Methods of manufacturing reduced graphene oxide (RGO) and graphene oxide / reduced graphene oxide (go / RGO) composites

Abstract

Reduced Graphene Oxide was synthesized by treatment of Graphite using only H2SO4 acid, KMnO4, H2O2 and H2O, adopting the method disclosed herein, where the mix of Graphite, H2SO4 acid and KMnO4 was subjected to sonication and stirring while maintaining the temperature at < 80°C for GO / rGO composite and > 80°C for rGO followed by addition of water and H2O2 to stop the oxidation / reduction reaction. A separate reductant was not used during reduction stage and vibrational energy of the ultrasonic cell disruptor caused the detachment of functional group from GO at given temperature to form rGO. Analytical techniques such as XRD, XPS, Raman and FTIR were used to confirm the presence of reduced Graphene Oxide and Graphene Oxide.

Description

[0001] TITLE OF THE INVENTION:

[0002] METHODS OF MANUFACTURING REDUCED GRAPHENE OXIDE (RGO) AND GRAPHENE OXIDE / REDUCED GRAPHENE OXIDE (GO / RGO) COMPOSITES

[0003] TECHNICAL FIELD

[0004] The present invention relates to reduced Graphene Oxide and Graphene Oxide / reduced Graphene Oxide composites, more specifically methods of manufacturing reduced Graphene Oxide and Graphene Oxide / reduced Graphene Oxide composites.

[0005] BACKGROUND ART

[0006] Graphene is a remarkable material with several unique properties, which have garnered significant attention from the scientific community and industry. Some of its notable characteristics include exceptional strength, excellent conductivity and thermal conductivity. In most cases, vein graphite is used as the starting material in the production of Graphene. During this process, two intermediate derivatives of Graphene are formed namely Graphene Oxide and reduced Graphene Oxide.

[0007] Graphene Oxide and reduced Graphene Oxide have attracted a lot of attention from various research fields such as sensing, nanoelectronics, energy storage, catalyst construction nanotechnology etc. In recent years reduced Graphene Oxide have attracted a lot of attention due to its optical, electro-mechanical, thermal, mechanical and catalytic properties.

[0008] Usually reduced Graphene Oxide (rGO)1is synthesized from Graphite of purity >98% via synthesis of graphene oxide. The synthesized graphene oxide is reduced in the presence of reducing agents such as H2S2, hydrazine3, NaBPU4, hydrohalic acid5, metal iodide6, ascorbic acid7, amino acid8etc. to produce reduced graphene oxide. The properties of the final product rGO varies with the reductant and the mechanism used. Their oxygen content in terms of C / O ratio had varied from 30 to 2.2 and conductivities had changed from 3.2 x IO5to 55,088 s / m6 9.

[0009] Prior art LK / P / 18157 discloses a method of preparing graphene oxide and few layer graphene wherein graphite oxide is synthesized from graphite of purity above 97% using an acid-based oxidation method followed by ultrasonic agitation for a period of 30 to 120 min at 30 °C - 60 °C in water to synthesize graphene oxide. Dried graphene oxide is fired at 100 °C to 250 °C in air for a period of 30 to 120 min to obtain reduced graphene oxide. Reduced graphene oxide is heated at 400 °C - 1000 °C in Argon for a period of 30 to 120 min for further removal of oxygen to obtain few layers graphene. Therefore, the end product is few layers graphene and not a stable product of Reduced Graphene Oxide (rGO) or Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composites.

[0010] TECHNICAL PROBLEM

[0011] In view of prior art, the problem that further remains to be solved is thus how to provide a cost effective, environmentally friendly method to obtain high purity reduced Graphene Oxide with high C / O ratio.

[0012] It is also required to develop a cost effective and environmentally friendly method to obtain composites of Graphene Oxide / Reduced Graphene Oxide (GO / rGO) for various applications.

[0013] TECHNICAL SOLUTION

[0014] The objective of the present invention is to provide a cost-effective, environmentally friendly method to obtain high purity reduced Graphene Oxide with high C / O ratio.

[0015] The invention provides a method of manufacturing reduced Graphene Oxide using vein graphite from Sri Lanka Bogala and Kahatagaha mines.

[0016] The invention further provides a method for manufacturing Graphene Oxide / reduced Graphene Oxide (GO / rGO) composites having various compositions of Graphene Oxide (GO) from 5 - 95% from vein graphite. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1: XRD pattern of the reduced Graphene Oxide obtained according to an example embodiment of the present invention.

[0018] Figure 2: XPS spectrum of the reduced Graphene Oxide obtained according to an example embodiment of the present invention.

[0019] Figure 3: Raman spectrum of the reduced Graphene Oxide obtained as of the present invention.

[0020] Figure 4: FTIR spectrum of reduced Graphene Oxide obtained as per the present invention.

[0021] Figure 5: XRD pattern of the synthesized Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite.

[0022] Figure 6: Raman spectrum of Graphene Oxide / Reduced Graphene Oxide GO / rGO composite.

[0023] Figure 7 : XRD pattern of Graphene Oxide (GO) prepared by the same method.

[0024] ADVANTAGEOUS EFFECTS

[0025] The present method is cost effective and environmentally friendly method for manufacturing reduced Graphene Oxide. In the present invention use of reductants are avoided, final product with high Raman ID / IG value product is obtained, confirming defect free pure rGO.

[0026] MODE OF INVENTION

[0027] During the manufacture of single phase reduced Graphene Oxide (rGO) or a composite of GO / rGO comprising a composition of Graphene Oxide (GO) varying from 5% - 95% by mass, graphite of purity > 98% is used as the starting material. The graphite is obtained from Bogala and Kahatagaha mines in Sri Lanka. Said graphite is dispersed in H2SO4 acid using an ultrasonic cell disruptor of variable power rating from 300 watts to 2000 watts and a frequency of 20 KHz. Here H2SO4 acid can be partially substituted with HNO3 acid or H3PO4 acid. This ultrasonic treatment helps to delaminate the flaky graphite. Said dispersion is carried out while stirring and maintaining the temperature of the dispersion at 5 °C and not more than 35 °C for a duration of more than 30 minutes and not more than 120 minutes. The dispersion is followed by addition of KMnO4, while maintaining the temperature of the graphite dispersion below 25 °C and continued mixing for a period of greater than 30 minutes but not more than 180 minutes. In addition to KMnO4, K2Cr2O? can also be used as an oxidizing agent.

[0028] The next step is sonication using the above ultrasonic cell disruptor, the high-energy waves of the ultrasonic cell disruptor in the presence of oxidizing agents like KMnO4 promotes the oxidation of graphite leading to formation of reduced Graphene Oxide or Graphene Oxide / reduced Graphene Oxide composite.

[0029] The extent or the proportion of reduced Graphene Oxide formed depends on the temperature the of the mixture is subjected to. Increase of temperature of the mixture can be achieved either by increasing the power rating of the ultrasonic cell disruptor or by providing an external heater such as a hot plate or a gas burner.

[0030] To obtain a composite of GO / rGO comprising different proportions of GO the temperature of the mixture to be maintained > 35 °C but not more than 80°C, but for single component rGO, the temperature of the mix to be maintained >80 °C but not more than 180°C, for durations > 5 minutes but not more than 180 minutes.

[0031] On completion of reaction time at particular temperature H2O is added while stirring to stop the oxidation reaction followed by H2O2. The resultant slurry is filtered and washed with HC1 and H2O until the pH of the filtrate is 5 - 6. After washing the residue is made in to a slurry with water and sonicated using the ultrasonic cell disruptor till it forms a stable suspension.

[0032] The end product can be either in a water dispersion form or a dried powder form of black colour. To get the powder form, the slurry is dried at 70 °C. For identification and characterization of the final product the following analytical techniques XRD, XPS, Raman, FTIR, and TG are used. These techniques help to confirm the phases present, the proportions in the final product, and their expected properties. The synthesis of a cost-effective, environment- friendly and simple process consists of the steps of pre-exfoliation of Graphite, oxidation of Graphite, ceasing the oxidation reaction by adding water and H2O2, washing resultant slurry and further exfoliation by ball milling / sonicating.

[0033] During the manufacturing of reduced Graphite Oxide, graphite having purity > 98% was provided. Said graphite was dispersed in H2SO4 acid under sonication and stirring while maintaining the temperature of the graphite and sulfuric acid mixture below 25 °C and followed by addition of KMnO4. On completion of adding KMnO4, the temperature of the mixture was raised to > 80 °C, under sonication and stirring and maintained this temperature for a period greater than 5 minutes but not more than 180 minutes and followed by addition of H2O and H2O2. The resultant slurry was filtered and washed with HC1 and water to adjust the pH 5-6.

[0034] The washed slurry was dried in an oven at 70 °C. The end product was a black color powder. The analytical techniques XRD, XPS, Raman, FTIR and TG were followed to confirm the presence of only reduced Graphene Oxide.

[0035] The invention described below is for the manufacturing of reduced Graphene Oxide in a laboratory scale and the same proportion was used for scaling up the synthesis process.

[0036] Example 1: Synthesis of Reduced Graphene Oxide

[0037] About 5.0 g of Graphite obtained from Kahatagaha mines Sri Lanka, was slowly added to 200 ml of concentrated H2SO4 acid while stirring / sonicating using an ultrasonic cell disruptor of power rating 300 watts to 2000 watts and frequency of 20 KHz. The power rating was adjusted to 1000 watts and Titanium Horn was used to get an amplified mechanical vibration and to prevent acid leaching. The temperature of the Graphite dispersion was maintained at 20 °C using an ice bath. The sonication using the ultrasonic cell disruptor was continued for one hour for complete exfoliation of Graphite flakes. Then 15.0 g of KMnO4 was added slowly to the H2SO4 / Graphite mix while stirring and maintaining the temperature at 20 °C. One completion of addition of KMnO4 the mix was stirred for another 60 minutes followed by sonication using the same ultrasonic cell disruptor under power rating of 1000 watts until the temperature of the mix rise to 125 °C. At this point the colour of the mix was turned to black.

[0038] The temperature of the mix was maintained at 125°C for 30 minutes and 75 ml of chilled H2O was added to seize the oxidation reaction which was followed by addition of 3 ml of H2O2. At this stage the temperature of the mixture was maintained below 150 °C.

[0039] After the temperature of the mix reached room temperature, the reduced Graphene Oxide slurry was separated by filtration and washed with 500 ml of 5 % HC1 solution followed by deionized water until the pH of the filtrate was 5.5. The pH adjusted slurry was sonicated using the same ultrasonic cell disruptor at a power rating of 1500 W for 1 hour to get a stable nano rGO dispersion. To obtain the rGO in powder form the slurry was dried at 70 °C for 12 hours. The powder of rGO is black in colour.

[0040] The rGO synthesized was characterized by XRD, XPS, Raman and FTIR which are given in figures 1,2,3, and 4 respectively.

[0041] The XRD pattern of rGO synthesized given in Fig. I shows only a broad peak at 2 theta =24.24 degrees corresponding to reduced Graphene Oxide.

[0042] The below table of XPS analysis shows the composition of synthesized Reduced Graphene Oxide.

[0043] Table 1: The composition of synthesized reduced Graphene Oxide

[0044] The C / O ratio is 5. 1 Table 2: Functional groups composition of reduced Graphene oxide synthesis obtained from XPS analysis as per the Figure 2.

[0045] High C-C / C=C group content and a high C / O ratio of 5.1 indicate a very low percentage of oxygen functional groups.

[0046] The D band and G band of Raman Spectra (Fig 3) were broadened and shifted to round 1300 cm1and 1500 cm1respectively. The I(DJ / I(GJ ratio was 1.51, which confirmed the presence of defects-free Reduced Graphene Oxide.

[0047] FTIR spectrum given in Figure 4 indicates the complete disappearance of the hydroxy stretching band (3000 - 4000 cm1) and the decrease of intensities of C=O and C-0 vibrations. This is an evident of the formation of reduced Graphene Oxide with very low oxygen containing functional groups.

[0048] Example 2: Synthesis of GO / rGO composites at temperature below 80 °C

[0049] About 5.0 g of Graphite obtained from Kahatagaha mines, Sri Lanka was slowly added to 200 ml of concentrated H2SO4 acid while stirring / sonicating using an ultrasonic cell disruptor of power rating 300 watts to 2000 watts and frequency of 20 KHz. The power rating was adjusted to 1000 watts and Titanium Horn was used to set an amplified mechanical vibration and to prevent acid leaching. The temperature of the graphite dispersion was maintained at 20 °C using an ice bath. The sonication using the ultrasonic cell disruptor was continued for one hour for complete exfoliation of graphite flakes.

[0050] Then 15.0 g of KMnO4 was added slowly to the H2SO4 / Graphite mix while stirring and maintaining the temperature at 20 °C. On completion of addition of KMnO4 the mix was stirred for 45 minutes followed by sonicating using the ultrasonic cell disruptor under power rating of 600 watts until the temperature of mix reaches to 70 °C. At this point the colour of the mix is brownish black and the mix was allowed to cool below 50 °C and 75 ml of chilled water was added to seize the oxidation of Graphite, followed by addition of 3 ml of H2O2. The temperature of mix was maintained below 75 °C. On reaching the temperature of the mix below 40 °C, the mixture was filtered and washed with 5 % HC1 and de-ionized water until the pH of the filtrate reaches around 2.5.

[0051] The residue was dispersed in water and sonicated under power rating of 1000 watts for 50 minutes. The slurry obtained can be used in dispersed form or as a powder form. To obtain the powder form the slurry was dried at 70°C for 12 hours.

[0052] The GO / rGO composite obtained was characterized by XRD and Raman which are given in figure 5 and 6 respectively. The XRD pattern given in Fig. 5 shows 2 peaks at 2 theta 10.337 degrees and 2 theta = 22.61 degrees corresponding to the presence of Graphene Oxide (GO) and reduced Graphene Oxide (rGO).

[0053] The quantification was carried out by obtaining the intensities of two peaks and equivalent to GO 60 % and rGO 40 %. By varying the temperature of reaction, using the power rating of the ultrasonic cell disruptor or an external power source the GO to rGO ratio of the composite could be varied to suit the required application.

[0054] Figure 7 shows the XRD pattern of pure GO prepared by the same method by maintaining the reaction temperature at 40 °C. Peak at two theta 9.96 corresponds to GO.

[0055] INDUSTRIAL APPLICABILITY

[0056] The above disclosed methods of manufacturing of reduced Graphene Oxide and reduced Graphene Oxide / Graphene Oxide (rGO / GO) composites can be used in manufacturing reduced Graphene Oxide and reduced Graphene Oxide / Graphene Oxide (rGO / GO) composites for various industrial applications.

[0057] REFERENCES

[0058] 1) U.S. Patent 16499407

[0059] 2) Zang,C. et.al - Adv. Energy Materials 2014,4,1301565

[0060] 3) Park,S. et.al - Carbon 2011,49, 3019-3023

[0061] 4) Shin, H. et. al - Adv. Funct. Mater.2009, 19, 1981- 1992

[0062] 5) Moon, I. et.al- Nat. Commun. 2010,1, 73

[0063] 6) Liu, C. et. al - Sei. Rep. 2014,4, 3965

[0064] 7) Zhang, J. et.al - Chem. Commum 2010,46, 1112-1114

[0065] 8) Chen, D. et.al - Nano technology, 2011,22,32560

[0066] 9) Chua, C. et.al - Chem. Soc. Rev. ,2014,43,291-312

Claims

METHODS OF MANUFACTURING REDUCED GRAPHENE OXIDE (RGO) AND GRAPHENE OXIDE / REDUCED GRAPHENE OXIDE (GO / RGO) COMPOSITESWhat is claimed is,1. A method of manufacturing reduced Graphene Oxide, comprising the steps of:(a) providing vein graphite having a purity greater than 98 percent;(b) treating the vein graphite with a concentrated acid in an ultrasonic cell reactor while stirring;(c) adding an oxidizing agent to the reactor while maintaining temperature of the reactor below 25 °C and stirring for 60 minutes while sonicating;(d) raising the temperature of the reactor to greater than 80 °C but not more than 125 °C for reaction to complete by sonicating;(e) adding an element to stop the reaction;(f) filtering resulted reduced Graphene Oxide; and(g) washing the resulted reduced Graphene Oxide with HC1 and deionized water.

2. The method of claim 1 wherein the step (b) the concentrated acid is sulfuric acid.

3. The method of claim 2 wherein the sulfuric acid can be partially substituted by HNO3 or H3PO4 acids.

4. The method of claim 1 wherein the step (b) the graphite is sonicated for one hour in the ultrasonic cell disruptor for complete exfoliation of graphite.

5. The method of claim 1 wherein the ultrasonic cell disruptor having power rating of 300 watts to 2000 watts and frequency of 20 KHz.

6. The method of claim 1 wherein the step (b) the treating with the acid is done under an ice bath.

7. The method of claim 1 wherein steps (b), (c), (d) are conducted under sonication and stirring.

8. The method of claim 1 wherein the oxidizing agent of step (c) is KMnCk9. The method of claim 1 wherein the oxidizing agent of step (c) is KaCraO .

10. The method of claim 1 wherein the temperature of the reactor is maintained at 125 °C for 30 minutes before adding the element to stop the reaction.

11. The method of claim 1 wherein the added element of step (e) to stop the reaction are water and H2O2.

12. The method of claim 1 wherein the resulted reduced Graphene Oxide is washed with HC1 and deionized water until pH reaches 5.5 for purification.

13. The method of claim 12 wherein the purified reduced Graphene Oxide is further sonicated for 30 minutes to obtain a stable nano reduced Graphene Oxide (rGO) dispersion.

14. The method of claim 13 wherein the purified reduced Graphene Oxide is oven dried at 70 °C for 12 hours to obtain a powder form of rGO.

15. A method of manufacturing Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composites, comprising the steps of:(a) providing vein graphite having a purity greater than 98 percent;(b) treating the vein graphite with a concentrated acid in an ultrasonic cell reactor while stirring;(c) adding an oxidizing agent to the reactor while maintaining temperature of the reactor below 25 °C and stirring for 45 minutes while sonicating;(d) raising the temperature of the reactor to less than 80 °C for reaction to complete by sonicating;(e) adding an element to stop the reaction;(f) filtering resulted Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite; and(g) washing the resulted Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite with HC1 and deionized water.

16. The method of claim 15 wherein the step (b) the concentrated acid is sulfuric acid.

17. The method of claim 15 wherein the sulfuric acid can be partially substituted by HNO3 or H3PO4 acids.

18. The method of claim 15 wherein the step (b) the graphite is sonicated for one hour in the ultrasonic cell disruptor for complete exfoliation of graphite.

19. The method of claim 15 wherein the ultrasonic cell disruptor having power rating of 300 watts to 2000 watts and frequency of 20 KHz.

20. The method of claim 15 wherein the step (b) the treating with the acid is done under an ice bath.

21. The method of claim 15 wherein steps (b), (c), (d) are conducted under sonication and stirring.

22. The method of claim 15 wherein the oxidizing agent of step (c) is KMnO4.

23. The method of claim 15 wherein the oxidizing agent of step (c) is KaCraO .

24. The method of claim 1 wherein the temperature of the reactor is maintained at 70 °C before adding the element to stop the reaction.

25. The method of claim 15 wherein the added element of step (e) to stop the reaction are water and H2O2.

26. The method of claim 15 wherein the resulted Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite is washed with HC1 and deionized water until pH reaches 2.5 for purification.

27. The method of claim 26 wherein the purified Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite further sonicated for 50 minutes to obtain a stable nano Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite dispersion.

28. The method of claim 26 wherein the purified Graphene Oxide / Reduced Graphene Oxide (GO / rGO) composite is oven dried at 70 °C for 12 hours to obtain a powder form of GO / rGO composite.

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