Experimental and theoretical study of electronic and hyperfine properties of hydrogenated anatase (TiO2): defects interplay and thermal stability
Abstract
In this study we report on the results from emission 57Fe M\"ossbauer Spectroscopy experiments, using dilute 57Mn implantation into pristine (TiO2) 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 Fe2+ and Fe3+ are observed in pristine anatase, where the latter demonstrates the spin-lattice relaxation. The spectra obtained 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 57Fe 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 presence of both Ti3+ and Ti2+ 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.