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Rational synthesis of nanostructures with desired properties critically depends on our understanding of the growth mechanism. In addition to the traditional mechanism involving atomic addition, oriented attachment (OA) has received increasing attention in recent years. Employing nanocrystallites as building blocks, OA offers an important route to anisotropic growth, inclusion of defects, and formation of nanostructures with branched morphology. With a focus on metals, here we offer a brief account of recent progress in understanding OA and how it can be adapted for the colloidal synthesis of nanostructures with diverse compositions and morphologies. We start with a discussion on the current understanding of OA based on computational simulations and experimental studies, followed by typical examples of metal nanostructures produced through OA. Finally, we showcase the catalytic and plasmonic applications enabled by those nanostructures, together with perspectives on the challenges and opportunities.

Bimetallic Janus nanocrystals have received considerable interest in recent years owing to their unique properties and niche applications. The side‐by‐side distribution of two distinct metals provides a flexible platform for tailoring the optical and catalytic properties of nanocrystals. First, a brief introduction to the structural features of bimetallic Janus nanocrystals, followed by an extensive discussion of the synthetic approaches, is given. The strategies and experimental controls for achieving the Janus structure, as well as the mechanistic understandings, are specifically discussed. Then, a number of intriguing properties and applications enabled by the Janus nanocrystals are highlighted. Finally, this article is concluded with future directions and outlooks with respect to both syntheses and applications of this new class of functional nanomaterials.

A relatively unexplored aspect of noble‐metal nanomaterials is polymorphism, or their ability to crystallize in different crystal phases. Here, a method is reported for the facile synthesis of Ru@Pd core–shell nanocrystals featuring polymorphism, with the core made of hexagonally close‐packed (hcp)‐Ru while the Pd shell takes either anhcpor face‐centered cubic (fcc)phase. The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking thehcpphase whereas the thicker ones revert tofcc. The injection rate provides an experimental knob for controlling the phase, with one‐shot and drop‐wise injection of the Pd precursor corresponding tofcc‐Pd andhcp‐Pd shells, respectively. When these nanocrystals are tested as catalysts toward formic acid oxidation, the Ru@Pd_hcpnanocrystals outperform Ru@Pd_fccin terms of both specific activity and peak potential. Density functional theory calculations are also performed to elucidate the origin of this performance enhancement.

In addition to the conventional knobs such as composition, size, shape, and defect structure, the crystal structure (or phase) of metal nanocrystals offers a new avenue for engineering their properties. Various strategies have recently been developed for the fabrication of colloidal metal nanocrystals in metastable phases different from their bulk counterparts. With a focus on noble metals, we begin with a brief introduction to their atomic packing, followed by a discussion about five major synthetic approaches to their colloidal nanocrystals in unconventional phases. We then highlight the success of synthesis in terms of mechanistic insights and experimental controls, as well as the enhanced catalytic properties. We end this Minireview with perspectives on the remaining issues and future opportunities.

We report a robust method for effectively removing the chemisorbed Br⁻ions, a capping agent, from the surface of Pd nanocubes to maximize their catalytic activity. The Br⁻ions can be removed by simply heating the sample in water, but the desorption of Br⁻ions will expose the underneath Pd atoms to the O₂from air for the formation of a relatively thick oxide layer. During potential cycling, the oxide layer evolves into detrimental features such as steps and terraces. By introducing a trace amount of hydrazine into the system, the Br⁻ions can be removed by heating without forming a thick oxide layer. The as‐cleaned nanocubes show greatly enhanced activity toward formic acid oxidation. This cleaning method can also remove Br⁻ions from Rh nanocubes and it is expected to work for other combinations of nanocrystals and capping agents.

Surface capping agents have been extensively used to control the evolution of seeds into nanocrystals with diverse but well‐controlled shapes. Here we offer a comprehensive review of these agents, with a focus on the mechanistic understanding of their roles in guiding the shape evolution of metal nanocrystals. We begin with a brief introduction to the early history of capping agents in electroplating and bulk crystal growth, followed by discussion of how they affect the thermodynamics and kinetics involved in a synthesis of metal nanocrystals. We then present representative examples to highlight the various capping agents, including their binding selectivity, molecular‐level interaction with a metal surface, and impacts on the growth of metal nanocrystals. We also showcase progress in leveraging capping agents to generate nanocrystals with complex structures and/or enhance their catalytic properties. Finally, we discuss various strategies for the exchange or removal of capping agents, together with perspectives on future directions.

We report a new catalytic system by partially covering the uniform Pt nanocrystals on a carbon support with an ultrathin film derived from polyacrylonitrile (PAN). The use of Pt nanocrystals uniform in both size and shape effectively suppresses Ostwald repining, while partially covering them with a PAN‐derived film prevents migration, aggregation, and detachment from the support. In addition, the pyridinic N atoms on the edges of the thermally‐treated PAN film can also weaken the O=O bond, accelerating the reduction of oxygen. Upon optimization, the new catalyst exhibits a mass activity of 0.51 mA ⋅ μg⁻¹_Pttoward oxygen reduction, substantially enhanced relative to the same catalyst without PAN (0.22 mA ⋅ μg⁻¹_Pt) and a commercial Pt/C (0.41 mA ⋅ μg⁻¹_Pt). The mass activity is essentially retained after 10,000 cycles of accelerated durability test between 0.6 V and 1.1 V in oxygen‐saturated HClO₄. Even after aging in H₃PO₄at 220 °C for one week, the electrochemical surface area of the catalyst is still maintained. This catalytic system holds great promise for use in various types of fuel cells with a long lifetime.

Despite the pivotal roles played by halide ions (e. g., Cl⁻and Br⁻) in directing the evolution of seeds into metal nanocrystals with diverse shapes, it is still unclear how halides affect the reduction kinetics of a salt precursor and thus the outcome of a synthesis. Here we report a quantitative analysis of the multiple roles played by halides in controlling the growth behaviors of Pd seeds with cubic and octahedral shapes, respectively. Our quantitative measurements clearly indicate the existence of a transition point around 10⁻³ mM min⁻¹for the reduction rate, which separates the reduction into two distinctive pathways (solutionversussurface) for the formation of completely different products. More significantly, we demonstrate that the speciation, reduction kinetics, and reduction pathway of a Pd(II) precursor can all be manipulated by varying the type and/or amount of halides introduced into a synthesis for the deterministic formation of a specific product. This work represents a critical step forward in achieving a quantitative understanding of the multiple roles of halides involved in the shape‐controlled synthesis of Pd nanocrystals, with the knowledge potentially extendible to other noble metals and their alloys.

This paper presents a facile synthesis of Cu twin cubes, with a yield ofca. 70%, from seeds based on Pd hexagonal nanoplates. The lattice mismatch, capping agent, and number of planar defects in the seeds all play important roles in directing the shape evolution of Cu on the Pd seeds. Initially, the Cu atoms are only deposited on one of the two basal planes of a Pd nanoplate in the form of small islands. As the growth continues, Cu {100} facets developed in the presence of hexadecylamine and Cl⁻, two capping agents with selectivity towards the Cu(100) surface. When switched to Pd triangular nanoplates, Cu right bipyramids instead of cubes are obtained and only three {100} facets are created from each side of the seed. Atomic‐resolution transmission electron microscopy analysis indicates that the correspondence between the type of the seed and the shape of the final product can be attributed to the number of planar defects along the vertical direction of the plate‐like seed, with two and one twin planes corresponding to cube and right bipyramid, respectively. By adjusting the experimental condition, this synthetic method can also be extended to Pd−Ag and other bimetallic systems.

This article reports a facile method for the synthesis of Pd‐Ru nanocages by activating the galvanic replacement reaction between Pd nanocrystals and a Ru(III) precursor with I‐ions. The as‐synthesized nanocages feature a hollow interior, ultrathin wall of ≈2.5 nm in thickness, and a cubic shape. Our quantitative study suggests that the reduction rate of the Ru(III) precursor can be substantially accelerated upon the introduction of I‐ions and then retarded as the ratio of I‐/Ru³⁺is increased. The Pd‐Ru nanocages take an alloy structure, with the Ru atoms in the nanocages crystallized in a face‐centered cubic structure instead of the hexagonal close‐packed phase taken by bulk Ru. Using Pd nanocubes with different edge lengths, the dimensions of the nanocages in the range of 6−18 nm can readily be tuned. When tested as catalysts toward the electro‐oxidation of ethylene glycol and glycerol, respectively, the Pd‐Ru cubic nanocages prepared from 18 nm Pd cubes exhibit 5.1‐ and 6.2‐fold enhancements in terms of mass activity relative to the commercial Pd/C. After 1000 cycles of accelerated durability test, the mass activities of the nanocages are still 3.3 and 3.7 times as high as that of the pristine commercial Pd/C catalyst, respectively.