Effects of aging conditions on the structural properties of mesoporous SiO u2 /TiO u2 composite materials with crystalli
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Well-organized mesoporous SiO2/TiO2 materials with crystallized framework were synthesized. The resultant materials showed high surface area (180–300 m2/g), narrow pore-size distribution (3.5–6 nm), and nanocrystalline framework. The influences of aging conditions on the mesoporous 80TiO2–20SiO2 materials were investigated. The water in the initial solution is beneficial for the mesostructural organization of the as-prepared sample while air moisture (relative humidity) during the aging process plays a key role in the crystallization of the calcined sample. Low aging temperature is another decisive factor to the formation of the mesostructure. I. INTRODUCTION
Titanium dioxide is one of the most studied semiconductors for a variety of applications such as photocatalysis, gas sensors, and photoelectrodes.1–5 For many applications, large surface area mesoporous TiO2 materials are desired. Various synthesis strategies have been developed to synthesize TiO2 mesoporous materials even with crystalline frameworks. However, reproducibility in obtaining high-quality mesoporous materials for these reported preparations is still a weak point in many cases.6 Apart form less explored and more diverse chemistry of TiO2, high reactivity toward hydrolysis and condensation, easy crystallization, and speed of solvent evaporation etc. are all reasons for concern. One suitable method of controlling the high reactivity of transition metals is to add complex molecules or acids as stabilizing agents or to use nonaqueous solvents to decrease the reactivity of metal ions.7–10 To avoid the collapse of mesostructure caused by crystallization/grain growth of TiO2, template removal by washing is also used, but an irreversible deterioration of the mesostructure is still associated with the processing due to an insufficiently condensed inorganic framework. Evaporation-induced self-assembly (EISA)11 is one of the most promising approaches.12 In this process, the preferential evaporation of solvent (usually alcohol) concentrates the initial diluted solution in the nonvolatile surfactant and inorganic species before equilibration with atmosphere12 during which a liquid crystalline mesophase is gradually formed. However, as far as mesoporous TiO2 materials are concerned, there
are different opinions on the parameters’ effects even for the same formula, especially on that of water.12–15 Besides, the polymerizing ability of anions and cations,16 speed of solvent evaporation,17 pH,18 calcining,6,17–19 the ratio of surfactant/inorganic precusor18,20,21 etc., were studied in detail. In these reports, they achieved different, even opposite results, though they used the same ratio of initial agents or heat-treatment. In fact, in the EISA process, the mechanism of formation is kinetically controlled and is often influenced by the aging conditions including aging temperature, relative humidity etc., which have typically been neglected during the experiments. On the other hand, most articles paid more attention to the organization of mesostructure and less att
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