Cation Doped Titanum Dioxygen Thin Films Prepared by Reactive Sputtering: Synthesis, Characterization, and Applications for Environmental Catalysis

Direct current reactive magnetron sputtering (DCMS) is a synthesis technique to produce mixed phase (anatase/rutile) titania thin films with high interfacial density, preferred crystal orientation, and controlled defects. Previous work has shown that phase proportions and oxygen deficiency can be optimized to produce a relative maximum in photoactivity. The goal of this research is to utilize DCMS to investigate Nb substitution in mixed phase films to enhance their visible light harvesting and improve photoefficiency for solar-driven environmental catalysis. It was my hypothesis that Nb substitution in the TiO2 lattice will introduce defect energy levels within the host bandgap and lead to visible light activation.;I have investigated how Nb substitution changes structural and functional properties of TiO2 films. The addition of Nb during synthesis favored rutile formation and increased the energy input required to produce crystalline films. Nb was substituted as a mixture of Nb5+/Nb 4+ and cation vacancies formed to compensate for n-type doping. Mixed-phase Ti1-xNbyO2 showed a strongly red-shifted photoresponse and higher charge carrier densities under UV and visible light compared to undoped films. At Nb concentrations ≥∼5 at.%, photocurrent was diminished due to higher recombination rates facilitated by cation vacancies. Nanoscale characterization was used to determine the distribution of Nb and crystal phases in the films. Anatase and rutile were found to be homogeneously dispersed in the films, indicating that high densities of antase/rutile interfaces are formed. At lower concentrations, Nb is homogeneously disturbed in the films, while at higher concentrations, Nb preferentially substitutes into rutile. Despite a high charge carrier density, Ti1-xNbyO 2 thin films showed diminished photooxidative activity and showed no activity for photoreduction of CO2. Nb doped films were found to have a different crystalline orientation and higher rates of charge recombination compared to undoped films, which accounted for their lower activity. Composites incorporating Ti1-xNbyO2 were investigated where the Nb doped film is used as a photosensitizer with other active TiO 2-based materials.;This work identified an optimal phase distribution and dopant concentration (<5 at.%) to produce a visible light activated film with a high charge carrier density which may find potential use in solar harvesting applications.