Fast high-shear exfoliation of natural flake graphite with temperature control and high yield

High-shear exfoliation of graphite is a cost-effective liquid-phase exfoliation (LPE) method for producing few-layer graphene dispersions. However, the drawbacks of LPE such as low graphene concentrations, long processing times and solvent residuals hamper the scalability of the process and applicability of the dispersions. Here, we demonstrate a fast graphene synthesis where concentrations as high as 3 mg/ml with a 3% yield are produced only after 2 h of shear exfoliation in environmentally friendly aqueous medium using sodium cholate as surfactant. The produced graphene is of high quality and mostly less than 5 layers thick. The high concentration is attributed to a proper choice of effective mixing parameters, low exfoliation temperature (10 °C) and an appropriate surfactant concentration. In addition, we noticed that larger graphite flakes (250 μm-1 mm) increased the lateral dimensions of the exfoliated sheets while the yield and concentration were independent of the flake size. Furthermore, using larger graphite flakes improved the conductivity of the subsequent graphene films. Prior to film fabrication, dialysis removed excess surfactant from the dispersions improving the film conductivity (17 000 S/m) by ca 40%.