Atomistic Insights into Early Stage Oxidation and Nanoscale Oxide Growth on Fe(100), Fe(111) and Fe(110) Surfaces

Abstract

Reactive molecular dynamics (MD) simulations with dynamic charge transfer between atoms is used to investigate the oxidation kinetics during the early stages of nanoscale oxide growth on Fe(100), Fe(111), and Fe(110) surfaces. The growth rate of the oxide layer was found to follow logarithmic time dependence, with limiting thicknesses ranging from 1 to 2 nm depending on the crystal orientation. Temperature and pressure effects were studied for the three surface geometries, with the (110) surface exhibiting a stronger dependence compared to the (100) and (111) counterpart. Structure and dynamical correlations in the metal/oxide/gas environments are used to gain insights into the evolution and morphology of the growing oxide film. The surface structure is found to strongly influence not only the morphology of the oxide but also the stoichiometry of the oxide layer formed. Stoichiometry of the oxide layer formed at room temperature shows evidence for the presence of a nonstoichoimetric oxide layer consisting of two phases: a surface layer dominant with mixed oxide (FexOy with y/x ≈ 1.3–1.5) oxides and a bulk layer of FexOy with y/x ≈ 0.7–0.8. This is found to be directly related to the propagation of the oxide growth through the thin film. The relative fractions and near surface distribution of the mixed oxides are dictated by the differences in cationic/anionic diffusivities which are strongly dependent on the crystal surfaces, consistent with previously established experimental observations. At any given oxidation condition, the activation energy barrier for oxidation was found to be lowest for Fe(110) (7.44 KJ/mol) compared to the other two surfaces (23.69 KJ/mol for Fe(100) and 19.88 KJ/mol for Fe(111)). The differences in oxide formation in the early stages of oxidation are explained in terms of the transport characteristics of the anion/cation for the various crystal orientations. The simulation findings agree well with previously reported experimental observations of oxidation on Fe surfaces.