Graphene Production Method

Abstract

A method whereby a mixture of expanded graphite and a liquid is subjected to high-shear mixing in order to exfoliate the expanded graphite and produce graphene. The exfoliation may be performed for be

Description

Technical Field of the Invention The present invention relates to a method for producing graphene, to the graphene produced by the method and to a coating or composite comprising graphene. 5 Background to the Invention Few-layer graphene may be produced by following a “top-down” method which involves the physical exfoliation of graphite. In the past this has been achieved by mechanically exfoliating 2D materials in solid-state, e.g., by micromechanical cleavage and ball milling. However, these methods fail to provide a sufficient yield 10 and satisfactory flake sizes while remaining defect free. It is also known to exfoliate graphite in the liquid phase by dispersing graphite in a solvent (aqueous or organic), exfoliating the graphite into smaller fragments, partially exfoliated graphite (graphene nanoplatelets) and few-layer graphene, e.g., by high shear mixing and then separating few-layer graphene from the other fragments. Although liquid phase exfoliation of 15 graphite enables greater yields of few-layer graphene to be obtained relative to the solid-state exfoliation of 2D materials, the yields of few-layer graphene are still limited because it is typically produced using a batch production method. Therefore, there is a need for a low-cost method for producing defect-free few-layer graphene in greater yields. 20 It is an object of embodiments of the invention to provide a method for increasing the yield of graphene, particularly few-layer graphene. It is an object of embodiments of the invention to provide a method for producing defect-free graphene in high yields. 15 05 25 It is an object of embodiments of the invention to a provide a cost-effective method for producing graphene in high yields, particularly few-layer graphene. Summary of the Invention According to a first aspect of the invention there is provided a method for the 5 production of oxide-free graphene, the method comprising the steps of: a. intercalating a graphite with a quaternary ammonium sulphate inert intercalation compound and thermally treating the intercalated graphite to produce oxide-free expanded graphite; b. mixing the expanded graphite and a liquid in a container to provide a 10 mixture, and c. exfoliating the expanded graphite by exposing the mixture to an in-line high shear mixing assembly to produce graphene, wherein the mixture is drawn from the container through the in-line high shear mixing assembly and recirculated to the container from the in-line high shear 15 mixing assembly. It has been found that significant increases in the yield of few-layer graphene are obtainable by subjecting expanded graphite to a high shear mixing treatment. Moreover, the few-layer graphene obtained is defect-free or substantially defect-free and can be produced at low cost. Accordingly, the above method is a low-cost and 20 scalable method for producing few-layer graphene with no or few defects. In some embodiments a high shear mixing assembly may be located in a container for dispersing the expanded graphite in the liquid. The temperature of the 15 05 25 container may be adjusted in order to avoid or at least minimise the number of defects in the exfoliated graphene. Suitably, the container may be kept at an ambient temperature. The container may be provided with a water jacket which is operable to achieve the desired temperature. In some embodiments the water jacket may be 5 provided within the container. The high shear mixing assembly may be located in a device connected to and located downstream of the container. The device may comprise an in-line high shear mixer. Accordingly, the mixture is exposed to an in-line high shear mixing assembly. In operation, actuation of the device causes the mixture to be drawn from the 10 container through the in-line high shear mixing assembly. This in-line assembly ensures that a significant proportion, if not all, of the expanded graphite in the mixture is subjected to mechanical and hydraulic shear forces. As a result, further increases in the yield of few-layer graphene can be obtained. In some embodiments the temperature of the mixture may be adjusted prior to 15 exposing the mixture to the in-line high shear mixing assembly. Suitably, a device for reducing the temperature of the mixture may be provided between the container and the in-line high shear mixing assembly. The device may comprise a chiller unit. Once the mixture has passed through the high shear mixing assembly it is recirculated back to the container or directed elsewhere for further processing. 20 Recirculating the mixture back to the container ensures that increased quantities of expanded graphite and / or exfoliated products thereof are exposed to the in-line high shear mixing assembly, which in turn enables high yields of few-layer graphene to be obtained. 15 05 25 In some embodiments the temperature of the mixture may be adjusted after it has passed through the in-line high shear mixing assembly, e.g.. during recirculation of the mixture back to the container. Suitably, a device for reducing the temperature of the mixture may be provided between an in-line high shear mixing assembly outlet 5 and a container inlet. The device may comprise a chiller unit. In some embodiments a high shear mixing assembly may be located in the container and in the device located downstream of the container. In this way, the expanded graphite is subjected to shear forces in the container and when passing through the device which enables greater yields of graphene to be obtained. 10 The high shear mixing assembly may comprise a rotor-stator assembly. The rotor may be rotated at least 6000 rpm. A rotational speed of less than 6000 rpm results in reduced yields of few-layer graphene which is undesirable. Suitably, the rotor may be rotated at 6000-9000 rpm. It was found that while a rotational speed of more than 9000 rpm does not further increase the yield, it may compromise the lateral 15 size of few layer graphene particles. The expanded graphite may be exfoliated for at least 8 hours. If the expanded graphite is exfoliated for less than 8 hours then the resulting mixture contains more graphene nanoplatelets (GNP) than few-layer graphene (FLP) which is undesirable. By increasing the exfoliation period to at least 8 hours increased quantities of few-20 layer graphene can be obtained. Suitably, the expanded graphite may be exfoliated for between 8 and 24 hours. Exfoliating the expanded graphite for more than 24 hours increases the yield of few-layer graphene, however, its lateral size may be reduced significantly. 15 05 25 The expanded graphite is produced by intercalating graphite with an intercalation compound and thermally treating the intercalated graphite. Expanded graphite may be obtained by heating the intercalated graphite at a temperature of 600 °C to 1200 °C. The intercalated graphite may be heated at a 5 temperature of at least 850°C, and in some embodiments the intercalated graphite may be heated at a temperature of 850°C to 1100°C. Suitably, the intercalated graphite may be heated at a temperature of 900°C to 1000°C. At temperatures below 600°C, only partial expansion of graphite occurs because the thermal energy provided is not substantial enough to weaken the van der Waal’s forces between adjacent graphene 10 layers. At temperatures above 1200 °C, there is an increased risk that the graphite will thermally decompose unless the heat treatment is canned out under a carefully controlled environment. The temperature boundaries indicated above achieve a high expansion rate under a reduced oxygen environment. At temperatures above 1200°C, an inert gas environment may be required to avoid thermal decomposition of graphite. 15 In some embodiments, the thermal expansion of intercalated graphite is performed for a period of from at least 10 seconds to not more than 1 min. The thermal expansion of intercalated graphite may be performed for a period of from at least 20 to 30 seconds. The intercalated graphite may be heated in an inert atmosphere in order to 20 prevent or at least reduce the formation of oxides. In some embodiments, the intercalated graphite may be heated using infrared radiation or by induction. The graphite from which the expanded graphite is produced may comprise natural graphite such as or highly oriented pyrolytic graphite ("HOPG"). Suitably the natural graphite may comprise vein graphite. 15 05 25 The graphite from which expanded graphite is produced may have a lateral size of more than 50 pm. In some embodiments the graphite may have a lateral size of from at least 100 pm and up to 500 pm. Suitably, the lateral size of the graphite may be from at least 180 pm up to 300 pm. The use of graphite having a larger lateral size 5 increases the rate of expansion in the presence of intercalating agents and heat. This in turn enables increased yields of FLG to be obtained after high shear mixing. Moreover, due to the increased rate of expansion less energy may be required to produce FLG so that the exfoliation time and shear force can be reduced. In order to obtain defect free products, inert intercalants such as quaternary 10 ammonium sulphate or ammonium sulphate may be used. The expanded graphite may be defect-free or substantially defect-free. The expanded graphite may be subjected to a mechanical milling treatment prior to the step of high shear mixing. In particular, the expanded graphite may be provided in a ball milling vessel and subjected to a ball milling treatment, or it may be 15 subjected to a fluidised bed air milling treatment to reduce the lateral size of the expanded graphite layers. In some embodiments the ball milling treatment may be performed for a period of from at least 4 hours to not more than 10 hours. Hie ball milling of expanded graphite may be performed for a period of from at least 5 hours to not more 20 than 8 hours. Suitably, ball milling may be performed for a period of from at least 6 hours to not more than 7 hours. In some embodiments the ball milling vessel may be rotated at a speed of from at least 200 rpm to not more than 600 rpm. In other embodiments the ball milling 15 05 25 vessel may be rotated at a speed of at a speed of from at least 300 rpm to not more than 500 rpm. Suitably, the ball milling vessel may be rotated at a speed of at a speed of from at least 350 rpm to not more than 450 rpm. The expanded graphite may be screened after the mechanical milling treatment 5 in order to obtain expanded graphite having a pre-determined lateral size. By mechanically milling expanded graphite and then screening it to obtain expanded graphite having a pre-determined size, improved yields of FLG can be obtained after high shear mixing relative to expanded graphite which has not been mechanically milled or screened but is of substantially the same size. 10 The expanded graphite may have a mean lateral particle size of 5-30 microns. In some embodiments, the expanded graphite may have a mean lateral particle size of 10-20 microns. The use of expanded graphite that has been mechanically milled and screened to obtain expanded graphite having a mean particle size of 10-20 microns enables greater yields of quality FLG to be obtained after high shear mixing. Suitably, 15 the expanded graphite may have a mean lateral particle size of 15-20 microns. The liquid may compxise water and / or a solvent. Suitably, the liquid may comprise de-ionised water. The solvent may comprise cyrene (dihydrolevoglucosenone), N-methyl-2-pyrrolidone (NMP), N - dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), dimethyl sulfoxide (DMSO), y-20 butyrolactone (GBL), ortho dichloro benzene (ODAC), butyl acetate, toluene or a combination thereof. 15 05 25 The ratio of expanded graphite to liquid may be from 0.01:1 to 0.1 to 1. Suitably, the ratio may be from 0.03:1 to 0.08:1. In some embodiments the ratio may be 0.05:1. The mixture may comprise a surfactant. The mixture may comprise surfactants 5 and dispersing agents. The mixture may comprise anionic surfactants, non-ionic surfactants, cationic surfactants, amphoteric surfactants, silicone surfactants, fluorosurfactants, polymeric surfactants and combinations thereof. In particular, the mixture for water based dispersions may comprise one or more of Disperbyk 2010, Disperbyk 2080, Disperbyk 2012, Antiterra 250, Disperbyk 190 (manufactured by BYK), a salt 10 of cholic acid such as sodium deoxycholate (SDC), sodium dodecylbenzenesulfonate (SDBS), cetyltrimethylammonium bromide (CATB) and for solvent based dispersions Disperbyk 2050 , Anti-Terra-U 100, BYK-9076, Disperbyk-110 and Disperbyk-2025. “Cyrene”, “Disperbyk” and “Anti-Terra-U” are registered trademarks. The ratio of expanded graphite to surfactant may be from 2:1 to 10:1. Suitably, 15 the ratio may be from 4:1 to 8:1. In some embodiments the ratio may be 5:1. When the ratio of expanded graphite: surfactant is below 2:1, expanded graphite is not adequately dispersed and stabilised in the liquid which can lead to agglomeration of expanded graphite, smaller fragments such as graphene nanoplatelets and few-layer graphene. If the expanded graphite: surfactant ratio is greater than 10:1 then the 20 concentration of the expanded graphite particles is too low and the impact of the shear force felt by individual particles is insignificant which leads to an inefficient exfoliation process. The method may comprise the steps of separating graphene nanoplatelets from 15 05 25 few-layer graphene and exposing a dispersion of the separated graphene nanoplatelets to the high shear mixing assembly to produce oxide-free few-layer graphene. Alternatively, the separated graphene nanoplatelets may be freeze dried and stored. 5 The method may be a continuous method for producing oxide-free graphene. The cost of producing oxide-free graphene may be reduced when it is produced using a continuous or a semi-continuous production method rather than a batch-wise production method. Detailed Description of the Invention 10 In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows Raman spectra of graphene produced by passing expanded graphite through an in-line high shear mixing assembly. 15 Figure 2 shows Raman Spectra of graphene produced by passing non-expanded graphite through an in-line high shear mixing assembly. Figure 3 shows a schematic of apparatus including an in-line high shear mixier for exfoliating expanded graphite. To obtain expanded graphite, 3 g of vein graphite (99 % purity) having a particle size 20 range of 100 - 500 um is soaked in an intercalant solution for at least 4 hours. The intercalant solution is prepared by mixing 1 g of an oxidiser such as KMnO4 with 16 mL of a strong acid such as H2SO4. Accordingly, the ratio of expanded graphite: 15 05 25 oxidiser: acid is 3:1:16. Intercalate compounds such as KMnO4 and strong acids such as H2SO4 can cause defects in graphene and do not do not fall within the scope of the claimed invention. In order to obtain graphene that is free from oxide defects, graphite is intercalated with inert quaternary ammonium sulphate intercalants. 5 The intercalated graphite is filtered, dried, and then heated at 1000 °C under a reduced oxygen environment for 30 seconds. Accordingly, the expansion of graphite is facilitated by intercalation and the subsequent heat shock. The expanded graphite particles have a large lateral size and in order to improve the exfoliation process efficiencies (reduced energy consumption and 10 increased exfoliation rate), the particle size was reduced. This was achieved by ball milling the expanded graphite particles in a ball milling vessel for 6 hours using a 1-2 L Vertical Benchtop Square Planetary Ball Mill, (0.75 to 1.5 KW ) containing 1 to 2 mm diameter ceramic balls. The ball milling vessel was rotated at 400 rpm and the process was stopped for 30 min at every 30 min to avoid overheating. The material is 15 then sieved in a vibratory sifting screen to obtain expanded graphite with an average lateral particle size of 15 to 20 microns. In embodiments where larger volumes (> 2L) of expanded graphite are to be mechanically milled the expanded graphite is fed into a Hosokawa Opposed Jet Mill AFG, (2 kW, 17000 - 22000 rpm) to reduce the lateral particle size. The particles 20 were subjected to milling and screening at 20000 rpm for 6 hrs. The resulting product has a particle size distribution, d50, of about 5-20 microns. In one embodiment, expanded graphite, a liquid and surfactants are added to a mixing vessel 1 to form a mixture. The expanded graphite has an average particle size 15 05 25 of 15-20 microns and a layer spacing of >0.4nm. The liquid may be water or an organic solvent and the choice of liquid and surfactants will depend on the application in which the obtained few-layer graphene is to be used. In this embodiment 1000 g of expanded graphite is mixed with 20 L de-ionised water and 200 g of Disperbyk 2010, 5 a surfactant manufactured by BYK. The mixing vessel 1 comprises a mixing element 2 which is operable to mechanically mix the expanded graphite, liquid and surfactants so that the expanded graphite remains dispersed in the liquid. The mixing element 2 rotates at 2000 rpm. An in-line high shear mixing apparatus 3 by Silverson (RTM) is provided 10 downstream of the mixing vessel 1. A pipe 4 extends between a mixing vessel outlet 5 and a high shear mixing apparatus inlet 6 so that in use the mixture is continuously drawn from the mixing vessel 1 into the high shear mixing apparatus 3. The high shear mixing apparatus 3 comprises a rotor-stator assembly (not shown) for high shear mixing the expanded graphite in the liquid. The stator in this embodiment comprises a 15 plurality of square perforations for the mixture to flow through and the rotor comprises one or more blades that is / are arranged to be rotated relative to the stator. In this embodiment the rotor is rotated at 8000 rpm for 8 hours. The in-line high shear mixing apparatus 3 comprises an in-line high shear mixing apparatus outlet 7. The in-line high shear mixing apparatus outlet 7 is 20 connected to a mixing vessel inlet 8 via a recirculation pipe 9. Once the mixture has passed through the rotor-stator assembly of the in-line high shear mixing apparatus 3 it is recirculated back into the mixing vessel 1 via the recirculation pipe 9. In this way, expanded graphite and graphite fragments are continuously subjected to high 15 05 25 mechanical and hydraulic shear forces which enables greater yields of few-layer graphene to be obtained. The recirculation pipe 9 may comprise a valve 10 connected to a conduit 11 which leads to another container 12 or apparatus for respectively storing or further processing the exfoliated graphene once high shear mixing of the 5 mixture is complete. The exfoliated graphene is allowed to settle for 24 hours. Graphene nanoplatelets in the mixture containing exfoliated graphene settle to the bottom of the container while the few-layer graphene remains dispersed in the aqueous supernatant which lies above the graphene nanoplatelets. Separation of few-layer graphene from 10 the graphene nanoplatelets is achieved by decanting the supernatant from the container. To enable 100 % conversion of expanded graphite into few-layer graphene, the obtained graphene nanoplatelets may be re-introduced into the mixing vessel 1 for further exfoliation by the in-line high shear mixing apparatus 3. Alternatively, the 15 graphene nanoplatelets themselves may be used in the manufacture of composite materials for example. The supernatant was diluted 1:5 ratio with deionised water and applied on a silicon grid. The film formed on the grid was exposed to a temperature of 300°C in a vacuum oven to remove the surfactants. Raman Spectroscopy was used to characterise 20 few- layer graphene and the results are shown in Table 1 below. The results confirmed that exposing expanded graphite to an in-line high shear exfoliation treatment enables few-layer graphene to be obtained and that it is characterised by D, G and 2D peaks at 1348 cm4, 1575 cm4 and 2692 cm4 respectively. Table 1 also 15 05 25 shows the results of a comparative experiment in which expanded graphite was replaced by non-cxpandcd graphite (T1MREX® SFG 75 Graphite by Imerys). Both graphene samples have a 2D peak at closer wavelength positions. The shape, position and the intensity of 2D peak indicates that the materials is graphene 5 rather than graphite. Higher values for the intensity ratio of I2D / IG and sharpness of the 2D peak indicates that the exfoliation rate of expanded graphite is higher and that the number of layers in few-layer graphene is lower compared to graphene obtained from the exfoliation of non-expanded graphite. The ID / IG ratio is a measure of the extent of defects in graphene layers, and 10 the values for the graphene materials produced from the exfoliation of expanded and non-expanded graphite are 0.64 and 0.54 respectively. These values are similar, but higher than the ID / IG value obtained for graphite which indicates that a similar level of defects or structural disorders have been introduced into both graphene materials. The extent of the defects introduced during the high shear exfoliation can be reduced 15 by optimising the operating parameters such as cycle length, rotor speed and the temperature of the high shear mixing vessel. D band (cnr1) G band (cnr1) 2 D band (cnr1) I2D / IG Id / Ig Graphite 1350 1576 2717 0.52 0.05 Non-expanded 1349 1575 2702 0.65 0.54 Expanded 1349 1577 2690 1.17 0.64 15 05 25 Table 1: Raman spectra parameters of few layer graphene obtained from the in-line high shear mixing of non-expanded graphite and expanded graphite. The yield of few-layer graphene was also detennined. This was achieved by 5 separating few-layer graphene from the graphene nanoplatelets in the manner described above, drying the separated graphene nanoplatelets by freeze drying or vacuum drying and then weighing the dried graphene nanoplatelets. The results are summarised in Table 2 which shows that a 5% yield of few-layer graphene was obtained from the in-line high shear exfoliation of non-expanded graphite, whereas a 10 70% yield of few-layer graphene was obtained when expanded graphite was subjected to an in-line high shear mixing treatment. Sample Graphite Dry GNP Yield Non-expanded 1000g 950 g 5% Expanded 1000g 300 g 70% Table 2: Yield of few layer graphene obtained from the in-line high shear mixing of 15 non-expanded graphite and expanded graphite. From the above results it can be seen that the in-line high shear mixing of expanded graphite enables few-layer graphene with no or few defects to be obtained in very high yields. Accordingly, the method provides a low-cost and scalable 20 production method for graphene. LD The one or more embodiments are described above by way of example only. Many variations are possible without departing from the scope of protection afforded by the appended claims. 24

Claims

1. A method for the production of oxide-free graphene, the method comprising the stepsof:a. intercalating a graphite with a quaternary ammonium sulphate inert intercalation compound and thermally treating the intercalated graphite to produce oxide-free expanded graphite,b. mixing the expanded graphite and a liquid in a container to provide a mixture, andc. exfoliating the expanded graphite by exposing the mixture to an in-line high shear mixing assembly to produce oxide-free graphene, wherein the mixture is drawn from the container through the in-line high shear mixing assembly and recirculated to the container from the in-line high shear mixing assembly.

2. A method according to claim 1, wherein the high shear mixing assembly comprises a rotor-stator assembly.

3. A method according to claim 2, wherein the rotor is rotated at 6000-9000 rpm.

4. A method according to any preceding claim, wherein the expanded graphite isexfoliated for between 8 and 24 hours.

5. A method according to any preceding claim, wherein the graphite from which expanded graphite is produced has a lateral size of >50-500 um.

6. A method according to any preceding claim, wherein the thermal treatment of intercalated graphite is performed for a period 10 seconds to not more than 1 min.

7. A method according to claim 6, wherein the intercalated graphite is heated from 600-1200 °C.

248. A method according to any of claims 5 to 7, wherein the expanded graphite is produced by intercalating graphite with the intercalation compound and exposing the intercalated graphite to micro waves.

9. A method according to any of claims 5 to 8, wherein the expanded graphite is subjected to a mechanical milling treatment.

10. A method according to claim 9, wherein the expanded graphite is provided in a ball milling vessel and subjected to a ball milling treatment.

11. A method according to claim 10, wherein the ball milling treatment is performed for a period of from at least 4 hours to not more than 10 hours.

12. A method according to claim 11, wherein the ball milling vessel is rotated at a speed of from at least 200 rpm to not more than 600 rpm.

13. A method according to claim 8, wherein the expanded graphite is subjected to a fluidised bed air milling treatment.

14. A method according to any of claims 10 to 13, wherein the expanded graphite obtained after ball milling or fluidised bed air milling is screened to obtain expanded graphite having a mean particle size of 10-20 microns.

15. A method according to claim 14, wherein the expanded graphite has a mean particle size of 15-20 microns.

16. A method according to any preceding claim, wherein the liquid comprises water and / or an organic solvent.

17. A method according to claim 16, wherein the solvent comprises N-methyl-2-pyrrolidone, Dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, y-butyrolactone, ortho dichloro benzene, dimethyl sulfoxide, butyl acetate and toluene.

18. A method according to any preceding claim, wherein the mixture comprises a surfactant.

19. A method according to claim 18, wherein the surfactant comprises anionic surfactants, non-ionic surfactants, cationic surfactants, amphoteric surfactants, silicone surfactants, fluoro-surfactants, polymeric surfactants or combinations thereof.

20. A method according to claim 18 or claim 19, wherein the ratio of expanded graphite to surfactant is from 2:1 to 10:1.

21. A method according to any preceding claim, comprising the steps of separating graphene nanoplatelets from few-layer graphene and exposing a dispersion of the separated graphene nanoplatelets to the high shear mixing assembly.

22. A method according to any preceding claim, wherein the graphene is produced using a continuous or semi-continuous process.24

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