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2008, 112, 15102–15105 Published on Web 09/09/2008
Sonochemical Synthesis of Au-Ag Core-Shell Bimetallic Nanoparticles Sambandam Anandan,‡ Franz Grieser,† and Muthupandian Ashokkumar*,† School of Chemistry, UniVersity of Melbourne, Vic 3010, Australia, and Nanomaterials & Solar Energy ConVersion Laboratory, Department of Chemistry, National Institute of Technology, Trichy 620 015, India ReceiVed: August 4, 2008; ReVised Manuscript ReceiVed: August 27, 2008
Au-Ag bimetallic nanoparticles were prepared by the sonochemical co-reduction of Au(III) and Ag(I) ions in aqueous solutions. TEM images indicate that the co-reduction of Au(III) and Ag(I) produces bimetallic nanoparticles with a Au core-Ag shell morphology. It is suggested that the formation of core-shell morphology is due to the reduction order of the metal ions and the involvement of a polymer-Ag ion complex. Introduction In recent years, intensive research has been undertaken in the field of metal nanoparticles because of their unique optical properties and their applications in electronic devices, ultrafast data communication, optical data storage, and so forth. Perfectly monodispersed metal nanoparticles are, of course, ideal, but special properties are to be expected even if the “ideal” properties cannot be achieved. The mass production of these nanoparticles with enough uniformity is also important for industrial-scale applications, and an understanding of how to control their particle size is crucial. A number of methods have been used to prepare nanoparticles from metal ion precursors, including alcohol reduction,1,2 citrate reduction,3,4 polyol reduction,5 borohydride reduction,6 sonochemically,7,8 photolytic reduction,9,10 radiolytic reduction,11,12 laser ablation,13,14 and metal evaporation-condensation.15 The preparation conditions of the nanoparticles have a direct effect on the size, structure, and composition and, as a consequence, on the optical and electrochemical properties of these particles. The sonochemical synthetic method has been found to generate nanomaterials of a much smaller size range and higher surface area than those reported by other methods.16 Further, the focus on core-shell bimetallic nanoparticles is of great interest from both fundamental scientific and technological points of view because of the fact that one of the metals (the shell) determines the surface properties of the particles while the other (the core) may be responsible for specific functional (optical, catalytic, magnetic, etc.) properties of the system. Successive reduction of two different metal ions can be considered as one of the most suitable methods for the preparation of core-shell structured bimetallic particles compared to co-reduction/simultaneous reduction of the precursor ions. However, Harada et al.17 produced Pd core-Au shell nanoparticles by the co-reduction method instead of through a successive reduction path. The difference in the structure was argued to be due to the difference in the reduction potentials of PdII and AuIII ions. When AuIII ions were added in the presence of Pd nanoparticles, some Pd0 atoms of the nanoparticles were oxidized and reduced AuIII ions; the * To whom correspondence should be addressed: E-mail: masho@ unimelb.edu.au. Tel.: +61-3-83447090. Fax: +61-3-93475180. † University of Melbourne. ‡ National Institute of Technology.
10.1021/jp806960r CCC: $40.75
Figure 1. Absorption spectra of Au, Ag and Au-Ag core-shell bimetallic nanoparticles.
oxidized Pd ions were reduced again by the reductants, such as alcohols. This process led to the formation of particles with a core-shell structure in the co-reduction process. As Belloni et al.18 have described in their excellent review, the kinetics of competitive reduction in mixed systems has a dominant control on the final form of the mixed metal particles. Similarly, several preparation methods are also available for bimetallic nanoparticles, and for the particular case of Au-Ag nanoparticles, a number of proposals have been put forward to explain the nanoparticle formation processes.19-23 Sato et al.10 have prepared bilayered Ag-Au composites, with the core being composed of gold and with a silverrich shell layer, by a photochemical method. Papavassiliou15 prepared gold-silver alloy nanoparticles by evaporation and condensation of the alloys. Silver particles having a gold layer were prepared by Henglein using γ-radiolysis.24 In this article, we report the synthesis of gold-silver bimetallic nanoparticles with core-shell morphology by the sonochemical co-reduction of AuIII and AgI ions using 20 kHz ultrasound in aqueous solutions. Whereas similar observations were reported by Mizukoshi et al.,7 the current study is different in that the role of the stabilizing polymer in producing the core-shell morphology during the sonochemical reduction process is considered in some detail. Experimental Details All chemicals were of the highest purity available and were used as received without further purification. Chloroauric acid 2008 American Chemical Society
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J. Phys. Chem. C, Vol. 112, No. 39, 2008 15103
Figure 2. TEM images of sonochemically co-reduced Au-Ag core-shell bimetallic nanoparticles (a and b). FFT pattern (c) of Au-Ag core-shell bimetallic nanoparticles. The SAED spectrum (d) confirmed the presence of gold and silver in the nanoparticle. HRTEM image (e) of the sonochemically co-reduced Au-Ag core-shell bimetallic nanoparticles.
trihydrate (HAuCl4 · 3H2O), silver nitrate (AgNO3), polyethylene glycol (PEG, MW ) 29,000 g/mol), and ethylene glycol were purchased from Aldrich. Using a 20 kHz (23-47 W cm-3) horn
sonifier (Branson), gold-silver bimetallic nanoparticles were made by the co-reduction procedure as follows. An aqueous solution (70 mL) of HAuCl4 · 3H2O (0.2 mM) and AgNO3 (0.2
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mM) containing polyethylene glycol (0.1 wt %)25 and ethylene glycol (0.1 M) was sonicated at room temperature under an argon atmosphere for 30 min. The pH of the solutions was ∼3.5. Under similar experimental conditions, PEG-stabilized pure monometallic gold and silver nanoparticles were prepared separately to compare the rate of formation of nanoparticles. The surface morphology and particle size of the sonochemically prepared gold-silver bimetallic nanoparticles with core-shell geometry were analyzed by transmission electron microscopy (TECNAI G2 model). Energy-dispersive X-ray analysis was used to determine the elemental composition of the bimetallic nanoparticles. Results and Discussion The reduction of both AuCl4- ions to metallic gold and Ag+ ions to metallic silver was accomplished at room temperature by ultrasonic irradiation in the presence of alcohols.26-28 The mechanism of gold chloride reduction and silver nitrate reduction by the primary and secondary radicals generated during acoustic cavitation has been dealt with in detail elsewhere.26-29 In short, hydrogen atoms and alcohol radicals reduce gold and silver ions to generate gold-silver bimetallic nanoparticles. In the presence of a polymer as the stabilizer, it is also possible that polymeric radicals, produced by the reaction of primary radicals with the polymer, reduce the metal ions to generate metal particles. The polymeric radicals could also be generated by thermal decomposition at the bubble/solution interface. The absorption spectra of Au-Ag bimetallic nanoparticles and Au and Ag monometallic particles are shown in Figure 1. The solution containing the Au-Ag bimetallic nanoparticles was dark purple in color, and the absorption spectrum showed a band at λmax ) 536 nm. The solution containing only gold nanoparticles was purple in color, and the absorption spectrum showed a band at λmax ) 530 nm. The solution containing only silver nanoparticles was yellow in color, and the absorption spectrum showed a band at λmax ) 437 nm. A significant change in the absorption spectrum is observed for Au-Ag bimetallic particles compared to the spectra observed for the individual metal nanoparticles. Figure 2 shows the transmission electron microscopy (TEM) micrographs and size distribution of Au-Ag bimetallic nanoparticles prepared by sonochemical irradiation. The average diameter of the bimetallic clusters prepared by the simultaneous reduction is about 20 nm. This is slightly larger than the individual metal nanoparticles (∼15 nm in size; see Supporting Information). This increase in size may be due to the formation of Au-Ag bimetallic nanoparticles with core-shell geometry. The FFT pattern (Figure 2c) of the Au-Ag core-shell confirms the plane (110) of silver and the plane (111) of gold nanoparticles. Indeed, most of the prepared Au-Ag bimetallic nanoparticles with a core-shell geometry showed interlayer spacing and an electron diffraction (ED) pattern that evidenced the crystalline nature of the particles. Figure 2d shows the selected area electron diffraction (SAED) measured for the Au-Ag bimetallic nanoparticles shown in Figure 2a. The SAED results related to the Au-Ag bimetallic nanoparticles indicate the presence of both Au and Ag nanoparticles. These results provide support for the formation of core-shell nanoparticles, gold core with silver shell, upon simultaneous sonochemical irradiation of gold and silver ions. In addition, the HRTEM image (Figure 2e) indicates the crystalline nature of the bimetallic particles. The contrast seen at the outer surface of the particles suggests that they are gold particles covered with thin silver layers. In order to understand the core-shell structure formation process, the growth rates of individual metal nanoparticles under
Figure 3. The absorbance versus time plot indicates that the rate of Au nanoparticle formation is significantly greater than the rate of Ag nanoparticle formation.
similar sonochemical irradiation conditions were monitored. The growth rate of Au could be followed by monitoring the absorption changes at 530 (Au plasmon absorption band) and 265 nm (AuCl4- absorption band). The rate of reduction of Ag+ was monitored at 303 nm. A comparison of the rate of formation of individual metal clusters indicates (Figure 3) that the rate of formation of gold nanoparticles is higher compared to that of silver nanoparticles. In addition, TEM images (see Supporting Information) indicate that the number of silver nanoparticles formed was much less than that of the gold nanoparticles under similar experimental conditions. What these results suggest is that the formation of core-shell morphology is most likely due to the difference in the reduction rates of the individual metal ions. Since gold ions are easily reduced under the sonochemical conditions, it may be suggested that gold particles are first generated followed by the reduction of Ag+ ions on the surface of the gold particles. Vinodgopal et al.8 reported similar observations during the synthesis of Pt-Ru core-shell nanoparticles. However, in their study, they used a sequential reduction method, that is, first the sonochemical reduction of Pt was performed prior to the addition of Ru ions. In order to further support the above discussion, the amount of AuCl4reduced was calculated in a separate experiment. We found that about 90% of AuCl4- was reduced in about 20 min of sonication, which is consistent with the data shown in Figure 3. We are currently in the process of quantifying the amount of unreacted Ag+ ions in the presence of Au-Ag particles. This will allow us to calculate the thickness of the Ag coating on Au particles. There is also another mechanism that could be responsible for the formation of core-shell morphology. Our experimental investigation (data not shown) shows that the rate of reduction of silver ions in polyvinylpyrrolidone (PVP) is negligible compared to that in PEG. This indicates that the nature of the stabilizing polymer may play some role in the reduction process. Toshima and Yonezawa2 reported the role of stabilizing polymers in detail during the formation of bimetallic nanoparticles. They have suggested that the stabilizing polymers can coordinate to metal ions before the reduction. This interaction between the polymer and the metal ions provides a different growth pathway leading to the formation of smaller size core-shell nanoparticles with a narrow size distribution. The core-shell morphology observed in our study could also be due to such an interaction between the metal ions and the polymer used. In summary, a simple sonochemical co-reduction method has been developed for synthesizing Au-Ag core-shell nanopar-
Letters ticles. This methodology could be used for synthesizing a number of bimetallic systems involving other metals with core-shell morphology. Acknowledgment. The research described herein was supported by the Department of Innovation, Industry, Science and Research, Australia, the Australian Research Council funded Particulate Fluids Processing Centre and the Department of Science and Technology, India. S.A. thanks the University of Melbourne for the Visiting Academic appointment during May-July 2008. Supporting Information Available: The TEM images of individually prepared Au and Ag particles indicate that the concentration of the Au particles is higher than that of the Ag particles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (2) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (3) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (4) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. B 1996, 100, 718. (5) Silvert, P. Y.; Vijayakrishnan, V.; Vibert, P.; Herrara-Urbina, R.; Elhsissen, K. T. Nanostruct. Mater. 1996, 7, 611. (6) Liz-Marzan, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120. (7) (a) Mizukoshi, Y.; Okitsu, K.; Maeda, Y.; Yamamoto, T. A.; Oshima, R.; Nagata, Y. J. Phys. Chem. B 1997, 101, 7033. (b) Mizukoshi, Y.; Takagi, E.; Okuno, H.; Maeda, Y.; Nagata, Y. Ultrason. Sonochem. 2001, 8, 1. (8) Vinodgopal, K.; He, Y.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2006, 110, 3849.
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15105 (9) Remita, H.; Mostafavi, M.; Delcourt, M. O. Radiat. Phys. Chem. 1996, 47, 275. (10) Sato, T.; Kuroda, S.; Takami, A.; Yonezawa, Y.; Hada, H. Appl. Organomet. Chem. 1991, 5, 261. (11) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (12) Treguer, M.; de Cointet, C.; Remita, H.; Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J.; de Keyzer, R. J. Phys. Chem. B 1998, 102, 4310. (13) Hodak, J. H.; Henglein, A.; Giersig, M.; Hartland, G. V. J. Phys. Chem. B 2000, 104, 11708. (14) Chen, Y. H.; Yeh, C. S. Chem. Commun. 2000, 371. (15) Papavassiliou, G. C. J. Phys. F: Met. Phys. 1976, 6, L103. (16) Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: Weinheim, Germany, 1988. (17) Harada, M.; Asakura, K.; Toshima, N. J. Phys. Chem. 1993, 97, 5103. (18) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J. L.; Delcourt, M. O. New J. Chem. 1998, 1239. (19) Liu, M. Z.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 5882. (20) Raveendran, P.; Fu, J.; Wallen, S. L. Green Chem. 2006, 8, 34. (21) Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir. 2000, 16, 9722. (22) Bright, R. M.; Walter, D. G.; Musick, M. D.; Jackson, M. A.; Allison, K. J.; Natan, M. J. Langmuir. 1996, 12, 810. (23) Lu, L.; Wang, H.; Zhou, Y.; Xi, S.; Zhang, H.; Hu, J.; Zhao, B. Chem. Commun. 2002, 144. (24) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (25) The main reason for choosing polyethylene glycol (PEG) as the stabilizer here instead of polyvinylpyrrolidone (PVP) is that the rate of formation of pure silver nanoparticles is very high in PEG compared to that in PVP. Further, no silver nanoparticle formation could be observed after 30 min sonication in the presence of PVP as the stabilizer. (26) Yeung, S. A.; Hobson, R.; Biggs, S.; Grieser, F. J. Chem. Soc., Chem. Commun. 1993, 378. (27) Nagata, Y.; Watananabe, Y.; Fujita, S.; Dohmaru, T.; Taniguchi, S. J. Chem. Soc., Chem. Commun. 1992, 1620. (28) Salkar, R. A.; Jeevanandam, P.; Aruna, S. T.; Koltypin, Y.; Gedanken, A. J. Mater. Chem. 1999, 9, 1333. (29) Caruso, R. A.; Ashokkumar, M.; Grieser, F. Langmuir 2002, 18, 7831.
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