Pure Tungsten Oxide Research Articles

Electrospun Tungsten Oxide Nanofibers: Fabrication and Characterization

AbstractTungsten oxide nanofibers have been successfully fabricated in a way based on electrospinning technique. A DMF solution of Poly(Vinyl Acetate)(PVAc, Mw=500,000) was mixed with tungsten isopropoxide to form a viscous precursor solution. Composite nanofibers were obtained by electrspinning this viscous solution. By calcination of the composite fibers, pure tungsten oxide nanofibers were obtained with controllable diameters of around 100 nm. Morphology of the fibers has been characterized by SEM and TEM. The relationship between solution concentration and ceramic nanofiber morphology has been studied. The detailed structure evolution process has been investigated by synchrotron based in-situ XRD. Specific phases of the oxide nanofibers at various calcination temperatures have been obtained from the in-situ XRD spectrum.

Gas-sensing properties of catalytically modified WO/sub 3/ with copper and vanadium for NH/sub 3/ detection

Ammonia gas detection by pure and catalytically modified WO/sub 3/-based gas sensors was analyzed. Sensor response of pure tungsten oxide to NH/sub 3/ was unsatisfactory, probably due to the unselective oxidation of ammonia into NO/sub x/. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH/sub 3/ detection. The introduction of Cu and V as catalytic additives improved the sensor response to NH/sub 3/. Possible reaction mechanisms of NH/sub 3/ over these materials are discussed. Sensor responses to other gases like NO/sub 2/ or CO and interference of humidity on ammonia detection were also analyzed so as to choose the best sensing element.

Sensitivity and selectivity improvement of rf sputtered WO 3 microhotplate gas sensors

Active layers consisting of rf sputtered WO 3 were deposited on microhotplate substrates. The films were doped with seven different materials (Pt, Au, Ag, Ti, SnO 2, ZnO and ITO (indium tin oxide)). The eight types of sensors (including pure tungsten oxide ones) were tested in the presence of ammonia, hydrogen sulphide, nitrogen dioxide, carbon monoxide and methane. It was found that gold improved the sensitivity to H 2S. On the other hand, doping with Ag and Pt led to higher responses to NO 2 and NH 3, respectively. No response to CH 4 was observed. The sensitivity to CO was very low. The influence of the working temperature on the sensor response was also studied. Our study proves that selective gas detection is possible combining a few tungsten oxide sensors with different dopants.

Proton Conducting Electrolyte Membranes based on Tungsten Oxide and Sulfonated Polyether Ether Ketone Hybrid Composites

ABSTRACTComposite membranes, prepared by mixing sulfonated polyether ether ketone (SPEEK) and WO3·2H2O in dimethylacetamide, were characterized by Thermogravimetry, Electrochemical Impedance Spectroscopy and Water Uptake measurements to evaluate their possible performance as proton exchange membranes (PEM). The body of results indicated the existence of a coordinative interaction between the water molecules of tungsten oxide and the sulfonic acid (-SO3H) groups of SPEEK. Moreover, EIS data demonstrated that the proton conductivity of the composite membranes is higher than both that of pure SPEEK and pure tungsten oxide, suggesting the presence of ion-rich regions where the proton transfer is favored. The SO3H - H2O interaction not only lead to enhancement of the proton conductivity of the membranes but also to improvement of their heat resistance as well as to decrease their water solubility.

Combinatorial electrochemical synthesis and characterization of tungsten-molybdenum mixed metal oxides

Automated systems for electrochemical synthesis and high throughput screening of photoelectrochemical materials were developed and used to prepare tungsten-molybdenum mixed metal oxides, WO3-MoO3. Two-dimensional arrays (library) of diverse composition were synthesized by automated cathodic electrodeposition on Ti foil substrate. Electrolytes were prepared by mixing W-peroxo complex and Mo-peroxo complex. The compositions in films were found to strongly depend on the composition of electrolyte, and found to be relatively independent of deposition voltage. The morphology was found to be very homogeneous. The electrodeposited oxides typically showed n-type behavior. Automated measurement of photocurrent by using a scanning photoelectrochemical cell showed that mixed oxides showed higher photocatalytic activity than pure tungsten oxide or pure molybdenum oxide. Photoactitvities of the mixed oxides were strongly dependent on the film composition.

Production of pure tungsten oxide from scheelite concentrates

Scheelite concentrates, obtained from Uludag, Bursa (Turkey), containing roughly 50% W, are treated by the chelate‐added acidic leaching route. Dissolved tungsten ions are then selectively precipitated by the addition of ‘ammonia ion‐containing’ reagents – either organic or inorganic – and thus are purified from other ions present in the solution. Selective precipitation of pure (> 99%) tungstate salt was realized from the polytungstate solution at 25°C by adding a ‘chelate‐breaking agent’ at a NH4+/WO3 weight ratio of ≥ 1/4. The formula of the complex salt is indicated by (NH4)3PW12O40·9H2O. The optimum precipitation conditions are investigated with the aim of making pure tungsten oxide via a thermal breakdown process. Tri‐ammonium‐dodeka‐tungstate‐phosphate salt can be easily converted to nanometer‐sized WO3 particles by a simple calcination operation carried out above 650°C. Weight loss ratios of the samples vary between 8.7 and 9.6% at the end of this 2‐h thermal breakdown process. Tungsten trioxide obtained can be suitably employed in tungsten‐related powder metallurgy applications with its extremely low impurity content of 0.04% Mo, 0.15% SiO2, 0.06% Fe and 0.03% Al.

Electrochromism of Mixed Tungsten-Vanadium Oxide Thin Films Grown by Pulsed Laser Deposition

Mixed vanadium tungsten oxide thin films were grown in oxygen atmosphere using pulsed laser deposition. With the oxygen pressure fixed to an optimized value of the influence of the substrate temperature and the film composition (i.e., vanadium content) on the electrochromism properties were studied. Whatever the film composition, films deposited on coated glass substrate were amorphous when deposited at room temperature while W-V-O (V<20%) films, deposited at 400°C, exhibited X-ray characteristics similar to those of monoclinic suggesting a solid solution. As a general trend, when V-based films cycle in protonic medium, both the cell capacity and the coloration efficiency decrease as the vanadium content increases. Upon very early cycling, a preferred reduction of the vanadium ions is observed with no further oxidation. Films containing a low amount of vanadium were more neutral in color than pure tungsten oxide. © 2001 The Electrochemical Society. All rights reserved.

Optical properties of photochromic organic–inorganic composites

We prepared several photochromic composite films based on a hybrid organic–inorganic matrix in which metal heteropolyoxometallates are entrapped. Infrared Fourier transform and UV-Visible spectroscopic ellipsometry methods have been used for the first time to study the coloring/bleaching process of the composite films. Data on all samples were acquired using a J.A. Woollam Co. VASE instrument and WVASE software. The optical absorption of these spin-coated films changed markedly by a reversible process, in the presence or absence of UV irradiation. The VIS-NIR transmission data showed that the films containing tungsten heteropolyoxometallate (HPOM) had strong absorptions at about 720 nm, 1108 nm and the films containing molybdenum HPOM had a strong absorption at 720 nm. We have also extracted for the first time the visible and infrared optical functions of the films. For the colored films, the Lorentz and Gaussian oscillators were used to describe the optical behavior in the 400–1700 nm wavelength range and 700–4000 cm−1 wavenumber range, respectively. The composite films containing tungsten heteropolyoxometallate (HPOM) showed faster coloration and bleaching than pure tungsten oxide films. The composite films with molybdenum HPOM showed faster coloration and much slower bleaching than tungsten HPOM.

Sputtering of beryllium, tungsten, tungsten oxide and mixed W–C layers by deuterium ions in the near-threshold energy range

An experimental method of determination of sputtering yield for current-conducting materials under ion bombardment of light gases in the near-threshold energy range has been developed. Such an information is very important in both the purely scientific and applied aspects. This method is based on the use of special regimes of field ion microscopic analysis. The procedure of measuring the sputtering yield includes cleaning of the surface in situ by desorption and evaporation of atoms by the field in order to make atomic-clean and atomic-smooth surface. This method permits to observe single vacancies in the irradiated surface, i.e., directly to count the single sputtered atoms. It has been used for beryllium, technically pure tungsten, tungsten oxide and mixed W–C layer on the tungsten irradiated by deuterium ions. The energy dependence of sputtering yield of those materials by deuterium ions at energies ranging from 10 to 500 eV is investigated. Experimental results for beryllium are in a satisfactory agreement with the calculations of Eckstein et al. Substantial connection between threshold energy of the sputtering and condition of oxidized surface of tungsten has been ascertained. The threshold energy for sputtering of oxidized tungsten surface is equal to 65 eV. The threshold energy for sputtering of mixed W–C layer has almost the same value as for the pure W.

IR Spectroscopic Study of the Skeletal Isomerization of Butene by Alumina-Supported Tungsten Oxide Catalysts

The surface properties of supported tungsten oxide catalysts, pure tungsten oxide, and pure alumina were studied by IR spectroscopy, using pyridine and carbon monoxide as probes. The interaction of these catalysts with n-butenes, molecules which are isomerised selectively by the catalysts to isobutene, has also been studied by IR spectroscopy. A correlation is found between the catalytic properties of the catalysts and the strength and nature of the acidic sites found on their surface.

Recovery of pure tungsten oxide from low grade wolframite concentrates by adsorption-desorption on activated charcoal

This paper reports laboratory work on the recovery of tungsten values from low grade wolfframite concentrates, typically analysing 26.7% WO 3, by adsorption-desorption on activated charcoal. The process essentially consists of bringing tungsten into aqueous solution as sodium tungstate by alkali fusion - leaching and then adsorbing the tungsten values on activated charcoal at a pH of 1.8 or less. Tungsten values from loaded carbon are then desorbed with NaOH as eluate, and pure tungstic oxide recovered by a chemical precipitation route. The paper also includes the results of detailed investigations on adsorption-desorption studies using activated charcoal, first with synthetic solutions to arrive at the optimum adsorption and desorption conditions and then with the leach solutions of low grade concentrates.

Direct trace element analysis of tungsten powders, alloys and related materials by inductively coupled plasma atomic emission spectrometry (ICP-AES)

A method for the direct analysis of pure tungsten metal and oxide powders and related materials by inductively coupled plasma atomic emission spectrometry (ICP-AES) is described with emphasis on line selection. Metal samples are decomposed using hydrofluoric and nitric acids, oxides are dissolved employing hydrogen peroxide and related complex materials using a sodium peroxide fusion or a mixed-acid technique. Solutions are injected into a HF-resistant sample introduction system and torch. It will be shown that the use of a tungsten ion line as an internal reference for simultaneous multi-element analysis of pure tungsten can compensate for fluctuations in background intensity, resulting in the improvement of the limits of detection. For the analysis of complex materials, the use of a high-resolution sequential monochromator facilitates line selection. Trace element analytical data compare favourably with recommended values when the concentration is 10–20 times the limit of detection in a tungsten solution. Limits of detection and analytical data are compared with those obtained by direct current plasma, flame and furnace atomic absorption spectrometries, and direct current carbon arc emission spectrography.