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 x 10(-4) min(-1), respectively.