Defect Complexes Interplay and its Influence on the Hyperfine Structure of Hydrogenated TiO$_2$

Determination of factors, that govern the kinetics of photocatalytic processes and the knowledge of their roles in transition metal oxides is a critical component to consider when devising efficient materials. Previous studies have shown that point defects, such as substitutional atoms, interstitials and vacancies account remarkably for the electronic structure and chemical properties, that influence the performance of these material in various applications. The expectation that a fundamental understanding of such defects will benefit to elucidate the influence that they have on the system’s functionality is the driving force for pursuing experimental and theoretical research on doped and reduced materials. This dissertation aims to ascertain how electronic structure of rutile and anatase TiO$_2$, which is considered the model system in transition metal oxides, changes when the material contains even subtle amount of dopants (H, Fe and Cd) and point defects. Consequently, time-differential $\gamma-\gamma$ perturbed angular correlation technique, along with Mössbauer spectroscopy and tracer diffusion studies have been applied and mostly complemented with theoretical studies and standard techniques. Furthermore, the author presents a newly developed emission Mössbauer set-up (eMIL), which has been developed and constructed during the current study. Obtained results demonstrate that dopant behaviour is not straight-forward. In monocrystalline rutile, Cd resides not only at the cationic site, but also at impurity-vacancy configurations near the probe. The probe in both environments seem to withstand high temperature annealing/measurements, although associated with alterations in fractions ratio. During emission Mössbauer studies, thin films of anatase show the temperature dependent behaviour throughout the whole temperature range. This results in two annealing stages caused by the vacancy movements and their interaction with Ti interstitials. Substitutional Fe$^{3+}$, at Ti sites showing spin-lattice relaxation transforms upon hydrogenation, implying that hydrogen behaves as a shallow donor. eMS experiments performed in a temperature range of 300 - 700 K reveal that vacancies and their agglomerations may govern the hydrogen motion. Further perturbed angular correlation studies at the temperature when the hydrogen motion starts, show that depending on the hydrogenation degree the dopant (hydrogen) acts differently and could form a unique coupling with Cd. Hydrogenation for prolonged times demonstrates that doping/reduction is followed by the recovery processes. The current work clearly indicates that experimental techniques based on hyperfine interactions can unravel a wealth of information about the nature and behaviour of defects in transition metal oxides, that may be readily evaluated and complemented with detailed density functional theory calculations.