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D. Halwidl, W. Mayr-Schmölzer, M. Setvin, D. Fobes, J. Peng, Z. Mao, M. Schmid, F. Mittendorfer, J. Redinger, U. Diebold:
"A full monolayer of superoxide: oxygen activation on the unmodified Ca₃Ru₂O₇(001) surface";
Journal of Materials Chemistry A, 6 (2018), 5703 - 5713.

Activating the O
2
molecule is at the heart of a variety of technological applications, most prominently in energy
conversion schemes including solid oxide fuel cells, electrolysis, and catalysis. Perovskite oxides, both
traditionally-used and novel formulations, are the prime candidates in established and emerging energy
devices. This work shows that the as-cleaved and unmodi
fi
ed CaO-terminated (001) surface of Ca
3
Ru
2
O
7
,
a Ruddlesden
-
Popper perovskite, supports a full monolayer of superoxide ions, O
2

,whenexposedto
molecular O
2
. The electrons for activating the molecule are transferred from the subsurface RuO
2
layer.
Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA),
predict the adsorption of O
2

with
E
ads
¼
0.72 eV and provide a thorough analysis of the charge transfer.
Non-contact atomic force microscopy (nc-AFM) and scanning tunnelling microscopy (STM) are used to
resolve single molecules and con
fi
rm the predicted adsorption structures. Local contact potential
di
ff
erence (LCPD) and X-ray photoelectron spectroscopy (XPS) measurements on the full monolayer of
O
2

con
fi
rm the negative charge state of the molecules. The present study reports the rare case of an
oxide surface without dopants, defects, or low-coordinated sites readily activating molecular O
2