Hydrothermal synthesis of monodispersed octahedral gold nanocrystals with five different size ranges and their self-assembled structures

Here we report the hydrothermal synthesis of octahedral gold nanocrystals from an aqueous solution of HAuCl₄, trisodium citrate, and cetyltrimethylammonium bromide (CTAB) surfactant. By heating the mixture at 110 °C for 6, 12, 24, 48, and 72 h, gold octahedra with approximate average sizes of 30, 60, 90, 120, and 150 nm can be obtained. The percent yield of octahedral nanocrystals is more than 90% for all the samples. Products formed at shorter reaction times were also examined. Structural characterization confirmed that the octahedra are bounded by entirely {111} faces. As particle size increases, the surface plasmon resonance (SPR) absorption band red-shifts from 543 to 635 nm. These monodisperse gold octahedra can spontaneously self-assemble into long-range ordered packing structures upon water evaporation. Three types of self-assembled structures with faces, edges, or corners of the octahedra contacting the substrates have been identified. Particle size can affect the type of packing structure they adopt. The detailed packing arrangements are described. Significant red-shifted SPR coupling absorption bands in the near-infrared region are observed as a result of the formation of these assembled structures. These gold octahedra have been shown to serve as effective surface-enhanced Raman scattering substrates using thiophenol as the test molecule.