Molecular Structure Engineering of Graphitic Carbon Nitride for Photocatalytic Hydrogen Evolution: Recent Advances and Perspectives.

Photocatalytic hydrogen evolution has emerged as a sustainable strategy to address the global energy crisis and environmental challenges. Among various photocatalysts, graphitic carbon nitride (g-CN) has garnered significant attention due to its visible light responsiveness and tunable electronic structure. However, its intrinsic limitations, including rapid charge recombination and insufficient light harvesting capability, have hindered its practical applications. To overcome these constraints, molecular structure engineering of g-CN has emerged a pivotal approach for modulating its physicochemical properties at the molecular level. This review systematically elucidates advanced strategies for molecular-level modulation of g-CN, such as functional group grafting, defect engineering, element doping, morphology regulation, and crystallinity regulation. The synergistic effects of these strategies in enhancing charge separation efficiency and surface redox dynamics are thoroughly discussed, with a particular emphasis on the structure-activity relationships revealed through in situ characterization and theoretical calculations. Furthermore, this article delineates the challenges and future directions for designing high-performance g-CN photocatalysts. This comprehensive review aims to provide a holistic framework for understanding the molecular structure-performance correlations of g-CN and to inspire innovative solutions in the field of solar-driven hydrogen production.