Abstract
Colloidal solid‐solution‐like Au‐Ag alloy nanoclusters of different compositions were synthesized through citrate reduction of mixed metal ions of low concentrations, without using any other protective or capping agents. Optical absorption of the alloy nanoclusters was studied both theoretically and experimentally. The position of the surface plasmon resonance (SPR) absorption band of the nanoclusters could be tuned from 419 nm to 521 nm through the variation of their composition. Considering effective dielectric constant of the alloy, optical absorption spectra for the nanoclusters were calculated using Mie theory, and compared with the experimentally obtained spectra. Theoretically obtained optical spectra well resembled the experimental spectra when the true size distribution of the nanoparticles was considered. High‐resolution transmission electron microscopy (HREM), high‐angle annular dark field (HAADF) imaging, and energy dispersive spectroscopy (EDS) revealed the true alloy nature of the nanoparticles with nominal composition being preserved. The synthesis technique can be extended to other bimetallic alloy nanoclusters containing Ag.
Highlights
Muchattention has been paid to the synthesis and characterizationof bimetallic nanoparticles due to their unique catalytic, electronic,optical, structural, and thermalproperties [1,2,3,4,5] and subsequent technological applications as catalysts, sensors, nanoelectronic devices [6,7,8,9,10,11,12,13], and biosensors [14]
AuAg alloy nanoparticles show a single, composition-sensitive absorption band located at an intermediate position between pure Au and Ag nanoparticles’ surface plasmon resonance (SPR) peaks, which results in amplification of light-induced processes (e.g., Raman scattering) undergone by molecules localized on their surfaces, giving rise to surface-enhanced Raman scattering [21]
While the SPR peaks for the bimetallic colloids prepared with Au/Ag = 9/1 and 3/1 appeared at the same position as that of monometallic Au, for the colloids with Au/Ag = 1/3, the SPR peak position appeared around 418 nm, which is very close to the SPR position of monometallic Ag
Summary
Muchattention has been paid to the synthesis and characterizationof bimetallic nanoparticles due to their unique catalytic, electronic,optical, structural, and thermalproperties [1,2,3,4,5] and subsequent technological applications as catalysts, sensors, nanoelectronic devices [6,7,8,9,10,11,12,13], and biosensors [14]. The properties and the applicability of these nanoparticles depend on their size and shape, and on the combination of the component metals (composition) and their fine structure, either as alloy or core-shell structures. Bimetallic nanoparticles with well-defined alloy structures of noble metals like PtRu, Cu-Pd, Pt-Mo, Pt-W, Pt-Ni, and Au-Ag provide practical examples for the influence of metals’ composition and their structures on their catalytic properties [15,16,17,18,19]. Au-Ag bimetallic nanoparticles show different optical responses for alloy and core-shell configurations, even when they have the same Au and Ag contents. Relatively few methods produce true alloy nanoparticles due to phase separation at the atomic level leading to the formation of core-shell particles [31,32,33,34,35]. Considering effective dielectric constant of the alloy, optical absorption spectra for the nanoclusters were calculated using Mie theory, and compared with the experimentally obtained spectra
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