在電弧系統中以苯蒸氣碳源合成 石墨包裹奈米金屬顆粒

Synthesis of graphite encapsulated metal nanoparticles in an arc-discharge system: using benzene vapor as carbon source

蔡敦皓(Tun-Hao Tsai)

指導教授 ： 鄧茂華

國立臺灣大學/理學院/地質科學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201901876

摘要

石墨包裹奈米金屬顆粒（graphite encapsulated metal nanoparticles, GEM）是一種具特殊核殼結構的奈米複合材料，其組成為石墨質外殼包覆內部的奈米級金屬顆粒，尺寸大約介於5到100奈米之間。由於GEM具有特殊的核殼結構，使其可同時展現石墨以及內部奈米金屬的性質，如：石墨的穩定物化性質、生物相容性、以及奈米金屬的鐵磁性、微波吸附能力等，因此GEM在很多領域的應用上皆極具潛力，如：在電子及國防工業可做為微波吸收劑，在應用地質領域可做為良好地下水示踪劑，而於醫療產業可當藥物載體或進行熱治療等。雖然GEM在許多的應用上都極具潛力，但以目前的技術仍無法將其大量生產，因此產量不足的問題也成了GEM所面臨的最大挑戰。
本團隊對於GEM的製程改善進行了多年研究，在2012年開始以液態醇類做為主要碳源，自此，GEM的產量及良率都有大幅度的提升，但使用液態形式碳源進行實驗仍具有許多的缺點，如：碳源加入時容易使實驗中斷、無法估計碳源的實際使用量、對於其合成機制不了解等，而為了改善原有碳源輸入法的問題，並深入探討其合成機制，本研究設計一種新的液態碳源合成方式-蒸氣法(vapor method)，並以此法進行各種不同金屬GEM的合成。
蒸氣法成功解決了電弧容易熄滅的問題，同時保有原本方法高效率合成的優點，更使Fe-GEM的良率從以往的最高50%提升達80%以上。除此之外，藉由比較三種鐵磁性金屬(Fe, Co, Ni)在利用蒸氣法時所得的產物型態，可以發現在電弧系統中鐵磁性金屬催化非晶質碳形成石墨的能力依序為： Ni > Co >> Fe，而在比較鐵磁性金屬的催化能力後，本研究進而提出一合成假說模型，並稱其為「催化加溫循環」模型，以解釋使用蒸氣法合成Ni-GEM時產量極高的原因，此循環的提出可以作為未來新製程設計的基礎，並使產率能夠獲得大幅提升。
最後，透過比較傳統的液態碳源合成法(液滴法)與新設計蒸氣法所合成出產物的合成效率、外觀形貌、石墨殼層、以及殘留在坩堝內的金屬塊切面，成功建立出新的液態碳源模型，其較為完整的解釋了使用液態碳源合成GEM時的機制，並提出傳統的液滴法主要由「相分離機制」主導，蒸氣法則由「催化反應機制」所主導。藉由此新模型也提供了許多在前人文獻中所遇到的問題一個較合理的解釋，如：2002年鄭啟煇使用氣體碳源的高良率，以及2016年許舜婷使用微量進樣的高產率等。

關鍵字

石墨 ； 奈米金屬顆粒 ； 核殼結構 ； 電弧法 ； 苯

並列摘要

Graphite encapsulated metal (GEM) nanoparticle is a core-shell structured nanocomposite material composed of a stable outer graphitic shell and inner nano-scaled metal core; its size is about 5 to 100 nm. Due to its core-shell structure, GEM can simultaneously exhibit the properties of both graphite and the metal core, such as the biocompatibility, ferromagnetism, and microwave adsorption ability. Therefore, GEM has huge potential for applications in various fields, such as a microwave absorbent in the electronics industry and military applications, groundwater tracer in geosciences, drug carrier in biomedical engineering, hydrogen storage in energy engineering, etc.
However, GEM has so far been unable to serve in mass production due to various difficulties in the synthesis processes, especially for the Fe-GEM. Even though our research team has significantly increased the yield of GEM by using alcohols as carbon sources in 2012, the mechanism behind the processes (conventional droplet method) is still not fully understood.
In addition, there are many disadvantages in using this method, such as the arc being prone to extinction when droplets fall into arc plasma, and it’s difficult to calculate how much carbon sources actually drop into the crucible.
To further understand the mechanism of using liquid carbon sources, a new design (vapor method) for using vapor-form carbon sources has used the synthesis of GEM. In this study, the new design method succeeds in overcoming the problems caused by employing the conventional method, and enhancing the yield of various GEMs. Futhermore, some interesting results can be observed when using vapor-form carbon sources to synthesize various GEM in a modified tungsten arc-discharge system. For example, the encapsulation efficiency of Fe-GEM can be raised from less than 50% to nearly 90%, and the production rate of Ni-GEM is twice as high as that of droplet method.
Additionally, by observing the products form of GEM, the graphite catalytic ability of ferromagnetic metal in the arc system is Ni > Co >> Fe when using the vapor method in an arc discharge system. The results also reveal the reason why Ni-GEM has an extremely high yield via the vapor method. This study further proposes a reaction hypothesis model called the "catalytic heating cycle". The catalytic heating cycle model can also provide the fundamental framework for a new design to greatly improve the production rate.
Finally, by means of comparing the droplet method and the vapor method, the encapsulation efficiency, morphology of the graphite shell, and the remaining metal in the crucible, we can successfully establish a new working model. It shows that the mechanism of the droplet method is dominated by the "phase segregation", while the mechanism of vapor method is dominated by the “catalysis reaction”. The new working model can help to resolve the problems which were still not fully explained in the previous study, such as the high encapsulation efficiency by using gas carbon source (2002) and the high production rate by minor injection (2016).

並列關鍵字

graphite ； metal nanoparticles ； core-shell structure ； arc discharge ； benzene

參考文獻

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