Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (√2 × √2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace.

To explore the catalytic properties of cobalt oxide at the atomic level, we have studied the interaction of CO and O 2 with well-ordered Co 3 O 4 (111) thin films using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HR-XPS), infrared reflection absorption spectroscopy (IRAS), and temperature-programmed desorption spectroscopy (TPD) under ultrahigh vacuum (UHV) conditions. At low coverage and temperature CO binds to surface Co 2+ ions on the (111) facets. At larger exposure a compressed phase is formed in which additional CO is located at sites in between the Co 2+ ions. In addition a bridging carbonate species forms which is associated with defects such as step edges of Co 3 O 4 (111) terraces or the side facets of the (111) oriented grains. Preadsorbed oxygen neither affects CO adsorption at low coverage nor the formation of the surface carbonate but it blocks formation of the high coverage CO phase. Desorption of the molecularly bound CO occurs up to 180 K, whereas the surface carbonate decomposes in a broad temperature range up to 400 K under the release of CO and, to a lesser extent, of CO 2 .Upon strong loss of crystalline oxygen the Co 3 O 4 grains eventually switch to the CoO rocksalt structure.

We report on the self-organized growth of monatomic transition-metal oxide chains of (3×1) periodicity and unusual MO_{2} stoichiometry (M=Ni, Co, Fe, Mn) on Ir(100). We analyze their structural and magnetic properties by means of quantitative LEED, STM, and density functional theory (DFT) calculations. LEED analyses reveal a fascinating common atomic structure in which the transition-metal atoms sit above a missing-row structure of the surface and are coupled to the substrate only via oxygen atoms. This structure is confirmed by DFT calculations with structural parameters deviating by less than 1.7 pm. The DFT calculations predict that the NiO_{2} chains are nonmagnetic, CoO_{2} chains are ferromagnetic, while FeO_{2} and MnO_{2} are antiferromagnetic. All structures show only weak magnetic interchain coupling. Further, we demonstrate the growth of oxide chains of binary alloys of Co and Ni or Fe on Ir(100), which allows us to produce well-controlled ensembles of ferromagnetic chains of different lengths separated by nonmagnetic or antiferromagnetic segments.

Cobalt oxide nanomaterials show high activity in several catalytic reactions thereby offering the potential to replace noble metals in some applications. We have developed a well-defined model system for partially reduced cobalt oxide materials aiming at a molecular level understanding of cobalt-oxide-based catalysis. Starting from a well-ordered Co3O4(111) film on Ir(100), we modified the surface by deposition of metallic cobalt. Growth, structure, and adsorption properties of the cobalt-modified surface were investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and infrared reflection absorption spectroscopy (IRAS) using CO as a probe molecule. The deposition of a submonolayer of cobalt at 300 K leads to the formation of atomically dispersed cobalt ions distorting the surface layer of the Co3O4 film. Upon annealing to 500 K the Co ions are incorporated into the surface layer forming ordered two-dimensional CoO islands on the Co3O4 grains. At 700 K, Co ions diffuse from the CoO islands into the bulk and the ordered Co3O4(111) surface is restored. Deposition of larger amounts of Co at 300 K leads to formation of metallic Co aggregates on the dispersed cobalt phase. The metallic particles sinter at 500 K and diffuse into the bulk at 700 K. Depending on the degree of bulk reduction, extended Co3O4 grains switch to the CoO(111) structure. All above structures show characteristic CO adsorption behavior and can therefore be identified by IR spectroscopy of adsorbed CO.

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