Exfoliation and Chemical Modifications of Natural Flake Graphite

Graphene, a two-dimensional sheet of atomically thin carbon, has recently gained attention due to its excellent electronic, mechanical, and optical properties. It has the potential for use in a variety of applications such as circuits, solar cells, and transparent conducting electrodes. In order to feasibly produce graphene, practical methods for its preparation need to be developed. Current methods, such as micromechanical exfoliation (Scotch tape™), solvent exfoliation and chemical vapor deposition (CVD) are far from ideal. Micromechanical exfoliation produces minute amounts of very small graphene flakes, and CVD is expensive and lacks scalability. This work presented in this thesis focuses on producing graphene sheets from natural flake graphite. Graphite, composed of stacked graphene sheets, is naturally available at low costs. Our use of graphite as a source for graphene will allow for the large-scale production of inexpensive graphene. This thesis presents two approaches for the preparation of graphene from graphite: chemical modification and direct exfoliation. Chemically modifying graphite with oxygen functional groups to form graphite oxide (GO) increases the interlayer spacing and reduces the van der Waals attractive forces holding the stacked sheets together. Oxidation increases the water dispersibility of the graphene and subsequent reduction restores some of the electrical and mechanical properties of the sheets. We study the oxidation process in order to optimize the resulting GO material and study the diazonium functionalization of the dispersed GO. Diazonium salt addition allows for the addition of nitro groups, increasing the water solubility of the graphene sheets and increasing the processing options of the sheets. Direct exfoliation of graphite to graphene sheets is studied using both experimental and computational techniques. First, we demonstrate the use of an equimolar mixture of benzene and hexafluorobenzene (B/HFB). This solvent mixture self assembles into alternating B/HFB stacks due to strong quadrupolar interactions. We find this stacked morphology can be templated by graphene sheets during sonication, resulting in order 30 Å from the basal plane of the graphene. This ordering suspends graphene in solution by preventing re-aggregation and the result is a suspension of exfoliated, pristine graphene in a highly volatile, easily removable solvent. A second system, using a water/heptane solvent mixture, is shown stabilize the graphene sheets at the interface of the phase-separated solvents in order to minimize interfacial energy. This stabilization is then used to form thin, transparent and conductive films of graphene as the sheets spread at the interface and climb the walls of the sample vials