Colloidal nanocrystals are nanometer-sized, solution-grown inorganic particles stabilized by a layer of surfactants attached to their surface. The inorganic cores exhibit useful properties controlled by composition as well as size and shape, while the surfactant coating ensures that these structures are easy to fabricate and process. It is this combination of features that makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. But their full potential can only be exploited if we achieve exquisite control over their composition, size, shape, crystal structure and surface properties. Here we review what is known about nanocrystal growth and outline strategies for controlling it.

Colloidal nanocrystal synthesis and the organic-inorganic interface

Achieving mastery over the synthesis of metal nanocrystals has emerged as one of the foremost scientific endeavors in recent years. This intense interest stems from the fact that the composition, size, and shape of nanocrystals not only define their overall physicochemical properties but also determine their effectiveness in technologically important applications. Our aim is to present a comprehensive review of recent research activities on bimetallic nanocrystals. We begin with a brief introduction to the architectural diversity of bimetallic nanocrystals, followed by discussion of the various synthetic techniques necessary for controlling the elemental ratio and spatial arrangement. We have selected key examples from the literature that exemplify critical concepts and place a special emphasis on mechanistic understanding. We then discuss the composition-dependent properties of bimetallic nanocrystals in terms of catalysis, optics, and magnetism and conclude the Review by highlighting applications that h...

Bimetallic Nanocrystals: Syntheses, Properties, and Applications

This Perspective provides a contemporary understanding of the shape evolution of colloidal metal nanocrystals under thermodynamically and kinetically controlled conditions. It has been extremely challenging to investigate this subject in the setting of one-pot synthesis because both the type and number of seeds involved would be changed whenever the experimental conditions are altered, making it essentially impossible to draw conclusions when comparing the outcomes of two syntheses conducted under different conditions. Because of the uncertainty about seeds, most of the mechanistic insights reported in literature for one-pot syntheses of metal nanocrystals with different shapes are either incomplete or ambiguous, and some of them might be misleading or even wrong. Recently, with the use of well-defined seeds for such syntheses, it became possible to separate growth from nucleation and therefore investigate the explicit role(s) played by a specific thermodynamic or kinetic parameter in directing the evolution of colloidal metal nanocrystals into a specific shape. Starting from single-crystal seeds enclosed by a mix of {100}, {111}, and {110} facets, for example, one can obtain colloidal nanocrystals with diversified shapes by adjusting various thermodynamic or kinetic parameters. The mechanistic insights learnt from these studies can also be extended to account for the products of conventional one-pot syntheses that involve self-nucleation only. The knowledge can be further applied to many other types of seeds with twin defects or stacking faults, making it an exciting time to design and synthesize colloidal metal nanocrystals with the shapes sought for a variety of fundamental studies and technologically important applications.

Shape-Controlled Synthesis of Colloidal Metal Nanocrystals: Thermodynamic versus Kinetic Products

In this work we have carried out systematic studies and identified the critical role of hydrogen peroxide instead of the generally believed citrate in the well-known chemical reduction route to silver nanoplates. This improved understanding allows us to develop consistently reproducible processes for the synthesis of nanoplates with high efficiency and yields. By harnessing the oxidative power of H(2)O(2), various silver sources including silver salts and metallic silver can be directly converted to nanoplates with the assistance of an appropriate capping ligand, thus significantly enhancing the reproducibility of the synthesis. Contrary to the previous conclusion that citrate is the key component, we have determined that the group of ligands with selective adhesion to Ag (111) facets can be expanded to many di- and tricarboxylate compounds whose two nearest carboxylate groups are separated by two or three carbon atoms. We have also found that the widely used secondary ligand polyvinylpyrrolidone can be replaced by many hydroxyl group-containing compounds or even removed entirely while still producing nanoplates of excellent uniformity and stability. In addition to the general understanding of NaBH(4) as a reducing agent, it has also been found to act as a capping agent to stabilize the silver nanoparticles, prolong the initiation time required for nanoplate nucleation, and contribute to the control of the thickness as well as the aspect ratio of silver nanoplates. The improved insight into the specific roles of the reaction components and significantly enhanced reproducibility are expected to help elucidate the formation mechanism of this interesting nanostructure.

http://pubs.acs.org/doi/pdf/10.1021/ja2080345

A systematic study of the synthesis of silver nanoplates: is citrate a "magic" reagent?

Platinum nanoparticles are widely used as the primary catalysts in a myriad of industrial processes such as CO/NOx oxidation in catalytic converters, nitric acid production, petroleum cracking, as well as hydrogen (or alcohol) oxidation and oxygen reduction reactions in fuel-cell technology. For most catalytic reactions, it has been shown that high-index planes, which are associated with large numbers of atomic steps, edges, and kinks hold the key to the enhancement of catalytic performance in terms of activity and/or selectivity. A number of protocols have been demonstrated for generating Pt nanoparticles enclosed by high-index facets, including those based on electrochemical reduction and heat treatment. For example, Sun and co-workers have reported the synthesis of tetrahexahedral (THH) Pt nanocrystals with high-index facets, such as {730}, {210}, and {520}, by applying a square-wave potential to polycrystalline Pt microspheres supported on a glassy carbon electrode. Although these Pt nanocrystals have been shown to have high catalytic activity, their sizes are still relatively too large and the method of preparation is rather limited in terms of production volume. It still remains a challenge to produce Pt nanocrystals with high-index facets by using a simple, scalable route based on wet chemical reduction. Over the past several years, kinetic control has been demonstrated as a simple and versatile approach to the shapecontrolled synthesis of noble-metal nanocrystals in the solution phase. In general, kinetic control can be achieved by: 1) substantially slowing down the formation rate of atoms, 2) using a weak reducing agent, 3) introducing an oxidation process, and 4) taking advantage of Ostwald ripening. When the concentration of metal atoms in the solution is low, the atoms tend to add to the edges and corners of a seed rather than the entire surface, thus leading to the formation of nanocrystals with thermodynamically unfavorable morphologies, including rods, plates, multipods, and dendritic structures. In recent years, nanocrystals with concave rather than flat faces have attracted attention because of their high-index facets. To this end, Zheng and co-workers have demonstrated the synthesis of concave Pd polyhedral nanocrystals with high electrocatalytic activity for formic acid oxidation. Mirkin and co-workers have also reported the synthesis of concave cubic Au nanocrystals, and demonstrated higher chemical activity compared to octahedra enclosed by lowindex {111} facets. Herein we report the first synthesis of Pt concave nanocubes enclosed by high-index facets including {510}, {720}, and {830} by slowly adding an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 into deionized water by using two syringe pumps. In this synthesis, the formation of a Pt pyrophosphato complex (that is formed by mixing K2PtCl4 and Na2H2P2O7) and the slow addition of this precursor by a syringe pump are believed to play a key role in the formation of Pt concave nanocubes. In this case, the seeds selectively overgrow from corners and edges, and the Br ion serves as a capping agent to block the growth of the h100i axis. The Pt concave nanocubes exhibited substantially enhanced specific activity (per unit surface area) relative to those of Pt nanocubes, cuboctahedra, and commercial Pt/C catalysts that are bounded by low-index facets such as {100} and {111} toward the oxygen reduction reaction (ORR), which is the ratedetermining step in a proton-exchangemembrane (PCM) fuel cell. In a typical synthesis, an aqueous NaBH4 solution and a mixture containing K2PtCl4, KBr, and Na2H2P2O7 were prepared separately and then injected simultaneously at an injection rate of 67 mLmin 1 by using two syringe pumps into deionized water maintained at 95 8C. The color of the solution immediately turned from light pink to black upon the addition of the reactant solutions, thus indicating rapid reduction of PtCl4 2 into elemental Pt by NaBH4. Figure 1a shows a typical transmission electron microscopy (TEM) image of the product that contains Pt nanocubes with a concave structure. [*] Dr. T. Yu, D. Y. Kim, Prof. H. Zhang, Prof. Y. Xia Department of Biomedical Engineering Washington University Saint Louis, MO 63130 (USA) E-mail: xia@biomed.wustl.edu D. Y. Kim Department of Chemical and Biomolecular Engineering (BK21 graduate program) Korea Advanced Institute of Science and Technology (KAIST) 335 Gwahangro, Yuseong-gu, Daejeon 305-701 (Korea)

Platinum concave nanocubes with high-index facets and their enhanced activity for oxygen reduction reaction

The world on a plate: In addition to their role as a capping agent that selectively binds to the {l_brace}111{r_brace} faces of silver, the carboxylate group can coordinate with Ag+ ions to form complexes and thus substantially reduce the reduction rate of Ag+ ions, leading to the formation of silver seeds with both twin planes and stacking faults.

A mechanistic study on the formation of silver nanoplates in the presence of silver seeds and citric acid or citrate ions.

The applications of nanostructures critically depend on their morphologies. Although significant progress has been made in the chemical synthesis of nanostructures with a variety of different morphologies, it is still highly desired to develop an approach that allows one to quickly identify the best set of parameters for nanostructure syntheses. Herein, an on-chip approach to the rapid screening of experimental conditions pivotal to the production of nanostructures with different morphologies is reported. The key component of this approach is an array of reactors containing solutions with a one- or two-dimensional gradient in reagent concentration, pH value, or reaction temperature. In the proof-of-concept experiments, the parameters needed for the production of Au and Pd nanostructures with various morphologies are quickly identified. In principle, this approach can be extended to other systems for rapid screening and optimization of experimental conditions involved in the syntheses of different types of nanostructures.

On-chip screening of experimental conditions for the synthesis of noble-metal nanostructures with different morphologies

T he applications of nanostructures critically depend on their morphologies. Although signifi cant progress has been made in the chemical synthesis of nanostructures with a variety of different morphologies, it is still highly desired to develop an approach that allows one to quickly identify the best set of parameters for nanostructure syntheses. Herein, an on-chip approach to the rapid screening of experimental conditions pivotal to the production of nanostructures with different morphologies is reported. The key component of this approach is an array of reactors containing solutions with a one- or two-dimensional gradient in reagent concentration, pH value, or reaction temperature. In the proof-of-concept experiments, the parameters needed for the production of Au and Pd nanostructures with various morphologies are quickly identifi ed. In principle, this approach can be extended to other systems for rapid screening and optimization of experimental conditions involved in the syntheses of different types of nanostructures.

/pdf/on-chip-screening-of-experimental-conditions-for-the-biwbbn8k52.pdf

On-Chip Screening of Experimental Conditions for the Synthesis of Noble-Metal Nanostructures with Different

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A mechanistic study on the formation of silver nanoplates in the presence of silver seeds and citric acid or citrate ions.

On-chip screening of experimental conditions for the synthesis of noble-metal nanostructures with different morphologies

On-Chip Screening of Experimental Conditions for the Synthesis of Noble-Metal Nanostructures with Different