Small Hole Polarons in Rutile TiO2

Hole transport and localization through small polarons is essential to the performance of TiO2 in photocatalysis applications. The existence of small hole polarons in bulk rutile TiO2 has been controversial with contradicting evidences from theory and experiments. We find that a fraction of exchange exists satisfying Koopmans' compliance for the polaron state and reproducing the band gap provided that charge correction is used. We clearly show that first principles computations indicate that the small hole polaron is unstable in bulk for rutile and stable for anatase TiO2. We apply our method to surface hole polarons on TiO2 and find stable surface hole polarons that are observed with transient reflectance spectroscopy.