The nature of high surface energy sites in graphene and graphite

Variations in the adsorption enthalpies of acetone to few-layer graphene and graphite nanopowders were analyzed as a function of surface coverage. The adsorption enthalpies were measured by inverse gas chromatography at low monolayer coverage levels (0.1-20%). The adsorption enthalpies increased from −13 kcal/mol at the lowest coverage to −7.5 kcal/mol. We fitted the measured adsorption enthalpies as a function of coverage using a two-state model and estimated the number of high-energy sites on both materials. The graphite powder had seven times more high-energy sites than the few-layer graphene, which explains why the adsorption enthalpies for graphite increased more slowly with increasing coverage. We also performed a theoretical study based on density functional theory calculations using a functional that accounts for dispersive interactions to elucidate the nature of the high-energy adsorption sites. The calculated adsorption enthalpies ranged from −16 to −1 kcal/mol while the adsorption enthalpy to a plain graphite surface was −9 kcal/mol. The high-energy adsorption sites were localized on surface steps and edge-cavities. The adsorption enthalpies at very low coverage therefore corresponded to adsorption on steps and edge cavities, while those measured at coverage levels of ∼4% or more reflected adsorption to the flat surface.