Impact of Co–Fe Overlayers on Charge Carrier Dynamics at WO₃/BiVO₄ Heterojunctions: A Picosecond-to-Second Spectroscopic Analysis

Surface modification with CoFe-based overlayers has been widely studied to improve the performance of WO3/BiVO4 photoanodes for photoelectrochemical water oxidation because such overlayers can increase the photocurrent and shift the onset potential to more favorable values. Herein, we present a transient absorption spectroscopic analysis of WO3/BiVO4 photoanodes coated with cobalt iron oxide or cobalt iron Prussian blue overlayers, designed to establish the underlying mechanisms for these enhancements on the picosecond-to-second time scale. The data reveal that the overlayer suppresses recombination of trapped holes in BiVO4, with free and trapped electrons, and accepts photogenerated holes. These results show that the observed boost in efficiency for water oxidation can be explained by the dual role of the overlayer in inhibiting charge recombination and enhancing charge extraction.