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18; iron oxide model catalysts: preparation and characterization
Abstract:
Epitaxial Fe3O4(111) films were grown onto a Pt(111) substrate by repeated cycles of iron deposition and subsequent oxidation in 10-6 mbar oxygen. A previous low energy electron diffraction (LEED) intensity analysis revealed the regular Fe3O4(111) surface to expose ¼ monolayer Fe atoms over a close-packed oxygen layer underneath. With scanning tunneling microscopy (STM) a hexagonal lattice of protrusions with a 6 Å periodicity is observed. The protrusions are assigned to the topmost layer Fe atoms, which agrees with the dominating Fe3d electron density of states near the Fermi level related to these surface atoms, as revealed by ab-initio spin density functional theory calculations. The most abundant type of point defects observed by STM are attributed to iron vacancies in the topmost layer, which was confirmed by LEED intensity calculations where different types of vacancy defects have been simulated. For oxidation temperatures around 870 K the regular Fe3O4(111) surface coexists with several different surface structures covering about 5% of the films, which expose ¾ ML iron atoms or close-packed iron and oxygen layers, resulting in surface domains that are FeO(111) and Fe3O4(111) in nature. These domains are arranged periodically on the surface and form ordered biphase superstructures. At 1000 K oxidation temperature they vanish and only the regular Fe3O4(111) surface remains.
