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碳納米管的合成

維基百科,自由的百科全書
碳納米管與基底結合

生產碳納米管的技術有很多,包括電弧放電、激光燒蝕、高壓一氧化碳歧化和化學氣相沉積(CVD)。這些過程大多是在真空或工藝氣體[a]中進行的。碳納米管的CVD生長[b]可以在真空或大氣壓下進行,這些方法可以合成大量的碳納米管。催化和連續生長方面的進步使碳納米管的合成成本更加低。[1]

方法

電弧放電

1991年,在100安培的電弧放電過程中,在石墨電極的碳煙塵中觀察到了碳納米管,儘管其目的是為了產生富勒烯[2]然而,1992年,日本電氣基礎研究實驗室的兩位研究人員首次從宏觀上生產出了碳納米管[3]。他們同樣使用了高安培電弧放電。在這個過程中,由於放電溫度較高,負極中含有的碳會升華。

這種方法的產率按重量計可達30%。它能生產出長度達50微米的單壁和多壁納米管且結構缺陷少。[4]與其他方法相比,電弧放電技術在較高的溫度(1700℃以上)進行碳納米管合成,通常能使其膨脹,結構缺陷少。[5]

激光燒蝕

在激光燒蝕中,脈衝激光在高溫反應器中汽化石墨靶,同時將惰性氣體引入反應室。當汽化的碳冷凝時,納米管在反應器的冷卻表面上形成。系統中可包括水冷表面以收集納米管。

這個過程是由萊斯大學理查德·斯莫利博士和同事開發的,在發現碳納米管的時候,他們正在用激光對金屬進行爆破,以產生各種金屬分子。當他們聽說納米管的存在時,他們用石墨代替金屬,製造出多壁碳納米管。[6] 當年晚些時候,該團隊使用石墨和金屬催化劑顆粒的複合體來合成單壁碳納米管,而的混合物催化劑產率最高[7]

激光燒蝕法的產率在70%左右,主要為單壁碳納米管。碳納米管的直徑由反應溫度決定。它比電弧放電或化學氣相沉積都要貴。[4]

在知道碳納米管介質的空間調製折射率的情況下,由半導體碳納米管的導帶電子的玻爾茲曼無碰撞方程可以獲得很少的循環光脈衝動力學的有效方程解。[8]

熱等離子體法

使用的等離子體炬的特寫。通常,火焰呈淺紫色,但由於觀察窗是為了減少高能輻射,所以呈綠色。

單壁碳納米管也可以通過熱等離子體法合成,該方法由Olivier Smiljanic於2000年在加拿大瓦倫尼斯的國家科學研究所英語Institut national de la recherche scientifique首次發明。這種方法重現了電弧放電和激光燒蝕方法中普遍存在的條件,但使用含碳氣體代替石墨蒸汽來提供必要的碳元素。這樣做,單壁碳納米管的生長效率更高,分解氣體所消耗的能量可以比石墨蒸發少10倍。這個過程也可連續進行且成本較低。氬、乙烯和二茂鐵的氣態混合物加入微波等離子體炬中,在大氣壓等離子體的作用下霧化,形成強烈 "火焰"形態。「火焰」產生的煙霧中含有碳納米管、金屬和碳納米粒子以及無定形碳。[9][10]

另一種用等離子炬生產單壁碳納米管的方法是使用感應熱等離子體法英語Induction plasma,該方法由舍布魯克大學和加拿大國家研究委員會於2005年發現。[11] 該方法與電弧放電類似,因為兩者都利用電離氣體達到汽化含碳物質和隨後納米管生長所需的金屬催化劑的高溫。 熱等離子體是由線圈中的高頻振盪電流產生的,並在流動的惰性氣體中維持。 通常,將炭黑和金屬催化劑顆粒的原料加入等離子體中,然後冷卻以合成不同直徑的單壁碳納米管。

感應熱等離子體法每分鐘可生產多達2克的納米管材料,高於電弧放電法或激光燒蝕法。[11]

化學氣相沉積

通過等離子體生長的碳納米管可以增強沉積程度

碳的催化氣相沉積技術在1952年[12]和1959年[13]就已經被發現並發表,但直到1993年才通過這種方法合成了碳納米管。[14] 2007年,辛辛納提大學的研究人員開發了一種在FirstNano ET3000碳納米管生長系統上生長長度為18mm的對齊碳納米管陣列的工藝。[15]

在CVD過程中,用金屬催化劑顆粒層(最常見的是或它們的組合[16])製備襯底。[6]金屬納米顆粒也可以通過其他方式生產,包括還原氧化物或氧化物固溶體。在襯底表面生長的納米管的直徑與金屬顆粒的尺寸有關。這可以通過金屬的圖案化(或掩膜)沉積,退火或金屬層的等離子蝕刻來控制。在合成過程中基板需要加熱到約700°C。工藝氣體[c]和含碳氣體[d]加入反應器後,碳納米管在金屬催化劑的位置上開始生長。含碳氣體在催化劑顆粒的表面被分解,並且碳被運輸到顆粒的邊緣形成納米管。該過程的機理仍在研究中。[17]催化劑顆粒可以在生長過程中停留在正在生長的納米管的頂端,也可以停留在基體上,具體取決於催化劑顆粒與基底之間的附着力。[18]碳氫化合物在催化劑下的熱解已成為研究的一個活躍領域,並且可能成為大量生產碳納米管的途徑。流化床反應器是碳納米管制備中應用最廣泛的反應器。擴大其反應堆規模是研究人員的主要挑戰。[19][20]

生產碳納米管大部分使用化學氣相沉積法。[21]將金屬納米顆粒與催化劑載體如MgO或Al2O3混合以增加表面積,可使碳原料與金屬顆粒的催化反應產率更高。這種合成路線的一個問題是通過酸處理去除催化劑載體,有時可能會破壞碳納米管的原始結構。然而,可溶於水的替代催化劑載體已被證明對納米管的生長是有效的。[22]

如果在碳納米管生長過程中施加強電場產生等離子體[e],則納米管的生長方向將與電場方向相同。[23]通過調整反應器的幾何形狀,可以合成垂直排列的碳納米管英語Vertically aligned carbon nanotube arrays[24](即垂直於基底)。 如果不使用等離子體,產物取向將無法確定。 在某些反應條件下,即使沒有等離子體,間距緊密的碳納米管也會保持垂直生長方向,從而形成類似地毯或森林的密集管陣。

在各種合成碳納米管的方法中,CVD最有希望實現工業規模化沉積。它的價格/單位比較低,並且CVD能夠直接在所需的基底上生長,而其他生長技術必須分離並提純納米管。 特殊沉積方式產生的催化劑的碳納米管生長位點可控。[25]2007年,名城大學的一個團隊演示了一種樟腦中生長碳納米管的高效CVD技術。[26]萊斯大學的研究人員由理查德·斯莫利Richard Smalley)領導,一直致力於尋找生產大量、純淨、特定類型的碳納米管的方法。 他們的方法是從單根納米管上切下許多小種子生長出長纖維,結果發現所有長纖維的直徑與原始納米管的直徑相同,類型也可能相同。[27]

超增長氣相沉積

超增長CVD(水輔助化學氣相沉積)是由日本國立先進產業科學技術研究所英語National Institute of Advanced Industrial Science and Technology的羽田賢治、飯島澄男和他的同事開發的。[28]在該方法中,向CVD反應器中加入水可顯著提高催化劑的活性和壽命。反應器中產生了垂直於基材排列的緻密的毫米高垂直排列的納米管陣列。這些陣列的高度方程可表示為:

[29]

在該方程中,β代表碳納米管的生長速率,代表催化劑特徵壽命。[30]

其比表面積超過1000 m2/g(封頂)或2200 m2/g(無封頂)[31],超過了HiPco樣品400-1,000 m2/g的表現。[32]超增長氣相沉積合成效率是激光燒蝕英語laser ablation法的100倍左右。2004年,用該方法製造高度為2.5mm的單壁碳納米管「森林」所需時間為10分鐘。該方法得到的產物容易從催化劑中分離,純度>99.98%,無需進一步提純;而在相同條件下生長的HiPco碳納米管含有大約5-35%的金屬雜質。超增長技術避免了分離與離心提純所導致碳納米管的損壞。該方法同樣可以製造圖案化的高組織單壁碳納米管結構。[31]

超增長碳納米管的密度約為0.037 g/cm3[33][34]它遠低於傳統的碳納米管粉末(〜1.34 g/cm3),可能是因為後者包含金屬和無定形碳

超生長法是化學氣相沉積法的一種變體,可以生長含有單壁碳納米管、雙壁碳納米管和多壁碳納米管的材料,並通過調整生長條件來改變它們的比例。[35] 它們的比例會因催化劑的厚度而改變。管的直徑很寬,因其包含多壁碳納米管。[34]

垂直排列的納米管森林將其浸入溶劑中並乾燥後,會產生「拉鏈效應」。拉鏈效應是由溶劑的表面張力和碳納米管之間的范德華力引起的。 它將納米管排列成緻密的材料,通過在此過程中施加較小的壓力,可以將其製成各種形狀,例如片狀和條狀。 緻密化使維氏硬度提高約70倍,密度為0.55 g/cm3。 堆積後的碳納米管長度超過1毫米,並且碳純度為99.9%或更高; 它們還保留了納米管森林所需的排列特性。[36]

與多壁碳納米管相比,單壁碳納米管的低生長效率使其生產成本高;而超增長技術為單壁碳納米管的商業化提供了一個機會。[37]

液體電解法

2015年,喬治華盛頓大學的研究人員發現了通過熔融碳酸鹽電解合成多壁碳納米管的新途徑。[38]其機理與CVD類似。一些金屬離子被還原成金屬態,並附着在陰極上作為碳納米管生長的成核點。 陰極上的反應是:

如方程式所示,所形成的氧化鋰可以吸收二氧化碳[f]並形成碳酸鋰。

總反應為:

換句話說,反應物僅是溫室氣體中的二氧化碳,而產物卻是高價值的碳納米管。Science[39][40],BBC新聞[41],MIT技術新聞[42],等着重指出了這一發現,並將其視為一種二氧化碳捕獲和轉化的技術。

火焰環境

富勒烯碳納米管不一定是高科技實驗室的產品,它們也可在燃燒甲烷[43],乙烯[44],和苯[45]產生的普通火焰中形成[46],在室內和室外空氣的煙灰中也發現了它們。[47]但是,這些天然存在的品種在尺寸和質量上可能是極不規則的,因為它們的生產環境高度不受控。[48][49][50] 因此,儘管它們可以在某些應用中使用,但是它們可能缺乏研究和工業所必需的高度均勻性。研究人員的努力主要在於控制火焰的環境以生產理化性質更均勻的碳納米管。[51]基於理論模型,這種方法有希望進行大規模、低成本的納米管合成,儘管它們必須與快速發展的大規模化學氣相沉積競爭。[52]

提純

帶有碳納米管溶液的離心管,使用密度梯度超離心法英語Differential centrifugation,以直徑為標準分離。[53]

催化劑分離

納米級金屬催化劑是固定床和流化床[g]化學氣相沉積合成碳納米管的重要成分。它們可以提高碳納米管的生長效率,並控制其結構和手性。[54]在合成過程中,催化劑可以將碳前體轉化為管狀碳結構,但也可以在外圍形成一層碳塗層,與金屬氧化物載體一起附着或結合到產品中。[55]金屬雜質的存在會產生許多應用上的問題,尤其是之類的催化劑金屬可能會引起毒理學問題。[56]未封裝的催化劑金屬很容易通過酸洗去除,而封裝的催化劑則需要進行氧化處理才能打開碳外殼。[57]如何在保留碳納米管結構的同時有效去除催化劑,尤其是封裝的催化劑是一項挑戰。[58][59]一種打破碳質催化劑包封的新方法是快速熱退火。[60]


表徵 

在合成、提純完成後,表徵是檢測碳納米管純度與摻雜情況的最佳方法。透無線電子顯微鏡,拉曼光譜,熱重分析和近紅外光譜是表徵的常用儀器。[61]

應用相關問題

碳納米管的許多電子應用主要依賴於有選擇性地生產半導體或金屬型碳納米管的技術,最好是具有特定手性的碳納米管。[62] 現在有幾種分離半導體和金屬型碳納米管的方法,但大部分尚不適用於大規模技術過程。最有效的方法是基於密度梯度超離心法,通過微小密度差異將包覆表面活性劑的納米管分離。這種密度差異通常會導致納米管直徑和(半)導電性的差異。[53]另一種分離方法是使用嵌入瓊脂糖凝膠中的單壁碳納米管進行冷凍、解凍和壓縮的序列過程。這個過程會得到一個含有70%金屬型單壁碳納米管的溶液,並留下一個含有95%半導體型單壁碳納米管的凝膠。通過這種方法分離的稀釋溶液會呈現出各種顏色。[63][64]使用這種方法分離的碳納米管已經應用於電極,例如雙層電容器。[65]此外,SWNT可以通過柱色譜法來分離。半導體型 SWNT 的產率為 95%,金屬型 SWNT 的產率為 90%。[66]

In addition to separation of semiconducting and metallic SWNTs, it is possible to sort SWNTs by length, diameter, and chirality. The highest resolution length sorting, with length variation of <10%, has thus far been achieved by size exclusion chromatography (SEC) of DNA-dispersed carbon nanotubes (DNA-SWNT).[67] SWNT diameter separation has been achieved by density-gradient ultracentrifugation (DGU)[68] using surfactant-dispersed SWNTs and by ion-exchange chromatography (IEC) for DNA-SWNT.[69] Purification of individual chiralities has also been demonstrated with IEC of DNA-SWNT: specific short DNA oligomers can be used to isolate individual SWNT chiralities. Thus far, 12 chiralities have been isolated at purities ranging from 70% for (8,3) and (9,5) SWNTs to 90% for (6,5), (7,5) and (10,5)SWNTs.[70] Alternatively, carbon nanotubes have been successfully sorted by chirality using the aqueous two phase extraction method.[71][72][73] There have been successful efforts to integrate these purified nanotubes into devices, e. g. FETs.[74]

An alternative to separation is development of a selective growth of semiconducting or metallic CNTs. Recently, a new CVD recipe that involves a combination of ethanol and methanol gases and quartz substrates resulting in horizontally aligned arrays of 95–98% semiconducting nanotubes was announced.[75]

Nanotubes are usually grown on nanoparticles of magnetic metal (Fe, Co), which facilitates production of electronic (spintronic) devices. In particular, control of current through a field-effect transistor by magnetic field has been demonstrated in such a single-tube nanostructure.[76]

備註

  1. ^ 例如,氦氣就是一種在工業中廣泛使用的惰性氣體。
  2. ^ CVD生長是將基底暴露在一種或多種不同的前趨物下,在基底表面發生化學反應或化學分解來產生沉積碳納米管。
  3. ^ 例如
  4. ^ 例如乙炔乙烯乙醇甲烷
  5. ^ 如本章節附圖所示,等離子體態可增強化學氣相沉積程度
  6. ^ 如果反應器中的碳元素產生了二氧化碳
  7. ^ 流化床是將一定量的固體顆粒物質(通常存在於容器中)置於適當條件下以使固體/流體混合物表現流體性質時發生的物理現象。

參考資料

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