Definitive effects of chloride ions on the formation of spherical hematite particles in a forced hydrolysis reaction

Abstract

The effects of chloride ions on the morphology and structure of hematite particles, produced from a forced hydrolysis reaction of acidic Fe(III) solution, were investigated. Spherical hematite particles were precipitated under all conditions employed which changed the molar fraction of coordinative Cl⁻ to non-coordinative NO₃⁻ ions to Fe(III) ions in the solutions. The size of the particles was increased with increase in the molar fraction of FeCl₃, FeCl₃/[FeCl₃+Fe(NO₃)₃] (X_FeCl3) although no remarkable change was observed in varying that of HCl, HCl/(HCl+HNO₃) (X_HCl). TEM and XRD measurements suggested that the hematite particles formed at X_FeCl3⩽0.2 exhibited a single-crystal nature, although those produced at X_FeCl30.4 are polycrystalline with an enlarged c edge length in a unit cell possessing OH⁻ ions in the lattice. The rate of phase transformation from β-FeOOH to hematite decreased with increase in X_FeCl3 and was closely related to the crystal lattice distortion. The results obtained were explained by an aggregation mechanism taking into account the difference in the coordination properties of Cl⁻ and NO₃⁻ ions to Fe(III) ions. The hydrolysis reaction of Fe(III) was slowed in the presence of Cl⁻ by its strong coordination to produce electrically neutral, extremely low molecular weight polynuclear (PN) species, [FeO_2/3(OH)_4/3(H₂O)_xCl_1/3]_p. Therefore, this system gave a large spherical particle through the aggregation of the neutral small PNs with less mutual electrostatic repulsion force. In contrast, NO₃⁻ ions exhibited a fast hydrolysis reaction owing to their non-coordinative nature and produced positively charged, high molecular weight PNs, [FeO_2/3(OH)_4/3(H₂O)_x]_p^(p/3)+, resulting in less aggregated small hematite particles owing to their repulsion force. This aggregation mechanism was further confirmed by gas adsorption, TEM and addition of NaCl.