Nitrogen vacancies in graphitic carbon nitride and their role in heterogeneous photocatalysis

Graphitic carbon nitride (g-C₃N₄) is a promising metal-free photocatalyst whose activity is often enhanced by nitrogen vacancies, though their microscopic role remains unclear. Using advanced ab initio calculations with large periodic supercells, we show that long-range buckling is essential to correctly evaluate defect energetics and thus determine the stability of distinct vacancy configurations. The most stable defects are found to introduce localized in-gap states corresponding to shallow acceptor and deep donor levels. These features explain (i) the experimental red-shifted absorption and (ii) suppressed photoluminescence observed in N-deficient g-C₃N₄ samples. Most importantly (iii) energy-level alignment at the water-semiconductor interface explains the enhanced photocatalytic reduction and reduced oxidation activity reported experimentally. Overall, our results provide a unified microscopic picture that quantitatively connects defect-induced electronic structure changes and experimental observables, offering a concrete predictive strategy for designing defect engineered carbon nitride and related metal-free photocatalysts.