Hierarchical S-scheme heterojunction systems: A comprehensive review on outstanding photocatalysts

Hierarchical heterojunction systems have recently exhibited promising potential for environmental remediation and fuel generation due to their wide photoresponse, higher charge separation, large surface area, and powerful redox capacity. Moreover, the construction of hierarchical nanostructures offers control over the charge carriers' kinetics and surface reaction kinetics for different photocatalytic reactions. Herein, the basic principles of heterogeneous photocatalysts as well as the advantages and limitations of hierarchical nanostructures for photocatalytic applications are first discussed. Subsequently, a state-of-art description of different types of heterojunction systems, their operational mechanisms, and critical conditions are introduced to promote the separation and photocatalytic performance. In addition, this review offers a compressive and critical review of design approaches of hierarchical S-scheme systems, and their applications in pollutant removal, Cr (VI) reduction, CO₂ conversion, and H₂ evolution. Finally, the main bottlenecks and outlooks of hierarchical systems are discussed to highlight their brilliant scope in the future. This review offers novel insights and effective solutions to design novel hierarchical S-scheme photocatalysts with well-defined morphology and remarkable photocatalytic performances to promote photocatalytic efficiency for diverse applications.