On the Relationship between Nonstoichiometry and Passivity Breakdown in Ultrathin Oxides: Combined Depth-Dependent Spectroscopy, Mott−Schottky Analysis, and Molecular Dynamics Simulation Studies

Abstract

Understanding the relationship between nonstoichiometry and physical properties of ultrathin oxides is of great importance from both scientific and technological aspects. A specific example includes the onset of passivity breakdown in an ultrathin oxide film in aqueous medium leading to the onset of corrosion. In this work, using the model system of ultrathin oxide of alumina on aluminum synthesized by natural oxidation and photon-assisted oxidation processes, we demonstrate a direct correlation between passivity and quality of the oxide film quantitatively. Depth-dependent high-resolution X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and nuclear reaction analysis (NRA) have been performed to characterize the physical and chemical properties of the oxide films, while detailed impedance measurements and Mott−Schottky studies have been performed to understand electronic transport. Combined NRA and TEM analysis reveal an 18% increase in oxygen density (for oxide films with near identical thicknesses ∼3.8 nm) in the case of photon-assisted oxidation. The denser oxide film results in a ∼34% more blockage of chloride ions transport as indicated by XPS analysis. Mott−Schottky measurements on these oxide films indicates a 43% reduction of defect levels for UV-synthesized alumina when compared to native one, suggestive of chloride ion transport via oxygen vacancies. Additionally, molecular dynamics simulations have been performed to provide insights into the structure of the oxides at the atomic level to correlate with the experimental measurements. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al (100) surfaces because of atomic and molecular oxygen. Oxidation using molecular and atomic oxygen resulted in an amorphous oxide scale with self-limiting thickness of ∼16 and 22 Å, respectively, at 300 K. Structural and dynamic correlations indicate significant charge transfer to exist in the oxide film in both the cases. The oxide growth in both the cases occurs due to the inward oxygen and outward cation diffusion. The calculated in-plane and out-of-plane atomic diffusivities are 40−70% higher in case of atomic oxidation. In the presence of atomic oxygen, the O/Al ratio is more uniform and varies from 1.37 at the oxide−gas interface to 1.30 at the metal−oxide interface, whereas that formed by natural oxidation was substoichiometric and oxygen deficient with O/Al values varying from 1.27 (oxide−gas interface) to 1.05 (metal−oxide interface) at room temperature. The simulation results are consistent with the reported experimental investigations.