Several aspects of the results suggest that the pre-heated silica NPs must have retained significant silica cores, or at least significant subsurface oxygen content. As discussed above, the fact that the silica initial EEO2 values were substantially higher than those for Si NPs under the same conditions, is tentatively taken as evidence that the silica NPs either didn't melt completely during pre-heating (Tmelt(silica) = 1986 K, Tmelt(Si) = 1687 K), or that there was differential contraction of a core/shell structure during cooling, such that the surface layer was rough or strained. More persuasive evidence that the NPs retained significant core oxygen content comes from comparing the passivation timescales for pre-heated silica and Si NPs under identical conditions. The silica NPs passivated much more rapidly, and based on the mechanism developed in modeling Si NP passivation, the most likely explanation is participation of subsurface oxygen in the passivation process. O-rich subsurface material would reduce the oxygen-sink capacity of the subsurface, and O from the subsurface would also tend to diffuse to the surface, with both effects tending to increase the surface O concentration, thus hastening SiO2 nucleation and growth.