Fabrication of thermally stable and active bimetallic Au-Ag nanoparticles stabilized on inner wall of mesoporous silica shell

Article abstract of DOI:10.1039/c3dt51546j

A general method has been developed for the fabrication of highly dispersed and thermally stable bimetallic Au-Ag nanoparticles (NPs) stabilized on the inner wall of a mesoporous silica shell. In our approach, gold particles were formed in the first step on carbon spheres decorated with Sn²⁺ cations. Upon Ag⁺ adsorption and reduction by l-ascorbic acid in the second step, specific nanoparticles with a gold-silver alloy core and a silver nanoshell have been formed. Important evidence of the core-shell configurations of the bimetallic Au-Ag nanoparticles were clearly characterized by UV-vis, TEM and HAADF-STEM observations combined with elemental mapping and line scans. The mesoporous silica outer shell was obtained through the hydrolysis and condensation of the precursors tetraethoxysilane (TEOS) in a basic condition and cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. On this basis, the nanoreactors were fabricated after calcination, which further serves as a nanoreactor for the reduction of p-nitrophenol. Furthermore, such particles have been found to be thermally stable and their sizes remain substantially unchanged even upon calcination in air at 500 °C and a reduction treatment in H₂. Potentially, the method can be developed into a general approach to synthesize other highly dispersed and thermally stable bimetallic nanoparticles stabilized on the inner wall of a mesoporous silica shell.

Full text of DOI:10.1039/c3dt51546j

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Fabrication of thermally stable and active bimetallic
Au–Ag nanoparticles stabilized on inner wall of
mesoporous silica shell†
Cite this: Dalton Trans., 2013, 42, 13940
Yu Chen,a,b Qihua Wang*a and Tingmei Wanga
A general method has been developed for the fabrication of highly dispersed and thermally stable
bimetallic Au–Ag nanoparticles (NPs) stabilized on the inner wall of a mesoporous silica shell. In our
approach, gold particles were formed in the first step on carbon spheres decorated with Sn2+ cations.
Upon Ag+ adsorption and reduction by L-ascorbic acid in the second step, specific nanoparticles with a
gold–silver alloy core and a silver nanoshell have been formed. Important evidence of the core–shell
configurations of the bimetallic Au–Ag nanoparticles were clearly characterized by UV-vis, TEM and
HAADF-STEM observations combined with elemental mapping and line scans. The mesoporous silica
outer shell was obtained through the hydrolysis and condensation of the precursors tetraethoxysilane
(TEOS) in a basic condition and cetyltrimethylammonium bromide (CTAB) as a structure-directing agent.
On this basis, the nanoreactors were fabricated after calcination, which further serves as a nanoreactor
for the reduction of p-nitrophenol. Furthermore, such particles have been found to be thermally stable
and their sizes remain substantially unchanged even upon calcination in air at 500 °C and a reduction
treatment in H2. Potentially, the method can be developed into a general approach to synthesize
other highly dispersed and thermally stable bimetallic nanoparticles stabilized on the inner wall of a
mesoporous silica shell.
Received 11th June 2013,
Accepted 15th July 2013
DOI: 10.1039/c3dt51546j
www.rsc.org/dalton
us, especially when using a thermal treatment for the remov-
ing the protective agents.
Introduction
Bimetallic core–shell nanoparticles (NPs) have attracted
tremendous attention in recent years owing to their unique wonderful candidate to solve such
Monodispersed mesoporous hollow spheres should be an
problem.17–19 For
a
and novel optical, electrical and catalytic properties.1–9 instance, when functional noble-metal nanoparticles are
Recently, more and more studies focus on the catalysis of located into a hollow silica shell, these composite spheres can
bimetallic core–shell NPs because the catalytic activity of such not only effectively isolate noble metal NPs and protect them
structured NPs can be greatly influenced by their size, struc- against aggregation, but also allow the diffusion of small mole-
ture and compositions.10–13 Up to now, different methods have cules in and out of the reactors.20–26 So far, the fabrication
been developed for fabricating well-dispersed bimetallic of monodispersed mesoporous hollow spheres can be broadly
NPs.14–16 A typical route of synthesizing bimetallic core–shell divided into two categories: template-free and templating
NPs is by reducing a second metal onto the presynthesized approaches.27–29 Although the process of the first method is
cores. Although such a method can effectively control the size simple, templating approaches offer important advantages,
and shape of the NPs, the problem of aggregation has plagued including narrow size-distribution products with well-defined
structural features. However, there are, nevertheless, inherent
drawbacks in the template removal process. The multiple
metal particles encapsulated in the hollow spheres tend
to sinter and grow into larger particles after calcination,
aState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Science, Lanzhou 730000, China. E-mail: wangqh@licp.cas.cn;
especially in the cases where they are subjected to a thermal
treatment for the removal of a surfactant or template due to
heat- and atmosphere-induced sintering and restructuring. To
prevent or reduce sintering, many attempts have been made,
including confining gold particles in the nanopores of a meso-
porous support and so on.30–32 Generally, aminated organic/
Fax: +86-0931-4968180; Tel: +86-(0)931-4968180
bGraduate University of the Chinese Academy of Sciences, Beijing, 100049, China
†Electronic supplementary information (ESI) available: The TEM of Au–Ag@SiO2
after the reduction treatment by H2; calculating the mean diameters of the (Au–
Ag)/C@mSiO2 composite microspheres; the reaction almost does not occur
before adding catalysts; the TEM of the Au@mSiO2 composite microspheres and
the distribution of the pore size. See DOI: 10.1039/c3dt51546j
13940 | Dalton Trans., 2013, 42, 13940–13947
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inorganic nanoparticles are selected as the core material to Synthesis of the carbon nanoparticles (CNs)
load the synthesized metal nanoparticles.
Glucose (4.5 g) was dissolved in 50 ml to form a clear solution
and then transferred into a Teflon-sealed autoclave. The auto-
clave was maintained at 180 °C for 4 h.33 The precipitation
was isolated by centrifugation, cleaned by four cycles of
centrifugation–washing–redispersion in water and in ethanol
and finally oven-dried at 60 °C for further use.
In this work, we have successfully used a successive two-
step reduction approach to prepare (Au–Ag)/C composite
spheres in which Au–Ag bimetallic NPs are well-anchored on
the carbon surfaces. On this basis, a nanoreactor composed of
a mesoporous silica shell and bimetallic NPs residing in the
silica shell was fabricated. The nanoreactor framework with
metal NPs act as a core anchored on the inner wall and the
mesoporous silica acts as a shell to not only stabilize the metal
particles, but also allow the diffusion of small molecules in
and out of the reactors. A synthesized route is illustrated in
Scheme 1. Firstly, monodisperse CNs with a specific particle
size were prepared as the support and Au NPs were further
loaded onto the surfaces of the CNs. Then, the CNs decorated
with Au NPs were dispersed in water and a certain amount
of PVP and L-ascorbic acid were responsible for the creation
of the Au–Ag bimetallic heterostructures supported on the
carbon spheres. After the addition of a AgNO3 solution, the
(Au–Ag)/C composite NPs were obtained. Herein, the CNs not
only provide a platform for loading metal nanoparticles but
also serve as a sacrificial hard template for the fabrication of a
nanoreactor with a yolk–shell structure. Secondly, the meso-
porous silica outer shell is obtained through the hydrolysis
and condensation of the precursors tetraethoxysilane (TEOS)
in a basic condition and cetyltrimethylammonium bromide
(CTAB) as a structure-directing agent. Finally, a simple exten-
sion of this process yields (Au–Ag)@mSilica yolk–shell nano-
structures via calcination in air at 550 °C for 4 h.
Synthesis of the bimetallic core–shell nanoparticles on the
surfaces of the CNs
Synthesis of the Au/C nanoparticles (NPs). In a typical
process, 0.1 g of carbon NPs dispersed in 50 ml of distilled
water was added to a three-necked flask and 0.1 g of SnCl2 dis-
solved in 20 ml of a 0.02 M HCl solution was added for sensit-
ization. The mixture was supersonicated for 15 min, aged for
3 h and subjected to three centrifugation–water wash–redisper-
sion cycles and dispersed in 50 ml of distilled water. Then, a
solution of HAuCl4 (2.9 mM, 5 ml) was added into the solution
and further stirred for 1 h. The final products was subjected to
five centrifugation–water wash–redispersion cycles and oven-
dried at 60 °C overnight.
Synthesis of the (Au–Ag)/C NPs. The Au/C composite NPs
obtained from the last step were first dispersed in a solution
containing 50 ml of H2O, 10 ml of PVP (K30, 0.05 g) and 20 ml
of L-ascorbic acid under stirring. Then, 10 ml of AgNO3 was
injected into the above-mentioned mixture using a pipette.
The molar concentration of L-ascorbic acid and AgNO3 in the
final solution was 1.76 mM and 0.21 mM, respectively. The
final products were subjected to two centrifugation–water
wash–redispersion cycles and dried at 60 °C for 12 h.
Synthesis of the (Au–Ag)@mesoporous SiO2 nanoreactor.
The obtained (Au–Ag)/C NPs were dispersed in a solution con-
taining 100 ml of distilled water, 60 ml of ethanol and 1.45 ml
of an ammonia aqueous solution and then 0.3 g of CTAB dis-
persed in 15 ml of ethanol was added to the solution with vig-
orous stirring for 30 min. Afterward, 0.3 ml of TEOS was added
to the solution drop by drop and further stirred for 6 h.34 The
products were separated by five centrifugation–water–ethanol
wash–redispersion cycles. Finally, calcination was carried out
in air at 550 °C with a heating rate of 5 °C min−1 and main-
tained at that temperature for 4 h to eventually remove the
CTAB and carbon template.
Experimental methods
Materials
Hexadecyl trimethyl ammonium bromide (CTAB) and tetra-
ethyl orthosilicate (TEOS) were purchased from Sigma-
Aldrich. Glucose, stannous, gold chloride, silver nitrate and
vitamin C were purchased from Sinopharm Chemical Reagent
Co., Ltd.
Details for the catalytic reaction of the reduction of
p-nitrophenol (4-NP)
The reduction of 4-NP by NaBH4 was chosen as a model reac-
tion to test the catalytic activity of the catalysts. Typically, the
reaction proceeded under room temperature conditions. To a
mixture of 184 mg of NaBH4 dispersed in 2.3 ml of H2O,
16.7 ml of 0.18 mmol L−1 4-NP and 10 ml of distilled water
were added. Then, a certain amount of the catalyst (Au–Ag)-
@mSiO2 or Ag@mSiO2 microspheres were added into the
mixture under stirring. The mixture was kept stirring and
we took 2 ml of the mixture by pipette every 5 min.
The samples were analyzed immediately on an analyticjena
Scheme 1 The procedure of fabricating (Au–Ag)@mSiO2 composite
microspheres.
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specord 50 UV-visible spectrometer. The concentration change
of p-nitrophenol was determined by the absorbance at its
maximum peak. In the presence of the catalysts, the absorp-
tion of p-nitrophenol at 400 nm decreases along with a con-
comitant increase of the ∼300 nm peak of p-aminophenol. The
absorption spectra of the solution were measured in the range
of 250–550 nm.
Characterization
Structural and morphological investigations of the samples
were performed by a JSM-6701F scanning electron microscope
(SEM) and transmission electron microscope (TEM, Tecnai-G2-
F30). A Lambda 750 ultraviolet and visible (UV-vis) spectrophoto-
meter was employed for the analysis of the 4-NP reduction
and to investigate in detail the structural evolution during
each step of the preparation. X-ray diffraction using Cu–Kα
radiation (XRD, Panalytical X’Pert Pro) and Brunauer–Emmett–
Teller (BET, ASAP 2020M) was employed for the analysis of the
mesoporous structure.
Fig. 1 The low and high magnification TEM image of the (a, b) Au/C NPs and
(c, d) (Au–Ag)/C NPs and their corresponding size distribution (c, f).
Results and discussion
(Fig. 1d). The NPs increased slightly and a wide distribution of
particle diameters was observed from the histogram, but
aggregation barely occur (Fig. 1e). The mean size of the par-
ticles was 15 nm and the largest diameter was about 20 nm.
Both investigations revealed that the products were well-dis-
persed as individual spherical nanoparticles immobilized on
carbon surfaces. Taking into account the presence of a hydroxy-
methyl group on the surface of the carbon sphere, we
have to recognize that trace monometallic Ag particles could
arguably exist in our method, but, observed from our results,
bimetallic particles are predominant because of strong
bimetallic interactions and a very low concentration of Ag+
cations.33,37
As shown in Fig. 2a, the surfactant-templating process
results in a CTAB/SiO2 mesophase coated on the (Au–Ag)/C
composite NPs. This shows that the average size of the spheres
were about 450 nm. In order to avoid the collapse of the shell
during the process of calcination, we believe that a relatively
thick shell is more suitable. Fig. 2b shows a silica shell with a
thickness was about 120 nm. As can be seen in Fig. 2c, the syn-
thesized (Au–Ag)/C@SiO2 composite microspheres were visual-
ized by STEM imaging. This technique enables high-contrast
imaging of high-Z elements (such as Au or Ag) against
the lower-Z background (such as SiO2). Fig. 2d is a high
As shown in Fig. 1a, Au NPs were well-anchored onto the sur-
faces of carbon spheres after the purification and collection of
the synthesized Au/C NPs processes. Actually, carbon/metal
composite particles can also be used for the catalytic reaction.
However, due to stability and reusability considerations,
carbon/metal composite particles are not suitable because the
negatively charged Au NPs are absorbed onto the surface of
the carbon spheres through electrostatic attraction. We specu-
lated that the mechanism of depositing bimetallic NPs onto
the surfaces of carbon spheres can be summarized in two suc-
−
cessive steps. Firstly, the negatively charged AuCl4 ions were
strongly adsorbed on the positively charged Sn2+ cations
absorbed on the surface of the carbon spheres. Then, the Sn2+
cations served as a strong reductant for the reduction of local
AuCl4− (Scheme 2). Secondly, the negatively charged Au NPs were
absorbed on the surface of the carbon spheres through electro-
static attraction. Au NPs with an average size of 8 nm (Fig. 1c)
were synthesized and no agglomeration could be observed
(Fig. 1b). Following that, Ag+ cations were allowed to adsorb
preferentially on the negatively charged gold particle surface
via electrostatic attractions and metallic bonding.35,36 Follow-
ing the reduction by L-ascorbic acid, a silver nanoshell was pro-
duced on the gold core. The Au–Ag structured bimetallic NPs
were well-anchored to the surface of the carbon spheres
Scheme 2 The mechanism of depositing Au NPs onto the surfaces of the carbon spheres.
13942 | Dalton Trans., 2013, 42, 13940–13947
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inset. The STEM image of the hollow (Au–Ag)@mSiO2 NPs
looks like a bale of cotton (Fig. 3b), which can further demon-
strate their hollow structures. Compared with the thickness of
the silica shell shown in Fig. 2 and 3, we found that the shell
thickness drastically reduced. This phenomenon was caused
by shrinkage of the silica shell during the process of calcina-
tion. Fig. S1† shows the TGA curves of (Au–Ag)/C@SiO2 and
that the residual mass of the microspheres after calcination
was about 52%, which presents the mass of Au–Ag@mSiO2.
A series of UV-vis spectra were observed and analyzed to
investigate in detail the structural evolution during each step
of the preparation. As shown in Fig. 3d, when Sn2+ was
adsorbed on the carbon spheres surfaces, the UV-vis spectrum
of the sample at this stage shows a very broad peak centered at
436 nm. When the AuCl− ions adsorbed on the carbon sphere
surfaces were reduced by Sn2+ cations, uniform gold particles
were produced. The UV-vis spectrum of this sample at this
stage shows a peak centered at 572 nm. The characteristic of
the surface plasma resonance (SPR) of gold nanoparticles
changed from about 525 nm to after 572 nm, as a result of the
modulation effect of the carbon spheres. Subsequently, Ag+
cations were predominantly adsorbed on the Au NPs. Follow-
ing the reduction by L-ascorbic acid, a silver nanoshell was pro-
duced on the gold core. The silver atoms in the nanoshell still
remain as a pure silver phase and are in close contact with
gold surface. They will diffuse into the gold core to form a
gold–silver alloy phase due to the stronger bimetallic
bonding.38,39 Thus, there are two distinct SPR peaks, 541 nm
and 450 nm, respectively. One has a blue shift of 31 nm com-
pared to the C/Au SPR peak, which can be assigned to the SPR
of the Au–Ag alloy phase. The other is centered at 450 nm and
is characteristic of C/Ag. After the formation of the silica shell,
a calcination treatment was carried out to remove the carbon
spheres. The adsorption peak of Au@SiO2 was 526 nm, while
the peak of Au–Ag@SiO2 should show a blue-shift pheno-
menon after the formation of the Ag nanoshell.35 However, the
Au–Ag@SiO2 shows a peak centered at 528 nm, indicating a
small amount of aggregation of the Au–Ag nanoparticles
during the calcination process. The disappearance of the SPR
band at 450 nm is most likely due to the oxidation of silver by
air to form silver oxide, which is transparent to the incident
light. It is well known that a high temperature will induce, in
most cases,40 severe sintering of nanoparticles for thermo-
dynamic reasons. However, in our case, a small amount of
Fig. 2 The low and high magnification TEM (a, b) and STEM image (c, d) of the
(Au–Ag)/C@SiO2 NPs.
Fig. 3 (a) TEM image (a) and STEM image (b) of the hollow (Au–Ag)@mSiO2
NPs; the morphology after the reduction treatment by H2 (c); a series of UV-vis
spectra during each step of the preparation.
magnification of the STEM imaging of the synthesized nano- particles appear to reunite but serious aggregation was suc-
composite NPs. The contrast clearly shows that the (Au–Ag)/ cessfully avoided (observed from Fig. 3a and 3b). We attribute
C@SiO2 composite microspheres were successfully obtained the reasons to a synergistic effect of the chemical bonding
through the hydrolysis and condensation of the precursor tetra- interaction and the large specific surface area of the meso-
ethyl orthosilicate (TEOS).
porous silica shell as well as the limiting effect of the Ag
We fabricated hollow (Au–Ag)@mSiO2 composite micro- shell.22,41,42 After the reduction treatment by H2 (Fig. 3c),
spheres after calcination at 550 °C for 4 h to remove the because the silver oxide is reduced to metallic silver,
carbon core and surfactant, containing Au–Ag bimetallic NPs which drives more silver atoms to be alloyed with gold, the
anchored onto the inner wall. As shown in Fig. 3a, even with a SPR band of the sample had a blue-shift from 528 nm to
high temperature treatment, the size distribution of the metal 521 nm. Satisfactorily, the vast majority of particle sizes
nanoparticles did not show an obvious change. The majority remain essentially unchanged even through a series of harsh
of particle diameters are less than 25 nm, observed from the treatments.
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Fig. 4 EDS elemental mapping of the Au–Ag NPs (a); STEM-EDS line-scan across a single Au–Ag NPs (b) and the corresponding EDS spectra (c).
In order to provide support for the structural characteristics
of the bimetallic metal NPs anchored on the mesoporous
silica shell, elemental mapping by EDS analysis was carried
out to provide proof of the core/shell geometry of the nano-
particles. The elemental mapping (Fig. 4a) confirms the presence
of a core–shell structure and the Ag content is also rich in the
core region. We also employed the corresponding EDS spectra
shown in Fig. 4c, but it was unable to provide definitive evi-
dence for structural characteristics of the metal particles. The
STEM-EDS line-scan EDS spectra across the Au–Ag NPs was
carried out to further provide independent proof of the core/
shell geometry of the nanoparticles. Fig. 4b displays an
STEM-EDS line scan across a single core–shell structured par-
ticle. This result reflects the higher atomic number of Au in
the center of the particle and the higher atomic number of Ag
in the outer shell. While Fig. 4b does appear to show enrich-
ment of Ag along the edge of the bimetallic particles, it
obviously shows a significant amount of Ag in the central
region, which suggests the particle has more of an Au–Ag alloy
character. All the results are consistent with the results of the
UV-vis spectra that Au and Ag are homogeneously mixed in the
core region. So, the presynthesized core–shell structured bi-
metallic particle consists of a Au–Ag alloy acting as a core and
Ag acting as a shell.
Fig. 5 SEM images of the (a) (Au–Ag)/C@SiO2 and (b) (Au–Ag)@mSiO2 NPs
and their corresponding histogram analysis (insets). (c) TEM image of some par-
ticles on the external surface of the silica shell; (d) TEM image of some broken
microspheres.
meant their metal nanoparticles dispersed into solution,
causing the metal particles to be adsorbed on the external
surface of the mesoporous silica during calcination (shown in
Fig. 5d).
The uniform and nonaggregated nature of the composite
microspheres can be more easily observed in scanning electron
microscopy (SEM) images, as shown in Fig. 5a and b. The his-
togram analysis shows that the (Au–Ag)/C@SiO2 and (Au–Ag)-
@mSiO2 NPs are about 450 nm and 320 nm, respectively
(shown in the insets). These results are generally consistent
with the result of the TEM images. The diameters of the com-
posite microspheres were estimated by calculating the mean
diameters of the (Au–Ag)/C@mSiO2 and (Au–Ag)@mSiO2 NPs
by measuring at least 100 particles from the SEM images, as
shown in Fig. S2.† In our experiment, the absolute majority of
the particles were trapped inside the mesoporous silica shell,
while very few particles were on the outlayer of the SiO2
(shown in Fig. 5c†). We infer that some broken microspheres
The mesoporous structure can not only stabilize the
bimetallic NPs but also allow the diffusion of small molecules
in and out of the reactors. Fig. 6a shows the mesoporous
nature of the silica wall. Fig. 6b shows the nitrogen adsorp-
tion–desorption isotherm of the composite microspheres with
a narrow hysteresis loop, both of which indicate the presence
of mesopores. Fig. 6c shows the BJH pore size distribution,
showing a sharp peak at 3.2 nm. The small-angle XRD pattern
depicted in Fig. 6d shows a broad peak at 2.4°, indicating a
worm-like mesopore with a uniform pore size. The BET surface
area of the composite is 1167 m2 g−1, similar to normal meso-
porous silica.
Heterogeneous catalysis is one of the most important appli-
cations of bimetallic NPs.10,43–46 It is well-known that the
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Fig. 6 (a) TEM of the mesoporous silica shell; (b) nitrogen adsorption–desorption isotherms and (c) pore size distribution of the (Au–Ag)@mSiO2 composite micro-
spheres; small-angle XRD pattern of the composite nanoreactor (d).
Fig. 7 UV-vis spectra showing the gradual reduction of 4-NP over (a) no catalysts, (b) (Ag–Au)@mSiO2 nanoreactors, (c) concentration changes of 4-NP as a func-
tion of reaction time by using different concentrations of Au–Ag@SiO2 as a catalyst, (d) reusability of the hollow (Au–Ag)@mSiO2 and bare Au nanoparticles as cata-
lysts, respectively.
reduction of p-nitrophenol by NaBH4 almost does not proceed adding catalysts (Fig. 7b). In the presence of catalysts, the
in the absence of catalysts but proceeds rapidly with cata- absorption of p-nitrophenol at 400 nm decreases along with a
lysts.47,48 As shown in Fig. 7, the reaction barely occurs concomitant increase of the ∼300 nm peak of p-aminophenol.
(Fig. 7a) but the reaction rate was significantly improved by Although the outermost Ag shells were oxidized by air to form
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silver oxide, they still exhibit good catalytic properties in the
reduction of p-nitrophenol by NaBH4. The BET surface area
and BJH pore size distribution of the Au–Ag@SiO2 catalysts is
1167 m2 g−1 and 3.8 nm. Fig. 7c presents the effect of the Au–
Ag@SiO2 concentration on the reduction of 4-NP. With the
8 K. W. Hu, C. C. Huang, J. R. Hwu, W. C. Su, D. B. Shieh
and C. S. Yeh, Chem.–Eur. J., 2008, 14, 2956–2964.
9 B. Lim, K. H. Kim, T. Yu, J. G. Wang, M. J. Kim, Z. Y. Li,
M. Rycenga and Y. N. Xia, J. Am. Chem. Soc., 2010, 132,
2506–2507.
increase in the amount of the catalyst, the catalytic rate 10 H. L. Jiang, T. Akita, T. Ishida, M. Haruta and Q. Xu, J. Am.
becomes faster. Furthermore, the nanoreactors can be easily Chem. Soc., 2011, 133, 1304–1306.
recovered from the reaction solution by centrifugation and 11 M. J. Weber, A. J. M. Mackus, M. A. Verheijen,
show excellent reusability, an important characteristic of such
a nanoreactor. Compared to the (Au–Ag)@SiO2 composite
C. V. D. Marel and W. M. M. Kessels, Chem. Mater., 2012,
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13 J. F. Huang, S. Vongehr, S. C. Tang, H. M. Lu and
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nanoparticles stabilized on the inner wall of a mesoporous Commun., 2010, 46, 2085–2087.
silica shell has been demonstrated. This approach effectively 17 J. Liu, S. Z. Qiao, S. B. Hartono and G. Q. Lu, Angew. Chem.,
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mesopores on the silica shell made the interior of the Au– 18 X. J. Wu and D. S. Xu, J. Am. Chem. Soc., 2009, 131, 2774–
Ag@SiO2 composite microspheres an ideal nanoreactor for 2775.
heterogeneous catalytic reactions. In addition, these catalysts 19 K. Kamata, Y. Lu and Y. Xia, J. Am. Chem. Soc., 2003, 125,
could be easily separated from the reaction solution and 2384–2385.
exhibit good reusability. Furthermore, the vast majority of the 20 S. Wu, J. Dzubiella, J. L. Kaiser, M. K. Drechsler, X. H. Guo,
particle sizes remain essentially unchanged even through a
series of harsh treatments. We believe that the process is versa-
M. Ballauff and Y. Lu, Angew. Chem., Int. Ed., 2012, 51,
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tile since it not only allows for independent adjusting of the 21 I. Lee, J. B. Joo, Y. D. Yin and F. Zaera, Angew. Chem., Int.
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applied to other bimetallic and alloy NPs.
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24 S. H. Liu, S. Q. Bai, Y. G. Zheng, K. W. Shah and M. Y. Han,
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Acknowledgements
The financial support from the National Science Foundation
for Distinguished Young Scholars of China (grant no. 25 Z. Ma and S. Dai, ACS Catal., 2011, 1, 805–818.
51025517) and the National Defense Basic Scientific Research 26 Q. Zhang, I. Lee, J. B. Joo, F. Zaera and Y. D. Yin, Acc. Chem.
Project (A1320110011) are duly acknowledged.
Res., 2012, DOI: 10.1021/ar300230s.
27 Y. Chen, H. Chen, L. Guo, Q. He, F. Chen, J. Zhou, J. Feng
and J. Shi, ACS Nano, 2010, 4, 529–539.
28 J. Lee, J. C. Park and H. Song, Adv. Mater., 2008, 20, 1523–
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