Monolithic TiO₂ with Controlled Multiscale Porosity via a Template-Free Sol−Gel Process Accompanied by Phase Separation

Abstract

This article describes the fabrication of multiscale porous nanocrystalline TiO 2 monoliths through a one-step method that combines a sol-gel process and phase separation in template-free conditions. A large-dimension monolith with well-defined macropores and a mesostructured anatase-type TiO 2 gel skeleton is spontaneously obtained by controlling the solution pH during the hydrolysis and polycondensation reactions of titanium alkoxides. The size of the macropores is adjusted by the starting composition, and a crystallized anatase TiO 2 skeleton is formed without heat treatment. The use of titanium alkoxide strengthens the gel network by the formation of chemical bonding in the condensation reaction, which yields porous monoliths with higher mechanical strength than for the case of porous monoliths derived from colloidal TiO 2 using freeze drying to maintain the porous morphology. The average crystallite size of anatase TiO 2 nanocrystals was found to be about 3.6 nm for the dried gel and about 5.0 nm for the gel calcined at 300 °C. As a result of the growth of the anatase TiO 2 nanoparticles, the mesopores with a median size of 5.0 nm are obtained. The high surface area (∼150 m 2 /g) is maintained even after the heat treatment at 300 °C.

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