【作者】 黄承焕；

【导师】 杨华明；

【作者基本信息】 中南大学 ， 材料学， 2005， 硕士

【摘要】 本论文首先简要地介绍了微波加热机理和三种微波合成技术应用,以及微波液相合成无机纳米材料和微波固相烧结陶瓷材料的研究进展。 采用微波液相加热技术,先后直接制备出具有闪锌矿结构的ZnS和立方相的CdS纳米晶材料,以及的SnS₂超细粉体。TEM结果表明ZnS颗粒有团聚现象,CdS颗粒尺寸达到纳米级,形貌均匀。采用硫代乙酰胺（TAA）和聚乙烯吡咯烷酮（PVP）为原料,通过微波加热,合成出300～500 nm的六方相SnS₂颗粒。 研究ZnS的光致发光性能（PL）表明:（1）微波加热时间对PL发射光谱强度有影响,这可由Stokes位移解释。（2）相对于纯的ZnS而言,掺杂离子Ag⁺,Cu²⁺和Sn⁴⁺（掺杂摩尔量分别为0.2%）使得发射光谱的强度减弱,这有可能是这些离子对光复合辐射过程有猝灭作用。掺0.2%Ce³⁺的ZnS的PL发射光谱强度最强,这可能增强了光复合辐射过程;但当掺杂量提高到0.4%时,发射光谱强度变弱。（4）掺杂ZnS的发射峰中心位置相对于ZnS的没有变化,说明掺杂离子没有在ZnS禁带中形成发光中心。 研究了在紫外光光照之下产物CdS对甲基橙溶液的光催化性能,研究结果表明:（1）在酸性条件下（pH=2.0）和温度为20～30℃的条件下,CdS有利于甲基橙溶液的光催化。随着溶液的pH值和温度的不断增高,CdS光催化能力下降。这是由于温度高时,CdS溶解程度加大,更多的金属隔沉积在半导体表面,阻止空位与溶液的接触,也就阻碍了光催化反应;另一方面,CdS溶解程度加大,也就造成了更多的空位载流子的消耗,导致半导体缺少足够的空位载流子来促进有机物的氧化。而在强酸性下,空位载流子的氧化能力可能变得更强。（2）回收利用的CdS光催化性能降低;这是由于CdS不稳定,在空位离子的攻击下,金属单质Cd不断地沉积在CdS颗粒表面,从而阻碍了半导体的光催化活性。（3）其他的条件,如半导体剂量,搅拌速率,光源等都影响CdS半导体光催化效率。 本论文研究了PVP阻止SnS₂颗粒团聚的机理。PVP聚合物骨架上的N和O原子可能首先与锡离子发生杂化,生成一种复杂化合物。聚合物的乙烯骨架的空间效应可能阻止了SnS₂颗粒间的团聚。 采用微波加热辅助方法,间接地制备了纳米二氧化铈。实验发现通过500℃2h热处理后,样品的形貌由热处理前的棒状转为球状,而且颗粒的大小达到纳米级。随着温度的增大,晶粒也在长大;温度从300增加到700℃时,纳米晶平均的粒径从5nm变到27nm。温度在500～600℃期间,纳米晶生长迅速。 微波液相加热技术为合成硫化物纳米晶材料和超微细粉体提供了一种方便、更多还原

【Abstract】 In this thesis, the field of microwave chemistry is firstly reviewed. Both theories of microwave heating, which are polarization and conduction mechanisms, and development of microwave synthesis technique, which includes microwave refluxing system, microwave high-pressure system, microwave continuous flow system, as well as the applications of microwave radiation to prepare inorganic nano-materials in liquid phase and ceramics in solid state are briefly introduced.In this work, a domestic microwave oven （Galanz, WP900） was modified with refluxing system and used to directly prepare three metal sulfides, such as, ZnS and CdS nanocrystals, and well-dispersed ultrafine SnS₂ particles in one step. .Both of Zinc-blende ZnS nanocrystals and cubic-phase cadmium sulfide nanocrystals with crystal sizes of less than 10 nm, have been successfully synthesized via microwave irradiation in liquid phase, using corresponding metal salts and sodium sulfide as starting materials. Moderate agglomeration for ZnS powder with the size as large as ca. 20 nm has been observed by transmission electron microscope （TEM）. Morphologies of CdS powder are homogeneous. Using thioacetamide （TAA） as sulfide source and polyvinylpyrrolidone （PVP） as protective reagent, well-dispersed PVP-capped SnS₂ ultrafine particles with narrow size distribution at ca. 300⁵00 nm, have been successfully prepared in ethylene glycol solvent via this simple and convenient method.Physical properties of as-prepared ZnS have been studied. The Ultraviolet-visible （UV-vis） absorption spectrum shows that the size of ZnS nanoparticles is not significantly influenced by the microwave irradiation time and however there is blue shift existence for product’s absorption edges. The photoluminescent properties of pure and doped ZnS nanocrystals can be concluded as following: （1） The photoluminescent properties of pure ZnS nanocrystals are affected by the microwave irradiation time, which may be related to its surface states and can be also explained by Stokes shift. （2） It has been found that when ZnS nanocrystals doped with such metallic ions as Ag⁺, Cu²⁺ and Sn⁴⁺（0.2% molar ratio）, respectively, their emission intensities are weaker than that of pure ZnS due to the enhancement of nonradiative recombination in luminescent process. （3） The intensity of ZnS nanocrystals doped with 0.2% Ce3+ is stronger than that of ZnS nanocrystals doped with the above metallic ions at the same molar ratio and that of pure ZnS nanocrystals. But when the Ce3+ molar ratio is up to 0.4%, the relative emission intensity was found to decrease, also due to the enhancement of non-radiative recombination. （4） All peaks’ emission wavelength is 450 run, indicating that the luminescent centers of various metallic ions are not formed in the doped ZnS nanocrystals obtained by this method.The photocatalytic oxidation of methyl orange （MeO） in suspensions of CdS powder under UV-illumination was studied. The photocatalytic mechanism is explained that the direct oxidation of MeO by positive holes absorbed on the surface of CdS was expected to be the main oxidation pathway. The photocatalytic results show as following: （1） The low pH value （pH=2.0）, and the temperature between 20-30°C are suitable for the degradation of MeO solution. However, the degradation efficiency decreases with the increase of pH value and solution temperature. It is explained that high temperature enhances the dissolution of CdS. As a result, more amount of Cd will deposit on the surface of CdS to prevent photocatalytic reaction and at the same time, there are so many positive holes consumed that it will lead to reduce photocatalytic reaction’s degree. On the other hand, lower pH value may enhance oxidation of positive holes. （2） Efficiency of recycled CdS semiconductor becomes lower due to the elemental Cd deposit on the surface of CdS, which blocks the photocatalytic reactivity. （3） Other factors, such as concentration of initial MeO solution and semiconductor dosage, speed of stirring, as well as light source influencing on the photocatalytic reaction are also studied.The mechanism of PVP preventing SnS2 particles from aggregation and the role of ethylene glycol solvent on the crystal growth have been investigated. PVP has a structure of a polyvinyl skeleton with polar groups. It is assumed that the donated lone pairs electrons of bothnitrogen and oxygen atoms in the polar groups of one PVP unit may occupy one sp2 orbital of the tin ion to form a complex compound. The steric effect from polyvinyl skeleton of PVP on the surface of SnS2 particles might contribute to the anti-agglomeration.Microwave heating is an assisted-synthesis method to prepare metal oxide powder. Nanocrystalline CeC>2 particles have been successfully prepared by microwave process and subsequent thermal treatment. It has been found that the rod-like CeO2 nanoparticles ean completely change to sphere-like nanoparticles with narrow size distribution by conventional thermal treatment. The average crystal size of the CeO2 nanoparticles ranges from about 5 run to 27 nm at 300⁷00°C. Nanocrystallites grow rapidly in the range of 500⁶00°C. The calculation of the activation energy （￡’=20.57 kJ/mol） indicates that CeC>2 nanocrystallites grow during thermal treatment mainly by means of an interfacial reaction.The method of microwave-heating synthesis has been proved to be a convenient, efficient and environmentally friendly one step route to produce metal sulfide nanocrystals and untrafine particles. It posses the advantages of avoiding thermal gradient offering homogeneous environment for growth of crystal nuclei in liquid phase. Combination with further conventional heating, microwave-heating, as an assisted -synthesis method, can be used to prepare metal oxide. This method shows great potentials in the preparation of high-performance metal compounds.更多还原