Growth of magnetic nanowires and nanodumbbells in liquid polyol

Cobalt and cobalt-nickel nanoparticles were synthesized by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propanediol. The particle shape depends strongly on the sodium hydroxide concentration and the Co/Ni composition of the particles. For cobalt-rich content, agglomerated rods, nanowires with a mean diameter of about 8 nm, and platelets were successively observed when [NaOH] was increased in the range 0-0.2 M. For the Co50Ni50 composition and [NaOH] in the range 0.1-0.18 M, nanodumbbells are formed that consist of a central column richer in cobalt capped with two terminal platelets richer in nickel. The shape of the dumbbells strongly depends on the basicity; long dumbbells are obtained for the lowest NaOH concentration, and short dumbbells and diabolos for the highest. To understand the role of the sodium hydroxide concentration and the different reactivities of cobalt and nickel, we analyzed the equilibrium between the Co2+ and Ni2+ ions in solution and the intermediate unreduced solid phase. For the Co80Ni20 composition, we show that increasing the sodium hydroxide amount lowers the Co2+ and Ni2+ ions in solution through the precipitation of the intermediate solid phase, suggesting that the nanowires are obtained with a higher growth rate than the platelets. The analysis of the solid intermediate phases revealed a Co(II) alkoxide and a Ni(II) hydroxy-acetate showing strong differences in the chemistry of the these two ions in basic solutions of 1,2-propanediol. These differences can explain the two well-separated growth steps originating the Co50Ni50 nanodumbbells.