PNNL

Abstract

In this work we describe our use of Marcus theory to model the electron transfer process in TiO2. Electron transport is described by a polaron model, whereby a photo-excited electron is localized at a Ti4+ site and hops to an adjacent Ti4+ site. We obtained the relevant parameters in Marcus theory (namely the activation energy ?G*, the reorganization energy ?, and the electronic coupling matrix elements Vab) for selected crystallographic directions in rutile and anatase, using periodic DFT+U and Hartree-Fock cluster calculations. The DFT+U method was necessary to correct for the ubiquitous electron self-interaction problem? in DFT. Our results give non-adiabatic activation energies of similar magnitude in rutile and anatase, all near 0.3 eV. The electronic coupling matrix element, Vab, was determined to be largest for electron transfer parallel to the c direction in rutile, with a value of 0.20 eV, while the other directions investigated in both rutile and anatase gave Vab values near 0 eV. The results are indicative of adiabatic transfer (thermal hopping mechanism) in rutile and of diabatic transfer (tunneling mechanism) in anatase. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.