Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 51–57

Synthesis of Au, Ag and Au–Ag alloy nanoparticles in aqueous polymer solution Angshuman Pal, Sunil Shah, Surekha Devi ∗ Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390002, Gujarat, India Received 13 September 2006; received in revised form 2 January 2007; accepted 29 January 2007 Available online 8 February 2007

Abstract Gold, silver and gold–silver alloy nanoparticles were prepared under mild conditions using microwave heating, polyacrylamide as a stabilizing agent and hydrazine hydrate as a reducing agent. The UV–vis spectroscopy revealed the formation of silver and gold nanoparticles by exhibiting surface plasmon absorption maxima at 410 and 548 nm, respectively. For Au–Ag bimetallic particles the surface plasmon absorption maxima appeared in between the peaks corresponding to pure silver and pure gold. The surface plasmon absorption maxima for bimetallic nanoparticles changes linearly with increasing gold content in the alloy. Transmission electron micrograph (TEM) showed presence of spherical particles in the range of 5–50 nm size. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Polyacrylamide; Surface plasmon; Alloy

1. Introduction Metallic and bimetallic nanoparticles are becoming increasingly interesting in nanoscience and technology due to their unique optical, electrical and catalytic properties. The high surface-to-volume ratios of nanoparticles lead to dramatic changes in their properties. As the size and the dimension of particles are reduced, their electronic properties change drastically, because the density of states and the spatial length scale of the electronic motion are reduced with decreasing size [1]. The energy eigenstates are determined by the system’s boundaries, and therefore surface effects become very important [2,3]. In metal nanoparticles a large number of available research papers have focused on controlling the particle size and shape for tuning their physical, chemical and optical properties. For metallic nanoparticles, interesting optical and electronic effects are expected on the 10–100 nm scale because the mean free path of an electron in a metal is 10–100 nm [4]. The physical origin of the absorption of light by metallic nanoparticles is the coherent oscillations of the conduction band electrons, induced by

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the interacting electromagnetic field [5]. Many methods have been reported for the preparation of metallic and bimetallic nanoparticles, such as laser irradiation [6], sonochemical deposition [7,8], photochemical reduction [9,10], electrochemical method [11,12] and chemical reduction of metal salts [13]. In most of these cases, the surface passivation reagents, including surfactant and polymer play a major role in prevention of the aggregation of nanoparticles. Metal nanoparticles have a common tendency of agglomeration and need a stabilizing agent. Polyvinylpyrrolidone (PVP) has been widely used as a stabilizing agent in the synthesis of silver and other metallic nanoparticles [14,15]. The formation of stable metallic nanoparticles in polymer matrix is achieved by a combination of a low concentration of solute and adhereness of polymeric monolayer on the growing surface. Both low concentration and polymeric monolayer would hinder the diffusion of growing species from the surrounding solution to the growing surface. As a result the diffusion process becomes the rate-limiting step of subsequent growth of the initiating nuclei, resulting in the formation of uniformly sized nanoparticles. The bimetallic nanoparticles have unique catalytic, electronic, and optical properties distinct from those of the corresponding metallic particles. The structure of the bimetallic combinations depends on the preparation conditions, miscibility, and kinetics of reduction of metal ions. Silver

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and gold have almost identical lattice constants (0.408 for Au and 0.409 for Ag) and are completely miscible over the entire composition range [16], which leads to a strong tendency toward alloy formation. Hence single-phase alloys can be achieved with any desired composition and the absorption spectra of alloy nanoparticles exhibit only one surface plasmon band, whose absorption maximum depends on the alloy composition [17]. The optical properties of metal nanoparticles depend on surface plasmon resonance. The origin of surface plasmon resonance in noble metal nanoparticles is the free conduction electrons in the metal surface (d electrons in silver and gold). The mean free path of the electrons in the gold and silver is 50 nm. Therefore for the particles smaller than 50 nm, no scattering is expected from the bulk [18]. Hence all the spectral properties are the function of surface and not of bulk. According to Mie’s theory the total extinction coefficient of small metallic particles is the summation of all electronic and magnetic multipole oscillations, contributing to the absorption and scattering of the interacting electromagnetic field [19]. Now for the particles much smaller than the wavelength of the absorbing light only the dipole term is assumed to contribute to the absorption [5]. The electric field of the incoming electromagnetic radiation induces the formation of a dipole in the nanoparticles. A restoring force in the nanoparticles tries to compensate for this, resulting in a unique resonance wavelength [20]. Alloy nanoparticles have received a special attention due to the possibility of tuning their optical and electronic properties over a broad range by simply varying the alloy composition. Here in this paper we report a simple method for preparation of Au, Ag, and Au–Ag alloy nanoparticles by using polyacrylamide as a stabilizing agent and microwave as a heating device. Microwave dielectric heating is a new promising technique for preparation of size controlled metallic nanostructures due to its rapid heating and penetration. Microwaves are electromagnetic waves and these high-frequency electromagnetic radiations interact with the dipole moment of molecules which causes the heating. Water has a very high dipole moment, which makes it one of the best solvents for microwave heating and hence water soluble polyacrylamide is used as a stabilizing agent for the preparation of these metallic and bimetallic nanoparticles. To our knowledge there are no reports on the use of polyacrylamide as a stabilizing agent in the synthesis of silver, gold and their alloy nanoparticles using microwaves. Hence a systematic study of synthesis and characterization is undertaken.

2.2. Microwave a rapid heating device For the synthesis of Au, Ag and Au–Ag alloy nanoparticles LG make microwave oven (model: MG 605 AP) was used. The desired solution mixture in closed vessel was reacted in a microwave oven that operated at the 100% power of 1350 W and frequency 2450 MHz. Over the conventional synthetic methods the advantage of microwave-mediated synthesis is the improved kinetics of the reaction generally by one or two order of magnitude, which is due to rapid initial heating and the generation of localized high-temperature zones at reaction sites [21]. 2.3. Preparation of silver nanoparticles For the preparation of silver nanoparticles, silver nitrate solution (0.01 M) and 10% (v/v) polyacrylamide (MW 10,000) were used as a metal salt precursor and a stabilizing agent, respectively. Hydrazine hydrate (1%, w/v) was used as a reducing agent. To the polyacrylamide solution equal volume of silver nitrate and hydrazine hydrate (3% of polyacrylamide volume) were added and reaction was carried out under microwave for 1 min. The transparent colourless solution was converted to the characteristic pale yellow colour, indicating the formation of silver nanoparticles. The stability of the solution at room temperature was observed to be more than one month. 2.4. Preparation of gold nanoparticles

2. Experimental

For the preparation of gold nanoparticles gold chloride solution (0.01 M) and 10% (v/v) polyacrylamide (MW 10,000) were used as a metal salt precursor and a stabilizing agent, respectively. Hydrazine hydrate (1%, w/v) and tri-sodium citrate (2%, w/v) were used as reducing agents. To the polyacrylamide solution equal volume of gold salt solution and tri-sodium citrate (3% of polyacrylamide volume) were added and the reaction was carried out under microwave for 10 min. The pale yellow colour transparent solution was converted to the characteristic red rose colour, indicating the formation of gold nanoparticles. The solution remained as it is without changing its physicochemical properties at room temperature for more than 6 months. Gold nanoparticles were also prepared following the same procedure but using hydrazine hydrate. The reaction was carried out only for 1 min. On addition of hydrazine the total solution immediately turns into transparent blue in colour which slowly turns into purple on standing at room temperature for 24 h and gradually turns red after 10 days prolonged standing. However, if the reaction is carried out under microwaves solution turns purple within 1 min.

2.1. Materials

2.5. Preparation of alloy nanoparticles

Silver nitrate (AgNO3 ) was purchased from Merck Mumbai, India. Tetrachloroauric(III)acid (HAuCl4 ) and polyacrylamide (MW 10,000) was purchased from Sigma–Aldrich, Steinheim, Germany. Hydrazine hydrate was obtained from S.D. FineChemicals, Mumbai, India. All the solutions were prepared using double-distilled deionised water.

Bimetallic alloy nanoparticles were prepared following the above mentioned procedure, using different ratios of silver and gold ion concentrations and hydrazine hydrate as a reducing agent. The solutions were kept under microwave for 1 min. The change in the colour of the solution indicates the formation of alloy nanoparticles with different composition.

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2.6. Characterization Size, shape and optical properties of nanoparticles were determined by using Perkin-Elmer Lambda 35 UV–vis spectrophotometer and transmission electron microscope Philips Model CM-200 operated at voltage 200 kV with 0.23 nm resolutions. Samples were prepared for TEM analysis by placing a drop of the solution on a polymer coated copper grid and then dried under electric bulb for 30 min. The particle size distribution was measured using 90 Plus, Dynamic light scattering (DLS) by Brookhaven, Holtsville, USA.

3. Results and discussion Use of microwave radiation in the synthesis is showing promise not only due to faster heating but it also gives internal uniform heating resulting into uniformly distributed particles. The transmission electron microscopic (TEM) images of the silver and gold nanoparticles, synthesized using this method are represented in the Fig. 1 and indicate monodispersed particles. The UV–vis absorption spectra of silver and gold nanoparticles synthesized using hydrazine as a reducing agent and gold nanoparticles prepared using tri-sodium citrate as a reducing agent are given in Fig. 2A–C. UV–vis spectra (Fig. 2A and C) reveal the formation of silver and gold nanoparticles by showing surface plasmon absorption maxima at 410 and 548 nm, respectively. The position and shape of the plasmon absorption depends on the particle size and the dielectric constant of the surrounding medium. For gold nanoparticles if the reducing agent is hydrazine the blue/purple coloured dispersion (Fig. 2C) is obtained and for tri-sodium citrate it was red in colour (Fig. 2B). The surface plasmon absorption maximum for blue/purple gold particles was observed at 548 nm (Fig. 2C) whereas for red gold particles it was at 521 nm (Fig. 2B). The shifting of surface plasmon absorption maximum is due to the difference in particle size. As we know from the literature that the gold nanoparticles change colour from red to blue with increasing particle size and red shift is observed in surface plasmon absorption maxima. The

Fig. 2. UV–vis spectra of silver and gold nanoparticles. (A) Ag particles prepared using hydrazine hydrate as a reducing agent. (B and C) Au particles prepared using tri-sodium citrate and hydrazine hydrate as reducing agents, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

formation of metallic nanoparticles in polymer solution depends on the concentration of solute and adherness of the polymeric monolayer on the growing surface. At low metal ion concentration diffusion becomes rate-limiting step for subsequent growth of the initiating nuclei. Both low concentration and polymeric monolayer hinder the diffusion of growing species from the surrounding solution to the growing surface. This results in the formation of uniformly sized nanoparticles. Hence weak reducing agent or slow reduction kinetics favours more uniformly distributed and smaller particles. In case of tri-sodium citrate red gold solution (Fig. 3) was obtained after 10 min microwave irradiation where as on use of hydrazine hydrate the pale yellow colour Au[III] solution turns in to blue in no time. The kinetics of reduction is very fast for hydrazine and that is why in polyacrylamide solution blue/purple colour gold nanoparticles were

Fig. 1. TEM image of silver nanoparticles (a) and gold nanoparticles (b) prepared using hydrazine as a reducing agent.

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Fig. 3. Photograph of red Au nanoparticles prepared using tri-sodium citrate as a reducing agent 15 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

obtained through hydrazine reduction and red coloured smaller size particles were obtained by using sodium citrate. Fig. 4 represents the histogram of the red coloured gold nanoparticles, with effective diameter of 15 nm.

Fig. 4. Particle size distributions of gold nanoparticles prepared through sodium citrate reduction.

The synthesis and growth of the Au–Ag alloy nanoparticles in polyacrylamide solution is monitored by UV–vis spectroscopy. The resulting nanoparticles dispersion exhibited different colours based on the composition, indicating the formation of Au–Ag alloy nanoparticles. The dispersions containing

Fig. 5. Photographs of the Ag, Au and Au–Ag alloy nanoparticles with various compositions. (a) After 1 min microwave irradiation. (b) After 10 days of storage compositions 1: Ag; 2–5: Au–Ag; 6: Au.

Fig. 6. (a) UV–vis absorption spectra of Ag, Au and Au–Ag alloy nanoparticles with varying mole ratios. All the spectra have been normalized at the plasmon absorption maxima. The Au/Ag mole ratios: a (0:1); b (0.16:0.83); c (0.33:0.66); d (0.5:0.5); e (0.66:0.33); f (0.83:0.16); g (1:0). (b) Plot of the plasmon absorption maximum against the Au mole ratio for the various alloy composition (a–g, respectively).

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Fig. 7. Particle size distributions of metallic and bimetallic nanoparticles. The Au/Ag mole ratios: (a) 0:1; (b) 0.16:0.83; (c) 0.33:0.66; (d) 0.5:0.5; (e) 0.66:0.33; (f) 0.83:0.16; (g) 1:0.

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Fig. 8. Possible mechanism for formation of gold nanoparticles in polyacrylamide solution.

only silver or gold particles exhibited yellow and purple colour, respectively. The compositions containing various ratios of silver and gold solutions resulted in exhibiting colours between yellow and purple as can be seen from the digital photographs in Fig. 5. It is well known that the surface plasmon bands are the characteristic for the metal and alloy nanoparticles. The core shell nanoparticles give rise to two surface plasmon absorption bands and the individual band intensities depend on the initial composition of the metal ions [22]. If the particles are not homogeneous alloy particles then the same situation will arise from a dispersion containing separate gold and silver nanoparticles. From the Fig. 6a, it is seen that only a single absorption band is obtained for each composition with the absorption maxima varying between that for gold and silver nanoparticles dispersions. This observation supports the formation of homogeneous alloy particles. The plot of the plasmon absorption maximum against the Au mole ratio for the various alloy composition is given in Fig. 6b. A linear relationship is observed between surface plasmon absorption maxima value and the Au mole ratio in various alloy compositions. The synthesized particles are not a physical mixture of silver and gold nanoparticles. If the synthesized silver and gold particles are mixed physically, it gives two surface plasmon absorption bands corresponding to the individual silver and gold particles. The Fig. 7 represents the particle size distribution of silver gold, and Au–Ag alloy nanoparticles. All the represented histograms show that the particles obtained through these methods are in the range of 5–50 nm. The effective diameter of the particle of the system

presented in histograms corresponds to the maximum scattering intensity 100 in Fig. 7. The possible mechanism for the formation of gold nanoparticles in polyacrylamide solution is given in Fig. 8. 4. Conclusions Microwave-mediated synthesis provides an effective environment for the preparation of stable aqueous dispersions of Ag, Au and Au–Ag alloy nanoparticles, which can be achieved by reducing the corresponding metal ions using water soluble polymer, polyacrylamide as a stabilizing agent and hydrazine as a reducing agent. It is also shown that stable red gold nanoparticles were obtained through this method by using tri-sodium citrate as a reducing agent. The metal and alloy nanoparticles prepared through this method are highly stable and do not show any sign of agglomeration even after storage for months. Microwave rapid heating process can be a good alternative approach for the preparation of metallic and bimetallic alloy nanoparticles in aqueous polymer solution. Polyacrylamide being a biodegradable polymer the proposed method can be readily integrated for biological applications. Acknowledgement The authors are thankful to GUJCOST (Gadhinagar, Gujarat) for the financial support.

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