SnO₂一维纳米材料的制备、表征以及特性研究

【作者】 赵鹤云；

【导师】 赵怀志；

【作者基本信息】 昆明理工大学 ， 材料学， 2006， 博士

【摘要】 本文采用新工艺和新方法制备了SnO₂纳米棒、Cd²⁺掺杂SnO₂纳米棒和Zn²⁺掺杂SnO₂纳米棒三种SnO₂一维纳米材料。对材料的制备工艺、材料合成、材料结构的表征、晶体生长机理等进行了探讨；与SnO₂纳米颗粒粉体和SnO₂（SiO₂）壳（核）结构纳米粒子两种零维纳米SnO₂材料相对照，对比研究了SnO₂一维纳米材料的红外吸收光学特性、氧化还原特性和气体敏感特性，制备出了具有高灵敏度和高选择性的几种新型气敏元件。论文取得了多项新颖的研究结果。1、SnO₂纳米棒的新制备方法与表征在微乳液法制备SnO₂一维纳米材料的基础上，发展了采用表面活性剂包裹下的室温固相还原反应合成单分散的前驱体粒子再在熔盐中生长SnO₂一维纳米材料的新工艺制备出了SnO₂纳米棒，获得了采用新方法制备SnO₂纳米棒的条件与工艺。采用新方法制备SnO₂一维纳米材料，可获得直径为5nm～100nm，长度为几百纳米～10μm的单晶SnO₂纳米棒。SnO₂纳米棒Sn和O原子浓度比随着在熔盐中的晶体生长温度的高低而发生变化，生长温度越低，O原子缺位越严重；生长温度越高，SnO₂纳米棒中O原子缺位越少。2、Cd²⁺掺杂或Zn²⁺掺杂SnO₂纳米棒新型一维纳米材料的合成与表征采用微乳液法，在熔盐介质中生长制备了掺杂Cd²⁺或掺杂Zn²⁺的SnO₂纳米棒新型SnO₂一维纳米材料。通过XPS和XRF测量表明，在Cd²⁺掺杂的SnO₂纳米棒晶体中CdO的含量约为3％，Zn²⁺掺杂的SnO₂纳米棒晶体中ZnO的含量约为1.5％。SnO₂纳米棒的晶体结构是Cd²⁺、Zn²⁺代替部分Sn⁴⁺构成替位式固溶体的晶体结构。这种掺杂的SnO₂一维纳米材料未见报导。3、研究了SnO₂纳米棒在熔融盐中的晶体生长过程，提出了熔融盐中SnO₂一维纳米晶体生长机制的新见解SnO₂纳米棒在熔融盐中的生长是一个从最初的SnO₂纳米颗粒前驱物，在一定生长温度形成细小纤维，纤维生长纳米棒成型，在较高的生长温度下纳米棒逐步长大、长粗，最终生长成为纳米棒。这一生长期过程的主要特点是在熔盐介质中同一种物质（SnO₂）从零维的纳米颗粒到一维的纳米棒的固态转变。SnO₂纳米棒在熔融盐中的固态转变生长机制是：在熔融盐提供的液态环境中，单分散前驱体SnO_2-δ纳米颗粒在熔融盐中作热扩散，以熔融盐中的空洞线或隙缝线形成的位错线为“软模板”，单分散SnO_2-δ纳米颗粒中SnO发生歧化反应形成SnO₂，按气—液—固（VLS）晶体生长机制，以降低表面能为驱动力，沿着“软模板”按一定的晶面进行“自组装”取向生长，最终在熔融盐形成的封闭体系中生长成SnO₂一维纳米材料。SnO₂纳米棒在熔融盐中的固态转变生长机理是一种新的一维纳米材料的生长机制。这种SnO₂一维纳米材料生长机制尚未见报道。4、对SnO₂一维纳米材料的红外吸收光谱特性进行了研究，发现SnO₂纳米棒、掺杂Cd²⁺或掺杂Zn²⁺的SnO₂纳米棒的奇异红外光谱演变现象在400-1000cm^-1低波数范围，SnO₂纳米棒材料Sn-O振动带的特征红外吸收峰呈双峰形式，分别位于680.5cm^-1和475.2cm^-1的位置。SnO₂纳米棒Sn-O红外吸收特性与纳米棒直径大小密切相关，表现出明显的尺寸效应。随着SnO₂纳米棒直径的减小，吸收峰出现明显宽化；475.2 cm^-1位置的吸收峰，随着SnO₂纳米棒直径减小，吸收峰出现蓝移；680.5cm^-1位置的吸收峰，随着SnO₂纳米棒直径减小，吸收峰却出现了红移。这种红外收吸峰蓝移和红移并存的奇异光谱现象，在一维纳米材料中也是首次观察到。在400-1000 cm^-1低波数范围的Sn-O的红外吸收振动带，Cd²⁺掺杂和Zn²⁺掺杂SnO₂纳米棒的红外吸收与SnO₂纳米棒的红外吸收相似，也是呈双红外吸收峰。但是随着SnO₂纳米棒直径的减小，吸收峰出现明显宽化外，其Sn-O红外吸收峰只出现红移现象。这种红移由Cd²⁺、Zn²⁺在SnO₂纳米棒晶体中形成的替位式晶体结构引起。5、SnO₂一维纳米材料的氧化还原特性及新的还原反应热力学机理分析通过H₂-TPR研究表明，SnO₂纳米棒具有由表面吸附的活性氧导致的较好的表面活性和氧化还原性质。纯SnO₂纳米棒在320℃左右的低温下就可以使H₂发生缓慢氧化，纯SnO₂纳米棒的H₂还原反应机理是“表面脱氧反应+歧化反应”；Cd²⁺掺杂和Zn²⁺掺杂的SnO₂纳米棒的H₂还原反应与纯SnO₂纳米棒的H₂还原反应类似，但由于纳米CdO和ZnO的催化作用使H₂还原反应温度大大降低，并形成H₂消耗反应峰。掺杂Cd²⁺和Zn²⁺的SnO₂纳米棒的H₂还原反应机理是“脱氧反应+歧化反应+纳米催化还原反应+Cd²⁺(或Zn²⁺)的还原反应”。6、SnO₂纳米棒一维纳米材料的气敏特性及气敏机理探讨首次以SnO₂纳米棒集群材料为气体敏感材料，设计制作了二种新型材料气体传感器：（?）SnO₂一维纳米集群材料传感器（?）SnO₂一维纳米集群材料+SnO₂纳米粉体混合材料传感器两种新型传感器对乙醇（C₂H₅OH）气体具有很好的灵敏度和稳定性，并具有较好的恢复-响应特性。其中以掺杂Zn²⁺的SnO₂纳米棒对乙醇气体的敏感性最好，纯SnO₂纳米棒次之，而掺杂Cd²⁺的SnO₂纳米棒对乙醇气体的敏感性再次之。这种SnO₂一维纳米集群气敏材料的气敏特性及元件设计，还未见报道。根据纳米SnO₂气敏材料偏离化学计量比及相关缺陷，材料的表面气体吸附特性，晶粒尺寸及比表面积大小，以及界面理论和导电通道理论，讨论了SnO₂一维纳米材料的气敏机理，提出了SnO₂一维纳米材料的气敏机理模型。用该气敏机理能很好地解释相关的实验结果。更多还原

【Abstract】 Nanoscale one-dimensional （1-D） materials of SnO₂ nanorods, SnO₂ nanorods doped Cd²⁺ and SnO₂ nanorods doped Zn²⁺ were produced by a new method and technology of preparation. It was researched that the method and the technology of preparation, the structures, the growth mechanism of SnO₂ nanorods were studied. Comparing SnO₂ nanorods materials with SnO₂ nano-powders and SnO₂ nano-particles coating on SiO₂ ball prepared, the properties in infrared absorption spectra （IR）, redox, gas sensitive of SnO₂ nanorods materials were studied. High sensitivity and high selectivity new gas sensors of SnO₂ nanorods materials were investigated too. Some new results were obtained in this paper. The results are as follows:1. Preparation and characterizations of SnO₂ nanorodsA new synthesis method of SnO₂ nanorods developed based on the preparing way of SnO₂ nanorods via annealing the precursor powders, which were prepared by microemulsion system consisting of oil phase, water phase and surfactant. SnO₂ nanorods were produced via the growth in molten salt medium of precursors, which prepared applying by solid-state reaction coated surfactants at room temperature. It was obtained that the preparing techniques of SnO₂ precursors and SnO₂ nanorods.It is found that SnO₂ nanorods prepared via the new synthesis method were single crystal materials. The diameter and length of the SnO₂ nanorods are in the range of 5nm to 100nm and several micrometers depended on the growth temperature and time. The atomic composition of Sn and O was calculated by using peak area sensitivity factors. The results showed that the atomic ratio of Sn/O was changed from 1:1 to 1:2. The deviation of composition from stoichiometry caused by oxygen vacancies was strongly affected by the growth temperature. Lower growth temperature, higher atomic ratio of Sn and O. Contradictorily, higher growth temperature, closer to the stoichiometry of atomic ratio of Sn and O.2. Synthesis and characterizition of SnO₂ nanorods doped Cd²⁺ or Zn²⁺In molten salt medium, two kinds of SnO₂ nanorods doped Cd²⁺ or Zn²⁺ were obtained via annealing the precursors of SnO₂ powders doped Cd²⁺ or Zn²⁺, which were prepared by redox reaction in microemulsion system. The structure and chemical composition of SnO₂ nanorods doped Cd（2+） or Zn（2+） were characterized by means of XPS and XFS（X-ray fluorescence spectrometry）. It is found that the content of CdO in SnO₂ nanorods materials doped Cd²⁺ is up to 3%, and the content of ZnO in SnO₂ nanorods materials doped Zn²⁺ is up to 1.5 %. The crystalline structure of SnO₂nanorods doped Cd²⁺ or Zn²⁺ is a substitutional solid solution crystalline, which was formed by Sn⁴⁺ replaced with Cd²⁺ or Zn²⁺. The work was not reported.3. Growth processes and mechanism of SnO₂ nanorods in molten salt mediumThe growth processes and mechanism of SnO₂ nanorods in molten salt medium were investigated. The growth of SnO₂ nanorods is a process of homogeneously dispersed particles of SnO₂ precursors growing into SnO₂ whiskers via self assembly, forming thin SnO₂ nanorods, and gradually growing up in molten salt medium. The main feature of this procedure is a solid transformation forming process of SnO₂ crystalline from nano-particles of 0-D to nanorods of 1-D in molten salt medium.In molten salt, the growth mechanism of SnO₂ nanorods is that molten salt provide the liquid environment, in which dispersed particles of SnO_2-δ nano-crystal can diffuse along the dislocations in tubes. SnO₂ was formed by divergent reaction of SnO contained in SnO_2-δ, and take the tube dislocations as "template" oriented growth via self assembly derived by decreasing the surface energy of homogeneously dispersed SnO_2-δ particles, and controlled by VLS growth mechanism. Eventually, SnO_2-δ particles grow into SnO₂ nanorods in the tube dislocations of molten salt. The growth mechanism of SnO₂ nanorods in molten salt medium is a new mechanism of 1-D nano-materials.4. Study on infrared absorption characteristics of SnO₂ nanorodsThe infrared absorption characteristics of SnO₂ nanorods with different diameters were investigated by infrared absorption spectra （IR）. It is found that there existdouble peaks at 475.2cm^-1 and 680.5cm^-1 of the vibrational modes of Sn-O band in the low frequency range 400-1000 cm^-1. The IR modes of Sn-O band depend on the diameter of SnO₂ nanorods closely. The smaller the diameter is, the wider the IR peaks become. The active IR mode at 475.2cm^-1 shifts to high frequency（blue shift） in accordance with decrease of diameter of SnO₂ nanorods. On the other hand, the active IR mode at 680.5cm^-1 shifts to low frequency（red shift） in accordance with decrease of diameter of SnO₂ nanorods. This novel phenomenon about the infrared absorption spectra of SnO₂ nanorods with different diameters was investigated for the first time.Being similar to the infrared absorption spectra of SnO₂ nanorods, the infrared absorption spectra of SnO₂ nanorods doped Cd²⁺ or Zn²⁺ have double peaks of the vibrational modes of Sn-O band in the low frequency range 400-1000 cm^-1, too. But the two active IR modes shift to low frequency（red shift） in accordance with decreasing of nanorods diameter at the same time. It is different from the infrared absorption spectra of SnO₂ nanorods due to the substitutional solid solution crystalline structure of SnO₂ nanorods doped Cd²⁺ or Zn²⁺.5. Redox properties and new reaction thermodynamics mechanism of SnO₂ nanorodsThe redox properties of SnO₂ nanorods materials were investigated by means of H₂-TPR. The results showed that the better redox properties of SnO₂ nanorods ascribed to higher oxidizing activity induced adsorbed oxygen on the surface of SnO₂ nanorods. The slow-reaction temperature of H₂ consumption was started at 320℃. The reaction mechanism of SnO₂ nanorods is in the employ of H₂ consumption reaction of the desorbed oxygen and the divergent reaction of SnO₂.The redox properties of SnO₂ nanorods materials doped Cd²⁺ or Zn²⁺ were similar to redox properties of SnO₂ nanorods materials. But the started slow-reaction temperature of H₂ consumption were reduced in a large degree due to the catalytic effect of CdO or ZnO. The reaction was started at 270℃and the H₂ consumption peak was at 510℃in H₂-TPR profile of SnO₂ nanorods materials doped Cd²⁺. In the meanwhile, the reaction was started at 180℃and the H₂ consumption peak was at 420℃in H₂-TPR profile of SnO₂ nanorods materials doped Zn²⁺. The reaction mechanism submits to catalytic reaction of CdO or ZnO cooperated with H₂ consumption reaction of the desorbed oxygen, the divergent reaction of SnO₂ and reduction reaction of Cd²⁺ or Zn²⁺. This work of reaction mechanism was reported for the first time.6. Research on gas sensing properties and new gas sensitive mechanism of SnO₂ 1-D nano-materials.Two kinds of gas sensor have been desired with new gas-sensing materials based on SnO₂ nanorods and mixture materials of SnO₂ nanorods with SnO₂ nano-powders for the first time. It was discovered that sensors exhibited higher sensitivity, better selectivity, better stability, and better response and reversion to ethanol （C₂H₅OH）. Sensors based on SnO₂ nanorods materials doped Zn²⁺ illustrated the best sensitivity to ethanol and sensors based on SnO₂ nanorods materials was inferior to it. Sensitivity to ethanol of the sensors based on SnO₂ nanorods materials doped Cd²⁺ followed them. The work was not still reported.The gas sensitive mechanism of SnO₂ nanorods was put forward according to structure characteristics of SnO₂ nanorods gas sensitive materials with deviated from stoichiometry, to adsorption properties and interface principle, to grain size and surface-to-volume ratios associated with 1-D nanostructures, and to the tunnel of electrical conductivity theory. Relative experiment results of the gas sensing properties of SnO₂ nanorods materials can be explain very well with using the mechanism pattern.更多还原