Does Polaronic Self-Trapping Occur at Anatase TiO₂ Surfaces?

Abstract

The\nbehavior of electron and hole charge carriers at surfaces of\ntitanium dioxide (TiO₂) controls performance for important\napplications, including photocatalysts and solar cells. Although anatase\nTiO₂ exhibits high electron mobility in the bulk, a commonly\nheld belief is that strong coupling between electrons and phonons\ncan lead to electron trapping at anatase surfaces. However, direct\nevidence is scarce, and the nature of the trapping sites and electronic\nproperties remains unclear. To address this question, we investigate\nthe trapping of electrons and holes at low and high index surfaces\nof anatase TiO₂ using an accurate hybrid density functional\ntheory approach. We find that, as in the bulk, electrons do not trap\non the low index planes (001, 100, 101, 110, 112) of anatase crystals.\nFor the higher index planes (103, 105, 107) that contain structural\nstep defects, we find that electrons do trap at the low-coordinated\nTi cations present on the steps. The trapping of holes at the surfaces\nof anatase TiO₂ is a more complicated picture, as the distribution\nof hole traps is facet-dependent. The (001) and (100) surfaces as\nwell as 105 and 107 surfaces, which have 001-type terraces, have the\nstrongest affinity to trap holes. Hole trapping for the 101, 110,\n112, and 103 surfaces is found to be favored in the subsurface layers\nand not at the surface facet. These results provide crucial insights\ninto the behavior of electrons and holes in TiO₂ relevant\nfor applications in photocatalysis and challenge the common perception\nthat electrons trap at low index surfaces of anatase TiO₂.