Tungsten Trioxide Thin Films Research Articles

Photoresponsive and wetting performances of sheet-like nanostructures of tungsten trioxide thin films grown on wood surfaces

Tungsten trioxide films with sheet-like nanostructures coated on wood substrates possessed photoresponsive behavior and superhydrophobic performance after OTS treatment.

Effect of solution concentration on physicochemical and gas sensing properties of sprayed WO3 thin films

Sub-stoichiometric tungsten trioxide (WO3) thin films are deposited onto the glass substrates by spray pyrolysis technique using ammonium metatungstate. Effect of solution concentration on structural, morphological, optical, electrical and NO2 sensing properties of WO3 thin films is studied. Films are polycrystalline with monoclinic crystal structure and sub-stoichiometric as observed form the XRD and XPS studies, respectively. The SEM and AFM images show micro grained structure and surface roughness increases with increase in solution concentration. The PL studies revealed that the majority of the defects are the oxygen vacancies. From XPS and PL studies it is observed that, oxygen vacancies decrease with increase in solution concentration. The dielectric constant of the films as a function of frequency is in concurrence with resistivity measurements. Films show reproducible and reversible gas response at various operating temperatures and gas concentrations. Highest sensor response (38%) towards 200 ppm NO2 concentration is observed for the film with 15 mM solution concentration at moderate operating temperature (200 °C). Pd sensitization enhanced gas response to 68% and improved kinetics of the sensor. Films are highly selective towards NO2 as compared with the various gases such as SO2, LPG, NH3 and H2S.

Nanostructured tungsten trioxide thin films synthesized for photoelectrocatalytic water oxidation: a review.

The recent developments of nanostructured WO3 thin films synthesized through the electrochemical route of electrochemical anodization and cathodic electrodeposition for the application in photoelectrochemical (PEC) water splitting are reviewed. The key fundamental reaction mechanisms of electrochemical anodization and cathodic electrodeposition methods for synthesizing nanostructured WO3 thin films are explained. In addition, the effects of metal oxide precursors, electrode substrates, applied potentials and current densities, and annealing temperatures on size, composition, and thickness of the electrochemically synthesized nanostructured WO3 thin films are elucidated in detail. Finally, a summary is given for the general evaluation practices used to calculate the energy conversion efficiency of nanostructured WO3 thin films and a recommendation is provided to standardize the presentation of research results in the field to allow for easy comparison of reported PEC efficiencies in the near future.

Electrochemical Analysis of Photoelectrochromic Device Combined Dye-Sensitized Solar Cell

In this study, the photoelectrochromic device (PECD) is combination of dye-sensitized solar cell with electrochromic device (ECD) on flexible ITO/PET substrate. First, the presintering titanium dioxide was fabricated on flexible ITO/PET substrate by a screen-printing technique. Second, the tungsten trioxide (WO 3 ) thin film was fabricated by radio frequency sputtering as electrochromic layer with different working pressures, which obtain optimal parameters in order to improve property and applied to PECD. The transmittance variation (ΔT%), optical density change (ΔOD), and coloration efficiency of optimal WO3 thin film which applied to flexible ECD and PECD were investigated. In addition, the electron transfer property of flexible PECD was analyzed by electrochemical impedance spectroscopy under dark and illumination. The Nyquist diagram indicates that the difference between dark and illumination is a diffusion phenomenon, and the charge transfer resistance between electrode and electrolyte can be observed. According to Bode plot, the phase value of flexible PECD is decreased under illumination. It indicates that the mobility of charge increases at the interface which is between titanium dioxide (TiO 2 ) layer and electrolyte. Through Nyquist diagram and Bode plot, we can assert that the performance of the WO 3 thin film which sputtered at 30 mtorr and applied to flexible ECD and PECD could obtain the optimal characteristics.

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.

Effect of Pt nanoparticles on the optical gas sensing properties of WO3 thin films.

Thin films of tungsten trioxide were deposited on quartz substrates by RF magnetron sputtering. Different annealing temperatures in the range from 423 to 973 K were used under ambient atmosphere. The influence of the annealing temperature on the structure and optical properties of the resulting WO3 thin films were studied. The surface morphology of the films is composed of grains with an average size near 70 nm for the films annealed between 773 and 973 K. Some of the WO3 thin films were also coated with Pt nanoparticles of about 45 nm in size. Spectrometric measurements of transmittance were carried out for both types of WO3 samples in the wavelength range from 200–900 nm, to determine the effect of the exposure to two different gases namely H2 and CO. Films showed fast response and recovery times, in the range of few seconds. The addition of Pt nanoparticles enables reducing the operation temperature to room temperature.

Effects of Annealing Treatment on WO<sub>3</sub> Thin Films Prepared by DC Reactive Magnetron Sputtering

Tungsten trioxide (WO3) thin films were prepared by a DC reactive magnetron sputtering technique. The thin film fabrication process used tungsten (99.995%) as the sputtering target, the mixture of argon and oxygen as sputtering and reactive gases, and silicon (100) and glass slides as the substrates. The effects of annealing temperature in the range of 200-400°C on physical and optical properties of the WO3 thin films were investigated. The nanostructures and morphologies of these films were characterized by grazing-incident X-ray diffraction (GIXRD) and field-emission scanning electron microscopy (FE-SEM). The optical properties were analyzed by variable-angle spectroscopic ellipsometry (VASE) and spectrophotometer. From the XRD results, the as-deposited and annealed WO3 thin films up to 300°C were all amorphous. Only the WO3 thin film annealed at 400°C exhibited a polycrystalline monoclinic phase. The FE-SEM cross-sections and surface topologies demonstrated nearly identical thin-film thickness and physical grain sizes. The SE analyses showed that the thin films were all homogeneous dense layers with additional surface roughness. With the annealing treatment, the thin film thickness was slightly decreased. The SE physical model was best optimized with the Cauchy optical model. The results showed that the refractive index at 550 nm was increased from 2.17 to 2.23 with the increased annealing temperature. The results from the spectrophotometer confirmed that the optical spectra for the WO3 thin films were decreased. This study demonstrated that, the thin film annealed at 400°C exhibited the slightly lower transparency, which corresponded to the results from the GIXRD and SE analyses.

Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering

The optical properties of tungsten trioxide thin films prepared by DC magnetron sputtering, with different oxygen vacancy (Vo) concentration, have been studied by spectrophotometry and photoluminescence (PL) emission spectroscopy. Absorption and PL spectra show that the films exhibit similar band gap energies, Eg ≈ 2.9 eV. The absorption spectra of the films show two pronounced absorption bands in the near-infrared region. One peak (P1) is located at approximately 0.7 eV, independent of Vo concentration. A second peak (P2) shifts from 0.96 eV to 1.16 eV with decreasing Vo concentration. Peak P1 is assigned to polaron absorption due to transitions between tungsten sites (W5+ → W6+), or an optical transition from a neutral vacancy state to the conduction band, Vo0 → W6+. The origin of peak P2 is more uncertain but may involve +1 and +2 charged vacancy sites. The PL spectra show several emission bands in the range 2.07 to 3.10 eV in the more sub-stoichiometric and 2.40 to 3.02 eV in the less sub-stoichiometric films. The low energy emission bands agree well with calculated optical transition energies of oxygen vacancy sites, with dominant contribution from neutral and singly charged vacancies in the less sub-stoichiometric films, and additional contributions from doubly charged vacancy sites in the more sub-stoichiometric films.

Effect of substrate on the electro chromic properties of tungsten trioxide thin films

Thin films of tungsten trioxide (WO3) were deposited onto glass and Indium Tin Oxide (ITO) coated glass substrates using thermal evaporation technique under various deposition conditions. In the present work the effect of substrate on the structure and EC performance of WO3 thin films has been examined using a dry lithiation method. The structural and optical changes occurring in these films with lithium insertion have been studied. It is found that the ITO coated substrate promotes the granulation and higher coloration efficiencies of the films. The peak efficiency is observed at higher infrared wavelengths where it is principally the reflectance modulation that brings about optical changes. Keywords: WO3 Thin Films, Electrochromism, Optical and Structural Properties, Effect of ITO Substrate.

Influence of annealing and processing conditions on nano-structured thin films of tungsten trioxide

Transition metal oxides represent a novel class of compounds which have attracted a considerable interest in the recent literature. Among these materials, tungsten trioxide has shown great potential due to photo-oxidation of water with visible light, high photocurrent with nano-crystals and good sensing properties towards several gases. The purpose of this study is to investigate the influence of conditions of heat treatment on properties of WO3 thin films prepared by thermal evaporation under vacuum. Physico-chemical properties of WO3 thin layers for different heat processing conditions were determined by X-ray diffraction XRD, microprobe electronics and scanning electron microscopy (SEM). Optical measurement yielded transmission and reflection measurements. The study of the physicochemical properties of thin layers of thermally post-treated tungsten trioxide showed that layers processed under vacuum have an unidentifiable structure than those annealed in air and crystallized under different crystallographic structures depending on processing temperature. Layers annealed in oxygen had monoclinic crystalline structures. It has been recorded that crystallinity and transmission of these films were drastically improved.

NO2 Sensing Properties of WO3 Thin Films Deposited by Rf-Magnetron Sputtering

Tungsten trioxide (WO3) thin films were deposited by Rf-magnetron sputtering onto Pt interdigital electrodes fabricated on corning glass substrates. NO2 gas sensing properties of the prepared WO3 thin films were investigated by incorporation of catalysts (Sn, Zn, and Pt) in the form of nanoclusters. The structural and optical properties of the deposited WO3 thin films have been studied by X-ray diffraction (XRD) and UV-Visible spectroscopy, respectively. The gas sensing characteristics of all the prepared sensor structures were studied towards 5 ppm of NO2 gas. The maximum sensing response of about 238 was observed for WO3 film having Sn catalyst at a comparatively lower operating temperature of 200°C. The possible sensing mechanism has been highlighted to support the obtained results.

Preparation and characterization of electrochromic tungsten oxide–titania composite thin films with different tungsten/titanium ratios

Tungsten oxide–titania composite thin films with different W/Ti atomic ratios were prepared and characterized in this study. Surface morphology, composition and structure of the composite thin films were investigated by scanning electron microscope with an energy dispersive spectrometer, X-ray diffraction and atomic force microscope analyses. Electrochromic properties including transmittance modulation ability, coloration efficiency and coloring/bleaching response time of these thin films were examined using cyclic voltammetry and double-potential-step experiment with in-situ ultraviolet–visible spectroscopy measurement. The presence of titania in the composite thin film could inhibit the crystal growth of tungsten oxide, which enhances the transmittance modulation ability and coloration efficiency but prolongs the coloring/bleaching response time of the composite thin film. The tungsten oxide–titania composite thin film (WTi-30) with a W/Ti atomic ratio of 78/22 exhibited better transmittance modulation ability (57.5%) and coloration efficiency (49.2cm2/C) than a pristine tungsten trioxide thin film (53.6%, 36.2cm2/C).

Electrochromic performance of the mixed V2O5–WO3 thin films synthesized by pulsed spray pyrolysis technique

Vanadium pentoxide (V2O5) mixed tungsten trioxide (WO3) thin films have been synthesized by a novel pulsed spray pyrolysis technique (PSPT) on glass and fluorine doped tin oxide (FTO) coated glass substrates at 400 °C. Aqueous solutions of equimolar vanadium chloride and ammonium tungstate were mixed in volume proportions (5%, 10% and 15%) for the deposition of V2O5–WO3 thin films. The structural, morphological, optical and electrochemical properties of V2O5–WO3 thin films were investigated by FT-IR, XRD, SEM, cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The results showed that the electrochemical properties of V2O5 were altered by mixing WO3. All the films exhibited cathodic electrochromism in lithium containing electrolyte (0.5 M LiClO4 + propylene carbonate (PC)). Maximum coloration efficiency (CE) of about 49 cm2 C−1 was observed for the V2O5 film mixed with 15% WO3. The electrochemical stability of the sample was examined and it was found to be stable up to 1000 cycles.

Development of an antenna coil type sensor device for hydrogen leakage detection

An antenna coil type hydrogen gas sensor device was fabricated and evaluated for applying to radio frequency identification (RFID) tag which can easily realize highly reliable and multipoint hydrogen leakage detection system. An antenna coil composed of platinum thin film was formed by photolithography onto a quartz-glass substrate. Then, a platinum-supported tungsten trioxide thin film was deposited on that coil by spin coating method. A frequency dependence of electrical properties (S-parameters) of the sensor device in air and in 4vol.% hydrogen balanced with nitrogen gas was investigated in the range from 300kHz to 3GHz. The considerably change in frequency dependence of S11 with the exposure to hydrogen gas was observed. The result indicated high sensitivity of this sensor device and the possibility of wireless transmission of the information about hydrogen leakage. The 90% response time was about 3s at 188MHz. The sensitivity to hydrogen in air was lower than that in nitrogen atmosphere. It could result from the competitive oxidation reaction of tungsten bronze by oxygen. However, the sensor device was able to detect hydrogen gas in concentration range below lower explosion limit. In addition, the change in S21 between the sensor device and a conventional antenna coil was also demonstrated.

Influence of annealing temperature on nanostructured thin films of tungsten trioxide

Tungsten trioxide thin films of ~300nm thickness have been deposited on indium tin oxide coated glass and silicon substrates by thermal evaporation technique. Influence of annealing temperature on the structural, vibrational, morphological, optical and gas sensing properties of these films has been extensively studied to search out the possible applications in opto-electronic and gas sensing devices. From the studies of optical transmittance spectra it is observed that optical band gap decreases from 3.24 to 2.72eV with increase in annealing temperature. It is also observed that because of annealing the photoluminescence yield of the films increases. All films, especially the annealed films have shown reasonably good gas sensing behavior in acetylene environment. The film annealed at 500°C shows better optical as well as gas sensing behaviors and hence can have good device applications.

Electrochemical Properties and Electrochromic Behaviors of the Sol–Gel Derived Tungsten Trioxide Thin Films

Electrochemical Properties and Electrochromic Behaviors of the Sol–Gel Derived Tungsten Trioxide Thin Films

Gallium-doped tungsten trioxide thin film photoelectrodes for photoelectrochemical water splitting

A sol–gel method was used to synthesise different compositions of Ga-doped tungsten trioxide thin films using tungstic acid and gallium(III) nitrate as starting materials. The precursor solutions were drop-casted on FTO glass and annealed at 500 °C for 30 min, and the resulting materials were characterised with SEM, XRD, UV/Vis spectrophotometry and photoelectrochemical analysis. The Ga-doped WO3 samples exhibited a greater grain than undoped WO3, and a monoclinic structure was observed for all WO3 samples. The band gap reduction of WO3 from 2.74 to 2.60 eV indicated the red-shift of light absorption towards the visible light region in the solar spectrum. The donor carrier density for the doped WO3 increased, and the conduction band edge position exhibited a positive shift. The photoactivity of WO3 increased threefold when the photoanode was 20% doped with gallium.

Nanostructured tungsten oxide gas sensors prepared by electric field assisted aerosol assisted chemical vapour deposition

Nanostructured thin films of tungsten trioxide were deposited on to gas sensor substrates at 600 °C from the aerosol assisted chemical vapour deposition reaction of tungsten hexaphenoxide solutions in toluene under the influence of electric fields. The electric fields were generated by applying a potential difference between the inter-digitated electrodes of the gas sensor substrates during the deposition. The deposited films were characterised using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The application of an electric field, encouraged formation of enhanced nanostructured morphologies, with an increase in needle length and reduction in needle diameter being observed. The film gas sensor properties were also examined; it was found that the highest response of 110 to 800 ppb NO2 was given by a sensor grown under the influence of a 1.8 × 104 V m−1 electric field and operated at 250 °C, a 2.5 times enhancement compared to a sensor grown in the absence of an electric field under its optimal operating conditions.

Photoelectrochemical Properties of WO3 Thin Films Prepared by Electrodeposition

Tungsten trioxide (WO3) thin films were synthesised by electrodeposition using peroxotungtic acid as the precursor electrolyte solution for use as photoanodes in a photoelectrochemical cell for solar hydrogen applications. The films were coated at deposition potentials varying from 0.30 to 0.90V versus Ag/AgCl in order to study the effect of the potential on the mineralogical, morphological, optical, and photoelectrochemical properties of the nanoparticulate films. The films were composed of monoclinic WO3, the degree of crystallinity and preferred orientation of the orthogonal planes of which increased with deposition potential and associated film thickness. In contract, increasing the deposition potential had a minimal impact on the particle sizes, which were in the range ∼80-90nm. While films deposited at the potential range of 0.30 to 0.60V showed controlled nanostructures with thicknesses in the range 168-431nm, increasing the deposition potential from 0.70 to 0.90V resulted in rapid increase in film, which led to cracking from drying stress. Linear voltammetry data suggested that the optimal potential for the deposition of stable films was in the range 0.37 to 0.60V. However, the data for the film deposited at a potential of 0.30V indicated a film of high quality. Further, the data for the film deposited at a potential of 0.60V indicated a film of a low quality and so this voltage represents a transition point for stable-unstable film growth and unstressed-stressed nanostructure from drying. The trends in the optical transmission properties showed that the photocatalytic activity of the films could be expected to decrease with increasing potential, thickness, and crystallinity. That is, the band gap and projected absorption edge exhibited a red shift. This was attributed to the relative effects of the surface and volume band gaps, in which the latter would increase relative to the former with increasing thickness. The photocurrent densities reflected the effects of increasing solid volume and decreasing band gap with increasing film thickness. The exception was the reduced performance of the film deposited at the potential of 0.60 V. This outcome was attributed to the competition between the photogeneration and recombination of electron-hole pairs, where the potential of 0.60V represents the transition point described above.

Evaporation-assisted high-power impulse magnetron sputtering: The deposition of tungsten oxide as a case study

The deposition rate during the synthesis of tungsten trioxide thin films by reactive high-power impulse magnetron sputtering (HiPIMS) of a tungsten target increases, above the dc threshold, as a result of the appropriate combination of the target voltage, the pulse duration, and the amount of oxygen in the reactive atmosphere. This behavior is likely to be caused by the evaporation of the low melting point tungsten trioxide layer covering the metallic target in such working conditions. The HiPIMS process is therefore assisted by thermal evaporation of the target material.