Dynamic Atomic-Scale Fundamental Mechanisms of the Initial Stages of Cu Oxidation Revealed by Environmental TEM*

Surface oxidation is an important process for corrosion, which costs a few percent of the U.S. Gross Domestic Product (GDP) each year. Much is known about oxygen interaction with metal surfaces and the macroscopic growth of thermodynamically stable oxides. At present, however, the transient stages of oxidation - from nucleation of the metal oxide to formation of the thermodynamically stable oxide - represent a scientifically challenging and technologically important terra incognito. These issues can only be understood through a detailed study of the relevant microscopic processes at the nanoscale in situ. We have previously demonstrated via in situ transmission electron microscopy (TEM) that the formation of epitaxial Cu₂O islands during the transient oxidation of Cu(100), (110), and (111) films bear a striking resemblance to heteroepitaxy, where the initial stages of growth are dominated by oxygen surface diffusion and strain impacts the evolution of the oxide morphologies.  To deepen our understanding of the atomic-scale dynamic processes of Cu₂O island formation on Cu during oxidation in situ, we are presently using correlated in situ high-resolution environmental TEM (ETEM) and atomistic theoretical simulations. As an example of this approach, preferential monolayer-by-monolayer growth along Cu₂O(110) planes was noted instead of along Cu2O(100) planes. Correlated Density Functional Theory (DFT) simulations on the surface and diffusion energies during Cu₂O growth on various Cu₂O surface orientations and terminations were carried out. Our DFT results show that monolayer formation of Cu₂O along Cu₂O(110) was both thermodynamically and kinetically preferred over that of Cu₂O(100) during Cu₂O growth, which explains the observed phenomenon. These results shed new light on the epitaxial oxide growth mechanism and provide a deeper understanding of the dynamic processes involved in initial oxidation, which will ultimately help to precisely predict, design, and control nanostructured oxide growth for either corrosion-protection or creating nano-oxides for their functional properties.