Mechanism of the Shape and Structure Control of Monodispersed alpha-Fe2O3 Particles by Sulfate Ions.

The characteristics of the adsorption of sulfate ions to hematite (alpha-Fe2O3) particles have been studied to elucidate the anisotropic growth of hematite particles in the presence of sulfate ions. Sulfate ions were actually found to be most strongly adsorbed to crystal faces parallel to the c-axis of the hexagonal crystal system from the adsorption isotherms of sulfate to hematite particles of different crystal habits, in accord with the retardation of their growth in the direction normal to the c-axis. It was found from FTIR spectroscopy that the adsorbed sulfate ions on the faces parallel to the c-axis, such as the {110} and {100} faces, or on {012} faces took the bidentate structure to Fe ions on the surfaces, while a monodentate structure was suggested for sulfate ions on the c-planes ({001} faces). The adsorbed amount of sulfate ions to the {012} faces of hematite at 100 degreesC decreased with increasing pH and became almost zero at pH >/=5, suggesting that the shape control with sulfate is possible only in the acidic media at pH < 5. EDX and chemical analysis revealed that sulfate ions were almost uniformly incorporated into the ellipsoidal or peanut-type particles during their growth in the presence of sulfate ions. On the other hand, sulfate ions in the solution phase with a high concentration of chloride ions such as 3.0 mol dm-3 were likely to be present in the form of free ions without forming a complex with Fe3+, as suggested from UV spectroscopy at 25 degreesC. When the concentration of sulfate ions in the solution phase exceeded the level for shape control of hematite, precipitation of acicular alpha-FeOOH (goethite) was observed. If hematite particles were present in such a system, alpha-FeOOH was grown as whiskers on the hematite particles. When [SO2-4] in the solution phase was higher than 0.1 mol dm-3, precipitation of basic ferric sulfate was also observed in addition to the growth of alpha-FeOOH whiskers on hematite. Copyright 1998 Academic Press.