Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our ...aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.
Far‐field scattering of randomly deposited Au nanoparticles (NPs) is demonstrated as a physically unclonable optical function for anti‐counterfeit applications in which the scattering patterns are ...easily produced yet impractical to replicate. Colloidal metal NPs are superb components for nanoscale labels owing to their small dimensions and intense far‐field scattering visible at wavelengths that depend on colloidal size, shape, composition, and their local environment. The feasibility of Au NP depositions as nanofingerprints is presented using a simple pattern matching algorithm. These NPs offer extended functionality as environmental sensors. Taking advantage of the local refractive index dependent scattering wavelengths of metal NPs, a detectable color change is also demonstrated from a nanofingerprint comprised of Au and Ag NPs when placed in media with different refractive index. The facile deposition method coupled with the intense scattering and optical response of metal NPs provides physically unclonable tags (nanofingerprints) with the ability to serve as tamper‐evident and aging labels.
An optical, physically unclonable function using a random distribution of Au nanoparticles is presented, with the additional functionality of refractive index based environmental sensing using both Au and Ag nanoparticles. This system provides facile fabrication of anti‐counterfeit labels, with total dimensions on the microscale for applications in pharmaceutical and electronic counterfeit prevention.
There are few methods yielding oxynitride crystals with defined shape, yet shape‐controlled crystals often give enhanced photoactivity. Herein, single‐crystalline SrTaO2N nanoplates and polyhedra are ...achieved selectively. Central to these synthetic advances is the crystallization pathways used, in which single‐crystalline SrTaO2N nanoplates form by topotactic nitridation of aerosol‐prepared Sr2Ta2O7 nanoplates and SrTaO2N polyhedra form by flux‐assisted nitridation of the nanoplates. Evaluation of these materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) showed improved performance for the SrTaO2N nanoplates, with a record apparent quantum efficiency (AQE) of 6.1 % for OER compared to the polyhedra (AQE: 1.6 %) and SrTaO2N polycrystals (AQE: 0.6 %). The enhanced performance from the nanoplates arises from their morphology and lower defect density. These results highlight the importance of developing new synthetic routes to high quality oxynitrides.
Keep in shape: Topotactic nitridation of aerosol‐prepared Sr2Ta2O7 nanoplates gives single‐crystalline SrTaO2N nanoplates. They exhibit higher photoactivity for water oxidation: four‐ and ten‐times more than SrTaO2N polyhedra prepared by flux‐assisted nitridation and a SrTaO2N polycrystals reference, respectively. The high photoactivity of the nanoplates arises from their morphology and lower defect density.
Eloquent routes to colloidal metal nanostructures have emerged in recent years, and a central component to any successful nanosynthesis is the initial selection of metal complexes with an appropriate ...ligand environment. This local ligand environment may be predetermined by the coordination complex selected as the metal precursor; however, recent studies reveal that the ligand environment of coordination complexes can be modified through exchange with other components for the synthesis that include solvent molecules, capping agents, anions, and even reducing agents. Importantly, ligands can often play multiple roles in a synthesis and direct the outcome by manipulating the rates of precursor reduction and particle coalescence, providing colloidal and facet stabilization and even serving as reducing agents themselves. This Feature Article highlights examples in which the ligand environments of metal precursors and nanoparticles contribute to product formation in multiple ways. Acknowledgment of the dual roles of ligands in nanomaterial synthesis will enable new strategies for nanostructures by decoupling the often contradictory roles of ligands.
Under the irradiation of light, the free electrons in a plasmonic nanoparticle are driven by the alternating electric field to collectively oscillate at a resonant frequency in a phenomenon known as ...surface plasmon resonance. Both calculations and measurements have shown that the frequency and amplitude of the resonance are sensitive to particle shape, which determines how the free electrons are polarized and distributed on the surface. As a result, controlling the shape of a plasmonic nanoparticle represents the most powerful means of tailoring and fine-tuning its optical resonance properties. In a solution-phase synthesis, the shape displayed by a nanoparticle is determined by the crystalline structure of the initial seed produced and the interaction of different seed facets with capping agents. Using polyol synthesis as a typical example, we illustrate how oxidative etching and kinetic control can be employed to manipulate the shapes and optical responses of plasmonic nanoparticles made of either Ag or Pd. We conclude by highlighting a few fundamental studies and applications enabled by plasmonic nanoparticles having well-defined and controllable shapes.
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