The Gas Sensing Property And Echanism Of LaFeO₃Based Oxide To Reduce Gases

Recently due to the application in the giant magneto resistance effect, catalysis, high temperature superconductivity and gas sensing, ABO3perovskite rare-earth oxides have attracted more and more attention. Especially as a gas sensing material, perovskite rare-earth compounds have the good gas sensitivity, selectivity and stability, and its gas sensing properties could be controlled by doping A or B site. The doped materials still keep the perovskite structure, and its conductivity and gas sensing performance will be greatly improved. Therefore, studies of these materials have particularly meaningful in order to find new and practical gas-sensitive materials.Generally, lower valance-cation elements such as Ca, Sr and Pb are the best elements used to doping LaFeO3to improve its gas sensing performance. Ba is the same main group as Ca and Sr in the periodic table. However as so far, we have not found the reports about the gas sensing performance of the Ba doped LaFeO3materials. In this paper, the La1-xBaxFeO3nanopowders were prepared using the sol-gel method and their conduction and gas sensing performances were studied. We found that proper Ba-doping could improve the sensitivity of LaFeO3to alcohol gas. To comprehend gas sensing mechanism of the perovskite oxide more deeply, we have chosen the LaFeO3and CO molecule as representatives, using first-principle calculation to simulate the dissociation process of the adsorbed O2from the LaFeO3surface. We found that the dissociation of the adsordrd O2was indeed caused by the adsoption of CO, meanwhile in the CO adsorption process, electron transfer could change the conductance of the LaFeO3, so that the LaFeO3material could show gas sensing to the CO gas. After that we calculated the Ca2+doped LaFeO3material and found that the appropriate Ca2+doping not only could improve the transferred electrons in the CO adsorption process, but also could reduce the dissociation energy of the adsorbed O2molecule. These may be the reason Ca2+doing could improve the gas sensing of the LaFeO3material.In order to further understanding the reaction between the gas molecules and the surface of LaFeO3-based material, we calculated a series of gas molecules adsorption on the surface of the pure and doped LaFeO3based on the first-principles. The results showed that in general the surface Fe site plays a leading role in the adsorption process. Through the calculated results of the adsorption energy and the transferred electrons in the adsorption process, we determined the best adsorption configuration when the gas molecules adsorption on the surface of the pure and doped LaFeO3. Meanwhile, we also studied the influence of O vacancy on the LaFeO3adsorption CO molecule.The abstract of our results as follows:1. In the doped La1-xBaxFeO3(x≤0.3) materials, X-ray diffraction pattern (XRD) showed that all the samples are perovskite structure, indicating that Ba2+replaces La3+position. Because Ba2+ionic radius (135pm) is bigger than the La3+ionic radius (106.1pm), the cell volume increases with the increase of the x. When x <0.1, the resistance of the La1-xBaxFeO3reduces with the increase of the x; however when x>0.1, the resistance of the La1-xBaxFeO3increases with the increase of the x. These were mainly caused by the mutual compensation of the tariff and oxygen vacancy. The gas sensing performance of the Lao.75Bao.25Fe03to500ppm alcohol gas is better than pure LaFeO3material and other doped materials, and its optimum operating temperature is240℃. Although the optimum operating temperature of La0.75Ba0.25FeO3gas sensor was slightly higher compared to LaFeO3, its gas sensitivity to alcohol gas at the working temperature range175℃-360℃was higher than LaFeO3gas sensor in its optimum operating temperature point. When x=0.25, the doping Ba2+create the maximum amount of defect for the oxygen molecules adsorbed in the surface of the material. The gas sensor based on the La0.75Ba0.25FeO3exhibits good selection properties to the alcohol gas and it also has a good working stability in air.2. The nanocrystalline material La0.875Ba0.125FeO3powders were prepared by sol-gel method, followed by calcinations at800℃for3h. XRD pattern showed that the sample material is perovskite phases with the orthorhombic structure. The gas sensor based on La0.875Ba0.125FeO3nanocrystalline has P-type semiconductor properties and the activation energy we calculated from the image of the resistance and temperature in the air is0.18eV. The La0.875Ba0.125FeO3nanocrystalline material has good selectivity to alcohol gas and its sensitivity at170℃to500ppm alcohol gas reaches58. We calculated the adsorption of O2molecule on the La0.875Ba0.125FeO3(010) surface using first-principles calculation. Through the analysis results of surface La, O and Fe site, we found that the O2molecule adsorption on the surface of Fe site is most stable. By the length and vibration frequency of the O-O bond, we could know the superoxide02-was generated after adsorption. The adsorbed O2molecule prefers the vertical configuration compared to the parallel adsorption configuration, meanwhile when O2molecule adsorbed on the surface Fe site by the vertial configuration, the electrons transferred from surface to O2molecule was more.3. We studied the CO molecule adsorption on the O2molecule pre-adsorbed LaFeO3(010) surface using first-principles calculation method. Compared to the other adsorption positions in the surface, CO molecule prefers to react with the pre-adsorbed O2molecule to form O-C-O substance. In the CO adsorption process, electrons tranfreed from the CO molecule to the surface, reducing the surface holes, improving the resistance of the LaFeO3, which is consistant to the results of the previous experiment. The adsorption of CO does not change the bonding mechanism between the adsorbed oxygen and Fe atom, but it can make the HOMO-LUMO band gap narrowing, which is mainly caused due to the redistribution of the surface electron. After the adsorption of CO molecule, the electrons of Fe-3d were changed, which causes the HOMO-LUMO band gap becoming narrow. When CO molecule adsorbed on the O2pre-adsorbed La1-xCaxFeO3(010), CO molecule still preferentially reacted with the pre-adsorbed O2molecule, causing the dissociation of pre-adsorbed O2molecule. The appropriate Ca2+doping not only could improve the transferred electrons in the CO adsorption process, but also could reduce the dissociation energy of the adsorbed O2molecule. These may be the reason Ca2+doing could improve the gas sensing of the LaFeO3material.4. We studied the adsorption of NO on the LaFeO3(010) surface using first-principles calculations. By comparison of several adsorption configurations, we found that the adsorption of NO molecule on the Fe site is relatively stable, and the Fe-NO configuration is the most stable. This indicates that Fe site still plays a leading role in the adsorption of NO molecule. In the adsorption process, the electron transfer from the surface to the NO molecule, Weaken the N-O bond length. The analysis results show that in the adsorption process, although the Fe-s, p and d orbits have undergone varying degrees of change, the greatest change still happen in the d orbit. The main hybridization occurs between the NO and Fe d orbit.5. We studied the adsorption of CO, NH3and O2molecules on the La0.875Sr0.125FeO3(010) surface using first-principles calculation method. For the adsorption of CO, the calculation results show that the C atom of CO downward to the surface Fe site is more stable than the other adsorption configuration. The hybridization between the C-s, p orbits and Fe-d orbit is the main source of the bonding mechanism. The N-down configuration for NH3molecule adsorption is more stable. The angle of the H-N-H after adsorption becomes large and the strong orbital hybridization occurs between the N atom and the Fe atom. When O2molecule is adsorbed on the La0.875Sr0.125FeO3(010) surface, the configuration that the angle between the O2molecule and the Fe atom is about120is the most stable. 6. We studied the adsorption of formaldehyde molecules on the Fe site of the LaFeO3(010) surface. When formaldehyde molecule is adsorbed on the Fe site of the LaFeO3(010) surface, the0-down configuration is most stable. After H2CO molecule adsorption, two partial energy lines appear in the LaFeO3(010) surface, one at the middle of the band gap, another at the bottom of the valence band. This indicates that the adsorption of H2CO molecule could change the electronic structure of the LaFeO3(010) surface. The strong hybridization occurs between H2CO2p orbit and the Fe atom3d orbit, which is the main reason that the H2CO molecule can be adsorbed on the Fe site.7. We studied the adsorption of CO molecule on the LaFeO3(010) surface using first-principles calculation method, meanwhile we contrasted the influence of Ca doping and surface oxygen vacancy to the adsorption of CO molecule on the LaFeO3(010) surface. For the pure LaFeO3(010) surface, Fe site is the most suitable for CO molecule adsorption, and the Fe-CO adsorption configuration is most stable. In the adsorption process, the electron transfer from CO molecule to the LaFeO3(010) surface. Although the Ca doping can improve the adsorption energy of the Fe-CO configuration, it reduces the amount of electron transfer in the adsorption process. When the LaFeO3(010) surface contain oxygen vacancy, the best adsorption position transfer from the Fe site to the O vacancy, the most stable adsorption configuration is the vacancy-CO configuration. The CO molecule takes electrons from the surface. Overall, the oxygen vacancy influences the adsorption more than the Ca doping.In summary, from the studies of the Ba doped LaFeO3material we found that the appropriate Ba doping can improve the conductivity and gas sensing performance of the LaFeO3material. This provides the basis for the preparation of high sensitivity, low cost, good selectivity and gas-sensitive gas sensor in the practical application. By simulate the gas sensing mechanism of the LaFeO3perovskite oxide, we verify the previous experimental results and have a clearer understanding to the reason that lower valance-cations elements doping could improve the gas sensing properties of the LaFeO3. At the same time, the studies of gas molecules adsorption on the LaFeO3surface using first-principles calculation method can make us understanding the rection between the LaFeO3surface and the gas molecules from the micro.