Atom-by-atom electrodeposition of single isolated cobalt oxide molecules and clusters for studying the oxygen...

We report an electrodeposition protocol for preparing isolated cobalt oxide single molecules (Co₁O_x) and clusters (Co O_y) on a carbon fiber nanoelectrode. The as-prepared deposits are able to produce well-defined steady-state voltammograms for the oxygen evolution reaction (OER) in alkaline media, where the equivalent radius (r_d) is estimated by the limiting current of hydroxide oxidation in accordance with the electrocatalytic amplification model. The size of isolated clusters obtained from the femtomolar Co²⁺ solution through an atom-by-atom technique can reach as small as 0.21 nm (r_d) which is approximately the length of Co-O bond in cobalt oxide. Therefore, the deposit was close to that of a Co₁O_x single molecule with only one cobalt ion, the minimum unit of the cobalt-based oxygen-evolving catalyst. Additionally, the size-dependent catalysis of the OER on Co O_y deposits shows a faster relative rate on the smaller cluster in terms of the potential at a given current density, implying the single molecular catalyst shows a superior OER activity.