Preparation of few-layer and single-layer graphene by exfoliation of expandable graphite in supercritical N,N-dimethylformamide

Few-layer graphene (FG) is produced in supercritical N,N-dimethylformamide (DMF) using expandable graphite (EG) as starting material in less than 15 min. Monolayer graphene is produced by exfoliation of FG in supercritical DMF. The samples were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), and Raman spectroscopy. AFM result reveals that the average thickness of FG is about 3 nm and monolayer graphene is about 1.2 nm. Raman result indicates that the ID/IG of FG is 0.3, which proves a small proportion of defects in it. Moreover, we have studied the effect of process parameters on the yield of FG. Results show that a yield of 7 wt% can be obtained at an optimum condition (with concentration being 2 mg/ml, temperature being 673 K, and volume ratio being 0.67). Besides, the temperature of FG formation is very low (473 K). Finally, these results show that EG is an excellent precursor for the fabrication of FG and monolayer graphene.