The Oxidation of Cobalt Nanoparticles into Kirkendall-Hollowed CoO and Co₃O₄: The Diffusion Mechanisms and Atomic Structural Transformations

Abstract

We report on the atomic structural changes and diffusion processes during the chemical transformation of ε-Co nanoparticles (NPs) through oxidation in air into hollow CoO NPs and then Co₃O₄ NPs. Through XAS, XRD, TEM, and DFT calculations, the mechanisms of the transformation from ε-Co to CoO to Co₃O₄ are investigated. Our DFT calculations and experimental results suggest that a two-step diffusion process is responsible for the Kirkendall hollowing of ε-Co into CoO NPs. The first step is O in-diffusion by an indirect exchange mechanism through interstitial O and vacancies of type I Co sites of the ε-Co phase. This indirect exchange mechanism of O has a lower energy barrier than a vacancy-mediated diffusion of O through type I sites. When the CoO phase is established, the Co then diffuses outward faster than the O diffuses inward, resulting in a hollow NP. The lattice orientations during the transformation show preferential orderings after the single-crystalline ε-Co NPs are transformed to polycrystalline CoO and Co₃O₄ NPs. Our Co₃O₄ NPs possess a high ratio of {110} surface planes, which are known to have favorable catalytic activity. The Co₃O₄ NPs can be redispersed in an organic solvent by adding surfactants, thus rendering a method to create solution-processable colloidal, monodisperse Co₃O₄ NPs