Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Chemical and Biochemical Engineering


Prof. Paul A Charpentier

Second Advisor

Prof. Hugo deLasa

Third Advisor

Prof. Amin Rizkalla


One-dimensional nanostructures (e.g. nanotubes, nanowires, nanobelts) of titania (TiO2) are receiving considerable attention due to their unique physical properties, high activity, strong photooxidation capability, chemical stability and numerous potential applications in solar energy systems. These nanostructured materials bear tremendous promise in the effective utilization of solar energy based on photovoltaic and water-splitting devices. However, the available preparation techniques have limitations. In this respect, a sol-gel process using supercritical carbon dioxide (scCO2) as solvent offers a viable alternative to obtain these nanostructured materials, due to the unique enabling properties of scCO2. This research has focused on the synthesis and modification of TiO2 nanomaterials via an acid modified sol-gel process in scCO2. In this regard, the doping agents used were Fe, Zr, and N. Because of its zero surface tension, scCO2 helps to form and maintain nanostructures and high surface areas of the synthesized nanomaterials. The low dielectric constant results in lower solubility with LA-LB interactions helping to stabilize the hydrolysis products, resulting in new and desirable morphologies. The synthesis was carried out in a batch reactor, i.e. in a view cell equipped with sapphire windows, for observation of the phase changes. The synthesis process was also studied using in situ FTIR spectrometry with the resulting nanomaterials being characterized using electron microscopy, N2 physisorption, FTIR, XRD, XPS, thermal analysis, TPD and UV-Vis analysis. A kinetic model for the phase changes involved during heat treatment of the synthesized nanomaterials was developed, and the performance of the Hi synthesized nanomaterials was evaluated as a photocatalyst to degrade methylene blue in aqueous solution under UV light. The results showed that the type of dopant significantly altered the morphology as well as the surface properties of the hybrid nanomaterials. For instance, while being synthesized under similar experimental conditions in scCO2 pure TiO2 formed nanofibers of 20-50 nm dia, Zr doped TiO2 samples formed sheets with 100-300 nm width and nanotubes with a diameter of 50-140 nm, depending on the initial concentration of precursor(s). However, Fe doped samples showed a flake type flat structure while zirconia modified N doped TiO2 samples showed a sheet type structure. Crystalline structures were obtained when the prepared materials were calcined at 500 °C. Anatase TiO2 nanocrystallites with a size range ca. 6-14 nm were obtained depending on the amount and type of doping. The resulting materials exhibited a mesoporous structure and a higher surface area compared to pure TiO2 nanomaterials. This work revealed that the direct sol-gel process in scCO2 is a promising technique for synthesizing and modifying TiO2 nanomaterials. The formed modified TiO2 nanostructured materials had a higher surface area, smaller crystallite size, and greater thermal stability, which are all desirable features for catalysts, support materials, semiconductors, and electrodes in dye-sensitized solar cells (DSSC). The simple synthesis procedure, which is scalable, used mild reaction conditions with a green solvent, providing a high yield and high quality nanomaterials, making this route potentially attractive for commercial employment.



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