Electrochemical and Optical Properties of WO3 Prepared by Sol-Gel Coating 10.4028

Electrochemical and Optical Properties of WO 3 Prepared by Sol-Gel Coating

Chang-Yeoul Kim 1a, Seong-Geun Cho 1,2 , Seok Park 1,2 , Tae-Yeoung Lim 1b, and Duck-Kyun Choi 2c 1. Korea Institute of Ceramic Engineering and Technology, Seoul 153-801, KOREA E-Mail ; a [email protected], [email protected], [email protected] 2. Hanyang University, 133-791, Seoul, Korea

Keywords : electrochromic, sol-gel, electrochemical, optical, current density

Abstracts Electrochromic WO 3 thin film was prepared by using tungsten metal solution in hydrogen peroxide as a starting solution and by a sol-gel dip coating method. The thermal analysis was conducted by DSC/TG method. A DSC/TG analysis and the XRD patterns showed that a tungsten oxide crystal o o phase was formed at 400 C. WO 3 thin film when heat-treated at 300 C was amorphous and had a better electrochemical property than that of the crystalline phase. Crystallization of tungsten oxide decreased active sites of ion intercalation so that the current density decreased with heat-treatment temperature.

I. INTRODUCTION Electrochromism in amorphous tungsten oxide has been extensively studied since it was discovered by Deb in 1969 1) . Tungsten oxide is reduced in electrolyte by applying voltage in which proton/lithium ion is inserted to form H xWO 3/Li xWO 3 (blue) and oxidized to the original form of WO 3 (transparent) by applying reverse voltage. Tungsten oxide electrochromic material has a variety of applications such as smart window, antireflection rear-view mirror, displays, and electrochromic glasses.

Tungsten oxide thin film has been prepared by a variety of methods such as evaporation, sputtering, chemical vapor deposition, electrodeposition, sol-gel technique, etc 2) . Sol-gel coating method has the advantages of controlling stoichiometry and film structure. The chemical composition, crystal structure and microstructure of the film affect the electrochemical and electrochromic properties of tungsten oxide film. The tungsten oxide coating solutions have been synthesized by using many 3), 4) 5) 6) 7), 8) 9) kinds of starting materials of Na 2WO 4 , WOCl 4 , WCl 6 , W metal , tungsten alkoxide . The tungsten oxide thin film prepared by sol gel method has been reported to be comprised of micro-crystallites with a hexagonal-like structure when it is annealed at 190 oC 10) . The film prepared by sol gel method contains loosely bound water molecules when annealed at low temperature below 200 oC. Structurally bound water evaporates at 200-300 oC 11) .

In this research, tungsten oxide thin film was prepared by a peroxotungstic acid precursor method. The thermal analysis of peroxotungstic acid precursor, crystal phase and microstructural development observations according to heat treatment temperature was conducted. In most of the sol-gel coating methods, tungsten oxide thin film has been prepared below 200 oC up to date. For this reason, we prepared a tungsten oxide thin film at 200-500 oC. The electrochemical and electrochromic properties of the tungsten oxide thin film heat-treated at various temperatures were analyzed

II. EXPREIMENTAL PROCEDURE Tungsten metal powder or tungstic acid (H 2WO 4) used as starting materials was dissolved into 30% of hydrogen peroxide, which was so exothermically reactive that it was conducted at 70 o C. Ethanol was added to that solution whose molar concentration was varied from 0.1 to 1 M.

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WO 3 thin film was coated by dipping ITO-coated glass in the WO3 coating solution and o withdrawing. The withdrawing speed was 100mm/min. The WO 3 coated glass was dried at 100 C for 10min and then heat-treated at 200 to 500 oC for 2hrs in air. A thermal analysis of orange peroxotungstic solution was carried out by differential scanning calorimetry analysis (DSC) and thermogravimetric analysis (TG) method. The crystal phase development of WO 3 thin film with heat-treatment temperature was identified by X-ray diffraction (XRD) analysis. The cyclovoltammetry of WO 3 thin film was analyzed by Autolab PGASTA12 by applying voltage from –1V to 1V in 0.1M H 2SO 4 liquid electrolyte, of which scan speed was 20mV/s. Ultraviolet/Visible transmission properties of WO 3 thin film were measured at the state of colored by applying –1 V and bleached by applying 1V.

III. RESULTS and DISCUSSIONS

Orange powder 0.5 100 100 o 98.06% at 99 C Ar atmosphere 1E-8 H2O 0.4 95 90 89.95% at 178 oC 120.7 oC 0.3 87.60% at 295 oC 90 80 o 86.0% at 381 C HO 1E-9

0.2 TG (%) o 85 O 170.4 C H 2 357.9 oC 2 DSC/(mW/mg) 70 H TG (%)

0.1 o 80 O 203.0 C Current intensity (A) 445.0 oC 60 1E-10 0.0 o 75 479.8 C H O 2 2 -0.1 70 50 100 200 300 400 500 0 100 200 300 400 500 Temperture / oC Temperature ( oC)

Fig.1. DSC and TG analysis of orange-colored Fig.2. TG and mass analysis of organge peroxotungstic peroxotungstic acid powders dried at 100 oC

A DSC/TG analysis was conducted to investigate the thermal property of the peroxotungstic acid starting solution dried at 100 oC in oven. Fig. 1 shows the DSC/TG results of orange peroxotungstic acid powder. An endothermic peak in peroxotungstic acid solution appeared at 114 oC which was accompanied by weight losses. The weight loss below 200 oC was thought to be from water evaporation and hydrogen peroxide dissociation, measured by mass spectroscopy (Fig.2). The quantitiy of weight loss was 10% until 178oC. The small endothermic peak at 235 oC resulted from water dissociation from tungstic acid. The weight loss of 1.6 % until 350 oC corresponded to 1/4 12) H2O to WO 3·1/4H2O. It is assumed from DSC/TG analysis results that two steps of water and hydrogen peroxide elimination from perxotungstic acid occurred, where WO 3·xH 2O2·yH 2O was o o dissociated to WO 3·1/2H2O at 178 C, and then finally to WO 3 at 235 C. As is seen in Fig.1, an endothermic peak appeared at 445 oC and an exothermic peak was shown at 480oC which accompanied no weight loss. The peak at 445 oC corresponded to the crystallization of amorphous tungsten oxide to monoclinic phase, and the peak at 480oC was attributed to the crystalline phase transition from monoclinic to orthorhombic tungsten oxide. The crystal phase changes of WO 3 thin film on ITO glass was shown in Fig.3. As is seen, WO 3 thin o o film was amorphous at 300 C, but monoclinic phase WO 3 crystal peaks appeared at 400 C and the intensities of that peaks increased at 500 oC, which means that crystallinity increased with increasing o heat-treatment temperature. At 500 C, orthorhombic phase of WO 3 also appeared. The crystallization temperature of WO 3 was thought to be located between 300 and 400 , which was also expected at the results of DSC/TG analysis.

Materials Science Forum Vols. 544-545 1083

ITO Fig. 4 shows the cyclic voltamograms of WO3-monoclinic o o

o WO thin film heat-treated at 200 C, 300 C, 500 C 3 400 oC and 500 oC. The cyclovoltametric measurement of the WO 3 thin film was

400 oC conducted between –1V and +1V vs. Ag/AgCl in 0.1M H 2SO 4 solution and its scan

300 oC speed was 20mV/s. When –1 V was applied, Intensity (arb.unit) proton ion was inserted into WO 3 thin film and it became to H xWO 4 which is reduction

o 200 C state, but it was oxidized to WO 3 when +1V 10 20 30 40 50 60 70 80 was applied. CV curve of tungsten film heat- o 2θ (degree) treated at 200 C was smaller than that at 300 oC, which is thought to have resulted Fig. 3. XRD patterns of tungsten oxide film on ITO from the impurities and the remaining organics in the tungsten oxide film. The according to heat-treatment temperature amorphous phase heat-treated at 300 oC had – 2.6mA/cm 2 of current density at reduction state and +1.3mA/cm 2 at oxidation state higher than – 2 2 o 0.8mA/cm and 0.5mA/cm of the WO 3 thin film heat-treated at 500 C. This phenomenon occurred probably because proton ion was diffused more easily into the amorphous phase of WO 3 thin film than the crystal phase. For this reason, current density of tungsten oxide thin film by applied voltage tented to decrease with heat-treatment temperature. It is known that WO 3 thin film has a higher current density when it is heat-treated below 200 oC, but its electrochemical stability is degraded. 13) The results of the cyclic voltametric measurement in this research demonstrated that heat-treatment o above 300 C decreased the CV properties of WO 3 thin film so that the optimum heat-treatment temperature was 300 oC. Fig. 5 shows the optical transmission spectra of colored and bleached-state WO 3 thin film according to heat-treatment temperature. The transmittance of colored thin film o increased with heat-treatment temperature, 15.9% for colored-stat WO 3 thin film at 200 C, 14.6% for 300 oC, 16.3% for 400 oC, and 19.9% for 500 oC at 550nm wavelength of light. In the colored o state, the WO 3 thin film heat-treated at 300 C showed the lowest transmission. In the bleached state, o o the optical transmission of the WO 3 thin film was 69.9% for 200 C, 75.6% for 300 C, 73.3% for o o 400 C, 40.8% for 500 C. The most suitable optical transmission property was that of WO 3 thin film heat-treated at 300 oC. As mentioned before, heat-treatment of 200 oC was too low to evaporate water molecule or organics from the thin film so that its optical property was not good, while heat- o treatment at 400 and 500 C crystallized WO 3 thin film and reduced active sites for intercalation.

90 200 oC 80 o 1.5 300 C o Bleached 70 400 C 1.0 o 60 500 C Extraction

) 0.5

2 50

0.0 40

30 -0.5

-1.0 o Transmittance(%) Insertion 200 C 10 Colored o -1.5 300 C 0 400 oC -2.0 o Currentdensity (mA/cm 500 C -10 200 300 400 500 600 700 800 900 -2.5 Wavelength (nm) -3.0 -1.0 -0.5 0.0 0.5 1.0

Electrical potential (V vs. Ag/AgCl) Fig. 5. Visible light transmission of WO 3 thin film heat- treated at different temperatures at bleached state and

Fig. 4. Cyclic voltammetry of WO 3 thin film according colored state 1084 Eco-Materials Processing and Design VIII

o These findings led us to conclude that the WO 3 thin film heat-treated at 300 C had the most suitable electrochromic property. The optical transmittance difference of the film heat-treated at 300 oC between colored and bleached state at 550nm was 61%.

IV. CONCLUSIONS WO 3 thin film was coated in a sol-gel process from tungsten metal-based and tungstic acid-based solution precursors. The crystallization and the film thickness changes of WO 3 thin film were o o investigated. Monoclinic phase WO 3 crystals appeared at 400 C, while it was amorphous at 300 C. 200nm thick WO 3 thin film was obtained by using 1M of the coating solution in this research. An electrochemical analysis revealed that the heat-treatment temperature and film thickness strongly affected its cyclovoltametric property and electrochromic property. When the WO 3 thin film was heat-treated above 300 oC, its electrochemical property and electrochromic property were degraded by the crystallization of WO 3 thin film. The electrochromic property of WO 3 thin film heat-treated o o below 300 C was also deteriorated. Therefore, WO 3 thin film heat-treated at 300 C had the most suitable electrochemical and electrochromic properties in this research.

Acknowledgment The authors are appreciated that this research was conducted by financial support of Korean Ministry of Commerce, Industry and Energy.

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Electrochemical and Optical Properties of WO3 Prepared by Sol-Gel Coating 10.4028/www.scientific.net/MSF.544-545.1081