Defect Engineering and Surface Polarization of TiO₂ Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution

The relatively low photo-conversion efficiencies of semiconductors greatly restrict their real-world practices toward photoelectrochemical water splitting. In this work, we demonstrate the fabrication of TiO_2-x nanorod arrays enriched with oxygen defects and surface-polarized hydroxyl groups by a facile surface reduction method. The oxygen defects located in the bulk/surface of TiO_2-x enable fast charge transport and act as catalytically active sites to accelerate the water oxidation kinetics. Meanwhile, the hydroxyl groups could establish a surface electric field by polarization, for efficient charge separation. The as-optimized TiO_2-x nanorod photoanode achieves a high photocurrent density of 2.62 mA cm⁻² without any cocatalyst loading at 1.23 V_RHE under 100 mW cm⁻², which is almost double that of the bare TiO₂ counterpart. Notably, the surface charge separation and injection efficiency of the TiO_2-x photoanode reach as high as 80% and 97% at 1.23 V_RHE, respectively, and the maximum incident photon-to-current efficiency reaches 90% at 400 nm. This work provides a new surface treatment strategy for the development of high-performance photoanodes in photoelectrochemical water splitting.