High temperature bulk Si oxidative etching has been proposed to involve the following basic steps, the importance of which vary with temperature and PO2: or reversing passivation. 6. Oads diffusion to subsurface sites, increasing the amount of oxygen adsorbed at saturation. For example, saturation was observed at one monolayer of oxide coverage at room temperature, but at 800 K several monolayers-worth of oxide were present in the XPSaccessible top few nanometers at saturation. In our experiments (TNP ≥ 1200 K), diffusion should be faster, presumably increasing the subsurface oxygen absorption capacity and suppressing passivation. On the other hand, if the pre-heated silica NPs retain significant O concentration in their cores, diffusion may also bring O from the core to the surface, speeding passivation.