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.

Cobalt oxide films were prepared by oxidation of different amounts of cobalt deposited on Ir(100)-(1 × 1), where oxygen rich conditions were applied during deposition. The resulting oxide films with thicknesses of up to about 40 Å were investigated as regards their crystallographic structure and morphology, applying quantitative low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). It can be unequivocally shown that the spinel-type Co(3)O(4) phase develops, for which an excellent fit between measured and calculated LEED intensity spectra is achieved (Pendry R-factor R = 0.124). In spite of the quadratic unit cell of the substrate the oxide films are in the polar (111) orientation. Also, the native lattice parameter of the material is assumed, i.e. there is no pseudomorphic relation to the substrate. However, by means of orientational epitaxy, one of the unit-mesh vectors of the oxide and one of those of the substrate layer are aligned, leading to two mutually orthogonal domains in the oxide. The oxide is terminated by a sublayer of cobalt ions which in the bulk were tetrahedrally coordinated Co(2+) ions. There are drastic relaxations of layer spacings at and near the surface. As a consequence, the bond length between the surface terminating cobalt ions and oxygen ions below is considerably reduced, indicative of a substantial change of the ionicity of the cobalt and/or oxygen ions. This is interpreted as accounting for polarity compensation of the film, as surface reconstruction, oxygen vacancies and species adsorbed can be ruled out.

Epitaxial cobalt oxide films on Ir(100) exhibit a rich scenario of different structural phases which are reviewed in this paper. The great majority of phases could be, as a rare case, crystallographically described by the joint application of atomically resolved STM and quantitative LEED, whereby structural surprises were more the rule than the exception. So, the oxide grows in the polar (111) orientation for both the Co3O4 and CoO stoichiometry on the bare Ir substrate in spite of the latter's square symmetry. Moreover, the film orientation can be tuned to non-polar (100) growth when one or several pseudomorphic Co layers are introduced as an interface between oxide and Ir substrate. By using the nanostructured Ir(100)-(5 × 1)-H phase as a template a nanostructured Co film can be formed whose oxidation leads to a nanostructured oxide. The nominally polar films circumvent the polarity problem by appropriate surface terminations. That of CoO(111) is, again as a surprise, realized by a switch from rocksalt-type to wurtzite-type stacking near the surface, by which the latter becomes metallic. The stepwise oxidation of a pseudomorphic Co layer on the bare Ir substrate leads to the sequential formation of rocksalt-type tetrahedral Co-O building blocks (with intermediate BN-type blocks) whereby the Co species more and more assume positions determined by the inner-oxidic binding.

Cobalt oxide (CoO) films epitaxially grown on Ir(100) in (111) orientation were investigated by means of quantitative low-energy electron diffraction and scanning tunneling microscopy. We find with high crystallographic precision that in the bulk of the films the rocksalt structure prevails while near the surface there is a switch towards the wurtzite structure. As a consequence, nanosized CoO cannot be considered as a single structural phase. The film surfaces prove to be metallic, apparently connected with polarity compensation.

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.