Preparation of silver, gold and silver–gold bimetallic nanoparticles in w/o microemulsion containing TritonX-100

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Abstract

Silver, gold and silver–gold alloy nanoparticles were prepared in w/o microemulsion containing TritonX-100 and cyclohexane. Silver nitrate and tetrachloroauric(III) acid (HAuCl4) were taken as the metal precursors and sodium borohydride (NaBH4) was used as a reducing agent. The formation of Ag and Au nanoparticles was confirmed from the appearance of surface plasmon absorption maxima at 404 ± 2 and 525 ± 2 nm for Ag and Au nanoparticles, respectively. For bimetallic nanoparticles absorption peak was observed between the two maximas of corresponding metallic particles. The surface plasmon absorption maxima for bimetallic nanoparticles changes linearly with increasing Au mole ratio content in various alloy compositions. The transmission electron microscopy (TEM) showed formation of particles of 5–50 nm diameter. Dynamic light scattering (DLS) data gave narrow particle size distribution.

Introduction

Metal nanoparticles, ranging from colloidal sols to organometallic clusters, have been widely investigated since the pioneering studies of colloidal gold by Faraday in 1857 [1]. Increased interest in metallic nanoparticles is due to the variation in their optical, magnetic and electrical properties, which are dependent on their size, surface plasmon, surface free energy and surface area. As a result, the size-dependent properties of metal nanoparticles have recently been exploited in technological applications varying from electronics [2], [3], optics [4], sensing [5], [6], [7] to catalysis [8]. In addition, nanoparticles, and more particularly bimetallic alloy nanoparticles, are very important for their catalytic properties [9], [10] and unique electronic and optical properties. Metals like Au and Ag have almost identical lattice constants (0.408 for Au and 0.409 for Ag) which are responsible for a strong tendency towards alloy formation. The bimetallic particles will be in core–shell or alloy form, depending on the preparation conditions, miscibility and kinetics of reduction of metal ions. Bimetallic particles like Au–Pd [11], [12] and Au–Pt [13], [14] are reported to exhibit a core–shell type of structure, while Au–Ag are reported to form homogeneous alloy when reduced simultaneously [15]. Many methods have been reported for the preparation of Au–Ag alloy nanoparticles such as reduction of supported metal salts using NaBH4 [16], citrate [17], hydrazine [18] and laser ablation [19]. Preparation of metallic nanoparticles using microemulsion has also been well reported where ionic and nonionic surfactants are used [20]. But preparation of bimetallic alloy nanoparticles in microemulsion is not that well reported. Microemulsions are colloidal “nano-dispersions” of water in oil or oil in water, stabilized by a surfactant film. In water in oil microemulsion the size of the water droplet can be tuned by changing only one parameter such as water-to-surfactant ratio (W0). The size of nanoparticles is controlled by the size of the droplet of the microemulsion. Here in this paper we report a reverse micelle method using nonionic surfactant TritonX-100 for the preparation of Ag, Au and Au–Ag alloy nanoparticles. The advantage of this system is we are able to synthesize the alloy nanoparticles at room temperature (25 °C) and removal of particles from surfactant solution is relatively less tedious. To our knowledge there are no reports for the preparation of Ag and Au–Ag alloy nanoparticles by using this reverse micelle system. Hence a systematic study of synthesis and characterization of Ag, Au and Au–Ag nanoparticles is undertaken.

Section snippets

Materials

Silver nitrate (AgNO3) was purchased from Merck, Mumbai, India and tetrachloroauric(III) acid (HAuCl4; MW 339.79) was purchased from Sigma–Aldrich, Steinheim, Germany. Sodium borohydride, cyclohexane and 1-hexanol were obtained from S.D. Fine-Chemicals, Mumbai, India. TritonX-100 was purchased from Sisco Research Laboratories Pvt. Ltd. (SRL), Mumbai, India. All these chemicals were used without further purification. Double distilled deionised water was used throughout the work.

Experimental procedure

The metallic and

Results and discussion

Silver, gold metallic and bimetallic nanoparticles were prepared by reducing metal compounds in reverse micelle medium using sodium borohydride reducing agent. Water-to-surfactant ratio (W0) in microemulsion was varied from 1 to 7. With increase in W0, concentration of particles formed increases but stability decreases. At W0 = 5 and 7 particles were stable only for 1–2 h and were precipitated out thereafter. At W0 < 3 particles were stable but their concentration was very low. Hence, further study

Conclusion

Silver, gold and silver–gold alloy nanoparticles synthesized in w/o microemulsion containing TritonX-100, cyclohexane and water were observed to be stable for more than 6 months. Appearance of surface plasmon absorption maxima at 404 ± 2 and 525 ± 2 nm, respectively, for Ag and Au confirmed formation of Ag and Au nanoparticles. Appearance of single absorption maxima proved the alloy nature of bimetallic particles. With increase in metal ion concentration no distinguishable change was observed in

Acknowledgement

The authors are thankful to GUJCOST (Gandhinagar, Gujarat) for the financial support.

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