Showing papers on "Tungstate published in 2008"

Synthesis and Optimum Luminescence of CaWO4-Based Red Phosphors with Codoping of Eu3+ and Na+

Abstract: Scheelite nanostructures Ca1−2x(Eu,Na)2xWO4 (0 < x ≤ 0.135) were prepared from 5 nm Ca0.968(Eu,Na)0.032WO4 by hydrothermal treatment. The preparation of 5 nm Ca0.968(Eu,Na)0.032WO4 at room temperature and subsequent hydrothermal treatment allow control over chemical compositions and particle size of CaWO4-based red phosphors that has not yet possible when using traditional preparation methods. By careful structural and electronic characterization, it is shown that simultaneous substitutions of Eu3+ and Na+ at Ca2+ sites were possible using this methodology, which allows one to vary the local symmetry surrounding Eu3+ and moreover the energy transfer from O2− to Eu3+ and tungstate groups to Eu3+ for optimum luminescence. As a consequence, the obtained CaWO4-based nanocrystals displayed excellent luminescence properties as demonstrated by luminescence lifetimes of milliseconds, abnormally narrowed emissions, and maximum quantum efficiencies of 92%. The results reported in this work show that it is possible ...

Size‐ and Shape‐Controlled Conversion of Tungstate‐Based Inorganic–Organic Hybrid Belts to WO3 Nanoplates with High Specific Surface Areas

Abstract: Two-dimensional monoclinic WO(3) nanoplates with high specific surface areas are synthesized through a novel conversion process using tungstate-based inorganic-organic hybrid micro/nanobelts as precursors. The process developed involves a topochemical transformation of tungstate-based inorganic-organic hybrid belts into WO(3) nanoplates via an intermediate product of H(2)WO(4) nanoplates, utilizing the similarity of the W-O octahedral layers in both H(2)WO(4) and WO(3). The as-obtained WO(3) nanoplates show a single-crystalline nanostructure with the smallest side along the [001] direction. The WO(3) nanoplates are 200-500 nm x 200-500 nm x 10-30 nm in size, and their specific surface areas are up to 180 m(2) g(-1). Photocatalytic measurements of visible-light-driven oxidation of water for O(2) generation in the presence of Ag(+) ions indicate that the activity of the as-obtained WO(3) nanoplates is one order of magnitude higher than that of commercially available WO(3) powders.

Preparation and Characterization of Bismuth Tungstate Polycrystalline Flake-Ball Particles for Photocatalytic Reactions

Abstract: This chapter has discussed the preparation of micrometer-sized spherical particles of bismuth tungstate (Bi2WO6) with the hierarchical architecture of “flake-ball” shape for photocatalytic application. The particles, which are assemblies of polycrystalline flakes composed of square-shaped laminar plates with a lateral size of a few hundred nanometers and thickness of 20–35 nm, are prepared by a facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. An excess amount of a tungstate precursor (10 %) and an acidic condition (pH 1.2) during the hydrothermal reaction are required to obtain a high yield of uniform particles with the flake-ball architecture. The control of the hierarchical assemblies of two-dimensional nanostructures provides high crystallinity, large surface area (19 m2 g−1), and large pore volume. The flake-ball particles are promising as a photocatalyst for oxidative decomposition of organic pollutants to carbon dioxide, for example, in water purification systems because of the high level of photocatalytic activity, the response to visible light, and feasible separation from suspensions by sedimentation and filtration.

Exfoliated nanosheet crystallite of cesium tungstate with 2D pyrochlore structure: synthesis, characterization, and photochromic properties.

Abstract: Layered cesium tungstate, Cs(6+x)W(11)O(36), with two-dimensional (2D) pyrochlore structure was exfoliated into colloidal unilamellar sheets through a soft-chemical process. Interlayer Cs ions were replaced with protons by acid exchange, and quaternary ammonium ions were subsequently intercalated under optimized conditions. X-ray diffraction (XRD) measurements on gluelike sediment recovered from the colloidal suspension by centrifugation showed a broad pattern of a pronounced wavy profile, which closely matched the square of calculated structure factor for the single host layer. This indicates the total delamination of the layered tungstate into nanosheets of Cs(4)W(11)O(36)(2-). Microscopic observations by transmission electron microscopy and atomic force microscopy clearly revealed the formation of unilamellar crystallites with a very high 2D anisotropy, a thickness of only approximately 2 nm versus lateral size up to several micrometers. In-plane XRD analysis confirmed that the 2D pyrochlore structure was retained. The colloidal cesium tungstate nanosheet showed strong absorption of UV light with sharp onset, suggesting a semiconducting nature. Analysis of the absorption profile provided 3.6 eV as indirect band gap energy, which is 0.8 eV larger than that of the bulk layered precursor, probably due to size quantization. The nanosheet exhibited highly efficient photochromic properties, showing reversible color change upon UV irradiation.

Tungsten, the Surprisingly Positively Acting Heavy Metal Element for Prokaryotes

Abstract: The history and changing function of tungsten as the heaviest element in biological systems is given. It starts from an inhibitory element/anion, especially for the iron molybdenum-cofactor (FeMoCo)-containing enzyme nitrogenase involved in dinitrogen fixation, as well as for the many "metal binding pterin" (MPT)-, also known as tricyclic pyranopterin- containing classic molybdoenzymes, such as the sulfite oxidase and the xanthine dehydrogenase family of enzymes. They are generally involved in the transformation of a variety of carbon-, nitrogen- and sulfur-containing compounds. But tungstate can serve as a potential positively acting element for some enzymes of the dimethyl sulfoxide (DMSO) reductase family, especially for CO(2)-reducing formate dehydrogenases (FDHs), formylmethanofuran dehydrogenases and acetylene hydratase (catalyzing only an addition of water, but no redox reaction). Tungsten even becomes an essential element for nearly all enzymes of the aldehyde oxidoreductase (AOR) family. Due to the close chemical and physical similarities between molybdate and tungstate, the latter was thought to be only unselectively cotransported or cometabolized with other tetrahedral anions, such as molybdate and also sulfate. However, it has now become clear that it can also be very selectively transported compared to molybdate into some prokaryotic cells by two very selective ABC-type of transporters that contain a binding protein TupA or WtpA. Both proteins exhibit an extremely high affinity for tungstate (K(D) < 1 nM) and can even discriminate between tungstate and molybdate. By that process, tungsten finally becomes selectively incorporated into the few enzymes noted above.

One-pot synthesis of twist-like helix tungsten–nitrogen-codoped titania photocatalysts with highly improved visible light activity in the abatement of phenol

Abstract: The twist-like helix W,N-codoped TiO2 photocatalysts were prepared by a simple one-pot synthesis route to hydrolysis of titania tetrachloride using ammonium tungstate as tungsten and nitrogen sources. The morphology and microstructure characteristics of W,N-codoped titania photocatalysts with different amount of tungsten doping were characterized by means of BET, TEM, SEM, XPS, UV–vis DRS, PLS and XRD. The probable mechanism of codoping effect is proposed. It is presumed that cooperation of nitrogen and tungsten ions leads to produce new states and narrow the band gap between the valence band and conduction band effectively, which will greatly improve the photocatalytic activity in the visible light region. On the other hand, the tungsten ions with changing valences in the W,N-TiO2 samples are considered to act as trapping sites, which will effectively decrease the recombination rate of photo-induced electrons and holes and then increase the photo-oxidation efficiency of the catalysts. The metal and nonmetal codoped 1%-W,N-TiO2 sample shows the best photocatalytic activity, which is much superior to P25 under both visible and ultraviolet light irradiation. The superior activity of W,N-TiO2 photocatalysts can also be ascribed to the special twist-like helix structure with regular holes on the wall, high surface area, large pore volume and well-crystallized anatase phase.

Growth, characterization, and high-pressure optical studies of CuWO4

Abstract: Copper tungstate (CuWO4) crystals grown by the top-seeded solution growth method were characterized by X-ray diffraction, Raman scattering, and optical measurements. CuWO4 has a triclinic structure (P 1¯) with a = 4.709 A, b = 5.845 A, c = 4.884 A, α = 88.3°, β = 92.5°, and γ = 97.2°. It consists of corner-linked CuO6 and WO6 octahedra, the former having a pseudo-tetragonally elongated geometry caused by the Cu2+ Jahn–Teller effect. Fifteen out of the eighteen Raman modes of CuWO4 are reported, discussed, and compared with those of other tungstates. We also determined the indirect band-gap energy of CuWO4 (2.3 eV) and its negative pressure coefficient up to 25 GPa. The pressure evolution of the band-gap is discussed in terms of the electronic structure of CuWO4. Finally, no clear evidence of structural changes were found in our high-pressure experiments, but large amounts of defects are apparent beyond 18 GPa.

Room-Temperature Preparation, Characterization, and Photoluminescence Measurements of Solid Solutions of Various Compositionally-Defined Single-Crystalline Alkaline-Earth-Metal Tungstate Nanorods

Abstract: The current report describes the systematic synthesis of single-crystalline alkaline-earth-metal tungstate AWO4 (A = Ca, Sr, Ba) nanorods as well as a series of their crystalline solid−solution analogues Sr1-xCaxWO4 and Ba1-xSrxWO4 (0 < x < 1) with controllable chemical composition and morphology using a modified template-directed methodology under ambient room-temperature conditions. Extensive characterization of the resulting nanorods has been performed using diffraction, X-ray photoelectron spectroscopy, electron microscopy, and optical spectroscopy. The composition-modulated luminescence properties of these alkaline-earth-metal tungstate solid−solution nanorods provide for a fundamental understanding of the intrinsic optical and optoelectronic properties of these systems, suggesting, therefore, the possibility of their rational incorporation into functional nanoscale devices.

Geochemical parameters influencing tungsten mobility in soils.

Abstract: The biogeochemistry of tungsten and its effects on mobility have recently gained attention due to the existence of human cancer clusters, such as in Fallon, NV. Tungsten exists in many environmental matrices as the soluble and mobile tungstate anion. However, tungsten can polymerize with itself and other anions, creating poly- and heteropoly-tungstates with variable geochemical and toxicological properties. In the present work, geochemical parameters are determined for tungstate species in a model soil that describe the potential for tungsten mobility. Soluble tungsten leached from a metallic tungsten-spiked soil after six to twelve months aging reached an equilibrium concentration >150 mg/L within 4 h of extraction with deionized water. Partition coefficients determined for various tungstate and polytungstate compounds in the model soil suggest a dynamic system in which speciation changes over time affect tungsten geochemical behavior. Partition coefficients for tungstate and some poly-species have been observed to increase by a factor of 3 to 6 over a four month period, indicating decreased mobility with soil aging.

Charge Transport by Polyatomic Anion Diffusion in Sc2(WO4)3

Abstract: Discussions about the nature of the charge carriers in the scandium tungstate and other isostructural tungstates and molybdates have persisted in the literature since a variety of experimental indi

Polymorphism in the negative thermal expansion material magnesium hafnium tungstate

Abstract: Magnesium hafnium tungstate [MgHf(WO4)3] was synthesized by high-energy ball milling followed by calcination. The material was characterized by variable- temperature neutron and x-ray diffraction. It crystallized in space group P21/a below 400 K and transformed to an orthorhombic structure at higher temperatures. The orthorhombic polymorph adopted space group Pnma, instead of the Pnca structure commonly observed for other A2(MO4)3 materials (A = trivalent metal, M = Mo, W). In contrast, the monoclinic polymorphs appeared to be isostructural. Negative thermal expansion was observed in the orthorhombic phase with αa = −5.2 × 10−6 K−1, αb = 4.4 × 10−6 K−1, αc = −2.9 × 10−6 K−1, αV = −3.7 × 10−6 K−1, and αl = −1.2 × 10−6 K−1. The monoclinic to orthorhombic phase transition was accompanied by a smooth change in unit-cell volume, indicative of a second-order phase transition.

Exploring the Room-Temperature Synthesis and Properties of Multifunctional Doped Tungstate Nanorods

Abstract: Uniform Mn-doped alkaline-earth metal tungstate—AWO4 (A = Ca, Sr, Ba)—nanorods of reproducible size, shape, and composition have been methodically prepared using a modified template-directed methodology under ambient, room-temperature conditions. The dopant ion distribution within the nanostructures does not appear to adversely affect either the structural or crystalline integrity of our as-prepared compounds, as determined by microscopy and diffraction studies. What is much more important is the fact that the presence of Mn2+ not only substantially increases the photoluminescent potential of a pristine tungstate material but also reinforces its versatility by adding a desirable magnetic component to its repertoire of properties. In so doing, we have created multifunctional one-dimensional nanorods with exciting opto-magnetic behavior, which should become important for the future incorporation of these materials into functional nanoscale devices, with various potential applications in a number of diverse ...

Removal of ammonia from air on molybdenum and tungsten oxide modified activated carbons.

Abstract: Microporous coconut-based activated carbon was impregnated with solutions of ammonium metatungstate or ammonium molybdate and then calcined in air in order to convert the salts into their corresponding oxides. The surface of those materials was characterized using adsorption of nitrogen, potentiometric titration, Fourier-transform infrared spectroscopy, X-ray diffraction, and thermal analysis. The results indicated a significant increase in surface acidity related to the presence of tungsten or molybdenum oxides. On the materials obtained, adsorption of ammonia from either dry or moist air was carried out. The oxides distributed on the surface provided Lewis and/or Bronsted centers for interactions with ammonia molecules or ammonium ions. Water on the surface of carbon or in the gas phase increased the amount of ammonia adsorbed via involvement of Bronsted-type interactions and/or by leading to the formation of molybdate or tungstate salts on the surface. Although the amount of ammonia adsorbed is closely related to the number of moles of oxides and their acidic centers, the carbon surface also contributes to the adsorption via providing small pores where ammonia can be dissolved in the water film.

Potassium Tungsten Bronze Nanowires: Polarized Micro-Raman Scattering of Individual Nanowires and Electron Field Emission from Nanowire Films

Abstract: Nanostructures are known to possess large surface-area-tovolume ratios and possible quantum-confinement effects They have currently been intensely researched because these special properties can give rise to potential applications in nanotechnology, such as the fabrication of nanometer-scale devices In the family of nanostructures, metal oxide nanostructures have significantly been applied in areas such as chemical/biological sensing, lasering, and displays, and are the most important and widely studied nanostructures Many methods have been developed for the fabrication of nanostructures, including applying electrochemical techniques, porous aluminum templates, vapor–liquid–solid (VLS) growth and vapor–solid (VS) reactions Heat-oxide methods have been shown to provide an alternative route for synthesis of metal oxide semiconductor (MOS) nanostructures, with the advantages of mild synthetic conditions, simple manipulation, and large-scale production The hotplate method, one of the simplest heat-oxide methods in particular, has been successfully applied for the synthesis of various metal oxide (CuO, a-Fe2O3, Co3O4, and ZnO) nanostructures [7-10] In the past few decades, much attention has been focused on tungsten bronzes owing to their Drude-type optical behavior, photochromic, and superconducting properties In particular, the hexagonal alkali tungsten bronzes AxWO3 (HTBs, A = K, Rb, Cs, and NH4; 0 < × <1/3) have been the subject of numerous studies The traditional methods for the preparation of the hexagonal tungsten bronze structure require high temperature reaction of metal tungstate, tungsten oxide, and metal tungsten powder or the electrochemical reduction of tungstate Here, we demonstrate a simple and convenient process for the synthesis of K033W0944O3 nanowires Our scanning electron microscopy (SEM) investigations directly confirm that the growth mechanism is that of the tip-growth vapor–solid mechanism Raman scattering is a well-established technique for elucidating structural properties of nanostructures More recently, micro-Raman studies on individual nanowires such as SiC, ZnSe, and CuO successfully demonstrate their impressive potential for probing crystal properties of basic building blocks such as single nanowires However, to the best of our knowledge, there is no polarized micro-Raman scattering study on individual potassium tungsten bronze nanowires to date For the first time, the electron field emission measurement of potassium tungsten bronze nanowire film was performed in this work A field-emission scanning electron microscope (FE-SEM) was used to directly observe the morphology of the as-grown samples After being heated at 450 °C for 10 hours in air, the pretreated tungsten foil was found to be covered with a layer of randomly oriented nanowires with diameters and lengths in the range of approximately 50–200 nm and 5–10 lm (Fig 1a), respectively The highest aspect ratio was established to be 100, and the average aspect ratio of these nanowires was around 50, which may give rise to promising field-emission properties It was noted that the tungsten bronze nanostructures are able to be synthesized into nanorods, nanowires, or nanosheets by changing the heating temperatures The detailed morphology-controlled synthesis of potassium doped tungsten bronze nanostructures will be discussed in our future work Figure 1b shows the XRD pattern of the as-prepared samples All the reflections can be indexed to the hexagonal potassium tungsten bronze K033W0944O3 (Joint Comittee on Powder Diffraction Standards (JCPDS) file No 81-0005) and no impurities were identified in the XRD pattern The crystal structural characterization of nanowires was performed by means of high-resolution transmission electron microscopy (HRTEM) The fringe spacing of 038 nm (Fig 2b) concurs well with the interplanar spacing of (0 0 1) plane of hexagonal K033W0944O3 (JCPDS file No 81-0005), revealing the single crystalloid of the synthesized tungsten bronze nanowires and the growth direction of [0 0 1] The selected area electron diffraction (SAED) pattern with the zone axis [0 2 0] (inset of Fig 2a) further confirmed the single crystalline property of nanowires The ideal hexagonal symmetry structure of AxWO3 (A = K, Rb, and Cs) is schematically shown in the inset of Figure 2c The WO6 octahedra are formed into a sixmember ring Consequently, the maximum alkali-metal content is 033 if the composition ratio of W is set to be 1 The C O M M U N IC A TI O N

Cyclic microwave-assisted synthesis and characterization of nano-crystalline alkaline earth metal tungstates

Abstract: Nano-crystalline MWO4 (M = Ca, Sr and Ba) were successfully produced using cyclic microwave radiation in propylene glycol. XRD, TEM, SAED and EDX analyses revealed the presence of nano-crystalline MWO4 containing the corresponding alkaline earth metals, W and O. Their lattice parameters and crystallite sizes were determined, and enlarged with the increase in the ionic radii of the corresponding divalent metals. Six different vibrations were detected using Raman spectroscopy and very strong W-O stretching of [WO4]2- tetrahedrons using FTIR.

Tungsten oxides used to increase the heat-input amount of near infrared radiation

Abstract: The invention relates to the use of tungsten oxide or of tungstate to increase the heat-input amount of near infrared radiation in processes selected from laser welding of plastics, NIR curing of coatings, drying of printing inks, fixing of ink toners to a substrate, heating of plastic preforms, laser marking of plastics or paper.

Preparation of hydrogenation catalyst

Abstract: The invention discloses a preparation method for a hydrogenation catalyst. The metal components of the catalyst comprise one or two metals from Co and Ni as well as one or two metals from Mo and W; carbamide or ammonia is used as a reaction addition agent; a method of reaction in-situ in the hole of a carrier is adopted to lead the metal active components to generate nickel molybdate (cobalt) or nickel tungstate (cobalt) compounds, thereby being capable of avoiding the reaction between the metals and the carrier, simultaneously leading the metal active components to be easier to be sulfurized and being capable of improving the activity of the hydrogenation catalyst.

Synthesis of MgHf(WO4)3 and MgZr(WO4)3 using a non-hydrolytic sol-gel method

Abstract: Magnesium zirconium tungstate and magnesium hafnium tungstate were successfully synthesized using a non-hydrolytic sol–gel method. Crystalline materials could be obtained at temperatures as low as 540 °C after as little as 3 h. The samples were composed of micron-size particles with defined morphology. Highly crystalline material was formed after a 0.5 h heat treatment of 1050 °C.