A comprehensive review on the boosted effects of anion vacancy in the photocatalytic and photoelectrochemical water-splitting: Focus on oxygen vacancy

Abstract

An attractive solar energy-driven hydrogen production technology is the semiconductor-based photocatalytic (PC) and photoelectrochemical (PEC) water splitting. The advantages of the mentioned strategy are the high yield of storable H2 fuel via clean, sustainable, and eco-friendly energy sources and the decreasing global warming effects of fossil fuels. Several highly stable, inexpensive, earth-abundant, and highly catalytic active metal oxide photocatalysts are interested. Still, their application limitations are the intense photogenerated charge carrier’s (electron-hole (e/h)) pairs recombination, high overpotential, and slow surface reactions’ kinetics. Among strategies used to overcome these, introducing oxygen vacancy (OV) is one of the most influencing strategies for exploring photocatalytic activity via promoting light harvesting, better e/h separation, tuning electronic structure, and capability of reactive radicals’ generation. Here, we summarize the recent water-splitting progress by OV-mediated metal oxide photocatalysts by briefly introducing the defects and OV concepts, illustrating the introduced and controlled OV concentration strategies, and the OV formation mechanism. The review investigates the combined boosted OV and other factors' roles in light absorption, band-gap narrowing, conductivity, and e/h separation. Furthermore, it also evaluates the vital effects of surface and bulk defects in photocatalytic water splitting and OV in the interfacial heterojunction systems. Finally, this review proposes some incoming challenges and essential roles in synthesizing OV-enriched photocatalysts for PC/PEC water-splitting processes.