WO2017025973A1

WO2017025973A1 - Graphene manufacturing method

Info

Publication number: WO2017025973A1
Application number: PCT/IL2016/050883
Authority: WO
Inventors: Matat BUZAGLO; Oren Regev; Michael SHTEIN
Original assignee: B.G. Negev Technologies And Applications Ltd., At Ben-Gurion University
Priority date: 2015-08-13
Filing date: 2016-08-11
Publication date: 2017-02-16

Abstract

Provided herein a process of graphite exfoliation from graphite dispersion in a liquid medium in the presence of a surfactant, comprising intermittently applying sonication energy to produce graphene in the liquid, wherein said intermittently applying sonication comprises holding at least a portion of said dispersion at static conditions during the intermissions. An apparatus for carrying out the process is also described.

Description

Graphene manufacturing method

Field of the invention

The present invention relates to processes and systems for producing pristine graphene. More specifically, the invention relates to a process of graphite exfoliation with the aid of ultrasound, to form graphene.

Background of the invention

Graphene is a material with unique properties that are very appealing in the field of material engineering. Graphene is a planar crystalline form of carbon; stacked graphene sheets are known as graphite.

Various methods were developed to obtain graphene from graphite, including exfoliating graphite sheets with adhesive tape, chemically modifying (oxidizing) graphite to graphite oxides which are more easily stabilized in solution, and many others. Chemical vapor deposition methods have also been evaluated in graphene production. Exfoliating graphite into graphene by ultrasonication in presence of various solvents was investigated as well .

Graphene was also obtained by ultrasonication of graphite in aqueous solutions of surfactants. For example, US patent number 7,824,651 describes direct conversion of graphite into graphene in water in presence of polyethoxylated perfluoroethanol (Zonyl™ FSO) or polyethoxylated isooctyl phenol (Triton™ X-100) . A similar process is described in US patent application 2014/0235123, with somewhat higher sonication energies. A slightly more elaborate process is described in US patent application 2009/0022649. Graphite is first turned into an intermediate thin nano-scaled graphite platelets (NGP) material at low sonication energy, which in turn is then converted into graphene with further sonication at higher energy.

Somewhat similarly, WO 2014/122465 discloses processes of exfoliation of graphite into 5-10-layered graphene in a variety of media, including aqueous surfactant solutions, using at least two ultrasonic frequencies differing by almost an order of magnitude, consecutively. The present inventors have also previously reported ( Phys . Chem. Chem. Phys . , 2013, 15, 4428) exfoliation of graphite into few-layer graphene sheets using low-energy (ultrasonic bath) and high-energy (ultrasonic tip) sonication, to produce dispersion with high graphene content.

A further aspect of aqueous ultrasonic exfoliation of graphite is disclosed in WO 2013/010211. The significant emphasis is placed on the process of preserving low surface tension and thereby ensuring the constant supply of a surfactant to freshly- exfoliated platelets for prevention of their successive re- aggregation. The processes disclosed include continuous or noncontinuous addition of a surfactant at a predetermined efficacious rate, or discrete surfactant replenishment controllable by the surface tension measurements, during the sonication process.

Summary of the invention

The invention provides a process of graphite exfoliation in a liquid medium (preferably an aqueous medium) in the presence of a surfactant, under the intermittent application of sonication energy, to produce a dispersion of graphene in the liquid. It was found that the concentration of the graphene in the product dispersion is increased significantly when the sonication energy needed for exfoliating the graphite is delivered discontinuously, with alternating irradiation and repose steps, as compared with the case where the same amount of energy is delivered continuously. During a repose step at least a portion of the dispersion is not irradiated, and preferably is held under static conditions, e.g. it does not flow and/or is not in contact with moving elements.

Therefore, the invention is primarily directed to a process of graphite exfoliation from graphite dispersion in a liquid medium in the presence of a surfactant, comprising intermittently applying sonication energy to produce graphene in the liquid, wherein said intermittently applying sonication comprises holding at least a portion of said dispersion at static conditions during the intermissions. Preferably, the intermission consists of a repose interval, during which interval at least a portion of the dispersion is not subjected to ultrasonic irradiation, flow and agitation. The repose interval is preferably not less than 15 minutes, e.g. not less than 30 minutes, or not less than 45 minutes, or not less than 60 minutes .

The process of the present invention may be carried out by a) providing graphite dispersion comprising a graphite, at least one surfactant and water; b) applying to said graphite dispersion one or more cycles (e.g. at least two cycles, e.g., from 2 to 20 cycles), each cycle comprising a period of ultrasonic irradiation and a repose interval; and c) optionally applying to said graphite dispersion a final period of ultrasonic irradiation to furnish graphene dispersion. On completion graphite residues may be removed from the graphene dispersion to furnish a purified graphene dispersion. The removal of graphite residues may be achieved by centrifugation, sedimentation, or both. Ultimately, graphene may be recovered from the graphene dispersion or from the purified graphene dispersion .

The present invention is also directed to a process of graphite exfoliation, comprising combining graphite and at least one surfactant with water to form an aqueous graphite dispersion, circulating a stream of said graphite dispersion through a circulation line provided with at least one sonication chamber, periodically holding (e.g. for a duration of the repose interval as described herein) at least a portion of the ultrasonically irradiated dispersion under static conditions before it is returned back to said at least one sonication chamber, and collecting graphene dispersion. The circulation line is preferably in fluid communication with at least a first tank and a second tank, such that the process may be carried out by circulating the stream alternately through said first tank or said second tank while holding at least a portion of the ultrasonically irradiated dispersion under static conditions in the other tank. Additionally, the process may further comprise removing graphite residues from the graphene dispersion, returning the removed graphite to the circulating stream and collecting a purified graphene dispersion.

The present invention is also directed to a system for performing the above-disclosed process, as described in greater detail hereinbelow. For example, the apparatus for production of graphene comprises at least a first tank and a sonication chamber, which are in fluid communication with one another via a circulation loop, said sonication chamber comprising a vessel and at least one sonotrode operably linked to an ultrasound generator, said apparatus further comprises downstream processing device including a separation unit having at least one of a centrifuge, a lyophilizer, and/or a cross-flow filtration assembly, said separation unit being coupled via a discharge line to a storage container for holding graphene and optionally connected via a flow line to said circulation loop. The apparatus may further comprise at least a second tank and an array of valves to enable the circulation alternately through said first and second tanks.

The present invention is also directed to graphene obtainable by the disclosed processes.

Brief description of the drawings

Figure 1 schematically represents a system for practicing a process according to an embodiment of the present invention.

Figure 2 schematically represents a system for practicing a process according to another embodiment of the present invention .

Figure 3 graphically represents the dependence of graphene concentration in the graphene dispersion produced according to an embodiment of the present invention, on the repose time.

Figure 4 graphically represents the dependence of graphene concentration in the graphene dispersion produced according to an embodiment of the present invention, on the energy per volume (the energy density) delivered. Figure 5 schematically represents a system for practicing a continuous process according to yet further embodiment of the present invention.

Figure 6 graphically represents the results of experiments testing various graphite starting materials.

Detailed description of the invention

In its most general form, the invention relates to a process for graphite exfoliation in a liquid medium, preferably an aqueous medium in presence of at least one surfactant, wherein said process comprises an intermittent sonication of the graphite dispersion. The process of the invention is characterized in that it involves an intermittent sonication consisting of alternating irradiation and repose steps. During a repose step at least a portion of the dispersion is not exposed to sonication. Preferably, during the repose step at least a portion of the dispersion is not exposed also to flow, agitation and/or moving elements.

For example, one aspect of the invention provides a process comprising dispersing graphite in an aqueous surfactant solution and subjecting the graphite dispersion to intermittent sonication consisting of alternating steps of irradiation and repose, wherein during the ultrasonic irradiation steps sonication is applied to the dispersion, and during the repose steps the dispersion is placed under conditions whereat no ultrasonic irradiation is delivered to the dispersion.

The processes of the invention may be carried out by either a batch or a continuous mode of operation. In the batch mode of operation the repose step may be accomplished by switching off the sonication and agitation, if employed. In a continuous mode of operation a graphite dispersion, i.e. an aqueous graphite dispersion comprising at least one surfactant, is allowed to circulate between at least one holding tank and at least one sonication zone, and the repose step is accomplished by switching off the sonication and collecting the dispersion in the at least one holding tank, or by allowing a portion of the dispersion to flow into a separate repose tank and retain there for a sufficient time interval.

The term "graphene" as used herein should be construed to include essentially two-dimensional sheets of monoatomic carbon- layer particles, e.g. containing preferably from 1 to 10, and more preferably 1 to 5 carbon layers (so-called "few-layer graphene") .

The term "repose", more specifically "repose step" as used herein, is a step of placing and/or maintaining at least a portion of the graphite dispersion under sonication-free conditions for an effective time interval.

The term "graphite dispersion" as used herein, should be construed as a suspension of graphite in a liquid, preferably water, further comprising at least one surfactant. The graphite dispersion as used herein refers to the starting and the intermediate material of the processes. The graphite dispersion may further comprise varying amounts of pristine graphene.

The term "graphene dispersion", conversely, as used herein, refers to the final product of the sonication, as disclosed herein, e.g. the dispersion obtained after the alternating steps of irradiation and repose. Nevertheless, it may further comprise significant amounts of unexfoliated graphite and/or other impurities. The graphene dispersion may be subjected to work-up to recover graphene .

The process utilizes graphite as the source of graphene. Graphite may be supplied in a variety of particle forms and sizes, such as flakes or ground powder, with at least 75% of the bulk being of the 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 graphite is above 98% w/w, and sometimes above 99 %w/w, above 99.5 %w/w, above 99.9 %w/w, above 99.95 %w/w or above 99.99 %w/w. For example, commercially available graphite may also be used. The graphite may have a melting point of above 3,000°C. The graphite may have a density of about 1.9 grams per cubic centimeter, or up to 2.23 grams per cubic centimeter. The suitable graphite types include crystalline flake graphite, and hydrothermal graphite (known also as lump or vein graphite, e.g. as manufactured by Zenyatta, Canada) .

The liquid suitable for graphite dispersion of the present inventions is preferably water. Water suitable for the processes of the present invention may usually be deionized water. In some embodiments, the water is regular distilled water. Generally, the quality of water may be readily adjusted according to the process needs based on the compatibility with the process variables, requirements of the final product and economic considerations. Sometimes the liquid may be an organic solvent, or a hydro-organic solution.

The surfactants suitable for the processes of the present invention may be water-soluble surfactants. Suitable surfactants include, but not limited to, cationic surfactants, anionic surfactants and non-ionic surfactants. Illustrative cationic surfactants include quaternary ammonium salts, such as didodecyl dimethyl ammonium bromide, cetyl trimethyl ammonium bromide, quaternary pyridinium salts, and others. Examples of suitable anionic surfactants include alkyl sulfonate salts, such as sodium lauryl sulfate, carboxylic acid salts, sterane derivatives' salts, such as sodium cholate, and others. Examples of suitable non-ionic surfactants include polyethoxylated compounds, fatty acids and alcohols derivatives, sorbitan derivatives, and amphiphilic polymers. Preferably, the surfactant is a non-ionic surfactant. Sometimes, non-ionic surfactants include aromatic materials. Examples of polymeric surfactants are poloxamers, e.g. poloxamer 407, poloxamer 234, poloxamer 333, poloxamer 403, poloxamer 185; examples of aromatic surfactants are polyethoxylated isooctyl phenols, such as Triton™ X materials, e.g. Triton ™X (TX)-15, TX35, TX45, TX100, TX102, TX114, TX165, TX305, TX405 and TX705. Typically, at least one surfactant is used. However, the term "surfactant" should be construed to include also a mixture of surfactants as defined herein.

The ratio between graphite and the surfactant may have a significant impact on the process. The exact graphite/surfactant ratio is dependent on the nature of the surfactant and the needs of the process. The ratio may be, for example, from about 20 parts of graphite per 1 part of surfactant, to about 0.1 part of graphite per 1 part of surfactant, preferably from 12:1 to 0.5:1. In a preferred embodiment, the surfactant is an aromatic surfactant, e.g. polyethoxylated isooctyl phenols, such as Triton™ X100 and the ratio is from 12:1 to 8:1, e.g. around 10:1 graphite to surfactant. In another preferred embodiment, the surfactant is poloxamer, e.g. poloxamer 407 and the ratio is from 2:1 to 0.5:1, e.g. around 1:1.

Providing the graphite dispersion in a surfactant solution may be effected by a variety of means. Graphite may be dispersed in a pre-prepared surfactant solution; conversely, graphite and surfactant may be consecutively or concomitantly added into water and mixed until the surfactant is dissolved. The mixing/dispersing may be conveniently performed by the means known in the art, for example, by a reactor equipped with suitable agitation equipment. Alternatively or additionally, the dispersion may be performed by feeding the graphite into the surfactant solution and subjecting it to circulation through a system having a suitable flow rate to prevent the settling of the graphite. The concentration of graphite in the dispersion may be adjusted according to the needs of the specific process and the specific surfactant used; the concentration may generally be above about 0.2 %w/w.

The volume of the graphite dispersion may define the batch size. It has been long known in the art that the sonication processes are hard to upscale. The present inventors, however, have surprisingly found that the processes of the present invention may be conveniently scaled-up. With suitable adaptations to the process, the volume of the dispersion may therefore vary from about 10 milliliters to about 1,000 liters. More specifically, the volume may be from about 1 liter to about 10 liters, and may further be up to 50 cubic meters.

The graphite dispersion is exposed to at least two steps of sonication, with said steps being separated by a repose period. In some embodiments, the whole bulk of graphite dispersion is exposed simultaneously to the sonication for the duration of the sonication step. Alternatively, the graphite dispersion may be continuously or discontinuously fed through at least one zone subjected to continuous or discontinuous ultrasound irradiation. The repose step, for the purpose of these embodiments, should be construed as placing the dispersion for an effective time interval at static conditions, e.g. whereat the graphite dispersion is neither sonicated nor circulated through a system. Sometimes, the repose period may be specific to a quantum of the graphite dispersion. Generally, the repose step may be characterized in that at least one sonication zone, or all sonication zones, accordingly, are inactive, and essentially no flow exists in the system, for a sufficient time interval. Alternatively or additionally, the graphite dispersion may be split into at least two parts, wherein a first part of the graphite dispersion is being exposed to sonication and flow, and a second part being at repose.

A repose step may last longer than about 15 minutes. Preferably, the repose step lasts longer than about 30 minutes, and sometimes even longer than about 60 minutes. Consequently, placing the dispersion at the repose conditions for shorter than about 15 minutes, and sometimes shorter than 10 minutes, 5 minutes, 2 minutes or even 1 minute, should not be construed as repose steps for the purpose of the present invention. Therefore, two time intervals of irradiation separated by a short repose time constitute a single irradiation step.

At least two discrete ultrasound irradiation steps are separated by a repose step, as described above. The total number of irradiation steps may be adapted according to the needs of specific processes, materials and parameters, to furnish the suitable final product yield. The total number of irradiation steps may be 2 and more per a batch processing time, e.g. above 3 steps in 24 hours, separated by at least 2 repose steps. The duration of the sonication steps may vary from about 15 minutes to about 6 hours. The irradiation steps may have varying duration one from another, or may have an essentially constant duration. Similarly, the repose steps may be of equal or varying duration. Generally, the duration of the irradiation steps may be expressed as percentage of total irradiation time. The duration of a sonication step, may be, for example, over 1% of total irradiation time, sometimes over 2%, over 5%, over 10%, over 15%, over 20%, over 25%, or over 30% of the total sonication time; concomitantly, the duration of the sonication step may be no longer than 50% of total sonication time, preferably no longer than 35%, and may be less than 30%, 25%, 20%, 15% or 10%, thereby splitting the total irradiation process into at least 3 steps, and up to 100 steps. Therefore, the duration of a sonication step (either of equal or unequal duration) may be from 1% to 30%, from 5% to 20%, or from 7.5% to 15% of the total sonication time.

The ultrasonic irradiation steps may be characterized by the total energy delivered per volume unit (the energy density) over the irradiation time interval . The exemplary ranges of energy density per step may vary from about 50 kilo-Joules (kJ) per liter to about 10 mega-Joules (MJ) per liter. However, it may be more convenient to define the total ultrasound energy of the process. The exemplary values of the total energy may vary from about 0.5 MJ/L to about 20 MJ/L, and may reach 50 MJ/L. Sometimes a surfactant may not be stable at high sonication energies. For example, poloxamer 407 was found to degrade at sonication energies of above 10-12 MJ/L. Therefore, in some embodiments, where the surfactant is poloxamer 407, the total energy density of sonication is below 12 MJ/L. The skilled artisan should follow established techniques to determine the stability of a surfactant at high-end sonication energies.

The ultrasound irradiation may be generally provided by an ultrasound generator equipped, inter alia, with a suitable sonotrode. The ultrasound generator generates ultrasonic waves of a defined frequency. The present inventors have found that it may be sufficient to use a single frequency of ultrasound in conjunction with the processes of the present invention to furnish significantly superior graphene yields. Therefore, the ultrasonic irradiation may be performed in the at least one sonication areas by ultrasound of essentially similar or even identical frequencies. The term "essentially similar" in reference to the ultrasound frequencies should be construed as the variance of the ultrasound frequencies is lower than 20% from the average thereof. The exemplary frequencies may vary from about 10 kHz, e.g., 15kHz, to about 35 kHz, preferably about 20 kHz .

The processes of the present invention may be performed at ambient temperature and pressure. It may however, sometimes be convenient to control either the temperature, the pressure or both. Sometimes the temperature is kept constant, about 15 degrees Celsius (°C), but the processes may also be performed at other temperatures, for example from about 4°C to about 60°C, or at any other point below the cloud point of the surfactant used in the process, or below the boiling point of water, the lowest of these. Likewise, the pressure of the processes may be suitably controlled to perform the process either at ambient pressure, or at elevated pressure. The elevated pressure may be, for example, above 0.5 bar gauge (barg) . Preferably, the pressure is above 1.5, or above 2.0 barg. More specifically, the pressure may range between about 0.5 barg to 4.0 barg.

Upon completion of the last sonication step, a graphene dispersion is obtained. The graphene dispersion may further contain graphite residues. The residues may be removed by a variety of processes know to the skilled artisan. Sometimes the graphene dispersion may be separated by sedimentation over a suitable sedimentation time interval. Alternatively or additionally, the graphene dispersion may be separated from the graphite residues by means of centrifugation . For the centri fugation step a variety of equipment may be used. The process may be characterized by the multiples of the gravity force (g) and the duration. Usually, the centrifugation is at 4,000 g for about 20 minutes. However, the present inventors have surprisingly found that for the graphene obtainable by the processes of the present invention, it may be sufficient to centrifuge the graphene dispersion at significantly lower g- force, for example, lower than 3,000 g or lower than 2,000 g. More preferably, the centrifugation is at about 800-1,200 g. The centrifugation times required may vary from about 20 minutes to about 300 minutes. Preferably, the centrifugation duration is about 120-180 minutes. A stable dispersion with little or no further sedimentation may be thus obtained, for example, by centrifugation at 1,000 g for 150 minutes, at the graphene concentration higher than that obtained by centrifugation at 4,000 g for 20 minutes. Moreover, even lower g-force values may be used for centrifugation, such as from 400 g to 700 g, if the obtained graphene suspension is subjected to further sedimentation for a period ranging from 1 day to about 14 days.

The centri fugation and/or sedimentation processes may furnish pristine graphene suspension and graphite sediment. The graphite sediment may be further recycled and used as additional starting material for further exfoliation. Similarly, at least part of the surfactant may be recovered by techniques known to the skilled artisan, for example by recovery of the material by cross-flow filtration of the graphene dispersion, and likewise recycled in a consecutive process.

The obtained graphene suspension may be used for further applications as is, or it may be further processed into a form suitable for further application. The suspension may be filtered onto a suitable filter substrate to obtain recovered graphene supported on said filter. The filter may be any suitable water- resistant or organic-resistant membranes, e.g. cellulose, nylon or PTFE membranes. The suspension may also be lyophilized, to obtain recovered powder comprising graphene. For example, the suspension may be directly lyophilized when the surfactant is a poloxamer, for example, poloxamer 407, or sodium cholate.

The graphene obtainable by the processes of the present invention may be further used in a variety of applications, such as potential applications as electrodes, additives for composites or thermal management, and others. The use of graphene in these applications may be according to the protocols known to the skilled artisan according to a specific application . In another aspect of the present invention, there is provided a system or an apparatus for carrying out the processes of the present invention, comprising parts which are in liquid communication with one another; the apparatus may also comprise a closed flow loop. The system preferably comprises a reactor for mixing and/or dissolving at least one surfactant and/or graphite. Sometimes at least one working tank is provided to hold and supply pre-prepared graphite dispersion. One or more pumps may be used to drive the dispersion through the system, particularly through at least one sonication chamber. Each of the sonication chambers contains a sonotrode that conducts ultrasound generated by an ultrasound generator. Sometimes the frequencies of the ultrasound provided in each of the ultrasonic chambers are identical. In some embodiments, the frequencies of the ultrasound provided in each of the ultrasonic chambers are essentially similar. The pressure in the system may be controlled by suitably located valves. The system may further comprise at least one second tank (e.g. a repose tank) in liquid communication with the system. The second tank may comprise at least one inlet and at least one outlet, each of said inlets and outlets being controllably closable to restrict the flow into or from the repose tank. At least one pump may be situated between the second tank and the system. Alternatively or additionally, suitably controllable valves may switch the flow from into and from a first tank, to into and from the second tank, thereby cutting off the first tank from the circulation and plugging in the second tank to the circulation. Thereby the first tank may become the repose tank, and the second tank may become the working tank. The system may further comprise a controlling unit to control the parameters of the system, for example, the temperature of the at least one sonication chamber, temperature of the at least one repose tank and/or at least one working tank, the pressure of various parts of the system, the sequence of the steps, especially the number of ultrasound irradiation and repose steps, and others. The system may further comprise a separation tank to separate graphite residues from the graphene suspension. The system may also further comprise a centrifuge. The system may further comprise a cross-flow filtration assembly. The system may also comprise a lyophilizer.

Now referring to Fig. 1, a schematic representation of an embodiment of the present invention is given. A holding tank 10 contains graphite dispersion. The tank is in liquid communication with the pump 20. The graphite dispersion stream 15 is pressurized to yield pressurized graphite dispersion 25, and is fed into the sonication cell 30, which is in liquid communication with said pump 20. The sonication cell 30 is equipped with a sonotrode 35. The sonication cell 30 is further in liquid communication with said holding tank 10. A control valve 40 is placed between the sonication cell 30 and the holding tank 10, to control the pressure in the system. The irradiated graphite dispersion 37 flows through the pressure control valve 40 and is returned into the feeding tank 10 as depressurized stream 45. The sonotrode 35 is active, implying that the system is performing the sonication step.

Now referring to Fig. 2, a schematic representation of an embodiment of the present invention is given. A sonication system with a repose tank is demonstrated. Two holding tanks 10 and 15 contain graphite dispersion. The tanks are in liquid communication with the pump 20 via the selector 14, which regulates the efflux of the graphite dispersion stream 18 from either the tank 10 or 15. The graphite dispersion stream 18 is pressurized to yield pressurized graphite dispersion 25, and is fed into the sonication cell 30, which is in liquid communication with said pump 20. The sonication cell 30 is equipped with a sonotrode 35. The sonication cell 30 is further in liquid communication with said holding tanks 10 and 15 via the selector 12, controlling the flow to either the holding tank 10 or 15. A control valve 40 is placed to control the pressure in the system, between the sonication cell 30 and the selector 12. The irradiated graphite dispersion 37 flows through the pressure control valve 40 and is returned into the feeding tank 10 or 15 via the selector 12, as depressurized stream 45. The sonotrode 35 is active, implying that the system is performing the sonication step on the portion of the graphite dispersion held in either tank 10 or tank 15, whereas the other is concomitantly at repose.

Referring now to Fig. 5, provided is a system for continuous production of graphene according to one embodiment of the present invention. A sonication system is demonstrated. Water with surfactant dissolved therein is fed into an inlet of the feed tank 1, which is in liquid communication with the working tank 2. The working tank 2 comprises graphite prior to operation of the system and graphite dispersion during the operation, and is in liquid communication with the sonicator comprising a flow cell, via the pump. The graphite dispersion is pressurized by the pump into a pressurized graphite dispersion, which is fed into the flow cell of the sonicator, to furnish the irradiated graphite dispersion. The sonicator is further in liquid communication with the separation tank 3 and the working tank 2, via the selector and the faucet. The irradiated graphite dispersion is fed either into the working tank 2 to continue the circulation, or into the separation tank 3. The separation tank 3 is fitted with two outlets, the upper outlet in liquid communication with the storage tank 4, and the lower outlet, which is in liquid communication with the working tank 2. The graphene dispersion is decanted from the separation tank 3 into the storage tank 4, whereas the graphite sediment is fed back into the working tank 2 for further processing. In this way, irradiated graphite that was not completely exfoliated is recycled to produce graphene therefrom.

Examples

General procedure for preparation of graphene dispersion, 1.5-L batches

An exemplary experimental set-up is shown in the Figure 1. In a feeding tank of 5.0 L volume, 15 g of graphite flakes (Sigma, CAS 7782-42-5) were dispersed in a solution of 1.5 g of a surfactant (e.g. Triton™ X100 or poloxamer 407) in 1.5 L of water. The feeding tank was connected via stainless steel tubing to a pump (Progressive cavity, Seepex) and further to a stainless steel ultrasonic chamber with a volume of 250 mL . The total volume of the tubing was 500 mL . The ultrasonic chamber was fitted with high power tip sonicator (UlPlOOOhd, Hielscher Germany), maximum power 1000W, frequency 20 kHz, equipped with a 34-mm titanium sonotrode. The ultrasonic chamber was back connected with the feeding tank via a ball faucet to control the pressure .

The sonication chamber temperature was controlled using a cooling unit to maintain the temperature of 15 °C. The ultrasound output was set to 700W power. The pump was operated at a rate 3 L/min. The faucet was adjusted to generate a pressure of about 2 barg.

The sonication was operated continuously for the comparative processes, and was operated in cycles each consisting of 1-hour irradiation step followed by a repose step, for the processes of the present invention. During the repose step the sonication and the pump were turned off. The processes continued until the desired total energy per volume was delivered. The resultant dispersion was centrifuged using Megafuge™ 1.0 (Heraues), at varying conditions as set out below. About 85% of the supernatant was decanted to furnish the graphene dispersion. The concentration of graphene in the graphene dispersion was determined spectrophotometrically with detection at 660 nm.

Example 1

The effect of repose period on the graphene yield

The materials, methods and system were used as described in the general procedure above.

The sonication was carried out continuously with no interruptions (the comparative process) and intermittently with cycles each consisting of 1 hour irradiation step followed by repose step. For a given intermittent process the duration of repose steps was constant throughout the process (either 30, 60 or 120 minutes) . The total energy delivered was 4 MJ/L. The dispersions were centrifuged at 4,000g for 20 minutes. The results are graphically presented in Figure 3. The ordinate y axis is the concentration of graphene obtained by the process, denominated C, in grams per liter (g/L) . The abscissa x axis is the repose time, designated as "Rep.", in minutes. The legend entry TX-100 signifies the surfactant type Triton™ X-100.

As indicated therein, the concentrations of graphene were as follows: for the continuous process (comparative) - 0.152 g/L, for an intermittent process with repose steps of 30 minutes - 0.194 g/L, and for intermittent processes with repose steps lasting 60 or 120 minutes - 0.230 and 0.237 g/L, respectively. It can be seen that switching from the continuous process to the intermittent process improves the efficacy of graphene production . Example 2

Comparison between intermittent and continuous sonication processes on varying the amount of total energy density applied

A comparison between the two processes - an intermittent sonication and a continuous sonication, was performed using the materials, methods and system of the general procedure.

For continuous sonication, the sonication continued until the contemplated sonication energy was delivered. For intermittent sonication, the process consisted of alternating 1-hour irradiation steps with repose intervals of 1 hour, and the process continued until the same contemplated sonication energy was delivered. Centrifugation was performed in both cases at 4,000 g for 20 minutes.

The results are presented graphically in Figure 4, showing the concentration of the graphene in the product dispersion as function of total sonication energy density applied. The ordinate y axis is the concentration of graphene obtained by the process, denominated "Cone.", in grams per liter (g/L) . The abscissa x axis is the volume energy delivered, designated as "Enrg . /Vol . " , in kilo-Joules per milliliter (kJ/mL) . The concentrations of graphene from continuous process are designated with filled triangle A (legend entry "Cont."), and from intermittent process with a filled square ■ (legend entry "Interm.") .

The results indicate that delivering a fixed amount sonication energy intermittently, rather ;han continuousl leads to an increase of the concentration of graphene in the dispersion product.

Example 3

The effect of centrifugation on the graphene concentration

The materials, methods and system were used as described in the general procedure above. A comparison between two centrifugation settings was performed.

The sonication consisted of ten 1-hour irradiation steps, with 1 hour break (repose) between each two consecutive steps. Therefore, the total irradiation time was 10 hours and the total repose time was 9 hours. The total energy per volume delivered was 16.8 MJ/L.

When the dispersion was centrifuged at 1,000 g for 150 minutes, the final concentration of graphene obtained was 1.8 g/L. When the dispersion was centrifuged at 4, 000 g for 20 minutes, the final concentration of graphene was 0.88 g/L.

Example 4

Lyophilization with poloxamer 407

A graphene dispersion obtained from procedure of example 1 performed with poloxamer 407 instead of Triton™ X100, was lyophilized as follows: following decantation, the aqueous dispersion was placed into plastic flask of 40 mL volume and 3 cm in diameter, frozen in liquid nitrogen and placed in a lyophilizer (Labconco Freezone 4.5) at 0.027 mBar for 48 h. Example 5

5.5 L batch

In a feeding tank of 8 L volume, 82.5 g of graphite flakes were dispersed in a solution of 8.25 g of Triton™ X100 in 5.5 L of water. The other system components and parameters were as in the Example 1.

The sonication continued until the energy level was delivered, with alternating 1-hour irradiation steps and 1-hour repose intervals. Centrifugation was performed at 4,000 g for 20 minutes. For a total delivered energy per volume of 4.58 MJ/L, graphene concentration obtained was 0.362 g/L.

Example 6

Various graphite types

Graphene dispersions were prepared according to the general procedure and the intermittent setup described in the Example 3, with Triton™ X100 as surfactant. Graphite source varied to include graphite powder, supplied by Alfa Aesar (Cat. #10129) or Zeynatta, graphite flakes as in the general procedure, and recycled graphite separated from the process of the Example 2. The total delivered energy varied from 1.68 MJ/L to 10.08 MJ/L. Centrifugation was performed for 20 minutes at 4,000 g.

The results are presented in Figure 6. The ordinate y axis is the concentration of graphene obtained by the process, denominated "Cone", in grams per liter (g/L) . The abscissa x axis is the volume energy delivered, designated as "Enrg/Vol.", in kilo-Joules per milliliter (kJ/mL) . The concentrations of graphene from graphite powder are designated with filled diamond ♦ (legend entry "GrPO"), from recycled graphite with a filled circle · (legend entry "Re-Gr") , from Zenyatta graphite with a filled diamond A (legend entry "Zen"), and from fresh flakes with an open circle ° (legend entry "Gr-Fl") .

Higher graphene concentrations were obtained for the Zenyatta graphite, similar to graphite powder.

Example 7

Characterization of graphene

Graphene dispersions from the Examples 2 and 3 were used.

Micrographs of transmission electron microscopy (TEM) were obtained by FEI Tecnai 12 G2 TWIN TEM, operated at 120 kV. Dry samples were prepared on holey-carbon-coated copper grids (300 mesh, lacey carbon, Ted Pella) by placing a drop of dispersion on a grid and allowing it to dry at ambient conditions before storage. The microscope was operated at 120 kV in low electron dose mode (to reduce radiation damage) and with a few micrometers under focus to increase phase contrast. Images were recorded on a Gatan 794 CCD camera and analyzed by Digital Micrograph 3.6 software.

Raman spectra of the GS were measured by Jobin-Yvon HR LabrRam micro-Raman at 514 nm on quartz slide. The samples were dried out on the slide from a 0.2 mL drop before the measurement.

TEM micrographs have revealed the presence of stacks of few layers and some folded individual graphene sheets. The "average length" was defined as the square root of individual patch's area (min ^χ max lengths) . The average length was 4001200 nm regardless of sonication energy. The Raman spectrum's so-called the 2D band at ~2700 cnr¹ was used to differentiate between graphite and graphene, as it splits due to bilayer interaction and the high energy sub-peak becomes considerably stronger for graphite compared to few-layer graphene. For the tested specimens the intensity of the two doublets was similar in a marked contrast to the graphite reference. This indicates that their thickness did not exceed 5 layers, in line with the TEM results.

Claims

1. A process of graphite exfoliation from graphite dispersion in a liquid medium in the presence of a surfactant, comprising intermittently applying sonication energy to produce graphene in the liquid, wherein said intermittently applying sonication comprises holding at least a portion of said dispersion at static conditions during the intermissions.

2. The process of claim 1, wherein the intermission consists of a repose interval, during which interval at least a portion of the dispersion is not subjected to ultrasonic irradiation, flow and agitation.

3. The process of any one of claims 1 or 2, said process comprising

a) providing graphite dispersion comprising a graphite, at least one surfactant and water;

b) applying to said graphite dispersion one or more cycles, each cycle comprising a period of ultrasonic irradiation and a repose interval; and

c) applying to said graphite dispersion a final period of ultrasonic irradiation to furnish graphene dispersion.

4. The process according to any one of the preceding claims, wherein said repose interval is at least 15 minutes.

5. The process according claim 4, wherein said repose interval is at least 30 minutes.

6. The process according to any one of claims 3-5, wherein said number of cycles is from 2 to 20.

7. The process of any one of the preceding claims, further comprising removing graphite residues from the graphite dispersion to furnish purified graphene dispersion.

8. The process according to claim 7, wherein said removing is by centri fugation, sedimentation, or both.

9. The process according to the claim 8, wherein said centri fugation is at less than 3,000 g-force.

10. The process according to any one of the preceding claims, further comprising recovering graphene from the graphene dispersion or the purified graphene dispersion.

11. The process according to claim 10, wherein said recovering comprises lyophilizing said graphene dispersion or said purified graphene dispersion, or filtering said graphene dispersion or said purified graphene dispersion onto a suitable filter .

12. A process of graphite exfoliation, comprising combining graphite and at least one surfactant with water to form an aqueous graphite dispersion, circulating a stream of said graphite dispersion through a circulation line provided with at least one sonication chamber, periodically holding at least a portion of the ultrasonically irradiated dispersion under static conditions before it is returned back to said at least one sonication chamber, and collecting graphene dispersion.

13. The process according to claim 12, wherein the circulation line is in fluid communication with at least a first tank and a second tank, the process comprises circulating the stream alternately through said first tank or said second tank while holding at least a portion of the ultrasonically irradiated dispersion under static conditions in the other tank.

14. A process according to claim 12 or 13, which further comprises removing graphite residues from the graphene dispersion, returning the removed graphite to the circulating stream and collecting a purified graphene dispersion.

15. Graphene obtainable by a process of any one of the preceding claims.

16. An apparatus for production of graphene, comprising at least a first tank and a sonication chamber, which are in fluid communication with one another via a circulation loop, said sonication chamber comprising a vessel and at least one sonotrode operably linked to an ultrasound generator, said apparatus further comprises downstream processing device including a separation unit having at least one of a centrifuge, a lyophilizer, and/or a cross-flow filtration assembly, said separation unit being coupled via a discharge line to a storage container for holding graphene and optionally connected via a flow line to said circulation loop.

17. An apparatus according to claim 1, further comprising at least a second tank and an array of valves to enable the circulation alternately through said first and second tanks.