LaFeO_3基氧化物对还原性气体的气敏性与机制研究
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摘要
近年来由于在巨磁电阻效应、催化、高温超导以及气敏等方面的广泛应用,钙钛矿稀土化合物(具有结构ABO3)材料吸引了人们越来越多的注意。尤其是当其作为气敏材料应用时,钙钛矿稀土化合物不仅表现出了良好的气敏性、选择性和稳定性,而且它的各项气敏性能还可以通过对A位或者B位的掺杂来进行调节。掺杂不仅不会影响这类化合物本身所具备的钙钛矿结构,还会大大提高其导电和气敏性能。因此为了寻找实用的新型气敏材料,对这种材料所进行的研究就显得特别的有意义。
通常来说,低价阳离子元素比如Ca、Sr和Pb是用来掺杂LaFeO3以提高其气敏性能的最佳元素。Ba和Ca、Sr在元素周期表中属于同一主族,但是到目前为止,我们并未发现有关于Ba掺杂LaFeO3材料气敏性能的研究报道。本文用溶胶凝胶法制备了La1-xBaxFeO3(x≤0.3)纳米晶体并对其气敏性能进行了一系列的测试研究,发现适量Ba2+的掺杂能提高LaFeO3材料对酒精气体的气敏性能。为了更加深入了解钙钛矿氧化物的气敏机理,我们选用LaFeO3和CO分子作为代表,采用第一性原理的方法模拟计算了吸附氧由LaFeO3表面解离的过程,发现CO分子的吸附确实引起了吸附氧的解离,同时在吸附过程中的电子转移能改变LaFeO3的电导,从而使LaFeO3材料对CO气体产生气敏效应。之后,我们对Ca2+掺杂的LaFeO3材料进行了计算研究,发现适量Ca2+的掺杂不仅能提高CO分子吸附过程中的电子转移数量,还能降低吸附氧解离时候需要的能量,这也是Ca2+掺杂能提高LaFeO3材料气敏性能的原因。
为了在微观上研究气体分子和LaFeO3基材料表面的反应情况,我们采用第一性原理方法模拟计算了多种还原性气体分子在LaFeO3基材料表面的吸附情况。计算结果表明一般情况下Fe位在气体分子的吸附过程中起着主导作用。通过对吸附能和电子转移等计算结果的分析对比,我们确定了气体分子在LaFeO3基材料表面的最佳吸附构型。同时,我们也研究了氧空位对LaFeO3材料表面吸附CO气体分子的影响。
本论文的主要结论如下：
1.在掺杂材料La1-xBaxFeO3(x≤0.3)中,XRD图像表明所有的样品材料都具有钙钛矿结构,这说明样品中的Ba2+取代了La3+的位置。由于Ba2+的半径(135pm)大于La3+的半径(106.1pm),随着掺杂量的增加,样品的晶胞体积也随之增大。当x<0.1时,La1-xBaxFeO3气敏元件的电阻随着掺杂量的增加而逐渐的降低；而当x>0.1时,La1-xBaxFeO3气敏元件的电阻随着掺杂量的增加而逐渐的升高,这主要是由于表面电价补偿和氧空位补偿的相互影响造成的。基于La1-xBaxFeO3材料制造的气体传感器对于500ppm酒精的气敏性能远远好于纯净的LaFeO3材料和其他几种掺杂材料,同时其最佳工作温度点在240℃。虽然La0.75Ba0.25FeO3气敏元件的最佳工作温度相比LaFeO3略有增高,但是在175℃-360℃的工作温度范围内,它对酒精的气敏性能都要比LaFeO3气敏元件在其最佳工作点要高。当x=0.25时,Ba2+的掺杂在材料表面制造了最大量的适合氧气分子吸附的空位,从而使得LaFeO3气敏元件对酒精气体的气敏性能最高。LaFeO3气敏元件对酒精气体表现出了良好的选择性能,与此同时它在空气中也具有良好的工作稳定性。
2.用溶胶凝胶法制备了LaFeO3纳米粉体,并将之在800℃下退火3个小时。XRD图像表明La0.875Ba0.125FeO3纳米粉体具备典型的钙钛矿结构。基于La0.875Ba0.125FeO3纳米材料制造的气敏元件具备典型的P型半导体性质。根据空气中电阻与温度的图像,我们可以计算得到它的激活能为0.18eV。La0.875Ba0.125FeO3气敏元件对酒精气体具有良好的选择性,它在170℃下对500ppm酒精气体的灵敏度达到了58。我们采用第一性原理计算的方法研究了02分子在La0.875Ba0.125FeO3(010)表面上的吸附。通过对表面La、O和Fe位的对比分析,我们发现O2分子吸附在表面Fe位的时候最稳定。由吸附后O2分子O-O键的长度和振动频率可以知道,吸附后产生了超氧化物02-。相比较于平行的吸附构型,02分子更喜欢以竖直的构型吸附在表面Fe原子上,同时当02分子以竖直的构型吸附在表面Fe原子上时,由表面转移到02分子的电子数量最多。
3.用第一性原理计算的方法研究了CO分子吸附在O2分子预吸附的LaFeO3(010)表面。相比较于表面的其他吸附位置,CO分子更适合与表面预吸附氧发生反应并形成O-C-O物质。在CO分子的吸附过程中,电子由CO分子转移到表面中,使得表面空穴的数量减少,从而导致了LaFeO3电阻的升高。这与之前的实验结果是相符的。CO分子的吸附并没有改变吸附氧与Fe原子之间的成键机制,但是能使得LaFeO3(010)表面的HOMO-LUMO带隙变窄。这主要是由于CO分子的吸附引起了LaFeO3(010)表面的电子重新分布。CO分子吸附前后表面Fe-3d轨道电子数量的变化是引起HOMO-LUMO带隙变化的主要原因。当CO分子吸附在O2分子预吸附的La1-xCaxFeO3(010)表面时,CO分子依然优先与吸附氧发生反应,引起了预吸附O2分子的解离。适量Ca2+的掺杂不仅能降低O2分子解离时所需的能量,还能大大提高在吸附过程中电子的转移数量,这也是Ca2+的掺杂能提高LaFeO3材料气敏性能的主要原因。
4.用第一性原理计算的方法研究了NO分子吸附在LaFeO3(010)表面。通过对多种吸附构型的比较,我们发现NO分子在表面Fe位的吸附比较稳定,其中Fe-NO的吸附构型是最稳定的。这表明Fe位在NO分子的吸附中依然起着主导的作用。在吸附过程中,电子由表面转移到NO分子,削弱了N-O键的长度。分析结果表明虽然在吸附过程中Fe的s、p和d轨道都发生了不同程度的变化,但是变化最大的依然发生在d轨道。NO分子与Fe位成键的主要的原因来自NO分子和Fe-d轨道之间的杂化。
5.用第一性原理计算的方法研究了CO、NH3和O2分子在La0.875Sr0.125FeO3(010)表面上的吸附。对于CO气体分子的吸附,计算结果表明CO喜欢以C原子向下的构型吸附在表面Fe原子上,其中C-s、p轨道与Fe-d轨道之间的杂化是CO分子与Fe位成键的主要来源。NH3分子的N原子向下的吸附构型更稳定。吸附后的H-N-H的夹角变大。N原子与Fe位之间发生了强烈的轨道杂化。当O2分子吸附在La0.875Sr0.125FeO3(010)表面Fe位时,对两种吸附构型进行的结构优化结果表明,O2分子与表面Fe原子之间成大约120°角时是最稳定的。
6.用第一性原理计算的方法研究了H2CO分子吸附在LaFeO3(010)表面的Fe位。当H2CO分子吸附在LaFeO3(010)表面的Fe位时,O原子向下的吸附构型是最稳定的。在H2CO分子吸附以后,LaFeO3(010)表面出现了两个局部的能量线,一个位于带隙的中间,另一个位于价带的底部。这表明H2CO分子的吸附改变了LaFeO3(?)勺电子结构。H2CO分子的2p轨道与Fe原子的3d轨道之间发生了强烈的杂化,这也是H2CO分子能吸附在Fe位置的主要原因。
7.用第一性原理计算的方法研究了CO分子吸附在LaFeO3(010)表面,并对比了Ca掺杂和表面氧空位对LaFeO3(010)表面吸附CO分子的影响。对于纯净的LaFeO3(010)表面,表面Fe位是最适合CO分子吸附的位置,其中Fe-CO吸附构型是最稳定的。在吸附过程中,电子由CO分子转移到LaFeO3(010)表面。Ca的掺杂虽然能提高Fe-CO吸附构型的吸附能,但是却减少了在吸附过程中的电子转移量。当LaFeO3(010)表面含有氧空位的时候,最佳的吸附位置由Fe位转移到了O空位上面,最稳定的吸附构型是空位-CO构型。CO分子在此时从表面得到电子。总的来说,氧空位对LaFeO3(010)表面吸附CO分子的影响要比Ca的掺杂显著的多。
总之,通过对Ba掺杂的LaFeO3一系列材料的实验研究,我们发现Ba的适量掺杂能很好的改变LaFeO3材料的导电性和对酒精气体的气敏性能。这对于在实际应用中制备廉价实用的气敏元件提供了重要的依据。通过对LaFeO3钙钛矿氧化物气敏机理第二步的理论模拟,我们验证了以前实验结果,并对低价阳离子的掺杂为何能提高LaFeO3材料的气敏性能有了更清楚的了解。同时,通过对一系列气体分子吸附在LaFeO3基材料表面进行的第一性原理计算研究,我们能从微观上对LaFeO3基材料和气体分子之间的反应有更清楚的认识。
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.
通常来说,低价阳离子元素比如Ca、Sr和Pb是用来掺杂LaFeO3以提高其气敏性能的最佳元素。Ba和Ca、Sr在元素周期表中属于同一主族,但是到目前为止,我们并未发现有关于Ba掺杂LaFeO3材料气敏性能的研究报道。本文用溶胶凝胶法制备了La1-xBaxFeO3(x≤0.3)纳米晶体并对其气敏性能进行了一系列的测试研究,发现适量Ba2+的掺杂能提高LaFeO3材料对酒精气体的气敏性能。为了更加深入了解钙钛矿氧化物的气敏机理,我们选用LaFeO3和CO分子作为代表,采用第一性原理的方法模拟计算了吸附氧由LaFeO3表面解离的过程,发现CO分子的吸附确实引起了吸附氧的解离,同时在吸附过程中的电子转移能改变LaFeO3的电导,从而使LaFeO3材料对CO气体产生气敏效应。之后,我们对Ca2+掺杂的LaFeO3材料进行了计算研究,发现适量Ca2+的掺杂不仅能提高CO分子吸附过程中的电子转移数量,还能降低吸附氧解离时候需要的能量,这也是Ca2+掺杂能提高LaFeO3材料气敏性能的原因。
为了在微观上研究气体分子和LaFeO3基材料表面的反应情况,我们采用第一性原理方法模拟计算了多种还原性气体分子在LaFeO3基材料表面的吸附情况。计算结果表明一般情况下Fe位在气体分子的吸附过程中起着主导作用。通过对吸附能和电子转移等计算结果的分析对比,我们确定了气体分子在LaFeO3基材料表面的最佳吸附构型。同时,我们也研究了氧空位对LaFeO3材料表面吸附CO气体分子的影响。
本论文的主要结论如下：
1.在掺杂材料La1-xBaxFeO3(x≤0.3)中,XRD图像表明所有的样品材料都具有钙钛矿结构,这说明样品中的Ba2+取代了La3+的位置。由于Ba2+的半径(135pm)大于La3+的半径(106.1pm),随着掺杂量的增加,样品的晶胞体积也随之增大。当x<0.1时,La1-xBaxFeO3气敏元件的电阻随着掺杂量的增加而逐渐的降低；而当x>0.1时,La1-xBaxFeO3气敏元件的电阻随着掺杂量的增加而逐渐的升高,这主要是由于表面电价补偿和氧空位补偿的相互影响造成的。基于La1-xBaxFeO3材料制造的气体传感器对于500ppm酒精的气敏性能远远好于纯净的LaFeO3材料和其他几种掺杂材料,同时其最佳工作温度点在240℃。虽然La0.75Ba0.25FeO3气敏元件的最佳工作温度相比LaFeO3略有增高,但是在175℃-360℃的工作温度范围内,它对酒精的气敏性能都要比LaFeO3气敏元件在其最佳工作点要高。当x=0.25时,Ba2+的掺杂在材料表面制造了最大量的适合氧气分子吸附的空位,从而使得LaFeO3气敏元件对酒精气体的气敏性能最高。LaFeO3气敏元件对酒精气体表现出了良好的选择性能,与此同时它在空气中也具有良好的工作稳定性。
2.用溶胶凝胶法制备了LaFeO3纳米粉体,并将之在800℃下退火3个小时。XRD图像表明La0.875Ba0.125FeO3纳米粉体具备典型的钙钛矿结构。基于La0.875Ba0.125FeO3纳米材料制造的气敏元件具备典型的P型半导体性质。根据空气中电阻与温度的图像,我们可以计算得到它的激活能为0.18eV。La0.875Ba0.125FeO3气敏元件对酒精气体具有良好的选择性,它在170℃下对500ppm酒精气体的灵敏度达到了58。我们采用第一性原理计算的方法研究了02分子在La0.875Ba0.125FeO3(010)表面上的吸附。通过对表面La、O和Fe位的对比分析,我们发现O2分子吸附在表面Fe位的时候最稳定。由吸附后O2分子O-O键的长度和振动频率可以知道,吸附后产生了超氧化物02-。相比较于平行的吸附构型,02分子更喜欢以竖直的构型吸附在表面Fe原子上,同时当02分子以竖直的构型吸附在表面Fe原子上时,由表面转移到02分子的电子数量最多。
3.用第一性原理计算的方法研究了CO分子吸附在O2分子预吸附的LaFeO3(010)表面。相比较于表面的其他吸附位置,CO分子更适合与表面预吸附氧发生反应并形成O-C-O物质。在CO分子的吸附过程中,电子由CO分子转移到表面中,使得表面空穴的数量减少,从而导致了LaFeO3电阻的升高。这与之前的实验结果是相符的。CO分子的吸附并没有改变吸附氧与Fe原子之间的成键机制,但是能使得LaFeO3(010)表面的HOMO-LUMO带隙变窄。这主要是由于CO分子的吸附引起了LaFeO3(010)表面的电子重新分布。CO分子吸附前后表面Fe-3d轨道电子数量的变化是引起HOMO-LUMO带隙变化的主要原因。当CO分子吸附在O2分子预吸附的La1-xCaxFeO3(010)表面时,CO分子依然优先与吸附氧发生反应,引起了预吸附O2分子的解离。适量Ca2+的掺杂不仅能降低O2分子解离时所需的能量,还能大大提高在吸附过程中电子的转移数量,这也是Ca2+的掺杂能提高LaFeO3材料气敏性能的主要原因。
4.用第一性原理计算的方法研究了NO分子吸附在LaFeO3(010)表面。通过对多种吸附构型的比较,我们发现NO分子在表面Fe位的吸附比较稳定,其中Fe-NO的吸附构型是最稳定的。这表明Fe位在NO分子的吸附中依然起着主导的作用。在吸附过程中,电子由表面转移到NO分子,削弱了N-O键的长度。分析结果表明虽然在吸附过程中Fe的s、p和d轨道都发生了不同程度的变化,但是变化最大的依然发生在d轨道。NO分子与Fe位成键的主要的原因来自NO分子和Fe-d轨道之间的杂化。
5.用第一性原理计算的方法研究了CO、NH3和O2分子在La0.875Sr0.125FeO3(010)表面上的吸附。对于CO气体分子的吸附,计算结果表明CO喜欢以C原子向下的构型吸附在表面Fe原子上,其中C-s、p轨道与Fe-d轨道之间的杂化是CO分子与Fe位成键的主要来源。NH3分子的N原子向下的吸附构型更稳定。吸附后的H-N-H的夹角变大。N原子与Fe位之间发生了强烈的轨道杂化。当O2分子吸附在La0.875Sr0.125FeO3(010)表面Fe位时,对两种吸附构型进行的结构优化结果表明,O2分子与表面Fe原子之间成大约120°角时是最稳定的。
6.用第一性原理计算的方法研究了H2CO分子吸附在LaFeO3(010)表面的Fe位。当H2CO分子吸附在LaFeO3(010)表面的Fe位时,O原子向下的吸附构型是最稳定的。在H2CO分子吸附以后,LaFeO3(010)表面出现了两个局部的能量线,一个位于带隙的中间,另一个位于价带的底部。这表明H2CO分子的吸附改变了LaFeO3(?)勺电子结构。H2CO分子的2p轨道与Fe原子的3d轨道之间发生了强烈的杂化,这也是H2CO分子能吸附在Fe位置的主要原因。
7.用第一性原理计算的方法研究了CO分子吸附在LaFeO3(010)表面,并对比了Ca掺杂和表面氧空位对LaFeO3(010)表面吸附CO分子的影响。对于纯净的LaFeO3(010)表面,表面Fe位是最适合CO分子吸附的位置,其中Fe-CO吸附构型是最稳定的。在吸附过程中,电子由CO分子转移到LaFeO3(010)表面。Ca的掺杂虽然能提高Fe-CO吸附构型的吸附能,但是却减少了在吸附过程中的电子转移量。当LaFeO3(010)表面含有氧空位的时候,最佳的吸附位置由Fe位转移到了O空位上面,最稳定的吸附构型是空位-CO构型。CO分子在此时从表面得到电子。总的来说,氧空位对LaFeO3(010)表面吸附CO分子的影响要比Ca的掺杂显著的多。
总之,通过对Ba掺杂的LaFeO3一系列材料的实验研究,我们发现Ba的适量掺杂能很好的改变LaFeO3材料的导电性和对酒精气体的气敏性能。这对于在实际应用中制备廉价实用的气敏元件提供了重要的依据。通过对LaFeO3钙钛矿氧化物气敏机理第二步的理论模拟,我们验证了以前实验结果,并对低价阳离子的掺杂为何能提高LaFeO3材料的气敏性能有了更清楚的了解。同时,通过对一系列气体分子吸附在LaFeO3基材料表面进行的第一性原理计算研究,我们能从微观上对LaFeO3基材料和气体分子之间的反应有更清楚的认识。
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.
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