Graphitic carbon nitride nanosheets via acid pretreatments for promoted photocatalysis toward degradation of organic pollutants

Acid treatment serves as an effective engineering strategy to modify the structure of graphitic carbon nitride (g-C 3 N 4 ) for enhanced metal-free photocatalysis, while their lacks a comprehensive understanding about the impacts of different acid species and acid treatment approaches on the intrinsic structure and properties of g-C 3 N 4 and structure–activity relationships are ambiguous. Employing inorganic/organic acids including hydrochloric acid (HCl), nitric acid (HNO 3 ), acetic acid (HAc), sulphuric acid (H 2 SO 4 ), or oxalic acid (H 2 C 2 O 4 ) as treatment acids, herein, we compare the impacts of different acid pretreatment approaches on the structure and properties of g-C 3 N 4. Due to different acid-melamine interaction modes and the activation roles of various acids, the obtained g-C 3 N 4 samples exhibit varied structures, physiochemical properties and photocatalytic activities. Compared with bulk graphitic carbon nitride (BCN), g-C 3 N 4 prepared by acid pretreatment show enhanced photocatalytic performance on bisphenol A (BPA) degradation. The photocatalytic degradation rates of BPA by g-C 3 N 4 prepared by HNO 3, HAc, H 2 SO 4, H 2 C 2 O 4, or HCl pretreatment are about 2.2, 2.7, 2.8, 3.2 and 3.8 folds faster than that by BCN. HCl pretreatment proves to be the optimal approach, with the derived g-C 3 N 4 (HTCN) showing more intact heptazine structural units, and increased specific surface area, which promote the exposure of more active sites, accelerate charge transfer, and give rise to a notable improvement in photocatalysis, eventually. Mechanistic investigations through quenching experiments and electron paramagnetic resonance (EPR) characterization unveil that superoxide ion radical (O 2 − ) and photo-induced holes (h + ) worked principally in the photodegradation reaction. This work provides new insights for the rational selection of acid types and treatment methods to synthesize metal-free carbon nitrides with improved activity for photocatalytic applications.