A Sustainable Approach to the Preparation of MoO2 Quantum Dots and the Pseudocapacitive Performance before and after Calcination

Abstract

The present preparation methods of molybdenum dioxide (MoO2) quantum dots (QDs) are often cumbersome and not environmentally friendly, and more importantly, the prepared MoO2 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 MoO2 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 MoO2 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 MoO2 QDs into MoO3 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 MoO2 QDs is about 1.4 times larger than that of MoO2 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.