The influence of inorganic electrolyte on the properties of carbon quantum dots in electrochemical exfoliation

Carbon quantum dots (CQDs) with highly graphitic structure are of great importance for electronic performance in applications of optoelectronics or photocatalysis. A facile method to obtain CQDs is the electrochemical exfoliation of graphitic materials. In our previous work, CQDs have been exfoliated from graphite rods in electrolyte of distilled water, organic and inorganic solutions, and the results show that inorganic electrolyte is the best choice in economy and efficiency. According to previous conclusions, the type of electrolyte has a prominent influence on the property and morphology of CQDs in process of exfoliation. However, to the best our knowledge, the type of inorganic electrolyte on behaviors of CQDs has not been researched and reported. Herein, a series of common inorganic salt solution are used as electrolyte and are compared to each other for the best choice of electrochemical exfoliation. Nitrate solution for electrochemical exfoliation of CQDs is better than any others, not only due to its similar photoluminescence, but also possessing common average size of 3.5 nm, which lead to regular and easily distinguishing morphology in TEM image. The lattice spacing of 0.21 nm observed from HRTEM is indicative of graphitic nanostructure. The blue photoluminescence of as-exfoliated CQDs is relative weak with quantum yield of 5.6%. Thus, we are inclined to believe that its photoluminescence mechanism is attribute to quantum confinement effects of carbon core rather than surface functional groups. It can also be supported by XPS and FT-IR results that only some simple groups (-OH and C=O) were form on surface of CQDs during exfoliation. Therefore, the electrolyte of nitrate shows the best suitability for the electrochemical exfoliation of CQDs. Besides, it can provide a way to tune the size of CQDs by electing different electrolyte.