The promoted photocatalytic performance of in-situ surface alkalinization of g-C3N4 for methylene blue and Cr(VI) in dye wastewater

In this paper, a series of surface-alkalinized graphitic phase carbon nitride (g-C3N4) were synthesized via a one-step strategy employing dicyandiamide as the precursor. The effects of single salts (KCl, NH4Cl) and complex salts (KCl with NH4Cl) on the morphological structure, compositional composition, optical properties, and photocatalytic performance of alkalized CN samples were investigated. The IR and XPS analyses revealed that new structural units, C≡N and (N)2C-OH, were formed in the CN structure, indicating that the surface alkalinization of g-C3N4 had been successful. After in-situ surface alkalinization, the energy band structure of CN was optimized, the light absorption range was widened, and the photocatalytic activity was enhanced. Among them, the compound salt alkalinized sample (CNK2.0) at twice the amount of KCl as dicyandiamide showed the most excellent photocatalytic degradation performance. Its degradation efficiency of methylene blue within 60 min could reach 83.39 %, which was 2.05 times higher than that of pure g-C3N4. With a 47.1 % reduction of Cr(VI) at 60 min, the CNK1.5 sample demonstrated the prime photocatalytic activity, which was 34.2 % higher than that of pure g-C3N4. The surface hydroxylation and the optimization of the energy band structure are the main reasons for the enhanced photocatalytic activity of carbon nitride after complex salt alkalinization. [Display omitted] •Double salt surface-alkalinized g-C3N4 was synthesized via a facile one-step strategy with enhanced photocatalytic performance for MB degradation as well as Cr(VI) reduction.•K+ plays an important role in the surface alkalinization process, which induced the generation of oxygen-containing functional groups (CO, CO) and nitrile groups (-CN) on the surface of modified CN.•The surface hydroxylation and the optimization of the energy band structure are the primary reasons for the enhanced photocatalytic activity of carbon nitride after double salt alkalinization.•The alkalization effect of double salt is superior to that of single salt in terms of photoreduction Cr(VI) activity.