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.

This study examines the facet-dependent optical properties of size-tunable Ag−Cu 2 O core−shell nanocrystals with 38, 42, and 50 nm cubic Ag cores. The Ag cores were prepared from octahedral Au seeds. The Cu 2 O shells are single-crystalline. In the case of Au@Ag−Cu 2 O 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 Cu 2 O 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−Cu 2 O nanocrystals. Facetdependent 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.