Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that have been enabled and/or enhanced by precisely controlling the synthesis of bimetallic nanocrystals.

This Perspective provides a contemporary understanding of the shape evolution of colloidal metal nanocrystals under thermodynamically and kinetically controlled conditions. It has been extremely challenging to investigate this subject in the setting of one-pot synthesis because both the type and number of seeds involved would be changed whenever the experimental conditions are altered, making it essentially impossible to draw conclusions when comparing the outcomes of two syntheses conducted under different conditions. Because of the uncertainty about seeds, most of the mechanistic insights reported in literature for one-pot syntheses of metal nanocrystals with different shapes are either incomplete or ambiguous, and some of them might be misleading or even wrong. Recently, with the use of well-defined seeds for such syntheses, it became possible to separate growth from nucleation and therefore investigate the explicit role(s) played by a specific thermodynamic or kinetic parameter in directing the evolution of colloidal metal nanocrystals into a specific shape. Starting from single-crystal seeds enclosed by a mix of {100}, {111}, and {110} facets, for example, one can obtain colloidal nanocrystals with diversified shapes by adjusting various thermodynamic or kinetic parameters. The mechanistic insights learnt from these studies can also be extended to account for the products of conventional one-pot syntheses that involve self-nucleation only. The knowledge can be further applied to many other types of seeds with twin defects or stacking faults, making it an exciting time to design and synthesize colloidal metal nanocrystals with the shapes sought for a variety of fundamental studies and technologically important applications.

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

We report an approach based on a combination of inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy for quantitative analysis of the role played by Br(-) ions in the synthesis of Pd nanocrystals. The Br(-) ions were found to adsorb onto Pd{100} facets selectively with a coverage density of ca. 0.8 ion per surface Pd atom. The chemisorbed Br(-) ions could be removed via desorption at an elevated temperature under reductive conditions. They could also be gradually released from the surface when Pd cubic seeds grew into cuboctahedrons and then octahedrons. On the basis of the coverage density information, we were able to estimate the minimum concentration of Br(-) ions needed for the formation of Pd nanocubes with a specific size. If the concentration of Br(-) ions was below this minimum value, not all of the {100} facets could be stabilized by the capping agent, leading to the formation of nanocubes with truncated corners. The quantitative analysis developed in this study is potentially extendable to other systems involving chemisorbed capping agents.

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.

Palladium has been recognized as the best anodic, monometallic electrocatalyst for the formic acid oxidation (FAO) reaction in a direct formic acid fuel cell. Here we report a systematic study of FAO on a variety of Pd nanocrystals, including cubes, right bipyramids, octahedra, tetrahedra, decahedra, and icosahedra. These nanocrystals were synthesized with approximately the same size, but different types of facets and twin defects on their surfaces. Our measurements indicate that the Pd nanocrystals enclosed by {1 0 0} facets have higher specific activities than those enclosed by {1 1 1} facets, in agreement with prior observations for Pd single‐crystal substrates. If comparing nanocrystals predominantly enclosed by a specific type of facet, {1 0 0} or {1 1 1}, those with twin defects displayed greatly enhanced FAO activities compared to their single‐crystal counterparts. To rationalize these experimental results, we performed periodic, self‐consistent DFT calculations on model single‐crystal substrates of Pd, representing the active sites present in the nanocrystals used in the experiments. The calculation results suggest that the enhancement of FAO activity on defect regions, represented by Pd(2 1 1) sites, compared to the activity of both Pd(1 0 0) and Pd(1 1 1) surfaces, could be attributed to an increased flux through the HCOO‐mediated pathway rather than the COOH‐mediated pathway on Pd(2 1 1). Since COOH has been identified as a precursor to CO, a site‐poisoning species, a lower coverage of CO at the defect regions will lead to a higher activity for the corresponding nanocrystal catalysts, containing those defect regions.

Iridium nanoparticles have only been reported with roughly spherical shapes and sizes of 1-5 nm, making it impossible to investigate their facet-dependent catalytic properties. Here we report for the first time a simple method based on seed-mediated growth for the facile synthesis of Ir nanocrystals with well-controlled facets. The essence of this approach is to coat an ultrathin conformal shell of Ir on a Pd seed with a well-defined shape at a relatively high temperature to ensure fast surface diffusion. In this way, the facets on the initial Pd seed are faithfully replicated in the resultant Pd@Ir core-shell nanocrystal. With 6 nm Pd cubes and octahedra encased by {100} and {111} facets, respectively, as the seeds, we have successfully generated Pd@Ir cubes and octahedra covered by Ir{100} and Ir{111} facets. The Pd@Ir cubes showed higher H2 selectivity (31.8% vs 8.9%) toward the decomposition of hydrazine compared with Pd@Ir octahedra with roughly the same size.

We report the use of seed-mediated growth as a simple and versatile approach to the synthesis of penta-twinned Cu nanorods with uniform diameters and controllable aspect ratios. The success of this approach relies on our recently demonstrated synthesis of Pd decahedra as uniform samples, together with tightly controlled sizes in the range of 6-20 nm. When employed as a seed, the Pd decahedron can direct the heterogeneous nucleation and growth of Cu along the five-fold axis to produce a nanorod with a uniform diameter defined by the lateral dimension of the original seed.Due to a large mismatch in lattice constant between Cu and Pd (7.1%), the deposited Cu is forced to grow only along one side of the Pd decahedral seed, generating a nanorod with an asymmetric distribution of Cu, with the Pd seed being situated at one of the two ends. According to their extinction spectra, the as-obtained Cu nanorods could form a stable colloidal suspension in water and be stored in a capped vial under the ambient conditions for at least 6 months without noticeable degradation. This excellent stability allows us to systematically investigate the sizedependent surface plasmon resonance properties of the penta-twinned Cu nanorods. With their transverse modes being positioned at 560 nm, their longitudinal modes can be readily tuned from the visible to the near-infrared region by controlling their aspect ratios.