Monoclinic WO3 Phase Research Articles

Structural, photoluminescence, electrical, anti cancer and visible light driven photocatalytic characteristics of Co doped WO3 nanoplates

In this study, soft chemical route has been adopted for the synthesis of Co doped WO3 nanoplates. The prepared samples are crystallized into monoclinic phase of WO3 and composed of two dimensional (2-D) nanoplates. The substitution of Co2+ ions on the sites of W6+ ions has been confirmed through X-ray photoelectron spectroscopy (XPS) analysis. The presence of functional groups and chemical bonding has been verified through FTIR and Raman spectra. Narrowing of the optical gap with Co doping has been observed which is linked with formation defects levels in the band of WO3. The Co doping has been found to be very effective in enhancing the visible light driven photodegradation activity of WO3 nanoplates up to 90% which is attributed to trapping photo-generated electrons by defects. Furthermore, Co doped WO3 nanoplates have also shown good anticancer activities against human breast (MSF-7) and lever (Hep-2) cancer cells.

Effect of oxygen pressure on pulsed laser deposited WO3 thin films for photoelectrochemical water splitting

Tungsten oxide (WO3) thin films on fluorine-doped tin oxide (FTO) coated glass substrates are fabricated by pulsed laser deposition for photoelectrochemical water splitting. The effects of oxygen pressure during deposition on the structure, morphology, optical properties and photoelectrochemical performance of the films have been systematically evaluated. Columnar growth characteristic of WO3 thin films is observed at oxygen pressure higher than 8 Pa. The grain size increases with increasing oxygen pressure in the range of 1–13 Pa then decreases with further increasing oxygen pressure from 13 Pa to 30 Pa. Pure monoclinic phase of WO3 has been obtained for samples deposited above 13 Pa. Decreasing oxygen pressure below 8 Pa leads to an oxygen deficient surface with a substoichiometric phase. The film prepared at 13 Pa exhibits the best photoelectrochemical performance with a photocurrent density as high as 1.9 mA/cm2 under a xenon lamp illumination in 0.1 M Na2SO4 electrolyte. Furthermore, hydrogen evolution conducted with a three-electrode configuration and a two-electrode configuration is presented and discussed.

Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination

Advanced oxidation technologies are a friendly environmental approach for the remediation of industrial wastewaters. Here, one pot synthesis of mesoporous WO3 and WO3-graphene oxide (GO) nanocomposites has been performed through the sol–gel method. Then, platinum (Pt) nanoparticles were deposited onto the WO3 and WO3-GO nanocomposite through photochemical reduction to produce mesoporous Pt/WO3 and Pt/WO3-GO nanocomposites. X-ray diffraction (XRD) findings exhibit a formation of monoclinic and triclinic WO3 phases. Transmission Electron Microscope (TEM) images of Pt/WO3-GO nanocomposites exhibited that WO3 nanoparticles are obviously agglomerated and the particle sizes of Pt and WO3 are ~10nm and 20–50nm, respectively. The mesoporous Pt/WO3 and Pt/WO3-GO nanocomposites were assessed for photocatalytic degradation of Methylene Blue (MB) as a probe molecule under visible light illumination. The findings showed that mesoporous Pt/WO3, WO3-GO and Pt/WO3-GO nanocomposites exhibited much higher photocatalytic efficiencies than the pure WO3. The photodegradation rates by mesoporous Pt/WO3-GO nanocomposites are 3, 2 and 1.15 times greater than those by mesoporous WO3, WO3-GO, and Pt/WO3, respectively. The key factors of the enhanced photocatalytic performance of Pt/WO3-GO nanocomposites could be explained by the highly freedom electron transfer through the synergetic effect between WO3 and GO sheets, in addition to the Pt nanoparticles that act as active sites for O2 reduction, which suppresses the electron hole pair recombination in the Pt/WO3-GO nanocomposites.

Facile synthesis of tungsten trioxide 3D architectures by a simple chemical solution route and photodegradation of Rhodamine B: structural, thermal, optical and impedance studies

A facile chemical precipitation method for slab and flower like WO3 microstructure having three-dimensional (3D) architecture is reported. The as-prepared WO3 powder specimens were characterized by X-ray diffraction (XRD) combined with Rietveld refinements, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy and UV–vis spectroscopy. SEM and XRD results revealed that the hierarchical WO3 microstructures with uniform flower-like morphology are congregated by number of interleaving nanosheets which have thickness of 150–180 nm and are crystallized in a single monoclinic WO3 phase. Thermo-gravimetric analysis shows a major weight loss of 8% due to evaporation of adsorbed water. The impedance spectroscopy of compact WO3 pellets showed that all samples were semiconducting with a minute difference in electrical properties. The photo-catalytic activity of the as-prepared WO3 samples was evaluated by the degradation of Rhodamine B (RhB) under ultra-violet light irradiation. The results showed that 3D hierarchical architectures exhibits high photo-catalytic efficiency compared with the slab like structure due to their porous hierarchical structures. The optical band gap energy of slab like structure is found to be lowered compared to flower-like WO3 structures. These tunable optical and transport features allow the development of new materials for potential applications as photo-catalysts, transparent conducting electrodes, electrochromic and sensor devices.

Characterization of PLD grown WO3 thin films for gas sensing

Tungsten trioxide (WO3) thin films were grown by pulsed laser deposition (PLD) with the aim to be applied in gas sensors. The films were studied by atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and profilometry. To study the gas sensing behavior of these WO3 films, they were deposited on quartz resonators and the quartz crystal microbalance (QCM) method was applied to analyze their gas sensitivity. Synthesis of tetragonal-WO3 films starting from a target with predominantly monoclinic WO3 phase was observed. The films deposited at 300°C presented a surface topology favorable for the sorption properties, consisting of a film matrix with protruding craters/cavities. QCM prototype sensors with such films were tested for NO2 sensing. The PLD grown WO3 thin films show good sensitivity and fast reaction at room temperature, even in as-deposited state. With the presented technology, the manufacturing of QCM gas sensors is simple, fast and cost-effective, and it is also suitable for energy-effective portable equipment for on-line monitoring of environmental changes.

Influence of annealing conditions on anodic tungsten oxide layers and their photoelectrochemical activity

The nanoporous tungsten oxide films having an amorphous structure were prepared in an electrolyte containing fluoride ions via an anodization process. The as-synthesized anodic oxide layers can be easily converted to the monoclinic WO3 phase upon annealing in air. The as-synthesized and annealed WO3 layers were investigated by using X-ray diffraction, scanning electron microscopy, and photocurrent spectroscopy. The effect of annealing temperature and annealing time on the oxide morphology, crystal structure and electrochemical properties were studied. The samples were annealed in air at the temperatures ranging from 400 to 600°C, and it was found that the original porous morphology of oxide is completely lost after annealing at 600°C. The changes in the average crystallite sizes upon annealing were confirmed by XRD measurements. The photoelectrochemical performance of the annealed WO3 layers were studied under pulsed UV illumination, and the highest photocurrents were observed at the incident light wavelength of 350nm for the sample annealed at 500°C for 2h. The band gap energy and the positions of conduction and valence band edges were determined for all studied samples.

A facile and one-pot synthesis of pure and transition metals (M = Co & Ni) doped WO3 nanoparticles for enhanced photocatalytic performance

Cobalt (Co) and Nickel (Ni) doped tungsten oxide (WO3) nanoparticles were successfully synthesized by simple household microwave irradiation method without employing hydrothermal process. The samples have been characterized by various advanced techniques such as; powder X-ray diffraction (XRD), Transmission electron microscopy (TEM), UV–Vis spectra, Photoluminescence and Fourier transform infrared spectroscopy (FT-IR) analysis. The X-ray diffraction reveals that all samples are monoclinic phase of WO3 and the average crystalline size was observed to vary from 42 nm to 21 nm as the dopant content was introduced. The TEM images confirm the spherical shaped morphology and the size of the nanoparticles in nanoscale. The optical property of the pure WO3 was significantly improved by dopant content, which is confirmed by UV and PL studies. The photo catalytic activity of the nanoparticles was studied in the degradation reaction of Crystal Violet (CV) and Methylene blue (MB) under UV light irradiation. It was found that Ni doped WO3 showed superior performance of the photocatalytic activity of the pure WO3. The improved photocatalytic mechanism by Ni doping was also discussed.

Synthesis of Fe8V10W16O85 by a solution method

Phase of block-type structure and formula Fe8V10W16O85 has been synthesized by a solution method using Fe(NO3)3·9H2O, NH4VO3 and (NH4)6W12O39·xH2O as a metal precursors. Brown solid obtained after evaporation of water at 60 °C has been heated in air at temperatures selected based on results of measurements with the aid of differential thermal analysis and thermogravimetric analysis up to 1000 °C. Products obtained after selected stages of heating have been investigated with the help of X-ray diffraction, infrared spectroscopy and diffusion reflectance spectroscopy in the near-infrared range. Results of investigations have revealed several stages of thermal decomposition of precursor with the FeVO4, monoclinic WO3 and unknown phase as intermediate products. Single-phase sample of Fe8V10W16O85 has been obtained after heating stage at 800 °C. Tungsten precursor (NH4)6W12O39·xH2O has been subjected to thermoanalytical and spectroscopic study as well. Process of thermal decomposition of tungsten precursor runs with formation of hexagonal WO3 and ammonium bronze (NH4)0.33−xWO3, with monoclinic WO3 as a final product of decomposition.

Enhanced visible light photocatalytic activity of pristine and silver (Ag) doped WO3 nanostructured thin films

In this report, pristine and silver (Ag) doped tungsten trioxide (WO3) thin films were deposited on FTO-coated glass substrate by using simple chemical bath deposition method for the first time. The structural, morphological, and optical properties of pure and Ag-doped WO3 thin films have been systematically studied. The as-deposited films were annealed at 600 °C for 2 h in the ambient atmosphere in order to improve the crystallinity. The structural properties revealed that both pure and Ag-doped WO3 films exhibited the monoclinic phase of WO3. The surface morphology revealed that pure films showed the dense surface and films contained agglomerated grains which were uniformly distributed on the surface of the substrate. The optical transmittance decreased from 95 to 85 % for pure and Ag-doped WO3 films, respectively. The photocatalytic activities of the films were evaluated by degradation of methyl orange and Phenol in an aqueous solution under visible light irradiation. The photocatalytic activity of WO3 nanostructures could be extremely improved by doping with Ag. The improved photocatalytic mechanism by Ag also discussed. The films were further characterized by Fourier Transform Infrared (FTIR) spectra and photoluminescence spectra analysis.

WO3/TiO2 composite coatings: Structural, optical and photocatalytic properties

WO3/TiO2 and TiO2 coatings were prepared on titania substrates using facile and cost-effective plasma electrolytic oxidation process. The coatings were characterized by X-ray diffraction, scanning electron microscopy, Raman, UV⿿vis diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. With increasing duration of PEO process, the monoclinic WO3 phase became dominant and new monoclinic WO2.96 phase appeared. The optical absorption edge in the WO3/TiO2 samples, enriched with WO3/WO2.96 phase, was shifted to the visible region. The photocatalytic efficiency of WO3/TiO2 and pure TiO2 samples was evaluated by performing the photodegradation experiments in an aqueous solution of Rhodamine 6G and Mordant Blue 9 under the visible and UV light. The WO3/TiO2 catalysts are much more efficient than pure TiO2 under visible light and slightly better under UV light. The improvement of photocatalytic activity in the visible region is attributed to better light absorption, higher adsorption affinity and increased charge separation efficiency.

Synthesis of WO3−x nanomaterials with controlled morphology and composition for highly efficient photocatalysis

Abstract

Tungsten Oxide Film and Nanorods Grown by Aerosol-Assisted Chemical Vapor Deposition Using κ2-β-Diketonate and β-Ketoesterate Tungsten (VI) Oxo-Alkoxide Precursors

Deposition of continuous, dense WOx films and nanorods was accomplished by aerosol-assisted chemical vapor deposition (AACVD) using the recently synthesized precursors WO(OCH3)3(acac) (1), WO(OCH2C(CH3)3)3(tbac) (2), WO(OCH2C(CH3)3)3(dpm) (3), WO(OC(CH3)3)3(tbac) (4), and WO(OCH2C(CH3)3)3(tbpa) (5). This works seeks to define the deposition conditions and precursors that yield C-free tungsten oxide and the potential to control the stoichiometry and phase of deposited WOx. In addition, the systematic variation of the ligand chemistry provides insight into precursor design. Variation of the precursor and growth temperature during deposition revealed a window where C-free WOx was deposited using 3, 4, and 5. The surface morphology of the WOx varied from amorphous thin film to crystalline nanorods to dendrites as temperature was increased. Films grown between 150 and 350°C in pure N2 atmosphere are sub-stoichiometric, amorphous and contaminated with carbide species (3–9 at.%). As the deposition temperature increased (400–550°C), the tungsten became more oxidized, the sub-stoichiometric crystalline W18O49 monoclinic phase formed, and increased surface bound C was detected. Material was also deposited under oxidizing conditions (1–2% O2 in N2) as well as annealing in air at the deposition temperature. The material grown in N2/O2 carrier gas at low temperatures (200 and 300°C) is amorphous WOx, similar to that grown in N2. At higher temperature (350 and 550°C), however, GIXRD results reveal WOx corresponding to a sub-stoichiometric tetragonal phase transitioning to the monoclinic WO3 phase for samples grown at 550°C. This demonstrates that crystalline structure of WOx is affected by the growth temperature and introducing O2 in the carrier gas. Air annealing samples grown in pure N2 also produced structural and compositional changes, but not identical to those grown in a N2/O2 carrier gas. Notably, annealing samples grown at 250 and 350°C at the same temperature, produced C-free material with unchanged amorphous morphology. The measured stoichiometry and crystallinity showed a dependence on the precursor structure. The growth rate of deposited material was measured as a function of temperature and activation energies were estimated for growth of amorphous and nanostructured material. The systematic variation in activation energies is consistent with initial dissociation of the alkoxide C-O bonds and modifications of the steric bulk of the β-diketonate ligand.

Fabrication of Ag/WO3 nanobars for Baeyer–Villiger oxidation using hydrogen peroxide

We report the preparation of Ag/WO3 nanobars, mediated by cationic surfactant CTAB through hydrothermal route. XRD revealed the formation of metallic Ag supported on monoclinic WO3 phase and TEM diagram showed the formation of bar-like structure, where supported Ag nanoparticles are in the range between 2 and 7nm. The catalyst exhibited high activity for selective oxidation of cyclohexanone to caprolactone with H2O2. A cyclohexanone conversion of 97% with 99% caprolactone selectivity was achieved over this catalyst at 80°C temperature. Moreover, the catalyst did not show any significant activity loss even after 5 reuses and proved its efficiency in the oxidation of other cycloalkanones also.

Crystallization and stability of electrospun ribbon- and cylinder-shaped tungsten oxide nanofibers

Tungsten oxide (WO3) nanofibers with cylindrical (400±200nm diameter) and ribbon-like (80±10nm thick ribbons with the width-to-thickness ratio of up to 50:1) shapes were prepared by annealing electrospun polyvinylpyrrolidone/ammonium metatungstate (PVP/AMT) fibers with 1:1 PVP/AMT weight ratio. The effects of annealing temperature (573–1173K), time (1–12h), and fibers׳ shape on their crystalline structure and stability were investigated. Only a polycrystalline monoclinic phase of WO3 was observed in all experiments, and the grain size increased with temperature and time from a few nanometers to ~1μm. Results have shown that thermal stability of fibers depend on their shape and whether or not the fibers are clamped to the substrate. Loose fibers of both types were stable to at least 873K. Ribbon-like structures degraded between 873K and 973K, whereas some fraction of cylindrical fibers survived annealing at 1173K. When both types of fibers were clamped to silicon substrate, annealing above 873K led to their partial disintegration with the formation of large needle and lamellar WO3 crystals.

Structural, morphological, optical and electrical properties of spray-deposited Sb-doped WO3 nanocrystalline thin films prepared using ammonium tungstate precursor

Spray-deposited Sb-doped WO3 thin films have been investigated using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray system, and atomic force microscopy (AFM) in order to study structural and morphological properties of the films. The XRD and Raman studies confirm the monoclinic phase of WO3 with preferred orientation along the (200) plane. The SEM and AFM analyses show that with the increase of Sb-doping concentration, the surface becomes more and more compact, and the surface roughness increases. Both the direct and indirect band gap energies have been found to decrease upon Sb doping. PL emission spectra show two major peaks—one blue emission at 424 nm and another green emission at 500 nm. The electrical studies reveal that the room temperature resistivity decreases with the increasing Sb doping concentration.

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.

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.

Erratum to: Sol–gel synthesis of mesoporous WO3–TiO2 composite thin films for photochromic devices

Mesoporous WO3–TiO2 composite films were prepared by a sol gel based two stage dip coating method and subsequent annealing at 450, 500 and 600 °C. An organically modified silicate based templating strategy was adopted in order to obtain a mesoporous structure. The composite films were prepared on ITO coated glass substrates. The porosity, morphology, and microstructures of the resultant products were characterized by scanning electron microscopy, N2 adsorption–desorption measurements, μ-Raman spectroscopy and X-ray diffraction. Calcination of the films at 450, and 500 °C resulted in mixed hexagonal (h) plus monoclinic phases, and pure monoclinic (m) phase of WO3, respectively. The degree of crystallization of TiO2 present in these composite films was not evident. The composite films annealed at 600 °C, however, consist of orthorhombic (o) WO3 and anatase TiO2. It was found that the o-WO3 phase was stabilized by nanocrystalline anatase TiO2. The thus obtained mesoporous WO3–TiO2 composite films were dye sensitized and applied for the construction of photochromic devices. The device constructed using dye sensitized WO3–TiO2 composite layer heat treated at 600 °C showed an optical modulation of 51 % in the NIR region, whereas the devices based on the composite layers heat treated at 450, and 500 °C showed only a moderate optical modulation of 24.9, and 38 %, respectively. This remarkable difference in the transmittance response is attributed to nanocrystalline anatase TiO2 embedded in the orthorhombic WO3 matrix of the WO3–TiO2 composite layer annealed at 600 °C.

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.

Sol–gel synthesis of mesoporous WO3–TiO2 composite thin films for photochromic devices

Mesoporous WO3–TiO2 composite films were prepared by a sol gel based two stage dip coating method and subsequent annealing at 450, 500 and 600 °C. An organically modified silicate based templating strategy was adopted in order to obtain a mesoporous structure. The composite films were prepared on ITO coated glass substrates. The porosity, morphology, and microstructures of the resultant products were characterized by scanning electron microscopy, N2 adsorption–desorption measurements, μ-Raman spectroscopy and X-ray diffraction. Calcination of the films at 450, and 500 °C resulted in mixed hexagonal (h) plus monoclinic phases, and pure monoclinic (m) phase of WO3, respectively. The degree of crystallization of TiO2 present in these composite films was not evident. The composite films annealed at 600 °C, however, consist of orthorhombic (o) WO3 and anatase TiO2. It was found that the o-WO3 phase was stabilized by nanocrystalline anatase TiO2. The thus obtained mesoporous WO3–TiO2 composite films were dye sensitized and applied for the construction of photochromic devices. The device constructed using dye sensitized WO3–TiO2 composite layer heat treated at 600 °C showed an optical modulation of 51 % in the NIR region, whereas the devices based on the composite layers heat treated at 450, and 500 °C showed only a moderate optical modulation of 24.9, and 38 %, respectively. This remarkable difference in the transmittance response is attributed to nanocrystalline anatase TiO2 embedded in the orthorhombic WO3 matrix of the WO3–TiO2 composite layer annealed at 600 °C.