Light induced formation of a surface heterojunction in photocharged CuWO₄ photoanodes

Photocharging has recently shown the ability to significantly improve the performance of several metal oxide photoanodes, similar to the enhancements achieved with co-catalysts and passivation overlayers. Herein, we demonstrate the effect of photocharging on CuWO4 photoanodes for the first time, with prolonged AM 1.5 illumination under open-circuit conditions. The photocharging treatment on CuWO4 samples doubled the photocurrent obtained at 1.23 VRHE. This enhancement is attributed to the light induced formation of a surface bound copper complex with the solution anion species in the electrolyte. This thin semiconducting copper borate layer forms a heterojunction with the CuWO4, improving the charge separation near the surface and thus suppressing the recombination of charge carriers in the space charge region. The striking similarities in photocharging of different metal oxide semiconductors highlights that the metal oxide semiconductor-electrolyte interface is more complex than previously understood. The formation of this time-dependent light induced surface layer should therefore be considered in all experimental studies on photo-electrochemistry with metal oxide semiconductor photoanodes.