Protective Agent-Free Synthesis of Colloidal Cobalt Nanoparticles

Herein, we report a facile low-temperature (80 °C) synthesis route for colloidal cobalt (Co) nanoparticles by Co2+ reduction using hydrazine (N2H4) in a basic solution of ethylene glycol (EG). The colloidal particles were spherical and monodispersed, with mean diameter ranging from 2-7 nm increasing with the Co chloride concentration. However, precipitation of the nanoparticles resulted in weakly agglomerated spherical Co structures of submicron size. X-ray diffraction (XRD) of the precipitated powder revealed hexagonal close-packed-Co (hcp-Co). But since the major peaks of hcp and face-centered cubic-Co (fcc-Co) are overlapping, the presence of fcc-Co cannot be ruled out. Besides precursor concentration, it was found that pH, reaction temperature, and molar ratio of N2H4 to Co2+ also influence the reduction rate. An alkaline medium, elevated temperature (80°C) and high molar ratio of N2H4 to Co2+ promoted the formation of colloidal Co nanoparticles. Nonetheless, when the molar ratio of N2H4 to Co2+ was greater than 12, the mean particle size remained almost constant.