FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patents [Amendment] Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
I. TITLE OF THE INVENTION
Stable Dispersion Of Reduced Graphene Oxide, Process For Preparation And Use
Thereof
2. APPLICANT
NAME : Indian Oil Corporation Limited
NATIONALITY : IN
ADDRESS : G-9, Ali Yavar Jung Marg, Bandra (East), Mumbai-400 051, India
3. PREAMBLE TO THE DESCRIPTION
COMPLETE
The following specification describes the invention and the manner in which it is to be
performed

FIELD OF THE INVENTION
The present invention relates to reduced graphene oxide dispersion and a process of preparing the same. More particularly, the present invention relates to an environment friendly route to synthesize stable dispersion of reduced graphene oxide.
BACKGROUND OF THE INVENTION
Allen M J, Tung V C and Kanner R B 2010, Chem. Rev. 110 132 and Choi W, Lahiri I, Seekaboyina R and Kang Y S 2010 Crit. Rev. Solid St. Mater Sc. 35, 52 disclose graphene as the building block of various kinds of carbon analogues, the most sought after material in recent years. The unique and improved mechanical, electrical and thermal properties supported by its comparatively cost-effective and plausible methods of preparation have attracted the scientists and technologists to explore hard in this area of research. However, the main challenge is effective and scalable protocol to get single to few layers of graphene with a thorough control over the number of layers.
According to Xu Y, Bai H, Lu G, Li C and Shi G 2008, J. Am. Chem. Soc. 130, 5856, graphene oxide (GO) was synthesized from natural graphite powder by the modified Hummers method [Hummer et.al]. Hummer's method has been widely used for preparation of graphite oxide from graphite. However, it was found that, prior to the GO preparation an additional graphite oxidation procedure was needed to avoid the unreduced graphite.
According to Park S and Ruoff R S 2009 Nat Nanotechnol. 4,217; Zhao W, Fang M, Wu F, Wu H, Wang L and Chen G 2010, J. Mat Chem. 20, 5817 and Murugan A V, Muraliganth T, Manthiram A 2009 Chem. Mater, 21, 5004 a number of chemical, photochemical, microwave assisted and mechanical methods have been introduced, though the first one mentioned attracted considerable interest. In most of the cases for a chemical reduction of graphene oxide, hydrazine hydrate or its derivatives have been used. Li W, Tang X, Zanng H, Jian Z, Yu Z, Du X and Mai Y, 2011 Carbon 49,4724 discloses heating a mixture of graphene oxide and hydrazine in a water bath at 95 °C to get graphene dispersion. They used ammonia solution to maintain the pH, where
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the stability constant of the dispersion increased with alkalinity. The negative charge developed on the surface of graphene sheets keep them separated due to electrostatic repulsion.
Tung V C, Allen M J, Yang Y and Kanner R B 2009 Nat. Nanotechnol. 4, 25; Liu S, Tian J, wang L and Sun X 2011 Carbon 49, 3158]. Moreover, as observed from Stankvich S, DikinD A, Piner R D, Kohlhass K A, Jia Y, Yue W, Nguyen S T and Ruoff R. S. 2007, Carbon 45, 1558, disclose another high-throughput process able method for dispersed graphene oxide film into 98% anhydrous hydrazine solution and allowed to stir for one week. Similarly, in a number of reports hydrazine and its derivatives are used to reduce graphene oxide. However, being toxic in nature and a potential explosive, they may call disaster when used in large scale that highly hinders the possibility of scalability. The prime challenge and a major drawback of this method is that with time the sheets separated by Van der Waal's interaction or electrostatic repulsion tend to form irreversible aggregates.
In recent times few works have been reported using either surfactant or polymer f unctionalization route to improve the stability. In this regard Kuila T, Bose S Hong C E, Uddin M E, Khanra P, Kim N H and Lee J H 2011, Carbon, 49,1033 discloses use of dodecyl amine to wrap the graphene oxide followed by reduction with hydrazine monohydrate to get modified graphene sheets for better dispersion. Li et. al. used long chain surfactant octadecyl amine to functionalize and reduce the graphene oxide with no external reducing agent.
In an attempt for polymeric modification, Salavagione H J, Gomez M A, Martinez G 2009 Macromolecules 42, 6331 has reported soluble graphene covalently functionalized with poly vinyl alcohol (PVA) by simple esterification reaction of carboxylic groups in graphite oxide. However, using the polymer or any other surfactant to increase the stability has a concern over final material properties. None the less surfactants and polymers are unwanted in most of the graphene applications and better to be avoided. Hence, the issue of avoiding hydrazine in the reaction medium has been addressed in some of the recent reports.
An ideal protocol would be a single entity acting as reducing and stabilizing agent. In this regard, Zhang et. al reported a method using L-ascorbic acid to remove the oxygen from graphene oxide.
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They disclosed that the method was simple, scalable and environment friendly. Various other reducing agents such as benzyl amine, oligo thiophene, amino acids, polyphenols and vitamin C have also been used as a good replacement for hydrazine [ Liu S, Tian J, Wang L and Sun X 2011, Carbon 49, 3158; Liu Y, Zhou J, Zhang X, Liu Z, X. Wan, Tian J, Wang T and Chjen Y 2009 Carbon 47, 3113; Chen D, Li L, Gou L, 2011 Nanotechnolol. 22,325601; R. Liao, Z. Tang, Y. Lei, B. Guo, J. Phys. Chem. C 2011, 115, 20740 and Fernandez-Merino M J, Guardia L, Paredes J 1, Villar-Rodil S, Soils-Fernandez P, Martinez-Alonso A and Tascon J M D 2010, J. Phys. Chem. CI 14, 6426].
On a different note, industrial thermal management has been a big issue in the recent years and this is explained by the practical needs that heat removal is now a crucial issue for the continuing progress in industry. Thermal conductivity in nano dimension has truly shown the intriguing features, carbon allotropes occupying a special place in terms of their ability to conduct heat |Baladin A.A. 201 INat. Mater. 10, 569]. It was recently discovered experimentally from Baladin A. A., Gosh S, bao W, Calizo I, Teweldebrhan D, Miao F and Lau C N 2008 Nano Lett. 8, 902 and Ghosh S, Calizo I, Teweldebrhan D, Pokatilov E P, Nika D L, baladin A A, Bao W, Miao F and Lau C N 2008 Appl. Phys. Lett 92, 15911 that the near room-temperature (RT) thermal conductivity of partially suspended single-layer graphene is in the range K ~ 3000-5000 W/mK, depending on the graphene flake size and is superior to CNTs [ D. L. Nika, S. Ghosh, E. P. Pokatilov and A. A. Baladin 2009 Appl. Phys. Lett., 94, 203103] that graphene reveals an extremely high thermal conductivity.
In light of the above processes and the use of graphene in thermal management, there exists a need for a simple, scalable and environment friendly method to produce a stable dispersion of reduced graphene oxide.
STATEMENT OF THE INVENTION
Accordingly, the present invention provides a process for preparing a stable dispersion of graphene, said process comprising the step of reacting graphene oxide dispersion with an excess amount of reducing cum stabilizing agent, characterized in that the reducing cum stabilizing agent is of the general formula:
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N(H).(Ca HbX)m
wherein a > 2, b > 4;
n is an integer having a value of 1 or 2;
m is an integer having a value of 1 or 2; each of n & m has a value of 1 or 2, wherein n +
m = 3;
X is a group capable of forming hydrogen bonding with a solvent of the graphene oxide
dispersion,
In an embodiment of the present invention, X is OH or a group capable of hydrogen
bonding with water provided the whole molecule is soluble in water.
In an embodiment of the present invention, "a" is in the range of 2 to 6 and "b" is in the
range of 4 to 15.
In an embodiment of the present invention, the reducing cum stabilizing agent is selected
from mono ethanol amine or di-ethanol amine.
In another embodiment of the present invention, the graphene oxide dispersion is in a
polar protic solvent.
In yet another embodiment of the present invention, the solvent is selected from water,
ethylene glycol or a mixture thereof.
In still an embodiment of the present invention, the graphene oxide dispersion is
containing graphene oxide contains 0.25 mg/mL to 1 mg/mL of graphene oxide in the
solvent.
In yet another embodiment of the present invention, the reducing cum stabilizing agent
added to the dispersion at least 500 uL/10 mg of graphene oxide.
In still another embodiment of the present invention, the graphene oxide dispersion is
refluxed with the reducing cum stabilizing agent for about 6 to 12 hours at 90 °C.
SUMMARY OF THE INVENTION
The present invention relates to a process for the preparation of a stable dispersion of reduced graphene oxide by using a reducing cum stabilizing agent. In the process the graphene oxide solution is reacted with a reducing cum stabilizing agent and then refluxed to obtain a stable dispersion of reduced graphene oxide.
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BRIEF DESCRIPTION OF DRAWINGS
Figure 1 shows schematic representation for the process and stabilization.
Figure 2 shows photographs of dispersion at different intervals for stability evaluation.
Figure 3 shows the digital photographs and the corresponding UV-Vis spectra of GO and rGO.
Figure 4 shows Raman spectra of GO and reduced GO.
Figure 5 shows XRD pattern of GO and Reduced GO.
Figure 6 shows TEM images of the reduced graphene oxide sample.
Figure 7 shows Zeta potential of the rGO dispersion.
Figure 8 shows FTIR spectra of GO and rGO
Figure 9 shows thermal diffusivity of water and rGO dispersions.
DESCRIPTION OF INVENTION
The present invention describes a method of preparing a dispersion of reduced graphene through an environmental friendly route by using a suitable agent having reducing and stabilizing property. According to the present invention, an organic amine is used as a reducing agent as well as a stabilizing agent for synthesizing reduced graphene oxide. The amine is a water soluble amine containing at least one group which can make hydrogen bonding with the solvent and particularly a hydroxyl group.
The stable dispersion of graphene oxide is prepared in aqueous medium. However, other polar mediums such as ethylene glycol or mixture thereof with water can also be used. The dispersion contains a dispersion of graphene oxide, solvent and an organic amine which acts as a reducing cum stabilizing agent wherein, the hydrogen attached to the amine at one end reduces the functionalized graphene and the other end of the organic amine is capable of forming hydrogen boding with the solvent. With the help of the organic amine the graphene layers are kept separated and they do not adhere to each other as a result of which the agent acts as a spacer also which stabilizes the dispersion. Particularly, the organic amine used is diethanol amine (DEA) and it is always preferred to use organic amine in excess amount to form a stable dispersion.
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The process for producing the stable dispersion comprises dispersing graphene oxide powder in a suitable medium followed by addition of reducing agent with sonication, refluxing to obtain the stable dispersion. The stable dispersion so prepared was centrifuged and finally dried to characterize for its physical and chemical properties by using various techniques.
Morphological characterization of the stable dispersion shows formation of mixture of single and few layers of graphene sheets. Thermal diffusivity study reveals substantial increase compared to base fluid and so has a potential to be used as coolant in industries and in solar thermal generation also.
Further, the present invention discloses a method of using the dispersion of reduced graphene of the present invention for applications in thermal management.
The following non-limiting examples illustrate in details about the invention. However, they are not intended to be limiting the scope of present invention in any way,
Example 1
Preparation of graphite oxide (GO)
The graphite powder (1 g) was put into an 80 °C solution of concentrated sulfuric acid (2 mL), potassium persulphate (0.5 g), and phosphorus pentoxide (0.5 g). The resultant dark blue mixture was thermally isolated and allowed to cool to room temperature over a period of 4 h. The mixture was then diluted with distilled water and centrifuged repeatedly till the product became nearly neutral. The product was dried at 60 °C in an oven at ambient atmosphere. This preoxidized graphite was then subjected to oxidation by Hummers' method. The oxidized graphite powder (0.98 g) was put into cold (0 °C) concentrated sulfuric acid (23 mL). Potassium permanganate (3 g) was added gradually with stirring very slowly, so that the temperature of the mixture was not allowed to reach 20°C. The mixture was then stirred at 35°C for 2 h, and distilled water (50 mL) was added drop wise. The solution was stirred for 30 mins and the whole was added in to 150 mL of water. 30 % hydrogen peroxide solution was added till the color of the mixture changed to bright yellow. The mixture was centrifuged and washed with 1:10
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hydrochloric acid solution in order to remove metal ions and dried in ambient conditions to get the GO powder.
Example 2
Reduction of GO to form graphene (rGO)
lOmg of graphene oxide powder obtained in the above step was dispersed in distilled water (0.25mg/mL) with help of a sonic horn operating at amplitude 35 for 40 mins effective time. To the dark brown dispersion 500uL of diethanol amine (DEA) was added and sonicated for another 2 mins. The whole dispersion was put into round bottom flask and refluxed at 90 °C for 6h. The resultant dark gray dispersion could be physically distinguished to be the reduced graphene oxide or graphene.
Example 3 Characterization techniques
The dispersion was centrifuged at 8500 RPM with water twice and dried. The dried dispersion of rGO obtained in the above step was further characterized using UV-Vis spectrometer. The UV-Vis absorption spectra of the aqueous solutions were collected using a Perkin Elmer X 36 spectrometer. Raman spectra of the drop casted and dried solution on a glass slide were collected from a SEKI 750 Raman analyzer using argon ion laser at A. 514.5 nm. XRD of the powder samples were done on a Bruker X-ray diffractometer using CuKa radiation (X = 1.54 A°) from angle 2 to 35° at a scanning speed of 5° / min. TEM and HRTEM were done from a drop casted and dried sample on carbon coated grid using JEOL 2100 electron microscope. FTIR spectra of GO and rGO were taken on a Perkin Elmer spectrum BX-II with KBr method. Zeta potential of the solution was done on a Malvern zetasizer at 150 V taken a standard average of 20 cycles. Thermal diffusivity of the 0.025 % solution and the base fluid was measured with help of Microflash instrument LFA 457 from NETSCZH using the laser flash technique. The samples were taken in an aluminum pan, coated with conductive graphite. 1. Schematic representation of the process.
Graphite on oxidation by modified Hummer's method gives graphite oxide with increased inter layer space. The layers are now separated by week van der Waal's force that exfoliate on sonication to give single or few layered graphene oxide sheets. Upon refluxing the
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dispersion with DEA, the amino part of the molecule is attached to functional groups of GO, reducing the oxygen from the carbon network. The functional hydroxyl groups on the other side of the molecule form hydrogen bonding with the aqueous medium providing stability to the dispersion. The whole process is represented in figure 1. The photograph of the prepared dispersion at different time intervals is shown in figure 2 to compare the stability. There could be seen no separation or sedimentation even after one year of storage that boasts our claim for long term stability of the RGO dispersion prepared by the present protocol.
2. UV-Vis spectra of GO and rGO.
The reduction of GO is indicated by a color change from dark brown in case of GO to dark gray for graphene and that can be seen in the respective digital photographs of the two shown in figure 3. It is to be noted that the change in color was noticeable after 2h of reaction duration and the reaction was complete after 6h. The corresponding UV-Vis spectra of the dispersions are shown in the same figure to further confirm the reduction. The peak for GO at 235 is characterized for TC-TT* transition of the C=C plasmon resonance. The peak shifts from 235 nm to 264 nm confirming the reduction of GO and formation of graphene. This red shift reflects increase in 7r electron concentration and structural reordering, which is consistent with restoration of sp carbon as well as the aromatic structure. However, the peak for graphene is observed at lower wavelength with respect to other reports suggesting possible unreduced oxygen present.
3. Raman spectra of GO and reduced GO.
Raman spectroscopy is highly sensitive to electronic structure, and so far is the best technique for characterization of carbon based materials. Spectra of both GO and rGO shown in figure 4 depicts the D and G bands with position of both shifting towards left. Graphene is generally characterized by two peaks: G band arising from first order scattering of the E2g phonon from sp2 carbon and D band arises from breathing mode of k-point photons of Aig symmetry [Khanra et. al]. The peak of 4G' band in case of graphene oxide is around 1602 cm'1, which shifts to 1593 cm"1 upon reduction. This is most probably due to formation of more hydrocarbon double bonds upon reduction and removal of oxygen from the system. Formation of more hydrocarbon double bonds is in conformity with structural restoration as observed in UV-vis spectra. After oxidation,
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the D band of GO also shifts from 1356 to 1347 cm"1, which results from decrease in in-plane sp2 domain due to extensive reduction. The shifting of D band indicates functionalization of the reduced GO that creates defects. The most interesting observation here is relative increase in intensity of 'D' band compared to the 'G' band, which usually reveals the change of electronic conjugation state. The relative ratio (ID/IG) of the both increase from 0.99 to 1.12, which suggests proper reduction of GO and formation graphene. All our observations are in conformity with the previous reports on Raman study[Liu et. al, Zhang et. al].
4. XRD pattern of GO and Reduced GO
As mentioned earlier in experimental section, graphite oxide initially prepared by modified Hummer's method was subsequently reduced by help of DEA to get functionalized graphene. As the destacking and number of layers is the main featured difference between graphite and graphene, structural information is an important parameter. Graphite has a signature peak around 26°, which vanishes with oxidation as it can be seen in case of prepared graphite oxide given in figure 5. A new broader peak appears at 10.4° due to formation of graphite oxide. Such high increase in d-spacing (3.5 nm to 7.96 nm) is due to inter-lamellar water molecules entrapped in between the layers [Ruoff et.al]. This indicates the resultant solid is randomly ordered with low Van der Waal's force and can be de-stacked easily. On sonication, the loosely bound layers de-laminate forming graphene oxide that could be reduced further to get graphene sheets. The XRD for the sonicated and reduced sample i.e. graphene shows no peak at 10.4° is and appearance of broad peak centered around 21°. This indicates increase in the d-spacing in comparison to GO, but certainly larger than the d002 spacing of graphite. This is due to higher packing of the reduced sample in comparison to the unreduced sample and confirms formation of graphene [Chen et.al; Ruoff et.al]. However, this higher basal spacing compared to initial precursor may be due to presence of residual oxygen functional groups. The broad d002 spacing also suggests the samples are poorly stacked and comprises largely free graphene sheets [Rajamathi et.al]. The broadness of peak indicates proper delamination and formation of few layer or single sheets.
5. TEM images of the reduced graphene oxide sample.
The structural characterization was further done by TEM analysis illustrated in figure 6. The low magnification image shown in figure 6a shows diffused sheet like structures with lateral
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dimension of around few hundred nanometers. The SAED pattern of the sample is shown in the inset shows concentric rings with well defined spots. Hence, the sample can be identified to be crystalline in nature. This suggests good quality of the reduced graphene oxide synthesized, which would be beneficial for some of the applications, where phonon transport would be better. As it can be seen in the figure 6b, folding of the edges occur in case of some structures, which suggest the sheets have been folded due to high stress from the edges. However, in most cases the distinct edges of the sheets could be observed, suggesting formation of single layered graphene. The featureless transparent region in the image indicates the possible formation of single layered graphene sheet. From the overall TEM study it can be concluded that the dispersion contains a mixture of few layered and single layered graphene sheets having few hundreds to micron size in lateral dimension. The figure 6c shows the HRTEM image of a single layered graphene sheet, absence of any layers or lattice fringes even at very high magnification confirms the same.
6. Zeta potential of the rGO dispersion.
Stability is the major deciding factor for application of graphene dispersion, as with time they tend to agglomerate and loose the desired properties. Hence, the same needs to be checked physically as well as by analytical techniques. Zeta potential of the prepared dispersion was measured at 150 V for 20 cycles and the value found to be -45 eV (figure 7). In general, the value falling below -30 or above +30 is considered to be stable. Hence, the zeta potential suggests the dispersion is quite stable and this has been confirmed by physical observation also.
The dispersion is stable for several months and also the interesting fact is that it is reversible, an advantage over the hydrazine derivative assisted reduction. In most of cases where hydrazine is used as a reductant, the dispersion is irreversible; once the sheets agglomerate it is not possible to get them re-dispersed. This is due to decrease in electrostatic repulsion with time and due to the high surface energy of the sheets they tend to agglomerate [R.S Ruoff et.al ]. However, in case the industries, where the thermic fluid is in continuous flow for years, needs to be reasonably stable. This particular attribute can be exploited for solar thermal applications or rolling mill applications in industries. The super stability of the dispersion can be explained on the basis of possible hydrogen bonding owing to presence of functional groups in the DEA.. However, the
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main interaction between amine or hydroxyl with GO could be the ring opening reaction of epoxides. The free ethyl hydroxyl groups present in the DEA works like spacer to keep the sheets apart and hydrogen bonding with the aqueous medium.
7. FTIR spectra
The FTIR spectra taken further to confirm the nature of functionalization developed during the reduction process and the spectra for both graphene oxide and reduced graphene oxide is shown in figure 8. Typical peaks of GO appear at 1724 cm"1 (C=O carboxyl stretching), 1620 cm"1 (C=C aromatic stretching), and 1390 cm"1 (C-OH stretching) [Sun et.al]. Another two signature peaks at 1220 cm"' and 1052 cm"1 corresponds to epoxy C-0 stretching and alkoxy C-0 stretching respectively [Guo et.al; Bangal et.al]. Interestingly the peak for carboxyl stretching decreased substantially suggesting the reduction and formation of reduced GO. The bands for epoxy stretching also decreased confirming the interaction of DEA with epoxy ring. These observations confirm reduction of GO and removal of most oxygen functionalities. However, the peak for C-O stretching for alkoxy group is not decreased reasonably due to presence of functional groups form DEA. Two new peaks appearing at 2862 cm"1 and 2926 cm-1 are resulting from -CH2 stretching of alkyl chain [Cai et.al.] The FTIR was performed on the sample centrifuged twice with distilled water and dried thereafter. Hence, we believe that most of the unreacted DEA, if any has been removed and the presence of these groups suggests functionalization of the GO with DEA.
8. Thermal diffusivity of water and rGO dispersions.
The thermal conductivity of the graphene dispersion in a medium has been reported less and needs to be addressed for specific industrial applications. Hence, the prepared dispersion was characterized for thermal diffusivity study in order to know possibility of being used as a thermic fluid. As it can be observed in figure 9, thermal diffusivity of 0.025 % dispersion has around 60-65 % increase over the base fluid water and the 0.05 % dispersion has enhancement around 90%. Thermal conductivity data having components of density, specific heat & thermal diffusivity; in our system water is the major component density & specific heat value for the said temperature range remains same. So increase in thermal diffusivity reflects the increase in thermal conductivity value in same ratio. Hence, the increase in thermal conductivity of the dispersion at
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such a low concentration from 0.37 Watt/m.K for base fluid (water) to 0.75 Watt/m.K in the present case quite promising. In general a method of chemical reduction to derive graphene increases the defect levels that hinder the transport of phonons. However, this increase in thermal diffusivity is promising and can further be exploited for application in solar thermal or thermic fluid applications.
ADVANTAGES
1. The present invention provides an environmentally safe process for the preparation of a dispersion of reduced graphene.
2. The present invention provides a process for the preparation of a dispersion of reduced graphene which is very stable.
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We Claim:
1. A process for preparing a stable dispersion of graphene, said process comprising the step
of reacting graphene oxide dispersion with an excess amount of reducing cum stabilizing
agent, characterized in that the reducing cum stabilizing agent is of the general formula:
N(H).(Ca HbX)m
wherein a > 2, b > 4;
n is an integer having a value of 1 or 2; m is an integer having a value of 1 or 2; each of
n & m has a value of 1 or 2, wherein n + m = 3;
X is a group capable of forming hydrogen bonding with a solvent of the graphene oxide
dispersion.
2. The process as claimed in Claim 1, wherein X is OH or a group capable of hydrogen bonding with water provided the whole molecule is soluble in water
3. The process as claimed in Claim 1, wherein "a" is in the range of 2 to 6 and "b" is in the range of 4 to 15.
4. The process as claimed in Claim 1, wherein the reducing cum stabilizing agent is selected from mono ethanol amine and di-ethanol amine.
5. The process as claimed in Claim 1, wherein the graphene oxide dispersion is in a polar protic solvent.
6. The process as claimed in Claim 5, wherein the solvent is selected from water, ethylene glycol or a mixture thereof.
7. The process as claimed in Claim 1, wherein the graphene oxide dispersion is containing graphene oxide contains 0.25 mg/mL to 1 mg/mL of graphene oxide in the solvent.
8. The process as claimed in Claim 1, wherein the reducing cum stabilizing agent added to the dispersion at least 500 µL/10 mg of graphene oxide.
9. The process as claimed in Claim 1, wherein the graphene oxide dispersion is refluxed with the reducing cum stabilizing agent for about 6 to 12 hours at 90 °C.