B. Dillmann

Hydroxyl groups on oxide surfaces: NiO(100), NiO(111) and Cr2O3(111)

Abstract Hydroxyl groups at the surfaces of NiO(100), NiO(111), and Cr2O3(111) have been studied using different surface sensitive spectroscopies. The OH groups are readily formed by the interaction of the oxide surfaces with the residual gas atmosphere or by dosing of water. They can be removed by annealing at temperatures T ⩾ 600 K (NiO) or T ⩾ 540 K (Cr2O3). OH does not bond to regular NiO(100) sites so that for a cleaved NiO(100) single crystal surface no OH adsorption could be observed. For the more defect containing NiO(100)/Ni(100) film the existence of OH could be verified by isotope exchange with OD. As indicated by TDS (thermal desorption spectroscopy) of an NO adsorbate, OH groups fully block the (111) oriented surface of NiO for NO adsorption which indicates that OH groups bond to regular NiO(111) surface sites. For Cr2O3(111) thermal decomposition of water at defect sites and photochemical dissociation is observed. The latter path seems to involve water molecules in the second layer and leads most likely to an occupation of regular surface sites.

Adsorption on a polar oxide surface: O2, C2H4 and Na on Cr2O3(0001)/Cr(110)

A polar Cr2O3(0001) surface is prepared as an epitaxial film on a Cr(110) substrate. The film is thick enough to represent the bulk surface. Applying a variety of surface sensitive techniques [thermal desorption spectroscopy (TDS), reflection absorption infrared spectroscopy (RAIRS), electron energy loss spectroscopy (EELS) and photoelectron spectroscopy (PES)] we have studied adsorption of molecular oxygen, ethene and sodium.

Adsorption and reaction of molecules on surfaces of metal—metal oxide systems

Abstract Molecular adsorption on oxide surfaces is gaining increasing interest both experimentally and theoretically. Adsorption studies on model systems, where well ordered thin oxide films grown on a metal substrate to avoid sample charging in connection with electron spectroscopic measurements, were used, are reported. Two oxide systems are compared: (i) a reactive transition metal oxide surface of Cr2O3(111) where it is shown that the surface contains Cr2+ ions which trigger its reactivity; (ii) a non-reactive simple metal oxide surface of γ-Al2O3(111) which is used as a support model surface. The adsorption of various molecules on both surfaces has been examined, and how the properties of the surface are modified when metals are deposited on the oxide surface have been studied. The results of alkali metal deposits on Cr2O3(111) and Pt deposits on γ-Al2O3(111) are presented. The applied methods include LEED, STM, TPD, ARUPS, ELS, XPS, HREELS and ISS.

Molecules on oxide surfaces

Abstract Metal oxides may be prepared as thin (5–50 A) 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 Cr 2 O 3 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, O 2 , CO 2 , H 2 O and C 2 H 4 . 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.

Electronic and geometric structure of adsorbates on oxide surfaces

Abstract We have investigated the electronic and geometric structure of surfaces of transition metal oxides and simple metal oxides applying electron spectroscopic methods. In order to avoid charging problems, we have resorted to the preparation of thin (5 – 50 A) metal oxide films grown on metallic substrates via several oxidation techniques. We have studied NiO, CoO, Cr 2 O 3 , and Al 2 O 3 . The thin films have the advantage that they may be easily cooled to liquid nitrogen and liquid helium temperatures. Another interesting feature of the thin films is the possibility to prepare thermodynamically unstable surfaces, such as (111) surfaces of ionic rock salt structures, and study the adsorption and reaction at such surfaces. Adsorption and reaction of molecules has not only been investigated on the clean oxide substrates but also on the surfaces modified through deposited ultrathin metal films. Such systems may be considered as models for heterogeneous catalysts.

Electronic Excitations at Oxide Surfaces

The present paper summarizes the possibilities to use electron energy loss spectroscopy (EELS) to investigate the electronic properties of systems with highly localized electronic states, such as oxide surfaces. Surface excitations may be clearly distinguished from bulk excitations in EELS, and eventually a ligand field spectroscopy may be developed on this basis. This ligand field spectroscopy is used to study surface phase transitions at oxide surfaces. Adsorbates influence the surface properties of oxides dramatically. Examples are discussed.