WO3 Films Research Articles

Improved isoprene detection performance of Si-doped WO3 films deposited by sputtering and post-annealing.

Si-doped WO3 films were sputtered at room temperature and then annealed in air at 500 °C. The Si doping resulted in structural distortion from space group P21/n to Pc. A high density of pores with a diameter of ∼20 nm was observed in the films, which is ideal for gas sensing applications because of the easy diffusion of gas. Isoprene sensitivity, which is defined as the resistance ratio measured in pure air and in air containing 5 ppm isoprene, was greatly improved by the Si doping. The films containing 6.3 at% Si showed the highest sensitivity of 7.7 at a working temperature of 325 °C. However, despite a lower sensitivity of 6.9 measured at 350 °C, the films exhibited better gas selectivity for isoprene over a range of reference gases, including methanol, ethanol, acetone, CO and CO2. The response and recovery times of the films were very short, being less than 1.5 and 3.0 seconds, respectively. Detailed characterization with a range of techniques verified that the increase in gas sensitivity in the Si-doped films was related to better oxygen adsorbability as a consequence of an increase in positively-charged oxygen vacancies introduced by the aliovalent substitution of W6+ by Si4+.

Achieving neutral-tinted and energy-efficient electrochromic device enabled by sputtered tungsten-niobium-oxygen film for smart windows

WO3 based electrochromic device (ECD) is becoming the mostly-used one for smart windows, however, in WO3 tinted state, the intrinsically deep blue and strong infrared (IR) light absorption, resulting in a habitant discomfortableness and also an enhanced thermal gain from the heated device surface, severely restrict its diversified applications. Herein, we firstly conduct a kind of columnarly-grown and amorphous WO3 film with high concentration of niobium doping (Nb/(Nb+W) = 54.1 at.%). This film is fabricated via sputtering a home-made ceramic target, and subsequently improved by a rapid thermal annealing (RTA) process. Comparing with the conventional WO3, the newly-established system owns not only basic EC properties comprising large light modulation range, fast response, and long-term electrochemically cyclic stability, but also a neutral-tinted color (a* = −1.5, b* = 1.0) as well as a relatively lower IR absorption (63.7 %) in tinted state. These outstanding performances enable the assembled ECD with neutral color (a* = 0.6, b* = −2.7) and a higher energy efficiency in cutting down the interior space air temperature about 4.3 °C in its fully-tinted state, underscoring the potential of the tungsten-niobium-oxygen for commercially viable ECDs.

Powering Smart Windows: Unveiling Crystal Structure and Al-Ion Storage Correlation in WO3 Electrode Via XRD for Aqueous Al-Ion Battery with Real-Time Energy-Level Indication

The potential for using aqueous-based electrochromic batteries in colour indicator energy storage devices, particularly energy-efficient buildings and wearable technology with visible energy level performance, is significant. Unfortunately, many materials that are able to change colour upon ion intercalation have low optical contrast and capacity. To address this, multivalent ions can be utilized, providing multiple electrons and thus increasing optical contrast and capacity. This work presents a novel Al-ion electrochromic battery with WO3 as the cathode material, Al metal as the anode and AlCl3 as the aqueous electrolyte. X-ray diffraction results demonstrate the high structural stability of the optimized WO3 film (~170 nm), which only experienced a 0.69 pm change during Al-ion intercalation and de-intercalation. A DC-EES device with two cathodes on either side of an Al metal film anode exhibited superior performance, with an open circuit potential of 1.2 V and a capacity of 200 mAh/m2 at 0.5 A/m2. Additionally, the device displayed excellent electrochromic behaviour upon discharge, achieving an optical contrast of ~70%. These results illustrate the promising potential of Al-ion electrochromic batteries as a cost-effective and safe option for applications such as smart windows and energy storage displays in modern infrastructures. Figure 1

Electrochromic properties of Ni or Ti single-doped and Ni-Ag or Ti-Ag binary-doped WO3 thin films

In this paper, the single-doped (Ni-WO3 and Ti-WO3) and binary-doped (Ni-Ag-WO3 and Ti-Ag-WO3) amorphous WO3 thin films were respectively prepared on indium-doped tin oxide conductive glass substrates by radio-frequency magnetron sputtering. Among the single-doped WO3 thin films, the electrochromic (EC) properties of films can be enhanced by the presence of Ni or Ti. However, the low conductivity of WO3 limits its application in EC devices. A novel approach was proposed to ameliorate this disadvantage which has not been explored, that is, the highly conductive Ag was introduced into WO3 films with Ni or Ti. The experimental results show that the addition of Ag reduces the charge transfer resistance and shortens the ion diffusion path length, thereby improving the EC response time (Ni-Ag-WO3: tb=1.16 s; Ti-Ag-WO3:tc=4.14 s), electrochemical active area, and ion diffusion coefficients (Ni-Ag-WO3: Da =3.032 × 10−9 cm2/s; Ti-Ag-WO3: Dc=7.737 × 10−9 cm2/s). Based on these work, the feasibility of Ni-Ag and Ti-Ag binary-doped WO3 films was confirmed. These discoveries contribute to the exploration of preparing high-performance WO3 EC films for the practical applications of EC devices such as in automatic anti-glare rearview mirrors.

SiCw strengthened WC-Y2O3 composite and its electrochemical corrosion in aggressive medium

The mechanical properties and corrosion resistance of binderless tungsten carbide largely determine its performance and lifespan in complex industrial environments. Herein, WC-1Y2O3-xSiCw (x = 0, 0.5, 1, and 2 wt%) was prepared using the hot-press sintering. The mechanical properties of the composite, as well as the corrosion resistance of optimized mechanical properties in an aggressive medium (0.05 M H2SO4 and 1 M NaOH) were investigated. Results showed that SiCw could significantly increase mechanical properties of the composite, but excessive content (2 wt%) leads to whiskers aggregation and lower properties. The flexural strength and fracture toughness are all increased to varying degrees with whisker content. WC-1Y2O3-0.5SiCw possesses the optimal mechanical properties. The Vickers hardness, fracture toughness, and flexural strength are 2331.24 HV30, 8.59 MPa·m1/2, and 1727 MPa, respectively. Electrochemical tests show that the corrosion resistance of the WC-1Y2O3-0.5SiCw in H2SO4 is higher than that in NaOH solution, and its corrosion resistance in H2SO4 solution increases, but that of the NaOH solution decreases with exposure time. Detailed characterization demonstrated that a dense WO3 film forms in the H2SO4 solution, resulting in a flatter corrosion surface. In contrast, the WC phase in NaOH solution dissolves, thus leading to the accelerated corrosion rate.

Amorphous/ordered porous crystalline stacked WO3 films fabricated by electrodeposition-assisted sol–gel and its application to electrochromic devices

WO3 is currently the most widely used and promising electrochromic material, but the preparation of WO3 thin films with excellent optoelectronic properties is a formidable challenge. The amorphous WO3/ordered porous crystalline WO3 stacked films (a-WO3/OP-c-WO3) were prepared by electrodeposition-assisted sol–gel method with polystyrene (PS) spheres as the templates. The microstructure and properties of a-WO3/OP-c-WO3 films were significantly influenced by electrodeposition time (150 s ∼ 300 s). The optimized a-WO3/OP-c-WO3 films exhibited even better electrochromic properties (at 633 nm) with high cycling stability, superior optical modulation (53.9 %), fast response time (3.9 s/6.1 s for bleaching /coloration), and especially high coloration efficiency (51.3 cm2/C), compared with OP-c-WO3 film (25.1 cm2/C) and a-WO3 film (35.2 cm2/C), which is unique synergistic effect between the top layer of a-WO3 and the bottom layer of OP-c-WO3 film. The ordered porous structure allows the film to fully contact with the electrolyte, thereby providing a greater specific surface area and more active sites for the electrochemical reaction. The electrochromic device (EDC) prototype assembled with a-WO3/OP-c-WO3 laminated film as the electrochromic layer has better electrochromic properties.

Femtosecond laser thinning for resistivity control of tungsten ditelluride thin-films synthesized from sol-gel deposited tungsten oxide

In this work we present a route for fabricating WTe2 thin-films together with femtosecond laser post processing, enabling to finely control the conductivity. First, we produce amorphous films of WO3 on Si by spin-coating a sol-gel precursor followed by a consolidating annealing and a reduction process in partial H2 atmosphere, leading to porous metallic tungsten cluster layers. To achieve WTe2, the films were exposed to the chalcogen vapours by isothermal closed space vapor transport. The formation of a tungsten ditelluride film composed of piled crystals could be confirmed and a gradient of surface rich Te identified through hard X-ray photoelectron spectroscopy. Finally, it is demonstrated that resistivity can be changed from 0.2 mΩ.m to 1 mΩ.m, while keeping the material characteristics. An anisotropic conductivity can be induced by direct selective thinning with fs laser writing (350 fs pulse duration, 515 nm laser wavelength) of 1D stripes. The obtained results, demonstrate that laser processing is a promising thin-film post-processing technique that can be applied to 2D transition metal dichalcogenide thin films.

Optical and Electrical Characterization of Crystallized M:WO3 (Cu, Fe, Ni) Films in Acidic Medium

Pure and metal (M) doped [copper (Cu), nickel (Ni) and iron (Fe)] WO3 films have been produced on In:SnO2 (ITO) slides by using facile chemical bath deposition and then annealed at 500 oC for 2 h. Structural, morphological, electrical and optical properties of the produced WO3-based films were examined. Monoclinic WO3 phase were observed in all the samples, and the peak intensities were decreased by metal inclusion with heterogeneous film growth on ITO substrate. Slight shifts from defect related emission peaks (blue and green) were observed in metal-substituted WO3 samples from PL study. An optical band gap was observed to decrease in M:WO3 samples. The surface resistance values were significantly reduced by metal additives compared to its pure counterpart, especially by the inclusion of nickel ions in WO3. The results indicated that nucleus growth and thereby impurity/defect-related surfaces had a serious effect on the optical and electrical properties of M:WO3 films.

External magnetic field: Enhancing electrochromic efficiency of magnetic metals composited WO3 films prepared by sparking method

This study investigated the impact of magnetic metals (Fe, Ni, and Co) composited WO3 films on the enhancement of electrochromic (EC) performance. These films were fabricated using a one-step sparking method with and without the presence of external magnetic fields (B). The FE-SEM images show that the fluffy morphology (without B) gradually shifts to form a denser surface (with B). This shift is most likely ascribed to the rearrangement of the deposited particles under the magnetic field, leading to an improvement in film reversibility. The Co/W nanocomposite films, among the tested films, exhibited the most significant enhancement, as demonstrated by their reversibility, which increased from 14.4 % (without B) to 73.3 % (with B), and coloration efficiency, which improved from 27.1 cm2/C (without B) to 131.3 cm2/C (with B). These improvements are crucial in the practical application of these materials. Moreover, a minimum resistivity of 0.45 × 10-3 Ωcm and a maximum mobility of 183.12 cm2/Vs were observed in this sample.Subsequently, the cobalt-composited tungsten oxide (CoW) films were deeply investigated in term of their morphological, structural, optical, and electrochromic properties under the influence of various magnetic fields at 0–500 mT, labeled as CoW without B, CoW-200, CoW-300, CoW-400, and CoW-500, respectively. Raman spectroscopy confirmed that the Co3O4 crystalline structure in the CoW-300 sample underwent transformation into a robust WO3 monoclinic phase in the CoW-400 sample. In addition, the XPS analysis of the CoW-400 sample revealed an oxygen vacancy peak, which facilitated electron transport and improved the conductivity. Finally, an excellent 176.0 cm2/C coloration efficiency and 87.2 % reversibility was obtained for the Co/W composite film prepared by the sparking method under a magnetic field of 400 mT, which is a potential material for electrochromic devices.

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