Influence of ternary hydroalcoholic solvent on direct exfoliation concentration of graphite

The direct exfoliation of graphite to form graphene was investigated in a ternary solvent system consisting of ethylene glycol, isopropanol, and water in various volume ratios. We found that a previously reported fractional cohesion parameters map was applicable to estimating the graphene concentration obtained using the ternary solvent system. Almost all of the solvent composition that afforded medium or high graphene dispersion concentrations were found to lie within the limiting boundary region, and the solvent composition that led to high graphene dispersion concentrations were located at the center of this region. Furthermore, the thermodynamics of the exfoliation process were examined in an effort to use the free energy to explain the influence of the initial graphite concentration and exfoliation temperature on the obtained graphene concentration and yield. The results revealed that the graphene yield first increased with increasing initial graphite concentration and reached a maximum at an initial graphite concentration of approximately 14.5 mg/mL, beyond which the graphene yield decreased with increasing initial graphite concentration. The graphene concentration and yield also increased with decreasing temperature, and maximum values of the graphene concentration (72.9 μg/mL) and yield (4.99 μg/mg) were obtained at 10 °C. The fractional cohesion parameters map therefore provides additional method when selecting ternary solvents for the preparation of graphene dispersions, allowing improved graphene concentrations and yields to be obtained using lower temperatures and the appropriate initial graphite concentrations.