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.

Atomic-resolution observations of silver segregation in a [111] tilt grain boundary in copper

Alloying a material and hence segregating solutes to grain boundaries is one way to tailor a material to the demands of its application. Direct observation of solute segregation is necessary to understand how the interfacial properties are altered. In this study, we investigate the atomic structure of a high-angle grain boundary both in pure copper and upon silver segregation by aberration-corrected scanning transmission electron microscopy and spectroscopy. We further correlate the experiments to atomistic simulations to quantify the local solute excess and its impact on grain boundary properties. We observe that the grain boundary structure remains intact upon silver segregation and up to five different positions within a structural unit serve as segregation sites. By combining the atomic-resolution observation with atomistic modeling, we are able to quantify the local silver concentration and elucidate the underlying segregation mechanism.

Atom Probe Analysis of Nanoparticles Through Pick and Coat Sample Preparation.

The ability to analyze nanoparticles in the atom probe has often been limited by the complexity of the sample preparation. In this work, we present a method to lift–out single nanoparticles in the scanning electron microscope. First, nanoparticles are dispersed on a lacey carbon grid, then positioned on a sharp substrate tip and coated on all sides with a metallic matrix by physical vapor deposition. Compositional and structural insights are provided for spherical gold nanoparticles and a segregation of silver and copper in silver copper oxide nanorods is shown in 3D atom maps. Using the standard atom probe reconstruction algorithm, data quality is limited by typical standard reconstruction artifacts for heterogeneous specimens (trajectory aberrations) and the choice of suitable coatings for the particles. This approach can be applied to various unsupported free-standing nanoparticles, enables preselection of particles via correlative techniques, and reliably produces well-defined structured samples. The only prerequisite is that the nanoparticles must be large enough to be manipulated, which was done for sizes down to ~50 nm.

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.

Sputter-Deposited Cr–Ag Films for Environmental Antimicrobial Applications

Chromium–silver films with ≤ 39.2 at.% Ag were deposited by magnetron sputtering for antibacterial applications. X-ray diffraction and scanning electron microscopy analyses indicate that the films consist of Cr and Ag, and silver segregation is not obvious at the surface. The films are hard (628–968 HV0.001) and hydrophobic with low surface energy (12.8–26.8 mJ/m2). The drying time of water droplets is dependent on ambient conditions, material wettability and droplet size. The test against Escherichia coli indicates antibacterial ratios of 100% for the Cr–Ag films (action time 3 h). However, bacteria died within 15 min due to quick drying of the bacterial suspension in open ambient conditions. The Cr–Ag films would have potential antimicrobial applications in public environmental facilities.

PdAu and PdAuAg composite membranes with reduced film thickness using YSZ as a stainless-steel support modifier

PdAu and PdAuAg alloy membranes were synthesized on top of YSZ-modified planar porous stainless steel supports by electroless deposition. A uniform, continuous and smooth coverage was observed after three YSZ deposition calcination cycles, with neither uncovered zones nor detachment after calcination at 673 K. Through this modification procedure, we could obtain thinner and more robust PdAu and PdAuAg alloy membranes. These membranes showed higher hydrogen permeance during the evaluations (623–723 K) and high thermal stability during the long-term stability test. The PdAu and PdAuAg membranes exhibited a good performance during the exposure to CO/H2O steam mixture. Contrary to the PdAu membrane, where no significant change on the atomic surface composition was observed, silver segregation was evidenced on the PdAuAg membrane after exposure to CO/H2O steam mixture.

Microstructural Features and Properties of Al-Cu-Mg Alloy Containing Silver and Cerium

The article presents the results of an investigation of microstructural features and mechanical characteristics of Al-5.0Cu-0.5Mg alloy containing up to 0.4 wt. % Ag and up to 0.1 wt. % Ce. The experiment was conducted using optical microscopy, Scanning Electron Microscopy as well as an electron probe micro-analyzer and Differential Scanning Calorimetry. Samples in cast condition and after heat treatment were examined. The melting temperatures of non-equilibrium eutectics (non-equilibrium solidus), equilibrium solidus and liquidus were determined. The optimal temperature of the homogenizing heat treatment was determined, which was 500°C. Using this heat treatment mode resulted in the elimination of dendritic segregation and complete dissolution of silver in aluminum. Injection of cerium into the Al-Cu-Mg-Ag system during crystallization of the melt is accompanied by the formation of a coarse four-component phase, which has the morphology of polyhedrons, is on the grain boundaries. The estimation of the relation between microstructure characteristics and mechanical properties of the alloy has been made.

Pd3Ag(111) as a Model System for Hydrogen Separation Membranes: Combined Effects of CO Adsorption and Surface Termination on the Activation of Molecular Hydrogen

The co-adsorption of hydrogen and carbon monoxide on Pd3Ag(111) alloy surfaces has been studied as a model system for Pd-Ag alloys in membrane and catalysis applications using periodic density functional theory calculations (PW91-GGA). We explored the effects of Pd–Ag surface composition, since segregation of silver towards and away from the surface has been suggested to explain the experimentally observed changes in H2 activation, CO inhibition and reactivity. We found that CO pre-adsorbed on the surface weakens the adsorption of H on Pd3Ag(111) alloy surfaces irrespective of whether the surface termination corresponds to the bulk Pd3Ag composition, or is purely Pd-terminated. A higher coverage of H with CO present is obtained for the Pd-terminated surface; this surface also exhibits a larger range of chemical potentials for co-adsorbed hydrogen and CO. The barrier for H2 activation increases with increasing CO coverage, but the surface composition has the largest impact on H2 activation at intermediate CO coverage. The results imply that Pd-based membranes with typically ~ 23 wt% Ag are less prone to CO poisoning if the surface becomes Pd-terminated.

New nanomaterials and nanoarchitecture of oxygen membranes for clean energy

A nanocermet-based oxygen disk membrane 60δ-Bi2O3/40(60Ag@40P), (P = La0.5Y0.3Pr0.2Cr0.5Fe0.3Mn0.2O3) that has passed the total degradation test at 500 °C for 500 h requires further optimization due to silver segregation, which is accelerated by the presence of a Pd catalyst in the contact. The solution to the problem of recrystallization and segregation of silver is based on optimizing the architecture of the membrane and replacing silver with a smart composite. A thin protective porous layer of mixed conductive ceramic is introduced between the catalytic and selective layers in the membrane architecture. As a smart composite that simultaneously performs various functions at different stages of the creation and operation of the membrane, a dual-core nanocomposite of the core-shell type is proposed using a homogeneous AgCu-alloy with a lower melting temperature as a matrix: AgCux@y(CuMn2)z(P*), where P* = La0.7Pr0.2Tb0.1Mn0.4Co0.3Ni0.3O3.

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.

Fabrication, characterization and catalytic activity measurements of nano-crystalline Ag-Cr-O catalysts

This research paper describes the preparation, characterization and the hydrogen peroxide decomposition activity of a series of nano-crystalline Ag-Cr-O catalysts obtained by calcining a parent mixture of silver acetate and ammonium dichromate. This parent has been calcined for 1 h, in air, at the temperature range of 200–700 °C. The characterization of the nano-crystalline Ag-Cr-O catalysts was performed using a series of physico-chemical analyses including TGA-DTA, XRD, FTIR, SEM, TEM, and XPS. The results revealed that heat treatment brought about a significant-phase formation, Ag2Cr2O7 is formed at 200 °C and a mixture of Ag2CrO4 and Cr2O3 are formed over the calcination temperature range of 300–500 °C. AgCrO2 stats to form at 500 °C and becomes a major phase at the range 600–700 °C with a noticeable segregation of metallic silver. The activity of the different nano-crystalline Ag-Cr-O catalysts was tested for hydrogen peroxide decomposition at the temperature range 35–50 °C. The kinetic data showed a direct dependence on the calcination temperature and it was correlated with the formed surface redox-couples.

Interphase boundary segregation of silver and enhanced precipitation of Mg17Al12 Phase in a Mg-Al-Sn-Ag alloy

Aging-hardening response of Mg-7Al-2Sn (wt%) alloy can be remarkably accelerated by a small addition of silver (Ag). Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with high resolution energy dispersive X-ray spectroscopy (EDS) mapping, for the first time, segregation of solute atoms (Ag in this case) was observed at the interphase boundary between continuous Mg17Al12 precipitate and magnesium matrix in Mg-7Al-2Sn alloy with 0.7 wt% Ag addition. Substantial size refinement and increased number density of Mg17Al12 precipitates may be responsible for the enhanced aging-hardening response in Mg-7Al-2Sn alloy with Ag addition.

Tungsten film as a hard and compatible carrier for antibacterial agent of silver

Silver-containing tungsten (W–Ag) films for antibacterial applications were deposited on glass, silicon, and 316L stainless-steel substrates by magnetron sputtering with the silver target current of 0–2.0 A. The addition of silver improves adhesion of the films on glass substrate due to the reduced residual stress in the films. SEM and EDX analyses reveal Ag-rich tiny dots (~ 20 nm) at the surface of W–Ag films with high silver contents. In XRD patterns, silver peaks are present for the samples deposited at 1.5 and 2.0 A, and tungsten grain size is decreased from ~ 23 to 10 nm by silver addition. XPS analysis shows that tungsten is slightly oxidized (WO3) at the top surface of the film, and silver presents mainly in metallic state. The low Ag/W ratios and the small surface roughness (< 8 nm) indicate that silver segregation at the film surface is not obvious. Microhardness of the samples with ≤ 6.7 at.% silver is nearly seven times that of the stainless steel (~ 250 HV). The coated samples are hydrophobic tested by contact angle measurement. The potentiodynamic polarization and the soaking test simulating the inflammatory state show that even corrosion occurs and silver addition decreases corrosion resistance of the films. The antibacterial ratio of the coated samples increases with silver content, being 91% at 4.2 at.% silver content tested by agar plate counting method. In agar disk diffusion assay, no inhibition zone is observed for all samples. The antibacterial property of the W–Ag films is localized, long-lasting, and reusable, which would be beneficial for their potential biomedical and environmental applications.

Effects of silver segregation on sputter deposited antibacterial silver-containing diamond-like carbon films

Silver-containing diamond-like carbon (DLC) films for antibacterial applications were prepared on 316L stainless steel and slide glass substrates by magnetron sputtering at different silver target current. Scanning electron microscopy and atomic force microscopy analyses show that the film surface becomes rough due to the formation of nanoparticles/clusters and even a surface layer with the increase of silver target current. The Ag content was measured as 0, 2.1, 6.1 and 14.3 at.%, respectively by energy dispersive X-ray analysis. X-ray diffraction and X-ray photoelectron spectroscopy reveal the dominant metallic nature of silver for the Ag-containing films. Silver segregation from the carbon matrix to film surface is confirmed by the above analyses. Microhardness indentation, contact angle measurement and potentiodynamic polarization test in a Ca-free Hank's balanced salt solution indicate that the addition of silver has decreased hardness, elastic modulus, H/E and H3/E2 ratios, wettability, surface energy and corrosion resistance of the DLC films, respectively. The antibacterial tests by agar disk diffusion assay and agar plate counting method against Escherichia coli demonstrate long-lasting and reusable antibacterial activity of the Ag-containing films. The study shows that silver segregation plays a crucial role in microstructure, mechanical, chemical and biological properties of the antibacterial Ag-containing DLC films.

A first principles study on the electronic origins of silver segregation at the Ag-Au (111) surface

The special electronic structure of gold gives rise to many interesting phenomena, such as its color. The surface segregation of the silver-gold system has been the subject of numerous experimental and theoretical studies, yielding conflicting results ranging from strong Ag surface enrichment to Au surface segregation. Via a combined approach of density functional theory (DFT) and statistical physics, we have analyzed the segregation at the Ag-Au (111) surface with different Ag bulk concentrations. Interestingly, we observe a moderate Au surface segregation, which is due to a charge transfer from the less electronegative Ag to Au. Canonical Monte Carlo simulations suggest that the calculated concentration profile with a Au-enriched surface layer remains stable up to higher temperatures. However, the presence of adsorbed oxygen reverses the segregation behavior and leads to strong Ag enrichment of the surface layer.

How silver segregation stabilizes 1D surface gold oxide: a cluster expansion study combined with ab initio MD simulations

Gold has many unique properties, some of which continue to be uncovered, such as the rich chemistry of gold at the nanoscale. In this study, gold surprises us again by the unusual stability of one-dimensional gold oxide structures on the surface of gold, which enhances in the presence of silver impurities. We employ first-principles calculations to investigate the surface segregation of silver in the presence of atomic-oxygen adsorbates arranged in chains on the Au(321) surface. Such 1D oxide chains have previously been suggested as the most stable form of adsorbed oxygen on gold. Although Ag-O bonds are expected to be generally stronger than Au-O bonds, we show that this does not hold for 1D oxide chains, where Au-O bonds seem to be at least as strong as Ag-O bonds. Remarkably, we find that up to very high surface concentrations of silver, the Ag atoms do not occupy positions within the oxide chain, but prefer locations next to it. Ab initio molecular dynamics simulations support this picture and reveal how oxide chains and silver atoms rearrange on the surface toward a lower-energy configuration.

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.

Evidence for a diamondlike electronic band structure of Si multilayers on Ag(111)

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a $s{p}^{3}$ diamondlike structure. This refutes the interpretation of silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers.

Chemical and structural characterization of Zr[sbnd]C[sbnd]N[sbnd]Ag coatings: XPS, XRD and Raman spectroscopy

In this report ZrCNAg coatings produced by dual magnetron sputtering are characterized by X-ray photoelectron spectroscopy (XPS) and complemented by X-ray diffraction, scanning electron microscopy, electron-probe microanalysis and Raman spectroscopy, in order to determine the chemical bonds and phases formed during the production process. The results demonstrate the possibility of producing coatings with different silver content (0–16at.%), in which zirconium carbonitrides (70–95at.%), amorphous carbon nitride phases (0–16at.%) and residual zirconium oxide phases (∼5at.%) coexists with the metallic silver. The profile analysis evidenced a highly oxidized surface due to the affinity of Zr to form ZrO2, and silver segregation to the surface. The composition after Ar+ bombardment revealed the contribution of silver clusters and metallic silver, together with the mentioned phases that vary in content depending on the deposition parameters utilized. STEM images revealed silver nanoparticles distributed in the ZrCN matrix with sizes around 5–20nm, as well as silver surface agglomeration with sizes <80nm.

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.