Showing papers by "Michael H. Huang published in 2013"

Facet‐Dependent Catalytic Activity of Cu2O Nanocrystals in the One‐Pot Synthesis of 1,2,3‐Triazoles by Multicomponent Click Reactions

Abstract: We report the highly facet-dependent catalytic activity of Cu2O nanocubes, octahedra, and rhombic dodecahedra for the multicomponent direct synthesis of 1,2,3-triazoles from the reaction of alkynes, organic halides, and NaN3. The catalytic activities of clean surfactant-removed Cu2O nanocrystals with the same total surface area were compared. Rhombic dodecahedral Cu2O nanocrystals bounded by {110} facets were much more catalytically active than Cu2O octahedra exposing {111} facets, whereas Cu2O nanocubes displayed the slowest catalytic activity. The superior catalytic activity of Cu2O rhombic dodecahedra is attributed to the fully exposed surface Cu atoms on the {110} facet. A large series of 1,4-disubstituted 1,2,3-triazoles have been synthesized in excellent yields with high regioselectivity under green conditions by using these rhombic dodecahedral Cu2O catalysts, including the synthesis of rufinamide, an antiepileptic drug, demonstrating the potential of these nanocrystals as promising heterogeneous catalysts for other important coupling reactions.

The influence of shell thickness of Au@TiO2 core-shell nanoparticles on the plasmonic enhancement effect in dye-sensitized solar cells.

Abstract: Plasmonic core–shell nanoparticles (PCSNPs) can function as nanoantennas and improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve maximum enhancement, the morphology of PCSNPs needs to be optimized. Here we precisely control the morphology of Au@TiO2 PCSNPs and systematically study its influence on the plasmonic enhancement effect. The enhancement mechanism was found to vary with the thickness of the TiO2 shell. PCSNPs with a thinner shell mainly enhance the current, whereas particles with a thicker shell improve the voltage. While pronounced plasmonic enhancement was found in the near infrared regime, wavelength-independent enhancement in the visible range was observed and attributed to the plasmonic heating effect. Emission lifetime measurement confirms that N719 molecules neighboring nanoparticles with TiO2 shells exhibit a longer lifetime than those in contact with metal cores. Overall, PCSNPs with a 5 nm shell give the highest efficiency enhancement of 23%. Our work provides a new synthesis route for well-controlled Au@TiO2 core–shell nanoparticles and gains insight into the plasmonic enhancement in DSSCs.

Formation of Diverse Supercrystals from Self-Assembly of a Variety of Polyhedral Gold Nanocrystals

Abstract: Cubic, rhombic dodecahedral, octahedral, and corner-truncated octahedral gold nanocrystals with sizes of tens of nanometers have been used as building blocks to form micrometer-sized supercrystals by slowly evaporating a water droplet on a substrate placed in a moist environment. Drying the droplet at 90 °C was found to yield the best supercrystals. Supercrystals were evenly distributed throughout the entire substrate surface originally covered by the droplet. Diverse supercrystal morphologies have been observed. Nanocubes formed roughly cubic supercrystals. Rhombic dodecahedra were assembled into truncated triangular pyramidal supercrystals. Rhombic dodecahedral, octahedral, and hexapod-shaped supercrystals were generated through the assembly of octahedra. Corner-truncated octahedra formed tetrapod-shaped supercrystals at room temperature, but octahedral, truncated triangular pyramidal, and square pyramidal supercrystals at 90 °C. Nanocrystal assembly was found to be strongly shape-guided. Expulsion of excess surfactant to the surfaces of supercrystals suggests that responsive adjustment of surfactant concentration during particle assembly mediates supercrystal formation. Transmission X-ray microscopy and optical microscopy have been employed to follow the supercrystal formation process. Surprising rotational water current near the droplet perimeter carrying the initially formed supercrystals has been observed. Supercrystals appear to grow from the edge of the droplet toward the central region. Supercrystals assembled from octahedra inherently contain void spaces and possibly connected channels. The mesoporosity of these supercrystals was confirmed by infiltrating H(2)PdCl(4) into the supercrystal interior and reducing the precursor to form Pd nanoparticles. The embedded Pd particles can still catalyze a Suzuki coupling reaction, demonstrating the application of these supercrystals for molecular transport, sensing, and catalysis.

Achieving polyhedral nanocrystal growth with systematic shape control

Abstract: From our series of studies on the syntheses of polyhedral gold, Cu2O, Ag2O, PbS, Au–Cu2O, and Au–Pd nanocrystals with systematic shape evolution, we have gained important insights into the growth mechanism and factors controlling the particle morphologies. In the formation of metal nanocrystals, dendritic or branched structures have consistently been observed at the initial moments of the metal nanoparticle growth process. All evidence has indicated that tuning the reaction rates can lead to particle shape changes. Varying the amount of reducing agent used and partial halide ligand replacement of the metal precursors can tune the reduction rate and yield nanocrystals with shape evolution. For nanocrystals formed by precipitation reactions, lowering the reaction rates by decreasing the equilibrium constants through the use of suitable metal precursors and equilibrium shifting reagents, polyhedral nanocrystals can be obtained. These nanocrystals exposing different surfaces have shown very interesting facet-dependent physical and chemical properties.

Seed-mediated growth of ultralong gold nanorods and nanowires with a wide range of length tunability.

Abstract: This study reports a systematic approach to synthesize ultralong gold nanorods and nanowires using a seed-mediated growth approach. In the first series, the effect of growth solution pH on the lengths of nanorods prepared was investigated. Interestingly, although shorter rods (230–310 nm) were produced in a basic solution environment than in an acidic condition (330–410 nm), the nanorod yield is greatly improved with relatively few nanoplate byproducts formed. Nanorod growth proceeds quickly in a basic solution as evidenced by the fast solution color changes. By adjusting several experimental parameters with the aim to elongate the nanorod length in a tunable fashion, gold nanorods and nanowires with average lengths from 580 to 2850 nm can be synthesized by progressively increasing the HNO3 concentration in the final growth solution. Nanowire growth in a highly acidic solution is slower, and a substantially longer time is needed to reach long lengths. Further extension of the nanowire length can be achiev...

Investigation of facet effects on the catalytic activity of Cu2O nanocrystals for efficient regioselective synthesis of 3,5-disubstituted isoxazoles

Abstract: Cubic, octahedral, and rhombic dodecahedral Cu2O nanocrystals bound by respectively {100}, {111}, and {110} facets were successfully employed to catalyze the [3 + 2] cycloaddition reaction for the regioselective synthesis of 3,5-disubstituted isoxazoles. Surfactant-free nanocrystals having the same total surface area were used for the catalysis. Strongly facet-dependent organocatalytic activity has been observed. Rhombic dodecahedra with fully exposed surface copper atoms on the (110) planes are the most efficient catalysts, followed by octahedra and the least active nanocubes. The particles are also recyclable catalysts. Cu2O rhombic dodecahedra were also used for the syntheses of 3,5-disubstituted isoxazoles from a wide variety of aromatic imidoyl chlorides and terminal alkynes in ethanol at 50 °C with excellent yields. Furthermore, a one-pot multi-component synthetic approach was demonstrated to form isoxazoles directly from readily available aldehyde precursors. This work clearly shows that precise facet engineering of Cu2O crystals can lead to significantly improved organocatalytic efficiency.

Fabrication of Diverse Cu2O Nanoframes through Face-Selective Etching

Abstract: Two approaches have been employed to generate Cu2O nanoframes. Novel edge-truncated cubic nanoframes with empty {110} edges can be obtained directly by growing Cu2O nanocrystals in the presence of HCl etchant. After 1 h of reaction for particle growth, introduction of ethanol and sonication of the mixture effectively removes surface-adsorbed sodium dodecyl sulfate (SDS) surfactant to facilitate HCl etching. Crystal structures of the nanoframes have been examined. The entire nanoframe formation process was captured by recording the complete solution color changes. By injecting precise volumes of HCl solution to a solution of presynthesized {100}-truncated and all-corner-truncated Cu2O rhombic dodecahedra, nanoframes with etched {110} faces and hollow interior were produced in 10 min. Observations made on some partially etched particles suggest etching rapidly proceeds from the surface {110} faces into the interior regions of the rhombic dodecahedra. The strong light scattering feature observed for solid rh...

Formation of Ag2S Cages from Polyhedral Ag2O Nanocrystals and their Electrochemical Properties

Abstract: Ag2O nanocubes, rhombicuboctahedra, octahedra, and hexapods have been employed as templates for the generation of Ag2O-Ag2S core-shell structures through a rapid sulfidation process in a basic solution. Addition of an ammonia etching solution quickly removes the Ag2O cores, thereby resulting in the formation of Ag2S cages with morphologies that resemble the starting templates. The composition of the Ag2S shells and cages has been extensively determined by various analytical techniques including X-ray and electron diffraction and X-ray photoelectron spectroscopy. The Ag2S shells have a monoclinic crystal structure and are polycrystalline with some amorphous and porous regions. The nanocage formation process has been captured by transmission electron microscopy (TEM). Gap spaces are formed initially between the cores and the shells owing to uniform etching of the Ag2O cores on all of the faces. No linkages connecting the cores to the shells have been observed. Depending on the potential scanning ranges applied, four types of electrochemical redox behavior have been identified for the Ag2O and Ag2O-Ag2S cubes in a basic solution. The ability to easily fabricate thin sheets of Ag2S over different Ag2O surfaces should extend the applications of Ag2S nanostructures.

Influence of morphology on the plasmonic enhancement effect of Au@TiO 2 core-shell nanoparticles in dye-sensitized solar cells

Abstract: Plasmonic core-shell nanoparticles (PCSNPs) can function as nanoantennas and improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve maximum enhancement, the morphology of PCSNPs needs to be optimized. Here we precisely control the morphology of Au@TiO 2 PCSNPs and systematically study its influence on the plasmonic enhancement effect. Enhancement mechanism was found to vary with the thickness of TiO 2 shell. PCSNPs with thinner shell mainly enhance the current, whereas particles with thicker shell also improve the voltage. While pronounced plasmonic enhancement was found in the near infrared regime, wavelength-independent enhancement in the visible range was observed and attributed to plasmonic heating effect. Emission lifetime measurement confirms that N719 molecules neighboring nanoparticles with TiO 2 shell exhibit longer lifetime than those in contact with metal cores. Overall, PCSNPs with 5-nm shell give highest efficiency enhancement of 23%. Our work provides a new synthesis route for well-controlled Au@TiO 2 core-shell nanoparticles and gains insight into the plasmonic enhancement in DSSCs.

Influence of morphology on the plasmonic enhancement effect of Au@TiO2 core-shell nanoparticles in dye-sensitized solar cells

Abstract: Plasmonic core-shell nanoparticles (PCSNPs) can function as nanoantennas and improve the efficiency of dye-sensitized solar cells (DSSCs). To achieve maximum enhancement, the morphology of PCSNPs need to be optimized. Here we precisely control the morphology of Au@TiO2 PCSNPs and systematically study its influence on the plasmonic enhancement effect. Enhancement mechanism was found to vary with the thickness of TiO2 shell. PCSNPs with thinner shell enhance the current due to plasmonic effect, whereas particles with thicker shell improve the voltage due to increasing semiconducting character. Wavelength-independent enhancement in the visible range was observed and attributed to plasmonic heating effect. PCSNPs with 5-nm shell give highest efficiency enhancement of 23%. Our work provides a new synthesis route for well-controlled Au@TiO2 core-shell nanoparticles and gains insight into the plasmonic enhancement in DSSCs.