Macroporous Morphology Induced by Phase Separation in Sol-Gel Systems Derived from Titania Colloid
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Macroporous Morphology Induced by Phase Separation in Sol-Gel Systems Derived from Titania Colloid Junko Konishi, Koji Fujita, Kazuki Nakanishi, and Kazuyuki Hirao Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyotodaigakukatsura, Nishikyo-ku, Kyoto 615-8510, Japan. ABSTRACT Macroporous titania gels have been prepared by the sol-gel method starting from aqueous colloidal dispersion of titania. The use of titania sol instead of highly reactive titanium alkoxide allows us to control the rate of gelation. Macroporous morphology is formed when the transitional structure of phase separation is fixed as permanent structures by the sol-gel transition. The domain size can be controlled reproducibly by changing the starting composition. The crystal structure of the titania gel is transformed from anatase to rutile through the heat treatment at 900°C, while the macroporous morphology remains unchanged. INTRODUCTION Porous ceramics have been attracting significant attention because of their applications in various engineering fields, such as separation media for liquid solutions and gas mixtures, catalyst supports, optical devices, and so forth. They can be also applied to the field of biotechnology as supports of biological catalysts such as enzymes or microbes. A sol-gel method has emerged as a very promising technique for the fabrication of porous materials. Macroporous morphology can be obtained by inducing the phase separation parallel to the sol-gel transition, and the pore size and density are controlled through physical and chemical interactions among the constituents of the reaction mixture. Based on this concept, Nakanishi et al. [1] found that monolithic silica gels having well-defined pores in the size range of micrometers can be obtained from systems containing silicon alkoxide. By appropriately selecting the starting materials, including alkoxide, organic polymer and/or other additives, it is possible to fabricate macroporous gels with desired morphology. In contrast to the silica-based sol-gel systems, the formation of three-dimensionally interconnected macroporous structure has not been achieved in transition-metal alkoxide-derived sol-gel systems, so far. The transition-metal alkoxides, M(OR)x, are very reactive toward hydrolysis and polycondensation because of the presence of highly electronegative OR groups that render M very susceptible to nucleophilic attack. Due to the rapid polymerization reaction, it is generally difficult to control the structural development until the gelation. Since TiO2 possesses novel functions, including photocatalytic activity and oxygen-gas-sensing properties, macroporous TiO2 monoliths, if developed, are expected to have applications to various technologies due to the dual merits of porous morphology and TiO2. In the present paper, an attempt has been made to obtain monolithic macroporous TiO2 gels from aqueous titania colloid instead of titanium alkoxide. The aggregation and gelation of titania colloid are controlled by the pH inc
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