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.

Nanomaterials possess unusual chemical and physical properties than their bulk counterparts because of their large surface to volume ratio. This benefit has found applications in the fields of optics, electronics catalysis and biomedicine. Over the past two decades cerium oxide based materials have been extensively studied and used in applications such as glass and ceramics, phosphor/luminescence and in various catalysis and chemical applications. Nanocrystalline cerium oxide materials can benefit not only these applications, but they also possess some unique properties such as blue shift in ultraviolet absorption spectra, shifting and broadening of Raman allowed modes and lattice expansion. Unfortunately, the high specific surface area of the nanocrystalline particles also results in a stronger tendency of the particles to agglomerate. The problem of agglomeration is of prime concern for the particles smaller than 5nm and the beneficial effects of the nanosized particles are usually lost due to the agglomeration problem. Therefore synthesis of non-agglomerated nanocrystalline cerium oxide particles is highly important in improvement of properties for various applications. The present study investigates the use of microemulsion for synthesis of monodispersed, non-agglomerated nanocrystalline cerium oxide particles. Sodium bis(2-ethylhexyl sulphosuccinate (AOT) was used as a surfactant in the microemulsion system used in this study. It was found that the use of hydrogen peroxide as a precipitating agent gives a very stable sol of cerium oxide containing nanocrystalline particles of 3nm in size. The particle morphology and chemical state study was done for these particles and it was found that cerium oxide nanoparticles consist of both Ce(+3) and Ce(+4) valence states while the micron sized cerium oxide particles consist of only Ce(+4) valence state. Different applications of the synthesized cerium oxide nanoparticles were also studied. The beneficial effects of the synthesized nanocrystalline ceria to improve the high temperature oxidation resistance of stainless steel were investigated using oxidation kinetics measurements. For comparing the size effect on the improvement, comparative coatings of 10 μm and 20nm-sized cerium oxide were also studied. It was found that the 3nm-sized ceria gave the best results in improving high temperature oxidation resistance of stainless steel even in cyclic heating conditions. It results in a fine grained scale morphology with improved scale adhesion to the substrate and changed the scale growth mechanism from cation outward to oxygen inward. The role of Ce(+3) valence state in nanocrystalline cerium oxide particles to improve the oxidation resistance is proposed and discussed. Another application of the synthesized nanocrystalline ceria was found in improving lifespan of in vitro cell cultures in collaboration with Molecular Biology and Microbiology Department. Although this is the not main part of this thesis, however, it is worth mentioning that cerium oxide nanoparticles prolonged brain cell longevity by 2-3 fold. Further, these nanoparticles reduced hydrogen peroxide and UV light induced cell injury by over 60%. It is hypothesized that the cerium oxide nanoparticles act as free radical scavengers due to their unique structure, with respect to valence and oxygen defects, to promote cell longevity. Thus nanotechnology plays a vital role at the interface of materials science and molecular and microbiology.

Central to the practical use of nanoscale materials is the controlled growth in technologically meaningful quantities. Many of the proposed applications of the nanomaterials potentially require inexpensive production of the building blocks. Solution-based synthetic approach offers controllability, high throughput, and scalability, which make the process attractive for the potential scale-up. Growth kinetics could be readily influenced by chemical interactions between the precursor and the solvent. In order to fully utilize its benefits, it is therefore pivotal to understand the decomposition chemistry of the precursors used in the reactions. Supercritical fluids were used as solvent in which high temperature reactions could take place. Silicon nanowires with diameters of 20~30 nm was synthesized in supercritical fluids with metal nanocrystals as seeds for the nanowire growth. To unravel the effect of silicon precursors, several silicon precursors were reacted and the resulting products were investigated. The scalability of the system is discussed based on the experimental data. The nanowires were characterized with various characterization tools, including high-resolution transmission electron microscopy and electron energy loss spectroscopy. The crystallographic signatures were analyzed through the transmission electron microscopic study, and fundamental electrical and optical properties were probed by electron energy loss spectroscopy. Carbon nanotubes were prepared by reacting carbon-containing chemicals in supercritical fluids with organometallic compounds that form metal seed particles in-situ. A batch reaction, in which the temperature control was relatively poor, yielded a mixture of multiwall nanotubes and amorphous carbon nanofilaments with a low selectivity of nanotubes in the product. When reaction parameters were translated into a continuous flow-through reaction, nanotube selectivity as well as the throughput of the total product significantly improved. Magnetic properties of various metal nanocrystals were also studied. Colloidal synthesis enables the growth of FePt and MnPt3 nanocrystals with size uniformity. The as-synthesized nanocrystals, however, had compositionally disordered soft-magnetic phases. To obtain hard magnetic layered phase, the nanocrystals must be annealed at high temperatures, which led to sintering of the inorganic cores. To prevent sintering, the nanocrystals were encapsulated with silica layer prior to annealing. Interparticle magnetic interactions were also explored using particles with varying silica thickness. / text

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--State University of New York at Binghamton, Department of Chemistry, 2005. / Includes bibliographical references.

Nanocrystals are an important field of study in the 21st century. Crystallites that are nanometers in size have very different properties from their bulk analogs because quantum mechanical effects become dominant at such small length scales. When a crystallite becomes small enough, the quantum confinement of electrons in the material manifests as a size-dependence of the nanocrystal's properties. Electrical and optical properties such as absorbance, surface plasmon resonance, and photoluminescence are sensitive to the size of the nanocrystal and proffer an array of technological applications for nanocrystals in such fields as biological imaging, laser technology, solar power enhancement, LED modification, chemical sensors, and quantum computation.The synthesis of size-controlled nanocrystals is critical to using nanocrystal in applications for their size-dependent properties. The development of nanocrystal synthesis techniques has been its own entire field of study for two decades or more, and several successes have established novel, utilitarian protocols for the mass-production of nanocrystals with controlled size and very low polydispersity. However, the experimental successes are generally poorly understood and no theoretical framework exists to explain the dynamics of these processes and how to better control or optimize them. It is the goal of this thesis to develop novel theories of nanocrystal synthesis processes to describe these phenomena in theoretical detail and extract meaningful correlations and driving forces that provide the necessary insight to improve the technology and enhance our understanding of nanocrystal growth. Chapter 4, 5 and 6 comprise all the novel research conducted for this thesis, with Chapters 1, 2 and 3 serving as necessary background to understanding the current state of the art. In Chapter 4, we develop a quantitative describe of the process of size focusing, in which a population of polydisperse nanocrystals, which are useless for applications, can be made more monodisperse by the injection of new crystallizable material. We derive mass balance equations that relate the rate of new-material generation to changes in the growth patterns of the nanocrystals. Specifically, we determine that only when the rate of crystal-material production is sustained at a high level can size focusing occur and a monodisperse sample of nanocrystals be produced. Quantitative criteria are provided for how high the rate of production must be, and the quantitative effects on the nanocrystal size distribution function for various magnitudes of the production rate. The effect of the production rate on every facet of the size distribution function is evaluated analytically and confirmed numerically. Furthermore, through comparison of the theory to experimental data, it is determined that a typical nanocrystal synthesis accidentally correlates two variables that are critical to the phenomenon of size focusing. The unknowingly correlated variables have frustrated experimental investigations of the same insights we provided with theory. We recommend a new synthesis protocol that decouples the critical variables, and thus permit the quantitative control of nanocrystal size and polydispersity through theoretical relations, which can also be generalized for the a priori design and optimization of nanocrystal synthesis techniques. In Chapter 5, a theoretical investigation of the growth of surfactant-coated nanocrystals is undertaken. The surfactants create a layer around the nanocrystal that has different transport properties than the bulk solution, and therefore has a strong effect on diffusion-limited growth of nanocrystals. This effect of a surfactant layer is investigated through the lens of the LSW theory of Ostwald ripening as well as through the lens of our own theory of size focusing from Chapter 4. The quantitative effect of a surfactant layer on the various growth processes of spherical nanocrystals is determined, with the result that size focusing can potentially be enhanced by the choice of an appropriate surfactant for a particular nanocrystal material. In addition to the kinetic studies of Chapter 4 and 5, a thermodynamic investigation of surfactant-coated nanocrystals is conducted in Chapter 6, with the goal of understanding the process known as "digestive ripening". In digestive ripening, a population of polydisperse gold nanocrystals is exposed to a strongly binding surfactant, at which point the nanocrystals spontaneously shrink and become highly monodisperse. Different surfactants and different crystal materials can exhibit digestive ripening. Those same materials also have the capacity to be digested further from nanocrystals into molecular clusters that eliminate all crystalline material in favor of surfactant-crystal coordination. The outstanding question is, why does the spontaneous digestive ripening process appear to make large nanocrystals shrink to small nanocrystals, but it does not force small nanocrystals to shrink further to molecular clusters? We construct a full Gibbs free energy model, which we minimize under multiple constraints to obtain quantitative relations for what thermodynamic properties (such as the surfactant binding energy and the crystal-solvent surface energy) govern the existence and size-dependence of a thermodynamically stable nanocrystal. Through our model, we determine that a finite-size nanocrystal is only stable under two possible conditions: either the surfactant-crystal binding is stronger than the crystal-crystal binding and the system contains too few surfactants to form molecular clusters and thus "surfactant-lean" nanocrystals are created, or the surfactantsurfactant intermolecular interactions are sufficiently strong that the nanocrystal core is treated as a swollen micelle in a microemulsion and is stabilized by the surfactant tails' interactions. Quantitative equations are provided that establish what trends and values are expected for experimental results. The results are inconclusive: there is no evidence supporting either conclusion because the available experimental data is insufficient. More accurately, many thermodynamically critical parameters (like the crystal surface energy) are unknown and are practically immeasurable in experimental systems. Speaking generally, the evidence for the surfactant-lean condition is moderately better than the evidence for the microemulsion condition, but in both cases the evidence is insufficient to make a solid conclusion. We therefore use our quantitative results of the thermodynamic investigation to make recommendations to experimentalists as to what trends and what nanocrystal growth processes we expect to observe in either thermodynamic case. While our results are inconclusive in and of themselves, they will be used to highlight the exact thermodynamic driving forces of the experimental systems. We conclude by giving an overview of two new fields of study for theoretical descriptions of nanocrystal growth, specifically the growth of anisotropic nanocrystals and a practical theory for nanocrystal nucleation. Preliminary relations are constructed, with comments on what directions we expect the research to take and how the results would be useful in enhancing our understanding of nanocrystal growth behavior.

No description available.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The investigation of the barrier properties of highly filled polymer-clay hybrid latex films is described. Montmorillonite (MMT) clay contents ranging from 10–30 wt.% were effectively incorporated into polystyrene-butyl acrylate (PSBA) random copolymers, via miniemulsion polymerization. The optical properties of the films were evaluated using UV-Vis spectroscopy. Compared to the neat films, the PSBA nanocomposites retained remarkable visual properties. The light transmittance for PSBA films with styrene/n-butyl acrylate (S/BA) comonomer contents of 40:60 and 50:50 (mol.%) only decreased from 70% in the neat films to 50% in the nanocomposite films containing 30 wt.% clay. The best optical properties were observed in the films with S/BA comonomer contents of 30:70 (mol.%), the light transmittance only decreased from 85% (neat film) to 60% in the nanocomposite films containing 30 wt.% clay. The improved optical properties for the PSBA-30:70 films (compared to the PSBA-40:60 and PSBA-50:50 counterparts) were attributed to an increase in the low UV-absorbing butyl acrylate component of the copolymer, which at the same time has a low Tg that probably facilitated dispersion of the rigid MMT platelets in the matrix. In this study, the overall water vapour transport behaviour was governed by the MMT clay presence and less affected by the copolymer composition variation. The lower diffusion coefficients in the polymer clay nanocomposites (PCNs) were a result of the impermeable clay platelets which forced the water vapour molecules to follow longer and more tortuous paths to diffuse through the nanocomposite films. The irregular shape in the PSBA-40:60 and PSBA-30:70 neat latex particles was lost in the hybrid particles and well defined, dumb-bell shaped particles were observed. This was because of the faceting effect of the rigid MMT clay platelets. The MMT clay platelets were predominantly adhered to the surface of the PSBA latex particles because MMT clay particles have a larger size than the effective size of the copolymer particles. The stable overall transport coefficients in the PSBA-30:70-MMT films were attributed to the morphological organization of clay platelets in the matrix. The storage modulus of the materials decreased with an increase in clay content. This was attributed to the dual role played by the organoclay, firstly as nanofiller and reinforcing agent leading to the increase in storage modulus, and secondly as a plasticizer leading to a decrease of storage modulus. / AFRIKAANSE OPSOMMING: Die versperringseienskappe van hoogsgevulde polimeer-klei saamgestelde latekslae is beskryf. „n 10–30 wt % Montmorilloniet (MMT) klei inhoud is inkorporeer in polistireenbutielakrilaat (PSBA) onreëlmatige kopolimere, via miniemulsie polimerisasie. Die optiese eienskappe van die lae is bepaal m.b.v. UV-Vis spektroskopie. In vergelyking met die lae sonder klei (sogenaamde „neat films‟), het die PSBA nanosamestellings interressante visuele eienskappe getoon. Die ligtransmissie van die PSBA lae met „n stireeen/n-butielakrilaat (S/BA) komonomeerinhoud van 40:60 en 50:50 (mol %) het slegs afgeneem vanaf 70% in die „neat films‟ tot 50% in the nanosaamgestelde lae wat 30% klei bevat het. Die beste optiese eienskappe is waargeneem vir die lae wat „n 30:70 (mol %) S/BA komonomeerinhoud bevat het; die transmissie het slegs afgeneem vanaf 85% in die „neat films‟ to 60% in the nanosaamgestelde lae wat 30% klei bevat het. Die verbeterde optiese eienskappe van die PSBA-30:70 films (in vergelyking met die -40:60 and -50:50 films) is toegeskryf aan „n toename in die lae UV-absorberende butielakrilaat komponent van die kopolimeer. Terselfdetyd het laasgenoemde „n lae Tg-waarde, wat dispersie van die onbuigbare MMT kleiplaatjies in die matriks gefasiliteer het. In hierdie studie is die algehele waterdampvervoer deur die teenwoordigheid van die MMT klei beheer; dit is minder geaffekteer deur variasie in die samestelling van die kopolimeer. Die lae diffusiekoëffisiënte in die polimeer-klei nanosamestellings is as gevolg van die ondeurdringbare kleiplaatjies, wat die waterdampmolekules dwing om langs langer en meer gekronkelde paaie te diffundeer deur die nanosaamgestelde lae. Die onreëlmatige vorm wat gesien is in die PSBA-40:60 and PSBA-30:70 latekspartikels (sonder klei) het geleidelik verdwyn in die saamgestelde partikels, en goed-gedefineerde partikels met die vorm van handgewigte is waargeneem (in TEM beelde). Die rede hiervoor is die sogenaamde „faceting‟ effek, wat deur die onbuigbare MMT kleiplaatjies veroorsaak is. Die MMT kleiplaatjies sit hoofsaaklik aan die oppervlaktes van die PSBA latekspartikels. Die rede hiervoor is dat die MMT kleipartikels groter is as die effektiewe grootte van die kopolimeerpartikels. Die stabiele vervoerkoëffisiënte in die PSBA-30:70-MMT films is aan die unieke morfologiese eienskappe toegeskryf. Die bergingsmodulus van die materiale het monotonies afgeneem met „n toename in klei-inhoud. Dit is toegeskryf aan die tweedelige rol wat die organoklei speel – eerstens as 'n nanovuller en versterkingsmiddel, wat „n toename in bergingsmodulus tot gevolg het, en tweedens as „n plastiseerder, wat „n afname in bergingsmodulus tot gevolg het.

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Mechanical Engineering / Master / Master of Philosophy