College of Science

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Now showing 1 - 5 of 19
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    Syntheses And Characterization Of Bismuth And Lead Chalcogenides And Their Ternary Alloys From Single Source Precursors
    (KNUST, 2018-07) Selina Ama Saah
    This research describes the syntheses and characterization of lead and bismuth chalcogenide nanoparticles and thin films from single source precursors. The single source precursors used were dithiocarbonates, dithio- and diselenoureas, and dithiocarbamates. These precursors were characterized using micro-elemental analyses, thermogravimetric analyses, infrared spectroscopy, nuclear magnetic resonance spectrometry and mass spectrometry. The single crystal structures of bismuth ethyl xanthate and its 1, 10 phenanthroline adduct were determined. The complexes were used for the syntheses of nanoparticles using the hot injection and thin films using the spin coating technique. The as-synthesized nanoparticles and thin films were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, inductively coupled plasma, energy dispersive X-ray spectroscopy and absorption measurements. Optical band gaps were estimated from the absorption data using Tauc’s plots. For the PbS thin films from lead dithiocarbonates, the effect of the alkyl chain length on the morphology and size of the nanocrystals were investigated. There was a change in morphology from cubes to fibrous structures as the alkyl chain changed from ethyl to dodecyl, thus confirming the anisotropic nature of PbS. SEM analyses of Bi2S3 thin films also showed a change in shape from nanowires of the bismuth ethyl xanthate to clusters of nanorods (triethylamine, pyridine) and nanodots (1,10 phenanthroline) depending on the choice of nitrogen adduct. Ternary PbSxSe(1-x) thin films deposited from lead xanthate and lead selenourea showed a gradual shift in p-XRD pattern to lower 2 theta values as the mole fraction of the lead selenourea complex in the coating mixture changed from 0 to 1 at an interval of 0.1. Estimated optical band gaps for the parent binaries (PbS and PbSe) and their ternary alloys (PbSxSe(1-x)) were all blue shifted from their respective bulk materials. PbSxSe(1-x) nanoparticles synthesized from lead thio- and selenourea complexes also exhibited a similar trend in both p-XRD and optical band gap. EDX analyses confirmed the presence of Pb, S and Se in all the PbSxSe(1-x) alloys formed at different atomic percentages depending on the ratios of the starting materials. The effects of different concentrations of Bi doping on the optical and structural properties of PbS nanoparticles have been investigated. Morphological studies showed cubes at 50% Bi doping suggesting the incorporation of the Bi atoms into the crystal lattice of the PbS nanoparticles. However, an interplay between rods and cubes were seen at 80% Bi doping. The estimated band gaps for PbS and Bi2S3 were 0.72 and 1.94 eV respectively with the alloy having band gaps in between the two binaries.
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    A Modal Approach To Price An Option In Continuous Time
    (KNUST, 2018) Osei Antwi
    Average value options or Asian options have been priced using geometric and arithmetic averages of the underlying asset. However, these methods do not give accurate results especially in very low volatility regimes. In this study, we develop a new option pricing model based on the modal average of the underlying asset to price options. Using data from the NASDAQ in the United States of America we use the proposed model to price options sold on some stocks listed on the exchanges using software. The results consistently showed that for volatilities less than 3% of the underlying asset, the modal average option pricing model gives a better option price when compared to existing average option pricing models. Moreover, the modal average consistently does better at all levels of volatility when compared to the Black-Scholes model. We further proved analytically that the modal average model indeed does better than the geometric or arithmetic average models especially for low volatility stocks.
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    On The Trapped Surface Characterization Of Black Hole Regions In The Spherically And Axially Symmetric Spacetimes
    (KNUST, 2018-04) Mohammed Kumah
    The characterization of black holes by means of classical event horizon is a global concept since one need to have the knowledge of the whole spacetime in order to locate a black hole region and the event horizon. To surmount these issues, we investigate alternative approach based on the concept of trapped surfaces in a variety of spacetimes. Specifically, to compute the expansions of the appropriate null vectors in both spherically and axiallysymmetric spacetimes and thus explicitly determine the existence of trapped and marginally trapped surfaces in their respective black hole regions
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    Green Approach To Metal Chalcogenide Semiconductor Nanomaterials: Syntheses And Characterization Of Cds, Pbs, Zns, Mns And Mnxzn1-Xs Nanoparticles
    (KNUST, 2018-07) Micheal Baah Mensah
    Metal chalcogenide nanomaterials such as CdS, PbS, ZnS, MnS and MnxZn1-xS are very useful compound semiconductors with potential applications in sensing, bioimaging, drug delivery, data storage and photovoltaic devices as well as in catalysis. However, the syntheses of good quality nanomaterials usually include toxic chemical reagents and therefore limit their application and adaptation to large scale production. Castor oil and single source precursors (SSPs) are suitable alternative green reagents for the syntheses of undoped and doped metal chalcogenide nanocrystals. In this thesis, green syntheses of (i) CdS nanoclusters and PbS nanorods/belts via thermal decomposition of xanthate complexes in castor oil, and (ii) ZnS, MnS and MnxZn1-xS nanocrystals via the thermal decomposition of diricinoleate carboxylate and heterocyclic dithiocarbamate complexes in oleylamine are reported. The dopant (Mn) feed concentration was varied in the range 0 ≤ x ≤ 0.1. The CdS displayed a band gap of 2.63 – 2.84 eV and a strong emission in the near-UV (λmax = 393 nm) region and formed small clusters of nanocrystals with average particle size of 6.78 1.89 nm. The PbS nanocrystals showed variety of morphologies. The average width and length of the PbS nanorods observed were 22.2 ( 2.3) nm and 225.9 ( 23.3) nm respectively. The activation energy obtained for the growth of ZnS nanoparticles in oleylamine was 53.41 kJ/mol. Powder-X-ray diffraction (p-XRD) patterns showed that the ZnS and MnxZn1-xS nanocrystals synthesized by different precursor routes were sphalerite and the shift in the lattice parameter indicated distortion due to the incorporation of the Mn(II) into the ZnS. The MnS nanocrystals were pure alabandite. The p-XRD patterns of the nanocrystals synthesized by the dithiocarbamate route showed some wurtzitic characteristics of ZnS and MnxZn1-xS. The band gap energy was found to decrease with increasing Mn(II) dopant concentration which indicated the introduction of trap states into the forbidden zone of the host ZnS. Luminescence was observed for MnxZn1-xS (for x ≤ 0.01) nanocrystals at 587 – 599.7 nm characteristic of Mn 4T16A1 transition suggesting possible incorporation of the Mn(II) into the ZnS host lattice. Electron paramagnetic resonance (EPR) studies confirmed the presence of Mn(II) ions dispersed at substitutionary sites for x ≤ 0.01 MnxZn1-xS nanocrystals. However, at high concentrations of Mn dopant (≥ 0.01), the Mn(II) ions were found bound to the surface/interstitial sites in ZnS. Thus, useful morphologies and interesting properties of important metal chalcogenide semiconductors were achieved by the use of environmentally benign materials.
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    Dehydration Characteristics, Quality Evaluation And Consumer Assessment Of Solar Dried Tomato.
    (KNUST, 2018-11-29) Mavis Owureku Asare
    Tomato (Lycopersicon esculentum) is an important vegetable used in cooking most local foods in Ghana. At the peak season of harvesting, high loses are incurred because of the absence of facilities to process fresh tomato and extend the shelf life. Solar drying has been proven to be a more efficient and low cost method of enhancing quality and adding value to tomato and other vegetables. However, there are concerns on the usage, functionality and sensory appeal of the dried products by consumers. In this study a natural mixed mode solar dryer suitable for drying tomato was adapted and used to investigate the dehydration characteristics, quality and consumer acceptability of the dried products. An initial baseline survey was conducted using semi structured questionnaires administered to 395 randomly selected respondents in the Accra Metropolis. Information was obtained on the demographics, consumption pattern, knowledge and acceptance of tomato processing technologies and assessment of quality attributes of tomato. The efficiency of a passive solar dryer was evaluated and used in the processing of fresh tomato to powder. The processing involved the pre-treatment of 6mm slices of fresh roma tomato by dipping in (a) 1% potassium metabisulphite solution (b) 1 % ascorbic acid solution (1:1) for 10 minutes respectively. Untreated tomato slices served as control. Samples were then dried in the passives and in the open sun, with the open sun drying serving as control. The moisture content, moisture ratio and dehydration rate of solar dried tomato was assessed. The quality of dried tomato was assessed for their physicochemical, nutritional and microbiological characteristics. Physicochemical analysis involved determination of pH, total titratable acidity, dry matter, ash, tristimulus colour, total soluble solids, water activity and moisture. Nutritional quality was assessed for lycopene, beta-carotene, total carotenoid acids using high performance liquid chromatography (HPLC) procedures. Flowability, smoothness and compressibility (or packing porosity), particle size and shape distribution of the tomato powder was determined using a Morphologi G3-ID. Scanning electron microscopy of pre-treated solar and sundried tomato samples were imaged with an FEI Quanta 3D FEG scanning electron microscope. Quantitative Descriptive Analysis (QDA) was carried out to compare the sensory descriptive profiles of solar dried tomato powder with existing products on the market using a trained descriptive panel of nine (9). Home Use Test (HUT) was conducted using a trained panel to assess the acceptability of solar dried tomato powder in local foods. The results showed that, most consumers (74%) preferred tomato powder that was conveniently packaged to retain the characteristic intense taste and the flavour. The first day dryer efficiency of 24.2 % facilitated the drying process of tomato (final moisture content of 12-14%). The ash content was slightly higher in the sun-dried tomato (9.3 -10.14 %) compared with the solar dried tomato (9.4 - 9.68 %), an indication of potential contamination with extraneous materials from the environment. Water activity for solar dried tomato powder were significantly lower (0.35 - 0.38) than sundried tomato powder (0.53 - 0.57) and water activity lower than 0.6 is considered microbiologically safe for storage. Lower tristimulus colour L* values (37.81 - 40.31) observed for sun-dried tomato samples indicated that these samples were darker in colour than the solar dried samples with L* values (50.35 - 46.44). Aerobic mesophile counts were lower in solar dried (3.90 CFU/g) tomato pre-treated with potassium metabisulphite compared with sun-dried samples (4.85 CFU/g). Sulphur dioxide content (740.8 ppm) of solar dried tomato pre-treated with potassium metabisulphite was lower than the maximum legal limit (2000 ppm) recommended in fruits and vegetables, indicating its safety for human consumption. A strong, negative correlation between sulphur dioxide 740.9 ppm concentration and microbial load was observed for solar dried tomato. QDA results indicated a strong tomato aroma intensity (scored 127 out of 150) for solar dried tomato powder with the market samples having an extremely low intensity score of 0.7 out of 150 for tomato. Sensory profiling of the two products differed extremely in aroma, appearance and texture. Sensory characteristics of reconstituted solar dried tomato powder was similar to that of fresh tomato and tomato paste (two products commonly used in cooking). It had a very coarse appearance and texture (as predicted by the particle size distribution and shape profile and parameters of convexity and circularity) and a strong boiled/cooked aroma compared to tomato paste which had a higher intensity of red colour, metallic and stewed tomato concentrate aroma and flavour. Most of the participants used tomato powder to prepare local dishes such as “jollof “rice, tomato stew and light soup because of the good swelling characteristics of the product. The mixed mode solar dryer developed in this study was efficient in processing tomato powder which appealed to consumers and had varied uses in food production. The dryer has the potential of enhancing post-harvest loses, extending the shelf life of tomato and creating an alternative processing method which is simple and convenient.