The Synthesis, Assembly, And Property Of Core-shell Structured Noble Metal Nanomaterials

Noble metals have received wide interest due to their excellent physical and chemical properties, which have been applied in optical, mechanical, magnetic, electrical, catalytic and sensing fields. It is well-known that changing the shape of a metal nanometerials can profoundly alter its properties and thus its performance in a given application. The synthesis of metal nanometerials with well-controlled shapes and sizes has been motivated by both the potential enhancements new materials which may bring to industrial applications and the prospect for new technology development. On the other hand, core-shell structured noble metal nano-composites possess excellently catalytic, optical properties and high stability compared to the bulk metal nanoparticle. Especially, the nanostructures were used in the field of surface-enhanced Raman scattering, single molecule detection and biosensors, thus the preparation of core-shell structure nanocomposite became the latest hot research field of nanomatericals. In this paper, silver and silver/silica nonmaterials were prepared by sol-gel method, the structure, morphology and properties of as-prepared nanomaterials have been studied. The main contents and results achieved in this paper were listed as follows:1. The nanoparticles, nanorods and other morphologies of silver nanomaterials have been synthesized by polyol process under different reaction times, temperatures and proportion of the reactants. In this reaction, the polyvinylpyrrolidine (PVP) served as protective and structure directing agent, the ethylene glycol as solvent and reducing agent, reducing the silver ions to silver nanoparticles in the solution. The morphologies, sizes and crystalline structures of as-prepared samples were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD). And the possible formation mechanism of five-fold twinned nanorods was discussed.2. Core-shell structured SiO₂@Ag microspheres were prepared by a sol-gel method. Firstly, the uniform size of 230nm SiO₂ microspheres were synthesized by St?ber method, and then the chemical reduction method was used to obtain the silver layer. In this reaction, PVP as protective agent and reducing agent, this two-step method is the simplest method in preparing core-shell structure. The as-prepared sample was characterized by TEM and XRD, which showed that the surface of SiO₂ were coated by a silver layer with size in 5-20nm, and the silver was face-centered cubic structure. In addition, centrifugal deposition, evaporation-induced method and vertical deposition method were used to prepare SiO₂ microspheres and core-shell structured SiO₂@Ag microsphere. Compared with these three methods, the vertical deposition method is the best method to achieve high quality photonic crystals.3. Ag@SiO₂@Ag sandwiching nanostructures were prepared by a facile one-pot synthesis method. In this reaction, polyvinylpyrrolidone (PVP) served as the gentle reducing and protective agent. The PVP, as a smart reducing agent, can reduce the Ag+ to Ag core at high temperature and reduce Ag(NH3)2+ to Ag nanoparticle shell at room temperature separately in a one-pot reaction. Further, the PVP can prevent the aggregation of the silver nanoparticles. The SiO₂ shell and outer-layer Ag nanoparticles were obtained when tetraethyl orthosilicate and ammonia were added into the silver core solution. Ammonia accelerates the hydrolysis of the tetraethyl orthosilicate into SiO₂ as the catalyst. Moreover, when aqueous ammonia was added into the solution, the Ag(NH3)2+ ions were formed, which can promote the PVP reduced Ag(NH3)2+ ions to small Ag nanoparticles on the surface of the Ag@SiO₂ at the ambient temperature, and then Ag@SiO₂@Ag sandwich structures were formed. We have prepared Ag@SiO₂@Ag nanoparticles in different size by controlling the amount of TEOS, and its optical properties and catalytic activity were studied at last. Because the outer layer of silver nanoparticles are small, which were less than 5 nm, this structure shows high catalytic activity.