Anatase and rutile thick films generated from the same source of titania we
re 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, wh
ereas 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 oxyg
en 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 Ti
O2 lattice should lead to creation of interstitial titanium defects. During
anatase to rutile conversion at temperatures of 1000 degreesC, the titaniu
m 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 stu
died. It was found that samples with < 50% rutile upon CO exposure exhibite
d resistance changes similar to that of anatase. The sample with 75% rutile
also showed n-type behavior, though the change in resistance was diminishe
d 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, t
he 75% rutile sample showed no change upon exposure to CH4, while exhibitin
g an n-type response to CO, indicative of a selective CO sensor at temperat
ures of 600 degreesC. A polychromatic percolation model was developed to ex
plain the electrical data. Two independent, parallel pathways involving the
n-type anatase and p-type rutile were considered to be important in the co
nductivity. 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 perc
olate, resulting in the observed diminished changes in resistance. (C) 2001
Elsevier Science B.V. All rights reserved.