Fabrication And Gas Sensing Properties Of LaFeO₃ Based Sensors

Metal oxide semiconductor has been playing an important role in the field of gas sensors. But single metal oxide material has the defects of high working temperature and low sensitivity, which limit its application in the market. ABO₃ perovskite rare-earth oxide is one of good catalysts due to its stable structure which will not be changed by doping suitable A and B atoms in certain proportions at the same time, which makes it good gas sensitive materials. As gas sensitive material, it has high stability, sensitivity and good selectivity. So in recent years, researches on these gas sensitive materials are very active.As a typical perovskite oxide, LaFeO₃ has been investigated a lot by scientific staff on its gas-sensing properties. Compared to the single metal oxide, its gas-sensing performance improves to a certain extent, but still can’t meet the requirements of modern science and technology development. In this paper, we synthesized various LaFeO₃-based nano-materials by sol-gel method, and prepared indirect-heating gas sensors in ceramic-tube structure. In order to further improve its gas-sensing properties, we used Ba, Sr to partly replace La in LaFeO₃ structure and Mn, Cu to replace Fe in the structure, aiming to study the impact of the bivalent cation replacement of A site and transition metal element replacement of B site on the electrical conductivity and gas-sensing properties of LaFeO₃; We used Na2CO₃ and NaCl as raw materials to study the impact of the monovalent cation Na replacement of A site and co-existence of halogen element Cl replacement of O site and monovalent cation Na replacement of A site on the electrical conductivity and gas-sensing properties of LaFeO₃; We adjusted the atomic ratio of La/Fe in the structure to investigate the impact of nonstoichiometry on the electrical conductivity and gas-sensing properties of LaFeO₃. The impact of different elements doping and atomic ratio of La/Fe on the crystal structure, element valence state and ratio, and the concentration of surface oxygen adsorption were studied by XRD and XPS.The summary of our results are as follows:1、Sr, Ba replacing La site and Mn, Cu replacing Fe in the structure do not improve the gas-sensing properties, but on contrary reduce its sensitivity to ethanol, which do not agree with the experimental results of other researchers. The reason is ascribed to the different preparation processes which leads to different crystallite size of material and affects the performance.2、Na doping does not change the basic perovskite structure of LaFeO₃, but improves the electrical conductivity, and reduces the best working temperature from 140 degrees to 120 degrees, however, the gas sensitivity to ethanol is reduced at the same time. Compared to only doping of Na, doping Na and Cl at the same time further improves the electrical conductivity, and increases the sensitivity to ethanol, which can be 28 under the working temperature of 120 degrees, exceeding that of pure LaFeO₃ at the same temperature. This may be ascribed to the introduction of Cl element which inhibits the growth of the crystallite size3、LaFexO₃-δ series shows high sensitivity, good stability and short response time to ethanol, and the best sensitivity of 132 is obtained by LaFexO₃-δ（x=0.8） at 140 degrees. XPS analysis on O1 s and C1 s spectra verifies the existence of La-carbonate on the surface. The variation of surface concentration of adsorbed oxygen and La-carbonate, atomic ratio of Fe⁴⁺/Fe³⁺ and Fe/La with x value are discussed in combination, which indicate that the release of electrons trapped by native active oxygen due to the decomposition of monodentate La-carbonate also makes contribution to the response of LaFexO_3-δ sensors to ethanol, in addition to the major contribution from adsorbed oxygen species.