Abstract

A method for the preparation of acicular hydrogoethite (α-FeOOH·xH₂O, 0.1 < x < 0.22) particles of 0.3–1 μm length has been optimized by air oxidation of Fe(II) hydroxide gel precipitated from aqueous (NH₄)₂Fe(SO₄)₂ solutions containing 0.005–0.02 atom% of cationic Pt, Pd or Rh additives as morphology controlling agents. Hydrogoethite particles are evolved from the amorphous ferrous hydroxide gel by heterogeneous nucleation and growth. Preferential adsorption of additives on certain crystallographic planes thereby retarding the growth in the perpendicular direction, allows the particles to acquire acicular shapes with high aspect ratios of 8–15. Synthetic hydrogoethite showed a mass loss of about 14% at ∼280 °C, revealing the presence of strongly coordinated water of hydration in the interior of the goethite crystallites. As evident from IR spectra, excess H₂O molecules (0.1–0.22 per formula unit) are located in the strands of channels formed in between the double ribbons of FeO₆ octahedra running parallel to the c-axis. Hydrogoethite particles constituted of multicrystallites are formed with Pt as additive, whereas single crystallite particles are obtained with Pd (or Rh). For both dehydroxylation as well as H₂ reduction, a lower reaction temperature (∼220 °C) was observed for the former (Pt treated) compared to the latter (Pd or Rh) (∼260 °C). Acicular magnetite (Fe₃O₄) was prepared either by reducing hydrogoethite (magnetite route) or dehydroxylating hydrogoethite to hematite and then reducing it to magnetite (hematite–magnetite route). According to TEM studies, preferential dehydroxylation of hydrogoethite along <010> leads to microporous hematite. Maghemite (γ-Fe₂O_3 − δ, 0 < δ < 0.25) was obtained by reoxidation of magnetite. The micropores are retained during the topotactic transformation to magnetite and finally to maghemite, whereas cylindrical mesopores are formed due to rearrangement of the oxygen sublattice from hexagonal to cubic close packing during the conversion of hydrogoethite to magnetite and then to maghemite. Accordingly, three different types of maghemite particles are realized: strongly oriented multicrystalline particles, single crystalline acicular particles with micropores or crystallites having mesopores. Higher values of saturation magnetization (σ_s = 74 emu g⁻¹) and coercivity (H_c = 320 Oe) are obtained for single crystalline mesoporous particles. In the other cases, the smaller size of particles and larger distribution of micropores decreases σ_s considerably (<60 emu g⁻¹) due to relaxation effects of spins on the surface atoms as revealed by Mössbauer spectroscopy.