Synthesis and characterization of large colloidal cobalt particles.

Large colloidal environmentally stable silica-coated cobalt particles were synthesized by combining the sodium borohydride reduction in aqueous solution and the Stöber method. Low size polydisperse cobalt spheres with an average size of 95 nm were synthesized by using a borohydride reduction method and were subsequently coated with a thin layer of silica by means of hydrolysis and condensation of tetraethylorothosilicate (TEOS) in an aqueous/ethanolic solution. The large uniform cobalt spheres consist of smaller metallic Co clusters, explaining the superparamagnetic behavior of the spheres. The particles were investigated by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM).