Abstract
BackgroundReproducible growth of narrow size distributed ε-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 (bar{D}mean) and median (bar{D}median) particle diameter using dichlorobenzene (DCB), Co2(CO)8 and oleic acid (OA) as the reactant matrix is lacking.MethodsA series of solution-based ε-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 (bar{D}mean) and median (bar{D}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.Resultsε-Co NPs in the size range ~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 ~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 (bar{D}mean) and median (bar{D}median) particle diameter.ConclusionsThe described synthesis procedure towards ε-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 ε-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 ε-Co NPs, when the metal to surfactant molar ratio left( {frac{[Co]}{[OA]}} right) is fixed at ~12.9.Electronic supplementary materialThe online version of this article (doi:10.1186/s13065-016-0156-1) contains supplementary material, which is available to authorized users.
Highlights
Reproducible growth of narrow size distributed ε-Co nanoparticles with a specific size requires full understanding and identification of the role of essential synthesis parameters for the applied synthesis method
The hydrodynamic diameters are larger than the measured mean diameters from transmission electron microscopy (TEM) analysis (i.e. 4–10 nm, see particle diameter control section below) because of the contribution of the chemisorbed surfactant (OA) on the particles surface, as well as coordinated solvent molecules
The polydispersity index (PDI) for the analysed samples was in all cases lower than 0.20, indicating near monodispersed particles [21]
Summary
Reproducible growth of narrow size distributed ε-Co nanoparticles with a specific size requires full understanding and identification of the role of essential synthesis parameters for the applied synthesis method. Zacharaki et al Chemistry Central Journal (2016) 10:10 consist of Co NPs dispersed on an oxide support [5–7], prepared by impregnation, and followed by drying, calcination and activation steps. This way of preparation yields normally non-uniform Co NPs with respect to size and shape, which hinders the study of size-dependent catalytic properties. Systematic single parameter studies to correlate particle properties such as size, shape, atomic arrangement and chemical composition to magnetic behavior or catalytic performance, require highly refined and reproducible synthesis procedures. Metallic cobalt crystallizes in hexagonal- and cubic close packed (hcp and ccp) structures, wherein the hcp variant is the stable modification below ~693 K [9]. The εCo phase transforms irreversibly during annealing in a non-oxidative atmosphere into hcp and ccp at ~573 and 773 K, respectively [1, 10, 13]
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