Fabrication of hcp-Co Nanocrystals via Rapid Pyrolysis in Inverse PS-b-PVP Micelles and Thermal Annealing

Magnetic colloidal nanocrystals tend to aggregate in solution because of magnetic dipolar interactions. A diblock copolymer was used as a stabilizer to limit these effects, especially those leading to aggregation in solution. Cobalt nanoparticles have been synthesized within inverse micelles of polystyrene- block -poly(2-vinylpyridine) copolymer in toluene by the pyrolysis of dicobalt octacarbonyl at 115 °C. The nanoparticle structure at different reaction times was investigated using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). At early reaction stages, the nanoparticles were found to be noncrystalline from TEM, and FT-IR showed that the precursor was only partially decomposed. After 15 min of reaction, the nanoparticles became crystalline, forming chains due to magnetic interactions. The noncrystalline nanoparticles could be crystallized upon heating to 420 °C on grids in the transmission electron microscope. This produced nearly monodisperse single nanocrystals inside each micelle, with limited aggregation, but such annealing led to the degradation of the polymer.