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.