Time-Resolved Spectroelectrochemical Observation of Overlayer-Induced Charge Carrier Dynamics in Water Photooxidation

Surface and interface engineering is essential for constructing efficient and stable photoelectrodes for photoelectrochemical (PEC) solar fuel production. Despite the recent advances in photoelectrode optimization for the practical application, the corresponding interfacial reaction mechanism has not been elucidated owing to a lack of suitable measurements at the semiconductor-electrolyte interface (SEI). Herein, the key factor for an interfacial reaction in a model system (WO₃ photoanode coated with amorphous TiO₂ overlayers) is elucidated using operando spectroelectrochemistry. The thin TiO₂ overlayers are shown to enhance n-type semiconductor characteristics and heal the excessive oxygen vacancies on the WO₃ photoanode surface, which suggests reduced bulk and surface charge carrier recombination and 1.5-fold increase in Faradaic efficiency. Operando transient absorption spectroscopy measurements reveal that the overlayers accelerate the transfer of photogenerated electrons from the electrode to the external circuit and increase the population of trapped holes by promoting band bending, which reveals that the kinetic connection between ultrafast phenomena and real water oxidation reaction. Thus, our work elucidates band bending in the space-charge region as a key factor and provides a major strategy for designing surface-modified PEC devices.