Imaging the surface potential at the steps on the rutile TiO₂(110) surface by Kelvin probe force microscopy

Although step structures have generally been considered to be active sites, their role on a TiO₂ surface in catalytic reactions is poorly understood. In this study, we measured the contact potential difference around the steps on a rutile TiO₂(110)-(1 × 1) surface with O₂ exposure using Kelvin probe force microscopy. A drop in contact potential difference was observed at the steps, indicating that the work function locally decreased. Moreover, for the first time, we found that the drop in contact potential difference at a < 1-11 > step was larger than that at a < 001 > step. We propose a model for interpreting the surface potential at the steps by combining the upward dipole moment, in analogy to the Smoluchowski effect, and the local dipole moment of surface atoms. This local change in surface potential provides insight into the important role of the steps in the catalytic reaction.