Catalyst Deposition on Photoanodes: The Roles of Intrinsic Catalytic Activity, Catalyst Electrical Conductivity, and Semiconductor Morphology

Semiconducting oxide photoanodes are used to drive the oxygen evolution reaction (OER) in water-splitting systems. The highest-performing systems use nanostructured semiconductors coated with water-oxidation catalysts. Despite much work, the design principles governing the integration of catalysts with semiconductors are poorly understood. Using hematite as a model system, we show how semiconductor morphology and electrical conductivity of the catalyst affect the system photoresponse. Electrically conductive catalysts can introduce substantial "shunt" recombination currents if they contact both the semiconductor surface and the underlying conducting-glass substrate, leading to poor performance. This recombination can be largely eliminated by using pinhole-free semiconductors, using selective photoassisted electrodeposition of thin catalyst layers on the semiconductor surface, using electrically insulating catalyst layers, or adding an intermediate insulating oxide layer. The results of this study are used to clarify the mechanisms behind several important results reported in the literature.