Organic colloid method to prepare ultrafine cobalt nanoparticles with the size of 2 nm

Ultrafine Co nanoparticles with the size of ca. 2 nm were prepared by an organic colloid method, in which sodium formate acted as reducing agent, ethylene glycol acted as solvent, sodium citrate acted as both complexing agent and stabilizing agent, respectively. X-ray diffraction (XRD) analysis indicated that the as-prepared Co nanoparticles were in hexagonal close-packed phase, and transmission electron microscope (TEM) images revealed that the size of the well-dispersed Co nanoparticles was as small as 2 nm, and the sizes were distributed in a very narrow region. The hysteresis loop of the as-prepared Co sample measured at room temperature showed a superparamagnetic behavior due to the extremely small size of the products. It was revealed that sodium citrate played a crucial role in decreasing the particle size and narrowing the size distribution.