Doctor of Philosophy
Dr. J Clara Wren
This thesis aims to develop the mechanistic understanding of the formation of iron oxide nanoparticles by γ-irradiation. Under irradiation water breaks down into a wide array of redox active species (H•, •OH, H2O2, •e-(aq)) uniformly throughout the solution. These species are capable of rapidly interconverting any soluble FeII and insoluble FeIII species resulting in the formation of nanoparticles with a narrow size distribution. The initial concentration of Fe2+ in the solution, the solution pH, the presence of any radical scavengers, the temperature of the reaction, and the dose rate of the Co60 source were all parameters investigated to elucidate this mechanism. UV-Vis spectrographic methods, gas chromatography, pH, and transmission electron microscopy were used to study the kinetics of the solution and growing particles. Raman, Fourier transform infrared spectroscopy, x-ray absorption near edge spectroscopy were employed to study the final composition of particles. Computer modelling was employed to model the water radiolysis speciation.
It was found that the growth follows a three stage mechanism. The first stage involves the rapid oxidation of Fe2+ to FeIII by •OH which forms the initial nucleate sites. Lower initial pH values will favour larger particles because the FeIII species will be more soluble and fewer nucleates will form. Lower dose rates will also favour larger particles because fewer nucleates are generated initially. In the second stage, H2O2 is responsible for the bulk oxidation of Fe2+ which adsorbs on the surface of these initial nucleates. The nucleates convert to mixed oxide FeOOH intermediates. The second stage ends when the reverse reduction reactions are capable of competing with the forward oxidation reactions. Solutions with higher initial Fe2+ concentrations stay in the growth stage for longer periods of time resulting in larger particles. The final stage is the pseudo steady-state in which continuous cycling of FeII and FeIII species by H2O2 converts the oxide to magnetite with residual mixed oxides incorporated in the system. The system continues to undergo radiation-assisted Ostwald ripening. Temperature promotes this ripening by rapidly converting the residual mixed oxides in the system allowing the particles to agglomerate.
Sutherland, Thomas I., "Model of the Formation of Iron Oxide Nanoparticles by Irradiation in Aqueous Media" (2016). Electronic Thesis and Dissertation Repository. 4178.