Direct in Situ Measurement of Charge Transfer Processes During Photoelectrochemical Water Oxidation on Catalyzed Hematite.

Electrocatalysts improve the efficiency of light-absorbing semiconductor photoanodes driving the oxygen evolution reaction, but the precise function(s) of the electrocatalysts remains unclear. We directly measure, for the first time, the interface carrier transport properties of a prototypical visible-light-absorbing semiconductor, α-FeO, in contact with one of the fastest known water oxidation catalysts, NiFeO , by directly measuring/controlling the current and/or voltage at the NiFeO catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-FeO directly transfer to the catalyst film over a wide range of conditions and that the NiFeO is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-FeO. The NiFeO therefore acts as both a hole-collecting contact and a catalyst for the photoelectrochemical water oxidation process. Separate measurements show that the illuminated junction photovoltage across the α-FeO|NiFeO interface is significantly decreased by the oxidation of Ni to Ni and the associated increase in the NiFeO electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement.