The transition from surface to bulk oxide growth on Pt(1 0 0): Precursor-mediated kinetics

We investigated the kinetics governing the transition from surface (2D) to bulk (3D) oxide growth on Pt(1 0 0) in ultrahigh vacuum as a function of the surface temperature and the incident flux of an oxygen atom beam. For the incident fluxes examined, the bulk oxide formation rate increases linearly with incident flux (Φ_O) as the oxygen coverage increases to about 1.7 ML (monolayer) and depends only weakly on the surface temperature in the limit of low surface temperature (T_S < 475 K). In contrast, in the high temperature limit (T_S > 525 K), the bulk oxide formation rate increases with for oxygen coverages as high as 1.6 ML, and decreases with increasing surface temperature. We show that the measured kinetics is quantitatively reproduced by a model which assumes that O atoms adsorb on top of the 2D oxide, and that this species acts as a precursor that can either associatively desorb or react with the 2D oxide to form a 3D oxide particle. According to the model, the observed change in the flux and surface temperature dependence of the oxidation rate is due to a change in the rate-controlling steps for bulk oxide formation from reaction at low temperature to precursor desorption at high temperature. From analysis of flux-dependent uptake data, we estimate that the formation rate of a bulk oxide nucleus has a fourth-order dependence on the precursor coverage, which implies a critical configuration for oxide nucleus formation requiring four precursor O atoms. Considering the similarities in the development of surface oxides on various transition metals, the precursor-mediated transition to bulk oxide growth reported here may be a general feature in the oxidation of late transition metal surfaces.