Coal derived highly fluorescent N-Doped graphene quantum dots with graphitic and chemisorbed nitrogen

Abstract

Graphene quantum dots (GQDs) constitute a novel category of quantum dots distinguished by their distinctive properties. The introduction of nitrogen heteroatom in GQD is an effective strategy for tuning its intrinsic properties and band gap toward its optoelectronic application. This work explores the synthesis and characterization of nitrogen-doped graphene quantum dots (N-GQDs) derived from low-cost precursor coal. The synthesized N-GQD contains both graphitic and chemisorbed nitrogen states along with pyrollic and pyridinic nitrogen states which is the key responsible factor for excellent photoluminescence properties. The excitation dependent photoluminescence of the synthesized N-GQD exhibited excellent emission at 520 nm in neutral and basic pH due to the additional conjugation provided by the doped nitrogen on the surface and functional groups as well. The pH dependent photophysical properties support the existence of doped graphitic and chemisorbed nitrogen states which is also well supported by the high resolution N1s XPS peaks at 402 eV (graphitic N) and 406 eV (chemisorbed N/N2). DFT calculations further showed the decrease in fermi energy level by introducing chemisorbed-N. TEM analysis evident the formation of quantum dots with an average diameter of 3 - 7 nm with d-spacing of 0.20-0.34 nm. Additionally, the cyclic voltammetry analysis of the synthesized N-GQDs sheds light on the participation of doped nitrogen in redox reactions.