http://repository.mut.ac.ke:8080/xmlui/handle/123456789/2879 2026-04-07T13:13:25Z 2026-04-07T13:13:25Z Keru, Godfrey K. http://repository.mut.ac.ke:8080/xmlui/handle/123456789/4541 2024-02-17T11:14:44Z 2015-07-01T00:00:00Z

Boron- and Nitrogen-Doped Carbon Nanotubes: Synthesis, Characterization and Application in Solar Cells Keru, Godfrey K. The investigations in this project focused on the synthesis and characterization of boron- and nitrogen-doped carbon nanotubes (B- and N-CNTs), and the subsequent application of the doped-CNTs in organic solar cells (OSCs). The CNTs (B-CNTs and N-CNTs) were synthesized by using a floating catalyst chemical vapour deposition method with either ferrocene or its derivatives as the catalysts. A novel ferrocenyl derivative, namely, (4-{[(pyridin-4-yl)methylidene]amino}phenyl)-ferrocene, was synthesised by a mechanochemical solvent-free method and its crystal structure was also determined. The ferrocenyl derivative was obtained in high yields (94%) within a short reaction time (30 min). To synthesise N-CNTs, this ferrocenyl derivative was used as a catalyst, nitrogen source as well as an extra source of carbon. To synthesize B-CNTs, ferrocene was used as the catalyst, triphenylborane was the boron source as well as an extra source of carbon at 900 °C. The main carbon source was either toluene or acetonitrile, where applicable, in all CNT synthesis reactions. The shaped carbon nanomaterials formed were characterized by means of transmission electron microscopy, scanning electron microscopy, high resolution transmission electron microscopy, electron dispersive X-ray spectroscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy (XPS). In addition, a vibrating sample magnetometer, four probe conductivity measurement instrument, and an inverse gas chromatography surface energy analyser were used for B-CNT analysis. Formation of bamboo compartments in N-CNTs and cone structures in B-CNTs was a preliminary indication that boron and nitrogen were successfully doped in the hexagonal carbon network of carbon nanotubes (CNTs). XPS was used to ascertain the bonding environment of boron and nitrogen in the carbon network. Boron in the B-CNTs was quantified with inductively coupled plasma-optical emission spectroscopy, while the nitrogen content in the N-CNTs was determined by elemental analysis. The amount of boron incorporated was found to be directly proportional to the percentage by weight of the boron-containing precursor used. At the same time, it was also observed that the amount of boron incorporated had an effect on the conductivities, dispersive surface energies and ferromagnetic properties, among other physicochemical properties, of the B-CNTs. For N-CNTs, the amount of nitrogen incorporated was found to be dependent on synthesis temperature and the amount of nitrogen in the precursors. A temperature of 850 °C and acetonitrile as a carbon source, as well as an extra source of nitrogen, were found to be the best conditions in this study. This resulted in high nitrogen incorporation of about 17.57 at.% and high yield of ≈85% of carbon nanotubes in the total mass of the products obtained. The B-CNTs or N-CNTs were then used to synthesize nanocomposites with poly(3-hexylthiophene) [P3HT] by either in situ polymerization or direct solution mixing. The nanocomposites were characterized with electron microscopy, UV-Vis spectrophotometry, and photoluminescence spectrophotometry. These nanocomposites were subsequently mixed with a fullerene derivate, [6,6]-phenyl C61 butyric acid methyl ester, to form donor-acceptor components in the photoactive layer in OSCs. Nanocomposites that were synthesized by in situ polymerization method performed better than those by direct solution mixing. Organic photovoltaic cell devices were fabricated on indium tin oxide coated glass substrates which were coated with a very thin layer of a hole transport layer, namely, 3,4-ethylenedioxythiophene:poly (styrenesulphonate). The nanocomposite photoactive layer of the devices was spin coated from a chloroform based solution. Finally, an ultra-thin layer of lithium fluoride and a 60 nm aluminium counter electrode were thermally evaporated in a vacuum. The electrical properties of the fabricated solar cell devices were characterized by using a standard solar simulator operating at air mass (AM) of 1.5 at a light intensity of 100 mW cm-2. Important cell parameters, such as, fill factor (FF) and efficiency (ɳ) were determined from the current-voltage characteristics of the devices. The effects of B-CNTs or N-CNTs in the photoactive layer were studied and then compared with a standard device structure without doped-CNTs. Several techniques such as transient absorption spectroscopy (TAS) and atomic force microscopy (AFM) were used to understand the effects of B-CNTs or N-CNTs as part of the photoactive layer. The TAS technique was used to determine the yield and lifetime of the photo-generated charge carriers. This was compared with the power output (or ɳ) of the cell devices. AFM was used to study the morphology of the photoactive layer with B-CNTs or N-CNTs films on the substrates. Doped-CNTs were also tested as charge extracting layer under various device configurations. The position of B- or N-CNTs in the photoactive layer was altered whereby a thin film of B- or N-CNT/P3HT was spin coated next to the hole or electron collecting electrodes. B-CNTs had a positive Hall coefficient and their film was coated close to the ITO electrode, while N-CNTs had negative coefficient and the film was placed close to the Al counter electrode. The short circuit current density and ɳ improved by 31% and 141%, respectively, for the devices with B-CNTs. While, in the case of N-CNTs, the respective values changed by 35% and 38%, respectively. B- or N-CNTs in the photoactive layer were found to improve the absorption of the polymer, the lifetime of the photo-generated free charge carriers and, also, the charge transport properties. The open circuit voltage was found to decrease in some devices which was attributed to recombination. This work has shown that incorporation of doped-CNTs in the photoactive layer, markedly improves the photovoltaic properties of organic solar cells. Doctor of Philosophy (School of Chemistry and Physics, College of Agriculture, Engineering and Science) University of KwaZulu-Natal, Westville campus, 2015

2015-07-01T00:00:00Z Opiyo, Sylvia A. http://repository.mut.ac.ke:8080/xmlui/handle/123456789/2846 2024-02-17T11:11:57Z 2011-01-01T00:00:00Z

Detection of sweet potato viruses in Western Kenya, development of a multiplex PCR technique for simultaneous detection of major viruses and evaluation of medicinal plants for antifungal and antibacterial activities against the crop pathogens Opiyo, Sylvia A. Sweet potato is an important food crop worldwide since it is drought tolerant and acts as a famine relief crop. However, its production is limited by viral, fungal and bacterial infections. Lack of rapid and sensitive techniques for detection of infections inhibits their control. Use of synthetic chemicals to manage microbial infections causes is discouraged since they ar nonbiodegradable. The aim of this study was to identify viruses infecting sweet potato in western Kenya, develop a multiplex PCR protocol for detection of major viruses of the crop, and evaluate efficacy of medicinal plants for antimicrobial activity against the crop pathogens. Symptomatic sweet potato vines obtained from farmers’ fields were testes for ten viruses using NCM-ELISA. The mPCR protocol for detection of SPFMV and SPCSV was optimized through variation of test parameters under standard PCR conditions. Extracts from Warburgia ugandensis, Elaeodendron schweinfurthianum and Terminalia brownii, which are traditionally used to manage microbial infections, were fractionated using chromatographic methods to obtain pure compounds. Structures of isolates were determined using spectroscopic and physical methods. Extracts and isolates were tested for antimicrobial activity against Alternaria spp, Aspergillus niger, Fusarium oxysporum, F. solanum, Rhizopus stolonifer (fungi), Ralstonia solanacearum and Streptomyces ipomoeae (bacteria). Five viruses namely SPFMV, SPCSV, SPMMV, SPCFV and CMV were detected. Eighty nine percent of samples had viral infection with over 80% showing multiple infections. Occurrence of CMV in Kenya was recorded for the first time. An mPCR protocol was successfully developed that detects SPFMV and SPCSV. Phytochemical studies afforded 26 compounds including 7α-acetylugandensolide that was isolated from W. Ugandensis for the first time. All the methanol, ethyl acetate and n-hexane extracts of the three plants were active against one or more of the test organisms. Methanol extract of W. ugandensis exhibited significantly higher activity (24.50 mm) than positive controls against R. stolonifer. Eighteen out of the twenty six compounds isolated exhibited antimicrobial activity against one or more of the test pathogens at concentrations ≤200 µg/ml. Polygodial, warbuganal, mukaadial, ugandensidial, ugandensolide, deacetoxyugandensolide and muzigadial, exhibited low MIC (MIC ≤ 50 µg/ml) against one or more pathogens that was comparable to those exhibited by standard drugs. This study provided a quantitative assessment of viruses infecting sweet potato in western Kenya and a simplified mPCR protocol for routine rapid detection of SPFMV and SPCSV. Scientific proof for the efficacy of extracts of W. ugandensis, E. schweifurthianum and T. brownii as antimicrobial agents against sweet potato pathogens and perhaps other crop pathogens was also provided. Doctor of Philosophy in Chemistry, 2011

2011-01-01T00:00:00Z Mwangi, Benson http://repository.mut.ac.ke:8080/xmlui/handle/123456789/343 2024-02-17T11:07:07Z 1992-01-01T00:00:00Z

Habitat Preferences, Feeding Habits, Leght-Weight Relationship and Relative Condition Factors of Juvenile Tilapias in Lake Naivasha, Kenya. Mwangi, Benson

1992-01-01T00:00:00Z Arero, Jaro http://repository.mut.ac.ke:8080/xmlui/handle/123456789/120 2024-02-17T11:03:14Z 2016-01-01T00:00:00Z

Design and Synthesis of Organic Molecular Models of Artificial Photosynthetic Reaction Center Arero, Jaro A clean and sustainable alternative to fossil fuels is solar energy. For efficient use of solar energy to be realized, artificial systems that can effectively capture and convert sunlight into a usable form of energy have to be developed. In natural photosynthesis, antenna chlorophylls and carotenoids capture sunlight and transfer the resulting excitation energy to the photosynthetic reaction center (PRC). Small reorganization energy, λ and well-balanced electronic coupling between donors and acceptors in the PRC favor formation of a highly efficient charge-separated (CS) state. By covalently linking electron/energy donors to acceptors, organic molecular dyads and triads that mimic natural photosynthesis were synthesized and studied. Peripherally linked free base phthalocyanine (Pc)-fullerene (C60) and a zinc (Zn) phthalocyanine-C60 dyads were synthesized. Photoexcitation of the Pc moiety resulted in singlet-singlet energy transfer to the attached C60, followed by electron transfer. The lifetime of the CS state was 94 ps. Linking C60 axially to silicon (Si) Pc, a lifetime of the CS state of 4.5 ns was realized. The exceptionally long-lived CS state of the SiPc-C60 dyad qualifies it for applications in solar energy conversion devices. A secondary electron donor was linked to the dyad to obtain a carotenoid (Car)-SiPc-C60 triad and ferrocene (Fc)-SiPc-C60 triad. Excitation of the SiPc moiety resulted in fast electron transfer from the Car or Fc secondary electron donors to the C60. The lifetime of the CS state was 17 ps and 1.2 ps in Car-SiPc-C60 and Fc-SiPc-C60, respectively. ii In Chapter 3, an efficient synthetic route that yielded regioselective oxidative porphyrin dimerization is presented. Using Cu2+ as the oxidant, meso-β doubly-connected fused porphyrin dimers were obtained in very high yields. Removal of the copper from the macrocycle affords a free base porphyrin dimer. This allows for exchange of metals and provides a route to a wider range of metallporphyrin dimers. In Chapter 4, the development of an efficient and an expedient route to bacteriopurpurin synthesis is discussed. Meso-10,20- diformylation of porphyrin was achieved and one-pot porphyrin diacrylate synthesis and cyclization to afford bacteriopurpurin was realized. The bacteriopurpurin had a reduction potential of – 0.85 V vs SCE and λmax, 845 nm.

2016-01-01T00:00:00Z