巨型鐵鉑奈米粒子之製備及性質探討

Abstract

由於具有特殊的磁學性質，如何製備高品質的 FePt 奈米粒子，並藉由控制尺寸及形狀以改變其磁性上的表現，為現今材料科學界的依個重要課題。近年來不少研究團隊致力於 FePt 奈米粒子的尺寸及形狀控制，其中Yang et al以先形成 Pt/Fe2O3 core-shell 結構後，再通入惰性氣體以還原外層氧化鐵的方式成功製備出了現今文獻所發表過尺寸最大的 FePt 奈米粒子。在本篇論文中，我們以 Fe(acac)3 及 Pt(acac)2 作為前驅物， oleic acid 及 oleyl amine 作為介面活性劑，透過多元醇還原的方式製備出了大於 60 nm 的 FePt 奈米粒子。在我們的反應系統下，先在 120 ℃ 下使金屬前驅物與介面活性劑反應形成錯合物，之後在 240 ℃ 下加入還原劑以還原生成大尺寸的 FePt 奈米粒子。我們以穿透式電子顯微鏡 (TEM) 觀察合成出的大尺寸的 giant FePt (g-FePt) 奈米粒子的形貌；以 X-ray 繞射儀 (XRD) 鑑定其晶體結構，確認所合成之 g-FePt 奈米粒子為 f.c.c.（face-centered-cubic）結構。我們藉由反應參數的調控 (如介面活性劑的比例、反應溫度、反應時間、金屬前驅物的比例等)，試圖對 giant FePt (g-FePt) 奈米粒子的生成機制能有更進一步的了解。與文獻比對後，我們認為 g-FePt 奈米粒子的生成機制與”成核和生長”的概念有極密切的關係。在成核階段，成核速率的快慢會影響生成之奈米晶體的尺寸。當成核速率快時，有較高濃度的晶核，會形成較小的奈米粒子；成核速率慢時，晶核濃度較低，進而生成較大尺寸的奈米粒子。在我們的合成系統中，金屬前驅物及介面活性劑在低溫下反應形成錯合物，進而抑制了成核速率，促使晶核濃度較低，導致最終生成了較大尺寸的 g-FePt 奈米粒子。

The size control of FePt nanoparticles has been investigated extensively for nanomagnetism. Comparing to previous works, recently, the biggest FePt nanoparticle of 17nm was prepared by reduction of Pt/Fe2O3or FePt/ Fe2O3 core-shell nanoparticle. In our work, we successfully synthesize FePt nanoparticles larger than 60 nm by polyol reduction of platinum (II) acetylacetonate and iron (III) acetylaetonate in presence of oleic acid and oleyl amine. In this one-pot synthesis, the giant FePt nanoparticles (g-FePt NPs) were synthesized with uniform size and rough surface. By adjusting several experimental parameters, including surfactant ratio, reaction time, reaction temperature, precursor, etc., we also prepare FePt with different size and morphology. According to results of TEM images, the mechanism of g-FePt NPs formation is proposed .Based on the concept of “nucleation and growth”, the nuclei concentration serves as the main factor in g-FePt NPs formation. At the nucleation stage, the nucleation rate could affect the size of formation nanocrystals. Fast nucleation provides high nuclei concentration and yields small particles; slow nucleation provides low concentration of seeds and result in large particles. In our synthetic system, the interaction of metal precursors and surfactants induces to the formation of metal complexes which inhibit the nucleation rate. Therefore, it leads to the formation of g-FePt NPs.