Construction of complex nanostructures in solution phase
Date of Issue2013
School of Physical and Mathematical Sciences
In this thesis, the main work explored a new methodology for preparation of complex nanostructures in solution phase. There are two strategies, namely 1) block copolymer assisted nanoparticle (NP) self-assembly and 2) oil/water in-terface assembly. Based on these strategies, we successfully developed 1) core-shell system, which utilized amphiphilic block copolymer as shell and a wide range of inorganic NPs as core materials; 2) emulsion system, which uti-lized emulsified oil droplets as soft templates to compress nanofilaments into nanoring structures. At first, we described core-shell systems in chapter 2. Various NPs includ-ing metal, metal oxide and semiconductor NPs, could be encapsulated within uniform polymer shells. Mechanism studies demonstrated that one block of the polymer is extremely hydrophobic and the other is extremely hydrophilic, so that the polymer forms micelles including hydrophobically functionalized NPs. The micelles are covered with long ionic PAA chains in extended conformation, which introduces charge and steric repulsion against aggregation. After encap-sulation, we further demonstrated that the presence of polymer shell maintained structural integrity of nanoclusters, allowing them to be isolated, enriched and then characterized. Anisotropic polymer coating was also achieved for both hy-drophobic and hydrophilic NPs. Triblcok copolymer P4VP-PS-P4VP encapsu-lation achieved demonstrated our polymer encapsulation was applicable to other block copolymers. In chapter 3, based on the established core-shell system, we successfully prepared Au nanosprings by manipulating ultrathin AuNWs inside polymer shells. The ultrathin AuNWs were dispersed in THF/DMF/water mixture and coated with a layer of PSPAA shell. A large amount of water was then added to the solution to de-swell the polymer shells. This led to a significant increase of polymer/solvent interfacial tension and the contraction of the polymer na-no-droplet. Thus, as the cylindrical polymer shells transformed to become a spherical droplet, the incorporated AuNWs were forced to coil into rings. The obtained Au nanosprings uncoiled when the polymer shells were removed or swelled by organic solvent. Obviously, these results suggested that elastic po-tential energy of the nanosprings was converted back to kinetic energy. This was the first evidence of energy storage in colloidal NPs. Emulsion system was also utilized for assembly of complex nanostructures (Chapter 4). We found that the oil droplets could exert sufficient force to straight bundles of AuNWs to compress them into perfect nanorings. The me-chanism could be explained as solvent-shifting process. Surfactant greatly in-fluenced the morphologies of final rings. As a proof of concept, we successfully achieved Au@Ag@Ppy nanorings by using as-synthesized Au nanorings as templates. Furthermore, Au nanoring composed of close-packed monolayer free-standing films were achieved by simply using Ppy modified Au nanorings self-assembly at water/oil interface. Finally, in chapter 5, we demonstrated that the as-synthesized Au nanor-ings could be fused into coherent Au nanorings. The method was very simple. Heating as-synthesized Au nanorings together with reductant in SDS aqueous solution for a short time gave fused rings. Importantly, we found that the fused nanorings had single-crystalline dominant structure, and a few crystal defects presented within small range. Further studies revealed that single crystal parts of nanoring had the same lattice orientation with as-synthesized Au nanorings. Zipper mechanism was proposed for explanation of lattice orientation preserved fusion, which was confirmed by perpendicular growth of Ag nanorods on fused nanorings.