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Nanotechnology 22 (2011) 365306 Z Shen et al The designed JCD has prepared graphene with ease and the force generated by jet cavitation has successfully exfoliated crystal graphite into graphene sheets. Though the force exerted on graphite flakes is so complex during jet cavitation that it is difficult to precisely define what kind of is force exfoliating the flakes, the mechanism about how jet cavitation prepares graphene can be described in a general way as follows. Firstly, bubbles caused by jet cavitation distribute around graphite flakes. When these bubbles collapse, micro-jets and shock waves will act on the graphite surfaces instantly, resulting in compressive stress waves which propagate throughout the graphite body. According to the theory of stress waves [27], once the compressive wave spreads to the free interface of graphite, a tensile stress wave will be reflected back to the body. Hence it is one of the principal ways that cause tensile stress wave effectively exfoliating graphite. As such, collapses of numerous micro-bubbles will lead to intensive tensile stress in the graphite flakes; just as intensive ‘sucking discs’ exfoliate the flakes. We know that the interlayer binding force of layered graphite is a van der Waals force which is relatively weak, while the tensile stress induced by jet cavitation is around several kPa, which can exfoliate graphite easily. In addition, a subordinate process is likely in that the unbalanced lateral compressive stress can also separate two adjacent flakes by a shear effect. Also, micro-jets may split graphite flakes just as a wedge is driven into the interlayer. The above results from SEM, AFM, TEM and Raman spectroscopy have confirmed the presence of graphene sheets prepared by jet cavitation; the graphene yield by our method is estimated as ∼4 wt%. Though the domain size of graphene captured in the AFM is several hundred nanometers, TEM and Raman spectroscopy results present proof of much graphene with a lateral size of several micrometers. In addition, HRTEM shows that the monolayer is generally free of defects. Statistical Raman analysis suggests at least 61% graphene to be largely defect free. Due to the mechanical nature of the jet cavitation method, no chemical reaction occurs and the prepared graphene is not oxidized. So in the view of defects, oxidation and chemical post-treatments, this method is superior to the chemical oxidation and reduction method. Though the lateral size of graphene here is smaller than that prepared by CVD, this method does not require the high temperatures and special pressures needed for CVD. And it has advantages in terms of yield and throughput compared to traditional micromechanical cleavage. Also we have noted recent reports for large scale production of graphene by microwave irradiation [28, 29] and ultrasonication [14, 15, 17]. The microwave irradiation technique takes graphite intercalation compounds as a starting material, and involves labor-intensive preparation with ultrasonication and ultracentrifugation in a high boiling and expensive solvent [28]. Though newly developed, solid-state microwave irradiation could synthesize high-quality graphene nanosheets, it uses graphite oxide as precursor and needs a hydrogen-containing atmosphere [29]. So it suffers from oxidation defects and unfitness for room conditions. As for the reported ultrasonication methods which are always combined with special organic solvents and ultracentrifugation, they can give defect-free monolayer graphene at a large scale [14, 15, 17]. However, these processes are not without their drawbacks because these solvents, generally with a high boiling point, are expensive and require special care when handling. Nevertheless, by using water as the solvent and crystal graphite as precursor, the jet cavitation method reported here is safe, mechanical, non-oxidative, user-friendly, timesaving, insensitive to the environment, etc. Moreover, the intensity of jet cavitation and the force exerted on the graphite flakes can be easily controlled by modulating the difference in jet pressure between the front and rear of the nozzle, though the control knob is not gained now and will be found in a further study. In addition, the yield of graphene can be potentially enhanced by repetitive jet cavitation treatments within a short time. In short, the jet cavitation method, with low cost and high efficiency, could be potentially scalable to mass production. In particular, it has significant advantages for the cheap production of graphene powder which has enticing applications in composites, electric batteries, supercapacitors, field emitters, coatings, etc. 4. Conclusions In summary, we have demonstrated the feasibility of preparing graphene in liquid phase with a water solvent by jet cavitation, which is green, low cost, laborsaving, timesaving and insensitive to the environment. Moreover, the innovative jet cavitation device, the satisfactory quality of graphene and the graphene yield, estimated as ∼4 wt% here, show that this method is promising for mass-production of graphene. We believe that the work presented here marks a significant step forward to bring graphene materials much closer to real- world applications needing liquid-phase production, especially in the fields of nanocomposites, batteries, supercapacitors, catalysisetc. Acknowledgments This works was partially funded by the Special Financial Support of Joint Building Project of the Beijing Education Committee and the ‘985’ Project of Ministry of Education of China. References [1] Geim A K and Novoselov K S 2007 Nature Mater. 6 183 [2] Geim A K 2009 Science 324 1530 [3] SegalM2009NatureNanotechnol.4612 [4] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V and Firsov A A 2004 Science 306 666 [5] Lee C, Wei X, Kysar J W and Hone J 2008 Science 321 385 [6] Balandin A A, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F and Lau C H 2008 Nano Lett. 8 902 [7] Stankovich S, Piner R D, Chen X, Wu N, Nguyen S T and Ruoff R S 2006 J. Mater. Chem. 16 155 [8] EdaG,FanchiniGandChhowallaM2008Nature Nanotechnol. 3 270 [9] Land T A, Michely T, Behm T, Hemminger J C and Comsa G 1992 Surf. Sci. 264 261 6PDF Image | Preparation of graphene by jet cavitation
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