Photoelectrophoresis of colloidal iron oxides 1. Hematite (α-Fe 2O 3)

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 Fe 2O 3 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 α-Fe 2O 3 particle preparation from 2 to 1.4 was found to alter the resultant surface from one comprising mostly α-Fe 2O 3, α-FeOOH and λ-FeOOH with an isoelectric point (i.e.p.) of 7.4, to one whose behaviour was dominated by the presence of δ-FeOOH with an i.e.p. of 1.5. The α-Fe 2O 3 particles whose surfaces are found to be mostly “Fe(OH) 3”/Fe 2O 3· nH 2O 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 α-Fe 2O 3 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 α-Fe 2O 3 surface or, possibly, the formation of hydrogen from the reduction of H + ions. The photoelectrophoretic mobility—illumination wavelength spectrum of colloidal α-Fe 2O 3 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 TiO 2.