Mechanochemical Exfoliation of 2D Crystals in Deep Eutectic Solvents

Research Article pubs.acs.org/journal/ascecg

Mechanochemical Exfoliation of 2D Crystals in Deep Eutectic Solvents A. M. Abdelkader*,†,‡ and I. A. Kinloch‡,§ †

School of Physics and Astronomy, University of Manchester, Oxford Road, M13 9LP Manchester, United Kingdom National Graphene Institute (NGI), University of Manchester, Booth Street East, M13 9Q Manchester, United Kingdom § School of Materials, University of Manchester, Oxford Road, M13 9LP Manchester, United Kingdom ‡

S Supporting Information *

ABSTRACT: We report a method for the large-scale production of 2D materials through the mechanochemical exfoliation of graphite or other layered compounds in deep eutectic solvents. The exfoliation process is based on the double intercalation of Li+ and Et4N+ ions within the layered crystals using the shear forces in the milling chamber as the driving force. The produced materials composed in majority of few-layer, large diameter flakes. The extent of exfoliation was found to be strongly dependent upon the energy input from the milling process, expressed in terms of the milling duration and the rotation speed. KEYWORDS: Graphene, 2D materials, Mechanochemical, Deep eutectic solvents

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INTRODUCTION Since its initial isolation by micromechanical cleavage, graphene, a single-atom-thick (or thin) sheet of hexagonally arrayed sp2-bonded carbon atoms, has attracted tremendous interest and shown great promise for potential applications in nanoscience and technology.1 Graphene exhibits an ambipolar electric field effect, ballistic conduction of charge carriers, and the quantum Hall effect at room temperature.2,3 Additionally, it possesses a high transparency, high elasticity, excellent thermal conductivity, unusual magnetic properties, and a high degree of charge transfer with other molecules.4 The interest in graphene has led to an intense study of other inorganic 2D materials. The study of inorganic 2D materials was initially dominated by research into clays and other layered oxides but now extends to materials such as hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs), transition metal carbides (MAXen), and black phosphorus.5 The common feature of all these 2D nanomaterials is that in their bulk form they have strong, in-plane covalent bonding but weak, interlayer van der Waals bonding. Upon exfoliation, single-layer inorganic nanosheets exhibit attractive properties for applications in nanoelectronics, optoelectronics, catalysis, thermal managements, etc. For example, MoS2 goes from an indirect to direct bandgap semiconductor (∼1.8 eV) when exfoliated down to a single layer thickness.6,7 There has been significant research into developing low-cost, large-scale production methods for graphene and other 2D materials. Bottom-up approaches such as epitaxial growth,8 and chemical vapor deposition,9 produce monolayer graphene with © 2016 American Chemical Society

quality comparable to that of mechanical cleavage. These methods have adapted for other inorganic 2D materials, and are now scaled up to produce films with areas over 1 m2.10−12 However, such techniques fail to provide the kilogram quantities required for applications such as composites, conductive inks, energy storage, and water treatment. Topdown techniques, in which energy is used to exfoliate graphite, and other bulk inorganic layered materials particles, has been proved most promising for large quantities of 2D materials.13−15 These production techniques include the sonication in organic solvents,16 surfactants assisted processes using either ultrasound or high shear,17,18 mechanical delamination,19−24 and the formation of graphite intercalated compounds.25 However, these methods tend to produce either very small and/or highly defective flakes. For decades, graphite known to be able to host a number of different atoms and small molecules between its graphene layers with this particular feature making graphite an ideal electrode for rechargeable batteries. These graphite intercalation compounds decompose readily in water metal/organic hydroxides and free-standing graphene sheets. This principle was recently introduced as a route for the scalable production of graphene and could theoretically be used to produce other inorganic 2D materials.14,26,27 However, because of the slow kinetic nature of the intercalation process, the intercalate was Received: May 30, 2016 Revised: July 9, 2016 Published: July 11, 2016 4465

DOI: 10.1021/acssuschemeng.6b01195 ACS Sustainable Chem. Eng. 2016, 4, 4465−4472

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ACS Sustainable Chemistry & Engineering only hosted in regions of the graphite close to the edges of the grains. Upon exfoliation in water, graphite with expanded edges was produced, and thus further intercalation, water decomposition or sonication steps were needed to achieve full exfoliation. Swager and Zhong proposed a method to intercalate graphite with initially Li electrochemically and then further intercalate it with ammonia in two separate steps.28 However, due to the expanding nature of the cathode, the initial distance between the electrodes had to be large, hence a high voltage had to be applied to overcome the IR drop. As a result, the organic solvent used as the electrolyte dissociated during the process, hindering the intercalation process. Therefore, an additional sonication step was necessary to achieve reasonable exfoliation. We recently report the successful exfoliation of graphite through the electrochemical ammonia−graphite intercalated compound.15,29,30 Without sonication or necessarily repeating the intercalation/decomposition steps, the product was few-layer graphene with a particle size of the order of a micron. However, the electrochemical intercalation methods need conductive electrodes, and hence successful with graphite. Most of the layered inorganic materials are insulating and hence conductive additives are required to assemble the electrode, which may contaminate the final product of the 2D materials. However, herein, the mechanical power of the ball mill rather than the electrochemical potential is used to drive the exfoliation process. This switch of energy source makes the route applicable to nonconductive inorganic layered materials and also easier to scale than an electrochemical cell. We used ammonium based deep eutectic solvents (DES) as the source of ammonium and the wet grinding media in combination with alumina spheres, which replaces the organic solvents traditionally used in all the mechanical exfoliation of graphite. The use of ammonium-based ionic liquid satisfies high activity of the Et4N+ ions, which accelerates the kinetics of the intercalation process. Also, DES are known as green solvents having the advantages of nonflammability, high thermal stability, wide liquid phase range, negligible vapor pressure and easy recycling; which will eliminate the safety problems associated with the organic solvents and also promote to an eco-friendly process. The present work has selected choline chloride−urea system due to its ability to dissolve a reasonable amount of lithium ions (2.5 wt % for LiCl) and also its biocompatibility, which allows the produced 2D nanosheets to be used in bioapplications. The process can be considered as a modification of the Birch Reduction, and it is also resemble the principles of the metallothermic reduction of oxides using molten halide as flux.31,32

thermodynamically possible, the slow kinetics of the intercalation process usually means that the process has to be accelerated by an external driving force. We have shown in previous work that the kinetic of intercalation reaction is enhanced by applying an electrochemical potential and, when the intercalated compound decomposes, the bulk layered materials can be exfoliated down to the single layer of graphene-like sheets.15 In the present work, we use the kinetic energy provided by the high-speed rotation of the ceramic balls during ball milling to open the edges of the graphite grains. In the presence of ammonium ions and another strong reducing agent such as Li or Na metals, active carbon species (mostly carbon radicals, carbocations and carbanions) formed at the broken edges.15,34 This caused further opening at the edge of the graphite gallery and facilitate the intercalation of both lithium and ammonium ions to a deeper extended on the graphite grain. The reaction between Li, ammonium ions and graphene result in forming a charged graphene sheets that significantly weaken the van der Waals forces and, coupling with the ammonia and hydrogen gases bubbles produced from the interlayer reaction, the graphene sheets can no longer hold together and dispersed as individual thin sheets in the solution. A similar story can be predicted for layered inorganic material. Exfoliation of Graphite: Evidence of the Formation of Graphene. Graphite was mixed with 1.5 times its weight of Li and the mixture was suspended in choline chloride−urea DES by ball milling. The solid exfoliation product was recovered from the DES by a series of washing and filtration steps. The produced powder was first subjected to XRD analysis. It is evident from the XRD pattern (Figure 1a) that the transversal

RESULTS AND DISCUSSION Graphite and many of the layered inorganic materials such as TMDs are known to be good lubricants, and it is hard to achieve its mechanical delamination in a ball mill down to graphene or graphene-like flakes. This is mainly because the random ordination of the graphite grain makes the applied forces by the milling balls a mixture of shearing and compressing forces. The two forces can cancel each other and even where there are some exfoliated graphene flakes they have a high tendency to restack together via van der Waals bond. Intercalating these layered materials with different chemical species is known to reduce the van der Waals forces that hold the graphene-like flakes together.33 Although the formation of many of the intercalating compounds is

dimension of the graphite grains as well as the number of graphene sheets alongside the c-axis are sharply decreased as a result of the mechanochemical treatment in DES. The intensity of the (002) peak decreases and the full width at half-maximum of the (002) peak increases as a result of Scherrer broadening. The reduction of the (002) intensity indicates that there are a sharp weakening in the π−π-stacked layers and reflects the decrease in the graphite particle size in a direction perpendicular to the basal plane.28 Also, the intensity of the (002) peak decreased with increasing rotation speed of the ball mill indicating that the exfoliation depends on the energy provided by the milling process. Significantly, it was found that the pattern of the product obtained by milling using pure DES without the Li, was almost identical to that of the original

Figure 1. (a) XRD pattern of the initial graphite and result product after ball-milling in urea−choline chloride DES using different rotating speed.

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materials are not homogeneous in term of the flakes thickness and defect disruption. Comparing the 2D regions of different represented spectra, it is clear that the wide asymmetrical peak of graphite at 2680 cm−1 replaced by a more symmetrical peak at positions ranged between 2645 and 2660 cm−1, which can be attributed to 2−5 layers of graphene.36−38 From the defects formation point of view, the appearance of a shoulder on the G band (usually called the D′ band) and the increase in the intensity of the D band both suggested the presence of structural disorder in the mechanochemically exfoliated graphene. The ratio of ID/IG is often used to evaluate the density of defects of graphene sheets.39,40 For more than 70% of the flakes the ID/IG is ∼0.60, much smaller than that of chemically produced GO and comparable with that produced via the electrochemical exfoliation.29 Interestingly, the ID/IG value decreased with decreasing the rotation speed of the ball mill (see the Supporting Information). Increasing the time of the milling to 48 h resulted in a more homogeneous thickness of the obtained flakes, more than 85% of the spectra recorded were similar to that in Figure 2a. The value ID/IG significantly increased with increasing the milling time, possibly due to decreasing flake size. To confirm the thickness of the produced graphene, atomic force microscopy (AFM) analysis was used. The flakes were deposited from a diluted ethanol suspension on a Si substrate. Figure 3 shows representative examples of the flakes produced after 20 h of mechanochemical treatment at 500 rpm rotating speed. The height profile of the flakes indicated a thickness between ∼0.9 and ∼2.5 nm. The statistical analysis of 15 flakes also showed the thickness range for more than 95% of the flakes was between 2 and 5 layers, in good agreement with the Raman results. The lateral particles size ranged from 5 to 20 μm. This is a particular advantage of the present work as the majority of the literature showed that ball milling of graphite, either in dry or wet conditions, produce small diameter flakes with lateral sizes of few tens of nm to a few micrometers.41 It is also important to evaluate the chemical purity of the exfoliated graphene. Hence X-ray photoelectron spectroscopy (XPS) was used to probe the chemical composition of the mechanochemically exfoliated graphene. Figure S4 in the Supporting Information shows the XPS survey scan spectrum of the graphene product after 20 h of milling at 500 rpm. The spectrum shows a strong C 1s peak at 284.5 eV, a small O 1s peak at 532.6 eV and a weak OKLL Auger band between 955 and 985 eV. The exfoliated graphene spectrum also showed a small N 1s peak at ∼400 eV, no other elements such as Cl, or Li, are found in the sample. The concentration of elements N and O in graphene is calculated to be about 2.8% and 4.6 at%, respectively. This value of oxygen is very close to the oxygen content value of the starting materials (3.6 at. %) suggesting that the exfoliation process is not oxidative. This was further confirmed by the high-resolution scan of the C 1s of graphene (Figure S3 in the Supporting Information), which was almost identical to that of the starting graphite flakes. The thermal gravimetric analysis in air (TGA) also confirmed the absence of any functional group bonded to the graphene sheets with the freshly exfoliated graphene was almost featureless before the decomposition. (Figure S1 in the Supporting Information). Interestingly the decomposition temperature of graphene is about 150 °C lower than that of graphite, which is also seen for other exfoliated systems.14 We postulate this low thermal stability is a result of the high reactivity of graphene and the presence of defects.

source graphite (Supporting Information). This result indicates clearly the essential role of the Li in the exfoliation process. Raman spectroscopy is a well-known technique for evaluating carbonaceous materials. It is also a standard technique for estimating the level of defects in graphene. For that purpose, thin films of graphene were made via vacuum filtration of dispersions produced at different milling rotation speed. Approximately 30 Raman spectra were recorded at various spots of the film. The Raman spectrum of the original natural graphite was also recorded and is presented in Figure 2 for

Figure 2. (a) Raman spectra for graphite, different flakes produced after 24 h of milling (spectra a−f) and typical spectrum for the flakes after 48 h. (b) Statistical analysis of the ID/IG Raman peaks ratio of the graphene sample produced after 20 h of the milling at 500 rpm.

comparison. In general, the pristine defect-free graphite possessed two bands: one at ∼1580 cm−1 (the G band) arises from the first order scattering of the E2g phonon of sp2-bonded carbon atoms; and a band at ∼2700 cm−1 (the 2D band) corresponding to the double-resonance process.35 Upon introducing defects either edges or topological defects in the sheet, another band (the D Band) arose at about 1350 cm−1. The Raman spectrum obtained from the edge area of pristine graphite shows a very weak D band indicating a highly ordered structure with a low level of defects and a flake size larger than the Raman spot (a few microns diameter). The Raman spectra recorded for the graphene samples after mechanochemical treatment at 500 rpm for 20h indicate that the obtained 4467


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Figure 3. AFM image and height profile for two flakes of the graphene produced after 20 h of milling.

Figure 4. TEM images and diffraction pattern for graphene flakes.

monolayer graphene.16 On the other hand, the same ratio of the pattern in Figure 3d is about 0.75, as expected for trilayer graphene.1 Synthesis of hBN Nanosheets. The route we followed to exfoliate layered inorganic materials is similar to that of exfoliating graphite. Typically, 1 g of hBN is mixed with 1.5 g of lithium and ball milled in the same DES for 20 h at 500 rpm. Figure 5a and Figure S8 (Supporting Information) show a series of scanning electron microscopy (SEM) images of the obtained hBN nanosheets, which possess different lateral sizes varying from hundreds to thousands of nanometers. This lateral

Transmission electron microscopy provided further evidence for graphene exfoliation. Figure 4 shows representative TEM images of the samples as well as the electron diffraction patterns that have been collected from different spots on the TEM images. Although the electron diffraction patterns for graphite and graphene present the same hexagonal structure, it has been established that relative intensities of the Miller−Bravais (hkil) can give an unambiguous local identification of monolayer versus multilayer graphene.16,42 The diffraction pattern in Figure 3c shows that the intensity ratio of the inner spots (i.e., −1010) to that of the outer spots (i.e., −2110) is >1, indicating 4468


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Figure 5. BN nanosheets produced by the mechanochemical process. (a) SEM image, (b) tapping mode AFM image, (c) statistic of the flakes lateral size measured by the SEM, (d) statistic of the flakes thickness measured by the AFM, (e) Raman, and (f) XRD.

results for hBN exfoliation to monolayer flakes.44−46 XRD spectrum of the BN after ball milling (Figure 5f) indicated the materials retrains its hexagonal in-plane structure. However, the intensity of the (002) peak at 2θ ∼26.5, which characterize the π−π stacking of the BN sheets in the bulk gallery, has significantly decreased, and the peak clearly widened.47 This data is analogous to the results observed when graphite is exfoliated to graphene. Exfoliation of TMDs. The concept of combining the sheer force of the ball milling with the chemical intercalation reaction was extended further to include TMDs. MoS2 and WS2 were selected as examples of the TMDs due to the large number of published work on these two materials.48,49 The milling was run for a duration of 20 h and at a speed of 500 rpm. Raman analysis proved the exfoliation of both MoS2 and WS2 into nanosheets; the Raman spectra of bulk WS2 has two vibration modes of A1g at ∼420 cm−1 and E12g at ∼354 cm−1, representing the out-of-plane W−S phonon mode and the in-plane W−S phonon mode, respectively. The ratio between the intensity of the two peaks E12g/A1g are in the range of 0.55 for the bulk

size is 3 to 10 orders of magnitude larger than the hBN nanosheets obtained by the sonication method.5,43 Considering the initial particles size of the bulk hBN, less than 2 μm, the nondistractive nature of the present process is clearly demonstrated. Undisputable evidence for the effective exfoliation arose from the AFM analysis. Tapping mode image (Figure 5b) revealed flakes with thickness ∼1 nm, close to the theoretical thickness value of single layer hBN. Statistics relating to the thickness and lateral size of the exfoliated BN nanosheets are shown in Figure 5c,d. Most of the produced hBN have a thickness less than 3 nm, corresponding to flakes of less than 10 layers. Further evidence of the exfoliation was obtained from the Raman spectroscopy and XRD analysis. As can be seen from Figure 5e, bulk hBN exhibits a characteristic Raman peak from E2g phonon mode (B−N vibration mode), which is analogous to E2g mode (G band) in graphene. The typical E2g band position of bulk hBN is around 1366 cm−1. After exfoliation, the E2g band shifted to 1369 cm−1, and the intensity of the peak reduced significantly, in agreement with previously reported 4469


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Figure 6. Evidence of the exfoliation of MoS2 and WS2, (a) and (c) Raman spectra for the bulk and the exfoliated nanosheets of WS2 and MoS2 respectively, AFM images of the exfoliated WS2 (c) and MoS2 (d). conducted in vertical laboratory attrition mill (PE 075, Netzsch Feinmahltechnik GmbH, Selb, Germany). The grinding chamber is a double wall 750 mL tank with silicon nitride inner wall. Wear resistant commercially available yttria stabilized zirconia milling media with diameters of 2 mm were charged with 2 g of natural graphite and 4 g of battery grade lithium chips into the grinding chamber inside a glovebox. The DES was then poured into the chamber until the liquid covered the grinding media (about 2/3 of the chamber volume). The ball milling was run under a flow of N2 gas with different rotation speeds. The grinding process was paused for 30 min every 2 h to allow the system to cool. The exfoliation product was washed with DMSO several times. In the final step, the sample was sonicated for 15 min and then centrifuged at 1500 rpm to remove the thick graphite flakes. The samples for SEM, Raman and AFM, were dropped from the DMSO suspension. Dry powder and membrane were obtained by filtration using Anodisc alumina membranes with 100 nm pore size and then dried at 100 °C under vacuum. Characterization of the Produced Powder. X-ray photoelectron spectra (XPS) were collected using a Kratos Axis Ultra Xray photoelectron spectrometer, equipped with an aluminum/ magnesium dual anode and a monochromated aluminum X-ray source. The samples were ground with potassium bromide and then pressed into discs. Raman spectra were obtained using a Renishaw system 1000 with an excitation wavelength of 633 nm for the graphene samples and 514 nm for other 2D materials. The laser spot size was ∼1−2 μm, and the power was about 1 mW when the laser is focused on the sample using an Olympus BH-1 microscope. The Raman bands were fitted using the Lorentzian function. Atomic force microscopy (AFM) images were obtained using a Multimode Nanoscope V scanning probe microscopy (SPM) system (Veeco, USA) with Picoscan v5.3.3 software. The tapping mode was used to obtain the images under ambient conditions. TGA was performed in air using a Jupiter Netzsch STA 449 C instrument. The sample was placed into an alumina crucible and heated at a rate of 10 °C/min from 30 °C up to 800 °C in air. Scanning electron microscopy (SEM) was performed using a Philips XL30 FEG SEM, operating at an accelerating voltage of 5 kV. The TEM analysis used a FEI Tecnai F20 microscope. The samples were supported on a 3 nm ultrathin carbon film supported on Cu TEM grid (G3347N, Agar Scientific). The XRD analysis was conducted using a Philips X’PERT APD powder X-ray diffractometer (λ = 1.54 Å, Cu Kα radiation).

sample and it increases with reducing the number of layers. From Figure 6a, it is clear that the ratio E12g /A1g increased to ∼1.2, suggesting a successful exfoliation.50−52 Similarly, the bands of the exfoliated MoS2 nanosheets are different from that of the bulk material both in terms of Raman frequency and signal intensity. For the exfoliated nanosheets two strong Raman bands, deconvoluted by a single Lorentzian centered at ∼380 cm−1 and ∼407 cm−1, were assigned to in-plane E12g and out-of-plane A1g vibrational modes.5 There was no sign of structural distortion in the Raman spectra, suggesting the absence of structure damage and/or covalent bond formation upon the mechanochemical exfoliation. Unlike the bulk MoS2, the A1g and E12g modes for exfoliated MoS2 had similar intensities, indicating a weaker coupling between the electronic transition at the K point with the A1g phonon existing in MoS2 nanosheets.53,54 The peak positions difference between A1g and E12g was measured to be 24 cm−1. The value of this difference for the bulk material was found to be 27 cm−1, signifying the success of exfoliation and the existence of MoS2 in few layers.38,50,54 The AFM images confirmed the exfoliation down to few layers as can be concluded from Figure 6c,d.

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CONCLUSIONS In conclusion, we have shown that the Li−Et4N+ intercalation chemistry, which was used previously for electrochemical exfoliation of graphene and other 2D materials, can be adapted for utilization in a planetary ball mill, using a deep eutectic salt as the wet media. The use of the ball mill gives an alternative and potentially easier route for scale-up, while still maintaining a reasonable flake diameter.

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EXPERIMENTAL DETAILS

Choline chloride was recrystallized from ethanol absolute. The crystals were then filtered out from the solution and dried overnight at 70 °C under vacuum. Urea was separately dried under vacuum before mixing. The dry salts were then mixed in their eutectic composition (2:1 mol ratio of urea:choline chloride), and the deep eutectic solvent was synthesized as described elsewhere.55 The grinding process was 4470


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electrochemical exfoliation of graphene materials from bulk graphite. Nanoscale 2015, 7, 6944−6956. (15) Sole, C.; Drewett, N. E.; Liu, F.; Abdelkader, A. M.; Kinloch, I. A.; Hardwick, L. J. The role of re-aggregation on the performance of electrochemically exfoliated many-layer graphene for Li-ion batteries. J. Electroanal. Chem. 2015, 753, 35−41. (16) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. High-yield production of graphene by liquid-phase exfoliation of graphite. Nat. Nanotechnol. 2008, 3, 563−568. (17) Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z.; McGovern, I. T.; Duesberg, G. S.; Coleman, J. N. Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. J. Am. Chem. Soc. 2009, 131, 3611−3620. (18) Paton, K. R.; Varrla, E.; Backes, C.; Smith, R. J.; Khan, U.; O’Neill, A.; Boland, C.; Lotya, M.; Istrate, O. M.; King, P.; Higgins, T.; Barwich, S.; May, P.; Puczkarski, P.; Ahmed, I.; Moebius, M.; Pettersson, H.; Long, E.; Coelho, J.; O’Brien, S. E.; McGuire, E. K.; Sanchez, B. M.; Duesberg, G. S.; McEvoy, N.; Pennycook, T. J.; Downing, C.; Crossley, A.; Nicolosi, V.; Coleman, J. N. Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids. Nat. Mater. 2014, 13, 624−630. (19) Kun, P.; Wéber, F.; Balázsi, C. Preparation and examination of multilayer graphene nanosheets by exfoliation of graphite in high efficient attritor mill. Cent. Eur. J. Chem. 2011, 9, 47−51. (20) Zhao, W.; Fang, M.; Wu, F.; Wu, H.; Wang, L.; Chen, G. Preparation of graphene by exfoliation of graphite using wet ball milling. J. Mater. Chem. 2010, 20, 5817−5819. (21) Aparna, R.; Sivakumar, N.; Balakrishnan, A.; Sreekumar Nair, A.; Nair, S. V.; Subramanian, K. R. V. An effective route to produce fewlayer graphene using combinatorial ball milling and strong aqueous exfoliants. J. Renewable Sustainable Energy 2013, 5, 033123. (22) Knieke, C.; Berger, A.; Voigt, M.; Taylor, R. N. K.; Röhrl, J.; Peukert, W. Scalable production of graphene sheets by mechanical delamination. Carbon 2010, 48, 3196−3204. (23) Wu, H.; Zhao, W.; Chen, G. One-pot in situ ball milling preparation of polymer/graphene nanocomposites. J. Appl. Polym. Sci. 2012, 125, 3899−3903. (24) León, V.; Rodriguez, A. M.; Prieto, P.; Prato, M.; Vázquez, E. Exfoliation of Graphite with Triazine Derivatives under Ball-Milling Conditions: Preparation of Few-Layer Graphene via Selective Noncovalent Interactions. ACS Nano 2014, 8, 563−571. (25) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved synthesis of graphene oxide. ACS Nano 2010, 4, 4806−4814. (26) Abdelkader, A. M. Electrochemical synthesis of highly corrugated graphene sheets for high performance supercapacitors. J. Mater. Chem. A 2015, 3, 8519−8525. (27) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558−1565. (28) Zhong, Y. L.; Swager, T. M. Enhanced electrochemical expansion of graphite for in situ electrochemical functionalization. J. Am. Chem. Soc. 2012, 134, 17896−17899. (29) Abdelkader, A. M.; Kinloch, I. A.; Dryfe, R. A. W. Continuous electrochemical exfoliation of micrometer-sized graphene using synergistic ion intercalations and organic solvents. ACS Appl. Mater. Interfaces 2014, 6, 1632−1639. (30) Abdelkader, A. M.; Patten, H. V.; Li, Z.; Chen, Y.; Kinloch, I. A. Electrochemical exfoliation of graphite in quaternary ammoniumbased deep eutectic solvents: a route for the mass production of graphane. Nanoscale 2015, 7, 11386−11392. (31) Abdelkader, A. M.; Daher, A. Preparation of hafnium powder by calciothermic reduction of HfO2 in molten chloride bath. J. Alloys Compd. 2009, 469, 571−575.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01195. Other experimental details, other characterizations, more SEM images, and statistical analysis of the effect of the process parameters (PDF).

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AUTHOR INFORMATION

Corresponding Author

*A. M. Abdelkader. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.M.A. thanks the Faculty of Engineering & Physical Sciences, University of Manchester for financial support. Further grant funding from the EPSRC (EP/I023879/1) is acknowledged by I.A.K.

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REFERENCES

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