metallic catalysts. The advantages for the sonicationassisted method as compared with the conventional methods, includes high dispersion of metal nanoparticles on the catalyst support, much higher deposition efficiency (DE) than those of the depositionprecipitation (DP) and coprecipitation (CP) methods, and the much faster preparation, which only lasts 1020 s for the deposition [11]. Nowadays, design of multicomponent Au nanocatalyst architectures has been realized by modifying the parent oxide (i.e., CeO2 and TiO2) supports either by forming their binary composite/mixed oxide solid solutions or by dispersing them over another support. The ratio of parent oxides (usually the component which is taken in larger amount compared to the other) to the dopant materials results in alteration of the structural and physicochemical properties of the binary systems. It also has significant effect on the overall catalytic efficiency of the nanosized Au-decorated catalysts. Hence, finding out a suitable ratio for devising such architectures is immensely crucial in order to achieve optimum catalytic activity. A large variety of such novel architectures have been revealed until now and some recent reports among them are enlisted in Tables 24.1A and 24.1B (Fig. 24.1).

24.3 CATALYTIC APPLICATIONS 24.3.1 CO Oxidation The first major discovery in the field of catalysis by Au NPs was their high potency towards the oxidation of CO into CO2 [22]. 24.3.1.1 Over CeO2-Based Au Nanocatalysts Oxidation of CO is the most investigated reaction for evaluating the catalytic activity of CeO2-based Au catalysts. It has succeeded in gaining attention due to following reasons: (i) the catalyst are highly active in the oxidation of CO at low-temperature level; (ii) exposure of CO to a few hundred ppm can cause permanent health damage or even death due to its high toxicity; (iii) catalytic oxidation of CO is practically very useful, since it eliminates the traces of CO in H2 streams for the purification of feeds to hydrogen fuel cells for automobiles (the fuel cell catalysts are poisoned by small amounts of CO); (iv) catalytic CO oxidation occurs at subambient temperatures, and it can be investigated by methods such as infrared and X-ray absorption spectroscopies; and (v) CO as a reactant offers the advantages of a good probe of catalyst surface structures, as adsorbed CO is easily characterized by numerous spectroscopic methods, notably including infrared spectroscopy [23].

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TABLE 24.1(A)

CeO2-Based Multicomponent Au Nanocatalyst Reduction

Catalyst Au/CeO2ZrO2

Support

Au Particle Calcination Size (nm)

Preparation Method

CeO2ZrO2



DPU

Air; 400 C 

Application

Reference

4

CO oxidation [12,13]

Au/CeO2ZrO2

CeO2ZrO2

Anion exchange

300 C for 4 hrs

15

CO oxidation [14]

Au/CeO2SiO2

CeO2SiO2

Incipient wetness impregnation followed by depositionprecipitation

200 C, 400 C & 600 C for 4 hours

1015

CO oxidation [15]

Au/CeO2SBA-15 CeO2SBA-15 Liquid-phase depositionprecipitation method

300 C for 2h

5

CO oxidation [16]

Au/CeO2KIT

CeO2KIT

do

do

2.5

do

do

Au/CeO2HMS

CeO2HMS

do

do

2.5

do

do



Au/CeO2TiO2

CeO2TiO2

Depositionprecipitation with urea (DPU) method

400 C for 12 h

4

Reduction of [14] 4-NP and organic dyes

Au/CeO2TiO2

CeO2TiO2 NRs & NPs

Colloidal deposition

250 C for 4h

Less than 5

CO oxidation [17]

Au/CeO2ZnO/ Al2O3

CeO2ZnO/ Al2O3

Coprecipitation

350 C for 4h

35

CO free H2 production

[18]

TABLE 24.1(B) TiO2-Based Multicomponent Au Nanocatalyst Reduction Catalyst

Support

Preparation Method

Au Particle Calcination Size (nm) Application

Au/TiOx/SBA15

TiOx/ SBA-15

Photodeposition

__

10

H2 evolution

[19]

Au/CeO2TiO2

CeO2TiO2

Depositionprecipitation with urea (DPU)

400 C for 12 h

4

Reduction of 4NP and Organic Dyes

[14]

Au/CeO2TiO2

CeO2TiO2 NRs & NPs

Colloidal deposition

250 C for 4 h

Less than 5

CO Oxidation

[17]

Au/ZrO2TiO2

ZrO2TiO2

Depositionprecipitation with urea (DPU)

550 C for 4 h

Less than 5

Oxidation of methanol

[20]

Au/TiO2SiO2

TiO2SiO2

Depositionprecipitation

946 C

36

Propene epoxidation

[21]

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FIGURE 24.1 Catalytic reductive degradation of organic dye by Au/CeO2TiO2 nanohybrid. Reproduced from P. Saikia, et al., Highly efficient catalytic reductive degradation of various organic dyes by Au/CeO2TiO2 nano-hybrid. J. Chem. Sci. 129 (2017) 8193 with permission.

Numerous factors have been studied, including the Au particle size and its size distribution, Au oxidation state, nature of the support, and the preparation method to elucidate low-temperature CO oxidation activity of Au catalysts. It is revealed that Au particles (particularly ,5 nm) with high degree of dispersion is a very fundamental requirement for low-temperature CO oxidation and particles larger than 5 nm are less active in the low-temperature range, and hence the catalytic activity is shifted to the high-temperature region. Nature of supporting oxide is influential because interactions between Au and oxides play a significant role in the CO oxidation [24]. Carabineiro et al. studied comprehensively the different driving factors including effect preparation methods, Au particle size, and nature of supports on CO oxidation [25]. Investigating the variation observed in CO oxidation activity, the different metal oxides are categorized into two types, viz., reducible metal oxide (RMO) and irreducible oxide (IRO), depending on the ease of reducibility of the supports. RMO supports (e.g., CeO2, TiO2, ZrO2, Fe2O3, Co3O4, etc.) are easily reduced and they are generally able to store and release oxygen under oxygen-rich and oxygenlean conditions, respectively. In contrast, the IRO materials (e.g., SiO2, Al2O3, MgO, ZnO, etc.) are comparatively difficult to be reduced [26]. Literature reveals that Au/RMOs catalyze the oxidation of CO at lower temperatures compared to Au/IROs. Therefore, the RMOs are termed as active and IRMOs are as inert support for CO oxidation. The difference between active and inert supports can be attributed to their O2 adsorption capacity in the CO oxidation [24,26]. An inert oxide possesses poor O2 adsorption ability and the oxidation of CO occurs by dissociative adsorption of O2 on the surface of the Au particles. On the contrary, an active oxide has a strong ability to adsorb O2 where it may or may not be dissociated before reacting with CO adsorbed on the Au. At this stage, oxidation of CO is sensitive to the microcrystalline structure of the Au/support interface. However, reducibility can be tuned by adding CeO2 to other oxide supports, as investigated by Idakiev et al. in the case of CeO2-modified meso-macroporous TiO2ZrO2 supported gold nanocatalysts [27].

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Among different metal oxides, CeO2 has attracted noteworthy attention, due to the presence of oxygen vacancy defects generated by Ce41/Ce31 redox cycle, which facilitates O2 adsorption and activation for CO oxidation [28]. CeO2 itself is catalytically active for the oxidation of CO only at high temperature region which causes sintering of ceria particles due to its poor thermostability. Due to this, catalytic activity of pure ceria is observed to be lowered. In this regard, the high temperature requirement with pristine ceria is not convenient for practical applicability, which restricts its catalytic worth [29]. Interestingly, the thermal stability of CeO2 is robustly improved upon doping of appropriate metal ions into the CeO2 lattice, which simultaneously enhances its unique structural and redox properties [29,30]. Gold-supported CeO2 (Au/CeO2) is well-recognized for its excellent activity in lowtemperature CO oxidation. Han et al. studied CO oxidation at low-temperatures over Au/CeO2 hybrid nanocatalysts prepared by different solution-based routes (namely, hydrothermal depositionprecipitation (HDP), hydrothermal precipitation (HP), coprecipitation hydrothermal (co-PH), coprecipitation (co-P), and depositionprecipitation (DP)) [31]. The extent of CO oxidation activity of the catalysts was found to be dependent on the preparation methods. It was reported that catalytic activities differed according to the morphology (preparation method) of CeO2. For example, Yi et al. experienced that by using Au/CeO2 nanohybrids that the CO conversion depended on the shape (polyhedra, cube, or rod), i.e., the crystal planes of CeO2 [32]. In recent years, two component nonceria-supported Au catalysts are modified with CeO2 to fabricate three-component catalysts. In these catalysts, the CeO2 component acts like a promoter for various catalytic applications. The idea of post-addition of CeOx modifier to Au catalysts was introduced by Senanayake et al. [33]. Literature reveals that introduction of CeO2 to inert oxide-supported Au catalysts, such as Au/Al2O3, Au/SiO2 (SBA-15, KIT-6, HMS), Au/ZnO, results in remarkable enhancements in the CO oxidation activity [3436]. Qian et al. studied the influences of various CeO2 microstructures on the structure and activity of Au/CeO2/SiO2 catalysts in CO oxidation [15]. Au NPs were dispersed on the CeO2 aggregates on SiO2 as well as on the pure SiO2 surface. Au supported on CeO2/SiO2 prepared by wet impregnation and calcined at only 200 C was the most active sample. Au (I) species on CeO2 alone was not active in CO oxidation giving the impression that Au NPs in contact with CeO2 in Au/CeO2/SiO2 catalysts were vital in catalyzing CO oxidation reaction. Castano et al. experienced an inhomogeneous distribution of Au NPs on CeO2-decorated hexagonal mesoporous silica (HMS) support [37]. Crystalline CeO2 domains were detected on the mesoporous silica with Au particles (introduced by DP) on their surfaces; however, many Au particles remained isolated from CeO2 particles of 410 nm size. When mesoporous silica modified by CeO2 loading was done by direct synthesis or impregnation, the impregnated sample exhibited better catalytic activity. The suggested reasons were closely related to higher Au dispersion, highest content of metallic Au, and larger extent of CeO2 coverage on HMS (more effective oxygen mobility, higher redox ability) [38]. In view of the abovementioned facts, it can be concluded that the preparation method and the pretreatment conditions are very important when the effect of CeO2 loading on the properties of AuCeO2 interface is studied using inactive oxide (SiO2) support

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to provide high surface area against sintering. If there are Au particles not in contact with CeO2 but SiO2, then the overall activity measured will be the sum of activities originating from Au/SiO2 and Au/CeO2SiO2 are as present in the sample [39]. 24.3.1.2 Over TiO2-Based Au Nanocatalysts Prof. M. Haruta’s legendary discovery transformed catalytically inactive bulk gold into a highly active low temperature CO oxidation catalyst with the formation of nanosized Au dispersed on TiO2 [1]. The crystalline phases of TiO2 (i.e., anatase, rutile, brookite, and commercial P25) significantly influence the particle size, stability, and CO oxidation efficiency of Au nanocatalysts under the identical pretreatment and reaction conditions [40]. Experimental evidences (XRD and TEM results) demonstrate that the Au particles supported on brookite are smaller than those on the other supports following the reaction and pretreatment sequences [40]. Among different catalyst types, the brookite-supported Au catalyst maintains the highest catalytic activity after all treatments. Au/TiO2 catalysts prepared by arc plasma deposition (APD) revealed higher CO oxidation activity than that obtained by solgel method. A direct particle size-dependent activity was established, i.e., catalytic activity regularly increased with decreasing size of the Au particles. The inferior activity of the solgel Au/TiO2 hybrid nanocatalyst is attributed to the formation of larger Au (10 nm) particles. However, an opposite picture was disclosed for Pt/TiO2 APD nanocatalyst despite being associated with smaller Pt particles (2 nm) than the Au in TiO2 APD (2.73.7 nm). The Pt/TiO2 APD nanocatalyst exhibited even much poorer activity than the solgel Au/TiO2 nanocatalyst under identical conditions [41]. It has investigated by Mohapatra and his coworkers that the activity of titania supported gold catalysts is greatly enhanced by sulfate ion addition. Nitrate, phosphate, fluoride, and chloride promotion of Au/TiO2 catalyst for CO oxidation have all been observed, particularly at relatively low anion contents [42]. The activity for CO oxidation is strongly dependent on the preparation method in Au/TiO2. The titania-supported gold catalyst prepared by depositionprecipitation is found to be more active towards CO oxidation [43]. It has been recently reported that the addition of iron between 1 and 3.5 wt.% to high-area TiO2 (Degussa P25, a 80:20 mixture of anatase and rutile) increases the catalytic activity of Au/TiO2 towards the lowtemperature CO oxidation [44].

24.3.2 VOCs Oxidation Gold nanoparticles supported on metal oxides are efficient catalysts for important oxidation process, including selective oxidation of hydrocarbons and oxidation of various volatile organic compounds (VOCs), such as CO, CH3OH, and HCHO, at moderately elevated temperatures. 24.3.2.1 Over CeO2-Based Au Nanocatalysts The SPR absorption and the catalytic activity of gold nanoparticles presents an important opportunity: if the heated gold NPs could activate the organic molecules on them to

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induce oxidation of the organic compounds, then oxidation on gold catalysts can be driven by visible light at ambient temperature [45]. Scire et al. studied the catalytic combustion of some representative volatile organic compounds (VOCs) (2-propanol, methanol, and toluene) on gold/cerium oxide catalysts prepared by coprecipitation (CP) and depositionprecipitation (DP). It has been found that the presence of gold enhances the activity of cerium oxide towards the oxidation of the selected volatile organic compounds [46]. Effect of preparation conditions of CeO2 support on the catalytic combustion of HCHO over Au/CeO2 was studied. It was found that modification of CeO2 via the addition of dispersing agent (PEG-6000) and microwave irradiation treatment (10 min) during aqueous precipitation method enhanced the interaction between CeO2 and Au and thus improved the catalytic activity [47]. The unmodified CeO2 supported catalyst required 146 C for complete conversion of HCHO, which was lowered to 98 C with the modified CeO2 supported catalyst. Au0 and Au1 species coexisted in the Au/CeO2 catalysts and the activity of catalysts correlated with Au1/Au0 ratio. Temperature-programmed reduction (TPR) results demonstrated that the reduction peak appeared between 100 C and 170 C with the influence of ionic gold. The dependence of activity on the reduction peak temperature leads to the fact that ionic gold was a catalytically active component for HCHO oxidation. Among three preparation methods (CP, DP, and MCD) employed, Au/CeO2 hybrid nanocatalyst prepared by DP method was found to be the most efficient and CP method produced the least efficient catalyst [48]. 24.3.2.2 Over TiO2-Based Au Nanocatalysts AuTiO2 NPs shows a strong absorption of the visible light due to the surface resonance plasmon (SPR) of their free electrons [49,50]. Due to which the AuTiO2 plasmonic photocatalyst exhibited high efficiency in UV or visible light photoactivated reactions such as 2-propanol degradation [51,52], chemoselective oxidation of alcohols [53], CO2 reduction [54], and water splitting for H2 and O2 generation [55,56]. Au/CeO2 is a significant catalytic material for VOC oxidation, but single ceria would be sintered after calcination above 750 C. To avoid this discrepancy, antisintered oxide TiO2 is mixed, which forms an outstanding catalytic material for total oxidation of VOCs [57]. Santos and his coworker investigated the oxidation of CO, ethanol, and toluene on noble metal catalysts supported on TiO2. It was observed that the Au/TiO2 hybrid nanocatalyst prepared by liquid phase reduction deposition (LPRD) is very active for the CO oxidation, and for VOC oxidation also it was satisfactorily active [58].

24.3.3 Organic Transformations 24.3.3.1 By CeO2 Supported Au Nanocatalysts Nanosized gold supported ceria are highly active and selective heterogeneous catalysts for aerobic oxidation of alcohols. Au/CeO2 nanocatalysts effectively oxidize primary alcohols to aldehydes under solvent free conditions at 100 C in the presence of O2 [59]. The nanoscale ceria surface stabilizes the positive valence states of gold by creating Ce(III) and oxygen-deficient sites in ceria due to the electronic interaction between them. Primary

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aliphatic alcohols were more disinclined to undergo oxidation in the absence of solvent, and predominantly gave the corresponding ester with high selectivity. The esters were directly formed via the hemiacetal intermediate, which was dehydrogenated [60]. 1-Octen-3-ol (an allylic alcohol) undergoes chemoselective oxidation with Au/CeO2 catalysts to the corresponding ketone without oxidizing or isomerizing the C 5 C double bond [61]. A mesoporous crystalline CeO2 film decorated with gold particles of size B5 nm, exhibited excellent catalytic activity and stability for aerobic benzyl alcohol oxidation in the absence of solvent or base [62]. Several factors, including the Au particle size, its valence state, CeO2 unique properties, preparation method, etc., influence the catalytic performance of Au/CeO2 in the alcohol oxidation [61,63,64]. Sudarsanam et al. studied solvent- and alkali-free benzyl alcohol oxidation with green oxidant O2 over Au/CeO2 catalysts prepared by two different methods, namely, direct anion exchange (DAE) and homogeneous depositionprecipitation (HDP) [64]. Au/CeO2 catalyst prepared by the HDP method exhibited better catalytic performance owing to smaller Au NPs (B5.3 nm) and plentiful oxygen vacancies. Meanwhile, pure CeO2 exhibited nominal activity and only 3.4% of benzyl alcohol conversion was obtained. Apart from the aforementioned factors, selection of solvent is indispensable when the reaction is carried out in solvent because it plays a major role in alcohol oxidation. Haider et al. studied the benzyl alcohol oxidation over Au/CeO2 catalyst using different solvents. The optimum benzyl alcohol conversion and benzaldehyde selectivity were found to be B24% and 98%, respectively, in mesitylene solvent at 373K of reaction temperature for 3 h of reaction time [63]. In a recent study, Zhang et al. investigated visible-light-driven photoelectrocatalytic (PEC) selective aerobic oxidation of benzyl alcohols to corresponding aldehydes by using Au/CeO2TiO2 nanotubes as the photocathodes [65]. The excellent performance obtained was attributed to the synergistic PEC process combining the advantages of both photocatalytic and electrocatalytic activity. Under catalytic effect of ceria-based materials, the organic compounds are reduced via the different pathways like, hydrogenation of C 5 C double bonds (e.g., hydrogenation of phenol, 1,3-butadiene, acetylene, acrylonitrile, mesityl oxide, and sunflower oil, benzene, etc.), hydrogenation of C 5 O bonds (e.g., hydrogenation of α,β-unsaturated aldehydes, carboxylic acids, aldehydes, esters, etc.), and hydrogenation of CN bonds (e.g., gas-phase hydrogenation of acetonitrile, o-chloronitrobenzene, etc.) [59,66]. Traditional coupling reactions such as Knoevenagel condensation, aldol condensation, Mannich reaction, SuzukiMiyaura, or Sonogashira cross couplings have also been reported [66]. Corma et al. discovered an excellent activity of TiO2-supported Au NPs for the highly selective reduction of substituted nitroarenes to their corresponding anilines in the presence of H2 [67,68]. Mesostructured ceria-supported gold (Au/meso-CeO2) is a highly versatile and flexible catalytic system for controlled and selective transfer reduction of nitroarenes with 2-propanol as the hydrogen source [69]. Ceria and ceria-based oxide-supported Au NPs with different morphologies were employed for the reduction of 4-notrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH4 as the reducing agent. Yolkshell AuCeO2@ZrO2 nanoreactors obtained via the replacement of SiO2 shell with porous ZrO2 of AuCeO2@SiO2 template were three times more

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active than the Au@ZrO2 nanoreactors in 4-NP reduction. The enhanced activity of AuCeO2 nanoreactors was explained by feasible electron transfer between CeO2 and Au NPs as well as strong affinity of CeO2 for 4-NP adsorption. The CeO2-decorated yolkshell nanoreactors showed rather improved CO oxidation activity (B1.5 times higher) in comparison to the nondecorated ones [70]. Au NPs (B5 nm in diameter), fabricated by pulsed laser ablation in liquid (PLAL), were assembled onto the surface of CeO2 nanotubes (NTs) to synthesize novel PLALAu NP/CeO2NT composite catalyst for reduction of 4-NP [71]. It exhibited remarkably higher reduction rate due to the unique, relatively “bare” surface of the PLALAu NPs as well as oxidized Au species induced by the strong interaction between the “barrier-free” surface of PLALAu NPs and surface defects (oxygen vacancies) of CeO2 NTs. 24.3.3.2 By TiO2-Supported Au Nanocatalysts TiO2-supported gold catalysts have been utilized in various organic transformation reactions. A highly efficient and selective reaction for the synthesis of 2-substituted benzoxazoles and benzimidazoles catalyzed by Au/TiO2 has been developed through two hydrogen-transfer processes. Different templates were used as support with metal nanoparticles to selectively synthesize 2-phenylbenzoxazole. The desired product was selectively obtained in a good yield of 90% under the catalysis of Au/TiO2, while very poor yields were obtained with other supports. These results showed that the nature of the support had a strong influence on the activity of the Au catalysts and TiO2 was the best support for this reaction [72]. In an another investigation, the catalytic performances of the silica-supported gold catalyst and titania doped silica-supported gold catalysts were tested in a reaction time of 1 h where it was revealed that the Au/SiO2 catalyst showed a conversion of 0.9%, while in comparison, the Au/TiO2/SiO2 catalyst showed a conversion of 3.2% with higher selectivities of cyclohexanone and cyclohexanol. This means a small quantity of titania doping can effectively increase the catalytic performance of the silicasupported gold catalyst [73]. Abundant research works have been found on propene or propylene epoxidation. Direct epoxidation of propene to propene oxide using O2 over a Au/TiO2 hybrid nanocatalyst was studied by Kanungo et. al. The role of the oxygen atoms of the titania support was studied by quantum-chemical studies which shows that the mechanism which involves CO as a coreactant proceed via surface oxygen vacancies, whereas with H2 the pathway involving OOH is favored [74]. Supported gold nanoparticles are surprisingly active and selective catalysts for several green oxidation reactions of oxygen-containing hydrocarbons using molecular oxygen as the stoichiometric oxidant. Klitgaard employed bifunctional goldtitania catalysts to facilitate the oxidation of amines into amides with high selectivity. It was revealed that AuTiO2 nanocatalyst is capable of catalyzing the oxidation of amines into amides. The reaction was demonstrated for n-hexyl amine and 1,6-hexanediamine which could be oxidized into amides, N-hexyl hexanoic amide and caprolactam, respectively. These oxidation procedures could eventually find application as new green routes to nylon-6 precursors [75].

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24.3.4 Photocatalysis 24.3.4.1 Au/CeO2 nanohybrids as Photocatalyst It is well documented that visible-light photocatalytic activity of semiconducting oxide (TiO2, Nb2O5, CeO2, and ZnO) materials is remarkably enhanced upon decoration of nanosized coinage metal particles over them. This is attributed to localized surface plasmon resonance (SPR) effect caused by the coinage MNPs [76]. This opened up the gateway of plasmonicphotocatalysts into the realm of selective redox transformations under visible-light irradiation. Moreover, metal oxide-supported coinage MNPs are excellent heterogeneous catalysts for thermally induced redox conversions, which can facilitate the improvement of the plasmonic photocatalysis. Conventional ceria with a large particle size acts as insulator and lacks photocatalytic activity under visible-light irradiation. In contrast, the visible-light photocatalytic activity is significantly improved upon deposition of Au NPs on ceria. Presence of Au NPs effectively improves the photoconversion efficiency and light harvesting ability of ceria [7779]. In addition, Au NPs can effectively suppress the recombination of photoexcited charge carriers (e2h1 pairs) and enhance the photocatalytic activity of Au/CeO2 nanocomposite [79,80]. A novel and efficient ceria-based gold photocatalyst with visible-light activity has been developed recently by Primo et al. with comparable or higher efficiency than those currently known [78]. Au/CeO2 nanocomposite photocatalyst prepared using electrochemically active biofilms (EABs) showed much higher visible-light degradation of MO (methyl orange) and MB (methylene blue) than pure ceria. The superior visible-light activity of the nanocomposite was due to the reduced electronhole recombination, enhanced light harvesting capability of the Au NPs, and presence of a large number of interfaces between Au NPs and ceria, which triggered charge transfer [79]. The nature of charge or energy transfer processes between MNPs and semiconductor nanocrystals depends on the electrical and optical properties of the two materials, surface properties of the nanocrystals, and wavelength of the irradiated light [79,81,82]. Photochemically deposited Au NPs on commercial ceria (1 wt.% Au/CeO2) powder in the presence of citric acid as the reducing agent do better than other gold catalysts in the mineralization of organic acids (formic acid, acetic acid, and oxalic acid) in an aqueous suspension under visible-light irradiation [83]. The same Au/CeO2 catalyst also selectively oxidizes aromatic alcohols to corresponding aldehydes almost quantitatively in an aqueous suspension under the irradiation of green-light in the presence of O2 [84]. Meanwhile, formation of aldehydes over pristine CeO2 is negligible under the parallel reaction conditions. The same kind of result is also noted under dark (with Au/CeO2) and/or in the absence of Au/CeO2 (with light irradiation) catalyst, implying that Au/CeO2 and visiblelight irradiation are crucial for the investigated reaction. The outstanding photocatalytic activity of Au/CeO2 sample was attributed to the stronger SPR absorption at 550 nm, which increased with increase in the external surface area of Au deposited on CeO2, and the rate of aldehydes formation under visible-light irradiation exhibited linear dependency on the external surface area of the supported Au. Besides oxidation reactions, ceria-based gold photocatalysts have recently been employed to promote the reduction of organic compounds under visible-light irradiation with high conversion and selectivity. Ke et al. showed that Au/CeO2 nanohybrid

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FIGURE 24.2 Solar light-induced catalytic reduction of nitrophenol by Au/CeO2TiO2 catalyst. Reproduced from A. Taleb Miah, et al., Catal. Lett. 146 (2016) 291303.

efficiently catalyzes the selective reductions of organic compounds under visible-light irradiation at ambient temperatures [85]. Later, the same research group studied the effect of supporting oxides for Au/support photocatalyst (supports: CeO2, TiO2, ZrO2, Al2O3, and zeolite Y) in the reduction of nitro-aromatics to azo compounds, hydrogenation of azobenzene to hydroazobenzene, reduction of ketones to alcohols, and deoxygenation of epoxides to alkenes [86]. The SPR absorption caused by the Au NPs was demonstrated to be vital for the catalytic activity, and the active AuH species reacted with N 5 O, N 5 N and C 5 O double bonds or epoxide bonds depending on the Au NPs excited electrons energy, which can be manipulated by changing the wavelength [85,86] (Fig. 24.2). 24.3.4.2 Au/TiO2 nanohybrids as Photocatalyst TiO2, due to its robustness, outstanding photochemical stability, availability, affordability, and lack of toxicity has massive photocatalytic utilization. To improve the photosensitization of TiO2, Au is deposited on the surface of TiO2. Titania-on-gold “inverse nanocomposites” were formed by loading smaller TiO2 nanoparticles onto larger gold nanospheres by combining seed-mediated method with solvothermal method. This noble nanoarchitecture exhibits excellent properties in hydrogen evolution from water splitting under visible-light irradiation. The enhanced performance was attributed to the synergistic effect between ultrafine TiO2 nanoparticle and localized surface plasmon resonance effect of gold nanoparticles, which induce the faster separation of photogenerated electrons from Au nanospheres to TiO2 nanoparticles. Moreover, the nanocomposites have a strong absorption at around 550 nm, corresponding to the maximum irradiation intensity of the solar spectrum. Owing to these outstanding properties, the titania-on-gold nanoarchitectures showed promising and great potential for practical

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529

applications in the photocatalytic field [87]. Ga¨rtner et al. also prepared Au/TiO2 catalysts which contain gold nanoparticles with a size of 315 nm that showed stable hydrogen generation for more than two days, with good activities [88]. Different organic pollutants including phenols and carboxylic acids have been found to be degraded by using various photocatalyst. Iliev and coworkers studied oxalic acid degradation using Au/TiO2 photocatalysts of different particle sizes. They found that the photocatalytic activity of Au/TiO2 increases as the particle size of gold decreases [89]. The key factor of the use of the Au/TiO2 is the possibility to introduce response in the material upon irradiation with visible light by excitation on the surface plasmon band of gold. Kowalska and coworkers have examined the action spectra of a series of 15 Au/TiO2 samples (2% Au wt.) prepared by photodeposition method for the degradation of simplecarboxylic acids which were tested for the photooxidation of the acetic acid and 2-propanol to form CO2 and acetone, respectively. It was revealed that gold deposition enhanced significantly the UV-induced photocatalytic activity and also Au deposition renders all the photocatalyst active under visible-light irradiation, showing an increase of the photocatalytic degradation efficiency along an increase in the Au particle size. The most significant observation was a clear correspondence between the surface plasmon band absorption and the efficiency for 2-propanol oxidation [90]. Zheng et al. developed a simplistic in situ procedure of preparing noble metal plasmonic photocatalysts M@TiO2 (M 5 Au, Pt, Ag). Among the three photocatalysts M@TiO2 (M 5 Au, Pt, Ag), Au@TiO2 exhibits a high yield (63%) and selectivity (91%) for the oxidation of benzene to phenol in aqueous phenol [91].

24.4 CONCLUSIONS Supported nanostructured metal oxides are of significant importance to both the basic understanding of size- and shape-dependent properties and various applications. At the same time, development of efficient synthetic methodologies are also equally important. We have tried to provide an overview of the different synthetic methods and probable applications of ceria- and titania-based Au hybrid nanocatalysts in this chapter. However, newer techniques and diverse applications of these fascinating materials are emerging very rapidly and therefore, interest on this subject is expected to intensify further in the coming days.

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C H A P T E R

25 Radiolabeled Theranostics: Magnetic and Gold Hybrid Nanoparticles Ayuob Aghanejad1 and Yadollah Omidi1,2 1

Research Center for Pharmaceutical Nanotechnology, Tabriz University of Medical Sciences, Tabriz, Iran 2Department of Pharmaceutics, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran

25.1 INTRODUCTION Hybrid nanoparticles (HNPs) composed of a targeting agent such as antibody (Ab) or aptamer (Ap), imaging agent (e.g., radionuclide, fluorophore), and therapeutic agent have been developed as theranostic nanocarriers for targeted detection and therapy. Several types of HNPs (e.g., metal/metal oxide, polymer/metal, metal/graphene, lipid/ polymer, radioisotope/metal, etc.) have extensively been explored for the development of improved drug delivery systems (DDSs) with maximal therapeutic effects and minimal side effects [13]. The physicochemical features (e.g., morphology, size, surface area, multivalency, and unique optical characteristics) of various metal-based NPs, in particular, gold NPs (AuNPs) and magnetic NPs (MNPs), have widely been exploited to improve the efficiency of target-guided imaging/therapy molecular probes [4]. Further, so far a number of diagnostic procedures has been used for simultaneous diagnosis and therapy of diseases/malignancies using theranostics. Among them, nuclear imaging approach using radiolabeled tracers has been well-established in medical practice and research for decades [5,6]. Various radiolabeled NPs have been employed as hybrid NSs. Accordingly, a number of potent diagnostic modalities, whose applications depend on the radionuclide’s property, has been used as hybrid NPs in imaging techniques such as single-photon emission computed tomography (SPECT) and positron emission tomography (PET) [7]. Overall, the potential of radioisotope-containing NPs (i.e., radiolabeled HNPs) in the accurate medical imaging procedures make them very valuable diagnostic and therapeutic agents. Moreover, radiolabeled hybrid NPs (RHNPs) provide great

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00025-5

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TABLE 25.1 Selected Targeted Nanohybrids for PET and SPECT Imaging Nanohybrid

Radionuclide Half-life

Targeting Agent

Application

Ref.

Magnetic

64

12.70 h

C-type atrial natriuretic factor peptide

PET imaging of prostate cancer cells

[9]

59.49 d

3H11 mAb

SPECT/MR imaging of gastric cancer models

[10]

6.02 h

Cyclic ArgGlyAspDPheLys (cRGDfK) peptide

SPECT/MR imaging of glioblastoma

[11]

78.41 h

Cetuximab

PET imaging of the epidermal cancer

[12]

Tc

6.02 h

GGC peptide

SPECT imaging of lymph node

[13]

I

59.49 d

cRGD peptide

SPECT/CT imaging of small cell lung cancer models

[14]

Cu

125

I

99m

Gold

89

Tc

Zr

99m 125

possibility for whole-body imaging with high quantitative and sensitivity, and hence are considered as one of the most reliable and robust methods for the in vitro and in vivo molecular imaging [8]. Table 25.1 shows some selected radiolabeled nanohybrids used for CT, SPECT, PET, and MR imaging. In fact, development of molecular nanoprobes for the target-guided imagining of biological events in the living body without any disruption at molecular and cellular levels can remarkably be beneficial not only for the detection of disease mechanisms but also for the improvement of targeted therapy of designated cells/tissue, in particular in different types of solid tumors.

25.2 IMAGING MODALITIES Different imaging modalities such as isotopic PET and SPECT imaging, computed tomography (CT), and magnetic resonance imaging (MRI), as well as optical imaging by means of various fluorophores have been employed for the accurate diagnosis of a number of formidable diseases, such as malignancies.

25.2.1 PET and SPECT Imaging Systems Undoubtedly, PET is one of the most valuable quantitative imaging technologies with the great detection ability of abnormalities at the molecular and cellular levels [15,16]. In this molecular imaging system, extremely minimized doses of the radiopharmaceuticals with high specific activity are used to acquire high-resolution images—an imaging process that is considered to be safe to other healthy tissues/organs [17,18]. SPECT is another interesting technique with unique features that can be employed for the imaging of emerging biological phenomena at molecular/cellular levels. In addition,

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the progression of malignancy, infections, or inflammations and even the delivery of radiopharmaceuticals have been imaged by SPECT and hybrid imaging techniques such as SPECT/CT [19,20]. Moreover, PET, SPECT, and their hybrid imaging systems (e.g., MRICT hybrid imaging system) were shown to have significant potential in the simultaneous detection of various biological functions. For the probing of different molecular processes by these imaging systems, NP-based probes have successfully been used for multimodal bioimaging using radioisotopes with different emission energies [21,22].

25.2.2 MRI, CT, and Optical Imaging Systems MRI is a nonionization approach, which spots water molecules in the tissues/organs at a magnetic field. This imaging approach has a significant potential for the detection and status of diseases such as tumor progression and even responses to treatment modalities. So far, a number of contrast agents (e.g., MNPs and hybrid NPs) have been used for the implementation of this powerful diagnostic technique [23]. CT as a diagnostic and costeffective tool has been developed for the X-ray-based monitoring of various diseases including different types of solid tumors. Currently, the most common contrast agents of CT are surprisingly used nonspecifically. They are not able to target the desired cells/tissues specifically, failing to detect cancer biomarkers and biological components. Additionally, different types of optical imaging methods based on hybrid NPs (e.g., nearinfrared, fluorescence, etc.) have been developed for monitoring of the biological events, in large part due to their significant complementary imaging features [24,25]. Hence, to provide images with significantly high specificity and resolution, the multimodal imaging systems which merge functional imaging techniques (e.g., SPECT, PET) with anatomically three-dimensional (3D) modalities (e.g., CT, MRI) can be effectively employed and result in improved imaging in comparison with the single modality operating systems [26,27]. The PETCT/MRI and SPECTCT, as remarkable dual-modality imaging techniques, have been employed to improve the medical applications, particularly in diagnosis and therapy of cancers, providing much more functional information, an enhanced soft-tissue contrast, and a lower radiation exposure [28,29]. Fig. 25.1 represents different imaging techniques and multimodal nanohybrids (NHs) as imaging agents.

25.3 RADIOLABELED HYBRID AuNPs Recently, various types of NHs such as superparamagnetic nanoparticles (SPIONs), quantum dots (QDs), and AuNPs have been developed for targeted molecular detection of tumor cell/organs. The promising potential of NPs for functionalization with various ligands make them unique tools for a target-guided imaging. Different moieties can be conjugated to NPs/NHs, including antibodies (Abs), aptamers (Aps), peptides, nucleic acids, and proteins. Based on these facts, radiolabeled functionalized metal NPs, such as magnetic/gold NPs, as imaging probes play an important role in cancer detection by PET, SPECT, and their combined imaging modalities (e.g., SPECT/MR, PET/MR, SPECT/CT, and PET/CT), resulting in reliable information about the cellular and molecular

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FIGURE 25.1 Radiolabeled nanohybrids (NHs) for multimodality in vitro and in vivo targeted imaging of cancer. (A) Schematic representation of a radiolabeled nanohybrid conjugated with targeting agent. (B) Various imaging techniques for the diagnosis of cancer. SPECT: Single-photon emission computed tomography; PET: Positronemission tomography; CT: Computed tomography; MRI: Magnetic resonance imaging.

interactions from increasing the signal to noise ratios in desired organs/cells [30,31]. These RHNPs are able to target the diseased organs/cells and provide comprehensive physiologic data in both in vivo and in vitro for diagnosis and treatment of cancers.

25.3.1 Radiolabeled Hybrid AuNPs for PET Imaging The AuNPs distinctive physicochemical properties (e.g., biocompatibility, large surface area, and high stability and photoacoustic features) make them as significant NHs for biomedical applications. Various gold-based nanostructures have been preclinically investigated for imaging/therapy in the cancer cells/tissue, including nanocages, nanoshells, nanorods, and nanospheres [32]. Biologically, the AuNPs are considered as nontoxic and inert materials, which can be used for passive and/or active targeting of desired cell/ organs. PET imaging by means of radiolabeled AuNPs at the target cell/tissue provides key quantitative molecular/cellular imaging information on the diseases with high sensitivity and efficiency. In one study, PET/CT imaging of dendritic cell (DC) using adenine-rich oligonucleotide conjugated 124I-AuNPs was performed for tracking of the DCs migration towards lymph nodes. The DCs were labeled with radionuclide-grafted AuNps and their antitumor immune responses examined in mice models. Interestingly, the results revealed the mice that immunized with Lewis lung carcinoma (LLC) lysates and labeled DCs exhibited a strong antitumor activity [33]. In another study, the 64Cu-labeled gold nanoclusters (NCs) functionalized with plerixafor (AMD3100) were used for the detection of the CXCR4 upregulation in the lung metastasis and primary tumors in the breast cancer models using PET imaging. These radiopharmaceuticals provided great capability for the specific detection of tumors expressing CXCR4, which were then proposed as a target-specific RHNPs for the early and accurate detection of the breast cancer with lung metastasis [34]. The 68Ga-labeled gold NHs (AnNHs) are the dual-modality PET/surface-enhanced resonance Raman scattering (SERRS) imaging probe that was developed for the imaging of both pre-operative and intraoperative profile of diseased lymph nodes. As a result, these

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PETSERRS nanohybrids were proposed for the tracking of lymph node, and hence, imaging of the status of the disease after treatment strategies [35]. In one study, PET/CT imaging by means of 64Cu-labeled PEGylated AuNHs was developed for the tracking of T cells in patients with B-cells’ malignancy. For this purpose, the T cells were modified with a CD19 specific chimeric antigen receptor (CAR) and the firefly luciferase (ffLuc) through electrotransferring of DNA plasmids. As a result, these PEGylated GNP64Cu were proposed as a target-specific radiolabeled NHs for the detection and immunotherapy of cancer [36]. In another study, the PEGylated AuNPs were conjugated with arginylglycylaspartic acid (RGD) peptide and radiolabeled with 68Ga. The NH was then used as PET/MRI imaging tracer to study the overexpressing integrin αvβ3 receptor in U87MG cancer cells. The NHs showed specific accumulation in the U87MG glioma cells and appear to offer efficient potentials for integrin-related tumor imaging [37]. Similarly, the PEGylated hollow AuNPs were radiolabeled with 64Cu and decorated with cyclic RGD to develop a target-guided PET/CT imaging agent. The engineered NHs were examined for intratumor accumulation on the VX2 tumor-bearing rabbits, resulting in specifically targeting of the integrin αvβ3 expressing tissues/cells [38].

25.3.2 Radiolabeled Hybrid AuNPs for SPECT Imaging To investigate the vulnerability of atherosclerotic plaques, in a study, the technetium99m labeled AuNPs were conjugated with Annexin V and used as targeted dual modality SPECT/CT imaging nanoprobe. The engineered AuNHs were shown to serve as a potential radiopharmaceutical for specific diagnosis of apoptotic macrophages in ApoE knockout mice by means of SPECT/CT system [39]. In another study, to provide SPECT nanotracer, AuNPs were functionalized with bombesin (BBN) peptide and labeled with 67Ga and used for the preclinical studies on prostate tumor-bearing mice. These NHs were reported to be largely taken up by the gastrinreleasing peptide (GRP) receptor-expressing tumor cells. In the prostate tumor-bearing mice, an intraperitoneal administration of the NHs resulted in pancreas uptake due to the GRP receptor-mediated mechanism, while the enhanced permeability and retention (EPR) effect seemed to play the major role in the intravenous administration. Taken together, these NHs were proposed to serve as promising radiotracers for imaging of cancer cells [40]. Auger electron-emission is an effective approach for the diagnosis and therapy of diseases such as malignancies. In a study, the PEGlyted 125IAuNHs were developed to serve as a diagnostic and therapeutic agent for the delivery of radionuclides to the neoplasms. Based on the Auger-electron energies, these NHs showed a great potential for the imaging and therapy of cancer cells [41]. It should be noted that targeted radionuclide therapy of solid tumors provides the delivery of a high dose of radioactivity to target cells/tissues with decreased side effects to the surrounding normal tissues, and is considered as one of the important strategies for the detection and therapy of cancer cells/tissues. In a study, indium-111 (111In)-labeled PEGylated epidermal growth factor (EGF)-armed AuNHs were engineered to serve as a

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FIGURE 25.2 The whole-body SPECT images in xenograft mice bearing 231-H2N (H2N) and MDA-MB-468 (468) tumors 72 h after injection of 111In-Au-EGF, 111In-Au-EGF-PEG6000 and 111In-Au-EGF-PEG6000 plus 30 μg or 15 μg of EGF bulking dosages (T: tumor; S: spleen; L: liver; K: kidney; B: bladder). Source: Image was adapted with permission from a published work conducted by L. Song, S. Able, E. Johnson, K.A. Vallis, Accumulation of (1 1 1) Inlabelled EGF-Au-PEG nanoparticles in EGFR-positive tumours is enhanced by coadministration of targeting ligand, Nanotheranostics 1 (2017) 232243.

potential radiopharmaceutical for the detection of EGFR-positive breast cancer. The in vivo micro-SPECT/CT studies of the engineered nanosystem (NS) showed somewhat liver uptake with limited tumor accumulation, in large part because of the moderate expression of EGFR by the hepatocytes as well as the rapid clearance of NSs through the mononuclear phagocyte system. The coadministration of unlabeled EGF to reduce the liver uptake might enhance the accumulation of the NSs in the tumor sites. As a result, these NHs were proposed as the target-specific radiolabeled NSs for the detection of EGFR-positive cancer (Fig. 25.2) [42]. Similarly, the conjugation of cetuximab with 131I-labeled AuNHs was performed to target the EGF receptor-expression on the human lung cancer models. The 131I-labeled immune AuNHs exhibited the specific uptake in tumor cells and demonstrated significant accumulation of radiotracers in the tumor tissue. These radiolabeled NHs revealed a high specificity and sensitivity towards EGFR-expressing cells including cancer lung cells. As a result, it was proposed as a promising radiolabeled nanoprobe for the SPECT/CT imaging of the human lung tumors [43]. In a study, to provide targeted and radiolabeled NH for the SPECT/CT imaging, AuNPs were conjugated with the cyclic ArgGlyAsp (cRGD) and labeled with iodine-125 for the in vivo tracking of NHs as the nanoprobe. The SPECT/CT imaging of 125IcRGDAuNHs revealed substantial uptake of these NHs in the tumor cells/tissues, which indicates that they can serve for targeted imaging and radiotherapy of cancer [14]. The preclinical studies for tracking of RGD-modified 111In-labeled AuNHs were developed for target-guided imaging of αvβ3 integrin in the glioblastoma and melanoma models. The in vivo studies and SPECT/CT imaging demonstrated the notable uptake of RGD-modified AuNHs in integrin-expressing tumors. As a result, the NH was suggested for the targetguided detection of the integrin-expressing cancer cells/tissues [44].

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25.4 RADIOLABELED HYBRID MNPs 25.4.1 Radiolabeled Hybrid MNPs for PET Imaging Radiolabeled hybrid MNPs have also been used in PET imaging. Peripheral artery disease (PAD) is a disease in which narrowed arteries can reduce the blood flow to the limbs that may indicate ischemia in the tissues/organs. Recently, 64Cu-labeled MNPs were developed for the visualization of the ischemic tissues of hindlimb ischemia models. In this study, the PEGylated composite of reduced graphene oxide (rGO)-MNPs was radiolabeled with 64Cu to serve for the photoacoustic (PA) and PET imaging and also benefited from the passive-targeting capabilities of MNPs through EPR effect. The significant localization of 64CurGO-MNPs in the ischemic tissue were determined by PET imaging that indicated the potential of these radiolabeled magnetic nanohybrids (MNHs) for PA/PET dual imaging [45]. In another study, the silica-coated MNHs were engineered and labeled with 64Cu. The potential of this nanotracer was evaluated for the PET/MR imaging of cancer cells/tissues. Optimal in vivo accumulation of radiolabeled MNHs in organs and significant stability in serum substantially proved their usefulness for the PET/MR imaging of cancers [46]. In an interesting study, the PEGylated manganese oxide NPs were conjugated with anti-CD105 antibody, TRC105, and further labeled with 64Cu. The examination of the NHs in the 4T1 breast cancer model by PET/MR imaging revealed great PET/MR imaging specificity and sensitivity in terms of breast cancer detection [47]. Furthermore, solid tumors form irregular vasculature through angiogenesis. Tumor microvasculature (TMV) is a pathological process which favors the solid tumors’ progression and invasion. A number of TMV-based molecular markers can be targeted. Angiogenesis can also occur in plaque vulnerability and hemorrhage. Several peptide receptors have been considered as molecular targets for the early detection of angiogenesis using radiolabeled systems. For example, a nine-amino acid cyclic peptide (CTKNSYLMC, GEBP11) was shown to target the neovascularization of endothelial cells. To use its targeting capacity, a dual-modality imaging nanoprobe has recently been engineered by conjugating 2,3-dimercaptosuccinnic acid (DMSA)-coated MNPs (DMSA-MNPs) and 68Ga chelator 1,4,7-triazacyclononane-N,N’,N”-triacetic acid (NOTA) to the GEBP11 peptide (68Ga-NOTAGEBP11DMSA-MNPs). The 68Ga-NOTAGEBP11DMSA-MNPs nanoprobe was examined for the dual-modality PET/MR imaging of angiogenesis in the atherosclerosis models. The affinity of peptide was investigated by Prussian blue and immunofluorescence staining. As shown in Fig. 25.3, the findings of this study indicated that the 68Ga-MNHs could serve as a promising PET/MR imaging nanoprobe for the visualization of the vulnerable plaques [48]. The Gallium-68 labeled MNHs were shown to be selectively accumulated in the plaque vasculature, and hence, the NH was proposed as a suitable nanoprobe for in vivo molecular imaging of progressive plaque angiogenesis through dual-modality PET/MR imaging. Similar approaches using MNHs have also been introduced for molecular imaging of malignancies [49]. To develop the dual modality NHs for PET/MRI imaging, Chakravarty et al. reported on the fabrication of the PEGylated 69Ge-SPIONs as one of the biocompatible and safe imaging NHs used for the detection of sentinel lymph nodes. The cellular uptake and

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FIGURE 25.3

Nanohybrid for PET imaging. (A) Schematic representation of the production and structure of Ga-NOTAGEBP11DMSA-MNPs (68Ga-NGD-MNPs). (B) The micro-PET images of 68Ga-NGD-MNPs and 68 Ga-NUD-MNPs on rabbits 2 h after injection. (C) The 68Ga-NGD-MNPs group exhibited significantly more plaque uptake than control group. Source: Image was adapted with permission from a study published by T. Su, Y.B. Wang, D. Han, et al., Multimodality imaging of angiogenesis in a rabbit atherosclerotic model by GEBP11 peptide targeted nanoparticles, Theranostics 7 (2017) 4791804. 68

biocompatibility of the engineered NHs were evaluated in tumor models. The 69 Ge-labeled NHs showed a substantial stability in the serum and a time-dependent accumulation in tumors, indicating its potential as a multifunctional PET/MR imaging nanoprobe [50].

25.4.2 Radiolabeled Hybrid MNPs for SPECT Imaging MNPs armed with various homing devices and γ-emitting radionuclides can serve for the simultaneous target-guided SPECT imaging and treatment of cells/tissues. Further, acquiring the nuclear and MR imaging using a single dose of contrast agent is one of the significant challenges in the diagnosis of the different diseases/cancers. In a study,

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25.5 RADIOLABELED AuFe3O4 HYBRID NANOPARTICLES

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I-labeled magnetoferritin nanocages (M-HFn) NHs were engineered to serve for the imaging of cancerous cells/tissues. The radiolabeled M-HFn nanoprobes were shown to be robustly internalized through a tumor-specific H-ferritin (HFn)-transferrin receptor 1 (TfR1) pathway. These 125I-conjugated M-HFn nanoprobes were shown to provide simultaneous morphological and functional tumor detection by a single dose injection via SPECT/MR imaging [51]. The SPION-based theranostic nanohybrids employed for the diagnosis and targeted α-particle therapy offer a great opportunity for the simultaneous multimodality imaging and treatment of diseases, including different types of tumors. The in vitro results of an investigation indicated that 223Ra-labeled Fe3O4 NPs can be considered as a potential nanoprobe for the theranostic applications [52]. In one study, the PEGylated silica-coated MNPs were functionalized with rituximab and radiolabeled with Rhenium-188 (188Re) as a therapeutic radionuclide for the imaging targeted therapy of cancer cells/tissues. The theranostic efficiency of these NHs was investigated with in vitro and in vivo models, results of which highlighted the promising potential and perspectives of the NHs for the targeted cancer diagnosis/therapy [53]. Lee et al. developed the dextran-coated SPIONs, which were conjugated with technetium-99m (99mTc) as dual modality diagnosis nanoprobe for the SPECT/MR imaging in the liver cancer models. Preclinical studies confirmed the significant uptake of the engineered radiotracer in the cells/tumor sites, and hence nominated them as 99mTc-labeled nanoprobes for the target-guided detection of the hepatic cancers [54]. In a study, the magnetic polymeric NHs were developed via loading of SPIONs into the PEGylated poly(lactic-co-glycolic acid) (PLGA) NPs. Following the labeling of polymeric MNHs using indocyanine green (ICG) and indium-111, the MNHs were examined as multimodal imaging (fluorescence/nuclear imaging/magnetic resonance) nanoprobe on the colon cancer models. Upon the in vitro and in vivo SPECT/CT, optical imaging, and MRI findings, it seems that this polymeric MNH as a multimodal targetguided nanoprobe can be translated into clinical applications for diagnosis of cancers such as colon cancer [55]. Currently, development of the NP-based vaccines to treat and prevent emerging new pathogens, cancers, and infections remain as one of the most challenging issues in the field of biomedicine [56]. In one study, lipid-coated 67Ga-labeled magnetic hybrids were developed for the SPECT/CT tracking of 67Ga-labeled vaccine to the antigen presenting cells. Some preclinical studies have emphasized the potential targeted lymphatic delivery of the radiolabeled nanovaccines to the lymph nodes. All these findings provide compelling evidence that the MNHs can serve as a theranostic vaccine for image-guided applications [57].

25.5 RADIOLABELED AuFe3O4 HYBRID NANOPARTICLES In order to improve the accuracy of diagnosis and targeted therapy, the multimodal imaging systems are required [5860]. Having both AuNPs and MNPs within one nanostructure could offer great capability for multimodal target-guided imaging. Yang et al. developed the goldiron oxide heteronanostructures for tumor PET, optical and MR imaging [59]. The authors reported the use of 48 nm AuFe3O4 dumbbell-like nanoparticles

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as PET/Optical/MRI probes for cancer imaging in small living animals. Their findings demonstrated that AuFe3O4 hybrid nanoparticles were the promising nanoplatform for multimodality imaging. Similarly, Zhao et al. reported the use of strawberry-like Fe3O4Au nanoparticles as CT-MR dual-modality contrast agents for detection of the progressive liver disease [60]. In their study, the in vitro phantom experiment indicated that their hybrid nanoparticles had the superior contrast enhancement for both CT and MR imaging, thus significantly increasing the accuracy of disease detection. Li et al. also synthesized the AuNPs (3.5 nm) decorated Fe3O4 NPs (16.7 nm) for dual-mode MR/CT imaging applications [61]. In case of Fe3O4Au coreshell hybrid nanoparticles, with inner Fe3O4 NP core as an MRI agent, their applications for targeting and multimodal imaging were reported [58,62]. Zhou et al. found that these coreshell NPs not only enhanced MR contrast but also were the multimodal contrast probe for both microwave-induced thermoacoustic imaging and photoacoustic imaging [62]. Similarity, Li et al. fabricated the gold-coated Fe3O4 nanoroses with five different functions, such as integrating aptamer-based targeting, MRI, optical imaging, photothermal therapy, and chemotherapy [58].

25.6 CONCLUSION Development of the target-guided radiolabeled NHs to serve as contrast agents for the diagnosis and therapy of diseases such as various malignancies is an ongoing challenge among a large number of scientists. Indeed, simultaneous active targeting and multimodality imaging of defects within the diseased cells/tissues needs implementation of targetguided NHs as contrast agents to be combined with the imaging techniques such as SPECT, PET, MRI, and CT and the integrated imaging techniques. The implementation of radiolabeled target-guided NHs as contrast agent provide key molecular imaging information—important for diagnosis and prognosis of various diseases, including different types of cancers. If coupled with a therapeutic agent, the NHs can be also used as image-guided delivery system for delivery of drug molecules specifically onto the diseased cells/tissue. These NHs can be designed in a way to be responsive to the internal/external stimuli for an on-demand release of drugs. Regarding the AuNPs and Fe3O4 NPs, their hybridization offers great capability for multimodal target-guided imaging. It can be envisioned that many smart NHs, as personalized nanomedicines/theranostics, will be devised in the near future to serve for the simultaneous detection and therapy of diseases. However, despite some advances in the preclinical diagnosis using RNHs, the development of personalized radiolabeled hybrid nanoprobes remains as a significant challenge.

Acknowledgment The authors like to acknowledge the financial support of the Research Center for Pharmaceutical Nanotechnology, Biomedicine Institute, Tabriz University of Medical Sciences.

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REFERENCES

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[24] A. Abdukayum, C.X. Yang, Q. Zhao, J.T. Chen, L.X. Dong, X.P. Yan, Gadolinium complexes functionalized persistent luminescent nanoparticles as a multimodal probe for near-infrared luminescence and magnetic resonance imaging in vivo, Anal. Chem. 86 (2014) 40964101. [25] S.K. Sun, L.X. Dong, Y. Cao, H.R. Sun, X.P. Yan, Fabrication of multifunctional Gd2O3/Au hybrid nanoprobe via a one-step approach for near-infrared fluorescence and magnetic resonance multimodal imaging in vivo, Anal. Chem. 85 (2013) 84368441. [26] A. Samarin, F.P. Kuhn, F. Brandsberg, G. von Schulthess, I.A. Burger, Image registration accuracy of an inhouse developed patient transport system for PET/CT 1 MR and SPECT 1 CT imaging, Nucl. Med. Commun. 36 (2015) 194200. [27] R. Abgral, M.R. Dweck, M.G. Trivieri, et al., Clinical utility of combined FDG-PET/MR to assess myocardial disease, JACC Cardiovasc. Imaging 10 (2017) 594597. [28] J.R. Lamb, J.P. Holland, Advanced methods for radiolabelling nanomedicines for multi-modality nuclear/ MR imaging, J. Nucl. Med. 59 (2018) 382389. [29] A. Kumar, S. Zhang, G. Hao, et al., Molecular platform for design and synthesis of targeted dual-modality imaging probes, Bioconjug. Chem. 26 (2015) 549558. [30] J. Pellico, J. Ruiz-Cabello, M. Saiz-Alia, et al., Fast synthesis and bioconjugation of 68Ga core-doped extremely small iron oxide nanoparticles for PET/MR imaging, Contrast Media Mol. Imaging 11 (2016) 203210. [31] J. Koziorowski, A.E. Stanciu, V. Gomez-Vallejo, J. Llop, Radiolabeled nanoparticles for cancer diagnosis and therapy, Anticancer Agents Med. Chem. 17 (2017) 333354. [32] Y. Qian, M. Qiu, Q. Wu, et al., Enhanced cytotoxic activity of cetuximab in EGFR-positive lung cancer by conjugating with gold nanoparticles, Sci. Rep. 4 (2014). Article number 7490, 8 pages. [33] S.B. Lee, S.B. Ahn, S.-W. Lee, et al., Radionuclide-embedded gold nanoparticles for enhanced dendritic cellbased cancer immunotherapy, sensitive and quantitative tracking of dendritic cells with PET and Cerenkov luminescence, NPG Asia Mater. 8 (2016) e281. [34] Y. Zhao, L. Detering, D. Sultan, et al., Gold nanoclusters doped with 64Cu for CXCR4 positron emission tomography imaging of breast cancer and metastasis, ACS Nano 10 (2016) 59595970. [35] M.A. Wall, T.M. Shaffer, S. Harmsen, et al., Chelator-free radiolabeling of SERRS nanoparticles for wholebody PET and intraoperative Raman imaging, Theranostics 7 (2017) 30683077. [36] P. Bhatnagar, Z. Li, Y. Choi, et al., Imaging of genetically engineered T cells by PET using gold nanoparticles complexed to Copper-64, Integr Biol (Camb). 5 (2013) 231238. [37] C. Tsoukalas, G. Laurent, G. Jimenez Sanchez, et al., Initial in vitro and in vivo assessment of Au@DTDTPARGD nanoparticles for Gd-MRI and 68Ga-PET dual modality imaging, EJNMMI Phys. 2 (2015) A89. [38] M. Tian, W. Lu, R. Zhang, et al., Tumor uptake of hollow gold nanospheres after intravenous and intraarterial injection: PET/CT study in a rabbit VX2 liver cancer model, Mol. Imaging Biol. 15 (2013) 614624. [39] X. Li, C. Wang, H. Tan, et al., Gold nanoparticles-based SPECT/CT imaging probe targeting for vulnerable atherosclerosis plaques, Biomaterials 108 (2016) 7180. [40] F. Silva, A. Zambre, M.P. Campello, et al., Interrogating the role of receptor-mediated mechanisms: biological fate of peptide-functionalized radiolabeled gold nanoparticles in tumor mice, Bioconjug. Chem. 27 (2016) 11531164. [41] R. Clanton, A. Gonzalez, S. Shankar, G. Akabani, Rapid synthesis of 125I integrated gold nanoparticles for use in combined neoplasm imaging and targeted radionuclide therapy, Appl. Radiat. Isot. 131 (2018) 4957. [42] L. Song, S. Able, E. Johnson, K.A. Vallis, Accumulation of 111In-labelled EGF-Au-PEG nanoparticles in EGFRpositive tumours is enhanced by coadministration of targeting ligand, Nanotheranostics 1 (2017) 232243. [43] H.W. Kao, Y.Y. Lin, C.C. Chen, et al., Evaluation of EGFR-targeted radioimmuno-gold-nanoparticles as a theranostic agent in a tumor animal model, Bioorg. Med. Chem. Lett. 23 (2013) 31803185. [44] Q.K. Ng, C.I. Olariu, M. Yaffee, et al., Indium-111 labeled gold nanoparticles for in-vivo molecular targeting, Biomaterials 35 (2014) 70507057. [45] C.G. England, H.J. Im, L. Feng, et al., Re-assessing the enhanced permeability and retention effect in peripheral arterial disease using radiolabeled long circulating nanoparticles, Biomaterials 100 (2016) 101109. [46] J.A. Barreto, M. Matterna, B. Graham, H. Stephan, L. Spiccia, Synthesis, colloidal stability and 64Cu labeling of iron oxide nanoparticles bearing different macrocyclic ligands, New Journal of Chemistry 35 (2011) 27052712. [47] Y. Zhan, S. Shi, E.B. Ehlerding, et al., Radiolabeled, antibody-conjugated manganese oxide nanoparticles for tumor vasculature targeted positron emission tomography and magnetic resonance imaging, ACS Appl Mater Interfaces 9 (2017) 3830438312.

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[48] T. Su, Y.B. Wang, D. Han, et al., Multimodality imaging of angiogenesis in a rabbit atherosclerotic model by GEBP11 peptide targeted nanoparticles, Theranostics 7 (2017) 47914804. [49] M.-A. Karageorgou, S. Vranjeˇs, et al. Gallium-68 labeled iron oxide nanoparticles coated with 2,3-dicarboxypropane-1,1-diphosphonic acid as a potential PET/MR imaging agent: A proof-of-concept study. Contrast Media Mol. Imaging 2017 (2017) Article ID 6951240, 13 pages. [50] R. Chakravarty, H.F. Valdovinos, F. Chen, et al., Intrinsically germanium-69-labeled iron oxide nanoparticles: synthesis and in-vivo dual-modality PET/MR imaging, Adv Mater. 26 (2014) 51195123. [51] Y. Zhao, M. Liang, X. Li, et al., Bioengineered magnetoferritin nanoprobes for single-dose nuclear-magnetic resonance tumor imaging, ACS Nano 10 (2016) 41844191. [52] O. Mokhodoeva, M. Vlk, E. Ma´lkova´, et al., Study of 223Ra uptake mechanism by Fe3O4 nanoparticles: towards new prospective theranostic SPIONs, J. Nanopart. Res. 18 (2016) 301. [53] B. Azadbakht, H. Afarideh, M. Ghannadi-Maragheh, A. Bahrami-Samani, M. Asgari, Preparation and evaluation of APTES-PEG coated iron oxide nanoparticles conjugated to rhenium-188 labeled rituximab, Nucl. Med. Biol. 48 (2017) 2630. [54] I.J. Lee, J.Y. Park, Y.I. Kim, et al., Image-based analysis of tumor localization after intra-arterial delivery of technetium-99m-labeled SPIO using SPECT/CT and MRI, Mol. Imaging 16 (2017). 1536012116689001. [55] J. Bai, J.T. Wang, N. Rubio, et al., Triple-modal imaging of magnetically-targeted nanocapsules in solid tumours in vivo, Theranostics 6 (2016) 342356. [56] M.M. Pourseif, G. Moghaddam, N. Saeedi, A. Barzegari, J. Dehghani, Y. Omidi, Current status and future prospective of vaccine development against Echinococcus granulosus, Biologicals 51 (2018) 111. [57] A. Ruiz-de-Angulo, A. Zabaleta, V. Gomez-Vallejo, J. Llop, J.C. Mareque-Rivas, Microdosed lipid-coated 67 Ga-magnetite enhances antigen-specific immunity by image tracked delivery of antigen and CpG to lymph nodes, ACS Nano 10 (2016) 16021618. [58] C. Li, T. Chen, I. Ocsoy, G. Zhu, E. Yasun, M. You, et al., Gold-coated Fe3O4 nanoroses with five unique functions for cancer cell targeting, imaging, and therapy, Adv. Funct. Mater. 24 (12) (2014) 17721780. [59] M. Yang, K. Cheng, S. Qi, H. Liu, Y. Jiang, H. Jiang, et al., Affibody modified and radiolabeled goldIron oxide heteronanostructures for tumor PET, optical and MR imaging, Biomaterials 34 (2013) 27962806. [60] H.Y. Zhao, S. Liu, J. He, C.C. Pan, H. Li, Z.Y. Zhou, et al., Synthesis and application of strawberry-like Fe3O4Au nanoparticles as CT-MR dual-modality contrast agents in accurate detection of the progressive liver disease, Biomaterials 51 (2015) 194207. [61] J. Li, L. Zheng, H. Cai, W. Sun, M. Shen, G. Zhang, et al., Facile one-pot synthesis of Fe3O4@Au composite nanoparticles for dual-mode MR/CT imaging applications, ACS Appl. Mater. Interfaces 5 (2013) 1035710366. [62] T. Zhou, B. Wu, D. Xing, Bio-modified Fe3O4 core/Au shell nanoparticles for targeting and multimodal imaging of cancer cells, J. Mater. Chem. 22 (2012) 470477.

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C H A P T E R

26 Noble MetalManganese Oxide Nanohybrids Based Supercapacitors Thuy T.B. Hoang School of Chemical Engineering, Hanoi University of Science and Technology, Hanoi, Vietnam

26.1 INTRODUCTION A supercapacitor is a high-capacity energy storage device, which exhibits high power density, long cyclic stability, and rapid charging/discharging efficiency. The power density of supercapacitors is about one thousand times higher than that of rechargeable batteries. In addition, the supercapacitors can store much more energy than dielectric capacitors [1]. The capable applications of supercapacitors are the power sources for camera flash equipment, lasers, pulsed light generators, memory back-up, portable power supplies, hybrid electric vehicles, etc. [2]. Based on the charge storage mechanism, supercapacitors can be classified into three main categories, such as (i) electrochemical double-layer capacitors; (ii) pseudocapacitors; and (iii) hybrid capacitors (Fig. 26.1). In the case of the electrochemical double-layer capacitors (EDLCs), they store energy electrostatically by adsorption/desorption of electrolyte ions at the electrode/ electrolyte interfaces. Therefore, their specific capacitance is proportional to the surface area of exposed electrode. EDLC electrodes are typical made of highly porous carbon materials, which have a large specific surface area (up to 3000 m2/g), such as activated carbon [35], carbon nanotube [6], graphene [79], and mesoporous carbon [10]. Several types of these EDLC electrodes have been tested, with an organic electrolyte, for industrial application [3,11,12]. Regarding the pseudocapacitors (PCs), their charge storage mechanism depends on fast redox reactions on the electrode surface. Accordingly, their charge storage originates mainly from Faradaic electron-transfer mechanisms with redox reactions, intercalation, or electrosorption [13]. PC electrodes are typical made of the transition metal oxides/hydroxides and conducting polymers, such as RuO2 [14,15], MnO2 [1620], Mn2O3 [2125], Fe3O4 [26], Co3O4 [27,28], NiO [29,30], Ni(OH) [31,32], nanocomposites [3336], polyaniline and its composites [3739], polypyrrole and its composites [4043]. Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00026-7

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FIGURE 26.1

Classification of electrochemical supercapacitors.

In term of the hybrid capacitors (HCs), they often consist of composite or asymmetric electrodes, which exhibit both behaviors of electrostatic capacitance and pseudocapacitance. Thus, HCs often enable achievements of higher cell voltage, and higher energy and power densities than their corresponding symmetrical devices [4447]. Recently, many studies have focused on developing electrode active materials for high pseudocapacitive performance and high cycling stability. Among transition metal oxides, RuO2 and MnO2 are the two most auspicious pseudocapacitance materials, which have attracted much attention from researchers. In particular, RuO2 exhibits the electrochemical signature of a capacitive electrode, with a high specific capacitance (720 F/g), an ultrahigh specific power (4320 kW/kg), and a high specific energy (7.5 Wh/kg) [15,48]. However, application of RuO2 is very limited due to its high cost and toxicity [49]. Meanwhile, MnO2 and MnO2-based composites have been shown to be the best potential electrode materials for supercapacitors [16,5055], by the following advantages: (i) high theoretical specific capacitance (1370 F/g) calculated based on a one-electron transfer reaction between Mn(III) and Mn(IV) [56,57]; (ii) wide potential window (0.81.0 V) in aqueous electrolytes [50,58]; (iii) less corrosion issues by using near neutral electrolytes [34]; and (iv) low cost and eco-friendliness [59]. However, manganese oxide suffers relatively poor electrical conductivity, which limits its energy and power density for supercapacitor applications. A prospective approach for solving this problem is the incorporation of conductive materials (carbon, silver, and gold) into the active materials [55,60,61,62]. In this chapter, we review the effect of noble metal nanoparticles (Ag nanoparticles and Au nanoparticles) on the properties of MnO2-based supercapacitors.

26.2 AgMnO2 NANOHYBRIDS-BASED SUPERCAPACITORS MnO2 materials could be synthesized using several methods, such as wet-chemical, solgel, thermal, hydrothermal, and electrochemical methods [2,33,45]. It was reported II. APPLICATIONS

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that MnO2 had the high theoretical specific capacitance. However, the semiconducting nature of the bulk manganese oxides limited their specific surface area and their electrical conductivity. Recently, various nanostructures of MnO2 including nanoparticles, nanorods, nanowires, nanotubes, mesoporous and branched nanostructures have been used as electrode materials for energy storage [8,51,54,55,63,64]. On the other hand, the incorporation of silver into the MnO2 matrices (silver-doped or silver-loaded MnO2 materials) is a hopeful approach to enhance their electrical conductivity as well as their electrochemical performance through the electron-transferring channels during charge-discharge process. AgMnO2 nanohybrids materials can be synthesized by three main methods as follows: (i) wet chemical redox method; (ii) hydrothermal and solvothermal method; and (iii) electrochemical deposition method.

26.2.1 Wet-Chemical Redox Method Zhang et al. synthesized the Ag NPs-loaded MnO2 nanosheets by using the delaminated birnessite manganese oxide (BirMO) [52]. The fabrication process involved a reassembling reaction between Ag1 ions and delaminated manganese oxide nanosheets, followed by a reduction treatment. The authors informed that the synthesization of Ag NPs-loaded MnO2 nanosheets from the BirMO had considerably improved the specific capacitance of the electrode. The specific capacitance of the AgMnO2 nanohybrids was 272 F/g, which was three times higher than that of the MnO2 nanosheet materials (90 F/g). The larger specific capacitance was credited to the high electrical conductivity of AgMnO2 electrodes, which facilitated the access of the electrolyte ions onto the MnO2 surface and maximized the use of manganese oxide pseudocapacitance. Accordingly, silver-loaded manganese dioxide electrode could remarkably improve the electrical performance. Similarly, for battery application, Abuzeid et al. successfully prepared the Ag-coated MnO2 or Ag-doped MnO2 nanohybrids by using the wet-chemical redox method [55]. The hybrid materials also demonstrated notable enhancement of electrochemical performance. Xia et al. reported a simple one-pot method to fabricate the hierarchical heterostructure of Ag NPs-decorated MnO2 nanowires (Ag/MnO2 nanowires) [51]. The one-dimensional hierarchical Ag/MnO2 nanowires were prepared by simply immersing Ag foil into a mixed solution of KMnO4 and H2SO4. Their chemical method is schematically illustrated in Fig. 26.2. It was observed that after 1 week of immersion, Ag nanoparticles (B10 nm) were uniformly decorated on MnO2 nanowires (width of 1020 nm), which significantly improved the electrical conductivity and the supercapacitive performance of MnO2-based electrodes. It was also found that these one-dimensional hierarchical Ag NPs-decorated MnO2 hybrid nanowires showed a large specific capacitance (293 F/g), which was two times higher than that of the bare MnO2 nanowires electrode (B130 F/g), at a scan rate of 10 mV/s. In addition, the layer of highly conductive Ag nanoparticles enabled improving the rate capability and the cycle performance of Ag/MnO2 nanowire electrode. This electrode could deliver a high specific energy density of 17.8 Wh/kg and power density of 5000 W/kg at an extremely high current density of 10 A/g with a capacitance retention of 96.8% after 5000 cycles.

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FIGURE 26.2 Schematic illustration of the formation of the hierarchical heterostructures of Ag NPs-decorated MnO2 nanowires. Source: Reprinted with permission from H. Xia, et al., Hierarchical heterostructures of Ag nanoparticles decorated MnO2 nanowires as promising electrodes for supercapacitors, J. Mater. Chem. A 3 (2015) 12161221 (2015). Copyright 2014 Royal Society of Chemistry.

In another approach, Ma et al. studied the effect of graphene and silver codoping on the electrochemical performance of manganese dioxide materials [65]. In their study, the ternary nanocomposite of Ag/MnO2/RGO was synthesized by the decoration of Ag and MnO2 nanoparticles on the reduced graphene oxide (RGO) sheets. The authors described that Ag and MnO2 nanoparticles, with sizes of several nanometers, were homogeneously distributed on the surface of RGO sheets. The resultant Ag/MnO2/RGO nanocomposites showed excellent capacitive performance with a specific capacitance of 467.5 F/g at a scan rate of 5 mV/s, which was much higher than that of MnO2/RGO nanocomposites (293.2 F/g). Moreover, Ag/MnO2/RGO composites possessed an excellent cycling stability. Their specific capacitance did not show any decay after 1000 cycles at the scan rate of 80 mV/s. The authors supposed that this enhancement of capacitive performance was mainly attributed to the incorporation of Ag nanoparticles, which increased the electrical conductivities and promoted the electron transfer between the active components.

26.2.2 Hydrothermal and Solvothermal Methods By using the solvothermal method, Li et al. synthesized one-dimensional (1D) tubular Ag/MnOx nanocomposites via the Kirkendall effect between potassium permanganate and silver nanowire templates in an acidic environment (pH 0.76) [66]. The authors found that their hierarchical tubular Ag/MnOx nanosheets exhibited a good electrochemical performance, with a specific capacitance of 180 F/g (at a current density of 0.1 A/g), and still maintained 80% of the initial capacity after 1000 cycles (at a current density of 1 A/g). These findings were attributable to the better transportation of ions and electrons through the ultrathin nanosheets assembled to tubulars (Fig. 26.3).

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FIGURE 26.3 FESEM image, EDS spectra, and TEM images of (AD) pristine Ag nanowires, (EH) Ag/MnO2 nanotubes prepared at pH 7.00, and (IL) tubular Ag/MnOx nanosheets produced at pH 0.76. Source: Reprinted with permission from Y. Li, H. Fu, Y. Zhang, Z. Wang, X. Li, Kirkendall effect induced one-step fabrication of tubular Ag/MnOx nanocomposites for supercapacitor application, J. Phys. Chem. C 118 (2014) 6604 2 6611. Copyright 2014 American Chemical Society.

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Wang et al. successfully synthesized the Ag-doped δ-MnO2 using the hydrothermal method [67]. The authors described that when MnO2 was doped with Ag at the concentration of up to 10 wt.%, the microstructure of MnO2 did not change from the original petal-like morphology. Unfortunately, after addition of Ag the specific capacitance of the synthesized AgMnO2 slightly reduced, but the cycle life of the hybrid was improved remarkably. AgMnO2 hybrid showed an insignificant attenuation with an average rate of 0.028% after 1000 chargedischarge cycles. Besides, some other authors also used the similar hydrothermal technique to synthesize Ag-decorated α-MnO2 nanostructured composite and Ag-loaded MnO2 nanohybrids for other applications [54,68]. Guan et al. fabricated the core/shell nanorods of MnO2/C embedded with Ag nanoparticles by using the in situ hydrothermal and reduction method [69]. The core/shell structure consisted of a conductive material (as a core) with easy diffusion of ion, and a redox active material (as a shell) with high electrical conductivity, thus enhancing the electrochemical performance of supercapacitors. The as-synthesized core/shell nanorods structure performed a very high specific capacitance (628 F/g) and superior cycle durability (B98.5% retention after 2000 cycles).

26.2.3 Electrochemical Deposition Method As regards electrochemical synthesis of AgMnO2 nanohybrids, the main influencing factors are surfactant, pulse potential, and current density. Sawangphruk et al. successfully produced the Ag-doped MnO2 electrodes with dendrite and foam-like structures using an electrodeposition method, wherein sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were employed as structure-directing agents [53] (Fig. 26.4). Ag-doped MnO2 hybrids were produced by the cathodic electrodeposition at

FIGURE 26.4 Scanning electron micrographs of Ag-doped MnO2 films produced using (A) CTAB, (B) SDS, (C) without surfactant, and (D) the pure MnO2 film. Source: Reprinted with permission from M. Sawangphruk, S. Pinitsoontorn, J. Limtrakul, Surfactant-assisted electrodeposition and improved electrochemical capacitance of silver-doped manganese oxide pseudocapacitor electrodes, J. Solid State Electrochem. 16 (2012) 26232629. Copyright 2012 Springer.

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a current density of 1 mA/cm2 for 30 min in an aqueous solution containing KMnO4, NaNO3, and AgNO3 at 25 C. The surfactants (SDS or CTAB) were added to control the surface morphology of samples. The authors specified that resultant nanohybrids enhanced electronic conductance and ion transportation. It is also indicated that the Agdoped MnO2 films produced in the presence of SDS and CTAB had the specific capacitance values of 551 and 557 F/g, respectively (measured in 0.5 M Na2SO4 solution at a scan rate of 5 mV/s). These values are about 2.7 times higher than that of the pure MnO2 film and about 1.4 times higher than that of the Ag-doped MnO2 film in the absence of surfactants. The authors stated that the foam-like structure of Ag/MnO2 (CTAB) and the dendrite structure of Ag/MnO2 (SDS) with their high stability played an important role in enhancing ion transport and lowering resistance for chargedischarge and electrolyte diffusion. A pulsed potential electrodeposition technique was developed by Kim et al. to synthesize ternary Ag/MnO2/PANI (silver/manganese oxide/polyaniline) nanocomposite thin films [70]. The electrolyte utilized for electrodeposition consisted of the aniline, KMnO4, and AgNO3 precursors. To fabricate the ternary composite film, three different deposition potentials (0.75 V, -0.2 V, 0.35 V) were applied to deposit PANI, Ag, Mn, respectively. These deposition potentials were then applied in a set of four pulses, at 0.75 V/-0.2 V/0.75 V/0.35 V with the duration of 30 s/45 s/30 s/45 s, respectively, for the PANI/Ag/PANI/Mn nanocomposites. It was observed that the agglomerated nanoscale-vermicular-like structure of the pure PANI was converted into more uniform vermicular morphology containing Ag and MnO2. The ternary Ag/MnO2/PANI nanocomposite delivered very high specific capacitance of 621 F/g (from cyclic voltammogram) and 800 F/g (from charge-discharge measurements) with its retention of B83% after 700 cycles. Recently, three-dimensional (3D) electrode structure has been constructed for supercapacitor application. It is well known that 3D structure favors not only the electron conduction, but also the ion diffusion due to its high specific area which notably influences supercapacitive performance of the electrodes. By using electrodeposition with sonication growth in AgNO3 solution, Usman et al. fabricated the 3D vertically aligned Ag nanoplates on nickel foamgraphene substrate (Ag@NFG) [71]. Enhancement of surface area was caused by vertical deposition of Ag NPs (with irregular polyhedron shapes) on the surface of NFG. This novel architecture of Ag@NFG nanocomposite exhibited a high specific capacitance of 900 F/g (at current density of 0.5 A/g), which was six times higher than that of nickel foamgraphene. Moreover, the Ag@NFG nanocomposite showed good long-term cycle stability with capacitance retention of approximately 99% compared to the initial value after 5000 cycles. In summary, the hybridization of Ag NPs and MnO2 enhanced the supercapacitive performance of MnO2-based electrodes. Silver nanoparticles not only enhanced the electron conductivity, but also improved the cation diffusion throughout the electrode. Therefore, the incorporation of Ag NPs into MnO2 structure improved the total conductivity and consequently enhanced its supercapacitive performance characteristics. The AgMnO2 nanohybrids delivered larger specific capacitances and better cycle durability than that of their pure MnO2 counterparts.

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26.3 AuMnO2 NANOHYBRIDS-BASED SUPERCAPACITORS As far as MnO2-loaded electrode is concerned, a large thickness of the oxide layer influences not only the surface area/mass ratio, but also the participation of electrolyte in the electrochemical chargedischarge process. A thick MnO2 layer causes an increase in the contact resistance and, consequently, a decrease in the specific capacitance of the electrode. The incorporation of gold nanoparticles into the manganese oxide layer could significantly increase the electrical conductivity of electrodes. In addition, by their excellent electronic conductivity, the presence of Au NPs in MnO2 matrix could also enhance the electrical contacts between active materials (MnO2) and current collector through the electron transfer channels during the charge-discharge process of supercapacitors. Au NPs decorated MnO2 nanowires (AuMnO2 NWs) were synthesized by Khandare et al. [60]. The authors indicated that AuMnO2 NWs exhibited the better electrochemical performance than MnO2 nanowires (MnO2 NWs). These AuMnO2 NWs delivered a high specific capacitance of 267 F/g (at current density of 1 mA/g) and a maximum energy density of 37.08 Wh/kg, which are three times higher than the values for MnO2 NWs (83 F/g and 11.52 Wh/kg, respectively). The assynthesized AuMnO2 NWs showed good cyclic stability with retention of 97% after 1000 cycles. In order to maximize the conductivity of electrode, carbon nanotubes or graphene were introduced into AuMnO2 nanohybrids [72,73]. However, in these nanocomposites, Au NPs still played an important role in enhancement of the electrochemical performance, as reported. Chen et al. successfully synthesized the high density nanosized MnO2 spines on Au stems (NMSAS), all which were electrochemically deposited directly on flexible conductive substrates (Au/Ti/polyethylene terephthalate) [73]. These NMSAS electrodes demonstrated high specific capacitance, high specific energy value, high specific power value, and long-term stability. Their maximum specific capacitance was 1130 F/g (at a scan rate of 2 mV/s) and 225 F/g (at a current density of 1 A/g). At an extremely high charge/discharge rate of 50 A/g, their specific capacitance remained a high value of 165 F/g. To evaluate their supercapacitive performance, a highly flexible solid-state device was fabricated. It was found that the flexible electrodes demonstrated a high specific energy (15 Wh/kg) and specific power (20 kW/kg) at 50 A/g, with capacitance retention of 90% after 5000 cycles at a current density of 10 A/g. The coreshell and ordered 3D nanostructure materials are also highly attractive for energy storage application, particularly for high-performance flexible supercapacitors. Qiu et al. developed a novel Au@MnO2 coreshell nanomesh structure on a flexible polymeric substrate as transparent flexible electrodes (TFE) for future electronic devices [74]. In the study, MnO2 was directly grown on the well-conformed Au nanomesh, which acted as the TFE with high transparency. It was stated that a high areal capacitance of 4.72 mF/cm2 and ultrahigh rate capability up to 50 V/s had been achieved on these hybrid films.

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26.3 AuMnO2 NANOHYBRIDS-BASED SUPERCAPACITORS

557

Another design of coreshell structures has been presented by Lu et al. They fabricated the novel hybrid WO3x@Au@MnO2 coreshell nanowires (NWs) on flexible carbon fabric [75]. Cyclic voltammetry measurement revealed the box-like shapes with a mirror-image feature, implying the ideal capacitive behaviors and good high-rate capabilities of WO3x@Au@MnO2 NWs. These nanohybrids gained a high specific capacitance of 588 F/g at a scan rate of 10 mV/s (1.7 times higher than that of WO3x@MnO2 NWs), or 1195 F/g at a current density of 0.75 A/g. Gao et al. reported a unique MnO2/Au/MnO2 nanospike (MAMNSP) supercapacitor based on free-standing 3D gold nanospike (NSP) films [76]. The NSP films were highly flexible and transferable onto an arbitrary flexible substrate to enable applications that require high flexibility. The large surface area of this unique structure led to a remarkable enhancement in capacitance, which was 1.9 and 4.26 times higher than MnO2/Au NSP and MnO2/planar electrodes, respectively. The solid state symmetric supercapacitors based on MAMNSP electrodes were then fabricated. The as-prepared devices exhibited a high volumetric capacitance (20.35 F/cm3) and high specific energy (1.75 3 1023 Wh/cm3). In addition, the bendability test indicated that these supercapacitors are highly flexible and reliable. The supercapacitors performed high cycling stability with capacitance retention of approximately 94% after 1000 cycles and 88% after 5000 cycles. Such electrodes with 3D structures are believed to be a potential candidate for portable and flexible energy storage systems. Another 3D structure, AuMnO2 nanohybrids were reported by Lv et al. [77]. The authors prepared a series of Au-doped-α-MnO2 (ADM) nanocomposites, which were electrodeposited on ZnO nanorods (ZNs) by cyclic voltammetry. These ZNs served as 3D scaffolds for binder-free electrodes and provided a large surface area and shorter ion diffusion. The as-prepared electrodes of ZnO-nanorods/Au-doped-α-MnO2 (ZNs/ADM) nanocomposites exhibited excellent electrochemical performances. Their maximum specific capacitance (1305 F/g) was 2.2 times higher than that of ZNs/αMnO2 electrode. This value was alsovery close to the theoretical value of MnO2(B1370 F/g). For supercapacitor application, the asymmetric supercapacitor based on ZNs/ADM (positive electrode) and grapheneCNTs (negative electrode) hybrid materials was then configured. This supercapacitor could be cycled reversibly in a wide potential window and exhibited a high energy density (101 Wh/kg), high power density (33.6 kW/kg), and good cycling stability. The authors supposed two effects of Au on the electrochemical performance of ADM hybrids including: (i) some Au atoms were incorporated into the α-MnO2 lattice during the electrochemical redox process, thus enhancing the electronic conductivity of α-MnO2; (ii) the distributed gold nanoparticles in α-MnO2 nanothin films provided abundant Au/MnO2 interfaces with direct and stable pathways for rapid charge transport and collection. In addition, these Au NPs played a role in keeping Au doping into the lattice of α-MnO2 during the energy storage process to ensure cycle life as well. In another approach, Zhu et al. synthesized the mesoporous Au@MnO2 nanohybrids by using a facile self-decomposition process [78]. Their Au@MnO2 electrode

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558

26. NOBLE METALMANGANESE OXIDE NANOHYBRIDS BASED SUPERCAPACITORS

exhibited a high specific capacitance (906.4 F/g), with outstanding rate capability and excellent cycle performance (97.2% cyclic stability after 5000 cycles). The twoelectrode cell made of Au@MnO2 and activated graphene oxide/Au nanohybrids could deliver a high energy density of 62.8 Wh/kg corresponding to the power density of 1000 W/kg. In summary, the hybridization with Au NPs has remarkably enhanced the electrochemical performance of manganese dioxide materials. Au NPs notably enhanced both the electronic conductivity (inside the electrode) and the electrical contacts (between the electrode and current collectors). Therefore, Au NPs play an important role as an electron transfer channel during the charge and discharge process of MnO2. The presence of Au NPs in MnO2 matrix could: (i) provide the large interfacial areas for electrochemical reactions; (ii) shorten the ion diffusion pathways; (iii) increase the electron transport rate; and (iv) increase the charge/discharge performance.

26.4 CONCLUDING REMARKS Despite the various advantages of the pure MnO2 material, such as abundancy, low cost, eco-friendliness, and wide potential window, the poor electrical conductivity largely limits its electrochemical performance and practical applications. The hybridization of Ag/Au nanoparticles with manganese dioxide materials is a new promising approach to enhance the performance of MnO2-based supercapacitors. Performance characteristics of AgMnO2 and AuMnO2 nanohybrids-based supercapacitors in the previous reports are presented in the Table 26.1 and illustrated in Fig. 26.5. It can be seen that the hybridization of MnO2 with noble metals improves the specific capacitance as well as the cycling stability for the electrodes. These enhancements are attributed to the fact that silver and gold nanoparticles not only accelerate the electron conduction but also enhance the proton diffusion throughout the electrode. Ragone plots of energy density versus power density (Fig. 26.5) indicate that the AuMnO2 nanohybrid electrodes deliver higher power density than AgMnO2 nanohybrid electrodes. However, their improvement in the supercapacitive performance characteristics substantially depends not only on the hybridization, but also on the morphology and structure of electrodes. Nanorods showed higher activity than nanoparticles as a result of the numerous active sites and crystal planes. Threedimensional structure nanocomposites could deliver an ultrahigh specific capacitance close to the theoretical value of MnO2 (1305 F/g recorded for ZnO-nanorods/Au-dopedα-MnO2 [77]). The Ag/AuMnO2 nanohybrid materials could provide the binder-free electrodes with superior properties as follows: (i) large interfacial areas for electrochemical reactions; (ii) short ion diffusion pathways; (iii) high electron transport rates; (iv) high capacitive performance; and (v) high charge/discharge performance. This enables noble metalMnO2 nanohybrids to become promising electrode materials for a new generation of supercapacitors to meet the various requirements for specific practical applications in near future.

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TABLE 26.1

Performance Characteristics of AgMnO2 and AuMnO2 Nanohybrids-Based Supercapacitors Reported in the Literature Specific Capacitance (F/g)

Energy Density (Wh/kg)

Power Density (W/kg)

Cycling Stability/ Lifetime

Ref.

Electrolyte

Scan Rate/ Current Density

Ag NPs loaded MnO2 nanosheets

1 M Na2SO4

10 mV/s

-0.05 to 0.95 V (SCE)

272







[52]

Ag2OMnO2

Ag2O NPs loaded MnO2 nanosheets

1 M Na2SO4

10 mV/s

-0.05 to 0.95 V (SCE)

232







[52]

3

Ag/MnO2 NWs

Wet-chemical method with 1 M Na2SO4 an Ag foil/one dimensional hierarchical Ag/MnO2 nanowires

10 mV/s (10 A/g)

0 to 1.0 V (Ag/AgCl)

293

17.8

5000

Retention of 96.8% after 5000 cycles

[51]

4

Ag/MnO2/RGO

In situ growth of MnO2 NPs on GO sheets, then coreduction of Ag1 and GO

3 M KOH

5 mV/s

0 to 0.5 V (SCE)

467.5





Remain 100% after 1000 cycles at 80 mV/s

[65]

5

Ag/MnOx

Solvothermal/1D tubular

1 M Na2SO4

0.1 A/g

0 to 1.0 V (SCE)

180





Retention of 80% after 1000 cycles

[66]

6

AgMnO2

Hydrothermal/petal-like morphology

0.5 M Na2SO4

50 mA/g

0 to 0.8 V (SCE)

177





0.019% loss after 1000 cycles

[67]

7

MnO2/CAg

Hydrothermal and reduction/coreshell nanorod

3 M KOH

1 A/g

0 to 1.0 V (SCE)

628

48.3

851.7

Retention of 98.5% after 2000 cycles at 1 A/g

[69]

8

Ag-doped MnO2

SDS assisted 0.5 M Na2SO4 electrodeposition/foam-like structures

5 mV/s

0 to 1.0 V (Ag/AgCl)

557





1.4% loss after 2000 cycles

[53]

9

Ag-doped MnO2

CTAB assisted 0.5 M Na2SO4 electrodeposition/foam-like structures

5 mV/s

0 to 1.0 V (Ag/ AgCl)

551





1.4% loss after 2000 cycles

[53]

10

Ag-doped MnO2

Electrodeposition/foamlike structures

0.5 M Na2SO4

5 mV/s

0 to 1.0 V (Ag/ AgCl)

393







[53]

11

AgMnO2PANI Pulsed potential electrodeposition/thin film

0.5 M LiClO4 1 PC

100 mV/s

-1.0 to 1.0 V (SCE)

621





Retention of 83% after 700 cycles

[70]

12

Ag@NFG

Electrodeposition with sonication/3D vertically aligned Ag nanoplates

6 M KOH

0.5 A/g

-0.15 to 0.38 V (Ag/AgCl)

900

35

910

Retention of 99% after 5000 cycles

[71]

13

Ag NPs

Electroless deposited/ nanoparticles

0.5 M NaOH

2 mV/s

-0.2 to -1.1 V (Ag/ AgCl)

452

27.8

4100

Retention of 63% after 10,000 cycles

[79]

Sample No.

Materials

1

AgMnO2

2

Synthesis Method /Structure

Potential Window

(Continued)

TABLE 26.1

(Continued) Specific Capacitance (F/g)

Energy Density (Wh/kg)

Power Density (W/kg)

Electrolyte

Scan Rate/ Current Density

Wet chemical/Au NPs decorated on MnO2 nanowires

1 M Na2SO4

1 mA/g

0 to 1.0 V (Ag/ AgCl)

267

37

458

Retention of 97% after 1000 cycles

[60]

AuMnO2/CNT

Electrodeposition, infiltration and chemical vapor deposition/coaxial arrays

0.1 M Na2SO4

6.6 A/g

0 to 0.7 V (Ag/ AgCl)

68

4.5

33,000

Good stability after 1000 cycles

[72]

16

AuMnO2

Electrochemical deposition/nanocomposite

0.5 M NaOH

2.5 A/g

-0.4 to 0.5 V (Ag/ AgCl)

575





26% loss after 1500 [80] cycles

17

MnO2 on Au stems (NMSAS)

Electrochemical/ultrathin MnO2 on Au nanowire stems

1 M Na2SO4

2 mV/s (50 A/ g)

0 to 0.8 V (Ag/ AgCl)

1130 (165)

15

20,000

Retention of 90% after 5000 cycles

[73]

18

Au@MnO2 coreshell nanomesh

Lithography and electrodeposition/novel Au@MnO2 coreshell nanomesh

1 M Na2SO4

0.56 A/g

0 to 1.0 V (Ag/ AgCl)

524





Retention of more than 95% after 1000 cycles

[74]

WO3-

Anodic electrodeposition of 0.1 M Na2SO4 MnO2 onto WO3-x@Au/ coreshell nanowires on carbon fabric

10 mV/s (0.75 A/g)

0 to 0.8 V (SCE)

588 (1195)

106.4 (78.1)

23,600 (30,600)

Almost constant at 110% after 5000 cycles

[75]

Sample No.

Materials

14

AuMnO2

15

19

x@Au@MnO2

NWs

Synthesis Method /Structure

Potential Window

Cycling Stability/ Lifetime

Ref.

20

MnO2/Au/MnO2 (NSPs)

Nanoimprint with anodization/3D MnO2/ Au/MnO2 nanospike arrays

1 M Na2SO4

10 mV/s

0 to 0.8 V (Ag/ AgCl)

581.3 (20.35 F/ cm3)

-(1.75 3 1023 Wh/  cm3)

Retention of 88% after 5000 cycles

[76]

21

Au-doped α-MnO2

Cyclic voltammetry/3D electrode nanomaterials on ZnO-nanorods

1 M Na2SO4

5 mV/s

0 to 0.8 V (SCE)

1305

101

6100

Retention of 97% after 5000 cycles

[77]

22

Au@MnO2

Self-decomposition/ mesoporous Au@MnO2 network

1 M Na2SO4

2.5 A/g

-0.2 to 0.8 (SCE)

906.4

62.8

1000

Retention of 97.2% after 5000 cycles

[78]

Note: CNT, carbon nanotube; NSPs, nanospikes; GO, graphene oxide; NWs, nanowires; NFG, nickel foamgrapheme; PANI, polyaniline; NMSAS, nanosized MnO2 spines on Au stems; RGO, reduced graphene oxide; NPs, nanoparticles.

REFERENCES

561 FIGURE 26.5 Ragone plot for AgMnO2 and AuMnO2 nanohybridsbased electrodes. Data taken from Table 26.1.

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[64] T.T. Mai, T.B.T. Hoang, T.T.H. Le, Metal doped manganese oxide thin films for supercapacitor application, J. Nanosci. Nanotechnol. 15 (2015) 69496956. [65] L. Ma, X. Shen, Z. Ji, G. Zhu, H. Zhou, Ag nanoparticles decorated MnO2/reduced graphene oxide as advanced electrode materials for supercapacitors, Chem. Eng. J. 252 (2014) 95103. [66] Y. Li, H. Fu, Y. Zhang, Z. Wang, X. Li, Kirkendall effect induced one-step fabrication of tubular Ag/MnOx nanocomposites for supercapacitor application, J. Phys. Chem. C 118 (2014) 66046611. [67] J.W. Wang, Y. Chen, B.Z. Chen, Effects of transition-metal ions on the morphology and electrochemical properties of δ-MnO2 for supercapacitors, Met. Mater. Int. 20 (2014) 989996. [68] L. Wang, et al., Antimicrobial activity of silver loaded MnO2 nanomaterials with different crystal phases against Escherichia coli, J. Environ. Sci. (China) 41 (2016) 112120. [69] Y. Guan, et al., Core/shell nanorods of MnO2/carbon embedded with Ag nanoparticles as high-performance electrode materials for supercapacitors, Chem. Eng. J. 331 (2018) 2330. [70] J. Kim, et al., Synthesis and enhanced electrochemical supercapacitor properties of AgMnO2polyaniline nanocomposite electrodes, Energy 70 (2014) 473477. [71] M. Usman, L. Pan, A. Sohail, Z. Mahmood, R. Cui, Fabrication of 3D vertically aligned silver nanoplates on nickel foam-graphene substrate by a novel electrodeposition with sonication for efficient supercapacitors, Chem. Eng. J. 311 (2017) 359366. [72] A.L.M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, Multisegmented AuMnO2/Carbon nanotube hybrid coaxial arrays for high-power supercapacitor applications, J. Phys. Chem. C114 (2010) 658663. [73] Y.-L. Chen, P.-C. Chen, T.-L. Chen, C.-Y. Lee, H.-T. Chiu, Nanosized MnO2 spines on Au stems for highperformance flexible supercapacitor electrodes, J. Mater. Chem. A1 (2013) 1330113307. [74] T. Qiu, et al., Au@MnO2 coreshell nanomesh electrodes for transparent flexible supercapacitors, Small 10 (2014) 41364141. [75] X. Lu, et al., WO3-x@Au@MnO2 coreshell nanowires on carbon fabric for high-performance flexible supercapacitors, Adv. Mater. 24 (2012) 938944. [76] Y. Gao, et al., Highly flexible and transferable supercapacitors with ordered three-dimensional MnO2/Au/ MnO2 nanospike arrays, J. Mater. Chem. A3 (2015) 1019910204. [77] Q. Lv, et al., Ultrahigh capacitive performance of three-dimensional electrode nanomaterials based on ˚ -scale channels, Nano Energy 21 (2016) 3950. α-MnO2 nanocrystallines induced by doping Au through A [78] S. Zhu, Q. Shan, F. Dong, Y. Zhang, L. Zhang, Fabrication of mesoporous gold networks@MnO2 for highperformance supercapacitors, Gold Bull. 50 (2017). [79] B. Pandit, V.S. Devika, B.R. Sankapal, Electroless-deposited Ag nanoparticles for highly stable energyefficient electrochemical supercapacitor, J. Alloys Compd. 726 (2017) 12951303. [80] V. Veeramani, B. Dinesh, S.-M. Chen, R. Saraswathi, Electrochemical synthesis of AuMnO2 on electrophoretically prepared graphene nanocomposite for high performance supercapacitor and biosensor applications, J. Mater. Chem. A4 (2016) 33043315.

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27 Palladium-Based Hybrid Nanocatalysts: Application Toward Reduction Reactions Biraj Jyoti Borah, Manoj Mondal and Pankaj Bharali Department of Chemical Sciences, Tezpur University, Napaam, Assam, India

27.1 INTRODUCTION Palladium-based hybrid nanoparticles (NPs) have generated significant interest as heterogeneous catalysts due to their versatile compositional and structural properties [1]. The fabrication of these hybrid NPs delivers improved catalytic activity, selectivity, and stability of metal NPs [2]. Compared to their monometallic analogous, the improved activity of the bimetallic hybrid NPs is due to the synergistic character of two kinds of metal atoms involve in a catalyst. The catalytic activity of hybrid NPs is tuned or modified by controlling the electronic structure of two metals, surface elemental distribution, intermetallic charge transfer, and lattice strain [3]. Hybrid NPs of palladium and nonprecious metals are especially important from the economic and environmental perspective for lowering production costs and attaining sustainability. Apart from displaying improved catalytic ability, the hybridization of palladium with nonprecious metals also increases their resistance towards poisonous ingredients [4]. Palladium can be hybridized with various transition metals (Cu, Ni, Co, Fe) and metal oxides to improve its catalytic properties [5]. Specifically, the PdNi hybrid displays excellent catalytic efficiency due to their profound synergistic effect [6]. This is primarily due to the good miscibility between Pd and Ni due to their identical electron configurations and crystal structures [7]. Moreover, the density functional theory calculations shows d-band shifts in Pd, which improves the catalytic activity [8]. The idea of hybridizing two different metals was devised during the 1970s [3a], and latterly extended by Toshima [9] who employed PVP to stabilize bimetallic

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PdAu NPs. Conventionally, hybrid NPs are synthesized using various methods such as thermal annealing [10], galvanic replacement [11], microwave-assisted reduction [12], and template growth process [13]. Nowadays, the use of supported metal NPs are gaining much attention, as the metal immobilized on solid supports remains insoluble in solvents, thereby reducing residual metal contamination in final products [14], thus improving efficiency/quality as well as decreasing the production costs. Thus, various biopolymers and inorganic materials were investigated as supports for the immobilization of palladium NPs to develop hybrid materials as catalyst for improving catalytic performance [15]. In recent years, Pdmetal oxide hybrid nanoparticles have been extensively explored for various catalytic and sensing applications including photocatalytic and electrocatalytic applications [2cg,16,17]. Because of the increasing population and rapid industrialization, the fossil-fuel resources are decreasing day by day, hence there is a critical need for the use of nonfossil-fuel resources. Sustainable alternative, such as wind, solar, wave energy power, geothermal, and fuel cells (FCs) are being explored [18]. Along with other such alternatives, FCs are being actively developed for the purpose as they have several advantages like zero emissions, high power density, high efficiency, noiseless operation etc. Among all the existing FCs, the proton exchange membrane fuel cell (PEMFC) has been actively developed for use in vehicles and portable electronics systems due to its simplicity, high power density, quick start-up, and low working temperature [19]. However, PEMFC, faces several difficulties, primarily the sluggish kinetics of the oxygen reduction reaction (ORR), and thermodynamic instability of cathodes under unsympathetic electrochemical conditions [20]. On the other hand, the catalytic reduction [21] of organic substrates such as nitroarenes [22], carbonyl compounds [23], α,β-unsaturated carbonyl compounds [24], and reductive amination [25] of aldehydes are considered as the most significant reactions in chemical industry as these are the key steps in various industrially important organic transformations. Metal NPs involving elements such as Ni, Pd, Ru, and Pt [26,27] have been extensively studied as catalysts for a variety of nonidentical reduction reactions. This chapter is broadly categorized into two major sections. The first section discusses the application of hybrid Pd-based nanocatalysts toward ORR in FCs and the second section elaborates the reduction reactions of various organic substrates that are most important in chemical and pharmaceutical applications.

27.2 OXYGEN REDUCTION REACTION (ORR) ORR is a multielectron reaction that proceeds via several elementary steps involving various intermediates and suitable mechanisms on the cathode surface of FCs. In an acidic medium, O2 reduction proceeds as follows: i. A direct four-electron reduction pathway where O2 is converted into H2O. O2 1 4H1 1 4e2 -2H2 O

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ii. A partial two-electron reduction pathway that involves formation of H2O2 as intermediate. O2 1 2H1 1 2e2 -H2 O2 H2 O2 1 2H1 1 2e2 -2H2 O Similarly, O2 reduction in alkaline medium also proceeds via four-electron pathway to give OH2 as follows: O2 1 2H2 O 1 4e2 -4OH2 Or, by a two-electron pathway to give HO22 as intermediate and then OH2 as shown below: O2 1 H2 O 1 2e2 -HO2 2 1 OH2 HO2 2 1 H2 O 1 2e2 -3OH2 The slow kinetics of the ORR is among the most limiting factors in the energy conversion efficiency of FCs. For these reasons the Pt-based catalysts have been extensively used for the desired ORR [28]. However, the high-ceiling price and low abundance of Pt in natural resources leads to the high cost of its corresponding devices [29,30]. Moreover, when Pt is used as a cathode catalyst, the kinetic limitations of the ORR, as well as the low alcohol tolerance in direct alcohol fuel cells, become pertinent obstacles in the commercialization of low-temperature fuel cells [31]. Therefore it is an urgent necessity to develop nonplatinum-based ORR catalysts with low cost, superior efficiency, and high alcohol tolerance. In this context, Pd- and Pd-based nanomaterials have become an area of intense attention [32] as capable alternatives for ORR due to Pd’s low price, high abundance, and comparable properties with Pt, such as the identical crystalline structure and close lattice parameters [33]. Although, the inherent catalytic property of Pd for the ORR is still inferior to that of Pt [34], recent works that utilize Pd-based hybrid catalysts show much promise. For electrocatalytic ORR, the catalysts are supported onto a variety of materials. Carbon black is the most common support material, due to its unique functional properties like high conductivity, large surface area, and low price. Other materials such as carbon nanotubes (CNTs), graphene, graphitic carbon nitride, metal oxides, biochar, etc., have also been used in recent years. The support material can control the efficiency of the catalysts in several directions. For example, it prevents the agglomeration of NPs, may change conductivity of supported metal via electronic effects, and the stability of a catalyst is greatly influenced by preventing the metal leaching. Kumar et al. [35] showed the influence of chemical pretreatment of Vulcan XC-72R carbon support for ORR on Pd NPs in acidic electrolyte. The kinetics of ORR on these electrocatalysts predominantly involves a 4e2 process. However, the observed ORR activity is greatly influenced by the pretreated carbon support and is superior to that of standard E-Tek 20% Pd/C. Moreover, Pd/Ketjen black and Pd/Vulcan also exhibit good ORR activity and provide stability during chronoamperometric tests [36]. To study the morphology-dependent ORR activity, Pd/C with various morphologies using the carbonyl chemical route with various metal loading were explored, which resulted in variable electrocatalytic activity [37]. These materials exhibited similar Tafel slope (260 mV) at low current densities in acid solution, but the slope varied

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in KOH solution. Pd/C is also synthesized by using NaBH4 and NH3 as reducing and complexing reagent, respectively. Twinned and polycrystalline structures of Pd/NaBH4NH3 showed high activity for ORR with a maximum power density of 508 mW/cm2. High activity of the catalyst is ascribed to the uniformly dispersed NPs and crystalline lattice defects [38]. Carbon paper is also employed as support for the improved ORR activity. Rego et al. used electroless deposition methods for deposition of Pd NPs on porous carbon paper. The new material exhibits superior ORR activity compared to the standard Pt/carbon paper [39]. In another report, an oxygen diffusion cathode has been successfully synthesized via electroless deposition of Pd directly onto carbon paper support, and this showed good ORR activity [40]. Pd in oxide form supported on carbon is inactive towards ORR in perchloric acid solution, however, a recent study shows that activity rises when the oxides are reduced. In oxide form peroxide formation is higher than the metallic surface. Kar et al. also reported a similar result in alkaline solution [41]. Pd/C synthesized by the oxide route shows a significant improvement of ORR activity as compared to Pd/C prepared by the usual methods [42]. This improvement is due to the interaction between Pd and PdO as the oxide layer may inhibit the peroxide formation or catalyze the formed peroxides into water [43]. The typical carbon materials are easily oxidized to CO2 electrochemically. Therefore, long-term durability of the catalyst under fuel cell operating condition reduces drastically [44]. To overcome these problems, 2D materials, like graphene, graphitic carbon nitride (g-CN), have been used for the dispersion of NPs for their superior electrocatalytic properties [45]. Carrera-Cerritos et al. synthesized reduced graphene oxide (rGO)-supported Pd nanocatalyst by the polyol method. The electrocatalytic activity of the catalyst is improved by the rGO-supported Pd in comparison to Pd/C. This result shows that the ORR activity strongly depends on the support of the metal [46]. Liu et al. observed superior activity of Pd/graphene hybrid than the conventional Pd/C towards ORR. The high catalytic activity is due to the large number of physical defects, which facilitated large active sites of Pd nanocatalyst due to its higher surface area, and thus improved ORR activity [47]. Fig. 27.1 presents the schematic illustration of Pd cubes on RGNs, prepared via an electrostatic attraction between CTAB-capped Pd nanocubes and SDS-modified RGNs. Doping of nonmetal, like N, P, B, in graphene is another strategy to improve ORR activity [48]. Ramaprabhu and his coworkers synthesized triangular-shaped Pd NPs-decorated

FIGURE 27.1 Schematic illustration of Pd cubes on RGNs, prepared via an electrostatic attraction between CTABcapped Pd nanocubes and SDSmodified RGNs. Reproduced from 47.

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nitrogen-doped graphene (N-G) by pyrolysis method. It showed high ORR activity with high methanol tolerance in acidic media due to the strong cooperation between Pd NPs and N-G support [49]. Pd/graphene quantum dot (GQDs) hybrid nanocatalyst is synthesized using thermolytic reduction of PdCl2 in 1,2-propanediol and this exhibited good ORR activity in alkaline medium [50]. Barman and his coworkers observed superior ORR activity of Pd NPs-carbon nitride (PdCNx) hybrid in both acidic and alkaline media compared to benchmarked Pt/C. Furthermore, this porous PdCNx exhibits brilliant methanol tolerance and has better long-term stability than Pt/C [45]. Fig. 27.2 shows the ORR performance over PdCNx hybrid nanocatalyst. Photo-assisted method has been employed to synthesize Pd/g-C3N4 which exhibits ORR activity via the 4e2 pathway in 0.1 M KOH solution. This improved ORR activity might be credited to synergetic effects between Pd and g-C3N4 with significant methanol tolerance as well as enhanced stability in comparison to the benchmark Pt/C [51]. Carbon nanotube is also used as support for ORR, due to its various advantageous properties like high surface area, porous structure, good conductivity, stability, etc. [52]. Jukk et al. prepared Pd nanoparticle/multiwalled carbon nanotube (PdNP/MWCNT) hybrids by

FIGURE 27.2 ORR on PdCNx composite in 0.5 M KOH medium: (A) cyclic voltammograms of PdCNx composite in O2 and N2 saturated 0.5 M KOH solution with scan rate 100 mV/s; (B) comparison of LSV curves of porous PdCNx, Pt/C, and Pd/C modified GC electrode in O2 saturated 0.5 M KOH with 1600 rpm rotation at 100 mV/s scan rate; (C) LSV curves of PdCNx modified electrode in O2 saturated 0.5 M KOH solution with rotating speed varying from 600 rpm to 2700 rpm; (D) the corresponding KL plots at different potentials; (E) mass transfer corrected Tafel slope of PdCNx composite in comparison with Pt/C and Pd/C catalyst in basic medium; (F) the mass activity and specific activity of PdCNx, Pt/C, and Pd/C modified electrode at different potentials; (G) steady state chronoamperometric response of PdCNx composite, Pt/C and Pd/C at a constant potential of 0.7 V. Source: Reproduced from 45.

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magnetron sputtering and applied for ORR in both acidic and alkaline solution. The ORR activity is more facile in alkaline solution. From the RDE measurement it is found that at low overpotentials the Tafel slope is close to 260 mV dec21 and at higher current densities the slope is approximately 2100 mV dec21 [53]. Kishida and his group synthesized Pd/CNT and silica-coated Pd/CNT (SiO2/Pd/CNT) of which the SiO2/Pd/CNT catalyst showed greater ORR activity and outstanding durability [54]. In another report, Takenaka et al. used silica coat on Pd/CNT which exhibits similar ORR activity Pd/CNT. Additionally, the silica-coated Pd/CNT was highly stable during potential cycling between 0.05 and 1.20 V (vs RHE) in aqueous HClO4 while Pd/CNT undergoes severe deactivation under the identical conditions [55]. Transition metal oxides are also used to improve the ORR activity and methanol tolerance. The hypo d-electron transition metal oxides have good corrosion resistance, high chemical stability, high surface area ,and strong metalsupport interaction, which can improve the activity performance of the catalysts. Metal oxides, as catalyst supports, are able to stabilize and disperse effectively a number of active phases, as well as to retain a high surface area [56,57]. Sun et al. synthesized Pd@MnO2 hybrid nanocatalysts by depositing Pd on the surface of β-MnO2 nanorod in aqueous solutions at room temperature. The activities of the Pd@MnO2 nanocatalysts for ORR were 2.5 times higher than that of the Pd/carbon black catalyst. The better activity of the catalyst is due to the Pd layer on the surface of the MnO2 nanorod [17a]. Another report using different forms of MnO2 as support material has also found enhanced activity compared to the commercial Pt/C catalysts [17b,c]. Tungsten oxide (WO3) and titanium oxide (TiO2) are also explored for fabrication of hybrid catalysts with superior ORR activity and methanol tolerance ability than the benchmark Pt/C [17df]. Chen and his coworker developed a strategy for the synthesis of Pd tetrahedron/tungsten oxide nanosheet hybrid nanomaterials (Pd/W18O49) by using organopalladium(I) complexes containing PdPd bonds as precursors [17g]. They have found that Pd nanoparticles or W18O49 hybrids exhibited unexpectedly high ORR activity and larger stability than commercial Pt in alkaline solutions. These results recommend that the activity enrichment is a result of electronic structure change of Pd upon its synergistic interaction with the W18O49 nanosheet support. Very recently, Wang and coworkers prepared Pd/TiO2VO (VO means oxygen vacancy) by pyrolysis and observed superior ORR activity compared to the 20 wt.% commercial Pt/C [17h]. The prepared 10 wt.% Pd/TiO2VO catalyst reveals a 30 mV positive shift of half-wave potentials, better durability with less loss (3.8% vs 34.9%) in the current density after 10 h, and higher methanol tolerance ability in comparison to the 20 wt.% commercial Pt/C. They have also performed DFT calculation to establish the improved ORR performance. The strong metalsupport interactions (SMSIs) are mainly responsible for the enriched ORR activity of the 10 wt% Pd/TiO2VO. Fig. 27.3 presents the TEM and EDS elemental images of Pd/TiO2VO. Yousaf et al. synthesized Pt3Pd1 2 CeO2/C through a facile surfactant-free method [17i]. The synthesized hybrid nanocatalysts follow a 4e2 ORR process which is verified by RDE experiments. The onset potential of ORR over Pt3Pd1 2 CeO2/C catalyst shifted toward more positive values than that of Pt/C catalyst, and Pt 2 CeO2/C is comparable to Pt/C.

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FIGURE 27.3 TEM images of Pd/TiO2VO with a scale bar of (A) 50 and (B) 20 nm and (C) Pd particle size in the Pd/TiO2VO catalyst. (D) EDX pattern of the Pd/TiO2VO catalyst. (E) HRTEM image of Pd/TiO2VO with a scale bar of 5 nm. (FI) EDS elemental mapping of Pd, Ti, and O in Pd/TiO2VO. Source: Reproduced from 17h.

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27.3 REDUCTION OF ORGANIC SUBSTRATES The catalytic reduction [21] of organic substrates such as nitroarenes [22], carbonyl compounds [23,24], and reductive amination [25] of aldehydes are considered as the most important reactions in the chemical industry as these are key steps in various important organic transformations (Scheme 27.1). Palladium-based catalysts are highly active towards reduction reactions due to their superiority in dissociating H2 molecules into atomic H and forming activated PdH bond. In the “volcano” relationship of metalH bond energy with H2-dissociation ability, the location of Pd metal in the top area of the volcano plot implies its exceptional ability towards catalyzing reduction reactions [58]. This can be illustrated with the example of catalytic hydrogenation of benzene to cyclohexane, a significant industrial process, which is the primary ingredient for caprolactam production. Various transitions metals are known to be active towards the hydrogenation of benzene in a typical reactivity order of Rh . Ru . Pt . Ni . Pd . Co, attributed to the fundamental interactions of metallic NPs with substrates (benzene and hydrogen) [59,60]. According to Yoon et al. the Rh/CNT (CNT 5 carbon nanotube) catalyst is highly efficient in the hydrogenation of benzene into cyclohexane at room temperature (Scheme 27.2), whereas under identical conditions Pd is almost inactive due to poorer adsorption of benzene [61]. However, on using the alloy of Pd with Rh the hydrogenation of benzene proceeds with an activity almost twice compared to the monometallic Rh catalyst. This increase in reactivity is attributed to the unique electronic arrangement of PdRh alloy, which synergistically activates both benzene and dihydrogen. The reduction of nitro groups to aromatic or aliphatic amines represents another most straightforward approach [62]. The conventional method includes the reductions via hydrogenation with classical methods (Pd/C, PtO2, Raney nickel, or transition metal SCHEME 27.1

Some significant reduction reactions.

SCHEME 27.2 Hydrogenation of benzene.

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catalysts [63,64], SnCl2 [65,66] or for metal-dissolving reductions involving Zn, Fe, In, or Sm [67,68], or under transfer hydrogenation conditions [69]. However, a major problem with these protocols is limited functional group compatibility, use of harsh (e.g., hightemperature and high-pressure equipments) and hazardous reagents (e.g., hydrazine), and/or the presence of nonreusable transition metal catalyst, which often generates waste (a frequent environment concern) or otherwise contaminates the final products. To remove these drawbacks, the combination of Pd and Au is an excellent choice as they exhibit strong bimetallic electronic ligand interactions between their constituent atoms [3a,70]. In a typical example reported recently [71] Pd NPs supported over AuAl2O3 were prepared for the gas-phase selective hydrogenation of 4-chloronitrobenzene, resulting in the sole formation of 4-chloroaniline. PdAu NPs of specific tailored sizes were decorated over alumina in a controlled manner by depositionprecipitation with urea (DP) and impregnation (IMP) methods with different molar ratios of Au/Pd (8, 20, and 88). They successfully achieved the inclusion of Pd (at Au/Pd 5 20, IMP and DP) resulted in a threefold increase in the catalytic activity. DRIFTS measurements employing CO as a probe molecule suggests the formation of bimetallic entities and surface PdAu interactions. The surface PdAu synergism is attributed for the enhanced and selective production of 4-chloroaniline. In another report, Corbos et al. developed an efficient method for the preparation of PdAu NPs via a one-spot colloidal approach [3b]. The prepared bimetallic nanoparticle displayed high activity in the selective hydrogenation of 2chloronitrobenzene. It was also established that the superior selectivity and reaction rates with bimetallic structure of PdAu is due to the presence of a Pd-rich surface. To further enhance the catalytic activity and selectivity of PdAu bimetallic system, Paul and coworkers have selected cellulose and silica to construct a hybrid layer for the stabilization of PdAu NPs [72]. Herein, cellulose acts as biomaterial component and SiO2 as inorganic component providing an organic/inorganic hybrid support. This combination was found to be very important for the high dispersibility that suppresses the loss of Au and Pd NPs during catalytic cycles thereby providing highly enhanced activity and selectivity to PdAu NPs in comparison to their monometallic catalysts. The prepared hybrid catalyst was employed in the selective hydrogenation of various nonidentical substrates, viz. α,β-unsaturated carbonyl compounds, carbonyl compounds, nitroarenes, and in the onepot reductive amination of aldehydes with nitroarenes under H2 atmosphere and mild reaction conditions. Pal and his group reported that extremely small mesoporous leafy nanostructures of gold and palladium can be prepared by employing 1,4-dihydropyridine ester (DHPE) as a potential reducing agent. These AuPd nanoleaves were found to be active catalytic systems for the reduction of 4-nitrophenols in the presence of hydrazine hydrate as reducing agent (Scheme 27.3A) [73]. The activity of Pd can also be enhanced by stabilizing Pd NPs on mesoporous cerium oxide (CeO2) NPs. Recently, we have synthesized and utilized these Pd/CeO2 NPs in the reduction of substituted nitroarenes in presence of NaBH4 as reductant (Scheme 27.3B) [74]. The prepared CeO2-based nanocatalyst exhibits excellent catalytic activity with chemoselectivity for the synthesis of amine derivatives at room temperature. Based on kinetic studies the reaction is found to follow a pseudo-first order rate equation. With the same principle, we successfully prepared CuPd alloy NPs which synergistically catalyze the aqueous phase reduction of nitroarenes in the presence of NaBH4 as reductant (Scheme 27.3C) [75]. Moreover, PdRu

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NO2

NH2

SCHEME 27.3A

Hydrogenation of 4-nitrophenol.

Au-Pd nanoleaves H2O, N2H6.H2O OH

OH

NO2

SCHEME 27.3B

NH2 R

Pd/CeO2 NPs NABH4

R

SCHEME 27.3C Hydrogenation of nitroarenes.

NH2

NO2 R

CuPd NPs NABH4

NO2

Hydrogenation of nitroarenes.

R

NH2

SCHEME 27.3D

Hydrogenation of 4-nitrophenol.

Pd-Ru MAF film NaBH4 OH

OH

SCHEME

27.4

Selective

hydrogenation

of

cinnamaldehyde.

nanoalloys supported on alumina films also display high activity toward the reduction of 4-nitrophenols in the presence of NaBH4 (Scheme 27.3D) [76]. The selective catalytic C 5 O bond hydrogenation of cinnamaldehyde to cinnamyl alcohol is an important reaction due to its significance as a precursor in various industrial processes. Although, Pd-based catalysts are highly active towards C 5 C bond hydrogenation, they are quite unreactive in the reduction of C 5 O bond due to the lower adsorption tendency of Pd for carbonyl groups [77]. To improve its reactivity, Qiu et al. have amalgamated Pd with a small amount of Ru over carbon nanotube support which enhances the selective hydrogenation of the C 5 O group for the production of cinnamyl alcohol (Scheme 27.4) [78]. This shift in catalytic selectivity was attributed to the synergistic effect between Pd and Ru. Moreover, in accordance with the “volcano” relationship between the catalytic activity of Pd and Ru, and their binding energy with C 5 O species [79], the amalgamation of Pd with Ru strengthens their interaction with the C 5 O group into an appropriate position, and gives higher catalytic activity compared to either of the monometallic species. Liu et al. [80] showed that Pd/Ni bimetallic nanocatalyst exhibits much higher

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SCHEME 27.5

Chemoselective hydrogenation of activated

benzaldehydes.

(S,S)

(R,S)

SCHEME 27.6

Selective catalytic hydrogenation of imine to chiral benzylic primary amine.

SCHEME 27.7

Probable explanation of the observed selectivity towards reductive amination.

activity towards chemoselective reduction of active benzaldehydes to benzyl alcohols than the corresponding monometallic catalysts analogues (Scheme 27.5). ˘ et al. have reported a highly active PdCu/C catalyst as an alternaMu¨slehiddinoglu tive to Raney Ni for the diastereoselective hydrogenation of imines (a prochiral ketones and α-phenylethylamines derivative) (Scheme 27.6) [81]. Compared to conventional Pd/C catalyst, which afforded a diastereomeric excess (de) of 72%, the use of PdCu/C delivers a de up to 94% of the corresponding chiral amine. The results also revealed that catalysts with a ratio of 4:1 (Pd:Cu) exhibited higher levels of activity. In contrast to Raney Ni, the key advantage with this bimetallic PdCu/C system is improved safety, consistent results on scale, and wide substrates scope. The unique ability of metal NPs (Pd, Pt) to mediate tandem reactions has been exploited on many occasions for the reductive CN coupling reactions of carbonyl compounds with nitroarenes [82,83]. In this regard, the ability of Pd1Ag1.7 as catalyst in the tandem reductive amination reactions between nitroarenes and aldehydes is a significant approach (Scheme 27.7) [84]. Under ambient conditions a wide variety of substrates were coupled with high catalytic activity and selectivity and excellent recyclability. Moreover, a clear composition-dependent activity/selectivity order was observed upon varying the composition of the Pd1xAgx (x 5 01) catalysts during this study.

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SCHEME 27.8

Selective hydrogenation of phenylacetylene.

In literature, various monometallic catalytic systems are available for the selective hydrogenation of arenes, as reviewed by Claver and coworkers [85]. Selective hydrogenation of multiunsaturated bonds is often associated with unwanted side-products due to multihydrogenation steps and certain other parallel pathways. For instance, the selective synthesis of ethylene (C2H4) from acetylene (C2H2) is one of the most significant industrial goals due to its widespread application in polymer synthesis, a process chiefly mediated by supported Pd-based catalysts [86,87]. However, Pd-based catalysts are often less selective towards ethylene production due to the accumulation of ethylene hydride intermediates over bulky Pd ensembles, which thereby lead to their conversion into ethane [88,89]. In 2003, Choudhary et al. identified that the incorporation of Pd with Au atoms (PdAu alloy NPs) dramatically enhances the selectivity of Pd towards partial hydrogenation of acetylene to ethylene [90]. Recently, the group of Han has reported that the ethylene selectivity could be increased to 80% on using a Pd@Ag coreshell nanoparticle [91]. Whereas, in an another report, Schlo¨gl and coworkers have found that among various Pd alloybased catalysts (PdAu, PdAg, and PdGa) PdGa exhibits the highest selectivity towards ethylene [92]. A possible explanation for the selectivity of these Pd-based bimetallic catalysts is due to the presence of strong electron-charge transfer from modifier atoms (Au, Ag, Ga) to Pd, and isolated Pd-modifier moieties. This synergistic interaction between Pd and the modifier atoms alters the electronic structure of Pd to reduce its density states at its Fermi edge, thereby decreasing the position of the d-band center of the corresponding Pd-alloy. This effect will reduce the ability of Pd to form a metal hydride bond (reducing the rate of hydrogen molecules dissociation) and also lowers the active H-atom availability, leading to the selective hydrogenation of acetylene to ethylene. Moreover, the isolation of Pd by the modifier atoms restricts the extensive formation of Pd ensembles required for the production of ethylene hydride, thereby reducing ethane formation. In consistence with the above findings, the combination Pd25Ag75 with 20% CO2 system delivers high catalytic activity and selectivity (97.5%) towards partial hydrogenation of phenylacetylene (Scheme 27.8) [93]. Tan and coworkers reported that PdAg bimetallic colloids can be prepared in CO2-expanded hexane from the corresponding isostearate salts, Pd(C18H35O2)2 and Ag(C18H35O2)2. The key factor for success was the addition of CO2 which inhibited the enlargement of the NPs and controlled small particle sizes.

27.4 CONCLUSIONS In this chapter we have reviewed the application of various Pd-based hybrid nanocatalysts in a series of important reduction reactions. These include the oxygen reduction reactions in fuel cells, and the catalytic reduction of arenes, nitroarenes, carbonyl compounds, imines, alkynes, amination of aldehydes, and chemoselective reduction of α,β-unsaturated carbonyl compounds. However, this field still suffers from certain drawbacks and needs further research. For instance, the development of a common and controlled procedure for

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the synthesis of a wide range of Pd-based hybrid nanoparticles might be more productive than specific methods restricted to the preparation of only one or a few catalysts. Moreover, explorations to develop catalytic systems with simpler and effective conditions need to be addressed in the near future.

Acknowledgments The authors thank Tezpur University, Council of Scientific and Industrial Research (CSIR No: 01(2813)/14/EMRII), New Delhi and Science and Engineering Research Board (SERB-DST No: SB/FT/CS-048/2014), New Delhi for generous financial support. M.M. is also grateful to DST-SERB, New Delhi, for a National Post-Doctoral Fellowship (PDF/2016/003650).

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C H A P T E R

28 Photoelectrochemical Water Splitting Prabhakarn Arunachalam and Abdullah M. Al Mayouf Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia

28.1 INTRODUCTION One of the most essential concerns for human society is perceived with the increasing awareness in producing a low-cost, clean, and copious source of energy to ensure minimal impact to the environment. In fact, the solar energy with its infinite quantity is likely to be the clean and low-cost renewable energy resource. The solar power arriving the earth’s surface corresponds to that delivered by 130 million power plants of 500 MW [16]. Presently, solar energy-based technology is realizing great recognition owing to its numerous properties, which include its ability to operate without noise, toxicity, or greenhouse gas release [711]. The viability for the advancement of solar energy at the TW scale depends on discovering economically compatible solutions to the intrinsic variability of solar power supply problems [12]. Nature elucidated these problem by means of photosynthesis, the method that transforms solar energy into chemical energy stored in the form of atomic bonds of the adenosine triphosphate molecule. Likewise, water photoelectrolysis through semiconductor materials is recognized to be the most essential challenge for the transformation and storage of solar energy [13]. To accomplish this “artificial photosynthesis” an eco-friendly PEC cell must be established, comprised of stable semiconductor materials and designed in such a way that the appropriate PEC reactions take place at the semiconductor/solution interface [14]. Out of anxiety for natural source exhaustion and ecological debates, the visible-light driven electrolysis of water to generate oxygen and hydrogen fuels and the conversion to electricity, inspire the advancement of clean energy systems for producing energy.

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FIGURE 28.1 Scheme of a basic photoelectrochemical cell.

The simplest structure of a PEC cell that concurrently executes both oxygen evolution reaction (OER) and H2 evolution reactions (HER) is shown in Fig. 28.1. At the cathode the reduction reaction takes place (2H1 1 2e2-H2) and at the photoanode the reaction of O2 evolution is carried out (2H2O-4H1 1 O2 1 4e2). The hydrogen economy is increasingly investigated to substitute the fossil fuel resources. However, storing hydrogen therefore offers a means to store electrical energy, e.g., surplus electricity produced by power stations controlled by the weather, wind farms, or by solar farms. To achieve this, PEC water splitting driven by solar energy creates an eco-friendly methodology to store energy in the covalent bonds of H2 [15]. To effectively transform solar photons into such a chemical fuel, semiconductorbased materials skilled at absorbing a great part of the solar spectral region, with a suitable energy level position, and low overpotential to perform a water splitting reactions are essential. In this line, exhaustive research works have been carried out to advance competitive n-type semiconducting materials employed as water splitting photoanodes, as the oxygen evolution reaction (OER) is both kinetically and thermodynamically more challenging, and the photoanode materials are damaged by severe oxidizing circumstances [16]. Various kinds of semiconductor oxide photoelectrodes (TiO2 [17], Fe2O3 [18,19], WO3 [20,21], BiVO4 [2224], etc.) have been engaged for fabricating PEC devices to oxidize water and generate molecular oxygen. In particular, titanium dioxide (TiO2) has been recognized to be the most suitable contender for PEC water splitting due to its band-edge levels, greater optical stability, great chemical inertness, photostability, and cost-efficiency [2528]. Various efforts have been devoted to improving TiO2 light absorption behaviors and making this catalytic material sensitive to visible-light irradiation, which also offers benefits of hindering recombination rate of charge carriers and higher anatase crystallinity [29,30]. Alternatively, BiVO4 has fascinated researchers worldwide as the most appropriate water splitting photoanode materials for PEC water splitting [3134]. BiVO4 gratifies numerous desires, like n-type semiconductor with a direct band gap size of 2.4 2 2.5 eV (have the tendency upwards of B7.5 mA/cm2 photocurrent), engrosses full visible-light region of solar spectrum, and has a friendly nature with basic and neutral

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conditions, nontoxic, and relatively cheap [35]. In particular, BiVO4 grasps the greatest of catalytic performance with 8.1% solar to hydrogen (STH) efficiency via double-junctioned GaAs/InGaAsP photovoltaic (PV) device [36]. Quite recently, Qiu et al. reported the tandem configuration with a single perovskite solar cell and achieved unaided water splitting with a STH conversion efficiency of up to 6.2% for more than 10 h [37]. Subsequently, there are few concerns on the availability of bismuth in the Earth crust [38], and various kinds of compositional alterations have been investigated to attain an inexpensive metal vanadate built on this method [3941]. In this line, BiVO4 photoanodes must satisfy many of the requirements; their sluggish electronic properties produce low solar conversion efficiencies, inhibiting their commercial usage in PEC systems at present. In particular, the incorporation of an effective, stable, and inexpensive water oxidation catalyst on the photoactive semiconductor material is crucial to attain the directed techno-economical necessities. Recently, Prussian Blue (PB)-type electrocatalyst materials were engaged as supporting cocatalysts and a hole-storage layer to safeguard unstable BiVO4 anode against photocorrosion in PEC water splitting [4244]. To address this issue, various approaches have been reported, including ion doping [4547], nanostructuring [48], surface modification with passivation layers or electrocatalysts [32,49,50], and combinatorial synthesis [51]. It is highly in demand to look for nonoxide-based semiconductors for PEC water oxidation. Oxynitride-based photoanodes have the potential to substitute oxide-based anodes for light absorption to generate H2 and O2 from H2O by a stoichiometric ratio [5254]. Scaife described that it is intrinsically tougher to progress an oxide photocatalyst which has a properly negative conduction band (CB) and a narrow band gap (i.e., ,3 eV) for light absorption owing to the greater positive valence band (VB) (at c. 13.0 V vs NHE) produced by the O 2p orbital [5556]. In this book chapter, we describe recent progress in the growth of earth-abundant electrocatalysts for water splitting in the context of their probable use in PEC solar-tohydrogen devices. Moreover, we describe various examples of complete and functional PEC solar-to-hydrogen devices and critically evaluate the continuing disputes in this field. Finally, this is a well-timed review to investigate the numerous renewable hydrogen generation schemes and also to give greater consideration to the visible-light driven solardriven water electrolysis.

28.2 PRINCIPLES OF PEC WATER SPLITTING PROCESS The free energy change for the transformation of one molecule of H2O into H2 and 1/2O2 in standard conditions is ΔG 5 237.2 kJ/mol, which according to the Nernst equation, agrees to ΔE 5 1.23 V per transported electron. Accordingly, the usage of a semiconductor that conveys this reaction with the backing of light, means that the semiconductor must absorb photons with energy .1.23 eV (light absorption of 1008 nm) and transform this energy into H2 and O2, as schematically presented in Fig. 28.2. The actual processes involved in PEC processes are described in Fig. 28.3. To achieve this, there are four main physiochemical steps that need to be satisfied to complete PEC water splitting reaction. These processes include: (i) capture of photons;

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Water splitting (Artificial photosynthesis)

(A)

CB

H2

e–

H2O

Energy

Band gap H2O

h+

O2

H2 + O2

VB Chemical energy ΔG0 = 237 kJ/mol

H2O Powdered photocatalyst

Water

(B) –2.0 SiC

2.8eV

H+/H WO3 Fe2O3 2

2.0

2.3eV

MoS2 1.75eV

1.0

1.1eV

Si 3.0eV

1.7eV

3.6eV

TiO2

2.4eV

Cds Cdse 3.0eV

3.2eV

0

3.4eV

KTaO3 SrTiO3 5.0eV

V vs. NHE (pHD)

–1.0

GaP

2.25eV

Zns ZrO2

O2/H2O

3.0 4.0

FIGURE 28.2 Photocatalytic water splitting: (A) graphical representation of water splitting via semiconductor photocatalyst; (B) band structure of various kinds of semiconductors and its corresponding redox potentials of water splitting. Source: Reprinted with permission from K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 440 (2006) 295295. Copyright from Royal Chemical Society.

(ii) creation/separation of electronhole pairs; (iii) separated charge carriers that need to be transported via charge transfer; which must be catalytically active. During the last two processes, induced electron/hole pairs can either recombine in the bulk, and therefore both effective charge separation and higher transportation of electron/hole pairs are desired; and (iv) surface chemical reactions. Both the potential of the induced electron/ hole pairs and appropriate water oxidation kinetics are vital for effective water splitting reaction. Usually, the ideal photoanode material must satisfy a number of criteria to carry out the water photoelectrolysis. These are: (1) to display a strong (visible) light absorption with a band gap varying between 1.8 and 2.4 eV; (2) to display great chemical inertness both in the darkness and under illumination; (3) band edge energy position that supports the water redox potentials; (4) to effectively separate/transport the charge carriers within

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FIGURE 28.3 A basic energy diagram for a photoanode (n-type semiconductor). Various stages of process are presented, namely: (i) light absorption; (ii) chargetransfer; (iii) transportation of charge carriers; and (iv) surface chemical reactions.

the semiconductor material to keep the rate of water-splitting reaction faster; (5) to show low charge transfer resistance at the liquid/semiconductor interface (low overpotentials); and (6) to have low cost.

28.3 PHOTOANODE MATERIALS During last decades, several kinds of photocatalyst materials for PEC water splitting reaction have been demonstrated, and huge research expenses have been devoted in this area. This section reviews the most favorable economical and efficient anodes materials engaged so far, and puts forward prospective approaches to enhance their PEC behaviors.

28.3.1 TiO2-Based Photocatalysts TiO2-based photocatalysts have been recognized to be the most suitable applicant for visible-light driven water splitting, and have been extensively considered since 1972, owing to its suitable band-energy levels, its nontoxicity, as well as its photostability [57]. However, owing to its larger band-gap (B3.2 eV: anatase; B3.1 eV: rutile phase), merely 5% of the solar spectrum can be absorbed, which reduces its widespread use and results in a very low maximal theoretical STH efficiency. In recent years, several research works have been undertaken to incorporate TiO2 with various kinds of anions or cations to lengthen its operating range into the visible-light region in order to advance the whole absorption as well as preserving its good photostability and low cost [29,58]. Until now, sensitization of TiO2 electrode surface with a smaller band-gap semiconductor/dyes [59,60], nonmetal, and metal nanoparticles doping [61,62] has been mostly employed to improve the PEC performance of TiO2 materials. Though, in most of these doping strategies because there is no significant band gap change, no considerable improvement in PEC performances has been reported. While the doping approach can spread the

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absorption range into the visible light region, the optical absorption is still very restricted beyond 450 nm. Despite these facts, Mao et al. advanced black-colored hydrogenated TiO2 nanocrystals, relating to a band gap energy of 1.0 eV rather than the 3.2 eV usually observed for pure TiO2, favoring a much superior PEC efficiency [63]. In recent years, surface plasmon resonance (SPR) has been used in PEC water electrolysis process with extended light absorption in the whole UVvisible region of the solar spectrum [64,65]. Additionally, SPR is an inherent stuff of metal nanoparticles, where the collective oscillation frequency is extremely sensitive to the size and shape of the metals. For example, Auincorporated TiO2 nanowire electrodes showed superior photocurrent generation at 710 nm and improved photoactivity, which is credited to the excitation of SPR of Au [66]. Similarly, significant research works have been undertaken on Ag doping on TiO2 electrode; the Ag nanoparticles act as an electron sink in the role of Ag SPR effect for the photoinduced electronhole pairs and thus results in improved PEC performance [6769]. Quite recently, we reported the incorporation of Ag onto mesoporous TiO2 (meso-TiO2), fabricated by the evaporation-induced self-assembly method and the improved PEC over various ranges of Ag nanoparticles over mesoporous TiO2 is shown in Fig. 28.4. Moreover, the PEC results demonstrated that the maximal photocurrent density of Ag/meso-TiO2 nanospheres photoanodes reaches 1.0 mA/cm2 (for [AgNO3] 5 1 mM) which is nearly a two-fold enhancement over that of meso-TiO2 photoanodes. Under illumination condition, the enhanced photocurrent at lower potential shows that the incorporation of Ag particles reduced the recombination of electron/hole pairs. Loading TiO2 photoanode materials with other lower band gap semiconductors to create a heterojunction are alternative favorable methods to produce visible light, whereas the loaded semiconductors assist as a photosensitizer and builder for internal electric field through the interface. The heterojunctioned materials tends to have internal potential bias, which considerably encourages the excited hole and electrons pairs separation and transport via the interface, resulting in a reduced recombination. Recently, Choi et al. developed a heterojunction CdTe/TiO2 photoelectrodes, enhancement in PEC performance credited to the optimization of Fermi level, band positions, and the conductivity of CdTe layer [70]. Similarly, TiO2 nanotube arrays were modified with Cu2O semiconductors [71,72].

28.3.2 BiVO4-Based Photocatalysts The functional properties of BiVO4 photoanodes have been advanced by heterostructuring strategies with Fe2O3. For example, the PEC behavior of BiVO4 has been greatly enhanced when loaded on nanostructured WO3 layers, credited to the synergistic interface between the BiVO4 (offering good light gathering behaviors) and WO3 (enhanced charge carrier transport) [7375]. Moreover, various semiconductor combinations have been effectively employed and established with significant effects for water splitting reactions, like Si/TiO2/BiVO4 [76], SnO2/BiVO4 [77], WO3/Fe2O3 [78], and Ag3PO4/BiVO4 [79]. Cai et al. reported the decoration of BiVO4 with Fe2O3 nanoparticles, where it is advised that Fe2O3 behaves as a proficient supportive catalyst for the photocatalytic degradation of organic pollutants [80]. Similarly, various kinds of Fe-based cocatalysts have been effectively engaged to decorate BiVO4 photoanodes, such as FeOOH [81] and NiFeOx [82].

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FIGURE 28.4 (A) Linear sweep voltammograms (LSV) during in situ 1.5 AM light pulsing for mesoporous TiO2 photoanodes incorporated with various loading of Ag nanoparticles deposited in 0.5 M Na2SO4 solution at pH 13. (B) Expanded zone of LSV in (A) with various loading of Ag that photodeposited via numerous concentration of AgNO3 solution (from 0 to10.0 mM). (C) Comparison for the LSV at 50 mV/s for meso-TiO2 and Ag/mesoTiO2 nanospheres photoanodes in in 0.5 M Na2SO4 solution at pH 13 and in visible-light illumination condition, (D) and consistent relative investigation in UV-light illumination. Source: Reprinted with permission from P. Arunachalam, M.S. Amer, M.A. Ghanem, A.M. AlMayouf, D. Zhao, Int. J. Hydrogen Energy 42 (2017) 1134611355. Copyright 2017 Elsevier.

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Recent years, Co-based WOCs have received plentiful consideration owing to the greater efficiency under near neutral pH conditions, and production cost of Co [83]. Moreover, the transport of photoinduced holes from the valence states of BiVO4 to the Co31/21 species through the water oxidation reaction significantly lower the overpotential [8487]. There are several works carried out on Co-based catalysts on several kinds of photoanodes containing WO3, α-Fe2O3, and ZnO that have shown improved PEC water-splitting efficiency [8890]. Moreover, significant enhancements were noticed predominantly for BiVO4 photoanodes upon the incorporation of cocatalysts. Long et al. fabricated powdered composite of BiVO4/Co3O4 on conducting substrates with four-fold enhanced photon to current efficiency related to bare BiVO4 [91]. Zhong et al. attained a photocurrent of 1.4 mA/cm2 at 1.23 VRHE with CoPi catalyzed W-loaded BiVO4 [92]. They revealed that the cobalt phosphate (CoPi)OECs enhanced the PEC efficiency of the water electrolysis to approximately 100% and that the total PEC behaviors of CoPi catalyzed W:BiVO4 photoelectrodes is restricted by its bulk recombination processes. However, bulk-recombination process limitations need to be rectified to achieve the efficient BiVO4-based photoelectrodes. Recently, Abdi et al. investigated the CoPi-catalyzed BiVO4 photoanodes and attained external quantum efficiencies of 90%. Moreover, they rule out the possibility of bulk recombination process and illustrated that the sluggish electron transport is vital factor to be investigated in BiVO4 materials. In addition, Abdi et al. attained the photocurrent of 1.7 mA/cm2 at 1.23 VRHE for co-catalyzed undoped BiVO4 [93]. In recent years, there have been two conflicting views to understanding the mechanism of the photocurrent enhancement in the CoPi-loaded photoanodes. Barroso et al. described improved photoinduced charge carriers separation and carrier lifetime of semiconductor by hosting CoPi, which was credited to the improved band bending (Fig. 28.5) [94]. On the other hand, Klahr et al. showed that there is no alteration in the band bending, and in its place advised that CoPi quickly collects photoinduced holes from the semiconductors, thereby decreasing the carrier recombination at the surface [95]. Zacha¨us et al. examined the surface carrier dynamics of CoPi-loaded BiVO4 photoanodes by the intensity modulated photocurrent spectroscopy (IMPS) method. The detailed investigation illustrated that the loading with CoPi OECs reduced the recombination behavior of BiVO4 with a factor of 1020, without considerably affecting the charge transfer kinetics. In addition, the major part of the CoPi is passivation of the surface of BiVO4 and the photocurrent of BiVO4 photoanodes is restricted by surface recombination instead of charge transfer [96]. Lichterman and coworkers employed CoOx/BiVO4 photoelectrodes to achieve a photocurrent of 1.49 mA/cm2 at 1.23 VRHE under AM 1.5G [97]. Wang et al. revealed 17-fold enhanced PEC water electrolysis performance upon the deposition of Co3O4 for BiVO4 [98]. Recently, Maged and coworkers reported a combination of ZrO2 and α-Fe2O3 nanoparticles deposited on the surface of a BiVO4 photoanodes films. Subsequently, an incredible fivefold enhancement of the photocurrent for optimized BiVO4ZrFe photoanodes was reported, which can be credited to the supportive catalytic part of monoclinic ZrO2 and a-Fe2O3 nanoparticles dispersed on the surface of BiVO4 [99]. Very recently, Hegnaer et al. integrated the photoactive BiVO4 materials and electrocatalytic materials of CoFe Prussian blue (CoFePB) electrocatalysts. Moreover, this mixture cathodically shifts the onset potential of BiVO4 by 0.8 V and raises the photovoltage

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28.3 PHOTOANODE MATERIALS

593 FIGURE 28.5 Basic model of elementary process in a BiVO4 photoanodes (A) without cocatalyst; (B) with a surface passivating cocatalyst (e.g., CoPi); (C) with a nonpassivating cocatalyst (e.g., RuOx). Source: Reprinted with permission from F. Abdi, R. van de Krol, J. Phys. Chem. C 116 (2012) 9398 copyright from Royal Chemical Society; C. Zacha¨us, F. Abdi, L.M.P. Peter, R. van de Krol, Chem. Sci. 8 (2017) 37123719 published by The Royal Society of Chemistry.

by 0.45 V. In addition, an astonishing sixfold enhancement of the photocurrent at 1.23 V vs RHE is attained for BiVO4/CoFePB photoelectrodes [98] (Table 28.1).

28.3.3 Fe2O3 Oxide Photocatalysts Another auspicious photocatalyst for PEC water electrolysis and visible-light responsivity is hematite (α-Fe2O3), which has been established to challenge the 4 electron oxidation reaction of water [100102]. In particular, α-Fe2O3 provides more advantages over other material owing to its greater chemical inertness, low toxicity, and also due to its high natural abundance. Furthermore, it has a band gap value between 1.9 and 2.3 eV, permitting visible-light absorption which transforms to a maximal theoretical STH efficiency [103]. Though, α-Fe2O3 holds a CB position considerably more positive proton reduction potential and has a capability to be utilized for PEC water oxidation with an external bias. Additionally, α-Fe2O3 has other shortcomings, including: (1) minor charge carrier lifetime (in the order of 10212), resulting in a rapid charge carriers combination in the bulk; (2) a comparatively low absorption coefficient demanding thick film (B400500 nm) for optimal light absorption; (3) a sluggish minority charge carrier (hole) mobility; and (4) poor water oxidation kinetics, which result in a higher recombination rate at the surface.

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TABLE 28.1 Review of Deposition Metal Oxide/Phosphate Electrocatalyst into BiVO4 Photocatalyst for Water Splitting Reaction

Catalysts

Electrolyte, Conc., pH

Nature of the Structure

Photocurrent Density (mA/cm2) at 1.23 VRHE

Ref.

CoPi 0.1 M KPi buffer at pH 8 catalyzed W: BiVO4 photoelectrodes

Polycrystalline with primary 1.4 features having dimensions of B100200 nm

[92]

BiVO4/CoPi

0.5 M K2SO4 (99%, Alfa Aesar) solution buffered to pH B5.6 with 0.09 M KH2PO4 (99.5%, Fluka)/ 0.01 M K2HPO4 (99%, J.T. Baker).

30 nm thick CoPi catalyzed BiVO4 photoelectrode

1.7

[93]

BiVO4/ Fe2O3/Zr

Phosphate buffer solution at pH 7.6

500 nm for pristine BiVO4; 200 nm for the optimized BiVO4Zr electrode

1.2

[88]

BiVO4/ CoFePB

0.1 M KPi buffer (solid lines) and after addition of the hole scavenger Na2SO3

BiVO4 photoanode, with a thickness of about 200250 nm

1.4

[99]

pn BiVO4/Co3O4 heterojunction

0.5 M KPi buffer/1 M Na2SO3, pH 7 Discrete Co3O4 particles B10 nm

2.7

[100]

BiVO4/ ZnO/CoPi

0.2 M Na2SO4 solution (pH 6.5)

3

[101]

BiVO4 on 1D ZnO rods with CoPi on surface

To overcome these shortcomings, α-Fe2O3 photoanodes have been effectively doped with heteroatoms like Sn [104], Ti [105], Zr [47,106], Si [107], and Nb [46]. In particular, Si incorporated α-Fe2O3 films can reveal a superior PEC behavior, with photocurrent densities of nearly 2.7 mA cm22 at 1.23 VRHE under AM 1.5G [49]. Furthermore, cocatalyst modification, for instance, with CoPi or FeOOH, has been revealed to speed up the surface water oxidation kinetics [108]. Lastly, thin metal oxide under/overlayers have been incorporated onto α-Fe2O3 resulting in substantial enhancement in PEC performances; these layers influence surface state passivation and result in an upsurge in charge carrier concentration and mobility [109].

28.3.4 Oxynitride-Based Photocatalysts Among the oxide-based photoanodes, the top edge of the VB involves O 2p orbitals and is positioned at about 13.0 V versus NHE at pH 0. Additionally, the CB position is more negative than the water reduction potential, resulting in a larger band gap of nearby 3.0 eV, meaning the material is inactive under visible-light region [55]. In recent years, certain oxynitrides-based photocatalysts have been demonstrated as replacements to oxide

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595

photocatalysts to absorb visible-light photons to photogenerate H2 and O2 from water at the stoichiometric ratio [49,56,109]. In recent years, various kinds of oxynitride-based photoanodes have been established for PEC water oxidation, with minimal externally applied forces [53,54]. Domen et al. reported the TaON/cocatalyst photoanodes with an IPCE of 76% at 400 nm with a minimal external applied force [110]. Afterwards, oxynitrides, such as LaTiO2N [111,112], SrNbO2N [110], ZnTaO2N [113], BaNbO2N [114], among other materials, have been recently advanced as photoanodes [115118]. Perovskite-based oxynitride and related semiconductor photoanodes modified with appropriate cocatalysts have been shown to be an new pathway in enhancing PEC water splitting reaction [119,120]. In recent years, extensive research works have been proceeding to develop nonoxide-based photoanodes incorporated with low-cost cobalt and/or nickel oxides. Low-cost nickel and cobalt-based oxides have been applied in various kinds of applications [6,121,122]. In PEC applications, photoanodes can be fabricated via electrophoretic deposition [109], squeegee [53], spraying [54], and particle transfer methods [123]. Despite its capacity to absorb visible-light and chemical stability, the PEC activity of oxynitride is remarkably limited by its poor photon absorption, high recombination rate of photoexcited charge carriers, and poor OER kinetics. To overcome these limitations, oxynitride photoanodes may be incorporated with cocatalyst materials to advance the visible light photons absorption. In recent years, oxynitride photoanode materials such as LaTiO2N/CoOx, BaTaO2N/BaZrO3, and BaNbO2N [52,124–126], have been demonstrated to employ the light photons absorption with the assistance of appropriate sacrificial reagents. In particular, more consideration has been devoted to developing visible light-active low band gap semiconductor photoanodes (,2 eV). Quite recently, Maeda et al. demonstrated SrNbO2N-based photoanodes with a band gap of 1.8 eV and these showed IPCE efficiency of 10% at 400 nm at 1.23 V vs RHE [127]. Similarly, Zhang et al. demonstrated LaTaO2N photocatalysts materials fabricated by a one-step flux method and it also showed enhanced PEC performance for water oxidation reaction [128]. Very recently, we described the PEC nature of CoPi/La(Ta, Nb)O2N and CoPi/ZnTaO2N photoanodes for water oxidation in alkaline media [4,129], which succeeded with up to a three- to fourfold enhancement in the PEC performances at a lower oxidation potential. The incorporation of the CoPi OER cocatalyst could magnify the charge separation and carrier collection produced at the electrode surface, consequently enhancing PEC performance of the photoanodes (Fig. 28.6).

28.3.5 Cocatalyst Selection Water electrolysis demands the requirements of potential .1.23 V vs RHE between the electrodes owing to its kinetic barriers that are generally observed in executing multielectron oxidation/reduction reactions. For instance, OER from water demands four electrons and produces four intermediate species [130]. On the bare photoanodes surfaces, the production of intermediate species generates a huge energy barrier to OER/HER and thus an overpotential is highly essential to overcome the kinetically rate-limiting multistep reactions [131]. In most of these investigated photoanodes, the electrocatalytic performances

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(A)

(B) 40 Current density (mA/cm2)

Current density (mA/cm2)

40 w/o CoPi w CoPi

30

20

10

0

w/o H2O2 w H2O2

30

20

10

0 0.0

0.4 0.8 Applied potential (V vs SCE)

1.2

0.0

0.3 0.6 0.9 1.2 Applied potential (V vs SCE)

FIGURE 28.6 Cyclic voltammogram spectra of ITO/La(Ta, Nb)O2N photoanodes (A) with and without incorporation of CoPi under illumination and dark condition and (B) in the presence and absence of sacrificial electron donor of H2O2. Dashed light-under dark; solid line-illumination conditions. Source: Reprinted with permission from P. Arunachalam, A. Al-Mayouf, M.A. Ghanem, M.N. Shaddad, M.T. Weller, Int. J. Hydrogen Energy 41 (2016) 1164411652. Copyright 2016 Elsevier.

happen at the semiconductorliquid surface and appropriate cocatalysts need to be incorporated to decrease the overpotential (activation energy) and reduce recombination of charge carriers at the photoanode surface by performing as electronhole acceptors. The application of PEC water oxidation reaction of bare photoanodes is severely restricted owing to its inappropriate CB edge level, shorter carrier lifetimes, and sluggish oxygen evolution kinetics [132]. To overcome these setbacks, i.e., to enhance water oxidation kinetics with related to surface recombination, a usual approach is to incorporate the photoanodes surface with a suitable water oxidation cocatalyst (WOC) [133]. Therefore, the efficiency and kinetics of the overall PEC water oxidation reaction will be absolutely improved. Without a rapid and robust water oxidation process, solar fuels will certainly not have commercial value. The Ir (or Ru)-based oxide cocatalysts are amongst the maximum acting OER catalysts, but are highly expensive and scarce owing to their low earth-abundance, impeding large technological impact. During the last decades, various kinds of research efforts have been focused on creating low-cost first-row transition metal oxide OER cocatalysts. More research efforts have been carried out on examining Co-based electrocatalysts for the electrooxidation of alcohols or other related applications [134137]. Among these, cobalt oxide has been recognized as a robust, effective WOCs that can work in neutral conditions. Nocera et al. fabricated cobalt oxide films from cobalt salts in phosphate buffer (CoPi) solutions at applied potentials .1.1 V vs NHE [138]. In recent years, the incorporation of CoPi cocatalyst enhanced the charge separation and carrier collection at the photoanodes surface, which resulted in enhancement in PEC efficiency. In particular, the thin CoPi layer on the surface of photoanodes surface is the most appropriate way to capture the photoholes to generate the catalytic species of cobalt (Co41), which is vital process for water oxidation reaction [83]. However, when the CoPi layer turn out to be thicker, the photoinduced holes have to transport between many CoPi molecules and

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597

CoPi/electrolyte interface, which results in sluggish hole transfer and leads to a poor PEC water oxidation behavior. Anyhow, the simple grouping of cocatalyst with photoanodes does not assure success. Indeed, in-depth investigations on the mechanism of WOCs on photoanodes have pointed out that most “catalysts,” even though enhancing PEC behavior, do not behave as true catalysts, i.e., do not offer an efficient hole transport path to enhance the rate of water oxidation. In most of the cases, the creation of the semiconductor/catalyst interface does “only” improve the lifetime of surface recombination either by performing as a capacitive layer or by passivation surface states [139141]. Very recently, Hegner et al. reported the cobalt hexacyanoferrate water oxidation catalyst, it performed as a genuine cocatalyst on top of photoactive BiVO4 photoanodes [142].

28.4 NOBLE METALMETAL OXIDE NANOHYBRIDS-BASED PHOTOANODE In the case of noble metalTiO2 nanohybrids, by acting as an antenna that localizes the optical energy, noble metal nanoparticles have been suggested to sensitize nano-TiO2 to light with energy below the bandgap, thus generating additional charge carriers for water oxidation. Liu et al. [143] reported that the presence of Au NP in TiO2 films, under visible light illumination, had 66-fold enhanced PEC water splitting. The authors explained this enhancement by the fact that SPR could induce the electric field amplification near the TiO2 surface, which enhanced the photon absorption rate of TiO2. Zhang et al. [141] also argued that there was a matching of Au (nanocrystals) SPR wavelength (under visiblelight illumination) with the photonic band gap of TiO2, which significantly increased the SPR intensity to boost hot electron injection. Similarity, the plasmon-enhanced photocatalytic activity of iron oxide on gold nanopillars was reported by Gao et al. [142]. Table 28.2 presents the summary of noble metal hybrid strategies used to enhance the photocurrent density of oxide photoanodes. As can be seen in this table, the hybridization with noble metals (AuNP, AgNP) enhanced significantly the photocurrent density of TiO2 (and ZnO)-based photoanodes.

28.5 CONCLUSION This book chapter reviews the fundamental principles of PEC water splitting, overviews the various kinds of photoanodes materials and the scientific challenges of visible lightdriven water splitting. In principle, for water splitting via a bare semiconductor to succeed independently, the band gap energy levels need to overlap the reduction and oxidation potentials of water, and subsequently photoinduced charge carriers have appropriate overpotential for the HER and OER, respectively. Among the reviewed semiconductors, TiO2 photoanodes are low-cost and stable, but not suitable for visible-light absorption. However, Fe2O3 and BiVO4 are recognized to have relatively broader absorption, but failed to achieve theoretical maximum photocurrent, owing to its rapid recombination of photoinduced charge carriers and sluggish water oxidation kinetics. To enhance the

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TABLE 28.2 Review of Photoanodes Based on Noble MetalSemiconducting Oxide Nanohybrids Photocurrent Density (µA/cm2) Photoanode Electrode

Counter Reference Electrode Electrolyte Electrode

Applied Potential (V)

Control Hybrid Light Electrode Electrode Illumination Ref.

AgNpsZnO nanorods hybrids

Pt

0.5 M Na2SO4

Ag/AgCl

0.28 and 0.34 (Voc)shortcircuit

89

616

UV

[144]

AgZnO nanocomposite

Graphite

0.5 M Na2SO4

SCE

0.22 and 0.5 (Voc) shortcircuit

20

249

[145]

59

303

Visible light UV

AuNpsZnO nanopencil

Pt

0.5 M Na2SO4

Ag/AgCl

1

700

1500

AM 1.5G

[146]

AuNpsZnO nanocomposites

Pt

0.1 M NaOH

SCE

0.5

1500

2600

Visible light

[147]

AgNpsZnO nanocomposites

Pt

0.1 M NaOH

SCE

0.5

1500

2100

Visible light

[147]

AuNPsZnO nanorods hybrids

Pt

0.1 M Na2SO4

Ag/AgCl

1

330

9110

AM 1.5G

[148]

0.5

200

350

0.5 M Na2SO4

Ag/AgCl

1/RHE

700

1450

AM 1.5G

[63]

Arrays hybrids

AuNPsZnO nanowires hybrids AgTiO2 nanocomposite

Pt

0.1 M KNO3

Non

Non

0.005

0.015

Visible light

[149]

AuTiO2 nanocomposite

Pt

0.05 M NaOH

Non

Non

120

280

UV

[150]

AuNPsTiO2 nanocomposite

Pt

0.05 M NaOH

SCE

0.75

0.04

0.15

UV

[151]

Au NPsTiO2 nanowires

Pt

1M NaOH

Ag/AgCl

0

820

1490

AM 1.5G

[64]

AuNpsTiO2 nanotube hybrids

Pt

1 M KOH

Ag/AgCl

1.23/RHE

3

150

AM 1.5G

[141]

0.5 M Na2SO4

Ag/AgCl

1.23/RHE

8001000 2500

Visible light

[152]

Pt AuNPTiO2 nanorod hybrids

photocurrent density of these oxide photoanodes in visible light, a new approach was developed by hybridization with noble metal nanoparticles. It is also a foremost mission in the cast of semiconductor photoanodes to regulate the morphology and semiconducting nature to permit sufficient light absorption and charge

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separation. This progress of developing highly active photoanode materials will definitely continue, and confident band gap design will soon be a certainty.

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C H A P T E R

29 Theranostic Application of Fe3O4Au Hybrid Nanoparticles S. Rajkumar and M. Prabaharan Department of Chemistry, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu, India

29.1 INTRODUCTION In the past two decades, even though cancer therapies have received much advancement, still their performance is far from satisfactory due to their nonspecific drug delivery, ineffectiveness against metastatic disease, and deficiency of an effective modality for monitoring the treatment [1]. In recent years, enormous efforts have been taken by researchers to develop theranostics for effective cancer treatment [2]. The development of theranostics for advanced cancer therapy affords new strategies for combined diagnostics and treatment. In this context, many metal and metal oxide nanoparticles (NPs) with suitable functionalities have been established as theranostics for the execution of various targeted imaging modalities, such as computed tomography (CT) scan, T1 and T2 magnetic resonance (MR) imaging, photoacoustic (PA) imaging, ultrasound (US) imaging, and fluorescence optical (FO) imaging for cancer therapies [3]. Recently, considerable efforts have been given to analyze the biomedical applications of Fe3O4 NPs, such as MR image contrast enhancement, hyperthermia, immunoassay, targeted drug delivery, and protein purification [4]. To improve the chemical stability, biocompatibility, and multifunctionality for broader applications, different types of functional materials such as polymers, Au, and silica have been coated on the surface of Fe3O4 NPs. Among the coating materials, Au is considered as the most preferred one for fabricating Fe3O4Au hybrid NPs because of its superior optical properties, catalytic activity, and surface functionality [5]. Due to the diverse physicochemical properties and ability to tune the optical and magnetic properties by changing the particle size, Au shell thickness, shape, charge, and surface modification, Fe3O4Au hybrid NPs have greatly been considered as theranostics for the improved

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00029-2

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photothermal therapy and MR imaging of cancer in recent years [6]. Until now, various approaches, which include sonochemical reduction, laser irradiation, self-assembly, seedmediated growth, chemical reduction, reverse micelle method, etc. have been practiced to develop Fe3O4Au hybrid NPs for biomedical application. This chapter summarizes the recent progress in design, synthesize, and theranostic applications of Fe3O4Au hybrid NPs. Further attention has been given to the strategies that are involved in the synthesis of coreshell Fe3O4Au hybrid NPs, dumbbell-like Fe3O4Au hybrid NPs, and coresatellite Fe3O4Au hybrid NPs. The theranostic applications of Fe3O4Au hybrid NPs in the fields of hyperthermia therapy, photodynamic therapy, targeted drug delivery, and molecular imaging are discussed in detail.

29.2 DESIGN AND SYNTHESIS OF Fe3O4Au HYBRID NPs The synthesis of Fe3O4Au hybrid NPs with the desired design, size, and properties are challenging, and this has received much attention from material science researchers. In general, Fe3O4Au hybrid NPs can be divided into three categories based on their design, such as coreshell Fe3O4Au hybrid NPs, dumbbell-like Fe3O4Au hybrid NPs, and coresatellite Fe3O4Au hybrid NPs. The design of Fe3O4Au hybrid NPs can be tuned by choosing the appropriate method and controlling the process variables during the preparation.

29.2.1 CoreShell Fe3O4Au Hybrid NPs The coreshell Fe3O4Au hybrid NPs possess a single hybrid nanostructure that comprises a full layer coating of reduced Au precursors on the surface of Fe3O4 NPs. The design of coreshell Fe3O4Au hybrid NPs could not only deliver the biocompatibility but can also convey the particle’s surface with the suitable chemical and biological interfacial activities [7]. The uniformly covered Au shell on the surface of Fe3O4 NPs could be easily functionalized with sulfur-based ligands to establish a well-defined novel structure. For past two decades, a substantial amount of work has been carried out to develop Fe3O4Au coreshell hybrid NPs using the different techniques, which include direct coating, coprecipitation, thermal decomposition, and seed-mediated growth methods. The direct coating of Au shell on the surface of Fe3O4 NPs is a most common technique to prepare coreshell Fe3O4Au hybrid NPs. In this technique, two approaches have been followed for the formation of Au shell on the surface of Fe3O4 NPs as shown in Fig. 29.1. The first approach is the one-pot synthesis, in which Au ions grow on the surface of Fe3O4 NPs to form the shell. In the second approach, the Au NPs are grown outside and then seeded into Fe3O4 NPs suspension to form coreshell Fe3O4Au hybrid NPs [8]. It is observed that the direct coating of Au shell could be more effective on the surface of Fe3O4 NPs capped with small ligands like citric acid and ascorbic acid [911]. Hu et al. developed a facile coprecipitation method to synthesize coreshell Fe3O4Au hybrid NPs [12]. In this work, polyethyleneimine stabilized Au NPs were initially prepared using sodium borohydride reduction method. Thereafter, in the presence of

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FIGURE 29.1 Schematic representation of the preparation of coreshell Fe3O4Au hybrid NPs.

FIGURE 29.2 Schematic diagram of the construction of Fe3O4@SiO2Au hybrid NPs. Source: Reprinted with permission from X. Jin, H. Li, S. Wang, N. Kong, H. Xu, Q. Fu, et al., Nanoscale 6 (2014) 1436014370. Copyright 2014 Royal Society of Chemistry.

polyethyleneimine stabilized Au NPs, controlled coprecipitation of Fe(II) and Fe(III) salts (molar ratio 5 1:1.25) was performed to obtain coreshell Fe3O4Au hybrid NPs. Wang et al. demonstrated the preparation of coreshell Fe3O4Au hybrid NPs using thermal decomposition method [13,14]. In this study, monodispersed Fe3O4 NPs were prepared by the reduction of Fe(acac3)3 using 1,2-hexadecanediol in the presence of oleic acid and oleylamine at 210oC in phenyl ether as a solvent. Thereafter, the presynthesized Fe3O4 core % was treated with Au(OOCCH3)3 in the presence of 1,2-hexadecanediol, oleic acid, and oleylamine at 180-190 oC under the argon atmosphere to form coreshell Fe3O4Au % hybrid NPs. To develop coreshell Fe3O4Au hybrid NPs, many researchers have adopted the seed-mediated shell growth process that leads to multilayer Fe3O4Au hybrid NPs [15,16]. This method is the most desirable to obtain the coreshell Fe3O4Au hybrid NPs with higher colloidal stability for in vivo biomedical applications. In the seed-mediated method, polymer/SiO2 functionalized Fe3O4 NPs were seeded with Au31 ions and then dropped into the Au growth solution to form coreshell Fe3O4Au hybrid nanostructure. In the formed nanostructure, the inner shell between the Fe3O4 core and Au shell could be made up of polymer/SiO2. Due to the presence of metal binding groups, the inner shell could bind the Au seeds to lead the homogeneous nucleation for uniform growth of Au shell. In recent years, SiO2 has been widely considered as a coating agent for Fe3O4 NPs due to its thermal stability and biocompatibility. For instance, Jin et al. constructed a coreshell nanostructure composed of a Fe3O4 core, Au shell and SiO2 inner layer as shown in Fig. 29.2 [17]. In their study, the presynthesized Fe3O4 NPs were coated by SiO2 and then surface-modified with (3-aminopropyl)triethoxysilane (APTES) for amine functionalization. The Au-seeded Fe3O4@SiO2 NPs were obtained by mixing APTES coated Fe3O4@SiO2 NPs and tetrakis(hydroxymetyl)phosphonium chloride (THPC) in presence of Au solution. The Au seeds attached to the surface of APTES-coated Fe3O4@SiO2 NPs acted as

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nucleation sites for the formation of a uniform Au shell on APTES-coated Fe3O4@SiO2 NPs by HAuCl4 reduction with formaldehyde. Recently, Jin et al. developed coreshell Fe3O4Au hybrid NPs using poly-L-histidine and phospholipid-polyethylene glycol having the terminal carboxylic acid group as coating agents [18]. Here, presynthesized Fe3O4 NPs were initially coated with coating polymers through a layer-by-layer technique and then treated with Au solution to form the core-shell Fe3O4Au hybrid NPs. In recent years, different types of polymers such as polyphosphazene, polyethyleneimine, polyaniline, polypyrrole, and chitosan have been used as coating agents to develop the coreshell Fe3O4Au hybrid NPs [1924]. Due to the presence of reducing amine groups, these polymers can interact with Au to form the uniform and highly stable Au shell on the surface of Fe3O4 NPs.

29.2.2 Dumbbell-Like Fe3O4Au Hybrid NPs Dumbbell-like NPs comprise of a strongly interacting heterostructure with one nanoparticle at one end and another at the other end. The two different NPs in close contact with each other look dumbbell or peanut-like particles. Unlike coreshell Fe3O4Au hybrid NPs, in which the Fe3O4 core is surrounded by the Au shell, in the dumbbell-like Fe3O4Au hybrid NPs the functional surfaces of both Fe3O4 and Au NPs and their active interface are wide-open; this improves their applications as theranostics for diagnostic and therapeutic applications [25]. The dumbbell-like Fe3O4Au hybrid NPs have distinct advantages such as (i) simultaneous optical and magnetic detection, (ii) ability to attach various functionalities for target-specific imaging and delivery applications, and (iii) capability to tune the magnetic and optical properties by controlling the size of Fe3O4Au hybrid NPs [26]. Generally, dumbbell-like Fe3O4Au hybrid NPs can be prepared by epitaxial growth of one type NP on another type NP, which is called the seed NP. During the process, the nucleation must be controlled properly to create the heterogeneous nucleation on one specific crystal plane around the seeding NPs [27]. Yu et al. prepared the dumbbell-like Fe3O4Au hybrid NPs by the decomposition of iron pentacarbonyl (Fe(CO)5) over the surface of the presynthesized Au NPs followed by air oxidation as shown in Fig. 29.3 [28]. In their study, the Au NPs were prepared by reducing the HAuCl4 solution in the presence of oleylamine. Thereafter, the presynthesized Au NPs were reflexed with Fe (CO)5 in the presence of oleic acid and oleylamine in octadec-1-ene at 300oC followed by room-temperature air oxidation to form the dumbbell-like Fe3O4Au hybrid% NPs. The size of the Au NPs was altered by controlling the temperature or by controlling the HAuCl4/ oleylamine ratio. Felber et al. have also reported a similar approach to synthesize FIGURE 29.3 Schematic diagram of the preparation of dumbbell-like Fe3O4Au hybrid NPs.

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FIGURE 29.4 Synthesis of chitosan-coated dumbbell-like Fe3O4Au hybrid NPs. Source: Reprinted with permission from N. Kostevsek, E. Locatelli, C. Garrovo, F. Arena, I. Monaco, I.P. Nikolov, S. Sturm, K.Z. Rozman, V. Lorusso, P. Giustetto, P. Bardini, S. Biffic, M.C. Franchini, Chem. Commun. 52 (2016) 378381. Copyright 2015 Royal Society of Chemistry.

dumbbell-like Fe3O4Au hybrid NPs by refluxing a mixture of Fe(CO)5 and presynthesized Au NPs in the presence of oleic acid and oleylamine in 1-octadecene at 310 C [29]. Using the similar approach, dumbbell-like Fe3O4Au hybrid NPs have also been prepared by simple thermal decomposition of iron-oleate and Au-oleylamine complexes [30,31]. Kostevsek et al. developed chitosan-coated dumbbell-like Fe3O4Au hybrid NPs using a two-step method as shown in Fig. 29.4 [32]. In the first step, dumbbell-like Fe3O4Au hybrid NPs were prepared by the reduction of gold acetate and thermal decomposition of Fe(CO)5 in the presence of 1,2-hexadecanediol, oleylamine, and oleic acid simultaneously. During the reaction, Au NPs were found to form initially in the reaction mixture due to the higher difference in the reduction potential between Au and Fe. Thereafter, these Au NPs were utilized to decompose Fe(CO)5 at a higher temperature in order to produce dumbbell-like Fe3O4Au hybrid NPs. In the next step, the surface of the presynthesized Fe3O4Au hybrid NPs was modified with thioglycolic acid (TGA) and hydrocaffeic acid (HCA)-conjugated chitosan in order to obtain highly biocompatible chitosan-coated dumbbell-like Fe3O4Au hybrid NPs.

29.2.3 CoreSatellite Fe3O4Au Hybrid NPs Coresatellite is one of the common structures of Fe3O4Au hybrid NPs. This structure possesses a single Fe3O4 core with the attachment of numerous satellite-like Au NPs by covalent bonds or supramolecular interactions. The coresatellite Fe3O4Au hybrid NPs consists of a residually uncovered Fe3O4 core surface, which is appropriate for MR imaging properties and further functionalization. Moreover, this hybrid structure comprises of

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FIGURE 29.5 Synthetic route of coresatellite Fe3O4Au hybrid NPs. Source: Reprinted with permission from Y. Wang, Y.H. Shen, A.J. Xie, S.K. Li, X.F. Wang, Y. Cai, J. Phys. Chem. C 114 (2010) 42974301. Copyright 2010 Royal Society of Chemistry.

several peripheral satellite Au NPs possessing a high surface area of satellite material, which is beneficial for imaging and photothermal abilities [33]. Coresatellite Fe3O4Au hybrid NPs have been prepared using the various approaches. Xuan et al. developed the coresatellite Fe3O4Au hybrid NPs by treating the positively charged ammonium-functionalized Fe3O4@SiO2 NPs with the negatively charged citrate-stabilized Au NPs through electrostatic interactions [23]. The more stable coresatellite Fe3O4Au hybrid NPs were also prepared by treating the positively charged polymer (aniline, allylamine, etc.)-coated Fe3O4 NPs with the negatively charged citrate-coated Au NPs [34]. Wang et al. synthesized coresatellite Fe3O4Au hybrid NPs using lysine-functionalized Fe3O4 NPs as a template for in situ formation of Au NPs in the presence of HAuCl4 and sodium borohydride [35]. Using the similar approach, Caruntu et al. developed the coresatellite Fe3O4Au hybrid NPs using the positively charged APTES functionalized Fe3O4 NPs and the negatively charged citrate-coated Au NPs [36]. Using cysteine-functionalized Fe3O4 NPs as a template, Wang et al. prepared the coresatellite Fe3O4Au hybrid NPs. Due to the presence of thiol group in cysteine, the Au NPs were effectively stabilized on the surface of cysteine-functionalized Fe3O4 NPs [37]. In another study, Zhang et al. reported a method to synthesize coresatellite Fe3O4Au hybrid NPs using polydopamine as an anchor for Au NPs. In their work, due to the reduction of Au31 ions, satellite-like Au NPs were assembled on the surface of polydopamine functionalized Fe3O4 as depicted in Fig. 29.5 [38]. Recently, Zhao et al. developed coresatellite Fe3O4Au hybrid NPs by the reversible additionfragmentation chain transfer (RAFT) technique [39]. Fe3O4@SiO2-embedded poly (styrene) was initially prepared by polymerizing styrene using Fe3O4@SiO2-RAFT initiator. Thereafter, the residual RAFT terminal was reduced to the thiol group, which acts as a ligand for the immobilization of Au NPs via an AuS bond. Since the Au NPs were attached on the flexible poly(styrene) segments surrounded on Fe3O4@SiO2, they were found to be effectively separated from each other to avoid self-aggregation.

29.3 THERANOSTIC APPLICATION OF Fe3O4Au HYBRID NPs In recent years, theranostic nanomedicine, consisting of both diagnosis and therapy within a single nanosystem, has received much attention due to its capability to perform various functions in the biomedical application. By integrating imaging, detecting, and

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therapeutic functions into a single platform, the combined disease diagnosis, tissue imaging, and real-time monitoring of drug delivery have been achieved [40,41]. This approach could be important for assessing biodistribution, drug release, and therapeutic effect of the nanomedicine. In this context, various types of Fe3O4Au hybrid NPs have been developed for simultaneous diagnosis and treatment due to their magnetic, optical, and photothermal properties [42]. These NPs have the large potential to be used as theranostics in different biomedical fields such as hyperthermia therapy, photodynamic therapy, targeted drug delivery, and molecular imaging.

29.3.1 Hyperthermia Therapy In hyperthermia therapy, body tissue is exposed to high temperatures (4248oC) in order to damage and kill cancer cells or to make cancer cells more susceptible to the %effects of radiation and anticancer drugs [43]. Recent studies showed that hyperthermia induced by the photomagnetic NPs has the potential to attain localized tumor heating [44,45]. Over the past two decades, different types of Fe3O4Au hybrid NPs have been analyzed for their effectiveness as hyperthermic agents to generate the cytotoxicity towards tumor cells. Due to the presence of Fe3O4 and Au NPs, the Fe3O4Au hybrid NPs could follow both magnetic-induced hyperthermia through oscillating magnetic fields and photoinduced hyperthermia through infrared radiations. Mohammad et al. developed coreshell Fe3O4Au hybrid NPs and demonstrated their heat releasing ability using a sinusoidal time-varying magnetic field generated at the center of a conductor coil by passing an alternating electric current (AC) through it [46]. In the presence of low frequency oscillating magnetic fields (44430 Hz), a five-fold increase in the amount of heat released with Fe3O4Au hybrid NPs in comparison with bare Fe3O4 NPs was detected. Mohammad et al. also reported the doxorubicin (DOX)-conjugated Fe3O4Au hybrid NPs for a combined therapy of cancer by means of both hyperthermia and drug delivery [47]. It was observed that magnetic hyperthermia of the developed NPs is strongly influenced by the applied frequency and the solvents used. The drug release studies showed that the DOX release efficacy is enhanced at acidic pH conditions and the oscillatory magnetic fields. These observations proved that the frequency-dependent drug and heat releasing property can be used for the improved cancer therapy efficiency by increasing the drug bioavailability and local tumor temperature, respectively. Recently, Han et al. developed the multifunctional Fe3O4Au hybrid NPs for free prostate-specific antigen detection, MR imaging, and magnetic hyperthermia using a surface-enhanced Raman scattering-assisted theranostic strategy [42]. These Fe3O4Au hybrid NPs showed superparamagnetic property, high r2 relaxivity, adequate magnetic heating effect, and less toxicity. Hence, this multifunctional Fe3O4Au hybrid NPs could have an importance for the real-time diagnosis-therapy of cancer. Jingchao et al. synthesized hyaluronic acid-conjugated coresatellite Fe3O4Au hybrid NPs for multimodal imaging and photothermal ablation of tumor cells [48]. The developed NPs were found to be water-dispersible, colloidally stable, and biocompatible. Moreover, they showed high r2 value (144.39/mM/s) and brighter CT image with the increase in Au concentration. Photothermal studies indicated that the developed NPs are able to produce

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heat quickly and effectively upon laser irradiation. These results confirmed that the hyaluronic acid-conjugated coresatellite Fe3O4Au hybrid NPs have potential to be used as a multifunctional nanoplatforms for MR/CT imaging and photothermal therapy of cancer cells. Recently, Hou et al. prepared multifunctional coreshell Fe3O4Au hybrid NPs using seed growth method for MR/CT dual-imaging and photothermal therapy of tumor [49]. These NPs displayed high stability with no significant change of photothermal effect after five laser cycles and high photothermal conversion capability with a temperature rise of 40oC under 808 nm laser irradiation at 2 W/cm2 for 10 min at the concentration of % 160 μg/mL. Cytotoxicity studies demonstrated that these NPs could reduce the viability of HePG2 cancer cells up to 40% under 808 nm laser irradiation at 2 W/cm2 for 3 min at the concentration of 200 μg/mL.

29.3.2 Photodynamic Therapy In recent years, photodynamic therapy has emerged as a promising method for a variety of cancer types. This is based on the concept that photosensitizer molecules transfer the absorbed light to the surrounding oxygen to produce reactive oxygen species such as singlet oxygen or free radicals under laser irradiation that can irreversibly degrade the treated cancer tissues [50]. Recently, theranostics based on Fe3O4Au hybrid NPs have been extensively considered as carriers for singlet oxygen photosensitizers for the efficient photodynamic therapy of cancer because of their unique inherent properties. For instance, Li et al. prepared the heparin (H)/pheophorbide A (PhA)-conjugated coreshell Fe3O4Au hybrid NPs as a multifunctional carrier for photodynamic therapy of cancer (Fig. 29.6) [51]. These NPs revealed effective quenchability of the surface conjugated phA in aqueous media, thereby destroying the photoactivity of the phA. However, the photoactivity was reestablished when the H/phA molecules were released from the hybrid NPs by a glutathione-mediated ligand-exchange reaction within the tumor cells. The glutathione-mediated dequenchability of the H/phA-conjugated Fe3O4Au hybrid NPs was also detected in the cell culture medium, in which the internalized NPs efficiently produced strong fluorescence signals along with the generation of a significant amount of singlet oxygen upon light treatment. These results indicated that the heparin/pheophorbide FIGURE 29.6 Schematic representation of synthesis and working method of H/PhAconjugated core-shell Fe3O4Au hybrid NPs. Source: Reprinted with permission from X. Hou, X. Wang, R. Liu, H. Zhang, X. Liu, Y. Zhang, RSC Adv. 7 (2017) 18844. Copyright 2014 Royal Society of Chemistry.

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A-conjugated coreshell Fe3O4Au hybrid NPs could have great potential to be used as theranostics for photodynamic therapy of cancer. Recently, Rosa-Pardo et al. have developed the coreshell Fe3O4AuSiO2 hybrid NPs encapsulated with a photosensitizer, namely Rose Bengal [52]. These NPs displayed strong absorption bands in the visible spectrum, low fluorescence quantum yield, high triplet quantum yield, stable triplet state, and singlet oxygen with high quantum yield compared to that of free Rose Bengal or SiO2 conjugated Rose Bengal. The enhanced singlet oxygen generation efficiency could be due to the existence of the covalent bond between Rose Bengal and SiO2 in the hybrid NPs.

29.3.3 Targeted Drug Delivery The multifunctional Fe3O4Au hybrid NPs have a potential to be used as drug delivery carriers and diagnostic imaging agents simultaneously due to their improved drug loading ability, target ability, blood circulation time, magnetic and optical properties. Hence, in recent years, considerable efforts have been taken to develop multifunctional Fe3O4Au hybrid NPs using various strategies. Xu et al. developed dumbbell-like herceptin-conjugated Fe3O4Au hybrid NPs that can act as targeted drug delivery carriers to deliver the anticancer drug, cisplatin, to Her2-positive breast cancer cells (Fig. 29.7A) [53]. Due to the presence of herceptin, these NPs were found to have preferred targeting of Sk-Br3 cells (Her2-positive breast cancer cells) (Fig. 29.7B and C). TEM analysis revealed that the herceptin-conjugated Fe3O4Au hybrid NPs were taken up into endosome/lysosome compartment of the cells through the endocytosis process. The in vitro drug release studies showed that the release of cisplatin from the NPs is pH-dependent. At pH 6, the amount of cisplatin released from the NPs was found to be about 70% for 10 h, whereas at pH 8, this value was only about 40%. This observation indicates that the acidic condition that encounters in endosome/lysosome compartments of cancer cells could accelerate the release of cisplatin from the herceptin-conjugated Fe3O4Au hybrid NPs once they are taken inside the cells through endocytosis. FIGURE 29.7 (A) Schematic representation of dumbbell-like herceptin/cisplatin-conjugated Fe3O4Au hybrid NPs. Reflection images of (B) Sk-Br3, and (C) MCF-7 cells after 24 h incubation with herceptin/ cisplatin-conjugated Fe3O4Au hybrid NPs at 37 C. Source: Reprinted with permission from L. Li, M. Nurunnabi, M. Nafiujjaman, Y.Y. Jeong, Y. Lee, K.M. Huh, J. Mater. Chem. B 2 (2014) 29292937. Copyright 2009 American Chemical Society.

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Li et al. developed trisoctahedral coreshell Fe3O4Au hybrid NPs covered with mSiO2 using poly-L-lysine-coated Fe3O4 NPs as a template for a near-infrared (NIR)responsive remote control drug delivery [54]. In this study, the bondable oligonucleotides (dsDNA) were utilized as pore blockers of the mSiO2 shell that permitted the controlled release, resulting in a NIR responsive DNA-gated Fe3O4AumSiO2 hybrid NPs. By taking the advantage of the magnetic properties, in this study, remotely controlled drug release was achieved by magnetic attraction accompanied with the introduction of NIR radiation. Due to the presence of magnetic target, MR imaging diagnosis, and combination therapy through the manipulation of magnet and NIR laser, the developed DNA-gated Fe3O4AumSiO2 hybrid NPs can be effectively used for cancer therapy. Recently, Fe3O4Au hybrid NPs were prepared using poly(styrene-alt-maleic acid) polymer encapsulation reaction for the combined NIR-triggered chemophotothermal therapy [55]. The photothermal therapy of HT-29 tumor-bearing nude mice and chemophotothermal therapy of HT-29 cells under 808 nm laser irradiation using the developed NPs were found to be improved due to their intense NIR absorption ability. The DOX-loaded Fe3O4Au hybrid NPs showed phototriggered drug release to increase antitumor responses to cancers. The molecular imaging study revealed that the DOX-loaded NPs can alter the lipid and amide structures in DOX-resistant cancer cells.

29.3.4 Molecular Imaging Molecular imaging has been extensively employed for diagnosing the cancer tissues. Due to the technological advancements, different types of imaging techniques such as MR imaging, CT imaging, FO imaging, and PA imaging have been developed and utilized for clinical diagnostics using Fe3O4 and Au-NPs. However, each of these methods has its own advantages and limitations (Table 29.1). Hence, in recent years, dual-mode imaging modalities such as MR/CT imaging, MR/FO imaging, and MR/PA imaging using Fe3O4Au hybrid NPs have been widely considered for accurate cancer diagnosis [56]. Table 29.2 summarizes the properties of Fe3O4Au hybrid NPs that have been developed for the dual-mode molecular imaging. 29.3.4.1 Dual-Mode MR/CT Imaging Fe3O4Au hybrid NPs have been considered as dual-mode MR/CT imaging agents since they contain two different radio-dense elements. In the hybrid structure, Fe3O4 NPs are capable of shortening the T2 relaxation time of water protons, resulting in enhanced imaging contrast and sensitivity; while Au NPs are used as a powerful contrast agent for CT imaging because of their higher X-ray attenuation intensity than that of conventional iodine-based CT contrast agents [57]. Li et al. fabricated Fe3O4Au hybrid NPs via the facile one-pot approach using hydrothermal method for dual-mode MR/CT imaging application [58]. These NPs were characterized with in vitro cell viability assay, cell morphology observation, flow cytometry, and hemolysis assay. The results showed that the formed Fe3O4Au hybrid NPs are noncytotoxic and hemocompatible in the concentration range studied. MR and CT imaging data revealed that the developed hybrid NPs have a relatively high r2 relaxivity (146.07/mM/s) and good X-ray attenuation property. Cai et al.

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TABLE 29.1

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Advantages and Disadvantages of Different Types of Imaging Techniques

Modality

Advantages

Disadvantages

Ref.

CT imaging

Unlimited tissue depth penetration

Radiation exposure

[1]

High spatial and density resolution

Poor soft tissue

Used for whole body imaging Low-slung acquisition time 3D Anatomical imaging MR imaging

Unlimited tissue depth penetration

Expensive

Nonionizing radiation

Long acquisition time

High soft tissue contrast

Limited sensitivity

[8]

High spatial resolution (50 μm) FO imaging

High spatial resolution

Not for the whole body

Real-time imaging

Limited depth (,1 cm)

[3]

Highly sensitive and inexpensive PA imaging

Acoustically visualize the tissues

No 3D imaging

High optical contrast

Limited depth (,1 cm)

[50]

High ultrasonic resolution

prepared folic acid-conjugated Fe3O4Au hybrid NPs for targeted dual-mode MR/CT imaging of tumors [56]. These hybrid NPs were found to be cytocompatible, hemocompatible, and taken up specifically by folic acid receptor-overexpressing cancer cells through the folate-mediated endocytosis. Moreover, these NPs showed a relatively high r2 relaxivity (92.67 3 1023/M/s) and good X-ray attenuation property. In another study, tetramethylammonium hydroxide-modified Fe3O4Au hybrid NPs were reported as a dual-model contrast agent for CT/MR imaging [59]. These NPs exhibited the strong MR/CT imaging contrast enhancement in a rabbit model. Recently, well-defined multifunctional Fe3O4Au hybrid NPs composed of a superparamagnetic Fe3O4 inner core, SiO2 as the middle layer, and Au nanoshell were developed by seed-mediated growth method for MR/CT dualimaging and photothermal therapy [49]. These NPs showed a relatively high r2 relaxivity (41.077/mM/s) and CT imaging contrast abilities at different concentration ranges studied (0, 0.22, 0.44, 0.88, and 1.76 mM of Fe). These studies suggested that the multifunctional Fe3O4Au hybrid NPs can be used as an efficient nanoprobe for targeted dual-mode CT/ MR imaging of a xenografted tumor model. 29.3.4.2 Dual-Mode MR/FO Imaging Recently, a remarkable attention has also been given to the development of multifunctional Fe3O4Au hybrid NPs tagged with fluorescent dyes for dual-mode MR/FO

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TABLE 29.2 Properties of Fe3O4Au Hybrid NPs Developed for Dual-Mode Molecular Imaging Imaging Modes

Materialsa

Particle Size (nm)

r2 (/mM/s)

Field (T)

Ref.

Dual-mode MR/CT imaging

Fe3O4/Au/PEI

50

264.16

1.5

[12]

Fe3O4/Au/AcFA

20

92.67

0.5

[56]

Fe3O4/Au

35

124.2

1.5

[57]

Fe3O4/AumPEG

16.7

146.07

0.5

[58]

AuFe3O4 (Dumbbell)

14

136.4

1.5

[59]

Fe3O4/SiO2/AuPEG

100

41.077

9.4

[49]

Fe3O4/Au NC

70

28.117

3

[60]

Fe3O4/PPy/Au

65

119.35

0.5

[61]

AuFe3O4 (Dumbbell)

20

245.12

3

[62]

PS/CS/Au/Fe3O4FA

369

326.2

0.55

[63]

Fe3O4/Au/SiO2/ICG

120

390

1.5

[64]

Fe3O4/SiO2/Au

20

393.8

3

[65]

Fe3O4/CS/Au

335.8



3

[66]

Fe3O4/PL/PEG/Au

150



3

[18]

Fe3O4/Au

90

1.5

[67]

Dual-mode MR/FO imaging

Dual-mode MR/PA imaging

208

a

Ac, acetic anhydride; NC, nanocage; mPEG, methoxy polyethylene glycol; PPy, polypyrrole; PS, polystyrene; CS, chitosan; FA, folic acid; ICG, indocyanine green; PL, phospholipid; PEG, polyethylene glycol.

imaging. For instance, Wang et al. reported indocyanine green-conjugated Fe3O4Au hybrid NPs for MR/FO dual-mode imaging-guided targeted photothermal therapy [63]. Indocyanine green is an FDA-approved fluorescent dye, which shows strong optical absorption at B780 nm and NIR fluorescence absorption at B800 nm. Hence, it has been conjugated/encapsulated with Fe3O4Au hybrid NPs for photothermal therapy and imaging applications. The indocyanine green-conjugated Fe3O4Au hybrid NPs were found to have good biocompatibility, enhanced stability, and desirable MR/FO multimode imaging performance as shown in Fig. 29.8. In addition, these NPs were found to accumulate effectively at the tumor site due to the active targeting of folic acid and convert NIR light into thermal energy to kill tumor cells. 29.3.4.3 Dual-Mode MR/PA Imaging PA imaging is another convenient tool in biomedical imaging, which is acquired from acoustic waves that are produced by the thermal expansion of the target tissues due to absorption of the laser pulse. As the delivered energy can be transformed into thermal energy, the PA imaging agents with high absorption produce the apparent thermal effect to kill the tumor cells with high discrimination and minimal intrusiveness. Over the past few decades, Au NPs have been extensively exploited as PA imaging agents due to their

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FIGURE 29.8 Multimodal in vivo imaging of HeLa tumor-bearing mouse before and after intravenous injection of indocyanine green-conjugated Fe3O4Au hybrid NPs. (A) CT scan images showing the tumor in a marked red circle, (B) T2 weighted MR, and (C) FO image of the tumor after treatment with (i) hybrid NPs and (ii) free indocyanine green for the different time period. Source: Reprinted with permission from Y. Wang, X. Liu, G. Deng, Q. Wang, L. Zhang, Q. Wang, J. Lu, J. Mater. Chem. B 5 (2017) 42214232. Copyright 2017 Royal Society of Chemistry. FIGURE 29.9

Dual-mode imaging of xenograft Balb/C nude mice before and after the intravenous injection of folic acid-conjugated Fe3O4Au hybrid NPs. (A) T2 weighted MR image (orange-colored spots corresponds to the T2 contrast). (B) Representative PA image. Source: Reprinted with permission from R. Bardhan, W. Chen, M. Bartels, C. Torres, M.F. Botero, R.W. McAninch, A. Contreras, R. Schiff, R.G. Pautler, N.J. Halas, A. Joshi, Nano Lett. 10 (2010) 49204928. Copyright 2017 American Chemical Society.

adjustable optical properties and satisfactory signal-to-noise ratio of PA [50]. Recently, dual-mode molecular imaging by the combination of MR and PA has been considered to acquire the structural and functional information of cancer cells with high resolution and sensitivity. For instance, Zhou et al. developed multifunctional Fe3O4Au hybrid NPs as MR/PA imaging agents for photothermal ablation therapy of cancer [67]. Due to strong magnetic properties and high NIR optical absorbance, the developed NPs showed an excellent r2 relaxivity (208/mM/s) and PA imaging ability. Moreover, after the intravenous administration of Fe3O4Au hybrid NPs, the visualization of tumor blood vasculature was found to be improved. Similarly, Monaco et al. fabricated folic acid-conjugated Fe3O4Au hybrid NPs for dual-mode MR/PA imaging [68]. The in vivo MR/PA imaging studies revealed that the Fe3O4Au hybrid NPs induce tumor contrast threefold higher than the control system as shown in Fig. 29.9.

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29.4 CONCLUDING REMARKS Multifunctional Fe3O4Au hybrid NPs have a great potential to be used as theranostics for cancer therapy and imaging due to their unique magnetic and optical properties. The discrete surface property of Fe3O4Au hybrid NPs enables the functionalization of NPs with either a targeting ligand and/or a therapeutic agent for target-specific medical diagnostic and therapy. Due to the advancements in synthetic methods, different types of Fe3O4Au hybrid NPs such as coreshell NPs, dumbbell-like NPs, and coresatellite NPs have been exploited in recent years. The physicochemical properties of these hybrid NPs have been tuned by controlling the size, shape, composition of each individual NP, and interparticle interactions according to the requirement of specific needs. Due to the presence of dissimilar functional materials, the multifunctional Fe3O4Au hybrid NPs have been widely considered as theranostic agents for cancer-specific applications that include hyperthermia therapy, photodynamic therapy, targeted drug delivery, and molecular imaging. However, the progress of Fe3O4Au hybrid NPs as theranostics is still in the infant stage and is encountering many challenges and concerns. It is highly challenging to design more efficient, smart, and safe Fe3O4Au hybrid NPs for theranostic application. Even though a variety of Fe3O4Au hybrid NPs have been developed, the translation of these materials to the real clinical application has not yet been realized. To overcome these limitations, efforts should be taken to develop the multifunctional Fe3O4Au hybrid NPs, in which each functionality works in a combined manner without compromising other functionalities. Moreover, these materials should be rigorously subjected to pharmacokinetics and biodistribution analysis, acute and long-term toxicity studies, and other preclinical tests. In spite of these challenges, theranostics based on Fe3O4Au hybrid NPs will certainly find the real-time application due to their unique features. Through the combined efforts of researchers with multidisciplinary backgrounds, the success of Fe3O4Au hybrid NPs as theranostics can be improved.

Acknowledgments The authors thank DST-Nano Mission (SR/NM/NS-1260/2013), Department of Science and Technology, Government of India for financial support.

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C H A P T E R

30 Synthesis and Application of AuFe3O4 Dumbbell-Like Nanoparticles Xueping Zhang1,2 and Shaojun Dong1,2 1

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, P.R. China 2University of Chinese Academy of Sciences, Beijing, P.R. China

30.1 INTRODUCTION Hybrid nanoparticles consisting of noble metals and metal oxides nanoparticles often show improved physical/chemical properties over those of the individual component nanoparticles, or exhibit new properties that are not present in the individual component nanoparticles [15]. This could be attributed to the interfacial interactions that originate from electron transfer across the nanometer contact at the interface of these two nanoparticles [610]. For example, Au nanoparticles are usually chemically inert, but Au nanoparticles deposited on a metal oxide support exhibit superior catalytic activity for the CO oxidation reaction [1113]. Besides, Au nanoparticles are known to have attractive optical properties with a well-defined plasmon resonance peak [14], while this peak can show a red shift by association with metal oxide nanoparticles [1517]. On the other hand, magnetic Fe3O4 nanoparticles often show high magnetic response to an externally applied magnetic field and can be used for selective capture of targeting substrates, recyclable nanocatalysis, and magnetic-photonic purposes [1823]. Thus, the combination of both Au and Fe3O4 into a single entity would lead to a hybrid nanostructure with advantageous and serendipitous properties from both individual Au and Fe3O4 nanoparticles. AuFe3O4 dumbbell-like nanoparticles have recently undergone intensive investigation [3,24,25]. The dumbbell-like nanoparticles are heterogeneous nanostructures with two

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different functional nanoparticles in intimate contact with each other [26,27]. In contrast to other structures, AuFe3O4 dumbbell-like nanoparticles have distinct advantages. First, in such a structure, each side of the nanoparticles is restricted to the nanometer scale, and a small variation in electron transfer across the interface between these two limited electron “nanoreservoirs” may lead to a drastic property change for each nanoparticle [3]. Second, AuFe3O4 dumbbell-like nanoparticles contain both a magnetic (Fe3O4) and an optically active plasmonic (Au) unit and are suitable for simultaneous optical and magnetic detection [28]. Third, they offer two functional surfaces, and this can enhance the catalytic activity and facilitate the attachment of different chemical functionalities, making them especially attractive as multifunctional probes for target-specific imaging and delivery applications [29]. We therefore prefer to devote this chapter to summarizing the recent research progress in the synthesis of AuFe3O4 dumbbell-like nanoparticles, and some significant factors that influence the preparation process, which would help shed light on the development of dumbbell-like nanoparticles with various components. Then, we illustrate the interesting optical and magnetic properties found in these hybrid nanoparticles, and highlight the potential applications of these nanohybrids in catalysis and biomedicine.

30.2 SYNTHESIS OF AuFe3O4 DUMBBELL-LIKE NANOPARTICLES 30.2.1 General Strategy Sun’s group reported a general synthesis strategy for the AuFe3O4 dumbbell-like nanoparticles [30]. As illustrated in Fig. 30.1, the AuFe3O4 dumbbell-like nanoparticles were prepared by the decomposition of iron pentacarbonyl, Fe(CO)5, over the surface of the Au nanoparticles, followed by oxidation in air. The Au nanoparticles are either premade in the presence of oleylamine or synthesized in situ by injecting a HAuCl4 solution into the reaction mixture. Mixing Au nanoparticles with Fe(CO)5 in 1-octadecene in the presence of oleic acid and oleylamine and heating the mixture to reflux (B300 C) for 45 min followed by room-temperature air oxidation leads to the formation of AuFe3O4 dumbbell-like nanoparticles. The size of the particles could be tuned from 2 to 8 nm for Au and 4 to 20 nm for Fe3O4. 30.2.1.1 Formation Mechanism This process is similar to the seed-mediated growth to form core/shell nanoparticles, while the difference is that the nucleation and growth is anisotropically centered on one

FIGURE 30.1 Schematic illustration of the growth of AuFe3O4 dumbbell-like nanoparticles. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional AuFe3O4 nanoparticles, Nano Lett. 5 (2005) 379382. Copyright (2005) American Chemical Society.

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specific crystal plane around the seeding nanoparticles, and not uniformly distributed as in the synthesis of core/shell structures [3]. The epitaxial growth of Fe3O4 onto the Au nanoparticles surface is favored by the lattice mismatch between both materials since the ˚ ) is almost double that of the Au (4.08 A ˚ ). The conlattice parameter of the Fe3O4 (8.345 A trolled nucleation and growth of only one Fe3O4 on each Au seeding nanoparticle can be ascribed to the suitable electron transfer. As the Fe3O4 nanoparticles nucleate on Au nanoparticles, the polarized plane induces a change in the charge at the interface, and the free electrons from the Au nanoparticles will compensate for that charge change. However, the Au particle has only a very limited source of electrons, so this compensation makes all other facets of the Au nanoparticle electron deficient and unsuitable for multinucleation. Therefore, only dumbbell-like nanoparticles can be produced. 30.2.1.2 Characterization Fig. 30.2A shows the transmission electron microscopy (TEM) image of the AuFe3O4 dumbbell-like nanoparticles with Fe3O4 at around 14 nm and Au at 8 nm. The Au particles appear black and Fe3O4 are light colored in the image because Au has a higher electron density and allows fewer electrons to transmit. Fig. 30.2B is the high angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image of the dumbbelllike nanoparticles. The brightness in the image reflects the intensity of scattered electrons from different substance, which is proportional to the atomic number (Z) [31]. In Fig. 30.2B, the brighter dots refer to the Au particles since they have higher Z compared to the Fe3O4 particles. Fig. 30.2C displays a typical high-resolution TEM (HRTEM) image of a dumbbell-like particle with Fe3O4 at 12 nm and Au at 8 nm. In the structure, a Fe3O4 (1 1 1) plane grows onto an Au (1 1 1) plane, giving the dumbbell-like structure.

30.2.2 Other Modified Strategies The AuFe3O4 dumbbell-like nanoparticles were also synthesized by a modified Sun’s method, replacing highly toxic Fe(CO)5 with a safe Fe precursor (Fe oleate, Fe(OL)3). Doong et al. synthesized AuFe3O4 dumbbell-like nanoparticles by simply refluxing a mixture which contains Fe(OL)3, Au colloid dispersion, oleic acid, oleylamine, and

FIGURE 30.2 (A) TEM image of the 814 nm AuFe3O4 particles; (B) HAADF-STEM image of the 89 nm AuFe3O4 particles; and (C) HRTEM image of one 812 nm AuFe3O4 particle. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional AuFe3O4 nanoparticles, Nano Lett. 5 (2005) 379382. Copyright (2005) American Chemical Society.

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1-octadecene at 310 C for 30 min [32]. The size of the synthesized dumbbell-like nanoparticles ranges from 12 to 16 nm. Our group prepared AuFe3O4 dumbbell-like nanoparticles by mixing pre-made Au nanoparticles with Fe(OL)3 in a solution of oleylamine and oleic acid in 1-octadecene and heating the mixture to reflux (320 C) for 40 min followed by room-temperature air oxidation [33]. Interestingly, the Fe3O4 domain presents a squarelike shape (Fig. 30.3), which is different from the spherical shape when Fe(CO)5 was used as Fe-precursor.

30.2.3 Influence Factors The size and morphology of the AuFe3O4 dumbbell-like nanoparticles are closely related to the synthetic conditions, so in this section we discussed several important factors that influence the synthesis of AuFe3O4 dumbbell-like nanoparticles. 30.2.3.1 The Molar Ratio of Au and Fe Precursors Grzybowski et al. discovered that different molar ratio of Au and Fe precursors led to a gradual change of morphology from dumbbell-like to flower-like [34]. When more Fe precursor was used relative to Au precursor, the formation of nanoflowers was preferred, and much larger flower petals were observed when the relative amount of Fe precursor increased. Fig. 30.4 shows the AuFe3O4 nanoparticles prepared by refluxing different amounts of Fe(CO)5 in 1-octadecene at the presence of Au nanoparticles, oleic acid, and oleylamine for 50 min. When the molar ratio of Au and Fe precursors was 1:1 (Fig. 30.4A), the nanoparticles are dumbbell-like dimers comprising a smaller (58 nm) Fe3O4 domain attached to a larger (10 nm) Au part. The size of the Fe3O4 domain increases with the increase of the molar ratio of Au and Fe precursors and, at 1:2.5, is similar to that of the Au particle (Fig. 30.4B). At higher values of Au:Fe, multiple Fe3O4 domains form on each Au nanoparticle. For example, at 1:6 (Fig. 30.4C), about 60% of the composite particles have two and about 20% have three Fe3O4 domains around the Au core, while at 1:10 (Fig. 30.4D), about 80% of the particles have three or four Fe3O4 leaves per individual Au nanoparticle. 30.2.3.2 The Reaction Temperature The reaction temperature could influence the sizes of Au seeds. For example, Sun et al. observed that injecting HAuCl4 solution into the reaction mixture containing Fe(CO)5 at FIGURE 30.3 TEM images of AuFe3O4 nanoparticles in water with different magnification. Source: Reproduced with permission from Y.M. Zhai, L.H. Jin, P. Wang, S. Dong, Dual-functional AuFe3O4 dumbbell nanoparticles for sensitive and selective turn-on fluorescent detection of cyanide based on the inner filter effect, Chem. Commun. 47 (2011) 82688270. Copyright (2011) Royal Society of Chemistry.

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FIGURE 30.4

TEM images of AuFe3O4 nanoparticles prepared in 1-octadecene at the presence of oleic acid and oleylamine with Au:Fe initial molar ratios of (A) 1:1, (B) 1:2.5, (C) 1:6, and (D) 1:10. The dark portions of AuFe3O4 nanoparticles correspond to Au while lighter ones to Fe3O4. All scale bars are 20 nm. Source: Reproduced with permission from Y. Wei, R. Klajn, A.O. Pinchuk, B.A. Grzybowski, Synthesis, shape control, and optical properties of hybrid Au/Fe3O4 “nanoflowers”, Small 4 (2008) 16351639.

120 C led to B2 nm Au particles, while injection at 160 C or 180 C gave 4 or 6 nm Au particles [30]. In a facile synthesis of monodisperse Au nanoparticles, they prepared Au nanoparticles with sizes tunable from 1 to 10 nm through a burst nucleation by carefully controlling the reaction temperature at which the reducing solution was injected into the precursor solution [35]. The nanoparticle size and the reaction temperature had a linear correlation, as listed in Table 30.1. This can be explained by the classic La Mer theory [36,37]: injection of the reducing agent into the precursor solution at a relatively high temperature results in the burst nucleation consuming most of the precursors, leading to fast nucleation and growth processes, and ultimately produces smaller Au nanoparticles. 30.2.3.3 The Refluxing Time Velasco et al. investigated the influence of the refluxing time in the reaction, and discovered that the morphology of AuFe3O4 nanoparticles evolved from dumbbell-like to flower-like and finally a coreshell structure with a prolonged refluxing time [38]. The synthesis of AuFe3O4 nanoparticle is based on the method developed by Chen et al. for the preparation of FeAu nanoparticles [39], which was adapted from Sun et al. for the synthesis of monodisperse FePt nanoparticles [40]. But, in this case, the iron precursor, Fe (CO)5, was injected when the solution of oleylamine and oleic acid in dioctyl ether was heated to 200 C. With different refluxing time of 30, 90, and 180 min, the authors observed that the morphology of hybrid nanoparticles changed from dumbbell-like to flower-like and finally a coreshell structure. As is explained above, after the addition of Fe(CO)5, Fe3O4 nanoparticles start to nucleate epitaxially onto one single Au surface, leading to the formation of AuFe3O4 dumbbell-like nanoparticles. As the refluxing time increases, the

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TABLE 30.1 Average Size of Au Nanoparticles (Au NPs) Synthesized at Different Temperatures Reaction temperature ( C)

Avg. Au NP size (nm)

2

9.5

10

8.1

15

7.3

20

6.4

25

5.3

35

3.3

40

2.4

Reproduced with permission from S. Peng, Y. Lee, C. Wang, H. Yin, S. Dai, S. Sun, A facile synthesis of monodisperse Au nanoparticles and their catalysis of CO oxidation, Nano Res. 1 (2008) 229234. Copyright (2008) Springer Nature.

number of Fe3O4 nanoparticles increases and they nucleate onto the gold free facets. After 90 min, flower-like nanoparticles are formed with 5 nm Fe3O4 nonuniform petals linked to a 7 nm Au core, whereas after 180 min, most of the Au nanoparticles seem to be completely surrounded by an Fe3O4 layer forming a coreshell structure with a diameter of about 12 nm, where the Au core was estimated to be about 6 nm [38]. 30.2.3.4 The Solvent Polarity As we mentioned above, Sun et al. prepared AuFe3Oh4 dumbbell-like nanoparticles with 1-octadecene as solvent in the presence of oleic acid and oleylamine [30]. However, when the solvent was changed from 1-octadecene to diphenyl ether, which has a higher polarity, they obtained flower-like AuFe3O4 nanoparticles. Fig. 30.5 shows the TEM images of such flower-like nanoparticles with the faceted Au core being B8 nm and radial length of the Fe3O4 at B4 nm, indicating clearly the multinucleation of Fe3O4 on the faceted Au seeds. This observation is in accordance with the formation mechanism: if the Au nanoparticle has more electrons to compensate for the charge at the plane, the nucleation of Fe3O4 nanoparticles on Au seeds would occur at more facets of the Au nanoparticle, while these extra electrons could be offered by high polarity solvents in the synthesis.

30.3 OPTICAL AND MAGNETIC PROPERTIES The epitaxial linkage between Au and Fe3O4 in the AuFe3O4 dumbbell-like nanoparticles has a significant effect on the optical and magnetic properties of individual nanoparticles.

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30.3.1 Optical Properties The AuFe3O4 dumbbell-like nanoparticles show a red shift in the plasma resonance of Au because of the intimate contact between the Au and Fe3O4 [30]. It is known that for Au particles with sizes ranging from 5 to 20 nm in diameter, freely mobile electrons are trapped in the small Au metal box and show a characteristic collective oscillation frequency of the plasma resonance, giving rise to the plasma resonance band at around 520 nm [14]. The exact absorption position varies with particle morphology and particle surface coating. Fig. 30.6 shows the UVvis spectra of the Au and AuFe3O4 dumbbell-like nanoparticles dispersed in hexane. The peaks at 520 nm for Au nanoparticles are independent of the size and concentration of the particles but the width increases with the decreased nanoparticles size (Fig. 30.6A and B). However, once attached to Fe3O4, the Au particles show plasmon resonance absorption at 538 nm (Fig. 30.6C and D), an 18 nm red shift from that of the pure Au nanoparticles. As the dielectric environment for all the nanoparticles in this measurement is the same, the only difference comes from the size and dumbbell-like

FIGURE 30.5 (A) Low resolution TEM and (B) HRTEM images of flower-like AuFe3O4 nanoparticles. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional AuFe3O4 nanoparticles, Nano Lett. 5 (2005) 379382. Copyright (2005) American Chemical Society.

FIGURE 30.6 UVvis spectra of the Au and AuFe3O4 dumbbell-like nanoparticles dispersions in hexane: (A) 8 nm Au; (B) 4 nm Au; (C) 714 nm AuFe3O4; and (D) 314 nm AuFe3O4. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional AuFe3O4 nanoparticles, Nano Lett. 5 (2005) 379382. Copyright (2005) American Chemical Society.

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structure. Previous studies demonstrate that excess electrons on the Au particles can cause the plasmon resonance absorption shift to shorter wavelength; whereas electron deficiency will shift the absorption to longer wavelength [14]. The red shift of the plasmon resonance spectra for AuFe3O4 dumbbell-like nanoparticles indicates that the Au nanoparticles in the AuFe3O4 dumbbell-like nanoparticles are electron deficient, which is likely caused by the interface communication between Au and Fe3O4.

30.3.2 Magnetic Properties The interface communication between the nanoscale Au and Fe3O4 also leads to the change of magnetization behaviors of the Fe3O4 nanoparticles, especially for those smaller than 8 nm. Fig. 30.7 shows the hysteresis loops measured at room temperature for AuFe3O4 dumbbell-like nanoparticles with Au being 3 nm and Fe3O4 14 nm (Fig. 30.7A) and 6 nm (Fig. 30.7B), respectively. Like pure Fe3O4 nanoparticles, the AuFe3O4 dumbbell-like nanoparticles are superparamagnetic at room temperature. The 314 nm dumbbell-like nanoparticles show loops similar to the 14 nm Fe3O4 nanoparticles with saturation moment reaching 80 emu/g (Fig. 30.7A), a value that is close to the related Fe3O4 nanoparticles due to the negligible weight percentage of 3 nm Au in the nanocomposite. The 36 nm dumbbell-like nanoparticles, however, show a loop of slow increase in moment with the field up to 5 T (Fig. 30.7B). The slope is likely caused by surface spin canting of the small Fe3O4 nanoparticles [41], since the pure Fe3O4 nanoparticles of the same size do not show such a magnetization behavior. This spin canting is further aggravated upon the connection with Au nanoparticles [42].

FIGURE 30.7 Hysteresis loops of the AuFe3O4 dumbbell-like nanoparticles measured at room temperature: (A) 314 nm AuFe3O4 and (B) 36 nm AuFe3O4 nanoparticles. Source: Reproduced with permission from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbelllike bifunctional AuFe3O4 nanoparticles, Nano Lett. 5 (2005) 379382. Copyright (2005) American Chemical Society.

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30.4 POTENTIAL APPLICATIONS The unique properties make AuFe3O4 dumbbell-like nanoparticles show high potential for applications in the fields of catalysis, sensors, and biomedicine [24,4348].

30.4.1 Catalysis AuFe3O4 dumbbell-like nanoparticles have recently undergone intensive research as highly efficient catalysts for carbon monoxide oxidation [43,49,50], reduction of nitrophenols [32], and reduction of hydrogen peroxide [51]. The superparamagnetic nature of the AuFe3O4 dumbbell-like nanoparticles enables trouble-free separation of the catalysts from the reaction mixture for recycling by using an external magnet [52]. Furthermore, the hybrid material matrix significantly stabilizes the Au nanoparticles from agglomeration and leaching, leading to a catalytic lifetime enhancement [53,54]. 30.4.1.1 Catalysis of Carbon Monoxide Oxidation Dai et al. reported the solution/suspension deposition of AuFe3O4 dumbbell-like nanoparticles on amorphous carbon supports with further calcination at 300 C in 8% O2/He for 1 h to remove the residue of organic surfactant on the particle surface [43]. The treated nanoparticles exhibited significant catalytic activity for the CO oxidation reaction at room temperature. Fig. 30.8 summarizes the light-off curves for the CO oxidation reaction catalyzed by AuFe3O4 dumbbell-like nanoparticles deposited on various supports. The reactivity shown on AuFe3O4/C is especially interesting as it is well-known that Au nanoparticles loaded directly on the carbon supports are inactive for the CO oxidation reaction [55]. In contrast, the AuFe3O4 nanoparticles supported on carbon are highly active for this reaction, showing 100% CO conversion at 50 C. This enhanced catalytic activity arises clearly from the local modification of the electronic structure of Au by Fe3O4 through the interface interaction. 100

CO conversion (%)

80

60

40 a) b) c) d)

20

0 –100

–50

0 50 Temperature (ºC)

Au-Fe3O4 Au-Fe3O4/SiO2 Au-Fe3O4/TiO2 Au-Fe3O4/C 100

150

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FIGURE 30.8 CO oxidation conversion light-off curves of (A) AuFe3O4: AuFe3O4 nanoparticles calcined at 300 C for 1 h; (B) AuFe3O4/SiO2: AuFe3O4 deposited on SiO2 was calcined at 500 C for 1 h; (C) AuFe3O4/TiO2: AuFe3O4 deposited on TiO2 was calcined at 300 C for 1 h; (D) AuFe3O4/C: AuFe3O4 deposited on carbon was calcined at 300 C for 1 h. Source: Reproduced with permission from H. Yin, C. Wang, H. Zhu, S.H. Overbury, S. Sun, S. Dai, Colloidal deposition synthesis of supported gold nanocatalysts based on AuFe3O4 dumbbell nanoparticles, Chem. Commun. (2008) 43574359. Copyright (2008) Royal Society of Chemistry.

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30.4.1.2 Catalysis of Nitrophenol Reduction Lin and Doong reported the synthesis of AuFe3O4 dumbbell-like nanoparticles by thermal decomposition of Fe(OL)3 at Au seeds at 310 C for the catalytic reduction of p-nitrophenol and 2,4-dinitrophenol [32]. They discovered that the AuFe3O4 dumbbelllike nanoparticles show higher catalytic efficiency than the pure Au nanoparticles, which was attributed to the synergetic effect that occurs at the interface of Au and Fe3O4. It is believed that the electronic structures of both the metal and metal oxide support are modified by electron transfer across the interface, giving rise to oxygen vacancies on the interfacial metal oxide support that become active sites for oxygen absorption and activation [24]. The as-prepared AuFe3O4 dumbbell-like nanoparticles with magnetic properties can be easily recycled by an external magnet after the catalytic reduction. Fig. 30.9 shows the magnetically recyclable reduction of nitrophenols in the presence of AuFe3O4

FIGURE 30.9

Catalytically recyclable reduction of (A) p-nitrophenol and (B) 2,4-nitrophenol by AuFe3O4 dumbbell-like nanoparticles in the presence of NaBH4. Conversion efficiency of (C) p-nitrophenol in six successive cycles of reduction and (D) 2,4-nitrophenol in seven successive cycles of reduction by AuFe3O4 and citratestabilized Au nanocatalysts. Source: Reproduced with permission from F.-H. Lin, R.-A. Doong, Bifunctional AuFe3O4 heterostructures for magnetically recyclable catalysis of nitrophenol reduction, J. Phys. Chem. C 115 (2011) 65916598. Copyright (2011) American Chemical Society.

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dumbbell-like nanoparticles. The catalysts can be successfully recycled and reused for at least six successive cycles of reaction with stable conversion efficiency of around 100%. In contrast, the conversion efficiency of nitrophenols by citrate-stabilized Au nanoparticles drops dramatically after the second cycle. It is obvious that the presence of Fe3O4 nanoparticles makes the dumbbell-like heterostructures a promising bifunctional probe for magnetically recyclable catalytic reduction. 30.4.1.3 Catalysis of Hydrogen Peroxide Reduction Sun et al. proposed a unique protocol to investigate the synergetic effect in AuFe3O4 dumbbell-like nanoparticles for catalyzing the reduction of H2O2 [51]. The strategy started with the synthesis of AuFe3O4 dumbbell-like nanoparticles, and then single component Au and Fe3O4 nanoparticles were obtained from the AuFe3O4 dumbbell-like nanoparticles by a controlled etching of the composite particles (Fig. 30.10A), which ensured that the individual Au and Fe3O4 nanoparticles have the same structural features as their corresponding domains in the AuFe3O4 dumbbell-like nanoparticles. The catalytic reduction (A)

H 2SO 4

Au

KI/I2

Au-Fe3O4 Fe3O4

(B) (b) –1

40 0

I/mA (mg Fe3O4)

I/mA (mg Au)

–1

(a)

–40 –80 –120 –160

10 0 –10 –20 –30 –40

–0.4 –0.2 0.0

0.2

0.4

0.6

V/V vs. Ag/AgCl

0.6

–0.4 –0.2 0.0 0.2 0.4 V/V vs. Ag/AgCl

0.6

(d) 0.1

0.1

0.0

0.0

–0.1

–0.1

I/mA

I/mA

(c)

–0.4 –0.2 0.0 0.2 0.4 V/V vs. Ag/AgCl

–0.2

–0.2

–0.3

–0.3 –0.4 –0.2 0.0 0.2 0.4 V/V vs. Ag/AgCl

0.6

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FIGURE 30.10 (A) Selected etching of AuFe3O4 dumbbelllike nanoparticles for the preparation of the Au nanoparticles and dented Fe3O4 nanoparticles. (B) IV curves of (a) AuFe3O4 (black) and Au (gray) nanoparticles, (b) AuFe3O4 (black) and Fe3O4 (gray) nanoparticles normalized by Au and Fe3O4 weight, respectively, (c) AuFe3O4 catalyst (black) and carbon support (gray), and (d) AuFe3O4 catalyst with (black) and without (gray) the addition of 4 mM of H2O2. Recorded in N2-saturated 0.1 M PBS with 4 mM of H2O2; scan rate: 50 mV/s. Source: Reproduced with permission from Y. Lee, M.A. Garcia, N.A.F. Huls, S. Sun, Synthetic tuning of the catalytic properties of AuFe3O4 nanoparticles, Angew. Chem. Int. Ed. 49 (2010) 12711274.

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of H2O2 demonstrated that the AuFe3O4 dumbbell-like nanoparticles had a higher catalytic activity than that of either Au or Fe3O4 nanoparticles (Fig. 30.10B). The enhanced catalytic activity of the AuFe3O4 dumbbell-like nanoparticles for H2O2 reduction reaction resulted from the electronic interaction between Au and Fe3O4 domains in the composite nanoparticles.

30.4.2 Sensors Wei et al. reported the application of AuFe3O4 dumbbell-like nanoparticles in the detection of the cancer biomarker, prostate specific antigen (PSA) [56]. As shown in Fig. 30.11, a sandwich-type structure was formed by immobilizing the primary anti-PSA antibody (Ab1) onto the graphene sheet (GS) surface, the PSA in the sample captured, and the Ab2AuFe3O4 label. The AuFe3O4 dumbbell-like nanoparticles showed a synergetic effect in catalyzing H2O2 reduction, which was more active than either Au or Fe3O4 nanoparticles alone. The electrochemical signal emanating from AuFe3O4 nanoparticles was in accordance with the concentration of PSA, which could be quantified for the detection of PSA. The immunosensor showed a wide linear range (0.0110 ng/mL), low detection limit (5 pg/mL), good reproducibility, and stability. Our group demonstrate for the first time that the bifunctional AuFe3O4 dumbbell-like nanoparticles can be used for sensitive and selective turn-on fluorescent detection of cyanide with Rhodamine B (RB) as the fluorescent probe based on the inner filter effect (IFE), and a “magnetic concentration-washing process” is proposed to effectively reduce the interference of dye pollution [33]. As shown in Fig. 30.12, AuFe3O4 dumbbell-like nanoparticles can be mixed into a larger volume of sample solution and magnetically concentrated to the original volume after the reaction with cyanide. By doing so, a higher fluorescence recovery signal can be obtained. More importantly, if dye pollution exists in the sample, this fluorescent detection method can also be used. Most real sample solution can be removed during the magnetic concentration process. Then, a certain amount of buffer prepared with pure water was added to dilute the rest of the real sample, followed by another magnetic concentration (this procedure was named a “magnetic concentrationwashing process”). Repeating such procedures, very little interference from the real sample would be left. Cyanide can dissolve Au domains of AuFe3O4 nanoparticles in the

FIGURE 30.11 Schematic representation of the immunosensor. Ab1, primary anti-PSA antibody; Ab2, secondary anti-PSA antibody; GC, glassy carbon electrode; GS, graphene sheet; PSA, prostate specific antigen. Source: Reproduced with permission from Q. Wei, Z. Xiang, J. He, G. Wang, H. Li, Z. Qian, et al., Dumbbell-like AuFe3O4 nanoparticles as label for the preparation of electrochemical immunosensors, Biosens. Bioelectron. 26 (2010) 627631. Copyright 2010 Elsevier.

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FIGURE 30.12

Schematic depiction of the turn-on fluorescent detection of cyanide based on the dual-functional AuFe3O4 dumbbell-like nanoparticles. Source: Reproduced with permission from Y.M. Zhai, L.H. Jin, P. Wang, S. Dong, Dualfunctional AuFe3O4 dumbbell nanoparticles for sensitive and selective turn-on fluorescent detection of cyanide based on the inner filter effect, Chem. Commun. 47 (2011) 82688270. Copyright (2011) Royal Society of Chemistry.

presence of oxygen, forming a stable Au(CN)22 complex. With the erosion of the Au domains, there will be more light available to excite the RB and also more emission light be detected, because the Au domains can effectively reduce the emission of RB. As a result, the signal of fluorescence was enhanced gradually by increasing the concentration of cyanide. The proposed method obtained a good linear relationship between the fluorescence enhancement ratio and the cyanide concentration in the range of 4.0 3 1027 M to 1.2 3 1024 M (R2 5 0.994), and can be applied for real samples analysis. Huang et al. developed an effective sensitive interface to detect As(III) by using AuFe3O4 dumbbell-like nanoparticles [57]. Fig. 30.13 illustrates the detection mechanism of As(III) on AuFe3O4 dumbbell-like nanoparticles. According to their previous report [58], the synergistic effect of the excellent catalytic properties of Au nanoparticles and the good adsorption ability of 400 nm Fe3O4 nanoparticles can significantly improve the detection of As(III), and the adsorbed As(III) on 400 nm Fe3O4 nanoparticles will be directly reduced and oxidized on the Au surface (left side of Fig. 30.13). The adsorption capacity toward As(III) increases as the size of Fe3O4 nanoparticles decreases (B10 nm). Besides, active Fe(II) exposed on the surface of the AuFe3O4 nanoparticles also has a role. The concentration of As(III) near the electrode surface gradually increases as a result of the adsorption of B10 nm Fe3O4 nanoparticles. The surface-activated Fe(II) can denote an electron to form Fe(III) in the reduction of As(III) to As(0) (right side of Fig. 30.13). The generated Fe(III) will then get an electron from the electrode or oxidation of As(0) to As(III) during the process of square wave anodic stripping voltammetry (SWASV), which completes the Fe(II)/Fe(III) cycle (right side of Fig. 30.13). The Fe(II) works as an electrocatalyst to mediate electron transfer between electrode and As(III). This mediation of the Fe(II)/Fe (III) cycle as well as the catalyst of Au nanoaprticles will efficiently enhance the electrochemical sensitivity toward As(III). As a result, the modified AuFe3O4 dumbbell-like nanoparticles modified screen-printed carbon electrode (SPCE) serves as an efficient sensing interface for As(III) detection with an excellent sensitivity of 9.43 μA/ppb and a low detection limit of 0.0215 ppb.

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FIGURE 30.13 Ultrahighly sensitive electroanalysis of As(III) based on the adsorption of B10 nm Fe3O4 nanoparticles and the catalyst of B7 nm Au nanoparticles as well as the redox mediation by surface-active Fe(II) at AuFe3O4 SPCE. Source: Reproduced with permission from S.S. Li, W.Y. Zhou, M. Jiang, Z. Guo, J.H. Liu, L. Zhang, et al., Surface Fe(II)/Fe(III) cycle promoted ultra-highly sensitive electrochemical sensing of Arsenic(III) with dumbbell-like Au/Fe3O4 nanoparticles, Anal. Chem. 90 (2018) 45694577. Copyright (2018) American Chemical Society.

30.4.3 Biomedicine Magnetically and optically active AuFe3O4 dumbbell-like nanoparticles containing two different chemical surfaces are particularly suitable for selected nanoparticle functionalization with both targeting agents and drug molecules, which facilitates their application as multifunctional probes for target specific imaging [47,48] and delivery [59]. 30.4.3.1 Cell Imaging Sun et al. reported that the AuFe3O4 dumbbell-like nanoparticles were suitable probe for A431 (human epithelial carcinoma cell line) cell imaging [60]. By functionalizing the surface of Fe3O4 and Au domains in AuFe3O4 dumbbell-like nanoparticles with the epidermal growth factor receptor antibody (EGFRA) and HS-PEG-NH2, respectively, the nanohybrids could remain stable against aggregation in phosphate-buffered saline (PBS) or PBS containing 10% fetal bovine serum (FBS) at 37 C for 12 h. They then incubated the functionalized dumbbell-like nanoparticles with A431 cells in Dulbecco’s Modified Eagle’s Medium (DMEM) containing 10% FBS for 1 h. In their study, A431 cells labeled with 820 nm AuFe3O4 hybrid nanoparticles were visualized with a scanning confocal microscope. The wavelength used for this image was 594 nm, which is close to the strong reflectance of the nanoparticles. Furthermore, A431 cells labeled with these hybrid nanoparticles can be also manipulated by using an external magnetic field, which is readily tracked under the optical microscope. To demonstrate the specific targeting, the authors incubated A431 cells and the 820 nm AuFe3O4 hybrid nanoparticles without EGFRA. In this case, the reflection signal was much weaker, and the signal-to-noise ratio was much higher, thus indicating that the EGFRA-labeled nanoparticles had higher specificity in their attachment to A431 cells [60]. II. APPLICATIONS

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30.4.3.2 Drug Delivery Sun et al. developed a drug nanocarrier by using AuFe3O4 dumbbell-like nanoparticles with herceptin (Her2 antibody) attached on Fe3O4 nanoparticles and the anticancer drug cisplatin attached on the Au nanoparticles [61]. Such a structure takes the advantages of multitasking during therapy and also two kinds of functional ligands can work independently without disturbing each other. The specificity of the platinAuFe3O4Herceptin composite nanoparticles was examined through their preferred targeting of Sk-Br3 cells that are Her2-positive breast cancer cells with Her2-negative breast cancer cells (MCF-7) [62] as a control. Fig. 30.14 shows the reflection images of Sk-Br3 cells (Fig. 30.14A) and MCF-7 cells (Fig. 30.14B). The brighter image shown in Fig. 30.14A demonstrates that more composite nanoparticles are conjugated to the Sk-Br3 cells, indicating that under the same incubation concentration, Herceptin helps the preferred targeting of Sk-Br3 cells as

FIGURE 30.14 Reflection images of (A) Sk-Br3 and (B) MCF-7 cells after incubation with the same concentration of platinAuFe3O4Herceptin NPs. (C) Cisplatin and platin release curves at 37 C (pH 5 7). (D) pHdependent Pt release from platinAuFe3O4Herceptin at 37 C. Source: Reproduced with permission from C.J. Xu, B.D. Wang, S. Sun, Dumbbell-like AuFe3O4 nanoparticles for target-specific platin delivery, J. Am. Chem. Soc. 131 (2009) 42164217. Copyright (2009) American Chemical Society.

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opposed to MCF-7 cells. The drug release profile was analyzed by using the dialysis bag in PBS, 37 C at pH 5 7. As shown in Fig. 30.14C, 80% of the free cisplatin diffused through the dialysis bag in 1 h, while for the nanoparticle conjugate, this release was reduced to only B25% in the same incubation time. The Pt release was pH dependent (Fig. 30.14D). It can be seen that the release of platin from the platinAuFe3O4Herceptin nanoparticles increases when the pH becomes lower. The comparison of the cell viability after incubation with the composite nanoparticles, control composite nanoparticles (without antibody), and cisplatin showed that the cell viability of the composite nanoparticles was lowest for its therapeutic effect. 30.4.3.3 Gene Transfection Gene therapy has the great potential for treating many diseases such as cancer. However, at present gene therapies in the clinic are still limited due to the low efficiencies of nonviral gene vectors and the receptor-dependent host tropism of adenoviral or low titers of retroviral vectors [63]. Recently, “Magnetofection” has been used to overcome these deficiencies by associating gene vectors with magnetic nanoparticles and application of a magnetic field [64]. For example, Xu et al. reported the usage of AuFe3O4 dumbbell-like nanoparticles in gene transfection as well as micro-optical coherence tomography (μOCT) imaging [65]. As illustrated in Fig. 30.15, negatively charged plasmids can be conjugated with AuFe3O4 nanoparticles premodified with cationic polymer, PEI. Given the presence of a magnetic component (Fe3O4), under a magnetic field, these nanocomposites provided a higher efficiency in transfecting adherent mammalian cells. Moreover, AuFe3O4 nanoparticles (especially the Fe3O4 core) enabled the visualization of this transfection process through micro-optical coherence tomography (μOCT) technology. Although the Au didn’t show any specific contribution to the μOCT imaging and magnetofection here, considering the multirole applications of Au nanoparticles already demonstrated in multimodal imaging [66], the properties reported here FIGURE 30.15 Schematic illustration of AuFe3O4 dumbbell-like nanoparticles for gene delivery and μOCT tracking. Source: Reproduced with permission from W. Shi, X. Liu, C. Wei, Z.J. Xu, S.S.W. Sim, L. Liu, et al., Micro-optical coherence tomography tracking of magnetic gene transfection via AuFe3O4 dumbbell nanoparticles, Nanoscale 7 (2015) 17249-17253. Copyright (2015) Royal Society of Chemistry.

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are expected to trigger more cross-disciplinary research on AuFe3O4 nanoparticles. Furthermore, by increasing the size, Au nanoparticles might dominate and greatly enhance the μOCT signal [67].

30.5 CONCLUSIONS AuFe3O4 dumbbell-like nanoparticles offer an interesting platform to investigate the physical/chemical properties of materials based not only on each particle dimension and morphology, but also on the communication between the two different components. The interfacial interaction that exists between two different nanoparticles can induce significant change in the physical and chemical properties of both nanoparticles in the structure. As a result, the AuFe3O4 dumbbell-like nanoparticles have shown interesting catalytic, optical, and magnetic properties. The distinct surface chemistry presented in the AuFe3O4 dumbbell-like nanoparticles also facilitates the selected nanoparticles functionalization with either a targeting agent and/or a therapeutic agent, making these nanohybrids especially important for target-specific medical diagnostic and therapeutic applications. The research of AuFe3O4 dumbbell-like nanoparticles could be categorized as follows: (i) developing general strategies to prepare the nanohybrids with controlled size and morphology of each component; (ii) understanding the synergetic effect and interface boundary sites, which play an important role in the modulation of physical and chemical properties; and (iii) exploring potential applications of these nanohybrids.

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Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Aberration-corrected scanning transmission electron microscopy (ac-STEM), 72 Aberration-free high-angle annular dark field (HAADF), 66 Absorbing boundary conditions (ABCs), 440441 Absorption and emission spectroscopy, 80 Absorption enhancement (AE), 272273, 277278 effect of the active layer thickness on, 275276 effect of the nature of the dielectric shell material on, 276 effect of the shell thickness on, 273275 influence of ZnO optical spacer layer and active layer material on, 277278 Active sensing coatings, 342343, 346349 corrosion-sensing coatings, 346348 pressure-sensing coatings, 348349 Advanced oxidation processes (AOPs), 377 Ag@Fe3O4 coreshell nanoparticles, 55, 186 Ag@Fe3O4@cellulose nanocomposites, 306307 Ag/MnO2 nanocomposite, 110, 419422, 424t Ag/MnO2 nanotubes, 110, 424t, 553f Ag/TiO2 hybrid nanoparticles, 373, 463, 463f coreshell hybrid nanoparticles, 148 energy production mediated by, 382386 for environmental application, 376382 atmospheric pollution abatement by photocatalytic Ag/TiO2 hybrid nanoparticles, 380382 photoactive Ag/TiO2 hybrid nanoparticles for water treatment, 376380 for quality life improvement, 386389 bactericidal coatings, 386388 photoactive Ag/TiO2 hybrid nanoparticles for odor-control filters, 388389 Ag(TiO2/SnO2) material, ethanol sensing mechanism by, 298f Ag/ZnO hybrid nanocomposites, synthesis of, 184, 184f Ag2OTiO2 coated on CS-based polypropylene fibers (ATCPF), 378379 AgAl hybrid nanoparticles, 208213, 218221 absorption spectra of, 208f electron diffraction pattern of, 212f polymer effect on the synthesis of, 214217 TEM images of, 209f, 215f, 218f, 219 UVvisible spectra of, 218f, 219

AgAu alloy hybrid nanoparticles experimental and theoretical spectra of, 226227, 226f Ag-coated Fe3O4@SiO2 composite microspheres, synthesis route of, 470471, 470f Ag-doped lanthanum ferrite (LaFeO3), 108109 AgFe3O4 coreshell nanowires, 181 AgFe3O4 dimer nanoparticles, 55 AgFe3O4 magnetic SERS substrate, 471 AgFe3O4 nanoparticles, role of Fe3O4 in antibacterial action of, 305309 AgFe3O4GO nanocomposite, 309 Agmetal oxide hybrid nanoparticles, role of oxides in the antibacterial action of, 303 role of Fe3O4 in antibacterial action of AgFe3O4 nanoparticles, 305309 role of MnO2 in antibacterial action of AgMnO2 nanoparticles, 309310 AgMnO2 nanohybrids-based supercapacitors, 550555, 559t electrochemical deposition method, 554555 hydrothermal and solvothermal methods, 552554 wet-chemical redox method, 551552 AgMnO2 nanoparticles, role of MnO2 in antibacterial action of, 309310 AgPtFe3O4 heterotrimers, 7072, 71f AgSnO2/SBA-15 nanohybrids-based humidity sensors, 286292 AgSnO2/SBA-15(X), 286288 HRTEM image of, 288290, 289f low-angle XRD (LAXRD), 287f AgTiO2 nanocomposites, in SERS, 463f Agtransition metal oxides (TMOs)/TMHs composites, 419422 AgZnO composite nanoarrays, 466 Air pollution, 380 AiroCide technology, 388 Al2O3@AgAu alloy hybrid nanoparticles, 226230 size distribution of, 229f TEM image, 229f UVvisible spectra, 229f Al2O3@Au@Ag coreshell hybrid nanoparticles, 230232 HRTEM image of, 232, 233f TEM image and size distribution of, 231f, 232 AlAg hybrid nanoparticles, 213

645

646

INDEX

AlAg hybrid nanoparticles (Continued) EDX of, 212f HRTEM image of, 215217, 216f TEM image of, 219f UVvisible spectra of, 219f AlAu hybrid nanoparticles EDAX spectrum of, 224, 225f formation in de-ionized water, 224225 HRTEM image of, 224f TEM image of, 222f, 224f UVvisible spectra, 222f AlAuAg trimetallic HNs UVvisible spectra of, 230f Alcohol oxidation, 322 Alkoxysilanes, 2122 Alkyl phosphates, 3334 Alloy hybrid nanoparticles, 203 Allyldiphenylphosphine oxide (ADPPO), 360 α-MoO3, 407408 SEM micrographs of, 408f Alternating magnetic field (AMF), 254 Alumina, 211, 224, 226 Amine functional group, ligands with, 1920, 3445 Amine-functionalized silica nanopowder, synthesis of, 183 Amino propyl tetra ethoxy silane, 183 N-(6-Aminohexyl)-aminopropyltrimethoxysilane (AHAPS), 3033 3-Aminopropyltrimethoxysilane (3-APTMS), 2122, 3033 2-Aminoethyl-aminopropyltrimethoxysilane (AEAPS), 2122 3-(Aminopropyl)-triethoxysilane (APTES), 2122, 609610 Aminoterephthalic acid, 2629 4-Aminothiophenol (4-ATP), 466 Amorphous manganese oxide (AMO), 324328 Ampicillin (AMP), 378379 Anisotropy, 247 Antibacterial behavior of noble metals in their nanohybrids, 141 challenges and perspective, 149 metal oxide nanoparticles’ effect on, 145 noble metal nanoparticles effect on, 145149 Antibacterial coatings, 356358, 358f Antibiotic-resistant bacteria, 142143, 376377 Antibiotics, 141142, 145, 149 Antifingerprint coatings, 360363 Antifogging coatings, 362363 Antifouling coatings, 354356, 355f Antigraffiti coatings, 354 Anti-icing coatings, 361362

Antimicrobial interactions of Ag nanoparticles, 142, 144f Antireflective coatings, 361 Anti-Stokes scattering, 457458 Aqueous asymmetric supercapacitor (AAS), 411413 Artificial photosynthesis, 585 Atmospheric pollution, 380382 Atom beam cosputtering method, 57 Atomic number contrast scanning transmission electron microscopy (Z-STEM), 7072 ATS-h sensor, 294297 Au@Ag coreshell nanohybrids, 180, 180f Au@Fe3O4 heterodimers, 82 Au@In2O3 coreshell hybrid nanoparticles, 499 Au metal NPs in improved hydrogen gas-sensing of, 511512 characterizations, 502507 chemical sensitization, 512 high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, 503505, 504f hydrogen gas-sensing application, 507512 gas sensor device fabrication and measurements, 507508 hydrogen gas-sensing studies, 508511 powder X-ray diffraction (XRD) studies, 505, 506f synthesis, 501502, 502f Au@In2O3/In2O3 NPs sensor devices, reaction mechanisms of, 512 Au@TiO2 coreshell nanoparticles, 55 Au metal nanoparticles in hydrogen gas-sensing of hybrid nanoparticles, 511512 Au(I)-thiolate species, 1519 Au/Ag/Pt (trimetallic nanocomposites), synthesis of, 182 Au/CeO2 nanohybrids as Photocatalyst, 527528 Au/TiO2 catalysts, 523 Au/TiO2 hybrid nanoparticles, 5253, 52f AuAl hybrid nanoparticles, 221225 TEM image of, 221f UVvisible spectra, 221f Au-decorated hierarchical NiO nanostructures, 424t AuFe3O4 dumbbell-like nanoparticles, 424t, 625 characterization, 627 formation mechanism, 626627 hysteresis loops of, 632f influence factors, 628630 molar ratio of Au and Fe precursors, 628 reaction temperature, 628629 refluxing time, 629630 solvent polarity, 630 magnetic properties, 632

INDEX

modified strategies, 627628 optical properties, 631632 potential applications, 633641 biomedicine, 638641 carbon monoxide oxidation, catalysis of, 633 cell imaging, 638 drug delivery, 639640 gene transfection, 640641 hydrogen peroxide reduction, catalysis of, 635636 nitrophenol reduction, catalysis of, 634635 sensors, 636637 schematic illustration of, 626f, 640f synthesis of, 626630 characterization, 627 formation mechanism, 626627 TEM images of, 628f, 629f AuFe3O4 heterostructured nanocrystals (HNCs), 9698 Auger electron-emission, 539 Aumetal oxide hybrids, 55, 181 AuMnO2 hybrid nanowall film, 474475 FDTD simulations for, 476f AuMnO2 nanohybrid electrodes, 558 AuMnO2 nanohybrids-based supercapacitors, 556558, 559t AuNiO nanohybrids, 110, 422423 Au-seeded Fe3O4@SiO2 NPs, 609610 AuTiO2 hybrid Janus-like nanoparticles, 45 AuTiO2 nanohybrid, 5556, 149 as photocatalyst, 528529 AuZnO NPs, 132 2,2’-Azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS), 321322 Azobenzenes, 322323, 346, 347f

B Bacillus subtilis, 145, 305 Bacteria, 141142, 149, 356358, 387 toxicity of nanoparticles (NPs) against, 143f Bacterial pathogens, removal of, 321322 Bactericidal coatings, 386388 Band gap energy, 105107, 597598 Bariumferrite nanoparticles, 246 Benzene, 320, 381, 529, 572 hydrogenation of, 572f Benzene carboxylic acids, 2629 Benzotriazole (BTA), 345346 17β-estradiol, 321322 Bimetallic hybrid nanoparticles, 218 Bioactive coatings, 354358 antibacterial coatings, 356358 antifouling coatings, 354356

647

Bioheat transfer model for heat distribution, 253254 Biomedicine, 543, 638641 cell imaging, 638 drug delivery, 639640 gene transfection, 640641 Biosensors, hybrid nanoparticles in, 431 applications of, 443450 discrete dipole approximation (DDA) method, 448449 finite-difference time-domain (FDTD) method, 446447 finite element methods (FEM), 450 Mie theory, 443446 fundamental theory of hybrid nanoparticles, 432437 fundamental theory of LSPR and Mie theory, 435437 Maxwell’s equations in matter and dielectric constants, 432435 simulation methods, 437443 discrete dipole approximation (DDA), 441443 finite-difference time-domain method (FDTD), 439441 finite element methods (FEM), 443 generalized Mie theory, 437438 Birnessite manganese oxide (BirMO), 551 Bis(4-pyridyl) ethylene (BPE) molecules, 104 BiVO4-based photocatalysts, 586587, 590593 Bloch eigenstates, 132133 Blue-shift phenomenon, 107, 146 Bohr radius, 8081, 131, 133134 Booster biocides, 354355 Bottom-up chemical synthesis, 179180 Brownian losses, 250 Brownian motion, 12, 252 Brownian relaxation, 252253 BrustSchiffrin method (BSM), 1519 Burgundy Cukote, 364t

C Carbon monoxide (CO) oxidation, 316318 catalysis of, 633 Carbon nanotubes (CNTs), 257258, 346348, 569570 Carbon paper, 567568 Carboxylate functional group, ligands with, 45 Catalysis, 633636 of carbon monoxide oxidation, 633 of hydrogen peroxide reduction, 635636 of nitrophenol reduction, 634635 Cavity quantum electrodynamics (QED), 159 Cell imaging, 638 Cellulose, 572574

648

INDEX

Ceria (CeO2) and titania (TiO2)-based Au nanocatalysts, 517 catalytic applications, 519529 CO oxidation, 519523 over CeO2-based Au nanocatalysts, 519523 over TiO2-based Au nanocatalysts, 523 multicomponent Au nanocatalyst, 520t Au/CeO2 nanohybrids as photocatalyst, 527528 Au/TiO2 nanohybrids as photocatalyst, 528529 organic transformations, 524526 CeO2 supported Au nanocatalysts, 524526 TiO2-supported Au nanocatalysts, 526 photocatalysis, 527529 synthesis of, 518519 VOCs oxidation, 523524 over CeO2-based Au nanocatalysts, 523524 over TiO2-based Au nanocatalysts, 524 Cetyl pyridinium chloride (CPC), 1920 Cetyltrimethylammonium bromide (CTAB), 1920, 459, 554555 Chemical methods for synthesis of hybrid nanoparticles, 179 coprecipitation method, 181 hydrothermal/solvothermal method, 185186 photochemical method, 184 seed growth method, 180 solgel method, 182183 amine-functionalized silica nanopowder, synthesis of, 183 trimetallic Au/Ag/Pt nanocomposites, synthesis of, 182 trimetallic Au/Pt/Ag nanocomposites-doped amine-functionalized silica nanopowder (Au/ Pt/Ag@SiO2), synthesis of, 183 sonochemical synthesis, 181182 Pdmetal oxide hybrid nanoparticles, 182 synthesis of (Pd, Co)@Pt nanohybrids, 181 wet-chemical synthesis, 185 Chemical pollutants, 376377 Chemical reduction (CR) and photoreduction (PR) methods of synthesis of hybrid nanoparticles, 5254 Chemical vapor deposition (CVD), 348349, 397 Chemistry of manganese oxides, 314315 Chemotherapy, 241, 260 Chitosan (CS), 378379 Chitosan-coated dumbbell-like Fe3O4Au hybrid NPs, 611, 611f Chitosan-functionalized end-capped Ag NPs, 306307 Citrate reduction method, 3435 ClausiusMossotti formulation, 442 CNT@PPy@MnO2 core-double-shell sponge fabrication process from CNT sponge to, 417f

supercapacitor application of, 418f Co3O4@Pt@MnO2 nanowire arrays, 424t Co3O4 NSWAs, electrochemical characterization of, 401f Coalescence, 210211 Coating plasmonic NPs/nanostructures, 104105 Cobalt phosphate (CoPi)OECs, 592 Cobaltferrite nanoparticles, 246 Co-based WOCs, 592 Conduction band (CB) Bloch eigenstates, 132133 Continuous hyperthermic peritoneal perfusion (CHPP) technique, 242 Controlled potential electrolysis (CPE) curves, 330331 CoPi cocatalyst, 596597 Coprecipitation method, 181, 186 of synthesis of hybrid nanoparticles, 55 Core/shell Pt/MnO2 nanotubes, 424t Coresatellite Fe3O4Au hybrid NPs, 611612, 612f Coreshell configuration, 377, 383, 385 Coreshell Fe3O4Au hybrid NPs, 608610, 609f, 613 Coreshell hybrid nanoparticles, 204205, 501507 morphological transformation from sintered to, 213f Coreshell nanoparticle (CSNP), 4344, 266, 269, 279 metaldielectric CSNP, 268f Coreshell nanostructures, 256257 Corresponding chloride (CTAC), 1920 Corrosion-sensing coatings, 346348, 364365 CoSn (OH)6 nanostructure, 413419 Coulomb interactions, 131 Coupling reactions, 322323 Crystal phase characterization through x-ray techniques, 7577 CS2, 1519 Cubic nanostructure (CNS), 411413 TEM image of, 415f Cumulative Equivalent Minutes (CEMs), 244 Cyclic voltammograms of water oxidation oxygen reduction, 327330

D Deletum 5000 & 3000, 364t Dendritic cell (DC), 538 Density of states (DOS), 158159, 173 Deposition technique, 413419, 464465 DescartesSnell’s laws, 277278 Diamagnetism, 250 Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), 8284 p,p-Dimercaptobenzene (DMAB), 479480 2,3-Dimercaptosuccinnic acid (DMSA), 541 Dimethylformamide (DMF), 1920, 4344 Discrete dipole approximation (DDA), 441443, 448449

INDEX

Disodium salt of ethylenediaminetetraacetic acid (Na2EDTA), 2629 Disulfides, 1519 Dithiocarbamate (DTC) ligands, 1519, 16f Dithiols, 1519 Dodecyl dimethyl ammonium bromide (DDAB), 6768 Dodecylamine (DDA), 1920, 2326 Double-shell hollow nanoparticle, 113 Doxorubicin (DOX)-conjugated Fe3O4Au hybrid NPs, 613 Doyle formulation, 442 Draine formulation, 442 Drift velocity, 135 Drude model, 202203, 267, 433 Drug delivery systems (DDSs), 535536, 639640 Dual magnetic photothermal cancer therapy application of noble metalFe3O4 hybrid nanoparticles for, 258259 Dual-mode MR/CT imaging, 616617 Dual-mode MR/FO imaging, 617618 Dual-mode MR/PA imaging, 618619 Dulbecco’s Modified Eagle’s Medium (DMEM), 638 Dumbbell-/tipped rod-like NPs, 113 Dumbbell-like Fe3O4Au hybrid NPs, 610611, 610f, 611f Dumbbell-shaped nanoparticles, 4445 Dye-sensitized solar cells (DSSCs), 278279, 384, 385f, 386

E Easy-to-clean coatings, 342343, 350354 antigraffiti coatings, 354 self-cleaning coatings, 350354 ECONTROL, 364t Eddy currents losses, 250 Effective mass theory (EMT), 132 Efros and Shklovskii ES-VRH model, 129 Elastic light scattering, 249 Electrical double layer capacitor (EDLC), 395396 Electrocatalysis, 326330 oxygen evolution reaction (OER), 330 oxygen reduction reaction (ORR), 328330 water oxidation, 326328 Electrochemical deposition method, 554555 Electrochemical double-layer capacitors (EDLCs), 549 Electrochemical supercapacitor (ES), 395396 classification of, 550f Electrodeposition method, 398400, 554555 Electromagnetic energy, 250 Electromagnetic near-field resonance, 266268 Electromagnetic radiation, 244, 254255, 373375, 382 Electromagnetic theory, 103, 458 Electron energy loss spectroscopy (EELS), 72

649

Electronic sensitization, 512 Electronic transport by excitons in hybrid NMMO NP systems, 130137 electronic transport by excitons in NMMO NP systems, 131133 theoretical modeling of electronic transport in 2D NMMO NP assemblies, 133137 in nanoparticle assemblies, 127130 electronic characteristics of a singular nanoparticle, 127128 electronic transport across NP assemblies, 128130 Electrostatic stabilization, 1213, 14f Electrosteric stabilization, 13 Energy dispersive X-ray spectroscopy (EDS), 66 Energy Efficient Window Coatings, 349 Environmentally sensitive coatings. See Smart coatings Escherichia coli, 144f, 305 Ethanolamine, 2326, 5455 Exciton hopping mechanism, 130 Excitonplasmon interactions, 157 applications, 168173 femtosecond absorption, 163167 free space spontaneous emission, 159161 spontaneous emission in cavities, 161163 strong coupling, 162163 weak coupling, 162 Excitons, electronic transport by in NMMO NP systems, 131133 Explosive ablation mechanism, 199 Explosive boiling, 199 Extinction cross section, 492493, 495 wavelength-dependent, 494f, 495f

F Far-field scattering, 268269 Fe3O4, in antibacterial action of AgFe3O4 nanoparticles, 305309 Fe2O3 oxide photocatalysts, 593594 Fe3O4@SiO2 coreshell nanostructures, 307309 Fe3O4Ag coreshell nanoparticles, 145 Fe3O4Ag coreshell nanostructures, 305306 Fe3O4Ag dumbell-like hybrid nanoparticles, 145 Fe3O4Au@Agstreptomycin, 307309 Fe3O4Au coreshell hybrid nanoparticles, 544 Fe3O4Au hybrid nanoparticles, 613, 616617 design and synthesis of, 608612, 609f coresatellite Fe3O4Au hybrid NPs, 611612, 612f coreshell Fe3O4Au hybrid NPs, 608610, 609f dumbbell-like Fe3O4Au hybrid NPs, 610611, 610f, 611f

650

INDEX

Fe3O4Au hybrid nanoparticles (Continued) theranostic application of, 612619 dual-mode MR/CT imaging, 616617 dual-mode MR/FO imaging, 617618 dual-mode MR/PA imaging, 618619 hyperthermia therapy, 613614 photodynamic therapy, 614615 targeted drug delivery, 615616 Fe3O4-decorated Au nanoparticles synthesis, 473f Fe3O4noble metal nanohybrids for SERS, 470473, 476f Fe-based oxides and oxy-hydroxides, 410 Femtosecond absorption, 163167 FePtiron oxide HNCs, 8485 Fermi’s golden rule, 160 Ferromagnetic materials, 250251 Ferromagnetic nanoparticles, 255256 Ferromagnetism, 250 Finite difference time-domain (FDTD) method, 270, 439441, 446447 Finite element methods (FEM), 443, 450 Fire-retardants coatings, 358360 intumescent coatings, 358359 nonintumescent coatings, 359360 Fluorescence spectroscopy, 8081 Fluorine-based fouling release coatings (FRC), 354355 Fluoropolymers, 354 Fly ash-based nanocomposite, 309 Fogging, 362363 Folic acid, 2629, 616617 Formaldehyde, 381 Fourier transformation, 8284 Fourier transforms infrared spectroscopy, 8284 Free space spontaneous emission, 159161 Frolich condition, 490 Fuel cells (FCs), 566567, 576577 Full width at half-maximum (FWHM), 202, 227229

G Gas sensor, 284286, 499500 device fabrication and measurements, 507508 mesoporous AGTiO2/SnO2 nanohybrids-based, 292297 Gaussian spectrum, 135 Gene transfection, 640641 Generalized Mie theory, 437438 GibbsThomson effect, 10 Glutathione, 1519, 614615 3-Glycidoxypropyltrimethoxy silanes (3-GPTMS), 2122 Gold, 517, 611 dispersion curves for, 434f experimental and fitted dielectric functions of, 434f

Gold hybrid nanoparticles, 535 Gold nanaparticles (AuNPs), 1519, 113, 142, 257, 501, 523, 535536, 550 -decorated MnO2 nanowires, 424t -based hybrids, 9698, 424t LSPR spectra of, 222223, 223f Gold nanorods, 257 thiol-mediated assembly of, 18f Gold nanoshells, 4344, 257, 259 Gold nanostructures, 257 Gold-coupled binary catalysts, 317318 Gold-supported CeO2 (Au/CeO2), 522 Gram-negative bacteria, 142, 145, 304306, 309310 Gram-negative bacterial cultures, 305307, 310311 Gram-positive bacteria, 141142, 304 Gram-positive bacterial cultures, 145 Graphene oxide (GO)-based magnetic nanocomposites, 309

H H2 evolution reactions (HER), 585586 H2 gas-sensing, 501, 512f H2O2 decomposition, 319, 331332 HaberBosch process, 316 Hamiltonian dipole interaction, 161163 Hard and Soft Acids and Bases (HSAB) theory, 14 Hard/soft-templating methods, 197 Haruta’s assertion, 316317 Heat generation model based on Ne´elian and Brownian relaxation, 251253 Heatingmeltingevaporation mechanism, 199200 Her2-negative breast cancer cells (MCF-7), 639640 Herceptin, 615, 639640 1,2-Hexadecanediol, 608609, 611 Hexadecylphosphonic acid (HDPA), 3334 Hierarchical nanostructure (HNS), 411413 TEM image of, 415f High resolution transmission electron microscopy (HRTEM), 66, 206, 503505 High-angle annular dark field (HAADF), 66, 503505 Highest occupied molecular orbital (HOMO), 461462 Hopping mechanism, 127, 130, 137138 Hot electrons, 157158, 163164, 375376, 384 Humidity and gas sensors, materials for, 284286 mesoporous silica, 284285 semiconductor metal oxide (MOx), 285286 Hyaluronic acid-conjugated coresatellite Fe3O4Au hybrid NPs, 613614 Hybrid capacitors (HCs), 550 Hybrid catalysts, 569570 Hybrid materials, 413419 Hybrid nanoparticles (HNPs) photochemical properties of, 111113

INDEX

photoelectrochemical (PEC) properties of, 110111 Hybrid nanostructures, 45 different types of, 4f Hybrid noble metalmetal oxide nanoparticles (NMMO NP), 137 Hybridization strategies to enhance the specific capacity of noble metalsbased supercapacitors, 424, 424t Hydrocaffeic acid (HCA)-conjugated chitosan, 611 Hydrocarbon oxidation, 322 Hydrogen, 382 clean energy carrier, 501 Hydrogen gas-sensing studies, 508511 Hydrogen peroxide reduction, catalysis of, 635636 Hydrothermal and solvothermal methods, 552554 Hyperthermia therapy, 613614 Hyperthermia treatments, 239 challenging problems, 259260 magnetic-induced thermal cancer therapy, 254257 noble metalFe3O4 hybrid nanoparticles, application of, 258259 photo-induced thermal cancer therapy, 257258 physical fundamentals of, 243254 magnetic-induced heating, 245247 photo-induced heating, 248250 Hysteresis curves, 251, 251f Hysteresis heating, 250251

I ICECOAT project, 361362 125 I-labeled magnetoferritin nanocages (M-HFn) NHs, 542543 Imaging modalities, 536537 MRI, CT, and optical imaging systems, 537 PET and SPECT imaging systems, 536537 Immunosensor, 636f Indium oxide (In2O3), 500 Indium tin oxide (ITO), 271 Inductively coupled plasma mass spectrometry (ICPMS), 7375 Inductively coupled plasma optical emission spectra (ICP-OES) analysis, 7375 Inelastic scattering, 249 Inherently safer process, 189 Inhomogeneous polarization of the nanoparticle, 202203 Inner filter effect (IFE), 636637 Intelligent coatings. See Smart coatings Intensity modulated photocurrent spectroscopy (IMPS) method, 592 Intercept 8500 LPP, 364t Interfacial growth method, 11 Interferogram, 8284

651

Intumescent coatings, 358359 Inverse Bremsstrahlung (IB) absorption coefficient, 199200 Ion implantation method, of synthesis of hybrid nanoparticles, 5859 Ion-exchange resin method, 4041 Ir (or Ru)-based oxide cocatalysts, 596597 Iron oxide, 6970, 246 Iron oxide nanoparticles, esterification of, 2629, 26f Iron pentacarbonyl, 610611 Ironcobalt nanoparticles, 246 Ironplatinum nanoparticles, 1920, 246 Irreducible oxide (IRO), 521 3-Isocyanatopropyltrimethoxysilane (IPTMS), 3033 ITO-RuO2 nanopillars, 397

J Janus-like nanoparticles, 45 JaynesCummings model, 160

K Kirkendall effect, 197, 208211, 552 Klebsiella pneumoniae, 145, 305

L LaMer burst nucleation, 7 LaMer theory, 78, 8f LangmuirHinshelwood model, 318319 Laplace equation, 489490 Laser ablation, 198199 Laser-induced heating synthesis of hybrid nanoparticles, 57, 195 AgAl hybrid nanoparticles, 208213 polymer effect on the synthesis of, 214217 alloy hybrid nanoparticles, 203 coreshell hybrid nanoparticles, 204205 experimental methodologies, 205207 laser heating, 205206 two-step laser ablation method, 206207 hybrid nanoparticles synthesized by two-step laser ablation, 218225 AgAl hybrid nanoparticles, 218221 AuAl hybrid nanoparticles, 221225 localized surface plasmon resonance (LSPR), 201 pulsed laser ablation in liquid (PLAL), 198199 fundamentals of, 199200 spherical nanoparticles, 201203 trimetallic hybrid nanoparticles, 225232 Al2O3@AgAu alloy hybrid nanoparticles, 226230 Al2O3@Au@Ag coreshell hybrid nanoparticles, 230232 Laser-induced plasma, 211

652

INDEX

Laser-induced plasma plume, 227 Lewis acids (LA), 14 Lewis bases (LB), 14 LifshitzSlyozovWagner (LSW) theory, 912 Ligands interactions with amine functional group, 1920, 3445 interactions with carboxylate functional group, 45 interactions with phosphate/phosphine functional group, 2021, 3334 interactions with silanol functional group, 2122, 3033 interactions with thiol functional group, 1519 Light-responsive coatings, 346 Linear scan voltammetry (LSV), 8586 Linear sweep voltammograms (LSV), 87f, 591f Lipopolysaccharide (LPS), 142 Liquid phase reduction deposition (LPRD), 524 Local density of states (LDOS), 162 Local hyperthermia, 242243 Localized surface plasmon (LSPs), 96, 157, 201 Localized surface plasmon resonance (LSPR), 80, 157158, 195196, 201, 208210, 222223, 226227, 231232, 265266, 431432, 435f, 458 absorption, 146 coupling, 9698, 105, 107 fundamental theory of, 435437 Lorentz oscillator model, 434 Lowest unoccupied molecular orbital (LUMO), 461462 Luminescent coatings, 348

M Magnetic and gold hybrid nanoparticles. See Radiolabeled theranostics Magnetic concentration-washing process, 636637 Magnetic fluid hyperthermia (MFH), 245246 Magnetic hyperthermia, 250, 254255, 258259 Magnetic iron oxide nanoparticles, 258. See also Superparamagnetic iron oxide nanoparticles (SPIONs) Magnetic nanohybrids (MNHs), 541, 543 Magnetic nanoparticles (MNPs), for hyperthermia treatment, 243, 245246, 250, 254259, 535536 Magnetic relaxation, 250 Magnetic-induced heating physical fundamentals of, 245247 anisotropy, 247 magnetic field strength and frequency, 247 particle size and concentration, 247 type of material, 246 viscosity, 247 physical models of, 250254

Magnetic-induced thermal cancer therapy, 254257 Magnetisms, 250 Magnetofection, 640641 Magneto-Optical Resonance Hyperthermia (MORH) method, 259 Manganese, 314315, 324326 Manganese oxide, 313319, 324326, 330, 550 chemistry of, 314315 Marine antifouling coating approaches, 354355, 355f Maxwell’s equations, 439, 441 in matter and dielectric constants, 432435 4-Mercaptobenzoic acid (4-MBA), 6970, 463 16-Mercaptohexadecanoic acid (MHA), 185 3-(Mercaptopropyl)-trimethoxysilane (MPTMS), 2122 Mesoporous Ag/TiO2 composite nanofilms, 386 Mesoporous AGTiO2/SnO2 nanohybrids-based gas sensors, 292297 Mesoporous silica, 284285 Mesoporous silica materials (MSM), 284 Metal@dielectric CSNPs, 269 Metal@metal oxide coreshell, 111113 Metal nanoparticles (MNPs), 201, 257, 266, 269, 517518 for SERS applications, 460f Metal oxide and hydroxide nanostructures, 397398 Metal oxide nanoparticles effect on localized surface plasmons of noble metal NPs, 96101 AgNPs-based hybrids, 99101 AuNPs-based hybrids, 9698 effect on the antibacterial behavior of noble metals, 145 interactions between ligands and surface of, 2334 wet-chemical synthesis of, 4143 Metal oxide nanostructures in SERS, 461462 Metal oxide semiconductor nanoparticles, 145146 Metal oxide semiconductors, 500 Metal oxides, 4143, 7375, 499500 Metal-based NPs, 535536 Metaldielectric oxide CSNPs, 279 Metal-loaded octahedral molecular sieves, 316317 Metalmetal oxides hybrid nanoparticles, 67f Metalsemiconductor Schottky junction, 158 Metalsupport interaction (MSI), 501, 517 Methacrylated phenolic melamine (MAPM), 359360 Methyl orange (MO), 378379 Methylimidazolium dodecylphosphonic acid (MImDPA), 3334 Micro-optical coherence tomography (μOCT) imaging, 640641 Microwave (MW)polyol method, 180 Mie scattering theory, 249, 269 Mie theory, 9698, 201203, 443446

INDEX

fundamental theory of, 435437 generalized Mie theory, 437438 Mixed calcium manganese (III) oxides, 327328 Mixed transition metal hydroxides (MTMHs), 411413 Mixed transition metal oxides (MTMOs), 411413 Mn(CH3COO)2, 6970 MnO2, role of in antibacterial action of AgMnO2 nanoparticles, 309310 MnO2 materials, 550551 MnO2 nanomaterials, 396398 MnO2 nanorod/AuNPs, 424t MnO2/Au/MnO2 nanospike (MAMNSP) supercapacitor, 557 MnO2-protected Ag nanoparticles, 474475 Modulation excitation spectroscopy (MES), 8284 Monoclinic CuO nanoflowers, 2326 MoS2Ag NP, 165166 Multi-functional coating, 363 Multifunctional nanomaterials, 6566 Multilayered nanoshells/nanomatryushkas, 195196 Multiply twinned particles (MTPs), 3638, 38f

N Nafion matrix, 324326 Najafpour group, 327328 NanoChar, 364t Nanocoatings market, 341342 Nanocomposite coating, 356 Nanocontainers, 343345, 365 Nano-Fe3O4 based hybrids, 186 Nanohybrids, 554555, 597, 598t for PET imaging, 542f quantification of metal content in, 7375 Nanomedicine, 544 theranostic, 612613 NANOMYTE, 364t Nanoparticle heating techniques, 243 Nanoparticle hyperthermia, 244 Nanoscale characterization, 65 crystal phase characterization through x-ray techniques, 7577 electrochemical characterization, 8586 morphological characterization, 6773 atomic number contrast scanning transmission electron microscopy (Z-STEM), 7072 scanning tunneling microscopy (STM), 7273 transmission electron microscopy (TEM), 6770 quantification of metal content in nanohybrids, 7375 spectroscopic characterization, 7985 Fourier transforms infrared spectroscopy, 8284

653

nuclear magnetic resonance (NMR) spectroscopy, 8485 UVVis and photoluminescence spectroscopy, 7982 surface characterization, 7779 Nanoshells, 257 Nanosized MnO2 spines on Au stems (NMSAS), 424t, 556 Nanosoldering, in laser ablation, 205206 Nanostructured coatings, 341342 Nano-TiO2-based hybrids, 5455, 186 Natural polymers, 378 Natural self-cleaning surfaces, 351f Nearest neighbor hopping (NNH) mechanism, 129 Ne´elian and Brownian relaxation heat generation model based on, 251253 Ni(OH)2 electrochemical performances of, 405f pseudocapacitance-behavior of, 402403 SEM images of, 404f Ni-based oxides and hydroxides, 402403 NiCo-based oxides or hydroxides, 411413 NiO, 403407 pseudocapacitance-behavior of, 402403 3D nanochannels of, 402403 4-Nitrobenzenethiol (4NBT), 171 Nitrogen-doped graphene (N-G), 568569 Nitrophenol reduction, catalysis of, 634635 NMMO NP systems, electronic transport by excitons in, 131133 Noble metal (NM) nanoparticles, 52, 5657, 265266, 268, 478479 Noble metalmanganese oxide hybrid nanocatalysts, 313, 315 applications in catalysis, 316331 alcohol oxidation, 322 CO oxidation, 316318 coupling reactions, 322323 decompositon of ozone, 319 electrocatalysis, 326330 epoxidation of olefines, 323 H2O2 decomposition, 319 hydrocarbon oxidation, 322 oxidation of VOCs, 319320 oxidative dehydrogenation, 323324 photocatalysis, 324326 photoelectrocatalysis, 330331 removal of bacterial pathogens, 321322 selective reduction and decomposition of NOx and SOx, 318319 sensing of VOCs, 320 chemistry of manganese oxides, 314315 future outlook, 331332

654

INDEX

Noble metalsemiconductor oxide hybrid nanostructures, excitonplasmon interactions in, 157 applications, 168173 femtosecond absorption, 163167 free space spontaneous emission, 159161 spontaneous emission in cavities, 161163 strong coupling, 162163 weak coupling, 162 Noble metalsFe3O4 hybrid nanoparticles, 109 Noble metalTiO2 nanohybrids, 597 Nonintumescent coatings, 359360 Nuclear magnetic resonance (NMR) spectroscopy, 8485 Nucleation and growth of nanoparticles, 78 GibbsThomson effect, 10 LaMer theory, 78, 8f LifshitzSlyozovWagner (LSW) theory, 1112 Numerical model, 269272 finite difference time-domain (FDTD) method, 270 simulation parameters, 271272 Numerical simulations, 431432

O Octadecylamine (ODA), 1920 1-Octadecene, 55, 610611, 627628, 630 Odor gases, 388 Odor-control filters photoactive Ag/TiO2 hybrid nanoparticles for, 388389 Olefines, epoxidation of, 323 Oleylamine, 1920, 5556 1D nanochain, 400402 1D nanostructured materials, 396 Optical absorption enhancement, in plasmonic organic solar cells, 266269 electromagnetic near-field resonance, 266268 far-field scattering, 268269 Optical imaging systems, 537 Optical spectroscopic technique, 80 Optically-active coatings, 349 Organic photovoltaic (OPV) devices, 127 Organic solar cells (OSCs), 265266 power conversion efficiency (PCE) of, 265266 Organic substrates, catalytic reduction of, 572 Organic transformations, 524526, 572 Ostwald ripening process, 910, 197, 210 Oxidative dehydrogenation, 323324 Oxide metal (OM) nanoparticles, 52 Oxides, noble metal nanohybrids for SERS, 474478 Oxides-based semiconductor, 8586 Oxygen evolution reaction (OER), 330, 585586 Oxygen reduction reaction (ORR), 328330, 566571

Oxynitride-based photoanodes, 586587 Oxynitride-based photocatalysts, 594595 Oxynitrides, 594595 Ozone, decompositon of, 319

P Palladium-based hybrid nanocatalysts, 565 oxygen reduction reaction (ORR), 566571 reduction of organic substrates, 572576 Palladiummetal oxide hybrid nanoparticles, sonochemical synthesis of, 189 chemical reactions involved in, 191193 PdCuO hybrid nanoparticles, 190191 Para-aminothiophenol (PATP), 478480 Paramagnetism, 250 Para-nitrothiophenol (PNTP), 478480 Pd/Ni bimetallic nanocatalyst, 574575 PdCu/C catalyst, 575 PdCuO hybrid nanoparticles, 189190 synthesis of, 190191, 190f PdCuO nanohybrids, 55, 182 PdFe3O4 nanohybrids, 186 PdTiO2 nanohybrid, 5556 Pennes’ bioheat transfer equation (PBHTE), 253254 Pennes’ model, 253254 Peptidoglycan, 141142 Perfectly matched layer (PML), 440441 Peripheral artery disease (PAD), 541 Perovskite solar cells (PSCs), 278279 Perovskite-based oxynitride, 594595 Phase sensitive detection (PSD), 8284 Phenylacetylene, hydrogenation of, 576, 576f Phosphate/phosphine functional group, ligands with, 2021, 3334 Phosphates/phosphonic acid salts, 3334 Phospholipid-polyethylene glycol, 610 Phosphonates, 3334 Photoactive Ag/TiO2 hybrid nanoparticles for odor-control filters, 388389 for water treatment, 376380 Photoactive nanosized heterostructures, 384 Photoanodes based on noble metalsemiconducting oxide nanohybrids, 598t Photocatalysis, 324326, 527529 Photocatalytic water splitting, 588f Photochemical method for nanohybrid synthesis, 184 Photochemical properties of HNPs, 111113 Photodynamic therapy, 614615 Photoelectrocatalysis, 330331 Photoelectrochemical cell, 8586 Photoelectrochemical characterizations, 8586 Photoelectrochemical properties of HNPs, 110111

INDEX

Photoelectrochemical (PEC) water splitting, 585 noble metalmetal oxide nanohybrids-based photoanode, 597 photoanode materials, 589597 BiVO4-based photocatalysts, 590593 cocatalyst selection, 595597 Fe2O3 oxide photocatalysts, 593594 oxynitride-based photocatalysts, 594595 TiO2-based photocatalysts, 589590 principles of, 587589 Photo-induced heating, physical fundamentals of, 248250 Photo-induced thermal cancer therapy, 257258 Photothermal therapy, 257258 Physical properties of HNPs, 96109 metal oxide NPs, effect of on band gap energy of semiconducting oxide NPs, 105107 on localized surface plasmons of noble metal NPs, 96101 on magnetic behavior of noble metalmagnetic oxide HNPs, 108109 on the specific capacitance of noble metalmetal oxide based supercapacitors, 109110 surface-enhanced Raman scattering (SERS) effect of HNPs, 101105 Plasmon frequency, 492493 Plasmonexciton coupling, 159, 169171, 173, 450 Plasmonic metal nanoparticles, 35 Plasmonic metals, 201 Plasmonic nanostructures, 35 Plasmonic organic solar cells, optical absorption modeling of, 265 dye-sensitized solar cells (DSSCS), 278279 mechanism of, 266269 electromagnetic near-field resonance, 266268 far-field scattering, 268269 metal nanoparticles (MNPs), 269 numerical model, 269272 finite difference time-domain (FDTD) method, 270 simulation parameters, 271272 perovskite solar cells (PSCs), 278279 results and discussion, 272278 effect of bare Ag NSs size versus period on absorption enhancement, 272273 effect of the active layer thickness on absorption enhancement, 275276 effect of the nature of the dielectric shell material on absorption enhancement, 276 effect of the shell thickness on absorption enhancement, 273275 influence of ZnO optical spacer layer and active layer material on absorption enhancement, 277278

655

Plasmonic perovskite solar cells utilizing noble metalmetal oxide hybrid nanoparticles, 487 results and discussion, 492495 theoretical analysis, 489492 dielectric constant of metal, 491492 polarizability of noncoated and coated nanosphere, 489491 Plasmonicsemiconductor photocatalytic systems, 45 PlatinAuFe3O4herceptin nanoparticles, 639640 Platinum and palladium nanostructures, 103104 Platinum nanoparticle, 113, 423424 -based hybrid materials, 424t Platinum-based catalysts, 567 Poisson PDF, 135 Poly(vinylidenedifluoride) (PVDF) polymer, 362363 Polydimethylsiloxane (PDMS), biodegradability measurements of, 357f Polydopamine (PDA), 471472, 612 Polyethyleneimine (PEI), 608609 -mediated synthesized Fe3O4 nanoparticles, 2629 -stabilized Fe3O4 nanoparticles, 4344 Polyethylene-oxide (PEO), 284285 Poly-L-histidine, 610 Polymer electrolyte membrane fuel cells (PEMFCs), 328, 566 Polymer/oxide nanocomposite coatings, 354 Polymeric stabilization, 13 Polyol method, 3638, 36t Polypropyleneoxide (PPO), 284285 Polyvinyl alcohol (PVA), 459 Polyvinylpyrrolidone (PVP), 3638, 6970, 206, 214 POSS-Ag/Ti materials, 387388 Potentiodynamic polarization technique, 352353 Pressure-sensing coatings, 348349 Probability distribution function (PDF), 134135 Propagating surface plasmon polaritons (PSPPs), 157158 Propagating surface plasmons (PSPs), 201 Prostate specific antigen (PSA), 636 Proton exchange membrane fuel cell (PEMFC), 328, 566 Prussian Blue (PB)-type electrocatalyst materials, 586587 Pseudocapacitors (PCs), 395396, 425, 549 P-type oxide semiconductors, 500 Pulsed laser ablation in liquid (PLAL), 57, 197200 Pyromellitic acid, 2629

Q Quadrupole mass spectrometry (QMS), 8689 Quantum dots. See Semiconductor nanocrystals Quartz crystal microbalance (QCM), 8689 Quasi-static Mie theory, 201203

656 R Rabi splitting, 450 Radiolabeled hybrid AuNPS, 538539 Radiolabeled hybrid MNPs, 541542 Radiolabeled hybrid NPs (RHNPs), 535536 Radiolabeled theranostics, 535 imaging modalities, 536537 MRI, CT, and optical imaging systems, 537 PET and SPECT imaging systems, 536537 radiolabeled AuFE3O4 hybrid nanoparticles, 543544 radiolabeled hybrid AuNPS, 537540 for PET imaging, 538539 for SPECT imaging, 539540 radiolabeled hybrid MNPs, 541543 for PET imaging, 541542 for SPECT imaging, 542543 Radiotherapy, 241 Raman scattering, 101. See also Surface-enhanced Raman scattering (SERS) Raman spectroscopy, 457458 Raman-based strain sensors, 348349 Rattle-structure particles, 197 Rayleigh scattering, 249, 457458 Reactive oxygen species (ROS), 373 Redox transmetallation method, 3940 Reducible metal oxide (RMO), 521 Reduction of organic substrates, 572576, 572f Regional hyperthermia, 242 Regional perfusion technique, 242 Remnant magnetization, 250251 Resorcinarene tetrathiol, 1519 Reversible additionfragmentation chain transfer (RAFT) technique, 612 Rhodamine 6G (R6G) as a model analyte, 464 Rietveld refinements, 76, 76f Roughened polydopamine (rPDA), 356357 RuO2 nanomaterials, 396397, 398f

S Salmonella abony, 305 Santa Barbara Amorphous (SBA-15), 284285 Saturation magnetization, 250 SBA-15, 286288 HRTEM image of, 288290, 289f low-angle XRD (LAXRD), 287f Scanning tunneling microscopy (STM), 7273 Scanning vibration electrode technique (SVET), 346 Schottky barrier, 131132, 163 Schottky barrier height (SBH), 512 Schro¨dinger equation, 132133 Screen-printed carbon electrode (SPCE), 637 Secondary ablation, 214

INDEX

Secondary phosphine oxide (SPO), 2021 Seed growth method, 5657, 180 Seed-mediated method, 39 Selective reduction and decomposition of NOx and SOx, 318319 Self-assembled monolayer (SAM), 1519 Self-cleaning coatings, 350354, 363 Self-cleaning foul release coatings, 356 Self-healing anticorrosive coatings, 346 Self-healing coatings, 343346 Self-polishing coatings (SPC), 354355 Semiconducting metal oxide (SMOx), 284286 Semiconducting oxide NPs, band gap energy of noble metal NPs’ effect on, 105107 Semiconducting oxides, hybridization of with noble metals, 146 Semiconductor metal oxide (MOx), 285286 Semiconductor nanocrystals, 34 Semiconductor/metal hybrid NPs, 375376 Semiconductors, 373374 Sensors, 508, 636637 Shell-isolated plasmonic NPs, 104105 Shift of plasmon resonance (SPR) band, 96 Sick Building Syndrome (SBS), 380381 Silanol functional group, ligands with, 2122, 3033 Silica, 572574 mesoporous silica, 284285 -coated silver CSNPs, 269 Silicone-based fouling release coatings, 354355 Silver nanaparticle (AgNPs), 3435, 5354, 99, 110, 142, 147148, 286288, 290291, 297, 304, 464, 466, 470471, 550552, 554555, 589590 antibacterial activity of, 358 -based hybrids, 99101, 424t -decorated MnO2 nanowires, 424t hierarchical heterostructures of Ag NPs-decorated MnO2 nanowires, 552f -loaded MnO2 nanosheets, 424t Simulation methods, 437443 discrete dipole approximation (DDA), 441443 finite-difference time-domain method (FDTD), 439441 finite element methods (FEM), 443 generalized Mie theory, 437438 Single particle mode, 7375 Single transition metal oxides/hydroxides, 397411 Single-particle or particle-mode ICPMS (spICP-MS), 7375 Single-photon emission computed tomography (SPECT), 535540 radiolabeled hybrid AuNPS for, 539540 radiolabeled hybrid MNPs for, 542543 Single-walled CNTs, 257258

INDEX

Singular nanoparticle, electronic characteristics of, 127128 Smart coatings, 341 applications and commercial viability of, 363, 364t classification of, 342363 active sensing coatings, 346349 antifingerprint coatings, 360363 antifogging coatings, 362363 anti-icing coatings, 361362 antireflective coatings, 361 bioactive coatings, 354358 easy-to-clean coatings, 350354 fire-retardants coatings, 358360 optically-active coatings, 349 self-healing coatings, 343346 Smart solution, 364t Smart window coatings, 349 Smart-repair coatings. See Self-healing coatings SnO2/SBA-15, 286288 HRTEM image of, 288290, 289f low-angle XRD (LAXRD), 287f Sodium 3-sulfonatemercaptopropane, 54 Sodium dodecyl sulfate (SDS), 554555 Solar energy-based technology, 585 Solgel method, 182183 synthesis of amine-functionalized silica nanopowder, 183 of synthesis of hybrid nanoparticles, 54 synthesis of trimetallic Au/Pt/Ag nanocompositesdoped amine-functionalized silica nanopowder (Au/Pt/Ag@SiO2), 183 synthesis of trimetallic nanoparticles Au/Ag/Pt, 182 Solgel-based electrospinning method, 403407 Sonochemical synthesis, 181182 of (Pd, Co)@Pt nanohybrids, 181 of hybrid nanoparticles, 5556 of Pdmetal oxide hybrid nanoparticles, 182 SPD-SmartGlass, 364t Specific absorption rate (SAR), 244, 257, 260 Spherical nanoparticles, 201203 Spontaneous emission in cavities, 161163 strong coupling, 162163 weak coupling, 162 Stabilization of nanoparticles, 1213 electrostatic stabilization, 1213 electrosteric stabilization, 13 polymeric stabilization, 13 Staphylococcus aureus, 305, 321322 Stimuli-responsive coatings. See Smart coatings Stokes scattering, 457458 Streptococcus epidermis, 305 Strong coupling, 161163 Strong metalsupport interactions (SMSIs), 569570

657

Subcutaneous Ehrlich carcinoma cells, 259 Sulfur-containing ligands, 1519 Supercapacitor, defined, 549 Superhydrophobic surfaces, 350, 365 Supermagnetism, 251252 Superparamagnetic iron oxide nanoparticles (SPIONs), 2629, 255256, 537538 SPION-based theranostic nanohybrids, 543 Superparamagnetism, 34 Supported catalyst, 190 Surface catalytic reaction, 158, 170 Surface functionalization of the nanoparticles, 256 Surface plasmon (SP) absorption, 104 Surface plasmon polaritons (SPPs), 487488 Surface plasmon resonance (SPR), 157, 196, 199200, 248, 374375, 383, 471472, 487488, 589590 Surface plasmons (SPs), 157, 257 Surface-enhanced Raman scattering (SERS), 34, 157, 431432, 447, 457 hybrids nanoparticles, SERS effect of, 101105 mechanism of, 458459 metal oxide nanostructures in, 461462 noble metalmetal oxide nanohybrids-based SERS substrates, 462480 in controlling selectivity of photocatalytic reactions monitored by SERS, 478480 Fe3O4, 470473 TiO2, 463465 ZnO, 466470 noble metal nanoparticle based SERS platforms, 459460 Suspended particle devices (SPD), 349 Synthesis of hybrid nanoparticles, 51 chemical synthesis methods, 5257 chemical reduction (CR) and photoreduction (PR) methods, 5254 coprecipitation method, 55 hydrothermal and thermal decomposition processes, 5455 seeding growth method, 5657 solgel method, 54 sonochemical synthesis, 5556 future trend, 59 physical fabrications, 5759 atom beam cosputtering method, 57 ion implantation method, 5859 laser-induced heating process, 57 Synthesis of SBA-15, 285f

T Targeted drug delivery, 615616 TDBC, 443444 Terephthalic acid, 2629

658

INDEX

Tert-butylamine borane (TBAB), 2021 Tetra(N-methyl) aminomethyl resorcinarene (TMAR), 1519 Tetraethoxysilane (TEOS), 183 Tetraethyl ortho silicate (TEOS), 3033 Tetrakis(hydroxymetyl)phosphonium chloride (THPC), 609610 Tetraoctylammonium bromide (TOAB), 1520 Theranostic application of Fe3O4Au hybrid nanoparticles, 612619 hyperthermia therapy, 613614 molecular imaging, 616619 dual-mode MR/CT imaging, 616617 dual-mode MR/FO imaging, 617618 dual-mode MR/PA imaging, 618619 photodynamic therapy, 614615 targeted drug delivery, 615616 Theranostic nanomedicine, 612613 Theranostics, 544, 607608 radiolabeled. See Radiolabeled theranostics Thermal ablation, 241242 Thermoregulation system, 242 Thermotherapy, 241242 Thermotolerance, 244 Thioglycolic acid (TGA), 611 Thiol capped gold nanoparticles, synthesis of, 15f Thiol functional group, ligands with, 1519 Thiol-functionalized Au NPs, 82 Thiol-functionalized ionic liquids (TFILs), 1519 Thiol-protected gold nanoclusters, 18f Three-dimensional (3D) porous nanomaterials, 396 Time-resolved photoluminescence spectroscopy, 81 TiO2 nanoparticles, 3033, 158, 373374 TiO2/Ag nanosponge materials, synthetic process for, 378f TiO2-based photocatalysts, 589590 TiO2noble metal nanohybrids for SERS, 463465 TiO2-supported gold catalysts, 526 Tip-enhanced Raman scattering (TERS), 157 Titanium dioxide (TiO2) nanocrystals, 2326 Titanium oxide, 276, 278 Titanium tetraisopropoxide (TTIP), 2629 Transition metal hydroxides (TMHs), 395396, 425 fundamentals of, 396 as the pseudocapacitor, 396397 Transition metal oxides (TMOs), 395411, 425, 569570 fundamentals of, 396 as the pseudocapacitor, 396397 Transmission electron microscopy (TEM), 6770, 191193 Transparent flexible electrodes (TFE), 556557 Tributyltin (TBT), 355

3-(Triethoxysilyl) propylsuccinic anhydride (SSA), 3033 Trimanganese tetraoxide (Mn3O4)silver (Ag) nanocomposites, 320 Trimesic acid, 2629 Trimetallic Au/Pt/Ag nanocomposites-doped aminefunctionalized silica nanopowder (Au/Pt/ Ag@SiO2), synthesis of, 183 Trimetallic hybrid nanoparticles, 225232 Trimetallic nanoparticles Au/Ag/Pt, synthesis of, 182 2,4,6-Trinitrotoluene reduction products, 322323 Trioctyl phosphine (TOP)-stabilized monodispersed Pd nanoparticles, 2021 Trisoctahedral coreshell Fe3O4Au hybrid NPs, 616 Trisodium citrate, 459 Trithiols, 1519 Tumor microvasculature (TMV), 541 2C Marine Sealant PRO, 364t 2D nanosheet, 400402, 411413 2D nanostructured materials, 396 2D noble metalmetal oxides NP assemblies theoretical modeling of electronic transport in, 133137 Two-step laser ablation, hybrid nanoparticles synthesized by, 218225 AgAl hybrid nanoparticles, 218221 AuAl hybrid nanoparticles, 221225 Two-step laser ablation method, 206207, 233

U Ullmann-type cross-coupling reactions, 322323 Ultra-Ever Dry, 363, 364t Ultrafast pumpprobe transient absorption spectroscopy, 164 Ultrasound-assisted process, 189, 193 Ultraviolet photoelectron (UPS) reactions, 181 UTHSCSA image processing program, 207 UV light irradiation, 373374, 382 UVVis and photoluminescence spectroscopy, 7982 UVvisible absorption spectroscopy, 80

V V2O5 nanoporous network (VNN), 409f charge/discharge curves for, 410f cycling behavior of, 410f Vacuum Rabi frequency, 159160 Valence band maxima (VBM) levels, 131 Van der WaalsLondon (VDWL) attraction, 1213 Vancomycin-modified Fe3O4@SiO2@Ag microflower, 307309 Variable range hopping mechanism (VRH), 129 Vibrational spectroscopy, 80

INDEX

Volatile organic compounds (VOCs), 380381 oxidation of, 319320 sensing of, 320 Volatile Organic Contents (VOC) emissions, 341342

659

ion-exchange resin method, 4041 polyol method, 3638, 36t redox transmetallation method, 3940 seed-mediated method, 39 Whole-body hyperthermia, 242

W Wastewater treatment plants (WWTP), 376377 Water oxidation, 324328, 596 Water photoelectrolysis, 585 Water splitting, 382, 383f photocatalytic, 588f photoelectrochemical. See Photoelectrochemical (PEC) water splitting Water-oxidation catalysts (WOCs), 327328 Water-soluble nanoparticles, 2021, 21f Weak coupling, 162 Wet-chemical redox method, 551552 Wet-chemical synthesis of metal oxide nanoparticles, 4143 of nanohybrids, 185 of noble metal-metal oxide hybrid nanoparticles, 4345 coreshell nanoparticles, 4344 dumbbell-shaped nanoparticles, 4445 Janus-like nanoparticles, 45 of noble metal nanoparticles, 3441 citrate reduction method, 3435 interfacial growth method, 11

X Xanthates, 1519 X-ray absorption near edge spectroscopy (XANES), 7778, 79f X-ray photoelectron spectroscopy (XPS), 77 X-ray techniques, crystal phase characterization through, 7577

Y Yee cells, 270, 439440, 440f

Z 0D nanoparticles, 396 Zinc oxide (ZnO), 2329 ZnCo2O4 nanorods/nickel foam synthesis, 413f ZnCo2O4 nanowire, 419f ZnO nanorods (ZNs), 557 ZnOMgO nanocomposites, 181 ZnO-nanorods/Au-doped-α-MnO2 (ZNs/ADM) nanocomposites exhibited, 557 ZnOnoble metal nanohybrids for SERS, 466470