Open structure tungstates: Synthesis, reactivity and ionic mobility

doi:10.1016/0167-2738(92)90393-4

Solid State Ionics

Volumes 53–56, Part 1, July–August 1992, Pages 305-314

https://doi.org/10.1016/0167-2738(92)90393-4 Get rights and content

Abstract

Sodium tungstates with the hexagonal tungsten bronze and the pyrochlore structure have been synthesized at around 150°C using hydrothermal methods. The phase formed is a function of the pH of the reaction medium. Their structures have been determined using Rietveld analysis on X-ray and neutron powder data. The pyrochlore phase readily undergoes ion-exchange with a wide range of monovalent cations giving the compounds, $M_{x} W_{2} O_{6+x2} ·y H_{2} O$ ; the value of y is strongly dependent on the identity of the cation, M. Thermogravimetric analysis was used to determine the water content and the thermal stability of the cation exchanged compounds. WO₃ with the pyrochlore structure could be formed from the hydronium and ammonium complexes. Lithium can be readily intercalated both chemically and electrochemically into both phases. Surprisingly more lithium is incorporated in most cases in the hexagonal than in the pyrochlore phase. Silver is ion-exchanged most rapidly into the pyrochlore structure, and this compound also reacts with the most lithium. The ionic mobility has been determined by complex impedance methods, with the hydronium ion showing the greatest mobility.

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Wide research interest has been focused on the investigation of its genuine color switching for electrochromic devices, photocatalysts and gas sensors [21–28]. Importantly, interstitial sites from ordered stacking WO6 octahedra are effective accommodations for guest ions, rendering the absorption and desorption of ions at the surface, as well as insertion-deinsertion behavior inside [29–33]. Attributed to its intriguingly fast and reversible surface redox reactions between W6+ and W5+, recent years many progress about EESDs based on WO3 nanomaterials with large effective area has been reported [34–40].
Asymmetric supercapacitors (ASCs) represent an effective electrochemical energy storage device with wider potential windows, which may lead to a substantial increase in the energy density and power density of the device. Unfortunately, their development is severely restricted by the carbon-based negative electrodes with low energy density. In this work, we demonstrate the facile synthesis of engineering WO_3-x electrode on carbon cloth via hydrogenation, which achieves excellent electrochemical performance as a negative electrode for ASCs. The enhanced crystallinity and introduction of oxygen vacancy synergistically contribute to more reversible redox reactions, faster ion diffusion and charge transfer rates. The as-obtained WO_3-x electrode reaches a high areal capacitance of 1.83 F cm⁻² at 1 mA cm⁻², and an outstanding electrochemical durability with high capacity retention of 74.8% after 10000 cycles. All these results open a new way to the construction of high-performance anodes for ASCs.
Enhancing the photodegradation activity of n-type WO<inf>3</inf>·0.33H<inf>2</inf>O by proton-intercalation via negative potential biases
2014, Applied Catalysis B: Environmental
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But due to its lower stability and activity compared with TiO2, n-type WO3 is rarely employed as a photocatalyst for organic degradation. Among the various crystal structures of WO3, hexagonal WO3 leads to form large tunnels, which can be intercalated with cations to obtain hexagonal tungsten bronzes MδWO3 (M = H+, Li+, Na+, etc.) with a blue color [18,19]. This phenomenon implies an enhanced solar efficiency for colored hexagonal tungsten bronzes in capturing visible light.
It is here demonstrated that proton-intercalation via negative potential biases enhance the rate constant of methylene blue (MB) photo-electrocatalytic de-colorization on n-type tungsten trioxide hydrate (WO₃·0.33H₂O as prepared by hydrothermal synthesis), rather than the positive potential bias in promoting the charge separation of photo-generated electrons/holes. Due to the generation of W(V) and W(IV) hydroxyl species with smaller band gaps (in comparison with W(VI)) and the electrochemical activation of WO₃·0.33H₂O, negatively biased proton intercalation into WO₃·0.33H₂O clearly enhances its photo-electrocatalytic activity for MB de-colorization, including its visible-light photo-electrocatalytic activity. A possible scheme for promoting the photocatalytic activity of WO₃·0.33H₂O through proton intercalation under constant potential biases is proposed and discussed. The photocatalytic de-colorization rate of MB on UV100-TiO₂ is significantly decreased under the same potential biases due to the difficulty in proton-intercalation
Hexagonal hydrated tungsten oxide nanomaterials: Hydrothermalsynthesis and electrochemical properties
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The second [WO5(OH2)] octahedral type contains one coordinated water molecule bonded along the c-axis opposite the WO bond [12,13]. In the WO3 hexagonal structure, the octahedra form six-membered rings in the equatorial plane (0 0 1) and the stacking of such planes along the c-axis leads to the formation of large tunnels [14,15]. The tunnel structure gives these materials the ability to be intercalation hosts for monovalent ions (such as H+, Li+, Na+, etc.) by electrochemical oxidation or reduction [16,17].
Nanocrystalline hexagonal hydrated tungsten oxide h-WO₃·1/3H₂O has been synthesized by hydrothermal process using sodium tungstate as inorganic precursor and three n-alkyl chain sodium sulfate surfactants (n = 10, 12 and 14) as structure-directing templates. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the structure, the morphology and the composition of the material. The length of the alkyl chain of the surfactant molecules has a marked effect on the morphology and, particularly, on the particle size of the material. Nanofibers (about 50 nm in diameter) are obtained when sodium decyl sulfate is used as surfactant. Whereas, nanoneedles are obtained with sodium dodecyl (about 60 nm in diameter) or sodium tetradecyl sulfate (about 80 nm in diameter) as surfactant.
Thin films of h-WO₃·1/3H₂O deposited on ITO substrates were electrochemically characterized by cyclic voltammetry; they show the same behavior whatever the surfactant. The voltammograms show reversible redox behavior with doping/dedoping process corresponding to reversible cation intercalation/de-intercalation into the crystal lattice of the nanofibers. This process is easier in propylene carbonate than in aqueous solvent and is easier for the small Li⁺ cation than larger ones, Na⁺ and K⁺. This is attributed to probable presence of two different tunnel cavities in the h-WO₃·1/3H₂O lattice.
Hexagonal nanorods of tungsten trioxide: Synthesis, structure, electrochemical properties and activity as supporting material in electrocatalysis
2011, Applied Surface Science
Citation Excerpt :
All the WO3 structures could be described as deformations of the ReO3 cubic perfect model. Among various crystal structures of WO3, hexagonal form was of great interest owing to its well-known structural tunnel cavities in which WO6 octahedrons shared their corners with each other thus forming hexagonal tunnel cavities along c-axis [25]. Hexagonal tungsten oxide (h-WO3) was widely investigated, especially as an intercalation host to obtain hexagonal tungsten bronzes MxWO3 (M = K+, Li+, etc.) and as a promising material for negative electrodes of rechargeable lithium batteries [26–28].
Tungsten trioxide, unhydrated with hexagonal structure (h-WO₃), has been prepared by hydrothermal method at a temperature of 180 °C in acidified sodium tungstate solution. Thus prepared h-WO₃ has been characterized by X-ray diffraction (XRD) method and using electrochemical techniques. The morphology has been examined by scanning and transmission electron microscopies (SEM and TEM) and it is consistent with existence of nanorods of 50–70 nm diameter and up to 5 μm length. Cyclic voltammetric characterization of thin films of h-WO₃ nanorods has revealed reversible redox behaviour with charge–discharge cycling corresponding to the reversible lithium intercalation/deintercalation into the crystal lattice of the h-WO₃ nanorods. In propylene carbonate containing LiClO₄, two successive redox processes of hexagonal WO₃ nanorods are observed at the scan rate of 50 mV/s. Such behaviour shall be attributed to the presence of at least two W atoms of different surroundings in the lattice structure of h-WO₃ nanorods. On the other hand, in aqueous LiClO₄ solution, only one redox process is observed at the scan rate of 10 mV/s. The above observations can be explained in terms of differences in the diffusion of ions inside two types of channel cavities existing in the structure of the h-WO₃ nanorods. Moreover, the material can be applied as active support for the catalytic bi-metallic Pt–Ru nanoparticles during electrooxidation of ethanol in acid medium (0.5 mol dm⁻³ H₂SO₄).
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Hexagonal tungsten oxide nanorods have been synthesized by hydrothermal strategy using Na₂WO₄·2H₂O as tungsten source, aniline and sulfate sodium as structure-directing templates. Techniques X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy have been used to characterize the structure, morphology and composition of the nanorods. The h-WO₃ nanorods are up to 5 μm in length, and 50–70 nm in diameter.
Novel materials based on organic-tungsten oxide hybrid systems II: Electronic properties of the W-O framework
2004, Current Applied Physics
In this paper we report experimental and theoretical results concerning the band structure of tungsten trioxide and how this system relates to a new class of layered organic–inorganic hybrid materials based on a tungsten oxide framework. UV–visible spectroscopy was performed on various tungsten trioxide samples. Diffuse reflectance on powder samples indicates a single absorption edge centred at 2.8 eV, while transmission and reflectance of thin films show a strong feature at 4.0 eV. This is supported by ab initio calculations, in which a strong peak is seen at 4.0 eV in the density of states. For the hybrid system, 1,n-diaminoalkane tungsten oxide, a change in the calculated band structure is seen compared with the tungsten oxide system; a peak appears just above 3.0 eV in the density of states with a low intensity `tail' below this. UV–visible spectra of powder reflectance of hybrid samples indicate a dominant absorption edge centred at 4.0 eV.

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Open structure tungstates: Synthesis, reactivity and ionic mobility

Abstract

J. Solid State Chem.

J. Less Common Met.

Mat. Res. Bull.

Solid State Ionics

Prog. Solid State Chem.

Solid State Ionics

Solid State Ionics