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We review the current knowledge of the geometric and electronic structure of oxide surfaces. In particular, material published during the last five years and going beyond the latest book by Henrich and Cox, Surface Science of Oxide Surfaces, is documented. In addition to the discussion of effects on and in bulk oxide single crystal samples, and in fact mainly, we also refer to work on thin oxide films epitaxially grown on metallic substrates. Surface states on oxide surfaces as well as band structure effects are discussed. Reconstruction and the local electronic structure are other aspects covered. We review experimental as well as theoretical approaches. Adsorption on oxide surfaces is an important subject covered in this review, and oxide modification by metal adsorption is also mentioned.

Our combined experimental and theoretical analysis of the shifts, with particle size, of core-level binding energies (BE's) of metal nanoparticles on insulating supports, shows that these shifts have an important initial state contribution arising, in large part, because of lattice strain. This contribution of BE shifts has not been recognized previously. Lattice strain changes the chemical bonding between the metal atoms and this change induces BE shifts. DOI: 10.1103/PhysRevLett.93.026805 PACS numbers: 73.22.-f, 79.60.-i Photoelectron spectroscopy (PES) is often used to deduce information on the electronic structure of molecules, solids, and at surfaces [1]. There is increasing interest in nanostructured materials, especially in clusters grown on inert surfaces since they may have high catalytic activity [2]. Thus, the question of how the PES of deposited nanoparticles reflects their electronic and geometric structures is quite important. It is known that the corelevel BE's of metal clusters on insulating supports [3][4][5][6][7][8][9][10][11][12] shift to lower BE with increasing size by BE 1 eV from small clusters to bulk metal. However, there is substantial disagreement over the assignment of these shifts to initial or final state effects; see Ref.[13] for a rigorous definition of these two contributions. Mason [7] argued that changes in the electron configuration of the atoms in smaller clusters, not relaxation energies, were primarily responsible for the shift. On the other hand, Wertheim and collaborators [4,5] assigned the shift as due to final state screening effects and related the magnitude of BE to an effective cluster radius. For Cu clusters grown on thin Al 2 O 3 films, Wu et al. [11] concluded from an Auger parameter analysis that initial state contributions are small and may be significant only for very small clusters. This separation is quite important since the initial state BE reflect changes in the electronic structure before ionization of a core level and, hence, are of direct interest for materials properties. In the present work, we show that the initial and final state contributions to the BE are of comparable magnitude. Further, we relate the initial state BE to a lattice strain that exists because the average bond distances in small clusters are shorter than in the bulk [14,15]. The chemical bonding is different at the shorter bond distances and this, in turn, leads to changes in the core-level BE's. The chemical changes important for the BE involve an increased d to sp promotion, or hybridization, for shorter bond distances.Our work is based on a combination of experimental and theoretical methods that allow us to decompose the BE into initial and final state contributions. Our theoretical approach involves the calculation of electronic wave functions (WF's) for clusters and the determination of the BE's where relaxation in response to ionization is excluded, a pure initial state BE, and where this relaxation is allowed [13]. Our experimental approach involves use of an Auger para...

Figure 1. Thermal desorption spectra of NO on a cleaved singlecrystal NiO(100) surface and on NiO(100)/Ni(100). Adapted with permission from ref 1.

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The interaction of hydrogen with reduced ceria (CeO2−x) powders and CeO2−x(111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce3+ species to hydrogen, which results in the formation of Ce4+, and thus in oxidation of ceria.

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