Size control and characterization of colloidal magnetic cobalt nanoparticles

The overall goals of this research were to synthesize magnetic cobalt nanoparticles and characterize their structural and magnetic properties. Cobalt nanoparticles were synthesized via high temperature reduction of cobalt chloride utilizing two different surfactants, tributylphosphine and trioctylphosphine, and varying the surfactant-to-reagent molar concentration for each. X-ray diffraction results indicate that each synthesis condition yielded nanoparticles with an &egr;-cobalt crystal structure, a resemblance to the beta-manganese structure. Transmission electron micrographs indicate the formation of discrete hexagonal networks among the particles with inter-particle spacing of 2--5 nm. Sizing analysis found that utilizing a bulkier surfactant yields smaller mean particle size. Furthermore, utilizing a surfactant at a higher surfactant-to-reagent molar ratio also yields a smaller mean particle size with a tighter distribution. Normalized vibrating sample magnetometer results indicate synthesized cobalt nanoparticles exhibit ferromagnetic behavior at room temperature and increasing coercivity with mean particle size.