GB2512711A

GB2512711A - Graphene production method

Info

Publication number: GB2512711A
Application number: GB1402044.0A
Authority: GB
Inventors: Toultchinski Iana
Original assignee: CARBONLAB Ltd
Current assignee: CARBONLAB Ltd
Priority date: 2013-02-07
Filing date: 2014-02-06
Publication date: 2014-10-08
Anticipated expiration: 2034-02-06
Also published as: GB2512711B; GB201302149D0; WO2014122465A1; GB201402044D0

Abstract

A method for the production of graphene by exfoliation of graphite, said method comprising: providing a mixture of graphite in a solvent (e.g. water), subjecting the mixture to sonication at at least one frequency in the range of 15 to 30 kHz, and subjecting the mixture to sonication at at least one frequency in the range of 50 to 250 kHz. The two sonications can be performed in two different reactors, with one at 85 degrees C and the other at 23 degrees C.

Description

GRAPHENE PRODUCTION METHOD

[0001] This invention relates to a method fol the production of graphene. The invention also relates to graphene obtainable by such a method.

BACKGROUND

[0002] Graphene can be viewed as a two dimensional sheet composed of sp2 carbons in a honeycomb structure. Graphene layers are the building blocks for all the other graphitic carbon allotropes. For example, graphite (3-D) is composed of graphene layers stacked on top of each other with an interlayer spacing of approximately 3.4 Angstroms. Carbon nanotubes, on the other hand, can be viewed as graphene layers rolled into tubes.

[0003] Graphene has very attractive physical, optical and mechanical properties, including high charge carrier mobility, high thermal conductivity and stiffness. It can be used for a wide range of applications, for example, in the electronics industry as well as for an additive in polymer production.

[0004] Various methods are known for the production of graphene. For example, layers of graphene can be exfoliated from graphite using adhesive tape or obtained by reducing layers of graphite oxide. Graphite can also be exfoliated in the liquid phase in an appropriate solvent. For example, J.N. Coleman (Advanced Functional Materials, Volume 19, Issue 23, pages 3680 -3695, December 9, 2009) describes a lab-scale experiment in which a dispersion of graphite powder is sonicated in an aqueous solution of sodium dodecyl benzene sulphonate. After mild centrifugation, a grey dispersion having a graphene concentration approaching 0.1 mg/mI was obtained. The flakes obtained, however, were relatively large, ranging from 100 nm to 3 Rm in size. This method also has low productivity while demanding lengthy ultrasound treatment.

[0005] It is among the objects of embodiments of the present invention to develop a new method for producing graphene.

BRIEF SUMMARY OF THE DISCLOSURE

[0006] According to the present invention, there is provided a method for the production of graphene, said method comprising: [0007] a) providing a mixture of graphite in a solvent, b) subjecting the mixture to sonication at at least one frequency in the range of 15 to kHz, and c) subjecting the mixture to sonication at at least one frequency in the range of 50 to 250 kHz, for example, 50 to 100 kHz or 100 to 250 kHz.

[0008] The present invention also provides graphene obtained or obtainable by the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] An embodiment of the invention are further described hereinafter with reference to the accompanying drawings, in which: [0010] Figure 1 is a schematic diagram of an apparatus that is suitable for carrying out a method according to one embodiment of the present invention.

DETAILED DESCRIPTION

[0011] As noted above, the present invention provides a method for the production of graphene, said method comprising: a) providing a mixture of graphite in a solvent, b) subjecting the mixture to sonication at at least one frequency in the range of 15 to kHz, and c) subjecting the mixture to sonication at at least one frequency in the range of 50 to 250 kHz, for example, 50 to 100 kHz or 100 to 250 kHz.

[0012] Without wishing to be bound by any theory, it is believed that, by subjecting the mixture by sonication at at least two different frequencies in steps b) and c), the exfoliation of graphite may be improved. In particular, the present inventors have found that the frequencies employed in step c) may be used to separate larger graphene particles (e.g. 20 to lOOnm), while the frequencies employed in step b) may be used to separate smaller graphene particles (e.g. less than 20 microns, for example, less than 20 nanometres). By controlling the duration and time interval between steps b) and c), therefore, it may be possible to tailor the process to achieve graphene of a desired particle size. The mixture may also be circulated between steps b) and c) in a loop to optimise the process.

[0013] As mentioned above, step b) of the method is performed by sonicating the mixture at to 30 kHz. Preferably, the mixture is subjected to ultrasound at at least one frequency in the range of 20 to 25 kHz. In a preferred embodiment, the ultrasound treatment occurs at a frequency of 18 to 22 kHz, for example, 20 kHz. Step b) may be performed at a power of 300 to 2000W, preferably 400 to 800W, more preferably 400 to 500 W. [0014] In step c), the mixture is subjected to sonication at at least one frequency in the range of 50 to 250 kHz, for example, 50 to 100 or 100 to 250 kHz. In one embodiment, the ultrasound treatment in step c) occurs at a frequency of 50 to 100 kHz. In a preferred embodiment, the ultrasound treatment occurs at a frequency of 55 to 90 kHz, preferably 55 to 80 kHz, for example 60 kHz. In another embodiment, the ultrasound treatment occurs at a frequency of 100 to 250 kHz, preferably 120 to 180 kHz, more preferably 140 to 160 kHz, for example, 150 kHz. Step c) may be performed at a power of 500 to 4000W, preferably 3500 to 4000 W. [0015] On average, the volume of mixture that is treated may be subjected to a power of 300 to 700W per litre, preferably 350 to 500W per litre. In another embodiment, the volume of mixture that is treated may be subjected to a power of 25 to 200 W per litre, preferably to 150W per litre.

[0016] Steps b) and c) may be carried out in any order in the process. In one embodiment, step c) is carried out prior to step b). As mentioned above, however, the mixture is preferably circulated between steps b) and c). For example, in one embodiment, a first reactor is provided for performing step b), while a second reactor is provided for performing step c). The mixture may then be passed from the first reactor to the second reactor or vice-versa, or preferably circulated between the reactors using, for example, an appropriate piping or connection means. In one embodiment, the mixture may be passed from one reactor to another and thereafter to a holding tank. Preferably, the holding tank is positioned downstream of the second reactor, such that the mixture is introduced into the holding tank after treatment in both the first and second reactors. The temperature and pressure of the mixture may be monitored and/or controlled, for example, by withdrawing a sample from the holding tank and, optionally, heating or cooling it as necessary. In one embodiment, the mixture is circulated through the first reactor, second reactor and holding tank. The circulation of the mixture may be regulated through the use of pumps. The reactors used in steps b) and/or c) may be continuous flow reactors.

[0017] In one example, a summary of the operation of the reactors is as follows. Acoustic energy with a working frequency between 15-30 kHz in the first reactor and 50-250 kHz or 100-250 kHz in the second may be transmitted from an ultrasonic generator to a magnetostrictive transducer. A waveguide may be attached to the magnetostrictive transducer. The waveguide may be a concentrator with an amplification coefficient k of between 7 and 10, which produces axial and radial emission within the reactor. The normal working displacement amplitude of the concentrator face may be 70-120 micron. The waveguide may be adjacent to the bottom section of the reactor, to which the input nozzle for the working mixture may be attached. The clearance between the waveguide and bottom section of the reactor may be equal to the amplitude of displacement of the waveguide face.

Thus, the waveguide and the bottom section of the reactor may together constitute a hydrodynamic valve, operating at the frequency of the magnetostrictor. The static pressure in the region of cavitational emission may fluctuate between 0 and a set working pressure, P of the magnetostrictive transducer. This may generate supplementary vibrational energy within the cavitation zone, in addition to the acoustic energy. This process may be used to assist in the dispersion of solids.

[0018] Step b) may be carried out at a temperature of 0 to 200 degrees C, preferably 15 to degrees C. In one embodiment, the temperature is 10 to 90 degrees C, for example, 10 to 20 degrees C. The temperature may be selected depending on the nature of the solvent.

Step b) may also be carried out at a pressure of 0.01 to 0.97 MPa. In one embodiment, the pressure and/or temperature of the first reactor is controlled such that the reaction temperature and pressure are within the desired ranges.

[0019] Step c) may be carried out at a temperature of 0 to 200 degrees C, preferably 15 to degrees C. In one embodiment, the temperature is 10 to 90 degrees C, for example, 10 to 20 degrees C. The temperature may be selected depending on the nature of the solvent.

Step c) may also be carried out at a pressure of 0.01 to 0.97 MFa. In one embodiment, the pressure and/or temperature of the second reactor is controlled such that the reaction temperature and pressure are within the desired ranges.

[0020] The temperature and pressure conditions may be the same in both reactors.

[0021] Where the mixture is held in a holding tank, it may be desirable to maintain the temperature of the holding tank at a temperature of 15 to 90 degrees C, preferably 60 to 70 degrees C. The pressure may be maintained at 0.01 to 0.97MPa. In one embodiment, a mixture (e.g. slurry) of graphite and solvent may be formed and mixed in the holding tank.

[0022] The concentration of graphite in the solvent can be varied to maintain the desired viscosity of the solution. For example, the concentration of graphite in the solvent may be from 5 to 18%.

[0023] As mentioned above, the graphite is provided as a mixture in a solvent. Preferably, the graphite is provided as a dispersion in a solvent. The mixture of graphite in a solvent may form a homogenized solution. Any suitable form of graphite may be used. The graphite may be prepared by crushing (e.g. in a ball mill) prior to being added to the solvent.

Examples of suitable forms of graphite include graphite powder and graphite flakes.

Preferably, graphite flakes are used having a high carbon content (e.g. above 99%, preferably 99.95%). The flakes are preferably crushed (e.g. in a ball mill) into particles, for example, of 50 microns in size.

[0024] The graphite may be washed in a solvent prior to forming the mixture. The solvent for used washing the graphite may be the same or different to the solvent used for forming the mixture.

[0025] Any suitable solvent may be employed. Examples of such solvents include organic solvents and aqueous solvents, including water. Suitable organic solvents include alcohols, such as C2 to C6 alcohols, and hydrocarbon solvents, such as alkanes. Suitable alcohols include ethanol, propanol, butyl alcohol, pentanol and hexanol. Suitable hydrocarbon solvents include C5 to C10 alkanes and mixtures thereof. An example of a suitable alkane mixture is petroleum ether. Advantageously, the present invention may be carried out using non-corrosive solvents. Specific examples include butyl alcohol, ethanol, petroleum ether and water. Without wishing to be bound by any theory, it is believed that the efficient exfoliation afforded by steps b) and c) of the present invention allows milder solvents to be used to achieve desirable results.

[0026] A surfactant may also be used in the mixture, particularly where aqueous solvents or water are used as the solvent. An example of a suitable surfactant is sodium dodecyl sulphate.

[0027] The graphene produced from the method of the present invention may be separated using any suitable method. Examples include decanting, centrifugation and filtration (e.g. membrane filtration). To separate the graphene, a portion of the mixture may be removed from the process, for example, after a particular reaction time. In one embodiment, the sample may be continuously withdrawn from the process, for example, via a holding tank.

The reaction time of the process can be controlled to determine the particle size of the graphene produced. Generally speaking, the longer the reaction time, the thinner the graphene particle size. In one embodiment, the reaction time for a single cycle is 60 minutes. More than one cycle may be carried out, for example to obtain thinner graphene.

The specific choice of solvent may also affect the graphene particle size.

S

[0028] Following separation, the graphene produced from the method of the present invention may be dried using any suitable method. An example of a suitable drying method is vacuum drying.

[0029] In one embodiment, the graphene produced from the method of the present invention may be separated from the mixture by decanting and centrifugation, followed by vacuum drying.

[0030] The graphene produced may have an average particle size of 100 nni or less, preferably 80 nm or less, more preferably 60 nm or less, for example, 20 nm or less. By particle size, it is meant the thickness of the graphene particles rather than diameter. In one embodiment, the particle size (thickness) is ito 15 nm, preferably ito 10 nni, for example, 1 to 5 nm or 5 to 10 nm. The diameter of the particles may range from 5 to 20 microns, for example, 5 to 10 microns. In one embodiment, the graphene particles produced have an average particle size (thickness) of 1 to 5 nm and a diameter of 5 to 7 microns. In another embodiment, the graphene particles produced have an average particle size (thickness) of 1 to 10 nm and a diameter of 5 to 15 microns.

[0031] For the avoidance of doubt, the graphene produced or obtainable by the method of the present invention includes single layer graphene as well as few-layer graphene of, for example, up to 10 layers of graphene thick. In other words, the term "graphene" as used in relation to the present invention covers graphene flakes and graphite nanoplatelets, and nanographite materials. The number of layers of graphene in nanographite materials is dependent on the time and parameters used. In one embodiment, the nanographite material comprises between 5 and 10 layers of graphene.

[0032] Particle size may be measured using any known method. For example, an electron dual beam microscopy or scanning probe microscopy may be used. Raman spectroscopy, X-ray diffraction or an atomic force microscope may also be used to measure the particle size. The resulting thickness of the graphene is determined by treatment time and type of solvent.

[0033] In one embodiment, a prevelance (up to 98%) of the particles have an average particle size (thickness) of ito 5 nm. In one embodiment, the effective particle diameter is 3 to 5 microns.

[0034] The graphene produced or obtainable using the method of the present invention may be used for a wide range of applications. For example, the graphene may be used as an additive for polymers, such as polyethylene and polystyrene. The graphene may also be used for electrical or electronic applications, such as an electrode additive for, for example, lithium accumulators, or in the manufacture of supercapacitors. The graphene may also be formulated into inks and coatings to achieve desired thermal and electrical properties.

Graphene materials obtained using described method may also be used in a thermal interface material, such as a thermal grease. The thermal conductivity of the thermal grease may be in the range of up to 300 Wm1 K1.

[0035] The method of the present invention may allow graphene to be produced in high yield in a relatively time and cost efficient process. The specific surface area of the graphene produced may range from 200 to 2400 m2/g. In one embodiment, the specific surface area of the grapheme produced is from 200 to 1000 m2/g.

[0036] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0037] These and other aspects of the present invention will now be described with reference to Figure 1.

[0038] Figure 1 depicts an apparatus for performing a method according to one embodiment of the present invention. The apparatus comprises a first reactor 10, a second reactor 20 and a holding tank 30. The apparatus also comprises a control unit 40 for monitoring the temperature and pressure of liquid in the holding tank 30.

[0039] In use, a mixture 50 of graphite in a solvent of, for example, butyl alcohol is circulated through the apparatus. The mixture 50 is treated in the first reactor 10 where it is subjected to sonication at a frequency in the range of 15 to 30 kHz. The mixture is then introduced into the second reactor 20 where it is subjected to sonication at a frequency in the range of 50 to 250 kHz, for example, 50 to 100 kHz or 100 to 250 kHz. Thereafter, the mixture 50 is introduced into the holding tank 30, where it is circulated through the control unit 40 via pump 60 to monitor its pressure and temperature. From the holding tank 30, the mixture 50 is also recycled to the first reactor via pump 70. Graphene may be separated from a sample of the mixture 50 withdrawn (not shown), for example, from the holding tank after a predetermined period of treatment.

[0040] The temperature of the first reactor 10, second reactor 20 and cooling tank 30 may be controlled, for example, through the use of a circulating heat control medium 80.

[0041] EXAMPLES

[0042] Example 1

g crystalline flake graphite (50 mesh) was dissolved in 1000 ml water with addition of 2% SDS surfactant. The solution was subjected to ultrasonication over the course of 60 minutes using the following system parameters: * Solution temperature 85°C * Reactor 1 -frequency 22.5 kHz; power density 2.1 W/cm3 * Reactor 2 -frequency 190 kHz; power density 6.2 W/cm3 * System pressure 5.4 kg/cm2 95% yield of NGPs comprising between 5-10 layers graphene was obtained. After washing in acetone, the product was used as an additive in polymer composites and thermal greases.

[0043] Example 2

g crystalline flake graphite (50 mesh) was dissolved in 1000 ml of a 50/50 pentane/hexane mixture. The solution was subjected to ultrasonication over the course of 50 minutes using the following system parameters: * Solution temperature 23°C * Reactor 1 -frequency 25 kHz; power density 3 W/cm3 * Reactor 2 -frequency 220 kHz; power density 7.2 W/cm3 * System pressure 10.5 kg/cm2 95% yield of NGPs comprising between 5-10 layers graphene was obtained. The product did not require further purification and was utilised in conductive inks and high-efficiency thermal greases.

Claims

Claims 1. A method for the production of graphene, said method comprising: providing a mixture of graphite in a solvent, subjecting the mixture to sonication at at least one frequency in the range of 15 to 30 kHz, and subjecting the mixture to sonication at at least one frequency in the range of 50 to 250 kHz.
2. A method as claimed in claim 1, wherein, in step b), the mixture is subjected to sonication at at least one frequency in the range of 20 to 25kHz.
3. A method as claimed in claim 1 or 2, wherein, in step b), the power is maintained at 300 to 2000W.
4. A method as claimed in any one of the preceding claims, wherein, in step c), the mixture is subjected to sonication at at least one frequency in the range of 50 to 100 kHz, or in the range of 100 to 250 kHz.
5. A method as claimed in any one of the preceding claims, wherein, in step c), the power is maintained at 500 to 4000W.
6. A method as claimed in any one of the preceding claims, wherein step b) is carried out prior to step c).
7. A method as claimed in any one of the preceding claims, which comprises providing a first reactor for subjecting the mixture to sonication at at least one frequency in the range of 15 to 30 kHz, providing a second reactor for subjecting the mixture to sonication at at least one frequency in the range of 50 to 250 kHz, and treating the mixture in the first reactor and the second reactor in either order.
8. A method as claimed in claim 7, wherein the mixture is circulated between the first reactor and the second reactor.
9. A method as claimed in claim 7 018, wherein the first reactor is operated at a temperature of 10 to 20 degrees C.
10. A method as claimed in any one of claims 7 to 9, wherein the second reactor is operated a temperature of 20 to 90 degrees C.
11. A method as claimed in any one of claims 7 to 10, wherein the first reactor is operated at a pressure of 0,01 to 0,1 MPa.
12. A method as claimed in any one of claims 7 to 11, wherein the second reactor is operated at a pressure of 0,1 to 0,7 MPa.
13. A method as claimed in any one of the preceding claims, wherein the mixture is transferred to a holding tank after step b) or c) and maintained at a temperature of 60 to 70 degrees C and/or a pressure of 0,01 to 0,97 MPa.
14. A method as claimed in any one of the preceding claims, wherein the solvent comprises a solvent selected from at least one of butyl alcohol, ethanol, petroleum ether and water.
15. A method as claimed in any one of the preceding claims, wherein the mixture comprises graphite, water and a surfactant.
16. A method as claimed in any one of the preceding claims, wherein the graphene particles have a particle size of lOOnm or less.
17. A method as claimed in any one of the preceding claims, which comprises controlling the duration of steps b) and c) and the time interval between these steps, so as to achieve a target graphene or graphite nanoplatelet structure thickness.
18. A method as claimed in any one of the preceding claims, which further comprises separating the graphene produced from the mixture.
19. Graphene obtainable or obtained using a method as claimed in any one of claims 1 to 18.