Facet-dependent electrical conductivity properties of Cu2O crystals.

It is interesting to examine facet-dependent electrical properties of single Cu2O crystals, because such study greatly advances our understanding of various facet effects exhibited by semiconductors. We show a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is nonconductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets.

Facet-dependent surface plasmon resonance properties of Au-Cu2 o core-shell nanocubes, octahedra, and rhombic dodecahedra.

Au-Cu2 O core-shell nanocubes, octahedra, and rhombic dodecahedra display facet-dependent optical properties. Furthermore, different-sized Au-Cu2 O octahedra with 58, 65, 68, and 73 nm octahedral gold cores clearly show a transition from the shell thickness-independent gold surface plasmon resonance band for octahedra with smaller gold cores to progressive red-shifts of the band with increasing shell thickness in octahedra with larger gold cores.

Poly[bis-(µ(2)-4,4'-bipyridine)-bis-(3-nitro-benzoato)nickel(II)].

The crystal structure of the title complex, [Ni(C(7)H(4)NO(4))(2)(C(10)H(8)N(2))(2)](n), exhibits a two-dimensional network, which is built up from slightly distorted NiN(4)O(2) polyhedra (2 symmetry), bipyridine ligands, and carboxyl-ate anions. The Ni(II) atoms are six-coordinated by two O atoms of two monodentate carboxyl-ate anions and four N atoms from bipyridine ligands and are connected into layers by the 4,4'-bipyridine ligands.

Poly[bis-(µ(2)-4,4'-bipyridine)bis-(3-nitro-benzoato)cobalt(II)].

The hydro-thermal reaction of cobalt nitrate with 4,4'-bipyridine and 3-nitro-benzoic acid lead to the formation of the title complex, [Co(C(7)H(4)NO(4))(2)(C(10)H(8)N(2))(2)](n). In the crystal structure, the Co(II) atoms are coordinated by two terminal carboxyl-ate anions and four 4,4'-bipyridine ligands within slightly distorted octa-hedra. The Co(II) atom and one of the two independent 4,4'-bipyridine ligands are located on a twofold rotation axis, while the second independent 4,4'-bipyridine mol-ecule is located on a centre of inversion. One of the two rings of one 4,4'-bipyridine ligand is disordered over two orientations and was refined using a split model [occupancy ratio 0.68 (2):0.32 (2)]. The Co(II) atoms are connected by the 4,4'-bipyridine ligands into layers, which are located parallel to the ab plane.

catena-Poly[[tetraaquanickel(II)]-µ(3)-benzene-1,3,5-tricarboxylato-3':1:2-κO:O,O:O-[tetraaquanickel(II)]-µ(2)-benzene-1,3,5-tricarboxylato-2:3κO:O-[tetraaquanickel(II)]].

The microwave solvothermal reaction of nickel nitrate with trimesic acid provided the title compound, [Ni(3)(BTC)(2)(H(2)O)(12)](n) (BTC = benzene-1,3,5-tricarboxyl-ate anion, C(9)H(3)O(6)), which is a metal coordination polymer composed of one-dimensional zigzag chains. The crystal under investigation was ramecically twinned with an approximate twin domain ratio of 1:1. In the asymmetric unit, there are two types of Ni atoms. One of the NiO(6) groups (2 symmetry) is coordinated to only one carboxyl-ate group and thus terminal, the other is bridging, forming the coordination polymer. The extended chains are connected by the organic BTC anions via µ(2)-linkages. O-H⋯O hydrogen bonds and π-π inter-actions between the chains [centroid-centroid distance 3.58 (1) Å] induce the complex to mimic a three-dimensional structure.

Photothermal effects from Au-Cu2O core-shell nanocubes, octahedra, and nanobars with broad near-infrared absorption tunability.

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.

Facet-dependent optical properties of Pd-Cu2O core-shell nanocubes and octahedra.

Pd-Cu2O core-shell nanocubes and truncated octahedra with six average sizes for each particle shape have been synthesized from 29 nm Pd nanocubes. The nanocubes have average edge lengths of 64-124 nm, while the truncated octahedra are 107-183 nm in the opposite tip distance. The core and shell composition and lattice orientation have been determined, showing the formation of single-crystalline Cu2O shells. The surface plasmon resonance (SPR) band from the Pd nanocrystal cores is barely visible. However, the Cu2O shells display facet-dependent optical properties. The Cu2O absorption band for smaller Pd-Cu2O cubes is consistently more red-shifted than somewhat larger Pd-Cu2O truncated octahedra. This work again shows that the observed facet-dependent optical phenomenon in metal-Cu2O core-shell nanocrystals is derived from the Cu2O shells. The use of 40 nm Pd cubes as cores gave uniform and size-tunable Pd-Cu2O nanocubes and truncated octahedra that display the Pd SPR band. The Pd SPR band is consistently located at 650 nm for Pd-Cu2O truncated octahedra, and 670 nm for the cubes despite large variation in the shell thickness. Both the Cu2O absorption and the Pd plasmonic band exhibit facet-dependent optical properties.

Facet-dependent properties of polyhedral nanocrystals.

Syntheses of metal and oxide nanocrystals with cubic crystal structures and well-controlled polyhedral morphologies such as cubic, octahedral, and rhombic dodecahedral shapes exposing, respectively, {100}, {111}, and {110} surfaces enable a more accurate determination of their facet-dependent properties. So far molecular adsorption, photocatalytic, organocatalytic, and electrical conductivity properties have been demonstrated to be surface-related or facet-dependent. Chemical etching and metal nanoparticle deposition can also be face-selective. Examples of these surface properties are presented. In general, ionic solids such as Cu2O nanocrystals exhibit more sharply different surface properties than those seen in metal nanoparticles. A better understanding of these facet-dependent properties is necessary to prepare nanomaterials with enhanced properties such as their catalytic activities.