Oxygen vacancies in metal oxides: recent progress towards advanced catalyst design

Energy crisis and environmental problems urgently drive the proposal of new strategies to improve human wellbeing and assist sustainable development. To this end, scientists have explored many metal oxides-based photocatalysts with high stability, low cost, earth abundance, and potentially high catalytic activity relevant for key applications such as H2O splitting, CO2 reduction, N-2 fixation, and advanced oxidation of pollutants. In these metal oxides, oxygen vacancies (OVs) are ubiquitous and intrinsic defects with pronounced impacts on the physicochemical properties of the catalysts, which may open new opportunities for obtaining efficient metal oxides. The thorough understanding of the structural and electronic nature of OVs is necessary to determine how they serve as catalytically active sites. In this review, we summarize the origin of OVs, the strategies to introduce OVs, as well as the fundamental structure-activity relationships to relate these crystal defects to catalyst properties including light absorption, charge separation, etc. We emphasize the mechanism of OVs formation and their effects on the intrinsic catalytic characteristics of the metal oxides. We also present some multicomponent catalytic platforms where OVs contribute to catalysis via synergy. Finally, opportunities and challenges on engineering defects in photocatalysts are summarized to highlight the future directions of this research field.