Tungsten Oxide Thin Films Research Articles

Determination of the reduction depth of tungsten oxide thin films under the effect of proton irradiation

This work is devoted to experimental determination of the limiting reduction depth of tungsten oxide to metal by the measurement of volumetric variations in a thin film of WO3 under the effect of proton irradiation with an energy of 1.5 keV. The method of radiation-induced reduction of tungsten from WO3 can be used for preparation of conducting structures in a dielectric matrix and obtaining an inorganic mask for carrying out different ion-beam processes. It is shown experimentally that the effect of a proton beam with an energy of 1.5 keV provides complete reduction of a tungsten oxide layer up to 138 nm thick. The experimental ratio of the thickness of the reduced layer to the thickness of the starting oxide film was 0.31. It is shown that the limiting reduction depth of tungsten oxide is determined by the path of protons in tungsten.

Synthesis of W17O47 nanothick plates with preferred orientation and their photocatalytic activity

AbstractA simple method for synthesis of W17O47 nanothick plates by annealing sol‐gel‐deposited tungsten oxide thin films on soda lime substrate has been reported. After heat treatment of the dried thin films at 700 °C in N2 ambient for 60 min, W17O47 nanothick plates with [100] orientation were obtained. The synthesized product was characterized and analyzed by, X‐ray diffraction (XRD), SEM, XPS and ultraviolet‐visible spectrophotometery. According to SEM observations, nanothick plates grew with random orientations on the surface. In addition, it was observed that some of the nanothick plates were constituted from several nanosheets. XRD analysis determined that the nanothick plates were mainly composed of monoclinic W17O47 phase with strong (500) diffraction peak and about 50 nm average crystalline size. Basing on XPS analysis, the surface composition of the nanothick plates was determined WO2.78. It was found that the 60 min annealed sample has the maximum number of nanothick plates per unit area over the surface as compared to the samples annealed for 15, 45 and 80 min. In addition, the photocatalytic degradation of methylene blue over this sample exhibited the highest photoactivity rate. A mechanism for photocatalytic activity of the W17O47 nanothick plates was also proposed. Copyright © 2011 John Wiley & Sons, Ltd.

Porous orthorhombic tungsten oxide thin films: synthesis, characterization, and application in electrochromic and photochromic devices

Porous orthorhombic tungsten oxide (o-WO3) thin films, stabilized by nanocrystalline anatase TiO2, are obtained by a sol–gel based two stage dip coating method and subsequent annealing at 600 °C. An Organically Modified Silicate (ORMOSIL) based templating strategy is adopted to achieve porosity. An asymmetric electrochromic device is constructed based on this porous o-WO3 layer. The intercalation/deintercalation of lithium ions into/from the o-WO3 layer of the device as a function of applied coloration/bleaching voltages have been studied. XRD measurements show systematic changes in the lattice parameters associated with structural phase transitions from o-WO3 to tetragonal LixWO3 (t-LixWO3) and a tendency to form cubic LixWO3 (c-LixWO3). These phase transitions, induced by the Li ions, are reversible, and the specific phase obtained depends on the quantity of intercalated/deintercalated Li. Raman spectroscopy data show the formation of t-LixWO3 for an applied potential of 1.0 V and the tendency of the system to transform to c-LixWO3 for higher coloration potentials. Optical measurements show excellent contrasts between colored and bleached states. An alternate photochromic device was constructed by sensitizing the o-WO3 layer with a ruthenium based dye. The nanocrystalline anatase TiO2 in the o-WO3 layer has led to an enhanced photochromic optical transmittance contrast of ∼51% in the near IR region. The combination of the photochromic and electrochromic properties of the synthesised o-WO3 layer stabilized by nanocrystalline anatase TiO2 opens up new vista for its application in energysaving smart windows.

Electrochromic performance of WO3 thin films with solvent-free viscous electrolytes based on polyethylene glycol-titanium oxide nanocomposites

ABSTRACTPolymeric nanocomposites of polyethylene glycol (PEG) with titanium oxide compound, PEG-Ti, have been prepared by sol-gel method from liquid PEG and titanium isopropoxide in acidic medium. Lithium salt (LiX) has been added into PEG-Ti to form PEG-Ti-LiX polymeric electrolytes. Electrochromic devices based on tungsten oxide thin films and PEG-Ti-LiX electrolyte may show excellent optical transmittance transient, however it depends on the type of lithium salt used during the sol-gel process. With LiI, the color change speed of the devices is very fast but they show a yellow color at bleaching state. The use of LiClO4 makes the devices totally transparent in visible region but the optical contrast is small. Possible molecular structure model of these polymeric electrolytes have been analyzed to explain the relation between electrochromic performance of tungsten oxide and electrolyte chemical composition.

Optical and Electrochromic Properties of Gold or Palladium Doped Tungsten-Oxide Thin Film Prepared with RF Reactive Sputtering

Gold- and palladium-doped tungsten oxide thin films were prepared by RF reactive sputtering in a mixture of argon and oxygen at room temperature and optical properties and electrochromic characteristics of the films were investigated. XRD patterns indicated that the films were amorphous. SEM imaging indicated that the grain size of gold- and palladium- doped thin films was smaller than that of the non- doped tungsten oxide thin films. By gold and palladium doping, the current- voltage characteristics show a significant change, even in the a bsorption of the film wearing the color reaction consumption, resulted in major changes.

Manipulation of Work Function and Surface Free Energy of Tungsten Oxide Hole Injection Layer Modified with a Self-Assembled Monolayer

As a hole injection layer for organic devices, a tungsten oxide (WOx) thin film was vapor-deposited on an indium–tin oxide (ITO) substrate, on which a self-assembled monolayer (SAM) of either 3-aminopropyltrimethoxysilane (APS), phenyltrimethoxysilane (PTMS), or octadecyltrimethoxysilane (ODS) was prepared to modify the surface characteristics. The deposition of WOx substantially increased the ionization potential (Ip) of the substrate surface, which was effective in enhancing hole injection. The formation of SAM on WOx reduced Ip, but enabled the control of the surface free energy so as to modify the growth morphology of an organic film deposited on its surface. A hole-only device was prepared using a hole transport material of N,N '-diphenyl-N,N '-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD). In the space-charge-limited region, a high current was drawn by using an anode that has a high Ip. At low driving voltages, however, the current flow was considerably influenced by the surface free energy. It was found that the PTMS-SAM on WOx gives a satisfactory accommodation of both the work function and the surface energy.

Electrochromic properties of N-doped tungsten oxide thin films prepared by reactive DC-pulsed sputtering

In depositing nitrogen doped tungsten oxide thin films by using reactive DC-pulsed magnetron sputtering process, nitrous oxide gas (N 2O) was employed instead of nitrogen (N 2) as the nitrogen dopant source. The nitrogen doping effect on the structural and electrochromic properties of WO 3 thin films was investigated. X-ray diffraction (XRD) results show that the films are amorphous. Morphological images reveal that the films are characterized by a hybrid structure comprising nanoparticles embedded in amorphous matrix and open channels between the agglomerated nanoparticles, which promotes rapid charge transport through the film. Increasing the nitrogen doping concentration is found to decrease the nanoparticle size and the band gap energy. The electrochromic properties were studied using cyclic voltammetric and spectroeletrochemical techniques. The film with N content of ~ 5 at.% exhibits higher optical modulation and coloration efficiency as well as faster ion transport kinetics. The results reveal that electrochromic and lithium ion transport properties are moderately enhanced relative to the un-doped tungsten oxide thin films by appropriate content of dopant, due to the effects of nitrogen doping.

Spectroscopic analysis of tungsten oxide thin films

We present a detailed study of the morphology and composition of tungsten oxide (WO3) thin films, grown by radio frequency magnetron reactive sputtering at substrate temperatures varied from room temperature (RT) to 500 °C, using infrared (IR) absorption, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). This work includes valuable new far-IR results about structural changes in microcrystalline WO3. Both IR absorption and Raman techniques reveal an amorphous sample grown at RT and initial crystallization into monoclinic structures for samples grown at temperatures between 100 and 300 °C. The Raman spectra of the samples grown at high temperatures indicate, apart from the monoclinic structure, a strain effect, with a distribution revealed by confocal Raman mapping. XPS indicates that the film surface maintains the stoichiometry WOx, with a value of x slightly greater than 3 at RT due to oxygen contamination, which decreases with increasing temperature.

The effect of Ar plasma bombardment upon physical property of tungsten oxide thin film in inverted top-emitting organic light-emitting diodes

This study examined the mechanism of performance degradation induced by the plasma sputtering process for the formation of the ITO top anode in inverted top-emitting organic light-emitting diodes (ITOLEDs) with WO 3 thin film as the hole injection layer (HIL) and the buffer layer. The degradation is mainly caused by the changes in the physical properties of the WO 3 thin film as well as the damages of organic layers, which are caused by the bombardment of energetic particles induced by the plasma sputtering process. The number of oxygen vacancies in the WO 3 thin film is increased with the increase of the bombardment energy of energetic particles such as Ar ions, resulting in the decrease in the work function and optical band gap and the dramatic increase in conductivity. Above all, the increase in the energy potential difference between WO 3 HIL and NPB HTL, which was attributed to the reduced work function of the WO 3 HIL, reduces the efficiency of carrier injection and dominantly affects the device performance. These results show that the plasma damage effect, induced by the sputtering process for ITO deposition, could not be effectively removed, even when the metal oxide thin film is used as the buffer layer in ITOLEDs.

Electron Beam Evaporation of Tungsten Oxide Films for Gas Sensors

Pure and iron incorporated nanostructured Tungsten Oxide (WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) thin films were investigated for gas sensing applications using noise spectroscopy. The WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sensor was able to detect lower concentrations (1-10 ppm) of NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , CO, CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , and Acetaldehyde gases at operating temperatures between 100°C to 250°C. The iron-doped Tungsten Oxide sensor ( WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Fe) showed some response to Acetaldehyde gas at relatively higher operating temperature (250°C) and gas concentration of 10 ppm. The sensitivity of the WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sensor towards NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and Acetaldehyde at lower operating temperatures (50°C-100°C) was significant when the sensor was photo-activated using blue light-emitting diode (Blue-LED). From the results, photo-activated WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin film that operates at room temperature appeared to be a promising gas sensor. The overall results indicated that the WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sensor exhibited reproducibility for the detection of various gasses and the WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> :Fe indicated some response towards Acetaldehyde gas.

Synthesis and evaluation of bis-β-diketonate dioxotungsten(VI) complexes as precursors for the photodeposition of WO 3 films

In this paper non-stoichiometric tungsten oxide thin films have been successfully prepared by direct UV irradiation of bis-β-diketonate dioxotungsten(VI) precursor complexes spin-coated Si(1 0 0) substrates. Photodeposited films were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the surface morphology examined by Atomic Force Microscopy (AFM). The results of XRD analysis showed that the as-photodeposited WO 3− x films are amorphous and have a rougher surface than thermally treated films. Post-annealing of the films in air at 500 °C transforms the sub-oxides to a monoclinic WO 3 phase.

Amorphous copper tungsten oxide with tunable band gaps

We report on the synthesis of amorphous copper tungsten oxide thin films with tunable band gaps. The thin films are synthesized by the magnetron cosputtering method. We find that due to the amorphous nature, the Cu-to-W ratio in the films can be varied without the limit of the solubility (or phase separation) under appropriate conditions. As a result, the band gap and conductivity type of the films can be tuned by controlling the film composition. Unfortunately, the amorphous copper tungsten oxides are not stable in aqueous solution and are not suitable for the application of photoelectrochemical splitting of water. Nonetheless, it provides an alternative approach to search for transition metal oxides with tunable band gaps.

Optical Emission Spectroscopy of Low-Discharge-Power Magnetron Sputtering Plasmas Using Pure Tungsten Target

To study the mechanism of a tungsten oxide (WO3) thin film using an RF magnetron sputtering method, optical emission spectroscopic (OES) measurements for the RF plasma of a pure W target have been performed. We also examined the crystalline structure and atomic composition rate of the prepared thin film using X-ray photoelectron spectroscopy (XPS). Experimental results indicate that Ar I emission peak intensity slightly increased with increasing Ar gas mixture. On the other hand, W I emission peak intensity rapidly increased with increasing Ar gas mixture. The increase rates of these two emission peak intensities are different, which may be due to the difference in emission mechanism. O I emission peak intensity decreased with increasing Ar gas mixture, indicating that O I emission intensity increased with increasing O2 gas mixture. Electron density and deposition rate increased with increasing Ar gas mixture, and their dependences on Ar gas mixture were very similar to that of Ar I emission. XPS analyses indicate that the oxidation ratio of the prepared film was slightly decreased with decreasing Ar gas mixture. These results suggest that the W sources of the WO3 film on the substrate are W atoms and WOx molecules sputtered from the W target. The plasma phase reaction between W and O atoms and the intermediate-energy O atomic and/or O2 molecular reaction on the surface of the substrate are considered to be important for WO3 film production in low-energy magnetron sputtering deposition.

Synthesis and characterization of chemically grown electrochromic tungsten oxide

Tungsten oxide thin films, which are cathodic coloration materials that are used in electrochromic devices, were prepared by a chemical growth method and their electrochromic properties were investigated. The thin films of WO3 were deposited onto electrically conducting substrates: fluorine doped tin oxide coated glass (FTO) with sheet resistance of about 10 Ω/cm. Transparent, uniform and strongly adherent thin film samples of WO3 were studied for their structural, morphological, optical and electrochromic properties. The XRD data confirmed the monoclinic crystal structure of WO3 thin films. The direct band gap Eg for the films was found to be 2.95 eV which is good for electrochromic device application. The electrochromism of WO3 thin film was evaluated in 0.5 M LiClO4/propylene carbonate for Li+ intercalation. Electrochromic properties of WO3 thin films were studied with the help of Cyclic Voltammetry (CV), Chronoamperometry (CA) and Chronocoulometry (CC) techniques.

Flower-Shaped Tungsten Oxide with Inorganic Fullerene-like Structure: Synthesis and Characterization

Relatively thick (1.2 μm), novel, flower-like nanostructured tungsten oxide thin films are obtained by electrochemically anodizing tungsten foil in a fluoride containing acidified electrolyte solution. X-ray diffraction analysis reveals the presence of monoclinic hydrated tungstite (WO3·2H2O) in the as-prepared samples, while films annealed at 400 °C for 4 h contain predominantly orthorhombic WO3 phase. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and UV−vis spectroscopy are utilized to determine the surface morphology, crystal structure, and optical properties of these WO3 films. An inorganic fullerene-like WO3·2H2O structure is observed, with the water molecules acting as the coordination solvent and allowing crystallographically specific growth of crystallized WO3·2H2O through oriented attachment. We propose here that the formation of the flower-like structured hydrated tungstite film occurs through an anodization/precipitation−recrystallization...

Tungsten oxide (WO3) thin films for application in advanced energy systems

Inherent processes in coal gasification plants produce hazardous hydrogen sulfide (H2S), which must be continuously and efficiently detected and removed before the fuel is used for power generation. An attempt has been made in this work to fabricate tungsten oxide (WO3) thin films by radio-frequency reactive magnetron-sputter deposition. The impetus being the use of WO3 films for H2S sensors in coal gasification plants. The effect of growth temperature, which is varied in the range of 30–500 °C, on the growth and microstructure of WO3 thin films is investigated. Characterizations made using scanning electron microscopy (SEM) and x-ray diffraction (XRD) indicate that the effect of temperature is significant on the microstructure of WO3 films. XRD and SEM results indicate that the WO3 films grown at room temperature are amorphous, whereas films grown at higher temperatures are nanocrystalline. The average grain-size increases with increasing temperature. WO3 films exhibit smooth morphology at growth temperatures ≤300 °C while relatively rough at &amp;gt;300 °C. The analyses indicate that the nanocrystalline WO3 films grown at 100–300 °C could be the potential candidates for H2S sensor development for application in coal gasification systems.

Synthesis of tungsten oxide thin film by liquid phase deposition

High purity and well crystallized tungsten acid hydrates (H 2WO 4·H 2O) thin films were prepared from H 2WO 4–HF(aq.) and H 3BO 3 as precursors by the liquid phase deposition method. The crystal structure was indexed as monoclinic with unit cell lattice constants a = 7.517 Å, b = 6.907 Å, c = 3.694 Å and β = 89.58°. The monoclinic phase was transformed into orthorhombic WO 3·H 2O after heating at 100 °C. Further heating from 300 to 500 °C resulted in an anhydrous monoclinic WO 3 films. The effects of the composition and the reaction time on the deposition and the microstructures of the deposited films were studied by the means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The film formation showed strong dependence upon the composition, whereas the amount of deposition, the shape and the films thickness could be controlled by the reaction time. Cross-sectional TEM image of WO 3 film deposited on Au wire indicated that the epitaxial growth of the film was maintained after calcination at 500 °C. XPS analysis also revealed the existence of W 6+ ions in both the deposited and calcined films.

Structural transformation induced changes in the optical properties of nanocrystalline tungsten oxide thin films

Nanocrystalline tungsten oxide (WO3) films were grown by reactive magnetron sputter-deposition. The structure and optical properties of WO3 films were evaluated using grazing incidence x-ray diffraction and optical spectroscopic measurements. The effect of ultramicrostructure was significant on the optical characteristics of WO3 films. The band gap decreases from 3.25 to 2.92 eV with increasing grain-size from ∼9 to 50 nm while the films exhibit a transition from monoclinic to tetragonal phase. A direct microstructure-property relationship found suggests that tuning properties of WO3 films for desired applications can be achieved by tuning the conditions and controlling the phase.

Preparation and characterizations of tungsten oxide electrochromic nanomaterials

Nanoscaled tungsten oxide thin films were fabricated by galvanostatic electrodeposition. The effect of preparation parameters such as tungsten ions concentration, pH, current density and annealing on the properties and performance of WO3 thin films electrochromic materials was investigated. XRD, SEM–EDS, TEM, FTIR, UV–VIS spectrophotometry, and electrochemical measurements were utilized to characterize the structural and compositional properties as well as the electrochromic behaviour of the prepared thin films. Triclinic WO3 structure was prepared at 0.1 M W+ and current density of 0.5 mA cm−2, while at 0.2 M W+ and 1 mA cm−2, orthorhombic structure was revealed. High energy gap of 3.5 eV with diffusion coefficient of 6.81 × 10−11 cm2 S−1 and coloration efficiency of 62.68 cm2 C−1 were obtained for the films prepared at pH 2, 1 mA cm−2, and 0.1 M W+.

Effects of film density on electrochromic tungsten oxide thin films deposited by reactive dc-pulsed magnetron sputtering

The correlation between the electrochromic performance and the film density of the tungsten oxide thin films sputtered by reactive dc-pulsed magnetron sputtering with varying working pressure was investigated. It is found that the optical modulation and coloration efficiency of films are strongly affected by the amount of tungsten oxide present in the film. The coloration kinetic is also found to be sensitive to the film density, with the maximum value of 3.48 × 10 −10 cm 2/s for the porous film deposited at higher pressure. X-ray diffraction and TEM showed that the films were amorphous. By using spectroscopic ellipsometer to anaylsis the surface roughness and refractive index of the sputtered films, we found that the film at higher working pressure was rough and lower density, which offered fast electrochromic response.