Preparation and Characterization of a Dip-Coated SnO2 Film for Transparent Electrodes for Transmissive Electrochromic Devices Paulo Olivi, Ernesto C. Pereira, Elson Longo, Josd A. Varella," and Luis Otavio de S. Bulh6es* Laboratdrio Interdisciplinar de Eletroqufmica e Cer~mica, Departamento de Quimica, Universidade Federal de S&o Carlos, 13560 Silo Carlos SP, Brazil ABSTRACT A new method for the synthesis of SnO2 is proposed and thin films are prepared by a dip-coating method. In the present paper we repo that these SnO2 films exhibit a reversible electrochemical insertion of lithium ions while maintaining high optical transmissivity. These film can be used as transparent counterelectrodes in electrochromic transmissive devices and in gas sensors. Considerable attention has been directed to using dip-coating methods for the production of single and multilayer coatings. This technique offers some advantages compared to other methods such as chemical vapor deposition or sputtering. Many fundamental studies of the synthesis of monodispersed metal oxide particles by solution routes have been carried out due to the need of ceramic materials with improved physical proper- tiesJ 2 Investigators have also been very active in the field of elec- trochromic devices in order to develop transparent electrodes for electrochromic windows. 3 Tin oxide is widely used as active sensor material for the detec- tion of toxic gases due to its ability to undergo gas-induced conduc- tivity changes. 4,~ Recently SnO2 has been prepared by a sol-gel- type condensation of tin(IV) ethoxide under basic solution to produce spherical, submicrometer SnO2 particles. 6 In this paper we describe a new method to prepare SnO2 films deposited by a dip-coating technique onto indium tin oxide (ITO) coated glass. We analyze the performance of this electrode as a transparent counterelectrode for a transmissive electrochromic device. The starting solution to produce SnO2 films was prepared by dissolving 50% w/w citric acid in ethylene glycol at 60~ Next tin citrate in a 3:1 molar ratio (acid:tin) was added to this solution with rapid stirring. A transparent pale yellow solution was obtained by * Electrochemical Society Active Member. a IQ-UNESP, Araraquara SP, Brazil. o-f 0 m O - - T2o A | | -B adding few drops of concentrated nitric acid. The presence of nitric acid solubilizes tin citrate and catalyzes the estherefication reaction between citric acid and ethylene glycol. The solution was heated at 110~ to eliminate water and nitric acid. In a few minutes the es- therefication reaction occurs resulting in a more viscous solution. The ITO coated glass was carefully cleaned, rinsed with ethanol and Milli-Q water, then dried with hot air. The ITO was dipped into the solution and withdrawn vertically at a low speed, then dried in an oven at 140~ for 1 h. After this step the ITO plate with the organometallic tin film was calcinated at 500~ for 1 h yielding a transparent and homogeneous film. The x-ray diffraction analysis carried out on the powder showed the presence of a crystalline SnO2 material. The infrared spectra shows the 600 cm -1 band characteristic of the Sn-O bonding. No bands associated with the organic molecules were present in the IR spectra of the powder treated at 500~ Figure 1 shows the cyclic voltammograms in acetonitrile (AN) 0.1M LiCIO4 for the ITO electrode (Fig. 1A) and for the ITO elec- trode with the 1.0 ~,m thick SnO2 film (Fig. 1B and 1C). During the reduction of Sn(IV) the insertion of lithium ions probably occurs. The cathodic and anodic processes are characteristic of an elec- trochemically reversible process. The charge associated with the cathodic and anodic processes at a sweep rate of 10 mV/s is 4 i~C/cm 2. There are no changes in the transmittance of the films on ITO electrode in the oxidized and reduced form as shown in Fig. 2. Impedance data for the SnO2 film electrode are shown in Fig. 3. The linear region observed at lower frequencies suggests that the rate of lithium injection be diffusion controlled. The diffusion coeffi- cient calculated using the model proposed by Ho et aL 7 was 5.8 • 10 -9 cm2/s after polarizing the electrode at 0.6 V. This paper presents the first investigation on the preparation, electrochemical and optical response of SnO2 films prepared by dip-coating using a solution. This system appears to be a very attractive material for the transparent counterelectrode in trans- 100 I I I I - 0 . 5 0 0.5 ~ l 0 E/V Fig. 1. Cyclic voltammograms (A) of an ITO electrode at 50 mV/s; (B) of a SnO2 film at 50 mV/s; and (C) of a SnO2 film at 200 mV/s in AN with 0.1M LiClO4. J. Electrochem. Soc., Vol. 140, No. 5, May 19939 The Electrochemical Society, Inc. 8C I.-- .N 4o ~ 2o 0~50 400 450 ] i i i I I 500 550 600 650 700 750 800 WAVELENGTH (nm) Fig. 2. Transmittance spectra in the visible region of a SnO~ thin film electrode: (- --) oxidized at 1,1 V and (--) reduced at 0.2 V. L81 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.89.24.43 Downloaded on 2014-10-11 to IP