Adsorption and dissociation of water on oxygen pre-covered Cu (110) observed with scanning tunneling microscopy

The adsorption and dissociation of water on the oxygen pre-covered Cu(110) surface are studied with scanning tunneling microscopy (STM). At room temperature, oxygen atoms are adsorbed on the Cu(110) surface and self-assembled into ordered (21) Cu-O chains along the[001] direction. The relative proportion of clean and (21) O-strips can be tuned by the sample exposure time to oxygen gas. When the oxygen pre-covered Cu(110) sample is exposed to water molecules at 77 K, the water molecules are adsorbed at the edges and on the top of the Cu-O chains. On the bare Cu(110) surface, we observe the formation of a hexagonal structure right next to the Cu-O stripes at 77 K. This is different from the water molecule adsorption on the clean Cu(110) surface, in which water molecules are adsorbed and self-assembled into ordered zig-zag chains along the[001] direction. While on oxygen pre-covered Cu(110) surface, water molecules prefer to hydrogen bond with oxygen atoms inside the Cu-O chains and then bond with the other water molecules, forming stable hexagonal network. From our earlier STM results, we find that water forms zig-zag chains only when oxygen pre-coverage is lower than 0.125 ML. On the top of hexagonal network, we observe the bright spots and attribute them to the 2nd layer water clusters. The fact that the 2nd layer clusters form on the top of the hexagonal water-hydroxyl regions rather than at the other locations on the Cu(110) surface indicates that the mixed hexagonal network may have more H-dangling bonds that facilitate the 2nd layer growth. In order to remove the upper layer water molecules, we apply a 5 V bias voltage for scanning, for which the tunneling electrons provide enough energy for overcoming the water desorption and dissociation barrier (0.5-0.55 eV at UHV and low temperature). With the excitation of tunneling electrons from the tip, the water molecules in the hexagonal network react with oxygen atoms inside the Cu-O chains (H2O+O2OH). According to Forster proposed Bjerrum defect model, the hexagonal network is formed by water donating hydrogen to hydroxyl, in which two hydrogen atoms are located between two adjacent oxygen atoms. Our results demonstrate that the oxygen atoms pre-adsorbed on the Cu(110) surface act as nucleation centers for water adsorption and catalyze its dissociation, which is important in water gas shift reaction study. However, we still need more X-ray photoelectron spectroscopy experiments to certify whether the water molecules react with the pre-covered oxygen atoms at low temperature (below 100 K).