Abstract: Graphitic carbon nitride (g-C₃N₄), as a fascinating polymeric lamellar semiconductor, has garnered substantial attention in the photocatalytic field benefiting from its non-toxicity, cost-effectiveness, and suitable bandgap. Generally, a thermal polymerization method is utilized to prepare the conventional g-C₃N₄ by using the various organic precursors. However, the traditional g-C₃N₄ usually displays low crystallinity and inadequate surface-active centers, leading to the fast recombination of photoinduced charges and poor interfacial reaction efficiency, which can severely restrict its photocatalytic performance. Recently, to overcome these drawbacks, increasing the crystallinity and anchoring metal single atoms emerge as the serviceable strategies to effectively boost the flow and separation of photogenerated charges and provide surface-active centers for improving the photocatalytic activity. Herein, this review outlines the current advancements in various metal-single-atom anchored highly crystalline g-C₃N₄ (MSA-HCCN) photocatalysts, involving the Cu, Co, Fe, Mg, Mn, Pt, Au, Ni, and others. Initially, the features of highly crystalline g-C₃N₄ (HCCN) are meticulously presented compared to traditional g-C₃N₄. Second, the typical characteristics of MSA-HCCN photocatalysts are systematically emphasized. Subsequently, various synthetic methods of MSA-HCCN photocatalysts are presented in detail, containing the calcination method, ion exchange method, and freezing-assisted photodeposition method. After that, the photocatalytic applications of MSA-HCCN are described, encompassing energy photocatalysis (H₂ evolution, H₂O₂ production, etc.), environmental photocatalysis (CO₂ reduction, pollutant degradation, etc.), and organic synthesis (CH₄ oxidation, alkylaromatic oxidation, etc.). Lastly, the perspectives of MSA-HCCN photocatalysts are described to supply new thoughts for boosting their further development.

Key words: Photocatalysis, Metal single atom, Highly crystalline g-C₃N₄, Synthesis, Applications