H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

ELS study on initial oxidation of Ni(100) surfaces

Electron energy loss spectra of clean and oxygen-covered Ni(100) surfaces were observed with concomitant measurements of LEED, work function change, and Auger peak height ratio O(KL_{2, 3}L_{2, 3})/Ni(L_{2, 3}VV). The observed electronic transitions are interpreted on the basis of primary election energy dependence, and of comparison with the loss spectrum for a UHV-cleaved NiO(100) surface and optical data of Ni. The observed loss peaks at 9.1, 14, and 19 eV in the clean surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electrons, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Three oxygen-derived peaks at 6.0, 8.0, and 10.3 eV in the low oxygen exposure region (?4 L) are ascribed to the O 2p(e) → Ni 3d, O 2p(a₁) → Ni 3d, and O 2p → Ni 4s transitions, respectively. In the high oxygen exposure region (?50 L), the spectra become quite similar to that of the UHV-cleaved NiO(100) surface. The oxidation process consistent with LEED, Auger peak height ratio and work function change measurements is discussed.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

The initial oxidation of Cu(100) single crystal surfaces: an electron spectroscopic investigation

The total energy distribution of electrons emitted from clean Cu(100) and oxygen covered surfaces is analysed. A primary electron energy of 400 eV enabled the investigation of characteristic losses (ELS), Cu MVV Auger transitions and true secondary electrons in a single spectroscopic run. Oxygen exposure up to 10⁸ L at elevated temperature (~400 K) results in a Cu density of states (DOS) strongly affected by O(2p) electrons. The Auger lines of Cu, atomic-like for clean surfaces, reveal DOS effects after some 10⁷ L oxygen exposure: all MVV transitions shift down by ~2 eV in spite of a fixed M₂₃ level; the M₂₃VV Auger line splitting is vanishing due to a broadened valence band maximum allowing the deexcitation of the final two-hole state of intraatomic transitions. Heating the oxygen covered crystal to 820 K is accompanied by the removal of much surface oxygen and an electronic state resembling an earlier oxidation state without DOS effects in the Cu Auger spectrum.     

Oxygen chemisorption and initial oxidation of Cr(110)

The initial stages of the interaction of oxygen with a Cr(110) surface have been investigated at 300 K by LEED, AES, electron energy loss spectroscopy (ELS), secondary electron emission spectroscopy (SES) and work-function change measurement (Δφ). In the exposure region up to 2 L, the clean-surface ELS peaks due to interband transition weakened and then disappeared, while the ～5.8 and 10 eV loss peaks attributed to the O 2p → Cr 3d transitions appeared, accompanied with a work-function increase (Δφ = +0.19 eV at2L). In the region 2–6 L the work function decreased to below the original clean-surface value (Δφ_min = ?0.24 eV at6L), and five additional ELS peaks were observed at ～2, 4, 11, 20 and 32 eV: the 2 and 4 eV peaks are ascribed to the ligand-field d → d transitions of a Cr³⁺ ion, the 11 eV peak to the O 2p → Cr 4s transition, the 20 eV peak to the Cr 3d → 4p transition of a Cr³⁺ ion and the 32 eV peak probably to the Cr 3d → 4f transition. A new SES peak at 6.1 eV, being attributed to the final state for t he 11 eV ELS peak, was observed at above 3 L and identified as due to the unfilled Cr 4s state caused by charge transfer from Cr to oxygen sites in this region. In the region 6–15 L the work function increased again (Δφ_max = +0.32 eV at15 L), the 33 and 46 eV Auger peaks due to respectively the M_2,3(Cr)L_2,3(O)L_2,3(O) cross transition and the M_2,3VV transition of the oxide appeared and the 26 eV ELS peak due to the O 2s → Cr 4s transition was also observed. Above 10 L, the ELS spectra were found to be practically the same as that of Cr₂O₃. Finally, above 15 L, the work function decreased slowly (Δφ = +0.13 eV at40L). From these results, the oxygen interaction with a Cr(110) surface can be classified into four different stages: (1) dissociative chemisorption stage up to 2 L, (2) incorporation of O adatoms into the Cr selvedge between 2–6 L, (3) rapid oxidation between 6–15 L leading to the formation of thin Cr₂O₃ film, and (4) slow thickening of Cr₂O₃ above 15 L. The change in the Cr 3p excitation spectrum during oxidation was also investigated. The oxide growth can be interpreted on the basis of a modified coupled current approach of low-temperature oxidation of metals.     

The adsorption of SO2 on iron surfaces studied by x-ray photoelectron spectroscopy

The interaction of SO₂ with evaporated iron surfaces in the temperature range 80–450 K was investigated by using X-ray photoelectron spectroscopy. At 300 K, SO₂ decomposed at the initial stage of the interaction and gave adsorbed S with the S2p peak at 161.9 eV and adsorbed O with the O1s at 530.0 eV. Further exposure of SO₂ gave adsorbed SO₄ with S2p at 166.8 eV O1s at 531.3 eV, being different in binding energies from ionic SO₄^2?. This indicates the two stage reaction Of SO₂ with iron surface; SO₂(gas) → S(ads) + 20(ads), SO₂(gas) + 2O(ads) → SO₄(ads). The first reaction did not occur at low temperature or in the presence of adsorbed O. The adsorbed SO₄ formed at 80 K showed a quantitative decomposition reaction into S(ads) and O(ads) in the temperature range 200–350 K.     

Atomic and molecular oxygen adsorption on Ag(111)

The adsorption of O₂ on Ag(111) between 150 and 650 K has been studied with thermal desorption spectroscopy, Auger and photoelectron spectroscopies, and low-energy electron diffraction. A molecularly adsorbed O₂ species is populated with extremely low sticking probability (~ 5 X 10^?6) at 150 K. This species desorbs, with little dissociation, at 217 K. An atomically adsorbed species, with an O(1s) BE of 528.2 eV, is populated at 490 K with a sticking probability near 10^?6. This species exists in islands of local coverage θ_O ? 0.41, displaying a p(4 X 4)-O LEED pattern. It associatively desorbs at 579 K as O₂, and can be titrated at room temperature with CO to produce CO₂. There is also evidence for a subsurface oxygen species which reactivates below 600 K. Surface carbonate (CO_3,a) can be produced from O_a and CO₂ gas. These results are compared with similar species on Ag(110). A kinetic model is developed which describes the interaction of O₂ with these surfaces over a broad range of temperatures, and provides energetic values for the O₂/Ag interaction potential.     

Electron energy-loss spectroscopy study of oxygen chemisorption and initial oxidation of Fe(110)

The initial stages of the interaction of oxygen with an Fe(110) surface have been studied at 300 K by electron energy-loss spectroscopy with in-situ combined low energy electron diffraction, Auger electron spectroscopy and work-function change measurement. From all the results, four different stages of the oxygen interaction are distinguished: (I) a first dissociative chemisorption up to 3 L, characterized by the c(2×2)-O structure, (II) a second dissociative chemisorption between 3 and 7 L, characterized by the c(3×1)-O structure, (III) incorporation of O adatoms into the selvage between 7 and 30 L, and (IV) oxidation above 30 L leading to the formation of FeO(111), characterized by the diffuse hexagonal diffraction pattern. The sticking probability was found to be initially near unity and fall off rapidly to a minimum value of ≈0.05 at ≈1 L. Particular emphasis was placed upon the investigation of the change in surface electronic properties from those characteristic of them metal to those of the oxide. In stage (I) an energy-loss peak, being attributed to the transition from the 2p orbital of the chemisorbed oxygen, was observed at 6.0 eV, while in stage (II) two additional peaks of the same origin appeared at 7.5 and 9.3 eV due to the formation of the O 2p band. The energy-loss spectrum in the oxide phase was characterized by the peaks at 4.8 and 7.5 eV due to the O²⁻ 2p → Fe²⁺3d charge-transfer transitions and by a peak at 2.4 eV due to the ligand-field d → d transitions of an Fe²⁺ ion in FeO. It is shown that the Fe 3d_yz,zx and 4sp electrons play a major role in the chemisorption bond (O adatoms located in the long-bridge site), and that for the incorporation process the Fe 3d_y² electrons are also involved in bonding by the symmetry breaking. The change in the Fe 3p-excitation spectrum during oxidation was also investigated. The differences between the experimental results on Fe(100) and (110) surfaces are summarized.     

A LEED/UPS study on the interaction of oxygen with a Ni(111) surface

Exposure of a Ni(111) surface to oxygen leads at first to the formation of a chemisorbed overlayer which is characterized by a 2 × 2-superstructure and a maximum in the photoemission spectrum (hv = 40.8 eV) centered at 5.6 eV below the Fermi level E_F. The emission from the Ni d-states is nearly unaffected at this stage of interaction. After high oxygen exposures the epitaxial growth of NiO can be identified from the LEED pattern. The corresponding photoelectron spectrum is strongly altered and exhibits close agreement with the transition energies as calculated by Messmer et al. for a NiO₆^10- -cluster.     

Initial stages of hydrogen reduction of NiO(100)

The reduction of single crystal NiO(100) under hydrogen has been followed by AES, XPS and LEED for the pressure range of 1.0 × 10^?7 to 1.3 × 10^?6 Torr and for substrate temperatures of 150–350°C. The kinetics of reduction are controlled both by the rate of removal of lattice oxide at the surface and by the diffusion of subsurface oxygen to the oxygen-depleted surface. The rate of oxygen removal is first-order in surface oxide concentration and in hydrogen pressure. An induction period precedes the reduction reaction and its length is postulated to be controlled by surface defect concentration. The stoichiometric and reduced lattice oxygen species appear to be chemically identical and give a single symmetric XPS peak at 529.4 eV. Nickel spectra indicate a shift in XPS binding energies from those expected of the oxide to those of nickel metal early in the reduction process, although LEED indicates the NiO(100) surface lattice to remain the stable structure for surface reduced to approximately 20% of the stoichiometric oxygen concentration. Ni(100) island formation is observed, with Ni 〈010〉 and 〈001〉 directions along the NiO 〈010〉 and 〈001〉, respectively, but only after the NiO surface is severely depleted in oxygen.     

Oxygen adsorption on an alkali metal-covered Ni(100) surface

The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of θ_O^cwhose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(100), which suggests that alkali metals cause no change in the character of the Ni-O bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.     

In situ XPS study of Pd(1 1 1) oxidation. Part 1: 2D oxide formation in 10 mbar O2

The oxidation of the Pd(1 1 1) surface was studied by in situ XPS during heating and cooling in 3 × 10⁻³ mbar O₂. A number of adsorbed/dissolved oxygen species were identified by in situ XPS, such as the two dimensional surface oxide (Pd₅O₄), the supersaturated O_ads layer, dissolved oxygen and the R 12.2° surface structure.Exposure of the Pd(1 1 1) single crystal to 3 × 10⁻³ mbar O₂ at 425 K led to formation of the 2D oxide phase, which was in equilibrium with a supersaturated O_ads layer. The supersaturated O_ads layer was characterized by the O 1s core level peak at 530.37 eV. The 2D oxide, Pd₅O₄, was characterized by two O 1s components at 528.92 eV and 529.52 eV and by two oxygen-induced Pd 3d_5/2 components at 335.5 eV and 336.24 eV. During heating in 3 × 10⁻³ mbar O₂ the supersaturated O_ads layer disappeared whereas the fraction of the surface covered with the 2D oxide grew. The surface was completely covered with the 2D oxide between 600 K and 655 K. Depth profiling by photon energy variation confirmed the surface nature of the 2D oxide. The 2D oxide decomposed completely above 717 K. Diffusion of oxygen in the palladium bulk occurred at these temperatures. A substantial oxygen signal assigned to the dissolved species was detected even at 923 K. The dissolved oxygen was characterised by the O 1s core level peak at 528.98 eV. The “bulk” nature of the dissolved oxygen species was verified by depth profiling.During cooling in 3 × 10⁻³ mbar O₂, the oxidised Pd²⁺ species appeared at 788 K whereas the 2D oxide decomposed at 717 K during heating. The surface oxidised states exhibited an inverse hysteresis. The oxidised palladium state observed during cooling was assigned to a new oxide phase, probably the R 12.2° structure.     

Angle-resolved AES study on initial oxidation of Ni (100) surface

The Ni-M_2,3VV Auger electron angular distributions have been measured from oxygen-adsorbed Ni (100) surfaces and from cleaved NiO (100) clean surfaces. Significant variations in the angular profiles have been observedin the second reaction stage of initial oxidation process. Present results support the island growth model of NiO layers. It is also shown that the new information on the ratio of domain area of NiO to that of c (2 × 2)-O on Ni (100) surface can be obtained by angle-resolved AES method.     

X-ray photoelectron spectroscopic studies of nickel-oxygen surfaces using oxygen and argon ion-bombardment

We have studied the surface chemistry of the nickel-oxygen system using both temperature changes and ion bombardment as techniques for elucidating the surface structure. The spectra of metallic Ni, NiO and Ni₂O₃ were characterized from samples prepared directly in the spectrometer. The Ni₂O₃ species could be distinguished from an authentic Ni(OH)₂ sample from both the X-ray photoelectron lines and the Auger transitions. The oxides of NiO and Ni₂O₃ could be prepared by bombardment with low energy (400eV) O₂⁺ ions as well as by exposure of Ni to oxygen at reduced pressure (～ 100 torr). The Ni₂O₃ was found to be present on most nickel-oxygen surfaces except those prepared by exposing Ni to air for many hours at high temperature (> 600°C), indicating that the stability of Ni₂O₃ decreased as the temperature increased. Exposure of both NiO and Ni₂O₃ to 400 eV Ar⁺ ion bombardment caused reduction to metallic Ni. This observation has also been noted for several other oxides and a prediction of whether or not reduction should be observed is presented by examining the free energy of formation of the molecule.     

The reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature

Ultraviolet photoemission spectroscopy using hv = 21.2 eV and filtered 40.8 eV radiation as well as temperature programmed thermal desorption spectroscopy are used to investigate the chemical reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature. Striking crystallographic effects and several coexisting phases are observed and found to be coverage and temperature dependent. A methodology is described and used to predict the relative energy levels for a variety of adsorbed hydrocarbon fragments on Ni surfaces. Such levels together with the thermal desorption spectra are used to identify the observed species. In particular, CH and CCH species are isolated on Ni(100) and Ni(110) surfaces, respectively, via low temperature adsorption and subsequent pulsed sample warming experiments. The room temperature adsorption phases are deduced using these ionization levels together with those of chemisorbcd acetylene, atomic hydrogen and carbon. At room temperature on Ni(100), H, C, CH and C₂H₂ species form together below 2 L exposure while CH species form thereafter, up to a saturation exposure of ~10 L. On Ni(110), H and CCH species form below 1.5 L exposure followed by the formation of CH₂ and likely CH species. The relative stabilities of these species at elevated temperatures is: C₂H₂ < CCH ? CH < CH₂. A model for the bonding of acetylene and its reaction to form CCH species on Ni(110) is proposed.     

Water adsorption on the (001) plane of Fe2O3: An XPS,UPS, Auger,and TPD study

Thermal programmed desorption, ultraviolet photoelectron spectroscopy, and X-ray photoelectron spectroscopy have been used to study the reaction of H₂O with stoichiometric and partially reduced single crystals of α-Fe₂O₃. On the stoichiometric surface only ice condensation below 200 K was observed. Oxygen deficient surfaces were prepared by Ar bombardment giving rise to a decrease in the work function of the crystal of up to 1 eV. On these surfaces OH species were formed as detected by UPS that were stable up to 320 K. Annealing the defective surfaces between 475 and 700 K increased the work function by values between 0.5 and 0.7 eV respectively. These surfaces contained reduced Fe²⁺ species in subsurface layers as shown by UPS and XPS, but were inactive towards H₂O chemisorption. The Fe²⁺ species were stable for long periods of time at temperatures of up to 775 K. Potassium deposited on the surface forms a strongly bound monolayer compound. With H₂O it produced a complex that resulted in H₂ evolution upon annealing.     

Investigation by X-ray photoelectron spectroscopy of the transient oxidation of NiAl

X-ray photoelectron spectroscopy (XPS) has been used to study the oxidation of NiAl in oxygen at atmospheric pressure. Prior to oxidation, the native oxide scale on the specimen was removed by ion sputtering and the specimens were (pre-)heated in vacuum before exposure to oxygen. At low oxidation temperatures (<750 K) scales consisted of Al₂O₃ and NiAl₂O₄, with a thin surface layer of NiO, but at higher temperatures were of Al₂O₃, apart from about 0.5 at % Ni. The Ni content in the latter case was constant throughout the scale and did not increase dramatically near the alloy/oxide interface. In the experimental conditions used in this study, initial formation of NiO and NiAl₂O₄ seems to be avoided at the higher oxidation temperatures ( > 750 K).     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

The adsorption and decomposition of N2O on nickel (100)

The adsorption and decomposition of N₂O on clean and oxygen covered Ni(100) surfaces has been studied using a combination of Auger electron spectroscopy (AES) and molecular beam relaxation spectroscopy (MBRS) techniques. As observed in a previous study of this reaction on the Ni(110) surface, N₂O decomposes to yield N₂ gas and adsorbed O at temperatures between 200 and 800 K. Measurements at temperatures below 200 K led to the identification of two weakly adsorbed precursor species, one on clean surfaces and the other on surfaces covered with 0.25 ML of adsorbed O. The adsorption rate constants measured for these two species are consistent with values inferred indirectly in the previous study.     

Nanoscale modification of Ni/Al interfaces by low-energy $\mathrm{O}_{2}^{+}$ reactive ion beam mixing

The kinetics of growth, composition and electronic structure of thin oxide films formed by reactive ion beam mixing (IBM) of Ni/Al interfaces bombarded with low-energy (3-keV) O₂⁺\mathrm{O}_{2}^{+} ions have been studied at room temperature using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and factor analysis. Initially, NiO species are formed but, later, with increasing ion dose, Ni–Al mixed oxide species appear due to Al incorporation in the near-surface region. These changes are accompanied by a slight increase of the oxygen concentration and a decrease of the Ni/Al ratio in the thin oxide films formed. Angle-resolved X-ray photoelectron spectroscopy shows that Ni–Al mixed oxide species are located nearer the surface than NiO species. Experimental results have been compared with Monte Carlo TRIDYN simulations, suggesting that processes driven by residual defects or the reaction with oxygen predominate over pure ballistic mechanisms during reactive IBM of Ni/Al interfaces.