Preparation of graphene nanosheets by shear-assisted supercritical CO2 exfoliation

We have developed a procedure of shear-assisted supercritical CO2 exfoliation (SSCE) for producing high-quality and large-quantity graphene nanosheets in this work. Supercritical CO2 fluid can diffuse in between the interlayer of graphite due to its high diffusivity and low viscosity. When high-speed shear stress was applied to CO2 fluid, graphite powder was expanded and delaminated effectively into graphene sheets. Our experiments indicate that high temperature and high rotation speed of CO2 fluid accelerate the exfoliation process, consistent with our molecular dynamics simulation results that the shock velocity of CO2 molecules should be large enough for stable separation of graphene layers. It is found that the graphene synthesized by SSCE consists of 90% exfoliated sheets with less than 10 layers and similar to 70% between 5 and 8 layers, confirmed by atomic force microscopy and transmission electron microscopy characterizations. The conductivity of graphene is up to 4.7 x 10(6) S/m, much higher than that of chemically reduced graphene oxide. Therefore, SSCE provides a simple, fast, and economical method of producing large-scale and high-quality graphene. (C) 2015 Elsevier B.V. All rights reserved.