The effect of oxygen stoichiometry on the humidity sensing characteristics of bismuth iron molybdate

The sensitivity of the four probe electrical conductance of compressed bismuth iron molybdate (Bi3FeO4(MoO4)(2)) powder to humid air was measured under conditions of varying degrees of chemical reduction at 24 degrees C, 40 degrees C and 60 degrees C. It is expected that the oxygen vacancies created will alter the water adsorption sites and have a big effect on a conductance dominated by proton hopping, hydronium ion diffusion, or vacancy donor traps releasing electrons into the conduction band. The data was analyzed as the logarithm of the conductance change due to humid air vs. the logarithm of the humidity. Slopes were calculated as limiting values at the two ends of the graphs and were interpreted in terms of the Fleming model of surface conductance, where they represent the average number of physisorbed molecules per cation site. This analysis implies that a Grotthuss chain reaction through multi-layers of surface water is the correct mechanism for the oxidized surface at both low and high humidity. A maximum slope of four argues against significant micropore condensation and a minimum slope of one argues against electronic effects. But for the chemically reduced surfaces, electronic conduction in the semiconductor is increasingly important at low humidity, as seen by slopes less than one. If this is the case, chemical reduction may increase the material's sensitivity to fermi level shifts due to adsorbed water emptying vacancy surface traps, as has been claimed for other materials. (C) 2000 Elsevier Science B.V. All rights reserved.