Cesium adsorption on TiO2(110)

The adsorption of Cs on a TiO2(110) rutile surface was investigated at 130-800 K using x-ray photoelectron spectroscopy, x-ray excited Auger electron spectroscopy, temperature-programmed desorption, work-function, and band-bending measurements. Below room temperature, the Cs displays a Stranski-Krastanov growth mode, with the completion of a uniform monolayer (ML) containing (6+/-2)x10(14) Cs adatoms per cm(2), followed by the growth of three-dimensional clusters of Cs that cover only a small fraction of the surface. The Cs in the first similar to 1/2 ML is very cationic, donating electron density to the TiO2. Most of this charge is localized near the topmost atomic layers, with Ti4+ ions being reduced to Ti3+. This gives rise to a local dipole moment of the adsorbate-substrate complex of similar to 6D at similar to 0.1 ML. However, a small part of the charge transferred to the substrate also goes much deeper into the solid, giving rise to downward band bending of similar to 0.2-0.3 eV. This band bending nearly saturates at similar to 0.05 ML. The local dipole moment of the alkali-metal-substrate complex decreases smoothly with coverage in the first ML, due to dipole-dipole repulsions and their consequent mutual depolarization, similar to transition-metal surfaces. This gives rise to a rapid and smooth decrease in the heat of adsorption with coverage from >208kJ/mol down to similar to 78kJ/mol.