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

Abstract

A polymeric carbon nitride is a promising metal-free photocatalyst for the synthesis of solar fuels. However, its poor light-harvesting ability and severe charge recombination lead to low photocatalytic efficiency. In this work, a modified carbon nitride (CNBD) featuring a benzene ring and a nitrogen vacancy has been synthesized via co-polymerization and subsequent thermal treatment under a hydrogen flow, presenting an excellent photocatalytic hydrogen evolution rate of 1252.76 μmol g⁻¹ h⁻¹, which is 6.7 times higher than that of the pristine carbon nitride. The incorporation of a benzene ring and a nitrogen vacancy tunes the band structure and enables the redistribution of an electron cloud on the carbon nitride framework, resulting in a decrease in the bandgap to expand the photoresponsive range as well as the spatial separation of electrons and holes to reduce 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 a midgap state in the bandgap and provides an active site for hydrogen adsorption. Moreover, the Gibbs free energy of hydrogen adsorbed onto the nitrogen vacancy site is significantly close to zero (≈0.37 eV) compared with pristine PCN, suggesting that 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 improving photocatalytic activity.