Adsorption of cobalt on gamma-Fe{u2082}O{u2083}

Abstract

The treatment of gamma-Fe₂O₃ with cobalt(II) is an important commercial process since the product is used extensively as a magnetic material for magnetic recording. The preparation of cobalt-doped gamma-Fe₂O₃ consists of interacting Co(II) with particulate gamma-Fe₂O₃ in solution under alkaline conditions.

The surface of cobalt-treated gamma-Fe₂O₃ was characterized using the surface analysis techniques of x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). Characterization was also accomplished using infrared spectroscopy and quantitative analysis.

Surface analysis results suggest that the Co(II) is initially adsorbed as a hydrous precipitate during base addition (NaOH or NH₄OH). Following base addition, the reaction suspension was heated to 90-95°C. Results from surface analysis indicate that during this warm up period there is a conversion from the hydrous surface to a surface with a composition near CoFe₂O₄ (cobalt ferrite). No oxidation of Co(II) to Co(III) was observed. Surface analysis also suggests that cobalt-treated γ-Fe₂O₃ prepared using NaOH has a different surface chemistry than cobalt-treated γ-Fe₂O₃ prepared with NH₄OH.

Following the adsorption of Co(II) on γ-Fe₂O₃ the product underwent a thermal treatment to enhance the coercivity. Surface analysis results indicate that the thermal treatment causes a significant diffusion of Co(II) into the bulk γ-Fe₂O₃. The results also suggest that the coercivity enhancement following thermal treatment is largely due to the inward diffusion of Co(II) and not a change in the surface composition of the cobalt-treated γ-Fe₂O₃.