Molecules on oxide surfaces

Metal oxides may be prepared as thin (5–50 Å) films on top of metallic substrates. By such means oxide substrates with properties identical to bulk oxides may be formed which can be studied via electron spectroscopies without being hindered by charging, as well as cooling problems. We report here on results on NiO and on Cr2O3 surfaces. We discuss some structural aspects of oxide surfaces such as surface reconstruction of polar rock salt-type surfaces, and structural phase transitions on corundum type structures. The nature of the phase transition will be discussed with respect to the magnetic properties of the oxide. Furthermore we report on the interaction of those surfaces with molecules from the gas phase. In particular we study the interaction with small molecules such as CO, NO, O2, CO2, H2O and C2H4. We observe via various surface sensitive techniques such as thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), angle resolved photoemission (ARUPS), electron energy loss spectroscopy (HREELS), infrared-reflection-absorption-spectroscopy (IRAS), and near-edge-X-ray-absorption-fine-structure spectroscopy (NEXAFS), associative as well as dissociative adsorption and in the case of ethylene also polymerization reactions. Via isotopic labelling techniques combined with IRAS we study in detail the interaction of oxygen with the oxide surfaces, a process of general interest in connection with oxidation reactions

Adsorption of CO and NO on NiO and CoO: a comparison

Polar (111) and non-polar (100) thin epitaxial NiO and CoO films were prepared on suitable Ni and Co single-crystal metal surfaces. On these oxide films, CO and NO were adsorbed to probe the local electronic and geometric structure of the substrates using electron spectroscopic methods, especially HREELS and NEXAFS. On all oxide surfaces, the N-O stretching frequencies exhibit a red shift due to the chemical bonding to the surface, whereas the C-O stretching frequencies all lie in the vicinity of the C-O gas phase value with a tendency for a blue shift because of the purely electrostatic interaction with the substrates. An evaluation of these data together with NEXAFS data show that the NiO(111) film, which undergoes octopolar reconstruction upon heating, exhibits microfacets with fourfold sites tilted about 55° away from the surface normal even at room temperature. The situation on CoO(111), which cannot be heated sufficiently to prepare a reconstructed surface, seems to be somewhat different; a model of the possible structure of the unreconstructed CoO(111) surface is proposed

Autoionization spectroscopy of CO on metal oxide surfaces

Angular resolved autoionization spectra of CO after Cls → 2π∗ excitation on different epitaxially grown metal oxide surfaces are presented. The spectra of the adsorbates on the oxide substrates differ with variation of the substrate. The spectra are compared with those obtained both for gas phase CO and for CO adsorbed on different metals, and differences in the bonding mechanisms are discussed. Qualitative calculations of the angular dependence are presented together with the measured spectra. Valence CI calculations of the states reached by the decay are also given

Stability of Polar Oxide Surfaces

The structures of the polar surfaces of ZnO are studied using ab initio calculations and surface x-ray diffraction. The experimental and theoretical relaxations are in good agreement. The polar surfaces are shown to be very stable; the cleavage energy for the (0001)-Zn and (0001 ¯ )-O surfaces is 4.0J/m 2 comparable to 2.32J/m 2 for the most stable nonpolar (1010) surface. The surfaces are stabilized by an electronic mechanism involving the transfer of 0.17 electrons between them. This leads to 2D metallic surface states, which has implications for the use of the material in gas sensing and catalytic applications