Direct Formation of Anatase (TiO 2 )/Silica (SiO 2 ) Composite Nanoparticles with High Phase Stability of 1300 °C from Acidic Solution by Hydrolysis under Hydrothermal Condition

Anatase (TiO 2 )/silica (SiO 2 ) composite nanoparticles that were directly synthesized from solutions of TiOSO 4 and tetraethyl orthosilicate by hydrolysis under hydrothermal conditions at 200 °C showed far more enhanced phase stability than pure anatase-type TiO 2 . By the presence of 48.2 mol % SiO 2 as mostly amorphous phase, the crystallite growth of anatase was completely suppressed to maintain the crystallite size (14−16 nm) of the as-prepared condition after heating at 1000 °C, and the anatase-to-rutile phase transformation was retarded to maintain the anatase-type structure up to 1300 °C with no trace of rutile phase. The critical crystallite size of anatase in the composite samples for the anatase-to-rutile phase transformation was suggested to be around 70−100 nm. The silica, which contained -OTi groups around tetrahedrally coordinated Si and was regarded to exist mostly as amorphous phase, was considered to play an important role in suppressing diffusion between anatase crystallites, which led to the retardation of the crystallite growth of anatase and consequently resulted in the suppression of the phase transformation.