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Chemistry Letters Vol.37, No.2 (2008)
Preparation of Highly Ordered Mesoporous TiO2 Materials with Crystalline Framework from Different Mesostructured Silica Templates via Nanoreplication Sung Soo Kim,1 Hyung Ik Lee,1 Jeong Kuk Shon,1 Jae Young Hur,2 Min Suk Kang,1 Sang Soo Park,1 Soo Sung Kong,1 Ji Ae Yu,1 Miran Seo,1 Donghao Li,1;3 Santosh Singh Thakur,1 and Ji Man Kim1 1 Department of Chemistry, BK21 School of Chemical Materials Science and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Korea 2 Central Research Institute, Hyosung Corporation, Anyang 431-080, Korea 3 Analysis and Inspection Center, Yanbian University, Yanji 133000, P. R. China (Received October 22, 2007; CL-071167; E-mail:
[email protected]) Highly ordered mesoporous TiO2 materials, exhibiting crystalline frameworks and various mesostructures, have been successfully obtained via nanoreplication using various kinds of mesoporous silicas as the rigid templates.
Since the first report of ordered mesoporous silica, MCM41,1 there have been extensive researches on the synthesis and application of mesoporous materials.2–4 Among these researches, the diversity of framework composition has been regarded as one of the most essential points in material science considering their targeting applications. In this regard, various strategies for the synthesis of nonsiliceous mesoporous materials have been reported.5,6 Especially, mesoporous anatase TiO2 is expected to be very useful for applications such as photocatalysts, sensors, photovoltaic cells, photochromic devices, and so on.7–9 However, the sol–gel synthesis using supramolecular assembly of surfactants as ‘‘soft template’’ for the mesoporous TiO2 has been less successful,10 compared to the case of mesoporous silicas. Although there are some successful reports of mesoporous TiO2 via the evaporation-induced self-assembly strategy and the acid–base pair concept,11,12 the preparation of mesoporous TiO2 with crystalline framework is still challenging. Recently, nanoreplication route, which uses mesoporous silica as ‘‘rigid template,’’ is regarded as a promising method for the preparation of mesoporous materials with novel framework compositions such as carbons, metal oxides, metals, and so on.13–18 Recently, highly ordered mesoporous metal oxides such as Fe2 O3 , Co3 O4 , and CeO2 are synthesized by this method using mesoporous silica templates.16–18 In the present work, we report on a facial synthesis method for mesoporous TiO2 materials with highly ordered mesostructures and crystalline frameworks using different mesoporous silica templates such as SBA-15 (2-D hexagonal P6mm), MSU-H (2-D hexagonal P6mm), and KIT-6 (bicontinuous cubic Ia3d).19–21 It is difficult to create TiO2 materials which exhibit both of highly ordered mesoporosity and crystalline framework because of the too fast hydrolysis and condensation rates of TiO2 precursors. Therefore, we have utilized a modified nanoparticle route22 where the TiO2 precursor is first hydrolyzed and subsequently transformed to nanoparticles under the controlled acidic conditions. The mesoporous silica templates (KIT-6, SBA-15, and MSU-H) were synthesized following methods in references.19–21 A triblock copolymer (Pluronic 123, EO20 PO70 EO20 , Mav ¼ 5800) was used as the structure-directing agent for the silica materials. Tetraethylorthosilicate (TEOS, Aldrich) was used
as the silica source for the SBA-15 and KIT-6 materials, and a sodium silicate solution (10 wt % of SiO2 , Na/Si = 2.5) was used as the silica source for MSU-H material. After calcination, these mesoporous silica materials were used as templates for the preparation of TiO2 replica. Titanium tetraethoxide (Ti(OEt)4 , Aldrich) was used as TiO2 precursor. In a typical nanoreplication, 0.6 g of Ti(OEt)4 and 30 mL of water are mixed, giving a white precipitate. The precipitate was collected by centrifuge and decantation of supernatant, and dissolved with 0.8 g of HCl (35 wt %) at room temperature. This clear TiO2 precursor sol was impregnated into 1.0 g of the mesoporous silica templates by simple incipient wetness method, and the composites were dried at 433 K for 10 min. This impregnation–drying process is repeated nine times in order to maximize the amounts of TiO2 precursor within the mesopores of silica templates. Subsequently, the samples were dried at 373 K for 24 h. The materials were heated to 723 K for 3 h under ambient conditions. Finally, the silica templates were removed by using 1 M aqueous NaOH solution, resulting in the removal of more than 99% of SiO2 , which was confirmed by an elemental analysis. Figure 1 shows powder X-ray diffraction (XRD) patterns for the mesoporous TiO2 materials obtained from the various mesoporous silica templates. The small-angle XRD patterns in Figure 1 indicate that the mesostructures of TiO2 replica materi-
Figure 1. Small-angle and wide-angle XRD patterns for the mesoporous TiO2 materials obtained from mesoporous silica templates: (a) SAB-15, (b) MSU-H, and (c) KIT-6 ( rutile phase).
Copyright Ó 2008 The Chemical Society of Japan
Chemistry Letters Vol.37, No.2 (2008)
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Table 1. Physical properties of mesoporous materials Samplea
SBET b /m2 g1 Vtot c /cm3 g1 Pore sized /nm
SBA-15 627 0.98 8.5 MSU-H 560 0.76 7.2 KIT-6 652 0.81 6.4 TiO2 -SBA-15 258 0.30 2.7 TiO2 -MSU-H 150 0.44 5.1 TiO2 -KIT-6 220 0.41 3.9 a TiO2 -X means the mesoporous TiO2 material obtained from silica template X. b BET surface areas calculated from the N2 adsorption. c Total pore volumes measured at p=p0 ¼ 0:99. d Calculated by BJH method from N2 adsorption branches. als exhibit highly ordered and similar to those of silica templates (See Figure S1 in Supporting Information).23 The mesoporous TiO2 materials obtained from SBA-15 and MSU-H show one intense peak and relatively weak two peaks, which are characteristics of 2-D hexagonal structures (P6mm). The wellresolved three peaks in Figure 1c can be indexed to (211), (220), and (332) of bicontinuous cubic Ia3d symmetry. The framework structures of mesoporous TiO2 materials are highly crystalline with mainly anatase phases, as shown in the wideangle XRD patterns in Figure 1. Quite small portion of rutile phases is shown in the TiO2 replicas. Figure 1 shows that the wide-angle XRD peaks are somewhat broad because the frameworks of mesoporous TiO2 materials are formed within the confined mesopores of silica templates. The sizes of crystalline TiO2 frameworks are estimated to be about 11 nm by using the Scherrer equation, which are correspond to the pore sizes of silica templates (Table 1). Figure 2 shows the SEM and TEM images of mesoporous TiO2 materials obtained from the mesoporous silica templates. The SEM images in Figure 2 clearly demonstrate that the overall particle morphologies of mesoporous TiO2 materials are similar to those of silica templates in Figure S2 (Supporting Information).23 The TEM images of the mesoporous TiO2 materials in Figure 2 indicate that the materials consist of TiO2 nanorods which are inverse-mesostructures of silica templates, as expected by XRD patterns in Figure 1. The HRTEM image (inset of Figure 2b) also reveals that the TiO2 nanorods are highly crystalline and that the crystallite size is about 10 nm. The N2 adsorption–desorption isotherms and the corresponding BJH pore size distribution curves also indicate that the present TiO2 materials exhibit well-ordered mesostructures (Figure S3 in Supporting Information).23 All the N2 sorption isotherms in Figure S323 are typically type IV isotherms with hysteresis loops. Well-defined steps in the adsorption–desorption curves appear between the relative pressures, p=p0 , of 0.4–0.7, indicating the presence of a narrow distribution of mesopores. The physical properties of mesoporous silica templates and corresponding mesoporous TiO2 replica materials are listed in Table 1. In summary, the highly ordered mesoporous TiO2 materials with crystalline frameworks have been successfully synthesized via the nanoreplication route from various mesoporous silica templates using the modified nanoparticle route. The mesoporous TiO2 materials, exhibiting the well-developed regular mesopores, high surface areas and crystalline anatase frameworks, are expected to give great potentials for various kinds of applications.
Figure 2. SEM and TEM images for the mesoporous TiO2 materials obtained from mesoporous silica templates: (a) SAB15, (b) MSU-H, and (c) KIT-6. We thank to Korea Science and Engineering Foundation (KOSEF, M10503000291 and R-01-2006-000-10283-0) and Korea Research Foundation Grant (MOEHRD, KRF-2005005-J11901). We also thank to Pohang Light Source for the measurement of small-angle XRD at BL8C2 beam line. References and Notes 1 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartulli, J. S. Beck, Nature 1992, 359, 710. 2 S. Shen, A. E. Garcia-Bennett, Z. Liu, Q. Lu, Y. Shi, Y. Yan, C. Yu, W. Liu, Y. Cai, O. Terasaki, D. Zhao, J. Am. Chem. Soc. 2005, 127, 6780. 3 J. E. Lim, C. B. Shim, J. M. Kim, B. Y. Lee, J. E. Yie, Angew. Chem., Int. Ed. 2004, 43, 3839. 4 J. M. Kim, Y. Sakamoto, Y. K. Hwang, Y. U. Kwon, O. Terasaki, S. E. Park, G. D. Stucky, J. Phys. Chem. B 2002, 106, 2552. 5 U. Ciesla, S. Schacht, G. D. Stucky, K. K. Unger, F. Schu¨th, Angew. Chem., Int. Ed. Engl. 1996, 35, 541. 6 D. M. Antonelli, J. Y. Ying, Angew. Chem., Int. Ed. Engl. 1996, 35, 426. 7 M. Wei, Y. Konishi, H. Zhou, M. Yanagida, H. Sugihara, H. Arakawa, J. Mater. Chem. 2006, 16, 1287. 8 P. W. Morrison, Jr., R. Raghavan, A. J. Timpone, C. P. Artelt, S. E. Pratsinis, Chem. Mater. 1997, 9, 2702. 9 F. Campus, P. Bonhote, M. Gratzel, S. Heinen, L. Walder, Sol. Energy Mater. Sol. Cells 1999, 56, 281. 10 P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature 1998, 396, 152. 11 C. J. Brinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mater. 1999, 11, 579. 12 G. J. de A. A. Soler-Illia, A. Louis, C. Sanchez, Chem. Mater. 2002, 14, 750. 13 R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B 1999, 103, 7743. 14 K. Zhu, B. Yue, W. Zhou, H. He, Chem. Commun. 2003, 98. 15 B. Tian, X. Liu, L. A. Solovyov, Z. Liu, H. Yang, Z. Zhang, S. Xie, F. Zhang, B. Tu, C. Yu, O. Terasaki, D. Zhao, J. Am. Chem. Soc. 2004, 126, 865. 16 F. Jiao, A. Harrison, J. C. Jumas, A. V. Chadwick, W. Kockelmann, P. G. Bruce, J. Am. Chem. Soc. 2006, 128, 5468. 17 Y. Wang, C. M. Yang, W. Schmidt, B. Spliethoff, E. Bill, F. Schu¨th, Adv. Mater. 2005, 17, 53. 18 S. C. Laha, R. Ryoo, Chem. Commun. 2003, 2138. 19 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredirckson, B. F. Chemlka, G. D. Stucky, Science 1998, 279, 548. 20 S. S. Kim, A. Karkamkar, T. J. Pinnavaia, J. Phys. Chem. B 2001, 105, 7663. 21 F. Kleitz, S. H. Choi, R. Ryoo, Chem. Commun. 2003, 2136. 22 Y. K. Hwang, Y. U. Kwon, K. C. Lee, Chem. Commun. 2001, 1738. 23 Supporting Information is available electronically from the CSJJournal Web site; http://www.csj.jp/journals/chem-lett/.
Published on the web (Advance View) December 22, 2007; doi:10.1246/cl.2008.140
