Physiochemical phase transformations in Co/CoO nanoparticles prepared by inert gas Condensation

We present results of the studies of structural and chemical transformations in Co/CoO nanoparticles prepared by inert gas condensation. The effect of the morphology and agglomeration on the phase transformation reaction path in self-oxidation and in controlled reduction processes are discussed in detail. As-prepared samples show self-oxidation related to the non-core/shell morphology of the particles. Annealing of particles at 250 °C in reducing atmosphere leads to the oxidation of the particles showing coexistence of CoO and Co3O4 structures. This is explained by the diffusion of oxygen from the amorphous oxide surface to the bulk of the nanoparticles. Upon increasing the reaction temperature beyond 250 °C, reductive transformation of the samples occurs systematically, from CoO/Co3O4 to CoO to Co (HCP + FCC) and eventually to Co (FCC). We have presented X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and magnetic data to track the structural and chemical transformation paths. We found strong correlation between structural and magnetic properties. Thermodynamic stability as a function of reaction temperature on the phase/chemical transformation is also discussed.