Separation And Transformation Of Gold Nanostructures

Close interaction between the outer electrons makes gold chemicallystable and hard to be oxidized. Therefore, gold always exits as elementalstate in the minerals. The position of gold in the periodic table indicatesits special properties. In the1800s, Michael Faraday found that a solutionof nanometer-sized gold particles was red in color, unlike the yellowcolor of bulk gold, and that varying the size of the particle s could changethe color of the colloid. With the development of nanotechnology, manyspecial properties of gold nanostructures such as surface plasmonresonance (SPR), surface-enhanced Raman scattering (SERS), electricaland catalytic properties have been demonstrated, which make goldnanostructures potential candidates in many areas such as nanoelectronics,sensors, bio-medicine, drug delivery, biological imaging, photothermaltherapy, catalysis, analysis, conductivity and heat transfer, etc. Recently,research on gold nanostructures has been a hotspot. The properties of gold nanostructures closely related to their composition, size andmorphology, however, fabrication of monodisperse gold nanostructurealways need strict synthetic conditions and synthesis of goldnanocomposite lacks accurate control of the composition. Thus, researchon the fabrication of monodisperse gold nanoparticles with controlledcomposition has been a research direction. In this thesis, we demonstratea density gradient rate separation method to obtain monodisperse goldnanoparticles along with a chemical transformation of goldnanocomposite. Details are illustrated as follows:1. Density gradient rete separation of gold nanostructures inorganic mediaWe expanded a pure liquid phase, no damage and wide applicableseparation method, density gradient rete separation, into organic media.Through this method, non-pretreatment gold nanoparticles with surfactantmolecules attached on the surface could be isolated by size and severalmonodisperse gold nanoparticles with certain diameters could be obtained.This method could also be applied to ultra-fine goldnanowire/nanoparticle system. Large aspect ratio of ultra-fine nanowireincreased its friction and made it stayed on the top of the gradient, whilethe nanoparticles fell down and thus separation achieved. Themonodisperse gold nanoparticles obtained after separation could assembleinto superlattice at the interface of ethanol and cyclohexane. Furthermore, the success on separation of gold nanostructures could be expaneded tolots of other nanostructures like noble metals (Ag), semiconductors (CdSe,CdS) and magnetic nanoparticles (Fe₃O₄, Au/Co, Au/Ni) and so on.Specially, in the separation CdSe nanoparticles, we used relativelymonodisperse nanoparticles, after separation, the half peak width of theoriginal "monodisperse" samples in the fluorescence was successfullyreduced from100nm to50nm, which confirmed the efficiency of densitygradient rete separation. We have also introduced polymers into thegradients to increase the viscosity; results indicated that viscosity increasecould enhance the separation efficiency. Finally, CdSe nanoparticles wereused as probe molecule to study the influence of viscosity and slope ofthe gradient.2. Transformation of gold nanostructuresAu–Ni spindly nanostructures were synthesized using the methodreported by Wang and Li. Many reports have shown that halogen ionsplay important roles in synthesis and conversion of noble metalnanoparticles by interfering with the reaction process. Here we introducedDDAB and OA into the Au-Ni system to study the chemicaltransformation of Au-Ni heteronanostructures. Different ligands results indifferent reactions. When increasing quantities of OA-containingHAuCl4·4H2O solutions were gradually added to solutions of the Au–Ninanoparticles, the Au tips grew larger while the Ni tails gradually dissolved. However, when OA was replaced by DDAB, the results werecompletely different. As the amount of DDAB-containing HAuCl4·4H2Osolution was increased, the Au tips first shrank and then vanished leavinga bowl-like structure. Subsequently the Ni tails dissolved leaving asemi-filled sphere within which new Au cores were generated. Finally,the newly generated Au cores mostly disappeared again, leaving hollownanospheres. HRTEM, UV-Vis spectroscope, EDS and a series of controlexperiments helps us to propose a mechanism of the two differenttransformation routes: we propose that the rapid reaction between Au–Niand the DDAB-containing HAuCl₄·4H₂O solution leads to the initialdissolution of the Au tips, while the NiO layer prevents any redoxreaction between Au³⁺and Ni, resulting in selective etching of the Au.However, such etching of Au at the "neck" sections of the original Au–Niheterostructures eventually exposes fresh Ni metal not protected by alayer of NiO, and when larger quantities of DDAB-containingHAuCl4·4H2O solution were added, etching of the exposed Ni metal leadsto a "bowl-like" Ni tail with a NiO layer. The new Au cores generatedfrom the galvanic replacement reaction between Ni metal and Au³⁺aremostly located at the openings/cores of the "bowls". Eventually the Nitail dissolves completely leaving Au cores inside or attached to the NiOshell. Finally, the redox reaction between the Au cores and theDDAB-containing HAuCl₄·4H₂O solution restarts until most of the regenerated Au cores redissolved again. The ability to tailor thecompositions and structures of heterostructures with low symmetryshould benefit finely manipulation on their properties and lead to newbuilding blocks for the construction of new functional nanomaterials.