Gas-sensitive resistors: surface interaction of chlorine with semiconducting oxides

Abstract

The electrical response of WO₃ and FeNbO₄ to the presence of small concentrations of Cl₂ in air, in the presence and absence of water vapour, is reported. The response of WO₃ was very large and fast; that of FeNbO₄ was smaller and slower. The electrical conductivity of the niobate could be remarkably decreased by making it slightly niobium-deficient, but this had little effect on the chlorine response. A simple, surface-trap-limited model describes reasonably well the variation of electrical resistance with gas concentration. A competitive, dissociative chemisorption of chlorine and oxygen to form the surface-trap states O_ads^– and Cl_ads^– along with OH_ads^–(from water vapour) is postulated and the enthalpy of displacement of surface oxygen by chlorine derived from the temperature dependence of response: ΔH_ads=(–152 ± 28) kJ mol^–1 on FeNbO₄. The different behaviour of the different materials is rationalised in terms of differing trap-state energies for O_ads^–, Cl_ads^– and OH_ads^–.