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.     

Optical properties of electrochromic all-solid-state devices

We have investigated the optical properties of an all-thin-film electrochromic device, with a thin film of ZrO₂ acting as an ion conductor. The device also employed electrochromic layers of amorphous or crystalline WO₃ and NiV_xO_yH_z. Transmission (T) and reflection (R) spectra were recorded in the wavelength range 300–2500 nm at different intercalation levels, both for single films and complete devices. The results show that T decreases significantly upon intercalation in the WO₃ thin films as well as in the devices. The reflectance only shows minor changes.     

Electrochromic behavior of Ni---W oxide electrodes

A new electrochromic material, Ni---W oxide, was fabricated by a reactive sputtering method and the effects of tungsten concentration in Ni---W oxide thin films on the electrochromic behavior were investigated. It has been found that the charge transfer resistance which has been known to be very high in NiO_x becomes significantly low with the addition of tungsten in NiO_x. It turned out that the charge transfer density during the cyclic voltammetry increased considerably with the addition of tungsten in NiO_x such that at an optimum concentration of tungsten (atomic ratio of W/Ni = 0.33), a fast optical switching with the insertion and removal of lithium in Ni---W oxide thin films could be expected. The electrochromic display device composed K_0.3WO_3.15 and Ni---W oxide was fabricated and its optical switching characterized.     

WO3−x nanowires based electrochromic devices

A simple method was developed to fabricate tungsten oxide (WO_3−x) nanowires based electrochromic devices. The WO_3−x nanowires are grown directly from tungsten oxide powders in a tube furnace. The WO_3−x nanowires have diameters ranging from 30 to 70 nm and lengths up to several micrometers. The WO_3−x nanowires based device has short bleach-coloration transition time and can be grown on a large scale directly onto an ITO-coated glass that makes it potential in many electrochromic applications. The structure, morphology, and composition of the WO_3−x nanowires were characterized using the scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometer. The optical and electrochromic performance of the nanowires layer under lithium intercalation was studied in detail by UV–VIS–NIR spectroscope and cyclic voltameter.     

In situ Raman spectroscopy of the electrochemical reduction of WO3 thin films in various electrolytes

With in situ micro-Raman measurements during the electrochemical reduction of WO₃ thin films, the influence of the intercalated cation (H⁺/Li⁺) and an addition of water to the aprotic lithium electrolyte was investigated. The Raman spectra of lithium bronzes Li_xWO₃ show two main results: (i) the intercalation of hydrogen can be clearly distinguished in situ from the intercalation of lithium with this technique and (ii) even with an addition of 500 ppm of water to the lithium electrolyte no hydrogen intercalation was observed.     

The electrochromic response of tungsten bronzes MxWO3 with different ions and insertion rates

The electrochemical intercalation of Li⁺ and Na⁺ into thin WO₃ films prepared by evaporation (130 nm thick) has been performed starting from liquid electrolytes with very low water content (< 20 ppm). Transmittance and reflectace measurements have been performed ‘in-situ’ during the electrochemical formation of the tungsten bronzes under controlled current. For inserted charges larger than 20 mCcm⁻² there is an optical transition responsible for a pale yellow coloration instead of the typical dark blue. This transition is irreversible since the subsequent bleaching of the film is not complete and a part of the inserted charge cannot be extracted again. When the insertion rate is high (0.4 mAcm⁻²) the irreversible transition occurs earlier (around 10 mCcm⁻²) in the case of Na⁺, which is known to diffuse slowly into the film. This limit composition for reversible electrochromic reaction is therefore reached earlier near the film surface, where the accumulation of the Na ions is responsible for an optical transition seen in the reflectance and for a phase change shown by a typical inflection in the electrode potential curve.     

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.     

Electrochromism in NiOx and WOx obtained by spray pyrolysis

Electrochromic films of NiO_x and WO_x were produced by the spray pyrolysis technique. The nickel-oxide-based coatings were obtained from an aqueous solution of nickel nitrate. Those obtained below 300° C did not show any diffraction peak when subjected to X-ray diffraction analysis, and those obtained above 400° C showed a diffraction pattern corresponding to cubic NiO. Films obtained below 300° C showed an electrochromic effect with an electrochromic efficiency of 30 cm²/C.Tungsten-oxide-based coatings were obtained from a solution of H₂WO₄ in aqueous ammonia. The films were grown at 150° C, and they showed a diffraction pattern corresponding to monoclinic WO₃ when subjected to a post-heat treatment at 400° C during ten minutes. The WO_x films showed a noticeable electrochromism under cation insertion, and presented an electrochromic efficiency of 42 cm²/C. Both as-deposited and heat-treated samples showed good electrochromism.     

Step potential analysis of cobalt oxide-based electrochromic devices

The characterization of electrochemical behavior of electrochromic intercalation device based on cobalt oxide thin film was carried out using the step potential excitation method. A method based on generating plots of current density as a function of passed charge has been applied for characterization of electrochromic cobalt oxide thin films using an aqueous KOH electrolyte. The device resistance and the intercalation capacity of the material are calculated. Dynamic built-in potential estimated from step potential experiment and plots of the built-in potential as function of the passed charge, V_bi(ΔQ), are generated for intercalation process. The intercalation efficiency curve is obtained to confirm the nature of energy distribution of intercalation sites in electrochromic cobalt oxide.     

Analysis of the influence of the gas pressure during the deposition of electrochromic WO3 films by reactive r.f. sputtering of W and WO3 target

Tungsten trioxide is the most accepted material for electrochromic devices. In the work thin films of WO₃ were deposited by reactive r.f. sputtering of both metallic (W) and ceramic (WO₃) targets to study the correlation between the electrochromic properties and the structures of the films. Samples were grown at different pressures of Ar+O₂ in order that the energy regimes of the sputtered particles on the condensing surface could be set either below or above the thermalisation diffusion limit. Lithium ions were intercalated in the films in an aprotic electrolyte and the colouring/bleaching behaviour as a function of the intercalated amount of lithium was detected in the 1st and 10th cycle. From these measurements, the electrochromic properties of the films were worked out. The optical and morphological characteristics were analysed respectively, by spectrophotometric and X-TEM measurements. The amount of water present in the films, detected by IR spectroscopy, turned out to be well correlated to the film morphology and also to the porosity.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

Pre-existence of HxWO3 in e-beam deposited WO3 films

Structural and optical properties of e-beam deposited tungsten trioxide (WO₃) films in as-deposited and electrochemically coloured states were investigated by spectrophotometric and XRD techniques. These investigations have shown the as-deposited WO₃ films to be porous and with small amount of H_xWO₃ pre-existing in them. The films further facilitate insertion of H⁺ ions on colouration resulting in tetragonal H_xWO₃ with a = 4.74Å and c = 3.19Å.     

V0.50Ti0.50Ox thin films as counterelectrodes for electrochromic devices

Thin films of V_0.50Ti_0.50O_x have been deposited by RF sputtering from metallic targets. Their use as potential counterelectrodes in electrochromic devices has been investigated. It is found that although they are slightly yellow looking in transmission, the films can reversibly store relatively large amounts of charge, whilst showing a reasonably low electrochromic colouration efficiency. The electrochemistry of V_0.50Ti_0.50O_x is found to be simple, in fact rather similar to that of WO₃, making it an almost ideal material to use in such a variable transmission device.     

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.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Study on the WO3 dry lithiation for all-solid-state electrochromic devices

In this report, a simple WO₃ dry lithiation is proposed for fabrication of all-solid-state electrochromic devices and characterized completely by X-ray photoelectron spectroscopy and electrochemical method. Lithiation is carried out by electron-beam evaporation of metal lithium, and the lithiated films have different components and electrochromic properties with different lithiation degrees. It is found that if Li/W ratio is less than 0.25, tungsten bronze Li_xW0₃ is formed and the lithiated by wet method. Finally, a lithium-based all-solid-state electrochromic device with proper lithiation degree is fabricated using this dry method.     

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.     

Improved electrochromic films of NiOx and WOxPy obtained by spray pyrolysis

Electrochromic films of NiO_x and WO_xP_y were produced by the spray pyrolysis technique. The nickel-oxide-based coatings were obtained from both an alcoholic solution of nickel nitrate and aqueous solution of the mixture nickel nitrate/cobalt nitrate. Coatings obtained from alcoholic solutions showed a noticeable contrast of optical transmittance from fully bleached to colored state. X-ray diffraction analysis showed a slight crystallization in NiO_x after electrochemical treatment: one diffraction peak for as-deposited films turned to three diffraction peaks for electrochemical treated samples. Coatings obtained from aqueous solution of mixture nickel nitrate/cobalt nitrate showed an optimized electrochromic behavior at a Ni:Co proportion of 90:10. At this condition an optical contrast of 50% is found. X-ray diffraction showed that these samples comprised a phase mixture of Co₃O₄ and NiO.WO_xP_y samples were obtained from polytungsten gel in which H₃PO₄ was added. We found that for 8.3 at% of P:W, the electrochromism was optimized. Pyrolytic coatings of WO_xP_y show superior behavior than those of WO_x obtained by spray pyrolysis, both in optical contrast and durability.     

Electrochromic and photoelectrochemical behavior of electrodeposited tungsten trioxide films

Electrodeposited WO₃ films exhibit reversible photochromic and electrochromic behavior (blue coloration with absorption in the near-IR region). The W-4f core level of WO₃ film at different levels of coloration has been investigated using XPS. The W-4f peaks become broader after coloration. Photoelectrochemical measurements have shown a low photoelectric conversion, efficiency of WO₃ film.     

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.