A NEW AND SIMPLE METHOD FOR MANUFACTURING ELECTROCHROMIC TUNGSTEN OXIDE FILMS

A new and simple method for the preparation of electrochromic tungsten oxide film is presented. This novel approach can be realized by introducing a nanoporous textured polyacrylic acid (PAA) layer between tungsten oxide layer and indium tin oxide (ITO) one by dip-coating process. Homogeneous film with a thickness of about 0.8 μm is made by single dipping a PAA-coated ITO glass in the WO3-NH4OH solution and shows an X-ray amorphous structure. During the cathodic and anodic processes by chronopotentiometry, the present films exhibit highly reversible electrochemical insertion of lithium ions; the corresponding inserted charge of 12.5 mC/cm2 gives rise to a marked blue color yielding the change in optical density (AOD) of 0.65 at 550 nm and a coloration efficiency (η) of 51.8 cm2/C.


INTRODUCTION
Electrochromic coatings have been extensively investigated due to their potential application in a broad range of devices including smart windows and flat panel displays [1][2][3][4].Various kinds of inorganic or *Corresponding author.organic materials for electrochromism have been proposed and their coloration properties have been studied in relation to their application for electrochromic devices.Among them, W oxide becomes one of the most promising materials for electrochromic devices because of its advantages of a good perceived contrast, memory effect and low switching voltage.Therefore several techniques have been used to prepare W oxide films, such as sputtering, evaporation, chemical vapor deposition, electrodeposition, and sol-gel method [5].Recently, the last method has attracted much attention since this approach allows not only for the formation of oxides using relatively low temperature, much higher deposition rates and low-cost coating technique compared to other techniques but also for a large-area electrochromic coating.
In order to obtain a suitable coating solution by the sol-gel method, such parameters as temperature, nature and concentration of electrolyte (acid and base), nature of the solvent, and type of precursor material should be adjusted during hydrolysis and polycondensation processes [6].Furthermore, some of starting materials in the sol-gel method are alkoxylates which are more or less expensive and do not afford satisfactory coating results if directly dissolved in typical solvent.In this context, a conventional sol-gel approach for electrochromic coating seems to be complicated and impractical.
The modified sol-gel methods [7, 8] concerning electrochromic W oxide film have been proposed, where the peroxotungstic species obtained by reacting W metal with aqueous H202 were used as a source of coating solution.These methods, however, call for some several steps to obtain a proper coating solution, and the viscosity of coating solution should be controlled upon mixing the intermediate of peroxotungstic acid or ester powder with water or alcohol.Therefore, it is considered that a more efficient preparative method with easier route toward electrochromic W oxide film is being required in order to fabricate a low-costable large-area electrochromic coating.
Here we report the preparation of W oxide film by a new and simple method in which a polyacrylic acid (PAA) layer is introduced as a buffer layer before making W oxide film.Electrochromic phenomena are examined by chronopotentiometry and UV-Vis-NIR spectro- photometry.To our knowledge, the present method is the first example of producing electrochromic W oxide film by incorporating inorganic oxy-anions into a nanoporous textured polymer layer and provides an easily fabricable way for a large-area electrochromic coating.

Preparation of Coating Solutions
The coating solution for polyacrylic acid (PAA) was prepared by dissolving the PAA powder in absolute ethanol (EtOH).To make 1% PAA solution, 3g ofPAA (Aldrich, average molecular weight 450,000) was dissolved in 300 ml of EtOH at room temperature, which was stirred for day in order to obtain a transparent clear solution.The coating solution for tungsten oxide (WO3) film was prepared by dissolving 21 g of WO3 powder (99 + %, Aldrich) in 60 ml of NH4OH (30% solution in H20, Aldrich) at 100C for 4h; then 240 ml of H20 was added.The solution was further stirred at room temperature for day, and was finally filtered in order to remove the undissolved WO3 powder.

Film Preparation
The ITO glass (109t/[-l, Samsung)with 10 x 10 cm 2 size was rinsed with ethanol and distilled water, then dried with air blowing just before coating.The ITO glass was first dipped into the 1% PAA-EtOH solution and withdrawn vertically at 30 mm/sec, then dried at room temperature.After this step, the ITO glass with a PAA layer was dipped into the WO3-NH4OH solution and withdrawn vertically at 30 mm/sec.The coated film was dried at 100C in an oven for 5 min, where the glass was tilted with respect to the vertical line to obtain a homogeneous film.Before using the film in lithium electrolyte, proton and electron were electrochemically injected/extracted into/from the as-prepared film in 1N HC1 electrolyte under a charge density of 10 mC/cm 2 to induce polycondensation; a Pt wire was used as counter electrode.This electrochemically treated film was heated in the deintercalated state at 60C for hr under vacuum to remove the weakly adsorbed water contaminating the film surface.In order to achieve electrochemical and optical measurements, the 10 10 cm 2 film was cut into appropriate small pieces.

Film Characterizations
X-ray diffraction measurements were carried out using CuKc radiation to investigate the film structure.Scanning Electron Micro- scopy (SEM), JEOL JSM-840A apparatus, was used to determine the film thickness and composition.

Electrochemical and Spectrophotometric Measurements
Chronopotentiometric experiments were performed with a computer- controlled potentiostat/galvanostat (TACUSSEL, PGS 201 T model) for the electrochemical cell ofPt LiTFSI + 1, 3-EtMeImTFSI WO3-PAA-ITO.All the measurements were advantageously performed at room temperature in air owing to the hydrophobic character of the lithium electrolyte.The electrochemical lithium insertion/deinsertion was processed under the charge density of 12.5 mC/cm 2, which was repeated automatically.Optical properties of the colored and bleached state were investigated in the wavelength range between 300 nm and 2000 nm using a UV-Vis-NIR spectrophotometer (Varian Cary 2415 Spectrophotometer equipped with DS-15 Data Station).

RESULTS AND DISCUSSION
The chemical species in the dissolution of inorganic material in aqueous solution and its dissolved chemical species depend on the pH of the solution and the oxidation state of the metal cation M z+ 10.
High-valent cations with z > 4 gives oxy-anions [MOn] (2n-z)-at high pH.The oxidation state of tungsten in WO3 is hexavalent; therefore, the tungsten oxy-anions of [WO,] (2n-z)-can be formed at high pH domain.According to the voltage equilibrium-pH diagram 1 WO]species are easily formed at the pH domain higher than 7.It is found that the utilization of a PAA layer provides a unique opportunity to prepare W oxide coating.Though PAA is an electronic-nonconducting polymer, the W oxide layer formed on such an electrically insulating PAA layer exhibits electrochromic property, which implies that the PAA layer may have a nanoporous textured matrix in order to carry electrons between ITO and W oxide layer.
For the as-prepared WO3-PAA-ITO film and the proton and electron injected one, X-ray diffraction analysis has been performed.The X-ray diffraction diagrams, reported on Figure 1, show not only the expected narrow peaks corresponding to well crystallized ITO, but also a broad peak, observed for low angle, which accounts for the amorphous structure of the W oxide layer.In order to determine the sample thickness, SEM analysis is carried out for the cross sectioned area.Figure 2 shows the WO3-PAA film coated on the ITO layer.The average film thickness is determined to be about 0.8 tm and 0.3 tm for the WO3-PAA layer and the ITO one, respectively.According to elemental analysis by energy dispersive mode with 15 kV, WO3-PAA layer has tungsten and oxygen with small amount of carbon, and ITO one has indium, tin and oxygen.
To study the electrochromic property of the present WO3-PAA-ITO film, the change of potential for the electrochemical cell with the Pt anode and the WO3 cathode assembly is monitored with time at room temperature by chronopotentiometry, where the charge of 12.5 mC/cm 2 is inserted and extracted automatically over 100 cycles.Figure 3 shows the E=flt) diagram for the electrochemical cell with Pt/LiTFSI + Time (min.) FIGURE 3 E(V) f(t) cycling diagram of the cell with Pt/LiTFSI + 1, 3-EtMelm TFSI WO3-PAA-ITO during 0 to 20 cycles (top) and 80 to 100 cycles (bottom), where each cathodic and anodic process is carried out at constant current of 0.5 mA for 150 sec (corresponding to Q= 12.5 mC/cm2).
found that the present WO3 films exhibit reversible electrochemical and electrochromic cycling behaviors.
Figure 4(a) shows the UV-Vis-NIR spectra for the bleached state of the WO3-PAA-ITO film together with the ITO and PAA-ITO films as a reference.The spectrum of the well crystallized ITO film shows little spectral interferences, which accounts for the small film thickness.The PAA-ITO structure shows no spectral interference because of the nanoporous structure of the PAA thin film.On the other hand, the spectrum of the X-ray amorphous WO3-PAA-ITO film exhibits many 400 600 800 100012001400160018002000 Wavelength (nm) FIGURE 4 Comparison of spectral interferences among the ITO film, the PAA film coated on ITO, and the WO3 films coated on PAA-ITO (a) and the UV-Vis-NIR spectra of colored state (--) and bleached one (...) for the WO3-PAA-ITO film after 100 cycles of inserting and extracting charge of Q 12.5 mC/cm=(b).
spectral interferences due to the dense WO3 layer and large thickness.
In Figure 4(b), the colored and bleached spectra are obtained after 100 cycles of inserting and extracting charge of Q= 12.5 mC/cm 2 by chronopotentiometry; the transmittance (%) is determined to be 19.47 for the colored state while the bleached state has a transmittance (%) of 86.30.The change in optical density (AOD ODcoor-ODbleach In [Tbleach/Tcoor], where OD is defined as log T-) at 550 nm is determined to be 0.65, and the coloration efficiency (r/ AOD/Q) is estimated to be 51.8 cm=/C.It is found that there is no destructive problem in surface adhesion and optical property ofthe WO3-PAA-ITO film during cathodic and anodic processes.
FIGUREX-ray diffraction diagram for the ITO film (a); the PAA-coated ITO film (b); the as-prepared WO3-PAA-ITO film (c); the colored film after injection of Q= 10 mC/cm in N HC1 (d).
1,3-EtMelmTFSI/WO3-PAA-ITO.One coloration and one subse- quent bleaching phenomenon constitute a cycle.During the cathodic process, the WO3 electrode gets colored with a deep blue color.It is