abstract
The performance of TiO2-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 Fe-57 Mossbauer spectroscopy experiments, using dilute 57Mn implantation into pristine (TiO2) 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 Fe2+ and Fe3+ are observed in pristine anatase, where the latter demonstrates the spin-lattice relaxation. The spectra recorded 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 Fe-57 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 the presence of both Ti3+ and Ti2+ 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.
keywords
SURFACE; TITANIUM; ABSORPTION; DIFFUSION; STORAGE; RUTILE; OXYGEN; LIGHT; NANOCRYSTALS; EFFICIENCY
subject category
Chemistry; Science & Technology - Other Topics; Materials Science
authors
Zyabkin, DV; Gunnlaugsson, HP; Goncalves, JN; Bharuth-Ram, K; Qi, B; Unzueta, I; Naidoo, D; Mantovan, R; Masenda, H; Olafsson, S; Peters, G; Schell, J; Vetter, U; Dimitrova, A; Krischok, S; Schaaf, P
our authors
João Nuno Santos Gonçalves
Junior Researcheracknowledgements
The authors are indebted to the ISOLDE team, particularly to J.G.M. Correia, for technical assistance during the beamtime. This work was supported by the German Federal Ministry of Education and Research (BMBF) Projects 05K16SI1, 05K19SI1, and 05K16PGA. Additional funding was from the European Union's Horizon 2020 research and innovation program under grant agreement no. 654002 (ENSAR2). The authors are also grateful to CERN/ISOLDE for support of the experiment IS653. B.Q. and S.O '. acknowledge the support from the Icelandic University Research Fund. I.U. acknowledges the support of the Ministry of Economy and Competitiveness (MINECO/FEDER) under the project RTI2018-094683-B-C55 and Basque Government Grant IT1005-16. D.N., K.B.-R., H.M., and G.P. acknowledge support of the DST/NRF, South Africa. We are thankful for research funding by the state of Thuringia and the European Union within the frame of the European Funds for Regional Development (EFRD) under grant 12021-715.