The dependence of the measured surface energy of graphene on nanosheet size

Abstract

The surface energy of graphene nanosheets is surprisingly poorly known, probably due to size effects and energetic heterogeneities. Here we use finite-dilution inverse gas chromatography to measure the surface energy of liquid-exfoliated, few-layer graphene nanosheets of different sizes as a function of probe coverage. In all cases, the surface energy falls with probe coverage from a defect-controlled, low-coverage value to a value that approaches the basal plane surface energy at high coverage. We find an intrinsic basal plane dispersive surface energy of 61 ± 4 mJ m−2, close to the value of 63 mJ m−2 found for graphite. By comparison with similar data measured on graphite and using simple models, we can use the length dependence of the low coverage surface energy to differentiate between the effects of edge and basal plane defects, finding these to contribute ∼130 and 180 mJ m−2 to the surface energy respectively. From this data, we estimate a basal plane defect content of ∼6 × 1014 defects m–2 for both graphite and graphene in reasonable agreement with Raman data. This work shows that, in terms of surface energetics, few-layer graphene nanosheets behave exactly like graphite with the only differences associated with platelet dimensions.