The lack of significant mass gain implies that even when first exposed to O 2 at 1500 K, processes resulting in loss of C, Si, or Hf atoms dominated over processes that might have added mass, such as chemisorption of O 2 on the NP surfaces. For carbon NPs, the absence of mass gain is unsurprising because the oxidized surface functional groups that form from O 2 chemisorption on carbon (e.g. ethers, carbonyls) are unstable in this temperature range, desorbing as O or O 2 or as CO or CO 2 , the latter pair resulting in mass loss. For Si, O 2 adsorbs dissociatively to form a variety of O ads species, however, at 1500 K most O ads desorbs as SiO, resulting in net mass loss.