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

The performance of TiO$_2$-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 $^{57}$Fe Mössbauer spectroscopy experiments, using dilute $^{57}$Mn implantation into pristine (TiO$_2$) 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 Fe$^{2+}$ and Fe$^{3+}$ are observed in pristine anatase, where the latter demonstrates the spin-lattice relaxation. The spectra recorded for hydrogenated anatase show no Fe$^{3+}$ 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 $^{57}$Fe 14.4 keV state. The surrounding oxygen vacancies favor the high-spin Fe$^{2+}$ 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 Ti$^{3+}$ and Ti$^{2+}$ 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.