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

Abstract

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.

Introduction

Cobalt is a widely investigated magnetic material due primarily to its various current and potential future uses in several applications, such as dilute magnetic semiconductor based spintronics [1], recording media [2], [3], etc. Bulk cobalt is allotropic, exhibiting face centered cubic (FCC) to hexagonal close-packed (HCP) structural transformation at 420 °C [4]. However, the structure dependence of Co nanoparticles on the specific synthetic approach is not very well understood. Also, the explanation about the structural transformation temperature dependence on the size of the small Co particles remains controversial. Specifically, in the case of physical routes of synthesis, for example, vacuum evaporation [5], sputtering [6], and plasma (or arc) discharge deposition [7], nanosized metastable FCC Co particles have been produced at ambient temperature. Occasionally a mixture of FCC and HCP phases is also obtained [8]. It is reported that below 420 °C only HCP phase Co particles are obtained by laser pyrolysis or by the reduction of oxides under H₂ flow [9]. Medium temperatures, around 420 °C, resulted in a mixture of HCP and FCC phases. Pure FCC phase formed only when temperatures above 500 °C were used during the synthesis or annealing of Co particles. This transformation into FCC at high temperature was irreversible and the FCC structure was maintained upon cooling to ambient temperature. Huang et al. reported that phase transformation between the two phases in Co nanoparticles can be controlled by adjusting the variables in the ball milling technique [10].

Cobalt has two primary oxides, CoO and Co₃O₄, which are also used in various applications. Poizot proposed a class of new anode materials which are composed of nanosized transition metal oxides, such as nickel oxide, cobalt oxide and iron oxide, for lithium ion batteries [11]. Wang et al. [12] demonstrated CoO electrode with a stable reversible lithium storage capacity. Liu et al. reported cathodic deposition of Co₃O₄ as the activated materials for lithium-ion battery [13]. Riva et al. [14] reported phase transformation study of bulk Co₃O₄ during reduction of unsupported and supported samples on SiO₂ and TiO₂ particles in 3% H₂-He atmosphere. For unsupported samples, Co₃O₄ remained stable up to 200 °C and completely reduced to CoO at 300 °C when annealed for 2 h. However, reduction to pure cobalt only occurred when Co₃O₄ sample was heated in reducing atmosphere for 32 h.

Since cobalt is a widely used material, both in metallic and oxide forms, it is important to study the oxidation and reduction processes in Co–CoO_x system, especially in the nanostructured form. In this paper, we report oxidation in Co/CoO nanoparticles which is attributed to the morphology of the particles. Detailed analyses of the effects of heat treatment under reducing atmosphere are presented. We observed strong effects of the size of particles and their morphology on the chemical transformations. We systemically studied chemical and structural transformation of as-prepared samples into different forms like Co₃O₄, CoO, Co (HCP + FCC) and Co (FCC) upon annealing at different temperatures in forming gas (Nitrogen with 5 vol.% hydrogen). Structural, morphological and surface studies are carried out by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. Magnetic data for all the samples was also obtained which correlated well with the structural transformations.