School of Chemistry - Theses
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ItemBiomaterial porous networks of hydroxyapatite and titanium dioxide( 2014)A gap in a bone that exceeds 2.5 times the bone radius is termed a critical size bone defect and will not heal naturally. The defect needs to be filled with a synthetic bone substitute (a biomaterial scaffold). \(\textit{In situ}\) delivery of medicinal drugs may assist with treating a bone defect, but current drug delivery vehicles (DDVs) are neither able to controllably release drug molecules nor allow for targeted delivery. Porous networks of either hydroxyapatite (HA) or anatase titanium dioxide could be used as biomaterial scaffolds or DDVs. Using sol-gel chemistry and a templating technique, the preparation of such networks, with potential as materials for biomedical applications, forms the research objective. Polyurethane sponge (PU), polyethylenimine-modified polyurethane sponge, polyurethane-agarose gel composite sponge (AG) and natural sea sponge were used as templates for open-celled, porous HA networks. Two concentrations of sol-gel precursor solutions were employed; the higher concentration obscured the template structure in the final network. The choice of template, multiple sol-gel coating, and the rate of temperature increase when removing the template by calcination led to evolution of the HA fibre surface. PU-templated and AG-templated HA networks were contrasted against each other, with the agarose gel component influencing results. All final networks were HA, but other calcium species were present as well. As an initial alternative to the HA networks, titanium dioxide networks were templated on PU sponge, but these lacked both high surface area and mesoporosity. Next, a Type I collagen gel was employed as a template for anatase titanium dioxide networks composed of mesoporous fibres. A standard method for titanium dioxide network preparation is firstly described, where selective solvothermal treatment preceded calcination. This is followed by modified preparations exploring the morphological transition from the collagen to titanium dioxide network structures, and solvothermal fluids containing varying solvent ratios or ammonia. The collagen fibres were 50-100 nm thick, while the titanium dioxide fibres had walls up to 300 nm thick but retained the collagen structure. Compared to networks that only underwent calcination, solvothermal treatment altered the fibre morphology and enhanced the textural properties (surface area, mesopore diameter and total pore volume). Three titanium dioxide networks, previously templated on collagen gel, and spanning a large surface area range were studied for possible biomedical applications. Biomineralisation took place in a simulated body fluid. Apatite grew on each network indicating in vitro bioactivity, but surface area may affect sustained biomineralisation. Ibuprofen drug delivery was monitored by two methods, with a loading of 58.9 mg/g achieved on the highest surface area network. The drug release was modelled as a sustained diffusion mechanism. Ibuprofen could be stored in mesopores or adsorb to the titanium dioxide fibre surface. Overall, HA and titanium dioxide porous networks were fabricated by sol-gel chemistry and templating. In general, morphology and textural properties were influenced by the choice of template, precursor concentrations and processing conditions, including the rate of heating, calcination time and solvothermal treatment. The collagen-templated titanium dioxide networks have potential as materials for biomedical applications.
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ItemCore-shell structures in dye-sensitised solar cells( 2014)The growing energy demand in the world combined with global warming and pollution is increasing the need for new energy sources. Energy production through capturing sun light is emerging as a good and feasible alternative to fossil fuels. An alternative to the conventional silicon-based devices is the dye-sensitised solar cells (DSCs) with a photoelectrode generally made from nanoparticles of titania. This type of solar cell offers many advantages such as low materials and manufacturing cost, light weight, flexibility and transparency. However, so far only 12% efficiency has been achieved for laboratory cells with a liquid hole conductor. The cell efficiency can be increased by enhancing the internal light-scattering ability of the porous semiconductor photoelectrode. The overall aim of this thesis was to increase the light scattering in the photoelectrode of the DSC by introducing core-shell structures into the photoelectrode. The objective was to improve the light-scattering ability of current mesoporous structures by synthesising core-shell structures where a core of different refractive index was introduced into mesoporous spheroids to increase their light-scattering power. A semi-batch coating method for coating particles with amorphous titania was developed with which particles of varying sizes (80-300 nm) and materials could be successfully coated. Under optimal conditions the thickness of the coating (50-220 nm) could be controlled to a great extent by varying the feed time of the water and precursor feed solutions as well as by varying the concentration of the core particles. Coating thicknesses of up to 550 nm could be achieved when four coating layers were grown on the same particles. A batch method for the synthesis of monodisperse amorphous titania was developed to produce titania cores with low porosity. A solvothermal treatment method was developed where the morphology of the crystallised particles together with their pore size distribution was controlled by the oven temperature and the ammonia concentration. Successful synthesis of core-shell structures of crystalline titania core-particles and a thick amorphous titania coating was obtained by first priming the crystalline particle surface with a thin layer of amorphous titania. A solvothermal treatment method was developed for the crystallisation of the thick titania coating. The specific surface area of the solvothermally treated coatings was significantly higher (higher than 60 m2 g-1) than for the calcined coatings. The specific surface area, pore size and morphology of the solvothermally treated coatings were dependent on ammonia concentration, treatment time and temperature. The silica core-particles dissolved during the solvothermal treatment. The amount of Si incorporated into the titania coating depended on the treatment temperature. Six different core-shell structured materials were synthesised (three dense-core and three hollow-core materials) for testing of their performance in DSCs. All the core-shell structured materials had a better light-scattering ability than the control photoelectrode consisting of only nanoparticles. The dense-core materials had the lowest solar cell performance with no difference in diffuse reflectance observed for the different core-shell structures with dense cores for a 6.5 µm photoelectrode film. The similarity in the diffuse reflectance was attributed to the low structural contrast between the core and the mesoporous coating. For a 6.5 µm film, the hollow-core materials displayed a higher diffuse reflectance compared to the dense-core materials with the silica-templated material reaching more than 75% for light with a wavelength of 400 nm. The better light-scattering ability was attributed to the higher structural contrast between the core and the shell compared to the dense-core materials. The polystyrene-templated material with a thick mesoporous coating (240 nm) had the best photovoltaic performance of all the tested materials. The cell efficiency of 8.0 ± 0.3% was attributed to the high incident-photon-to-current conversion efficiency given by a high dye concentration, good light-scattering properties and low mass transport limitations for the electrolyte.