Preparation, Structure And Performance Of Graphene Composites

Graphene sheet is a two-dimensional carbon material, which is composed of monolayers of carbon atoms tightly packed into hexagonal symmetry. Due to its low bulk density, large specific surface area, extraordinary mechanical, thermal, electrical and optical properties, graphene has become a hot topic in the fields of physics, chemistry and material science. Unfortunately, inherent high-surface energy graphene sheets usually result in an irreversibly agglomerated structure in water and common organic solvents, which has complicated their uses and processing in liquid phase and thus greatly limited their integration in potential applications. Therefore, exploring suitable materials, which can simultaneously stabilize and form complexes or hybrids with graphene in solvents, as well as being able to endow graphene with additional performance, has become a new challenge tor non-covalent approaches toward solubilizing graphene sheets. Currently, the hybridization modifications of graphene still face several challenges that need to be solved. For example, how to design and fabricate graphene-based hybrids with hierarchical and ordered nanostructures via simple methods; how to achieve a homogeneous and (or) oriented nanostructure of graphene and graphene-based hybrids within polymer matrix. In this regard, we focused on the design and construction of graphene-based hybrids via non-covalent approaches, the preparation-structure-property relationship of graphene-based hybrids with hierarchical and ordered nanostructures, and the further applications of the graphene-based hybrids in polymer nanocomposites and supercapacitors. The main results obtained in this dissertation are as follows:1. Graphene oxide (GO) sheets, considered as "soft" two-dimensional dispersants containing multiple aromatic regions, can adsorb the pristine carbon nanotubes (CNTs) through π-π stacking interactions, thus resulting in the pristine CNTs to be stably dispersed and fractionated in aqueous media. Solubility results indicate that the GO sheets are prone to stabilize the CNTs with larger diameters. This can be probably developed into a practical method to fractionate CNTs with different outer diameters in water, which are important for non-covalent approaches toward solubilizing graphene and thus open a new way for GO applications in colloidal chemistry and other fields.2. A simple and practical strategy based on vacuum-assisted self-assembly has been used here to fabricate flexible GO-CNT hybrid films with highly ordered nanostructures. The reduced GO-CNT (G-CNT) hybrid films have been used as novel substrates for the immobilization of electroactive polyaniline (PANI) nanoparticles. The as-obtained flexible and electrically conductive G-CNT-PANI ternary hybrid film shows a much improved electrochemical performance than G-CNT hybrid films. These intriguing nanostructures and electrochemical performances make the G-CNT-PANI ternary hybrid films very promising electrode materials for supercapacitors.3. Direct reduction of GO in water in the presence of acid-treated CNTs (t-CNTs) results in a homogeneous dispersion of reduced graphene oxide (r-GO) and t-CNT (G-CNT) hybrids. The three-dimensional G-CNT hybrids possess unique properties, making them ideal reinforcing fillers for the reinforcement of poly(vinyl alcohol)(PVA) composites using a simple casting method. Accompanied by assisted-dispersion of r-GO with t-CNTs, the incorporation of r-GO that acts as a CNT "carrier" plays an important role in assisted-dispersion of t-CNTs, and thus a uniform dispersion for both t-CNTs and r-GO in matrix has been achieved and evidenced. By using such hybridization and co-exfoliation method, PVA composites reinforced with the as-prepared G-CNT hybrids show remarkable enhancement in mechanical properties, and an improved thermal stability of PVA composites is also achieved.4. The aqueous colloids of r-GO sheets have been synthesized in an alkaline solution. By using NaCl salts as flocculants, the resultant r-GO aggregates can be easily redispersed in appropriate organic media as individual sheets with the aid of sonication, which is a facile and economical approach for the large-scale production of organically dispersible graphene sheets without the addition of complicated dispersing agents or the need for functionalization of the starting GO. The homogeneous graphene organic dispersion facilitates solution-processing procedures for incorporating graphene into novel thermoplastic polyurethane (TPU) composites with much improved mechanical, electrical and thermal properties.5. Lightweight and highly flexible composite films consisting of r-GO sheets and carbon spheres (CSs) have been fabricated by vacuum-filtration of a homogeneous suspension containing graphene-CS composites followed by calcination. Under filtration-induced directional flow, graphene-CS composites can be assembled to form highly ordered microscopic structures throughout the composite film, where uniformly dispersed CSs are immobilized in the space between the parallel arranged graphene frameworks. The as-obtained composite film is particularly promising for flexible electrode materials, exhibiting good cycle stability and capacitances larger than that of graphene and CS alone.6. Aqueous colloidal dispersion of r-GO has been obtained by direct reduction of GO in the presence of exfoliated montmorillonite (MMT) nanoplatelets. The formation of stable dispersion of r-GO sheets in water is a result from two contributing factors, i.e. the hydrogen-bonding interaction and the crosslinking effects originated from sodium ions acting as "crosslinkers" between r-GO sheets and MMT nanoplatelets. Through filtration, the two-dimensional and positively charged MMT and r-GO can self-assemble to form highly oriented hybrid films, where,MMT and r-GO stack with each other in a fashion of interlocking arrangements. The as-obtained hybrid films show excellent flexibility, electrical conductivity and fire-retardant properties.