Fabrication of TiO2 Nanotubes Using Electrochemical ...

Fabrication of TiO2 Nanotubes Using Electrochemical Anodization

Republic of Iraq Ministry of Higher Education and Scientific Research University of Baghdad College of Science

Fabrication of TiO2 Nanotubes Using Electrochemical Anodization A Thesis Submitted to the University of Baghdad, College of Sciences, Department of Physics as a Partial Fulfillment of the Requirements for the Degree of Master of Science in Physics

By

Haidar H. Hamdan Al-Eqaby B.Sc., Al Mustansiriyah University, 2007

Supervised by

Prof. Dr. Harith I. Jaafar

Lect. Dr. Abdulkareem M. Ali

2012 AD

1433 AH

1

‫‪Fabrication of TiO2 Nanotubes Using Electrochemical Anodization‬‬

‫( َوي َ ْسأَلون ََك َع ِن‬ ‫ُّالروحِ‬ ‫وح ِم ْن َأ ْم ِر قُ ِل‬ ‫ُّالر ُ‬ ‫َر ِّب َو َما ُأوتِ ُ ْيُت ِم َن‬ ‫الْ ِع ْ ِْل ا اَّل قَ ِلي ال)‬ ‫ِ‬ ‫صدق اهلل العلي العظيم‬ ‫سورة ا إَّلرساء‬ ‫الآية ‪58‬‬ ‫‪2‬‬


DEDICATION To: My mother My father My Brothers My Sisters My Uncle Mr. Jabbar My close friends My country beloved Iraq The martyrs of Iraq with all the love and appreciation.

Haidar

3


ACKNOWLEDGEMENTS Praise be to ALLAH, his majesty for his uncountable blessings, and best prayers and peace be unto his best messenger Mohammed, his pure descendant, and his family and his noble companions. First I would like to thank my family. Without their love and support over the years none of this would have been possible. They have always been there for me and I am thankful for everything they have helped me achieve. Next, I would like to thank my supervisors Prof. Dr. Harith I. Jaafar and lect. Dr. Abdulkareem M. Ali, Dr. Harith your help and guidance over the years which is unmeasurable and without it I would not be where I am today. Dr. Harith, what can I say, as graduate students we are truly fortunate to have you in the department. I thank you so much for the knowledge you have passed on and I will always be grateful for having the opportunity to study under you. I would like to thank Dr. Kamal H. Lateif, Dr. Baha T. Chiad, Dr. Shafiq S. Shafiq, Dr. Fadhil I. Shrrad, Dr. Kadhim A. Aadim, Dr. Issam, Dr. Sadeem, Dr. Qahtan, Dr. Muhammad K., Mr. Muhammad U., Mr. Issam Q., Mr. Muhammad J., Ms. Duaa A. and Ms. Hanaa J. for their assistance. This work would not have been possible without their help and input. I would also like to express my thanks to the deanery of the College of Sciences and head of Physics Department for their support ship to the student of higher education, the faculty is irreplaceable and their generosity to the student body is incomparable. Thank to Prof. Dr. Moohajiry (Tehran University) and his research group to provide me an Opportunity to work in his respectable laboratory (SEM Technician).

4


To my fellow graduate students, thank you for the good times throughout our years. Whether it was late nights studying or in the University, it was always a good time. I wish everyone good luck in the future and hope our paths cross again. In addition, I would like to thank my friends from Al-Mustansiriya University, especially the Assistant Lecturer Ms. Marwa A. Hassan. From the times that “escalated quickly” to showing me the way to “victory lane,” it seems like we've never missed a beat. Finally I would like to thank all of the other friends that I developed over the years. I am a lucky person to have the friendships that I have.

Haidar

5


Supervisor Certification We certify that this thesis titled “Fabrication of TiO2 Nanotube Using Electrochemical Anodization” was prepared by Mr. (Haidar H. Hamdan), under our supervision at Department of Physics, College of Science, University of Baghdad, as a partial fulfillment of the requirements for the degree of Master of Science in Physics.

Signature:

Signature:

(Supervisor)

(Supervisor)

Name: Dr. Harith I. Jaafar

Name: Dr. Abdulkareem M. Ali

Title: Professor

Title: Lecturer

Date: 5 / 3 / 2012

Date: 5 / 3 / 2012

In view of the available recommendation, I forward this thesis for debate by the Examination Committee.

Signature: Name: Dr. Raad M.S AL- Haddad Title: Professor Address: Head of Physics Department, Collage of Science, University of Baghdad. Date: 5 / 3 / 2012

6


Examination Committee Certification We certify that we have read this thesis entitled “Fabrication of TiO2 Nanotube Using Electrochemical Anodization” as an examine committee, examined the student Mr. (Haidar Hameed Hamdan) in its contents and that, in our opinion meets the standard of thesis for the degree of Master of Science in Physics. Signature:

Signature:

Name: Dr. Ikram A. Ajaj

Name: Dr. Raad S. Sabry

Title: Assistant Professor


Address: University of Baghdad

Address: Al-Mustansiriyah University

Date: 25 / 4 /2012

Date: 25 / 4 /2012

(Chairman)

(Member)

Signature:

Signature:

Name: Dr. Inaam M. Abdulmajeed

Name: Dr. Dr. Harith I. Jaafar


Title: Professor

Address: University of Baghdad Date: 25 / 4 /2012 (Member)

Address: University of Baghdad Date: 25 / 4 /2012 (Supervisor)

Signature: Name: Dr. Abdulkareem M. Ali Title: Lecturer Address: University of Baghdad Date: 25 / 4 /2012 (Supervisor) Approved by the Council of the College of Science. Signature: Name: Dr. Saleh M. Ali Title: Professor Address: Dean of the Science College, University of Baghdad Date: 27 / 4 /2012 7


Abstract This thesis describes the synthesis of self-organized titanium dioxide nanotube layers by an electrochemical anodization of Ti at different conditions (time, voltage, concentration of NH4F in electrolyte with glycerol, conductivity and water content) at room temperature (~25ºC) were investigated. In the current study, self-organized, vertically-oriented TiO2 nanotubes were successfully prepared by anodization method of a pure Titanium sheet (99.5%) using anodization cell is designed for first time in Iraq (Homemade) from Teflon material according to our knowledge to produce self-ordered Titanium nanotube in organic based electrolytes (glycerol based electrolyte) an electrolyte solution containing (0.5, 1, 1.5 and 2 wt.% NH4F) then added water (2 and 5Wt.% H2O) to (0.5wt.% NH4F) only with 15V. The range of anodizing time and potential were between 1-4hr. and 5-40V, where Wt.% represent weight percentage. Scanning electron microscopy (SEM), Atomic force microscopy (AFM) and (XRD) X-Ray diffraction were employed to characterize the morphology and structure of the obtained Titania templates, optical interferometer (Fizeau frings) method to tubes length measurement. For TiO2 nanotubes fabricated in non-aqueous electrolyte, the influence of the NH4F concentration on characteristics of nanotubes was studied. The results showed that when the NH4F concentration increased from 0.5 to 2wt.%NH4F, the tubes diameter, tubes length and roughness of TiO2 surface increased. Also the effects of the anodizing time and anodizing potential were studied. The formation of TiO2 nanotubes was very sensitive to the 8


anodizing

time.

Length

of

the tube increases with

increasing anodizing time and anodizing potential significantly. Either water content (2 and 5wt.%) with 0.5wt.% NH4F and the conductivity of electrolyte it is increasing the diameter, tube length and roughness of TiO2 surface increased, but simple increasing and formation of less homogenized TiO2 nanotube. The optimal conditions for TiO2 formation was found 15V at 4hr with 0.5wt.% NH4F due to we obtain on best results for tube diameter, tube wall thickness, tube length and more homogenized of TiO2 nanotubes.

9


Contents Title Dedication Acknowledgments Supervisor Certification Examination Committee Certification Abstract Contents List of Figures List of Tables List of Abbreviation

Page 3 4 6 7 8 10 13 15 16

Chapter One (Introduction and Literature Review) Paragraph 1-1 1-2 1-3 1-4 1-5

Title Physical and Chemical Science and Nanotechnology Nanomaterials Types of nano materials Literature Review Aim of this Work

Page 19 19 20 21 26

Chapter Two (Theoretical Part) Paragraph 2-1 2-2 2-2-1 2-2-2 2-2-3 2-3 2-4 2-5 2-6 2-6-1 2-6-1-1 2-6-1-2 2-6-1-3 2-7 2-8

Title Introduction to Nanotechnology Quantum Confinement in Semiconductors Quantum Dot Quantum Wire Quantum Well Summary of Quantum Confinement Effect Micro to Nano Materials Perspective Strategies of Making Nanostructures Properties of Titanium Dioxide (TiO2) Crystal Structure of Titanium Dioxide (TiO2) Titanium Dioxide (TiO2) in Rutile Stable Phase Titanium Dioxide (TiO2) in Anatase Metastable Phase Titanium Dioxide (TiO2) in Brookite Structure Synthesis Techniques of TiO2 nanotube Electrochemical Anodization processes 11

Page 28 30 30 30 30 31 32 33 34 35 35 36 37 39 39


2-9 2-10 2-11 2-11-1 2-11-2 2-11-3 2-11-4 2-11-5

Electrochemical Anodization of Metals Mechanism of TiO2 nanotubes array formation Factors affecting the formation of (TiO2) nanotube The effect of anodization potential The effect of electrolyte The effect of temperature The effect of annealing before and after anodizing The effect of distance between electrodes

40 42 46 46 47 48 49 49

Chapter Three (Experimental and Methods) Paragraph 3-1 3-2 3-2-1 3-2-2 3-2-3 3-3 3-3-1 3-4 3-4-1 3-4-2 3-5 3-5-1 3-5-2 3-5-3

Title Introduction Chemicals and Instrumentations Chemicals Instrumentations Processes flow chart of template synthesis Electrochemical Anodization system Electrochemical Anodization Cell Design Samples preparation Pretreatment of Ti samples TiO2 Nanotube preparation Characterization measurements X-Ray diffraction (XRD) pattern Atomic Force Microscopy (AFM) Scanning Electron Microscopy(SEM)

Page 52 52 52 53 54 55 55 56 56 57 59 59 60 61

3-5-4

Thickness measurement

62

Chapter Four (Results and Discussions) Paragraph 4-1 4-2 4-2-1 4-2-2 4-2-3 4-2-4 4-3 4-3-1

Title Introduction (I-V) characteristics of the electrochemical anodization process Effect of NH4F concentration Effect of anodizing potential Effect of water content Effects of conductivity Characterization of Titania nanotubes Structural and morphological characterization 11

Page 65 65 65 66 70 71 72 73


of Titanium nanotubes (TiO2) in (SEM and AFM) measurement Effect of NH4F concentration Effect of anodization time Effect of anodizing potential Effect of water content Effects of conductivity Structural characterization of Titania in (XRD) measurement Results of thickness measurement

4-3-1-1 4-3-1-2 4-3-1-3 4-3-1-4 4-3-1-5 4-3-2 4-5-3

73 77 80 84 89 89 97

Chapter Five (Conclusions and Future Work) Paragraph 5-1 5-2

Title Conclusions and Perspectives Suggestions for Future Research References

12

Page 100 101 103


List of Figures Figure (2-1) Figure (2-2) Figure (2-3) Figure (2-4) Figure (2-5) Figure (2-6) Figure (2-7)

Figure (2-8)

Figure (3-1) Figure (3-2) Figure (3-3) Figure (3-4) Figure (3-5 a, b) Figure (3-6 a, b) Figure (4-1)

Figure (4-2) Figure (4-3)

Figure (4-4)

Figure (4-5)

Figure (4-6) Figure (4-7) Figure (4-8)

Figure (4-9)

Figure (4-10)

Density of states as a function of energy for bulk material, quantum well, quantum wire and quantum dot. Schematic of nanostructure making approaches Rutile structure for crystalline TiO2 Anatase metastable phase for crystalline TiO2 Brookite structure for crystalline TiO2 Schematic set-up of anodization experiment Schematic diagram of the evolution of (TiO2) nanotubes in anodization: (a) oxide layer formation; (b) pore formation on the oxide layer; (c) climbs, formation between pores; (d) growth of the pores and the climbs; (e) fully developed (TiO2) nanotubes arrays Schematic representation of processes in (TiO2) nanotube formation during anodization: a) in absence of fluorides; b) in presence of fluorides Flow chart of Titanium nanotube synthesis Schematic and photograph of set-up illustrates of the anodization experiment with Teflon cell Schematic diagram of homemade Teflon cell Block diagram of atomic force microscope Set-up and Photograph illustrates the SEM Experimental arrangement for observing Fizeau fringes The current transient recorded during anodization during 2 hours at 15V in the glycerol + 0.5Wt. %NH4F and glycerol + 1.5Wt. %NH4F The current transient recorded during anodization during 2 hours at 15 and 40V in the glycerol + 0.5Wt. %NH4F Optical images of TiO2 grown on a Ti metal substrate during 2hr of anodization at 5V (a), 10V (b), 15V (c), 25V (d) and at 40 V (e) in 0.5wt. % NH4F. The current transients recorded during 2 hours of Ti anodization at 15V in glycerol / water / 0.5wt. %NH4F electrolytes with different weight ratios of glycerol: water The current transient recorded during anodization during 4 hours at 15V in the glycerol + 0.5Wt. %NH4F at a different conductivity of electrolyte SEM image of Ti anodized in (0.5 wt. % NH4F) in glycerol electrolyte at 15V for 2 h SEM image of Ti anodized in (1.5 wt. % NH4F) in glycerol electrolyte at 15V for 2 hr. AFM images of Ti anodized in (0.5 wt. % NH4F + glycerol) electrolyte at 15V for 2 h, (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chart AFM images of Ti anodized in (1.5 wt. % NH4F + glycerol) electrolyte at 15V for 2 hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chart SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 15V for 2hr 13

31 34 36 36 37 44 45

45

54 55 56 61 62 63 66

67 69

71

72

74 74 75

76

78




Figure (4-16)


Figure (4-20)

Figure (4-21)


SEM image of Ti anodized in (1.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 15V for 4hr AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 15V for 4hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chart SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 15V for 2hr SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 40V for 2hr AFM images of Ti anodized in in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 15V for 2hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chart AFM images of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 40V for 2hr. , (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chart SEM image of Ti anodized in (0.5 Wt. % NH4F + 99.5 Wt. % glycerol) electrolyte at 15 V for 2h SEM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. % H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h SEM images: (a) top-views and (b) cross-sectional images of Ti anodized in (0.5 Wt. % NH4F + 5 Wt. % H2O + 94.5 Wt. % glycerol) electrolyte at 15 V for 2h AFM images of Ti anodized in (0.5 Wt. % NH4F + 2 Wt. % H2O + 97.5 Wt. % glycerol) electrolyte at 15 V for 2h, (a) The 2D, cross section, (b) 3D and (c) porosity normal distribution chart AFM images of Ti anodized in (0.5% NH4F + 5% H2O + 94.5% glycerol) electrolyte at 15 V for 2h, (a) The 2D, cross section, (b) 3D and (c) porosity y normal distribution chart XRD pattern of Titania before and after annealing at temperatures 450°C and 3hr on Ti foil substrate XRD pattern of Titania before and after annealing at temperatures 530°C and 3hr on Ti foil substrate

14

78 79

81 81 82

83

85 86 86

87

88

91 94


List of Tables Table (2-1) Table (3-1) Table (3-2) Table (3-3) Table (3-4) Table (4-1) Table (4-2) Table (4-3) Table (4-4) Table (4-5) Table (4-6) Table (4-7) Table (4-8) Table (4-9) Table (4-10) Table (4-11)

Physical and chemical properties of the three TiO2 structures The chemicals and materials which used in process Origin, function and specification devices The condition of experimental work without water added The condition of experimental work with water added Color as a variable of anodizing TiO2 thickness The average Roughness and Pores diameter of TiO2 nanotubes under different proportion of NH4F The average Roughness and Pores diameter of TiO2 nanotubes under different anodization time The average Roughness and pores diameter of TiO2 nanotubes under different anodization voltage The results of TiO2 nanotubes under different proportion of glycerol and water content The average Roughness and pores diameter of TiO2 nanotubes under different proportion of glycerol and water content XRD results for Titania before annealing XRD results for Titania after annealing at temperatures 450°C and 3hr on Ti foil substrate XRD results for Titania before annealing XRD results for Titania after annealing at temperatures 530°C and 3hr on Ti foil substrate Result Titania thickness measurement by optical interferometer method

15

38 52 53 58 59 69 77 79 84 85 89 92 93 95 96 97


List of Abbreviations Symbols 0D 1D 2D 3D AAO ATO Ti TiO2 Pt NH4F Al Al2O3 Si Hf Zr Ta Nb N2 ZrO2 H2SO4 NaF Ta2O5 Na2SO4 HF H3PO4 H2O F ZnO NaOH DNA PH DI SEM AFM XRD ASTM t X ΔX λ

Meaning Zero-dimensional One-dimensional Two-dimensional Three-dimensional Anodic Aluminum Oxide Anodic Titanium Oxide Titanium Titania Platinum Ammonium Fluoride Aluminum Alumina Silicon Hafnium Zirconium Tantalum Niobium Nitrogen Gas Zirconia Sulfuric Acid Sodium Fluoride Tantalum Pentoxide Sodium Sulfate Hydrofluoric Acid Phosphoric Acid Water Fluoride Zinc Oxide Sodium Hydroxide Deoxyribonucleic Acid Acidity number Deionized Water Scanning Electron Microscopy Atomic Force Microscope X-ray Diffraction American Society of Testing Materials Thickness of Film Fringes Spacing Displacement Wavelength 16


UV UV–vis PVD CVC CVD M RF wt.% d n a 2Ɵ OCP SWNT MWNT

Ultraviolet rays Ultraviolet–visible Spectroscopy Physical Vapor Deposition Chemical Vapor Condensation Chemical Vapor Deposition Molarity Roughness Weight Percentage The Spacing Between Atomic Planes Refractive Index Lattice Constant Bragg Diffraction Angle Open-circuit Potential Single Wall Nanotube Multi Wall Nanotube

17


18


Chapter One Introduction and Literature Review 1-1- Physical and Chemical Science and Nanotechnology Over the last ten years, the physics, chemistry and engineering scientists interested in formation of self-organized nanostructures and nanopatterns which attracted a great scientific and technological interest due to its far-reaching and innumerable applications. Apart from these facts, the popularity and significance of these self- arranged nanostructures stem from the nature of their fabrication that relies on self- regulation processes (often called self-assembly). The main advantage of these processes is that it can represented a ``smart´´ nanotechnique. Therefore, it is not surprising that a large part of material's science nowadays targets these nano-scale fabrication techniques. Nanotechniques are a natural consequence of the necessity of achieving smaller and smaller electronic and photo-devices that satisfy the actual requests of the technological evolution. Within materials science, a highly promising approach to form selforganized nanostructured porous oxides is essentially based on a very simple process – electrochemical anodic polarization. Some important findings in this particular field include the growth of ordered Titanium dioxide (TiO2), nanoporous Aluminum oxide (Al2O3, Alumina) Silicon

[2]

[1]

and ordered macroporous

. Synthesis of all these materials has stimulated considerable research

efforts and given rise to many other materials to be processed in a similar fashion.

1-2- Nanomaterials Nanomaterials: A materials with dimensions below 100nm and they have at least one unique properties that is different than the bulk material and the characteristics can be applied in different fields such as nanoelectronics, pharmaceutical and cosmetic. Several methods have been studied in fabricating 19


these nanostructures, which include laser ablation [3], chemical vapor deposition (CVD) [3] and template-directed growth [4]. In order to integrate one dimensional nanomaterial into a device, a fabrication method that enables well-ordered nanomaterials with uniform diameter and length is important. Template-directed growth is a nanomaterials fabrication method that uses a template which has nanopores with uniform diameter and length

[5]

. Using chemical solutions or

electro deposition, nanomaterials are filled into the nanopores of the templates and, by etching the template, nanowires or nanotubes with similar diameter and length as the template nanopores are obtained. Because the size and shape of the nanomaterial depends on the nanoholes of the template, fabricating a template with uniform pore diameters is very important. TiO2 nanotube is particularly interested with its high potential for use in various applications, e.g., being used as gas-sensor [6], self-cleaning materials [7], and photoanode in dye-sensitized solar cells [8].

1-3-Types of nano materials Nanomaterials can be classified by different approaches such as; according to the X, Y and Z dimension, according to their shape and according their composition. The more classification using is the order of dimension into 0D (quantum dot), 1D (nanotube, nanowire and nanorod), 2D (nanofilm), and 3D dimensions such as bulk material composited by nanoparticles [9]. Nanotubes are made, sometimes, from inorganic materials such as oxides of metals (Titanium oxide, Aluminum oxide), are similar in terms of his structure to the carbon nanotubes, but the heaviest of them, not the same strong as carbon nanotube. Titanium nanotube can be described as a particles of Titania is requested about an axis, to take a cylindrical shape where both ends of the atoms associated with each slide to close the tube. 21


Be one of the ends of the tube is often open and one closed in the form of a hemisphere, as might be the wall of the tube individual atoms and is called in this case the nanotubes and single-wall (single wall nanotube) SWNT, or two or more named multi-wall tubes (multi wall nanotube) MWNT The tube diameter ranges from less than one nm to 100 nm (smaller than the width head of hair by 50,000 times), and has a length of up to 100 micrometers to form the nanowire. Of several forms of nanotubes may be straight, spiral, zigzag, or conical bamboo and so on. The properties of these tubes are unusual in terms of strength and hardness and electrical conductivity, and others [10]. Titania nanotube is 1D type nanomaterails that is means existing only one micro or macro dimension which represented by the length of the tube.

1-4- Literature Review Since its commercial production in the early twentieth century, Titanium dioxide (TiO2) has been widely used as a pigment [11] and in sunscreens paints [12]

, ointments [13], toothpaste [14], etc. In 1972, Fujishima and Honda discovered

the phenomenon of photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light [15]. Since then, enormous efforts have been devoted to the research of TiO2 material, which has led to many promising applications in areas ranging from photovoltaics and photocatalysis to photo-electrochromics and sensors

[16]

. These applications can be roughly divided into “energy” and

“environmental” categories such as water purification, pollution prevention, antibacterial, and purify the air. Many of which depend not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 material host (e.g., with inorganic and organic dyes) and on the interactions of TiO2 materials with the environment. An exponential growth of research activities has been seen in nanoscience and nanotechnology in the past decades

[17]

. New physical and chemical

properties emerge when the size of the material becomes smaller and smaller, 21


and down to many serious environmental and pollution challenges. TiO2 also bears tremendous hope in helping ease the energy crisis through effective utilization of solar energy based on photovoltaic and water-splitting devices [18]. As continued breakthroughs have been made in the preparation, modification, and applications of TiO2 nanomaterials in recent years, especially after a series of great reviews of the subject in the 1990s. We believe that a new and comprehensive review of TiO2 nanomaterials would further promote TiO2-based research and development efforts to tackle the environmental and energy challenges that we are currently face it. Here, we focus on recent progress in the synthesis, properties, modifications, and applications of TiO2 nanomaterials [19]. In 1991, Zwilling et al.

[20]

first reported the porous surface of TiO2 films

electrochemically formed in fluorinated electrolyte by Titanium anodization. In 1999 it was reported that porous TiO2 nanostructures could be fabricated by electrochemically anodizing a Ti sheet in an acid electrolyte containing a small amount of hydrofluoric acid (HF) [21]. Since then, many research groups have paid considerable attention to this field, because anodization opens up ways to easily produce closely packed tube arrays with a self-organized vertical alignment. A decade later Gong and co-workers

[22]

synthesized the uniform and

highly-ordered Titanium nanotube arrays by anodization of a pure Titanium sheet in a hydrofluoric acid (HF) aqueous electrolyte. They obtained nanotubes directly grew on the Ti substrate and oriented in the same direction perpendicular to the surface of the electrode, forming a highly ordered nanotubearray surface architecture. In 2001 Dawei Gong et al. [23] fabricated Titanium dioxide nanotubes by anodization of a pure Titanium sheet in an aqueous solution containing 0.5 to 3.5 wt. % hydrofluoric acid. These tubes are well aligned and organized into high-density uniform arrays. While the tops of the tubes are open, the bottoms of 22


the tubes are closed, forming a barrier layer structure similar to that of porous Alumina. The average tube diameter, ranging in size from 25 to 65 nm, was found to increase with increasing anodizing voltage, while the length of the tube was found independent of anodization time. Later in 2003 Oomman K. Varghese et al. [24] used anodization with a time-dependent linearly varying anodization voltage and made films of tapered, conical-shaped Titania nanotubes. The tapered, conical-shaped nanotubes were obtained by anodizing Titanium foil in a 0.5% hydrofluoric acid electrolyte, with the anodization voltage linearly increased from 10–23 V at rates varying from 0.43- 2.0 V/min. The linearly increasing anodization voltage results in a linearly increasing nanotube diameter, with the outcome being an array of conical-shaped nanotubes approximately 500 nm in length. Evidence provided by scanning electron-microscope images of the Titanium substrate during the initial stages of the anodization process enabled them to propose a mechanism of nanotube formation. In 2005 Seung-Han Oh et al. [25] a vertically aligned nanotube array of Titanium oxide fabricated on the surface of titanium substrate by anodization. The nanotubes were then treated with NaOH solution to make them bioactive, and to induce growth of hydroxyapatite (bone-like calcium phosphate) in a simulated body fluid. Such TiO2 nanotube arrays and associated nanostructures can be useful as a well-adhered bioactive surface layer on (Ti) implant metals for orthopaedic and dental implants, as well as for photocatalysts and other sensor applications. In 2006 Aroutiounian et al. [26] the semiconductor photoanodes made of thin film Titanium oxide were prepared by anodization of Titanium plates in hydrofluoric acid solution at direct voltage at room temperature. The influence of the change of Titanium oxide film growth conditions (concentration of hydrofluoric acid, voltage, duration of anodization process) and subsequent heat 23


treatment of films on a photocurrent and current-voltage characteristics of photoelectrodes were investigated. In 2007 V. Vega et al.

[27]

synthesized Self-aligned nanoporous TiO2

templates synthesized via dc current electrochemical anodization have been carefully analyzed. The influence of environmental temperature during the anodization, ranging from 2ºC to ambient, on the structure and morphology of the nanoporous oxide formation, has been investigated, as well as that of the (HF) electrolyte chemical composition, its concentration and their mixtures with other acids employed for the anodization. Arrays of self-assembled Titania nanopores with inner pores diameter ranging between 50 and 100 nm, wall thickness around 20–60 nm and 300 nm in length, are grown in amorphous phase, vertical to the Ti substrate, parallel aligned to each other and uniformly disordering distributed over all the sample surface. In 2008 Hua-Yan Si et al. [28] studied the effects of anodic voltages on the morphology, wettability and photocurrent response of the porous Titanium dioxide films prepared by electrochemical oxidation in a hydrofluoric acid (HF)/chromic acid electrolyte have been studied. The porous Titanium dioxide films showed an increased surface roughness with the increasing anodizing voltages. By controlling the films morphology and surface chemical composition, the wettability of the porous Titanium dioxide films could be easily adjusted between superhydrophilicity and superhydrophobicity. X-ray diffraction (XRD), Raman and UV–vis spectroscopy revealed that the obtained Titanium dioxide films were in anatase phase. The Titanium dioxide films showed clear photocurrent response, which decreased dramatically with the increase of the anodizing voltages. This study demonstrates a straightforward strategy for preparing porous Titanium dioxide films with tunable properties, and

especially

emphasizes

the

importance

morphology/properties relationship. 24

of

understanding

their


In 2009 Michael et al.

[29]

anodized Titanium-oxide containing highly

ordered, vertically oriented TiO2 nanotube arrays is a nanomaterial architecture that shows promise for diverse applications. An anodization synthesis using HFfree aqueous electrolyte solution contains 1 wt.% (NH4)2SO4 plus 0.5 wt.% NH4F. The anodized TiO2 film samples (amorphous, anatase, and rutile) on Titanium foils were characterized with scanning electron microscopy and X-ray diffraction. Additional characterization in terms of photocurrent generated by an anode consisting of a Titanium foil coated by TiO2 nanotubes was performed using an electrochemical cell. A Platinum cathode was used in the electrochemical cell. In 2010 Hun Park et al. [30] studied the properties of TiO2 nanotube arrays which are fabricated by anodization of (Ti) metal. Highly ordered TiO2 nanotube arrays could be obtained by anodization of (Ti foil in 0.3 wt.% NH 4F contained ethylene glycol solution at 30°C. The length, pore size, wall thickness, tube diameter etc. of TiO2 nanotube arrays were analyzed by field emission scanning electron microscopy. Their crystal properties were studied by field emission transmission electron microscopy and X-ray photoelectron spectroscopy. In 2011 S. Sreekantan et al. [31] formed Titanium oxide (TiO2) nanotubes by anodization of pure Titanium foil in a standard two-electrode bath consisting of ethylene glycol solution containing 5 wt.% NH4F. The PH of the solution was ∼ 7 and the anodization voltage was 60 V. It was observed that such anodization condition results in ordered arrays of TiO2 nanotubes with smooth surface and a very high aspect ratio. It was observed that a minimum of 1 wt. % water addition was required to form well-ordered TiO2 nanotubes with length of approximately 18.5 μm. As-anodized sample, the self-organized TiO2 nanotubes have amorphous structure and annealing at 500oC of the nanotubes promote formation of anatase and rutile phase.

25


1-5- Aim of this Work Fabrication of forest of Titania nanotubes via electrochemical anodizing of pure Titanium foil using electrochemical Teflon cell designed for first time in Iraq according to our knowledge to produce self-ordered Titanium nanotubes. Investigate the effects of some process parameters such as; time, voltage and electrolyte composition on the diameter and length of fabricated nanotubes by nanoscopic instrument atomic force microscopy (AFM), scanning electron microscopy (SEM), (XRD) spectroscopy and optical interferometer method.

26


27


Chapter Two Theoretical Part 2-1- Introduction to Nanotechnology Nanotechnology or nanoscale science is concerned with the investigation of matter at the nanoscale, generally taken as the 1 to 100 nm range. The breakthrough in both academic and industrial interest in these nanoscale materials over the past ten years has been interested because of the remarkable variations in solid-state properties

[32]

. The “nano” as word means dwarf (small

man) in Greek, nano as SI unit refers amount of 10-9, such as nanometer, nanolitter and nanogram [33]. As such a nanometer is 10-9 meter and it is 10,000 times smaller than the diameter of a human hair. A human hair diameter is about 50000 nm (i.e., 50× 10-9 meter) in size, meaning that a 50 nanometer object is about 1/1000th of the thickness of a hair [33]. Nanoparticels are considered to be the building blocks for nanotechnology and referred to particles with at least one dimension less than 100nm. Particles in these size ranges have been used by several industries and humankind for thousands of years [34]. The nanotechnology deals with the production and application of physical, chemical, and biological system at scales ranging from individual atoms or molecules to submicron dimension, as well as the integration of the resulting nanostructures into larger system [35]. Nanometer–scale features are mainly built up from their elemental constituents. Examples in chemical synthesis, the spontaneous self –assembly of molecular clusters (molecular self- assembly) from simple reagents in solution. The biological molecules (e.g., DNA) are used as building blocks for the 28


production of zero- dimensional nanostructure, and the quantum dots (nanocrystals) of arbitrary diameter (about 10 to 10 5 atoms). When the dimension of a material is reduced from a large size, the properties remain the same first, and then small changes occur, until finally, when the size drops below 100nm, dramatic changes in properties occur [35]. At the nanoscale, objects behave quite differently from its behave at larger scales, such as increased hardness values of metallic materials and their alloys as well as increase the strength to face the stresses of different loads, located it, either the ceramic material increases the durability and tolerance to stresses impact. As for the electrical properties have a great ability to connect and increase the diffusion and interactions in nano-seconds and the speed of ion transport [36]. Nanotechnology manipulates matter for the deliberate fabrication of nanosized materials. These are therefore “intentionally made” through a defined fabrication process. The definition of nanotechnology does not generally include “non-intentionally made nanomaterials”, that is, nano-sized particles or materials that belong naturally to the environment (e.g., proteins, viruses) or that are produced by human activity [36]. A nanomaterial is an object that has at least one dimension in the nanometre scale

[36]

. Nanomaterials are categorized according to their

dimensions into three classes [37]: 1. Zero-dimension confinement (quantum dot). 2. One-dimension confinement (quantum wire). 3. Two-dimensions confinement (quantum well). 4. Three -dimensions confinement (bulk).

29


2-2- Quantum Confinement in Semiconductors In the last few years a great effort has been devoted to the study of low dimensional semiconductor structures. The reduction of the dimensionality causes several changes in the electronic and excitonic wave functions and these features can be used, at least in principle, to produce novel microelectronics [38]. In bulk semiconductor materials, the energy levels of both conduction band and valence band are continuous, with electrons and holes moving freely in all directions. As the dimensions of the material shrink, effect of quantum confinement will be seen, this effect is seen in the objects, when size of object is less than de Broglie wavelength of electrons. Here, classical picture of electrons trapped within hard wall boundaries is not unrealistics. Three different types of confinement that have been realized among semiconductors materials are described below [39].

2-2-1- Quantum Dot Typically, the dimension is ranging from 1 to 100 nanometers. A quantum dot has the most restricted confinement in all three dimensions of the electrons and holes. It is working under the condition (λF >>Lx, Ly, Lz), where λF represent the Fermi wavelength [40]. As shown in figure (2-1). An important property of a quantum dot is the large surface to volume ratio [39].

2-2-2- Quantum Wire A quantum wire is a structure in which the electrons and holes are confined in two dimensions, as shown in figure (2-1) such confinement allows free electrons and holes behavior in only one direction, along the length of the wire [39]. These properties give rise to produce many nanoproductions which can be considered as a quantum wire (λF> Lx, Ly and Lx, Ly