Experimental and theoretical study of electronic and hyperfine properties of hydrogenated anatase (TiO${}_2$): defects interplay and thermal stability

Abstract

In this study we report on the results from emission ${}^{57}$Fe M$\ddot{o}$ssbauer Spectroscopy experiments, using dilute ${}^{57}$Mn implantation into pristine (TiO${}_2$) and hydrogenated anatase held at temperatures between 300-700 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 obtained 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 presence of both Ti${}^{3+}$ and Ti${}^{2+}$ states. This could imply that a significant amount of oxygen vacancies and -OH bonds are present in the samples. Theory suggests that in the anatase sample implanted with Mn(Fe), probes were located near equatorial vacancies as next-nearest-neighbours, whilst a metastable hydrogen configuration is responsible for the annealing behavior.