School of Chemistry - Theses
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ItemPorous titania-based composite materials and their high-throughput photocatalytic evaluation for environmental remediation( 2016)Semiconductor-mediated photocatalysis is a promising technology for environmental remediation. Among various materials, titania is a well-known photocatalyst, yet much improvement is still required to further improve its activity. This thesis presents some approaches used to optimize the photocatalytic activity of porous titania-based materials that are physically viable for practical operations. Specifically, the effect of the addition of nitrogen during synthesis and silver nanoparticles combined with various templating methods were examined. A high-throughput testing system based on parallelization and miniaturization of methylene blue photodegradation reactions was also developed to facilitate the photocatalytic evaluation in an efficient manner. Hierarchically porous, anatase titania thin films of varied thickness were fabricated by a one-pot, soft-templating technique combined with a phase separation route. The pore structure was readily tuned by adjusting the concentration of the polymeric components added during the sol-gel synthesis. Poly(vinylpyrrolidone) (PVP) altered the three dimensional pore structure, generating macroporous networks within the films. The highest photocatalytic activity under UV irradiation normalized by the accessible surface area was obtained by porous titania thin films prepared using 1:1:0 poly(ethylene glycol):PVP:F127. The addition of F127 did increase the overall photocatalytic activity, but lowered the activity per unit area because of obstructed light penetration. In order to effectively utilize the visible light, mesoporous anatase titania with nitrogen doping was prepared by a template-free, sol-gel synthesis route. The effect of calcination conditions and the type of titania precursor were investigated, highlighting their profound influence upon the adsorption and visible light activity. The titania crystallization in the presence of nitrogen was also studied using in situ synchrotron powder diffraction. The nitrogen modified titania prepared from titanium (IV) butoxide and diethanolamine calcined at 350 °C for 10 h exhibited a high methylene blue adsorption capacity (85 mg g-1) and high photocatalytic activity under visible light. The prominent photocatalytic performance was attributed to the synergetic effect from the abundant nitrogen content (10.91 at. %), relatively high specific surface area (154.8 m2 g-1), and enhanced surface acidity (isoelectric point ≈ 2.7). To further enhance the practicality of the titania composites with nitrogen modification, the synthesis method was then extended to obtain porous monolithic structures. The goal of this study was to investigate the relationship between the photocatalytic activity and the diverse porous morphologies produced using the phase separation route and agarose gel templating. The amount of polymer used in the phase separation induced monoliths and the infiltration method in the preparation of agarose templated monoliths were shown to affect both the physicochemical and optical properties. This comparative study showed that the highest UV and visible light activity for methylene blue removal was achieved by the agarose-templated monoliths that were infiltrated at 60 °C. This was accredited to their higher surface area and higher nitrogen content compared to those of the monoliths obtained from phase separation. The addition of nitrogen and silver nanoparticles was carried out simultaneously with a hard templating technique using silica spheres packed into a three dimensional “opal” structure to further optimize the performance of titania under visible light. All of the opal templated samples in this work performed better than the commercial titania, P25. The highest photocatalytic enhancement, showing more than eight times higher activity than the non-modified titania, was achieved by the opal templated sample prepared with 1.0 mol % of silver. Although both the nitrogen and silver addition and templating enhanced the visible light activity, the most significant improvement was afforded by the utilization of the silica opal template that gave rise to high surface area (>100 m2 g-1) and promoted the surface charge interaction.
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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.