臨場液相穿透式電子顯微鏡研究尖晶石結構氧化物奈米結構成長動力學

Abstract

本論文利用臨場液相穿透式電子顯微鏡針對功能性氧化物奈米結構做一系統性動力學觀測與分析，探討從溶液中利用電子束激發生成氧化物晶體之原理與成長行為、奈米核殼結構成長之觀測，到奈米樹枝狀結構的發展與分支行為分析，對材料科學工程與化學合成氧化物技術在多方面皆提供了更深一層次的了解。 我們知道，利用溶液相成長並調控各式各樣奈米結構形貌及其物理特性之技術已成為新穎材料科學研究舉足輕重之一環，為了瞭解其成長行為並進一步控制材料生成，材料成長動力學一直是主要的探討議題。在過去，理論模擬以及非臨場觀測是取得資訊的主要工具；近年來，臨場液相穿透式電子顯微鏡技術的突破，成功地在材料成長過程中提供高解析度的空間與時間資訊。在此趨勢下，本論文針對溶液相成長功能性鐵系尖晶石氧化物奈米結構之動力學做一系統性探討。首先，論文第一部分探討了溶液中鐵系尖晶石氧化物藉由電子束激發所引發的成長行為以及成長機制；和以往金屬材料在電子束下生成機制不同的是，我們發現電子束加熱造成前驅物與表面活性劑的熱分解可能是成長氧化物最主要的機制；因此，本論文第二部份便是利用金屬與氧化物成長機制之不同，觀察了溶液相中核殼結構的成長動力學：成長前期，鉑鐵前驅物在電子束照射下因為還原反應率先生成金屬奈米顆粒，而剩餘的鐵前驅物在成長後期因為電子束造成的熱分解而生成氧化鐵殼包覆；整個成長過程以及結構變化皆利用高分辨電子顯微鏡成像紀錄並分析，我們發現，異質結構的成長過程中能障非常低，且兩相晶體結構彼此應力係藉由介面缺陷而釋放。在分析完較簡單的奈米結構形貌之後，本論文第三部分探討了鐵系尖晶石氧化物樹枝狀結構的形貌動力學。由於樹枝狀結構是在非平衡狀態下快速生成，用高解析時間的影像紀錄形貌成長行為成為本部分主要探討項目，我們觀測到樹枝狀分岔時寬化的現象，並對其動力學做一量化的探討。功能性氧化物奈米結構應用廣泛，本論文提出之實驗方法以及研究成果不僅奠定臨場液相穿透式電子顯微鏡領域在氧化物奈米結構上觀察之基礎，也將對材料科學工程與化學合成領域有重要貢獻。

The formation process and kinetics of nanostructures in solution have long been a myth with only theoretical and ex situ experimental supports in the past. Not until recently could we be able to directly monitor the materials nucleation and growth in solution with extraordinary temporal and spatial resolution by the revolution of in situ liquid cell TEM. As a relatively new technique with great potential, huge efforts have been put into the disclosure of fundamental growth kinetics in various systems. In this thesis, we first studied the oxide nanoparticles formation in a reductive environment under electron beam irradiation. Different from previous reports about noble metal nanoparticles formation, the growth of oxides were studied. We proposed that not only reduction of solvated electrons but electron beam heating effect is considerably important for the growth initiation. Additionally, we found that the reduction potential and the thermal decomposition temperature of loaded solution play a decisive role in the determination of final product. On top of the fundamental understanding about formation pathways in first part, in the second part of this thesis, we addressed the growth kinetics of core-shell nanostructure with complex composition. The formation and structural evolution of PtFe3- Fe2O3 core-shell nanostructure was captured in real time with atomic resolution. The results suggest that the sequential formation of metallic core and oxide sheath was associated with the reduction potential difference and the Pt catalytic effect. In the last part, the complex pattern development was traced in real time. Using the material systems similar to first part, the study in the last part nails the entire development of dendritic nanostructures at nanoscale. The growth kinetics between non-splitting tip and splitting tip were analyzed. In addition, we quantified the characteristic wavelengths to visualize an oscillatory behavior during nanostructure development. Overall, this thesis covers mostly the development process of various functional oxide nanostructures in real time using liquid cell TEM technique, enriching the fundamental understanding of material growth kinetics critical to nanomaterials engineering.