Studies On Gas Sensing Properties Of Mg-Doping Ln (Fe,Co)O₃ Materials And Electronic Structure Of LaFeO₃ Based On First-Principles

Perovskite-type compound oxides LnTO₃(Ln=rear earth,T=transition metal) are important functional materials.Their functional properties can be changed by changing Ln or T site ions,or partially substituting both A(Ln) or B(Fe) cations by other kind of cations.The doped LnTO₃ still keep the perovskite structure,and the gas sensing,catalytic and conduction properties will be improved.So studies on these materials have important theory significance and utility value.Generally,gas-sensing properties of LaFeO₃ can be improved by substituting La ions by lower valance-cations,such as Ca,Sr,Ba,Pb,or substituting Fe by Cu,Co,Ni,Mn and so on.Mg is in the same main group as Ca,Sr,Ba,Pb,however, studies on gas sensing properties of Mg-doping LaFeO₃ materials have not been reported.Meanwhile,there is controvercy about the substitution site of Mg.Some people reported that Mg could substitute Ln site,and some people reported that Mg could only substituted T site.In this paper,Mg-doping LnFeO₃ and LaCoO₃ nano powders were prepared by sol-gel method,and the conduction and gas sensing properties were also studied.We found that proper Mg-doping could improve conduction and sensitivity.Up to now,few modeling calculations about gas sensing mechanism of LaFeO₃ have been reported.To comprehend its gas sensing mechanism more deeply in theory, we calculated the oxygen adsorption on LaFeO₃ surface based on first-principle calculation.We found that Fe ions dominated the oxygen adsorption.In addition,we studied the effect of vacancy defects on electron structure and magnetism of LaFeO₃, and found that oxygen vacancy caused weak ferromagnetism in the antiferromagnetism LaFeO₃.Calculations about Mg-doping LaFeO₃ are carrying out.The abstract of our results as follows:1.Among the Mg-doping LaFeO₃ materials La_1-xMg_xFeO₃,when x＜0.1,only perovskite phase peaks could be seen;when x≥0.2,peaks of MgFe₂O₄ appeared except that of LaFeO₃ and became stronger with an increase in x.This indicated that Mg was prone to occupy the Fe site.when x＜0.1,the resistance of La_1xMgxFeO₃ were depended on[VLa^x],[MgFe^x]and oxygen vacancy defects. Their resistances were much smaller than that of LaFeO₃.When x＞0.1,due to p-La_1-xMg_yFe_1-yO₃/n-Mg_zFe_1-zFe₂O₄ heterojunctions in the samples,the resistance of La_1-xMg_xFeO₃ increased with x and then became bigger than LaFeO₃.The responses to ethanol and methane of proper Mg-doping LaFeO₃ were much bigger than that of undopped LaFeO₃.La_0.92Mg_0.08FeO₃-based sensor exhibited excellent response and selectivity to alcohols,the response increased with an increase in gas concentration,the optimal working temperature decreased with an increase in gas concentration.In addition,we found that the response of the sensor increased with increasing the hydrocarbon chain of alcohols.This is because that the steric effect and electron donating effect of gas molecule are the two factors affecting the gas adsorption.La_0.9Mg_0.1FeO₃-based sensor exhibited good response,selectivity,stability and low optimal temperature(200℃) to methane.2.Ultramicro powders of the solid solution LaMgxFe_1-xO₃(x=0,0.1,0.2,0.3) were prepared by sol-gel method,and then followed by calcinations at 800℃for 4h. The single perovskites phases could be obtained when x＜0.2.The lattice perturbation caused by larger Mg²⁺ ions and octahedral distorted Fe⁴⁺ ions.This material showed p-type semiconducting properties,The combined effect of electrovalence compensation and oxygen vacancy compensation resulted in that Mg-doping decreased resistance of LaFeO₃,but for samples with x＞0.1,its resistivity increased with increasing Mg²⁺-doping amount.The LaMg_0.1Fe_0.9O₃-based sensor showed higher response and better selectivity to C₂H₅OH.Great difference on the conductance-temperature curves of LaMg_0.1Fe_0.9O₃-based sensor in ethanol gas and air or other gas were also found. The conductance in ethanol gas decreased with temperature from 130℃to 200℃. But in air and other gas the conductance increased all the time.It indicated that at about 200℃the conductance difference between in air and ethanol was the biggest and the response reached the maximum.3.We used 10%Mg to dope LnFeO₃(Ln=Nd,Sin,Gd,Dy),and all the prepared materials showed orthorhombic perovskite structure.It was observed that of all the materials the SmFe_0.9Mg_0.1O₃-based sensor was extremely sensitive to ethanol,with good selectivity.The reason has not been well understood.One explanation is possibly the amount of oxygen vacancies.In the process of synthesis,because of the deviation of the mole ration of Ln/Fe from chemical stoichiometric proportion, they lost the metal atoms at the crank points of the cells and produced metal vacancies.To maintain the charge balance,oxygen vacancies should form resulting in the increase of oxygen absorbed.The results indicated that the largest number of oxygen vacancies in LnFe_0.9Mg_0.1O₃ crystals was got for Ln=Sm.Thus, SmFe_0.9Mg_0.1O₃-based sensor has the highest response because of the largest amount of adsorption sites for oxygen on surface.The gas response is believed to be due to the surface catalytic reaction of alcohol vapors with the adsorbed oxygen at working temperature.Meanwhile,we studied the effect on the material structure of annealing temperature,and found the optimal annealing temperature was 800℃.The maximum response of the sample annealed at 600℃is as low as 12.3 at an operating temperature of 300℃.As the annealing temperature increases from 600℃to 800℃,the maximum response increases to 109.6 while the operating temperature shifts to 200℃.However,the maximum response decreases to 56.4 when the sample annealed at 900℃.The sensitivity of this perovskite-type material is related to the Fe-O bond strength.The stronger the bond strength,the lower the sensitivity.We calculated the average(Fe/Mg)-O bond length and found the sample annealed at 800℃had the longest(Fe/Mg)-O bond length and the weakest bond strength,so it exhibited the best gas sensing properties.4.It is found that a substitutional solid solution SmFe_1-xMg_xO₃ was formed with orthorhombic perovskite structure when x<0.3.The Mg²⁺-doping decreased the mean grain size of SmFeO₃ and the resistance of SmFe_1-xMg_xO₃.The energy barrier of conduction became smaller with an increase in the Mg²⁺-doping amount. The SmFe_0.9Mg_0.1O₃-based sensor showed high sensitivity at low operating temperature and good selectivity to acetone gas.The highest response to 300 ppm acetone gas reached 353 for SmFe_0.9Mg_0.1O₃-based sensor at 260℃.The overall reaction of acetone vapor with chemisorbed oxygen may take place as below:CH₃COCH₃(gas)+O^-→CH₃C⁺O +CH₃0^-+e^-CH3C⁺O→C⁺H₃+COCO+O^-→CO₂+e^-5.The compounds LaCo_1-xMg_xO₃(x=0,0.1,0.2,0.3) crystallized as a perovskite phase with rhombohedral structure with x≤0.1,Since the radius of Mg²⁺ ion(0.72 (?)) is larger than that of Co³⁺(0.61(?)),when Mg²⁺ substituted Co³⁺ in B-site the lattice parameters and unit cell volume become larger than non-substituted LaCoO₃ when x≤0.1;The compounds LaCo_1-xMg_xO₃ crystallized as a perovskite phase with cubic structure with x＞0.1,and the lattice parameter of cubic structure reduce slightly.The reason is the oxygen vacancies produced by the substitution of Co³⁺ to Mg²⁺ for keeping charge neutrality.Creation of oxygen vacancies is also driving the rhombohedral structure toward the cubic perovskite structure. LaCo_1-xMg_xO₃ material all showed p-type semiconductor.The influence on resistance of Mg-doping depends on the combined effect of electrovalence compensation and oxygen vacancy compensation.The LaCo_0.8Mg_0.2O₃-based sensor showed higher response and better selectivity to C₂H₅OH.The relationship between concentration of ethanol gas and resistance accords with the formula R = kC^αc₂H₅OH well,withα=1.03 much larger than that of LaCoO₃.What is more, we found that the LaCo_0.7Mg_0.3O₃-based sensor had the best response to CO gas and had a wide operating temperature range.6.The clean LaFeO₃(0 1 0) surface and O₂ adsorption on it have been investigated at the level of density functional tvdheory.The surface states of LaFeO₃(0 1 0) 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 show no direct dissociation adsorption of O₂ on the LaFeO₃(0 1 0) surface.It indicates that O₂ dissociation on LaFeO₃ surface belongs to chemisorbed-precursor mechanism.7.We have studied the ferromagnetism of vacancy-defective LaFeO₃ using first-principle calculation.The calculations showed that O₂ vacancy had the biggest net magnetic moment of 5.96μ_B than O₁(0.12μ_B) and La(0.84μ_B) vacancy.The O₂- defective LaFeO₃ not only has the biggest net magnetic moment, but also shows character of half-metal DOS at Fermi level.From the spin-density distribution map,we can see that the spin-density dose not appearances at O1and La vacancy,but at O2 vacancy obviously appearance spin-density.The magnetism of O2- defective LaFeO₃ can be explained by F-center model.In summary,from the studies on Mg-doping LnFeO₃ and LaCoO₃ we found that proper Mg-doping could improve their conduction and gas sensing properties well. We got a series of gas sensors with simple technics,low cost,high and quick response and good selectivity sensors to ethanol,carbon monoxides and acetone gases which had promising applications.In addition,modeling calculations about LaFeO₃ and oxygen adsorption provided a new way to study properties and gas sensing mechanism of materials in detail from micro point of view.They also provided theory basis and guidance for seeking new gas sensing materials.