Phase Composition of Nanocrystalline Titania Synthesized under Hydrothermal Conditions from Different Titanyl Compounds

Data are presented on the phase composition and physicochemical properties of nanocrystalline TiO2 powders prepared via hydrothermal treatment of aqueous titanyl sulfate (TiOSO4), titanyl nitrate (TiO(NO3)2), and aqua complex titanyl oxalate acid (H2TiO(C2O4)2) solutions and amorphous titanyl hydroxide (TiO2 · nH2O) gel (synthesized by precipitating H2TiCl6 with an excess of aqueous ammonia) at 423 and 523 K for 10 min to 6 h at solution concentrations from 0.07 to 0.5 M. The synthesized samples were characterized by x-ray diffraction, thermogravimetry, transmission electron microscopy, and nitrogen BET surface area measurements. It is shown that, independent of the precursor, short-term (10 min) hydrothermal treatment leads to the formation of nanocrystalline anatase (crystallite size d = 10–30 nm), a metastable form of titania. Upon an increase in hydrothermal-treatment time to 6 h, the systems studied exhibit different behaviors. Nanocrystalline anatase may persist (titanyl sulfate solutions and amorphous titanyl hydroxide gel) or transform into rutile (d = 50–100 nm), the thermodynamically stable form of TiO2, through recrystallization processes (titanyl nitrate solutions; 0.07 M H2TiO(C2O4)2 solutions, partial conversion at 423 K and full conversion at 523 K; 0.28 M H2TiO(C2O4)2 solutions, full conversion at 423 K). Also possible is the formation of mesoporous anatase (0.28 M H2TiO(C2O4)2 solution, 523 K, 70- to 90-nm aggregates of crystallites, 10- to 20-nm closed pores containing the solution). A model is proposed according to which the formation of mesoporous oxides is possible at comparable rates of anatase nucleation and growth.