Boosting CO production from visible-light CO2 photoreduction via defects-induced electronic-structure tuning and reaction-energy optimization on ultrathin carbon nitride

Abstract

Polymeric carbon nitride is a promising photocatalyst for CO₂ reduction; however, its efficiency is limited by the rapid recombination of photogenerated charges and weak CO₂ activation ability. In this study, we present the synthesis of g-C₃N₄ with carbon vacancy and oxygen doping (Vc-OCN) through a facile formaldehyde-assisted thermal polycondensation of molten urea. Comprehensive investigations were carried out and established that the oxygen doping site substituted the 2-coordinated nitrogen site while a carbon vacancy (Vc) was formed at the position of C_3N in the heptazine structure. The incorporation of Vc and oxygen doping could efficiently modify the band structure and electronic structure, leading to enhanced optical absorption and charge separation. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and DFT calculations demonstrated that the Vc-OCN reduced the energy of the rate-determining step of CO₂ conversion, with the Vc serving as the active site for CO₂ activation. Combining thermodynamics and kinetics optimization, Vc-OCN achieved a high CO generation rate of 13.7 μmol g⁻¹ h⁻¹ under visible-light irradiation without the help of any cocatalysts or sacrificial agents, displaying a 7.6-fold improvement over GCN. This study provides a deep understanding of the synergistic effect of doping and vacancies on CO₂ photoreduction and provides novel insights into the design of high-efficiency polymer semiconductors obtained through defects engineering.