零維至準二維奈米碳－其模板合成及鑑定

Zero to quasi two-dimensional nanocarbons － their template-synthesis and characterization

林正得(Cheng-Te Lin)

指導教授 ： 金重勳 ； 李紫原 ； 裘性天

國立清華大學/工學院/材料科學工程學系/博士(2008年)

https://doi.org/10.6843/NTHU.2008.00066

摘要

近年來，模板合成法已經成為一種普遍應用於製作各類奈米碳材料的技術。經由各種具有奈米級微孔隙的氧化物模板及不同的碳氫化合前驅物，可得到零維的奈米微粒、一維的碳奈米線、奈米碳管、二維的石墨薄片及三維介孔相碳材。雖然以模板合成法已經能得到形貌各異及具有不同特殊功能的奈米碳材料，隨著新的理論提出及技術進步，新型態碳材料仍不斷地以模板合成方式被製作出來。本研究以具有奈米通道狀結構的陽極處理氧化鋁為模板，並以固態瀝青及乙醇蒸汽為前驅物，可製備出數種未曾被發現或研究過的新型奈米碳材料，如：錐狀及盤狀形貌的碳奈米粒、具有魚脊狀微結構的碳纖維、以及厚度約5奈米的準二維尺度碳薄帶等。錐狀碳奈米粒具有特別的石墨層撓曲微結構，其堆疊形式並不能用目前現有的理論模型完美解釋，因此我們提出一個修改過的模型以說明該種特殊的堆疊結構。由雜環芳香烴系瀝青為前驅物，以模板法可得到形貌均勻具有魚脊狀微結構的碳纖維，其魚脊狀微結構的夾角會隨著軸向位置不同而規律地改變為此種碳纖維最大特色，不同於以組成均勻的多環芳香烴系瀝青為前驅物得到的層狀堆疊的產物。此種由芳香烴系瀝青在奈米孔道中熱裂解所得到的一維材料，其微結構型態可由近年來所發展出來的表面錨定效應加以解釋。該效應說明熔融瀝青中的液晶態多環芳香烴與接觸基板的作用力會決定其裂解後的石墨排列結構。先前未曾有過系統化研究雜環芳香烴系瀝青與不同結晶度碳基板的結果，因此我們利用數種鍍膜技術製備了各類奈米級中空管狀碳基板，並研究其與雜環芳香烴系瀝青的表面錨定效應。結果顯示該效應不僅僅受基板種類影響，不同石墨化程度的基板也會影響其分子排列方向。其排列取向可由比較前驅物及碳基板彼此的非晶程度加以預測。超薄的準二維尺度碳薄帶是經由乙醇在高溫下裂解，並與模板進行表面反應所製備而得，由筒道狀的模板卻能的到薄帶狀產物，突破了既有的關於氧化鋁模板合成法的觀念。該種薄帶狀產物具有極大的長寬比，邊緣增強效應使其具有良好的場發射性質，量測結果顯示其起始電壓約為3.25 V/μm，較其他類似型態的準二維碳材料為低。另外，自行合成由單壁碳奈米管所組成的巨觀陣列結構並量測其片電阻及垂直方向電導度等性質。由於單壁碳奈米管是以彼此平行並垂直於基板方向成長，因此預期該陣列應該具有明顯的電導異向性。由二點及四點探針量測結果，得到垂直成長的單壁碳奈米管陣列其電導率不隨垂直方向而改變，約固定在1.11 Ω-1mm-1左右，然而表面電阻會隨著厚度增加而減少。因此單壁碳奈米管陣列不但具有電導異向性，隨著厚度增加，其兩方向的電阻比值亦隨之增大，較其他類似的碳管樣品顯示出更明顯的異相性質。

關鍵字

模板合成 ； 多環芳香烴化合物 ； 多環芳香烴化合物 ； 魚脊狀碳纖維 ； 陽極處理氧化鋁 ； 垂直成長單壁奈米碳管陣列

並列摘要

In recent year, template synthesis method has been extensively adopted for fabricating a variety of carbon nanomaterials ranging from zero- to three-dimensional structures. Such as carbon nanoparticles, nanotubes, nanofibers, graphite films, and meso-porous carbons can be obtained using nanoporous templates and various hydrocarbon precursors. Although there have been so many template-synthesized carbon materials, the possibility of this technique still has not been brought into full play. In this study, several kinds of nanocarbons, like conical and disc-like nanoparticles, unusual herringbone-type nanofibers, and ultra-thin nanobelts, are produced using anodic alumina (AAO) as template and petroleum pitch or ethanol vapor as precursors. All the products with characteristic morphologies or microstructures are prepared and characterized for the first time.
Microstructural study on carbon nanocones obtained by mechanical treatment of graphitic nanofilaments indicates that their graphene stacking form does not fully agree with the current theoretical models and has to be expounded by a modified model. Using petroleum pitch composed of irregular PAHs (polycyclic aromatic hydrocarbons) as carbon source, herringbone-type carbon nanofibers constructed by conical graphenes with progressively increasing apex angles can be synthesized due to the metastable anchoring effect. The surface anchoring state at C/C boundary has also been studied by observing the interfacial lattice arrangement between pitch and carbonaceous substrates in nanocomposites. The results demonstrate that the surface anchoring state is not only dominated by the substrate species but the degree of graphitization of carbon substrates is also a key factor. A quantitative method for predicting the resulting orientation at C/C interface has been developed using average ID/IG ratio from Raman spectra. Ultra-thin carbon nanobelts with extremely-high aspect ratio of 1000 : 100 : 1 (length : width : thickness) are fabricated by decomposing ethanol in the AAO via a surface reaction route. Such morphology can be taken into account in one kind of quasi-2D nanomaterials. Their turn-on voltage obtained from field emission experiment is 3.25 V/μm, which is lower than many of current quasi-2D nanocarbons.
In addition, anisotropic electrical properties of vertically-aligned single-walled carbon nanotube (VA-SWNTs) films have been measured by two- and four-probe system. The conductivity of VA-SWNT films along the vertical direction is demonstrated in independence of the film thickness (conductivity: 1.11 Ω-1mm-1), whereas their sheet conductivity along the in-plane direction decreases progressively with increasing film thickness. The ratio between vertical and sheet conductivities increases from 10s to 100s in proportion to the film thickness, exhibiting the anisotropic electrical phenomenon.

並列關鍵字

Template-synthesis ； Polycyclic aromatic hydrocarbons ； PAHs ； Herringbone-type carbon nanofibers ； Anodic aluminum oxide ； Vertically-aligned single-walled carbon nanotubes

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