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 degrees 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 O-2 range of 2-21% and CO from 10 to 2000ppm at 600 degrees C, with the response disappearing beyond 700 degrees C. On the other hand, the rutile films showed an entirely different electrical behavior. Until 1000 degrees C, p-type response was observed to oxygen concentrations from 2 to 21%. With CO concentrations up to 2000ppm, p-type behavior was observed in 2-21% oxygen concentrations between 600 and 700 degrees C. However, when the temperature was above 750 degrees C, an n-type behavior to CO with concentrations ranging from 200 to 2000 ppm was observed, and response was observed until 850 degrees 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 degrees C. With the Cr-rutile sample, p-type behavior was observed for both CO and O-2. 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. (C) 2009 Elsevier B.V. All rights reserved.