Electrochromic properties of tungsten oxide nanowires fabricated by electrospinning method

In this work, we report electrochromic properties of polycrystalline WO₃ nanowire electrodes fabricated on an indium tin oxide (ITO)-coated substrate by electrospinning method. The electrochromic and electrical properties of the electrospun WO₃ nanowire electrodes are investigated and compared with those of conventional WO₃ thin film electrodes. As a result, the one-dimensional WO₃ nanowires show faster charge transfer and optical responses with a bleaching time of 1.0 s and a coloring time of 4.2 s than the thin film electrodes. The coloration efficiency of the electrospun WO₃ nanowires is also greater (56 cm²/C) by 21% than the thin film along with an improved memory effect after coloring process.     

Stability of gasochromic WO3 films

Gasochromic films consist of an electrochromic layer such as WO₃ and a very thin catalyst coating, like Pt. Hydrogen is dissociated on the catalyst into H atoms, which color the electrochromic film. A complete bleaching can be achieved in oxygen, whereas flushing with argon or evacuating the sample leads to a comparatively slow and incomplete bleaching. We discuss two kinds of aging processes, i.e. catalysed poisoning by reactants in air and a change in the water content of the WO₃. Poisoning by air increases the time needed for coloring in H₂ and bleaching in O₂ or in Ar. From results with variable WO₃ film thicknesses, we conclude that poisoning results from adsorption of a blocking species on the interior surfaces of the WO₃ pores and not on the catalyst. The adsorption process is accelerated by the catalyst. After drying the device at 100°C in vacuum, there was a severe decrease in the coloring and bleaching rates due to a reduction of the diffusion in the WO₃. Furthermore, the coloration at steady state was more intense. The variation of the water content of the WO₃ was attempted by exposing it to dry or humid atmospheres and was investigated by IR spectroscopy. No changes in water content could be detected, and no significant change in the coloration velocity could be found. To demonstrate the long-term stability of the film, a 1.1 m×0.6 m large window was switched 20,000 times at 20°C over 2 yr without any significant change in performance.     

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO₃) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm² C⁻¹ (pure WO₃) to 27 cm² C⁻¹ for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO₃ films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.     

Highly selective hydrogen sensing of Pt-loaded WO3 synthesized by hydrothermal/impregnation methods

Unloaded and 0.25–1.0 wt% Pt-loaded WO₃ nanoparticles were synthesized by hydrothermal method using sodium tungstate dihydrate and sodium chloride as precursors in an acidic condition and impregnated using platinum acetylacetonate. Pt-loaded WO₃ films on an Al₂O₃ substrate with interdigitated Au electrodes were prepared by spin-coating technique. The response of WO₃ sensors with different Pt-loading concentrations was tested towards 0.01–1.0 vol% of H₂ in air as a function of operating temperature (200–350 °C). The 1.0 wt% Pt-loaded WO₃ sensing film showed the highest response of ∼2.16 × 10⁴ to 1.0 vol% H₂ at 250 °C. Therefore, an operating temperature of 250 °C was optimal for H₂ detection. The responses of 1.0 wt% Pt-loaded WO₃ sensing film to other flammable gases, including C₂H₅OH, C₂H₄ and CO, were considerably less, demonstrating Pt-loaded WO₃ sensing film to be highly selective to H₂.     

Opportunities and challenges in science and technology of WO3 for electrochromic and related applications

Since the discovery of the electrochromic (EC) effect in transition metal oxides in the mid-1960s, intense research and development work spanning four decades has led to many exciting developments in the science and technology of this class of materials. Tungsten oxide (WO₃) has emerged as one of the key materials, not only for EC devices, but also for many other related applications. After many years of technology development efforts, WO₃-based EC “smart windows” have finally emerged as a viable commercial product. In spite of enormous progress being made on the structural, electrical, and optical properties of amorphous and crystalline WO₃, a detailed understanding of the EC effect in this material still remains somewhat qualitative. Although theoretical models based on intervalence charge transfer and polaron formation have been widely accepted, these models are still unable to explain some of the experimental results on the coloration phenomena. The coloration in WO₃ is a structure-sensitive phenomenon, and excess electrons can be either localized or delocalized. The presence of structural defects such as oxygen vacancies, impurities, and degree of disorder plays a crucial role in determining the coloration efficiency. Although significant progress has been made in recent years on the calculation of electronic structure and defect properties of both amorphous and crystalline WO₃, the structural complexity of the material presents many challenges and opportunities for theoretical computation. The unique ability to induce bistable optical and electrical properties in WO₃ by a variety of excitation sources has led to many devices of significant technological interest. Some of the applications currently being pursued include the photoelectrochemical cell for solar energy conversion and storage; photoelectrochemical splitting of water to generate hydrogen; chemical and biological sensors based on the gasochromic effect; photo- and electrocatalysts for a variety of chemical reactions; demonstration of high-temperature (91 K) superconductivity in WO₃ doped with H, Na, and K; synthesis of a new class of hybrid organic/inorganic (WO₃) materials; and application in ultra-high-resolution electron beam lithography. The emergence of nanostructured WO₃ in recent years will undoubtedly provide new opportunities and significant impact to many of these technologies. This paper presents a brief overview of some of the key research issues the author believes will impact the science and technology of this exciting material.     

Comparative studies of “all sol–gel” electrochromic windows employing various counter-electrodes

Electrochromic (EC) “smart” windows for buildings represent an effective way to modulate the intensity of incoming solar radiation. While it is accepted that WO₃ films represent the best option for the working electrode, the choice of the best counter-electrode is still debatable. Optical properties of counter-electrodes such as Ce, Fe, V and Sn oxides are presented. Electrochromic windows were made with a sol–gel WO₃ active colouring film (150°C), Ce, Fe, V oxide counter-electrodes and a sol–gel organic–inorganic hybrid (Li⁺ormolyte) ion conductor. The electrochromic responses of these devices predicted from the charge capacities, photopic transmittances and coloration efficiencies of individual films are compared with measured values.     

Integrated low-emittance–electrochromic devices incorporating ZnS/Ag/ZnS coatings as transparent conductors

We have prepared and tested integrated low-emittance–electrochromic devices using ZnS/Ag/ZnS coatings as transparent electrodes and WO₃ films as electrochromic layers. These devices exhibit adequate coloration and can withstand more than 1000 bleaching-coloration cycles, provided that the metal layer is protected from the liquid electrolyte by a combination of thick dielectric films (ZnS/WO₃). We have also predicted the optimum configuration of the WO₃/ZnS/Ag/ZnS/Glass stack that maximizes transmission in the visible. Integration of low emittance and electrochromic films into one device could improve the performance and reduce the cost of electrochromic windows.     

Study on photocurrent of bilayers photoanodes using different combination of WO3 and Fe2O3

Bilayer photoanodes were prepared onto glass substrates (FTO) in order to improve generated photocurrents using UV-vis light by water splitting process. A comparative study of photocatalytic was performed over the films surface using Fe₂O_3, WO₃ and mixture of bicomponents (Fe₂O₃:WO₃). Different types of films were prepared using Fe₂O_3, WO₃ and bicomponents (mixture) on FTO substrates. The films were grown by sol gel method with the PEG-300 as the structure-directing agent. The photo-generated of the samples were determined by measuring the currents and voltages under illumination of UV-vis light. The morphology, structure and related composition distribution of the films have been characterized by SEM, XRD and EDX respectively. Photocurrent measurements indicated surface roughness as the effective parameter in this study. The deposited surfaces by bicomponents or mixture are flat without any feature on the surface while the deposited surfaces by WO₃ appears rough surface as small round (egg-shaped particles) and cauliflower-like. The surface deposited by Fe₂O₃ show rough no as well as WO₃ surface. The deposited surfaces by WO₃ reveal the higher value of photocurrent measurement due to surface roughness. Indeed, the roughness can be effective in increasing contact surface area between film and electrolyte and diffuse reflection (light scattering effect). The solution (Fe₂O₃:WO₃) shows the low photocurrent value in compare to WO₃ and Fe₂O₃ hat it may be due to decomposition the compound at 450 ± 1 °C to iron-tungstate Fe₂(WO₄)₃.     

Electrochromism in amorphous lithium tungsten oxide films

A small ac-signal impedance analysis was utilized to characterize the surface property of an ITO electrode and Li transport in an Li_xWO₃ electrode. The as-deposited ITO film has a surface layer with extremely high carrier density, which could be easily removed by mechanical polishing. Based on the kinetic model of Li_xWO₃, the diffusion coefficient of Li transport was obtained. It depends on the various deposition conditions of WO₃ films and the composition of the electrolyte used. The response of Li_xWO₃ film showed a gradual decrease and reached a certain equilibrium after repeated cycling (more than 10⁴ times). The examination of such a degradation phenomenon leads to the conclusion that there are two active sites in the WO₃ structure, which are available for the Li ion; one for coloration (xLi⁺ + WO₃ + xe⁻ ↔ Li_xWO₃), and the other for an ion exchange reaction expressed by WO-H+Li⁺ ↔ WO-Li+H⁺. The mechanism of these phenomena are further discussed. Impedance analysis has been proved to be very sensitive and applicable enough to the quantitative characterization of ITO and Li_xWO₃ electrodes involved in the electrochromic device.     

Application of WO3 thin films for enhancement of photolysis in AgCl

A double-layer AgCl–WO₃ structure was employed to produce photochemical hydrogen for doping of an AgCl film. Atomic photochemical hydrogen, detached under the action of light from hydrogen donor molecules, previously adsorbed on the WO₃ surface, migrated through the WO₃ film into the AgCl film, which provided doping of the AgCl surface and yielded hydrogen sensitization simultaneous to illumination and yielded the enhancement of photochromism in the AgCl films. The atomic hydrogen played the role of a reducing agent and triggered the formation of sensitization centers on the halide surface, which in turn facilitated the growth of silver clusters and colloids under the action of light. The double-layer AgCl–WO₃ structure realizes the idea of two-stage catalysis: first the oxide surface catalyses hydrogen production under the action of light, then the photochemical hydrogen atoms act as catalysts during the photolysis of the halide.     

Ag grid induced photocurrent enhancement in WO3 photoanodes and their scale-up performance toward photoelectrochemical H2 generation

The hydrogen generation from photoelectrochemical (PEC) water splitting under visible light was investigated using large area tungsten oxide (WO₃) photoanodes. The photoanodes for PEC hydrogen generation were prepared by screen printing WO₃ films having typical active areas of 0.36, 4.8 and 130 cm² onto the conducting fluorine-doped tin oxide (FTO) substrates with and without embedded inter-connected Ag grid lines. TiO₂ based dye-sensitized solar cell was also fabricated to provide the required external bias to the photoanodes for water splitting. The structural and morphological properties of the WO₃ films were studied before scaling up the area of photoanodes. The screen printed WO₃ film sintered at 500 °C for 30 min crystallized in a monoclinic crystal structure, which is the most useful phase for water splitting. Such WO₃ film revealed nanocrystalline and porous morphology with grain size of ∼70-90 nm. WO₃ photoanode coated on Ag grid embedded FTO substrate exhibited almost two-fold degree of photocurrent density enhancement than that on bare FTO substrate under 1 SUN illumination in 0.5 M H₂SO₄ electrolyte. With such enhancement, the calculated solar-to-hydrogen conversion efficiencies under 1 SUN were 3.24% and ∼2% at 1.23 V for small (0.36 cm²) and large (4.8 cm²) area WO₃ photoanodes, respectively. The rate of hydrogen generation for large area photoanode (130.56 cm²) was 3 mL/min.     

Fast, self-supplied, all-solid photoelectrochromic film

This paper describes the fabrication and the characterization of a new nanostructured and self-supplied photoelectrochromic device. The main properties of this film are its all-solid nature, its fast coloration time as well as its fast bleaching time. The photoelectrochromic film was manufactured by coating dye functionalized TiO₂ nanoparticles (dye-TiO₂) on a layer of WO₃ nanoparticles. In order to improve their electrical conductance, both the dye-TiO₂ and WO₃ layers were properly doped with single wall carbon nanotubes (SWNT) bearing COOH groups. A layer of PEDOT/PSS was cast between the dye-TiO₂ layer and the ITO counter electrode, without the use of any fluid component. When exposed to the light, SWNT doped dye-TiO₂ layer generates electrons that reduce the WO₃ layer. As a consequence of this redox reaction, the film changes its color if the two external electrodes are not short-circuited. On the contrary, a fast bleaching of the device can be achieved by shortcircuiting the electrodes.     

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO₃/TiO₂ nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO₂ and WO₃ layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO₃ microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO₂ nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO₃ and TiO₂ films show thickness and microstructure dependence. Among individual WO₃ layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO₂ nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO₃-wedge type layer with a layer of TiO₂ nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO₃ layers. The highest photocurrent that is measured in the combinatorial library of porous WO₃/TiO₂ films is as high as 0.11 mA/cm². Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.     

Residual charges and infrared absorption in electrochromic WO3 films prepared by hydrogen-introduced sputtering

Electrochromic WO₃ films were prepared by rf−sputtering in atmosphere consisting of Ar/H₂/O₂ mixed gas. The as-sputtered films require several times of injection/extraction of ions (the aging) for obtaining reversible coloration/bleaching. After the aging, there are ions (protons) remaining in the films, namely residual charges. From the results of IR absorption of the as-sputtered and aged films, the residual charges contribute to create OH and HOH bonds. Hydrogen introduced in the films during sputtering is transformed only into OH bonds combining with unstable oxygen in the films. The introduced hydrogen is considered to suppress the growth of grain in WO₃ films from AFM observation.     

Improved electrochromic devices with an inorganic solid electrolyte protective layer

The durability problem of electrochromic devices (ECD) in Li ion-conducting electrolytes may be due to the degradation of the ECD electrode matrix and irreversibly reacting Li ions during cycling. With this hypothesis, we investigated the performance of WO₃ film with an inorganic solid electrolyte, lithium phosphorous oxynitride (LiPON), protective layer and of the ECD composed of WO₃ and V₂O₅ with a LiPON protective layer prepared by RF magnetron sputtering. WO₃ and ECD (glass/ITO/V₂O₅/LiPON/electrolyte/LiPON/WO₃/ITO/glass) with a protective layer not only showed improved durability with continuous potential cycling, but also demonstrated a good memory effect under voltage-off, good response times and high coloration efficiency (CE). Our results demonstrated that LiPON layers are electrochemically stable, enhance the electrochromic properties in Li ion-conducting electrolytes, and can be used as a protective layer for ECD.     

Electrochromic properties of nanocomposite WO3 films

A new nanocomposite WO₃ (NWO) film-based electrochromic layer was fabricated by a spray and electroplating technique in sequence. An indium–tin oxide (ITO) nanoparticle layer was employed as a permanent template to generate the particular nanostructure. The structure and morphology of the NWO film were characterized. The optical and electrochromic properties of the NWO films under lithium intercalation are described and compared to the regular WO₃ film. The NWO films showed an improved cycling life and an improved contrast with compatible bleach-coloration transition time, owing to the larger reactive surface area. The nanocomposite WO₃ film-based electrochromic device (NWO-ECD) was also successfully fabricated. Most importantly, the NWO film can be prepared on a large scale directly onto a transparent conductive substrate, which demonstrates its potential for many electrochromic applications, especially, smart windows, sunroof and displays.     

Sensitive and rapid hydrogen sensors based on Pd–WO3 thick films with different morphologies

WO₃ loaded with noble metals is well-known to be sensitive to reducing gases and can be used as hydrogen sensor. This paper presents a simple and attractive method concerning the preparation of hydrogen sensors based on Pd-loaded WO₃ nanocomposites with different morphologies. The influences of the morphology of WO₃ and the palladium growth on its surface on the hydrogen sensing performances are studied. WO₃ nanospheres, nanowires and nanolamellae were synthesized by different methods starting from the same precursor (H₂WO₄·nH₂O) which has been obtained by acidification of sodium tungstate (Na₂WO₄). The prepared WO₃ nanostructures were modified with the Pd by dispersing them in a PdCl₂ containing solvent using sonication (giving Pd-WO₃ inks). The sensors were prepared by screen-printing thick films (∼10 μm) of these inks on alumina substrates fitted with gold electrodes. The response of Pd-loaded WO₃ sensors to hydrogen was checked for the different morphologies at working temperatures ranging from 180 to 240 °C. The sensors prepared from nanolamellae showed the highest response while the nanowires presented the shortest response time to hydrogen.     

Preparation conditions of sputtered electrochromic WO3 films and their infrared absorption spectra

Tungsten oxide films were prepared by rf sputtering in an argon-oxygen atmosphere from W and WO₃ targets. To bring about reversible electrochromic (EC) characteristics, as-deposited films require an aging process (i.e. cycles of injection/ejection of charge carriers). The infrared absorption band at around 3300 cm⁻¹ increases during the aging process, and it is assigned as OH stretching vibrations of absorbed water.By coloration after aging, the 3300 cm⁻¹ band decreases, and a new band appears at 2400 cm ⁻¹. The latter band is considered to be to the stretching mode of radicals incorporated in the WO₃ matrix. At low coloration levels, the 2400 cm⁻¹ band increases slightly with injected charge, and a coloration mechanism other than the usual double injection model may be considered.The coloration efficiency depends on the preparation conditions. Its maximum value is the same for films prepared from W and WO₃ targets, and is 60 cm²/C at a wavelength of 600 nm. When a tungsten target is used, the substrate temperature is low and the deposition rate is high compared with a WO₃ target.     

Hydrogen evolution reaction on single crystal WO3/C nanoparticles supported on carbon in acid and alkaline solution

Single crystal tungsten oxide (WO₃) nanoparticles were prepared via a microwave-assisted method. Electrochemical activity for hydrogen evolution reaction (HER) on WO₃ supported on carbon black (WO₃/C) electrocatalyst was first studied in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH) at room temperature. The overall experimental results revealed that the electrocatalytic activity for HER on WO₃/C is one order magnitude higher than those obtained with carbon black in 0.5 M H₂SO₄ and is six times than in the case of carbon black in 1.0 M KOH. These results demonstrated that WO₃ could enhance the electrocatalytic activity for hydrogen evolution reaction in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH). On the other hand, the kinetic reaction mechanisms were discussed on WO₃/C electrocatalysts and carbon black in acid solution and alkaline solution for HER. Consequently, the rate-determining step changed from Tafel to Volmer due to the incorporation of WO₃ in acid solution. However, the rate-determining step carries through Tafel reaction on both electrocatalysts in alkaline solution though WO₃ was introduced.     

An all-solid-state electrochromic device based on NiO/WO3 complementary structure and solid hybrid polyelectrolyte

An all-solid-state electrochromic (EC) device based on NiO/WO₃ complementary structure and solid polyelectrolyte was manufactured for modulating the optical transmittance. The device consists of WO₃ film as the main electrochromic layer, single-phase hybrid polyelectrolyte as the Li⁺ ion conductor layer, and NiO film as the counter electrochromic layer. Indium tin oxide- (ITO) coated glass was used as substrate and ITO films act as the transparent conductive electrodes. The effective area of the device is 5×5 cm². The device showed an optical modulation of 55% at 550 nm and achieved a coloration efficiency of 87 cm² C⁻¹. The response time of the device is found to be about 10 s for coloring step and 20 s for bleaching step. The electrochromic mechanism in the NiO/WO₃ complementary structure with Li⁺ ion insertion and extraction was investigated by means of cyclic voltammograms (CV) and X-ray photoelectron spectroscopy (XPS).