Cyanamide-defect-induced built-in electric field in crystalline carbon nitride for enhanced visible to near-infrared light photocatalytic activity

Abstract

Carbon nitride materials have achieved great accomplishments in solar-to-hydrogen energy conversion under visible light. However, the weak kinetics and rapid recombination of the photogenerated charge carriers result in low photocatalytic efficiency, which hinders its practical utilization. Herein, terminal cyanamide groups were grafted on K⁺-inserted crystalline carbon nitride (KCCN-cya) via a potassium-salt (KSCN and KOCN)-assisted one-step crystallization process. The cyanamide groups, which have high electron accepting ability, contribute to the formation of a built-in electric field (BIEF). The enhanced charge separation and dislocation was verified through linear sweep voltammetry (LSV) and electron paramagnetic resonance (EPR). Kelvin probe force microscopy (KPFM) measurements revealed the increased fluctuations of localized potential, which induced the formation of a BIEF in KCCN-cya. Moreover, the HOMO and LUMO distribution obtained for KCCN-cya using DFT computations was more highly separated than that of KCCN materials, indicating the dislocation of generated electron–hole pairs and the effective BIEF construction after the implantation of electron-withdrawing cyano terminals. As a result, the constructed BIEF promoted the separation of photogenerated electrons and holes in KCCN-cya samples, which thus exhibited an approximately three-fold improvement in hydrogen evolution activity under visible light. This finding demonstrated that introducing an electron-withdrawing group (i.e., –CN) into an organic photocatalyst would be a promising way to improve photocatalytic activity and solar energy conversion.