Temperature-dependent differences in wettability and photocatalysis of TiO2 nanotube arrays thin films

TiO2 nanotubes arrays (TiO2-NTAs) layers were successfully formed by electrochemical anodization on a titanium (Ti) surface in fluorine-containing electrolytes under different processing voltages and annealing temperatures. The formed TiO2-NTAs layers annealed at different temperatures were compared in terms of their morphologies, structures and band gaps. Their wetting abilities, photo-induced hydrophilicities and photocatalytic performances were also evaluated. It was found the sample anodized at 40 V for 2 h have the most uniform surface morphology. The TiO2-NTAs structures annealed at 600 °C for 2 h in air show multiphase crystallinity and have the narrowest band gap (2.89 eV). These structures have noticeable nanowires on the top of the nanotubes. Their photocatalytic activity, tested by degradation of aqueous methylene orange solutions under UV light illumination for 2 h, is the highest among all samples (degradation efficiency was 89.05%). Specimens annealed at 200, 400 and 800 °C show light-induced hydrophilicity. However, small contact angle change was observed when samples were annealed at 600 °C.