A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Chemistry.
University of the Witwatersrand, Johannesburg. May 2016 / In this research project, the morphogenesis and polymorphism of calcium carbonate (CaCO3) and its subsequent use as a template in the fabrication of hollow carbon spheres (HCS) is reported. A series of ratios (i.e. 5:0, 5:1, 5:2, 5:3, 5:4, 5:5, and 0:5) of binary solvent mixtures consisting of polar aprotic (dimethylformamide and dimethyl sulfoxide) and polar protic (methanol, ethanol, isopropanol, and 2-butanol) solvents, with 10% PEG200 as a crystal modifier, were used to influence the morphogenesis and polymorphism of precipitated CaCO3 (PCC). The PCC products were characterised using scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), and laser Raman spectroscopy. An increase in the ratio of the polar protic solvent (methanol, ethanol, isopropanol, and 2-butanol) relative to the polar aprotic solvent (DMF & DMSO) within the binary solvent mixture favored the formation of vaterite particles of different morphologies, while an increase in the ratio of polar aprotic solvent (DMF & DMSO) within the binary solvent mixture favored the precipitation of rhombohedral calcite crystals. Time-resolved ex situ PXRD and SEM measurements revealed that the nucleation and phase transformation of the CaCO3 under polar protic and aprotic solvents followed the dissolution-reprecipitation mechanism. The major phase transformation occurred within 3 hours after mixing the precursor solutions.
The effect of poly (4-styrenesulfonic acid) (PSSA) as an additive in the crystallisation of CaCO3 at different temperatures (i.e. 30, 40, 75, and 100 °C) and different crystallisation times (3, 6, 12, and 24 hrs) was investigated. The as-synthesised CaCO3 products were subjected to: SEM, laser Raman spectroscopy, PXRD, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The crystallisation of CaCO3 in the presence of PSSA resulted in the self-assembly of vaterite particles into spherical bulk crystals. Varying the crystallisation temperature led to different particle attachment (CPA) pathways, which in turn resulted in bulk crystal morphologies that varied. Changes in the crystallisation temperature were found to not have changed the polymorphism of the precipitated CaCO3 due to the kinetic stabilisation effects of PSSA, instead hollow vaterite spheres formed at 75 and 100 °C. The possibility to synthesise HCS using CaCO3 as a template under chemical vapour deposition (CVD) at different temperatures (i.e. 600, 700 and 800 °C) was, for the first time, demonstrated. The evolution of CO2(g) from the decomposition of the template during CVD resulted in the formation of a rough surface topography on the carbon shell of the HCS. This surface roughness
increased with the increase in the reaction temperature due to the increased rate of CaCO3 decomposition. The structural integrity of the spherical template was not affected by the CO2(g) evolution during carbonisation at all the reaction temperatures. The as-synthesised HCS at 600, 700, and 800 °C gave specific BET surface areas of: 193, 55, and 51 m2/g, respectively. / MT2016
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/21075 |
| Date | 19 September 2016 |
| Creators | Makgae, Ofentse Alfred |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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