Experimental and Theoretical Study of Electronic and Hyperfine Properties of Hydrogenated Anatase (TiO2): Defect Interplay and Thermal Stability

The performance of TiO2-based materials is highly dependent on the electronic structure and local defect configurations. Hence, a thorough understanding of defect interaction plays a key role. In this study, we report on the results from emission Fe-57 Mossbauer spectroscopy experiments, using dilute 57Mn implantation into pristine (TiO2) and hydrogenated anatase held at temperatures between 300 and 700 K. Results of the electronic structure and local environment are complemented with ab initio calculations. Upon implantation, both Fe2+ and Fe3+ are observed in pristine anatase, where the latter demonstrates the spin-lattice relaxation. The spectra recorded for hydrogenated anatase show no Fe3+ contribution, suggesting that hydrogen acts as a donor. Due to the low threshold, hydrogen diffuses out of the lattice, thus showing a dynamic behavior on the time scale of the Fe-57 14.4 keV state. The surrounding oxygen vacancies favor the high-spin Fe2+ state. The sample treated at room temperature shows two distinct processes of hydrogen motion. The motion commences with the interstitial hydrogen, followed by switching to the covalently bound state. Hydrogen out-diffusion is hindered by bulk defects, which could cause both processes to overlap. Supplementary UV-vis and electrical conductivity measurements show an improved electrical conductivity and higher optical absorption after the hydrogenation. X-ray photoelectron spectroscopy at room temperature reveals that the sample hydrogenated at 573 K shows the presence of both Ti3+ and Ti2+ states. This could imply that a significant amount of oxygen vacancies and -OH bonds is present in the samples. Theory suggests that, in the anatase sample implanted with Mn(Fe), probes were located near equatorial vacancies as next-nearest neighbors, while a metastable hydrogen configuration was responsible for the annealing behavior. The obtained information provides a deep insight into elusive hydrogen defects and their thermal stability.