Growth mechanism and structural control of catalytically grown carbon nanotubes using nano-sized metal particles

在本项目中，我们通过准确理解纳米级金属颗粒在纳米管生长过程中的关键功能，成功地用催化化学气相沉积方法制备了高纯度的碳纳米管，并对其进行了结构表征。主要研究成果如下：(1)通过对碳纳米管在催化化学气相沉积（CVD）中生长机理的理解，开发了高纯度双壁碳纳米管（DWNTs）的合成方法。具体来说，我们已经阐明了支撑材料的重要性，因为它们的大小和组成直接有助于形成纳米尺寸的金属颗粒，用于生长碳纳米管。(2) 850℃的反应温度比单壁碳纳米管（SWNTs）更有利于薄碳纳米管（约0.8 nm）的生长。(3)在2000℃氩气条件下热处理DWNTs时，相邻的外壳在结构上融合，形成聚结纳米结构。(4)低碳纳米管基纸具有较高的抗氧化性能，且1 nm以下的纳米孔占比较大。(5)在1400 ~ 1600℃的热处理过程中，硼(B)原子在双壁碳纳米管（DWNTs）之间的掺入导致了共价纳米管“Y”结、DWNT聚结和扁平多壁碳纳米管（MWNTs）的形成。这些过程是通过相邻管的合并发生的，这是由B间隙原子触发的。(6)通过使用甲烷+硫混合气体代替甲烷，我们发现得到的纳米管的壳数增加，这可能是由于支撑材料外表面金属颗粒的尺寸增加所致。(7)通过与氟气体在200℃下直接反应5h，成功地将氟原子附着在DWNTs的外壳侧壁上。XPS结果表明，氟化后的DWNTs的化学计量学为CF_<0.30>，而HR-TEM结果表明，氟化后的DWNTs仍保持双层结构。通过调整仅外壳的电子特性和控制DWNTs的可有可无性，而不会通过氟化破坏双层同心壳的形态，我们设想这些化学修饰的DWNTs可用于制造新型传感器，纳米复合材料和电子器件。少

In this project, we successfully produced high purity carbon nanotubes in a catalytic chemical vapor deposition method through an exact understanding of the critical function of nano-sized metal particles during nanotube growth, and have carried out their structural characterizations.The summarized research results are as follows :(1) We have developed the method of synthesizing high purity double walled carbon nanotubes (DWNTs) through an understanding of long-discussed growth mechanism of carbon nanotubes in a catalytic chemical vapor deposition (CVD) method. Specifically, we have clarified the importance of supporting materials because their size and compositions directly contributed to the formation of nano-sized metal particles for growing carbon nanotubes.(2) We found that the reaction temperature of 850℃ was the optimum condition for preferentially growing thin DWNTs (ca. 0.8 nm) over single walled carbon nanotubes (SWNTs).(3) When DWNTs were thermally treated at 2000℃ in argon, … More the adjacent outer shells were structurally merged to form a coalesced nanostructure.(4) DWNT-derived buckypaper exhibited higher resistance to oxidation, and furthermore, had large fraction of nano-sized pores below 1 nm.(5) We demonstrate that the incorporation of boron (B) atoms between double walled carbon nanotubes (DWNTs) during thermal annealing (1400 -1600℃) results in covalent nanotube "Y" junctions, DWNT coalescence and the formation of flattened multi-walled carbon nanotubes (MWNTs). These processes occur via the merging of adjacent tubes which is triggered by B interstitial atoms.(6) By using the mixed gases (methane + sulfur) instead of methane, we found that the obtained nanotubes exhibited the increased shell numbers, possibly due to the increased size of metal particles on the outer surface of supporting materials.(7) We successfully attached fluorine atoms to the sidewall of outer shell in DWNTs through direct reaction with fluorine gas at 200℃ for 5 hrs. The stoichiometry of the fluorinated DWNTs is CF_<0.30>, based on XPS study, while double-layered structure is sustained after fluorination according to HR-TEM observation. By tuning electronic property of only outer shell and controlling dispensability of DWNTs without disrupting double-layered concentric shell morphology through fluorination, we envisaged these chemically modified DWNTs to be useful in the fabrication of novel sensors, nanocomposites and electronic devices. Less