Chem. Mater. 1999, 11, 2813-2826
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Block Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Framework Peidong Yang,† Dongyuan Zhao,†,‡ David I. Margolese,† Bradley F. Chmelka,‡,§ and Galen D. Stucky*,†,‡ Departments of Chemistry and Materials, Materials Research Laboratory, and Department of Chemical Engineering, University of California, Santa Barbara, California 93106 Received April 2, 1999. Revised Manuscript Received July 1, 1999
A simple and general procedure has been developed for the syntheses of ordered largepore (up to 14 nm) mesoporous metal oxides, including TiO2, ZrO2, Nb2O5, Ta2O5, Al2O3, SiO2, SnO2, WO3, HfO2, and mixed oxides SiAlOy, Al2TiOy, ZrTiOy, SiTiOy, ZrW2Oy. Amphiphilic poly(alkylene oxide) block copolymers were used as structure-directing agents in nonaqueous solutions for organizing the network-forming metal oxide species. Inorganic salts, rather than alkoxides or organic metal complexes, were used as soluble and hydrolyzable precursors to the polymerized metal oxide framework. These thermally stable mesoporous oxides have robust inorganic frameworks and thick channel walls, within which high densities of nanocrystallites can be nucleated. These novel mesoporous metal oxides are believed to be formed through a mechanism that combines block copolymer self-assembly with alkylene oxide complexation of the inorganic metal species.
Introduction Self-assembly of block copolymers,1,2 surfactants,3 colloidal suspensions4 and proteins5 provides a versatile approach to the creation of nanostructures with potential applications in biomaterials, optoelectronics, and nanotechnology.6 Since the discovery of mesoporous silicates based on amphiphilic supramolecular templates,7,8 the use of surfactant species to organize mesoporous silica has been explored over a wide range of conditions. Such conditions have relied on various interactions including electrostatic, hydrogen-bonding, covalent bonding, and van der Waals forces that have been balanced to achieve self-assembly and thereby direct mesostructure formation.3,9-11 Recently, block * To whom correspondence should be addressed. † Departments of Chemistry and Materials. ‡ Materials Research Laboratory. § Department of Chemical Engineering. (1) Muthukumar, M.; Ober, C. K.; Thomas, E. L. Science 1997, 277, 1225. (2) Fredrickson, G. H.; Bates, F. S. Annu. Rev. Mater. Sci. 1996, 26, 501. (3) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (4) Van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (5) Ghadiri, M. R.; Cranja, J. R.; Buehler, L. K. Nature 1997, 369, 301. (6) Yang P.; Deng T.; Zhao D.; Feng P.; Pine D.; Chmelka B. F.; Whitesides G. W.; Stucky G. D. Science 1998, 282, 2244. (7) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (8) Inagaki S.; Fukushima Y.; Kuroda K. J. Chem. Soc., Chem. Commun. 1993, 8, 680. (9) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (10) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (11) Antonelli, D. M.; Ying, J. Y. Chem. Mater. 1996, 8, 874.
copolymers have been increasingly used to organize mesostructured composite solids, because the architectures of the amphiphilic block copolymers can be rationally adjusted to control the interactions between the inorganic and organic species, self-assembly, and especially processibility more than possible with low molecular weight surfactants.12 For example, using amphiphilic block copolymers as the structure-directing agents, aluminosilicate mesostructures,13 and mesoporous silica14-17 with large ordering lengths (>15 nm) and a variety of macroscopic morphologies (e.g., thin film and fiber) have been synthesized. Extension of the surfactant templating procedure to the formation of non-silica mesoporous oxides, however, has been less widespread, although these mesoporous metal oxides hold more promise in applications that involve electron transport and transfer or magnetic interactions.18 Mesoporous MnO2,19 Al2O3,20,21 TiO2,22,23 Nb2O5,11 Ta2O5,11 ZrO2,24-28 HfO2,29 and SnO230 and (12) Fo¨rster, S.; Antonietti, M Adv. Mater. 1998, 10, 195. (13) Templin, M.; Franck, A.; Chesne, A. D.; Leist, H.; Zhang, Y.; Ulrich, R.; Scha¨dler, V.; Wiesner, U. Science 1997, 278, 1795. (14) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D Science 1998, 279, 548. (15) Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 1998, 10, 1380. (16) Yang, P.; Zhao, D.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033. (17) Kramer E.; Forster S.; Goltner C.; Antonietti M. Langmuir 1998, 14, 2027. (18) Sayari, A.; Liu, P. Microporous Mater. 1997, 12, 149. (19) Tian, Z.; Tong, W.; Wang, J.; Duan, N.; Krishnan, V. V.; Suib, S. L. Science 1997, 276, 926. (20) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (21) Vaudry F.; Khodabandeh S.; Davis M. E. Chem. Mater. 1996, 8, 1451. (22) Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1995, 34, 2014.
10.1021/cm990185c CCC: $18.00 © 1999 American Chemical Society Published on Web 09/29/1999
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reduced Pt31 have been synthesized using low molecular weight surfactants over the past few years, although often thermally unstable mesostructures have resulted.3,32,33 Stucky and co-workers developed a surfactant templating strategy for the synthesis of non-silicabased mesostructures, mainly metal oxides,3,9 in which both positively and negatively charged low molecular weight surfactants were used in the presence of water soluble inorganic species. It was found that charge density matching between the surfactant and the inorganic species is important for the formation of the organic-inorganic mesophases. Unfortunately, many of these non-silica mesostructures are not thermally stable. Pinnavaia and co-workers used nonionic surfactants to synthesize mesoporous alumina in neutral media and suggested that the resulting mesoporous materials with wormlike channels are assembled by hydrogen-bonding interactions of the inorganic species with the surfactant agents.20 Antonelli and Ying prepared the stable mesoporous transition metal oxide, TiO2, using a modified sol-gel method, in which an organometallic precursor was hydrolyzed in the presence of alkyl phosphate surfactants.22 Mesoporous ZrO2 was prepared using long-chain quaternary ammonium, primary amines, or amphoteric cocamidopropyl betaine as structure-directing agents.26,27 A scaffolding process was developed by Knowles et al. for the preparation of mesoporous ZrO2.25 Porous HfO2 has been synthesized using cetyltrimethyllammonium bromine as the structure-directing agent.29 Suib et al. prepared mixed-valent semiconducting mesoporous maganese oxide with hexagonal and cubic structures and showed that these materials are catalytically active.19 A ligand-assisted templating approach has been successfully used by Ying and co-workers for the synthesis of mesoporous Nb2O5 and Ta2O5.11,22 Dative coordinate bond interactions between the inorganic and surfactant species were utilized in this process to assemble the mesostructure. Recently, Attard et al. have extended the surfactant templating strategy to produce mesoscopically ordered porous reduced platinum metal.31 For all of these mesoporous non-silica materials, low molecular weight surfactants were used for the assembly of the mesostructures and the resulting mesoporous materials generally have small pore sizes (
