Thus, if O2 sticking were controlled mainly by the availability of O-free surface sites, then the initial EEO2 for pre-heated Si NPs should be greater than, or equal to the initial EEO2 for preheated silica NPs. Furthermore, since the surface oxygen content of the NPs can only increase during O2 etching (eventually leading to passivation), we might expect EEO2 to decrease monotonically with etching time as increasing oxygen coverage blocks O2 adsorption sites.