1,721,184 research outputs found
Designing boron nitride nanotubes and their macroscopic assemblies via sacrificial templating
Get PDFHexagonal boron nitride nanotubes (BNNTs) have properties advantageous for the
development of next generation materials and advanced composites that exhibit improved mechanical strength, high temperature resistance and thermal conductivity.
The future exploitation of BNNTs depends on the development of economically
accessible and safe production methods. These should also present a scope for BNNT structural tunability, and output a high purity of BNNTs versus other possible
hexagonal boron nitride (h-BN) structures. In this thesis, a novel synthesis technique
is validated that is primarily motivated by a demand to address these issues.
Here, atmospheric pressure chemical vapour deposition (APCVD) is employed in
conjunction with a safe and easy-to-handle compound, ammonia borane (H3N−BH3),to deposit h-BN coaxially onto the surface of multi-wall carbon nanotubes (MWCNTs). Subsequently, thermal oxidation is demonstrated to remove/sacrifice the MWCNTs, thereby leaving BNNTs as shells that exhibit diameter dependence on their MWCNT templates. By this method, BNNTs with walls 2-3 nm thick are synthesised, and nanotube outer diameters 16±2 nm and 40±16 nm are obtained using separate grades of MWCNT templates. The achieved BNNT diameter control exemplifies the versatility of MWCNT as a template material, which is justified by its structural compatibility to BNNTs, commercial scale availability and established morphological diversity. It is also argued here that use of H3N−BH3 precursor favours the van der Waals isolation of the deposited h-BN from the MWCNT template, which is advantageous for the h-BN purity of the BNNT shell.
The work presented here also establishes that the proposed APCVD and
sacrificial templating method, when implemented in combination with vacuum
filtration, is a novel and adaptable means of fabricating macroscopic assemblies of
BNNTs. This is achieved at the cm-scale for BNNTs randomly entangled within a
low density, self-supporting and sheet-like architecture with thicknesses <100µm.
Furthermore, platelet crystals discovered on the surface of these BNNT assemblies,
revealed to be boric acid, provide evidence in support of a new theory on the
oxidation behaviour and moisture sensitivity of BNNTs.
Throughout this work, newly synthesised nanomaterials (non-assembled and
assembled) are characterised by multiple techniques including electron microscopy,
spectroscopy and thermogravimetry, in order to determine their qualities such as
dimensionality, crystallinity, chemical composition and thermal stability
Tailoring the synthesis of graphene and other 2D materials towards applications
Get PDFThis thesis focuses on using Chemical Vapour Deposition (CVD) technique to produce various classes of flat 2D materials and also their three-dimensional architectures. Graphene, hexagonal boron nitride (hBN) and tungsten disulfide (WS2) are the main examples presented in the study: a conductor, an insulator and a semiconductor respectively. Such materials are elementary "building blocks" in a variety of applications. Many novel advances to CVD synthesis of 2D materials are identified, developed and discussed in this thesis
Biocompatible carbon nanotube/β-titanium alloy composite materials
Get PDFThe thesis describes a study of the modifications of orthopaedic Ti-based substrates using nanomaterials, and the evaluation of their biocompatibility for further use as implant material, with the aim to develop new, biocompatible β-Ti/CNT composite materials. Traditionally, CNTs require the presence of a transition metal catalyst such as Fe, Ni, Co, for successful growth. Different aspects of a catalyst-assisted CVD MWCNTs growth on various Ti-based substrates including bulk, thin films and 3D porous scaffolds, have been investigated. Low concentrations of catalyst were deposited using spin coating on titanium substrates of various forms and shapes. A strong influence of the surface topography was observed. In contrast, no effect of the elemental composition of the substrate could be detected.
To evaluate the biocompatibility of the newly created materials, cell culture studies using fetal human osteoblasts (fHobs) were performed. It was shown that β-Ti/MWCNTs samples possess good initial osteoblast attachment, but no long-term osteoblast activity. Hence the biocompatibility of isolated (i.e. without a Ti substrate) MWCNTs was studied, using MWCNT carpets and various types of MWCNTs buckypapers. All the samples revealed very low cell activity. While β-Ti/MWCNTs samples did not exhibit good biocompatibility, alternative β-Ti/TiC samples were synthesized with a simple CVD method and revealed good osteoblast response with increased mineralization. Moreover, good corrosion resistance and mechanical properties of β-Ti/TiC samples have been reported.
Finally, successful method for non-catalytic CVD MWCNTs growth on Ti substrates was developed for the first time, thereby excluding potentially toxic catalysts from the implant material. CVD was performed with acetylene precursor on bulk titanium substrates etched with Piranha solution, which generated an appropriate surface to foster MWCNTs growth. A combination of the change in the surface roughness, improved hydrophilicity, and elemental composition of the surface as a result of the Piranha etching is likely to be responsible for the successful formation of MWCNTs.</p
Production and properties of two dimensional borocarbonitrides using chemical vapor deposition growth: Rational design and synthesis of 2D borocarbonitrides via chemical vapor deposition
Get PDFGraphene and hexagonal boron nitride (hBN) are two of the most widely studied two-dimensional (2D) nanomaterials and their development is considered mature in the field. Although they possess a similar structure, their opto-electronic properties vary significantly – graphene is a zero-bandgap material while hBN has a wide band gap. Recently, synthesis of hybrid structures of these materials, comprised of boron, carbon and nitrogen, has piqued interest due to the tuneability of the opto-electronic and chemical properties of the system by its atomic composition and their arrangement within the network. Formally known as borocarbonitrides (BCN), synthesis of these structures is a promising field of research due to potential applications in catalysis, energy storage, and opto-electronics. However, cost-effective, scalable methods to produce high quality BCN materials are required for adoption. This thesis aims to address this research gap by establishing a low-cost synthesis protocol for 2D BCN structures, using safe-to-handle precursors, methane and ammonia borane, and facile synthesis apparatus, atmospheric pressure chemical vapor deposition (APCVD).
Pristine metal substrates of Cu and Fe are investigated and reviewed for the APCVD growth of 2D BCN. It is found that for surface-mediated growth of carbon doped hBN (hBN-C) on Cu, different growth mechanisms are present at various partial pressures of methane, affecting the material’s morphology and opto-electronic properties in differing ways. For modulation of the bandgap of hBN, C doping is restricted to low partial pressures of CH4 only. At higher partial pressures, C segregation and graphene formation occurs, which restores the bandgap of hBN, something which has not been reported in the literature. Performing complementary Density Functional Theory (DFT) calculations, the first comparative study on the atomic processes of B, C and N on Cu reveals insights into the behaviour of the precursors on the substrate, and sheds light on the different growth mechanisms observed at the various methane partial pressures employed during our hBN-C CVD study on Cu.
Mechanical robustness of films is a consideration for certain applications and so multi-layer growth of hBN-C films is investigated for the first time via a segregation-mediated growth mechanism on Fe substrates. Even at significantly higher partial pressures of methane, no sign of C and BN segregation is observed, while a tuneable optical bandgap from 5.92 eV to 5.31 eV is found, suggesting the segregation growth mechanism on Fe is more accommodating when synthesising carbon doped hBN than surface-mediated growth on Cu.
In parallel to CVD investigations, DFT methods are employed to study the opto-electronic properties of hBCN materials. Due to the propensity for graphene and hBN to segregate, graphene-hBN lateral heterostructures are chosen as the system for study, assessing their viability as photo-catalysts in the water-splitting reaction. Seven systems are investigated, starting from pure hBN and increasing the C content of the carbon domain at the centre until a pure graphene domain is obtained. Three systems are found to have suitable opto-electronic properties of band-gap and band-edge potentials for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with two of these showing photo-activity in the solar spectrum, and are proposed as potential photo-catalysts for further future investigations.
Over the course of this study, challenges were identified in using aged or low-purity ammonia-borane for hBN growth. Experiments using commercially available, low-purity precursor were compared with results from syntheses using high purity ammonia-borane, which has not been carried out in the literature previously. The low purity AB precursor resulted in ubiquitous growth of silica nanoplatelets on the Cu surface, interfering with hBN deposition. The origin of these structures is investigated, highlighting the importance of appropriate precursor and CVD apparatus selection in hBN synthesis. A growth mechanism for the structures is presented based on the diffusion limited aggregation model of growth, which occurs as a result of SiO2 contamination via the quartz CVD reactor and BxOy activation by the low-purity precursor. Improved CVD protocols are outlined to avoid their formation in future studies.
Finally, a simple, cost-effective and up-scalable method to produce high-quality porous BCN-metal composites is developed based on prior BCN growth studies on Cu pristine metal foils. Using Cu metal powders, porous metal scaffolds were realised via thermal annealing, which can offer a significant cost reduction compared to commercially available alternatives. 3D BCN@Cu foam composites were realised via simultaneous and sequential deposition techniques, providing BCN materials with different morphologies and level of B,N doping which can add functionality to the Cu foam for many future far-reaching applications
Application of nanostructured emitters for high efficiency lighting
Get PDFThis is the first study comparing morphologies of CNT films on Kanthal wire, with their field emission properties, and as such offers ways to design better cylindrical emitter devices. A low turn-on field was achieved (0.35 V/µm), the field emission results have been explained using a simple model, and a fluorescent lamp was fabricated. Whilst previous work has been done on the link between “as grown” CNT films and their respective field emission properties on flat substrates, very little work has been done on linking morphology to emission performance on wire substrates, where the morphology can be very different. Microscopic structures such as towers, ridges and clumps consisting of many aligned or entangled CNTs were grown using an aerosol chemical vapour deposition (a-CVD) technique. Hydrogen added to the carrier gas resulted in a decrease in defect density in the growth of undoped CNTs, and an increase in defect density in the growth of nitrogen doped CNTs (N-CNTs) and boron doped CNTs (BCNTs). In-situ transmission electron microscopy (TEM) studies show that damage to CNT tips results in a significantly higher turn-on field compared to undamaged tips. This can be recovered by making the CNT emit current for several minutes which makes the tip recrystallize due to heat caused by the Nottingham effect. The field emission properties of the “as grown” CNT films are dominated by protruding CNTs found at the edges of ridge and tower microscopic structures. The field emission properties are also related to the dimensions of these structures with the longest ridges (hence those with the longest protruding CNTs) resulting in the lowest turn-on electric field. The ridge and tower structures act to accommodate protruding CNTs at their edges and their physical dimensions (mainly width) act to separate these emitters so that screening is minimised. This work shows that efficient emitters can be fabricated effectively from simple a-CVD techniques and microscopic structures act to improve, not degrade, field emission properties
Carbon Nanotube Growth on Perovskite Substrates
Get PDFThis thesis reports on the chemical vapour deposition (CVD) growth of carbon nanostructures (mainly carbon nanotubes (CNTs)) on perovskite oxide surfaces with the aid of various catalysts. Two types of perovskite oxide, single crystal SrTiO3 (001) and polycrystalline BaSrTiO3, have been used as catalyst supports (in metal-catalyst-involved CVD routes) or as catalysts (via metal-catalyst-free CVD routes) for the growth of carbon nanostructures. In metal-catalyst-involved cases, SrTiO3 (001) single crystal has been proven, for the first time, to serve as a substrate for the growth of CNTs. Fe and Ni catalysts can be tailored in a controllable manner on SrTiO3 (001) surfaces prior to the CNT synthesis, forming truncated pyramid shaped nanocrystals with uniform size distributions. The growth of vertically aligned CNT carpets was realised with the aid of Fe on SrTiO3 (001) surfaces, and it was further found that the CNTs grow via a base growth model. Furthermore, it is possible to grow helical carbon nanostructures on BaSrTiO3 substrates by introducing a Sn catalyst into the system. The synthesised helical carbon nanostructures follow a tip growth mode, where the structural and chemical aspects of catalyst particles gave rise to a wide range of carbon morphologies. CNTs were also grown on single crystal SrTiO3 (001) and polycrystalline BaSrTiO3 substrates via metal-catalyst-free routes. The surface-roughness-tailored growth of CNTs was surprisingly achieved on a series of engineered SrTiO3 (001) surfaces, where a correlation between the surface roughness/morphology of the substrates and the relevant catalytic activity was revealed. The growth of CNTs arises because the catalyst fabrication methods lead to the formation of SrTiO3 asperities with nanoscale curvatures, over which the CNTs are generated throughout a lift-off process. Facet-selective growth of CNTs was observed on polycrystalline BaSrTiO3 surfaces, where BaSrTiO3 (110) facets lead to the growth of CNTs on them, whereas the (001) facets result in no growth at all. This observation was further analysed in the content of the adsorption and diffusion of carbon species on distinct BaSrTiO3 facets, before reaching the conclusion that the formation of CNTs occurs through a metal-free, stack-up process driven by the assembly of the carbon fragments
Chemical vapour deposition growth of large-area graphene on metals
Get PDFGraphene has unrivalled properties and is heralded as a revolutionary material for the 21st century. Chemical vapour deposition (CVD) on metals is a promising method to produce large-area graphene. Controlling the properties of CVD graphene is vital for its integration in a wide-range of future applications. Many factors can influence the CVD growth of graphene and its properties, therefore further investigations will be beneficial to fully understand and control this technique. In this thesis I expand the knowledge about the growth of pure and heteroatom-doped graphene by low pressure chemical vapour deposition (LPCVD) and atmospheric pressure chemical vapour deposition (APCVD) on commercially available Cu and Pt foils. Using a range of characterisation techniques, I investigate the influence of the substrate’s properties and the synthesis conditions on the growth of graphene, in pursuit of improved, controlled or optimised production, which can promote high quality, large-area, single-layer graphene, or other as desired. By characterising the topography, surface roughness, crystallographic orientations, and chemical composition of six Cu foils, I find that their properties vary greatly and this influences the growth of CVD graphene. I elucidate that the commonly used 99.8 % Alfa Aesar Cu foil has a surface coating composed of calcium, chromium, and phosphorus, which detrimentally influences graphene growth. Cleaning Cu foils with CH3COOH is shown to reduce the concentration of surface contaminants, consequently reducing the nucleation density and increasing the growth rate of CVD graphene. I also demonstrate that the shape, orientation, edge-geometry and thickness of CVD graphene domains can be controlled by the Cu crystallographic orientations. Single layer LPCVD graphene domains align with zigzag edges parallel to a single direction on Cu{111} and Cu{101}, while bilayer domains align to two directions on Cu{001}. Hexagonal APCVD domains also preferentially align with edges parallel to the direction(s). This discovery resolves a key challenge of controlling the orientation of individual graphene domains and opens a new avenue for tailored production of large-area CVD graphene with improved properties. By controlling the synthesis conditions of APCVD graphene on Pt foils I optimise production of ~0.5 mm single layer graphene domains with reduced nucleation density and increased growth rate of ~100 μm/min by synthesis at 1150°C, a higher temperature than previously reported. The absence of large, hexagonal, single-crystal domains on pristine Pt foil, and observation of a reaction between quartz and Pt that promotes hexagonal domains, suggests that a silicon or platinum silicide surface layer may be advantageous for improved growth of graphene. Finally, I demonstrate that the dopant concentration of nitrogen-doped graphene is increased at lower synthesis temperatures and higher NH3 concentration, up to 1.3 %, but with an associated decrease in the growth rate. Direct visualisation, elemental confirmation, and electronic characterisation of individual nitrogen atoms is shown for the first time using aberration corrected scanning transmission electron microscopy and electron energy loss spectroscopy. Boron-doped graphene is also synthesised. The implications of these findings, and many additional minor contributions, are wide-ranging and of considerable importance for the future understanding of CVD growth of graphene on metals, and more generally for the advancement of scientific knowledge for manufacturing large-area graphene. Collectively, these discoveries represent a significant body of work that can improve the efficiency of production and assist with controlling the properties of large-area CVD graphene
Tailoring nanomaterials for energy applications
Get PDFSilicon-based materials present high theoretical capacities (4200 mAh/g), making them among the most promising materials for Li-ion battery anodes. However, the major problem with them is massive volume change (∼ 300%) that occurs during cycling, resulting in pulverisation and significant capacity drop of these anode materials. Besides, the use of additives and metallic current collectors to make the anodes reduces the overall battery energy density as these components could not contribute any capacities to the battery. In this work, free-standing silicon-coated multi-wall carbon nanotube (MWCNT) anodes were developed without any polymeric binders and carbonaceous additives being added. These materials were achieved through exploiting atmospheric pressure chemical vapour deposition as well as advanced nanomaterials fabrication and processing methods. Electrodes were prepared utilising MWCNT papers (so-called buckypapers) as robust, electrically conductive flexible (22.2 S/cm) and free-standing scaffolds since the capacity of these buckypapers is too low to use as a new active material for the battery (0.3 mAh/cm2 in the 300th cycle). Hence, these buckypapers were functionalised with silicon nanoparticles attached through a strong silicon carbide interface and coated with a protective amorphous carbon layer. The SiC interface was partially developed between Si NPs and MWCNTs as excessive SiC could reduce electrochemical capability on the material due to its low electroactivity. The coin cell battery test demonstrated a high areal capacity of up to 2.01 mAh/cm2. These free-standing Si-CNT-SiC-C materials showed good rate capability and stability performance (0.55 mAh/cm2 after 300 cycles with coulombic efficiency of 99.7%). As a result, the material interface design, which is the key of this work, enhances the connection of Si NPs to the MWCNT walls and addresses the issue of Si detachment. A post-mortem result also revealed the structural stabilisation of the electrode after the repeated cycling test. In addition, the pouch-cell battery was fabricated to confirm the use of this free-standing Si-CNT-SiC-C electrode without the influence of the battery component. This new anode architecture performs better than the buckypaper, thereby increasing the areal capacity by 83.3%, allowing better functionality for this material to be implemented in a full-cell battery system
Development of an aerosol-CVD technique for the production of CNTs with integrated online control
Get PDFThis dissertation summarises the study of different aspects of the aerosol-assisted chemical vapour deposition (AACVD) technique for the production of multi-wall carbon nanotubes (MWCNTs). Upscaling the synthesis while retaining the quality of MWCNTs has been a prime objective throughout the work. A key aspect of this work was the study of different growth parameters and their influence on the homogeneity of the products across the reactor. The effect of the precursor composition on the yield and quality of MWCNTs were also investigated. It was shown that the synthesis rate can be significantly (60 – 80 %) increased by tuning the composition of the precursor. Moreover, by optimising the synthesis recipe and using a larger reactor, the synthesis rate and efficiency of the precursor were increased fivefold (up to 14 g/hr) and twice (up to 88 %) respectively. Large area (up to 90 cm2), mm-thick carpets of MWCNTs which were both free-standing and on substrate were produced. The carpets could withstand normal handlings without tearing apart, making them suitable for macroscopic characterisations and applications. By in-situ qualitative and quantitative gas analysis of the atmosphere of the reactor, the thermocatalytic cracking behaviour of 25 precursors was investigated and a mechanism for successive formation of different hydrocarbon fragments inside the reactor was proposed. A number of dedicated gas analysis methods and apparatuses such as a probe for zone-by-zone gas analysis of reactor and a heated chamber for preparation of standard gas analysis samples were developed to explore some of the least investigated aspects of the thermocatalytic cracking of precursors. Mapping the reactor revealed that some single-wall and double-wall carbon nanotubes (SWCNTs and DWCNTs) were also produced near the exhaust of the reactor. The SWCNTs were partly covered by fullerene-like species and resembled different forms of carbon nanobuds. In addition, the effect of the electron beam on the interaction of the SWCNTs and the fullerene-like species was studied in situ using high-resolution transmission electron microscopy (HRTEM)
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