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Solvent-free infiltration method for mesoporous SnO2 using mesoporous silica templates
Microporous and Mesoporous Materials 120 (2009) 441–446
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Solvent-free infiltration method for mesoporous SnO2 using mesoporous silica templates Jeong Kuk Shon a, Soo Sung Kong a, Yoon Sung Kim b, Jong-Heun Lee b,*, Won K. Park a, Seung C. Park a, Ji Man Kim a,* a b
Department of Chemistry, BK21 School of Chemical Materials Science, Institute of Basic Science and SKKU Advanced Institute of Nanotechnology, Suwon 440-746, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea
a r t i c l e
i n f o
Article history: Received 9 September 2008 Received in revised form 17 November 2008 Accepted 22 December 2008 Available online 1 January 2009 Keywords: SnO2 Mesoporous material Nano-replication Silica template Solvent-free infiltration
a b s t r a c t Mesoporous tin oxide (SnO2) materials, exhibiting high surface areas, crystalline frameworks and various mesostructures, were successfully obtained by a facile solvent-free infiltration method from mesoporous silica templates. Various kinds of mesoporous silica materials, such as KIT-6 (bicontinuous 3-D cubic, Ia3d), SBA-15 (2-D hexagonal, p6mm), SBA-16 (3-D cubic with cage-like pores, Im3m) and spherical mesoporous silica (disordered), were utilized as the hard templates. Tin precursor (SnCl2 2H2O, m.p. 310– 311 K) was infiltrated spontaneously within the mesopores of silica templates by melting the precursor at 353 K without using any solvent. The heat-treatment of SnCl2-infiltrated composite materials at 973 K under static air conditions and subsequent removal of silica templates by using HF result in the successful preparation of mesoporous SnO2 materials. The mesostructures as well as the morphologies of mesoporous SnO2 materials thus obtained were very similar with those of the mesoporous silica templates. The mesoporous SnO2 materials exhibit high surface areas of 84–121 m2/g as well as high pore volumes in the range of 0.22–0.35 cm3/g. The present solvent-free infiltration method is believed to be a simple and facile way for the preparation of mesoporous materials via nano-replication from mesoporous silica templates. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Tin oxide (SnO2), with an n-type semiconducting property and a wide band gap (Eg = 3.6 eV), is one of the most promising materials for the applications such as gas sensing, photo-electrochemical devices, energy conversion, etc. [1–3]. The surface properties as well as surface area of the materials are very important for such their practical applications, because they include the adsorption or reaction of chemicals on the SnO2 surface. There have been a lot of approaches to obtain the SnO2 nanostructured materials with various morphologies such as nanoparticles, nanorods, nanowires and hollow spheres, in order to increase the surface areas and maximize their performances [4–7]. An alternative way to increase the surface areas is the generation of pores in nanometric scales. Porous structures also give several desired properties such as high and fast adsorption capability of chemicals, facilitated materials transport, and increased thermal and mechanical stability. Mesoporous materials are of great interest in various applications, due to their high surface areas, tunable pore sizes, adjustable
* Corresponding authors. Tel.: +82 (31) 290 5930; fax: +82 (31) 299 4174 (J.M. Kim), tel.: +82 (2) 3290 3282; fax: +82 (2) 928 3584 (J.-H. Lee). E-mail addresses: [email protected] (J.-H. Lee), [email protected] (J.M. Kim). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.022
framework compositions and diverse surface properties [8]. Since the reports on the syntheses of mesoporous silica materials, there have been lots of developments in their syntheses, structural characterizations, and applications [8]. Recent research direction of such the mesoporous materials has faced from the silica-based materials to the non-siliceous metal oxide materials that are expected to be more useful for wide range of applications [9]. The successful synthesis of mesoporous SnO2 materials with improved long-range orders and thermal stability at relatively high temperature has been developed by using surfactants and block copolymers [10]. Although there have been some reports on the syntheses of mesoporous metal oxides by synergetic self-assembly between organic templates and inorganic precursors [11], those are less successful compared to the syntheses of mesoporous silica-based materials. One difficulty lies in the structural collapse during the mesostructure formations and during the removal of organic templates which are probably due to high lattice energies of most metal oxides. An alternative way to produce the mesoporous metal oxides is a nano-replication method where the pre-synthesized mesoporous silica materials are utilized as the hard templates [9]. Generally, a precursor solution for the targeted metal oxide is infiltrated into the mesopores of silica templates [9,12]. After drying the solvent, the composite is heated under air conditions, and finally the silica template is removed by chemical etching technique using HF or
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NaOH, giving ordered mesoporous metal oxides. However, this nano-replication method also has some difficulties such as inconvenient infiltration process of precursor, crystal growth outside mesoporous silica particles during the heat treatment, and proper choice of chemical etching agent. Especially, it is necessary to fill the void volume of mesoporous silica template with the metal precursor as completely as possible, in order to obtain a highly ordered mesoporous metal oxides. Therefore, if the solubility of metal precursor is low, one needs to carry out the infiltration step several times to achieve the optimum loading of precursors within the mesopores [13]. Also, there is the other limitation when the metal precursors are not stable in a solution enough to infiltrate them within the mesopores of silica template, due to their fast hydrolysis and precipitation. For example, the utilization of aqueous solution of SnO2 precursors such as SnCl2 and SnCl2 2H2O is very difficult, because the precursors are readily hydrolyzed to form precipitates within few minutes after the dissolution of precursors in water, according to our preliminary experiments. In the present work, the mesoporous SnO2 materials have been successfully obtained from various kinds of mesoporous silica templates such as KIT-6 (bicontinuous 3-D cubic, Ia3d), SBA-15 (2-D hexagonal, p6mm), SBA-16 (3-D cubic with cage-like pores, Im3m) and spherical mesoporous silica (SMS, disordered) by the nano-replication. Here, we have developed a simple and facile solvent-free infiltration method by using a tin precursor (SnCl2 2H2O) with low melting point around 310 K. The mesoporous SnO2 materials, thus obtained, exhibit very similar mesotructures as well as morphologies, compared with those of mesoporous silica templates.
Table 1 Energy dispersive X-ray spectroscopy data for the mesoporous SnO2 materials obtained from SEM-EDX spectra. Materials
SnO2(KIT-6) SnO2(SBA-15) SnO2(SBA-16) SnO2(SMS)
Oxygen
Tin
O/Sn ratio
wt.%
at.%
wt.%
at.%
23.3 23.0 24.1 23.0
69.3 68.9 70.2 68.9
76.7 77.0 75.9 77.0
30.7 31.1 29.8 31.1
2.25 2.21 2.35 2.21
also indicated that the replicated materials consisted of tin and oxygen where the atomic ratios of O/Sn are around 2 as shown in Table 1. Finally, the mesoporous SnO2 thus obtained was dried at 353 K. The mesoporous SnO2 materials, obtained from KIT-6, SBA-15, SBA-16 and SMS, are denoted as SnO2(KIT-6), SnO2(SBA-15), SnO2(SBA-16) and SnO2(SMS), respectively. 2.3. Characterization of the materials X-ray diffraction patterns were obtained in the reflection mode using a Rigaku D/MAX-2200 ultima equipped with Cu Ka at 30 kV and 40 mA. Scanning electron microscope (SEM) images and EDX
2. Experimental 2.1. Synthesis of mesoporous silica template In the present work, various kinds of mesoporous silica materials such as KIT-6, SBA-15, SBA-16 and SMS were used as the silica templates for mesoporous SnO2 materials. The mesoporous silica templates were synthesized following methods in references with slightly modified method [14–18]. A triblock copolymer (Pluronic 123, EO20PO70EO20, Mav = 5800) was used as the structure-directing agent for the KIT-6 and SBA-15. In the cases of SBA-16 and SMS, the other triblock copolymer (Pluronic F127, EO106PO70EO106, Mav = 12600) and hexadecylamine were utilized as the organic template, respectively. Tetraethylorthosilicate (TEOS) was utilized as the silica source for the syntheses of all the mesoporous materials in the present work. 2.2. Synthesis of mesoporous SnO2 The mesoporous SnO2 materials were synthesized by the nanoreplication method using the above mesoporous silica materials as the templates. Typically, 2.0 g of the calcined mesoporous silica template was heated at 373 K for 1 h. The pre-heated silica template was poured into a glass bottle containing 3.06 g of tin(II) chloride dihydrate (SnCl2 2H2O, Aldrich, 98%, m.p. 310–311 K) that was melted to liquid phase at 383 K. The bottle containing the mixture was closed and shaken vigorously to mix the SnCl2 and silica template. Subsequently, the bottle was put in an oven at 353 K overnight in order for the spontaneous infiltration of tin precursor within the mesopores of silica templates. The composite materials, then, were heated to 973 K under static air conditions for 3 h. The resulting material was stirred in an aqueous solution of HF (20 wt%), filtered, washed with doubly distilled water and acetone, in order to remove the silica template. This removal process was carried out once more, resulting in the removal of more than 99.9% of SiO2 that was confirmed by an elemental analysis using an energy dispersive X-ray spectroscopy (EDX). The EDX data
Fig. 1. Low-angle XRD patterns for the mesoporous SnO2 materials: (a) SnO2(KIT-6), (b) SnO2(SBA-15), (c) SnO2(SBA-16) and (d) SnO2(SMS). Insets are the XRD patterns of mesoporous silica templates.
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data were taken by LEO Supra 55 field emission scanning microscope operating at accelerating voltage of 15 kV. Transmission electron microscope (TEM) images were obtained using JEOL JEM
443
3010 at accelerating voltage of 300 kV. N2 adsorption–desorption isotherms were collected on a Micromeritics Tristar system at liquid N2 temperature.
Fig. 2. Wide-angle XRD patterns for the mesoporous SnO2 materials.
Fig. 3. SEM images for the mesoporous silica templates and mesoporous SnO2 materials.
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3. Results and discussion Fig. 1 shows small-angle XRD data for the various kinds of mesoporous silica templates (insets of Fig. 1) and the corresponding mesoporous SnO2 materials obtained by the nano-replication technique. The XRD patterns in Fig. 1 indicate that the mesostructures of SnO2 materials are similar very to those of mesoporous silica templates. The KIT-6 gives typical XRD peaks, which can be indexed to [2 1 1], [2 2 0], and [3 3 2] and are the characteristics of 3-D cubic (Ia3d) mesostructure (inset of Fig. 1a). However, the
SnO2(KIT-6) exhibits a new XRD peak at low angle region as shown in Fig. 1a, which corresponds to the position of the [1 1 0] reflection for Ia3d symmetry. The presence of [1 1 0] reflection is known to be a phase transition from the cubic Ia3d mesostructure to the tetragonal I41/a (or lower) mesostructures after the removal of silica template [19]. Very similar phase transformation upon the removal of silica template was reported for the synthesis of mesoporous carbon (CMK-1) from mesoporous silica MCM-48 [20] and for the synthesis of mesoporous silver from KIT-6 [21]. The XRD patterns in Fig. 1b show the 2-D hexagonal mesostructures (p6mm) of both
Fig. 4. TEM images for the mesoporous SnO2 materials.
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the SBA-15 and SnO2(SBA-15), which exhibit well resolved peaks corresponding to [1 0 0], [1 1 0], and [2 0 0] planes. In the case of SBA16, the cubic Im3m symmetry is clearly shown (inset of Fig. 1c), whereas the replicated SnO2(SBA-16) gives one intense peak and broadened peaks as shown in Fig. 1c. The broadening of [2 0 0] and [2 1 1] peaks, compared to the SBA-16 template, indicates the somewhat loss of mesostructural orders in the SnO2(SBA-16). A broad XRD peak is obtained for both the SMS and SnO2(SMS) materials (Fig. 1d) due to their disordered mesostructures. The framework structures of mesoporous SnO2 materials are highly crystalline with tetragonal rutile phases (JCPDS 41-1445, P42/mnm) [12], as shown in the wide-angle XRD patterns in Fig. 2. All the XRD patterns in Fig. 2 are somewhat broad peaks, and there are no significant differences before and after the silica removal. The wide-angle XRD results indicate that the frameworks of mesoporous SnO2 materials are formed within the confined mesopores of silica templates during the infiltration and aging process, and that the nanostructured frameworks thus formed are maintained during the silica removal. The average crystalline domain sizes of SnO2 frameworks are estimated to be about 6.5 nm, 7.5 nm, 8.3 nm and 5.3 nm by using the Scherrer equation for the SnO2(KIT-6), SnO2(SBA-15), SnO2(SBA-16) and SnO2(SMS), respectively, which are comparable to the pore sizes of silica templates. Fig. 3 shows SEM images for the mesoporous silica templates and the replicated mesoporous SnO2 materials. As shown in Fig. 3, the KIT-6 exhibits very large particles (10 lm) with irregular morphologies, whereas the SBA-15, SBA-16 and SMS materials give regular rod-type (100 nm in diameter and 1 lm in length), spherical (10 lm) and relatively small spherical (700 nm) morphologies, respectively. The SEM images in Fig. 3 clearly demonstrate that the overall particle morphologies and sizes of mesoporous SnO2 materials are very similar to those of silica templates. The conventional nano-replication methods, reported in the literatures [9,12,13], involved the impregnation of precursors within the mesopores of silica templates after the dissolution of precursors in a proper solvent such as water, acetone, or ethanol. Therefore, one needs to carry out the infiltration process several times after the solvent evaporation, in order to maximize the filling degree of mesopores with the precursors. Otherwise, the resulting materials may not exhibit mesostructures after the removal of silica templates, and the morphologies are quite different from those of the mother silica templates even though the mesostructures are obtained [13]. In the present work, a tin precursor with low melting point (SnCl2 2H2O, m.p. 310– 311 K) is infiltrated within the mesopores of silica template without using any solvent by simply heating a physical mixture of precursor and silica template. Only once is necessary to fill the mesopores with the precursors completely, following the simple solvent-free infiltration process. Moreover, the present method can shorten the time-consuming replication process that needs the solvent-drying step. TEM images in Fig. 4 clearly show that all the mesoporous SnO2 materials exhibit well-distinguishable mesostructures and highly crystalline frameworks, indicating the successful replication from the silica templates. The domain sizes of the SnO2 frameworks are about 7 nm which are very similar to those estimated by Scherrer equation from the XRD patterns. The mesostructures of SnO2(KIT-6) and SnO2(SBA-15) are highly ordered and very similar to those of KIT-6 and SBA-15 templates. The TEM image of SnO2(SBA-16) material indicates that it consists of lots of spherical nano-particles with regular sizes and there are interconnection between the nano-particles, which is because the silica template SBA-16 has spherical cavities of about 9.8 nm diameter and the cavities are connected through mesoporous opening of about 3.5 nm. In case of SnO2(SMS), the overall morphologies are spherical and its mesostructure is disordered.
Fig. 5 shows N2 adsorption–desorption isotherms and the corresponding BJH pore size distribution curves for the mesoporous SnO2 materials. All the N2 sorption isotherms in Fig. 4, except of SnO2(SMS), are typical type IV isotherms with hysteresis loops, which indicate that the present SnO2 materials exhibit well-defined mesostructures. The physical properties of mesoporous silica templates and corresponding mesoporous SnO2 replica materials are listed in Table 2. As shown in Fig. 4a, the SnO2(KIT-6) material exhibits dual porous structures (4 nm and 18 nm). One should be come from the silica frameworks (4 nm) after the replication process, and the other may be generated by the structural transformation form the cubic Ia3d mesostructure to the tetragonal I41/a (or lower) mesostructures [19–21]. The pore size distribution of SnO2(SMS) material is much broader than those of other mesoporous SnO2 materials, which is probably due to its irregular and disordered mesostructures.
Fig. 5. (a) N2 adsorption–desorption isotherms, and (b) the corresponding BJH pore size distribution curves for the mesoporous SnO2 materials.
Table 2 Physical properties of mesoporous silica templates and mesoporous SnO2 materials. Materials
Lattice parametera / nm
SBETb / m2 g
KIT-6 SBA-15 SBA-16 SMS SnO2(KIT-6) SnO2(SBA-15) SnO2(SBA-16) SnO2(SMS)
23.25 11.07 13.71 – 23.00 10.84 13.00 –
814 569 738 365 114 84 121 117
a
1
DBJHc / nm
Vtotd / cm3 g
7.3 7.9 3.5 e/9.8 2.8 3.4/ 18.0 2.8 4.1 7.1
0.95 0.92 0.45 0.50 0.32 0.22 0.28 0.35
1
Lattice parameters calculated from XRD peaks for the materials. BET surface areas calculated in the range of relative pressure (p/p0) = 0.05–0.20. BJH pore sizes obtained from the adsorption branches. d Total pore volume measured at p/p0 = 0.99. e BJH pore size calculated from the desorption branch in order to obtain the pore opening size of SBA-16 material. b
c
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4. Conclusion The different mesostructured SnO2 materials have been successfully obtained by using mesoporous silica templates, KIT-6, SBA-15, SBA-16 and spherical mesoporous silica, through the solvent-free infiltration method. The mesopores of silica templates can be infiltrated simply by mixing them with tin precursor (SnCl2 2H2O) and melting the precursor at 353 K without using any solvent. The mesostructures and morphologies of the silica templates are basically maintained in the mesoporous SnO2 materials after the present nano-replication process. The present solvent-free infiltration method to nano-replication is believed to be very useful for the preparation of various kinds of nanostructured materials via the templated synthesis.
[7]
[8] [9]
[10]
Acknowledgements [11]
We thank to Korea Science and Engineering Foundation (KOSEF, M10503000291, R-01-2006-000-10283-0, and R0A-2008-00020032-0) and Korea Research Foundation Grant (MOEHRD, KRF-2005-005-J11901, KRF-2005-005-J11903 and KRF-2008-013C00050). We also thank to Pohang Light Source for the measurement of small-angle XRD at BL8C2 beam line.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Solvent-free infiltration method for mesoporous SnO2 using mesoporous silica templates Jeong Kuk Shon a, Soo Sung Kong a, Yoon Sung Kim b, Jong-Heun Lee b,*, Won K. Park a, Seung C. Park a, Ji Man Kim a,* a b
Department of Chemistry, BK21 School of Chemical Materials Science, Institute of Basic Science and SKKU Advanced Institute of Nanotechnology, Suwon 440-746, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea
a r t i c l e
i n f o
Article history: Received 9 September 2008 Received in revised form 17 November 2008 Accepted 22 December 2008 Available online 1 January 2009 Keywords: SnO2 Mesoporous material Nano-replication Silica template Solvent-free infiltration
a b s t r a c t Mesoporous tin oxide (SnO2) materials, exhibiting high surface areas, crystalline frameworks and various mesostructures, were successfully obtained by a facile solvent-free infiltration method from mesoporous silica templates. Various kinds of mesoporous silica materials, such as KIT-6 (bicontinuous 3-D cubic, Ia3d), SBA-15 (2-D hexagonal, p6mm), SBA-16 (3-D cubic with cage-like pores, Im3m) and spherical mesoporous silica (disordered), were utilized as the hard templates. Tin precursor (SnCl2 2H2O, m.p. 310– 311 K) was infiltrated spontaneously within the mesopores of silica templates by melting the precursor at 353 K without using any solvent. The heat-treatment of SnCl2-infiltrated composite materials at 973 K under static air conditions and subsequent removal of silica templates by using HF result in the successful preparation of mesoporous SnO2 materials. The mesostructures as well as the morphologies of mesoporous SnO2 materials thus obtained were very similar with those of the mesoporous silica templates. The mesoporous SnO2 materials exhibit high surface areas of 84–121 m2/g as well as high pore volumes in the range of 0.22–0.35 cm3/g. The present solvent-free infiltration method is believed to be a simple and facile way for the preparation of mesoporous materials via nano-replication from mesoporous silica templates. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Tin oxide (SnO2), with an n-type semiconducting property and a wide band gap (Eg = 3.6 eV), is one of the most promising materials for the applications such as gas sensing, photo-electrochemical devices, energy conversion, etc. [1–3]. The surface properties as well as surface area of the materials are very important for such their practical applications, because they include the adsorption or reaction of chemicals on the SnO2 surface. There have been a lot of approaches to obtain the SnO2 nanostructured materials with various morphologies such as nanoparticles, nanorods, nanowires and hollow spheres, in order to increase the surface areas and maximize their performances [4–7]. An alternative way to increase the surface areas is the generation of pores in nanometric scales. Porous structures also give several desired properties such as high and fast adsorption capability of chemicals, facilitated materials transport, and increased thermal and mechanical stability. Mesoporous materials are of great interest in various applications, due to their high surface areas, tunable pore sizes, adjustable
* Corresponding authors. Tel.: +82 (31) 290 5930; fax: +82 (31) 299 4174 (J.M. Kim), tel.: +82 (2) 3290 3282; fax: +82 (2) 928 3584 (J.-H. Lee). E-mail addresses: [email protected] (J.-H. Lee), [email protected] (J.M. Kim). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.022
framework compositions and diverse surface properties [8]. Since the reports on the syntheses of mesoporous silica materials, there have been lots of developments in their syntheses, structural characterizations, and applications [8]. Recent research direction of such the mesoporous materials has faced from the silica-based materials to the non-siliceous metal oxide materials that are expected to be more useful for wide range of applications [9]. The successful synthesis of mesoporous SnO2 materials with improved long-range orders and thermal stability at relatively high temperature has been developed by using surfactants and block copolymers [10]. Although there have been some reports on the syntheses of mesoporous metal oxides by synergetic self-assembly between organic templates and inorganic precursors [11], those are less successful compared to the syntheses of mesoporous silica-based materials. One difficulty lies in the structural collapse during the mesostructure formations and during the removal of organic templates which are probably due to high lattice energies of most metal oxides. An alternative way to produce the mesoporous metal oxides is a nano-replication method where the pre-synthesized mesoporous silica materials are utilized as the hard templates [9]. Generally, a precursor solution for the targeted metal oxide is infiltrated into the mesopores of silica templates [9,12]. After drying the solvent, the composite is heated under air conditions, and finally the silica template is removed by chemical etching technique using HF or
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NaOH, giving ordered mesoporous metal oxides. However, this nano-replication method also has some difficulties such as inconvenient infiltration process of precursor, crystal growth outside mesoporous silica particles during the heat treatment, and proper choice of chemical etching agent. Especially, it is necessary to fill the void volume of mesoporous silica template with the metal precursor as completely as possible, in order to obtain a highly ordered mesoporous metal oxides. Therefore, if the solubility of metal precursor is low, one needs to carry out the infiltration step several times to achieve the optimum loading of precursors within the mesopores [13]. Also, there is the other limitation when the metal precursors are not stable in a solution enough to infiltrate them within the mesopores of silica template, due to their fast hydrolysis and precipitation. For example, the utilization of aqueous solution of SnO2 precursors such as SnCl2 and SnCl2 2H2O is very difficult, because the precursors are readily hydrolyzed to form precipitates within few minutes after the dissolution of precursors in water, according to our preliminary experiments. In the present work, the mesoporous SnO2 materials have been successfully obtained from various kinds of mesoporous silica templates such as KIT-6 (bicontinuous 3-D cubic, Ia3d), SBA-15 (2-D hexagonal, p6mm), SBA-16 (3-D cubic with cage-like pores, Im3m) and spherical mesoporous silica (SMS, disordered) by the nano-replication. Here, we have developed a simple and facile solvent-free infiltration method by using a tin precursor (SnCl2 2H2O) with low melting point around 310 K. The mesoporous SnO2 materials, thus obtained, exhibit very similar mesotructures as well as morphologies, compared with those of mesoporous silica templates.
Table 1 Energy dispersive X-ray spectroscopy data for the mesoporous SnO2 materials obtained from SEM-EDX spectra. Materials
SnO2(KIT-6) SnO2(SBA-15) SnO2(SBA-16) SnO2(SMS)
Oxygen
Tin
O/Sn ratio
wt.%
at.%
wt.%
at.%
23.3 23.0 24.1 23.0
69.3 68.9 70.2 68.9
76.7 77.0 75.9 77.0
30.7 31.1 29.8 31.1
2.25 2.21 2.35 2.21
also indicated that the replicated materials consisted of tin and oxygen where the atomic ratios of O/Sn are around 2 as shown in Table 1. Finally, the mesoporous SnO2 thus obtained was dried at 353 K. The mesoporous SnO2 materials, obtained from KIT-6, SBA-15, SBA-16 and SMS, are denoted as SnO2(KIT-6), SnO2(SBA-15), SnO2(SBA-16) and SnO2(SMS), respectively. 2.3. Characterization of the materials X-ray diffraction patterns were obtained in the reflection mode using a Rigaku D/MAX-2200 ultima equipped with Cu Ka at 30 kV and 40 mA. Scanning electron microscope (SEM) images and EDX
2. Experimental 2.1. Synthesis of mesoporous silica template In the present work, various kinds of mesoporous silica materials such as KIT-6, SBA-15, SBA-16 and SMS were used as the silica templates for mesoporous SnO2 materials. The mesoporous silica templates were synthesized following methods in references with slightly modified method [14–18]. A triblock copolymer (Pluronic 123, EO20PO70EO20, Mav = 5800) was used as the structure-directing agent for the KIT-6 and SBA-15. In the cases of SBA-16 and SMS, the other triblock copolymer (Pluronic F127, EO106PO70EO106, Mav = 12600) and hexadecylamine were utilized as the organic template, respectively. Tetraethylorthosilicate (TEOS) was utilized as the silica source for the syntheses of all the mesoporous materials in the present work. 2.2. Synthesis of mesoporous SnO2 The mesoporous SnO2 materials were synthesized by the nanoreplication method using the above mesoporous silica materials as the templates. Typically, 2.0 g of the calcined mesoporous silica template was heated at 373 K for 1 h. The pre-heated silica template was poured into a glass bottle containing 3.06 g of tin(II) chloride dihydrate (SnCl2 2H2O, Aldrich, 98%, m.p. 310–311 K) that was melted to liquid phase at 383 K. The bottle containing the mixture was closed and shaken vigorously to mix the SnCl2 and silica template. Subsequently, the bottle was put in an oven at 353 K overnight in order for the spontaneous infiltration of tin precursor within the mesopores of silica templates. The composite materials, then, were heated to 973 K under static air conditions for 3 h. The resulting material was stirred in an aqueous solution of HF (20 wt%), filtered, washed with doubly distilled water and acetone, in order to remove the silica template. This removal process was carried out once more, resulting in the removal of more than 99.9% of SiO2 that was confirmed by an elemental analysis using an energy dispersive X-ray spectroscopy (EDX). The EDX data
Fig. 1. Low-angle XRD patterns for the mesoporous SnO2 materials: (a) SnO2(KIT-6), (b) SnO2(SBA-15), (c) SnO2(SBA-16) and (d) SnO2(SMS). Insets are the XRD patterns of mesoporous silica templates.
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data were taken by LEO Supra 55 field emission scanning microscope operating at accelerating voltage of 15 kV. Transmission electron microscope (TEM) images were obtained using JEOL JEM
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3010 at accelerating voltage of 300 kV. N2 adsorption–desorption isotherms were collected on a Micromeritics Tristar system at liquid N2 temperature.
Fig. 2. Wide-angle XRD patterns for the mesoporous SnO2 materials.
Fig. 3. SEM images for the mesoporous silica templates and mesoporous SnO2 materials.
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3. Results and discussion Fig. 1 shows small-angle XRD data for the various kinds of mesoporous silica templates (insets of Fig. 1) and the corresponding mesoporous SnO2 materials obtained by the nano-replication technique. The XRD patterns in Fig. 1 indicate that the mesostructures of SnO2 materials are similar very to those of mesoporous silica templates. The KIT-6 gives typical XRD peaks, which can be indexed to [2 1 1], [2 2 0], and [3 3 2] and are the characteristics of 3-D cubic (Ia3d) mesostructure (inset of Fig. 1a). However, the
SnO2(KIT-6) exhibits a new XRD peak at low angle region as shown in Fig. 1a, which corresponds to the position of the [1 1 0] reflection for Ia3d symmetry. The presence of [1 1 0] reflection is known to be a phase transition from the cubic Ia3d mesostructure to the tetragonal I41/a (or lower) mesostructures after the removal of silica template [19]. Very similar phase transformation upon the removal of silica template was reported for the synthesis of mesoporous carbon (CMK-1) from mesoporous silica MCM-48 [20] and for the synthesis of mesoporous silver from KIT-6 [21]. The XRD patterns in Fig. 1b show the 2-D hexagonal mesostructures (p6mm) of both
Fig. 4. TEM images for the mesoporous SnO2 materials.
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the SBA-15 and SnO2(SBA-15), which exhibit well resolved peaks corresponding to [1 0 0], [1 1 0], and [2 0 0] planes. In the case of SBA16, the cubic Im3m symmetry is clearly shown (inset of Fig. 1c), whereas the replicated SnO2(SBA-16) gives one intense peak and broadened peaks as shown in Fig. 1c. The broadening of [2 0 0] and [2 1 1] peaks, compared to the SBA-16 template, indicates the somewhat loss of mesostructural orders in the SnO2(SBA-16). A broad XRD peak is obtained for both the SMS and SnO2(SMS) materials (Fig. 1d) due to their disordered mesostructures. The framework structures of mesoporous SnO2 materials are highly crystalline with tetragonal rutile phases (JCPDS 41-1445, P42/mnm) [12], as shown in the wide-angle XRD patterns in Fig. 2. All the XRD patterns in Fig. 2 are somewhat broad peaks, and there are no significant differences before and after the silica removal. The wide-angle XRD results indicate that the frameworks of mesoporous SnO2 materials are formed within the confined mesopores of silica templates during the infiltration and aging process, and that the nanostructured frameworks thus formed are maintained during the silica removal. The average crystalline domain sizes of SnO2 frameworks are estimated to be about 6.5 nm, 7.5 nm, 8.3 nm and 5.3 nm by using the Scherrer equation for the SnO2(KIT-6), SnO2(SBA-15), SnO2(SBA-16) and SnO2(SMS), respectively, which are comparable to the pore sizes of silica templates. Fig. 3 shows SEM images for the mesoporous silica templates and the replicated mesoporous SnO2 materials. As shown in Fig. 3, the KIT-6 exhibits very large particles (10 lm) with irregular morphologies, whereas the SBA-15, SBA-16 and SMS materials give regular rod-type (100 nm in diameter and 1 lm in length), spherical (10 lm) and relatively small spherical (700 nm) morphologies, respectively. The SEM images in Fig. 3 clearly demonstrate that the overall particle morphologies and sizes of mesoporous SnO2 materials are very similar to those of silica templates. The conventional nano-replication methods, reported in the literatures [9,12,13], involved the impregnation of precursors within the mesopores of silica templates after the dissolution of precursors in a proper solvent such as water, acetone, or ethanol. Therefore, one needs to carry out the infiltration process several times after the solvent evaporation, in order to maximize the filling degree of mesopores with the precursors. Otherwise, the resulting materials may not exhibit mesostructures after the removal of silica templates, and the morphologies are quite different from those of the mother silica templates even though the mesostructures are obtained [13]. In the present work, a tin precursor with low melting point (SnCl2 2H2O, m.p. 310– 311 K) is infiltrated within the mesopores of silica template without using any solvent by simply heating a physical mixture of precursor and silica template. Only once is necessary to fill the mesopores with the precursors completely, following the simple solvent-free infiltration process. Moreover, the present method can shorten the time-consuming replication process that needs the solvent-drying step. TEM images in Fig. 4 clearly show that all the mesoporous SnO2 materials exhibit well-distinguishable mesostructures and highly crystalline frameworks, indicating the successful replication from the silica templates. The domain sizes of the SnO2 frameworks are about 7 nm which are very similar to those estimated by Scherrer equation from the XRD patterns. The mesostructures of SnO2(KIT-6) and SnO2(SBA-15) are highly ordered and very similar to those of KIT-6 and SBA-15 templates. The TEM image of SnO2(SBA-16) material indicates that it consists of lots of spherical nano-particles with regular sizes and there are interconnection between the nano-particles, which is because the silica template SBA-16 has spherical cavities of about 9.8 nm diameter and the cavities are connected through mesoporous opening of about 3.5 nm. In case of SnO2(SMS), the overall morphologies are spherical and its mesostructure is disordered.
Fig. 5 shows N2 adsorption–desorption isotherms and the corresponding BJH pore size distribution curves for the mesoporous SnO2 materials. All the N2 sorption isotherms in Fig. 4, except of SnO2(SMS), are typical type IV isotherms with hysteresis loops, which indicate that the present SnO2 materials exhibit well-defined mesostructures. The physical properties of mesoporous silica templates and corresponding mesoporous SnO2 replica materials are listed in Table 2. As shown in Fig. 4a, the SnO2(KIT-6) material exhibits dual porous structures (4 nm and 18 nm). One should be come from the silica frameworks (4 nm) after the replication process, and the other may be generated by the structural transformation form the cubic Ia3d mesostructure to the tetragonal I41/a (or lower) mesostructures [19–21]. The pore size distribution of SnO2(SMS) material is much broader than those of other mesoporous SnO2 materials, which is probably due to its irregular and disordered mesostructures.
Fig. 5. (a) N2 adsorption–desorption isotherms, and (b) the corresponding BJH pore size distribution curves for the mesoporous SnO2 materials.
Table 2 Physical properties of mesoporous silica templates and mesoporous SnO2 materials. Materials
Lattice parametera / nm
SBETb / m2 g
KIT-6 SBA-15 SBA-16 SMS SnO2(KIT-6) SnO2(SBA-15) SnO2(SBA-16) SnO2(SMS)
23.25 11.07 13.71 – 23.00 10.84 13.00 –
814 569 738 365 114 84 121 117
a
1
DBJHc / nm
Vtotd / cm3 g
7.3 7.9 3.5 e/9.8 2.8 3.4/ 18.0 2.8 4.1 7.1
0.95 0.92 0.45 0.50 0.32 0.22 0.28 0.35
1
Lattice parameters calculated from XRD peaks for the materials. BET surface areas calculated in the range of relative pressure (p/p0) = 0.05–0.20. BJH pore sizes obtained from the adsorption branches. d Total pore volume measured at p/p0 = 0.99. e BJH pore size calculated from the desorption branch in order to obtain the pore opening size of SBA-16 material. b
c
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4. Conclusion The different mesostructured SnO2 materials have been successfully obtained by using mesoporous silica templates, KIT-6, SBA-15, SBA-16 and spherical mesoporous silica, through the solvent-free infiltration method. The mesopores of silica templates can be infiltrated simply by mixing them with tin precursor (SnCl2 2H2O) and melting the precursor at 353 K without using any solvent. The mesostructures and morphologies of the silica templates are basically maintained in the mesoporous SnO2 materials after the present nano-replication process. The present solvent-free infiltration method to nano-replication is believed to be very useful for the preparation of various kinds of nanostructured materials via the templated synthesis.
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Acknowledgements [11]
We thank to Korea Science and Engineering Foundation (KOSEF, M10503000291, R-01-2006-000-10283-0, and R0A-2008-00020032-0) and Korea Research Foundation Grant (MOEHRD, KRF-2005-005-J11901, KRF-2005-005-J11903 and KRF-2008-013C00050). We also thank to Pohang Light Source for the measurement of small-angle XRD at BL8C2 beam line.
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