Co-reduced Ag/Pd Bimetallic Nanoparticles: Surface Enrichment of Pd Revealed by Raman Spectroscopy

In the hope of developing metal nanoparticles that are expected to possess simultaneously higher surface-enhanced Raman scattering (SERS) activity and also catalytic activity, we have prepared Ag/Pd alloy nanoparticles by coreduction of Ag and Pd ions using citrate and then examined their characteristics in terms of their composition. Ag/Pd (2% in atomic percent) particles, about 53 nm in diameter, are alloy-like, with their surfaces being composed of Ag and Pd atoms. Ag/Pd(5%) particles, about 29 nm in diameter, are also alloy-like, but their surfaces appear to be covered fully with Pd atoms of similar to 2 nm thickness. The surface enrichment of Pd atoms were clearly identified by the SERS spectra of 2,6-dimethylphenylisocyanide, 4-nitrobenzenethiol, and 4-aminobenzenethiol, as well as by high-angle annular dark-field scanning transmission electron microscopy analysis, even though the presence of Pd atoms was hardly detectable by X-ray diffraction. Owing to the borrowing effect from Ag, the SERS activity of Ag/Pd(5%) particles was at least an order of magnitude higher than that of pure Pd particles: the SERS activity deteriorated rapidly when the Pd content in Ag/Pd was increased above 5%. However, the catalytic activity of Ag/Pd(5%) nanoparticles was the highest among Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd nanoparticles in the reduction reaction of 4-nitrophenol to 4-aminophenol by NaBH4, suggesting the presence of a synergistic interaction between the Ag and Pd atoms. Ag/Pd(5%) particles appeared thus to be the optimum type, possessing both higher SERS activity and also higher catalytic activity simultaneously.