Surface Segregation In Alloys Research Articles

Trends of Pd3Au(111) Alloy Surface Segregation in Oxygen, Carbon, and Nitrogen Environments

Trends of Pd₃Au(111) Alloy Surface Segregation in Oxygen, Carbon, and Nitrogen Environments

Modelling of metal nanoparticles’ structures and dynamics under reaction conditions

Metal nanoparticles (NPs) are widely used in heterogeneous catalysis. Their performance in catalytic reactions is closely related to the shape, surface structure, and composition. In the past two decades, a number of in-situ experiments have observed that under real reaction conditions, the structures of metal NPs are not static, but undergo remarkable dynamic evolution due to the presence of gas molecules. Therefore, it is imminent to develop reliable theoretical models to provide accurate predictions and comprehensive understandings of the structure reconstructions and dynamic behaviors of catalysts in response to the different reactive environments. In this review, we summarize a series of progress and achievements made by first-principle-based theoretical models in analyzing the shape evolution of metal NPs and surface segregation of alloys under different gas conditions in recent years. We also discuss the understanding of the catalytic performance of NPs by considering the reaction-condition-dependent structures. In addition, the real-time dynamic simulation methods of catalysts under reaction conditions are introduced. The perspective of simulating the kinetic process of in situ structural change is provided at last.

Atomistic Mechanisms of Binary Alloy Surface Segregation from Nanoseconds to Seconds Using Accelerated Dynamics.

Although the equilibrium composition of many alloy surfaces is well understood, the rate of transient surface segregation during annealing is not known, despite its crucial effect on alloy corrosion and catalytic reactions occurring on overlapping timescales. In this work, CuNi bimetallic alloys representing (100) surface facets are annealed in vacuum using atomistic simulations to observe the effect of vacancy diffusion on surface separation. We employ multi-timescale methods to sample the early transient, intermediate, and equilibrium states of slab surfaces during the separation process, including standard MD as well as three methods to perform atomistic, long-time dynamics: parallel trajectory splicing (ParSplice), adaptive kinetic Monte Carlo (AKMC), and kinetic Monte Carlo (KMC). From nanosecond (ns) to second timescales, our multiscale computational methodology can observe rare stochastic events not typically seen with standard MD, closing the gap between computational and experimental timescales for surface segregation. Rapid diffusion of a vacancy to the slab is resolved by all four methods in tens of nanoseconds. Stochastic re-entry of vacancies into the subsurface, however, is only seen on the microsecond timescale in the two KMC methods. Kinetic vacancy trapping on the surface and its effect on the segregation rate are discussed. The equilibrium composition profile of CuNi after segregation during annealing is estimated to occur on a timescale of seconds as determined by KMC, a result directly comparable to nanoscale experiments.

Improved Al-Mg alloy surface segregation predictions with a machine learning atomistic potential

Various industrial/commercial applications use Al-Mg alloys, yet the Mg added to Al materials, to improve strength, is susceptible to surface segregation and oxidation, leaving behind a softer and Al-enriched bulk alloy. To better understand this process and provide a systematic methodology for investigating dopants that can mitigate corrosion, we have developed a robust atomistic deep neural net potential (DNP) using a dataset generated with first-principles density-functional theory (DFT). The potential, validated systematically against DFT values, has been shown to have a high fidelity in calculating different elemental and intermetallic Al-Mg systems' properties. Our calculations predict a linear trend in the formation energy of the Al-Mg alloy and its density as a function of temperature, consistent with experimental literature. Employing the DNP within a hybrid Monte Carlo and molecular dynamics (MC/MD) approach, we predict anisotropic surface segregation for Al-Mg alloys such that (111)(100)(110), with (111) surfaces displaying the lowest segregation enthalpies and Mg enrichment. Furthermore, we model the segregation tendencies by adapting a recently introduced isotherm model for grain boundary segregation. Our results show that this model describes the MC/MD segregation profiles with higher fidelity than the McLean and Fowler-Guggenheim isotherm models.

Atomistic Mechanisms of Binary Alloy Surface Segregation From Nanoseconds to Seconds Using Accelerated Dynamics

Although the equilibrium composition of many alloy surfaces is well understood, the rate of transient surface segregation during annealing is not known, despite its crucial effect on alloy corrosion and catalytic reactions occurring on overlapping timescales. In this work, CuNi bimetallic alloys representing (100) surface facets are annealed in vacuum using atomistic simulations to observe the effect of vacancy diffusion on surface separation. We employ multi-timescale methods to sample the early transient, intermediate, and equilibrium states of slab surfaces during the separation process, including standard MD as well as three methods to perform atomistic, long-time dynamics: parallel trajectory splicing (ParSplice), adaptive kinetic Monte Carlo (AKMC), and kinetic Monte Carlo (KMC). From nanosecond (ns) to second timescales, our multiscale computational methodology can observe rare stochastic events not typically seen with standard MD, closing the gap between computational and experimental timescales for surface segregation. Rapid diffusion of a vacancy to the slab is resolved by all four methods in tens of ns. Stochastic re-entry of vacancies into the subsurface, however, is only seen on the microsecond timescale in the two KMC methods. Kinetic vacancy trapping on the surface and its effect on the segregation rate are discussed. The equilibrium composition profile of CuNi after segregation during annealing is estimated to occur on a timescale of seconds as determined by KMC, a result directly comparable to nanoscale experiments.

In-situ Atomic-scale Imaging of Surface Segregation in Alloys

In-situ Atomic-scale Imaging of Surface Segregation in Alloys

Surface Composition Evolution of Bimetallic Alloys under Reaction Conditions

Surface composition is a critical factor determining the chemical and physical properties of bimetallic alloys, which highly depends on the alloy surface segregation behaviors. Recent in situ exper...

NiMo alloy surface segregation induced by water adsorption

近年来人们发现纳米尺度下金属/合金颗粒结构与性质很容易受外界环境影响. 众多实验表明反应气体能够极大地影响合金纳米材料的表面偏析倾向, 并进而影响其表面化学组分. 但是另一方面, 地球上最普遍的环境之一的水环境对于合金纳米材料的影响却少有人问津. 在本工作中我们对NiMo合金在水分子、水团簇吸附情况下的表面偏析现象进行了第一性原理计算与蒙特卡洛模拟两方面的研究. 通过第一性原理计算发现水分子、水团簇吸附都对NiMo合金偏析能有显著改变, 并有可能改变其表面偏析倾向. 通过进一步的蒙特卡洛模拟发现水环境对NiMo合金表面组分有着不可忽视的影响. 通过这个工作我们希望人们能够重新审视水环境对合金偏析现象的影响. 同时本工作对正确理解水-固界面相互作用和从微观角度理解合金腐蚀现象有着深远意义.

Surface segregation of AuCu3 by He+ and Au+ irradiation

Surface segregation is a significant phenomenon due to its influence on many surface processes, such as corrosion, oxidation and catalysis. Defects and vacancies produced by ion irradiation in alloys used in reactors or other radiation environments may also induce surface segregation. In this work, we deposit AuCu3 film on a Si(111) substrate by magnetic sputtering. He+ and Au+ produced by pelletron are used to simulate radiation fields in reactors, and surface segregation induced by ion irradiation is investigated. SRIM software is used to simulate ion range and displacements produced in sample. Rutherford backscattering spectrometry is used to determine concentration changes near the surface of sample before and after irradiation. The results show that two kinds of ion irradiations lead to different surface segregation trends. When irradiated by 2 MeV He+, Au elements are segregated at the surface of sample. Oppositely, when irradiated by 1 MeV Au+, Cu elements are observed at the surface of sample. After analysis and discussion, we consider that this phenomenon is induced by different vacancy distributions by He+ and Au+ irradiation. 2 MeV He+ produced Au and Cu vacancies are distributed in whole film from surface to substrate smoothly, except very near the surface the concentration of vacancies has an obvious reduction. As a result, a gradient of the vacancy concentration is formed between the surface and the interior of the film. As the concentration of vacancies on the surface is lower than in interior, it would lead to vacancy diffusion from interior to surface, equivalent to diffusions of Cu and Au atoms along the opposite directions. Because of lighter atomic mass, Cu atom has a faster diffusion rate than Au atom. As a result, the concentration of Au atoms near the surface increases. Unlike He+, Au+ produces a mass of vacancies near the surface of the film, consistent with the Bragg peak by energy deposition of Au+, but decreases rapidly inside the film. It leads to a gradient of the vacancy concentration from surface to interior of the film. When vacancies diffuse from surface to interior, Cu and Au atoms diffuse from interior to surface, the lighter Cu atom concentration increases faster than Au atom concentration. Our research results explain the different segregation trends by light ion with higher energy and heavy ion with lower energy. It may help to understand the surface segregation of alloys used in complex irradiation field.

Density Functional Theory Study of Pt3M Alloy Surface Segregation with Adsorbed O/OH and Pt3Os as Catalysts for Oxygen Reduction Reaction

Using quantum mechanics calculations, we have studied the segregation energy with adsorbed O and OH for 28 Pt3M alloys, where M is a transition metal. The calculations found surface segregation to become energetically unfavorable for Pt3Co and Pt3Ni, as well as for the most other Pt binary alloys, in the presence of adsorbed O and OH. However, Pt3Os and Pt3Ir remain surface segregated and show the best energy preference among the alloys studied for both adsorbed species on the surface. Binding energies of various oxygen reduction reaction (ORR) intermediates on the Pt(111) and Pt3Os(111) surfaces were calculated and analyzed. Energy barriers for different ORR steps were computed for Pt and Pt3Os catalysts, and the rate-determining steps (RDS) were identified. It turns out that the RDS barrier for the Pt3Os alloy catalyst is lower than the corresponding barrier for pure Pt. This result allows us to predict a better ORR performance of Pt3Os compared to that of pure Pt.

Ab initio based multiscale modeling of alloy surface segregation

A fully integrated ab initio based multiscale model for analysis of segregation at alloy surfaces is presented. Major components of the model include a structure-energy analysis from the first-principles density functional theory (DFT), a Monte Carlo/molecular dynamics (MC/MD) hybrid simulation scheme for atomic transport, and a reactive force field formalism that binds the two. The multiscale model accurately describes the atomic transport processes in a multi-component alloy system at finite temperature, and is capable of providing quantitative predictions for surface compositions. The validity of the model was demonstrated by investigating the temperature-dependent segregation behavior of B2 FeAl binary alloy surfaces with a detailed description of the segregation mechanism. Based on the model’s prediction capabilities, potential extension of the model to the analysis of systems undergoing rapid chemical reactions is discussed.

High-Throughput Characterization of Surface Segregation in CuxPd1–x Alloys

A high throughput methodology for the study of surface segregation in alloys has been developed and applied to the Cu x Pd 1―x system. A novel offset-filament deposition tool was used to prepare Cu x Pd 1―x composition spread alloy films (CSAFs), high throughput sample libraries with continuous lateral composition variation spanning the range x = 0.05―0.95. Spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the films' top-surface and near-surface compositions, respectively, as functions of alloy composition, x, and temperature. Electron backscatter diffraction (EBSD) was used to identify the bulk phases in the CSAF as a function of alloy composition, x. Films equilibrated by annealing at temperatures ≥ 700 K displayed preferential segregation of Cu to their top-surfaces at all bulk compositions; segregation patterns did not, however, depend on local structure. The Langmuir―McLean thermodynamic model was applied to segregation measurements made in the temperature range 700―900 K in order to estimate the enthalpy (ΔH seg ) and entropy (ΔS seg ) of segregation as a function of bulk Cu x Pd 1―x composition. Segregation measurements at x = 0.30 on the CSAF compare well with results previously reported for a bulk, polycrystalline Cu 0.30 Pd 0.70 alloy, demonstrating the utility of the CSAF as a high throughput library for study of segregation.

The effect of adsorbed sulfur on surface segregation in a polycrystalline Pd70Cu30 alloy

Surface segregation in alloys can be influenced by the presence of adsorbed species. In this work, the effect of adsorbed sulfur on surface segregation in a Pd70Cu30 alloy was studied for sulfur coverages from zero through saturation and for temperatures from 400 to 1000K. X-ray photoemission spectroscopy (XPS) was used to determine the alloy composition in the near-surface region (∼7 atomic layers) and low energy ion scattering spectroscopy (LEISS) was used to probe the composition of the topmost atomic layer of the alloy. Surface segregation was observed to depend on both the presence of adsorbed sulfur and heat-treatment history. The near-surface region of the clean alloy was enriched in Pd relative to the bulk, but the topmost atomic layer was enriched in Cu. Adsorbed sulfur caused a reversal of the Cu enrichment of the topmost surface, resulting in a top layer that contained only Pd and S atoms. Segregation reversal may be driven by the formation of thermodynamically favored Pd–S bonds at the terminating surface of the alloy.

Time‐of‐flight atom‐probe field ion microscope studies of surface‐related phenomena

AbstractThe time‐of‐flight atom‐probe field ion microscope is both a single‐atom detection sensitivity mass‐to‐charge ratio analyser and an ion energy analyser, provided that the instrument resolution is good enough. Its major applications are in material analysis. However, it has been used also for studying surface‐related phenomena such as surface segregation of alloys, mechanisms of ion formation in field desorption, novel ion and cluster ion formation in laser‐stimulated field ion emission, field dissociation of compound ions and its novel isotope effects and analysis of the site‐specific binding energy of surface atoms, etc. Some of these studies are briefly reviewed here. Copyright © 2004 John Wiley & Sons, Ltd.

Anisotropy in thermal oscillations of CoNi3 surface atoms

The anisotropy of thermal oscillations of atoms in the outermost layers has been studied using LEED. For the first time, data is presented on lateral θ⟂s and longitudinal θ‖s components of the effective Debye temperature and mean square displacement (MSD) amplitudes (〈U⟂2〉 and 〈U‖2〉 are mean square displacement amplitudes of thermal oscillations in the direction normal and parallel to the surface, accordingly) for CoNi3 alloys. It is suggested, using the present study, that surface segregation in alloys strongly affects the oscillation anisotropy. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface and sub-surface segregation at the Pt 25Rh [formula omitted] surface: a medium energy ion scattering study

Using 100 keV incident He + ions, medium energy ion scattering (MEIS) has been used to determine the layer-by-layer composition of the outermost three atomic layers of a Pt 25Rh 75(1 1 1) crystal after annealing to temperatures in the range 900–1300 K. Highly layer-specific compositional information was obtained using three different double-alignment scattering geometries, although full analysis of the data included simulation of blocking curves recorded around the double alignment geometry. The results provide independent confirmation of the conclusions of an earlier quantitative low energy electron diffraction (LEED) investigation of the same crystal, notably strong Pt segregation to the outermost layer, enhanced average Pt segregation with increasing temperature, and Pt depletion of the second layer. MEIS also confirms the presence of a weak rumpling of the outermost atomic layer, with Pt atoms lying at slightly larger layer spacing than the surrounding Rh atoms. The quantitative agreement is significantly better than in the only previous comparison of LEED and MEIS for the determination of layer-dependent alloy surface segregation. Possible reasons for anomalously large surface vibrational amplitudes are discussed.

On the estimation of SRO effects on surface segregation

The free-energy concentration expansion method (FCEM) is introduced and applied to the elucidation of short-range order (SRO) effects on surface segregation in alloys. This statistical-mechanical analytical approach, based on the Ising model Hamiltonian energetics, is verified by Monte Carlo (MC) simulations. In particular, calculations performed for the (100) surface of fcc solid solution show that the FCEM agrees with MC results much better than the Bragg-Williams (BW) theory, including the prediction of an increase in equilibrium segregation level with temperature, associated with SRO/segregation competition. FCEM is quite accurate and simple to apply, demanding much less computational effort compared to MC simulations or the cluster variation method. It can replace the corresponding BW formulae in theoretical evaluation of various surfaces as well as bulk phenomena, allowing systematic studies of SRO effects in alloys.

Alloy surfaces: segregation, reconstruction and phase transitions

Surface segregation in alloys, i.e., concentration modulation in the surface selvedge at thermodynamical equilibrium, can be viewed as resulting from two kinds of competition or synergy. The first one is between surface and bulk interactions, i.e., surface energy versus bulk alloying interactions, whereas the second one is between these chemical forces and the atomic size-mismatch. Modelling the phenomenon then requires to account for all these forces on the same level. This leads to use both realistic energetic models derived from electronic structure and efficient statistical tools, from mean–field approximation to Monte Carlo simulations. A particular attention must be payed to the possible multisolution character of the problem. Actually, the possible coexistence of stable and metastable solutions may be at the origin of superficial phase transitions, in which the surface acts as a precursor of bulk order–disorder transitions: layering transitions, concentration profile transitions, surface induced order or disorder. All these phenomena are illustrated here on some specific systems and analyzed with tools derived from tight-binding formalism. Finally, the strong coupling between surface segregation and atomic reconstructions is illustrated in the case of strong size-mismatch by means of molecular dynamics calculations.

Surface segregation in multicomponent systems: Modeling of surface alloys and alloy surfaces

The study of surface segregation, though of great technological importance, has been largely restricted to experimental work due to limitations associated with theoretical methods. However, recent improvements in both first-principles and semiempirical methods are opening the doors to an array of new possibilities for surface scientists. We apply one of these techniques, the BFS method for alloys, which is particularly suitable for complex systems, to several aspects of the computational modeling of surfaces and segregation, including alloy surface segregation, structure and composition of alloy surfaces, and the formation of surface alloys. We conclude with the study of complex NiAl-based binary, ternary and quaternary thin films (with Ti, Cr and Cu additions to NiAl). Differences and similarities between bulk and surface compositions are discussed, illustrated by the results of Monte Carlo simulations. For some binary and ternary cases, the theoretical predictions are compared to experimental results, highlighting the accuracy and value of this developing theoretical tool.

LOCAL ORDER AT THE SURFACE OF BINARY ALLOYS IN RELATION TO THEIR CHEMICAL REACTIVITY

The behavior of binary alloys with respect to catalysis can be understood in terms of the geometric “ensemble effect” and/or the electronic “ligand effect.” For understanding, one has consequently to know precisely the local concentration and arrangement of both components at the very surface (in contact with the reactants), and also in the sublayers which influence electronically the outer atoms. Two kinds of well-defined surface are considered: binary alloys and well-controlled deposits of a given metal on a foreign metallic substrate. After a brief description of the driving forces for surface segregation and of metal-on-metal growth modes, the relations between surface composition/structure and reactivity of binary alloys are illustrated by some striking examples. In order to limit the discussion, the presentation is focused on platinum-based and palladium-based alloys. The ensemble effect is discussed first on the basis of the influence of inactive Bi, Pb or Sn deposits on Pt single crystal surfaces. For alloys, the PtCu3 (111), which exhibits a p (2×2) ordered surface structure in which the Pt surface atoms are all isolated by Cu atoms, offers a unique model sample for studying the dilution (of Pt by Cu) influence. The electronic influence of elements present in the sublayers is illustrated on PtNi (111) and Pt 3 Fe (111) samples, which present a quasicomplete Pt surface layer (with more or less Ni or Fe in the sublayers) and strong modifications of their chemisorptive properties and catalytic performances for some simple reactions. The properties of surfaces largely concentrated into Pd (either by large surface segregation in alloys containing low bulk Pd content, or by Pd deposition on various single crystal metal substrates) are discussed in the light of electronic modifications induced by the substrate atoms, with consequences on their chemical reactivity.