Top-Down, Scalable Graphene Sheets Production: It Is All about the

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Top-Down, Scalable Graphene Sheets Production: It Is All about the Precipitate Matat Buzaglo,*,† Efrat Ruse,‡ Idan Levy,† Roey Nadiv,† Guy Reuveni,† Michael Shtein,§ and Oren Regev*,†,∥ †

Department of Chemical Engineering and ∥Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel ‡ Department of Chemistry, Nuclear Research Center, Negev, Israel § Dotz Nano Limited, Tel-Aviv 6438123, Israel S Supporting Information *

ABSTRACT: Graphene production has been widely explored and developed in the past decade. Most research has aimed at developing scalable, environmentally friendly and cheap procedures to produce defect-free graphene that can be used in a variety of applications such as mechanical properties enhancement and energy storage. Top-down graphene production approaches (from graphite) in liquid, which include high-shear mixing and sonication, were recently scaled up. Nevertheless, their production yields have remained low ( 1 μm for all precipitated GS.

supernatant nor the precipitate product. This can be assumed since the starting graphite flake materials have a lateral size much larger than the laser spot (MLD ≫ 1 μm). Therefore, the D-band is dominated by contributions from basal plane defects of the parent graphite material, which also remain in the obtained GS.26 We further support these findings and eliminate sonicationdriven GS functionalization by X-ray photoelectron spectroscopy (XPS, Figure S3b, inset, in the Supporting Information). The carbon 1s peak binding energy and shape are not affected by the production method. This indicates that the surface of the as-synthesized graphene in all methods is similar to those of the raw material, i.e., not damaged or functionalized due to the sonication procedure. Thus, for graphene produced using sonication the precipitate phase also contains graphene whose characteristics are similar to those of the product found in the supernatant but with a larger MLD as indicated by TGA and Raman measurements (Figures 4 and 5, respectively). This finding has substantial implications for the overall graphene yield: instead of a negligible yield of a few percent from only the supernatant, one can obtain a yield of 100% by collecting the graphene also formed in the precipitate. Remarkably, the shear mixing and ball milling procedures exhibited similar graphene production behavior, as described in the following section.

As expected, as the specific sonication energy increased, the particle size (as reflected in the T1/2 value) in the precipitate decreased. Remarkably, for both the lab- and the large-scale sonication procedures, T1/2 decreased rapidly in the low-energy range (0−60 kJ/g of graphite) and moderately in the highenergy range (>60 kJ/g graphite; see change in slope in Figure 4b). This behavior corresponds to an exfoliation−fragmentation mechanism: in the low-energy range, a transition from graphite to GS, namely, exfoliation, takes place, and therefore, both graphite and GS were present in the product. The change in the slope in the T1/2−energy curve for sonication occurs at 60 kJ/g graphite (Figure 4b, green arrow), which is the sonication energy required to exfoliate all of the graphite to GS (according to the T1/2 values, see above). Increasing the sonication energy to above 60 kJ/g graphite will result in reduced GS size (fragmentation). The change to moderate slope above 60 kJ/g of graphite also indicates that the exfoliation rate was much higher than the fragmentation rate. A possible explanation for this finding is that the energy required to exfoliate and fragment the isotropic, large graphite particles is lower than that required to fragment the smaller GS particles. At energies that exceed 60 kJ/g graphite, therefore, the value of T1/2 slowly decreases. We characterized the defect density and the size of the GS obtained in the supernatant and in the precipitate using Raman spectroscopy (Figure 5). The intensity ratio ID/ID′ indicates the defect type:23 edge defects, ID/ID′ ≈ 3, vacancy basal plane point defects, ID/ID′ ≈ 7, and sp3 defects, ID/ID′ ≈ 13,24 and ID/ IG indicates the total defect density.25 The precipitate was characterized by an energy-independent value of ID/IG, which is constant with the sonication energy and equal to the measured ID/IG value of graphite (0.14, Figure 5a). On the other hand, the supernatant product was characterized by an average ID/IG value of ∼0.19 and ID/ID′ ≈∼ 2−3 (Figure 5b), indicating that only edge defects were present.24 This implies that the GS in the supernatant are relatively small and have MLD below 1 μm, which remain constant above 250 kJ/g graphite (Figure 5a; edges are detected by the Raman laser with spot diameter of 1 μm), while the GS in the precipitate are larger and have MLD greater than 1 μm (most edges are not detected). Hence, in terms of in-plane defects, the sonication damaged neither the

3. COMPARISON TO OTHER TOP-DOWN PRODUCTION METHODS We compared the T1/2 sonication curve (Figure 4b) with those of other top-down liquid- and solid-based production methods, namely, shear mixing and ball milling of graphite, respectively. Similar to sonication, shear mixing is also performed in a dispersant-assisted aqueous solution and the product is obtained in both the supernatant and the precipitate. Likewise, the bulk of the product formed during the shear mixing procedure is found in the precipitate. Therefore, we used only the precipitate product for this methodological comparison. Ball milling, in contrast, is a solid-based method, and as such, only one product is obtained. 3.1. Shear Mixing. Shear mixing (a.k.a., batch dispersion) was used to exfoliate and disperse particles such as single-walled carbon nanotubes with minimal defect density,27 which was possible due to the low power of the shear mixer (1.4 W; section S1 in the Supporting Information). Nevertheless, this shear-based method is also expected 10002

DOI: 10.1021/acs.chemmater.7b03428 Chem. Mater. 2017, 29, 9998−10006

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Figure 7. Fragmentation mechanism of GS above the critical specific energy (60 kJ/g of graphite, red circle) in shear mixing precipitate products. (Top) AFM micrograph of the GS products at various shear mixing energies (a, b, and d) reveals holes that penetrate the GS layers as shown in c; enlargement of b. These holes grew with increasing specific energy, which led to the fragmentation of the GS. (Bottom) Side-view schematic (not to scale) of the fragmentation process with increasing shear mixing energy.

Figure 8. Ball milling:17 (a) T1/2 vs specific energy of the products obtained in pyrene-protected milled graphite. (Inset) SEM micrograph of the GS obtained in the precipitate (with a specific energy of 200 kJ/g graphite) indicates a lateral dimension larger than 2 μm. (b) Raman spectroscopy: ID/ IG of the obtained products in ball milling vs specific energy. to be effective in top-down production of graphene, as was shown for high-shear mixing (80−250 W).13 Figure 6 summarizes the effect of energy input in shear mixing on T1/2 and ID/IG of the precipitate, which presents a very similar behavior to the one obtained by sonication (Figures 4b and 5a), indicating a similar GS production mechanism from graphite. Moreover, the critical specific energy required for total conversion of graphite to graphene in shear mixing (change in slope in the T1/2specific energy curve, Figure 6a and section S3 in the Supporting Information) is 60 kJ/g of graphite, i.e., the same value as obtained for sonication (Figure 4b). The shear mixing precipitate is characterized by energy-independent ID/IG value (0.14, Figure 6b), similar to the findings in the sonication (Figure 5a). These observations indicate that both shear mixing and sonication do not damage the precipitate products in terms of in-plane defects, and most of the obtained GS in the precipitate has a lateral dimension above 1 μm as imaged by SEM (Figure 6a, inset), AFM (section S3 in the Supporting Information), and confirmed by

spectroscopy measurements (Figure 6b and Figure S3b in the Supporting Information). Unlike sonication (700 W) and ball milling (12.5 W) in this study, the low power in shear mixing (1.4 W) enables one to obtain temporal resolution required to discern intermediate events in the fragmentation mechanism (Figure 7), that is, above the critical specific energy (60 kJ/ g of graphite, Figure 6a) the weak shear forces pierce the graphitic plane of the GS (average hole diameter of 2 μm). As the shearing energy increases, these holes widen and deepen to expose the layered GS. At even higher shearing energy, the keep growing holes decompose the graphitic plane of the GS into small-size GS (MLD of ∼1 μm and six layers, Figure 7c and section S4 in the Supporting Information). This intermediate form of graphene was recently termed “holey graphene”,28 a structural derivative of graphene formed by removing a large number of atoms from the graphitic plane to produce holes distributed on and through the atomic thickness of the GS. It can be used for energy storage,29 as a membrane,30 and even for improving mechanical properties31 in composite materials. 10003

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Chemistry of Materials 3.2. Ball Milling. Graphite ball milling (in the presence of a protective agent such as pyrene) has been reported as an effective method for producing defect-free GS from graphite, and based on its T1/2−energy curve (Figure 8a), it exhibits the same slope change behavior as sonication and shear mixing (Figures 4b and 6a, respectively).17 In the ball milling procedure, graphite flakes are premixed with solid diluents to prevent the GS that is subsequently obtained from re aggregating and to minimize the formation of amorphous carbon during the dry milling process. Because the diluent partially adsorbs the impact forces (low milling energies) it enables the graphite to be exfoliated to GS (due to shear forces), after which the GS are fragmented at higher milling energies. To extract the graphene product from the diluent−graphene mixture, the diluent is then completely removed via filtration with specific solvents. In this solid-phase process there is no phase separation to supernatant and precipitate. The effects of graphite ball milling input energy on T1/2 and ID/IG (Figure 8) demonstrate a very similar behavior to that observed in liquid-based methods (sonication and shear mixing), indicating a similar exfoliation−fragmentation mechanism of GS from graphite. Nevertheless, the critical specific energy required for the total conversion (change in slope in the T1/2-specific energy curve, Figure 8a and section S3 in the Supporting Information) of graphite to graphene is higher compared to sonication and shear mixing (300 compared to 60 kJ/g of graphite, respectively). This implies that the liquid-based methods (sonication and shear mixing) are more efficient in the graphite-to-GS conversion process than the solid-based one (ball milling) and therefore require less energy. One possible explanation for this finding could be the nature of the forces in each method. In the liquid-based methods, shear forces are dominant, while the principal forces in ball milling are impact forces, leading to a greater loss in energy in the latter method during the graphite-tographene conversion process. In addition, the increase in ID/IG with specific energy (Figure 8b) is due to the formation of edge defects solely,17 a consequence of the decrease in the GS MLD32 (fragmentation due to dominance of the impact forces), in line with the sp2 percentage trend measured by XPS (Figure S3b in the Supporting Information). In summary, we show that the three top-down methods tested here proceed via similar GS formation mechanisms (section S3 in the Supporting Information) and could provide GS yields from graphite of up to 100%. Moreover, we showed for liquid-based methods that most of the GS product was present in the precipitate, which is usually not considered a product,13,33,34 while the amount of GS in the supernatant, the fraction that is typically reported on, was negligible. Therefore, the scaling laws presented in eq 3 (and in previous studies) correctly predict the concentration of the GS in the supernatant but fail to deliver the total quantity of GS produced during sonication or shear mixing or the yield. Finally, a low-power production method (e.g., shear mixing) makes it possible to control the graphene structure, which is fundamental to its utilization in various applications in the electrocatalysis35 and energy storage fields.36

fundamentally alter existing notions of shear-based top-down production methods in liquid media. This approach could be easily applied to other carbon raw materials (e.g., coal or graphite powder) to produce GS. Moreover, it could easily be applied in the exfoliation of other 3D layered materials, such as boron nitride and tungsten disulfide to 2D materials with only a few layers.

5. METHODS Sonication. Semi-Industrial Scale. High-power continuous tip sonication (TS) was performed in UIP1000hd (1000 W 20 kHz, 22 mm and 34 mm sonotrods; “the tip”, Hielscher, Germany). A closed water cooling system maintained the dispersion temperature at 15 °C. The minimal volume that can be used in this system is 1.5 L: The minimal volume in the feeding tank is 1 L in addition to 0.5 L in the pipeline. Lower volumes would lead to cavitation in the pump. Lab Scale. A batch TS procedure was performed in Qsonica Q-500 (500 W 20 kHz, 1/8“ μtip, Qsonica; maximal sonication volume of 10 mL). The dispersion temperature was kept at 0 °C by an ice bath. The tip surface was maintained in smoothly polished form. Dispersion Procedure. Graphite flakes (1 wt %) were mixed with dispersant (0.1−1 wt %) in water and sonicated using tip sonication (TS, lab or semi-industrial scales). Our tests of several dispersants and graphite:dispersant weight ratios and in order to avoid extensive foaming of the dispersant found that when TX-100 was the dispersant the optimal ratios were 10:1 in the large-scale and 1:1 in the lab-scale procedure.12 After sonication the dispersion was centrifuged (Megafuge 1.0 (Heraues) operated at 4000g for 20 min) and 85 vol % (∼5 mm above the precipitate) of the supernatant was carefully collected and kept under ambient conditions. Before characterization (e.g., SEM, Raman, XPS) of the precipitate GS product was washed with water and filtered on a membrane (Sartorius 0.2 μm pores) to completely remove the soluble dispersant (TX-100) from the carbonaceous product. Shear Mixing. Graphite flakes (1 wt %) were mixed with dispersant (1 wt %) in water and mixed using a shear mixer (PT 4000 Stand Dispersion Unit). After mixing, the dispersion was centrifuged (the same procedure as in the sonication process), and 85 vol % (∼5 mm above the precipitate) of the supernatant was carefully collected and kept under ambient conditions. Before characterizing the precipitate product it was washed with water and filtered on a membrane (Sartorius 0.2 μm pores) to completely remove the soluble dispersant (TX-100) from the carbonaceous product. Ball Milling. Graphite flakes (36 mg) were ground with pyrene as a protecting agent in a fixed weight ratio (diluent/GF = 28) for various milling times and rotational speed (RPM) values in air. The number of zirconia balls, 10 mm in diameter (10 balls), was fixed, with a balls-topowder weight ratio of 23. The ground powder was washed and filtered on a membrane (Sartorius 0.2 μm pores) with acetone to remove completely the soluble diluent (pyrene) from the carbonaceous product.

4. CONCLUSION We show that sonication is a valid and scalable method for producing defect-free graphene from graphite and that the graphene production rate increases with volume (Figure 3d and eq 4). In this procedure, two types of products are present: GS dispersed in aqueous solution (supernatant) and GS in powder form that can be isolated from the precipitate, leading to an overall conversion (above the critical specific energy) of 100%. We found that shear mixing and solid-based milling production exhibit the same thermogravimetric signature, which implies that other top-down methods could also yield a rate of graphite conversion to graphene of 100%. In our view, these results, which indicate unequivocally that the precipitate must be examined as part of the product,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03428. Energy calculation and elaborated comparison of the GS products of the different top-down methods (including TGA, XPS, AFM, TEM, and Raman spectroscopy); materials and additional details on the methods for the characterization of the GS (PDF) 10004

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Matat Buzaglo: 0000-0003-2119-3393 Efrat Ruse: 0000-0001-9073-9471 Roey Nadiv: 0000-0003-2663-7846 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from Dotz Nano Ltd. and the excellent technical support of Ran Atias, Jurgen Jopp, and Dor Gershkovich.

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