Etching was done at TNP = 1200 K except for one silica NP (Minitial = 22.9 MDa -green circles) which was studied at 1300 K, however, as shown in In contrast, carbon NPs behave very differently, as shown by the graphene NP example included in Fig. . That NP had initial EEO2 in the same range as the Si-containing NPs, and during the first ~30 minutes of etching, EEO2 decreased smoothly by a factor of ~30, but after that point, EEO2 began to have order-or-magnitude fluctuations superimposed on the continuing slow decline. We found that all graphitic NPs studied showed waves of fast etching interspersed with periods when the NP was much less reactive. Oxidized carbon surface groups (carbonyls, epoxides, etc.) are unstable with respect to decomposition/desorption in this TNP range, thus passivation cannot be due to oxide layer formation. Fully coordinated sp 2 carbon, such as that found in graphite basal planes, is unreactive toward O2 in this temperature range, thus carbon NP passivation is attributed to isomerization to form surface layers with few under-coordinated reactive sites. Examples of fully coordinated surface structures include carbon nano-onions (multiwall fullerenes), which are known to form when carbon NPs are annealed, and looped edge structures (carbon connecting across basal plane edges) observed to form when graphite is annealed. The observation of strong fluctuations in reactivity as the NP begins to passivate, implies that during the isomerization process, the NP surface goes through time periods with high and low numbers of reactive surface sites.