新型催化剂控制生长单壁碳纳米管及其机理研究

IMR OpenIR

	新型催化剂控制生长单壁碳纳米管及其机理研究
	刘碧录
学位类型	博士
导师	成会明 ; 任文才
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	碳纳米管化学气相沉积可控生长生长机理非金属催化剂 Carbon Nanotubes Chemical Vapor Deposition Controlled Growth Growth Mechanism Metal-catalyst-free
摘要	"单壁碳纳米管（SWCNT）具有独特的一维结构及优异的性质，具备很高的理论研究价值和实际应用前景。SWCNT的性质由其结构决定，控制SWCNT的结构是实现其应用的前提。然而，目前对SWCNT的生长机理并未完全理解，这极大地阻碍了对其结构的精细控制。催化剂是决定SWCNT结构的最关键因素之一，本论文以研究化学气相沉积法（CVD）生长SWCNT的催化剂为出发点，通过探索和发展不同种类的新型催化剂，深入研究SWCNT的生长机理，以期实现其精细结构控制生长。取得的主要结果包括：一、对比研究了几种新型金属催化剂，发现碳在其中的溶解度并非决定SWCNT能否生长的关键因素；发展了CoPt合金催化剂实现了高温下高质量（6, 5）型SWCNT的选择生长。探索了两种碳溶解度和金属碳化物形成能力截然不同的金属Mn和Ag为催化剂生长SWCNT。研究发现它们均可作为高效生长SWCNT的催化剂，表明碳在催化剂中的溶解度并非决定SWCNT能否生长的关键因素，极大地拓宽了探索新型催化剂控制生长SWCNT的思路。进一步发展出一种新型的CoPt合金催化剂，实现了在800 °C的较高温度下选择生长（6, 5）型富集的SWCNT。与前期研究相比，CoPt合金催化剂可在高温下生长手性富集的SWCNT，因此产物的质量和结晶性更高，这主要源于Pt的加入提高了合金催化剂的高温稳定性。这种高质量、手性富集的SWCNT在纳电子器件应用方面更具吸引力。二、提出了采用非金属催化剂生长SWCNT、发现其极慢的生长速度、结合原位透射电子显微镜（TEM）和密度泛函理论（DFT）计算提出了非金属催化剂生长SWCNT遵循新的气-固-固生长机理，并揭示了氧的重要作用。提出了非金属氧化物催化剂（SiOx）生长高质量SWCNT的方法，发展出SiO2/Si基片表面刻划法和SiO2镀膜法，实现了高质量、高密度SWCNT的图案化生长。非金属催化剂生长SWCNT不仅为研究SWCNT的本征性质和应用提供了材料保障，也对研究SWCNT的生长机理和控制生长提供了新的视角。系统对比研究了SiOx催化剂和Co催化剂生长SWCNT的生长速度及催化剂活性时间。发现SiOx生长SWCNT的速度为8.3 nm/s，仅为传统Co催化剂速度的1/300。DFT计算表明CH4的慢热解速度及其分解产物-CHx在SiO2表面的吸附解离是SiOx生长SWCNT速度很慢的根源。基于SiOx生长SWCNT速度很慢这一特点，实现了SWCNT的长度控制、高质量短SWCNT的直接控制生长和高空间精度图案化生长SWCNT，为研究短SWCNT的本征性质及其应用提供了材料基础。研究还发现，SiOx催化剂较传统的Co催化剂在生长SWCNT时具有更长的催化活性时间。结合原位TEM观察、CVD实验及DFT计算，研究了非金属催化剂生长SWCNT时催化剂的状态及氧的作用。发现催化剂为固态、非晶SiOx纳米颗粒，据此提出SiOx生长SWCNT遵循气-固-固生长机理，而非传统铁族催化剂遵循的气-液-固生长机理，并指出固体催化剂更有利于实现SWCNT的结构控制生长。研究还揭示了氧对SWCNT生长的重要作用，一方面，DFT计算指出，SiOx中的氧提高了-CHx在SiOx表面的吸附能；另一方面，实验发现SiOx具有形成石墨碳结构的能力。三、研究了SWCNT与O2的反应活性与其手性的依赖关系，通过简单O2处理选择获得了大直径、大螺旋角的半导体性SWCNT。利用电子衍射技术，研究了CVD法制备的原始高质量、单根、悬空的SWCNT的反应活性与其结构和性质的依赖关系。发现SWCNT与O2的反应活性与SWCNT的直径、螺旋角、导电属性等均有关。直径越小、螺旋角越小的SWCNT与O2的反应活性越高；相同直径与螺旋角情况下，金属性SWCNT的反应活性高于半导体性SWCNT。通过简单O2处理，可以选择获得大直径、大螺旋角、半导体性SWCNT富集的产物。大螺旋角SWCNT稳定性更高这一发现，解释了目前选择生长SWCNT总是优先生长大螺旋角产物的实验结果。以上研究可为通过后处理方法实现SWCNT的手性分离和原位控制生长SWCNT提供重要指导。"
其他摘要	"Single-walled carbon nanotubes (SWCNTs) are regarded as a landmark of one-dimensional nanomaterials, which possess unique structures with superb properties and are of great importance for both fundamental studies and vast technological applications. The properties of SWCNTs strongly depend on their geometric structures. Therefore, realizing the structure-controlled growth of SWCNTs is a prerequisite for many practical applications. However, the growth mechanism of SWCNTs is not completely understood till now, which greatly hinders the selective growth of SWCNTs with specific structures and properties. Catalyst is one of the key factors that govern the structures of as grown SWCNTs. In this dissertation, we focus on the exploration and development of new catalysts for the chemical vapor deposition (CVD) growth of SWCNTs, aim at an in-depth understanding of the growth mechanism of SWCNTs and realize their fine-structure-controlled growth. The main results are as follows: 1. New metal catalysts were explored for SWCNT growth and it was found that carbon solubility in catalyst is not a must for SWCNT growth. Furthermore, a new CoPt alloy catalyst was developed to realize the selective growth of high quality (6, 5)-dominated SWCNTs at high temperature. We first studied two metals, Mn and Ag, which possess distinct properties in terms of carbon solubilities and metal carbides formation, for the CVD growth of SWCNTs. We demonstrate that both metals can grow SWCNTs efficiently, indicating that the carbon solubility in catalyst is not a prerequisite for SWCNT growth. This contributes to the growth mechanism understanding of SWCNTs and greatly broadens the horizon for exploring new catalysts for the controlled growth of SWCNTs. Moreover, we have devised a new CoPt bimetallic catalyst, which is capable of growing narrow chirality (6, 5)-predominated SWCNTs at a high growth temperature of 800°C. Compared with other selective growth results reported recently, CoPt bimetallic catalyst can grow SWCNTs with a narrow chirality distribution at higher temperature, originated from the improved stalibity of Co catalyst with Pt adding via alloy formation, which renders the SWCNTs with much better quality and crystallinity. Such high quality and narrow chirality distributed SWCNTs are attractive for nanoelectronics. 2. A metal-catalyst-free CVD method was proposed for the growth of SWCNTs, extremely low growth velocity of SWCNTs grown from SiOx was observed, and the vapor-solid-solid growth mechanism and the importance of oxygen for SWCNT growth on SiOx were elucidated. We have developed a metal-catalsyt-free process for the growth of SWCNTs. By using a “scratched” SiO2/Si substrate or SiO2 film sputtering deposited substrate, we realized SWCNT growth without any metal contamination. The method can grow high quality and high density SWCNTs and realize their patterned growth. Such metal-free SWCNTs provide an ideal sample for investigating the intrinsic physical and chemical properties of SWCNTs and their applications in many fields where metal residue are harmful. In addition, the ability of nonmetal species for SWCNTs growth opens up a new perspective to understand the SWCNT growth mechanism and their controlled synthesis. We systematically compared the growth behavior of SWCNTs between SiOx and traditional Co catalyst in terms of SWCNT growth velocity and catalyst lifetime. We found that SiOx grow SWCNTs with an extremely slow velocity of 8.3 nm/s, 300 times slower than that of commonly used Co catalyst. Based on density functional theory (DFT) calculations, we attribute this slow growth velocity to the following two facts: 1) SiOx does not have functions for catalytic decomposition of CH4 and 2) the absorption energy of -CHx on SiO2 is lower than the bond dissociationg energy of H-CH3, and thus -CHx prefers to desorb from SiO2 surface. Based on this slow growth velocity of SWCNTs from SiOx catalyst, we realized the direct length controlled growth of short SWCNTs and patterned growth of SWCNT with much better spatial resolution. Such pristine, high quality, and short SWCNTs provide a material platform for the investigation of intriguing properties of short SWCNTs and their practical applications. We have also found that SiOx catalyst possesses much longer catalytic lifetime than Co catalyst for SWCNT growth. Combining in situ transmission electron microscopy (TEM) and CVD experiments with DFT calculations, we have investigated the state of SiOx catalyst during SWCNT growth and revealed the importance of oxygen for SWCNT growth. Experimental results show that it is solid and amorphous SiOx nanoparticles that act as active catalyst for SWCNT growth. This suggests that SWCNT grown on SiOx catalyst follows a vapor-solid-solid mechanism, instead of the well-known vapor-liquid-solid mechanism for traditional iron group metal catalysts. Solid catalyst should be an advantage for the chirality-controlled growth of SWCNTs. Further studies revealed the importance of oxygen in SiOx for SWCNT growth. On the one hand, DFT calculations show that oxygen can enhance the -CHx absorption ability on SiOx. On the other hand, we have observed the graphitization ability of SiOx nanoparticles. 3. The chirality-dependent reactvitity of SWCNTs to O2 was studied and found that large diameter, high chiral angle, and semiconducting SWCNTs can be selectively obtained after simple O2 treatment. By using electron diffraction technique, we have investigated the structure- and property-dependent reactivity of the as grown high quality, individual, and suspended SWCNTs. By tracking the same SWCNT at different states, we have observed that the reactivity of SWCNTs to O2 is sensitive to their diameter, chiral angle, and metallicity. The smaller the diameter and the chiral angle of SWCNTs, the higher the reactivity of the SWCNTs to O2. In addition, metallic SWCNTs are more reactive than semiconducting ones with similar diameters and chiral angles. Through simple O2 treatments, we can selectively enrich large diameter, high chiral angle, and semiconducting SWCNTs. Chiral-angle-dependent reactivity of SWCNTs may have important implications to explain the current selective growth experiments on why high chiral angle SWCNTs is specifically enriched. These results can also guide the postsperation and in situ selective growth of SWCNTs with specific chiralities."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64457
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘碧录. 新型催化剂控制生长单壁碳纳米管及其机理研究[D]. 北京. 中国科学院金属研究所,2012.