Preparation Of Mesophase Pitch- Derived Graphene Quantum Dots And Speculate Of Their Photoluminescence Mechanism

Due to the increasing use of fluorescent materials in many fields, high property requirements of these materials are raised, especially in biological field. Conventional fluorescent materials always show strongly biotoxicity,thus they were not capable of meeting the requests. Therefore, carbonaceous fluorescent materials with high biocompatibility have powerful reasons to be a great alternative of traditional ones. Though carbonaceous fluorescent materials have been investigated for a long time, they have not shown attractive properties and they failed to draw enough attention from each area until the emergence of graphene quantum dots (GQDs).Herein, we used mesophase pitch as the precursor to prepare GQDs by chemical oxidation using the mixture of concentrated sulfuric acid and 30% perhydrol solution. We further developed a simple route to separate each type of GQDs by modulating the pH value of the solutions. Finally, GQDs with different size and different content of oxygen-bearing functional groups were obtained. By study the key impact parameters, we found the optimized conditions for preparation. And we discussed in depth to speculate the PL mechanism of the as-prepared GQDs.The main conclusions were as follows:1. GQDs could be efficiently obtained by using the mixture of concentrated H2SO4 and H2O2 (30%). And they could be separated effectively by tuning pH values of solutions.2. Comparing with other two oxidation methods, H2SO4+HNO3 and H2SO4+NaNO3+KMnO4, H2SO4 and H2O2 (30%) solution we utilized is the safest, the most efficient, and most environmental method to prepare GQDs.3. With the increasing of reaction temperature (0??5??10?), the oxidation activity improves significantly. Along with this change, more oxygen-bearing groups are depleted because of the oxidation process and more small pieces are newly generated. It is why the mean oxidation degree of samples obtained at 5? and 10? is lower than that of ones at 0?. However, higher temperature also will lead the oxidation of newly obtained pieces, and a higher content of -COOH is received.4. GQDs derived from mesophase pitch are actually a mixture of GQDs in different sizes, containing different content of oxygen-bearing functional groups. We separated different GQDs using electrostatic repulsion which produced by oxygen-bearing groups'negative charge after ionization which strongly influenced by pH value of the solutions.By comparing with different separation pH values conditions, we finally determined that pH=0,1.5,2, 2.5,3 and 4 was the best one to distinguish GQDs. And these products hold different PL properties.5. With deeply research of GQDs'PL properties, we discovered that GQDs' PL spectra are all contained with ?450nm and?520 nm PL peaks. More than that, we speculate that ?450 nm peak would strongly influenced by size effects and ?520 nm one might be mainly effected by oxygen-bearing functional groups on the plant or at the edge of GQDs.