Synthesis of monodispersed hematite particles from highly concentrated metal hydroxide gels

The synthesis of uniform hematite particles with various morphologies from highly concentrated metal hydroxide gels was investigated. The investigations were focused on the effects of cation (trivalent and divalent), anions (arsenate and selenate), and organic additives (alkyl sufates) on particle formation.; Monodispersed aluminum-substituted hematite particles were synthesized from concentrated mixed solutions of FeCl₃ and AlCl₃. With increasing aluminum contents, the product particles exhibited ellipsoidal, rod- and peanut-like shapes. Lattice parameter a decreased with increasing Al incorporation. The particles grew anisotropically along c crystal axis according to TEM and XRD analysis.; Chromium substitution had a similar influence on particle shape modification. With increasing chromium content, hematite particles exhibited ellipsoidal and peanut-like morphologies. The incorporation of Cr in hematite increases with increasing Cr addition in the starting solution. The particles grew anisotropically along c crystal axis according to XRD analysis.; The size of hematite particles decreased when divalent Cu or Mn contents increased in initial solutions. The cubic shape was well maintained with Cu addition. Hematite particles produced with the existence of Mn2+ were irregular in shape and porous in structure. Copper substitution in hematite was verified by both chemical analysis and lattice parameter calculation. However, the incorporation of Mn is negligible.; Ellipsoidal and peanut-shaped hematite particles were synthesized with the addition of arsenate and selenate ions. Product particles were more elongated upon the addition of more anions. The shape modification effects of these anions were interpreted in terms of bond matching due to similar Fe-Fe and O-O distances of hematite and anions, respectively.; The addition of alkyl sulfates produced peanut-shaped hematite particles. Uniform particles with the same morphology were always formed despite the differences in carbon chain lengths. Further study showed that alkyl sulfates were hydrolyzed at different speed upon aging, releasing inorganic sulfate ions gradually into the system. The rate of hydrolysis increased with longer carbon chains. The formation of peanut-shaped hematite particles was attributed to the adsorption of inorganic sulfate ions produced upon the hydrolysis of alkyl sulfates and the adsorption of smaller alkyl sulfate ions as well.