COADSORPTION OF ZN AND S ON MO(110) - WEAKENING OF THE ZN-MO BOND AND ZN-PROMOTED SULFIDATION OF MO

The coadsorption of Zn and S on Mo(110) has been investigated using TDS, XPS and XAES. Zn atoms supported on clean Mo(110) desorb at 670 (first layer), 510 (second layer) and 480 K (multilayer). The Mo-Zn bond is considerably stronger (similar to 10 kcal/mol) than the Zn-Zn bond. The strong Mo <--> Zn interaction leads to shifts of similar to 0.35 eV toward lower binding energy in the Zn 3d and 2p levels, and in the ZnL(3)M(45)M(45) Auger transition. At submonolayer coverages (theta(S) + theta(Zn) < 1 ML), Zn and S adatoms do not react to form zinc sulfides on top of Mo(110). The Zn and S adatoms compete for making bonds with the Mo(110) substrate. This competition produces a significant (5-10 kcal/mol) weakening in the strength of the Mo-Zn bond. Surfaces with submonolayer coverages of Zn and S show desorption of Zn at temperatures between 450 and 650 K, with S still remaining on the surfaces at 1200 K. The exposure of Zn/Mo(110) surfaces to large amounts of S-2 gas (theta(S) > 1 ML) at 300 K produces zinc sulfides and chemisorbed sulfur, without forming molybdenum sulfides. At 600-700 K, zinc promotes the formation of molybdenum sulfides by favoring the migration of sulfur from the surface into the lattice of the Mo substrate. In the ZnxS/MoyS/Mo(110) systems, the zinc sulfides decompose from 750-950 K, whereas the molybdenum sulfides dissociate at much higher temperatures (1200-1350 K).