For many two-dimensional (2D) materials,low coordination edges and corner sites offer greatly enhanced catalytic performance compared to basal sites,motivating the search for new synthetic approaches towards ultrathin and ultrafine 2D nanomaterials with high specific surface areas.To date,the synthesis of catalysts that are both ultrathin (monolayer) and ultrafine (lateral size ＜ 10 nm) has proven extremely challenging.Herein,using a facile ultrasonic exfoliation procedure,we describe the successful synthesis of ultrafine ZnCo-LDH nanosheets (denoted as ZnCo-UF) with a size ～3.5 nm and thickness ～0.5 nm.The single layer ZnCo-UF nanosheets possess an abundance of oxygen vacancies (Vo) and unsaturated coordination sites,thereby affording outstanding electrocatalytic water oxidation performance.DFT calculations confirmed that Vo on the surface of ZnCo-UF enhanced H2O adsorption via increasing the electropositivity of the nanosheets.