Growth mechanism of vapor phase CVD-grown multi-walled carbon nanotubes

The formation mechanisms involved in the growth of carbon nanotubes (CNTs) by spray pyrolysis was studied. Both iron and nickel were used as catalysts for growth, and nanotubes were also produced using thermal chemical vapor deposition for comparison. Transmission electron microscopy was used to analyze the encapsulated metal catalyst particles found within the tubes, and the dimensions and location of these particles was recorded. CNTs grown by spray pyrolysis were found to have encapsulated particles in both the middle and end of tubes, with large length to diameter ratios. As a result of these observations, it is concluded that nanotubes grown using spray pyrolysis are formed via an open-ended, root growth mechanism. Additionally, the presence of multiple, high aspect ratio particles within single tubes is explained by an additional growth theory. During the continued growth of these CNTs, metal atoms or nanoscale metal catalyst particles deposit in the open ends of growing tubes, forming new particles and helping to prevent tube closure. CNTs grown with thermal CVD did not contain similar elongated particles or particles along the middle of the tubes, indicating that this new growth mechanism is only applicable in the case of tubes grown via spray pyrolysis or other vapor phase CVD growth methods. (C) 2005 Elsevier Ltd. All rights reserved.