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Preparation and properties of electrochromic iridium oxide thin film by sol-gel process
Thin Solid Films 350 (1999) 96±100
Preparation and properties of electrochromic iridium oxide thin ®lm by sol-gel process K. Nishio*, Y. Watanabe, T. Tsuchiya Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda-shi, Chiba 278-8510, Japan Received 18 May 1998; received in revised form 22 March 1999; accepted 13 April 1999
Abstract We established a method for preparation of iridium oxide thin ®lm by the sol-gel dip-coating process where iridium chloride was used as a starting material. The coating solution was prepared by reacting iridium chloride, ethanol and acetic acid. Iridium oxide coating was formed at 2.0 cm/min withdrawing rate. The coating ®lms heat treated at 3008C did not contain impurities. Iridium oxide crystallized at temperatures above 4508C. Both crystalline and amorphous iridium oxide thin ®lms showed electrochromism. The change in transmittance of the crystalline Ir2O3 ®lm is larger than that of the amorphous Ir2O3 under the same experimental conditions. The transmittance of the crystalline thin ®lm (®lm thickness 200 nm, measured at 400 nm) decreased 13.0% on application of 3 V for 1 s. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Sol-gel process; Electrical properties and measurements; Iridium; Optical properties; Oxides
1. Introduction Electrochromism is a phenomenon in which the color of material changes on application of a voltage. Many electrochromic materials have been reported (e.g. tungsten oxide, nickel oxide, molybdenum oxide, etc.) [1±7]. Electrochromic materials are expected to be applied for smart windows and displays (electrochromic display: (ECD)). Application to window glass makes it possible to change the solar optical permeation in summer and winter. As a result, the electric power consumption of air conditioners can be reduced. At present, cathode-ray tubes or liquid crystal panels are generally used in displays. But cathode-ray tubes do not readily produce pale patterns and can not be manufactured with light weight. ECDs can display pale patterns and are light in weight, and also solve problems of image degradation at the corners of the screen. Iridium oxide is one of electrochromic materials. Iridium oxide is a hydrated crystal. When a proton is removed from the H2O, the electric charge of iridium is changed. The color
* Corresponding author. Fax: 181-471-23-9362. E-mail address: [email protected] (K. Nishio)
of iridium oxide then changes from transparent to brown [7]. ÿ Ir2 O3 ´xH2 O transparent 2 2H1 2 2e2 ) Ir2 O4 ´ x 2 1H2 O brown
1
A sol-gel process can be used to prepare ceramics and glasses at lower temperatures than conventional sintering processes [8,9]. In general, the sol-gel process uses metal alkoxides or metal salts as raw materials. Some kinds of alcohol are used as organic solvents. But metal alkoxides are more expensive than metal salts and are dif®cult to handle. Usually, iridium oxide is prepared by a sputtering method [10,11]. Few reports have described preparation of iridium oxide from solution, since iridium alkoxide cannot be so prepared. Moreover, the solubility of iridium compounds, including the oxide, is low in most solvents. In 1992 Michalak [12] reported the preparation of iridium alkoxide from an ethanol solution of iridium chloride by irradiation with g -rays. However, it is not easy to use g -rays. We studied preparation of iridium oxide thin ®lm by a solgel process from a metal salt and organic solvent. The structure of the coating solution was investigated. An ECD consisting of a liquid electrolyte and iridium oxide thin ®lm on indium-tin oxide (ITO) coated glass substrate was
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0029 0-4
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prepared. The electric and optical properties of the iridium oxide thin ®lms were investigated. 2. Experimental procedure Fig. 1 shows the fabrication procedure of iridium oxide thin ®lms. Coating solution for preparing iridium oxide thin ®lms was prepared from anhydrous iridium chloride (IrCl4) and ethanol (C2H5OH). IrCl4 1 g (High Purity Chemetals Laboratory Co., Ltd.) was dissolved in C2H5OH 42 ml (Wako Pure Chemical Industries, Ltd.) in a dry box ®lled with dry N2 gas at room temperature. Acetic acid (CH3COOH: Wako Pure Chemical Industries, Ltd.) was added to the solution (from 0 to 6: IrCl4 molar ratio). Thin ®lms were prepared by dip coating at withdrawing rate of 2.0 cm/min from the solution on indium tin oxide (ITO) thin ®lm, RA 20 V (sheet resistance) coated glass substrates. These coated substrates were heat treated in the furnace at different temperatures in air. Structural analysis of the coating solutions and thin ®lms were performed using a Shimazu UV-3000 spectrometer and Shimazu Fourier transform infrared spectrometer (FTIR) 4200. Crystal structure of the ®lms were examined by using X-ray diffractometer (Rigaku model CN4148) with a thin ®lm attachment. Film thicknesses were measured using a high precision surface pro®lometer (Kosaka model ET-10S). Residual chlorine on the surface and inside of the thin ®lm was investigated by X-ray photoelectron spectroscopy system (JEOL-JPS-90SX) using Mg Ka (1253.6 eV) radiation. Au (1 mmf ) was deposited on center of thin ®lm by vacuum evaporation. Au 4f7/2 (83.8 eV) was used as reference [13]. The thin ®lm was etched by Ar ion sputtering. The ECD consisted of transparent electrodes (ITO), electrochromic material (Ir2O3) and electrolyte (H2SO4 mixed with ethylene glycol). The electrical and electrochromic coloration properties of these thin ®lms were measured.
Fig. 2. XPS spectra of the prepared iridium oxide thin ®lms heat treated at 3008C. (a) prepared from the solution without addition of CH3COOH, (b) prepared from the solution with addition of CH3COOH.
For electrical property measurements, an ITO coated glass substrate was used as the counter electrode. The space between two electrodes was kept at 1 mm by a rubber spacer. The solution electrolyte was H2SO4 mixed with ethylene glycol (0.1 mol/l:H2SO4). Cyclic voltammetric experiments were performed at a scanning speed of 100 mV/s by using the HZ-1A cyclic voltammetric system (Hokuto Denkou) with HC-205C (TOA Electronics Ltd.) as a reference electrode. The coloration properties of these thin ®lms were measured by UV-spectrophotometer over the range 400± 800 nm. An ITO coated glass substrate was used as reference. 3. Results
Fig. 1. Procedure for preparation of iridium oxide thin ®lm.
For preparation of iridium oxide, IrCl4 was selected as a raw material, since Ir2O3 or Ir2O3 z xH2O hardly dissolves in ethanol or water. IrCl4 was dissolved in ethanol by extensive stirring at room temperature for 30 min. As the IrCl4 dissolved, the solution became brown, and this color never changed. Iridium was con®rmed to exist as an oxide by XRD analysis of the thin ®lm prepared from the solution without addition of CH3COOH. Fig. 2a,b shows the XPS
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Fig. 3. Transmittance spectra of the coating solution; (ÐÐ) IrCl4 : CH3 COOH 1 : 0 (molar ratio) ; (ÐÐ - - -) 1:1; (ÐÐ) 1:3; (- - -) 1:6.
wide range scan of iridium oxide thin ®lm prepared from the solution without and with the addition of CH3COOH, respectively. This spectrum was measured after the surface of the thin ®lm was etched by Ar ion sputtering. It was con®rmed that chlorine remained within the thin ®lm in Fig. 2a. It is known that iridium ions and organic solvents with a carbonyl group form carbonyl complex ions. A greater than equimolar quantity of CH3COOH was added to the IrCl4 and ethanol solution, and the solution was stirred for 24 h. Fig. 2b shows the XPS spectrum of iridium oxide thin ®lm prepared from a solution with added CH3COOH by the ratio IrCl4 : CH3 COOH 1 : 6. The etching condition is the same as for Fig. 2a. The peaks of chlorine (Cl 2p3/2 at 198.4 eV and Cl 2s1/2 at 270.3 eV [13]) observed in the spectrum of the iridium oxide thin ®lm prepared from the solution without addition of CH3COOH are completely absent in the spectrum of Fig. 2b. Fig. 3 shows transmittance spectra of the solution. The color of solution was changed as the amount of CH3COOH was increased. Without addition of CH3COOH the solution was brown, and showed absorption on the short wavelength range. The transmittance of the IrCl4 ethanol solution (IrCl4 : CH3 COOH 1 : 1; molar ratio) was higher than that of the solution without addition of CH3COOH. Furthermore, when the quantity of added CH3COOH was increased, the transmittance was decreased. The IrCl4 : CH3 COOH 1 : 6 solution became black. Fig. 4 shows FTIR spectra (C2H5OH mixed with CH3COOH (a), magnesium chloride dissolved in C2H5OH with added CH3COOH (magnesium chloride does not react with CH3COOH), and the coating solution with CH3COOH (c)). Absorption due to stretching vibration of the CvO bond formed only a single peak in solution (a) and solution (b). The absorption peak at 1710 cm 21 agreed with stretch-
ing vibration of the CvO bond of non-reacted CH3COOH [14]. The absorption peak of the (c) solution spectrum split into two. The splitting is explain to form something complex. Fig. 5 shows XRD patterns of the iridium oxide thin ®lms which were prepared from the solution with added CH3COOH. Iridium oxide thin ®lms were prepared on ITO coated glass substrate, and were heat-treated at different temperatures. Crystallization began from 4008C. The crystal structure of the prepared iridium oxide agreed with the Ir2O4 crystal structure [JCPDS card 15-870]. As deposited amorphous and crystalline iridium oxide thin ®lms were brown. The crystalline thin ®lm was darker than the amorphous ®lm. Fig. 6 shows cyclic voltammogram of an amorphous and crystalline Ir2O3 ®lms (®lm thickness 200 nm) cycled at 100 mV/s. The current density of the crystal thin ®lm is almost equal to that of amorphous thin ®lm in this voltage range. The color of crystalline or amorphous thin ®lm changed from brown to transparent with increasing positive voltage, and from transparent to brown with increasing negative voltage. Although the I±V curve is asymmetric, total charge for the bleaching by positive applied voltage was equal to that for the coloring. In these measurement, gas was observed to evolve at surface of counter electrode. Fig. 7 shows transmittance spectra of these thin ®lms in the range from 400±800 nm. The transmittance of amorphous thin ®lm was greater than that of crystalline thin ®lm for the bleached ®lms. The relative transmittance
Fig. 4. IR spectra of the coating solution: (a) C2H5OH and CH3COOH solution, (b) Magnesium chloride dissolved in C2H5OH and CH3COOH solution, (c) Iridium chloride dissolved in C2H5OH and CH3COOH solution.
K. Nishio et al. / Thin Solid Films 350 (1999) 96±100
Fig. 5. XRD patterns of the iridium oxide thin ®lms prepared on ITO coated glass substrate, heat treated at different temperatures (peaks marked by W indicate the presence of the crystalline phase of iridium oxide).
change for crystalline thin ®lm was slightly greater than that for amorphous thin ®lm. It is also observed that the ®lms showed large absorption in the short wavelength range.
4. Discussion Iridium oxide (Ir2O3, Ir2O4) is a stable material, and hardly dissolves in organic solvent or H2O. However, iridium oxide can dissolve in several organic solvents by addition of hydrochloric acid. Iridium chloride has higher solubility than iridium oxide in H2O or organic solvents. As for iridium chloride dissolved in H2O or organic solvents, iridium exist
Fig. 6. Cyclic voltammogram of the amorphous thin ®lm (W) and the crystalline thin ®lm (X) cycled at 100 mV/s (®lm thickness 200 nm).
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Fig. 7. Transmittance spectra of the colored amorphous thin ®lm and the crystalline thin ®lm. (®lm thickness 200 nm, application of 3 V for 1 s): (ÐÐ) bleached amorphous thin ®lm; (Ð Ð ± ±) colored amorphous thin ®lm; (- - - -) bleached crystalline thin ®lm; (± ± ±) colored crystalline thin ®lm.
in the solution as chloride complex ions. These chloride complex ions are very stable in the H2O or organic solution [15]. While iridium was con®rmed to exist as oxide by XRD analysis of thin ®lm prepared from solution without added CH3COOH, a large quantity of chlorine was detected at the surface and within the thin ®lm by XPS. However, iridium chloride could not be observed in XRD patterns. It is thought that chlorine ions remained in the ®lm during solvent evaporation and iridium chloride was reformed, but detection in the crystal grains or in the amorphous state is very dif®cult by XRD analysis. The transmittance spectra of solution without added CH3COOH showed high transparency on the long wavelength range, and showed large absorption on the short wavelength range. The transmittance spectra of solution was changed with addition of CH3COOH. The color changed from brown to black when the CH3COOH content was increased. The splitting of the absorption peak due to CvO stretching vibration was observed in FTIR spectra. This splitting of the CvO stretching vibration shows the formation of carbonyl complex. It is reported that CH3COOH forms complex ions with many elements [15]. It is known that iridium forms a carbonyl complex ion with carbonyl groups. Chlorine was not detected by XPS at the surface and within the thin ®lms prepared from solution with added CH3COOH, therefore it is thought that CH3COOH can displace chlorine of iridium chloride complex in the solution. In other words, it is thought that carbonyl group may take the place of chlorine sites. The color change of the solution from brown to black is further evidence that carbonyl groups take the place of chlorine sites. This black is called iridium black, and results when iridium is complexed with carbonyl groups [13]. It is thus possible that there was no chlorine in the thin ®lm. The prepared crystalline thin ®lms were Ir2 O4 ´ x 2 1H2 O
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as shown by XRD. The amorphous thin ®lm was brown, and the crystalline thin ®lm was dark brown. As deposited amorphous and crystalline thin ®lms showed electrochromism. The current density of crystalline thin ®lm is almost equal to that of amorphous thin ®lm in the range of applied voltages. The color of crystalline or amorphous thin ®lm changes reversibly from brown to transparent. The color of crystalline thin ®lm was darker than that of amorphous thin ®lm. Iridium oxide shows electrochromism through the insertion and removal of protons and electrons from water of crystallization, which is coordinated with iridium oxide. In an iridium oxide crystal, atom ordering is greater than in amorphous material. It is thought that the difference in the transmittance change is due to the number of H2O molecules coordinated with Ir2O3, which is larger in crystalline Ir2O3 than in amorphous. Two causes for the gas evolution at surface of ITO counter electrode are thought. One is decomposition of the electrolyte solution. The electrochromism of iridium oxide is due to insertion and removal of protons and electrons. When a negative voltage is applied to the ECD, electrons and protons are pulled out from the iridium oxide, and transferred to the electrolyte solution. The decomposition of the electrolyte solution is caused by the supplying electrons and protons, and gas evolution may result. The other is reduction of ITO counter electrode. If all electrons and protons can not be accepted by electrolyte, or some electrons and protons are supplied to ITO. We have reported properties of fully solid state ECD (ITO/iridium oxide/gel electrolyte/tungsten oxide/ITO) [16]. The fully solid state ECD system did not cause the gas evolution because both tungsten oxide and iridium oxide are electrochromic materials. Tungsten oxide can accept protons and electrons from iridium oxide. ECD consisted with these materials inhibits the decomposition of the electrolyte solution. 5. Conclusions Iridium oxide thin ®lm was prepared by a sol-gel process
from metal chloride and ethanol. A stable iridium oxide coating solution was successfully prepared. We found that addition of CH3COOH to an ethanol solution of iridium chloride improved the coating quality. It is thought that the addition of the CH3COOH displace chlorine of iridium chloride complex in the solution. Iridium oxide ®lms could be prepared by heat treatment at a temperature of 3008C. Chlorine was not detected by XPS at the surface or within the thin ®lm prepared from CH3COOH added solution. Iridium oxide ®lms crystallized at 4508C and above. Anodic electrochromism was observed in both crystalline and amorphous iridium oxide ®lms. The transmittance change of crystalline thin ®lm (13.0% at 400 nm) was larger than that for amorphous thin ®lm (8.5% at 400 nm) under the same conditions. References [1] C.G. Grangvist, Handbook of Inorganic Electrochromic, Elsevier, Amsterdam, 1995. [2] P.M.S. Monk, T. Ali, R.D. Partridge, Solid State Ionics 80 (1995) 75. [3] T. Maruyama, T. Kanagawa, J. Electrochem. Soc. 143 (1996) 1675. [4] H. Kanoh, T. Hirotsu, K. Ooi, J. Electrochem. Soc. 143 (1996) 905. [5] M.J. Sienko, Adv. Chem. 39 (1963) 224. [6] O. Bohnke, G. Robert, Solid State Ionics 6 (1982) 115. [7] S.F. Cogan, T.D. Plante, R.S. McFadden, R.D. Rauh, Solar Energy Mater. 16 (1987) 371. [8] T. Tsuchiya, Y. Iitani, H. Nakajima, J. Ceram, Soc. Jpn. 96 (1988) 625. [9] M.I. Yanovskaya, I.E. Obvintseva, V.G. Kessler, B. Sh. Galyamov, S.I. Kucheiko, R.R. Shifrina, N.Ya. Turova, Galyamov, J. Non-Cryst. Solids 124 (1990) 155. [10] M. Watanabe, Y. Koike, T. Yoshimura, K. Kiyota, Proc. Jpn. Disp. 83 (1983) 372. [11] R.D. Rauh, S.F. Cogan, J. Electrochem. Soc. 140 (1993) 378. [12] F. Michalak, L. Rault, P. Aldebert, SPIE 1728 (1992) 278. [13] N. Ikeo, Y. Iijima, N. Niimura, et al., Handbook of X-ray Photoelectron Spectroscopy, JEOL (in Japanese), 1991. [14] H. Horiguti, Sekigaikyukouzu Kaisetusouron, (in Japanese) Sankyousyttupan, Tokyo, 1989. [15] S. Kubo, R. Nagakura, H. Iguchi, H. Ezawa, Rikagakujiten (in Japanese), Iwanamishyoten, Tokyo, 1987 p. 85. [16] K. Nishio, T. Sei, T. Tsuchiya, Proc. Sol-gel Opt. 4 (1997) 419.
Preparation and properties of electrochromic iridium oxide thin ®lm by sol-gel process K. Nishio*, Y. Watanabe, T. Tsuchiya Department of Materials Science and Technology, Faculty of Industrial Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda-shi, Chiba 278-8510, Japan Received 18 May 1998; received in revised form 22 March 1999; accepted 13 April 1999
Abstract We established a method for preparation of iridium oxide thin ®lm by the sol-gel dip-coating process where iridium chloride was used as a starting material. The coating solution was prepared by reacting iridium chloride, ethanol and acetic acid. Iridium oxide coating was formed at 2.0 cm/min withdrawing rate. The coating ®lms heat treated at 3008C did not contain impurities. Iridium oxide crystallized at temperatures above 4508C. Both crystalline and amorphous iridium oxide thin ®lms showed electrochromism. The change in transmittance of the crystalline Ir2O3 ®lm is larger than that of the amorphous Ir2O3 under the same experimental conditions. The transmittance of the crystalline thin ®lm (®lm thickness 200 nm, measured at 400 nm) decreased 13.0% on application of 3 V for 1 s. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Sol-gel process; Electrical properties and measurements; Iridium; Optical properties; Oxides
1. Introduction Electrochromism is a phenomenon in which the color of material changes on application of a voltage. Many electrochromic materials have been reported (e.g. tungsten oxide, nickel oxide, molybdenum oxide, etc.) [1±7]. Electrochromic materials are expected to be applied for smart windows and displays (electrochromic display: (ECD)). Application to window glass makes it possible to change the solar optical permeation in summer and winter. As a result, the electric power consumption of air conditioners can be reduced. At present, cathode-ray tubes or liquid crystal panels are generally used in displays. But cathode-ray tubes do not readily produce pale patterns and can not be manufactured with light weight. ECDs can display pale patterns and are light in weight, and also solve problems of image degradation at the corners of the screen. Iridium oxide is one of electrochromic materials. Iridium oxide is a hydrated crystal. When a proton is removed from the H2O, the electric charge of iridium is changed. The color
* Corresponding author. Fax: 181-471-23-9362. E-mail address: [email protected] (K. Nishio)
of iridium oxide then changes from transparent to brown [7]. ÿ Ir2 O3 ´xH2 O transparent 2 2H1 2 2e2 ) Ir2 O4 ´ x 2 1H2 O brown
1
A sol-gel process can be used to prepare ceramics and glasses at lower temperatures than conventional sintering processes [8,9]. In general, the sol-gel process uses metal alkoxides or metal salts as raw materials. Some kinds of alcohol are used as organic solvents. But metal alkoxides are more expensive than metal salts and are dif®cult to handle. Usually, iridium oxide is prepared by a sputtering method [10,11]. Few reports have described preparation of iridium oxide from solution, since iridium alkoxide cannot be so prepared. Moreover, the solubility of iridium compounds, including the oxide, is low in most solvents. In 1992 Michalak [12] reported the preparation of iridium alkoxide from an ethanol solution of iridium chloride by irradiation with g -rays. However, it is not easy to use g -rays. We studied preparation of iridium oxide thin ®lm by a solgel process from a metal salt and organic solvent. The structure of the coating solution was investigated. An ECD consisting of a liquid electrolyte and iridium oxide thin ®lm on indium-tin oxide (ITO) coated glass substrate was
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0029 0-4
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prepared. The electric and optical properties of the iridium oxide thin ®lms were investigated. 2. Experimental procedure Fig. 1 shows the fabrication procedure of iridium oxide thin ®lms. Coating solution for preparing iridium oxide thin ®lms was prepared from anhydrous iridium chloride (IrCl4) and ethanol (C2H5OH). IrCl4 1 g (High Purity Chemetals Laboratory Co., Ltd.) was dissolved in C2H5OH 42 ml (Wako Pure Chemical Industries, Ltd.) in a dry box ®lled with dry N2 gas at room temperature. Acetic acid (CH3COOH: Wako Pure Chemical Industries, Ltd.) was added to the solution (from 0 to 6: IrCl4 molar ratio). Thin ®lms were prepared by dip coating at withdrawing rate of 2.0 cm/min from the solution on indium tin oxide (ITO) thin ®lm, RA 20 V (sheet resistance) coated glass substrates. These coated substrates were heat treated in the furnace at different temperatures in air. Structural analysis of the coating solutions and thin ®lms were performed using a Shimazu UV-3000 spectrometer and Shimazu Fourier transform infrared spectrometer (FTIR) 4200. Crystal structure of the ®lms were examined by using X-ray diffractometer (Rigaku model CN4148) with a thin ®lm attachment. Film thicknesses were measured using a high precision surface pro®lometer (Kosaka model ET-10S). Residual chlorine on the surface and inside of the thin ®lm was investigated by X-ray photoelectron spectroscopy system (JEOL-JPS-90SX) using Mg Ka (1253.6 eV) radiation. Au (1 mmf ) was deposited on center of thin ®lm by vacuum evaporation. Au 4f7/2 (83.8 eV) was used as reference [13]. The thin ®lm was etched by Ar ion sputtering. The ECD consisted of transparent electrodes (ITO), electrochromic material (Ir2O3) and electrolyte (H2SO4 mixed with ethylene glycol). The electrical and electrochromic coloration properties of these thin ®lms were measured.
Fig. 2. XPS spectra of the prepared iridium oxide thin ®lms heat treated at 3008C. (a) prepared from the solution without addition of CH3COOH, (b) prepared from the solution with addition of CH3COOH.
For electrical property measurements, an ITO coated glass substrate was used as the counter electrode. The space between two electrodes was kept at 1 mm by a rubber spacer. The solution electrolyte was H2SO4 mixed with ethylene glycol (0.1 mol/l:H2SO4). Cyclic voltammetric experiments were performed at a scanning speed of 100 mV/s by using the HZ-1A cyclic voltammetric system (Hokuto Denkou) with HC-205C (TOA Electronics Ltd.) as a reference electrode. The coloration properties of these thin ®lms were measured by UV-spectrophotometer over the range 400± 800 nm. An ITO coated glass substrate was used as reference. 3. Results
Fig. 1. Procedure for preparation of iridium oxide thin ®lm.
For preparation of iridium oxide, IrCl4 was selected as a raw material, since Ir2O3 or Ir2O3 z xH2O hardly dissolves in ethanol or water. IrCl4 was dissolved in ethanol by extensive stirring at room temperature for 30 min. As the IrCl4 dissolved, the solution became brown, and this color never changed. Iridium was con®rmed to exist as an oxide by XRD analysis of the thin ®lm prepared from the solution without addition of CH3COOH. Fig. 2a,b shows the XPS
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Fig. 3. Transmittance spectra of the coating solution; (ÐÐ) IrCl4 : CH3 COOH 1 : 0 (molar ratio) ; (ÐÐ - - -) 1:1; (ÐÐ) 1:3; (- - -) 1:6.
wide range scan of iridium oxide thin ®lm prepared from the solution without and with the addition of CH3COOH, respectively. This spectrum was measured after the surface of the thin ®lm was etched by Ar ion sputtering. It was con®rmed that chlorine remained within the thin ®lm in Fig. 2a. It is known that iridium ions and organic solvents with a carbonyl group form carbonyl complex ions. A greater than equimolar quantity of CH3COOH was added to the IrCl4 and ethanol solution, and the solution was stirred for 24 h. Fig. 2b shows the XPS spectrum of iridium oxide thin ®lm prepared from a solution with added CH3COOH by the ratio IrCl4 : CH3 COOH 1 : 6. The etching condition is the same as for Fig. 2a. The peaks of chlorine (Cl 2p3/2 at 198.4 eV and Cl 2s1/2 at 270.3 eV [13]) observed in the spectrum of the iridium oxide thin ®lm prepared from the solution without addition of CH3COOH are completely absent in the spectrum of Fig. 2b. Fig. 3 shows transmittance spectra of the solution. The color of solution was changed as the amount of CH3COOH was increased. Without addition of CH3COOH the solution was brown, and showed absorption on the short wavelength range. The transmittance of the IrCl4 ethanol solution (IrCl4 : CH3 COOH 1 : 1; molar ratio) was higher than that of the solution without addition of CH3COOH. Furthermore, when the quantity of added CH3COOH was increased, the transmittance was decreased. The IrCl4 : CH3 COOH 1 : 6 solution became black. Fig. 4 shows FTIR spectra (C2H5OH mixed with CH3COOH (a), magnesium chloride dissolved in C2H5OH with added CH3COOH (magnesium chloride does not react with CH3COOH), and the coating solution with CH3COOH (c)). Absorption due to stretching vibration of the CvO bond formed only a single peak in solution (a) and solution (b). The absorption peak at 1710 cm 21 agreed with stretch-
ing vibration of the CvO bond of non-reacted CH3COOH [14]. The absorption peak of the (c) solution spectrum split into two. The splitting is explain to form something complex. Fig. 5 shows XRD patterns of the iridium oxide thin ®lms which were prepared from the solution with added CH3COOH. Iridium oxide thin ®lms were prepared on ITO coated glass substrate, and were heat-treated at different temperatures. Crystallization began from 4008C. The crystal structure of the prepared iridium oxide agreed with the Ir2O4 crystal structure [JCPDS card 15-870]. As deposited amorphous and crystalline iridium oxide thin ®lms were brown. The crystalline thin ®lm was darker than the amorphous ®lm. Fig. 6 shows cyclic voltammogram of an amorphous and crystalline Ir2O3 ®lms (®lm thickness 200 nm) cycled at 100 mV/s. The current density of the crystal thin ®lm is almost equal to that of amorphous thin ®lm in this voltage range. The color of crystalline or amorphous thin ®lm changed from brown to transparent with increasing positive voltage, and from transparent to brown with increasing negative voltage. Although the I±V curve is asymmetric, total charge for the bleaching by positive applied voltage was equal to that for the coloring. In these measurement, gas was observed to evolve at surface of counter electrode. Fig. 7 shows transmittance spectra of these thin ®lms in the range from 400±800 nm. The transmittance of amorphous thin ®lm was greater than that of crystalline thin ®lm for the bleached ®lms. The relative transmittance
Fig. 4. IR spectra of the coating solution: (a) C2H5OH and CH3COOH solution, (b) Magnesium chloride dissolved in C2H5OH and CH3COOH solution, (c) Iridium chloride dissolved in C2H5OH and CH3COOH solution.
K. Nishio et al. / Thin Solid Films 350 (1999) 96±100
Fig. 5. XRD patterns of the iridium oxide thin ®lms prepared on ITO coated glass substrate, heat treated at different temperatures (peaks marked by W indicate the presence of the crystalline phase of iridium oxide).
change for crystalline thin ®lm was slightly greater than that for amorphous thin ®lm. It is also observed that the ®lms showed large absorption in the short wavelength range.
4. Discussion Iridium oxide (Ir2O3, Ir2O4) is a stable material, and hardly dissolves in organic solvent or H2O. However, iridium oxide can dissolve in several organic solvents by addition of hydrochloric acid. Iridium chloride has higher solubility than iridium oxide in H2O or organic solvents. As for iridium chloride dissolved in H2O or organic solvents, iridium exist
Fig. 6. Cyclic voltammogram of the amorphous thin ®lm (W) and the crystalline thin ®lm (X) cycled at 100 mV/s (®lm thickness 200 nm).
99
Fig. 7. Transmittance spectra of the colored amorphous thin ®lm and the crystalline thin ®lm. (®lm thickness 200 nm, application of 3 V for 1 s): (ÐÐ) bleached amorphous thin ®lm; (Ð Ð ± ±) colored amorphous thin ®lm; (- - - -) bleached crystalline thin ®lm; (± ± ±) colored crystalline thin ®lm.
in the solution as chloride complex ions. These chloride complex ions are very stable in the H2O or organic solution [15]. While iridium was con®rmed to exist as oxide by XRD analysis of thin ®lm prepared from solution without added CH3COOH, a large quantity of chlorine was detected at the surface and within the thin ®lm by XPS. However, iridium chloride could not be observed in XRD patterns. It is thought that chlorine ions remained in the ®lm during solvent evaporation and iridium chloride was reformed, but detection in the crystal grains or in the amorphous state is very dif®cult by XRD analysis. The transmittance spectra of solution without added CH3COOH showed high transparency on the long wavelength range, and showed large absorption on the short wavelength range. The transmittance spectra of solution was changed with addition of CH3COOH. The color changed from brown to black when the CH3COOH content was increased. The splitting of the absorption peak due to CvO stretching vibration was observed in FTIR spectra. This splitting of the CvO stretching vibration shows the formation of carbonyl complex. It is reported that CH3COOH forms complex ions with many elements [15]. It is known that iridium forms a carbonyl complex ion with carbonyl groups. Chlorine was not detected by XPS at the surface and within the thin ®lms prepared from solution with added CH3COOH, therefore it is thought that CH3COOH can displace chlorine of iridium chloride complex in the solution. In other words, it is thought that carbonyl group may take the place of chlorine sites. The color change of the solution from brown to black is further evidence that carbonyl groups take the place of chlorine sites. This black is called iridium black, and results when iridium is complexed with carbonyl groups [13]. It is thus possible that there was no chlorine in the thin ®lm. The prepared crystalline thin ®lms were Ir2 O4 ´ x 2 1H2 O
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as shown by XRD. The amorphous thin ®lm was brown, and the crystalline thin ®lm was dark brown. As deposited amorphous and crystalline thin ®lms showed electrochromism. The current density of crystalline thin ®lm is almost equal to that of amorphous thin ®lm in the range of applied voltages. The color of crystalline or amorphous thin ®lm changes reversibly from brown to transparent. The color of crystalline thin ®lm was darker than that of amorphous thin ®lm. Iridium oxide shows electrochromism through the insertion and removal of protons and electrons from water of crystallization, which is coordinated with iridium oxide. In an iridium oxide crystal, atom ordering is greater than in amorphous material. It is thought that the difference in the transmittance change is due to the number of H2O molecules coordinated with Ir2O3, which is larger in crystalline Ir2O3 than in amorphous. Two causes for the gas evolution at surface of ITO counter electrode are thought. One is decomposition of the electrolyte solution. The electrochromism of iridium oxide is due to insertion and removal of protons and electrons. When a negative voltage is applied to the ECD, electrons and protons are pulled out from the iridium oxide, and transferred to the electrolyte solution. The decomposition of the electrolyte solution is caused by the supplying electrons and protons, and gas evolution may result. The other is reduction of ITO counter electrode. If all electrons and protons can not be accepted by electrolyte, or some electrons and protons are supplied to ITO. We have reported properties of fully solid state ECD (ITO/iridium oxide/gel electrolyte/tungsten oxide/ITO) [16]. The fully solid state ECD system did not cause the gas evolution because both tungsten oxide and iridium oxide are electrochromic materials. Tungsten oxide can accept protons and electrons from iridium oxide. ECD consisted with these materials inhibits the decomposition of the electrolyte solution. 5. Conclusions Iridium oxide thin ®lm was prepared by a sol-gel process
from metal chloride and ethanol. A stable iridium oxide coating solution was successfully prepared. We found that addition of CH3COOH to an ethanol solution of iridium chloride improved the coating quality. It is thought that the addition of the CH3COOH displace chlorine of iridium chloride complex in the solution. Iridium oxide ®lms could be prepared by heat treatment at a temperature of 3008C. Chlorine was not detected by XPS at the surface or within the thin ®lm prepared from CH3COOH added solution. Iridium oxide ®lms crystallized at 4508C and above. Anodic electrochromism was observed in both crystalline and amorphous iridium oxide ®lms. The transmittance change of crystalline thin ®lm (13.0% at 400 nm) was larger than that for amorphous thin ®lm (8.5% at 400 nm) under the same conditions. References [1] C.G. Grangvist, Handbook of Inorganic Electrochromic, Elsevier, Amsterdam, 1995. [2] P.M.S. Monk, T. Ali, R.D. Partridge, Solid State Ionics 80 (1995) 75. [3] T. Maruyama, T. Kanagawa, J. Electrochem. Soc. 143 (1996) 1675. [4] H. Kanoh, T. Hirotsu, K. Ooi, J. Electrochem. Soc. 143 (1996) 905. [5] M.J. Sienko, Adv. Chem. 39 (1963) 224. [6] O. Bohnke, G. Robert, Solid State Ionics 6 (1982) 115. [7] S.F. Cogan, T.D. Plante, R.S. McFadden, R.D. Rauh, Solar Energy Mater. 16 (1987) 371. [8] T. Tsuchiya, Y. Iitani, H. Nakajima, J. Ceram, Soc. Jpn. 96 (1988) 625. [9] M.I. Yanovskaya, I.E. Obvintseva, V.G. Kessler, B. Sh. Galyamov, S.I. Kucheiko, R.R. Shifrina, N.Ya. Turova, Galyamov, J. Non-Cryst. Solids 124 (1990) 155. [10] M. Watanabe, Y. Koike, T. Yoshimura, K. Kiyota, Proc. Jpn. Disp. 83 (1983) 372. [11] R.D. Rauh, S.F. Cogan, J. Electrochem. Soc. 140 (1993) 378. [12] F. Michalak, L. Rault, P. Aldebert, SPIE 1728 (1992) 278. [13] N. Ikeo, Y. Iijima, N. Niimura, et al., Handbook of X-ray Photoelectron Spectroscopy, JEOL (in Japanese), 1991. [14] H. Horiguti, Sekigaikyukouzu Kaisetusouron, (in Japanese) Sankyousyttupan, Tokyo, 1989. [15] S. Kubo, R. Nagakura, H. Iguchi, H. Ezawa, Rikagakujiten (in Japanese), Iwanamishyoten, Tokyo, 1987 p. 85. [16] K. Nishio, T. Sei, T. Tsuchiya, Proc. Sol-gel Opt. 4 (1997) 419.