PNNL

Abstract

The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO2(110) is presented. The sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting a presence of long lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. The density functional theory calculations show that the lowest energy diffusion path involves a simple hydrogen motion along the BBO from one BBO to its neighbour. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via two step mechanism, the initial diffusion of localized charge followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.