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 this work, self-assembly monolayers of faceted gold nanocrystals were employed as surface-enhanced Raman scattering (SERS) substrates for the examination of facet-dependent SERS performance. In particular, cubes, concave cubes, octahedra, and rhombic dodecahedra with three sizes have been synthesized via a seed-mediated growth method. All 12 samples with high purity and size uniformity were assembled into densely arranged monolayers by a robust liquid/liquid interface self-assembly technique, and their localized surface plasmon resonance properties were comparatively investigated. Furthermore, the SERS activity of these monolayers on a Si wafer was analyzed, and rhombic dodecahedra were found to possess the strongest enhancement as excellent SERS substrates. Meanwhile, the simulation of electromagnetic field distribution based on a diverse faceted gold nanoparticle array was performed by FDTD solutions software, which supports our SERS behavior results. Finally, rhombic dodecahedral gold nanocrystals were effectively applied to the analysis of two pesticides in orange juice. As a result, rhombic dodecahedra are expected to serve as highly sensitive SERS substrates for the real-world detection of a variety of molecules.

Metrics & MoreArticle RecommendationsCONSPECTUS: As a metal that can occur in nature in the elemental form, copper (Cu) has been used by humans since ca. 8000 BC. With most properties matching those of Ag and Au, Cu has played a more significant role in commercial applications owing to its much higher (the 25th among all elements) abundance in Earth's crust and thus more affordable price. In addition to its common use as a conductor of heat and electricity, it is a constituent of various metal alloys for hardware, coins, strain gauges, and thermocouples. Bulk Cu is also widely utilized as a building material. When downsized to the nanoscale, Cu and Cu-based structures have found widespread use in applications ranging from electronics to optoelectronics, plasmonics, catalysis, sensing, and biomedicine. Besides Ag and Au, for example, Cu is another metal known for its localized surface plasmon resonance (LSPR) in the visible and nearinfrared regions when prepared as nanocrystals. As a potential replacement for indium−tin oxide (ITO) films, polymer coatings containing Cu nanowires are strong candidates for the fabrication of transparent and flexible electrodes key to touchscreen display and related applications. The commercial catalysts for water−gas shift and gas detoxification reactions are also based on Cu nanoparticles. Most recently, Cu nanocrystals have attracted considerable interest for their superior selectivity toward hydrocarbons and multicarbon species during the electrochemical reduction of CO 2 . The success of all these applications critically depends on our ability to control the shape and surface structure of the nanocrystals. Relative to Ag and Au, it is more challenging to generate Cu-based nanocrystals using colloidal methods due to its lower reduction potential and greater vulnerability to oxidation.In this Account, we discuss recent progress in the colloidal synthesis of Cu nanocrystals with controlled shapes for plasmonic, biomedical, and catalytic applications. With glucose serving as a reducing agent, Cu nanocrystals bearing a twinned or single-crystal structure can be synthesized using an aqueous system with the assistance of hexadecylamine (HDA). In this synthetic protocol, HDA not only passivates the surface to protect the nanocrystals from oxidation but also manipulates the reduction kinetics of Cu(II) precursor through coordination and an increase of solution pH. Typical products include nanocubes and penta-twinned nanowires whose surfaces are dominated by {100} facets. When seeds produced either in situ or ex situ are introduced, Cu-based nanocrystals featuring a singly twinned, core−shell, or Janus structure can be readily synthesized. Aside from segmented structures, Cu-based alloys with various noble metals can be synthesized through coreduction or a galvanic replacement reaction with preformed Cu nanocrystals. By controlling the size and/or shape of Cu nanocrystals, their LSPR peaks can be tuned into the near-infrared region, making them promising candidates for optical imaging con...

The dissolution of a polymeric solid typically starts with the absorption of solvent molecules, followed by swelling and volume expansion. Only when the extent of swelling reaches a threshold can the polymer chains be disentangled and then dissolved into the solvent. When the polymeric solid is encapsulated in a rigid shell, the swelling process will be impeded. Despite the widespread use of this process, it is rarely discussed in the literature how the polymeric solid is dissolved from the core for the generation of colloidal hollow particles. Recent studies have started to shed light on the mechanistic details involved in the formation of hollow particles through a template‐directed process. Depending on the nature of the material used for the template, the removal of the template may involve different mechanisms and pathways, leading to the formation of distinct products. Here, a number of examples are used to illustrate this important phenomenon that is largely neglected in the literature. This article also discusses how the swelling of a polymeric template encapsulated in a rigid shell can be leveraged to fabricate new types of functional colloidal particles.