Given that the pre-heated silica NPs have Si-rich surface layers on a silica core, another passivation mechanism might be thought to be fast initial etching of the Si-rich surface layers, followed by passivation as etching exposed the silica core. In that limit, the passivated NPs would essentially be pure silica, but that possibility is excluded by the fact that the passivated NPs retained high 532 nm absorptivity and visible/near IR emissivity, i.e., the passivated NPs were still "black". Silica NPs would be transparent, as shown by the experiment shown in Fig. .