Graduate Thesis Or Dissertation


Exploring the Iron Keggin Ion as a Prenucleation Cluster for Iron Oxide Nanoparticles Formation Public Deposited

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  • Iron oxide materials including ferrihydrite and magnetite play essential roles in contaminant transport, pH control of surface and groundwater, and microbial activity. They have various applications ranging from data storage to cancer therapy. Recent studies showed that the NPs growth does not involve atom by atom nucleation but starts by formation and aggregation of pre-nucleation clusters. In case of ferrihydrite and magnetite, the building block is believed to be the iron Keggin ion, but the synthesis was undiscovered for many years since ferric clusters are too reactive (acidic) and form polymeric species due to the inability of forming terminal oxo ligand. In this thesis, we used bismuth ion to isolate the reactive Keggin ion in discrete form and crystalize that in THF. This work is the first report on isolating iron Keggin ion with oxo ligands. However, the cluster is not stable and converts to ferrihydrite NPs in solution. This instability happens because by aging, the bismuth ions detach from the cluster and they start to aggregate and form Fh. These solubility and stability limitations challenge further fundamental studies or applications of the molecular iron Keggin ion, such as determining the stepwise mechanism of the transition from the iron Keggin ion to ferrihydrite or other related iron oxide materials. As the other part of this thesis, we were able to synthesize a water-soluble and stable Keggin ion using the same strategy but a different ligand (TFA instead of TCA). Chemical reduction of this cluster by hydrazine forms magnetite nanoparticles and bismuth particles while gentle reduction with electron beam forms magnetite and ferrihydrite nanoparticles. Although this strategy was a robust method for isolating unstable clusters, the mechanism, and intermediate species were not identified and needed to be investigated thoroughly. Therefore, as the last part of this thesis, we managed to isolate a tetramer intermediate using milder pH and temperature than Keggin ion synthesis. This cluster converts to Keggin ion at high temperatures and ultimately to Fh nanoparticles. This thesis clarifies the pathway for formation of iron oxide nanoparticles from iron nitrate to tetramer, Keggin ion and finally nanoparticles. Isolating these PNCs helps us to have a better understanding of iron oxide NPs formation, and to use that information to control the size and/or shape of these NPs.
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