Engineering Charge Transfer Pathways in Photocatalysis: A Comprehensive Review of Z-, S-, R-, and C-scheme Mechanisms

Photocatalysis is a promising strategy for sustainable energy conversion, yet its large-scale deployment is constrained by persistent challenges in charge generation, separation, and utilization. Recent advances in interfacial design have led to four distinct charge-transfer architectures, namely Z-, S-, R-, and C-schemes that reshape how electrons and holes migrate, interact, and drive redox reactions. Each scheme provides unique benefits: Z-schemes retain strong redox ability through selective recombination, S-schemes exploit internal electric fields for directional carrier flow, R-schemes utilize donor–acceptor frameworks for controlled charge migration, and C-schemes accumulate charges to enable multi-electron processes. Therefore, this review critically evaluates these architectures, highlighting both their mechanistic principles and trade-offs in efficiency, cost, and durability. Special attention is given to heterojunctions based on metal–metal oxides/metal-metal oxide and their integration with carbonaceous materials, which enhance interfacial charge dynamics, broaden light absorption, and improve structural robustness. While Z- and S-schemes are relatively mature and experimentally validated, R- and C-schemes remain at an early stage but show strong potential for selective and complex catalytic processes. Future directions are outlined for designing efficient, stable, and scalable photocatalytic systems aligned with sustainable energy goals.