Cu-Au Alloys Research Articles

AuCu/ZnO heterogeneous photocatalysts: Photodeposited AuCu alloy effect on product selectivity in alkene epoxidation

AuCu metal alloy nanoparticles were photodeposited on ZnO nanorods (ZnO_NRs) which proved to be effective photocatalysts for alkene epoxidation. The alloy nanoparticles were photodeposited onto ZnO nanorods with controlled ratios of gold and copper, with the deposition monitored in situ by UV-Vis spectroscopy. The alloy catalyst hybrids were tested for activity toward styrene epoxidation and HMF oxidative esterification, where the photoreactions were both optimized for time, temperature, and metal ratio content of the catalyst. The Au0.54Cu1/ZnO_NR alloy catalysts showed excellent photocatalytic activity and were most effective for conversion of styrene to styrene epoxide, where the product selectivity could be controlled by varying the metal ratio. Cu content in the alloy NP was essential to this process, as shown by the extrema in terms of metal content, using Au/ZnO only, where 100% benzaldehyde was obtained as the product. Au/ZnO evidenced best photocatalytic activity for HMF esterification, with conversion rapidly diminishing upon alloying of Au with Cu. A detailed XPS study was carried out to investigate reaction mechanism based on these studies, in particular, mechanisms are proposed for styrene epoxidation and oxidation cycles using the AuCu/ZnO_NR photocatalysts.

High-Efficiency Biocidal Solution Based on Radiochemically Synthesized Cu-Au Alloy Nanoparticles.

The use of nanotechnologies in the applied biomedical sciences can offer a new way to treat infections and disinfect surfaces, materials, and products contaminated with various types of viruses, bacteria, and fungi. The Cu-Au nanoparticles (NPs) were obtained by an eco-friendly method that allowed the obtaining in a one-step process of size controlled, well dispersed, fully reduced, highly stable NPs at very mild conditions, using high energy ionizing radiations. The gamma irradiation was performed in an aqueous system of Cu2+/Au3+/Sodium Dodecyl Sulfate (SDS)/Ethylene Glycol. After irradiation, the change of color to ruby-red was the first indicator for the formation of NPs. Moreover, the UV-Vis spectra showed a maximum absorption peak between 524 and 540 nm, depending on the copper amount. The Cu-Au NPs presented nearly spherical shapes, sizes between 20 and 90 nm, and a zeta potential of about −44 mV indicating a good electrostatic stability. The biocidal properties performed according to various standards applied in the medical area, in dirty conditions, showed a 5 lg reduction for Staphylococcus aureus, Pseudomonas aeruginosa, and Enterococcus hirae, a 5 lg reduction for both enveloped and non-enveloped viruses such as Adenovirus type 5, Murine Norovirus, and human Coronavirus 229E, and a 4 lg reduction for Candida albicans, respectively. Thus, the radiochemically synthesized Cu-Au alloy NPs proved to have high biocide efficiency against the tested bacteria, fungi, and viruses (both encapsulated and non-encapsulated). Therefore, these nanoparticle solutions are suitable to be used as disinfectants in the decontamination of hospital surfaces or public areas characterized by high levels of microbiological contamination.

A Study on the Quantitative Analysis of Portable XRF for the Components Analysis of Metal Cultural Heritage

In this study we conducted component analyses of portable XRF detectors using four Au-Cu alloy standard samples to improve their accuracy by drawing up a calibration curve based on ICP-OES standard values. The portable XRF analysis found absolute errors of 0.3 to 3.7 wt% for Au and 0.2 to 8.2 wt% for Cu, confirming that the error range and standard deviation differed from one detector to another. Furthermore, the calibration curve improved their accuracy, such that the relative error rates of Au and Cu decreased from 9.8% and 14% to 3.5% and 3.7%, respectively. Accordingly, an experiment to confirm the calibration curve was conducted using unknown samples, finding that the measured values of the unknown samples fell on the calibration curve. Therefore, to accurately analyze the components of metal cultural heritage items, it is necessary to prepare a calibration curve for each element after checking whether the detector is suitable for the artifact.

Electrochemical Filling of Nanoporous Gold with Silver via Surface-Limited Redox Replacement

Nanoporous (np) metals feature a unique network of interconnected pores and ligaments structured at a length scale of a few to hundreds of nanometers. As a result, np metals have an inherently higher surface area-to-volume ratio resulting in lower melting temperature compared to their bulk metal counterparts. Most applications of np metals have been associated with electrocatalysis. However, in recent years, a growing interest in the use of np metals in microelectronics have been pursued. Among these applications is the development of np-Cu-based chip-to-substrate interconnections that is achieved by Cu-on-Cu bonding. np-Cu is an ideal candidate for solder alternatives because of its relatively lower melting and sintering temperatures. Furthermore, the incorporation of a thin film of Sn in the pores and/or the surface of the np-Cu ligaments could further lower its sintering temperature while maintaining low resistivity. In order to fabricate these composites, there must be strict control over the uniform and flat deposition of the Sn thin film on np-Cu.Here we present a proof-of-concept study on the development of an all-electrochemical metal-filling of np structures. Conventional methods like electron beam induced deposition and overpotential deposition (OPD) have been developed for the filling of porous structure. However, these approaches showed only partial filling that are also accompanied by the growth of three-dimensional protrusions forming outside of the pores. In order to overcome these complications, we developed an approach based on electrochemical atomic layer deposition by surface-limited redox replacement (SLRR). This method allows for a layer-by-layer deposition of metals that has been shown to form relatively flat and uniform deposits. In this study, the model porous structure is np-Au, which is prepared by the selective de-alloying of pre-deposited Au-Cu alloys. The np-Au structure was then filled with Ag via SLRR (in a one-cell configuration) that is realized by successive cycles of the adsorption of a Pb monolayer, serving as a sacrificial metal, that is immediately replaced by Ag via electroless galvanic displacement.A comparative study suggests that the SLRR approach results in a more uniform Ag-filling compared to a stepwise potentiostatic OPD approach that was also done independently. Morphological characterization via surface and cross-sectional scanning electron microscopy (SEM) reveals progressive filling of np-Au with Ag. Additionally, electrochemical BET characterization via cyclic voltammetry of Pb-underpotential deposition (Pb-UPD) was also used to monitor the filling process by measuring the electrochemical surface area of the Ag-filled np-Au composite. The findings from this study will be discussed in detail. Reference:Castillo, Y. Xie, and N. Dimitrov “Filling in nanoporous gold with silver via bulk deposition and surface-limited redox replacement approaches” Electrochimica Acta, 2021, 380, 138196.

The Impact of Ceria Loading on the CuOx−CeO2 Interaction and Performance of AuCu/CeO2−SiO2 Catalysts in CO‐PROX Reaction

AbstractThe use of CeO2 as a promoter for non‐reducible supports, such as SiO2, enhances the catalytic activity by facilitating the activation of species such as O2 and H2O. However, in the case of the bimetallic nanoparticles (NPs), the presence of promoters may also influence the migration of the metallic species, dramatically impacting the stabilization of catalytic sites and as consequence, the overall catalytic performance. In the case of AuCu alloy supported catalysts, the reduced form, i. e., AuCu alloy, or the oxidized form, i. e., Au−CuOx species, present different activities/selectivities. Moreover, several parameters, such as reactional conditions, AuCu particle size, support nature, and pretreatments dictate which species are stabilized on the catalyst surface. The preferential oxidation of CO (CO‐PROX) reaction, that takes place under the H2‐rich stream, favors the stabilization of the reduced form of the AuCu catalysts. However, the Cu distribution in the crystal lattice and the final composition of the AuCu NPs are dependent on the mobility of the Cu species on the support. In this work, we show that the loading of the CeO2 as a promotor in AuCu/SiO2 catalysts impacts the CeOx species and the Cu reincorporation into the Au lattice and consequently, the activity and stability of the catalysts under CO‐PROX reaction.

Synergistic Integration of AuCu Co-Catalyst with Oxygen Vacancies on TiO2 for Efficient Photocatalytic Conversion of CO2 to CH4

Photocatalytic reduction of CO2 toward eight-electron CH4 product with simultaneously high conversion efficiency and selectivity remains great challenging owing to the sluggish charge separation and transfer kinetics and lack of active sites for the adsorption and activation of reactants. Herein, a defective TiO2 nanosheet photocatalyst simultaneously equipped with AuCu alloy co-catalyst and oxygen vacancies (AuCu-TiO2-x NSs) was rationally designed and fabricated for the selective conversion of CO2 into CH4. The experimental results demonstrated that the AuCu alloy co-catalyst not only effectively promotes the separation of photogenerated electron-hole pairs but also acts as synergistic active sites for the reduction of CO2. The oxygen vacancies in TiO2 contribute to the separation of charge carriers and, more importantly, promote the oxidation of H2O, thus providing rich protons to promote the deep reduction of CO2 to CH4. Consequently, the optimal AuCu-TiO2-x nanosheets (NSs) photocatalyst achieves a CO2 reduction selectivity toward CH4 up to 90.55%, significantly higher than those of TiO2-x NSs (31.82%), Au-TiO2-x NSs (38.74%), and Cu-TiO2-x NSs (66.11%). Furthermore, the CH4 evolution rate over the AuCu-TiO2-x NSs reaches 22.47 μmol·g-1·h-1, which is nearly twice that of AuCu-TiO2 NSs (12.10 μmol·g-1·h-1). This research presents a unique insight into the design and synthesis of photocatalyst with oxygen vacancies and alloy metals as the co-catalyst for the highly selective deep reduction of CO2.

Melting and phase diagram of Au-Cu alloy at nanoscale

The Au-Cu nanoalloys (NPs) were produced by NaBH4 co-reduction from the mixture precursor solution of component Au and Cu, under the protection of sodium citrate stabilizer. The Au-Cu NPs encapsulated in SiO2, Au5Cu5@SiO2 NPs, were then synthesized by the Stöber method based on the obtained Au-Cu NPs. The microstructure and particle sizes were characterized by TEM observation. The phase transition temperature of Au-Cu@SiO2 was determined by DSC measurement. The surface tension, surface segregation and surface energy of Au-Cu NPs as a function of particle size and temperature were calculated by the theoretical model based on the thermodynamic properties of pure component of Au and Cu NPs, respectively. The thermodynamic parameters of Au-Cu system at various nanoscales were optimized on the basis of experimental and theoretical results in this work and previous results in literature. Based on the obtained the thermodynamic data set, the nano phase diagrams of the Au-Cu system at nanoscale were calculated, and the thermodynamic properties of Au-Cu nanometer system were obtained. The results show that the calculated results agree well with experimental work.

Compensation Effect Exhibited by Gold Bimetallic Nanoparticles during CO Oxidation.

While CO oxidation catalyzed by gold nanoparticles has been practiced academically for several decades, there are still important discoveries to be made. One area of current interest is to pair Au with another alloying metal and observe the catalytic consequences of the presence of the other metal. In this work, TiO2-supported bimetallic Au nanoparticles are alloyed with Cu, Co, Ni, Pd, and Ru and used as catalysts for CO oxidation. Two synthetic methods for the alloys are presented: a strong electrostatic adsorption (SEA) method and a sterically demanding ligand synthesis (SDLS) method which uses triphenylphosphine (TPP) as the ligand. The catalytic performance of the materials synthesized with the SEA and SDLS methods is compared in CO oxidation. The results indicate that the materials tested present an enthalpy–entropy compensation effect. Interestingly, both the enthalpy of activation, ΔH‡, and the entropy of activation, ΔS‡, generally decrease with particle size. AuCo and AuRu materials exhibit a decrease in the overall activity as compared to Au and the other Au alloys when synthesized via SEA. Au face-centered-cubic alloys AuCu, AuNi, and AuPd prepared via SEA show an improvement in activity compared to monometallic Au in our reaction conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy presents two distinct regions for Au bimetallics where AuCo and AuRu show peak positions in the region of 2070–2050 cm–1, indicating a weaker interaction for AuCo and AuRu with CO when compared to that of the other alloys. For the SDLS method samples, the hypothesis is that TPP would enhance the CO oxidation rate by enhancing the charge transfer to the metallic surface. The results indicate that SDLS samples have lower CO oxidation rates and if any charge transfer occurs, it is masked by the lateral interactions of the CO π bonds and the phenyl groups of TPP.

Synergistic Effect of Cu Single Atoms and Au-Cu Alloy Nanoparticles on TiO2 for Efficient CO2 Photoreduction.

The synergy between metal alloy nanoparticles (NPs) and single atoms (SAs) should maximize the catalytic activity. However, there are no relevant reports on photocatalytic CO2 reduction via utilizing the synergy between SAs and alloy NPs. Herein, we developed a facile photodeposition method to coload the Cu SAs and Au-Cu alloy NPs on TiO2 for the photocatalytic synthesis of solar fuels with CO2 and H2O. The optimized photocatalyst achieved record-high performance with formation rates of 3578.9 for CH4 and 369.8 μmol g-1 h-1 for C2H4, making it significantly more realistic to implement sunlight-driven synthesis of value-added solar fuels. The combined in situ FT-IR spectra and DFT calculations revealed the molecular mechanisms of photocatalytic CO2 reduction and C-C coupling to form C2H4. We proposed that the synergistic function of Cu SAs and Au-Cu alloy NPs could enhance the adsorption activation of CO2 and H2O and lower the overall activation energy barrier (including the rate-determining step) for the CH4 and C2H4 formation. These factors all enable highly efficient and stable production of solar fuels of CH4 and C2H4. The concept of synergistic SAs and metal alloys cocatalysts can be extended to other systems, thus contributing to the development of more effective cocatalysts.

Robust Cu-Au alloy nanowires flexible transparent electrode for asymmetric electrochromic energy storage device

Flexible electrochromic energy storage (EES) systems have attracted tremendous attention because of their combined advantages of color-changing and energy-storing. Although copper nanowires (Cu NWs) flexible transparent electrodes (FTEs) have been considered as one of the most promising candidates for next-generation flexible electronics, its instinct electrochemical instability hindered its application in flexible electrochemical energy system, including EES device. Herein, we proposed a highly conductive electrochemically stable FTE based on Cu-Au alloy NWs network synthesized in a facile and simple electrochemical method. Compared with the pristine Cu NWs network that breaks into disconnected nanorods less than 10 s in 1 M H2SO4 solution during electrochemical anodic corrosion, the Cu-Au NWs network remained stable after 4000 s under the same condition. Furthermore, thanks to the forming of firm joints between the stacked NWs during inducting Au atoms, the optical-electrical performance (a sheet resistance of 23.2 Ohm/sq at a transmittance of 87%) and mechanical flexibility were highly improved. Based on these improvements, anodic PANI and cathodic WO3 layer were successfully deposited onto the Cu-Au NWs FTE by electrochemical method. An asymmetric-EES (AEES) device was assembled utilizing these complementary electrodes, which concurrently exhibited excellent electrochromic (coloration efficiency: 153.77 cm2/C at 633 nm) and energy storage performance (areal capacitance: 2.29 mF/cm2), demonstrating their potential for smart multifunctional flexible electronics.

First-principles study on the Cu-Au alloy monolayer supported on WC for hydrogen evolution

Rational design of supported catalysts with alloy monolayer on suitable substrate is of crucial importance for reducing the cost and enhancing the catalytic activity. In this work, the reactivity of Cu-Au alloy monolayer supported on WC for HER is investigated using density functional theory (DFT) calculations, and the results are compared with those of the supported pure monolayer ones. It is found that the Cu-Au alloy monolayers on WC exhibit enhanced stability and tunable HER activity. Superior activity of Cu2Au7d@WC(0001) for HER (ΔG = 0.007 eV) is identified according to the calculated Gibbs free energy and the exchange current density. Geometric and electronic analyses enable to a deep understanding on the important role of the specific configuration and ratio of Cu and Au for moderate H adsorption. Cu doping leads to the synergistic ligand and stress effects in the alloy systems, which further optimize the adsorption characters and the charge redistribution around the dopants, which is essential to the high HER activity. The present study sheds light on the significance of alloy effects on tuning the geometric stability and electronic properties as well as the catalytic activity, which may benefit to the rational design and development of supported alloy materials.

In situ atomic-scale observation of AuCu alloy nanowire with superplasticity and high strength at room temperature

Metallic nanowires usually exhibit ultrahigh strength but suffered low ductility. Previous studies on pure metals suggested this strength-ductility trade-off results from limiting the dislocation activities. However, it is unclear whether such deformation model is valid for a solid solution alloy as well. Here, for the first time, the atomic-scale deformation process of AuCu nanowires with size of ∼16 nm was investigated in situ. The results show the NWs exhibit superplasticity (∼185%) and high strengths (∼2.98 GPa) at room temperature. It was discovered that superplasticity originates from continuous full dislocation nucleation and disappearance, as well as dislocation dipole formation and annihilation etc., which differ from the previous studies in pure metals. The observed full dislocation activities, also different from the ones in the previous studies, suggested that, as the size of the metals is below ∼100 nm, their deformation should be governed by partial dislocation and twinning.

Thermodynamic properties of Cu3Au alloy

Based on the general expression of the Helmholtz free energy of alloys with Ll2 structure obtained in previous works, we calculate the equations of the long-range order (LRO) parameter, and order-disorder transition (ODT) temperature in the Cu3Au alloy, and obtained thermodynamic quantities of Cu3Au alloy as simple functions of temperature and LRO-parameter, at pressures P = 0. Numerical results of the linear thermal expansion coefficient, isochoric and isobaric heat capacities, isothermal and adiabatic bulk modulus, are in good agreement with experiments.

Lattice stability of ordered Au-Cu alloys in the warm dense matter regime

In the warm dense regime, where the electron temperature is increased to the same order of the Fermi temperature, the dynamical stability of elemental metals depends on its electronic band structure as well as its crystal structure. It has been known that phonon hardening occurs due to an enhanced internal pressure caused by electron excitations as in close-packed simple metals, whereas phonon softening occurs at a specific point in the Brillouin zone as in body-centered cubic metals. Here, we investigate the dynamical stability of binary ordered alloys (Au and Cu) in the L1$_0$ and L1$_2$ structures. By performing first principles calculations on phonon dispersions, we demonstrate that warm dense Au-Cu systems show phonon hardening behavior except the lowest frequency phonon mode at point R of AuCu in the L1$_0$ structure. We show that when a phonon mode is stabilized by long-range interatomic interactions at ambient condition, such a phonon will be destabilized by the short-range nature of the warm dense matters.

Localized surface plasmon resonance of Au–Cu alloy nanoparticles enhances the performance of polymer photovoltaic devices for outdoor and indoor applications

Au-Cu alloy nanoparticles (NPs) are synthesized for triggering localized surface plasmon resonance (LSPR) in organic photovoltaic devices (OPVDs). Because Cu is readily oxidized, alloying with Au enhances the chemical stability of the NPs, thereby simplifying the fabrication processes. The electrical characterizations indicate that the alloy NPs improve the device performance under both one-sun illumination and indoor lighting conditions due to the effects of LSPR. Finally, the result of the stability test reveals that the use of the Au-Cu NPs would not affect the device stability. We anticipate that the results in this work open up a new avenue for plasmonic-enhanced OPVDs featuring low cost, stable nanostructures.

Theoretical insights into the selective and activity of CuAu catalyst for O2 and CO2 electroreduction

By using first principles calculations, CuAu alloy catalysts were shown to exhibit a high catalytic activity at low overpotential. The proposed approach is promising for explorations of the selectivity of both the oxygen reduction (evolution) reaction (ORR/OER) and the carbon dioxide reduction reaction (CO2RR) via the low-index (100) and (110) facets on the CuAu grain boundary (GB) surface, leading to a high selectivity toward CH4 on CuAu (110) and indicating that the CuAu is a highly efficient catalyst that can improve the catalytic performance of CO2 reduction to generate hydrocarbon fuels. The CO2 reduction reaction refers to the transfer of a series of successive protons/electrons (H+/e−) from the CuAu surface by different reaction mechanism pathways. We found that CuAu exhibits a promising catalytic activity for CO2 reduction that can be used for CH4 selective conversion energy storage. The results predicted that intermediate species form during the hydrogenation steps by spontaneous reactions. The atomic level can be used to gain insight into the high-performance catalysts and electrochemical mechanisms involved in the reduction of CO2 to different hydrocarbon fuels, which is an important method of revealing the reaction mechanism of the elementary steps for CO2RR and should be examined in future studies.

Non-aqueous synthesis of AuCu@ZnO alloy-semiconductor heteroparticles for photocatalytical degradation of organic dyes

Heteroparticles with anisotropic structure have emerged as a focus of research. They contain two distinct sides with different composition, structures, ionization potential and surface chemistry. The asymmetric structure allows a tuning of chemical, optical, electrical, magnetic, mechanical and thermodynamic properties within a single particle by controlling composition, size, shape and organization at the nanoscale. Here we report the preparation of AuCu@ZnO heteroparticles using non-aqueous solution chemistry by in situ alloying of the metal domains of AuCu@ZnO nanoparticles. The size, shape and optical properties of the AuCu@ZnO hybrid nanoparticles were characterized by transmission electron microscopy and UV–visible spectroscopy. The nanocrystals have a multipod-like morphology with ZnO domains connected to an AuCu alloy core. The AuCu@ZnO nanoparticles showed a pronounced red-shift of the plasmon band compared to Au@ZnO heteroparticles. The crystal structure and phase purity were confirmed by X-ray powder diffraction. Surface-functionalization with imidazole-type ligands rendered the AuCu@ZnO nanoparticles water soluble. The AuCu@ZnO alloy heteroparticles showed an enhanced activity compared to Au@ZnO for the photocatalytic degradation of organic pollutants, as demonstrated with the model compound rhodamine B.

The Impact of the Manufacturing and Corrosion Steps of the AuCu Master Alloy on the Catalytic Activity of Nanoporous Gold for CO Oxidation

Nanoporous gold (npAu) fabricated from a AuCu alloy is a complex procedure mostly because of the passivated outer surface and the inhibited corrosion phenomena observed in those materials as a consequence of the formation of a Pm¯3m (AuCu3). To further deepen our understanding of the leaching of Cu in these materials, a series of experiments were performed to study the corrosion process and finally start giving an answer to this problem. Here we compare different corrosion methods, beginning with free corrosion (as the standard synthetic route for npAu) and finishing with an optimized potentiostatic dealloying (four-step method) procedure. In order to distinguish between the structural and chemical effects affecting the catalytic activity, a series of experiments employing scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) coupled with Energy-dispersive X-ray spectroscopy (EDXS), and X-ray photoelectron spectroscopy (XPS) were performed before and after catalytic CO oxidation. The data reveals a clear influence in key synthetic steps in the master alloy synthesis (thermal treatment), as well as in the dealloying procedure.

Effects of aging on the electrical properties of Au/n-Si/Ti, Cu/n-Si/Ti and AuCu/n-Si/Ti Schottky diodes

In this study, it is aimed to produce Au/n-Si/Ti, Cu/n-Si/Ti and AuCu/n-Si/Ti Schottky diodes and to calculate the diode parameters such as ideality factor, barrier height and series resistance by using time-dependent current–voltage (I-V) measurements. First, pure Ti metal was evaporated on the matt surface of the n-Si semiconductors and annealed at 420 ͦ C in nitrogen atmosphere. Secondly, Au and Cu metals and AuCu alloy were evaporated on the other surface of n-Si semiconductors. So, Au/n-Si/Ti, Cu/n-Si/Ti and AuCu/n-Si/Ti Schottky diodes were obtained. Finally, I-V measurements of the Schottky diodes were taken at specified time intervals (immediately,1, 7, 15, 30, 90, 180 and 365 days) and time-dependent diode parameters were calculated using the Thermionic Emission, Cheung and Norde methods.

First-Principles Calculations of Structural and Electronic Properties of Au-Cu Alloys

This article studies the effect of Au:Cu ratios on the structural properties, electronic structures, and the formation of Au-Cu bulk alloys using density functional calculations. The structural properties are determined by the lattice constants (a, b, c), the crystal systems, and the total energy of the unit cells (Etot). The electronic structures are discussed through band structures, projected density of states (PDOS), and the total density of states (DOS) of materials. The obtained results show, there is an influence of the pseudo-field on the lattice constants of Au, Cu. In which, with the LDA-PWC pseudo-field, the result a = b = c = 4.0910 A with Au, a = b = c = 3.5710 A with Cu, results consistent with the experimental results. When increases concentration of Cu impurity in Au , resulting in the lattice constant of Au: Cu alloy decreases from a = b = c = 4.0910 A to 3.5710 A, the total energy of the cell decreases from Etot = -22952 eV to Etot = -24124 eV, an increase in the electronic density leads to an increase in the conductivity of the Au: Cu alloy. With the obtained results will serve as the basis for future experimental studies.