Effect of dopant level on environmental behavior of doped nanoparticles : a case study of indium tin oxide
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Novel engineered nanomaterials (ENMs) continue to be synthesized and adopted for commercial and industrial applications. Currently, the classes of ENMs utilized most in consumer products are metal oxides, metals, and carbonaceous materials. An emerging subset of metal oxide ENMs with potential in many applications are doped metal oxides, which are binary metal oxides (MO [subscript x] ) with some amount of another element, metal or non metal, inserted into the crystal lattice. This research focused on the environmental fate and transport of a major doped metal oxide, indium tin oxide (ITO), that is currently widely produced for applications in electronics. Specifically, this dissertation investigated the particle stability, solubility, and production of reactive oxygen species (ROS) by ITO nanoparticles in aqueous systems. The stability of ITO particles in electrolyte solutions and the effect of Sn level was investigated in a series of homoaggregation studies. In order to better compare colloidal stability, a novel method, called the TAA-logistic method, for estimating the critical coagulation concentration (CCC) from dynamic light scattering data was developed and tested with experimental and literature data. Using the new method, particle aggregation kinetics were compared for a range of solution conditions including pH, electrolyte valency, ionic strength, and presence of natural organic matter (NOM). Aggregation kinetics were determined for a set of synthesized particles coated with PAA-PEO polymer and for a set of bare, commercially-obtained particles. Aggregation experiments indicated inclusion of Sn in In₂O₃ decreased the aqueous stability of the nanoparticle, largely due to decreases in the magnitude of surface charge. However, the surface charge and aqueous stability did not always trend linearly with Sn content, indicating other factors, such as the distribution of Sn within the ITO crystal, were also important. Lastly, Suwannee River aquatic natural organic matter (NOM) significantly increased the aqueous stability of ITO nanoparticles through charge reversal and electrostatic stabilization. Dissolution of ITO in dilute, inert electrolyte was studied in batch and flowthrough experiments. Slow dissolution kinetics were shown in both experimental con- figurations. Sn was not appreciably leached from ITO at either pH = 4 or pH = 6. Inclusion of Sn appeared to reduce In solubility relative to In₂O₃ at pH = 6 but increased In leaching at pH = 4. The discrepancy between dissolution behavior at the two pH values relative to the In₂O₃ end-member indicated more complex solubility than explained by simple ideal solid solution aqueous solution behavior. Lastly, the electronic band structure of ITO was determined for multiple levels of Sn using ultraviolet photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy. Inclusion of tin resulted in an increase of the optical band gap and a shift of the conduction band minimum, Fermi level, and valence band maximum to more oxidizing potentials relative to un-doped In₂O₃. From these findings, ITO would thermodynamically be able to produce hydroxyl radicals from water by photocatalysis under UVB irradiation, regardless of the level of Sn doping. However, the ITO with the highest doping level investigated, which is the ITO currently produced commercially, was able to produce hydroxyl radicals under UVB illumination at a significantly faster rate than lesser- and un-doped ITO. This study showed that numerous characteristics related to the transport, transformation, and toxicity of ITO nanoparticles in aqueous environmental matrices were affected by the amount of Sn in ITO. However, the behaviors exhibited by ITO were not easily predicted by simply considering ITO as a mixture of varying amounts of the In₂O₃ and SnO₂ end-members. Therefore, further study of the environmental fate and transport of a more extensive set of doped metal oxides is needed to develop more complex models for assessing the environmental fate and transport of doped metal oxides.