Advances in 2D‐2D Heterostructures for Photocatalytic and Photoelectrocatalytic H ₂ Evolution, CO ₂ Reduction, and N ₂ Fixation: Design Strategies and Mechanistic Insights

ABSTRACT The increasing global demand for sustainable energy and environmental remediation has accelerated research on photocatalytic and photoelectrocatalytic systems for hydrogen evolution, CO 2 reduction, and N 2 fixation. Among emerging material platforms, 2D‐2D heterojunctions have demonstrated exceptional potential in overcoming intrinsic bottlenecks such as rapid charge recombination, limited charge‐separation efficiency, and narrow visible‐light absorption. This review highlights recent advances involving key 2D materials, including g‐C 3 N 4 , MXenes, black phosphorus, and MoSe 2 integrated with complementary 2D counterparts such as layered double hydroxides, graphene, 2D‐WO 3 , and 2D‐ZnCo‐MOFs. Special emphasis is placed on heterojunction configurations such as type‐II, Z‐scheme, S‐scheme, and Schottky architectures, which exploit interfacial band alignment to enhance directional charge flow and catalytic activity. The article further discusses the fundamental principles governing 2D‐2D interfaces and outlines major synthesis approaches including hydrothermal/solvothermal processing, calcination, ultrasonication, and electrostatic self‐assembly, capable of tailoring morphology, crystallinity, and interfacial coupling with high precision. Despite recent progress, challenges remain in achieving long‐term stability, simplifying multistep synthesis routes, and establishing standardized evaluation metrics. Future research should prioritize scalable, environmentally benign fabrication, robust interfacial engineering, and unified characterization protocols. Integrating hybrid charge–transfer mechanisms and real‐time operando analysis will be essential for fully exploiting 2D‐2D heterojunctions in next‐generation sustainable energy and environmental technologies.