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°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°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.