【作者】 冯庆；

【导师】 王新强；

【作者基本信息】 重庆大学 ， 凝聚态物理， 2009， 博士

【摘要】 环境保护是人类面临的共同问题,运用气敏传感器制作而成的气体检测与监控警报系统为环境保护提供了一种有效的监控手段。金属氧化物半导体作为制作气敏传感器的新型材料而备受关注。TiO2是一种具有良好物理化学性质的n型金属氧化物半导体,近年来,TiO2材料与纳米科技和微电子技术相结合发展起来的TiO2固体薄膜型气敏传感器适应了低能耗与微型化的发展要求,并不断运用许多办法来实现对其微观结构和电学特性的优化,从而达到对环境更有效的监控。本文通过不同的实验方法制备TiO2传感器薄膜并对其在气体环境中的气敏特性进行了系统的实验研究,运用缺陷化学的原理定性的分析薄膜表面性质、表面反应过程以及气体分压与薄膜表面电导率的关系。同时,还利用基于密度泛函理论的第一性原理方法定量的对TiO2的缺陷、掺杂以及吸附情况下的薄膜传感性质进行研究,主要内容如下:1.通过对Ti/W合金靶的反应射频溅射制备出W/TiO2薄膜,运用原子力显微镜和伏特安培计研究其结构特性与气敏传感特性,发现在600℃的退火温度下,Ti与W的浓度比为Ti:W=80%:20%时,薄膜具有最小的电阻系数。在退火温度为700℃时,实现了薄膜的P型掺杂。该方法制备的薄膜实现了对低浓度NO2与SO2气体的探测,并具有高灵敏度与低响应时间和恢复时间,甚至可以实现对100ppb浓度的NO2气体的探测,在汽车尾气检测方面具有很重要的应用价值。该实验研究是本论文的创新点之一。2.溶解乙酰丙酮化钛在纯甲醇溶液里,通过超声雾化热分解法制备了氧敏TiO2薄膜,并利用扫描电镜分析薄膜晶体的微观结构。实验发现,钌(Ru),铑(Rh),铱(Ir)掺杂的TiO2薄膜对氧气很敏感。纯TiO2薄膜在400℃的操作温度下对1500ppm到15000ppm的浓度的氧气具有弱传感性,而催化剂注入后,薄膜对氧气具有更高的灵敏度与更低的操作温度(250℃)。3.通过运用火焰喷雾高温分解法制备的纯TiO2和Cu、Nb掺杂的TiO2颗粒并沉积成薄膜,测试在400℃时对CO和酒精气体的气敏特性,并通过透射电镜与X射线衍射研究粉末的微观特性。测试结果发现Nb的掺入阻碍了在高温下TiO2从锐钛矿向金红石的转变。掺杂物改善了薄膜对CO的传感灵敏度,但是只有Nb的掺入能够改善薄膜对酒精气体的传感能力。该实验研究是本论文的创新点之二。4.通过溶胶-凝胶和旋涂法制备了TiO2-KTaO3单层和多层薄膜,并在不同的温度下退火一小时,通过扫描电镜和X射线衍射研究薄膜的形貌特性,发现薄膜具有纳米尺度并具有多孔特性。对薄膜的湿敏传感特性测试表明,在相对湿度变化为12%-97%,退火温度为500℃时,TiO2/KTaO3复合薄膜具有高灵敏度的湿敏传感特性。相比单层薄膜,双层薄膜的电导变化范围更大,其原因在于TiO2/KTaO3双层薄膜比表面积更大,薄膜表面对水的电解能力更强。5.通过电子束蒸发的办法制备了TiO2-WO3复合传感器薄膜并进行退火处理,该薄膜型传感器对丙酮和SO2气体有较强的敏感性,电阻变化率大,工作温度低,并且气体响应与复合浓度和操作温度有密切的关系。另外,该薄膜对于酒精气体只有在400℃左右,复合浓度较低时才有较高的灵敏度。实验表明,薄膜的复合浓度和工作温度对传感特性有很大影响。6.对TiO2的表面性质和表面反应过程及杂质对TiO2传感特性的影响进行了定性分析,研究TiO2对氢气、氧气和水的吸附过程,并通过K-V符号和点缺陷理论来讨论金属氧化物的气敏机理,总结出了电导率随氧分压变化的一般规律,在此基础上总结出TiO2在接近大气氧分压条件下的电导率与氧分压的? 16次方成正比,而在较低氧分压环境下,电导率与氧分压的? 14次方成正比。7.运用基于密度泛函理论(DFT)的第一性原理方法来计算金红石和锐钛矿缺陷引起的晶体能带结构、键能键长、电荷布居数及态密度变化。研究发现,氧空位缺陷使晶体的费米能量升高,在导带底产生杂质能级。钛空位缺陷使晶体的费米能量降低,在价带顶部产生了一个杂质能级,该能级与费米能级相差分别约0.4eV(金红石)和0.25eV(锐钛矿)。对TiO2缺陷的理论研究是本论文的创新点之三。8.在对N、S和C替代O原子掺杂锐钛矿相TiO2晶体的电子结构进行了研究,发现这三种掺杂均会在不同程度上使锐钛矿相TiO2晶体的费米能级向上移动从而导致体系的带隙变小,有利于提高TiO2电导率,提高材料的传感特性。而Si掺杂金红石TiO2后能带结构中出现了一条杂质能级,对光生电子-空穴对的分离非常有利,但带隙宽度没有明显减小,电子的非局域化性质更加明显。9.金红石型TiO2(110)的表面原子和电子的结构也通过DFT进行了研究。研究发现,金红石型TiO2(110)表面的原子结构发生了弛豫和重构,并随着原子层的增加相应的弛豫距离减小。金红石型TiO2(110)表面禁带宽度降低,具有准金属性。金红石型TiO2(110)表面的HOMO和LUMO轨道分别是由Ti4+原子的p轨道和d轨道构成,其表面活性位为Ti4+。10.运用第一性原理方法模拟金红石TiO2(110)表面对H2分子的吸附,发现吸附后H-H键变长,晶体内部Ti-O键断裂,H原子与晶格O形成H-O键。通过能带结构和态密度分析发现,表面禁带宽度明显减小,电导增大,形成sp杂化轨道,并在价带内形成一个类sp带,晶体表面电子局域化运动明显,从而证明了表面化学反应使电导率变化。通过对气体在晶体表面吸附过程的理论研究来揭示传感效应的本质是该论文的创新点之四。更多还原

【Abstract】 The environmental protection is the common problem which the mankind faces. The alarm system for gas detection or regulation be manufactured by gas sensors provides a kind of effective supervision method to protect environment. Metal-oxide semiconductor which is regard as a new-style materials is subjected to close concern. TiO2 has been known as an n-type metal oxide semiconductor and an important inorganic function material. In recent years, TiO2 solid thin film sensor combining micro-electronics technical adapt the development request of low energy consumption and miniaturization. A lot of effective ways proposed to optimize the microstructure and electrical property of TiO2 solid thin film sensor in order to control the environment effectively.In this paper, TiO2 solid thin film was fabricated by different ways and the character in different gas environment was studied systematically. The property and process of TiO2 solid thin film surface and the influence of impurity upon the material function was been analysed. Simultaneity, the character of point defects, impurity defects, crystal surface and adsorption process of TiO2 was investigated by first-principles pseudopotential calculations based on density-functional theory (DFT). The main content is as follows:1.W/TiO2 thin film, deposited by RF reactive sputtering from a composite target of W and Ti, was investigated through AFM for structural characterization and by volt-amperometric technique for electrical and gas-sensing properties. It was found that the film with Ti and W concentration ratio of 80%:20% has the lowest resistivity at the 600℃annealing temperature. P type doped had been implemented with the 700℃annealing temperature. The film, which has high sensitivity, short response time and recovery time, could detect the low concentration gas of to NO2 and SO2. A detection limit lower than 100 ppb of NO2 was expected.2.TiO2 thin film was prepared on substrate by ultrasonic spray pyrolysis (USP) method from dissolving titanium oxide acetylacetonate in pure methyl alcohol. Scanning electron microscopy (SEM) was employed to analyze the crystalline and microscopic structure of the film. Results showed that the films doped Ru, Rh, Ir be sensitive to oxygen. The effects of catalyst incorporation on semiconductor gas sensors described in terms of the operation temperature and the response magnitude. It was conclude that pure TiO2 thin film measured at 400℃displayed a low response of oxygen of 1500ppm and 15000ppm, whereas TiO2 thin film using impregnated catalysts proved to have the highest response to O2 at an optimal operating temperature of 250℃.3.Pure TiO2 and Nb-doped and Cu-doped TiO2 nanoparticles were produced by flame spray pyrolysis (FSP) and tested for sensing of CO and ethanol at 400℃in dry air. The powders were characterized by transmission electron microscopy and X-ray diffraction. Niobium stabilized the anatase phase and retarded anatase change toward rutile. Both dopants improved the sensitivity towards CO over that of pure TiO2. In contrast, for ethanol a high increase in sensitivity was observed only for Nb/TiO2.4.TiO2 compound films also were fabricated in this study. Bilayered and monolayered TiO2-KTaO3 films had been fabricated by sol-gel spin-coating method and heated at deferent temperature for an hour. The humidity-sensing characterizations of the films were investigated. Morphological characterization had been studied by SEM and XRD. It proved that the films possessed nanosized gains and capillary nanoporous structure. During the relative humidity variation from 12% to 97%, TiO2/KTaO3 films annealed at 500℃exhibited the highest humidity sensitivity. Compared to monolayered films, the bilayered films showed the large change in impedance. The reasons of humidity sensing properties of TiO2-KTaO3 films also had been discussed.5.TiO2-WO3 composite thin films had been synthesized by E-beam evaporation and heated at 500℃for two hours. Morphological characterization had been studied by SEM and XRD. It showed that the films have very high sensitivity to alcohol, acetone and SO2. The influence of temperature, composite concentration and gas concentration on the gas sensing capability of the films had also been studied.6.Surface property and surface reaction process of TiO2 had been investigated and the influence of impurities upon the TiO2 had been researched. TiO2 adsorbing O2 H2 and H2O have been analyzed. K-V symbol and point defects had been used to discuss the mechanism of gas-sensor of metal-oxide. It was concluded that the electric conductivity coefficient of TiO2 thin film is proportional to oxygen pressure ? 16 square in normal atmosphere pressure and is proportional to oxygen pressure ? 14 square in low pressure.7 . In this paper, first-principles calculations within the Local Density Approximation (LDA) was carried out on studying the character of the band structure, bond length, density of states and population of TiO2 with defect. By comparing with perfect crystal, it was found that in the case of oxygen vacancy Fermi energy goes up and no defect level appears in the gap and in the case of titanium vacancy Fermi energy goes down and a defect level appears in the gap, which is about 0.4eV above Fermi energy in rutile and about 0.25eV above Fermi energy in anatase.8.The electronic structure of N, S, C doping TiO2 would made band gap narrow in different extent, which is advantageous to decrease the gap width and improve the sensor activity of TiO2.9.The atomic and electronic structure of rutile TiO2 (110) surface was given using DFT. It was found that the relaxation and restructure occur on (110) surface, and the corresponding distance of the relaxation decreases with increasing in the atomic layer thickness. The width of forbidden band gap at the surface decreases so that it shows the quasi-metal property. The highest occupied and the lowest unoccupied molecular orbits are composed of the p orbit s and d orbit s of the Ti atoms. The active sites are localized at the surface Ti4+ sites.10.Rutile TiO2(110) surface with absorption H2 were studied by using DFT. It was discovered that H-H band and Ti-O band split and O-H band combined. By analyzing band structure and density of states, it was found that the gap decreased and the conductance increased because of adsorption of H2. s-p hybrid orbital and similar s-p band was formed in value band. It was evidenced that electron moved locally in rutile (110) surface.更多还原