碳奈米管之高效率離子與化學表面改質製程及其對碳奈米結構之影響

Abstract

為了提高碳奈米管(CNTs)應用之可能性，本研究主要目的為針對不同多壁碳奈米(MWCNTs)之表面改質技術進行研究，同時檢視其對碳奈米結構之影響。此製程包括利用提供不同氫/氧混合比例之氣體進行離子處理以及/或是使用兩種不同酸溶液進行酸處理。此實驗之離子乃藉由發散磁場以及施加於電漿區下28 cm處之樣品台偏壓自電漿萃取出，而此電漿由電子迴旋共振微波化學氣相沉積系統(ECR-MPCVD)所產生。而酸處理則使用0.25 M之稀釋硝酸溶液以及濃硝酸與濃硫酸之混合溶液(HNO3:H2SO4 = 1:3 (v/v))，同時以不同的超音波震盪時間進行酸處理。經過處理後之碳奈米管，則藉由X射線光電子能譜儀(XPS)，掃瞄式電子顯微技術(SEM)，穿透式電子顯微技術(TEM)，拉曼光譜技術(Raman spectroscopy)，以及熱重分析法(TGA)進行結構及性質分析。 理想碳奈米管之官能基化本質上為藉由鍵結含氧官能基於碳管表面以便改變表面性質，而對奈米結構無太大之破壞。表面處理可能包括非結晶碳之消除、結構缺陷之形成或破壞。酸處理之效應，基本上藉由氧化碳管表面以產生自由基鍵並且稼接官能基於此鍵上，比較以濃硝/硫酸與稀釋硝酸處理之碳奈米管官能基化程度與結構之破壞，結果顯示以強酸進行酸處理，可使碳奈米管具較高的官能基化程度(高[O]/[C]值，最高可至52.7%)，但結構破壞過大(較高的ID/IG值至0.96以及較低的熱分解溫度至638 oC)，由於XPS及Raman探測媒介的穿透深度限制，所量測之[O]/[C]、sp2以及ID/IG值，僅代表接近碳奈米管表面之量測結果。此外，傳統之酸處理亦具有汙染及處理時間過長(將近9小時)之缺點。 離子處理之效應本質上藉由使用高負偏壓值以便以較多陽離子轟擊碳奈米管並於碳管表面形成較多自由基鍵。同時，萃取自電漿具高還原電位之氧陽離子則易於此自由基鍵上鍵結而將碳管表面官能基化。實驗結果顯示，經離子處理後之碳奈米管，其[O]/[C]、sp2值對於氫/氧氣體流量比之曲線具有最大值而ID/IG具有最小值。最大官能基化程度之產生端賴於碳管表面之自由基鍵形成與萃取自電漿之氧化陽離子含量兩者間的競合關係。總言之，本研究之離子處理對於碳奈米管結構並無明顯之破壞，同時在中間的氫/氧氣體流量比( = 25/25 (sccm/sccm))下，當離子處理時間為5分鐘及20分鐘時，官能基化程度之[O]/[C]值分別可達31.1%及59.8%. 而利用結合5分鐘之離子前處理與稀釋硝酸處理之兩階段製程來處理碳奈米管，結果顯示出，在中間的氫/氧氣體流量比( = 25/25 (sccm/sccm))下，經離子處理之碳奈米管，再經過稀釋硝酸處理兩小時後，由於可提高碳管提高管徑分布之尺寸，故可將碳奈米管之分解溫度自~ 595 oC提高到至 684 oC，同時不會犧牲其官能基化之結果([O]/[C] =52.4%)。比較不同的製程方法，在不使用離子處理下，濃硝/硫酸與稀釋硝酸製程處理皆可藉由減小雜質與減小小管徑之碳奈米管以提高管徑分布之尺寸來達到提升碳奈米管之分解溫度，但會造成碳奈米管之結構破壞過大或是製程時間過長的問題。總言之，結合離子前處理與稀釋硝酸後處理乃為一簡單且有效之官能基化方法，同時在不產生過大之結構破壞下提高碳奈米管之分解溫度。

For extending the potential applications of carbon nanotubes (CNTs), various processes to modify the surface of the multi-walled carbon nanotubes (MWCNTs) were studied, and effects on carbon nanostructure were examined. The processes include the ion treatment by using various flow ratios of H2/O2 gas mixtures and/or acid treatment of two different compositions. The ions for treatment were extracted from the plasma, generated by an electron cyclotron resonance assisted microwave plasma chemical vapor deposition (ECR-MPCVD) system, through the divergent magnetic flux and the bias voltage application on the specimen stage at the position 28 cm below the plasma zone. The solutions for acid treatment include 0.25 M nitric acid or nitric/sulfuric (HNO3:H2SO4=1:3 (v/v)) acids. The acid treatments were conducted under various sonication times. The MWCNTs after each processing step were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and thermogravimetric analysis (TGA). Optimum functionalization of CNTs is essentially to bond more oxygen-containing functional groups on the surface to vary their surface properties without too much damage to the nanostructures. The surface treatment may include amorphous carbon elimination, structure defect formation or damage. Effect of acid treatment is basically to oxidize the surface of the nanotubes to create free radical bonds and to graft polar functional groups to the free bonds. By comparing the degree of functionalization and structure damage of CNTs of nitric/sulfuric acid with the dilute nitric acid treatments, the results show that former treatment results in a greater functionalization (i.e. higher [O]/[C] values, up to 52.7%) but too much structure damage (i.e. higher ID/IG ratios and lower decomposition temperatures, up to 0.96 and down to 638 oC), though the values of [O]/[C], sp2 and ID/IG merely represent the near surface features due to limitation of penetration depth of XPS and Raman probes. Other drawbacks of the traditional acid treatment are its pollution issue and too long treating time (up to 9 h). Effect of the ion treatment by using high negative substrate bias (-250 V) is essentially to bombard CNTs by more positive ions and create more free radical bonds on their surface. Meanwhile, the oxygen cations extracted from plasma can readily be bonded with these free radical bonds to functionalize the surface. The results indicate that there are existence of maximum values of [O]/[C] and sp2, and minimum values of ID/IG values at medium H2/O2 ratios. The existence of maximum functionalization is due to the competition between the amount of free radical bonds on nanotube surface and oxygen cations in the plasma stream. In summary, at medium H2/O2 ratio (= 25/25 (sccm/sccm)), the ion treatment in the present cases causes no significant structure damage, and at treatment times of 5 and 20 minutes, the [O]/[C] values of the functionalization degree, are 31.1% and 59.8% , respectively. For process combining the 5 min ion pretreatment and a post dilute nitric acid treatment, the results show that the ion-treated MWCNTs at medium H2/O2 ratio (= 25/25 (sccm/sccm)) can be further treated by the dilute acid to increase the decomposition temperature from ~ 595 oC up to 684 oC without sacrificing the functionalization ([O]/[C] =52.4%) owing to the increase of size distribution change of the nanotube. By comparing different process methods, both nitric/sulfuric and dilute nitric acid treatment, without the ion treatment can also enhance decomposition temperature by eliminating the impurities and the smaller CNTs to vary the size distribution of the tubes but it causes either too much structure damage or too long treating time. In summary, the process with the ion pretreatment and followed by a dilute acid treatment is relative simple and efficient to functionalize CNTs, simultaneously enhance the decomposition temperature and cause no significant structure damage.