The reasons why metals catalyze the nucleation and growth of carbon nanotubes and other carbon nanomorphologies

Carbon nanotubes (CNTs) and other carbon nanomorphologies are grown using "typical" (Ni, Co, and Fe) and "atypical" (Al, In, Pt, Ti, Mg, Pd, K, Cs, Na, W, Mn, Mo, Ir, and Ni3C) catalysts by chemical vapor deposition. X-ray diffraction analysis reveals the formation (and decomposition) of metal carbides at different stages of these growth reactions. Based on these results and various pieces of evidence gathered from the literature, we present a model that explains why different metals catalyze the nucleation and growth of CNTs or other carbon nanomorphologies, and in particular, why Ni, Co, and Fe display the highest catalytic activity. The catalytic activity of a metal strongly depends on its electronic structure. This property of the catalyst not only controls the decomposition of the carbon source, but also the formation and stability of metal carbides and, more importantly, the release of carbon atoms. Another property of significance is the enthalpy of formation of the carbon source because it enhances the activation of the catalyst. (C) 2008 Elsevier Ltd. All rights reserved.