Interactions between graphene sheets and ionic molecules used for the shear-assisted exfoliation of natural graphite

Water-soluble graphene suspensions have been prepared directly from natural graphite powder. Graphite is mechanically ground with small ionic organic molecules or polyelectrolytes, followed by sonication. The grinding produces a mechanical shearing force, which is supposed to expose new surface and help the molecules to adsorb on it and intercalate into the graphite lattice. The graphene composites, their aqueous suspensions and, especially, the interactions between the adsorbates and the graphene sheets, were characterized using UV-vis and luminescence spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, particle size distribution and zeta potential measurements, and atomic force microscopy. The sign of the zeta potential of the colloidal graphene sheets was determined by the charge of the adsorbate. Highest zeta potentials were obtained with polyelectrolytes and taurodeoxycholic acid but the stability of the pyrene and naphthalene based suspensions was also good or even excellent. Changes in the aggregation of the molecules upon adsorption were observed and a specific donor/acceptor interaction between the hydroxy group of the naphthalene derivative and graphene was evidenced by the fluorescence spectra. In most cases the suspensions contained significant amounts of single or few-layer graphene.