Physico-chemical processes and kinetics of sunlight-induced hydrophobic ↔ superhydrophilic switching of transparent N-doped TiO₂ thin films.

Sunlight-responsive anatase N-doped TiO2 thin films undergoing reversible and switchable hydrophobic to superhydrophilic transition were synthesized by ultrasonic spray pyrolysis in a single step. Film thickness, roughness, surface morphology, crystal structure, composition, band gap, and wetting properties were studied using surface profilometry, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis spectroscopy, and water contact angle measurements, respectively. Surface X-ray photoelectron spectroscopy before and after sunlight irradiation revealed the major physicochemical process responsible for the hydrophobic → hydrophilic transition as surface hydroxylation and that responsible for hydrophilic → superhydrophilic transition as destruction of surface adsorbed organic species. The kinetic rates of the hydrophobic → superhydrophilic transition under sunlight and superhydrophilic → hydrophobic transition when kept under dark are found to be 0.215 min(-1) and 2.03 × 10(-4) min(-1), respectively.