Preparation of magnetic fine particles by borohydride reduction

Abstract Binary (CoB) and ternary (FeCoB) particles were prepared by the chemical reduction of corresponding metal salts with sodium borohydride in aqueous solutions. The reduction of Co 2+ (aq) by NaBH 4 has been studied with mechanistic and stoichiometric factors as the focus. The primary product of this reduction is nanoscale Co 2 B particles, which are formed possibly through the intermediacy of [(H 2 O) 5 CoOH(BH 2 )HOCo(H 2 O) 5 ] 3+ , and this complexion is reduced by electrons provided by three more moles of NaBH 4 . However, a side reaction where NaBH 4 reacts directly with H 2 O becomes important if sufficient time is available before Co 2+ addition or if Co/Co 2 B particles are allowed to catalyze it. In this way, NaBO 2 can be formed that can react with Co 2+ to give Co(BO 2 ) 2 as a product. The secondary product is metallic Co particles, which are formed by heat treatment of Co 2 B/Co(BO 2 ) 2 mixtures, or by sacrificial oxidation of Co 2 B. A tertiary product is Co 3 B formed by heating a mixture of Co 2 B and Co. The FeCoB particles were prepared with varying composition and particle size. The as-made samples are amorphous for lower Fe/Co ratios and crystalline for higher Fe/Co ratios.