Synthesis, mechanisms, and applications of g-C₃N₅-based type-II advanced heterojunctions in photocatalysis

With the accelerating pace of global economic development, the extensive consumption of traditional energy resources such as coal and oil has led to substantial emissions of greenhouse gases, including carbon dioxide. Compounding this issue, the irregular discharge of wastewater from industrial, agricultural, manufacturing, and domestic sources further exacerbates environmental degradation. As a result, energy shortages and environmental pollution have emerged as critical challenges that urgently threaten sustainable human development. Semiconductor-based photocatalytic technology has gained widespread recognition as a promising approach to mitigating energy crises and environmental issues, leveraging solar energy for chemical conversion processes. Among various candidates, nitrogen-rich graphitic carbon nitride (g-C₃N₅) has attracted significant research interest due to its narrow and suitably positioned band structure, which enables superior visible-light harvesting and photocatalytic activity. This review comprehensively summarizes the synthesis strategies of g-C₃N₅-based photocatalytic materials and elucidates the mechanisms underlying heterojunction strategies for enhancing photocatalytic performance. Moving beyond the conventional type-II system, the review places a particular emphasis on the more advanced Z-scheme and S-scheme heterojunctions, which are designed to overcome the drawback of weak redox ability while maintaining efficient charge separation. We systematically discuss the applications of these g-C₃N₅-based heterojunctions—encompassing type-II, Z-scheme, and S-scheme architectures—in diverse photocatalytic processes, including H₂ evolution, CO₂ reduction, environmental remediation, and N₂ fixation. The challenges associated with the practical application of g-C₃N₅ photocatalysts are also discussed, along with prospective directions for future research. This review is expected to offer valuable perspectives on the physicochemical properties of g-C₃N₅ and inspire innovative approaches for the rational design of advanced g-C₃N₅-based photocatalytic systems for heterogeneous applications.