We report the growth of ultralong ZnO nanowires on silicon (100) substrates via the gold-catalyzed vapor transport approach. An ample supply of zinc vapor generated through carbothermal reduction of ZnO powder at 917 °C and a suitable amount of oxygen facilitate the rapid growth of nanowires. These ZnO nanowires are extremely long with lengths of 85−100 μm, and exhibit an overall vertical orientation. The nanowires have largely diameters of 250−400 nm. Crystal structure analysis indicates typical ZnO nanowire growth along the [0001] direction. The band gap of these nanowires was determined to be 3.22 eV. These nanowires show a relatively weak near-band-edge emission peak at 390 nm, and a significant oxygen vacancy-related emission band at 495 nm. Good photocatalytic activity of these nanowires on substrates toward the photodegradation of rhodamine B and 4-chlorophenol was demonstrated. Furthermore, we showed that these nanowires on substrates can serve as effective and convenient recyclable photocatalysts. Only a slight decrease in the photodecomposition rate was observed after 10 cycles of the photocatalysis experiment. The photocatalysts also work well under natural sunlight.

Unusual ZnO microspheres constructed of interconnected sheetlike nanostructures were prepared by the hydrothermal synthesis approach. These microspheres possess high surface areas (28.9 m(2)/g) and are amorphous. Trisodium citrate plays a key role in directing the formation of these microstructures. By increasing the reaction time, these microspheres gradually dissolved to form short hexagonal microrods with stacked nanoplate or nanosheet structure. The microrods were also formed under the influence of trisodium citrate. They are crystalline and show a strong (002) X-ray diffraction peak of wurtzite ZnO structure. Both microsphere and microrod samples show near-band-edge emission at approximately 385 nm, but only the microrod sample exhibits yellow luminescence at approximately 560 nm. Due to their high surface areas, these ZnO microstructures were examined for their ability to photodecompose phenol. The as-prepared samples did not display photocatalytic activity due to possible surface adsorption of solution species. However, microspheres with heat treatment to 300 degrees C can substantially enhance the photodecomposition of phenol under direct sunlight irradiation and still maintain their high surface area nanosheet structure.

We report a new approach for the seed-mediated synthesis of gold nanorods with high aspect ratios by the addition of nitric acid. Various amounts of nitric acid were added during nanorod growth to significantly enhance the production of high aspect ratio nanorods. The resulting nanorods have uniform diameters of 19-20 nm and can reach lengths of 400-500 nm to yield nanorods with average aspect ratios of 21-23. The nanorods represent a large fraction of the gold nanostructures synthesized, with triangular, truncated triangular, and hexagonal nanoplates about 120-200 nm in width making up rest of the gold nanostructures formed. These nanorods spontaneously self-assemble into a high-density three-dimensional packing structure with the nanorods arranged side-by-side and end-to-end. These nanorods possess a 5-fold twinned structure with ten {111} end faces and five {100} side faces. UV-vis absorption spectrum shows a transverse plasmon absorption band at 508 nm. The longitudinal plasmon absorption band should appear beyond 2500 nm due to the extremely long length of these nanorods. The presence of nitrate ions, rather than the slight pH change caused by nitric acid, is believed to have a greater effect on the formation of these nanorods.

We describe the formation of gold sulfide (Au 2 S) nanoparticle superstructures via hydrothermal synthesis approach at 175°C using HAuCl 4 and Na 2 S reagents and cetyltrimethylammonium bromide (CTAB) surfactant. Uniform 2-4 nm Au 2 S nanoparticles were found to assemble into a densely packed lamellar phase structure, and the resulting material displays a cross-sheet-like morphology. The sheets are 125-250 nm in length and can be suspended in solution. XRD, TEM, and EDS characterization of the nanoparticle samples confirmed the composition of the nanoparticles as Au 2 S. Some isolated faceted gold nanoparticles were also observed. XPS data also support the formation of Au 2 S nanoparticles. UV-vis absorption spectra showed only absorption features from the Au 2 S nanoparticles, and an indirect band gap value of 1.77 eV was obtained. Sufficiently high concentrations of Na 2 S and CTAB in the reaction mixture were concluded to be necessary to promote the growth of Au 2 S nanoparticles, and reduce the production of gold nanocrystals. Maintenance of the superstructures after boiling in water suggests the absence of CTAB molecules on the particle surfaces. FT-IR, XPS, and NMR results indicate thermolysis of CTAB, and long alkyl chains interact with the Au 2 S nanoparticles and direct their assembly into a lamellar phase structure.

We report the growth of ultralong β-Ga(2)O(3) nanowires and nanobelts on silicon substrates using a vapor phase transport method. The growth was carried out in a tube furnace, with gallium metal serving as the gallium source. The nanowires and nanobelts can grow to lengths of hundreds of nanometers and even millimeters. Their full lengths have been captured by both scanning electron microscope (SEM) and optical images. X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images have been used to study the crystal structures of these nanowires and nanobelts. Strong blue emission from these ultralong nanostructures can be readily observed by irradiation with an ultraviolet (UV) lamp. Diffuse reflectance spectroscopy measurements gave a band gap of 4.56 eV for these nanostructures. The blue emission shows a band maximum at 470 nm. Interestingly, by annealing the silicon substrates in an oxygen atmosphere to form a thick SiO(2) film, and growing Ga(2)O(3) nanowires over the sputtered gold patterned regions, horizontal Ga(2)O(3) nanowire growth in the non-gold-coated regions can be observed. These horizontal nanowires can grow to as long as over 10 µm in length. Their composition has been confirmed by TEM characterization. This represents one of the first examples of direct horizontal growth of oxide nanowires on substrates.