Phase-Controlled Deposition of Ru on Pd Nanocrystal Templates: The Effects of Particle Shape and Size

Template-directed growth has been widely used for the phase-controlled synthesis of metal nanocrystals, but much remains to be discovered about the mechanistic details. In this work, we systematically investigate the roles played by the shape and size of Pd nanocrystal templates in controlling the crystal phase taken by the deposited Ru overlayers. For Pd cubic nanocrystals, a face-centered cubic (<i>fcc</i>) Ru shell is always favored when the particle size is varied in the range of 6−25 nm. In the case of Pd octahedral nanocrystals, we observe a size dependence, with 14-nm octahedra giving <i>fcc</i>-Ru but 20- and 26-nm octahedra resulting in hexagonal close-packed (<i>hcp</i>) Ru. This trend can be attributed to their difference in surface atomic arrangement, as the {100} facets on a cubic template cannot be matched by any facet of <i>hcp</i>-Ru, forcing the deposited Ru to take the <i>fcc</i> phase. In contrast, the {111} facets on an octahedral template can be matched by the {0001} facets of <i>hcp</i>-Ru, allowing the deposited Ru to take either an <i>hcp</i> or <i>fcc</i> phase. The crystal phase taken by the Ru shell is determined by the relative contributions from the surface and bulk energies of the deposited <i>fcc</i>- and <i>hcp</i>-Ru phases. Additionally, the water content in the reaction mixture also plays an important role in affecting the crystal phase taken by the Ru shell by altering the reduction kinetics. When tested as catalysts toward ethylene glycol oxidation, the <i>fcc</i>-Ru outperforms <i>hcp</i>-Ru while the cubic shape is advantageous over the octahedral counterpart.