Preparation Of Fluorescent Graphene Quantum Dots Through Electron Beam Irradiation

Graphene quantum dots(GQDs) assume numerous novel chemical/physical properties, and would broadly apply in the fields of energy, environment and biomedicine. Therefore, GQDs have emerged and ignited tremendous research interest in the recent years. However, the preparation methods of GQDs exit disadvantages. Thus, the method needs further investigation, and their luminescent property also needs improvements. In our study, for the first time, the method of electron beam irradiation was carried out to improve the GQDs’ fluorescence. The irradiated GQDs were carefully characterized and the potential mechanism on enhancing the quantum yield of GQDs was also expored.(1) The as-GQDs were prepared by the oxidation of carbon fiber using nitric acid and sulfuric acid. Their average size is 4.9 nm and the thickness is about from 1 to 4 nm. On surface of GQDs, there are many oxygen-containing functional groups. However, these as-GQDs almost have none luminescence when excited at different wavelength, with the quantum yields of 0.12%.(2) The methods of electron beam irradiation were carried out to improve the fluorescence of GQDs, and the best irradiation condition was explored. Five different kinds of solution including pure water solution, the DMF-water solution, the acetonitrile-water solution, the methanol-water solution and acetone-water solution, were used to explore which solution is the best system to enhance the quantum yields of GQDs. The results show that the DMF-water solution and methanol-water solution were the optimal two solutions for the quantum yields of GQDs.(3) We systematically explored to enhance the fluorescence of GQDs by the method of electron beam irradiation in the methanol-water solution. The average size of irradiated-GQDs was 5.5 nm, and average thicknesswas 1.8 nm. A reduction reaction happened in the process of irradiation, and a large number of hydroxyl methyl(-CH2OH) formed on the surface of GQDs. The quantum yields can be as high as 10.8%.(4) We systematically explored to enhance the fluorescence of GQDs by the method of electron beam irradiation in the DMF-water solution. The average size of irradiated-GQDs was 4.7 nm, comparable to the as-GQDs. Interesting, the N element was doped into the skeleton of GQDs, whose quantum yields can increase to 3.5% from 0.2%. Furthermore, the reduction process before irradiation can also improve the quantum yields of GQDs.