传感器表面纳米敏感材料微结构的调控及其气敏性能研究
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摘要
随着生活水平的提高,人们对自身健康和周围生存环境的要求日益提高,尤其是对空气中污染气体浓度的实时检测也更为迫切。因此,能实现这一愿望的金属氧化物气敏传感器就受到了人们的强烈关注和重点研究。目前,研究者们关注较多的是气敏材料的可控合成,而对气敏材料制成传感器的过程中所遇到的科学和技术问题却研究的很少,所占比例不足5%。于是本文着重针对金属氧化物纳米结构在制备气敏传感器过程中的聚集、破坏以及活性下降等问题,发展材料-器件一体化制备的新技术,通过在传感器表面原位调控纳米敏感材料的微结构包括形态和缺陷来提高传感器的气敏性能。
首先采用浸渍提拉法在管状传感器表面合成了直径为8nm的ZnO纳米棒,这些纳米棒平躺着,主要暴露面为{10-10}京面。接着在这些纳米棒组成的薄膜上采用溶液处理法成功得到了直径为100nm的纳米棒阵列,暴露面主要为(0001)晶面,更重要的是气敏性能获得了明显的提高：3倍的敏感度,少于10s的快速响应和低到1ppm的检测限。实验结果也揭示了暴露京面的影响要大于尺寸效应,ZnO(0001)京面的气敏性能优于{10-10}京面,同时暴露京面上的原子结构对提高ZnO纳米棒阵列传感器的吸附氧含量及其气敏性能具有重要的影响。
上述工艺比较复杂耗时,为此采用可以大批量制备的四针状ZnO粉末作为气敏材料,通过改变热处理温度来调控四针状ZnO的形态和缺陷。随着热处理温度的增加,四针状ZnO的针逐渐变短变粗,最后演变成四面体形态,颗粒之间的接触状况也发生了变化,并建立了热处理温度与ZnO颗粒形态的对应关系,同时发现间隙锌的数量减小而氧空位增多。在烧结温度为450℃下的ZnO传感器表现出了对甲醛气体最高的响应值。这主要是因为该传感器具有最好的颗粒接触,较多的间隙锌缺陷和较大的比表面积。
不过人工涂覆制造传感器这一工艺比较低效。为此采用丝网印刷技术在平板传感器上印刷四针状ZnO厚膜,再结合镧离子溶液滴注法和后续热处理,来调控四针状ZnO颗粒的表面缺陷。随着镧离子浓度的增加,缺陷Lazn所占比例增大,而氧空位所占比例减小,同时ZnO传感器的最佳工作温度一直降低至250℃。最佳工作温度的降低主要是由于掺杂缺陷Lazn增多的缘故。在这一工艺中,ZnO等金属氧化物的形态难以调控。于是改进工艺,将CNTs丝网印刷到平板传感器上,微滴注各金属氧化物的前驱体溶液于表面,采用热处理工艺将CNTs模板去除,就可以同时得到多种不同金属氧化物的多孔网状膜,从而在提高传感器敏感度的同时改善其选择性,并揭示了金属氧化物的微结构及组分对其气敏性能的重要作用。
With the improvement of living standard, the requirements of the health and living environment around are increasing day by day. Especially, it becomes more urgent for real-time monitoring of the air pollution gas concentration. Therefore, metal oxide based gas sensors which can realize this goal has aroused great attention and focused investigation. At present, researchers has focused on the controllable synthesis of gas-sensing materials while the study on the science and technique problems during the fabrication process of gas sensors from gas-sensing materials has been reported rarely. In this paper, it is important to solve the problems including aggregation, damage and decreased activity of metal oxide nanostructures during the fabrication process of gas sensors. For this purpose, the control of microstructures including morphology and surface defects of metal oxide nanomaterials on the sensor surface by using new material-device intergration techniques is developed to enhance the gas sensing properties.
At first, thin film made from8-nm-diameter and exposed-{10-10}-facet ZnO nanorods which were self-aligning in a straight line on the surface of Al2O3tube-like sensor was prepared by a simple dip-coating method. On this surface, we successfully synthesized a ZnO nanorods array with exposed (0001) plane and100nm in diameter in situ by a facile solution-processing technique. More importantly, the gas-sensing properties showed an obvious improvement:a high sensitivity (3-fold prefactor Ag), fast response (less than10s) and low detection limit (1ppm) to benzene and ethanol. On basis of these results, the effect of exposed facet is dominant rather than the size effect, and the order of gas-sensing properties of ZnO crystal face is (0001)>{10-10}. Moreover, it was found that the surface structure at atomic level was a key factor in improving the oxygen adsorption and, consequently, the gas-sensing performance of a ZnO nanorods array based gas sensor.
The above method is very complicated and time-consuming. To solve this problem, ZnO nanotetrapods (T-ZnO) which can be prepared on a large scale were used as gas-sensing materials. To control the morphology and surface defects of T-ZnO, the sintering temperature was varied. With an increase in the sintering temperature from350to750℃, the feet and cross of T-ZnO became gradually shorter and bigger, respectively, and subsequently tetrahedron-shaped ZnO nanoparticles were produced instead of T-ZnO at850℃. The morphological evolution was explained by a new physical model based on Thomson effect, leading to a decrease in the surface area and an obvious variation in the grain contact. Meanwhile, the surface defects were also changed:the amount of zinc interstitial (Zni) was decreased while oxygen vacancy (Vo) showed an inverse trend. Moreover, the best gas-sensing performance towards formaldehyde and methanol was obtained after sintering at450℃. This was mainly attributed to the synergetic effect between the best grain contact (meaning that more nanoparticles can make contributions to the sensor response) and more zinc interstitial as well as larger surface area (supplying more chemisorbed oxygen).
However, the above coating method is usually inefficient. To fabricate gas sensor on a large scale, T-ZnO were made into thick sensing film by a facile screen-printing technique. On the surface of flat-type gas sensor, the surface defects of T-ZnO thick film including intrinsic defects and extrinsic defects can be controlled by injecting several microlitres of La ions solution and post treatment to improve gas-sensing properties. With increasing the concentration of La ions, the ratio of Lazn was increased while the relative ratio of oxygen vacancy showed an inverse trend, leading to a decrease in the optimal working temperature of ZnO sensor. The lowest optimal working temperature was250℃when the concentration of La ions was0.05mol/L. These results indicated that the increased ratio of Lazn played a dominant role in decreasing the optimal working temperature. In such fabrication of gas sensors, the morphology of metal oxide was difficult to control because metal oxide were very fragile. To solve the problem, CNTs were at first made into thick film on the coplanar gas sensors by a screen printing technique and used as a template. Then, several different metal oxide precursor solutions could be injected into the thick film. After sintering process, CNTs were removed and these metal oxide net-like films with porous structures were obtained, leading to an increased sensitivity as well as better selectivity, indicating that metal oxide microstructures and composition played an important role in the gas sensing properties.
首先采用浸渍提拉法在管状传感器表面合成了直径为8nm的ZnO纳米棒,这些纳米棒平躺着,主要暴露面为{10-10}京面。接着在这些纳米棒组成的薄膜上采用溶液处理法成功得到了直径为100nm的纳米棒阵列,暴露面主要为(0001)晶面,更重要的是气敏性能获得了明显的提高：3倍的敏感度,少于10s的快速响应和低到1ppm的检测限。实验结果也揭示了暴露京面的影响要大于尺寸效应,ZnO(0001)京面的气敏性能优于{10-10}京面,同时暴露京面上的原子结构对提高ZnO纳米棒阵列传感器的吸附氧含量及其气敏性能具有重要的影响。
上述工艺比较复杂耗时,为此采用可以大批量制备的四针状ZnO粉末作为气敏材料,通过改变热处理温度来调控四针状ZnO的形态和缺陷。随着热处理温度的增加,四针状ZnO的针逐渐变短变粗,最后演变成四面体形态,颗粒之间的接触状况也发生了变化,并建立了热处理温度与ZnO颗粒形态的对应关系,同时发现间隙锌的数量减小而氧空位增多。在烧结温度为450℃下的ZnO传感器表现出了对甲醛气体最高的响应值。这主要是因为该传感器具有最好的颗粒接触,较多的间隙锌缺陷和较大的比表面积。
不过人工涂覆制造传感器这一工艺比较低效。为此采用丝网印刷技术在平板传感器上印刷四针状ZnO厚膜,再结合镧离子溶液滴注法和后续热处理,来调控四针状ZnO颗粒的表面缺陷。随着镧离子浓度的增加,缺陷Lazn所占比例增大,而氧空位所占比例减小,同时ZnO传感器的最佳工作温度一直降低至250℃。最佳工作温度的降低主要是由于掺杂缺陷Lazn增多的缘故。在这一工艺中,ZnO等金属氧化物的形态难以调控。于是改进工艺,将CNTs丝网印刷到平板传感器上,微滴注各金属氧化物的前驱体溶液于表面,采用热处理工艺将CNTs模板去除,就可以同时得到多种不同金属氧化物的多孔网状膜,从而在提高传感器敏感度的同时改善其选择性,并揭示了金属氧化物的微结构及组分对其气敏性能的重要作用。
With the improvement of living standard, the requirements of the health and living environment around are increasing day by day. Especially, it becomes more urgent for real-time monitoring of the air pollution gas concentration. Therefore, metal oxide based gas sensors which can realize this goal has aroused great attention and focused investigation. At present, researchers has focused on the controllable synthesis of gas-sensing materials while the study on the science and technique problems during the fabrication process of gas sensors from gas-sensing materials has been reported rarely. In this paper, it is important to solve the problems including aggregation, damage and decreased activity of metal oxide nanostructures during the fabrication process of gas sensors. For this purpose, the control of microstructures including morphology and surface defects of metal oxide nanomaterials on the sensor surface by using new material-device intergration techniques is developed to enhance the gas sensing properties.
At first, thin film made from8-nm-diameter and exposed-{10-10}-facet ZnO nanorods which were self-aligning in a straight line on the surface of Al2O3tube-like sensor was prepared by a simple dip-coating method. On this surface, we successfully synthesized a ZnO nanorods array with exposed (0001) plane and100nm in diameter in situ by a facile solution-processing technique. More importantly, the gas-sensing properties showed an obvious improvement:a high sensitivity (3-fold prefactor Ag), fast response (less than10s) and low detection limit (1ppm) to benzene and ethanol. On basis of these results, the effect of exposed facet is dominant rather than the size effect, and the order of gas-sensing properties of ZnO crystal face is (0001)>{10-10}. Moreover, it was found that the surface structure at atomic level was a key factor in improving the oxygen adsorption and, consequently, the gas-sensing performance of a ZnO nanorods array based gas sensor.
The above method is very complicated and time-consuming. To solve this problem, ZnO nanotetrapods (T-ZnO) which can be prepared on a large scale were used as gas-sensing materials. To control the morphology and surface defects of T-ZnO, the sintering temperature was varied. With an increase in the sintering temperature from350to750℃, the feet and cross of T-ZnO became gradually shorter and bigger, respectively, and subsequently tetrahedron-shaped ZnO nanoparticles were produced instead of T-ZnO at850℃. The morphological evolution was explained by a new physical model based on Thomson effect, leading to a decrease in the surface area and an obvious variation in the grain contact. Meanwhile, the surface defects were also changed:the amount of zinc interstitial (Zni) was decreased while oxygen vacancy (Vo) showed an inverse trend. Moreover, the best gas-sensing performance towards formaldehyde and methanol was obtained after sintering at450℃. This was mainly attributed to the synergetic effect between the best grain contact (meaning that more nanoparticles can make contributions to the sensor response) and more zinc interstitial as well as larger surface area (supplying more chemisorbed oxygen).
However, the above coating method is usually inefficient. To fabricate gas sensor on a large scale, T-ZnO were made into thick sensing film by a facile screen-printing technique. On the surface of flat-type gas sensor, the surface defects of T-ZnO thick film including intrinsic defects and extrinsic defects can be controlled by injecting several microlitres of La ions solution and post treatment to improve gas-sensing properties. With increasing the concentration of La ions, the ratio of Lazn was increased while the relative ratio of oxygen vacancy showed an inverse trend, leading to a decrease in the optimal working temperature of ZnO sensor. The lowest optimal working temperature was250℃when the concentration of La ions was0.05mol/L. These results indicated that the increased ratio of Lazn played a dominant role in decreasing the optimal working temperature. In such fabrication of gas sensors, the morphology of metal oxide was difficult to control because metal oxide were very fragile. To solve the problem, CNTs were at first made into thick film on the coplanar gas sensors by a screen printing technique and used as a template. Then, several different metal oxide precursor solutions could be injected into the thick film. After sintering process, CNTs were removed and these metal oxide net-like films with porous structures were obtained, leading to an increased sensitivity as well as better selectivity, indicating that metal oxide microstructures and composition played an important role in the gas sensing properties.
引文
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