觸媒前趨物輔助合成單壁奈米碳管之製程及其性質

Abstract

為了探討觸媒前趨物，前處理方法與基材再成長對於成長單壁奈米碳管製程之影響，本研究在矽基材上鍍上CoCrPtOx與CoCrOx為觸媒前趨物，在氫氣與甲烷氣氛下以微波電漿輔助化學氣相沈積法(MPCVD)成長單壁奈米碳管。製程首先濺鍍緩衝層及觸媒前趨物在矽基材上。接著使用氫電漿前處理(在MPCVD系統中)或局部雷射加熱前處理(在DVD動態測試機中)使觸媒變成均勻分佈的奈米觸媒顆粒，然後在MPCVD系統中沈積碳奈米結構。前處理後的奈米觸媒顆粒與沈積後的碳奈米結構使用掃描電子顯微技術(SEM)、穿透電子顯微技術(HRTEM)、拉曼光譜技術(Raman spectroscopy)、光電子能譜儀(XPS)、熱重分析儀(TGA)以及場發射J-E測量法加以分析探討。從本研究的結果可獲得下列結論。 使用CoCrPtOx觸媒前趨物並配合氫電漿前處理可基底成長出準直的單壁奈米碳管。其直徑為2~3奈米，長度為40微米。觸媒前趨物的爆炸式化學還原反應有效的使奈米觸媒顆粒變小。其機制為PtO2的高能量還原反應使觸媒前趨物薄膜爆散成奈米觸媒顆粒，而Cr2O3則有效的阻止奈米觸媒顆粒的聚集。 將成長後的單壁奈米碳管使用超音波震盪取下後，其矽基材可以再一次的使用來成長單壁奈米碳管。此矽基材再一次經過氫電漿前處理與化學沈積後，成長出的單壁奈米碳管之形貌與性質幾乎與第一次成長出單壁奈米碳管的完全相同，具有高品質高密度。此製程提供一個量產單壁奈米碳管的方法。 若我們使用局部雷射加溫前處理來幫助觸媒前驅物爆散為奈米觸媒顆粒，可以降低我們整體製程的溫度，並且在373 oC的低溫下成長出單壁奈米碳管。相對於氫電漿前處理，局部雷射加熱只是局部的單點加熱使PtO2還原反應進行，讓觸媒爆散成奈米顆粒，整個矽基材並沒有經過高溫的處理，提供了一個在低溫下觸媒前處理與成長單壁奈米碳管的方法。 關於成長出的準直單壁奈米碳管的性質，場發射分析指出其起始電壓為4.6 V/□m (在電流密度0.01 mA/cm2時)而電流密度為6 mA/cm2 (在7.2 V/□m的電壓時)。拉曼分析結果顯示其IG/ID比高達43。而熱重分析顯示其在空氣中的抗氧化溫度高達586°C ~ 691°C，此結果與雷射法成長出並且純化過的單壁奈米碳管相似。

To examine effects of catalyst precursors, pretreatment methods and substrate recycling on the single-walled carbon nanotubes (SWNTs) formations, the processes were developed to synthesize various carbon nanostructures on Si wafer by the microwave plasma chemical vapor deposition (MPCVD) with CH4 and H2 as source gases, with CoCrPtOx and CoCrOx as catalyst precursors. The processes include; First, the buffer layer (AlON) and then catalyst precursors or catalysts (Co) were deposited on Si wafer by sputtering. The coated substrates were followed by H-plasma (in MPCVD system) or scanning local laser heating (in air by DVD dynamic tester) pretreatments to produce the well-distributed catalyst nanoparticles before nanostructures deposition by MPCVD method. The microstructures at each processing step and the as-deposited nanostructures and their properties were characterized by FE-SEM, TEM, HRTEM, Raman spectroscopy, XPS, TGA and I-V measurements. From the experimental results, the following conclusions can be drawn. The results indicate that the well-aligned and base-growth SWNTs can be synthesized by using CoCrPtOx as catalyst precursor under H-plasma pretreatment. The as-deposited SWNTs are about ~ 2.6 nm in diameter and 40 □m in length. The function of the catalyst precursor is essentially to promote miniaturization of the sizes of the catalyst nanoparticles down to about ~ 2.4 nm in diameter due to explosive chemical reactions during pretreatment process. This mechanism is based on the explosion associated with the reduction of PtOx in the CoCrPtOx film, and Cr2O3 is employed to inhibit the agglomeration of nanoparticles during pretreatment and the initial stage of nanostructure deposition. Effect of substrate recycling process was studied by removing the as-deposited nanostructures from the substrates, and the substrates were then pretreated in H-plasma to reactivate the catalyst particles on the substrate. The well-aligned SWNTs can be obtained by substrate recycling for several times. In other words, the process gives the advantage to synthesize SWNTs in mass production by cycling the catalyst-coated substrates like a template. Regarding to the effect of the precursor-coated substrate pretreatment methods, the results show that the scanning local laser heating method gives rise to lower deposition temperatures ( ~ 373oC) for SWNTs formation. By comparing with the traditional H-plasma pretreatment method, it has advantages of promoting the miniaturization of the catalyst particles at local area without high temperature heating on the whole substrate. About properties of the well-aligned SWNTs, field emission properties indicate a turn on voltage of 4.6 V/□m (for current density 0.01 mA/cm2) and the current density of 6 mA/cm2 (for field strength 7.2 V/□m). The Raman spectra display the IG/ID ratio of the synthesized SWNTs can be reached to a value of ~ 43. The results of TGA analysis in air show that the as-deposited SWNTs can resist oxidation up to 586°C ~ 691°C, which are much higher than the reported temperatures (~ 350°C) in the literature, and are comparable with that for the purified SWNTs synthesized by a laser-oven method.