Room temperature hydrogen sensing of multiple networked ZnO/WO3 core–shell nanowire sensors under UV illumination

ZnO/WO₃ core–shell nanowires were synthesized by thermal evaporation of a mixture of ZnO and graphite powders (ZnO:C = 1:1) followed by sputter-deposition of WO₃. The sensing properties of multiple networked ZnO-core/WO₃-shell nanorod sensors toward H₂ gas was examined. The responses of pristine ZnO and ZnO-core/WO₃-shell nanorods to 1000 ppm H₂ at room temperature under UV illumination were ∼236% and ∼645%, respectively. The responses of the core–shell nanowires increased from ∼118 to ∼645% with increasing the UV illumination intensity from 0 mW/cm² to 1.2 mW/cm². The enhanced sensing performance of the ZnO-core/WO₃-shell nanowires induced by encapsulation with WO₃ was explained based on a combination of surface depletion and potential barrier-controlled carrier transport models. The origin of the enhanced sensing properties of ZnO-core/WO₃-shell nanorods toward H₂ under UV illumination was also discussed.     

Structural and electrical properties of core–shell structured GaP nanowires with outer Ga2O3 oxide layers

This paper presents a review of our current experimental research on GaP nanowires grown by a vapor deposition method. Their structural, electrical, opto-electric transport, and gas-adsorption properties are reviewed. Our structural studies showed that a GaP nanowire consisted of a core–shell structure with a single-crystalline GaP core and an outer Ga₂O₃ layer. The individual GaP nanowires exhibited n-type field effects. Their electron mobilities were in the range of about 6 to 22 cm²/V s at room temperature. When the nanowires were illuminated with an ultraviolet light source, an abrupt increase of conductance occurred resulting from carrier generation in the nanowire and de-adsorption of adsorbed OH^- or O₂ ^- ions on the Ga₂O₃ surface shell. Using an intrinsic Ga₂O₃ shell layer as a gate dielectric, top-gated GaP nanowire field-effect transistors were fabricated and characterized. Like other metal oxide nanowires, the carrier concentration and mobility of GaP nanowires were significantly affected by the surface molecular adsorption of OH or O₂. The GaP nanowire devices were fabricated as sensors for NO₂, NH₃, and H₂ gases by using a simple metal decoration technique. PACS 73.63.-b; 72.80.Ey; 85.35.-p     

Preparation and photocatalytic activity of Mo-doped WO3 nanowires

Mo-doped WO₃ nanowires were fabricated by a hydrothermal method in the presence of K₂SO₄. The physical properties of prepared nanowires were characterized by X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the obtained products are nanowires with diameters ranging between 10 and 20 nm, and lengths of about 600 nm. Its photoactivity was evaluated through the photodegradation of methylene blue (MB) in aqueous solution. Effects of the molybdenum concentration on the photoactivity of the obtained samples were investigated detailedly. The experimental results indicated that the Mo-doping enhanced the photoactivity of WO₃ nanowires.     

Ultraviolet photodetection properties of indium oxide nanowires

Photoconducting properties of In₂O₃ nanowires were studied. Devices based on individual In₂O₃ nanowires showed a substantial increase in conductance of up to four orders of magnitude upon exposure to UV light. Such devices also exhibited short response times and significant shifts in the threshold gate voltage. The sensitivity to UV of different wavelengths was studied and compared. We have further demonstrated the use of UV light as a “gas cleanser” for In₂O₃ nanowire chemical sensors, leading to a recovery time as short as 80 s. Received: 8 January 2003 / Accepted: 9 January 2003 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-213/740-8677, E-mail: chongwuz@usc.edu     

Morphologically different WO3 nanocrystals in photoelectrochemical water oxidation

Different morphologies of WO₃ nanocrystals such as nanorods and nanoplates have been obtained under hydrothermal conditions using ammonium metatungstate as the precursor in presence of different organic acids such as citric, oxalic, and tartaric acid in the reaction medium. Detailed characterization of the crystal structure, particle morphology, and optical band gap of the synthesized powders have been done by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and solid-state UV–visible spectroscopy study. The as-synthesized materials are WO₃ hydrates with orthorhombic phase which transform to the hexagonal WO₃ through dehydration upon heating at 350 °C. The resultant products are crystalline with nanoscale dimensions. Finally, the photoactivity of the synthesized materials annealed at 500 °C has been compared employing in photoelectrochemical water oxidation under the illumination of AM 1.5G simulated solar light (100 mWcm⁻²). The photocurrent measurements upon irradiation of light exhibit obvious photocatalytic activity with a photocurrent of about 0.77, 0.61, and 0.65 mAcm⁻² for the WO₃ film derived with the oxalic acid, tartaric, and citric acid assisting agents, respectively, at 1.8 V versus Ag/AgCl electrode.     

Unprecedented photocatalytic activity of carbon coated/MoO3 core–shell nanoheterostructurs under visible light irradiation

We reveal that nano-scale carbon layer deposited by hydrothermal process on molybdenum oxide (MoO₃) nanowires surface significantly improve the light absorption range. Furthermore, the graphene-carbon coated MoO₃ nanocopmosite (rGO/C-MoO₃ nanocomposite) exhibits excellent chemical stability and enhanced photocatalytic activity for methylene blue in aqueous solution under visible light irradiation compared to the bare MoO₃ nanowires and carbon coated MoO₃ nanowires (C-MoO₃ nanowires). The enhanced photocatalytic activity of rGO/C-MoO₃ nanocomposite could be attributed to the extended light absorption range, better adsorptivity of dye molecules and efficient separation of photogenerated electrons and holes. Overall, this work provides new insights that the as synthesized rGO/C-MoO₃ nanocomposite can be efficiently used as high performance photocatalysts to improve the environmental protection issues under visible light irradiation.     

Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting

Tungsten trioxide (WO₃) is a technologically important material for photoelectrochemical (PEC) water-splitting for the solar production of hydrogen fuel from water. For PEC water-splitting, high aspect ratio WO₃ nanostructures such as nanowires (NWs) and nanotubes (NTs) are superior to planar WO₃ films because they orthogonalize the directions of light absorption (along the long axis) and charge transport (across the short radius), leading to both efficient light absorption and charge carrier collection. However, PEC water-splitting requires the growth of WO₃ on delicate transparent conducting oxide (TCO) substrates that cannot tolerate high temperature processing. To date, the large-scale, rapid, economical synthesis of high aspect ratio WO₃ nanostructures on these delicate TCO substrates remains a major challenge. Previously, we synthesized WO₃ NW arrays by a rapid, atmospheric and scalable flame vapor deposition (FVD) method, in which a flame oxidizes and evaporates tungsten metal to produce tungsten oxide vapors that condense onto a colder substrate in the form of NWs. Nevertheless, at substrate temperatures low enough to ensure the health of the TCO, the growth of WO₃ NW arrays was non-uniform and sparse due to limitations of the experimental design. Herein, we significantly improve the FVD design to grow uniform and densely packed WO₃ nanostructures on TCO substrates, thereby enabling the application of these WO₃ nanostructures to PEC water-splitting. The morphology of the nanostructures varied from densely packed multi-shell NTs and single-shell NTs to NWs as we increased the substrate temperature in the range 530–700 °C. Importantly, the WO₃ NTs synthesized by FVD had higher areal number density and longer length than state-of-the-art WO₃ NW photoanodes grown by chemical vapor deposition and hydrothermal methods, resulting in stronger light absorption and superior PEC water-splitting performance. Thus, in addition to being scalable, rapid and economical, the FVD method also synthesizes materials of high quality.     

Vibrational studies of hexagonal bronze systems: phonon calculation and high pressure induced phase transformation

Hexagonal bronzes and valence‐balanced hexatungstates (VBHT) have attracted great attention for presenting rich polymorphism and displaying superconductor or ferroelectric properties. In the present work, structural and vibrational properties of RbBi_1/3W_8/3O₉ (RBW) crystal (VBHT type) were investigated by high‐pressure Raman scattering experiments. The results suggest the existence of a reversible pressure‐induced structural phase transition at about 4 GPa. This transformation is most likely related to rotations of octahedral units along the c‐axis with no abrupt changes of apical O W O bonds length. In order to get further understanding of RBW vibrational properties, we performed phonon calculations, by using classical lattice dynamics, in the related hexagonal structure of the K_0.26WO₆ system. These calculations revealed that most of vibrational properties of K_0.26WO₆ are governed by tubular ‘like’ vibrations of the hexagonal cavity, which resemble similar vibrations of tubular nanostructures. The pressure dependence of Raman modes is understood in terms of calculated phonon eigenvectors at ambient pressure. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermochemistry of the cubic and hexagonal lithium tungsten bronze phases LixWO3

The enthalpies of formation of hexagonal WO₃, hexagonal and cubic Li_xWO₃ have been determined by solution calorimetry. The ambient-temperature materials h-Li_0.44WO₃ and c-Li_xWO₃ (x=0.51, 0.71) were prepared by n-butyl lithiation in hexane. The thermochemistries of the lithium insertion compounds were examined and related to structural features.     

Sensors for the nitrogen oxides, NO2, NO and N2O, based on In2O3 and WO3 nanowires

Sensing characteristics of ZnO, In₂O₃ and WO₃ nanowires have been investigated for the three nitrogen oxides, NO₂, NO and N₂O. In₂O₃ nanowires of ∼20 nm diameter prepared by using porous alumina membranes are found to have a sensitivity (defined as the ratio of the sensor resistance in the gas concerned to that in air) of about 60 for 10 ppm of all the three gases at a relatively low temperature of 150 °C. The response and recovery times are around 20 s. The sensitivity of these In₂O₃ nanowires is around 40 for 0.1 ppm of NO₂ and N₂O at 150 °C. WO₃ nanowires of 5–15 nm diameter, prepared by the solvothermal process show a sensitivity of 20–25 for 10 ppm of the three nitrogen oxides at 250 °C. The response and recovery times are 10 s and 60 s, respectively. The sensitivity is around 10 for 0.1 ppm of NO₂ at 250 °C. The sensitivity of In₂O₃ and WO₃ nanowires is not affected by humidity even up to 90% relative humidity. The study also reveals that the sensing mechanism for the three nitrogen oxides have a commonality in that the desorption of oxygen is a crucial step in all the cases. PACS 07.07.Df; 85.35.-p; 82.35.Np     

Soft X-ray spectroscopic study on the electronic structure of WO3 thin films fabricated under various annealing temperature and gas flow conditions

The electrical properties of WO₃ thin films vary significantly depending on the growth conditions. In this work, the influence of O₂ gas on the band gap of WO₃ thin films during growth was investigated via electronic structure characterization using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). A substantial decrease in the electrical conductivity of the WO₃ films was observed with an increase in the O₂ partial pressure during growth. Spectral differences in the peak energy and intensity were apparent for WO₃ films grown under only Ar and those grown in Ar:O₂. It is difficult to explain the acquired spectrum of WO₃ with oxygen defects through the rigid-band model in terms of the simple addition of electrons to the conduction band of WO₃. Our results show that an oxygen deficiency in WO₃ moves the conduction band to the Fermi edge.     

Vibrational spectroscopy of alkaline tungsten oxyfluoride crystals: structure,lattice dynamics,ordering processes,and phase transitions

Raman scattering (RS) and infrared absorption (IR) of ammonium oxyfluoride crystals (NH₄)₃WO₃F₃, (NH₄)₂KWO₃F₃, and Cs₂(NH₄)WO₃F₃are compared. Conformation of the WO₃F₃ octahedral groups has been established; anomalies have been found close to the transition temperatures in the internal vibrational regions of ammonium and WO₃F₃ groups. The phase transition in (NH₄)₃WO₃F₃ is associated mostly with the ordering of octahedral groups and formation of W O···H N hydrogen bonds. In (NH₄)₂KWO₃F₃ crystal, the transition is not related to the ordering processes; Cs₂(NH₄)WO₃F₃ retains its disordered structure down to 10 K. Copyright © 2010 John Wiley & Sons, Ltd.     

Structural characterization of ZnO/ Er2O3 core/shell nanowires

Zinc oxide/erbium oxide core/shell nanowires are of great potential value to optoelectronics because of the possible demonstration of laser emission in the 1.5 μm range. In this paper we present a convenient technique to obtain structures of this composition. ZnO core nanowires were first obtained by a vapor–liquid–solid (VLS) method using gold as a catalyst. ZnO nanowires ranging from 50 to 100 nm in width were grown on the substrates. Erbium was incorporated into these nanowires by their exposure to Er(tmhd)₃ at elevated temperatures. After annealing at 700 C in air, the nanowires presented 1.54 μm emission when excited by any of the lines of an Ar⁺ laser. An investigation of nanowire structure by HRTEM indicates that indeed the cores consist of hexagonal ZnO grown in the 001 direction while the surface contains randomly oriented Er₂O₃ nanoparticles. EXAFS analysis reveals that the Er atoms possess a sixfold oxygen coordination environment, almost identical to that of Er₂O₃. Taken collectively, these data suggest that the overall architectures of these nanowires are discrete layered ZnO/ Er₂O₃ core/shell structures whereby erbium atoms are not incorporated into the ZnO core geometry.     

Ultraviolet-induced erasable photochromism in bilayer metal oxide films

We demonstrate that the optical transmittance of bilayer samples consisting of pyrolytically coated amorphous Mg-Sn-O and metal oxide films such as In₂O₃ and SnO₂ decreases upon ultraviolet illumination, but can be recovered by annealing in air at ∼300 ^°C. Spectral, structural, and compositional studies suggest that this photochromic phenomenon is induced by photoelectronic excitation in the Mg-Sn-O film, electron injection into the metal oxide, which becomes negatively charged, and subsequent formation of metallic particles, which absorb and/or scatter visible light.     

Production of ultrafine particles of high-temperature tetragonal WO<Subscript>3</Subscript> by dc arc discharge in Ar–O<Subscript>2</Subscript> gases

Ultrafine particles of WO₃ are successfully produced by dc arc discharge in Ar-O₂ gases. Particle sizes are distributed from 10 nm to 1 m depending on production conditions: gas pressure, collection position and discharge current. Observations of the cooled particles by electron microscopy indicate that the WO₃ particles are tetragonal, a phase that is usually only stable above 725 °C. The octahedral crystals are bounded by eight {1 0 1} faces and occasionally truncated by {1 0 0} and/or {0 0 1} faces. This method of producing WO₃ by dc arc discharge therefore affords a high-temperature phase that is preserved upon cooling to room temperature.     

单根砷掺杂氧化锌纳米线场效应晶体管的电学及光学特性

采用化学气相沉积（CVD）的方法在砷化镓基底上合成直径为20 nm左右、长约数十微米的氧化锌纳米线,然后采用热扩散的方法,将生长于砷化镓基底之上的氧化锌纳米线通过600 ℃,30 min的有氧退火处理后,获得了砷掺杂的氧化锌纳米线.将获得的掺杂后的氧化锌纳米线采用电子束曝光以及真空溅射镀膜的方法将钛/金合金作为接触电极引出,从而构建成场效应晶体管.文中研究了单根氧化锌纳米线砷掺杂前后的电学特性,证实了通过砷掺杂来获得p型的氧化锌纳米线的可行性.构建的p型砷掺杂氧化锌场效应晶体管的跨导为35 nA/V,载流 关键词： p型ZnO纳米线 砷掺杂 场效应晶体管 光致发光     

Adsorption behaviors of V2O5 nanowires on binary mixed self-assembled monolayers

The adsorption behaviors of V₂O₅ nanowires on binary mixed self-assembled monolayers (SAMs) were investigated with variation of the mixing ratio of two differently terminated thiolates on Au. Hydroxyl-covered V₂O₅ nanowires showed a preferential adsorption on amine (NH₂)-terminated thiolates over methyl (CH₃)-terminated ones. However, on the binary mixed SAM of NH₂- and CH₃-terminated thiols, the adsorption behavior did not follow a simple expectation based upon the electrostatic interaction. The total number of adsorbed V₂O₅ nanowires increased with the mole fraction of NH₂-terminated thiolates up to χNH₂∼0.5, then it decreased with further increase of χNH₂. The height distribution of adsorbed nanowires showed that the relative portion of the agglomerated wires thicker than 3.5 nm to individual wires thinner than 3.5 nm increased up to χNH₂∼0.75 and then it decreased with further increase of χNH₂. The dispersion of molecules with polar-functional groups as well as the molecular ordering of mixed SAMs is attributed to such adsorption behaviors of V₂O₅ nanowires.     

Structure and electrical conduction of WO3 nanorods epitaxially grown on mica

Tunsten oxide nanorods are grown on mica substrate in air from WO₃ vapor at 590 °C. They are epitaxially oriented on the substrate in three directions according to the hexagonal symmetry of the mica. Their morphology was investigated by Atomic Force Microscopy (AFM), their structure by Electron Diffraction (ED) and High Resolution Electron Microscopy (HRTEM). The energy dispersive X-ray analysis (EDX) associated with the TEM revealed the presence of potassium in the first step of the nanostructures growth suggesting the formation of a tungsten bronze. According to their thickness the nanorods have a structure either hexagonal or monoclinic. The structural investigations showed that numerous kinds of planar defects parallel to the growth direction are formed. The electrical conduction was analyzed with a Conductive Atomic Force Microscopy (CAFM) method which allows to obtain either an image of the resistance variations along the nanorods or a current-voltage response. The result demonstrates that the WO₃ nanorods form electrically networks suitable for gas sensing experiments.     

The WO3/WS2 nanostructures: Preparation,characterization and optical absorption properties

The WO₃/WS₂ nanostructures were successfully prepared using a two-step hydrothermal/gas phase method. The physical properties of the nanostructures were characterized using XRD, SEM, TEM, UV–visible spectroscopy. The WO₃/WS₂ nanostructures obtained were coexistence of WO₃ and WS₂ in the same particle. The WO₃/WS₂ nanostructures contained a wide and intensive absorption in the UV–visible light region of 245–750 nm, which showed that the WO₃/WS₂ nanostructures may have a potential application as an UV–visible photocatalyst.     

Synthesis of TiO2/WO3 nanoparticles via sonochemical approach for the photocatalytic degradation of methylene blue under visible light illumination

Through an ultrasound assisted method, TiO₂/WO₃ nanoparticles were synthesized at room temperature. The XRD pattern of as-prepared TiO₂/WO₃ nanoparticles matches well with that of pure monoclinic WO₃ and rutile TiO₂ nanoparticles. TEM images show that the prepared TiO₂/WO₃ nanoparticles consist of mixed square and hexagonal shape particles about 8–12 nm in diameter. The photocatalytic activity of TiO₂/WO₃ nanoparticles was tested for the degradation of a wastewater containing methylene blue (MB) under visible light illumination. The TiO₂/WO₃ nanoparticles exhibits a higher degradation rate constant (6.72 × 10⁻⁴ s⁻¹) than bare TiO₂ nanoparticles (1.72 × 10⁻⁴ s⁻¹) under similar experimental conditions.