Anomalous dependence of particle size on supersaturation in the preparation of iron nanoparticles from iron pentacarbonyl

Iron nanoparticles were prepared by decomposing iron pentacarbonyl (Fe(CO) ₅) at 170-220°C in the presence of amine surfactant and alkane solvent and under 1-12bar carbon monoxide (CO) pressure. It was found that the amine not only acted as a stabilizer for the growing particles but also had a critical role as a promotor in the decomposition reaction. Relatively small changes in the CO pressure had anomalous effects on the particle size distribution. Typically, monodisperse particles were obtained at 1bar, while pressures in the 2-6bar range led to wider and even bimodal size distributions due to an emergence of smaller particles. At still higher pressures, the larger particle size disappeared leaving the distribution monodisperse again. The CO pressure, at which the bimodal transition took place, increased with the reaction temperature. Polycrystalline particles were formed at lower pressures and monocrystalline particles at higher pressures. This indicates that increased CO pressure inhibits aggregation.