S-Scheme Shapes Heterojunction Photocatalysis

ConspectusA persistent obstacle in heterogeneous photocatalysis is the rapid recombination of photogenerated electrons and holes, a consequence of the strong Coulombic attraction between carriers within conventional semiconductors. This intrinsic limitation significantly constrains the efficiency of solar-to-chemical conversion processes. Heterojunction engineering has therefore become a central strategy for promoting charge separation by coupling semiconductors with complementary electronic structures. Such systems typically outperform their single-component analogues because the interfacial electronic configuration promotes directional charge migration and suppresses bulk recombination losses.Within this context, S-scheme heterojunctions (SH) offer a mechanistically robust framework that reconciles efficient carrier separation with strong redox capability. An S-scheme couples a reduction photocatalyst (RP) and an oxidation photocatalyst (OP) in a staggered configuration. Under illumination, electrons in the OP selectively recombine with holes in the RP, while the high-energy electrons in the RP and high-energy holes in the OP are spatially retained and directed to catalytic sites. This selective recombination preserves redox power, enhances charge utilization, and accelerates surface reactions.Since introducing the S-scheme concept in 2019 with the WO₃/g-C₃N₄ system supported by in situ irradiated X-ray photoelectron spectroscopy (ISIXPS)─we have expanded its material scope across multiple dimensional architectures, including perovskite materials, semiconducting quantum dots (QDs), conjugated polymers (CP), metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs). To validate the S-scheme mechanism, elucidate charge transfer dynamics, and resolve reaction mechanisms, we have employed an array of state-of-the-art characterization techniques, such as light-irradiated Kelvin probe force microscopy (KPFM), in situ electron paramagnetic resonance (EPR), in situ X-ray absorption spectroscopy (XAS), and femtosecond-transient absorption spectroscopy (fs-TAS).Our most recent efforts focus on composition tuning, defect modulation, and interfacial bonding engineering to optimize the separation and lifetime of photogenerated carriers. Through these strategies, we aim to reinforce the internal electric field, regulate band bending, and precisely control charge flow pathways, ultimately maximizing photocatalytic efficiency. This Account provides a concise yet comprehensive overview of the evolution of SH, with emphasis on the design principles and advanced characterization techniques developed and adopted by our group. We summarize key strategies for engineering SH tailored for enhanced charge carrier separation and highlight their applications in major photocatalytic reactions. Finally, we outline promising future directions for the field.