The review on photoactive C(2–7)N(1–9) carbon nitrides for the photocatalytic applications

The family of carbon nitrides (CNs), including compounds such as C2N, C3N, C3N2, C3N3, C3N4, C3N5, C3N6, C3N7, C4N, C4N3, C5N, C5N2, C6N7, C6N9H3, and C9N5H3, has garnered growing attention for their tunable optoelectronic properties and applications in photocatalysis. While g-C3N4 remains the most widely studied member, emerging CN phases offer distinct structural motifs, nitrogen configurations, and electronic band structures that may outperform conventional systems. This review provides a comprehensive analysis of photoactive CN materials, with a special emphasis on less-explored CxNy compounds. It highlights their synthesis routes, classification based on active sites and solid-state behavior, and structural characteristics such as pore topologies, surface terminations, and nitrogen doping patterns. Furthermore, the diverse roles of CNs in photocatalysis as electron donors, sensitizers, redox mediators, and co-catalyst supports are critically evaluated. By correlating structure–property–performance relationships, this review offers a framework to guide the rational design of advanced CN-based photocatalysts for solar-driven energy conversion.