Macroporous morphology of titania films prepared by sol-gel dip-coating method from a system containing poly(ethylene gl
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Kazuki Nakanishi Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan (Received 7 June 2000; accepted 1 September 2000)
Macroporous titania (TiO2) films were prepared by a sol-gel dip coating method from a system containing poly(ethylene glycol) (PEG) and poly(vinylpyrrolidone) (PVP). The thickness of the macroporous films increased with an increase in PVP concentration, but the excess incorporation of PVP suppressed the macroscopic phase separation and enhanced the formation of macroscopic cracks. The porosity and the domain size were simply determined by PEG concentration. Although both PEG and PVP are hydrogen-bonding polymers having proton-accepting ability, preparation of macroporous TiO2 films was unsuccessful in systems containing only PVP as a polymer. Macroporous TiO2 films having interconnected pore structure as thick as 1 m were successfully prepared by repeating the deposition several times.
I. INTRODUCTION
Sol-gel methods are generally recognized as efficient routes in fabricating ceramic thin films.1 One field of special interest has been preparing homogeneous, transparent, and thicker films without forming macroscopic cracks during the drying and heating stages. On the other hand, the sol-gel method is suitable for preparing porous ceramics because the reaction proceeds with the coexistent solvent phase of which evaporation leaves a lot of cavities. The size and distribution of pores are determined by physical and chemical interactions among the constituents of the reaction mixture. The phase separation parallel to the polycondensation, which is induced by the reduction of mixing entropy owing to the formation or incorporation of polymeric species, can be utilized to tailor the pore structure on a macroscopic scale. Based on this concept, macroporous silica gels having controlled pore structure have been successfully prepared from systems containing water-soluble organic
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Address all correspondence to this author. Present address: Hosono Transparent Electro-Active Materials Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Corporation, KSP C-1232, 3-2-1 Sakado Takatsu-ku, Kawasaki 213-0012, Japan. [email protected]
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J. Mater. Res., Vol. 16, No. 1, Jan 2001 Downloaded: 14 Mar 2015
polymers.2–5 Macroporous titania (TiO2) films have also been prepared by dip-coating methods from systems containing poly(ethylene glycol) [PEG; HO(–CH2CH2O–)nH], in which the macroporous morphology is formed concurrently with solvent evaporation during the dipping operation.6–11 However, the macroporous films were not thick enough to realize the three-dimensionally interconnected pore structure as realized in the silica systems2–5 because the films were too thin. Thicker macroporous semiconductive films having larger surface area due to the interconnected pore structure are widely applicable to electrode materials,12,13 gas sensors,14 and thin-film chromatographies. One
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