The successful synthesis of noble-metal nanocrystals with controlled shapes offers many opportunities to not only maneuver their physicochemical properties but also optimize their figures of merit in a wide variety of applications. In particular, heterogeneous catalysis and surface science have benefited enormously from the availability of this new class of nanomaterials as the atomic structure presented on the surface of a nanocrystal is ultimately determined by its geometric shape. The immediate advantages may include significant enhancement in catalytic activity and/or selectivity and substantial reduction in materials cost while providing a well-defined model system for mechanistic study. With a focus on the monometallic system, this review article provides a comprehensive account of recent progress in the development of noble-metal nanocrystals with controlled shapes, in addition to their remarkable performance in a large number of catalytic and electrocatalytic reactions. We hope that this review article offers the impetus and roadmap for the development of next-generation catalysts vital to a broad range of industrial applications.

compounds, core-shell, core-satellite (or core-frame), and Janus nanocrystals. [2,10] Figure 1 shows schematic illustrations of these four structures at an atomic level. Both alloys and intermetallic compounds are formed when two metals are homogeneously mixed at the atomic level. [10] While alloys feature a random distribution of atoms in space, intermetallic compounds take a highly ordered structure with the atoms of each metal precisely positioned at specific sites. The atomic fractions of the two metals are typically limited by a well-defined stoichiometry in intermetallic compounds. When the two metals are spatially segregated, they will form core-shell, -satellite, and -frame structures, as well as bimetallic Janus nanocrystal. [2] Although the metals in all these structures are separated by a heterogeneous interface, bimetallic Janus nanocrystals are particularly interesting for their simplicity associated with a side-by-side, asymmetric arrangement. This interface also supports the charge separation between two metals with different work functions and inner electron transfer, offering a means to maneuver their electronic structures and thus bestowing superior catalytic and photocatalytic capabilities upon the Janus nanocrystals. [11][12][13] Furthermore, the plasmonic coupling associated with the side-by-side elemental distribution of a Janus nanocrystal offers unique optical properties (e.g., bonding and antibonding plasmon modes, as well as Fano resonance profiles) for various applications. [9,[14][15][16][17] The past few decades have witnessed tremendous advances in the colloidal synthesis of metal nanocrystals with well-controlled compositions, sizes, shapes, and structures. [1,2,18] However, the fabrication of bimetallic Janus nanocrystals has been hindered by the difficulties in achieving a side-by-side distribution of two different metals in a single particle. [19] In many cases, alloying may take place during the synthesis, especially when the two metals share similar atomic radii, crystal lattice constants, valencies, and electronegativities. [1] Besides, the nucleation and growth of a different metal from a preformed seed often proceed in a symmetric pattern for the generation of core-satellite, -frame, and -shell structures rather than a Janus structure with reduced symmetry. [19][20][21] To this end, synthetic strategies capable of disrupting the symmetric growth while preventing the formation of alloys are critical to the successful preparation of bimetallic Janus nanocrystals. A good example can be found in the synthesis of Pd-Ag or Pd-Au Janus nanocrystals, in which the asymmetric growth of Ag on preformed Pd cubic seeds could be induced and maintained by manipulating the reduction kinetics. [22][23][24][25] This Review presents recent progress related to the syntheses, properties, and applications of bimetallic Janus nanocrystals.Bimetallic Janus nanocrystals have received considerable interest in recent years owing to their unique properties and niche applications. The sideby-side distri...

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.

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.

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.