We report an approach for extracting the optical constants of bimetallic Ag–Au nanoparticles from the measured surface-plasmon (SP) extinction spectra. The dielectric function of the metal is expressed as an analytic function of the wavelength in which the interband (and all other non-Drude) contributions to the dielectric function are represented by a sum of Lorentz functions. This expression is then used to fit the experimental extinction spectra to appropriate functions based on Mie theory. Three Lorentz functions (plus a Drude term) were found to be sufficient to reproduce the dielectric functions of Ag and Au [P. B. Johnson and R. W. Christy, Phys. Rev. B 6, 4370 (1972)] over the entire 0.6–6.5 eV range reported. With functions of this type, an excellent multiparameter fit of the measured extinction spectrum of colloidal Ag was obtained. Extinction spectra of a series of (Ag)Au hydrosols, prepared by reducing a gold precursor in the presence of previously synthesized silver seed particles with Au mole fractions ranging from 0.1 to 0.8, were measured. The extinction spectra show a single band (attributed to the surface plasmon) for all of the colloids produced, suggesting alloy formation. Transmission electron microscopy (TEM) images, however, indicate clear core–shell contrast for nanoparticles with Au mole fractions 0.4 and higher. With a presumed particle structure consisting of Ag core and Ag/Au alloy shell, very good fits were obtained for all of the measured extinction spectra by using a fitting strategy that restricted the number of parameters allowed to vary freely in the aforementioned dielectric function. The values of the dielectric function of the presumed shells were extracted in this manner as a function of wavelength. For particles with Au mole fraction 0.1–0.3, the results suggest an incompletely formed shell. For particles with higher Au mole fractions, the dielectric function of the shell gradually approaches that of Au. Overall, the results are consistent with a particle structure that consists of a Ag-rich core and Ag/Au alloy shell whose composition is gradually enriched in Au with increasing overall Au mole fraction. The dielectric function of the alloy shells cannot be written merely as a weighted average of the corresponding dielectric functions of Ag and Au. An almost linear decrease of the electron scattering rate in the particle’s shell with the increasing Au mole fraction was determined through this fitting analysis. This was ascribed both to the confinement of the electrons within the shell (which increases in thickness with increasing Au mole fraction) and the gradual change of the composition, and, consequently, of the electrical conductivity of the shell.

Pyrazinamide (pza) and 2-mercaptopyridine (2-MPy) are shown to form Ag colloid-adsorbate films at the interface between an aqueous Ag colloid and a solution of an adsorbate in dichloromethane. By contrast, under the same conditions, 4-mercaptopyridine (4-MPy) forms Ag organosol aggregates by the interface transfer of adsorbate-covered Ag colloidal particles from the aqueous phase to the organic phase. This result confirms that formation of Ag colloid-adsorbate films is an adsorbate specific process. Transmission electron and optical microscopy and surface plasmon adsorption measurements are used to characterize the morphology of the deposited Ag colloid-adsorbate structures while SERS spectra of the incorporated adsorbates probe their internal structure. Furthermore, we demonstrate that, for a particular adsorbate, the process of Ag colloid-adsorbate film formation, the morphology of the deposited films, and the SERS spectra are substantially influenced by the Ag content of the parent colloid (Ag-pza and Ag-4-MPy films) and/or by the concentration of the adsorbate in the organic phase (Ag-2-MPy films). We propose that the observed differences in the rates of formation of the films, their morphology, and the orientation of the adsorbate with respect to the Ag surface are related to differences in the actual surface coverage of Ag colloidal particles by adsorbate molecules.

Layered core−shell bimetallic silver−gold nanoparticles were prepared by overdeposition of Au over Ag seeds by the seed-growth method using tetrachloroauric acid, with hydroxylamine hydrochloride as the reductant. The effects of pH, reduction rate, and seeding conditions on the morphology and surface plasmon extinction of the bimetallic nanoparticles were investigated. Nanoparticles prepared by a rapid reduction in the neutral ambient and assembled into two-dimensional nanoparticulate films by adsorption of 2,2‘-bipyridine were characterized by energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, surface-enhanced Raman scattering spectroscopy, and transmission electron microscopy. The results are consistent with Ag core and Ag/Au-alloyed shell composition of the nanoparticles. Evidence of the presence of Ag on the surface of the nanoparticles, of enrichment of the Ag/Au alloy shell by Ag toward or at the nanoparticle surface, and of modification of the nanoparticle surface by adsorbed chlorides is also provided. Reduction of the size of the Ag seeds, alloying of Ag and Au in the shell of the nanoparticles, and modification of their surfaces by adsorbed chlorides are tentatively attributed to positive charging of the nanoparticles during the electrocatalytic overdeposition of Au over Ag seeds.