【作者】 胡培广；

【导师】 刘宏；

【作者基本信息】 山东大学 ， 材料学， 2011， 硕士

【摘要】 在过去的十几年中,由于环保、现代生活、工业及国家安全等方面的需求,气体探测与监测的研究与开发受到了人们空前地重视。气敏传感器在各个领域中有着广泛的应用,比如化学、生物医药、食品工业、汽车工业、酒精质量监测、空气质量监控以及呼吸气体分析等。这些应用可以分为两个比较重要的类型,第一种是对单一气体(如NOx、NH3、O3、CO、CH4、H2、SO2等)的探测,第二种是气体(味)的辨别或者说是环境变化的监测。比如单一气体传感器可以用做火灾探测器、泄漏探测器、汽车和飞机上的通风控制器和工作车间中危险气体浓度超标的报警装置等;而用于气体(味)辨别或环境监测的传感器可以在食品工业和室内空气质量控制中对易挥发有机化合物(VOCs)和食物或家用物品产生的气味进行识别和监测。因此,具有优良性能的各种气敏传感器的研究和制备具有很重要的理论意义和实际意义。二氧化钛是一种气体敏感的n型金属氧化物半导体,对很多气体都有很好的敏感特性,且具有良好的化学稳定性和耐腐蚀性。以往的研究主要集中在二氧化钛颗粒或者薄膜材料上,对于一维二氧化钛纳米材料的研究相对比较少。二氧化钛颗粒或者薄膜材料存在的主要问题就是材料的敏感度低、选择性差。一维TiO2纳米材料不但保持了纳米材料的很多特性,而且与纳米颗粒相比,由于载流子沿着一维长轴方向传递,减少了由于大量晶界的存在而损失电子的可能,具有高的表面载流子传输速率,因此是比上述材料更为理想的气敏材料。TiO2纳米带是一种准一维纳米材料,它既有纳米材料的特性,又具有较大的尺寸和可操作的表面空间,这一新型的纳米结构在气敏传感器的应用上具有巨大的潜能。然而由于二氧化钛纳米带完美的表面结构和单一的相结构,其气敏性能具有一定的局限性,表面改性如表面酸腐蚀、组装表面异质结构是进一步地提升纳米带气敏性能的重要途径。本文的主要内容是在水热法批量制备TiO2纳米带的基础上,通过酸辅助水热法实现了对TiO2纳米带的表面粗化,利用光催化还原法和液相合成技术在纳米带表面组装了异质结构,并且对表面粗化纳米带及纳米带表面异质结构材料的气敏性能进行了研究,旨在为TiO2基气敏元件的实际应用提供理论依据。本论文的主要研究内容如下：1、通过水热法和酸辅助水热法大批量地合成了二氧化钛纳米带和表面粗化二氧化钛纳米带；以P25及两种纳米带作为敏感材料制作了气敏元件,研究分析了三者的气敏性能。水热法制备的纳米带的宽度约为200纳米,宽度约为50纳米,长达数微米至数十微米,纳米带表面光滑,结晶性好。通过酸辅助的水热法合成了表面粗化二氧化钛纳米带,表面粗化二氧化钛纳米带在三个维度上尺寸都有所减小,光滑完美的表面被破坏,形成了粗糙的表面,并出现了大量的表面缺陷。以P25、二氧化钛纳米带和表面粗化二氧化钛纳米带三种材料为敏感材料制作的气敏传感器都表现出了良好的气敏性能：三种传感器对酒精蒸汽和丙酮蒸汽都有响应,对CO和甲烷都没有响应,由TiO2纳米带和表面粗化TiO2纳米带制作的气敏传感器对氢气有微弱的响应；三种传感器的响应和恢复时间都很短,在3s之内；三种传感器的响应都具有很好的稳定性；其中由表面粗化二氧化钛纳米带制作的传感器相对于用另外两种材料制作的传感器表现出更好的气敏性能,前者相对于两外两种传感器具有更高的灵敏度,更低的最低工作温度和最佳工作温度；P25基气敏元件的敏感特性最差。综合了三种材料的微观结构及所形成薄膜的表面形貌对三种传感器的性能差异进行了解释。利用表面电荷转移模型(表面电荷耗尽层模型)解释了气敏机理,同时解释了酸腐蚀对气敏性能的影响。总之,在三种材料中,由于表面粗化TiO2纳米带具有更好的表面结构,因此基于该材料的气敏元件也展现出了最优异的气敏性能。2、通过光催化还原法在表面粗化TiO2纳米带表面合成了贵金属(Ag、Au)-TiO2异质结构,并通过改变紫外光照时间调整了异质结构中贵金属(Ag、Au)纳米颗粒的直径,获得了具有不同粒径的贵金属(Ag、Au)-TiO2纳米异质结构材料,研究了这些异质结构材料的气敏性能。随着光照时间的延长,贵金属(Ag、Au)纳米颗粒的直径逐渐增大。Ag-TiO2异质结构有效地提升了表面粗化TiO2纳米带对酒精蒸汽的敏感性,降低了对丙酮蒸汽的敏感性,非常有希望成为酒敏元件的敏感材料；而Au-TiO2异质结构则削弱了表面粗化TiO2纳米带对酒精蒸汽和丙酮蒸汽的敏感性而提升了纳米带对氢气的敏感性,有希望通过进一步地改性后应用在氢敏器件上。随着贵金属粒径的增大,异质结构材料对相应气体的敏感性有所降低。最后,通过化学作用机理解释了贵金属(Ag,Au)-TiO2异质结构对表面粗化TiO2纳米带气敏性能的影响,而且基于此机理说明了金属纳米颗粒直径大小对异质结构材料气敏性能的调控机理。一方面,贵金属颗粒作为氧分子的吸附中心,加速了电子从TiO2纳米带导带到氧分子上的转移,增加了吸附氧的浓度,一定程度上提高了材料的敏感度,另一方面贵纳米颗粒有利于TiO2纳米带对特定还原性气体分子(比如对与Ag来说是酒精分子)的吸附,促进了这种气体分子与吸附氧的反应,同时抑制了其他气体分子吸附及其与吸附氧的反应。而贵金属纳米颗粒的直径的增加带来的比表面积的下降使得贵纳米颗粒的对气体分子的吸附及催化作用有所降低,最终减弱了异质结构材料的气敏性能。3、利用液相法合成了半导体(ZnO、CdS)-表面粗化TiO2纳米带异质结构材料,研究了这类异质结构材料的气敏性能。ZnO在纳米带的表面形核并生长成花瓣形颗粒,分布比较均匀,花瓣形颗粒宽大约为30-50nm,长大约为50-80nm。ZnO-TiO2异质结构提升了纳米带对氢气的敏感度,同时大幅削弱了对酒精蒸汽和丙酮蒸汽的敏感度,提高了对氢气的选择性。CdS在纳米带的表面形核并生长成颗粒,分布比较均匀,颗粒直径大约为3nm。CdS-TiO2异质结构的气敏性能与ZnO-TiO2异质结构材料的气敏特性类似,而且比后者对氢气的敏感性和选择性更好,在制作氢敏传感器方面前景更好。最后讨论了半导体(ZnO、CdS)-TiO2异质结构对纳米带气敏性能的影响机理。一方面,这类异质结构的形成改变了纳米带的表面特征,对纳米带表面的活性位置的种类和数量产生了影响,有利于H2分子吸附和发生表面化学反应的活性位置增多,而有利于酒精蒸汽和丙酮蒸汽分子吸附和发生表面化学反应的活性位置减少,从而增强了纳米带对氢气的敏感度,改善了纳米带对氢气的选择性。更多还原

【Abstract】 In the last decade, the gas detection and monitoring became greatly valued as the increase of the demand for environment protection, modern life, industry and national security, etc. Gas sensors are applied widely in many areas, such as chemistry, biomedical, food industry, wine-quality monitoring, and breath analysis. Those applications can be classified into two groups which are the detection of single gases (as NOx, NH3,O3, CO, CH4, H2, SO2) and odor discrimination or the monitoring of changes in the ambient. For example, gas sensor for single gas can be used as fire detectors, leakage detectors, controllers of ventilation in cars and planes, alarm devices warning the overcoming of threshold concentration values of hazardous gases in the work places, while sensors for odor discrimination or monitoring changes in the ambient can be applied to detect volatile organic compounds (OVCs) or smells generated from food and household products in food industry and indoor air quality control. Therefore, research on gas sensors with high qualities as well as sensor fabrication are of great importance in both theory and practice.TiO2 is a kind of n-type metal oxide semiconductor with excellent chemical stability and erosion resistance, which is sensitive to several kinds of gases. Previous research has mainly focused on titanium dioxide particles and thin films, but there are very few studies on 1-D titanium dioxide nanobelts. However, the sensing performance of titanium dioxide particles or thin films was found to be limited by such two main drawbacks as low sensitivity and inferior selectivity.1-D titanium dioxide nanomaterials not only keep many specific properties of nanomaterials, but also maintain a much higher carrier transferring rate since the chance for electron loss is decreased because interior carriers transfer along 1-D long axis direction without crossing grain boundaries, so they are much better gas sensor materials candidates than particles and thin films. Titanium dioxide nanobelt is a kind of quasi 1-D nanomaterials with a much bigger size and operable surface area, which also keeps the specifics of nanomaterials. However, the gas sensing property of titanium dioxide nanobelt is limited by the fact that TiO2 nanobelt has a perfect surface structure and a single phase without any defects. Surface modification, such as surface acid corrosion, and assembly of surface heterostructures, is one of the most important ways to further enhance the gas sensing property of TiO2 nanobelt.In the thesis, surface-coarsened TiO2 nanobelts were successfully prepared by acid-assisted hydrothermal method based on the mass production of TiO2 nanobelts through hydrothermal method, and several kinds of surface-coarsened TiO2 nanobelts based heterostructures were also designed and fabricated by photocatalytic reduction method or the liquid phase method. Moreover, the gas sensing properties of surface-coarsened TiO2 nanobelts as well as the heterostructures were also studied in order to provide a theoretical basis for future practical applications of TiO2 based gas sensors.The main research contents of the thesis are as follows:1. TiO2 nanobelts and surface-coarsened TiO2 nanobelts were synthesized via hydrothermal method or acid-assisted hydrothermal method, and three kinds of gas sensors were fabricated based on P25, TiO2 nanobelts and surface-coarsened TiO2 nanobelts, through which the gas sensing properties of those three materials above were studied and analyzed. TiO2 nanobelt prepared by hydrothermal method is a kind of 1-D nanostructure with the length of several or several tens of micrometers, width of more than 100 nm, and thickness of about 50nm. Its surfaces are very smooth and of good crystallinity. The size of surface-coarsened TiO2 nanobelts is decreased in all three dimensions, the smooth surfaces were destroyed and the coarsened surfaces were formed with plenty of defects on. The three types of sensors based on P25, TiO2 nanobelt and surface-coarsened TiO2 nanobelt showed good gas sensing performance.Three sensors all respond to ethanol vapor, acetone vapor, and hydrogen, but not to CO and methane. The response time and recovery time are shorter than 3 s, and the response are all very stable. Among the three sensors, surface-coarsened TiO2 nanobelts based sensor displayed the best performance which means higher sensitivity, lower working temperature and optimal working temperature, while P25 based sensor did the worst. The differences in the properties of three sensors were explained on the base of the microstructure of three materials and the surface morphology of the films on sensors. The sensing mechanism was interpreted with the carrier transferring model or the surface carrier depletion model, and the influence of acid corrosion on nanobelt’s gas sensing property was also explained. In brief, surface-coarsened TiO2 nanobelts possess the best gas sensing property in this experiment due to the best surface structures.2. Noble metal (Ag, Au)-TiO2 heterostructures were synthesized on the surface of surface-coarsened TiO2 nanobelts by photocatalytic reduction method, the size of the metal particles was modified by changing the illuminating time, and the gas sensing property of the noble metal-TiO2 heterostructure materials were also studied. The diameter of metal particles (Ag, Au) increases as the extension of the illuminating time. Ag-TiO2 heterostructures have efficiently improved the sensing property of TiO2 nanobelts to ethanol vapor but reduced that to acetone vapor, which means Ag-surface-coarsened TiO2 nanobelt heterostructures are potential to act as ethanol sensing materials. Au-TiO2 heterostructures remarkably enhance the sensing property of TiO2 nanobelts to hydrogen and weaken that to ethanol and acetone vapor, which shows that it is promising to fabricate hydrogen sensing devices based on Au-surface-coarsened TiO2 nanobelt heterostructures after further modification. In addition, the sensitivitis of both heterostructures decrease as the metal particles grow. The chemical interaction mechanism was used to interpret the improvement of gas sensing property of noble metal (Ag, Au)-surface-coarsened TiO2 nanobelts, on the basis of which it was also interpreted about the influence of the metal particle size on the sensing propert. On one hand, metal nanoparticles act as the absorption center of oxygen and accelerate the transfer of electrons from nanobelts to oxygen, which both improve the sensing property of nanobelts;on the other hand, metal nanoparticles favor the absorption of specific reducing gas(es) molecules (e.g. ethanol molecules for Ag) on the surface of nanobelts, and catalyze the reaction betweem such molecules and absorbed oxygen, meanwhile they inhibit the absorption of other kinds of reducing gas molecules and their reaction with absorbed oxygen. The absorption of reducing gas molecules and their reaction with absorbed oxygen will be both weakened as the decrease of specific surface area of metal particles resulting from the increase of the diameter, which finally leads to the reduced gas sensing property.3. Semiconductor (ZnO, CdS)-TiO2 heterostructures were assembled on the surface of surface-coarsened TiO2 nanobelts by liquid phase method, and the gas sensing properties of those heterostructure materials were also studied. ZnO nucleated on the surface of surface-coarsened TiO2 nanobelts and grew up with a petal shape. The particles, which are wide of 30-50 nm and long of 50-80 nm, distribute uniformly on the surface. The ZnO-TiO2 heterostructures enhance the sensitivity and selectivity of surface-coarsened TiO2 nanobelts to hydrogen, and reduce that to ethanol and acetone vapor. CdS particles deposited on the surface of surface-coarsened TiO2 nanobelts with a diameter of 5 nm, and distribute uniformly on the surface. The gas sensing property of CdS-TiO2 heterostructures is similar with that of ZnO-TiO2 heterostructures. Furthermore, CdS-TiO2 heterostructures are more sensitive and selective to hydrogen, which makes it a much better candidate for hydrogen sensor materials. The mechanism of the influence of semiconductor (ZnO, CdS)-TiO2 heterostructures on the sensing property of nanobelts was also discussed. The formation of semiconductor (ZnO, CdS)-TiO2 heterostructures has changed the surface characteristics of the nanobelts, and altered the type and number of active sites on the surface. Those active sites, which favor the absorption of hydrogen molecules and the reaction with absorbed oxygen on the surface, have increased, and those, which are favorable to the absorption of ethanol or acetone molecules and their reactive with absorbed oxygen, have decreased. Therefore, the hydrogen sensing property of such heterostructures is improved.更多还原