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.     

Optical properties of sol–gel deposited Al2O3 films

Highly transparent, uniform and corrosion resistant Al₂O₃ films were prepared on stainless-steel and quartz substrates by the sol–gel process from stable coating solutions using aluminum-sec-butoxide, Al(OBu^s)₃ as precursor, acetylacetone, AcAcH as chelating agent and nitric acid, HNO₃, as catalyzer. Films up to 1000 nm thick were prepared by multiple spin coating deposition, and were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), optical spectroscopy and micro Vickers hardness test. XRD of the film heat treated at 400°C showed that they had an amorphous structure. XPS confirmed that they were stoichiometric Al₂O₃. The refractive index (n) and extinction coefficient (k) were found to be n=1.56±0.01 and k=0.003±0.0002 at 600 nm, respectively. The surface microhardness and corrosion resistance investigations showed that Al₂O₃ films improved the surface properties of stainless-steel substrates.     

Optical properties of sol–gel dip-coated Ta2O5 films for electrochromic applications

Amorphous Ta₂O₅ films were prepared by sol–gel dip process on different substrates. The dip-coating technique was used to prepare amorphous Ta₂O₅ films by hydrolysis and condensation of tantalum ethoxide, Ta(OC₂H₅)₅, precursor. Stable coating solutions were prepared using acetic acid as a chelating ligand and catalyzer. Single layer and multi-layered Ta₂O₅ films were fabricated at a dipping rate of 107 mm/min. The microstructure, stoichiometry and optical properties of these films were investigated as a function of the film thickness. Room temperature CV measurements clearly revealed a protonic conductor behavior for Ta₂O₅ films. Optical properties such as refractive index, extinction coefficient and optical band gap value of the Ta₂O₅ films were calculated from optical transmittance measurements. It was found that the refractive index and extinction coefficient values were affected by the thickness of the coatings. The refractive index at a wavelength of 550 nm increased from 1.70 to 1.72 with increasing film thickness. The optical band gap value (3.75±0.12 eV) of the coating was unaffected by the film thickness. These results indicate that sol–gel-deposited Ta₂O₅ films have a promising application as proton conductors in electrochromic devices.     

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.     

Electrochromic properties of Nb2O5 sol–gel coatings

Sol–gel niobium oxide coatings are promising electrochromic materials. The sols have been prepared by a sonocatalytic mixing of NbCl₅ powder, butanol and acetic acid. Thermal analysis (DTA/TG) coupled to mass spectrometry has been performed on Nb₂O₅ precipitates to quantitatively analyse the effluents. Transparent and defect-free single and multilayers coatings have been deposited on ITO-coated glass by a dip-coating process and then calcined between 400°C and 600°C. The coatings structure change from amorphous to crystalline (TT form) and the later ones are highly textured. The films present a reversible and fast insertion/extraction kinetics for Li⁺ ions. After insertion the amorphous coatings present a grey-brown color, while the crystalline ones are dark blue. The maximum charge density exchanged with a three-layer 200 nm thick coating sintered at 600°C was 16 mC/cm² with a corresponding spectral transmission change practically wavelength-independent varying from 80% to 20%. The coloring efficiency determined at λ=600 nm was 22 cm²/C.     

Modeling the optical properties of WO3 and WO3-SiO2 thin films

The optical properties and surface morphology of sol-gel spin coated WO₃ and WO₃-SiO₂ composite films annealed at 250 and are investigated. For the purpose of extracting the optical parameters of the films, a novel form for the dielectric function is introduced, consisting of two Tauc-Lorentz oscillators and an Urbach tail component, which is suited for amorphous multi-transition materials with substantial subgap absorption. The evolution of the refractive indices, transmittances, and band gaps with doping is marked by sizable shifts at 2.0-2.5% SiO₂ doping for the films, and 4.0-4.5% doping for the films. In addition, pronounced changes in the surface roughness of the films occur at these doping values.     

Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO3) thin films

Thin films of tungsten oxide (WO₃) were deposited onto glass, ITO coated glass and silicon substrates by pulsed DC magnetron sputtering (in active arc suppression mode) of tungsten metal with pure oxygen as sputter gas. The films were deposited at various oxygen pressures in the range 1.5×10⁻²−5.2×10⁻² mbar. The influence of oxygen sputters gas pressure on the structural, optical and electrochromic properties of the WO₃ thin films has been investigated. All the films grown at various oxygen pressures were found to be amorphous and near stoichiometric. A high refractive index of 2.1 (at λ=550 nm) was obtained for the film deposited at a sputtering pressure of 5.2×10⁻² mbar and it decreases at lower oxygen sputter pressure. The maximum optical band gap of 3.14 eV was obtained for the film deposited at 3.1×10⁻² mbar, and it decreases with increasing sputter pressure. The decrease in band gap and increase in refractive index for the films deposited at 5.2×10⁻² mbar is attributed to the densification of films due to ‘negative ion effects’ in sputter deposition of highly oxygenated targets. The electrochromic studies were performed by protonic intercalation/de-intercalation in the films using 0.5 M HCl dissolved in distilled water as electrolyte. The films deposited at high oxygen pressure are found to exhibit better electrochromic properties with high optical modulation (75%), high coloration efficiency (CE) (141.0 cm²/C) and less switching time at λ=550 nm; the enhanced electrochromism in these films is attributed to their low film density, smaller particle size and larger thickness. However, the faster color/bleach dynamics is these films is ascribed to the large insertion/removal of protons, as evident from the contact potential measurements (CPD) using Kelvin probe. The work function of the films deposited at 1.5 and 5.2×10⁻² mbar are 4.41 and 4.30 eV, respectively.     

Physicochemical properties and photocatalytic hydrogen evolution of TiO2 films prepared by sol–gel processes

Anatase TiO₂ films were obtained on glass substrates using a sol–gel method using titanium isopropoxide as a precursor. The thickness of the film was about 140 nm for one coating, and the thickness is controlled by the number of coating cycles. The spectra of UV-VIS absorption indicated that the absorption edge of the TiO₂ films is ca. 385 nm, corresponding to the band gap energy of 3.20 eV. We obtained TiO₂ films having a high activity for the hydrogen evolution from photocatalytic water cleavage. By loading with 0.3 wt% Pt rate of hydrogen production increases. No influence of film thickness and calcination temperature on the photocatalytic property is observed.     

The kinetic behaviour of ion injection in WO3 based films produced by sputter and sol–gel deposition: Part II. Diffusion coefficients

The use of AC impedance spectroscopy for kinetic study of the ion intercalation into WO₃ films is reviewed, and methods for extracting the diffusion coefficient of the ion diffusion process from AC impedance spectroscopy data are described. These are applied to several different electrochromic thin films, all based on tungsten oxide, and the electrochromic performance is correlated with the diffusion coefficient. The results are also compared with results of a previous paper which concentrated on modelling the voltage response of films coloured and bleached using constant current charge injection techniques. Several examples of non-ideal behaviour of the impedance spectra are observed, including depressed semicircles and evidence of two semicircles. A full discussion of these effects is given in a following paper.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

The kinetic behaviour of ion transport in WO3 based films produced by sputter and sol–gel deposition:: Part I. The simulation model

This paper develops a new simulation model for charge injection and extraction in electrochromic WO₃ films under conditions of constant current during the charge injection process. The model is applied to the constant current coloration and bleaching of electrochromic films, and values for the diffusion coefficient and other model parameters have been obtained by fitting the model to experimental data. Application of the model to coloration and bleaching of electrochromic devices is discussed, in particular the use of the model in the design of electrochromic switching schemes.     

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.     

Pd–Pt alloy as a catalyst in gasochromic thin films for hydrogen sensors

We have used Pd–Pt alloy as the catalyst in the hydrogen sensor thin film. Palladium and platinum were co-sputtered on top of a tungsten oxide layer grown by reactive sputtering. Both the sensitivity and the durability were dramatically improved over the case of a palladium single-component catalyst. The fractional change in the optical absorption on exposure to 1% hydrogen gas was increased by a factor larger than 2, and the fractional change decreased only a little after more than 1000 cycles of repeated exposure to 1% hydrogen and air. Moreover, the sensor film exhibited good selectivity to other organic vapors.     

Reversible gasochromic hydrogen sensing of mixed-phase MoO3 with multi-layered Pt/Ni/Pt catalyst

Visual detection of hydrogen is important for hydrogen-powered vehicles and hydrogen fuel stations. However, there are few studies on such the sensing approach, particularly solving challenges about the endurance or reusability of the sensors. Here, our development of superior reversible gasochromic hydrogen sensors based on a novel combination of a multi-layered Pt–Ni catalyst and a mixed-phase MoO₃ active layer is introduced. The mixed α and β phases in the MoO₃ layer can provide more high-energy sites for gasochromic reactions. The Pt–Ni catalyst, where Ni serves as a modifier of surface atoms’ diffusivity, successfully converts the mostly irreversible gasochromic sensing mode of MoO₃ to the reversible gasochromic mode. Our hydrogen sensor shows coloration from grey to navy blue within 30 s, and its recovery occurs within 50 s. Furthermore, it can detect H₂ gas (5–1000 ppm) in both optical and electrical modes. Notably, our sensor is all constructed through dry processes, raising its potential for large-scale manufacture and integration.     

Palladium nanoparticle deposition onto the WO3 surface through hydrogen reduction of PdCl2: Characterization and gasochromic properties

Nowadays, gasochromic Pd/WO₃ coatings as optically switchable materials have become more applicable for hydrogen sensors and smart windows. In this study, WO₃ films were prepared by Pulsed Laser Deposition (PLD) and spin-coating sol-gel techniques. For deposition of Pd, first a layer of PdCl₂ was obtained via a simple drop-drying process by dropping PdCl₂ solution onto WO₃ substrates and drying them at room temperature. Then Pd nanoparticles were synthesized via hydrogen gas exposure that causes reduction of the PdCl₂ layer. According to Scanning Electron Microscope (SEM) observations before hydrogen reduction, many individual nanoparticles or fractal-like constructions of palladium were formed in the PdCl₂ layer in which the fractal branches were distorted after hydrogen treatment. Surface chemistry of the observed Pd nanoparticles was studied using X-ray Photoelectron Spectroscopy (XPS) at different stages of the reduction process. The results showed that after hydrogen treatment, the chlorine atoms were desorbed from the PdCl₂ layer and a metallic Pd layer remained on the surface of WO₃. Gasochromic properties in the presence of H₂ or O₂ gases for different PdCl₂ amounts revealed that the rate and saturated level of coloring depends on the PdCl₂ amounts as well as on the preparation method of the WO₃ substrates due to different porosities.     

Kinetics and thermodynamic behavior of WO3 and WO3:P thin films

 There is a considerable interest in the research and development of materials and devices, that can be used for optical switching of large-scale glazings. Several potential switching technologies are available for glazings, including those based on electrochromic, thermochromic and photochromic phenomena. One of the most promising technologies for optical switching devices is electrochromism (EC). In order to improve the electrochromic properties of tungsten oxide, we have investigated the effect of phosphorous insertion on the electrochromic behavior of oxide films prepared by the sol–gel process.The kinetics and thermodynamics of electrochemical intercalation of lithium into Li_xWO₃ and Li_xWO₃:P films prepared by the sol–gel process were investigated. The standard Gibbs energy for lithium intercalation was calculated. The chemical diffusion coefficients, D, of lithium intercalation into oxide, were measured by galvanostatic intermittent titration technique (GITT), as functions of the depth of lithium intercalation.     

Optimisation of the electrochromic properties of Nb2O5 thin films produced by sol–gel route using factorial design

Electrochemical and electrochromical properties of oxide films are dependent on their microstructure and morphological properties. Thus, the effects of three preparation variables on the electrochemical and electrochromical properties of Nb₂O₅ thin films prepared by the Pechini method were investigated. In order to minimise the number of experiments, a factorial design 2³ was used. The effects of the following variables: CA/EG molar ratio, CA/[Nb] molar ratio and calcination temperature were evaluated. Films prepared with the resin composition CA/EG=1 : 4, CA/[Nb]=10 : 1 and calcined at 500°C, showed the highest values of intercalation charge, transmittance variation and coloration efficiency, 22 mC/cm², 84% and 23 cm² C⁻¹, respectively.     

Photoresponse and H2 production of topographically controlled PEG assisted Sol–gel WO3 nanocrystalline thin films

WO₃ thin films were fabricated by sol–gel method using polyethylene glycol (PEG) as dispersing agent. Physical and photoelectrochemical properties of the synthesized nanocrystalline films were studied by varying weight ratio of PEG to tungsten precursor (x). Based on AFM observations and statistical modeling of the WO₃ surface, the thickness of the films increased by increasing the amount of x with a nearly linear fashion while the surface roughness reached to a saturated value. However, the film synthesized with x = 4 showed a chaotic surface behavior. Optical analysis revealed that by increasing the x, transmittance of the films decreased while their band gap energies remained unchanged. According to XRD results, variation of x did not change structure of the nanocrystalline film while XPS analysis indicated a better stoichiometry for the films with higher x values. A less charge transport life time was obtained for films with higher x values, but an enhanced photoresponse of the films and also hydrogen production via water splitting reaction were observed by increasing the amount of x. On the other hand, the charge transfer resistance of the samples reduced from 6.5 kΩ to 1.2 kΩ by addition of PEG to the sol from x = 0 to x = 2.     

Structure–property relationships in electrochromic WO3 films deposited by reactive sputtering

WO_x electrochromic (EC) films deposited by DC magnetron sputtering technique were investigated by XRD and STM measurements. The reversible microstructure changes of the WO_x film between the bleached and colored EC states were revealed. The study indicates that the amorphous as-deposited WO_x film (a-WO_x) is of amorphous microstructure both in bleached and colored states; however, the crystalline WO_x (c-WO_x) is stoichiometric triclinic lattice WO₃ in bleached state (the lattice parameters: a=7.2944 Å, b=7.4855 Å, c=3.7958 Å, α=89.38°, β=90.42°, γ=90.80°), and changes into nonstoichiometric tetragonal lattice WO_2.9 in colored state (a=b=5.336 Å, c=3.788 Å, α=β=γ=90°). The surface morphologies of the colored WO_x films are very different from those of the bleached WO_x films.