花状纳米氧化锌光电气敏性质的研究
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摘要
随着科学技术和经济的发展，生活水平的不断提高，人们在工业生产、家庭安全等领域对气体传感器的精度、性能、稳定性方面的要求越来越高。因此，研究和开发高性能的气体传感器已经成为一项重要的工作。目前应用最广泛的是传统的加热型传感器，这种传感器需要在高温条件下（一般温度不小于200℃）工作。它不仅耗能高，而且在一定程度上限制了对易燃、易爆气体的检测。近年来发现在低温条件下，用光激发半导体替代加热也可以实现对目标气体（特别是易燃易爆气体）的检测。在众多半导体中，氧化锌作为一种直接带隙宽禁带半导体材料，具有良好的光电导性、较强的表面活性以及对环境的敏感性等为其在光电传感器方面提供了广阔的应用前景。因此，我们选用纳米氧化锌作为主体材料，通过对其进行掺杂、修饰以及与窄带隙半导体复合等方法进行改性，并对其在紫外光或可见光激发下的表面光电特性、气体选择性以及气敏响应强度等光电气敏性能进行研究。
本论文研究内容主要包括以下几个方面：
1.采用水热方法分别合成由纳米棒组装的花状ZnO和纳米片组装成的花状ZnO，并对其光电性质以及对各种气体的光电气敏响应强度和选择性进行研究。结果表明由纳米棒组装成的ZnO对气体具有良好的选择性；而由纳米片组装成的花状ZnO具有较强的气敏响应，这可能是由于在合成过程中加入的表面活性剂，经过煅烧后，仍有少量的C存在ZnO于中引起的。
2.借鉴可见光催化的研究思路，通过用水热方法合成不同铁掺杂量的花状ZnO，使其光响应范围扩展到可见光区。研究在532nm可见光激发下，铁掺杂ZnO对甲醛的气敏响应。结果表明当氧化锌中掺入1.0mol%铁离子时，具有较好的气敏响应。当掺杂量过多时，由于ZnO外表面形成了第三种物质ZnFe_2O_4，反而降低了氧化锌的气敏响应。除此之外，我们还研究了当铁掺杂量为1.0mol%时，不同的煅烧温度对ZnO光电气敏响应强度的影响。结果表明随着煅烧温度的增加，气敏响应逐渐增强。当煅烧温度为600℃时，气敏响应达到了一个最高值。当煅烧温度超过600℃时，气敏响应反而减弱。这主要是由于在高温煅烧过程中，离子发生了热运动，使部分铁离子溢出到ZnO的外表面，和氧化锌结合形成ZnFe_2O_4引起的。
3.我们通过在花状氧化锌的外表面修饰In_2O_3来研究室温下In_2O_3/ZnO对甲醛的光电气敏响应。发现被In_2O_3修饰的ZnO在可见光区对甲醛有着良好的气敏响应。其原因是由于In_2O_3的修饰，一方面使该复合物的吸收范围扩展到可见光区。另一方面，抑制了光生电子空穴的复合，增强了光生载流子的利用率，因此气敏响应也相应增强。但是当外表面修饰过多的In_2O_3，在一定程度上减小了氧化锌表面的活性位点，反而降低了氧化锌的光电气敏响应。除此之外，我们还进行了铟离子掺杂氧化锌的光电气敏性能的研究，结果表明，铟离子的掺入提高了氧化锌在可见光区对HCHO的气敏响应，当掺入量为0.75mol%时，气敏响应最强。这主要是因为掺入适当的铟离子，可以有效的降低氧化锌的禁带宽度，增加ZnO在可见光的光电性质，从而提高对甲醛的气敏响应强度。
With the development of technology and economic, the improvement of living standards, demand for the gas sensor accuracy, performance, stability increasingly in the filed of industrial production, household security etc. Therefore, the studying of gas sensor with high performance becomes more important. At present, the traditional heat-treatment gas sensor has been wild used in many areas, but it working at high temperature conditions (working temperature more than200℃normally). It not only consumes energy, but also the high operation temperature restricts the application of gas sensors in many areas, such as an explosive environment and a low-temperature environment. Recently many people report that the gas, especially for the flammable and explosive gas, can be detected under light irradiation at lower temperature. Among the variety of semiconductor, Zinc oxide (ZnO), with a wide direct band gap, has been considered as one of the promising materials in the field of gas sensors for organic pollutants under UV light irradiation due to its well conductance, surface properties and photoelectric performance. In this paper, the sensitized and doped are applied to improve the properties of ZnO, in order to fabricate the photoelectric gas sensor with high response under visible light irradiation.
The main results are illuminated as follows:
1. The photoelectric property of ZnO influenced by the morphology, on one hand we fabricated the ZnO nanoflowers that assembled from nanorods by a simple hydrothermal method. On the other hand, we synthesized the ZnO nanoflowers that assembled from nanoplates by addition of vitamin C. The photoelectric property of the both nanoflower-like ZnO, the gas response and selectivity to various kinds of gas were studied. It was found that the ZnO nanoflowers assembled by nanorods have a better selectivity, and the ZnO nanoflowers assembled by nanoplates have a better response. This may because that added surfactant in the during the synthesis process, after annealed in the muffle furnace, there is still has a small amount of C exists in ZnO.
2. Pure ZnO only can be excited by UV light to produce photo-generated carrier. To extend the photoresponse of ZnO into the visible light region, the formation of a new dopant energy level below the conduction band for the ZnO by the doping of Fe ions. We synthesized Fe-doped flowerlike ZnO powders with various doping contents by a hydrothermal method and study the influence of doping contents under532nm light irradiation. It was found that The1.0mol%dopant sample has a much better sensing performance than the others. However, the response of the sample deduced with containing more Fe. The main reason may be the formation of a second phase with the increase of the Fe dopant. Besides that, we study the impact of annealed temperature on ZnO gas response. We found that the response increasing with the increasing annealed temperature. When the annealed temperature arrived600℃, the sample has a best response. When the annealed temperature above600℃, the gas response deduced due to the formation of ZnFe2O4. Thermal motion can happen in the condition of high temperature, and apart of Fe ions spill over into the surface of ZnO.
3. Expect to form a new dopant energy level below the conduction band by the doping ions, composite system can improve the photoelectric activity under visible light irradiation. On one hand, composite system can inhibit the recombination of photo-generated electron-holes pairs and improve the utilization of photo-generated electron. On the other hand, it can improve the photo-response of ZnO under visible light irradiation. We synthesized In2O3-sensitized flowerlike ZnO with various sensitized contents by a facile two-step process. It was found that ZnO sensitized with In2O3could enhance the gas response to HCHO under the visible light illumination. This may be due to the fact that the composite structure of In2O3/ZnO extends the photo absorbing range to visible light area, inhibits the recombination of photo-generated electrons and holes, and thus increases the utilization of photo-generated carriers in photoelectric gas detection, resulting in the higher sensing response in some extent. But when the amount of the sensitizers is added too much, In2O3particles might be seriously agglomerate. This leads to a decrease in the number of the active sites on the surface of ZnO and induce the gas response. Besides that, we study the photo-response of ZnO dopant with In3+under visible light irradiation. It was found that ZnO dopant with In3+can improve the HCHO response under visible light region. This because that the dopant In3+can induce the wide band gap, increase the photoelectric property of ZnO in the visible light region.
本论文研究内容主要包括以下几个方面：
1.采用水热方法分别合成由纳米棒组装的花状ZnO和纳米片组装成的花状ZnO，并对其光电性质以及对各种气体的光电气敏响应强度和选择性进行研究。结果表明由纳米棒组装成的ZnO对气体具有良好的选择性；而由纳米片组装成的花状ZnO具有较强的气敏响应，这可能是由于在合成过程中加入的表面活性剂，经过煅烧后，仍有少量的C存在ZnO于中引起的。
2.借鉴可见光催化的研究思路，通过用水热方法合成不同铁掺杂量的花状ZnO，使其光响应范围扩展到可见光区。研究在532nm可见光激发下，铁掺杂ZnO对甲醛的气敏响应。结果表明当氧化锌中掺入1.0mol%铁离子时，具有较好的气敏响应。当掺杂量过多时，由于ZnO外表面形成了第三种物质ZnFe_2O_4，反而降低了氧化锌的气敏响应。除此之外，我们还研究了当铁掺杂量为1.0mol%时，不同的煅烧温度对ZnO光电气敏响应强度的影响。结果表明随着煅烧温度的增加，气敏响应逐渐增强。当煅烧温度为600℃时，气敏响应达到了一个最高值。当煅烧温度超过600℃时，气敏响应反而减弱。这主要是由于在高温煅烧过程中，离子发生了热运动，使部分铁离子溢出到ZnO的外表面，和氧化锌结合形成ZnFe_2O_4引起的。
3.我们通过在花状氧化锌的外表面修饰In_2O_3来研究室温下In_2O_3/ZnO对甲醛的光电气敏响应。发现被In_2O_3修饰的ZnO在可见光区对甲醛有着良好的气敏响应。其原因是由于In_2O_3的修饰，一方面使该复合物的吸收范围扩展到可见光区。另一方面，抑制了光生电子空穴的复合，增强了光生载流子的利用率，因此气敏响应也相应增强。但是当外表面修饰过多的In_2O_3，在一定程度上减小了氧化锌表面的活性位点，反而降低了氧化锌的光电气敏响应。除此之外，我们还进行了铟离子掺杂氧化锌的光电气敏性能的研究，结果表明，铟离子的掺入提高了氧化锌在可见光区对HCHO的气敏响应，当掺入量为0.75mol%时，气敏响应最强。这主要是因为掺入适当的铟离子，可以有效的降低氧化锌的禁带宽度，增加ZnO在可见光的光电性质，从而提高对甲醛的气敏响应强度。
With the development of technology and economic, the improvement of living standards, demand for the gas sensor accuracy, performance, stability increasingly in the filed of industrial production, household security etc. Therefore, the studying of gas sensor with high performance becomes more important. At present, the traditional heat-treatment gas sensor has been wild used in many areas, but it working at high temperature conditions (working temperature more than200℃normally). It not only consumes energy, but also the high operation temperature restricts the application of gas sensors in many areas, such as an explosive environment and a low-temperature environment. Recently many people report that the gas, especially for the flammable and explosive gas, can be detected under light irradiation at lower temperature. Among the variety of semiconductor, Zinc oxide (ZnO), with a wide direct band gap, has been considered as one of the promising materials in the field of gas sensors for organic pollutants under UV light irradiation due to its well conductance, surface properties and photoelectric performance. In this paper, the sensitized and doped are applied to improve the properties of ZnO, in order to fabricate the photoelectric gas sensor with high response under visible light irradiation.
The main results are illuminated as follows:
1. The photoelectric property of ZnO influenced by the morphology, on one hand we fabricated the ZnO nanoflowers that assembled from nanorods by a simple hydrothermal method. On the other hand, we synthesized the ZnO nanoflowers that assembled from nanoplates by addition of vitamin C. The photoelectric property of the both nanoflower-like ZnO, the gas response and selectivity to various kinds of gas were studied. It was found that the ZnO nanoflowers assembled by nanorods have a better selectivity, and the ZnO nanoflowers assembled by nanoplates have a better response. This may because that added surfactant in the during the synthesis process, after annealed in the muffle furnace, there is still has a small amount of C exists in ZnO.
2. Pure ZnO only can be excited by UV light to produce photo-generated carrier. To extend the photoresponse of ZnO into the visible light region, the formation of a new dopant energy level below the conduction band for the ZnO by the doping of Fe ions. We synthesized Fe-doped flowerlike ZnO powders with various doping contents by a hydrothermal method and study the influence of doping contents under532nm light irradiation. It was found that The1.0mol%dopant sample has a much better sensing performance than the others. However, the response of the sample deduced with containing more Fe. The main reason may be the formation of a second phase with the increase of the Fe dopant. Besides that, we study the impact of annealed temperature on ZnO gas response. We found that the response increasing with the increasing annealed temperature. When the annealed temperature arrived600℃, the sample has a best response. When the annealed temperature above600℃, the gas response deduced due to the formation of ZnFe2O4. Thermal motion can happen in the condition of high temperature, and apart of Fe ions spill over into the surface of ZnO.
3. Expect to form a new dopant energy level below the conduction band by the doping ions, composite system can improve the photoelectric activity under visible light irradiation. On one hand, composite system can inhibit the recombination of photo-generated electron-holes pairs and improve the utilization of photo-generated electron. On the other hand, it can improve the photo-response of ZnO under visible light irradiation. We synthesized In2O3-sensitized flowerlike ZnO with various sensitized contents by a facile two-step process. It was found that ZnO sensitized with In2O3could enhance the gas response to HCHO under the visible light illumination. This may be due to the fact that the composite structure of In2O3/ZnO extends the photo absorbing range to visible light area, inhibits the recombination of photo-generated electrons and holes, and thus increases the utilization of photo-generated carriers in photoelectric gas detection, resulting in the higher sensing response in some extent. But when the amount of the sensitizers is added too much, In2O3particles might be seriously agglomerate. This leads to a decrease in the number of the active sites on the surface of ZnO and induce the gas response. Besides that, we study the photo-response of ZnO dopant with In3+under visible light irradiation. It was found that ZnO dopant with In3+can improve the HCHO response under visible light region. This because that the dopant In3+can induce the wide band gap, increase the photoelectric property of ZnO in the visible light region.
引文
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