Anatase and brookite are robust materials with enhanced photocatalytic properties. In this study, we used density functional theory (DFT) with a hybrid functional and the Hubbard on-site potential methods to determine electron- and hole-polaron geometries for anatase and brookite and their energetics. Localized electron and hole polarons were predicted not to form in anatase using DFT with hybrid functionals. In contrast, brookite formed both electron and hole polarons. The brookite electron-polaronic solution exhibits coexisting localized and delocalized states, with hole polarons mainly dispersed on two-coordinated oxygen ions. Hubbard on-site potential testing over the wide 4.0–10 eV range revealed that brookite polarons are formed at U = 6 eV, while anatase polarons are formed at U = 8 eV. The brookite electron polaron was always localized on a single titanium ion under the Hubbard model, whereas the hole polaron was dispersed over four oxygen atoms, consistent with the hybrid DFT studies. The anatase electron polarons were dispersed at lower on-site potentials but were more localized at higher potentials. Both methods predict that brookite has a higher driving force for the formation of polarons than anatase.

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Often the choice of semiconductor material for light‐chemical energy conversion in solar cell technology is TiO2 polymorphs. However, quantum‐mechanical phenomena of electron self‐trapping (polaron) in these polar semiconductors present a challenge to optimize their performance for absorption of solar radiation. The electron trapped in the defect site of the lattice may suppress the recombination of charge carriers and improve the efficiency of light‐chemical energy conversion. Therefore, it is crucial to study polarons in transition metal oxides and semiconductors using first‐principles methods to elucidate the nature of charge carriers and trapping sites for ameliorating the performance of energy conversion. In this work, a comprehensive review is presented using selected literature of polaron studies in TiO2 from first‐principles methods. Overview of the Landau–Pekar model to the recent development of ab initio theory of polaron is presented. The popular DFT+U approach and hybrid functional method are discussed to show the general way of studying polaron using ab initio methods. Introduction of electron‐phonon interaction and the ab initio theory of polaron are briefly presented Therefore, this review presents the development of first‐principles methods from Landau theory to the state‐of‐the‐art to study polarons using TiO2 as the toy model. Finally, conclusions and future perspectives are outlined.