School of Chemistry - Theses

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    Interfacial effects on aqueous sonochemistry and sonoluminescence
    Sostaric, Joe Zeljko ( 1999-06)
    The dissolution of quantum sized CdS and MnO2 particles in water was conducted using 20 kHz ultrasound. CdS particles were found to dissolve chemically via an oxidation process while MnO2 particles dissolved via a reductive process. It was found that the dissolution of the colloids could be controlled via the addition of surface active chemicals to solution and by varying the saturation gas type. In the presence of Na2S or propan-2-ol and argon gas, the dissolution of CdS was inhibited, whereas the addition of alcohols (methanol, ethanol, propan-2-ol, butan-1-ol and pentan-1-ol) to the MnO2 system led to an increase in the amount of dissolution for a given time of sonication. This increase in dissolution was found to be dependent on the ability of the surface active radical scavenger to accumulate around the bubble interface during the cavitation process. Eventually, at higher alcohol concentration there was a plateau or a limiting value reached for the efficiency of colloid dissolution which was common for each alcohol. (For complete abstract open document)
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    Optical properties of silica-coated metal particles
    Ung, Thearith H. ( 1999)
    The significance of silica coating technology for stabilising metal semiconductor nanoparticles was demonstrated. The physicochemical properties of silica coated colloidal silver (Ag@Si02) and the optical properties of films containing uncoated and silica coated colloidal gold (Au@Si02) were investigated. The factors governing the deposition of silica onto silane-primed silver particles were examined and the deposition conditions (pH and reagent concentrations) optimised. Thin silica shells deposited from aqueous/ethanolic sodium silicate solutions were porous and non-rigid. The shells remained insensitive to boiling in an aqueous-ethanolic solution but were substantially dissolved when boiled in pure water. The silver cores participated in a variety of chemical reactions with charged and neutral molecules to form new structures such as hollow silica nonobubbles, semiconductor cores and silica coated alloys of silver and gold. The rate of the reactions of the core with solution phase substrates decreased with increasing silica shell thickness. The silver cores of Ag@Si02 still remained catalytically active and colloidally stable during the reduction of water. Homogeneous films containing uncoated and silica coated gold particles were prepared using a self-assembly technique. The optical properties of the films were investigated as a function of film thickness and the separation between core particles. For interparticle separations below a silica shell thickness of ≈ 6.3 nm, the absorption band red-shifted and narrowed with increasing film thickness. Moreover, with increasing interparticle separation, the band blue-shifted, and both the transmittance and reflectance spectra increasingly began to resemble those of the same isolated particles in solution, in agreement with the Maxwell-Garnett model. In contrast, above a shell thickness of ≈ 6.3 nm, the absorption peak position remained insensitive to film thickness and interparticle separation, suggesting that the gold cores were no longer electromagnetically coupled. For silica shell thicknesses above 12.2 nm, the dependence of the reflectance spectra o interparticle separation and film thickness still needs to be quantified. Possible scientific and industrial applications associated with the Ag@Si02 particles and Au@Si02 particle films are suggested.