【作者】 王文寿；

【导师】 甄良；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 空心微纳米结构由于具有低密度、高比表面积和良好的单分散性、及空心部分能容纳其它材料等优点,使其在微尺度反应器、药物传输载体、光子晶体、催化剂和能量存储等诸多领域都有着重要的应用,已成为当前纳米材料科学的前沿和热点。目前空心微纳米结构的关键科学问题主要包括以下三个方面:一是如何发展无模板法可控合成出多元成分及难以合成材料的空心微纳米结构;二是探索空心微纳米结构的空心化形成机制,为无模板法可控合成技术提供理论依据;三是实现空心微纳米结构的功能化,为其实际应用奠定基础。本论文围绕上述三个方面的科学问题展开研究。论文的主要研究成果概括如下。首先是采用无模板法水热合成了CaWO4和CaMoO4空心微球。通过对合成工艺的系统研究,实现了对目标产物的形貌、尺寸、几何均匀性等的精确控制。提出了“十二烷基磺酸钠(SDS)诱导Ostwald熟化”及“溶解-形核-长大-Ostwald熟化”分别是形成CaWO4和CaMoO4空心微球的主要机制;反应起始阶段形成中间产物的结构不稳定是诱发随后Ostwald熟化的关键。该研究结果证明Ostwald熟化机制可以用来合成多元化合物空心微纳米结构,为无模板法合成多元化合物空心微纳米结构提供了一条切实可行的途径。利用“热注入法”,在较低温度下(100○C)水相合成了具有不同厚度和直径的“面包圈状”CaMoO4微米结构,该微米结构由纳米片沿厚度方向堆积而成。通过改变反应体系的酸碱度、钼源及混合溶剂,制备出了不同直径的微球状、纺锤体状、面包圈状及复杂超结构状CaMoO4微米结构,实现了CaMoO4微米结构的可控合成。通过酸性溶液腐蚀,获得了空心“面包圈状”CaMoO4微米结构。在室温液相条件下合成了CdMoO4空心微球,其球壳由沿微球径向排列的纳米棒组成。该法仅使用CdCl2和Na2MoO4水溶液在室温常压下反应,不需要加热或使用表面活性剂,为直接研究空心化过程提供了良好的条件。中间产物的研究结果表明,“NaCl诱导Ostwald熟化”是形成CdMoO4空心微球的主要机制。在空心微球的形成过程中,NaCl对早期形成的CdMoO4实心微球起到稳定作用并包覆在微球表面,使得后续反应生成物CdMoO4沉积在“NaCl-稳定CdMoO4实心微球”表面上,从而形成核-壳结构;随后发生Ostwald熟化过程形成空心微球。在此基础上,通过向反应体系添加一定量的SDS,可以制备出同质CdMoO4核-壳型微球。反应过程中间产物研究表明核-壳微球的形成受“三步生长过程”控制,SDS和CdMoO4间的相互作用是形成核-壳型微球的主要原因。CdMoO4空心微球的UV-Vis结果表明,微球在200到370nm波段内有吸收,其带隙为3.48eV。CdMoO4空心微球的发射光谱在320-600nm波段内出现一个发射峰,峰值位于354nm,其发射光谱主要来源于[MoO42-]的电荷-迁移转变。以CdMoO4为基质,采用Eu离子掺杂,获得了一种新型红色发光体。发光光谱表明发光体在535、590及613nm处出现发射峰,且613nm处的发射峰显著强于590和535nm。研究还发现,CdMoO4空心微球在紫外光作用下具有光催化降解污染物的能力。经500oC热处理2h能显著提高CdMoO4空心微球的光催化效果。CdMoO4催化剂是通过O2p轨道的电荷向Mo4d轨道转移来实现光催化降解污染物。在相同条件下,CdMoO4空心微球的光催化效果优于CdMoO4实心微球,其原因源于空心微球具有更大的比表面以及壳层存在大量孔隙。提出了“化学转变诱导Ostwald熟化”机制,用来合成直接反应难以制备物质的空心微纳米结构。分别采用CaWO4和CdMoO4实心微球作为前驱体,合成了CaF2及Cd(OH)2空心微球。空心微球的形成主要通过三个过程:首先在室温水溶液条件下获得CaWO4和CdMoO4实心微球;实心微球经过化学转变,生成CaF2及Cd(OH)2实心微球;刚形成的CaF2及Cd(OH)2实心微球结构不稳定,导致Ostwald熟化发生,实心微球逐渐转变成空心微球。“化学转变诱导Ostwald熟化”机制可以用来合成一系列新型空心微纳米结构,为制备直接反应难以合成化合物的空心微纳米结构提供了新思路和途径。进一步的研究结果表明,Eu离子掺杂CaWO4实心微球可通过该机制转变成Eu离子掺杂CaF2空心微球。Eu离子掺杂CaF2空心微球表现出优异的发光性能。“化学转变诱导Ostwald熟化”机制为无机空心微纳米结构的形成机制、性能及应用研究提供了更为广阔的空间。更多还原

【Abstract】 Inorganic hollow micro/nanostructures have been one of the research frontiers and focuses for nanomaterials field due to their low density, high specific surface area, monodispersity, stability, and novel multifunctional properties arising from combining different materials into the hollow interiors and potential applications in micro/nanoscale chemical reactors, drug-delivery carriers, photonic building blocks, efficient catalysts, energy-storage media and so forth. The key scientific questions in hollow micro/nanostructures include i) controlled synthesis of hollow micro/nanostructures with multicompositions and materials which are difficult to prepare by template-free methods; ii) investigating the formation mechanism of hollowing process, which providing theoretic evidence for the controlled synthesis; iii) realizing the functionality of hollow micro/nanostructures for the applications. In this work, we have studied the above three questions and obtained the following conclusions.We have developed a template-free hydrothermal process for CaWO4 and CaMoO4 hollow microspheres. Experimental results show that the morphology, size and homogenization are fine controlled by changing the reaction conditions.“Sodium dodecyl sulfate (SDS) induced Ostwald ripening”and“dissolution-nucleation-growth-Ostwald ripening”mechanisms are the main reason for the formation of CaWO4 and CaMoO4 hollow microspheres, respectively. Structural instability of intermediates formed at initial reaction process is the key step for the subsequently hollowing process. The results demonstrate that Ostwald ripening can be used for preparing ternary metal oxides with hollow interiors, which provide an efficient pathway for the synthesis of multicompositional hollow structures.A solution-phase hot-injection-based route has been developed for the synthesis of CaMoO4 microstructures with well-defined shapes and sizes at 100○C. The CaMoO4 doughnuts are composed of numerous nanosheets along thickness direction. Shapes such as microspheres, spindles, and complex three-dimensional structures can be produced by controlling the reaction conditions including the concentration of reactants, molar ratio between reactants, molybdenum source, pH, and the volume ratio of the mixed solvents. CaMoO4 doughnuts with hollow interiors can be obtained by etching in acetic condition.CdMoO4 hollow microspheres have been prepared via a template-free aqueous solution method at room temperature. The shell is composed of nanorods along diameter direction. Using CdCl2 and Na2MoO4 as the starting reaction reagents without surfactants and heating, this method provides a suitable way to investigate the formation mechanism of the hollowing process. The results of intermediates obtained at different reaction times show that“NaCl induced Ostwald ripening”is account for the hollow structures. It is believed that core-shell CdMoO4 microspheres are formed first by a two-step precipitation process: precipitation of solid cores and subsequent precipitation of the outer shell onto ionic species (Na+ and Cl-)-stabilized cores. The core has a strong tendency to dissolve because of high surface energies compared to those in the outer shells, providing the driving force for Ostwald ripening. By adding a suitable amount of SDS, homogenous CdMoO4 core-shell microspheres are prepared. A possible three-step growth mechanism is presented for the formation of homogenous core-shell CdMoO4 microspheres. The mutual effect between SDS and CdMoO4 is the main reason for core-shell microspheres.UV-Vis spectrum of CdMoO4 hollow microspheres shows that there is a strong absorption peak in the wavelength of 200-370 nm. The Eg is 3.48eV. PL spectrum shows a strong and broad emission band centered at 354 nm, originating from the charge-transfer transitions within the [MoO42-] complex. A new Eu-doped CdMoO4 red phosphor was prepared by a similar aqueous solution process at room temperature. The phosphor exhibits three emissions at 535, 590 and 613 nm, while a dominant red emission at 613 nm at room temperature.We report for the first time the photocatalytic activity of the hollow CdMoO4 microspheres in the degradation of Rhodamine B (RhB) under UV light irradiation. Calcination the CdMoO4 microspheres at 500 oC for 2 h significantly enhance the photocatalytic activity. The photooxidation of the pollutant over CdMoO4 photocatalyst occurs from the charge transfer from the O 2p orbitals to empty Mo 4d orbitals. The CdMoO4 hollow microspheres after calcinating at 500 oC for 2 h exhibit higher photocatalytic efficiency than that of corresponding solid microspheres under the same conditions. The effective photocatalytic activity of hollow microspheres is related to their hollow structures, which possess plenty of meso- and macro-pores in the shells. “Chemical conversion induced Ostwald ripening”is developed for the synthesis of hollow microstructures which are difficult to prepare by general methods. CaF2 and Cd(OH)2 hollow microspheres are prepared by using CaWO4 and CdMoO4 solid microspheres as precursor, respectively. It has been shown that the formation process includes three important steps: the first formation of CaWO4 or CdMoO4 solid microspheres by direct precipitation process; the chemical conversion of CaWO4 or CdMoO4 solid microspheres to CaF2 or Cd(OH)2 solid microspheres; and finally hollowing process via Ostwald ripening in the CaWO4 or Cd(OH)2 solid microspheres to form corresponding hollow spheres.“Chemical conversion induced Ostwald ripening”is a universal method, providing a new strategy and pathway for the synthesis of hollow micro/nanostructures which are difficult to prepare by general methods. Further study shows that Eu-doped CaWO4 solid microspheres can be tranferred to Eu-doped CaF2 hollow microspheres, which exhibit excellent optical properties. These results indicate that the“chemical conversion induced Ostwald ripening”method opens new insights for the formation mechanism, properties and applications of hollow micro/nanostructures.更多还原