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 xiii 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.

M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / 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%.

This thesis focuses on the development of method to the photochemical synthesis of niobium nanoparticles (NbNP) using Irgacure 907 (I-907) photoinitiator. This investigation is composed of two parts; whereas the mechanistic study of formation of particles was investigated first, and then followed by particles property characterization. By studying the mechanism of formation we were able to obtain knowledge on how to control the size of NbNP. This knowledge provided us with ability to generate a library of nanoparticles with the varying sizes. Furthermore, the study of I-907 photoproducts has given insight for an alternative method for the synthesis of NbNP using a milder reducing agent, 4-(methylthio)benzaldehyde (MSBA). Exposure of NbNP to air causes their oxidation leading to the formation of niobium oxide nanoparticles (NbONP). The oxidation of NbONP was characterized with variety of techniques (XPS, EDS and HRTEM) that demonstrated the core-shell structure of the nanoparticles. These methods indicated that the core is metallic Nb0 and the shell is the niobium oxide, Nb2O5. Since Nb2O5 is known be a strong Brønsted acid, we tested the Brønsted activity of NbONP with pH sensitive dye coumarin-6 (C6) (monitored using fluorescence and UV-vis). The results of these spectroscopic experiments indicated that NbONP can protonate C6, thus serving as confirmation for the acidity of NbONP. Furthermore, particles with varying sizes were tested with C6 to check if the difference in size affects the acidity. It was observed that the particles with the larger sizes have the strongest acidity and the particles of smaller sizes are less acidic.

Thesis (Ph.D.) - University of St Andrews, June 2007. / Restricted until 14th June 2008.

Thesis (Ph. D.)--University of Oregon, 2007. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 158-183). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.

Thesis (M.S.)--Missouri State University, 2008. / "August 2008." Includes bibliographical references (leaves 51-53). Also available online.

Thesis (Ph. D. in Physics)--Vanderbilt University, Dec. 2007. / Title from title screen. Includes bibliographical references.

This thesis focuses on the development of method to the photochemical synthesis of niobium nanoparticles (NbNP) using Irgacure 907 (I-907) photoinitiator. This investigation is composed of two parts; whereas the mechanistic study of formation of particles was investigated first, and then followed by particles property characterization. By studying the mechanism of formation we were able to obtain knowledge on how to control the size of NbNP. This knowledge provided us with ability to generate a library of nanoparticles with the varying sizes. Furthermore, the study of I-907 photoproducts has given insight for an alternative method for the synthesis of NbNP using a milder reducing agent, 4-(methylthio)benzaldehyde (MSBA). Exposure of NbNP to air causes their oxidation leading to the formation of niobium oxide nanoparticles (NbONP). The oxidation of NbONP was characterized with variety of techniques (XPS, EDS and HRTEM) that demonstrated the core-shell structure of the nanoparticles. These methods indicated that the core is metallic Nb0 and the shell is the niobium oxide, Nb2O5. Since Nb2O5 is known be a strong Brønsted acid, we tested the Brønsted activity of NbONP with pH sensitive dye coumarin-6 (C6) (monitored using fluorescence and UV-vis). The results of these spectroscopic experiments indicated that NbONP can protonate C6, thus serving as confirmation for the acidity of NbONP. Furthermore, particles with varying sizes were tested with C6 to check if the difference in size affects the acidity. It was observed that the particles with the larger sizes have the strongest acidity and the particles of smaller sizes are less acidic.

No description available.

Challenges in treating injuries, together with an increased need for repair of damaged tissues and organs, have made regenerative medicine a major research area today. Biomaterials such as silk fibroin (SF) have proven to be excellent tissue scaffolds possessing properties essential in tissue engineering such as biocompatibility, biodegradability and exceptional mechanical properties. SF nanofibres are especially attractive due to their large surface-to-volume ratio and high porosity which is beneficial in regenerative medicine. However, to design biomaterial scaffolds, chemical and physical properties of SF have to be sufficiently known. The thesis aims to contribute to knowledge by characterizing silk fibroin from the African wild silkworm species Gonometa rufobrunnae, Gonometa postica, Argema mimosae, Epiphora bahuniae and Anaphe panda. Moreover, the feasibility of producing nanofibrous biomaterial scaffolds from these fibroins is explored. The chemical composition of degummed fibres was investigated using Capillary electrophoresis whilst Infrared (IR) and Raman spectroscopic techniques were utilized to determine structural characteristics of the fibroin. In addition, thermal behaviour and mechanical properties of the fibroins were also investigated. Nanofibres were fabricated via electrospinning. The effects of solution concentration, voltage, polymer flow rate and tip to collector distance were studied to give optimum electrospinning conditions. IR spectroscopy was also utilized to observe the conformational structure of the degummed and electrospun fibres whilst scanning electron microscopy (SEM) provided information on the size and morphology of the fibres. The use of the nanofibres as biomaterials was evaluated using cytotoxicity tests. Results showed that glycine, alanine and serine constituted over 70% of the amino acid composition of all the fibroins. Gonometa fibroin had more glycine than alanine whilst the opposite was true for Argema mimosae, Epiphora bahuniae and Anaphe panda fibroin. The abundance of basic amino acids in Gonometa rufobrunnae, Gonometa postica, Argema mimosae and Epiphora bahuniae fibroin makes them prime candidates for cell and tissue culture. The amino acid composition of the fibroins influenced secondary structure as the β-sheet structure. Anaphe panda, Argema mimosae and Epiphora bahuniae silks was made up of mostly alanine-alanine (Ala-Ala)n polypeptides whilst Gonometa fibroin had an interesting mixture of both glycine-alanine (Gly-Ala)n and (Ala-Ala)n units. The unique structures impacted the mechanical and thermal properties of the fibroins. Production of Gonometa nanofibres was mainly dependent on fibroin solution concentration. A minimum of 27 % w/v was needed to produce defect free nanofibres. Diameters of the electrospun fibres produced ranged from 300 to 2500 nm. IR spectroscopy data highlighted that the β-sheet conformation of degummed fibroin was degraded during the formation of the nanofibres rendering them water soluble. It was however possible to regenerate the β-sheet structure in the nanofibres by exposing them to various solvents. Cytotoxicity tests using Sulforhodamine B (SRB) assay demonstrated that the nanofibres were not toxic to cells, a major prerequisite for use as a biomaterial. This thesis successfully provides useful data in an area that has been minimally explored. Results suggest that SF from African silkworm species offers diversity in properties and are therefore attractive for use as biomaterials, especially in cell and tissue engineering. As far as we could determine, we are the first to extend the use of fibroin from African silk species by producing Gonometa SF nanofibres that are of potential use as biomaterial scaffolds. / Chemistry / D. Phil. (Chemisty)