Self-healing of oxygen vacancies and phase transition-induced built-in electric field regulate H₂ and H₂O₂ production

Oxygen vacancies in photocatalysts are an important parameter for improving their catalytic activities. However, this study found that oxygen vacancies do not always help with photocatalytic properties. MXene nanosheets were annealed at different temperatures to prepare nanosheets of titanium dioxide (TiO₂) with anatase or rutile phase. Photo-induced self-healing of oxygen vacancies in MXene nanosheets annealed at 400 °C promoted the efficient photocatalytic evolution of hydrogen (H₂) with a ∼ 6.8 mmol/g optimized yield. In contrast, the photo-induced increase of oxygen vacancies in MXene nanosheets annealed at 500 °C corresponded to an optimized ∼ 1.8 mmol/g/h yield of hydrogen peroxide (H₂O₂). Spectroscopic and electrochemical methods revealed that the built-in electric field between the anatase and rutile TiO₂ facilitated the fast charge separation from the anatase to rutile TiO₂. The photo-induced self-healing of oxygen vacancies and the phase transition-induced built-in electric field regulated the photocatalytic selectivity of H₂ and H₂O₂ production.