A Comprehensive Review of the Role of Overlayers in Photocatalytic Overall Water Splitting and Related Reactions

Photocatalytic overall water splitting using cocatalyst-modified photocatalysts faces a significant challenge to achieve highly efficient performance in pure water, as the back reaction─where the evolved H₂ and O₂ recombine back into H₂O─is also catalyzed by the cocatalysts. Overlayers on modified photocatalysts have been reported to prevent the back reaction and significantly enhance the overall water splitting activity. This review provides a comprehensive overview of the use of overlayers in suppressing the back reaction in photocatalytic overall water splitting and their multifunctional roles in facilitating charge separation and transport, and stabilizing cocatalysts and photocatalysts. The application of overlayers is also explored in suppressing the back reactions in other photocatalytic reactions and systems, including Z-scheme systems, electrochemical and photoelectrochemical water splitting systems, pollutant degradation reactions, and photocatalytic CO₂ conversion. Furthermore, this review addresses the challenges facing overlayers, particularly their durability during photoreaction, and suggests future research areas. Overall, the knowledge gained to date emphasizes the potential of overlayers to enhance the efficiency and stability of cocatalyst-modified photocatalysts in the field of photocatalysis.