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.

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

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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.

enhanced catalytic activities toward various structure-sensitive reactions. [3] In the case of Pt, for example, its activity toward oxygen reduction can be enhanced by five times by switching from cubic nanocrystals enclosed by {100} facets to the octahedral counterparts covered by {111} facets. [4] This and many other examples demonstrate that one can increase the figures of merit of metal nanocrystals in catalytic applications by controlling the arrangement of atoms. [5,6] Most of the prior studies, however, only dealt with metal nanocrystals in their native crystal structures without looking into polymorphism, the ability of a solid to crystallize in metastable phases distinct from the native, thermodynamically stable one. [7] Owing to the corresponding changes in surface and electronic structures, phase-controlled synthesis would offer another viable avenue to maneuver the catalytic properties of metal nanocrystals. [6,[8][9][10] Metastable phases have been achieved for a number of metals, including face-centered cubic (fcc) Co, hexagonally close-packed (hcp) -Ag and Au, 4H-Ag and Au, as well as fcc-Ru and hcp-Rh. [10,11] Recently, hcp-Pd was also reported, but phase-controlled synthesis of Pd nanocrystals remains in its infancy. [12] In general, phase control can be achieved through a variety of methods, including kinetically controlled growth, pressure-induced phase transition, and template-directed synthesis. [7] For kinetically controlled growth, the slow reduction kinetics achieved through control over the reaction temperature and reagents help induce unusual atomic stacking in the resultant nanocrystals, but it is difficult to rationally extend this method to different systems. [7] Pressure-induced phase transition relies on the application of high pressure to nanocrystals for the removal of stacking faults and shifting of the crystal structure, but it is mainly of theoretical interest because the nanocrystals revert to their native phases when the pressure is removed. [7] In template-directed synthesis, nanocrystals with a specific crystal structure can serve as seeds to dictate the nucleation and growth of a metal featuring a different crystal structure. [13] Under proper conditions (such as reduction kinetics, temperature, and use of stabilizing ligands), the crystal structure of the seed can be extended to the shell, resulting in the formation of a metastable core-shell nanocrystal. [7,13,14] As a major advantage, template-directed synthesis can be readily 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 an hcp or face-centered cubic (fcc) phase.The polymorphism shows a dependence on the shell thickness, with shells thinner than ≈1.4 nm taking the hcp phase whereas the thicker ones revert to fcc. The injection rate pro...