Relating Photoelectrochemistry and Wettability of Sputtered Cu- and N-Doped TiO₂ Thin Films via an Integrated Approach

We present here an integrated study of the photoelectrochemical and hydrophilic properties of sputtered TiO₂ thin films, enhanced by means of nitrogen (N) and copper (Cu) doping. We investigated the effect that doping has on both photoelectrochemical efficiency and surface properties by employing a variety of techniques spanning from impedance electrochemical impedance spectroscopy to static contact angle and atomic force microscope (AFM) force spectroscopy before and after UV irradiation through a comprehensive approach able to connect photelectrochemical and hydrophilic performance. Namely, Cu doping was observed to worsen TiO₂ photoelectrochemical efficiency, unlike N-doping, which instead improved it, whereas both doping enhanced the surface chemistry. Both doping resulted in anodic shift of the flat band potential and in an increase in the donor density with the occurrence of surface defects beneficial for the separation of charge carriers in N-TiO₂ on one side, and more recombination centers in Cu-TiO₂ on the other. On the other hand, macroscopic wettability characterization indicated that Cu-TiO₂ and N-TiO₂ had a much lower contact angle than TiO₂ (static contact angle ≈ 20 and 10° for Cu-doped and N-doped films, respectively, as compared to 50° in the bare film) and became superhydrophilic after UV irradiation; AFM corroborated the contact angle data, pointing out that the enhanced hydrophilicity in doped films can be ascribed to an alteration in the surface chemistry because of a greater number of surface defects, such as oxygen vacancies, acting as binding sites for water molecules.