The interaction of Pt with TiO2(110) surfaces: A comparative XPS, UPS, ISS, and ESD study

The interaction of platinum with (110) single crystal surfaces of titanium dioxide (TiO2) has been studied by means of X-ray and ultraviolet photoemission spectroscopies (XPS and UPS), ion backscattering spectroscopy (ISS), Auger electron spectroscopy (AES), and electron-stimulated desorption (ESD) for Pt coverages in the sub- and monolayer range. The latter have been evaporated thermally under ultra-high vacuum conditions. We found that Pt overlayers are thermodynamically stable at room temperature with respect to the formation of Ti suboxides. At submonolayer coverages, Pt atoms adsorb preferentially on top of five-fold coordinated Ti surface atoms which act as nucleation sites for the subsequent three-dimensional growth of Pt clusters. Metallic Pt clusters are formed at the surface at elevated temperatures. UPS difference spectra show additional Pt-derived electronic states below the Fermi energy. Chemical bonds between Pt and TiO2 surface atoms are not formed under these conditions. On pre-reduced TiO2(110) surfaces, in contrast, a localized electronic charge transfer is found between Ti3+ states and Pt surface atoms.