Surface-Enhanced Raman Scattering of a Cu/Pd Alloy Colloid Protected by Poly(N-vinyl-2-pyrrolidone)

Abstract

A novel series of poly(N-vinyl-2-pyrrolidone)-protected nanometer-sized Cu/Pd alloy colloids have been prepared by a polyol reduction method. These colloids, having high molar fractions of Cu (x = 0.99−0.80), show remarkable stability, as good as a Pd colloid. Transmission electron microscopy and high-resolution transmission electron microscopy analyses indicate that the Cu-rich particles spanning from 2−10 nm in size display an increase in the (111) lattice spacings compared to the bulk state. X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure analysis confirm the alloy phase nature in the Cu/Pd [Cu:Pd = 4:1 (mol/mol)] nanoparticles with Cu at the zero-oxidation state. The Raman scattering behavior of several types of molecules adsorbed on the Cu/Pd(4/1) colloids has been examined for the first time. Significant band shifts in the ring modes of p-aminobenzoic acid, thiophenol, and bis(3-carboxy-4-nitrophenyl) disulfide in the colloid surface spectra reveal their particular adsorption interactions and preserve the typical traits, when compared with some of the previously reported surface-enhanced Raman scattering (SERS) spectra on pure Ag, Au, or Cu colloids or metal films. The Raman signal intensities from the molecules adsorbed on the Cu/Pd(4/1) alloy colloid are about 10−102-fold more intense than normal Raman scattering in an aqueous solution. The implied surface adsorption configurations are also discussed in terms of the surface selection rule, with the facilitation of reliable assignments of normal modes of p-aminobenzoic acid and thiophenol.