Au nanoparticles followed by dealloying

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

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

One-pot synthesis of porous gold nanoparticles by preparation of Ag/Au nanoparticles followed by dealloying Jing Cheng a , Romain Bordes a,∗ , Eva Olsson b , Krister Holmberg a a b

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

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Gold–silver true alloys were prepared in reverse microemulsion.

• Dealloying by treatment with nitric acid microemulsion was carried out and proved by UV–vis spectroscopy. • Porous gold nanoparticles were obtained in a two steps reaction, monitored by UV–vis spectroscopy.

a r t i c l e

i n f o

Article history: Received 23 May 2013 Received in revised form 6 August 2013 Accepted 7 August 2013 Available online 17 August 2013 Keywords: Nanoparticles Dealloying Microemulsion Silver Gold Nanoporous

a b s t r a c t Porous gold nanoparticles were obtained from nanoparticle alloys of gold and silver. The alloy nanoparticles were prepared by reducing the gold and silver precursors, HAuCl4 and AgNO3 , respectively with NaBH4 . The reduction was made in a microemulsion of water-in-oil type and the precursors were contained in the water droplets. The droplet size of the microemulsion was 25–30 nm and the size of the mixed nanoparticles was 5–7 nm. The position of the plasmon band in the absorption spectrum, as well as analysis by energy-dispersive X-ray spectroscopy linked to high resolution scanning transmission electron microscopy showed that the nanoparticles were true alloys, not of core–shell type. The mixed nanoparticles were subsequently dealloyed by treatment with nitric acid, which dissolved silver much more readily than gold. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Noble metal particles of sizes below 10 nm are of interest for a variety of purposes, partly because many physical characteristics, such as optical and magnetic properties, are different from those of the bulk material and partly because of the large surface area that the nanoparticles provide. For many applications a large surface area is decisive of proper performance. Using specific preparation techniques, such as use of water-in-oil microemulsions in which the small water pools can be regarded as minireactors for the

∗ Corresponding author. Tel.: +46 317722976. E-mail address: [email protected] (R. Bordes). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.08.023

reaction, we [1,2] and others [3,4] have prepared noble metal particles with diameters in the 5–10 nm size. Uniform and spherical gold particles of 10 nm size give a surface area of 31 m2 /g. Such a particle contains 24,000 gold atoms and a sizable fraction of these atoms will be at the surface. This is very attractive for applications such as heterogeneous catalysis and sensor applications. An even higher surface area and, thus a higher fraction of surface atoms, can be obtained if the nanoparticles are made porous. To our knowledge very few examples of a simple, direct, and one-step synthesis of porous nanoparticles have been reported [5]. In this communication we present a two-stage procedure to synthesize gold nanoparticles with a high–and tuneable–porosity. The principle is simple. Gold–silver alloy nanoparticles are first prepared by the microemulsion technique. The nanoparticles in the form

824

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

0.00

of a suspension are then treated with a microemulsion that contains nitric acid in the water pools. Nitric acid dissolves silver at a much higher rate than gold and the result will be that the solid alloy nanoparticles will be transformed into porous gold nanoparticles.

1.00

0.25

0.75

0.50

2.1. Materials Silver nitrate (Sigma–Aldrich, 99.9999%), chloroauric acid (Sigma–Aldrich, 99.999%), sodium borohydride (Sigma–Aldrich, 99.99%), cyclohexane (Sigma–Aldrich, ≥99.7%) and nitric acid (Sigma–Aldrich, 69%) were used as purchased. The aqueous solutions were prepared in Milli-Q water (resistance > 18 M cm). The surfactants employed, tri(ethylene glycol)monoundecyl ether (C11 E3 ) and penta(ethylene glycol)monoundecyl (C11 E5 ) ether, were kindly provided by AkzoNobel Surface Chemistry (Stenungsund, Sweden). 2.2. Light scattering The size of the water droplets in the microemulsion was determined with a Zetasizer Nano ZS from Malvern at 23 ◦ C. The refractive index of cyclohexane was taken at 1.4235 [6]. The temperature of the sample was allowed to stabilize for 10 min in the chamber prior to the measurement. 2.3. UV–visible spectroscopy The UV–visible spectroscopy was carried out on a double beam spectrophotometer GBC UV/Vis 920. The microemulsion without metal salt was used as reference. The spectra were recorded between 290 nm and 750 nm at a speed of 2 nm/s. 2.4. Transmission electron microscopy Samples for transmission electron microscopy (TEM) were prepared by placing a droplet of suspension on a Cu grid covered by a carbon support film. The routine TEM was done on a JEOL JEM-1200 EX II TEM operated at 120 kV. A FEI Titan 80-300 TEM/Scanning TEM (STEM) with a high energy resolution Tridium GIF and an Oxford Instruments energy dispersion X-ray detector were used for high resolution imaging and spectroscopy. High angle annular dark field (HAADF) STEM images were acquired using a 19.7 mrad beam convergence angle and ∼40–200 mrad detector collection angle. Prior to observation, the TEM grids were cleaned for 1 min using a Fischione Model 1020 plasma cleaner to remove remaining organics left after the evaporation of the droplet. 2.5. Synthesis of pure nanoparticles Microemulsions were prepared by mixing the surfactant, cyclohexane and an aqueous solution of either the metal salt, i.e., HAuCl4 or AgNO3 , or the reducing agent, i.e., NaBH4 and stirring for 5 min. The concentrations used are given in Section 3. For a typical sample, a total mass of 5 g of microemulsion was prepared by mixing 1.35 g of the surfactant system (C11 E3 :C11 E5 (60:40)) with 3.30 g of cyclohexane. 0.35 g of aqueous solution of the metal salt or reducing agent was then added. The microemulsion containing the metal salt was mixed under stirring with the microemulsion containing the reducing agent. The ratios are given in Section 3. The mixture was vigorously stirred and analysis by UV–visible spectroscopy was done after two hours.

Oil

Wa te

r

2. Experimental 0.50

0.75

0.25

1.00 0.00

0.25

0.50

Surfactant

0.75

1.00

0.00

Fig. 1. Partial phase diagram of the system C11 E3 :C11 E5 (60:40)/water/cyclohexane at 25 ◦ C. The area to the right of the line is the isotropic microemulsion region and on the left side is a two-phase region.

2.6. Synthesis of alloy nanoparticles The alloy nanoparticles were prepared as a suspension by first making a microemulsion by mixing a solution of the metal salts in water at the required stoichiometry with the oil component in which the surfactant was dissolved, using the same protocol as for the pure nanoparticles. The mixture was shaken for 10 min and then added to solid NaBH4 , which was used in a 10-fold molar excess. The mixture was stirred intensively for two hours and then analysed with UV–visible spectroscopy. 2.7. Dealloying The dealloying was conducted using a microemulsion with a 69% nitric acid solution constituting the aqueous component. The ratio to cyclohexane and surfactant remained unchanged. The ratio between the nitric acid microemulsion and the alloy nanoparticle suspension was 1:2 (w/w). The microemulsion and the suspension were 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 with the gold and the silver precursors, HAuCl4 and AgNO3 , respectively, present in the water pools. Sodium borohydride, NaBH4 , a water soluble salt, was used as reducing agent. Cyclohexane constituted the oil component of the microemulsion. The generation of the nanoparticles was monitored by UV–vis. spectroscopy, following the increase of the surface plasmon absorption peaks of gold nanoparticles at 500–525 nm [7] and of silver nanoparticles around 400 nm [8]. Fig. 1 shows the partial phase diagram of the system surfactant/water/cyclohexane, where the surfactant component is a mixture of C11 E3 (60%) and C11 E5 (40%). The isotropic microemulsion region is to the right of the line. It has previously been shown for an anionic surfactant that for water-in-oil microemulsions an increase 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 contact with NaBH4 , precipitation of AgCl competes with the formation of the mixed metal nanoparticles [10]. This has previously been

J. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823–829 1.2 Silver Nanoparticles Gold Nanoparticles

1

0.8

Absorbance

described in the literature and it has been shown that a way to avoid this side reaction is to first prepare Au nanoparticles by reduction of HAuCl4 , then remove the chloride ions by ultracentrifugation and subsequently reduce an Ag precursor in the presence of the Au nanoparticles [11]. This procedure will definitely eliminate formation of AgCl but it will lead to core–shell nanoparticles, which is not what we are aiming for in this work. We now demonstrate that precipitation of AgCl can be avoided also when the two metal precursors are reduced in a one step reaction provided the experimental conditions are right. It was found that not only is a large excess (10–20 times) of NaBH4 necessary in order to avoid AgCl formation; it was also important to use a large surfactant to water ratio in the formulation. A large surfactant to water ratio means that the water droplets of the water-in-oil microemulsion become small. The preferred formulation, which contained only 0.7% aqueous component and had a surfactant to water weight ratio of 16, consisted of water droplets with a diameter of 27 ± 4 nm according to dynamic light scattering. This formulation gave gold nanoparticles of 5.7 nm size and silver nanoparticles with a diameter of 9.6 nm. The mixed nanoparticles obtained when the water component of the microemulsion contained both HAuCl4 and AgNO3 had sizes in the intermediate range. After completed reaction a noble metal sol was obtained; thus, in a formal sense the reaction product was a suspension of metal nanoparticles coexisting with a water-in-oil microemulsion. The pure silver nanoparticles were yellow and the pure gold nanoparticles bright red. The mixed nanoparticles had intermediate colours; the higher the fraction of gold in the nanoparticles, the more red was the solution. Gold, silver and mixed gold–silver nanoparticles have been prepared before by a similar procedure starting with a waterin-oil microemulsion containing the metal precursors, as well as the reducing agent, in the water pools. In previous work the microemulsions have been based on ionic surfactants, usually sodium bis(2-ethylhexyl)sulphosuccinate, often referred to as AOT [10,12–17]. In this work anionic and cationic surfactants were avoided in order not to introduce any element that could be detrimental to catalytic activity, such as sulphur, phosphorus, halide ions, etc. The intention is to use the final product as a catalyst for selective CO oxidation. We therefore chose a microemulsion entirely based on nonionic surfactants of fatty alcohol ethoxylate type. Nonionic surfactants also have the advantage over ionic surfactants of being much less sensitive to variations of the ion strength in the aqueous domains. Even if the over-all concentrations of HAuCl4 , AgNO3 and NaBH4 in the microemulsion are low, the local electrolyte concentration in the water pools is relatively high. A 60:40 molar ratio of tri(ethylene glycol)monoundecyl ether (C11 E3 ) and penta(ethylene glycol)monoundecyl (C11 E5 ) ether was found optimal for the purpose. Gold–silver alloy nanoparticles have also been obtained without the use of surfactants. Both wet chemical methods, such as co-reduction of AgNO3 and HAuCl4 with citrate [18] and dry methods, such as laser ablation of a solid Au–Ag alloy [19], yield such mixed nanoparticles but the particles tend to be large and the size distribution broad compared to the microemulsion route of preparation. The polyoxyethylene chain of the nonionic surfactants used in this work can also serve as reducing agent for AgNO3 (and probably for HAuCl4 as well). We have previously seen that surfactants containing a polyoxyethylene chain as polar head group are efficient in reducing a silver salt to metallic silver [20]. The surfactant undergoes oxidative degradation generating formaldehyde and other aldehydes. The initial step is most likely abstraction of a hydrogen atom from the methylene groups of the polyoxyethylene chain. All these methylene groups are alpha to an ether bond and therefore reactive.

825

0.6

0.4

0.2

0

-0.2

350

400

450

500 550 600 Wavelength (nm)

650

700

750

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

Thus, there are in practice two reducing agents in the formulation used, NaBH4 and the polyoxyethylene headgroup of the nonionic surfactant that is forming a palisade layer around the water pools of the microemulsion. As mentioned above, the size of the water droplets was found to be critical when both HAuCl4 and AgNO3 were present in the formulation. When the drops were large, 20–30 nm, AgCl precipitated along with formation of mixed metal nanoparticles. When the drops were small, 3–4 nm, no AgCl precipitate was detected. In such small water pools the polyoxyethylene chains will more or less reach across the whole aqueous domain. The water pools will then consist of highly hydrated polyoxyethylene chains in which the reactants, i.e., HAuCl4 , AgNO3 and NaBH4 are dissolved. The combined reducing power of NaBH4 and the polyoxyethylene chains is evidently enough to transform Ag(I) to Ag(0) so rapidly that precipitation of AgCl does not occur. The concentration of the reactants was also found to be critical for formation of both the pure nanoparticles and the alloy nanoparticles. For the synthesis of the gold nanoparticles, using a 1:1 ratio between the metal salt microemulsion and the microemulsion containing the reducing agent, nanoparticle suspensions stable for at least one week could only be obtained up to concentrations of HAuCl4 and NaBH4 in the water pools of 0.03 and 0.05 M, respectively. The resulting suspension exhibited a plasmon peak at 520 nm (Fig. 2). The gold nanoparticles were analysed by transmission electron microscopy (TEM) and the size and size distributions were determined. As it can be seen from Table 1, the mean particle size was 5.7 nm. Individual particles were also analysed by scanning transmission electron microscopy STEM using high angle annular dark field (HAADF) including high resolution STEM (HR-STEM) and an image is shown in Fig. 3. For the preparation of silver sols, the concentration of the reactants needed to be even lower in order to give a nanoparticle suspension with proper stability. Fig. 2 shows a UV–visible

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

Mean (nm)

Variance (PI)

Au Au–Ag 75:25 Au–Ag 50:50 Au–Ag 25:75 Ag

5.7 5.5 6.7 6.9 9.6

1.1012 1.5338 1.7838 2.7334 14.939

826

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

Fig. 3. Scanning transmission electron microscopy images of gold nanoparticles prepared using a reducing agent to metal salt ratio of 1.7 (left) and silver nanoparticles prepared using a reducing agent to metal salt ratio of 10 (right).

absorption spectrum of a silver sol prepared using a AgNO3 concentration of 0.005 M and a NaBH4 concentration of 0.05 M in the water pools. The mean size of the particles is 9.6 nm, as can be seen from Table 1. Fig. 3a shows a STEM image of the nanoparticles generated. Increasing the water to surfactant ratio, which results in larger microemulsion droplets and also in an increased size of the nanoparticles, gave a shift of the peak in the UV–visible spectrum to higher wavelength. This is in accordance with previous observations [21]. There was a pronounced difference in the ratio of reducing agent to noble metal salt needed to prepare stable suspensions of gold and silver nanoparticles. Whereas a molar ratio of 10 was required for silver, a ratio of only 1.7 was needed for gold. This probably reflects the more “noble” character of gold, i.e., reduction of Au(III) to Au(0) is more facile than reduction of Ag(I) to Ag(0), based on the difference in standard electrode potential [5]. 4. Formation of gold–silver alloy nanoparticles As was the case for the pure metal nanoparticles, a high stability of alloy nanoparticles could only be achieved with a high surfactant to water ratio, which means small water droplets in the microemulsion, which, in turn, yields small nanoparticles. A microemulsion

Ag-Au 100:0 Ag-Au 90:10 Ag-Au 80:20 Ag-Au 70:30 Ag-Au 60:40 Ag-Au 50:50 Ag-Au 40:60 Ag-Au 30:70 Ag-Au 20:80 Ag-Au 10:90 Ag-Au 0:100

Absorbance

0.8

0.6

0.4

540 520 500 480

λmax (nm)

1

with the composition water:surfactant:oil of 0.007:0.111:0.882 by weight was used for the synthesis of the alloy nanoparticles. The total concentration of precursors (HAuCl4 + AgNO3 ) was 0.1 M and the ratio metal:reducing agent was 1:20. In order to avoid formation of AgCl the reducing agent, NaBH4 , had to be added immediately after mixing the microemulsions containing the gold and the silver salts. A series of nanoparticles of different composition was prepared with proportions ranging from pure silver to pure gold. The UV vis. spectra were recorded and are shown in Fig. 4. As can be seen, there is a smooth shift of the absorption maximum from 400 nm for the pure silver nanoparticles to 520 nm for the pure gold nanoparticles. The peak maximum plotted versus the atom percentage of gold in the nanoparticles gave a linear relationship (Fig. 5). As can be seen from Table 1, the sizes of the mixed nanoparticles were in the same range as those of the pure metal nanoparticles. Fig. 4 also shows that while a suspension of pure silver nanoparticles gave a sharp peak (at 400 nm), the suspension of pure gold nanoparticles exhibited a broader peak (at 520) and a large shoulder at lower wavelength. This is in accordance with the literature [21]. The suspensions of the mixed nanoparticles gave intermediate absorption curves and there was a smooth transition from one extreme to the other.

460 440

0.2 420 400

0 300

350

400

450

500 550 600 Wavelength (nm)

650

700

750

Fig. 4. UV–visible spectra of suspensions of the different alloys with absorption maxima normalized to 1.

380

0

10

20

30 40 50 60 70 Atom percent Au in the alloy

80

90

100

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

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

827

1.2

540

1

520 500

0.8

λmax (nm)

Absorbance

480 0.6

460 0.4

440 0.2

420

0

Fig. 6. A high resolution scanning transmission electron microscopy image recorded using high angle annular dark field showing atom columns in nanoparticles with an Au to Ag atom ratio of 50:50.

In principle, the simultaneous reduction of HAuCl4 and AgNO3 present in the water pools of a water-in-oil microemulsion may lead to either - a physical mixture of gold nanoparticles and silver nanoparticles - mixed nanoparticles which contain a core of one metal surrounded by a shell of the other metal; it is likely that gold, which is the more noble of the metals and therefore the one most easily generated from its precursor, would constitute the core - mixed nanoparticles with a homogeneous distribution of the two metal atomsor to mixtures of the three extremes. Since the objective of our work is to synthesize porous gold nanoparticles by leaching out silver from mixed gold–silver nanoparticles, we are aiming for nanoparticles with a homogeneous distribution of gold and silver, i.e., a true alloy. Gold and silver have very similar lattice constants, 0.408 and 0.409 nm, respectively [22], and it is known since long back that they can form true alloys in bulk phase. Also for nanoparticles incorporation of silver atoms into gold need not give rise to a lattice mismatch, as has been demonstrated by TEM [23]. If the reaction conditions are right, true alloy nanoparticles will form [24–26]. A physical mixture of two one-metal nanoparticles gives two peaks in the UV spectrum rather than one peak with a maximum in-between those of the individual metals. Thus, a 50:50 mixture of pure gold nanoparticles and pure silver nanoparticles has been shown to give two separate plasmon bands, one at 523 nm (from gold) and one at 399 nm (from silver) [27]. This is obviously not what we are seeing. More or less pronounced core–shell structures, with gold dominating the core and silver being enriched in the shell, have also been reported [13]. The spectroscopic results of this work, with a smooth transition of the plasmon peak from that of silver to that of gold as the ratio of gold to silver is increased are not consistent with the core–shell option, however. There are several reports in the literature that core–shell, and other non-alloy, nanoparticles display the peaks of the individual components with the relative absorbance corresponding to the relative amount of the metal [28–35]. The spectroscopy results presented above strongly indicate that the particles obtained are alloy nanoparticles and this is also consistent with results from analysis by energy-dispersive X-ray spectroscopy (EDX) combined with HR-STEM. A HR-STEM image of nanoparticles obtained from a 50:50 molar ratio of Au and Ag nanoparticles is shown in Fig. 6. EDX analysis, at a resolution of around 1 nm, indicated that the ratio of the two metals remained approximately constant through the particles. It should be noted that the linear correlation between ␭max and gold content (Fig. 5) is in accordance with calculations based on

-0.2

400

0

0.5

1

1.5

2 2.5 Time (hours)

3

3.5

380 4

Fig. 7. Change in relative absorbance of suspensions of pure gold nanoparticles (at 520 nm; squares) and of pure silver nanoparticles (at 400 nm; rings) on exposure to nitric acid, left axis. The dotted lines show the positions of the surface plasmon band (gold: squares; silver: circles), right axis.

the Mie theory for gold–silver alloys [29,36]. The Mie theory also tells that the absorbance of the alloy nanoparticles should be lower than that of the pure gold and silver nanoparticles [30]. This has been 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 of two or more metals is a well-known procedure. The literature contains many examples of preparation of porous gold by dealloying Au/Ag alloys by the use of nitric acid as leaching agent [37–40]. In the majority of cases the procedure has been applied on macroscopic alloys, e.g. white gold leafs, or on Au/Ag alloy surfaces and the main application for the porous gold obtained has been heterogeneous catalysis. The methodology has also been extended to make nanoporous Au/Pt alloys by leaching out Cu from Au/Pt/Cu alloys [41]. The procedure used in this work, to leach out Ag from Au/Ag alloy nanoparticles by adding a microemulsion containing concentrated nitric acid in the water pools to a freshly made suspension of the nanoparticles seems not to have been described before. The procedure is attractive for generation of ultra small porous gold nanoparticles because the microemulsion procedure for making the alloy nanoparticles can yield a suspension of small nanoparticles with good control of the particle size. It is reasonable to assume that the porous nanoparticles obtained after the dealloying step will be similar in size–or somewhat smaller due to some loss also of gold–as the alloy nanoparticles. Nanoparticles with three different ratios of Au to Ag were used for the dealloying: 75:25, 50:50 and 25:75. As reference, the same procedure was also applied to pure Au and pure Ag nanoparticles. The dissolution of nanoparticles of pure gold and silver was monitored by UV–vis. spectroscopy following the decline of the surface plasmon band of Au at 525 nm and of Ag at 400 nm. As can be seen from Fig. 7, the silver plasmon band disappeared much more rapidly than the plasmon band of gold. After 4 h exposure to the nitric acid-containing microemulsion the silver plasmon band was down to almost zero absorbance while the gold peak maintained approximately two thirds of its absorbance. The figure also shows that the positions of the surface plasmon band for both gold and silver were unaffected by the leaching procedure.

828

J. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823–829 534

515

532 560

510

530

540 505

524 522

520

λ max (nm)

526

λmax (nm)

λmax (nm)

528

500

495

520

500 480 460

518 490

440

516 514

0

2

4

6

8 10 Time (hours)

12

14

16

18

485

0

2

4

6 8 Time (hours)

10

12

14

420

0

0.5

1

1.5

2

2.5 3 3.5 Time (hours)

4

4.5

5

Fig. 8. Change in position of the surface plasmon band vs. time for nitric acid treatment of alloy nanoparticles based on Au to Ag atom ratios of 75:25 (left), 50:50 (middle) and 25:75 (right).

Fig. 8 shows dealloying results for the gold–silver nanoparticles. The increase in ␭max is in accordance with a gradual increase in gold content in the nanoparticles. As can be seen, the ␭max for the 25:75Au–Ag nanoparticles increased well beyond the value of 525 obtained for pure gold nanoparticles. This is the composition with the highest content of silver, which means that after leaching it will be the nanoparticles with the highest porosity and the largest gold surface area. It is important to realize that the reduction in absorption intensity of the gold plasmon band seen during the dealloying process cannot be taken as a quantitative measure of the amount of gold that has leached out of the particles (see Fig. 7). It has been pointed out that the Au plasmon absorption intensity decreases as Ag ions are removed from mixed nanoparticles. The reason for this is not clear but it has been suggested that the phenomenon is due to a structural change of the nanoparticles. Surface plasmon resonance bands are known to be influenced by the size, shape, composition and dielectric properties of the nanoparticles [7,18]. The wavelength is not changed, however. In our case it is conceivable that the porosity will influence the position of the maximum in the surface plasmon band. 5. Conclusions We have developed a method to prepare mixed nanoparticles of gold and silver by adding solid NaBH4 in large excess to waterin-oil microemulsions containing the gold and silver precursors, HAuCl4 and AgNO3 , respectively in the water pools. A combination of two fatty alcohol ethoxylates were used as surfactant system and TEM analysis showed that the mixed nanoparticles were in the 5–7 nm size range. In a series of syntheses from pure silver, via incrementally increasing atomic ratios of gold to silver, to pure gold, there was a smooth shift of the absorption maximum from 400 nm for the pure silver nanoparticles to 520 nm for the pure gold nanoparticles. The peak maximum plotted versus the atom percentage of gold in the mixed nanoparticles gave a linear relationship. This observation, together with EDX analysis combined with HR-STEM, indicated that the particles were true alloys, not of core–shell type. The suspensions of mixed nanoparticles were subsequently dealloyed using nitric acid as dealloying reagent. Reference experiments on suspensions of pure gold and pure silver nanoparticles showed that the rate of dissolution differed considerably between the two metal nanoparticles. After 4 h exposure to nitric acid in the form of a microemulsion essentially all metallic silver was dissolved while 2/3 of the gold remained intact. Exposure of alloy nanoparticles of different composition to the nitric acid microemulsion gave a continuous increase of the position of the surface plasmon band. This is consistent with a gradually increasing ratio of gold to silver, eventually arriving at porous gold nanoparticles.

References [1] M. Yashima, et al., Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion, J. Colloid Interface Sci. 268 (2) (2003) 348–356. [2] H. Härelind Ingelsten, et al., Deposition of platinum nanoparticles: synthesized in water-in-oil microemulsions, on alumina supports, Langmuir 18 (5) (2002) 1811–1818. [3] M.P. Pileni, Reverse micelles as microreactors, J. Phys. Chem. 97 (27) (1993) 6961–6973. [4] K. Holmberg, Surfactant-templated nanomaterials synthesis, J. Colloid Interface Sci. 274 (2) (2004) 355–364. [5] E. Detsi, et al., Direct synthesis of metal nanoparticles with tunable porosity, J. Mater. Chem. 22 (11) (2012) 4588–4591. [6] CRC Handbook of Chemistry and Physics, CRC Press: Boca Raton. p. 3–138. [7] P.K. Jain, et al., Calculated absorption and scattering properties of gold nanoparticles of different size: shape, and composition: applications in biological imaging and biomedicine, J. Phys. Chem. B 110 (14) (2006) 7238–7248. [8] N. Shirtcliffe, U. Nickel, S. Schneider, Reproducible preparation of silver sols with small particle size using borohydride reduction: for use as nuclei for preparation of larger particles, J. Colloid Interface Sci. 211 (1) (1999) 122–129. [9] P.D.I. Fletcher, et al., Activity of lipase in water-in-oil microemulsions, J. Chem. Soc. Faraday Trans. 1: Phys. Chem. Condens. Phases 81 (11) (1985) 2667–2679. [10] M. Husein, E. Rodil, J. Vera, Formation of silver chloride nanoparticles in microemulsions by direct precipitation with the surfactant counterion, Langmuir 19 (20) (2003) 8467–8474. [11] S. Tokonami, et al., Novel synthesis, structure, and oxidation catalysis of Ag/Au bimetallic nanoparticles, J. Phys. Chem. C. 114 (23) (2010) 10336–10341. [12] R.P. Bagwe, K.C. Khilar, Effects of intermicellar exchange rate on the formation of silver nanoparticles in reverse microemulsions of AOT, Langmuir 16 (3) (2000) 905–910. [13] D.-H. Chen, C.-J. Chen, Formation and characterization of Au–Ag bimetallic nanoparticles in water-in-oil microemulsions, J. Mater. Chem. 12 (5) (2002) 1557–1562. [14] M.-L. Wu, L.-B. Lai, Synthesis of Pt/Ag bimetallic nanoparticles in water-in-oil microemulsions, Colloids Surf. A: Physicochem. Eng. Aspects 244 (1–3) (2004) 149–157. [15] M.-L. Wu, D.-H. Chen, T.-C. Huang, Synthesis of Au/Pd bimetallic nanoparticles in reverse micelles, Langmuir 17 (13) (2001) 3877–3883. [16] M.-L. Wu, D.-H. Chen, T.-C. Huang, Preparation of Pd/Pt bimetallic nanoparticles in water/AOT/isooctane microemulsions, J. Colloid Interface Sci. 243 (1) (2001) 102–108. [17] G. Hota, S. Jain, K.C. Khilar, Synthesis of CdS–Ag2 S core–shell/composite nanoparticles using AOT/n-heptane/water microemulsions, Colloids Surf. A: Physicochem. Eng. Aspects 232 (2–3) (2004) 119–127. [18] M.M. dos Santos, M.J. Queiroz, P.V. Baptista, Enhancement of antibiotic effect via gold:silver-alloy nanoparticles, J. Nanopart. Res. 14 (5) (2012) 1–8. [19] S. Dengler, et al., Near- and off-resonant optical limiting properties of gold–silver alloy nanoparticles for intense nanosecond laser pulses, J. Opt. 14 (7) (2012) 075203. [20] F. Currie, M. Andersson, K. Holmberg, Oxidation of self-organized nonionic surfactants, Langmuir 20 (10) (2004) 3835–3837. [21] S. Link, M.A. El-Sayed, Spectral properties, relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods, J. Phys. Chem. B 103 (40) (1999) 8410–8426. [22] C. Kittel, Introduction to Solid State Physics, Wiley, NY, 1996. [23] S. Link, Z.L. Wang, M.A. El-Sayed, Alloy formation of gold–silver nanoparticles and the dependence of the plasmon absorption on their composition, J. Phys. Chem. B 103 (18) (1999) 3529–3533. [24] Z.Y. Li, et al., Structures and optical properties of 4–5 nm bimetallic AgAu nanoparticles, Faraday Discuss. 138 (2008) 363–373. [25] P. Raveendran, J. Fu, S.L. Wallen, A simple and green method for the synthesis of Au:Ag, and Au–Ag alloy nanoparticles, Green Chem. 8 (1) (2006) 34–38.

J. Cheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 823–829 [26] C. Wang, et al., One-pot synthesis of oleylamine coated AuAg alloy NPs and their catalysis for CO oxidation, Chem. Mater. 21 (3) (2009) 433–435. [27] M.P. Mallin, C.J. Murphy, Solution-phase synthesis of sub-10 nm Au@Ag alloy nanoparticles, Nano Lett. 2 (11) (2002) 1235–1237. [28] P. Mulvaney, M. Giersig, A. Henglein, Electrochemistry of multilayer colloids: preparation and absorption spectrum of gold-coated silver particles, J. Phys. Chem. 97 (27) (1993) 7061–7064. [29] J. Sinzig, et al., Binary clusters: homogeneous alloys and nucleus–shell structures, Z. Phys. D Atoms, Mol. Clusters 26 (1) (1993) 242–245. [30] P. Mulvaney, Surface plasmon spectroscopy of nanosized metal particles, Langmuir 12 (3) (1996) 788–800. [31] L. Rivas, et al., Mixed silver/gold colloids: a study of their formation, morphology, and surface-enhanced Raman activity, Langmuir 16 (25) (2000) 9722–9728. [32] I. Srnová-Sloufová, et al., Core–shell (Ag) Au bimetallic nanoparticles: analysis of transmission electron microscopy images, Langmuir 16 (25) (2000) 9928–9935. [33] C.S. Ah, S.D. Hong, D.-J. Jang, Preparation of Au-core Ag-shell nanorods and characterization of their surface plasmon resonances, J. Phys. Chem. B 105 (33) (2001) 7871–7873.

829

[34] K. Mallik, et al., Seed mediated formation of bimetallic nanoparticles by UV irradiation: a photochemical approach for the preparation of “core–shell” type structures, Nano Lett. 1 (6) (2001) 319–322. [35] L. Lu, et al., Seed-mediated growth of large, monodisperse core–shell gold–silver nanoparticles with Ag-like optical properties, Chem. Commun. (2) (2002) 144–145. [36] M. Moskovits, I. Srnova-Sloufova, B. Vlckova, Bimetallic Ag–Au nanoparticles: extracting meaningful optical constants from the surface-plasmon extinction spectrum, J. Chem. Phys. 116 (23) (2002) 10435–10446. [37] Y. Ding, M. Chen, J. Erlebacher, Metallic mesoporous nanocomposites for electrocatalysis, J. Am. Chem. Soc. 126 (22) (2004) 6876–6877. [38] C. Xu, et al., Low temperature CO oxidation over unsupported nanoporous gold, J. Am. Chem. Soc. 129 (1) (2006) 42–43. [39] V. Zielasek, et al., Gold catalysts: nanoporous gold foams, Angew. Chem. Int. Ed. 45 (48) (2006) 8241–8244. [40] J. Snyder, K. Livi, J. Erlebacher, Dealloying silver/gold alloys in neutral silver nitrate solution: porosity evolution, surface composition, and surface oxides, J. Electrochem. Soc. 155 (8) (2008) C464–C473. [41] C. Xu, et al., Dealloying to nanoporous Au/Pt alloys and their structure sensitive electrocatalytic properties, Phys. Chem. Chem. Phys. 12 (1) (2010) 239–246.