Carbon Coating, Carburization, and High-Temperature Stability Improvement of Cobalt Nanorods

The reactivity of highly crystalline hcp cobalt nanorods (NRs) with organic solvents at high temperature was studied. Cobalt NRs with a mean diameter of 15 nm were first synthesized by the polyol process and then heated to 300 degrees C in octadecene (ODE), oleylamine (OA), or mixtures of these two solvents. The surface and structural modifications of the Co NRs were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM). A disordered carbon shell was formed at the cobalt rod surface, the thickness of which can be tuned from 5 to 25 nm by increasing the amount of oleylamine in the solvent mixture. This carbon shell partially reduced the native cobalt oxide observed at the surface of the NRs and drastically improved their temperature stability as inferred from in situ XRD study and TEM. The shape anisotropy and the crystallite anisotropy of the hcp phase are both preserved up to 400 degrees C for the carbon-coated cobalt rods, whereas the uncoated NRs lose their anisotropy at 225 degrees C. Treatments at 300 C in ODE/OA mixtures for different durations allowed the progressive carburization of Co to Co2C. The crystallographic orientation of the Co2C grains within the cobalt NRs combined with the different carbon-shell thickness on the {10 (1) over bar0} and (0001) facets of the rods suggested a preferential carburization from the lateral facets of the hcp cobalt rods.