Phosphorus Doping of Graphitic Carbon Nitride: Structural Corrugation and Midgap Electronic States

Graphitic carbon nitride (g-C₃N₄) is a promising metal-free photocatalyst, yet its efficiency remains limited by rapid charge recombination. Heteroatom doping offers an effective means to tailor its electronic structure and enhance photocatalytic performance. In this study, structural and electronic modifications in triazine-based g-C₃N₄ (g-CN1) induced by phosphorus incorporation were systematically investigated through a combination of experimental characterization and density functional theory (DFT) calculations, providing complementary insights into the atomic-scale bonding and electronic properties of pristine and phosphorus-doped g-C₃N₄ (P@g-CN1). Both pristine g-CN1 and P@g-CN1 were synthesized via thermal polycondensation of melamine using H₃PO₄ as the dopant source. Transmission electron microscopy, X-ray diffraction, and photoluminescence spectroscopy reveal that phosphorus doping preserves the fundamental lattice topology while inducing lattice relaxation, surface corrugation, and flake extension—features that are well rationalized by the Topology Conservation Theorem. Phosphorus incorporation markedly suppresses charge recombination and enhances charge separation efficiency in the P@g-CN1 composite. DFT analysis confirms that phosphorus atoms induce downward shifts of the valence and conduction bands and introduce localized midgap states near the Fermi level, thereby enhancing electronic delocalization and facilitating carrier transport. The P@g-CN1 system retains its semiconducting character with pronounced σ–π hybridization between carbon and nitrogen 2p orbitals. The strong agreement between experimental results and theoretical analysis underscores the high degree of complementarity between these approaches. This provides a coherent understanding of the structure–property relationships in phosphorus-doped g-C3N4, thereby guiding the rational design of next-generation, metal-free two-dimensional photocatalysts and photovoltaic materials.