Single‐atom catalysts in photocatalysis: unveiling the essential roles of water

Abstract This article systematically reviewed the applications of single‐atom catalysts (SACs) in the domain of photocatalytic reactions, with a particular emphasis on the indispensable role of H 2 O in these processes. SACs, due to their distinct active sites and superior catalytic efficacy, found their applications in the fields of energy conversion and environmental protection. The review elaborated on the potential carriers, preparation methods, and characterization techniques for single‐atom photocatalysts. Subsequently, the article provided an in‐depth explanation of the crucial role of H 2 O in photocatalytic reactions, serving as an important green solvent and an oxygen/proton source. The adsorption of water could also change the surface energy structure and charge distribution of the photocatalyst. Conversely, the presence of H 2 O might also inhibit the target reaction. Additionally, the distinct roles of water in both liquid and gas phases were discussed. Furthermore, the review systematically summarized the applications of single‐atom photocatalysts in H 2 generation, CO 2 reduction, N 2 fixation, H 2 O 2 production, and environmental remediation. It delved into the mechanisms by which water molecules participated in photocatalytic processes and their interactions with competing pathways, thereby revealing the complexity and critical importance of water in photocatalytic reactions. Finally, the article discusses the opportunities and challenges of SACs in photocatalytic reactions with H 2 O. This article provides a comprehensive perspective for understanding the role of SACs in water‐involved photocatalytic reactions.