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

[Image: see text] 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-Fe(2)O(3), in contact with one of the fastest known water oxidation catalysts, Ni(0.8)Fe(0.2)O(x), by directly measuring/controlling the current and/or voltage at the Ni(0.8)Fe(0.2)O(x) catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-Fe(2)O(3) directly transfer to the catalyst film over a wide range of conditions and that the Ni(0.8)Fe(0.2)O(x) is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-Fe(2)O(3). The Ni(0.8)Fe(0.2)O(x) 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-Fe(2)O(3)|Ni(0.8)Fe(0.2)O(x) interface is significantly decreased by the oxidation of Ni(2+) to Ni(3+) and the associated increase in the Ni(0.8)Fe(0.2)O(x) electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement.