1,720,964 research outputs found
Electrochemical detection of 4-chlorophenol on phthalocyanine-metal organic framework sensors
Get PDFDissertation (MSc (Chemistry))--University of Pretoria, 2022.Metal organic frameworks (MOFs) have been used as electrochemical sensors owing to their high surface area and porosity. Unfortunately, the low conductivity and redox activity of MOFs limit their application in electrochemical sensing. To address this problem redox-active materials can be incorporated in MOFs to produce MOF composites. In this work we report on the synthesis and characterization of Zeolitic imidazolate framework-8 (ZIF-8), Zirconium based MOF (UiO-66) and their corresponding phthalocyanine (Pc) composites. Solvothermal synthesis was used to obtain ZIF-8, UiO-66, and their composites in powder form. The morphology and elemental composition were attained using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis, respectively. The composites were composed of agglomerated rods surrounded by irregular polyhedral structures. The presence of MOF and Pc elements confirmed that the Pcs were within the MOFs. The crystallographic properties of the composites were probed using powder X-ray diffraction (PXRD) which showed that the composites were more crystalline than the pristine MOFs. The increase in order of the structure of the composites confirmed incorporation of the Pcs onto the MOFs. The surface area and pore size distribution of the composites were determined using Brunauer-Emmett-Teller (BET) analysis. The pore volume ranged from 0.10 to 0.37 cc/g. The decrease in surface area and pore volume of the composites indicated that the Pcs were adsorbed onto the MOF pores. The optical properties of the composites were obtained using ultraviolet-visible (UV-vis) spectroscopy. The composites exhibited the characteristic B and Q bands of phthalocyanines and showed metal-to-metal charge transfer between MOFs and Pcs confirming that the phthalocyanines were incorporated within the MOF structure. ZIF-8, UiO-66 and their corresponding phthalocyanine composites were immobilized on a glassy carbon electrode (GCE) using the drop drying method. The modified electrodes were used for the detection of 4-chlorophenol, and the electrochemical responses were recorded using cyclic voltammetry. The electrocatalytic behaviour of the electrodes modified with MOF composites showed improved oxidation compared to the ZIF-8 or UiO-66 modified electrodes. The FePc@ZIF-8 modified electrode showed the best catalytic behaviour with an anodic peak potential at 0.54 V while FePc@UiO-66 behaved best with respect to enhancing catalytic currents. The limit of detection ranged from 0.05 mM to 2.8 mM for all redox active composites. This work shows that phthalocyanine-MOF composites are effective electro-catalysts and may be developed for the detection of other environmental pollutants.National Research Foundation (NRF) of South Africa.ChemistryMSc (Chemistry)Unrestricte
Computational studies of doped tin disulphide monolayer for photoelectrochemical water splitting
Get PDFDissertation (MSc (Physics))--University of Pretoria, 2023.In this ab initio study, density functional theory, including a Hubbard U correction term (DFT+U), calculations were performed to investigate the photoelectrochemical (PEC) water splitting possibility of SnS2 monolayer. Initially, the thermodynamic stability and photocatalytic (PC) properties of pristine SnS2 were studied and it was found that it suffers poor electrical conductivity and the bottom of conduction band minimum (CBM) is below the reduction potential of H^(+/) H_2 level, limiting the PC performance. To improve performance for PEC water splitting, various doping strategies were performed on a large 5×5 SnS2 supercell. These doping strategies are: C adsorbed onto an interstitial position, C adsorbed on a S atom, C adsorbed on a Sn atom, C substituting a S atom and C substituting a Sn atom.Department of Physics (University of Pretoria)National Research Foundation (NRF)PhysicsMSc (Physics)UnrestrictedFaculty of Natural and Agricultural Science
Two-dimensional semiconductor transition metal dichalcogenide nanosheets as charge carriers in metallophthalocyanine based solar cells
Get PDFDissertation (MSc (Chemistry))--University of Pretoria, 2020.Two-dimensional (2D) materials have received widespread attention as prime candidates for scientific research in the fields of optoelectronics and photonic applications. This thesis focuses on the use of transition metal dichalcogendes (TMDs) for solar cell application.
TMDs are compounds with the general formula MX2, where M is transition metal and X is chalcogen and most of them are intrinsically n-type. These materials are arranged as layers stacked together by weak van der Waals forces. They have found application in solar cells due to their high carrier mobility, chemical stability and flexibility. Exfoliation of the bulk TMDs maintains these properties and further imparts more properties such as switching the bandgap from indirect to direct and this further increases the carrier mobility of the material.
In this work we explore the use of molybdenum sulfide (MoS2) and tungsten (WS2) TMDs as electron transporting layers for solar cell application. The TMDs were successfully synthesised using the chemical exfoliation method and they were further doped with gold (Au) in order to enhance charge carrier mobility. The doped and undoped TMDs were characterised using X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM).
These TMDs were used together with metallophthalocyanines (MPc’s) as photon absorbers. MPc’s are known to be thermally and chemically stable in addition to having a relatively high extinction coefficient in the visible range. In our study we show the photovoltaic properties of TMDs combined with a variety of MPc’s. The utilisation of these TMDs with MPc’s has led to the generation of cost-effective solar cells that can be exploited for future technological applications.NRFChemistryMSc (Chemistry)Unrestricte
Effects of irradiation-induced defects in germanium
No full textDissertation (MSc (Physics))--University of Pretoria, 2023.In this study, we investigated the electrical characteristics of defects in germanium produced by alpha particle radiation using conventional deep level transient spectroscopy (DLTS) and Laplace deep-level transient spectroscopy (L-DLTS). Resistive deposition was used to fabricate palladium (Pd) and silver-gold (Ag/Au) Schottky contacts. I-V and C-V measurements were used to determine the suitability of the device by calculating the ideality factor (n) and carrier concentration (Nd). The Pd/n-Ge Schottky diodes were of high quality with ideality factor n = 1.159 before irradiation. After irradiation, ideality factor increased to n = 1.383, showing that exposing the device to irradiation, Schottky diodes of the device degraded. The carrier concentration of the devices from the C-V graphs, where the plot representing the samples before irradiation, was observed to be steeper compared to the plot before irradiation, indicating a decrease in free carrier density from 1.52×1016 cm-3 for an unirradiated sample to a 6.37×1015 cm-3 irradiated sample.
A DLTS spectrum of the unirradiated germanium did not show any electrically active defects in detectable concentrations. After irradiation, DLTS spectrum illustrated the presence of several defects: E07+04, E10, E16+17, E23+25, and E37 (E+Eʹ). Peaks E and E’ were separated at 185 K by L-DLTS using manual regularization parameters, allowing the inversion routine to take into consideration the possibility of two or more closely spaced peaks. Conventional DLTS spectrum were also recorded for different pulse widths, and the peak height was reduced with a shorter pulse width, indicating that partial trap recharge was hardly observed for defects with pulse widths of 100 ns and 1 µs. The peak height started intensifying from 10 µs to 1 ms, which is a longer pulse width until it reaches saturation, meaning that all traps were now filled. The activation energies for all observed defects were calculated from the Arrhenius plot. Defects E25, E23, E10, E0.07, and E0.04 were observed after irradiation, but they were not all fully characterized. AuAg/Ge samples measurements illustrated the presence of 6 peaks after irradiation, which revealed an additional peak with an activation energy of 0.21 eV, defect E21 when compared to Pd/Ge samples.
The electrical properties of E and E’ defects in germanium (Ge) introduced by alpha particle radiation were studied using high-resolution Laplace deep-level transient spectroscopy (L-DLTS). From the Arrhenius plot, it was observed that the E-centre consists of two components with similar DLTS signatures, but they have different properties, given that they have different activation energies of Et 0.375 eV and 0.370 eV. Electric field dependence of the E defect was measured at different temperatures to distinguish between Poole-Frenkel and phonon-assisted tunnelling, it was observed that the emission of carriers was described by phonon-assisted tunnelling for all measurements at different temperatures. The defect's depth profile measurements for the E-centre showed that as we probe deeper into the bulk of the semiconductor, the concentration of the E-centre defect decreases. The DLTS amplitude of the combined E and E' defects increased as the filling pulse width increased from a few microseconds to a millisecond. However, the DLTS amplitude of E was observed to be 4 times bigger than the DLTS amplitude of E' and both defects are structurally differentPhysicsMSc (Physics)RestrictedFaculty of Natural and Agricultural Science
Synthesis and characterization of tin dioxide coated gold nanocomposites for applications in thermal heat conduction
Get PDFDissertation (MSc (Physics))--University of Pretoria, 2023.In this work, we synthesized three nanofluids based on tin dioxide-coated gold
nanocomposites (Au@SnO2) using a two-step method. First, we synthesized gold (Au)
nanomaterials, then encapsulated them with tin dioxide (SnO2) through spontaneous
hydrothermal encapsulation. Finally, we dispersed them into an ethylene glycol base fluid.
The materials were analysed using various physicochemical techniques, such as Powder X-
Ray diffraction (XRD), Transmission and Scanning electron microscopy (TEM and SEM),
and Ultraviolet-Visible spectroscopy (UV-Vis). The diffraction patterns of the materials
showed that the composite structures consisted of an FCC Au core and a mesoporous SnO2
shell.
TEM and SEM micrographs showed that the Au@SnO2 nanocomposites had
spherical, rod-like, and prism-like shapes. To assess the structural stability of these different
types of Au@SnO2 nanocomposites, their TEM micrographs were collected and analysed
over a three-month period. The results showed that the Au@SnO2 nanocomposites had better
structural stability than their counterparts that were not coated with SnO2 and were exposed
to the same conditions.
During an additional evaluation of the materials' structural stability, their ultraviolet-
visible absorption spectra were analysed over a three-month period. The results indicated
that by encapsulating the Au nanostructures with SnO2 with an appropriate coating thickness
size in the range from 50 nm to 150 nm, both the structural stability and optical properties
of the materials can be significantly improved. The study was based on the observation of
localized surface plasmonic resonance absorption peaks at a wavelength of 550 nm, which
showed an increase in intensity and a narrower bandwidth upon the encapsulation of the gold
nanostructures with SnO2. In contrast, the pure uncoated Au nanostructures showed low
intensity and broad absorption peaks at the same wavelength. Furthermore, for the last two
months of the study, a red shift to a higher wavelength of 600 nm was observed for the high
intensity absorption peaks of the coated nanostructures, while the uncoated ones did not
show any such shift.
Thermal conductivity of the Au-based nanofluids, both coated and uncoated with tin
dioxide, was measured using the transient hot-wire method. The results showed that the
thermal conductivity of the nanofluids made up of SnO2-coated gold nanostructures
increased by more than 10%. Additionally, the thermal conductivity of the SnO2-coated Au
2
nanofluids was observed to rise with an increase in the thickness of the SnO2 layer
encapsulating the Au nanostructures. These findings suggest that the Au-based nanofluids
coated with SnO2 have potential applications in thermal energy management and electronic
cooling systems.National research foundation (NRF)PhysicsMSc (Physics)UnrestrictedFaculty of Natural and Agricultural Science
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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