It is known that the two polymorphs of TiO2, rutile and anatase, show different n-type doping behaviour: while heavy Nb-doping makes anatase a transparent conductor, rutile remains insulating. The reason for that is electron self-trapping by Ti(4+/3+) transitions in rutile, which is absent in bulk anatase. Using HSE06 calculations on a (101) anatase slab we show that electron self-trapping is possible on the anatase surface. Therefore, near the surface both Nb dopants and oxygen vacancies give rise to localized polaronic states with band gap levels deeper than in the bulk. These low lying levels stabilize the near surface vacancies, and make the bridging surface vacancy significantly more favourable energetically than its bulk counterpart. We do not find subsurface vacancies with formation energy lower than at the surface bridging site. The calculated ionization energy of the surface bridging vacancy is in excellent agreement with recent observations.
Nb prefers a sixfold-coordinated site on the anatase (101) surface and, unlike in the bulk, gives rise to a deep, well localized electron–polaron state. Nb is the black sphere, Ti atoms are blue and O atoms red. The yellow lobe shows the wave function square of the occupied gap state, with dominating contribution from a nearby Ti(3+) site. Electrons of an oxygen vacancy at or near the surface can be trapped in such states, too.
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