【作者】 葛春桥；

【导师】 谢长生；

【作者基本信息】 华中科技大学 ， 材料学， 2008， 博士

【摘要】 金属氧化物气体传感器由于结构简单、廉价、反应迅速等优点,因而成为应用最广、发展最快的气敏传感器之一。同时,金属氧化物气体传感器能够检测气体的种类也在逐渐增加,目前已经广泛应用于气体检测仪、报警仪以及食品鉴别。但是,金属氧化物气体传感器普遍存在着工作温度高、热稳定性差、可靠性、选择性和抗干扰性不够强等缺点与不足,因而限制了金属氧化物气体传感器的进一步的广泛应用。本文以ZnO与WO₃为基本体系,从提高灵敏度、改善选择性以及降低工作温度的角度出发,着重研究了稀土掺杂ZnO、ZnO与WO₃纳米复合材料的制备以及纳米材料的形态控制等对湿法制备的金属氧化物气体传感器气敏性能的影响。首先,本文介绍了半导体金属氧化物气敏元件的缺陷与不足,以及气敏材料与器件的发展方向。随后阐述了半导体金属氧化物的气敏机理,分析和研究了影响气敏性能的因素,并论述了当前国内外所报道的改善气敏传感器的气敏性能的方法。最后详细介绍了纳米ZnO颗粒与薄膜的各种湿法制备方法,并全面综述了当前国内外围绕纳米ZnO形态的可控制备与气敏性能的改善两大方面进行的大量的研究工作。其次,本文采用sol-gel方法制备了稀土掺杂的ZnO基纳米颗粒与纳米薄膜,并讨论了掺杂浓度、工作温度以及光激发等因素对ZnO基元件的气敏性能的影响。在稀土La掺杂的ZnO纳米颗粒与Ce掺杂的纳米薄膜中,随着掺La或Ce浓度的逐渐上升,元件对乙醇与苯的敏感度也增大;但是过多的La或Ce掺杂又会导致敏感度下降。随着工作温度的上升,元件对乙醇与苯等挥发性有机物的敏感度先逐渐增大,随后又迅速降低;光激发能提高ZnO基纳米材料对挥发性有机物（VOCs）的灵敏度并降低工作温度。X射线衍射（XRD）结果表明,稀土La与Ce的掺入,产物中分别出现了La2O₂CO₃相与CeO₂相。场发射扫描电镜（FESEM）结果表明,ZnO大多呈颗粒形,晶粒尺寸处于20-75nm之间,并且随着掺La浓度的增加,晶粒大小逐渐减小。在采用浸渍-提拉法制备的稀土Ce掺杂的ZnO纳米薄膜中,薄膜的厚度达到5μm左右,并且组成薄膜的圆形颗粒尺寸多数处于40-65nm之间。新相CeO₂的产生对其气敏性能的改善具有一定的积极作用,同时建立了掺杂剂浓度对气敏性能影响机制的理论模型。第三,分别采用水热法与蒸发自组装法制备了两种具有特殊形态的纳米ZnO,并研究了气敏性能。在水热反应制备ZnO的过程中,在较低的反应温度下得到六方长柱状的ZnO,而在较高的反应温度下得到六方棱锥形的ZnO。较低的反应温度使得溶液具有较强的碱性,因而在溶液中形成具有负电特征的[Zn（OH）₄]^2-配位离子基团作为生长基元,此生长基元容易在氧化锌晶体的正极面叠合,且生长速度最快,而在负极面、柱面上生长则比较困难,最终结晶的纳米ZnO晶粒呈六方棱柱形貌,而在较高的温度下生长成六方棱锥形的ZnO。在蒸发自组装法制备中,由于嵌段共聚物F₁₂₇的的两亲性能,ZnCl₂发生醇解反应而形成的醇盐Zn（Cl）_2-x（OEt）_x互相结合而形成胶束。在蒸发过程中溶胶中的醇盐与F₁₂₇浓度的增加,导致溶胶中的Zn（Cl）_2-x（OEt）_x沿着嵌段共聚物的长链方向单向排列并逐渐形成无定形的纳米棒簇单元。F₁₂₇的加入使得ZnO的尺寸更加细小,因而能够提高灵敏度。第四,本文分别采用sol-gel方法与表面修饰技术制备了Zn-W-O体系纳米复合材料并讨论了气敏性能。X射线衍射（XRD）、场发射扫描电镜（FESEM）以及气敏测试结果表明,采用这两种方法制备的Zn-W-O体系纳米复合材料中,均出现了具有白钨矿结构的高电阻的ZnWO₄相。由于这种具有白钨矿结构的ZnWO₄相能够产生更多的新的Lewis酸中心,因而能够加速碳氢氧化物的脱氢而提高ZnO的灵敏度。但是,过量的高电阻的ZnWO₄相也会恶化其气敏性能。最后,本文采用水热法以PEG-400为模板剂制备了长方体形的WO₃薄片,并讨论了其对NO₂等有毒气体的气敏性能。X射线衍射（XRD）、场发射扫描电镜（FESEM）以及透射电镜（TEM）结果表明,在水热条件以及PEG的作用下,短时间内首先快速形成WO₃·H₂O,随着时间的延长,WO₃·H₂O逐渐脱水形成WO₃;随后时间的延长,WO₃沿二维方向生长成长方体形的WO₃薄片。气敏测试表明,这种长方体形的WO₃薄片能够实现对ppb浓度水平的NO₂进行检测,并且具有较高的灵敏度。WO₃在气敏元件的工作范围内发生相变的特性使得其气敏性能对温度的反应曲线与其它氧化物的反应曲线存在很大的不同。更多还原

【Abstract】 Metal oxide gas sensors are one of the most widely researched and used metal oxide semiconductor sensors due to their advantageous features, such as high sensitivity under ambient conditions, low power consumption, low price, prompt response and simple structure. At the same time, fathomable gas types have been increasing by metal oxide gas sensors. At present, metal oxide gas sensors have been widely used in some fields, such as gas detectors, annunciators, food discriminators, etc. However, metal oxide gas sensors have the same defects and deficiency as other metal oxide semiconductor sensors, for example, poor thermal stability, reliability, selectivity, anti-interference, high operating temperature, which limit the further development and widely application of metal oxide gas sensors. In allusion to these defects and deficiency, the effect of rare earth-dopant, the fabrication of nano-composite materials, shape controls of nano-materials on wet-fabricated metal oxide gas sensors were discussed in this paper, on the basis of ZnO and WO₃ systems.Firstly, defects or deficiency of metal oxide semiconductor sensors and development trend of gas-sensing materials and sensors were introduced in this paper. Gas-sensing mechanism metal oxide semiconductor sensors was expatiated, and factors to affect gas-sensing property were analyzed and researched. In addition, ways, reported in the world, to improve gas-sensing property were reviewed. At last, all kinds of wet-chemical ways to prepare ZnO nano-materials and films were introduced in detail, moreover, the current research evolvements on the controllable fabrication and gas-sensing property improvements of ZnO nano-materials were summarized across-the-board.Secondly, Re-doped ZnO nano-materials and films were prepared by sol-gel method, and the effect of the dopant concentration, operating temperature, UV- irritation, etc. on gas-sening property were discussed. The results show that the appropriate concentration of La and Ce dopant could improve gas-sensing property. Sensitivities, to VOCs, have been increasing with the increase of La and Ce dopant concentration, but the superabundant dopant concentration will worsen gas-sensing property. At the same time, sensitivities, to VOCs, will increase and then decrease with the increase of the operating temperature. UV-irritation could enhance sensitivity and reduce the operating temperature. XRD results show that the addition of La and Ce lead to new La₂O₂CO₃与CeO₂ phase. FESEM results show almost uniform spherical grains are about 20–75 nm in diameter, and the grain size tends to decrease with the increase of the concentration of the additives. The cross-section SEM image show the surface of the films, with the thickness of about 5 um, is rough. A new physical model of the CeO₂ dopant influence on the gas-sensing properties of ZnO thin films is proposed.Thirdly, two kinds of special-shape ZnO nano-materials were prepared by the hydrothermal method and evaporation-induced self-assembly （EISA）, respectively, and their gas-sensing property were also discussed. In view of hydrothermal fabrication of nano-ZnO, hexagonal pillar nano-ZnO was formed at lower temperature, but hexagonal pyramidal nano-ZnO was formed at higher temperature. Higher temperature results in stronger alkalescent in solution, which induces the solution produce [Zn（OH）4]2--groups with electronegative characteristic. These [Zn（OH）₄]^2--groups as growth units could easily superpose on anodic plane of ZnO single crystals, but difficultly grow on cathodal plane of ZnO single crystals. Therefore, nano-ZnO formed finally is hexagonal and pyramidal shape. In EISA, due to the amphipathic property and the superfluous addition of F₁₂₇, the chloro-alkoxide Zn（Cl）_2-x（OEt）_x nano-entities connected each other via a hydroxy-bonding interaction to form arrays of amorphous nano-entities surrounded by the F₁₂₇ molecules. After heattreatment, the surfactant removal causes the formation of ZnO nanorod-bundle structures. In addition, the addition of F₁₂₇ could improve their gas-sensing property.Fourthly, Zn-W-O nano-composite materials were fabricated by sol-gel and surface-coated methods, respectively, and their gas-sensing property to VOCs was discussed. XRD, FESEM and gas-sensing test results show that ZnWO₄ phase with a monoclinic structure appears in the whole concentration range （1–99 at. %） by two methods. The ZnWO₄ phase with high resistance almost did not respond to PVOCS. The ZnWO₄ phase with a scheelite structure can increase the number of Lewis acid centers on the surface of ZnO and WO₃. Therefore, their ability of oxidizing dehydrogenation can be strengthened, and thus the gas-sensing properties can be improved further. But the superabundant ZnWO₄ phase can congregate on the limited Lewis acid centers on the surface of ZnO and WO₃ and decrease the number of available acid sites, and thus worsen the gas-sensing properties.Finally, Rectangular WO₃ nanosheets were fabricated by a facile hydrothermal process employing PEG400 as the structure-directing agent, and their gas-sensing property to NO₂ was discussed. XRD, FESEM, TEM and HRTEM results reveal the whole production process of WO₃ phase, namely, WO₃·H2O phase forms quickly and is subsequently dehydrated to yield WO₃ phase under the hydrothermal condition. Rectangular WO₃ nanosheets continue to grow along 2-dimension with the increase of reaction time. Gas sensing tests show that rectangular WO₃ nanosheets could promptly response to NO₂ at ppb level.更多还原