This hypothesis was tested by experiments with surface-reduced silica NPs. Silica is transparent in the visible with low thermal emissivity. Thus, freshly introduced silica NPs are not easily heated by 532 nm laser irradiation and are undetectable in our experiments because they emit so weakly. When freshly-trapped silica NPs were briefly CO 2 -laser heated, however, they began to absorb strongly at 532 nm and emit blackbody-like thermal spectra, implying that they became Si-rich, i.e., that heating drove desorption of excess oxygen in addition to SiO and/or SiO 2 . Furthermore, upon O 2 exposure, the preheated silica NPs initially etched with kinetics essentially identical to those for pure Si NPs, suggesting that their surface layers were almost pure Si. However, the surface-reduced silica NPs were found to passivate more quickly, and after much less mass loss than similar size Si NPs. Rapid passivation was attributed to the presence of substantial sub-surface oxygen in the surface-reduced silica NPs, inhibiting O ads diffusion into the sub-surface, leading to O ads buildup on the surface, and triggering SiO 2 formation.