Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

Atomic and nanostructures of monolayer c(2 × 2)NiN on Cu(0 0 1)

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.     

Investigation of the doped transition metal promotion effect on CO2 chemisorption on Ni (1 1 1)

The chemisorption of CO₂ on the pure Ni (1 1 1) and doped Ni (1 1 1) by transition metal (Co, Rh, Cr, Ce, La) were investigated by using the generalized gradient approximation (GGA) and the Perdew–Burke–Emzerhof (PBE) functional. The optimized structure of doped metal surface showed that Rh, Cr, Ce, La atoms upward shift from the surface of Ni (1 1 1) plane, while the atom radius of Co is the minimum offset which lead to the height is ?0.03 Å. The ability of CO₂ chemisorption follows the order of La/Ni (1 1 1) > Ce/Ni (1 1 1) > Cr/Ni (1 1 1) > Co/Ni (1 1 1) > pure Ni (1 1 1). It is exothermic when CO₂ chemisorbed on Cr/Ni (1 1 1) Ce/Ni (1 1 1) and La/Ni (1 1 1), while it is endothermic on the Co/Ni (1 1 1) and pure Ni (1 1 1). CO₂ molecular chemisorbed on all the metal surfaces are negatively charged, result from the electron transfer between the metal surfaces and the CO₂ molecular. The transition metals La, Ce and Cr can promote the transformation of electron and make the CO bonds longer than the pure Ni (1 1 1). We also analyzed the dissociation of CO₂ on the Ni-based surface and found that the La/Ni (1 1 1) surface is the preference surface for the dissociation of CO₂, which improved the ability to hinder carbon deposition.     

Structural origin of perpendicular magnetic anisotropy in Ni–W thin films

The magnetic properties and microstructure of electrodeposited Ni–W thin films (0–11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0–2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2–7 at% W), and an amorphous phase (>7 at% W). In the transition region, (4–5 at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni–W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni–W films with PMA is composed of isolated columnar crystalline grains (27–36 nm) with the FCC phase surrounded by the Ni–W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni–W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni–W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of 〈1 1 1〉 columnar grains and its shape magnetic anisotropy. It is concluded that Ni–W electrodeposited films (4–5 at% W) may be applicable for perpendicular magnetic recording media.     

Coverage dependence of glycine adsorption on the Ge(100) − 2 × 1 surface

A combination of infrared spectroscopy, X-ray photoelectron spectroscopy and density functional theory has been used to investigate the adsorption behavior of glycine at the Ge(100) ? 2 × 1 surface under ultrahigh vacuum conditions. Comparison of experimental and simulated IR spectra indicates that at 310 K, glycine adsorbs on Ge(100) ? 2 × 1 via O–H dissociation, with some fraction of the products also forming an N dative bond to a neighboring germanium atom. O–Ge dative bonding is not observed. As coverage increases, the surface concentration of the monodentate O–H dissociated adduct increases, while that of the N dative-bonded species appears constant. XPS data support and clarify the IR findings and reveal new insights, including the presence at higher coverage of a minor product that has undergone dual O–H and N–H dissociation. These findings are supported by the calculated energy diagrams, which indicate that the reaction of a glycine molecule on the Ge(100) ? 2 × 1 surface via O–H dissociation and interdimer N dative bonding is both kinetically and thermodynamically favorable and that N–H dissociation of this adduct is feasible at room temperature given incomplete thermal accommodation along the reaction pathway.     

Growth mode of Pt1−xNix films biased dc-sputter-deposited on MgO(0 0 1)

A study is made by TEM, XRD and by measuring electrical/magnetic properties, of growth mode and properties of Pt_1−xNi_x alloy films deposited on MgO(0 0 1) at 250°C by dc-sputtering at 2.5–2.7 kV in Ar. A bias voltage V_s≤−160 V was applied to the substrate during deposition. It was confirmed that the Pt film was polycrystalline with the texture of Pt(1 1 1)/MgO(0 0 1) while the films of Pt_0.14Ni_0.86 and Pt_0.19Ni_0.81 were epitaxially grown with Pt–Ni(0 0 1)[1 0 0]/MgO(0 0 1)[1 0 0] similarly to the case of Ni/MgO(0 0 1). Thus the growth mode transformation between Pt–Ni(1 1 1)/MgO(0 0 1) and Pt–Ni(0 0 1)/MgO(0 0 1) may be induced at x less than 0.81 for Pt_1−xNi_x alloy films. The temperature coefficient of resistance TCR from 100 to 300 K of Pt_0.14Ni_0.86 films was estimated to be 0.0044–0.0053 K⁻¹ and saturation magnetization at 300 K to be 1.7–3.2 kG, respectively, while TCR of Pt films was estimated to be 0.0035–0.0048 K⁻¹.     

Growth-induced perpendicular magnetic anisotropy and clustering in NixPt1−x alloys

Polycrystalline and epitaxial (1 0 0), (1 1 0), and (1 1 1)-oriented Ni₃Pt, NiPt, and NiPt₃ films were deposited over a range of growth temperatures from 80°C to 700°C. Films grown at moderate temperatures (200–400°C) exhibit growth-induced properties similar to Co–Pt alloys: enhanced and broadened Curie temperature, perpendicular magnetic anisotropy and large coercivity. As in Co–Pt, the magnetic properties suggest a clustering of Ni into platelets on the growth surface, as the films are being grown. Unlike Co–Pt, however, NiPt films exhibit a strong orientational dependence of anisotropy and enhanced Curie temperature, possibly resulting from different types of surface reconstructions which affect the growth surface.     

One-step sonochemical syntheses of Ni@Pt core–shell nanoparticles with controlled shape and shell thickness for fuel cell electrocatalyst

We demonstrate a facile one-step method to synthesize Ni@Pt core–shell nanoparticles (NPs) with a control over the shape and the Pt-shell thickness of the NPs. By adjusting the relative reactivity of the Pt and Ni reagents in ultrasound-assisted polyol reactions, two Ni@Pt NP samples of the same composition (Ni/Pt = 1) and size (3–4 nm) but with different particle shape (octahedral vs. truncated octahedral) and different Pt-shell thicknesses (1–2 vs. 2–3 monolayer) are obtained. The control is achieved by using different Ni reagents, Ni(acac)₂ (acac = acetylacetonate) and Ni(hfac)₂ (hfac = hexafluoroacetylacetonate). A reaction mechanism that can explain all of the observations is proposed. The Ni@Pt NPs show up to threefold higher mass activity than pure Pt NPs in oxygen reduction reaction. Between the two Ni@Pt NP samples, the one composed of octahedral NPs with the thicker Pt-shell has higher activity than the other.     

Fast atom diffraction at metal surface

Fast atoms with energies from 300 eV up to 1.7 keV are scattered under a grazing angle of incidence from a clean and flat Ni(1 1 0) surface. For scattering under ”axial surface channeling” conditions, we observe – as reported recently for insulator and semiconductor surfaces – defined diffraction patterns in the angular intensity distributions for scattered fast ³He and ⁴He atoms. We have investigated the domain of scattering conditions where decoherence phenomena are sufficiently small in order to observe for metal targets quantum scattering of fast atomic projectiles. As a consequence, fast atom diffraction appears to be a general technique with a wide range of applicability in surface science.     

Electrodeposition of nickel-tungsten alloys under ultrasonic waves: Impact of ultrasound intensity on the anticorrosive properties

Electrodeposited Ni–W alloy assisted by high-intensity ultrasound was evaluated considering the nominal power effect on the anticorrosive property. Temperature profiles demonstrated that using a nominal power of 400 W, the electrolytic bath at 30 °C reached values of 39 ± 1 °C. The maximum acoustic power corresponded to 6.7% of the nominal power value at 400 W. Increasing the nominal power from 0 to 400 W; the Ni content decreased from 85.3 to 75.2 wt%, and the W content increased from 15.1 to 25.1 wt%. The deposited coating at 200 W and 300 W had a smooth, homogeneous, and uniform surface. At 400 W, the acoustic cavitation promoted erosion, affecting the coating surface. X-ray diffraction analysis indicated that the nominal power of 200 W promoted electrodeposition of the Ni₁₇W₃ structure with the plane (1 1 1) as a preferred orientation. The crystallite size decreased for the planes (1 1 1) and (2 0 0) when increased nominal power from 100 to 200 W. The optimum condition for the improved corrosion resistance occurred with the nominal power of 200 W, providing a polarization resistance of 23.42 kΩ cm².     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Relative reactivities of amino and ethenyl groups in allylamine on Si(100)2 × 1: Temperature-dependent X-ray photoemission and thermal desorption studies of a common linker molecule

The room-temperature adsorption and thermal evolution of allylamine on Si(100)2 × 1 have been investigated by using temperature-dependent X-ray photoelectron spectroscopy (XPS) and thermal desorption spectrometry (TDS). The presence of a broad N 1 s feature at 398.9 eV, attributed to a N―Si bond, indicates N―H dissociative adsorption. On the other hand, the presence of C 1 s features at 284.6 eV and 286.2 eV, corresponding to C═C and C―N, respectively, and the absence of the Si―C feature expected at 283.2 eV shows that [2 + 2] C═C cycloaddition does not occur at room temperature. These XPS data are consistent with the unidentate staggered and eclipsed allylamine conformer adstructures arising from N―H dissociation and not [2 + 2] C═C cycloaddition. The apparent conversion of the N 1 s feature for Si―N(H)―C

at 398.9 eV to that for Si―N(H) at 397.7 eV and the total depletion of C 1 s feature for C―N at 286.2 eV near 740 K indicates cleavage of the C―N bond, leaving behind a Si―N(H) radical. Furthermore, the C═C C 1 s feature at 284.6 eV undergoes steep intensity reduction between 740 K and 825 K, above which a new C 1 s feature at 283.2 eV corresponding to SiC is found to emerge. These spectral changes suggest total dissociation of the ethenyl fragment and the formation of SiC. Moreover, while the total N 1 s intensity undergoes a minor reduction (24%) upon annealing up to 1090 K, a considerable reduction (43%) is found in the overall C 1 s intensity. This observation is consistent with our TDS data, which shows the desorption of C-containing molecules including propene and ethylene at 580 K and of acetylene at 700 K. The lack of N-containing desorbates suggests that the dissociated N species are likely bonded to multiple surface Si atoms or diffused into the bulk. Interestingly, both the staggered and eclipsed N―H dissociative adstructures are found to have a less negative adsorption energy than the [N, C, C] tridentate or the [2 + 2] C═C cycloaddition adstructures by our DFT calculations, which suggests that the observed formation of N―H dissociative adstructures is kinetically favored on the Si(100)2 × 1 surface.     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks: I. Configurations,energies, and hydrogen bonding

Adsorption and rotation of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum were studied by density functional theory calculations using the PW91 approximation to the energy functional. On the (111) terrace, water monomer and the donor molecule of the dimer and trimer adsorb at atop sites. The per-molecule adsorption energies of the monomer, dimer, and trimer are 0.30, 0.45, and 0.48 eV, respectively. Rotation of monomers, dimers, and trimers on the terrace is facile with energy barriers of 0.02 eV or less. Adsorption on steps and kinks is stronger than on the terrace, as evidenced by monomer adsorption energies of 0.46 to 0.55 eV. On the (221) stepped surface the zigzag extended configuration is most stable with a per-molecule adsorption energy of 0.57 eV. On the (322) stepped surface the dimer, two configurations of the trimer, and the zigzag configuration have similar adsorption energies of 0.55 ± 0.02 eV. Hydrogen bonding is strongest in the dimer and trimer adsorbed on the terrace, with respective energies of 0.30 and 0.27 eV, and accounts for their increased adsorption energies relative to the monomer. Hydrogen bonding is weak to moderate for adsorption at steps, with energies of 0.04 to 0.15 eV, as the much stronger water–metal interactions inhibit adsorption geometries favorable to hydrogen bonding. Correlations of hydrogen bond angles and energies with hydrogen bond lengths are presented. On the basis of these DFT/PW91 results, a model for water cluster formation on the Pt(111) surface can be formulated where kink sites nucleate chains along the top of step edges, consistent with the experimental findings of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

Metastable helium atom scattering from Ni(1 1 0) surface

The survival probability (SP) of metastable helium atoms (He¹) during scattering from the clean, alkalated and oxygen-adsorbed Ni(1 1 0) surfaces has been examined in the kinetic energy range of 50–400 meV. The measurements were carried out using a time-of-flight technique and a pulsed-discharge type metastable helium atom source. The SP is nearly constant for a kinetic energy (E_kin) of 50–100 meV and decreases exponentially with the increase in E_kin at 100–400 meV. It has been shown that the SP at E_kin=100–400 meV depends on the repulsive part of the He¹-surface interaction potential.     

Influence of rapid thermal annealing on electrical and structural properties of double metal structure Au/Ni/n-InP (1 1 1) diodes

The effect of rapid thermal annealing on the electrical and structural properties of Ni/Au Schottky contacts on n-InP have been investigated by current–voltage (I–V), capacitance–voltage (C–V), auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The Au/Ni/n-InP Schottky contacts are rapid thermally annealed in the temperature range of 200–500 °C for a duration of 1 min. The Schottky barrier height of as-deposited Ni/Au Schottky contact has been found to be 0.50 eV (I–V) and 0.86 eV (C–V), respectively. It has been found that the Schottky barrier height decreased with increasing annealing temperature as compared to as-deposited sample. The barrier height values obtained are 0.43 eV (I–V), 0.72 eV (C–V) for the samples annealed at 200 °C, 0.45 eV (I–V) and 0.73 eV (C–V) for those at 400 °C. Further increase in annealing temperature to 500 °C the barrier height slightly increased to 0.46 eV (I–V) and 0.78 eV (C–V) compared to the values obtained for the samples annealed at 200 °C and 400 °C. AES and XRD studies showed the formation of indium phases at the Ni/Au and InP interface and may be the reason for the increase in barrier height. The AFM results showed that there is no significant degradation in the surface morphology (rms roughness of 1.56 nm) of the contact even after annealing at 500 °C.     

Adsorption of Xe atoms on the TiO2(1 1 0) surface: A density functional study

The interaction of xenon atoms with the TiO₂(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO₂(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO₂(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance.     

Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni₃Al and NiAl films, obtained respectively below and above a critical Al initial coverage θ_c, differ markedly from those of corresponding bulk alloys.The Ni₃Al ordered films form in a concentration range larger than the stability domain of the L1₂ Ni₃Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases.It also appears that the value of θ_c depends on the morphology of the Ni(1 1 1) substrate, increasing from θ_c = 4.5 ML for a flat surface to θ_c = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.     

Ultrasonic degradation of N-di and trihydroxy benzoyl chitosans and its effects on antioxidant activity

Modified chitosans with 3,4-dihydroxy benzoyl groups (CS-DHBA) and 3,4,5-trihydroxy benzoyl groups (CS-THBA) were synthesized and their chemical structures were determined by Fourier transform infrared (FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy. Then, ultrasonic degradation of CS, CS-DHBA and CS-THBA in 1% acetic acid solution was investigated. The kinetics studies of degradation were followed by gel permeation chromatography (GPC). The results indicated that the weight-average molecular weight of chitosan decreased obviously after ultrasound treatment, but molecular weights of CS-DHBA and CS-THBA decreased slowly with increasing sonication time. Degradation kinetics model based on 1/M_t−1/M₀ = kt was used to estimate the degradation rate constant. It was found that the rates of degradation of CS-DHBA and CS-THBA are lower than CS, and follow the order: CS₄ > CS₈ > CS₁₂ > CS-THBA₄ > CS-THBA₈ ≈ CS-DHBA₄ > CS-THBA₁₂ > CS-DHBA₈ > CS-DHBA₁₂. The antioxidant activity of the CS, CS-DHBA and CS-THBA before and after sonication was investigated by the radical scavenging activity method using 1,1-diphenyl-2-picrylhydrazyl (DPPH). The DPPH scavenging free radical capacity of CS-THBA and CS-DHBA increased up to 89% and 74% respectively, when the concentration reached 6 μg/ml. The ultrasonic treatment of CS-DHBA and CS-THBA after 30 min decreased the DPPH free radical scavenging activity but ultrasonic treatment of CS increased the DPPH free radical scavenging activity.     

The initial growth structure of Ni1-xFex (x=0.6–0.8) films dc-biased plasma-sputter-deposited on Ni/MgO(0 0 1), and on Fe/MgO(0 0 1)

Cross sectional and plane-view transmission electron microscopy (X- and PV-TEM) were used to investigate the initial growth phase of 5, 10, 20 and 40 nm thick Ni_1-xFe_x (x=0.6–0.8) films, prepared on MgO(0 0 1) covered with a buffer layer of Fe or Ni as well as on naked MgO(0 0 1). The 100 nm thick buffer layers of Fe and Ni were pre-grown on MgO(0 0 1). All of Ni_0.20Fe_0.80, Ni_0.40Fe_0.60, Fe and Ni films could be epitaxially grown at 250°C by dc-biased plasma sputtering at 2.9 kV in pure Ar gas.The films of Ni_0.20Fe_0.80 and Ni_0.40Fe_0.60 were grown in their own stable phase, bcc and fcc on MgO(0 0 1), respectively. However, Ni_0.20Fe_0.80 film could be grown in fcc phase pseudomorphic with Ni(0 0 1) up to 20 nm thick on Ni/MgO(0 0 1), while Ni_0.40Fe_0.60 film in bcc phase pseudomorphic with Fe(0 0 1) up to 10 nm thick on Fe/MgO(0 0 1). With increasing thickness, their growth phases transformed into their own stable phases. Whether or not the pseudomorphic phase may be induced and what its critical thickness may be should depend primarily on the lattice misfit between the crystal planes in contact. The growth mode of Ni_0.40Fe_0.60 film was investigated more in details to be compared with the simulations of the average strain energy versus thickness and with those of the critical thickness of the pseudomorphic films versus the lattice misfit between the contacted crystal planes.