【作者】 佟文明；

【导师】 李广社；

【作者基本信息】 内蒙古大学 ， 化学， 2010， 硕士

【摘要】 纳米材料结构和物理化学性质与其晶粒尺寸和晶粒表面状态密切相关。晶粒尺寸的变化会带来晶格结构和物理化学性质相应的改变,从而使纳米材料表现出常规体相粉末所没有的特殊性能。此外掺杂效应可改变晶格内部结构,从而改变材料的性质,因此研究材料的尺寸效应及掺杂效应具有重要的科学意义。ABO4型氧化物是一类重要的化合物,在闪烁体材料、多铁材料、激光基质材料和显示设备等领域具有广泛应用。本文选取ABO4类二元氧化中具有普遍代表性的钨酸盐(AWO4)作为研究对象,以“尺寸效应和掺杂效应对纳米AWO4(A=Mn、Cd、Ca、Zn)结构及性质的影响”为题,研究纳米颗粒尺寸效应、掺杂对化合物的相结构及性质的影响,通过粒度控制和掺杂实现对其晶格结构的调变和性能优化。主要结果如下：(1)MnWO4纳米晶的生长动力学控制及其纳米结构调控。采用水热法成功合成了尺寸为8-29 nm的MnWO4纳米晶,并确定在反应时间t和反应温度T条件下的颗粒尺寸D的生长动力学方程为。系统的结构表征和分析表明晶粒尺寸的减小导致纳米MnWO4晶格膨胀,晶格对称性降低,能带变宽。与块体MnWO4不同,纳米尺寸下MnWO4红外和拉曼光谱出现新的振动模式,分别位于913和930 cm-1,其峰强度随着晶粒尺寸的减小而增大,这一现场主要与纳米晶粒表面畸变层有关。我们通过核壳模型拟合计算出MnWO4纳米颗粒的表面畸变层厚度约为1.8 nm,相当于三个MnWO4晶胞厚度。(2)CdWO4纳米晶的粒度调控与光催化性能优化。以柠檬酸为络合剂通过调节反应温度合成了一系列不同晶粒尺寸和结晶度的CdWO4纳米晶。X射线粉末衍射、透射电镜、红外光谱、紫外漫散射光谱、荧光发射光谱和BET比表面积测试等系统表征和分析表明随着反应温度由160℃升高至220℃纳米CdWO4的晶粒尺寸从11 nm增大为21 nm。随着晶粒尺寸的减小CdWO4晶格体积膨胀,晶格对称性降低,能带变宽,红外Au振动模式红移。254 nm紫外光催化降解甲基橙测试结果显示21 nm CdWO4光催化活性为11 nm CdWO4纳米晶的三倍以上。上述结果表明通过调控CdWO4纳米晶结晶度和晶粒尺寸可达到优化其光催化性能的目的,同时此合成方法也可推广至其它半导体光催化剂体系。(3)Ca1-xZnxWO4纳米晶的合成及Zn掺杂效应对CaWO4能带、发光及导电性能的影响。以有机小分子柠檬酸为络合剂在室温下由共沉淀法制备了一系列Ca1-xZnxWO4纳米晶。ICP元素测试结果表明Zn含量在0-0.104之间。系统的结构表征及数据分析表明随着Zn掺杂含量的增加纳米Ca1-xZnxWO4的晶格体积收缩,紫外吸收边红移,荧光发射强度降低,导电性能增强。初步估计Zn在CaWO4中的固溶线为10%左右。更多还原

【Abstract】 The lattice structure and physi-chemical properties of nanomaterials are closely related to their particle size and surface modification. Indeed, properties are always different from the bulk when the physical dimension comes down to nanoscale. ABO4 type materials had a great application in many areas, such as phosphors, laser crystals, catalysis, and multiferroic materials. In this thesis, we conducted a detailed study of size and doping effect on lattice structure and properties of AWO4 (A=Mn, Cd, Ca, Zn) nanoparictls. The main results can be summarized as follows:(1) Using citric acid as the capping agent, MnWO4 nanocrystals were prepared to show particle sizes ranging from 8 to 29 nm under hydrothermal conditions. The grain-growth kinetics for MnWO4 nanoparticles was determined to quantitatively follow the equation, where D is the particle size at given reaction time,t, and reaction temperature, T. Systematic sample characterization using combined techniques of X-ray diffraction, transmission electron microscope, selected area electron diffraction, Barret-Emmett-Teller technique, Fourier transformed infrared spectra, UV-visible diffuse reflectance spectra, and Raman spectra indicates that particle size reduction led to an apparent lattice expansion, which is followed by lowering of lattice symmetry and band-gap broadening. Different from the bulk counterparts, new vibration modes were observed in both Infrared and Raman spectra at about 913 cm-1 and 930 cm-1, respectively, which intensified monotonously with particle size reduction, leading to a picture that MnWO4 nanoparticles were terminated by a surface disordered layers. All these size-dependent physical properties were closely related to the surface disorder and the relevant absorbates.(2) CdWO4 nanocrystals with controlled particle size and crystallinity were successfully synthesized via a simple hydro thermal method using citric acid as the capping agent. By systematic sample characterization using X-ray powder diffraction, transmission electron microscope, selected area electron diffraction, Barret-Emmett-Teller technique, Fourier transformed infrared spectra, UV-visible diffuse reflectance spectra, and photoluminescence spectra, all as-prepared CdWO4 samples were demonstrated to crystallize in a pure-phase of monoclinic wolframite structure. With varying the reaction temperature from 160 to 220℃, particle size was controlled to grow from 11 to 21 nm. With particle size reduction, CdWO4 nanostructure showed a lattice expansion, as is followed by a surprisingly lowered lattice symmetry, band gap broadening, and redshift of Au vibration mode. Photocatalytic activity of CdWO4 nanocrystals was examined by monitoring the degradation of methyl orange dye in an aqueous solution under UV radiation of 254 nm. High crystallinity CdWO4 nanostructures with relatively small particle size showed an optimum photocatalytic performance. Consequently, systematic control over semiconductor nanostructures is proved to be useful, in some cases likely general, in achieving the advanced photocatalytic properties for technological uses.(3) Ca1-xZnxWO4 nanocrystals were prepared at room temperature by co-precipitation method using citric acid as a capping agent. As prepared samples have a scheelite structure, and the average sizes of spherical-like particles were about 6nm. With increasing the zinc content, the lattice volume of Ca1-xZnxWO4 nanocrystals decreased, and band edge shift from 4.99 eV for x=0 to 3.98 eV for x=0.104. IR spectra revealed that nanocrystals surfaces were hydration and also caped with citric acid. The photo luminescence spectra show that emission intensity was decreased dramatically with increasing of zinc content. Impedance spectroscropy of Ca1-xZnxWO4 nanocrystals shows that the resistivity of the samples decreased with increasing of zinc content. The Solid solution limit of znic in CaWO4 was estimated to be around 10%.更多还原