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-Cu₄ O₃ → Cu₂ O is demonstrated and identifies interfacial reaction fronts at the atomic scale, where the Cu₂ O/m-Cu₄ O₃ interface shows a diffuse-type interfacial transformation; while the lateral flow of interfacial ledges appears to control the m-Cu₄ O₃ /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 Cu₂ O and m-Cu₄ O₃ , 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.