Reduced Ti–Nb–O Nanotube Photoanode with Bulk-Phase Nb Doping and Surface Oxygen Vacancy Engineering for Enhanced Photoelectrochemical Water Splitting

Improving optical absorption and photogenerated electron–hole separation–transport performances of TiO2-based nanostructures are considered as key strategies for realizing high-efficiency photoelectrochemical (PEC) water splitting. In this work, we successfully prepared VO/Ti–Nb–O nanotubes through bulk-phase Nb doping with surface electrochemical reduction-introduced oxygen vacancies for an enhanced PEC performance. Microstructure analysis combined with density functional theory calculation revealed that synergistic Nb doping with oxygen vacancy self-doping could drive the Fermi level shift toward the conduction band and introduce shallow donor energy levels below the conduction band. This could not only improve electrical conductivity of the reduced Ti-based system but also enhance the transport efficiency of photogenerated carriers. Compared with that of pristine TiO2 (0.38 mA/cm2), the photocurrent density of the VO/Ti–Nb–O increased to 1.72 mA/cm2. The solar-to-hydrogen efficiency was 0.67%, which was 2.8 times higher than that of intrinsic TiO2. This work introduced oxygen vacancies into Ti–Nb–O nanotubes by one-step facile electrochemical reduction, which may provide a potential synergistic strategy for developing new-type and high-efficiency Ti-based nanostructure photoanodes for boosted PEC water splitting.