Structure and Stability of Au-Supported Layered Cobalt Oxide Nanoislands in Ambient Conditions

Cobalt oxide is a promising earth-abundant electrocatalyst for water splitting, however the structural complexity of oxides coupled with the difficulty of characterizing it in its operating environment mean that fundamental understanding of its catalytic properties remains poor. In this study, we go beyond vacuum studies and investigate the morphological evolution of a CoOx/Au(111) model system from intermediate to high pressures of H2O vapor, by means of Scanning Tunneling Microscopy (STM) and Near-Ambient Pressure (NAP)- and vacuum X-ray Photoelectron Spectroscopy (XPS). At elevated H2O pressure, we describe the formation of a well-defined Co(OH)2 nanoisland morphology with cobalt in the 2+ oxidation state. In contrast, the presence of O2, in air and liquid water, results in only partially hydroxylated Co3+ phases comprising sheets of CoO(OHx) trilayer, corresponding to a single sheet of cobalt(III) oxyhydroxide. We conclude that the oxyhydroxide structure, known to be the catalytically active phase for the oxygen evolution reaction (OER) is stabilized by aerobic conditions, which inhibits further transformation into the catalytically inactive cobalt (II) hydroxide.