Ambient-Pressure Near-Edge X-ray Absorption Fine Structure Study of the Photothermal Water Splitting Process on the Cu:CeO2 Nanostructure Surface

The modification of the electronic properties of CeO2 thin films through Cu doping is a promising approach to enhancing their performance in photoinduced water splitting (WS). In this study, undoped and Cu-doped CeO2 films of 5 nm thickness with 5 and 11% Cu atomic concentrations were characterized under ambient pressure conditions by near-edge X-ray absorption fine structure (NEXAFS) to provide insights into the film modification and the water splitting process during exposure to water and to laser light at different temperatures. The analysis of NEXAFS data, acquired at the Ce M5 and Cu L3 edges, reveals temperature-dependent changes in the oxidation states of Ce and Cu. Notably, a temperature-dependent evolution of the Ce3+ concentration is observed during water exposure, accompanied by spectral changes consistent with a partial reduction of Cu2+ to Cu1+ in the film with the highest dopant concentration. The ambient-pressure NEXAFS measurements highlight the role of Cu dopant ions in modifying the electronic structure of CeO2 and the catalytic response in the presence of water and illumination, enabling more efficient hydrogen production. In parallel, micro gas chromatography analysis revealed a marked increase in hydrogen production from the 11% Cu-doped CeO2 film compared to the undoped CeO2 and the 5% Cu-doped films. These results provide crucial insights into the structure–function relationship in Cu-doped CeO2, offering pathways to optimize the design of materials for hydrogen production and related applications.