New Experimental Evidences of Pt–Pd Bimetallic Nanoparticles with Core–Shell Configuration and Highly Fine-Ordered Nanostructures by High-Resolution Electron Transmission Microscopy

Abstract

In our facile synthesis method, poly(vinylpyrrolidone)-protected Pt and Pt–Pd bimetallic nanoparticles with controllable polyhedral core–shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol with silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the size range of 20 nm. Important evidences of core–shell configurations of the Pt–Pd core–shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core–shell Pt–Pd nanoparticles in the size range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of ∼3 nm in thickness as the atomic Pd layers grown on the Pt cores. Very interesting characteristics of surface structure of Pt nanostructures and Pt–Pd core–shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt–Pd bimetallic nanoparticles showed that the Frank–van der Merwe and Stranski–Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. It is predicted that the FM growth becomes the main favorable growth compared with the SK growth in the formation of the thin Pd shells of Pt–Pd core–shell nanoparticles. The experimental evidence of the deformations of lattice fringes and lattice-fringe patterns was found in Pt and Pt–Pd core–shell nanoparticles. The interesting renucleation and recrystallization at the attachments between the nanoparticles are revealed to form a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and promising utilization of them are proposed for next generations of various fuel cells with low cost. Finally, the products of Pt–Pd core–shell nanoparticles can be potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane fuel cell and direct methanol fuel cell using the very low Pt loading with better cost-effective design.