Green synthesis of nitrogen-doped graphene quantum dots by recycling waste graphite and impact of oxygen adsorption on enhancing photoluminescence

Graphene quantum dots (GQDs) are fluorescent materials highly suited for biomedical applications owing to their excellent biocompatibility and low cytotoxicity. However, the chemical substances used in recent GQDs syntheses, such as polycyclic aromatic hydrocarbons and some aromatic organics, are causing severe environmental pollution. For sustainable research, we report a method for synthesizing recycled nitrogen-doped GQDs using waste graphite anode scrap (WGAS) extracted from retired lithium-ion batteries. Interestingly, increasing the inner cylinder temperature in a Couette–Taylor flow reactor from 37 to 44 °C during graphene oxide (GO) synthesis enhanced oxygen adsorption by ≈3 %. This led to a ≈ 20 % increase in photoluminescence (PL) intensity in GQDs synthesized hydrothermally. This indicates that the increased oxygen adsorption in GO enhances the density of the resulting oxidized debris, leading to a greater number of GQDs per unit volume. Our approach demonstrates an eco-friendly route to produce blue-emitting N-GQDs with enhanced optical properties.