Phonon confinement in individual titanium dioxide nanowires

Size effects on the phonon modes have been unambiguously observed on single individual titanium dioxide (TiO₂) nanowire using Raman spectroscopy. A template-free oxidation process using ethanol vapor was developed to control the growth and the density, length, and size distribution of the TiO₂ nanowires. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to characterize the morphology, crystalline phases and microstructures. Using a confocal laser probe, Raman spectroscopy revealed in the individual rutile TiO₂ nanowire, the E_g, A_1g vibration modes which are confined in the radial directions. Along the single-crystalline nanowire axis, the systematic size-dependent variations in the center-shift, broadening as well as the shape asymmetry of the Raman peaks can be well explained in terms of the phonon confinement model.     

Thermal evaporation synthesis of zinc oxide nanowires

High quality ZnO nanowires were synthesized at high temperature without using heterogenous catalysts. The nanowires had a uniform prismatic shape and were grown in a cacti-like morphology. Characterizations of the products by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy showed that the ZnO nanowires were single crystalline and of high purity. An intensive exciton emission was observed around 3.25 eV from the ZnO nanowires at room temperature. The growth mechanism was discussed based on the experimental conditions and the ZnO crystal growth habits. This growth method can be used to prepare other metal oxide nanowires. PACS 61.46.+w; 81.16.-c; 81.07.De, 81.05.Hd     

A novel one-step electrochemical method to obtain crystalline titanium dioxide films at low temperature

A crystalline titanium dioxide (TiO₂) thin film on various metal substrates such as hot-dip-galvanized (HDG), electrochemically galvanized (EG) and cold rolled steel (CRS) has been successfully prepared for the first time by simple one-step electrodeposition method from alkaline aqueous solution containing potassium titanium oxalate and hydroxylamine. The as-grown titanium dioxide films are composed of a mixture of anatase, rutile and brookite structures and have a good crystalline state. The present investigation reveals that the electrochemical deposition of crystalline titanium dioxide on different metal substrates from single electrochemical bath is possible and very promising as a preparation method for industrial applications.     

Growth and mechanism of titania nanowires

Anatase and rutile-phase titania nanowires have been prepared via an efficient molten salt-assisted and novel pyrolysis route, respectively. The growth of anatase nanowires is parallel to [010] direction. The anatase titanium oxide nanowires are obtained by exchange reaction between Na₂TiO₃ and HCl, whereas the formation of rutile titania nanowires is conventional vapor-liquid-solid growth mechanism.     

Alumina template-assisted electrodeposition of Bi2Te2.7Se0.3 nanowire arrays

Bi₂Te_2.7Se_0.3 nanowire arrays have been fabricated by electrodeposition into the pores of an anodic aluminum oxide (AAO) template followed by annealing at 300 °C under Ar atmosphere. The as-prepared nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The nanowires are uniform single crystalline with diameter of ∼14 nm.     

Self-assembly of Sb2O3 nanowires into microspheres: Synthesis and characterization

Sb₂O₃ nanowires with diameters of ∼233 nm and microspheres assembled by these nanowires were successfully synthesized by a simple poly-(vinylpyrrolidone) (PVP) assisted hydrothermal method. The morphologies, nano/microstructures and optical properties of the as-grown nanowires and microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis diffuse reflection spectrum. It has been found that the experimental parameters, such as mineralizers, played crucial roles in the morphological control of Sb₂O₃ nanowires. The possible growth mechanism of microspheres has been proposed.     

Preparation and photoluminescence properties of silica-coated CuO nanowires

We have fabricated cupric oxide (CuO)-core/silica (SiO _x )-shell nanowires by using a two-step process: thermal oxidation and sputtering. The structure and photoluminescence (PL) properties of the core/shell nanowires has been investigated by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and PL analysis techniques. The CuO cores and the SiO _x shells of the as-synthesized nanowires have crystalline monoclinic CuO and amorphous SiO _x structures, respectively. The PL emission intensity of the CuO-core/SiO _x -shell nanowires has been increased but the emission peak position has not been nearly shifted by annealing in a nitrogen atmosphere, whereas the emission peak position has been shifted a lot from 510 to around 650 nm as well as the emission intensity has been increased by annealing in an oxygen atmosphere. In addition, the origin of the PL enhancement in the CuO-core/SiO _x -shell nanowires by annealing and the growth mechanism of the CuO nanowires have been discussed.     

Fabrication and characterization of CuO-core/TiO2-shell one-dimensional nanostructures

We have fabricated CuO-core/TiO₂-shell one-dimensional nanostructures by coating the CuO nanowires with MOCVD-TiO₂. The structure of the core/shell nanowires has been investigated by using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis techniques. The CuO-cores and the TiO₂-shells of the as-synthesized nanowires have been found to have crystalline monoclinic CuO and crystalline tetragonal anatase TiO₂ structures, respectively. The CuO-core/TiO₂-shell nanowires are winding and has rougher surface, whereas the CuO nanowires are straight and have smoother surface.Influence of the substrate temperature and the growth time on the structure such as the morphology, size, and crystallographic orientation of CuO nanowires synthesized by thermal oxidation of Cu foils have also been investigated. All the nanowires have only the CuO phase synthesized at 600 °C, whereas those synthesized at 400 °C have both CuO and Cu₂O phases. The highest density of CuO nanowires with long thin straight morphologies can be obtained at 600 °C. In addition, the growth mechanism of the CuO nanowires has been discussed.     

Synthesis and characterization of GaN nanowires by ammoniating Ga2O3/Cr thin films deposited on Si(1 1 1) substrates

GaN nanowires have been successfully synthesized on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga₂O₃/Cr thin films at 950 °C. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), FT-IR spectrophotometer, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and photoluminescence (PL) spectrum were carried out to characterize the microstructure, morphology, and optical properties of GaN samples. The results demonstrate that the nanowires are single-crystal GaN with hexagonal wurtzite structure and high-quality crystalline, have the size of 30-80 nm in diameter and several tens of microns in length with good emission properties. The growth direction of GaN nanowires is perpendicular to the fringe of (1 0 1) plane. The growth mechanism of GaN nanowires is also discussed in detail.     

Fabrication and annealing effects of SnO2/SiOx nanocables sheathed by the sputtering technique

We reported an approach, in which we have produced the nano-sized crystalline tin oxide (SnO₂) particles with rutile structure. SnO₂ nanowires were coated with a shell layer of SiO_x via a sputtering method. Transmission electron microscopy and elemental mapping investigations revealed that the nanostructures consisted of a crystalline SnO₂ core surrounded by an amorphous SiO_x sheath. The annealing effects on the core-shell nanowires were investigated, revealing that the outer surface became rougher by the thermal annealing. For core-shell nanowires, a room-temperature PL measurement with a Gaussian fitting showed yellow, blue, and violet light emission bands, with the relative intensity of the yellow band showing an increase after thermal annealing. Possible PL emission mechanisms are discussed. This study reveals that the sputtering is effective for preparing the shell layers of nanocables.     

The structure and photoluminescence properties of TiO<Subscript>2</Subscript>-coated ZnS nanowires

The structure and photoluminescence properties of TiO₂-coated ZnS nanowires were investigated. ZnS nanowires were synthesized by thermal evaporation of ZnS powder and then coated with TiO₂ by using the metal organic chemical vapor deposition (MOCVD) technique. We performed scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, and photoluminescence (PL) spectroscopy to characterize the as-synthesized and TiO₂-coated ZnS nanowires. TEM and XRD analyses revealed that the ZnS core and the TiO₂ coatings had crystalline zinc blende and crystalline anatase structures, respectively. PL measurement at room temperature showed that the as-synthesized ZnS nanowires had two emissions: a blue emission centered in the range from 430 to 440 nm and a green emission at around 515 nm. The green emission was found to be dominant in the ZnS nanowires coated with TiO₂ by MOCVD at 350°C for one or more hours, while the blue emission was dominant in the as-synthesized ZnS nanowires. Also the mechanisms of the emissions were discussed.     

Growth of undoped and Fe doped TiO2 nanostructures and their optical and photocatalytic properties

Undoped and Fe-doped TiO₂ nanostructures have been successfully grown on Pt-coated quartz and Si (100) substrates using vapor-liquid-solid (VLS) growth method. The scanning electron microscopy (SEM) image showed that TiO₂ grew in nanowires (NWs) with diameters of 200–400 nm and lengths greater than 12 μm. However, the morphology of Fe-doped TiO₂ consists of chunk shaped nanoparticles (NPs). The X-ray diffraction analysis for undoped TiO₂ NWs clearly showed the formation of tetragonal rutile TiO₂, whereas for the Fe-doped TiO₂ NPs it showed orthorhombic TiO₂ phase and there are no crystalline peaks for iron or iron oxide. The refractive index and extinction coefficient values of undoped and Fe-doped TiO₂ nanostructures were derived from the ellipsometric measurements. Enhanced photocatalytic activities were obtained for undoped and Fe-doped TiO₂ nanostructures. The obtained results may find potential applications in optical devices and degradation of organic wastes.     

Synthesis of GaN nanowires by Tb catalysis

Rare earth metal seed Tb was employed as catalyst for the growth of GaN wires. GaN nanowires were synthesized successfully through ammoniating Ga₂O₃/Tb films sputtered on Si(1 1 1) substrates. The samples characterization by X-ray diffraction and Fourier transform infrared indicated that the nanowires are constituted of hexagonal wurtzite GaN. Scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy showed that the samples are single-crystal GaN nanowire structures. The growth mechanism of the GaN nanowires is discussed.     

Catalyst-free synthesis of GeO2 nanowires using the thermal heating of Ge powders

Germanium dioxide (GeO₂) nanowires have been synthesized by means of the simple evaporation of solid Ge powders, without using metal catalysts. The nanowires, with a diameter of about 90–200 nm, were characterized using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The obtained GeO₂ nanowires were crystalline with a hexagonal structure. The growth mechanism was discussed with respect to the vapor–solid process. The photoluminescence measurement revealed two emission peaks at about 2.45 eV and 2.91 eV at room temperature, opening up a route to potential applications in future optoelectronic nanodevices. Raman measurement of as-synthesized GeO₂ nanowires was made at room temperature.     

Synthesis of β-Ga2O3 nanowires through microwave plasma chemical vapor deposition

In this study, we demonstrate the large-scale synthesis of beta gallium oxide (β-Ga₂O₃) nanowires through microwave plasma chemical vapor deposition (MPCVD) of a Ga droplet in the H₂O and Ar atmosphere at 600 W. Unlike the commonly used MPCVD method, the H₂O, not mixture of gas, was employed to synthesize the nanowires. The ultra-long β-Ga₂O₃ nanowires with diameters of about 20-30 nm were several tens of micrometers long. The morphology and structure of products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). The growth of β-Ga₂O₃ nanowires was controlled by vapor-solid (VS) crystal growth mechanism.     

Studies on the effect of titanium addition on LiCoO2

The lithiated transition metal oxide has been used as the cathode materials for lithium ion rechargeable batteries. Among the various cathode materials, LiCoO₂ has been widely used. There are lot of reports on the substituted LiCoO₂ replacing small amount of Cobalt with other transition and nontransitional metals. Here, we focus on to a tetravalent transition metal atom such as titanium, as an addition in LiCoO₂ and studied its performance. The titled cathode material was synthesized by solid-state reaction method. Thermogravimetric/differential thermal analysis, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and particle size analysis were carried out to assess the effect of addition of titanium on LiCoO₂. Electrochemical studies were carried out by cyclic voltammetry and life cycle analyzer.     

Large-scale synthesis of In2O3 nanowires

In₂O₃ nanowires have been successfully fabricated on a large scale from indium particles by thermal evaporation at 1030 °C. The as-synthesized products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images show that these nanowires are uniform with diameters of about 60–120 nm and lengths of about 15–25 μm. XRD and selected-area electron diffraction analysis together indicate that these In₂O₃ nanowires crystallize in a cubic structure of the bixbyite Mn₂O₃ (I) type (also called the C-type rare-earth oxide structure). The growth mechanism of these nanowires is also discussed. Received: 29 June 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

Sol–gel template synthesis and characterization of LaCoO3 nanowires

Densely packed LaCoO₃ nanowires of the rare-earth perovskite-type composite oxide were synthesized within a porous anodic aluminum oxide (AAO) template by means of the sol–gel method using nitrate as raw the material and citric acid as the chelating agent. The results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the LaCoO₃ nanowires possessed a uniform length and diameter, which were controlled by the thickness and the pore diameter of the applied AAO template, respectively. The results of X-ray diffraction (XRD) and the selected area electron diffraction (SAED) indicated that the LaCoO₃ nanowires had a rhombohedral perovskite-type crystal structure. Furthermore, X-ray photoelectron spectroscopy (XPS) demonstrated that LaCoO₃ nanowires were formed. Finally, the formation mechanism of nanowires was also discussed. PACS 61.66.Fn; 61.46.-W; 81.20.Fw     

Effect of ambient pressure on the crystalline phase of nano TiO2 particles synthesized by a dc thermal plasma reactor

The synthesis of nanoparticles of titanium dioxide (TiO₂) with varying percentages of anatase and rutile phases is reported. This was achieved by controlling the operating pressure in a transferred-arc, direct current thermal plasma reactor in which titanium vapors are evaporated, and then exposed to ambient oxygen. The average particle size remained around 15 nm in each case. The crystalline structure of the as-synthesized nanoparticles of TiO₂ was studied with X-ray diffraction analysis; whereas the particle morphology was investigated with the help of transmission electron microscopy. The precursor species responsible for the growth of these nanoparticles was studied with the help of optical emission spectroscopy. As inferred from the X-ray diffraction analysis, the relative abundance of anatase TiO₂ was found to be dominant when synthesized at 760 Torr, and the same showed a decreasing trend with decreasing chamber pressure. The study also reveals that anatase TiO₂ is a more effective photocatalytic agent in degrading methylene blue by comparison to its rutile phase.     

Compositional and structural evolution of the titanium dioxide formation by thermal oxidation

Titanium oxide films were prepared by annealing DC magnetron sputtered titanium films in an oxygen ambient. X-ray diffraction （XRD）, Auger electron spectroscopy （AES） sputter profiling, MCs^＋-mode secondary ion mass spectrometry （MCs^＋-SIMS） and atomic force microscopy （AFM） were employed, respectively, for the structural, com- positional and morphological characterization of the obtained films. For temperatures below 875 K, titanium films could not be fully oxidized within one hour. Above that temperature, the completely oxidized films were found to be rutile in structure. Detailed studies on the oxidation process at 925K were carried out for the understanding of the underlying mechanism of titanium dioxide （TiO2） formation by thermal oxidation. It was demonstrated that the formation of crystalline TiO2 could be divided into a short oxidation stage, followed by crystal forming stage. Relevance of this recognition was further discussed.