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.

This Review offers a comprehensive review of the colloidal synthesis, mechanistic understanding, physicochemical properties, and applications of onedimensional (1D) metal nanostructures. After a brief introduction to the different types of 1D nanostructures, we discuss major concepts and methods typically involved in a colloidal synthesis of 1D metal nanostructures, as well as the current mechanistic understanding of how the nanostructures are formed. We then highlight how experimental studies and computational simulations have expanded our knowledge of how and why 1D metal nanostructures grow. Following specific examples of syntheses for monometallic, multimetallic, and heterostructured systems, we showcase how the unique structure− property relationships of 1D metal nanostructures have enabled a broad spectrum of applications, including sensing, imaging, plasmonics, photonics, display, thermal management, and catalysis. Throughout our discussion, we also offer perspectives with regard to the future directions of development for this class of nanomaterials.

Ruthenium nanocrystals with both a face-centered cubic (fcc) structure and well-controlled facets are attractive catalytic materials for various reactions. Here we report a simple method for the synthesis of Ru octahedral nanocrystals with an fcc structure and an edge length of 9 nm. The success of this synthesis relies on the use of 4.5 nm Rh cubes as seeds to facilitate the heterogeneous nucleation and overgrowth of Ru atoms. We choose Rh because it can resist oxidative etching under the harsh conditions for Ru overgrowth, it can be readily prepared as nanocubes with edge lengths less than 5 nm, and its atoms have a size close to that of Ru atoms. During the seed-mediated growth, the atomic packing of Ru overlayers follows an fcc lattice, in contrast to the conventional hexagonal close-packed (hcp) lattice associated with bulk Ru. The final product takes an octahedral shape, with the surface enclosed by {111} facets. Our in situ measurements suggest that both the octahedral shape and the fcc crystal structure can be well preserved up to 400 °C, which is more than 100 °C higher than what was reported for Ru octahedral nanocages. When utilized as catalysts, the Ru octahedral nanocrystals exhibited 4.4-fold enhancement in terms of specific activity toward oxygen evolution relative to hcp-Ru nanoparticles. We also demonstrate that Ru{111} facets are more active than Ru{100} facets in catalyzing the oxygen evolution reaction. Altogether, this work offers an effective method for the synthesis of Ru nanocrystals with an fcc structure and well-defined {111} facets, as well as enhanced thermal stability and catalytic activity. We believe these nanocrystals will find use in various catalytic applications.

Copper nanostructures are promising catalysts for the electrochemical reduction of CO 2 because of their unique ability to produce alarge proportion of multi-carbon products. Despite great progress,t he selectivity and stability of such catalysts still need to be substantially improved. Here,w e demonstrate that controlling the surface oxidation of Cu nanowires (CuNWs) can greatly improve their C 2+ selectivity and stability.S pecifically,w ea chieve af aradaic efficiency as high as 57.7 and 52.0 %f or ethylene when the CuNWs are oxidized by the O 2 from air and aqueous H 2 O 2 ,r espectively, and both of them showh ydrogen selectivity below1 2%.T he high yields of C 2+ products can be mainly attributed to the increase in surface roughness and the generation of defects and cavities during the electrochemical reduction of the oxide layer. Our results also indicate that the formation of arelatively thick, smooth oxide sheath can improve the catalytic stability by mitigating the fragmentation issue.

Despite extensive efforts devoted to the synthesis of Pt−Co bimetallic nanocrystals for fuel cell and related applications, it remains a challenge to simultaneously control atomic arrangements in the bulk and on the surface. Here we report a synthesis of Pt−Co@Pt octahedral nanocrystals that feature an intermetallic, face-centered tetragonal Pt−Co core and an ultrathin Pt shell, together with the dominance of {111} facets on the surface. When evaluated as a catalyst toward the oxygen reduction reaction (ORR), the nanocrystals delivered a mass activity of 2.82 A mg −1 and a specific activity of 9.16 mA cm −2 , which were enhanced by 13.4 and 29.5 times, respectively, relative to the values of a commercial Pt/C catalyst. More significantly, the mass activity of the nanocrystals only dropped 21% after undergoing 30 000 cycles of accelerated durability test, promising an outstanding catalyst with optimal performance for ORR and related reactions.

We report ah ighly active and durable water oxidation electrocatalyst based on cubic nanocages with ac omposition of Ir 44 Pd 10 ,t ogether with well-defined {100} facets and porous walls of roughly 1.1 nm in thickness.S uch nanocages substantially outperform all the water oxidation electrocatalysts reported in literature,w ith an overpotential of only 226 mV for reaching 10 mA cm À2 geo at al oading of Ir as low as 12.5 mg Ir cm À2 on the electrode in acidic media. When benchmarked against ac ommercial Ir/C electrocatalyst at 250 mV of overpotential, such an anocage-based catalyst not only shows enhancements (18.1-and 26.2-fold, respectively) in terms of mass (1.99 Amg À1 Ir )a nd specific (3.93 mA cm À2 Ir ) activities,b ut also greatly enhanced durability.T he enhancements can be attributed to ac ombination of multiple merits, including ahigh utilization efficiency of Ir atoms and an open structure beneficial to the electrochemical oxidation of Ir to the active form of IrO x .Water oxidation plays ac ritical role in many promising energy conversion devices,s uch as water electrolyzers and metal-air batteries,b ut its slow kinetics has greatly hindered commercialization of these devices. [1] Especially,i th as been am ajor challenge to develop an efficient, cost-effective,a nd long-lived water oxidation electrocatalyst for the acidic medium that is preferred for water electrolysis. [2] At the moment, Ir, an extremely scarce element in earthsc rust, is known to be ak ey component of the most effective water oxidation electrocatalyst because of its optimal balance in terms of activity and stability in an acidic medium. [3] To make the Ir-based electrocatalysts cost-effective,one has to ensure ah igh utilization efficiency (UE) for the Ir atoms.R educing the particle size has been ac ommonly used strategy for increasing the UE of Ir atoms due to the increased proportion of atoms exposed on the surface.A saresult, most of the Irbased water oxidation electrocatalysts are typically based on ultrafine particles with as ize of only af ew nanometers,w ith the exact forms varying from IrO x clusters to IrO x nanoparticles with irregular shapes,a sw ell as Ir-M (M = Ni, Co, Fe,o racombination of them) alloys and core-shell nanostructures. [4] In an effort to enhance the activity,s uch an approach, although extensively practiced, is not ideal for the elucidation of the relationship between electrocatalytic properties and the composition or surface structure of the catalytic particles.Moreover,such nanoparticles are prone to aggregation, dissolution, and/or detachment from the support during operation, leading to poor durability.Noble-metal nanocages are advantageous for the development of highly efficient and cost-effective electrocatalysts because of their ultrathin wall thickness (typically,afew atomic layers), ahighly open structure,and thus substantially increased UE of atoms.W hent he wall thickness is reduced down to four atomic layers without breaking the cage structure,t he UE can be as high as 50 %b ym aking full...

A simple strategy for developing a cost-effective and efficient Irbased catalyst toward the oxygen evolution reaction (OER) is to construct a core− shell structure with most of the Ir atoms serving as reactive sites on the surface. However, it has been challenging to achieve a precise control over the thickness of the Ir shell from one to several atomic layers and thus optimize the OER performance. Here, we report a facile synthesis of Pd@Ir nL (n: the number of Ir atomic layers) core−shell nanocubes with the shell thickness controlled from one to four atomic layers. Their OER activities showed a volcano-type dependence on the number of Ir atomic layers, with a maximum point corresponding to n = 3, which can be attributed to Pd−Ir intermixing, and possible ligand and/or strain effects. Owing to the better passivation for the Pd cores and the formation of a more stable phase during electrolysis, the Pd@Ir nL nanocubes with thicker Ir overlayers exhibited greater OER durability. The Pd@Ir 3L nanocubes delivered the best activity and durability toward OER with η as low as 245 mV at 10 mA•cm geo −2 and a mass activity of 3.33 A•mg Ir −1 at η = 300 mV. Both values were much better than those of commercial Ir/C and represent the best set of data among the Ir-based core−shell OER catalysts in acidic media.

Bimetallic nanocrystals often outperform their monometallic counterparts in catalysis as a result of the electronic coupling and geometric effect arising from two different metals. Here we report a facile synthesis of Pd–Cu Janus nanocrystals with controlled shapes through site-selected growth by reducing the Cu­(II) precursor with glucose in the presence of hexadecylamine and Pd icosahedral seeds. Specifically, at a slow reduction rate, the Cu atoms nucleate and grow from one vertex of the icosahedral seed to form a penta-twinned Janus nanocrystal in the shape of a pentagonal bipyramid or decahedron. At a fast reduction rate, in contrast, the Cu atoms can directly nucleate from or diffuse to the edge of the icosahedral seed for the generation of a singly twinned Janus nanocrystal in the shape of a truncated bitetrahedron. The segregation of two elements and the presence of twin boundaries on the surface make the Pd–Cu Janus nanocrystals effective catalysts for the electrochemical reduction of CO2. An onset potential as low as −0.7 VRHE (RHE: reversible hydrogen electrode) was achieved for C2+ products in 0.5 M KHCO3 solution, together with a faradaic efficiency approaching 51.0% at −1.0 VRHE. Density functional theory and Pourbaix phase diagram studies demonstrated that the high CO coverage on the Pd sites (either metallic or hydride form) during electrocatalysis enabled the spillover of CO to the Cu sites toward subsequent C–C coupling, promoting the formation of C2+ species. This work offers insights for the rational fabrication of bimetallic nanocrystals featuring desired compositions, shapes, and twin structures for catalytic applications.