Tungsten Trioxide Thin Films Research Articles

Facile fabrication of WO 3 crystalline nanoplate on FTO glass and their application in electrochromism

Tungsten trioxide (WO3) thin films are of great interest as counter electrodes in electrochromic (EC) devices such as ‘smart window’ for energy-efficient buildings. Uniform WO3 nanoplates filmed as EC working electrodes were fabricated on seed-free fluorine-tin-oxide (FTO) coated glass via a facile and addictive agent-free hydrothermal process. The WO3 nanoplates were characterised by scanning electron microscopy and the X-ray photoelectron spectroscopy. The morphological analysis of the film showed that the WO3 nanoplates lying on the FTO glass uniformly. Fourier transform infrared spectroscopy was applied to study the vibrational information of the sample. Furthermore, uniform WO3 nanoplates exhibit well performance of EC properties. Owing to the highly two-dimensional nanostructure, a fast switching speed of 12 and 3 s for colouration and bleaching are achieved for WO3 film. These properties of the WO3 nanoplates film endow its promising practical applications in smart windows.

Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy and optical spectrophotometry

Nanocrystalline tungsten trioxide (WO3) thin films prepared by DC magnetron sputtering have been studied using soft x-ray spectroscopy and optical spectrophotometry. Resonant inelastic x-ray scattering (RIXS) measurements reveal band gap states in sub-stoichiometric γ-WO3−x with x = 0.001–0.005. The energy positions of these states are in good agreement with recently reported density functional calculations. The results were compared with optical absorption measurements in the near infrared spectral region. An optical absorption peak at 0.74 eV is assigned to intervalence transfer of polarons between W sites. A less prominent peak at energies between 0.96 and 1.16 eV is assigned to electron excitation of oxygen vacancies. The latter results are supported by RIXS measurements, where an energy loss in this energy range was observed, and this suggests that electron transfer processes involving transitions from oxygen vacancy states can be observed in RIXS. Our results have implications for the interpretation of optical properties of WO3, and the optical transitions close to the band gap, which are important in photocatalytic and photoelectrochemical applications.

Evaluation of microstructural and electrical properties of WO3-x thin films for p-Si/n-WO3-x/Ag junction diodes

The ln(J)-V-T characteristics of the p-Si/n-WO3-x/Ag junction diode have been analyzed by thermionic emission (TE) mechanism with Gaussian distribution (GD) of the barrier heights. The ultra-fine homologous phases of tungsten oxide (WO3-x) thin films on the glass substrate can be prepared simply via sol-gel spin coating method. The oxygen reduction of the tungsten trioxide (WO3) thin films was experimentally controlled by various organic acid additives. The organic acid-treated films have hexagonal and monoclinic crystallographic Magneli phases of WnO3n-2 series (WO2.92, WO2.9 and WO2.89). The morphological changes in the plate-like structure under the strong influence of the organic acids were observed from SEM analysis. In the temperature dependent dc electrical conductivity, the charge transport mechanism of the WO3-x thin films was analyzed by Arrhenius, Mott’s variable hopping, small polaron mechanisms. From the J-V-T characteristics of the p-Si/n-WO3-x/Ag diode, the increasing of barrier height (ΦB) and decreasing of ideality factor (n) reveal that barrier inhomogeneities at the interface, which is assumed by Gaussian distribution. The mean barrier height (ΦB), the standard deviation (σ02) and Richardson constant (A*) values were investigated in the temperature range 303−423K. The better diode performance was acquired to the Si/WO2.89/Ag device with the experimentally intended A* value of 34.81A/cm2/K2 which is near to the well-known value of 32A/cm2/K2 for p-type Si.

Impact of annealing treatment on structural and dc electrical properties of spin coated tungsten trioxide thin films for Si/WO3/Ag junction diode

We report on the effect of the thermal annealing on structural and electrical properties of p-Si/n-WO3/Ag junction diode. According to the XRD pattern, the WO3 films exposed that the crystalline phase transformation of monoclinic to orthorhombic structure for an increasing annealing temperature. The SEM images show an abrupt change in the plate like grain growth and surface morphology. From the UV–visible analysis, the band gap energy decreases for the higher annealing temperature. The dc electrical characterization shows that the conductivity (σdc), activation energy (Ea) and pre-exponential factor (σ0) values vary function of temperature. The Si/WO3/Ag contact junction diode parameters of ideality factor (n), barrier height (ΦB), leakage current density (J0) and series resistance (Rs) were examined by the J-V method, Cheung's and Norde functions as a function of annealing temperature according to the thermionic emission method (TE). The values of n and ΦB decrease with increasing annealing temperature and better the device performance on an optimized annealing temperature at 873K. The temperature dependent of experimental n and ΦB revealed the presence of inhomogeneity at WO3-Ag interface. This behavior is modeled by assuming the existence of Gaussian distribution (GD) of barrier heights in temperature range 303–423K.

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.

The Structural, Optical and Electrical Properties of Spin Coated WO 3 Thin Films Using Organic Acids

Tungsten trioxide (WO3) thin films were prepared incorporating various organic acid additives by the sol-gel spin coating technique. They were characterized by X-ray diffraction (XRD), UV-Visible analysis, scanning electron microscopy (SEM) and dc electrical conductivity. From XRD, the crystal phase, average grain size and structural parameters of WO3 thin films were found to vary owing to different water dissolved organic acid additives. The variation of optical conductivity and band gap energy was calculated from the UV-Visible analysis. The SEM studies revealed that the organic acids influenced the surface morphology of the microsized plates of tungsten oxides. The electrical conductivity at various temperatures correlated with the average grain size of the nanocrystallites of WO3 thin films.

Direct laser deposition of nanostructured tungsten oxide for sensing applications

Nanostructured tungsten trioxide (WO3) thin films are deposited by pulsed laser deposition (PLD) and radio-frequency (RF) assisted PLD onto interdigitated sensor structures. Structural characterization by x-ray diffraction and Raman spectroscopy shows the WO3 films are polycrystalline, with a pure monoclinic phase for the PLD grown films. The as-fabricated WO3 sensors are tested for ammonia (NH3) detection, by measuring the electrical response to NH3 at different temperatures. Sensors based on WO3 deposited by RF-PLD do not show any response to NH3. In contrast, sensors fabricated by PLD operating at 100 °C and 200 °C show a slow recovery time whilst at 300 °C, these sensors are highly sensitive in the low ppm range with a recovery time in the range of a few seconds. The microstructure of the films is suggested to explain their excellent electrical response. Columnar WO3 thin films are obtained by both deposition methods. However, the WO3 films grown by PLD are porous, (which may allow NH3 molecules to diffuse through the film) whereas RF-PLD films are dense. Our results highlight that WO3 thin films deposited by PLD can be applied for the fabrication of gas sensors with a performance level required for industrial applications.

Preparation and Study the Characteristics of Tungsten Trioxide Thin Films for Gas Sensing Application

In this study, Tungsten Trioxide thin films were successfully synthesized at different substrate temperatures by pulse laser deposition. Structural, morphological and electrical properties of WO3 thin films, were investigated by X-ray diffraction (XRD), Atomic Force Microscope AFM), Scanning Electron Microscope (SEM), Hall Effect and sensing measurements. The results was indicated that WO3 thin films prepared at 450ᵒC was optimum condition where sensitivity toward H2S gas has been measured, sensitivity was higher than other films preparation at (250ᵒC,350ᵒC) temperature.

Toward Plastic Smart Windows: Optimization of Indium Tin Oxide Electrodes for the Synthesis of Electrochromic Devices on Polycarbonate Substrates.

Plastic smart windows are becoming one of the key elements in view of the fabrication of inexpensive, lightweight electrochromic (EC) devices to be integrated in the new generation of high-energy-efficiency buildings and automotive applications. However, fabricating electrochromic devices on polymer substrates requires a reduction of process temperature, so in this work we focus on the development of a completely room-temperature deposition process aimed at the preparation of ITO-coated polycarbonate (PC) structures acting as transparent and conductive plastic supports. Without providing any substrate heating or surface activation pretreatments of the polymer, different deposition conditions are used for growing indium tin oxide (ITO) thin films by the radiofrequency magnetron sputtering technique. According to the characterization results, the set of optimal deposition parameters is selected to deposit ITO electrodes having high optical transmittance in the visible range (∼90%) together with low sheet resistance (∼8 ohm/sq). The as-prepared ITO/PC structures are then successfully tested as conductive supports for the fabrication of plastic smart windows. To this purpose, tungsten trioxide thin films are deposited by the reactive sputtering technique on the ITO/PC structures, and the resulting single electrode EC devices are characterized by chronoamperometric experiments and cyclic voltammetry. The fast switching response between colored and bleached states, together with the stability and reversibility of their electrochromic behavior after several cycling tests, are considered to be representative of the high quality of the EC film but especially of the ITO electrode. Indeed, even if no adhesion promoters, additional surface activation pretreatments, or substrate heating were used to promote the mechanical adhesion among the electrode and the PC surface, the observed EC response confirmed that the developed materials can be successfully employed for the fabrication of lightweight and inexpensive plastic EC devices.

Pyrolysis preparation of WO3 thin films using ammonium metatungstate DMF/water solution for efficient compact layers in planar perovskite solar cells**Project supported by the National Natural Science Foundation of China (Nos. 51472071, 512720616, 51072043), and the National Basic Research Program of China (No. 2011CBA00700).

The tungsten trioxide (WO3) thin films were firstly prepared by spin-coating-pyrolysis methods using the ammonium metatungstate ((NH4)6H2W12O40) DMF/water solution, and successfully applied as the efficient compact layers for the planar perovskite solar cells. The influence of the WO3 film thickness and the rinsing treatment of CH3NH3PbI3 thin film with isopropanol on the photovoltaic performance of the corresponding perovskite solar cells was systematically investigated. The results revealed that the perovskite solar cell with a 62 nm thick WO3 compact layer achieved a photoelectric conversion efficiency of 5.72%, with a short circuit photocurrent density of 17.39 mA/cm2, an open circuit voltage of 0.58 V and a fill factor of 0.57. The photoelectric conversion efficiency was improved from 5.72% to 7.04% by the isopropanol rinsing treatment.

Effects of electrodeposition synthesis parameters on the photoactivity of nanostructured tungsten trioxide thin films: Optimisation study using response surface methodology

The main aim of this study was to synthesize and characterise nanostructured tungsten trioxide (WO3) thin films via electrodeposition and subsequently, optimise the electrodeposition synthesis parameters using response surface methodology (RSM). Statistical Box–Behnken RSM design was used to investigate and optimise the effects of four independent electrodeposition synthesis parameters, namely: deposition time, precursor tungsten (W) concentration, annealing temperature and pH. In addition, the synergistic interaction between different electrodeposition synthesis parameters was identified and quantified in enabling a higher photoactivity achievable by nanostructured WO3 thin films. Resultant nanostructured WO3 thin films were characterised using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and photocurrent density measurements under one-Sun irradiation. From the electrodeposition synthesis process, it was found that there was a gradual increase in the nanocrystallites WO3 size from 30 nm to 70 nm when the annealing temperature was varied between 400 °C and 600 °C. XRD results verified the existence of the same photoactive phase of monoclinic WO3 with increasing annealing temperature with the preferred growth orientation along the {002} planar. Whilst from the Box–Behnken RSM design, it was found that the optimum deposition time, precursor W concentration, annealing temperature and pH were: 60 min, 0.15 mol/L, 600 ℃, and pH 1.0, respectively. The highest photocurrent density of 120 µA/cm2 was measured at 1 V (versus Ag/AgCl) for nanostructured WO3 thin film synthesized at the optimum conditions as informed by the Box–Behnken RSM. Further analysis and validation of the Box–Behnken RSM model using analysis of variance (ANOVA) revealed that the RSM-derived statistical predictive model was robust, adequate and representative to correlate the various electrodeposition synthesis parameters to photocurrent density. This study highlights the importance to systematically optimise the electrodeposition synthesis parameters in order to achieve a higher photocurrent density on nanostructured WO3 thin film for sustainable hydrogen production from photoelectrochemical water splitting reaction.

Electrodeposition-Based Fabrication and Characteristics of Tungsten Trioxide Thin Film

In this study, tungsten trioxide (WO3) thin films were electrodeposited on indium tin oxide (ITO) glass to form WO3-coated glass. The electrodeposition (ED) time (tED) and ED current (IED) were varied to control the film thickness and morphology. Furthermore, the crystallization of the thin films was controlled by annealing them at 250°C, 500°C, and 700°C. The results showed that the thickness of the WO3thin films increased withtEDandIED. The as-deposited thin films and those annealed at 250°C were amorphous, whereas the WO3thin films annealed at 500 and 700°C were in the anorthic phase. Moreover, the amorphous WO3-coated glass exhibited high transmittance in visible light and low transmittance in near-infrared light, whereas the anorthic WO3-coated glass had high transmittance in near-infrared light. An empirical formula for determining the thickness of WO3thin films was derived through multiple regressions of the ED process parameters.

Temperature dependence of the electrical conductivity of dry lithiated Tungsten trioxide thin films

A great deal of basic and applied research has been dedicated in the past couple of decades to the transition metal oxide (TMO) thin films such as those of tungsten trioxide (WO3). This is due to the interesting reversible optical and electrical switching that occurs in these materials under various external influences. These switching properties are interesting both from the physics point of view as well as for their application. In particular, the tungsten trioxide (WO3) thin films exhibiting electrochromic properties have been studied extensively. While their optical switching properties are very well studied, not much study is focussed on their electrical properties, especially as a function of temperature. In this work we have studied the temperature coefficient of resistance (TCR) behavior of lithium atom intercalated WO3 thin films. Using the variable range hopping model we have calculated the density of states at the Fermi level of samples prepared by thermal evaporation and lithiated by a dry process. The TCR measurements were performed in the temperature range between 298 and 353K. The understanding of this temperature dependent electrical behavior is expected to enhance our knowledge of the electrochromic process in these films and lead to better design WO3 based electrochromic (EC) devices.

Synthesis and Characterization of Tungsten Trioxide Films Preparedby a Sol-Gel Method for Electrochromic Applications

Abstract Tungsten trioxide thin films are successfully synthesized by a sol-gel method using tungsten hexachlorideas precursors. The structural, chemical, and optical properties of the prepared films are characterized by scanning elec-tron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV-Vis spectrophotometry. The electrochem-ical and electrochromic properties of the films before and after heat treatment are also investigated by cyclicvoltammetry, chronoamperometry, and in situ transmittance measurement system. Compared to as-prepared films, heat-treated tungsten trioxide thin films exhibit a higher electrochemical reversibility of 0.81 and superior coloration effi-ciency of 65.7 cm 2 /C, which implies that heat treatment at an appropriate temperature is a crucial process in a sol-gelmethod for having a better electrochromic performance.Keywords: Electrochromism, Tungsten trioxide, Sol-gel, Heat treatment, Coloration efficiency ······························································································································· ·································································································

Structural, morphological, gas sensing and photocatalytic characterization of MoO3 and WO3 thin films prepared by the thermal vacuum evaporation technique

Thin films of molybdenum trioxide and tungsten trioxide were deposited on glass substrates using a simplified thermal evaporation under vacuum method monitored by heat treatment in flowing oxygen at 500°C for 1h. The structural and morphological properties of the films were investigated using X-ray diffraction, Raman spectroscopy, atomic force microscopy and scanning electron microscopy. The X-ray diffraction analysis shows that the films of MoO3 and WO3 were well crystallized in orthorhombic and monoclinic phase respectively with the crystallites preferentially oriented toward (200) direction parallel a-axis for both samples. In literature, we have shown in previous papers that structural and surface morphology of metal thin films play an important role in the gas detection mechanism. In this article, we have studied the response evolution of MoO3 and WO3 thin films sensors ethanol versus time, working temperature and the concentration of the ethanol. It was found that these films had high sensitivity to ethanol, which made them as a good candidate for the ethanol sensor. Finally, the photocatalytic activity of the samples was evaluated with respect to the degradation reaction of a wastewater containing methylene blue (MB) under UV–visible light irradiation. The molybdenum trioxide exhibits a higher degradation rate than the tungsten trioxide thin films under similar experimental conditions.

Electrochemically synthesized tungsten trioxide nanostructures for photoelectrochemical water splitting: Influence of heat treatment on physicochemical properties, photocurrent densities and electron shuttling

Abstract The primary aim of this study was to investigate the influence of heat treatment on the physicochemical properties, photocurrent densities and capacity for electron shuttling of nanostructured tungsten trioxide (WO 3 ) thin films prepared via an electrochemical deposition method. The as-deposited amorphous WO 3 films were further subjected to heat treatment at various annealing temperatures to transform the amorphous material into polycrystalline WO 3 nanostructures. X-ray diffraction (XRD) spectra indicated the existence of polycrystalline WO 3 nanostructures on the surfaces of the fluorine-doped tin oxide (FTO) electrodes. Through XRD analysis, a clear and distinctive phase transition from amorphous to monoclinic WO 3 was observed with elevated heat treatment. This WO 3 phase transition was further examined by field emission-scanning electron microscopy (FE-SEM) imaging whereby the surface morphologies of the nanostructured WO 3 thin films were observed to progress through four major physical transformation stages during the heat treatment process. Further Fourier transform infrared (FTIR) spectroscopy analysis reaffirmed that the appearance and disappearance of key functional groups during heat treatment coincided with the thermally induced phase transitions of the polycrystalline WO 3 nanostructures. In addition, the influence of heat treatment on the intrinsic electron shuttling ability of nanostructured WO 3 thin films synthesised at different annealing temperatures was studied using electrochemical impedance spectroscopy (EIS). EIS results indicated that the separation of photogenerated charge carriers in the nanostructured WO 3 thin films was greatly enhanced when the films were annealed at 600 °C, exhibiting recombination times of 3.65 ms. This was thermodynamically linked to the increase in average WO 3 crystallite size and the reduction of WO 3 grain boundaries during the thermally induced phase transition, which led to the suppression of the electron-hole pair recombination rate.

Synthesis of ALD Tungsten Trioxide Thin Films from W(CO)6 and H2O Precursors

Tungsten trioxide (WO3) thin films are extensively investigated for their remarkable semiconducting properties in electrochromics, microelectrics, selective catalysts, solar cells, gas-sensors and environmental engineering. In addition, WO3 has been particularly developed for electrochromic applications, such as smart windows for green buildings and airplanes, rear mirrors in automobiles, displays, and electro-devices.WO3 thin films have been fabricated by various methods, including physical vapor depostion (PVD), pulsed vapor deposition (PLD), RF magnetron sputtering deposition, and electrodeposition [1, 2]. Atomic layer deposition (ALD) is a novel process to synthesize variaous thin films, where the sequential alternating vapor pulses of chemical precursors are introduced into the ALD reaction chamber, one at a time. Each precursor pulse was separated by inert gas, such as Ar or N2. The excess precursors and reaction by-products were evacuated by vacuum pump from the ALD reaction chamber. The precursor is fully reacted with only one monoatomic layer on the reacting surface through a surface saturating self-terminationg process. Therefore, the ALD technique produces extremely uniform and conformal films in atomic level thickness. In general ALD has been shown to be able to generate reproducible and well-defined thin films of binary oxides, nitrides, and sulfides. In recent years ALD has been used to deposit pure elemental metals, new types of ternary and quarternary materials. Presently only a few processes have been reported on the synthesis of WO3 films by atomic layer deposition (ALD) so far. ALD WO3 thin films have been deposited using WF6 and H2O as precursors. WO3 films have also been deposited by ALD using tungsten hexacarbonyl (W(CO)6) and ozone as reactant precursors for tungsten and oxygen. In this work, we report on the synthesis of WO3 thin films on Si and quartz substrates by thermal ALD using a cross-flow reactor, with W(CO)6 and DI water as the chemical ALD precursors for tungsten and oxygen, respectively. This was followed by 600 oC furnace annealing in regular atmosphere. Characterization of WO3 thin films has been performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), UV-VIS-NIR spectrometer, Raman spectroscopy (RS). Figure 1 shows the XRD pattern of the ALD WO3 samples obtained after furnace annealing at 600 oC under atmospheric conditions. All the diffraction peaks can be indexed into the monoclinic structure of the WO3 phase [space group: P21/n (14)], which is consistent with PDF 83-0950.

Size-Dependent Photoelectrochemical Properties of Nanostructured WO<sub>3</sub> Thin Films Synthesized via Electrodeposition Method

The main aim of this study was to investigate size-dependent effect on the photoelectrochemical properties of nanostructured tungsten trioxide (WO3) thin films synthesized via electrochemical method. Firstly, the nanostructured WO3 thin films of different crystalline sizes were synthesized on fluorine-doped tin oxide (FTO) glass working electrodes followed by controlled annealing treatment at temperature of 100-600°C. The resultant nanostructured WO3 thin films were further characterized using field emission-scanning electron microscopy (FE-SEM) and photocurrent density measurements. Through FE-SEM analysis, it was found that the WO3 crystalline size increases with increasing annealing temperature that resulted in elevated photocurrent per unit area of the synthesized nanostructured WO3 thin films. Finally, it was observed that the highest photocurrent density of up to 35μA/cm2 was attained for WO3 crystallines size of 86nm that formed at the annealing temperature of 600°C.

Preparation of nanostructured tungsten trioxide thin films by high pressure sublimation and condensation

Thin films of tungsten trioxide (WO3) have gained increasing importance due to their interesting chromogenic properties and for their high application potential in electrochromic devices. It is very well known that their electrochromic switching properties depend very sensitively on their nanostructure. Hence, a vast majority of the research work carried out in this domain at present is dedicated to the various techniques of controlled inducing of a nanostructure in these WO3 thin films in order to enhance their electrochromic performance.In the present work we have carried out a systematic study of the nanostructured WO3 thin films by using a novel technique of varying the source–substrate distance in a high pressure sublimation and condensation method. This technique has been found to be very efficient in controlling the grain size and thus the nanostructure of the deposited films. A correlation is established between the optical and electrochromic properties of the WO3 films and the induced nanostructure. The electrochromic properties are studied by a dry lithiation process developed in our laboratory. The results indicate a strong dependence of the film nanostructure on the source–substrate distance which influences quite sensitively the electrochromic properties. These results are expected to help design electrochromic devices suitable for different applications.

Nanomechanical and Morphological Characterization of Tungsten Trioxide (WO3) Thin Films Grown by Atomic Layer Deposition

This study investigates the nanomechanical properties and surface morphology of tungsten oxide (WO3) thin films deposited on p-type Si (100) substrates using atomic layer deposition (ALD) technology with 2000 ALD deposition cycles at a growth temperature of 300°C and annealed at different temperatures. The samples were further furnace annealed at 500, 600 and 700°C for 60 min. The influence of the deposition process on the structure and properties of the WO3 films is discussed, presented and correlated to the characteristic features of the ALD technique. The results depict significant difference in the hardness and modulus measurements between the as deposited sample and the annealed ones. The hardness and modulus drop from 14 and 170 GPa for the as deposited sample to 10 and 140 GPa for the annealed ones respectively. Surface roughness was observed to increase with annealing temperature and the initially amorphous as deposited sample reached complete recrystallization and transformed into polycrystalline films as indicated by XRD.