Postsynthesis Stabilization of Free-standing Mesoporous Silica Films R. Vogel, †,‡ C. Dobe, † A. Whittaker, § G. Edwards, † J. D. Riches, | M. Harvey, ‡ M. Trau, † and P. Meredith* ,‡ Department of Chemistry & Centre for Nanotechnology and Biomaterials, Centre for Magnetic Resonance, and Centre for Microscopy and Microanalysis and Department of Physics & Centre for Biophotonics and Laser Science, The University of Queensland, Qld 4072, Australia Received September 25, 2003. In Final Form: January 21, 2004 Mixed ammonia-water vapor postsynthesis treatment provides a simple and convenient method for stabilizing mesostructured silica films. X-ray diffraction, transmission electron microscopy, nitrogen adsorption/desorption, and solid-state NMR ( 13 C, 29 Si) were applied to study the effects of mixed ammonia- water vapor at 90 °C on the mesostructure of the films. An increased cross-linking of the silica network was observed. Subsequent calcination of the silica films was seen to cause a bimodal pore-size distribution, with an accompanying increase in the volume and surface area ratios of the primary (d ) 3 nm) to secondary (d ) 5-30 nm) pores. Additionally, mixed ammonia-water treatment was observed to cause a narrowing of the primary pore-size distribution. These findings have implications for thin film based applications and devices, such as sensors, membranes, or surfaces for heterogeneous catalysis. Introduction In 1992 researchers at Mobil discovered a new class of ordered porous adsorbents, M41S. 1 These novel, silica- based materials possessed adjustable, uniform pore-sizes 2 and large surface areas. Mesoporous silicates can be produced by either an alkaline route or an acidic route 3 using surfactants as templates. Silicates produced under alkaline conditions exhibit good thermal stability. This is improved by postsynthesis hydrothermal treatment. 4-6 Under acidic conditions, silicon alkoxide precursors are typically used, forming a more labile and soft network during the hydration process. 7 Postsynthesis hydrothermal treatment in boiling ammonia-water improves the order and stabil- ity of these acid-synthesis mesoporous silica powders. 8-10 An important feature of mesoporous silica materials is their ability to form thin films as well as bulk powders. 11 For many applications and devices, such as membranes, sensors, or surfaces for heterogeneous catalysis, meso- porous thin films are required. Silica-based mesoporous thin films have been prepared by methods such as dip- coating, 12 spin-coating, 13 and growth on hydrophilic substrates 14 as well as via unsupported growth at air/ liquid as well as liquid/liquid interfaces. 15 Free-standing silica films produced under acidic condi- tions were reported to be labile and thermally unstable, as demonstrated by a partial collapse of the mesostructure during calcination. 16 The use of a postsynthesis treatment involving boiling the film in ammonia-water would be wholly inappropriate for the stabilization of silica-based mesoporous thin films because their macrostructure, especially in the case of labile free-standing films, would be destroyed. For these reasons, Wu et al. 17 and Grosso et al. 18,19 recently reported the use of an ammonia atmosphere to treat silica films on glass substrates. Wu et al. applied a mixed ammonia-water atmosphere at approximately 400 °C to increase the scratch resistance of nanoporous silica films. These films were produced by a sol-gel process without using surfactants. Grosso et al. posttreated dip- coated thin silica films produced from a silica precursor (TEOS) and either cetyltrimethylammonium bromide (CTAB) 18 or a pluronic triblock copolymer (F127) 19 under an ammonia atmosphere at room temperature. This * To whom correspondence should be addressed. E-mail: meredith@physics.uq.edu.au. † Department of Chemistry & Centre for Nanotechnology and Biomaterials. ‡ Department of Physics & Centre for Biophotonics and Laser Science. § Centre for Magnetic Resonance. | Centre for Microscopy and Microanalysis. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Sayari, A.; Yang, Y.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 1999, 103, 3651. (3) Huo, Q.; Margoleses, S. I.; Ciesla, U.; Feng, P.; Gier, D. E.; Sieger P.; Leon, B. F. R.; Petroff, P. M.; Schueth, F.; Stucky, G. D. Nature 1994, 368, 317. (4) Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, 2499. (5) Chen, L.; Horiuchi, T.; Mori, T.; Maeda, K. J. Phys. Chem. B 1999, 103, 1216. (6) Kruk, M.; Jaroniec, M.; Sayari, A. Micropor. Mesopor. Mater. 1999, 27, 217. (7) Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Garces, J.; Olken, M. M.; Coombs, N. Adv. Mater. 1995, 7, 842. (8) Lin, H. P.; Mou, C. Y.; Liu, S. B. Chem. Lett. 1999, 1341. (9) Mou, C. Y.; Lin, H. P. Pure Appl. Chem. 2000, 72, 137. (10) Lin, H. P.; Mou, C. Y.; Liu, S. B.; Tang, C. Y.; Lin, C. Y. Micropor. Mesopor. Mater. 2001, 44-45, 129. (11) Pevzner, S.; Regev, O.; Yerushalmi-Rozen, R. Curr. Opin. Colloid Interface Sci. 2000, 4, 420. (12) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky G. D. Adv. Mater. 1998, 10, 1380. (13) Martin, J. E.; Anderson, M. T.; Odinek, J.; Newcomer, P. Langmuir 1997, 13, 4133. (14) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (15) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (16) Yang, H.; Coombs, N.; Ozin, G. A. J. Mater. Chem. 1998, 8, 1205. (17) Wu, G.; Wang, J.; Shen, J.; Yang, T.; Zhang, Q.; Zhou, B.; Deng, Z.; Bin, F.; Zhou, D.; Zhang, F. J. Non-Cryst. Solids 2000, 275, 169. (18) Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Lavergne, M.; Mazerolles, L.; Babonneau, F. J. Mater. Chem. 2000, 10, 2085. (19) Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Ayral, A.; Amenitsch, H.; Babonneau, F. Chem. Mater. 2001, 13, 1848. 2908 Langmuir 2004, 20, 2908-2914 10.1021/la035788o CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004