Growth Kinetics of Vertically Aligned Carbon Nanotube Arrays in Clean Oxygen-free Conditions

Abstract

Vertically aligned carbon nanotubes (CNTs) are an important technological system, as well as a fascinating system for studying basic principles of nanomaterials synthesis; yet despite continuing efforts for the past decade many important questions about this process remain largely unexplained. We present a series of parametric ethylene chemical vapor deposition growth studies in a "hot-wall" reactor using ultrapure process gases that reveal the fundamental kinetics of the CNT growth. Our data show that the growth rate is proportional to the concentration of the carbon feedstock and monotonically decreases with the concentration of hydrogen gas and that the most important parameter determining the rate of the CNT growth is the production rate of active carbon precursor in the gas phase reaction. The growth termination times obtained with the purified gas mixtures were strikingly insensitive to variations in both hydrogen and ethylene pressures ruling out the carbon encapsulation of the catalyst as the main process termination cause.

Effect of Hydrogen Concentration on the Growth of Carbon Nanotube Arrays for Gecko-Inspired Adhesive Applications

“…When the H2 concentration is gradually increased to 35%, there is an evident decrease in the growth height of the VACNTs. It can be explained that excess hydrogen reacts with the carbon product during deposition, hindering the effective growth of CNTs [41]. These results show that VACNTs synthesized under a H2 concentration of 15% feature a good vertical orientation, and the tops of these arrays form small and homogeneous clusters.…”

“…In addition, maintaining a balance between carbon and hydrogen radicals in the reaction system is very important for CNT growth [40]. Insufficient hydrogen causes the oxidized catalyst Fe to be reduced incompletely, while excessive hydrogen converted the carbon production in the gas phase into the reagents, thus decreasing the overall driving force of the process [41]. Joshi et al [37,42,43] experimentally observed the role of water and hydrogen in terms of their influence on CNT growth, and point out that hydrogen controls the cracking of the carbon precursor ethylene, thereby controlling the purity as well as the number of walls of the CNTs grown.…”

Recent Progress on Electrochemical Capacitors Based on Carbon Nanotubes

“…Two of the most common orientations of CNTs are displayed schematically in Figure 3. In general, films of vertically oriented carbon nanotubes exhibit much better electrical and capacitance properties in comparison to films formed from randomly oriented nanotubes [6,50,[54][55][56][57]. Figure 4 shows the structure of the SWCNT layer formed under CVD growth procedure.…”

Growth and characterization of vertically aligned centimeter long CNT arrays

“…Especially, centimeter long CNT arrays are attractive for environmental sensor to monitor toxic chemicals and gases and for biomedical sensor to detect dopamine due to the excellent electrical, mechanical, electrochemical and thermal properties of CNT [6,[10][11][12][13]. There are variety of methods used to grow long, well-aligned CNT arrays such as oxygen assisted CVD, hot filament CVD, point arc plasma CVD, water assisted CVD, but most of them produce short CNTs (less than a few millimeters) [14][15][16][17][18][19][20][21][22][23]. Nevertheless, vertically aligned, centimeter long CNT arrays have rarely been reported since the window of growth conditions and catalyst composition is very narrow and the substrate design is challenging.…”