Production of graphene quantum dots by ultrasound-assisted exfoliation in supercritical CO2/H2O medium

Abstract

In this research, three kinds of graphene quantum dots (GQDs)-pristine graphene quantum dots (PGQDs), expanded graphene quantum dots (EGQDs) and graphene oxide quantum dots (GOQDs)-were produced from natural graphite, expanded graphite, and oxide graphite respectively in an ultrasound-assisted supercritical CO (scCO)/HO system. The effects of aqueous solution content ratio, system pressure, and ultrasonic power on the yields of different kinds of GQDs were investigated. According to these experiment results, the combination of the intense knocking force generated from high-pressure acoustic cavitation in a scCO/HO system and the superior penetration ability of scCO was considered to be the key to the successful exfoliation of such tiny pieces from bulk graphite. An interesting result was found that, contrary to common experience, the yield of PGQDs from natural graphite was much higher than that of GOQDs from graphite oxide. Based on the experimental analysis, the larger interlayer resistance of natural graphite, which hindered the insertion of scCO molecules, and the hydrophobic property of natural graphite surface, which made the planar more susceptible to the attack of ultrasonic collapsing bubbles, were deduced to be the two main reasons for this result. The differences in characteristics among the three kinds of GQDs were also studied and compared in this research. In our opinion, this low-cost and time-saving method may provide an alternative green route for the production of various kinds of GQDs, especially PGQDs.

Synthesis of graphene quantum dots and their applications in drug delivery

“…Gao et al [71] prepared three kinds of GQDs of pristine graphene quantum dots (PGQDs), expanded graphene quantum dots (EGQDs) and graphene oxide quantum dots (GOQDs) using natural graphite, expanded graphite, and oxide graphite as the raw materials in a supercritical CO 2 /H 2 O system assisted by ultrasound. The experimental results show that this method is an environmentally friendly, low-cost, fast and large-scale synthesis method of GQDs, which it can provide an alternative green route for the production of various GQDs, especially PGQDs.…”

“…4 Illustration of the exfoliation process of pristine graphite, expanded graphite and graphite oxide in ultrasonic-assisted scCO 2 process (reprinted/reproduced with the permission of Ref. [ 71 ], copyright 2017, Ultrasonics Sonochemistry) …”

“…Illustration of the exfoliation process of pristine graphite, expanded graphite and graphite oxide in ultrasonic-assisted scCO 2 process (reprinted/reproduced with the permission of Ref [71],. copyright 2017, Ultrasonics Sonochemistry) The electrolytic process of graphene paper in a ammonia and b NaOH solutions under different reaction times (reprinted/reproduced with the permission of Ref [76],.…”

Graphene Platelets and Their Polymer Composites: Fabrication, Structure, Properties, and Applications

“…A mixture of water and CO 2 SCF was recently used as the solvent to exfoliate graphite under ultrasonication . Graphite suspension and CO 2 SCF were, respectively, pumped into a reactor, and an ultrasonication generator turned on with specific pressure and temperature maintained inside the reactor.…”

“…Graphite flakes prior to ultrasonication either flowed on the interface between water and CO 2 SCF or dispersed in water. Once ultrasound was applied, water mixed with CO 2 SCF to form microemulsions which intercalated into the layer spacing . Approximately 93% of the product had few‐layer thicknesses with lateral dimensions below 1 µm.…”