Influence of the defects of a thin NiO(100) film on the adsorption of NO

Defects often play an important role for the adsorption and reaction behavior of a surface. This, however, is not the case with the adsorption of NO on NiO(100), as our experiments with a thin NiO(100) film grown on a Ni(100) substrate demonstrate. This film possesses a three‐dimensional band structure, which is comparable to that of a NiO(100) single crystal. A spot profile analysis low‐energy electron diffraction investigation shows that the film consists of crystallites, which are tilted with respect to the Ni substrate. The film must exhibit a high defect density where the crystallites border on each other. Moreover, x‐ray photoelectron spectroscopic (XPS) measurements indicate the presence of O− or OH species on the surface. We have studied the adsorption of NO on a NiO(100) film via high‐resolution electron energy‐loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and XPS. With HREELS and TDS we could only detect one kind of NO species on the surface. Comparing the TD and XP spectra of NO adsorbed on the film and on a bulk NiO(100) surface, which exhibits a much lower defect density, we can show that this species is NO adsorbed on regular, nondefect NiO sites

CO on NiO(100): orientation and bonding

We use thermal desorption spectroscopy to estimate the adsorption energy of CO on NiO(100) to be 7.0–8.8 kcal mol−1. NEXAFS is employed to determine the orientation of the CO axis. The molecule is oriented perpendicular to the NiO(100) surface. In the present case we have resorted to angle-resolved photoelectron spectroscopy (ARUPS) to find indications that the CO molecule interacts with the surface through its carbon lone pair. The experimental analysis is in agreement with theoretical predictions that CO is held to NiO(100) mainly via electrostatic multipolar forces

Hydroxy1 driven reconstruction of the polar NiO(111) surface

We have studied the reconstruction of the polar NiO(111) surface predicted recently by D. Wolf with low energy electron diffraction. Thin NiO films (10–20 Å) are used as substrates. As prepared, the films with a p(1×1) NiO(111) structure are covered with hydroxyl groups, which may be removed through a simple heat treatment. As the hydroxyl groups are desorbed, the surface reconstructs, exhibiting a diffuse p(2×2) structure. Readsorption of water onto the reconstructed surface lifts the reconstruction and again leads to the formation of the p(1×1) hydroxyl covered surface

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

The structure of thin NiO(100) films grown on Ni(100) as determined by low-energy-electron diffraction and scanning tunneling microscopy

A Ni(100) surface exposing terraces of approximately 100 Å width which are separated from each other by monatomic steps descending along the [010] direction has been oxidized above room temperature. Via intermediate formation of the well-known p(2×2) and c(2×2) chemisorbed phases, which are identified by LEED (low energy electron diffraction) and STM (scanning tunneling microscopy) in the present study, a thin film of 4–5 layers of NiO(100) builds up on the surface. The NiO layer consists of crystallites with a typical lateral extension of 50 Å as revealed by the STM data. SPA-LEED (LEED spot profile analysis) measurements allowed us to determine that the crystallite surfaces are tilted preferentially along the [011] and [01̅1] directions of the Ni(100) plane by an average angle of 8° with a half width of the angular distribution of 6°. We show that the development of the oxide islands most probably starts at the terrace edges of the metal surface. While the islands grow in size the strain between oxide and metal increases due to the large differences in the lattice constants of Ni and NiO. Part of the strain is compensated by a tilt of the islands induced via migration of Ni atoms from the step edges underneath the oxide islands. The generated NiO surface is characterized by two types of regions, namely the regions on the islands which are basically flat and contain regular NiO sites, covering 75–80% of the crystal surface, and the regions between the islands with many defect sites (20–25% of the surface area). The consequences of the structural properties of the NiO film on the adsorption of molecules, i.e., NO, are discussed in line with results of a previous study

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

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

Electronic surface state of NiO (100)

The electronic structure of the (100) surface of NiO has been studied using EELS (electron energy loss spectroscopy) and ab initio calculations. In addition to the previously documented bulk excitations of NiO, two new states at energies of 0.57 and 1.62 eV have been found. These states are attributed to d-d transitions of the nickel surface ions. As expected for surface states, they are affected by the interaction with an adsorbate, i.e. adsorption of NO leads to a shift to higher energy. Ab initio cluster calculations show that electronic structure of the surface is considerably different from that of the bulk which is a result of the lower symmetry of the crystal field at the surface (Oh→C4v). The nature of the observed surface states has been identified by a comparison of the experimental data with theoretical results