Graphene-based assemblies from colloidal suspensions

This thesis explores the properties, morphologies and formation mechanisms of graphene-based assemblies from colloidal suspensions. The influence of pH on colloidal graphene oxide (GO) liquid crystals was investigated, and the phase transition plots were sketched. A crossing point, where suspensions at pH > pKa of the GO sheets have different behavior than suspensions at pH < pKa, was found. This was obtained by broadening of the bi-phasic region in the isotropic to nematic transition, in different shape and internal configuration of the nematic droplets, and in the magnitude of fractionation between large and small sheets in the nematic and isotropic phases. A unique structure of spherical GO liquid crystals was shown for the first time, and their formation explained by a mechanism that is triggered by polydispersity in surface charge density, which enhances dissimilarity between large and small sheets at pH > pKa. The formation of rGO and GO films by dip-coating from colloidal suspension was examined, and capillary-force-assisted self-assembly (CAS) as a method for preparation of thin films of rGO was reported. Assisted by capillary forces at the contact line, the particles in the isotropic suspension become aligned with the film growth direction as the contact line moves across the substrate surface. The degree of order in rGO films and assemblies was determined by birefringence and di-attenuation imaging. The slow axis of the rGO platelets within the CAS films displayed a narrow angular distribution (±3°) within a film area of 1 mm2, resulting in a high order parameter (S) of ∼1 with 8-fold enhancement of electrical conductivity compared to films formed by traditional techniques such as filtration. The influence of pH and withdrawal rate on the texture of GO films formed by dip-coating from colloidal suspensions was investigated. A criss-cross pattern of wrinkles was obtained and three competing mechanisms were suggested, the dominancy of which changes under different conditions. The use of the LC-PolScope system, which is based on polarized light microscopy and sophisticated acquisition and analysis processes, in the characterization of graphene materials is presented for the first time. The system was found to be a powerful tool for characterization of both GO and rGO architectures, such as liquid crystals and thin films. The effect of cross-linking of graphene-sheets by tetrazonium chemistry on the porosity and the nanostructure of graphene-based materials was investigated for both GO and rGO sheets. The porous structure’s performance as electrodes in super-capacitors using cyclic voltammetry and electrochemical impedance spectroscopy was examined. Most surprisingly, the cross-linked GO material after chemical reduction becomes extremely dense with minimal surface area, whereas a fluffy, highly porous product is obtained when rGO sheets are cross-linked. Because the grafting relies on the presence of sp2 domains, the reaction is more efficient for rGO sheets and prevents restacking and creates a 3-D structure with very high proportion of exposed edge planes. The exposed edge planes improve the electrochemical capacitance from 60 F/g for graphene sheets that were not cross-linked to 95 F/g for the cross-linked architecture, as measured by cyclic voltammetry.