Modifying the bridging N atoms of polymeric carbon nitride to achieve highly enhanced photocatalytic hydrogen evolution

Abstract Based on density functional theory calculations, bridging N atoms hinder the transmission and separation of electrons, resulting in a high recombination rate of the photogenerated electron–hole pairs and poor photocatalytic activity of pristine polymeric carbon nitride (PCN). One of the keys to improving the photocatalytic activity of PCN is modifying the bridging N atoms. Herein, dicyandiamide was hydrothermally treated in triethanolamine aqueous solution; then, bridging N-modified PCN was obtained via condensation polymerization. The optimal bridging N-modified PCN sample, HCN-0.4, exhibited a negative shift of the conduction band, reduced photoluminescent emission, and longer carrier lifetime in comparison with the pristine PCN (BCN). According to the theoretical calculations, after methyl modification of the bridging N atoms in HCN-0.4, the carrier migration rate increased owing to the electron rearrangement. The hydrogen evolution rate of HCN-0.4 was 31.38 mmol/(h × g), which was 56.04 times higher than that of the pristine PCN (0.56 mmol/(h × g)) under visible light irradiation. The apparent quantum efficiency of HCN-0.4 was 28.89% at 420 nm and 0.31% at 670 nm, the latter of which was due to the presence of midgap state.