WO3 Phase Research Articles

Controllable Synthesis of WO3 Nanowires by Electrospinning and Their Photocatalytic Properties Under Visible Light Irradiation

Large amounts of WO3 nanowires were prepared on silicon substrates by electrospinning followed by appropriate calcinations in air using ammonium metatungstate (AMT) as WO3 source. Tunable densities and diameters of WO3 nanowires were achieved by changing the electrospinning time and the concentration of AMT in precursor solution. TG/DSC analysis was used to direct the heating process. The effects of both solvent ratio and heating process on the morphology of the obtained nanowires were investigated. The morphology, structure, and chemical compositions of the tungsten oxide were characterized by SEM, XRD, and EDX, respectively. Results showed that monoclinic phase WO3 nanowires with diameters ranging from 100 to 200 nm were obtained after the appropriate heating process when the AMT concentration of the precursor solution increased from 10 to 20 wt%. The photocatalytic performance of the obtained WO3 nanowires under visible light irradiation (>420 nm) was investigated in the degradation of Rhodamine B at room temperature in air.

Structural, optical, and surface properties of WO3 thin films for solar cells

Transparent back contacts can be used in thin film solar cells facilitating their potential application in tandem cells, bifacial devices and solar windows. In this study, tungsten oxide (WO3) thin films were deposited by Radio Frequency (RF) reactive sputtering in Ar+O2 ambient. The effects of post deposition anneals in air on the structural, optical, and surface properties of the deposited films were investigated using X-ray diffraction, UV/Vis/NIR spectrophotometry, and X-ray photoelectron spectroscopy, respectively. As-deposited films exhibited amorphous structures with no change after annealing at 300°C. Samples annealed at 400 and 500°C were crystallized and identified as pure monoclinic WO3 phase with (200) and (002) preferred orientation, respectively, determined by XRD fiber texture analysis. Scherrer analysis of excess broadening indicated a coherency length of 50 and 65nm for the 400 and 500°C annealed films, respectively. High resolution XPS studies showed the presence of W6+ (WO3) oxidation states at the surface of the as-deposited and the 300°C annealed films. Annealing at 400–500°C led to an oxygen deficient surface with a sub-stoichiometric WO3−x phase. UV/Vis/NIR spectrophotometry revealed that post processing decreased the optical bandgap from 3.30eV for the as-deposited films to 3.17eV for the 500°C annealed films. Results are compared with MoO3 films deposited by a similar technique.

Structural, optical and electrochromic properties of RF magnetron sputtered WO3 thin films

Thin films of tungsten trioxide (WO3) have been prepared by RF reactive magnetron sputtering of tungsten target at different substrate temperatures in the range 303–673K and at fixed oxygen partial pressure of 6×10−2Pa and sputter pressure of 4Pa. The effect of substrate temperature on the structural, morphological, optical and electrochromic properties of WO3 films was systematically studied. The films formed at 303K were of X-ray amorphous, while those deposited at substrate temperatures ≥473K were crystallized into orthorhombic phase WO3. The crystallite size of the films increased from 17 to 24nm with increase of substrate temperature from 473 to 673K. Raman studies confirmed that the presence of O–W–O and W=O bonds in WO3 films. The surface morphology of the films was significantly varied with substrate temperature. The optical transmittance data revealed that the optical band gap increased from 3.08 to 3.48eV and refractive index increased from 2.18 to 2.26 with increase of substrate temperature from 303 to 673K respectively. The WO3 films formed at substrate temperature of 473K exhibited better optical transmittance modulation of 40% between colored and bleached state with a color efficiency of 33.8cm2/C and diffusion coefficient of 1.85×10−11cm2/s.

Mechanism for the Growth of Thin Films of WO3 and Bronzes from Suspensions of WO3 Nanoparticles

WO3 is an oxide with semiconducting and electrocatalytic properties, of high interest in a number of applications. It may exist in different hydrates, like WO3.2H2O and WO3.H2O. In aqueous solution, it may be reduced reversibly to bronzes (WO3Hx) and lower valence oxides (WO3-x), exhibiting different physical and chemical properties, like colour, hydrophobicity, and conductivity (protonic and electronic). In combination with platinum, it forms a mixed compound, Pt-WO3, with interesting possibilities as catalyst for fuel cell electrodes.Thin films of WO3 can be grown electrochemically from aqueous solutions containing dissolved W precursors. Another possibility is the growth from WO3 suspensions formed from insoluble tungsten compounds. The electrochemical growth from WO3 suspensions is a process that requires the diffusion and sticking of the suspended (nano-) particles on the electrode surface.In this communication, the growth of WO3 films and derived compounds, from aqueous suspensions of WO3 is studied with the help of the quartz crystal microbalance (EQCM). WO3 suspensions are prepared from acidic WO4 2- solutions, and deposited on gold and Pt substrates. Film growth rates by stepping and cycling the electrode potential are analysed with EQCM.Fig. 1 contains steady state deposition rate and electrode current on a Pt substrate. For potentials above 0.15 V exists just an attachment of WO3 particles leading to a constant, low, growth rate. Film grown in this regime tends to come off the electrode surface after some minutes. Below 0.15 V, growth rates increases linearly up to -0.2 V. The same figure shows that the film growth rate has close resemblance with the logarithm of stationary reductive current density due to reduction of water (H+ + e- --> 0.5H2). The acceleration of film growth rate in parallel with water reduction appears to indicate that hydrogen reduced species on the Pt surface participate in the electrochemical reduction of WO3 suspensions. Such possibility is probable in the light of the well known H-spillover mechanism from Pt to WO3 phase. Reactions for film growth can be proposed in this way: Pt + H+ + e- --> Pt-Hsp [WO3]sus + xPt-Hsp --> WO3Hx + Pt The growth mechanism is analysed on the light of EQCM results, thin films are characterized (SEM, XPS), and the stability of the films in aqueous solutions is also analysed in view of their possible application as an electrocatalyst component for fuel cell electrodes.AcknowledgementsThis work is financed by the project ELECTROFILM, MAT2011-27151 from Ministry of Science and Innovation of Spain.

Structures and optical properties of tungsten oxide thin films deposited by magnetron sputtering of WO3 bulk: Effects of annealing temperatures

Tungsten oxide thin films were deposited on glass substrates by the magnetron sputtering of WO3 bulk at room temperature. The deposited films were annealed at different temperatures in air. The structural measurements indicate that the films annealed below 300 °C were amorphous, while the films annealed at 400 °C were mixed crystalline with hexagonal and triclinic phases of WO3. It was observed that the crystallization of the annealed films becomes more and more distinct with an increase in the annealing temperature. At 400 °C, nanorod-like structures were observed on the film surface when the annealing time was increased from 60 min to 180 min. The presence of W=O stretching, W—O—W stretching, W—O—W bending and various lattice vibration modes were observed in Raman measurements. The optical absorption behaviors of the films in the range of 450–800 nm are very different with changing annealing temperatures from the room temperature to 400 °C. After annealing at 400 °C, the film becomes almost transparent. Increasing annealing time at 400 °C can lead to a small blue shift of the optical gap of the film.

Annealing dynamics of WO3 by in situ XRD

Tungsten trioxide is a semiconductor with distinct applications in gas sensors, catalysis, batteries and pigments. As such the transition between its different crystal structures during its annealing are of interest, especially for sensor applications. Here, WO3 nanoparticles with closely controlled crystal and grain size (9–15nm) and phase composition are made by flame spray pyrolysis and the formation of different WO3 phases during annealing is investigated. Most notably, the dynamic phase transition and crystal size evolution of WO3 during heating and cooling is monitored by in situ X-ray diffraction revealing how metastable WO3 phases can be captured stably. The effect of Si-doping is studied since it is used in practise to control crystal growth and phase transition during metal oxide synthesis and processing. Finally the influence of annealing on the WO3 sensing performance of NO, a lung inflammation tracer in the human breath, is explored at the ppb-level.

Microstructure of a commercial W–1% La2O3 alloy

W–1% La2O3 alloy is considered as the most promising material for plasma-facing components of fusion reactors. The microstructure of a commercial W–1% La2O3 alloy was investigated using optical and transmission electron microscopes. The microstructure of pure tungsten can be improved significantly by fabrication of W–1% La2O3 alloys. W–1% La2O3 alloys can be produced with no porosities and cracks, and with various oxide phases dispersed in alloy matrix. La2O3 with different crystal structures, La6W2O15, WO2, WO3 and W3O8 phases were identified in as-forged W–1% La2O3 alloy. Long strip-like La2O3 has a very large size, whereas spherical La6W2O15, navicular WO3, hexagonal W3O8 and short rod-like La2O3 are smaller particles. Most identified phases have a heterogeneous distribution. Forging leads to a more dispersive distribution of large-sized La2O3 particles but not of fine WO3 particles compared with rolling. The mechanical properties of the alloys are also discussed.

Fabrication of positively and negatively charged, double-shelled, nanostructured hollow spheres for photodegradation of cationic and anionic aromatic pollutants under sunlight irradiation

A novel approach to prepare double-shelled, nanostructured hollow spheres with positively and negatively charged was proposed. If the difference in isoelectric points between MxO and NyO was large enough, at a certain pH range, the inner and outer shell of double-shelled MxO@NyO hollow spheres were positively and negatively charged, respectively. A proof-of-concept was shown, using WO3 and TiO2 as anionic and cationic building subunits, respectively. The nanoparticles of WO3 and TiO2 were coated successively onto the functionalized polystyrene (PS) template beads, the resulted PS@WO3@TiO2 nanocomposites were calcined at elevated temperature, and then double-shelled WO3@TiO2 hollow spheres were obtained. The dispersity, morphology, size, and lattice of these hollow spheres were tested by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The presence and purity of both WO3 and TiO2 phases were confirmed by electron energy loss mapping analysis and X-ray diffraction analysis (XRD). The specific surface area and the average pore diameter of these hollow spheres were 53.04m2/g and 8.042nm, respectively. The photocatalytic activity for the degradation of cationic and anionic aromatic pollutants was in the order of double-shell hollow spheres WO3@TiO2>single-shell WO3-TiO2>P25, i.e., the synergistic effect of coupling TiO2 with WO3 on photocatalytic performance was proved by us.

Multi-techniques investigation of mesoporous zinc and tungsten titanates materials

We have investigated the preparation of zinc and tungsten mesoporous titanates materials from a sol–gel method using a block copolymer as amphiphile. The mesoporous network is created through a soft templating method. Samples have been characterized by SAXS, XRD, nitrogen adsorption–desorption, XPS, TEM, EDX, Raman and UV–Vis spectroscopies. The presence of tungsten induces the formation of orthorhombic crystal phase WO3 besides the titania network and inhibits the formation of anatase, which is the predominant structure in the Zn–TiO2 materials. No trace of ZnO is detected by XRD and Raman. However XPS suggests the presence of zinc ions in the mesoporous network and their linkage with oxygen atoms. The zinc oxide could therefore either be randomly dispersed or amorphous on the material surface. XPS and EDX also reveal that the tungsten clusters are located at the surface of the tungsten mesoporous titanates. Studies of the Kubelka–Munk function showed that at low Zn content, the band gap energy is in the range of the anatase one and the photocatalytic efficiency of the corresponding zinc titanates is similar to the one of the pure titania mesoporous material. Beyond the addition of 1mol.% of tungsten, the titanates present a band gap energy in the range 2.7–2.8eV, close to the value of the orthorhombic WO3. For both kinds of titanates, the decrease of the band gap reduces their capacity for the photodegradation of methyl orange.

Influence of Operation Parameters on the Response of a PEMFC with Electrodeposited Pt‐WO3 Cathode

AbstractPt‐WO3 electrodes for proton exchange membrane fuel cells (PEMFC) have been prepared by electrodeposition on microporous carbon substrates. The electrodes have been studied as cathode in single cells under different operation conditions, including gas humidification, stoichiometry, and cell temperature. Polarization curves show a response qualitatively different from that observed with standard Pt/C cathodes, dominated by properties of the WO3 phase. At low current densities, below ca. 400 mA cm–2, the response is limited by protonic resistance and electrochemical kinetics in the cathode catalyst layer; at high current densities, large water retention by the hydrophilic cathode catalyst layer may cause anode flooding. Chemical changes in WO3 phase accelerate at high temperature and under dry cathode conditions, compromising cell stability in the long term. Therefore, continuous and stable operation requires low humidification in the anode and high in the cathode. Also, a pronounced transient voltage response to current demand steps is observed at low cell voltage, which we attribute to the transition of the cathode potential to a mixed potential value, laying between the oxygen reduction and the hydrogen adsorption/desorption equilibrium mediated by spillover from Pt to the WO3 phase. Basic kinetics considerations show how the height of the voltage transient is related with H spillover rate. Pt‐WO3 cathode catalyst layer may have a practical interest as cathode for PEMFC when dry hydrogen inlet conditions are imposed, for instance, by the use of a metal hydride hydrogen storage system.

Optimizing the Band Edges of Tungsten Trioxide for Water Oxidation: A First-Principles Study

Tungsten trioxide (WO3) is a promising photoanode material for water oxidation, due to its stability in aqueous environments under different pH conditions. However, the optical gap of monoclinic WO3—the stable form of the solid at ambient conditions—is larger than desired for optimal sunlight absorption, and the position of its valence band maximum with respect to the water oxidation potential requires a large overpotential to drive the oxidation reaction. Using first-principles calculations we showed that several phases of WO3 found either at high temperature or under pressure exhibit a valence band maximum higher in energy than that of room temperature monoclinic WO3 by ∼0.35 eV and smaller electronic gaps, better suited to efficiently absorb sunlight. In addition, we found that orthorhombic WO3, stable at high temperature, has also improved carrier mobilities for both holes and electrons. We propose that epitaxial growth on appropriate substrates may be a promising way to stabilize metastable phases of...

Formation and electrochemical characterization of anodic ZrO2–WO3 mixed oxide nanotubular arrays

ZrO2–WO3 mixed oxide nanotubes were synthesized by a simple electrochemical anodization route. The oxide nanotubes contained a mixture of metastable hexagonal WO3 and monoclinic (and orthorhombic) ZrO2 phases, as well as a mixed-oxide ZrW2O8 phase that showed a metastable tetragonal symmetry. Evaluation of photo-activity of the materials showed generation of photo-potentials of −85mV and −230mV in the as-anodized and annealed conditions. Because of the mismatch in the band edge positions of the WO3 and ZrO2 phases and the resultant relaxation of photo-generated charge carriers, no significant photo-current density could be observed. The arrays of oxide nanotubes are considered for electrochemical capacitor application because of their morphology-assisted fast charge/discharge kinetics and large surface area. Presence of a large concentration of charge defects (on the order of 1021cm−3) and the reported high proton conductivity of the ZrO2–WO3 mixed oxide rendered high capacitance, which decreased with an increase in the scan rate of cyclic voltammetry. The highest measured capacitance was 40.03mF/cm2 at a scan rate of 10mV/s and the lowest was 1.93 mF/cm2 at 1V/s in 1M sulfuric acid solution.

Gas sensing of ruthenium implanted tungsten oxide thin films

Different amounts of Ru were implanted into thermally evaporated WO3 thin films by ion implantation. The films were subsequently annealed at 600°C for 2h in air to remove defects generated during the ion implantation. The Ru concentrations of four samples have been quantified by Rutherford Backscattering Spectrometry as 0.8, 5.5, 9 and 11.5at.%. The un-implanted WO3 films were highly porous but the porosity decreased significantly after ion implantation as observed by Transmission Electron Microscopy and Scanning Electron Microscopy. The thickness of the films also decreased with increasing Ru-ion dose, which is mainly due to densification of the porous films during ion implantation. From Raman Spectroscopy two peaks at 408 and 451cm−1 (in addition to the typical vibrational peaks of the monoclinic WO3 phase) associated with Ru were observed. Their intensity increased with increasing Ru concentration. X-ray Photoelectron Spectroscopy showed a metallic state of Ru with binding energy of Ru 3d5/2 at 280.1eV. This peak position remained almost unchanged with increasing Ru concentration. The resistances of the Ru-implanted films were found to increase in the presence of NO2 and NO with higher sensor response to NO2. The effect of Ru concentration on the sensing performance of the films was not explicitly observed due to reduced film thickness and porosity with increasing Ru concentration. However, the results indicate that the implantation of Ru into WO3 films with sufficient film porosity and film thickness can be beneficial for NO2 sensing at temperatures in the range of 250°C to 350°C.

Solvothermal synthesis of spindle-like WO3–TiO2 particles with enhanced photocatalytic activity

Spindle-like WO3–TiO2 composite particles were synthesized through a facile one-step solvothermal method. The as-synthesized spindle-like WO3–TiO2 composite particles with center diameter of ca. 150nm and length of ca. 300nm were assembled by numerous WO3–TiO2 nanorods of ca. 20nm in diameter. It is found that the photo-responding range was extended and the specific surface area of the composite was increased. Such spindle-like WO3–TiO2 composite exhibited much higher photocatalytic activity than that of single phase TiO2 or WO3 in the degradation of methyl orange dye under illumination with simulated sunlight.

Electrical and optical properties of mixed phase tungsten trioxide films grown by laser pyrolysis

AbstractLaser pyrolysis was chosen to synthesize tungsten trioxide starting with tungsten ethoxide precursor. The film was found to have a thickness that varied from 205 nm to 1 µm. X‐ray diffraction and Raman spectroscopy confirmed the presence of a mixture of hexagonal and tetragonal phase WO3 in the synthesized film, as well as tungsten bronzes. It was evident that annealing greatly influenced the phases and types of structures formed, and EDXS was carried out in an attempt to quantify the tungstate bronzes to tungsten oxide. I‐V curves of the films showed n‐type semiconducting behaviour, but the mixed phase appeared to cause a similar behaviour of dopants in a semiconductor. The refractive index decreased with increasing wavelength and gave values of up to 21 at low wavelengths. The average optical band gap was found to be 3.6 eV from UV/Vis spectroscopy. Scanning Electron Microscopy (SEM) showed a mixture of nano‐ and microstructures after annealing. Nanorod structures were isolated and Pt‐contacted using FIB for possible applications such as an active sensing medium in gas sensors. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Alkali Metals Interstitial Doping on Structural and Electronic Properties of WO3

The band gap engineering of semiconducting oxide to shift the light absorption edge to lower energies in the visible spectrum is often achieved by doping the bulk material. Intercalation of atomic and small molecular species in the cavities of WO3 is a viable approach to introduce foreign elements in the lattice (Mi, Q.; Ping, Y.; Li, Y.; Cao, B.; Brunschwig, B. S.; Khalifah, P. G.; Galli, G. A.; Gray, H. B.; Lewis, N. S.; J. Am. Chem. Soc. 2012, 134, 18318). In this work we show that introduction of alkali metal cations in room-temperature (RT) monoclinic WO3 interstices causes an anisotropic distortion of the lattice parameters (b/c ratio is smaller than that of the pure bulk value) which is the major cause of a considerable band gap reduction (about 0.5 eV). Analogous intercalation of neutral atoms (i.e., Ne and Xe) induces only a tiny b/c ratio variation in the opposite direction with no effective band gap reduction. Structural differences in WO3 (e.g., from RT monoclinic to cubic phase WO3) are known...

Electrochemical Response for Spherical and Rod Shaped WO3 Nanoparticles

A rapid and new technique describes synthesis technique of spherical and rod shaped tungsten trioxide (WO3) nanoparticles with similar band gap at visible wavelength. Acid catalyzed exothermic reaction and structure directing reagent follows the formation of two different morphologies and monoclinic WO3 phase. Rod shaped WO3 nanoparticle coated ITO glass electrode exhibits high current density at identical low voltage and scan rate due to its better adherence and coating uniformity in comparison with spherical nanoparticles. WO3-ITO electrode alters to blue tungsten bronze in color at low voltage, and further the color restores after removing the same applied voltage.

Crystallization and thermochromism of annealed heterostructures containing titanium and tungsten oxide films

Crystalline phases in heterostructures containing titanium and tungsten oxide films are studied after step annealing in vacuum at temperatures between 500 and 750°C. The films are deposited on a silica glass substrate by dc reactive magnetron sputtering. It is found that crystalline phases in single layers and bilayer structures form in a different way. In the latter, crystallization is influenced by the order of layer arrangement on the substrate. Thermochromism in structures annealed in vacuum is due to the oxygen-deficient phase WO3 − x belonging to the hexagonal syngony. This phase intensely grows as the temperature rises from 650 to 750°C.

Soft-chemical synthesis, characterization and humidity sensing behavior of WO3/TiO2 nanopowders

Tungsten oxide/titania (WO3/TiO2) nanopowders were synthesized by the polymeric precursor method which varied the WO3 content between 0 and 10mol%. The powders were thermally treated in a conventional furnace and their structural, microstructural and electric properties were evaluated by X-ray diffraction (XRD), Raman spectrometry, N2 physisorption, NH3 chemisorption, temperature-programmed reduction (TPR), X-ray absorption near-edge spectroscopy (XANES) in situ XANES and extended X-ray absorption fine structure spectroscopy (EXAFS) and transmission electron microscopy (TEM). XRD and Raman spectrometry confirmed the homogeneous distribution of an amorphous WO3 phase in the TiO2 matrix which stabilized the anatase phase through the generation of [TiO5·V0] or [TiO5·V0] complex sites. Conventional TPR-H2 (temperature programmed reduction) along with XANES TPR-H2 and XANES TPR-EtOH showed that WO3/TiO2 sample reduction occurs through the formation of these complex clusters. Moreover, the addition of WO3 promoted an increase in the surface acidity of doped samples as revealed by NH3 chemisorption. The WO3/TiO2 bulk-ceramic samples were further used to estimate their potential application in a humidity sensor in the range of 15–85% relative humidity. Probable reasons that lead to the different humidity sensor responses of samples were given based on the structural and surface characterizations. Correlation between the sensing performance of the sensor and its structural features are also discussed. Although all samples responded as a humidity sensor, the W2T sample (2mol% added WO3) excelled for sensitivity due to the increase in acid sites, optimum mean pore size and pore size distribution.

Synthesis and thermoelectric properties of CoSb3/WO3 thermoelectric composites

In this study, nano-sized WO3 powder was dispersed into CoSb3 powder by ball milling and CoSb3/WO3 thermoelectric composites were fabricated using hot-pressing sintering. The results showed that the WO3 phase distributed uniformly in the form of clusters and the average size of cluster was lower than 4 μm. As the content of WO3 increased, the electrical conductivity and Seebeck coefficient of CoSb3/WO3 composites decreased. The thermal conductivity of composites decreased obviously which resulted from the phonon scattering by the WO3 inclusions locating on the grain boundaries of CoSb3 matrix. The highest thermoelectric figure of merit ZT = 0.40 was achieved at 650 K for CoSb3/2%WO3 composite.