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

Abstract

A single nanoparticle (NP) mass spectrometry method was used to measure sublimation rates as a function of nanoparticle temperature (TNP) for a number of individual graphite and graphene NPs. Initially, the NP sublimation rates were ~400 times faster than that for bulk graphite, and there were large NP-to-NP variations. Over time, the rate slowed substantially, though remaining well above the bulk rate. The initial activation energies (Eas) were correspondingly low and doubled as a few monolayer's worth of material were sublimed from the surfaces. The high initial rates and low Eas are attributed to large numbers of edge 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, most frequently increasing. The emissivity and sublimation rates were anti-correlated, leading to the conclusion that high densities of low-coordination sites on the NP surfaces enhances sublimation but suppresses emissivity

High Temperature Oxidation of Single Carbon Nanoparticles: Dependence on Surface Structure, and Probing Real-Time Structural Evolution via Kinetics.

“…Once TNP had stabilized at 1900 K, the slope of M vs. time was used to determine the raw mass loss rate, -1773 Da/sec. Sublimation dominates the mass loss at 1900 K, 4 and while we have no information about the product species, for bulk graphite a distribution of small Cn species is observed. [30][31][32][33] The nominal (spherical) surface area of this NP was ~4463 nm 2 , giving Rsublimation of ~0.4 Da/sec/nm 2 , and the total mass sublimed was ~1.4 MDa -roughly a monolayer's (ML's) worth of carbon for a spherical NP of this mass.…”

“…Our experimental protocols for single NP kinetics measurements have been reported previously [4][5] along with the method for TNP determination, and analysis of the uncertainties in NP mass and reaction rates (~1.3% absolute, ~0.2% relative) and TNP (~6.2% absolute, ~2% relative). Further details are given in the SI.…”

“…This behavior was attributed to the NPs initially having large, but highly variable numbers of undercoordinated surface sites, and that preferential sublimation at such sites reduces the numbers of such sites over time, reducing the rates, and modifying the optical properties. 4 We also reported a study of O2 oxidation of graphitic NPs. 5 Etching rates peaked in the 1200 -1500 K range, dropping at higher TNP.…”

“…S5, and then TNP was set to 1500 K. Rbase was near zero (~constant mass) because 1500 K is well below the range where graphite sublimation is significant. 4 At the ~1.39 hour point, 4.3 x 10 -5 Torr of O2 was added (cyan background), resulting in a significant mass loss rate, attributed to etching reactions in which CO (and possibly CO2) desorb. The raw etching rate accelerated during the ~10 minute oxidation period, from -680 Da/sec to -1239 Da/sec after a few minutes, corresponding to an EEO2 of ~1.44 x 10 -3 Da/O2 collision, averaged over the oxidation period.…”

“…To enable study of these effects, we have developed an approach to measuring surface reaction kinetics for individual NPs, held in an electrodynamic trap. [4][5] For example, we previously found that sublimation of graphitic NPs at temperatures (TNP) > 1700 K was substantially faster than the rates for bulk graphite, [6][7][8] varied by up to an order of magnitude from NP-to-NP, and slowed significantly over time, correlated with increases in optical emissivity. This behavior was attributed to the NPs initially having large, but highly variable numbers of undercoordinated surface sites, and that preferential sublimation at such sites reduces the numbers of such sites over time, reducing the rates, and modifying the optical properties.…”