Microscopy investigations of the microstructural change and thermal response of cobalt-based nanoparticles confined inside a carbon nanotube medium

Abstract

Faceted cobalt–cobalt oxide based nanoparticles (NPs) with high density and narrow size distribution (50 ± 5 nm) were selectively cast inside the channels of multi-walled carbon nanotubes (CNTs) through the controlled thermal decomposition of cobalt stearate in the presence of oleic acid as surfactant. The total loading of the NPs is ca. 60 wt% due to the confinement effect of CNTs, which play the role of “nanoreactors” for the cobalt complex filling and its further decomposition. The cast Co-based NPs consist of a Co–CoO core–shell structure with a relatively high contribution of the metallic phase, thanks to the confinement effect which prevents excessive oxidation. The Co–CoO NPs were transformed into highly porous cobalt aggregates constituted by small cobalt clusters (5 nm) after reduction with a high effective surface area. These cobalt clusters display an extremely high resistance towards oxidation, thanks to the confined medium. The evolution of the Co-based NP microstructure of the as-synthesized and the reduced composites, as a function of temperature, was evaluated by in situ heating the systems inside the TEM. On the as-synthesized sample with a mixture of Co and CoO the heating process leads to a rapid shape modification and coalescence of the NPs at a temperature of 400 °C along with the formation of a cobalt carbide interface. In contrast, the reduced sample displays a much higher sintering resistance upon annealing under the same conditions as cobalt NPs with a mean diameter of 10 nm, still present at an annealing temperature up to 800 °C. The work reported here underlines the benefit of the confinement effect to improve the oxidative and sintering resistance of cast metal NPs and also in the synthesis of a new porous structure containing small metal NPs which could be of great interest for catalytic and magnetic applications.