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Synthesis of Cu3Au Nanocluster Alloy in Reverse Micelles Claudio Sangregorio, Monica Galeotti, Ugo Bardi, and Piero Baglioni* Department of Chemistry, University of Florence, Via G. Capponi 9, 50121 Florence, Italy Received May 13, 1996. In Final Form: August 12, 1996X Gold and gold-copper particles of sizes of the order of nanometers were prepared in reverse micelles, with different sizes of the water pool, by controlled chemical reduction of metal cations. The nanoclusters were characterized by a combination of dynamic light scattering (QELS), transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). The results indicate that it is possible to obtain a uniform or nearly uniform particle size distribution and that compound metallic nanoparticles of Cu3Au can be prepared by this method. At present we have no evidence on the mechanism that lead to mixing at the atomic level; nevertheless, it appears possible to exploit the method in order to prepare mixed bimetallic phases of various composition.
Introduction Reverse micelles are aggregates formed by dissolving amphiphilic molecules in organic solvents.1 Water is solubilized in the polar core of these aggregates forming a so-called “water pool” characterized by the watersurfactant molar ratio w ) [water]/[surfactant]. The water pool radius can be varied, changing w. The shape of these aggregates depends on several factors and generally changes from spherical to cylindrical to more complex structures according to the nature of the surfactant counterion (for ionic surfactants), the surfactant, and the water concentration. Monodisperse or nearly monodisperse particles of size on the order of a few nanometers can be prepared in the water pool of reverse micelles.1-5 In principle, by varying the dimensions of the water pool and the composition of the water phase, a wide variety of compounds of controlled size, including pure metals, oxides, borides, and other compounds,6-14 can be prepared by this method. Regarding transition metals, however, so far alloy formation could be evidenced only for particles of relatively large size.15 In the present work, we have addressed the question of whether alloy phases can be synthesized in the form of nanoparticles (i.e. particles of radius in the range 1-10 nm) in reverse micelles. As a test system we used the Au-Cu system, where it is well-known that intermetallic phases of various composition exist. Among these, the most stable is the Cu3Au phase, that crystallizes with the * To whom correspondence should be addressed. E-mail: Colloid@ sirio.cineca.it. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (2) Barnickel, P.; Wokaun, A.; Sager, W.; Eicke, H. F. J. Colloid Interface Sci. 1992, 148, 80. (3) Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3757. (4) Steigerwald, M. L.; Brus, L. E. Annu. Rev. Mater. Sci. 1989, 19, 471. (5) Nagy, J. B. Colloids Surf. 1989, 35, 201. (6) Marigner, J. L.; Belloni, J. J. Chim. Phys. 1988, 85, 21. (7) Bowen Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98, 4109. (8) Dye, J. L.; Tsai, K.-L. J. Am. Chem. Soc. 1991, 113, 1650; J. Chem. Soc., Faraday Discuss. 1991, 92, 42. (9) Wiesner, J.; Wokaun, A. Chem. Phys. Lett. 1989, 157, 569. (10) Borgarello, E.; Lawless, D.; Serpone, N.; Pelizzetti, E.; Meisel, D. J. Phys. Chem. 1990, 94, 5048. (11) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. Chem. Phys. 1993, 98, 9933. (12) Sager, W.; Eicke, H.-F.; Sun, W. Colloids Surf. A 1993, 79, 199. (13) Schlag, S.; Eicke, H.-F.; Mathys, D.; Guggenheim, R. Langmuir 1994, 10, 3357. (14) Schlag, S.; Eicke, H.-F. Solid State Commun. 1994, 91, 883. (15) Lopez-Quintela, M. A.; Rivas, A. J. J. Colloid Interface Sci. 1993, 158, 446.
S0743-7463(96)00462-3 CCC: $12.00
Figure 1. Histograms for the particle diameters evaluated from TEM (left side) and by LS (right side) for Au metallic nanoparticles prepared in reverse micelles with different values of w.
ordered fcc L12 structure. We have attempted to prepare this phase by chemical reduction of the cations in a water in oil microemulsion, to obtain uniformly dispersed particles of sizes of the order of a few nanometers. X-ray photoelectron spectroscopy data indicate that the Cu3Au intermetallic phase is formed. Experimental Section For the synthesis of gold and gold-copper particles, we used sodium bis(2-ethylhexyl)sulfosuccinate (AOT) or the Cu2+, Au3+ sodium-exchanged surfactant, in a water/isooctane system. Gold and copper ions were also introduced as aqueous solutions of HAuCl4 and CuCl2/HAuCl4. Reduction of the metal salts was obtained by mixing the solutions with an equal volume of water/ isooctane microemulsion containing hydrazine as reducing agent. The w reported in this paper refers to the amount of water added to the M bis(2-ethylhexyl)sulfosuccinate surfactant (M ) Na+, Cu2+, Au3+). It is worth recalling that the surfactant polar headgroups may have associated some water molecules that crystallize with the surfactants, usually from 2 to 4 depending on the counterions.3 Therefore the “true” w value is obtained by taking into account this additional amount of water that participates in the microemulsion system. TEM measurements were performed using a conventional transmission electron microscope with a nominal resolution of 3 Å. The average diameter and the relative standard deviation have been calculated for each sample from the experimental distribution. The ratio is taken as a measure of dispersion (polydispersity). The particle size distribution in solution has also been obtained by dynamic
© 1996 American Chemical Society
Cu3Au Nanocluster Alloy in Reverse Micelles
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Figure 2. Transmission electron micrographs of Au nanoparticles obtained from AOT reverse micelles with w ) 5 and 10. Table 1. Average Diameter and Polydispersity Obtained from TEM (〈D〉 (Å)) and Light Scattering (〈Dh〉 (Å)) Measurements and the Standard Deviation σ (TEM) w
〈D〉 (Å)
1 2.5 5 10 15
