Preparation of hierarchically porous nanocrystalline CaTiO₃, SrTiO₃ and BaTiO₃ perovskite monoliths

Hierarchically porous nanocrystalline CaTiO₃, SrTiO _3, and BaTiO₃ ceramics have been produced by impregnating corresponding alkaline-earth metal ions into preformed macroporous TiO₂ monoliths in a solution containing urea, followed by calcination. The macroporous TiO₂ had been obtained via the sol-gel process accompanied by phase separation utilizing a chelating agent, ethyl acetylacetonate (EtAcAc), together with mineral salt and ammonium chloride, to decrease the reactivity of titanium alkoxide. Formations of CaCO₃, SrCO₃, and BaCO₃ on the surface of TiO₂ monoliths are promoted by CO₂ generated by the concurrent two processes during impregnation; hydrolysis, and decarbonation of the chelating agent EtAcAc, and hydrolysis of urea at 60°C. The latter also raises pH of the impregnating solution which further promotes the mineralization of the carbonate salts. Calcination of the resultant monolithic composite of metal carbonate/TiO₂ allows the crystallization of metal titanate. The addition of urea to the impregnating solution is found to be an effective strategy for the formation of perovskite monoliths by the impregnation process. This study provides a versatile approach to the preparation of hierarchically porous titania-based perovskites.