Revolutionizing carbon nitride-based photocatalysts: design strategies for energy conversion and environmental applications.

Carbon nitride (CN) has been recognized as a promising photocatalyst for sustainable energy conversion and environmental remediation due to its moderate band gap (2.7-2.8 eV), facile synthesis, favorable band-edge positions, and high physicochemical stability. Despite numerous efforts in defect engineering, the development of CN-based photocatalysts still lacks unified design principles that correlate defect types and configurations with photocatalytic performance across different reactions. In addition to summarizing recent progress, this review emphasizes emerging design paradigms that elevate defect engineering from trial-and-error optimization to descriptor-driven and predictive strategies for next-generation CN-based photocatalysts. A comprehensive overview of defect-engineering strategies is put forward, including vacancy formation, cyano and amino modifications, as well as interstitial, substitutional, and anti-site defects, and how these structural modifications regulate the electronic structure and local coordination environment of CN is discussed. The influence of defects on key photocatalytic processes, light absorption, charge separation and transport, and surface redox reactions, is systematically analyzed, revealing how defect-induced electronic descriptors govern catalytic activity. Representative applications, such as hydrogen evolution, CO2 reduction, and organic pollutant degradation, are discussed to illustrate the structure-activity relationships. Insights into the advances and challenges of this promising metal-free photocatalyst are provided, along with approaches for further exploring the immense potential to develop efficient CN-based photocatalysts.