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.

Metal sols are often stabilized in solution by the presence of a charged double layer surrounding each colloidal nanoparticle that produces a coulomb barrier to aggregation. The nanoparticles can be induced to aggregate by replacing the charged surface species by uncharged adsorbate. We present a simple analysis that produces an expression for the initial rate constant describing the aggregation process, which is shown to depend nontrivially on the adsorbate concentration. The expression is tested experimentally by following and analyzing the time rate of decrease of the surface plasmon resonance absorption of isolated Ag nanoparticles in aqueous solution. The experimental result accords well with theory, producing a rough estimate of the (adsorbate-concentration dependent) barrier to aggregation that in the absence of added adsorbate is approximately equal to 0.08 eV.

Excitation profiles of SERS (surface-enhanced Raman scattering) and/or SERRS (surface-enhanced resonance Raman scattering) spectral bands of two forms of a Ag-bpy (bpy = 2,2'-bipyridine) surface complex and of [Ru(bpy)3]2+ on Ag nanoparticle (hydrosol) surfaces were determined from the spectra excited in the 458-600 nm region and are reported together with the FT-SERS spectra of the Ag-bpy surface complex and FT Raman spectra of [Ru(bpy)3] Cl2. Seven of the observed 11 fundamentals as well as their first overtones and combination bands are selectively enhanced in SERS of the Ag-bpy surface complex formed in the Ag colloid/HCl/bpy system. The profiles of these bands show a common maximum at approximately 540 nm. The selectively enhanced bands of the Ag-bpy surface complex have nearly the same wavenumbers as those enhanced in the SERRS and resonance Raman spectra of [Ru(bpy)3]2+ upon excitation close to the 453 nm maximum of its MLCT absorption band. Moreover, the intensity patterns of the bpy vibrations of the two species match both in resonance (541 nm excitation for Ag-bpy, 458 nm for [Ru(bpy)3]2+) and in off-resonance (458 and 1064 nm for Ag-bpy, 1064 nm for [Ru(bpy)3]2+). The distinct band shapes of the excitation profiles of the selectively enhanced vibrational modes of the Ag-bpy surface complex, as well as the observation of overtones and combination bands in the SERS spectra upon excitation into this "band", are interpreted in terms of a charge-transfer resonance contribution to the overall SERS enhancement. In view of the near-coincidence of the vibrational modes coupled to the resonant electronic transition of Ag-bpy with those coupled to the MLCT transition of [Ru(bpy)3]2+, the resonant electronic transition is tentatively assigned to a Ag metal to bpy (pi*) CT transition.

Surface-enhanced resonance Raman scattering (SERRS) spectra of free base 5,10,15,20-tetrakis(4-carboxypheny1)porphyrin (TPPC4) and of its silver complex, Ag(II)TPPC4, have been investigated under conditions of (I) the direct adsorption of the porphyrin on the surface of Ag colloid and (11) the attachment of the porphyrin to the surface of Ag colloid via a nonionic surfactant, Triton X-100 (p-( 1,1,3,3-tetramethylbutyl)phenylply-(oxyethylene)), functioning as a molecular spacer. For both types of porphyrin adsorption, the mechanisms of the SERS-active system formation have been elucidated. In the former case (I), the system is formed by a two-step process involving (1) preaggregation of Ag colloid by adsorption of phosphate introduced to the system by phosphate buffer and (2) elimination of phosphate from the surface by chemisorption of free base TPPC4.Step 2 of the process is effectively accomplished provided that the concentration of free base (f.b.) TPPC4 in the system is higher than 5 X lo-' M. Formation of theAg(II)TPPC4 surface complex by chemisorption of f.b. TPPC4 on the surface of Ag colloid was proven by SERRS and UV-vis absorption spectroscopy. For the synthetically prepared Ag(II)TPPC4, the second step of the SERS-active system formation is absent, and the SERRS spectrum of the metalloporphyrin is not obtained. In contrast to that, high-quality SERRS spectra of both the free base TPPC4 and Ag(1I)TPPCd were obtained by the spectral study of organized Ag colloid/ Triton X-100/porphyrin systems (11) assembled by a different type of the two-step adsorption process: (1) adsorption of Triton X-100 on the surface of Ag colloid by its hydrophilic part and (2) attractive, hydrophobic interaction between the hydrophobic part of adsorbed Triton X-100 and the porphyrin. SERRS spectra of Ag(II)TPPC4 surface complex, of Ag(II)TPPC4 (synthetic), and of free base TPPC4 were interpreted. The observed coincidence of the vibrational frequencies of Ag(I1)TPPCs surface complex with those of its synthetically prepared analogue signifies a mutual structural equivalence of the porphyrin core parts of the two species. The minor differences in relative band intensities originate from the effect of the surface on the orientation of phenyl rings with respect to the porphyrin macrocycle as well as on the electronic structure of the surface complex.

No abstract

No abstract