【作者】 张新峰；

【导师】 韦世强；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2008， 博士

【摘要】 本论文主要利用同步辐射X射线吸收精细结构（XAFS），结合透射电镜（TEM）和X射线衍射（XRD）等实验方法研究三种纳米体系：（1）具有可控光电性能的Ⅱ-Ⅵ族CdSe半导体纳米晶；（2）具有室温铁磁性的过渡金属Co掺杂ZnO纳米晶；（3）具有电化学储氢性能的一维Se纳米管。在微观局域结构的基础上分别探讨了CdSe纳米晶的生长机理、Co掺杂ZnO纳米晶的结构特点和磁学性能以及Se纳米管的形成机理，从而为高性能半导体纳米晶发光材料、稀磁半导体纳米材料和电化学储氢纳米材料的可控制备提供理论的研究基础。1．CdSe纳米晶熟化过程的结构演变研究利用XAFS、XRD和高分辨透射电镜（HR-TEM）等方法研究CdSe半导体纳米晶熟化生长过程中的结构演化。利用尺寸选择分离的方法制备尺寸分别为1．9、2．7和3．1nm，且尺寸分布较窄（<10％）的纳米晶。XAFS结果表明，随着CdSe颗粒尺寸从1．9nm增加到2．7和3．1nm，其Se-Cd第一近邻配位的结构无序度σ_S²从0．0005（?）增加到0．0012和0．0034（?）。1．9nm的CdSe颗粒具有很小的σ_S²表明生长到该阶段的时候其结晶程度很好，内部缺陷很少。σ_S²随着颗粒尺寸的增加而增加，这一反常变化是由于其内部结构缺陷的增加导致的。颗粒的内部缺陷随着熟化生长过程的进行而不断累积，从而导致大颗粒的CdSe纳米晶的结构无序度迅速增加。2．Co掺杂ZnO纳米晶的结构和磁性研究利用HR-TEM、XRD、XAFS和SQUID等方法研究了具有明显室温铁磁性的Zn_1-xCo_xO纳米晶的结构和磁性。尽管XRD和HR-TEM都没有探测到样品中含有杂相或包覆在ZnO晶格中的颗粒，但XAFS结果清晰的表明样品中同时存在金属Co相和掺杂进ZnO晶格中的Co离子相：在Zn_0.98Co_0．02O和Zn_0．95Co_0.05O中仅有64％和63％的Co掺杂进入ZnO晶格；而其他部分形成了平均尺寸小于10nm的金属Co纳米颗粒。样品的铁磁性并不是来源于掺杂相，而是来源于沉淀的金属Co纳米颗粒。样品的矫顽力随着Co含量的增加而降低，表明其铁磁性来源不同于纯金属相Co单畴颗粒，而可能是来源于Co纳米颗粒与表面包覆层的相互作用。3．Se纳米管的形成机理研究利用XAFS结合TEM等方法研究了可见光辐照辅助液相法制备的Se纳米管的形成机理。结果表明，Se纳米管的形成过程包含了三个典型阶段：（1）化学还原Se⁴⁺为Se⁰，并形成非晶Se微球；（2）部分非晶Se晶化为三方相Se（t-Se）纳米颗粒；（3）剩余的非晶Se继续晶化，但此阶段晶化沿着t-Se晶核的[001]晶向进行，形成管状结构。管状结构的形成是非晶Se链的扩散速度与反应速度相竞争的结果，该竞争导致了纳米管内部结构呈独特的楔形状。可见光辐照大大增加了Se原子链与晶核的反应速度，对Se纳米管的形成起着关键作用。更多还原

【Abstract】 This thesis presents the investigation on nanomaterials by X-ray absorption fine structure （XAFS）, combined with X-ray diffraction （XRD）, transmission electron microscopy （TEM）. It involves three different kinds of nanomaterials: （1）Ⅱ-Ⅵgroup CdSe semiconductor nanocrystals with controlled optoelectric property; （2） transition metal （cobalt） doped-ZnO nanocrystals with room temperature ferromagnetism; and （3） one-dimensional Se nanotubes with electrochemical hydrogen storage potential. This work provides structural aspect of nanomaterials, and will be helpful to controlled synthesis of nanomaterials as well as to better understanding of their properties.1.Structural evolutions of CdSe nanocrystals in ripening processThe structural evolutions of CdSe nanocrystals （NCs） in the ripening process were investigated by high-resolution transmission electron microscopy （HR-TEM）, XRD, and XAFS. A size-selective separation method was used to synthesize CdSe NCs with different grain sizes. The XAFS results indicate that the structural disorderσ_S² of the first nearest neighbor coordination Se-Cd shell in CdSe NCs rises from 0.0005 to 0.0012 and 0.0034 （?） as the size of NCs increases from 1.9 to 2.7 and 3.1 nm. The smallσ_S² of the 1.9 nm NCs unambiguously reveals that at this stage the CdSe NCs are well crystallized and almost free of interior defects. The unusual increase ofσ_S² with size can only be interpreted by the interior defects rather than surface defects of NCs. The interior defects produced in the core of the CdSe NCs through the ripening process are accumulated. This leads to a rapid increase in their structural disorders for the large CdSe NCs.2. Structures and magnetism of Cobalt doped-ZnO nanocrystalsThe structures and magnetism of Cobalt-doped ZnO nanocrystals (Zn_1-xCo_xO, x=0.02, 0.05) were investigated by HR-TEM, XRD, XAFS and SQUID. The Zn_1-xCo_xO nanocrystals demonstrate strong room-temperature ferromagnetism （FM） behavior from SQUID measurement. Although neither secondary phase nor imbedded cluster was detected by XRD and HR-TEM, XAFS has clearly shown the coexistence of metallic Co phase and Co²⁺ ionic phase incorporated in to ZnO matrix. EXAFS fittings indicate that only 64% and 63% of Co has effectively incorporated into wurtzite ZnO crystal lattice for the Zn_1-xCo_xO （x=0.02, 0.05） samples, while the rest precipitated out as Co clusters with average size well below 10 nm. The incorporated Co²⁺ ions in the highly crystalline ZnO nanocrystals did not contribute to FM signals, while the FM was assigned to the Co clustering in the samples. The precipitate of Co nanoparticle would result in RT-FM due to the interaction with the coatings/matrix, even its size is within the critical size for superparamagnetism.3. Growth mechanism of Se nanotubesFluorescence XAFS and TEM were used to study the formation mechanism of trigonal selenium （t-Se） nanotubes synthesized by photothermally assisted solution method. The results demonstrate that the formation process of t-Se nanotubes contains three distinctive stages: （1） reduction of Se⁴⁺ to form amorphous Se （α-Se） phase, （2） partialα-Se crystallized into t-Se nanoparticles, （3） the crystallization of t-Se preferentially along [001] direction to form nanotubes. The formation of tubular structures is the outcome of competition between diffusion rate ofα-Se chains and reaction rate. This competition results in the cone-shaped hollow of inner tubular structures. Light irradiation plays an important role in the formation of tubular structures by accelerating the reaction rate of Se chains.更多还原