Clean Ni Surface Research Articles

Exchange splitting of the threeΓ¯surface states of Ni(111) from three-dimensional spin- and angle-resolved photoemission spectroscopy

The valence-band electronic structure of a clean Ni(111) surface is investigated by spin-resolved photoemission. At room temperature the orientation of the photoelectron spins on the Bloch sphere and the exchange splitting of surface and bulk states along the surface normal $(\overline{\ensuremath{\Gamma}})$ are determined. All investigated states are found to have a sizable exchange splitting $g50\text{ }\text{meV}$. Since the splitting is smaller than the intrinsic line width in the spin-integrated spectrum this is only seen with a spin-resolved technique. At room-temperature photoemission reaching above the Fermi level directly shows that the Shockley type surface state ${S}_{1}$ has an occupied majority and an unoccupied minority band with a splitting $\ensuremath{\Delta}{E}_{ex}=62\ifmmode\pm\else\textpm\fi{}15\text{ }\text{meV}$. The surface states below the Fermi energy show a larger exchange splitting for in-plane hybridization $[\ensuremath{\Delta}{E}_{ex}({S}_{3})=160\text{ }\text{meV}]$ than for out-of-plane hybridization $[\ensuremath{\Delta}{E}_{ex}({S}_{2})=55\text{ }\text{meV}]$.

Effect of adsorbates on the formation of doubly excited He atoms during impact of He 2+ ions on a Ni(1 1 0) surface

The formation of doubly excited states of He atoms during impact of He 2+ ions with projectile energies of 60–1000 eV under near-grazing angles of incidence of 5°–20° on clean and adsorbate-covered Ni(1 1 0) surfaces is studied by means of Auger electron spectroscopy. Pronounced dependencies of electron spectra from autoionization of atoms in doubly excited 2 s 2, 2 s2 p and 2 p 2 configurations on the coverage of the target surface with adsorbates are observed. These are directly related to work function changes, which are studied for the controlled adsorption of oxygen. Changes of the electron spectra on the target temperature are found for adsorbate-covered surfaces only, which puts into question recent interpretations of similar electron spectra in terms of a high local electron spin polarization of Ni(1 1 0) by an alternative interpretation based on thermal desorption or dissolution into bulk of surface contaminations. The formation of doubly excited states is studied for the oxygen p(2 × l) and p(3 × l) superstructures on Ni(1 1 0) in order to provide well-defined experimental data for theoretical investigations.

Water adsorption on clean Ni(111) andp(2×2)-Ni(111)-Osurfaces calculated from first principles

A systematic and comprehensive analysis of the adsorption of water monomers, small water clusters, and water thin films on clean and oxygen-predosed Ni(111) surfaces is performed. For the clean Ni surface, the formation of water hexamers (and possibly also incommensurate icelike bilayers) is favored. The total-energy results as well as the calculated vibrational frequencies for water adsorbed on the $p(2\ifmmode\times\else\texttimes\fi{}2)\text{-Ni}(111)\text{-O}$ surface suggest a reinterpretation of recent experimental data [M. Nakamura and M. Ito, Phys. Rev. Lett. 94, 035501 (2005)] in terms of single water monomers adsorbed on top of Ni and the formation of (islands of) an icelike bilayer.

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.

Effect of spin polarization of Ni(1 1 0) surface on Auger neutralization for grazing scattering of He+ ions

The survival of ions during grazing scattering of keV He+ ions from a clean Ni(110) surface is studied as function of target temperature. We observe ion fractions in the scattered beams of typically 10−3 which show a slight increase with temperature of the target surface. From computer simulations of projectile trajectories we attribute this enhancement for ion fractions to effects of thermal vibrations of lattice atoms on the survival of ions in their initial charge state. Based on concepts of Auger neutralization, we discuss the role of the spin polarization of target electrons on charge transfer. We do not find corresponding signatures in our data and conclude that in the present case of Ni(110) the spin polarization has to be small.

Hidden surface states on pristine and H-passivated Ni(111): Angle-resolved photoemission and density-functional calculations

By means of angle-resolved photoemission, we have uncovered surface related states on the pristine and hydrogen saturated Ni(111) surfaces. Near normal emission spectra were recorded at room temperature as a function of photon energy. A hidden Tamm surface state is found on the clean Ni(111) surface at a binding energy of $\ensuremath{\sim}1.19\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, completely masked by the ${\ensuremath{\Lambda}}_{1}$ bulk $d$ band. The existence of this surface state is in agreement with the density-functional theory calculations presented here. On the other hand, a surface state related to hydrogen adsorbed on the surface has been identified at a binding energy of $\ensuremath{\sim}0.22\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Under hydrogen exposure, it grows at the same rate as the three other surface states of the clean Ni(111) vanish.

A possible source of spin-polarized electrons: The inert graphene/Ni(111) system

We report on an investigation of spin-polarized secondary electron emission from the chemically inert system: graphene/Ni(111). An ordered passivation graphene layer (monolayer of graphite) was formed on Ni(111) surface via cracking of propylene gas. The spin polarization of secondary electrons obtained from this system upon photoemission is only slightly lower than the one from the clean Ni surface but does not change upon large oxygen exposure. These results suggest to use such passivated Ni(111) surface as a source of spin-polarized electrons stable against adsorption of reactive gases.

Angle-Resolved Photoemission Study of Electronic States in Ni(111) Surface with Oxygen Adsorption

We have investigated the surface electronic states of clean and oxygen-adsorbed Ni(111) surfaces by angle-resolved photoemission spectroscopy. Upon oxygen adsorption, c(2×2) and p(1×1) structures appear on 0.51 and 0.88 ML O/Ni(111) surfaces, respectively. By comparing the experimentally obtained energy dispersion of the surface electronic states with molecular-orbital and energy-band calculations, 3 d x z –2 p x and 3 d y z –2 p y antibonding π states are shown to be responsible for the electronic states at the interface and for the enhancement of the perpendicular magnetic anisotropy on oxygen-adsorbed Ni(111) surfaces.

Angle-resolved photoemission of orderedPb∕Ni(111)surface phases

$\mathrm{Pb}∕\mathrm{Ni}(111)$ surface phases were investigated by angle-resolved photoemission spectroscopy. Annealing of the $\mathrm{Pb}∕\mathrm{Ni}(111)$ surface led to the formation of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction characterized by a topmost layer consisting of a substitutional alloy. The transformation to the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure was accompanied by a modification of the surface band structure confirming the formation of a new phase in the Ni(111) surface layer, characterized by a transformation from ferromagnetic to a nonmagnetic state. The modification was displayed as a disappearance of the $\mathbit{k}$-dependent exchange splitting in the topmost alloy layer that was observed on the clean Ni surface due to the energy difference between spin-up and spin-down states. Fermi surface mapping showed partial disappearance of splitting of bands crossing the Fermi level.

Adsorbed hydrogen on Ni(111) surface studied by slow positron beam

AbstractReemitted slow positrons from clean Ni(111) surface and hydrogen‐adsorbed Ni(111) one have been measured. We have estimated experimentally the positron work functions, ‐0.60±0.05 and ‐0.82±0.07 eV of clean Ni(111) surface and the hydrogen‐adsorbed Ni(111) surface, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computational study of sulfur–nickel interactions: A new S–Ni phase diagram

Prediction of the interactions between H 2S-contaminated hydrogen fuel and Ni surfaces under conditions similar to those for solid oxide fuel cell (SOFC) operation using DFT (density function theory) calculations (with thermodynamic corrections) has resulted in a new S–Ni phase diagram, which suggests the existence of an intermediate state between clean Ni surfaces and nickel sulfides – sulfur atoms adsorbed on Ni surfaces. This prediction is consistent with many experimental observations relevant to sulfur poisoning of Ni-based anodes in SOFCs, which cannot be explained using the existing S–Ni bulk phase diagram from classical thermodynamics. The accurate prediction of the adsorption phase is vital to a fundamental understanding of the sulfur poisoning mechanism of Ni-based anodes under SOFC operating conditions.

Hydrogen-assisted reduction of the Ni(110)(3×1)–O surface studied by metastable-induced electron spectroscopy

Hydrogen-assisted reduction reactions at oxidized Ni(1 1 0) surfaces have been studied by metastable-induced electron spectroscopy (MIES). We prepared an O/Ni(1 1 0) surface with the high oxygen-coverage (3 × 1) surface phase, “HC-(3 × 1)”. This surface has been known to show no reactions to molecular hydrogen gas at RT, which differs from other surface phases, low-coverage (3 × 1) and (2 × 1), that undergo prompt reduction reactions at RT. We exposed the HC-(3 × 1) surface to molecular hydrogen while elevating the substrate temperature. In the temperature range from 450 to 550 °C, MIES showed a decrease in the emission intensity of the O2p-induced peak in parallel with a recovery of the Ni3d-induced peak intensity. Evidence of reactions of the HC-(3 × 1) surface to the incident molecular hydrogen was obtained on the MIES spectrum only at temperatures above 500 °C, where a H1s-induced peak appeared at the same energy as the case of initial H adsorption on the clean Ni(1 1 0) surface. This result means that molecular hydrogen takes effect on the HC-(3 × 1) O/Ni(1 1 0) surface but at reduced oxygen-coverages. A surface-structure dependency of the hydrogen-assisted reduction reactions at the O/Ni(1 1 0) surface is discussed.

Adsorbed hydrogens and their behavior on Ni(111) surface studied by slow‐positron beam

AbstractReemitted slow positrons from the clean Ni(111) surface and the hydrogen‐saturated Ni(111) surface were measured. We suggest that the disordered hydrogen adsorbed structure of the β1 phase contains vacancy‐type defects, and that the number of these defects will grow with time in the case of hydrogen‐saturated Ni(111) surface at 180 K. Copyright © 2006 John Wiley & Sons, Ltd.

The transition from the adsorbed state to a surface alloy in the Sn/Ni(1 1 1) system

Conventional X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and high resolution photoelectron spectroscopy with synchrotron radiation (SRPES) were used in order to study the overlayer to surface alloy transition in the Sn/Ni(1 1 1) system, as a function of temperature and initial Sn coverage. The TPD of CO adsorbed at 150 K, shows complete blocking of CO adsorption on the perfect surface alloy (0.33 ML Sn), whereas for less than 0.33 ML Sn, CO adsorbs on the alloy-free regions with the same local saturation coverage as on the clean Ni(1 1 1) surface. For RT deposition of 0.1 to 2.6 ML Sn/Ni(1 1 1), only a new sharp c(4 × 2) LEED structure was observed at exactly 0.25 ML, whereas Sn in excess of 1 ML yielded a c(2 × 2) LEED structure, when flashed at 700 K, and the sharpest (√3 × √3) R30° alloy structure, when annealed at 1000 K. A series of high resolution Sn4d SRPE spectra with 50 eV photon energy taken upon short heating of 1.2 ML Sn/Ni(1 1 1) at increasing temperatures, showed the gradual transition from the overlayer (Sn4d 5/2 BE 24.00 eV) to the alloyed (Sn4d 5/2 BE 23.73 eV) state of Sn. The combination of LEED and SRPES results suggests that the overlayer-to-alloy transition occurs via Sn adsorption at the threefold hollow Ni(1 1 1) sites. A theoretical estimation of the Sn4d 5/2 chemical shift using data from ab initio electronic structure calculations yielded a value of −0.36 eV, in reasonable agreement with the experiment.

Density-functional theory calculations for the carbon-monoxide and hydrogen co-adsorbed on Ni(111) surface

In this article a first-principles total energy calculation of the atomic geometry and electronic structure of carbon-monoxide and hydrogen co-adsorbed on Ni(111) p(2×2) surface at a coverage of 0.25 ML is reported. The optimized atomic geometry is in good agreement with the most recently reported LEED experiment on Ni(111) p(2×2)/(CO+H) co-adsorbed system. The carbon-monoxide is predicted to be adsorbed on the hcp site while hydrogen atom occupies the fcc site, and both adsorbates locate on opposite p(1×1) cells within a p(2×2) supercell. The adsorption energy of CO and H in this configuration is 2.81 eV with respect to the CO and H2 molecule and the surface work function is calculated to be 6.28 eV in comparison with 5.23 eV of clean Ni(111) surface. The bond lengths of C—O, C—Ni, and H—Ni are estimated to be 1.19?, 1.96?, and 1.71?, respectively, and the substrate interlayer distance variations, Δd12 and Δd23, are calculated to be +1.6% and +0.4%, respectively. The chemisorptions of CO and H atom on the Ni(111) p(2×2) surface were calculated in the same way and the adsorption heights of CO and H were evaluated to be 1.33? and 0.90?, respectively. The interaction energy between CO and H in the best fit geometry of Ni(111) p(2×2)/(CO+H) system is about 0.60 eV, which could be taken as the essential factor for the realization of Fisher-Tropsch reaction.

Adsorption and Thermal Decomposition of 2-Iodoethanolon Ni(100)-Intermediates of Oxidation of Hydrocarbons: Hydroxyalkyls and Oxametallacycles

Based on the study of the adsorption and thermal reactions of 2-iodoethanol on clean Ni(100) surface, further research and discussion were conducted on the results of experimental data, and the mechanism of chemical reaction was analyzed in detail. Two potential intermediates (2-hydroxyethyl and oxametallacycle surface species) in catalytic oxidation hydrocarbons were studied. 2-iodoethanol adsorbed molecularly on Ni(100) at 100 K starts chemical reaction around 140 K and produces small amounts of ethylene and water, due to the concerted decomposition or disproportionation of the adsorbed molecular species. Around 150 K with an initial C-I bond scission, 80% of 2-iodoethanol decompose to form-0(H) CH2CH2-, while 20% of 2-iodoethanol transform to 2-hydroxyethyl intermediates. The subsequent reaction of the 2-hydroxyethyl species around 160 K concerns two competing reactions: a reductive elimination with surface hydrogen to yield ethanol, and a β-H elimination to from surface vinyl alcohol. At the same temperature, the-O(H)CH2CH2-intermediate dehydrogenates to a -OCH,CH2-oxametallacycle species. Both 2-hydroxyethyl and oxametallacycle species tautomerize to acetaldehyde, around 210 K and above 250 K, respectively. Some of that acetaldehyde desorbs while the rest decomposes to hydrogen and carbon monoxide. The implications of this chemical process to catalysis are discussed.

Comparative Study ofC−HStretch and Bend Vibrations in Methane Activation on Ni(100) and Ni(111)

State-resolved measurements on clean Ni(100) and Ni(111) surfaces quantify the reactivity of CH4 excited to v = 3 of the nu4 bend vibration. A comparison with prior data reveals that 3nu4 is significantly less effective than the nu3 C-H stretch at promoting dissociative chemisorption, even though 3nu4 contains 30% more energy. These results contradict statistical theories of gas-surface reactivity, provide clear evidence for vibrational mode specificity in a gas-surface reaction, and point to a central role for C-H stretching motion along the reaction path to dissociative chemisorption.

Estimation of adsorbed hydrogen on Ni(1 1 1) surface by slow-positron beam

We have measured reemitted slow-positrons from clean Ni(1 1 1) surface and hydrogen saturated one, and have observed the reduction of reemitted slow-positron by adsorbed hydrogen atoms in the case of saturated coverage. The origin of the reduction has been discussed, taking into account of electrostatic repulsion between hydrogen adatoms.

The surface chemistry of vapor phase lubricants: tricresylphosphate on Ni(1 0 0)

Tricresylphosphate (TCP) is known to serve as an excellent high temperature vapor phase lubricant with some metals such as Fe but not with others such as Ni. The surface chemistry of m-TCP has been studied on clean and phosphorous covered Ni(1 0 0) surfaces in order to understand the differences between its reactivity on Fe and Ni. Our results show that upon heating to 800 K m-TCP decomposes on the clean Ni(1 0 0) surface to deposit carbon and phosphorous with the evolution of H 2, CO, benzene, and toluene into the gas phase. During further heating to 1000 K, all the carbon on the surface dissolves into the Ni bulk leaving only phosphorous. The adsorption and heating of m-TCP on the phosphorous-modified Ni surface does not result in significant decomposition. The clean Ni substrate is able to activate TCP decomposition in much the same way that it decomposes on the clean Fe surface. The lack of significant differences in the chemistry of TCP on the clean metal surfaces cannot explain the fact that TCP vapor will form thick lubricating films on Fe but not on Ni.

Thermal Chemistry of NH3 on Oxygen-Pretreated Ni(111) Studied by Metastable Induced Electron Spectroscopy and Ultraviolet Photoelectron Spectroscopy

Ammonia adsorption on Ni(111) and on oxygen-pretreated Ni(111) has been studied using metastable induced electron spectroscopy and ultraviolet photoelectron spectroscopy at room temperature. Both techniques allowed us to report on the electronic structure of the valence band. The nickel oxidation leads to the existence of an oxygen-adsorbed phase before NiO growth. So we compared the ammonia reaction toward three types of surfaces: Ni(111), NiO/Ni(111), and O−Ni(111). We observed different ammonia reactivities on these three types of surfaces at room temperature: ammonia molecules were molecularly adsorbed on Ni(111), and were decomposed on oxygen-pretreated Ni(111). After surface heating, ammonia products were dissociated, and adsorbed hydroxyl groups were also seen on O−Ni(111). Above 450 K, no ammonia decomposition products were observed on NiO/Ni(111), whereas Ni(111) and the O−Ni(111) were covered by atomic nitrogen. Its coverage is higher on the O−Ni(111) surface than on the clean Ni(111) surface. This observation proves the high ammonia reactivity toward adsorbed oxygen. This study shows that the reactivity of oxygen atoms on Ni(111) toward ammonia depends on their characteristics (adsorbed oxygen or oxygen embedded into NiO).