Preparation Of The Graphene-based Functional Aerogels And Their Applications In Environmental Treatment

With the rapid development of industrialization and urbanization of the world,various environmental pollution problems such as water pollution,air pollution,soil pollution,noise pollution,and electromagnetic pollution are becoming more and more serious,which bring threat to the natural environment and human health.Therefore,it is urgent to explore effective treating methods and materials.Graphene,as a two-dimensional material,has shown excellent performance comparing with traditional materials,due to its large specific surface area,good chemical stability,and high electrical conductivity.The application potential of graphene in environmental treatment has been revealed.Among them,graphene aerogel(GA)with a three-dimensional(3D)network structure,which can be constructed by the assembly of graphene flakes,has become one of the research hotspots.Compared with two-dimensional(2D)graphene materials,the properties and application of GA can be further widened.In recent years,using the graphene oxide(GO)produced from graphite powder as the precursor for the preparation of GA with low cost,has laid a good foundation for the application of GA.But there still exist some problems and challenges in the preparation and application processes.Firstly,the random physical connection and stacking between GO flakes during the assembly process,which leads to poor electrical conductivity and mechanical properties of GA;secondly,the precise regulation of pore structures is hard to achieve due to the disorderly building and assembling between GO flakes;besides,compounding GO with other materials is one of the efficient ways to realize the functionalization of GA,while owing to the lack of clear understanding about the microscopic-mechanisms of the assembly process,the functionalization of composite materials and the practical applications of GA were limited.Aiming at finding approaches to solve the above problems,in this thesis,firstly,the influence of the functional groups of GO flakes on the formation of 3D network structure was explored,and the optimization of 3D network structure and the synthesis of the photocatalytic nanoparticles were realized simultaneously by the controlled reduction of oxygen-containing functional groups on GO flakes,which reveals an efficient way for the functionalization and precise assembly of GA;thereafter,by using bubble and nanoparticle templates,the pore structure of GA was optimized,and the regulation of surface polarity and the enhancement of mechanical property on GO flakes were achieved by combining GO with surfactants and metallic-carbon nanotubes(metallic-CNTs),respectively;and an approach of cross-linking the GO flakes by using nanoparticles was also proposed.To sum up,this thesis provided various meaningful strategies for the construction of GA with good mechanical performance and excellent functionalization,thus providing theoretical guidance for the development of GA.In addition,the performance of the constructed GA in the adsorption of pollutants,photocatalytic degradation,and microwave absorption was investigated,revealing its promising application potential in environmental pollution treatments.This thesis including as follows:In Chapter 1,the basic structure and properties of graphene and GO were introduced briefly.The preparation methods and problems of GA were summarized,and the necessity of the preparation of graphene-based composites was discussed.The mechanism,advantages,and prospects of GA in the treatment of environmental pollution were analyzed.Finally,the research contents and proposed works of this thesis are outlined.In Chapter 2,based on the controlled reduction strategy,the formation of 3D graphene structure,the loading of photocatalysis nanoparticles(NPs),N-doping are realized simultaneously,and the constructed GA could be used for the treatment of the large-scale organic pollutants under sunlight.GO was prepared by the improved Hummers method by using graphite as the raw materials.Based on the basic principle of thermodynamics,ethylenediamine(EDA)was selected as the reducing agent.By decreasing the reaction temperature and time,GO flakes were gradually reduced and deoxidized,thus achieving the precise regulation of the self-assembly process.The selective retention of oxygen-containing functional groups of reduced graphene oxide(rGO)hydrogel promotes the adsorption of Zn2+.Therefore,the reduction of rGO hydrogel,nitrogen doping,and the growth of NPs can be realized simultaneously during the hydrothermal reaction.The N-doped graphene aerogel loaded with zinc oxide(ZnO)NPs(ZnO/N-GA)not only has good mechanical stability,but also shows excellent adsorption performance towards organic pollutants.Moreover,owing to the heterojunction interface formed between ZnO NPs and graphene,as well as the improved electrical conductivity by N-doping and 3D structure of GA,ZnO/N-GA shows excellent photocatalytic performance towards organic pollutants under visible light,providing new materials for the treatment of water and soil pollution.In Chapter 3,GA with hierarchical pore structure and excellent mechanical properties was constructed based on the bubble/nanoparticle template to achieve the efficient adsorption and recovery of leaked organic solvents or crude oil.In the assembly process,the bubble template was introduced to form the micro-scale pore structure,which enhances the connectivity between pores and the adsorption capacity towards organic solvents.Meanwhile,Cu2+is pre-adsorbed on the GO flakes,and CuO NPs were grown in situ during the hydrothermal reaction to promote the formation of nano-scale pore structure.The construction of hierarchical pore structure greatly increased the specific surface area of GA,while the mechanical properties were improved significantly.Owing to the hydrophilic functional groups/NPs exposed on the GA surface,the constructed GA shows good adsorption performance towards a variety of organic solvents with different polarities.Moreover,due to its excellent mechanical stability,the organic solvents can be recovered by extrusion.The GA constructed by the dual-template strategy not only has a good application prospect in dealing with a large number of leaked organic solvents,but also presents the characteristics of viscous adsorption on water droplets,which also has application potential in water collection.In Chapter 4,based on the above work,the bubble/nanoparticle templates are further combined with strategies of the regulation of surfactant and pH to construct GA with super-amphipathicity,which has good performance in the treatment of compounding pollutants.On the one hand,the introduced surfactant plays a role in enhancing the stability of the bubble template,and on the other hand,the hydrophilicity of the composites was enhanced due to hydrophilic head groups of surfactant are arranged outward.While the metal cations promote the cross-linking of GO flakes to bubble templates,and the generated NPs in the hydrothermal reduction process also increase the roughness of the aerogel surface,thus further increases the wettability of the aerogel surface.Therefore,the aerogel shows super-amphiphilic characteristics and presents good treatment performance for compounding pollutants such as water-soluble dyes,organic solvents,emulsified wastewater,etc.In addition,since the interaction between GO and surfactant can be regulated by pH value,the material shows remarkable pH responsiveness and the wettability transitions between super-amphiphilic to hydrophobic can be achieved by pH regulation,so it also has potential application in the field of oil-water separation.In Chapter 5,metallic-CNTs were compounded with GO to enhance the mechanical properties and electrical conductivity of GO flakes,and Fe3O4 NPs were used to promote the interfacial cross-linking of GO flakes,the GA with high electrical conductivity,excellent mechanical stability,and microwave absorption properties was constructed.Molecular dynamics simulation was used to investigate the interaction between metallic-CNTs and GO flakes.It was found that metallic-CNTs adsorbed on the unoxidized area of the GO surface.The continuous network structure formed by metallic-CNTs enhances the mechanical properties of GO flakes,at the same time,the Fe3O4 NPs adsorbed on the edge of GO flakes due to the steric effect,while enhancing the connectivity of flakes.It was found that the effective absorption bandwidth of GA is 4.4?18 GHz,and the maximum reflection loss is-49 dB,thereby showing good application prospects in microwave adsorption.In summary,this thesis explored several strategies to construct GA with good mechanical properties and efficient functionalization by using GO as the precursor.The theoretical mechanisms were explored to provide clearer theoretical guidance for the assembly and find more effective strategies for the functionalization of GA,meanwhile solving the problems of the poor mechanical properties and disordered pore structure of GA.The constructed composite aerogels show excellent mechanical stability,and present good performance in the adsorption of pollutants,photocatalytic degradation,and microwave absorption respectively,thus providing broad application prospects in the treatment of environmental pollutions.The research work in this paper has both theoretical guidance and practical application value.