School of Chemistry - Theses
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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.