【作者】 范会涛；

【导师】 张彤；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2011， 博士

【摘要】 一维纳米材料由于高的比表面积、大的长径比、对电子传输的限域等优点在催化、电子等领域一直受到人们关注。一维半导体纳米材料可有效避免纳米颗粒敏感材料中团聚现象的发生,对提高半导体气体传感器的灵敏度,响应恢复特性,改善元件的选择特性具有重要作用。目前对二元及三元纳米纤维网络的气敏特性研究较频繁,有序排列纳米纤维的研究只停留在有机纳米纤维材料的制备阶段。探索多元固溶体及有序排列的纳米纤维敏感材料的制备工艺,揭示它们气敏机制的特异性是一维敏感材料的有意义的研究方向。本论文围绕着α-Fe2O3内米带,三元固溶体LaFeO3纳米带和纳米纤维、四元固溶体LaxSr1-xFeO3纳米纤维以及有序排列的In2O3纳米纤维的制备、表征、气敏特性和敏感机理方面展开研究,得到了一些有意义的结果。利用静电纺丝方法,经过600℃高温烧结工艺制备出α-Fe2O3纳米带材料,SEM表征得知纳米带的表面粗糙,带的宽度约为200-400 nm,厚度约为25-30nm。基于a-Fe2O3纳米带的气敏元件在285℃对浓度500 ppm的乙醇气体灵敏度为4.9,响应时间在10 s以内,恢复时间在24 s以内。利用静电纺丝的方法,经过600℃高温煅烧制备出LaFeO3内米带和纳米纤维材料。通过SEM照片可知LaFeO3内米带的表面光滑,纳米带的宽度200-300LaFeO3纳米纤维的直径均匀,约80-90 nm。并利用化学共沉淀的方法制备出直径在30 nm左右的LaFeO3纳米颗粒材料。通过三种不同形貌的LaFeO3材料气敏特性的对比测试得出纳米带和纳米纤维相对纳米粒子都具有较低的电阻,并且这些一维纳米材料对乙醇气体的最佳工作温度较低,分别为170℃和200℃。在各自的最佳工作温度时LaFeO3纳米纤维材料对500 ppm乙醇的灵敏度为47,而LaFeO3纳米带对500 ppm的乙醇灵敏度仅为8.6。在工作温度均为200℃时,LaFeO3纳米纤维材料与纳米带和纳米颗粒材料相比具有快速的响应恢复速度,此时LaFeO3纳米纤维对200 ppm乙醇的响应和恢复时间分别为11 s和19 s。LaFeO3纳米纤维材料与纳米带和纳米粒子材料相比对乙醇气体具有更高的灵敏度和快速的响应恢复速度,这与纳米纤维的一维几何形貌有着密切关系。纳米纤维具有高的比表面积,引起外界气体分子大量迅速吸附,提高了纳米纤维材料的灵敏度；由于吸附气体产生的表面空间电荷层沿纤维轴向方向重叠,形成连续的载流子传输通道,导致纳米纤维材料表现出快速的响应恢复特性。首次制备出四元固体溶LaxSr1-xFeO3 (x=0.6,0.7,0.8)内米纤维,这些纳米纤维的直径均匀,在50-80 nm之间。通过对乙醇气敏特性的测试,La0.7Sr0.3FeO3纳米纤维材料对乙醇的灵敏度明显高于其它比例时的灵敏度。La0.7Sr0.3FeO3纳米纤维在工作温度为185℃时对500 ppm乙醇的灵敏度为40.4,此时它的响应恢复时间分别为11 s和10 s。实验中对比测试了La0.7Sr0.3FeO3纳米纤维元件和市售的MQ-3乙醇气敏元件的敏感特性。Lao.7Sro.3FeO3纳米纤维元件的灵敏度峰值出现在170℃,而MQ-3乙醇气体传感器灵敏度的峰值出现在315℃,同时Lao.7Sro.3FeO3纳米纤维在250 s以内就完成了一个响应恢复周期的测试,而MQ-3乙醇气体传感器在脱离乙醇气体时却无法恢复到正常的空气阻值。通过与市售传感器的对比测试,可以看出一维纳米材料具有低的工作温度和高的灵敏度以及快速的响应恢复速度,在气体传感器领域有着非常可观的应用前景。采用两块平行的导电磁铁(8 cm×5 cmxl.5 cm)作为静电纺丝中的收集装置,在静电纺丝过程中带电喷射流受到磁场力的牵引和静电荷的排斥作用,纳米纤维被有序的搭接在平行磁铁的空隙上。In2O3纳米纤维经过煅烧后纤维的表面光滑,直径均匀,大约在80-90 nm范围内。实验测得有序排列的In2O3纳米纤维气体传感器的在工作温度为275℃时对1 ppm的NO2的响应时间和恢复时间分别问0.6 s和4 s,在工作温度为245℃时对1 ppm的C12的响应时间和恢复时间分均为0.7 s。相比之下,此时无序排列的In2O3纳米纤维传感器对NO2气体的响应和恢复时间分别为1.3 s和13.1 s,而对Cl2气体的响应和恢复时间分别为1 s和7 s,明显比有序排列的纳米纤维所需的时间长。当有序排列的纳米纤维吸附空气中的氧分子时导致纳米纤维表面形成一层空间电荷区,在沿着纤维轴向的方向这些空间电荷区发生重叠,甚至出现“平带”现象,在这个连续的载流子的传输通道中,由于势垒高度较小,电子的传输过程中受到的散射几率变小,所以有序排列的纳米纤维材料表现出快速的响应恢复速度。通过水热法合成ZnSnO3空心微球材料,它的直径在400-600 nm范围内。此种材料在工作温度为380℃时对500 ppm的正丁烷灵敏度值为5.79,响应时间为0.3 s,恢复时间为0.65 s,灵敏度和响应恢复速度明显高于实心ZnSnO3微球材料。这主要因为ZnSnO3空心微球材料表面相对比较疏松,有利于气体的吸附和扩散,所以材料相对实心微球表现出较好的灵敏度和响应恢复特性。总之,本论文通过上述实验和相关结果,证明了纳米纤维优异的气敏特性；不但首次获得了具有敏感特性的多元固溶体,还首次将有序排列的纳米材料应用到气体传感器领域,进一步证实了有序纤维可有效的提高元件的响应恢复特性。更多还原

【Abstract】 One dimensional (1D) nano-materials have driven much attention in catalyst and electronics due to their large specific surface area, large aspect ratio and electronic-transport confinement effect. 1D semiconductor nano-materials overcome the nanoparticles aggregation phenomenon and have important effect on enhancing response, shortening response and recovery time and improving selectivity of gas sensors. To date, researches are focused on binary and ternary-component solid solutions semiconductor materials, and the study on aligned nanofibers are still in the preparation of organic materials stage. The study on multi-compound-system and aligned sensitive nanofibers and their gas sensing properties is significative direction in 1 D sensitive meterials.This paper introduces the fabrication, characterization, gas sensing properties and sensing mechanism ofα-Fe2O3 nanobelts, ternary-component solid solution LaFeO3 nanobelts and nanofibers, quaternary-component solid solution LaxSr1-xFeO3 nanofibers and aligned In2O3 nanofibers, and lists some meaningful results.α-Fe2O3 nanobelts were fabricated by an electrospinning method followed by a calcination in air at 600℃process, the SEM photographs showed that the surface of belts are rough, the width and thickness of the belts are about 200 to 400 nm and 25 to 30 nm, respectively. Gas sensors based onα-Fe2O3 nanobelts have a response of 4.9, a response time less than 10 s and recovery time less than 24 s to 500 ppm ethanol at 285℃.LaFeO3 nanobelts and nanofibers were also fabricated by an electrospinning method followed by a calcination in air at 600℃process, the SEM photographs showed that the surface of belts are smooth and the width of the belts range from 200 to 300 nm; the diameters of LaFeO3 nanofibers are uniform in the range of 80 to 90 nm. LaFeO3 nanoparticles with diameters of around 30nm were prepared by a chemistry co-precipitation method. Contrastive experiment of gas sensing properties of these three morphologies LaFeO3 materials showed that LaFeO3 nanobelts and nanofibers have lower resistance compare with LaFeO3 nanoparticles and show lower optimum operating temperatures (170℃and 200℃, respectively). Under each optimum operating temperature, the response of LaFeO3 nanofibers material was 47 to 500 ppm ethanol, while that of LaFeO3 nanobelts material was only 8.6. When the operating temperature is 200℃and the concentration of ethanol is 200 ppm, the response and recovery time of LaFeO3 nanofibers were 11 s and 19 s, respectively, which were faster compared with LaFeO3 nanoparticles and nanobelts. The reason why LaFeO3 nanofibers showed higher response and faster response/recovery properties compared with LaFeO3 nanoparticles and nanobelts is closely allied to the geometric morphologies of the nanofibers. The high specific surface area property of nanofibers result in the rapid speed and large amounts of adsorption of gas molecules and this follows the enhancing of the response; the overlap of the surface space-charge layer along the fiber direction leads to the formation of a continuous electron transport tunnel, and this result in the improving of response/recovery property.Quaternary-component solid solution LaxSr1-xFeO3 (x=0.6,0.7,0.8) nanofibers were successfully prepared for the first time, the diameters of the gained nanofibers are uniform and in the range of 50 to 80 nm. When x takes different values,0.6,0.7 and 0.8, the contrastive experiment of gas sensing properties of LaxSr1-xFeO3 nanofibers showed that La0.7Sr0.3Fe03 nanofibers have higher response than the other ratios. At 185℃, to 500 ppm ethanol, the response of La0.7Sr0.3FeO3 nanofibers was 40.4, and the response and recovery time were 11 s and 10 s, respectively. Compared with commercial available product MQ-3 ethanol gas sensor, La0.7Sr0.3FeO3 nanofibers based sensors showed highest response at 170℃, while MQ-3 ethanol gas sensor showed highest response at 315℃, at the same time, La0.7Sr0.3FeO3 nanofibers based sensors completed a response and recovery period in 250 s, while MQ-3 ethanol gas sensor could not recover when break away ethanol. The above results show that the new one dimensional materials have low operating temperature, high response and fast response/recovery and are promising materials in gas sensor.Two conductive parallel-positioned magnets (8 cm×5 cmxl.5 cm) were used as collector in electro spinning process. During electro spinning, the electrostatic repulsion force and magnetic field which the charged jet experienced strech the fibers across the gap to form aligned arrays. The calcinated aligned In2O3 nanofibers had smooth surface, uniform diameter of 80 to 90 nm. The experiments results showed that at 275℃, the response and recovery time of aligned In2O3 nanofibers to 1 ppm NO2 are 0.6 s and 4 s, respectively, and at 245℃, the response and recovery time of aligned In2O3 nanofibers to 1 ppm Cl2 are both 0.7 s. While, the response and recovery time of randomly deposited In2O3 nanofibers to NO2 are 1.3 s and 13.1 s, respectively, and that to Cl2 are 1 s and 7 s, respectively. These results show that the aligned nanofibers have shorter response and recovery time. To aligned nanofibers, when oxygen molecules adsorption takes place, there are surface space-charge layers fonned at the surface of the nanofibers, these surface charge spaces overlap along the fibers, sometimes, even form flat band, in this continuous electron transport tunnel, due to the relatively lower potential barriers, the electron scattering probability decreases during transport process, and result in the fast response/recovery property of aligned nanofibers. Hollow ZnSnO3 microspheres were prepared by hydrothermal method, the diameter of the hollow ZnSnO3 microspheres was in the range of 400 nm to 600 nm. To 500 ppm butane, the reponse of the hollow ZnSnO3 microspheres materials was 5.79 at 380℃, and the response and recovery time were 0.3 s,0.65 s, respectively. The response and response/recovery properties of the gained hollow ZnSnO3 microspheres materials are sufficient better than that of solid ZnSnO3 microspheres materials, this can be explained as the surface of hollow ZnSnO3 microspheres are relatively loosen, and this is conducive to adsorption and desorption of gas molecules, so the hollow ZnSnO3 microspheres materials showed better response and response/recovery properties.In conclusion, in this paper, the experiments results showed the outstanding gas sensing properties of nanofibers, multi-component solid solutions with sensitive characteristics were successfully prepared for the first time, and applied aligned nanofibers to gas sensor, and demonstrated that aligned nanofibers can improve the response/recovery property.更多还原