【作者】 程国娥；

【导师】 黄开勋；

【作者基本信息】 华中科技大学 ， 无机化学， 2007， 博士

【摘要】 纳米半导体材料以其新异的光学特性、电学特性、光电转换特性、光催化性能等引起了广泛的研究兴趣。合理设计纳米结构的合成,从材料的尺寸、形貌、维度、组成、晶体结构等方面进行调控,研究纳米结构与性能的关系,是纳米科技发展的重要组成部分,对实现按照人们的意愿设计合成功能材料具有重要的意义。本论文以SnO₂为研究对象,采用多种化学方法,以简单的无机盐为前驱体,在液相反应体系中通过引入聚合物做辅助剂、在SnO₂晶体中掺杂Zn、改变溶剂组成以及以天然棉花做模板等方法,得到了一些新的形貌和纳米结构,通过X射线衍射（XRD）、场发射扫描电镜（FESEM）、透射电镜（TEM）及高分辨透射电镜（HRTEM）、红外光谱（FT-IR）、拉曼光谱、紫外-可见吸收光谱（UV-Vis）、光致发光光谱（PL）、X射线光电子能谱（XPS）等手段对产物进行了表征,讨论了纳米结构的形成机理,并对其性能进行了研究。主要内容如下:（1）分别以聚丙烯酸（PAA）、聚乙二醇（PEG 200）、聚乙烯醇（PVA 1799）三种聚合物为辅助剂,制备了几种不同的SnO₂纳米结构:首先,在溶液中利用聚丙烯酸（PAA）的辅助作用,通过溶剂热过程得到了平均长度为1.0μm、底部直径为100-500nm的金红石型SnO₂纳米锥形结构以及由纳米锥自组装成的空心球结构,考察了溶液碱性、溶剂组成、聚合物的加入量等参数对形貌的影响,晶体的生长经历了PAA辅助下纳米颗粒的定向聚集过程,通过取向附生机制,得到了单晶纳米锥结构,由于纳米锥的顶部尺寸小以及其粗糙的凹凸不平的表面,使其更进一步组装成空心球结构。纳米锥的拉曼光谱结果显示,出现了由纳米材料微结构而产生的新的振动吸收;其次,采用高温加热的方法,以聚合物聚乙二醇（PEG 200）为溶剂和表面修饰剂,得到了长径比大、结晶较好的单晶SnO₂纳米棒,并且发现,晶体的生长方向为较少报道的[11?2]方向。反应温度、反应时间、溶液的碱度、反应物浓度等生长参数对产物的尺寸有较大的影响,纳米棒的形成机制为取向附生机制,其荧光光谱显示了较强的激子发光、缺陷能级发光;另外,以聚乙烯醇（PVA 1799）为基质,通过加热回流及氧化过程,首先得到了SnO₂纳米颗粒分散其中的复合物,高温培烧除去聚合物后,得到了由纳米颗粒组成的空心球结构等纳米结构。（2）通过在SnO₂晶体中掺杂Zn,有效的控制了晶体的形貌和尺寸。在乙醇-水溶液中,引入少量无机金属离子Zn²⁺作掺杂,采用溶剂热法,得到了具有枝状结构的Zn掺杂的、具有金红石结构的SnO₂纳米棒团簇,而且随着产物中Zn含量的增加,纳米棒的直径减小,长度增加。考察了溶剂组成、溶液的碱度、摩尔比[Zn²⁺]/[Sn⁴⁺]、聚合物的引入等对产物形貌和晶相的影响。详细考察了晶体生长过程中的晶相转变和形貌的形成过程,提出了可能的晶体生长机理。产物的拉曼光谱显示,除了传统的SnO₂晶体的振动峰以及由SnO₂纳米结构所引起的振动峰,另外还出现了纯的SnO₂晶体没有出现的新振动峰。研究了该产物对乙醇、丙酮和苯蒸汽的气体敏感性。（3）研究了在两种或多种溶剂混合组成的均相体系中进行溶剂热过程时,SnO₂晶体的可控生长:首先,以乙二胺和蒸馏水为混合溶剂,通过调节其体积比、溶液的碱度和反应温度等,制备了多种不同形貌、尺寸以及聚集状态的SnO₂纳米颗粒、纳米棒、微米棒等结构,比如像花一样聚集的纳米棒团簇、纳米棒组成的弧形或环状结构、小尺寸的纳米棒及纳米颗粒、铅笔形状的微米级SnO₂棒等。所得到的纳米棒生长方向为[11?2],纳米棒中存在有高浓度的氧空位和明显的位错缺陷,讨论了纳米棒及其多级结构的形成机理。并对不同比例的溶剂组成时所得到的不同尺寸和形貌的产物进行了光学性能测试和分析,其FT-IR、UV-Vis以及PL均出现了强度或位移的变化;在以上结果的基础上,设计了多种溶剂组成,如乙二胺:乙醇:蒸馏水（1:1:1）、二乙烯三胺:蒸馏水（1:1）等进行了实验和比较,通过调节溶剂的组成可进一步调控纳米SnO₂晶体的形貌和尺寸,溶剂固有的性质以及溶剂分子所含有的功能基团在晶体生长过程中起了关键作用。（4）以天然棉纤维为模板,通过化学方法,得到了具有多孔结构的棉花状SnO₂,此纤维状SnO₂由直径约14nm的SnO₂纳米颗粒相互连接而成。所得产物除了尺寸有一些缩小外,从宏观到纳米尺寸上几乎完全复制了天然棉纤维的结构。纤维状SnO₂的多孔结构有利于被测气体的扩散,对乙醇气体有非常高的敏感性,是一种优良的气敏材料。最后,对本论文的实验结果和创新之处进行了总结,提出了相关的工作展望。更多还原

【Abstract】 Owing to their unique optical, electric, photoelectric transition, and photocatalytic properties, semiconductor nanomaterials have drawn wide research attentions. Designing reasonable synthetic routes of nanostructures, controlling their sizes, morphologies, dimensions, components, and crystal structures as well as studying the relationship between nanostructures and their properties are the important sections of nanoscience and nanotechnology. They are also of great significance in realizing the manipulated synthesis of functional materials to our will. In this dissertation, semiconductor SnO₂ was chosen as the target and several kinds of chemistry methods were conducted to prepare new morphologies and nanostructures. The main strategies were involved in using ordinary tin salts as the precursors to synthesize SnO₂ nanocrystals in solution-based reaction system by introducing polymers, doping Zn in the SnO₂ nanocrystals, tuning the components of solvents, and using natural cotton as templates. The obtained products were characterized by XRD, FESEM, TEM, HRTEM, FT-IR, UV-Vis, Raman spectra, PL, XPS, et al. The formation mechanisms of the obtained nanostructures were discussed and their properties were also investigated. The main work is summarized as follows:1. Sevaral kinds of SnO₂ nanostructures were prepared assisted by polymers Poly（acrylic acid） （PAA）, polyethlene glycol （PEG） 200, and polyvinyl alcohol （PVA） 1799: Firstly, single-crystalline SnO₂ nanocones with average 1.0μm in length and 100-500nm in root size and their self-assembly morphologies of hollow spheres were obtained through a solvothermal process assisted by PAA. The influence of crystal growth parameters such as alkalinity of the solution, components of the solvents, dosage of PAA on the morphologies and sizes were investigated. The crystal growth process experienced a self-assembly of nanoparticles directed by PAA and oriented attachment mechanism to form the single-crystalline nanocones. With nanometer-sized tips and rugged surfaces, the nanocones have high surface free energy and further assemble into hollowspheres. The Raman spectrum of the nanocones shows new group of shifts related to the crystal microstructures; Secondly, single-crystalline SnO₂ nanorods were obtained in hot PEG 200. PEG 200 was crucial in this anisotropic growth process and believed to act both as a solvent and a surfactant. The preferred growth direction of the nanorods was along [11-2] which has seldom reported by now. In this reaction system, the growth parameters such as temperature, time, alkalinity of the solution, and concentration of initial ion were all important to the size of nanorods. The growth process experienced an oriented attachment process. The photoluminescence spectrum displayed strong excitonic PL and defects PL; Finally, PVA 1799 was used as the substrate and PVA-SnO₂ nanocomposite was firstly obtained by fluxing at high temperature and oxidation process. When this nanocomposite was subjected to calcination, the polymer was removed and the pure SnO₂ crystals were obtained. Nanostructures such as SnO₂ hollow spheres composed of nanoparticles were obtained.2. The morphologies and sizes were well controlled by Zn doped into SnO₂ crystals. Branched Zn-doped SnO₂ nanorods clusters were prepared through a facile solvothermal process in ethanol-water media. The introduction of a small quantity of Zn2+ was used as dopant and the obtained products exhibited pure SnO₂ rutile structures. With the content of Zn in the products, the diameters of the nanorods were decreased while length increased. The growth parameters such as component of solvent, alkalinity of the solution, molar ratio [Zn²⁺]:[Sn⁴⁺], and the introduction of polymer PAA can greatly influence the crystal phase and morphologies. The possible growth mechanism has been proposed by investigating the transition of crystal phase and formation of the nanorods during the growth process. These Zn-doped SnO₂ nanorods exhibited unique Raman spectra in contrast with the undoped SnO₂ nanostructures and some new shifts were observed. The gas sensing properties of these nanorods to ethanol gas, acetone gas, and benzene gas were also investigated.3. Controlled growth of SnO₂ crystals was investigated in homogenous solutions composed of two or several kinds of solvents through sovlothermal processes: At first, SnO₂ nanostructrues such as flower-like nanorod clusters, nanorod microrings, and individual nanorods were obtained via a solvothermal approach in a mixed solution of ethylenediamine and distilled water. By just simply adjusting the volume ratio of the above two solvents, alkalinity of the solution, and temperature, the size and hierarchical structures of SnO₂ nanocrystals could be easily varied. The preferential growth direction of the nanorods was [11- 2] and there were high concentration of oxygen vacancy and lattice defects in the nanorods. The formation mechanism of the nanorods and their hierarchical structures were also discussed. The optical spectra including FT-IR spectra, UV-vis absorption spectra, and photoluminescence spectra of the samples prepared in different volume ratios of ethylenediamine and distilled water all showed changes of intensities and shifts. On the base of the above results, other solvent component such as ethylenediamine:ethanol:distilled water（1:1:1）, diethenetriamine: distilled water （1:1）, et al. were also used as reaction media to control the sizes and morphologies of SnO₂ nanocrystals. A conclusion can be drawn that the inherent properties and functional groups of the solvent molecules are vital for the crystal growth during the solvothermal process.4. A novel chemistry strategy was conducted to prepare cotton-like SnO₂ using natural cotton as templates. The obtained fiber-like SnO₂ were composed of nanoparticles with the average size of 14nm which interconnected each other and formed porous microstructures. The original morphology of cotton fibers was found to be replicated by the SnO₂ crystals from nanometer to micrometer regimes. However, the size of the obtained cotton-like SnO₂ became smaller. Because the porous microstructures were in favor of the diffusion of the target gases, these fiber-like SnO₂ showed high sensitivities to ethanol gas and were good candidates to gas sensing materials.Finally, the main experimental results and innovations in this dissertation were summarized and the following expectations were proposed.更多还原