Capping Agent Controlled Syntheses And Properties Research Of Several Kinds Of Nanoparticles

Magnetic and semiconducting nanoparticles with a size of 2-120 nm are of great importance in the fields of fundamental research and practical applications since their unique physical and chemical properties, and they have been studied interestingly. In this thesis, the morphologies controllable syntheses of hematite nanomaterials with sizes below 20 nm were achieved, meanwhile, the reaction process was monitored, and the magnetic and optical properties were studied. In addition, a novel manner was developed to obtain water-soluble cubic Ag₂Se semiconductor nanocrystals by cation-exchange reaction under ambient conditions, using citrate as a stabilizer, finally, the various complex Bi₂S₃ nanostructures were synthesized, and the effect of different experimental parameters on the evolution of the morphologies was explored in ethylene glycol solution with polyvinylpyrrolidone (PVP) as a stabilizer, thiourea and mercaptopropionic acid (MPA) acting as sulfur sources, respectively. The main results can be suammarized as follows:1 hematite nanocrystals with various shapes and sizes (naocubes, nanorods and nanoparticles, etc.) were obtained by hydrothermal and microwave-heating treatment. The experimental results showed that the reaction conditions (the molar ratio of oleate/Fe source , temperature, etc.) had an important influence on the preparation of hematite nanocubes. It favored the formation of nanorods or nanowires if the mixtured capping agents of oleic acid and hexadecylamine were used under hydrothermal conditions, and uniform hematite nanoparticles of ca. 4 nm could be obtained by microwave-heating treatment. the magnetic and optical properties of the nanoparticles below 20 nm were related to their sizes and shapes, the UV-Vis absorption of theirs mainly lay in the far-ultraviolet band, and focuses around 230 nm and 270 nm with the morphology turning into regular nanocubes. The blocking temperature (T_B ) was proportional to the average size of the nanoparticles, T_B was 20 K for 4 nm, and 80 K for 14.7 nm, the nanoparticles were supermagnetic at room temperature.2 Water-soluble cubic structure Ag₂Se (α-Ag₂Se ) nanocrystals less than 5 nm could be obtained by cation-exchange reaction at room temperature, using water-dispersed ZnSe nanocrystals as precursors, which was achieved by controlling the speed of the injection of AgNO₃ solution via a syringe pump in the presence of the stabilizer of trisodium citrate. Meanwhile, the thermal stability of the product Ag₂Se nanocrystals was studied. The results showed that the mean size and shape of the precusor ZnSe and product Ag₂Se nanocrystals were similar, and Se anion sublattices between them were topotaxial. In addition, no phase transition was observed for the product Ag₂Se (cubic structure) nanocrystals below 180℃. This cation-exchange manner could also be applied to the syntheses of PbSe and CuSe nanocrystals.3 complex Bi₂S₃ nanostructures were synthesized, and the effect of various experimental parameters on the evolution of the morphologies were investigated in ethylene glycol solution with polyvinylpyrrolidone (PVP), thiourea and mercaptopropionic acid (MPA) acting as stabilizer and sulfur sources, respectively. When thiourea was used as sulfur source, the products with sea urchin-like nanostructure composed of nanorods were obtained , and if pH value in the solution was increased, the nanostructure evolved into nanobelts with the change of the nanocrystals growth direction from [001] to [010], meanwhile, the purity of the products was also influenced. When MPA serves as sulfur source, the addition of the chelating agent ethylenediamine (EDA) made the flower-like product consisting of short nanorods (30 nm×1μm) turn into quasi-dispersed relative long nanorods (40 nm×30μm), and these nanorods grew along [110] direction with no change of the growth direction, which showed the chelating EDA was more favorable for the growth of the nanorods along the long axis. In addition, the UV-Vis absorption spectra of the corresponding products were characterized.