Fabrication and characterization of hierarchical nanostructure materials
Graphene, a one-atom-thick planar sheet of graphite, has demonstrated high electrical conductivity, charge carrier mobility, specific surface area, as well as excellent mechanical and thermal properties. The graphene nanosheets have been integrated with various nanoparticles (NPs) to form NPs/graphene hierarchical nanostructure materials. The hierarchical nanostructure materials exhibit excellent properties and new functionalities due to the synergetic effects between graphene nanosheets and the nanoparticles, and are regarded as one of the most promising materials for energy storage and conversion. However, to date, the wide application of NPs/graphene nanostructures has been hindered due to several technical challenges. The first challenge involves the effective functionalization of graphene with nanoparticles. The second one is a lack of the understanding of the processing-structure-property relationship of the hierarchical nanostructure. In this dissertation, two types of nanoparticles (metallic silver nanoparticles and non-metallic C60 nanoparticles) were functionalized onto graphene nanosheets. The processing-structure relationship of the hierarchical structures was well studied and analyzed. Moreover, the specific capacitance of the silver NPs/graphene hierarchical nanostructures and the power factor of the C60/graphene/polymer nanocomposites were quantitatively compared and analyzed. First, the processing-structure-property relationship of silver nanoparticles (Ag NPs)/graphene nanostructure was studied. Silver nanoparticles were deposited onto graphene nanosheets through electrostatic attraction and subsequent reduction. Characterizations by X-ray diffraction and transmission electron microscopy (TEM) have confirmed the formation of Ag NPs/graphene nanostructure. The TEM images and TGA test results showed that the concentration of the silver salt solution can effectively tune the graft density and morphology of the silver nanoparticles on graphene. The nitrogen adsorption/desorption tests indicated that the Ag NPs prevented the restacking of graphene sheets, resulting in larger surface area. The electrical conductivity and the specific capacitance of silver-deposited graphene were improved by adjusting the concentration of the silver salt solution. Particularly, the electrical conductivity and capacitance increased by 3 times and 2 times, respectively, when compared with the as-fabricated graphene nanosheets. Secondly, the processing-structure relationship of C60/graphene nanostructure was investigated. In this study, a novel covalent method and a unique non-covalent method were presented to synthesize the nanostructure. The resultant materials were characterized by FT-IR spectroscopy, UV-Vis spectroscopy, atomic force microscopy, and TEM. The characterization results confirmed that fullerene was successfully attached onto graphene nanosheets through Fisher esterification reaction and liquid-liquid interfacial precipitation, respectively. Through particle analysis, it was found that the particle size (from 5 to 85 nm), size distribution, and morphology can be tuned by adjusting the concentration of the C60 solution. Finally, the enhancement effect of C60/graphene hierarchical structure on the power factor of composites was investigated. The C60/graphene hierarchical materials were incorporated into epoxy resin and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PSS). The electrical conductivity of epoxy resin was significantly increased by incorporating the hierarchical structures. An optimal power factor of 1.1 μW/K2m was achieved with the addition of 45 wt% C60/graphene hierarchical nanostructure. When incorporated the hierarchical materials with the PEDOT:PSS, the Seebeck coefficient was increased through an energy-filtering effect between the hierarchical nanostructure and the PEDOT:PSS. A power factor of 32.4 μW/mK2, which was ~50% higher than the value of pure PEDOT:PSS, was achieved by properly adjusting the weight ratio between the C60, graphene and PEDOT:PSS. This dissertation developed three novel methods to functionalize graphene with nanoparticles. The processing-structure relationship of Ag NPs/graphene and C60/graphene hierarchical nanostructures was well studied and analyzed. Through these investigations, the specific capacitance and power factor of the hierarchical nanostructures and their composites were improved. This dissertation will provide guidance for design and fabrication of the NPs/graphene hierarchical nanostructures to achieve a better performance on energy storage and conversion.