A review on Type-I, Type-II, Z and S-scheme mechanisms involved in photocatalysis of inorganic and organic pollutant

This review evaluates the evolution from Type-I/II to advanced Z-scheme and S-scheme architectures for the remediation of toxic Cr(VI) and industrial dyes. Heterojunction photocatalysis represents a critical advancement in overcoming the charge recombination and narrow light-harvesting limits of single-component semiconductors. We systematically analyzed the thermodynamic and kinetic principles of charge transfer, providing a diagnostic roadmap for identifying these mechanisms through surface potential mapping and in-situ spectroscopy. By integrating seminal literature with recent breakthroughs in material design, we highlight how strategic band alignment preserves high redox potentials for superior quantum efficiency. Finally, we address the technical and economic bottlenecks—including interfacial resistance, photo-corrosion, and scalability—hindering the transition from laboratory-scale research to industrial wastewater treatment. This work provides a rigorous framework for developing high-performance, stable, and scalable photocatalytic systems for a sustainable environmental future.