Adsorption of ethylene on stoichiometric and reduced NiO(100)

The adsorption of ethylene has been studied on stoichiometric NiO(100) and on surfaces reduced to 40% of the stoichiometric oxygen content. The adsorption process was followed with XPS, Auger spectroscopy and LEED at substrate temperatures of 200 to 500 K and at ethylene pressure of 5 × 10^?7 Torr. At 200 K, two distinct ethylene species are observed on stoichiometric NiO(100). The first species saturates at 0.02 ML after 200 L and is adsorbed molecularly, interacting with both nickel and oxygen sites. A condensed species then forms which does not saturate for exposures up to 2100 L. Both adsorb reversibly with all traces of carbon absent by 270 K. At 200–300 K, reduced NiO(100) also adsorbs two molecular ethylene species, although with a preference for nickel sites. However, the uptake of ethylene increases only slightly with surface reduction. Adsorption is no longer reversible for the reduced surface and increasing the substrate temperature causes fragmentation of the adsorbed ethylene with a concomitant reduction in lattice oxygen content.     

Structure analysis of an oxidized nickel (110) surface using low energy ion scattering

Low energy (6 keV) argon and neon ion scattering in the low angle mode (θ = 30°) has been used to investigate changes in the surface structure of a Ni(110) surface caused by the adsorption of oxygen at low exposures (10^?6 Torr s). The experimental energy spectra indicate that due to adsorption of oxygen, the interatomic distance in the 〈1̄10〉 direction increases while in the 〈001̄〉 direction this distance seems to decrease. This represents strong evidence that a reconstruction process is taking place during the early stages of oxidation of the Ni(110) face, in which the interatomic distances in the 〈1̄10〉 direction doubles. The oxygen atoms were found to lie in or close to the nickel 〈001̄〉 rows. These results are not in agreement with recently published dynamical LEED calculations.     

Surface and subsurface oxide formation on Ni(1 0 0) and Ni(1 1 1)

The oxidation of Ni(1 0 0) and Ni(1 1 1) at elevated temperatures and large oxygen exposures, typical of the methods used in the preparation of NiO(1 0 0) films for surface studies, has been investigated by medium energy ion scattering (MEIS) using 100 keV H⁺ incident ions. Oxide film growth proceeds significantly faster on Ni(1 1 1) than on Ni(1 0 0), but on both surfaces oxide penetration occurs to depths significantly greater than 100 Å with total exposures of 1200 and 6000 L respectively. The metal/oxide interface is extremely rough, with metallic Ni extending to the surface, even for much thicker oxide films on Ni(1 1 1). On Ni(1 1 1), NiO growth occurs with the (1 0 0) face parallel to the Ni(1 1 1) surface and the close-packed 〈1 1 0〉 directions parallel. On Ni(1 0 0) the MEIS blocking curves cannot be reconciled with a single orientation of NiO(1 0 0) (with the 〈1 1 0〉 directions parallel) on the surface, but is consistent with the substantial orientational disorder (including tilt) previously identified by spot-profile analysis LEED.     

Ultrathin nickel oxide films grown on Ag(0 0 1): a study by XPS, LEIS and LEED intensity analysis

The structure of a nickel oxide film 2 ML thick has been investigated by LEED intensity analysis. The NiO film was prepared by evaporating Ni in presence of O₂ at a pressure in the 10⁻⁶ mbar range. The growth of the oxide film was followed by XPS, LEIS and LEED. In the early stages of deposition, the film shows a (2 × 1) superstructure in LEED. After deposition of 2 ML of NiO, a sharp (1 × 1) LEED pattern is observed. The intensity versus electron energy curves of the LEED spots were measured for this NiO(1 × 1) film and analysed by means of the tensor LEED method. A good level of agreement of the experimental LEED intensities with those calculated for a pseudomorphic NiO(0 0 1) film was obtained. We found that oxygen atoms at the oxide-substrate interface are on-top silver atoms. The interlayer distance in the oxide does not differ significantly from that in bulk NiO(0 0 1), within the accuracy of the analysis. An outward displacement (0.05 ± 0.05 Å) of oxygen atoms with respect to nickel atoms was found at the oxide film surface. The interlayer distance at the silver-nickel oxide interface is 2.43 ± 0.05 Å.     

A quantitative ion-scattering study of the Ni(110) surface during the early stages of oxidation

The adsorption of oxygen on clean Ni(110) has been studied at room temperature and at 475 K by Rutherford backscattering, using the effects of channeling and blocking, and lowenergy electron diffraction. At both temperatures successive LEED structures are formed at low oxygen coverage (?0.5 monolayer). With increasing oxygen content stoichiometric NiO is formed on top of the Ni(110) surface, at room temperature as an amorphous layer and at 475 K as patches of crystalline oxide, oriented with the NiO(100) planes parallel to the Ni(110) surface plane. At 475 K the nickel atoms in the interface region between oxide and substrate are displaced over a thickness of less than 2 monolayers. Based on the measurement of the oxide composition as function of coverage we suggest a modification of the island growth model as proposed by Holloway and Hudson for the Ni(100) and (111) surfaces.     

Vibrational properties of the adsorbed monolayer on face-centered cubic crystals

Calculations of mean-square displacements 〈u²〉 of the atoms in adsorbed monolayers on fcc crystals are presented and compared with LEED experimental results. This text is restricted to the case of a C(2 × 2) adsorbed layer on a (100) surface [experimental case of Ni(100) with adsorbed sulfur, sodium, cesium or oxygen]. 〈u²〉's perpendicular to and parallel to a (100) surface are calculated for the adsorbed atoms and the atoms of the first surface layer of the crystal. The values obtained are compared with those for a clean (100) surface and the volume of the crystal. Every possibility for force constants between adsorbate and substrate atoms is examined. It is shown that the measurement of 〈u²〉 perpendicular to the (100) surface yields the adsorbate-substrate force constants and that 〈u²〉 parallel to the (100) surface yields the adsorbate-adsorbate force constants.     

Stoichiometric and oxygen deficient MoO3(010) surfaces

The (010) surface of single crystal MoO₃ has been prepared and examined using LEED, XPS, UPS, and ELS. Three methods yield the stoichiometric surface: scraping in UHV and annealing, ion etching followed by reoxidation (770 K, 10² Pa O₂), or oxygen treatment to remove carbon contamination. LEED shows the surface periodicity is the same as that of the bulk (010). The MoO₃ valence band is 7 eV wide with density of states maxima at 1.5, 3.6, and 5.6 eV below the top of the valence band. Heating MoO₃ in vacuum reduces the surface region. XPS indicates the O/Mo atomic ratio decreases to 2.85 ± 0.12 on heating to 600 K. Ar ion bombardment disorders the surface and reduces the surface O/Mo atomic ratio to 1.6. Annealing of reduced surfaces at > 770 K incompletely reoxidizes them by diffusion of oxygen from the bulk. UPS of reduced and annealed MoO₃ exhibits two new emission features in the bandgap at 0.9 and 2.0 eV above the top of the valence band. These features originate from Mo derived states of a defect involving two or more Mo atoms, such as crystallographic shear planes. Because of the insulating nature of MoO₃, surface charging and electron beam induced damage were substantial hindrances to electron spectroscopic examination.     

On the origin of the substrate-induced oxidation of Ni/NiO(001) studied by X-ray Photoelectron Spectroscopy and Molecular Dynamics Simulations

Metallic Ni, vapor-deposited on NiO(001) near room temperature, could be gradually oxidised upon annealing between 800 K and 940 K in Ultra High Vacuum (UHV), as evidenced by X-ray Photoelectron Spectroscopy for initial Ni coverage of 1.6, 3.8 and 7.5 equivalent monolayers (ML). The time dependence of the oxidation process was consistent with a diffusion mechanism, supplying oxygen via the NiO crystal to a coalesced particulate deposit and resulting in an oxide shell, which grew over the entire surface and enclosed a shrinking metallic core. Similar to the well known behaviour upon gas phase oxidation, the process was fast within a depth of two atomic layers of Ni, limited by the diffusive supply of oxygen from the substrate. Molecular Dynamics Simulations for 0.06, 0.11 and 0.22 ML of Ni ions deposited on a model NiO(001) substrate indicated the formation of NiO islands via oxygen ions transferred from the surface and near-surface layers of the crystal. A significant atomic concentration of oxygen vacancies of the order of 10 to 20% could be created in each underneath layer, before the next one started donating lattice anions. This suggests a possible explanation for the aforementioned NiO-substrate-induced oxidation of deposited Ni, whereby the formation of oxygen vacancies inside the crystal supplies the necessary oxygen.     

Relationship between vibrational states of O-Ni systems and their superficial structure on (100) face of nickel single crystal

High resolution energy loss spectra of 4 eV electrons reflected in the specular direction from Ni(100) surface clean or covered by the ordered structures obtained in the different stages of the metal oxidation, are analysed with reference to LEED patterns. At room temperature, the successive p(2 × 2) and c(2 × 2) structures associated with the chemisorption of oxygen have been observed without modification of the energy loss spectra, in respect of the clean nickel surface. Surface phonons are known to occur in the case of the c(2 × 2)S ordered layer and their absence in the case of Ni-O corresponding system is discussed. After short exposures to oxygen between 200 to 500° C, the surface exhibits a so called “intermediate oxide”. It is identified by its hexagonal unit mesh (～5 Å) with two equivalent orientations along the [100] and [110] directions of the substrate and its vibrational spectra characterized by a loss peak at ? 112.5 meV (± 2.5 meV). Subsequent exposures to oxygen lead to the formation of the (100) face of NiO (in epitaxy on the Ni(100) face) accurately identified by its LEED pattern. The obtained typical multiple loss spectra with spacing 67.5 meV (± 15 meV) reveal a scattering of low energy electrons by long wavelength optical phonons associated to the oxide. The characteristic energy loss (67.5 meV) is in relative good agreement with the energy of the Fuchs-Kliewer surface phonon calculated from the optical constants of the nickel oxide.     

Gas adsorption properties of stoichiometric and reduced NiO(100)

The gas adsorption properties of stoichiometric and surface reduced NiO(100) single crystals are presented in their interactions with CO, ethylene and H₂O at gas pressures of 1×10^-7 to 3×10^-6 Torr and substrate temperatures of 200 to 500 K. At temperatures below 500 K, gas adsorption is sluggish or even completely undetectable, depending upon the degree of surface oxidation and gas of interest. At temperatures of ⪆ 450 K, however, comparatively vigorous reaction occurs between the oxide lattice atoms and the adsorbing gas, which results in surface reduction for CO and ethylene exposures and surface hydroxylation for the case of H₂O exposure. These data suggest that the mechanisms dominating metal oxide catalytic behavior are fundamentally different from those currently proposed for their pure metal counterparts. Significant amounts and rates of gas uptake occur only with a concomitant destruction and/or transformation of the oxide surface, itself. For interactions involving reducing gases, the surface can be described as truly catalytic only from the viewpoint of the metal component, with surface oxygen requiring replenishment either through migration of subsurface oxide or through adsorption of an oxidizing gas.     

Surface properties of LiCoO2, LiNiO2 and LiNi1−xCoxO2

The surface composition and chemical environment of LiCoO₂, hexagonal LiNiO_2, cubic LiNiO₂, and the mixed transition metal oxide LiNi_0.5Co_0.5O₂ have been determined by Auger electron and X-ray photoelectron spectroscopies. While the LiCoO₂ surface properties can easily be extrapolated from bulk composition, the nickel-containing materials are less straightforward. Their surface concentration tends to be depleted in lithium relative to that of the bulk and shows an atypical chemical environment for the constituent elements. The Ni 2p XPS photoemission suggests a near “ NiO-like” selvedge through the XPS binding energies and satellite structure which are essentially identical to that of NiO; the spectrum appears fairly insensitive to lithium concentration. Although there is little evidence for higher binding energy Ni³⁺ species or for an electron poor Ni^2.δ+-derived band structure in the XPS, the lattice oxygen is very electron-rich and yields among the lowest binding energies reported for a transition metal oxide. The nickel-containing lithium oxide selvedge is thus not simply “NiO” and the surface lithium cations have a measurable effect on the electronic structure even in their more highly depleted levels. This is explained in the context of the charge-transfer model of the oxide band structure.     

The condensation of gold onto tantalum (100) single crystal surfaces: LEED,AES analysis

The condensation of gold onto clean and contaminated, single crystal, tantalum (100) surfaces has been followed by using LEED and AES. On a contaminated surface gold condenses as crystallites in a (211) surface orientation with some degree of preferred, azimuthal orientation. On a clean surface gold condenses in an ordered overlayer. Up to approximately $\text{3}\text{4}$ monolayer the structure conforms to the (1 × 1) tantalum surface. Beyond this, the observed LEED structure may be interpreted initially in terms of a TaAu overlayer made up of 90° rotated domains with (001)TaAu//(100)Ta and 〈 10 〉 TaAu// 〈 11 〉 Ta, and then in terms of a gold overlayer in a “distorted (111)” orientation. Annealing of these gold films always results in the formation of a (1 × 1) TaAu overlayer of small crystallite size.     

Distorted surface oxygen structure on UO2(100)

Low-energy electron diffraction (LEED) has been combined with ion-scattering spectroscopy (ISS) measurements of He⁺ at 500 eV to characterize experimentally the surface structure formed by oxygen atoms on UO₂(100). Insight into the surface geometry required to generate the LEED features was gained via laser transform simulation and kinematical diffraction analysis of two-dimensional arrays. Integrating the above approaches leads to a UO₂(100) surface model consisting of a monolayer of oxygen atoms arranged in distorted bridge-bond, zig-zag chains along 〈100〉 directions. Configurational energies were calculated which support the distorted UO₂(100) zig-zag structure.     

Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.     

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

The initial stages of the interaction of oxygen gas with a clean Ni (100) surface have been studied by a combination of LEED, AES, work function change and ion bombardment sectioning techniques. The reaction could be divided into three reaction regions: a fast dissociative chemisorption leading to surface structures based on the initial nickel interatomic spacing and resulting in an oxygen coverage of approximately 0.4 monolayers; a rapid oxidation leading to epitaxial NiO, two layers thick ; and a final slow thickening of bulk NiO. The first two regions were dependent only upon oxygen exposure. The third region was observed only at high gas-phase oxygen pressures or very low surface temperatures. Kinetics analyses are developed to explain the rate of oxygen chemisorption and the rate of oxide nucleation and growth.     

An investigation of the kinetics of structural changes during the early oxidation stages of a Ni(100) surface using low energy ion scattering (LEIS)

Low energy ion scattering has been used to investigate the early stages of the oxidation of a Ni(100) surface. This technique allows simultaneous study of the oxygen uptake in the surface and the development of surface structures. Bombardment induced surface damages was minimised by performing the experiments with low ion doses, while keeping the target at 200–300°C. The measured kinetics of the oxygen uptake are in good agreement with recent work, using different techniques. It is concluded that during the early chemisorption, a two stage process takes place: an initial oxygen adsorption during which the O atoms probably reside within the fourfold surface hollows, followed by a reconstruction process, caused by the combined action of at least two nearest neighbour O atoms, trapping mobile Ni adatoms, after which the O atoms stabilise at a site in or close to the reconstructed 〈001̄〉 row. Observed structural changes at higher exposures are compatible with a transition into a (3 × 1) structure and subsequently NiO, but cannot, as yet be positively identified.     

Structural models of the reconstructed W{001} surface

A LEED intensity analysis of 5 beams from the low-temperature W{001}c(2×2) structure indicates that the surface reconstruction involves shifts of the surface atoms along 〈110〉 directions within the plane of the surface, as suggested by Debe and King. At temperatures 100–140K the shifts are in the range 0.15–0.3 Å, with the first interlayer spacing 1.48–1.58 Å (bulk value 1.58 Å). Similar analysis of the room-temperature W{001}c(2×2)-H phase indicates: (i) none of the models proposed, which ascribe the c(2×2) structure directly to ordered hydrogen adsorption, can explain the experimental data; (ii) the W{001}c(2×2)-H structure is probably impurity stabilized by H at room temperature in the same W lattice as the low-temperature reconstructed phase.     

Angle-resolved ups measurements in a modified LEED system

A LEED chamber has been modified to include a differentially pumped discharge lamp (He or Ne) and an additional retarding grid electron energy analyser for UPS. This small analyser is located at right angles to the LEED analyser and does not interfere with normal LEED and Auger operations. The UPS signal is amplified by a channel plate multiplier and accelerated onto a phosphor-coated screen. Directional information is obtained by scanning this screen with a collimated photomultiplier detector. A phase-lock amplifier is used to differentiate the signal from the photomultipler. Alternatively the phosphor screen can be used as a collector to measure a total spectrum. The acceptance angle of the UPS analyser is 90°. In the angular resolving mode it is possible to observe emission from a (100) fcc crystal in the 〈100〉, 〈110〉 and 〈111〉 directions with a fixed incident photon angle in the range 20–40° to the normal. The acceptance angle of the detector was usually ~7° but this can be varied by changing the collimating tube on the photomultipler. The direction dependent features of the d-band spectrum of clean nickel with a (100) surface have been examined. Characteristic features were observed for each of the 〈100〉, 〈111〉 and 〈110〉 directions. These are compared with those reported for crystals with the corresponding surface orientations. The effects resulting from the chemisorption of nitric oxide on this nickel crystal have also been investigated.     

Initial growth of platinum on oxygen-covered Ni(1 1 0) surfaces

The initial growth of Pt on the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) surfaces has been studied by coaxial impact collision ion scattering spectroscopy (CAICISS), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). Prior to Pt deposition, the atomic structure of the near-surface regions of the Ni(1 1 0)-(3 × 1)-O and NiO(1 1 0) structures were studied using CAICISS, finding changes to the interlayer spacings due to the adsorption of oxygen. Deposition of Pt on the Ni(1 1 0)-(3 × 1)-O surface led to a random substitutional alloy in the near-surface region at Pt coverages both below and in excess of 1 ML. In contrast, when the surface was treated with 1800 L of atomic oxygen in order to form a NiO(1 1 0) surface, a thin Pt layer was formed upon room temperature Pt deposition. XPS and LEED data are presented throughout to support the CAICISS observations.     

Structure study of oxygen-adsorbed Ni(111) surface by high energy ion scattering

Surface atomic structures of clean, oxygen-adsorbed, and oxidized (111) nickel have been studied quantitatively by using MeV ion scattering in combination with AES and MEED. We show that; the clean (111) nickel surface has the bulk-like structure with reconstruction or relaxation less than 0.02 Å, the surface thermal vibration amplitude is enhanced by ~20% compared to the bulk value, adsorbed oxygen results in surface lattice expansion by ~0.15 Å which is closely correlated to the p(2 × 2) or (√3 × √3) R30° superstructure, and oxidation at room temperature saturates at a stage which incorporates ~ 3 monolayers of nickel in a stoichiometric amorphous film of NiO whereas at temperatures higher than ~200° C thicker oxide films are produced. Our study indicates that each oxygen atom adsorbed on the Ni(111) surface interacts with and relaxes three nearest neighbor nickel atoms, and after saturation of the relaxation, oxidation of three monolayers takes place abruptly after which the oxide layer on the surface apparently blocks further reaction.