TiO 2 nanoparticles were prepared by hydrothermal reaction of titanium alkoxide stabilized in acidic ethanol/water solution. The sizes of particles have been controlled to the range of 7-25 nm by adjusting the concentration of Ti precursor and the composition of the solvent system. The TiO 2 samples synthesized under this acidic ethanol/water environment were mainly primary particles in anatase phase without secondary structure. The suspension of as-prepared 7-nm-sized TiO 2 nanoparticles demonstrates long-term stability, and has been applied successfully for the fabrication of ultra-transparent particulate TiO 2 films. The photocatalytic efficiency of TiO 2 films prepared from the 7-nm-sized nanoparticles was 1.6 times of that of films derived from Degussa P25 in decomposing gaseous 2-propanol.

By monolayer coverage of highly acidic semiconductors, such as MoO 3 and WO 3 on the surface of TiO 2 nanoparticles (Degussa P25), the stability of colloidal suspensions in aqueous solution can be greatly improved. The average diameter of agglomerated MoO 3 /TiO 2 and WO 3 /TiO 2 particles in aqueous suspension is only 85-110 nm, respectively, which is about one-fifth to one-fourth of that of pure TiO 2 suspension. The optically transparent photocatalytic films have been prepared with deposition of these colloidal suspensions. The colloidal particles in aqueous solution and the prepared transparent photocatalytic films were characterized with TEM, SEM, AFM, and other spectroscopic techniques. It has also been found that the photocatalytic activity of WO 3 /TiO 2 film is 2.8-3 times that of pure TiO 2 film in decomposing gas-phase 2-propanol, while MoO 3 /TiO 2 film is less effective.

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