Spatial Control of Oxygen Vacancy Concentration in Monoclinic WO₃ Photoanodes for Enhanced Solar Water Splitting

Oxygen vacancies (OVs) are a mixed blessing for the photoelectrochemical (PEC) water oxidation performance of monoclinic tungsten trioxide (m-WO₃) photoanodes. Although it is widely accepted that a moderate concentration of OVs is beneficial for the PEC performance of the m-WO₃ photoanodes, this argument assumes a uniform distribution of OVs throughout the m-WO₃ crystal. In this case, only the overall concentration of OVs needs to be considered. However, the spatial non-uniformity of OV defects in m-WO₃ photoanodes has not been thoroughly examined. In this study, by employing a m-WO₃ nanorod array as a model photoanode, the aim is to show that a higher OV concentration near the surface of m-WO₃ compared to that in the bulk is advantageous for the PEC performances of this material. In addition, a laser-assisted defect control (LADC) process is employed to manipulate the spatial distribution of OVs in the m-WO₃ photoanodes to achieve enhanced PEC performances. Moreover, a one-step laser deposition process is introduced to obtain an ultrathin FeNi oxygen evolution catalyst overlayer on the defect-controlled m-WO₃ photoanodes, further improving PEC performance, photostability, and Faradaic efficiency.