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

A single
nanoparticle (NP) mass spectrometry method was used to
measure sublimation rates as a function of nanoparticle temperature
(TNP) 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 (Ea values) were correspondingly low and doubled, as a few
monolayers worth of material was sublimed from the surfaces. The high
initial rates and low Ea 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