Comprehensive Understanding of the Structural Modulation Principles for Highly Efficient Photoelectrochemical/Photocatalytic Processes via X‐Ray Absorption Spectroscopy

Abstract Photoelectrochemical (PEC) and photocatalytic (PC) reactions offer promising pathways toward the efficient conversion of intermittent solar energy into high value‐added chemicals. To improve the conversion efficiency of solar energy, rational modulation of both light absorbers and surface sites is required for high sunlight‐harvesting capacity and reaction activity, yet depending on a comprehensive understanding of the reaction process. In this regard, X‐ray absorption spectroscopy (XAS) plays a critical role in probing the electronic and geometric structures of active materials in PEC and PC reactions, thereby revealing the related structure‒activity relationships and providing a guide toward further structural modulation. This review begins with the principles of PEC/PC reactions. The fundamental knowledge of XAS is then introduced, including the basics of X‐ray absorption near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS), general experimental methods, and the in situ XAS technique. Furthermore, we provide several examples to illustrate how XAS can be utilized to reveal the structure‒activity relationships in PEC/PC reactions (e.g., water splitting, CO 2 reduction, and N 2 fixation). Finally, major challenges and future opportunities in this exciting field are highlighted.