Much less is known about the elementary reactions that occur during the initial stage of hafnium oxidation, but given the refractory nature of Hf (T melt = 2506 K, T boil = 4876 K) and HfO 2 (T melt = 3031 K, T boil = 5670 K) we might have expected HfO 2 to form a stable oxide layer at 1500 K. If that had occurred, however, we would have observed a small mass gain leading to almost immediate passivation. Instead, O 2 exposure resulted in prolonged etching, implying that the mass loss from desorption of Hf-containing species more than offsets any mass gain from possible oxygen adsorption or absorption. A key result for the interpretation of these experiments is the observation that HfO 2 powder heated in the 2250 to 2600 K range has substantial vapor pressure, and that the dominant Hf-containing species in the vapor is HfO. The O/O 2 partial pressures were not measured in this experiment, however, it was noted that the initially white HfO 2 powder darkened, implying that it became Hf-rich. shown in Fig. imply that the numbers of undercoordinated surface sites were initially high, resulting in fast initial etching. The subsequent >two orders-of-magnitude drop in reactivity implies that the NP surfaces transformed under etching conditions to have very few reactive sites. Multiwall fullerene-like structures, known as carbon nano-onions, are known to form spontaneously when carbon NPs are heated, and if the fullerene-like surface layers are closed and defect free, such NPs would have no undercoordinated surface sites. Carbon NP passivation, therefore, is attributed to isomerization leading to the formation of surface layers with fullerene-like structure. The isomerization has interesting and material-dependent dynamics, as can be seen from the fluctuating etching efficiencies for the graphite and carbon black NPs in Fig. and discussed in detail elsewhere. The absence of initial mass gains upon O 2 exposure clearly shows that reactions of Hf and Si that lead to mass loss (e.g. HfO or SiO desorption) initially outweigh reactions that might form stable oxides.