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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.
i
Table of Contents
Acknowledgement
i
Table of Contents
ii
Summary
iv
Chapter 1 Introduction ............................................................................... 1
1.1 Synthesis and Characterization of Nanostructured Materials.
2
1.1.1 Synthesis of Nanostructured Materials
2
1.1.2 Characterization of Nanostructured Materials
5
1.2 Synthesis of Metallic Nanostructured Materials in Solution.
10
1.2.1 Synthesis of Monodispersed Metallic Nanoparticles
10
1.2.2 Synthesis of Bimetallic Nanoparticles
13
References
16
Chapter 2 Synthesis of self-aligned Ag and Au monometallic
nanoparticles stabilized with beta-Cyclodextrin. ................................... 20
2.1 Introduction
20
2.2 Experimental Section
22
2.3 Results and Discussion
24
2.4 Conclusion
33
References
34
ii
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell
bimetallic nanoparticles ............................................................................ 36
3.1 Introduction
36
3.2 Experimental Section
38
3.3 Results and Discussion
40
3.4 Conclusion
48
References
49
iii
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.
iv
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.
1
Chapter 1: Introduction
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
2
Chapter 1: Introduction
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
Chapter 1: Introduction
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
Chapter 1: Introduction
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
5
Chapter 1: Introduction
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
Chapter 1: Introduction
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.
7
Chapter 1: Introduction
(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
8
Chapter 1: Introduction
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.
9
Chapter 1: Introduction
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
10
Chapter 1: Introduction
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
11
Chapter 1: Introduction
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.
12
Chapter 1: Introduction
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
Chapter 1: Introduction
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.
14
Chapter 1: Introduction
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.
15
Chapter 1: Introduction
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44. Sibbald, M.S.; Chumanov, G.; Cotton, T.M. J. Phys. Chem. 1996, 100, 4672–4678.
45. Gonsalves, K.E.; Rangarajan, S.P. Appl. Polym. Sci. 1997, 64, 2667-2671.
46. Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1982–
1992.
47. Chung, Y. M.; Rhee, H. K. J. Colloid Interface Sci. 2004, 271, 131–135.
48. Zaera, F. J. Phys. Chem. B 2002, 106, 4043–4052.
18
Chapter 1: Introduction
49. Chung, Y. M.; Rhee, H. K. Catal. Lett. 2003, 85, 159–164.
50. Ferrando, R.; Jellinek, J.; Johnston, R.L. Chem. Rev. 2008, 108, 845–910.
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52. Zhou, M.; Chen, S.; Zhao, S.; Ma, H. Physica E 2006, 33, 28–34.
53. Mallin, M.P.; Murphy, C.J. Nano Letters, 2002, 2, 1235–1237.
54. Raveendran, P.; Fu, J.; Wallen, S.L. Green Chem. 2006, 8, 34–38.
55. Sun, S.H.; Murray, C.B.; Weller, D.; Folks, L.; Moser, A. Science, 2000, 287,
1989–1992.
56. Howard, L.E.M.; Nguyen, H.L.; Giblin, S.R.; Tanner, B.K.; Terry, I.; Hughes,
A.K.; Evans, J.S.O. J. Am. Chem. Soc. 2005, 127, 10140–10141.
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Weller, H. J. Am. Chem. Soc. 2002, 124, 11480–11485.
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19
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
(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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
(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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
withCD 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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
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.
33
Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin.
References
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A.; Aussenegg, F.R. Phys. Rev. Lett. 2000, 84, 4721–4724.
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35
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
(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
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
(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.
47
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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.
48
Chapter 3 Preparation of Ag-Au alloy and Au@AgI core-shell bimetallic nanoparticles.
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[...]... 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 Chapter 2: Synthesis of self-aligned Ag and Au. .. Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin 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... once Re-suspension of the nanoparticles was done with further dilution using deionised water and sonication Synthesis of monometallic copper nanoparticles 22 Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin 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... excellent candidate to demonstrate the self-alignment property of CD27 Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin 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. .. 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... periods of six months Figure 2.1 UV-visible absorption spectra of Ag nanoparticles colloidal solution 24 Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin 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... 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 Chapter 2: Synthesis of self-aligned Ag and Au. .. 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. .. 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, ... 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 Chapter 2: Synthesis of self-aligned Ag and Au monometallic nanoparticles stabilized with betaCyclodextrin Figure 2.4 UV-visible absorption spectra of Au nanoparticles colloidal ... appearance of 416nm, 43 Chapter Preparation of Ag -Au alloy and Au@ AgI core-shell bimetallic nanoparticles AgI nanoparticles, and 556nm of Au nanoparticles The absorption peak of the Au core was... Preparation of Ag -Au alloy and Au@ AgI core-shell bimetallic nanoparticles (c) (d) Figure 3.6 TEM images of Au@ IAg nanoparticles colloidal solution (a) and (b)wide size distribution of particles (c) and. .. 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