Facile synthesis of nanocrystalline hexagonal tungsten trioxide from metallic tungsten powder and hydrogen peroxide

A hexagonal form of tungsten trioxide (h‐WO₃, particle size: 15.9‐57.1 nm) was found to be formed by a direct reaction between metallic tungsten powder (W, particle size: 0.45‐0.59 μm) and 15%‐30% hydrogen peroxide (H₂O₂) aq solution. Oxide film on the powder surface having the similar crystal structure as h‐WO₃ was essential for the formation, and the surface oxide film was formed by aging the powder in air at 45°C, a relative humidity of 100% (P_H2O 96 hPa) for 3‐28 days or in ambient atmosphere at room temperature for 12 years. The Rietveld analysis performed in the space group P6₃/mcm (Z = 6) indicated the crystal structures were the same as those of the reported h‐WO₃ and that the crystallographic characteristic was as follows: a = 0.74219 nm, c = 0.77198 nm for h‐WO₃ from the 28‐day aged powder, and a = 0.74538 nm, c = 0.77194 nm for h‐WO₃ from the 12‐year aged powder.     

Hollow Sodium Tungsten Bronze (Na0.15WO3) Nanospheres: Preparation, Characterization, and Their Adsorption Properties

Abstract  

We report herein a facile method for the preparation of sodium tungsten bronzes hollow nanospheres using hydrogen gas bubbles as reactant for chemical reduction of tungstate to tungsten and as template for the formation of hollow nanospheres at the same time. The chemical composition and the crystalline state of the as-prepared hollow Na_0.15WO₃ nanospheres were characterized complementarily, and the hollow structure formation mechanism was proposed. The hollow Na_0.15WO₃ nanospheres showed large Brunauer–Emment–Teller specific area (33.8 m² g⁻¹), strong resistance to acids, and excellent ability to remove organic molecules such as dye and proteins from aqueous solutions. These illustrate that the hollow nanospheres of Na_0.15WO₃ should be a useful adsorbent.     

Behavior of the monophosphate tungsten bronzes (PO2)4(WO3)2m (m = 7 and 8) in the course of electrochemical lithium insertion

The electrochemical lithium insertion process has been studied in the family of monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m, where m = 7 and 8. Structural changes in the pristine oxides were followed as lithium insertion proceeded. Through potentiostatic intermittent technique the different processes which take place in the cathode during the discharge of the cell were analyzed. The nature of the bronzes Li_x(PO₂)₄(WO₃)_2m formed was determined by in situ X-ray diffraction experiments. These results have allowed establishing a correlation with the reversible/irreversible processes detected during the electrochemical lithium insertion.     

Electrochemical lithium insertion in (PO2)4(WO3)2m (2 ≤ m ≤ 10): Relation among the electrochemical insertion process and structural features

In this work it is presented a review of the main results obtained during the electrochemical lithium insertion in the family of monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m (2 ≤ m ≤ 10). This family of oxides is a good system in order to study the relation among the electrochemical processes observed in the course of lithium insertion and the changes of bronzes structures. By means of X-ray diffraction experiments, the nature of Li_x(PO₂)₄(WO₃)_2m phases has been elucidated and a correlation with the reversible/irreversible processes observed during the electrochemical insertion has been established. The electrical properties of the inserted Li_x(PO₂)₄(WO₃)_2m phases were measured and a relation with the amount of lithium inserted and m was also found.     

Oxygen vacancies and pseudo Jahn-Teller destabilization in cesium-doped hexagonal tungsten bronzes

Structural and compositional changes of Cs-doped hexagonal tungsten bronzes (HTB) with respect to variations in oxygen deficiency and alkali content have been investigated in detail through x-ray diffraction Rietveld analysis, x-ray photoelectron spectroscopy, and Raman spectroscopy. Cs-HTB crystallized in a reductive atmosphere is evidenced to generally contain plenty of oxygen defects, and a general formula, Cs_xWO_3−y (0.20 ≤ x ≤ 0.32, 0 < y ≤ 0.46), is proposed. Lattice parameters of Cs-HTB are observed to vary according to the relation, c (Å) = −3.436a (Å) + 33.062. The coordinated modification of W–O octahedral dimensions suggests the origin of the structural change with increasing x and y to be a destabilization of the pseudo Jahn-Teller distortion due to donated electrons. The dimensional change of lattice due to electrons emitted from oxygen defects is appraised only 1/18 as that due to electrons from doped alkali ions, suggesting that most electrons from oxygen defects should be localized in Cs-HTB.     

Synthesis of tungsten carbide nanopowders by direct carbonization of tungsten oxide and carbon: Effects of tungsten oxide source on phase structure and morphology evolution

In the paper, WC nanopowders are successfully prepared by carbothermal reduction method, and the effect of tungsten oxide source on the phase structure evolution and products properties of the as-synthesized WC nanopowders has been investigated. Four tungsten oxide powders are chosen as tungsten oxide sources, e.g., rods-like WO₃ , WO₃ nanopartiles, WO₃ micro-particles and WO_2.9 micro-particles. Compared with other three tungsten oxide sources, the WO₃ micro-particles possesses small particle size, less agglomerates and good dispersity and the uniform tungsten oxide-carbon mixture after ball milling can be easily obtained. The appropriate tungsten oxide source can result in lower processing temperature (≤1200 °C) and shorter holding time (≤3 h). Single-phase WC powders with average particle size of 100 nm and uniform particle distribution can be achieved by micro-particle-like WO₃ at 1100 °C for 3 h. The as-prepared WC products by other three tungsten oxide sources exhibit problems of more aggregates, non-uniform particle size and large particle size (250 nm), respectively. In addition, the method can provide a facile, low-cost, efficient, and industrially feasible pathway for large scale preparation of WC nanopowders.     

Synthesis and characterization of mesostructured tungsten nitride by using tungstic acid as the precursor

Mesostructured tungsten nitride was firstly prepared from tungstic acid via the temperature programmed reaction with ammonia. The N₂ adsorption isotherm of as-synthesized tungsten nitride was of type IV with a type H-3 hysteresis loop. BJH pore size distribution was of bimodal distribution (2.5 and 3.5 nm), among which the latter was the main channel of tungsten nitride. The surface area of as-synthesized tungsten nitride was up to 89 m² g⁻¹. XRD pattern showed that the crystal phase of the product was β-W₂N. The effect of synthesis parameters on the surface area of tungsten nitride was investigated extensively. The nitridation mechanism was investigated by in-situ XRD and N₂ adsorption analysis. It was found that H₂WO₄ was initially transformed into WO₃ by eliminating the axial water molecules, and WO₃ retained the layered and porous structure of H₂WO₄. Below 773 K, WO₃ was just partially reduced to W₂₀O₅₈ and W₂₀O₄₀. Above 773 K, β-W₂N phase could be detected. It indicated that during nitridation, WO₃ was gradually reduced and then the homogeneous substitution of oxygen vacancies in the reduced oxides with nitrogen atoms occurred. Based on the experimental results, a reduction-nitridation mechanism was firstly proposed.     

New Methods for Preparing Submicrometer Powders of The Tungstate‐Ion Conductor Sc2(WO4)3 and its Al and In Analogs

Two new methods for preparing submicrometer powders of M₂(WO₄)₃, M = Sc, In, and Al via combustion synthesis are reported. Stoichiometric combinations of trivalent metal nitrates, ammonium metatungstate, and either urea or carbohydrazide as the fuel were reacted at 550°C, producing amorphous or poorly crystallized powders with an average particle size ranging from 164 to 350 nm. Calcining the powders at 800°C for 1 h produced well‐crystallized, phase‐pure powders with an average particle size ranging from 210 to 711 nm. Powders sintered at 1000°C for 14 h resulted in pellets that were 87%–95% of the theoretical density, which is notably higher than typically obtained from powders prepared by solid‐state reaction. Whereas there was little difference in the microstructure of Al₂(WO₄)₃ pellets prepared with the two different powders, the carbohydrazide‐derived powders resulted in In₂(WO₄)₃ and Sc₂(WO₄)₃ pellets with a larger grain size than those prepared with urea‐derived powders. The electrical conductivity of the sintered pellets, while comparable to that reported for polycrystalline M₂(WO₄)₃ prepared by solid‐state reaction, was strongly influenced by grain‐boundary effects.     

Self Assembly and Properties of C:WO3 Nano-Platelets and C:VO2/V2O5 Triangular Capsules Produced by Laser Solution Photolysis

Laser photolysis of WCl₆ in ethanol and a specific mixture of V₂O₅ and VCl₃ in ethanol lead to carbon modified vanadium and tungsten oxides with interesting properties. The presence of graphene’s aromatic rings (from the vibrational frequency of 1,600 cm⁻¹) together with C–C bonding of carbon (from the Raman shift of 1,124 cm⁻¹) present unique optical, vibrational, electronic and structural properties of the intended tungsten trioxide and vanadium dioxide materials. The morphology of these samples shows nano-platelets in WO _x samples and, in VO _x samples, encapsulated spherical quantum dots in conjunction with fullerenes of VO _x . Conductivity studies revealed that the VO₂/V₂O₅ nanostructures are more sensitive to Cl than to the presence of ethanol, whereas the C:WO₃ nano-platelets are more sensitive to ethanol than atomic C.     

Preparation of phosphate promoted Na2WO4/Al2O3 catalyst and its application for oxidative desulfurization

Phosphate promoted Na₂WO₄/Al₂O₃ catalyst with 10 wt.% tungsten was prepared by simple impregnation method. Analytical characterization results showed that tungstate and phosphate were uniformly dispersed in alumina matrix and its structural properties were preserved. The effect of phosphate as promoter in catalyst activity was studied using dibenzothiophine (DBT) as model oil and the results reveal that it plays an important role in oxidation activity of Na₂WO₄/Al₂O₃ catalyst, in addition, the catalytic activity of Na₂WO₄/Al₂O₃ was increased gradually with increasing phosphorus contents up to 2.5 wt.%. The catalyst was recycled and the results show that no significant decrease in catalyst activity was observed even after five recycled runs. We also applied our catalyst in oxidative desulfurization (ODS) of FCC diesel oil (with sulfur contents 4100 ppm), and more than 92% of sulfur was removed from diesel oil under mild reaction conditions.     

A macrokinetic study of the WO3/Zn reaction diluted with NaCl

In this work, experiments with stoichiometric WO₃ + 3Zn mixture, diluted with NaCl, were conducted for nanostructured tungsten synthesis. The reaction samples, preheated until 720 K, were self-ignited and reacted in the steady combustion regime. The temperature–time profiles in the combustion wave were collected over the NaCl interval from 1 to 6 mol, and the values of the combustion parameters (T_c, U_c) were evaluated. From these profiles the spatial distributions of heat generation rate (x) and degree of conversion η(x) in the combustion wave were received at different k values. The calculated activation energy for the combustion process was E = 55 ± 2 kJ mol⁻¹. After the reduction experiments, pure tungsten nanopowder with particle size from 200 to 50 nm was obtained depending on NaCl concentration.     

Negative thermal expansion coefficient of Sc-doped indium tungstate ceramics synthesized by co-precipitation

Co-precipitation was successfully applied to synthesize the Sc³⁺ doped In_2-xSc_x (WO₄)₃ (x = 0, 0.3, 0.6, 0.9 and 1.2) compounds. The composition- and temperature-induced structural phase transition and thermal expansion behaviors of Sc³⁺ doped In₂(WO₄)₃ were investigated. Results indicate that In_2-xSc_x (WO₄)₃ crystalizes in a monoclinic structure at 300 °C for x ≤ 0.3 and changes into hexagonal structure for x ≥ 0.6. Hexagonal In_1.1Sc_0.9(WO₄)₃ displays negative thermal expansion (NTE) with an average linear coefficient of thermal expansion (CTE) of −1.85 × 10⁻⁶ °C ⁻¹. After sintering at 700 °C and above, a phase transition from hexagonal to orthorhombic phase was observed in In_2-xSc_x (WO₄)₃ (x ≥ 0.6). Sc³⁺ doping successfully reduce the temperature-induced phase transition temperature of In_2-xSc_x (WO₄)₃ ceramics from 250 °C (x = 0) to room temperature (x = 0.9). When x = 0.9 and 1.2, the average linear CTEs of In_2-xSc_x (WO₄)₃ ceramics are −5.45 × 10⁻⁶ °C⁻¹ and -4.43 × 10⁻⁶ °C⁻¹ in a wider temperature range of 25–700 °C, respectively.     

Optical and electrical properties of thin films of WO3 electrochemically coloured

Strong blue colouration has been induced in rf sputtered thin films of WO₃ by electrochemical injection of H⁺, Li⁺, and Ag⁺ ions from various solid and liquid electrolytes. Electrical conductivity and optical properties of the coloured films are reported. Comparison of these properties with those of single crystal tungsten bronzes of equivalent composition is made. Evidence, electrical and optical, for a non-uniform distribution of injected ions produced by relatively fast diffusion down grain boundaries in these polycrystalline WO₃ films is presented. A model for the optical absorption consisting of two components, due to (i) conduction electron intra-band transitions (in states close to crystalline surfaces) and (ii) transitions from unionized donor states to the conduction band (in the grain boundary phase), is tentatively proposed.     

Synthesis of cubic sodium tungsten bronze NaxWO3 in air

A solid‐state reaction route was used to synthesis cubic sodium tungsten bronze, Na_xWO₃. The Na_xWO₃ crystals were successfully prepared through reducing Na₂WO₄ with boron carbide (B₄C) in air condition, followed by washing with hot‐water. The solid‐state reaction between Na₂WO₄ and B₄C occurred at 600‐700°C, and the insertion amount of sodium ion is as high as 0.82. The obtained Na_xWO₃ crystals present an orange color, and the crystal size can grow to ~40 μm in just 20 minutes. The advantage of the method lies in the potential energy and cost savings over other conventional synthetic routes.     

One-step ball-milling synthesis of cesium tungsten bronze nanoparticles and near-infrared shielding performance

Tungsten bronze (M_xWO₃) materials have been widely used as thermal insulation for architectural glass because of their higher near-infrared (NIR) light shielding capacity. To solve the problems encountered in solvothermal preparation with high costs and low yields, this study has developed a facile, eco-friendly, effective, but low-cost method, with no need for any annealing process, for promoting the large-scale fabrication of cesium tungsten bronze (Cs_xWO₃, x = 0.32) nanomaterials for potential thermal insulation windows applications. In the proposed ball-milling process, the tungstic acid material could be reduced to hydro tungsten bronze (H_xWO₃) by cellulose, while the cesium ions (Cs⁺) could be gradually incorporated into the interspace until the formation of Cs_0.32WO₃ (Cs/W atomic ratio = 0.32), with an average particle size of ∼33 nm after a 15 h ball-milling process. The reaction mechanism has been investigated in detail via particle structural analysis and optical performance characterization. The obtained Cs_0.32WO₃ nanoparticles are dispersed in a solution composed of surfactant (s) and polymer (s) which can form a thin film with a thickness of about 2 μm on a glass substrate, by a spinning coating or casting method, to exhibit high visible (Vis) light transmittance (T_566nm = 72.6%), and excellent NIR-shielding capability (T_1388nm = 5.3%), reflected by good heat-insulating performance in practice use. This work will pave a new path for large-scale production of Cs_0.32WO₃ nanomaterials with low costs and high performance, beneficial for practical applications in energy-saving glass coatings.     

Thermoelectric properties of tungsten-titanium-phosphate glass-ceramics

The thermoelectric properties of tungsten-titanium phosphate glass-ceramics, which were described in previous articles, were measured as a function of composition, ceram temperature, and measurement temperature. The glass-ceramics comprise tungsten monophosphate crystals, (PO₂)₄(WO₃)_2 m, in a matrix of TiP₂O₇. The glass-ceramics behave as n-type semiconductors with nearly metallic behavior. Conduction occurs along percolating networks of primarily m6 and m7 crystals. The highest electrical conductivities and (absolute) Seebeck coefficients were measured on a glass-ceramic containing large, interconnecting, prismatic, m7 crystals. At 777°C, the electrical conductivity reached nearly 5500 S/m and the Seebeck coefficient was −60 µV/K. Thermal conductivity was in the 2.5-3 W/m.K range, and the maximum ZT obtained was 0.007. This ZT is two orders of magnitude lower than those of the best bulk, polycrystalline, n-type, oxide thermoelectric materials. The exceptional property of the tungsten-titanium phosphate glass-ceramics is their high electrical conductivity when compared with oxide glasses.     

Tungsten oxide in polymer electrolyte fuel cell electrodes—A thin-film model electrode study

Thin films of WO_x and Pt on WO_x were evaporated onto the microporous layer of a gas diffusion layer (GDL) and served as model electrodes in the polymer electrolyte fuel cell (PEFC) as well as in liquid electrolyte measurements. In order to study the effects of introducing WO_x in PEFC electrodes, precise amounts of WO_x (films ranging from 0 to 40 nm) with or without a top layer of Pt (3 nm) were prepared. The structure of the thin-film model electrodes was characterized by scanning electron microscopy and X-ray photoelectron spectroscopy prior to the electrochemical investigations. The electrodes were analyzed by cyclic voltammetry and the electrocatalytic activity for hydrogen oxidation reaction (HOR) and CO oxidation was examined. The impact of Nafion in the electrode structure was examined by comparing samples with and without Nafion solution sprayed onto the electrode. Fuel cell measurements showed an increased amount of hydrogen tungsten bronzes formed for increasing WO_x thicknesses and that Pt affected the intercalation/deintercalation process, but not the total amount of bronzes. The oxidation of pre-adsorbed CO was shifted to lower potentials for WO_x containing electrodes, suggesting that Pt-WO_x is a more CO-tolerant catalyst than Pt. For the HOR, Pt on thicker films of WO_x showed an increased limiting current, most likely originating from the increased electrochemically active surface area due to proton conductivity and hydrogen permeability in the WO_x film. From measurements in liquid electrolyte it was seen that the system behaved very differently compared to the fuel cell measurements. This exemplifies the large differences between the liquid electrolyte and fuel cell systems. The thin-film model electrodes are shown to be a very useful tool to study the effects of introducing new materials in the PEFC catalysts. The fact that a variety of different measurements can be performed with the same electrode structure is a particular strength.     

High crystalline tungsten trioxide thin layer obtained by SPD technique

A nanostructured layer of tungsten oxide, WO₃, was prepared by a spray pyrolysis deposition (SPD) method using (NH₄)₂WO₄ precursor. The films were investigated in order to determine the electrical behaviour (impedance, Mott–Schottky and I–V) and the morphological characteristics (SEM). The XRD analysis reveals that tungsten oxide is present in monoclinic phase but the orthorhombic phase is also expected to be present in the structure of WO₃ crystals. Changes in conductivity of the WO₃ films have been observed after immersion in water.     

Intergranular properties of polycrystalline YBa2Cu3O7−δ superconductor added with nanoparticles of WO3 and BaTiO3 as artificial pinning centers

Pure (x = 0.0 wt%) superconducting YBa₂Cu₃O_7−δ (YBCO) sample and added YBCO sample with 0.1 wt% artificial barium titanate (BTO) and 0.1 wt% tungsten trioxide (WO₃) nanoparticles were prepared using the solid-state reaction route. Phase purity was analyzed via the X-rays diffraction technique. Scanning electron microscopy showed a high density of isolated W-rich secondary phases embedded within the YBCO added sample. Furthermore, both WO₃ and BTO nanoparticles tend to reside at the grain boundaries and play the role of bridges connecting the YBCO superconducting granules. Quantitative analysis performed on the areas where nanosized entities induced by BTO and WO₃ phases was evidenced by EDX analysis equipped with SEM instrument. The values of H_c2 increased from 1.6 T for pristine to 3.4 T for BTO/WO₃ added samples. The superconducting parameters determined by AC susceptibility measurements also showed an improvement with WO₃ and BTO nanoparticles co-addition. The value of J_cinter(0) increases from 1.18 kA/cm² for the pristine sample to 5.10 kA/cm² for BTO/WO₃ co-added sample. Hence, the incorporation of artificial BTO and WO₃ nanoparticles into the YBCO superconducting phase could be a useful way to make such compounds available in practical applications.     

Facile preparation of electrodes based on WO3 nanostructures modified with C and S used as anode materials for Li-ion batteries

An appropriate morphological and structure matrix configuration where lithium ions could insert and de-insert is essential for lithium-ion batteries (LiB). Tungsten oxides (WO₃) are especially attractive materials for this aim. In this research, the effects of the morphology and composition of WO₃ nanostructures on the charge/discharge behavior for Li-ion batteries are methodically examined. On the one hand, nanostructured WO₃ thin film was effectively synthesized by an electrochemical procedure. Then, an annealing treatment at 600°C in air environment for 4 h was carried out. In the second electrode synthesized, a carbon layer was uniformly deposited on WO₃ nanostructures to obtain a WO₃/C electrode. Finally, WO₃/WS₂ electrodes were prepared by means of in situ sulfurization of WO₃ one-step solid-state synthesis using tungsten trioxide (WO₃) and thiourea as precursor material. By using X-ray photoelectron spectroscopy, X-ray diffraction analysis, transmission electron microscopy, Raman spectra, and field-emission scanning electron microscopy, the three electrodes have been morphologically characterized. Electrochemical properties were analyzed by cyclic voltammogram, galvanostatic charge/discharge cycling, and electrochemical impedance spectroscopy. Among all the synthesized samples, WO₃/C nanostructures reveal the best performance as they exhibit the greatest discharge capacity and cycle performance (820 mA h g⁻¹).