We report a facile synthesis of multiply twinned Pd@Pt core-shell concave decahedra by controlling the deposition of Pt on preformed Pd decahedral seeds. The Pt atoms are initially deposited on the vertices of a decahedral seed, followed by surface diffusion to other regions along the edges/ridges and then across the faces. Different from the coating of a Pd icosahedral seed, the Pt atoms prefer to stay at the vertices and edges/ridges of a decahedral seed even when the deposition is conducted at 200 °C, naturally generating a core-shell structure covered by concave facets. The nonuniformity in the Pt coating can be attributed to the presence of twin boundaries at the vertices, as well as the {100} facets and twin defects along the edges/ridges of a decahedron, effectively trapping the Pt adatoms at these high-energy sites. As compared to a commercial Pt/C catalyst, the Pd@Pt concave decahedra show substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). For the concave decahedra with 29.6% Pt by weight, their specific (1.66 mA/cm(2)Pt) and mass (1.60 A/mgPt) ORR activities are enhanced by 4.4 and 6.6 times relative to those of the Pt/C catalyst (0.36 mA/cm(2)Pt and 0.32 A/mgPt, respectively). After 10,000 cycles of accelerated durability test, the concave decahedra still exhibit a mass activity of 0.69 A/mgPt, more than twice that of the pristine Pt/C catalyst.

Despite the pivotal role played by the reduction of a salt precursor in the synthesis of metal nanocrystals, it is still unclear how the precursor is reduced. The precursor can be reduced to an atom in the solution phase, followed by its deposition onto the surface of a growing nanocrystal. Alternatively, the precursor can adsorb onto the surface of a growing nanocrystal, followed by reduction through an autocatalytic process. With Pd as an example, here we demonstrate that the pathway has a correlation with the reduction kinetics involved. Our quantitative analyses of the reduction kinetics of PdCl and PdBr by ascorbic acid at room temperature in the absence and presence of Pd nanocubes, respectively, suggest that PdCl was reduced in the solution phase while PdBr was reduced on the surface of a growing nanocrystal. Our results also demonstrate that the reduction pathway of PdBr by ascorbic acid could be switched from surface to solution by raising the reaction temperature.

Engineering the elemental composition of metal nanocrystals offers an effective strategy for the development of catalysts or electrocatalysts with greatly enhanced activity. Herein, we report the synthesis of Pt-Ag alloy nanocages with an outer edge length of 18 nm and a wall thickness of about 3 nm. Such nanocages with a composition of PtAg could be readily prepared in one step through the galvanic replacement reaction between Ag nanocubes and a Pt(II) precursor. After 10 000 cycles of potential cycling in the range of 0.60-1.0 V as in an accelerated durability test, the composition of the nanocages changed to PtAg, together with a specific activity of 1.23 mA cm toward oxygen reduction, which was 3.3 times that of a state-of-the-art commercial Pt/C catalyst (0.37 mA cm) prior to durability testing. Density functional theory calculations attributed the increased activity to the stabilization of the transition state for breaking the O-O bond in molecular oxygen. Even after 30 000 cycles of potential cycling, the mass activity of the nanocages only dropped from 0.64 to 0.33 A mg, which was still about two times that of the pristine Pt/C catalyst (0.19 A mg).

We report a quantitative understanding of the reduction kinetics responsible for the formation of Pd-Pt bimetallic nanocrystals with two distinctive structures. The syntheses involve the use of KBr to manipulate the reaction kinetics by influencing the redox potentials of metal precursor ions via ligand exchange. In the absence of KBr, the ratio between the initial reduction rates of PdCl4(2-) and PtCl4(2-) was about 10.0, leading to the formation of Pd@Pt octahedra with a core-shell structure. In the presence of 63 mM KBr, the products became Pd-Pt alloy nanocrystals. In this case, the ratio between the initial reduction rates of the two precursors dropped to 2.4 because of ligand exchange and, thus, the formation of PdBr4(2-) and PtBr4(2-). The alloy nanocrystals took a cubic shape owing to the selective capping effect of Br(-) ions toward the {100} facets. Relative to the alloy nanocubes, the Pd@Pt core-shell octahedra showed substantial enhancement in both catalytic activity and durability toward the oxygen reduction reaction (ORR). Specifically, the specific (1.51 mA cm(-2)) and mass (1.05 A mg(-1) Pt) activities of the core-shell octahedra were enhanced by about four- and three-fold relative to the alloy nanocubes (0.39 mA cm(-2) and 0.34 A mg(-1) Pt, respectively). Even after 20000 cycles of accelerated durability test, the core-shell octahedra still exhibited a mass activity of 0.68 A mg(-1) Pt, twice that of a pristine commercial Pt/C catalyst.

Exploring effective electrocatalysts for oxygen evolution reaction (OER) is a crucial requirement of many energy storage and conversion systems, involving fuel cells, water splitting, and metal–air batteries. Herein, a heterogeneity metal‐organic framework (MOF) is prepared by the assembling of metals, terephthalic (A) and 2‐aminoterephthalic ligands (B), defined as A2.7B‐MOF‐FeCo1.6. More importantly, A2.7B‐MOF‐FeCo1.6 exhibits excellent OER activity with an ultralow overpotential of 288 mV at 10 mA cm−2 and a Tafel slope of 39 mV dec−1. The high electrocatalytic performance for OER is attributed to the optimized electronic structure of the intrinsic catalytic center in MOFs via the engineering of the metal node and linkers. The work offers not only a benchmark for pure MOFs in electrocatalysis but also a new efficient strategy to improve electrocatalytic performance by electronic structure engineering of catalytic active centers in MOFs.

By taking advantage of the anisotropy of AuNRs, we design different bifunctional PEG molecules to selectively bind to either the end or side face and simultaneously protect other faces of individual AuNRs. In this way, we successfully achieve orientation-controllable assemblies of AuNRs into side-by-side (SS), end-to-end (EE) and end-to-side (ES) orientations based on the electrostatic interaction between carboxylic PEG and CTAB capping on AuNRs. Furthermore, we find that the different orientations of assembledmotifs in these three types of AuNRs assemblies exhibited different near field coupling between the surface plasma of the neighboring AuNRs, leading to different surface-enhanced Raman signals. Undoubtedly, the current rational design of oriented assembly can be potentially useful for directing anisotropic nanoparticles into well-defined orientations, which provides a powerful route in designing families of novel nanodevices and nanomaterials with programmable electrical and optical properties.National Natural Science Foundation of China[20725310, 90923042]; Research Fund for the Doctoral Program of Higher Education of China[20100121120038]; Natural Science Foundation of Fujian Province of China[2010J01046]; Fundamental Research Funds for the Central Universities[2010121023]; key laboratory of Biomedical Material of Tianji

We describe a new strategy to enhance the catalytic durability of Pt-Ni octahedral nanocrystals in the oxygen reduction reaction (ORR) by conformally depositing an ultrathin Pt shell on the surface. The Pt-Ni octahedra were synthesized according to a protocol reported previously and then employed directly as seeds for the conformal deposition of ultrathin Pt shells by introducing a Pt precursor dropwise at 200 °C. The amount of Pt precursor was adjusted relative to the number of Pt-Ni octahedra involved to obtain Pt-Ni@Pt1.5L octahedra of 12 nm in edge length for the systematic evaluation of their chemical stability and catalytic durability compared to Pt-Ni octahedra. Specifically, we compared the elemental compositions of the octahedra before and after treatment with acetic and sulfuric acids. We also examined their electrocatalytic stability toward the ORR through an accelerated durability test by using a rotating disk electrode method. Even after treatment with sulfuric acid for 24 h, the Pt-Ni@Pt1.5L octahedra maintained their original Ni content, whereas 11 % of the Ni was lost from the Pt-Ni octahedra. After 10 000 cycles of ORR, the mass activity of the Pt-Ni octahedra decreased by 75 %, whereas the Pt-Ni@Pt1.5L octahedra only showed a 25 % reduction.

Shape-controlled synthesis of colloidal metal nanocrystals has traditionally relied on the use of an approach that involves the reduction of a metal precursor by a single reductant. Once the concentration of atoms surpasses supersaturation, they will undergo homogeneous nucleation to generate nuclei and then seeds, followed by further growth into nanocrystals. In general, it is a grand challenge to optimize such an approach because the kinetic requirement for nucleation tends to be drastically different from what is needed to guide the growth process. In this work, we overcome this difficulty by switching to a dual-reductant approach, in which both strong and weak reductants are added into the same reaction solution. By controlling their amounts to program the reduction kinetics, the strong reductant only regulates the homogeneous nucleation process to generate the desired seeds, and once consumed, the weak reductant takes over to control the growth pattern and thereby the shape of the resulting nanocrystals.