Lattice orientation-dependent circular photogalvanic effect in hydrogenated TiO2

Abstract

TiO2 is a widely utilized semiconductor material. This study demonstrates that hydrogenation can induce spin–orbit coupling (SOC) in TiO2, thereby generating a circular photogalvanic effect (CPGE). A significant CPGE is observed in hydrogenated TiO2 (H:TiO2) under illumination at wavelengths of 450, 532, and 780 nm. The crystal orientation is found to significantly influence the CPGE in H:TiO2. Regardless of hydrogenation duration or illumination wavelength, the H:TiO2 (001) exhibits the strongest CPGE. This effect is attributed to the larger lattice interstices along the [001] direction, which facilitate faster hydrogen diffusion. For H:TiO2 subjected to short-term hydrogenation, the role of hydrogen predominates. However, for H:TiO2 subjected to prolonged hydrogenation, oxygen vacancies are inevitably introduced, which alters the relative strength order of the CPGE among the H:TiO2(100), (110), and (111). X-ray photoelectron spectroscopy measurements reveal that a larger shift in the valence band maximum corresponds to a stronger CPGE. This study demonstrates that easily fabricated H:TiO2 exhibits pronounced SOC and can serve as a promising platform for both fundamental research and practical applications.