A full monolayer of superoxide: oxygen activation on the unmodified Ca₃Ru₂O₇(001) surface

D. Halwidl, W. Mayr-Schmölzer, M. Setvin, D. Fobes, J. Peng, Z. Mao, M. Schmid, F. Mittendorfer, J. Redinger, U. Diebold

J. Mater. Chem. A 6 (2018) 5703-5713

Activating the O₂ 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 unmodified CaO-terminated (001) surface of Ca₃Ru₂O₇, a Ruddlesden-Popper perovskite, supports a full monolayer of superoxide ions, O₂^-, when exposed to molecular O₂. The electrons for activating the molecule are transferred from the subsurface RuOO₂ layer. Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA), predict the adsorption of O₂^- with E_ads = 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 confirm the predicted adsorption structures. Local contact potential difference (LCPD) and X-ray photoelectron spectroscopy (XPS) measurements on the full monolayer of O₂^- confirm 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 O₂.

Corresponding author: Ulrike Diebold (diebold).

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