School of Chemistry - Theses
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ItemMetal-anilate coordination polymers( 2018)Crystalline coordination polymers are a class of materials which hold promise as adsorbents, sensors, electronic materials and magnets, as catalysts and separation matrices, or as batteries and electro-catalysts. Many of these applications depend on materials which offer an unusual combination of properties, resulting from the combination of a ligand and metal in a periodic lattice. These properties include stability and permanent porosity upon desolvation, crystallinity, monodisperse pores, open metal sites, flexibility, uptake of and interaction with guest molecules. Coordination polymers derived from redox-active ‘anilate’ ligands such as 2,5-dihydroxy-p-benzoquinone (H2dhbq) and chloranilic acid (H2can) have been studied as a possible mechanism for the introduction of electroactive properties into coordination polymers. No systematic study of the coordination polymers derived from substituted anilate ligands has previously been undertaken. In Chapter 2, the syntheses and crystal structures of novel anilate molecules and derived mononuclear and dinuclear compounds are reported. Ligand syntheses are achieved in one or two steps from commercially available starting materials, achieving sufficient purity upon recrystallisation, and many are observed to form isostructural dinuclear metal complexes. The bond distances within these compounds and similar compounds in the literature are observed to be dependent on oxidation state, and are independent of metal coordination geometry, allowing for the assignment of oxidation state from crystallographic data. In Chapter 3, a number of novel one-dimensional coordination polymers are described with metal centres linked by anilate ligands. These one-dimensional chains are shown to have a few different shapes, dependent on the relative coordination geometries around sequential metal centres in a chain. Two strategies for the formation of porous materials by cross-linking of one-dimensional chains are described, but synthetic attempts ultimately resulted in the formation of non-porous materials. In Chapter 4, structures are shown in which each metal centre links to three bridging anilate ligands, forming two- and three-dimensional coordination polymers, the majority of which showed a two-dimensional hexagonal grid network structure. Of these compounds, anionic coordination polymers of the form [M2(anilate)3]2- were found to show open-type structure, and were investigated for host-guest properties via gas adsorption. Reversible, gas-specific structural transformations were observed with the loading of CO2 and N2O into some of these compounds; by changing the cation, the structure was rigidified and did not collapse on desolvation. Compounds containing iron exhibited metal-to-ligand charge transfer, resulting in enhanced conductivity. This MLCT was found to be a temperature-dependent process in some cases. In Chapter 5, structures are shown in which each metal centre links to four bridging anilate ligands, forming two- dimensional square-grid coordination polymers and three-dimensional diamond-type coordination polymers. The square grid compound (Et4N)[Y(can)2] is shown to maintain structure on loss of solvent, and is investigated for interactions with solid, liquid and gaseous intercalants. Particularly strong interactions with hydrogen, methane and CO2 are investigated further via in situ crystallographic methods. Finally, this thesis is concluded, and these individual results are examined as a whole, including commentary on the limitations of this work and potential future work in this area. Appendices 1–5 contain additional data that supplement the thesis.
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ItemHierarchically porous titania nanostructures with high crystallinity: synthesis and photocatalytic application( 2017)Water pollution is one of the most pressing issues affecting society, consequently using titanium dioxide (TiO2) as a photocatalyst for the treatment of polluted water has attracted immense attention over past decades. However, low photocatalytic performance as a result of the fast recombination of photogenerated electron-hole pairs, few active sites and poor light utilization has restrained its real application. This thesis reports the synthesis of various novel TiO2 photocatalysts with high crystallinity and tailored nanostructures obtained by sol-gel chemistry, templating, self-assembly, solvothermal treatment and calcination. Mixed-phased hierarchically porous TiO2 networks (PTN) were prepared through sol-gel chemistry and a templating technique, followed by calcination. The PTN materials possessed reduced contact areas between TiO2 nanocrystals, significantly retarding the anatase to rutile transformation and rutile crystal growth. Compared to control samples prepared without the template, hierarchical PTN materials showed enhanced photocatalytic activity towards the degradation of methylene blue (MB) under UV light illumination. The material calcined at 600 °C for 6 h contained 15.4 % rutile and had a specific surface area of 32.2 m2 g-1, giving the highest photocatalytic activity. This enhancement was attributed to optimal rutile content and increased active sites resulting from the high surface area. Micrometer-size, monodisperse amorphous TiO2 spheres with controllable sizes were fabricated through a sol-gel process. The monodispersity, spherical shape and size were tuned by varying experimental parameters including the amount of structure-directing hexadecylamine, salt species and concentration, water amount and reaction temperature. The diameter of the spheres was determined by a competitive process between the solubility of Ti oligomers and the hydrolysis rate of titanium isopropoxide, the TiO2 precursor. Spheres with diameters up to 5.39 ± 0.68 um were achieved. The amorphous TiO2 spheres were readily converted by a solvothermal treatment and calcination process to anatase TiO2 spheres with three fascinating morphologies: ‘fluffy’ core/shell, yolk/shell and hollow nanostructures. Direct evidence was found that a surface seeding and subsequent inwards hollowing through an Ostwald ripening process lead to the formation of diverse nanostructures. The hollow microsphere calcined at 650 °C displayed a higher degradation MB rate than the benchmark, commercial Degussa (Evonik) P25. The superior photocatalytic activity of the anatase hollow structures resulted from the unique hollow structure, hierarchically porous shell and high crystallinity. The amorphous TiO2 spheres were also readily converted by a solvothermal process to pure anatase TiO2 with high thermal stability. The resultant microspheres were composed of well-crystallized anatase nanocrystals with a uniform size of 24 nm and a 77 nm pore after calcination at 900 °C. The superior thermal stability was primarily attributed to increased Ti-O-Ti bond strength and narrow crystal size distribution. Microspheres calcined at 800 or 900 °C displayed higher photocatalytic performance than P25 treated at the same temperatures. The excellent performance of the microspheres was attributed to the retention of anatase phase, presence of large pores, high crystallinity and high surface area. Overall, TiO2 photocatalyst nanostructures were fabricated by sol-gel chemistry, templating, self-assembly, solvothermal and calcination processes, and exhibited UV light photocatalytic activity that surpassed P25.