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The photocatalytic activity of GaN nanowires was investigated for the use of GaN nanowires as photocatalysts in harsh environments. GaN nanowires with diameters of 20-50 nm and lengths of 4-6 microm were prepared by Ni catalyst-assisted metal-organic chemical vapor deposition. Comparisons of GaN nanowires with GaN submicron dot arrays and thin films showed that GaN nanowires exhibit much better photocatalytic activity, resulting from a high surface-to-volume ratio. In addition, GaN nanowires exhibited good ability to photodegrade organic dye at various pHs, even under strong acidity and alkalinity. The photocatalytic activity of GaN nanowires was also compared with that of ZnO and TiO(2) nanowires.
ACS Nano 2008 Apr
PMID:Photocatalysis using GaN nanowires. 1920 93

ZnO nanowires (NWs) are grown on a bulk copper half-transmission electron microscopy grid by chemical vapor deposition in a high temperature tube furnace. Photoluminescence (PL) microscopy revealed band gap emission at 380 nm and a more intense visible emission around 520 nm due to defect states in these NWs. High-resolution transmission electron microscopy shows that the ZnO NWs are single crystalline with hexagonal structure. Auger electron spectroscopy (AES) and energy dispersive X-ray spectroscopy reveal that copper atoms are present along the length of the NW. AES also found that the surface of the NWs is oxygen rich. The surface concentration of zinc increases moving from the tip toward the base of the NW while the concentration of oxygen decreases. The copper in this system not only remains at the tip of the growing NW but also acts as a dopant along the length of the NW, leading to a decrease in the intensity of the band gap PL of these NWs.
ACS Nano 2008 Feb
PMID:Analysis of copper incorporation into zinc oxide nanowires. 1920 39

The solid-state reaction within a coaxial Al2O3/ZnO/Al2O3 multilayered microtubular structure can be used to prepare discrete microtube-in-microtube ZnAl2O4 spinel assemblies through a Kirkendall void production mechanism at 700 degrees C. In contrast with previous studies of the nanoscale Kirkendall effect, the reaction observed here proceeds through a bi-directional vacancy diffusion mechanism wherein ZnO species diffuse into inner- and outer-Al2O3 concentric layers, thereby resulting in vacancy supersaturation and void production between two isolated spinel microtubes. Low-temperature atomic layer deposition (ALD) of Al2O3 and ZnO enables the fabrication of complex coaxial multilayered microtubes with precise control of the starting film thicknesses and relative composition. When a molar excess of ZnO is present between two Al2O3 layers, electron microscopy images reveal incomplete ZnO consumption after annealing at 700 degrees C. At higher initial Al2O3 concentrations, however, complete reaction with ZnO is observed, and the size of the Kirkendall gap between isolated spinel microtubes appears to be directly influenced by the thickness of the intermediate ZnO layer.
ACS Nano 2009 Mar 24
PMID:Bi-directional Kirkendall effect in coaxial microtube nanolaminate assemblies fabricated by atomic layer deposition. 1922 33

Vertically aligned, dense ZnO nanorod arrays were grown directly on zinc foils by a catalyst-free, low-temperature (450-500 degrees C) oxidization method. The zinc foils remain conductive even after the growth of ZnO nanorods on its surface. The success of this synthesis largely relies on the level of control over oxygen introduction. By replacing zinc foils with zinc microspheres, unique and sophisticated urchin-like ZnO nanorod assemblies can be readily obtained.
ACS Nano 2009 Feb 24
PMID:Aligned ZnO nanorod arrays grown directly on zinc foils and zinc spheres by a low-temperature oxidization method. 1923 61

A detailed study of kinetic and thermodynamic aspects in the microwave-assisted synthesis of ZnO nanoparticles from zinc acetate and benzyl alcohol is presented. The use of a nonaqueous sol-gel approach provides the unique opportunity to investigate simultaneously the organic reaction, that is, the esterification between acetate and benzyl alcohol, and the inorganic process, represented by the growth of the ZnO nanoparticles. Monitoring both the formation of the organic species as well as ZnO crystal size with time makes it possible to directly correlate the kinetics of the organic side reaction with the growth kinetics of the ZnO nanoparticles. The esterification reaction, which is the chemical basis for producing the monomers for ZnO formation, was found to be first order. The growth of the ZnO nanoparticles followed the Lifshitz-Slyozov-Wagner model for coarsening, pointing to a diffusion-limited process. Comparison of the microwave-mediated route with conventional heating showed that microwave irradiation greatly accelerates nanoparticle formation by (a) facilitating the dissolution of the precursor in the solvent, (b) increasing the rate constants for the esterification reaction by 1 order of magnitude, resulting in faster production of monomer and consequently in an earlier nucleation event, and (c) increasing the rate constants k(growth) for the crystal growth from 3.9 nm(3)/min (conventional heating) to 15.4 nm(3)/min (microwave heating).
ACS Nano 2009 Feb 24
PMID:Kinetic and thermodynamic aspects in the microwave-assisted synthesis of ZnO nanoparticles in benzyl alcohol. 1923 87

We report the fabrication and characterization of ZnO nanowire memory devices using a ferroelectric Pb(Zr(0.3)Ti(0.7))O(3) (PZT) film as the gate dielectric and the charge storage medium. With a comparison to nanowire transistors based on SiO(2) gate oxide, the devices were evaluated in terms of their electric transport, retention, and endurance performance. Memory effects are observed as characterized by an eminent counterclockwise loop in I-V(g) curves, which is attributed to the switchable remnant polarization of PZT. The single-nanowire device exhibits a high (up to 10(3)) on/off ratio at zero gate voltage. Our results give a proof-of-principle demonstration of the memory application based on a combination of nanowires (as channels) and ferroelectric films (as gate oxide).
ACS Nano 2009 Mar 24
PMID:Ferroelectric transistors with nanowire channel: toward nonvolatile memory applications. 1924 45

Nanowires with twinned morphology have been observed in many cubic-phase materials including spinel. We study systematically the formation of multitwinned Zn(2)TiO(4) nanowires based on a solid-solid reaction of ZnO nanowires with a conformal shell of TiO(2), which is deposited by atomic layer deposition (ALD). By varying the solid-state reaction temperature, reaction time, and TiO(2) shell thickness, the formation process is carefully analyzed with the help of transmission electron microscopy. It is found that the multitwins develop through an oriented attachment of initially separated spinel nanobricks and a simultaneous Ostwald ripening process. The oriented assembly of the individual bricks is strongly dependent on annealing conditions, which is required to favor the motion and interaction of the bricks. This mechanism differs dramatically from those proposed for twinned nanowires grown with the presence of metal catalysts. Our result provides new insights on controlling the morphology and crystallinity of designed 1-D nanostructures based on a solid-state reaction route.
ACS Nano 2009 Mar 24
PMID:Multitwinned spinel nanowires by assembly of nanobricks via oriented attachment: a case study of Zn2TiO4. 1925 79

We report the growth of ultrathin diamond nanorods (DNRs) by a microwave plasma assisted chemical vapor deposition method using a mixture gas of nitrogen and methane. DNRs have a diameter as thin as 2.1 nm, which is not only smaller than reported one-dimensional diamond nanostructures (4-300 nm) but also smaller than the theoretical value for energetically stable DNRs. The ultrathin DNR is encapsulated in tapered carbon nanotubes (CNTs) with an orientation relation of (111)diamond//(0002)graphite. Together with diamond nanoclusters and multilayer graphene nanowires/nano-onions, DNRs are self-assembled into isolated electron-emitting spherules and exhibit a low-threshold, high current-density (flat panel display threshold: 10 mA/cm2 at 2.9 V/microm) field emission performance, better than that of all other conventional (Mo and Si tips, etc.) and popular nanostructural (ZnO nanostructure and nanodiamond, etc.) field emitters except for oriented CNTs. The forming mechanism of DNRs is suggested based on a heterogeneous self-catalytic vapor-solid process. This novel DNRs-based integrated nanostructure has not only a theoretical significance but also has a potential for use as low-power cold cathodes.
ACS Nano 2009 Apr 28
PMID:Self-assembled growth, microstructure, and field-emission high-performance of ultrathin diamond nanorods. 1934 50

We present a model to calculate particle size distributions (PSDs) of colloidal ZnO nanoparticles from their absorbance spectra. Using literature values for the optical properties of bulk ZnO and correlating the measurement wavelengths in the UV-visible regime with distinct particle sizes by a tight binding model (TBM), an algorithm deconvolutes the absorbance spectra into contributions from size fractions. We find an excellent agreement between size distributions determined from TEM images and the calculated PSDs. For further validation, bimodal PSDs have been investigated and an approach to determine not only particle size but also solid concentration is introduced. We will show the applicability of our model by the determination of temperature-dependent ripening rates, which enables the calculation of solubilities, surface tensions, and the activation enthalpy of ripening. In principle, our methodology is applicable to different semiconductor nanoparticles in various solvents as long as their bulk properties are known and scattering is negligible.
ACS Nano 2009 Jul 28
PMID:Analysis of optical absorbance spectra for the determination of ZnO nanoparticle size distribution, solubility, and surface energy. 1950 65

Controlling the morphology of the as-synthesized nanostructures is usually challenging, and there lacks of a general theoretical guidance in experimental approach. In this study, a novel way of optimizing the aspect ratio of hydrothermally grown ZnO nanowire (NW) arrays is presented by utilizing a systematic statistical design and analysis method. In this work, we use pick-the-winner rule and one-pair-at-a-time main effect analysis to sequentially design the experiments and identify optimal reaction settings. By controlling the hydrothermal reaction parameters (reaction temperature, time, precursor concentration, and capping agent), we improved the aspect ratio of ZnO NWs from around 10 to nearly 23. The effect of noise on the experimental results was identified and successfully reduced, and the statistical design and analysis methods were very effective in reducing the number of experiments performed and in identifying the optimal experimental settings. In addition, the antireflection spectrum of the as-synthesized ZnO NWs clearly shows that higher aspect ratio of the ZnO NW arrays leads to about 30% stronger suppression in the UV-vis range emission. This shows great potential applications as antireflective coating layers in photovoltaic devices.
ACS Nano 2009 Jul 28
PMID:Optimizing and Improving the Growth Quality of ZnO Nanowire Arrays Guided by Statistical Design of Experiments. 1953 70


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