Toward monodispersed silver nanoparticles with unusual thermal stability.
ABSTRACT A novel in situ autoreduction route has been developed, by which monodispersed silver nanoparticles with tunable sizes could be easily fabricated on silica-based materials, especially inside the channels of mesoporous silica (MPS). 13C CP/MAS NMR spectroscopy was employed to monitor the whole assembly process. It was demonstrated that the amino groups of APTS (aminopropyltriethoxyl silane)-modified MPS can be used to anchor formaldehyde to form novel reducing species (NHCH2OH), on which Ag(NH3)2NO3 could be in situ reduced. Monodispersed silver nanoparticles were thus obtained. In situ XRD and in situ TEM experiments were used to investigate and compare the thermal stabilities of silver nanoparticles on the external surface of silica gels (unconfined) and those located inside the channels of SBA-15 (confined). It was observed that unconfined silver nanoparticles tended to agglomerate at low temperatures (i.e., lower than 773 K). The aggregation of silver nanoparticles became more serious at 773 K. However, for those confined silver nanoparticles, no coarsening process was observed at 773 K, much higher than its Tammann temperature (i.e., 617 K). Only when the treating temperature was higher than 873 K could the agglomeration of those confined silver nanoparticles happen with time-varying via the Ostwald ripening process. The confinement of mesopores played a key role in improving the thermal stabilities of silver nanoparticles (stable up to 773 K without any observable coarsening), which is essential to the further investigations on their chemical (e.g., catalytic) properties.
Toward Monodispersed Silver Nanoparticles with Unusual
Junming Sun,†Ding Ma,†He Zhang,†Xiumei Liu,†Xiuwen Han,†Xinhe Bao,*,†
Gisela Weinberg,‡Norbert Pfa ¨nder,‡and Dangsheng Su‡
Contribution from the State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian 116023, People’s Republic of China, and Department of
Inorganic Chemistry, Fritz-Haber Institute of the Max Planck Society, Berlin D-14195, Germany
Received July 10, 2006; E-mail: email@example.com
Abstract: A novel in situ autoreduction route has been developed, by which monodispersed silver
nanoparticles with tunable sizes could be easily fabricated on silica-based materials, especially inside the
channels of mesoporous silica (MPS).13C CP/MAS NMR spectroscopy was employed to monitor the whole
assembly process. It was demonstrated that the amino groups of APTS (aminopropyltriethoxyl silane)-
modified MPS can be used to anchor formaldehyde to form novel reducing species (NHCH2OH), on which
Ag(NH3)2NO3 could be in situ reduced. Monodispersed silver nanoparticles were thus obtained. In situ
XRD and in situ TEM experiments were used to investigate and compare the thermal stabilities of silver
nanoparticles on the external surface of silica gels (unconfined) and those located inside the channels of
SBA-15 (confined). It was observed that unconfined silver nanoparticles tended to agglomerate at low
temperatures (i.e., lower than 773 K). The aggregation of silver nanoparticles became more serious at 773
K. However, for those confined silver nanoparticles, no coarsening process was observed at 773 K, much
higher than its Tammann temperature (i.e., 617 K). Only when the treating temperature was higher than
873 K could the agglomeration of those confined silver nanoparticles happen with time-varying via the
Ostwald ripening process. The confinement of mesopores played a key role in improving the thermal
stabilities of silver nanoparticles (stable up to 773 K without any observable coarsening), which is essential
to the further investigations on their chemical (e.g., catalytic) properties.
Ultra fine metal (or metal nanoparticles) catalysts are widely
used in industry as well as in laboratory.1-3With the develop-
ment of catalysis science, increasing attention has been paid to
the size-dependent chemistry of nanocatalysis,4-5which eluci-
dates the correlations between the activity and particle size.6
As an example, gold nanoparticles show amazingly high activity
toward low-temperature oxidation of carbon monoxide.7Quan-
tum size effect that emerged from the reduction of gold particle
size, especially when the diameter is less than 3 nm, is
responsible for the high activity. However, it must be kept in
mind that most of the catalytic reactions are high-temperature
processes, where bare metal nanoparticles (those without the
protection of protective agents) tend to agglomerate.8At the
same time, sintering of metal is strongly temperature-dependent
and closely related to the so-called Tammann temperature
(TTammann) 0.5TF; TFrepresents the melting point). When the
Tammann temperature is reached, bulk atoms tend to move,
which leads to interparticle diffusion and therefore coalescence
of metal particles. It is reported that when the particle size comes
into nanorange, the melting point of those metal nanoparticles
decreased drastically with the decrease of the particle size,
resulting in a much lower thermal stability.9The low thermal
stability of metal nanoparticles limits their applications inevi-
tably. Traditionally, inert inorganic matrix hosts (e.g., SiO2,
Al2O3, etc.) were often used to suppress the sintering of the
metal nanoparticles. Unfortunately, as most of those hosts are
mainly structurally nonuniform, the size and shape of the
obtained metal nanoparticles were difficult to be controlled and
thus in most cases randomly distributed. As a result, the catalytic
properties of these supported nanocatalysts are not well under-
Recently, the discovery of mesoporous silicas, such as M41s10
and SBA-15,11has initiated intensive research of inclusion
chemistry inside the channels of mesoporous silicas.12-14Due
to their uniform mesostructures, high surface areas, and tunable
†Chinese Academy of Sciences.
‡Fritz-Haber Institute of the Max Planck Society.
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Published on Web 11/17/2006
15756 9 J. AM. CHEM. SOC. 2006, 128, 15756-15764
10.1021/ja064884j CCC: $33.50 © 2006 American Chemical Society
pore sizes,11,15these ordered mesoporous silicas have been found
to be the promising templates to control the shape and size of
the occluded metal nanoparticles,16-32which have potential
applications in catalysis.33Current methodologies for the
assembly of metal nanoparticles inside the channels of meso-
porous silica include conventional incipient wetness impregna-
tion,16-17metal-organic chemical vapor deposition (MOCVD),18
and schemes making use of the silanols on the internal
channels,23-27etc. Metals such as Au, Pd, Pt, etc., have been
successfully encapsulated inside the channels of mesoporous
As a relatively inexpensive metal, silver has been extensively
studied in the field of catalysis (e.g., selective oxidation of
methanol, oxidative coupling of methane, and ethylene epoxi-
dation, etc.).34-36However, due to the low melting point (ca.
1233 K) and thus the low TTammann(ca. 617 K) of silver, sintering
of silver nanoparticles during the high-temperature reactions
(e.g., methanol oxidation) becomes one of the scientifically most
important and challenging problems. To use a nanocatalyst in
high-temperature reactions, one solution is to confine the
nanoparticles within a host (i.e., mesoporous materials) that
could prevent those particles from sintering. However, due to
the strong diffusion ability of ionic metals (in this case, Ag+)
via hydroxyl groups,29encapsulation is much more difficult
inside mesoporous channels, especially in terms of particle
homogeneity. Although many efforts have been made,24,27,29,31
metal nanowires or a mixture of nanowires and nanoparticles
were often the result because of the limitation of the meso-
structures themselves or the lack of an effective control over
the synthesis.28,31Herein, we present a general route by which
nearly monodispersed silver nanoparticles could be fabricated
on the surface (internal or external) of silica-based materials,
and noteworthy, monodispersed silver nanoparticles can be
selectively assembled inside the channels of mesoporous silicas.
13C CP/MAS NMR spectroscopy was employed to monitor the
whole assembly process. It was observed that the amino groups
of APTS (aminopropyltriethoxyl silane)-modified MPS can be
used to anchor formaldehyde to form NHCH2OH species, on
which Ag(NH3)2NO3could be in situ reduced. In situ XRD and
in situ TEM experiments were used to compare the thermal
stabilities of unconfined and confined silver nanoparticles. It
was observed that silver nanoparticles located inside the
mesopores showed unusual thermal stabilities (stable up to 773
K, which is far higher than its Tammann temperature (i.e., 617
K)). The confinement of ordered mesopores played a crucial
role in improving the thermal stabilities of silver nanoparticles,
which is essential to the further investigations on their chemical
(e.g., catalytic) properties.34-36
2. Experimental Section
2.1. Materials Synthesis. Mesoporous silicas (MPS, such as SBA-
15) were synthesized according to the published procedure.11,15,37An
amount of 2.0 g as-synthesized MPS (herein, MPS represents SBA-
15, MCM-41, and FDU-12) was suspended in 150 mL pure toluene
under flowing N2, and then 10 mL TMCS (trimethylchlorosilane) was
added dropwise under stirring. The mixture was recovered by filtration
with toluene and ethanol after stirring continuously at 353 K for 8 h.
After that, templates (e.g., P123) that occluded inside the samples can
be removed by solvent extraction or by calcination at 623-673 K for
6 h. Here, the calcinations method was used. Thus, the mesoporous
silicas with the external surface functionalized with Si(CH3)3(assigned
as TMCS-MPS) were obtained. An amount of 2.0 g TMCS-MPS was
dried at 373 K for 12 h before dispersion in 150 mL toluene. Then 6.0
mL APTS (aminopropyltriethoxyl silane) was added under stirring. The
mixture obtained was stirred for another 12 h at room temperature and
refluxed at 353 K for 8 h. The recovered solid was washed with toluene
and then with ethanol intensively to remove the physically adsorbed
APTS and toluene. The selectively functionalized sample was denoted
as APTS-TMCS-MPS (if SBA-15 was used, then the sample was
denoted as APTS-TMCS-SBA-15), after being vacuum-dried at 353
K for more than 6 h. Reducing species were introduced by stirring 1.0
g APTS-TMCS-MPS in a 105 mL mixture of formaldehyde, ethanol
and water (formaldehyde/ethanol/water ) 5:20:80, v/v/v) at 313 K for
30 min. The filtered solid was dried at 323 K for 12 h and named as
HCHO-APTS-TMCS-MPS. To introduce Ag, 1.0 g HCHO-APTS-
TMCS-MPS was added into a mixture of ethanol and Ag(NH3)2NO3-
(aq) (ethanol/Ag(NH3)2NO3(aq) ) 1:4, v/v). Ag(NH3)2NO3(aq) solution
(0.005 M) was prepared by dropwise addition of NH3(aq) into AgNO3
aqueous solution until the formation of a clear colorless solution. The
HCHO-APTS-TMCS-MPS/Ag(NH3)2NO3mixture was then stirred at
313 K for 30 min before filtration. The filtered yellowish or brown
product was rinsed with large amounts of deionized water and dried at
323 K for 12 h to get Ag-1/MPS (in the case of SBA-15, Ag-1/SBA-
15). In the control experiments, the same procedure was followed except
for the HCHO treatment. The sample complexed with silver ions was
calcined at 573 K to get the metallic Ag inside channels, which was
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Monodispersed Silver Nanoparticles
A R T I C L E S
J. AM. CHEM. SOC. 9 VOL. 128, NO. 49, 2006 15757
denoted as Ag-2/MPS. A sample without TMCS treatment but following
the same procedure to Ag-1/MPS, was also prepared and denoted as
In the control experiments, silica gel (SiO2) was bought from
Qingdao Ocean Chemical Ltd. Surface areas is ca. 238 m2/g, and
average pore size is more than 20 nm (Figure S1). Preparation of Ag/
SiO2follows that of Ag-3/MPS. It should be mentioned that the porosity
and surface area of the silica gel are created by the aggregated primary
nonporous silica particles.38Therefore, the obtained monodispersed
silver nanoparticles are actually located on the external surface of SiO2
spheres (in the current case, the diameters of the primary silica particles
are smaller than 50 nm). Thus, those silver nanoparticles are called
unconfined silver nanoparticles
2.2. Characterization. NMR spectra were recorded on a Varian
infinity-plus 400WB solid-state NMR spectrometer. SEM images were
taken on the Hitachi S4000 and S4800 scanning electron microscopes.
All samples were observed directly without coating any metal or carbon.
In addition, to get clearer contrast of silver nanoparticles, 15-70%
BSE signals are introduced in the secondary electrons. The TEM images
were obtained with a Philips CM 200 Transmission Electron Microscope
equipped with a CCD camera. The samples were embedded in epoxy
resin and cut into slices for TEM characterization.
X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX
2400 diffractometer equipped with a Cu KRX-ray source operating at
40 kV and 100 mA (40 kV and 50 mA for small-angle X-ray scanning).
In situ XRD experiments were carried out as follows: The powder
sample was first pressed into a self-supporting wafer and then fixed
on the sample holder equipped with a temperature-programmed heater.
After the room-temperature X-ray scanning, sample was heated to a
specific temperature at 5 K/min ramping rate and kept at that
temperature for 1 to 4 h. XRD experiments were performed at each
temperature or different time intervals of a specific temperature to
observe the agglomeration behavior of silver nanoparticles.
In situ TEM experiments were done on a Philips CM200 Transition
Electron Microscope. Sample was first dispersed on a copper grid and
then fixed in a special holder equipped with the temperature pro-
grammed heater. When a proper region was selected, all experiments
were in situ performed. The temperature of the grid was increased from
room temperature to specific temperature (keeping at that temperature
for 30-40 min) until 873 K. TEM images were taken regularly at given
heating temperatures or different time intervals to observe the ag-
glomeration behavior of silver nanoparticles. In order to avoid any
possible destructive effect of the long time irradiation on the particles
and mesoporous walls, the electron beam was switched off during
3. Results and Discussion
3.1. Controlled Fabrication of Monodispersed Silver
Nanoparticles within the Channels of SBA-15. In order to
produce monodispersed silver nanoparticles, the amino func-
tional groups were grafted to the internal pore walls of SBA-
15 using APTS (aminopropyltriethoxyl silane), while silanol
groups on the external surface of the silica were terminated/
blocked by TMCS (trimethylchlorosilane) treatment. Formal-
dehyde was then grafted over these amino groups (for details,
see Experimental Section). When reacting with adjacent metal
precursors such as Ag(NH3)2NO3, the in situ autoreduction
reaction occurred within the mesopores, which enables us to
get the monodispersed Ag nanoparticles confined inside the
mesopores (Scheme 1).
Figure 1 shows the HRSEM and TEM images of 8 wt %
Ag-3/ SBA-15 (Figure 1a,c) and Ag-1/SBA-15 (Figure 1b,d).
Silver nanoparticles are present on both the external and internal
surface of SBA-15 when the external surface was not treated
with TMCS (Figure 1a,c). However, different from that prepared
by the traditional incipient impregnation methods (Figure S2),
no large silver particles were observed on the external surface
of SBA-15 (Figure S3). Instead, they are nearly monodipersed
nanoparticles (Figure 1a), indicating that the in situ autoreduction
route has a good control over the formation of highly dispersed
silver nanoparticles, whether they are on the internal or external
surface of silica. After deactivation of the silanol groups on the
external surface by TMCS, the number of Ag particles on the
external surface decreased immediately (Figure 1b), and almost
all the monodispersed Ag nanoparticles stayed inside the
channels of the mesoporous silica (Figure 1d). This result
suggests that by selectively grafting the amino groups, mono-
dispersed silver nanoparticles can be introduced into the
mesoporous matrix as we need.
The small-angle XRD pattern of APTS-TMCS-SBA-15
(Figure 2a) indicates that the grafting of amino groups inside
the channels did not affect the long-range ordering of the
mesostructures. After the loading of Ag into APTS-TMCS-SBA-
15, a decrease in intensities of the lower angle peaks was
observed (Figure 2b), suggesting that the Ag nanoparticles have
been encapsulated inside the pores, which is similar to the case
of Pt/SBA-15.25The broadening of the Ag diffraction peaks
demonstrates that the size of the Ag nanoparticles is in the
nanometer range (Inset in Figure 2), which is in good agreement
with the TEM observations (Figure 1a).
In the control experiments, if HCHO was not used (Ag-2/
SBA-15, i.e., instead a relatively high-temperature treatment was
used to get metallic Ag), a mixture of various sized Ag particles
and Ag nanowires is observed (Figure 3). If silver ions were
first anchored by a complex (SiCH2CH2NH2) inside the meso-
pores, followed by formaldehyde reduction (it is named the
(38) Unger, K. Angew. Chem., Int. Ed. 1972, 11, 267.
Scheme 1. Schematic Representation of Assemblage of Highly Dispersed Ag Nanoparticles Inside the Channels of SBA-15
A R T I C L E S
Sun et al.
15758 J. AM. CHEM. SOC.9VOL. 128, NO. 49, 2006
complex-reduction process),27silver loading after one cycle of
this process is much lower than in that prepared by the in situ
autoreduction method (Figure S5). At the same time, the
multicycle complex-reduction process will lead to diverse
distribution of silver nanoparticles in the channels of SBA-15
3.2. Fabrication of Highly Dispersed Silver Nanoparticles
on Other Silica-Based Materials. It has been proved that the
in situ autoreduction route is a facile method for the assembly
of highly dispersed silver nanoparticles within 2-D mesoporous
SBA-15 silica. In fact, it is also suitable to other silica-based
materials. Figure 4 shows the typical TEM and HRSEM images
of monodispersed silver nanoparticles confined within the
channels of various mesoporous silicas (e.g., MCM-41, FDU-
12) and on the external surface of silica gels. Indeed, in situ
autoreduction is an efficient method to synthesize monodispersed
silver nanoparticles on silica-based materials (especially, within
the channels of the mesoporous silica materials). Significantly,
by varying the pore size of mesoporous silicas, it is possible to
fabricate monodispersed silver nanoparticles with different
particle sizes, which is essential to the investigation of their
3.3. Formation Mechanism of Highly Dispersed Silver
Nanoparticles in the in Situ Autoreduction Process. To
investigate the formation mechanism of metal nanoparticles
within the channels of SBA-15,13C CP/MAS NMR spectros-
copy (Figure 5) was employed to monitor the whole assembly
process. Figure 5a shows the13C CP/MAS NMR spectrum of
as-synthesized SBA-15. The signals at 16.3 ppm, 74.5 ppm and
its shoulder peak at 70 ppm indicate the existence of the P123
template in the pores.39After the functionalization of as-
synthesized SBA-15 with TMCS, a new methyl signal at 1.7
ppm appeared (Figure 5b),40which suggests that TMCS
successfully reacted with external silanol, while the internal
silanols remain unaffected due to the protection of occluded
templates. The methyl groups on the external surface of SBA-
15 are relatively stable upon calcination at 623 K (Figure S7),
with a slightly upper field shift which might be related to the
slight changes of the circumjacent Si species after the removal
of the surfactants.At the same time, the P123 template within
the pore channels has been removed completely (Figure 5c),
and consequently the exposed internal silanols are accessible
to further modification. With APTS treatment, new peaks at
9.5, 16.5, 25.7, 43.9, and 58.3 ppm are observed (Figure 5d).
The three stronger signals at 9.5, 25.7, and 43.9 ppm can be
ascribed to the resonances of the three methylene groups in the
grafted aminopropyl moieties.41,42The presence of two weaker
peaks at 16.5 and 58.3 ppm indicates that there are some
unreacted ethoxy groups left. Combined with the29Si CP/MAS
(39) Yang, C. M.; Zibrowius, B.; Schmidt, W.; Schu ¨th, F. Chem. Mater. 2003,
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Figure 1. HRSEM (a, b) and TEM images (c, d) of 8 wt % Ag-3/SBA-15 (a, c) and 8 wt % Ag-1/ SBA-15 (b, d).
Figure 2. Small-angle XRD patterns of (a) APTS-TMCS-SBA-15; (b) 8
wt % Ag-1/ SBA-15; inset is the wide-angle XRD pattern of 8 wt % Ag-
Monodispersed Silver Nanoparticles
A R T I C L E S
J. AM. CHEM. SOC. 9 VOL. 128, NO. 49, 2006 15759
NMR spectrum (Figure S8), it can be concluded that the amino
groups have been grafted to the internal pore walls of SBA-15,
which is consistent with the nitrogen results (Figure S4 and
After the introducion of formaldehyde, another new peak at
ca.78.1 ppm appears in the13C CP/MAS NMR spectrum (Figure
5e). Meanwhile, the signals of C3 and C2 shift to lower and
higher fields, respectively (Figure 5). This suggests that
Figure 3. TEM images of Ag-2/SBA-15.
Figure 4. TEM or HRSEM images of (a, b) 8 wt % Ag-1/MCM-41; (c, d) 8 wt % Ag-1/FDU-12 and (e, f) 8 wt % Ag/SiO2. Bright dots in Figure 4f are
A R T I C L E S
Sun et al.
15760 J. AM. CHEM. SOC.9VOL. 128, NO. 49, 2006