- Title
- Heterogeneous catalytic processes for carbon nanotube growth control
- Creator
- Eveleens, Clothilde Amelia
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Single-walled carbon nanotubes (SWCNTs) possess extraordinary electrical, optical and mechanical properties which are determined by their (n, m) chirality. For these properties to be exploited, synthetic methods capable of producing single (n, m) SWCNTs on an industrial scale need to be developed. So-called "chirality-controlled" growth techniques are a major goal in SWCNT growth today. The most popular technique for synthesising SWCNTs is chemical vapour deposition (CVD). Often, chemical etchants are added to the CVD feedstock to remove amorphous carbon from the catalyst surface and prolong the catalyst lifetime, however, there is recent experimental evidence that etchants serve another purpose; they can change (n, m) SWCNT distributio ns. Nevertheless, very little is known about how etchants influence CVD mechanisms and (n, m) distributions. This thesis addresses this knowledge gap, using quantum chemica l methods. In Chapter 3, the (n, m) dependent chemical reactivity of SWCNT cap structures with ammonia-based etchant radicals (NH₂, NH and H) is investigated with density functional theory (DFT). It is revealed that the selective destruction of particular (n, m) SWCNTs by chemical etchants during growth is an important component of the etching process. A thermochemical model for predicting the abundances of (n, m) SWCNTs as a function of the concentration of the etchant species is presented based on the propensity for H and NH₂ to etch near-zigzag SWCNTs, and NH to etch near-armchair SWCNTs. In Chapter 4, competing reactions between OH as an etchant, and C₂H as an active growth species at the reactive edge of SWCNT caps are examined with DFT. Both radicals are shown to preferentially react with near-zigzag SWCNTs. By applying a “Goldilocks principle” it is proposed that chirality-control via etching is possible by balancing growth and etchant reactions, through careful selection of the etchant and feedstock species. Quantum chemical molecular dynamics (QM/MD) simulations detailing how ammonia affects surface chemistry during methane CVD and SWCNT nucleation on transition metal catalysts are presented in Chapters 5 and 6. Ammonia primarily etches carbon from Fe and Ni catalyst surfaces via one main pathway, viz. the formation and dissociation of hydrogen (iso-)cyanide. On Fe, ammonia also provides an additional source of hydrogen which passivates carbon dangling bonds and inhibits ring condensation, and hence SWCNT nucleation. Similarly, nucleation on Ni was shown to be hindered in the presence of ammonia, however, the mechanism differs from that on Fe. Methane decomposition proceeds by disproportionation on Fe, whereas hydrogen interacts weakly with Ni and evolves from the surface. As such, methane decomposes more rapidly on Ni. The additional hydrogen generated from Ni-catalysed ammonia activation does not passivate carbon-dangling bonds, but lowers the surface carbon chemical potential to slow nucleation. Patterns of ring formation differ, and ammonia induced changes in ring growth kinetics vary between transition metals. Thus, the catalyst metal itself influences the manner in which nitrogenous species alter CVD surface chemistry. In Chapter 7, experimentally-observed diameter-modulating effects of acetonitrile are evaluated using QM/MD. Like ammonia, HCN was created in noticeable quantities from reactions between surface hydrocarbons and cyano radicals. Otherwise-passivating hydrogen was removed from the surface chemistry via this process, and ring growth kinetics increased accordingly in the presence of acetonitrile. More curve-inducing pentagon rings were introduced to nucleating pre-SWCNT cap structures at a faster rate, hence, it was predicted that smaller-diameter SWCNTs would be produced from these highly-curved caps. Lastly, Chapter 8 presents a summary of the main conclusions gained from this thesis, and discusses potential avenues for future research.
- Subject
- carbon nanotubes; chirality; chemical vapour deposition; carbon nanotube growth; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1403624
- Identifier
- uon:35196
- Rights
- Copyright 2019 Clothilde Amelia Eveleens
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 19 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 401 KB | Adobe Acrobat PDF | View Details Download |