Synthesis Of Gold Nanomaterials Using Quaternary Ammonium Gemini Surfactants By Wet-Chemical Method

Gold nanomaterials are of great interest due to their optical, electronic, magnetic properties (quantum size effect), and promising wide ranges of applications, such as material science, catalysis and biological medicine. Therefore, gold nanomaterials will be the building blocks in21st century. In current synthesis of gold nanomaterials in aqueous phase, the surfactants used as directing agents are very limited, and CTAB is almost the only choice. However, considering that different shapes of nanomaterials need to be synthesized, it cannot meet the demand by using only CTAB as directing agent. Due to the advanced properties of the gemini surfactants, we exploit the possibility of a improved synthesis of gold nanomaterials by utilizing them as directing agents. In this dissertation, gold nanoparticles (GNPs) with relatively broad range of sizes, gold nanowires (GNWs), ultrahigh aspect ratio gold nanorods (GNRs) and gold nanoplates (GNLs) with both ultrahigh yields and tunable sizes were synthesized using quaternary ammonium gemini surfactants by wet-chemical method. The followings are the main point in each chapter.1. The synthesis of the GNPs, GNRs, GNWs, GNLs, the growth mechanism of GNRs and GNLs, and the optical properties and corresponding applications are summarized. In addition, the synthesis and properties of the gemini surfactants, and the synthesis of gold nanomaterials using gemini surfactants as stabilizing or directing agents are also summarized.2. According to the method previously reported in the literature, we synthesized the quaternary ammonium gemini surfactants with different tail lengths, spacer lengths, and alkyl lengths of the head groups. The chemical structure and purity of products were checked by1HNMR spectra, Electrospray Ionization mass spectra and element analysis.3. Gold nanoparticles with diameters ranging from1.3to16.8nm have been synthesized by in situ reduction of HAuCl4with NaBH4in aqueous phase using Gemini surfactants as ligands. Their properties/behaviors were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and zeta potential spectroscopy. FTIR and zeta potential measurements suggest that surface aggregates adsorb on the nanoparticle surface at high surfactant surface coverage. Above the CMC, micelles could serve as stabilizers that lead to the formation of ultrasmall nanoparticles. It is interesting to note that at low surfactant concentrations, surface aggregates adsorbed on gold nanoparticles might bridge small particles, which results in the formation of larger nanoparticles. However, when the surfactant concentration was further reduced to1μM, the AuNPs precipitated, indicating that0.1mM is the lowest concentration to obtain stable nanoparticles in this study. In addition, the average particle size decreases at a given surfactant concentration as the tail length of the Gemini surfactant is increased. On the other hand, when increasing the number of alkyl groups on the head group, the particle size decrease at first. However, with further increasing the number of alkyl groups, the gemini surfactants with butyl groups on the head groups allow for the increase of the particle sizes, especially at high surfactant concentrations. In other words, the particle size exhibit an U shape increase with increasing the alkyl group on the head group. As a result, the particle size can be controlled by the variations in concentration and/or hydrocarbon part of the Gemini surfactants.4. Crystalline gold nanowires (NWs) and nanorods (NRs) using quaternary ammonium gemini surfactants as directing agents by three-step seed-mediated method are prepared. For this, each of five different gemini surfactants are utilized as directing agents, and the length and/or aspect ratio of the NWs/NRs can be tuned by the variations in the hydrocarbon chain lengths of the gemini surfactants. Additionally, anisotropic nano structures are formed at ultralow surfactant concentration (0.01M), which can be attributed to the high viscosity of the gemini surfactants. Interestingly, the gemini surfactants with relatively short spacers promote two-dimensional growth of the gold nanostructures. Both single and twinned crystalline structures are elucidated by the selective-area electron diffraction as well as high resolution transmission electron microscope studies. Compared to the NRs with relatively low aspect ratios (typically below20) obtained using cetyltrimethylammonium bromide as directing agent, the NWs obtained in this investigation can reach up to4.2μm, and the largest aspect ratio was calculated to be210. Furthermore, the diameters of both the NWs and NRs are around30nm, which is similar to that of the NRs produced using CTAB as directing agents, thus leading to relatively high aspect ratios.5. The seed-mediated method was modified to synthesize the gold nanoplates. High yield synthesis of gold nanoplates with tunable size were achieved using16-4-16as directing agent by the modified method, in which the gold seeds were oxidized by HAuCl4. By simply varying the added amount of seeds, the edge length or the size of the nanoplates can be tuned, ranging from50to200nm, and at the same time the thickness of the nanoplates maintain below20nm, thus leading to ultrahigh aspect ratios of the large nanoplates. Compared to the seed-mediated method that initially used to synthesize gold nanorods, the yields of the nanoplates increased significantly. By using16-4-16as directing agent, the yields of nanoplates increased further. The yields of large nanoplates (500-2000nm) are above98%, and that of smaller nanoplates are above85%. It is interesting to note that the byproducts obtained using16-4-16are in the shape with well defined{111} facets. By contrast, small amount of quasispherical nanoparticles are obtained using16-6-16. It is thus possible that the16-4-16prefer to the adsorption on the{111} facets of the gold nanocrystals. In addition, by extending one-step growth to two-step growth, the edge length of nanoplates increased further, and the monodispersity were improved either.