Polyol synthesis and characterization of magnetic nanocrystalline cobalt and cobalt platinum

Abstract

The present work addresses the systematic study of the synthesis and characterization of cobalt nanoparticles produced in polyol solutions. The kinetics of the metal forming reaction strongly influenced the stability conditions of cobalt phases. The control of the reaction kinetics was achieved through a suitable selection of the solution chemistry in the Polyol–Co (II) system. The addition of hydroxyl ions into the cobalt polyol solution under boiling conditions not only accelerated the formation of the magnetic phase but also affected the stability conditions of the metallic precipitate. Depending on the OH- /Co mole ratio and the type of cobalt salt, hexagonal close packed (hcp), face centered cubic (fcc), and metastable pseudo-cubic epsilon cobalt (ε-Co) phases were formed. Highly monodisperse ε-Co nanoparticles were produced when cobalt acetyl-acetonate salt was used instead of the acetate alone. Furthermore, the presence of Pt ions –added to promote heterogeneous nucleation- caused a dramatic shortening of the time required for the complete formation of the magnetic phase. The shortening in reaction time was conducive to the formation of the ordered face centered tetragonal (fct)-CoPt nanoparticles although co-existing with fcc and ε-Co structures. Excess of platinum was present as fcc-Pt. The room-temperature saturation magnetization (Ms) and coercivity (Hc) of the nanocrystalline cobalt powders ranged from 80 to 110 emu/g and 101 to 211 Oe, respectively. The coercivity was as high as 248 Oe when CoPt was present. The coexistence of Pt ions in starting solutions not only promoted the nucleation rate and accelerated the cobalt reduction but also induced the formation of ordered fct-CoPt nanocrystals at low temperature (487 K).