Modeling based on the mechanistic steps outlined above suggested that this qualitative difference between NP and bulk passivation resulted from the effects of the limited NP volume on the disposition of Oads. In our TNP range, most Oads is lost by SiO (or O/O2) desorption, but diffusion to subsurface sites should be a significant sink for Oads, inhibiting surface SiO2 formation and passivation. For NPs, the high surface-area-to-volume ratio means that the subsurface O concentration would tend to saturate more readily during etching than for bulk Si, favoring surface passivation in several ways. As the subsurface approaches saturation, this would block further Oads diffusion to subsurface sites, leaving more Oads to nucleate and grow surface SiO2. At high O concentrations, SiO2 may also nucleate and grow in the subsurface region, becoming surface SiO2 as overlying material etches away. Finally, higher subsurface O concentrations should also stabilize surface SiO2 against loss by reaction with underlying Si (SiO2 + Si → 2 SiO(g)).