Stable cobalt nanoparticles passivated with oleic acid and triphenylphosphine

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Abstract
Monodisperse cobalt nanoparticles (NPs) are synthesized via a high-temperature thermal decomposition method in the presence of oleic acid and triphenylphosphine. The as-synthesized NPs are stable against further deep oxidation when they are kept in heptane (C7H16). Time-dependent x-ray photoelectron spectroscopy studies indicate that the oxidation of the as-synthesized cobalt NPs by air is slow and the valence change of cobalt from the Co NP sample kept in heptane under air is not observed for 120 days. The cobalt NPs have β-manganese type structure (also called ε-cobalt). Annealing the NPs at 500 °C under Ar (95%)+H2 (5%) converts these particles from ε-Co to fcc-Co. Two-dimensional (2D) and three-dimensional (3D) self-assembled superlattices of passivated cobalt NPs are formed by slow evaporation of the carrier solvent.