Novel photocatalyst oxide nanostructures engineered by oblique angle and glancing angle deposition
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This thesis reports on fundamental aspects of TiO2 photocatalysis for water purification and water splitting to create hydrogen as an alternative fuel source. Although TiO2 becomes active only under UV illumination, when coupled with WO3 the photoactivation can be shifted to the visible region, and photocatalytic performance can be increased due to a charge-separation effect. Our approach has been unique in creating single-layer (TiO2) and two-layer (TiO2/WO3) nanostructures that are uniform and easily reproducible utilizing oblique angle deposition (OAD) and glancing angle deposition (GLAD). An experimental relationship was developed between TiO2 surface area and its photocatalytic performance by using OAD to fabricate uniform arrays of TiO2 nanorods with varying nanorod length. The experimental results showed a direct correlation between the surface area and photodecay rate, and a theoretical first-order reaction model was developed that agreed well with our results. Using consecutive OAD and GLAD depositions, a series of two layer TiO2/WO3 nanostructures were fabricated and their photocatalytic performance was measured. The performance of the two-layer nanostructures was found to depend on the photocatalyst surface area, the relative crystal structure of each material, and the interfacial area between the TiO2 and WO3 layers. The interfacial area between TiO2 and WO3 can be maximized by utilizing a modified GLAD technique to create core-shell nanorods, which has shown significant photocatalytic enhancement under UV and visible irradiation. TiO2 is also used in photoelectrochemical cells (PECs) to photo-dissociate water for hydrogen production. TiO2 nanorods have been fabricated by OAD and shown the ability to split water at an applied overpotential of 1.0 V, with photoresponse beginning at λ = 400 nm. In addition, TiO2/WO3 core-shell nanorods are utilized in this PEC system and have the ability to dissociate water and create photocurrent with no applied overpotential, and shown a photoresponse out to λ = 600 nm. In summary, TiO2 and WO3 nanostructures fabricated by OAD and GLAD have shown promising results for effective water photo-dissociation and photocatalytic reactions. The unique properties of these nanostructures have great potential for further improvement.