Coreduced Pt/Ag Alloy Nanoparticles: Surface-Enhanced Raman Scattering and Electrocatalytic Activity

In an effort to develop highly effective Pt-based substrates for surface-enhanced Raman scattering (SERS) and electrocatalysis, we have synthesized 10 nm-sized Pt/Ag alloy nanoparticles, preparing first 7 nm-sized seed particles of Pt, followed by the coreduction of Ag and Pt precursors onto them. The formation of Pt core-Pt/Ag alloy shell nanoparticles was evident from their UV-visible extinction characteristics, X-ray diffraction patterns, and high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy images. According to the phase diagram, an alloy is possible only with a very high atomic content of either Ag or Pt, but the formation of alloys herein, even with an apparent composition of Pt0.70Ag0.30, can be understood by presuming that due to the presence of seed Pt particles, the reduction potentials of Pt and Ag are modified to allow the formation of Pt/Ag alloys onto them. The formation of alloys of Pt with Ag has resulted in the enhancement of not only the SEAS activity but also the electrocatalytic activity of Pt alone. The SERS activity was confirmed to increase as more Ag was incorporated into Pt to form Pt/Ag alloy nanoparticles. As a methanol electro oxidation catalyst, a pure Pt surface was poisoned by incompletely oxidized species such as CO and was thus unable to recover its electrocatalytic activity. In contrast, methanol oxidation peaks were observed in repeated cycles when Pt/Ag alloyed electrodes were used. In particular, the ratio of the forward oxidation current peak to the reverse current peak was as large as 2.52 for the Pt0.95Ag0.05 electrode, which is more than 3 times larger than that of a commercially available pure Pt catalyst, suggesting that the Pt/Ag alloy catalysts are superior in their ability to tolerate poisoning species.