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We mixed high-pressure supercritical carbon dioxide (scCO 2) 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 CO 2 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. [Display omitted] [ABSTRACT FROM AUTHOR]

High quality and high quantity few-layer graphene was successfully prepared using a new impinging jet method. Natural graphite flakes were first agitated in N-methyl pyrrolidone (NMP) with the assistance of supercritical CO2, then the half-exfoliated graphite was further stripped using the shear stress derived from the impinging jets. After the energy conversion and stress analysis of the graphite particles during the whole exfoliation process, it was revealed that the size of the target mesh, the distance between the nozzle and the target, the decompression rate, and the size of the raw materials had a significant influence on the exfoliation process. Additionally, a microscopic view of the exfoliation and dispersion mechanism of graphene in the CO2–NMP system was investigated using molecular dynamics simulation, and CO2 was found to be beneficial for the penetration of NMP into the graphite sheets. Finally, the concentration and quality characteristics of the prepared graphene were characterized using ultraviolet–visible spectroscopy, transmission electron microscopy, Raman spectroscopy, and atomic force microscopy. The maximum concentration was as high as 0.689 mg ml−1, the thickness of 68% of the product was less than 2.5 nm, and the lateral dimension was from 0.5 to 3.0 μm. These results indicate that this impinging jet method is promising for large-scale industrial production. [ABSTRACT FROM AUTHOR]