Shape Evolution Synthesis of Monodisperse Spherical, Ellipsoidal, and Elongated Hematite (α-Fe₂O₃) Nanoparticles Using Ascorbic Acid

Spherical, ellipsoidal, and elongated hematite particles have been obtained via a simple chemical precipitation reaction of FeCl₃ and NaOH in the presence of ascorbic acid (AA). The effects of pH, molar ratio of AA/Fe­(III), and time on the formation and shape of the hematite particles were investigated. The optimal conditions to well obtain crystalline hematite are 0.1 mol/L FeCl₃, 6 mol/L NaOH, pH 7, and AA/Fe­(III) ratios of 0.5–2.0%. The presence of AA catalyzed the formation of hematite by reductive dissolution of ferrihydrite and the molar ratio of AA/Fe­(III) determined the crystal structure and morphology of hematite. As the ratio of AA increased from 0.5 to 2%, the morphology changed from spherical to ellipsoidal particles and then to elongated particles. The dissolution of Fe­(II) from the ferrihydrite precursor is enhanced by AA, and this leads to the formation of hematite by precipitation and crystallization. The effect of AA on the particle shape can be explained by the difference in AA adsorption on the various crystal planes. The hematite samples with different morphologies enhanced the photodegradation of methylene blue in an acid solution with peroxide; the elongated particles that had the highest specific surface area were most effective with the methylene blue degradation