Physical-chemical exfoliation of pristine graphite flakes

Abstract

Two-dimensional (2D) nanomaterials, especially graphene, have received increasing attention due to their mechanical, electronic, optical, chemical, and thermal properties. A set of physical-chemical techniques was applied to increase the exfoliation effect's promotion and characterization over the natural graphite flakes. More than 30 natural impurities were determined in raw graphite flakes by instrumental neutron activation analyses. A possible graphite exfoliation mechanism combining ultrasonication and low temperatures in liquid nitrogen was postulated using density functional theory (DFT) calculation. The sonication shock waves act on the graphite surfaces resulting in compressive stress propagating throughout the graphite bulk. The most observed effect was the corner's damage, promoting gas interlayer adsorption pathways, followed by explosive desorption. The applied sequence of treatments results in an increasing defect level. Intense exfoliation was observed with several graphite layers, visible after gamma radiation treatment in doses ranging from 0.1 MGy to 6.0 MGy on standard ambient temperature and pressure. The increasing irradiation dose mostly affects the surface of the flakes. The pristine Graflake shows a homogeneous carbon surface with no signs of oxidation. At the presence of oxygen peroxide under oxygen atmosphere, samples irradiated at 0.3 MGy already show more brick surface signs. The irradiation dose associated with the oxidation generates a much less homogeneous surface, the result is a more bricked and somehow smaller flakes. Under the same oxidizing atmosphere at a higher irradiation dose, the effect has mostly increased in all surfaces compromised by oxidation, favoring the flakes' dispersion. The treatment association results in a bricked surface with much smaller particles of the original Graflake, favoring the exfoliation.