Kinetics and Physicochemical Process of Photoinduced Hydrophobic ↔ Superhydrophilic Switching of Pristine and N-doped TiO₂ Nanotube Arrays

Visible-light-active near superhydrophobic ↔ superhydrophilic switching vertically aligned anatase TiO 2 and TiO 1.84 N 0.14 nanotube array thin films were synthesized by anodizing Ti foils in ethylene glycol + NH 4 F + water electrolyte-containing urea as nitrogen source. Compared with pristine TiO 2, N-doped TiO 2 nanotube arrays underwent hydrophobic to superhydrophilic transition faster under sunlight. The UV-light-induced hydrophobic-to-superhydrophilic conversion rates in the hydrophobic and hydrophilic regimes for pristine and N-doped samples are 0.2178 min –1, 0.9534 min –1 and 0.5185 min –1, 1.376 min –1 respectively. The corresponding sunlight induced conversion rates for the pristine and N-doped samples are 0.015 min –1, 0.061 min –1 and 0.013 min –1, 0.152 min –1 respectively. The reverse hydrophobic conversion, when kept in dark, was found to follow a single curve with rates 8.94 × 10 –5 and 4.06 × 10 –5 min –1 for the pristine and doped samples respectively. The major physicochemical process behind the hydrophobic ↔ superhydrophilic transition is found to be decomposition of surface carbonaceous species and their adsorption, respectively, by surface X-ray photoelectron spectroscopy before and after photoirradiation.