Synthesis, FTIR studies and sensor properties of WO3 powders

Several synthetic approaches were used to obtain nano-sized porous and nonporous monoclinic WO₃ (m-WO₃) powders. All of these methods begin with a standard preparative method where H₂WO₄ is first generated by passing a Na₂WO₄ solution through a cation-exchange resin. It is shown that high surface area particles are produced by dripping the H₂WO₄ exiting from the ion-exchange column into a solution containing oxalate and acetate exchange ligands or alternatively, into a water-in-oil (W/O)-based emulsion. Porous materials are produced using surfactant-templating architectures. The surface properties were investigated by IR spectroscopic studies during thermal evacuation and the use of chemical probes. The nature of the surface depends on the initial evacuation temperature of the WO₃ surface as this alters the relative number of the Lewis and Brønsted acid sites along with the amount of adsorbed water. Infrared studies of the adsorption of various molecules on the powders led to a new size-selective approach to improve selectivity in semiconducting metal oxide (SMO) sensors. The key for achieving high selectivity is based on using a dual sensor configuration where the response on a porous WO₃ powder sensor was compared to the response on a nonporous WO₃ powder sensor. Detection selectivity between methanol and dimethyl methylphosphonate (DMMP) is obtained because the access of a gas molecule in the interior pore structure of WO₃ is size-dependent leading to a size-dependent magnitude change in the conductivity of the SMO sensor.     

Growth and structure of PbWO<Subscript>4</Subscript> films

Lead tungstate films have been grown on silicon by magnetron sputtering followed by heat treatment at various temperatures. The thermal oxidation of metal-oxide (Pb/WO₃/Si and W/PbO₂/Si) and metal-metal (Pb/W/Si) bilayer systems at temperatures above 870 K yields films that are dominated by monoclinic PbWO₄ and contain WO₃, also monoclinic. The optimal configuration for PbWO₄ synthesis is Pb/WO₃/Si because, even during lead deposition onto tungsten oxide, we observe the formation of lead tungstate, PbWO₄, and subsequent heat treatment increases the percentage of this phase in the film.     

Fabrication study of proton injection layer suitable for electrochromic switchable mirror glass

An electrochromic (EC) switchable mirror glass can change between a reflective state and a transparent state with voltage application. The conventional device has a multilayer of Mg₄Ni/Pd/Al/Ta₂O₅/WO₃/indium-tin oxide on a transparent substrate. A palladium thin film was used as the proton injection layer. For practical use, we attempted to reduce the amount of palladium thin film from the viewpoints of the reduction in total fabrication cost and the efficient use of resources. The thickness of the film was related to the optical switching properties of the device. Although the device with a 1-nm-thick palladium film showed a high transmittance of 63% in the transparent state, its low switching durability was not suitable for practical application.Moreover, we were able to adapt a palladium-based alloy (Pd_0.8Ag_0.2) which is a well-known hydrogen permeation membrane as the proton injection layer to reduce the amount of palladium thin film. As a result, we found that a 4-nm-thick Pd-Ag thin film has good adaptability to the EC switchable mirror.     

Soft-combustion (wet-chemical) synthesis of a new 4-V class cathode-active material,LiVMoO6, for Li-ion batteries

A new, 4-V class, lithiated transition metal oxide cathode, LiVMoO₆, has been synthesized by a novel soft-combustion (wet chemical) low temperature (LT) method that presents advantages compared to the classical ceramic method, namely, in terms of phase purity, surface texture and size, preparation time, costs and electrochemical performances of the resulting products. The structural properties of the newly synthesized product have been examined by means of X-ray diffraction studies (XRD). The thermal reactions which occur during the soft-combustion of the precursor mixture have been examined by DTA/TG techniques. It has been found that the layered LiVMoO₆ can only be obtained upon calcining the precursor at 540°C, beyond which the compound will thermally be reduced to LiVMoO₅ which exhibits inferior structural characteristics for the intercalation/deintercalation reactions. The product (LiVMoO₆) thus prepared exhibits submicrometre spherical grains (<1 μm) whose specific surface area is 5.01 m²/g. The intercalation/deintercalation (redox) kinetics of the above product has been studied and its suitability as cathode material in actual electrochemical cells is discussed in the light of electrochemical properties.     

Structural properties of TiO2–WO3 thin films prepared by r.f. sputtering

TiO₂–WO₃ thin films were prepared by radio frequency (r.f.) reactive sputtering from metallic target. Structural and morphological properties of the thin films have been studied through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The influence of the annealing on the phase composition TiO₂–WO₃ system was studied. The binding energies of titanium and tungsten are characteristic for Ti⁴⁺ and W⁶⁺. The influence of tungsten on anatase–rutile phase transition in TiO₂ was observed. The structural modeling has been performed to account the preferred orientation in tungsten doped titanium oxide.     

Crystal modulation of tungsten oxide gels and films prepared by the sol-gel process using 2,4-pentanedione as an organic ligand

Tungsten oxide gels and films were prepared by the sol-gel process using 2,4-pentanedione (PTN) as an organic ligand. WO₃ powders were obtained by peeling the films from the quartz glass substrates. The crystal structure of both the gels prepared with and without PTN and fired at 700 °C was monoclinic. WO₃ powders peeled from film samples prepared both with and without PTN showed only the monoclinic crystal structure, whereas the crystal phases of both types of films were cubic and a mixture of cubic and monoclinic crystals, respectively. These results indicate that the cubic crystals of the WO₃ in the films were transformed to the monoclinic crystals of the powders after the films were peeled from the quartz glass substrates. It is concluded that the cubic crystals in the films are transformed to stable monoclinic crystals by the peeling process, whereas PTN and the quartz glass substrate can control the crystal phase of WO₃ films selectively to form the cubic structure.     

Novel microwave assisted sol–gel synthesis (MW-SGS) and electrochromic performance of petal like h-WO3 thin films

Use of domestic microwave oven is first time employed for chemical deposition of nanocrystalline hexagonal WO₃ (h-WO₃) thin films. Low cost precursors like sodium tungstate, hydrochloric acid, oxalic acid and potassium sulfate signifies cost effectiveness of this thin film fabrication route. Scanning electron microscopy images reveal formation of petal like nanodisks. A number of analytical techniques were used to characterize the WO₃ petal like nanodisks, including X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy, FT-IR spectroscopy, Raman scattering spectroscopy, UV–visible spectrophotometry and cyclic voltammetry (CV). The X-ray photoelectron spectroscopic studies revealed 2.89 O/W atomic ratio. The electrical transport studies on WO₃ thin films show semiconducting behavior with n-type semiconductivity. The value of determined coloration efficiency is 57.90 cm²/C. The mechanism of Li⁺ intercalation and deinercalation in h-WO₃ matrix is proposed for enhanced electrochromism.     

Facile microwave synthesis of Sn-doped WO3 for pseudocapacitor applications

In the present work, we have successfully synthesized pure tungsten oxide (WO₃) and Sn (3 and 5 wt%)-doped WO₃ nanoparticles using facile microwave irradiation method and studied about the electrochemical performances for supercapacitor electrode material. Structural and morphological studies of the prepared nanomaterials were investigated systematically. The powder XRD analysis reveals that pure WO₃ and Sn-doped WO₃ have monoclinic crystal structure and also crystallite size of the material decreases from 38 to 30 nm with increasing dopant concentration. Micro-Raman analysis confirms the formation of monoclinic phase with υ(O–W–O) stretching and δ(O–W–O) bending mode of vibration. SEM and micrographs show the elongation of the plate-like nanostructure of WO₃ for the doping of Sn. High-resolution transmission electron microscope images depict the morphological change and increased porosity in doped samples. The supercapacitive performance and the electrochemical conductivity of the samples were analysed using cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy measurements. The results demonstrate that the 5 wt% Sn-doped WO₃ electrode has the enhanced electrochemical performance in 1 M KOH with a maximum specific capacitance of 418 F g^?1 at low current density of 1 A g^?1. Also, it shows the increase in energy density from 4.88 to 11.77 Wh kg^?1 with respect to the Sn concentration at the power density of 225 W kg^?1_.

     

Electrochromic performance of NiVxOy thin films deposited onto flexible PET/ITO substrates by reactive plasma sputtering for flexible electrochromic devices

An investigation was conducted on the electrochromic properties of plasma sputtered-nickel-vanadium oxide thin films on 40 Ω/□ flexible polyethylene terephthalate/indium tin oxide substrates. Metallic Ni_0.93V_0.07 target, sputtered by radio frequency power with argon gases and reacted with oxygen gases at room temperature (23 °C), was proven to provide extraordinary electrochromic performance. Cyclic voltammetry switching measurements found that only low driving voltages from − 1 V to 1 V were needed to provide reversible Li⁺ ion intercalation and deintercalation. The light modulation with up to 52% of transmittance variation, optical density change of 0.446 and color efficiency of 63.8 cm²/C at a wavelength of 550 nm was obtained for 200 cycles of Li⁺ intercalation and deintercalation in a 1 M LiClO₄-propylene carbonate electrolyte.     

Phosphorus and nitrogen co-doped titania photocatalysts with a hierarchical meso-/macroporous structure

Visible light-active phosphorus and nitrogen co-doped meso-/macroporous titania materials were prepared by a simple two-step approach of the direct phosphation with the use of phosphoric acid solution and the succedent nitridation with the use of the urea solution. The prepared materials were characterized by UV–vis, solid-state ³¹P MAS NMR, FT-IR, XPS, XRD, SEM, TEM, and N₂ adsorption analysis. Direct synthesis of phosphorus-doped meso-/macroporous titania materials could inhibit the formation of brookite phase and increase the surface area significantly, resulting in the hierarchical porous framework of nanocrystalline anatase phase with enhanced thermal stability and large porosity, and these features retained during the subsequent nitridation. The incorporation of P and N in the anatase titania lattice in the form of O–Ti–N, O–P–N, and Ti–O–P linkages was evidenced, and the extension of the absorption edges into the visible region and the corresponding narrowing of band gaps were observed in these N and P co-doped meso-/macroporous titanias, giving a higher photocatalytic activity in the degradation of Rhodamine B dye under visible-light irradiation than the samples doped with only N or P. The beneficial effect of hierarchical meso-/macroporous structure is also examined.     

Synthesis and characterization of TiO2/WO3 composite nanotubes for photocatalytic applications

TiO₂/WO₃ composite nanotubes were synthesized in an anodic aluminum oxide (AAO) template by a sol–gel method. The prepared nanotubes were characterized by transmission electron microscopy, scanning electron microscopy, powder X-ray diffraction, and Brunauer–Emmett–Teller surface area. Using the nanotubes embedded in the AAO templates as catalysts, photocatalytic degradation of methyl orange aqueous solution was carried out under UV light irradiation. The results showed that the TiO₂/WO₃ composite nanotubes with the thickness about 50 nm could be successfully synthesized by this method. TiO₂ showed anatase phase and WO₃ displayed monoclinic phase. The composite nanotubes (TiO₂/WO₃) exhibited higher photocatalytic activity than the pure nanotubes (WO₃ or TiO₂). The possible reason for improving the photocatalytic activity was also discussed.     

Flexible electrochromic devices based on crystalline WO3 nanostructures produced with hot-wire chemical vapor deposition

Crystalline WO₃ nanoparticles are employed in the development of flexible electrochromic (EC) devices. The nanoparticles are synthesized at high-density with a hot-wire chemical vapor deposition process where the hot filament provides the source of the tungsten metal. Polyethylene terephthalate coated with indium tin oxide is employed as a transparent flexible substrate. A simple electrophoresis technique is employed to deposit the WO₃ nanoparticles on the polymer, resulting in a uniform thin film. The EC performance is optimized for WO₃ particles that were baked at ~ 300 °C for 2 h prior to electrode fabrication. The transmittance is modulated between ~ 94% and ~ 28% without degradation for 100 cycles.     

WO3 thin film prepared by PECVD technique and its gas sensing properties to NO2

Tungsten oxide films have been successfully fabricated from tungsten oxychloride (WOCl₄) precursor by using plasma enhanced vapor deposition (PECVD) technique. The films were deposited onto silicon substrates and ceramic tubes maintained at 100°C under a constant operating pressure of He-O₂ gas mixtures. The compositions and the structures of the thin films have been investigated by means of anaysis methods, such as XRD, XPS, UV and IR. The as-deposited WO₃ thin films were amorphous state and became crystalline after annealing above 400°C. The surface analysis of the films indicates that stoichiometry O/W is 2.77 : 1. The gas sensing measurements of the WO₃ thin film sensors indicate that these sensors have a high sensitivity, excellent selectivity and quick response behavior to NO₂.     

Surface-oxidized tungsten for energy-storable dye-sensitized solar cells

Tungsten oxide electrodes were investigated as charge-storage materials for energy-storable dye-sensitized solar cells (ES-DSSCs). The electrochemical and structural properties of the surface-oxidized tungsten (so-WO_3 − x) and monoclinic nanocrystalline WO₃ (nc-WO₃) were studied on the difference of the charge-discharge properties. Although, the electromotive force (EMF) curve of the so-WO_3 − x was associated with structural change, the so-WO_3 − x did not show the significant structural change indicated by X-ray diffraction (XRD) patterns. On the other hand, the nc-WO₃ showed crystal transformation from monoclinic phase to tetragonal phase. The Li⁺ diffusion coefficients of the so-WO_3 − x with different Li⁺ content ratios obtained by the galvanostatic intermittent titration technique (GITT) did not fall down up to 0.3 of Li/W ratio, whereas the diffusion coefficients of nc-WO₃ decreased about two orders of magnitude in the vicinity of phase transitions. The different electrochemical properties could be explained by the less structural change of so-WO_3 − x compared with the nc-WO₃. The large-sized ES-DSSCs with the so-WO_3 − x were fabricated for the first time, and their photocharge-discharge performances were studied.     

Preparation and characterization of spray deposited n-type WO3 thin films for electrochromic devices

The n-type tungsten oxide (WO₃) polycrystalline thin films have been prepared at an optimized substrate temperature of 250 °C by spray pyrolysis technique. Precursor solution of ammonium tungstate ((NH₄)₂WO₄) was sprayed onto the well cleaned, pre-heated fluorine doped tin oxide coated (FTO) and glass substrates with a spray rate of 15 ml/min. The structural, surface morphological and optical properties of the as-deposited WO₃ thin films were studied. Mott-Schottky (M-S) studies of WO₃/FTO electrodes were conducted in Na₂SO₄ solution to identify their nature and extract semiconductor parameters. The electrochromic properties of the as-deposited and lithiated WO₃/FTO thin films were analyzed by employing them as working electrodes in three electrode electrochemical cell using an electrolyte containing LiClO₄ in propylene carbonate (PC) solution.     

Synthesis of uniform WO3 square nanoplates via an organic acid-assisted hydrothermal process

Two kinds of tungsten oxide (WO₃) square nanoplates have been prepared by a simple hydrothermal method using l(+)-tartaric acid or citric acid as assistant agents. The products are characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). XRD, SEM and TEM images of the products illustrate that WO₃ square nanostructures prepared in the presence of l(+)-tartaric acid have a hexagonal phase, length of ∼ 200 nm and thickness of ∼ 100 nm, while WO₃ nanostructures synthesized in the presence of citric acid have an orthorhombic phase, length of ∼ 500 nm and thickness of ∼ 100 nm. Selected area electron diffraction (SAED) suggests that both of the as-prepared WO₃ square nanoplates are single crystalline. The plausible growth mechanism for the formation of WO₃ square nanostructures is also proposed.     

Tantalum oxide film prepared by reactive magnetron sputtering deposition for all-solid-state electrochromic device

Inorganic-solid-state electrolyte tantalum oxide thin films were deposited by reactive DC magnetron sputtering to improve the leakage and deterioration of traditional liquid electrolytes in electrochromic devices. O₂ at 1–20 sccm flow rates was used to deposit the tantalum oxide films with various compositions and microstructures. The results indicate that the tantalum oxide thin films were amorphous, near-stoichiometric, porous with a loose fibrous structure, and highly transparent. The maximum charge capacity was obtained at an oxygen flow rate of 3 sccm and 50 W. The transmission change of the Ta₂O₅ film deposited on a WO₃/ITO/glass substrate between colored and bleached states at a wavelength of 550 nm was 56.7%. The all-solid-state electrochromic device was fabricated as a multilayer structure of glass/ITO/WO₃/Ta₂O₅/NiO_x/ITO/glass. The optical transmittance difference of the device increased with increasing applied voltage. The maximum change was 66.5% at an applied voltage of ± 5 V.     

Study of Semimagnetic Mn-Doped WO<Subscript>3</Subscript> Nanoparticles Synthesised by Precipitation Method: Hydrogenation Creates a Promising DMS

Abstract Tungsten oxide (WO₃) nanoparticles doped with different amounts of manganese ions (W_1?x Mn _x O₃, where x =?0.011, 0.022 and 0.044) were synthesised by hydraulic acid-assisted precipitation, followed by thermal calcinations. The powders were characterised by X-ray fluorescence (XRF), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and magnetic measurements. The monoclinic structure at room temperature (～293 K) found for un-doped WO₃ was preserved even with Mn doping. However, doping with Mn ions caused decease in unit-cell volume and slight increase in crystallite size (CS) of host WO₃. The hydrogenation was observed to corrode the crystallites without changing in crystalline structure. Controllable room-temperature ferromagnetic (RT-FM) properties were obviously observed with hydrogenated WO₃ doped with Mn. In addition, there existed an optimum doping concentration of Mn in WO₃ to obtain superior FM properties. Therefore, Mn-doped WO₃ nanopowders, owning to these amazingly tunable magnetic properties, could be considered a potential candidate for many applications partially required FM properties such as optical phosphors and catalysts.     

A facile method to enhance electrochemical performance of high-nickel cathode material Li(Ni<Subscript>0.8</Subscript>Co<Subscript>0.1</Subscript>Mn<Subscript>0.1</Subscript>)O<Subscript>2</Subscript> via Ti doping

The cycle stability of Li(Ni_0.8Co_0.1Mn_0.1)O₂ is enhanced obviously by titanium doping via a facile solid-state method. The property of crystal structure is evaluated by XRD, which illustrates the samples possessed a layered α-NaFeO₂ structure with R-3m space group. According to the charge/discharge studies, the capacity retention of pristine sample is around 51% after 125 cycles at 5 C, and the sample with Ti dopant displays a good cyclic stability, after 125 cycles, the capacity retention increases to 75% under 5 C, suggesting it could be possibly applied in fast charge Lithium-ion battery area. The superb electrochemical performance might be attributed to the Ti⁴⁺ occupy the layer structure to broaden the Lithium-ion channel, which is benefit to lithium intercalation and deintercalation during cycling.     

Synthesis and characterization of novel nanorod superstructures and twin octahedral morphologies of WO3 by hydrothermal treatment

Controlled WO₃ morphologies, such as nanorods and octahedral structures, were synthesized by the hydrothermal technique using sulfate salts based structure directing agents (SDAs). The role of the sulfate salts’ cation in controlling the shape, size and phase of WO₃ nanomaterials was investigated by choosing sulfate salts whose cations are from d-bloc elements (FeSO₄, (NH₄)₂Fe(SO₄)₂, CoSO₄, CuSO₄, ZnSO₄), an alkaline earth metal (MgSO₄) and a non-metal ((NH₄)₂SO₄). In addition chloride (MnCl₂) and acetate (Zn(CH₃CO₂)₂) anion based SDAs were also used in order to clarify the role of sulfate ions in the growth of WO₃ nanostructures. We controlled the pH of the reaction medium with oxalic acid. The obtained WO₃ samples were investigated by SEM, micro-Raman, and XRD. At pH = 1, the WO₃ samples exhibit novel superstructures consisting of aligned hexagonal nanorods, whereas at pH = 5.25, novel twin octahedral morphology with a cubic structure is obtained. The results demonstrate that the phase and morphology change is influenced by the pH and both the anion and the cation of the SDA. A growth mechanism for the obtained novel WO₃ morphologies is presented.