Electron spectroscopic studies of tin surface segregation on iron single crystal surfaces

AES, ELS, LEED and XPS investigations of the surface segregation of tin dissolved in a Fe-4wt%Sn alloy were performed in ultra-high vacuum at elevated temperatures. The three low indexed surface orientations (100), (110) and (111) were studied. In all cases, no dependence of the maximum tin surface coverage on temperature was detected within the temperature range from 450 to 650°C. An order-disorder transition was observed by LEED, AES and XPS for the (100) oriented surface during tin segregation. The binding state for the segregated tin atoms abruptly changes at the order-disorder transition as determined by XPS. Similar results were obtained for the (111) surface. A deviating behaviour was observed for the (110) surface orientation, where two different ordered hexagonal surface structures were detected by LEED during tin surface enrichment. The first structure is similar to the diamond structure of pure tin, and the second one corresponds to the formation of a thin layer of the intermetallic compound FeSn on the (110) surface. The electron binding energies of the segregated tin atoms determined by XPS increase with increasing tin coverage on the (110) oriented surface. ELS studies on (100) and (111) oriented surfaces saturated with segregated tin show in comparison with literature data of pure tin a surface plasmon loss peak but no signal for the corresponding bulk loss. An energy loss signal found only for the (110) surface at Sn saturation coverage seems to be characteristic of an intermetallic FeSn surface phase.     

Segregation and adsorption of sulphur on Ni50Fe50(100) alloy: Studies of the surface composition and structure and of the effects on the passivation

The surface composition of Ni₅₀Fe₅₀(100) alloy was studied and the segregation and adsorption of sulphur were investigated by AES, LEED and radiotracer (³⁵S) techniques. Ion etching produces a surface composition identical to the matrix composition (Ni:Fe=1:1). In the temperature range 300–600°C iron segregates on the surface and nitrogen was detected. Heating to higher temperatures (>600°C) also causes the segregation of iron as well as the segregation of sulphur. The durface reaches a stable composition that does not depend on further changes of the temperature in the range 25–800°C. It consists of an almost complete monolayer of iron segregated on the alloy. The segregation of sulphur leads to the formation of a c(2×2) structure. A sulphur coverage of 45 ng cm^-2, consistent with the c(2×2) structure, was measured by the radiotracer method after chemisorption in gaseous H₂S/H₂ mixtures at 550° and 200 Torr. This sulphur monolayer is stable in a range of p_H2S/p_H2=7.4 × 10^-5-6×10^-4. Above this pressure, a preferential sulphidation of iron is observed. The effects of sulphur on the anodic dissolution and passivation of the alloy in acid solution were studied. Adsorbed sulphur promotes the dissolution and delays the passivation. When the alloy is doped with sulphur, bulk sulphur accumulates on the surface during the dissolution of Ni and Fe. This anodic segregation leads to the formation of an adsorbed layer of sulphur, followed by the growth of a sulphide which blocks the formation of the protective oxide film.     

Segregation of sulphur on a molybdenum surface studied by auger electron spectroscopy

The kinetic behaviour of sulphur segregation was studied during heat treatment of a molybdenum ribbon between 750°C and 1350°C by using an Auger electron spectroscopic technique. When the specimen is heated to high temperatures, the sulphur present as an impurity in the polycrystalline molybdenum diffuses on to the surface along grain boundaries. The intensity changes of Auger electron signals of sulphur and molybdenum showed that the rate of the diffusion remained constant until the surface was covered with a first sulphide layer in the temperature range studied. The activation energy in the initial stage of the diffusion was 26kcal/mol. High resolution Auger spectra of the sulphur were measured and explained from the density of states of MoS₂.     

A theoretical study of poisoning in heterogeneous catalysis; discussion of the role of electronegativity and a comparison with experimental results of Goodman et al. on CO adsorption and methanation on Ni(100)

The influence of a range of electronegative poisons, (C, S, P, and Cl) on the catalytic activity is considered. Calculations, in which the atoms are described by muffin tin potentials, use a Green function method to study the effects of poisons on the local density of states (LDOS) at the CO adsorption site of a nickel (100) cluster. All poisons studied influence nearest neighbour sites, as indicated by a marked reduction of the LDOS around the Fermi level. S, P, and Cl also affect next-nearest neighbour sites. The effective range of S and Cl, the strongest poisons studied, is shown to be < 5 Å. The importance of electronegativity in determining poisoning strength is shown to be strongly modified by the size of the poison atom, reflected by its vertical distance (d⊥) above the Ni(100) surface. A simple model is developed to compare calculations with experimental studies on nickel by Goodman et al. [Acc. Chem. Res. 17 (1984) 194, Surface Sci. 108 (1981) 64]. The model yields good agreement with experimental results for suppression of CO adsorption activity by carbon and sulphur. The model suggests that the influence of sulphur on methanation should be significant, but less dramatic than observed. Substantial changes to the model fail to generate quantitative agreement with experiment and reasons for the discrepancy are discussed.     

Surface segregation in a Ni-1 at% Pd alloy

The surface composition of a Ni-1 at% Pd alloy has been determined by Auger electron spectroscopy after equilibration of the alloy at several temperatures in the range 550 to 800°C. Moderate segregation of Pd to the surface of the alloy has been observed, with concentrations of up to ~～30 at% being detected at the surface after equilibration at 550°C. The heat of segregation (or adsorption) of Pd has been found to agree reasonably well with the predictions of a previously developed unified model of surface segregation in alloys. The kinetics of Pd segregation to the surface have also been measured between 500 and 700°C. Analysis of the kinetic data yields apparent diffusivities of Pd through the alloy that are suggestive of a dominant grain boundary transport mechanism.     

An auger investigation of oxygen-enhanced tin segregation on a liquid Pb−Sn alloy

Summary Auger measurements of the surface composition have been performed on solid and molten Pb-5at%Sn alloys. They confirm the theoretical predictions that, in the absence of oxygen, liquid or solid alloys exhibit no significant surface segregation. On the contrary, surface composition measurements and simultaneous surface tension measurements clearly show that oxygen adsorption strongly affects the segregation of tin at the liquid-vapour interface. This surface enrichment is driven by the contribution of the reaction free energy and of the products surface tension.     

Hydrogen, sulphur and chlorine coadsorption on Pd(111): a theoretical study of poisoning and promotion

First principles calculations for the coadsorption of hydrogen with sulphur and chlorine on Pd(111) are presented. Individually, both sulphur and chlorine poison hydrogen adsorption, sulphur being the more efficient poison. The observed sulphur poisoning is a structural effect. The adsorption energy decreases and the diffusion barrier increases for hydrogen atoms in the vicinity of sulphur adatoms. A sulphur coverage of 0.33 ML is sufficient to completely poison hydrogen adsorption on Pd(111). The presence of chlorine adatoms on the sulphur-poisoned surface is found to stabilise localised hydrogen adsorption. The possible promotional effects of chlorine on sulphur-poisoned catalysts are discussed.     

E´tude par la diffraction dese´lectrons lents et la spectroscopie auger de l'adsorption du soufre sur l'or

A study has been made of the adsorption of sulphur on the (111), (100) and (110) faces of gold using LEED and AUGER spectroscopy combined with a radioactive tracer technique employing ³⁵S. The content of the surface unit mesh can thus be determined. A preliminary adsorption state with localization of sulphur atoms at sites of maximum coordination of the metal is observed. This state is transformed by reconstruction into a surface compound, or adsorbed two-dimensional sulphide. The results are compared with previous results from HEED and experiments of reversible chemisorption. On the (110) face the equilibrium between different adsorbed phases of sulphur is studied under conditions for which sulphur dissolution or evaporation is negligible. A temperature- coverage diagram is determined which shows the regions where sulphur is present at equilibrium in one or two phases.     

Measuring grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy: Further developments

A technique for the quantification of surface and grain boundary segregation using Wavelength Dispersive X-ray Spectroscopy (WDS) in the Scanning Electron Microscope (SEM) is proposed. As an example, the case of sulphur segregation in nickel is considered. The sulphur segregation can be quantified using a single voltage measurement (20 kV) of the sulphur Kα line intensity. The quantification is made from a simple proportionality equation derived from the XPP microanalysis model of correction. The result of segregation quantification by WDS is a surface concentration expressed in g.cm^-2.Special attention was paid to the quantification of grain boundary segregation on fracture surfaces, taking into account the off-normal incidence of the electron beam on the analyzed surface. A simple technique that allows determination of the tilt angle from the specimen absorbed current is proposed. The influence of the azimuth angle of the analyzed surface with respect to WDS spectrometer is also discussed.The results of the WDS technique are shown to be repeatable within 5% with reasonable counting times (a few minutes per facet). As an example, the kinetics of sulphur grain boundary segregation in nickel at 750 °C was measured using WDS to document the quantitative capabilities of the technique.As already pointed out in a previous paper, it is confirmed that WDS is insensitive to surface contamination, which allows the analysis of ex-situ fractured samples.     

Temperature-dependent modifications in the surface composition of Finemet: A ToF-SIMS study

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed as an in situ tool to study the temperature-induced alteration of the surface composition of amorphous Finemet, Fe₇₃Si_15.8B_7.2Cu₁Nb₃. Temperature was changed reversibly by cooling from room temperature to 118 K and warming back to room temperature. As a general result, the ion intensities and, consequently, the surface concentrations of the alloy constituents were found to vary non-monotonously. Therefore segregation processes were in operation the extent of which was element-specific. Most importantly, while cycling the temperature hysteresis behaviour was observed with concentration of Fe developing just opposite to that of the alloying elements. Accordingly, on cooling the alloy, the surface enrichment with B, Si, Nb, Cu attained first a maximum in the range of 248-193 K before the segregation changed the trend to establish appreciable depletion of these elements at 118 K (as compared to room temperature). By contrast, the surface iron content developed inversely and decreased first to a minimum at ∼223 K before reaching enrichment at 118 K. During warming, a maximum segregation of boron and silicon was observed at about 223 K - similar as on cooling - so that this temperature can be considered characteristic of the segregation process. Dissociative adsorption of water from the residual atmosphere occurring at low temperatures was responsible for the formation of surface hydroxides of iron, silicon and niobium; an enhanced adsorption of molecular water was observed at temperatures below 153 K. The temperature-dependent segregation and adsorption-desorption processes were found to be largely reversible, so that the surface composition of Finemet was practically restored after finishing the cooling-warming cycle. The processes and factors governing the non-monotonous temperature dependence of the surface segregation in the amorphous alloy are discussed within the frame of segregation theory and the influence of temperature-induced tensile stress on segregation.     

A model of the kinetics and equilibria of surface segregation in the monolayer regime

A kinetic model of surface segregation based on foreign atom movement in an appropriate potential is developed. It is used to calculate the time and temperature dependence of surface concentration and the concentration distribution perpendicular to the surface for the case of structure limited surface enrichment. The results give an explanation for Auger electron spectroscopy obervations of the segregation of tin at the surface of copper single crystals. In-depth profiles of composition obtained by sputtering are in agreement with predictions from the model. Deviations from other models of segregation kinetics are briefly discussed.     

Directional UV photoemission from clean and sulphur saturated (100), (110) and (111) nickel surfaces

Angle resolved photoemission energy distribution curves (EDC's) were obtained on clean and sulphur saturated (100), (110) and (111) nickel surfaces for excitation energies equal to 10.2, 13.5, 16.8 and 21.2 eV. The EDC's of clean surfaces are weakly structured at θ = 0° and become more rich in features for oblique angles. In the explored energy range, adsorption of sulphur produces two extra-structures at initial energies depending on surface orientation. One of which situated at about ?4.5 eV below the Fermi level is in good agreement with Hagstrum ion neutralization spectroscopy results, the other at around ?1.8 cV has never been observed before. A remarkable similarity of adsorption effects on the three surfaces is found. These results are compared with experimental data obtained previously on equivalent sulphur saturated surfaces by INS and discussed in relation to recent theoretical calculations on chalcogen adsorption on nickel.     

On the cleaning of monocrystalline metallic samples from impurities

The problem of sample cleaning is essential for all the scientists using ultra-high vacuum (UHV) techniques. The paper explores the issue of how the real structure of the monocrystalline sample affects its cleaning procedure. The mosaic structure of a monocrystalline sample should be taken into account in the interpretation of segregation and adsorption phenomena studies carried out under UHV conditions. Some examples of the cleaning of an Fe(1 1 1) sample from sulphur and carbon impurities are presented in the paper. Cleaning may result in obtaining two different states: clean surface and clean defects or clean surface but filled defects. According to these studies, the number of adsorption sites in the defects equals approximately 50% of the number of adsorption sites on the surface.     

Precipitation and re-solution of impurities at the surface of indium on traversing the melting-point

In the course of preparing a clean indium surface for solid-versus-liquid studies, changes in the surface concentrations of sulphur and oxygen were observed by AES and XPS while the metal was heated and cooled through its melting-point. Both impurities disappeared on melting, and reappeared on solidification, over a very narrow temperature range; the disappearance and reappearance were to a certain extent reproducible. The effect was found to be similar in characteristics to that observed for the behaviour of carbon on a nickel surface by Shelton et al., and the same Bragg-Williams model is invoked to explain the sharpness of the impurity concentration changes with temperature. Although the maximum temperature reached by the indium was only 200°C, traces of platinum were also observed on the indium surface after melting, in both the AES and XPS spectra, probably as a result of solution from the platinum boat.     

Carbon segregation to single crystal surfaces of Pt,Pd and Co

Results are reported on the surface segregation behaviour of carbon from dilute solid solutions in Pt, Pd and Co. With Pt(100) no preferential surface segregation was observed; this is similar to previous results for Pt(111). For Pd(lOO), Pd(111) and Co(0001) segregation was observed with evidence for a surface phase transition of the type previously reported for Ni(111). These observations suggest that the strong carbon-carbon interactions within a graphite monolayer are of more importance in producing the transition than a good epitaxial fit to the substrate. A comparison of the kinetics of carbon segregation to Co(0001) with those predicted by a simple diffusion model suggest that surface processes such as nucleation or lateral diffusion may play important roles.     

Surface composition and electronic properties of indium tin oxide and oxynitride films

The surface properties of indium tin oxynitride films prepared by rf-sputtering in nitrogen atmosphere were investigated by X-ray and ultraviolet photoelectron spectroscopy as well as electron energy loss spectroscopy and Auger electron spectroscopy depth profiling. The results are compared to reference measurements on conventional rf-sputtered indium tin oxide films. The incorporated nitrogen is present in different chemical environments. Employing these different spectroscopic techniques, it was found that desorption of nitrogen from the ITON structure upon annealing is the origin of the observed drastical changes in the surface composition and electronic structure. The formation of oxygen vacancies and Sn surface segregation upon annealing is linked to improvements in the physical properties (larger spectral range of transmittance and higher conductivity) of the films.     

Evaluation of the inhibitive effect of benzotriazole on archeological bronze in acidic medium

An archaeological bronze artefact was a Punic coin excavated from the north east of Tunisia in 2001. The composition of the copper alloy revealed a content of 3.5 % of tin and 1.4 % of lead with the presence of some sulphur heterogeneity. The surface presents some roughnesses and cracks and is covered by a corrosion layer of 20–40 μm thickness. The use of benzotriazole (BTA) as an inhibitor has become a standard element for the preservation of cuprous-based metals. In order to investigate the behaviour of BTA in an acidic medium, an Electrochemical Impedance Spectroscopy (EIS) investigation was performed to characterize the electrochemical behaviour of the interface of the archaeological bronze sample/acidic medium without and with BTA addition. Impedance diagrams obtained at different immersion times show that the presence of the inhibitor prevents the diffusional process observed in the absence of BTA. The inhibition of the pre-polarized bronze surface revealed that the mechanism of action of the benzotriazole molecule in an acidic medium is governed by the chemisorption process.     

Research of the behaviour of O chemisorption on the (110) surface of Rhx--Pt1－x alloy

An atomic group model of the disordered binary alloy Rhx-Pt1-x has been constructed to investigate surface segregation. According to the model, we have calculated the electronic structure of the Rhx-Pt1-x alloy surface by using the recursion method when O atoms are adsorbed on the Rhx-Pt1-x （110） surface under the condition of coverage 0.5. The calculation results indicate that the chemical adsorption of O changes greatly the density of states near the Fermi level, and the surface segregation exhibits a reversal behaviour. In addition, when x 〈 0.3, the surface on which O is adsorbed displays the property of Pt; whereas when x 〉 0.3 it displays the property of Rh.     

V(100)表面上S和O偏析的LEED—AES研究

用AES,LEED等表面分析手段,对V(100)表面上杂质S,O偏析作了详细研究。明确了S,O偏析关系;发现(8×1)-O,(4×1)-O两个新的表面结构;系统观察和分析了V(100)表面在不同S,O偏析量情况下的各种表面超结构,并获得这些表面超结构的相互关系。 关键词：     

V(001)表面上(4×1)-O,(2×2)-S两超结构的角分辨光电子谱

用角分辨紫外光电子谱详细研究了V(001)表面上,S和O偏析引起的(4×1)-O,(2×2)-S两超结构,确定了S,O各自引起的吸附态的峰位和对称性;得到了O占据V(001)表面上空位的实验证据。所得实验结果与理论计算相等。 关键词：