PNNL

Abstract

We describe a DFT+U study of the (110) rutile surface with oxygen vacancies (Ov). Oxygen vacancies leave behind two excess unpaired electrons per Ov the formation of two Ti3+ ions. We investigate the location of the Ti3+ ions within the first three surface layers. In total we obtained 49 unique solutions of possible Ti3+ pairs, to examine the stability of all Ti types (e.g. five-coordinated surface Ti, six-coordinated surface Ti, sub-surface sites, etc.). Our results show that sub-surface sites are preferred, but that many configurations are close in energy, up to 0.3-0.4 eV of each other. In contrast to findings in previous work, we show that sites directly adjacent to the Ov are unstable. Analysis of our results show that the two Ti3+ ions within a pair behave independent of each other, as there are little electronic interactions between the excess electrons associated with these sites. We also examined the migration of Ti3+ sites from the surface into the bulk and find the surface locations to be preferred by ~ 0.5 eV relative to the bulk. Our systematic results provide a comprehensive picture of excess electrons that indicates that they are not trapped or localized at specific sites, but are distributed across several sites due to nearly degenerate Ti3+ states. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.