| Summary: | 博士 === 國立臺灣大學 === 高分子科學與工程學研究所 === 99 === “Dispersion technology” is considered as the key step in Bottom-Up process, for self-assemblies and fabricating nanomaterial devices. Herein, nanohybrid materials, including silver nanoparticles decorated on the silicate clay and carbon nanotube, were fabricated by ionic excharge reaction and non-covalent method. These materials were investigated on dispersing ability, particle size and distribution, and electrical behavior. The thesis is divided into three parts:
Part 1. Silver nanoparticles (AgNPs) of narrow size distribution and low melting point were synthesized from the reduction of silver nitrate in the presence of inorganic silicate clays. The natural clays with a lamellar geometric shape provided a high surface area for immobilizing AgNPs with nanometer diameter in the range of 17-88 nm. At a 1/1 equivalent ratio of Ag+ to clay counter ions, the generated particles had a narrow size distribution (polydispersity of Dw/Dn = 1.2 at 26 nm Dn by SEM) and a UV absorption at 420 cm-1. Without organic dispersants, the colloidal clays could complex with Ag+ in the initial stage of mixing and subsequently stabilized the generated Ag0 particles. It seems that the high surface area stabilizes the clay rather than the Ag metal intercalation into the layered structure since the basal spacing was only slightly enlarged (12.0 Å versus 13.9 Å by XRD). The resulting AgNPs were highly stable and maintained their particle size after several cycles of drying at 80 oC and re-dispersion in water. Moreover, the AgNPs on the clay surface melted at a low temperature (110 oC), observed by SEM. Such AgNPs may have potential applications for fabricating silver arrays or conductors at low temperature.
Part 2. Nanohybrids of silver nanoparticles (AgNPs) decorated on the surface of multiwalled carbon nanotubes (Ag/MWNT) were synthesized via the in situ reduction of AgNO3 in N,N-dimethylformamide (DMF) and water mixtrues. The process required the presence of a poly(oxyethylene)-backboned oligoimide (POE-imide), which stabilized the dispersion of MWNTs and AgNO3 initially, and subsequently the reduced Ag0 nanoparticles. AgNPs in the range of 8–30 nm diameter were generated and some of these were directly attached to the MWNT surfaces, as observed by transmission electron microscopy (TEM). Without the presence of POE-imide, AgNO3 can only be reduced into Ag0 mirror by DMF slowly and deposits on the side of the reactor wall. The kinetic formation of these nanohybrids was characterized by UV-visible (UV-vis) absorption for MWNTs at 550 nm and AgNPs at 420 nm. The single MWNT tubes of decorated with AgNPs (20–30 nm) were isolated by washing off the dispersant and free AgNPs. The synthesis involving an organic dispersant provides a convenient and facile method for preparing Ag/MWNT nanohybrids from the unmodified MWNTs.
Part 3. We fabricate a flexible and surface conductive films by hybridizing silver nanoparticles on multi-walled carbon nanotubes (Ag/CNT) via an in situ silver nitrate reduction in poly(oxyethylene)-imide (POE-imide) dispersion. The POE-imide copolymers provided dual functions for homogenizing CNT dispersion in DMF/H2O mixture and subsequently stabilizing the Ag/CNT nanohybrids during the solvent evaporation into films. By simple coating on polyimide substrate and heating to 160 oC, the generated silver nanoparticles (AgNPs) migrated to surface and aggregated to larger size of 100−150 nm. Continuing heating at 170 oC and 350 oC, the film surface appeared to have color changes from golden to milky-white with lower sheet resistance of 2.2 x 10-1 Ohm/sq and 2.7 x 10-2 Ohm/sq, respectively. The enhancement of surface electrical conductivity was attributed to the AgNPs migration through CNT network and melt into silver granule connection while simultaneously annealing at the preferable temperature of 350 oC. The mechanistic aspects were elucidated by surface observation on a scanning electronic microscope, measurement of organic degradable temperature by thermal gravimetric analyzer and silver characterization by wide-angle X-ray diffraction. The synthesis is viable for making flexible polyimide film exhibiting an unprecedented high conductivity that easily lighting a light-emitting diode lamps.
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