Method for producing graphene and new form of graphene

Abstract

The invention provides a method for preparing graphene which method comprises the steps of: (a) forming a graphite/water mixture; and (b) introducing the graphite/water mixture into a cavitation reactor using at least two offset nozzles; a cavitation reactor for use in the method wherein the cavitation reactor has a cavitation chamber wherein the cavitation chamber has at least two offset inlet nozzles which are directed towards the centre of the cavitation chamber and at least one outlet; and graphene having a carbon content of at least about 98 wt %.

Claims

1. A method for preparing graphene which method comprises the steps of: (a) forming a graphite/water mixture; and (b) introducing the graphite/water mixture into a cavitation reactor using at least two offset nozzles.

2. The method as defined in claim 1 wherein step (b) comprises pulsing the graphite/water mixture at a frequency of from 10-200 Hertz.

3. The method as defined in claim 1 wherein step (b) is carried out at a temperature not exceeding 50° C.

4. The method as defined in claim 1 wherein step (b) comprises introducing counter streams of the water/graphite mixture using the at least two nozzles wherein the counter streams have substantially the same velocity and substantially the same pressure.

5. The method as defined in claim 1 which comprises a step (c) to dry the product of step (b).

6. A cavitation reactor for use in the method defined in claim 1 wherein the cavitation reactor has a cavitation chamber wherein the cavitation chamber has at least two offset inlet nozzles which are directed towards the centre of the cavitation chamber and at least one outlet.

7. The reactor as defined in claim 6 which has one or more reflecting surface to help the formation of a cavitation field.

8. Graphene having a carbon content of at least about 98 wt %.

9. Graphene as defined in claim 8 which has an electrical conductivity of 5000 S/m to 85000 S/m.

10. Graphene as defined in claim 8 which has a ratio of lateral size to thickness of up to 10000.

11. Graphene as defined in claim 8 which has a layer content of from 1 to 10.

12. Graphene as defined in claim 8 which has a carbon content of at least about 99 wt % carbon.

13. Graphene as defined in claim 8 which has substantially no edge and structural defects, as determined by Raman spectrum.

14. Graphene as defined in claim 8 which has a thickness of up to 2 nm.

15. Graphene as defined in claim 8 which has a lateral size of up to 20 μm.

16. Graphene as defined in claim 8 which has a specific surface area of from 100 to 300 m.sup.2/g.

17. The method as defined in claim 5 wherein the drying step (c) comprises use of micro-filtration.

Description

[0035] The invention will now be illustrated with reference to the following Figures of the accompanying drawings which are not intended to limit the scope of the claimed invention:

[0036] FIG. 1 shows a schematic cross-sectional view of a cavitation reactor according to the invention;

[0037] FIG. 2 shows a FTIR spectra for graphene according to the invention;

[0038] FIG. 3 shows a Raman spectra for graphene according to the invention; and

[0039] FIG. 4 shows a scanning electron microscope image of graphene platelets.

[0040] A reactor according to the invention is indicated generally at 10 on FIG. 1. Reactor 10 is in the form of an elongate cuboid sealed chamber 3 which has reflecting surfaces 1, 2, a cavitation void 8, inlet nozzles 6,7, and outlet nozzles 4,5. The inlet nozzles 6,7 are arranged towards the centre of the cavitation void 8 but are offset such that they are not coaxial. It has been found that this provides better conditions for the reaction. The outlet nozzles 4,5 are arranged in two of the corners of the cavitation void 8. The cavitation void 8 is formed by the sealed chamber 3 and the reflecting surfaces 1,2. The reflecting surfaces 1,2 are each arranged obliquely across diagonally opposing corners of the sealed chamber 3 such that reflecting surface 1 is arranged above inlet nozzle 7 and reflecting surface 2 is arranged below inlet nozzle 6. In an alternative embodiment, the reactor 10 may have one or two further reflecting surfaces 1,2 to cover the other two corners or vertices of the cavitation void 8.

[0041] The invention will now be illustrated with reference to the following Examples which are not intended to limit the scope of the claimed invention.

EXAMPLE

[0042] The following example illustrates how the reactor 10 can be used in the production of graphene.

[0043] A working solution, consisting of water and graphite with a weight ratio of 90:10, is fed through inlet nozzles 6,7 into the cavitation chamber 8 at a pressure of 15 MPa to 20 MPa and at a flow rate of 250 m/s. At the same time, the flow of the solution is pulsed at a frequency of 50-100 hertz.

[0044] As described above, the inlet nozzles are not aligned co-axially inside the reactor. This is required to create a collision of jets of the working solution in a tangential direction. The distance between the centres of the inlet nozzles 6,7 is equal to half the cross-sectional area of one jet:

s=G/2V

[0045] Where s represents the distance between the inlet nozzles 6 and 7; G is the total consumption of the mixture, l/min. V is the flow rate of the mixture from each inlet nozzle 6,7 in metres/second. The cross-sectional areas of typical jets which are suitable for use in the method of the invention are from 1.2 mm.sup.2 to 10 mm.sup.2. The typical distance between the jets is from 0.6 to 5 mm. The cross-sectional area of the jet and the distance between the jets are selected depending on the performance of the pumps and the particle size of the original graphite, which is used to produce graphene.

[0046] As a result of such a tangential interaction of the jets of the working solution from the inlet nozzles 6,7, cavitation starts in cavitation chamber 8. The energy flux density transferred to the mixture in cavitation chamber 8 is:

FD=W/s; or FD=G*P/s,

where FD represents the energy flux density in W.Math.m.sup.−2; W represents the rate of consumption of energy in Watts; P is the total pressure in the nozzles in N/m.sup.2. The pressure in the reactor is maintained at 0.2P. After the cavitational interaction, the resultant mixture flows out from the outlet nozzles 4,5. The direction of movement of the resultant mixture in the reactor is provided by the reflectors 1,2.

[0047] After processing in the reactor, the resultant mixture is dehydrated using micron-sized filters until a water-graphene paste with a graphene content of 20-40 wt % is obtained. The resulting paste is used in anode materials of lithium batteries with an aqueous binder as well as for the production of acrylic-based coatings to provide conductive and/or shielding properties. For other applications, the aqueous paste is dried to obtain a powder with a bulk density of 120-150 kg/m.sup.3.

[0048] It should be noted that the method of the invention has ecological purity and is a safe process. This method does not use any chemicals or additives. The process takes place at a low temperature less than 60° C. Water after treatment of the mixture may be filtered and used repeatedly.

[0049] The graphene obtained was analysed and was found to have the following parameters: [0050] a. Thickness—up to 2 nm; [0051] b. Specific surface area (SSA)—180 m.sup.2/g; [0052] c. Lateral Size—up to 20 micron; [0053] d. Electrical Conductivity—80000 S/m; and [0054] f. Carbon—99.7 wt %.

[0055] The electrical conductivity of the graphene powder measured at a pressure of 500 Bar using a four points method at currents over 200 Amperes.

[0056] The graphene obtained was analysed using FTIR and Raman spectra as shown in FIGS. 2 and 3. The FTIR spectra shows peaks for CH2, CO2 and C═C which show the purity and lack of contamination of the graphene. The Raman spectra is indicative of a lack of graphene defects and contaminants. The wave number data indicate that the sample had 1-2 graphene layers. In particular, in FIG. 3, the following bands are shown: [0057] 1350 sm.sup.−1—D band [0058] 1556 sm.sup.−1—G band [0059] 2674 sm.sup.−1—2D band

[0060] The I.sub.D/I.sub.G ratio, which can be calculated from the intensity of peaks D and G on the Raman spectrum, indicates the presence of defects or the oxidation state of graphene. As can be seen from FIG. 3, the I.sub.D/I.sub.G ratio is very small, indicating virtually no defects in the graphene prepared in the method according to the invention.

[0061] The graphene platelets obtained were imaged using a scanning electron microscope as shown in FIG. 4. The image shown in FIG. 4 shows flakes of graphene without impurities, having different thickness and showing no signs of heat deformation.