- Title
- Growth mechanisms of boron nitride nanotubes during chemical vapour deposition
- Creator
- McLean, Ben
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Boron nitride nanotubes (BNNTs), analogues of carbon nanotubes (CNTs), exhibit remarkable thermal, optical and mechanical properties and are primarily synthesised by chemical vapour deposition (CVD). Compared to the CVD synthesis of CNTs, there is very little understood regarding the growth mechanism of BNNTs. To utilise BNNTs in application, synthesis techniques require first optimisation, then upscaling. In order to optimise the CVD synthesis, the growth mechanism must be understood. This thesis addresses this knowledge gap using nonequilibrium molecular dynamics and first principles calculations for a range of BNNT precursors and catalysts. Chapter 3 presents a comprehensive review of the CVD synthesis and growth mechanism of low dimensional carbon and boron nitride nanomaterials including CNTs and BNNTs. This review highlights the synergy between experiment and theory in the advancement of carbon nanoscience where such a synergy is lacking for the field of boron nitride, hence providing motivation for the original works of this thesis. In Chapter 4, first principles density functional theory calculations of the BNNT-catalyst interface are performed. Compared to CNTs, only a select few transition metals have been tested and utilised as catalysts for BNNT synthesis. Through calculation of the interfacial energy of BNNT-transition metal contacts, the origins of catalytic activity can be probed. Both zigzag and armchair BNNTs of different diameter are examined on 1st, 2nd and 3rd row transition metal catalyst nanoparticles. The primary factor contributing to the strength of interaction is revealed to be the charge transfer at the BNNT-catalyst interface. The BNNT zigzag edge is always preferred over the armchair edge with Mg and Group III-VI metals preferring the N-terminated edge and Group VII-XII metals preferring the B-terminated edge. This work provides insight into why Fe and Ni have proved effective for BNNT growth, how tuning the catalyst can influence the BNNT growing edge and growth mode, and suggests Mn, Co, Rh, Ru, Pd, Re and W as potential catalysts for future experiments. In Chapter 5, nonequilibrium molecular dynamics (MD) simulations detail the BNNT growth mechanisms during CVD on a Ni nanoparticle catalyst from an ammonia borane precursor. The Ni-catalysed mechanisms of ammonia borane decomposition, H2 production and BN ring condensation from BN chains are revealed. The Ni catalyst selectively activates N-H bonds to free adsorbed BN fragments of H2 as they oligomerise to form chains and rings. The Ni catalyst also facilitates the cleavage of homoelemental B-B and N-N bonds to afford defect-free ring networks on the surface, comprising entirely of hexagons. BN ring networks grow perpendicular to the surface via surface diffusion and a base growth mode to form h-BN sheets which undergo direct network fusion to nucleate a BNNT cap structure. The cap structures observed are consistent with experimentally observed caps and this mechanism provides evidence for their sharper, flatter morphology compared to the rounded, CNT caps. In Chapters 6 and 7, nonequilibrium MD simulations examine the nucleation of BN during boron oxide CVD (BOCVD) on a B nanoparticle and a Ni nanoparticle catalyst respectively. In both cases, the presence of O is the primary hindrance to the nucleation of extended BN ring networks. Following previous experimental work proposing the by-products to BN formation to be H2 and H2O, the chemical roles of these species are investigated by removing these species from the simulations. When a B nanoparticle is present, the natural production and manual removal of H2 both promote the clustering of B-containing species to form B clusters. The Ni nanoparticle catalyst activates N-H bonds and naturally produces far greater numbers of H2 compared to the B nanoparticle. In either case, the O remaining restricts the formation of extended BN ring networks. The removal of H2O rarely occurs naturally and manual removal results in the nucleation of BN rings in significant quantity both in the gas-phase and at the respective B and Ni nanoparticle surfaces. The B nanoparticle acts more as a solid support structure rather than a catalyst and the growth of amorphous, defective BN networks is observed. The Ni catalyst, similar to the behaviour observed in Chapter 5 for ammonia borane CVD, facilitates the cleavage of homoelemental bonds following H2O removal such that hexagons and defect-free h-BN are selectively nucleated. Subsequently, the same BNNT cap nucleation mechanism elucidated in Chapter 5 is observed. In Chapter 8, a summary of the main scientific findings from this thesis is presented and the resulting avenues of future investigation are discussed.
- Subject
- boron nitride nanotubes; carbon nanotubes; chemical vapour disposition; nonequilibrium MD simulations; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1413303
- Identifier
- uon:36607
- Rights
- Copyright 2020 Ben McLean
- Language
- eng
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 25 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 360 KB | Adobe Acrobat PDF | View Details Download |