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Controllable Synthesis Of The Citrate Caped Silver Nanoparticles

Posted on:2010-01-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y DongFull Text:PDF
GTID:1101360302465842Subject:Polymer Chemistry and Physics
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Noble metal nanoparticles have attracted a great deal of attention during the past decades due to their unique electronic and optical properties and good chemical stabilities. The synthesis of noble metal nanparitcles and the utility of them as building block for bio-sensor and other devices are major subject of nanotechnology researches today.Silver nanoparticles, as an important member of the noble metal nano-materials, have also been in-depth studied. Great efforts have been devoted to control over the size and shape of silver nanoparticles since it is well documented that their properties are both size- and shape-dependent. Though various shapes and sizes of silver nanoparticles could be well synthesized in nonaqueous solution, the synthesis in aqueous system is still a tremendous challenge. From a green chemistry standpoint, synthesis of silver nanoparticles in aqueous solution is necessary. Synthesis of silver nanoparticles by citrate reduction is an ideal model system, this method had been widely investigated and developed since it was first invented by Turchevich because the citrate is well consistent with the bio-molecules. On the other hand, citrate is a weak ligand, and it is facile to be exchanged with other ligands. Synthesis of gold nanoparticles by the citrate reduction method has been well developed, while the synthesis on silver nanoparticles is still under exploring.In this dissertation, we selected the citrate reduction of silver nitride method as a model system. The kinetic factors which affecting the particle growth were studied in details. New synthesis strategy has been designed according to the nucleation-growth-ripening theory, and shape/size-controlled citrate caped silver nanoparticles were obtained.This dissertation includes three parts:1. we first did research on the following factors, such as reductant concentration, pH value, and reaction temperature, which affect the reaction rate and final shape\size of nanoparticles. Ultimately, we demonstrated that pH value played a crucial role in controlling the reaction rate and the final particle shape and size. We also identified there reaction steps of this synthesis, which are nucleation, growth and particle ripening. pH value can affect the redox rate of this system and through which affect the nucleation and growth rate. According to the well-known LaMer model, a stepwise reduction method, in which the nucleation and growth stages were carried out at high and low pH, respectively, is proposed. The shape control over the spherical silver nanoparticles is improved greatly due to the improved balance between nucleation and growth stages in the stepwise method.2. Encouraged by the stepwise reduction method, we adopted NaBH4 to control over the nucleation stage of reaction, and using sodium citrate as a reducing agent in the growth process. The particle size was decreased with the increasing of NaBH4 concentration due to the increasing concentration of silver nuclei reduced by NaBH4. By adjusting the molar ratio of the two reducing agents, the silver particles with various sizes (from 20 nm to 50 nm) were obtained. TEM images showed that the size distribution of the silver nanoparticles was also improved to around 6%.3. Using a co-reduction of NaBH4 and citrate under the lower AgNO3 concentration of 1×10-4 M, we prepared triangular silver nanoprisms with uniform thickness and various edge lengths. Through the study of thermo synthesis method, we proposed triangular sheet transmission mechanism. Also, we ascertained the two decisive factors that impact the transformation from spherical silver nanoparticles transforming into triangular nanoprisms: spherical particle size and the residual silver precursor. The transformation started when the concentration of AgNO3 was about 40% left, and kept 5×10-6 M for all through the transformation process during the 20 to 50 hours. Meaning an inter-particle Ostwald ripening process took place. Further investigation shows that the triangular silver nanoprisms could be synthesized only in the range of the NaBH4 concentration of 5×10-7 M to 5×10-5 M. Moreover, by adjusting the ratio of reductant, we could control the side length of triangular silver nanoparticles, and obtain a series of triangular silver nanoparticles with various sizes.
Keywords/Search Tags:citrate, NaBH4, spherical silver nanoparticles, triangular silver nanoprisms
PDF Full Text Request
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