Operando X-ray absorption spectroscopy of WO3 photoanodes

In this work we demonstrate the feasibility of hard X-rays operando XAS in photoelectrochemistry. WO3, one of the most studied photoanodes for water splitting and for environmental remediation, is here studied at the W LIII-edge. This guarantees the direct observation of the W 5d band. The material, that is preliminary fully characterized in terms of its photoelectrochemical features, is studied in a three-electrode spectroelectrochemical cell, while X-ray absorption is measured in the X-ray absorption near edge structure (XANES) region. The recording of differential spectra and the monitoring of X-ray absorption at constant energy are used to compensate for the little XANES differences expected in the dark and under visible light illumination, which otherwise risks to be masked by experimental errors and/or by signal manipulation for data analysis. The results point to the filling of the W t2g orbitals under illumination, that is followed by a structural rearrangement that compensates for the accumulation of electrons in the conduction band under open circuit (OC) conditions.