THE ROLE OF THE SURFACE ELECTRONIC STRUCTURE IN THE OXIDATION OF IRON

Abstract

The Fromhold–Cook coupled-currents theory of metal oxidation states that below ~ 150° C , a saturation thickness is reached, which is independent of temperature, as a result of the diminution of the electron tunnel current. However, Fe and Co show a strong temperature dependence of the oxide saturation thickness. In this study, we combine ellipsometry, X-ray Photoelectron Spectroscopy (XPS) and the high energy ion beam technique Elastic Recoil Detection (ERD) for the detailed determination of oxidation rate and oxide thickness, composition, and electronic structure on Fe(100) in O 2. At room temperature, first a 1 nm thin FeO film forms, followed by the growth of a 1 nm mixed Fe 2+/ Fe 3+ layer, which forms the effective barrier against further oxidation. By vacuum annealing at 200°C, the Fe 3+ is reduced to Fe 2+ and the oxidation kinetics through a single Fe 2+ containing layer can be studied, yielding results in excellent agreement with the FC theory. At T<150° C , the transport of metal ions to the surface is rate-limiting and the saturation thickness is independent of temperature (3.2 nm of FeO). At T ≥ 150° C , the thermionic emission of electrons determines the oxidation rate. Thus, a complete and consistent picture of the initial thin oxide layer growth on Fe single crystals is obtained.