SYNTHESIS AND CHARACTERIZATION OF

SYNTHESIS AND CHARACTERIZATION OF CYCLODEXTRIN CAPPED AU AND AG NANOPARTICLES

YANG JIEXIANG (B.Sc. (Hons.), NUS)

A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE 2009

ACKNOWLEDGEMENT

I wish to express my heartfelt gratitude to Assoc. Prof. Fan Wai Yip, my supervisor and mentor, who has provided much advice and guidance in my graduate work. I am grateful for the assistance and supervision that he has provided in the most encouraging and sincere manner throughout my term as his student.

I am thankful for the help rendered by my fellow members of our research group, and would like to extend my graitude to them; Li Shu Ping, Tan Sze Tat, Ng Choon Hwee Bernard, Chong Yuan Yi, Kee Jun Wei, and Toh Chun Keong.

I am also appreciative of the support given by Mdm. Loy Gey Luan of the Electron Microscopy Unit in experiments of TEM imaging; Mdm Adeline Chia nd Mdm Patricia Tan from the Physical Chemistry Laboratory for their invaluable aid in my daily work.

Finally, I wish to acknowledge the National University of Singapore for awarding me a research scholarship and granting me the opportunity to attain my master degree.

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Table of Contents Acknowledgement

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Table of Contents

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Summary

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Chapter 1 Introduction ............................................................................... 1 1.1 Synthesis and Characterization of Nanostructured Materials.

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1.1.1 Synthesis of Nanostructured Materials

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1.1.2 Characterization of Nanostructured Materials

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1.2 Synthesis of Metallic Nanostructured Materials in Solution.

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1.2.1 Synthesis of Monodispersed Metallic Nanoparticles

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1.2.2 Synthesis of Bimetallic Nanoparticles

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References

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Chapter 2 Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with beta-Cyclodextrin. ................................... 20 2.1 Introduction

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2.2 Experimental Section

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2.3 Results and Discussion

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2.4 Conclusion

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References

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Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles ............................................................................ 36 3.1 Introduction

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3.2 Experimental Section

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3.3 Results and Discussion

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3.4 Conclusion

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References

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SUMMARY

This thesis illustrates the studies made on synthesis and characterization of nanoparticles. Chapter 1 provides an introductory outline on the various preparation and characterization of nanoparticles with attention given to materials that contain metals. Preparation of silver, gold and copper monometallic nanoparticles protected by beta-cyclodextrin was attempted and presented in Chapter 2. The synthetic steps involve the effective use of beta-cyclodextrin as both a reducing and stabilizing agent, and results in self-assembled chains. The factors that cause such self-assembly were explored and discussed. In Chapter 3, stable silver-gold monodispersed alloy nanoparticles were synthesized via a co-reduction method. These alloyed nanoparticles were used as precursor for the formation of gold core and silver iodide shell particles, with the addition of iodine. The synthesis works on a diffusion mechanism, where silver atoms move towards the surface to form silver iodide shell in reaction with the iodine. Characterization of these materials prepared was made using TEM, UV-visible absorption spectroscopy, and electron diffraction.

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Chapter 1: Introduction

Chapter 1 Introduction

The interests in the field of nano science and technology have been the focus of research by scientists and engineers, and stem from the discovery of unique properties that belong to particles of this scale. These properties differ from those observed from bulk materials, which offer a new direction for chemists to explore. Nanostructured materials are defined by the average size of the particles in the order of nanometer (10-9 m) 1, with common studies centred on particles with size around 5nm to 100nm. Historically, colloidal gold solution prepared from chemical reduction was one of the first scientific observations of nanoparticles, which was reported in 1857, by Farady2. However, the beginning of modern studies of nanoscale science is credited to Richard Feynman3, who suggested a branch of science which is dedicated in the manipulation of smaller units of matter. This has been regarded as the pioneer scientific discussion and proposition of nanotechnology and science. Since then, numerous researchers continue to focus their investigations in the area of nanotechnology, with emphasis on preparation, characterization and application of nanomaterials. This introductory chapter‟s purpose is to provide an insight into the developments in nanoscale science, especially on the synthesis and characterization of nanostructured metal particles.

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1.1 Synthesis and Characterization of Nanostructured Materials. 1.1.1 Synthesis of Nanostructured Materials

Numerous methods have been used for the synthesis of nanomaterials to yield particles of various size and shapes. Generally, all of the reported preparation methods can be divided into two distinct categories, top-down and bottom-up approaches. The bottom-up approach focuses mainly on chemical methods of preparation, whereby the particles are “built” from individual atoms or ions. As for the top-down approach, the routes involves the utilisation of bulk material as starting reagents, and are broken down to nanoparticles typically with physical methods. Figure 1.1 illustrates both approaches4.

Figure 1.1 Two approach to prepare nanoparticles. A comparison of physical(Topdown) and chemical (bottom-up) methods.4

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A common top-down preparation is attrition or milling under controlled conditions, which can produce nanoparticles of sizes in the range of few hundred to tens of nanometers. This method makes use of rigorous physical deformation to generate nanostructured materials from large bulk compounds induced by the application of high mechanical energy. It involves grinding down the starting material to form “ultrafine powder” of individual size in the nanoscale. Although this method has much value in the production of large quantities of nanoparticles for commercial use, it suffers issues of contamination from the process of grinding as a serious disadvantage5. Another physical method that is often used, is the procedure based on evaporating metal and mixing it with a flow of inert gas in a confined chamber maintained at a certain temperature. The metal vapour cools through collisions with the inert gas, and nucleates to form the nanoparticles. This method has a variety of adaptation that has been examined in literature6. However, top-down preparations often suffer from poorer control on the particle size and shape distribution, and this result in lesser precision of the synthesis. Synthetic route that takes the bottom-up approach typically involve the use of wet chemical methods that can control size and growth of the particle, with suitable agents used to stabilize the colloid formed. Liquid or solution phase is often preferred as the medium for chemical fabrication routes, as true nanoscale systems can be established due to the higher rate of diffusion as compared to solid phase, allowing controlled growth and isolation of individual particles to occur. The key advantage of chemical processes in nano synthesis is the result of excellent homogeneity of the particles that can be readily dispersed in a suitable chemical environment. Furthermore, there is the flexibility of designing and preparing new nanoparticles as precursor, and later be refined to the final desired product. The ability of bottom-up 3


approach through chemical methods to synthesize monodispersed particles and achieve good stability makes it a preferred strategy for preparing nanomaterials in scientific research. Chemical reduction is one of the suitable bottom-up approaches, used most extensively in the liquid phase preparation of well dispersed nanoparticles, in both aqueous and organic solution7. It involves a precursor which is often a multi-element compound that contains the component of the final product. Mixing of suitable reactant will result in the reduction of the precursor and lead to the formation of the nanoscale materials as insoluble precipitate which can be collected or suspended in the medium. This strategy focuses on constructing at the molecular or atomic level, allowing ions and molecules to be directed into the nanoparticles of the preferred morphology. However, there are significant challenges in this approach that scientists have seek to resolve. Nucleation of the precipitate can be a key concern, with possible formation of undesired agglomeration. Hence, the reaction conditions have to be tuned to avoid the unwanted aggregation of nanoparticles. Temperature, pH, reaction time, concentration and used of stabilizing agent have been shown as factors to ensure the reproducibility of the performed synthesis8. Moreover, to form monodispersed particles of narrow size distribution, it is important to allow the initial reduction to occur at almost the identical time6, which require use of suitable mixing and experimental techniques to ensure this. The degree of dispersion and resultant stability overtime is often a factor of the solvent and capping agent used. There also exists the issue of contamination of the final particles from the reagents used, as typical wet chemical synthesis involves the reaction occurring in a single medium. This has been mitigated with procedures to purify the product with repeated dilution and extraction, and other tactics such as phase-transfer. Thus chemical reduction have progressed on 4


to an attractive method for the preparation of monodispersed nanoparticles, due to the availability of chemical reactants, simplicity in steps and reproducibility. It also provide many opportunities to finetune the parameters to form numerous novel nanomaterials that exhibit many interesting properties for further application. Another viable and innovate fabrication method is the application of photolysis. It can be used as a pure physical method, such as laser-ablation of conventional metal or alloy material to form nanparticles9, or utilized as a source to induce reduction in chemicals during preparation. Selected precursor can also be dissolved in solution and decomposed through photolysis in a photochemical process to form nanoparticles with the use of capping agene as stabilizers. The use of photolysis eliminates the concern of contamination from the reducing agent and reactants and provides reproducible results, thus have gained much acceptance progressively in synthesis of nanoscale materials7.

1.1.2 Characterization of Nanostructured Materials The ability to observe and analyse experimental results is essential in the process of making new scientific findings. Particularly, the characterization of nanoparticle is a key requirement in the evidence for nanoscale material, and crucial in understanding of the individual particles formed. Further analysis will be useful in comprehension and modification of the synthesis process and applications. Traditionally, scientist often described the unique colour displayed by suspended nanoparticles, which is a clear difference from its opaque bulk material. These appearance of colour is attributed to the surface plasmon effect of the extremely small particles, and it‟s a characteristics affected by size, medium and elements present in the nanoparticles. This optical property can be easily probed by UV-visible

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spectroscopy, and have found application for nanoparticles as simple probe of other chemicals10. Although UV-visible spectroscopy can be used to determine the formation of nanoparticles, it does not satisfy the need of “direct” observation of individual particles to validate the formation, actual size and shape. In order to achieve direct imaging, microscopes have to be used, and it is the considered a major technique for determining the nanoparticle size11. Conventional microscopic probe that make use of visible light is inadequate to observe objects smaller than the wavelength of visible spectrum, thus electron microscope, which uses accelerated electrons instead of photons to produce the required image, is primary used for observing and analysing nanoparticles. Electron microscope is commonly divided into two types, the scanning electron microscope (SEM) and transmission electron microscope (TEM). While both types have similar working principle of electrons as illumination source, they differ in the operating principles and are generally used for different forms of samples. SEM uses the energy lost by the incident electron to form the image, with the electron beam scanning through a selected area of the sample. It has the versatility of imaging samples in a wide range of size, and provides a three-dimensional shape of the particles. However, it has a reduced resolution as compared to TEM, and less suitable of observing monodispersed nanoparticles. TEM on the other hand uses the electrons that are deflected or absorbed by the particles as a basis to form the “silhouette” image, while the rest of the electrons will be transmitted and form the bright field background. This image can be observed real-time through a phosphorous screen, or electron sensitive video sensors. Capturing of the image can be done through traditional films or using charge-coupled device (CCD) as image sensors. These recorded image Although TEM primary produces a two-dimensional image, it offers 6


better resolution and incorporates other analytical tools such as electron diffraction (ED) and energy dispersive x-ray (EDX) while the sample is under observation. It has been extensively reviewed and proposed that TEM is one of the most efficient and versatile tools for the characterization of nanoscale materials, and essential in size and shape analysis of the prepared particles11,12. Besides the observation of the particles, there is a necessity to accurately identify determine the elements that are present in the nanoparticles. During the TEM imaging, two common microanalysis methods, EDX and ED, can be carried out to survey the targeted area. The electron beam used can interact with the nano material, and emit characteristic X-rays. EDX is a form of spectroscopy that detects the energy and intensity of these X-ray that are released, and record them as spikes or peaks in the spectrum. Elements that are suspected to be present can have their characteristic X-ray lines matched to position of these peaks, with the composition derived from the relative intensity and area of the peaks. This analytical tool is very useful as an evidence of the proposed material present, and provides accurate information on the ratio of elements present as well. ED is performed by directing the electron beam at the selected area in higher intensity to observe the resultant diffraction patterns. Measurement and analysis of these patterns will give the d spacing of the material, and in turn provide an insight to the degree of crystallinity and supporting evidence of the exact compound present. The exact indices can be matched with the individual rings of the pattern, and highlight the characteristic of the particles. Examples of TEM images, ED patterns and EDX spectrum are presented in Figure 1.2.

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(a)

(b)

(c)

(d)

Figure 1.2 TEM images (a)-(b) and EDX spectrum (c) of the palladium nanoparticles13. ED pattern of face-cubic copper nanoparticles14.

X-ray diffraction (XRD) was originally targeted at analysing singe crystals, with near perfect structures and sizes of around 0.1mm, by exposing the sample with X-ray radiation, and the peaks in the spectrum indicates the angles of the scattering. These angles can be compared with standards in literature and identify the phase and

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molecular structure of the crystals. In analysis of nanoscale particles, powder diffractometer is used with a convergent beam, resulting in higher sensitivity and resolution. At the same time, the sample has to be finely grounded to ensure all the possible crystal planes are exposed to the X-ray beam. However, particles that are less than 50nm in diameter can experience peak broadening, and may not be easily differentiated from the background15,16. The bulk powder sample that is examined may contain amorphous and poorly ordered particles, which render the analysis difficult as well. Fortunately, the broadening is less pronounced at low angle peaks, and these can be used to elucidate the identity of the molecular formula and crystal structure of the synthesized material. In addition to microscopy and diffraction, spectroscopy is also widely applied as analytical method for nano measurements. UV-visible absorption spectroscopy has been briefly discussed above, and remains the most common and useful technique to initially identify nanoparticles. Research in the theory of surface plasmon bands and experimental studies on the factors affecting UV-visible absorbance of dispersed nanoparticles are available in the literature18,19. There are restrictions of this analytical tool, as surface plasmon frequency of most metal are in the UV region, and no colouration is observed for such suspension. Conversely, coinage metals and its compounds exhibit d-d band transitions in the visible spectrum, and UV-visible absorption spectroscopy acts as a powerful tool to identify and characterize these nanoparticles.

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1.2 Synthesis of Metallic Nanostructured Materials in Solution. 1.2.1 Synthesis of Monodispersed Metallic Nanoparticles

Metallic nanoparticles have wide application in many scientific fields, and many studies have been conducted to investigate on the synthesis these materials through a variety of methods20,21. In particular, they can be achieved in solid, gaseous and aqueous medium, but for the discussion of monodispersion among metallic nanoparticles, we will restrict to preparation based on solution phase. Precursors to forming metallic nanoparticles can either be inorganic or organometallic compounds that contain the required metal element. In well reported synthesis, inorganic salts can be employed, and water used as the solvent. Many chemical reductions can readily occur in water, to reduce the salts into individual atoms. Commonly used reducing agents include sodium borohydride, sodium citrate, and alcohols20. However other simple organic reactants, such as glucose have also been exploited for this purpose. Upon reduction, the individual atoms initially agglomerate together to form nanoparticles, and if these particles are not stabilized, further aggregation will occur with the eventual growth into undesirable precipitates. Hence, another important reactant in wet chemical synthesis of monodispersed nanoparticles is stabilizing or capping agents, which functions as barriers to prevent uncontrolled growth processes. Good choice of these agents, also known as surfactants or stabilizers, should involve organic molecules that have a suitable hydrophobic end that bind to the metal nanoparticles covalently, and solubility on the other end as represented in Figure 1.3 (a). Some surfactants have an ionic tail that aid the solution of the nanoparticles and

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exhibit columbic repulsion of similar surfactants on neighbouring nanoparticles, which aid dispersion, as exemplified in Figure 1.3(b) For example, gold hydrogen tetrachloroaurate, HAuCl4 were reduced by sodium citrate to form gold nanoparticles, in the range of 7 to 100nm since several decades ago22. The citrate acts as ionic stabilizer as well, and prevented aggregation of the particles formed. Citrate reduction has also been used in preparation of other metallic nanoparticles, such as silver, and studied extensively on the factors that influence the particles formed23. In recent developments, thiol, nitrate salts, and glucose have also been used in such manner as a simultaneous role of reducing and stabilizing agent.

(a)

(d)

Figure 1.3 (a) A stabilizing shell composed of either covalently bound ligands24. (b) The tightly bound layer (surfactant layer) prevents aggregation by electronic repulsion, while the ionic charge promotes solubility in the solvent environment25.

Reduction of metals may require stronger reducing agents such as metal borohydrides (MBH4) salt, as the standard reduction potential of metallic cations lies in the typical range of 0.1V to 1.0V, while MBH4 has standard potential of 1.24V in an alkaline medium. This has been demonstrated in preparation of a variety of coinage

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and transition metals nanoparticles, such from metal salts in either aqueous or organic solvent26-32.

Figure 1.4 TEM images of gold nanoparticles prepared by NaBH4 as reducing agent32.

Organometallic complexes are another convenient and viable source of starting material used to prepare monodispersed nanoparticles. For example, iron and cobalt carbonyls have been used as the precursors to form iron and cobalt nanoparticles in the literature33,34. The processes involve injection of the metal carbonyls into organic solvents at high temperature for thermal decomposition of the complex to occur. This method have significant advantages of precursors already containing metal elements already at zero oxidation state, hence requiring less reactants. This will help to reduce contaminants (e.g. anions from metal salts and reducing agents) which may be difficult to remove from the resultant products. At the same time, the organic solvent helps to disperse the nanoparticles formed and perform a stabilizing role. Similar methods have also been directed at copper, silver and gold organometallic materials to form the desired spherical nanoparticles35.

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Finally, photochemical synthesis of metallic nanoparticles is possible through either reduction or decomposition induced by photolysis, and stabilized by suitable chemicals in the aqueous medium. As irradiation affects a large amount of ions or molecules at a single instant, it easily satisfies the condition simultaneous nucleation to achieve monodispersed and homogenous nanoparticles6. In previous reports13,37-42, metal salts and organometallic precursor has been subjected to either UV photolysis or laser in preparation of silver, gold, palladium, and iron nanoparticles. Laser ablations of metal plates or foils in solution have also been employed, with the production of suspended nanoparticles that are stabilized with suitable capping agents43,44.

1.2.2 Synthesis of Bimetallic Nanoparticles

Intermetallic nanoparticles are those that have different metallic elements in a single particle, and complex material of four unique metals, have been isolated and investigated45. The simplest and most common form of intermetallic nanoparticles is bimetallic nanoparticle, which contains two different metal components, and further subdivision provides two possible structures of either alloy or core-shell. The properties, synthesis and applications of bimetallic nano materials have generated much interest among scientists46-50. Bimetallic alloy nanoparticles are basically homogeneous solid mixture of two different metals in a single nanoparticle, which can be well stabilized. TEM analysis of these particles should provide images of particles that have even contrast over a single particle, and no differentiation of individual metals. UV-visible absorption spectroscopy is another useful method to identify alloy nanoparticles, as the resultant 13


sample should exhibit an absorbance that is between that of the individual atoms. The characteristic surface plasmon of Ag-Au is the most widely investigated among alloy, and provides reliable information on the ratio of the metals51, 52. Co-reduction of two precursors that each contains the target metal is an efficient and straightforward method to prepare alloy nanoparticles. Metal ions that are selected, needs to have similar standard reduction potentials and thoroughly mixing are required for the ions to be in a homogenous distribution. As reduction is initiated, both metals are reduced simultaneously, and undergo nucleation and growth in the same site, forming alloyed bimetallic particles. This strategy is well documented53-55 for the formation of Ag-Au alloy nanoparticles, which have been an ideal method to modify the exact composition that is required. Instead of chemical co-reduction, thermal decomposition may be used for one of the metal precursor in the situation when there is a significant difference in the reduction potentials. Fe-Pt and Co-Pt3 have both been synthesized using organometallic precursors and thermolysis46,56,57. Bimetallic core-shell nanoparticles can be expressed as M@X, where M is the core metal and X is the shell metal. Au@Ag core-shell nanoparticles were prepared in 1964, and opened up a new type of nano materials that exhibited interesting properties58. TEM imaging will indicate two areas of different contrast, with the heavier metal, which is usually the core, allowing less electrons to pass through. At the same time, only the metal in the shell will retain its surface plasmon band during UV-visible spectroscopy, while the surface plasmon band of the core metal is pacified. Thus during synthesis, only initial colour will correspond to the monometallic suspension of the shell material, but upon TEM imaging, the core-shell structure would be obvious.

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Similar to preparation of alloy nanoparticles, core-shell can be formed through the co-reduction method, but there must be some differences in the electrode potential of the metals used. In a typical solution, the metal which can be reduced easier will form the core, while the other metal will form the shell, for example Pd@Ag and Pd@Au was formed in such a manner59,60. The major disadvantage of this method is the reliability to form only core-shell structure, as alloy particles may be formed simultaneously in the same solution. Successive reduction helps to eliminate this issue, with the intial core metal formed by reduction first, before the addition of the metal precursor to form the shell. Various studies61-63 have shown the application on this synthetic process to form Au@Ag and Ag@Au nanoparticles. Other novel methods include -ray irradation, and sonochemistry, as demonstrated in literature64-66 for the synthesis of Au@Pd, Pt@Au and Au@Pt. Hence, it is of interest to explore various novel preparation method and the focus of our work was placed on the synthesis of monometallic, alloy and core-shell nanoparticles protected by beta-cyclodextrin.

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Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.

Chapter 2 Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with beta-Cyclodextrin.

2.1 Introduction Nanomaterials are known to have size-dependent physical properties that provide many opportunities for innovative applications. In particular, there is a wide variety of interest from catalytic to biomedical fields1,2.

These have also led to further

research in the preparation of monodispersed nanoparticles and their role as building units of nanoscale device3-5. Cyclodextrin (CDs) are a class of cyclic oligosaccharides six (alpha), seven (beta), or eight (gamma) α(1,4)-linked glucopyranose units6. These non-toxic cyclic rings, forms a cone-like molecular structure with primary alcohol directed to the narrow side, and secondary alcohol on the wider side of the torus. The interior side or cavity of the cone is hydrophobic due to this arrangement, and has been widely investigated as a molecular host that allows small organic molecules to act as guest and form complex compounds with CD. As complexing agents, CDs can be applied to enhance solubility, act as protection or carrier for the smaller guest compounds, and

20


selectively remove substances from a given mixture. Among the CDs, betaCyclodextrin is most widely employed due to the ideal cavity diameter of 6 to 6.5Å7. There have been numerous studies of CDs‟ application as stabilizing agents for various metallic nanoparticles such as gold8, silver9, and platinum10. They were initially proposed to be used in the modified form11,12, mostly as thiolated-CD for stronger attachment to gold particles. However in recent studies, unmodified CD had been used with reducing agents information of silver and gold nanoparticles9,13. In attempts for a more convenient and environmental friendly approach, simple sugars had been used simultaneously as a capping and reducing agent in a single-step method in an earlier study14. Similarly, in literature unmodified CD was employed in the same manner due to its similar functional group as glucose and solubility in water15. In this work, the formation and stabilization of water-soluble Ag and Au nanoparticles by beta-Cyclodextrin (CD) via aqueous self-reduction methods were performed separately, resulting in particles of size ~10nm, and significant evidence of self-alignment. Under similar conditions, Cu nanoparticles preparation was also attempted and studied. In alignment to the increasing focus on green chemistry and processes16, the use of non-toxic, glucose like surfactants and water as solvent medium was selected. The extent of hydrophobic-hydrophobic attractions of the

CD and metal nanoparticles was accounted for. Hydrogen bonding interactions between the surfactants, oxidized CD, is believed to drive the self alignment of the nanoparticles into necklace and chains. The degree of this attraction between Ag and Au is discussed, with the evidence of length of the nano “chain” formed.

21


2.2 Experimental Section All chemicals used were of reagent grade, obtained from Sigma Aldrich and used without further purification. Beta-Cyclodextrin (CD) was used as pure solid at the start of synthesis and diluted immediately before addition of metal salt. While of silver, gold and copper metal salts were prepared as stock solution and kept in the dark, with further dilution freshly prior to the synthesis. Synthesis of monometallic silver and gold nanoparticles The synthesis was modified from previously reported in the literature15, to ensure the successful preparation of the nanoparticles. 5.0 ml of deionised water and 0.0396g CD (3.5 x 10-5 mol) was mixed thoroughly for 10 minutes to form a clear solution of CD. After which, 40μl of the precursor salt solution, either AgNO3 (15mM) or HAuCl4 (15mM) was added and a further 10 minutes of stirring was required to ensure that the solution was homogenous. 50μl of NaOH (1M) was added to the solution while stirring continued, which activated the reductive capability of

CD, and a faint yellow (Ag) or purple (Au) solution was observed. The solution was heated in a 600C water bath for 20 minutes, and the colour of the solution intensified, indicating the complete formation of the nanoparticles. Purification of the nanoparticles was made by dilution to 15ml with deionised water and centrifuged at 2000rpm for 60 minutes. This resulted in a bottom layer of nanoparticles and excess colourless solution above, which was removed. The resulting solution was diluted to 15ml again, and the purification process repeated once. Re-suspension of the nanoparticles was done with further dilution using deionised water and sonication. Synthesis of monometallic copper nanoparticles

22


A solution of 5.0 ml deionised water and 0.0396g CD (3.5 x 10-5 mol) was mixed for 20 minutes and constant supply of N2 gas was bubbled into to the solution in order to remove any dissolved oxygen. After which, 20μl of CuNO3 solution (15 mM) was added and a further 10 minutes of stirring and 50μl of NaOH (1M) was added to the solution. The solution was heated in an 800C water bath for 20 minutes, but colour change was absent, indicating that reduction Cu2+ to form nanoparticles did not occur. NaBH4 a common reducing agent was added to aid the reduction, which resulted in an instant formation of a dark red solution. Throughout the preparation, N2 was continuously bubbled into the solution, and the Cu nanoparticles were considered stable under these conditions with a persistent dark red coloration. However, upon purification, the solution lost the red colouration and quickly turned into dark grey suspension, which was an indication of copper oxide formation. Characterization All UV absorption spectra were recorded with the use of a UV-Visible absorption spectrometer (Shimadzu UV-2550) using diluted solutions in a 1-cm quartz cell. Transmission Electron Microscopes (TEM, JEOL-2010 or JEOL-3010) were used to capture images and electron diffraction(ED) pattern of the samples, which were suspended on carbon coated copper grids. Energy Dispersive X-ray (EDX) spectra was taken during TEM (JEOL-3010) imaging and observation. Sample preparation for TEM imaging involved suspension of a single drop of suitably diluted solution on the carbon coated copper grid, and evaporation of the solvent(H2O) was carried out under vacuum condition (< 0.01 Torr) for a duration of 6 to 12 hours.

23


2.3 Results and Discussion Synthesis of monometallic silver nanoparticles The resultant silver colloidal solution was characterized with UV-visible spectroscopy, and the absorption spectrum shown in Figure 2.1 corresponds to those previously recorded in literature9,

15

. The surface plasmon band at λmax = 400nm

indicates the successful preparation of stable and well dispersed silver nanoparticles. This synthesis is easily reproducible, as repeated experiments under this specific conditions produces the same well-imposed absorption profile. The nanoparticles are also fairly stable, showing no significant aggregation in low concentration over periods of six months.

Figure 2.1 UV-visible absorption spectra of Ag nanoparticles colloidal solution.

24


The synthesis involves the reduction of metal salts, accompanied with oxidation of the primary alcohol at the hydrophobic face of the CD, as proposed in the earlier study16. It has also been identified that the secondary alcohols do not undergo oxidation in an alkaline solution17. Hence, these oxidized CD which contains carboxylic acid are present as surfactants on the nanoparticles to stabilize the nanoparticles. The carboxylic acids are formed in the alkaline solution which will be deprotonated as in Figure 2.2 (b), resulting in a negative charge which increases the stabilization of Ag due to the electrostatic repulsion between neighbouring nanoparticles. Therefore, the main stabilization of the silver particles is likely the hydrophobic interaction between the cavity of CD and the Ag nanoparticles with the secondary alcohol groups pointing towards the metal centre, Figure 2.2(c). It is known that CDs can increase the solubility of large hydrocarbons through such hydrophobic interactions. At the same time, the deprotonated carboxylic groups would be orientated towards the water molecule, and greatly increase the solubility of the nanoparticles. Absence of self-alignment in the aqueous phase is also supported by the lack of near IR absorbance as displayed in silver nanowire or nanorods18,19, due to the elongation of the particles and resulting surface plasmon resonance. However, upon drying, it can be suggested that the nanoparticles can self-align with favourable interaction between the carboxylic acid, similar to those of inter-molecular hydrogen bonding during gaseous dimerization (Figure 2.2 (d)).

25


(a)

(b)

(c)

(d)

Figure 2.2 (a) Normal CD, (b) Oxidized CD in alkaline solution, (c) Ag nanoparticles stabilized by oxidized CD, (d) hydrogen-bonding interactions between oxidized CD after drying.

Mono-dispersed spherical silver nanoparticles which are around 10nm in diameter are observed in the TEM images (Figure 2.3). It can be observed that the general distribution of the particles follow a somewhat self-aligned arrangement, whereby there are examples of nano “chain” or “necklace”, where 20 and up to 100s of nanoparticles are aligned without application of external force. The prior studies of cyclodextrin as surfactants for either silver or gold nanoparticles did not report any self-alignment property9,13-15.

26


(a)

(c)

(b)

(d)

Figure 2.3 TEM images of Ag nanoparticles colloidal solution. (a) Nano-“Necklace” (b) high magnification of individual nano particles. (c), (d) nano-“chain”.

Factors affecting the Role of CD in self-alignment of nanoparticles As CD is the least soluble among the three common CDs (alpha, beta, gamma), there is a higher tendency for the molecules to form weaker complex with water molecules as compared to the other CDs. At the same time, the hydrogen bonding between the CD would be more favourable as drying occurred. Thus,

CD is an excellent candidate to demonstrate the self-alignment property of CD27


capped nanoparticles. The concentration of nanoparticles has to be carefully regulated as well, where the ratio of metal:surfactants is kept at 1:60 during synthesis, and further dilution can be done before TEM analysis. If the nanoparticles are overly concentrated, self-alignment would not occur as the arrangement of the particles would be crowded before drying, and little space is allowed for alignment to occur. However, if dilution is done at a larger extent, the nanoparticles would be too dispersed and surfactant-surfactant attractions would not be strong enough to direct the alignment. The 1-D arrangement instead of 2-D or 3-D alignment is attributed to the unfavourable steric hindrance in formation of 2-D or 3-D alignment, while the diluted nanoparticles allow enough particles for favourable liner interaction, and not enough for more complicated network alignments. Hence, it can be observed that the choice of capping agent that has strong intermolecular interactions, with the right ration and dilution can lead to self-alignment of nanoparticles. Synthesis of monometallic gold nanoparticles In the synthesis of Au nanoparticles, the successful formation was indicated by the observation the characteristic deep purple colour of Au colloidal solution. This was further confirmed with the use of UV-visible spectroscopy, and the spectrum shown with λmax = 527nm in Figure 2.4, correspond to those previously recorded in literature13,15.

28


Figure 2.4 UV-visible absorption spectra of Au nanoparticles colloidal solution.

This synthesis can be easily reproduced, as proven with identical UV-visible absorption spectrum was recorded with repeated experiments under this specific conditions. The nanoparticles are very stable, showing no significant aggregation at suitable dilution over periods of six to twelve months. In a similar trend as that of Ag, absence of far infra-red absorption suggest that the nanoparticles are monodispersed and self-alignment did not occur in the aqueous phase. The confirmation of Au nanoparticles was also supported with the EDX characterization, Figure 2.5(d). During the TEM imaging, the similar occurrence of self-aligned nano chains had been observed, with small individual nanoparticles aligned in chains, as shown in Figure 2.5 (b) and (c). It was noted that the size of Au was similar to those of Ag, at around 10nm, which was expected with the similar ratio of metal salt to surfactants used. However, the Au nano-chains observed in the TEM images were significantly 29


shorter ranging from 5 to 20 nanoparticles in a “chain”. It can be suggested that as the van der Waals interaction between the gold nanoparticles and CD is stronger than that of silver nanoparticles, there is higher stabilization of surfactants and lesser attraction between surfactants between nanoparticles and result in shorter chain formation.

(a)

(b)

(c) (d)

Figure 2.5 TEM images of Au nanoparticles colloidal solution. (a) high magnification of individual nanoparticle. (b) and (c) nano-“chain”. (d) EDX spectrum of Au nanoparticles. 30


Synthesis of copper nanoparticles In contrast, the synthesis of Copper nanoparticles had much more difficulties. Copper (II) ions reduction to copper metals was not achievable with CD in alkaline solution alone. The E0 value of copper and silver are listed below; Cu2+(aq) + 2e− → Cu(s)

+0.34V

Ag+(aq) + e− → Ag(s)

+0.80V

Our studies have placed the reductive strength of alkaline CD between that of silver and copper, thus after many attempts at elevated temperature there was no formation of copper nanoparticles without addition of reducing agents. Similarly, in earlier attempts when copper salt precursor was used in preparation of nanoparticles withCD as surfactants, only copper oxide was successfully isolated20. From our experimental observation, the initial formation of Copper nanoparticles was observed with the addition of NaBH4 as reducing agent, with the characteristics of dark red coloration of copper nano colloidal solution. The UV spectrum as shown in Figure 2.6(a), act as further evidence of the presence of Cu with λmax = 475nm. Although most literature21,22 give the λmax of Cu(0) nanoparticles to be around 500nm to 600nm, there are good amount of litereature23-25 in recent investigation to suggest a blue shift to aroud 420 to 480nm. This may be due to different medium used for the suspension or when the size of the nanoparticles is smaller. However, the oxidation of these pure Cu nanoparticles quickly occurred, and black suspension of copper oxide nanoparticles were observed during the synthesis. In the UV spectroscopy analysis, in Figure 2.6(a), a broad absorption at λmax = 750nm which correspond to literature assignment of CuO nanoparticles was noted. In the TEM imaging, Figure 2.6(b) no indication of nano “chain” was identified, which 31


suggest that the interaction between surfactants in this colloidal solution was unfavourable. Furthermore, without prior oxidation of CD, the driving force for chain formation would be hydrogen bonding between the OH, alcohol group which is not as extensive as the hydrogen bonding of COOH, carboxylic acid in the case of Au. (a)

(b)

Figure 2.6 (a) UV-visible spectrum of colloidal mixture in reduction of Cu with

CD as surfactants. (b) TEM images of colloidal of CuO.

The experimental results from our attempted synthesis indicates that CD is unsuitable for self-reduction preparation of pure copper nanoparticles, and the interaction between CD and copper is much weaker than that of silver and gold. Hence, CD is unable to protect copper from oxidation and there is no evidence of self-alignment of the resultant CuO nanoparticles.

32


2.4 Conclusion We have demonstrated an effective method to separately prepare 10nm Ag nanoparticles and Au nanoparticles in aqueous solution. The CD is used simultaneously as the reducing agent and surfactants, which result in more straightforward synthesis and purification. The oxidized CD acts as surfactants through hydrophobic-hydrophobic interaction with the metal nanoparticles. At the same time, in aqueous phase, the negative COO- group of the oxidized CD results in electrostatic repulsion, and prevents coalescence. The COO- groups also greatly increase the solubility of the stabilized nanoparticles, with positive ion-dipole interactions with the water molecules. In addition, upon drying, the nanoparticles selfaligned to form “chains” or “necklaces”. Further investigations suggest that the strength of interaction between metal and

CD follow the trend of Au > Ag > Cu, which is evident from unstable Cu nanoparticles. As a result of the stronger surfactant-metal interaction of Au stabilized by CD , there is weaker interactions between surfactants of neighbouring Au particles, resulting in shorter nano-“chain” as compared to Ag nanoparticles prepared under identical conditions. Future works using different concentrations and other CDs can be embarked to further elucidate the self-alignment mechanism and stabilization of the nanoparticles. The successful syntheses of metal nanoparticles protected by CDs suggest the possibility of further self-alignment in two or three dimensional, which can be useful in applications in development of photonic devices.

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References 1. Daniel, M.; Astruc, D. Chem. Rev. 2004, 104, 293–346. 2. Chen, D.; Wang, G.; Li, J. J. Phys. Chem. C 2007, 111, 2351–2367. 3. Djalali, R.; Samson, J.; Matsui, H. J. Am. Chem. Soc. 2004, 126, 7935–7939. 4. Ren, J.; Tilley, R.D. J. Am. Chem. Soc. 2007, 129, 3287–3291. 5. Yu, H.; Gibbons, P. C.; Kelton, K. F.; Buhro, W. E. J. Am. Chem. Soc. 2001, 123, 9198–9199. 6. Szejtli, J. Chem. Rev. 1998, 98, 1743–1754. 7. March, J. Advanced Organic Chemistry, 4th ed.; John Wiley and Sons: New York, 1992. 8. Liu, J.; Ong, W.; Roman, E.; Lynn, M.J.; Kaifer, A.E. Langmuir 2000, 16, 30003002. 9. He, B.; Tan, J.J.; Liew, K.Y.; Liu, H. J. Mol. Catal. A Chem. 2004, 221, 121–126. 10. Giuffrida, S.; Ventimiglia, G.; Petralia,S.; Conoci,S.; Sortino, S. Inorg. Chem. 2006, 45, 508–510. 11. Alvarez, J.; Liu, J.; Roman, E.; Kaifer, A.E. Chem. Commun. 2000, 1151–1152. 12. Liu, J.; Alvarez, J.; Ong, W.; Roman, E.; Kaifer, A.E. J. Am. Chem. Soc. 2001, 123, 11148–11154. 13. Liu, Y.; Male, K.B.; Bouvrette, P.; Luong, J.H.T. Chem. Mater. 2003, 15, 4172– 4180. 14. Panigrahi, S.; Nath, S.; Praharaj, S.; Ghosh, S. K.; Kundu, S.; Basu, S.; Pal, T. Colloids Surf. A: Physicochem. Eng. Aspects 2005, 264, 133–138. 15. Pande, S.; Ghosh, S.K.; Praharaj, S.; Sudipa, P.; Basu, S.; Jana, S.; Pal, A.; Tsukuda, T.; Pal, T. J. Phys. Chem. C 2007, 111, 10806–10813. 16. Dahl, J.A.; Maddux, B.L.S.; Hutchison, J.E. Chem. Rev. 2007, 107, 2228–2269

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17. Fraschini, C.; Vignon, M. R. Carbohydr. Res. 2000, 328, 585–589. 18. Lamprecht, B.; Schjider, G.; Lechner, R.T.; Ditlbacher, H.; Krenn, J.R.; Leitner, A.; Aussenegg, F.R. Phys. Rev. Lett. 2000, 84, 4721–4724. 19. Maier, S.A.; Brongersma, M.L.; Kik, P.G.; Atwater, H.A. Phys. Rev. B 2002, 65, 193408–193411. 20. Premkumar, T.; Geckeler, K.E. J. Phys. Chem. Solids 2006, 67, 1451–1456. 21. Lisiecki, I.; Pilleni, M. P. J. Phys. Chem. 1995, 99, 5077-5082. 22. Salzemann, C.; Lisiecki, I.; Brioude, A.; Urban, J.; Pileni M. P. J. Phys. Chem. B 2004, 108, 13242-13248. 23. Panigrahi, S.; Kundu, S.; Ghosh, S. K.; Nath, S.; Praharaj, S.; Basu, S.; Pal, T. Polyhedron 2006, 25, 1263-1269. 24. Guo, L.; Wu, Z. H.; Ibrahim, K.; Liu, T.; Tao, Y.; Ju, X. Eur.Phys. J. D 1999, 9, 591-594. 25. Panigrahi, S.; Kundu, S.; Basu, S.; Praharaj, S.; Jana, S.; Pande, S.; Ghosh, S.K.; Pal, A.; Pal, T. J. Phys. Chem. C 2007, 111, 1612-1619

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Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.

Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles

3.1 Introduction In nano science research, bimetallic nanoparticles in the structure of either alloy or core-shell have often been studied due to the various interesting physical and chemical properties.1-7 Bimetallic nanoparticles have a variety of uses due to its superior spectral3,4 and catalytic5-7 properties, as compared to monometallic nanoparticles. Therefore, as compared to monometallic nanoparticles, bimetallic species received greater attentions in its synthesis, properties and application8-10. Many strategies have been employed in synthesis of bimetallic particles during past years, resulting in either homogenous bimetallic alloy structure, or those of coreshell with monometallic core, and a secondary metallic shell11,12 or metal compound shell13,14. Preparation route varied from those of seed growth or successive reduction8,11 and co-reduction15,16 in aqueous media, to those that require photolytic17,18 treatments and gas-phase synthesis13. In addition to achieve these homogenous particles, different capping agent8,14,19 is also generally applied to stabilize the resulting colloid. Co-reduction is often the most ideal method in preparation of alloy nanoparticles, with good control over the proportion of metals, size and morphology. In co-reduction,

36


a mixture of the metal precursors, in the form of metallic salt is simultaneously reduced either in an aqueous or non-aqueous medium. During this simultaneous reduction, the metal ions are required to be well mixed in order to achieve the desired homogeneous alloy structure. In comparison, the successive reduction approach is commonly adopted for the formation of core-shell structure, with the initial reduction of the metal ions that is to form the core, followed by addition of the metal ions for the further reduction in formation of the shell. Successive reduction method requires a suitable surfactant, or choice of metals, so that the incoming metal ions (shell) would prefer to coat the existing metal nanoparticle (core) before reduction is performed. Another route to core-shell nanoparticles involves the formation of either monometallic or alloy nano core through various methods, followed by surface oxidation14,20, or diffusion driven reaction13 to form a suitable shell respectively. These steps allow the coating of the initial nanoparticles with the resultant metal oxide or metal salts, forming a uniform shell around the metallic core. Specifically, for diffusion driven reaction by addition of reactant has been demonstrated in some studies that involved the formation of Ag@AgI core-shell structure, with the use of iodine in both aqueous17 and gaseous21 medium. Diffusion driven synthesis of Au@AgI core-shell particle have also been prepared recently attempted13, with Ag@Au as the precursor resulting in a core-shell inversion. The novel synthesis of AgI as a shell in nanoparticles is driven by its semi-conductor22 and optical properties23. Furthermore, AgI shell may have the potential application as carriers that can release the core on demand. This is supported by the proposal of AgI as a tight shell13, with the ability to „protect‟ the core, but at the same time, AgI can be decomposed easily by photolysis24.

37


In this part, the novel preparation and stabilization of Ag-Au alloy (size about 10nm) by beta-Cyclodextrin (CD) via aqueous self-reduction methods were performed. CD is chosen as the reactant and surfactants due to its similar molecular composition as simple sugar, and its ability to promote water solubility of such alloy nanoparticles. Furthermore, there have been attempts to use glucose25 to synthesis AgAu alloy, in accordance to growing trends of greener nano synthesis26,27. Characterization of the prepared alloy colloidal was done using UV-visible absorption spectroscopy and Transmission Electron microscope (TEM) imaging. Energy Dispersive X-ray and the resulting elemental analysis were used to elucidate the possible ratio of the composite metals.

The prepared alloy nanoparticles were

employed as the template to synthesis Au@AgI core-shell nanoparticles, with the addition of aqueous iodine. This synthetic approach is driven by the diffusion mechanism, whereby the Ag atoms diffuses out from the nanoparticles to react and form the AgI shell, while the unreacted Au atoms will form the core. The successful preparation of Au@AgI core-shell particles was verified with UV-visible spectroscopy, TEM and electron diffraction imaging.

3.2 Experimental Section All chemicals used were of reagent grade, obtained from Sigma Aldrich and used without further purification. Beta-Cyclodextrin (CD) and iodine was used as pure solid at the start of synthesis and diluted immediately before addition of metal salt. While of silver, gold and copper metal salts were prepared as stock solution and kept in the dark, with further dilution freshly prior to the synthesis. Preparation of bimetallic silver and gold alloy nanoparticles

38


5.0 ml of deionised water and 0.0020g CD (1.75 x 10-6 mol) was mixed thoroughly for 10 minutes to form a clear solution of CD. 40μl of AgNO3 (15mM) and 40μl of HAuCl4 (15mM) was then added with another 10 minutes of stirring. 50μl of NaOH (1M) was introduced to the solution while stirring continues, and it was heated in a 600C water bath for 30 minutes. The solution turned pink in colour, indicating the formation of bimetallic Au-Ag nanoparticles. The nanoparticles were purified by dilution to 15ml with deionised water and centrifuged at 2000rpm for 60 minutes. The resultant mixture contained a residual layer of nanoparticles and excess solution above, which was removed. Dilution to 15ml was performed again with deionised water, and the process was repeated thrice. Re-suspension of the nanoparticles was done with further dilution using deionised water and sonication. Preparation of bimetallic gold core-silver iodide shell nanoparticles 0.5ml of a solution of the silver and gold alloy nanoparticles capped by CD, as prepared above was diluted to 5ml with deionised water. Aqueous iodine was prepared by addition of 0.0010g (4 x 10-6 mol) of iodine crystals into 5ml of solution. Addition of aqueous to the alloy colloidal solution was performed in a drop-wise manner, and monitored after every drop with UV absorption spectroscopy. The experiment was reaction to form AgI shell is considered complete with the UV spectrum of AgI observed and stayed consistent with further addition of iodine solution. Characterization UV-Visible absorption spectrometer (Shimadzu UV-2550) was used to observe and record all UV absorption spectra with the use of a 1-cm quartz cell filled with the diluted solutions. Transmission Electron Microscopes (TEM, JEOL-2010 or JEOL39


3010) were used to capture images and electron diffraction(ED) pattern of the samples, which were suspended on carbon coated copper grids. Energy Dispersive X-ray (EDX) spectra was taken during TEM (JEOL-3010) imaging and observation. Sample preparation for TEM imaging involved suspension of a single drop of suitably diluted solution on the carbon coated copper grid, and evaporation of the solvent(H2O) was carried out under vacuum condition (< 0.01 Torr) for a duration of 6 to 12 hours.

3.3 Results and Discussion Preparation of bimetallic silver and gold alloy nanoparticles UV-visible absorption spectroscopy was used to analyse the purified resultant alloy colloidal, and the spectrum shown in Figure 3.1 indicates a surface plasmon band at 444nm which is evident of Au-Ag alloy as described in various earlier investigations3,28,29.

Figure 3.1 UV-visible absorption spectra of Ag-Au alloy nanoparticles. (1:1 Ag+:Au3+) 40


The single absorbance peak validate that alloy formation has successfully occurred, as the alternative individual silver and gold nanoparticles will exhibit 2 separate peaks instead29. The position of the alloy surface plasmon band with close reference to investigations in the literature28, further suggest a composition of around 45% to 50% gold in the alloy nanoparticles, which agrees closely to the 1:1 metal salts used as precursors. (a)

(c)

(b)

(d)

Figure 3.2 TEM images: (a)-(c) Ag-Au alloy nanoparticles stabilized by oxidized CD, (d) Higher magnification of single Ag-Au nanoparticle.

41


Observation under TEM, represented in Figure 3.2 (a) to (d), indicated that the majority of alloy nanoparticles were around 10nm, with good extent of dispersion. EDX was performed as well during the TEM imaging, and indicated distinctly both silver and gold peaks (Figure 3.3(a)). The elemental analysis of the EDX (Figure 3.3(b)) revealed a molar ratio of 36:64 for Ag:Au, which suggest a higher composition in Au as compared to that suggested by the UV-visible absorption. This can be expected as Au has a standard electrode value that is twice higher than that of Ag, which would suggest that when both Ag and Au salt of similar concentration is being reduced by a limited among of reductant,

-CD, more Au(0) will be formed. (a)

(b) Element

Peak Area

Weight%

%

CK

6707

Cu K

345718 41.37

58.48

Ag L

86958

13.51

11.25

Au L

192577 43.73

19.94

Totals

1.38

Atomic

10.33

100.00

Figure 3.3 (a) EDX of Ag-Au alloy nanoparticles (b) Elemental analysis of the EDX spectrum (Carbon and Copper are elements inherent from the TEM grid used)

Although the literature28 UV spectrum had attributed that 450nm is the surface plasmon band of Ag-Au alloy, with 1:1 ratio of salt precursor, the actual percentage of each metal was not measured directly from the product formed. Thus it is reasonable to accept that the fraction of Au in this alloy is 64% under the described procedures and give an UV-visible absorption profile with λmax = 444nm. Cyclodextrin had been 42


used in earlier attempt8, simultaneously as both reducing and capping agent in synthesis of monometallic silver, gold and bimetallic core-shell nanoparticles. However, this is the first attempt in preparation of Au-Ag alloy nanoparticles with the use of -CD as both surfactant and reductant. Despite the low ratio of -CD: metal salts at 1:1, the alloy nanoparticles exhibited good stability with little aggregation, and can be easily redispersed through sonication after period of six months, with an identical UV-visible absorption spectrum. Preparation of bimetallic gold core-silver iodide shell nanoparticles The preparation of Au@AgI was performed successfully, with the Au-Ag alloy nanoparticles as starting reactants, and addition of I2. It has been well documented14,30 that energy barriers to diffusion is much lower for nanoparticles, and the rate increases with decreasing size of particles. Furthermore, there are some prior examples in literature that suggest the inversion of core-shell nanoparticles, due to such diffusion driven mechanism, whereby the core metal has diffused out to the surface to form a shell, with suitable reactant or adsorbate. Our approach in the preparation was to utilize the diffusion of Ag atoms out from the Au-Ag alloy, when aqueous iodine, I2 (aq) reacted with the alloy to form AgI shell. Ag-Au(Alloy) + I2(aq)  Au@AgI (core-shell) As observed from the UV-visible spectrum (Figure 3.4), the growth of AgI is clearly characterized by the sharp distinct surface plasmon band at 416nm, which had been reported in earlier studies of AgI colloidal solutions. An identical absorbance, in Figure 3.5 was yielded from exposure of silver nanoparticles stabilized by CD to iodine. At the same time in Figure3.4 the initial peak representative of the Au-Ag alloy at 444nm, had gradually disappeared with the concurrent appearance of 416nm, 43


AgI nanoparticles, and 556nm of Au nanoparticles. The absorption peak of the Au core was shifted from the typical 520nm to a longer wavelength, which is predicted with the formation of the AgI shell, as it has a comparatively higher refractive index. Hence the absorption feature at 416nm can be confidently attributed to the formation of AgI shell from the Ag in the alloy core, resulting in pure Au@AgI core-shell nanoparticles.

Figure 3.4 UV-visible absorption spectra of Au@AgI nanoparticles formation, beginning with A: pure Au-Ag alloy nanoparticles, and progressed till B: Au@AgI nanoparticles.

44


Figure 3.5 UV-visible absorption spectrum of pure AgI nanoparticles.

The size distribution of the resultant core-shell nanoparticles is wider compared to that of the precursor alloy nanoparticles, with a range of diameter from 3 to 10nm, as inferred from the TEM images obtained, Figure 3.6 (a) and (b). This observation is similar to those of AgI nanoparticles, formed from chemical reduction method in other studies17, 31.

(a)

(b)

45


(c)

(d)

Figure 3.6 TEM images of Au@IAg nanoparticles colloidal solution. (a) and (b)wide size distribution of particles. (c) and (d) Higher magnification highlighting “core-shell” feature. (a)

(b)

Figure 3.7 ED pattern of pure (a) Ag-Au alloy and (b) Au@AgI core-shell nanoparticles. The ED pattern, Figure 3.7, suggest that the nanoparticles are polycrystalline, and displayed significant difference from that of Ag-Au alloy, with the occurrence of distinct indices (220) and (331) that corresponds to data of -AgI in literature31. At higher magnification, Figure 3.6 (c) and (d), it was observed that a good number of particles had a darker spot within itself, indicating the core-shell structure, with Au

46


(darker due to lower electron permeability) as the core. Nanoparticles observed without the presence of dark spots, are likely pure AgI, which have dissociated from the precursor alloy nanoparticles during addition of iodine, and did not form part of the shell. Hence, these non-shell AgI nanoparticles are predominantly smaller size compared to the desired Au@AgI core-shell particles.

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3.4 Conclusion In this chapter, CD was shown to be a suitable reactant, as a capping and reducing agent in the aqueous preparation of 10nm Ag-Au alloy nanoparticles. In addition this alloy was employed as a precursor to prepare Au@AgI core-shell nanoparticles with addition of iodine, and through a diffusion mechanism whereby the silver atoms diffuses out to react and form the AgI shell. The characterisation of both species of nanoparticles was also carried out using UV-visible absorption spectroscopy, TEM, EDX and ED analysis. The described synthesis steps provide a simple and effective method of preparation for such bimetallic and core-shell nanoparticles. Future work can be devoted on the preparation of other alloy particles, synthesized with Ag and CD as capping agent, which can result in novel core-shell structure.

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