Electrically Tunable Reactivity of Substrate-Supported Cobalt Oxide Nanocrystals.

First-row transition metal oxides are promising materials for catalyzing the oxygen evolution reaction. Surface sensitive techniques provide a unique perspective allowing the study of the structure, adsorption sites, and reactivity of catalysts at the atomic scale, which furnishes rationalization and improves the design of highly efficient catalytic materials. Here, a scanning probe microscopy study complemented by density functional theory on the structural and electronic properties of CoO nanoislands grown on Au(111) is reported. Two distinct phases are observed: The most extended displays a Moiré pattern (α-region), while the less abundant is 1Co:1Au coincidental (β-region). As a result of the surface registry, in the β-region the oxide adlayer is compressed by 9%, increasing the unoccupied local density of states and enhancing the selective water adsorption at low temperature through a cobalt inversion mechanism. Tip-induced voltage pulses irreversibly transform α- into β-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.