Synthesis of Carbon Nitride Nanosheets with n→π* Electronic Transition for Boosting Photocatalytic CO2 Reduction

Awakening n ->pi* electronic transition in graphitized carbon nitride can extend the visible light absorption range of the original g-C3N4, which will contribute to improve the photocatalytic activity of carbon dioxide reduction. Here we report that the n ->pi* transition in g-C3N4 is activated by the cooperation of steam etching and alkali treatment. The CO and CH4 evolution yields of the NaOH/Vc-CN sample are 4.3 and 16 times higher than those of original g-C3N4, respectively. The planar asymmetry structure of heptazine was fabricated due to the hydroxyl groups reacting with terminal N-H content produced by the construction of carbon vacancy and the Na+ ions insert into the interlayer. Therefore, n ->pi* electronic transition in g-C3N4 was awakened, extending the optical absorption range with light wavelengths longer than 470 nm. At the same time, the ability of CO2 chemisorption and activation was improved due to the NaOH modification. Therefore, the extended visible light absorption, the improved crystallinity and the increased active sites are beneficial to optimizing the utilization efficiency of photogenerated carriers and enhancing photocatalytic activity.