Au In Cu Research Articles

(Invited) Fundamentals of Single Atom Alloy Catalysts for Electrochemical Nitrate Reduction to Ammonia

Electrochemical conversion of toxic nitrate into useful ammonia provides a route to remediating the most frequent water pollutant while simultaneously generating one of the most produced bulk chemicals. While highly desirable, this conversion is plagued by complications including a highly branching reaction pathway - which can produce highly toxic undesirable products, the need to reduce a negatively charged species, a relatively low nitrate concentration, and competition for electrons between nitrate reduction and parasitic hydrogen evolution. Therefore, electrocatalysts must be developed which are highly selective not only for nitrate over other species in the water matrix, but also selective to the desired ammonia product. Single atom alloy catalysts, where a single atom of one element is embedded in a solvating phase of a secondary element, provides an opportunity to engineer catalysts with atomic level precision. In this presentation, we will outline the fundamental behavior of single atom alloy for nitrate reduction. We explore this topic through density functional theory calculations of the reaction mechanisms, including thermodynamics and kinetic aspects. Particularly, we will discuss the effect of single atom identity, effect of matrix element, and pH and potential. We show that the ability of the single atom to localize the N* species drives the reduction towards ammonia formation, as exemplified by Ru in Cu, while facile exchange of N* between the single atom site and the matrix leads to the formation of N2 or other undesirable products, as exemplified by Pd in Cu and Au in Cu, respectively. We also discuss the importance of the selection of the solvating element; for example, Cu is a good solvating metal because it is a good conductor, while minimizing the hydrogen evolution reaction. Further, we discuss the limits of pH and potential on the performance. In general, higher pH and more moderately negative potentials favor nitrate reduction to NH3 or N2, while lower pH and more negative potentials drive H2 evolution, and generation of NOx species.

Study of inter-diffusion of gold in copper in the presence of single layer graphene

• Graphene acts as a diffusion barrier for interdiffusion of Au in Cu upto 220°C. • From 240°C to 280°C, grain boundary diffusion is dominant in Au/Gr/Cu. • Slower ion beam mixing is observed in Au/Gr/Cu compared to Au/Cu sample. • Disorder is more in graphene upon annealing and amorphization upon irradiation. Rutherford backscattering spectrometry (RBS) has been used to examine the inter-diffusion behaviour in Au thin films (∼30 nm) deposited on CVD grown monolayer Graphene on copper (Au/Gr/Cu) with reference to gold film deposited on copper (Au/Cu), under thermal annealing and ion beam irradiation processes. Graphene is acting as a diffusion barrier in Au/Gr/Cu upto 220 °C for the inter-diffusion of Au in Cu in thermal annealing process, while interdiffusion occurs below 200 °C in Au/Cu. In high temperature regime, bulk diffusion mechanism is dominant in Au/Cu system whereas grain boundary diffusion is dominant in Au/Gr/Cu system. The dominant grain boundary diffusion in Au/Gr/Cu indicates that grain boundary defects in graphene play a major role in inter-diffusion process. Upon 9 MeV Si 6+ ion irradiation, there is no ion beam mixing upto 5 × 10 15 ions/cm 2 and at the ion fluence of 3 × 10 16 ions/cm 2 , ion beam mixing is observed in both Au/Cu and Au/Gr/Cu samples, with lower ion beam mixing rate and mixing efficiency for Au/Gr/Cu sample. This implies that graphene interlayer can prevent ion beam mixing to a certain extent due to its impermeable nature. The blue shift of Raman spectroscopy peaks G and 2D and the splitting of G-peak from graphene barrier layer signifies the compressive strain in Au deposited and thermally annealed Gr/Cu samples. Even though interlayer graphene can reduce the interdiffusion in Au/Cu system modestly, it becomes disordered with the formation of sp 3 carbon bonds with increase in annealing temperatures and ion fluences, thereby loosing its impermeable nature.

Renewable redox couple system for sustainable precious metal recycling from e-waste via halide-regulated potential inversion

Precious metal (PM) retrievement from e-waste is of great significance for reducing virgin mining activity and promoting rare resource sustainability. However, current PM recycling methods rely mainly on caustic aqua regia or unstable sulfur-based ligand, which has caused severe environmental damage and process inefficiency. Here, we propose an environmentally friendly halide-regulated strategy, utilizing milder and renewable oxidant—cupric/ferric ion for facile PM dissolution. This is realized by the synergistic effect of enhanced oxidizing ability of Cu(II) and reduced oxidation potential of PM with halide addition. Electrochemical tests and leaching experiment results show that Cu(II)/Cu(I) redox potential experiences great change with bromide, increasing from 0.4 to 0.75 V. Fast corrosion feature was observed for Au in Cu(II)/Fe(III)-Br- and Pd in Cu(II)/Fe(III)-Cl-, and it can be accelerated by increasing oxidant and halide concentration. Our proposed strategy outperforms traditional methods with stable and fast dissolution, where 2.5 mol/L Br- is appropriate for Au dissolution. Moreover, selective dissolution of base metal, Pd/Ag, and Au can be achieved via ligand alteration and be further combined with electrodeposition technique for multi metal recovery and oxidant regeneration. This halide-regulated strategy can lead PM recycling from pollutive status towards environmentally friendly road.

Gold-in-copper at low *CO coverage enables efficient electromethanation of CO2

The renewable-electricity-powered CO2 electroreduction reaction provides a promising means to store intermittent renewable energy in the form of valuable chemicals and dispatchable fuels. Renewable methane produced using CO2 electroreduction attracts interest due to the established global distribution network; however, present-day efficiencies and activities remain below those required for practical application. Here we exploit the fact that the suppression of *CO dimerization and hydrogen evolution promotes methane selectivity: we reason that the introduction of Au in Cu favors *CO protonation vs. C−C coupling under low *CO coverage and weakens the *H adsorption energy of the surface, leading to a reduction in hydrogen evolution. We construct experimentally a suite of Au-Cu catalysts and control *CO availability by regulating CO2 concentration and reaction rate. This strategy leads to a 1.6× improvement in the methane:H2 selectivity ratio compared to the best prior reports operating above 100 mA cm−2. We as a result achieve a CO2-to-methane Faradaic efficiency (FE) of (56 ± 2)% at a production rate of (112 ± 4) mA cm−2.

The structural, elastic, thermodynamic, and electronic properties of (Cu6-xAux)Sn5 (x = 0, 0.5, 1, 1.5, 2) intermetallic compounds

In this paper, the effects of substitution of Au for Cu on the structural, mechanical, thermodynamic, and electronic properties of Cu6Sn5 intermetallic compound (IMC) are studied based on first-principles calculations. The results manifest that doping Au in Cu6Sn5 IMC can form a thermodynamically more stable structure than the pure phase. It is also found that, with the increasing in Au concentration, the volume, anisotropic property, and ductility of Cu6Sn5 IMC increase. However, doping Au weakens the shear modulus, Young’s modulus, hardness, and Debye temperature of Cu6Sn5 IMC. The electronic density of states displays that the Au-doped Cu6Sn5 IMC has higher thermodynamic stability than the pure phase.

Effect of Au Doping on Elastic, Thermodynamic, and Electronic Properties of η-Cu6Sn5 Intermetallic

The effects of substitution of Au for Cu on the elastic, thermodynamic, and electronic properties of hexagonal η-Cu6Sn5 intermetallic compound (IMC) are investigated by first-principles calculations. The results show that Au atoms preferentially occupy Cu4 or Cu3 site to form η-Cu5Au1Sn5 or η-Cu4Au2Sn5 IMC, respectively. Doping Au in η-Cu6Sn5 IMC forms a more stable thermodynamic structure than pure phase. The ductility of η-Cu6Sn5 IMC increases after substitution of Au for Cu. However, doping Au weakens the Young’s modulus, shear modulus, hardness, and Debye temperature of η-Cu6Sn5 IMC. The results for the charge density difference, total density of states, and partial density of states show that doping Au atoms can stabilize the structure of η-Cu6Sn5 IMC.

Vacancy-mediated structural changes in Au–Cu nanoparticles

ABSTRACTWe report the formation of new phases in bimetallic Au–Cu nanoparticles. These phases were observed in nanoparticle synthesised by adopting a three-step protocol in a single pot. Nanoparticles at 180°C for 1 h led to the formation of single-phase solid solution of Cu in Au. Subsequent heat treatment at 290°C for 2 h of these Au–Cu nanoparticles revealed three new phases. One of them relates to the modification of occupancy of Cu in an ordered AuCu tetragonal phase (tP4). This cell although retains tetragonal symmetry but displays metrical properties akin to that of a cube. The other two relates to vacancy ordering along <111> directions in the {111} planes of an ordered AuCu3 cubic phase (cP4). On the one hand, statistical occupancy of vacancy on Cu site in this cell leads to the reduction of cell size from ∼3.75 Å to ∼3.5 Å whereas ordering of vacant layer on the other hand gives rise to symmetry breaking. Former continues to display cubic symmetry whereas latter transforms to a trigonal cell.

Electrochemical determination of arsenic(III) with ultra-high anti-interference performance using Au–Cu bimetallic nanoparticles

Bimetallic nanoparticles provide a new opportunity for enhancing electrocatalytic activity because of some possible synergetic effects. In this study, different compositions of Au–Cu bimetallic nanoparticles are prepared via a simple hydrothermal method. The structure of Au–Cu bimetallic nanoparticles is characterized by using X-ray absorption fine structure (XAFS) techniques, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The correlations between electrochemical performance and the compositions of Au–Cu bimetallic nanoparticles on the determination of arsenic(III) are investigated through square wave anodic stripping voltammetry (SWASV). It is found that the amount of copper (Cu) in Au–Cu bimetallic nanoparticles is critical to the detection of arsenic(III). The XAFS results indicated that the AuAu bond length (RAu–Au) can be influenced by the Cu concentration in the materials. Different RAu–Au of Au–Cu nanoparticles lead to the different electrochemical catalytic activity toward arsenic(III), further revealing the different electrochemical behavior. Compared with Au nanoparticles and commercial Au electrode, the Au–Cu bimetallic nanoparticles showed enhanced electrochemical performance with high sensitivity at ppb level, low detection limit. Moreover, the Au89Cu11 bimetallic nanoparticles exhibited ultra-high anti-interference performance on the detection of arsenic(III).

Crystal structure and thermodynamic properties of NdCu4Au compound

We report the synthesis of the antiferromagnet cubic-type structure NdCu4Au derived by substituting Au for Cu in the parent binary NdCu5 compound. The room temperature X-ray diffraction analysis indicates a cubic MgCu4Sn-type structure with space group F4¯3m (No. 216) for the NdCu4Au compound. The thermodynamic properties of NdCu4Au have been probed by magnetic susceptibility, χ(T), magnetization, M(μ0H), and specific heat, Cp(T), measured down to 1.8K. The low temperature χ(T) data shows probably an antiferromagnetic (AFM)-like anomaly associated with a Néel temperature TN=3.9K. In the paramagnetic region, χ(T) data follows the modified Curie–Weiss law with an effective magnetic moment μeff=3.547(5)μB and Weiss temperature θp=−10.19(8)K. The value for μeff is close to the value of 3.62μB expected for the Nd3+-ion. No evidence of metamagnetic transition was observed from the isothermal M(μ0H) results. Cp(T) data confirm the AFM phase transition at TN=3.5K close to the value of 3.9K observed in χ(T). The 4f-electron specific heat shows a Schottky-type anomaly around 20K associated with crystalline-electric-field (CEF), with energy splitting Δ1=62(5)K and Δ2=109(9)K of the Nd3+ (J=9/2) multiplet, that are associated with the first and second excited state of Nd3+-ion. From the results of the 4f-electron magnetic entropy, it is speculated that the CEF ground state of Nd3+ (J=9/2) ions is the Γ6 doublet for NdCu4Au.

Composition-selective fabrication of ordered intermetallic Au–Cu nanowires and their application to nano-size electrochemical glucose detection

Bimetallic nanostructures can provide distinct and improved physicochemical properties by the coupling effect of the two metal components, making them promising materials for a variety of applications. Herein, we report composition-selective fabrication of ordered intermetallic Au–Cu nanowires (NWs) by two-step chemical vapor transport method and their application to nano-electrocatalytic glucose detection. Ordered intermetallic Au3Cu and AuCu3 NWs are topotaxially fabricated by supplying Cu-containing chemicals to pre-synthesized single-crystalline Au NW arrays. The composition of fabricated Au–Cu NWs can be selected by changing the concentration of Cu-containing species. Interestingly, Au3Cu NW electrodes show unique electrocatalytic activity for glucose oxidation, allowing us to detect glucose without interference from ascorbic acid. Such interference-free detection of glucose is attributed to the synergistic effect, induced by incorporation of Cu in Au. We anticipate that Au3Cu NWs could show possibility as efficient nano-size electrochemical glucose sensors and the present fabrication method can be employed to fabricate valuable ordered intermetallic nanostructures.

Comparative study of the alloying effect on the initial oxidation of Cu-Au(100) and Cu-Pt(100)

Using in situ transmission electron microscopy, we show that the oxidation of the Cu-Au(100) results in the formation of Cu2O islands that deeply embed into the Cu-Au substrate while the oxidation of the Cu-Pt(100) leads to the formation of Cu2O islands that highly protrude above the Cu-Pt substrate. Their difference is attributed to the different mobilities of Pt and Au in the Cu base alloys for which the sluggish mobility of Pt in Cu results in trapped Pt atoms at the oxide/alloy interface while the faster mobility of Au in Cu leads to enhanced rehomogenization of the alloy composition.

Kinetics and thermodynamics associated with Bi adsorption transitions at Cu and Ni grain boundaries

The grain boundary diffusivity of Au in Cu and Cu-Bi, and Cu in Ni and Ni-Bi are characterized by secondary ion mass spectroscopy depth profiling. Samples are equilibrated in a Bi containing atmosphere at temperatures above and below the onset of grain boundary adsorption transitions, sometimes called complexion transitions. A simple thermo-kinetic model is used to estimate the relative entropic contributions to the grain boundary energies. The results indicate that the entropy term plays a major role in promoting thermally and chemically induced grain boundary complexion transition.

Scaling effects on grain boundary diffusivity; Au in Cu

This work quantifies the effect of grain size on grain boundary diffusivity. Diffusion of Au into Cu thin films of varying grain size is measured using secondary ion mass spectroscopy depth profiles. Appropriate models for fitting the composition profiles to analytical solutions to diffusion equations are identified by characterizing the microstructure both before and after the diffusion anneal. Finite-element numerical simulations are also employed to extract the diffusion coefficients directly from fits to experimental data. The results indicate that factors such as non-columnar grains, migrating grain boundaries, dislocations and stress gradients must be appropriately accounted for in order to obtain meaningful results. Overall, a strong scaling effect is not observed, which suggests that the atomic structure of boundaries in nanocrystalline thin-film materials probably resembles that of their coarse-grained counterparts.

Stopping power of Au for Cu ions with energies below Bragg’s peak

The stopping power of Au for Cu in the energy range 6 < E < 25 MeV was measured using a secondary beam of low velocity heavy ions produced by elastic scattering of an energetic primary beam (typically 28Si or 16O) on a natural Cu target. The results were compared to predictions of the Lindhard, Scharf and Schiott (LSS) theory, the binary theory (BT), and the unitary convolution approximation (UCA) and also to semi-empirical predictions such as the Northcliffe and Schilling tables and the SRIM2003 computer program.

Electronic Tongue for Qualitative Analysis of Aqueous Solutions of Salts Using Thick-film Technology and Metal Electrodes

An electronic tongue for the qualitative analysis of aqueous solutions of salts hasbeen developed. The following set of electrodes was used: RuO2, Ag, and Cu in thick-filmtechnology and Au, Pb, Zn and Ni as small bars of the corresponding metal. The response ofthe designed “electronic tongue” was tested on a family of samples containing pure salt andcomplex mixtures. The electrodes were used as potentiometric un-specific sensors and thee.m.f. of each electrode in contact with a certain aqueous solution was used as input signalfor a PCA analysis. The study showed that the set of electrodes were capable to discriminatebetween aqueous solutions of salts basically by their different content in anions and cations(the anions SO42-, Cl-, PO4H2-, CO3H-, NO3- and cations Na+ and K+ were studied). In orderto better analyze the basis for the discrimination power shown by the electronic tongue, aquantitative analysis was also envisaged. A fair estimation of the concentrations of thedifferent ions in the solutions studied appeared to be possible using this electronic tonguedesign.Keywords:

Effect of Minor Au Addition on Glass-Forming Ability and Mechanical Properties of Pd&amp;ndash;Cu&amp;ndash;Au&amp;ndash;Si&amp;ndash;P Alloys

The glass-forming ability (GFA), thermal stability and mechanical properties of bulk glassy Pd 79 Cu 6-x Au x Si 10 P 5 (x = 0-6 at%) alloys were studied. The results revealed that the minor substitution of Au for Cu strongly affects the thermal stability and GFA of Pd-Cu-Au-Si-P alloys. The alloy in which 2 at% Au was added to substitute Cu (Pd 79 Cu 4 Au 2 Si 10 P 5 ) exhibits the broadest supercooled liquid region (ΔT x = 80 K) and possesses the highest GFA among the alloys investigated. The critical diameter for glass formation of this alloy reaches ∼7 mm by copper mold casting. The GFA is dramatically reduced when the Au content is more than 5 at%, beyond which no bulk glasses could be formed. The Young's modulus, yield strength and maximum compressive strength of the best glass former (Pd 79 Cu 4 Au 2 Si 10 P 5 ) are 87 GPa, 1525 MPa and 1660 MPa, respectively, with a compressive plastic strain over 13%.

Structure analysis of Au-containing cuprate of Au 1+ xBa 2Ca 2Cu 3− xO 9 (Au-1223)

Single crystals of Au 1+ x Ba 2Ca 2Cu 3− x O 9 (Au-1223) were grown at 6 GPa and 1325 °C. This phase is the third member of the Au-containing cuprate family of AuBa 2Ca n−1 Cu n O 2 n+3 and shows a weak signal of superconductivity. The crystal structure was refined to R1 ( I>2 σ( I))=0.0188 and wR2=0.0485 with F 2 data. The Au-1223 phase crystallizes in the space group Pmmm (no. 47), a=3.8119(1) Å, b=3.8556(1) Å, c=15.4431(5) Å, Z=1. Partial substitution of Au for Cu in the CuO 2 planes was revealed. The Au incorporation to the conduction layers may be a reason for the suppression of superconductivity in Au-1223.

Microstructure-controlled interdiffusion of Cu/Co/Au thin films investigated by three-dimensional atom probe

The dependence of thin-film microstructure on the curvature of the substrate is investigated by field ion microscopy (FIM) and transmission electron microscopy (TEM). For this purpose, Cu–Au multilayers were deposited on W single crystals, which are prepared both as curved (radius of curvature ∼50 nm) and as planar substrates. 3D atom probe analysis shows that the curved substrates lead to heterogeneous reaction patterns by grain boundary reaction beside planar reaction patterns. This result is explained by local variations of microstructure. Grain boundary reaction is dependent on the atomic structure of the grain boundary and can lead to the early formation of intermetallics. At temperatures too low for significant volume diffusion, interdiffusion between enriched grain boundaries and neighbored grains is observed under the formation of ordered compounds. The elastic strain, which is caused by a considerable atomic size mismatch, is found to hinder interdiffusion and to cause alloying under grain boundary motion (DIGM). The introduction of a Co diffusion barrier between Au and Cu slows down interdiffusion, resulting in higher reaction temperatures. Thus, it is possible to observe that the grain boundary reaction in the Au layer disappears at elevated temperatures, at which volume diffusion of Cu in Au gets possible, whereas in the Cu layer the grain boundary reaction is still observed.

PHASE RELATIONS IN THE SYSTEM Au Cu Ag AT LOW TEMPERATURES, BASED ON NATURAL ASSEMBLAGES

The composition of some 85 particles of native gold from the 15 Mile rodingite in serpentinite showing in the Coquihalla gold district and the Wheaton Creek placer in the Dease Lake district, British Columbia, falls mainly within the Cu-rich portion of the Au–Ag–Cu phase diagram. The particles display a variety of textures, including exsolution-induced textures. The textures and phase compositions are used to construct a low-temperature phase diagram for the system Au–Ag–Cu. This diagram represents a system that equilibrated for a longer period of time at a lower temperature than is accessible in synthetic systems. In this diagram, the composition AuCu is nearly stoichiometric, a three-phase region is found among AuCu (Au1.0Cu0.971Ag0.003), Au2Cu (Au2.0Cu0.962Ag0.045), and Au3.0Ag0.71Cu0.23, and there is a two-phase region on the Au side of the Au2Cu–Au3.0Ag0.71Cu0.23 join. There are insufficient data to determine the phase relationships that exist between the compositions Au and Au2Cu, which includes Au3Cu. Au2Cu may or may not be part of a solid solution centered around Au3Cu. The sequence of formation of extensive exsolution-induced textures and their relation to the other textures seen are explained by combining the phase diagram with a significant shift in the position of the solvi with temperature in this system. Low values of Ag in Au–Cu alloy and of Cu in Au–Ag alloy are reported. Grains of gold alloy from deposits where no Au–Cu alloy is found appear to be undersaturated with respect to Cu.

Electric dipole polarizabilities of negative ions of the coinage metal atoms

The dipole polarizability of the coinage metal anions is calculated in different approximations for both the electron correlation and relativistic effects. The large polarizabilities of these ions give a good illustration of the performance of different computational methods. The spin-averaged Douglas-Kroll no-pair scheme combined with the CCSD(T) treatment of the electron correlation predict the following values of polarizabilities: 243 au for Cu −, 213 au for Ag − and only about 93 au for Au −. The non-relativistic CCSD(T) results are 278, 299 and 256 au, respectively. Both the non-relativistic and relativistic results agree with the QCISD(T) pseudopotential data, though serious discrepancies occur at the level of the QCISD approximation. The use of the scalar Pauli approximation for the estimation of the relativistic contribution to the dipole polarizabilities of the coinage metal ions is limited to Cu − and Ag −. For Au − this approximation strongly overestimates the magnitude of the relativistic effect.