Ultrathin LiCoO<sub>2</sub> Nanosheets: An Efficient Water-Oxidation Catalyst

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO<sub>2</sub>, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO<sub>2</sub> nanosheets with 5–6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO<sub>2</sub> nanosheets provided a current density of 10 mA cm<sup>–2</sup> at a small overpotential of a mere 0.41 V and a small Tafel slope of ∼88 mV/decade, which is strikingly followed by an excellent cycle life. The findings reported in this work suggest that ultrathin cation-exchanged layered metal oxides could be a next generation of advanced catalysts for oxygen evolution reaction.