Expanding the defect and doping chemistry of indium oxide and titanium dioxide through colloidal synthesis
Date
2025
Authors
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Publisher
University of Delaware
Abstract
Metal oxide nanoparticles are attractive materials for photocatalytic processes. With the thrust into alternative and green energies, the use of broad-spectrum solar illumination to drive the photocatalytic process is extremely attractive. Typical metal oxides of interest such as titanium dioxide (TiO2) and indium oxide (In2O3) possess many qualities desirable in a photocatalyst but fall short in terms of broad absorbance due to their wide band gaps exhibiting excitation strictly in the ultraviolet spectrum. Much research has been developed in the way of expanding the photon harvesting capabilities of wide band gap metal oxides in the way of defect and doping engineering. Nanoparticles prepared through non-aqueous, colloidal means offer a synthetic handle to prepare doped and defective metal oxides through relatively simple means. However, the mechanistic understanding of these deceptively simple systems requires further work to access more elaborate and challenging materials to promote broad spectrum photon absorbance. ☐ In the first part of this work, I identified a solution-based synthesis for the preparation of black, oxygen deficient In2O3. Forming an understanding of the relation between precursor, solvent/ligand, and reaction environment is a crucial step in designing synthesis to rationally target and tune oxygen deficient metal oxides for various applications. I used a wide range of solid-state and solution-based techniques to further justify the material as being oxygen deficient. We performed an in-depth analysis of the reaction supernatant relying on a combination of organic analysis and structural/morphological characterization techniques at various stages of the reaction and reaction conditions, I was able to propose a mechanism for the formation of the black oxide based on the precursor/solvent reactivity relationship. ☐ In the second part of this work, I identified a literature procedure capable of directly preparing bismuth and phosphorus doped In2O3 through non-aqueous colloidal means, which to my knowledge has not been previously reported. When doping with bismuth I focused on two different bismuth precursors to demonstrate dopant incorporation can impacted by the precursor identity. When doping with phosphorus I targeted one phosphorus precursor which we hypothesized would promote anionic P3-doping which in turn promoted the opposite in the form of cationic 5P+ doping. We performed the gambit of standard nanoparticle analysis to confirm the presence and quantitation of either dopant. We moved to further probe the doped systems to gain an improved understanding of the nature of the dopants. Performing Rietveld refinement on the doped diffraction patterns led us to suspect selectivity in the bismuth doped series, while the phosphorus doped material presented in a manner which was unexpected for 5P+ incorporation. This then led us to analyze the material with solid state NMR (ssNMR) presented promising results to support our dopant incorporation deviations which will require further computational collaboration to corroborate our findings. ☐ In the third part of this work, I developed a new method to prepare TiO2 which proved beneficial in preparing bismuth doped TiO2, again a material which to my knowledge has not been prepared through non-aqueous, colloidal techniques. Through trial and error, I identified an impact on the premature reduction of bismuth or the failure to produce material depending on the metal precursor. I then found that the presence of an alkyl chloride proved beneficial in both the formation of TiO2 and the incorporation of bismuth in the absence of a metal chloride precursor. Through NMR supernatant analysis we moved to try an identify the reaction mechanism and the promoting effect the alkyl chloride had on the formation of the material. Inspired by the success reported in other challenging systems I included lithium stearate in the reaction which proved beneficial in the improved incorporation of bismuth. Using a host of typical solid state and solution-based analysis for nanoparticles I justified the presence of bismuth as well as an improved bismuth uptake when utilizing the lithium stearate. This effort has laid the groundwork for further analysis to elucidate the overarching effect on the mechanism induced by the alkyl halide and lithium stearate which we hope will allow for an improved understanding of the reaction dynamics to promote higher bismuth incorporation and target new and challenging materials.
Description
Keywords
Doping, Enhanced absorption, Metal oxides, Nanoparticles, Oxygen vacancy, Synthesis