Facile one-pot green synthesis of Au-Ag alloy nanoparticles using sucrose and their composition-dependent photocatalytic activity for the reduction of 4-nitrophenol

Article abstract of DOI:10.1039/c7dt03850j

Au-Ag alloy nanoparticles (NPs) less than 10 nm in size were synthesized using sucrose as a reductant and surfactant. Au-Ag alloy NPs with a homogeneous composition were continuously obtained by changing the synthesis time from 2 to 40 min in one pot. Bas

Full text of DOI:10.1039/c7dt03850j

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Facile one-pot green synthesis of Au-Ag alloy nanoparticles using
sucrose and their composition-dependent photocatalytic activity
for the reduction of 4-nitrophenol
Li Sun, a* Yuechao Yin, a Fa Wang, a Wenxian Su a and Lixin Zhang a
Au-Ag alloy nanoparticles (NPs) less than 10 nm in size were synthesized using sucrose as reductant and surfactant. Au-Ag
alloy NPs with homogeneous composition were continuously obtain by changing the synthesis time from 2 to 40 min in
one-pot. Based on the UV-vis, ICP, TEM, HR-TEM, EDX and SAED analyses, the synthesis mechanism of Au-Ag alloy NPs was
deduced. Under hydrolysis conditions, sucrose showed a stronger reducibility compared with single glucose, fructose and
their mixture. And as-prepared Au-Ag alloy NPs exhibited a superior photocatalytic activity and stability for the reduction
of 4-nitrophenol due to the abundance hydroxyl groups of sucrose and the synergistic effect between Au and Ag elements.
The rate constant of 4-nitrophenol reduction could be linearly controlled by the composition of Au-Ag alloy NPs or their
synthesis time. It was indicated that the photocatalytic activity of Au-Ag alloy NPs could be predetermined as early as their
synthesis process. The above methods to control the rate constant provide promising routes for other photocatalytic
reactions using bimetallic NPs as photocatalysts.
Received 00th October 20xx,
Accepted 00th October 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
been no definite correlations between the rate constant and the
morphology, microstructure and composition of Au-Ag alloy NPs.
The above characteristics of Au-Ag alloy NPs are dependent on their
Introduction
As of today, Au-Ag bimetallic nanoparticles (NPs) have drawn
attention due to their unique catalytic,1-3 optical,4,5 and electronic
properties.6 Especially in the field of catalysis, Au-Ag bimetallic NPs
have been applied into the low-temperature CO oxidation reaction
and revealed a high catalytic activity because of the synergistic
effect between Au and Ag elements.7 In the aspect of
photocatalysis, it was found that Au-Ag alloy NPs had an obvious
photocatalytic activity for the degradation of many organic
pollutants in the wastewater.2,8,9 Compared with semiconductor
photocatalysts, Au-Ag alloy NPs can make full use of solar photons
in the visible spectral region.10,11 Generally, the reduction of 4-
nitrophenol (4-NP) as a model reaction is used to confirm the
photocatalytic activity of biosynthesized noble metal NPs.9,12-14 4-NP
is relatively stable in biological systems and can lead to health
hazards in humans and animals.15 The single product of 4-NP
reduction is non-toxic 4-aminophenol (4-AP), which is an important
intermediate in drug synthesis.3,15 Therefore, it is necessary to
converse 4-NP to 4-AP for the environmental remediation and the
industrial production. Up to now, although Au-Ag alloy NPs have
shown an obvious photocatalytic activity for the reduction of 4-NP,
the rate constant of 4-NP reduction is not very high. And there have
synthesis method.
The methods, employed to synthesize Au-Ag alloy NPs, can be
divided into two groups: physical and chemical methods.16-22 Most
of these reported methods rely on complex process, high cost, or
strong ligands, which can pose potential environmental and
biological risks and then limit the application of Au-Ag alloy NPs as
photocatalysts in wastewater treatment. The increase of the
awareness towards green chemistry has resulted in a desire to
develop an eco-friendly method for the synthesis of Au-Ag alloy
NPs. In recent years, green synthesis methods, especially using the
plant extracts as reductants, have been considered as an eco-
friendly alternative to conventional physical and chemical methods
because of their obvious advantages in safety, cost-effectiveness
and biocompatibility.3,23-25 Based on the fruit juice of pomegranate,
the Aloysia triphylla plant extract, and the Silybum marianum seed
extract, Au-Ag alloy NPs were synthesized without the addition of
any chemical agent.9,26,27 However, in these methods, the ratio of
Au and Ag element in Au-Ag alloy NPs was nearly equal to the
Au3+/Ag+ molar ratio in the raw materials, which implied it was
difficult to flexibly adjust the composition of Au-Ag alloy NPs in one-
pot. Up to now, the continuous synthesis technology of Au-Ag alloy
NPs based on the synthesis time in one-pot was carried out only by
co-reduction of Au and Ag salts using chemical agents (oleylamine
and octadecene) as reductant and surfactant.20,28
a
School of Energy and Power Engineering, University of Shanghai for Science and
Technology, Jun Gong Road 516, Shanghai 200093, China
E-mail: marybrother@yeah.net; Fax: +86 21 55272376; Tel: +86 21 55272740
†Electronic Supplementary Information (ESI) available. See
10.1039/x0xx00000x
.
The saccharide, widely exists in fruits and vegetables, is often
considered as an important kind of reducing agent in green
synthesis methods.29,30 Obviously, the research on the saccharide
DOI:
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plays a guiding role in the selection of the plants for the green 25 mL of 0.5 M sucrose aqueous solution in a flask was slowly
synthesis of NPs. In the present work, monosaccharide (glucose and heated to boiling with vigorous stirring. After a hydrolysis time of 10
fructose), disaccharide (sucrose and maltose), and polysaccharide min, the mixture of chloroauric acid solution and silver nitrate
(soluble starch with different structure) as reductants have been solution was injected into above hot solution and then kept boiling
used to prepare noble metal NPs.29-33 Most of these synthesis until the end of the reaction. The Au-Ag alloy NPs were continuously
reactions needed long synthesis time and occurred only under the obtained in one-pot only by adjusting the synthesis time from 2 to
conditions of acid/alkali treatment, microwave irradiation or high 60 min. Similar process was employed to synthesize Au-Ag alloy NPs
pressure thermal treatment.29,32,34 To the best of authors’ individually using glucose, fructose and the mixture of glucose and
knowledge, few works in literature reported
a totally green fructose (1:1) instead of sucrose as reductant with or without the
synthesis of Au-Ag alloy NPs just using saccharide as reductant and addition of 100 mL of 1 M NaOH solution.
surfactant without the addition of any other chemical agent. Due to
Characterization
the weak capping property of monosaccharide, they often need to
combine with macromolecular substances to increase the stability
of the synthesis system.35 Raveendran et al synthesized Au-Ag alloy
NPs in a glucose-starch combination scheme.29 In the process, alkali
was required to enhance the reducibility of glucose and starch was
demanded to improve the stability of obtained NPs. And the
composition of Au-Ag alloy NPs couldn’t be continuously adjusted in
one-pot. For disaccharide, Filippo et al reported monodisperse Ag
NPs with a small size of 5.2 nm were prepared only using sucrose as
reductant and surfactant under mild hydrolysis conditions.30 It
indicated that the sucrose had excellent reducing and capping
properties. Generally, the noble metal NPs with a small size possess
a high photocatalytic activity because of their high specific surface
area.3 Accordingly, the sucrose can have a potential application in
the green synthesis of Au-Ag alloy NPs with a high photocatalytic
activity.
The zeta potential (Zeta PLAS, Malvern Instruments) was used to
analyze the potential stability of Au-Ag alloy NPs synthesized by the
sucrose solution. The molar ratio of Au and Ag element in Au-Ag
bimetallic NPs was measured by means of Varian 710 inductively
coupled plasma spectrometry (ICP-MS). The samples for ICP were
solubilized in aqua regia (HCl/HNO3) in advance. Ultraviolet visible
spectrum (UV-vis) analysis was carried out on a spectrophotometer
(Cary 50, Varian) using deionized water as the dispersing agent. The
relationship between the SPR wavelength and the composition of
Au-Ag alloy NPs was investigated by UV-vis absorption spectra and
ICP. High resolution transmission electron microscope (HR-TEM)，
energy-dispersive X-ray spectroscopy (EDX) and selected area
electron diffraction (SAED) analyses were performed on an electron
microscope (JEM-2100F, JEOL). The sample was prepared by
depositing a drop of Au-Ag alloy NPs hydrosol on an amorphous
carbon-copper grid and then drying at room temperature in air. For
each sample, the average diameter and the standard deviation were
determined by averaging measurements of more than 100 NPs. X-
Here, we tried the green synthesis of Au-Ag alloy NPs in one-pot
using sucrose as reductant and surfactant without any toxic or
polluting chemical agent. The synthesis parameters were
systematically investigated to flexibly control the surface plasmon
resonance (SPR) wavelength, the composition and the
microstructure of Au-Ag alloy NPs. Au-Ag alloy NPs were
characterized by UV-vis, ICP, TEM, HR-TEM, EDX, SAED, XPS, and
XRD, and a possible synthesis mechanism was further deduced.
Obtained Au-Ag alloy NPs were used as photocatalysts for the
ray photoelectron spectroscopy (XPS) was acquired by
a
spectrometer (ESCALAB 250Xi, Thermo Fisher Scientific). X-ray
diffraction (XRD) patterns were recorded on the X-ray
diffractometer (D/MAX 2550 VB/PC, Rigaku). The samples for XPS
and XRD were made by depositing NPs on silicon wafers.
reduction of 4-NP and their photocatalytic activity and stability Photocatalytic reduction of 4-nitrophenol
were studied in details.
2.5 mL of 1 M freshly prepared aqueous solution of NaBH4 was
mixed with 2.5 mL of 10 mM 4-NP in a standard quartz cuvette.
Then 500 mL of 0.1 mM Au-Ag alloy NPs hydrosol as photocatalyst
was added into the above reaction mixture. After that, the UV-vis
spectra were recorded at every 30 s interval in the range of 600-200
Experimental section
Materials
Silver nitrate (AgNO3, GR), chloroauric acid (HAuCl4·3H2O, AR), nm at room temperature under sunlight irradiation. The reduction
sucrose (C12H22O11), glucose (C6H12O6), fructose (C6H12O6), sodium of 4-NP was followed by monitoring the changes in the absorbance
borohydride (NaBH4, AR), 4-nitrophenol (C6H5NO3, AR), sodium of the initially observed peak at 400 nm (4-nitrophenolate anion) as
hydroxide (NaOH) and ethanol (C2H5OH, GR) were purchased from a function of time. Au-Ag alloy NPs were stored at 20 oC and then
Aladdin Reagent Inc. (Shanghai, China) and used directly as the raw the stability of their photocatalytic properties was measured at
materials without further treatment. Deionized water was used every 15 days up to 3 months.
throughout this work.
Results and discussion
Synthesis of Au-Ag Alloy NPs
Synthesis and Characterization of Au-Ag alloy NPs
Typically, 0.1 M chloroauric acid solution and 0.01 M silver nitrate
solution were added into 5 mL of deionized water under sonication
for 5 min. The quantity of chloroauric acid solution was fixed at 25
mL. The Au3+/Ag+ molar ratio in the raw materials was varied from
1:5 to 1:40 through changing the amount of silver nitrate solution.
The optical behavior of Au-Ag bimetallic NPs directly reflects their
composition, microstructure, morphology and size distribution,
which are dependent on their synthesis parameters (the synthesis
temperature, the sucrose concentration, the synthesis time and the
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Au3+/Ag+ molar ratio in the raw materials).36 The synthesis
temperature is constant at the boiling point of the sucrose solution Here, Yw is the SPR wavelength, and
Yw = 117
X
+ 413
(2)
X
is the Au molar fraction in Au-
for the generation of homogeneous Au-Ag alloy NPs.20,28 According Ag alloy NPs. Based on the linear fashion, it can be inferred that as-
to the synthesis process of Au NPs (Fig. S1 and Fig. S2a in SI), the prepared Au-Ag bimetallic NPs have homogeneous alloy
sucrose concentration is fixed at 0.5 M in order to obtain Au-Ag microstructure.
alloy NPs with a small size. The zeta potential of as-prepared Au-Ag
alloy NPs suspension is -38.7 mV (Fig. S3 in SI), which suggests that
0.5 M sucrose solution is suitable for the synthesis of stable Au-Ag
alloy NPs.37 Then, the other two main synthesis parameters were
investigated in details. Figure 1 exhibits the UV-vis absorption
spectra of Au-Ag bimetallic NPs synthesized at different synthesis
time and Au3+/Ag+ molar ratio in the raw materials. In Fig. 1a, b and
c, the SPR wavelength of Au-Ag bimetallic NPs synthesized at 10 min
is longer than 500 nm, which implies the reduction rate of Au3+ ions
is faster than that of Ag+ ions in the boiling sucrose solution. After
that, the SPR wavelength is blue-shifted between 530 and 430 nm
with the increase of the synthesis time from 10 to 50 min, which
indicates the amount of reduced Ag+ ions with respect to reduced
Au3+ ions is increased due to a less loss of Ag+ ions at the initial
reaction stage and its higher molar ratio in the raw materials.
Generally, Au-Ag alloy NPs present a single SPR wavelength, while
the mixture of Au, Ag and Au-Ag bimetallic NPs possesses two to
Fig. 1 UV-vis absorption spectra of Au-Ag bimetallic NPs synthesized at
four wavelengths between 530 and 415 nm.28,38 As shown in Fig. 1a,
different Au3+/Ag+ molar ratio in the raw materials. (a) 1:5, (b) 1:10, (c) 1:20,
the UV-vis absorption spectrum of Au-Ag bimetallic NPs synthesized
(d) 1:40. The synthesis time was changed from 10 to 50 min.
at 50 min displays two peaks (438 nm, 520 nm), which illustrates as-
obtained product contains various NPs. With the decrease of the
Au3+/Ag+ molar ratio in the raw materials from 1:5 to 1:40, the UV-
vis absorption spectrum of Au-Ag bimetallic NPs synthesized at 50
min gradually shows only one wavelength (in Fig. 1b, c, d). It was
suggested that pure Au-Ag alloy NPs could be obtained only when
the Au3+/Ag+ molar ratio in the raw materials was lower than 1:5. In
Fig. 1d, when the Au3+/Ag+ molar ratio is decreased to 1:40, two SPR
peaks (400 nm, 515 nm) appear in the UV-vis absorption spectrum
of Au-Ag bimetallic NPs synthesized at 10 min. It was implied that
Ag+ ions was excess in the raw materials. Based on above discussion,
the optimal Au3+/Ag+ molar ratio was considered as 1:20 (Fig. 1c). At
Fig. 2 The SPR wavelength dependence of the synthesis time from 2 to 60
min. The Au3+/Ag+ molar ratio in the raw materials was kept at 1:20. Inset is
the corresponding photographs.
the optimal ratio, the UV-vis absorption spectrum of Au-Ag
bimetallic NPs exhibited only one SPR wavelength and their SPR
wavelength could be continuously adjusted from 520 to 430 nm by
increasing the synthesis time from 2 to 40 min (Fig. 1c and Fig. 2). In
Fig. 2, the color of Au-Ag bimetallic NPs transforms from purple to
dark yellow and the approximately linear relationship between the
SPR wavelength and the synthesis time is shown as follows:
ꢀꢁ = -2.45tꢂ522
(1)
Here,
t is the synthesis time from 2 to 40 min and Yw is the SPR
wavelength. According to the formula, the SPR wavelength of Au-Ag
bimetallic NPs can be linearly controlled only by changing the
synthesis time in one-pot.
Generally, the homogeneous alloy microstructure of Au-Ag
bimetallic NPs can be confirmed through examining a single SPR
wavelength and a linear relationship between the SPR wavelength
of Au-Ag bimetallic NPs and the Au molar fraction in Au-Ag
bimetallic NPs.19,38 In Fig. 1c, Au-Ag bimetallic NPs exhibits only one
SPR wavelength in their UV-vis absorption spectrum. In order to
obtain the exact composition of Au-Ag bimetallic NPs, ICP analysis
was performed. In Fig. 3A, the relationship between the SPR
wavelength and Au molar fraction is fitted linearly as follows:
Fig. 3 (A) Plot of the SPR wavelength against the Au molar fraction in Au-Ag
bimetallic NPs. (B) Plot of the Au molar fraction in Au-Ag bimetallic NPs
against the synthesis time. Au-Ag bimetallic NPs were synthesized at the
Au3+/Ag+ molar ratio of 1:20 at the different time (a: 10 min, b: 20 min, and c:
30 min).
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According to Fig. 3B, the relationship of the composition of Au- monosaccharide and carboxylic acids.30 Among these thick repelling
Ag alloy NPs and their synthesis time can be fitted linearly as layers, the strong electrostatic repulsion forces were generated,
follows:
which could offset the Van Der Waals forces responsible for atom
= -0.023 (3) deposition. When the electrostatic repulsion force was strong
is the Au molar fraction in Au-Ag alloy NPs, and t is the enough to overcome the Van Der Waals force, the deposition of Au
X
t + 0.975
Here,
X
synthesis time ranged from 2 to 40 min. Obviously, it is facile to and Ag atoms would be prevented and then new nuclei (Au-Ag
control the composition of Au-Ag alloy NPs based on their synthesis bimetallic, Au or Ag nuclei) would be generated. Based on these
time in one-pot.
new nuclei, new NPs with a small size were produced, which could
In order to further evaluate the exact size and microstructure of result in an increasing standard deviation. It can also be seen from
Au-Ag alloy NPs, the morphological analysis is indispensable. TEM Fig. 4(d-1), these new NPs have a small average size of 4.3 nm.
images, displayed in Fig. 4a-c, are used to observe the morphology According to Mie, the variety of the size distribution could also be
of Au-Ag alloy NPs. It clearly shows that Au-Ag alloy NPs are sphere- reflected from the half width at half maximum (HWHM) of the UV-
like, monodisperse, and less than 10 nm in size, which illustrates the vis absorption spectrum.39,40 As shown in Fig. 4e, the HWHM of Au-
sucrose is suitable to act as reductant and surfactant to obtain small Ag alloy NPs follows the same change trend with their size
and homogeneous NPs. The above morphology is similar as that of distribution obtained from TEM images. EDX elemental line
Au-Ag alloy NPs synthesized by chemical agents, which suggests scanning was used to observe the composition of those small NPs
obtained Au-Ag alloy NPs can have a comparable photocatalytic synthesized at 50 min. It can be found from Fig. 4f that the
activity.7,20 When Au-Ag alloy NPs are synthesized at 10 min, their observed small NP contains both Ag and Au elements and the
SPR wavelength is at 498 nm (Fig. 4e) and their average size is about distribution of the two elements in the NP is similar. It implies the
6 nm (Fig. 4a). Increasing the synthesis time to 40 min, these Au-Ag small NP also has an alloy microstructure.
alloy NPs grow to 9 nm (Fig. 4c) and their SPR wavelength is blue-
Fig. 4g is the HR-TEM image, which shows Au-Ag alloy NPs tend
multiple-twinned crystal microstructure.41 The
shifted to 430 nm (Fig. 4e). Although the difference of the synthesis to adopt
a
time reached up to 30 min, the average size had no obvious change. interplanar distance is approximately 0.24 nm, corresponding to the
Obviously, the SPR wavelength of Au-Ag alloy NPs was mostly (111) lattice planes of the face-centered cubic (fcc) Au or Ag.42 In fcc
depended on their composition rather than their average size.
As exhibited in the insets of Fig. 4a-d, the standard deviation (
metal NPs with a small size, the multi-twinned crystal
microstructure is frequently observed.43 As displayed in Fig. 4h, the
σ)
of the size distribution of Au-Ag alloy NPs follows the order:
polycrystalline nature of Au-Ag alloy NPs is further identified via the
σ20min < σ40min < σ10min. σ20min and σ40min were smaller than σ10min, which SAED analysis. The electron diffraction pattern presents four
30
could be attributed to the Ostwald ripening effect.
σ
was diffraction rings indexed to the (111), (200), (220), and (311) Bragg
20min
smaller than
σ40min, which could be ascribed to the growth of NPs reflections from the fcc structure of Au or Ag, revealing the
with increasing synthesis time. When the NPs grew to a critical size, polycrystalline nature of Au-Ag alloy NPs.
their surface was covered by a thick repelling layer of sucrose,
Fig. 4 TEM images and size histograms (insets) of Au-Ag alloy NPs synthesized at the Au3+/Ag+ molar ratio of 1:20 at different time. (a) 10 min, (b) 20 min, (c)
40 min and (d) 50 min; (e) The UV-vis absorption spectra of Au-Ag alloy NPs corresponding to (a)-(d); (f) Distributions of Au and Ag components along the
cross-sectional line profile of a single Au-Ag alloy NP; (g) HR-TEM image of a single Au-Ag alloy NP; (h) Selected area electron diffraction pattern of Au-Ag
alloy NPs.
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Proposed synthesis mechanism of Au-Ag alloy NPs
In the growth stage, the synthesis time shows a significant influence
on the size distribution. Initially, the size distribution gets narrower
and narrower with the increase of the synthesis time because of the
Ostwald ripening effect. As synthesis time goes on, size distribution
becomes broader and broader and the heterogeneous alloy NPs
and even the corresponding monometallic NPs (Au, Ag) will appear
due to the electrostatic repulsion forces produced by the repelling
layers covered on the surface of NPs.30 Therefore, in our
experiment, in order to obtain Au-Ag alloy NPs with uniform
composition and narrow size distribution, the synthesis time is
limited to less than 40 min. Meanwhile, the microstructure of Au-Ag
alloy NPs is also formed, which is determined by the relative rate
between the reduction rate of ions and the mutual diffusion rate of
Au and Ag elements in Au-Ag bimetallic NP. Only is the diffusion
rate higher than the reduction rate, Au-Ag alloy NPs with
homogeneous alloy microstructure can be obtained. Otherwise,
synthesized Au-Ag alloy NPs can have a gradient or disordered alloy
microstructure.46 Both the reduction rate and the diffusion rate are
dependent on the synthesis temperature.47 In our experiment, it is
clear that the boiling point of the sucrose solution is a suitable
temperature for the synthesis of Au-Ag alloy NPs with
homogeneous alloy microstructure. Through analyzing the synthesis
mechanism of Au-Ag alloy NPs, a promising route can be offered for
the continuous synthesis of other alloy NPs in one-pot.
Sucrose is a non-reducing disaccharide, whose reducing action is
considered to be derived from its hydrolysis products (glucose and
fructose).30 In order to further investigate the hydrolysis process,
glucose, fructose and the mixture of glucose and fructose (1:1) were
individually employed instead of sucrose as reductants and
surfactants to synthesize Au-Ag alloy NPs. As shown in Fig. 5, only in
the presence of NaOH, Au-Ag alloy NPs can be continuously
obtained in a boiling solution. If just under the boiling condition
without NaOH, the SPR wavelength of more than 535 nm is
exhibited in the UV-vis spectra. It indicates the formation of
monometallic Au NPs.29 Clearly, the hydrolysis products of sucrose,
not a simple combination of glucose and fructose, possess a much
superior reducibility compared with the monosaccharide. Generally,
the open-chain monosaccharide shows
a
higher reducibility
small
compared to the cyclic monosaccharide while only
a
percentage of monosaccharide has the open-chain structure in the
state of nature.29 In the case of hydrolysis or alkaline degradation,
the ratio of open-chain monosaccharide is increased, which can
lead
to
the
enhancement
of
the
reducibility
of
monosaccharide.29,31,33 Therefore, it can be inferred that sucrose
can provide much more monosaccharide with the open-chain
structure with respect to glucose and fructose during the mild
hydrolysis process. In our experiment, 25 mL of 0.5 M sucrose
solution was kept boiling for 10 min in order to ensure the efficient
hydrolysis of sucrose and obtain abundant monosaccharide with the
open-chain structure.
Based on the characterization, a possible formation mechanism
of Au-Ag alloy NPs is illustrated schematically in Fig.6. The whole
formation process can be divided into three stages: reduction,
nucleation and growth.28 During the reduction stage, both Au3+ ions
and Ag+ ions are reduced simultaneously and the reduction rate of
Au3+ ions is higher than that of Ag+ ions, which results that the Au-
Ag alloy NPs with a long SPR wavelength (near the SPR wavelength
of Au NPs) are obtained at the beginning.44,45 The key to realize the
continuous blue-shift of the SPR wavelength from 520 to 430 nm is
that the molar ratio of Ag+ ions must be higher than that of Au3+
ions in the raw materials. In the nucleation stage, there can be four
kinds of nuclei: Au nuclei, Ag nuclei, and Au-Ag bimetallic nuclei
(Au-rich or Ag-rich in the surface of the nucleus). When the molar
ratio of Au3+ ions is relatively higher, the Au nuclei and Au-rich
bimetallic nuclei can appear. Contrarily, the Ag nuclei, Ag-rich and
Au-rich bimetallic nuclei will be generated (Fig. 1d). Therefore, the
Au3+/Ag+ molar ratio in the raw materials has an optimal value,
which is equal to 1:20 under our experimental conditions. Based on
the optimal value, single Au-rich nuclei can be obtained, which
ensure the synthesized Au-Ag alloy NPs have uniform composition.
Fig. 5 Normalized UV-vis absorption spectra of NPs synthesized at the
Au3+/Ag+ molar ratio of 1:20 in the boiling solution using fructose, glucose
and the mixture of glucose and fructose (1:1) as reductants, respectively.
(a1), (b1) and (c1): without NaOH; (a2), (b2) and (c2): with 100 mL of 1 M NaOH.
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PAPER
Fig. 6 Schematic illustration of possible synthesis mechanism of Au-Ag alloy NPs.
Photocatalytic activity of Au-Ag alloy NPs
0.045, 0.034, 0.024, 0.018 and 0.007 s-1, respectively. The
photocatalytic activities of Au-Ag alloy NPs are located between the
activities of Au and Ag NPs. With increasing Au molar fraction in Au-
Ag alloy NPs, the photocatalytic activity of alloy NPs is similar to the
activity of Au NPs. On the contrary, their activity is close to the
activity of Ag NPs. Usually, noble metal NPs with a small size can
exhibit a high catalytic activity due to their big specific surface
area.3 In Fig. S2 and Fig. 4a-c, Au, Au-Ag alloy and Ag NPs display
comparable average sizes. Thus, the difference in photocatalytic
activities of above NPs cannot be attributed to their average sizes.
Kumari reported Au-Ag alloy NPs showed enhanced photocatalytic
activity with increasing size and Au content, which suggested the
composition of NPs could have a more significant influence on their
photocatalytic activity.9 Therefore, it could be inferred that the
photocatalytic activities of Au, Au-Ag alloy, and Ag NPs were mainly
dependent on their composition. Nevertheless, up to now, few
works in literature reported the correlation between the
composition of Au-Ag alloy NPs and their photocatalytic activity.48
In this paper, the relationship between the Au molar fraction in
Au-Ag alloy NPs and the rate constant of 4-NP reduction was
depicted in Fig. 8a. A linear fashion is shown as follows:
Generally, the reduction of 4-NP as a model reaction is used to
confirm the photocatalytic activity of Au-Ag alloy NPs.9 In order to
systematically investigate the photocatalytic activities of Au-Ag
alloy NPs, Au0.75Ag0.25, Au0.50Ag0.50 and Au0.28Ag0.72 alloy NPs were
individually chose as photocatalysts for the reduction of 4-NP using
NaBH4 as reductant. Upon the addition of NaBH4 to the 4-NP, the
UV-vis absorption peak of 400 nm was appeared immediately,
implying the formation of 4-nitrophenolate anions. In the absence
of the photocatalyst, the peak at 400 nm remained undiminished
even for several hours. However, when Au-Ag alloy NPs were added
into the reaction system, the absorbance of the peak at 400 nm
declined quickly and a new peak emerged immediately at 300 nm
indicating the formation of 4-AP.48 As shown in Fig. 7a-c and Fig. S4
(SI), based on Au, Au0.75Ag0.25, Au0.50Ag0.50, Au0.28Ag0.72 and Ag NPs,
4-NP can be thoroughly reduced in 75, 90, 120, 150 and 240 s,
respectively.
The rate constant is often considered as an important
parameter to evaluate the activity of the photocatalyst.48 When the
amount of NaBH4 is in excess, the rate constant can be calculated
according to the pseudo-first order equation as follows: 3,9,49
K
= 0.034 + 0.008
X
(5)
ꢃ
⁄ ꢄ
ln Ct C0 = -K t
is the rate constant, is the reaction time, and Ct and C0 is
the concentration of 4-NP at time and the initial concentration,
respectively. The Ct C0 ratio is equal to the absorbance At A0 ratio.
In Fig. 7d, the rate constants are given, which are corresponding to
the slopes of fitted linear relationships between -ln(Ct C0) and the
(4)
Here,
K
is the rate constant, and X is the Au molar fraction in Au-Ag
Here,
K
t
alloy NPs. According to the equation (5), the rate constant can be
linearly controlled only by adjusting alloy composition. Generally,
the rate constant of 4-NP reduction using monometallic NPs (eg.
Au) as photocatalysts is tuned by the addition amount of NPs.50
Obviously, the control of the rate constant based on alloy
composition is simple and flexible, which provides a new route for
t
/
/
/
reaction time. On the basis of Au, Au0.75Ag0.25, Au0.50Ag0.50 and
Au0.28Ag0.72 and Ag NPs, the rate constants of 4-NP reduction are
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other photocatalytic reactions using bimetallic NPs as Ag alloy NPs. According to the Scherrer formula, the calculated
photocatalysts. Equation (3) reveals the composition of Au-Ag alloy mean crystallite sizes of Au-Ag alloy NPs are 7.28 and 7.27 nm,
NPs can be linearly controlled based on the synthesis time at the respectively. There is no distinct variation with the increase of the
Au3+/Ag+ molar ratio of 1:20. Combining with the equation (5), the store time. In order to further observe the change in nanoparticle
relationship equation between the rate constant and the synthesis size, TEM image of Au-Ag alloy NPs used as photocatalysts after 3
time to prepare Au-Ag alloy NPs can be established as follows:
= -8.0×10-4t ꢂ 0.041
months is displayed in Fig. 11. Compared with the fresh Au-Ag alloy
(6) NPs (Fig. 4c), the morphology of Au-Ag alloy NPs after 3 months has
K
Here,
t is the synthesis time to prepare Au-Ag alloy NPs, whose no obvious difference and there are in absence of agglomerates,
range is from 2 to 40 min. Accordingly, the variation range of the which further confirms the stability of Au-Ag alloy NPs.
rate constant is from 0.039 to 0.009 s-1. Based on the above
equation, the rate constant can be predetermined only by setting
the synthesis time, which implies the photocatalytic activity of Au-
Ag alloy NPs can be predicted as early as their preparation process.
The normalized rate constant Knor (Knor=K/mcat, where mcat is the
mass of photocatalyst) is often used to compare the activity of the
photocatalyst.3 According to Fig. 7d, the Knor values for Au and Ag
NPs synthesized in the sucrose solution were calculated as 4568
and 1296 s-1g-1, respectively. For Au-Ag alloy NPs, the controllable
range of the Knor values was from 4133 to 1594 s-1g-1 (equation (6)).
Clearly, as-prepared NPs exhibited a superior photocatalytic activity
compared to hollow porous Au NPs (742 s-1g-1), monometallic Ag
NPs (654 s-1g-1), green synthesized Au-Ag alloy NPs (1007 s-1g-1), and
Pd-based alloy NPs (less than 1355 s-1g-1).3,48,49,51 Li et al reported
both the reductant and the surfactant had strong effects on the
photocatalytic activity of obtained Ag NPs.52 Ag NPs synthesized by
β
-cyclodextrin exhibited a higher photocatalytic activity than those
synthesized by sodium citrate, which was attributed to the hydroxyl
groups of
-cyclodextrin.52 The interaction between the hydroxyl
Fig. 7 UV-vis absorption spectra for the reduction of 4-NP solution mixed
with different NPs, (a) Au0.75Ag0.25, 498 nm, (b) Au0.50Ag0.50, 470 nm, and (c)
Au0.28Ag0.72, 446 nm. (d) The plots of -ln (Ct/C0) versus the reaction time
based on different photocatalysts (Au, Au-Ag alloy and Ag NPs).
β
groups and 4-NP improved the diffusion of 4-NP to Ag NPs surface,
leading to the enhanced photocatalytic activity of Ag NPs.52 In our
experiment, the sucrose and its hydrolysis products have
abundance hydroxyl groups, which can play the same role with
β-
cyclodextrin. Therefore, it can be inferred that the synthesis system
using sucrose as reductant and surfactant is suitable for the
preparation of photocatalysts with high activities for 4-NP
reduction.
It is well-known that Ag NPs are easy to be oxidized whereas Au
NPs are stable. For this reason, the stability of Au-Ag alloy NPs must
be researched for practical engineering applications. Guo et al
reported the photocatalytic performance of hollow porous Au NPs
(80 nm) had no significant change after 30 days.3 In Fig. 9, Au-Ag
alloy NPs also show an excellent stability after 3 months, which
indicates the stability of Au-Ag alloy NPs with a small size less than
10 nm is comparable with that of Au NPs with a large size of 80 nm.
The high-resolution XPS spectrum of Ag 3d and XRD patterns are
used to analyze the stability of Au-Ag alloy NPs (Fig. 10). In Fig. 10a,
the peaks of Ag 3d appear at 368.0 and 374.0 eV, which correspond
to Ag 3d5/2 and 3d3/2 orbits, respectively. The gap between the 3d5/2
and 3d3/2 peaks of Ag is 6.0 eV, which is exactly the same as the
value for zero-valent Ag.41 The tiny shift in the peak binding energy
of Ag 3d can be ascribed to the formation of Au-Ag alloy.48 In Fig.
10b, XRD patterns exhibit diffraction peaks at 38.2o, 44.4o, 64.6o,
and 77.6o, which can be indexed to (111), (200), (220), and (311)
planes of crystalline Au or Ag, respectively. Clearly, the combination
of Au and Ag elements overcome the chemical instability of
monometallic Ag NPs, which is the other obvious advantage of Au-
Fig. 8 The relationships between the rate constant for the reduction of 4-NP
and Au molar fraction in Au-Ag alloy NPs (a) as well as the synthesis time of
Au-Ag alloy NPs (b).
Fig. 9 The conversion efficiency of 4-NP to 4-AP using Au-Ag alloy NPs
synthesized at 40 min as photocatalyst in three months.
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Fig. 10 (a) The high-resolution XPS spectrum of Ag 3d based on the Au-Ag alloy NPs used as photocatalyst after three months. (b) XRD patterns of fresh Au-
Ag alloy NPs and Au-Ag alloy NPs used as photocatalyst after three months.
temperature and the sucrose concentration were fixed at the boiling
point of the sucrose solution and 0.5 M, respectively. Based on the
optimal Au3+/Ag+ molar ratio of 1:20, the SPR wavelength of Au-Ag
alloy NPs with homogeneous composition and alloy microstructure
could be continuously blue-shifted from 520 to 430 nm only by
increasing the synthesis time from 2 to 40 min in one-pot. Moreover,
the composition of Au-Ag alloy NPs could also be linearly controlled
by their synthesis time in one-pot. For the reduction of 4-NP to 4-AP,
obtained Au-Ag alloy NPs exhibited
a superior photocatalytic
activity and stability for 3 months. The controllable variation range
of the Knor value was from 4133 to 1594 s-1g-1. A linear equation
between the rate constant of 4-NP reduction and the Au molar
fraction in Au-Ag alloy NPs was established. Based on the
continuous synthesis of Au-Ag alloy NPs, the rate constant could
even be linearly adjusted only by changing the synthesis time of Au-
Ag alloy NPs. Obviously, as-prepared Au-Ag alloy NPs may have
great potential in the field of photocatalysis.
Fig. 11 TEM image and size histogram of Au-Ag alloy NPs synthesized at 40
min used as photocatalyst after three months.
Acknowledgements
This work was financially supported by National Natural Science
Foundation of China (Grant No. 11404210).
Conclusions
The spherical Au-Ag alloy NPs with the fcc polycrystalline
microstructure were synthesized using sucrose as reductant and
surfactant without any toxic or expensive chemical agent. The
morphology, microstructure and composition of as-prepared NPs
were dependent on their synthesis conditions (the synthesis
temperature, the sucrose concentration, the synthesis time and the
Au3+/Ag+ molar ratio in the raw materials). In order to obtain
homogeneous Au-Ag alloy NPs with a small size, the synthesis
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A table of contents entry
Graphical abstract
Homogeneous Au-Ag alloy nanoparticles, controllably synthesized using sucrose in
one-pot, showed high stability and photocatalytic activity dependent on their
composition.
Highlights