THIN OXIDE LAYERS ON CLEAN IRON SURFACES - FORMATION UNDER VACUUM AND CHARACTERIZATION BY PHOTOELECTRON-SPECTROSCOPY AND ELECTROCHEMICAL REACTIONS OF PROBE MOLECULES AT THE OXIDE ELECTROLYTE INTERFACE

Exposure of atomically clean polycrystalline iron surfaces to low pressures of O2 produces oxide films which UV photoelectron spectroscopy and X-ray photoelectron spectroscopy reveal are predominantly bilayers of FeO and Fe3O4. FeO is the principal product indicated in the first 10 L of oxygen exposure. Fe3O4 formation is initiated following this exposure level. The Fe3O4 layer appears to constitute up to 25% of the total oxide composition, which is predominantly FeO. Following the growth of several oxide thin films on iron under vacuum, at different O2 exposure times and pressures, transfer under ultrahigh vacuum to an electrochemical cell was carried out, whereupon electron transfer rates (ETR) of benzoquinone (BQ) reduction and ferrocene (Fc) oxidation were explored in acetonitrile. Minimal chemical reaction of the iron or oxidized iron surfaces with acetonitrile was demonstrated. The voltammetrically determined ETR of BQ and Fc both showed a different dependence on oxide thickness than predicted for a tunneling-controlled process.