多樣性鈷材料之製備及其特性分析

Abstract

本論文將討論多樣性鈷粒子-六角片(hexagonal shape)、圓餅(nanodisk)、玉米鬚(corn tassel)及長達微米級狀似毛毛蟲之鈷線(wire)之製備方法、生長機制、結構分析及其特性分析。1997年，Osamu Kitakami等人，利用物理濺鍍方式製備出不同尺寸(20 ~ 150 nm)與不同形狀之鈷奈米粒子，又鑒於鈷除了已知之於穩定狀態時為六方最密堆積結構(hcp-P63/mmc)，又稱α-鈷；當溫度達700K，其結構產生相變化，轉變為面心立方結構(fcc-Fm3m)，亦稱為β-鈷；近幾年亦發現以濕式化學法合成之鈷粒子傾向另一種複雜立方結構(cubic-P4132)，是故具磁化易軸之鈷金屬具多變型態之潛力。 目前，化學法僅V. F. Puntes等人，能以高溫於有機溶液環境，熱裂解Co2(CO)8並同時添加兩種以上分散劑製備鈷奈米片(Co nanodisks)，故本研究主要製備方法為兼具低成本與簡易操作特色之濕式化學法建立水相還原系統，以其能於水相系統簡易地控制實驗參數如溫度、雙還原劑、濃度、酸鹼值、分散劑、溶劑…等，改善熱烈解之繁複實驗設計，有系統地探討以化學還原方式，合成不同形狀之鈷奈米粒子。 實驗結果顯示，粒子形狀與反應物濃度與界面電位與酸鹼值之間有相當微妙之關係，酸鹼值主導界面電位之改變，界面電位將操控粒子之排列，同時於本實驗系統，亦可觀察選擇具鐵磁性之材料，其粒子成長不僅受到凡得瓦力影響，磁性更是主導粒子間的堆積。本研究製備了零維、一維及二維鈷奈米材料，探討不同維度粒子之成長變因，並將此系統推展至亦具鐵磁性之鐵、鎳材料，深究磁性奈米粒子成長之控制機制。 此外，我們利用穿透式電子顯微鏡及量測比表面積，分析其縱截面孔洞構造，以掃描式電子顯微鏡觀察其形貌與粒徑，並藉由界面電位分析儀獲得粒子表面電位分布。並同時利用所合成之各種形狀之鈷奈米粒子進行多項磁特性之量測，亦以同部輻射之X光印證其成分及特性結構。本研究合成多樣性鈷奈米結構，重點在於建立水相還原系統，探討不同維度磁性粒子成長機制，預期將可更精確地掌握磁性粒子之成長，控制磁特性，提供磁性維奈米元件應用上之參考依據。

The Objectives of the study are to synthesize and investigate various cobalt materials of different shapes like hexagonal flake, nanodisk, corn tassel and cobalt wire like caterpillar from cobalt nanoparticles.. The study also proposes a plausible mechanism of the formation of these cobalt materials. In 1997, a new physical sputtering method has been reported to produce different shapes cobalt nanomaterials. On the other side, cobalt has three kinds of phases such as hexagonal close packing (hcp-P63/mmc), face center close packing (fcc-Fm3m) and cubic structure (cubic-P4132). As cobalt has superior potential for fabrication into various shapes, cobalt is chosen as the subject of this study. However, nowadays most systems are used in organic environment. The present study establishes an aqueous system which is having advantages of lower cost and easier control of experimental conditions such as the concentration, temperature, pH and capping agent. The results show that the variation reactant concentration, magnetic force direction and zeta potential of cobalt materials lead to various shapes. For example this may be due to the increase of reactant concentration lowers the zeta potential of materials while slight variation of pH results in level of aggregation. TEM and SEM characterization techniques have been employed to study the interior porous structure of cobalt wire and to observe topography and size of materials, respectively. The study has also done MCD and EXAFS experiment in NSRRC to prove the nanostructure of inner cobalt wires. The present study establishes an aqueous system to synthesize cobalt nanomaterials of different shapes and can be a promising technique to produce other magnetic materials in future.