Continuous syntheses of Pd@Pt and Cu@Ag core–shell nanoparticles using microwave-assisted core particle formation coupled with galvanic metal displacement

Abstract

Continuous synthesis of Pd@Pt and Cu@Ag core–shell nanoparticles was performed using flow processes including microwave-assisted Pd (or Cu) core–nanoparticle formation followed by galvanic displacement with a Pt (or Ag) shell. The core–shell structure and the nanoparticle size were confirmed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation and EDS elemental mapping. The Pd@Pt nanoparticles with a particle size of 6.5 ± 0.6 nm and a Pt shell thickness of ca. 0.25 nm were synthesized with appreciably high Pd concentration (Pd 100 mM). This shell thickness corresponds to one atomic layer thickness of Pt encapsulating the Pd core metal. The particle size of core Pd was controlled by tuning the initial concentrations of Na₂[PdCl₄] and PVP. Core–shell Cu@Ag nanoparticles with a particle size of 90 ± 35 nm and an Ag shell thickness of ca. 3.5 nm were obtained using similar sequential reactions. Oxidation of the Cu core was suppressed by the coating of Cu nanoparticles with the Ag shell.