Abstract

The precise determination of the surface structure of iron oxides (hematite andmagnetite)
is a vital prerequisite to understand their unique chemical and physical properties
under different conditions. Here, the atomic structure evolution of the hematite (0001)
surface under reducing conditions was tracked by polarization-resolved infrared reflection
absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The
frequency and intensity of the CO stretch vibration is extremely sensitive to the valence
state and electronic environments of surface iron cations. Our comprehensive IRRAS
results provide direct evidence that the monocrystalline, stoichiometric a-Fe2O3(0001)
surface is single Fe-terminated. The initial reduction induced by annealing at elevated
temperatures produces surface oxygen vacancies, where the excess electrons are
localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface
restructuring occurs upon further reduction by exposing to atomic hydrogen followed by
Ar+-sputtering and annealing under oxygen poor conditions. The restructured surface
is identified as a Fe3O4(111)/Fe1−xO(111)-biphase exposing both, Fe3+ and Fe2+
surface species. Here the well-defined surface domains of Fe3O4(111) exhibit a Feoct2-
termination, while the reduced Fe1−xO(111) is Fe2+(oct)-terminated. These findings are
supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and
(001) monocrystalline surfaces.