The interesting point is not that the inert NPs continued to lose mass slowly, but rather that they were able to lose thousands of atoms without creating surface defects that restored the NPs' reactivity. Perfect graphite basal planes are inert to O2, but if the surface is sputtered to create defects, O2 reactions at the defects create etch pits that expand and consume the defective basal plane. Analogous behavior for the onionized surface layer of an NP would result in an etching wave removing the surface layer. Isomerization within the surface layer allow accommodation of some carbon loss without creating reactive sites, but clearly there there must be a limit. The graphite NP in Fig. had M =12.42 MDa at the point when it finally became inert. The NP then lost ~6700 atoms (~80 kDa) without regaining reactivity, corresponding to more than 10% of the surface layer for a spherical NP of this mass. Furthermore, when PO2 was increased at the end of the oxidation period, an additional ~250 kDa was lost without EEO2 ever rising above ~3% of its initial value, i.e., the NP was able to lose mass corresponding to ~half of the surface layer, without triggering fast etching. Finally, in the flash heating experiments (Figs. and), even larger mass losses occurred on time scales of a few seconds without restoring reactivity. As outlined in the SI, similar considerations make it unlikely that the "inert" structures formed at long etching times could have spiral layers.