Amorphous WO3 Thin Films Research Articles

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.

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.

Optimization of Optical Modulation in Amorphous WO3 Thin Films

Optimization of Optical Modulation in Amorphous WO3 Thin Films

Amorphous WO3 thin films designed as gigahertz/terahertz dielectric lenses

Herein, tungsten oxide thin films comprising excess oxygen are treated as optical resonator suitable for gigahertz/terahertz applications. WO3 thin films which are prepared by the thermal evaporation technique under a vacuum pressure of 10− 5 mbar are structurally, compositionally and optically evaluated. The amorphous WO3 films which showed high transparency permit electronic transitions within an indirect allowed energy band gap of 3.05 eV. The band gap comprised energy band tails of width of 190 meV. Four dominant dielectric resonators centered in the infrared (IR), visible (VIS) and ultraviolet (UV) ranges of light are detected. Analysis of the optical conductivity in accordance with the Drude-Lorentz approaches have shown that the drift mobility of free holes in this amorphous layer can be as large as 5.61 cm2/Vs an as low as 1.59 cm2/Vs when exposed to IR and UV light signals, respectively. In addition, the gigahertz/terahertz cutoff frequency (\({f}_{co}\)) spectra demonstrated \({f}_{co}\) values in the gigahertz frequency domain when exposed to IR light. Excitations with light signals in the VIS and UV spectral ranges allow \({f}_{co}\) values that extends from 0.7 to 40.0 THz. The wide range of tunability of the WO3 dielectric resonators nominates them as dielectric lenses suitable for optical communications.

Fast responding and highly reversible gasochromic H2 sensor using Pd-decorated amorphous WO3 thin films

For the safe use of H2 gas, an eco-friendly energy source in the spotlight but highly explosive at the same time, the need for gas sensors for early H2 detection has been highly increasing. Although promising H2 sensors have been developed with different sensor principles, most of them accompany bulky and large measurement systems which limit their further application to everyday life. In this study, we propose gasochromic H2 sensors based on Pd nanoparticle-decorated amorphous WO3 film (Pd-aWO3) that can visualize H2 detection by the rapid color change of the film with high reversibility using dissociated H+ ions. In specific, extremely fast H+ diffusion coefficient through amorphous WO3 film prepared by electron beam evaporation enables rapid and highly reversible operation that could not be achieved by previously reported crystalline WO3 film. As a result, Pd-aWO3 could exhibit a transmittance change of 57% and super rapid recovery time (16 s), which is the fastest compared to previously reported gasochromic H2 sensors to the best of the authors’ knowledge. Moreover, in-situ surface analysis using ambient pressure X-ray photoelectron spectroscopy revealed detailed coloring and beaching mechanisms. This study can provide promising perspectives to future advances in gasochromic H2 sensors toward everyday life application.

Characterization of nanoparticulated WO3 electrochromic thin films prepared via precipitation reaction of peroxotungstic acid solution

Amorphous WO3 thin films with a nanoparticulated morphology were fabricated using a combination of electrodeposition and electrophoretic deposition via the precipitation reaction of a peroxotungstic acid (PTA) solution. Due to the presence of the precipitated WO3 nanoparticles in the aged PTA solution, the electrophoretic deposition and electrodeposition occur simultaneously, which results in a grainy surface morphology in which nano-sized WO3 particles are embedded on a smooth WO3 thin film. The WAT-5 sample prepared with a PTA solution aged for 5 days exhibited faster ion intercalation/de-intercalation kinetics (coloration time of 3 s and bleaching time of 1 s), a greater transmittance modulation (68.36%), and a higher coloration efficiency (41.29 cm2/C) with better cyclic stability (91% retention during 300 cycles) than a WO3 thin film fabricated using the PTA solution without the aging process. The nanoparticulated morphology enhanced the active reaction area by expanding the interface between the electrolyte and the electrochromic material. Such results indicate that the electrochromic characteristics of WO3 thin films can be improved simply by adjusting the aging period of the PTA solution.

Electrochromic response and porous structure of WO3 cathode layers

Maximizing the electrochromic response of tungsten oxide-based systems demands highly porous electrode layers that facilitate the incorporation of electrolyte cations during the reduction process. In this work, amorphous and porous WO3 thin films were grown on indium tin dioxide glass substrates by magnetron sputtering at oblique angles at two different plasma gas pressures. Remarkably, the film that showed higher porosity presented a worse electrochromic response in terms of durability, time response and charge density capacity. This result is analyzed and explained on the basis of the features of the porous structure of the films: While the typical nanostructure developed at low pressures possesses large and connected pore voids with few ramifications, the nanostructure generated at a higher pressure presents a rather sponge-like porous structure with numerous and small well-connected voids. A general discussion on the role of the porous structure and, particularly, on the accessible pore volume and area is carried out. It is concluded that not only the accessible pore volume, defining the volume of electrolyte that stays inside the layer, but also the accessible pore area, which defines the efficiency of the incorporation/release of Li+ cations within the electrode material, determine the efficiency and reversibility of the electrochromic response.

Towards enhanced durability of electrochromic WO3 interfaced with liquid or ceramic sodium-based electrolytes

The reversible intercalation of sodium ion into tungsten oxide WO3 appears as an interesting alternative to hydrogen or lithium ion reduction in order to get the characteristic transition from clear transparent to bluish coloration in electrochromic devices, but it has been comparatively less considered. In order to address further viable all-ceramic devices based on sodium ion intercalation and overcome the issue of WO3 degradation in aqueous media, three configurations of WO3 thin film-based electrochromic half-cells were tested, namely in (i) aqueous acidified Na2SO4 electrolyte, (ii) room temperature ionic liquid BEPipTFSI electrolyte and (iii) aqueous acidified Na2SO4 electrolyte associated with an amorphous NASICON-cap onto WO3 film. We compared their electro-optical characteristics during 100 voltammetry cycles, including the Na+ diffusion coefficient calculated through electrochemical method. It is found that sputter-deposited amorphous WO3 thin films on transparent conductive substrates is promising for electrochromic all-ceramic devices based on Na ion insertion. Electrochemical characterization in aqueous medium is not relevant to extract relevant data when WO3 is in direct contact with the electrolyte as the electrochromic film is progressively dissolved. In contrast, WO3 capped with oxide amorphous Na-ion conductor readily operates over 100 cycles, the capping layer preventing degradation by the aqueous medium. Alternatively, ionic liquid does not degrade the WO3 film and can be employed to efficiently characterize the electro-optical performances.

Characteristics of amorphous WO3 thin films as anode materials for lithium-ion batteries

Amorphous WO3 thin films were fabricated on copper substrate by the magnetron sputtering technique. X-ray diffraction and X-ray diffraction pattern confirm the formation of phase is pure WO3. The as-prepared WO3 thin films as anode materials exhibit good electrochemical properties such as high reversible capacity, good cyclic stability and high rate capability in lithium-ion batteries. Specially, the WO3 electrode can exhibit high discharging capacity of about 142 μA h cm−2 (585 mA h g−1) even at the 100th electrochemical cycle at the current density of 24.3 μA h cm−2 (100 mA g−1). The redox couples, as shown in the CVs analysis, at around 1.02/2.14, 0.41/1.41 and 0.15/1.09 V in the 2nd and 3rd cycles are possibly attributed to the formation of the transition compound LixWO3 during lithium-ion inserting into and extracting from the WO3 thin film electrode. The good electrochemical properties of WO3 thin film make it become an excellent Li-ion storage material for rechargeable lithium batteries.

Conversion of WO3 thin films into self-crosslinked nanorods for large-scale ultraviolet detection.

We heat-treated an amorphous large-area WO3 thin film to synthesize high-density, high-quality WO3 nanorods. The WO3 nanostructures were effective, especially in reducing gas (hydrogen and helium) atmospheres. By electron microscopy analysis, we confirmed that the thermodynamic energy for forming oxygen vacancies in the [020] direction was low. We could apply self-crosslinked WO3 nanostructures to practical sensor device fabrication by simply placing the electrodes without complex processes such as transfer and e-beam lithography. It was used for the production of a UV detector, which reacted very fast (∼0.316 s) and was very sensitive to the actual UV-C (261 nm) wavelength. Also, plasmon-based light absorption through the Ag nanoparticle coating resulted in more than 350-fold improvement in the on/off process during UV-C irradiation.

Preparation of Orthorhombic WO3 Thin Films and Their Crystal Quality-Dependent Dye Photodegradation Ability

Direct current (DC) magnetron sputtering deposited WO3 films with different crystalline qualities were synthesized by postannealing at various temperatures. The in-situ DC sputtering deposited WO3 thin film at 375 °C exhibited an amorphous structure. The as-grown WO3 films were crystallized after annealing at temperatures of 400–600 °C in ambient air. Structural analyses revealed that the crystalline WO3 films have an orthorhombic structure. Moreover, the crystallite size of the WO3 film exhibited an explosive coarsening behavior at an annealing temperature above 600 °C. The density of oxygen vacancy of the WO3 films was substantially lowered through a high temperature annealing procedure. The optical bandgap values of the WO3 films are highly associated with the degree of crystalline quality. The annealing-induced variation of microstructures, crystallinity, and bandgap of the amorphous WO3 thin films explained the various photoactivated properties of the films in this study.

In Situ Monitoring of Lateral Hydrogen Diffusion in Amorphous and Polycrystalline WO3 Thin Films

AbstractThin films of electrochromic materials such as tungsten trioxide (WO3) are essential components of electrochromic devices such as “Smart Windows.” The transport of charge carriers in WO3 thin films has been investigated intensively over the past decades, yielding a wide spread of diffusion coefficients of hydrogen in WO3. Most of those analyses are based on measurements by electrochemical methods. However, such analyses are only applicable and the results reliable in cases where diffusive transport is the rate‐determining process. Here, WO3 thin films of different morphology are prepared by electron‐beam evaporation of WO3 powder. The combined lateral diffusion of protons and electrons is investigated inside amorphous and polycrystalline WO3 thin films by in situ transmission spectroscopy during electrochemical hydrogen insertion into the film. The insertion of the protons is carried out locally and lateral diffusion of H takes place in the film plane. The induced change of the film absorbance at a wavelength of (637 ± 15) nm is monitored and spatially and temporally resolved. The results reveal a concentration‐dependent diffusion process of hydrogen in both, amorphous and polycrystalline WO3, which are critically discussed in the context of existing sata from literature.

HRTEM Microstructural Characterization of β-WO3 Thin Films Deposited by Reactive RF Magnetron Sputtering.

Though tungsten trioxide (WO3) in bulk, nanosphere, and thin film samples has been extensively studied, few studies have been dedicated to the crystallographic structure of WO3 thin films. In this work, the evolution from amorphous WO3 thin films to crystalline WO3 thin films is discussed. WO3 thin films were fabricated on silicon substrates (Si/SiO2) by RF reactive magnetron sputtering. Once a thin film was deposited, two successive annealing treatments were made: an initial annealing at 400 °C for 6 h was followed by a second annealing at 350 °C for 1 h. Film characterization was carried out by X-ray diffraction (XRD), high-resolution electron transmission microscopy (HRTEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) techniques. The β-WO3 final phase grew in form of columnar crystals and its growth plane was determined by HRTEM.

Electrochromic response of WO3 and WO3-TiO2 thin films prepared from water-soluble precursors and a block copolymer template

Electrochromic tungsten trioxide (WO3) thin films are attracting renewed attention as transmittance-controllable windows for use in automobile, aircraft, and building applications. In order to achieve high electrochromic performance, high cycle stability, and high reliability, the microstructure and compositional homogeneity of WO3 thin films have to be optimized. In this study, non-doped WO3 and TiO2-doped WO3 thin films were fabricated from water-soluble precursors of tungsten and titanium, and their electrochromic response was investigated. Amorphous WO3 and TiO2-doped WO3 thin films were fabricated by calcining the spin-coated films at 573 K. The use of a PEO-PPO-PEO block copolymer as a porogen facilitated the redox reactions occurring on the thin film/electrolyte interface. Although the effect of TiO2-doping on the cycle stability of WO3 thin films has not been fully elucidated, this study demonstrated that TiO2 doping up to 15 mol% effectively enhanced the cycle stability.

Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films.

Amorphous WO3 thin films are of keen interest as cathodic electrodes in transmittance-modulating electrochromic devices. However, these films suffer from ion-trapping-induced degradation of optical modulation and reversibility upon extended Li+-ion exchange. Here, we demonstrate that ion-trapping-induced degradation, which is commonly believed to be irreversible, can be successfully eliminated by constant-current-driven de-trapping, i.e., WO3 films can be rejuvenated and regain their initial highly reversible electrochromic performance. Pronounced ion-trapping occurs when x exceeds ~0.65 in LixWO3 during ion insertion. We find two main kinds of Li+-ion trapping sites (intermediate and deep) in WO3, where the intermediate ones are most prevalent. Li+-ions can be completely removed from intermediate traps but are irreversibly bound in deep traps. Our results provide a general framework for developing and designing superior electrochromic materials and devices.

Electrochromic properties of amorphous and crystalline WO3 thin films prepared by thermal evaporation technique

The tungsten trioxides thin films were deposited by thermal evaporation method onto indium tin oxides coated onto glass substrates. The structural properties of the films were investigated by X-ray diffractometer and atomic force microscopy techniques. The as-deposited film shows a polycrystalline nature related to indium tin oxides planes and the WO3 planes appears by annealing film to 773K for 2h. The 2D and 3D images were carried out using atomic force microscopy techniques. The film color is converted from transparent to deep blue color after applying electric field, and bleaching occurs by applying reverse electric field. The F-like color centers model was used to investigate the coloration in amorphous films. On the other hand, the crystalline films were characterized by the free carrier absorption mechanism. Both coloration response and coloration efficiency for the amorphous films are greater than crystalline. The coloration efficiency is found to be 31.25cm2/C for the as-deposited films and decreases to 18.3cm2/C by annealing films at 723K.

Correlation between lithium storage and diffusion properties and electrochromic characteristics of WO3 thin films

As essential electrochromic (EC) materials are related to energy savings in fenestration technology, tungsten oxide (WO3) films have been intensively studied recently. In order to achieve better understanding of the mechanism of EC properties, and thus facilitate optimization of device performance, clarification of the correlation between cation storage and transfer properties and the coloration performance is needed. In this study, transparent polycrystalline and amorphous WO3 thin films were deposited on SnO2:F-coated glass substrates by the pulsed laser deposition technique. Investigation into optical transmittance in a wavelength range of 400–800 nm measured at a current density of 130 μA · cm−2 with the applied potential ranging from 3.2 to 2.2 V indicates that polycrystalline films have a larger optical modulation of ∼ 30% at 600 nm and a larger coloration switch time of 95 s in the whole wavelength range compared with amorphous films (∼ 24% and 50 s). Meanwhile, under the same conditions, polycrystalline films show a larger lithium storage capacity corresponding to a Li/W ratio of 0.5, a smaller lithium diffusion coefficient (2×10−12 cm2·s−1 for Li/W=0.24) compared with the amorphous ones, which have a Li/W ratio of 0.29 and a coefficient of ∼2.5×10−11 cm2·s−1 as Li/W=0.24. These results demonstrate that the large optical modulation relates to the large lithium storage capacity, and the fast coloration transition is associated with fast lithium diffusion.

Double-Side Digital Display Devices Based on the Solid-State Electrochromic Effect of the Amorphous WO<sub>3</sub> Thin Films

A double-side digital display device has been fabricated based on the solid-state electrochromic effect of amorphous WO3 thin film in this article. The device simply consists of a transparent anode, a transparent cathode, an amorphous WO3 thin film and a solid electrolyte. The transparent electric wiring has been achieved by the electrochemical corrosion of the designed region of ITO (indium tin oxide) film. The amorphous WO3 thin film as an electrochromic layer is prepared on the ITO film by thermal evaporation deposition. The complex of polyethylene and LiClO4 as a solid electrolyte is used in the devices. This double-side digital display can work well under the normal mobile phone battery of 3.7 V, which is compatible with the traditional driven circuit technology. Our results prove the potential application of the amorphous WO3 thin film on the double-side display devices.

Write-Once Optical Recording Property of Amorphous WO<sub>3</sub> Thin Film and its Relative Mechanism

Write-once optical recording property of the amorphous WO3 (a-WO3) thin film prepared on the glass substrate by thermal evaporation deposition, has been demonstrated by focusing the laser of 514.5 nm with a variable power to a spot with a size of 1-2μm in diameter on the surface of thin film. The mechanisms responsible for the refractive index change of thin film after the laser recording, has also been studied by monitoring the Raman signal change with the increase of the incident laser power, and by characterizing the micro-structures of the pits induced by laser irradiation. As a result, two mechanisms, the laser-induced phase transition and the laser-induced micro-explosion, are proposed to be responsible for the write-once optical recording.

Synthesis of WO3 Nanogranular Thin Films by Hot‐Wire CVD

AbstractBy resistively heating tungsten filaments in a constant air flow under a reduced pressure, nanogranular amorphous WO3 thin films are deposited on glassy carbon substrates. The substrate surface temperature is monitored by a thermocouple. For deposition times of 15 min and longer, the films show crystallization into a monoclinic nanocrystalline WO3 phase due to thermal radiation from the filaments. The thin films thus deposited consist of very small grains with sizes of 8–16 nm for a film deposited in 5 min, and to sizes of 20–42 nm for a film deposited in 60 min. By heating the bottom of the substrate to 950 °C, the grains sinter to nanorods with diameters of 146 ± 26 nm with lengths of up to 750 nm.The morphology, the film thickness, the grain and the nanorod sizes of the depositions are characterized by high‐resolution scanning electron microscopy (HRSEM), and the atomic species present in the films by energy‐dispersive X‐ray spectroscopy (EDS). The number of atoms in the films is determined by Rutherford backscattering spectrometry (RBS). The results are combined with measurements of the physical thicknesses, and thus the mass densities of the deposited films are obtained. X‐ray diffraction (XRD) and Raman spectroscopy are used to characterize the deposited films crystallographically and chemically, respectively.