Growth and Oxidation of Zinc on Cu(111)

Zinc and zinc oxide films on Cu(111) have frequently been used as inverse model catalysts to study various chemical reactions. In the present work the growth and subsequent oxidation of zinc films are investigated by scanning tunneling microscopy (STM), low-energy ion scattering (LEIS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). Up to monolayer coverage zinc grows as a two-dimensional film with zinc atoms continuing the face-centered cubic lattice of the copper substrate. At higher coverages layer-by-layer growth is less strictly obeyed. Stacking faults are introduced as well, indicating a transition toward a hexagonal-close packed structure of zinc. At 300 K intermixing is found to be slow but rises when the temperature is increased. Accordingly, the continuous downshift and broadening of the zinc desorption peak with increasing submonolayer coverage are attributed to different levels of intermixing formed during the TPD temperature ramp. Postoxidation of zinc films starts at steps of zinc islands and is most effective in the temperature range from 430 to 500 K, although at temperatures beyond approximate to 450 K partial desorption of zinc has to be taken into account, too. At low oxygen exposures zinc forms small ZnO clusters along Zn steps, while at higher exposures continuous "bands" of ZnO develop around metallic Zn islands. These ZnO bands decompose around 650 K into zinc atoms dissolving into copper bulk and oxygen atoms forming a surface oxide with copper. Finally, bulk dissolved zinc atoms desorb at temperatures of approximate to 850 K and above.