Tungsten oxide bronzes with alkali metals

Single-phase samples of tungsten bronzes M _x WO₃ (M = K⁺, Rb⁺, Cs⁺) are prepared by solid-state synthesis. The reversibility of the M_0.33WO₃/M⁺-solid electrolyte interface is studied subject to the alkali metal nature and humidity over a wide temperature interval. The exchange current density at 24°C and 58%-relative humidity is 3.6 × 10^?4 A/cm² for the Rb_0.33WO₃/Rb⁺-solid electrolyte interface; 2.2 × 10^?4 A/cm² for the Cs_0.33WO₃/Cs⁺-solid electrolyte interface; and 1.3 × 10^?4 A/cm² for the K_0.33WO₃/K⁺-solid electrolyte interface. A correlation between the reversibility of the bronze|solid electrolyte interface, which is characterized by the exchange current density, and the rate of potential equilibration in sensor systems, where the bronze is a reference electrode, is revealed. Ionic component of the conductivity of the synthesized tungsten oxide bronzes is measured at a background of the predominant electronic conductivity. The ionic conductivity is three orders of magnitude lower than the electronic conductivity; it decreases in the series Rb_0.33WO₃ > Cs_0.33WO₃ > K_0.33WO₃, amounting to 2.3 × 10^?2, 2.1 × 10^?3, and 2 × 10^?4 S cm^?1, respectively. The working capacity of the M_0.3WO₃ bronzes as reference electrodes in sensor systems for carbon dioxide detection is evaluated. The plots of the cell potential vs. the CO₂ concentration in the electrochemical cells are linear, their slopes (59 ± 1 mV/decade) are characteristic for one-electron process. The fastest response to changes in the CO₂ concentration was obtained with the sensor system that used Rb_0.33WO₃ as reference electrode.     

Structural study of tungstate fluorophosphate glasses by Raman and X-ray absorption spectroscopy

Transparent glasses were synthesized in the NaPO₃-BaF₂-WO₃ ternary system and several structural characterizations were performed by X-ray absorption spectroscopy (XANES) at the tungsten L_I and L_III absorption edges and by Raman spectroscopy. Special attention was paid to the coordination state of tungsten atoms in the vitreous network.XANES investigations showed that tungsten atoms are only six-fold coordinated (octahedra WO₆) and that these glasses are free of tungstate tetrahedra (WO₄).In addition, Raman spectroscopy allowed to identify a break in the linear phosphate chains as the amount of WO₃ increases and the formation of P-O-W bonds in the vitreous network indicating the modifier behavior of WO₆ octahedra in the glass network. Based on XANES data, we suggested a new attribution of several Raman absorption bands which allowed to identify the presence of W-O⁻ and WO terminal bonds and a progressive apparition of W-O-W bridging bonds for the most WO₃ concentrated samples (?30% molar) due to the formation of WO₆ clusters.     

TeO2-WO3 Glasses: Infrared, XPS and XANES Structural Characterizations

Transparent (1−x)TeO₂-xWO₃ glasses with 0≤x≤0.325 mol were synthesized by the fast quenching technique. Several complementary techniques as infrared, X-ray photoelectron and X-ray absorption spectroscopies were used to approach the structure of these tungsten oxide-tellurite glasses. Special attention was paid to the oxidation state and the coordination state of tungsten atoms. The structural results show that (1−x)TeO₂-xWO₃ glasses present characteristic tellurium environments which vary with their chemical composition while tungsten ions always adopt an octahedral configuration.     

Crystal structure and charge carrier concentration of W18O49

The electrical resistivity of the tungsten oxide, W₁₈O₄₉, is 1.75 · 10^?3 Ω cm along the needle axis. The charge carrier density, as determined by reflectivity measurements, is 1.87 · 10²² cm^?3, thereby indicating that most of the charge carriers are delocalized. Hence the smaller conductivity along the needle axis than that expected for such charge carrier concentrations must be found in the structure, which has been refined using the data collected with an automatic diffractometer. The structure consists of WO₆ and WO₇ polyhedra which are linked along edges and/or corners. However, as the linkage parallel to b takes place only by sharing corners, an anisotropy in the electrical conductivity may be expected. Another explanation for the smaller conductivity may be found in the occurrence of defects such as tunnels in the structure, which may scatter the electrons. The refinement shows that the tungsten positions, determined by Magneli (Arkiv Kemi1, 223 (1950)), are essentially correct; but the positions of the oxygens, especially two of them, differ considerably. This results in one of the tungsten atoms getting an additional coordinating oxygen, the coordination number thereby becoming seven.     

Thermochemistry of hydrogen tungsten bronze phases HxWO3

The enthalpies of formation of two hydrogen tungsten bronze phases H_0.35WO₃ and H_0.18WO₃ have been determined by solution calorimetry. Values obtained for formation from H₂(g) and WO₃(s) at 298.15 K were H_0.35WO₃(s), ?9.6 ± 0.8 kJ mole^?1 and H_0.18WO₃(s), ?4.8 ± 0.6 kJ mole^?1. The stabilities of these phases towards decomposition, disproportionation and oxidation are discussed.     

介孔氧化钨电色薄膜的制备与性能研究

采用一种新的非离子型gemini表面活性剂结构导向模板, 成功制备了介孔氧化钨薄膜. 通过SAXRD, TEM和N₂吸附-脱附等方法考察薄膜的制备和微结构特性, 发现获得的产物具有三维蠕虫介孔结构, 比表面积可达145.5 m²• g^－1. 测定了该薄膜在无水高氯酸锂/碳酸丙烯酯电解质溶液中的循环伏安和电致变色性能, 并与无模板薄膜进行了对比研究. 研究表明, 由于具有更大的电化学活性比表面, 纳米介孔氧化钨薄膜表现出增强的电色性能, 在633 nm波长处的透过率调制幅度可达60%以上, 着色效率为51.7 cm²•C^－1.     

Heat capacity measurement of cubic lithium tungsten bronzes from 200 to 800°K

Heat capacities of three cubic lithium tungsten bronze samples (Li_xWO₃) with x values of 0.363, 0.437, and 0.478 were measured from 200 to 800°K. Heat capacities per gram-atom at the same temperature of Li_0.363WO₃ and Li_0.437WO₃ were equal within experimental error and also equal to those of Na_0.485WO₃, Na_0.698WO₃, and Na_0.794WO₃, regardless of the difference of the composition. λ-type heat capacity anomalies were observed around 330, 460, and 590°K in Li_0.363WO₃ and around 330 and 460°K in Li_0.437WO₃ and Li_0.478WO₃, showing the existence of second-order phase transitions. The entropy increments of the transitions were obtained as 1.36, 0.45, and 1.68 J mole^?1 K^?1 for Li_0.363WO₃, 1.09 and 0.59 J mole^?1 K^?1 for Li_0.437WO₃, and 1.42 and 0.50 J mole^?1 K^?1 for Li_0.478WO₃.     

123 Phases of composition CaBa2Cu2WO8 with substitutions of sodium and potassium for calcium and barium

Tetragonal (123) phases of compositions Ca_0.7Na_0.3Ba₂Cu₂WO₈, CaBa_1.7Na_0.3Cu₂WO₈, and CaBa_1.7K_0.3Cu₂WO₈, where tungsten occurs in Cu(1) positions and Cu³⁺ is in Cu(2) positions, were prepared from CaWO₄, BaCuO₂, and sodium and potassium nitrates at 1073 K in an oxygen flow. These phases constitute at least 90% of the samples; their unit call parameters are as follows: a = b = 0.4177(3) nm, c = 1.2561(8) nm; a = b = 0.4182(3) nm, c = 1.2564(8) nm; and a = b = 0.4194(3) nm, c = 1.2603(8) nm, respectively. Their X-ray diffraction patterns feature peaks that indicate a superstructure with all three unit cell parameters being doubled. The samples have weak conductive properties (room-temperature electrical conductivity ranges between 10^?6 and 10^?7 Ohm^?1 cm^?1). Conductivity versus temperature curves are indicative of the semiconductor character of the samples. Superconducting transitions do not occur up to 65 K.     

Surface modification of semiconductor photoelectrodes for better photoelectrochemical performance

The present paper describes the modification and solar hydrogen production studies employing a new semiconductor-septum (SC-SEP) photoelectrode ns-TiO₂/In₂O₃ based photoelectrochemical solar cell. The current-voltage characteristics of the above SC-SEP cell revealed that an enhancement in short-circuit current (I_SC) up to three times (5 ～ 14.6 mA cm^?2). The optimum hydrogen production rate was found to be 11.8 lh^?1 m^?2 for 5M H₂SO₄ and with a further increase in H₂SO₄ concentration, the hydrogen production rate was found to be invariant. In yet another part of our study instead of using new SC-SEP solar cell design, we used another new oxide material form such as ns-TiO₂/WO₃. The ns-TiO₂/WO₃ exhibited a high photocurrent and photo-voltage of 15.6 mA cm^?2, 960 mV, respectively. The ns-TiO₂/WO₃ electrode exhibited a higher hydrogen gas evolution rate of 13.8 lh^?1 m^?2. Evidences and arguments are put forward to show that, whereas for the bare ns-TiO₂ electrode, the improvement in the performance of this photo-electrode compared with its original form was due to the higher quantum yield. In the case of ns-TiO₂/In₂O₃ and ns-TiO₂/WO₃ photo-electrodes, the improvement is due to the improved spectral response resulting from decrease of energy band gap.     

Solution Processing of Topochemically Converted Layered WO3 for Multifunctional Applications

Solution processing of nanomaterials is a promising technique for use in various applications owing to its simplicity and scalability. However, the studies on liquid-phase exfoliation (LPE) of tungsten oxide (WO₃) are limited, unlike others, by a lack of commercial availability of bulk WO₃ with layered structures. Herein, a one-step topochemical synthesis approach to obtain bulk layered WO₃ from commercially available layered tungsten disulfide (WS₂) by optimizing various parameters like reaction time and temperature is reported. Detailed microscopic and spectroscopic techniques confirmed the conversion process. Further, LPE was carried out on topochemically converted bulk layered WO₃ in 22 different solvents; among the solvents studied, the propan-2-ol/water (1 : 1) co-solvent system appeared to be the best. This indicates that the possible values of surface tension and Hansen solubility parameters for bulk WO₃ could be close to that of the co-solvent system. The obtained WO₃ dispersions in a low-boiling-point solvent enable thin films of various thickness to be fabricated by using spray coating. The obtained thin films were used as active materials in supercapacitors without any conductive additives/binders and exhibited an areal capacitance of 31.7 mF cm⁻² at 5 mV s⁻¹. Photo-electrochemical measurements revealed that these thin films can also be used as photoanodes for photo-electrochemical water oxidation.     

Thermochemistry of ammonium tungsten bronze, (NH4)0.25WO3

The enthalpy of formation of ammonium tungsten bronze, (NH₄)_0.25WO₃(s), at 298.15 K has been determined by solution calorimetry. The value obtained for formation from NH₃(g), H₂(g) and WO₃(s) was ?25.7 ± 0.8 k¹ mol^?1. The stability of the bronze towards decomposition and oxidation is discussed.     

Fabricating polypyrrole/tungsten oxide hybrid based electrochromic devices using different ionic liquids

A facile approach of polypyrrole (PPy)/tungsten oxide (WO₃) composites electrosynthesized in ionic liquids for fabrication of electrochromic devices is discussed. The electrochromic properties of PPy/tungsten oxide nanocomposite films (PPy/WO₃) prepared in the presence of four different ionic liquids, 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIMBF₄), 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆), 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl) imide (BMIMTFSI), and 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl) imide (BMPTFSI) were investigated in detail. Cyclic voltammetry results revealed that PPy/WO₃ nanocomposite films have much more electrochemical activity than those of WO₃ and PPy film. The electrochromic contrast, coloration efficiency, and switching speed of the composite films were determined for electrochromic characteristics. The maximum contrast and the maximum coloration efficiency values were measured as 33.25% and 227.89 cm²/C for the PPy/WO₃/BMIMTFSI composite film. Copyright © 2015 John Wiley & Sons, Ltd.     

IR spectroscopic investigations on the determination of the water content in glasses

The IR spectra of glasses of the composition 16 Cs₂O?·?10 CaO?·?74 SiO₂ with different water contents were measured and quantitatively analyzed with regard to the water content. Nuclear reaction analysis (NRA) was used for calibration. The practical extinction coefficients were determined for the OH vibrational bands at 3500 and 2800 cm^–1. The two-band method of Scholze was applied to IR spectra of three different alkali lime silica glasses containing Na⁺, K⁺, or Cs⁺ as network modifier cations. After correcting the extinction coefficients, this method is very suitable for determination of total water concentrations in glasses.     

Application of heating microscopy to the study of thermal behaviour of ZnO–P2O5–WO3 glasses

The effect of WO₃ on thermal behaviour and thermal stability of ZnO–P₂O₅–WO₃ glasses prepared in compositional series (100 ? x)[0.5ZnO–0.5P₂O₅] ? xWO₃ (x = 0–60) was investigated by heating microscopy and the results were correlated with the results determined by conventional thermodilatometry and differential thermal analysis. Thermoanalytical studies showed that the glass transformation temperature and dilatation softening temperature increase with increasing WO₃ content while thermal expansion coefficient decreases. The highest stability towards crystallization possess glasses containing 20–30 mol% WO₃. Major compounds formed by the crystallization of the glasses were Zn(PO₃)₂, WO₃ and W₁₈P₂O₅₉. The values of sphere temperature, hemisphere temperature and flow temperature obtained using heating microscopy were strongly influenced by the degree of crystallization process at the sintering.     

Ion conductivity of poly(ethylene oxide)-based polyurethane networks containing alkali metal salts

Ion conductivity of poly(ethylene oxide) (PEO)-based polyurethane networks containing alkali metal salts has been investigated. Consequently, it has been revealed that the conductivity is dependent on the following parameters: lattice energy of the alkali metal salt, concentration of alkali metal salt, and the cross-linking density of the network polymer (which is a function both of the amount of cross-linking agent and the molecular weight of PEO). Under optimal conditions, the conductivity at ambient temperature corresponded to 2.51 × 10^?5 Scm^?1, which is greater than that of a typical alkali metal-PEO system by a factor of about 10² to 10³. Moreover, from the standpoint of the application to electrochromic displays (ECD), tensile bond strength between the polymer electrolytes and tungsten trioxide (WO₃), which is the most promising electrochromic material, has been evaluated. The bonding strength of the bond of WO₃ with the present electrolyte has been found to be much larger than that of the alkali metal-PEO system.     

Effect of tungsten oxide loading on metathesis activity of ethene and 2-butene over WO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> catalysts

The metathesis of ethene and 2-butene to propene was studied over WO₃/SiO₂ catalysts with various WO₃ loadings (2, 4, 8, 12, 16, and 24 wt%). The 2-butene conversion and propene selectivity increased greatly with WO₃ loading increasing from 2 to 8 wt%, reached maximum at 8–12 wt% WO₃ loading, and then decreased when the WO₃ loading was higher than 12 wt%. From the above results and taking the economics into account, the optimal amount of WO₃ loading was ~8 wt%. The catalysts were characterized by physico-chemical and spectroscopic techniques to elucidate the effect of different tungsten oxide loadings on the metathesis reactivity of ethene and 2-butene. The characterization data indicated that three types of tungsten species (i.e., surface tetrahedral tungsten species, surface octahedral polytungstate species, and WO₃ crystallites) were present in the catalysts. It was found that WO₃ was not the active centers, and surface tetrahedral tungsten species might be more active than octahedral polytungstate species in metathesis reaction. The reduced form of tungsten species [W⁺⁴, W⁺⁵, and W^+(6−y) (0 < y < 1)] may be the suitable state of W species acting as metathesis active centers.     

Chemical reactions in the W-Al2O3 system near the melting point of aluminum oxide under low vacuum

The W-Al₂O₃ system is thermodynamically analyzed at T = 2400 K and P = 1 × 10^?3 bar, and principal chemical reactions and their directions are determined from the sign of ??G _react. Although solid tungsten does not directly react with Al₂O₃ melts, its oxidation (dominantly, to WO₃) is established to occur via the reaction with aluminum-containing dissociative evaporation products. This reaction is shown to be a multistage process, in which both lower tungsten and aluminum oxides and gaseous W and Al₂ may act as intermediates.     

Pulsed NMR study of proton mobility in a hydrogen tungsten bronze

Crystalline hydrogen tungsten bronze H_0.46WO₃ was prepared by reduction of WO₃ single crystals. NMR relaxation times T₂, T₁, and T_1? were measured for 80 K < T < 450 K at 16 MHz and second moments for 160 K < T < 450 K at 100 MHz. The relaxation data were analyzed in terms of proton diffusion to give an activation energy of about 16 kJ mole^?1 and a correlation time preexponential factor of about 70 nsec for the process.     

Zum chemischen Transport von Wolfram mit HgBr2 – Experimente und Modellrechnungen

On the Chemical Transport of Tungsten using HgBr₂ – Experiments and Thermochemical Calculations Using HgBr₂ as transport agent tungsten migrates in a temperature gradient from the region of higher temperature to the lower temperature (e.g. 1 000 → 900°C). The transport rates were measured for various transport agent concentrations (0.64 ? C(HgBr₂) ? 11.74 mg/cm³; T? = 950°C) and for various mean transport temperatures (800 ? T? ? 1 040°C). Under these conditions tungsten crystals were observed in the sink region. To observe the influence of tungsten dioxide (contamination of the tungsten powder) on the transport behaviour of tungsten, experiments with W/WO₂ as starting materials were performed. According to model calculations the following endothermic reactions are important for the migration of tungsten: In the presence of H₂O or WO₂ other equilibria play a role, too. Using a special “transport balance” we observed a delay of deposition of tungsten (e.g. T? = 800°C; 15 h delay of deposition) with W and W/WO₂ as starting materials. The heterogeneous and homogeneous equilibria will be discussed and an explanation for the non equilibrium transport behaviour of tungsten will be given.     

Effect of N5+ ion implantation on optical and gas sensing properties of WO3 films

In this paper we report the optical and gas sensing behaviours of tungsten oxide (WO₃) films, implanted with 45‐keV N^{5 +} ions of different fluences in the range 1 × 10¹⁵ to 1 × 10¹⁷ cm^–2. The film with fluence 1 × 10¹⁵ cm^–2 shows the most intense PL spectrum with two prominent peaks near UV and blue regions. The morphological changes because of ion implantation are also investigated by atomic force microscopy. Because of implantation the gas sensitivity of the film, in exposure of methane, is found to increase with reasonably fast response and recovery times. With the increase of the concentration of methane, the sensors show better result. Present work also includes the effect of N^{5 +} ion implantation on the structural property of WO₃ films. Copyright © 2015 John Wiley & Sons, Ltd.