Photocatalytic properties of TiO2 films deposited by reactive sputtering in mid-frequency mode with dual cathodes

Titanium dioxide (TiO2) films were deposited on unheated non-alkali glass substrates by reactive mid-frequency (mf) magnetron sputtering using dual cathodes with two Ti metal targets. In order to keep the very high deposition rate, the depositions were successfully carried out stably in the "transition region" between the metallic and the reactive (oxide) sputter modes using the plasma control units (PCU). Very high rate depositions of 12-70 nm/min were successfully achieved to deposit TiO2 films in the whole "transition region". The as-deposited films deposited in the "oxide mode" showed polycrystalline anatase structure and performed both photo-induced hydrophilicity and photo-decomposition of acetaldehyde. Whereas, all the as-deposited TiO2 films deposited in the "transition region" showed amorphous structure, which did not perform the photocatalytic activity. Such amorphous films deposited in the transition region were crystallized by post-annealing in air at 200 oC or 300 oC and then performed the photocatalytic activities. The very thin TiO2 films with the thickness of 25 nm deposited in the transition region and post-annealed at 300°C showed photo-induced hydrophilicity, whereas there was a clear thickness dependence for the photo-decomposition and the rather thick films of 300nm showed the high photo-decomposition performance.