Growth of carbon nanotubes on controlled sized metal nanoparticles by manipulation of supersaturation

Abstract

Carbon nanotubes are unique materials that exhibit extraordinary electrical properties which are determined by their chirality and diameter. They have great potential for use in electronic and energy related applications; however, a major roadblock for their practical usage is the limitation in understanding of how synthesis parameters modulate the diameter, length, and the type of carbon nanotubes (SWCNTs or MWCNTs). These parameters can be outlined as the catalyst and support material, the type of carbon precursor, the carbon introduction rate, and growth temperature. Theoretically, the latter two modulate the supersaturation and thus the thermodynamics and kinetics of the nanocarbon nucleation and growth. However, their role on the formation of the nanotube product has never been reported in the literature. The investigation of the role of catalyst, substrate and the supersaturation on the nature of the nanocarbon product is the broad aim of this Thesis. The first part of this study involved the investigation of metal particles deposited onto silicon surfaces. This was achieved by using various Si surfaces (i.e. smooth Si, smooth oxidised Si and porous Si) onto which metal nanoparticles (Fe and Co) were deposited using the respective metal acetates via a dip coating method. By doing this we have studied the solvent influence on the size of the metal nanoparticles produced. On smooth substrates, single-component solvents give rise to smaller catalyst nanoparticles with much narrower particle size distributions than mixed solvents did. Deposition of the nanoparticles on nano-porous substrates not only further improved the control over the particle sizes but also minimised the negative effects when mixed solvents were used. Optimisation of the nanocatalyst deposition was then followed into examination of how the controlled nano-gram quantities per unit time of the hydrocarbon precursor material can be introduced in the nanotube growth chamber over a fixed period of time. In doing this we initially examined the evaporation kinetics of ethanol under dynamic gas conditions typically employed for carbon nanotube formation. This was achieved through the use of thermal gravimetric analysis in conjunction with gas IR spectroscopy analysis. The results showed as expected that as the temperature increased there was an increase in the rate of evaporation. In addition, it was confirmed that the evaporation rate was greatly affected by the carrier gas type with nitrogen causing a higher evaporation rate than argon. The gas-IR analysis also showed that the molecular state of the ethanol changes when in the vapour state giving rise to the formation of monomers, small clusters and large clusters of ethanol. Next it was important to examine how the ethanol pyrolysis was affected during isothermal decomposition at a series of temperatures > 500 oC i.e temperatures at which the decomposition of ethanol vapours would occur. This experiment was achieved through the use of a flow-through chemical vapour deposition system where the pyrolysis products were characterised by gas IR and by GC-MS analysis. For the first time, it was found that as the temperature increases there is a shift from non-aromatic products (e.g. ethylene) to smaller aromatics (e.g. benzene, toluene) to much larger fused aromatic structures (e.g. pyrene) under conditions typically employed for nanotube formation. Finally, using the results obtained from the previous sections a series of carbon nanotube growth experiments were designed. This was done in order to examine the effect of carbon introduction rate, i.e supersaturation, on the nucleation and growth of carbon nanotubes and their rates in accordance with classical nucleation theory. Through this we have determined that supersaturation can have a large effect on the nucleation of carbon nanotubes with the nucleation rate increasing quickly as the carbon introduction rate increases. The results and discussion outlined in this Thesis are useful for further optimisation of the nanotube formation process by chemical vapour deposition.

Date of Award	2013
Original language	English