Study On Preparation And Photocatalytic Performance Of Graphene Quantum Dots/Metal Sulfide

Most metal sulfides semiconductors have the advantages of strong photo-generated carrier separation performance and narrow band gap energy,which benefit it use sunlight for environmental remediation and realize solar energy conversion.However,a single metal sulfide has limited activity and stability for the photocatalytic reduction of hexavalent chromium and the photolysis of water to hydrogen evolution,which is far from reaching the standard for commercial application.Therefore,the metal sulfide should be modified.In this paper,a simple hydrothermal method was used to construct a metal sulfide-supported graphene quantum（GQDs）composite.Under metal halide lamps or xenon lamp light source.the performance of the GQDs/MSx（M=Zn,Bi or Sn）composite catalyst for the photocatalytic reduction of hexavalent chromium and hydrogen evolution was evaluated.The main research contents are as follows:1.Using sodium citrate as the carbon source of graphene quantum dots,thiourea as the sulfur source,and zinc acetate as the zinc source,GQDs/ZnS was synthesized under hydrothermal conditions at 200℃for 24 h.By adjusting the ratio of sodium citrate and zinc acetate,GQDs/ZnS catalysts with different GQDs content were prepared.When the ratio of the amount of GQDs and ZnS is 2:10,GQDs/ZnS shows the best photocatalytic activity.Under the irradiation of a metal halide lamp,10 ppm of hexadecane can be reduced within 12 minutes.The valence Cr（VI）is completely reduced,and the reduction rate of Cr（VI）over pure ZnS is 12.2%.Under xenon lamp irradiation,the hydrogen evolution rate over GQDs/ZnS reached 2122.1μmol/g/h,which was more than 7 times that of pure ZnS（284.4μmol/g/h）.The test of photocatalytic reduction of chromium by adjusting the p H indicates that the catalyst has better photocatalytic activity under acidic conditions,while the photocatalytic activity will be inhibited under alkaline conditions.2.Using sodium citrate as the carbon source,thiourea as the sulfur source,and bismuth nitrate pentahydrate as the bismuth source,GQDs/Bi₂S₃was hydrothermally synthesized at 200℃for 24 h.The performance of GQDs/Bi₂S₃composite catalyst prepared by traditional method was explored.Studied shows that GQDs/Bi₂S₃prepared by hydrothermal method shows more efficient reduction activity of Cr（VI）.When the ratio of the amount of GQDs and Bi₂S₃is 1.0,the catalytic reduction performance of GQDs/Bi₂S₃is the best,and the reduction efficiency for 10 mg/L Cr（VI）reaches 70.3%in 80 minutes.While the reduction rate of hexavalent chromium over pure Bi₂S₃catalyst is 17.3%.The improvement in performance of the GQDs/Bi₂S₃composite catalyst can be attributed to that the introduced GQDs accelerates the transfer of photogenerated electrons and holes on Bi₂S₃,and the load of GQDs changes the microscopic appearance of Bi₂S₃.The increased specific surface area of??the catalyst could provide more reactive sites,thereby facilitating the adsorption of reaction substrates and the progress of photocatalytic reduction.Therefore,the improvement of photocatalytic performance is achieved.3.The same hydrothermal preparation method was adopted to prepare GQDs/SnS₂.The effect of different carbon sources on the performance of the catalyst was mainly explored.0.8 GQDs/SnS₂（sodium citrate）and 0.8 GQDs/SnS₂（citrate acid）were prepared by different carbon sources.The reduction of Cr（VI）experiment shows that the performance of 0.8 GQDs/SnS₂（sodium citrate）is better than that of0.8 GQDs/SnS₂（CA）.0.8 GQDs/SnS₂displayed the best catalytic performance.Under the conditions of metal halide lamp irradiation,10 ppm of hexavalent chromium can be completely reduced in 60 minutes,which is about twice the activity of pure SnS₂.In addition,the cycle performance of the composite catalyst was also explored.After three recycles,the catalytic performance remained basically unchanged,indicating that the composite catalyst has good photocatalytic stability.The research in this paper shows that graphene quantum dots（GQDs）are successfully loaded into metal sulfides by a simple hydrothermal method.The loading of GQDs can increase the surface area of the catalyst,expand the light absorption range of the catalyst,accelerate the separation of photogenerated electrons and holes.and extend the lifetime of photogenerated carriers.Therefore,the reducibility of the catalyst was improved.This work provides a new idea for the design of metal sulfide-supported graphene quantum dot photocatalysts.