Co3O4 quantum dots (QDs) with diameter of 3-5 nm were successfully prepared by a facile surfactant-free large-scale synthesis strategy at room temperature in ethanol. The formation of Co3O4 QDs follows the sequence of Co2+ +2OH(-) -> Co(OH)(2) -> CoOOH -> Co3O4. It is demonstrated that the dissolved oxygen and the bubbling of compressed air are the two keys for the synthesis of Co3O4 QDs. The produced Co (OH)(2) was oxidized to CoOOH by the oxygen dissolving in ethanol. In order to synthesize pure phase Co3O4 QDs, compressed air has to be bubbled into the solution vigorously to promote the decomposition of CoOOH by removing the oxygen evolved from the decomposition reaction. Due to small particle size and inherent properties of Co3O4, the QDs have excellent electrochemical performance and high longterm electrochemical stability. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.