GAS-SENSITIVE RESISTORS - SURFACE INTERACTION OF CHLORINE WITH SEMICONDUCTING OXIDES

The electrical response of WO3 and FeNbO4 to the presence of small con centrations of Cl-2 in air, in the presence and absence of water vapou r, is reported. The response of WO3 was very large and fast; that of F eNbO4 was smaller and slower. The electrical conductivity of the nioba te could be remarkably decreased by making it slightly niobium-deficie nt. but this had little effect on the chlorine response. A simple, sur face-trap-limited model describes reasonably well the variation of ele ctrical 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 postul ated and the enthalpy of displacement of surface oxygen by chlorine de rived from the temperature dependence of response: Delta H-ads = (-152 +/- 28) kJ mol(-1) on FeNbO4. The different behaviour of the differen t materials is rationalised in terms of differing trap-state energies for O-ads(-), Cl-ads(-) and OHads-.