H. Yumoto*, T. Inoue, T. Kano, S. J. Li and K. Nishiyama**

Department of Materials Science and Technology,
**Department of Mechanical Engineering
Science University of Tokyo, Noda, Chiba 278-8510, Japan
yumoto@rs.noda.sut.ac.jp

ITO (Indium Tin Oxide) is used for transparent electrode, and has gas-sensing property for NO2 [1,2]. The purpose of this study is to apply gas sensor based on ITO film to measure the photocatalysis of NO2. The typical photocatalysts are TiO2 and ZnO. The band gap of ITO (3.5eV) was near to those of TiO2 (3.0 eV) and ZnO (3.3eV), but ITO has not been studied as a photocatalyst. Therefore, the photocatalysis of ITO itself is also studied in this paper.

ITO films for NO2 gas sensor were prepared by magnetron sputtering (350 V, 0.25 mA). Vacuum pressure was 10-3 Torr (O2: 2 sccm, Ar: 25 sccm). Film thickness was 6 nm, and the surface area of the sensor was 1x10 mm. The operating temperature was 573 K. The resistance of the ITO film increased with increasing NO2 concentration. As the sensitivity was degraded, the calibration curve was obtained before each measurement of the photocatalysis. The photocatalysis of ZnO films (30x30 mm) prepared by electroplating was measured in a glass tube (0.4 l) by the gas sensor based on the ITO film. The NO2 concentration was decreased by the irradiation of ultraviolet lamp (0.6 W) exponentially from 200 to 150 ppm for 240 s, and the reproducible was good. It was possible to use the ITO film as a detector of NO2 photocatalysis. The rate, however, was depended on the roughness of ZnO films.

ITO whiskers were grown by electron shower method. Therefore, the surface was rough. When this ITO film replaced ZnO film, the concentration of NO2 was also decreased. This shows ITO also had photocatalysis. But the rate of photochemical transformation was below 1/3 of that of ZnO. In the case of the sputtering, the ITO film was consisted of fine plates, and had a lower transformation rate than that prepared by electron shower.

[1] G. Sberveglieri, G. Faglia, S. Groppelli and P. Nelli: Sensors Actuators B 8 (1992) 79
[2] H. Yumoto, S. Onozumi, et al.: Cryst.Res.Technol., 31 (1996) 159.