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 investi gated using photoemission and rib initio self-consistent-field (SCF) c alculations. On zinc films, the sticking probability of O-2 is extreme ly 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 oxyg en and the transformation of metallic zinc into zinc oxide. The presen ce 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 format ion of Zn-->O-2 dative bonds and breaking of the O-O bond. For the coa dsorption of Cs and O-2 on Zn(001). the larger the electron transfer f rom Zn into the O-2 (1 pi(g)) orbitals, the bigger the adsorption ener gy of the molecule and the elongation of the O-O bond. In general, ces ium does not promote the oxidation of copper. In the Cs/CuZn system, c opper withdraws electrons from zinc. The presence of copper in the Cs/ CuZn system inhibits the oxidation of the Zn component compared with t he Cs/Zn system by lowering the electron density on the Zn atoms. Afte r 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 see n in Cs/Zn surfaces. Molecular hydrogen is found to have no effect on oxidized Cut Zn and CuZn films. However, atomic hydrogen reduces ZnO t o metallic zinc and CuO to Cu2O. In the oxidized CuZn alloy, CuO is re duced first followed by the reduction of ZnO, A comparison of the beha vior of O-2/Cs/Zn and H2O/Cs/Zn systems shows that while O-2 causes se vere oxidation of Cs promoted Zn surfaces, H2O has little or net effec t. (C) 1997 Elsevier Science B.V.