Controllable synthesis of metallic nanocrystals (NCs) with tunable phase, uniform shape, and size is of multidisciplinary interests but has still remained challenging. Herein, a robust phase control strategy is developed, in which seeds with a given phase are added to guide the epitaxial growth of the target metal to inherit the seeds' phase. Through this strategy, M@Ru (M = Pt, Pd) NCs in the face-centered cubic (fcc) phase, a metastable phase for Ru under ambient conditions, were synthesized with the hydrothermal method. The Pt@ Ru NCs showed not only the pure fcc phase but also high morphology selectivity to tetrahedrons surrounded by {111} facets. As revealed by density function theory (DFT) calculations, the preferentially epitaxial growth of Ru atom layers on the nonclosest-packed facets of hetero fcc metal seeds led to the formation of fcc Ru shells. Furthermore, the fcc Pt@Ru tetrahedrons/C showed electrocatalytic activity enhancement with more than an order of magnitude toward hydrogen oxidation reaction (HOR) in acidic electrolyte compared with hydrothermally synthesized Ru/C. Electrochemical measurement combined with DFT calculations revealed that the optimum HOR activity should be achieved on well-crystallized fcc Ru catalysts exposing maximum {111} facets.The surface atomic arrangement of metallic nanocatalysts, which strongly affect the interaction between the substrate molecules and the catalysts' surface, and consequently their catalytic performance, is determined by both their phase and exposing facets. For instance, face-centered cubic (fcc) Ru nanocrystals (NCs) show higher catalytic activity to CO oxidation than hexagonal closest packed (hcp) ones, because fcc NCs exposed more the closest packed surface of Ru which exhibit higher activity to this reaction. 1,2 hcp Co NCs exhibited higher catalytic activity toward Fischer−Tropsch synthesis than fcc ones 3,4 since the {10−11} facets of hcp Co comprise active sites for CO activation. 5 hcp Co NCs exposing more {10−11} facets would predictably represent even higher activity. The exposing-facet-control, namely the shape-control synthesis of metallic NCs, has been widely investigated during the past decade, and the development of phase control strategies is attracting increasing attention. 1,2,6−10 The existing strategies to tune the phase of metallic NCs contain decreasing particle size, 2,6 varying composition, 7,8 introducing supports, 9 and using an intermediate compound to link up a phase transition. 3,10 However, through these strategies, it remains difficult to prepare metallic NCs in the metastable phase together with good crystallinity and uniform shape. Therefore, more robust phase control strategies of metallic NCs are still in demand.Introducing metal seeds with a given phase is a possible route to prepare core−shell NCs with shells inheriting the seeds' phase. Recently, a great deal of research attention was drawn to the heteronanostructures combined metals with different phases. 11−16 For instance, Ru shows an hcp structure under ambie...
