Adsorption of water at the SrO surface of ruthenates

TL;DR

This study investigates the molecular-level formation and behavior of water monolayers on SrO-terminated surfaces of layered ruthenates using microscopy, spectroscopy, and theoretical calculations.

Contribution

It provides new insights into water adsorption mechanisms on layered perovskite surfaces, highlighting differences from binary rocksalt oxides.

Findings

Water dissociates into hydroxide ions at the surface.

Hydroxide ions are trapped at Sr-Sr bridge sites.

Water forms chains and networks at higher coverage.

Abstract

Although perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Sr$_{n+1}$Ru$_n$O$_{3n+1}$ ($n$ = 1, 2) using low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighbouring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr-Sr bridge positions, circling the surface OH with a measured activation energy of 187 $\pm$ 10 meV. At higher coverage, dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites.