The liquid exfoliation of graphene in polar solvents

Highlights: • More than 300 graphene dispersions were prepared in solvents at short sonication time. • Exfoliation of graphene can be optimized in DMF at short sonication time. • The exfoliated graphene in DMF or ethanol is no more than ten layers. Graphene is the most attractive 2D nanomaterial with interesting applications from smart coating to high frequency electronics. Its production through liquid exfoliation of graphite in suitable solvents is the most practical approach to prepare graphene dispersions. In recent years, the production of graphene through a short sonication time has attracted huge attention, trying to find a balance between the quality and cost. However, little is known about how the technical parameters (solvent type, graphite amount, sonication time, centrifugation time and centrifugation speed) affect the exfoliation process. By combining different spectroscopical (UV–vis, EDS, IR, XRD and Raman) and morphological (SEM and TEM) characterizations, statistical analysis, and ab initio simulations, we present a detailed experimental and theoretical investigation seeking to explain the liquid exfoliation of graphene in polar solvents at short sonication time (up to 7 h). In particular, we demonstrate that the short-time exfoliation process can only be optimized by dispersing graphene in dimethylformamide (DMF). Raman measurements show that edge-type defects are most noticeable when graphene is exfoliated in ethanol, which is corroborated by SEM analysis. TEM results show well-exfoliated graphene nanosheets in DMF, but depend on the centrifugation speed. Computational studies (considering one- and two-graphene layers) predict that the orientation of the molecule and the number of layers affect the final adsorption energy.