NiO Islands Research Articles

Oxidation Kinetics of Cu-Ni Alloy Observed by in situ UHV-TEM

AbstractThe nucleation and growth of Cu2O and NiO islands due to oxidation of Cu-24%Ni(001) films were monitored at various temperatures by in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM). In remarkable contrast to our previous observations of Cu and Cu-Au oxidation, irregular-shaped polycrystalline oxide islands were observed to form with respect to the Cu-Ni alloy film, and an unusual second oxide nucleation stage was noted. Similar to Cu oxidation, the cross-sectional area growth rate of the oxide island is linear indicating oxygen surface diffusion is the primary mechanism of oxide growth.

Study of the growth of NiO on highly oriented pyrolytic graphite by X-ray absorption spectroscopy

In this work, we present a X-ray absorption spectroscopy (XAS) study of the growth of NiO on highly oriented pyrolytic graphite (HOPG). NiO has been grown by reactive evaporation of metallic Ni in an oxygen atmosphere (2 × 10 −5 Torr) at room temperature. We paid special attention to the study of the early stages of growth. Both, Ni 2p and O 1s core-level XAS spectra were measured. For large NiO coverages, the spectra resemble that of a NiO single crystal, thus indicating the formation of a stoichiometric NiO thin film on the HOPG substrate. The Ni 2p XAS spectra remain similar during the whole growth process, indicating that Ni atoms are present in the high spin Ni 2+ form, as supported by multiplet calculations. In contrast, for low coverages the line-shape of the O 1s XAS spectra differ strongly from that of bulk NiO. Cluster calculations of the spectra in octahedral and pyramidal symmetries support the formation of nanometric planar NiO islands at the graphite steps as previously observed by atomic force microscopy (AFM) in this system.

Strain relaxation and surface morphology of nickel oxide nanolayers

The surface morphology and the lattice constants of NiO overlayers in the thickness range of 1–20 monolayers (NiO nanolayers) on Pd(1 0 0) have been investigated by high-resolution spot profile low-energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). NiO islands grow epitaxially on Pd(1 0 0) on top of a c(4 × 2) Ni 3O 4 monolayer with a compressed strained lattice, which relaxes gradually attaining the bulk lattice constant at 10–12 monolayers. The strain relaxation is accompanied by the formation of small angle mosaic defect regions at the surface, which have been characterised quantitatively by following the behaviour of the satellites to the main Bragg diffraction rods. The analysis of the diffuse scattering intensity around the (0 0) diffraction spot reveals anisotropic NiO island shapes, whose orientation depends on the growth conditions. An incommensurate superlattice in LEED and STM at intermediate NiO coverages (∼2–6 monolayers) is observed and its origin is discussed.

Pressure-dependent Ni–O phase transitions and Ni oxide formation on Pt(111): An in situ STM study at elevated temperatures

Growth, atomic structure and O2 partial pressure dependent phase transitions of Ni-O structures and thin NiO films on Pt(111) have been studied using scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). In situ STM experiments were performed during film growth by reactive metal deposition at elevated temperatures (400-550 K) and variable O2 pressure. Depending on the substrate temperature, one-dimensional network-like Ni-O structures and islands with (7x1) and (4x2) reconstructions are formed during the initial stages of growth. These structures transform reversibly to a (2x2) reconstruction in a narrow O2 pressure range of 1.5-2x10(-6) mbar and can be monitored by in situ STM. Upon reduction of the O2 pressure to <10(-10) mbar pseudomorphic Ni monolayers are obtained. The defect-free ordering of Ni atoms on Pt(111) in a single stacking domain indicates an O-surfactant induced growth mode. The structural properties of the O2 pressure-dependent Ni-O phases are discussed in a simple model assuming NiO(001)-like atomic arrangements in the adsorbate overlayer. At higher coverage stable (111)-oriented NiO islands grow in a three-dimensional mode.

Reactive growth of NiO ultrathin films on Pd(1 0 0): a multitechnique approach

Reactive growth of NiO ultrathin films on Pd(1 0 0) has been performed by evaporating metallic Ni in an oxygen atmosphere. The evolution of the ultrathin film is followed by means of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and scanning tunnelling microscopy (STM). The first monolayer (ML) of the deposited oxide develops as a completely wetting 2D overlayer with a c(4 × 2) periodicity, as shown by sharp LEED patterns and atomically resolved STM images. Further NiO deposition results in 3D NiO(1 0 0) islands formation on-top the c(4 × 2) superstructure, as shown by STM images and XPD data. Two layers thick islands are shown to be pseudomorphic to the substrate, i.e. characterised by in-plane compressive strain and interlayer expansion. LEED, XPD and STM give independent yet converging evidence that partial strain relaxation occurs within the third monolayer.

Adsorption of oxygen on a nickel coveredSrTiO3(100) surface studied by means of Auger electron spectroscopy and work function measurements

The interaction of oxygen with evaporated Ni films on an Fe-dopedSrTiO3(100) substrate was investigated by means of LEED, AES and work function measurements(WF) at room temperature. The adsorption of oxygen takes place on the nickel overlayerfirstly by chemisorption on nickel step sites, accompanied by a reduction of the WF, andsecondly on terrace sites, followed by a WF increase. After the chemisorption phase, theoxidation of the nickel overlayer starts with NiO island formation followed by bulk NiOdevelopment, which is marked by a second WF reduction. The adsorption phases of oxygencorrespond closely to those of oxygen on single crystals of nickel. This indicates that thecharacter of the Ni predeposited layers on strontium titanate seems to be metallic.

Early Stages of NiO Growth on Ag(001): A Study by LEIS, XPS, and LEED

Ultrathin nickel oxide films were grown on the Ag(001) surface by evaporation of Ni in an O 2 atmosphere. The growth mechanism, composition, and structure of the NiO layers formed in the early stages of deposition have been investigated by LEIS, XPS, XPD, and LEED. We found that at O 2 pressures in the 10 - 7 - 10 - 6 mbar range and sample temperatures between room temperature and 373 K, a nickel oxide layer forms showing a (2 × 1) LEED pattern. This is a phase of monatomic thickness which gradually covers the substrate surface upon increasing the surface coverage. LEIS and XPS data show that before the (2 × 1) layer is completed NiO(001) islands start to nucleate. The substrate surface is completely covered by NiO(001) for coverages higher than 2 ML. When NiO is deposited with the sample held at 453 K, we found that in the early stages of deposition metallic Ni is present in the subsurface region of the substrate. Upon further NiO deposition the oxide becomes the prevalent species on the surface. Under these conditions, the (2 × 1) phase is not observed. We found that the (2 x 1) phase is not stable upon annealing at 473-600 K and transforms into NiO(001). LEIS and XPD results indicate the formation of NiO islands 2 ML thick upon annealing. The structure of the (2 × 1) phase has been investigated by LEED intensity analysis (tensor LEED method). The best agreement with the experimental data is obtained for a structural model similar to the (111) surface of NiO. The effects of surface temperature and O 2 pressure on nickel oxide growth are discussed in terms of the low sticking probability of O 2 on Ag(001) and of the interaction between nickel and silver.

Local morphology and correlation lengths of reactive NiO/Cu(111) interfaces

A combined experimental near-field and reciprocal space analysis was used to quantitatively characterize the complex reactive interaction between a NiO vapor phase and the stable Cu(111) surface. The results are discussed within a pair potential energetics model. It is shown that the incorporation of Ni in the substrate in the early stages of growth leads to a well-defined almost monodispersed assembly of holes (48 \AA{} in diameter). The Ni-rich corrals along the hole edges act as a template for the subsequent $(&gt;4\mathrm{\AA{}}$ NiO) growth of NiO islands showing a well-defined correlation length of again 48 \AA{}.

Reduction of oxygen pre-treated Ni(1 1 1) by H 2 exposure: UPS and MIES studies compared with Monte Carlo simulations

The reduction of oxygen pre-treated Ni(1 1 1) under hydrogen has been followed by metastable induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) for a substrate temperature of 600 K. Both techniques have allowed us to report on the evolution of the electronic structure of the valence band over hydrogen exposure. These investigations showed the fast formation and desorption of water molecules, but no hydroxyl groups. The oxygen coverage of the surface depends on the Ni 3d band intensity during hydrogen exposure. By using the MIES technique, we observed that the reduction reaction is preceded with a induction period. These periods are not seen with UPS data. This difference can be attributed to the fact that MIES gives information about the outermost layer of the surface, whereas UPS probes the first innerlayers. So it indicates that oxygen atoms are desorbed from the surface, and then subsurface oxygen atoms diffuse to the oxygen-depleted surface. To interpret these experimental results, we carried out simple Monte Carlo simulations based on the fact that the reduction takes place on the surface along the NiO island perimeters. This simulation fits well the experimental data when a lateral mobility of subsurface oxygen atoms is taken into account, i.e. the reaction leads to the reduction of the three-dimension NiO before the outermost layer oxygen desorption.

Ni/NiO(0 0 1) interface studied by X-ray photoelectron spectroscopy and molecular dynamics simulations

X-ray photoelectron spectroscopy (XPS) results referring to the interface formed upon vapor deposition of Ni atoms on NiO(0 0 1) indicate, that upon annealing the deposited Ni is oxidized forming a NiO overlayer. The oxidation process is activated and becomes very fast above 900 K, resulting in a surface that exhibits identical photoelectron spectrum with the clean substrate. In addition, it was found that during the oxidation process two extra components in the O 1s XPS peak spectra appear, with binding energies above and below the main lattice oxygen peak, manifesting the existence of oxygen atoms carrying charges that deviate from their formal ionic value. Moreover, by molecular dynamics simulations it was found that upon deposition of Ni ions, neighboring O surface atoms become adatoms and combine with the Ni ions forming a series of Ni x O y oxides, most of which eventually coalesce into small NiO islands. Consequently, the lower binding energy component of the O 1s XPS peak may be attributed to intermediate oxides with x> y, while the higher binding energy component to intermediate oxides with x< y.

Growth of NiO on Ag(001): Atomic Environment, Strain, and Interface Relaxations Studied by Polarization Dependent Extended X-ray Absorption Fine Structure

This paper reports a structural study of 3 and 10 monolayer (ML) thick NiO films grown on Ag(001). Polarization-dependent X-ray absorption spectroscopy at the Ni K edge allowed us to obtain an accurate description of the local atomic environment of the Ni atoms up to the seventh coordination shell, including the determination of the in-plane and out-of-plane strains for the NiO films (determined with an accuracy better than 1%, resulting from the determination of Ni-O bond distances with a statistical error lower than 0.01 A). Multishell multiple scattering calculations have been used to fit the data. Within the experimental errors, the 3 ML film has both in and out-of-plane Ni-O distances compatible with those expected in the case of perfect pseudomorphism on Ag(001). A rough film morphology, possibly including also the presence of NiO islands, is also suggested by the optimized coordination numbers. Conversely the 10 ML film is partially relaxed without significant deviation from ideal bidimensional film. The present EXAFS study allows us to rule out any significant atomic interdiffusion process between the substrate and the NiO film. Comparison with previous literature data allows us to make the hypothesis of a two step growth mechanism for NiO films on Ag(001). The NiO-Ag(001) interface distance determined in this EXAFS study (d i n t e r f a c e = 2.36 ′ 0.05 A) is in remarkable agreement with theoretical calculations (d i n t e r f a c e = 2.38 A).

Adsorption of NH 3 on oxygen pre-treated Ni(1 1 1)

The adsorption of NH 3 on oxygen pre-treated Ni(1 1 1) surfaces has been studied at room temperature using X-ray photoelectron spectroscopy (XPS). Oxygen pre-treatments have been performed at 650 K. This protocol leads to a “two-phase domain” (O-adsorbed phase + NiO islands) over a large range of oxygen exposures. The investigation of the surface reactivity towards NH 3 shows that ammonia is adsorbed provided that the O-adsorbed phase is present; the surface reactivity increases with the O-adsorbed phase coverage. Two N-adspecies have been detected from the N 1s core level peaks at 399.8 ± 0.2 and 397.8 ± 0.2 eV and assigned to molecular NH 3 and dissociated NH 2 species, respectively. The molecular adsorption results from direct impingement of the NH 3 molecules, whereas the dissociated one results from the dissociation of a part of the preadsorbed molecular species. At saturation, the dissociated species is the more abundant one (about 4/5 of the total N 1s peak) whatever the initial coverage (>0) of the surface by the O-adsorbed phase. The XPS data indicate that this dissociation is correlated to the formation of OH from oxygen of the adsorbed phase and hydrogen abstraction from the molecular ammonia. When the O-adsorbed phase is absent on the surface, i.e. for clean Ni(1 1 1) or the complete NiO layer, none of these surface reactions with ammonia occurs, under the same adsorption conditions.

Chemical effects in the Auger spectrum of lead, deposited on nickel oxide––an indirect characterization of the NiO growth mode on Ni( [formula omitted

The interaction of submonolayer lead deposits with oxygen pre-exposed Ni(1 0 0) has been investigated by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). By comparison with the Auger line shapes of metallic lead and orthorhombic lead oxide, chemical effects have been evidenced in the Pb Auger spectrum, for submonolayer lead deposits performed on the NiO phase (hydroxylated or not). Inversely, it has been shown that no chemical effect occurred for lead deposits performed on an O-adsorbed phase. This difference of chemical state has been used to investigate the growth mode of NiO on Ni(1 0 0). The nucleation and growth of NiO islands among an O-adsorbed phase have been proved through this lead deposit approach, which appeared distinctly more sensitive than LEED to evidence the beginning and the end of the “biphased” domain. Under annealing to T⩾500 K, the decomposition of a full oxide thin film, into a mixture of oxide islands and O-adsorbed phase, has also been evidenced through this lead deposit method.

Real time scanning tunneling microscopy study of the initial stages of oxidation of Ni(111) between 400 and 470 K

A real-time in situ study of the initial stages of the interaction of oxygen with Ni(111) in a high temperature scanning tunneling microscope (STM) at temperatures between 400 and 470 K is presented. Oxygen adsorption features, in particular the Ni(111)-p(2×2)O reconstruction, were identified in room temperature STM images. The process of oxide formation is investigated in detail at 470 K. Starting from the initial nucleation at the step edges, a gradual transformation of the terrace structure results in the formation of a three-domain NiO(001) layer finally covering the entire terraces as well as forming bulges at the steps due to surface diffusion of excess Ni. Well-resolved local oxide surface structures are discussed on the basis of a thin (001)-oriented NiO layer model on the Ni(111) substrate. Formation of triangular bulk NiO(111) islands is observed for higher oxygen exposures above 25 L.

Structural characterization of NiO films on Al 2O 3(0 0 0 1)

NiO films on clean α-Al 2O 3(0 0 0 1) were prepared by molecular beam epitaxy. Two parameters were considered: the growth temperature (320–700°C) and the film thickness (29–200 nm). Low energy electron diffraction (LEED) patterns of the deposits show a six-fold symmetry. The samples were ex situ characterized by atomic force microscopy (AFM) and by grazing incidence X-ray diffraction. For all samples, NiO islands with different sizes and shapes were observed. A layer of pyramidal NiO islands, with triangular facets is present. Depending on the sample, also hexagonal or square islands were observed. The angles of the triangular facets correspond to (1 0 0) surfaces of the NiO(1 1 1). X-ray diffraction evidences that the NiO film grows with the (1 1 1) plane parallel to the (0 0 0 1)Al 2O 3 one. Twinned and not-twinned face centered cubic (FCC) NiO(1 1 1) stacking are present in approximately equal quantities. The crystallographic quality of the NiO layers was investigated.

Study of the growth of ultrathin films of NiO on Cu(111)

We have studied the early stages of growth of the layers formed by Ni evaporation in an oxygen atmosphere at room temperature on a Cu(111) single crystal. The layers have been analysed by XPS and x-ray absorption spectroscopy as a function of the deposition time. A background analysis of the XPS spectra has also been performed. We have found a Stranski-Krastanov way of growth, i.e. the formation of a metallic Ni monolayer followed by the formation of defective NiO islands. Further deposition leads to the formation of stoichiometric NiO.

Atomic force microscope study of the early stages of NiO deposition on graphite and mica

An atomic force microscope (AFM) operating in air has been used to study the growth of evaporated nickel oxide on highly oriented pyrolytic graphite (HOPG) and mica. These substrates show atomically flat surfaces which can be imaged by the AFM with atomic resolution enabling, therefore, a clear identification of the deposit. On HOPG, spherical NiO structures (diameter ∼100 nm, height ∼10 nm) are observed to coalesce along the steps. Increasing the evaporation time and once the steps are occupied, a `dendritic' growth on the terraces is observed. On mica, a more homogeneous distribution of smaller NiO islands is obtained over the whole surface. Nevertheless, a weak adhesion to the substrate is found. The friction AFM images have allowed to establish the relative friction of the materials, which follow the decreasing sequence: graphite, NiO and mica.

Interaction of water with clean and oxygen precovered nickel surfaces

The adsorption of water on a Ni(111) single crystal surface, clean as well as precovered with oxygen, has been investigated with thermal desorption spectroscopy (TDS) and measurements of the adsorption-desorption equilibrium combined with XPS (X-ray photoelectron spectroscopy). The measurements have been carried out with water pressures up to 10−5 mbar on surfaces, which have been either clean or precovered with oxygen. On the clean Ni(111) surface the first adsorbate layer with a maximum coverage of 0.42 ML (monolayers) has a desorption energy of 52 kJ/mol and a preexponential factor of desorption of 1016s−1. A second water layer adsorbs with the desorption energy of the ice multilayer but with first order kinetics. On Ni(111) precovered with chemisorbed oxygen an additional state of molecular, more strongly bound water is found, but no dissociation. For higher oxygen precoverages where NiO islands are formed on the surface, also the water dissociation product OH is found adsorbed. On a sample covered with a closed NiO layer, adsorbed OH and molecular water in an energetically not well-defined state are found. High doses of water on oxygen-precovered Ni(111) induce a slow surface modification leading to water dissociation.

CO 2 adsorption on oxygen-modified Ni(111)

Adsorption of CO 2 on Ni(111) oxidised at 300 K has been studied using TPD, XPS and UPS. CO 2 does not adsorb on either clean Ni(111) or on the Ni(111)-p(2 × 2)O overlayer at 300 K. However, it does interact with two distinct binding sites on Ni(111) NiO(111) . The O(1s) and C(1s) XP spectra and the He II UP spectra indicate the formation of CO 3 δ− on the surface of NiO and the CO 3 2− and the CO 3 − species are suggested to be associated with anion vacancies and O −, respectively. The CO 3 − species decomposes during decomposition of the oxide, evolving CO 2 and CO; the latter process may involve dissociation of a CO 2 − intermediate to CO (ads) + O (ads). This occurs on the NiO island sites which are stable up to 690 K. The saturation CO 2 coverage is 0.14 ML with respect to the atomic density of Ni(111).

A fluorescence-yield near-edge spectroscopy (FYNES) investigation of the reaction kinetics of NiO/Ni(100) with hydrogen

Abstract The method of fluorescence-yield near-edge X-ray absorption spectroscopy (FYNES) has been applied to investigate the thermal stability of NiO films on a Ni(100) substrate and the reaction kinetics of this surface with hydrogen gas. The FYNES results indicate that the total oxygen content near the surface region remains constant after heating a NiO/Ni(100) layer above a phase separation temperature of 525 K, indicating that there is no significant thermally induced diffusion of oxygen into the bulk Ni(100) lattice. These results support conclusions from previous EELS and XPS experiments that the NiO/Ni(100) layer prepared at 300 K undergoes a thermally induced phase separation to produce a surface with mixtures of roughly 50 A NiO(100) crystalline clusters and c(2 × 2)−O chemisorbed phases. The FYNES investigation of the reaction kinetics of NiO/Ni(100) with H2 in the temperature range of 600–800 K indicates that the oxygen removal rate from NiO clusters is roughly zero order with respect to the initial oxygen content, and that the subsurface oxygen is removed completely. In addition, the removal rate of oxygen is found to be approximately first order with respect to H2 pressure. The reaction of NiO/Ni(100) with H2 has a rather weak temperature dependence, as characterized by an activation energy of 1.8 kcal/mol with a pre-exponential factor of 9.4 × 1013 molecules cm−2 s−1. These observations indicate that the rate-determining step for the removal of oxygen is most likely the initial HH bond breaking, and that the removal of oxygen does not occur preferentially at the edges of NiO islands.