▎ 摘　　要

NOVELTY - Preparing graphene quantum dot comprises e.g. (i) adding 1 g naphthalene and 80 ml 70% nitric acid slowly under stirring into closed reactor, then refluxing the reaction at 80 degrees C for 12 hours, and crystallizing by cooling, (ii) cooling the obtained dinitronaphthalene solid completely, removing acid solution using vacuum suction filtration method, and washing the solid on the filter membrane with deionized water for multiple times until the filtrate is neutral, (iii) washing the obtained solid, and drying at 60 degrees C for 12 hours, adding 20 mg dried solid into 10 ml 1% sodium hydroxide solution, ultrasonically mixing, sealing in hydrothermal kettle, reacting at constant temperature of 200 degrees C for 8 hours in oven, and cooling to room temperature naturally, and (iv) transferring obtained dark brown product solution into dialysis bag for dialysis, changing deionized water for every 5 hours after 48 hours of dialysis to obtain pure undoped graphene quantum dot solution, and freeze-drying. USE - The graphene quantum dot is useful in preparing nitrogen-doped graphene quantum dot, and nitrogen-sulfur doped graphene quantum dot (all claimed). ADVANTAGE - The graphene quantum dot: has stable structure, small size and rich hydroxy functional group on the surface; can be stably dispersed in aqueous solution, and doped with nitrogen or sulfur atoms; and is non-toxic, and environmentally friendly. The method: has simple process, mild reaction condition, high product yield, low energy consumption and less equipment; solves the problem of high production cost when compared to existing method; utilizes industrialized and easily available raw materials; and is suitable for large-scale industrial production. DETAILED DESCRIPTION - Preparing graphene quantum dot comprises (i) adding 1 g naphthalene and 80 ml 70% nitric acid slowly under stirring into closed reactor, then refluxing the reaction at 80 degrees C for 12 hours, and crystallizing by cooling to obtain dinitronaphthalene solid, (ii) cooling the obtained dinitronaphthalene solid completely, removing acid solution using vacuum suction filtration method, and washing the solid on the filter membrane with deionized water for multiple times until the filtrate is neutral, (iii) washing the solid obtained in step (ii), then placing in oven, and drying at 60 degrees C for 12 hours, adding 20 mg dried solid into 10 ml 1% sodium hydroxide solution, ultrasonically mixing, sealing in hydrothermal kettle, reacting at constant temperature of 200 degrees C for 8 hours in oven, and cooling to room temperature naturally to obtain dark brown product solution, and (iv) transferring obtained dark brown product solution into dialysis bag for dialysis, changing deionized water for every 5 hours after 48 hours of dialysis to obtain pure undoped graphene quantum dot solution, and freeze-drying to obtain final product as solid powder. INDEPENDENT CLAIMS are also included for: (1) preparing nitrogen-doped graphene quantum dot comprising (i) adding 1 g naphthalene, and 80 ml 70% nitric acid slowly under stirring into a closed reactor, then refluxing the reaction at 80 degrees C for 12 hours, and crystallizing by cooling to obtain dinitronaphthalene solid, (ii) cooling the obtained dinitronaphthalene solid completely, removing acid solution using vacuum suction filtration method, and then washing the solids on the filter membrane with deionized water for multiple times until the filtrate is neutral, and (iii) adding dry solid prepared in step (ii) in a ratio of 2 g/l into saturated ammonia solution, mixing ultrasonically, and sealing in hydrothermal kettle, reacting at constant temperature of 200 degrees C for 8 hours, cooing naturally to room temperature, then transferring the obtained dark brown product solution into dialysis bag for dialysis, changing the deionized water for every 5 hours after 48 hours of dialysis to obtain nitrogen-doped graphene quantum dot solution, and then freeze-drying to obtain nitrogen-doped graphene quantum dot solid powder; (2) preparing nitrogen-sulfur doped graphene quantum dot comprising (i) adding 1 g naphthalene, and 80 ml 70% nitric acid slowly under stirring into a closed reactor, then refluxing the reaction at 80 degrees C for 12 hours, and crystallizing by cooling to obtain dinitronaphthalene solid, (ii) cooling the obtained dinitronaphthalene solid completely, removing acid solution using vacuum suction filtration method, and then washing the solids on the filter membrane with deionized water for multiple times until the filtrate is neutral, and (iii) adding dry solid prepared in step (ii) in a ratio of 2 g/l into saturated ammonia containing 1% thiourea mixed solution, mixing ultrasonically, and sealing in hydrothermal kettle, reacting at constant temperature of 200 degrees C for 8 hours in an oven, cooing naturally to room temperature, then transferring the obtained dark brown product solution into dialysis bag for dialysis, changing the deionized water for every 5 hours after 48 hours of dialysis to obtain nitrogen-sulfur doped graphene quantum dot solution, and then freeze-drying to obtain final product as solid powder; (3) use of graphene quantum dot comprising adding graphite powder into undoped, nitrogen-doped or nitrogen-sulfur doped graphene quantum dot solution in a solid ratio of 1:1, carrying out ultrasonic treatment for 2 hours, and fully exfoliating the graphite powder into monoatomic layer of graphene material to obtain graphene quantum dot-van der waals heterojunction of graphene; and (4) use of graphene quantum dot comprising dispersing graphene quantum dot solid powder in 5% perfluorosulfonic acid in ethanol solution, then dropping the prepared conductive ink uniformly on nickel mesh, drying completely to obtain working electrode, then carrying out photo-electro-catalytic electrolyte solution reduction reaction under the condition of xenon lamp irradiation to produce hydrogen, and then carrying out photo-electro-catalytic oxidation reaction to produce oxygen.