Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of FexO (x ~ 0.90–0.95); Fe2O3 (α and γ); Fe3O4; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). FeII and FeIII states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below EF, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for FexO and Fe3O4. Fe surfaces (films, foils, (100) face) yield predominantly FeIII species when exposed to high exposures of oxygen or air, though there is evidence for some FeII also. At low exposures the FeII/FeIII ratio increases.