Investigation on a change in response direction of Ga doped ZnO nanoparticles resistive sensors on exposure to NO

Abstract: Semiconductor-based gas sensors have been used for a wide range of applications over the last few decades. In this thesis, sensing properties of pure ZnO and Ga doped ZnO are investigated. There are three types of tested gas species, H2, O2 and NO, and three test temperatures, 300oC, 400oC and 500oC. After measurements of response to exposure to H2 and O2, it is concluded that Ga doped ZnO and ZnO are both n-type metal oxides. In measurements of NO, two test conditions were considered, the case with background O2 (10%) in the gas flow and the case without background O2. NO can be oxidized to NO2 or reduced to N2 and O2. The resistance of Ga doped ZnO and ZnO sensors always decreases for all exposures to NO except for the case in which the Ga doped ZnO sensor was exposed to NO in a background of O2 at 500 oC. In this special case, the resistance of Ga doped ZnO actually increases during exposure to low concentrations of NO (< 30 ppm). It is not clear whether the change in response direction is due to an n-p transition or different reactions between gas molecules and Ga doped ZnO. Work function measurements were therefore conducted to understand more about the electron transfer during gas exposure. The work function measurements suggest that there are probably several stages of interactions between gas molecules and Ga doped ZnO during each gas pulse exposure.