Characterization of the “native” surface thin film on pure polycrystalline iron: A high resolution XPS and TEM study

The characterization of the “native” surface thin film on pure polycrystalline iron has been studied by high resolution X-ray photoelectron (XP) spectroscopy of Fe 2p and O 1s regions. The film was allowed to form by exposing the sample to atmosphere at ambient conditions for a period of 1 h. The systematic approach used here includes the determination of curve fitting parameters from external standards and their use in fitting the raw data for the surface thin film. The quantitative high resolution XPS analysis involved an angle resolved study of the surface to determine the chemical composition and thickness of this native film. The film was found to be a mixture of Fe3O4 and Fe(OH)2 with a thickness of 1.2 ± 0.3 nm. This conclusion is consistent with thermodynamics as indicated by the Pourbaix diagram for the Fe-H2O system and the phase diagram for the Fe-oxygen system. A detailed TEM study of the native surface film also supports this conclusion.