氧化物纳米材料对CO_2及还原性气体的气敏性研究
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摘要
随着工业生产规模的逐渐扩大,在生产过程中使用的气体原料和产生的气体种类、数量不断增多。这些气体中有些是易燃易爆的气体,有些是有毒的气体,它们的泄漏不仅会导致温室效应、酸雨、臭氧层破坏等环境污染的问题,而且容易产生爆炸、火灾、使人和生物中毒等危害人们的人身和财产安全的问题。同时随人们生活水平的提高,液化石油气、天然气及城市煤气作为家庭用燃料也迅速普及,这些气体的泄漏也会引起的爆炸、火灾和中毒事故。对这些有毒、有污染的气体进行有效的检测和报警是解决环境污染问题的必要前提。虽然目前Sn02和ZnO等半导体金属氧化物作为气敏材料已被广泛使用,但人们对其气敏机理的认识仍较为模糊。Sn02氧化物体系是国内外研究活跃的方向,作为n型半导体氧化物的代表,其气敏机理也引起了越来越多的关注。
目前人们已经发现了红外线、表面声波、固体电解质、电容型、电阻型和MOS型等种类的CO2气体传感器。红外吸收式传感器,测量精度高,但是装置庞大、价格高、普及实用比较困难；还有CO2电化学Severinghaus电极传感器,易受电磁干扰,目前主要应用于测血液中的二氧化碳；而NASICON固体电解质型CO2传感器也因为工艺要求十分苛刻,实际制造难度很大而增加了普及使用的难度。目前电导或电阻型的CO2气敏材料主要集中在p-n节型复合氧化物材料,例如CuO-BaTiO3复合材料或掺银的CuO-BaTiO3复合材料,该复合材料因为涉及CuC、BaTiO3两种物质,工艺要求也较为复杂。另外,BaTiO3成相温度高,CuO-BaTiO3复合材料CO2气敏传感器的最佳工作温度也高达400℃以上。CO2是一种化学性质比较稳定的气体,相比于其他氧化性或者还原性气体来说比较难以探测。复合的和单一相结构的金属氧化物材料可以用作电阻型CO2传感器,当暴露于CO2气体中时其电阻会发生变化。复合的氧化物薄膜(如：CuO-BaTiO3, La2O3-BaTiO3和ZrO2-BaTiO3)和单一相的半导体(如：LaOCl, Nd2O2CO3, SmCoO3, GdCoO3(?)PLa1-xSrxFeO3)都表现出了电阻的增大。CuO-BaTiO3p-n体系对CO2的气敏机理被认为与CuO上形成的碳酸盐有关。CuO-BaTiO3中的p-n势垒高度被形成的碳酸盐薄膜所改变。这种碳酸盐的形成也用于解释LaOCl、Nd2O2CO3和1a1-xSrxFeO3的CO2气敏特性。一项第一性原理计算的研究结果表明,在LaOCl晶格表面原子(如氧原子)吸附CO2分子之后可以形成桥式的或者多配位的碳酸盐。但是,该研究没有给出LaOCl与CO2之间电子转移的任何信息。还有一些实验结果表明半导体表面的氧吸附可能参与了干燥空气中La1-xSrxFeO3对CO2的气敏过程。到目前为止单一及复合氧化物对CO2的气敏机理仍然不是很清楚,还存在很多争议。所以说,对于CO2气体传感器的气敏机理还需要进行长期而深入的探究。
钙钛矿型氧化物ABO3作为气敏材料具有单一金属氧化物所不具有的优势,不仅对气体表现出了良好的选择性和灵敏性,而且其工作稳定性非常高。同时这类气敏材料对气体的灵敏性、选择性以及工作稳定性等不仅可以通过改变A、B位的元素种类来调控,还可以通过对A位或者B位进行部分替代来调控。钙钛矿型氧化物ABO3结构稳定性强,掺杂也不会改变其原来所具有的结构。目前钙钛矿气敏材料的研究仍需要进行反复的尝试实验,需要选取掺杂物的种类,改变掺杂物的量等,以提高其气敏性、选择性和传感器的稳定性。此外,还需要发展先进的制造技术以降低其成本,同时确保其可靠性、安全性和重复性等。由于缺乏普遍适用的气敏机理模型作为指导,目前的气敏实验工作还具有一定的盲目性。为进一步开发新型气敏材料提供理论指导,并对气敏实验中的各种实验结果提供理论解释,在本论文中,我们基于第一性原理,研究了气敏反应过程中气敏材料的原子及电子结构信息,建立完整的模拟机制。
本论文的研究主要包括以下结果：
1、实验中采用溶胶凝胶法制备的LaFeO3纳米晶粉体材料对二氧化碳表现出了明显的气敏性能。随着二氧化碳浓度的增加,LaFeO3气敏元件的电阻增大。在300℃下,LaFeO3气敏元件对100O、2000和4000ppm的CO2气体的气敏响应分别为1.74、2.19和2.74。同样在300℃下,LaFeO3气敏元件对2000ppm CO2气体的响应时间和恢复时间分别为4分钟和8分钟。我们的第一性原理计算结果表明,LaFeO3(010)表面预吸附了足够多氧气分子之后,CO2分子可以吸附在LaFeO3(010)表面晶格上,其中C原子与表面晶格O原子成键,并有0.331e的电荷从CO2中的C原子转移至LaFeO3(010)表面；CO2中的两个O原子分别与相邻的表面晶格Fe原子成键,并有0.161e和0.156e的电荷由LaFeO3(010)表面分别转移到两个O原子上。总共有0.021e的净电荷由C02传递给LaFeO3(010)表面,与实验中材料暴露在CO2气氛中电阻升高的现象相吻合。
2、采用溶胶凝胶法制备的La0.82sCa0.12sFeO3(?)米晶粉体材料具有单一正交钙钛矿相结构。La0.87sCa0.12sFeO3气敏元件的电阻随着环境湿度的升高而增大,而气敏响应却表现出相反的规律,随湿度升高而减小。在320℃,La0.875Ca0.125FeO3气敏元件在38%RH和70%RH的湿空气环境中对1000ppmCO2气体的响应分别为1.67和1.53。我们利用第一性原理计算探讨了湿度条件下La0.875Ca0.125FeO3纳米晶材料对CO2气体可能的气敏机制。计算结果表明,不管是以分子形式还是解离形式吸附在La0.875Ca0.125FeO3(010)表面的H2O,都会释放电子到材料表面。两种模式中转移的电荷分别为0.046e和0.025e。引入CO2分子后的计算结果表明CO2分子是从吸附了H2O的La0.875Ca0.125FeO3(010)表面获得电子。根据实验和计算结果我们推测,可能有两个因素导致了La0.875Ca0.125FeO3气敏元件对CO2气体的气敏响应随湿度升高而减小：一方面H2O分子的吸附占据了一部分吸附活性位置,CO2分子的可吸附位置减少可能导致材料对CO2气敏响应变小；另一方面CO2分子与吸附H2O的相互作用从La0.875Ca0.125FeO3材料表面夺取电子,减弱了元件电阻升高的趋势,导致材料对CO2气敏响应变小。
3、我们研究了100℃到340℃温度范围内纯净的LaFeO3和掺杂了不同比例Pd的LaFeO3钙钛矿氧化物对丙酮气体的气敏特性。X射线衍射(XRD)结果显示,我们采用溶胶-凝胶法制备的LaFeO3粉体及掺杂了不同比例Pd的LaFeO3粉体材料都具有正交钙钛矿结构。Pd掺杂量为0wt%、1wt%、2wt%、3wt%和5wt%的LaFeO3纳米晶材料的平均晶粒尺寸D分别为50.1nm,58.6nm,50.9nm,56.5nm和59.5nm。在Pd的掺杂比例为2wt%的LaFeO3气敏元件上,我们观测到了一个对500ppm丙酮气体的相当大的气敏响应,响应值约为729；同时,该气敏元件对1ppm的微量丙酮气体也表现出了明显的响应。2wt%Pd掺杂量的LaFeO3气敏元件对1ppm的微量丙酮气体的响应-恢复时间分别为4秒和2秒。此外,该气敏元件还表现出了对丙酮气体的良好的选择性。所以说,Pd掺杂的LaFeO3纳米晶材料可以作为一种新型的有潜质的检测丙酮的气敏材料。
4、利用棉花纤维作为模板制备了La1-xSrxFeO3(x=0～0.3)中空微米管。X射线衍射图谱表明所制备的样品都具有正交钙钛矿结构。利用扫描电镜(SEM)观测了样品的微观形貌,这些微米管的孔径大约在2～5微米之间。管壁由许许多多的纳米晶颗粒组成。对x=0,0.1,0.2和0.3的Lai-xSrxFeO3样品,其管壁上的纳米晶颗粒的平均晶粒尺寸分别为53nm、66nm.63nm和65nm。虽然管壁凹凸多孔,对材料的气敏性能有益,但是过厚的管壁又不利于材料的气敏性能。Lai-xSrxFeO3(x=0-0.3)对不同气体的气敏性研究表明,适量的Sr掺杂可以提高LaFeO3传感器对乙醇气体的响应。x=0.1(La0.9Sr0.1FeO3)的样品气敏性能最好,在260℃下对400ppm乙醇气体的气敏响应约为52.8。
5、我们的DFT计算结果显示,在引入CO分子之前,吸附在缺氧SnO2(110)表面上的O-2和O-主要是从Sn原子上夺取的电子。当引入CO分子之后,CO与预吸附的O-2、O-以及SnO2(110)表面上特定位置晶格原子之间的相互作用使得电子释放给半导体SnO2。对于O-2的情况,CO分子可以与O-2中距离表面较远的O原子结合生成CO2；当CO分子的初始位置放置于O-2中另一个O原子上方时,CO分子可以吸附在O-2形成碳酸盐形式的过渡态,这个过渡态最终可能会分解形成CO2。对于低氧气含量的环境中,CO分子可以与特定位置的晶格氧相互作用形成CO2,还可以吸附在表面特定位置的Sn原子上。对于O-的情况,当引入CO之后,CO分子可以与吸附氧O-反应生成CO2；还可以与吸附氧旁边的桥位氧反应生成CO2。当SnO2(110)表面暴露在还原性气体CO中时,CO分子与吸附氧(O-2,O-)或者某些特定位置的SnO2(110)表面晶格原子相互作用,使得一部分电子释放回半导体SnO2表面。我们的DFT计算结果给出了一个SnO2(110)表面对CO气敏机制的详细的描述,这是与实验结果相一致的。
As the gradually expand of industry scale, the gas type and quantity of raw gases and produced gases are growing. Some of these gases are flammable or explosive, some of them are poisonous. Their leakage not only causes environmental pollution problems such as the greenhouse effect, acid rain, ozone depletion et al. but also harms to people's personal and property security, prone to explosions, fires or biological poisoning. At the same time, as the rise of people's living standard, liquefied petroleum gas, natural gas and city fuel gas are also rapidly popularized as domestic fuel. These can also cause the leakage of gas explosion, fire and poisoning accidents. It is the necessary premise to solve the problem of environmental pollution to effectively detection and alarm these toxic, pollution gases. Although the semiconductor metal oxide such as SnO2and ZnO has been widely used as gas sensing materials, the understanding of the gas-sensing mechanism is still relatively vague. SnO2oxide system is the active direction all over the world. It can be seen as a representative of the n type semiconductor oxide. To study the gas-sensing mechanism of SnO2oxide system is worthwhile.
Several types of CO2sensors such as infrared, surface acoustic wave, solid electrolyte, capacitive, resistive and MOS have been found. The measurement accuracy of infrared absorption sensor is high, though meanwhile, the device is large, the price is high, and then the popularization is difficult. Electrochemical Severinghaus electrode sensor, which is susceptible to electromagnetic interference, mainly applied to measure the carbon dioxide in the blood. The NASICON solid electrolyte CO2sensor is also difficult to use widespread because of its actual manufacturing difficulty. The conductance or resistance type CO2gas sensing materials are mainly concentrated in the p-n type composite oxide materials, such as CuO-BaTiO3composites or mixed silver CuO-BaTiO3composites. Because of involving two substances (CuO and BaTiO3), the composite materials need complex preparation technics. In addition, BaTiO3needs higher temperature to crystallize, and the best working temperature of CuO-BaTiO3composites CO2gas sensor is400℃or more. It has been shown that some composite or single phase oxides can be used as the resistive CO2sensors, where there is a change in the electrical resistance of semiconductor upon exposure to CO2in air. The composite oxide films (such as CuO-BaTiO3, La2O3-BaTiO3and ZrO2-BaTiO3) and single phase semiconductors (such as LaOCl, Nd2O2CO3, SmCoO3, GdCoO3, and La1-xSrxFeO3) present an increase of resistance to CO2gas. It was suggested that the possible CO2sensing mechanism of CuO-BaTiO3p-n system was connected with the carbonates of CuO. The height of potential barrier of p-n junctions in CuO-BaTiO3could be modified through thin films of carbonation. The formation of carbonates was also suggested to be origin (or one of origins) of CO2sensing mechanisms for LaOCl and Nd2O2CO3and La1-xSrxFeO3. An ab initio calculation showed that for LaOCl, the bridged or polydentate species/carbonates could be formed, through the adsorption of CO2molecule on the surface atom (such as oxygen) of the lattice. However, it gave no information about electron transfer between LaOCl and CO2. Some experimental results also showed that the oxygen adsorbed on the surface of semiconductor may be involved in the sensing process of CO2in dry air for La1-xSrxFeO3. So far, the gas-sensing mechanism of the single and composite oxides to CO2is still not very clear. There are a lot of controversies. The CO2sensing mechanism needs long-term and in-depth exploration.
Compared with the single metal oxide, ABO3perovskite oxide as gas-sensing material has some advantages, not only showed better selectivity and sensitivity of gases but also had higher stability. In addition, the sensitivity, selectivity and stability of ABO3perovskite gas sensing material can be adjusted and controled not only by changing the element in A or B site, but also by partly replacing the element in A or B site with other elements. The structure of ABO3perovskite oxide is so stable that doping will not change its original structure. Because of the lack of the gas-sensing mechanism of universal model as a guide, the gas-sensing experiment has certain blindness until now. To provide a theoretical guidance for further development of new gas sensitive material, and a theoretical explanation for the gas sensitive experimental results, this article, we study the atomic and electronic structure informations of gas sensing material in the sensing reaction processes based on the first principles.
The studies of this paper mainly include the following results:
1. LaFeO3nanocrystalline powders prepared by sol-gel method can exhibit considerable sensing response to CO2gas. The resistance of the LaFeO3sensor increases with increasing concentration of CO2. At300℃, the responses of the sensor to1000,2000and4000ppm CO2are1.74,2.19and2.74. Its response and recovery times to2000ppm CO2at300℃are about4min and8min, respectively. Our first principles calculations results demonstrate that with adeuate oxygen molecules pre-adsorbed on the LaFeO3(010) surface, the CO2molecule can be absorbed on the LaFeO3(010) surface accompanied by releasing electrons to the surface. The C atom of CO2bonds with the lattice O atom of the LaFeO3(010) surface and releases0.331e charge to the surface, while the two O atoms of CO2bond with the nearest lattice Fe atoms with obtaining0.161e and0.156e charge from the surface. In other words, there is a net charge of0.021e transferring from CO2to the lattice of LaFeO3(010) surface. It means that the calculation result is likely responsible for the observed increase in resistance of LaFeO3when exposed to CO2in dry air.
2. The structure of the nano-La0.875Ca0.125FeO3powders prepared by sol-gel method has a single orthogonal perovskite phase. The resistance of Lao.875Cao.i25Fe03increases with increasing the moisture, while the gas response S decreases with increasing the moisture. At320℃, the responses to1000ppm CO2in38%RH and70%RH are1.67and1.53respectively. The possible CO2sensing mechanisms in moist air for La0.875Ca0.125Fe03sensor are investigated by first principles calculations. Calculated results demonstrate that there is a small charge transfer from H2O to La0.875Ca0.125FeO3(010) surface both in molecularly and dissociatively adsorption configuration with0.046e and0.025e respectively. CO2could gain electrons from the surface of La0.875Ca0.125FeO3(010) with pre-adsorbed H2O. The gas response S decreases with increasing the moisture can be elplained as follows:In dry air, there are adsorbed oxygen species on the La0.875Ca0.125FeO3surface. The resistance of the material will increase after exposed to CO2gas. With the presence of H2O, a part of adsorption sites on the surface are occupied, so that the site for CO2to adsorbed on the surface reduced. Besides, the reaction of CO2and pre-adsorbed H2O can capture electrons from the La0.875Ca0.12sFeO3(010) surface. Then the rise of the material resistance is reduced, in other words, the response to CO2is reduced.
3. The acetone-sensing properties of the pure and Pd doped perovskite-type oxides LaFeO3were investigated from100℃to340℃. X-ray diffraction (XRD) shows that LaFeO3is an orthorhombic structure. The obtained D values of LaFeO3powders doped with0wt%,1wt%,2wt%,3wt%and5wt%Pd were about50.1,58.6,50.9,56.5and59.5nm, respectively. A giant acetone-sensing response of729is observed when the Pd content in LaFeO3powders is about2wt%in500ppm acetone. An obvious response is also observed for1ppm acetone of the2wt%Pd doped LaFeO3sensor. The response and recovery time of the sensor to the1ppm acetone gas are4and2seconds, respectively. At the same time, it performs a good selectivity to acetone gas and may be a new promising material candidate for the acetone-sensor development.
4. The Biomorphic La1-xSrxFeO3(x=0～0.3) hollow fibers with porous walls were fabricated using cotton as biotemplates. XRD patterns indicated that all the materials exhibit perovskite phase with orthorhombic structure. The morphologies of the samples were observed by scanning electron microscopy (SEM). The pore diameter of the tube is basically in the range of2-5μm. The walls of La1-xSrxFeO3(x=0～0.3) tubes are made up of lots of nanocrystalline particles. The average particle sizes of La1-xSrxFeO3(x=0～0.3) are about53nm,66nm,63nm and65nm respectively. Appropriate Sr-doping can restrain improve the response of the LaFeO3based sensor to ethanol gas. The best response to ethanol gas was observed with Sr content equal to0.1mole ratio at the operating temperature260℃. The gas response is about52.8to400ppm ethanol gas. Even though the walls are uneven, bumped and porous, the thicker wall is harmful to the interaction between gas and perovskite molecules. The decreasing of wall thickness is beneficial to improve the gas sensitivity.
5. Our DFT calculations show that before the introduction of CO, the oxygen species O2-and O-adsorbed on the oxygen-deficient SnO2(110) surface grab electrons mainly from Sn atoms of SnO2. When CO is introduced, For the case of O2, the CO molecule can react with the oxygen atom of pre-adsorbed O2, which is bonded with one Sn atom, to form CO2. There is another possibility that CO molecule may be adsorbed on the oxygen atom of pre-adsorbed O-2, which is inserted into the initial oxygen vacancy and is bonded with two Sn atoms, to form an intermediate state which may finally transform into CO2. At low O2concentration with fewO2-, CO reacts with the lattice oxygen atom to form CO2, when CO is closed to the lattice oxygen atom, since bond strength of Sn-O for this lattice oxygen atom becomes weak, after the oxygen adsorption. There is another possibility that CO is adsorbed on Sn position. For the case of O-, when CO is introduced, the CO molecule can react with pre-adsorbed O-to form CO2. CO may also react with the bridging oxygen atom of lattice neighbored with the pre-adsorbed O-to form CO2. When SnO2(110) surface is exposed to CO reducing gas, the interactions between CO and pre-adsorbed oxygen species (O-2,0-) as well as some lattice atoms at certain sites on SnO2surface leads to the releasing of electrons back to semiconductor SnO2-The DFT calculation can provide a good description for CO sensing processes on SnO2(110) surface.
目前人们已经发现了红外线、表面声波、固体电解质、电容型、电阻型和MOS型等种类的CO2气体传感器。红外吸收式传感器,测量精度高,但是装置庞大、价格高、普及实用比较困难；还有CO2电化学Severinghaus电极传感器,易受电磁干扰,目前主要应用于测血液中的二氧化碳；而NASICON固体电解质型CO2传感器也因为工艺要求十分苛刻,实际制造难度很大而增加了普及使用的难度。目前电导或电阻型的CO2气敏材料主要集中在p-n节型复合氧化物材料,例如CuO-BaTiO3复合材料或掺银的CuO-BaTiO3复合材料,该复合材料因为涉及CuC、BaTiO3两种物质,工艺要求也较为复杂。另外,BaTiO3成相温度高,CuO-BaTiO3复合材料CO2气敏传感器的最佳工作温度也高达400℃以上。CO2是一种化学性质比较稳定的气体,相比于其他氧化性或者还原性气体来说比较难以探测。复合的和单一相结构的金属氧化物材料可以用作电阻型CO2传感器,当暴露于CO2气体中时其电阻会发生变化。复合的氧化物薄膜(如：CuO-BaTiO3, La2O3-BaTiO3和ZrO2-BaTiO3)和单一相的半导体(如：LaOCl, Nd2O2CO3, SmCoO3, GdCoO3(?)PLa1-xSrxFeO3)都表现出了电阻的增大。CuO-BaTiO3p-n体系对CO2的气敏机理被认为与CuO上形成的碳酸盐有关。CuO-BaTiO3中的p-n势垒高度被形成的碳酸盐薄膜所改变。这种碳酸盐的形成也用于解释LaOCl、Nd2O2CO3和1a1-xSrxFeO3的CO2气敏特性。一项第一性原理计算的研究结果表明,在LaOCl晶格表面原子(如氧原子)吸附CO2分子之后可以形成桥式的或者多配位的碳酸盐。但是,该研究没有给出LaOCl与CO2之间电子转移的任何信息。还有一些实验结果表明半导体表面的氧吸附可能参与了干燥空气中La1-xSrxFeO3对CO2的气敏过程。到目前为止单一及复合氧化物对CO2的气敏机理仍然不是很清楚,还存在很多争议。所以说,对于CO2气体传感器的气敏机理还需要进行长期而深入的探究。
钙钛矿型氧化物ABO3作为气敏材料具有单一金属氧化物所不具有的优势,不仅对气体表现出了良好的选择性和灵敏性,而且其工作稳定性非常高。同时这类气敏材料对气体的灵敏性、选择性以及工作稳定性等不仅可以通过改变A、B位的元素种类来调控,还可以通过对A位或者B位进行部分替代来调控。钙钛矿型氧化物ABO3结构稳定性强,掺杂也不会改变其原来所具有的结构。目前钙钛矿气敏材料的研究仍需要进行反复的尝试实验,需要选取掺杂物的种类,改变掺杂物的量等,以提高其气敏性、选择性和传感器的稳定性。此外,还需要发展先进的制造技术以降低其成本,同时确保其可靠性、安全性和重复性等。由于缺乏普遍适用的气敏机理模型作为指导,目前的气敏实验工作还具有一定的盲目性。为进一步开发新型气敏材料提供理论指导,并对气敏实验中的各种实验结果提供理论解释,在本论文中,我们基于第一性原理,研究了气敏反应过程中气敏材料的原子及电子结构信息,建立完整的模拟机制。
本论文的研究主要包括以下结果：
1、实验中采用溶胶凝胶法制备的LaFeO3纳米晶粉体材料对二氧化碳表现出了明显的气敏性能。随着二氧化碳浓度的增加,LaFeO3气敏元件的电阻增大。在300℃下,LaFeO3气敏元件对100O、2000和4000ppm的CO2气体的气敏响应分别为1.74、2.19和2.74。同样在300℃下,LaFeO3气敏元件对2000ppm CO2气体的响应时间和恢复时间分别为4分钟和8分钟。我们的第一性原理计算结果表明,LaFeO3(010)表面预吸附了足够多氧气分子之后,CO2分子可以吸附在LaFeO3(010)表面晶格上,其中C原子与表面晶格O原子成键,并有0.331e的电荷从CO2中的C原子转移至LaFeO3(010)表面；CO2中的两个O原子分别与相邻的表面晶格Fe原子成键,并有0.161e和0.156e的电荷由LaFeO3(010)表面分别转移到两个O原子上。总共有0.021e的净电荷由C02传递给LaFeO3(010)表面,与实验中材料暴露在CO2气氛中电阻升高的现象相吻合。
2、采用溶胶凝胶法制备的La0.82sCa0.12sFeO3(?)米晶粉体材料具有单一正交钙钛矿相结构。La0.87sCa0.12sFeO3气敏元件的电阻随着环境湿度的升高而增大,而气敏响应却表现出相反的规律,随湿度升高而减小。在320℃,La0.875Ca0.125FeO3气敏元件在38%RH和70%RH的湿空气环境中对1000ppmCO2气体的响应分别为1.67和1.53。我们利用第一性原理计算探讨了湿度条件下La0.875Ca0.125FeO3纳米晶材料对CO2气体可能的气敏机制。计算结果表明,不管是以分子形式还是解离形式吸附在La0.875Ca0.125FeO3(010)表面的H2O,都会释放电子到材料表面。两种模式中转移的电荷分别为0.046e和0.025e。引入CO2分子后的计算结果表明CO2分子是从吸附了H2O的La0.875Ca0.125FeO3(010)表面获得电子。根据实验和计算结果我们推测,可能有两个因素导致了La0.875Ca0.125FeO3气敏元件对CO2气体的气敏响应随湿度升高而减小：一方面H2O分子的吸附占据了一部分吸附活性位置,CO2分子的可吸附位置减少可能导致材料对CO2气敏响应变小；另一方面CO2分子与吸附H2O的相互作用从La0.875Ca0.125FeO3材料表面夺取电子,减弱了元件电阻升高的趋势,导致材料对CO2气敏响应变小。
3、我们研究了100℃到340℃温度范围内纯净的LaFeO3和掺杂了不同比例Pd的LaFeO3钙钛矿氧化物对丙酮气体的气敏特性。X射线衍射(XRD)结果显示,我们采用溶胶-凝胶法制备的LaFeO3粉体及掺杂了不同比例Pd的LaFeO3粉体材料都具有正交钙钛矿结构。Pd掺杂量为0wt%、1wt%、2wt%、3wt%和5wt%的LaFeO3纳米晶材料的平均晶粒尺寸D分别为50.1nm,58.6nm,50.9nm,56.5nm和59.5nm。在Pd的掺杂比例为2wt%的LaFeO3气敏元件上,我们观测到了一个对500ppm丙酮气体的相当大的气敏响应,响应值约为729；同时,该气敏元件对1ppm的微量丙酮气体也表现出了明显的响应。2wt%Pd掺杂量的LaFeO3气敏元件对1ppm的微量丙酮气体的响应-恢复时间分别为4秒和2秒。此外,该气敏元件还表现出了对丙酮气体的良好的选择性。所以说,Pd掺杂的LaFeO3纳米晶材料可以作为一种新型的有潜质的检测丙酮的气敏材料。
4、利用棉花纤维作为模板制备了La1-xSrxFeO3(x=0～0.3)中空微米管。X射线衍射图谱表明所制备的样品都具有正交钙钛矿结构。利用扫描电镜(SEM)观测了样品的微观形貌,这些微米管的孔径大约在2～5微米之间。管壁由许许多多的纳米晶颗粒组成。对x=0,0.1,0.2和0.3的Lai-xSrxFeO3样品,其管壁上的纳米晶颗粒的平均晶粒尺寸分别为53nm、66nm.63nm和65nm。虽然管壁凹凸多孔,对材料的气敏性能有益,但是过厚的管壁又不利于材料的气敏性能。Lai-xSrxFeO3(x=0-0.3)对不同气体的气敏性研究表明,适量的Sr掺杂可以提高LaFeO3传感器对乙醇气体的响应。x=0.1(La0.9Sr0.1FeO3)的样品气敏性能最好,在260℃下对400ppm乙醇气体的气敏响应约为52.8。
5、我们的DFT计算结果显示,在引入CO分子之前,吸附在缺氧SnO2(110)表面上的O-2和O-主要是从Sn原子上夺取的电子。当引入CO分子之后,CO与预吸附的O-2、O-以及SnO2(110)表面上特定位置晶格原子之间的相互作用使得电子释放给半导体SnO2。对于O-2的情况,CO分子可以与O-2中距离表面较远的O原子结合生成CO2；当CO分子的初始位置放置于O-2中另一个O原子上方时,CO分子可以吸附在O-2形成碳酸盐形式的过渡态,这个过渡态最终可能会分解形成CO2。对于低氧气含量的环境中,CO分子可以与特定位置的晶格氧相互作用形成CO2,还可以吸附在表面特定位置的Sn原子上。对于O-的情况,当引入CO之后,CO分子可以与吸附氧O-反应生成CO2；还可以与吸附氧旁边的桥位氧反应生成CO2。当SnO2(110)表面暴露在还原性气体CO中时,CO分子与吸附氧(O-2,O-)或者某些特定位置的SnO2(110)表面晶格原子相互作用,使得一部分电子释放回半导体SnO2表面。我们的DFT计算结果给出了一个SnO2(110)表面对CO气敏机制的详细的描述,这是与实验结果相一致的。
As the gradually expand of industry scale, the gas type and quantity of raw gases and produced gases are growing. Some of these gases are flammable or explosive, some of them are poisonous. Their leakage not only causes environmental pollution problems such as the greenhouse effect, acid rain, ozone depletion et al. but also harms to people's personal and property security, prone to explosions, fires or biological poisoning. At the same time, as the rise of people's living standard, liquefied petroleum gas, natural gas and city fuel gas are also rapidly popularized as domestic fuel. These can also cause the leakage of gas explosion, fire and poisoning accidents. It is the necessary premise to solve the problem of environmental pollution to effectively detection and alarm these toxic, pollution gases. Although the semiconductor metal oxide such as SnO2and ZnO has been widely used as gas sensing materials, the understanding of the gas-sensing mechanism is still relatively vague. SnO2oxide system is the active direction all over the world. It can be seen as a representative of the n type semiconductor oxide. To study the gas-sensing mechanism of SnO2oxide system is worthwhile.
Several types of CO2sensors such as infrared, surface acoustic wave, solid electrolyte, capacitive, resistive and MOS have been found. The measurement accuracy of infrared absorption sensor is high, though meanwhile, the device is large, the price is high, and then the popularization is difficult. Electrochemical Severinghaus electrode sensor, which is susceptible to electromagnetic interference, mainly applied to measure the carbon dioxide in the blood. The NASICON solid electrolyte CO2sensor is also difficult to use widespread because of its actual manufacturing difficulty. The conductance or resistance type CO2gas sensing materials are mainly concentrated in the p-n type composite oxide materials, such as CuO-BaTiO3composites or mixed silver CuO-BaTiO3composites. Because of involving two substances (CuO and BaTiO3), the composite materials need complex preparation technics. In addition, BaTiO3needs higher temperature to crystallize, and the best working temperature of CuO-BaTiO3composites CO2gas sensor is400℃or more. It has been shown that some composite or single phase oxides can be used as the resistive CO2sensors, where there is a change in the electrical resistance of semiconductor upon exposure to CO2in air. The composite oxide films (such as CuO-BaTiO3, La2O3-BaTiO3and ZrO2-BaTiO3) and single phase semiconductors (such as LaOCl, Nd2O2CO3, SmCoO3, GdCoO3, and La1-xSrxFeO3) present an increase of resistance to CO2gas. It was suggested that the possible CO2sensing mechanism of CuO-BaTiO3p-n system was connected with the carbonates of CuO. The height of potential barrier of p-n junctions in CuO-BaTiO3could be modified through thin films of carbonation. The formation of carbonates was also suggested to be origin (or one of origins) of CO2sensing mechanisms for LaOCl and Nd2O2CO3and La1-xSrxFeO3. An ab initio calculation showed that for LaOCl, the bridged or polydentate species/carbonates could be formed, through the adsorption of CO2molecule on the surface atom (such as oxygen) of the lattice. However, it gave no information about electron transfer between LaOCl and CO2. Some experimental results also showed that the oxygen adsorbed on the surface of semiconductor may be involved in the sensing process of CO2in dry air for La1-xSrxFeO3. So far, the gas-sensing mechanism of the single and composite oxides to CO2is still not very clear. There are a lot of controversies. The CO2sensing mechanism needs long-term and in-depth exploration.
Compared with the single metal oxide, ABO3perovskite oxide as gas-sensing material has some advantages, not only showed better selectivity and sensitivity of gases but also had higher stability. In addition, the sensitivity, selectivity and stability of ABO3perovskite gas sensing material can be adjusted and controled not only by changing the element in A or B site, but also by partly replacing the element in A or B site with other elements. The structure of ABO3perovskite oxide is so stable that doping will not change its original structure. Because of the lack of the gas-sensing mechanism of universal model as a guide, the gas-sensing experiment has certain blindness until now. To provide a theoretical guidance for further development of new gas sensitive material, and a theoretical explanation for the gas sensitive experimental results, this article, we study the atomic and electronic structure informations of gas sensing material in the sensing reaction processes based on the first principles.
The studies of this paper mainly include the following results:
1. LaFeO3nanocrystalline powders prepared by sol-gel method can exhibit considerable sensing response to CO2gas. The resistance of the LaFeO3sensor increases with increasing concentration of CO2. At300℃, the responses of the sensor to1000,2000and4000ppm CO2are1.74,2.19and2.74. Its response and recovery times to2000ppm CO2at300℃are about4min and8min, respectively. Our first principles calculations results demonstrate that with adeuate oxygen molecules pre-adsorbed on the LaFeO3(010) surface, the CO2molecule can be absorbed on the LaFeO3(010) surface accompanied by releasing electrons to the surface. The C atom of CO2bonds with the lattice O atom of the LaFeO3(010) surface and releases0.331e charge to the surface, while the two O atoms of CO2bond with the nearest lattice Fe atoms with obtaining0.161e and0.156e charge from the surface. In other words, there is a net charge of0.021e transferring from CO2to the lattice of LaFeO3(010) surface. It means that the calculation result is likely responsible for the observed increase in resistance of LaFeO3when exposed to CO2in dry air.
2. The structure of the nano-La0.875Ca0.125FeO3powders prepared by sol-gel method has a single orthogonal perovskite phase. The resistance of Lao.875Cao.i25Fe03increases with increasing the moisture, while the gas response S decreases with increasing the moisture. At320℃, the responses to1000ppm CO2in38%RH and70%RH are1.67and1.53respectively. The possible CO2sensing mechanisms in moist air for La0.875Ca0.125Fe03sensor are investigated by first principles calculations. Calculated results demonstrate that there is a small charge transfer from H2O to La0.875Ca0.125FeO3(010) surface both in molecularly and dissociatively adsorption configuration with0.046e and0.025e respectively. CO2could gain electrons from the surface of La0.875Ca0.125FeO3(010) with pre-adsorbed H2O. The gas response S decreases with increasing the moisture can be elplained as follows:In dry air, there are adsorbed oxygen species on the La0.875Ca0.125FeO3surface. The resistance of the material will increase after exposed to CO2gas. With the presence of H2O, a part of adsorption sites on the surface are occupied, so that the site for CO2to adsorbed on the surface reduced. Besides, the reaction of CO2and pre-adsorbed H2O can capture electrons from the La0.875Ca0.12sFeO3(010) surface. Then the rise of the material resistance is reduced, in other words, the response to CO2is reduced.
3. The acetone-sensing properties of the pure and Pd doped perovskite-type oxides LaFeO3were investigated from100℃to340℃. X-ray diffraction (XRD) shows that LaFeO3is an orthorhombic structure. The obtained D values of LaFeO3powders doped with0wt%,1wt%,2wt%,3wt%and5wt%Pd were about50.1,58.6,50.9,56.5and59.5nm, respectively. A giant acetone-sensing response of729is observed when the Pd content in LaFeO3powders is about2wt%in500ppm acetone. An obvious response is also observed for1ppm acetone of the2wt%Pd doped LaFeO3sensor. The response and recovery time of the sensor to the1ppm acetone gas are4and2seconds, respectively. At the same time, it performs a good selectivity to acetone gas and may be a new promising material candidate for the acetone-sensor development.
4. The Biomorphic La1-xSrxFeO3(x=0～0.3) hollow fibers with porous walls were fabricated using cotton as biotemplates. XRD patterns indicated that all the materials exhibit perovskite phase with orthorhombic structure. The morphologies of the samples were observed by scanning electron microscopy (SEM). The pore diameter of the tube is basically in the range of2-5μm. The walls of La1-xSrxFeO3(x=0～0.3) tubes are made up of lots of nanocrystalline particles. The average particle sizes of La1-xSrxFeO3(x=0～0.3) are about53nm,66nm,63nm and65nm respectively. Appropriate Sr-doping can restrain improve the response of the LaFeO3based sensor to ethanol gas. The best response to ethanol gas was observed with Sr content equal to0.1mole ratio at the operating temperature260℃. The gas response is about52.8to400ppm ethanol gas. Even though the walls are uneven, bumped and porous, the thicker wall is harmful to the interaction between gas and perovskite molecules. The decreasing of wall thickness is beneficial to improve the gas sensitivity.
5. Our DFT calculations show that before the introduction of CO, the oxygen species O2-and O-adsorbed on the oxygen-deficient SnO2(110) surface grab electrons mainly from Sn atoms of SnO2. When CO is introduced, For the case of O2, the CO molecule can react with the oxygen atom of pre-adsorbed O2, which is bonded with one Sn atom, to form CO2. There is another possibility that CO molecule may be adsorbed on the oxygen atom of pre-adsorbed O-2, which is inserted into the initial oxygen vacancy and is bonded with two Sn atoms, to form an intermediate state which may finally transform into CO2. At low O2concentration with fewO2-, CO reacts with the lattice oxygen atom to form CO2, when CO is closed to the lattice oxygen atom, since bond strength of Sn-O for this lattice oxygen atom becomes weak, after the oxygen adsorption. There is another possibility that CO is adsorbed on Sn position. For the case of O-, when CO is introduced, the CO molecule can react with pre-adsorbed O-to form CO2. CO may also react with the bridging oxygen atom of lattice neighbored with the pre-adsorbed O-to form CO2. When SnO2(110) surface is exposed to CO reducing gas, the interactions between CO and pre-adsorbed oxygen species (O-2,0-) as well as some lattice atoms at certain sites on SnO2surface leads to the releasing of electrons back to semiconductor SnO2-The DFT calculation can provide a good description for CO sensing processes on SnO2(110) surface.
引文
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