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

The electrical response of WO3 and FeNbO4 to the presence of small concentrations of Cl-2 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 OHads- (from water vapour) is postulated and the enthalpy of displacement of surface oxygen by chlorine derived from the temperature dependence of response: Delta H-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 OHads-.