Mg掺杂Ln（Fe，Co）O_3材料的气敏性能及LaFeO_3电子结构的第一性原理研究
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摘要
钙钛矿型复合氧化物LnTO_3(Ln为稀土元素,T为过渡金属元素)是一类重要的功能材料。他们的功能性质能够通过改变Ln、T位的元素来控制,还可以通过对Ln、T位元素进行掺杂来控制。掺杂后的LnTO_3材料不仅能仍保持稳定的钙钛矿结构,而且掺杂后材料的气敏性能、催化性能和导电性能都得到很大提高。因此对它们的研究具有重要的理论意义和实用价值。
人们通常会通过用低价阳离子元素比如Ca、Sr、Ba、Pb等来部分替代LaFeO_3中La的位置或用Cu、Co、Ni、Mn等来替代Fe的位置来改善他们的气敏性。Mg与Ca、Sr、Ba、Pb在同一主族,但是关于Mg对LnFeO_3掺杂的材料的气敏性研究还未见报道,同时,关于Mg在LnTO_3晶体中的替代位置也存在有争议。有人认为Mg可以替代Ln的位置,有人则认为Mg不能替代Ln位,只能替代T的位置。因此本文用溶胶凝胶法合成了Mg掺杂的LnFeO_3和LaCoO_3系列纳米粉体,并对其导电以及气敏特性进行了研究,我们认为Mg更倾向于占据Fe位,同时发现适量的Mg掺杂能提高材料的电导,很好的改善其气敏性。
目前相关的研究领域几乎没有关于LaFeO_3的气敏机理的模拟计算,为了能够从理论上更深刻地来理解它的气敏机理,在论文的最后我们基于第一性原理的计算模拟了LaFeO_3表面的氧吸附情况,发现Fe离子对氧吸附起着主导作用。此外我们还研究了空位缺陷对LaFeO_3电子结构和磁性能的影响,氧空位缺陷能够让反铁磁性的LaFeO_3产生一定的弱磁性。关于Mg掺杂的LaFeO_3的计算还在进行中。
本论文的工作主要得到如下结论:
1.Mg对LaFeO_3的掺杂材料La_(1-x)Mg_xFeO_3中,当Mg的掺杂量x小于0.1时,在XRD图像上只有钙钛矿性的特征衍射峰可以看见。当Mg的掺杂量x大于等于0.2时,除了钙钛矿相的峰外还出现了MgFe_2O_4的峰,并且MgFe_2O_4的峰强随着Mg掺杂量的增大而增强。这说明Mg更倾向于占据Fe的位置。当x＜0.1时,La_(1-x)Mg_xFeO_3的电阻由[V_(La)~x]和[Mg_(Fe)~x]缺陷与氧空位缺陷共同决定。这时其电阻比LaFeO_3的电阻要小很多。当x＞0.1时,由于样品中出现了p-La_(1-x)Mg_yFe_(1-y)O_3/n-Mg_zFe_(1-z)Fe_2O_4异质节,使得La_(1-x)Mg_xFeO_3气敏元件的电阻随着x的增大而增大并且超过了LaFeO_3的电阻。
适量的Mg掺杂的LaFeO_3对乙醇和甲烷的响应值要远大于未掺杂的LaFeO_3。其中La_(0.92)Mg_(0.08)FeO_3基的气体传感器对醇类表现出极好的响应和选择性,其响应值随着气体浓度的增加而增加,工作温度随着浓度的增加而降低。随着气体分子碳链的增长,响应值会变大。这是因为气体分子的位阻效应和电子施主效应是影响气体分子吸附的两个主要因素;La_(0.9)Mg_(0.1)FeO_3基的气体传感器对甲烷表现出良好的气敏性与稳定性,同时拥有比较低的工作温度(200℃)。
2.LaMg_xFe_(1-x)O_3(x=0,0.1,0.2,0.3)固溶体的超精细粉体由溶胶—凝胶方法制备,然后经过800℃四个小时的煅烧。当Mg的含量小于等于0.2时,可以得到单相钙钛矿结构。晶体的摄动是由于较大的Mg~(2+)和八面体变形的Fe~(4+)离子引起的。这种材料显示出P型半导体的性质。电价补偿和氧空位补偿的复合效应导致了Mg掺杂的LaFeO_3的电阻的降低。但是对于x＞0.1样品,它的电阻随着Mg掺杂量的增多而增大,这是由于随着Mg掺杂量的增加氧空位增多,导致氧空位补偿效应增强。其中LaMg_(0.1)Fe_(0.9)O_3基气敏元件显示出了对乙醇的较高的响应和较好的选择性。LaMg_(0.1)Fe_(0.9)O_3基气敏元件在乙醇气体中的电导-温度曲线与其在空气中和其他气体中的有很大的不同。在乙醇气体中,随着温度从130℃升到200℃时,电导将下降。但是在空气和其他气体中,其电导一直是升高的。这表明在在200℃左右,空气和乙醇气体中的电导差将会达到最大,响应值也就最大。
3.用10%的Mg对LnFeO_3(Ln=Nd,Sm,Gd,Dy)进行掺杂,所制备的的材料均是正交钙钛矿结构。在这些材料中,SmFe_(0.9)Mg_(0.1)O_3基的气敏材料对乙醇的响应、选择性最好。其原因还没有被很好的理解,一种可能的解释是归因于氧空位的数量。在制备的过程中,由于Ln/Fe的摩尔比可能会偏离化学式,他们会在晶胞的顶点处丢失原子产生金属空位。为了维持电荷平衡,氧空位将会产生,从而导致氧吸附的增加。结果表明当Ln=Sm时,在晶体中引起的的氧空位浓度最大。因此,由于SmFe_(0.9)Mg_(0.1)O_3基的气敏元件表面有最大数量的氧吸附位,使得其具有最高的响应。气敏机理正是在特定的工作温度下表面吸附氧与乙醇气体之间发生了催化反应。
同时我们研究了烧结温度对材料结构和性能的影响,发现在800℃烧结的样品表现出最好的气敏性能。对于200 ppm的乙醇气体,600℃煅烧的样品在300℃是达到的最大响应值仅为12.3。当烧结温度从600℃升到800℃时,最大响应值增加到109.6,其工作温度下降到200℃。当烧结温度升到900℃时,最大响应值降为56.4。这种钙钛矿结构的材料的气敏性是与Fe—O键强有关的,键强越弱,气敏性越好。我们计算了晶体的中平均的(Fe/Mg)-O键长,800℃烧结的样品有着最长的(Fe/Mg)-O键,因而其键强最弱,表现出最好的气敏性。
4.SmFe_(1-x)Mg_xO_3纳米粉体可以在x＜0.3时形成固溶体,具有正交钙钛矿的结构。Mg的掺杂可以降低SmFeO_3的平均粒径,可以降低SmFe_(1-x)Mg_xO_3的电阻,导电的能量势垒随着Mg掺杂量的增加而减小。其中SmFe_(0.9)Mg_(0.1)O_3基气敏元件对丙酮气体的气敏性最好,对300 ppm的丙酮气体在260℃时达到最高的响应值353。丙酮蒸汽与化学吸附氧的整个反应如下CH_3COCH_3(gas)+O~-→CH_3C~+O+CH_3O~-+e~-CH_3C~+O→C~+H_3+COCO+O~-→CO_2+e~-
5.对LaCo_(1-x)Mg_xO_3(x=0,0.1,0.2,0.3)化合物粉体的研究发现,当x≤0.1时是正交钙钛矿结构,而且由于Mg~(2+)的半径(0.72 A)大于Co~(3+)(0.61 A),使得晶胞体积变大。当x＞0.1时为立方结构,通过Mg~(2+)对Co~(3+)的替代,氧空位的将会产生,使得晶胞体积稍稍有些减小。同时,氧空位的产生也推动了由斜六面体钙钛矿向立方钙钛矿结构的转变。LaCo_(1-x)Mg_xO_3材料都显示了p型半导体的特性,气敏元件的电阻取决于电价补偿和氧空位补偿。LaCo_(0.8)Mg_(0.2)O_3基气敏元件对乙醇显示了良好的气敏性与选择性,乙醇气体的浓度与其相应的电阻满足R=kC_(C_2H_5OH)~α式子,并且α=1.03值要远大LaCoO_3的。另外我们发现LaCo_(0.7)Mg_(0.3)O_3基气敏元件对CO也具有较高的响应,并且有较宽的工作温度范围。
6.用基于密度泛函的第一性原理计算方法研究了LaFeO_3(0 1 0)表面和氧在其表面上的吸附。LaFeO_3(0 1 0)表面的表面态出现在费米能级的附近,主要由Fe3d的轨道组成。表面Fe离子对氧吸附起到主导作用。在Fe离子上的吸附氧比在La和O离子上的吸附要稳定的多,吸附氧在表面Fe离子上的成键机制是O2p和Fe 3d轨道之间的强相互作用。此外,我们的计算表面氧分子在LaFeO_3(0 1 0)表面上没有直接的解离吸附。这表明LaFeO_3(0 1 0)表面上氧分子的解离属于化学吸附-前躯体机制。
7.用第一性原理的计算研究了空位缺陷的LaFeO_3的电子结构和铁磁性。结果表明O2空位产生的净磁矩最大为5.96μ_B,O1和La空位产生的净磁矩很少分别为0.12μ_B和0.84μ_B,在O2空位的LaFeO_3中其净磁矩不仅最大,而且其态密度在费米能级处还显示出了半金属的态密度的特征。从自旋密度分布图中,在O1和La空位处没有发现自旋密度,而在O2空位处明显发现了自旋密度。O2空位的LaFeO_3的铁磁性可以用F心模型来解释。
总之,通过对Mg掺杂的LnFeO_3和LaCoO_3系列材料的研究,我们发现Mg的适量掺杂可以很好的改善他们的导电性和气敏性。制得了一系列制备简单、成本低、响应高、反应快和选择性好且很有应用前景的乙醇、一氧化碳和丙酮的气敏传感器。另外对LaFeO_3和氧吸附的模拟计算让我们从微观角度更详细地了解材料的性能和气敏机理,为探索新的气敏材料提供理论依据和指导。
Perovskite-type compound oxides LnTO_3(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 LnTO_3 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 LaFeO_3 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 LaFeO_3 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 LnFeO_3 and LaCoO_3 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 LaFeO_3 have been reported.To comprehend its gas sensing mechanism more deeply in theory, we calculated the oxygen adsorption on LaFeO_3 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 LaFeO_3, and found that oxygen vacancy caused weak ferromagnetism in the antiferromagnetism LaFeO_3.Calculations about Mg-doping LaFeO_3 are carrying out.
The abstract of our results as follows:
1.Among the Mg-doping LaFeO_3 materials La_(1-x)Mg_xFeO_3,when x＜0.1,only perovskite phase peaks could be seen;when x≥0.2,peaks of MgFe_2O_4 appeared except that of LaFeO_3 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_(1_x)MgxFeO_3 were depended on[VLa~x],[MgFe~x]and oxygen vacancy defects. Their resistances were much smaller than that of LaFeO_3.When x＞0.1,due to p-La_(1-x)Mg_yFe_(1-y)O_3/n-Mg_zFe_(1-z)Fe_2O_4 heterojunctions in the samples,the resistance of La_(1-x)Mg_xFeO_3 increased with x and then became bigger than LaFeO_3.
The responses to ethanol and methane of proper Mg-doping LaFeO_3 were much bigger than that of undopped LaFeO_3.La_(0.92)Mg_(0.08)FeO_3-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_(0.9)Mg_(0.1)FeO_3-based sensor exhibited good response、selectivity、stability and low optimal temperature(200℃) to methane.
2.Ultramicro powders of the solid solution LaMgxFe_(1-x)O_3(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 Mg~(2+) ions and octahedral distorted Fe~(4+) 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 LaFeO_3,but for samples with x＞0.1,its resistivity increased with increasing Mg~(2+)-doping amount.The LaMg_(0.1)Fe_(0.9)O_3-based sensor showed higher response and better selectivity to C_2H_5OH.Great difference on the conductance-temperature curves of LaMg_(0.1)Fe_(0.9)O_3-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 LnFeO_3(Ln=Nd,Sin,Gd,Dy),and all the prepared materials showed orthorhombic perovskite structure.It was observed that of all the materials the SmFe_(0.9)Mg_(0.1)O_3-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_(0.9)Mg_(0.1)O_3 crystals was got for Ln=Sm.Thus, SmFe_(0.9)Mg_(0.1)O_3-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-x)Mg_xO_3 was formed with orthorhombic perovskite structure when x<0.3.The Mg~(2+)-doping decreased the mean grain size of SmFeO_3 and the resistance of SmFe_(1-x)Mg_xO_3.The energy barrier of conduction became smaller with an increase in the Mg~(2+)-doping amount. The SmFe_(0.9)Mg_(0.1)O_3-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_(0.9)Mg_(0.1)O_3-based sensor at 260℃.The overall reaction of acetone vapor with chemisorbed oxygen may take place as below:
CH_3COCH_3(gas)+O~-→CH_3C~+O +CH_30~-+e~-
CH3C~+O→C~+H_3+CO
CO+O~-→CO_2+e~-
5.The compounds LaCo_(1-x)Mg_xO_3(x=0,0.1,0.2,0.3) crystallized as a perovskite phase with rhombohedral structure with x≤0.1,Since the radius of Mg~(2+) ion(0.72 (?)) is larger than that of Co~(3+)(0.61(?)),when Mg~(2+) substituted Co~(3+) in B-site the lattice parameters and unit cell volume become larger than non-substituted LaCoO_3 when x≤0.1;The compounds LaCo_(1-x)Mg_xO_3 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 Co~(3+) to Mg~(2+) for keeping charge neutrality.Creation of oxygen vacancies is also driving the rhombohedral structure toward the cubic perovskite structure. LaCo_(1-x)Mg_xO_3 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_(0.8)Mg_(0.2)O_3-based sensor showed higher response and better selectivity to C_2H_5OH.The relationship between concentration of ethanol gas and resistance accords with the formula R = kC~αc_2H_5OH well,withα=1.03 much larger than that of LaCoO_3.What is more, we found that the LaCo_(0.7)Mg_(0.3)O_3-based sensor had the best response to CO gas and had a wide operating temperature range.
6.The clean LaFeO_3(0 1 0) surface and O_2 adsorption on it have been investigated at the level of density functional tvdheory.The surface states of LaFeO_3(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 O_2 on Fe ion is much more stable than that on La and O ions,and the bonding mechanism of adsorbed O_2 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 O_2 on the LaFeO_3(0 1 0) surface.It indicates that O_2 dissociation on LaFeO_3 surface belongs to chemisorbed-precursor mechanism.
7.We have studied the ferromagnetism of vacancy-defective LaFeO_3 using first-principle calculation.The calculations showed that O_2 vacancy had the biggest net magnetic moment of 5.96μ_B than O_1(0.12μ_B) and La(0.84μ_B) vacancy.The O_2- defective LaFeO_3 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 LaFeO_3 can be explained by F-center model.
In summary,from the studies on Mg-doping LnFeO_3 and LaCoO_3 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 LaFeO_3 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.
人们通常会通过用低价阳离子元素比如Ca、Sr、Ba、Pb等来部分替代LaFeO_3中La的位置或用Cu、Co、Ni、Mn等来替代Fe的位置来改善他们的气敏性。Mg与Ca、Sr、Ba、Pb在同一主族,但是关于Mg对LnFeO_3掺杂的材料的气敏性研究还未见报道,同时,关于Mg在LnTO_3晶体中的替代位置也存在有争议。有人认为Mg可以替代Ln的位置,有人则认为Mg不能替代Ln位,只能替代T的位置。因此本文用溶胶凝胶法合成了Mg掺杂的LnFeO_3和LaCoO_3系列纳米粉体,并对其导电以及气敏特性进行了研究,我们认为Mg更倾向于占据Fe位,同时发现适量的Mg掺杂能提高材料的电导,很好的改善其气敏性。
目前相关的研究领域几乎没有关于LaFeO_3的气敏机理的模拟计算,为了能够从理论上更深刻地来理解它的气敏机理,在论文的最后我们基于第一性原理的计算模拟了LaFeO_3表面的氧吸附情况,发现Fe离子对氧吸附起着主导作用。此外我们还研究了空位缺陷对LaFeO_3电子结构和磁性能的影响,氧空位缺陷能够让反铁磁性的LaFeO_3产生一定的弱磁性。关于Mg掺杂的LaFeO_3的计算还在进行中。
本论文的工作主要得到如下结论:
1.Mg对LaFeO_3的掺杂材料La_(1-x)Mg_xFeO_3中,当Mg的掺杂量x小于0.1时,在XRD图像上只有钙钛矿性的特征衍射峰可以看见。当Mg的掺杂量x大于等于0.2时,除了钙钛矿相的峰外还出现了MgFe_2O_4的峰,并且MgFe_2O_4的峰强随着Mg掺杂量的增大而增强。这说明Mg更倾向于占据Fe的位置。当x＜0.1时,La_(1-x)Mg_xFeO_3的电阻由[V_(La)~x]和[Mg_(Fe)~x]缺陷与氧空位缺陷共同决定。这时其电阻比LaFeO_3的电阻要小很多。当x＞0.1时,由于样品中出现了p-La_(1-x)Mg_yFe_(1-y)O_3/n-Mg_zFe_(1-z)Fe_2O_4异质节,使得La_(1-x)Mg_xFeO_3气敏元件的电阻随着x的增大而增大并且超过了LaFeO_3的电阻。
适量的Mg掺杂的LaFeO_3对乙醇和甲烷的响应值要远大于未掺杂的LaFeO_3。其中La_(0.92)Mg_(0.08)FeO_3基的气体传感器对醇类表现出极好的响应和选择性,其响应值随着气体浓度的增加而增加,工作温度随着浓度的增加而降低。随着气体分子碳链的增长,响应值会变大。这是因为气体分子的位阻效应和电子施主效应是影响气体分子吸附的两个主要因素;La_(0.9)Mg_(0.1)FeO_3基的气体传感器对甲烷表现出良好的气敏性与稳定性,同时拥有比较低的工作温度(200℃)。
2.LaMg_xFe_(1-x)O_3(x=0,0.1,0.2,0.3)固溶体的超精细粉体由溶胶—凝胶方法制备,然后经过800℃四个小时的煅烧。当Mg的含量小于等于0.2时,可以得到单相钙钛矿结构。晶体的摄动是由于较大的Mg~(2+)和八面体变形的Fe~(4+)离子引起的。这种材料显示出P型半导体的性质。电价补偿和氧空位补偿的复合效应导致了Mg掺杂的LaFeO_3的电阻的降低。但是对于x＞0.1样品,它的电阻随着Mg掺杂量的增多而增大,这是由于随着Mg掺杂量的增加氧空位增多,导致氧空位补偿效应增强。其中LaMg_(0.1)Fe_(0.9)O_3基气敏元件显示出了对乙醇的较高的响应和较好的选择性。LaMg_(0.1)Fe_(0.9)O_3基气敏元件在乙醇气体中的电导-温度曲线与其在空气中和其他气体中的有很大的不同。在乙醇气体中,随着温度从130℃升到200℃时,电导将下降。但是在空气和其他气体中,其电导一直是升高的。这表明在在200℃左右,空气和乙醇气体中的电导差将会达到最大,响应值也就最大。
3.用10%的Mg对LnFeO_3(Ln=Nd,Sm,Gd,Dy)进行掺杂,所制备的的材料均是正交钙钛矿结构。在这些材料中,SmFe_(0.9)Mg_(0.1)O_3基的气敏材料对乙醇的响应、选择性最好。其原因还没有被很好的理解,一种可能的解释是归因于氧空位的数量。在制备的过程中,由于Ln/Fe的摩尔比可能会偏离化学式,他们会在晶胞的顶点处丢失原子产生金属空位。为了维持电荷平衡,氧空位将会产生,从而导致氧吸附的增加。结果表明当Ln=Sm时,在晶体中引起的的氧空位浓度最大。因此,由于SmFe_(0.9)Mg_(0.1)O_3基的气敏元件表面有最大数量的氧吸附位,使得其具有最高的响应。气敏机理正是在特定的工作温度下表面吸附氧与乙醇气体之间发生了催化反应。
同时我们研究了烧结温度对材料结构和性能的影响,发现在800℃烧结的样品表现出最好的气敏性能。对于200 ppm的乙醇气体,600℃煅烧的样品在300℃是达到的最大响应值仅为12.3。当烧结温度从600℃升到800℃时,最大响应值增加到109.6,其工作温度下降到200℃。当烧结温度升到900℃时,最大响应值降为56.4。这种钙钛矿结构的材料的气敏性是与Fe—O键强有关的,键强越弱,气敏性越好。我们计算了晶体的中平均的(Fe/Mg)-O键长,800℃烧结的样品有着最长的(Fe/Mg)-O键,因而其键强最弱,表现出最好的气敏性。
4.SmFe_(1-x)Mg_xO_3纳米粉体可以在x＜0.3时形成固溶体,具有正交钙钛矿的结构。Mg的掺杂可以降低SmFeO_3的平均粒径,可以降低SmFe_(1-x)Mg_xO_3的电阻,导电的能量势垒随着Mg掺杂量的增加而减小。其中SmFe_(0.9)Mg_(0.1)O_3基气敏元件对丙酮气体的气敏性最好,对300 ppm的丙酮气体在260℃时达到最高的响应值353。丙酮蒸汽与化学吸附氧的整个反应如下CH_3COCH_3(gas)+O~-→CH_3C~+O+CH_3O~-+e~-CH_3C~+O→C~+H_3+COCO+O~-→CO_2+e~-
5.对LaCo_(1-x)Mg_xO_3(x=0,0.1,0.2,0.3)化合物粉体的研究发现,当x≤0.1时是正交钙钛矿结构,而且由于Mg~(2+)的半径(0.72 A)大于Co~(3+)(0.61 A),使得晶胞体积变大。当x＞0.1时为立方结构,通过Mg~(2+)对Co~(3+)的替代,氧空位的将会产生,使得晶胞体积稍稍有些减小。同时,氧空位的产生也推动了由斜六面体钙钛矿向立方钙钛矿结构的转变。LaCo_(1-x)Mg_xO_3材料都显示了p型半导体的特性,气敏元件的电阻取决于电价补偿和氧空位补偿。LaCo_(0.8)Mg_(0.2)O_3基气敏元件对乙醇显示了良好的气敏性与选择性,乙醇气体的浓度与其相应的电阻满足R=kC_(C_2H_5OH)~α式子,并且α=1.03值要远大LaCoO_3的。另外我们发现LaCo_(0.7)Mg_(0.3)O_3基气敏元件对CO也具有较高的响应,并且有较宽的工作温度范围。
6.用基于密度泛函的第一性原理计算方法研究了LaFeO_3(0 1 0)表面和氧在其表面上的吸附。LaFeO_3(0 1 0)表面的表面态出现在费米能级的附近,主要由Fe3d的轨道组成。表面Fe离子对氧吸附起到主导作用。在Fe离子上的吸附氧比在La和O离子上的吸附要稳定的多,吸附氧在表面Fe离子上的成键机制是O2p和Fe 3d轨道之间的强相互作用。此外,我们的计算表面氧分子在LaFeO_3(0 1 0)表面上没有直接的解离吸附。这表明LaFeO_3(0 1 0)表面上氧分子的解离属于化学吸附-前躯体机制。
7.用第一性原理的计算研究了空位缺陷的LaFeO_3的电子结构和铁磁性。结果表明O2空位产生的净磁矩最大为5.96μ_B,O1和La空位产生的净磁矩很少分别为0.12μ_B和0.84μ_B,在O2空位的LaFeO_3中其净磁矩不仅最大,而且其态密度在费米能级处还显示出了半金属的态密度的特征。从自旋密度分布图中,在O1和La空位处没有发现自旋密度,而在O2空位处明显发现了自旋密度。O2空位的LaFeO_3的铁磁性可以用F心模型来解释。
总之,通过对Mg掺杂的LnFeO_3和LaCoO_3系列材料的研究,我们发现Mg的适量掺杂可以很好的改善他们的导电性和气敏性。制得了一系列制备简单、成本低、响应高、反应快和选择性好且很有应用前景的乙醇、一氧化碳和丙酮的气敏传感器。另外对LaFeO_3和氧吸附的模拟计算让我们从微观角度更详细地了解材料的性能和气敏机理,为探索新的气敏材料提供理论依据和指导。
Perovskite-type compound oxides LnTO_3(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 LnTO_3 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 LaFeO_3 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 LaFeO_3 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 LnFeO_3 and LaCoO_3 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 LaFeO_3 have been reported.To comprehend its gas sensing mechanism more deeply in theory, we calculated the oxygen adsorption on LaFeO_3 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 LaFeO_3, and found that oxygen vacancy caused weak ferromagnetism in the antiferromagnetism LaFeO_3.Calculations about Mg-doping LaFeO_3 are carrying out.
The abstract of our results as follows:
1.Among the Mg-doping LaFeO_3 materials La_(1-x)Mg_xFeO_3,when x＜0.1,only perovskite phase peaks could be seen;when x≥0.2,peaks of MgFe_2O_4 appeared except that of LaFeO_3 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_(1_x)MgxFeO_3 were depended on[VLa~x],[MgFe~x]and oxygen vacancy defects. Their resistances were much smaller than that of LaFeO_3.When x＞0.1,due to p-La_(1-x)Mg_yFe_(1-y)O_3/n-Mg_zFe_(1-z)Fe_2O_4 heterojunctions in the samples,the resistance of La_(1-x)Mg_xFeO_3 increased with x and then became bigger than LaFeO_3.
The responses to ethanol and methane of proper Mg-doping LaFeO_3 were much bigger than that of undopped LaFeO_3.La_(0.92)Mg_(0.08)FeO_3-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_(0.9)Mg_(0.1)FeO_3-based sensor exhibited good response、selectivity、stability and low optimal temperature(200℃) to methane.
2.Ultramicro powders of the solid solution LaMgxFe_(1-x)O_3(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 Mg~(2+) ions and octahedral distorted Fe~(4+) 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 LaFeO_3,but for samples with x＞0.1,its resistivity increased with increasing Mg~(2+)-doping amount.The LaMg_(0.1)Fe_(0.9)O_3-based sensor showed higher response and better selectivity to C_2H_5OH.Great difference on the conductance-temperature curves of LaMg_(0.1)Fe_(0.9)O_3-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 LnFeO_3(Ln=Nd,Sin,Gd,Dy),and all the prepared materials showed orthorhombic perovskite structure.It was observed that of all the materials the SmFe_(0.9)Mg_(0.1)O_3-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_(0.9)Mg_(0.1)O_3 crystals was got for Ln=Sm.Thus, SmFe_(0.9)Mg_(0.1)O_3-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-x)Mg_xO_3 was formed with orthorhombic perovskite structure when x<0.3.The Mg~(2+)-doping decreased the mean grain size of SmFeO_3 and the resistance of SmFe_(1-x)Mg_xO_3.The energy barrier of conduction became smaller with an increase in the Mg~(2+)-doping amount. The SmFe_(0.9)Mg_(0.1)O_3-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_(0.9)Mg_(0.1)O_3-based sensor at 260℃.The overall reaction of acetone vapor with chemisorbed oxygen may take place as below:
CH_3COCH_3(gas)+O~-→CH_3C~+O +CH_30~-+e~-
CH3C~+O→C~+H_3+CO
CO+O~-→CO_2+e~-
5.The compounds LaCo_(1-x)Mg_xO_3(x=0,0.1,0.2,0.3) crystallized as a perovskite phase with rhombohedral structure with x≤0.1,Since the radius of Mg~(2+) ion(0.72 (?)) is larger than that of Co~(3+)(0.61(?)),when Mg~(2+) substituted Co~(3+) in B-site the lattice parameters and unit cell volume become larger than non-substituted LaCoO_3 when x≤0.1;The compounds LaCo_(1-x)Mg_xO_3 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 Co~(3+) to Mg~(2+) for keeping charge neutrality.Creation of oxygen vacancies is also driving the rhombohedral structure toward the cubic perovskite structure. LaCo_(1-x)Mg_xO_3 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_(0.8)Mg_(0.2)O_3-based sensor showed higher response and better selectivity to C_2H_5OH.The relationship between concentration of ethanol gas and resistance accords with the formula R = kC~αc_2H_5OH well,withα=1.03 much larger than that of LaCoO_3.What is more, we found that the LaCo_(0.7)Mg_(0.3)O_3-based sensor had the best response to CO gas and had a wide operating temperature range.
6.The clean LaFeO_3(0 1 0) surface and O_2 adsorption on it have been investigated at the level of density functional tvdheory.The surface states of LaFeO_3(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 O_2 on Fe ion is much more stable than that on La and O ions,and the bonding mechanism of adsorbed O_2 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 O_2 on the LaFeO_3(0 1 0) surface.It indicates that O_2 dissociation on LaFeO_3 surface belongs to chemisorbed-precursor mechanism.
7.We have studied the ferromagnetism of vacancy-defective LaFeO_3 using first-principle calculation.The calculations showed that O_2 vacancy had the biggest net magnetic moment of 5.96μ_B than O_1(0.12μ_B) and La(0.84μ_B) vacancy.The O_2- defective LaFeO_3 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 LaFeO_3 can be explained by F-center model.
In summary,from the studies on Mg-doping LnFeO_3 and LaCoO_3 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 LaFeO_3 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.
引文
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