A Sustainable Approach to the Preparation of MoO 2 Quantum Dots and the Pseudocapacitive Performance before and after Calcination

The present preparation methods of molybdenum dioxide (MoO 2 ) quantum dots (QDs) are often cumbersome and not environmentally friendly, and more importantly, the prepared MoO 2 QDs are usually wrapped in surfactants, which are difficult to be removed and impact their intrinsic characterization and performance. Herein, we present a facile, scalable and sustainable approach to obtain clean and pure MoO 2 QDs (1.5~3 nm), which consists of a hydrothermal reaction and a further calcination process in vacuum following freeze drying. Furthermore, the influence of calcination temperature, atmosphere and time is also studied. The calcination at 400 °C for 2 h in vacuum can efficiently remove the surfactants wrapping on the MoO 2 QDs without obvious size change, whereas the calcination in nitrogen will lead to a slight increase in size, and the calcination in air could transform MoO 2 QDs into MoO 3 nanoplates. In addition, the pseudocapacitive performance of the as-prepared samples is tested and compared by electrochemical methods, and the specific capacitance of the clean MoO 2 QDs is about 1.4 times larger than that of MoO 2 QDs with surfactants at 5 mV s −1 . This study also demonstrates a possible way to efficiently remove the organic substance wrapping on quantum dot materials.