Cs promoted oxidation of Zn and Cu-Zn surfaces: a combined experimental and theoretical study

The interaction of O-2 with Zn, Cs/Zn and Cs/CuZn surfaces was investigated using photoemission and rib initio self-consistent-field (SCF) calculations. On zinc films, the sticking probability of O-2 is extremely low (10(-3)-10(-2)), and O-2 exposures in the range of 10(3) to 10(4) langmuirs are necessary to produce a significant adsorption of oxygen and the transformation of metallic zinc into zinc oxide. The presence of sub monolayer coverages of cesium enhances the oxidation rate of zinc by -3 orders of magnitude. In the Cs/Zn system, the alkali atom donates electrons to zinc. This charge transfer facilitates the formation of Zn-->O-2 dative bonds and breaking of the O-O bond. For the coadsorption of Cs and O-2 on Zn(001). the larger the electron transfer from Zn into the O-2 (1 pi(g)) orbitals, the bigger the adsorption energy of the molecule and the elongation of the O-O bond. In general, cesium does not promote the oxidation of copper. In the Cs/CuZn system, copper withdraws electrons from zinc. The presence of copper in the Cs/CuZn system inhibits the oxidation of the Zn component compared with the Cs/Zn system by lowering the electron density on the Zn atoms. After exposing the Cs/CuZn system to O-2, zinc is oxidized at a rate that is larger than that found for clean CuZn surfaces and smaller than seen in Cs/Zn surfaces. Molecular hydrogen is found to have no effect on oxidized Cut Zn and CuZn films. However, atomic hydrogen reduces ZnO to metallic zinc and CuO to Cu2O. In the oxidized CuZn alloy, CuO is reduced first followed by the reduction of ZnO, A comparison of the behavior of O-2/Cs/Zn and H2O/Cs/Zn systems shows that while O-2 causes severe oxidation of Cs promoted Zn surfaces, H2O has little or net effect. (C) 1997 Elsevier Science B.V.