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.     

An XPS and UPS study of the interaction of oxygen with sodium-dosed Ag(110)

The adsorption of Na and the coadsorption of Na and O₂ on Ag(110) have been studied by XPS and UPS. Adsorption of Na results in a rapid decrease in the work function. Δφ reaching a limiting value of ?2.0 eV at θ_Na = 0.5 and thereafter remaining constant. In the coverage range 0 < θ_Na <1 adsorption of O₂ onto the Na dosed surface always results in an increase in the work function to an almost constant value of Δφ ~ ?1 eV. At the same time the XPS data show that the Na/O stoichiometry of the oxygen saturated surface remains essentially constant and independent of the initial Na dose. Calibration experiments using sodium formate as a standard compound indicate that this surface phase has the stoichiometry Na₂O. For θ_Na > 1 there is a sharp change in behaviour; the work function of the oxygen saturated surface begins to decrease rapidly, and eventually falls below the value for the Na covered surface itself at θ_Na ~ 1.5. The XP spectrum now shows the appearance of a new oxygen peak which increases in intensity as θ_Na increases beyond unity. The UP spectra indicate that the binding energy of the surface orbital derived from Na (3s) is increased by ~6.7 eV as compared with the free atom value, and the emission at ~3 eV below E_F, which is associated with surface oxygen is not greatly affected by the presence of Na. These results are discussed against the background of information already available from LEED, Auger, and thermal desorption studies, and we attempt to give a consistent interpretation of the properties of the system at coverages both below and above one monolayer.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Investigation of the interaction of oxygen with an Al(100) surface by work function methods

The change in work function (Δφ) of an Al(100) surface due to oxygen adsorption at different pressures (10⁻⁶– 10⁻⁵ Torr) and different crystal temperatures (115–600 K) has been investigated. The data show an initially positive Δφ at low temperatures and negative Δφ values for temperatures above 160 K. The results are pressure independent during the first 500 L oxygen exposure but become pressure dependent at higher exposures. Measurements of Δφ in vacuum after exposure to 1040 L O₂ show a nearly exponential decrease of Δφ with time. The time constants of this exponential behaviour are temperature dependent and vary between 1.5 min for 370 K and 33 min at 115 K. These time dependent effects are believed to be related to the movement of adsorbed oxygen to sites below the aluminum surface.     

ESCA studies of Cr-Co alloys

ESCA examination of films formed on Cr-Co alloys after immersion in 0.1M NaCl for 24 h has shown that the thickness of passive films decreased with an increase in chromium content. Surface films consisted of chromium and cobalt oxides as Cr₂O₃ and CoO. The amount of CoO in the surface film of the alloy was decreased with an increase in chromium but Cr₂O₃ was found at a greater depth in the passive film at any composition. Cr₂O₃ was a major component of the surface film when the chromium content in the alloy was 10% or higher. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Both Co-10 wt.% Cr and Co-30 wt.% Cr alloys investigated showed a lower corrosion rate than the Co-5 wt.% Cr alloy. Corrosion rate measured could be correlated to the surface film composition and structure as determined by ESCA.     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

Interaction of O2 with low index faces of α-CuZn

The adsorption of O₂ and initial step of oxidation have been investigated, mainly at room temperature, for three different α-CuZn (75%Cu/25%Zn) surfaces ((110), (100) and (111)) by XPS. XAES, LEED, CPD and HREELS. No superstructures were detected on the LEED patterns during O₂ admission for the three faces, and from the beginning of adsorption Zn segregated to the surface. For (110), the interaction of oxygen follows the sequence: (1) dissociative chemisorption (up to ~ 20 L), accompanied by an increase of the work function and a single site occupancy as revealed by HREELS; (2) nucleation of ZnO only, indicated by a decrease of the work function, a shift of the Zn L₃M₄₅M₄₅ Auger transition and an emergence of a vibration at 550 cm^?1. corresponding to the surface phonon of ZnO. The (111) face follows the same scheme, except that the sticking coefficient for oxygen is very low. For (100), it is clear that two states of oxygen exist simultaneously, even at the beginning, as revealed by HREELS and CPD measurements. No copper oxides are ever detected, even after heat treatment. In addition, different bonding properties of OH groups on the three surfaces are reported.     

Energy threshold of Cs-induced chemisorption of oxygen on a GaAs(Cs,O) surface

The probability of Cs-induced chemisorption of oxygen on a p-GaAs(Cs, O) surface is experimentally shown to be close to unity only when the work function does not exceed ～3.1 ± 0.1 eV. The measured adsorption energy threshold likely corresponds to the energy of the unoccupied level of the antibonding 2π* orbital of the O₂ molecule in the preadsorption state on the semiconductor surface.     

Work function of indium sesquioxide thin film

This paper reports surface electrical properties of thin film of indium sesquioxide, In₂O₃, during oxidation and reduction at elevated temperatures (298 K and 873 K). The studies involved monitoring of surface potential during oxidation and reduction experiments using work function measurements. The obtained experimental data are considered in terms of the effect of temperature on reactivity between oxygen and In₂O₃, involving chemisorption and oxygen incorporation. The reactivity in the range 298 – 773 K is limited to chemisorption resulting in the formation of surface dipoles exhibiting positive surface charge. Oxidation at 873 K results in slow oxygen incorporation.     

Coadsorption of electropositive and electronegative elements: II. Cs and O2 on W (100)

The coadsorption of Cs and O₂ on W (100) surface has been studied systematically. The results were analogous to those of the CsH₂ system. Independent of the sequence of deposition, the adsorbates formed a double layer with the oxygen underneath the Cs layer. The presence of Cs caused a transformation of a(4 × 1) structure of O₂ to a c (2 × 2) structure. Cesium also caused submerged oxygen atoms in the crystal to diffuse to the surface. The initially adsorbed O₂ enabled an increase in saturation coverage of Cs. This effect was more drastic in the O₂ case than that found with preadsorbed hydrogen. The presence of oxygen caused an increase in the work function at the saturation coverage of the subsequently adsorbed Cs, and a shift of the Cs coverage at the work function minimum to lower values. But both of these changes were small compared to those found for the Cs + H₂ system. The sticking coefficient of O₂ on Cs covered W (100) was higher than that for O₂ on clean W. This was attributed to an active chemisorption of O₂ on the Cs film and a place exchange mechanism.     

Adsorption of oxygen on clean single crystal faces of aluminium

Auger electron spectroscopy and work function measurements have been used to study the interaction of clean Al(111) and Al(100) faces with oxygen at low pressure near room temperature. The results for the two faces differ strongly. Thus, the sticking probability of the (111) face decreases rapidly with coverage, while the work function increases slightly, by 0.1 eV at 200 L. In contrast, the sticking probability of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5–0.8 eV. The shape of the Al L_{2, 3}VV spectrum from oxidized Al(100) is independent of coverage, and it is in fact very similar to previously reported spectra from oxidized polycrystalline aluminium. The corresponding spectrum from Al(111) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 Å) islands of Al₂O₃-like oxide form on the (100) face.     

Quantum Monte Carlo study of the CO interaction with a dimer model surface for Cr(110)

The chemisorption of CO on a Cr (110) surface is investigated using the quantum Monte Carlo method in the diffusion Monte Carlo (DMC) variant and a model Cr₂CO cluster. The present results are consistent with the earlier ab initio HF study with this model that showed the tilted/near-parallel orientation as energetically favoured over the perpendicular arrangement. The DMC energy difference between the two orientations is larger (1.9 eV) than that computed in the previous study. The distribution and reorganization of electrons during CO adsorption on the model surface are analysed using the topological electron localization function method that yields electron populations, charge transfer and clear insight on the chemical bonding that occurs with CO adsorption and dissociation on the model surface.     

Variation du travail de sortie du W (100) par adsorption d'oxyge`ne,de ce´sium et co-adsorption d'oxyge`ne et de ce´sium

The work-function determination by the Kelvin method is used to study the adsorption of oxygen, cesium and co-adsorption of oxygen and cesium on a (100) tungsten surface, at room temperature. The work-function change of the clean surface with the oxygen exposure is used to estimate the sticking coefficient and the dipole moment of adsorbed oxygen. During cesium deposition on the clean surface, a minimum at 1.58 eV and a plateau at 1.80 eV are obtained. Starting from the minimum obtained with cesium, oxygen adsorption leads to a decrease of the work function down to 1.17 eV, when cesium adsorption on a previously oxygenated surface gives a 1.12 eV minimum and an increase of the plateau up to 2.20 eV. This last variation is shown to be consistent with the observed increase of the dipole moment of cesium adsorbed on a partially oxygenated surface, which accounts also for the lowering of the work function minimum.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Coadsorption of oxygen and sodium on Ru(001)

The interaction of oxygen with sodium predosed Ru(001) is studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficient of oxygen is found to increase from 0.45 for bare Ru(001) to 1 for Ru(001) with a 0.35 monolayer sodium coverage. The adsorption capacity of the sodium predosed Ru(001) surface towards oxygen is enhanced from θ_O = 0.5 for clean Ru(001) to θ_O = 1.4 for Ru(001) with a 0.7 monolayer sodium coverage. The work function, electron loss changes and thermal desorption data give evidence that as long as θ_Na is less than 0.25, the oxygen chemisorption phase is characterized mainly by oxygen-Ru bonds and by the absence of strong sodium-oxygen interactions. At high sodium coverages (θ_Na > 0.35), the experimental data indicate the formation of a Na-O compound in the second adsorption layer at high oxygen exposures. When Ru(100) is predosed with sodium (θ_Na ? 0.25), this leads to complete suppression of oxygen penetration into the bulk during heating, the latter process being observed for the oxygen-Ru(001) system.     

Corrosion behavior and composition analysis of chromate passive film on electroless Ni-P coating

A passive film was formed on electroless Ni-P coating (ENPC) in a bath of K₂Cr₂O₇ 30 g/l. XPS and electrochemical methods were employed to analyze its chemical compositions and corrosion behaviors. The potentiodynamic polarization tests indicated the corrosion current of the passivated sample was 1/30 that of as-plated ENPC. The XPS analysis illustrated the film comprised Cr, Ni and O. The film thickness was evaluated to be a few nanometers according to the sputtering rate of Ar⁺ ion. High-resolution XPS spectra suggested that the detected Cr in film was in the form of trivalent compounds, Cr₂O₃ and Cr(OH)₃.     

Etude par spectroscopie de photoelectrons et d'electrons auger des etapes initiales de l'oxydation du chrome polycristallin

The joint application of various techniques of surface analysis (XPS, UPS, AES and work function measurements) allowed us to pinpoint the initial stages in the oxidation of polycrystalline chromium in the temperature range 180–720 K. At weak exposures ( < 10 L) the oxygen was dissociatively adsorbed without providing any information on the amount of oxygen involved or the sites of adsorption (chemisorption vs. physical interaction due to the roughness of the surface). At stronger exposures the oxide Cr₂ O₃ is observed to form, and this is covered by a layer of chemisorbed oxygen at ordinary temperatures and below. The maximum thickness of the oxide layer depends on the temperature, as would be expected from the logarithmic oxidation kinetics. The change in shape of the Auger peaks of chromium during oxidation has been interpreted in terms of interatomic transitions made via the 2p levels of oxygen.     

Chemisorption of O2 and N2O on Cu/Ru(001)

The interaction of O₂ and N₂O are compared on the basal plane of Ru as a function of the coverage of copper. Dissociative N₂O uptake (N₂O(g) → O(a) + N₂(g)) on clean Ru was measured using AES and transient partial pressures. The initial dissociative reaction probability is a steadily declining function of temperature while the saturation uptake of oxygen remains constant from 300–900 K. The saturation oxygen AES signal for N₂O chemisorption was one half that observed for O₂ chemisorption. The presence of 0.02 monolayers of Cu retards the initial dissociative adsorption of N₂O by 40% but has little effect on the total uptake of oxygen. The initial dissociative sticking coefficient for O₂ is not retarded significantly by small amounts of copper. Deposition of larger amounts of Cu leads to the completion of a 2D overlayer before growth in 3D begins. Surfaces covered with less than 3 monolayers of Cu exhibit larger O₂ sticking coefficients and greater oxygen stability than pure Cu.     

A LEED-AES study of the initial stages of oxidation of Fe (001)

LEED and AES have been used to study the structural changes and kinetics of the initial interaction between Fe(001) and oxygen at room temperature. The AES oxygen signal was quantified by using a two-dimensional oxide layer as a calibration point. This reproducible oxide layer was prepared by the high temperature reaction of H₂O at 10^?6 torr with Fe(001). The initial oxygen sticking coefficient was observed to be close to unity, which suggests that the chemisorption is non-activated and involves a mobile adsorption step. The rate of chemisorption decreased as (1-Θ) and exhibited a minimum at Θ = 0.5. LEED data indicate that the minimum value of the sticking coefficient corresponded to the completion of a c (2 × 2) surface structure. Upon additional exposure to oxygen, an increase in the sticking coefficient was observed in conjunction with the disappearance of the c (2 × 2) and a gradual fade out of all diffraction features. After mild heating, epitaxial FeO (001) and FeO (111) structures were observed. The simultaneous appearance of a shifted M_2,3M_4,5M_4,5 iron Auger transition with the increase in the sticking coefficient and the disappearance of the c (2 × 2) indicated that oxide nucleated on the surface after the complete formation of the c (2 × 2) structure. The relatively high sticking coefficient during the initial oxidation indicates that formation of a mobile adsorbed oxygen state precedes the formation of oxide.     

Ellipsometric studies of chemisorption on GaP(110) single crystals

Ellipsometry has been used to study the room temperature adsorption of H₂, O₂, Cl₂, Br₂, HCl, HBr, HI, H₂S, CH₃SH, H₂Se, NH₃, PH₃ and AsH₃ on GaP(110) surfaces; CO, CO₂ and CH₄ did not adsorb. Initial sticking coefficients were determined from the change in the ellipsometric angle Δ with time at a constant gas pressure and found to lie between I and 10 ^?5. The change in both Δ and ψ at monolayer coverage was measured as a function of wavelength in the visible and near UV for several gases. The observed structure was at variance with theoretical expectation for non-absorbing layers and, further, appeared similar for each adsorbate. This is attributed to small changes in the optical properties of the substrate arising from the removal of intrinsic surface states by chemisorption. Analysis of the data reveals GaP surface states with a maximum interband transition probability at a greater than band gap energy of 3.5eV. The optical constants of film-free GaP were also measured between 2 and 5.5eV, and direct interband transitions between bulk states were located at 3.76 and 5.05eV.