School of Chemistry - Theses

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    Ultrafast spectroscopy of nanostructures
    Zeng, Peng ( 2017)
    This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.
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    Hierarchically porous titania nanostructures with high crystallinity: synthesis and photocatalytic application
    Cao, Lu ( 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.