Au Samples Research Articles

Combustion Methods for Measuring Low Levels of Carbon in Nickel, Copper, Silver, and Gold

Laboratory studies and a literature search indicate that there is no definitive procedure for combustion analysis of low levels of carbon in Cu, Ag, and Au. Literature data disagree by one to two orders of magnitude for solubility of carbon in Cu, near the melting point. Data for Ag and Au are very limited. This study develops a procedure for combustion analysis of ppm levels of carbon in high-purity Ni, Cu, Ag, and Au samples. For comparison, each sample is measured with glow discharge mass spectrometry. The study begins with Ni, as the procedure for this material is fairly well established. For the other metals, an optimum accelerator and sample-to-accelerate weight ratio is developed. Fine particle copper is a suitable accelerator for Cu and Ag samples, and also shows potential for Au samples

Recrystallization in severely deformed Ag, Au, and Fe studied by positron‐annihilation and XRD methods

Positron‐annihilation studies of severely deformed samples of pure Ag, Au, and Fe are reported. The observed changes of the positron‐annihilation parameters during isochronal annealing are linked to the recrystallization process confirmed by measurements of the XRD patterns and pole figures. The latter, being sensitive to the crystallographic texture, change significantly during the recrystallization process for all three metals. The measured annihilation line‐shape parameter dependencies are described by a model that takes into account the boundary migration of the new defect‐free grains and positron diffusion inside these grains. The model fairly well describes the observed dependencies and allows us to determine the activation energy for grain‐boundary migration. The usefulness of the positron‐annihilation studies for determination of the recrystallization temperature in deformed metals and alloys is discussed.

Interpreting anomalous low-strength and low-stiffness of nanoporous gold: Quantification of network connectivity

The strength and elastic modulus of macroscopic nanoporous Au samples are much lower than that predicted by using Gibson-Ashby scaling laws. Here we attribute this discrepancy to a lowered network connectivity in nanoporous structure, and propose to modify the scaling equations (for both strength and stiffness) by introducing a concept of effective relative density. It is presented that the effective relative density can be determined by measuring the elasticity of nanoporous Au; under this scenario, the corrected strengths of Au nano-ligaments agree (on the order of magnitude) well with the previous data for similar-sized Au nanowires or nanopillars, which confirms our hypotheses. This study also revealed a low power-law exponent of size-dependent strength for Au nano-ligaments (β = −0.34) compared with that of Au submicron-pillars machined by focused ion beam (FIB) (β = −0.61), which may be related to the critical roles of surface defects played in deformation. A ratio between effective relative density and relative density is suggested to quantify the network connectivity in nanoporous structure, which decreases rapidly during structure coarsening – it explains why most previous nanoporous Au samples, which more or less experienced coarsening, were unexpectedly soft. Current study also leads to some general strategies for syntheses of strong and stiff nanoporous metals.

Effect of Au on the Performance of Porous Silicon/V2O5 Nanorods Heterojunctions to NO2 Gas

A novel composite of Au-functionalized porous silicon (PS)/V2O5 nanorods (PS/V2O5:Au) was prepared to detect NO2 gas. PS/V2O5 nanorods were synthesized by a heating process of pure vanadium film on PS, and then the obtained PS/V2O5 nanorods were functionalized with dispersed Au nanoparticles. Various analytical techniques, such as field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), have been employed to investigate the properties of PS/V2O5:Au. Herein, the PS/V2O5:Au sample exhibited improved NO2-sensing performances in response, stability and selectivity at room temperature (25[Formula: see text]C), compared with the pure PS/V2O5 nanorods. These phenomena were closely related to not only the dispersed Au nanoparticles acting as a catalyst but also the p-n heterojunctions between PS and V2O5 nanorods. Whereas, more Au nanoparticles suppressed the improvement of response to NO2 gas.

Speciation of Gold Nanoparticles by Ex Situ Extended X-ray Absorption Fine Structure and X-ray Absorption Near Edge Structure.

A combined X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) methodology is here presented on a series of partially and fully reduced Au(III) samples. This allows monitoring the relative fraction of Au(III) and Au(0) in the studied samples, displaying a consistent and independent outcome. The strategy followed is based, for the first time, on two structural models that can be fitted simultaneously, and it evaluates the correlation among strongly correlated parameters such as coordination number and the Debye-Waller factor. The results of the present EXAFS and XANES approach can be extended to studies based on X-ray absorption spectroscopy experiments for the in situ monitoring of the formation of gold nanoclusters.

The effect of zinc and copper in gold catalysts supported on MCF cellular foams on surface properties and catalytic activity in methanol oxidation

MCF (Mesostructured Cellular Foams) and Zn/MCF were used as supports for gold and copper. The effect of zinc and copper on the surface properties, activity and selectivity of gold catalysts in the methanol oxidation was studied. The catalysts were characterized in detail by XRD, TEM, UV–Vis, XPS, ESR and FTIR-NO. The results showed that zinc oxide species play the role of a structural promoter which protects against agglomeration of gold species and an electronic promoter, decreasing reducibility of copper cations by the strong copper-zinc interaction. These interactions result in the incorporation of copper into the crystal structure of zinc oxide and lead to changes in the surrounding of gold in bimetallic (Au-Cu) catalysts compared to materials without zinc. The composition of the catalysts determines their activity and selectivity in methanol oxidation. Bimetallic (Au-Cu) catalysts, irrespective of the nature of the support (MCF vs Zn/MCF), are much more active than monometallic (Au) samples as a result of synergistic interactions between Au and Cu species. The samples containing zinc are highly selective to formaldehyde due to Au-Zn and/or Cu-Zn interaction, which weakens the adsorption of formaldehyde.

Fabrication and optical property of metal nanowire arrays embedded in anodic porous alumina membrane

Nanowires embedded in nanopores are potentially tough against surface scraping and agglomeration. In this study, we have fabricated Au and Ni nanowires embedded into anodic porous alumina (APA) and investigated their reflectance to study the effects of surface plasmon absorption properties and conversion from solar energy to thermal energy. Au nanowires embedded into APA show typical gold surface plasmon absorption at approximately 530 nm. On the other hand, Ni nanowires show quite a low reflectance under 600 nm. In the temperature elevation test, both Au and Ni nanowire samples present the same capability to warm up water. It means that Ni nanowires embedded into APA have almost the same photothermal activity as Au nanowires.

Au, Bi, Co and Nb cross-section measured by quasimonoenergetic neutrons from p + 7Li reaction in the energy range of 18–36 MeV

Au, Bi, Co and Nb samples were irradiated several times with quasi-monoenergetic neutrons from p + 7Li reaction in the energy range of 18–36 MeV. The activities of the samples were measured with the HPGe detector and the reaction rates were calculated. The cross-sections were extracted using the SAND-II method with the reference cross-sections from the EAF-2010 database. The uncertainties of the final results are discussed.

Thermally stable Ti/Al-based ohmic contacts to N-polar n-GaN by using an indium interlayer

The electrical properties of In/Ti/Al/Au contacts to N-polar n-GaN (nd = 5 × 1018 cm−3) for high-power vertical light-emitting diodes were investigated at various thicknesses of the In layer, and compared with those of Ti/Al/Au contacts. Before annealing, both the Ti/Al/Au and In/Ti/Al/Au contacts were ohmic. After annealing at 300 °C for 1 min, all of the samples exhibited some degradation of their electrical properties, although the In/Ti/Al/Au samples were more thermally stable. After annealing at 300 °C for 60 min, the Ti/Al/Au contacts became non-ohmic, while the In (5 nm)/Ti/Al/Au contacts remained ohmic with a contact resistivity of 2.6 × 10−4 Ω cm2. X-ray photoemission spectroscopy (XPS) results showed that, for all of the samples, annealing caused an increase in the content of interfacial oxygen. Based on the XPS and electrical results, the annealing dependence of the electrical characteristics of In/Ti/Al/Au contacts are described and discussed.

Experimental study on improving hohlraum wall reemission ratio by low density gold foam

It is important to improve the hohlraum radiation temperature for the research of high energy density physics, especially for study of inertial confinement fusion. Increasing the wall reemission ratio is an effective way to improve the temperature. It is found in theory that low density foam could reduce hohlraum wall energy loss, and then increase hohlraum temperature. In previous studies, experiments have shown that laser-to-X-ray conversion is enhanced by Au foam. However, improving reemission ratio is more important to increase hohlraum radiation temperature, because most of energy is lost in the wall.In this paper, we report our experiments carried out on SGⅢ prototype to compare the X-ray flux reemitted by Au foam and that by Au. For the experimental design, Au solid and Au foam are irradiated symmetrically along the axis by hohlraum radiation source Tr(t), which is assessed by broadband X-ray spectrometer flat-response X-ray diodes. The measured peak temperature is about 190 eV. Reemission flux from sample is measured by transmission grating spectrometer (TGS). The space-resolved image for pure Au sample shows that the hohlraum radiation is asymmetrical along the axis in the experimental conditions, temperature of top is higher than that at the bottom, which is consistent with simulation results obtained by using IRAD3D code. In order to compare the reemission flux from Au solid sample and that from Au foam sample in same conditions, we need to correct the symmetry of hohlraum radiation. By multiplying the ratio of top flux to bottom flux in pure Au target by the bottom flux in Au-Au foam target, where Au foam is on, we make sure that they are ablated by the same radiation source. The calculated results show that X-ray flux is increased by 20% by Au foam of 0.4 g/cc density when the hohlraum temperature is 190 eV. The typical observed time-integrated X-ray reemission spectra for Au solid and Au foam by TGS are also shown. We see that N-band and O-band reemission are clearly enhanced by Au foam, and the O-band reemission is almost the same as M-band reemission. The increased flux concentrates below 1 keV of the soft X-ray emission.The self-similar solution results and MULTI 1D simulation results show that the wall loss energy fraction is saved by Au foam, whose relation to reemission flux can be described by a simple expression. The theoretical solution shows that the emission flux increases about 10%, and the MULTI simulation indicates that the emission flux increases about 6.8%. They are in qualitative agreement with the experiments results. These results show an alluring prospect for Au foam to be used as hohlraum wall.

Synthesis and electrochemical double layer characterization of nanoporous gold

The present article highlights synthesis and electrochemical double layer characterisation of nanoporous Au. The electrochemical double layer studies were carried out on nanoporous Au samples immersed in a weakly adsorbing HClO4 electrolyte in-situ in three-electrode cell geometry. The specific-volumetric capacitance value obtained from the double layer charging experiments are discussed in terms of the variation in ligament diameter.

Defects evolution in nanoporous Au(Pt) during dealloying

The vacancies and small voids evolution within the ligaments of nanoporous (np) Au(Pt) in dealloying processes were investigated by positron annihilation spectroscopy. The np Au(Pt) samples with 5–8nm ligaments were prepared by electrochemical dealloying of (Au0.95Pt0.05)25Ag75 ingots. The positron lifetime of as-prepared np Au(Pt) consists of the two main components. One is assigned to the positron annihilation at vacancies within the nano-ligaments and the other to the positron annihilation in the enclosed voids in the ligaments. Vacancy concentration increased significantly when silver atoms leached out further from the np structure, which supports the vacancy-mediated lattice diffusion mechanism of porosity evolution.

Surface area of nanoporous gold: Effect on temperature

Nanoporous Au in two distinctly different surface states, namely, positively charged as-dealloyed and reduced clean metallic were investigated for temperature dependent properties like specific capacitance Cs and specific mass surface area αm. An electrochemical double layer capacitance ratio method was employed to carry out these studies. It is shown that for reduced clean metallic nanoporous Au sample, the variation in ln (Cs) versus 1/T plot is almost linear, which supports to the negative temperature coefficient predicted by the double layer model. This observation is in direct contrast with positively charged as-dealloyed nanoporous Au that did not reveal any such trend. We compare the specific-mass surface area αm of nanoporous Au values with the geometrical surface area αg, obtained by using the “ball-stick geometrical model” coupled with scanning electron microscopy images. Evolution of ultra small nanometric structures during dealloying renders positively charged as-dealloyed nanoporous Au vulnerable to oxidation and ligament coarsening up on its reduction. The validation of the specific-mass surface area obtained from the capacitance ratio method was confirmed by Brunauer-Emmett-Teller analysis and the process of adsorption/desorption of OH adsorbed species on pore surfaces in reduced clean metallic np-Au.

Expeditious Synthesis of Noble Metal Nanoparticles Using Vitamin B12 under Microwave Irradiation

A greener synthesis protocol for noble nanometals is developed using vitamin B12 as a reducing and capping agent in conjunction with the use of microwaves. Successful assembly of nanoparticles or microparticles with varied shapes and sizes have been demonstrated. The synthesized Ag, Au, and Pd samples were thoroughly characterized with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission microscopy, and UV-visible spectrophotometry, confirming that metallic Ag, Au, and Pd were synthesized by the green chemistry method.

The α ↔ γ transformation in Fe and Fe–Au thin films, micro- and nanoparticles – an in situ study

We studied the α↔γ phase transformation in Fe and Fe–Au thin films and nanoparticles employing in situ X-ray diffraction by varying the temperature (from room temperature up to 1100°C), film thickness, particle size and composition. In pure Fe particles the transformation did not occur, whereas in the respective thin films a “reverse size effect” was observed – only the thinnest and the thickest films underwent the transformation. In contrast, the γ-Fe phase was observed in all Fe–Au samples (bi-particles and thin films), and remained metastable at room temperature. γ-Fe particles as large as 300nm were observed by electron microscopy. The results are discussed in terms of the difference in available nucleation sites in the particles, bi-particles and films; critical nucleus size and nucleation energy barrier; and the facilitating presence of Fe/Au interfaces. The reverse size effect challenges the paradigm that phase stability at the nanoscale changes monotonously with a decrease of the system’s characteristic dimensions.

Coarsening of nanoporous Au: Relationship between structure and mechanical properties

Chemical dealloying was used to fabricate nanoporous Au samples with fine ligaments. The samples were subjected to isothermal annealing at relatively low temperature to induce slow, limited coarsening. This allowed obtaining a sequence of progressively coarsened nanoporous Au samples with correlated structure. Scanning and transmission electron microscopies were utilized to measure the characteristic lengths of nanoporous Au and investigate accurately its coarsening kinetics. Nanoindentation experiments were also performed to estimate Young’s modulus and yield strength. It is shown that mechanical properties undergo a gradual variation related to structural evolution. The observed mechanical behavior is tentatively rationalized by a simplified model able to describe the mechanical response of nanoporous metal structures with thick ligaments and massive nodes.

The effect of metal composition on the performance of Ir–Au/TiO2 catalysts for citral hydrogenation.

The effect of gold addition to iridium catalysts and the nature of active sites for citral hydrogenation were investigated over Ir–Au/TiO2 catalysts. All samples (Au/TiO2, Ir/TiO2 and Ir–Au/TiO2) were prepared by deposition-precipitation with urea (DPU). Bimetallic catalysts were synthetized by co-deposition at different Ir/Au atomic ratios (3, 1, 0.3). The catalysts were characterized by ICP, BET, H2–TPR, H2-Chemisorption, TEM, DRIFTS and XPS techniques. A partial coverage of iridium sites by gold atoms takes place as the gold amount in the catalyst increases as shown by chemisorption and XPS measurements. The latter also evidenced a trend consistent with modification of the electronic environment of iridium due to interaction with gold atoms. Also, compared to Ir/TiO2, the amount of Irδ+ species was higher in the bimetallic catalysts reaching a maximum in Ir–Au (1) sample. Characterization of the catalytic surface using DRIFTS of adsorbed CO evidenced a shift towards higher wavenumbers as a function of the gold content indicating, as the XPS results, a modification of the adsorption site. The catalytic activity for citral hydrogenation increased as a function of the gold content. The selectivity to unsaturated alcohol is related to the amount of Irδ+ species which in turns depends on the catalyst composition. For the most active sample Ir–Au (1), a suitable Irδ+/Ir0 ratio is already obtained when the catalyst is reduced at 573K and does not vary with reduction temperature.

Electromagnetic properties of scanning tunneling microscope tip–sample gap in the terahertz frequency range

We have investigated electromagnetic properties of the tip–sample gap of a scanning tunneling microscope (STM) in the terahertz (THz) spectral range. Light in the STM is emitted from the current source excited by tunneling electrons through dipole radiation processes. The antenna factor that measures the efficiency of the dipole radiation contains a factor proportional to the square of the angular frequency ω of the STM light. Hence, it might occur that STM light emissions in the THz spectral range become undetectably weak. The antenna factors for samples of Au and TiO2, whose plasma frequencies are in the UV and THz spectral ranges, respectively, were investigated as a function of the radius a of curvarure of the W tip. We have found that the effect of ω2 in the antenna factor can be compensated for using a tip having an a of approximately 1500 nm.

Physicochemical formation conditions of silver sulfoselenides at the Rogovik deposit, Northeastern Russia

The chemical compositions of acanthite, naumannite, and associated ore minerals have been studied from the samples of polychronous Au–Ag ores at the Rogovik deposit. The following admixtures have been detected: S in naumannite (0–2.9 wt %), Se in acanthite (0–7.45 wt %), argyrodite (~4.8 wt %), and galena (~3.1 wt %), and Fe in sphalerite (~1.2 wt %). The physicochemical parameters of ore formation have been reconstructed on the basis of mineralogical and geochemical data and thermodynamic calculations. Eh–pH (25°C, 1 bar), logfO2–pH, logfS2–T, logfSe2–T, and logfS2–logfSe2 (100–300°C, 1–300 bars) diagrams for the Ag–S–Se–H2O system with the stability fields of Ag sulfoselenides Ag2S1–x Se x of various composition (step x = 0.25, where 0 ≤ x ≤ 1) have been calculated for the first time. It has been established that Ag sulfoselenides of the naumannite series from polychronous ores of the Rogovik deposit precipitated below 70–133°C under reductive conditions (logfO2 =–65…–50) from near-neutral solutions containing elevated Se and relatively lowered S. It has been established that Ag sulfoselenides of acanthite series were formed later then naumannite but in the same range of logfO2 values at temperatures below 110–177°C from solutions with high S concentration and relatively lowered concentration of Se. The complex composition of the studied Au–Ag ores, whose characteristic feature is extremely variable mineralogy, is confirmed.

Effects of graded levels of tannin-containing tropical tree leaves on in vitro rumen fermentation, total protozoa and methane production.

This study was carried out to determine the effect of graded levels of tannin-containing tropical tree leaves, Autocarpus integrifolis, Azardirachta indica and Ficus bengalensis, on the in vitro rumen fermentation pattern, total protozoa and methane suppression in order to establish the optimum dose of these leaves for inclusion in the ruminant diets. The air-dried and ground samples of Au. integrifolis, Az. indica and Ficus bengalensis were subjected to in vitro incubation using 30 ml buffered rumen fluid at 0, 2.5, 5.0, 10.0, 15.0, 20.0, 25.0 and 30.0% (dry matter refers to moisture-free basis) of a total mixed ration (TMR: refers to mixture of roughage and concentrate containing cereals and oil cakes) devoid of tannin. The TMR for the experimental incubation was prepared by mixing 40 parts of ground Elusine coracana straw as roughage source with 60 parts of concentrate mixture. The leaves contained an average 130 g kg(-1) CP with 7·0 MJ of ME kg(-1) DM. The average neutral detergent fibre (NDF) content was <400 g kg(-1) DM. Highest total phenol (TP; g kg(-1) DM) was recorded in Az. indica (108) followed by F. bengalensis (103) and Au. integrifolis (76), and total tannin (TT) content also showed similar trend. However, condensed tannin (CT) was highest in F. bengalensis (260) followed by Au. integrifolis (186) and Az. indica (138). There was significant (P < 0.05) reduction in the NH3 concentration in the presence of all the leaves. Autocarpus integrifolis tannins did not cause inhibition of total volatile fatty acid (TVFA: refers to the concentration of volatile fatty acids, viz. acetic, butyric and propionic) concentration, whereas F. bengalensis and Az. indica tannins at higher level of incubation (>5.0%) reduced TVFA concentration. Protozoa (cells per mL) were similar at all levels of inclusion with Au. integrifolis, but reduced in case of F. bengalensis and Az. indica. As the level of tannin increased in the incubation medium, there was a linear reduction in methane concentration. Highest methane reduction (%) was recorded in incubations supplemented with Az. indica (61.5) followed by F. bengalensis (46.8) and Au. integrifolis (30.3). It was established from this study that tropical leaves of F. bengalensis, Au. integrifolis and Az. indica suppress methanogenesis. Ficus bengalensis, Au. integrifolis and Az. indica leaves are of interest in the enteric methane ameliorative strategies. Total mixed ration containing 10-15% ground F. bengalensis or Au. integrifolis or Az. indica leaves could be fed to ruminants to reduce enteric methanogenesis. However, in vivo trials need to be carried out to validate these in vitro results.