Study On Microbial And Non-Microbial-Mediated Synthesis Of Graphene-based Nanocomposites And Their Catalytic Performance

Under the pressure of environmental pollution and energy crisis,it is urgent to develop clean energy to solve the problem of environmental pollution,so as to ensure the sustainable development of human society.With all the advantages of green renewable energy,H₂ is the most promising environmentally friendly alternative to fossil fuels.Using photocatalyst to decompose hydrogen in aquatic products is a more green and environmental protection method in the current hydrogen energy production technology.Therefore,it is very important to develop efficient catalysts for water decomposition.Graphene oxide（GO）is an ideal support material for photocatalyst due to its large specific surface area,excellent carrier transfer rate and strong adsorption capacity,which contributes to the rapid transfer of photo-generated charges generated by semiconductors.However,it is difficult to be applied in engineering because of its easy agglomeration and no photocatalytic ability.As metal sulfides have high photocatalytic efficiency,cadmium sulfide（CdS）and silver sulfide（Ag₂S）are typical semiconductor materials,which not only have appropriate band gap,but also have good reduction potential.Therefore,on the basis of GO,we combined with semiconductor materials to improve the migration rate of semiconductor photogenerated carriers,increased the specific surface area of the composite materials,made up for the performance defects of single materials,and thus greatly enhanced the photocatalytic performance of the composite materials.In this paper,the photoelectric properties of nanocomposite catalytic materials synthesized by graphene were tested.The influencing factors of photocatalytic activity,the mechanism of photocatalytic H₂ production and the principle of synthesis of nanocomposites on GO surface by reducing bacteria Shewanella oneidensis MR-1 were discussed.The main research contents and results of this study are as follows:（1）Molybdenum disulfide（MoS₂）was the main active material.The MoS₂/rGO photocatalyst was prepared by hydrothermal method,and CdS nanoparticles（NPs）were supported on MoS₂/rGO nanosheets by in-situ synthesis.The research results showed that CdS NPs with a diameter of about 10 nm were uniformly grown on the surface of MoS₂/rGO and nanocomposites had high crystallinity.The addition of reduced graphene oxide（rGO）and MoS₂increased the separation rate of photogenerated electron-hole pairs in CdS,and enhanced the photocatalytic activity of the nanocomposites for H₂ production.The factors affecting the photocatalytic H₂ production efficiency of CdS/MoS₂/rGO nanocomposites were investigated under the conditions of changing the CdS loading capacity,catalyst dosage and photocatalytic reaction sacrificial agent ratio.The results showed that when lactic acid was used as a sacrificial agent and 0.35 wt%CdS was loaded in nanocomposites,the CdS/MoS₂/rGO catalyst could obtain the best rate of H₂ production,which is about 11331.65μmol·h^-1·g^-1.After 6 cycles of reaction,the CdS/MoS₂/rGO nanocomposites still had a high H₂ production efficiency,and its catalytic activity could still be maintained at about 91%,indicating that the photocatalyst has high stability.According to the calculation formula of apparent quantum efficiency（AQE）,the AQE of CdS（0.35 wt%）/MG under 420 nm light was 10.4%.（2）Under mild conditions,Ag₂S/rGO nanocomposites were synthesized in situ on the surface of GO using a gram-negative bacterium—S.oneidensis MR-1 as the biological reducing agent and Na₂S₂O₃as the electron acceptor.The results showed that Ag₂S nanoparticles（NPs）with a particle size of about 10 nm were uniformly distributed on reduced graphene oxide（rGO）,which proved the possibility of green synthesis of nanocomposites by S.Oneidensis MR-1.Fluorescence spectroscopy（PL）and current-time（I-T）experiments confirmed that the formation of Ag₂S/rGO can inhibit the recombination of photogenerated electrons and holes.In the experiment,Ag₂S/rGO and Ag₂S were respectively used as photoelectrode materials in photoelectrochemical reactions.Under the irradiation of visible light,Ag₂S/rGO nanocomposite showed better photocatalytic activity.Compared with pure Ag₂S,the photocurrent density of Ag₂S/rGO nanocomposites was increased by about 3 times,and the Ag₂S/rGO nanocomposites exhibited better photochemical properties,indicating that rGO promoted electron transfer and made the separation efficiency of photogenerated electron-hole pairs higher.（3）In the experiment,Au/rGO nanocomposites were prepared with GO as the substrate and chloroauric acid as the precursor.Au/rGO nanomaterials were prepared by chemical synthesis method and microbial method respectively,and sodium citrate and S.oneidensis MR-1 were used to reduce Au³⁺to Au.Then,CdS NPs were loaded by in-situ synthesis method to successfully prepare Au/CdS/rGO nanocomposites.In the photoelectric performance test,the photocurrent density of the Au/CdS/rGO nanocomposites was about twice as high as that of the CdS NPs,indicating that the nanocomposites had a better response to visible light.The H₂production rate of Au/CdS/rGO nanocomposites under visible light was 995.1μmol·h^-1·g^-1,which was much higher than that of CdS NPs(114.89μmol·h^-1·g^-1).The structure of ternary materials provided a more convenient way for electron conduction.The presence of rGO improved the separation efficiency of electron-hole pairs,and Auprovided an active site for catalytic H₂production.Biosynthesis and hydrothermal methods effectively controlled the morphology of nanoparticles in a low-cost manner,successfully satisfying the green synthesis and efficient recycling of graphene-based nanocomposites,and the nanocomposites had good catalytic activity and excellent H₂ production efficiency,the research results provided a new way for the development of photocatalytic technology.