The effect of carbon monoxide and methane on the electrical conductivity of TiO₂ single crystals

Abstract

Polycrystalline n-type semiconducting metal oxides such as ZnO and SnO₂ have been used in commercial gas-sensing devices for many years, the mode of operation being based on the decrease in surface electrical resistance in the presence of reducing gases. Although the polycrystalline materials are highly sensitive to these gases, a major disadvantage is their lack of selectivity. Studies of single crystals of ZnO and SnO₂ have shown that the single crystals are selective to carbon monoxide (CO) over methane (CH₄). In this paper we describe the results of electrical studies performed on single crystals of TiO₂ in the presence of gas mixtures. It is found that these crystals are also highly selective to CO in air but only in their stoichiometric form and the effect is to decrease the conductivity. The results can be explained in terms of the crystals behaving as extrinsic p-type semiconductors at the temperatures where they respond to the gases.