Alloy Formation of Gold-Silver Nanoparticles and the Dependence of the Plasmon Absorption on Their Composition S. Link, † Z. L. Wang, ‡ and M. A. El-Sayed* ,† Laser Dynamics Laboratory, School of Chemistry and Biochemistry, and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: February 4, 1999; In Final Form: March 12, 1999 Gold-silver alloy nanoparticles with varying mole fractions are prepared in aqueous solution by the co- reduction of chlorauric acid HAuCl 4 and silver nitrate AgNO 3 with sodium citrate. As the optical absorption spectra of their solutions show only one plasmon absorption it is concluded that mixing of gold and silver leads to a homogeneous formation of alloy nanoparticles. The maximum of the plasmon band blue-shifts linearly with increasing silver content. This fact cannot be explained by a simple linear combination of the dielectric constants of gold and silver within the Mie theory. On the other hand, the extinction coefficient is found to decrease exponentially rather than linearly with increasing gold mole fraction x Au . Furthermore, the size distribution of the alloy nanoparticles is examined using transmission electron microscopy (TEM). High- resolution TEM (HRTEM) also confirms the formation of homogeneous gold-silver alloy nanocrystals. Introduction The intense research in the field of nanoparticles by chemists, physicists, and materials scientists is motivated by the search for new materials in order to further miniaturize electronic devices as well as by the fundamental question of how molecular electronic properties evolve with increasing size in this inter- mediate region between molecular and solid-state physics. 1-7 Possible future applications include the areas of ultrafast data communication and optical data storage. 4-6 Semiconductor nanoparticles are also used in building solar cells 7 and metal nanoparticles are very important as catalysts because of their high surface-to-volume ratios. 4 Metal nanoparticles have mainly been studied because of their unique optical properties as especially nanoparticles of the alkali metals and the noble metals copper, silver, and gold have a broad absorption band in the visible region of the electromag- netic spectrum. 8-15 Solutions of these metal nanoparticles show a very intense color, which is absent for the bulk material as well as for the atoms. Their origin is attributed to the collective oscillation of the free conduction electrons induced by an interacting electromagnetic field. These resonances are also denoted as surface plasmons. Mie 16 was the first to explain this phenomenon by applying classical electrodynamics to spherical particles and solving Maxwell’s equations for the appropriate boundary conditions. The total extinction cross section composed of absorption and scattering is given as a summation over all electric and magnetic multipole oscillations. The Mie theory has the advantage of being conceptually simple and has found wide applicability in explaining experimental results. 8,9,14,15 However, all of the material properties are represented by a complex dielectric function of the absorbing metal nanoparticles thus obscuring somehow the underlying microscopic events, such as the possible decay mechanisms of the coherent motion of the free electrons. Gold and silver nanoparticles have been studied in great detail. 1,2,8,15,17 Especially, the size effect on the plasmon absorption in connection with the Mie theory and its modifica- tions has been of major interest. 8,17 Alloy nanoparticles, on the other hand, have mainly been studied because of their catalytic effects. 18,19 As gold has low catalytic activity compared to platinum or palladium, the structural and catalytic changes have been examined for the admixture of platinum or palladium to gold. 20-24 Studies on the Au-Ag system include the following. Pa- pavassiliou 25 prepared 10 nm gold-silver alloy nanoparticles in 2-butanol by evaporation and condensation of the alloys. A linear dependence of the plasmon absorption maximum on the composition of the nanoparticles was found. Alloy clusters with diameters between 9 and 15 nm were obtained in photosensitive glasses. 8,26 The disadvantage of this preparation method is, however, the fact that an increasing silver content also results in larger particles. Liz-Marzan et al. 27 used inorganic fibers in aqueous solution for the stabilization of gold-silver particles with diameters of 2 to 3 nm after the simultaneous reduction of gold and silver salts by sodium borohydride. Teo et al. 28 synthesized a 38 atom gold-silver cluster (Au 18 Ag 20 ). The solution of this cluster showed a single absorption peak at 495 nm. Mulvaney et al. 15,29 and also Sinzig et al. 8,26 prepared silver nanoparticles coated with an overlayer of gold (core-shell nanoparticles). These particles have two distinct plasmon absorption bands and their relative intensities depend on the thickness of the shell. But also alloy formation within the shell was suggested on the basis of the optical absorption spectra. Similarly, gold-silver composite colloids (30-150 nm in diameter) consisting of mixtures of gold and silver domains were obtained by irradiating aqueous solutions of gold and silver ions with 253.7 nm UV light. 30 Those nanoparticles also showed two plasmon absorption bands originating from the individual gold and silver domains. It is shown in this letter how gold-silver alloy nanoparticles can be obtained by simple co-reduction of chlorauric acid HAuCl 4 and silver nitrate AgNO 3 with sodium citrate in aqueous * Corresponding author. † Laser Dynamics Laboratory, School of Chemistry and Biochemistry. ‡ School of Materials Science and Engineering. 3529 J. Phys. Chem. B 1999, 103, 3529-3533 10.1021/jp990387w CCC: $18.00 © 1999 American Chemical Society Published on Web 04/16/1999