Study On The Synthesis And Plasmonicproperties Of Silver Nanoplates

Surface Plasmon Resonance (SPR) of noble metal nanostructures has been intensively studied for their wide applications. As the oscillation of electrons is sensitive to the size and shape of noble metal nanostructures, many efforts have been therefore devoted to the size and shape-controlled synthesis of noble metal nanostructures with desirable plasmonic property. However, a lot of researchers have been working on the controlled synthesis of silver nanoplates, an acceptable and detailed explanation about the experimental phenomena is not yet proposed. The lack of understanding in the synthetic system makes it difficult to reproduce most of the reported results with satisfactory yield and quality. In this thesis, we explored the plausible mechanisms underlying the controllable synthesis of silver nanoplates and control the SPR properties of silver nanomaterials. This thesis mainly includes the following aspects:We have carried out systematic studies in the direct chemical reduction route to silver nanoplates and identified the specific roles of each reagent. Contrary to the previous conclusion that trisodium citrate (TSC) is the crucial component, we instead found that hydrogen peroxide (H2O2) plays an even more important role in determining the shape evolution into plates. As a strong oxidant, H2O2favors the production of nanoplates by inducing planar twinned defects and removing other less stable structures. By controlling the oxidative strength of H2O2, various silver sources including metallic silver can now be directly converted to silver nanoplates with the assistance of an appropriate capping ligand. We also determined the list of ligands with selective adsorption to Ag (111) facets can be expanded from irreplaceable TSC to many di-and tricarboxylate compounds whose two nearest carboxylate groups are separated with two or three carbon atoms. We found that polyvinyl pyrrolidone (PVP) as surfactant can be replaced by many hydroxyl group-containing compounds or even removed entirely while still producing nanoplates of excellent uniformity and stability. In addition, sodium borogydride (NaBH4) can act as reducing agent, also capping agent to stabilize the Ag nanoplates. By tuning the concentration of NaBH4, we are able to control the thickness as well as the aspect ratio of silver nanoplates.By diluting the seed solution in the seed-growth stage, we explored the role of the aging process between nucleation and growth step. Aging process eliminates undecomposed NaBH4in the nucleation step and prevents free silver nanoparticles from producing in the growth step. By introducing H2O2into the seed-mediated growth reaction, the yield of silver nanoplates was improved and the aging time was reduced. We found that PVP can delay the reduction of silver ions, which deposit selectively on the (111) faces, leading to thicker silver nanoplates with lower aspect ratios. Based on these understanding, the synthesis procedure has been successfully improved to significantly shorten the reaction time and the synthetic efficiency was increased, thus enables a large-scale synthesis.The plasmonic properties of Ag nanostructures depend on their physical parameters. The optical characteristics of silver nanomaterials were analyzed by UV-vis spectroscopy. We tuned the SPR properties in the range of400-900nm of Ag nanoplates by controlling the size of Ag nanoplates using seed-mediated method after tuning the physical parameters including shape (nanoplate, nanowire and nanoparticle), the conner sharpness of silver nanoplates, the size (silver nanoparticles and nanoplates) and the composition (siver nanoparticles and core-shell structure).