Anatase TiO 2 Thin Films Based CO Gas Sensor
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Anatase TiO2 Thin Films Based CO Gas Sensor Ibrahim A. Al-Homoudi1, L. Rimai2, R. Naik3, R.J. Baird2, G.W. Auner2, G. Newaz1 (1) Mechanical Engineering Dept.; (2) Electrical and Computer Engineering Dept; (3) Physics and Astronomy Dept. Wayne State University, Detroit, Michigan, 48202. ABSTRACT: TiO2 anatase thin films with different thickness (100 – 1200 nm) have been deposited on glass, sapphire and Si(100) substrates using pulsed DC magnetron reactive sputtering. The thin films were exposed to carbon monoxide (CO) gas at different concentrations (20–100 ppm) in a nitrogen carrier, and the resistance was measured as a function of the CO concentration for films of different thicknesses for temperatures in the range of (100 - 300°C). The films with good crystalline order showed better response than amorphous films. The response increased monotonically with CO concentration. The thicker films showed higher sensitivity. The anatase films deposited on sapphire generally had a larger response than those deposited on glass or on silicon. The films on sapphire substrate showed good response at temperatures as high as 300ºC, while those deposited on glass and silicon had good responses only to 200ºC. Furthermore, the addition of different concentrations of O2 and H2 affected the response as expected. The response to CO in the presence of N2 showed good reversibility which is evidence that complete regeneration on turn off the CO does not require exposure to oxygen or air. The response and the recovery times are fast. INTRODUCTION: Gas sensors have wide applications ranging from automobiles, homes, and factories to food packaging. Most gas sensors materials are based on mixed metal oxides with a dominant crystal structure that may change electrical resistance when the stoichiometric proportion of the metal centers is altered. In gas sensors, for example, the sensor’s material changes when a gas is applied to this material, depending on the oxidative nature of the gas and the type of semiconductor under test. TiO2 is a markedly inert, stable compound found in nature in three crystalline forms rutile, anatase (both tetragonal) and brookite (orthorhombic), which differ in their opacity and other physical characteristics [1, 2]. There is a great deal of interest in anatase for thin film technological applications. It has been pointed out that the Fermi level in anatase film is higher than of rutile [3]. Moreover, it has been reported that anatase thin films have different electrical and optical properties than rutile film. The essential difference is that anatase thin films appear to have a wider energy gap and a smaller electron effective mass, resulting in a higher mobility for the charge carriers [3, 4]. These properties are beneficial to applications in gas sensing devices and other devices. Akbar et al. [3] studied a TiO2 (anatase)-based thick film CO (200 – 1000 ppm) sensor fabricated from high purity TiO2 powder, ball-milled with ZrO2 onto an alumina substrate, and they proposed a possible surface-controll
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