NiAl Surface Research Articles

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni 3Al, NiAl, and NiAl 3 alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p 3/2 peak for NiAl and NiAl 3 from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel–aluminum alloys.

Ferromagnetic one dimensional Ti atomic chain

Using the full potential linearized augmented plane wave method, we have explored the magnetic properties of one dimensional (1D) Ti atomic chain. Interestingly, we have observed that the 1D Ti atomic chain has ferromagnetic ground state even on NiAl(110) surface. In addition, it has been found that the magnetic dipole interaction is a key factor in the study of magnetic anisotropy, not the magnetocrystalline anisotropy arising from spin-orbit interaction.

Oxidation behavior of NiAl alloy at low temperatures

AbstractOxidation behavior of NiAl alloy at low temperatures was studied. A NiAl plate was oxidized by exposure to ambient atmosphere at room temperature, heated at 473 K in air, and heated at 773 K in air. The oxide formed on the NiAl surface was investigated by angle‐resolved X‐ray photoelectron spectroscopy (AR‐XPS). Chemical composition and atomic concentration in the oxide layer were analyzed with factor analysis of XPS spectra. Exposure of the NiAl plate to the ambient atmosphere resulted in the formation of an Al2O3 layer along with a small amount of NiO. Oxidation of the NiAl plate at 473 K in air formed a film of double‐layered oxide; the top layer consisted of NiAl2O4 and a small amount of NiO, and the second layer was Al2O3. Successive oxidation at 773 K only changed the oxide‐layer thickness without changing the structure. Formation of oxide observed in the present study corresponds to the thermodynamic prediction for the oxidation behavior of NiAl at 1373 K. Copyright © 2007 John Wiley & Sons, Ltd.

Nanoscopic nickel aluminate spinel (NiAl 2O 4) formation during NiAl(1 1 1) oxidation

Oxidation of a NiAl(1 1 1) single crystal surface was investigated using high resolution soft X-ray photoelectron spectroscopy (HRSXPS), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray mapping, and atomic force microscopy (AFM). After repeated oxygen exposure, annealing, and cleaning cycles under ultrahigh vacuum conditions, a new oxide phase in the form of tiny 3-dimensional surface structures was detected. These features are several micrometers long and ∼300 nm high and oriented along low index 〈 1 ¯ 0 1 〉 directions in the plane of the substrate; they have nickel aluminate spinel (NiAl 2O 4) stoichiometry. We propose that repeated cycles of oxygen dosing and annealing of the NiAl(1 1 1) surface leads to oxygen diffusion into the bulk and nucleation of spinel below the surface.

Epitaxial growth of cobalt clusters on the bare and the oxide film grown on NiAl(1 0 0) surfaces

The structure of the nano-sized cobalt clusters on bare NiAl(1 0 0) and an oxidized NiAl(1 0 0) surfaces have been investigated by AES, LEED and RHEED. The deposition of Co onto bare NiAl(1 0 0) at room temperature led to small crystalline Co grains and surface asperities of substrate. The latter is likely induced by replacement of surface Al, Ni atoms by Co deposit. At 800 K Co particles aggregate to form clusters, but incorporation of Co into bulk NiAl(1 0 0) could occur upon annealing at 900 K. On the other hand, pure face-centered cubic (fcc) phase of Co crystallites of ≈1 nm in diameter with inclusion of smaller-sized particles ( D < 1 nm) are observed on Θ-Al 2O 3 after Co deposition at room temperature. After annealing the Co nano-clusters grow larger at expense of small particles ( D ≈ 3 nm), where the [1 1 0] and [−1 1 0] axis of the Co(0 0 1) facets are parallel to the [1 0 0] and [0 1 0] directions of (0 0 1)oxide, respectively. The in-plane lattice constant of Co clusters is ca. 4% larger than that of bulk Co, yielding less strain at the Co/oxide interface. A 15° ± 10% random orientation of the normal to (0 0 1) facet of Co clusters with respect to (0 0 1)oxide surface was deduced from the “arc”-shape reflection spots in RHEED. These results suggest that both orientation and phase of Co clusters are strongly affected by the nature and structure of oxide surface.

CO methanation on Ni(1 1 1) and modified Ni 3Al(1 1 1) surfaces: A first-principle study

The adsorption energies of intermediates in CO methanation on the modified Ni 3Al(1 1 1) surface and the Ni(1 1 1) surface are calculated using density functional theory. A microkinetic analysis based on the calculated adsorption energies is performed to explain the different kinetics of CO methanation catalyzed by Ni 3Al and Ni powders. The electronic structures of different atoms on the modified Ni 3Al surface are also presented, and correlate well with the adsorption energies and geometries.

The effect of Pt on Ni 3Al surface oxidation at low-pressures

The fully-oxidized surface that forms on (1 1 1) oriented Ni 3Al single crystals, with and without Pt addition, at 300–900 K under oxygen pressures of ca. 10 −7 Torr was studied using XPS, AES, and LEIS. Two main types of surfaces form, depending upon oxidation temperature. At low-temperature, the predominant oxide is NiO, capped by a thin layer of aluminum oxide, which we refer to generically as Al x O y . At high-temperature (i.e., 700–800 K), NiO is replaced by a thick layer of Al x O y . By comparing samples that contain 0, 10 and 20 at.% Pt in the bulk, we find that the effect of Pt is to: (1) reduce the maximum amount of both NiO and Al x O y ; and (2) shift the establishment of the thick Al x O y layer to lower temperatures. Platinum also decreases the adsorption probability of oxygen on the clean surface.

Improvement of cyclic oxidation resistance of a NiAl-based alloy modified by Dy

The effect of the rare-earth-element dysprosium on the cyclic oxidation behavior of NiAl–31Cr–3Mo alloy at 1300 and 1400 K in static air atmosphere was investigated in this paper. The results revealed that doping 0.1.% atom fraction of Dy greatly improved the cyclic oxidation resistance at 1300 and 1400 K. The microstructure and composition of the oxidation layer were studied by means of X-ray diffractometer (XRD), scanning electron microscopy (SEM) with EDAX unit. The oxidation scale, which is located on the surface of NiAl–31Cr–3Mo alloy, mainly consisted of α-Al2O3 and a little Cr2O3. While the continuous and sole α-Al2O3 scale formed on the surface of the alloys doped with the dysprosium. The adherency of scale improved due to a Cr-rich phase decreased the internal stress by thermal cycling and transformation of Al2O3 or transverse growing induced for the testing alloy modified by Dy. The oxidation mechanism was discussed from phase constitution.

Engineering patterns of Co nanoclusters on thin film Al2O3∕NiAl(100) using scanning tunneling microscopy manipulation techniques

We present precise engineering of patterns of Co nanoclusters grown on ordered Al2O3∕NiAl(100) surface using the scanning tunneling microscopy (STM) manipulation technique. The clusters are attracted to the STM tip by lowering the bias below a threshold value and translated and relocated to another position by reversing the polarity. This facile manipulation technique in combination with the self-organized patterning on this system reported earlier might play a decisive role in nanotechnology for various applications where patterned nanoclusters are desired.

NiAl(110)∕Cr(110)interface: A density functional theory study

The optimal geometries, thermodynamic properties, and electronic structures of $\mathrm{Ni}\mathrm{Al}(110)∕\mathrm{Cr}(110)$ interface are studied using a first-principle density functional plane-wave ultrasoft pseudopotential method. Surface energies of different NiAl surfaces are compared with those obtained based on the classical broken-bond rule. Simulation results indicate that the structure of Ni and Al placed in the hollow sites of Cr atoms at the interface is more thermodynamically stable, and the NiCr bonding is dominated by $3d$ electrons of Ni and Cr. It is found that $\mathrm{Ni}\mathrm{Al}(110)∕\mathrm{Cr}(110)$ alloying could lower brittleness of NiAl compounds. With simulated values of adhesion work and interface energy for $\mathrm{Ni}\mathrm{Al}(110)∕\mathrm{Cr}(110)$ system, its mechanical and thermodynamic properties are also discussed.

Local tunneling barrier height observations of NiAl(1 1 0)

Local work functions at individual atoms on a compound metal surface have been experimentally examined. The simultaneously obtained STM and local tunneling barrier height (LBH) images of NiAl(1 1 0) show that LBH at geometrically lower Ni sites is much higher than that at geometrically higher Al sites, indicating that each individual atom on the NiAl(1 1 0) surface has a specific potential barrier for relevant electrons.

Atomic scale control of single molecule charging

A scanning tunneling microscope was used to study charging of single copper phthalocyanine molecules adsorbed on an ultrathin Al(2)O(3) film grown on a NiAl(110) surface. A double-barrier tunnel junction is formed by a vacuum barrier between the tip and the molecule and an oxide barrier between the molecule and the NiAl. In this geometry the molecule can be charged by the tunneling electrons. This charging was found to be strongly dependent on the position of the tip above the molecule and the applied bias voltage.

Translation-related domain boundaries form to relieve strain in a thin alumina film on NiAl (110)

We examine how translation-related domains form in thin alumina films synthesized by oxidizing a NiAl (110) surface. Low-energy electron microscopy observations reveal that translation-related domains (sometimes called antiphase domains in the literature) are created within isolated alumina islands as they grow or are annealed. Thus, the domains do not originate when islands with displaced lattices impinge, as frequently assumed in models of film growth. Even though the planar defects that bound the translation-related domains cost energy, the misfit dislocations that terminate the domain boundaries lower the film’s strain energy.

Low Temperature H2O and NO2 Coadsorption on θ-Al2O3/NiAl(100) Ultrathin Films

The coadsorption of H(2)O and NO(2) molecules on a well-ordered, ultrathin theta-Al(2)O(3)/NiAl(100) film surface was studied using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS). For H(2)O and NO(2) monolayers adsorbed separately on the theta-Al(2)O(3)/NiAl(100) surface, adsorption energies were estimated to be 44.8 and 36.6 kJ/mol, respectively. Coadsorption systems prepared by sequential deposition of NO(2) and H(2)O revealed the existence of coverage and temperature-dependent adsorption regimes where H(2)O molecules and the surface NO(x) species (NO(2)/N(2)O(4)/NO(2)(-),NO(3)(-)) form segregated and/or mixed domains. Influence of the changes in the crystallinity of solid water (amorphous vs crystalline) on the coadsorption properties of the NO(2)/H(2)O/theta-Al(2)O(3)/NiAl(100) system is also discussed.

Growth of Al 2O 3 thin films on NiAl(1 0 0) by gas-phase oxidation and electro-oxidation

The growth and structures of aluminum oxides on NiAl(1 0 0) have been investigated by RHEED (reflection high energy electron diffraction), complemented by LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and STM (scanning tunneling microscopy). Crystalline θ-Al 2O 3 phase grows through gas-phase oxidation on the NiAl(1 0 0) substrate with its a and b-axes parallel to [0 −1 0] and [0 0 1] direction of the substrate, respectively, forming a (2 × 1) unit cell. Whilst, three-dimensional nano-sized NiAl(1 0 0) protrusions and Al 2O 3, NiAl(0 1 1) clusters were found to co-exit at the surface, evidenced by extraordinary transmission spots superposed to the substrate reflection rods in the RHEED patterns. Particularly, the NiAl(0 1 1) clusters develop with their (0 1 1) plane parallel to the NiAl(1 0 0) surface, and [1 0 0] axis parallel to the [0 −1 1] direction of the substrate surface. STM observation combined with information from AES and TPD (temperature programmed desorption) suggest the formation of these 3D structures is closely associated with partial decomposition of the crystalline oxides during annealing. On the other hand, smoother (2 × 1) oxide islands with thickness close to a complete monolayer of θ-Al 2O 3 can be formed on NiAl(1 0 0) by electro-oxidation, in contrast with the large crystalline films formed by gas-oxidation.

Comparison of reducibility and stability of alumina-supported Ni catalysts prepared by impregnation and co-precipitation

Three Ni–Al 2O 3 catalysts, with nickel loadings of 10–13 wt.%, were prepared by co-precipitation (Ni–Al co-precip), impregnation on an in-house sol–gel derived alumina (Ni/sol–gel Al 2O 3), and impregnation on a commercial γ-Al 2O 3 (Ni/γ-Al 2O 3). The catalysts were characterized by N 2 physisorption, H 2 chemisorption, TPR, XRD, SEM and TEM. The Ni species, Ni particle size, and Ni reducibility depended on the preparation method. The Ni–Al co-precip and Ni/sol–gel Al 2O 3 catalysts contained NiAl 2O 4 species after calcination, while the Ni/γ-Al 2O 3 catalyst contained NiO and NiAl 2O 4 species after calcination. Although the Ni/γ-Al 2O 3 catalyst was the easiest to reduce, according to TPR, this catalyst had the lowest hydrogen uptake over a 100 h temperature-staged reduction experiment. The Ni–Al co-precip and Ni/sol–gel Al 2O 3 catalysts had Ni dispersions of over 7% with reduction at 550 °C for 31 h, and maximum dispersions of ∼10%, after reduction at 650 °C for 7 h. After reduction at 550 °C, the Ni particles were not evident by TEM examination. The results suggest that the formation of a surface NiAl 2O 4 spinel phase during preparation is beneficial for a high Ni dispersion in the reduced catalyst.

Hydrogen production by oxidative reforming of hexadecane over Ni and Pt catalysts supported on Ce/La-doped Al 2O 3

The activity and selectivity to hydrogen of alumina-supported nickel or platinum catalysts doped with Ce and La for the oxidative reforming of hexadecane were studied. The influence of both thermal stabilizer and activity promoter, such as lanthanum and cerium oxide, respectively, over hydrogen yield and catalyst durability was investigated. Catalytic activity was found to depend strongly on the type of metal, platinum showing lower specific activities per atom of metal exposed and a higher selectivity to combustion products than the Ni counterparts. The characterization results achieved with H 2 chemisorption, XRD, TPR and XPS showed differences in surface metal concentrations and metal–support interactions which depend on the presence of cerium and/or lanthanum in the support composition. For both based metal catalysts higher reforming activities were found when active metals were deposited on Ce–La–Al 2O 3 substrate. For Pt-based catalysts, the increase in activity observed for the sample using Ce–La-modified alumina as support is suggested to be related with a participation of lanthanum in the reaction more than modifications on coke resistance or dispersion and state of platinum-induced by lanthanum. In the case of Ni-based catalysts, a lower carbon deposition and a higher thermal stability of metallic Ni particles under reaction conditions were observed for catalyst using Ce–La-modified alumina as support. It is suggested that the higher number of Ni–Ce and Ni–Al surface interactions developed Ce–La–Al 2O 3-supported Ni catalysts are responsible of the better catalytic behaviour of this sample in the oxidative reforming of hexadecane.

Charging and Interaction of Individual Impurities in a Monolayer Organic Crystal

We use a scanning tunneling microscope to study the charging of Ag-doping centers in a monolayer of C(60) molecules supported on a thin Al(2)O(3) film grown on the NiAl(110) surface. Differential conductance spectroscopy shows that charging affects the conduction through C(60) molecules located around the doping centers. This effect is used to observe the electrostatic interaction of a pair of centers. Charging of one doping center affects the energy levels of the other, an analogue of the field-effect action.

Realization of a Particle-in-a-Box: Electron in an Atomic Pd Chain

Well-defined Pd chains were assembled from single atoms on a NiAl(110) surface with the tip of a scanning tunneling microscope. The electronic properties of the chains were determined by spatially resolved conductance measurements, revealing a series of quantum well states with parabolic dispersion. The particle-in-a-box states in Pd chains show higher onset energy and larger effective mass than those in Au chains investigated before, reflecting the influence of elemental composition on one-dimensional electronic systems. The intrinsic widths and spectral intensities of Pd induced states provide information on lifetime and spatial localization of states in the atomic chain.

Adsorption and scattering of H2 and D2 by NiAl(110)

We present quasiclassical dynamics calculations of H2 and D2 scattering by the NiAl(110) surface using a recently proposed six-dimensional potential-energy surface (PES) obtained from density-functional theory calculations. The results for dissociative adsorption confirm several experimental predictions using (rotationally hot) D2 beams, namely, the existence of a dissociation barrier, the small isotopic effect, the importance of vibrational enhancement, and the existence of normal energy scaling. The latter conclusion shows that normal energy scaling is not necessarily associated with weak corrugated surfaces. The results for rotationally elastic and inelastic diffractions are also in reasonable agreement with experiment, but they show that many more diffractive transitions are responsible for the observed structures than previously assumed. This points to the validity of the PES recently proposed [P. Riviere, H. F. Busnengo, and F. Martin, J. Chem. Phys. 121, 751 (2004)] to describe dissociative adsorption as well as rotationally elastic and inelastic diffractions in the H2NiAl(110) system.