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CSIRO PUBLISHING

Aust. J. Chem. 2008, 61, 66–71

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Preparation of Silver–Gold Alloy Nanoparticles at Higher Concentration Using Sodium Dodecyl Sulfate Angshuman Pal,A Sunil Shah,A and Surekha DeviA,B A Department

of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, India. B Corresponding author. Email: [email protected]

Highly stable gold–silver alloy nanoparticles with varying mole fractions were prepared in aqueous sodium dodecyl sulfate solution by simultaneous reduction of chloroauric acid (HAuCl4 ) and silver nitrate (AgNO3 ) using sodium citrate. The formation of alloy nanoparticles was confirmed by UV-visible spectroscopy and transmission electron microscopy. Particle size distribution was measured by dynamic light scattering. The surface plasmon absorption band of the Au–Ag alloy nanoparticles shows linear bathochromic shift with increasing Au content. Appearance of a single absorption peak in the visible region and lack of apparent core-shell structures in the transmission electron microscope images confirm the formation of homogeneous gold–silver alloy nanoparticles. Manuscript received: 19 May 2007. Final version: 11 November 2007.

Introduction In recent years, investigation of nanostructured materials, especially metal nanoparticles, has attracted much attention, because of their interesting chemical, mechanical, and physical properties, and potential applications in nanoelectronic and optoelectronic devices. Many methods have been reported in the literature for the synthesis of such novel materials. The wet-chemical synthesis of such novel materials opens a new area of research in the field of classical colloidal chemistry as well as materials science. The optical properties of these nanostructured silver, gold, and copper metals are markedly different from bulk materials. The optical properties of these nanomaterials are due to the surface plasmon resonance. The surface plasmons are collective electronic excitations at the interface between metal and dielectrics. There are two types of surface plasmon resonances, one localized and the other propagating. The localized surface plasmon resonance is a collective oscillation of conduction electrons of noble metals. The optical properties of Ag, Au, and their bimetallic nanoparticles change with size, shape, and dielectrics of the medium. Nowadays, much effort has been devoted to the fabrication of metal nanocomposites, including alloys, core-shell and mixed particles, owing to their valuable non-linear optical properties in optical switches,[1,2] in catalysis,[3,4] and in surface-enhanced Raman scattering (SERS).[5] In the literature, many methods have been reported for the preparation of Au– Ag bimetallic nanoparticles in the form of alloy or core-shell or a mixture of individual metal particles using various reducing agents. Alloy nanoparticles of silver and gold were prepared by simple coreduction of these metal ions using sodium citrate in aqueous medium by Link et al.[6] Murphy et al. reported the formation of Au–Ag alloy nanoparticles in aqueous medium using sodium borohydride as a reducing agent and sodium citrate as a capping agent.[7] Metal nanoparticles have a tendency to agglomerate. Hence, use of a protecting or passivating agent to prevent agglomeration is desirable. Polymers and surfactants

are commonly used as protecting or stabilizing agents. In our previous work, we have shown the formation of Au–Ag alloy nanoparticles stabilized in reverse micelles and in polyacrylamide solution.[8,9] Au–Ag alloy nanoparticles synthesized at higher concentrations using sodium citrate as a capping agent have been reported to be unstable by Michael et al.[7] Hence, here we report an attempt to improve the stability of Au, Ag, and Au–Ag alloy nanoparticles at more than 100 times the concentration reported earlier, using sodium dodecyl sulfate (SDS). Results and Discussion Au–Ag alloy nanoparticles were obtained through the simultaneous reduction of gold and silver ions using sodium citrate in SDS solution. Gold and silver form alloy nanoparticles when reduced simultaneously in the mixture, and this can be confirmed by optical spectroscopy. If pure gold and silver nanoparticles are mixed physically in the same solution, two plasmon bands are expected to be observed, whereas for alloys a single plasmon band is expected. Appearance of a single surface plasmon band in Fig. 1a, for colloidal solutions of alloys with varying gold content, confirms the formation of alloy particles. Figure 1b gives the comparative data at zero storage time, represented by capital letters (A–E), and data after 6 months’storage at 25 ± 2◦ C (room temperature) represented by lower case letters (a–d). Not much shift in surface plasmon absorption band was observed after 6 months’ storage, indicating the formation of highly stable alloy nanoparticles. The optical properties of these alloy nanoparticles vary with their composition, which is seen from the digital photographs in Fig. 2. The surface plasmon absorption band shows red shift and the colour of the colloidal solution changes from yellow to red with increasing gold content. From Fig. 1a, it is observed that for Ag and Au nanoparticles, the plasmon absorption maxima appeared at 404 and 537 nm, respectively. For the alloy nanoparticles, the plasmon

© CSIRO 2008

10.1071/CH07165

0004-9425/08/010066

Silver–Gold Alloy Nanoparticles

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(a) 1.2 [A] [B] [C] [D] [E] 1.0

Absorbance

0.8 0.6

0.4 0.2

0 300

400

500

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Wavelength [nm] (b)

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Immediate (A) 404 nm (B) 440 nm (C) 477 nm (D) 499 nm (E) 537 nm

0.9 A a

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After 6 months (a) 405 nm (b) 438 nm (c) 474 nm (d) 497 nm (e) 535 nm

E e

0.3 0.2 0.1 0 300

350

400

450

500

550

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650

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750

Wavelength [nm] Fig. 1. (a) UV-visible absorption spectra of Au, Ag, and Au–Ag alloy nanoparticles with varying mole ratios at t = 0 (where t is storage time). All the spectra have been normalized at the plasmon absorption maxima. A = Ag; E = Au; B, C and D for Au–Ag alloy nanoparticles synthesized at 0.4:0.6, 0.5:0.5, and 0.6:0.4 mol ratios. (b) UV-visible absorption spectra ofAu,Ag, andAu–Ag alloy nanoparticles with varying mole ratios. The spectra were taken at t = 0 and t = 6 months.

Au (a)

Au–Ag (b)

Au–Ag (c)

Ag (d)

Fig. 2. Digital photographs of the Ag, Au, and Au–Ag alloy nanoparticles with various compositions. Au mole fractions are (a) 1.0; (b) 0.6; (c) 0.5; (d) 0.0.

band appeared between 404 and 537 nm. The appearance of a single absorption band indicates that synthesized Au–Ag bimetallic particles are in alloy form rather than being a mixture of individual metal particles, whereas the physical mixture of synthesized Ag and Au nanoparticles showed two absorption bands corresponding to the individual metal nanoparticles (Fig. 3). Alloy formation can be attributed to similar lattice constants of 0.408 and 0.409 nm, respectively, for gold and silver. The difference in lattice constants being smaller than the amplitude of thermal vibrations of atoms favours alloy formation even at the nanometer scale.[6] The inset in Fig. 3 shows the positions of surface plasmon bands against the Au mole ratio in the various alloy compositions. A linear relationship was observed between the absorption maximum and the gold mole fraction in the alloy. The linear relationship is attributed to the formation of alloy particles rather than core-shell by Raveendran et al.[10] A linear behaviour between surface plasmon and gold mole fraction for Au–Ag alloy nanoparticles was predicted by Jian Zhu using the Drude model and quasi-static theory.[11] Figure 4 exhibits the transmission electron microscopy (TEM) photograph of pure metallic and alloy nanoparticles and Fig. 5 shows the particle size distribution of five different alloy compositions. From the

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A. Pal, S. Shah, and S. Devi

600

1

(a)

Wavelength [nm]

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0.8

500 450 400 350 300 250 200

Absorbance

[A]

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0.6 0.4 Au mole ratio

0.8

1

0.6 [C]

[B]

0.4

0.2

0 300

100 nm

400

500 600 Wavelength [nm]

700

(b)

Fig. 3. Surface plasmon absorption maximum for Ag [A], Au [B], and physical mixture of Au and Ag [C]. The inset shows the positions of surface plasmon bands plotted against the Au mole ratios for the various alloy compositions. The solid line is a linear fit of the absorption maximum for the increasing gold mole ratio.

TEM photograph and dynamic light scattering (DLS) analysis, it was observed that the particles are in the range of 10–50 nm in diameter. From Fig. 4c, the synthesized alloy particles were observed to be spherical in nature. From the results in Table 1, the red shift in surface plasmon absorption band and increase in particle size with increasing Au content in alloy composition were observed. The synthesized particles were stable up to 6 months. During this period, no change in surface plasmon absorption maxima and average particle size was observed, indicating the role played by SDS in stabilizing the particles at 5 × 10−4 M concentration. However, in the absence of SDS, at this concentration the nanoparticles formed were observed to be aggregated into black precipitate within 60 min owing to agglomeration. SDS molecules on the nanoparticles’ surface prevent the agglomeration. Figure 6 shows the infrared spectroscopy (FT-IR) spectrum of pure SDS (Fig. 6a) and SDS-coated gold nanoparticles (Fig. 6b). Symmetric stretching of S=O in the free SDS molecule was observed at 1379 cm−1 , which was shifted to 1400 cm−1 in the spectrum of SDS-capped gold nanoparticles, owing to the interaction between SDS molecules and metal nanoparticles. The role of SDS in stabilization of nanoparticles was further confirmed by adding 1% sodium chloride (NaCl) to the metal colloid. Figure 7a shows the UV-visible absorption spectra of gold–silver alloy nanoparticles recorded with time after addition of 1% sodium chloride. Figure 7b shows the UV-visible absorption spectra of gold–silver alloy nanoparticles in the presence of different concentrations of sodium chloride.After addition of sodium chloride to the colloidal solution, the colour of the solution changed from reddish to blue instantly, and the surface plasmon band showed a red shift. In addition, instead of one surface plasmon band, appearance of two bands and broadening of the bands was also observed. This observation also suggests that anisotropic agglomeration was taking place after the addition of salt. This can be attributed to the observation made by Krathovil et al.,[12] where they reported that in the case of SDS with increasing ionic

0.2 µm

(c)

100 nm 88.0 K

Fig. 4. Transmission electron microscopy images of: (a) Ag; (b) Au; (c) Au–Ag alloy nanoparticles.

strength, micelles undergo substantial transformations in size and shape, creating cylindrical micelles with large aggregation numbers, resulting in micelles with closely packed SDS headgroups. This supports the possible anisotropic agglomeration observed by us in Figs 7a and b.

Silver–Gold Alloy Nanoparticles

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100

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90 Au:Ag (0.4:0.6)

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81.5 143.2

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7 10.1 15.1 22.3 32.8 41.3 71.1 101 Diameter [nm]

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Au

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Intensity

60 50 40 30 20 10 0 1

1.4 1.9 2.7 3.7 5.1 7.1 9.3 13.5 15.3 16

36 40.9 69.1 95.7

Diameter [nm]

Fig. 5.

Particle size distributions of different metal ion compositions.

Table 1. Surface plasmon absorption maxima and particle sizes of different alloy compositions Au mole ratio 0.0 0.4 0.5 0.6 1.0

Wavelength [nm]

Particle size [nm]

404 440 477 499 537

20 ± 2 17 ± 1 25 ± 2 30 ± 2 40 ± 5

Conclusions The interesting aspect of the present study is the preparation of highly stable silver–gold alloy nanoparticles at high concentration by coreduction of metal salts using SDS as a stabilizer.

Tuning of optical properties of these alloy nanoparticles was achieved simply by changing the alloy composition. A good linear relationship was observed between the surface plasmon absorption maxima and Au content in the alloy. The optical properties of these nanoparticles can be used in colorimetric detection in future study.

Experimental Materials All chemicals used were of AR grade. Tetrachloroauric(iii) acid (HAuCl4 ; Mw 339.79) was purchased from Sigma-Aldrich (Steinheim, Germany). Silver nitrate (AgNO3 ) was purchased from Merck (Mumbai, India). Sodium citrate and SDS were

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%T

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1379

30 20 10 0 4000

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1]

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1594

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1500 Wavenumber [cm
1]

(a)

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1.5

Infrared spectroscopy spectrum of: (a) sodium dodecyl sulfate (SDS); and (b) SDS-bound Au nanoparticles.

Time [min] Without salt t2 t2 t3 SPB  surface plasmon band t  3 t4 t4 A  absorbance t5 t5 t9 t  17 t9 t  20 t  17 t  25 t  20 t  30 t  25 t  35 t  30 t  35

SPB 691 694 694 694 699 700 701 701 701 703

A 1.1088 1.0951 1.0836 1.0734 1.0372 0.9764 0.9530 0.9277 0.8942 0.8643

1.0

0.5 300

400

500 600 700 Wavelength [nm]

800

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690 nm 1.5 0.5% NaCl 1.0% NaCl 2.0% NaCl 3.0% NaCl

Absorbance

Fig. 6.

683 nm 678 nm 649 nm

1.0

0.5 300

400

500

600

700

800

Wavelength [nm]

Fig. 7. (a) UV-visible absorption spectra of gold–silver alloy nanoparticles recorded with time after addition of 1% NaCl. (b) UV-visible absorption spectra of gold–silver alloy nanoparticles recorded at different NaCl concentrations.

purchased from Qualigen (Mumbai, India). Distilled deionized water was used throughout the work. Instrumentation A Perkin–Elmer Lambda 35 UV-visible spectrophotometer was used to characterize the optical properties of gold nanoparticles.

Size and shape of the nanoparticles were determined by using TEM on a Philips CM-200 operating at 200 kV. The sample for TEM was prepared by putting one drop of the colloidal gold solution onto standard carbon-coated copper grids and then drying under an electric bulb for 30 min. The particle size distribution was measured using a 90 Plus DLS unit from Brookhaven

Silver–Gold Alloy Nanoparticles

(Holtsville, USA). FT-IR studies were carried out using a Perkin–Elmer RX1 IR spectrophotometer (Massachusetts, USA). Experimental Metallic and bimetallic alloy nanoparticles were prepared through the reduction of metal salts using sodium citrate (2% w/v) as a reducing agent. Throughout the reaction, 1% w/v aqueous SDS and 0.05 M metal ion concentrations were used. In 50 mL reaction volume, 0.5 mL 0.05 M metal ion concentration (total concentration 5 × 10−4 M) and 2.5 mL 2% v/v reducing agent concentrations were maintained for the preparation of all compositions of metallic and bimetallic particles. All the reactions were carried out at 90 ± 2◦ C with stirring for 1 h. Acknowledgement The authors are thankful to GUJCOST (Gandhinagar, Gujarat) for financial support.

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