Composite n–p semiconducting titanium oxides as gas sensors

Sensors and Actuators B 79 (2001) 17±27

Composite n±p semiconducting titanium oxides as gas sensors Nancy Savagea, Brian Chwierothb, Arwa Ginwallaa, Bruce R. Pattonb, Sheikh A. Akbarc, Prabir K. Duttaa,* a

Department of Chemistry, Center for Industrial Sensors and Measurements, The Ohio State University, 2041 College Road, Columbus, OH 43210-1178, USA Department of Physics, Center for Industrial Sensors and Measurements, The Ohio State University, 2041 College Road, Columbus, OH 43210-1178, USA c Department of Materials Science and Engineering, Center for Industrial Sensors and Measurements, The Ohio State University, 2041 College Road, Columbus, OH 43210-1178, USA

b

Received 10 December 2000; received in revised form 25 March 2001; accepted 4 April 2001

Abstract Anatase and rutile thick ®lms generated from the same source of titania were found to exhibit different types of conductivity upon exposure to CO and CH4 at 6008C in a background of 5% O2/95% N2. 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 nm dimensions, whereas rutile appeared as elongated rods of 1 mm 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 10008C, 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