Study On The Synthesis And Optical Properties Of Gold And Silver Nanostructures

The evolution and revolution of all modern technologies strongly depend on the improvement of existing materials and the development of new materials. In the hot research topic of nanomaterials, noble metal (especially for gold and silver) nanostructures have attracted particular attention because of their unique optical, electric, catalytic properties and the promising applications in the fields of new energy materials, optoelectronics, information storage, biomedicine and surface-enhanced Raman scattering. Recent investigations demonstrate that their properties are strongly depended on the size and shape of metal nanoparticles. Therefore, it is important to get the shape-controlled noble metal nanoparticles and design the functional materials. Although there have been a lot of reports on the synthesis of nanomaterials, the shape-controllable synthesis is still a challenge for materials and chemistry researchers. Based on the development of noble metal nanoparticles in the world, this dissertation presents a facile chemical solution method to prepare silver and gold nanoparticles with various morphologies and some nanostructures with novel shapes. Structures, morphology and spectra characterization were carried out by field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electronmicroscopy (HRTEM), X-ray diffraction (XRD) and UV-visible spectrometer (UV-Visble). The chemical reaction and possible growth mechanism were also proposed. The main results of this work are listed as follows:1. Silver nanocubes with perfect shape were prepared by a simple poly(vinyl pyrrolidone, K30, MW=44,000)-directed polyol synthesis process. Ag nanocubes with an average edge length of230nm were obtained successfully with sharp edges and corners under a precise synthesis condition of R=1and T=150℃. The effects of poly(vinylpyrrolidone)/AgNO3ratio R and reaction temperature T on the morphology and size of the products were investigated. The optical properties of Ag nanocubes show an attractive plasma resonance red-shift with size in a wide spectra region. The growth mechanism of the Ag nanocubes is proposed to be thermodynamically and kinetically controlled.2. Polyvinyl pyrrolidone (PVP) with different molecular weights was used as capping agent to synthesize silver nanowires through a polyol process. The results indicated that the yields and aspect ratios of silver nanowires were controlled by the chain length of PVP and increased with increasing the molecular weight (MW) of PVP. When the long-chain PVP-K90(MW=800,000) was used, the product was uniform in size and was dominated by nanowires with high aspect ratios. The growth mechanism and optical properties of the nanowires were studied. It is proposed that the chemical adsorption of Ag+on the PVP chains at the initial stage promotes the growth of Ag nanowires.3. A mixture of Ag nanowires and nanoparticles was obtained by an improved polyol process with the usage of PVP-K25(MW=38,000). Results of Surface-enhanced Raman scattering indicated that the enhancement factor of the wires/particles mixture is larger than that of Ag nanowires and nanoparticles of the mixture. This enhancement should ascribe to the coupling effect of nanowires and nanoparticles. A novel vertical growth process of silver nanowires was revealed by tracing the morphology evolution of Ag nanostructures, in which Ag nanoparticles undergo an important dissolution-recrystallization process and act as fuel for the lateral growth of nanowires. A plausible novel growth mechanism for the silver nanowires was proposed, in which the collection of Ag nanoparticles along the same direction leading to the vertical growth process of Ag nanowires.4. Gold nanoplates (tens of nanometers in thickness and micrometers in size) have been mass-synthesized through a polyol process. In general methods, most Au nanoplates are regular shapes of triangle and hexagon. The suitable growth conditions for gold nanoplates was studied by changing temperatures and the molar ratios of PVP/HAuCl4. Large sized gold nanoplates with size of～50um were fabricated by a solvent-thermal method.5. Au nanoplates with novel shapes, involving star-like and shield-like and other novel polygonal are found with introduction of obvious temperature variation in the early crystal growth stage. Structural studies demonstrate that the obtained Au nanoplates are single-crystalline with(111) planes as two growth of (111) plane along <110>,<211> and other high-index directions.