UV light-induced oxygen doping in graphitic carbon nitride with suppressed deep trapping for enhancement in CO2 photoreduction activity

• Oxygen (O) element is introduced in the bulk graphitic carbon nitride (CN) for the first time through a precursor pretreatment by ultraviolet (UV) light irradiation. • The doped O non-metal photocatalyst of CN increased visible light absorption and enhanced carrier density. • The optimized sample has lower charge recombination and suppressed electron deep trapping. • The optimized sample shows enhanced photoreduction activity of CO2 to CH4. While photoreduction of CO 2 to CH 4 is an effective means of producing value-added fuels, common photocatalysts have poor activity and low selectivity in photocatalytic CO 2 -reduction processes. Even though creating defects is an effective photocatalyst fabrication route to improve photocatalytic activity, there are some challenges with the facile photocatalyst synthesis method. In this work, an O element is introduced into a graphitic carbon nitride (CN) skeleton through a precursory ultraviolet light irradiation pretreatment to increase the visible light absorption and enhance the carrier density of this modified non-metal CN photocatalyst; the charge transfer dynamics thereof are also studied through electrochemical tests, photoluminescence spectroscopy, and nanosecond transient absorption. We verify that the optimized sample exhibits lower charge recombination and a suppressed 84 ns electron-trapping lifetime, compared to the 103 ns electron-trapping lifetime of the CN counterpart, and thereby contributes to robust detrapping and a fast transfer of active electrons. Through density functional theory calculations, we find that the improved light absorption and increased electron density are ascribed to O-element doping, which enhances the CO 2 adsorption energy and improves the CO 2 -to-CH 4 photoreduction activity; it becomes 17 times higher than that of the bare CN, and the selectivity is 3.8 times higher than that of CN. Moreover, the optimized sample demonstrates excellent cyclic stability in a 24-hour cycle test.