The Oxidation of Cobalt Nanoparticles into Kirkendall-Hollowed
CoO and Co<sub>3</sub>O<sub>4</sub>: The Diffusion Mechanisms and
Atomic Structural Transformations
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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 Co<sub>3</sub>O<sub>4</sub> NPs. Through XAS, XRD, TEM, and DFT calculations, the
mechanisms of the transformation from ε-Co to CoO to Co<sub>3</sub>O<sub>4</sub> 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 Co<sub>3</sub>O<sub>4</sub> NPs. Our Co<sub>3</sub>O<sub>4</sub> NPs possess a high ratio
of {110} surface planes, which are known to have favorable catalytic
activity. The Co<sub>3</sub>O<sub>4</sub> NPs can be redispersed in
an organic solvent by adding surfactants, thus rendering a method
to create solution-processable colloidal, monodisperse Co<sub>3</sub>O<sub>4</sub> NPs