Nucleation and growth of carbon nanotubes as a function of catalyst composition
Crouse, Christopher Alan
Barron, Andrew R.
Doctor of Philosophy
The nucleation and growth of carbon nanotubes (CNTs) occurs through a vapor-liquid-solid (VLS) mechanism and has been explored with an emphasis on the catalyst species used to initiate nucleation and sustain growth. Specific trends in growth density, diameter, length, and growth rates have been observed as features such as the catalyst source, concentration, and elemental composition were altered. The catalyst systems studied include the nanocluster [H xPMo12O40⊂H4Mo72Fe 30(O2CMe)15O254(H2O) 98] (FeMoC), spin-on-catalysts, and discrete bimetallic nanoparticles. Combining the catalytic properties of single elements into bimetallic combinations has proved be an effective approach towards increasing nanotube yields and establishing control over CNT diameters and lengths. Spin-on-catalyst studies with Fe, Co, and Fe/Co mixtures suggest that nanotube diameters and lengths can be controlled by varying the ratio of the two metals. A cobalt rich system yields high densities of short tubes with narrow diameters while iron rich systems favor longer nanotubes, lower densities, and larger nanotube diameters. Iron catalysts and Fe/Co mixtures are also active towards nanotube re-growth, including new tubes, nanotube amplification, double growth, and etching when re-exposed to a growth environment for a second time. Bimetallic combinations of Cu-Fe-O, Co-Fe-O, and Mn-Fe-O in the form of discrete nanoparticles with diameters ca. 5--7 nm have been shown to be catalytically active towards the formation of vertically aligned-CNT carpets from the decomposition of acetylene. Estimations for the average growth rates have been determined for each bimetallic combination by measuring the heights of the carpets which they produce. From these observations it has been shown that Cu-Fe-O and Mn-Fe-O nanoparticles are capable of producing CNTs at almost twice the rate observed for Fe nanoparticles alone under identical growth conditions. The synthesis of ca. 5 nm Co-Fe-O nanoparticles from organometallic precursors has also been explored as a route towards producing catalyst precursors with uniform size and composition for nanotube growth. Through investigating the properties associated with bimetallic catalysts of uniform size we have been able to obtain a better understanding of CNT growth dynamics.