Effect of phosphate on the morphology and size of α-Fe2O3 particles crystallized from dense β-FeOOH suspensions

Influence of phosphate on the crystallization of α-Fe2O3 at 160 °C from concentrated FeCl3 solution partially neutralized with concentrated NaOH solution was investigated by 57Fe Mössbauer, XRD, FT-IR and FE-SEM. In the reference precipitation system, without phosphate addition, very fine β-FeOOH particles were transformed into α-Fe2O3 ones in the form of double spheres with ring (1-3 days of autoclaving). After 24 h of autoclaving in the presence of 0.01 M Na2HPO4, α-Fe2O3 aggregates of different shapes were obtained. α-Fe2O3 subunits ∼20-25 nm in size were in laterally aggregated arrays. With further increase in Na2HPO4 concentration up to 0.05 M, the crystallization of α-Fe2O3 was strongly suppressed. A strong elongation of β-FeOOH and α-FeOOH particles was observed. In β-FeOOH and α-FeOOH particles phosphates were preferentially adsorbed on the surface of the crystallites, thus governing the preferential crystallite growth. Very long wires and/or rods of FeOOH were obtained. At high phosphate concentration the surface precipitation of phosphates on FeOOH particles is possible and in such way the dissolution of these particles is suppressed. For α-Fe2O3 particles, the incorporation of phosphorus into the α-Fe2O3 structure is suggested, taking into account the mechanism of the crystal growth on the iron(III) phosphate nuclei (probably amorphous) or some kind of iron(III) phosphate complexes serving as templates in the suspension. The incorporation of phosphorus into α-Fe2O3 particles retards the increase in their size, as well as that of the corresponding α-Fe2O3 subunits.