Surface Enrichment of Ag Atoms in Au/Ag Alloy Nanoparticles Revealed by Surface-Enhanced Raman Scattering of 2,6-Dimethylphenyl Isocyanide

Abstract

Surface-enhanced Raman scattering (SERS) of 2,6-dimethylphenyl isocyanide (2,6-DMPI) has been investigated with Au/Ag alloy nanoparticles as the adsorbing substrate. UV/visible spectra suggest the formation of alloy nanoparticles is taking place, in conformity with the mole fractions of gold and silver used initially in preparing the alloy sols; according to transmission electron microscopic images, the alloy sols are composed of spherical particles with diameters of ∼35 nm irrespective of their composition, from pure Ag to pure Au nanoparticles. We have confirmed that the SERS spectral feature of 2,6-DMPI on pure Ag is distinctly different from that on pure Au. Furthermore, the SERS spectral feature of 2,6-DMPI on Au/Ag alloy particles, even with a nominal composition of 95% Au and 5% Ag (Au0.95Ag0.05), is the same as that on pure Ag. The present observation supports our previous contention that the outermost layers of the Au/Ag alloy nanoparticles are highly enriched in Ag atoms compared with their interiors. SERS is thus proven to be an invaluable tool for the surface analysis of alloy metals.