Vanadium-based oxyhalide photocatalysts for visible-light-driven Z-scheme water splitting: advancing conduction band engineering

Abstract

Z-scheme water-splitting systems have garnered significant attention as a promising technology for producing hydrogen cleanly from water using solar energy. Layered oxyhalides have emerged as efficient oxygen-evolving photocatalysts (OEPs) for these splitting systems. However, the conduction band minimum (CBM) of these oxyhalides is excessively negative compared to the reduction potential of electron mediators. This limitation underscores the need for novel conduction band engineering approaches to narrow the band gap and enable the utilization of visible light across a broader spectrum. This study introduces V-based oxyhalides as OEPs in the Z-scheme system. The physicochemical properties and photocatalytic activities of the three lead-vanadium-based oxyhalides, Pb₁₄(VO₄)₂O₉Cl₄, Pb₅(VO₄)₃Cl, and PbVO₃Cl, were comprehensively characterized. The CBMs of these materials were found to be more positive than those of conventional oxyhalide photocatalysts and displayed significant variation. PbVO₃Cl exhibited the most positive CBM and the smallest band gap, enabling visible light absorption up to approximately 550 nm. Madelung site potential analysis on each vanadium cation highlighted the reasons for the significant difference in CBM positions among the Pb-V oxyhalides. Remarkably, PbVO₃Cl exhibited oxygen evolution activity under visible light irradiation, marking the first instance of a V-based oxyhalide as an OEP in a Z-scheme system. This exceptional activity of PbVO₃Cl was attributed to the superior carrier transport properties, owing to the interconnected VO₅ units, as revealed by time-resolved microwave conductivity (TRMC) measurements, as well as the extended visible light absorption.