Showing papers by "Michael H. Huang published in 2017"

Corrigendum: Diffraction data of core-shell nanoparticles from an X-ray free electron laser

Abstract: Scientific Data 4:170048 doi: 10.1038/sdata201748 (2017); Published 11 April 2017; Updated 24 October 2017. The Data Descriptor incorrectly states the number of normal incidences used to generate the plot in Fig. 4b as 209. This plot was generated from 32 normal incidence cases.

Facet-dependent photocatalytic properties of Cu2O crystals probed by using electron, hole and radical scavengers

Abstract: To understand the photocatalytic inactivity of Cu2O nanocubes and the superior photocatalytic activity of Cu2O rhombic dodecahedra compared to that of Cu2O octahedra, electron and hole scavengers, as well as hydroxyl and superoxide anion radical scavengers, were introduced in the photodegradation of methyl orange using these Cu2O crystals as the photocatalysts. Scavenger results suggest that photogenerated electrons and holes experience a large barrier at the {100} faces of cubes preventing charge migration to the particle surfaces, leading to the photocatalytic inactivity of Cu2O cubes. For Cu2O rhombic dodecahedra, both photogenerated electrons and holes are efficiently utilized to produce radicals, giving them excellent photocatalytic activity. In contrast, the photocatalytic activity of Cu2O octahedra mostly comes from photoexcited electrons migrating to the particle surfaces to yield superoxide anion radicals and hydroxyl radicals, while holes are largely unavailable for the photo-oxidation reaction. Ultraviolet photoelectron spectroscopy (UPS) and reflectance spectra were used to construct conventional band diagrams for these particle shapes, but such band diagrams cannot account for the large face-specific photocatalytic properties of Cu2O nanocrystals. A modified band diagram presenting different degrees of band bending at various crystal surfaces is more useful to explain the observed facet-dependent phenomena.

Strong Facet Effects on Interfacial Charge Transfer Revealed through the Examination of Photocatalytic Activities of Various Cu2O–ZnO Heterostructures

Abstract: Confirming the photocatalytic inactivity of Cu2O nanocubes through the formation of Au-decorated–Cu2O heterostructures, spiky ZnO nanostructures are grown on Cu2O cubes, octahedra, and rhombic dodecahedra to demonstrate that charge transfer across semiconductor heterojunctions is also strongly facet dependent. Unintended CuO formation in the growth of ZnO on perfect Cu2O cubes makes them slightly active toward methyl orange photodegradation. Under optimal ZnO growth conditions without CuO presence, Cu2O cubes remain inactive, while rhombic dodecahedra show an enhanced photocatalytic activity due to better charge transfer according to normal Cu2O–ZnO band alignment. Surprisingly, photocatalytically active Cu2O octahedra become inactive after ZnO deposition. An extensive interfacial microscopic examination reveals preferential formation of the ZnO (101) planes on the {111} surfaces of Cu2O octahedra, while different ZnO lattice planes are observed to deposit on Cu2O cubes and rhombic dodecahedra. The photocatalytic inactivity of ZnO-decorated Cu2O octahedra is explained in terms of an unfavorable band alignment arising from an unusual degree of band bending for the ZnO {101} face relative to the band energy of the Cu2O {111} surface. The efficiency of charge transfer across semiconductor heterojunctions strongly depends on the band edge energies of the contacting planes.

Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet-Dependent Optical Property, Photocatalytic Activity, and Electrical Conductivity Examinations

Abstract: This work has developed conditions for the synthesis of Ag3PO4 cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods by tuning the amount of NH4NO3, NaOH, AgNO3, and K2HPO4 solutions added. Use of a minimal amount of AgNO3 solution can form much smaller rhombic dodecahedra and tetrahedra. Submicrometer-sized Ag3PO4 cubes and rhombic dodecahedra with sizes larger than 300 nm do not exhibit the optical size effect, but ∼290 nm rhombic dodecahedra show a smaller band gap value than larger cubes, and tetrahedra show the most blue-shifted absorption edge. The optical facet effect is present in Ag3PO4 crystals. Ag3PO4 cubes are more photocatalytically active than rhombic dodecahedra toward photodegradation of methyl orange, but tetrahedra are inactive, showing clear presence of photocatalytic facet effects. Electron paramagnetic resonance results confirm much higher production of hydroxyl radicals from photoirradiated Ag3PO4 cubes than from rhombic dodecahedra, while tetrahe...

Silicon Wafers with Facet‐Dependent Electrical Conductivity Properties

Abstract: By breaking intrinsic Si (100) and (111) wafers to expose sharp {111} and {112} facets, electrical conductivity measurements on single and different silicon crystal faces were performed through contacts with two tungsten probes. While Si {100} and {110} faces are barely conductive at low applied voltages, as expected, the Si {112} surface is highly conductive and Si {111} surface also shows good conductivity. Asymmetrical I-V curves have been recorded for the {111}/{112}, {111}/{110}, and {112}/{110} facet combinations because of different degrees of conduction band bending at these crystal surfaces presenting different barrier heights to current flow. In particular, the {111}/{110} and {112}/{110} facet combinations give I-V curves resembling those of p-n junctions, suggesting a novel field effect transistor design is possible capitalizing on the pronounced facet-dependent electrical conductivity properties of silicon.

Metal-like Band Structures of Ultrathin Si {111} and {112} Surface Layers Revealed through Density Functional Theory Calculations.

Abstract: Density functional theory calculations have been performed on Si (100), (110), (111), and (112) planes with tunable number of planes for evaluation of their band structures and density of states profiles. The purpose is to see whether silicon can exhibit facet-dependent properties derived from the presence of a thin surface layer having different band structures. No changes have been observed for single to multiple layers of Si (100) and (110) planes with a consistent band gap between the valence band and the conduction band. However, for 1, 2, 4, and 5 Si (111) and (112) planes, metal-like band structures were obtained with continuous density of states going from the valence band to the conduction band. For 3, 6 and more Si (111) planes, as well as 3 and 6 Si (112) planes, the same band structure as that seen for Si (100) and (110) planes have been obtained. Thus, beyond a layer thickness of five Si (111) planes at ~1.6 nm, normal semiconductor behaviors can be expected. The emergence of metal-like band structures for the Si (111) and (112) planes are related to variation in Si‒Si bond length and bond distortion plus 3s and 3p orbital electron contributions in the band structure. This work predicts possession of facet-dependent electrical properties of silicon with consequences in FinFET design.

Facet-Dependent Electrical Conductivity Properties of Silver Oxide Crystals

Abstract: Ag2 O cubes, truncated octahedra, rhombic dodecahedra, and rhombicuboctahedra were synthesized in aqueous solution. Two tungsten probes were brought into contact with a single particle for electrical conductivity measurements. Strongly facet-dependent electrical conductivity behaviors have been observed. The {111} faces are most conductive. The {100} faces are moderately conductive. The {110} faces are nearly non-conductive. When electrodes contacted two different facets of a rhombicuboctahedron, asymmetrical I-V curves were obtained. The {111} and {110} combination gives the best I-V curve expected for a p-n junction with current flowing in one direction through the crystal but not in the opposite direction. Density of states (DOS) plots for varying number of different lattice planes of Ag2 O match with the experimental results, suggesting that the {111} faces are most electrically conductive. The thicknesses of the thin surface layer responsible for the facet-dependent properties of Ag2 O crystals have been determined.

Aqueous-Phase Synthesis of Size-Tunable Copper Nanocubes for Efficient Aryl Alkyne Hydroboration.

Abstract: Copper nanocubes with average sizes of 82, 95, and 108 nm have been synthesized in an aqueous mixture of cetyltrimethylammonium chloride (CTAC) surfactant, copper acetate, and sodium ascorbate reductant heated at 100 oC for 40 min. Copper nanowires with an average length of 25 m can also be prepared this way by simply increasing the volume of sodium ascorbate introduced. Small shifts in the plasmonic absorption band positions with tunable particle sizes have been observed. The copper nanocubes were employed to catalyze hydroboration of phenylacetylene and various substituted aryl alkynes with 100% (E)-product selectivity and 82‒95% product yields. The copper nanocubes are cheap to make and should catalyze a broad scope of organic coupling reactions.

Au–Cu core–shell nanocube-catalyzed click reactions for efficient synthesis of diverse triazoles

Abstract: Au–Cu core–shell nanocubes and octahedra synthesized in aqueous solution were employed to catalyze a 1,3-dipolar cycloaddition reaction between phenylacetylene and benzyl azide in water at 50 °C for 3 h. Interestingly, the nanocubes were far more efficient in catalyzing this reaction, giving 91% yield of a regioselective 1,4-triazole product, while octahedra only recorded 46% yield. The Au–Cu nanocubes were subsequently employed to catalyze the click reaction between benzyl azide and a broad range of aromatic and aliphatic alkynes. The product yields ranged from 78 to 99%. Clearly the Au–Cu cubes exposing {100} surfaces are an excellent and green catalyst for click reactions.

Tracing the Surfactant-Mediated Nucleation, Growth, and Superpacking of Gold Supercrystals Using Time and Spatially Resolved X-ray Scattering.

Abstract: The nucleation and growth process of gold supercrystals in a surfactant diffusion approach is followed by simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS), supplemented with scanning electron microscopy. The results indicate that supercrystal nucleation can be activated efficiently upon placing a concentrated surfactant solution of a nematic phase on top of a gold nanocrystal solution droplet trapped in the middle of a vertically oriented capillary tube. Supercrystal nuclei comprised of tens of gold nanocubes are observed nearly instantaneously in the broadened liquid-liquid interface zone of a steep gradient of surfactant concentration, revealing a diffusion-kinetics-controlled nucleation process. Once formed, the nuclei can sediment into the naoncrystal zone below, and grow efficiently into cubic or tetragonal supercrystals of ∼1 μm size within ∼100 min. Supercrystals matured during sedimentation in the capillary can accumulate and face-to-face align at the bottom liquid-air interface of the nanocrystal droplet. This is followed by superpacking of the supercrystals into highly oriented hierarchical sheets, with a huge number of gold nanocubes aligned for largely coherent crystallographic orientations.

Diffraction data of core-shell nanoparticles from an X-ray free electron laser

Abstract: X-ray free-electron lasers provide novel opportunities to conduct single particle analysis on nanoscale particles. Coherent diffractive imaging experiments were performed at the Linac Coherent Light Source (LCLS), SLAC National Laboratory, exposing single inorganic core-shell nanoparticles to femtosecond hard-X-ray pulses. Each facetted nanoparticle consisted of a crystalline gold core and a differently shaped palladium shell. Scattered intensities were observed up to about 7 nm resolution. Analysis of the scattering patterns revealed the size distribution of the samples, which is consistent with that obtained from direct real-space imaging by electron microscopy. Scattering patterns resulting from single particles were selected and compiled into a dataset which can be valuable for algorithm developments in single particle scattering research. Machine-accessible metadata file describing the reported data (ISA-Tab format)