Dual frequency ultrasonic liquid phase exfoliation method for the production of few layer graphene in green solvents.

In this work, we implement a dual frequency (24 kHz and 1174 kHz) ultrasonic assisted liquid phase exfoliation (ULPE) technique in deionized water (DIW) and other eco-friendly solvents, to produce a variety of high-quality few-layer graphene (FLG) solutions under controlled ultrasonication conditions. The resulting FLG dispersions of variable sizes (∼0.2-1.5 µm2) confirmed by characterisation techniques comprising UV-Vis spectroscopy, Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM). For the first time we demonstrate that high yield of FLG flakes with minimal defects, stable for 6 + months in a solution (stability âˆ¼ 70 %), can be obtained in less than 1-hour of treatment in either water/ethanol (DIW:EtOH) or water/isopropyl alcohol (DIW:IPA) eco-friendly mixtures. We also scrutinized the underlying mechanisms of cavitation using high-speed imaging synchronized with acoustic pressure measurements. The addition of ethanol or IPA to deionized water is proposed to play a central role in exfoliation as it regulates the extend of the cavitation zone, the intensity of the ultrasonic field and, thus, the cavitation effectiveness. Our study revealed that lateral sizes of the obtained FLG depend on the choice of exfoliating media and the diameter of a sonotrode used. This variability offers flexibility in producing FLG of different sizes, applicable in a wide spectrum of size-specific applications.

Temperature as a key parameter for graphene sono-exfoliation in water.

Graphene dispersions in water are highly desirable for a range of applications such as biomedicines, separation membranes, coatings, inkjet printing and more. Recent novel research has been focussed on developing a green approach for scalable production of graphene. However, one important parameter, which is often neglected is the bulk temperature of the processing liquid. This paper follows our earlier work where optimal sono-exfoliation parameters of graphite in aqueous solutions were determined based on the measured acoustic pressure fields at various temperatures and input powers. Here, we take the next step forward and demonstrate using systematic characterisation techniques and acoustic pressure measurements that sonication-assisted liquid phase exfoliation (LPE) of graphite powder can indeed produce high quality few layer graphene flakes in pure water at a specific temperature, i.e. 40 °C, and at an optimised input generator power of 50%, within 2-h of processing. UV-vis analysis also revealed that the exfoliation, stability and uniformity of dispersions were improved with increasing temperature. We further confirmed the successful exfoliation of graphene sheets with minimal level of defects in the optimized sample with the help of Raman microscopy and transmission electron microscopy. This study demonstrated that understanding and controlling processing temperature is one of the key parameters for graphene exfoliation in water which offers a potential pathway for its large-scale production.