Advances and challenges in multiple S-scheme heterojunction photocatalysts

Photocatalytic technology has emerged as a promising solution for addressing global energy shortages and environmental pollution. Despite extensive research, the practical application of conventional single photocatalysts remains constrained by their narrow light absorption range, insufficient redox capacity, and rapid recombination rates of photogenerated electrons and holes, hindering their photocatalytic efficiency. To overcome these bottlenecks, heterojunction strategies integrating two or more semiconductor materials are a promising approach to enhance the performance of individual photocatalysts. S-scheme heterojunction photocatalysts have provided a breakthrough by enabling efficient separation of photogenerated carriers and strong redox capabilities. However, traditional S-scheme systems are still constrained by high contact resistance and sluggish charge transfer at the interface between the oxidation and reduction components. Recent advances in multiple S-scheme heterojunction photocatalysts with reduced contact resistance offer a feasible solution by incorporating more than two semiconductors to facilitate multidirectional charge transfer pathways and improved carrier dynamics through the synergistic interaction of multiple components. Despite their promising performance, a comprehensive understanding of the design, mechanisms, and practical applications of multiple S-scheme heterojunction systems remains lacking, presenting a significant knowledge gap in the field. This work addresses this gap by systematically investigating the structural design, transfer models, and interfacial challenges unique to multiple S-scheme heterojunctions. This review first discusses the evolution of heterojunction mechanisms from type-II and Z-scheme to S-scheme. Then, it focuses on the novel principles governing multiple S-scheme systems, including their charge dynamics and interfacial optimization strategies. Highlighting the recent advancements in multiple S-scheme heterojunctions, their efficacy in diverse applications such as organic pollutant degradation, H₂ generation, disinfection, and CO₂ reduction have been demonstrated. Moreover, this study identifies key barriers to their practical application, such as scalable synthesis, stability, and unclear mechanisms, while suggesting a future roadmap, including advanced in-situ characterization techniques and machine learning-driven material design for next-generation systems. By bridging the gap between fundamental research and practical applications, this review provides a direction for advancing high-efficiency photocatalytic systems to support global sustainability.