Publication: Synthesis of cobalt nanoparticles by modified polyol method
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Date
2009-05-01
Authors
Sim, Ee Mei
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Abstract
Cobalt has a wide range of applications, such as data storage, in biomedical field and as catalyst using monodispersed nanoparticles. Liquid phase synthesis routes are the most successful in the preparation of monodipersed Co nanoparticles. Polyol method is the simplest, fastest and a cheap method to produce Co particles. Therefore, Co particles were produced by using modified polyol method. In this process, hydrazine was chosen as reducing agent and the solvent used was polyalcohol ethylene glycol. Cobalt acetate was used for the synthesis of Co nanoparticles following a homogeneous nucleation route. Hydroxyl groups of glycol play a major role in the reduction process, which in turn justifies the name modified polyol process. In this experiment, three parameters were investigated that were N2H4 to Co2+ mole ratio, Co2+ concentration and amount of PVP added. The crystal structures of the synthesized Co nanoparticles were FCC and HCP phase. The shape or morphology of Co nanoparticles varied from spherical shape to rod- like, thin sheet and hexagonal shape when PVP was added. The effect of N2H4 concentration on particles size and size distribution was not significant. Increasing the mole ratio of N2H4 to Co2+ from 2:1 to 3:1, 4:1 and 5:1, the mean diameter of particles obtained were 71.72 nm, 82.44 nm, 71.10 nm, and 110.40 nm respectively while the standard deviation for particles size are 19.93, 23.49, 19.03 and 33.28. The N2H4 to Co2+ mole ratio 4:1 was kept constant while the Co2+ was varied between 5 mmol and 2 mmol. The average diameters of Co particles obtained were 74.94 nm and 63.30 nm, as compared to the 4 mmol of Co2+ which was 71.10 nm in first parameter. Adding PVP, the particles size becomes larger reaching 500 nm. The reduction of Co2+ by using N2H4 was generally slow; for low concentration of N2H4, the reduction of Co2+ cannot be completed within 45 minutes. Thus, the reaction was enhanced by increasing N2H4 and Co2+ concentration. The Co nanoparticles are very sensitive to ambient atmosphere and the oxidation of Co nanoparticles was observed from the EDX analysis. From this experiment, increase in N2H4 and decrease in Co concentration could obtained smaller Co nanoparticles.