A density functional theory study of polarons on different TiO₂ surfaces

Polarons are widely considered to play a crucial role in the charge transport and photocatalytic performance of materials, but the mechanisms of their formation and the underlying driving factors remain a matter of controversy. This study delves into the formation of polarons in different crystalline forms of TiO₂ and their connection with the materials' structure. By employing density functional theory calculations with on-site Coulomb interaction correction (DFT + U), we provide a detailed analysis of the electronic polarization behavior in the anatase and rutile forms of TiO₂. We focus on the polarization properties of defect-induced and photoexcited excess electrons on various TiO₂ surfaces. The results reveal that the defect electrons can form small polarons on the anatase TiO₂(101) surface, while on the rutile TiO₂(110) surface, both small and large polarons (hybrid-state polarons) are formed. Photoexcited electrons are capable of forming both small and large polarons on the surfaces of both crystal types. The analysis indicates that the differences in polaron distribution are primarily determined by the intrinsic properties of the crystals; the structural and symmetry differences between anatase and rutile TiO₂ lead to the distinct polaron behaviors. Further investigation suggests that the polarization behavior of defect electrons is also related to the arrangement of electron orbitals around the Ti atoms, while the polarization of photoexcited electrons is mainly facilitated by the lattice distortions. These findings elucidate the formation mechanisms of different types of polarons and may contribute to understanding the performance of TiO₂ in different fields.