Showing papers by "Michael H. Huang published in 2010"

Au Nanocube-Directed Fabrication of Au-Pd Core-Shell Nanocrystals with Tetrahexahedral, Concave Octahedral, and Octahedral Structures and Their Electrocatalytic Activity

Abstract: In this study, we have successfully developed a facile method for the high-yield fabrication of Au-Pd core-shell heterostructures with an unusual tetrahexahedral (THH) morphology using Au nanocubes as the structure-directing cores. The lattice orientations of the Au nanocubes match those of the Pd shells. Structural analysis establishes that the THH nanocrystals are bounded by high-index {730} facets. A substantial lattice mismatch between Au and Pd, oxidative etching in the presence of chloride and oxygen, the use of cetyltrimethylammonium chloride (CTAC) surfactant, and the reaction temperature (30-60 °C) were identified to be key factors facilitating the formation of the THH core-shell nanocrystals. Intermediate products have also been examined to follow the growth process. By selecting cubic gold cores with sizes of 30-70 nm and varying the volume of the gold core solution used, THH Au-Pd core-shell nanocrystals with continuously adjustable sizes from 56 to 124 nm can be readily obtained. Their UV-vis spectra display progressive red-shifted bands. Interestingly, novel concave octahedral and octahedral Au-Pd core-shell nanocrystals can be prepared by lowering the reaction temperature and prolonging the reaction time. The concave octahedra show depressions on all the {111} faces. Electrocatalytic activity of the three Au-Pd core-shell structures for the oxidation of ethanol has been investigated. The THH nanocrystals with entirely high-index {730} facets were found to exhibit the best electrocatalytic activity. These size-tunable THH Au-Pd core-shell nanocrystals may be valuable for catalyzing other organic reactions.

Seed-mediated synthesis of gold nanocrystals with systematic shape evolution from cubic to trisoctahedral and rhombic dodecahedral structures.

Abstract: We report a seed-mediated synthesis method for the preparation of gold nanocrystals with systematic shape evolution from truncated cubic to cubic, trisoctahedral, and rhombic dodecahedral structures in aqueous solution for the first time. Nanocrystals with transitional morphologies were also synthesized. The combination of using cetyltrimethylammonium chloride (CTAC) surfactant and a very small amount of NaBr to control the bromide concentration in the growth solution was found to be critical to the formation of nanocubes. Variation in the volume of ascorbic acid added to the growth solution enabled the fine control of nanocrystal morphology. Nanocubes and rhombic dodecahedra with controlled sizes of 30-75 nm were prepared by adjusting the volume of the seed solution added to the growth solution. They can self-assemble into ordered packing structures on substrates because of their uniform sizes. XRD, TEM, and UV-vis absorption characterization of the different products synthesized have been performed. By increasing the bromide concentration 5-fold that used to make the nanocubes, unusual right bipyramids of gold bounded by six {100} faces were produced. The high product purity and excellent size control of this facile synthetic approach should make these novel gold nanostructures be readily available for a wide range of studies.

Synthesis of Ag2O Nanocrystals with Systematic Shape Evolution from Cubic to Hexapod Structures and Their Surface Properties

Abstract: We report the development of a facile method for the synthesis of Ag(2)O crystals with systematic shape evolution from cubic to edge- and corner-truncated cubic, rhombicuboctahedral, edge- and corner-truncated octahedral, octahedral, and hexapod structures by mixing AgNO(3), NH(4)NO(3), and NaOH at molar ratios of 1:2:11.8. A sufficient volume of NaOH solution was first added to a mixture of AgNO(3) and NH(4)NO(3) solution to promote the formation of Ag(NH(3))(2)(+) complex ions and the growth of Ag(2)O nanocrystals with good morphological control. The crystals are mostly submicrometer-sized. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy characterization has been performed to determine the crystalline surface facets. A band gap value of approximately 1.45 eV has been found for the octahedral Ag(2)O crystals. By changing the molar ratios of AgNO(3)/NH(4)NO(3)/NaOH to 1:2:41.8, corner-depressed rhombicuboctahedra and elongated hexapods were obtained as a result of enhanced crystal growth along the [100] directions. Smaller nanocubes with average sizes of approximately 200 and 300 nm and octapods can also be prepared by adjusting the reagent molar ratios and their added volumes. Both the octahedra and hexapods with largely silver atom-terminated {111} surface facets responded repulsively and moved to the surface of the solution when dispersing in a solution of positively charged methylene blue, but can be suspended in a negatively charged methyl orange solution. The cubes and octapods, bounded by the {100} faces, were insensitive to the molecular charges in solution. The dramatic facet-dependent surface properties of Ag(2)O crystals have been demonstrated.

Fabrication of truncated rhombic dodecahedral Cu 2 O nanocages and nanoframes

Abstract: We report a simple approach for the fabrication of cuprous oxide (Cu 2 O) nanocages and nanoframes. An aqueous solution of CuCl 2 , sodium dodecyl sulfate (SDS) surfactant, NH 2 OH·HCl reductant, HCl, and NaOH was prepared with reagents introduced in the order listed. Rapid seed particle aggregation and surface reconstruction of the intermediate structures resulted in the growth of type I nanoframes with just the {110} skeleton faces and empty {100} faces 45 minutes after mixing the reagents. Continued crystal growth for additional 75 min produced the 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 HCl via the addition of ethanol and then sonication of the solution led to the formation of type II nanoframes with elliptical pores on the {110} faces. The morphologies of these nanoframes have been carefully examined by electron microscopy. Without adding ethanol, random etching of the nanocages can occur at a slow rate. These composite materials should display interesting properties and functions.

Nanostructures, assembling method for nanowires, and device assembled from nanowires

Abstract: PROBLEM TO BE SOLVED: To provide one-dimensional nanostructures having diameters along a vertical axis wherein the diameters do not change nearly 10% or more at a section showing the maximum diameter change and the maximum diameters of less than approximately 200 nm, regarding substantially crystalline nanowire structures. SOLUTION: The one-dimensional nanostructures have the uniform diameters of less than approximately 200 nm. The new nanostructures called as "nanowires" include single-crystalline homostructures as well as heterostructures of two single-crystalline materials having different chemical compositions. The resulting heterostructures similarly become single crystals since the single-crystalline materials are used for forming the heterostructures. The nanowire heterostructures are generally based on a semiconducting wire wherein the doping and composition are controlled in either the longitudinal or radial directions, or in both directions, to yield a wire including different materials. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire (LOHN) and a coaxial heterostructure nanowire (COHN). COPYRIGHT: (C)2011,JPO&INPIT

Method of fabricating nanostructures and nanowires and device fabricated therefrom

Abstract: PROBLEM TO BE SOLVED: To provide a method of fabricating nanostructures and nanowires and a device fabricated therefrom. SOLUTION: A one-dimensional nanostructure has a uniform diameter of less than approximately 200 nm. Such new nanostructure which is called as "a nanowire" includes a single-crystal homostructure as well as heterostructure of at least two single crystal materials having different chemical compositions. Because the single crystal material is used to form the heterostructure, the resultant heterostructure may be a single crystal as well. The nanowire heterostructure is generally based on a semiconducting wire in which the doping and composition are controlled in either the longitudinal or radial directions, or in both directions, to yield a wire that comprises different materials. Examples of resulting nanowire heterostructures include a longitudinal heterostructure nanowire (LOHN) and a coaxial heterostructure nanowire (COHN). COPYRIGHT: (C)2010,JPO&INPIT

Plasmonic-enhanced polymer photovoltaic devices incorporating Au nanoparticles

Abstract: We have improved the device performance of polymer photovoltaic devices (OPVs) by blending Au nanoparticles (NPs) into the anodic buffer layer. The unique optical properties of localized surface plasmon resonance (LSPR), triggered by Au NPs, was beneficial for improving the photocurrent of OPVs. The excitation of LSPR increased not only the exciton generation rate but also the exciton dissociation probability. As a result, the power conversion efficiency of OPVs has been significantly enhanced from 3.57 to 4.24%.

Aqueous solution syntheses of palladium nanorods and gold octahedra and their assembly

Abstract: We have synthesized a variety of novel gold and palladium nanostructures by using simple aqueous solution growth approaches. In this conference, I will describe our recent work on the seed-mediated synthesis of palladium nanorods and branched nanocrystals. Hydrothermal synthesis of monodispersed octahedral gold nanocrystals and their self-assembled structures will be presented. The optical, SERS, and catalytic properties of these nanostructures will also be discussed.