Electrical conductivity in Fe2O3 and CoFe2O4 nanoparticle arrays and their application in gas sensing

Introduction Semiconducting metal oxides belong to the frequently used materials in gas sensing both in environmental protection and in medicine [1]. Amongst the broad variety of well established oxides, like SnO2, TiO2, WO3, ZnO, iron oxide ?-Fe2O3 and cobalt - iron oxide CoFe2O4 attract now attention because of complex magnetic and electric properties and high chemical reactivity. Moreover, innovative sensors are built from nanoparticle (NP) arrays. In comparison with continuous films these devices with high surface/volume ratio are more sensitive [2]. Our sensors are appropriate for oxidizing NO2 gas. With ?-Fe2O3 the response R = Iair/Igas (the ratio of the device current in dry air vs. in air mixed with the analysed gas) is 38 at NO2 concentration Cg = 500 ppb and working temperature Tw = 350oC [3]. This result is comparable with the top published sensitivities, e.g. R = 8 at Cg = 500 ppb and Tw =250oC [4]. With CO and acetone (studied as a marker of diabetes in the patient's breath) the sensitivities are lower [2]. With CO R = 2.8 at Cg = 100 ppm and Tw = 350oC, with acetone R = 1.8 at Cg = 5 ppm at Tw = 500oC. (CO and acetone are reducing gases, hence here R = Igas/Iair). High sensitivity of NP sensors to oxidizing gases and lower sensitivity to reducing gases was explained by charge carrier self-exhaustion of NPs by surface traps [5]. For the further progress in the field the mechanism of conductivity of NP arrays is of considerable interest. In this paper we summarize the results obtained as a by product of ?-Fe2O3 and CoFe2O4 sensors testing.