School of Chemistry - Theses
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ItemStudy of nanostructured metal oxides for electrochromic windows( 2018)Electrochromic materials have attracted the interest of researchers over the last several decades. They are able to change their color upon the application of a voltage. Tungsten oxide (WO3) and titanium oxide (TiO2) are the conventional electrochromic materials and have been studied most, due to their good electrochromic properties and electrochemical stability. Hence the research in this thesis is mainly based on tungsten oxide and titanium oxide electrochromic materials. By adjusting the precursor, nanostructured electrochromic materials were prepared on the substrates with improved electrochromic properties. A thin film of hexagonal WO3 nanowires was grown directly onto a bare fluorine-doped tin oxide glass (FTO) without the assistance of a WO3 seed layer via a facile and low-cost solvothermal method. Compared with the WO3 film synthesized with a seed layer, a faster switching time, higher coloration efficiency and more stable cycling were achieved due to direct nanowire contact with the substrate. In order to improve the contact between the WO3 and FTO, and therefore increase the stability of the electrode, it was annealed under N2 and the cycling life increased by almost a factor of 10. Hierarchical WO3 nanotree-like structures were grown from nanowires by a two-step non-seeded solvothermal technique on FTO. A study of the growth process revealed that the nanotrunks formed before the nanobranches grew, with the trunks orientated along the (002) plane of the hexagonal phase WO3, while the branches were orientated along the (100) and (200) planes. An electrochromic film prepared with WO3 nanotrees are expected to have a large active surface area, which enabled a large optical modulation of 74.7% at 630 nm at the low potential of -0.2 V, fast switching speeds of 2.64 s and 7.28 s for bleaching and coloration, respectively, and a high coloration efficiency of 75.35 cm2 C-1. A nanocomposite TiO2 film was prepared by decorating a rutile TiO2 nanorod (TiO2 NR) array with anatase TiO2 nanopyramids (TiO2 NPyr) via a facile two-step solvothermal method. The addition of NPyr to the TiO2 NR film is expected to increase the total amount of material, hence providing a greater number of sites for lithium ion insertion, and the NPyr are expected to increase the active surface area. As a result, the TiO2 NR–NPyr film displayed higher optical contrast and coloration efficiency compared with NR films, demonstrating that enhanced electrochromic properties could be improved by using the NR–NPyr composite structure. Anatase TiO2 nanowires were grown directly onto FTO by a solvothermal method. Different concentrations of precursor can easily control the length of the nanowires. The TiO2 nanowire films showed antireflective properties, increasing the transmittance of bare FTO by 7.2% at 550 nm. The electrochromic properties of the TiO2 nanowire sample was investigated in KCl electrolyte, giving a large optical modulation of 61.4% at 550 nm, a high coloration efficiency of 23.89 cm2 C-1 and good stability. The TiO2 nanowire film was modified by chemisorption of a monolayer of the redox chromophore 1,4-bis[((N-phosphono-2-ethyl)-4,4’-bipyridinium)-methyl]-benzene tetrachloride (viologen). The best viologen modified TiO2 nanowire film showed an optical modulation of 55.5% in 550 nm, coloration efficiency of 244.5 cm2 C−1 and good stability. Compared with the pure TiO2 nanowire thin film, the coloration efficiency of the viologen modified TiO2 nanowire thin film increased more than 10 times, showing its potential in smart window applications. Prussian blue was electrodeposited on bare FTO and a TiO2 nanowire thin film. The structure of the Prussian blue on FTO was a compact layer, whereas a core-shell (Prussian blue coating the TiO2) nanostructure was achieved on the TiO2 nanowire film. The core-shell nanostructure showed promising electrochromic properties with an optical modulation of 64% at 550 nm, a high coloration efficiency of 86.7 cm2 C-1 and a superior stability of 1000 cycles without significant decrease in optical modulation. The improvement in electrochromic performance was attributed to the support template of TiO2 nanowires increasing the Prussian blue contact with the electrolyte. Furthermore the presence of the TiO2 nanowires also lowered the current density, which improved the coloration efficiency and the stability. The electrochromic properties of the TiO2 nanowire thin films in different electrolytes were studied. The nanowire film in electrolyte containing multivalent ions, such as Al3+, showed a large optical modulation of 60.03% at 550 nm, fast insertion kinetics and good stability over 400 cycles. The electrochromic properties of a cycled TiO2 nanowire thin film in a LiClO4 propylene carbonate electrolyte can be recovered by water washing. This is an interesting concept and possible reasons for this behavior were discussed, however further work is required in this area. A range of nanostructured WO3 and TiO2 thin films have been prepared for this thesis with different morphologies and tested in electrochromic smart window application showing promising potential.
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ItemUltrasonic synthesis of advanced photocatalytic materials for use in continuous flow-through reactors( 2017)Water purification is a problem around the world. Every year, authorities introduce new standards on the amount of pollutants that could be released into the environment, and new compounds are continuously being added to the list of toxic waste. Some of these substances can be defined as persistent organic pollutants (POPs), and their degradation is generally difficult to achieve. In these cases, advanced oxidation processes are required for the decomposition of these hazardous molecules. In this thesis, the advanced oxidation process used is photocatalysis, which can be used to decompose almost any kind of organic pollutant. To enhance the photocatalytic activity of photocatalysts, researchers have focused their attention on reducing the size of the particles, reaching just a few nanometers. Unfortunately, no sufficient efforts were made in continuous flow studies, which are necessary if future industrial implementations are desired. In these systems, the photocatalyst can be used in two ways: dispersed or coated onto a surface. The former offers high activity, but its recovery at the end of the reaction can be difficult. Instead, the latter permits almost no effort in the catalyst retrieval, but its efficiency is significantly low. For these reasons, this thesis aims to investigate the conversion of nanosized catalysts into micron sized powders without a loss in activity. In this manner, the catalyst could be used as a dispersion, enhancing the degradation and reducing the costs involved in the filtration procedures. In order to accomplish this aim, microspheres were used as a template material. The studies on continuous flow systems and their comparison to batch systems, carried out in this thesis, could be useful for future industrial implementations. For the generation of microspheres, ultrasonic emulsification technique was utilized, and the fundamental principles of ultrasound, along with those of microspheres, photocatalysis, and continuous flow reactors are discussed in Chapter 1. In Chapter 2, a structured literature review examining ultrasonic emulsification, microencapsulation, photocatalysts, and studies on continuous flow reactors, is discussed. In this thesis, chitosan (a natural amino-polysaccharide used in a wide range of applications) was chosen as a shell material for the generation of the microspheres, while nano sized TiO2 and ZnO were used as model nanosized photocatalysts. In Chapter 3, materials, analytical methods, and e experimental details used in this thesis are discussed. Three continuous flow reactors are presented, along with a new type of ultra-bright LEDs used as a light source for the photocatalytic degradation of rhodamine B, metanil yellow and methylene blue. Chapter 4 is the first chapter of result and discussion section. The role of counter ions on controlling the properties of ultrasonically generated chitosan microspheres, produced via oilin- water emulsion technique, was investigated. Various acids were used to dissolve chitosan, and it was found that the conjugate bases of the acid used (which acted as counter ions to neutralize the positive charges of ammonium ions present in the chitosan backbone) played a significant role in controlling the size, size distribution, and stability of the chitosan microspheres. In Chapter 5, the development of micron sized photocatalysts was studied. Chitosan microspheres were used for the conversion of nano sized TiO2 and ZnO (25-50 nm) into micron sized particles, possessing a size of about 10 μm. The micron sized photocatalysts possessed a photocatalytic efficiency similar to that of the nano sized powders, which was investigated in both aqueous and gas phases. In addition, the mechanism on the formation of the micron sized structures was proposed. In Chapter 6, the comparison of the photocatalytic activity of batch and continuous flow systems was investigated, using the micron sized catalyst (TiO2) previously studied. It was found that the continuous flow system is able to increase the amount of decomposable dye of up to 110% compared to that reached by the batch system. In addition, the catalyst used was found to be suitable for such continuous flow studies, with no loss in activity over a period of 42 hours. In Chapter 7, the use of ultra-bright LEDs on continuous flow systems, and the ability to apply the theory of such systems on photocatalytic reactions, were studied. It was found that the consideration of the kinetics of the photocatalytic reaction being pseudo-first order is not entirely correct, and that the new type of light source is suitable for photocatalytic degradations. In Chapter 8, some concluding remarks have been provided.
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ItemMesoporous Ti-based nanomaterials for photocatalysis and energy storage( 2017)Inspired by the discovery of the photocatalytic phenomenon in splitting water, enormous efforts have been devoted to the research of TiO2 materials. This has led to various applications ranging from photovoltaics and photocatalysis to batteries and sensors, which can be roughly divided into ‘energy’ and ‘environmental’ categories. In general, the effectiveness in the practical applications depend not only on the intrinsic properties of the TiO2 material, but also on modification to the material, including composition, morphology, and the compositional modification. As a photocatalyst, TiO2 is a wide band gap semiconductor (3.0-3.2 eV) that can be used to decompose organic compounds under ultraviolet light irradiation. An efficient strategy to extend the light response to the visible range and thus improve photocatalytic activity is by designing a heterojunction semiconductor. In this thesis pristine anatase TiO2 microspheres were used to prepare mesoporous TiO2/g-C3N4 microspheres via a nano-coating procedure followed by calcination, where the porous TiO2 acts as the active supporting scaffold and g-C3N4 as the visible light sensitizer. The composite microspheres were 8.5 folds more active in degrading phenol under visible light irradiation than mesoporous g-C3N4. Furthermore, starting with mesoporous TiO2 hollow microspheres, mesoporous brookite/anatase TiO2/g-C3N4 hollow microspheres were prepared via a facile nanocoating procedure that showed mixed phases of brookite (48 %), anatase (44 %), and rutile (8 %), incorporated with a g-C3N4 coating layer. The mesoporous hollow microspheres exhibited a unique hollow shell morphology of packed TiO2/g-C3N4 nanosheets, and a remarkable 5-fold increase in degrading phenol under visible light irradiation compared to mesoporous g-C3N4. Besides visible light photocatalysis, TiO2 can be used as an anode material for lithium-ion batteries, as it shows good gravimetric performance (336 mAh g-1) and excellent cyclability. To overcome the poor rate behaviour, slow lithium-ion diffusion, and high irreversible capacity decay, TiO2 nanomaterials with tuned compositions and morphologies are being investigated. Here, a promising TiO2 material has been prepared that comprises a mesoporous ‘yolk-shell’ spherical morphology in which the core is anatase TiO2 and the shell is TiO2(B). The electrochemical results indicate high specific reversible capacity at moderate current (330.0 mAh g-1) and cyclability (98 % capacity retention after 500 cycles). Lithium-sulphur batteries have attracted considerable attention as they have high specific capacity (1675 mAh g-1) and the abundance of sulphur, makes them one of the more promising next-generation battery technologies. However, commercialization of LSBs has generally been hampered by low sulphur utilization and poor long-term cyclability. These issues can be addressed, in part, by producing cathodic additives to encapsulate sulphur and polysulphides during the charge/discharge process. Mesoporous Magnéli Ti4O7 microspheres were prepared via an in-situ carbothermal reduction that exhibited large pore volume (0.39 cm3 g-1) and high surface area (197.2 m2 g-1). Strong chemical bonding of the polysulphides to Ti4O7, along with effective physical trapping in the mesopores and voids of the matrix, give superior reversible capacity (1317.6 mAh g-1) and cyclability (88 % capacity retention after 400 cycles). Ti-based materials with carefully tuned compositions, porosity, and morphologies have been constructed and tested in photocatalytic and energy storage applications revealing promising potential.
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ItemFabrication of PVDF–TiO2 electrospun membranes incorporating with carbon nitride for solar fuel production and organic pollutant photodecomposition( 2016)Semiconductor–mediated photocatalysis for the decomposition of pollutants and production of industrially important species, i.e., methane by photoreduction of CO2 (g) is an emerging technology. However, problems, including low quantum efficiency, visible light inactivity and the difficulty to deploy and recover the photocatalyst, have to be mitigated. In order to enhance the photocatalytic activity of titanium dioxide, the sensitization of TiO2 with visible light active carbon nitrides (CNx) was proposed. Nonetheless, as an important step in the fabrication of a photocatalytic device, the integration of photocatalytic nanoparticles into a solid matrix, such as an electrospun fibrous membrane, forms a research objective in this thesis. A low temperature synthesis route to fabricate TiO2 nanoparticles with different crystal phase compositions was developed. The Ti–precursor concentration (9–45 mM) and the presence of Cl– during hydrothermal treatment influenced the TiO2 crystal phase composition. Overall, anatase–rich TiO2 samples showed higher photocatalytic decomposition activity than rutile–rich samples. However, all samples and a commercial TiO2 reference produced only trace amounts of methane during CO2 photoreduction. A polyvinylidene fluoride (PVDF)–TiO2 nanocomposite was fabricated by electrospinning followed by a low temperature hydrothermal treatment to induce the in situ growth of TiO2 nanoparticles on the electrospun PVDF nanofibres. The crystal phase composition of TiO2 was tuned by manipulating the concentration of the Ti–precursor (0.030–0.125 M) and acidity (pH <0–6.5) in the hydrothermal solution. The surface accessibility, crystal phase composition and the presence of Ti3+ within the nanocomposite significantly influenced the photocatalytic activity for CO2 reduction and organic pollutant decomposition. The maximum production of methane was 19.8 µmol per gram of photocatalyst per hour (quantum efficiency for the photomethanation reaction, Q. E.CH4 : 0.44 %) under UV irradiation. The visible light absorption of the PVDF–TiO2 nanocomposite was enhanced by the addition of CNx. A facile, low temperature wet–chemical synthesis was developed for CNx. The synthesized CNx possessed C=O functional groups that resulted in a negatively charged surface across pH 3–9, and led to an enhanced adsorption capacity and organic pollutant photodegradation under visible light irradiation. CNx also showed a relatively high capacity for heavy metal ion adsorption. Unfortunately, the CNx particles were too large for successful incorporation into the PVDF–TiO2 nanofibres. As an alternative, graphitic–CNx quantum dots (g–CNQDs) were synthesized by microwave heating, and were introduced into the PVDF–TiO2 nanofibres during electrospinning. The g–CNQDs were evenly distributed along the nanofibres, and significantly extended the photoresponse of the nanocomposite into the visible range. Methane production from CO2 photoreduction increased with the amount of g–CNQDs incorporated into the nanocomposite, with a maximum production of 39.8 µmol of methane per gram of photocatalyst per hour (Q.E.CH4 = 0.58%) under simulated sunlight irradiation.