Revealing Effect of Surface Hydroxyl Variation on Charge Spatiotemporal Dynamics in Photocatalysis

Abstract

Oxide semiconductor photocatalysts are widely used for solar energy conversion, and the abundant intrinsic hydroxyl groups as defect sites on their surfaces play a key role in photocatalytic performance. However, the nature of surface hydroxyl-related defect states and their effect on the behavior of photogenerated charges, especially if targeted for charge separation, and whether the electrons and holes facing these hydroxyl sites in the same temporal and spatial ranges compete and conflict are unknown. Understanding these may help us to reasonably control defect-induced charge separation. Here, we perform an energy-, time-, and space-resolved study to reveal the effect of surface hydroxyl variation of BiOCl photocatalyst particles on photogenerated charge dynamics. We reveal that picosecond-level trapping and millisecond-level stability of holes initiated in electron-occupied states induced by hydroxyl sites are the greatest contributor to hole separation but the culprits that hinder electron utilization. Eliminating them can reduce unnecessary recombination and introduce unoccupied states, resulting in electron trapping and stabilization on the surface. Guided by these findings, selectively removing and holding hydroxyl sites on specific crystal planes can achieve the effective spatial separation of electrons and holes and show the associated enhanced reaction performance, especially in photocatalytic reduction. Operando imaging indicates that the surface hydroxyl-related charge-transfer sites align with reaction sites. This study reveals the critical role of surface defect states related to surface hydroxyl variation on charge separation and transport, which helps to understand the pivotal role of surface states in the entire photocatalytic process, also providing a valuable reference for most oxide semiconductor photocatalysts with intrinsic surface hydroxyl groups.