Search

The article discusses a newly developed approach, which can be employed for the development of various metal-semiconductor core-shell heterostructures. The methodology is shown to be extremely efficient and effective, as it leads to exact morphological control in the heterostructures.

A simple approach is described for the fabrication of cuprous oxide ([Cu.sub.2]O) nanocages and nanoframes possessing an unusual truncated rhombic dodecahedral structure. Various gold nanostructures are encapsulated by nanocages and other metal and semiconductor nanostructures are also encapsulated by using [Cu.sub.2]O nanocages in order to provide enhanced properties and functions.

Metal nanocrystals (NCs) comprising rhodium are heterogeneous catalysts for CO oxidation, NO reduction, hydrogenations, electro-oxidations, and hydroformylation reactions. It has been demonstrated that control of structure at the nanoscale can enhance the performance of a heterogeneous metal catalyst, such as Rh, but molecular-level control of NCs comprising this metal is less studied compared to gold, silver, platinum, and palladium. We report an iodide-mediated epitaxial overgrowth of Rh by using the surfaces of well-defined foreign metal crystals as substrates to direct the Rh surface structures. The epigrowth can be accomplished on different sizes, morphologies, and identities of metal substrates. The surface structures of the resulting bimetallic NCs were studied using electron microscopy, and their distinct catalytic behaviors were examined in CO stripping and the electro-oxidation of formic acid. Iodide was found to play a crucial role in the overgrowth mechanism. With the addition of iodide, the Rh epigrowth can even be achieved on gold substrates despite the rather large lattice mismatch of ~7%. Hollow Rh nanostructures have also been generated by selective etching of the core substrates. The new role of iodide in the overgrowth and the high level of control for Rh could hold the key to future nanoscale control of this important metal's architecture for use in heterogeneous catalysis. [ABSTRACT FROM AUTHOR]

A general synthetic strategy for yolk-shell nanocrystal@ZIF-8 nanostructures has been developed. The yolk-shell nanostructures possess the functions of nanoparticle cores, microporous shells, and a cavity in between, which offer great potential in heterogeneous catalysis. The synthetic strategy involved first coating the nanocrystal cores with a layer of Cu2O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu2O surface assists in the formation of the ZIF-8 coating layer and is etched off spontaneously and simultaneously during this process. The yolk-shell nanostructures were characterized by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and nitrogen adsorption. To study the catalytic behavior, hydrogenations of ethylene, cyclohexene, and cyclooctene as model reactions were carried out over the Pd@ZIF-8 catalysts. The microporous ZIF-8 shell provides excellent molecular-size selectivity. The results show high activity for the ethylene and cyclohexene hydrogenations but not in the cyclooctene hydrogenation. Different activation energies for cyclohexene hydrogenation were obtained for nanostructures with and without the cavity in between the core and the shell. This demonstrates the importance of controlling the cavity because of its influence on the catalysis. [ABSTRACT FROM AUTHOR]

We report highly facet-dependent electrical properties of Cu2O nanocubes and octahedra and significant enhancement of gold nanocrystal cores to the electrical conductivity of Au-Cu2O core-shell octahedra. Cu2O nanocubes and octahedra and Au-Cu2O coreshell cubes and octahedra were synthesized by following our reported facile procedures at room temperature. Two oxide-free tungsten probes attached to a nanomanipulator installed inside a scanning electron microscope made contacts to a single Cu2O nanocrystal for the I-V measurements. Pristine Cu2O octahedra bounded by 1111 ) facets are 1100 times more conductive than pristine Cu2O cubes enclosed by ( 100) faces, which are barely conductive. Core-shell cubes are only slightly more conductive than pristine cubes. A 10000-fold increase in conductivity over a cube has been recorded for an octahedron. Remarkably, core-shell octahedra are far more conductive than pristine octahedra. The same facet-dependent electrical behavior can still be observed on a single nanocrystal exposing both (iii) and (100) facets. This new fundamental property may be observable in other semiconductor nanocrystals. We also have shown that both core-shell cubes and octahedra outperform pristine cubes and octahedra in the photodegradation of methyl orange. Efficient photoinduced charge separation is attributed to this enhanced photocatalytic activity. Interestingly, facet-selective etching occurred over the ( 100) corners of some octahedra and core-shell octahedra during photocatalysis. The successful preparation of Au-Cu2O core-shell heterostructures with precise shape control has offered opportunities to discover new and exciting physical and chemical properties of nanocrystals. [ABSTRACT FROM AUTHOR]

Abstract: Formation of metal-semiconductor core-shell heterostructures with precise morphological control of both components remains challenging. Heterojunctions, rather than core-shell structures, were typically produced for metal-semiconductor composites. Furthermore, growth of semiconductor shells with systematic shape evolution using the same metal particle cores can also present a significant challenge. Here, we have synthesized Au-Cu2O core-shell heterostructures using gold nanoplates, nanorods, octahedra, and highly faceted nanoparticles as the structure-directing cores for the overgrowth of Cu2O shells by a facile aqueous solution approach. The gold nanoparticle cores guide the growth of Cu2O shells with morphological and orientation control. Systematic shape evolution of the shells can be easily achieved by simply adjusting the volume of reductant added. For example, truncated cubic to octahedral Cu2O shells were produced from octahedral gold nanocrystal cores. Unusual truncated stellated icosahedral and star column structures have also been synthesized. The heterostructures were found to be formed via an unusual hollow-shell-refilled growth mechanism not reported before. The approach has potential toward the preparation of other complex Cu2O structures with well-defined facets. [ABSTRACT FROM AUTHOR]

Abstract: We report a simple approach for the fabrication of cuprous oxide (Cu2O) nanocages and nanoframes possessing an unusual truncated rhombic dodecahedral structure. An aqueous solution containing CuCl2, sodium dodecyl sulfate (SDS) surfactant, NH2OH•HCI reductant, HCI, and NaOH was prepared, with the reagents introduced in the order listed. Rapid seed-particle aggregation and surface reconstruction of the intermediate structure resulted in the growth of type-I nanoframes, which have only (110) skeleton faces and empty (100) faces, 45 mm after mixing the reagents. Continued crystal growth for additional 75 mm produced nanocages with filled (100) faces. The nanocages have diameters of 350-400 nm, and their walls are thicker than those of the nanoframes. Selective acidic etching over the (110) faces of the nanocages by HCI via the addition of ethanol followed by sonication of the solution led to the formation of type-Il nanoframes, which have elliptical pores on the (110) faces. The morphologies of these nanoframes were carefully examined by electron microscopy. Without addition of ethanol, random etching of the nanocages can occur at a slow rate. Octahedral gold nanocrystals and high-aspect-ratio gold nanorods were successfully encapsulated in the interiors of these Cu20 nanocages by adding the gold nanostructures into the reaction solution. The formation process for such core-cage composite structures was studied. These composite materials should display interesting properties and functions. [ABSTRACT FROM AUTHOR]