Ammonia Sensing Mechanism of Tungstated-Zirconia Thick Film Sensor

Abstract

A sensor element of interdigital capacitor (IDC) electrodes covering with tungstated-zirconia thick film shows a good response and cross-sensitivity in gaseous ammonia sensing. The sensing mechanism of the tungstated-zirconia thick film has been investigated by means of in situ FT/IR and H/D isotope effect. In the absence of ammonia, the role of protonic conduction was clarified by the H/D isotope effect of water vapor on the conductivity. In the presence of gas phase ammonia, the conductivity of the tungstated-zirconia thick film significantly increased. The conductivity in the presence of ammonia showed unique temperature dependence, i.e., the conductivity at first increased with a decrease in temperature, showed the maximum at 633 K, and then decreased with further decrease in temperature. In situ FT/IR spectra revealed the presence of both ammonium ion adsorbed on Brønsted acid site and ammonia coordinated to Lewis acid site on the tungstated-zirconia surface. Temperature dependence and dynamic response of the conductivity were in good accordance with the surface concentration of ammonium ion adsorbed on Brønsted acid site. The contribution of ammonium ion to electric conductivity was indicated. The role of ammonium ion as a migrating species for ammonia sensing was verified from negligible H/D isotope effect of ammonia and mobility of ammonium ion.