In Situ Growth of Amine-Rich g-C ₃ N ₄ with Carbon Defects for Boosting Visible-Light Photocatalytic CO ₂ Reduction Performance

Carbon dioxide (CO2) conversion into fuels and valuable chemicals driven by sunlight is a promising method for solving global warming and the shortage of fossil-derived chemicals simultaneously. Two-dimensional graphiticcarbon nitride (g-C3N4) has been viewed as a highly favorable material for photocatalytic CO2 reduction. However, its overall performance is relatively low due to poor charge separation. Engineering the g-C3N4 structure by introduction of a carbon (C) vacancy is an effective strategy to mitigate severe charge recombination, while amine groups (–NH2) could be beneficial for CO2 capture. Herein, we present a facile method for spontaneous growth of amine-functionalized g-C3N4 with rich C vacancies by adding trace Ni2+ ions during thermal treatment. Insightful investigation by various advanced characterizations revealed that the carbon defects could induce a microporous structure, resulting in high surface area. In addition, the resultant defects upshift the conduction band of g-C3N4 and promote charge carrier separation. Both of these contribute to enhanced CO2 reduction ability. Meanwhile, the exposed edge amino enhances the CO2 adsorption strength, as verified by both experimental and theoretical results. When coupled with the [Co(bpy)3]2+ molecular cocatalyst, the amine-rich g-C3N4 nanotubes with Ni2+ ion-induced carbon defects (Cv-CN) exhibit a superior visible-light-induced CO2-to-CO conversion rate of 330 μmol g–1 h–1, which is 66-fold and 3-fold higher than those of bulk g-C3N4 (bulk CN, 5 μmol g–1 h–1) and g-C3N4 nanotubes (CN, 120 μmol g–1 h–1), respectively, in a liquid–gas phase reaction. Density functional theory (DFT) calculations demonstrate that carbon vacancy introduction creates edge amino groups capable of forming strong hydrogen bonds with CO2 molecules, resulting in a 13-fold enhancement in adsorption energy from −0.01 to −0.13 eV. This work offers a facile and spontaneous strategy for synthesizing high-reactive g-C3N4 for photocatalytic CO2 reduction.