Surface Segregation Of Silver Research Articles

Alloying bulk-immiscible metals at the nanoscale: An XPS/STM study of bimetallic Ag-Pt/HOPG nanoparticles

The intentional tailoring of surface structure in bimetallic nanoparticles is of high interest for academic research and industry, and many attempts have been made to improve the control over the distribution mode of the constituent elements which ranges from homogeneously mixed nanoalloys to core–shell nanostructures. However, some metal combinations exhibit a wide miscibility gap in the bulk, which nonetheless could be overcome under definite conditions at the nanoscale due to size effects. Here we present the detailed XPS/STM investigation of alloying capability of bulk-immiscible metals on the example of the Ag-Pt/HOPG bimetallic system prepared by thermal vacuum deposition. We found out that partial alloying of Ag and Pt occurs already at the preparation stage, and the following thermal annealing of the Ag-Pt/HOPG samples at 300–400 °C does not help to enhance the alloying degree owing to thermodynamically driven surface segregation of silver. Increasing the annealing temperature ruins the structure of Ag-Pt bimetallic nanoparticles due to silver sublimation, while even longer annealing time leads to partial phase separation. Our results demonstrate a limited alloying capability of Pt and Ag at the nanoscale which will be relevant for further research on the design and functioning of bimetallic catalysts containing bulk-immiscible metals.

Influence of chemical vapor precursor flow rate on the structure and thermal stability of phase-modulated VSiCN/Ag coatings

Ceramic/silver coatings are of interest for tribological and antimicrobial applications. Here, the amorphous phase volume fraction in VSiCN/Ag coatings was varied and the microstructure, mechanical properties, and vacuum annealing behavior of the phase-modulated VSiCN/Ag coatings were compared with columnar VN/Ag coatings. The coatings were deposited by filament-assisted reactive magnetron sputtering in a gas mixture of argon, nitrogen, and hexamethyldisilazane (HMDS). HMDS vapor flow rates were varied to influence coating amorphous phase content (0 at. % ≤ Si ≤ 10 at. %, 0 at. % ≤ C ≤ 20 at. %), and the sputter power supplied to a V58Ag42 target was manipulated to control metal content (3 at. % ≤ Ag ≤ 8 at. %). Silver surface segregation did not occur during coating growth, nor immediately after deposition. VN/Ag coatings exhibited columnar growth structures with >100 nm B1-VNx grains and 10−50 nm silver inclusions, which preferentially decorated the column boundaries. Columnar growth was completely disrupted in the VSiCN/Ag coatings and discrete silver inclusions were not observed within the microstructure. The hardness of all coatings was similar (12.3–16.5 GPa), while the apparent Young's modulus decreased from 247 to 169 GPa with increasing amorphous phase content. Rockwell indentation tests revealed that coating adhesion began to suffer after deposition at higher HMDS flow rates. Silver surface segregation was observed on all coatings after vacuum annealing at 550 °C for 1 h. The size of the surface particles ranged between 50 and 200 nm for the VN/Ag coating but was below 20 nm for the VSiCN/Ag coatings. Silver segregation occurred preferentially at coating growth defects in the columnar VN/Ag coating while the particle distribution was uniform across the surface of the VSiCN/Ag coatings. The results of this study demonstrate that phase modulation of ceramic/silver coatings may enable high-diffusivity microstructural features to be tailored for improved thermal stability and control of silver surface segregation at elevated temperatures.

Stress effect on segregation and ordering in Pt–Ag nanoalloys

We performed a theoretical study of the chemical ordering and surface segregation of Pt–Ag nanoalloys in the range of size from 976 to 9879 atoms (3.12 to 6.76 nm). We used an original many-body potential able to stabilize the L11 ordered phase at equiconcentration leading to a strong silver surface segregation. Based on a recent experimental study where nanoparticles up to 2.5 nm have been characterized by high transmission electron microscopy with the L11 ordered phase in the core and a silver surface shell, we predict in our model via Monte Carlo simulations that the lower energy configuration is more complicated with a three-shell alternance of Ag/Pt/Ag from the surface surrounding the L11 ordered phase in the core. The stress analysis demonstrates that this structure softens the local stress distribution inside the nanoparticle which contributes to reduce the internal energy.

Tight-binding Ising modeling of the interplay between bulk ordering and surface segregation in Pt-Ag nanoalloys

We use a Tight-Binding Ising Model (TBIM) with effective pair interactions depending on concentration able to reproduce the bulk phase diagram of Pt-Ag system and in particular its strong asymmetry as a function of composition. This system presents both an L11 ordered structure around equiconcentration and a wide miscibility gap on the Pt-rich side. Using Monte Carlo simulations in canonical and semi-grand canonical ensembles, we succeed in reproducing qualitatively the bulk phase diagram and we study the equilibrium configurations of the (111) and (100) surfaces, and truncated octahedron clusters. The infinite surfaces reproduce sensibly the bulk phase diagram except that they present a strong silver surface segregation limited to the top surface layer. The clusters behave differently due to the reduction of the miscibility gap when decreasing the cluster size and present two typical ordered phases related to the L11 phase: one characterized by pure concentric atomic layers starting from silver surface layer which extends up to the center of the cluster, and the other one at higher silver concentration where the L11 phase is perfectly oriented along one (111) direction of the cluster. The later one has already been reported by recent experiments [1].

Study on the surface segregation evolution of Ag-TiOx nanocomposite coatings

The objective of this study is to systematically investigate the evolution of silver surface segregation of silver/titanium oxide nanocomposite coatings produced by reactive sputtering process. Different thickness barrier layers were deposited on the surface of Ag-TiOx two-dimensional (2D) and three-dimensional (3D) composite coatings at different deposition rates. In 2D mode, the SEM images and XPS patterns clearly show that the still strong silver segregation occurred even 50 nm thickness barrier coated with very low deposition rate (3 nm/min), while a sharp decrease of silver surface segregation can be achieved when high barrier deposition rate (20 nm/min) was applied. The high angle annular dark field (HAADF) STEM images reveal that the high silver mobility can be effectively suppressed under high barrier deposition rate. Similar phenomenon can be observed in 3D Ag-TiOx composite coatings. Moreover, the comparable study of the metallic titanium barrier layers show that a 30 nm barrier layer is enough to cover all the silver NPs, and silver atoms diffusion onto the surface was stopped under pure Ar atmosphere. The silver surface segregation evolution indicates that silver segregation could be the result of the combined effect of sputtering plasma and activated oxygen environment.

Hydrogen permeation and surface properties of PdAu and PdAgAu membranes in the presence of CO, CO2 and H2S

Using the electroless deposition technique with different deposition sequences, it was possible to obtain a single phase PdAgAu ternary alloy after treatment at 773 K during 7 days. Despite the order of deposition, the samples exhibited similar hydrogen permeation flux at the same temperature and pressure. In the presence of CO, the PdAgAu ternary alloy membranes showed a lower inhibition on the hydrogen permeance compared with the binary PdAu alloy at 623 and 673 K. On the contrary, under CO2 containing streams, the ternary alloy showed a higher inhibition with a complete hydrogen recovery after removing CO2 from the stream. When H2S was introduced in the 13% CO2/13% Ar/74% H2 stream, a higher permeance decrease was observed on both the PdAu and PdAgAu alloy membranes. After removal of H2S, a hydrogen recovery of 60% and 76% was measured for the PdAu and PdAgAu membranes, respectively. Pd surface segregation was observed on the Pd82Ag15Au3–873 K-5 d and the Pd78Au6Ag16–773 K-7 d samples upon exposure to CO containing streams. Instead, silver surface segregation was observed by XPS after exposing the PdAgAu ternary alloy to CO2 containing streams.

Core/shell formation and surface segregation of multi shell icosahedral silver-palladium bimetallic nanostructures: A dynamic and thermodynamic study

Core/shell formation and surface segregation of multi shell icosahedral bimetallic silver-palladium nanostructures with the size of 55 and 147 atoms were studied by using the Molecular Dynamics simulations, and calculating Helmholtz free energy changes in the penetration of palladium atoms from shell to core, core to shell transition of silver and melting temperatures by using statistical mechanical densities of states. In 55 atoms icosahedra, two core–shell motifs, Ag13Pd42 and Pd13Ag42 with their isomers Pd13(Pd29Ag13) and Ag13(Ag29Pd13) were considered. Similarly in 147 atoms icosahedra, all mutations corresponding to the occupations of either silver atoms or palladium atoms in the core, inner shell or outer shell and their isomers generated by interchanging thirteen core atoms with thirteen atoms of the other type in the inner and outer shells were considered. It is found that the palladium-core clusters are more stable than the silver-core clusters and cohesive energy increases with the palladium composition. Phase transition of each cluster was studied by means of constant volume heat capacity. The trend in variation of melting temperature is accordance with the energy trend. Helmholtz free energy changes in palladium penetration, core to shell transition of silver and in surface mixing and segregation revealed the thermodynamic stability of the formation of PdcoreAgshell structures especially at silver rich environment and the surface segregation of silver.

Silver surface segregation in Ag-DLC nanocomposite coatings

AgDLC coatings were deposited by magnetron sputtering in order to evaluate: i) the Ag nanoparticle size distribution along the coatings thickness and ii) the silver stability in DLC coatings. Four different coatings were deposited, two AgDLC nanocomposite coatings containing 20at.% of Ag, with thicknesses of 250nm and 1000nm, a 250nm AgDLC nanocomposite coating with carbon barrier layer of 75nm and a multilayer coating, consisting of a pure Ag layer with a carbon barrier layer of 75nm. The coatings were characterized with respect to their structure (by means of X-ray diffraction (XRD)), morphology (by scanning electron microscopy (SEM)) and Ag distribution in depth (by glow discharge optical emission spectroscopy (GDOES)). The AgDLC nanocomposite coatings are composed of Ag nanograins (with a size of about 3nm) dispersed in an amorphous carbon matrix. The SEM results suggest a bimodal size distribution along the coatings thickness, it was found that Ag forms nanoparticles and aggregates with sizes ranging from 14nm up to 52nm on the coating's surface, whereas in bulk, no Ag nanoparticles were visible by SEM. The monitoring of the Ag stability at room temperature conditions during a period of 6months showed that Ag segregates to the coatings surface, in the AgDLC nanocomposite coating with 1000nm forming Ag nanofibers, while the thinner AgDLC (250nm) as well as the multilayer (Ag+DLC and AgDLC+DLC) coatings were stable over time.

Controlling surface segregation of reactively sputtered Ag/TiOx nanocomposites

Reactive sputtering of silver/titania nanocomposites is potentially attractive for antibacterial, photocatalytic, and optical coatings. However, previous studies on such coatings showed strong segregation of silver towards the surface, which could lead to undesirable properties. Here we present results of our investigations of the mechanism of silver surface segregation and influence of sputter parameters on the morphology of reactively sputtered Ag/TiOx nanocomposites. It was observed that silver surface segregation depends on the silver filling factor of the composite and that it is largely independent of the oxygen partial pressure used in the sputtering process. At high total pressure even sputtering of pure TiOx barriers is—unlike non-reactive sputtering of TiOx—not sufficient to prevent the strong surface segregation effect. Surface segregation is only thoroughly suppressed by reduction of the total pressure during reactive sputtering, allowing the formation of a dense composite film. Based on this principle silver surface segregation can be further reduced by deposition of TiOx barriers with very high sputtering rates. Furthermore, the relation between the morphology of the nanocomposites and their optical properties is presented. Nanocomposites produced under these conditions show a controllable surface morphology for functional applications.

On the Promoting Role of Ag in Selective Hydrogenation Reactions over Pd−Ag Bimetallic Catalysts: A Theoretical Study

The surface structure of Pd−Ag alloy and its alteration in the presence of atomic hydrogen have been studied using density functional calculations on Pd1-xAgx(111) (x ≈ 0.2) models. In the absence of an adsorbate, silver atoms are found to segregate on the surface, in line with previous experimental observations under vacuum conditions. At equilibrium, the surface is predicted to expose mainly Ag atoms. Isolated Pd atoms incorporated in this Ag-rich layer appear to be slightly preferred over the Pd2 dimers. Increasing the coverage of adsorbed H atoms on the Pd−Ag substrate gradually suppresses surface segregation of silver, such that migration of all surface Ag atoms into the subsurface region becomes favorable at a H coverage of ∼0.25 ML. For the latter structures, with solely Pd atoms in the surface layer and Ag atoms in the subsurface layer, the propensity of H to be accommodated in interstitial sites below the surface layer essentially vanishes: subsurface H atoms are predicted to be energetically dr...

Equilibrium surface segregation of silver to the low-index surfaces of a copper single crystal

The equilibrium surface segregation of Ag to the low-index surfaces of Cu single crystals is presented. It is found that the close-packed surface of the (111) orientation allows a high interaction parameter resulting in a discontinuous transition in the surface concentration against temperature, as well as a step in the segregation kinetics. In contrast, the rougher surfaces of the (110) and (100) orientations yield smaller interaction parameter values and a smooth transition in surface coverage against temperature is observed. Copyright © 1999 John Wiley & Sons, Ltd.

Surface segregation in an Al–4.2 at% Ag alloy

The equilibrium surface composition of a polycrystalline Al–4.2 at% Ag alloy has been measured as a function of temperature using Auger electron spectroscopy. These results indicate that silver segregates to surfaces with an apparent enthalpy of segregation of about −38.5 kJ/mol. The equilibrium average surface composition of the alloy has been calculated as a function of surface orientation using the Lee–Aaronson discrete lattice plane model. A modified regular solution model originally due to Alexander et al. was used to represent the interactions between Al and Ag atoms. While this model successfully described the relevant portion of the Al–Ag phase diagram and the faceting behavior of GP zones in Al rich Al–Ag alloys, it predicted no surface segregation of Ag in the present studies. These results are in contrast to the results obtained experimentally. Possible reasons for the lack of agreement are considered. Finally, the surface segregation of silver measured by AES was compared with that predicted by the Gibbs adsorption isotherm, wherein the latter calculation made use of the results of ultra high vacuum measurements of the bulk composition dependence of the surface tension of α Al–Ag alloys (to be reported elsewhere). Qualitative agreement was found between the two experimental methods, both indicating enrichment of Ag at the free surfaces.

The influence of substrate temperature, substrate material and heat treatment on the micro structure of Ag/Si nanocomposite films prepared by r.f. co-sputtering

Nanocomposite films consisting of silver and silicon were manufactured by r.f. co-sputtering deposition. The influence of substrate temperatures varying from 20 to 200 °C and different substrate materials such as cleaved NaCl crystals, C-coated Cu grids and (100) Si single-crystal wafers was investigated. Subsequent heat treatment at different temperatures ranging from 500 to 800 °C and its effect on the microstructure of the deposited films was also investigated by using a combination of differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy, conventional transmission electron microscopy and high-resolution electron microscopy. The typical microstructure of the as-deposited film consisted of nanosized Ag particles embedded in an amorphous Si matrix. The crystallization temperature of the amorphous Si matrix was reduced from 550 to 490 °C with increasing Ag content in the films. The microstructure of the annealed Ag/Si film after crystallization consists of Si crystallites with sizes from several nanometres to hundreds of nanometres and Ag particles with average sizes from 12 nm (600 °C) to 28 nm (800 °C). Surface segregation of silver was also observed for the annealing temperatures of 600 °C and above. We suggest a possible explanation for the silver-induced lowering of the amorphous Si matrix crystallization temperature.

Surface segregation study of Ib-VIII single-crystal alloys

Surface segregation of AgPd, CuPd and CuRh single crystals was studied by AES. The samples were either massive alloys or epitaxial thin films deposited on MgO. A weak surface segregation of silver was detected in AgPd alloys while the surfaces of CuPd and CuRh alloys were enriched in copper. The diffusion of copper and silver to the surface during heat treatments can be easily shown by looking at the behaviour of the Auger low-energy peaks. Our results were compared with previously published data and with theoretical models. The element with the lower surface energy segregates but not as strongly as suggested by the ideal solution model.

Growth and properties of ultrathin silver layers on the Cu(111) face—the influence of the substrate temperature

Kinetics of Auger peak heights for Cu-M 2,3VV (62 eV) and Ag-MNN (360 eV) transition are presented for silver adsorption on the Cu(111) face performed at temperatures of 330–795 K and for annealing of 6 ML and 2.5 ML of silver at higher and higher temperatures. The shape of those kinetics is interpreted as a result of a competition between silver adsorption and dissolution of the deposited layer in the sample bulk according to the theoretical predictions concerning the silver surface segregation. The existence of 2 ML → 1 ML and 1 ML → O ML transitions predicted theoretically is experimentally confirmed. Inelastic mean free path for Cu-M 2,3VV Auger electrons λ = 0.48-0.50 nm is experimentally found. Arguments are found that the silver layer growth at 330 K is not the pure Frank-van der Merwe one.

Surface segregation induced by chemisorption at the alloy/solution interface

Au-Ag vacuum-deposited alloy films with a small content of gold (5–10 at %) were found to exhibit unusual potentiodynamic behaviour in alkaline media. The changes observed in the I- E profiles during the first 10–12 potential scans preceding the attainment of the stationary shape were assigned to a phenomenon of surface segregation of silver induced by the chemisorption of oxygen atoms, resulting in a HO - discharge step.

An X-Ray Photoelectron Spectroscopic Study of the Free Surface Segregation of Impurities in Platinum and Palladium

X-ray photoelectron spectroscopy has been used to observe the free surface segregation of impurities in samples of platinum and palladium. Heating in vacuo to 600°C promoted the surface segregation of silver, gold, and carbon in platinum and the surface segregation of silver and sulfur in palladium. All species segregated in their elemental form. The core level binding energies of the silver and gold segregants were lower than those measured for the corresponding pure metal states.

The variation of silver content in native gold particles as revealed by ESCA

Depth profiling analysis of native gold samples using X-ray photoelectron spectroscopy (XPS) shows the surface layers of lode gold to be significantly enriched with silver to a depth of 0.02–0.04 microns and the surface layers of placer gold to be depleted of silver up to a depth of several microns. The change in the surface composition of gold samples taken from oxidation zones is of more complicated character. Model experiments suggest the surface segregation of silver in lode gold samples to be due to outward diffusion and oxidation of silver at the surface. In placer gold the depletion of silver is caused by leaching of the latter from the surface layers.

The surface composition of the silver—Gold system by Auger electron spectroscopy

The surface composition of polycrystalline AgAu alloy foils is studied by Auger electron spectroscopy. The intensities of principal Auger peaks over a wide range of electron energies are obtained as a function of alloy bulk composition for the clean equilibrated surfaces. Their ratios are compared with ratios expected for a surface showing no surface segregation, and for a surface obeying the regular solution monolayer model that predicts silver surface segregation. The experimental results give evidence for enrichment of the surface with silver, but to an extent less than predicted by the regular solution model.