Water Adsorption on Cu2O(111) Surfaces: A Scanning Tunneling Microscopy Study

The interaction of water with well-ordered Cu2O/Au(111) thin films has been explored with thermal desorption spectroscopy (TDS) and low-temperature scanning tunneling microscopy (STM). A TDS peak at 1.75 K indicates weak binding of the D2O monolayer to the Cu2O surface, while additional water condenses in disordered multilayers desorbing at 155 K. STM reveals flat water islands that nucleate along oxide step,edges and grow up to a full monolayer at higher exposure. Molecules inside the islands take a local hexagonal order with 6 A periodicity, while no long-range structures are formed. The observed adsorption behavior is compatible with a cation free oxide termination that is Cu-deficient Cu2O(111) in the present case. For a stoichiometric film containing reactive Circus ions, higher adsorption energies and stronger template effects for water would be expected, in contrast to the experimental findings.