Direct Observation of Photoinduced Higher Oxidation States at a Semiconductor/Electrocatalyst Junction

Photoelectrochemical (PEC) water splitting devices using semiconductors and electrocatalysts rely on heterogeneous interfaces that drive charge separation, thus determining potential gradients that dictate the reaction efficiency. The PEC potential of the electrocatalyst depends on the chemical oxidation state of forming elements, which may strongly vary under the photoinduced charge flow. However, element-sensitive, real-time measurements of the oxidation state of the electrocatalyst are not generally possible using conventional X-ray absorption techniques. Here, we show that fixed-energy X-ray absorption voltammetry and chronoamperometry, which measure the X-ray absorption coefficient variations along with photocurrent, can follow in real time the redox kinetics of electrocatalysts. To demonstrate the validity, we investigate hematite (alpha-Fe2O3) photoanodes covered with a nickel hydroxide electrocatalyst and show that it is fully oxidized by photogenerated holes to nickel oxyhydroxide with Ni reaching a higher oxidation state (Ni-IV) than that observed under electrocatalytic oxygen evolution in dark conditions. Highly oxidized Ni results from charge accumulation in the overlayer and can be observed only in the case of thick layers (with low PEC performance). On the other hand, the average oxidation state of Ni reaches lower values, under operative conditions, for very thin layers, resulting in high PEC activity. We complete our study by presenting PEC activity and impedance spectroscopy analysis using different thicknesses of the electrocatalyst, thus giving a detailed picture of the multiple and complex charge transfer processes occurring at a semiconductor/electrocatalyst junction.