Effect of nitrogen-doping and post annealing on wettability and band gap energy of TiO2 thin film

Titanium dioxide film is one of the most promising self-cleaning materials. The self-cleaning performance is directly related to the photocatalytic activity and surface wettability, which, in turn, depends on the TiO2 film structure parameters. Nitrogen-doping and post annealing are commonly used for TiO2 film treatment. The present paper addresses the mechanisms of N-doping and annealing induced TiO2 film structure transition, band gap narrowing and wettability transition. It is shown that N-doping combined with annealing leads to anatase. rutile phase transition, formation of hierarchical topography, change of surface chemical composition, and consequently results in reduction of band gap energy and water contact angle. N-doping level and N-linkages are found to significantly affect the structure/properties of annealed TiO2 and N-doped TiO2 films. The proposed mechanisms might help optimize TiO2 film synthesis and post treatment procedures. Moreover, annealed N-doped TiO2 film with highest N-content, simultaneously exhibiting anatase-rutile polycrystalline structure, high roughness, as well as lowest band gap energy and water contact angle, is supposed to present optimal self-cleaning performance.