State of Transition Metal Catalysts During Carbon Nanotube Growth

Abstract

We study catalyst−support and catalyst−carbon interactions during the chemical vapor deposition of single-walled carbon nanotubes by combining environmental transmission microscopy and in situ, time-resolved X-ray photoelectron spectroscopy. We present direct evidence of what constitutes catalyst functionality by comparing the behavior of Ni, Fe, Pd, and Au model catalyst films on SiO 2 during preannealing in O 2 and NH 3 and during C 2 H 2 decomposition. The catalyst metal surface supplies sites to dissociate the hydrocarbon precursor and then guides the formation of a carbon lattice and the liftoff of a carbon cap. The catalysts are sharply distinguished by their reactivity toward activation of the hydrocarbon precursor, following trends known from heterogeneous catalysis. For Fe and Ni, the active state of the catalyst is a crystalline metallic nanoparticle. Graphitic networks do not form on oxidized Fe. Pd forms a silicide on SiO 2 under our reducing conditions. Pd (silicides) and Au nanocrystals are catalytically less efficient in terms of precursor dissociation, while the low adhesion of C on Au surfaces impedes nanotube nucleation.