Structural and optical properties of nanocrystalline WO<Subscript>3</Subscript> thin films

The nanocrystalline WO₃ thin films were deposited by r.f. magnetron sputtering on quartz and p- type Si (100) substrates at a constant power of 25 W and at three different sputtering pressures (0.05, 0.01 and 0.5 mbar) and post annealed at different temperatures. The deposited films were characterized by XRD, UV–VIS spectrophotometry, ellipsometry and atomic force microscopy (AFM). The structural studies from XRD spectra reveals that the films deposited at 0.05 mbar and post annealed at 573 and 673 K have the predominant orthorhombic phase, whereas at 0.1 mbar and 573, 673 K triclinic phase is predominant. When sputtering pressure is at 0.5 mbar the predominant phase is monoclinic when annealed at 473 K and triclinic at 673 K. The optical energy gap is influenced significantly by sputtering pressure and post annealing temperatures. The optical energy gap of the films deposited at higher sputtering pressures and post annealed at lower temperatures is high due to smaller crystallite sizes. The thickness of all deposited films at different conditions is around 200 nm.     

Structural and optical properties of Ag doped tungsten oxide (WO<Subscript>3</Subscript>) by microwave-assisted chemical route

The present study explores, the pure and silver (Ag) doped WO₃ nanoparticles synthesized by microwave irradiation method. Powder X-ray diffraction results reveal that the WO₃ doped with Ag concentration from 0 to 10 wt% crystallizes in monoclinic structure. TEM analysis shows both pristine and silver doped WO₃ nanoparticles. They are having spherical morphology with a average size from 30 to 40 nm. Scanning electron microscopy studies depicts that both the pristine and Ag doped WO₃ form in spherical shaped morphology with an average diameter of 40–30 nm, which is in proper agreement with the average crystallite sizes calculated by Scherrer’s formula. A considerable red shift in the absorbing band edge and a decrease in the band gap energy from 3.00 to 2.85 eV for Ag doped samples were observed by using UV–DRS spectra analysis. The defects in crystal and oxygen deficiencies were analyzed by photoluminescence spectra analysis.     

Investigation on electrical transport properties of nanocrystalline WO<Subscript>3</Subscript> under high pressure

The electrical transport properties of nanocrystalline tungsten trioxides (WO₃) under high pressures have been investigated by various electrical measurements up to 36.5 GPa. The discontinuous changes in direct-current resistivity under high pressures result from two electronic phase transitions at 4.3 and 10.5 GPa and two structural phase transitions at 24.8 and 31.6 GPa. Hall-effect measurement shows that the nanocrystalline WO₃ is n-type semiconductor within the whole investigated pressure range. The carrier concentration decreases monotonously with increasing pressure, but mobility increases first and then decreases at 10.4 GPa. Through alternate-current impedance measurement, it can be found that the variation of the ratio of grain boundary resistance to grain resistance synchronizes with that of the mobility under high pressures, indicating that the grain boundary plays more important role in the carrier transport process of nanocrystalline WO₃. The discontinuous changes of resistance and relaxation frequency of grain and grain boundary also provide the evidence for electronic phase transitions.     

Structure of WO<Subscript>3</Subscript>-TeO<Subscript>2</Subscript> glasses

WO₃-TeO₂ glasses have been studied by quantum-chemical simulation and Raman spectroscopy. The results have been used to develop a model for the network of tungstate-tellurite glasses. The model allows one to correlate the structure and optical properties (in particular, the position and intensity of Raman bands) of the glasses with their composition. The network of the glasses is shown to be made up, for the most part, of three types of structural groups: TeO₄ trigonal dipyramids, O=TeO₂ pyramids, and O=WO₅ octahedra. Any other structural units, in particular, WO₄ tetrahedra, are unnecessary. The model for the network of WO₃-TeO₂ glasses can be used to analyze the vibrational spectra of tungstate-tellurite glasses in a broad composition range. In particular, using this model we assigned the Raman spectra of the tungstate-tellurite glasses in the range 550–950 cm^?1.     

Synthesis of WO<Subscript>3</Subscript>/mesoporous ZrO<Subscript>2</Subscript> catalyst as a high-efficiency catalyst for catalytic oxidation of dibenzothiophene in diesel

The catalyst of WO₃ highly dispersed on mesoporous ZrO₂ was prepared by hydrothermal treatment and calcination. Ionic liquid templates with different cations and various calcination temperatures were taken into account to investigate factors that affect the structure of the catalysts. Wide-angle XRD, Raman, low-angle XRD, N₂ adsorption–desorption, SEM, EDS-mapping and TEM analyses were employed to characterize the composition, morphology and the dispersion of WO₃. It was found that [C₁₆H₃₃N(CH₃)₃]₄SiW₁₂O₄₀ and 700 °C were the optimal ionic liquid template and calcination temperature; the obtained catalyst, expressed as 700-C₁₆-WO₃/ZrO₂, exhibited the best catalytic activity in oxidation desulfurization. Dibenzothiophene (DBT) in the model oil could be oxidized to DBT sulfone (DBTO₂) completely, certified by GC-MS analysis, under optimum conditions. The oxidative desulfurization efficiencies of different substrates catalyze by 700-C₁₆-WO₃/ZrO₂, and the cycle performance on the oxidation of DBT were studied.     

Extrapure WO<Subscript>3</Subscript> for the preparation of CdWO<Subscript>4</Subscript> single crystals

A process has been developed for the preparation of extrapure WO suitable for crystal growth. The method involves thermal decomposition of ammonium paratungstate (APT) powder at 230–250° C, resulting in the formation of soluble ammonium metatungstate (AMT). The AMT solution is then purified and hydrolyzed. The resultant purified APT is converted to WO₃. The reaction yield is at a level of 95%. The contents of metallic impurities, especially those of polyvalent impurities (Fe, Mn, Cu, Pb, V, Cr, and others), capable of influencing the optical properties of crystals, was comparable to their detection limit: 10^?6 to 10^?5 wt %. The purification process was tested using a pilot-scale system. The purity was confirmed by the synthesis of high-quality CdWO₄ crystals.     

Thermoelectric properties of WO<Subscript>3</Subscript>-based ceramics doped with Co<Subscript>2</Subscript>O<Subscript>3</Subscript>

Tungsten trioxide (WO₃) doped with cobalt sesquioxide (Co₂O₃) was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 300 up to 1,000 K. The addition of Co₂O₃ to WO₃ resulted in an increase in both the grain size and porosity, indicating that Co₂O₃ promotes the grain grown of WO₃. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the Co₂O₃ content. As for the power factor (σS ² ), the 5.0 mol% sample has the maximum value of the power factor which is 0.12 μWm⁻¹K⁻² at 873 K.     

Low-loss,high-purity (TeO<Subscript>2</Subscript>)<Subscript>0.75</Subscript>(WO<Subscript>3</Subscript>)<Subscript>0.25</Subscript> glass

By melting a mixture of high-purity oxides in a platinum crucible under flowing purified oxygen, we have prepared (TeO₂)_0.75(WO₃)_0.25 glass with a total content of 3d transition metals (Fe, Ni, Co, Cu, Mn, Cr, and V) within 0.4 ppm by weight, a concentration of scattering centers larger than 300 nm in size below 10² cm⁻³, and an absorption coefficient for OH groups (λ ∼ 3 μm) of 0.008 cm⁻¹. The absorption loss in the glass has been determined to be 115 dB/km at λ = 1.06 μm, 86 dB/km at λ = 1.56 μm, and 100 dB/km at λ = 1.97 μm. From reported specific absorptions of impurities in fluorozirconate glasses and the impurity composition of the glass studied here, the absorption loss at λ ∼ 2 μm has been estimated at ≤100 dB/km. The glass has been drawn into a glass-polymer fiber, and the optical loss spectrum of the fiber has been measured.     

Defects Induced Magnetism in WO<Subscript>3</Subscript> and Reduced Graphene Oxide-WO<Subscript>3</Subscript> Nanocomposites

A series of reduced graphene oxide (rGO)-WO₃ nanocomposites were prepared by hydrothermal method using GO and tungsten complex. The nanocomposites were characterized by powder XRD, Raman spectroscopy, FT-IR spectroscopy, HRTEM, XPS, photoluminescence (PL), and magnetic studies. The structural analysis confirms the hexagonal crystal structure and formation of rGO-WO₃ nanocomposites. HRTEM images show rod-like shape WO₃ distributed on the wrinkle structure of rGO sheets. XPS results confirm the oxidation state and oxygen vacancies present in the samples. PL spectra of the samples show blue emission and indicate the existence of surface defects and oxygen vacancies. The M–H loop of rGO-WO₃ nanocomposites reveal that the co-existence of both ferro and antiferromagnetism at room temperature. The incorporation of rGO sheets notably increase magnetic behavior of composites due to extended C–C bond conducts much stronger coupling between the 5d and 6s orbitals of tungsten and carbon atoms.     

Preparation and photoelectrochemical characterization of WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanotube array electrode

WO₃/TiO₂ nanotube array electrode was fabricated by incorporating WO₃ with TiO₂ nanotube array via a wet impregnation method using ammonium tungstate as the precursor. TiO₂ and WO₃/TiO₂ nanotube arrays were characterized by field emission scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. In order to characterize the photoelectrochemical properties of WO₃/TiO₂ electrode, electrochemical impedance spectroscopy, and steady-state photocurrent (i _ss) measurement at a controlled potential were performed in the supporting electrolyte containing different concentrations of glucose. The photoelectrochemical characterization results reveal that WO₃/TiO₂ nanotube array electrode possesses a much higher separation efficiency of the photogenerated electron–hole pairs and could generate more photoholes on the electrode surface compared with the pure TiO₂ nanotube array electrode. The i _ss for glucose oxidation at WO₃/TiO₂ nanotube array electrode is much higher than that at the pure TiO₂ nanotube array electrode.     

Ca<Subscript>3</Subscript>WO<Subscript>6</Subscript>: a novel microwave dielectric ceramic with complex perovskite structure

Present work introduces a novel Ca₃WO₆ microwave dielectric ceramic with a complex perovskite structure. The Ca₃WO₆ ceramic was prepared by solid state reaction method and can be well densified at above 1,260 °C for 2 h in air. All the XRD patterns can be fully indexed as a single-phase monoclinic structure (space group P2₁/n). The sharp Raman vibration mode at 810 cm⁻¹ suggests the long range order in the Ca₃WO₆ structure. The best microwave dielectric properties can be obtained in ceramic sample sintered at 1,275 °C for 2 h with a permittivity ~15.3, a Qf value ~29,200 GHz and a TCF value about −30 ppm/°C. Applying the oxide additivity rule, the calculated permittivity agrees well with the measured value. This kind of ceramic might have some potential value for microwave application for its good microwave dielectric behavior. The (Ca_1/2W_1/2) complex cations holding the site of Ti⁴⁺ in perovskite structure would introduce many new systems in complex perovskite compounds in the future.     

Properties of nanocrystalline ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-CeO<Subscript>2</Subscript>-CoO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

We have studied the properties of nanocrystalline ZrO₂-Y₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).     

Tunable optical and nano-scale electrical properties of WO<Subscript>3</Subscript> and Ag-WO<Subscript>3</Subscript> nanocomposite thin films

WO₃ and Ag-WO₃ nanocomposite thin films have been synthesized from pure WO₃ and Ag-WO₃ composite pressed powder targets submitted to pulses generated by a frequency quadrupled Nd:yttrium aluminium garnet (YAG; λ = 266 nm, τ ~ 5 ns, ν = 10 Hz) laser source. The irradiations were performed in low pressure oxygen atmosphere. The obtained results proved the possibility to tailor the synthesized thin films optical properties in the UV–Visible spectral region and their nano-scale electrical characteristics through the process parameters, as ambient oxygen pressure value during the thin films deposition and Ag concentration of the Ag-WO₃ composite targets. The tunable optical and electrical features allow for the creation of new materials for future applications as photocatalysts, transparent conducting electrodes, electrochromic or chemical and biological sensor devices.     

Effects of sodium oleate on synthesis and photocatalytic activity of Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript>/Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>@RGO

In this study, the effects of sodium oleate on synthesis of Bi₂WO₆/Bi₂O₃ loaded reduced graphene oxide photocatalyst was studied. The as-prepared composites were characterized by X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, UV–visible diffuse reflectance and photoluminescence spectroscopy. The results suggested that addition of sodium oleate not only promoted synthesis of Bi₂O₃, but also enhanced the reduction of GO to graphene. When the amount of sodium oleate was 4 mol (Bi:SO?=?1:1), Bi₂WO₆/Bi₂O₃@RGO to the best visible-light photocatalytic activity can be synthesized by a facile one-step solvothermal process without further reduction reaction. Hence, it indicated that sodium oleate could affect the synthesis of the as-prepared composites and the photocatalytic activity for degradation of RhB. This study did provide not only a facile method to synthesize Bi₂WO₆/Bi₂O₃@RGO, but also a method to reduce graphene oxide to graphene.     

Gold/WO<Subscript>3</Subscript> nanocomposite photoanodes for plasmonic solar water splitting

A facile electron-charging and reducing method was developed to prepare Au/WO₃ nanocomposites for plasmonic solar water splitting. The preparation method involved a charging step in which electrons were charged into WO₃ under negative bias, and a subsequent reducing step in which the stored electrons were used to reductively deposit Au on the surface of WO₃. The electron-charged WO₃ (c-WO₃) exhibited tunable reducibility that could be easily controlled by varying the charging parameters, and this property makes this method a universal strategy to prepare metal/WO₃ composites. The obtained Au/WO₃ nanocomposite showed greatly improved photoactivity toward the oxygen evolution reaction (OER) when compared with WO₃. After Au decoration, the OER photocurrent was improved by a percentage of over 80% at low potentials (<0.6 V vs. SCE), and by a percentage of over 30% at high potentials (>1.0 V vs. SCE). Oxygen evolution measurements were performed to quantitatively determine the Faraday efficiency for OER, which reflected the amount of photocurrent consumed by water splitting. The Faraday efficiency for OER was improved from 74% at the WO₃ photoanode to 94% at the Au-8/WO₃ composite photoanode, and this is the first direct evidence that the Au decoration significantly restrained the anodic side reactions and enhanced the photoelectrochemical (PEC) OER efficiency. The high photoactivity of the composite photoanode toward OER was ascribed to the plasmon resonance energy transfer (PRET) enhancement and the catalytic enhancement of Au nanoparticles (NPs).

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Oxalic acid assisted hydrothermal synthesis and optical absorption property of WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanocomposites

The WO₃/TiO₂ nanocomposites were successfully prepared via a facile oxalic acid assisted hydrothermal process. The oxalic acid played a vital role on the preparation of WO₃/TiO₂ nanocomposites. Notably, it has been observed that the nanocomposites exhibited the wider absorption edge, and the higher photocatalytic activity, compared with pure TiO₂. In addition, the photocatalytic mechanism was proposed, and it elaborated that WO₃/TiO₂ nanocomposite promoted the separation of the photoproduction carriers, and improved photocatalytic activity. The WO₃/TiO₂ nanocomposite may have a potential application as a UV–visible photocatalyst.     

Effect of hydrothermal duration on synthesis of WO<Subscript>3</Subscript> nanorods

Among different type of transition metal oxides, tungsten trioxide (WO₃) is a suitable candidate for electronic device fabrication due to its n-type property and wide band gap. Herein, one-dimensional tungsten trioxide (WO₃) nanorods were achieved from an aqueous solution of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sodium chloride (NaCl) in an acidic media by a time-optimized hydrothermal synthesis in autoclave at 180°C or different synthesis durations. For studying morphology and size of obtained powder, X-ray diffraction (XRD), scanning electron microscope (SEM), and high resolution transmission electron microscope (HRTEM) were applied. Finally, WO₃ nanorods of about 2–3 μm in length and 100–200 nm in diameter were obtained during 3 h hydrothermal process.     

Thermal Analysis of the Ternary System Li<Subscript>2</Subscript>WO<Subscript>4</Subscript>-WO<Subscript>3</Subscript>-Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

Thermal analysis results indicate that the liquidus surface of the Li₂WO₄-WO₃-Li₂B₄O₇ system consists of the primary phase fields of Li₂WO₄, Li₂B₄O₇, WO₃, Li₂WO₄ · WO₃ (congruent melting), 3Li₂WO₄ · 2Li₂B₄O₇ (congruent melting), and Li₂WO₄ · 3WO₃ (incongruent melting). Low-melting-point compositions are selected that are potentially attractive for the low-temperature synthesis of lithium tungsten bronze powders.     

Solution route synthesis and characterization of nanocrystalline Bi<Subscript>2</Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript> for usage in resistor paste application

In this paper, we report synthesis of Bi₂Ru₂O₇ by solution route following the usual procedure, wherein, RuCl₃ solution was added drop by drop to a solution of bismuth nitrate while maintaining the pH of reaction at 6.75 with the help of dilute HCl and NaOH. Various physico-chemico-thermal analysis of these as -precipitated powders reveal that Bi & Ru precipitate as complex hydroxides with particle size ranging between 100–200 nm. The XRD analyses of the vacuum-dried (250°C) samples indicate conversion to Bi₂Ru₂O₇ (BRO) phase with low crystallinity having some RuO₂ impurity. Further heat treatment at 600°C for 4 h in an air atmosphere produces highly crystalline Bi₂Ru₂O₇ powder with a trace impurity of RuO₂. Resistor pastes were made using these BRO powders by intimately mixing them with other ingredients such as standard lead alumino-boro-silicate glass frit and polymeric thixotropic agents like ethyl cellulose and organic solvents like butyl cellosolve and -Terpinol. For all the paste samples, the inorganic part of the paste contained 43% glass and 57% BRO. These pastes were screen-printed and fired onto alumina substrates with pre-fired in-house Pd-Ag conductor terminations. The fired resistors had sheet resistance of 35–75 and hot TCR between 150 to 480 ppm/°C and zero cold TCR for resistors No.10 & 18 and for other resistors the cold TCR deviated from zero due to low scale processing of resistor paste.     

Synthesis and microstructure of nanocrystalline Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> powders

Nanocrystalline ytterbia powders have been synthesized using different precursors prepared by precipitation from nitrate solutions: ytterbium carbonates, oxalates, and hydroxides. The powders have been characterized by X-ray diffraction and scanning electron microscopy. The nature of the precursor has no effect on the crystallization temperature of ytterbia but influences its microstructure. The particles range in shape from spherical to platelike. The average crystallite size of the Yb₂O₃ powders is 20–25 nm. Raising the heat-treatment temperature from 600 to 1000°C increases the crystallite size to 33–46 nm. The highest thermal stability is offered by the ytterbia powders prepared through carbonate decomposition.