Nickel Overlayers Research Articles

In operando x-ray photoelectron spectroscopy studies of H2 oxidation and H2O electrolysis on gadolinia-doped ceria electrodes

In operando, ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has been used to evaluate surface states of gadolinia-doped ceria (GDC) thin-film electrodes during H2 oxidation and H2O electrolysis, on yttria-stabilized zirconia (YSZ)-supported solid oxide electrochemical cells (SOCs). Porous nickel (Ni) and gold (Au) overlayers deposited on separate GDC thin films served as current collectors and potential electrocatalysts to facilitate heterogeneous chemistry for H2 oxidation or H2O electrolysis. Electrochemical characterization of the GDC thin-film electrodes complemented in operando XPS measurements of the O 1s spectra to correlate electrochemical overpotentials with surface chemistry near the Ni/GDC and Au/GDC interfaces. Shifts in O 1s binding energies across the metal/GDC/YSZ interfaces signified changes of local surface potential and provided a means of estimating kinetic parameters associated with charge transfer reactions. Effective oxygen partial pressure and surface potential impacted oxide vacancy and ceria polaron concentrations in the GDC, resulting in different reactivities of the GDC under the tested conditions. Both the Ni/GDC and Au/GDC demonstrated much higher currents for H2O electrolysis vs. H2 oxidation for comparable metal/GDC overpotentials due to increased electronic conductivity of the GDC under positive potentials and associated spreading of the electrochemically active region away from the triple-phase boundary. Higher electrochemical activity of the Ni/GDC electrode is attributed to the increased H2 activation on Ni in promoting charge transfer reactions (particularly for H2 oxidation). These results provide a basis for developing more informed reaction mechanisms for both H2 oxidation and H2O electrolysis of GDC-based composite electrodes in SOCs.

Nickel Overlayers Modify Precursor Gases To Pattern Forests of Carbon Nanotubes

We analyzed the effect of nickel overlayers positioned in close proximity (bridges) or in contact (stencils) with the catalytic layer on the growth of vertically aligned carbon nanotubes (VACNTs) using thermal chemical vapor deposition (CVD). We studied the physical–chemical mechanisms, namely, the interaction of the overlayer with the gases and with the catalyst. We demonstrate that nickel inhibits CNT growth by adsorbing carbon to form graphene and by interacting with the gas precursors, leading to their modification into species that do not nucleate and grow CNTs. We demonstrate that the effect of the nickel bridge extends to the length of its boundary layer. We tested overlayer patterns and showed that the patterns were replicated during CNT growth. This facile method is a valid alternative to pattern CNT forests without the need for complex lithography and lift-off of the catalyst in applications where lithographic precision is not required.

Electrodeposition and characterization of nanostructured black nickel selective absorber coatings for solar–thermal energy conversion

Selective coatings consisting of a bright nickel interlayer and black nickel overlayer for solar-to-thermal energy conversion have been electrodeposited onto stainless steel substrates from a nickel(II) chloride bath. During electrodeposition of black nickel, multiple reduction processes combined with oxidative processes result in a complex film composition, containing α-Ni(OH)2, NiOOH, Ni2O3, NiO, water and metallic Ni, as indicated by a detailed X-ray photoelectron spectroscopy study. Reflectance measurements confirm that the selective coatings have a high absorbance in the wavelength range of the solar spectrum, and that the emissivity in the near to mid infrared is low. Electron microscopy showed that the films consist of nanostructured flakes, oriented perpendicular to the surface. The best coatings were obtained by using a two-pulse galvanostatic deposition method.

Ab initiostudy of Co and Ni under uniaxial and biaxial loading and in epitaxial overlayers

A detailed theoretical study of structural and magnetic behaviors of cubic cobalt and nickel along the bcc-fcc (Bain) transformation paths as well as of hcp cobalt and nickel loaded uniaxially along the [0001] direction at various atomic volumes is presented. The total energies are calculated by spin-polarized full-potential linearized augmented plane-wave method within the generalized gradient approximation and are displayed in contour plots as functions of tetragonal or hcp $c/a$ ratio and atomic volume; the borderlines between the ferromagnetic and nonmagnetic phases are shown. Stability of possible ferromagnetic phases of bcc nickel is analyzed. The calculated contour plots are used to explain and predict the lattice parameters and magnetic states of tetragonal and hcp cobalt and nickel overlayers on various (001) or (111) substrates, respectively. In case of tetragonally deformed structures, the stresses needed to keep the thin films coherent with the substrates are also determined and all Co and Ni overlayers on (001) cubic substrates are predicted to be ferromagnetic. The agreement of available experimental data for Co and Ni overlayers with the results of bulk calculations is remarkable and suggests that the geometrical effect of the substrate, i.e., imposing the lattice dimensions of the substrate in the plane of the film to the film material, is one of the most important factors determining the structure and properties of the film. In this way, the lattice parameters of Co and Ni overlayers may be very well understood in terms of properties of appropriately deformed bulk Co and Ni.

Growth of nickel nanoparticles on NiO/Ni(0 0 1): Evidence of adsorbed oxygen on metal particles by metastable induced electron spectroscopy (MIES)

We present results – obtained with X-ray as well as ultraviolet photoelectron spectroscopy (XPS and UPS) and metastable induced electron spectroscopy (MIES) – for the deposition of nickel on a NiO film of about 10 monolayers. It was found that up to a coverage of 0.5 monolayers single atoms or non-metallic clusters with up to 7 nickel atoms are formed. A MIES reference spectrum for this situation has been reconstructed from the experimental spectral series and the surface density of states has been obtained by deconvolution of this spectrum. Above 0.5 monolayer nominal nickel coverage a metallic nickel overlayer is formed as indicated by UPS. However, only MIES with its perfect surface sensitivity provides direct experimental evidence that on top of the metallic nickel a small amount of oxygen is adsorbed which originates from the underlying oxide.

Attenuation of photoelectrons and Auger electrons leaving nickel deposited on a gold surface

AbstractAttenuation lengths of signal electrons are very important in surface metrology. Contrary to inelastic mean free paths (IMFP), the attenuation length is determined by both inelastic and elastic scattering events of the signal electrons in the solid. Therefore, application of attenuation lengths leads to improved accuracy in measurements of length on a nanometer scale. In the present work, photoelectron and Auger‐electron spectra (AES) in regions of Au 4f, Ni 3p, Ni 2p, and Au MNN were recorded from nickel overlayers deposited in situ, step by step, on a gold surface. The experiment was coupled with advanced calculations of electron transport for the nickel/gold system. An overlayer growth mode has been elucidated by a line‐shape analysis of the Ni 2p spectra with the QUASES‐Tougaard software and from extensive Monte Carlo calculations of electron transport in the overlayer/substrate system with the SESSA software. The QUASES‐Tougaard analysis indicated three‐dimensional island growth followed by a continuous nickel overlayer for thicknesses exceeding 2 nm. The attenuation of electron intensities described by the effective attenuation length (EAL), calculated with the NIST SRD 100 database [Simulation of Electron Spectra for Surface Analysis (SESSA) software] and the NIST SRD 82 database, was found in close agreement for all electron transitions considered. The measured EAL values compared well with EALs calculated for Au 4f, Ni 3p, and Ni 2p photoelectrons, whereas the EAL derived from the measured Au MNN intensity–thickness plot was found to be about 20% less than the predicted values. Copyright © 2007 John Wiley & Sons, Ltd.

Substrate and Stabilization Effects on the Transport AC Losses in YBCO Coated Conductors

In support of second generation HTS conductor development for ac applications, transport ac loss measurements were conducted on a series of RABiTS-processed YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) coated conductors with different nickel alloy substrates and copper stabilization at 77 K. Each 1-cm wide sample had a critical current density between 1.0 and 2.0 MA/cm/sup 2/ and had either a 75 /spl mu/m Ni-5at%W substrate or a 75 /spl mu/m Ni-10%Cr-2%W substrate with 2-/spl mu/m nickel overlayer. Samples with copper stabilization had a 50-/spl mu/m strip of 1 cm wide copper laminated to a 3-/spl mu/m thick silver coated YBCO sample. Using thermal and electrical measurement techniques, the ac losses were measured as a function of the peak current ratio at 60 Hz. Experimental results were compared to the Norris thin strip and elliptical models to determine the influence of the ferromagnetic loss of the substrate and the copper lamination on the total ac loss.

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.

THE DEVELOPMENT OF NICKEL ULTRA-THIN FILMS AND THE INTERACTION WITH OXYGEN ON THE SrTiO3(100) SURFACE STUDIED BY SOFT X-RAYS PHOTOELECTRON SPECTROSCOPY

The electronic properties of very thin Ni films on the SrTiO 3(100)- Fe doped surfaces and their interaction with oxygen have been studied by soft X-rays photoelectron spectroscopy measurements. Nickel starts to become metallic on the surface in the very early adsorption stages. Oxygen adsorption on the nickel covered SrTiO 3(100) surface leads gradually to an almost complete oxidation of the nickel overlayer. The oxidation seems to take place through two different oxidation states, which according to the literature are due to the Ni 2+ and Ni 3+ species. The heating of the O / Ni / SrTiO 3 system at 850 K, causes a partial reduction of the nickel overlayer.

Transport ac loss studies of YBCO coated conductors with nickel alloy substrates

Transport alternating current (ac) loss measurements were performed on a series of rolling-assisted biaxially textured substrate (RABiTS) processed YBa2Cu3Ox (YBCO) coated conductors at 77 K. While each sample possessed a 1 µm layer of YBCO and a 3 µm silver cap layer, two different nickel alloy substrates were used and their impact on the ac loss was examined. Both substrates possessed a 75 µm Ni–5 at%W base, but one substrate also had a 2 µm nickel overlayer as part of the buffer layer architecture. The ac losses, which were determined by thermal and electrical measurements, contained two dominant contributions: superconductive hysteresis in the YBCO and ferromagnetic hysteresis in the substrates. The superconductive component followed the Norris elliptic model for the substrate with the nickel overlayer and the Norris thin strip model for the substrate without the nickel overlayer. The substrates' ferromagnetic loss was determined separately through magnetization measurements, which showed that this loss contribution was independent of the presence of the nickel overlayer for effective ac currents less than 50 A. While the overall loss was lower for the thin-strip-like conductor with no nickel overlayer, further research is necessary to strengthen this connection.

First-principles studies on bonding mechanism at Ni/X bimetallic interfaces (X=Ta, W, Re and Ru)

The atomic structures and bonding mechanism of Ni/X [X=Ta(110), W(110), Re(0001) and Ru(0001)] bimetallic surfaces have been studied using the local-density full-potential linearized augmented plane-wave (FLAPW) method with the atomic force approach. The strength of the overlayer–substrate bonding decreases in the sequence Ta>W>Re>Ru, which agrees with the thermal desorption mass spectroscopy results of Rodriguez et al. Charge polarization induced in the nickel layer by the substrate plays the key role in determining or altering the electronic and chemical properties of the nickel overlayer and the bonding of the bimetallic systems.

Field emission from a nickel adsorbed tungsten tip: Temperature dependent growth and its effects on current stability

Field electron emission from nickel adsorbed tungsten tip has been studied. The effect of equilibration temperature on the growth pattern of nickel overlayers has been investigated. Nickel doses have been equilibrated, in two separate sets of experiments at 760 and 960 K. Corresponding to these two cases the variations in the work function and the pre-exponential term with nickel coverage have been obtained using ten-point Fowler–Nordheim (F–N) plots and distinguishable growth features have also been observed in their respective field emission patterns. The field emission current-time characteristics have been recorded in the two cases. The emission current has been found to be remarkably stable for the case when nickel doses on tungsten tip have been equilibrated at higher temperature of 960 K. This stability in the field emission current has been attributed to almost negligible damage by ion bombardment on the hard and chemically stable nickel-adsorbed tungsten tip surface. The results have been discussed in the light of the existing literature.

Adsorption of nickel on tungsten: a probe-hole field emission microscopy study

In a probe-hole field emission microscopy experiment, submonolayer amounts of high-purity nickel were deposited on to a tungsten tip in clean, ultra high vacuum conditions and observations were made on W(1 1 0), (1 2 1) and (1 1 1) planes. The changes in the work function, Δ φ, and corresponding pre-exponential factors, ln B, have been obtained by ten point Fowler–Nordheim plots. The results show that the change in average work function of nickel covered tungsten surface increases and attains a saturation value of +0.52 eV. This is accompanied by a decrease in the pre-exponential factor. Adsorption of nickel on the W(1 1 0) plane causes a decrease in Δ φ in the lower coverage region and then exhibits an abrupt change to reach a saturation value of +0.5 eV as the coverage increases. This has been attributed to surface reconstruction. In the case of the W(1 2 1) plane Δ φ decreases with the number of doses to a value of −0.45 eV. On the W(1 1 1) plane, the Δ φ increases and saturates to a value of +0.50 eV after a brief initial decrease in the low-coverage region. The changes in the work function are explained in terms of atomic and electronic structure of nickel overlayers on various single-crystal planes of tungsten. A comparison of nickel with iron and cobalt adsorption on tungsten shows that the behaviour of nickel is similar to iron.

Diverse reactivity patterns observed in the interaction of oxygen with nickel deposited on gold and tungsten surfaces

Interaction of oxygen with nickel overlayers on gold and tungsten surfaces has been investigated by employing core-level spectroscopy. The Ni2p 3 2 binding energy at a monolayer coverage of nickel ( θ Ni ∼ 1 ML) is significantly differ compared to that at high coverages or of the bulk nickel. The Ni/Au surface showing a large negative shift in the binding energy ( ΔE c ∼ −0.5 eV) adsorbs oxygen on a prolonged exposure and gives rise to a high proportion of the O 1− species. In contrast, the Ni/W surface with ΔE c ∼ +0.3 eV reacts vigorously on exposure to oxygen, the O 2− species being predominant on the surface. Charge transfer across the NiAu(W) interface is suggested to be responsible for the observed diverse reactivity patterns with oxygen. Bimetallic surfaces formed by depositing palladium on silver and tungsten surfaces also exhibit a similar behaviour.

Core-level photoemission and alkali ion scattering structure analysis of Ni adsorption and surface alloying on W(001)

The growth and structural evolution of Ni overlayers deposited on W(001) are studied by low energy alkali ion scattering and W4 f 7 2 core-level photoemission. The composition and thermal stability of the interfacial layers are found to vary with nickel coverage. For sub-monolayer Ni coverages deposited near 300 K, annealing to 1000 K leads to a random alloy in the surface layer with W atoms protruding above the Ni atoms by approximately 0.022 nm. At Ni coverages near one monolayer, the surface layer is a nearly pure, pseudomorphic nickel overlayer 0.125 nm above the underlying W substrate. A second pseudomorphic nickel layer can be grown at 300 K but annealing results in intermixing at the buried interface. At 2 monolayers (ML) coverage, the uppermost two atomic layers are mostly nickel. Fitting of the W4 f 7 2 photoemission spectra requires three components: an invariant bulk component, a surface component at lower binding energy, and a third component at intermediate binding energy. The surface component intensity decreases linearly to zero as the nickel coverage approaches 1 ML, indicating that this component originates from coordinatively unsaturated W atoms in the alloyed top layer. The intermediate component, present at all coverages, is assigned to coordinatively saturated atoms in the layer below the alloyed top layer for coverages between 0–1 ML, and below the Ni layer at 1 ML. Variation in the binding energy of the intermediate component is observed during growth of the second pseudomorphic Ni overlayer. The correlation of structural and electronic information shows that coordination and interfacial strain dominate charge transfer in determining core-level shifts.

Observations of surface and projected bulk phonons in epitaxial overlayers: the Cu(100) and Cu(100)/Co system

The phonon spectra of both clean and cobalt covered Cu(100) have been studied with helium atom scattering, and the results compared with those previously obtained with EELS for nickel and iron overlayers on Cu(100) [1,2]. In the Γ-M direction on clean Cu(100), both Rayleigh and bulk band edge phonons were observed. In the Γ-X direction, surface Rayleigh modes with dispersion similar to that obtained in EELS [3] occurred together with a previously unobserved, higher energy mode. No change in phonon dispersion was found for cobalt covered Cu(100).

Final-state symmetry effects in photoemission of thin Gd overlayers

The 4f valence band and shallow core levels of ultrathin Gd overlayers on Cu(100) and on crystalline nickel overlayers on Cu(100) have been investigated using angle-resolved photoemission. The authors have observed a definite selection of final-state symmetries which, for one-half to four-monolayer-thick Gd films, indicates that the occupied Gd 5d band is predominantly 3z2-r2 and j=3/2 in character. The 5p3/2 appears to be predominantly z in character for thin Gd overlayers on Cu(100). This occurs as a consequence of final-state interactions between the Gd5d and 5p eigenstates. These results, together with the observed screening processes of the photoemission hole state, are discussed and applied to the understanding of photoemission resonance from the valance bands.

Fracture behavior of laser-modified ceramic materials

The healing of surface cracks in laser surface modified ceramic materials is analyzed. A partial or complete welding of the crack surfaces as a result of laser surface modification of silicon carbide with nickel overlayer is shown to be effective in improving the fracture toughness. The presence of the coherent second phase of nickel changes the nature of applied stress on the crack surfaces and welds the crack tip. Dislocation modelling of the partially closed crack surfaces is presented along with the development of constitutive energy equations associated with the crack in the two-phase medium. Crack configurations under either an applied tensile stress or an antiplane strain condition are analyzed and the results compared. These results show that the critical size of the crack at the interface in the laser surface modified materials is always larger than that of the surface crack. This leads to an improvement of fracture toughness, consistent with experimental observations.

Nickel monolayers on copper surfaces: CO adsorption and ni diffusion

Nickel monolayers on the Cu(111) surface have been studied in an attempt to understand metal-metal interface mixing. High resolution energy loss (HREEL) spectra of carbon monoxide is used to monitor the surface by distinguishing between CO adsorption on top and bridge sites of the nickel overlayer. Results are discussed for nickel deposition at temperatures in the range of 300–475 K. Room temperature nickel deposition leads to nickel island formation; at slightly elevated temperatures nickel and copper mix in the first layer ; and by 475 K, nickel becomes significantly covered by copper, the thermodynamically favored configuration.

Summary Abstract: A phase diagram for ultrathin nickel overlayers on Si(111)

Summary Abstract: A phase diagram for ultrathin nickel overlayers on Si(111)