Parviz, Dorsa (2016-08). Graphene Colloidal Dispersions: Production and Processing for Nanocomposites and Three-Dimensional Networks. Doctoral Dissertation. Thesis uri icon

abstract

  • Graphene, a two-dimensional layer of carbon atoms, is prized for its extraordinary properties including high electrical and thermal conductivities, surface area and exceptional mechanical strength. These supreme properties make it a promising material for applications such as electronics, nanocomposites, and energy storage devices. The scalable and repeatable production of high-quality graphene in large quantities is a challenging task. Among all the various production methods, the liquid-phase exfoliation from graphite is a promising technique due to its potential scalability, low cost, and simplicity of processing. Yet, the reaggregation of graphene sheets in the liquid, caused by the strong inter-sheets attractive forces, restricts the graphene yield of this method. One of the main goals of this thesis is to increase the yield of liquid-phase exfoliation method without compromising on the graphene quality. Non-covalent functionalization of graphene with specific dispersant molecules prevents reaggregation of the sheets and increases the graphene concentration in dispersions, while it preserves the ?-conjugated network of the nanosheets. Here, pyrene-derivatives are used as dispersants to stabilize pristine graphene in aqueous dispersions through non-covalent functionalization. We study the dependence of the graphene yield on the dispersant concentration, functional groups, counterions, solvent choice, and pH of solution. The graphene yield and graphene/dispersant ratio obtained by pyrene derivatives exceeds those obtained by polymers and surfactants. The pyrene-graphene interactions are then exploited for designing novel copolymer dispersants which can improve the graphene dispersion in polymer nanocomposites. Normally, the incompatibility of pristine graphene surface energy with polymers increases the interfacial tension within the nanocomposites and prevents proper dispersion of the graphene nanosheets. The pyrene-polysiloxane copolymers, synthesized through a hydrosilylation reaction, act both as graphene stabilizers in the dispersions and the host matrix of the resulting nanocomposite. The graphene/polysiloxane composite films are cast from the dispersion and their electrical properties and morphological structure is characterized by various techniques. Similar strategy is used to prepare pyrene-functional copolymers of polystyrene (PS) and poly(methylmethacrylate) (PMMA) as graphene dispersants. The graphene dispersions prepared by these dispersant are vacuum filtered to yield Janus graphene/PS and graphene/PMMA composite films with one electrically-conductive side and another electrically-insulating side. In order to prepare aggregation-resistant graphene powder, we crumple graphene nanosheets via rapid evaporation of the dispersions in an industrially scalable spray dryer. Morphological transition of 2D nanosheets to 3D crumpled particles is directly observed by sample collection within the spray dryer. The particle size and morphology of the crumpled sheets is tuned by adjusting the Peclet number of spray drying process. The unfolding of the crumpled particles upon rewetting depends on the sheet type and the solvent choice. The crumpled GO nanosheets are then used to prepare porous 3D networks of graphene using an aqueous sol-gel technique. The high surface area and electrical conductivity of these networks can be exploited in applications such as energy storage, chemical sensing, oil adsorption and catalysis.
  • Graphene, a two-dimensional layer of carbon atoms, is prized for its extraordinary properties including high electrical and thermal conductivities, surface area and exceptional mechanical strength. These supreme properties make it a promising material for applications such as electronics, nanocomposites, and energy storage devices. The scalable and repeatable production of high-quality graphene in large quantities is a challenging task. Among all the various production methods, the liquid-phase exfoliation from graphite is a promising technique due to its potential scalability, low cost, and simplicity of processing. Yet, the reaggregation of graphene sheets in the liquid, caused by the strong inter-sheets attractive forces, restricts the graphene yield of this method.

    One of the main goals of this thesis is to increase the yield of liquid-phase exfoliation method without compromising on the graphene quality. Non-covalent functionalization of graphene with specific dispersant molecules prevents reaggregation of the sheets and increases the graphene concentration in dispersions, while it preserves the ?-conjugated network of the nanosheets. Here, pyrene-derivatives are used as dispersants to stabilize pristine graphene in aqueous dispersions through non-covalent functionalization. We study the dependence of the graphene yield on the dispersant concentration, functional groups, counterions, solvent choice, and pH of solution. The graphene yield and graphene/dispersant ratio obtained by pyrene derivatives exceeds those obtained by polymers and surfactants.

    The pyrene-graphene interactions are then exploited for designing novel copolymer dispersants which can improve the graphene dispersion in polymer nanocomposites. Normally, the incompatibility of pristine graphene surface energy with polymers increases the interfacial tension within the nanocomposites and prevents proper dispersion of the graphene nanosheets. The pyrene-polysiloxane copolymers, synthesized through a hydrosilylation reaction, act both as graphene stabilizers in the dispersions and the host matrix of the resulting nanocomposite. The graphene/polysiloxane composite films are cast from the dispersion and their electrical properties and morphological structure is characterized by various techniques. Similar strategy is used to prepare pyrene-functional copolymers of polystyrene (PS) and poly(methylmethacrylate) (PMMA) as graphene dispersants. The graphene dispersions prepared by these dispersant are vacuum filtered to yield Janus graphene/PS and graphene/PMMA composite films with one electrically-conductive side and another electrically-insulating side.

    In order to prepare aggregation-resistant graphene powder, we crumple graphene nanosheets via rapid evaporation of the dispersions in an industrially scalable spray dryer. Morphological transition of 2D nanosheets to 3D crumpled particles is directly observed by sample collection within the spray dryer. The particle size and morphology of the crumpled sheets is tuned by adjusting the Peclet number of spray drying process. The unfolding of the crumpled particles upon rewetting depends on the sheet type and the solvent choice. The crumpled GO nanosheets are then used to prepare porous 3D networks of graphene using an aqueous sol-gel technique. The high surface area and electrical conductivity of these networks can be exploited in applications such as energy storage, chemical sensing, oil adsorption and catalysis.

publication date

  • August 2016