An experimental and theoretical investigation of the inversion of pd@pt core@shell dendrimer-encapsulated nanoparticles.

Bimetallic PdPt dendrimer-encapsulated nanoparticles (DENs) having sizes of about 2 nm were synthesized by a homogeneous route that involved (1) formation of a Pd core, (2) deposition of a Cu shell onto the Pd core in the presence of H2 gas, and (3) galvanic exchange of Pt for the Cu shell. Under these conditions, a Pd@Pt core@shell DEN is anticipated, but detailed characterization by in-situ extended X-ray absorption fine structure (EXAFS) spectroscopy and other analytical methods indicate that the metals invert to yield a Pt-rich core with primarily Pd in the shell. The experimental findings correlate well with density functional theoretical (DFT) calculations. Theory suggests that the increased disorder associated with <~2 nm diameter nanoparticles, along with the relatively large number of edge and corner sites, drives the structural rearrangement. This type of rearrangement is not observed on larger nanoparticles or in bulk metals.