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Paper RSC Advances Fig. 6 AFM images of monolayer and few layers of graphene sheets. (a) A graphene sheet with a thickness about $1 nm after 80-cycles, (b) the exfoliated graphene nanosheets large in lateral size after 60-cycles of treatment, and (c) a graphene sheet with defects, which were formed by exposure to excessive bubble collapse after 60-cycles. cycles of cavitation. However, it is worth pointing out that the calculated ID/ID0 ratios are lower than the ratios reported for the graphene nanosheets with sp3 and vacancy-like defects. The atomic force microscopy (AFM) characterization was used to determine the size and thickness of produced graphene nanosheets. The results complement the Raman spectroscopy data. AFM image and height prole of the produced graphene nanosheets aer 80-cycles of hydrodynamic cavitation further conrm the exfoliation of graphite akes into bi-layer graphene nanosheets having a thickness value of $1 nm (Fig. 6a). The thickness range of the produced graphene nanosheets aer 60- cycles is approximately between 1.2 and 2.5 nm, which is considered as #3 layer graphene (Fig. 6b).60 The lateral size of the analyzed nanosheets is in the range of 1–5 mm. Further- more, close inspections on the AFM image of the few-layer graphene nanosheets produced aer 60-cycles display large holes, which vary in sizes between 100 to 600 nm (Fig. 6c). The presence of these holes correlates well with the ID/IG ratios ob- tained by Raman spectroscopy, suggesting the formation of defects at the edges. These defects are not surprising since the exfoliated graphene nanosheets were subjected to intense cavitation energy. As in the LPE process, the formation of defects in the forms of edges and topological defects is unavoidable because these types of defects need lower forma- tion energy. Furthermore, the size of the defects is believed to be related to the size of bubbles, which varies from hundreds of nanometers to micrometers. UV-Vis spectroscopy was performed to assess the concen- tration of the isolated graphene nanosheets and the starting graphite dispersion (see ESI Fig. S1†). In agreement with the literature, the absorption spectra of the isolated graphene nanosheets (S3U-20 to 80) are featureless in the measurement range. Compared to the concentration of the starting graphite dispersion (25 mg mL1), the concentrations of the obtained graphene nanosheets are calculated as $2.1 (S3U-20), 1.1 (S3U- 40), 1.0 (S3U-60), and 1.1 (S3U-80) mg mL1 using the molar absorption coefficient of 3620 mL mg1 m1 at the wavelength of 660 nm for the graphene in water.61 Using the concentrations of the isolated graphene nanosheets, the process efficiencies aer 20-, 40-, 60-, and 80-cycles are calculated as 8.4%, 4.8%, 4%, and 4.4%, respectively. It is worth noting that the efficiency of the hydrodynamic cavitation-assisted graphene production process aer 40-cycles is higher than the previously reported study.58 However, the yield of exfoliated graphene showed a fast decrease aer 20-cycles. The observed concentration loss may be related to the trapping of exfoliated graphene in the cavities © 2021 The Author(s). Published by the Royal Society of Chemistry RSC Adv., 2021, 11, 17965–17975 | 17971 View Article Online Open Access Article. Published on 18 May 2021. Downloaded on 6/29/2021 1:36:46 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | graphene exfoliation hydrodynamic cavitation on a chip
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