Composite n-p semiconducting titanium oxides as gas sensors

Anatase and rutile thick films generated from the same source of titania were found to exhibit different types of conductivity upon exposure to CO and CH4 at 600 degreesC in a background of 5% O-2/95% N-2. Anatase behaved as a n-type semiconductor, with a decrease in resistance with reducing gas, whereas rutile exhibited p-type conductivity. The morphology of the particles were different, with anatase consisting of spherical particles of 100-200 mn dimensions, whereas rutile appeared as elongated rods of similar to1 mum lengths. The n-type behavior of anatase can be explained based on the oxygen vacancies. For explanation of the p-type behavior of rutile, impurities in the sample have to be taken into account. The impurity contents in both samples were similar, and doping of the lower valent impurities into the TiO2 lattice should lead to creation of interstitial titanium defects. During anatase to rutile conversion at temperatures of 1000 degreesC, the titanium interstitials can help incorporate excess oxygen, leading to formation of holes and p-type conductivity in the rutile phase. Resistance changes upon interaction of reducing gas with composites of anatase-rutile was also studied. It was found that samples with < 50% rutile upon CO exposure exhibited resistance changes similar to that of anatase. The sample with 75% rutile also showed n-type behavior, though the change in resistance was diminished as compared to anatase. Rutile samples showed p-type behavior indicating a crossover from n- to p-type response at a composition between 75% rutile and pure rutile. The resistance changes with CH4 followed a similar pattern. However, since the overall response of CH4 was smaller than that of CO, the 75% rutile sample showed no change upon exposure to CH4, while exhibiting an n-type response to CO, indicative of a selective CO sensor at temperatures of 600 degreesC. A polychromatic percolation model was developed to explain the electrical data. Two independent, parallel pathways involving the n-type anatase and p-type rutile were considered to be important in the conductivity. Using experimental data related to the extent of sintering, and appropriate particle sizes, the model predicted that n-n percolation would occur from 0 up to 94.5% rutile and p-p percolation would begin at 75.1 % rutile. In between 75.1 and 94.5% rutile, both n- and p-pathways would percolate, resulting in the observed diminished changes in resistance. (C) 2001 Elsevier Science B.V. All rights reserved.