Modified Semiconductor Band Diagrams Constructed from Optical Characterization of Size-Tunable Cu₂O Cubes, Octahedra, and Rhombic Dodecahedra

By making Cu₂O nanocubes, octahedra, and rhombic dodecahedra with tunable sizes and recording their light absorption and emission spectra, their absorption and emission bands shift steadily to longer wavelengths with increasing particle sizes from 10 nm to beyond 250 nm. Emission intensities are highest for the smallest nanocubes. Photoluminescence band shifts exceed 130 nm over this size range. For particles having the same volume, rhombic dodecahedra absorb light of shortest wavelength, while cubes show most red-shifted absorption with their band gaps differing by 0.17 eV (or 51.5 nm). They show obviously different colors. The presence of optical size and facet effects in semiconductors means that their emission wavelengths are tunable through facet control and use of nanocrystals much larger than quantum dots. A modified and general band diagram for Cu₂O crystals has been constructed incorporating their optical size and facet effects with surface band bending. In addition, a more complete understanding of the different orders of surface band bending for the {100}, {111}, and {110} facets used in explaining the facet-dependent photocatalytic activity, electrical conductivity, and light absorption properties of Cu₂O crystals is presented

Shape-Tunable Co₃O₄ Nanocrystals Possessing Surface-Dependent Optical and Magnetic Properties

Small Co3O4 cubes and cuboctahedra were synthesized at 110 and 150 °C in just 2 h, respectively. Size-tunable nanocubes were also prepared at 150 °C for 2 h. Despite their high crystalline quality as seen from X-ray and electron diffraction patterns, cubes and cuboctahedra formed at 150 °C present clear lattice deviations, while cubes synthesized at 110 °C show crystal perfection. Large amounts of Co3O4 cubes, octahedra, cuboctahedra, and truncated octahedra can also be obtained. These Co3O4 crystals exhibit both size- and facet-dependent absorption positions and band gaps. Remarkably, while smaller magnetization differences can be identified at 300 K, octahedra become much more magnetized than cubes and cuboctahedra at 2 K. Their Néel temperatures also vary. Octahedra have notably larger coercivity and retentivity values than cubes and cuboctahedra at 2 K. Thus, in addition to various facet-dependent properties of semiconductor materials, magnetic behaviors also reveal surface effects. This means that there is the potential to prepare more powerful magnets through surface control

Aqueous Phase Synthesis of Au–Cu Core–Shell Nanocubes and Octahedra with Tunable Sizes and Noncentrally Located Cores

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

Fabrication of Diverse Cu₂O Nanoframes through Face-Selective Etching

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 rhombic dodecahedra extending from the visible to the near-infrared regions largely disappears after their transformation into nanoframes

Formation of Free-Standing Supercrystals from the Assembly of Polyhedral Gold Nanocrystals by Surfactant Diffusion in the Solution

Gold supercrystals with polyhedral morphologies can be prepared from the ordered packing of octahedral and rhombic dodecahedral nanocrystals in the presence of a sufficient amount of surfactant by slow water droplet evaporation. The whole supercrystal formation process has been video-recorded using a specially designed chamber to enclose a substrate containing the nanocrystal droplet in a moist environment. Supercrystal growth from the assembly of octahedra is completed within a shorter time. The presence of cetyltrimethylammonium chloride (CTAC) within the supercrystals has been confirmed by small-angle X-ray diffraction analysis. Transmission electron microscopy examination reveals the tendency of two gold octahedra with face contact to fuse, a process frequently observed in the formation of octahedron-assembled supercrystals. Remarkably, we have developed a diffusional surfactant transport approach to make free-standing supercrystals in bulk aqueous solution by adding a concentrated CTAC solution to a concentrated particle solution with a lower CTAC concentration in an Eppendorf tube. Gradual diffusion of CTAC to the lower nanocrystal solution promotes the growth of polyhedral supercrystals. A solution with a sufficiently high surfactant concentration has been shown to be necessary for particle aggregation and supercrystal formation. This method allows the deposition of dense but evenly distributed supercrystals on a substrate. Supercrystals were also used to make a modified electrode for electro-oxidation of glucose. This simple and organic solvent-free approach to making a large quantity of supercrystals allows an ample supply of supercrystals for studies of densely assembled nanocrystal systems and for biomedical applications

Facet-Dependent Optical and Photothermal Properties of Au@Ag–Cu₂O Core–Shell Nanocrystals

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 wavelength as the particles become highly photothermally efficient to give solution temperatures of 80–95 °C within 3 min of laser irradiation

Cu₂O Pseudomorphic Conversion to Cu Crystals for Diverse Nitroarene Reduction

Cu₂O cubes, octahedra, and rhombic dodecahedra can be pseudomorphically converted to Cu crystals of the corresponding morphologies through the addition of ammonia borane. Nitroarene can be completely reduced during the compositional transformation with four equivalents of ammonia borane at 30 °C in 25 min. All the obtained polyhedral Cu crystals can give 100% nitroaniline conversion to <i>p</i>-phenylenediamine exclusively, but commercial Cu₂O powder shows a comparatively lower 4-bromonitrobenzene conversion and yields a mixture of products. Use of sodium borohydride as a reducing agent resulted in the formation of deformed Cu particles and a low nitroaniline conversion percentage. Cu₂O cubes cannot be converted to Cu particles with the addition of hydrazine, and nitroaniline conversion did not occur. Nitro group reduction is successful with high yields for diverse nitroarene molecules giving only a single product starting from a solution of the nitroarene compound, Cu₂O cubes and ammonia borane

Aqueous Phase Synthesis of Au–Ag Core–Shell Nanocrystals with Tunable Shapes and Their Optical and Catalytic Properties

In this study, rhombic dodecahedral gold nanocrystals were used as cores for the generation of Au–Ag core–shell nanocrystals with cubic, truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures. Gold nanocrystals were added to an aqueous mixture of cetyltrimethylammonium chloride (CTAC) surfactant, AgNO₃, ascorbic acid, and NaOH to form the core–shell nanocrystals. The nanocrystals are highly uniform in size and shape, and can readily self-assemble into ordered packing structures on substrates. Results from observation of solution color changes and variation in the reaction temperature suggest octahedra are produced at a higher growth rate, while slower growth favors cube formation. The major localized surface plasmon resonance (LSPR) band positions for these nanocrystals are red-shifted compared to those for pristine silver particles with similar dimensions due to the LSPR effect from the gold cores. By increasing the concentrations of reagents, Au–Ag core–shell cubes and octahedra with tunable sizes were obtained. Au–Ag cubes with body diagonals of 130, 144, and 161 nm and octahedra with body diagonals of 113, 126, and 143 nm have been prepared, allowing the investigation of size effect on their optical properties. Au–Ag octahedra with thinner Ag shells (12–16.5 nm) exhibit a blue-shifted major LSPR band relative to the LSPR band at 538 nm for the gold cores. For Au–Ag octahedra and cubes with thicker shells (22.5–37 nm), the major LSPR band is progressively red-shifted from that of the gold cores with increasing shell thickness and particle size. The Au–Ag octahedra show higher catalytic activity than cubes toward reduction of 2-amino-5-nitrophenol by NaBH₄ at 30 °C, but both particle shapes display significantly enhanced catalytic efficiency at 40 °C

Photocatalytic Activity Suppression of CdS Nanoparticle-Decorated Cu₂O Octahedra and Rhombic Dodecahedra

Wurtzite CdS nanoparticles have been lightly deposited on Cu₂O cubes, octahedra, and rhombic dodecahedra to examine facet effects on the interfacial charge transfer in a photocatalytic reaction. Instead of an expected photocatalytic activity enhancement on the basis of a favorable band alignment at the heterojunction, CdS-decorated Cu₂O octahedra and rhombic dodecahedra show drastically reduced photocatalytic activities. Further increasing the CdS deposition amount leads to complete suppression of photocatalytic activity. Cu₂O cubes remain inactive even after CdS deposition. Transmission electron microscopy analysis reveals epitaxial growth of the (101) planes of CdS on the (110) planes of a Cu₂O rhombic dodecahedron, whereas the (110) planes of CdS align parallel to the (111) planes of a Cu₂O octahedron. Because facet-dependent photocatalytic activity can be understood from different degrees of band bending at the crystal surfaces, significantly upward bending for the CdS-contacting planes can explain the observed photocatalytic inactivity. This work demonstrates that strong facet effects tuning the band energies of both semiconductors at the heterojunctions make the predictions of an enhanced photocatalytic activity, simply through bulk band energy alignment analysis, highly unreliable

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

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 tetrahedra yield essentially no radicals. A modified band diagram showing different degrees of band edge bending can explain these observations. All these Ag₃PO₄ crystals show poor electrical conductivity properties, but the {110} faces are slightly more conductive than the {100} faces. As a result, current rectifying <i>I</i>–<i>V</i> curves have been obtained, demonstrating that facet-dependent electrical properties are broadly observable in many semiconductor materials. This work reveals again that facet-dependent optical, photocatalytic, and electrical conductivity properties are intrinsic semiconductor properties