利用臨場穿透式電子顯微鏡在液態中觀測金奈米粒子成長之研究

Abstract

金奈米粒子因其光學特性、電學特性和生物相容性性質而被廣泛應用在催化劑、奈米感測器、光電元件以及生醫技術等各種方面，而其不同性質取決於不同形貌與尺寸，若我們能控制其性質將能設計出新型材料。然而目前對於金奈米粒子如何成長以及實際反應發生的現象尚未有顯微影像觀察，故本研究著重在使用液態試片搭配載具，對金奈米粒子合成過程進行液相臨場觀測，以便了解奈米金之成核成長機制並加以控制其性質。 研究中使用製程簡單且成本低之鹽類還原法製備奈米金，使用的前驅物為四氯金酸和檸檬酸鈉，之後將前驅物溶液封裝在液態試片並放置到液態載具中，以穿透式電子顯微鏡進行臨場觀測。在觀測過程中，電子束之加熱效應為合成反應的驅動力，而奈米金主要分成核與成長兩部分，成核主要以LaMer model來解釋，且其成長單體為Au3+，達過飽和後會逐漸析出並還原成Au0。另外，成長部分又可區分為形成雙晶結構為主之單顆粒子成長以及降低自由能之聚合成長，以單顆粒子成長來說，除了受雙晶面堆疊的方向及數量影響之外，液態較多之區域，易形成多雙晶十面體結構；液態較少之區域，易形成板狀結構。至於聚合成長方式，主要有兩粒子因晶格排列方向相同或相異，而形成單晶或多晶之新形貌，亦或是選擇性的在束縛力差的位置，像是尖端處進行聚合使其結構趨向穩定。另外，因電子束會使水輻射分解出活性自由基與其他副產物，而氫氣自由基彼此鍵結會產生氫氣分子，達過飽和會以氣泡的形式析出並影響觀測的品質。雖然臨場觀測的時間會因此有所限制，但本實驗仍成功觀察到多種形貌之金奈米粒子之生成，並提供了解實際反應與理論機制直接關聯的證據。

Gold nanoparticles(NPs) have been widely used in catalysis, sensor, electronics and biological application due to their unique optical, electrical, and biocompatible properties. These properties mainly related to the shape and size of Au nanoparticles. If we can control the properties during Au NPs synthesis, we can design and apply to many functional nanodevices. However, the lack of information that how NPs growing and reaction process are remain unclear. Therefore, the technology of liquid cells was used for in-situ TEM observation which can provide direct evident and extend the study of reaction kinetics for modifying the morphology of NPs. In this work, we used salt reduction method that preparing Au NPs by reducing HAuCl4 with citrate acid owing to its low cost. The solution was sealed in liquid cell and then the dynamic-growth process of Au NPs can be observed via in-situ TEM. The drive force for synthesis process resulted from the heating effect of electron beam. The synthesis process of Au NPs can be classified into nucleation and growth. The nucleation of Au NPs mainly followed LaMer model, the monomers of Au3+would reduce to Au0. Moreover, Au NPs grew as a result of either single nanoparticle growth which tended to form twin structures or aggregation of nanoparticles to minimize its free energy. In addition to the orientation of twin planes, the thickness of liquid space was also one of the parameters to affect the NPs growth. The thicker solution layer tended to form multi-twinned nanostructures; on the contrary, nanoplates were easy to form in the thinner solution layer. For aggregation, single crystalline NP generated when the lattice arrangement in two NPs almost in the same direction; on the other hand, if the direction is different, it would form polycrystalline NP. Furthermore, Au NPs would like to aggregate at the vertex sites where the surface atoms have low coordination number for stabilizing structures. However, the solution irradiated by electron beam would cause the formation of hydroxyl radical which resulted in the production of hydrogen bubble to hinder the in-situ observation. Although the bubble limited the observation time of liquid cell, we still successfully observed the nucleation and growth process. We revealed the synthesis process of Au NPs in thermodynamic and kinetic viewpoints and provided direct evident of the synthesis process associated with theoretical mechanism. These experimental results sheds light on the salt reduction method.