(289f) Using DFT+U for Modeling TiO2 Photocatalysis

The ability to model materials such as metal oxides has brought much insight to the areas of catalysis, gas sensing, and electronic materials development. Density functional theory (DFT), the most popular electronic structure method, unfortunately suffers from self-interaction problems such that for instance band gaps are often miscalculated. The localization of charge, due to defects or photoexcitation, is also difficult to model with DFT. The DFT+U method has made considerable advances towards correct description of the electronic structure of metal oxides.

We present results on modeling TiO₂, a model metal oxide used as catalyst support and photocatalyst, using DFT+U. We discuss two important aspects of the DFT+U method: (1) charge localization to form reduced Ti³⁺ centers (Ti is in a +4 state in stoichiometric TiO₂) and (2) optimal choice of U value. We discuss our strategies to control charge localization using DFT+U. Without proper control electrons may localize at a variety of Ti sites, often forming metastable states. By controlling the location of unpaired electrons we were able to determine preferred location of Ti³⁺ centers (e.g. surface versus sub-surface) as well as diffusion rates of these electrons. The procedures for controlled electron localization are not trivial but are necessary for a systematic study of charge localization. Finally, we show that U value choice is often property-dependent; a U value that may produce a "correct" experimental parameter (such as band gap) may not be suited for other properties. We also discuss the different phases of TiO₂ (anatase and rutile) and strategies for deciding a U parameter for each of the respective phases.