NiAl Surface Research Articles

Adsorption of S, O, and H on the NiAl(1 1 0)-(2×2) surface

We present results of ab initio calculations, based on pseudopotentials and density functional theory, for adsorption of sulfur (S), oxygen (O), and hydrogen (H) on the NiAl(1 1 0)-(2×2) surface. For the S, O, and H atoms, various adsorption sites are considered: (i) Ni–Ni short bridge, (ii) on top of the Ni atom, (iii) Al–Al short bridge, (iv) on top of the Al, and (v) between the Ni atom and Al atom regions. We find that adsorption site for S and H are found to be at the 2Ni–Al threefold minimum and O is found to be at the 2Al–Ni threefold minimum. The calculated key parameters are in good agreement with other calculations. We also present the reaction paths and adsorption energies for S, O, and H atoms on the NiAl(1 1 0) surface.

First‐principle studies of dissociative adsorption of H2O on NiAl(110)‐(2 × 2) surface

AbstractA theoretical study of the interaction of H2O with NiAl(110) surface has been carried out with density functional theory. We have studied that H2O molecule dissociates into OH and H on NiAl(110). We have assumed four possible locations for OH and H on the NiAl(110) surface. The minimum energy for OH molecule and H atom is at the 2AlNi site, but H atom towards a near by surface Ni atom, with the adsorption energy −0.79 eV. Bond length and bond angle of H2O molecule were found to be 0.99 Å and 103.9°, respectively, which are close to gas phase theoretical (exp.) values 0.97 Å (0.96 Å) and 104.8° (104.5° ). The calculated bond length between O and Al (H) is 1.97 Å (0.98 Å). Bond length between H and Ni is found to be 1.55 Å. We have also presented the reaction path for OHH on NiAl(110) surface. Copyright © 2010 John Wiley & Sons, Ltd.

X‐ray photoelectron spectroscopy study of the interaction of ultra‐thin alumina films on NiAl alloys with NaCl solutions

AbstractWe report a XPS study of the reactivity in 0.5 M NaCl aqueous solution of NiAl(100) surfaces covered by ultra‐thin aluminium oxide films grown by thermal oxidation of the alloy at 900 °C. Detailed analysis of the film thickness, the oxide stoichiometry and the alloy composition below the oxide was performed to characterize the drastic modifications induced by chloride exposure. Chloride entry in the oxide was observed by AR‐XPS that revealed the existence of two chemical states of chlorides, one associated to aluminium oxide, the other one to aluminium hydroxide. The film modifications were initiated in the inner part and characterized by the formation of chloride‐containing hydroxide species. The growth of these species spread later to the entire film, accompanied by a marked thickening of the modified alumina layer by selective oxidation of the alloy. Longer immersion times (>20 h) led to the formation of mixed nickel–aluminium chloride‐containing hydroxide species, indicating the loss of corrosion protection. Copyright © 2010 John Wiley & Sons, Ltd.

Catalytic Activity of Oxidation-Reduction Pre-Treated Ni<sub>3</sub>Al for Methane Steam Reforming

The catalytic activity of oxidation-reduction pre-treated Ni3Al powder for methane steam reforming was examined. The oxidation-reduction pre-treatment consisted of two steps: oxidation in air at various temperatures from 973 to 1373 K, and then followed by reduction in H2 at 873 K. It was found that the oxidation-reduction treatments significantly reduced the onset temperature of activity, i.e., improved the activity of Ni3Al powder at low temperatures. The characterization of Ni3Al surface showed that an outer surface layer of fine NiO particles were formed on the surface of Ni3Al after oxidation. These NiO particles were reduced to metallic Ni by the subsequent reduction treatment, resulting in the high activity for methane steam reforming. These results indicate that the Ni3Al can form highly active surface structure with oxidation-reduction treatment, having excellent heat resistance.

In situ x-ray study of the oxidation of a vicinal NiAl(6,7,1) surface

We present an in situ surface x-ray diffraction study of the clean, oxidized and subsequently annealed surfaces of regularly stepped NiAl(6,7,1). Our results show that the UHV stable, clean surface is not faceted and consists of a regular array of (1,1,0) terraces and (0,1,1) steps. The topmost Al and Ni atoms on the terraces exhibit a rippled relaxation while the step atoms are relaxed towards the bulk. Preferential Al oxidation at 540 K and 6×10−6 mbar O2 leads to the formation of a 5 Å thin, disordered alumina layer and induces Al vacancies and Ni anti-sites in the Al-depleted interfacial region. The terrace-step structure of the clean surface is maintained, but strong inward relaxations of the interfacial atoms change the strain field around the steps. Massive (1,1,0) faceting with facets up to 50 times larger than the original terraces occurs after high-temperature annealing, during which the surface oxide develops a complex long-range ordering. These results can be understood by the change of interfacial strain, which removes the energy barrier for mass transport. In addition, unlike in the case of low-index (1,1,0) surfaces, we find the step-induced suppression of twin domain formation in the alumina film grown on NiAl(6,7,1). Our results show that the interplay between oxidation and strain can have dramatic effects on the morphology of vicinal surfaces.

Giant perpendicular magnetic anisotropy of an Ir monolayer on a NiAl(001) surface

Using the state-of-the-art full potential linearized augmented plane-wave method, we have investigated the magnetic properties of Os and Ir monolayer (ML) film on NiAl(001) surface. It has been found that the one ML of Os and Ir film can have ferromagnetic ground state with magnetic moment of 0.35 and $0.64{\ensuremath{\mu}}_{B}$ on Ni terminated surface, whereas both films display no sign of magnetic state on Al terminated surface. In addition, the surface Ni atom has an induced magnetic moment of $0.26{\ensuremath{\mu}}_{B}$ in Ir/NiAl(001), while only $0.09{\ensuremath{\mu}}_{B}$ is observed in Os/NiAl(001). We attribute the existence of magnetism to the interaction between $5d$ of adlayer and $3d$ of surface Ni. Moreover, we have obtained that the Os/NiAl(001) and Ir/NiAl(001) films show a perpendicular magnetic anisotropy (PMA). Surprisingly, it appears that the Ir/NiAl(001) has a giant PMA energy of 7.18 meV.

Interaction of adsorbates with electric field fluctuations near surfaces: Influence of the STM tip and plasmonic effects

We discuss the interaction of adsorbates with fluctuating electric fields produced by zero-point charge fluctuations of electrons in the substrate. Zero-point motions associated with surface or interface plasmons are a source of such electric field fluctuations. When an adsorbate is on an oxidized surface, direct hybridization of the adsorbate electrons with those in the substrate can be ignored, and the long-ranged Coulomb field provides coupling of the adsorbate's electrons to those in the substrate through the electric field fluctuations. Earlier we developed a formalism that allows one to calculate adsorbate energy-level shifts along with the nonradiative lifetime of excited states with origin in these interactions. The present paper explores the influence of a nearby STM tip on the spectrum of electric field fluctuations. We also describe studies of the energy-level shifts of a Mg porphine molecule on the oxidized NiAl (110) surface, along with the nonradiative lifetime of its $\text{LUMO}+1$ state. The dependence of these effects on the distance between the tip and the oxide layer is studied. We find good agreement with data on the nonradiative lifetime of the $\text{LUMO}+1$ state of this molecule, along with its tip-induced shift in energy.

Point defects in the NiAl(100) surface

The stability of various point defects in NiAl(100) has been investigated by first-principlescalculations. For Al-rich surfaces, Ni vacancies within the first Al layer are energeticallymost favourable. For Ni-rich surfaces, so-called double defects, consisting of both Ni-antisiteatom in the first Al layer and a Ni vacancy within the second Ni layer, form theconfiguration of lowest energy, superior to singular Ni antisites. An additional andsignificant energy gain is found in both cases by mutual lateral interaction of the defects,when they are arranged in the diagonal direction. Respective ordered configurations were found as the most stable structures. A 50:50 mixture of bothdefect types turns out to be even lower in energy than the ideal Al-terminated NiAl(100)surface, proving the latter to be metastable only. This is in line with the often reportedinability in experiments to prepare ideal NiAl(100) surfaces.

Aligning one-dimensional arrays of nanoclusters on a thin film of Al 2O 3 grown on vicinal NiAl surfaces

We present a self-organised approach for the synthesis of one-dimensional (1D) arrays of supported nanoclusters. By oxidising NiAl surfaces vicinal to the (1 0 0) plane tilted along the crystallographic direction [0 1 0], we produced ordered thin films of θ-Al 2O 3 that exhibit uniform protrusion stripes propagating uniquely along direction [0 0 1] of the NiAl. These protrusions are preferential centres for nucleation of metal deposited from a vapour; the nanoclusters grown from such metal are aligned and form massive 1D cluster arrays along direction [0 0 1]. The arrays of Co nanoclusters exhibit a diameter as small as 3 nm and length exceeding a micrometer. The results imply prospective applications for which a patterned assembly of nanoclusters is desired.

Molecular dynamics simulation study of nanoscale passive oxide growth on Ni-Al alloy surfaces at low temperatures

Oxidation kinetics of Ni-Al (100) alloy surface is investigated at low temperatures (300--600 K) and at different gas pressures using molecular dynamics (MD) simulations with dynamic charge transfer between atoms. Monte Carlo simulations employing the bond order simulation model are used to generate the surface segregated minimum energy initial alloy configurations for use in the MD simulations. In the simulated temperature-pressure-composition regime for Ni-Al alloys, we find that the oxide growth curves follow a logarithmic law beyond an initial transient regime. The oxidation rates for Ni-Al alloys were found to decrease with increasing Ni composition. Structure and dynamical correlations in the metal/oxide/gas environments are used to gain insights into the evolution and morphology of the growing oxide film. Oxidation of Ni-Al alloys is characterized by the absence of Ni-O bond formation. Oxide films formed on the various simulated metal surfaces are amorphous in nature and have a limiting thickness ranging from $\ensuremath{\sim}1.7\text{ }\text{nm}$ for pure Al to 1.1 nm for 15% Ni-Al surfaces. Oxide scale analysis indicates significant charge transfer as well as variation in the morphology and structure of the oxide film formed on pure Al and 5% Ni-Al alloy. For oxide scales thicker than 1 nm, the oxide structure in case of pure Al exhibits a mixed tetrahedral $({\text{AlO}}_{4}\ensuremath{\sim}37%)$ and octahedral $({\text{AlO}}_{6}\ensuremath{\sim}19%)$ environment, whereas the oxide scale on Ni-Al alloy surface is almost entirely composed of tetrahedral environment $({\text{AlO}}_{4}\ensuremath{\sim}60%)$ with very little ${\text{AlO}}_{6}$ $(l1%)$. The oxide growth kinetic curves are fitted to Arrhenius-type plots to get an estimate of the activation energy barriers for metal oxidation. The activation energy barrier for oxidation on pure Al was found to be 0.3 eV lower than that on 5% Ni-Al surface. Atomistic observations as well as calculated dynamical correlation functions indicate a layer by layer growth on pure Al, whereas a transition from an initial island growth mode $(l75\text{ }\text{ps})$ to a layer by layer mode $(g100\text{ }\text{ps})$ occurs in case of 5% Ni-Al alloy. The oxide growth on both pure Al and Ni-Al alloy surfaces occurs by inward anion and outward cation diffusions. The cation diffusion in both the cases is similar, whereas the anion diffusion in case of 5% Ni-Al is 25% lower than pure Al, thereby resulting in reduced self-limiting thickness of oxide scale on the alloy surface. The simulation findings agree well with previously reported experimental observations of oxidation on Ni-Al alloy surface.

Stability of ultrathin alumina layers on NiAl(110)

By observing with low-energy electron microscopy whether individual alumina islands grow or shrink for different substrate temperatures and ${\mathrm{O}}_{2}$ pressures, we determine the stability of thin oxide layers on the NiAl(110) surface. At each temperature, a well-defined ${\mathrm{O}}_{2}$ pressure exists where islands do not change in size. Yet we conclude that the oxide cannot be in thermodynamic equilibrium with ${\mathrm{O}}_{2}$ gas and NiAl bulk, because the ${\mathrm{O}}_{2}$ pressures needed to attain this state are 20 orders of magnitude higher than expected. We discuss what kinetic processes can lead to the observed steady state, where the ${\mathrm{O}}_{2}$ pressure needed for stability differs greatly from thermodynamic predictions.

X-ray photoelectron spectroscopic studies on oxidation behavior of nickel and iron aluminides under oxygen atmosphere at low pressures

Oxidation behaviors of NiAl, Ni 3Al, and FeAl under oxygen atmosphere at low pressures were studied by X-ray photoelectron spectroscopy (XPS). Clean surfaces of these aluminides were prepared by fracturing in an ultra high vacuum, and then the fractured surfaces were oxidized by exposing to high-purity oxygen at pressures up to 1.3 Pa without exposing to air. The oxides formed on NiAl and FeAl surfaces were Al 2O 3, whereas the oxide on Ni 3Al was NiAl 2O 4. Aluminum, nickel, and iron on clean surfaces were oxidized even at a pressure of 1.3 × 10 −6 Pa. The oxidation evolves with an increase in the pressure of oxygen, and further oxidation of aluminum occurs prior to that of nickel or iron. The oxidation behaviors under such oxygen atmosphere were similar to those of the aluminides oxidized in air, and these behaviors could be predicted from thermodynamic consideration.

Chemical imaging of single 4,7,12,15-tetrakis[2.2]paracyclophane by spatially resolved vibrational spectroscopy

Single 4,7,12,15-tetrakis[2.2]paracyclophane were deposited on NiAl(110) surface at 11 K. Two adsorbed species with large and small conductivities were detected by the scanning tunneling microscope (STM). Their vibrational properties were investigated by inelastic electron tunneling spectroscopy (IETS) with the STM. Five vibrational modes were observed for the species with the larger conductivity. The spatially resolved vibrational images for the modes show striking differences, depending on the coupling of the vibrations localized on different functional groups within the molecule to the electronic states of the molecule. The vibrational modes are assigned on the basis of ab initio calculations. No IETS signal is resolved from the species with the small conductivity.

Surface structure and energy of B2 type intermetallic compound NiAl

The surface structure and energies for 22 surfaces of NiAl, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (1 1 0) surface belonged to the other kind of the surface consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (1 1 0) texture of NiAl appears mostly in the experiments.

Ferromagnetic one-dimensional tungsten atomic chain: Magnetic anisotropy and x-ray magnetic circular dichroism study

Using the full potential linearized augmented plane wave method, the magnetic properties of one-dimensional (1D) tungsten (W) atomic chain on NiAl(110) surface have been investigated. Very interestingly, we have obtained that the 1D $\mathrm{W}∕\mathrm{Ni}\mathrm{Al}(110)$ has a ferromagnetic ground state with magnetic moment of $0.81\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$, while the unsupported 1D W atomic chain shows an antiferromagnetic ground state with magnetic moment of $2.76\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$. This implies that the hybridization significantly alters the property of exchange interaction. We have obtained that the 1D W chain has a magnetization perpendicular to the chain axis, but the spins align in the in-plane direction. It has been found that the energy barrier for spin rotating in the in-plane direction is $3.509\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$/atom. Besides, the theoretically calculated x-ray absorption spectroscopy and x-ray magnetic circular dichroism have been presented.

Angular spectra of rainbow scattering at glancing keV He+ bombardment of NiAl(1 0 0) surface with transverse energies in the range 1–10 eV

Previous simulations of glancing incidence ion-surface interaction have demonstrated that classical dynamics using the row-model have successfully reproduced multimodal azimuthal and polar spectra. These studies have also shown considerable sensitivity to the form of the interatomic potential thus making it a strong test of the validity of such potentials and even allow deduction of the ion-surface potentials. In these simulations the individual pairwise interactions between the projectile and the target atoms have been replaced by cylindrical potentials.Comparison to numerous experimental studies have confirmed the existence of rainbow scattering phenomena and successfully tested the validity of the cylindrical potential used in these simulations. The use of cylindrical potentials avoids stochastic effects due to thermal displacements and allows faster computer simulations leading to reliable angular distributions.In the present work we extend the row-model to consider scattering from binary alloys. Using He+ scattered at glancing incidence from NiAl surfaces, Al or Ni terminated, a faster method has been developed to easily and accurately quantize not only the maximum deflection azimuthal angle but all the singular points in the angular distribution. It has been shown that the influence of the surface termination on the rainbow angle and the inelastic losses is small.

Growth of thin alumina films on a vicinal NiAl surface

Dramatic changes in the surface morphology have been observed during the oxidation of stepped NiAl(16, 14, 1) by LEED and STM. The initial sequence of identical (1 1 0) terraces is lifted in favor of large, triangular planes, whose mean size is determined by the mismatch-induced stress that accumulates in the thin alumina film. The asymmetry of the original step direction on NiAl(16, 14, 1) with respect to the orientation of the two alumina reflection domains favors the formation of one domain type, for which the stress relief via NiAl step edges is particularly efficient.

Evolution of a Reactive Surface via Subsurface Defect Dynamics

We find that the topography and composition of a reactive surface can evolve during epitaxy via motion of point and line defects within the material. We observe the response of a NiAl surface to an Al atom flux with low-energy electron microscopy. Initially, new NiAl layers grow as Al atoms exchange with bulk Ni atoms. When the surface is critically enriched in Al, condensation occurs at dislocations. They dissociate, move linearly, and leave tracks of altered composition and new atomic steps. We show how these dynamics depend on the identity and quantity of point defects near the surface.

First-principles calculations of the effect of Pt on NiAl surface energy and the site preference of Pt

Pt-modified NiAl is widely used as a coating material in industry. In this study, the surface energies of NiAl with and without Pt are investigated using first-principles calculations. The presence of Pt in NiAl takes the surface electronic states to higher energies, resulting in an increased surface energy, which explains some of the beneficial effects of Pt on the oxidation resistance of NiAl. The electronic structure of NiAl–Pt alloys is also analyzed in terms of the site preference of Pt in NiAl. Results show that Pt bonds strongly to Al, giving its site preference on the Ni site.

Interaction of adsorbates with electric-field fluctuations near surfaces: Nonradiative lifetimes and energy-level shifts

In the vacuum above surfaces, there are zero-point fluctuations in the electromagnetic field generated by zero-point charge fluctuations in the substrate. A molecule near the surface will couple to such fluctuations, with the consequence that excited states may decay via nonradiative transitions in which energy is transferred to the electronic excitations of the substrate (surface plasmons, particle hole pairs). In addition, coupling to the fluctuations will produce energy-level shifts not included in the standard version of density-functional theory. We develop a formalism that allows one to calculate the nonradiative lifetimes from this interaction, and also the energy-level shifts. We treat the electric-field fluctuations within the framework of a phenomenological theory that relates their amplitude and frequency spectrum to the optical dielectric constants of the materials from which the substrate complex is fabricated. We demonstrate that when the formalism is applied to a structureless, highly localized electron, the energy-level shift is just that given by the image potential of classical dielectric theory. In real systems one must include the effect of virtual transitions to excited states in the analysis of the level shifts. We present a quantitative study of the nonradiative lifetime of the lowest unoccupied molecular orbital $(\mathrm{LUMO})+1$ state of the magnesium porphine molecule, adsorbed on the oxide covered NiAl(110) surface, along with calculations of the energy-level shifts induced by the field fluctuations for this system. Our calculated nonradiative lifetime is in good accord with experimental data.