Controlling the morphology of multiwalled carbon nanotubes by sequential catalytic growth induced by phosphorus

Because of their exceptional electric, mechanical, and structural properties, carbon nanotubes are promising materials for a wide range of applications, such as electron field emitters,[1,2] composite additives,[3,4] electronic nanodevices,[5-7] and nano-machine components.[8,9] In view of these application potentials, intensive research has been conducted into control of the parameters crucial for modification of their desired properties. Areas of research include the morphology of the nanotubes,[10-14] insertion of metals or alloys during the synthesis,[15,16] and their positioning.[17,18] Concerning the morphology, it is generally assumed that, in catalytic decomposition, the nanotube's outer diameter correlates with the size of the catalyst particle that generates the tube,[12] while the inner diameter is mainly related to the chemical potential of the carbon atoms dissolved in the catalyst particle.[19,20] Bamboo morphologies, characterized by the periodic inclusion of graphitic layers,[13] are commonly observed, and the frequency of the inclusions appears to correlate with the diameter of the catalyst particle.[21] Coiled or helicoidal morphologies are also frequently synthesized (at temperatures of 500-800 deg C) by adding small amounts of sulfur or phosphorus to the transition-metal catalyst.[22-24]At higher temperatures, sulfur is also (known to promote filling of the nanotubes with metals and taietal sulfides.[16]