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.

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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.

Electrical conductivity measurements show that the {100} faces of a perfect SrTiO3 cube are insulating, but the {110} faces of a SrTiO3 truncated rhombic dodecahedron are considerably more conductive. Interestingly, compared to electrodes touching the proximal {110} faces, adjacent {110} face contacts give notably higher current. Unexpectedly, while the {110} faces of a truncated rhombic dodecahedron remain much more conductive than its {100} faces, the adjacent conductive {110} facets may cause the poorly conductive {100} facets to become more conductive through slight current leakage to the adjacent conductive faces. Consistent with previous insulating behavior observed for a {110}-bound Cu2O rhombic dodecahedron, the {110} faces of a Cu2O rhombicuboctahedral microcrystal remain insulating. Thus, the influence of adjacent conductive facet may be avoided using sharper electrodes. Still, the adjacent facet effect may always be present even with the use of sharp electrodes. Current-rectifying asymmetric I–V curves were recorded with electrodes contacting the {100} and {110} faces of a SrTiO3 truncated rhombic dodecahedron. The electrical facet effects can be understood with different degrees of band bending at these crystal surfaces and thus different barrier heights to charge carrier transport across these surfaces. Finally, high-resolution transmission electron microscopy (HR-TEM) images over the surfaces of SrTiO3 truncated rhombic dodecahedra and cubes were taken, showing notable shifts in atomic positions within the few layers of surface lattice planes relative to the atomic positions of the interior lattice, suggesting that the lattice deviations within the thin surface layer as predicted by density functional theory (DFT) calculations may be visually observable.

Cu2O cubes, octahedra and rhombic dodecahedra were employed to catalyze homo and heterocoupling reactions of aryl alkynes. X-ray diffraction (XRD) patterns of the Cu2O crystals show slight peak shifts for...