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.