Photoelectrophoresis of colloidal iron oxides. Part 2.—Magnetite (Fe3O4)

Abstract

Aqueous dispersions of colloidal magnetite have been prepared by aqueous precipitation and characterised using X-ray diffraction (XRD). Their surface chemistry was studied using photo-electrophoresis. Changes in the electrophoretic mobility of colloidal magnetite, indicative of the formation of net surface positive charge may be observed upon irradiation with ultra-band-gap energy photons at pH less than about 7. This was attributed to the hole driven photo-oxidation of surface > Fe—OH sites to form (> Fe—OH)⁺ sites. Photogenerated holes concomitantly oxidised the magnetite surface to maghaemite while the photogenerated electrons were removed from the particles by reductive dissolution of the Fe₃O₄ surface. Between pHs of ca. 7 and 12, the mobility change upon illumination was insignificant, reflecting reductive dissolution then being thermodynamically disallowed. At pH 12, photogenerated holes were removed from the particles by the photoanodic corrosion of the Fe₃O₄ surface. The remaining electrons reduce surface Fe^III_tet sites to Fe^II_tet, with an attendant change in the observed electrophoretic mobility of colloidal magnetite congruous with the observed formation of net surface negative charge.

The photoelectrophoretic mobility–illumination wavelength spectrum of colloidal Fe₃O₄ exhibited three distinct mobility change onsets: one each at 1.8, 2.2 and 3.1 eV, reflecting the band structure of magnetite. Comparison with the photoelectrophoretic mobility–illumination wavelength spectrum of α-Fe₂O₃ showed the latter two mobility change onsets to be common to both materials, a result of the structural similarities between magnetite and haematite.

The oxidation of surface > Fe—OH groups responsible for the increase in net positive surface charge of colloidal magnetite at pH less than about 7, was found to occur at the same rate as the corresponding reaction on colloidal haematite under identical conditions. Further, the oxidation of > Fe—OH on both materials was found to occur ten times more slowly than the corresponding reaction on colloidal TiO₂.