Enhancing the Photocatalytic Performance of Carbon Nitrides Through Controlled Local Structure Modification

Abstract Carbon nitrides are among the most efficient and extensively studied transition‐metal‐free photocatalysts, yet their industrial application is limited by high charge recombination, poor charge transport, and insufficient absorption above 460 nm. This study investigates how fine‐tuning the crystal structure of carbon nitrides helps to overcome these challenges and to enhance their photocatalytic performance. We used poly(heptazine imides) (PHIs) with various cations (M = H⁺, Na⁺, K⁺, Mg 2 ⁺) as a model system. Na‐PHI exhibits the highest activity among PHIs with monovalent cations, as the combination of solvated Na⁺ cations and rotational defects, experimentally observed in this study for the first time, optimizes interlayer charge transfer. Greater photocatalytic efficiency observed for Mg‐PHI is attributed to the preservation of rotational defects and the higher oxidation state of Mg 2 ⁺, which enhances charge density and facilitates charge transfer. Density functional theory (DFT) and spectroscopic analyses reveal that Na‐PHI and Mg‐PHI share a valence band dominated by nitrogens and a conduction band primarily influenced by carbons, with both cations contributing to n‐type doping. Mg‐PHI features sub‐gap impurity states, reducing the band gap and extending light absorption. Excited‐state molecular dynamic simulations further demonstrate that water molecules contribute more significantly to charge transfer. highlighting an additional key factor in optimizing photocatalytic performance.