WO3 Thin Films Research Articles

Origin of the Memory Effect in Electrochromic Sputtered WO3 Films: Composition, Structure, or Morphology?

AbstractThe memory effect property, described as a reversible color persistence while the potential is withdrawn, is of particular importance to reaching zero‐energy consumption electrochromic devices. Nevertheless, often observed in organic materials, it is poorly studied in oxides. In this study, electrochromic tungsten oxide thin films are elaborated at different working pressures by radiofrequency magnetron sputtering. The influence of the deposition pressure on the electrochemical properties of WO3 films as well as on their memory effect is investigated. Three kinds of WO3 films can be distinguished: with irreversible blue coloration, with reversible coloration presenting a high memory effect, and with reversible coloration with a low memory effect. The origin of these discrepancies is studied through the composition, the local atomic environment, and the morphology of the WO3 thin films. An increase of transmittance at 550 nm as low as 15.8% in 48 h in air and 5.2% in 24 h in the electrolyte is recorded. This study highlights a better understanding of the memory effect property of electrochromic oxides for low‐consumption energy electrochromic devices, pointing out morphology as a key parameter.

Correlation between the Experimental and Theoretical Photoelectrochemical Response of a WO3 Electrode for Efficient Water Splitting through the Implementation of an Artificial Neural Network

Since the discovery of photoelectrochemical (PEC) water splitting with titanium dioxide electrodes in the presence of ultraviolet light, much work has been conducted to build an effective PEC water splitting system and develop novel photoelectrodes. Using a facile and controllable electrodeposition method, a thin tungsten trioxide (WO3) film electrode onto a stainless steel (SS) substrate was synthetized. The effect of the deposition time on the structural, morphological, optical, and electrical properties of the as-grown WO3 thin films was assessed. XRD spectra of the obtained films reveal the polycrystalline nature of WO3 with a triclinic phase and exhibit a sharp transition to the (002) plane when the deposition time was extended beyond 10 min. The surface morphology showed a remarkable change in the grain size, thickness, and surface roughness when varying the deposition time. UV–Vis spectrophotometry revealed that the optical band gap values of WO3 decreased from 1.78 to 1.36 eV by extending the electrodeposition duration from 10 to 30 min, respectively. Notably, as indicated from the PEC measurements, the obtained photoelectrode exhibited the effects of the deposition time on the photocurrent density, and the maximum value obtained was around 0.07 mA cm−2 for the sample deposited at 10 min. Finally, this study presents for the first time an artificial neural network model to predict the PEC behavior of the prepared photoanode, with a highly satisfactory performance of less than 0.05% error. The low cost and simply synthetized WO3/SS electrode with superior electrochemical performance and the excellent correlation between the experimental and theoretical results demonstrate its potential for practical application in water splitting and hydrogen production.

Nb2O5 passivation of WO3 thin films to prevent electrochromic performance degradation in heated water

Electrochromic thin films must be highly stable against heat and moisture for their application to devices operating in air atmosphere. In this study, we discuss the degradation of the surface morphology of sputtered WO3 thin films in heated water. When the water temperature and soaking times increase, the flat surface of film undergoes a pronounced transition and becomes rough causing partial etching off the film. Although the surface area available for ion intercalation increases by etching, the electrochromic performance of films degrades. We propose stacking a passivation layer of Nb2O5 to prevent the etching off the WO3 thin films and changes in their surface morphology. The surface morphology of the bilayer thin films soaked in water at 70 °C for 30 and 180 s does not change noticeably when the thicknesses of the Nb2O5 layers are 25 and 100 nm, respectively. Both the thin films exhibit transmittance modulations and coloration efficiencies comparable to those of the WO3 thin film without etching, suggesting effective passivation. Moreover, the stacks are stable during cyclic tests up to 700 cycles in an H2SO4 solution. In contrast, the soaked WO3 thin films lose their electrochromic switching capability after the 35th cycle. In conclusion, Nb2O5 passivation of WO3 thin films can effectively prevent their degradation under hot and moist conditions.

Enhancing Electrical Properties and Seebeck Effects of WO3 Thin Films Using Spray Pyrolysis: Insights into the Conductivity and Carrier Type

Enhancing Electrical Properties and Seebeck Effects of WO3 Thin Films Using Spray Pyrolysis: Insights into the Conductivity and Carrier Type

Flake-Like WO3-Based Thin Films for Enhanced Ethanol Sensing Applications

Undoped and vanadium-doped tungsten trioxide (V–WO3) nanocrystalline thin films were prepared on glass substrates with spray pyrolysis. The synthesized films were characterized using XRD, Raman spectroscopy, AFM, FESEM, UV-visible spectroscopy, and XPS for micro-structural, topographical, optical and chemical properties. The results showed that XRD analysis confirmed that WO3-based thin films were hexagonal phases. The FESEM images showed the interconnected nanoflake-like morphology of the undoped and V–WO3 thin films. Atomic Force Microscopy (AFM) investigations confirmed that the average surface roughness of the thin films ranged from 3.36 nm to 14.2 nm. The optical energy bandgap of the films was estimated from the optical spectrum of absorption. The static liquid distribution method was utilized to examine the gas-sensing characteristics of deposited films. The results show that the 4 wt% vanadium-doped WO3 sensor responds to ethanol vapour at room temperature. The response and recovery times towards 50 ppm of ethanol gas were determined as 32 and 21 s, respectively. The 4 wt% V-WO3 film has shown a response of 2.8, which is 2.15 times more than the undoped WO3.

Optimization of Optical Modulation in Amorphous WO3 Thin Films

Optimization of Optical Modulation in Amorphous WO3 Thin Films

Electron Regulation in Pt-M (M = Cu, Co, and Ni) Decorated WO3 Thin Films for Photocatalytic Degradation Performance

In this study, Pt-M/WO3 (M = Cu, Co, and Ni) thin films are effectively synthesized by preparing homogeneous precursor sols, spin-coating, toluene-etching, and calcination. Furthermore, the microstructural, chemical, and electrochemical properties of the WO3, Pt-Cu/WO3, Pt-Co/WO3, and Pt-Ni/WO3 thin films are also systematically compared. The results demonstrate that when compared to the WO3 thin film, the photocatalytic capability for methylene blue (MB) solution degradation is greatly increased in the Pt-M/WO3 thin films. Transfer routes for photogenerated charges and an improved photocatalytic process are suggested based on the experimental results. Due to the large difference in the work function (Φ) between the bimetallic alloy Pt-M and WO3, a bending of the energy bands at the Pt-M/WO3 interface is presented. Furthermore, the introduction of transition metals such as Cu, Co, or Ni modifies the electronic structure of Pt-M/WO3 thin films, facilitating the separation and migration of electrons and holes. Specifically, the photogenerated electrons migrate from the CB of WO3 to Pt-Co or Pt-Ni nanoparticles in the samples of Pt-Co/WO3 or Pt-Ni/WO3 thin films, while the hot electrons from the localized surface plasmon resonance (LSPR) effect of Cu could transfer to the conduction band (CB) of WO3 and other electrons generated from the photoexcitation of the WO3 semiconductor itself in the sample of the Pt-Cu/WO3 thin film. In summary, this work proposes a unique strategy for creating electron regulation in Pt-M decorated WO3 thin films for photocatalytic application.

Physical Properties of SnO2/WO3 Bilayers Prepared by Reactive DC Sputtering

The combination of transparent and conductive SnO2 with colored WO3 thin films has interesting uses in electrochromic windows, photovoltaic cells and photocatalytic systems, where the SnO2/WO3 bilayer can improve the device efficiency by increasing the charge separation and extending the energy range of photoexcitation. In this work, SnO2 and WO3 thin films were prepared by reactive DC sputtering from Sn and W targets, respectively. Single layers and bilayers deposited on glass substrates have been analyzed by X-ray diffraction, atomic force microscopy, spectrophotometry, and electrical measurements. SnO2 crystallizes in the cassiterite structure, whereas amorphous WO3 is obtained on bare and SnO2-coated glasses, showing higher surface roughness on the SnO2 layer. Different oxygen vacancy defects have been identified in WO3 by analyzing photoconductivity transients. The oxygen vacancy defects are responsible for the sub-bandgap absorption that causes coloration in the WO3 films. Regarding SnO2, it shows a high transmittance of about 90% in the visible and near-infrared spectral ranges.

Reducing damage of sputtering and improving conductivity of transparent electrodes for efficient semi-transparent perovskite solar cells

Sputtered indium tin oxide (ITO) is widely used as an electrode in semi-transparent and tandem perovskite solar cells. However, damage from sputtering to under layers and the limited conductivity of ITO are still the two main obstacles that hinder further performance improvement of the devices. In this work, the effects and mechanism of sputtering damage and poor conductivity of ITO are investigated based on a traditional perovskite solar cell with bathocuproine (BCP) buffer layer. In order to suppress the sputtering damage, tin oxide (SnO2) is deposited on C60 to replace the BCP buffer layer. However, it is found that the deposition of SnO2 on the non-reactive C60 by atomic layer deposition will result in island growth of SnO2 film, which is the key reason for large dark current in solar cells. Fortunately, the phenomenon is inhibited by decorating C60 surface with WO3 thin film. In order to improve the conductivity of the transparent electrode, an ITO/Au/ITO multilayer architecture is designed. The fill factor (FF) and power conversion efficiency (PCE) of the semi-transparent solar cells (ST-PSCs) with the modified buffer layer and electrodes reached 76.4% and 17.62%, respectively, showing an improvement of FF and PCE when compared to the device with BCP buffer layer and ITO electrode. It is revealed that the optimization also increases the short circuit current of the solar cells. These results provide new strategies for damage reduction of sputtering and performance improvement of ST-PSCs.

Rapid synthesis and characterizations of defect-enhanced WO3-δ thin film electrode for effective energy storage application in asymmetric supercapacitor devices

Oxygen vacancy-doped WO3-δ thin film electrode with improved conductivity and high areal capacitance was synthesized via mild electrochemical oxygen de-intercalation of electrodeposited WO3 thin film. The X-ray diffraction (XRD) analysis revealed the presence of monoclinic phase W18O49 of the doped thin film electrode. Raman spectroscopy analysis confirmed the presence of lower valence W5+ in the WO3-δ film. Electrical characterizations of doped WO3-δ and undoped WO3 films show that the doped electrode exhibits far lower sheet resistance and resistivity than the undoped WO3 sample. Mott-Schottky analysis of the samples shows that the vacancy-doped WO3-x possesses a higher donor concentration than the stoichiometric WO3. Electrochemical characterizations by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD,) and electrochemical impedance spectroscopy (EIS) confirmed the superior pseudocapacitive charge storage metrics via its improved volumetric capacitance of 366.12 F·cm−3 (16.0 mF·cm−2) and reduced equivalent series as well as charge transfer resistance over WO3 thin film. This study demonstrated the synthesis of oxygen vacancy-doped WO3-δ thin film using a homemade two-electrode cell facility instead of the popular energy-consuming vacuum-assisted film deposition under controlled-atmosphere conditions. Besides, the vacancy-doped WO3-δ electrode shows a strong charge storage capability in the far negative operating potential range, indicating its suitability as an anode electrode for fabricating high-energy asymmetric supercapacitors.

Improved electrochromic performance of potentiostatically electrodeposited nanogranular WO3 thin films

Improved electrochromic performance of potentiostatically electrodeposited nanogranular WO3 thin films

Effect of post annealing on DC magnetron sputtered tungsten oxide (WO3) thin films for smartwindow applications

Tungsten oxide (WO3) thin films were deposited on Fluorine Doped Tin Oxide (FTO) and Corning Glass (CG) glass substrate by DC magnetron sputtering. The annealing temperature was varied to study its effect on surface morphology, structural, electrochromic (EC), and optical properties and these are investigated by Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, Cyclic voltammetry, and ultraviolet (UV) spectroscopy. From SEM analysis WO3 thin films annealed at 27 °C, 100 °C, 200 °C, and 300 °C were shown to crack free after that cracked film was observed for 400 °C. From the XRD investigation that the WO3 thin films annealed at 27 °C, 100 °C, 200 °C, and 300 °C were amorphous and crystallized at 400 °C. The optical band gap (Eg) of WO3 films was decreased from 2.98 eV to 2.30 eV with an increase in annealed temperature. The coloration efficiency (CE) was observed at 51.26 cm2/C at 300 °C and 35.06 cm2/C at 400 °C and the lowest diffusion coefficient was observed at 5.86 × 10−10 cm2/s at 400 °C. On coloring efficiency, which can be very important in electrochromic (EC) applications, post-annealing has been seen to have a strong influence.

Comparative analysis of the effect of post-annealing on CeO2 and DC Magnetron Sputtered WO3/ CeO2 nanorods thin films for smart windows

This study investigates and evaluates thin films of WO3 (Tungsten Oxide) coated CeO2 (Cerium Oxide) Nanostructure (NR) for electrochromic applications. Using CH4N2O (Urea) and Ce(NO3)3.6H2O (Cerium Nitrate Hexahydrate) on a fluorine-doped tin oxide (FTO) base, cerium oxide nanorods were produced hydrothermally. Cerium Oxide nanorods were coated with a thin coating of WO3 using DC magnetron sputtering under argon gas at room temperature. In a three-electrode electrochemical system, thin sheets of manufactured WO3/CeO2 nanorods (NRs) served as the working electrode, saturated Hg/HgCl2 served as the reference electrode, and platinum wire served as the counter electrode. To produce the 0.5 M electrolyte, the H2SO4 solution was diluted in deionized (DI) water. Electrochromic tests revealed a 12.15 cm2/C coloring efficiency for annealed 10 µL HCl CeO2/WO3 film. A scanning electron microscope (SEM), an electrochemical analyzer, an X-ray diffraction spectrometer, and an ultraviolet-visible spectrometer were used to study the optical properties, composition, surface morphology, and electrochromic property of the generated thin films. Due to its high charge capacity and optical transmission properties, CeO2 nanorods with WO3 coating have proven to be a practical material for electrochromic devices (ECD).

Transmittance correlated real-time resistivity modulation and insulator-metal transition of electrochromic WO3 thin films

A quantitative relationship between optical and electrical properties of crystalline WO3 films during Li + insertion has been established in this work. Detailly, the exponential relationship between transmittance and resistivity of crystalline WO3 thin films in visible and near-infrared wavebands is determined, and the real-time characterization of the resistivity of WO3 thin films during electrochromic process is realized. The experimental results show the resistivity of WO3 film can be regulated in the range of 0.078–0.98 Ω m with rapid response (∼6 s) when applied ±1 V voltage for 30 s. By analyzing the fitted resistivity-transmittance curve of WO3 thin film, a high-efficiency optical modulation of WO3 film in near-infrared 1000 nm can be achieved. Combining the IMT phenomenon during Li+ insertion, when x of LixWO3 changes from 0.08 to 0.13, the WO3 film is in a transition state from the insulating state to the metallic state with the tetragonal crystalline phase. The symmetry structural evolution of crystalline WO3 thin films during the Li + insertion is further demonstrated by transmission electron microscopy (TEM). These results contribute to the development of WO3 based optoelectronic bifunctional electrochromic devices.

WO3 Thin-Film Optical Gas Sensors Based on Gasochromic Effect towards Low Hydrogen Concentrations.

Hydrogen gas sensors have recently attracted increased interest due to the explosive nature of H2 and its strategic importance in the sustainable global energy system. In this paper, the tungsten oxide thin films deposited by innovative gas impulse magnetron sputtering have been investigated in terms of their response to H2. It was found that the most favourable annealing temperature in terms of sensor response value, as well as response and recovery times, was achieved at 673 K. This annealing process caused a change in the WO3 cross-section morphology from a featureless and homogenous form to a rather columnar one, but still maintaining the same surface homogeneity. In addition to that, the full-phase transition from an amorphous to nanocrystalline form occurred with a crystallite size of 23 nm. It was found that the sensor response to only 25 ppm of H2 was equal to 6.3, which is one of the best results presented in the literature so far of WO3 optical gas sensors based on a gasochromic effect. Moreover, the results of the gasochromic effect were correlated with the changes in the extinction coefficient and the concentration of the free charge carriers, which is also a novel approach to the understanding of the gasochromic phenomenon.

Synthesis and characterization of pure and Cu-doped WO3 thin films for high performance of toxic gas sensing applications

Oxygen vacancies and the high surface area of nanostructured materials offers stronger gas adsorption active sites for improved gas sensing performance. In the present work, we have deposited WO3-based thin films using a chemical spray pyrolysis technique at a substrate temperature of 400 °C with other optimized preparation parameters. Besides, this study reports the influence of Cu doping at various weight percentages on the structural, morphological, optical and toxic gas sensing properties of WO3 films in detail. The crystalline structure of the material was investigated using the X-ray diffraction technique. Each X-ray diffraction analysis of the films demonstrated a polycrystalline nature, matching the hexagonal phase of WO3 with the (200) plane of preferential orientation of crystal growth. The optical characteristics of the thin film's material were analyzed by UV–Vis absorption spectroscopy. The optical energy bandgap (Eg) of the deposited thin films was estimated using the Tauc relation, and the value was decreased along with the increase in the Cu-doping concentration. The surface roughness of the films has been investigated using atomic force microscopy. The TEM patterns of sprayed Cu-doped WO3 thin films exhibit nanocrystalline features. Detecting toxic gasses is of great importance across the globe due to an alarming enhancement in respiratory, ocular, skin, and lung diseases. Hence, cost-effective and room temperature-operated Cu-doped WO3 thin film sensors to trace low concentrations of toxic gasses have been investigated and systematically reported. It is observed that the sensor response is to be increased with the increase in Cu-doping concentration, particularly the 4 wt.% Cu-doped sensor exhibited a response value of 7.2 and fast response and recovery times in the presence of 50 ppm ammonia vapor.

Impact of top metal electrodes on current conduction in WO3 thin films

Abstract Using radio-frequency (rf) sputtering technique tungsten oxide (WO3) thin films (∼150 nm) were deposited in a gas mixture of Ar and O2 at 10 mTorr pressure on indium tin oxide (ITO) and corning glass substrates. The films were annealed at 400 °C. Structural and optical properties of films were studied. Metal–insulator–metal (MIM) structure was made by depositing metal electrode (Ag, Al, Au) on the prepared WO3 thin films (on ITO substrate) using thermal evaporation technique. Electrical properties of the MIM structure were studied by plotting current versus voltage (I–V) curves for Ag, Al and Au metal electrodes. Current conduction mechanism in WO3 film was determined by plotting and fitting I–V data in different equations of current mechanisms.

Enhanced electrochemical performance of WO3 thin films prepared from polyvinyl alcohol-modified nanoparticle ink

Tungsten oxide (WO3) is a potential material in catalysis, sensors, electrochromic devices, etc. In this research work, porous WO3 thin films are prepared on FTO-glass employing spin coating of WO3 nanoparticle's ink modified with polyvinyl alcohol (PVA) in the concentration range of 0 to 25.0 wt%. This study explored the change in structure and morphology of WO3 thin films with varying PVA concentration in ink composition and their effect on the ability to intercalate and de-intercalate Li+ ions as a function of applied bias. The wettability of ink on FTO-glass improved by raising its PVA content. All WO3 thin films exhibited a monoclinic structure. Scanning electron microscopy images indicated that the porosity of WO3 thin films gradually increased with increasing PVA concentration up to 25 wt% but the thin film became highly nonuniform at a very high concentration of additive. WO3 thin film prepared with ink having 10 wt% of PVA had optimum porosity and uniformity, therefore, yielded better electrochemical performance compared with all other samples. The addition of an optimum amount of PVA to ink is found to be an effective approach to modulating ink's characteristics and inducing porosity in thin films which eventually improves their redox reaction characteristics.

The Effect of Annealing Treatment on WO3 Thin Film Prepared by Reactive DC Magnetron Sputtering for Photo-electrochemical Water Splitting Application

In this work, WO3 thin films were fabricated by reactive DC magnetron sputtering, and then thermally annealed at 400°C for 2 h under air, low vacuum and high vacuum. After the annealing treatments, the morphology and crystallinity of the WO3 thin films were observed using FE-SEM, GI-XRD and Raman spectroscopy. The optical properties were analyzed by UV-Vis spectroscopy. The results showed the decrease of film thickness under different annealing conditions. In addition, the annealing conditions also affected the crystalline structure at diffraction planes (200) and (002). The transmittance of the WO3 thin films revealed that the annealing treatment at high vacuum led to lower transparency. Furthermore, the WO3 thin film annealed under air produced the highest PEC efficiency. Therefore, this approach offers an alternative strategy for photoelectrochemical (PEC) water splitting application.

Fabrication of Large-Area, Affordable Dual-Function Electrochromic Smart Windows by Using a Hybrid Electrode Coated with an Oxygen-Deficient Tungsten Oxide Ultrathin Porous Film.

Electrochromic (EC) devices are not commercialized extensively owing to their high cost. The best large-area devices in the market suffer from not reaching a distinct dark-colored state. These devices appear more like a blue tinted glass. While a better performance demands the use of appropriate components, the cost-effectiveness of such components is crucial for commercialization. Specifically, the utilization of cost-effective electrodes, thin WO3 coatings, and inexpensive electrolytes are essential for reducing the cost of EC devices. Here, we report a high-performing porous WO3 thin film (∼130 nm) achieved by optimizing the DC sputtering process parameters. This way, an affordable dual-function EC energy-storage device was fabricated, showing 84% transmittance modulation and a high power density of 3036 mW/m2, thus functioning simultaneously as a transparency switching energy-storage device. With a large-area (900 cm2) device, we have demonstrated that the need for expensive ITO electrodes and Li+ ion-based electrolytes can be eliminated by using a hybrid electrode (ITO/Al-mesh) and multivalent Al3+ ion-based electrolytes while not compromising the device performance. The findings of this study may revolutionize the EC device industry and their commercialization owing to inexpensive ingredients and scalable processing.