title: Interface controlled oxidation states in layered cobalt oxide nanoislands on gold.
abstract: Layered cobalt oxides have been shown to be highly active catalysts for the oxygen evolution reaction (OER; half of the catalytic "water splitting" reaction), particularly when promoted with gold. However, the surface chemistry of cobalt oxides and in particular the nature of the synergistic effect of gold contact are only understood on a rudimentary level, which at present prevents further exploration. We have synthesized a model system of flat, layered cobalt oxide nanoislands supported on a single crystal gold (111) substrate. By using a combination of atom-resolved scanning tunneling microscopy, X-ray photoelectron and absorption spectroscopies and density functional theory calculations, we provide a detailed analysis of the relationship between the atomic-scale structure of the nanoislands, Co oxidation states and substrate induced charge transfer effects in response to the synthesis oxygen pressure. We reveal that conversion from Co(2+) to Co(3+) can occur by a facile incorporation of oxygen at the interface between the nanoisland and gold, changing the islands from a Co-O bilayer to an O-Co-O trilayer. The O-Co-O trilayer islands have the structure of a single layer of β-CoOOH, proposed to be the active phase for the OER, making this system a valuable model in understanding of the active sites for OER. The Co oxides adopt related island morphologies without significant structural reorganization, and our results directly demonstrate that nanosized Co oxide islands have a much higher structural flexibility than could be predicted from bulk properties. Furthermore, it is clear that the gold/nanoparticle interface has a profound effect on the structure of the nanoislands, suggesting a possible promotion mechanism.
authors.affiliation: †Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark.
authors.forename: Alex S
authors.initials: AS
authors.lastname: Walton
authors.affiliation: †Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark.
authors.forename: Jakob
authors.initials: J
authors.lastname: Fester
authors.affiliation: ‡SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 San Hill Road, Menlo Park, California 94025, United States.
authors.forename: Michal
authors.initials: M
authors.lastname: Bajdich
authors.affiliation: §Division of Synchrotron Radiation Research, Department of Physics, Lund University, 221 00 Lund, Sweden.
authors.forename: Mohammad A
authors.initials: MA
authors.lastname: Arman
authors.affiliation: ⊥MAX IV Laboratory, Lund University, 221 00 Lund, Sweden.
authors.forename: Jacek
authors.initials: J
authors.lastname: Osiecki
authors.affiliation: §Division of Synchrotron Radiation Research, Department of Physics, Lund University, 221 00 Lund, Sweden.|⊥MAX IV Laboratory, Lund University, 221 00 Lund, Sweden.
authors.forename: Jan
authors.initials: J
authors.lastname: Knudsen
authors.affiliation: ‡SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 San Hill Road, Menlo Park, California 94025, United States.
authors.forename: Aleksandra
authors.initials: A
authors.lastname: Vojvodic
authors.affiliation: †Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark.
authors.forename: Jeppe V
authors.initials: JV
authors.lastname: Lauritsen
keywords: cobalt oxide; electrocatalysis; nanocatalysis; reducible metal oxides; two-dimensional materials; water splitting