Operando Imaging of Charge Transfer and Photoelectrochemical Water Oxidation at Defect-Rich Grain Boundaries

Abstract The microstructural properties of grain boundaries (GBs) and defects in semiconductor materials critically influence solar energy conversion. Yet, the precise role of GBs in charge separation and transport remains poorly understood and widely debated, particularly under reaction conditions. A key knowledge gap persists in linking charge separation and transfer to surface reactions at the GB-liquid interface, necessitating detailed operando nanoscale investigations. Here, we establish nanoscale, operando imaging of charge transfer and catalytic activity of a working BiVO4 photoanode using operando atomic force microscopy–scanning electrochemical microscopy (AFM-SECM) under illumination. We found distinct GB behaviors, with defect-rich GBs emerging as locally active regions for charge transfer and O2 generation, whereas other GBs impede charge transfer and serve as recombination sites. Specially for defect-rich GBs, operando electrochemical imaging and transient spectroscopy reveal that oxygen vacancies within GB interiors and on grain surfaces act synergistically on distinct time scales to trap photogenerated electrons and extend hole lifetimes into the millisecond range, thereby enhancing water oxidation activity locally at GBs. These insights underscore the pivotal role of defect-rich GBs in sustaining long-lived charge carriers, thereby highlighting the importance of modulating GB characteristics for efficient solar energy conversion.