Ph. D., Faculty of Science, University of the Witwatersrand, 2011 / Nitrogen doped carbon nanotubes (N-CNTs) have become a topic of increased importance in the
study of carbonaceous materials. This arises from the physical and chemical properties that are
created when N is embedded into a CNT. These properties include modified chemical reactivity,
modified conductivity and changed mechanical, electronic and magnetic properties.
This thesis covers the analysis of the catalytic growth of N-CNTs under well defined conditions
and the optimization of reaction conditions to produce N-CNTs. Herein, a range of
methodologies have been devised to synthesize N-CNTs. One of the procedures used in this
work uses a floating catalyst in which an organometallic complex is decomposed in the gas
phase in the presence of a nitrogen containing reactant to give the N-CNTs. This thesis focuses
on the use of ferrocene and ring substituted ferrocenes in the formation of N-CNTs and other
shaped carbon nanostructures. It talks of the effects that physical parameters such as temperature,
pressure, gas flow rates and the type and concentration of N source have on the N-CNT type,
size and yields as well as the nitrogen content incorporated into the tubes that are produced using
the organometallic complexes. Proposed growth models for N-CNT synthesis are also reported.
This work reveals that the N-CNTs produced are less stable (thermal gravimetric analysis
measurements), less graphitic and more disordered (transmission electron microscope
measurements) than their undoped counterparts. The ratio of the Raman D- and G-band
intensities increase with the nitrogen concentration used during the CNT growth. Furthermore,
the transmission electron microscopy (TEM) studies reveal that the CNTs are multi-walled, and
that the diameters of the N-CNTs can be controlled by systematically varying the concentrations
of the nitrogen source. Furthermore, X-ray photoelectron spectroscopy (XPS) and CHN analysis
demonstrate that substitutional N is indeed present in the CNTs mainly as pyridinic and pyrrolic
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N (and is sp2 and sp3 coordinated). The TEM analysis also revealed that when ferrocenylaniline
and ferrocene/aniline reactions are compared at similar Fe/N molar ratios, higher N doping levels
are achieved when ferrocenylaniline is the catalyst. Investigations of surface and interior imaging
of N-CNTs was carried out by high resolution TEM (HRTEM) and identification of N-rich
regions were performed by Energy filtered TEM (EFTEM).
We also investigated the solid state pyrolysis of ferrocenylmethylimidazole or a mixture of
ferrocene (FcH)/methylimidazole at 800 oC at different ratios in sealed quartz tubes. TEM
studies showed bamboo compartments are present in the CNTs. An investigation of the bamboo
structures revealed that three methylimidazole structural isomers led to tubes with different
individual bamboo compartment distances and different morphologies including different N
contents. It was observed that when diverse N containing hydrocarbons were used the amount of
N in the nitrogen containing reagent is more important than the source and type of the N atoms
used as revealed by trends in the morphology of the N-CNTs produced. We have also studied the
effect of arylferrocene ring substituents on the synthesis of CNTs and other shaped carbon
nanomaterials in subsequent chapters.
Magnetic properties of different N doped carbon structures produced in the earlier chapters were
investigated using electron spin resonance (ESR) spectroscopy. Most importantly, we observed a
large g-factor shift in samples of N-CNTs from that of the free electron. Further, the shift is
temperature dependant.
A facile method for attaching Au nanoparticles to the surface of pristine N-CNTs and
functionalized N-CNTs has been developed. The Au nanoparticles incorporated in the N-CNTs
have a wide range of diameters (10 – 35 nm) and possess different shapes. The method offers
certain advantages, such as providing Au nanoparticles in good yields and ease of use. The
Au/N-CNT nanohydrids are being employed in catalytic reactions e.g. the oxidation of styrene.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/10541 |
| Date | 12 October 2011 |
| Creators | Nxumalo, Edward Ndumiso |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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