Solid solutions and composite nanoparticles in the TiO2-SnO2 system were directly formed via the hydrothermal treatment of precursor solutions of TiCl4 and SnCl4 under weakly basic conditions in the presence of urea. The rutile-type (Ti, Sn)O-2 solid solutions were formed in the composition range of Ti 0- 70 mol%. The composite nanoparticles consisting of anatase- and rutile-type phases were formed at the composition of Ti 80 and Ti 90 mol%. The change in the lattice parameters a(0) and c(0) of the rutile-type solid solutions followed the Vegard Law. The crystallite size of the rutile-type solid solutions was in the range of 5-10 nm. The diffuse reflectance spectra varied with changing Ti content in the precipitates. The photocatalytic activity of composite nanoparticles synthesized at 240 degrees C was higher than that synthesized at 180 degrees C. The composite nanoparticles consisting of anatase- and rutile-type phases with compositions Ti0.90Sn0.10O2 and TiO0.80Sn0.20O2 showed improved photocatalytic activity. (C) 2011 Elsevier Ltd. All rights