Download Preparation and Characterization of a Dip-Coated SnO2

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 report
that these SnO2 films exhibit a reversible electrochemical insertion of lithium ions while maintaining high optical transmissivity. These films
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 propertiesJ 2 Investigators have also been very active in the field of electrochromic devices in order to develop transparent electrodes for
electrochromic windows.3
Tin oxide is widely used as active sensor material for the detection of toxic gases due to its ability to undergo gas-induced conductivity changes.4,~ Recently SnO2 has been prepared by a sol-geltype 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
T2oA |
O--
|
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 estherefication 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 electrode 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 electrochemically reversible process. The charge associated with the
cathodic and anodic processes at a sweep rate of 10 mV/s is
4 i~C/cm2.
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 coefficient 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
8C
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0 m
I.--
.N 4o
~ 2o
I
-0.5
I
I
I
0
0.5
~l 0
]
i
i
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I
WAVELENGTH (nm)
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.
i
0~50 400 450 500 550 600 650 700 750 800
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.
J. Electrochem. Soc., Vol. 140, No. 5, May 1993 9 The Electrochemical Society, Inc.
L81
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J. Electrochem. Soc., Vol. 140, No. 5, May 1993 9 The Electrochemical Society, Inc.
L82
Further characterization of these electrodes as well as the application of these films in liquid or solid state electrochromic devices
and sensors are in progress in this laboratory.
Acknowledgments
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The authors acknowledge the financial support from PADCT/
FINEP, CNPq and FAPESP.
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Manuscript submitted Dec. 9, 1992; revised manuscript received
Feb. 8, 1993.
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2.0
Universidade Federal de Sao Carlos assisted in meeting the
publication costs of this article.
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REFERENCES
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oi
0
I
I
2.0
4.0
Z' / .0. x 10-'I
Fig. 3. Complex plane plot for an SnO2 film electrode in AN
0,1 M LiCIO4 solution at 0,6 V.
missive electrochromic devices using nonaqueous solvents or
polymer electrolytes.
1. C. J. Bunker, D. E. Clark, and D. R. Ulrich, Better Ceramics
Through Chemistry II, Materials Research Soc. (1986).
2. G. L. Messing, K. S. Mazniyasni, J. McCauley, and R. Haber,
Ceramic Powder Science, American Ceramic Society, Westerville, OH (1987).
3. P. Baudry, A. C. M. Rodrigues, M. A. Aegerter, and L. O. S.
BulhSes, J. Non-Cryst. Solids, 121,319 (1990).
4. S. R. Morrison, Sensors and Actuators, 11,283 (1987).
5. S. Semancik and D. F. Cox, ibid., 12, 101 (1987).
6. E. A. Gulliver, J. W. Garvey, T. A. Wark, M. J. H. Smith, and A.
Datye, J. Am. Ceram. Soc., 74, 1091 (1991).
7. C. Ho, I. D. Raistrich, and R. A. Huggins, This Joumal, 127, 343
(1980).
Selective Etching of AIxGa _xAS and In(AIxGa _x)AS
Alloys in Succinic AcidHydrogen Peroxide Solutions
A. J. Tang, K. Sadra, and B. G. Streetman*
Department of Electrical and Computer Engineering and Microelectronics Research Center,
The University of Texas at Austin, Austin, Texas 78712
ABSTRACT
Succinic acid-hydrogen peroxide solutions were used to etch AIxGal_~As and In(AIxGal_x)AS alloys, and the etch rates were investigated with the varying alloy mole fractions. It was found that AIxGal_xAs (x -< 0.4) may be selectively etched over AlyGal_yAS (y -> 0.5),
with a selectivity better than 150. Extended surface exposure to air was found to affect the etch rates, most interestingly for AI04Ga0.6As.
In the In(AIxGal x)AS system, alloys with x -< 0.8 can be preferentially etched over Ino.~2AIo.48Aswith a selectivity of better than 20.
Selective etching plays a vital role in compound semiconductor
device fabrication. Recently, it has been reported that succinic acid
(HOOCCH2CH2COOH)-hydrogen peroxide solutions etch GaAs
over AlAs with a selectivity of better than 1000 and Ino.53Gao.4zAs
over Inos2AI0.48Aswith a selectivity of at least 200.1'2 It would be
useful to characterize this etchant for the entire range of
AIxGal_xAs and InP-lattice-matched In(AIxGal x)As alloys, as
many device applications employ the intermediate alloys as either
the etched or etch-stop layers. For AIxGal_xAS alloys, a few important selective etchants have been identified. Ammonium hydroxidehydrogen peroxide solutions have been used to selectively etch
GaAs over AIo.3Gao.~As,3 although such basic solutions often damage the photoresist. More recently, it has been found that citric
acid-hydrogen peroxide solutions can also perform the same function. 4 High-mole-fraction AIxGal_xAs (x -> 0.5) can be selectively
etched over low-mole-fraction AIxGa1 xAS (x -< 0.4) in hydrofluoric
acid with great selectivity. 5'6 The ability to selectively etch the lowmole-fraction AIxGal_xAS over the high-mole-fraction material
would be useful. There are also cases where two AIxGa1_xAs layers of differing mole fraction should be selectively recessed, the
* Electrochemical Society Active Member.
first with respect to the second and then the second with respect to
the first. 7 Similarly, such applications exist in the much less studied
In(AIxGal_,)As system. Here, we report on the selective etching
characteristics of succinic acid-hydrogen peroxide solutions for
both AIxGal_xAs and In(AIxGal_x)As systems as a function of alloy
composition.
Experiment
Several AIxGal_~As and In(AIxGal_x)AS samples were grown in
a Varian Gen II molecular beam epitaxy machine and used to
determine the etch rates. In all AIxGal_xAs samples with higher
mole fractions (x -> 0.5), 100 ,/~ GaAs caps were grown to prevent
the samples from oxidizing. All etch-rate data were obtained from
etching under constant stirring with a magnetic bar. The etch
depths were found by measuring the step height which resulted
from etching around a black-wax mask. Etch rates were obtained
from the etch depths for different etch times. This procedure was
repeated for the different mole fractions of both Al~Gal_xAS and
In(AIxGal ~)As samples. In all cases, the etched surface exhibited
featureless morphology. Of course, since the etching solution studied here is primarily intended for selective etching, and etch stop
layers naturally provide flat templates, the surface morphology of a
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