Abstract
The shape and porosity of hematite particles formed from the forced hydrolysis of acidic FeCl3 solutions could be controlled by the use of two types of tannic acid (hydrolysable and non-hydrolysable) within the concentration range 0–2 × 10−2 wt%. The system with hydrolysable tannic acid (AL) initially provided quite small spherical particles at concentrations up to 10−2 wt% and finally irregular small hematite particles at 2 × 10−2 wt%. In the system with condensed non-hydrolysable tannic acid (KT), on the other hand, the hematite particles became ellipsoidal in shape at concentrations above10−2 wt%. The formation of ellipsoidal particles was explained by the adsorption of KT molecules onto the growing polynuclear primary particles. TEM and XRD measurements revealed that all the hematite particles produced from FeCl3/HCl solutions with tannic acids are polycrystalline. The rate of phase transformation from β-FeOOH to hematite was accelerated as the concentration of AL increased, whereas no significant change was observed for the KT system. Analysis by t-plots revealed that the porosity of the hematite particles changed from mesoporous to microporous when the concentration of AL increased. In contrast, the presence of very low amounts of KT molecules (10−5 and 10−4 wt%) produced non-porous hematite particles via strong aggregation of polynuclear (PN) particles by hydrogen bonding between hydroxy and carboxy groups. Not only the morphology but also the pore size of hematite particles was controlled from non-porous to mesoporous by using different kinds of tannic acids.