Midgap-state-mediated two-step photoexcitation in nitrogen defect-modified g-C3N4 atomic layers for superior photocatalytic CO2 reduction

Abstract

Morphological control and surface vacancy modulation are effective strategies for promoting the readiness level of g-C₃N₄-based photocatalytic systems. In this work, we present an unprecedented proof-of-concept study on the doping effect of nitrogen vacancies in g-C₃N₄ atomic layers for enhanced CO₂ photoreductivity. A 3D bubbly architecture scaffolded by few-atomic-layer (ca. 1.6 nm) g-C₃N₄ nanosheets with the potential to facilitate charge transportation was successfully realized by inducing a disruption in the π–π interactions between adjacent layers of g-C₃N₄ through simple incorporation of an NH₄Cl gas template. Introduction of nitrogen vacancy defects into the ultrathin nanosheets further exerted remarkable tuning effects on their optoelectronic properties. The concomitant outcomes from the event of nitrogen vacancy introduction gave rise to several phenomena, including: (i) an increase in reducing ability with a higher nitrogen vacancy density, (ii) extended absorptivity to the long-wavelength visible light region due to the strategic band position of the midgap state below the CB, and (iii) prolonged radiative recombination of photogenerated electron–hole pairs, as the midgap state serves as an effective reservoir to temporarily trap electrons. As a result, the photoreduction performance of nitrogen defect-modified g-C₃N₄ atomic layers was synergistically improved with 3.16-fold and 5.14-fold enhancements in the CH₄ evolution yield over their respective pristine ultrathin counterpart and bulk form.