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Studies on Gas Sensing Properties of Divalent Element-Doping ABO3 Materials

Author: ZhaoMing
Tutor: QinHongWei
School: Shandong University
Course: Materials Physics and Chemistry
Keywords: Perovskite Sol-gel method Semiconductor Gas sensing First principles
CLC: O621.2
Type: Master's thesis
Year: 2010
Downloads: 78
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Abstract


ABO3 perovskite rare-earth oxides can be used in magneto-impedance materials, catalytic materials, high-temperature superconducting materials, but also can be served as gas-sensing meterials. As a gas sensor, it has sound stability, good selectivity and excellent sensitivity. Another excellence is that its characteristic can be controlled to get even better by electing suitable A and B atoms or chemical doping in ABO3. ABO3 material which has great theorical significance and utility value has aroused interest among the researchers.Generally, gas-sensing properties of LaFeO3 can be improved by substituting lower valance-cations, such as Ca, Sr, Ba, Pb for La, or substituting Fe by Cu, Co, Ni, Mn and so on. La1-xCaxFeO3 is a kind of very important photo catalysis material. Its gas sensing property of CO has not being studied yet. Since CO is a kind of very important toxic gas, in this article La1-xCaxFeO3 powders were prepared by the sol-gel method, and were characterized for the gas sensing property, stability and conductivity.For the further understanding of the ABO3 materials, we calculated the oxygen adsorption on LaFeO3 surface based on first-principle calculation. We found that Fe ions dominated the oxygen adsorption. Wherever intial position was placed, the optimized position all focused on the surface Fe ion, which illustrated B ion dominated chemisorption.The abstract of our results as follows:1. X-ray diffraction patterns (XRD) of La1-xCaxFeO3 show that La1-xCaxFeO3 nanocrystalline is perovskite phases with the orthorhombic structure, no obvious other phases are observed. It also illustrates that La ion is replaced by Ca ion. With an increase of x, the mean crystallite size decreases, cell volume decreases and also the lattice parameter. That is because calcium ion (100pm) is slightly smaller than lanthanum ion (103pm). When lanthanum ion is substitutd by calcium ion, the lattice parameter decreaces leading to a shrink of unit cell. 2. After measuring the conductivity of the gas-sensing device, we find that resistance of La1-xCaxFeO3 (x=0.1,0.2,0.4) in CO is bigger than resistance in air under the same temperature, which illustrates that La1-xCaxFeO3(x=0.1,0.2,0.4) is a kind of P-type semiconductor. La1-xCaxFeO3 (x=0.3) is found a transformation through p-type semiconductor to n-type semiconductor. The turnover temperature is 380℃. It is found that with an increase of x, the device resistance decreases at first then increases. That is because electrical valence compensation and vacancy compensation. Two kinds of mechanism both exist in La1-xCaxFeO3, and combat with each other. When the electrical valence compensation dominates, holes increase and resistance increases. On the other hand, the domination of oxygen vacancy compensation leads just opposite consequence.3. La1-xCaxFeO3 (x=0.1,0.2,0.3) have good sensitivity. La1-xCaxFeO3 (x=0.2) reachs the peak of sensitivity at 100 degrees Centigrade. The stability and respond time are detectived. The gas sensing device property parameters are pretty good. Lao.8Cao.2Fe03 could be used as a CO gas sensor in the future.4. Clean La7CaFegO24 (010) surface and adorption on it have been investigated at the level of density functional theory based on first-principle calculation. The surface states of LaFeO3 (010) surface appear near Fermi energy level mainly caused by Fe 3d orbital. The surface Fe ions dominate the oxygen adsorption process. The adsorbed O2 on Fe ion is much more stable than that on La and O ions, and the bonding mechanism of adsorbed O2 on surface Fe ions is the strong interaction between O 2p and Fe 3d orbital. In addition, our calculations indicate that O2 dissociation on LaFeO3 surface belongs to chemisorbed-precursor mechanism. The following equilibria show the adsorption process 5. La7CaFe7CoO24(010)surface and O2 adsorption on La7CaFe7CoO24(010)surface have been studied at the level of density functional theory based on first principles calculation using Dmol3. After doping Co into La7CaFegO24, La7CaFe7CoO24 maintained perovskite phase. In the La7CaFe7CoO24(01 0)surface, surface relaxation was found and surface reconstruction was not. Surface partial density of states suggested Fe and Co atom play an important role in O2 adsorption process.7 modes was built to simulate the different adsorption site of O2. Physisorption, molecular chemisorption and dissociative chemisorption were found in 7 modes. Co adsorption energy was smaller than other 4 kinds of molecular chempisorption. Dissociative chemisorption was found in O-site (O atom in Fe-O-Co) mode (O-O bonds length was 4.0474A.) and dissociative O atoms were both adsorbed on Fe atom. The result shows that Fe atom adsorption ability is stronger than Co in La7CaFe7CoO24(010)surface.Above all, perovskite nano-crystalline La1-xCaxFeO3 is studied using calculation and experimental method. The gas-sensing material has good stability and sensitivity. We gain further insight into the gas-sensing mechanism of La1-xCaxFeO3 material. The study foreshadows the seeking for new gas-sensing materials.

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