Microwave Synthesis And Properties Of Metal Sulfides And Oxide

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 (ï¿¡'=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.