Composition Of Au Research Articles

Effect of Ag–Au composition and acid concentration on dealloying front velocity and cracking during nanoporous gold formation

Nanoporous gold has many potential applications in various fields, including energy storage, catalysis, sensing and actuating. Dealloying of Ag–Au alloys under free corrosion conditions is a simple method to fabricate nanoporous gold. Here, we systematically investigate the dealloying rate of Ag–xAu alloy for a range of alloy compositions (x=20–40at.%) and nitric acid concentration (7.3–14.9M) using in situ transmission X-ray microscopy. High-resolution in situ X-ray projections and ex situ tomographic reconstructions allow imaging of the dealloying front position during dealloying. The dealloying front velocity is constant with time, and depends exponentially on the alloy Ag/Au atomic ratio and the acid molar concentration. Only the leanest alloy, Ag–20 Au, shows a large macroscopic shrinkage in sample diameter (∼38%) after dealloying, which leads to crack nucleation and growth observed in real time during dealloying. Finite element modeling is used to estimate dealloying-induced stresses and strains, and sheds light on the cracks created by the diameter shrinkage.

Effect of current density on composition and structure of electrodeposited Au–Ni alloy coatings

The effect of current density on composition and structure of galvanic Au–Ni alloy coatings electrodeposited from weakly acidic additive free electrolyte was investigated. At low current density (up to ∼10 mA cm−2), light yellow coloured alloys enriched with Au are deposited. At higher current density the Ni content is abruptly increased and the coatings become coarse and dark. Deposits with an average content of Ni up to ∼50 at-% were obtained. Only a small amount of Ni (up to 20 at-%) takes part in the formation of non-equilibrium super saturated Au–Ni solid solution. The remainder of the Ni is deposited in the interglobular space as an amorphous Ni oxide–hydroxide or as Ni bearing hydroxide salts. Besides Ni, the alloy coatings contain C, N, O, K and H.

Preparation and Characterization of Dendrimer-Derived Bimetallic Ir–Au/Al2O3Catalysts for CO Oxidation

The synthesis of alumina supported Ir–Au bimetallic catalysts using the dendrimer–metal nanocomposite (DMN) approach is reported. The surfaces and nanostructures of four bimetallic DMN-derived samples with similar Ir and Au loading and composition were correlated with their activity for CO oxidation. Results were compared to a catalyst prepared by conventional incipient wetness impregnation of metal salts. Scanning transmission electron microscopy and H2 chemisorption data reveal that these catalysts have different metallic dispersions with various particle sizes and distributions, depending on the preparation method. Moreover, in situ transmission Fourier transform infrared spectroscopy of CO adsorption allowed for identification of exposed metal surface area in the catalyst. DMN-derived catalysts were tested for CO oxidation, with turnover frequencies calculated in order to determine the intrinsic activity of the samples. Single-pass fixed-bed microreactor measurements show that the activity, measured under identical conditions, differ significantly for these catalysts and are correlated with the catalyst preparation method and with the Au role in the bimetallic catalysts. This points to a distinct Au effect and a direct participation of the Au in the reaction.

Localized plasmon resonances of bimetallic AgAuAg nanorods

We investigated the localized surface plasmon resonances of individual AgAuAg nanorods (NRs) using the dark-field spectro-microscopy technique. We find that the scattering spectra of such hetero-NRs show longitudinal resonance wavelengths that are nearly insensitive to the relative composition of Ag and Au. Instead, the resonance is mostly governed by the overall length of the nanorod. This shows that the plasmons oscillate along the entire length of the NR without the significant perturbation at the Ag-Au interfaces. The results demonstrate that the overall geometry as well as the composition determine the tunability of the hetero-metallic nanostructures, and provide an important design rule for the composition-tunable bimetallic plasmon structures.

Galvanic replacement approach for bifunctional polyacrylonitrile/Ag–M (M = Au or Pd) nanofibers as SERS-active substrates for monitoring catalytic reactions

SERS has been applied to monitor different types of catalytic reactions on the surface of metal NPs in recent years, which requires bifunctional metal structures with plasmonic properties as well as catalytic activity. Monitoring catalytic reactions with SERS technology in previous reports was usually performed in metal solution, using metal electrodes or on a glass/silica substrate with an immobilized metal. Here, we report a general approach for preparing novel SERS-active substrates which are used to monitor catalytic reactions. The polyacrylonitrile (PAN)/Ag–M (M = Au or Pd) bimetallic nanofibrous mats are prepared through galvanic replacement reactions of Ag with Au or Pd on the surface of electrospun PAN/Ag nanofibers. The composition of Ag and Au or Pd could be tuned by changing the concentration of the metal solution used for replacement. The PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats exhibit both excellent SERS and catalytic activities. The reduction of 4-nitrothiophenol (4-NTP) to the corresponding to 4-aminothiophenol (4-ATP) on PAN/Ag0.60Au0.40 and PAN/Ag0.90Pd0.10 bimetallic nanofibrous mats are monitored by SERS technology. The SERS signals of the reaction intermediate, 4,4′-dimercapto-azobenzene(4,4′-DMAB), are observed as the reaction proceeds with time.

Thiol‐Directed Synthesis of Highly Fluorescent Gold Clusters and Their Conversion into Stable Imaging Nanoprobes

Fluorescent gold clusters (FGCs) with tunable emission from blue to red and quantum yields in the range of 6-17% have been synthesized by simple modification of the conditions used for the synthesis of gold nanoparticles, namely by replacing the stronger reducing agent with a controlled amount of thiol. Various functional FGCs with hydrodynamic diameters of 5-12 nm have been successfully synthesized and used as cell labels. The results of our investigations strongly indicate that FGCs composed of Au(0) are more stable imaging probes than commonly reported red/NIR-emitting FGCs with a composition of Au(0)/Au(I), as this combination rapidly transforms into nonfluorescent large clusters on exposure to light. The FGC-based nanoprobes reported herein exhibit stable fluorescence upon continuous light exposure and can be used as imaging probes with low cytotoxicity.

One-Pot Synthesis of PdAu-Au Core-Shell Bimetallic Nanoparticles Using Electrodeposition and Their Optical Property

This study introduces a one-pot synthesis method of PdAu-Au core-shell nanoparticles using electrodeposition and displacement reaction. It was ascertained that Au and Pd atoms were well-distributed inside the nanoparticles, and Au shell was successfully formed via displacement reaction. The influences of Au ion concentration and current density on the particle size, the atomic fractions of Pd and Au, and the surface plasmon absorption were investigated. Although the current density affected both size and composition, the concentration of Au ions only changed the composition of Au and Pd. In addition, the behavior of surface plasmon absorption according to the atomic concentration of Au was figured out.

Effect of current density on composition and structure of electrodeposited Au–Co alloy coatings

The effect of current density on composition and structure of Au–Co alloy coatings deposited from weakly acidic additive-free electrolyte was investigated. With increasing current density, the co-deposition of Co is facilitated and the microstructure of the alloy changes from lamellar to columnar. Alloy coatings containing up to 70 mass-%Co can be obtained. The coatings have a uniform Au colour for alloying element contents of up to several per cent and turn to uniform grey when the Co content increases above 25–30 mass-%. Within the limiting current density region, periodic spatio-temporal structures of different alloying element contents are formed on the surface of the deposit. The cross-sections of the coatings give evidence of compositionally modulated lamellae with a thickness within the nanometric region. Two supersaturated solid solutions form in the system: fcc Au1−xCox; 0<x<0·18 and fcc Au1−yCoy; 0·18<y<0·87. Besides Co, the alloys contain C, N, O and H.

Shape- and Composition-Sensitive Activity of Pt and PtAu Catalysts for Formic Acid Electrooxidation

In this work, we have probed the shape- and composition-dependent activities of Pt and PtAu catalysts for formic acid electrooxidation. Pt-based hollow nanostructures such as Pt hollow nanospheres (Pt HNSs), Pt nanotubes (Pt NTs), and PtAu alloy nanotubes (PtAu NTs) with controlled Pt:Au compositions are successfully prepared via a galvanic replacement process using sacrificial Ag templates. The physicochemical properties of these Pt and PtAu catalysts are confirmed by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy techniques. The electrochemical activities of these hollow structured catalysts are compared for formic acid oxidation by cyclic voltammetry and chronoamperometry methods. Relative to the commercial Pt black and Pt/C nanoparticle catalysts, the hollow nanostructured Pt materials (Pt HNS, Pt NT) exhibit enhanced catalytic activities due to structural effects. Moreover, the bimetallic PtAu NT series shows improved catalytic activities over the monometallic Pt catalysts due to compositional effects. The present study demonstrates that the catalytic activity and the extent of surface poisoning are strongly dependent on both the structural and compositional variations of the nanostructured electrocatalysts, as shown by a systemically prepared series of nanostructured catalysts for formic acid oxidation.

First-principles calculations and XPS measurements of gold segregation at the Cu3Au(111) surface

A combination of first-principles calculations based on density-functional theory, pseudopotentials, and x-ray photoelectron spectroscopy (XPS) measurements is used in order to study Au segregation in Cu3Au(111) surfaces. Our theoretical results suggest Au compositions from 50% to 75% in the topmost layer, depending on the chemical potentials of the atomic species. This strong Au segregation is restricted to the topmost surface plane and it is supported by the XPS measurements in a semiquantitative manner.

Effect of gold composition on the orientations of oxide nuclei during the early stage oxidation of Cu-Au alloys

In situ environmental transmission electron microscopy is employed to study the effect of Au composition in Cu-Au alloys on the orientations of oxide islands during the initial-stage oxidation of Cu-Au(100) alloys. An orientation transition from nucleating epitaxial Cu2O islands to randomly oriented oxide islands is observed upon increasing the oxygen gas pressure. By increasing the Au composition in the Cu-Au alloys, both the oxide nucleation time and saturation density of oxide islands increase, but the critical oxygen pressure leading to nucleating randomly oriented Cu2O islands decreases. It is shown by a kinetic model that such a dependence of the critical oxygen pressure on the alloy composition is related to its effect on two competing processes, the oxide-alloy structure match and the effective collision of oxygen atoms, in determining the overall nucleation rate of oxide islands during the oxidation.

Influence of annealing temperature and Au concentration on the electrical properties of multilayered a-Ge/Au films

Multilayered a-Ge/Au was investigated to realize low-temperature formation of poly-Ge films on insulator. Increasing Au layer thicknesses, peak intensity of crystalline Ge–Ge TO mode increased and amorphous Ge–Ge TO mode decreased. When Au composition ratio is high, the samples are crystallized under eutectic temperature. Annealing temperature at 673K, all samples with Au layer are crystallized. Crystalline Ge has no strain evaluated by X-ray diffraction and Raman scattering spectroscopy. However, the Au is compressed or expanded in the films with various annealing temperature. It is thought that this phenomenon changes depending on the size of the space in the film. The behavior of electrical resistivity is changed at eutectic temperature.

Novel fabrication and catalytic application of poly(ethylenimine)-stabilized gold–silver alloy nanoparticles

Novel synthesis of amine-stabilized Au–Ag alloy nanoparticles with controlled composition has been devised using poly(ethylenimine) (PEI) as a reducing and a stabilizing agent simultaneously. The composition of Au–Ag alloy nanoparticles was readily controlled by varying the initial relative amount of HAuCl4 and AgNO3. Due to the presence of abundant amine functional groups in PEI, which could act as the dissolving ligand for AgCl, the precipitation problem of Ag+ in the presence of Cl− from the gold salt was avoided. On this basis, the relatively high concentrations of HAuCl4 and AgNO3 salts were used for the fabrication of Au–Ag alloy nanoparticles. The PEI thus plays triple roles in this study that include the co-reducing agents for HAuCl4 and AgNO3, the stabilizing agents for Au–Ag alloy nanoparticles, and even the dissolving agents for AgCl. As a novel material for use in catalysis, the Au–Ag alloy nanoparticles including pure Au and Ag samples were exploited as catalysts for the reduction of 4-nitrophenol in the presence of NaBH4. As the Au content was increased in the Au–Ag alloy nanoparticles, the rate constant of the reduction was exponentially increased from pure Ag to pure Au.

Development of high current Bi and Au beams for the synchrotron operation at the GSI accelerator facility

In this work, the latest results of developing high current ion beams of Au and Bi at GSI facility are described. The difficulties in the production of required charge state in vacuum arc discharge ion sources using the pure materials in the cathodes are discussed. As a possible solution, admix of a small amount of more refractory metal to the cathode material is considered. As a significant result, a dramatic improvement in the production of high charge state Bi ions using the mixed Bi-Cu cathodes (with 8%-15% of Cu admixed) compared to pure Bi cathodes is presented. The preliminary results of investigation of the material structure of Bi-Cu cathodes are discussed. As a next step, it is planned to test the composition of Au with Pd, Zr, and Fe as cathode materials.

Electrocatalytic Activity of PtAu Nanoparticles Deposited on TiO2 Nanotubes

We report on a facile photoassisted approach to directly deposit PtAu nanoparticles (NPs) with controllable compositions on TiO2 nanotubes (TiO2NTs), which were synthesized by electrochemical anodization of Ti substrates. The fabricated TiO2NTs and TiO2-supported Pt and PtAu NPs were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The electrocatalytic activity of the TiO2NT supported, variably composed PtAu NPs was investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). Our studies have shown that TiO2NT-supported PtAu nanoparticles with the Au composition between 30 and 50% exhibit very high electrocatalytic activity toward formic acid oxidation and that the direct dehydrogenation is the dominant reaction pathway in the electrochemical oxidation process.

Electrocatalytic Activity of PtAu/C Catalysts for Glycerol Oxidation

The electrocatalytic oxidation of glycerol on PtAu/C catalysts has been investigated by cyclic voltammetry. PtAu bimetallic nanoparticles are prepared by chemical reduction. Carbon-supported PtAu catalysts are found to exhibit high electrocatalytic activity for the oxidation of glycerol in alkaline solution in terms of oxidation potential and current density as well as stability, and PtAu/C catalysts with different Pt:Au composition ratios show no much difference in catalytic activity. In acidic solution, PtAu/C catalysts exhibit similar to Pt/C catalysts in activity, but the advantage of the PtAu/C catalysts in terms of per unit mass of platinum is still obvious. The PtAu/C catalysts, in a wide Pt:Au ratio range, show a remarkable enhancement in the mass specific activity of platinum with decreasing platinum content in both alkaline and acidic solutions. This is of significance for reducing the usage of platinum and indicates that though platinum acts as main active sites, gold also plays an important role in the function of PtAu/C catalysts.

Phase diagram and structural evolution of Ag–Au bimetallic nanoparticles: molecular dynamics simulations

We studied the structural evolution of a 270-atom Ag-Au bimetallic nanoparticle (2 nm in size) with varying composition and temperature. The liquid to solid transition region and the solid-state structure were investigated using molecular dynamics simulations. To determine the exact transition temperature region, we applied the mean square displacement and structure deviation methods, as well as the generally used caloric curve of potential energy versus temperature. The results showed that a complete solid-solution phase diagram of the binary Ag-Au system was obtained. Irrespective of the composition, the freezing temperature of a Ag-Au bimetallic nanoparticle was lower than that of the bulk state by a margin of several hundred degrees, and three different solid-state structures are proposed in relation to the Au composition. Our phase diagram offers guidance for the application of Ag-Au nanoparticles.

Electrochemical deposition of Au–Pt alloy particles with cauliflower-like microstructures for electrocatalytic methanol oxidation

Au–Pt alloy particles with cauliflower-like microstructures of varying Pt/Au ratios were electrodeposited on indium tin oxide (ITO) substrates by constant potential electrolysis at E = −0.25 V. The results of X-ray diffraction and X-ray photoelectron spectroscopy confirm that the bimetallic alloys can be obtained for different Pt/Au ratios including 4/1, 1/1 and 1/4. The formation of alloyed cauliflower-like microstructures may be the result of the fast formation of gold seeds as the core and subsequent simultaneous deposition of Au and Pt from cyclic voltammetric study. The effect of surface composition of Au–Pt alloy particles on electrocatalytic methanol oxidation were investigated in H2SO4 solution. The electrocatalytic abilities including electrochemical surface area, peak current density and the turnover number of methanol oxidation follow the order of Pt4Au1 > Pt > Pt1Au1. The results can be ascribed to that electronic effect may be prominent while bifunctional effect is insignificant for Au–Pt alloy systems because the electrocatalytic activity of Au is negligible in acidic media. Additionally, the Pt4Au1 electrode has superior kinetics of methanol electro-oxidation than monometallic Pt electrode by calculating the electron transfer coefficient (α).

Self-assembled dendritic nanowires of Au–Pt alloy through electrodeposition from solution under AC fields

Au–Pt dentritic nanowires (DNWs) bridging a pair of 10–40μm spacing electrodes were successfully self-assembled from the corresponding ions solution via deposition of an alternating current (AC). The morphology and the composition of Au–Pt DNWs were characterized by scanning electron microscopy (SEM) and energy-dispersion X-ray (EDX) spectrometer. In the electrolyte of various molar ratios of [AuCl4]−/[PtCl6]2−, Au–Pt DNWs with 8–95at% Pt were fabricated through applying a constant voltage 8–16Vpp and varied frequencies of 300Hz for nucleation and 1MHz for growth. The DNWs have the diameter range of 100–200nm. The X-ray diffraction (XRD) results confirmed that the DNWs of 20 or 80at% Pt have the alloy phase structure. Au–Pt DNWs are promising nanomaterials of chemical sensors, bioelectronic circuits and nanoelectronic interconnection.

Nano Structural and Thermoelectric Properties of SiGeAu Thin Films

SiGeAu thin films exhibit large thermoelectric power. Recently, the formation of SiGe nano crystal has been confirmed after annealing. In this study, we investigated the relationship between the size of the nano crystal and thermoelectric power. SiGe nano crystals were fabricated by the crystallization of amorphous SiGeAu thin films. The grain size of SiGe was around 6 nm when Au composition was 2.2 at. %. The grain size decreased with increasing Au composition up to 2.2 at. % due to metal-induced crystallization, and increased depending on Au composition over 2.2 at. % due to liquid phase crystallization. When the grain size was around 6 nm, the thin films exhibited large thermoelectric power (4.8 mV K-1) and power factor (2.4×10-2 W m-1 K-2). Therefore, the grain size of SiGe around 6 nm is important for obtaining the large thermoelectric power and power factor. These results indicate that the quantum size effect enhances the thermoelectric power and power factor of SiGeAu thin films.