Designing tandem S-scheme photo-catalytic systems: Mechanistic insights, characterization techniques, and applications

Tandem S-scheme heterojunctions have emerged as a highly promising innovation in photocatalysis, offering an effective solution for environmental remediation. Unlike traditional Z-scheme or type-II photocatalysts, the S-scheme architecture selectively retains high-energy photocarriers that actively participate in redox reactions. This unique mechanism enhances charge separation, strengthens internal electric fields, and enhance light absorption. However, the single junction of S-scheme suffers from low quantum efficiency. Therefore, engineering a multicomponent system with S-scheme effectively improve the photocatalytic properties. Tandem S-scheme systems consist of multiple semiconductors/materials with staggered energy band positions to create a stepwise or directional charge transferal mechanism. This stepwise potential gradient is responsible for more enhanced charge separation, light absorption, redox ability, stability, and overall photocatalytic activity. This article provides an in-depth overview of the principles governing tandem S-scheme heterojunctions, discussing the design of tandem S-scheme heterojunctions through semiconductor pairing, co-catalyst addition, and mediator inclusion for maximum charge mobility and minimum recombination. The various synthesis pathways are explored along with the kinetics and thermodynamics of tandem S-scheme heterojunction. A range of advanced characterization tools, including density functional theory (DFT) simulations, in-situ X-ray photoelectron spectroscopy (XPS), transient absorption spectroscopy (TAS), photoluminescence (PL), and electrochemical impedance spectroscopy (EIS) studies are discussed, which together offer valuable insight into electronic behaviours and interfacial dynamics. Applications of these heterojunctions are discussed across major domains such as carbon dioxide reduction, H₂ evolution, and degradation of organic pollutants. While the potential is clear, challenges such as complex synthesis procedures, material stability, and scalability still need to be addressed. To overcome the limitations, the article suggests future research paths. Overall, tandem S-scheme heterojunctions stand out as an excellent approach for building efficient and sustainable photocatalytic technologies.