Abstract
We mixed high-pressure supercritical carbon dioxide (scCO2) and graphite together without any auxiliary means for exfoliation. Combined with small-angle X-ray scattering (SAXS) and other characterizations, the influence of process parameters on the quantity and quality of graphene was studied. We calculated the Hansen solubility parameters and the critical conditions for exfoliation and used the response surface method to probe the effect of process parameters on the graphene exfoliation. The mechanism for exfoliation was critically discussed. The results showed that the highest (1.625 mg/mL) concentration was obtained after mixing at 45 MPa for 48 h and centrifuging at 1000 rpm for 30min. The number of layers and lateral dimensions of graphene sheets could be controlled by adjusting process parameters. Up to 75% of the produced graphene after mixing at 45 MPa for 48 h was about 3-6layers, and 90% of the lateral dimensions were less than 1.4 μm. The kinetic energy generated by the flowing CO2 was the main reason for exfoliation. The critical speed of exfoliation at 50 °C and 45 MPa was about 2.851–2.981 m/s. This method had no complicated operation steps, the obtained graphene surface was not oxidized, no solvent remained, and few-layer graphene did not need further purification.
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