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A full monolayer of superoxide: oxygen activation on the unmodified Ca₃Ru₂O₇(001) surface
- Halwidl, Daniel, Mayr-Schmölzer, Wernfried, Setvin, Martin, Fobes, David, Peng, Jin, Mao, Zhiqiang, Schmid, Michael, Mittendorfer, Florian, Redinger, Josef, Diebold, Ulrike
- Journal of materials chemistry 2018 v.6 no.14 pp. 5703-5713
- X-ray photoelectron spectroscopy, adsorption, atomic force microscopy, catalytic activity, density functional theory, electrolysis, electrons, energy conversion, fuel cells, ions, oxides, oxygen, scanning tunneling microscopy, superoxide anion
- 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 RuO₂ layer. Theoretical calculations using both, density functional theory (DFT) and more accurate methods (RPA), predict the adsorption of O₂⁻ with Eₐdₛ = 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.