Synthesis of Nanohybrids Involving Silicate Clays, Carbon Nanotubes and Silver Nanoparticles, and their Conductivity Studies
|關鍵字:||矽酸鹽黏土;奈米銀粒子;分散性;奈米碳管;非共價鍵鍵結;奈米材料;銀熔融;低電阻;催化熱降解;silicate clay;silver nanoparticles;dispersion;carbon nanotubes;noncovalent interactions;nanomaterials;silver melt;low resistance;catalytic thermal degradation||公開日期:||2010||摘要:||製造具自我排列特性的奈米材料，其分散技術為製程中之關鍵。於此，我們利用離子交換反應以及非共價鍵方式製造奈米混摻材料，如：將奈米銀粒子修飾在矽酸鹽黏土與奈米碳管上。這篇論文將分成三部份，分別探討奈米材料之分散性、粒徑大小、粒徑分佈及其電性探討。
第一部分 此部分是探討利用無機矽酸鹽黏土噹分散劑，可合成窄小尺寸分佈及具低熔點之奈米銀粒子。由於天然黏土是片狀幾何形狀，可提供其高表面積固定奈米銀粒子，使其尺寸維持在粒徑大小17−88 nm的範圍。當銀離子/黏土的離子交換當量比為1/1時，可透過掃描式電子顯微鏡及可見光吸收觀察生成之銀粒子具窄小分佈的尺寸(polydispersity of Dw/Dn = 1.2 at 26 nm)及波長為420nm時，有一特徵吸收峰。在無有機分散劑存在下，膠狀之黏土仍可和銀離子嵌合並穩定生成之銀粒子。由於那層間距離僅微小的擴張 (12.0 Å versus 13.9 Å)，銀粒子是穩定生成在黏土之表面。在經過80 的乾燥並再分散回水中後，銀粒子仍穩定且例子大小均一；更進一步藉由掃描式電子顯微鏡觀察得知，那黏土表面上之奈米銀粒子具有一低熔點 (110)性質。這類銀奈米粒子在低溫下製造銀導線及導管是有其潛在之應用性。
第二部分 經由將銀離子溶於DMF/water溶劑中，並以poly(oxyethylene)-imide (POE-imide)當為分散劑可將銀粒子修飾於碳管表面，而此高分子(POE-imide)可使碳管及銀粒子不會聚集於溶液之中並可將銀離子還原為銀粒子。由此法製備而成的奈米銀粒子其粒徑分佈為8−30nm並且經由TEM觀察可看到有部分的銀粒子會貼附於碳管表面之上。
第三部分 我們呈現一種直接合成poly(oxyethylene)-imide (POE-imide)複合奈米銀粒子/多壁奈米碳管方式，並利用加熱方法探討其混成薄膜之電性。藉由簡單塗層Ag/MWNT/POE-imide混合液於聚醯亞胺基材上並加熱至160 oC，奈米銀粒子遷移至表面且聚集成直徑為100−150 nm，當溫度增加至170 oC時，那複合薄膜外觀轉變成呈現乳白色，其片電阻值也大幅降低為2.2 x 10-1 Ohm/sq，當溫度持續加熱至350 oC時，此時複合膜顯示為白色之表面且具有最佳之片電阻值(2.7 x 10-2 Ohm/sq)。電性之提升是由於奈米銀粒子催化高分子降解並進而熔融，此機制可由掃描式電子顯微鏡(FE-SEM)、熱重分析儀(TGA)及廣角X-ray繞射(WAXRD)等儀器鑑定得知。就我們所知，於此低溫度下所測量之Ag/MWNT薄膜片電阻值是最低的且目前文獻上沒有報導。
“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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