Study On The Synthetic Mechanism And Coating Of Nanoparticles

Recent years,much effort has been taken on the modification of the nanoparticles in the material area with the development of nanotechnology.In the thesis,the nanocrystals such as Cu₂O,Au and CdTe were decorated with Au,SiO₂,TGA in aqueous solution,respectively.The formation mechanisms were proposed and their optical properties were investigated.The major contents are as follews:1.The Cu₂O nanocubes and nanospheres were firstly synthesized by using PVP as surfactant in aqueous solution under low temperature.Furthermore,we have synthesized Cu₂O@Au heterostructures by heterogenerous seeded growth process using Cu₂O nanocubes as seeds.The structure and morphology of the crystals were proposed and the optical properties were studied.The formation mechanisms of the Cu₂O nanocubes and nanospheres were as follows:The selective interaction between the oxygen atoms of the carbonyl group of PVP and the {100} planes of the Cu₂O nanocrystal could favor the preferential growth along＜111＞direction and make it far exceed that of＜100＞,thus {111} faces shrink.Cu₂O nanocubes were obtained.The increase of viscosity in the reaction system,which might cause the diffusion of ions more difficult and suppress the anisotropic growth of Cu₂O nanoparticles.Finally,the spherical nanocrystals were achieved.The reasons for the formation of Cu₂O@Au heterostructures were as follows:(1) The growth of Au on the Cu₂O nanocubes could decrease its surface energy,and thus the total free energy.(2) The {111} and {200} interplanar distances of Au with a cubic structure are about 2.3(?) and 2.0(?),which are close to the value(d₁₁₁=2.4(?) and d₂₀₀=2.1(?)) of the cubic Cu₂O,respectively.That is an advantage to the growth of Au on the Cu₂O nanocubes surfaces and the formation of the Cu₂O nanocubes-Au heterostructures.It is well known that the smaller sized particles will show a stronger absorption duo to the higher oscillator strength.The absorption spectra of Cu₂O nanocubes has a 36 nm blue shift towards higher energy compared to that of Cu₂O nanospheres.The broad absorption peak from 500～800 nm in the absorption spectra of Cu₂O@Au heterostructure confirms the formation of Au⁽⁰⁾. 2.The highly dispersed Au nanocrystals were synthesized by reducing[AuCl₄]^-with sodium citrate in the aquous solution and using SDS as the surfactant.The SiO₂ coated Au heterostructures were also prepared.The crystal structures were proposed and the optical properties were investigated.The main reaction may also be simplified and easily controlled at high temperature,because the reduction of gold precursor is a multistep process from Au(â…¢) to Au(â… ) and eventually to Au(0).The relatively higher temperature is an advantage to the growth of the Au nanocrystals by forming more nucleations and causing the diffusion of ions much easier in the solution.Hence,the Au nanocrystal of larger size and good morphology was obtained under higher temperature. The SiO₂ coated the surfaces of the Au nanocrystal improved its solubility in aquous solution and enhanced the optical property of Au nanocrystal.A silica shell coated on the bare Au nanoparticles also leads to an increase of the plasmon absorption and a red shift in the position of the maximum absorption.This is due to the fact that the local refractive index around the particles increase after the formation of silica shell.3.The TGA capped CdTe QDs were synthesized successfully in the aqueous solution.Various parameters in the synthesis process were studied.The absorption spectra of CdTe QDs showed a slightly red shift due to the increase of the size caused by a higher concentration of the reactants and a longer reacted time.An increase of pH value of the initial solution is followed by a considerable acceleration of the CdTe nanocrystals(NCs) growth.A relatively fast NCs growth leads to an insufficient quality reflected in low crystallinity of the resulting particles and a large amount of defects and surface states,thus,the absorption spectra of the CdTe QDs showed a red shift compared to the absorption spectra of CdTe QDs synthesized under a lower pH value. Moreover,we prepared the lindgrenite nanocrystas with various morphologies in aqueous solution under a relatively lower temperature by improving the synthesis process.The formation mechanisms were proposed and the optical properties were investigated.