PRODUCTION OF GRAPHENE

Abstract

Methods for the production in an electrochemical cell of graphene and graphite nanoplatelet structures having a thickness of less than 100 nm in a cell having a negative electrode which is graphitic and an electrolyte which consists of ions in a solvent, where the cations are sulfur-containing ions or phosphorus containing ions, wherein the method comprises the step of passing a current through the cell to intercalate ions into the graphitic negative electrode so as to exfoliate the graphitic negative electrode.

Claims

1. A method for the production in an electrochemical cell of graphene and graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the cell comprises: (a) a negative electrode which is graphitic; (b) a positive electrode which may be graphitic or another material; and (c) an electrolyte which consists of ions in a solvent, where the cations are sulfur-containing ions; and wherein the method comprises the step of passing a current through the cell to intercalate ions into the graphitic negative electrode so as to exfoliate the graphitic negative electrode.

2. A method according to claim 1, wherein the sulfur-containing cations are organosulfur cations.

3. A method according to claim 2, where the organosulfur cations are sulfonium ions.

4. A method according to claim 3, wherein the sulfonium ions are trialkyl sulfonium ions, suitably selected from triethyl sulfonium and trimethyl sulfonium.

5. A method according to claim 1, wherein the counteranions are selected from bis(trifluoromethylsulfonyl)imide, bromide, tetrafluoroborate (BF.sub.4.sup.?), perchlorate (ClO.sub.4.sup.?) and hexafluorophosphate (PF.sub.6.sup.?).

6. A method according to claim 1, wherein the solvent is a non-aqueous solvent, suitably selected from dimethyl sulfoxide, N-methyl-2-pyrrolidone, N,N-dimethyl formamide and mixtures thereof.

7. A method for the production in an electrochemical cell of graphene and graphite nanoplatelet structures having a thickness of less than 100 nm, wherein the cell comprises: (a) a negative electrode which is graphitic; (b) a positive electrode which may be graphitic or another material; and (c) an electrolyte which consists of ions in a solvent, where the cations are phosphorous-containing ions and wherein the electrolyte is substantially free of metal cations; and wherein the method comprises the step of passing a current through the cell to intercalate ions into the graphitic negative electrode so as to exfoliate the graphitic negative electrode.

8. A method according to claim 7, wherein the phosphorous-containing cations are phosphonium ions.

9. A method according to claim 8, where the phosphonium ions are tetraalkyl phosphonium ions.

10. A method according to claim 9, wherein the tetraalkyl phosphonium ions are selected from tetrabutyl phosphonium, tetraethyl phosphonium and tetramethyl phosphonium.

11. A method according to claim 7, wherein the counteranions are selected from hydroxide, bromide, tetrafluoroborate (BF.sub.4.sup.?), perchlorate (ClO.sub.4.sup.?) and hexafluorophosphate (PF.sub.6.sup.?).

12. A method according to claim 7, wherein the solvent is a non-aqueous solvent, suitably selected from dimethyl sulfoxide, N-methyl-2-pyrrolidone, N,N-dimethyl formamide, and mixtures thereof.

13. A method according to claim 7, wherein the negative electrode is an electrode comprising one or more selected from highly ordered pyrolytic graphite, natural graphite and synthetic graphite.

14. A method according to claim 7, which is carried out a temperature from 20? C. to 150? C.

15. A method according to claim 7, wherein the graphene or graphite nanoplatelet structures having a thickness of less than 100 nm are separated from the electrolyte by at least one technique selected from: (a) filtering; (b) using centrifugal forces to precipitate the graphene or graphite nanoplatelet structures; and (c) collecting the graphene or graphite nanoplatelet structures at the interface of two immiscible solvents.

16. A method according to claim 7, wherein the electrochemically exfoliated graphene or graphite nanoplatelet structures are further treated using ultrasonic energy and/or thermal energy.

17. A method according to claim 7, wherein the method further includes the step of isolating the graphene or graphite nanoplatelet structures.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0113] FIG. 1 shows a typical Raman spectrum of the product produced in Example 1;

[0114] FIG. 2 shows a typical Raman spectrum of the product produced in Example 2; and

[0115] FIG. 3 shows a typical Raman spectrum of the product produced in Example 3.

[0116] The present invention is described in more detail by way of example only with reference to the following Examples.

EXAMPLES

General Electrochemical Procedure

[0117] All the electrochemical experiments were conducted in 50 ml beakers. The beaker was sealed using rubber plug or custom-made plastic lid. The electrodes are fixed on the lid so that the electrode distance is fixed at 5 mm at the start of the run. To control the surface area of the electrodes, the electrodes were attached to stainless steel rods that are allowed to move vertically using a M4 screw threaded onto the lid.

Analysis of Graphene by Raman Spectroscopy

[0118] All the Raman spectroscopy was conducted using a 633 nm excitation laser.

[0119] It is well established in the literature that Raman spectroscopy can be used to determine the number of layers that a carbon flake possesses through the shape, intensity and position of the D (?1350 cm.sup.?1), G (?1580 cm.sup.?1) and 2D (?2700 cm.sup.?1) peaks (the 2D peak may be alternatively referred to as the G peak).

[0120] The exact positions of the Raman peaks depend on the excitation wavelength used and the level of doping in the sample [Ferrari 2006]. In general, the Raman spectrum for single layer graphene comprises a 2D peak which can be fitted with a single component and is similar or higher in intensity than the G peak. The 2D peak for monolayer graphene occurs at approximately 2637 cm.sup.?1 when measured using a 633 nm excitation laser. As the number of layers increase, the 2D peak decreases in relative intensity to the G peak.

[0121] The 2D peak would be expected to be centred at approximately 2637, 2663, 2665, 2675 and 2688 cm-1 for 1-layer, 2-layer, 3-layer, many-layer and graphite respectively using a 633 nm laser to measure graphene flakes deposited on an oxide-covered silicon wafer.

[0122] The intensity of the D peak relative to the G peak also provides an indication of the number of structural defects such as graphene edges and sub-domain boundaries in the material produced. A D peak to G peak ratio (ID/IG) of around 0.2 may be expected for pristine graphene and the lower the ratio the better the quality material produced [Malard 2009].

[0123] For comparison, pristine defect-free graphite generally shows two bands: one at 1580 cm.sup.?1 (the G band), which arises from the first order scattering of the E2g phonon of sp2-bonded carbon atoms; and a band at ?2680 cm.sup.?1 (the 2D band) corresponding to the double-resonance process.

Example 1

[0124] An electrochemical cell was assembled having a graphite rod as cathode, and Pt wire as anode. The electrolyte was 0.5 M tetrabutylphosphonium hydroxide in DMSO. A potential of 10 V was applied for 5 hours. Very small bubbles were observed on the cathode. After the electrolysis, the suspension produced was mixed with 2 L of water and then the powder was filtered out. The powder was then washed with water several times, and dried overnight at 60? C. under vacuum.

[0125] After electrochemical exfoliation, the Raman pattern shows characteristic graphene features: a large symmetrical peak at 2655 cm.sup.?1. This is indicative of 1- and 2-layer graphene.

Example 2

[0126] The cell and conditions were the same as Example 1 except the electrolyte was 1 M triethylsulfonium bis(trifluoromethylsulfonyl) imide in DMSO. After cleaning the sample Raman analysis was carried out. The Raman pattern (FIG. 2) shows the graphene features, i.e large G band at 1585 cm.sup.?1 and large symmetrical 2D band at ?2651 cm.sup.?1. Again, this is indicative of high quality 1- and 2-layer graphene.

Example 3

[0127] The cell and conditions were the same as Example 1 except the electrolyte was 0.5 M of trimethylsulfonium bromide in DMSO. The Raman analysis (FIG. 3) shows similar results to that of Examples 1 and 2, being indicative of high quality (few layer) graphene.

REFERENCES

[0128] The following documents are all incorporated herein by reference. [0129] [Novoselov 2004] Electric field effect in atomically thin carbon films, K. S. Novoselov et al., Science, 2004, 5296, pp 666-669. [0130] [Ruoff 2009] Chemical methods for the production of graphenes, S. Park and R. S. Ruoff, Nature Nanotechnology, 2009, DOI:10.1038/nnano.2009.58 [0131] [Bae 2010] Roll-to-roll production of 30-inch graphene films for transparent electrodes, S. Bae et al. Nature Nanotechnology, 2010, DOI: 10.1038/NNANO.2010.132 [0132] [Ang 2009] High-Throughput Synthesis of Graphene by Intercalation-Exfoliation of Graphite Oxide and Study of Ionic Screening in Graphene Transistor, P. K. Ang et al., ACS Nano, 2009, 3(11), pp. 3587-3594 [0133] [Wang 2010] Direct exfoliation of natural graphite into micrometre size few layers graphene sheets using ionic liquids, X. Wang et al., Chem. Commun., 2010, 46, pp. 4487-4489 [0134] [Liu 2008] N. Liu et al, One-Step Ionic-Liquid-Assisted Electrochemical Synthesis of Ionic-Liquid-Functionalized Graphene Sheets Directly from Graphite. Adv. Funct. Mater. 2008, 18, pp. 1518-1525 [0135] [Lu 2009] One-Pot Synthesis of Fluorescent Carbon Nanoribbons, Nanoparticles, and Graphene by the Exfoliation of Graphite in Ionic Liquids, ACS Nano, 2009, 3(8) pp. 2367-2375 [0136] [Kinloch, 2003] I. A. Kinloch et al, Electrolytic, TEM and Raman studies on the production of carbon nanotubes in molten NaCl, Carbon, 2003, 41, pp. 1127-1141 [0137] [Coleman 2008 & 2009] Y. Hernandez, et al, Nat. Nanotechnol., 2008, 3, 563; M. Lotya, et al, J. Am. Chem. Soc., 2009, 131, 3611. [0138] [Valles 2008] Valles, C. et al. Solutions of negatively charged graphene sheets and ribbons. J. Am. Chem. Soc. 130, 15802-15804 (2008). [0139] [Ferrari 2006] Ferrari, A. C. et al. Raman Spectrum of Graphene and Graphene Layers. Phys Rev Lett, 97 (2006), 187401 [0140] [Wang 2011] Wang, J., et al., High-yield synthesis of few-layer graphene flakes through electrochemical expansion of graphite in propylene carbonate electrolyte, JACS, 2011, 133, 8888-8891 [0141] [Simate 2010] The production of carbon nanotubes from carbon dioxide: challenges and opportunities, Simate, G. S. et al. Journal of Gas Chemistry, 2010, 19(5), 453; [0142] [Malard 2009] Malard L. M. et al., Raman spectroscopy in graphene, Phys. Rep. 473, 51-87 (2009). [0143] [Zhong 2012] Y. L. Zhong, T. M. Swager, J. Am. Chem. Soc. 2012, 134, 17896-17899 [0144] [Huang 2012] Huang et al, J. Mater. Chem., 2012, 22, 10452-10456 [0145] [Wang 2010] Wang Y, Xu X, Lu J, Lin M, Bao Q, ??zyilmaz B, Loh KP. Toward High Throughput Interconvertible Graphane-to-Graphene Growth and Patterning. ACS Nano 2010; 4:6146.