| AVS 60th International Symposium and Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS-TuM |
| Session: | Synthesis, Structure and Characterization of Oxides |
| Presenter: | C.J. Nelin, Consultant |
| Authors: | C.J. Nelin, Consultant F. Uhl, Ruhr-Universitat Bochum, Germany V. Staemmler, Ruhr-Universitat Bochum, Germany P.S. Bagus, University of North Texas H. Kuhlenbeck, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany M. Sterrer, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany H.-J. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany |
| Correspondent: | Click to Email |
We present theoretical and experimental results for the surface core-level shifts, SCLS, of MgO(100). Since MgO is an ideal ionic insulator, [1] one would expect shifts to higher binding energy, BE, for a surface Mg2+ cation and shifts to lower BE for a surface anion; furthermore, the magnitudes of the shifts of opposite sign should be comparable. This expectation follows from the electrostatics of the different coordination of the bulk and surface atoms. However, we have found that while the Mg(2p) core-level shift is ~+1 eV, as expected, the O(1s) SCLS is almost zero. This unexpected difference between the cation and anion SCLS has been found for a large range of theoretical models for MgO, which include extended embedded cluster models and periodic calculations. The experiments, which also show the different SCLS, were carried out for MgO films on Mo supports. While the electron density of the bulk Mg and O ions and the surface Mg ions have the dominantly spherical shape expected for closed shell species, the distortion of the surface O anions from spherical symmetry is the effect that leads to the different chemical shifts. Thus the SCLS provide another manifestation of the polarizability of the O anions that is associated with the catalytic activity of thin oxide films; see, for example, Ref. [2]. However, in this case the polarization is of the electronic charge density rather than of the geometric structure of the oxide. We analyze the nature of the polarization of the surface O anions and show how this leads to the anomalously small SCLS for these anions. We also consider the broadening of the XPS lines and we relate the Full Width at Half Maximum of the core-levels to the final state geometric modification of bond distances between Mg and O. [3]
1. C. Sousa, T. Minerva, G. Pacchioni, P. S. Bagus, and F. Parmigiani, J. Electron Spectrosc. Relat. Phenom. 63, 189 (1993).
2. H. J. Freund, Chem. Eur. J. 16, 9384 (2010).
3. C. J. Nelin, P. S. Bagus, M. A. Brown, M. Sterrer, and H.-J. Freund, Angew. Chem. Int. Ed. 50, 10174 (2011).