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

Titanium dioxide (TiO2) films were deposited on unheated nonalkali glass substrates by reactive midfrequency (mf) magnetron sputtering using dual cathodes with two Ti metal targets. In order to maintain the very high deposition rate, the depositions were successfully carried out stably in the "transition region" between the metallic and reactive (oxide) sputter modes using plasma control units (PCU). Very high-rate depositions of 12-70nm/min were successfully achieved in the deposition of TiO2 films throughout the whole "transition region". The as-deposited films deposited in the "oxide mode" had a polycrystalline anatase structure and exhibited both photoinduced hydrophilicity and photodecomposition of acetaldehyde. Whereas, all the as-deposited TiO2 films deposited in the "transition region" had amorphous structure, which did not exhibit photocatalytic activity. Such amorphous films deposited in the transition region were crystallized by postannealing in air at 200degreesC or 300degreesC and were then shown to have photocatalytic activity. Very thin TiO2 films with a thickness of 25nm deposited in the transition region and postannealed at 300degreesC exhibited photoinduced hydrophilicity, whereas there was a clear thickness dependence of photodecomposition and the rather thick films of 300nm exhibited pronounced photodecomposition.