Photoemission study of the (2 × 2) structure formed by H2O adsorption on the Zr(0 0 0 1) surface

We have studied adsorption of water on the Zr(0 0 0 1) surface at sub-monolayer coverage by means of LEED and photoemission spectroscopy. An ordered (2 × 2) structure is formed after adsorption of 0.6–1.4 Langmuirs at 473 K. The sharpest LEED pattern was observed at an exposure of 1.2 L implying a coverage of 0.5 ML of oxygen. The same exposure at 293 K gives only a weak and diffuse (2 × 2) pattern. In addition, the sharp (2 × 2) pattern obtained at 473 K can be reversibly weakened by cooling to 293 K and subsequently sharpened by heating. For the sharp (2 × 2) structure, valence band spectra indicated dissociation of water and showed a peak composed mainly of O 2p derived states with two components at 6.0 eV and 6.6 eV binding energy. On cooling to 293 K, the O 2p peak became narrower and a new state appeared at 7.9 eV. Two components of the O 1s core level were resolved for the (2 × 2) structure, assigned to oxide and hydroxyl groups. The hydrogen on the surface of Zr(0 0 0 1) resulting from the dissociation of water and from bulk segregation strongly influenced the formation of the (2 × 2) structure of oxygen, and caused a temperature instability of the structure.