New carbon nitride close to C6N7 with superior visible light absorption for highly efficient photocatalysis

The rational design and construction of novel two-dimensional (2D) carbon nitrides (CNs) beyond g-C3N4 is a hot topic in the fields of chemistry and materials. Inspired by the polymerisation of urea, we have prepared a series of novel C–C bridged heptazine CNs UOx (where x is the ratio of urea to oxamide, x = 1, 1.5, 2, 2.5, and 3), which are similar to (C6N7)n, upon the introduction of oxamide. As predicted using density functional theory (DFT) calculations, the conjugated structure of UOx was effectively extended from an individual heptazine to the entire material. Consequently, its bandgap was reduced to 2.05 eV, and its absorption band edge was significantly extended to 600 nm. Furthermore, its carrier transfer and separation were significantly enhanced, establishing its superior photocatalytic activity. The optimised UO2 exhibits a superior photocatalytic hydrogen production rate about 108.59 μmol h−1 (using 10 mg of catalyst) with an apparent quantum efficiency (AQE) of 36.12% and 0.33% at 420 and 600 nm, respectively, which is one of the most active novel CNs reported to date. Moreover, UO2 exhibits excellent photocatalytic activity toward the oxidation of diphenylhydrazine to azobenzene with conversion and selectivity reaching ~100%, which represents a promising highly efficient 2D CN material. Regarding phenols degradation, UO2 also displayed significantly higher activity and durability during the degradation of phenol when compared to traditional g-C3N4, highlighting its significant potential for application in energy, environment and photocatalytic organic reactions.