Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process

By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the modified tungsten hexachloride sol-gel techniques. Temperature was varied as it is an important factor for crystallization, surface morphology and microstructure of tungsten oxides, from the studies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy. The mesoporous sample calcined at 300 °C has tri-dimensional vermicular mesopores with nanocrystallites embedded in the pore wall, while such uniform structure would be destroyed by higher calcination temperature of about 400 °C. X-ray photoelectron spectroscopy was used for analyzing the surface-binding states and the stoichiometry for the oxides. Electrochromic characterization was implemented by simultaneous voltametric and spectrophotometric measurements of tungsten oxides/indium tin oxide (ITO) electrodes. The investigation results showed that organized pore-wall nanostructure has strong effects on the electrochemical and chromogenic properties depending on the specific surface area and the impacts from the evolved crystallization.     

Characterization of ion diffusion and transient electrochromic performance in PECVD grown tungsten oxide thin films

Electrochromic tungsten oxide thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). Film density and electrochromic performance were controlled by the degree of ion bombardment. A moderate degree of ion bombardment was optimal, and the refractive index was shown to be a sensitive indicator of electrochromic performance. Chronoamperometry in concert with optical transmission was used to determine diffusion and absorption coefficients using both H⁺ and Li⁺ containing electrolytes. The absorption coefficients were similar for both ions, scaling with the degree of intercalation to 50,000 cm⁻¹ in the opaque state. The diffusion coefficients for optimized films were found to be relatively insensitive to the degree of ion intercalation, with values of 10⁻⁹ and 10⁻¹⁰ cm²/s for H⁺ and Li⁺, respectively. These values are about an order of magnitude greater than values reported for vacuum-deposited films, which was attributed to low relative density in the PECVD films. The diffusion and absorption coefficients were incorporated into a model that successfully reproduced transient optical performance.     

Effect of humidity on structure and electrochromic properties of sol–gel-derived tungsten oxide films

The influence of varying relative humidity (RH55 and 75%) during thin film deposition from an oxalato-acetylated peroxotungstic acid sol by dip coating, on the microstructure and electrochromic properties of pristine tungsten oxide (WO₃) films obtained upon annealing is presented. The films fabricated under a relative humidity of 55% are amorphous whereas the ones cast under a substantially humid atmosphere (RH75%) are characterized by interconnected nanocrystallites with a triclinic phase and a nanoporous surface morphology as well. Upon lithium insertion, larger integrated values of transmission modulation and coloration efficiency are observed over the photopic and solar regions, for the films prepared under a RH75% as compared to that observed for the films deposited under a RH of 55%. Functional improvements are due to the larger surface area of nanocrystallites and a porous microstructure, a consequence of a higher degree of hydration and hydroxylation in the former films in contrast to the non-porous and a rather featureless structure of the latter films. Faster switching kinetics between the clear and blue states, a greater current density for lithium intercalation, a higher diffusion coefficient for lithium and a superior cycling stability, again shown by the film fabricated under a 75% RH confirm that the WO₃ film microstructure is most conducive for a more facile ion insertion–extraction process, which hints at its potential for electrochromic window applications.     

Performance of an electrochromic window based on polyaniline, prussian blue and tungsten oxide

In our laboratory various electrochromic windows (ECWs) have been investigated using mainly tungsten oxide (WO₃), polyaniline (PANI) and prussian blue (PB) as electrochromic materials in combination with poly(2-acrylamido-2-methyl-propane-sulphonic acid) (PAMPS) as a solid proton-conducting electrolyte. The ECWs have been characterized by AC-impedance, linear sweep voltammetry and spectroelectrochemical studies in the 290–3300 nm spectral region. The ECWs have the following general multilayered structure: Glass/ITO/EC1/IC/EC2/ITO/Glass, where ITO=indium oxide doped with tin, IC=ionic conductor, EC1 is either PANI or PANI including PB, and EC2 is WO₃. The best of these ECWs has been able to regulate up to 56% (typical 50%) of the transmission of the total solar energy in the 290–3300 nm spectral range. The combination of the two electrochromic materials PANI and PB has been shown to be mutually beneficial in such a way that the colouration of the window is enhanced by the addition of a layer of PB onto PANI, while the adhesion of PB is improved by the presence of PANI. The energy consumption of the ECW is about 0.01 Wh/m² for one complete cycle (−1.8 V/1.2 V). The switching time for 90% colouring/bleaching is typically 10–30 s. A PANI/PB//WO₃ window has been operated for about 50 days (3700 complete cycles) without substantial loss of transmission regulation, though with an increase in switching time (10 min.). Spectra from individual layers in the ECWs have been recorded by making holes in one or two of the electrochromic layers. In this way (the hole method), it has been possible to study the transmission regulation properties for each electrochromic material separately in complete solid state windows. In addition, spectra for complete windows have been simulated by adding contributions from individual electrochromic layers.     

The effect of precursor aging on the morphology and electrochromic performance of electrodeposited tungsten oxide films

Tungsten oxide films that have larger effective surface area or extensive grain boundaries tend to be more suitable for use in electrochromic devices. We propose in this paper a simple methodology for increasing the roughness and thus the effective surface area of WO₃ films. This method is based on the tendency of the peroxytungstate precursor to form large aggregates within its solution with time. To this aim, a systematic study of the precursor aging effect on the resulting WO₃ film properties was conducted. It was established that with increasing aging time of the precursor solution, more and larger aggregates are formed, which are then deposited on the film surface. The deposition of the aggregates causes the formation of large cracks on the film surface, thereby increasing its effective area.An optimum of the precursor aging time was found to be around 80 h. Films prepared with such an aged solution were found to have the highest Li⁺ diffusion coefficient and voltammetrically intercalated charge density per unit film thickness. It was also observed that the coloration efficiency of films prepared using the aforementioned method was higher than that of equivalent electron-gun deposited films throughout the visible spectrum and especially in the near infrared. The enhanced properties of these films indicate their improved electrochromic performance, which is mainly due to their increased surface area.     

Electrochromic properties of one-dimensional tungsten oxide nanobundles

In this study, one-dimensional (1D) tungsten oxide nanobundles (TNB) were synthesized via a simple solvothermal method. The phase of 1D tungsten oxide was W₁₈O₄₉, and the diameter and length of the building units (nanowires) were about 7 and 800 nm, respectively. TNB films were fabricated by the Langmuir–Blodgett (LB) method. The locally arranged domains of the long nanobundles form the LB films, but it is difficult for them to align perfectly owing to the inter-nanobundle interaction and dispersion problems. The electrochromic (EC) property of the TNB LB films was characterized by electrochemical potential cycling tests and in situ transmittance measurement. The deposition condition of the LB films influenced their EC property. The heat treatment and surface pressure of the TNB LB films plays an important role in the response time and transmittance of the TNBs.     

Thickness-dependent electrochromic properties of solution thermolyzed tungsten oxide thin films

Tungsten oxide (WO₃) thin films are prepared by using a simple and inexpensive solution thermolysis technique. Thin film samples of different thickness are obtained by varying quantity of ammonium tungstate solution sprayed onto the preheated conducting glass substrate. A simple three-electrode cell has been formed to study the electrochemical and electrochromic properties. The electrochemical parameters of the cell such as anodic peak current, anodic peak potential, threshold voltage, amount of H⁺ ions intercalated into and deintercalated out of the WO₃ samples are calculated. The effect of film thickness on these parameters are studied. The extent of electrochromism and reversibility of the colouration/bleaching processes of various WO₃ samples are described. The colouration efficiencies at 633 nm are calculated. The maximum colouration efficiency obtained for thicker film, is 56 cm²/C. The samples were found to be stable in 0.05N H₂SO₄ electrolyte up to 1×10³ colour/bleach cycles.     

Comparison of electrochromic amorphous and crystalline tungsten oxide films

A detailed systematic study of the tungsten oxide thin films has been carried out using WO₃ films after they were annealed at progressively increasing temperatures ranging from 350°C to 450°C in oxygen environments. The structural properties of the films were characterized using X-ray diffraction and Raman spectroscopy. The amorphous WO₃ films remain as an amorphous phase up to 385°C and begin to crystallize at 390°C and then are completely crystallized at 450°C. Absorption peaks of the films are found to shift to a higher energy side with increasing annealing temperature up to 385°C and then shift abruptly to a lower energy as the films begin to crystallize at 390°C. Deconvolution of the absorption spectra shows that there are two different polaron transitions in the amorphous WO₃ films.     

Nanostructured polyethylene glycol-titanium oxide composites as solvent-free viscous electrolytes for electrochromic devices

Composites of polyethylene glycol (PEG) and titanium oxide compounds have been prepared by sol-gel method using different molar ratio between PEG and titanium isopropoxide. Transmission electron microscope (TEM) images and UV-vis absorption spectra of these composites indicate that titanium oxide particles or clusters were formed inside the composite materials and their maximum sizes were between 1.8 and 7 nm for PEG:Ti molar ratio changing from 24:1 to 4:1. Fourier Transform Infrared spectroscopy analysis of the same PEG-Ti composites suggests the presence of tetragonal titanium oxide compounds and its association with ether oxygen atoms of PEG molecules. As lithium iodide salt was added into PEG-Ti composites, iodide ions were oxidized into iodine and tri-iodides and, at the same time, titanium oxide compounds should be reduced. Color change speeds of tungsten oxide thin films were significantly improved when PEG-Ti-LiI composites were used as electrolytes compared to salt-in-polymer one (PEG-LiI); the bleaching time of tungsten oxide was reduced from 22 to 2.5 s under +1.0 V polarization, and the coloring time under −1.5 V lowered from 16 to 2.2 s. The transfer of negative charges from smaller iodide ions onto longer or crosslinked PEG-Ti macromolecules could be the origin of faster lithium ion transport/insertion speeds in PEG-Ti composite electrolyte based electrochromic devices.     

A reversible inorganic electrochromic solution system

Electrochromic devices that respond to varied levels of applied electrical potential by changing light opacity are presently prepared from either insertion compounds or reversible electrodeposition of metals from solution. We report here another approach to electrochromics based on the reversible deposition/dissolution of silver oxide onto FTO-coated glass from an aqueous solution containing silver (I)-ammonia complexes. The basic redox chemistry underlying the functioning of this electrochromic system has been identified using traditional techniques such as X-ray diffraction, scanning electron microscopy, and cyclic voltammetry, as well as a quartz nanobalance for measuring the working electrode weight at different potentials. The AgNH₃⁺↔AgO reaction is responsible for the coloration/decoloration of the working electrode, which can be repeatedly cycled between various states of visual opacity. Optical measurements reveal that the transmittance of the working electrode in the visible region drastically drops from 80% without silver oxide deposition to 4% when it is coated with a silver oxide film.     

Cross-sectional high-resolution transmission electron microscopy of the microstructure of electrochromic nickel oxide

Electrochromic nickel oxide films on indium tin oxide (ITO) were investigated by cross-sectional high-resolution transmission electron microscopy and energy dispersive X-ray analysis. Microstructural features and the differences between high and poor-quality samples were studied and compared. The dominant phase of the NiO film has cubic structure, but selected area electron diffraction patterns revealed many extra diffraction spots for the high-performance samples which may result from additional hydrated nickel oxide phases. The NiO grains do not show clear shapes in the cross-sectional plane nor are there signs of a preferred orientation. The mean grain size is larger and there are more defects and superlattices in samples with good electrochromic properties. There was a clear correlation between grain size distribution and performance. The high-quality samples had a mean grain size of about 6.5 nm, whereas the poor-quality samples exhibited significantly smaller mean grain sizes of 5.0 and 3.8 nm.     

Towards electrochromic stability in sol–gel-derived tungsten oxide films: cyclic voltammetric and spectrophotometric investigations

Hitherto unexplored irreversible changes during initial coloration/bleaching cycles for sol–gel-derived tungsten oxide (WO₃) films have been investigated using cyclic voltammetric and spectrophotometric techniques. Non-ideal features appearing in the initial five anodic (deintercalation) cycles in the voltammogram with simultaneous decreased optical transmission of the bleached films have been explained in terms of possible stoichiometric variations affecting the coloration efficiency (CE) of the films and the associated mechanisms. Electrochromic stability attained thereafter manifests in retraceable voltammograms and almost invariant value of the CE.     

A complementary electrochromic device based on polyaniline-tethered polyhedral oligomeric silsesquioxane and tungsten oxide

In this paper we report a high-contrast complementary electrochromic device based on polyaniline-tethered polyhedral oligomeric silsesquioxane (POSS-PANI) and tungsten oxide (WO₃). The electrochromic properties, cyclic voltammetry behavior and coloration efficiency of the device are studied. Due to the loosely packed structure of POSS-PANI, it possesses more accessible doping sites and hence gives rise to a significantly higher electrochromic contrast than polyaniline (PANI). Furthermore, the replacement of PANI with POSS-PANI as the complementary layer for WO₃ leads to an enhanced complementary effect, for which the underneath mechanism is also discussed.     

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.     

The influence of the intercalate species on the quasi-static electrochromic behavior of tungsten-oxide-based devices

The influence of the intercalate species on the quasi-static electrochromic behavior of tungsten-oxide-based devices is investigated. Two different electrolytes are used in the devices: an aqueous sulfuric acid solution, from which it is assumed that intercalation of hydrogen occurs; and a solution of lithium perchlorate in propylene carbonate, from which it is assumed that intercalation of lithium occurs. Experiments are performed in which a step-current of small magnitude is imposed through the device, and the corresponding time-dependence of the electrical potential and optical transmission are measured simultaneously. The behavior of the optical efficiency is relatively insensitive to the nature of the intercalate species, but the device potential is appreciably more sensitive to lithium intercalation than to hydrogen intercalation. The disparity in electrical behavior is likely due to increased strain effects and/or a diminished availability of sites associated with the larger lithium intercalate. It is shown that the electrical and optical behavior of the two types of devices may be related by a single linear scaling relation, indicating that the fundamental processes involved in the operation of the devices are similar.     

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).     

Influence of sputter oxygen partial pressure on photoelectrochemical performance of tungsten oxide films

Polycrystalline tungsten oxide films of 1–1.2μm thickness were prepared by reactive sputtering at elevated substrate temperature (270 °C) and under different oxygen partial pressures in the range from 0.8 to 2.1 mTorr. At the lowest partial pressure the films were substoichiometric, showed increased disorder, and exhibited photocurrents of 0.6 mA/cm² at 1.8 V vs SCE in 0.33 M H₃PO₄. At partial pressures of 1.4 mTorr and greater, stoichiometric WO₃ films were produced which exhibited photocurrents of 2.4 mA/cm² at 1.8 V vs SCE. It has been determined that the photoelectrochemical performance of slightly substoichiometric films is adversely affected by changes in optical properties, while the photocurrents of severely substoichiometric films suffer additionally from poor carrier collection.     

Preparation and thermal properties of polyethylene glycol/expanded graphite blends for energy storage

Expanded graphite is a promising heat transfer promoter due to its high conductivity, which improves the thermal conductivity of organic phase change materials. Moreover, it can also serve as supporting materials to keep the shape of the blends stable during the phase transition. After various investigation, the results showed that the maximum weight percentage of polyethylene glycol was as high as 90% in this paper without any leakage during the melting period, with the latent heat of 161.2 J g⁻¹ and the melting point of 61.46 °C. It was found that the value of the latent heat was related to the polyethylene glycol portion, increased with the increase in polyethylene glycol content. Moreover, the measured enthalpy of the composite phase change materials was proportional to the mass ratio of the polyethylene glycol component. The melting temperatures were almost the same with different ratios of composites. The conductivity of blends was improved significantly with the high value of 1.324 W m⁻¹ K⁻¹ compared to the pure polyethylene glycol conductivity of 0.2985 W m⁻¹ K⁻¹.     

Electrodeposition and pseudocapacitive properties of tungsten oxide/polyaniline composite

Composite films of tungsten oxide (WO₃) and polyaniline (PANI) have been electrodeposited by cyclic voltammetry in a mixed solution of aniline and precursor of tungsten oxide. Surface morphology and chemical composition of WO₃/PANI composite are characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of H₂O₂ on the electrodeposition of WO₃/PANI composite film is also investigated. Cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) results show that WO₃/PANI composite film exhibit good pseudocapacitive performance over a wide potential range of −0.5 to 0.7 V vs. SCE with the specific capacitance of 168 F g⁻¹ at current density of 1.28 mA cm⁻² and energy density of 33.6 Wh kg⁻¹, which is 91% higher than that of similarly prepared PANI (17.6 Wh kg⁻¹). An asymmetric model capacitor using WO₃/PANI as negative and PANI as positive electrodes over voltage range of 1.2 V displays a specific capacitance of 48.6 F g⁻¹ and energy density of 9.72 Wh kg⁻¹ at the power density of 53 W kg⁻¹, which is two times higher than that of a symmetric capacitor modeled by using two PANI films as both positive and negative electrodes.     

Optical properties of tungsten and titanium oxide thin films prepared by plasma sputter deposition

Tungsten oxide and titanium oxide thin films were prepared by RF reactive magnetron sputter deposition. The stationary and rotating substrate holders were applied to analyze the rotating effect. The optical properties and thicknesses of oxide films were determined by a proposed optical model and the measured transmittance spectra. The dispersed refractive indices of thin films have a wide range distribution in different sputtering conditions. In the situation of rotating substrate holder, the refractive index was lower than that of the stationary substrate holder. Also, amorphous TiO₂ structure can be prepared by using rotating substrate holder. The transmittance spectrum of crystalline TiO₂ reveals that the textured structure on the film surface affects the transmittance characteristic.