Anatase (TiO 2 )/silica (SiO 2 ) composite nanoparticles over the compositional range of SiO 2 070.9 mol÷ were directly formed from acidic precursor solutions of TiOSO 4 and tetraethylorthosilicate by hydrolysis under mild hydrothermal conditions at 200 C. With increasing SiO 2 content, the specic surface area of the asprepared composite nanoparticles increased from 109 (SiO 2 0 mol÷) to 277 m 2 /g (SiO 2 70.9 mol÷), which was associated with an increase in the amorphous phase content. The composite nanoparticles with a unique microstructure, i.e., anatase crystallites 1416 nm in size that exist separately due to the presence of an amorphous silica phase in the TiO 2 /SiO 2 interface, showed enhanced photocatalytic activities for the decomposition of methylene blue (MB) in aqueous solution and better removal of NO gas in comparison with pure anatasetype TiO 2 . The highest photoactivity was obtained when the SiO 2 content in the composites was 25.1 mol÷ for MB decomposition and 48.2 mol÷ for NO oxidation.

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