Defining the Role of Excess Electrons in the Surface Chemistry of TiO2

Understanding and quantifying the principles governing surface-to-adsorbate charge transfer is of utmost importance because excess electrons in n-type oxides alter significantly surface binding and reactivity. We performed a systematic study using density functional theory (DFT) of the role of excess electrons in ruffle TiO2 which can result from point defects such as oxygen vacancies, bridging row hydroxyls, and interstitial Ti species. These defects create excess electrons within the Ti sublattice which can perform redox chemistry on adsorbates. We show the similarity of these defects in their ability to donate electrons to surface adsorbates, indicating that experimentally distinguishing the nature of the defects may be difficult. We examined the adsorption and reactivity of O-2 in detail and also present a generalization of these findings for a variety of species. A characterization of the redox properties of the surface/adsorbate complex indicates that when the electronegativity of the adsorbate is greater than the surface electronegativity, significant charge transfer from the reduced surface to the absorbate occurs. Surface defects do not participate in significant charge transfer for adsorbates with low electronegativity. Through variations of the U parameter in the DFT+U theory we modulated the position of the defect states in the band gap. Increased stability of the defect states leads to more difficult charge transfer to the adsorbates and a decrease in the adsorption energy. The present study offers insights on requirements with regard to modeling reduced TiO2 using electronic structure methods and an understanding of how to control the surface reactivity through degree of reduction or defect state location.