Cesium adsorption onTiO2(110)

Abstract

The adsorption of Cs on a ${\mathrm{TiO}}_{2}$(110) rutile surface was investigated at 130--800tK 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\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{14}$ Cs adatoms per ${\mathrm{cm}}^{2}$, followed by the growth of three-dimensional clusters of Cs that cover only a small fraction of the surface. The Cs in the first \ensuremath{\sim} ML is very cationic, donating electron density to the ${\mathrm{TiO}}_{2}$. Most of this charge is localized near the topmost atomic layers, with ${\mathrm{Ti}}^{4+}$ ions being reduced to ${\mathrm{Ti}}^{3+}$. This gives rise to a local dipole moment of the adsorbate-substrate complex of \ensuremath{\sim}6D at \ensuremath{\sim}0.1fML. 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 \ensuremath{\sim}0.2--0.3feV. This band bending nearly saturates at \ensuremath{\sim}0.05nML. 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 \ensuremath{\sim}78kJ/mol.