Showing papers by "Michael H. Huang published in 2016"
TL;DR: Au-Pd tetrahexahedra supported on activated carbon can act as a superior recyclable plasmonic photocatalyst for hydrogen evolution under light with 3-fold rate enhancement benefiting from lattice strain, modified surface electronic state, and a broader range of light absorption.
Abstract: Au-Pd core-shell nanocrystals with tetrahexahedral (THH), cubic, and octahedral shapes and comparable sizes were synthesized. Similar-sized Au and Pd cubes and octahedra were also prepared. These nanocrystals were used for the hydrogen-evolution reaction (HER) from ammonia borane. Light irradiation can enhance the reaction rate for all the catalysts. In particular, Au-Pd THH exposing {730} facets showed the highest turnover frequency for hydrogen evolution under light with 3-fold rate enhancement benefiting from lattice strain, modified surface electronic state, and a broader range of light absorption. Finite-difference time-domain (FDTD) simulations show a stronger electric field enhancement on Au-Pd core-shell THH than those on other Pd-containing nanocrystals. Light-assisted nitro reduction by ammonia borane on Au-Pd THH was also demonstrated. Au-Pd tetrahexahedra supported on activated carbon can act as a superior recyclable plasmonic photocatalyst for hydrogen evolution.
92 citations
TL;DR: This work confirms the presence of a large facet-dependent photocatalytic activity of Cu2 O crystals through sparse deposition of gold particles onCu2 O cubes, octahedra, and rhombic dodecahedra and shows greatly enhanced photodegradation rates of methyl orange resulting from a better separation of the photogenerated electrons and holes.
Abstract: This work confirms the presence of a large facet-dependent photocatalytic activity of Cu2O crystals through sparse deposition of gold particles on Cu2O cubes, octahedra, and rhombic dodecahedra. Au-decorated Cu2O rhombic dodecahedra and octahedra showed greatly enhanced photodegradation rates of methyl orange resulting from a better separation of the photogenerated electrons and holes, with the rhombic dodecahedra giving the best efficiency. Au–Cu2O core–shell rhombic dodecahedra also displayed a better photocatalytic activity than pristine rhombic dodecahedra. However, Au-deposited Cu2O cubes, pristine cubes, and Au-deposited small nanocubes bound by entirely {100} facets are all photocatalytically inactive. X-ray photoelectron spectra (XPS) showed identical copper peak positions for these Au-decorated crystals. Remarkably, electron paramagnetic resonance (EPR) measurements indicated a higher production of hydroxyl radicals for the photoirradiated Cu2O rhombic dodecahedra than for the octahedra, but no radicals were produced from photoirradiated Cu2O cubes. The Cu2O {100} face may present a high energy barrier through its large band edge bending and/or electrostatic repulsion, preventing charge carriers from reaching to this surface. The conventional photocatalysis model fails in this case. The facet-dependent photocatalytic differences should be observable in other semiconductor systems whenever a photoinduced charge-transfer process occurs across an interface.
91 citations
TL;DR: Although all Ag2O samples show significant etching during photocatalysis, metallic silver is not produced and tuning the concentration of NH3 in the solution can provide more particle morphologies including edge- and corner-truncated cubes, small rhombicuboctahedra, and edge-trunked octahedr to enrich Ag2 O shape diversity.
Abstract: Sub- to micrometer-sized Ag2O cubes, great rhombicuboctahedra, cuboctahedra, corner-truncated octahedra, octahedra, and rhombic dodecahedra have been synthesized at room temperature using simple molar ratios of NH4NO3, NaOH, and AgNO3 solutions with a short reaction time. In addition, tuning the concentration of NH3 in the solution can provide more particle morphologies including edge- and corner-truncated cubes, small rhombicuboctahedra, and edge-truncated octahedra to enrich Ag2O shape diversity. X-ray photoelectron spectroscopy (XPS) spectra indicate surface composition of various crystals as pure Ag2O. Diffuse reflectance spectra have been used to determine the band gap of Ag2O cubes. Ag2O cubes, octahedra, and rhombic dodecahedra having the same total particle surface area were used for facet-dependent photocatalytic activity comparison in the degradation of methyl orange. Cubes are comparably highly active for this reaction, while octahedra and rhombic dodecahedra give moderate and low catalytic act...
83 citations
TL;DR: In this paper, an octahedral gold nanocrystal cores and hexadecylamine (HDA) are added to the solution pH and act as a coordinating ligand to the copper ions to facilitate controlled copper shell growth.
Abstract: Copper nanocubes with tunable edge lengths over the range from 49 to 136 nm and ultrasmall octahedra with opposite corner distances of 45, 51, and 58 nm have been synthesized in aqueous solutions by reducing CuCl2 or copper acetate with ascorbic acid in the presence of octahedral gold nanocrystal cores and hexadecylamine (HDA) at 100 °C for 45 min to 1.5 h. Addition of HDA increases the solution pH and acts as a coordinating ligand to the copper ions to facilitate controlled copper shell growth. Due to ultralarge lattice mismatch between Au and Cu, nonuniform copper deposition yields cubes and octahedra with noncentrally located gold cores. The Au–Cu octahedra show little shift in the plasmonic band with increasing particle size. For Au–Cu nanocubes, the degree of absorption band red-shift gets smaller as cube size increases. The Au–Cu nanocubes have shown reasonable reactivity toward 4-nitrophenol reduction at 40 °C.
67 citations
TL;DR: These nanocrystals provide strong evidence of the existence of facet-dependent optical absorption properties of Cu2 O nanoparticles, showing nanocubes always have a more redshifted absorption band than that of octahedra having a similar volume by about 15 nm.
Abstract: Size-tunable small to ultrasmall Cu2 O nanocubes and octahedra are synthesized in aqueous solution without the introduction of any surfactant. These nanocrystals provide strong evidence of the existence of facet-dependent optical absorption properties of Cu2 O nanoparticles, showing nanocubes always have a more redshifted absorption band than that of octahedra having a similar volume by about 15 nm.
66 citations
TL;DR: In this article, a modified band diagram of PbS nanocrystals was constructed to account for the observed facet-dependent electrical conductivity behaviors, and an asymmetric I-V curve was recorded when electrical contacts were made simultaneously on the {110} and {111} facets of a truncated cube.
Abstract: Large PbS protruded cubes, edge- and corner-truncated cubes and octahedra, and perfect octahedra with sizes over 200 nm have been synthesized in aqueous solution. By using two surface oxide-free tungsten probes to contact a clean particle, these PbS nanocrystals displayed facet-dependent electrical conductivity behaviors. Both {110} and {100} faces are highly conductive at applied voltages beyond 4 V, but the {111} faces can remain nonconductive even at 5 V. An asymmetric I–V curve was recorded when electrical contacts were made simultaneously on the {110} and {111} facets of a truncated cube. A modified band diagram of PbS is constructed to account for the observed facet-dependent effect. Density of states plots for varying numbers of PbS surface planes show larger areas of conduction band electron occupancy for the (110) and (100) planes than that for the (111) planes at a layer thickness of 3.0–3.4 nm. The work represents that, for the first time, the facet-dependent electrical properties of an n-type ...
55 citations
TL;DR: These small Au-Cu2O nanocrystals represent the simplest nanostructure design to absorb light covering the entire NIR wavelengths and exhibit photothermal activity when illuminated by 1064 nm light.
Abstract: Other than the display of purely optical phenomenon, the recently-discovered facet-dependent optical properties of metal–Cu2O nanocrystals have become useful by illuminating Au–Cu2O nanocubes and octahedra having a surface plasmon resonance (SPR) absorption band in the near-infrared (NIR) region from octahedral Au cores with 808 nm light for heat generation. After 5 min of light irradiation, a solution of Au–Cu2O nanocubes can reach 65 °C with their Au SPR band matching the illuminating light wavelength. Photothermal efficiency has been found to be facet-dependent. In addition, short gold nanorods were employed to synthesize {100}-bound rectangular Au–Cu2O nanobars with a tunable longitudinal Au SPR absorption band covering a broad NIR range from ∼1050 to 1400 nm. Because the Au SPR bands can become fixed with relatively thin Cu2O shells of less than 15 nm, ultrasmall nanobars having a size of 61 nm directly red-shift the Au SPR band to 1047 nm. And 73 nm nanobars can give a Au SPR band at 1390 nm. Truncated nanobars exposing {100}, {110}, and {111} facets give a very blue-shifted Au SPR band. The nanobars also exhibit photothermal activity when illuminated by 1064 nm light. These small Au–Cu2O nanocrystals represent the simplest nanostructure design to absorb light covering the entire NIR wavelengths.
52 citations
TL;DR: In this paper, the facet-dependent optical properties of size-tunable Ag-Cu2O core-shell nanocrystals with 38, 42, and 50 nm cubic Ag cores were examined.
Abstract: This study examines the facet-dependent optical properties of size-tunable Ag–Cu2O core–shell nanocrystals with 38, 42, and 50 nm cubic Ag cores. The Ag cores were prepared from octahedral Au seeds. The Cu2O shells are single-crystalline. In the case of Au@Ag–Cu2O nanocrystals with 42 nm Ag cores, the Ag surface plasmon resonance (SPR) absorption band at 485 nm has been widely red-shifted to 730, 755, and 775 nm for rhombic dodecahedra, truncated octahedra, and cuboctahedra, respectively, after forming the Cu2O shells. The Ag SPR band positions are mostly fixed despite large changes in the shell thickness, showing the presence of facet-dependent optical properties. Because of the strong Ag SPR band absorption, all samples exhibit a better photothermal activity than that of Au–Cu2O nanocrystals. Facet-dependent heat transmission may be present for particles with a Ag SPR band much deviated from the laser wavelength, but this phenomenon is lost for particles with an SPR band approaching the excitation wavel...
43 citations
TL;DR: Two seed-mediated approaches for the growth of silver nanocubes in aqueous solution have been developed and the nanocages exhibited high and stable photothermal efficiency with solution temperatures quickly reaching beyond 100 °C when irradiated with an 808 nm laser for large heat and water vapor generation.
Abstract: Two seed-mediated approaches for the growth of silver nanocubes in aqueous solution have been developed. Addition of a silver-seed solution to a mixture of cetyltrimethylammonium chloride (CTAC), silver trifluoroacetate, and ascorbic acid and heating the solution at 60 °C for 1.5 h produces uniform Ag nanocubes with tunable sizes from 23 to 60 nm by simply adjusting the volume of silver-seed solution introduced. Alternatively, the silver-seed solution can be injected into a mixture of cetyltrimethylammonium bromide (CTAB), silver nitrate, copper sulfate, and ascorbic acid and heated to 80 °C for 2 h to generate 46 nm silver nanocubes. Plate-like Ag nanocrystals exposing {111} surfaces can be synthesized by reducing Ag(NH3 )2 (+) with ascorbic acid in a CTAC solution. Relatively large Ag nanocubes were converted to cuboctahedral Au/Ag and Au nanocages and nanoframes with empty {111} faces through a galvanic replacement reaction. The nanocages showed a progressive plasmonic band red-shift with increasing Au content. The nanocages exhibited high and stable photothermal efficiency with solution temperatures quickly reaching beyond 100 °C when irradiated with an 808 nm laser for large heat and water vapor generation.
40 citations