General Rules Governing the Highly Efficient Growth of Carbon Nanotubes

Throughout the history of materials, synthesis has always been a limiting factor, and often a breakthrough in synthesis has paved the way for breakthroughs in the entire field, which is exemplified by the arc-discharge method to synthesize fullerenes. In this respect, the history of single-walled carbon nanotubes (SWNTs) has been no exception. In the early years after their discovery, work on SWNTs was restricted to microscopic and theoretical research because of their limited availability. Only after the development of improved techniques, such as laser ablation and high-yield arc discharge, to synthesize SWNTs with higher yields were researchers able to ‘‘touch’’ the samples. This led to many breakthrough discoveries, such as metallic and semiconducting behaviors, a 1 TPa Young’s Modulus, single-electron transistors, and SWNT field-effect transistors. The next breakthrough was the synthesis of SWNTs by chemical vapor deposition (CVD), through which scalable, self-organized, and patterned carbon nanotubes (CNTs) could be realized. In CVD, CNTs grow from a carbon source, and by improving this source CVD has advanced significantly. CVD started with hydrocarbons, such as methane, ethylene, and acetylene, but these methods only attained low yields and low purities. To improve these characteristics, carbon monoxide was introduced at high pressure, well known as the HiPco process, enabling mass production of SWNTs with high selectivity. An alcohol carbon source was used to realize the first vertically standing SWNTs with a height of 4mm. The CVD concept was expanded by the addition of a second ingredient (growth enhancer) into the growth ambient of normal hydrocarbon CVD, which dramatically increased the growth efficiency to unprecedented levels. Specifically, with an almost background level of water (100–200 ppm), SWNTs have been shown to grow into millimeter-scale forests within 10min, with a SWNT/catalyst weight ratio exceeding 50 000%, and a carbon purity of over 99.98%. This water-assisted highly efficient growth, also called ‘‘super-growth’’, has become a very popular method to grow tall, vertically aligned forests. SWNTs grown by this process possess exceptional properties, such as high purity, high surface area, long length, and alignment. These properties have opened up new applications for CNTs, such as 3D microelectromechanical systems (MEMS), stretchable conductors, high-power and -density supercapacitors, fast-moving, lowvoltage actuators, and super-tough fibers. These examples highlight how a new growth ambient can open up new possibilities in the CNT field. Here, we demonstrate that highly efficient growth can be achieved not only using water, but also countless other growth enhancers, for instance, alcohols, ethers, esters, ketones, aldehydes, and even carbon dioxide. From our extensive investigation, we have elucidated general rules governing highly efficient growth of CNTs and the fundamental reasons behind them. Specifically, we found that the key for highly efficient growth includes two essential ingredients in the growth ambient: i) a growth enhancer containing oxygen and ii) a carbon source that does not contain oxygen. This result can be explained in general by the fact that a growth enhancer removes the carbon coating that kills the catalyst. This generalized formulation not only cultivates our understanding of CNT growth but also provides a new testbed for studies of innumerably new and totally unexplored growths. Consequently, we believe that the generalized formulation may act as a guide to inspire further transformation and expansion of the world of CNT synthesis. We first wanted to demonstrate that other compounds, such as acetone or CO2, could also invoke highly efficient growth, to prove that water was not the only growth enhancer (Fig. 1a,b). The experimental conditions were similar to those optimized for water-assisted CVD, and the details are described elsewhere. For both acetone and CO2 as growth enhancers with ethylene as a carbon source, CNTs grew efficiently into vertically aligned CNT forests as shown by the scanning electron microscopy (SEM) image in Figure 1a. In contrast, forest growth from ethylene alone was very limited (Fig. 1a,d). Growth curves of the forests (Fig. 1b,c) measured by an in situ telecentric height-monitoring system with a constant ethylene level and increasing acetone and CO2 level highlight several key features. First, as compared to normal ethylene CVD (no enhancer), even a small amount of acetone ( 120 ppm) or CO2 ( 1000 ppm) extended both the growth lifetime and increased the growth rate, and dramatically increased the growth yield (forest height). Optimization of all growth parameters, such as temperature, flow rates, etc., was not performed for each growth enhancer. As such, the results