Wednesday, December 23, 2015

Dancing water molecules at the SrO on surface of ruthenates

Here is a very recent publication from TU Wien in Nature on adsorption of H2O molecules on a SrO surface of strontium ruthenate that should be very interesting for all ALD guys working with this process and material for e.g. MIM Capacitors. There is also a recent highlight of the publication in EurekAlert!

This is a visualization of a dancing H2O molecule dissociating on the SrO crystal surface. (EurekAlert!, Credit : TU Wien)

EurekAlert! reports: "We studied strontium ruthenate - a typical perovskite material," says Ulrike Diebold. It has a crystalline structure containing oxygen, strontium and ruthenium. When the crystal is broken apart, the outermost layer consists of only strontium and oxygen atoms; the ruthenium is located underneath, surrounded by oxygen atoms.

A water molecule that lands on this surface splits into two parts: A hydrogen atom is stripped off the molecule and attaches to an oxygen atom on the crystal's surface. This process is known as dissociation. However, although they are physically separated, the pieces continue to interact through a weak "hydrogen bond".

It is this interaction that causes a strange effect: The OH group cannot move freely, and circles the hydrogen atom like a dancer spinning on a pole. Although this is the first observation of such behaviour, it was not entirely unexpected: "This effect was predicted a few years ago based on theoretical calculations, and we have finally confirmed it with our experiments" said Diebold

Adsorption of water at the SrO surface of ruthenates

Daniel Halwidl, Bernhard Stöger, Wernfried Mayr-Schmölzer, Jiri Pavelec, David Fobes, Jin Peng, Zhiqiang Mao, Gareth S. Parkinson, Michael Schmid, Florian Mittendorfer, Josef Redinger & Ulrike Diebold 
Nature Materials Published online, , doi:10.1038/nmat4512

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 Srn+1RunO3n+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 ± 10meV. 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.