WO2017056086A1 - A process for the production of few-layered graphene - Google Patents

Abstract

Disclosed herein a process for the manufacture of 1-10 layer graphene comprising dry ball-milling of graphite with at least one aromatic ring-containing grinding aid with a molecular weight less than 150,000 grams per mole.

Description

A PROCESS FOR THE PRODUCTION OF FEW-LAYERED GRAPHENE Field of the invention

The present invention relates to processes for the production of graphene nanoplatelets via solid phase milling with protective solid additives.

Background of the invention

Graphene is a 2D sheet of a mono atomic carbon layer with many desirable chemical, physical, and mechanical properties. These include high charge mobility, thermal and electrical conductivity, and mechanical breaking strength over 100 times greater than steel. Graphene has many potential applications such as in the manufacture of electrodes, additives for composites and thermal management .

Graphene nanoplatelets (GNPs) are particles having a lateral size in the sub-micron to over 50 micron range. Generally defined, GNPs are thin graphite particles with thickness of less than about 100 nm. "Few-layer graphene" (FLG) in reference to the present invention is graphene nanoplatelet consisting of 1-10 graphene layers. Due to high aspect ratio and chemical stability GNPs has many potential applications as mentioned above.

Graphene may be produced via bottom-up chemical processes or by top-down mechanical exfoliation of bulk graphite or graphite oxide. The chemical bottom-up preparation processes involve a substrate exposed to a carbonaceous volatile precursor, termed chemical vapor deposition (CVD) , which reacts on the substrate surface to produce the desired graphene deposit. The method is poorly scalable and the cost of the product is high. The exfoliation of graphite, on the other hand, requires energy to be invested to break Van der Waals (VDW) interactions between the layers. Oxidizing graphite or using intercalators facilitates the exfoliation, but typically results in poor- quality material with reduced conductivity and mechanical properties. Direct exfoliation with ultrasound in the presence of dispersants or organic solvents furnishes graphene of somewhat better quality and yield, but may require purification and tedious dispersant/solvent removal, and suffers from a low production rate and very low yields, and the presence of defects in the product. Similar drawbacks characterize also high-shear mixing exfoliation process of graphite in appropriate liquids.

Grinding of graphite particles as a synthetic approach for production of FLG, on the other hand, is restricted as a result of particle re-aggregation and of the formation of amorphous, e.g. non-crystalline non-graphitic carbon. In attempt to overcome this obstacle various liquid additives, such as dimethyl formamide (DMF) , ethanol or water, were introduced to the ground mixture. However, wet ball milling (WBM) usually also results in only low yields following a prolonged milling process.

While solid phase ball milling (SPM) of graphite and diluents' mixtures has not been extensively studied, ball milling of pure graphite, resulted in nano-graphene particles of wide polydispersity in size and a significant amount of amorphous carbon. SPM of graphite has been conducted previously with dry ice [Jeon Shin Proc .Acad . Sci . USA 2012, 109(15) 5588-5593]. The use of dry ice as the diluent resulted in the formation of edge-selectively functionalized graphene and is not applicable for synthesis of non-oxidized pristine GNPs. A ball milling procedure was reported with the addition of very small quantities of graphite and solid triazines and phenol derivatives that serve as an exfoliating agent. These additives effected the further dispersion of GNPs in DMF (Leon, V. , et al., Chem Commun (Camb) , 2011, 47(39): p. 10936-10938; and Leon, V. , et al., Acs Nano, 2014, 8(1): p. 563-571). No quantitative separation GNP product was reported in these publications. Milling was performed under very severe conditions (ratio of milling balls to powder approximately 1500:1)

Recently, Liu et al. [Liu, et al, Phys. Chem. Chem. Phys . , 2015, 17, 6913-6918, DOL: 10.1039/C4CP05864J) have described grinding graphite with sublimable grinding aids, such as ammonium carbonate or ammonium hydrochloride, and annealing the resultant product at 800°C (as termed therein "mechano-thermal activation") . The resultant product was characterized as graphene with a significant amount of defects, as seen from the Figure 3 of said paper, from the presence of Raman D peak indicative for defects. Notably, essentially the same process has been described in British patent GB 564,418 (1944) without the characterization of the product as graphene.

Another publication describes prolonged dry ball milling (many hours of milling) of graphite in presence of cellulose derivatives, corn starch or chitin (Sun et al, Cellulose (2014) 21: 2469-2478) . Isolation of graphene from the mixture was demonstrated by dissolving and removal of water-soluble derivatives ( carboxymethyl cellulose) . Again, the product suffered from defects, such as chemical modifications and had a morphology different from that of graphene produced by other methods. Graphene was characterized by microscopic techniques (SEM, AFM) and by indirect conductivity measurements, and not by bulk examination techniques such as Raman or XPS showing defects concentration and number of layers. The mixture of graphite and thermoplastic polymers, notably high-MW polystyrene, was also demonstrated but shown as ineffective, producing the same conductivity results as a control blend of separately ground components.

A yet further approach is described in Chinese patent application CN 103058176. The described process involves dry ball milling of a pre-granulated blend of expanded graphite with water-soluble grinding aids, mainly inorganic and some water-soluble organic salts. The separation of graphene was achieved by washing the ground mixture with water and filtering the slurry.

There is thus a need for a low-cost high-yield production method for GNPs of good quality, starting directly from graphite, preferably natural graphite. There further exists a need in the art for a process of production of good- quality GNPs under conditions that protect the exfoliated GNPs from defects, amorphization and re-aggregation. The present invention meets these needs by providing a dry ball-milling process of graphite with specific aromatic grinding aids . Summary of the invention

The invention provides a process for manufacturing a few- layer graphene (1-10 layers), preferably defect-free graphene, comprising dry ball-milling of graphite with at least one aromatic-ring containing grinding aid with a molecular weight less than 150,000 grams per mole. It has been unexpectedly found that some grinding aids are particularly suited for production of graphene, especially defect-free graphene. The grinding aids may be generally selected according to at least one of several criteria below. Preferably, the grinding aids are solid at processing conditions, i.e. at room temperature. Yet more preferably the grinding aids have a melting temperature above about 40°C, 50°C, 60°C, 70°C or above about 80°C. Preferably the grinding aids exhibit satisfactory solubility in at least one organic solvent, for example, at room temperature. The solubility in the at least one organic solvent may be, for example, above 1.0 g/L. Further preferably, the grinding aids may have water solubility of less than about 3.0 g/L, and more preferably below 2.0 g/L or 1.0 g/L, for example, at room temperature. Preferably the chemical additives suitable as grinding aids according to the present invention possess two or more aromatic groups, each two being connected with at least one linking moiety or fused together.

The suitable grinding aids include polyaromatic compounds, e.g. fused polyaromatic molecules. In some preferred embodiments, the grinding aids are fused polyaromatic molecules, preferably devoid of hetero atoms, such as pyrene, anthracene or naphthalene. In other preferred embodiments, the grinding aid is diphenyl acetylene. In other preferred embodiments, the grinding aid is diphenyl butadiyne. In other preferred embodiments, the grinding aid is dibenzo-18-crown-6. It was also unexpectedly found that certain aromatic thermoplastic polymers of relatively low molecular weight, e.g. low-MW polystyrene, may also be suitable for use as a grinding aid.

The present process is characterized with a use of graphite, preferably natural (i.e. non-expanded) graphite as a starting material, in a high quality of the graphene product, preferably defect-free graphene, in high yields of the product, usually above 90 %, and in that it does not involve preliminary oxidation, intercalation or excessive sonication. The product may be dispersed in various solvents. The grinding aid may be recovered for recycling by conventional dissolution in a suitable solvent and filtration of graphene.

Brief description of the drawings

Fig. 1 presents combined thermograms of pristine natural graphite flakes (GF) , graphite flakes following grinding with 1000 mg and without a grinding aid (pyrene) (36 mg graphite, 450 RPM, 2x30 min) .

Fig. 2 presents a scanning electron-microscopic (SEM) image of graphite flakes (GF) following grinding with a grinding aid .

Fig. 3 presents the statistical analyses of sizes for over 100 particles performed on the mixtures shown in Fig. 2. Fig. 4 presents an atomic force microscopic (AFM) image of graphite flakes (GF) following grinding with a grinding aid .

Fig. 5 presents a Raman spectrum of graphite flakes (GF) following grinding with a grinding aid.

Detailed description of the invention

In its broadest aspect, the present invention provides a process of manufacture of graphene nanoplatelets , preferably high-quality nanoplatelets, said process comprising ball-milling graphite with at least one aromatic-ring containing grinding aid with a molecular weight less than 150,000 grams per mole, preferably less than 100, 000 grams per mole, or less than 50, 000 grams per mole. The grinding aids may be generally selected according to at least one of several criteria below. Preferably, the grinding aids are solid at processing conditions, i.e. at room temperature. Yet more preferably the grinding aids have a melting temperature above about 40°C, 50°C, 60°C, 70°C or above about 80°C. Preferably the grinding aids exhibit satisfactory solubility in at least one organic solvent. The solubility in the at least one organic solvent may be, for example, above 1.0 g/L, preferably above 100 g/L. Further preferably, the grinding aids may have water solubility of less than about 3.0 g/L, e.g. below 2.0 g/L or 1.0 g/L. Preferably the structure of the grinding aids is characterized by two or more aromatic groups, each two being connected with at least one linker or fused together. The grinding aids are preferably fused polyaromatic compounds. Preferably, the fused polyaromatic compounds are devoid of hetero atoms, such as pyrene, anthracene or naphthalene .

In an additional aspect of the present invention, the grinding aid is an aromatic polymeric compound bearing a plurality of aromatic rings, for example, a polystyrene with a molecular weight lower than 100, 000, and even lower 50,000, for example, from 20,000 to 50,000.

In an additional aspect of the present invention, the grinding aid is a crown ether compound bearing at least one aromatic ring, for example, dibenzo-18-crown-6.

The present invention also encompasses the use of the described compounds in the manufacture of high-quality graphene nanoplatelets .

The ground mixture of graphite with the at least one grinding aid may be further separated to recover graphene nanoplatelets and the grinding aid. The separation may be effected by dissolving the grinding aid in a suitable solvent, followed by filtration to recover graphene nanoplatelets and a solution of the grinding aid in the solvent. Alternatively, the separation may also be effected by Soxhlet extraction technique to remove less soluble grinding aids . Definitions

The following terms should be construed as described below, unless the context clearly dictates otherwise.

The term "high-quality graphene" as used herein should be construed as graphene exhibiting few or no defects, in particular few or no basal defects, detectable by Raman spectroscopy, thermogravimetric analysis, or X-ray photoelectron spectroscopy (XPS) analysis. The term is particularly used to describe graphene nanoplatelets with no or very low degree of oxidation, basal defects, and low polydispersity of sizes, as generally demonstrated in the Examples section herein.

The term "ball-milling" as used herein should be construed as generally known in the art, i.e. a process of particle size reduction in an apparatus adapted to provide a plurality of impacts and shear actions on the particles to be ground with grinding media, e.g. balls, and/or the walls of the apparatus. The term includes, inter alia, the use of conventional rotational ball mills, planetary ball mills (i.e. high-energy ball mills) and attritors.

The terms "grinding aid", "diluents", "additive", and the like, as used interchangeably herein, should be construed as a material added to graphite before or during the ball milling process. More specifically, the grinding aid is a material capable of stabilizing exfoliated graphene sheets and/or preventing their agglomeration and controlling the milling energy. Preferably, the grinding aids are materials containing aromatic carbon systems as herein described. The ball milling of graphite, preferably a natural graphite, with an aromatic grinding aid may be performed as known to the skilled artisan. Generally, the adjustable parameters of the process include the diameter and the weight of the grinding media, i.e. balls, the hardness of the grinding media, usually influenced by the material of which the grinding media is made, the weight ratio between the powder to be ground and the grinding media, the rotation parameters specific to the apparatus, such as rotation of the ball mill shaft, planetary rotation of the ball mill bowl, and/or rotation of the attrition shaft, the grinding time and, in some cases, the gaseous atmosphere and the temperature. Due to the advantages of the present invention, the process may be performed at ambient atmosphere and temperature, e.g. as shown in the Examples section .

Generally, the balls will occupy from about 10% to about 50% of the bowl volume, and may account to any suitable number according to the process characteristics. For example, a 20-mL bowl may accommodate from about 7 to about 200 balls, each having a diameter ranging from about 3 to 20 mm and weight from about 0.06 to 12 grams, typically 10 balls, e.g. zirconia balls, of about 10 mm in diameter and weighing about 3 grams each. Generally, grinding media may be obtained in a variety of materials, shapes, sizes, sizes ranging from as low as 250 micrometers up to 25 millimeters. In a further exemplary embodiment, an attritor tank of 5.7 liters may accommodate about 3.8 liters of grinding media and about 3 liters of the grinding mixture. The grinding media may be made of any suitable material. The non-limiting examples of suitable materials for grinding media (e.g. milling balls, beads, ballcones or cylinders) are polypropylene, stainless or tempered steel, zirconia, ceramics, silicon nitride, corundum (alumina), tungsten carbide and agate. Likewise, the milling bowl may be coated or integrally formed with the same or different materials, including but not limited to, Teflon®, ceramics or stainless steel.

Usually the weight of the grinding media significantly exceeds the weight of powder (graphite and additives) to be ground. The weight ratio between the grinding media and powder may therefore be between 10:1 and 100:1, preferably between 10:1 and 50:1, more preferably between 25:1 and 35:1, adjustable according to the scale and the needs of specific equipment. Likewise, the rotation velocities of various parts of the ball mills may be adjusted according to the specific needs of the equipment. It is important that the velocities do not exceed a value at which the grinding media is carried away by the centrifugal force without creating an impact. This threshold value is specific for the materials of the mill chamber, the grinding media and the build of the mill, and can be determined by routine experimentation.

Graphite and the grinding aids may be ground for any suitable time interval. Generally, using the grinding aids of the present invention it was unexpectedly found that high-quality graphene nanoplatelets may be obtained after a variety of short processing times, ranging from several minutes to several hours, with very little or no oxidation. The process generally utilizes natural graphite flakes as the source of graphene nanoplatelets . Graphite may be supplied in a variety of particle forms and sizes, such as flakes or ground powder, e.g. with at least 75% of the bulk being of a size generally smaller than 150 micrometers (μπι) , but may also be smaller than 125 μπι, 100 μπι, 75 μπι, 45 μπι, and 20 μπι. The typical purity of natural graphite is 80-90% w/w, and sometimes above 92 %w/w, above 95 %w/w, above 98 %w/w, above 99 %w/w or above 99.5 %w/w. For example, commercially available graphite may be used, having the sublimation temperature of above 3,000°C and the density from about 1.5 to about 2.1 grams per cubic centimeter. Expanded or intercalated graphite may also be used, although the processes of the present invention do not require the addition investment into expansion or intercalation; moreover, some expansion techniques of graphite may increase the amount of defects in product graphene and should therefore be avoided.

Additionally or alternatively, low-quality graphene nanoplatelets may be used as a source of high-quality graphene nanoplatelets in the process of the present invention. It has been unexpectedly found that subjecting commercially available graphene nanoplatelet products to ball-milling with the grinding aids of the present invention improves its quality by lowering the polydispersity of the particle sizes, and reduction of the number of its layers. That is, the low-quality graphene nanoplatelets may be changed into a powder of smaller particle size (shown by changes of T½, as described below) with lower polydispersity (shown by changes of ΔΤ, as described below) and smaller thickness (shown by changes of A(2D) of Raman, as described below) . The term "low-quality graphene" as used herein in reference to a possible starting material, should be construed as graphene nanoplatelets with more than 10 carbon layers, e.g. 15-40 carbon layers.

While the use of grinding aids for dry ball-milling of graphite is known, the present inventors have discovered that many of the prior art grinding aids, especially water- soluble grinding aids, fail to furnish a high-quality graphene due to extensive oxidation and defects formation. The present inventors, however, have unexpectedly found that it is possible to overcome these drawbacks by using certain types of grinding aid as herein disclosed. Thus, the grinding aids suitable for use in the present invention may be characterized by several features: generally, they are generally insoluble in water, i.e. have a solubility in water that is lower than 3.0 g/L, e.g. less than 2.0 g/L or 1.0 g/L, preferably at room temperature, but preferably soluble in a variety of organic solvents, such as at least one of toluene, dichloromethane, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide and/or acetone, to a satisfactory extent. The solubility in at least one organic solvent may be, for example, above 1.0 g/L, preferably above 100 g/L, preferably at room temperature. Further, the grinding aids are generally aromatic compounds.

Generally, the suitable grinding aids contain at least two aromatic rings which are fused together, or connected with at least one linking moiety. More specifically, the suitable grinding aids may be described with a general formula I, and further selected using the solubility and the melting point criteria set forth above. The general Formula (I) is presented below:

(Rl_xAr_n)L_m

Formula (I)

wherein Ar is an aromatic ring, e.g. having 5, 6, 7 or 8 atoms, e.g. carbon or non-carbon members; preferably, Ar is six-membered aromatic ring, e.g. phenyl ring;

wherein Rl is a substituent on Ar ring and x is an integer between 0 and the number of atoms, e.g. carbon atoms, on said Ar aromatic ring; x is an integer, preferably 0, 1, 2 , 3 , 4 , 5 , or 6 ;

wherein each Rl is independently selected from a halogen, a carboxylic group, a linear or branched Ci-Cs alkyl, alkenyl, or alkynyl, an amine group, a sulfonic acid group;

wherein L is a linker connecting independently at least two Ar groups, each L being selected independently from null, a single bond, a multivalent radical containing 1 to 12 carbon or non-carbon members, wherein, for example, the non-carbon members comprise oxygen and sulfur, e.g. C1-C6 alkylene, alkenylene, alkynylene,

-0- (CH2) p-O- (CH2) q-O-, wherein p and q are integers from 1 to 3; optionally branched or substituted with a halogen;

wherein n and m are integers; wherein n is equal to or larger than 2; and wherein m is equal to or less than n.

Exemplary suitable grinding aids may be selected using the Formula I above. It is understood that for non-substituted aromatic molecules R1=H and x equals to the corresponding number of free valences of the aromatic compound. When the linker L is null, fused polyaromatic compounds are provided. The suitable exemplary compounds are naphthalene (n=2), anthracene (n=3), pyrene (n=4), dibenzanthracene (n=6), and diindenoperylene (n=9) .

When m is 1, and L linker is a single bond, the suitable exemplary compounds include biphenyl (n=2) .

When m is 1, and L linker is a multivalent alkane radical, e.g. bivalent (an alkylene), in which case the suitable exemplary compounds include diphenyl methane (n=2, L: - CH2 - ), diphenyl ethane (n=2, L: - CH2- CH2- ) ; or trivalent, e.g. triphenyl methane (n=3, L= -CH<) ; or tetravalent, e.g. tetraphenyl methane (n=4, L= >C<) .

When m is 1 and L linker is a multivalent alkene radical, e.g. bivalent (an alkenylene), the suitable exemplary compounds include diphenyl ethylene (n=2, L= -CH=CH-), and diphenyl butadiene (n=2, L= -CH=CH2 - CH2=CH- ) ; or trivalent, e.g. triphenyl ethylene (n=3, L= >C=CH- ) , or tetravalent, e.g. tetraphenyl ethylene (n=4, L= >C=C<) .

When m is 1 and L linker is a multivalent alkyne radical, e.g. bivalent (an alkynylene), the suitable exemplary compounds include diphenyl butadiyne (n=2, L= -C≡C-C≡C-) . When m=2 and L is a multivalent alkyne radical, the suitable exemplary compounds include 1,4-bis-

(phenylethynyl ) -benzene (n=3, L1=L2 = -C≡C-) .

While generally for linear compounds n=m+l, there are some special cases where n=m, e.g. when a cyclic structure is formed, for example when two Ar are connected with two linkers L. Thus, when m is 2, suitable compounds include fluorene (n=2, Ll= (-), L2= -CH2-), and dibenzo-18-crown-6 ether (n=2, L1=L2 = -O-CH2-CH2-O-CH2-CH2-O- ) . Another example is when m is 4, in which case suitable compounds include porphyrine (n=4, L= =CH-) .

It should be understood that formula I above also encompasses polymers, in which case m indicates the degree of polymerization, and L and Ar together form a repeat unit with single aromatic moiety (n=l for a repeat unit) . For example, L are linked to form the backbone of the polymer, and Ar are the pendant groups, e.g. polystyrene (L=-CH*-CH2- , Ar is -C6H5, and m=n) .

Some derivatives may be perfluorinated derivatives, such as perfluoro pentacene (n=5, L=null, R1=F) . Suitable derivatives may also include benzo- derivatives, i.e. wherein Rl is benzo- group. More complex derivatives may include pyrene butyric acid (n=4, L=null, Rli- (_X-i) =H, Rl_x= - (CH2)3COOH), perylenetetracarboxylic diimide (n=4, L1=L2= (- ) , Rli-(x-4)=H, Rl(x-3)-(x)= bridged - (C=0) -N- (C=0) - ) , and methyl orange (n=2, L= -N=N- , Rli= -N(CH₃)₂, Rl2= -S0₃Na, R1₃-_X=H) .

Without being bound by theory, it is believed that in addition to impact control by the grinding aid, the interaction between delocalized π-electrons of the grinding aid molecules and the freshly exposed surfaces of the graphene nanoplatelets stabilize the graphene layers, and the hydrophobic nature thereof repels oxygen and thus protects from the oxidation, amorphyzation and reagglomeration .

Particularly preferred grinding aids are pyrene, naphthalene, dibenzo-18-crown-6 ether, and low-MW polystyrene .

As shown above, the grinding aid suitable for the present invention may also contain a polymer bearing aromatic groups or fused aromatic groups as side chains or in the backbone. While it is known in the art that some such polymers, such as polystyrene with molecular weight 280,000 are unsuitable for dry ball-milling of graphite, the present inventors have unexpectedly found that utilizing low- and/or intermediate- molecular-weight polymers may be suitable for production of high-quality graphene by dry ball-milling of graphite. Such polymers are characterized by having molecular weight generally lower than 150,000, e.g. lower than 100,000 or lower than 50,000, and are exemplified by polystyrene.

The grinding aid suitable for the present invention may also contain crown ethers bearing at least one aromatic group (e.g. aromatic ring) or fused aromatic group. The examples of such crown ethers include dibenzo-18-crown- 6, dibenzo-15-crown-5, dibenzo-21-crown- 7 , dibenzo-24-crown-8 , and dibenzo-30-crown- 10. Preferably, the crown ether is dibenzo-18-crown- 6 ether.

The grinding aids may be used in any acceptable ratio with graphite, but a significant excess is particularly preferred. Thus the weight ratio between the grinding aid and graphite may be between about 7:1 to about 200:1, preferably between about 20:1 to about 50:1.

Whereas generally one grinding aid may be sufficient to produce adequate amounts of high-quality graphene nanoplatelets , it may sometimes be possible or even beneficial to use more than one grinding aid in the same process. When two or more grinding aids are used in the process, these may be added simultaneously or separately during the process, according to the requirements of the specific equipment and/or the process. Moreover, when two or more grinding aid is used in the process, it may not be necessary for all the grinding aids to meet the requirements as set forth above. For example, it may be sufficient that at least one grinding aid be selected from the group of planar polyaromatic compounds or crown ethers. Without being bound by theory, it is believed that it may be beneficial to use a mixture of grinding aids to augment or to confer the protection against chemical modification and/or defects formation in the high-quality graphene produced by the process, and to improve the process efficiency .

The high-quality graphene nanoplatelets obtained by the processes of the present invention may be separated from the grinding aid. This may be achieved, inter alia, by dispersing the resultant product in a sufficient amount of a solvent that dissolves the grinding aid, and by filtering off the solution of the grinding aid, retaining the high- quality graphene nanoplatelets on the filter. Alternatively, the ground mixture of graphene nanoplatelets and the grinding aid may be subjected to Soxhlet extraction to obtain the grinding aid in the solvent phase, and product graphene nanoplatelets in retained on a suitable filter in the thimble. The grinding aid may be further recovered from the solution by solvent evaporation, by crystallization, or by any other technique as known to the skilled artisan. When two or more grinding aids are used, these may be removed concomitantly by the same solvent, or consecutively, repeating the dispersing and filtering steps or Soxhlet extraction step for each solvent dissolving each of the respective grinding aids.

In order to differentiate between the starting material graphite and the product high-quality graphene nanoplatelets and to characterize the latter, electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis may be used.

Imaging includes scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) . These techniques are used for the characterization of lateral dimensions and thickness of GNPs particles. Raman spectroscopy assists in evaluating the concentration of defects and the number of graphene layers in FLG. X-ray photoelectron spectroscopy (XPS) provides information on the atomic composition of the surface of GNP powder. In addition XPS analysis of carbon band assists in differentiation of C-sp2 (e.g. aromatic carbon) , C-sp3 (non aromatic, amorphous carbon) and oxidized carbons.

Thermogravimetric analysis (TGA) enables rapid and simple characterization of bulk properties of materials. With relevance to graphene nanoplatelets , it is presently mostly used to determine the amount of impurities (e.g. residual grinding media, water, and inorganic residues), labile functional groups or traces of surfactants (following surfactant-assisted exfoliation) in nanocarbon powders. A novel TGA procedure for determination of the characteristics of graphene nanoplatelets was developed recently by us, and shows good correlation with spectroscopic and statistically significant bulk microscopic data (M. Stein, I. Pri Bar, M. Varenik and 0. Regev., Analytical Chemistry, Analytical Chem. ,2015, 87(8) 4076-80, DOI : 10.1021/acs . analchem.5b00228) . Briefly, a suitable amount, e.g. 2-6 mg of a sample, is placed into a suitable crucible, e.g. 70-μ1 alumina crucible, and heated at air flow 50 ml/min and thermal program from 40 to 500°C at 10°C/min, followed by either 500 to 1000°C at 5°C/min, or 500 to 800°C at 5°C/min and 800 to 1000°C at 10°C/min, optionally followed by isotherm of 1000°C for 30 minutes. The combustion temperature range (ΔΤ) and the "half- combustion" temperature (T½) (i.e. temperature in the middle of the combustion range) were found especially useful for distinguishing graphene from graphite and amorphous carbon. Generally, the materials with half- combustion temperature between about 550 and 630 °C may be attributed to activated amorphous carbon; between about 630 and 730 °C may be attributed to graphene nanoplatelets; and between 830 and 1000 °C may be attributed to graphite flakes .

Fig. 1, presents the combined thermograms of pristine graphite flakes (GF) , graphite flakes (GF) following grinding with and without grinding aids (0 or 1000 mg of the grinding aid, 36 mg GF, 450 rpm, 2x30 min) . TGA conditions: air flow 50 ml/min, thermal program: 40-500°C at 10°C/min, followed by 500-800°C at 5°C/min, followed by 800-1000°C at 10°C/min, followed by isotherm of 1000°C for 30 minutes. Note that following milling there are no traces of pristine graphite and that the milling with diluents results in a product of narrower range of combustion temperature and without amorphous carbon products which undergo combustion around 500 °C.

Fig. 2, provides a SEM image of natural graphite flakes (GF) following grinding with diluents. The micron size of the lateral faces of product should be noted. Statistical analyses of sizes for over 100 particles performed on such two mixtures are shown in Fig. 3. This figure shows the abundance of lateral sizes of products which were obtained by grinding with diluent - based on SEM image statistics. The lateral sizes are in the 0.4-1.0 microns range.

Figs. 4 present an AFM image of graphite flakes following grinding with diluent. The thickness of 4.27 nm and the lateral face of less than 200 microns are shown. The graph (Fig. 4A) and the image (Fig. 4B) are presented.

Figs. 5 show typical Raman spectra of graphite flakes following grinding with a diluent. The low wave number of 2D band at 2696 cnr¹ (Fig. 5A) assigned to few layer graphene is indicative of superior quality, as the 2D band for multilayered graphite is about 2727 cnr¹, while 2D band at about 2685 cnr¹ is indicative for single-layer graphene. The ratio of the intensity of the D band (around 1350 cnr¹) to G band (around 1550 cnr¹) (Fig. 5B) is indicative to the concentration of defects. The ID/IG should be low as possible, low ID/IG values of 0.11-0.22 were obtained with appropriate diluents.

For example, few layer graphene of the present invention has 1-10 layers of graphene, and preferably 1-5 layers, exhibiting one or more of the following characteristics when analyzed using described methodology: T1/2 of major component above 630°C, preferably between 630°C and 730 °C; and/or ΔΤ of the major component lower than 180 °C, preferably less than 150 °C; and/or amorphous carbon content of less than 10% by weight, e.g., less than 5 % by weight, preferably less than 3 % by weight; all as indicated by TGA; and/or ID/IG values of less than 0.25; and/or A(2D) lower than 2705 cnr¹; both as indicated by Raman spectroscopy. The aforementioned few-layers graphene forms another aspect of the invention.

Examples

Example 1 - preparation of graphene nanoplatelets by ball- milling with sodium chloride, comparative example

GNPs were produced by mixing natural graphite flakes (Aldrich Sigma) of a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content), 0.036 grams, and sodium chloride 1.0 gram, obtained from Sigma Aldrich. The powder was subjected to planetary ball milling in Fritch Pulverisette P7 premium line (140 mm diameter of main disk) ; grinding performed in a 20 ml Zirconia bowl with 10 Zirconia balls (10-mm diameter each, weighing about 3 g each), operated at 450 rpm, for two periods of 30 minutes each) .

The resulting powder was washed 4 times with 25 mL water for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman, and XPS . The isolated yield of the product was 94 wt% of the initial graphite and TGA indicated it contained 92 w% carbon.

TGA results: carbon yield 92 wt%, amorphous carbon 13 wt%,

AT=206°C, T½ = 637°C.

Raman λ (2D) ¾ 2700 cnr¹ I_D/IG=0.36

XPS: sp2 58 wt%, oxidized carbon 4 wt% .

These data show heterogeneous product comprising graphene of high polydispersity, containing oxidized graphene and amorphous carbon. Example la - preparation of graphene nanoplatelets by ball- milling without diluent , comparative example

GNPs were produced by mixing graphite flakes (Aldrich Sigma) of a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content), 0.18 grams. The powder was subjected to planetary ball milling in Fritch Pulverisette P7 premium line (140 mm diameter of main disk) ; grinding in a 20 ml Zirconia bowl with 10 Zirconia balls (10-mm diameter each, weighing about 3 g each) , operated at 450 rpm, for two periods of 30 min each) .

The resulting powder was washed 4 times with 25 mL water for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman and XPS . The isolated yield of the product was 94 wt% of the initial graphite and TGA indicated it contained 92 wt% carbon.

TGA results: carbon yield 92 wt%, amorphous carbon 36 wt%, ΔΤ>260°Ο, T½ = 650°C.

Raman λ (2D) ¾ 2700 cm^"1, I_D/IG= 0.36.

XPS: sp2 58 wt%, oxidized carbon 4 wt% .

These data show heterogeneous product comprising graphene of very high polydispersity, containing oxidized graphene and high percent of amorphous carbon.

Example 2 - Preparation of graphene by ball-milling with pyrene

GNPs were produced by mixing graphite flakes having a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content), 0.036 grams and pyrene 1.0 gram, obtained from Alfa Aesar. The powder was subjected to planetary ball milling in Fritch Pulverisette P7 premium line (140-mm diameter of main disk) ; grinding in a 20 ml bowl with 10 zirconia balls (10-mm diameter each and weighing about 3 g each, operated at 450 rpm for two periods of 30 min each) .

The resulting powder was washed 4 times with 25 mL acetone for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman, XPS, SEM image and AFM. The yield of the product was 95 wt% of the initial graphite and TGA indicated it contained 93 wt% carbon.

TGA results: carbon yield 93 wt%, AT=121°C, T½ =679°C, amorphous carbon >2 wt% .

Raman λ (2D) ¾ 2700 cm^"1, I_D/IG= 0.21.

XPS: sp2 75 wt%, oxidized carbon 0 wt% .

These data show uniform product consisting essentially of few-layered graphene with no oxidized sheets, amorphous carbon or unexfoliated graphite.

Example 3 - Preparation of graphene nanoplatelets by ball- milling with pyrene, additional procedure

A process for producing GNPs was performed by mixing graphite flakes having a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content) (Sigma Aldrich) 0.07 grams, and pyrene (Alfa Aesar) 2.0 grams, were subjected to planetary ball mill grinding in a 20 ml bowl with 10 balls as in the Example 2. Grinding was performed at 600 rpm for 30 min. The resulting powder was washed 4 times with 25 mL of acetone for 15 minutes and filtered on a membrane of 0.2 μηη pore size. The precipitate was dried at 100°C for 3 hr and analyzed by TGA, Raman spectra, XPS, SEM image and AFM. The yield of the product was 99 wt% of the initial graphite and TGA indicated it contained 95 wt% carbon.

Total process yield 99 wt% .

TGA: carbon yield 93 wt%, AT=121°C, T½= 678 °C, amorphous carbon >2 wt% .

Raman λ (2D) ¾2708 cm^"1, I_D/IG= 0.38.

XPS: sp2 70 wt%, oxidized carbon >1 wt% .

These data indicate a uniform product consisting essentially of few-layered graphene nano platelets, albeit somewhat thicker than in the Example 2, with small amount of oxidized graphene, and no amorphous carbon or unexfoliated graphite.

Example 4 _^ Preparation of graphene nanoplatelets by ball- milling with low-MW polystyrene

The process for producing GNPs was performed as described in Example 2. Graphite flakes, 0.036 grams, and ground polystyrene powder, 1.0 grams, (average mw 35000, supplied by Sigma Aldrich, cat #331651) . Grinding of mixture was performed twice at 450 rpm for two periods of 30 min each) . Ground polystyrene was prepared by grinding pristine polystyrene with 10 zirconia balls at 450 rpm for 15 minutes . The resulting powder mixture was washed 4 times with 25 mL of toluene for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman spectra, and XPS . The yield of the product was 86 w% of the initial graphite.

TGA results: carbon yield 96 wt%, amorphous carbon 9 wt%, AT=160°C, T½= 658°C.

Raman λ (2D) ~ 2704 cnr¹ I_D/IG= 0.24.

XPS: sp2 64 wt%, and oxidized carbon > lwt%

These data indicate a slightly less uniform product and more of amorphous carbon than that of Example 2, but still consisting essentially of few-layered graphene, with no oxidized graphene sheets, but some amorphous carbon and no residue of pristine graphite.

Example 5 - Preparation of graphene nanoplatelets by ball- milling with high-MW polystyrene - comparative

The process for producing GNPs was performed as described in Example 4 above, with the following alterations. Polystyrene with high molecular weight was used (~192,000, supplied by Sigma Aldrich, catalogue #430102) . Ground polystyrene was prepared by grinding pristine polystyrene with 10 zirconia balls at 500 rpm for 15 minutes.

The yield of the GNP product was only -48 %wt of the initial pristine graphite, about 28% leftover of graphite powder and 17% of the carbonaceous mixture containing active carbon and smaller GNP fragments. TGA results: Carbon yield 94 %wt, containing 45% of GNP AT=190°C, T½= 686°C; and 17 %wt of complex carbonaceous mixture of amorphous carbon and very small GNP fragments AT=420°C, T½= 535°C.

These data indicate a less efficient cleavage (e.g. graphite leftover) and less uniform fragmentation of graphite (e.g. more amorphous carbon and a complex mixture of very small GNP fragments) compared to that of Example 4, indicating an incomplete exfoliation due to higher molecular weight of polystyrene.

Example 6 - Preparation of graphene by ball-milling with low-MW polystyrene, additional procedure

The process for producing GNPs was performed as described in Example 4. Graphite flakes, 0.14 grams, and ground polystyrene powder, 4.0 grams were used.

The resulting powder mixture was washed 4 times with 50 mL of toluene for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman spectra, and XPS . The yield of the product was 90 w% of the initial graphite and TGA indicates it contains 88 w% carbon.

TGA results: carbon yield 88 wt%, ΔΤ=202°Ο, T½= 700°C, amorphous carbon 3 wt% .

Raman λ (2D) ¾ 2710 cm^"1, ID/I_G= 0.21.

XPS: sp2 70 wt% and oxidized carbon 1 wt%. Example 7 -- Preparation of graphene nano platelets by ball milling with dibenzo-18-crown-6

GNPs were produced by mixing graphite flakes of a size less than 100 mesh/150 μπι (¾95% carbon content), 0.036 grams, and dibenzo-18-crown- 6, 1.0 gram, obtained from Alfa Aesar. The powder was subjected to planetary ball mill grinding in a 20 ml bowl with 10 balls as in the Example 2.

The resulting powder was washed 4 times with 25 mL of dichloromethane for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman and XPS . The yield of the product was 85 wt% of the initial graphite and TGA indicates it contains 92 wt% carbon.

TGA results: carbon yield 92 wt%, AT=123°C, T½ = 665°C, amorphous carbon 7 wt% .

Raman λ (2D) ¾ 2690cm-¹, I_D/IG= 0.29.

XPS: SP² 76 wt% and oxidized carbon 2 wt% .

These data show uniform product consisting essentially of few-layered graphene with low quantity of oxidized carbon and of amorphous carbon and no unexfoliated graphite.

Example 8 - Preparation of graphene nano platelets by ball- milling with low-MW polystyrene, upscale procedure

GNPs were produced by mixing graphite flakes having a size of (>75%) over 100 mesh /150 μπι («95% carbon content), 0.36 grams and pyrene 5.0 gram, obtained from Alfa Aesar. The powder was subjected to planetary ball mill (Fritch Pulverisette P6 classic line) . Grinding performed in a 80 ml stainless steel bowl with 40 stainless steel balls (10- mm diameter each, weighing about 4 g) , operated at 450 rpm for 3 periods of 30 min each.

The resulting powder was washed 4 times with 75 mL of toluene for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 80°C for 3 hours and analyzed by TGA, Raman and XPS . The yield of the product was 95 wt% of the initial graphite and TGA indicated it contained 90 wt% of carbon.

TGA results: carbon yield 90 wt%, ΔΤ= 96°C, T½ =646°C, amorphous carbon 4 wt% .

Raman λ (2D) ¾ 2690 cnr¹ I_D/IG= 0.30.

XPS: sp2 66 wt%, oxidized carbon 3 wt% .

These data show very uniform product consisting essentially of few-layered graphene with some oxidized sheets, some amorphous carbon and no unexfoliated graphite.

Example 9 - Preparation of graphene nano platelets by ball- milling with anthracene

GNPs were produced by mixing graphite flakes having a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content), 0.036 grams and anthracene 1.0 gram, obtained from Alfa Aesar. The powder was subjected to planetary ball mill Fritch Pulverisette P7 premium line (140-mm diameter of main disk) grinding in a 20 ml bowl with 10 zirconia balls (10-mm diameter each and weighing about 3 g each, operated at 450 rpm for two periods of 30 min each) . The resulting powder was washed 4 times with 25 mL of dichloromethane for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman, XPS, SEM image and AFM.

TGA results: carbon yield 88 wt%, ΔΤ= 162.6 °C, T½ = 664.5

°C, amorphous carbon 3.8 wt% .

Raman λ (2D) ¾ 2703 cm^"1, I_D/IG= 0.28.

Example 10 - Preparation of graphene nano platelets by ball-milling with diphenyl butadiyne

GNPs were produced by mixing graphite flakes having a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content), 0.036 grams and diphenyl butadiyne 1.0 gram, obtained from Acros Organics. The powder was subjected to planetary ball milling with Fritch Pulverisette P7 premium line (140-mm diameter of main disk) grinding in a 20 ml bowl with 10 zirconia balls (10-mm diameter each and weighing about 3 g each, operated at 450 rpm for two periods of 30 min each) .

The resulting powder was washed 4 times with 25 mL of dichloromethane for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, and Raman.

TGA results: carbon yield 93 wt%, ΔΤ= 168.6 °C, T½ = 672.3

°C, amorphous carbon 10 wt% .

Raman λ (2D) ¾ 2700 cm^"1, I_D/IG= 0.20. Example 11 - Preparation of graphene nano platelets by ball-milling with naphthalene

GNPs were produced by mixing graphite flakes having a size of (>75%) over 100 mesh /150 μπι (¾95% carbon content), 0.036 grams and naphthalene 1.0 gram, obtained from Acros Organics. The powder was subjected to planetary ball milling in Fritch Pulverisette P7 premium line (140-mm diameter of main disk) grinding in a 20 ml bowl with 10 zirconia balls (10-mm diameter each and weighing about 3 g each, operated at 450 rpm for two periods of 30 min each) .

The resulting powder was washed 4 times with 25 mL of acetone for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, and Raman.

TGA results: carbon yield 89 wt%, ΔΤ= 135 °C, T½ = 661.3

°C, amorphous carbon 2.3 wt% .

Raman λ (2D) ¾ 2696 cm^"1, I_D/IG= 0.22

Example 11 - manipulation procedure of commercial GNP

GNPs were manipulated by mixing GNP Type H25 0.038 grams (obtained from XG Sciences) having a mean lateral diameter of 25 μπι and thickness of 15 nm) and pyrene 1.0 gram, obtained from Alfa Aesar. The powder was subjected to planetary ball milling in Fritch Pulverisette P7 premium line (140-mm diameter of main disk) grinding in a 20 ml zirconia bowl with 10 zirconia balls (10-mm diameter each), operated at 250 rpm for 15 min.

The resulting powder was washed 4 times with 25 mL of acetone for 15 minutes and filtered on a membrane of 0.2 μπι pore size. The precipitate was dried at 100°C for 3 hours and analyzed by TGA, Raman, and XPS . The yield of the product was 83 wt% of the initial GNP and TGA indicated it contained 99 wt% carbon.

TGA results: carbon yield 99 wt%, AT=152°C, T½ =694°C, amorphous carbon 9 wt%

Raman λ (2D) ¾ 2702 cm^"1, I_D/IG= 0.3.

XPS: sp2 79.8 wt%, oxidized carbon 3 wt% .

The pristine GNP (H25) has shown the following results in the same tests:

TGA: ΔΤ=220°Ο, T½ =752°C, amorphous carbon 4 wt%

Raman λ (2D) ¾ 2712 cm^"1, I_D/IG= 0.1.

XPS: sp2 78.8 wt%, oxidized carbon 2 wt% .

These data show that following the milling procedure the GNPs were changed into a powder of smaller particle size (shown by changes of T½) with lower polydispersity (shown by changes of ΔΤ) and smaller thickness (shown by changes of A(2D)) . A small increase in quantities of amorphous and oxidized carbon was detected.

Example 12 - recycling of grinding aid

The washings of several runs with same grinding aid (pyrene) were collected, united and dried using rotor evaporator. Pyrene was recovered quantitatively.

Claims

Claims

1. A process for the manufacture of 1-10 layer graphene comprising dry ball-milling of graphite with at least one aromatic ring-containing grinding aid with a molecular weight less than 150,000 grams per mole.

2. The process of claim 1, wherein the grinding aid has a solubility in water less than about 3.0 g/L at room temperature .

3. The process of any one of the claims 1-2, wherein the grinding aid has a solubility in at least one organic solvent of more than 1.0 g/L.

4. The process of any one of the claims 1-3, wherein the grinding aid contains at least two aromatic rings which are fused together, or connected with at least one linking moiety .

5. The process of claim 4, wherein the grinding aid has a structure according to the general Formula I:

(Rl_xAr_n)L_m

Formula (I)

wherein

Ar is an aromatic ring;

Rl is a substituent on Ar ring and x is an integer between 0 and the number of atoms on said Ar aromatic ring;

wherein each Rl is independently selected from a halogen, a carboxylic group, a linear or branched C1-C8 alkyl, alkenyl, or alkynyl, an amine group, a sulfonic acid group; wherein L is a linker connecting independently at least two Ar groups, each L being selected independently from null, a single bond, a multivalent radical containing 1 to 12 carbon or non-carbon members, optionally branched or substituted with a halogen;

wherein n and m are integers; wherein n is equal to or larger than 2; and wherein m is equal to or less than n.

6. The process of any one of claims 1-5, wherein the grinding aid is selected from the group consisting of a fused polyaromatic compound, a crown ether compound bearing at least one aromatic ring, diphenyl C1-C6 alkylene, diphenyl C1-C6 alkenylene, or diphenyl C1-C6 alkynylene, and mixtures of two or more of the above.

7. The process of claim 6, wherein the grinding aid is a fused polyaromatic compound.

8. The process of claim 7, wherein the fused polyaromatic compound is selected from the group consisting of pyrene, naphthalene or anthracene.

9. The process of claim 6, wherein grinding aid is diphenyl C1-C6 alkynylene.

10. The process of claim 9, wherein the diphenyl C1-C6 alkynylene is diphenyl butadiyne.

11. The process of any one of the claims 1-5, wherein the grinding aid is a polymeric compound.

12. The process of claim 11, wherein the polymeric compound is polystyrene having molecular weight lower than 100,000.

13. The process of claim 6, wherein the crown ether compound is dibenzo-18-crown- 6 ether.

14. The process of improvement of low-quality graphene of more than 10 carbon layers, comprising dry ball-milling of low-quality graphene with at least one aromatic ring- containing grinding aid with a molecular weight less than 150,000 grams per mole.

15. Few layer graphene of 1-10 carbon layers obtainable by a process of any one of claims 1-14.

16. Use of aromatic-ring containing compound with a molecular weight less than 150,000 grams per mole as a grinding aid in production of graphene by ball-milling of graphite or low-quality graphene.