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Paper RSC Advances method does not involve any surfactants or dispersion agents. According to the results, the suspensions with graphite akes led to an increase in the number of the sites of heterogeneous bubble nucleation and to a decrease in the upstream pressure needed for cavitation inception and a developed cavitating ow pattern. Fragmentation of akes and then exfoliation of layers was observed aer exposures of 60–80 hydrodynamic cavitation cycles inside a microuidic device and were rigorously charac- terized with different methods. With the implementation inside the reactor, it is possible to have a green, scalable, cost-effective, and energy-efficient process. The produced graphene nano- sheets (lateral size $500 nm; thickness $1.2–2.5 nm) meet the requirements well in many applications such as bioengi- neering, composites, and electronic devices. The results on hydrodynamic cavitation show a higher yield compared to the sonication and shear methods for graphene exfoliation. Data availability The data that support the ndings of this study are available from the corresponding author upon reasonable request. Conflicts of interest The authors declare that they have no conict of interest. Acknowledgements This work was supported by internal funding of the KTH Energy Platform. Equipment utilization support from the Sabanci University Nanotechnology Research and Applications Center (SUNUM) is gratefully appreciated. We gratefully acknowledge the Sabanci University Nanotechnology Research and Applica- tion Center (I.A.SN-19-00004) for research funding. We thank Milad Torabfam for preparing the graphite dispersions. References 1 X. Wang and G. Shi, Phys. Chem. Chem. Phys., 2015, 17, 28484–28504. 2 P. Tiwari, V. Sharma, N. Kaur, K. Ahmad and S. M. Mobin, ACS Sustainable Chem. Eng., 2019, 7, 11500–11510. 3 L. Jiang and Z. Fan, Nanoscale, 2014, 6, 1922–1945. 4 S. Goenka, V. Sant and S. Sant, J. Controlled Release, 2014, 173, 75–88. 5 J. Liu, L. Cui and D. Losic, Acta Biomater., 2013, 9, 9243–9257. 6 W. A. De Heer, C. Berger, E. Conrad, P. First, R. Murali and J. Meindl, in Technical Digest - International Electron Devices Meeting, IEDM, 2007, pp. 199–202. 7 S. Shintaro, Jpn. J. Appl. Phys., 2015, 54, 040102. 8 H. Yang, L. Geng, Y. Zhang, G. Chang, Z. Zhang, X. Liu, M. Lei and Y. He, Appl. Surf. Sci., 2019, 466, 385–392. 9 F. Sun, H. Tang, B. Zhang, X. Li, C. Yin, Z. Yue, L. Zhou, Y. Li and J. Shi, ACS Sustainable Chem. Eng., 2018, 6, 974–982. 10 B. Zhang and T. Cui, Appl. Phys. Lett., 2011, 98, 073116. 11 H. J. Yoon, D. H. Jun, J. H. Yang, Z. Zhou, S. S. Yang and M. M. C. Cheng, Sens. Actuators, B, 2011, 157, 310–313. RSC Adv., 2021, 11, 17965–17975 | 17973 Graphene exfoliation yield as a function of energy density with the use of shear, sonication, and hydrodynamic cavitation exfoliation. sizes of the isolated graphene sheets aer 60 and 80-cycles decrease to $4 and $3 mm, respectively. The treatment by the reactor under the developed cavitating ow pattern causes changes in the graphite lateral size, and the SEM results are in good agreement with the DLS size distribution as well as the AFM data. The residence time of the uid in the hydrodynamic cavita- tion device is rather short (2.9 106 s). Therefore, the energy density (E 1⁄4 J m3) can be calculated using the pressure differential along the channel.62 All of the cycles were performed at 650 psi (4.48 106 J m3) for having consistent results. However, the pressure sufficient to have graphene exfoliation decreases to 300 psi (2.06 106 J m3) with the number of cycles due to exfoliation and fragmentation of graphite powder in lower cycles, which provides active sites for nucleation of the cavitating bubbles. Fig. 9 shows the graphene exfoliation yield as a function of energy density and compares hydrodynamic cavitation with the sonication and shear methods reported in the literature.63,64 The results on hydrodynamic cavitation show a higher yield by consuming a lower energy density for graphene exfoliation. As a well-known technique for graphene exfoliation, sonication consumes 25–540 watts of power (5 times more than hydrodynamic cavitation) for an extensive amount of time (3000 times more than hydrodynamic cavitation) to exfoliate a frac- tion of 1 liter of graphite solution.61,65–68 Although the size of used graphite powders limits the hydrodynamic cavitation method, 300 times more energy is required to achieve the same yield for the sonication method. 4. Conclusions In this study, rst, the effects of the thermophysical properties of the working uid (presence of graphite akes) on cavitating ows were visualized and studied. Then, the impact of cavi- tating ows on graphite exfoliation was investigated. This © 2021 The Author(s). Published by the Royal Society of Chemistry Fig. 9 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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