Green preparationof visible-light-active TiO2 films

An environmental friendly method was developed to prepare transparent, uniform, crack-free and visible-light-active TiO2 thin films, such as N doped TiO2 (N-TiO2), Fe doped TiO2 (Fe-TiO2) and (C, N) codoped TiO2 (C-N-TiO2) films, without the use of organic Ti precursors and organic solvents. These films were prepared from heating aqueous peroxotitanate thin films deposited uniformly on superhydrophilic uncoated glass substrates. The pure glass substrates were superhydrophilic after being heated at 500 degreesC for 1 h. For the preparation of N-TiO2, the nitrogen concentrations in the TiO2 films were adjusted by changing the amount of ammonia solutions (pH value). The optimal photocatalytic activity of the N-TiO2 film was about 14 times higher than that of a commercial self-cleaning glass under the same visible light irradiation. The Fe- TiO2 films exhibited slight red-shift in absorbance and enhanced absorbance in the visible-light region compared to undoped TiO2. This was attributed to bandgap narrowing by successful Fe3+ doping into TiO2, supported by DOS calculations. For the preparation of C-N-TiO2 film, carbon black was employed as the carbon source as well as a template to increase surface roughness. Optimal visible light photocatalytic activity was observed for C-N-codoped TiO2 film at 10.0 wt.% C, which was more than double that of the N-doped TiO2 film. The enhancement in visible light photocatalytic activities of the C-N-TiO2 films was attributed to the synergistic effects of carbon and nitrogen dopants, and high surface roughness of the prepared films. The current reported preparative technique is generally applicable for the preparation of other thin films.