▎ 摘　　要

Low cost, gram scale and highly N doped graphene quantum dots (NGQDs) with a broad absorption spectrum was synthesized using hydrothermal method by citric acid and urea. The obtained NGQDs possess wide absorption spectrum from ultraviolet to visible and IR range of wavelength. Optical characterization revealed that NGQDs are highly photoluminescent in three region of light (i.e. UV, Vis and IR). The average thickness is lower than 1 nm, meaning that the NGQDs are single, two or three layers of graphene. Lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the NGQDs determined using a cyclic voltammetry (CV) method, and energy band gap was obtained similar to 1.91 eV. A mechanism of charge carrier generation, separation, transport and photodegradation of methylene blue under illumination was proposed using the energy states. Finally the effect of N concentration on the band gap of NGQDs was simulated and discussed.