New NOx sensors based on hematite doped with alkaline and alkaline-earth elements

Alkaline (lithium, potassium, rubidium) and alkaline-earth (magnesium, barium) doped hematite materials were studied for NO2 sensing application. The synthesized materials were characterized by laser granulometry, X-ray diffraction and scanning electronic microscopy. A temperature of 1300 degrees C was chosen as the optimal heat treatment in order to obtain the densest material. Humidity dependence of the electrical properties revealed a strong influence in the case of rubidium doped hematite material while the other doped materials were less sensitive. The AC impedance analyses underlined the n-type intrinsic semi-conduction of pure hematite. Alkaline-earth doped hematite materials showed two semi-conducting regions, below and above 500 degrees C, corresponding to extrinsic and intrinsic n-type semi-conduction, respectively. These electrical analyses associated with SEM observations suggested instability of the ferrites formed in rubidium and potassium doped materials. AC electrical measurements were performed in the 0-500 ppm NO2 partial pressure range. The alkaline-earth doped hematite materials exhibited the most promising behavior. (C) 2011 Elsevier Ltd. All rights reserved.