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In recent years, S-scheme oriented photocatalytic systems have shown tremendous potential and possibilities toward efficient conversion of solar energy to clear and sustainable chemical fuel owing to superior exciton separation and strong redox ability. This minireview summarizes the background history of the S-scheme mechanism along with a brief discussion of the type of band alignment and the journey from Z-scheme type to S-scheme systems. The review also elaborates the detailed operational mechanism and beneficial features of the S-scheme photocatalyst, including construction tactics, necessary conditions, band bending, and built-in electric field along with charge transfer dynamics. More importantly, we have described thoroughly the frequently used characterization techniques that demonstrate the interfacial charge transfer mechanism through S-scheme systems (XPS, in situ XPS, KPFM, and TRPL analysis) referring to the relevant reported literature with a brief computational interpretation. Additionally, the review highlights recent studies on the H2O2evolution reaction with particular emphasis on S-scheme-oriented systems. The conclusion and future prospective section focuses on associated challenges and insightful newly reported information for the research community to fully understand the chemistry of S-scheme-based charge flow and design, promising systems for achieving benchmark efficiency in the clean energy generation sector.

Step-scheme (S-scheme) heterojunctions comprising two semiconductors having two sets of charge carriers at different sites with an outstanding redox capability have emerged as a prospective tactic for H2O2production and antibiotic remediation. Herein, 0D/2D Fe2O3QD/B-g-C3N4(F-BN) was successfully fabricated via in situ nucleation of Fe2O3quantum dots (FQDs) over boron-doped g-C3N4(BCN) sheets for H2O2production and photo-Fenton amoxicillin (AMX) degradation. Empirical results demonstrate that the F-BN composite shows a superior catalytic activity compared to the parent material and the optimized 3F-BN attains the best activity for H2O2generation (729 μmol and solar-to-chemical conversion efficiency (SCC) of 0.12%) and photo-Fenton AMX degradation (93%) with a “k” of 0.0891 min–1, which is 3.34 and 7.01 times higher than those of the pristine materials. The outstanding activity could be attributed to effective separation and utilization of excitons through the S-scheme transfer pathway. Moreover, charge transfer through the S-scheme transfer corridor along with continuous Fe3+/Fe2+shuttling is responsible for the effective photo-Fenton activity. Additionally, the influence of the variation of experimental conditions is also studied in detail. The high photocurrent, lower EIS semicircle, and low PL intensity indicate effective separation efficiency of e–/h+in the 3F-BN material. Furthermore, the scavenging experiment and terephthalic acid (TA), nitro blue tetrazolium chloride (NBT), and EPR measurements not only evidence that the generated reactive oxygen species (•OH and •O2–) participated in photocatalytic activities but also validate the S-scheme charge-transfer mechanism which is further confirmed from in-situ XPS analysis.

Photocatalytic H2O2and NH3production are considered to be clean and encouraging technologies towards sustainable solar-to-chemical energy conversion. Exploiting the coupling of 0D–3D semiconductors, herein, we have developed a series of ZnCuInS–BiOI (ZCIS/BOI) bi-functional heterojunction composites for the first time with excellent morphological interface contact viaan in situreflux synthetic strategy. Additionally, the development of a heterojunction on the interface of two semiconductors with feasible band alignments, enriched defect sites and built-in-electric field play a pivotal role in the separation and transportation of charge carriers. The optimum Z-scheme heterojunction catalyst (ZCIS–BOI(1 : 1)) exhibits excellent activity towards H2O2and NH3generation, i.e., 2135.2 μmol h−1g−1(SSC = 0.24%) and 1610.5 μmol h−1g−1(SCC = 0.36%), respectively, with superb reusability (4 cycles). The observed enhancement in photocatalytic performance corresponds to superior photon absorption ability, feasible band alignment, greater exciton separation/transfer efficiency, and effective adsorption/activation. Furthermore, experiments investigating the influence of the sacrificial agent, pH, purging gas type, and scavenger were performed to determine the optimum conditions and mechanism. This research contributes the idea of constructing alloyed QD based Z-scheme heterojunctions with hierarchical defect sites towards robust photocatalytic hydrogen peroxide and ammonia production under ambient conditions.