以退火改善石墨包裹奈米鐵晶粒之包裹良率

Abstract

石墨包裹金屬奈米晶粒(Graphite Encapsulated Metal Nanoparticles, GEM)，為具有石墨層包裹於金屬內核的球狀複合奈米材料，粒徑介於5-100 nm。GEM之石墨外殼能保護內核金屬不受水解、酸蝕、及氧化環境的侵蝕，使GEM在多種極端環境下保持內核金屬原有的特質。例如：極易氧化的鐵金屬便可藉由石墨外層的保護維持在純金屬的狀態，保持其大部分的鐵磁性以及延展性，甚至可以避免奈米鐵氧化速率劇烈提升造成自燃的現象。由於上述優點，以及鐵相對於其他金屬而言較高的生物相容性以及鐵磁性，因此Fe-GEM在生醫相關科技運用有極大的潛力。例如藥物載體以及癌症治療等。 目前本研究室以Teng et al.與Dravid et al.於1995年所發展之改良式鎢-碳電弧法製備GEM，並以液態醇類作為石墨外層之碳源。成功將Co-GEM及Ni-GEM之包裹良率提升至80%。然而，相同製程所合成Fe-GEM之包裹良率卻僅有45%。為改善此情形，本研究利用鐵磁性金屬催化石墨的特殊性質，以退火之熱處理方式提升Fe-GEM之包裹良率。退火之實驗設計中，初產物分別以300℃、400℃、500℃、600℃、700℃、800℃、900℃等持溫溫度進行退火實驗。實驗結果顯示，當退火持溫溫度為300℃，Fe-GEM之包裹良率將自原本之~45%掉至~30%，而隨著持溫溫度的上升包裹良率也逐漸增加，並在700℃達到最高包裹良率80%，一旦退火持溫溫度超過800℃，則包裹良率便急遽降至~20%。 本研究以X光粉末繞射儀（X-ray Diffraction，XRD）分別分析經酸溶磁選與退火前後之產物。分析結果顯示，初產物之XRD分析光譜可見到α-Fe與γ-Fe之峰值，但經酸溶後產物之γ-Fe峰值會消失，顯示包裹不完全的顆粒中，其內核鐵金屬除少量常溫相之α-Fe外，絕大部分屬於高溫相的γ-Fe。而由退火後之XRD分析證實，高溫不穩定相之γ-Fe在退火後產生相轉變，並於低溫退火（300℃-500℃）中，傾向形成碳化鐵（Fe3C），且退火溫度越低，碳化鐵含量比例越高。 由於γ-Fe、α-Fe及Fe3C 之含碳量的差異（2.07%、0.02%、6.7%），造成Fe-GEM於不同退火持溫溫度下，其包裹良率相對改變。於低溫退火時，γ-Fe傾向轉變為高溶碳能力的碳化鐵（Fe3C），不僅無法於GEM外層形成碳殼，甚至使原本包裹於顆粒外的非晶質碳也轉而形成Fe3C，造成原本由非晶質碳包裹的顆粒成為不完全包裹，致使低溫退火後Fe-GEM的包裹良率下降。而針對退火持溫溫度超過 700℃時開始下降的包裹良率，本研究將純化後之顆粒以900℃退火，並進行SEM分析。於SEM影像中可於顆粒間發現不規則形狀之熔融金屬，推測為內核金屬由石墨外殼之裂隙擴散至外部造成。 根據實驗結果可知，影響Fe-GEM初產物退火後之包裹良率有兩種影響原因，一為初產物中非晶質碳的含量比例，非晶質碳表示初產物內於退火過程中可變動的碳，當非晶質碳越多，包裹良率的變化越大；第二個則是退火持溫溫度。過低的退火溫度會造成Fe3C含量比例上升，使產物中之碳源不足以完整包裹金屬內核，造成良率下降；而過高之退火溫度造成良率下降的原因則為內核金屬由石墨外殼之缺陷擴散至外部。因此Fe-GEM可於700℃退火持溫溫度可達最佳之包裹良率80%。

Graphite encapsulated metal (GEM) nanoparticles are spherical composite materials with core-and-shell structure, coated with several layers of graphite shells on its surface. And the shells will protect GEM in an acidic or oxidative environment. i.e., iron nanoparticles maintain most of its ferromagnetic and ductility, and would not combust spontaneously that causes by the oxidation of iron nanoparticles. With the alternative core materials, GEM can show either magnetic or other physical properties. Because of its variety and stable properties, GEM is useful in a number of applications, For example, Co-GEM with a fcc cobalt core exhibits excellent hydrogen storage ability, and Fe-GEM, which this study focus on, has extensive applications, from drug delivery agents to thermo seeds for hyperthermia therapy, mainly due to its excellent biocompatibility and magnetism. To improve the production rate and encapsulation efficiency of GEM, the modified tungsten arc discharge method developed by Teng et al. and Dravid et al. in 1995 has been used. Furthermore, the solid carbon source has been replaced by liquid alcohol to increase the encapsulation efficiency. Using the above process to synthesize ferromagnetic GEM, the encapsulation efficiency of Co-GEM and Ni-GEM is around 80 wt%, While for Fe-GEM, however, is only 45 wt%. To increase the encapsulation efficiency of Fe-GEM, the as-made powder of Fe-GEM were annealed under 1 atm 5%-H2/Ar at 300℃, 400℃, 500℃, 600℃, 700℃, 800℃ and 900℃ for two hours. Iron has been well documented possessing excellent catalytic ability that transforming amorphous carbon, i.e., carbon black, into well-crystallized graphite layers. Ideally, the amorphous carbon will first dissolve into the iron metal core of ill-encapsulated Fe-GEM nanoparticles, and then it will be catalyzed by iron nanocrystals core, forming small pieces of graphite flakes. Finally, the graphite pieces will move to the surface of iron nanocrystals and complete the encapsulation processes. According to the experimental results, the encapsulation efficiency of annealed Fe-GEM varies with annealing temperature. The as-made powder initial encapsulation efficiency of as-made Fe-GEM powders is around 45%, and it can be 80% after annealing temperature reaching 700℃. However, some annealing temperatures reduce the encapsulation efficiency instead, i.e., encapsulation efficiency is only around 30% when annealing at 300℃, and down to 20% when annealing at 900℃. XRD results shows that the amount of cementite (Fe3C) increases at lower annealing temperature (300℃-500℃). Unlike α-Fe, which can dissolve only a small amount of carbon (no more than 0.021 wt %), cementite contains 6.7% of carbon. That is, instead of graphite, much of amorphous carbon was used to form cementite. And this causes the low encapsulation efficiency at low annealing temperature. According to the TEM images, most of the well-graphitized outer shell of Fe-GEM were destroyed after annealing at 900℃. This causes the cracks between pieces of graphite of graphite shell. It is surmise that iron atom move from inner core to outside by diffusion through these cracks, and cause the low encapsulation efficiency of Fe-GEM at annealing temperature over 700℃. In conclusion, the encapsulation efficiency of Fe-GEM is controlled by the annealing temperature, and the amorphous carbon in as-made powder. When annealing temperature is 700℃, Fe-GEM has highest encapsulation efficiency (~80%).