Effect of Hydrogen Treatment on Anatase TiO2 Nanotube Arrays for Photoelectrochemical Water Splitting

Hydrogen (H2) treatment using a two-step TiO2 nanotube (TONT) film was performed under various annealing temperatures from 350 .deg. C to 550 .deg. C and significantly influenced the extent of hydrogen treatment in the film. Compared with pure TONT films, the hydrogen-treated TONT (H:TONT) film showed substantial improvement of material features from structural, optical and electronic aspects. In particular, the extent of enhancement was remarkable with increasing annealing temperature. Light absorption by the H:TONT film extended toward the visible region, which was attributable to the formation of sub-band-gap states between the conduction and valence bands, resulting from oxygen vacancies due to the H2 treatment. This increased donor concentration about 1.5 times higher and improved electrical conductivity of the TONT films. Based on these analyses and results, photoelectrochemical (PEC) performance was evaluated and showed that the H:TONT film prepared at 550 .deg. C exhibited optimal PEC performance. Approximately twice higher photocurrent density of 0.967 mA/cm2 at 0.32 V vs. NHE was achieved for the H:TONT film (550 .deg. C) versus 0.43 mA/cm2 for the pure TONT film. Moreover, the solar-to-hydrogen efficiency (STH, η) of the H:TONT film was 0.95%, whereas a 0.52% STH efficiency was acquired for the TONT film. These results demonstrate that hydrogen treatment of TONT film is a simple and effective tool to enhance PEC performance with modifying the properties of the original material