Burst nucleation by hot injection for size controlled synthesis of ε-cobalt nanoparticles

Background: Reproducible growth of narrow size distributed epsilon-Co nanoparticles with a specific size requires full understanding and identification of the role of essential synthesis parameters for the applied synthesis method. For the hot injection methodology, a significant discrepancy with respect to obtained sizes and applied reaction conditions is reported. Currently, a systematic investigation controlling key synthesis parameters as injection-temperature and time, metal to surfactant ratio and reaction holding time in terms of their impact on mean ((D) over bar (mean)) and median ((D) over bar (median)) particle diameter using dichlorobenzene (DCB), Co-2(CO)(8) and oleic acid (OA) as the reactant matrix is lacking. Methods: A series of solution-based epsilon-Co nanoparticles were synthesized using the hot injection method. Suspensions and obtained particles were analyzed by DLS, ICP-OES, (synchrotron) XRD and TEM. Rietveld refinements were used for structural analysis. Mean ((D) over bar (mean)) and median ((D) over bar (median)) particle diameters were calculated with basis in measurements of 250-500 particles for each synthesis. 95 % bias corrected confidence intervals using bootstrapping were calculated for syntheses with three or four replicas. Results: epsilon-Co NPs in the size range similar to 4-10 nm with a narrow size distribution are obtained via the hot injection method, using OA as the sole surfactant. Typically the synthesis yield is similar to 75 %, and the particles form stable colloidal solutions when redispersed in hexane. Reproducibility of the adopted synthesis procedure on replicate syntheses was confirmed. We describe in detail the effects of essential synthesis parameters, such as injection-temperature and time, metal to surfactant ratio and reaction holding time in terms of their impact on mean ((D) over bar (mean)) and median ((D) over bar (median)) particle diameter. Conclusions: The described synthesis procedure towards epsilon-Co nanoparticles (NPs) is concluded to be robust when controlling key synthesis parameters, giving targeted particle diameters with a narrow size distribution. We have identified two major synthesis parameters which control particle size, i.e., the metal to surfactant molar ratio and the injection temperature of the hot OA-DCB solution into which the cobalt precursor is injected. By increasing the metal to surfactant molar ratio, the mean particle diameter of the epsilon-Co NPs has been found to increase. Furthermore, an increase in the injection temperature of the hot OA-DCB solution into which the cobalt precursor is injected, results in a decrease in the mean particle diameter of the epsilon-Co NPs, when the metal to surfactant molar ratio ([Co]/[OA]) is fixed at similar to 12.9.