Synthesis of core–shell silver–platinum nanoparticles, improving shell integrity

Abstract

Silver–platinum core–shell (Ag@Pt) nanoparticles have been synthesized using various methods. In the case of Ag@Pt nanoparticles synthesized by the standard method based on the galvanic replacement reaction between Ag seeds and PtCl42−, transmission electron microscopy micrographs revealed well visible core–shell structure. However, electrochemical experiments showed that relatively large amount of silver can be easily stripped off from such nanoparticles. Significant improvement on the integrity of the deposited platinum shell can be achieved when nanoparticles are synthesized by the seeded growth reaction including reduction of PtCl42− with ascorbic acid at room temperature. To obtain pinhole-free platinum layers (where Ag oxidation is not observed) relatively large amount of platinum must be deposited. For example, to cover 11 nm Ag seeds, the number of moles of platinum in the formed Ag@Pt nanoparticles must be at least equal to the number of moles of Ag. It was also found that a similar seeded growth reaction may be used to form pinhole-free Au@Pt nanoparticles. The electrochemical behaviour of those two systems (Ag@Pt and Au@Pt nanoparticles) towards CO stripping was rather different. While the CO-stripping on Au@Pt occurred at typical potentials and without a significant reconstruction of the original surface, CO stripping voltammograms on Ag@Pt were very unusual and exhibited both exceptionally strong binding of CO to the surface, and such a reconstruction of the surface that silver atoms were no longer compactly covered by platinum.