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

The rational design and construction of novel 2D carbon nitrides (CNs) beyond g-C₃N₄ is a hot topic in the field of chemistry and materials. Inspired by the polymerisation of urea, we prepare the novel C−C bridged heptazine CNs UOₓ (x is the ratio of urea to oxamide, x = 1, 1.5, 2, 2.5 and 3), which closes to (C₆N₇)n for the first time by introducing part of oxamide. As predicted by density functional theory (DFT) calculation, the conjugated structure of UOₓ is effectually extended from individual heptazine to the entire material. Consequently, its bandgap is reduced to 2.05 eV, and its absorption band edge is greatly extended to 600 nm. Furthermore, its carrier transfer and separation are also significantly enhanced, thus establishing its superior photocatalytic activity. The optimised UO₂ exhibits a superior photocatalytic hydrogen production rate about 108.59 μmol h⁻¹ (10 mg of catalysts) with apparent quantum efficiency (AQE) of 36.12% and 0.33% for 420 and 600 nm, respectively, which is one of the most active novel CNs. Moreover, UO₂ exhibits superlative photocatalytic activity for the oxidation of diphenylhydrazine to azobenzene with conversion and selectivity reaching nearly100%, representing a promising highly efficient 2D CN. Regarding phenols degradation, UO₂ also displayed significantly higher activity and durability compared to traditional g-C₃N₄, highlighting its significant potential for applications in energy, environment and photocatalytic organic reaction. Copyright © 2021 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.