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RSC Advances Paper compared to the starting graphite dispersion (2D-band $2716 cm1). However, there is a signicant downshi ($25 cm1) in the maximum of the 2D-band for the S3U-80 and, the band appeared at 2692 cm1. In agreement with the litera- ture,52 the observed shi can be attributed to the formation of bilayer graphene nanosheets aer 80-cycles of hydrodynamic cavitation. Furthermore, the enhanced I2D/IG ratio aer 80- cycles further supports the formation of a few layer graphene nanosheets.57 When the defect density was analyzed, almost no defect was observed for the starting graphite dispersion. In contrast, the isolated graphene nanosheets have ID/IG ratios of 0.10, 0.48, 0.32, and 0.87 for the S3U-20, S3U-40, S3U-60, and S3U-80, respectively, suggesting a gradual defect formation.58 The nature of defects in graphene was previously studied,59 and it was shown that the intensity ratio between the D-band and D0-peak (at ca. 1620 cm1) could be used as a measure to probe the nature of the defects. In general, this ratio (ID/ID0) was found to be $13 for sp3-defects, while it was $7 and $3.5 for vacancy-like defects and boundaries in graphite, respectively. Aer the application of hydrodynamic cavitation, the intensity of D0-peak gradually increases with an increase in the number of cycles. In parallel, as above-mentioned, the intensity of D-band $1620 cm1 also gradually increases. The isolated graphene nanosheets have ID/ID0 of 1.93, 2.31, 1.91, and 3.63 for the S3U- 20, S3U-40, S3U-60, and S3U-80, respectively. From the observed ratios, it can be concluded that the hydrodynamic cavitation creates surface defects on the exfoliated graphene nanosheets, and the defect density becomes more pronounced aer 80- graphite suspension is thus treated with cavitating ows. Cavitating ows corresponding to inception and developed ow aer the 80th cycle are shown in Fig. 4. As shown in this gure, the cavitation inception decreases from 220 psi (rst cycle) to 140 psi at the 80th cycle, while fully developed cavitating ow is seen at the upstream pressure of 300 psi. This indicates that more heterogeneous sites as a result of the ne exfoliation are formed inside the introduced suspension, and the nucleation is triggered more vigorously aer the 80th cycle. 3.2. Characterization of graphene nanosheets In the hydrodynamic cavitation-assisted graphene production process, the starting graphite dispersion was circulated through the system to evaluate the effect of the number of cycles on graphene production. The pre-dened cycles of 20, 40, 60, and 80 were used to study this effect. For example, to prepare a graphene-containing solution via 20-cycles, the starting graphite dispersion was circulated 20 times through the hydrodynamic cavitation system, and the obtained solution of the graphene nanosheets was analyzed using spectroscopic and microscopic techniques. To maintain the homogeneity in the produced graphene nanosheets, a sequential centrifugation method was developed and applied for all samples. In this method, the graphene dispersions aer the hydrodynamic cavitation treatment were rst centrifuged at 2000 rpm for 1 hour; thus, the exfoliated graphene nanosheets and small fragments of graphite were obtained in the supernatant solu- tion (S2U). This supernatant was subjected to a second centri- fugation process at 3000 rpm for 1 hour to remove large particles and to isolate the highly exfoliated stable graphene nanosheets (S3U). Fig. 1b depicts a schematic for the isolation of the stable graphene nanosheets. The isolated graphitic materials and the starting graphite dispersion were rst characterized by Raman spectroscopy to evaluate the effect of the hydrodynamic cavitation on the exfo- liation of graphite akes. Raman spectroscopy is a versatile tool to analyze the structure of carbon nanomaterials, including carbon nanotubes49,50 and graphene.51,52 In a typical Raman spectrum of graphene, there are three commonly reported peaks as D, G, and 2D bands at around 1350, 1580, and 2700 cm1, respectively.53 The D band in the spectrum is related to the structural disorders, edges, and topological defects in the akes. The area ratio of D-band to G-band (AD/AG) is oen used to dene the relative amount of surface defects on the gra- phene.54,55 Besides, the 2D-band for graphene is attributed to two-phonon double resonance and can be used as a measure to evaluate the number of layers in the graphene nanosheets. More specically, the intensity ratio of 2D-band to G-band band (I2D/ IG) is an indication for the number of layers of graphene. Fig. 5 displays the Raman spectra of S3U-20, S3U-40, S3U-60, S3U-80, and the starting graphite dispersion. It is known that the position and shape of the 2D-band are highly sensitive to the number of graphene layers (less than 10 layers) because of the relations of peak activation parameters of Raman mode and band structure.56 No signicant change is observed in the maxima of the 2D-band of the S3U-20, S3U-40, and S3U-60 The normalized and offset Raman spectra (at G-band) of the produced graphene nanosheets after different cavitation cycles within the reactor. (a) D-, G- and D0-band region of the Raman spectra (b) 2D-band region. The dotted line in (b) shows the shift in the position of the 2D-band after 80-cycles of cavitation. 17970 | RSC Adv., 2021, 11, 17965–17975 © 2021 The Author(s). Published by the Royal Society of Chemistry Fig. 5 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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