Copper-palladium alloy surfacesII. Equilibrium surface compositions of dilute Pd/Cu alloys from a simple segregation model (original) (raw)
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Structure of vicinal surfaces of alloys: Cu–Pd(17%)(115)
Surface Science, 2001
An even single step structure is observed by scanning tunneling microscopy for the Cu±Pd(17%)(1 1 5) surface in the chemically ordered phase. This is in contrast with the paired step structure of vicinals of (0 0 1) and (1 1 0) surfaces with higher average step±step distance of A 3 B L1 2 alloys. The three possible structures for vicinal surfaces involving steps and subsurface antiphase boundaries (APBs) are compared. The observation of a paired step structure for Cu± Pd(17%)(1 1 11) and of a single step structure for (1 1 5) allows estimating the subsurface APB energy.
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2000
In the present work Cu 3 Au(1 0 0) alloy surface has been investigated with low energy ion scattering at temperatures below and above the bulk order±disorder transition temperature. The trajectories of Ar low energy ions scattered from atomic chains on the surface of Cu 3 Au(1 0 0) along á1 0 0ñ and á1 1 0ñ directions were computer simulated. The dependencies of the energy of scattered ions E on the scattering angle h and corresponding angular distributions were calculated. It has been shown that a comparison of the angular and energy distributions of ions scattered from the surface of an alloy in the process of ordering with similar distributions for pure targets made up of the alloy components permits a conclusion that two or more neighboring atoms in an alternating sequence are of the same species. The results obtained can be used to study short-range order in alloys undergoing ordering. Ó
Photoelectron diffraction studies of Cu on Pd(111) random surface alloys
Physical Review B, 2005
The study of surface alloys is motivated by their use in many applications of different segments of industry, such as in the search for new catalysts and sensors, in surface protection against corrosion, in lowering friction, and in testing electronic devices. An important aspect of surface alloys studies is that of the precise quantification of segregation and diffusion processes as well as the determination of surface structure. In this paper we report a combined low-energy electron diffraction and photoelectron diffraction ͑PED͒ ͑using synchrotron radiation͒ study of surface alloy formation when Cu ultrathin films are evaporated onto Pd͑111͒ single-crystal surfaces. We present results for two different coverages ͑1 and 3 ML͒ and three annealing temperatures ͑300, 600, and 800 K͒. For these preparation conditions, a random alloy phase with different concentrations seems to form in the first few layers. Through the analysis of PED data performed using a multiple scattering formalism and the average T-matrix approximation it was possible to determine the atomic structure and the atomic concentration of the first three layers.
Surface segregation of Pd–Cu alloy in various gas atmospheres
International Journal of Hydrogen Energy, 2020
h i g h l i g h t s Surface segregation of PdeCu was studied in vacuum and various gas atmospheres. Cu content of the surface region was measured by LEISS and XPS respectively. Opposite surface segregation behaviours were observed in vacuum and in H 2 or CO. Effect of gas adsorption on surface segregation were theoretically discussed.
The Journal of Physical Chemistry C, 2009
The microscopic details of how metals alloy have important consequences for both their material properties and their chemical reactivity. In this study, the initial stages of alloying of Pd and Ag with Cu(111) are compared. Low-temperature scanning tunneling microscopy reveals that physical vapor deposition of Pd and Ag at or above room temperature yields remarkably different surface alloys: Pd predominantly incorporates at the nearest ascending Cu step edge, whereas Ag appears to be able to traverse step edges rather easily and alloys into terraces both above and below its initial adsorption site. Density functional theory calculations reveal that even though Pd adatoms have a lower barrier than Ag for traversing step edges, unlike Ag they bind very strongly to ascending step edges and remain there permanently. This leads to a situation in which Pd atoms have at most a very small number of attempts to leave the terrace on which they are deposited before they are incorporated into the nearest ascending step edge. Ag adatoms, however, have many opportunities to cross step edges and can alloy at positions far from their initial starting point. This direct comparison demonstrates the importance in combining theory and experiment in order to understand complicated surface alloying mechanisms and illustrates how both the kinetics and the thermodynamics of the process must be considered to fully understand experimental observations.
Surface alloying of Pd on Cu(111)
Surface Science, 2004
A modeling analysis of the growth mode of submonolayer amounts of Pd on Cu(1 1 1) for different coverages and temperatures reproduces the known experimental behavior, including the formation of a surface alloy, the presence of subsurface Pd, formation of Pd bands off terrace steps, and weak short range surface ordering. An atom-by-atom analysis of the energetics using the BFS method for alloys provides a simple explanation of the underlying mechanisms leading to the observed behavior.
Pd on Cu(111) studied by photoelectron diffraction
Surface Science, 2002
The PdCu alloy system, which has important catalytic properties, has been the subject of many experimental and theoretical studies using a large number of different techniques. Theoretical and experimental structural studies converge in predicting ordered alloys for the Pd/Cu(1 1 0) and Pd/Cu(1 0 0) surfaces. No such agreement exists for the Pd/ Cu(1 1 1) surface, however; indeed, few structural studies have been performed for this surface. Here, we report the first application of X-ray photoelectron diffraction (XPD) (using synchrotron radiation) in combination with LEED to determine the structure of ultra-thin epitaxial Pd films ($1 ML) evaporated on Cu(1 1 1) single crystal surfaces. The analysis of the data was performed with the multiple scattering diffraction program of Chen and Van Hove. For the preparation condition used, a random surface alloy seems to form in the first three layers. The first interlayer distance expands whereas the second seems to contract. Ó : S 0 0 3 9 -6 0 2 8 ( 0 2 ) 0 1 0 9 7 -X
Diffuse scattering of electrons and lattice instabilities in a Cu-Pd alloy
Soviet Physics Journal, 1976
UDC 669. 112 The results of [1] indicate that by quenching a Cu alloy with 40 at.% Pd from the region above 600~ where the a phase is stable, down to room temperature the a-fl transformation can be suppressed (the aphase has a fcc lattice with a parameter a =3.730 A, and the ~ phase has a B2 structure with a parameter a =2.954 ,~). Thus, it is possible to study the a-phase structure on an electron microscope without heating the thin films on the microscope stage. It is important to note that the a phase is unstable, and the a-fi transformation can occur due to heating by the electron gun. An electron-microscope study of thin films [2] has shown that prolonged diffuse scattering effects appear in the electron diffraction patterns, but the geometry and the reason for the formation of diffuse scattering regions (DSR) have not been investigated. Consequently, the aim of the present paper is to determine the nature of the DSR and the reasons for their formation. An I~MV-100L microscope was used in the experiments.
Surface Science, 2007
In this work, surface segregation to Cu 3 Pt surfaces is studied with the modified embedded atom method (MEAM). This work is triggered by the catalytic importance of Cu-Pt alloys, together with the contradictory experimental results for the surface segregation in Cu 3 Pt alloys based on low energy ion scattering (LEIS) [Y.G. Shen, D.J. O'Connor, K. Wandelt, R.J. MacDonald, Surf. Sci. 328 (1995) 21] and low energy electron diffraction (LEED) [Y. Gauthier, A. Senhaji, B. Legrand, G. Tréglia, C. Becker, K. Wandelt, Surf. Sci. 527 (2003) 71].
Surface Science, 1987
The scattering of low-energy Na + ions (600 eV, ,9 = 50 o) off a Cu(ll0) surface along the [110] azimuthal direction shows a strong decrease of the intensity above 450 K. This observation cannot be explained by computer simulations using the regular lattice structure and, therefore, indicates a rearrangement of the surface. Experiments with Ne + ions (5 keV, 0 =164 ~ ) give a more direct picture of the geometrical surface structure. The results above 500 K suggest larger displacements of the surface atoms than expected from the extrapolation of the vibration amplitudes at lower temperatures. Additional structures can be interpreted as the creation of adatoms. The intensity of inverse photoemission transitions is found to decrease with temperature stronger than expected from a simple Debye-Waller factor. Taking scattering by defects into account gives a good fit of the experimental data.