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Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823– 829

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me page: www.elsev ier .com/ locate /co lsur fa

ne-pot synthesis of porous gold nanoparticles by preparationf Ag/Au nanoparticles followed by dealloying

ing Chenga, Romain Bordesa,∗, Eva Olssonb, Krister Holmberga

Chalmers University of Technology, Department of Chemical and Biological Engineering, SE-412 96 Göteborg, SwedenChalmers University of Technology, Department of Applied Physics, SE-412 96 Göteborg, Sweden

i g h l i g h t s

Gold–silver true alloys were preparedin reverse microemulsion.Dealloying by treatment with nitricacid microemulsion was carried outand proved by UV–vis spectroscopy.Porous gold nanoparticles wereobtained in a two steps reaction,monitored by UV–vis spectroscopy.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 23 May 2013eceived in revised form 6 August 2013ccepted 7 August 2013vailable online 17 August 2013

a b s t r a c t

Porous gold nanoparticles were obtained from nanoparticle alloys of gold and silver. The alloy nanopar-ticles were prepared by reducing the gold and silver precursors, HAuCl4 and AgNO3, respectively withNaBH4. The reduction was made in a microemulsion of water-in-oil type and the precursors were con-tained in the water droplets. The droplet size of the microemulsion was 25–30 nm and the size of themixed nanoparticles was 5–7 nm. The position of the plasmon band in the absorption spectrum, as wellas analysis by energy-dispersive X-ray spectroscopy linked to high resolution scanning transmission

eywords:anoparticlesealloyingicroemulsion

ilveroldanoporous

electron microscopy showed that the nanoparticles were true alloys, not of core–shell type. The mixednanoparticles were subsequently dealloyed by treatment with nitric acid, which dissolved silver muchmore readily than gold.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Noble metal particles of sizes below 10 nm are of interest for variety of purposes, partly because many physical characteris-ics, such as optical and magnetic properties, are different fromhose of the bulk material and partly because of the large surfacerea that the nanoparticles provide. For many applications a large

urface area is decisive of proper performance. Using specific prepa-ation techniques, such as use of water-in-oil microemulsions inhich the small water pools can be regarded as minireactors for the

∗ Corresponding author. Tel.: +46 317722976.E-mail address: bordes@chalmers.se (R. Bordes).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.08.023

reaction, we [1,2] and others [3,4] have prepared noble metal par-ticles with diameters in the 5–10 nm size. Uniform and sphericalgold particles of 10 nm size give a surface area of 31 m2/g. Such aparticle contains 24,000 gold atoms and a sizable fraction of theseatoms will be at the surface. This is very attractive for applicationssuch as heterogeneous catalysis and sensor applications.

An even higher surface area and, thus a higher fraction of sur-face atoms, can be obtained if the nanoparticles are made porous. Toour knowledge very few examples of a simple, direct, and one-stepsynthesis of porous nanoparticles have been reported [5]. In this

communication we present a two-stage procedure to synthesizegold nanoparticles with a high–and tuneable–porosity. The prin-ciple is simple. Gold–silver alloy nanoparticles are first preparedby the microemulsion technique. The nanoparticles in the form

8 ysicochem. Eng. Aspects 436 (2013) 823– 829

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24 J. Cheng et al. / Colloids and Surfaces A: Ph

f a suspension are then treated with a microemulsion that con-ains nitric acid in the water pools. Nitric acid dissolves silver at a

uch higher rate than gold and the result will be that the solid alloyanoparticles will be transformed into porous gold nanoparticles.

. Experimental

.1. Materials

Silver nitrate (Sigma–Aldrich, 99.9999%), chloroauric acidSigma–Aldrich, 99.999%), sodium borohydride (Sigma–Aldrich,9.99%), cyclohexane (Sigma–Aldrich, ≥99.7%) and nitric acidSigma–Aldrich, 69%) were used as purchased. The aqueous solu-ions were prepared in Milli-Q water (resistance > 18 M� cm).

The surfactants employed, tri(ethylene glycol)monoundecylther (C11E3) and penta(ethylene glycol)monoundecyl (C11E5)ther, were kindly provided by AkzoNobel Surface Chemistry (Ste-ungsund, Sweden).

.2. Light scattering

The size of the water droplets in the microemulsion was deter-ined with a Zetasizer Nano ZS from Malvern at 23 ◦C. The

efractive index of cyclohexane was taken at 1.4235 [6]. The tem-erature of the sample was allowed to stabilize for 10 min in thehamber prior to the measurement.

.3. UV–visible spectroscopy

The UV–visible spectroscopy was carried out on a double beampectrophotometer GBC UV/Vis 920. The microemulsion with-ut metal salt was used as reference. The spectra were recordedetween 290 nm and 750 nm at a speed of 2 nm/s.

.4. Transmission electron microscopy

Samples for transmission electron microscopy (TEM) were pre-ared by placing a droplet of suspension on a Cu grid covered by aarbon support film.

The routine TEM was done on a JEOL JEM-1200 EX II TEM oper-ted at 120 kV.

A FEI Titan 80-300 TEM/Scanning TEM (STEM) with a highnergy resolution Tridium GIF and an Oxford Instruments energyispersion X-ray detector were used for high resolution imagingnd spectroscopy. High angle annular dark field (HAADF) STEMmages were acquired using a 19.7 mrad beam convergence anglend ∼40–200 mrad detector collection angle.

Prior to observation, the TEM grids were cleaned for 1 min using Fischione Model 1020 plasma cleaner to remove remaining organ-cs left after the evaporation of the droplet.

.5. Synthesis of pure nanoparticles

Microemulsions were prepared by mixing the surfactant, cyclo-exane and an aqueous solution of either the metal salt, i.e., HAuCl4r AgNO3, or the reducing agent, i.e., NaBH4 and stirring for 5 min.he concentrations used are given in Section 3.

For a typical sample, a total mass of 5 g of microemulsion wasrepared by mixing 1.35 g of the surfactant system (C11E3:C11E560:40)) with 3.30 g of cyclohexane. 0.35 g of aqueous solution ofhe metal salt or reducing agent was then added. The microemul-

ion containing the metal salt was mixed under stirring with theicroemulsion containing the reducing agent. The ratios are given

n Section 3. The mixture was vigorously stirred and analysis byV–visible spectroscopy was done after two hours.

Fig. 1. Partial phase diagram of the system C11E3:C11E5 (60:40)/water/cyclohexaneat 25 ◦C. The area to the right of the line is the isotropic microemulsion region andon the left side is a two-phase region.

2.6. Synthesis of alloy nanoparticles

The alloy nanoparticles were prepared as a suspension by firstmaking a microemulsion by mixing a solution of the metal saltsin water at the required stoichiometry with the oil component inwhich the surfactant was dissolved, using the same protocol as forthe pure nanoparticles. The mixture was shaken for 10 min and thenadded to solid NaBH4, which was used in a 10-fold molar excess. Themixture was stirred intensively for two hours and then analysedwith UV–visible spectroscopy.

2.7. Dealloying

The dealloying was conducted using a microemulsion with a69% nitric acid solution constituting the aqueous component. Theratio to cyclohexane and surfactant remained unchanged. The ratiobetween the nitric acid microemulsion and the alloy nanoparticlesuspension was 1:2 (w/w). The microemulsion and the suspensionwere mixed and then shaken for several hours.

3. Results and discussion

3.1. The reaction system

The reaction system was a water-in-oil microemulsion withthe gold and the silver precursors, HAuCl4 and AgNO3, respec-tively, present in the water pools. Sodium borohydride, NaBH4, awater soluble salt, was used as reducing agent. Cyclohexane con-stituted the oil component of the microemulsion. The generationof the nanoparticles was monitored by UV–vis. spectroscopy, fol-lowing the increase of the surface plasmon absorption peaks of goldnanoparticles at 500–525 nm [7] and of silver nanoparticles around400 nm [8].

Fig. 1 shows the partial phase diagram of the system sur-factant/water/cyclohexane, where the surfactant component is amixture of C11E3 (60%) and C11E5 (40%). The isotropic microemul-sion region is to the right of the line. It has previously been shownfor an anionic surfactant that for water-in-oil microemulsions anincrease in the water to surfactant ratio gives larger water droplets,which, in turn, results in larger nanoparticles [9]. The same trend

was found in this work.

When the two metal salts are mixed and brought in contactwith NaBH4, precipitation of AgCl competes with the formationof the mixed metal nanoparticles [10]. This has previously been

ysicochem. Eng. Aspects 436 (2013) 823– 829 825

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Silver Nanoparticles

Gold Nanoparticles

For the preparation of silver sols, the concentration of thereactants needed to be even lower in order to give a nanopar-ticle suspension with proper stability. Fig. 2 shows a UV–visible

Table 1Size and size distributions of nanoparticles of gold and silver, as well of mixednanoparticles.

Composition Mean (nm) Variance (PI)

Au 5.7 1.1012

J. Cheng et al. / Colloids and Surfaces A: Ph

escribed in the literature and it has been shown that a way to avoidhis side reaction is to first prepare Au nanoparticles by reductionf HAuCl4, then remove the chloride ions by ultracentrifugationnd subsequently reduce an Ag precursor in the presence of theu nanoparticles [11]. This procedure will definitely eliminate for-ation of AgCl but it will lead to core–shell nanoparticles, which

s not what we are aiming for in this work. We now demonstratehat precipitation of AgCl can be avoided also when the two metalrecursors are reduced in a one step reaction provided the exper-

mental conditions are right. It was found that not only is a largexcess (10–20 times) of NaBH4 necessary in order to avoid AgClormation; it was also important to use a large surfactant to wateratio in the formulation. A large surfactant to water ratio meanshat the water droplets of the water-in-oil microemulsion becomemall. The preferred formulation, which contained only 0.7% aque-us component and had a surfactant to water weight ratio of 16,onsisted of water droplets with a diameter of 27 ± 4 nm accordingo dynamic light scattering. This formulation gave gold nanopar-icles of 5.7 nm size and silver nanoparticles with a diameter of.6 nm. The mixed nanoparticles obtained when the water com-onent of the microemulsion contained both HAuCl4 and AgNO3ad sizes in the intermediate range. After completed reaction aoble metal sol was obtained; thus, in a formal sense the reac-ion product was a suspension of metal nanoparticles coexistingith a water-in-oil microemulsion. The pure silver nanoparti-

les were yellow and the pure gold nanoparticles bright red.he mixed nanoparticles had intermediate colours; the higherhe fraction of gold in the nanoparticles, the more red was theolution.

Gold, silver and mixed gold–silver nanoparticles have beenrepared before by a similar procedure starting with a water-

n-oil microemulsion containing the metal precursors, as well ashe reducing agent, in the water pools. In previous work the

icroemulsions have been based on ionic surfactants, usuallyodium bis(2-ethylhexyl)sulphosuccinate, often referred to as AOT10,12–17]. In this work anionic and cationic surfactants werevoided in order not to introduce any element that could be detri-ental to catalytic activity, such as sulphur, phosphorus, halide

ons, etc. The intention is to use the final product as a catalystor selective CO oxidation. We therefore chose a microemulsionntirely based on nonionic surfactants of fatty alcohol ethoxy-ate type. Nonionic surfactants also have the advantage over ionicurfactants of being much less sensitive to variations of the iontrength in the aqueous domains. Even if the over-all concentra-ions of HAuCl4, AgNO3 and NaBH4 in the microemulsion are low,he local electrolyte concentration in the water pools is relativelyigh. A 60:40 molar ratio of tri(ethylene glycol)monoundecyl etherC11E3) and penta(ethylene glycol)monoundecyl (C11E5) ether wasound optimal for the purpose.

Gold–silver alloy nanoparticles have also been obtained with-ut the use of surfactants. Both wet chemical methods, such aso-reduction of AgNO3 and HAuCl4 with citrate [18] and dry meth-ds, such as laser ablation of a solid Au–Ag alloy [19], yield suchixed nanoparticles but the particles tend to be large and the size

istribution broad compared to the microemulsion route of prepa-ation.

The polyoxyethylene chain of the nonionic surfactants used inhis work can also serve as reducing agent for AgNO3 (and probablyor HAuCl4 as well). We have previously seen that surfactants con-aining a polyoxyethylene chain as polar head group are efficient ineducing a silver salt to metallic silver [20]. The surfactant under-oes oxidative degradation generating formaldehyde and other

ldehydes. The initial step is most likely abstraction of a hydrogentom from the methylene groups of the polyoxyethylene chain. Allhese methylene groups are alpha to an ether bond and thereforeeactive.

Fig. 2. UV–visible spectra of gold and silver nanoparticles.

Thus, there are in practice two reducing agents in the formu-lation used, NaBH4 and the polyoxyethylene headgroup of thenonionic surfactant that is forming a palisade layer around thewater pools of the microemulsion. As mentioned above, the size ofthe water droplets was found to be critical when both HAuCl4 andAgNO3 were present in the formulation. When the drops were large,20–30 nm, AgCl precipitated along with formation of mixed metalnanoparticles. When the drops were small, 3–4 nm, no AgCl precip-itate was detected. In such small water pools the polyoxyethylenechains will more or less reach across the whole aqueous domain.The water pools will then consist of highly hydrated polyoxyethy-lene chains in which the reactants, i.e., HAuCl4, AgNO3 and NaBH4are dissolved. The combined reducing power of NaBH4 and thepolyoxyethylene chains is evidently enough to transform Ag(I) toAg(0) so rapidly that precipitation of AgCl does not occur.

The concentration of the reactants was also found to be criticalfor formation of both the pure nanoparticles and the alloy nanopar-ticles. For the synthesis of the gold nanoparticles, using a 1:1ratio between the metal salt microemulsion and the microemul-sion containing the reducing agent, nanoparticle suspensions stablefor at least one week could only be obtained up to concentra-tions of HAuCl4 and NaBH4 in the water pools of 0.03 and 0.05 M,respectively. The resulting suspension exhibited a plasmon peak at520 nm (Fig. 2).

The gold nanoparticles were analysed by transmission electronmicroscopy (TEM) and the size and size distributions were deter-mined. As it can be seen from Table 1, the mean particle size was5.7 nm. Individual particles were also analysed by scanning trans-mission electron microscopy STEM using high angle annular darkfield (HAADF) including high resolution STEM (HR-STEM) and animage is shown in Fig. 3.

Au–Ag 75:25 5.5 1.5338Au–Ag 50:50 6.7 1.7838Au–Ag 25:75 6.9 2.7334Ag 9.6 14.939

826 J. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823– 829

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ig. 3. Scanning transmission electron microscopy images of gold nanoparticles prepared using a reducing agent to metal salt ratio of 10 (right).

bsorption spectrum of a silver sol prepared using a AgNO3 con-entration of 0.005 M and a NaBH4 concentration of 0.05 M in theater pools. The mean size of the particles is 9.6 nm, as can be

een from Table 1. Fig. 3a shows a STEM image of the nanoparticlesenerated. Increasing the water to surfactant ratio, which resultsn larger microemulsion droplets and also in an increased size ofhe nanoparticles, gave a shift of the peak in the UV–visible spec-rum to higher wavelength. This is in accordance with previousbservations [21].

There was a pronounced difference in the ratio of reducing agento noble metal salt needed to prepare stable suspensions of goldnd silver nanoparticles. Whereas a molar ratio of 10 was requiredor silver, a ratio of only 1.7 was needed for gold. This probablyeflects the more “noble” character of gold, i.e., reduction of Au(III)o Au(0) is more facile than reduction of Ag(I) to Ag(0), based onhe difference in standard electrode potential [5].

. Formation of gold–silver alloy nanoparticles

As was the case for the pure metal nanoparticles, a high stabilityf alloy nanoparticles could only be achieved with a high surfactant

o water ratio, which means small water droplets in the microemul-ion, which, in turn, yields small nanoparticles. A microemulsion

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ig. 4. UV–visible spectra of suspensions of the different alloys with absorptionaxima normalized to 1.

ed using a reducing agent to metal salt ratio of 1.7 (left) and silver nanoparticles

with the composition water:surfactant:oil of 0.007:0.111:0.882 byweight was used for the synthesis of the alloy nanoparticles.

The total concentration of precursors (HAuCl4 + AgNO3) was0.1 M and the ratio metal:reducing agent was 1:20. In order toavoid formation of AgCl the reducing agent, NaBH4, had to be addedimmediately after mixing the microemulsions containing the goldand the silver salts. A series of nanoparticles of different compo-sition was prepared with proportions ranging from pure silver topure gold. The UV vis. spectra were recorded and are shown inFig. 4. As can be seen, there is a smooth shift of the absorption max-imum from 400 nm for the pure silver nanoparticles to 520 nm forthe pure gold nanoparticles. The peak maximum plotted versus theatom percentage of gold in the nanoparticles gave a linear relation-ship (Fig. 5). As can be seen from Table 1, the sizes of the mixednanoparticles were in the same range as those of the pure metalnanoparticles.

Fig. 4 also shows that while a suspension of pure silver nanopar-ticles gave a sharp peak (at 400 nm), the suspension of pure goldnanoparticles exhibited a broader peak (at 520) and a large shoul-der at lower wavelength. This is in accordance with the literature[21]. The suspensions of the mixed nanoparticles gave intermedi-

ate absorption curves and there was a smooth transition from oneextreme to the other.

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Fig. 5. Position of the surface plasmon bands versus atom percent gold in thegold/silver nanoparticles.

J. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823– 829 827

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Fig. 7. Change in relative absorbance of suspensions of pure gold nanoparticles (at520 nm; squares) and of pure silver nanoparticles (at 400 nm; rings) on exposure

ig. 6. A high resolution scanning transmission electron microscopy image recordedsing high angle annular dark field showing atom columns in nanoparticles with anu to Ag atom ratio of 50:50.

In principle, the simultaneous reduction of HAuCl4 and AgNO3resent in the water pools of a water-in-oil microemulsion may

ead to either

a physical mixture of gold nanoparticles and silver nanoparticles mixed nanoparticles which contain a core of one metal sur-rounded by a shell of the other metal; it is likely that gold, whichis the more noble of the metals and therefore the one most easilygenerated from its precursor, would constitute the core

mixed nanoparticles with a homogeneous distribution of the twometal atomsor to mixtures of the three extremes.

Since the objective of our work is to synthesize porousold nanoparticles by leaching out silver from mixed gold–silveranoparticles, we are aiming for nanoparticles with a homoge-eous distribution of gold and silver, i.e., a true alloy. Gold and silverave very similar lattice constants, 0.408 and 0.409 nm, respec-ively [22], and it is known since long back that they can form truelloys in bulk phase. Also for nanoparticles incorporation of silvertoms into gold need not give rise to a lattice mismatch, as has beenemonstrated by TEM [23]. If the reaction conditions are right, truelloy nanoparticles will form [24–26].

A physical mixture of two one-metal nanoparticles gives twoeaks in the UV spectrum rather than one peak with a maximum

n-between those of the individual metals. Thus, a 50:50 mixturef pure gold nanoparticles and pure silver nanoparticles has beenhown to give two separate plasmon bands, one at 523 nm (fromold) and one at 399 nm (from silver) [27]. This is obviously nothat we are seeing.

More or less pronounced core–shell structures, with gold domi-ating the core and silver being enriched in the shell, have also beeneported [13]. The spectroscopic results of this work, with a smoothransition of the plasmon peak from that of silver to that of gold ashe ratio of gold to silver is increased are not consistent with theore–shell option, however. There are several reports in the litera-ure that core–shell, and other non-alloy, nanoparticles display theeaks of the individual components with the relative absorbanceorresponding to the relative amount of the metal [28–35].

The spectroscopy results presented above strongly indicate thathe particles obtained are alloy nanoparticles and this is alsoonsistent with results from analysis by energy-dispersive X-raypectroscopy (EDX) combined with HR-STEM. A HR-STEM imagef nanoparticles obtained from a 50:50 molar ratio of Au and Aganoparticles is shown in Fig. 6. EDX analysis, at a resolution of

round 1 nm, indicated that the ratio of the two metals remainedpproximately constant through the particles.

It should be noted that the linear correlation between �max andold content (Fig. 5) is in accordance with calculations based on

to nitric acid, left axis. The dotted lines show the positions of the surface plasmonband (gold: squares; silver: circles), right axis.

the Mie theory for gold–silver alloys [29,36]. The Mie theory alsotells that the absorbance of the alloy nanoparticles should be lowerthan that of the pure gold and silver nanoparticles [30]. This hasbeen observed previously [13] and is also found in this work.

4.1. Dealloying–formation of porous gold nanoparticles

Dealloying by selectively dissolving one metal from an alloy oftwo or more metals is a well-known procedure. The literature con-tains many examples of preparation of porous gold by dealloyingAu/Ag alloys by the use of nitric acid as leaching agent [37–40]. Inthe majority of cases the procedure has been applied on macro-scopic alloys, e.g. white gold leafs, or on Au/Ag alloy surfaces andthe main application for the porous gold obtained has been het-erogeneous catalysis. The methodology has also been extended tomake nanoporous Au/Pt alloys by leaching out Cu from Au/Pt/Cualloys [41].

The procedure used in this work, to leach out Ag from Au/Agalloy nanoparticles by adding a microemulsion containing concen-trated nitric acid in the water pools to a freshly made suspensionof the nanoparticles seems not to have been described before. Theprocedure is attractive for generation of ultra small porous goldnanoparticles because the microemulsion procedure for makingthe alloy nanoparticles can yield a suspension of small nanoparti-cles with good control of the particle size. It is reasonable to assumethat the porous nanoparticles obtained after the dealloying stepwill be similar in size–or somewhat smaller due to some loss alsoof gold–as the alloy nanoparticles. Nanoparticles with three differ-ent ratios of Au to Ag were used for the dealloying: 75:25, 50:50and 25:75. As reference, the same procedure was also applied topure Au and pure Ag nanoparticles.

The dissolution of nanoparticles of pure gold and silver wasmonitored by UV–vis. spectroscopy following the decline of the sur-face plasmon band of Au at 525 nm and of Ag at 400 nm. As can beseen from Fig. 7, the silver plasmon band disappeared much morerapidly than the plasmon band of gold. After 4 h exposure to thenitric acid-containing microemulsion the silver plasmon band wasdown to almost zero absorbance while the gold peak maintained

approximately two thirds of its absorbance. The figure also showsthat the positions of the surface plasmon band for both gold andsilver were unaffected by the leaching procedure.

828 J. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823– 829

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ig. 8. Change in position of the surface plasmon band vs. time for nitric acid treatnd 25:75 (right).

Fig. 8 shows dealloying results for the gold–silver nanoparti-les. The increase in �max is in accordance with a gradual increasen gold content in the nanoparticles. As can be seen, the �max for the5:75Au–Ag nanoparticles increased well beyond the value of 525btained for pure gold nanoparticles. This is the composition withhe highest content of silver, which means that after leaching it wille the nanoparticles with the highest porosity and the largest goldurface area. It is important to realize that the reduction in absorp-ion intensity of the gold plasmon band seen during the dealloyingrocess cannot be taken as a quantitative measure of the amountf gold that has leached out of the particles (see Fig. 7). It has beenointed out that the Au plasmon absorption intensity decreasess Ag ions are removed from mixed nanoparticles. The reason forhis is not clear but it has been suggested that the phenomenon isue to a structural change of the nanoparticles. Surface plasmonesonance bands are known to be influenced by the size, shape,omposition and dielectric properties of the nanoparticles [7,18].he wavelength is not changed, however. In our case it is conceiv-ble that the porosity will influence the position of the maximumn the surface plasmon band.

. Conclusions

We have developed a method to prepare mixed nanoparticlesf gold and silver by adding solid NaBH4 in large excess to water-n-oil microemulsions containing the gold and silver precursors,AuCl4 and AgNO3, respectively in the water pools. A combinationf two fatty alcohol ethoxylates were used as surfactant systemnd TEM analysis showed that the mixed nanoparticles were inhe 5–7 nm size range. In a series of syntheses from pure silver,ia incrementally increasing atomic ratios of gold to silver, to pureold, there was a smooth shift of the absorption maximum from00 nm for the pure silver nanoparticles to 520 nm for the pureold nanoparticles. The peak maximum plotted versus the atomercentage of gold in the mixed nanoparticles gave a linear rela-ionship. This observation, together with EDX analysis combinedith HR-STEM, indicated that the particles were true alloys, not of

ore–shell type.The suspensions of mixed nanoparticles were subsequently

ealloyed using nitric acid as dealloying reagent. Reference exper-ments on suspensions of pure gold and pure silver nanoparticleshowed that the rate of dissolution differed considerably betweenhe two metal nanoparticles. After 4 h exposure to nitric acid in theorm of a microemulsion essentially all metallic silver was dissolvedhile 2/3 of the gold remained intact. Exposure of alloy nanoparti-

les of different composition to the nitric acid microemulsion gave continuous increase of the position of the surface plasmon band.his is consistent with a gradually increasing ratio of gold to silver,ventually arriving at porous gold nanoparticles.

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of alloy nanoparticles based on Au to Ag atom ratios of 75:25 (left), 50:50 (middle)

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