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

• Published in: arXiv.org 2020-03
• Main Authors: Zyabkin, D V, Gunnlaugsson, H P, Goncalves, J N, Bharuth-Ram, K, B Qi, Unzueta, I, Naidoo, D, Mantovan, R, Masenda, H, Olafsson, S, Peters, G, Schell, J, Vetter, U, Dimitrova, A, Krischok, S, Schaaf, P
• Format: Article
• Language: English
• Subjects: Anatase; Defects; Electrical resistivity; Electronic structure; Emission analysis; Hydrogen; Hydrogen storage; Hydrogenation; Implantation; Lattice vacancies; Manganese; Photoelectrons; Room temperature; Spectroscopy; Spectrum analysis; Spin-lattice relaxation; Thermal stability
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2003.07600

In this study we report on the results from emission \(^{57}\)Fe M\({\"o}\)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.