The Photoelectrophoresis of Colloidal Iron Oxides I: Hematite (alpha-Fe2O3)

Boxall, Colin and Zhang, Zhenhong and KELSALL, G H (1993) The Photoelectrophoresis of Colloidal Iron Oxides I: Hematite (alpha-Fe2O3). In: Colloids in the Aquatic Environment :. Elsevier Science Ltd., Amsterdam, pp. 145-163. ISBN 978-1-85861-038-2

Abstract

Aqueous dispersions of colloidal hematite were prepared by aqueous precipitation and characterised using X-ray diffraction and Fourier transform infrared spectroscopy. Their surface chemistry was studied using (photo-)electrophoresis, in which electrophoretic mobilities were determined by laser Doppler electrophoresis, in the absence and presence of irradiation of photons from a xenon lamp and monochromator. Absorption of ultra-band-gap energy photons in Fe2O3 results in the generation of electron (e-)-hole (h+) pairs, which may then recombine, with the generation of heat/radiation, or react with lattice sites, solvent or solution species. Changing the pH of alpha-Fe2O3 particle preparation from 2 to 1.4 was found to alter the resultant surface from one comprising mostly alpha-Fe2O3, alpha-FeOOH and gamma-FeOOH with an isoelectric point (i.e.p.) of 7.4, to one whose behaviour was dominated by the presence of delta-FeOOH with an i.e.p. of 1.5. The alpha-Fe2O3 particles whose surfaces are found to be mostly ''Fe(OH)3''/Fe2O3.nH2O in character exhibit a continuum of i.e.p.s due to the non-crystalline nature of that phase. Large changes in the electrophoretic mobility of colloidal alpha-Fe2O3 at a pH of less than about 7-8 were observed upon irradiation with photons with ultra-band-gap energies, indicative of the formation of net surface positive charge, due to the hole-driven photo-oxidation of surface >Fe-OH sites to form (>Fe-OH)+ sites. Photogenerated conduction band electrons were removed from the particles via either the reductive dissolution of the alpha-Fe2O3 surface or, possibly, the formation of hydrogen from the reduction of H+ ions. The photoelectrophoretic mobility-illumination wavelength spectrum of colloidal alpha-Fe2O3 exhibits two distinct mobility change onsets, one at 2.2 eV and the other at 3 eV, reflecting the presence of an ''upper'' and ''lower'' valence band on hematite. The oxidation of surface >Fe-OH groups responsible for the change in net surface positive charge is found to proceed ten times more slowly than the corresponding reaction on colloidal TiO2.