Au Bimetallic Nanoparticles Research Articles

Synthesis of Hollow Ag–Au Bimetallic Nanoparticles in Polyelectrolyte Multilayers

Ag nanoparticles of ~20 nm size and rather uniform size distribution were synthesized in polyelectrolyte multilayers (PEMs) via an ion-exchange/reduction process in two stages (seeding and growth), which were used as sacrificial templates to fabricate Ag-Au bimetallic hollow nanoparticles via galvanic replacement reaction. The reaction process was monitored by UV-vis spectroscopy. The morphology and structure of the nanoparticles were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy, which confirmed the formation of hollow Ag-Au bimetallic nanoparticles. UV-vis absorbance spectroscopy and TEM results indicated that both size and optical properties of the Ag nanoparticles in the PEM can be controlled by manipulating ion content in the PEM and the number of the ion-exchange/reduction cycle, whereas that of Ag-Au bimetallic nanoparticles were dependent on size of the Ag templates and the replacement reaction kinetics. The hollow Ag-Au bimetallic nanoparticles exhibited a significant red shift in the surface plasmon resonance to the near-infrared region. The strategy enables facile preparation of hollow bimetallic nanoparticles in situ in polymer matrixes.

Investigation of factors influencing the catalytic performance of CO oxidation over Au–Ag/SBA-15 catalyst

Au–Ag bimetallic nanoparticles (NPs) supported on SBA-15 have been prepared by a two-step method and characterized by ICP-AES, XRD, UV–vis, TG-DTG, XPS and TEM. Au–Ag/SBA-15 bimetallic catalyst with a low metal loading of 1.26wt.% exhibited high catalytic performance for low temperature CO oxidation, which was governed by Au/Ag molar ratio and the pretreatment conditions (calcination and reduction). The Au–Ag/SBA-15 with an actual Au/Ag molar ratio of 5.4/1 showed the highest catalytic activity for CO oxidation (T100=20°C), and it has also been found that catalytic activity was strongly related with the calcination and reduction temperature of the bimetallic catalyst. The initial CO conversion was increased with the calcination temperature and then decreased above 500°C. It was necessary to activate the bimetallic catalyst and completely remove the amine groups in the catalyst at 500°C for the high activity of CO oxidation. The severe sintering of Au–Ag bimetallic NPs at 700°C resulted in an obvious loss of activity. H2 reduction following the calcination played an important role in the enhancement of catalytic activity for CO oxidation. The best catalytic performance was obtained in the activation temperature range of 500–600°C, and then decreased with a further increase of reduction temperature to 700°C. The reduction treatment induced the surface redistribution of gold and silver. The formation of a closer bulk value of the surface Au/Ag molar ratio after reduction at 500–600°C and a more random alloy resulted in the improvement in activity. However, the surface enrichment of Ag NPs and the severe aggregation of particles after high temperature reduction (>600°C) caused the activity decrease.

Bimetallic AuAg Nanoparticles: Enhancing the Catalytic Activity of Au for Reduction Reactions in the Liquid Phase by Addition of Ag

Teamwork of noble partners: The preparation and catalytic properties of small, highly stable, and monodisperse Au-, Ag- and bimetallic AuAg-nanoparticles with well-defined compositions are reported. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Au–Ag bimetallic nanoparticles, binary and ternary mixed micelles as active catalysts for the reduction of [Co(NH3)5Br](NO3)2 by iron(II)

The kinetics of reduction of [Co(NH3)5Br](NO3)2 by Mohr's salt has been studied in presence of Au–Ag bimetallic nanoparticle, binary, and ternary mixed micelles. The Au–Ag bimetallic nanoparticles with mole ratio of 0.75:0.25 show higher catalytic activity than that of Au nanoparticles of same particle size. The higher catalytic activity of the bimetallic nanoparticles is mainly for electronic charge transfer from adjacent Ag to Au atoms. The effects of binary mixed micellar environments of anionic with non-ionic surfactants (SDS/TX-100, SDS/Tween 80 and SDS/Brij 35) and ternary (SDS/Tween 20/Brij 35) mixed micellar environment on the electron transfer rate have been determined. These effects are attributed to the effect of the coulombic interactions and compared with that of pure ones. Above the CMC of the mixed micellar systems, the effects are explained quantitatively using a kinetic model.

ORR Activity Enhancement of MBE-Prepared Pt Monolayer on Au Single-Crystal Substrate

Oxygen reduction reaction (ORR) activities of Pt/Au(111) and Pt/Au(100) bimetallic surfaces prepared using molecular beam epitaxy were evaluated in this study. Surface structures of the bimetallic surfaces were verified using a scanning tunneling microscope under ultra-high vacuum conditions. The deposition of a 0.3-nm-thick Pt on a clean Au(111) at 300 K (300K-Pt0.3nm/Au(111)) generated a corrugated Pt(111) epitaxial layer on which monoatomic-height Pt islands are present. The ORR activity estimated from the kinetically controlled current density at 0.9 V vs. RHE was ca. two times greater than that of the clean Pt(111). In contrast, the 473K-Pt0.3nm/Au(111) as well as 300K-Pt0.3nm/Au(100) exhibited small hydrogen-related charges, thereby were less ORR active than the corresponding clean Pt substrate surfaces. These results suggest that atomic arrangements of surface Pt atoms, particularly the morphologies of the most dense plane of Pt(111), correlate with the enhancement in the ORR activity of Pt–Au bimetallic nanoparticles.

Core–Shell Bimetallic Nanoparticles Robustly Fixed on the Outermost Surface of Magnetic Silica Microspheres

The major challenges in practically utilising the immense potential benefits of nanomaterials are controlling aggregation, recycling the nanomaterials, and fabricating well-defined nanoparticulate materials using innovative methods. We present a novel innovative synthetic strategy for core–shell bimetallic nanoparticles that are well-defined, ligand-free, and robustly fixed on the outermost surface of recyclable magnetic silica microspheres. The strategy includes seeding, coalescing the seeds to cores, and then growing shells from the cores on aminopropyl-functionalised silica microspheres so that the cores and aminopropyl moieties are robustly embedded in the shell materials. The representative Au–Ag bimetallic nanoparticles fixed on the microsphere showed excellent catalytic performance that remained consistent during repeated catalytic cycles.

Effects of particle size, composition, and support on catalytic activity of AuAg nanoparticles prepared in reverse block copolymer micelles as nanoreactors

Block copolymer (BCP) micelles formed by polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) were used as nanoreactors for preparing stable bimetallic AuAg nanoparticles (NPs) with finely tunable composition and diameters. Micelles loaded with NPs were then deposited onto TiO2 and Al2O3 in order to investigate the catalytic properties of the NPs with respect to CO oxidation. The polymer matrix could be removed by thermal treatment at 400°C under H2 or O2. Different parameters, such as support, particle size, and metal composition were varied independently, so that their influence on the catalytic activity for CO oxidation could be separated. The atomic Au/Ag ratio was varied from 1:2 to 2:1, and the highest activity was obtained for an Au/Ag ratio of 1, showing synergy effects of both metals for catalyzing CO oxidation. Using nanosized TiO2 as support and an Au/Ag ratio of 1, the nanoparticle diameter was varied between ∼3 and ∼20nm which led to a variation of activity by a factor of ∼7 (in a continuous flow reactor at 70°C) with the smallest particles showing the highest turnover frequencies (TOFs). In comparison, regular TiO2 as catalyst carrier showed significantly lower performance than nanostructured TiO2 while no activity was found on non-reducible γ-Al2O3. Independently, all three parameters (metal particle size, Au/Ag atomic ratio, and the support) showed significant effects on the catalytic activity.

Fabrication of thermally stable and active bimetallic Au–Ag nanoparticles stabilized on inner wall of mesoporous silica shell

A general method has been developed for the fabrication of highly dispersed and thermally stable bimetallic Au-Ag nanoparticles (NPs) stabilized on the inner wall of a mesoporous silica shell. In our approach, gold particles were formed in the first step on carbon spheres decorated with Sn(2+) cations. Upon Ag(+) adsorption and reduction by l-ascorbic acid in the second step, specific nanoparticles with a gold-silver alloy core and a silver nanoshell have been formed. Important evidence of the core-shell configurations of the bimetallic Au-Ag nanoparticles were clearly characterized by UV-vis, TEM and HAADF-STEM observations combined with elemental mapping and line scans. The mesoporous silica outer shell was obtained through the hydrolysis and condensation of the precursors tetraethoxysilane (TEOS) in a basic condition and cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. On this basis, the nanoreactors were fabricated after calcination, which further serves as a nanoreactor for the reduction of p-nitrophenol. Furthermore, such particles have been found to be thermally stable and their sizes remain substantially unchanged even upon calcination in air at 500 °C and a reduction treatment in H2. Potentially, the method can be developed into a general approach to synthesize other highly dispersed and thermally stable bimetallic nanoparticles stabilized on the inner wall of a mesoporous silica shell.

Reactivity Analysis of the AunAgm (6 ≤ n + m ≤ 12) Bimetallic Clusters with Selected Proportions

ABSTRACTIt is well known that Au-Ag bimetallic nanoparticles have better performance in catalytic processes compared to their counterpart pure clusters. The improvement in their catalytic properties has been attributed to a kind of synergy between the gold and silver atoms that has not been fully understood. Unlike pure clusters, there are very few studies on the catalytic behavior of the Au-Ag binary nanoparticles. From the theoretical point of view, in the subnanometer regimen, the bimetallic Au-Ag clusters present a challenging problem, since by combining the different gold and silver relativistic effects, a variety of skeletal geometric structures and homotopic distributions are obtained. In particular, pure gold has favorable planar structure even up to 16 atoms, while silver begins to favor 3D arrangements from 5-7 atoms. This dissimilar behavior produces a diverse population of 2D and 3D coexisting binary clusters, whose properties strongly depend of the Au/Ag mixing ratio. In this work we use the relativistic approach ZORA-DFT to model the AunAgm (with 4 ≤ (n + m) ≤ 12) binary nanoclusters in selected proportions (1:1, 3:1, 5:1) in the gas-phase and we study their reactivity from the descriptors based in the condensed Fukui indexes obtained from an NBO electronic population analysis.

A stable bimetallic Au–Ag/TiO2 nanopaper for aerobic oxidation of benzyl alcohol

Well-dispersed small Au-Ag bimetallic nanoparticles (sub-3 nm) on one dimensional TiO2 nanobelts were synthesized by a facile successive photodeposition-galvanic replacement method. Based on these Au-Ag/TiO2-NB nanostructures, a porous paper-like monolithic catalyst was fabricated and exhibited synergistically enhanced activity and stability for catalyzing aerobic oxidation of benzyl alcohol.

AgAu Bimetallic Janus Nanoparticles and Their Electrocatalytic Activity for Oxygen Reduction in Alkaline Media

Bimetallic AgAu Janus nanoparticles were prepared by galvanic exchange reactions of 1-hexanethiolate-passivated silver (AgC6) nanoparticles with gold(I)-mercaptopropanediol complex. The AgC6 nanoparticles were deposited onto a solid substrate surface by the Langmuir-Blodgett method such that the galvanic exchange reactions were limited to the top face of the nanoparticles that was in direct contact with the gold(I) complex solution. The resulting nanoparticles exhibited an asymmetrical distribution not only of the organic capping ligands on the nanoparticle surface but also of the metal elements in the nanoparticle cores, in contrast to the bulk-exchange counterparts where these distributions were homogeneous within the nanoparticles, as manifested in contact angle, UV-vis, XPS, and TEM measurements. More interestingly, despite a minimal loading of Au onto the Ag nanoparticles, the bimetallic AgAu nanoparticles exhibited enhanced electrocatalytic activity in oxygen reduction reactions, as compared to the monometal AgC6 nanoparticles. Additionally, the electrocatalytic performance of the Janus nanoparticles was markedly better than the bulk-exchange ones, suggesting that the segregated distribution of the polar ligands from the apolar ones might further facilitate charge transfer from Ag to Au in the nanoparticle cores, leading to additional improvement of the adsorption and reduction of oxygen.

Fabrication of Au@Ag Core–Shell Nanoparticles Using Polyelectrolyte Multilayers as Nanoreactors

A new synthetic strategy has been developed for the fabrication of Au-Ag bimetallic core-shell nanoparticles (NPs) using polyelectrolyte multilayers (PEMs) as unique nanoreactors. Bimetallic NPs composed of Au core and Ag shell were successively incorporated into PEMs by repeating anion/cation exchange/reduction cycle multiple times in a stepwise manner. The strategy described here allows for the facile preparation of Au@Ag core-shell NPs with well-controlled core and shell dimensions and geometrically tunable optical properties by simply varying the number of ion-exchange/reduction cycles in the PEM matrix. The strategy can be extended to synthesize in situ other core-shell NPs in polymer matrix.

Fabrication of catalytically active AgAu bimetallic nanoparticles by physical mixture of small Au clusters with Ag ions

Catalytically highly active PVP-protected AgAu bimetallic nanoparticles (BNPs) less than 2nm in diameter were fabricated by simultaneous physical mixture of aqueous dispersions of Au clusters with Ag+ ions. The prepared AgAu BNPs, the dispersion of which was stably kept for more than 2 months under ambient conditions, were characterized by UV–vis, ICP, HR-TEM, and EDS in HR-STEM. The prepared BNP colloidal catalysts possessed a high activity for aerobic glucose oxidation. The highest activity of 3.77mol-glucoses−1mol-metal−1 was observed for the BNPs prepared with Ag/Au atomic ratio of 2/8, which was more than two times higher than that of Au nanoparticles with nearly the same particle sizes.

In situ biosynthesis of Ag, Au and bimetallic nanoparticles using Piper pedicellatum C.DC: Green chemistry approach

The synthesis of Ag, Au and Ag–Au bimetallic nanoparticles using Piper pedicellatum C.DC leaf extract is demonstrated here. The rapid formation of stable Ag and Au nanoparticles has been found using P. pedicellatum C.DC leaf extract in aqueous medium at normal atmospheric condition. Competitive reduction of Ag+ and Au3+ ions present simultaneously in solution during exposure to P. pedicellatum C.DC leaf extract leads to the synthesis of bimetallic Ag–Au nanoparticles in solution. Transmission electron microscopy (TEM) analysis revealed that the Ag nanoparticles predominantly form spherical in shape with the size range of 2.0±0.5–30.0±1.2nm. In case of Au nanoparticles, the particles are spherical in shape along with few triangular, hexagonal and pentagonal shaped nanoparticles also observed. X-ray diffraction (XRD) studies revealed that the nanoparticles were face centered cubic (fcc) in shape. Fourier transform infrared spectroscopy (FTIR) showed nanoparticles were capped with plant compounds. The chemical constituents, viz. catechin, gallic acid, courmaric acid and protocatechuic acid of the leaf extract were identified which may act as a reducing, stabilizing and capping agent. The expected reaction mechanism in the formation of Ag and Au nanoparticles is also reported.

Chloroplasts-mediated biosynthesis of nanoscale Au-Ag alloy for 2-butanone assay based on electrochemical sensor

We reported a one-pot, environmentally friendly method for biosynthesizing nanoscale Au-Ag alloy using chloroplasts as reducers and stabilizers. The prepared nanoscale Au-Ag alloy was characterized by UV–visible spectroscopy, X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM). Fourier transform infrared spectroscopy (FTIR) analysis was further used to identify the possible biomolecules from chloroplasts that are responsible for the formation and stabilization of Au-Ag alloy. The FTIR results showed that chloroplast proteins bound to the nanoscale Au-Ag alloy through free amino groups. The bimetallic Au-Ag nanoparticles have only one plasmon band, indicating the formation of an alloy structure. HR-TEM images showed that the prepared Au-Ag alloy was spherical and 15 to 20 nm in diameter. The high crystallinity of the Au-Ag alloy was confirmed by SAED and XRD patterns. The prepared Au-Ag alloy was dispersed into multiwalled carbon nanotubes (MWNTs) to form a nanosensing film. The nanosensing film exhibited high electrocatalytic activity for 2-butanone oxidation at room temperature. The anodic peak current (Ip) has a linear relationship with the concentrations of 2-butanone over the range of 0.01% to 0.075% (v/v), when analyzed by cyclic voltammetry. The excellent electronic catalytic characteristics might be attributed to the synergistic electron transfer effects of Au-Ag alloy and MWNTs. It can reasonably be expected that this electrochemical biosensor provided a promising platform for developing a breath sensor to screen and pre-warn of early cancer, especially gastric cancer.

Polyelectrolyte‐Multilayer‐Supported Au@Ag Core‐Shell Nanoparticles with High Catalytic Activity

Au-Ag bimetallic core-shell nanoparticles are synthesized in situ in polyelectrolyte multilayers via a sequential ion-exchange/reduction process. The Au-Ag bimetallic nanoparticles show a higher catalytic activity than Au and Ag monometallic ones for the reduction of 4-nitrophenol by NaBH(4) .

The IP6 micelle-stabilized small Ag cluster for synthesizing Ag–Au alloy nanoparticles and the tunable surface plasmon resonance effect

The stable small Ag seeds (size in diameter < 10 nm) were obtained in the presence of inositol hexakisphosphoric (IP6) micelles. Then Ag–Au bimetallic nanoparticles were synthesized through a replacement reaction with the rapid interdiffusion process between such small Ag seeds in nanoclusters and HAuCl4. Adjusting the dosage of HAuCl4 resulted in different products, which possessed unique surface plasmon resonances (SPR). The morphologies of the as-made nanoparticles were observed using transmission electron microscopy and field emission scanning electron microscopy and their compositions were determined by energy-dispersive x-ray spectroscopy. Among them, the Ag–Au alloy nanoparticles with the cauliflower-like structure had a suitable SPR for highly sensitive Raman detection application as a surface-enhanced Raman scattering (SERS) substrate with a long-term stability of six months.

A facile strategy to synthesize bimetallic Au/Ag nanocomposite film by layer-by-layer assembly technique

A facile strategy has been developed for the preparation of bimetallic gold–silver (Au–Ag) nanocomposite films by alternating absorption of poly-(ethyleneimine)–silver ions and Au onto substrates and subsequent reduction of the silver ions. The composition, micro-structure and properties of the {PEI–Ag/Au}n nanocomposite films were characterized by ultraviolet visible spectroscopy (UV–vis), transmisson electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), surface enhanced Raman scattering (SERS) and cyclic voltammetry (CV). The UV–vis characteristic absorbances of {PEI–Ag/Au}n nanocomposite thin film increase almost linear with the number of bilayers, which indicates a process of uniform assembling. Appearance of a double plasmon bands in the visible region and the lack of apparent core–shell structures in the TEM images confirm the formation of bimetallic Au–Ag nanoparticles. The result of XPS also demonstrates the existence of Ag and Au nanoparticles in the nanocomposite films. TEM and FESEM images show that these Ag and Au nanoparticles in the films possess sphere structure with the size of 20–25nm. The resulting {PEI–Ag/Au}n films inherit the properties from both the metal Ag and Au, which exhibits a unique performance in SERS and electrocatalytic activities to the oxidation of dopamine. As a result, the {PEI–Ag/Au}n films are more attractive compared to {PEI–Ag/PSS}n and {PEI/Au}n films.

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

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

Direct synthesis of metal nanoparticles with tunable porosity

Herein, we report a facile one-step synthesis route of porous bimetallic Au–Ag nanoparticles involving two parallel processes: alloying during nanocrystal growth and dealloying via galvanic replacement reaction. Further, we show that porosity in these nanoparticles can be tuned via their alloy composition. The specific surface area of the synthesized porous nanoparticles is significantly enhanced compared to that of their solid bulk counterparts.