WO3 Films Research Articles

Long-life Inorganic Electrochromic Device Based on WO3 and PB Films with Fast Switching Respond

Complementary electrochromic devices were fabricated by using tungsten trioxide (WO3) film as working electrode, Prussian blue (PB) film as counter electrode and 0.1 M lithium perchlorate in propylene carbonate as electrolyte. The XRD result presents that WO3 and PB films are amorphous, which is favourable for a fast ion intercalation-deintercalation process. The device was tested under potentiostatic control using ±1.3 V steps, and changed its color between blue and colorless. It has a fast-switching respond (about 1.5 s for colouring and 2 s for bleaching) and could withstand 100,000 cycles with little change in its optical contrast.

Surface morphology engineering of WO3 films for increasing Li ion insertion area in electrochromic supercapacitors (ECSCs)

Two different dry deposition methods were used to deposit electrode films with different surface morphologies in relation to the performance of electrochromic supercapacitors (ECSCs): direct current (DC) magnetron sputtering and the nanoparticle deposition system (NPDS). The NPDS method, which sprays powders onto the substrate using pressure differences, deposited WO3 film with its roughness of 36 nm and its thickness of 383 nm, while the sputtered WO3 film had its roughness of 3 nm and its thickness of 262 nm. Due to its dramatic difference in surface roughness, the NPDS-WO3 thin film had larger reaction surface area as well as Li-ion diffusion coefficient than those of sputtered-WO3 films. As a result, the ECSC device with NPDS-WO3 exhibited 19 % higher coloration efficiency with greater charge capacity compared to those with sputtered-WO3. However, the ECSC device with NPDS-WO3 showed different charge/discharge with trapped Li ions during the coloring(charging)/bleaching(discharge) processes respectively, leading to rapid degradation of the device. This research highlights the clear difference between its characteristics of the thin films fabricated using the NPDS vs. sputtering methods in relation to its surface roughness so that this study would be potentially beneficial in engineering various properties of ECSC devices.

Dynamic smart window over the entire solar spectrum using a dry deposited device for sustainable architecture

We developed a dynamic smart window capable of demonstrating transparent, black, and mirror modes across the entire solar spectrum. We demonstrated that the surface-roughness-controlled WO3 film deposited by nano particle deposition system (NPDS) can achieve the black mode with almost zero solar heat transmittance (Tsol). In the Reversible Metal Electrodeposition (RME), device can block heat similar to the mirror mode. NPDS is a unique dry deposition method to enhance the light scattering effect. As a result, the transmittance was measured in the range of the entire solar spectrum (λ ∼ 300 nm–2500 nm), obtaining 0.81% and 0.56% of Tsol for the black and mirror modes, respectively. However, 61% of Tsol was obtained for the transparent mode, confirming stark differences in Tsol. Furthermore, a dynamic smart window of 4 cm2 in size was fabricated to study the temperature change within the model house upon exposure to an infrared lamp to monitor any increase in temperature during transparent, black, and mirror modes. As a result, the temperature increases within the model house during transmittance mode increased by 19.2 °C, while the temperature increases during black and mirror modes slightly by 4.1 °C and 3.8 °C, respectively. The dynamic smart window was able to reduce the indoor temperature by more than 4.7 folds during the black/mirror modes. These findings confirmed that we developed an RME device capable of completely blocking sunlight to minimize indoor temperature change across the entire solar spectrum.

Spray deposited pristine and Mo doped WO3 thin films for acetaldehyde gas sensing at room temperature

In this paper, pure WO3 and molybdenum-incorporated WO3 were directly deposited in the form of thin films on the glass substrate. The films were deposited through the spray pyrolysis technique, and their gas sensing properties, structural, optical, electrical, and morphological properties were altered by adding a dopant to the pristine WO3. A direct, one-step approach is used to deposit metal oxide semiconductor materials as thin films, which enhances the ability to detect gas. A novel gas sensing response for acetaldehyde gas detection was observed from Molybdenum doped WO3 thin film at 25 °C. On investigating the sensing performance of both films for various volatile organic compounds (VOC), a high response (54.55 %) towards acetaldehyde was achieved for 2 % molybdenum-doped WO3 film. The XRD analysis demonstrated that the synthesized films are crystalline with triclinic phase. Decrease in crystallite size increases the grain boundaries that enhance the sensing response. The absorption spectra of UV–Visible spectroscopy showed an apparent change of increase in the absorbance and decrease in the optical band gap on addition of the dopant to the pure WO3 thin film. Filamentous structured porous thin films are discovered to have larger voids and greater surface roughness from FE-SEM and AFM, which aids in the adsorption of gas molecules. Mo-doped WO3 thin films showed a strong sensing response due to larger grain boundary scattering and wide pores abetting for more gas adsorption.

Rapid and low-temperature preparation of tungsten oxide electrochromic thin films by oxygen plasma treatment

Electrochromic materials have attracted considerable attention due to their ability to change color reversibly and rapidly under an external electrical stimulus, making them highly promising for use in various applications such as smart windows, displays, and optical memory devices. In particular, tungsten trioxide (WO3) thin films have shown excellent electrochromic properties, leading to their wide application in electrochromic devices. In this study, oxygen plasma treatment was used to convert WCl6 thin film to WO3 electrochromic thin film on ITO glass substrate. Surface morphology, element composition, phase, optical property, and electrochromic characteristics of WO3 film are analyzed. It has been proved that the conversion of precursor films is rapid and efficient, the mean roughness and root mean square of amorphous WO3 thin film are lower than 1.8 nm and 2.2 nm respectively, and the film exhibits good electrochromic performance parameters. The optical modulation range and coloration efficiency of the film can reach 58.5% and 80.2 cm2/C before cycle and 56.3% and 84.0 cm2/C after 100 cycles, coloration time and bleaching time are within 10 s. The method of plasma treatment avoids annealing and prepare WO3 film with good and stable performance. This method has the potential to expand to other electrochromic materials, and provides a new solution for the large area preparation and flexible application of electrochromic materials.

Plasmonic hot electron-induced WO3 films for a highly responsive visible photodetector

Visible photodetectors have found significant applications in various fields that require rapid and precise detection of visible light. These fields encompass optical communication, high-speed imaging, LIDAR systems, biomedical imaging, and industrial process monitoring. Semiconductors play a crucial role in photodetection applications by efficiently converting light into electrical signals, offering high sensitivity and rapid response time. Among various semiconductor metal oxides, WO3 is commonly used as a visible photodetector due to its unique properties like high chemical & thermal stability, and it possesses a direct bandgap making it suitable for efficient detection of visible light. Here we report the surface plasmon resonance induced visible light detection using Au NPs on WO3 film. The WO3 film was grown on SiO2 substrate by DC magnetron sputtering while monodispersed (⁓20 nm) Au NPs were chemically synthesised. MSM based Au NPs functionalized WO3 film was tested under 455 nm wavelength with changing optical power and applied bias. The interaction of Au NPs and visible light lead to generation of hot electrons leading to enhancement in the photocurrent. The fabricated device exhibits large bias-dependent responsivity, low NEP, and high EQE of 23.45 A/W, 2.6 × 10−13 WHz−1/2, and 126%, respectively. The study not only provides understanding of plasmon-enhanced visible light detection but also underscores the significance of Au NPs as a powerful tool for enhancing the performance of WO3-based visible photodetectors. The implications of this study extend beyond fundamental research, paving the way for novel applications in the field of optoelectronics appliances.

Effect of Annealing on Resistive Switching Properties of Glancing Angle Deposition‐Assisted WO3 Thin Films

Herein, the impact of postdeposition annealing on resistive switching behavior of radio frequency magnetron sputtered WO3 thin films is reported. Films are deposited under glancing angle deposition (GLAD) configuration of sputtering at varying GLAD angle from 65° to 80°. Structure transition from monoclinic to orthorhombic phase in deposited WO3 films is perceived after ex situ annealing at temperature of 400 °C. Resistive switching properties show shift from bipolar to unipolar switching on postdeposition annealing. WO3 films show unipolar switching behavior after ex situ annealing for all prepared samples. The value of resistance in high resistance state is lowered after ex situ heating treatment and interestingly switching voltage also reduces to 3 V from 7 V after annealing treatment. The ratio of high to low resistance state for annealed WO3 film fabricated at 70° GLAD angle is achieved to be maximum (≈219). A detailed charge transport mechanism shows that ohmic behavior is dominant current conduction mechanism at lower applied voltage, while space charge limited current and Child's law are dominant at higher applied voltages. Obtained results encourage utilization of prepared WO3 thin films toward a wide variety of applications in optoelectronics, microelectronics, and environmental engineering along with advanced electronics such as resistive memory devices.

Amorphous bismuth-doped WO3 film: Fast-switching time and high-performance proton-based aqueous electrochromic device

Tungsten trioxide (WO3) film, recognized as one of the most promising electrochromic materials due to its exceptional electrochemical characteristics, is extensively utilized in fabricating electrochromic devices. The use of bismuth-doped WO3 (Bi-WO3) films in photoelectrocatalysis has been extensively studied. However, its potential in electrochromism remains unexplored. Herein, amorphous Bi-WO3 films with enhanced electrochromic performance are synthesized through a simple sol–gel method. The effects of Bi doping amount and sol concentration on the electrochromic properties of Bi-WO3 films are investigated. The optimized Bi-WO3 film displays excellent electrochromic performance, encompassing high optical contrast (ΔT = 81% at 630 nm), fast switching time (2.4/1.2 s for coloring and bleaching), and high coloration efficiency (55.84 cm2C−1). An electrochromic device is assembled utilizing an amorphous Bi-WO3 film as the electrochromic layer with Cu foil as the counter electrode. The fascinating electrochromic properties of amorphous Bi-WO3 thin films and the high activity features of proton enable the assembled ECD to have a suitable working voltage range (1.1 V) in aqueous electrolytes. Besides, the electrochromic device demonstrates outstanding properties (ΔT = 70%, tc = 3 s, tb = 0.9 s) and provides a helpful strategy for the facile processing of high-performance electrochromic devices.

Enhanced photoelectrochemical water oxidation by Fe(II) modified nanostructured WO3 photoanode

Herein, we demonstrate a facile surface treatment of a nanostructured tungsten oxide (WO3) film by Fe2+ ions as a novel post-fabrication technique for enhanced photo-electrochemical water oxidation under sunlight. The structural, optical, morphological and elemental characterizations of the n-type WO3 nanoplates based electrodes have been done using X-ray diffraction, UV–visible diffuse reflectance spectroscopy, fluorescence spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy, respectively. The modified WO3 photoanodes have been subsequently examined for photo-electrochemical response by linear sweep voltammetry, photo-amperometry, electrochemical impedance spectroscopy (Nyquist plot) and Mott-Schottky analysis under AM 1.5 G solar simulated light. The optimized WO3 film with Fe:W ratio of ∼1:25, (Fe2+:Fe3+ ratio of ∼3:2) at the surface exhibited up to 3 times higher and more stable photocurrent under back illumination than that for untreated WO3 indicating the success of the surface treatment. Investigations revealed that the improved performance is a consequence of an improved charge transfer process at the interface resulting in lowering of unwanted recombination.

Influence of tungsten oxide concentration on structural, morphological, compositional, linear and non-linear optical properties, and vibrational properties on NiO:WO3 thin films

Mixed nickel-tungsten oxide, NiO:WO3 (90:10, 85:15) thin films with different tungsten oxide (WO3) concentrations of 10 and 15% were deposited using the radio frequency (RF) sputtering technique. An X-ray diffraction analysis reported the amorphous nature of the films. A SEM study showed smooth morphology. EDAX confirms the compositional purity, in which the nickel content decreases, while the tungsten and oxygen content increase with an increased WO3 concentration. The average transmittance of the NiO:WO3 (90:10, 85:15) films decreases from 79 to 75% as a consequence of increased thickness. The Urbach energy (Eu) enhances because of increased defects (localized states) inside the films induced by tungsten oxide addition. The energy bandgap decreases from 2.66 to 2.48 eV due to the formation of new localized energy states inside the band gap (Eg). The refractive index (n) acquired from different proposed models for the entire UV–Vis–NIR region has increased with increasing WO3 concentration, implying the escalation in densification (or packing density) of prepared films. The optical parameters like n, k, ε1, ε2, electronic polarizability (αp), optical conductivity (σop), electric conductivity (σe), volume and the surface energy loss function (VELF AND SELF) vary with WO3 concentration. The dispersion energy parameters (Eo, Ed), resulting from the Wemple and Didomenico (WD) model, decreases. The optical non-linear susceptibility, χ(3), decreased with increasing WO3 concentration. The photoluminescence (PL) emissions were perceived at 368 nm and 423 nm, assignable to 3 d8 Ni2+ ions transitions. Micro-Raman detected peaks at 570 cm−1 and 1100 cm−1 ascribing Ni–O bond vibrations, whereas the peak spotted at 870 cm−1 is ascribed to a W–O bond vibration. The deposited films are good enough for electrochromic and optoelectronic device fabrications.

Boosting electrochromic performance of WO3 film via interface-engineering with polyvinylidene fluoride as charge-balance regulator

Boosting electrochromic performance of WO3 film via interface-engineering with polyvinylidene fluoride as charge-balance regulator

WO3 work function enhancement induced by filamentous films deposited by resistive heating evaporation technique

Tungsten trioxide (WO3) films are of great interest due to their electronic properties which can be tuned by surface nanostructuring leading to enhanced efficiency for gas sensing, energy storage or electrochromic applications. Resistive heating of tungsten (W) filaments at pressures of few Pa in an oxygen O2 atmosphere has already demonstrated its capability to form porous, micro/nano-structured, cheap and fast films making it suitable for industrial applications. This work presents stoichiometric WO3 films produced by applying a current into a W filament for pressures in the range of 2–20 Pa. The increase of the pressure above 7.5 Pa led to amorphous WO3 films with a newly filamentous morphology. As a function of the pressure, the work function (WF) of the films was determined in-vacuo by aims of Ultraviolet Photoelectron Spectroscopy (UPS). In addition to the high surface to volume ratio of the films, the WF exhibited an increase from 5.8 eV, for a conventional WO3 film, to a maximum of 8.7 eV at 20 Pa. This change corresponds to an increase of the WF of about 50% compared to 5.8 eV making our films suitable for a large variety of applications. The highering of the WF is mainly induced by the change of morphology to filamentous structures, the defect induced by the amorphous structure and the nanometric size of the filaments leading to the confinement of the electrons in the valence band for our WO3 films. Additionally, a change from p to n-type polarity is reported in-vacuo when increasing the pressure from 7.5 to 20 Pa with the formation of an intermediate band at around 0.8 eV below the Fermi level. Additionally, molybdenum trioxide (MoO3) and vanadium pentoxide (V2O5) filamentous films were deposited via the resistive evaporation technique in an O2 atmosphere. Similarly, copper, aluminium and molybdenum wires were used, leading to the formation of islands having foam-like structures analogous to the WO3.

Preparation of ordered nanoporous WO3 thin films and the mechanism of large room-temperature ferromagnetism

Undoped porous WO3 nanofilms with large surface area were fabricated on porous anodic alumina substrates by magnetron sputtering. The nanofilms exhibited significant room-temperature ferromagnetism, at least one order of magnitude stronger than other reported undoped WO3 films. By controlling the oxygen partial pressure and the sputtering time during the deposition process, the saturation magnetization of the samples could be tuned over a large range, from 0.1 to 21.7 emu/cm3. Obvious magnetic anisotropy was found in the porous samples, indicating that the magnetic behavior is related to the regular pore structure perpendicular to the surface. Moreover, the first theoretical evidence that the Curie temperature of the sample is higher than room temperature was provided. When annealed in argon, the WO3 nanofilms turned black and showed a combination of a narrowed bandgap and enhanced ferromagnetism. These interesting phenomena are explained based upon the concept of hydrogen-like impurity states associated with oxygen vacancies.

A stability study of transparent conducting WO3/Cu/WO3 coatings with antimicrobial properties

WO3/Cu/WO3 coatings are transparent electrodes, but conductivity and transmittance have been observed to decrease with time. This paper reports the improved stability of WO3/Cu/WO3 coatings deposited by magnetron sputtering on glass and polyethylene terephthalate substrates. The stability issues due to Cu oxidation and migration can be addressed by adjusting the deposition parameters. Lowering the sputtering pressure results in denser WO3 films, confirmed by spectroscopic ellipsometry, and thus more stable coatings. The coatings retain their properties in an inert atmosphere, indicating that Cu oxidation is the main reason for the decrease in conductivity, rather than its migration observed by X-ray photoelectron spectroscopy. Optical property modeling is used to optimize the thickness of the three-layer coatings to obtain the highest figure-of-merit for a transparent electrode. A structure of glass/WO3 (70 nm)/Cu (10 nm)/WO3 (45 nm) gives a sheet resistance of 14 Ω/sq. and a light transmittance of 65% at 600 nm. In addition, the antimicrobial properties of these coatings are revealed. A decrease up to 105 of the gram-negative Escherichia coli and gram-positive Staphylococcus aureus bacterial colony formation units is found for several WO3/Cu/WO3-based coatings. In the case of the MS2 (Emesvirus zinderi) bacteriophage, a decrease in infectious particles for up to 104 plaque-forming units is obtained. The results indicate that more stable samples also had higher antimicrobial activity.

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.

Application of the [WO2(C5H7O2)2] Complex in Hydrothermal Synthesis of WO3 Film and Study of Its Electrochromic Properties

The goal of this work was the synthesis study of the [WO2(C5H7O2)2] complex and its application as a precursor for the growth of WO3 films in hydrothermal conditions, as well as evaluating the microstructural features and electrochromic properties of the formed materials. Dioxotungsten acetylacetonate was synthesized in an aqueous medium and purified. It was found that during hydrothermal treatment of the alcohol solution of the complex, acetylacetonate ligands undergo partial destructive substitution by alkoxyl groups, intensifying at temperatures above 140 °C. Considering these data and using a [WO2(C5H7O2)2] solution, WO3 films were grown on glass and glass/ITO substrates. The resulting films had different microstructures according to scanning electron microscopy (SEM) and atomic force microscopy (AFM): the former consisted of submicron spheres (~500 nm), distinct nanoparticles (60–160 nm), and submicron- and micron-sized ridges, while the latter consisted of 1D structures (length 350 ± 25 nm, width 110 ± 25 nm). Using Kelvin probe force microscopy (KPFM), the electron work function was determined for the film on glass/ITO substrate (4.77 eV). It was found that the electrochemical coloration process of the obtained WO3 film can proceed in two stages, and the optical contrast is about 17.5% (at the wavelengths of 600–1100 nm). The results obtained show the prospects of applying the proposed approach to obtaining WO3 electrochromic films with a hierarchical microstructure with the hydrothermal method using the [WO2(C5H7O2)2] complex as a precursor.

Large-Area Printed Oxide Film Sensors Enabling Ultrasensitive and Dual Electrical/Colorimetric Detection of Hydrogen at Room Temperature.

Commercial hydrogen (H2) sensors operate at high temperatures, which increases power consumption and poses a safety risk owing to the flammable nature of H2. Here, a polymer-noble metal-metal oxide film is fabricated using the spin-coating and printing methods to realize a highly sensitive, low-voltage operation, wide-operating-concentration, and near-monoselective H2 sensor at room temperature. The H2 sensors with an optimized thickness of Pd nanoparticles and SnO2 showed an extremely high response of 16,623 with a response time of 6 s and a recovery time of 5 s at room temperature and 2% H2. At the same time, printed flexible sensors demonstrate excellent sensitivity, with a response of 2300 at 2% H2. The excellent sensing performance at room temperature is due to the optimal SnO2 thickness, corresponding to the Debye length and the oxygen and H2 spillover caused by the optimized coverage of the Pd catalyst. Furthermore, multistructures of WO3 and SnO2 films are used to fabricate a new type of dual-signal sensor, which demonstrated simultaneous conductance and transmittance, i.e., color change. This work provides an effective strategy to develop robust, flexible, transparent, and long-lasting H2 sensors through large-area printing processes based on polymer-metal-metal oxide nanostructures.

Phase/Interfacial-Engineered Two-Dimensional-Layered WSe2 Films by a Plasma-Assisted Selenization Process: Modulation of Oxygen Vacancies in Resistive Random-Access Memory.

Here, we propose phase and interfacial engineering by inserting a functional WO3 layer and selenized it to achieve a 2D-layered WSe2/WO3 heterolayer structure by a plasma-assisted selenization process. The 2D-layered WSe2/WO3 heterolayer was coupled with an Al2O3 film as a resistive switching (RS) layer to form a hybrid structure, with which Pt and W films were used as the top and bottom electrodes, respectively. The device with good uniformity in SET/RESET voltage and high low-/high-resistance window can be obtained by controlling a conversion ratio from a WO3 film to a 2D-layered WSe2 thin film. The Pt/Al2O3/(2D-layered WSe2/WO3)/W structure shows remarkable improvement to the pristine Pt/Al2O3/W and Pt/Al2O3/2D-layered WO3/W in terms of low SET/RESET voltage variability (-20/20)%, multilevel characteristics (uniform LRS/HRS distribution), high on/off ratio (104-105), and retention (∼105 s). The thickness of the obtained WSe2 was tuned at different gas ratios to optimize different 2D-layered WSe2/WO3 (%) ratios, showing a distinctive trend of reduced and uniform SET/RESET voltage variability as 2D-layered WSe2/WO3 (%) changes from 90/10 (%) to 45/55 (%), respectively. The electrical measurements confirm the superior ability of the metallic 1T phase of the 2D-layered WSe2 over the semiconducting 2H phase. Through systemic studies of RS behaviors on the effect of 1T/2H phases and 2D-layered WSe2/WO3 ratios, the low-temperature plasma-assisted selenization offers compatibility with the temperature-limited 3D integration process and also provides much better thickness control over a large area.

Enhanced electrochromic and mechanical properties of WO3 film by substrate effect

In the present study, the electrochromic and mechanical properties of sputtered tungsten oxide (WO3) films deposited on substrates with different surface roughness were systematically investigated. The experimental results demonstrate that tungsten oxide films, which were deposited on fluorine-doped tin oxide with a surface roughness of 21.4 nm, exhibited superior electrochromic properties. The film's optical contrast reached 77.1% not only at 700 nm. Further, the colouring and bleaching speeds were 5.7 s and 5.9 s, respectively, while the coloration efficiency reached 157.9 cm2C−1. In addition, the peak current density of FRa21 was reduced by only about 5.6% from 4.23 mA/cm2 to 3.99 mA/cm2 after about 4000 cycles, and the coloured state transmittance reduced by only 28% without power supply during a 3600 s duration. The mechanical test results show that the elastic modulus was 79.7 ± 3.82 GPa, which was lower than the others. The finite element method was applied to explore the generation and propagation of lumps and cracks observed in SEM, and the film was found to have a greater ability to resist deformation. The present study can provide a reference for improvement of electrochromic properties especially for flexible electrochromic devices.

Porous V2O5 ion-storage film as the counter electrode of electrochromic WO3 film: Optimization via Response Surface Methodology (RSM)

Porous V2O5 ion-storage film as the counter electrode of electrochromic WO3 film: Optimization via Response Surface Methodology (RSM)