WO3 Lattice Research Articles

Enhanced Colouration Efficiency of Pulsed DC Magnetron Sputtered WO3 Films Cycled in H2SO4 Electrolyte Solution

In the present investigation, we report on DC power and pulsing frequency induced changes in electrochromic properties of pulsed DC magnetron sputtered WO3 films by intercalating/deintercalating H+ ions from 0.1 M H2SO4 electrolyte solution. The observed efficient colouration ↔ bleaching mechanism of WO3 films confirms the effective electrochromic nature of the films associated with the electrochemical intercalation/deintercalation of H+ ions and electrons into WO3 lattice. The higher optical modulation was observed in the visible region of the optical transmittance spectra of colored and bleached WO3 films. The maximum coloration efficiency of 79 cm2/C was observed the first time for the film deposited at a DC power of 150 W and a pulsing frequency of 25 kHz.

Tailoring the Microstructural, Optical, and Electrical Properties of Nanocrystalline WO3 Thin Films Using Al Doping

We have studied the influence of Al doping on the microstructural, optical, and electrical properties of spray-deposited WO3 thin films. XRD analyses confirm that all the films are of polycrystalline WO3 in nature, possessing monoclinic structure. EDX profiles of the Al-doped films show aluminum peaks implying incorporation of Al ions into WO3 lattice. On Al doping, the average crystallite size decreases due to increase in the density of nucleation centers at the time of film growth. The observed variation in the lattice parameter values on Al doping is attributed to the incorporation of Al ions into WO3 lattice. Enhancement in the direct optical band gap compared to the undoped film has been observed on Al doping due to decrease in the width of allowed energy states near the conduction band edge. The refractive indices of the films follow the Cauchy relation of normal dispersion. Electrical resistivity compared to the undoped film has been found to increase on Al doping.

Gold-modified N-doped TiO2 and N-doped WO3/TiO2 semiconductors as photocatalysts for UV–visible light destruction of aqueous 2,4,6-trinitrotoluene solution

The photocatalytic performance of TiO2, WO3/TiO2, Au/TiO2, and Au/WO3/TiO2 and their corresponding nitrogen-doped metal oxide samples in the photooxidation of aqueous 2,4,6-trinitrotoluene (TNT) solution was investigated. Oxidation was carried out using both visible and UV irradiation. The average particle size of the synthesized TiO2 and N-doped TiO2 photocatalysts was ≈20nm, while particles size of WO3/TiO2 and N-doped WO3/TiO2 was ≈17nm. The average size of the Au nanoparticles supported on the catalysts was ≈5nm. XPS and UV–vis diffuse reflectance spectroscopy measurements confirm the replacement of oxygen atoms by nitrogen atoms in the crystal lattice of TiO2 and WO3. Calculated rate constant values for 2,4,6-trinitrotoluene photooxidation by visible light on the different catalysts had the following order: kAu/N-(WO3/TiO2)>kAu/WO3/TiO2>kN-(WO3/TiO2)≈kAu/N-TiO2>kWO3/TiO2>kN-TiO2≫kAu/TiO2≈kTiO2. The rate constants of 2,4,6-trinitrotoluene photocatalytic degradation under visible light irradiation were lower by an order of magnitude compared to the rate constants under UV irradiation. In 2,4,6-trinitrotoluene photooxidation under UV light irradiation the rate constant kAu/N-(WO3/TiO2) was about 4 times larger than kN-doped TiO2, while for photooxidation under visible light irradiation the former was 8 times larger than the latter.

Incorporation of tantalum ions enhances the electrocatalytic activity of hexagonal WO3 nanowires for hydrogen evolution reaction

WO3 has been identified as a promising candidate electrocatalyst for hydrogen evolution reaction (HER), because it can form a tungsten bronze (HxWO3) which is highly electron and proton conducting. In this paper, we report that the electrocatalytic activity of WO3 for HER can be enhanced by incorporation of tantalum ions (Ta5+) into the lattice of WO3. The most active performance is achieved with the molar ratio of Ta/W being 0.01, which is two times more active than that of undoped WO3 at an overpotential of -0.52V. It is shown that incorporation of proper Ta5+ into WO3 can induce moderate defects and oxygen vacancies, as well as intercalate a higher amount of protons, which enhance the electron transfer and short the protons diffusion paths. These changes correlated positively with the enhanced catalytic HER activity. This study demonstrates, for the first time, that metal ions-doped WO3 nanowires are promising electrocatalysts for HER.

EDTA mediated microwave hydrothermal synthesis of WO3 hierarchical structure and its photoactivity under simulated solar light

In this paper, we report a facile microwave hydrothermal synthesis of WO3 hierarchical structure in presence of disodium salt of ethylenediaminetetraacetic acid (EDTA) as a complexing agent and its photoactivity for the degradation of rhodamine B (Rh B) under simulated solar light irradiation. The synthesized WO3 samples were structurally, morphologically and optically characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS) and Brunauer–Emmett–Teller (BET) surface area analysis. The result revealed that the addition of EDTA leads to the controlled aggregation of WO3 nanoparticles having high crystallinity with monoclinic structure and creates oxygen vacancy in the WO3 lattice. Moreover, at high concentration of EDTA, cauliflower like hierarchical structure was formed when the optimum concentration of EDTA reaches to 0.5mol% at pH 6 and exhibited best photoactivity for the Rh B degradation. These results indicate that the WO3 hierarchical structures with oxygen vacancies are potential materials for effective mineralization of Rh B, one of the hazardous pollutants.

Spray-deposited nanocrystalline WO3 thin films prepared using tungsten hexachloride dissolved in N-N dimethylformamide and influence of in doping on their structural, optical and electrical properties

Undoped and In-doped nanocrystalline tungsten oxide (WO3) thin films were prepared by chemical spray pyrolysis using tungsten hexachloride (WCl6) dissolved in N-N dimethylformamide as the host precursor solution and indium chloride (InCl3) as the source of dopant. XRD analyses confirm the monoclinic phase of the prepared films with the predominance of triplet (002), (020) and (200) in the spectra. On indium doping, the crystallinity of the films decreases and becomes minimum at 1.5 at. % doping. EDX analyses confirm the incorporation of In dopants into the WO3 lattice network. SEM micrographs show nonspherical grains over the surface and the average grain size decreases with higher In doping. AFM images of the films exhibit large nicely separated conical columnar grains (except in 1 at. %) throughout the surface with coalescence of some columnar grains at few places. UV-visible measurements reveal that the optical transmittance of the 1 at. % In-doped film increases significantly throughout the wavelength range 300–800 nm relative to that of the undoped film. Room temperature photoluminescence spectra show pronounced enhancement in the peak intensity of NBE emission on In doping. Electrical conductivity has been found to increase on In doping.

Mechanism of Hydrogen Spillover on WO3(001) and Formation of HxWO3 (x = 0.125, 0.25, 0.375, and 0.5)

Hydrogen spillover onto the WO3(001) surface and further into the bulk in the presence of a platinum catalyst was investigated by density functional theory calculations. The diffusion pathways are designed based on the energetic preference of hydrogen adsorption. Atomic H migrates from a platinum catalyst, where it is adsorbed upon H2 dissociative chemisorption, to terminal oxygen on WO3 (001) surface, followed by migration to in-plane oxygen and then diffusion across the surface or insertion in the lattice. We first theoretically corroborated the role of water in facilitating the hydrogen spillover process. The otherwise prohibitively high activation barriers were considerably reduced with the participation of water, which agrees well with experimental facts. The high mobility of H inside the WO3 lattice was also identified with low calculated diffusion barriers for several selected migration pathways. Finally, the effects of hydrogen concentration and electronic structures of HxWO3 were investigated. Th...

Gas Sensing Studies of Pulsed Laser Deposition Deposited WO<SUB>3</SUB> Nanorod Based Thin Films

WO3 nanorod based thin films were deposited via pulsed laser deposition onto quartz conductometric transducers with pre-patterned gold interdigitated transducers (IDT) employing the shortest wavelength (193 nm) ArF excimer laser. Micro-characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to study surface morphology and crystal structure. It was observed that the fabricated films showed nanocolumnar features perpendicular to the surface. The measured sizes of the nanorods were found to be approximately -50 nm in diameter. The high resolution TEM (HRTEM) image of the nanorods based WO3 showed the WO3 lattice spacing of 3.79 angstroms corresponding to the (020) plane of monoclinic WO3. Gas sensing characterizations of the developed sensors were tested towards hydrogen and ethanol at temperatures between room and 400 degrees C. The sensor exhibited high response towards H2 and ethanol at operating temperatures of 170 and 400 degrees C, respectively. The excellent sensing characteristics of WO3 films towards ethanol and H2 at low concentrations offer great potential for low cost and stable gas sensing.

Structural and alcohol response characteristics of Sn-doped WO3 nanosheets

Sn-doped WO3 materials were synthesized by chemical co-precipitation route using sodium tungstate dihydrate (Na2WO4·2H2O) as the host precursor and stannic tetrachloride hydrated (SnCl4·5H2O) as the source of Sn dopant. X-ray diffraction (XRD) analyses confirm that the prepared materials are polycrystalline WO3, possessing monoclinic structure with crystallite sizes in the range 22–44nm. The transmission electron microscopy (TEM) images of the samples show a sheet-like morphology embedded with small nanosized crystallites. The energy dispersive X-ray (EDX) analyses of the samples confirm incorporation of Sn ions into WO3 lattice network. The alcohol response characteristics of the samples were studied by measuring the change in its electrical resistance before and after exposure to test alcohol vapours at different operating temperatures (150–250°C) for various concentrations (10–50ppm) in air. On Sn doping, the response has been found to increase for all the test alcohols. It has been observed that at each operating temperature and concentration under investigation, the response to propan-2-ol is maximum, followed by those to ethanol and methanol. On Sn doping, the response and recovery times for all the test alcohols have been found to decrease relative to those observed in the case of undoped WO3 nanosheets.

Lattice polarization effects in electrochromic switching in WO3−x films studied by pulse-nanogravimetric technique

The interactions of various types of cations with the tungsten trioxide lattice have been investigated to evaluate possible intercalation of these cations and the occurrence of lattice polarization leading to the near-surface structural lattice damage. The interactions of cations, such as the large monovalent cations (K+, Et4N+, CtMe3N+ cations), transition metal dications (Ni2+), heavy metal ions (Cd2+), and representative lanthanides (La3+) and actinides (Th4+), in competition with intercalation of H+ ions have been investigated using pulse-nanogravimetric technique. The effects of these cations in electrochromic processes of WO3 proceeding during cathodic reduction have been assessed. For all of the metal ions studied, a large increase in the apparent mass uptake (up to eightfold) in comparison to pure H+ ion ingress was observed upon the film coloration induced by a cathodic potential pulse. The experiments indicate that the apparent mass gains, although large, are insufficient to confirm predominant contribution of metal ions in the ion transport along the channels in WO3 lattice. The lower decoloration rate in the case of Ni2+ ions, in comparison to H+ and other transition metal cations (Cd2+), has been attributed to a slow dissociation of Ni2+ ions from lattice-bound oxygen atoms. For et4N+ cation, which is too large to enter channels in WO3, a dissociative reduction of the WO3 and severe lattice damage was observed. Among the metal ions investigated, only K+ ions have been found to cause a dissociative reduction of WO3 and near-surface lattice damage. Strong lattice polarization effects and irreversible binding have been found for La3+ and Th4+ cations.

Photoelectron spectromicroscopy study of metal–insulator transition in NaxWO3

We have investigated the validity of percolation model, which is quite often invoked to explain the metal–insulator transition in sodium tungsten bronzes, NaxWO3 by photoelectron spectromicroscopy. The spatially resolved direct spectromicroscopic probing on both the insulating and metallic phases of high quality single crystals of NaxWO3 reveals the absence of any microscopic inhomogeneities embedded in the system within the experimental limit. Neither any metallic domains in the insulating host nor any insulating domains in the metallic host have been found to support the validity of percolation model to explain the metal–insulator transition in NaxWO3. The possible origin of insulating phase in NaxWO3 is due to the Anderson localization of all the states near EF. The localization occurs because of the strong disorder arising from random distribution of Na+ ions in the WO3 lattice.

Structural and photoelectrochemical investigation of boron-modified nanostructured tungsten trioxide films

We report a modification of nanostructured WO3 films by doping with boron. The films were obtained by a direct one-step sol–gel route involving tungstic acid/polyethelene glycol precursor. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the incorporation of boron results in the retention of a substantial amount of water and/or hydroxyl groups in the WO3 lattice and at the surface of nanoparticles occurring despite high temperature (550°C) annealing of the films. Another consequence of boron doping is the largely increased roughness factor revealed by atomic force microscopy (AFM) imaging. Both kinds of films are highly porous and consist of partly sintered particles with sizes in the range of tens of nanometers. The photoelectrochemical (PEC) studies performed under simulated solar AM 1.5 illumination showed significantly enhanced water oxidation photocurrents for B-WO3 photoanodes, by about 25% higher than those for the undoped WO3 films of similar thickness. The low extent of recombination of photogenerated charges was confirmed by incident photon-to-current conversion efficiencies (IPCEs) reaching 70% in the region of visible wavelengths at 420nm. The improved PEC properties were attributed to the increased surface hydroxylation of B-WO3 nanoparticles favoring water photo-oxidation reaction and to the larger surface area of the film exposed to the electrolyte.

A Self-Assembled Two-Layer Structured WO3/TiO2-x Mixed Film with Improved Electrochromic Capacities

Electrochromics are a promising avenue for energy savings in tropical climates where heating of buildings is largely by transmission of infrared light through windows, causing 40% of their electrical demand to be used on air-conditioning. One technique to improve the electrochromic properties of WO3 films is to dope with a second transition metal oxide. In the present work a mixed WO3/TiO2-x film was prepared by a single step hydrothermal method. XRD investigations suggested that the doping was achieved by replacement of W6+ by Ti4+ ions in the WO3 lattice structure, rather than discrete phases of WO3 and TiO2. The composite film presented a large light modulation ability of 67% at 632.8 nm, twice that of an equivalent pure WO3 film, was well adhered to the substrate and showed good electrochromic reversibility switching rates of less than 26 s. In the colored state the transmittance in the visible light range of the mixed transition metal oxide film was below 5%, whilst in the bleach state this exceeded 70%. Electrochemical impedance measurements suggested that the improvement in electrochromic properties was due to the TiO2 doping reducing both the film's electrical resistance and its resistance to diffusion of ions within the film.

Tungsten-Vanadium mixed oxides for the oxidehydration of glycerol into acrylic acid

In this paper we report on the one-pot transformation of glycerol into acrylic acid, catalyzed by W/V mixed oxides, with hexagonal tungsten bronze (HTB) structure. The reaction requires two different catalyst functions, i.e., an acid one, which is given by W oxide, and an oxidizing one, given by the V ions incorporated within the WO3 lattice. W–O bronze is very active and moderately selective in acrolein formation, but yields only traces of acrylic acid. The incorporation of increasing amounts of V inside the hexagonal tungsten bronze structure, with the development of a monophasic compound, allows the consecutive oxidation of acrolein into acrylic acid. An optimal atomic ratio between W and V equal to V/(W + V) = 0.12–0.21 made it possible to obtain an acrylic acid yield of 25% (with selectivity to residual acrolein of 11%). However, during reaction under the oxygen-containing feed, the V4+ incorporated into the hexagonal bronze structure underwent a slow oxidation into V5+, which caused a progressive decline of selectivity to acrylic acid and a concomitant increase of COx formation; the hexagonal structure however was stable during lifetime experiments. On the other hand, in the absence of oxygen a very rapid deactivation of the catalyst occurred, with a decrease in selectivity to acrolein and increase in heavy by-products.

From doping to phase transformation: Ammonia sensing performances of chloroalkoxide-derived WO3 powders modified with chromium

WO3 precursor solutions were prepared by methanolysis of WCl6 in presence of acetylacetone as a stabilizer. Chromium addition was achieved by mixing Cr 2-ethylhexanoate with the pure solutions, with Cr: W atomic concentrations ranging from 2% to 22%. Powders were prepared by drying the solutions and heat-treating the product up to 700°C. After heat-treating at 400°C, crystalline WO3 was obtained, and X-ray diffraction and Transmission Electron Microscopy showed that the powders were constituted by a mixture of the WO3 monoclinic and triclinic crystallographic phases. The Cr-modified samples, with a Cr concentration of at least 5%, presented the additional phase Cr2WO6. Structural investigations suggested that this phase was favored instead of chromium oxides due to the incorporation of Cr in the WO3 lattice in interstitial position. The sensing tests towards ammonia gas, in concentrations ranging from 50 to 500ppm, showed that, up to 5% concentration, Cr addition is beneficial in lowering the best operating temperatures and/or improving the response with respect to the pure powders. For higher Cr concentrations, the response severely decays. This result was interpreted in terms of the Cr2WO6 grains and of the influence of lowered concentration of interstitial Cr on the oxygen vacancies.

Large Cation Model of Dissociative Reduction of WO3-x Lattice Studied by EQCN and AFM

The intercalation of uncommon cations into WO3 lattice channels, including large monovalent cations (K+, Et4N+, CtMe3N+), transition metal dications (Ni2+) and heavy metal ions (Cd2+) in competition with H+ ions, have been investigated using pulse-nanogravimetric technique. For all metal ions, a large increase in the apparent mass uptake (up to 8-fold) in comparison to pure H+ ion ingress was observed upon film coloration, induced by a cathodic pulse. The experiments indicate that the apparent mass gains are insufficient for a predominant metal ion transport along the channels in WO3 lattice. The low de-coloration rate for Ni2+ ions has been attributed to slow dis-association of Ni2+ ions from lattice bound oxygen atoms. For the tetraethylammonium cation, a dissociative reduction of the WO3 and a severe lattice damage was observed. Among the metal ions, only K+ have been found to cause a dissociative reduction of WO3 and near-surface lattice damage.

Transparent and low resistive nanostructured laser ablated tungsten oxide thin films by nitrogen doping: I. Nitrogen pressure

Nitrogen incorporated tungsten oxide films are deposited on heated quartz substrates (973 K) for various nitrogen background pressures (namely, pN2 = 0.04, 0.08, 0.12, 0.17 and 0.20 mbar) by the pulsed laser deposition technique. The prepared films are characterized by using techniques such as x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), atomic force microscopy (AFM), scanning electron microscopy, micro-Raman spectroscopy, UV–visible spectroscopy and temperature dependent dc electrical resistivity studies. XRD measurements revealed the presence of tungsten sub-oxide and WO3 phases depending on the nitrogen ambient pressure. Nitrogen incorporation in the films is confirmed through EDX analysis. Morphological features consisting of densely packed, void-free grains with a distinct grain boundary are portrayed through the AFM images of the films. The WO3 lattice distortion due to nitrogen incorporation is discussed using micro-Raman spectra of the films. Band gap narrowing, moderate transmittance in the visible range and the very low resistivity exhibited by the films prepared at pN2 = 0.12 mbar are imperative as far as solar cell applications are considered. A comparably higher visible transmittance with a band gap of 3.141 eV and a low resistivity obtained for the N : WO3 films prepared at a nitrogen ambient of 0.17 mbar is among the best reported for the coexistence of transparency in the visible range and conductivity in tungsten oxide films, which make them significant for micro-electronic applications.

Results on the electrochromic and photocatalytic properties of vanadium doped tungsten oxide thin films prepared by reactive dc magnetron sputtering technique

In this investigation, vanadium doped tungsten oxide (V : WO3) thin films are prepared at room temperature by reactive dc magnetron sputtering employing a tungsten–vanadium ‘inlay’ target. In comparison with pure sputtered tungsten oxide thin films, 11% vanadium doping is observed to decrease the optical band gap, enhance the colour neutral property, decrease the coloration efficiency (from 121 to 13 cm2 C−1), increase the surface work function (4.68–4.83 eV) and significantly enhance the photocatalytic efficiency in WO3 thin films. These observations suggest that (i) vanadium creates defect levels that are responsible for optical band gap reduction, (ii) multivalent vanadium bonding with terminal oxygen in the WO3 lattice gives rise to localized covalent bonds and thus results in an increase in the work function, and (iii) a suitable work function of V : WO3 with ITO results in an enhancement of the photocatalytic activity. These results on electrochromic and photocatalytic properties of V : WO3 thin films show good promise in the low maintenance window application.

Microstructural characterization of a titanium-tungsten oxide gas sensor

Thin films of Ti–W–O were prepared from a W–Ti alloy target by rf magnetron sputtering in reactive atmosphere. Analysis devoted to investigate the microstructural properties of this material was carried out in order to explain the origin for the high sensing performance of a W–Ti-oxide gas sensor. Scanning and transmission electron microscopy techniques showed that after annealing the film consists of a polycrystalline layer, isostructural to tetragonal WO3, over which crystallites of pure WO3 are dispersed. The WO3 crystallites are insulated from each other and do not enter into the process of conduction of the layer. It was shown that Ti is soluted in the tetragonal WO3 lattice of the underlying layer. This layer exhibits fine granularity, which is an optimal feature for materials suited to gas sensing.

Microraman Spectroscopy and X-Ray Diffraction Studies of Ti-W-O Thin Films

AbstractThin films of the Ti-W-O system grown by r.f. reactive sputtering from a Ti-W (10%–90% weight) target have been studied by Raman and microraman spectroscopy, X-ray diffraction and scanning electron microscopy with the aim to investigate their microstructural and morphological properties. To this purpose, the kinetics of structural transformations at different temperatures (600 °C, and 800 °C) have been studied, and the effect of Ti on the WO3 lattice has been singled out. The results show that annealing at different temperatures induces a microstructural evolution from the amorphous phase of the as-deposited thin film to WO3 crystalline phases via an intermediate cubic disordered phase of WO3. The effect of Ti on this cubic phase and on the thin film morphology is also investigated with the aid of microraman and scanning electron microscopy analysis. The results show that two distinct phases arise upon long annealing treatments; namely, small crystallites belonging to the WO3 monoclinic phase are dispersed on a layer composed of a disordered cubic WO3 phase with a high Ti content.