Surface Composition Of Binary Alloys Research Articles

Surface composition of Pt25Ni75(111) probed by HREELS

High-resolution electron energy loss spectroscopy (HREELS) has been used to study vibrations of CO molecules chemisorbed at low coverage on a Pt25Ni75(111) sample after different surface treatments. In agreement with earlier work using other methods, we always observe that Pt is enriched in the outermost layer, with a chemical composition strongly dependent on the annealing temperature. We discuss to which extent HREELS of CO may be exploited to “titrate” the surface composition of binary alloys.

Application of an improved thermodynamic description of multilayer surface compositions of binary alloys

The previous developed new thermodynamic theory, the “modern thermodynamic calculation of interface properties” (MTCIP), in its first calculational approximation to a successful description of the surface composition of dilute binary alloys, has been improved in a multilayer description (using several surface atomic sublayers) by allowing for any bulk composition, accounting for farther than first neighbour effects, describing component partial molar surface areas as functions of the mixture surface composition, and including a treatment of surface reconstructions. The calculations for the {100}, {100} and {111} surfaces of Pt50Ni50 single crystal alloys at≈1000 K are presented, which are in good agreement with experimental results [1–3] for all surface atomic sublayers and all surface orientations.

ChemInform Abstract: Surface Composition of Binary Alloys Between Pd and Group IB Metals as Studied by Auger Electron Spectroscopy and Cyclic Voltammetry

The surface composition of alloys between Pd and Group IB metals (Pd+IB alloys) was investigated on the basis of Auger electron spectroscopy (AES) and cyclic voltammetry. A correction factor for the extinction of Auger peak intensities by surface contaminants, mainly carbon, in deducing the surface composition was determined first. The applicability of this method was substantiated in the case of Pd + Au alloys by the empirical rule in the electrochemical observation that the potential of the oxygen reduction peak in the voltammograms varies linearly with the surface composition. Both AES and electrochemical analysis revealed that in Pd + Au alloy specimens treated by electrochemical oxidation-reduction cycles in 0.1 M NaOH within the potential range 0.05–1.50 V (vs. RHE) the original surface composition was maintained whereas a significant amount of surface Au enrichment took place on those specimens which were anodically polarized in 0.5 M H2SO4. AES analyses of the Pd+Ag and Pd+Cu alloy systems revealed that a weak degree (less than 0.1 in atomic fraction) of surface enrichment in Ag and Cu took place when they were treated by oxidation-reduction cycles in the potential ranges 0.05–1.0 and 0.05–0.50 V (vs. RHE), respectively, in 1.0 M NaOH.

Surface composition of binary alloys between Pd and Group IB metals as studied by Auger electron spectroscopy and cyclic voltammetry

The surface composition of alloys between Pd and Group IB metals (Pd+IB alloys) was investigated on the basis of Auger electron spectroscopy (AES) and cyclic voltammetry. A correction factor for the extinction of Auger peak intensities by surface contaminants, mainly carbon, in deducing the surface composition was determined first. The applicability of this method was substantiated in the case of Pd + Au alloys by the empirical rule in the electrochemical observation that the potential of the oxygen reduction peak in the voltammograms varies linearly with the surface composition. Both AES and electrochemical analysis revealed that in Pd + Au alloy specimens treated by electrochemical oxidation-reduction cycles in 0.1 M NaOH within the potential range 0.05–1.50 V (vs. RHE) the original surface composition was maintained whereas a significant amount of surface Au enrichment took place on those specimens which were anodically polarized in 0.5 M H 2SO 4. AES analyses of the Pd+Ag and Pd+Cu alloy systems revealed that a weak degree (less than 0.1 in atomic fraction) of surface enrichment in Ag and Cu took place when they were treated by oxidation-reduction cycles in the potential ranges 0.05–1.0 and 0.05–0.50 V (vs. RHE), respectively, in 1.0 M NaOH.

Proposition of an algorithm for calculating the surface composition from auger electron intensities

AbstractFollowing the practice of electron probe microanalysis, a universal computer program is proposed which calculates the surface composition of binary alloys from the measured Auger electron intensities. The program can be applied for the usual experimental procedures, i.e. the relative sensitivity factor approach, and the direct comparison with a standard. The calculations are made for two models of the surface region; firstly, a uniform composition within the sampling depth of Auger electrons, and secondly, surface enrichment limited to the first monolayer. The relation of the Auger electron intensity to the surface concentration is expressed in terms of three matrix dependent correcting factors; the backscattering factor, the inelastic mean free path and the total number of atoms in a unit volume. The backscattering factor is assumed to vary linearly with the mass fraction of a given element in the bulk. The corrections for inelastic mean free path and number of atoms per unit volume are calculated for the bulk composition or the surface composition depending on the model of the surface region. The EXTENDED FORTRAN program, in addition to calculating the surface composition, has other auxiliary facilities such as plotting the calculated results, and calculating the extent of surface segregation when the model of monolayer enrichment is not valid.

Summary Abstract: Effect of ion sputtering on the surface composition of binary alloys of tin, lead, and indium

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Surface composition of binary alloys

A review is given of our present understanding of the laws which determine the surface composition of alloys in equilibrium. While the basic principles are fairly simple, their application is hampered by lack of numerical data on surface tensions and experimental verification of models was often unreliable because of unjustified neglect of the large contribution from subsurface atoms to, in particular, the peak intensity in Auger electron spectra. For perfect and regular solid solutions of atoms of equal size the “broken bond model” predicts a surface enriched with the element of lowest heat of atomization, the enrichment being strongest for faces of high coordinative unsaturation. In the case of slightly endothermic monophasic alloys this is accompanied by a smooth concentration gradient over several subsurface layers. For exothermic ordered alloys surface enrichment is counteracted by the energy associated with putting atoms in positions of the wrong sublattice in the interior. The subsurface is depleted of the element enriched in the outermost layer. In all monophasic alloys lattice strain effects have to be considered if atom sizes differ significantly. If such alloys are diluted, the strain effects tend to segregate solute atoms in the surface, but this phenomenon seems to be more pronounced for oversized than for undersized atoms. Also the broken bond model favours a surface enrichment with large atoms. Enrichment is, therefore, very large for alloys such as NiSn where these effects work together. This seems to be confirmed by new experimental data using inelastic ion scattering besides more conventional methods. For biphasic alloys a favourable arrangement is the “cherry model” where the phase of lower surface energy envelops the other phase. For all alloys the surface composition can be very strongly altered by adsorbing molecules, the element with highest heat of adsorption segregating in the surface. A brief review of the modern experimental methods is given and the geometrical and electronic effects of surface composition on chemisorption and catalysis are illustrated with one example.

Calculation of the surface composition of binary alloys by the regular solution theory; analysis of parameters involved and surface relaxation effects

It is shown in this paper that calculated surface composition of binary alloys and more particularly of CuNi alloys, using the regular solution theory, is in good quantitative agreement with experimental values as obtained by Auger spectroscopy or selective chemisorption. For this agreement to be obtained, surface relaxation effects must be included and taken into account in the manner proposed in this paper.

Estimation of backscattering factor for low atomic number elements and their alloys

The single elastic scattering theory of Everhart was adapted to determine the angular and energy distribution of electrons backscattered from low atomic number solid materials ( Z < 40–45). This distribution and the classical ionization cross-section expression of Gryziński were used in the calculations of the backscattering factor, r, in quantitative AES analysis. The values of r were found to be in reasonable agreement with the results of the Monte Carlo calculations and the existing experimental data. Everhart's theory was extended for the case of mixtures, and that made possible the determination of the backscattering factor for binary alloys. It was found that neglecting the concentration dependence of r in quantitative AES analysis apparently enriches the surface with the component having the lower atomic number. The experimental data on surface composition of binary alloys measured by AES are discussed in view of the presented theory.

Abstract: Determination of the surface vs bulk composition of Ag/Au alloys by low‐energy ion‐scattering spectroscopy

Recently there has been considerable interest in measuring the surface composition of binary alloys. This interest has been generated by a variety of other phenomena (such as catalysis) which are related to the surface composition. Thermodynamic arguments predict that the surface of a multicomponent system will be enriched in the constituent which has the lowest surface free energy.1,2 However, due to a number of experimental difficulties, many of the measurements of the surface enrichment which have been made by Auger electron spectroscopy (AES) have led to confusing results. By using a technique such as low‐energy ion‐scattering spectroscopy (ISS) which has monolayer surface sensitivity, many of the problems encountered in the AES measurements are eliminated.This paper is a report on the measurement of the surface composition of two Ag/Au alloys by ISS. The measurements were made with 1500‐V 4He+ and 20Ne+ as the ion probes. The ion beam was rastered over a 16‐mm2 area to minimize the sputtering which o...

Binary alloy surface compositions from bulk alloy thermodynamic data

The surface composition of binary alloys is determined by minimizing alloy surface free energy with respect to atom exchange between the surface and the bulk. The theory is developed using a pairwise bound model of the solid with a broken bond surface. Parametric predictions for the atom fraction in the first four atomic layers for ideal and regular alloy solutions are presented. The predictions are based on pure element vaporization enthalpies and alloy solution activity coefficients. The surface atom fraction is different from the bulk atom fraction for all bulk compositions. The alloy element with the lower heat of vaporization is enriched with respect to its bulk atom fraction. The effects of chemisorption of a foreign species and alloy particle size on surface composition are also quantified. The theory is in good agreement with existing data on nickel-copper surface compositions.

Quantitative study of the surface composition of binary alloys by Auger spectroscopy

An internal calibration method for Auger spectra of binary alloys is proposed. It comprises the in situ breaking of pre-notched, homogenized, alloys rods in ultrahigh vacuum and the immediate analysis of the fracture surface. An investigation of a wide range of compositions of each of the alloys PtSn, AgAu, AuSn and AgSn shows that the observed signal intensity ratio is proportional to the corresponding atomic ratio of the elements present, which would suggest that matrix effects do not play an important role.