【作者】 赵伟；

【导师】 孙思修；

【作者基本信息】 山东大学 ， 无机化学， 2009， 博士

【摘要】 无机纳米材料和纳米结构的合成方法研究是纳米科学技术发展的热点领域,也是制备复杂超微器件的前提条件。由于液相合成方法的诸多优点使其成为对基本纳米粒子进行裁剪、组装的重要方法之一。目前液相合成纳米材料的方法虽然很多,各具优势,但也都存在一些缺陷,获得尺寸可控、粒度均匀的纳米材料仍然存在一定的困难。探索设备简单、操作方便、成本低、产率高的液相合成方法,以实现对纳米材料形貌、尺寸、结构的选择控制,仍然是化学家和材料学家长期以来关心的课题之一。本课题组长期以来从事液相中合成无机纳米材料,在溶剂热合成等方面有较深入研究。本论文在课题组前人工作的基础上,选择具有优异物理性质的半导体材料氧化锌、钨酸锌和钨酸铅作为研究对象,发挥化学液相合成技术在控制材料的微结构、形貌和尺寸等方面的优势,探索在化学液相中制备纳米材料的新方法和控制条件,期望得到形貌和尺寸可控的纳米结构,并获得一些纳米材料的制备和形貌控制等方面的规律。企望论文对纳米材料液相合成过程,对开展新型、特殊、复杂结构纳米材料的制备提供借鉴。具体内容如下:1.在室温条件下,利用简单的液相法成功制备了ZnO纳米花状聚集体,且所得产物产率高、结晶好,反应时间短。ZnO花状结构直径约为1.5μm,由均匀的厚度大约5 nm的氧化锌纳米片自组装而成。在花状氧化锌的生长过程中,柠檬酸钾起了重要作用:首先,柠檬酸根与Zn²⁺络合,减缓了沉淀生成速度,有益于生成结晶性良好的产物;其次,柠檬酸根可被吸附在ZnO的（002）面上,降低了此晶面的生长速度,从而使氧化锌生长成片状形貌。根据时间调控实验分析,花状结构的生长过程为自组装过程,即首先生成ZnO纳米片,为了降低体系的表面能,继而纳米片开始聚集,并随着时间的延长自组装成花状结构。花状ZnO在357nm处有强的吸收峰,相对于ZnO块体材料有明显的蓝移,这是由于ZnO纳米片的厚度与氧化锌的波尔半径相近,吸收峰的蓝移是由于氧化锌纳米片的量子效应,这或许在光学器件方面有潜在的应用价值。2.通过水热合成方法,利用柠檬酸钾做形貌控制剂,成功制备了氧化锌微米花状结构。实验证明,体系pH值对氧化锌产物的形貌具有重要的影响,改变体系内NaOH的用量,可以在同一体系中得到多种形貌ZnO产物,如:实心圆球、六方块状、花状、花球状,以及纳米棒组成的聚集体。初步分析认为,产物形貌的不同源于不同pH值下Zn²⁺与OH^-络合方式不同,导致前躯体不同。3.在水热体系中,在不添加模板或形貌控制剂的情况下,通过改变反应物的种类,成功制备了哑铃状与梭状两种不同形貌的ZnO空心微米结构。实验证实,产物形貌的不同源于前躯体的不同,即Zn（OH）₄^2-易生成哑铃状产物,而Zn（NH₃）₄²⁺则生成梭状产物。对中空产物的生长过程的分析发现,“奥氏熟化”机理在中空的形成中起了重要作用,且两种形貌氧化锌的中间对称面起到了熟化过程活性中心的作用。对所得产物进行了荧光发射性质测试,两种产物几乎显示相同的发射光谱,均在可见光区400-500nm处显示较强的发射峰,明显强于其块体材料。4.通过水热合成方法,成功制备了钨酸锌纳米线/纳米带。通过添加表面活性剂聚乙烯吡咯烷酮（PVP）,钨酸锌纳米线转变为纳米带,PVP选择性吸附在（020）面上,从而得到钨酸锌带状结构。对产物进行的催化罗丹明B实验证明,钨酸锌的形貌和结构对其光催化活性有重要影响,钨酸锌纳米带较纳米线有更优异的光催化性质。钨酸锌纳米带的高催化活性可能来自两方面原因:一是带状结构具有更高的比表面积,增加了催化剂与被催化分子的接触面积;二是纳米带拥有更多的裸露在外的沿[100]晶轴方向的晶面,即钨酸锌的催化活性面。而且,带状产物的荧光发光性质也强于线状钨酸锌。5.在180℃水热体系中,制备得到由钨酸锌纳米棒组成而成的空心球状聚集体,这是第一次在纯水体系中制备钨酸锌空心结构。此空心球直径约为400 nm,组成空心球的纳米棒直径约10 nm,长为50 nm左右。对所得产物进行了催化罗丹明B的实验,证实此钨酸锌空心球具有较强的光催化性质,源自空心结构高的比表面积。6.在水热体系中,通过添加柠檬酸钾做形貌控制剂,成功制备了由纳米颗粒组成的钨酸铅球状聚集体。柠檬酸钾对产物的形貌具有重要影响,改变柠檬酸钾的用量,可以得到钨酸铅八面体、球状聚集体、胶囊状聚集体等产物。引入注意的是,所得聚集体在440 nm处具有强的荧光发射峰,与报道的500 nm发射峰不同。产物的发光性质的改变是因其由无数纳米棒组成,具有较大的比表面积,从而拥有更多的量子点。我们相信,所得钨酸铅独特的发光性质使其在应用方面具有更高的潜在价值。7.在水热合成PbWO₄体系中引入Zn²⁺作为添加剂,合成了多种未见报道的特殊形貌PbWO₄纳米材料,如棒状聚集体与花状聚集体。所得产物的荧光发射光谱显示,此类特殊形貌钨酸铅显示出了不同于块体材料的发光性质,在460nm与550nm处显示较强的发射峰,尚未见报道。更多还原

【Abstract】 The study of synthesis methods of inorgnic nanomaterials and nanostructures is a hot area of nanoscience, and is also the prerequisite of preparation of complex advanced microdevices. Due to so many advantages of the liquid-phase syntheses, it will be one of the important methods to assemble the basal nanoparticles in the future. Up to now, it is still an important task in the field of material that how to develop new methods for preparing nanomaterials. Although there are many liquid-phase methods reported for preparing materials, it is still difficult to obtain materials with controllable morphologies and sizes. Therefore, it is attracting a great deal of attention of the chemists and materials researchers to explore new liquid-phase methods for obtaining low-cost mild reaction.Our group has rich experience in the technology of preparation of inorganic nanoparticles in liquid-phase system. On the base of our group’s works in the past, this paper chooses ZnO, ZnWO₄, PbWO₄ as study objects, using liquid chemical synthesis technology’s advantages in controlling the materials microstructures, morphologies and size. We explore new methods and control conditions for liquid chemical synthesis of nanomaterials, and find some rules in synthesis and morphology control in nanomaterials. This study both enriches the liquid-phase methods of nanomaterials, and also provides a new thought for preparing complex materials with special nano-structures. The detailed information of the dissertation is listed as follows.1. A singularity flower-like ZnO nanostructure was prepared on a large scale through a very simple solution method at room temperature and under ambient pressure in a very short time. The flower-like ZnO nanostructures had a diameter of about 1.5μm, and were self-assembled by thin and uniform nanosheets, with a thickness of around 5 nm. Citrate played an important roal in the growth of flower structure. Firstly, citrate could coordinate with Zn²⁺, so the sedimentation rate was slowed down, resulting in better crystalline. Secondly, citrate could also control the ZnO crystal as it absorbed to the （002） surface and forced the crystal to grow into plates. Based on the time-dependent experiment, the possible growth mechanism was discussed, and it was the self-assemble process. At first, ZnO nanosheets were formed, and then they self-assembled in flower-like structures in order to reduce their high surface energy. UV-vis spectra of the ZnO nanostructures showed a strong exciton peak at 357nm, which has a blue shift compared to that of bulk ZnO. This should be mainly due to the quantum confinement of the ZnO nanosheets, since the man thickness of the sheets is comparable to the Bohr radius of ZnO. Hence, the products had advanced absorption of ultraviolet radiation; this may have extensive applications in optics devices.2. ZnO flower-like micro-structures were sucsessfully synthesized through a simple hydrothermal route, using potassium citrate as shape modifier. Based on the experament result, the pH value of the system played an important role on the morphology of the products. By changing amount of NaOH introduced into the system, many different morphologies of ZnO were obtained, such as solid spheres, hexahedrons, flowers, flower-clusters and aggregates made up of nanorods. Preliminary view was that different complex ways betweent Zn²⁺ and OH^-, that is different precursors, resulted in different morphologies.3. We synthesized two kinds of hollow twinning ZnO microstructures through a simple hydrothermal method without templates. Dumbbell-like and shuttle-like ZnO microstructures with hollows were obtained by changing the materials source. Experment results showed that different precursors resulted in different morphologies. That is Zn（OH）₄²⁺ prefered dumbbell-like products, and Zn（NH3）₄²⁺ resulted in shuttle-like products. Based on the time-dependent experiments, we investigated the growth process of these two hollow twinning structures and found the "Ostwald-ripening process" played an important role. The interesting part of this growth process was that the interface of the two twinning structure performed as the activate center where the Ostwald-ripening process carried out. We also investigated the luminescent properties of the as-obtained products by photoluminescence （PL） spectroscopy, and found that these two hollow structures both showed strong visible emission in the 400-500 nm regions, which is much stronger than bulk ZnO.4. ZnWO₄ nanowires/nanobelts were synthesized through a hydrothermal method. ZnWO₄ nanobelts were synthesized through a poly （vinylpyrrolidone） （PVP） assisted hydrothermal process. PVP molecules absorbed on some surface of the ZnWO₄ crystals could significantly decrease their growth rates and lead to highly anisotropic growth, resulting in nanobelts. We investigated the growth process of the products and found that the "Ostwald-ripening process" played an important role. Photo-decomposition experiments indicated that the morphology and crystallinity of ZnWO₄ photocatalyst had a significant influence on the photocatalytic activity for aqueous Rhodamine B, and ZnWO₄ nanobelts showed a much higher photocatalytic activity than nanowires. There were reasons for ZnWO₄ nanobelts’s high photocatalytic activity: firstly, nanobelts had higher surface, which provided large surface area for absorbing substrate; secondly, nanobelts had more bare planes containing W and O atoms, thus enhance the photocatalytic activity of nanobelts. Besides, ZnWO₄ nanobelts also exhibited a much stronger luminescence property than nanowires.5. ZnWO₄ hollow spheres made up of nanorods were successfully prepared through a tri-potassium citrate assisted hydrothermal process at 180℃. This is the first time that ZnWO₄ hollow structures are obtained in aqueous system. The hollow spheres’ diameter was about 400nm, and these spheres were made up of nanorods with a diameter of about 10 nm and a length of about 50nm. Based on experiments, the growth of these hollow spheres followed an aggregation-Ostwald ripening process. The photocatalytic activities for aqueous Rhodamine B of samples were investigated, and it was amazing that ZnWO₄ hollow spheres exhibited a strong photocatalytic activity, which was caused by hollow spheres’ high surface.6. Hierarchical PbWO₄ spheres assembled by nanoparticles were successfully synthesized through a tri-potassium citrate assisted hydrothermal process. It was found that citrate played a key role on the morphology of PbWO₄ products. By adjusting citrate’s concentration, PbWO₄ octahedrons, hierarchical spheres, hierarchical ellipses could be obtained. Based on time-dependent experiments, we found the growth of the hierarchical spheres followed a self-assembly process. The most interesting part was that the hierarchical spheres/ellipses showed a blue emission peak at 440nm, which differs from the typical green one at 500nm as reported. We believed that the PbWO₄ aggregates made up of nanorods exhibit high light-collection efficiency and enhanced luminescence performance due to their large surface area. The greatly enhanced luminescence performance is exciting and may have significant technological applications in the inorganic scintillating field.7. By introducing Zn²⁺ as shape modifier into the synthesis of PbWO₄ nanostructures under hydrothermal route, a series of various PbWO₄ nanostructures were successfully prepared, including rod-like and flower-like aggregates, most of them had not been reported. It is interesting that the products showed emission peaks at 460nm and 550nm, which was different from that of bulk PbWO₄, and this had not been reported.更多还原