As noted above, while HfO 2 is more refractory than SiO 2 , in both cases, sublimation of the more volatile monoxide (HfO), along with excess oxygen, occurs at temperatures well below T melt , and is believed to be responsible for the initial fast mass loss. The passivation behavior of Si and Hf NPs is also quite similar, thus we tentatively propose that passivation in both cases is triggered by buildup of subsurface O, leading to nucleation and growth of a passivating di-oxide layer.