Fast response of undoped and Li-doped titania thick-films at low temperature

The fast response of undoped and Li-doped TiO2 operating at low temperature to hydrogen and oxygen is investigated. The TiO2 sensors are fabricated using thick-film technique. The prepared materials exhibit the presence of only rutile phase of TiO2 but enlarged crystal lattice parameters were confirmed by X-ray diffraction (XRD). Scanning electron microscopy (SEM) shows that the grain size of the material has not obviously changed with different Li-doping (2–4 mol%), but the undoped is much smaller. Kroger–Vink model indicates that Li mainly substitutes for the lattice point of Ti. Because the material resistance decreases as the oxygen pressure increases, Li-doped samples can be regarded as a p-type semiconductor compared with pure TiO2. The operating temperature of the Li-doped TiO2 samples is found to be lower than that of pure TiO2 in H2 and O2 environment. At less than 3 mol% Li content, the response time of the Li-doped TiO2 gas sensors is much shorter than that of pure TiO2, at the same temperature under both H2 and O2 environment. Moreover, the sample of 3 mol% Li-doping exhibits the best response characteristics. The response mechanism is suggested to arise from the conduction holes ionized by Li and the surface potential barrier change in different gas environments.