Nonetheless, the NPs eventually do passivate, and in the bulk materials, passivation is clearly associated Curiously, the conditions of the Si NP etching experiment shown in Fig. are well within the high T/low P O2 range where bulk Si passivation is not observed; thus, the observation that Si NPs passivate under these conditions was unexpected. The etching/passivation kinetics for Si NPs were modeled based on kinetics developed for bulk Si, leading to the conclusion that the apparently anomalous passivation for NPs is a direct consequence of their nanoscale sizes. In addition to SiO desorption, surface O ads can be lost by diffusion into the bulk, reducing the O ads coverage and inhibiting SiO 2 formation. For NPs, the oxygen absorption capacity of the NP core is small, therefore the sub-surface O concentration builds up more rapidly than for bulk Si. We hypothesized that as the sub-surface region becomes O-saturated, SiO 2 nucleation and growth on the NP surface is triggered, passivating the NP.