COST Science Officer Materials, Physical and Nanosciences Caroline Whelan and (Hanret, BE) and Santiago Cruz Esconjauregui (Leuven, BE) found a method for massive carbon nanotube (CNT) growth by using metal-free catalyst nanoparticles, for example silicon (Si) or germanium (Ge) nanoparticles and a hot filament wire.
A wire was used as a hot filament and was located above the substrate comprising the catalyst nanoparticles. A flow of 0.1 l/min. of acetylene, ethylene or methane in addition to the other gases (N2:H2) was flown over the hot filament such that the Carbon source gas was cracked. The gas composition used for this experiment was N2:H2:C at a ratio 4:2:0.1 l/min. The Carbon source used was either one of acetylene, ethylene or methane. The CNTs were grown for half an hour at atmospheric pressure.
The hot filament may be a metallic filament such as a W filament or a Ta filament. The hot filament is maintained at a temperature suitable for decomposing or cracking the carbon source gas. For example when a hot filament is used for decomposing the carbon source gas, the filament may be kept at a temperature of 950.degree. C
One of the key issues in the growth mechanisms described in the prior art is the need for a metal catalyst particle to initiate the carbon nanotube growth. A disadvantage thereof is that the metal catalyst particles can lead to the presence of impurities in the grown CNTs. Before the CNTs can be used in many applications, these impurities have to be removed. A variety of chemical and thermal oxidative treatments are usually required to remove the unwanted metal impurities from the CNTs. For example, a multi-step purification procedure may be used which involves the use of nitric acid reflux and thermal oxidation.
Catalyst-free growth of CNTs has been achieved previously by using laser ablation and arc discharge CNT growth. However, these methods require very high temperatures, i.e. temperatures of above 3000.degree. C. Due to these high required temperatures, these methods are not suitable for in-situ CNT growth and consequently require an ex-situ approach. Furthermore, these methods may give low production yields compared to CVD methods that can be performed at relatively low temperatures (450-1100.degree. C.), can be in-situ or ex-situ, and give mass production yields.