Sublimation Kinetics for Individual Graphite and Graphene Nanoparticles (NPs): NP-to-NP Variations and Evolving...

A single nanoparticle (NP) mass spectrometry method was used to measure sublimation rates as a function of nanoparticle temperature (T_NP) for sets of individual graphite and graphene NPs. Initially, the NP sublimation rates were ∼400 times faster than those for bulk graphite, and there were large NP-to-NP variations. Over time, the rates slowed substantially, though they remained well above the bulk rate. The initial activation energies (E_a values) were correspondingly low and doubled, as a few monolayers worth of material was sublimed from the surfaces. The high initial rates and low E_a values are attributed to large numbers of edge, defect, and other low coordination sites on the NP surfaces, and the changes are attributed to atomic-scale "smoothing" of the surface by preferential sublimation of the less stable sites. The emissivity of the NPs also changed after heating, more frequently increasing. The emissivity and sublimation rates were anticorrelated, leading to the conclusion that high densities of low-coordination sites on the NP surfaces enhance sublimation but suppress emissivity.