Large-scale and green production of multi-layer graphene in deep eutectic solvents

To promote the industrial applications of graphene, it is crucial to develop a low-cost, green, and efficient production method. A practical and eco-friendly deep eutectic solvent-assisted ball milling technique was developed to prepare multi-layer graphene in this study. The expanded graphite was used as raw material, and the deep eutectic solvent was prepared by mixing urea and choline chloride. The obtained graphene was characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, thermogravimetric analysis, and Raman spectroscopy. After ball milling for 48 h, most of the graphite particles could be exfoliated into graphene sheets, and no new covalent bond or interaction was formed. Multi-layer graphene in N-methylpyrrolidone had better dispersibility and larger layers than that in N,N-dimethylformamide. The obtained multi-layer graphene exhibited few defects, high crystal integrity, 11–12 layers, 2–5 μm size at the lateral dimension, and superior thermal stability. This technique of low-energy ball milling in the nontoxic deep eutectic solvents provides a new idea for the efficient exfoliation of multi-layer graphene and also a certain reference value for the exfoliation of other two-dimensional nanosheets.