Influence of Sub-Band-Gap States on Light Induced Long-Lasting Super-Hydrophilic Behavior of TiO2

The photo-induced superhydrophilic behavior of nanocolumnar TiO2 layers was investigated by means of contact angle measurements and electrochemical impedance spectroscopy (EIS). Two types of layers were compared: the first type of layers showed long-lasting superhydrophilic behavior after UV exposure (even during storage in the dark), whereas in the second type of layers, no surface wettability conversion was observed. EIS measurements showed that the two layers were characterized by a high dielectric constant that still increased upon light exposure. A main difference between the two types of sample concerned the distribution of the trap states in the gap before UV exposure. Moreover, after UV exposure, new traps were generated in the gap but only for the superhydrophilic layer. These results are discussed, taking into account the well-admitted photoinduced hydrophilic mechanism, involving hole trapping at the surface of the layer and generation of oxygen vacancies. It clearly appeared that hole-trapping at the surface is determined by long-lasting electron-trapping in the bulk of the oxide layer, enabling efficient and long-lasting charge separation. A model is suggested for estimating the contact angle variation using the electrowetting theory.