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Abstract

Strontium titanate is an extensively studied material with a wide range of application,
for instance in photo-catalysis and most importantly, it is used as a substrate in growth of
functional oxides. The surface chemistry is crucial and hence understanding the surface
structure on atomic scale is essential for gaining insight into the fundamental processes in
the aforementioned applications. Moreover, there exist a lot of evidence that this surface
chemistry might be controlled to considerably by extrinsic species, such as residual hydrogen
and water.
Investigating the properties of water and oxygen on the strontium titanate surface is certainly
a natural starting point for a theoretical study based on density functional theory,
because these species are practically present on the surface on a wide range of experimental
conditions and they are computationally feasible.
For the oxygen and water adsorption the binding energy is controlled by long-range surface
relaxations leading to an effective repulsion of the adsorbed specimen. The isolated
oxygen ad-atom forms a covalently bonded “quasi-peroxide anion” in combination with
a lattice oxygen atom. Contrariwise, in all investigated configurations containing water
molecules and hydroxyl groups, the respective oxygen atoms assumed positions close to
the oxygen sites of the continued perovskite lattice of the substrate. Most remarkably, on
the strontium oxide termination, the water molecules adsorbs and dissociates effortlessly
leading to the formation of a pair of hydroxyl groups. For the titanium dioxide termination,
a coverage dependent adsorption mode is observed. Densely packings stabilize water
molecules, whereas at lower coverage and finite temperatures the formation of hydroxyl
groups is found. The energetics responsible for this behavior is consistent with recent experiments
by Iwahori and coworkers.