Effects of Template and Precursor Chemistry on Structure and Properties of Mesoporous TiO 2 Thin Films X. Shari Li, Glen E. Fryxell,* Jerome C. Birnbaum, and Chongmin Wang Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Received June 3, 2004. In Final Form: July 7, 2004 Mesoporous TiO2 thin films were synthesized by sol-gel processing using an aqueous-based, inexpensive, and environmentally friendly precursor and cationic surfactants as templates under mild reaction conditions. The films were prepared by spin-coating on glass substrates followed by calcination to remove the surfactant. N 2 sorption, X-ray diffraction, and transmission electron microscopy were used to characterize the porosity, pore size, and pore structure before and after calcination. Films were found to have wormlike pore structures after calcination and surface areas on the order of 200 m 2 /g. These results show that the mesostructure and porosity of the thin films can be controlled by the surfactant template chemistry such as surfactant/Ti ratio, pH, and rate of solvent evaporation. Introduction There has been a great interest in TiO 2 films for a variety of applications, for example, gas sensors, 1-3 photocataly- sis, 4 and photoelectrodes. 5-8 TiO 2 thin films are commonly prepared using a sol-gel process. 3,4,6,9 Sol-gel methodol- ogy is one of the most convenient technologies for preparation of oxide thin films due to its low cost, ease of execution, and low processing temperatures. For many applications, large surface area mesoporous TiO 2 films are desired. Mesoporous silica films with pore sizes in the range of 1-10 nm have been synthesized by the sol-gel process using surfactant templates, 11-13 in which the pores are formed in a spin-coated 14-17 or dip- coated film 18,19 after removal of the pore former. This molecularly templated synthetic strategy allows rational control of the porosity, pore size, pore shape, film texture, and thickness and can result in good mechanical properties in the film. 20 This methodology has been extended to the templated syntheses of various transition metal oxides. For example, porous TiO 2 films have been prepared using surfactant templating of a Ti(C 4 H 9 O) 4 as a precursor. 10 The micromorphologies and pore sizes of TiO 2 films can be controlled by changing the type of the surfactant species. Much of the interest behind making nanostructured titania phases stems from titania’s activity as a semi- conductor and photocatalyst. 4,7,8 Therefore, it is important to be able to prepare mesoporous titania phases that have been chemically modified. For example, Antonelli and co- workers have doped mesoporous titania with alkali metals, 21 potassium fulleride wires, 22 reduced Ti species, 23 and cobaltocene, 24 to create low valent reactive interfaces which undergo useful chemical processes, such as the fixation of dinitrogen to form ammonia. 23 Therefore, for maximum utility in this area, the chemistry involved in the synthesis of mesoporous titania thin films needs to be compatible with the incorporation of catalytically active species, photosensitizers, or nanoparticle adjuncts. In this paper, we present a convenient method to synthesize mesoporous TiO 2 thin films by spin-coating using surfactants as templates and using an aqueous- based, inexpensive, and environmentally friendly precur- sor, using mild reaction conditions, allowing this method to be tailored to include an assortment of modification chemistries (e.g., nanoparticle inclusions). Deposition solutions are composed of surfactant templates and titanium lactate in aqueous alcohol at modest pH. The mesostructure and porosity of the thin films can be controlled by the surfactant template chemistry, which is affected by parameters such as surfactant/Ti ratio, pH, and rate of solvent evaporation. * To whom correspondence should be addressed. (1) Yamazoe, N.; Miua, N. Chem. Sens. Technol. 1992, 4, 19-42. (2) Go ¨pel, W. Sens. Actuators 1996, 56, 83-102. (3) Atashbar, M. Z. IEEE-NANO 2001, 544-549. (4) Katoh, K.; Tsuzuki, A.; Torii, Y.; Taoda, H. J. Mater. Sci. 1995, 30, 837-841. (5) Regan, B. O.; Gra ¨ tzel, M. Nature 1991, 353, 737-739. (6) Ruhman, M. M.; Tanaka, H.; Soga, T.; Jimbo, T.; Umeno, M. IEEE 2000, 806-809. (7) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso ¨ rtel, F.; Salbeck, J.; Spereitzer, H.; Gra ¨ tzel, M. Nature 1998, 395, 583-585. (8) Gra ¨ tzel, M. Nature 2001, 414, 338-344. (9) Toko, T.; Yuasa, A.; Kamiya, K.; Saka, S. J. Electrochem. 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