Nanoscale Pt-Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt-Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt(-1) at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt(-1)). Our analysis suggests that this great enhancement of ORR activity could be attributed to the presence of a clean, well-preserved (111) surface for the Pt-Ni octahedra.

An effective strategy for reducing the Pt content while retaining the activity of a Pt-based catalyst is to deposit the Pt atoms as ultrathin skins of only a few atomic layers thick on nanoscale substrates made of another metal. During deposition, however, the Pt atoms often take an island growth mode because of a strong bonding between Pt atoms. Here we report a versatile route to the conformal deposition of Pt as uniform, ultrathin shells on Pd nanocubes in a solution phase. The introduction of the Pt precursor at a relatively slow rate and high temperature allowed the deposited Pt atoms to spread across the entire surface of a Pd nanocube to generate a uniform shell. The thickness of the Pt shell could be controlled from one to six atomic layers by varying the amount of Pt precursor added into the system. Compared to a commercial Pt/C catalyst, the Pd@PtnL (n = 1-6) core-shell nanocubes showed enhancements in specific activity and durability toward the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations on model (100) surfaces suggest that the enhancement in specific activity can be attributed to the weakening of OH binding through ligand and strain effects, which, in turn, increases the rate of OH hydrogenation. A volcano-type relationship between the ORR specific activity and the number of Pt atomic layers was derived, in good agreement with the experimental results. Both theoretical and experimental studies indicate that the ORR specific activity was maximized for the catalysts based on Pd@Pt2-3L nanocubes. Because of the reduction in Pt content used and the enhancement in specific activity, the Pd@Pt1L nanocubes showed a Pt mass activity with almost three-fold enhancement relative to the Pt/C catalyst.

Controlling the shape or morphology of metal nanocrystals is central to the realization of their many applications in catalysis, plasmonics, and electronics. In one of the approaches, the metal nanocrystals are grown from seeds of certain crystallinity through the addition of atomic species. In this case, manipulating the rates at which the atomic species are added onto different crystallographic planes of a seed has been actively explored to control the growth pattern of a seed and thereby the shape or morphology taken by the final product. Upon deposition, however, the adsorbed atoms (adatoms) may not stay at the same sites where the depositions occur. Instead, they can migrate to other sites on the seed owing to the involvement of surface diffusion, and this could lead to unexpected deviations from a desired growth pathway. Herein, we demonstrated that the growth pathway of a seed is indeed determined by the ratio between the rates for atom deposition and surface diffusion. Our result suggests that surface diffusion needs to be taken into account when controlling the shape or morphology of metal nanocrystals. seeded growth | shape control | noble metals S urface diffusion is a general process that involves the motion of adsorbed atoms (adatoms), molecules, or atomic clusters on the surface of a solid material (1, 2). Over the past decades, it has emerged as an important concept in many areas of surface science, including catalysis, epitaxial growth, and electromigration of voids (3-7). Here, we demonstrated that surface diffusion also plays a pivotal role in determining the growth pathway of a seed and thus the shape or morphology taken by the final product in a solutionphase synthesis of metal nanocrystals. Fig. 1 schematically illustrates four possible pathways for the growth of a cubic seed. As a model system, we focused on Pd nanocubes with slight truncation at corners and edges, together with six side faces passivated by chemisorbed Br -ions. In the following discussion, we refer to them as "Pd cubic seeds" for simplicity. We chose them as seeds for two major reasons: (i) they had a well-defined shape, together with a set of low-index facets on the surface (8, 9); and (ii) their side faces are blocked by Br -ions to ensure selective deposition of atoms onto the corner sites during seed-mediated growth (10-12). These two distinctive features allowed us to easily track the deposition of atoms and their surface diffusion during a growth process by analyzing the shape or morphology of the final product.The newly formed Pd atoms resulting from the reduction of a Pd precursor are expected to deposit at the corners of a cubic seed because the side faces are blocked by the chemisorbed Br -ions (Fig. 1A, 1). Upon deposition, there will be two different options for these adatoms: staying at the corner sites or migrating to other sites, including edges and side faces, through surface diffusion (Fig. 1A, 2 and 3). It should be pointed out that only surface diffusion was allowed here to move atoms from corners to edg...