The segregation and oxidation of silicon on Pt(111), OR: The question of the “platinum oxide”

Abstract

The appearance of so-called Pt oxide is correlated with the presence of oxide forming bulk impurities segregating to the surface during O 2 exposures at high temperature. This situation is illustrated by studying the enrichment of Si to the Pt(111) surface under oxidizing conditions. The equilibrium segregation of Si on a Pt(111) crystal which contained Si as an impurity was measured over the temperature range 770–1170 K using Auger electron spectroscopy (AES) as a monitor of the Si surface concentration. The energy of segregation was determined as ∾9 kcal mole . The segregated Si was exposed to molecular O 2 and atomized oxygen by using a hot W filament. The rate of oxidation in atomized O 2 at ~400 K was much faster than in molecular O 2 at various temperatures. The Si (LVV) Auger transition at 91 eV underwent drastic changes during oxidation of Si at ~400 K. In particular a strong decrease in its intensity erased any direct evidence for Si being present on the Pt surface. The possibility of confusing oxidized Si layers with “platinum oxide” is pointed out. The effect of O 2 exposure on the rate and maximum amount of Si segregation was investigated at 873 K. The oxidized Si accumulates on the Pt(111) surface under these conditions most likely in the form of SiO 2 clusters, characterized by an Si (LVV) Auger peak at 77 eV. The initial rate of Si segregation is not influenced by O 2. The amount of Si segregation in O 2 is no longer related to the equilibrium segregation curve of Si. In particular, there is no apparent saturation level for the accumulation of SiO 2 on Pt(111) under the present experimental conditions.