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Bimetallic nanoparticles present different properties than their monometallic counterparts, opening a wide range of possibilities for different applications. PtPd nanoparticles have raised interest for their many applications in fuel cells, ethanol and methanol oxidation reactions, hydrogen storage, etc. However, the cost of Pt makes it unpractical to use in big quantities; therefore, one of the big challenges is to synthesize very small catalysts in order to maximize the efficiency in their use. In this work, we synthesized polyhedral PtPd core-shell nanoparticles under 20 nm and characterized them by Cs-corrected scanning transmission electron microscopy. This technique allowed us to probe the structure at the atomic level of these nanoparticles revealing new structural information. We determined the structure of the three main polyhedral morphologies obtained in the synthesis: octahedral, decahedral and triangular plates. Decahedral PtPd core-shell nanoparticles are novel morphologies for this system. Morphology and defects present in the nanoparticles are shown and discussed.

The stability of decahedral shaped nanoparticles depends upon size. Ino and Marks introduced new mechanisms for the stabilization of decahedra nanoparticles which involves the faceting and the formation of surfaces different of {111}. These mechanisms have relevance for small size nanoparticles; however, they do not explain, thoroughly, how decahedral particles can grow up to 300 nm or more. Here, we report new mechanisms that help stabilize very large decahedral. With the use of aberration corrected scanning transmission electron microscopy, we observed the formation of high index facets, determined to be of the {511} family, on all five sides of the particles. Surface dislocations strings are also observed. In addition, surface reconstruction of the {001} surfaces can also be observed in two different orientation: with hexagonal strings along the [110] and along the [410] directions.