Terrace site hydroxylation upon water dimer formation on monolayer NiO/Ag(100)

Highlights: •Water dissociation at Ag supported NiO monolayers has been studied. •We show that the mechanism is different for water monomers and dimers. •The monomer can dissociate only through the polarization of the metal substrate. •For the dimer is the inter molecular hydrogen bond that drives the dissociation. -- Abstract: The interaction of water vapor with monolayer NiO/Ag(100) was examined using both experimental and computational techniques. Initial film growth was characterized by scanning tunneling microscopy and low energy electron diffraction showing the formation of NiO(1 × 1). X-ray photoelectron spectroscopy (XPS) reveals that the initial film was mainly composed of NiO oxide with a small amount of hydroxyl groups (OH) attributed to the dissociation of background water vapor at highly reactive edge sites. Density functional theory (DFT) reveals that the adsorption of a water monomer on NiO/Ag(100) terrace sites prefers to be in the molecular rather than the dissociate state. XPS results indicate that upon exposing the oxide film to high water vapor pressures (maximum 333.3 Pa), extensive hydroxylation occurs which is attributed to water dissociation at terrace sites. DFT reveals that upon aggregation of water monomers to dimers at the oxide interface the dissociated dimer is energetically stable. The results herein are consistent with previous MgO/Ag(100) studies, further revealing that for certain metal oxides the formation of water dimers at the metal oxide-vapor interface is a key mechanism leading to extensive terrace site hydroxylation.