Interactions between Cs and noble metals (Cu, Ag, Au) on ZnO surfaces: electronic perturbations and alloy formation

The adsorption of cesium on polycrystalline ZnO and the coadsorption of the alkali with noble metals (NM=Cu, Ag or Au) were investigated using X-ray and synchrotron-based photoelectron spectroscopies. At small coverages, Cs forms very strong adsorption bonds on polycrystalline ZnO with desorption temperatures that are higher than 700 K. The strength of the Cs-ZnO bond depends on the coverage of Cs, and for a saturated adlayer, the desorption of the alkali starts around 300 K. In the Cs/ZnO systems, there is a migration of oxygen toward the alkali-oxide interface, but the Cs does not become fully oxidized. When Cu or Au is deposited on Cs/ZnO, Cs moves from the alkali-oxide interface to the top of the noble metal overlayer. This migration substantially increases the thermal stability of Ca on the surface. In NM/ZnO surfaces, the metal overlayers exhibit valence bands that are significantly different from those of the corresponding bulk metals. These electronic perturbations do not facilitate the formation of Cs-NM alloys, and the Cs/NM/ZnO systems show a pattern of reactivity identical to that found for bulk samples: one can see the formation of Cs-Au alloys in Cs/Au/ZnO, whereas no alloying is found in Cs/Cu/ZnO and Cs/Ag/ZnO. The supported Cs-Au alloys are very reactive toward O-2. They decompose at temperatures between 400 and 500 K, with a fraction of the Cs desorbing into gas phase and the rest remaining chemisorbed on Au or ZnO. (C) 1998 Elsevier Science B.V. All rights reserved.