以高合成效率的製程方法合成石墨包裹奈米鐵、鈷、鎳以及銅晶粒之初步研究

Abstract

石墨包裹奈米金屬晶粒(Graphite Encapsulated Metal Nanoparticles, GEM)是一種粒徑介於5-100 nm的球狀複合材料，其內核為金屬、外殼為石墨層，其石墨外殼保護內核金屬不受水解、酸蝕、及氧化環境的侵蝕，即便將GEM曝露於多種極端環境下，仍可保存內核金屬原有特性，例如磁性、儲氫能力等。近年來，GEM的多種優異特性已被成功地應用於各種領域上，例如Co-GEM的高儲氫能力，可作為燃料電池的儲氫元件、鐵磁性GEM(Fe-GEM, Co-GEM, Ni-GEM)更可作為塗佈於戰機外殼的吸波塗層；生醫領域方面，因Fe-GEM具有高度生物相容性，可用作藥物載體或用於癌症熱治療，以上成果展示了這種材料的多方應用潛力。 目前GEM的製造方法以電弧法為主流，其中由Teng et al.以及Dravid et al.於1995年所發展出的改良式鎢-碳電弧法，最具工業化量產的潛力，此法雖可有效地改善以Krätschmer-Huffman電弧法合成GEM晶粒時，產物碳雜質過多的問題，但仍有產物包裹率極低、粒徑控制差的缺點，大幅地限制GEM的基礎科學研究以及相關應用。產物包裹良率不理想的問題，是由於以往實驗所採用的固態碳源，如石墨塊、鑽石粉，無法有效且均勻地與熔融金屬混合所致，導致蒸發的碳與金屬蒸氣比例過低。 根據GEM的形成機制－二步驟機制模型的指示，若能大幅提升合併區內的碳與金屬蒸氣比例，便可提升產物包裹良率。本研究的重點即為在電弧放電時，將液態碳源對艙內注入，利用電弧高熱蒸發並分解之，可大幅提升合併區內的碳蒸氣量，增加碳溶入金屬的機會，提升產物包裹良率。實驗結果顯示，利用此法可將原本包裹良率約20-30 wt%的鎳金屬以及鈷金屬提升至約80 wt%；鐵金屬的包裹良率亦可從原本不到10 wt%提升至42 wt%。而鐵金屬包裹良率不如其他兩者，可能是因為鐵的極強碳化物形成趨勢所致，碳化鐵形成後，可能使溶於金屬內的碳含量不足以包覆鐵。 為了探討碳源之碳含量多寡，對合成產物包裹良率的影響，本研究以三種具有不同含碳量的醇類作為碳源合成Co-GEM，實驗結果顯示，當分別以甲醇、乙醇以及正丙醇作為碳源時，合成產物的包裹良率分別為27%、80%以及85%，顯示隨著碳源含碳量升高，產物包裹率顯著地上升。這是因為當含碳量較高的碳源被電弧高熱分解後，可提供較多的碳蒸氣量以包覆金屬。另一方面，本研究在合成實驗中意外地發現，當液態碳源的含碳量升高，Co-GEM平均粒徑顯著地下降，可從約86.6 nm下降至16.2 nm，顯示使用液態碳源合成GEM時，不僅能大幅地提升產物包裹良率，更可藉由改變碳源種類，來有效地控制合成產物的粒徑；當使用正丁醇作為碳源合成GEM時，經由比表面積儀分析產物並轉換成粒徑大小，發現產物的平均粒徑更可縮小至約12 nm。 銅的熔點低，利用鎢-碳電弧法合成Cu-GEM時，銅的蒸發速率遠大於石墨，因此難以合成Cu-GEM。本研究以正丙醇作為碳源合成Cu-GEM，藉由大幅增加合併區內的碳濃度，便可在不犧牲銅金屬的蒸散速率下，有效地調控碳與金屬蒸氣比，成功地提升其合成效益，實驗結果顯示，產物包裹率以及產率可分別從4 %以及0.06 g/min提升至40.6 %以及0.7 g/min，顯示液態碳源注入法。綜合上述，本研究發展出的新方法不僅可控制合成產物的粒徑，並且可大幅提升鐵磁性GEM、甚至是低熔點金屬GEM，如Cu-GEM的包裹良率。

Graphite encapsulated metal (GEM) nanoparticle is a spherical composite material with diameter ranging from 5-100 nm, the core is metal and the outer shell is comprised of graphite. The outer graphite shells protect the inner core metal nanocrystals against degradation reactions, preserving its inherited properties even when exposed to severe environments. Due to its unique properties, GEM has been widely applied, including in hydrogen storage and electromagnetic energy absorption. In the biomedical field, Fe-GEM has extensive applications, ranging from drug delivery agents to thermoseeds for hyperthermia therapy, mainly due to its excellent biocompatibility and magnetism. These results have shown that GEM is a versatile material with great potential in various fields. The modified tungsten arc discharge method developed by Teng et al. and Dravid et al. in 1995 is definitely the most practical method, which routinely produces GEM in a relatively large quantity. Although it eliminates the ubiquitous problems confronting Krätschmer-Huffman arc synthesis of GEM, i.e., massive residual carbon debris, it suffers greatly from the unsatisfactory synthesis efficiency, which significantly hinders the progress of fundamental research and related applications of GEM. This poor synthesis efficiency was considered to result from the deficiency of evaporated carbon vapor, which could not be sufficiently concentrated since the solid carbon source could not uniformly dissolve and mix with metal during arcing. Under the guidance of a two-step mechanism model, it is clear that the carbon-to-metal ratio within the coalescence region need to be adequately controlled. Thus, a liquid injection method was developed in this study, whereby a liquid carbon source jet directed at the arc could effectively increase the carbon concentration within the coalescence region, and therefore enhance the synthesis efficiency. The experimental results show that the synthesis efficiency of ferromagnetic GEM increased dramatically, i.e., for Co-GEM and Ni-GEM, it was raised from 20-30 wt% to around 80 wt%; for Fe-GEM, it was raised from less than 10 wt% to 42 wt%. Since iron possesses greater carbon solubility, a large portion of the dissolved carbon in iron forms stable carbide instead of encapsulation layers; therefore, the encapsulation efficiency of Fe-GEM is lower than that of Co-GEM and Ni-GEM. The effect of carbon content on encapsulation efficiency was investigated by using methanol, ethanol and 1-propanol, as carbon sources to synthesize GEM. The experimental results show that the encapsulation efficiency apparently increases with increased carbon content, i.e., when adopting methanol, ethanol, and 1-propanol as carbon source, the encapsulation efficiencies were 27%, 80%, and 85% respectively. In addition, the size distribution of Co-GEM was surprisingly found to shift to a lower size value in response to the carbon content, with the average size reduced dramatically from 86 nm to 16 nm, attesting to the effectiveness of the liquid injection method in both raising the encapsulation efficiency as well as size control. Furthermore, by adopting 1-butanol as the carbon source, the average size of the as-received powders could be further reduced to around 12 nm. Due to the significant difference between the evaporation rates of Cu and of graphite, it has been a challenge to synthesize Cu-GEM by a modified tungsten arc discharge setup. In this study, 1-propanol was adopted as the carbon source to synthesize Cu-GEM; the experimental results show that without lowering the evaporating quantities of copper, the carbon-to-metal ratio has been successfully manipulated by simply increasing the carbon vapor concentration within the coalescence region. Unsurprisingly, the encapsulation efficiency and production rate were dramatically increased; i.e., for the encapsulation efficiency, it was raised from merely 4 wt% to 40.6 wt%, while for production rate, it was increased from unsatisfactory 0.06 g/min to 0.7 g/min. In this study, we have demonstrated by experimental results that the newly developed method is effective not only in controlling the size of GEM, but also in increasing the encapsulation efficiency of both ferromagnetic GEM and GEM with a low m.p. core metal such as Cu-GEM.