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15--Byunghwan Lee Influences of synthesis conditions and mesoporous structures on the gold nanoparticles supported on mesoporous silica hosts

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Microporous and Mesoporous Materials 122 (2009) 160–167
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso
Influences of synthesis conditions and mesoporous structures on the gold
nanoparticles supported on mesoporous silica hosts
Byunghwan Lee a,b,*, Zhen Ma a, Zongtao Zhang a,c, Chulhwan Park d, Sheng Dai a,*
a
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
Department of Chemical System Engineering, Keimyung University, Daegu 704-701, Republic of Korea
c
College of Chemistry, Jilin University, Changchun 130023, PR China
d
Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea
b
a r t i c l e
i n f o
Article history:
Received 6 November 2008
Received in revised form 2 February 2009
Accepted 23 February 2009
Available online 3 March 2009
Keywords:
Gold nanoparticle
Mesoporous silica
SBA-15
Pore structure
Thermal stability
a b s t r a c t
Loading gold on mesoporous materials via different methods has been actively attempted in the literature, but the knowledge about the influences of synthesis details and different mesoporous structures
on the size and thermal stability of gold nanoparticles supported on mesoporous hosts is still limited.
In this study, Au/HMS, Au/MCM-41, Au/MCM-48, Au/SBA-15, and Au/SBA-16 samples were prepared
by modifying a variety of mesoporous silicas by amine ligands followed by loading HAuCl4 and calcination. The influences of different amine ligands ((3-aminopropyl)triethoxysilane versus N-[3-(trimethoxysilyl)propyl]ethylenediamine), solvents (water versus ethanol), calcination temperatures (200 or 550 °C),
and mesoporous structures on the size of supported gold nanoparticles were systematically investigated
employing nitrogen adsorption–desorption measurement, X-ray diffraction (XRD), diffuse reflectance
UV–vis spectroscopy, and transmission electron microscopy (TEM). Interestingly, while big and irregular
gold particles situate on MCM-48 with bicontinuous three-dimensional pore structure and relatively
small pore size (2.4 nm) upon calcination at 550 °C, homogeneous and small gold nanoparticles maintain
inside SBA-15 with one-dimensional pore structure and relatively big pore size (6.8 nm). Apparently, the
pore structure and pore size of mesoporous silica hosts play a key role in determining the size and thermal stability of the supported gold nanoparticles. Our results may provide some useful clues for the
rational design of supported metal catalysts by choosing suitable mesoporous hosts.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
Highly dispersed gold nanoparticles have been demonstrated to
be very active for a number of reactions such as CO oxidation and
hydrogenation [1–5]. A prerequisite for their applications in many
catalytic reactions is the homogeneous distribution of small gold
nanoparticles with diameters between 2 and 5 nm. Several methods (e.g. impregnation, coprecipitation, deposition–precipitation,
anion adsorption, and chemical vapor deposition) have been developed for the preparation of supported gold catalysts [1–5], but one
of their main drawbacks is the difficulty in controlling both the
location and size of gold nanoparticles. Gold nanoparticles are normally either situated on external surfaces of oxide particles or
embedded in oxide matrixes. The gold nanoparticles on external
surfaces are susceptible to aggregation due to the decreased melting point of nanoparticles [6–8], the high surface free energy of
* Corresponding authors. Address: Department of Chemical System Engineering,
Keimyung University, Daegu 704-701, Republic of Korea. Tel.: +82 53 580 5239; fax:
+82 53 580 5165 (B. Lee).
E-mail addresses: [email protected] (B. Lee), [email protected] (S. Dai).
1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2009.02.029
small nanoparticles [4], and the lack of space confinements
[9,10], whereas those embedded in oxide matrixes may not be
accessible to reactants in catalytic reactions [11]. These deficiencies have prompted the development of alternative methodologies
for the synthesis of catalytic materials with better control over
both the size and location of supported gold nanoparticles [12–14].
Mesoporous silicas (e.g. MCM-41 and SBA-15) are an important
family of porous materials used as adsorbents and catalyst supports [15,16]. Their high surface areas, good thermal stability,
and ordered mesopores are the key advantages for engineering
nanoreactors with active components dispersed on their internal
surfaces [17–21]. However, it is difficult to load gold onto SiO2
via deposition–precipitation using HAuCl4 as the precursor, due
to the low isoelectric point of SiO2 [22,23]. Under the high-pH conditions adopted in regular deposition–precipitation, the negatively
charged Au(OH)xCl4 x species can not effectively adsorb onto the
negatively charged SiO2 surfaces. To overcome such barrier, Au/
mesoporous SiO2 samples have been synthesized via several alternative routes: (1) modification of mesoporous SiO2 by organic
functional groups followed by loading gold [24–29], (2) one-pot
synthesis of Au/mesoporous SiO2 involving both Au3+ and an SiO2
161
B. Lee et al. / Microporous and Mesoporous Materials 122 (2009) 160–167
source [30–34], (3) dispersing gold colloids on mesoporous SiO2
[35], (4) synthesizing mesoporous SiO2 in the presence of gold colloids [36–39], (5) liquid-phase grafting or chemical vapor deposition of gold precursors [40], and (6) using Au(en)2Cl3 as the
precursor [13]. The in the synthesis of Au/mesoporous SiO2 has
been reviewed recently [41]. However, regardless of the fact that
much effort has been placed on the development of these diversified methods, the knowledge on the influence of synthesis details
and different mesostructures on the size and thermal stability of
gold nanoparticles supported on mesoporous hosts is still limited
[42–44]. From a more general perspective, the thermal stability
of nanoparticles in mesoporous material hosts is of both fundamental interest and practical merit [45–47], especially considering
that there are numerous mesoporous materials available for hosting metal nanoparticles.
Our group has been interested in developing strategies that can
effectively load gold nanoparticles on mesoporous SiO2 [13,30,31].
In the current work, we systematically studied the influences of
synthesis conditions and mesoporous structures on the size and
thermal stability of supported gold nanoparticles upon calcination
at 200 or 550 °C. To this end, various kinds of mesostructured
materials, i.e., HMS [48] with bicontinuous wormhole structure,
MCM-41 [15] and SBA-15 [16] with one-dimensional hexagonal
structures, and MCM-48 [49] and SBA-16 [50] with bicontinuous
cubic structures, were adopted as hosts. These mesoporous materials were then grafted by bifunctional ligands that can not only
covalently bond to the porous silica matrix via siloxane groups
but also complex Au3+ via amine functional groups [51], leading
to the successful immobilization of Au3+ on the support surfaces
(Fig. 1). The influences of the types of functional ligands and sol-
vents used in preparation, calcination temperatures, and different
mesoporous structures on the size distributions of gold nanoparticles formed upon calcination were studied systematically. In particular, the good thermal stability of gold nanoparticles in SBA-15
was discussed in the context of gold particle size, melting point
of gold, and the pore size of SBA-15.
2. Experimental
2.1. Syntheses of mesoporous silicas
HMS [48,52]: A mixture of 1-hexadecylamine (99%, Aldrich),
tetramethyl orthosilicate (TMOS; 98%, Aldrich), deionized water,
and ethanol (99.5%, Aldrich) with a mole ratio of 1:4:200:50 was
stirred at room temperature for 20 h, filtered, and then vacuumdried at room temperature overnight. The dried sample was refluxed with ethanol three times to remove the surfactant and then
vacuum-dried at 80 °C for 6 h.
MCM-41 [15]: 2.97 g of cetyltrimethylammonium bromide
(CTAB; 95%, Aldrich) was dissolved in 40 ml of distilled water. To
this solution, 8.99 ml of ammonium hydroxide solution (ACS reagent, 28.0–30.0% as NH3, Aldrich) and 7.25 ml of tetraethyl orthosilicate (TEOS; 98%, Aldrich) was added, and the mixture was then
stirred for 24 h. The filtered material was vacuum-dried at room
temperature overnight, and then calcined at 550 °C for 6 h to remove the template.
MCM-48 [49]: 3.6 g of CTAB was dissolved in a mixture of 50 ml
of distilled water and 13 ml of ammonium hydroxide solution. To
this solution, 4 ml of TEOS was added and then refluxed for 4 h.
The filtered material was vacuum-dried at room temperature overnight, and then calcined at 550 °C for 6 h.
SBA-15 [16,52]: 2 g of block copolymer Pluronic P123
(EO20PO70EO20, Mav = 5800) was dissolved in a mixture of distilled
water (15 ml) and 2 M hydrochloric acid solution (60 ml). To this
solution, 4.25 g of TEOS was added and then stirred at 45 °C for
20 h. The mixture was then aged at 85 °C for 12 h. The filtered
material was vacuum-dried at room temperature overnight, and
then calcined at 550 °C for 6 h.
SBA-16 [50]: 3.3 g of Pluronic F127 copolymer (EO106PO70EO106)
was dissolved in a mixture of distilled water (15 ml) and 2 M
hydrochloric acid solution (60 ml). To this solution, 4.25 g of TEOS
was added and then stirred at 80 °C for 48 h. The filtered material
was vacuum-dried at room temperature overnight, and then calcined at 550 °C for 6 h.
2.2. Syntheses of gold nanoparticles supported on mesoporous silicas
Mesoporous silicas listed in Table 1 were grafted by amine functional groups [53]. First, mesoporous silica (0.5 g) was added to
50 ml of toluene and then stirred. To this solution, 0.5 ml of
(3-aminopropyl)triethoxysilane (APTS; 99%, Aldrich) or 0.5 ml of
N-[3-(trimethoxysilyl)propyl]ethylenediamine (DAPTS; 97%, Aldrich) was added and then refluxed for 24 h. The filtered materials
Table 1
Characteristics of the prepared mesoporous silicas.
Fig. 1. The synthetic procedure of gold nanoparticles using various types of
mesoporous silicas.
Sample
Surface areaa (m2/g)
Pore volume (cc/g)
Diameterb (nm)
HMS
MCM-41
MCM-48
SBA-15
SBA-16
1119
1980
1390
885
621
0.77
1.83
1.04
1.27
0.73
2.25
2.81
2.39
6.79
3.70
a
b
BET surface area.
Maximum diameter in BJH desorption pore size distribution.
162
B. Lee et al. / Microporous and Mesoporous Materials 122 (2009) 160–167
were washed several times with a large quantity of ethanol and
toluene, and then vacuum-dried at 80 °C for 6 h.
To introduce the gold precursor into the mesopores, 0.1 g of
functionalized mesoporous silica and 10 ml of 10 4 M gold(III)
chloride trihydrate (ACS reagent, Aldrich) dissolved in water or
ethanol solvent were mixed and then sonicated for 30 min. The
monoamine or diamine groups on silica surfaces are expected to
interact with the gold species [24–29], and the ultrasonic treatment is assumed to facilitate the homogeneous uptake of gold species. The resulting materials were then filtered and vacuum-dried
at room temperature overnight, and the obtained dry powders
were heated to 200 or 550 °C in a muffle furnace at a rate of
1 °C/min and then calcined at 200 or 550 °C for 1 h. The synthetic
procedure and conditions are shown in Fig. 1 and Table 2.
2.3. Characterization
Nitrogen adsorption–desorption measurements employing an
Autosorb-1 instrument were performed to determine the mesoporosity of the prepared samples. X-ray diffraction (XRD) patterns
were recorded using a SIEMENS D5005 X-ray diffractometer and
a CuKa source (wavelength = 0.154 nm). The average sizes of gold
nanoparticles were calculated using the half-width of the gold
peak at 2h = 38° by applying the Sherrer equation. The sizes and
size distributions of supported gold nanoparticles were directly observed using scanning transmission electron microscopy (STEM;
HD-2000, Hitachi). Additional diffuse reflectance UV–visible experiments were carried out on a Varian instrument (Cary 4E) using solid samples (Au/mesoporous SiO2). The purpose of the UV–vis
experiments was to quickly see the trends of particle size changes,
whereas the detailed sizes of supported gold nanoparticles were
solely determined by XRD and TEM (Table 2).
3. Results and discussion
3.1. Characterization of prepared mesoporous silica hosts
Five types of mesoporous silicas were used to study the effect of
mesoporous structures on the size and thermal stability of gold
nanoparticles supported on mesoporous hosts. These mesoporous
silicas are HMS [48] with bicontinuous wormhole structure,
MCM-41 [15] and SBA-15 [16] with one-dimensional hexagonal
structures, and MCM-48 [49] and SBA-16 [50] with bicontinuous
cubic structures.
Results of nitrogen adsorption–desorption measurements are
shown in Table 1. There is a relatively smooth step on the nitrogen
adsorption–desorption isotherm of HMS when the relative pressure P/P0 is between 0.2 and 0.4 (see Fig. S1 in the Supporting Information), and the pore diameter of HMS is 2.3 nm (Fig. S2A). There
are steep slopes on the isotherms of MCM-41 and MCM-48 at P/
P0 = 0.35 and 0.25, respectively (Fig. S1). The pore diameters of
MCM-41 and MCM-48 are 2.8 and 2.4 nm, respectively, and the
pore size distributions are sharp (Fig. S2B). SBA-15 and SBA-16
show typical hysteresis curves in their isotherms (Fig. S1). These
two materials have relatively big pore diameters of 6.8 and
3.7 nm, respectively. An additional TEM image shows the cubic
mesostructure of SBA-16 (inset of Fig. S2A).
3.2. Effect of amine ligand and solvent
To begin with, we examined the effects of the types of amine
functional ligands and the solvents used on the sizes of supported
gold nanoparticles. In these experiments, HAuCl4 was loaded onto
mesoporous silicas functionalized by either monoamine (APTS) or
diamine (DAPTS) using either water or ethanol solvent (Table 2).
The monoamine or diamine on silica surfaces is expected to interact with the gold species [24–29], and the ultrasonic treatment
was used to facilitate the homogeneous dispersion of gold species
in the pore channels. Fig. S3 shows a TEM image of the as prepared
Au/SBA-15 generated using monoamine and water. Below we focus
on the trends generalized by comparing the Au/mesoporous SiO2
samples calcined at 200 or 550 °C.
The sixth column in Table 2 shows the size range of gold nanoparticles formed in mesoporous silicas, as measured from TEM
experiments. Among these samples, Au/HMS and Au/SBA-15 were
studied more systematically by varying the synthesis conditions. A
general trend from these data is that bigger gold nanoparticles are
generated when using diamine instead of monoamine ligand, and
using ethanol versus water solvent. Here we first take Au/HMS
samples calcined at 200 °C as an example to show the case. The
Table 2
Bifunctional ligand and solvent used for loading gold, calcination temperature for the synthesis of gold nanoparticles on various types of mesoporous silicas, and the size of gold
nanoparticles synthesized.
Mesoporous
silica
Au/SiO2
sample
Bifunctional
ligand
Solvent used for
loading gold
Calcination
temperature (°C)
Au particle size from TEM
measurement (nm)
Au particle size calculated by Scherrer
equation (nm)
HMS
H-1
H-2
H-3
H-4
H-5
H-6
H-7
H-8
M41-1
M41-2
M48-1
M48-2
S15-1
S15-2
S15-3
S15-4
S15-5
S15-6
S15-7
S15-8
S16-1
S16-2
APTS
APTS
APTS
APTS
DAPTS
DAPTS
DAPTS
DAPTS
APTS
APTS
APTS
APTS
APTS
APTS
APTS
APTS
DAPTS
DAPTS
DAPTS
DAPTS
APTS
APTS
Water
Water
Ethanol
Ethanol
Water
Water
Ethanol
Ethanol
Water
Water
Water
Water
Water
Water
Ethanol
Ethanol
Water
Water
Ethanol
Ethanol
Water
Water
200
550
200
550
200
550
200
550
200
550
200
550
200
550
200
550
200
550
200
550
200
550
2.1–6.3
2.6–22.4
2.3–11.5
10.0–18.3
8.7–20.4
3.5–20.8
4.7–20.9
5.8–22.3
1.9–2.9
1.9–21.0
1.4–8.8
7.5–27.2
2.1–3.2
3.2–10.3
4.5–15.7
3.9–19.4
5.8–8.7
3.9–11.7
4.5–18.0
3.9–23.3
2.4–5.6
3.8–11.4
3.5
5.8
–
–
–
–
–
–
2.6
6.7
2.3
14.3
2.5
5.0
–
–
–
–
–
–
3.8
5.8
MCM-41
MCM-48
SBA-15
SBA-16
163
B. Lee et al. / Microporous and Mesoporous Materials 122 (2009) 160–167
gold particle size of sample H-1 prepared using APTS ligand and
water solvent is in the range of 2.1–6.3 nm, as observed by TEM
characterization, whereas that of sample H-3 prepared using APTS
ligand and ethanol solvent is in the range of 2.3–11.5 nm, indicating that the use of ethanol solvent leads to bigger gold nanoparticles. On the other hand, the gold particle size of sample H-5
prepared using DAPTS ligand and water solvent is in the range of
8.7–20.4 nm, bigger than the values (2.1–6.3 nm) corresponding
to sample H-1, demonstrating that the use of DAPTS ligand leads
to bigger gold nanoparticles.
Similar conclusions can be reached by examining Au/HMS samples calcined at 550 °C. The gold particle size of sample H-2 pre-
pared using APTS ligand and water solvent is in the range of 2.6–
22.4 nm, as observed by several TEM images, whereas that of sample H-4 prepared using APTS ligand and ethanol solvent is in the
range of 10.0–18.3 nm, and the gold particle size of sample H-6
prepared using DAPTS ligand and water solvent is in the range of
3.5–20.8 nm. The average sizes of gold nanoparticles of H-2, H-4,
and H-6 measured from the representative TEM images in Fig. 2
are 5.4 nm (S.D. 2.6 nm), 12.9 nm (S.D. 2.8 nm), and 11.3 nm (S.D.
6.9 nm), respectively.
Next, we performed diffuse reflectance UV–vis experiments
using 550 °C-calcined Au/HMS solid powders (Fig. 3A). The objective of our UV–vis experiments was to corroborate the trends obtained by TEM experiments, but not to measure out the actual
sizes of gold nanoparticles. In general, a strong absorbance in the
visible region around 520–540 nm is due to the excitation of surface plasmon vibrations [54], and a blue shift in the longitudinal
plasmon vibration is an indication of the extent of aggregation of
the gold nanoparticles [55]. Therefore, the relative peak positions
of Au/mesoporous SiO2 samples may be used to compare their relative gold particle sizes and show general trends. In Fig. 3A, H-2
was prepared using APTS ligand and water solvent, H-4 was prepared using APTS ligand and ethanol solvent, and H-6 was prepared using DAPTS ligand and water solvent, and these samples
Absorbance
A
c
b
a
400
500
600
700
Wavelength [nm]
Absorbance
B
f
e
d
400
500
600
700
Wavelength (nm)
Fig. 2. TEM images of prepared samples H-2 (A), H-4 (B), and H-6 (C). These
samples were prepared using APTS ligand and water solvent (H-2), APTS ligand and
ethanol solvent (H-4), and DAPTS ligand and water solvent (H-6), respectively, with
HMS as the host. All the samples were calcined at 550 °C.
Fig. 3. Results of diffuse reflectance UV–vis measurements of prepared samples: (a)
H-2, (b) H-4, (c) H-6, (d) S15-2, (e) S15-4, and (f) S15-6. These samples were
prepared using APTS ligand and water solvent (H-2 and S15-2), APTS ligand and
ethanol solvent (H-4 and S15-4), and DAPTS ligand and water solvent (H-6 and S156), with HMS and SBA-15 as hosts, respectively. All the samples were calcined at
550 °C.
B. Lee et al. / Microporous and Mesoporous Materials 122 (2009) 160–167
3.3. Thermal stability of gold nanoparticles on different mesoporous
silica hosts
The thermal stability of gold nanoparticles is of importance for
both fundamental interests [59,60] and practical applications [61].
Here we compared the thermal stability of a series of Au/mesoporous SiO2 samples by calcining them at 200 or 550 °C. The objective
of these experiments was to see whether the sizes of gold nanoparticles are larger than the pore sizes of their respective hosts, especially when calcining them at 550 °C.
We first take Au/MCM-48 samples as an example to show the
case. In Fig. 4A, the XRD peak at 2h = 22° corresponds to amorphous
silica host and those at 2h = 38, 44, 65, 78, and 82° correspond to
metallic gold [30]. For Au/MCM-48 calcined at 200 °C (denoted as
M48-1), the gold peaks are relatively broad, and the average gold
particle size is estimated by the Scherrer equation as 2.3 nm. It is
Au(111)
A
Au(200)
Intensity [relative units]
were all calcined at 550 °C. The absorbance peak position of H-4
and H-6 are at shorter wavelengths than that of H-2, indicating
that the gold nanoparticles in systems H-4 and H-6 are bigger. This
is in agreement with the TEM results highlighting the disadvantages of using ethanol solvent (H-4, Fig. 2B) and DAPTS ligand
(H-6, Fig. 2C), respectively.
Similar conclusions on the effects of ligands and solvents can be
reached when comparing the Au/SBA-15 samples either calcined at
200 °C (S15-1, S15-3, and S15-5) or 550 °C (S15-2, S15-4, and S156), although the ranges of gold nanoparticle sizes of different samples are not distinctly separated by TEM measurement (Table 2).
Hence, diffuse reflectance UV–vis experiments may provide direct
information on relative particle sizes of large quantities of samples.
Below we take the UV–vis experiments of 550 °C-calcined S15-2,
S15-4, and S15-6 samples as an example (Fig. 3B). S15-2 was prepared using APTS ligand and water solvent, S15-4 was prepared
using APTS ligand and ethanol solvent, and S15-6 was prepared
using DAPTS ligand and water solvent, and these samples were
all calcined at 550 °C. In this case, a blue shift is again observed
when using ethanol solvent or DAPTS ligand, but the extent of such
shift is less obvious, indicating that the agglomeration of gold
nanoparticles is less severe using SBA-15 as the host. This can be
understood, considering that SBA-15 has one-dimensional long
pore structure that may exert better space confinement whereas
HMS has bicontinuous pore structure that may facilitate the migration of gold nanoparticles. This point will be further addressed in
Section 3.3.
The question then arises as to why diamine ligand and ethanol
solvent lead to bigger gold nanoparticle. In general, the grafted
amine groups can interact with gold nanoparticles [25,28,29,56],
and the interaction is expected to be stronger when using diamine
instead of monoamine as the ligand. Therefore, the use of diamine
ligand may facilitate the uptake of gold, thus increasing the gold
particle size since the agglomeration of gold nanoparticles is related to the uptake or availability of gold on support surfaces. It
is supposed that more gold would lead to more severe agglomeration. As to why ethanol solvent tends to lead to bigger gold nanoparticles, it is known that ethanol can facilitate the reduction of
gold cations [57]. Okitsu and coworkers reported that the ultrasonic irradiation of an aqueous HAuCl4 solution containing a small
amount of 2-propanol leads to the formation of gold nanoparticle
due to the production of reducing radicals from 2-propanol [58].
In addition, the residual ethanol solvent that cannot be completely
removed by vacuum drying at room temperature may also facilitate the reduction of gold cations during ramping the temperature
for calcination. Because our above experiments have shown that
DAPTS ligand and ethanol solvent lead to bigger gold nanoparticles,
APTS ligand and water solvent were adopted consistently in our research below.
Au(220) Au(311)
c
b
a
20
40
2θ [degree]
60
80
B
Intensity [relative units]
164
f
e
d
20
40
60
80
2θ [degree]
Fig. 4. XRD patterns of prepared samples: (a) MCM-48, (b) M48-1, (c) M48-2, (d)
SBA-15, (e) S15-1, and (f) S15-2. Among them, M48-1 and S15-1 stand for Au/MCM48 and Au/SBA-15 calcined at 200 °C, respectively. M48-2 and S15-2 stand for Au/
MCM-48 and Au/SBA-15 calcined at 550 °C, respectively. See Table 2 for more
details.
known that small nanoparticles can not be easily detected by
XRD [4]. However, in our case the determination of gold particle
size as small as 2.3 nm by XRD is reasonable, considering that
the Au(1 1 1) peak is obvious and broad, without the interference
from the support. We previously successfully determined similarly
small gold particles on Au/SBA-15 synthesized using Au(en)2Cl3 as
the precursor [13,62]. In addition, according to the TEM image in
Fig. 5A, the gold particle size is in the range of 1.4–8.8 nm. For comparison, the gold peaks of Au/MCM-48 calcined at 550 °C (denoted
as M48-2) are very sharp. The average gold particle size is estimated by the Scherrer equation as 14.3 nm, and the gold particle
size is in the range of 7.5–27.2 nm (Fig. 5B). The comparison between the pore size of MCM-48 (2.4 nm) and the particle size of
gold nanoparticles (7.5–27.2 nm) suggests the extrusion of gold
nanoparticles to external surfaces as a result of calcination at
550 °C.
The size and thermal behavior of Au/SBA-15 upon calcination
was studied. S15-1 and S15-2 refer to the Au/SBA-15 samples calcined at 200 and 550 °C, respectively. The average gold particle size
of S15-1 is estimated by the Scherrer equation as 2.5 nm (Table 2),
and the gold particle size observed by TEM is in the range of
2.1–3.2 nm (Fig. 5C). On the other hand, the average gold particle
size of S15-2 is estimated by the Scherrer equation as 5.0 nm
(Table 2), and the gold particle size observed by TEM is in the range
165
B. Lee et al. / Microporous and Mesoporous Materials 122 (2009) 160–167
Fig. 5. TEM images of M48-1 (A), M48-2 (B), S15-1 (C), and S15-2 (D). M48-1 and S15-1 stand for Au/MCM-48 and Au/SBA-15 calcined at 200 °C, respectively. M48-2 and S152 stand for Au/MCM-48 and Au/SBA-15 calcined at 550 °C, respectively.
ger, the melting point of gold nanoparticles of 2.4 nm is below
500 °C [7]. The sample M48-2 was calcined at 550 °C, higher than
the calculated melting temperature of gold nanoparticles inside
MCM-48 mesopores. In addition, the mobility of gold may be facilitated by the well-ordered bicontinuous structure of MCM-48,
resulting in the aggregation of gold nanoparticles on the external
surface of MCM-48 (Fig. 5B). Others have found that cubic and 3D mesoporous structures can less efficiently control the sintering
15
Pore diameter or
Au particle size (nm)
of 3.2–10.3 nm (Fig. 5D). Although the gold particle size increases
with the calcination temperature, the extent of increase is not as
obvious as that observed with Au/MCM-48. Such difference can
be seen clearly when comparing the bottom panels of Figs. 4 and
5 with the corresponding top panels.
Gold nanoparticles can more easily agglomerate especially if the
metal-support interaction is weak [62]. In order to facilitate the
interaction between gold precursor and silica surface, we have
grafted aminosilane onto the surface of mesoporous silicas. Bifunctional amine ligands are stable on mesoporous silica up to approximately 300 °C [63]. Therefore, there is no dramatic difference
among the average particle sizes of H-1 (3.5 nm), M41-1
(2.6 nm), M48-1 (2.3 nm), S15-1 (2.5 nm), and S16-1 (3.8 nm).
The average gold particle sizes of these 200 °C-calcined samples
are all estimated by the Scherrer equation as within 2.3–3.8 nm,
and TEM results indicate that their particle sizes are all small
(Fig. 5A and C, and Table 2). However, the interaction between
amine ligands and gold may decrease and gold particles may easily
agglomerate at higher calcination temperatures (e.g. 550 °C
adopted in our experiments). It is known that the melting point
of gold particles decreases dramatically for particles smaller than
6–8 nm [7]. For instance, the melting point of bulk gold is
1064 °C, but the melting point can decrease below 550 °C if the
particle size is smaller than 4 nm [7]. Therefore, the size of nanopores may play a role in determining the thermal stability of gold
nanoparticles.
MCM-48 has a pore size of 2.4 nm, and the gold particle inside
such pores cannot be bigger than 2.4 nm due to the geometric confinement effect. According to the relation between particle size and
melting point of gold particles plotted by the method of Reifenber-
10
5
0
HMS
MCM-41
MCM-48
SBA-15
SBA-16
Fig. 6. Comparison of pore diameter of mesoporous silicas and size of gold
particles: pore diameter of mesoporous silicas (j) and sizes of gold particles formed
at 200 °C (Q) and 550 °C (h).
166
B. Lee et al. / Microporous and Mesoporous Materials 122 (2009) 160–167
of gold nanoparticles than 1-D mesoporous structures with cylindrical pores [44].
In contrast, SBA-15 had pore diameter of 6.8 nm, equivalent to a
gold melting point over 800 °C [7]. Therefore, gold nanoparticles
may grow to big particle size without melting within the mesopore
channels of SBA-15. Put another way, the gold nanoparticle size of
550 °C-calcined Au/SBA-15 (S15-2) is calculated by the Scherrer
equation as 5.0 nm, equivalent to the melting temperature of
approximately 700 °C, still well above the calcination temperature
of 550 °C. It should be mentioned that the correlation between gold
particle size and melting point refers to unsupported gold particles
[7] whereas here we are dealing with supported gold nanoparticles
with additional metal-support interaction, hence the absolute
melting point values may not be precise. Although we accept the
fact that the absolute melting point values may not be precise
when dealing with supported gold nanoparticles, we believe that
the correlation may show some general trends and qualitatively
justify our observations.
Considering that MCM-48 has bicontinuous interconnected
pore structure whereas SBA-15 has one-dimensional pore channels, it may be tempting to generalize that bicontinuous pore
structures always lead to the sintering of gold nanoparticles
whereas one-dimensional pore channels can mitigate the sintering
of gold nanoparticles. To see whether this is really the case, the
pore diameters of several mesoporous silicas and gold particle
sizes are summarized in Fig. 6. Interestingly, MCM-41 has onedimentional pore channels like SBA-15, but the sintering of gold
nanoparticles at 550 °C is quite obvious. This is probably due to
the small pore size of MCM-41 (2.8 nm). On the other hand, HMS
and SBA-16 have bicontinuous structures similar to MCM-48, but
the sintering of gold nanoparticles on SBA-16 is not particularly
obvious. This may be because the pore size of SBA-16 (3.7 nm) is
bigger than those of HMS (2.3 nm), MCM-41 (2.8 nm), and MCM48 (2.4 nm). As described before, the melting point of gold nanoparticles increases with its size [7]. Therefore, the size of mesopores plays an important role in the stabilization of gold
nanoparticles against sintering [42–44]. The observation that the
high-temperature sintering of Au nanoparticles on SBA-16 is less
prominent than that on MCM-48 could be induced by other structural factors. One of the key structural differences between these
two classes of the mesoporous materials lies in their microporosity. SBA-16 has a significant amount of micropores, which introduce a high-degree surface heterogeneity into the SBA type of
mesoporous supports, potentially leading to an enhanced metalsupport interaction.
These results furnish some interesting information, because
intuitionally, small mesopores (e.g. 2–3 nm) should physically confine gold nanoparticles more tightly, but our data show that this is
not the case. Small gold nanoparticles can indeed be placed in the
channels of mesoporous SiO2 with relatively small pores if the
samples are calcined at a relatively mild temperature (200 °C),
but they grow so as to be bigger than the sizes of these small mesopores when calcined at 550 °C. On the other hand, bigger nanopores of SBA-15 (6.8 nm) are better at confining gold nanoparticles
upon calcination. In fact, although our groups found that gold
nanoparticles can be entrapped in the channels of SBA-15 by using
Au(en)2Cl3 as a suitable gold precursor upon high-temperature calcination [13], our unpublished data indicated that the sizes of gold
nanoparticles on MCM-41 are much bigger than the pore size of
MCM-41, even using the same Au(en)2Cl3 precursor. In another
work, Datye and coworkers loaded gold on SBA-11, SBA-12,
HMM-2, MCM-41, and SBA-15, and calcined the Au/mesoporous
SiO2 samples at 400 °C [43]. They found that gold nanoparticles
are all bigger than the sizes of these mesoporous materials except
for SBA-15 with relatively bigger pores. Going beyond supported
gold nanoparticles, Bao and coworkers demonstrated that silver
nanoparticles have good thermal stability inside the SBA-15 host
[45]. Put together, these results underscore the importance of
using mesoporous materials with relatively larger pores as supports, especially when the sintering of metal nanoparticles constitutes a problem for practical applications.
4. Conclusions
The influences of synthesis conditions and mesoporous structures on the size and thermal stability of gold nanoparticles on different mesoporous silicas were studied systematically. HMS with
bicontinuous wormhole structure, MCM-41 and SBA-15 with
one-dimensional hexagonal structures, MCM-48 and SBA-16 with
bicontinuous cubic structures were grafted by APTS or DAPTS,
and HAuCl4 was loaded using water or ethanol solvent. These samples were then calcined at 200 or 550 °C to form gold nanoparticles. It was found that APTS is the preferred grafting ligand, and
water is the preferred solvent in order to get small gold nanoparticles. The size of gold nanoparticles all increases with the calcination temperature, but the extent of sintering is different with
different mesoporous hosts. When mesoporous silica such as
MCM-48 with bicontinuous pore structure and small pore size
(2.4 nm) was used as support material, the gold nanoparticles migrate out easily and agglomerate on the external surface of the
support. In SBA-15 with a bigger pore size of 6.8 nm, generated
gold nanoparticles remain stable within the mesoporous channels.
The thermal stability of gold nanoparticles on HMS (pore size
2.3 nm), MCM-41 (pore size 2.8 nm), and SBA-16 (pore size
3.7 nm) is in between that of Au/MCM-48 and Au/SBA-15. These
results enrich the literature data base on the thermal stability of
metal nanoparticles supported on mesoporous materials [42–46]
and provide information on the design of supported metal nanoparticles based on the rational choice of suitable mesoporous
supports.
Acknowledgments
B.L. thanks the Bisa Research Grant of Keimyung University in
2006. S.D. thanks the financial support from the Office of Basic Energy Sciences, US Department of Energy (Contract DE-AC0500OR22725).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.micromeso.2009.02.029.
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