Preparation and structure analysis of medium temperature coal tar pitch-based carbon quantum dots

The carbon quantum dots (CQDs) were prepared by HNO₃ pretreatment combined with ball milling and oxidative stripping by hydrogen peroxide with medium temperature coal tar pitch as the carbon source. In order to determine the suitable reaction conditions, the effects of oxidation time and the amount of hydrogen peroxide on the structure and properties of CQDs were investigated with the yield of CQDs and fluorescence quantum yield as targets. Under the optimum condition with an operating reaction time of 6 h and a H₂O₂ dosage of 100 mL, the respectively highest yield of CQDs and fluorescence quantum yield of 6.3% and 11.2% is obtained, and the prepared sample is denoted as c-CQDs. Meanwhile, the sample particle size is uniform and in the range of 4-14 nm. When the reaction time is extended to 8 h, the carbon quantum dots (a-CQDs) grow larger because of agglomeration. As the amount of H₂O₂ increases to 120 mL, the size of carbon quantum dots (b-CQDs) becomes smaller and disorder due to excessive oxidation. Then, the effects of reaction conditions on the structure of CQDs were investigated by XPS, TG-DTG, ¹³C NMR, Raman and XRD. The results show that the carbon content follows the order of a-CQDs > b-CQDs > c-CQDs, while the content of oxygen is b-CQDs > c-CQDs > a-CQDs. According to XPS analysis, the major form of carbon in CQDs is the aromatic carbon. It is noted that the maximum amount of C=O and O-C=O is obtained with the c-CQDs, while the higest value of C-O appears with the b-CQDs. The X_b characterized by ¹³C NMR characterization illustrates that the average aromatic ring size is about 0.5, and correspondingly, the average number of aromatic rings is about 3.