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Awakening n→π* 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→π* 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→π* 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. [ABSTRACT FROM AUTHOR]

A g-C3N4/TiO2 (CN/TiO2) nanocomposite was fabricated by depositing proton-functionalized g-C3N4 nanosheets onto the TiO2 nanotube array films with oxygen vacancies. Proton-functionalized g-C3N4 (p-CN) nanosheets was prepared by sonicationexfoliation of bulk g-C3N4 in the presence of hydrochloric acid. Protonation pretreatment of g-C3N4 reduce the size of g-C3N4 and increase the interfacial contact area of g-C3N4 nanosheets with TiO2 nanotube arrays. TiO2 nanotube arrays containing oxygen vacancy (OV-TiO2) was obtained by NaBH4 reduction, which results in the enhanced visible light absorption. As a result, the best photocurrent density of p-CN/OV-TiO2 nanocomposite is 5.3 times higher than that of CN/TiO2 samples. As further confirmed by photocatalytic degradation of methyl orange (MO), p-CN/OV-TiO2 heterostructure exhibit the highest photocatalytic activity among all the prepared samples. This improved photoelectrochemical (PEC) and photocatalytic performance can be attributed to the formation of a direct Z-Scheme heterostructure between protonated g-C3N4 and TiO2 containing oxygen vacanc [ABSTRACT FROM AUTHOR]