Transformation Of Au Research Articles

Controlling alloy to core-shell structure transformation of Au-Pd icosahedral nanoparticles.

The structure transformation between Au-Pd alloy and core-shell icosahedral nanoparticles was achieved by a one-step aqueous-phase strategy. This strategy provided a way to tune the structure and atomic distribution of Au-Pd icosahedral nanoparticles. It could modulate the electronic structure of Pd, achieving promoted electrocatalytic ability toward the hydrogen evolution reaction.

Roles of molecular weight-fractionated extracellular polymeric substance in transformation of Au(III) to Au nanoparticles in aqueous environments

Extracellular polymeric substance (EPS) is widely distributed in natural environments and plays important roles in the biogeochemical cycling of heavy metal. Earlier works reported that EPS could reduce metal ions such as Au(III) and Ag(I) to corresponding metal nanoparticles (NPs). EPS is a complex mixture of microbiogenic polymers with wide molecular weight (MW) distribution, and the specific components of EPS responsible for Au(III) reduction and AuNPs stabilization are still not well understood. In this study, the EPS of Shewanella oneidensis MR-1 was divided into six fractions with MW of <3, 3–10, 10–30, 30–50, 50–100, and >100 kDa, respectively through the ultrafiltration method and the roles of MW-fractionated EPS in the reduction of Au(III) to AuNPs were investigated. It was found that the low MW (<3 kDa) EPS was the major reducing agent in EPS but the fraction itself could not convert high concentration (>25 mg/L) of Au(III) to stable AuNPs due to its inferior AuNPs-stabilizing capacity. The high MW (>50 kDa) EPS could act as coating reagents to increase the stability of the formed AuNPs with sizes of 20–50 nm, but had low Au(III)-reducing activity. The carboxyl-containing substances in EPS may play crucial roles in stabilizing AuNPs. This finding is important for a better understanding of the differential roles of MW-fractionated EPS in the transformation and fate of Au(III) and AuNPs, as well as other metal ions and metal NPs in natural environments.

Isomer Structural Transformation in Au–Cu Alloy Nanoclusters: Water Ripple‐Like Transfer of Thiol Ligands

AbstractStructure isomerism is observed in noble metal nanoclusters; however, the mechanism study of this process is rarely reported. Herein, by using the biphosphine ligands instead of phosphine ligands, the high symmetry of the Au2Cu6(PPh3)2(SAdm)6 nanocluster is successfully broken and [Au4Cu4(L1)2(SAdm)5]Br (where, L1 is bis(diphenylphosphine)methane, DPPM) nanocluster is obtained with chiral arrangement. This newly obtained nanocluster contains a total of four isomers in crystal unit cell. Interestingly, these isomers undergo rapid isomorphism in solution, which is confirmed by 1H‐1H COSY spectrum. Density functional theory calculations demonstrate that the water ripple–like transfer of the thiol ligands results from the isomorphism of nanoclusters.

Shape Transformation of Bimetallic Au–Pd Core–Shell Nanocubes to Multilayered Au–Pd–Au Core–Shell Hexagonal Platelets

Transformation of metallic or bimetallic (BM) nanoparticles (NPs) from one shape to another desired shape is of importance to nanoscience and nanotechnology, where new morphologies of NPs lead to enhancement of their exploitable properties. In this report, we present the shape transformation of Au octahedral NPs to Au–Pd core–shell nanocubes, followed by their transformation to nanostars and finally to multilayered Au–Pd–Au core–shell hexagonal platelets in the presence of T30 DNA. The weaker binding affinity of T30 DNA directs the growth to favor the formation of lower energy {111} facets, changing the morphology from nanocubes to nanostar. The nanostars, exhibiting unusual intermediate morphologies, are comprised two sets of shell layers and have Au core, Pd intermediate shell, and Au outer shell. Similarly, the hexagonal platelets, which also have Au core and inner Pd shell, are encased in an external gold shell. The formation of multilayered Au–Pd–Au core–shell hexagonal platelets from Au–Pd core–shell nanocubes via the multilayered nanostars is monitored using scanning/transmission electron microscopy analysis.

Thermal and Photoinduced Reduction of Ionic Au(III) to Elemental Au Nanoparticles by Dissolved Organic Matter in Water: Possible Source of Naturally Occurring Au Nanoparticles

Naturally occurring Au nanoparticles (AuNPs) have been widely observed in ore deposits, coal, soil, and environmental water. Identifying the source of these naturally occurring AuNPs could be helpful for not only the discovery of Au deposits through advanced exploration methods, but also the elucidation of the biogeochemical cycle and environmental toxicity of ionic Au and engineered AuNPs. Here, we investigated the effect of natural/simulated sunlight and heating on the reduction of ionic Au by ubiquitous dissolved organic matter (DOM) in river water. The reductive process probed by X-ray photoelectron spectroscopy revealed that phenolic, alcoholic, and aldehyde groups in DOM act as reductive sites. Long-time exposure with thermal and photoirradiation induced the further fusion and growth of AuNPs to branched Au nanostructure as precipitation. The formation processes and kinetics of AuNPs were further investigated using humic acid (HA) as the DOM model, with comprehensive characterizing methods. We have observed that HA can reduce ionic Au(III) complex (as chloride or hydroxyl complex) to elemental Au nanoparticles under sunlight or heating. In this process, nearly all of the Au(III) could be reduced to AuNPs, in which HA serves as not only the reductive agent, but also the coating agent to stabilize and disperse AuNPs. The size and stability of AuNPs were highly dependent on the concentration ratio of Au(III) to HA. These results imply that, besides biological processes, this thermal or photochemical reduction process is another possible source of naturally occurring AuNPs in natural environments, which possibly has critical impacts on the transport and transformation of Au and engineered AuNPs.

Gold nanoclusters electrodeposited on multi-walled carbon nanotubes: enhanced electrocatalytic activity of hemoglobin

Three-dimensional structure domelike gold nanoparticle clusters (3D Au) are electrodeposited onto multi-walled carbon nanotubes (MWCNTs) through improved all-electrochemical three-step method, which involves (1) potential cycling which is performed in order to produce oxide functional groups (carbonyl, hydroxyl, and carboxyl) at the defect sites located at the ends and/or the sidewalls of the MWCNTs, (2) electrochemical oxidation of Au(III) complex to Au(V) complex by using potential-step method, and (3) electrochemical transformation of Au(V) complex to Au nanoparticle clusters on the surface of MWCNTs. This 3D fractal morphology is in marked contrast to the uniform nanoparticle dispersion of MWCNTs, which is achieved when the second step adopts cyclic voltammetry instead of a potential-step method. The 3D Au/MWCNTs-Nafion nanocomposite exhibits enhanced biocompatibility and expedites the electrical transfer from hemoglobin (Hb) to the surface of electrode. The average coverage (Γ) of Hb immobilized on the 3D Au/MWCNTs-Nafion electrode is calculated to be 7.65 × 10−9 mol cm−2 and testifies that a larger amount of Hb can access and be entrapped on the electrode surface. The electron transfer rate constant (kS) is calculated to be 1.8 s−1, which proves that the resulting nanocomposite facilitates Hb’s direct electrochemical behavior. Moreover, the Hb/Au/MWCNTs-Nafion electrode shows prominent electrocatalytic activity and good reversibility for the detection of hydrogen peroxide (H2O2) with a wide linear range from 0.5 to 1,000 μmol dm−3 and a low detection limit of 0.2 μmol dm−3. Meanwhile, the Michaelis–Menten constant (Km) is found to be 0.86 μmol dm−3. These attractive features are attributable to the unique 3D domelike structure of Au nanoparticle clusters on the MWCNTs with much higher electrochemically active surface areas. This provides an efficient platform for the development of mediator-free electrochemical biosensors.

Origin of tweed in Au–Cu–Al alloys

The premartensitic tweed in Au–Cu–Al alloys, contrary to previous thought that resort to defects, is confirmed to be associated with the coherent embryos of an intermediate phase (I phase) embedded in parent phase. The parent → I phase transformation temperature was measured by differential scanning calorimeter and dynamic mechanical analysers, which shifts from 82.3 to 557.6 °C depending on the alloy composition. X-ray diffraction and transmission electron microscopes (TEM) results show that the parent → I phase transformation is a charge density wave transition that cannot be suppressed even by melt-spun method, which shows obvious compositional inhomogeneity between I phase and parent. The results imply that the parent → I phase transition is a fast displacive transformation coupled with diffusion. Moreover, accompanying the parent → I phase transformation, alloys demonstrate diversified microstructure revealed by TEM observation, from tweed to chessboard nanowires or twins. These findings provide the experimental evidence for that parent → I phase transformation in Au–Cu–Al alloys is originated from pseudospinodal decomposition as theoretically predicted.

Crystallography of stress-induced [formula omitted] (trigonal) martensitic transformation in Au–49.5 at.%Cd alloy

The crystallography of B 2 - ζ 2 ′ (trigonal) stress-induced transformation in Au–Cd has been controversial. The present study was undertaken to clarify this issue by obtaining reliable experimental data, which enabled a critical test for the phenomenological crystallographic theory of martensitic transformation. The results revealed that the lattice invariant shear (LIS) was {1 0 0}〈0 1 1〉 C twinning in contrast to {0 1 1}〈1 0 0〉 C twinning, which was believed to be the case by all the previous investigators. The theoretical calculations using {1 0 0}〈0 1 1〉 C twinning agreed well with the observations in all respects. Therefore, the phenomenological crystallographic theory is shown to be well applicable to the present transformation. In addition to the stress-induced transformation, the morphologies of thermally induced martensites were examined, and two types of morphologies were observed: “roof-type” with LIS, and “herringbone-type” without LIS. Therefore, it is shown that transformations both with and without LIS are possible in the present transformation, and the previous discrepancy in LIS is rationalized.

Thermoelastic martensitic transformation in Au–Cu–Al alloys doped with Co or Ir

Thermoelastic martensitic transformations (TMT) were investigated in different Au 7+ x –Cu 5+ y –Al 4− z alloys, where x + y = z, doped with either Co or Ir. Transformation temperatures, heats of transformation, hysteresis cycles and microhardness were found strongly dependent on Au-content. The martensite self-accommodation patterns, detected using optical microscopy, after each martensite–parent phase–martensite (M–P–M) cycle only partially repeated in large grain size (≈1 mm) specimens, while the same pattern was found in small grain size (≈0.1 mm). Neither Co nor Ir apparently influence the nucleation sites of martensitic variants.

Quick XAFS Studies on the Y-Type Zeolite Supported Au Catalysts for CO−O2 Reaction

Au/zeolite catalysts prepared with a deposition-precipitation method were characterized with quick XAFS (QXAFS) in combination with IR. The data were correlated with the catalytic performance in the CO-O(2) reaction conducted at 273 K. On the basis of the XANES analysis of Au loaded on H-Y, the deposited Au(2)O(3) was observed at the initial stage. The transformation of Au(2)O(3) to form metal Au clusters was observed at 473 K in a H(2) atmosphere. The fact was supported by the IR measurement of adsorbed CO and the subsequent reaction with O(2). Detailed clustering process of Au supported catalysts could be directly followed by EXAFS analysis. The growth of metal Au proceeded via the formation of a Au(55) cluster at 473 K. Then it agglomerated to give metal Au with diameter of 2 nm at 723 K. The addition of H(2) was effective to retard the sintering of Au clusters. A similar phenomenon was observed over Au loaded on USY zeolite. In marked contrast to the H-Y and USY supports, significantly agglomerated Au particles generated on Na-Y zeolite, indicating the importance of the presence of acid sites in keeping the Au clusters with highly dispersed form. The performance of 5 wt % Au loaded on H-Y and USY in the CO-O(2) reaction was remarkably sensitive to the pretreatment temperature and the gas atmosphere. The catalyst pretreated with hydrogen showed a two-spike pattern with respect to the pretreatment temperature. Namely, the optimum activity was observed after the pretreatment at 373 and 723 K, where the temperatures corresponded to the generation of Au(2)O(3) and metal Au clusters with 2 nm diameter as evidenced by QXAFS analysis, respectively. The reason for enhancement of the activity of Au/H-Y by the addition of H(2) in the pretreatment step could be attributed to the formation of metal Au with appropriate size. In contrast to the H-Y and USY support, Au loaded on Na-Y prepared under the same condition was almost inactive in the reaction due to the formation of aggregated metal Au.

Kinetics of martensitic transformation in Au–Cd alloys

Martensitic transformation temperatures, M s, M f, A s, and A f, of an annealed Au–47.5at.% Cd–0.75at.% Cu alloy were obtained by X-ray diffraction method and electrical resistivity measurement. It is confirmed that the four values obtained by these two methods are in good agreement within an experimental error. The incubation time for the low temperature phase was observed during the isothermal holding above M s, from the time dependence of Bragg intensity. The incubation time becomes shorter when the set temperature is close to M s.

Aging effect in parent phase and martensitic transformation in Au–47.5at.%Cd alloys

Abstract Au–Cd alloy is one of the typical alloys which shows martensitic transformation. There are two martensites close to the 1:1 composition: one is γ′ 2 martensite and the other is ζ′ 2 martensite. When the phonon dispersion curve was measured in the composition for Au–47.5at.%Cd which produces γ′ 2 martensite, phonon softening was observed at the Brillouin zone boundary and at ζ=0.35 of the [ζζ0]TA 2 branch and a peculiar behavior was observed. One is that the M s temperature determined in this experiment was lower than the ordinary value. The other is the time dependence of the 1/3 elastic reflection, which was observed prior to the martensitic transformation. Electrical resistance measurements were performed in this alloy in order to clarify this peculiar behavior. A decrease of the M s temperature was observed after aging at 393 K, in the parent phase. The lower M s observed in neutron experiments can be explained by an aging effect in the parent phase. There are two possibilities of explaining the time-dependence of the 1/3 reflection; one is the transformation with diffusion (bainite transformation above M s ) and the other is embryo growing.

Martensitic Phase Transformation in Au&amp;ndash;Cu&amp;ndash;Zn Alloys Studied by &lt;SUP&gt;63&lt;/SUP&gt;Cu NMR

In order to obtain much information about the microscopic mechanism of martensitic transformations, 63 Cu nuclear magnetic resonance (NMR) and relaxation measurements have been performed at temperatures above and below the transformations in Au-Cu-Zn alloys with different compositions and transformation temperatures. The 63 Cu NMR spectra taken from the parent phase with a L2 1 type structure of the alloys comprising less and more Au contents can be decomposed into three and two components, respectively. It implies that the majority of Cu atoms occupy the corner sites, referring to the fundamental bcc lattice, and the minority of Cu atoms the body-centered sites with different Knight shifts. The component spectra reveal different distribution of the Cu atom, changing with Cu content. Although the NMR spectra taken from the martensite phase are too broad to decompose, we can obtain the average Knight shift with respect to the Cu atom distribution. The temperature dependence of the 63 Cu spin-lattice relaxation time follows the Korringa-like behavior in different ways by the alloys. The data on the relaxation rate are discussed in terms of the Knight shift value and the enhancement of electron-electron correlation. The transformation temperature has no clear relation with the NMR parameters.

Phase transformation in Au–Cd alloy (75 at.-%Cd)

AbstractThe Au–Cd alloy (75 at.-%Cd) undergoes a phase transformation ɛ′ to ɛ at 540 K. The crystal structure of the low-temperature ɛ-phase is reported to be hexagonal close packed. Olander evaluated the heat of transformation to be 100 cal mol−1 under certain assumptions. However, the exact nature of the phase transformation has not been studied. The purpose of the present investigation is to determine the characteristics of the transformation, including the crystal structures and lattice parameters, and also the thermodynamic properties of both phases near the transformation temperature.

Study of superelasticity associated with the thermoelastic martensitic transformation in Au—Cd alloys

Abstract Superelastic behaviour in Au-47·5 at.% Cd and Au-49·5 at.% Cd was studied using microscopic surface observation, an analysis of Laue photographs and measurement of the strain distribution. The superelasticity above the transformation temperature, T0, in Au—47·5 at. % Cd alloy was associated with the transformation, β(b.c.c.) β'(orthorhombic) upon loading and β'-β upon unloading. Below T0, superelasticity in a quenched and aged martensite was due to migration of twin boundaries in the martensite crystal. In Au—49-5 at.% Cd alloy, below T0, a significant superelasticity was observed when the specimen axis was near the [100]β pole. This superelasticity was accompanied by the transformation β“(hexagonal) β’ upon loading and β→β” upon unloading. Above T0, the superelasticity observed was identical to that found in Au-47-5 at.% Cd. In the stress-induced transformation, upon loading, a band consisting of alternate layers of the b.c.e. phase and the martensite phase was formed at one end of the specimen and advanced towards the other end. The superelastic strain δ∊τ was measured with a strain gauge.

The ultrasonic equation of state in the vicinity of the martensitic transformation in Au - 47.5 at.% Cd: Y. Gefen and M. Rosen, Nuclear Research Centre-Negev, P.O. Box 9001, Beer-Sheva, Israel; and Materials Engineering Department, Ben Gurion University, Beer-Sheva, Israel.

The ultrasonic equation of state in the vicinity of the martensitic transformation in Au - 47.5 at.% Cd: Y. Gefen and M. Rosen, Nuclear Research Centre-Negev, P.O. Box 9001, Beer-Sheva, Israel; and Materials Engineering Department, Ben Gurion University, Beer-Sheva, Israel.