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

Abstract

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 (fcc) 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 fcc-Ru but 20-and 26-nm octahedra resulting in hexagonal close-packed (hcp) 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 hcp-Ru, forcing the deposited Ru to take the fcc phase. In contrast, the {111} facets on an octahedral template can be matched by the {0001} facets of hcp-Ru, allowing the deposited Ru to take either an hcp or fcc phase. The crystal phase taken by the Ru shell is determined by the relative contributions from the surface and bulk energies of the deposited fcc-and hcp-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 fcc-Ru outperforms hcp-Ru, while the cubic shape is advantageous over the octahedral counterpart.

Elucidating the Role of Reduction Kinetics in the Phase-Controlled Growth on Preformed Nanocrystal Seeds: A Case Study of Ru

“…The interface between the hcp and fcc phases, indicated by a stacking fault, could be formed due to the symmetry alignment (C 3 ) of the fcc-{111} and hcp-{0001} facets. 18,20,27 The FFT pattern taken from the middle area of the core (blue boxed) matches the diffraction pattern of the hcp phase along the [21̅ 10] zone axis, while the FFT pattern acquired from the shell region (red boxed) is consistent with the diffraction pattern of the fcc phase along the [011] zone axis (Figure 3G, H). The deposition of fcc-Ru overlayers predominantly occurred on the top half of the Ru hcp seed, and this could be attributed to an asymmetrical growth mode initiated following the formation of the fcc phase in the first few layers.…”

“…The reaction temperature should also be taken into consideration since it affects not only the surface diffusion rate of adatoms but also the reduction kinetics, which will in turn influence the deposition rate of the Ru atoms. , When the reaction was conducted at a higher temperature of 200 °C, homogeneous nucleation was triggered as the concentration of the newly formed Ru atoms was elevated while island growth was initiated as a result of the fast deposition rate, producing Ru nanocrystals with a high density of branched arms due to the attachment of small particles (Figure S2B). In contrast, an optimal temperature of 180 °C led to products with uniform size and smooth shell since both deposition and diffusion rates of the newly generated Ru atoms were adequate (Figure A).…”

“…In recent years, seed-mediated growth has been established as a simple and versatile method for achieving phase-controlled synthesis. − Under optimal conditions, the deposited shell can replicate both the crystal and surface structures of the preformed seed, leading to the formation of a metastable phase. With the ability to simultaneously maneuver thermodynamic and kinetic factors in a colloidal synthesis, such phase-controlled growth allows for good reproducibility between batches, tight controls over the size and shape, and a systematic examination of parameters responsible for phase evolution. , Although many studies have delved into the effects of overlayer thickness, template shape (surface structure), and size (bulk versus surface energy) on phase evolution, − creating nanocrystals in a metastable phase of more than six atomic layers in thickness via epitaxial growth still presents a major challenge. The explicit role(s) played by reaction kinetics in affecting the packing of atoms in the bulk is yet to be resolved.…”

“…Most studies have focused on their thermodynamically stable hexagonal close-packed ( hcp ) phase and the metastable face-centered cubic ( fcc ) phase . The difference between these phases lies in the arrangement of the third atomic layer (ABCABC for fcc versus ABABAB for hcp ), a subtle variation that significantly influences catalytic performance by altering interactions between reaction intermediates and surface atoms. ,− As reported in the literature, various factors can impact the packing of Ru atoms. For the most robust polyol synthesis involving homogeneous nucleation, chemical selection was crucial for determining the crystal phase.…”

Bimetallic core–shell nanocrystals: opportunities and challenges

“…A notable example can be found in the development of Pd@Ru nanocrystals. 13,[29][30][31] As a noble metal high in cohesive energy, Ru is known to form very small (typically, o4 nm) particles during a solution-phase synthesis, making it a challenge to prepare nanocrystals with well-defined and controllable facets. 32 By introducing Pd nanocrystals with well-defined shapes as seeds, the deposition of Ru can be switched from an island to a layer-by-layer growth mode.…”