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 , and the lack of space conﬁnements [9,10], whereas those embedded in oxide matrixes may not be accessible to reactants in catalytic reactions . 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 , (4) synthesizing mesoporous SiO2 in the presence of gold colloids [36–39], (5) liquid-phase grafting or chemical vapor deposition of gold precursors , and (6) using Au(en)2Cl3 as the precursor . The in the synthesis of Au/mesoporous SiO2 has been reviewed recently . 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  with bicontinuous wormhole structure, MCM-41  and SBA-15  with one-dimensional hexagonal structures, and MCM-48  and SBA-16  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 , 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 : 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 : 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 : 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 . 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  with bicontinuous wormhole structure, MCM-41  and SBA-15  with one-dimensional hexagonal structures, and MCM-48  and SBA-16  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 , and a blue shift in the longitudinal plasmon vibration is an indication of the extent of aggregation of the gold nanoparticles . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . In contrast, SBA-15 had pore diameter of 6.8 nm, equivalent to a gold melting point over 800 °C . 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  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 . 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 , 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 . 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 . 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. 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