Defect-Electron Spreading on the TiO2(110) Semiconductor Surface by Water Adsorption

The dissociative adsorption of water at oxygen-vacancy defect sites on the TiO2(110) surface spatially redistributes the defect electron density originally present at subsurface sites near the defect sites. This redistribution of defect-electrons makes them more accessible to Ti(4+) ions surrounding the defects. The redistribution of electron density decreases the O(+) desorption yield from surface lattice O(2-) ions in TiO2, as excited by electron-stimulated desorption (ESD). A model in which OH formation on defect sites redistributes defect electrons to neighboring Ti(4+) sites is proposed. This switches off the Knotek-Feibelman mechanism for ESD of O(+) ions from lattice sites. Conversely, enhanced O(+) reneutralization could also be induced by redistribution of defect electrons. The redistribution of surface electrons by adsorption is further verified by the use of donor and acceptor molecules that add or remove electron density.