Optimizing crystallinity and defects in carbon nitride for enhanced visible-light photocatalytic hydrogen production

• K + -ions facilitate the formation of crystalline carbon nitride (KCN8). • N 2C defects introduced by reduction of crystalline carbon nitride with NaBH 4 (K-D0.5). • Nitrogen vacancies extend light absorption across the full visible spectrum . • K-D0.5 sample shows a superior H 2 evolution rate (569.3 μmol g −1 h −1 ) at λ = 500 ± 5 nm. Polymeric carbon nitride (PCN) is the most widely studied organic semiconductor photocatalyst. However, its photocatalytic activity is relatively low, primarily due to the rapid recombination of charge carriers and its limited response to the full visible light spectrum. In this study, crystalline carbon nitride (KCN) was synthesized for the first time by calcining dicyanodiamide (DCDA) in the presence of KCl, where K + -ions played a crucial role. This approach resulted in a significant enhancement of the H 2 evolution rate, increasing from 131.2 μmol g −1 h −1 (for PCN) to 2931.9 μmol g −1 h −1 (for the KCN8 sample, prepared with 8 g of KCl) under Xe lamp irradiation with a light cutoff filter (λ > 420 nm). Unfortunately, the H 2 evolution rate of KCN8 significantly decreased to only 71 μmol g −1 h −1 under the Xe lamp irradiation with a bandpass filter (λ = 500 ± 5 nm). However, after reducing KCN8 with NaBH 4 , the H 2 production rate increased to 569.3 μmol g −1 h −1 (for the K-D0.5 sample, prepared with 0.5 g of NaBH 4 ), representing an 8-fold improvement. This enhancement was attributed to the introduction of N 2C defects, which expanded the light-responsive range to nearly the entire visible spectrum. The outstanding photocatalytic performance of the defective crystalline carbon nitride (K-D0.5 material) in the visible region, particularly at longer wavelengths, can be attributed to the optimal balance achieved between crystallinity and defect regulation. This balance leads to a synergistic enhancement in light absorption and charge transfer/separation efficiency. Additionally, Density Functional Theory (DFT) calculations revealed that K + -ion doping and the introduction of N 2C vacancies facilitate the desorption of intermediate H*, significantly accelerating the release of H 2 from the surface of the crystalline g-C 3 N 4 photocatalyst.