Study of the resistance behavior of anatase and rutile thick films towards carbon monoxide and oxygen at high temperatures and possibilities for sensing applications

The resistance changes of thick films of titania were examined in the presence of CO at elevated temperatures ranging from 500 to 1000 °C. Three materials were examined, commercial samples of anatase and rutile and a Cr3+-doped rutile prepared in the laboratory. The anatase film exhibited a n-type response to both oxygen and CO in the tested O2 range of 2–21% and CO from 10 to 2000 ppm at 600 °C, with the response disappearing beyond 700 °C. On the other hand, the rutile films showed an entirely different electrical behavior. Until 1000 °C, p-type response was observed to oxygen concentrations from 2 to 21%. With CO concentrations up to 2000 ppm, p-type behavior was observed in 2–21% oxygen concentrations between 600 and 700 °C. However, when the temperature was above 750 °C, an n-type behavior to CO with concentrations ranging from 200 to 2000 ppm was observed, and response was observed until 850 °C. A coating of colloidal platinum on the rutile film induced a transition of the response of the thick film from p to n-type to both oxygen and CO at 600 °C. With the Cr-rutile sample, p-type behavior was observed for both CO and O2. The possible mechanisms for the observed p–n transition of the two rutile films are discussed. Strategies for developing CO sensors for high temperature applications based on the above observations are formulated.