Graphene quantum dots, graphene oxide, carbon quantum dots and graphite nanocrystals in coals

Six coal samples of different ranks have been used to prepare single-layer graphene quantum dots (S-GQDs). After chemical oxidation and a series of centrifugation separation, every coal could be treated into two fractions, namely, Coal(A) and Coal(B). According to the characterization results of TEM, AFM, XRD, Raman and FTIR, Coal(A) was revealed to be mainly composed of S-GQDs, which have an average height of about 0.5 nm and an average plane dimension of about 10 nm. The obtained S-GQDs showed excitation-dependent fluorescence and excellent electrochemiluminescence. Coal(B) was found to be some other carbon-based nanomaterials (CNMs), including agglomerated GQDs, graphene oxide, carbon quantum dots and agglomerated carbon nanocrystals. Generally, low-ranked coals might be more suitable for the preparation of S-GQDs. The production yield of S-GQDs from the six investigated coals decreased from 56.30% to 14.66% when the coal rank increased gradually. In contrast, high-ranked coals had high production yield of Coal(B) and might be more suitable for preparing other CNMs that were contained in Coal(B), although those CNMs were difficult to separate from each other in our experiment.