Comparison of oxygen-free graphene sheets obtained in DMF and DMF-aqua media

Abstract

N,N-Dimethylformamide (DMF) is often used as a liquid medium for the exfoliation of graphene for further use in a wide range of fields of science and technology: biomedical research, the technology of materials for electronics, and polymer composites. However, the nature of the processes of exfoliation of graphene sheets from the surface of micron-sized graphite particles and the subsequent stabilization of its dispersion has hardly been studied. In this work, the stages of graphene suspension formation in DMF and its mixture with aqua molecules under the action of ultrasonic irradiation were investigated. Characterization of graphene suspensions was carried out using transmission electron microscopy (TEM), dynamic light scattering (DLS), FTIR spectroscopy, Raman spectroscopy, and UV-Vis spectroscopy. The presence of aqua in the colloid system leads to the predominant formation of 3D graphene sheets with larger linear dimensions than in pure DMF. The defectiveness of the basal plane of graphene sheets obtained by exfoliation in pure DMF is significantly lower than in a mixture with aqua, while the edge defectiveness in both cases is rather low. A comprehensive analysis of the spectral data and TEM results with electron diffraction shows that in both cases graphene in suspensions is multilayer. However, if in pure DMF the graphene suspension was formed in one stage and its layering did not exceed five, then in the presence of aqua the formation of the dispersed phase occurred in two stages. At the second stage, a partial association of the sheets from finer fraction increases the sizes of graphene sheets, i.e., the particles with higher surface energy were attached to larger ones or associated with each other. Perhaps this is because aqua molecules included in the complexes and removed from the global hydrogen bond system, tend to restore these bonds and, thus, facilitate the fusion of the smallest and lightest graphene sheets.