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 TiO2 and TiO(1.84)N0(.14) nanotube array thin films were synthesized by anodizing Ti foils in ethylene glycol + NH4F + water electrolyte-containing urea as nitrogen source. Compared with pristine TiO2, Ndoped TiO2 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 x 10(-5) and 4.06 x 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.