Development of Hexagonal Closed-Packed Cobalt Nanoparticles Stable at High Temperature

Abstract

Cobalt metal particles have several stable phases depending on their size and the temperature employed in the preparation. The face-centred cubic (fcc) structure is thermodynamically stable above 450 °C, whereas the hexagonal-closed packed (hcp) phase is stable at lower temperatures. Cobalt oxide reduction carried out under H2 atmosphere leads to fcc-Co phase. This study shows that the reduction of cobalt oxide under CO or H2+CO mixtures giver rise to the formation of a hcp-Co phase, which still remains stable at high temperatures (up to 700°C). This circumstance is due to the encapsulation of hexagonal cobalt nanoparticles by carbon nanofibers that minimizes the surface energy of these cobalt structures. Moreover, this encapsulation effect prevents the sintering and oxidation reactions of hcp-Co metallic phase even at a high temperature (up to 700°C) Thus, the reduction atmosphere plays a key role in the formation and stabilization of these cobalt phases.