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.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

A novel flower-like Bi2WO6 superstructure was successfully realized by a facile hydrothermal process without any surfactant or template. Based on the evolution of this morphology as a function of hydrothermal time, the formation mechanism was proposed to be as follows: (1) self-aggregation of nanoparticles; (2) formation of crystalline nanoplates by Ostwald ripening; and (3) organization of the in situ-formed nanoplates into spherical superstructures. The pretty flower-like superstructure of Bi2WO6 was retained after calcination at 550 °C for 4 h. Both the uncalcined and calcined Bi2WO6 exhibited excellent visible-light-driven photocatalytic efficiencies for the degradation of Rhodamine B (RhB), up to 84 and 97% within 60 minutes, respectively, which were much higher than those of TiO2 (P-25) and Bi2WO6 sample prepared by solid-state reaction (SSR-Bi2WO6). Close investigation indicated that plenty of pores with different sizes existed in the Bi2WO6 superstructures, which could serve as hierarchical transport paths for small molecules and might greatly improve their photocatalytic activities.

Fabrication of flower-like Bi2WO6 superstructures as high performance visible-light driven photocatalysts

Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.

Adaptive Supramolecular Nanomaterials Based on Strong Noncovalent Interactions

Nanokristalle sind von grundlegender Bedeutung in der modernen Wissenschaft und Technologie. Die Eigenschaften eines Nanokristalls lassen sich uber seine Form beeinflussen und auf eine bestimmte Anwendung hin zuschneiden. Ziel dieses Aufsatzes ist es, einen umfassenden Uberblick uber den aktuellen Stand der Forschung auf dem Gebiet der formkontrollierten Synthese von Metallnanokristallen zu geben. Nach einer kurzen Einleitung uber die Prinzipien von Kristallkeimbildung und -wachstum diskutieren wir einzelne Arten von Nanokristallformen und betrachten verschiedene experimentelle Parameter, uber die die Keimbildung und das Wachstum von Metallnanokristallen in der Losungssynthese gezielt manipuliert werden kann. Wir erlautern diese Methoden anhand von Beispielen, wo zumindest ein Grundverstandnis fur die beobachtete Formkontrolle vorhanden ist. Schlieslich stellen wir Anwendungen vor, bei denen die Formkontrolle von Metallnanokristallen ein entscheidender Faktor ist, und beenden den Aufsatz mit einem personlichen Ausblick auf kunftig zu erwartende Entwicklungen auf diesem Gebiet.

Formkontrolle bei der Synthese von Metallnanokristallen: einfache Chemie, komplexe Physik?

In this work, a facile route using a simple solvothermal reaction and sequential calcinations to synthesize porous α-Fe2O3 flower-like nanostructures without employing templates or matrices for self-assembly is presented. The flower-like nanostructures are composed of nanosheets with a thickness of about 20 nm, which are characterized by field-emission scanning electron microscopy (FESEM). Influencing factors such as the dosage of reactants and the solvents are systematically investigated. A possible formation mechanism for the flower-like nanostructure is proposed. A BET test shows that the product is porous and has a large surface area. The electrochemical, magnetic, and photocatalytic properties of the as-obtained α-Fe2O3 3D nanostructure are systematically investigated. The result shows that these properties are greatly affected by the porous structure.

Facile Route for the Fabrication of Porous Hematite Nanoflowers: Its Synthesis, Growth Mechanism, Application in the Lithium Ion Battery, and Magnetic and Photocatalytic Properties

Reduction of aqueous RhCl3 with NaBH4 in the presence of poly(vinyl pyrrolidone) (PVP) yields dense spherical nanostructures. The spherical aggregates, which generally have diameters between 10 and 100 nm, are built from smaller 1−3 nm Rh particles. The dense nanostructures are thermally stable beyond 100 °C, and they have a tendency to form ordered superstructures upon drying. Combining sodium n-dodecyl sulfate (SDS) with PVP modifies the size and morphology of the primary 1−3 nm particles, but does not change the spherical shape of the aggregates except at high concentrations of SDS. Small-angle X-ray scattering measurements show that the large aggregates are formed directly in solution from small Rh particles, consistent with TEM and AFM results. Magnetic measurements indicate that the Rh nanoparticle aggregates are Pauli paramagnetic.

Chil Hung Cheng

Papers

Spontaneous Hierarchical Assembly of Rhodium Nanoparticles into Spherical Aggregates and Superlattices