Ambient energy dispersion and long-term stabilisation of large graphene sheets from graphite using a surface energy matched ionic liquid†

Abstract

Liquid phase exfoliation (LPE) is the most promising method for scalable graphene production, but conventionally requires significant energy input to break apart layers in the bulk material, introducing defects and limiting production of large, few-layer sheets. In this work, we exfoliated graphene under ambient conditions with minimal external energy input using the surface energy matched ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIm Ac), then isolated and characterised the exfoliated material. This method produced large and thin, graphene sheets with size up to 3 μm. Monolayer sheets were identified in samples that had been left undisturbed for 6 months prior to sampling, meaning the graphene sheets suspended in the ionic liquid are stabilized against restacking; to our knowledge, this is the longest period of time large (non-oxidised) graphene monolayers have been suspended in any liquid. Molecular dynamics simulations show the IL cation is enriched at the graphene surface. We postulate this leads to a positive charge on the graphene sheets in EMIM Ac leading to an electrostatic repulsion which contributes to colloidal stability. Strong adsorption of the EMIM cation to graphene is confirmed by energy dispersive X-ray spectroscopy images that show ionic liquid adsorbed to graphene sheets even after washing. Ambient energy exfoliation using ionic liquids offers simple, energy efficient method for producing large, high quality graphene sheets, with long-term stability in the ionic liquid. [Display omitted]