The adsorption of chlorine on TiO₂(1 1 0) studied with scanning tunneling microscopy and photoemission spectroscopy

The adsorption of chlorine, dosed from an electrochemical cell, on the (1 1 0)-surface of a bulk-reduced TiO₂ crystal was studied with scanning tunneling microscopy, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (UPS). At room temperature chlorine adsorbs on the fivefold coordinated Ti atoms and on the vacancies in the bridging oxygen rows. Chlorine exposure at 200 and 300 °C leads to adsorption at the position of bridging oxygen atoms. In addition, ∼15 Å wide protrusions are formed that contain several chlorine atoms. No long-range ordering was found in the adsorbed layer. UPS shows that adsorption of Cl on the hot surface leads to an increase in the Ti3d-related defect state. This indicates a replacement of bridging oxygen atoms by chlorine. Interestingly, the rate of the site exchange depends on the bulk reduction state of the TiO₂ crystal. After dosing Cl at 200 °C, negligible adsorption was found on a slightly reduced, light blue crystal. The data are compared and contrasted with similar experimental results on the S/TiO₂(1 1 0) system.