ETD Collection

Permanent URI for this collectionhttps://wiredspace.wits.ac.za/handle/10539/104


Please note: Digitised content is made available at the best possible quality range, taking into consideration file size and the condition of the original item. These restrictions may sometimes affect the quality of the final published item. For queries regarding content of ETD collection please contact IR specialists by email : IR specialists or Tel : 011 717 4652 / 1954

Follow the link below for important information about Electronic Theses and Dissertations (ETD)

Library Guide about ETD

Browse

Subject: search.filters.subject.Nanostructures

Search Results

Now showing 1 - 8 of 8
  • No Thumbnail Available
    Item
    Group III-V nanostructures application in gas sensing
    (2018) Nyembe, Sanele G.
    The advent of nanoscience and nanotechnology has made it possible to control several properties such as material shape, size and stability. However, different production approaches are often required. Engineered surfaces with tailor-made properties such as large surface area or specific reactivity are used routinely in a range of applications such as in fuel cells, catalysis, etc. Nanomaterials with unique morphologies have been developed and used in fields such as electronic device manufacture, chemistry and engineering. Synthesis of gold icosahedral (Ih) and decahedral (Dh) nanostructures was successfully achieved through a two-step heterogeneous nucleation process. Citrate stabilised seeds were used to grow these nanostructures. Cetyltrimethylammonium bromide (CTAB) was used to promote fast growth of Au nanostructure along the [111] crystal lattice plane. Dh and Ih nanostructures are known to be thermodynamically unfavourable above size of 5 nm. Successful growth of such nanostructures above this critical size limit was explained in terms growth kinetics. Gold nanostructures were found to have an average particle size of 50 nm and a narrow size distribution range of 45 nm to 55 nm. The seeds were fast-handled during growth to enhance the formation of these nanostructures. In-depth characterisation of these nanostructures confirmed that they formed via crystal twinning mechanism. Synthesis of gold nanostructures with different sizes was achieved through a three-step heterogeneous nucleation process. Different types of seeds were prepared using different stabilizers (Citrate and CTAB). Citrate and CTAB stabilized seeds were used to grow Au nanostructures separately. The CTAB stabilized seeds showed polydispersity, suggesting the presence of various shapes. These seeds produced agglomerated particles of various shapes with a wide particle diameter distribution ranging from 50 nm to 200 nm. The triangular Au nanostructures present in the mixture had a 3 dimensional morphology (i.e. pyramid shape) as confirmed by atomic force microscopy (AFM). The citrate stabilized seeds were monodisperse and they yielded well dispersed Au nanostructures with uniform morphologies. These Au nanostructures had an average particle size of 50 nm and a narrow size distribution range of 45 nm to 55 nm. iii Laser assisted synthesis of silicon nanowires (SiNWs) was achieved through the use of gold and nickel nanoparticles as catalysts. The diameter of the resulting SiNWs was found to be dependent on that of the catalyst. The gold catalysed silicon nanowires were unevenly curved and branched owing to the high kinetic energy possessed by gold nanoparticles (AuNPs) at relatively high processing temperature. The use of nickel as catalyst resulted in the formation of several SiNWs on a single nickel catalyst due to interconnection of the nickel metal particles at processing temperature. The morphology of SiNWs catalysed by both nickel and gold was controlled by optimising the laser energy during ablation. Indium phosphide nanowires (InPNWs) with an average diameter of 87 nm were successfully synthesized through thermal chemical vapour deposition (CVD) method. The smooth surface nanowires showed a relatively narrow size distribution of 70 nm to 105 nm. Temperature programmed deposition (TPD) was used to study the thermodynamic behaviour of gas desorption. The study revealed that gaseous CO and CH4 molecules bind to InPNW surface through chemical and physical adsorption. Redhead method was used to estimate the enthalpy energy of desorption for CO and CH4 to be 142 kJ/mol and 38 kJ/mol. The sorption temperature ranges were found to be 220 ̊C to 260 ̊C for CO and -50 ̊C to -20 ̊C for CH4. InPNWs were used to fabricate a gas sensor electronic device and were tested for performance. The device showed a quick response time of 29.19 s for CO at 250 ̊C. Indium arsenide nanowires (InAsNWs) with an average diameter of 45 nm were successfully synthesized using homogeneous catalysis approach and chemical vapour deposition method. Succesful synthesis of InAsNWs was achieved at a temperature of 700°C suggesting that solution-liquid-solid growth mechanism was involved. Synthesis conditions were optimised to minimise InAsNWs polytypism and stacking faults. The results showed that InAsNWs have significant adsorption sites and affinity for CO and H2S gases due to the formation of electron accumulating surface. The calculation of enthalpy energy of desorption revealed that interaction of InAsNWs and CO was through physisorption. InAsNWs showed a response time of 72s for CO at 250 ̊C. Characterization of the nanostructures was carried out using high resolution transmission electron microscope (HRTEM), Raman spectroscope, high resolution scanning electron microscope (HRSEM), UV-Vis spectrometer, X-ray diffractometer (XRD), temperature programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscope (DRIFTS).
  • No Thumbnail Available
    Item
    Use of chlorinated carbon materials to make nitrogen doped and un-doped carbon nanomaterials and their use in water treatment
    (2018) Maboya, Winny Kgabo
    Carbon nanomaterials (CNMs) and nitrogen doped CNMs (NCNMs) with different morphologies were obtained by decomposition of various chlorinated organic solvents using a chemical vapor deposition (CVD) bubbling and injection methods over a Fe-Co/CaCO3 catalyst. CNFs, CNTs with secondary CNT or CNF growth, bamboo-compartmented and hollow CNTs were obtained. Increasing the growth time to 90 min resulted in growth of ~ 90 % of secondary CNFs on the surface of the main CNTs, using dichlorobenzene (DCB) as source of chlorine. The secondary CNFs grew at defects sites of the CNT wall. Secondary CNFs were not observed at other studied temperatures, 600, 650. 750 and 800 °C. Using an injection CVD method, horn-, straw- and pencil-shaped closed and open-ended CNTs/CNFs were obtained from CH3CN/DCB solutions of various volume ratios. CNT growth was enhanced after addition of chlorine. Highly graphitic carbon materials were produced from feed solutions containing low and high DCB concentrations. CNTs with defects were obtained from solutions containing 66.7 vol.% DCB. Post-doping of the N-CNTs with chlorine and of the chlorinated CNTs with nitrogen resulted in production of highly graphitic materials. Using a bubbling CVD method, mixtures of CNMs namely, hollow and bamboo-compartmented CNTs with and without intratubular junctions and carbon nano-onions filled with metal nanoparticles were obtained from feed solutions containing TTCE. MWCNT/PVP composite nanofibers were successfully synthesized using an electrospinning technique. Adsorption capacities of 15–20 g/g were obtained in pure oil or in oil-water mixtures. The adsorption capability of the MWCNT/PVP composite depended on the type of oil and its viscosity.
  • No Thumbnail Available
    Item
    A parametric study of resin-gel synthesis to understand the formation mechanism of titanium-oxide nanoparticles
    (2018) Narrandes, Ashvir Ashwin
    The relatively unknown resin-gel synthesis technique has the potential to form multi-mode mixed metal oxide nanoparticles with differing stoichiometries. These oxides can be employed in a plethora of applications. In order to exploit these benefits, the mechanism of nanoparticle formation must be understood. To this end, this study embarked on a parametric investigation to gain insights on the formation of the less stable anatase and more stable rutile (titanium dioxide) using resin-gel synthesis. By adjusting parameters such as the type of polymer, solvent, acid, and metal ion precursor, and by varying other parameters such as the polymer chain length, polymer stoichiometry, and heating rate, a model for nanoparticle formation was developed and refined. This model considered the formation of hydroxylated metal ion species following the addition of a metal ion precursor to a hydroxyl-containing solvent. These species were protected and stabilised by the remaining fragments of solvent components. In addition, the size of the ligands attached to the metal ion precursor governed the amount of protection and stabilisation afforded to the hydroxylated species by the precursor. These complexes were coordinated to polymer chains that underwent degradation during the course of heating and ignition. Polymer degradation produced polymer reaction chambers. The formation, action, and interaction of these chambers with developing titania crystallites are a novel finding of this work. The sizes of these chambers were controlled largely by the quantity of polymer present in the reaction. The number of accessible oxygen sites on the precursor determined the degree of association between the metal ion complexes and the reaction chambers. If the association was intimate, the polymer reaction chambers served to stabilise and protect the newly nucleated anatase particles. If the combination of protection effects afforded by the solvent components, precursor ligands, and association of reaction chambers of appropriate sizes was insufficient to stabilise nucleated anatase, it readily converted into the rutile phase. Anisotropic growth along [0 0 1] then caused rutile to form nanorods. Rutile mesocrystals developed following sufficient polymer degradation. The association of nanoparticles with polymer fragments was viewed using TEM. Additionally, TEM investigations revealed the presence of polymer-derived superstructures containing reaction chambers. Reaction chambers presented with various morphologies and were composed of crystalline carbon.
  • No Thumbnail Available
    Item
    The influence of off-diagonal disorder on resonant transmission and emergent phenomena in nanostructured carbon thin films
    (2017) McIntosh, Ross William
    Nano-structured carbon lms, long studied due to the promise of exceptional quantum transport properties, present a signi cant problem in condensed matter due to the disorder which inherently forms in these materials. This work addresses the role of structural disorder in low dimensional carbon systems. The in uence of structural disorder on resonant transmission is studied in diamond-like carbon superlattices. Having established a model for disorder, this model for the structural changes is then applied to interpret experimental measurements of diamond-like carbon superlattices. The role of phonons on resonant transmission under a high frequency gate potential was also studied. This model for structural disorder in heterogeneous carbon lms was then applied to disordered superconductors close to the Anderson-Mott transition using the inhomogeneous Bogoliubov-de Gennes theory. This analysis is then used in support of experimental work to understand the superconductor-insulator transition in boron doped nano-crystalline diamond lms. Coherent quantum transport e ects were demonstrated in structurally-disordered diamondlike carbon (DLC) superlattices through distinct current modulation (step-like features) with negative differential resistance in the current-voltage (I-V) measurements. A model for these structurally disordered superlattices was developed using tight-binding calculations within the Landauer-B uttiker formalism assuming a random variation of the hopping integral following a Gaussian distribution. Calculations of the I-V characteristics for different con gurations of superlattices compliment the interpretation of the measured I-V characteristics and illustrate that while these DLC superlattice structures do not behave like conventional superlattices, the present model can be used to tailor the properties of future devices. Furthermore this tandem theoretical and experimental analysis establishes the validity of the model for structural disorder. The same model for the variation of disorder was then applied to interpret the electronic transport properties of disordered graphene-like carbon thin films. The influence of disorder on the activation energy in few layer graphitic lms was modelled and compared with experimental observations through collaboration. The lms, grown by laser ablation, allowed the speci c e ects of structural disorder in the sp2 - C phase to be probed. Defects acted as effective barriers resulting in localization of charge carriers. Electron transmission spectra, calculated with a tight-binding model, accounted for the change of localization length as a result of disorder in the sp2 - C phase. This theoretical study showed that the localization length of the thin graphitic lms can be tuned with the level of disorder and was shown to be consistent with experimental studies. The in uence of nitrogen incorporation on resonant transmission in DLC superlattices was then studied theoretically. This study illuminated the speci c role of the nitrogen potential in relation to the Fermi level (EF ) in nitrogen incorporated amorphous carbon (a- CN) superlattice structures. In a-CN systems, the variation of conductivity with nitrogen percentage has been found to be strongly non-linear due to the change of disorder level. The e ect of correlated carbon and nitrogen disorder was investigated in conjunction with the nitrogen potential through analysis of transmission spectra, calculated using a tight binding model, which showed two broad peaks related to these species. It was shown that the characteristic transmission time through nitrogen centres can be controlled through a combination of the nitrogen potential and correlated disorder. In particular, by controlling the arrangement of the nitrogen sites within the sp2 - C clusters as well as their energetic position relative to EF , a crossover of the pronounced transmission peaks of nitrogen and carbon sites can be achieved. Furthermore, it was shown that nitrogen incorporated as a potential barrier can also enhance the transmission in the a-CN superlattice structures. The strong non-linear variation of resistance and the characteristic time of the structures can explain the transport features observed experimentally in a-CN fi lms. This analysis was then partnered with measurements performed on nitrogen-incorporated carbon superlattices (N-DLC QSL) by Neeraj Dwivedi (National University of Singapore). The electrical characteristics of these nitrogen incorporated superlattice devices revealed prominent negative di erential resistance (NDR) behavior. The interpretation of these measurements was supported by 1D tight binding calculations of disordered superlattice structures (chains), which included signi cant bond alternation in sp3-hybridized regions. This analysis showed improved resonant transmission, which can be ascribed to nitrogendriven structural modi cation of the N-DLC QSL structures, especially the increased sp2-C clustering that provides additional conduction paths throughout the network. In order to determine the in uence of additional factors on coherent quantum states in molecular systems as an extension to the analysis on superlattices, a theoretical study of the electron-phonon interaction in double barrier structures under the in uence of a timedependent gate potential was undertaken. The Floquet theory was employed along with expansion in a polaron eigenbasis to render a multi-dimensional single body problem. An essentially exact solution was found using the Riccati matrix technique. It was demonstrated that optimal transmission can be achieved by varying the frequency of the gate potential. In addition, it was shown that the gate potential can be used to control the energy of the resonant states very precisely while maintaining optimal transmission. Having gained a deep understanding of the structural changes induced in carbon systems through the incorporation of nitrogen, a similar structural model was then applied to study the changes induced in diamond and nanocrystalline fi lms by boron incorpora- tion. Boron doped diamond provides an interesting superconductor with ongoing debate surrounding the nature of the impurity band and the effect on the superconducting phase transition of structural changes induced by boron incorporation. The in uence of disorder, both structural (non-diagonal) and on-site (diagonal), was studied through the inhomogeneous Bogoliubov-de Gennes (BdG) theory in narrow-band disordered superconductors with a view towards understanding superconductivity in boron doped diamond (BDD) and boron-doped nanocrystalline diamond (B-NCD) lms. We employed the attractive Hubbard model within the mean eld approximation, including a short range Coulomb interaction between holes in the narrow acceptor band. We studied substitutional boron incorporation in a triangular lattice, with disorder in the form of random potential uctuations at the boron sites. The role of structural disorder was investigated through non-uniform variation of the tight-binding coupling parameter where, following experimental ndings in BDD and B-NCD lms, we incorporated the concurrent increase in structural disorder with increasing boron concentration. Stark differences between the ffects of structural and on-site disorder were demonstrated and showed that structural disorder has a much greater e ect on the density of states, mean pairing amplitude and super uid density than on-site potential disorder. We showed that structural disorder can increase the mean pairing amplitude while the spectral gap in the density of states decreases, with states eventually appearing within the spectral gap for high levels of disorder. This study illustrated how the effects of structural disorder can explain some of the features found in superconducting BDD and B-NCD lms, such as a tendency towards saturation of the critical temperature (Tc) with boron doping and deviations from the expected Bardeen-Cooper-Shrie er (BCS) theory in the temperature dependence of the pairing amplitude and spectral gap. The variation of the super uid density considering only structural disorder was markedly different from the variation with on-site disorder only and revealed that structural disorder is far more detrimental to superconductivity and accounts for the relatively low Tc of BDD and B-NCD in comparison to the Tc predicted using the conventional BCS theory. This theoretical work was then used to interpret features in the measured transport properties of B-NCD lms with di erent doping concentrations and microstructures. The temperature dependence of a distinct local maximum in eld dependent magnetoresistance measurements showed suppression of the density of states as the system breaks up into superconducting regions separated by grain boundaries. Differential resistance measurements at different temperatures and magnetic fi elds showed a transition from a local minimum at zero applied current, indicative of persisting superconducting regions, to a local maximum. A power law dependence over a certain current range in the measured I-V characteristics at di erent magnetic elds suggests a Berezinski-Kosterlitz-Thouless (BKT) transition. In addition, features in the magnetoresistance clearly indicate additional phases. Together with features in current-voltage measurements, these signatures show the coexistence of superconductivity and additional competing phases close to the Anderson-Mott transition.
  • No Thumbnail Available
    Item
    Synthesis and characterization of solid, hollow, core-shell and worm-like carbon nanostructures for applications in organic photovoltaic devices and chemical sensors
    (2016) Mutuma, Bridget Kanini
    The synthesis of carbon spheres (solid and hollow) for application in organic photovoltaics and chemical sensors is a means of using inexpensive and readily processable carbons to eliminate global warming and to monitor harmful gases. The synthesis conditions used to make solid carbon spheres can also be used to tailor their structural, paramagnetic and thermal properties. More so, the ability to tailor the morphology, surface, structural and electronic properties of the hollow carbon spheres by a templating method is an added advantage to their applicability in electronic devices. Solid carbon spheres were synthesized by a vertically oriented chemical vapor deposition (CVD) reactor using acetylene as a carbon source and argon or hydrogen as the carrier gas. The flow rates of the acetylene or carrier gases determined the particle sizes of the carbon spheres. Annealing of carbon spheres in hydrogen resulted in an increase in thermal stability, fewer defects and narrower paramagnetic signals relative to the carbon spheres annealed in argon gas. In contrast, carbon spheres annealed in argon exhibited an increase in the number of defects, a decrease in thermal stability and broader paramagnetic signals. Doped carbon spheres portrayed an increase in ID/IG ratios, a decrease in thermal stability and stronger paramagnetic signals due to the presence of defects induced by nitrogen. The N doped carbon spheres synthesized in H2 comprised of 48% pyridinic-N, 22% pyrrolic-N and 24% quaternary -N while the N doped spheres obtained in the presence of Ar had 17% pyridinic- N, 20% pyrrolic-N and 49% quaternary-N. The presence of a higher percentage of pyridinic- N confirms the presence of more edge defects in carbon spheres synthesized under H2 gas corroborating with the stronger paramagnetic signal observed from the ESR spectra. Consequently, a higher N/C ratio was exhibited in the N doped CSs obtained in the presence of H2 (4.96) than in the presence of Ar (3.68). This could be attributed to the presence of edge defects in carbon spheres synthesized in the presence of H2 gas. The induction of edge defects in carbon spheres in the presence of H2 gas without the aid of a metal catalyst opens a platform for regulating surface and catalytic reactions using H2 gas. Pristine and mesoporous SiO2 spheres were synthesized using a modified Stober method. Carbonization of the pristine SiO2, pristine SiO2@PVP, mesoporous SiO2 and mesoporous SiO2@PVP spheres was carried out using a bubbling method with toluene as the carbon source and argon as the carrier gas in a CVD reactor for 1 h. Upon SiO2 removal, hollow carbon nanostructures of varying morphologies were obtained. The polyvinylpyrrolidone (PVP) adsorption time, PVP concentration, SiO2 mesoporosity, SiO2 particle size dispersion, and carbonization time played a role in the formation of unique hollow carbon nanostructures; complete HCSs, broken HCSs, deformed HCSs, edge connected, open ended, wormlike and bubble-like HCSs. The mesoporous broken HCSs and open ended HCSs portrayed a hierarchical structure with a bimodal pore size distribution. The surface area properties of these materials and the ease of control of the carbon morphology gives an insight into the application of these materials as dye adsorbents. The effect of the size dispersion of Au@SiO2 sphere templates for the synthesis of hollow carbon structures was evaluated using a CVD nanocasting method. The diameter of the template, the presence of the gold nanoparticles and the amount of PVP determined the size, thickness and shape of the synthesized carbon nanostructures. Carbonization (and SiO2 removal) of Au@polydispersed silica spheres for 1 h gave a graphene-like HCS layer while longer times (2-4 h) gave nanotube like (or worm like) HCSs. These results highlight the potential use of Au@carbon core shell structures for the generation of few layered graphene-like unusual nanostructures. As a proof of concept, the wormlike carbon structures were incorporated in organic solar cells and found to give a measurable photovoltaic response. The incorporation of Au nanospheres and nanorods in a hole transport layer (PEDOT:PSS) of a solar cell device increased the current density and the photo-conversion efficiency of the device due to the local surface plasmon resonance and enhanced light scattering effects of gold. However, high series resistance and leakage currents were obtained due to barrier centres created by uneven dispersion of Au nanaorods within the polymer matrix. The performance of bulk heterojunction organic photovoltaic cells based on poly(3-hexylthiophene- 2,5-diyl) (P3HT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) processed from chlorobenzene solution can be enhanced by solution heat treatment of the blend. The morphology of films spin coated from the heat treated blend solution reveals a more favourable diffusion of PCBM into the P3HT matrix than heating of the individual solutions separately. The films obtained from heat treated P3HT and PCBM solutions had a more homogeneous dispersion and enhanced light absorption than those obtained from solutions heat treated separately. There was a significant improvement in the performance for devices made from a solution heat treated blends relative to the non-treated blend; a maximum power conversion efficiency of 3.5% and a fill factor up to 43% was achieved under Air Mass 1.5 at 100 mW/cm2 illumination. This study also reports on the sensing characteristics of ammonia in humid environment by hollow carbon spheres, hollow carbon spheres-polyvinylpyrrolidone composite and annealed hollow carbon spheres, at 20°C and 40°C. For device fabrication, a surfactant assisted method was used to homogeneously disperse the hollow carbon spheres, allowing their deposition onto an interdigitated electrode by casting. An enhanced response and recovery time of the devices was observed at the higher working temperature. Annealing of the hollow carbon spheres resulted in a tremendous decrease in the humidity dependent ammonia sensing due to a decrease in the number of the oxygenated groups and defects in their structure. The presence of hydroxyl groups on the pristine hollow carbon sphere surface resulted in an enhanced proton conductivity. However, the ammonia sensitivity at high relative humidity in the pristine hollow carbon spheres is negligible due to the inhibition of ammonia adsorption sites by the high concentration of water molecules. The sensor response was investigated by varying both ammonia concentration and relative humidity, determining the topology of the response as a function of these two variables, and applying a tristimulus analysis in an attempt to determine the ammonia concentration independently of the relative humidity. This study demonstrates the critical role played by humidity and surface chemistry in the ammonia sensing properties of hollow carbon spheres. The studies reveal the day to day application of ammonia sensors, with temperature and humidity playing a critical role in the carbon based sensor response and recovery of the materials. These carbon based sensors that simultaneously measure ammonia and relative humidity could be applied in agricultural industries to monitor ammonia concentration in soils, fishponds and in food industries to monitor meat spoilage.
  • No Thumbnail Available
    Item
    Optical properties of vanadium oxide nanostructures synthesized by laser pyrolysis
    (2012-02-28) Shikwambana, Lerato David
    In this work, the primary investigation has been on the development of the laser pyrolysis setup and its optimization for the synthesis of nano-size VO2-x films. More specifically the focus was on making VO2-x depositions using various laser pyrolysis parameters and establish in this way (1) an optimum laser wavelength threshold for the photon induced dissociation of the molecular precursors while the thermal contribution was kept minimal by using low power density (laser energy of 30 W) and (2) the lower threshold for pure thermal contributions by working with wavelengths far from resonance in order to minimize pure photon induced contributions. The interest in synthesizing nano-size VO2-x materials stems from the low metal-insulator transition temperature at near room temperature with opto-electronic and thermo-electronic properties that can be used in specialised applications. A large number of samples were synthesized under various conditions and annealed under argon atmosphere for 17 hours. XRD analysis identified the VO2 (B) and/or β-V2O5 vanadium oxide phases characteristic for certain samples grown under optimum conditions. Raman spectroscopy also confirmed these vanadium oxide phases with bands observed at 175, 228, 261, 303, 422 and 532 cm-1. SEM analysis revealed a plethora of different nanostructures of various size and shapes. The particles have a range of sizes between 55 nm to 185 nm in diameter. The particles showed morphologies which included nano-rods, nanospheres and nano-slabs. An interesting phenomenon was observed on the samples synthesized with high power density, which was observed and reported by Donev iii et al. EDS analysis on the particles was also used to probe the elemental composition of the sample. Optical studies were performed on the samples which showed transitions in the visible and infrared region in accordance with the ones observed in the international literature using different nano-synthesis methods.
  • No Thumbnail Available
    Item
    CVD synthesis of nitrogen doped carbon nanotubes using iron pentacarbonyl as catalyst
    (2012-02-24) Ghadimi, Nafise
    In this dissertation, the synthesis of nitrogen doped carbon nanotubes (N-CNTs) was performed successfully, using a floating catalyst chemical vapour deposition (CVD) method. Fe(CO)5 was utilized as the catalyst and acetonitrile and toluene as nitrogen and carbon sources respectively. Two different procedures were used to add reagents to the reactor: an injection method and a bubbling method. The effect of nitrogen concentration and physical parameters such as reaction temperature, gas flow rate on the morphology, crystallinity and thermal stability of the tubes was studied. The synthesized materials were characterized by means of Raman spectroscopy, TGA and TEM analyses. The presence of nitrogen was confirmed by the presence of the bamboo formations in the tubes by TEM. A comparison of the data from the numerous reactions revealed that N-CNTs can be made from Fe(CO)5 and acetonitrile. Further the main conclusions achieved using the injection method were: i) the maximum number of tubes with bamboo structure were made using on acetonitrile concentration of 15%, ii) The best growth temperature to make N-CNTs was 850 oC, iii) An increase in acetonitrile concentration decreased the yield of NCNTs and iv) Tubes with the narrowest outer diameters were made using an acetonitrile concentration of 15%.
  • No Thumbnail Available
    Item
    Synthesis of Tungsten Oxide Nanostructures by Laser Pyrolysis
    (2012-02-01) Govender, Malcolm
    This dissertation discusses the synthesis method known as laser pyrolysis. The theory on laser pyrolysis has been inferred since 1975, but it is insufficient in predicting the products that can be formed. This is due to the use of a laser, which leads to indecisive reaction pathways from precursor to product. In this work, the laser wavelength and power are varied to initiate a starting point in understanding the complex nature of the laser–precursor interaction, in addition to studying the resulting nanomaterial that is formed by the corresponding laser pyrolysis parameters. The results are justified based on linear and nonlinear optical processes, as well as photophysical and photochemical processes. Experiments to produce tungsten trioxide nanowires were conducted, but similar products could not be achieved, due to the difficulty in emulating ‘sensitive’ variables such as gas pressure and flow rates. However, it was discovered for the first time using this method that six-sided tungsten oxide “stars” can be grown.