Charge transfer in bulk TiO2 and at aqueous TiO2 interfaces

In this thesis, we study the localization and transfer of extra charges in bulk TiO2 and at aqueous TiO2 interfaces. In a first part, we study the presence of self-trapped polarons, i.e. localized charges coupled with lattice distortions, in the two most common polymorphs of TiO2, anatase and rutile. We do so through the use of piecewise linear functionals, which allow for an accurate description of polarons without relying on empirical parameters. Our results highlight the presence of a hole polaron in anatase, both at the Hubbard-corrected semilocal (DFT+U) and hybrid levels of theory, while the electron polaron is found compatible with the piecewise linearity condition only at the DFT+U level. In rutile, we find that hole and electron polarons are compatible with the piecewise linearity condition at both the DFT+U and hybrid levels of theory. We determine the type of conduction of the positive carriers in both anatase and rutile through the use of the nudged elastic band method, finding that anatase is characterized by hopping conduction, while in rutile the conduction is band-like. We model the hopping conduction in anatase through a kinetic Monte Carlo approach, determining a hole mobility in agreement with typical values for this class of materials. In a second part, we study the process of deprotonation at the aqueous-anatase (101) interface through the use of machine-learning-driven molecular dynamics coupled with the on-the-fly probability enhanced sampling scheme (OPES). We train a machine learning potential at the hybrid functional level of theory, in the presence of a positive charge. Through nanosecond-scale OPES simulations and the reconstruction of the free energy surface of the deprotonation reaction as a function of suitable collective variables, we determine a free energy barrier for water deprotonation of 0.35 eV, in agreement with previous investigations. We verify that the deprotonation process leads to the hydroxylation of undercoordinated surface oxygen sites and, through the analysis of the interaction between the adsorbed water molecule and the solvent, we find that the abstracted proton is lost to the liquid. Through the reconstruction of the free energy surface of multiple proton transfer channels, we determine that the most favored mechanism involves the transfer of a proton from an adsorbed molecule to the undercoordinated oxygen sites via the mediation of a solvent water molecule. Through hybrid functional calculations, we find a correlation between the deprotonated state of the adsorbed molecule and the charge localization at the adsorbate.