New approaches to the synthesis of acicular α-FeOOH and cobalt-modified iron oxide particles

Abstract

Formation of acicular α-FeOOH particles from the reaction system of FeCl2 -NaOH is studied. It is found that acicular α-FeOOH particles with a particle size of about 0.5 μm and axial ratio of about 10 can be obtained by oxidizing the mixed solution of FeCl2 and NaOH with a molar ratio of NaOH/FeCl2 larger than 2. New acicular Co-γ-Fe2 O3 particles are synthesized by absorbing Co2+ ions on the surface of acicular α-FeOOH particles followed by dehydrating, reduction, and oxidation. We find that using N2 as dehydrating atmosphere is better than air or H2. After reduction, variations of the coercivity of Co-Fe3 O4 particles with various cooling rates are investigated. The transformation temperature of Co-Fe3 O4 →Co-γ-Fe2 O3 is about 300 °C for the particles with cobalt content (Co/Co+Fe)<12 mol %. Distributions of Co2+ and Fe2+ ions in the particles are measured by the dissolution method. Coercivity of the acicular Co-γ-Fe2 O3 particles between 400 and 1200 Oe can be controlled by the cobalt content and additional anneal in N2.