Atomic Dynamics of Multi‐Interfacial Migration and Transformations

Redox‐induced interconversions of metal oxidation states typically result in multiple phase boundaries that separate chemically and structurally distinct oxides and suboxides. Directly probing such multi‐interfacial reactions is challenging because of the difficulty in simultaneously resolving the multiple reaction fronts at the atomic scale. Using the example of CuO reduction in H2 gas, a reaction pathway of CuO → monoclinic m‐Cu4O3 → Cu2O is demonstrated and identifies interfacial reaction fronts at the atomic scale, where the Cu2O/m‐Cu4O3 interface shows a diffuse‐type interfacial transformation; while the lateral flow of interfacial ledges appears to control the m‐Cu4O3/CuO transformation. Together with atomistic modeling, it is shown that such a multi‐interface transformation results from the surface‐reaction‐induced formation of oxygen vacancies that diffuse into deeper atomic layers, thereby resulting in the formation of the lower oxides of Cu2O and m‐Cu4O3, and activate the interfacial transformations. These results demonstrate the lively dynamics at the reaction fronts of the multiple interfaces and have substantial implications for controlling the microstructure and interphase boundaries by coupling the interplay between the surface reaction dynamics and the resulting mass transport and phase evolution in the subsurface and bulk. In situ environmental transmission electron microscopy shows atomic processes of CuO‐surface‐reduction induced multi‐interface transformations, in which surface oxygen vacancies diffuse into deeper atomic layers and activate multi‐interface transformations. Cu2O/m‐Cu4O3 interface shows a diffuse‐type interfacial transformation controlled by lattice diffusion of oxygen vacancies whereas the m‐Cu4O3/CuO transformation exhibits ledge‐propogation interfacial transformation regulated by oxygen desorption from the interfacial ledges.