one-step synthesis of gold bimetallic nanoparticles with various metal-compositions

Journal of Alloys and Compounds 562 (2013) 74–83

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Journal of Alloys and Compounds

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One-step synthesis of gold bimetallic nanoparticles with variousmetal-compositions

Maria Antoaneta Bratescu a,⇑, Osamu Takai b, Nagahiro Saito c

a EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japanb Materials and Surface Engineering Research Institute, Kanto Gakuin University, Kanazawa-ku, 1-50-1 Mutsuura-Higashi, Yokohama 236-8501, Japanc Department of Materials, Physics and Energy Engineering, Green Mobility Collaborative Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

a r t i c l e i n f o

Article history:Received 12 November 2012Received in revised form 27 December 2012Accepted 5 February 2013Available online 19 February 2013

Keywords:Bimetallic nanoparticlesGold alloyPlasma in solutionElectrode erosionTEMPOL free radical

0925-8388/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2013.02.033

⇑ Corresponding author. Tel.: +81 052 789 5274.E-mail addresses: [email protected].

nagoya-u.jp (O. Takai), [email protected]

a b s t r a c t

A rapid, one-step process for the synthesis of bimetallic nanoparticles by simultaneous metal reductionand gold erosion in an aqueous solution discharge was investigated. Gold bimetallic nanoparticles wereobtained by alloying gold with various types of metals belonging to one of the following categories: diva-lent sp metals, trivalent sp metals, 3d or 4d metals. The composition of the various gold bimetallic nano-particles obtained depends on electrochemical factors, charge transfer between gold and other metal, andinitial concentration of metal in solution. Transmission electron microscopy and energy dispersive spec-troscopy show that the gold bimetallic nanoparticles were of mixed pattern, with sizes of between 5 and20 nm. A red-shift of the surface plasmon resonance band in the case of the bimetallic nanoparticles Au–Fe, Au–Ga, and Au–In, and a blue-shift of the plasmon band of the Au–Ag nanoparticles was observed. Inaddition, the interaction of gold bimetallic nanoparticles with unpaired electrons, provided by a stablefree radical molecule, was highest for those NPs obtained by alloying gold with a 3d metal.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

In this paper, we studied synthesis of gold nanoparticles (NPs)alloyed with various metals using a rapid, one-step process by adischarge in aqueous solution method, named Solution PlasmaProcessing (SPP), and we investigated the interaction of gold bime-tallic NPs with free radicals.

In material science, the properties of metallic NPs can be greatlyimproved and extended by making mixtures of elements to gener-ate complex intermetallic compounds and alloys. The unique fea-tures of structure, composition, segregation behavior, and thenovel optical, electrical, magnetic, and chemical characteristics ofalloyed nanomaterials have promoted the extensive experimentaland theoretical research efforts in this field [1,2]. The most impor-tant properties of nanoalloys used in applications are their surfaceplasmon resonance (SPR), their suitability for biodiagnostics, theirmagnetic properties, and their catalytic activity. SPR can be chan-ged by particle–particle interactions, by changing the NPs shapeand size, or by alloying with other metals, which may have poten-tial implications in solar cell systems to enhance the opticalabsorption of the visible light [3]. Historically, bimetallic NPs werefirst synthesized by alloying gold or silver with other rare metals

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jp (M.A. Bratescu), takai@(N. Saito).

and the SPR band tunability was experimentally and theoreticallystudied [4,5]. Recently, the catalytic activity of nanoalloys, espe-cially bimetallic ones, was demonstrated to be higher than in thecase of monometallic NPs. Different types and mixing patternstructures of bimetallic NPs display amazing catalytic properties,and has led to the design of new nanoalloys [6,7]. The giant mag-neto-resistance of mixed nonmagnetic metals with magnetic 3dmetal NPs demonstrates the possible applications of these bimetal-lic NPs in magnetic recording and sensors [8,9]. Bimetallic nano-rods have been used in biology as gene-delivery systems, sincedifferent metals have selective binding properties to biologicalmolecules [10].

Usually, bimetallic NPs are synthesized chemically by co-reduc-tion [4–6], thermal decomposition [9,11], and by a galvanicreplacement reaction [12,13]. A seed-mediated method was alsoused to synthesize binary metal nanocrystals [14–16]. Most chem-ical methods frequently introduce the toxic hydrogen tetrachloro-aurate precursor to form the gold NPs. Other methods previouslyemployed are co-deposition by using cluster beam sources of bothmetals [8] and electrodeposition from metal electrodes [10,17,18].

In this paper, gold bimetallic NPs were synthesized using a ra-pid, one-step process in SPP. Plasma in water has been known since1899, when different pairs of metal electrodes were used to gener-ate the discharge, and the optical emission spectra were collectedin order to explain different features of spectral lines observed inastronomy [19]. Now, in our group, SPP is a useful and simple

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M.A. Bratescu et al. / Journal of Alloys and Compounds 562 (2013) 74–83 75

method for metal NP synthesis, since this non-equilibrium plasmacan provide extremely rapid reduction of a metal ion to the neutralform without using a reducing agent, and offers the possibility tocontrol the size by controlling the surrounding chemistry, and, fur-thermore, it operates at room temperature and pressure [20–22].In the present work, we demonstrate another merit of the SPPmethod for the fabrication of bimetallic NPs using a combinationof the reduction reaction of the metal (M) ion to the neutral state,while simultaneously eroding the electrodes during the discharge,which generates the second metal in the structure of the bimetallicNPs. This combination of processes makes SPP a more useful meth-od, since no reducing agent or gold precursor is required in thereaction mixture, thus offering an ecologically friendly procedurefor nanostructure synthesis.

The gold bimetallic NPs were synthesized by alloying gold withvarious types of metals, which are divided in four categories: (i)divalent sp metals, Zn and Cd, (ii) trivalent sp metals, Ga and In,(iii) 3d metals Fe, Co, Ni, and Cu, and (iv) 4d metals Pd and Ag.We studied the optical properties of the gold nanoalloys, as wellas their crystallinity, composition, size and morphology, and wehave evaluated the interaction of various gold bimetallic NPs withfree radicals for possible consideration as catalysts. We found thatthe properties of the gold bimetallic NPs are connected with thecategory to which the metal belongs. We analyzed that the amountof the intermetallic compound in the composition of the goldbimetallic NPs depends on the electrochemical potential and theelectron density of the Wigner–Seitz cell.

Table 1Composition, size, morphology, and water solubility of gold bimetallic NPs synth

Au-M Composition from RIR methoda

Gold (wt%) Other compounds (wt%)

Divalent sp metalsAu–Zn 48 Zn(OH)2 46

Au3Zn 5Zn0.965Au0.035 1

Au–Cd 63 AuCd 10Au3Cd2.97 27

Trivalent sp metalsAu–Ga 3 (Au23Ga2)0.16 37

Au0.9Ga0.1 3(Au9Ga)0.4 57

Au–In 69 (Au23In2)0.16 31

3d MetalsAu–Fe 85 Au0.05Fe0.95 15

Au–Co 78 (Au9Co)0.4 5Au4Co 16Co0.975Au0.025 1

Au–Ni 95 Ni0.92Au0.08 1Au0.45Ni0.55 4

Au–Cu 80 Au3Cu 3Au0.8Cu0.2 4Cu2O 13

4d MetalsAu–Ag 29 Ag 26

AgAu 45

Au–Pd 73 AuPd 8Au1.1Pd0.9 8(AuPd9)0.4 10Pd(NO3)2 1

a The gold bimetallic NPs were prepared from a solution with an initial salt

2. Synthesis and analytical methods

2.1. Chemicals

Palladium (II) nitrate hydrate, Indium (III) nitrate hydrate, Gallium (III) nitratehydrate, Cadmium nitrate tetrahydrate, Copper (II) nitrate trihydrate, Iron (III)nitrate nonahydrate, Cobalt (II) nitrate hexahydrate were purchased from Sigma–Aldrich. Silver nitrate, Zinc nitrate hexahydrate, Nickel (II) nitrate hexahydrate,and potassium chloride were purchased from Kanto Chemicals Co. Inc., Japan. Allthe nitrates were used as received, without further purification, prepared as stocksolutions in deionized water to a 50 mM concentration, and kept in the dark be-tween experiments. Solutions with the desired concentration were prepared freshon the day of use.

For Electron Spin Resonance (ESR) experiments, we used 4-Hydroxy-2,2,6,6-tet-ramethylpiperidine 1-oxyl, TEMPOL, purchased from Sigma–Aldrich. TEMPOL wasdissolved in benzene (Kanto Chemicals Co. Inc., Japan) to a concentration of1 mM, and used on the day of preparation.

2.2. Synthesis

Detailed description of the SPP system has previously been reported [20–22].The reaction chamber in the SPP experiment was a 50 mL Teflon� vessel pro-

vided with two-rod gold electrodes with 1 mm diameter, covered with ceramictubes, and set at 0.5 mm separation distance. We used a pulsed bipolar high voltagesource with 5 kV and 10 A maximum peak voltage and current, respectively (Peku-ris, Kurita Seisakusyo Co. Ltd.). The solution plasma was produced under a constantpeak high voltage of 1 kV, and peak discharge current in the range from 1.34 to1.42 A, depending slightly on the solution conductivity, a fixed pulse width of1.5 ls, and a fixed frequency of 15 kHz. Synthesis of gold bimetallic NPs was per-formed in metal nitrate solutions with concentrations of 0.5, 1, and 2 mM, over5 min.

esized in SPP.

Average size (nm) Morphology aggregationsolubility in water

20 Chains of NPs which tend to aggregate

Insoluble

20 Chains of NPs which tend to aggregateInsoluble

5 SphereNo aggregationSoluble

3 SphereNo aggregationSoluble

5 Almost no aggregationSlightly soluble


Insoluble

10 Chains of NPs which trend to aggregateInsoluble


Insoluble

8 SphereNo aggregation.Soluble


Soluble

concentration of 1 mM.

Fig. 1. TEM image, with HRTEM image inserted in right-down corner (top left), EDS images with STEM-BF image (bottom) and EDS spectrum (middle right), and SAED pattern(top right) for bimetallic NPs synthesized with gold and one of the 3d metals: Fe, Co, Ni, and Cu. SAED patterns show identification of phases in NPs composition.

76 M.A. Bratescu et al. / Journal of Alloys and Compounds 562 (2013) 74–83

The presence of various excited species in the gas phase plasma was measuredby optical emission spectroscopy (OES) method, using an optical fiber coupled witha UV–visible spectrograph (USB4000, Ocean Optics), in the wavelength range from250 to 800 nm, with 2 nm spectral resolution.

The experimental conditions, solution pH and conductivity are indicated inTable IS. The values of peak current for the different conductivities are given inTable ISa of the Supplementary data.

2.3. Analytical methods

After the synthesis, the colloidal solutions of the gold bimetallic NPs were char-acterized by UV–vis spectroscopy (UV–vis–NIR 3600 spectrometer, Shimadzu) inthe spectral range 330–800 nm, with 1 nm spectral resolution, and an opticalabsorption path of 1 cm.

The morphology of the gold bimetallic NPs was observed by transmission elec-tron microscopy (TEM – JEM – 2500SE, Jeol) with 200 kV accelerating voltage. Thesamples for TEM analysis were prepared by dropping the as-prepared solution afterSPP on a copper or molybdenum (in the case of Au–Cu bimetallic NPs) TEM grid.High resolution TEM (HRTEM) analysis was performed using the previously re-ported procedure, recording the images close to the Scherzer defocus [21]. Calcula-tion of the distribution and diffraction pattern from selected area electrondiffraction (SAED) analysis were performed with the Process Diffraction V-6.2.0software [23]. The composition of the NPs was observed by energy dispersive spec-troscopy (EDS) of individual or a few bimetallic NPs.

The crystal structure of the gold bimetallic NPs was analyzed by X-rays diffrac-tometry (XRD, SmartLab, Rigaku), equipped with Cu Ka radiation source(k = 0.154056 nm), using an X-ray powder diffraction method. The NP powdersfor XRD analysis were prepared by centrifugation of the colloidal solutions contain-ing the bimetallic NPs, and drying them on the XRD glass holder, at room temper-ature, over night. The identification of the crystalline phases in the gold bimetallicNPs structure was carried out using Integrated X-ray Powder Diffraction Software –PDXL Qualitative analysis, from Rigaku Corporation. The quantitative analysis soft-ware, based on the reference intensity ratio (RIR) method [24], was applied to find

out the composition of the gold bimetallic NPs. The JCPDC card numbers, used forthe identification of the XRD diffraction peaks, together with the crystal phaseand the lattice constant of the compounds found in the NPs composition, are indi-cated in Table IIS of the Supplementary data. The XRD results were compared withthose of TEM–SAED analysis. In addition, for the NPs synthesized in a 1 mM nitratesolution, the composition determined by the XRD – RIR method was verified by Ve-gard’s law [11,25].

2.4. Interaction with free radicals

The interaction of the gold bimetallic NPs with free radicals of TEMPOL wasstudied by ESR spectroscopy, using an ESR Spectrometer, JES-FA 200, Jeol. Theparameters used in ESR spectroscopy were: 0.6 mW microwave power, sweepwidth of ±7.5 mT, modulation width 0.08 mT, and time constant 0.1 s. The goldbimetallic NPs were prepared as powders after centrifugation, removal the superna-tant liquid and drying for 1 day at room temperature. The NPs were dispersed byultrasonication in 1 mM TEMPOL solution in benzene to a concentration of10 mg mL�1. The colloidal solution was immediately measured by ESR. The ESR sig-nal of the gold bimetallic NPs in TEMPOL solution was compared with the ESR signalcorresponding to the same volume of TEMPOL solution in benzene. For the ESRmeasurements, we used quartz tubes with 5 mm diameter.

The ESR signal was evaluated using JEOL Data Processing software by integrat-ing the spectrum. The standard for the number of spins in TEMPOL solution was ta-ken from a 550 lL volume of 1 mM TEMPOL solution in benzene, which contains3.33 � 1017 spins.

3. Results

3.1. Composition, morphology and crystal structure

Table 1 shows the composition, size, morphology, and the watersolubility of gold bimetallic NPs synthesized by SPP, in the case of

Fig. 2. TEM image, with HRTEM image inserted in right-down corner (top left), EDSimages with STEM-BF image (bottom) and EDS spectrum (middle right), and SAEDpattern (top right) for bimetallic NPs synthesized with gold and one of the 4dmetals: Pd and Ag. SAED patterns show identification of phases in NPs composition.

Fig. 3. TEM image, with HRTEM image inserted in right-down corner (top left), EDSimages with STEM-BF image (bottom) and EDS spectrum (middle right), and SAEDpattern (top right) for bimetallic NPs synthesized with gold and one of the spdivalent metals: Zn and Cd. SAED patterns show identification of phases in NPscomposition.


an initial metal nitrate solution concentration of 1 mM. The mor-phology, TEM and HRTEM images, SAED patterns of a region con-taining the gold bimetallic NPs, EDS spectra and mapping imagesof gold and metal M are presented in Figs. 1–4, corresponding tovarious types of metals.

The crystalline phase identification, and the composition deter-mined by XRD – RIR method with an accuracy of ±1%, are shown inFigs. 5 and 6. Careful analysis was performed to check the presenceof metal oxide or nitrate compounds in the composition of the goldbimetallic NPs. The identification of the XRD diffraction peaks wasconfirmed by the analysis of the TEM–SAED patterns. The experi-mental lattice constant (aexp) was directly obtained from the(111) and (200) atomic reflections from the XRD spectra by usingBragg’s law. On the other hand, the calculated lattice constant(acalc) was evaluated from the Vegard’s law in order to confirmthe composition obtained by the XRD – RIR method. In this case,the calculated lattice constant, acalc, is given by the relation:a ¼ 1

100

Pncnan, where

Pncn ¼ 100, an represents the lattice con-

stant from JCPDC card, and cn is the composition in percent ofthe n-th component of the gold bimetallic NPs [11,25]. The valuesof the experimental and calculated lattice constant aexp and acalc,respectively, of the gold bimetallic NPs are given in Table 2.

3.2. Optical properties

Fig. 7 represents the dependence of the maximum wavelengthof the SPR band of the gold bimetallic NPs, on the initial metal saltconcentration in the SPP solution, in the case of Au–Fe, Au–Ga, Au–In, and Au–Ag particles. Pure gold NPs, synthesized by electrodeerosion in a 1 mM solution of KCl in water, were considered as ref-erence, with the SPR band at 520 nm.

The absorbance spectra of the gold bimetallic NPs synthesizedby SPP for different concentrations of the metal nitrate, are pre-sented in Fig. 1S of the Supplementary data. The errors in thedetermination of the maximum wavelength of the SPR band were±1%.


The relative decrease in the number of spins in a solution con-taining a stable free radical molecule is defined asRS ¼ ð1� N=N0Þ � 100; where N0 and N represent the number ofspins in TEMPOL solution before and after mixing with gold bime-tallic NPs, respectively. The accuracy in determining the number ofspins was within ±10%. Fig. 8 shows the dependence of RS on the

Fig. 4. TEM image, with HRTEM image inserted in right-down corner (top left), EDSimages with STEM-BF image (bottom) and EDS spectrum (middle right), and SAEDpattern (top right) for bimetallic NPs synthesized with gold and one of the sptrivalent metals: Ga and In. SAED patterns show identification of phases in NPscomposition.


initial metal salt concentration for various gold bimetallic NPs syn-thesized by SPP. The ESR signal of TEMPOL in benzene before andafter mixing with the gold bimetallic NPs is shown as an examplein Fig. 2S of the Supplementary data, in the case of Au–Ni bimetal-lic NPs. We have not noticed a change in the appearance of the ESRspectra, but only a decrease in the amplitude of the signal.

SPP characterization was performed by OES. The most impor-tant excited atoms and radicals detected in SPP by OES are shownin Fig. 3S of the Supplementary data.

4. Discussion

4.1. Mechanism of gold bimetallic nanoparticles formation in SPP

SPP is a useful and simple method for the synthesis of metal NPssince this non-equilibrium plasma can provide extremely rapidreactions due to the reactive chemical species, radicals, and UVradiation produced in an atmospheric pressure plasma [20–22].The most important merits of the SPP for the NPs synthesis, ascompared to chemical methods, are the short processing time (inthe range of a few minutes to several tens of minutes), and prepa-ration at room temperature and pressure conditions. Solution plas-ma offers a new reaction medium, where hydrogen, hydroxyl, and

oxygen radicals are produced, and where the hydrogen radical isthe most responsible for the reduction reaction of the metal ionto the neutral atom, and therefore a reducing agent is not neces-sary (Fig. 3S). The synthesis of gold bimetallic NPs in SPP demon-strates another useful merit of this method, due to the fact thatthe electrode material can be eroded in plasma, so furnishing oneof the element contained in the bimetallic NP. The other metal ofthe alloy is obtained by reducing the metal ion to the neutral atomfrom a metal salt present in the solution.

One of the most common methods for producing monometallicNPs is the reduction of metal salts in a proper solvent. This simplechemical method was also applied to the synthesis of bimetallicNPs starting from two kinds of metal salts, which were together re-duced in a solution [4–6]. However, in this strategy, it is difficult tosimultaneously control the reduction and nucleation processes oftwo types of metals which have different redox potentials andchemical behaviors.

In SPP, the gold nanoalloys were synthesized in a different way(Fig. 9). First, gold is produced in plasma by electrode erosion.There are various processes by which the metal electrodes canerode or wear away. First, there is the obvious process of a chem-ical reaction, such as oxidation or corrosion. This process in SPP hasa small contribution to the production of gold in the solution, be-cause gold is difficult to oxidize and the solution pH value doesnot fall below 3 for most of the metal salts used in the experiment(Table IS). Another process is the disintegration of the electrodestructure under atomic or ionic bombardment. This process, whichis in fact the cathode sputtering, leads to the highest amount ofgold released into the solution. And, thirdly, electrode erosion oc-curs by the action of an electrical discharge. It is well known thatat local regions on an electrode surface during discharge, hot spotsat very high temperature can be produced, from where the metal ofthe electrode can be released into the solution [26].

The anode undergoes bombardment by electrons, which are notso efficient to produce a high rate sputtering, while the cathodesuffers bombardment by positive ions, leading to a higher sputter-ing and evaporation rate than that at the anode surface.

Another process which occurs in SPP, at the anode surface is theanodic dissolution, which happens when the electrode surface iscovered with solution, and the current flows between the elec-trodes. Due to the instabilities of the solution plasma system, ithappens that the electrode surfaces are shortly covered with solu-tion. In this case, the electrode metal is released from the anode asions, which are rapidly neutralized by electrons from the plasmagas phase. Anodic dissolution can also produce free gold atomswhich can nucleate and generate clusters or gold NPs [18,27].

Gold sputtered from the cathode, or eroded from both elec-trodes, is present in solution plasma as free atoms, which agglom-erate as clusters or NPs, or directly as NPs.

At the same time, in SPP, hydrogen radicals (H�), which areformed from water dissociation, transferred into the solution phasefrom the plasma gas phase, produce the reduction reaction of themetal ion (Mx+) to the neutral form ðM0Þ : MðNO3Þx þ xH� !xHNO3 þM0, where M(NO3)x represents the metal nitrate, and xis the valence [22].

Gold is less reactive than all the metals used and a displacementreaction cannot take place.

The metal M adheres as atoms to already present gold atoms orseeds of gold NPs, or forms with gold intermetallic compounds. Ashas been found experimentally, the metal atoms do not nucleate asindependent NPs.

4.2. Composition of gold bimetallic NPs

The composition of the gold bimetallic NPs mainly depends onthe category to which the metal belongs, such as 3d or 4d metals,

Fig. 5. Crystalline phase identification and composition of gold bimetallic NPs determined by XRD. Dependence of the composition on the initial metal nitrate concentration.Each graph contains, on the right side, the XRD spectra (around 2H = 38� and 44�) of different gold bimetallic NP powders prepared with metal nitrate concentration of 0.5, 1,and 2 mM. Gold bimetallic NPs with (a) 3d and (b) 4d metals.


divalent or trivalent sp metals. From TEM–EDS analysis, we can ob-serve that the synthesized gold bimetallic NPs are disordered, andof mixed pattern, since both the intermetallic compounds and goldatoms are randomly arranged (Table 1 and Figs. 1–4). In terms ofdispersion in solutions, we may observe visually and also in TEMimages, that most of the bimetallic NPs alloyed with 3d and diva-lent sp metals are insoluble in water, and tend to aggregate. Thepreparation method influences the properties of bimetallic NPs.For example, Au–Cu NPs prepared by diffusion of Cu atoms intoas-prepared Au nanocrystals were highly dispersive in solvent,and their size was in the range from 4 to 6 nm [15]. The electrode-posited gold bimetallic NPs on amorphous carbon, Au–Fe, Au–Ni,and Au–Co had the tendency to agglomerate, similar to the corre-sponding bimetallic NPs prepared by SPP [17].

As mentioned above, we took much care on the identification ofthe crystal phases in the gold bimetallic NPs, especially we checkedthe presence of the metal oxide and nitrate in the composition. Inthe case of Au–Zn and Au–Cu bimetallic NPs, we found into the NPscomposition, 46% Zn(OH)2 and 13% Cu2O, respectively. Further-more, in the case of Au–Pd NPs, 1% Pd(NO3)2 was detected, whichmight be due to a quite low solubility of the palladium nitrate saltin water. The identification of the XRD diffraction peaks was con-firmed by the analysis of the TEM–SAED patterns, as shown inFigs. 1–4. We used Vegard’s law to verify the composition obtainedby the XRD – RIR method (Table 2). The crystal system of the com-ponents of the gold bimetallic NPs and the corresponding latticeconstant are indicated in Table IIS. In the case of the bimetallic

NPs made with trivalent sp, 3d, and 4d metals which belong tothe cubic crystal system, we can observe that the difference be-tween the experimental and calculated lattice constant, aexp andacalc, is in the range from 0.0017 to 0.0136 Å, proving that the crys-tal phase identification was correct.

The composition of the gold bimetallic NPs depends on the ni-trate concentration in solution, the erosion efficiency of gold, thereduction process of the metal ion to the neutral form in SPP,and metal properties, such as standard reduction potential andelectronegativity (Table 1).

First, in the reduction of the metal ion to the neutral form, wemust consider the initial concentration of the metal nitrate in solu-tion, which dictates the solution conductivity (Table IS). The initialsolution conductivity affects electron and hydrogen radical num-ber densities in the plasma gas phase. The increase in solution con-ductivity with salt concentration determines the decrease of peakdischarge current, and therefore the decrease of electron density,and consequently molecular dissociation and atomic excitationprocesses are fewer, leading to a smaller hydrogen radical numberdensity (Tables ISa and Fig. 3S) [22]. Solution conductivity in-creases with salt concentration, but also depends on the type ofmetal salt. In a salt concentration of 0.5 mM, we expect that the re-duced metal amount to be smaller than that in a 2 mM salt concen-tration solution. However, in the case of 3d metals, the amount ofmetal compounds in the bimetallic NPs synthesized starting from a2 and 0.5 mM salt concentration was almost the same, less than25%, a fact that can be explained by the small amount of reduced

Fig. 5. (continued)


metal M, due to a small density of hydrogen radicals in a 2 mM saltconcentration solution. The other metals from the present study,especially the trivalent sp metals form with gold a high amountof intermetallic compounds.

The highest amount of metal found in the gold bimetallic NPscomposition, mostly as intermetallic compounds, corresponds toa starting salt concentration of 1 mM. In a 1 mM metal salt concen-tration, solution plasma offers the optimum conditions for thebimetallic NPs formation. In the following, we will mainly referto the composition of gold bimetallic NPs synthesized from an ini-tial solution with 1 mM concentration (Table 1).

A complete explanation of the dependence of the intermetalliccompounds abundance in the alloy NPs, based on the initial metalsalt concentration must take into account the complex phenomenafrom the SPP, considering that plasma has a spatial distribution,the discharge current can be changed by the metal salt concentra-tion, and the amount of gold eroded depends on current and solu-tion conductivity.

The standard potential of the metal ion could have a role in thecomposition of the gold bimetallic NPs. All the used metals have astandard reduction potential smaller than that of gold, meaningthat the metal ions cannot be reduced by gold (Table IIIS) [28]. Alow and negative value of the standard reduction potential of 3dmetal ions, (Fe2+, Ni2+ and Co2+) shows that the reduction is notspontaneous and they have a low affinity for electrons. The contentof the intermetallic compounds of 3d metals was measured to beless than 25%, in the case of an initial salt concentration of 1 mM.In case of Cu2+, which, despite the positive value of the standardreduction potential, the yielded Au–Cu bimetallic NPs contain only7% intermetallic compounds. The 4d ion metals, Ag+ and Pd2+ havegreater electron affinity, the reduction reaction is spontaneous, and

consequently 45% and 26% intermetallic compounds, respectively,were contained in their bimetallic NPs.

To understand better the role of the electronegativity in thegold bimetallic NPs, we turn back to the rules of Hume-Rotheryin alloys formation [29–31]. The used parameters are the differ-ence in the work functions of the pure metals, D/ ¼ j/Au � /j;which determines the charge transfer, and the difference in elec-tron densities at the boundary of the Wigner–Seitz cell,Dnws ¼ jnAu

ws � nwsj, which determines a uniform charge distributionamong the alloy elements, where the upper notation Au refers togold atom values.

In terms for predicting the alloy stability, we analyzed the heatof formation of the gold bimetallic NPs. Fig. 10 represents a dia-gram of (D/, Dnws) for different metals used in the gold bimetallicNPs fabrication. This diagram can be divided into two distinct re-gions, corresponding to a positive and a negative value of the en-thalpy of formation of the gold bimetallic alloys [32,33]. InTable IVS of the Supplementary data, experimental work function,electron density at the Wigner–Seitz cell boundary of the metalsused in the gold bimetallic NPs syntheses, enthalpy for the bulkgold alloy formation and atomic radii of metals are indicated[30–34].

In the case of 3d metals, Ni, Fe, and Co, the solubility in gold islow, the content of these metals as compounds in the NPs is lessthan 25%, and the enthalpy for gold alloy formation is positive.Divalent and trivalent sp metals have a negative enthalpy of thegold bimetallic alloy formation and, as was measured, the contentof the intermetallic compounds in the NPs was higher than 30%(Table 1 and Figs. 5 and 6). The high content of In, Ga, and Cd inter-metallic compounds in the gold bimetallic NPs is explained by thegreatest difference in the electrochemical potential and the elec-tron density at the Wigner–Seitz cell boundary between these met-als and gold, as is shown in the diagram from Fig. 10. In the case ofAu–Zn NPs, zinc is easy to oxidize in SPP, and an amount of 46% ofZn(OH)2 can be found in their composition.

The high content of gold of 80% in the Au–Cu bimetallic NPs isnot easily explained following only the prediction based on the en-thalpy of formation. The phenomena are more complex, and othereffects should be also considered, such as the Brillouin zone effects,the coordination number changes and the size mismatch effects.Generally, for the formation of solid solutions and the stability ofa single phase system, the difference between the atomic radii ofthe two elements must not exceed 15% [29]. The gold atom radiusis favorable for Ag, Ga, Zn, and Pd, and less favorable for 3d transi-tion metals, Cu, Fe, Co, Ni, and In (Table IVS of the Supplementarydata) [34]. The surface energy of the bulk metal could be importantin surface segregation in binary alloys, when the element withlower surface energy tends to segregate to the surface. Lookingat the TEM–EDS analysis, we observe that the synthesized goldbimetallic NPs in SPP are mixed pattern.

4.3. Shift of the surface plasmon resonance band

Not all the synthesized gold bimetallic NPs present a shift of theSPR absorption band, which depends on the composition, or a dis-tinct SPR band. The Au–Ag bimetallic NPs present the highestabsorbance of the SPR band, which is blue-shifted depending onthe composition. The red shift of the SPR absorption band of theAu–Ga and Au–In bimetallic NPs is also influenced by the amountof the intermetallic compounds in the structure of NPs. Among 3dmetals, only in the case of the Au–Fe NPs we observed a shift of theSPR absorption band depending on the initial nitrate salt concen-tration (Figs. 7 and 1S).

The SPR absorption band also depends on the electronic struc-ture of metals, size, shape, and coupling (agglomeration) of NPs.The absence of a shift of the SPR absorption band in the case of

Fig. 6. Crystalline phase identification and composition of gold bimetallic NPs determined by XRD. Dependence of the composition on the initial metal nitrate concentration.Each graph contains, on the right side, the XRD spectra (around 2H = 38� and 44�) of different gold bimetallic NP powders prepared with metal nitrate concentration of 0.5, 1,and 2 mM. Gold bimetallic NPs with (a) divalent sp, and (b) trivalent sp metals.

Table 2Experimental lattice constant (aexp) of the gold bimetallic NPs directly obtained fromthe XRD spectra of the (111) and (200) atomic reflections. Calculated lattice constant(acalc) using Vegard’s law and the corresponding composition determined by the RIRmethod.

Au-M aexp (Å) (±0.0120 Å) acalc (Å) (±0.025 Å)

Divalent sp metalsAu–Zn 4.0702 –Au–Cd 4.0620 –

Trivalent sp metalsAu–Ga 4.0875 4.0739Au–In 4.0875 4.0854

3d MetalsAu–Fe 4.0877 4.0790Au–Co 4.0657 4.0640Au–Ni 4.0657 4.0635Au–Cu 4.0871 4.0920

4d MetalsAu–Ag 4.0846 4.0805Au–Pd 4.0877 4.0853

Fig. 7. Dependence of the maximum wavelength of the SPR band of gold bimetallicNPs on the initial salt concentration in SP solution, in the case of Au–Fe, Au–Ga, Au–In, and Au–Ag NPs.


Au–Co, Au–Ni, and Au–Cu NPs might be explained by the highamount of gold in the composition, and by the agglomeration ofthe NPs.

The SPR band shift is well known in the case of bimetallic Au–AgNPs [4,5,12]. A similar red-shift of the SPR band of the Au–Fe nano-alloys to those NPs obtained by SPP, was reported for water-solu-ble NPs prepared by a chemical synthetic method [11].


Nitroxyl free radicals, like TEMPOL and its derivative are used inESR as spin labels, spin traps, and antioxidant in biological applica-tions. For the interaction of free radicals with gold NPs, it wasfound that the ESR signal decreases after the adsorption of nitroxylfree radicals on the surface of gold NPs [35]. This interaction was

Fig. 8. Dependence of the relative decrease in the number of spins of TEMPOL inbenzene solution with 1 mM concentration, due to the interaction with the goldbimetallic NPs with 10 mg mL�1 concentration, in the case of gold alloying with (a)3d, (b) 4d, and (c) divalent sp.

ig. 9. Synthesis mechanism of gold bimetallic NPs in SPP. The diagram shows theuttering process on the grounded electrode, anodic dissolution, reduction of ionetal to the neutral form, and formation of intermetallic compounds. In this

iagram the gold atom is marked by the circle with Au, and the metal is marked bye circle with M. The intermetallic compound is marked as AuxMy.

Fig. 10. Diagram (D/, Dnws), where D/ ¼ j/Au � /j is the difference in the workfunctions of pure metals and Dnws ¼ jnAu

ws � nwsj is the difference in electrondensities at the boundary of the Wigner–Seitz cell. The upper notation Au refersto the gold atom. In the circle, the enthalpy of formation of the gold alloys isinserted.


stronger for 2.5 nm diameter gold NPs than for 15 nm diameterparticles. It was suggested that the interaction of gold NPs withfree radicals might be due by the exchange interaction of unpairedelectrons of the free radical with the conduction band electrons ofthe gold NPs.

We studied the interaction of gold bimetallic NPs with TEMPOLfree radicals in 1 mM solution in benzene. As can be observed fromFig. 8, the strongest interaction between the gold bimetallic NPswith the unpaired electrons of the TEMPOL molecule was mea-sured in the case of bimetallic NPs composed from Au and a 3d me-tal. We observed that the composition of the Au–Co and Au–Febimetallic NPs does not influence the relative decrease of numberof spins, and that, in the case of the Au–Ni and Au–Cu NPs, thehighest decrease of the ESR signal corresponds to those NPs syn-thesized from a starting solution with 1 mM initial concentration.Weak interaction was found in the case of the divalent sp metals

Fspmdth

and 4d metals, especially in the case of the Au–Pd NPs, where RS

was the smallest. In the case of trivalent sp metals, we could notobtain results due to the small quantity of powder NPs (as men-tioned in Table 1, the Au–Ga and Au–In NPs are highly water-sol-uble nanoparticles).

These results suggest that the decreasing of the ESR signal ofTEMPOL due to the interaction with the gold bimetallic NPs mightbe because of the electron exchange between the free radical andthe conduction band of the bimetallic NPs. With the exception ofAu–Pd NPs, all the other gold bimetallic NPs show a SPR absorptionband, that is actually the excitation by the visible light of the freeelectrons within the conduction band (Fig. 1S).

5. Conclusions

In this paper we showed that a SPP experimental procedurecould be applied to synthesize various gold bimetallic NPs, withmetals belonging to either 3d, 4d, sp divalent or trivalent catego-ries. SPP used for gold bimetallic NPs synthesis is based on simul-taneous reduction of the metal and erosion of gold from theelectrodes. This combination of processes makes SPP a moreadvantageous method, since fewer chemicals are used, as no gold


precursor or chemical reducing agents are required, thus offeringan ecologically friendly method for nanostructure synthesis.

The obtained gold bimetallic NPs, with sizes in the range from 5to 20 nm, contain a high percentage of gold, especially those NPsformed by alloying gold with a 3d metal. Most of the gold bimetal-lic NPs tend to aggregate, and are insoluble in water, except bime-tallic NPs with sp trivalent and 4d metals. The difference inelectronegativity between gold and the other metal was the mainreason for the formation of the intermetallic compounds found inthe composition of the gold bimetallic NPs. The composition ofthe bimetallic NPs depends on the category to which the metal be-longs and the initial metal salt concentration in solution.

We investigated the interaction with free radicals for possibleevaluation of the catalytic activity of the gold bimetallic NPs. Inthe future we will study the magnetic properties, especially forthose gold alloys NPs with 3d metals, and the change of the SPRabsorption band due to the interaction of the bimetallic NPs withthe free radicals.

Acknowledgments

This work was partially supported by ‘‘Tokai Region Nanotech-nology Manufacturing Cluster’’ sponsored by Ministry of Educa-tion, Culture, Sports, Science, and Technology (MEXT) and CoreResearch for Evolutional Science and Technology (CREST) of JapanScience and Technology (JST) Agency.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jallcom.2013.02.033.

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one-step synthesis of gold bimetallic nanoparticles with various metal-compositions

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