Selectivity and compositional dependence of response of gas-sensitive resistors .4. Properties of some rutile solid solution compounds

Solid solutions based on the rutile structure offer the possibility of independently varying the cation composition and dopant density in the lattice and hence exploring the effects that these have upon the electrical response to reactive gases. Thus, solid solutions of CrNbO4 (p-type response) with Nb-doped TiO2 and FeNbO4 (n-type response) have been investigated. The resistivities and activation energies increased in the order FeNbO4 < CrNbO4 < Ti0.975Nb0.025O2, which is explained in terms of the emptying and decreasing energy of the e(g) and t(2g) states within the bandgap. The n-p transitions can similarly be discussed in terms of the localised occupancy of these levels and the energy of the reactive oxygen surface state with respect to them. Sensitivity to CO decreased in the order Ti0.975Nb0.025O2 > CrNbO4, FeNbO4 and this was postulated to be due either to Nb5+ surface segregation or to the narrowing of the conduction bandwidth leading to valence trapping of electrons. An enhanced gas response at T < 300 degrees C was found for the compounds after they had been heated to ca. 700 degrees C in air. This was attributed to the desorption of surface hydroxy, carbonyl and lattice oxygen species to produce a vacancy around which the surface reconstructed. It is proposed that ionosorption of oxygen at this site produced a species able to react with CO and produce a resistance change at low temperatures.