Synergistic effect of supercritical CO₂ and organic solvent on exfoliation of graphene: Experiment and atomistic simulation studies

In this work, experiments and molecular dynamics (MD) simulations are carried out to explore the synergistic effect of supercritical CO₂ (scCO₂) and organic solvent on intercalation and exfoliation of graphene. Experimental characterizations via transmission electron microscopy, atomic force microscopy and Raman spectroscopy indicate that by combining scCO₂ and organic solvent (N-methylpyrrolidone, NMP), few-layer graphene is successfully exfoliated from graphite, among which over 30% is 1-4 layers, and 55% is 5-8 layers. Systematic experiments have shown that compared with pure scCO₂ or NMP, the mixed scCO₂ and NMP can significantly increase the amount of graphene and the rate of few-layer graphene, and the optimum volume fraction of NMP is 25%. Parallel MD simulations indicate that the scCO₂ molecules first diffuse into the interlayer of graphite, and then the larger NMP molecules insert as wedges and further expand interlayer spacing, promoting intercalation and exfoliation. The iteration of scCO₂ diffusion and the NMP wedge can generate positive feedback to improve the exfoliation productivity and efficiency. This work explores the synergistic effect of scCO₂ and NMP on the exfoliation of graphene, which may provide useful insights for exfoliation of other two dimensional materials.