Selectivity and composition dependence of response of gas-sensitive resistors .3. Properties of the solid solution series (CrNbO4)(x)(Sn1-ySbyO2)(1-x)(0<=x<=1, y=0,0.01,0.05)

The response of gas-sensitive resistors fabricated from solid solution compounds bf the form (CrNbO4)(x)(Sn1-ySbyO2)(1-x) (0 < x < 1; y = 0, 0.01, 0.05) to carbon monoxide, propane and water have been studied. An n-p transition occurred at a CrNbO4 stoichiometry that increased with the level of Sb5+ doping. XPS demonstrated the surface enrichment of Nb5+ in the CrNbO4 substituted-compounds and a Fermi level shift of 0.6 eV between the n-type and p-type materials. The n-type compounds exhibited higher sensitivity to CO and C3H8 that the p-type compounds, which was rationalised using a compensated semiconductor conduction model in which both the energy of the surface acceptor state relative to the conduction band, and the surface acceptor state density determined the sensitivity. Both the n-type and p-type compounds exhibited a resistance decrease upon exposure to water, and this was explained in terms of the relative energies of the trap states O-ads(-) and OHads:OHads lies higher in energy than O-ads(-) in the n-type but lower in the p-type compounds.