Ag, Au, Pd Nanoparticles--Ligand Design, Synthesis and Polymer Composites
Abstract
Metal nanoparticles, especially gold nanoparticles (AuNPs), have been extensively studied due to their interesting optical properties and potential applications in emerging technologies like drug delivery, cancer therapy, catalysis, chemical and bio-sensing and microelectronics devices. Alkyl thiol ligands in the form of self assembled monolayers are often used to stabilize and functionalize the gold nanoparticles while other types of ligands have been rarely employed and the properties of AuNPs protected by different types of ligands have not been studied comprehensively and comparatively. This dissertation reports the first comparative studies on the thermal and chemical stability of AuNPs protected by alkyl thiolates, alkyl selenolates, dialkyl dithiophosphinates, and dialkyl dithiophosphates (Chapters 2 and 3). AuNPs protected by dialkyl dithiophosphinates and dialkyl dithiophosphates are unprecedented. All AuNPs were prepared from amine protected precursor AuNPs by ligand exchange to ensure similar size, size distribution, and chemical composition. They were extensively characterized by solution 1H-NMR and UV-VIS spectroscopy, transmission electron microscopy (TEM), thermal analysis, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) analysis. For the first time, thermal stability was investigated by differential scanning calorimetry (DSC) that provided more accurate decomposition temperatures and enthalpies, whereas chemical stability was tested as the availability of the gold surface towards etching with cyanide in different solvents. Surprisingly, alkyl selenolate protected AuNPs are thermally less stable than alkyl thiolate protected AuNPs despite their proposed stronger binding to the gold surface and a much more crystalline monolayer, which suggests that different decomposition mechanisms apply to alkyl thiolate and alkyl selenolate protected AuNPs. Dialkyl dithiophosphinates and dialkyl dithiophosphates protected AuNPs are thermally least stable, although dialkyl dithiophosphates bind as bi-dentate ligands and are most stable towards cyanide etching. In contrast, dialkyl dithiophosphinates show mixed mono- and bi-dentate binding that generates loosely packed monolayers of low degree of crystallinity. Another part of this thesis (Chapter 4 and 5) employs AuNPs and silver particles as fillers to improve the electrical and thermal conductivities of polyurethane composites. High anisotropic electrical conductivity of thin composite films are obtained after curing at unprecedentedly low gold contents, which is reasoned with the coagulation of AuNPs to conductive gold networks in domains of high concentration of AuNPs. Silver particles and flakes of sizes between 20 nm and 1.5 micron were dispersed in polyurethane to investigate the effect of their size, morphology, aggregation, and dispersion on the thermal conductivity of the composites. Unexpectedly, composites filled with micron sized silver particles outperformed those filled with silver nanoparticles in thermal conductivity and stability. Finally, PdNPs were synthesized in the presence of thiolate ligands of different conical bulk (single phase surfactant free approach) to study the influence of the different ligands on their size (Chapter 6). No systematic effect was observed in contrast to a similar study on AuNPs, which is reasoned with a weaker binding of ligands to the Pd surface.