Low quantum efficiency has hampered the practical application of graphitic carbon nitride (g-C3N4). In this study, the effect of acid (H2SO4 and HF) on the photocatalytic degradation of Rhodamine B (RhB) over g-C3N4 was studied. It was found that the degradation of RhB was greatly enhanced in the presence of acid, and superoxide (O-2(center dot-)) is the predominant reactive oxygen species (ROSs) that are responsible for the efficient degradation of RhB. It is proposed that acidification of g-C3N4 results in the formation of a new surface state, which is 0.3 eV below the conduction band position of g-C3N4. The formed surface state can act as a trapping site for photo-generated electrons, which retards the recombination of the electron-hole pairs, enhancing the photocatalytic activity of g-C3N4. (C) 2015 Elsevier B.V. All rights reserved.