Electronic Supporting Information Title: The role of the CeO2/BiVO4 interface in optimized Fe-Ce oxide coatings for solar fuels photoanodes

X-ray fluorescence (XRF) spectroscopy was measured on duplicate libraries deposited on Si (without BiVO4) due to the overlap between the Ce Lα line (4.839 kV) from the coating and the V Kα line (4.952 kV) from the BiVO4. The x-ray beam was approximately 2 mm in diameter. Since the XRF sampling depth far exceeds the film thickness, XRF counts for each element were assumed to be proportional to the number of corresponding atoms present in the coating, and the sensitivity factor for each element was calibrated using commercial XRF calibration standards (MicromatterTM). We note that the XRF calibration may result in overestimation of the Fe-Ce loading because the sputtered coatings are on the order of 1000 times thinner than the calibration standards and thickness-dependent attenuation of XRF signals was not accounted for in data processing. The bulk Fe2O3 and CeO2 densities were used to calculate the nominal thickness of the coatings.