Production Of Pristine Graphene Quantum Dots Based On Supercritical CO₂

Graphene quantum dots(GQDs)are zero-dimensional luminescent graphene nanoparticles.They have attracted more and more attention since the discovery of graphene because they possess unique electronic and optical properties owing to their edge effects and quantum confinement.Their size is typically ranging from 2–20 nm in diameter and have broad applications like bio-imaging,drug delivery,biosensors and photovoltaic applications.A novel,facile and green approach for preparation of pristine graphene quantum dots(PGQDs)from graphite using a shear-assisted supercritical CO₂ approach was presented in this thesis.The fluorescence quantum yield of the PGQDs achieved 8.6%,and it was highly stable with blue light emission.The obtained PGQDs were around 4nm in lateral size with less than 7 graphene layers and displayed excitation-independent photoluminescence.During the exfoliation process,supercritical CO₂ pressure,graphite amount,shearing speed,and temperature were all proven to have significant influence on the PGQDs yields.The yield was enhanced by increasing the supercritical CO₂ pressure,shearing speed and,process temperature and,decreasing the graphite amount.The maximum yield of PGQDs was 1.74%in the condition of the pressure of 18MPa,starting graphite amount of 0.5g,the shearing speed of 3000rpm with the system temperature of45?.An ultrasonic-assisted supercritical CO₂ method was utilized to prepare PGQDs from graphite.The florescence of synthesized PGQDs was 2.8%with blue light luminescence.The as synthesized PGQDs had average size of 4nm and graphene layers were around 8.The ultrasonication power,ultrasonication time and,the supercritical CO₂pressure were all proven to have the impressive impact on the PGQDs yields,which all had direct impact.The maximum yield was 1.6%and achieved in the condition of 1KW sonication power for 120 minutes and system pressure of 16MPa.According to the experiment results,the combination of the intense knocking force produced by high-pressure acoustic cavitation in supercritical CO₂system and the extremely high penetration ability of supercritical CO₂ were considered to be significant to the successful exfoliation of PGQDs from bulk graphite.These newly developed approaches produced high-quality PGQDs without using acid,base and oxidant.Moreover,the synthesis procedure was simple and can be used for economical mass production.