Mg-doped Ln (Fe, Co) O_3 gas-sensing properties of materials and LaFeO_3 first-principles electronic structure study

Title: Mg-doped Ln (Fe, Co) O_3 gas-sensing properties of materials and LaFeO_3 first-principles electronic structure studyAuthor: Liu XingDegree-granting units: Shandong UniversityKeywords: semiconductor;; perovskite;; sol-gel method;; gas properties;; first-principlesSummary:Perovskite-type oxides LnTO_3 (Ln is rare earth, T is the transition metal elements) is an important class of functional materials. Their functional properties through changing the Ln, T-bit to control the elements, but also by Ln, T-bit to control the doping elements. Doped LnTO_3 material not only remained stable perovskite structure, and gas-sensing properties of doped materials, catalytic properties and electrical conductivity have been greatly improved. Magnetic lifter Therefore, their research has important theoretical significance and practical value.People often through the use of low-cost elements such as cation Ca, Sr, Ba, Pb, etc. to the partial substitution of La LaFeO_3 location or Cu, Co, Ni, Mn, etc. to replace the position of Fe to improve their gas sensitivity. Mg and Ca, Sr, Ba, Pb in the same main group, but on the Mg-doped materials for LnFeO_3 gas sensitivity study has not been reported, while the crystals on the Mg in LnTO_3 also controversial alternative location. Some people think that the location of Mg can replace Ln, while others believe that Mg is not a substitute Ln bit can only replace T's position. This article by Sol-Gel Synthesis of Mg-doped LnFeO_3 and LaCoO_3 series of nano-powders, and its electrical conductivity and gas sensing characteristics were studied, we believe that Fe Mg tend to occupy the position, also found that the amount of Mg-doped can improve the material's conductivity, good to improve the gas sensitivity.At present almost no field of study related to the sensing mechanism of LaFeO_3 the simulation, in order to theoretically understand it more deeply to the sensing mechanism, in the end of the paper we first-principles calculations based on simulated LaFeO_3 surface oxygen adsorption, and found that oxygen adsorption on the Fe ion plays a dominant role. In addition, we http://www.999magnet.com/products/131-magnetic-lifter also studied the electronic structure of vacancy defects on the LaFeO_3 and magnetic properties of oxygen vacancy defects can make the anti-ferromagnetic LaFeO_3 have some weak magnetism. On the calculation of Mg-doped LaFeO_3 still ongoing.This paper is mainly the following conclusions:1.Mg of LaFeO_3 doping material La_ (1-x) Mg_xFeO_3, when the Mg doping x less than 0.1, the only image in the XRD of the characteristics of perovskite diffraction peaks can be seen. When the Mg-doping x is greater than equal to 0.2, in addition to the perovskite phase peaks also appeared MgFe_2O_4 outside the peak, and MgFe_2O_4 peak intensity increases with the amount of Mg doping is enhanced. This shows that the Fe-Mg tend to occupy the position. When x <0.1, when, La_ (1-x) Mg_xFeO_3 resistance by the [V_ (La) ~ x] and [Mg_ (Fe) ~ x] defects and oxygen vacancy defects in the common decision. Then the resistance ratio LaFeO_3 resistance is much smaller. When x> 0.1, due to the emergence of the samples p-La_ (1-x) Mg_yFe_ (1-y) O_3/n-Mg_zFe_ (1-z) Fe_2O_4 heterogeneous section, making La_ (1-x) Mg_xFeO_3 gas components of the resistance as x increases and exceeds LaFeO_3 resistance.Amount of Mg-doped LaFeO_3 response to ethanol and methane is much larger than the value of undoped LaFeO_3. One La_ (0.92) Mg_ (0.08) FeO_3-based gas sensor for alcohol showed an excellent response and selectivity, the response value as the gas concentration increases, the working temperature as the concentration increases. With the growth of carbon chain gas molecules, the response value becomes large. This is because the gas molecules of the steric effect and electron donor effect of the adsorption of gas molecules that affect the two main factors; La_ (0.9) Mg_ (0.1) FeO_3-based gas sensor for methane gas of a good performance and stability, also have relatively low operating temperature (200 ℃).2.LaMg_xFe_ (1-x) O_3 (x = 0,0.1,0.2,0.3) ultra-fine powder solid solution by the sol - gel method of preparation, and then four hours after 800 ℃ calcination. When the Mg content is less than equal to 0.2, you can get single-phase perovskite structure. Crystals is due to a larger perturbation Mg ~ (2 +) and octahedral deformation of Fe ~ (4 +) ion-induced. This material shows that the nature of P-type semiconductor. Compensation and compensation for electricity combined effects of oxygen vacancies leads to the resistance of Mg-doped LaFeO_3 reduced. But for x> 0.1 sample, its resistance increases as the amount of Mg doping increases, which is due to the increase in the amount of Mg-doped with oxygen vacancies increase, leading to enhanced oxygen vacancy compensation effect. One LaMg_ (0.1) Fe_ (0.9) O_3-based gas sensor for ethanol showed higher response and better selectivity. LaMg_ (0.1) Fe_ (0.9) O_3-based gas sensor conductance in ethanol gas - temperature curve with air and other gases in the very different. In ethanol gas, as the temperature rose from 130 ℃ 200 ℃, the conductance will decrease. But in the air and other gases, its conductivity has been elevated. This indicates that at about 200 ℃, in air and ethanol gas will reach the maximum conductivity is poor, thus the maximum response value.3 with 10% of Mg on LnFeO_3 (Ln = Nd, Sm, Gd, Dy) doped, the prepared materials are orthogonal perovskite structure. In these materials, SmFe_ (0.9) Mg_ (0.1) O_3-based gas-sensing material for ethanol response, the best selectivity. The reason has not been well understood, one possible explanation is that due to the number of oxygen vacancies. In the preparation process, as Ln / Fe molar ratio may deviate from the chemical formula, they will lose in the unit cell of the vertex atoms of metal vacancies. In order to maintain charge balance, oxygen vacancies will be produced, leading to an increase in oxygen absorption. The results show that when Ln = Sm, in the crystal caused by the highest concentration of oxygen vacancies. Therefore, since SmFe_ (0.9) Mg_ (0.1) O_3-based gas sensor has the maximum number of oxygen surface adsorption sites, making it the highest response. Sensing mechanism is working in a specific temperature gas adsorption between oxygen and ethanol, the catalytic reaction.We studied the sintering temperature on structure and properties of materials, and found that samples sintered at 800 ℃ showed the best sensing properties. For 200 ppm of ethanol gas, 600 ℃ calcined samples at 300 ℃ is to achieve the maximum response of only 12.3. When the sintering temperature from 600 ℃ to rise to 800 ℃, the maximum response value increased to 109.6, its operating temperature down to 200 ℃. When the sintering temperature rose to 900 ℃, the maximum response reduced the value of 56.4. This perovskite material gas of the Fe-O bond is strong and relevant, the weaker the bond strength, the better gas. We calculated the average of the crystal (Fe / Mg)-O bond length, 800 ℃ sintered samples with the longest (Fe / Mg)-O bond, bond strength and thus its weakest, showing the best gas of.4.SmFe_ (1-x) Mg_xO_3 nanopowder in x <0.3, when the formation of solid solution with orthogonal perovskite structure. SmFeO_3 Mg doping can reduce the average particle size, can reduce the SmFe_ (1-x) Mg_xO_3 resistance, electrical conductivity of the energy barrier with the Mg doping increases. One SmFe_ (0.9) Mg_ (0.1) O_3-based gas sensor for acetone gas best gas sensitivity of 300 ppm acetone gas reached 260 ℃, the highest response value 353. Acetone vapor adsorption and chemical reaction of oxygen throughout the following CH_3COCH_3 (gas) + O ~ - → CH_3C ~ + O + CH_3O ~ - + e ~-CH_3C ~ + O → C ~ + H_3 + COCO + O ~ - → CO_2 + e ~ -5 of LaCo_ (1-x) Mg_xO_3 (x = 0,0.1,0.2,0.3) compound powder study found that when x ≤ 0.1 when orthogonal perovskite structure, and because Mg ~ (2 +) radius (0.72 A) is greater than the Co ~ (3 +) (0.61 A), makes the larger unit cell volume. When x> 0.1 when the cubic structure by Mg ~ (2 +) on the Co ~ (3 +) alternative, oxygen vacancies will have to make some of the unit cell volume decreases slightly. At the same time, also contributed to the generation of oxygen vacancies by the oblique hexahedral cubic perovskite to perovskite structure. LaCo_ (1-x) Mg_xO_3 material shows the characteristics of p-type semiconductor, the resistance of gas sensor depends on the price compensation and oxygen vacancy compensation. LaCo_ (0.8) Mg_ (0.2) O_3-based ethanol gas sensor shows good gas sensitivity and selectivity, the concentration of ethanol gas to meet its corresponding resistance R = kC_ (C_2H_5OH) ~ α formula, and α = 1.03 value to the great LaCoO_3. In addition, we found that LaCo_ (0.7) Mg_ (0.3) O_3-based gas sensor for CO also has a higher response, and has a wide operating temperature range.6 with the first-principles calculations based on density functional method of LaFeO_3 (0 1 0) surface and oxygen adsorbed on its surface. LaFeO_3 (0 1 0) surface of the surface states in the vicinity of the Fermi level, composed mainly by Fe3d track. Oxygen adsorption on the surface of Fe ions play a dominant role. The adsorbed oxygen on the Fe ion in La and O ions than on the more stable adsorption to the adsorption of oxygen on the surface of the Fe ions into the key mechanism is the O2p and Fe 3d orbitals of the strong interaction. In addition, our calculation of the surface oxygen molecules in LaFeO_3 (0 1 0) surface, no direct dissociation. This indicates that LaFeO_3 (0 1 0) surface, dissociation of oxygen molecules are chemisorbed - precursor mechanism.7 calculated using first-principles study of vacancy defects LaFeO_3 electronic structure and ferromagnetism. The results show that the net magnetic moment of O2 vacancies up to 5.96μ_B, O1 and La vacancies have little net magnetic moment, respectively 0.12μ_B and 0.84μ_B, at the O2 vacancies LaFeO_3 in the net magnetic moment is not only the largest, and its density of states at the Fermi level also shows a half-metallic density of states features. From the spin density distribution, in the La vacancy at O1 and did not find the spin density, and in the space at the O2 spin density was found. O2 vacancies LaFeO_3 ferromagnetic model can be used to explain the F center.In short, through the Mg-doped LnFeO_3 and LaCoO_3 series of materials research, we found that the amount of Mg doping can improve their good electrical conductivity and gas sensitivity. System had a series of simple preparation, low cost, high response, fast response and good selectivity and very promising ethanol, carbon monoxide and acetone gas sensors. In addition to the LaFeO_3 and simulation of oxygen adsorption from the microscopic point of view we learn more about material properties and sensing mechanism for exploring new gas sensing materials provide a theoretical basis and guidance.Degree Year: 2009