A consistent method for quantitative XPS peak analysis of thin oxide films on clean polycrystalline iron surfaces

We report a method for the quantitative analysis of the Fe(2p) and O(1s) core level XPS spectra which allows the determination of the Fe2+/Fe3+ ratio within a thin oxide film. The method involves the determination of the intensity of specific shake-up features within the Fe(2p) spectrum. This analytical method is demonstrated by comparing the Fe(2p) spectra of electrochemically modified Fe2+-rich and Fe3+-rich electrodes in a combined UHV-XPS-electrochemical system. Exposure of clean polycrystalline Fe surfaces to low pressures of O2 gas for different time intervals has also been carried out. Quantitative XPS analyses of the oxide films produced by O2 exposures reveal that the oxide films are predominantly trilayers of FeO, Fe3O4 and FeOOH phases, the latter due to ambient background contamination. The analyses demonstrate that a multiplicity of Fe2+ and/or Fe3+ chemical environments and binding energies is present in such films, and this must be explicitly accounted for during quantitative analyses.