Blue TiO nanotube arrays as semimetallic materials with enhanced photoelectrochemical activity towards water splitting.

In the past years there has been a great interest in self-doped TiO nanotubes (blue TiO nanotubes) compared to undoped ones owing to their high carrier density and conductivity. In this study, blue TiO nanotubes are investigated as photoanode materials for photoelectrochemical water splitting. Blue TiO nanotubes were fabricated with enhanced photoresponse behavior through electrochemical cathodic polarization on undoped and annealed TiO nanotubes. The annealing temperature of undoped TiO nanotubes was tuned before cathodic polarization, revealing that annealing at 500 °C improved the photoresponse of the nanotubes significantly. Further optimization of the blue TiO nanotubes was achieved by adjusting the cathodic polarization parameters. Blue TiO nanotubes obtained at the potential of -1.4 V (vs. SCE) with a duration of 10 min exhibited twice more photocurrent response (0.39 mA cm) compared to the undoped TiO nanotube arrays (0.19 mA cm). Oxygen vacancies formed through the cathodic polarization decreased charge recombination and enhanced charge transfer rate; therefore, a high photoelectrochemical activity under visible light irradiation could be achieved.