# Synthesis and Optical Properties of Au-Ag Alloy Nanoclusters with Controlled Composition

Authors: J. F. Sánchez-Ramírez; U. Pal; L. Nolasco-Hernández; J. Mendoza-Álvarez; J. A. Pescador-Rojas
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Colloidal solid-solution-like Au-Ag alloy nanoclusters of different compositions were synthesized through citrate reduction of mixed metal ions of low concentrations, without using any other protective or capping agents. Optical absorption of the alloy nanoclusters was studied both theoretically and experimentally. The position of the surface plasmon resonance (SPR) absorption band of the nanoclusters could be tuned from 419 nm to 521 nm through the variation of their composition. Considering effective dielectric constant of the alloy, optical absorption spectra for the nanoclusters were calculated using Mie theory, and compared with the experimentally obtained spectra. Theoretically obtained optical spectra well resembled the experimental spectra when the true size distribution of the nanoparticles was considered. High-resolution transmission electron microscopy (HREM), high-angle annular dark field (HAADF) imaging, and energy dispersive spectroscopy (EDS) revealed the true alloy nature of the nanoparticles with nominal composition being preserved. The synthesis technique can be extended to other bimetallic alloy nanoclusters containing Ag.


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## Body

## 1. Introduction

Recently, much attention has been
paid to the synthesis and characterization of bimetallic nanoparticles due to
their unique catalytic, electronic, optical, structural, and thermal properties
[1–5] and subsequent technological applications as catalysts, sensors, nanoelectronic devices [6–13],
and biosensors [14]. The properties and hence the applicability of these
nanoparticles not only depend on their size and shape, but also on the
combination of the component metals (composition) and their fine structure,
either as alloy or core-shell structures. Bimetallic nanoparticles particularly
with well-defined alloy structures of
noble metals like Pt-Ru, Cu-Pd, Pt-Mo, Pt-W, Pt-Ni, and Au-Ag provide practical examples for the
influence of metals’ composition and their structures on their catalytic
properties [15–19]. Au-Ag nanoparticles of alloy-type structure exhibit
high catalytic activities for low-temperature CO oxidation [12, 18] and aerobic
oxidation of alcohol [20].On the other hand, Au-Ag
bimetallic nanoparticles show different optical responses for alloy and
core-shell configurations, even when they have the same Au and Ag contents. Au-Ag
alloy nanoparticles show a single, composition-sensitive absorption band
located at an intermediate position between pure Au and Ag nanoparticles’
surface plasmon resonance (SPR) peaks, which results in amplification of
light-induced processes (e.g.,
Raman scattering) undergone by molecules localized on their surfaces, giving
rise to surface-enhanced Raman scattering [21]. Thus, the control of structural
type and composition of bimetallic nanoparticles comprised of Ag and Au have
been a subject of considerable interest; the development of simple and versatile methods for controlling the composition and
structure of Au-Ag nanoparticles is an important and challenging task.Bimetallic
Au-Ag nanoparticles have been obtained using different synthesis methods including
replacement reaction [22], biosynthesis [23], green synthesis [24], laser-assisted
[25], alcohol reduction [26], borohydride reduction [27], laser ablation [28], ultrasound irradiation [29], and
metal evaporation-condensation methods [30]. However, relatively few methods
produce true alloy nanoparticles due to phase separation at the atomic level
leading to the formation of core-shell particles [31–35].We have succeeded in synthesizing
true Au-Ag alloy nanoparticles of varying Au/Ag molar ratios in water, by
slightly modifying the citrate reduction method used by Link et al. [36].  Optical and structural properties of the samples
were studied using optical absorption spectroscopy in the UV-Vis range, transmission
electron microscopy (TEM), high-resolution TEM (HREM), and high-angle annular
dark field (HAADF) imaging techniques. Considering effective dielectric constant of the alloy, optical
absorption spectra for the nanoclusters were calculated using Mie theory, and
compared with the experimentally obtained spectra.


## 2. Experimental and Theoretical Calculations

### 2.1. Synthesis

Colloidal dispersions of bimetallic Au-Ag
nanoparticles of different molar ratios were synthesized with a slight
modification of conventional citrate reduction method without using any organic
stabilizer. Typically, the method of citrate reduction involves concentrated
solutions of metallic salts likeHAuCl4(s) and AgNO3(s) and their simultaneous reduction with sodium citrate. However, the presence of
high-concentration Cl− ions produced from the reduction of HAuCl4 does not permit a complete dissolution of silver salts, hence reduction of Ag+ ions due to spontaneous formation of AgCl(s). Considering the solubility
product (Kps) of AgCl(s) (1.8×10−10), to
produce complete solubility of AgNO3, it is necessary to use its solutions
of concentrations lower than the (Kps) of AgCl(s) [37]. To
verify this criterion, we prepared two sets of metal ion solutions with different
concentrations. In the first set, gold and silver ion solutions of 1.32 mM concentrations were
prepared by dissolving HAuCl4(s) and AgNO3(s) in water,
respectively. In the second set, metal ion solutions of 1.32×10−3 mM were
prepared in a similar way. Both solutions were mixed at room temperature at
various Au/Ag molar ratios (=9/1, 3/1, 1/1, 3/1, 1/9) to make a total volume
of 25 mL of bimetallic ion mixture. While for the first batch of ionic
solutions the solubility product was above the solubility product (Kps)
of AgCl(s), in the latter batch, the solubility product was below
the (Kps) of AgCl(s). To reduce the metal ions, 1 mL aqueous
solution of sodium citrate (3.5×10−5 m mol in 5 mL of H2O) was added to the mixture solutions at 100°C temperature under
magnetic agitation, and refluxed for 1 hour. The same procedure was followed to
prepare the monometallic Au and Ag colloids. Homogeneous colloidal dispersions
were formed after the addition of reductor in the metal ion solutions. On adding
the reductor, the color of the HAuCl4 solution changed from clear
yellow to red. In contrast, the AgNO3 solution changed from colorless
to light yellow. The intermediate compositions resulted in colors that are
varying between yellow and red. While the colloidal solutions prepared with the
first batch of solutions (with high metal ion concentrations) did precipitate
after a couple of days, we did not observe any flocculation or precipitation
for the second batch of samples (prepared with very low concentration of metal
ions) even after a couple of months.


### 2.2. Characterizations

Room-temperature
optical absorption spectra of the colloidal samples were recorded using a 10 mm path length quartz
cuvette in a UV-Vis-NIR scanning spectrophotometer (Shimadzu UV 3101PC double
beam). For transmission electron microscopic (TEM) observations, a drop of
colloidal solution was spread on a carbon-coated copper microgrid and dried in
vacuum. For electron microscopy analysis, two microscopes, a Jeol JEM200 and a
Tecnai 200 TEM with field-emission gun by FEI, were used for the low-magnification
and high-resolution observations
of the samples, respectively. High-resolution electron microscope (HREM) images
were digitally processed by using filters in the Fourier space. HAADF images
were recorded with a Jeol 2010F microscope in the STEM mode, using a dark field detector. Energy dispersive
spectroscopy of the samples was performed using a Jeol JSM6390 scanning
electron microscope with NORAN analytical system attached.


### 2.3. Theoretical Calculations

Formal solutions of the phenomena of light
absorption and scattering by small metal particles are obtained using Mie
theory [38–40]. The physical effect of light absorption by the metallic
nanoparticles suspended in liquids is the coherent oscillation of conduction
band electrons (SPR)
through the interaction with electromagnetic filed, where the electronic
transitions between the associated discrete energies give the extinction
(absorption + diffusion) of a part of incident light resulting in a coloring
effect in these systems. The absorption and dispersion of light in
nanoparticles depend on the nature of the metals, along with their chemical composition,
morphology, and sizes. In the case of spherical nanoparticles separated by long
distance, with no substance adsorbed on their surfaces, their absorbance (A) can be calculated as
(1)A=lCextCAuxAgx−1fr3,
where r is the particle radius, Cext is the extinction cross-section for a single particle in nm2, l is the optical path length  in nm, CAuxAgx−1 is
the concentration of the alloy AuxAgx−1 in
g cm−3, and f is a constant which depends on the average
bulk density ρ of the
alloy and is given by
(2)f=4πρ3.
In Table 1, the
concentrations of each of the alloy clusters used for our calculations and the corresponding f values in g cm−3 are presented.Table 1
Concentrations,f values, and bulk free-electron gas parameters for Au/Ag alloys of different compositions.
Molar ratio of Au and AgCAuxAgx−1×10−4 (g cm−3)f
(g cm−3)vf×106
(ms−1)ωd×1013 (s−1)ωp×1016
(s−1)1/02.600186.1831.3903.2251.3509/12.482177.6841.3903.1701.3523/12.306164.9361.3903.0861.3551/12.012143.6891.3902.9481.3601/31.718122.4421.3902.8091.3651/91.541109.6941.3902.7261.3680/11.424101.1951.3902.6701.370The extinction
cross-section of spherical particles embedded in a medium of refractive indexN can be calculated by
(3)Cext=2πk2∑n=1∞(2n+1)ℜ(an+bn),
where k=2πN/λ, and ℜ is the real part of the sum of the scattering
coefficients an and bn which are functions of particle radius and light
wavelength λ (in
terms of the Ricatti-Bessel functions). As the
absorption band of noble metal nanoparticles is dominated by the free-electron
behavior, the dielectric function can be described well through Drude’s model.
The magnitudes of the real and imaginary parts of the dielectric function of
the particles are affected when the particle size is smaller than the mean free
path of the conduction electrons. Considering this size dependence and based on
Drude’s model, size-dependent dielectric functions such as the damping
frequency of the particle ωr can be calculated using the relation [38, 39]
(4)ωr=ωd+Bvfr,
where vf is the electron speed at the Fermi level (Table 1), ωd is the bulk alloy damping constant (Table 1), and B is a theory-dependent factor, close
to one [38].The bulk dielectric functionε(ω)=ε1(ω)+iε2(ω) for bimetallic systems AuxAg(1−x) can be defined by
considering the weighted average for each component:
(5)εav(AuxAg(1−x))=εav(x,ω)=xε(Au)+xε(Ag),
where x is the volume fraction of one component. The size-dependent dielectric
functions of the alloy nanoclusters can be expressed considering Drude’s model as
(6)ε′(ω,r)=εbulk′+ωp2(ω2+ωd2)−ωp2(ω2+ωr2),ε′′(ω,r)=εbulk′′+iωp2ωr[ω(ω2+ωr2)]−iωp2ωd[ω(ω2+ωd2)],
where
ω, ωp, and ωd are the light frequency, plasmon frequency of the bimetallic Au-Ag
alloy, and bulk alloy damping constant, respectively.For calculations, a modified Mie
program developed by Kreibing and Vollmer [38] and compiled in Fortan 77 was
utilized. For this purpose, wavelength-dependent refractive indices of gold and
silver [41] were included in the data bank of the MieCalc program [42]. A fixed
refractive index value (1.33) [43] of water was used in the calculations. For
calculating the optical absorption spectrum of each bimetallic colloid, corresponding
experimentally obtained size distribution histogram (Figure 4) was used. First,
the whole size distribution histogram was divided into several equal size
ranges. For each size range, corresponding average particle size was used for
calculation of absorption spectrum. On obtaining the absorption spectrum for
each of the size ranges, the final absorption spectrum of the sample was
obtained by considering the normalized fraction of particles as the weighted
average of the corresponding sectional absorption spectrum. Throughout the
calculation, the true metal concentrations of the colloids were considered.


## 2.1. Synthesis

Colloidal dispersions of bimetallic Au-Ag
nanoparticles of different molar ratios were synthesized with a slight
modification of conventional citrate reduction method without using any organic
stabilizer. Typically, the method of citrate reduction involves concentrated
solutions of metallic salts likeHAuCl4(s) and AgNO3(s) and their simultaneous reduction with sodium citrate. However, the presence of
high-concentration Cl− ions produced from the reduction of HAuCl4 does not permit a complete dissolution of silver salts, hence reduction of Ag+ ions due to spontaneous formation of AgCl(s). Considering the solubility
product (Kps) of AgCl(s) (1.8×10−10), to
produce complete solubility of AgNO3, it is necessary to use its solutions
of concentrations lower than the (Kps) of AgCl(s) [37]. To
verify this criterion, we prepared two sets of metal ion solutions with different
concentrations. In the first set, gold and silver ion solutions of 1.32 mM concentrations were
prepared by dissolving HAuCl4(s) and AgNO3(s) in water,
respectively. In the second set, metal ion solutions of 1.32×10−3 mM were
prepared in a similar way. Both solutions were mixed at room temperature at
various Au/Ag molar ratios (=9/1, 3/1, 1/1, 3/1, 1/9) to make a total volume
of 25 mL of bimetallic ion mixture. While for the first batch of ionic
solutions the solubility product was above the solubility product (Kps)
of AgCl(s), in the latter batch, the solubility product was below
the (Kps) of AgCl(s). To reduce the metal ions, 1 mL aqueous
solution of sodium citrate (3.5×10−5 m mol in 5 mL of H2O) was added to the mixture solutions at 100°C temperature under
magnetic agitation, and refluxed for 1 hour. The same procedure was followed to
prepare the monometallic Au and Ag colloids. Homogeneous colloidal dispersions
were formed after the addition of reductor in the metal ion solutions. On adding
the reductor, the color of the HAuCl4 solution changed from clear
yellow to red. In contrast, the AgNO3 solution changed from colorless
to light yellow. The intermediate compositions resulted in colors that are
varying between yellow and red. While the colloidal solutions prepared with the
first batch of solutions (with high metal ion concentrations) did precipitate
after a couple of days, we did not observe any flocculation or precipitation
for the second batch of samples (prepared with very low concentration of metal
ions) even after a couple of months.


## 2.2. Characterizations

Room-temperature
optical absorption spectra of the colloidal samples were recorded using a 10 mm path length quartz
cuvette in a UV-Vis-NIR scanning spectrophotometer (Shimadzu UV 3101PC double
beam). For transmission electron microscopic (TEM) observations, a drop of
colloidal solution was spread on a carbon-coated copper microgrid and dried in
vacuum. For electron microscopy analysis, two microscopes, a Jeol JEM200 and a
Tecnai 200 TEM with field-emission gun by FEI, were used for the low-magnification
and high-resolution observations
of the samples, respectively. High-resolution electron microscope (HREM) images
were digitally processed by using filters in the Fourier space. HAADF images
were recorded with a Jeol 2010F microscope in the STEM mode, using a dark field detector. Energy dispersive
spectroscopy of the samples was performed using a Jeol JSM6390 scanning
electron microscope with NORAN analytical system attached.


## 2.3. Theoretical Calculations

Formal solutions of the phenomena of light
absorption and scattering by small metal particles are obtained using Mie
theory [38–40]. The physical effect of light absorption by the metallic
nanoparticles suspended in liquids is the coherent oscillation of conduction
band electrons (SPR)
through the interaction with electromagnetic filed, where the electronic
transitions between the associated discrete energies give the extinction
(absorption + diffusion) of a part of incident light resulting in a coloring
effect in these systems. The absorption and dispersion of light in
nanoparticles depend on the nature of the metals, along with their chemical composition,
morphology, and sizes. In the case of spherical nanoparticles separated by long
distance, with no substance adsorbed on their surfaces, their absorbance (A) can be calculated as
(1)A=lCextCAuxAgx−1fr3,
where r is the particle radius, Cext is the extinction cross-section for a single particle in nm2, l is the optical path length  in nm, CAuxAgx−1 is
the concentration of the alloy AuxAgx−1 in
g cm−3, and f is a constant which depends on the average
bulk density ρ of the
alloy and is given by
(2)f=4πρ3.
In Table 1, the
concentrations of each of the alloy clusters used for our calculations and the corresponding f values in g cm−3 are presented.Table 1
Concentrations,f values, and bulk free-electron gas parameters for Au/Ag alloys of different compositions.
Molar ratio of Au and AgCAuxAgx−1×10−4 (g cm−3)f
(g cm−3)vf×106
(ms−1)ωd×1013 (s−1)ωp×1016
(s−1)1/02.600186.1831.3903.2251.3509/12.482177.6841.3903.1701.3523/12.306164.9361.3903.0861.3551/12.012143.6891.3902.9481.3601/31.718122.4421.3902.8091.3651/91.541109.6941.3902.7261.3680/11.424101.1951.3902.6701.370The extinction
cross-section of spherical particles embedded in a medium of refractive indexN can be calculated by
(3)Cext=2πk2∑n=1∞(2n+1)ℜ(an+bn),
where k=2πN/λ, and ℜ is the real part of the sum of the scattering
coefficients an and bn which are functions of particle radius and light
wavelength λ (in
terms of the Ricatti-Bessel functions). As the
absorption band of noble metal nanoparticles is dominated by the free-electron
behavior, the dielectric function can be described well through Drude’s model.
The magnitudes of the real and imaginary parts of the dielectric function of
the particles are affected when the particle size is smaller than the mean free
path of the conduction electrons. Considering this size dependence and based on
Drude’s model, size-dependent dielectric functions such as the damping
frequency of the particle ωr can be calculated using the relation [38, 39]
(4)ωr=ωd+Bvfr,
where vf is the electron speed at the Fermi level (Table 1), ωd is the bulk alloy damping constant (Table 1), and B is a theory-dependent factor, close
to one [38].The bulk dielectric functionε(ω)=ε1(ω)+iε2(ω) for bimetallic systems AuxAg(1−x) can be defined by
considering the weighted average for each component:
(5)εav(AuxAg(1−x))=εav(x,ω)=xε(Au)+xε(Ag),
where x is the volume fraction of one component. The size-dependent dielectric
functions of the alloy nanoclusters can be expressed considering Drude’s model as
(6)ε′(ω,r)=εbulk′+ωp2(ω2+ωd2)−ωp2(ω2+ωr2),ε′′(ω,r)=εbulk′′+iωp2ωr[ω(ω2+ωr2)]−iωp2ωd[ω(ω2+ωd2)],
where
ω, ωp, and ωd are the light frequency, plasmon frequency of the bimetallic Au-Ag
alloy, and bulk alloy damping constant, respectively.For calculations, a modified Mie
program developed by Kreibing and Vollmer [38] and compiled in Fortan 77 was
utilized. For this purpose, wavelength-dependent refractive indices of gold and
silver [41] were included in the data bank of the MieCalc program [42]. A fixed
refractive index value (1.33) [43] of water was used in the calculations. For
calculating the optical absorption spectrum of each bimetallic colloid, corresponding
experimentally obtained size distribution histogram (Figure 4) was used. First,
the whole size distribution histogram was divided into several equal size
ranges. For each size range, corresponding average particle size was used for
calculation of absorption spectrum. On obtaining the absorption spectrum for
each of the size ranges, the final absorption spectrum of the sample was
obtained by considering the normalized fraction of particles as the weighted
average of the corresponding sectional absorption spectrum. Throughout the
calculation, the true metal concentrations of the colloids were considered.


## 3. Results and Discussion

Nanoparticles of noble metals such as gold and silver in the size scale smaller
than wavelengths of visible light strongly scatter and absorb light due to
surface plasmon resonance (SPR, collective oscillation of conduction electron
induced by incident light). The frequency and intensity of SPR band depend on
the size, shape, structure, and composition [38, 39, 44] of the metal nanoparticles.
While the techniques like TEM and HREM reveal the exact size and shape of those
nanoparticles, optical absorption spectra give a quick and gross idea on their
shapes and sizes. In this study, we used TEM, HREM, and HAADF, along with
optical absorption spectroscopy to study the size, size distribution, fine
structure, and composition homogeneity of bimetallic Au/Ag colloids of
different compositions.Absorption spectra of the bimetallic colloids prepared
with higher concentrations of metal ions (first set of metal ion solutions) are
presented in Figure1. The monometallic dispersions of Au and Ag revealed
absorption peaks at about 520 and 413 nm, respectively. While the SPR peaks for
the bimetallic colloids prepared with Au/Ag=9/1 and 3/1 appeared at the same
position as that of monometallic Au, for the colloids with Au/Ag=1/3, the SPR
peak position appeared around 418 nm, which is very close to the SPR position
of monometallic Ag. Appearance of no absorption peak in the case of Au/Ag=1/9
indicates the formation of neither type of nanoparticles. On the other hand,
the SPR peak positions (Figure 1(b)) of the colloidal solutions do not obey any
linear or quasilinear relationship with the Au mol fraction in the final
reaction mixtures. These characteristics demonstrate that with the utilization
of metal ion concentrations higher than the solubility product (Kps) of AgCl(s), it is not possible
to obtain Au/Ag alloy nanoparticles.Optical absorption spectra of the bimetallic
colloidal samples prepared at different Au/Ag molar ratios, using metal ion
solutions of high concentrations (first set of metal ion solutions). The
positions of the SPR peaks are plotted against Au mol fraction at the right.
(a)(b)In Figure2, the absorption
spectra of five bimetallic Au/Ag colloidal dispersions prepared with low concentration of metal ion
solutions are presented. For comparison, absorption spectra of monometallic Au and Ag colloids
are also presented. The peaks related to the SPR
of Au and Ag particles were revealed at about 519 nm and 407 nm, which are
consistent with the SPR peak positions of gold and silver nanoparticles, respectively
[36]. There appeared only one absorption peak for
each of the bimetallic colloids. The SPR absorption peaks for the bimetallic colloids
are revealed in between the SPR peak positions of monometallic Au and
monometallic Ag colloids. Such absorption spectra cannot be obtained either due
to the simple physical mixture of monometallic Au and Ag colloidal dispersions,
or due to formation of core-shell Au-Ag nanoparticles, where there appear two
characteristic absorption peaks [32, 45]. The SPR peak position was blue-shifted
in a quasilinear fashion with an increasing Ag
content due to the variation of composition of the bimetallic nanoparticles [36].
These observations strongly suggest that each bimetallic particle is a homogeneous Au/Ag alloy one and the novel absorption
bands are attributed to the SPR bands of Au-Ag alloy nanoparticles.Figure 2
Optical absorption spectra of bimetallic colloids prepared with different Au/Ag
molar ratios, using low metal ion concentrations (second set of metal ion solutions).In Figure3, the experimentally
obtained SPR peak positions are plotted against the molar ratios of Au and Ag
in the reaction mixtures. A quasilinear variation of the SPR peak position with
Au mole fraction infers that the composition of the alloy colloids corresponds
to the initial concentrations of gold and silver  ions in the reaction mixtures.
On the other hand, the SPR peak positions of the colloids obtained from  their
theoretically calculated absorption spectra (presented latter) are also presented
in Figure 3 (open squares). Close resemblance of the theoretically obtained SPR
position values with the corresponding experimentally obtained SPR peak
positions indicates that the bimetallic nanoclusters are of true alloy
character and of compositions very close to
their nominal compositions. A similar variation of SPR peak position with the
variation of Au mole fraction in bimetallic alloy clusters has been observed by
Peng et al. [25] for their Au-Ag colloids prepared by laser-assisted synthesis.
Therefore, by controlling the initial concentrations of gold and silver ions in
the reaction mixtures, we could control the SPR frequency of Au-Ag alloy nanoparticles.
It must be noted that the variation of our SPR peak position calculated
considering the bulk dielectric constants of Au and Ag is not exactly linear as
observed by Link et al. [36] considering the dielectric constant of alloy thin
film. However, the variation resembles extremely well the experimentally
observed variation. In the further parts of this article, we will discuss only
the second batch of samples (prepared with lower concentrations of metal ions in the reaction
mixture) for which nanoparticles of uniform composition were obtained.Figure 3
Relationship between Au content and surface
plasmon resonance peak position of Au/Ag alloy nanoparticles (for the second set of metal ion solutions):
(★) experimental and (□) calculated.Typical TEM micrographs and corresponding size
distribution histograms for the Au/Ag bimetallic nanoparticles (prepared using
second set of metal ion solutions) with different molar ratios of Au/Ag: (a) 1/0,
(b) 9/1, (c) 3/1, (d) 1/1, (e) 1/3, (f) 1/9, and (g) 0/1. Average particle sizeϕ and standard deviation σ are calculated from the Gaussian fittings of
the histograms.In order to determine the size of
the nanoparticles, TEM analysis has been performed. In Figure4, typical TEM micrographs
of the bimetallic nanoparticles prepared with different molar ratios of Au and Ag
(Au/Ag=9/1, 3/1, 1/1, 1/3, and 1/9) and their respective size distribution histograms
are presented. For the histograms, the size of more than 60 particles was
measured. The images reveal the formation of nearly spherical nanometric particles.
The average particle size varied from 19.4 to 43.2 nm depending on the
concentrations of two metals in them. However, the size dispersion decreases
with the increase of Au content in them. Absence of bimodal size distribution
in the size distribution histograms suggests that the nanoparticles obtained by
our synthesis process are not the simple mixture of monometallic particles of
Au and Ag. The average particle size increased with the increase of Ag contents
in bimetallic nanoparticles until about 75%. Further increase of Ag content reduced
the average diameter of the bimetallic nanoparticles. Dependence of particle size
on composition for bimetallic nanoparticles has been studied by several
researchers. Esumi et al. [46] and Wu
et al. [47] have also observed a positive deviation of mean diameter for Pd/Pt
bimetallic nanoparticles on composition.The increase of particle size on increasing
the Ag content in the bimetallic nanoparticles can be explained by considering
the reduction velocity of the individual metal ions. As the reduction potential
of Ag (0.8 V) is smaller than the reduction potential of Au (1.5 V), the
velocity of reduction of Au ions is faster than the velocity of reduction of Ag
ions. So, an increasing Ag ion content in the mixture solution inhibits the
nucleation rate of the bimetallic nanoparticles, and as a consequence the
bimetallic particles grow bigger and dispersed [48].For a closer observation on the crystallinity, composition, and fine
structure of the bimetallic nanoparticles, HREM images of two samples (Au/Ag=1/1 and 1/3) were recorded. From the HREM images (Figure 5), we can observe the
formation of well crystalline multiple twined nanoparticles for both samples.
In fact, most of the bimetallic nanoparticles formed in our bimetallic systems
were multiple twined. The lattice spacings in the nanoparticles are clearly
visible in the amplified images of the selected areas presented as insets. Interplanar spacing
calculated for both samples corresponds well with the interplanar spacing of the corresponding
alloy structures of fcc phase. However, due to very similar lattice constant values
of gold (0.408 nm) and silver (0.409 nm), it was not possible to confirm the
alloy nature of the particles with certainty from their HREM images. To confirm
the compositional homogeneity of the nanoparticles, we recorded the HAADF
images of the alloy particles.High-resolution electron micrographs of bimetallic colloidal particles: (a)Au/Ag=1/1 and (b) Au/Ag=1/3 (prepared using second set of metal ion
solutions). Amplified images of the marked portions are given as insets.
(a)(b)HAADF imaging uses high-angle
scattered electrons to obtain spatially resolved images which are able to show
suitable details about the compositional inhomogeneity and structural characteristic
of bimetallic nanoparticles. High-angle scattering is associated with electron
interaction close to the nucleus of the atoms which constitute the sample
(Rutherford scattering). So, the scattering phenomenon is strongly dependent on
the atomic number (Z). The HAADF image contrast is proportional to the 1.7 power
of the average atomic number in the atomic column.  As the Z of Ag (47) and Au (79) are amply different, the
Z-contrast imaging method should be able to determine the possible inhomogeneity
in chemical composition and structural conformation within the bimetallic
nanoparticles. Figure6 shows the typical HAADF images of our bimetallic
nanoparticles prepared with Au/Ag=1/1 and Au/Ag=1/3 compositions. We can
perceive only a homogeneous image contrast for both samples. No significant
variation in contrast/intensity over the entire volume of the nanoparticles
confirms a homogeneous distribution of metal atoms (Au and Ag) in them. On the contrary,
we could have observed HAADF images with inhomogeneous contrasts, as in the case
of core-shell nanoparticles [27]. Therefore, the bimetallic nanoparticles
obtained under the present synthesis conditions are of homogeneous
solid-solution alloys of Au and Ag.Typical HAADF
images of (a) Au/Ag(=1/1) and (b) Au/Ag (=1/3) bimetallic nanoparticles
(prepared using second set of metal ion solutions) showing homogeneous
elemental distribution.
(a)(b)To verify the composition of the nanoparticles, energy
dispersive spectroscopy (EDS) analysis on the particles was carried out inside
the transmission electron microscope for three sets of samples with
compositions ofAu/Ag=1/1, 1/3, and 1/9. We could not detect any particle of
monometallic nature in them. Though the composition of individual particles
varied from particle to particle for a given sample, the average composition
remained close to its nominal value. The global composition of the samples was
also monitored through EDS spectroscopy in a scanning electron microscope. In Figure 7, the EDS spectra of the samples Au/Ag=1/1 and 3/1 are presented. Along with
the Au and Ag emission peaks, there appeared carbon, oxygen, and copper
emissions in the EDS spectra of the samples. While the origin of the carbon,
oxygen, and copper emissions is related to the carbon film on the grid surface,
citrate salt, and copper microgrid, respectively, the obtained Au/Ag atomic ratios
for the samples fairly agree with their nominal molar ratios (Au/Ag=1.05 and
3.02 for the samples prepared with nominal Au and Ag molar ratios of 1/1 and 3/1,
resp.).EDS spectra of the colloidal samples (prepared with
second set of metal ion solutions) with nominal molar ratios: (a)Au/Ag=1/1 and (b) Au/Ag=3/1.
(a)(b)Calculated optical absorption spectra of the colloids with different molar
ratios of Au and Ag are presented in Figure8. The shape, intensity, and
position of the SPR peak in our calculated absorption spectra were in close
resemblance with the corresponding experimentally obtained absorption spectra. The
intensity of the SPR peak increases and shifts towards shorter wavelengths with
the increase of silver concentration.Figure 8
Calculated optical
absorption spectra for Au-Ag bimetallic colloidal solutions with different molar
ratios of Au and Ag.


## 4. Conclusions

Au-Ag nanoclusters of solid-solution
alloy type could be prepared by simultaneous reduction of gold and silver ions
using sodium citrate as reductor. For obtaining bimetallic clusters of true alloy
type while usingHAuCl4 and AgNO3 as precursors, the
solubility product of the metal ion solutions should be kept below the
solubility product (Kps) of solid AgCl(s). On the
contrary, chlorine ions from HAuCl4 react with silver ions forming solid AgCl, thereby inhibiting
the formation of bimetallic alloy clusters. While the composition of the
bimetallic alloy clusters could be maintained close to their nominal
compositions, the SPR absorption band positions could be tuned precisely close
to their theoretical values. Considering true size distribution of the
nanoparticles and a modified Mie theory, the optical absorption spectra of the
colloids could be reproduced theoretically. Bimetallic Au-Ag alloy
nanoparticles of any composition can be prepared using our synthesis method.



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*Source: 620412-2008-09-24.xml*