Water adsorption on Sr$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$Ru$_{2}$O$_{7}$

Perovskite oxides are promising materials for a wide range of applications as diverse as sensors, fuel cells and catalysts. Surprisingly an atomic scale knowledge of their surface chemistry is still rather poor. Here we present a combined DFT, low-temperature STM and XPS study of the first monolayer H$_{2}$O formation on the (001) surfaces of two Ruddelsden-Popper type compounds, Sr$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$Ru$_{2}$O$_{7}$. Both cleave nicely, yielding flat surfaces of rocksalt-like SrO and CaO type. Adsorbed H$_{2}$O monomers dissociate on both surfaces by transferring a proton to an apical O while the remaining OH resides at a nearby Sr-Sr or Ca-Ca bridge, respectively. The different tilting and rotations of the RuO$_{6}$ octahedra in the Sr and Ca perovskites cause different behaviour of the dissociated fragments: for Sr$_{3}$Ru$_{2}$O$_{7}$ a lone Sr-Sr bridge OH circles the apical OH with an activation energy of $\approx$187meV. At higher coverages dimers, followed by chains and cages are formed[1]. In contrast, the dissociated monomer fragments are immobile on Ca$_{3}$Ru$_{2}$O$_{7}(001)$ and only chains along the [010] direction occur. [1]D. Halwidl et. al., Nature Materials, 15(4), 450-455.