Defective polymer carbon nitride with expanding π-electron domains for efficient photocatalytic H2 evolution

Abstract

Polymeric carbon nitride is a promising metal-free photocatalyst for the synthesis of solar fuel. However, the poor light-harvesting ability and severe charge recombination lead to the low photocatalytic efficiency. In this work, a modified carbon nitride (CNBD) featured with benzene ring and nitrogen vacancy is synthesized via co-polymerization and subsequent thermal treatment in hydrogen flow, presenting an excellent photocatalytic hydrogen evolution rate of 1252.76 umol g-1 h-1, which is 6.7 times higher than the pristine carbon nitride. The incorporation of benzene ring and nitrogen vacancy tunes the band structure, and enables the redistribution of electron cloud on the carbon nitride framework, resulting in the decrease in the bandgap to expand photo responsive range as well as the spatial separation of electron and hole to reduce the charge recombination. The embedded benzene ring extends the delocalized mobility domains of π-conjugated electrons and downshifts the conduction band edge. The created nitrogen vacancy induces the formation of midgap state in the bandgap and provides active site for hydrogen adsorption. Moreover, the Gibbs free energy of hydrogen adsorbed on nitrogen vacancy site is significantly close to zero (≈0.37 eV) compared with pristine PCN, suggesting the more efficient photocatalytic hydrogen evolution reaction occurs in CNBD. This work shows a simple method to tune the band structure of carbon nitride for enhancing the light-harvesting ability and boosting the charge separation simultaneously, and thus improve photocatalytic activity.