WO2016063036A1

WO2016063036A1 - Methods for the production of 2-d materials

Info

Publication number: WO2016063036A1
Application number: PCT/GB2015/053124
Authority: WO
Inventors: Pagona PAPAKONSTANTINOU; Margherita SEPIONI; Branson BELLE; Arun Prakash ARANGA RAJU
Original assignee: 2-Dtech Limited; Innovation Ulster Limited
Priority date: 2014-10-21
Filing date: 2015-10-20
Publication date: 2016-04-28

Abstract

The present invention relates to a process for the production of graphene and other 2- dimensional materials. The process involves the step of mixing and/or grinding a bulk layered material whilst simultaneously an ionic liquid is added continuously. Continuous addition of the ionic liquid can provide improved yields of the desired products and improved consistency of the quality of that product.

Description

Methods for the production of 2-D materials

[0001] The present invention relates to a process for the production of graphene and other two-dimensional materials. The process involves the step of mixing and/or grinding a bulk layered material whilst simultaneously an ionic liquid is added continuously.

BACKGROUND

[0002] It has been found that when a small quantity of an ionic liquid is mixed with graphite flakes, a gel-like composite is formed. When shear forces are applied to graphene flakes in these gel-like composites, the shear forces detach the graphene layers from the graphite flakes. The ionic liquid then surrounds each layer preventing the detached graphene from restacking. It has been shown (see WO2012/117251) that the electronic structure of graphene in the ionic liquid gel remains unchanged and that there is no charge transfer between graphene and the ionic liquid. Furthermore, it has been found that graphene which has been prepared by grinding graphite in an ionic liquid has a lower oxygen content than graphene which has been prepared by sonicating graphite in an ionic liquid.

[0003] This process therefore offers a promising starting point for the development of an industrial process for the large scale production of graphene.

[0004] The process can work well but the ionic liquid gel mixture is prone to thickening after some time to form a dry paste. When this happens the mixture can be deposited on any stagnant regions of a milling process (e.g. the walls of the vessel) and on any mixing and grinding mechanisms which may be present as part of a particular milling process. The material which is deposited in this way does not undergo the exfoliation process with particularly high efficiency. Formation of a dry paste can also hamper the ability of the ionic liquid to lubricate between the layers of the graphite or other two-dimensional materials during the exfoliation process may be. If the process is not adequately lubricated this can also lead to a lowering of the exfoliation efficiency. The result is that large quantities of the bulk material can remain un-exfoliated and the yield of the exfoliated materials can be low.

[0005] The risk of thickening can be reduced by using larger quantities of ionic liquid from the outset of the process. However, the ionic liquid typically contains residual impurities which can become associated with the two-dimensional material, meaning that additional purification steps may be required to obtain the two-dimensional material in the desired purity for any given application. Such steps increase the cost of the process and the time it takes. They typically also lower the overall yield and make the process even more labour intensive.

[0006] It is an aim of certain embodiments of this invention to provide a process for the production of graphene and other two-dimensional materials which has a higher yield than prior art processes.

[0007] It is an aim of certain embodiments of this invention to provide a process for the production of graphene and other two-dimensional materials which is more predictable than prior art processes.

[0008] It is an aim of certain embodiments of this invention to provide a process for the production of graphene and other two-dimensional materials which is more industrially applicable than prior art processes, for example, which is automated.

[0009] It is an aim of certain embodiments of this invention to provide a process for the production of graphene and other two-dimensional materials which uses lower quantities of ionic liquid to prior art processes.

[0010] It is an aim of certain embodiments of this invention to provide a process for the production of graphene and other two-dimensional materials which provides graphene and other two-dimensional materials in higher purity and/or with fewer purification processes than prior art processes.

BRIEF SUM MARY OF THE DISCLOSURE

[0011] In a first aspect of the invention is provided a process for the production of graphene or an inorganic 2-dimensional material, the process comprising the step of: mixing and grinding a mixture of graphite or an inorganic multilayered material inone or more ionic liquids to form graphene or the inorganic 2-dimensional material; and

adding one or more ionic liquids continuously at one or more predetermined rates to the mixture;

wherein at least one of the mixing and grinding steps occurs simultaneously with the continuous addition of the ionic liquid for at least a period.

[0012] Thus, the invention may a process for the production of graphene or an inorganic two-dimensional material, the process comprising the step of:

mixing and grinding graphite or an inorganic multi-layered material in one or more ionic liquids to form graphene or the inorganic two-dimensional material; and adding one or more ionic liquids continuously at one or more predetermined rates to the mixture;

[0013] The graphene or the inorganic two-dimensional material obtained by the process of the invention can be described as being in the form of flakes.

[0014] The inorganic multi-layered material which is used in the process is formed of the same layered inorganic compound as the inorganic two-dimensional material formed in the process. The primary difference between the starting material and the product is the average number of layers in the particles.

[0015] The process could be described as an exfoliation.

[0016] Preferably, graphite is the starting material in the process and graphene is the product.

[0017] The inventors have found that more consistent yields for such exfoliation processes can be achieved by adding the ionic liquid continuously to the mixture by up to 60 hours after the initial process has begun (e.g. overnight). In part this arises because the continuous addition of liquid to the mixture prevents it from thickening and therefore minimises deposition of the mixture on the walls and stirrer mechanism of the gel- composite material.

[0018] It may be that a portion of the at least one ionic liquid is premixed with the graphite or an inorganic multi-layered material at or before the start of the process and a further portion of the at least one ionic liquid is added continuously to the mixture at a predetermined rate after the exfoliation process has begun. Typically from about 10% to about 90% of the total amount of ionic liquid is added continuously to the mixture.

Preferably from about 25% to about 60% (e.g. from about 40% to about 50%) of the total amount of ionic liquid is added continuously to the mixture.

[0019] Thus, it may be that a portion of the at least one ionic liquid is present in the vessel at the start of the process and a further portion is added continuously at a predetermined rate to the vessel. Typically from about 10% to about 90% of the total amount of ionic liquid is added continuously to the vessel. Preferably from about 30% to about 60% (e.g. from about 40% to about 50%) of the total amount of ionic liquid is added continuously to the vessel.

[0020] It may be that all of the ionic liquid is added continuously at a predetermined rate to the mixture, e.g. to the vessel. [0021] The total amount of ionic liquid mixed with the graphite or inorganic multi-layered material during the process will preferably be from 0.5 to 10 mL per gram of the graphite or inorganic multi-layered material. This range represents an optimal range in which the process is more efficient, due at least in part to an optimal consistency of the gel which is obtained at said range.

[0022] The total amount of ionic liquid may be from 0.5 mL to 4 mL (e.g. from 1 to 4 mL) per gram of the graphite or inorganic multi-layered material, or from 1.6 to 3.3 mL per gram. Even more preferably, the total amount of the ionic liquid is from 1.6 to 3 (e.g. from 1.6 to 2.6) mL per gram of the graphite or inorganic multi-layered material. This range is particularly preferred for the formation of graphene from graphite. For other materials it might be desirable to use more or less ionic liquid. For M0S2 for example, slightly more ionic liquid is typically used per gram of M0S2.

[0023] Above 0.5 mL per gram, it may be necessary to wash the obtained graphene or inorganic two-dimensional material to remove impurities from the ionic liquid which may become associated with it. In particular, above 2.6 mL per gram, it may be necessary to wash the obtained graphene or inorganic two-dimensional material to remove impurities from the ionic liquid which may become associated with it.

[0024] The one or more ionic liquids which are added continuously to the mixture may be different to the one or more ionic liquids which were in the mixture initially. Preferably, however, the one or more liquids which are added continuously to the mixture are the same as the one or more ionic liquids which were in the mixture initially. This can make it easier to recover and reuse the ionic liquids.

[0025] It may be that a single ionic liquid is in the mixture initially. It may be that a single ionic liquid is added continuously to the mixture. Preferably, the single ionic liquid which is added continuously is the same as the single ionic liquid which was in the mixture initially.

[0026] Typically the continuous addition will be achieved using an automated device, e.g. an automatic liquid dispenser machine. Such devices are known in the art and typically they can be calibrated, allowing the rate of addition (i.e. the volume of liquid delivered to the vessel within a specified time) to be controlled. An additional benefit of such machines is that the ionic liquids are less prone to contamination (either through incorporation of airborne impurities or by oxidative degradation of the ionic liquid itself) within the sealed syringes from which the liquid is often dispensed. This can reduce precipitation within the syringe and problems associated with this and can also result in fewer impurities in the final product. [0027] The predetermined rate of addition will be selected based on the amount of, and more particularly the total surface area of the graphite or inorganic multi-layered material which is being exfoliated, the total amount of ionic liquid used in the process and the proportion of the ionic liquid which is added continuously. The rate will be selected such that the gel does not thicken and little or no material is deposited at any stagnant part of the milling machine (e.g. the walls of a vessel) or on any stirring or mixing mechanism that may be present. The rate of addition may be from about 0.001 ml_ to about 1 ml_ per hour per gram of the graphite or inorganic multi-layered material. Preferably, the rate of addition is from 0.01 ml_ to about 0.06 ml_, e.g. from about 0.03 ml_ to about 0.04 ml_, per hour per gram of the graphite or inorganic multi-layered material.

[0028] The process may involve, before the mixing and grinding step, the step of suspending the graphite or inorganic multi-layered material in the one or more ionic liquids.

[0029] The graphite or inorganic multi-layered material may be ground for a duration in the range from about 3 hours to about 7 days. It may be ground for a duration from about 12 hours to about 5 days, e.g. from about 36 hours to about 4 days or from about 2 days to about 3 days. Preferably, the material was ground for 60 hours or less. The duration of the grinding process can be varied depending on how well exfoliated the product of the process is desired to be. Longer grinding times tend to result in smaller and/or thinner flakes. Shorter grinding times tend to result in larger and/or thicker flakes.

[0030] The process may be performed in an atmosphere of air. The process is thus both efficient and practical.

[0031] The mixing and grinding process will typically be performed at a temperature in the range from about 0 to about 50 °C, e.g. from about 15 to about 40 °C. It may be performed at room temperature. High temperatures are less desired because they can lead to undesirable effects on the rheology of the mixture, e.g. high temperatures can cause the mixture to become too thin to exfoliate efficiently, although high temperatures can be used, e.g. with larger amount of liquid relative to the material being ground. Low temperatures can slow down the exfoliation process as the mixture may become too thick. The mixture may therefore be cooled during the grinding and mixing step, e.g. by cooling the exfoliation mixture (e.g. by cooling the vessel). The exfoliation equipment may therefore be equipped with a cooling means.

[0032] Preferably the mixing and grinding steps are performed simultaneously, although one step may precede the other. Thus, the continuous addition of the ionic liquid may be conducted simultaneously with the mixing step or it may be conducted simultaneously with the grinding step. Preferably, however both the mixing and grinding steps occur simultaneously with the continuous addition of the one or more ionic liquid. [0033] The process may further comprise the step of washing the graphene or 2- dimensional inorganic material once it has been separated from the ionic liquid. Residual impurities from the ionic liquid can become associated with the graphene or inorganic two- dimensional material during the process. Thus, the washing step is more likely to be required in processes in which the ionic liquid: graphite or inorganic multi-layered material ratio is high (e.g. above 2 or above 2.6 ml_ per gram of graphite or inorganic multi-layered material).

[0034] The grinding can be achieved using any suitable apparatus known in the art. Examples of grinding methods include: a mortar and pestle, ball milling, planar milling, pressurised fluid milling, air jet milling in a liquid mixture, roll milling, spin milling, high speed revolution milling (e.g. using collision between particles and vessel pins or forcing the particles to pass through small holes at high energy), and bead milling). A preferred method is using a mechanical mortar and pestle.

[0035] In a further preferred method, the inorganic multi-layered material is mixed and ground using a roll mill. A roll mill typically comprises (in the grinding vessel) a number of parallel cylindrical rolls arranged sequentially, the first and other odd numbered roll after that, turn in the same direction (i.e. they each turn in a direction selected from clockwise or anticlockwise) and the second roll and every even numbered roll after that, turns in the opposite direction (i.e. if the odd numbered rolls turn clockwise, the even numbered rolls turn anticlockwise. Thus, in a five roll mill, if the first, third and fifth rolls turn anticlockwise, the second and fourth rolls turn clockwise or alternatively, if the first, third and fifth rolls turn clockwise, the second and fourth rolls turn anticlockwise). A roll mill may comprise (in the grinding vessel) three parallel cylindrical rolls arranged sequentially, the first and third of the rolls turn in the same direction (i.e. they both turn in a direction selected from clockwise or anticlockwise) and the second roll turns in the opposite direction (i.e. if the first and third rolls turn clockwise, the second roll turns anticlockwise and if the first and third rolls turn anticlockwise, the second roll turns clockwise). For the absence of doubt, the axis of rotation for each roll runs centrally along the length of the cylinders. The material (i.e. the mixture of one or more ionic liquids and graphite or inorganic multi-layered material) passes through a first gap between the first and second rolls and then through a second gap between the second and third rolls.

[0036] In a three roll mill, the gap between the first and the second rolls may be from 1 μηι to 250 μηι (e.g. from 1 μηι to 120 μηι). The gap between the second and third roll may be from <1 μηι to 250 μηι (e.g. from 1 μηι to 120 μηι). Roll mills can also be operated in force mode (e.g. with a gap between the second and third roll of <1 μηι), the force at the second gap can vary from 5N/mm to 500N/mm. The speed of rotation of the first roll may be from 1 rpm to 1000 rpm (e.g. from 3 rpm to 67 rpm). The speed of rotation of the second roll may be from 1 rpm to 1000 rpm (e.g. from 10 rpm to 200 rpm). The speed of rotation of the third roll may be from 1 rpm to 1000 rpm (e.g. from 30 rpm to 600 rpm). The temperature of the first roll may be from 15 °C to 50 °C. The temperature of the second roll may be from 15 °C to 50 °C. The temperature of the third roll may be from 15 °C to 50 °C.

[0037] The material may pass through the roll mill from 1 to 1000, e.g. from 1 to 100, times.

[0038] When the gap between the second and third roll falls below a lowest measurable distance (dependant on the mill model), the mill is said to operate in force mode where the force between the rolls is measured. Load values are typically from 0.1 N/mm to 500 N/mm. Load is the force applied by the roll to the material being exfoliated.

[0039] Typically the continuous addition of the ionic liquid when using a roll mill will be achieved using a nebulising device, e.g. an automated spraying device. This can be used to apply ionic liquid at a predetermined rate. Such devices are known in the art and typically they can be calibrated, allowing the rate of addition (i.e. the volume of liquid delivered to the vessel within a specified time) to be controlled. The ionic liquid will typically be sprayed onto the rolls of the roll mill.

[0040] The inventors have found that when using a roll milling technique with a nebulised ionic liquid addition, and specifically when using a three roll mill and an automated spray device, the graphene yield can be improved by up to 400% over the course of up to 1000 passes through the mill relative to the same process without continuous addition of ionic liquid. Thus an improvement in graphene yield from -20% of initial graphite weight to a graphene yield of -80% of initial graphite weight has been observed.

[0041] The method may comprise the step of sonicating the mixture of one or more ionic liquids and graphite or inorganic multi-layered material. The sonication step may be of one of up to 1 hour in duration; e.g. less than 10% of the time spent mixing and grinding the mixture). Preferably, however, the method of the invention does not comprise the step of sonicating the mixture of one or more ionic liquids and graphite or inorganic multi-layered material. The method of the invention may not comprise a sonication step at all.

[0042] It is conceivable that the suspension of graphene or the two-dimensional inorganic material in the ionic liquid could be used in certain applications in the form it is obtained from the mixing and grinding step, i.e. without even separating the graphene or the inorganic two-dimensional material from the ionic liquid.

[0043] Typically, however, the process will also comprise the step of separating the ionic liquid from the graphene or inorganic two-dimensional material. This may be achieved using a solvent, e.g. a polar solvent. It may be achieved by washing the mixture with a solvent to provide a mixture of the ionic liquid and the solvent on the one hand and solid graphene or inorganic two-dimensional material on the other. The solid graphene or inorganic two-dimensional material may be washed with further amounts of the solvent and the liquid fractions recombined. The solvent may be a polar aprotic solvent (e.g. DMF, NMP, acetone) or the solvent may be a polar protic solvent (e.g. water, ethanol, IPA or a mixture thereof). The solvent may be DMF, acetone or a mixture thereof.

[0044] The ionic liquid may be recovered for re-use. The step of recovering the ionic liquid may comprise separating the ionic liquid from the solvent (e.g. from acetone) by evaporation optionally under reduced pressure, e.g. by heating the mixture of ionic liquid and solvent. The solvent (which typically has a lower boiling point than the ionic liquid) leaves a vapour and the ionic liquid remains as a liquid.

[0045] The solvent may also be recovered for reuse, e.g. by condensing the vapour using, for example, a condenser.

[0046] The step of recovering the ionic liquid may also comprise one or more cleaning steps. Up to 99% of the mass of the ionic liquid can be recovered in this way and it is of sufficient quality to be reused.

[0047] If desired, a solvent exchange can be also performed in order to provide a suspension of the graphene or inorganic two-dimensional material in a second solvent (with the solvent which was used to remove the ionic liquid being a first solvent). This step can be performed immediately after the separation of the ionic liquid from the graphene or inorganic two-dimensional material or it can be performed after one or more further processing steps described herein (e.g. a dispersion step or a step for separating un- exfoliated graphene or multi-layered particles of the inorganic material)

[0048] The process may further comprise the step of dispersing the graphene or two - dimensional inorganic material in a dispersion solvent (either the first solvent or a second solvent, if a solvent exchange has been performed before the dispersion). This typically occurs after the ionic liquid has been removed. Thus, the solid graphene or two - dimensional inorganic material may be dispersed in the dispersion solvent.

[0049] The dispersion solvent may comprise a protic solvent, e.g. an C1-C4 alcohol solvent. The dispersion solvent may be acetone. The dispersion solvent may be a mixture of water and at least one of C1-C4 alcohol and acetone. The dispersion solvent may be a mixture of acetone and water. The dispersion solvent may be a mixture of acetone, a Ci- C₄ alcohol and water. [0050] Preferably, the dispersion solvent is a mixture of a C1-C4 alcohol and water. The volume ratio of the C1-C4 alcohol and water may be from 20:80 to 80:20, from 30:70 to 70:30 or from 60:40 to 40:60. The C1-C4 alcohol may be selected from methanol, ethanol, n-propanol, isopropanol and n-butanol. Preferably, the C1-C4 alcohol is selected from ethanol or isopropanol.

[0051] The inventors have found that dispersions of graphene in water/alcohol (e.g. water/I PA mixtures) are surprisingly stable, even at high concentrations of graphene. Dispersions having a graphene concentration of 4 or 5 g/L have been shown to be stable. This allows for the more space efficient transport and storage of graphene flakes. I PA and water are less harmful than solvents (such as NMP) which have traditionally been used to transport and store graphene flakes. Transport and storage of graphene dispersions in these solvent systems requires less chemical containment than with solvents which have traditionally been used to transport and store graphene thus reducing the cost of both.

[0052] Thus, the concentration of graphene or the inorganic 2-dimensional material in the dispersion may be greater than about 1 g/L, It may be that the concentration of graphene in the dispersion is greater than about 2 g/L. It may be that the concentration of graphene in the dispersion is greater than about 3 g/L. It may be that the concentration of graphene in the dispersion is greater than about 3.5 g/L.

[0053] It may be that the concentration of graphene in the dispersion is less than about 10 g/L. It may be that the concentration of graphene in the dispersion is less than about 8 g/L. It may be that the concentration of graphene in the dispersion is less than about 6 g/L.

[0054] The dispersion step may comprise applying sonication to the suspension of graphene or inorganic two-dimensional material in the solvent. Typically, this sonication has a duration of one day or less, but preferably has a duration of 3 hours or less, e.g. 2 hours or less or 1 hour or less. The suspension will typically be sonicated for a duration of 10 minutes or longer, e.g. 30 min or longer. The sonication process may also comprise stirring the dispersion, e.g. using a magnetic stirrer. This may occur simultaneously with at least a portion of the sonication process. It may occur either before or after the sonication process. It may be that the sonication process is carried out intermittently and that the dispersion is stirred during the periods in which the sonication is not occurring, If this is the case, the periods of stirring (and not sonicating) may be from 1 to 30 minutes long, from 5 to 20 minutes long or from 10 to 15 minutes long. Where the dispersion is sonicated intermittently the values given above for the duration of the sonication process represent the total length of time that the dispersion is undergoing sonication, i.e. the sum of the durations of the individual periods in which the sonication is applied. [0055] An alternative method of dispersing the graphene or inorganic two-dimensional material in a solvent (e.g. the first solvent or a second solvent) is simply to stir the suspension. This may be achieved using any suitable apparatus known in the art, e.g. a mechanical stirrer or a magnetic stirrer. The suspension may be stirred for a duration of 3 hours or less, 2 hours or less or 1 hour or less. The suspension will typically be stirred for a duration of 10 minutes or longer.

[0056] The process may also comprise the step of separating un-exfoliated graphene or inorganic multi-layered material from the graphene or inorganic two-dimensional material. This is typically performed on a suspension of the graphene or inorganic two-dimensional material in a solvent (e.g. the first solvent or a second solvent). Often this separation step is performed after the graphene or inorganic two-dimensional material has been dispersed in the solvent.

[0057] Un-exfoliated graphene or inorganic multi-layered material will typically be removed by centrifugation in the solvent, e.g. DMF, acetone, IPA and/or water. This process is described in WO2012/117251. Typically, the centrifugation will be performed after the graphene or inorganic two-dimensional material in the solvent has been dispersed.

[0058] Further centrifugation steps (e.g. in DMF only) may follow the first centrifugation step.

[0059] The centrifugation parameters can be selected, in combination with the duration of the grinding process, based on the desired flake size and/or thickness.

[0060] Each centrifugation step will typically be 1 h or less in duration, e.g. 15 minutes or less. Each centrifugation step will typically be 30 seconds or longer in duration, e.g. 2 minutes or longer.

[0061] The centrifugation conditions are typically fairly mild. Thus, the frequency of rotation will typically be from about 200 to about 1500 rpm, e.g. from about 250 to about 750 rpm. The frequency of rotation is preferably less than 1000 rpm and most preferably, the frequency of rotation is about 500 rpm.

[0062] Less mild centrifugation conditions (e.g. a frequency of rotation from about 1000 to 10000) may be required in certain circumstances. This might be the case, for example, where particularly small and/or thin flakes are desired.

[0063] An alternative method of separating un-exfoliated graphene or inorganic multi- layered material from the graphene or inorganic two-dimensional material is to allow a suspension to settle and separating the suspension from any sediment. Graphene and many inorganic two-dimensional materials form stable suspensions. Graphite and many multi-layered inorganic materials tend not to form stable suspensions and, if a suspension containing these bulk materials in left to settle, they will sink to the bottom as a sediment.

[0064] Where the exfoliation process has been achieved by roll milling it is preferred that un-exfoliated graphene or inorganic multi-layered material is separated from the graphene or inorganic two-dimensional material by centrifugation.

[0065] The solvent may also be recovered for re-use. The process may further comprise the step of washing the graphene or inorganic two-dimensional material once it has been separated from the ionic liquid. Residual impurities from the ionic liquid can become associated with the graphene or inorganic two-dimensional material during the process. Thus, the washing step is more likely to be required in processes in which the ionic liquid: graphite or inorganic multi-layered material ratio is high (e.g. above 3 ml_ per gram of graphite or inorganic multi-layered material).

[0066] The number of processing steps once the ionic liquid has been removed from the graphene or the inorganic two-dimensional material, the methods used for those steps and the duration of those steps will be selected depending on the quality of graphene or two- dimensional inorganic material desired. Production of a lower grade material will typically involve fewer steps. Indeed, graphene or inorganic two-dimensional material suitable for certain applications can be obtained without any further processing steps once the ionic liquid has been removed. Production of a higher grade material, in which all or

substantially all un-exfoliated graphene or inorganic multi-layered material has been removed from the graphene or inorganic two-dimensional material, will typically involve more processing steps.

[0067] The process may further comprise the steps of removing the solvent (e.g. the first solvent or the second solvent) from the graphene or the inorganic two-dimensional material. This may be achieved by evaporation, e.g. by distillation or by evaporation under reduced pressure. This may be achieved by filtration. This may be achieved by freeze drying.

[0068] Thus, the end product of the process may be graphene or the inorganic two- dimensional material in the form of a powder. It may be graphene or the inorganic two- dimensional material in the form of a suspension or gel in the ionic liquid. It may be graphene or the inorganic two-dimensional material in the form of a suspension in a solvent.

[0069] The size and thickness of the flakes can be selected by varying the exact method used. Factors which affect flake size include the starting particle size of graphite or layered inorganic material used, the duration of the mixing and grinding steps, and whether and to what extent the material is subjected to sonication or centrifugation steps after the material has been ground.

[0070] Typically, the graphene or inorganic two-dimensional material obtained from the process will be in the form of few layer flakes, i.e. predominantly from 2 to 8 layers thick, although it is possible to make material comprising monolayer graphene via the processes described herein. Typically, the flakes of graphene or inorganic two-dimensional material will have a longest lateral length from about 5 nm to 100 μηι. It may be that the flakes of graphene or inorganic two-dimensional material has a longest lateral length of up to 20 μηι

[0071] Thus, the size of the flake (i.e. the longest lateral length) might be from about 100 nm to about 5 μηι, e.g. from about 200 nm to about 10 μηι or from about 200 nm to about 1 μηι. A very small flake size might be desired, e.g. from 5-100 nm, or a very large flake size might be desired, e.g. from 5 to 100 μηι.

[0072] The process of the invention may be a batch process. Additionally or

alternatively, the process of the invention is a continuous process.

[0073] The process of the invention may be made continuous by incorporating a means for removing the gel gradually from the mixture, e.g. a screw conveyor. It is important in this case that the exfoliation is sufficiently efficient, and the rate of addition of the graphite or multi-layered inorganic material and the rate of removal of the gel are each such that the gel which is removed contains useful amounts of graphene or the two-dimensional inorganic material. It may be that the exfoliation process is conducted continuously but that the subsequent processing steps (e.g. ionic liquid separation, centrifugation etc) are conducted in a batchwise manner.

[0074] In a second aspect of the invention is provided graphene or an inorganic two- dimensional material obtainable or obtained according to the process of the first aspect. The graphene or the inorganic two-dimensional material may thus be in the form of a powder, in the form of a suspension or gel in the ionic liquid or in the form of a dispersion in a solvent.

[0075] In a third aspect of the invention is provided a dispersion of graphene flakes in a solvent mixture, wherein the solvent mixture is a mixture of a C1-C4 alcohol and water and wherein the concentration of graphene in the dispersion is greater than about 1 g/L.

[0076] The volume ratio of the C1-C4 alcohol and water in the solvent mixture may be from 20:80 to 80:20, from 30:70 to 70:30 or from 60:40 to 40:60. The C1-C4 alcohol may be selected from methanol, ethanol, n-propanol, isopropanol and n-butanol. Preferably, the C1-C4 alcohol is selected from ethanol or isopropanol. It may be that the total volume of both water and C1-C4 alcohol together in the solvent mixture is greater than about 90% of the total volume of the solvent mixture. It may be that the total volume of both water and C1-C4 alcohol together in the solvent mixture is greater than about 95% of the total volume of the solvent mixture. It may be that the total volume of both water and C1-C4 alcohol together in the solvent mixture is greater than about 98% or even 99% of the total volume of the solvent mixture.

[0077] As discussed above, the inventors have surprisingly found that dispersions of graphene in water/alcohol (e.g. water/I PA mixtures) can be surprisingly stable, even at high concentrations of graphene. Dispersions having a graphene concentration of 4 or 5 g/L have been shown to be stable for up to a week.

[0078] Thus, the concentration of graphene or the inorganic 2-dimensional material in the dispersion may be greater than about 1 g/L, It may be that the concentration of graphene in the dispersion is greater than about 2 g/L. It may be that the concentration of graphene in the dispersion is greater than about 3 g/L. It may be that the concentration of graphene in the dispersion is greater than about 3.5 g/L.

[0079] It may be that the concentration of graphene in the dispersion is less than about 10 g/L. It may be that the concentration of graphene in the dispersion is less than about 8 g/L. It may be that the concentration of graphene in the dispersion is less than about 6 g/L.

[0080] The dispersion may be obtainable or obtained by the process of the first aspect.

[0081]

BRIEF DESCRIPTION OF THE DRAWINGS

[0082] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 shows a photograph of a scriber containing a graphite/ionic liquid gel after 10 h of grinding in the absence of a continuous addition of an ionic liquid. The gel is thick and dry.

Figure 2 shows a photograph of a sample of graphene obtained following an ionic liquid exfoliation in the absence of a continuous addition of an ionic liquid. The sample has also been subjected to centrifugation (at low speed). Substantial amount of material has precipitated to the bottom of the tube, indicating that the precipitated material is graphite rather than graphene.

Figure 3A shows the Raman spectrum of the material obtained from ionic liquid exfoliation of graphite in the absence of a continuous addition of an ionic liquid. The spectra of three samples are superimposed on top of each other. The 2D band (around 2700 cm^-1 ; expanded in Figure 3B) shows a clear shoulder arising from the presence of graphite.

Figure 4 shows an optical spectroscopy image of material obtained from ionic liquid exfoliation of graphite in the absence of a continuous addition of an ionic liquid, once the material has been deposited on a silicon substrate. The flakes are not of uniform thickness or lateral size. Some flakes are thick (based on optical contrast) and some are over 20 μηι wide.

Figure 5 shows a photograph of a scriber containing a graphite/ionic liquid gel after 10 h of grinding with continuous addition of an ionic liquid. The gel is thin and less material is stuck to the scriber compared to Fig 1.

Figure 6 shows a photograph of a sample of graphene obtained following an ionic liquid exfoliation with continuous addition of an ionic liquid. The sample has also been subjected to centrifugation (at low speed). The suspension is stable and dark, indicating that the precipitated material is predominantly graphene rather than graphite.

Figure 7 A shows the Raman spectrum of the material obtained from ionic liquid exfoliation of graphite with continuous addition of an ionic liquid. The spectra of three samples are superimposed on top of each other. The 2D band (around 2700 cm^-1 ;

expanded in Figure 3B) lacks any shoulder arising from the presence of graphite. The breadth of the 2D band indicates that the material is few layer graphene (i.e. it is predominantly made up of material which is from 2 to 8 layers thick)

Figure 8 shows an optical spectroscopy image of material obtained from ionic liquid exfoliation of graphite with continuous addition of an ionic liquid, once the material has been deposited on a silicon substrate. The flakes are all thin and have a uniform lateral size.

Figures 9A and 9B shows scanning electron microscope images (SEM) of the material obtained from ionic liquid exfoliation of graphite with continuous addition of an ionic liquid. The flake sizes are predominantly between 200 nm and 1 μηι.

DETAILED DESCRIPTION

[0083] Throughout the present specification, the term 'added continuously' may mean that one or more ionic liquids are added as a stream or it may also mean that they are added dropwise or portionwise, in which discontinuous drops or portions are added at a predetermined rate over time.

[0084] The term 'few-layered' means a particle which is so thin that it exhibits different properties than the same compound when in bulk. Not all of the properties of the compound will differ between a few-layered particle and a bulk compound but one or more properties are likely to be different. A more convenient definition would be that the term 'few layered' refers to a crystal that is from 2 to 10 molecular layers thick (e.g. 2 to 5 layers thick). A molecular layer is the minimum thickness chemically possible for that compound. In the case of boron-nitride one molecular layer is a single atom thick. In the case of the transition metal dichalcogenides (e.g. M0S2 and WS2), a molecular layer is three atoms thick (one transition metal atom and two chalcogen atoms). Thus, few-layer particles crystals are generally less than 100 nm thick, depending on the compound and are preferably less than 20 nm thick, e.g. less than 10 or 5 nm thick.

[0085] The term 'inorganic multi-layered material' refers to a particle of an inorganic layered material which exhibits similar properties to the same compound when in bulk. A more convenient definition would be that the term 'multi-layered material refers to a particle that is more than 10 molecular layers thick.

[0086] The term 'inorganic two-dimensional material' means an inorganic layered material in a form which is so thin that it exhibits different properties than the same compound when present in bulk multi-layered form. Typically, inorganic two-dimensional compounds are in a form which is single- or few layers thick, i.e. up to and including 10 molecular layers thick. A two-dimensional crystal of a layered material (e.g. an inorganic compound or graphene) is a single or few layered particle of that material.

[0087] The term 'inorganic layered material refers to any compound made up of two or more elements which forms layered structures in which the bonding between atoms within the same layer is stronger than the bonding between atoms in different layers. Many examples of inorganic layered compounds have covalent bonds between the atoms within the layers but van der Waals bonding between the layers. The term 'inorganic layered compound' is not intended to encompass graphene.

[0088] Two-dimensional materials are not truly two dimensional, but they exist in the form of particles which have a thickness that is significantly smaller than their other dimensions. The term two-dimensional has become customary in the art. Many inorganic compounds exist in a number of allotropic forms, some of which are layered and some of which are not. For example boron nitride can exist in a layered graphene- or graphite-like structure (h-BN) in which the boron and nitrogen atoms have a have a trigonal planar orientation or as a diamond-like structure in which the boron and nitrogen atoms are tetrahedrally orientated.

[0089] Examples of layered inorganic compounds to which the present invention can be applied include: hexagonal boron nitride, bismuth strontium calcium copper oxide

(BSCCO), transition metal dichalcogenides (TMDCs), Sb₂Te₃, Bi₂Te₃ and Μη0₂. [0090] TMDCs are structured such that each layer of the compound consists of a three atomic planes: a layer of transition metal atoms (for example Mo, Ta, W...) sandwiched between two layers of chalcogen atoms (for example S, Se or Te). Thus in one embodiment, the TMDC is a compound of one or more of Mo, Ta and W with one or more of S, Se and Te. There is strong covalent bonding between the atoms within each layer of the transition metal chalcogenide and predominantly weak Van der Waals bonding between adjacent layers. Exemplary TMDCs include NbSe2, WS2, M0S2, TaS2, PtTe2, VTe₂.

[0091] A layer of graphene consists of a sheet of sp²-hybridized carbon atoms. Each carbon atom is covalently bonded to three neighboring carbon atoms (in a trigonal planar orientation) to form a 'honeycomb' network of tessellated hexagons. Carbon

nanostructures which have more than 10 graphene layers (e.g.. 20 atomic layers) generally exhibit properties more similar to graphite than to mono-layer graphene.

Conversely, carbon nanostructures which have up to 10 graphene layers (e.g. 2-7 atomic layers) generally exhibit properties more similar to mono-layer graphene than to graphite. Thus, throughout this specification, the term graphene is intended to mean a carbon nanostructure with up to 10 graphene layers. Graphene is often referred to as a two- dimensional structure because it represents a single sheet or layer of carbon of nominal (one atom) thickness. It may be that the graphene produced in the processes of the invention isis mono-layer graphene , but this will not typically be the case. Graphene which is not mono-layer graphene (i.e. few layer graphene) finds application in a range of contexts.

[0092] The term ionic liquid refers to an organic or partially inorganic salt which is liquid at a temperature below 100 °C. Ionic liquids are very effective at solvating a range of compounds and materials, including both polar and non-polar species. They also offer a number of attractive properties which render them useful in place of solvents in industrial processes: they have low volatility and low flammability. The ionic liquid used in the invention may comprise an organic ion, e.g. an ammonium, imidazolium or pyridinium ion. Thus, the ionic liquid may be one which contains an imidazolium ion. A preferred ionic liquid is 1-butyl-3-methyl-imidazolium hexafluorophosphate.

[0093] Graphene flakes formed according to the methods of the invention tend to be formed in irregular shapes. The term 'flake size' used throughout this specification is intended to mean the length of the longest lateral axis. It can conveniently be determined using transmission electron microscopy (TEM). A few drops of a dispersion of the graphene flakes is dropped on holey carbon grids and a large number (>100) of flakes are imaged using TEM. This technique is described in detail in J. N. Coleman et al., Small, 2010, 864-871. A similar technique can be carried out using scanning electron microscopy (SEM) (see Sepioni et al.; PRL; 105, 2010, 207205).

[0094] Where graphene is described in this specification as having a flake size which falls within a range it means that the mean average of the longest lateral axis of 500 randomly selected flakes when measured by TEM falls within the stated range.

[0095] The same techniques can be used to determine the thickness of graphene flakes formed according to the process of this invention. In TEM images the edges of individual layers are typically visible. The technique is described in detail in J. N. Coleman et al., Small, 2010, 864-871. Raman spectroscopy can also be used to determine the number of layers in a graphene sample (see Ferrari et al.; PRL; 97, 2006, 187401).

[0096] Where graphene is described in this specification as having a flake thickness which falls within a range it means that the mean average of the thicknesses (in layers) of 500 randomly selected flakes when measured by TEM falls within the stated range.

[0097] 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.

[0098] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0099] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Illustrative Process - mortar and pestle

[00100] The following process is illustrative of the processes of the invention and is not intended to be limiting.

[00101] Initially 30 mL of ionic liquid (e.g. butylmethylimidazoleum-hexafluorophosphate) is added to 30 g graphite into the vessel of the grinding machine. The grinding process is then started. The rest of the ionic liquid (an additional amount of about 20-30 mL) is added through a Fishnar Liquid Dispenser at a rate of about 0.5 mL every 30 mins.

[00102] For the grinding and mixing process, a mortar RM200 (from Retsch) is used, and the process is continued for about 55-90 hours (e.g. about 55-60 hours or about 60-90 hours).

Illustrative Process - roll mill

[00103] In an alternative example an ionic liquid (e.g. butylmethylimidazoleum- hexafluorophosphate; 10 mL) is added to graphite (10 g) and pre-mixed at low shear in order to coat the graphite with ionic liquid. The graphite/ionic liquid slurry is transferred to and then passed through a three-roll mill (EXAKT E80)a number of times until graphene is exfoliated. The mill is first operated in gap mode, where the first gap and second gap are set at a specific distance with a 3: 1 ratio (Gap1 :Gap2), for example where gapl is 15um and gap2 is 5um. Ionic liquid is applied to roll#2 using a Paasche A-JUAR automatic airbrush at a rate of 0.02ml/min, whilst the mill is in operation (i.e. the rolls are in motion and the slurry is passing through the gaps). The application of the ionic liquid should be sufficient to prevent the slurry from drying out and falling off the rolls, but not excessive which could cause slipping of the slurry on the rolls and preventing it passing through the gaps. Once the initial pass is completed the gaps are further reduced until gapl is 5um and gap2 is in the force setting of 5N/mm. The mill is then operated for 10 passes. After this gap 2 is reduced until the gap on this roll is reduced and operating in force mode at 26N/mm. The mill is then operated for a further 5 passes. After this Gapl is reduced over a further 10-15 passes until a force of 26N/mm is reached. The Mill is then operated under these force mode settings for a further 75 passes.

[00104] In a further example an EXAKT E80 three-roll mill is used and the process continued for 75 passes with Gapl at 26N/mm and Gap2 at 26N/mm.

Illustrative Processes - Isolation of obtained graphene [00105] The ionic liquid is removed from the graphene by washing the mixture with an acetone-DMF mixture (50:50 v/v) or, alternatively, with acetone.

[00106] The graphene is then collected and dispersed in the desired solvent (for example DMF or a IPA/water mixture) by applying sonication for 1 h or by stirring the material using a magnetic stirrer, until a homogeneous suspension is obtained.

[00107] The suspension can be either centrifuged at very mild speeds or left overnight to settle. This step is done to remove the graphitic sediment that has not been successfully exfoliated during the process.

[00108] This sediment is then separated from the rest of the suspension. The separation of the sediment from the suspended graphene is the final step if graphene in suspension is the desired product of the process.

[00109] A further step of centrifugation at very high speed (10K rpm or more) might follow if a very concentrated suspension is desired. In this case, after this further centrifugation step, the material from the very bottom of the tubes can be collected and re-dispersed in a smaller amount of solvent to the desired concentration).

[00110] Alternatively, this collected material can be freeze-dried in order to obtain a powder.

Claims

20 Claims

1. A process for the production of graphene or an inorganic 2-dimensional material, the process comprising the step of:

mixing and grinding a mixture of graphite or an inorganic multilayered material inone or more ionic liquids to form graphene or the inorganic 2-dimensional material; and

2. A process as claimed in claim 1 , wherein graphite is the starting material in the process and graphene is the product.

3. A process as claimed in claim 1 or 2, wherein a portion of the at least one ionic liquid is premixed with the graphite or an inorganic multi-layered material at or before the start of the process and a further portion of the at least one ionic liquid is added continuously to the mixture at a predetermined rate after the exfoliation process has begun.

4. A process as claimed in claim 1 or 2, wherein all of the ionic liquid is added continuously at a predetermined rate to the mixture.

5. A process as claimed in any preceding claim, wherein the total amount of ionic liquid mixed with the inorganic 2-dimensional material during the process is from 1 mL to 4 mL per gram of graphite or multilayered inorganic material.

6. A process as claimed in claim 5, wherein the total amount of the ionic liquid is from 1.6 to 2.6 mL per gram of the inorganic 2-dimensional material.

7. A process as claimed in any preceding claim, wherein the process is performed in an atmosphere of air. 21

8. A process as claimed in any preceding claim, wherein the process is performed at a temperature in the range from about 15 to about 40 °C.

9. A process as claimed in any preceding claim, wherein the mixing and grinding steps are performed simultaneously.

10. A process as claimed in any preceding claim, wherein the one or more ionic liquids which are added continuously may be the same as the one or more ionic liquids which were in the mixture initially.

11. A process as claimed in any preceding claim, wherein the process includes the step, before the mixing and grinding step, of suspending the graphite or multi-layered 2- dimensional material in the one or more ionic liquids.

12. A process as claimed in any preceding claim, wherein the process further comprises the step of washing the graphene or inorganic 2-dimensional material once it has been separated from the ionic liquid.

13. A process as claimed in any preceding claim, wherein the method does not comprise the step of sonicating the mixture of one or more ionic liquids and graphite or inorganic multilayered material.

14. A process as claimed in any preceding claim wherein the process further comprises the step of separating the ionic liquid from the graphene or two dimensional inorganic material.

15. A process as claimed in claim 14, wherein the step of separating the ionic liquid from the graphene or two dimensional inorganic material comprises washing the mixture with a solvent.

16. A process as claimed in any preceding claim, wherein the process further comprises the step of dispersing the graphene or two -dimensional inorganic material in a dispersion solvent.

17. A process as claimed in any preceding claims wherein the process also comprises the step of separating un-exfoliated graphene or inorganic multi-layered material from the graphene or two -dimensional inorganic material. 22

18. A process as claimed in any preceding claim, wherein the continuous addition is achieved using an automated device.

19. A process as claimed in any preceding claim, wherein the process is a continuous process.

20. A process as claimed in any preceding claims wherein the mixing and grinding step is performed using a roll mill.

21. A process as claimed in claim 20, wherein the continuous addition of the ionic liquid is achieved using a nebulising device.

22. Graphene or an inorganic two-dimensional material obtained according to the process of any one of claims 1 to 21.

23. A dispersion of graphene flakes in a solvent mixture, wherein the solvent mixture is a mixture of a C1-C4 alcohol and water and wherein the concentration of graphene in the dispersion is greater than about 1 g/L