Sensing Characteristics of Cubic and Rhombohedral Nanocrystalline ITO Thick Films
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Sensing Characteristics of Cubic and Rhombohedral Nanocrystalline ITO Thick Films Jeung-Soo Huh, Bong-Chull Kim, Jae-Yeol Kim and Jeong-Ok Lim1 Environmental Gas Monitoring Laboratory, Kyungpook National University, Taegu 702-701, Korea. 1 Medical Research Institute, Kyungpook National University, Taegu 702-701, Korea. Abstract Two kinds of nanocrystalline Tin doped indium oxide (or indium tin oxide: denoted ITO hereafter) powders with different crystal structures - rhombohedral and cubic - were prepared using a coprecipitation method through the control of pH of a mixing solution and aging time after coprecipitation. The two powders have the same particle size of 15 nm in diameter but different morphologies (spherical for rhomboheral and rectangular for cubic). The gaseous ethanol sensing characteristics of the sensors prepared by the two ITO powders were quite different. The sensitivity of rhombohedral ITO sensor was high compared to that of the cubic ITO sensor across all temperatures. The reason for this is explained through the viewpoint of the binding energy of XPS and the surface structure relating to the crystal structure. Introduction ITO is widely used in optical devices due to its high electrical conductivity and high transparency in the visible light range [1]. ITO is not used as a commercial material for gas sensors and publication papers on the gas sensing of ITO are relatively scarce [2]. The papers report only about ITO thin film that shows high sensitivity and good long-term stability to NO and NO2 but poor sensitivity to CO and CH4 [2]. The effects of grain size and noble metals on the gas sensing mechanism in the various semiconduting metals such as ZnO [3], SnO2 [4], In2O3 [5] and WO3 [6] have been well studied by many researchers since the 1950s. However, the research on the effect of crystal structures on the gas sensing has not been conducted as of yet. In the present study, nanocrystalline ITO powders were prepared using a coprecipitation method and the structures of the precipitates were controlled by varying the pH of solution during the coprecipitation reaction and the aging time after coprecipitation. As a result, after calcining the precipitate, cubic and rhombohedral nanocrystalline ITO powders were selectively produced. Gas sensing characteristics of the thick film prepared by nanocrystalline ITO powders with different crystal structures were examined. W9.35.1
Experimental In order to investigate the effects of crystal structures on the gas sensing properties, two different structured nanocrystalline powders, cubic and rhomohedral ITO, were used in this experiment. ITO powders were synthesized using a coprecipitation method which we have outlined in detail elsewhere [7,8]. The particle sizes of the powders were measured by X-ray diffraction. The morphologies of ITO powders were examined by transmission electron microscopy. In/Sn atomic ratios in synthesized powders were also analyzed by an X-ray fluorescence spectrometer. The ratios were not changed in the two ITO powders synthesized
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