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arXiv · 2003.07600

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

D. V. Zyabkin·H. P. Gunnlaugsson·J. N. Goncalves·K. Bharuth-Ram·B. Qi·I. Unzueta·D. Naidoo·R. Mantovan·H. Masenda·S. Olafsson·G. Peters·J. Schell·U. Vetter·A. Dimitrova·S. Krischok·P. Schaaf
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Abstract

In this study we report on the results from emission $^{57}$Fe M${ö}$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.

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D. V. Zyabkin, H. P. Gunnlaugsson, J. N. Goncalves, K. Bharuth-Ram, B. Qi, I. Unzueta, D. Naidoo, R. Mantovan, H. Masenda, S. Olafsson, G. Peters, J. Schell, U. Vetter, A. Dimitrova, S. Krischok, P. Schaaf. 2020-03-17. Experimental and theoretical study of electronic and hyperfine properties of hydrogenated anatase (TiO$_2$): defects interplay and thermal stability. https://doi.org/10.1021/acs.jpcc.0c00085

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