Preparation, characterization and thermal evolution of oxygen passivated nanocrystalline cobalt

Abstract

Nanocrystalline cobalt powders have been prepared by the inert gas evaporation method. After preparation the materials were passivated by pure oxygen and air exposure. In the present paper we describe the application of different techniques like transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) to achieve an exhaustive chemical and structural characterization of the nanocomposite material in its original state (passivated Co powder) and after thermal treatments in vacuum. After passivation the cobalt fcc nanocrystals (typical sizes 2-20 nm) are covered by an amorphous oxide passivation layer with a short range structural order similar to the cubic (fcc) CoO phase. Upon heating, crystallisation of the oxide layer is observed together with the formation of the Co ₃ O ₄ spinel phase. In situ XPS experiments allow the determination of the relative amounts of oxide and metal as a function of the oxygen dose. The present study is relevant due to the correlation between microstructure and the unusual magnetic properties of this type of material.