DESC:TITLE OF THE INVENTION: HIGH YIELD GRAPHENE PRODUCTION BY AQUEOUS-BASED EXFOLIATION

FIELD OF THE INVENTION
The present invention relates to the commercialization of quality graphene with fewer defects at low costs.
BACKGROUND OF THE INVENTION
Several different methods exist for graphene production, and each presents its own challenges. Micromechanical cleavage is the popular choice for many, not surprising given that it led to the ‘birth’ of graphene. Despite the high-quality graphene flakes that can be produced by this method, low yield and process expandability still remain the main drawbacks.
Graphene grown on metal substrates, via methods such as epitaxial growth or chemical vapor deposition, involves not only high energy consumption, but also requires subsequent transfer of graphene to other target substrates for a variety of applications, thus limiting the general applicability.
Chemical reduction of graphite oxide (GO) is one of the widely used methods for the production of graphene, and has shown high scalability potential. However, such harsh chemical treatment of graphite introduces hydroxyl or epoxide groups in the basal plane, and carbonyl and carboxylic moieties on the edges, which, in turn, disrupt the electronic structure of graphene, and makes GO essentially a semiconductor. Furthermore, it is impossible to remove all the defects completely by any known method. Therefore, reduced graphene oxide still retains some defects.
Direct exfoliation of graphite was first carried out by Coleman et al. using the liquid-phase exfoliation (LPE) method via sonication of graphite in organic solvents. Polymers and surfactants have also been used to enhance the exfoliation process and prevent against re-aggregation of the exfoliated sheets in LPE systems.
LPE encompasses mainly two different approaches for exfoliation of graphite: cavitation, and shear forces in sonication and high shear mixing. Compared to other methods, LPE is a simple method with high potential for large-scale production of graphene. The basic equipment needed for sonication or shear processing of graphite is generally available, and, besides, LPE does not require high temperature or vacuum systems. However, large-scale application of sonication-assisted LPE of graphite has been hindered due to high energy consumption and low concentration of produced graphene. For example, in the work made by Coleman and coworkers, they could only manage to produce graphene dispersion with a concentration of 0.3 mg/mL after 400 hours of bath sonication.
Moreover, particularly, organic solvents that have been shown to aid exfoliation of graphite to graphene under sonication-assisted LPE are required at sufficiently high concentrations to be effective; e.g., N-methyl-pyrrolidone (NMP) and N,N-dimethylformamide (DMF); these are considered to be not only toxic, but also expensive, making them inappropriate for large-scale industrial production.
Low energy consumption and easy availability of shear mixers have meant that shear exfoliation of graphite is increasingly being considered as an alternative to sonication for large-scale production of graphene dispersions. Shear mixing is an old technique that has been widely used in colloidal science, mostly for disintegration of agglomerates during dispersion. A few reports have also been shown where shear mixing has been incorporated into delamination of layered materials. However, most of these applications first involve intercalation of 2-dimensional materials with oxidants, sulfate ions, etc., which induces swelling of the material, followed by shear exfoliation into individual layers. The intercalation step also hinders the potential for large-scale graphene production.
Recently, it has been shown that shear mixing can be used for direct exfoliation of graphite without any intercalation. However, despite the potential for up scaling, Coleman and coworkers could only manage to produce graphene dispersions with a concentration of up to 0.07 mg/mL, which is very low and disadvantageous for most applications.
It is evident, therefore, that current methods have limitations when considering them for large-scale production of graphene, with the main drawbacks hindering their development being the low concentration of the resulting graphene dispersion and high energy consumption by sonication, and environmentally questionable chemical aid consumption by shear-induced exfoliation.
There is, therefore, a need in the art for a method of producing graphene, which overcomes the aforementioned drawbacks and shortcomings.
SUMMARY OF THE INVENTION
An eco-friendly process of producing defect-free, high specific surface area graphene by aqueous-based exfoliation is disclosed.
The process comprises the steps of: dissolving sodium cholate in 500 mL of distilled water under stirring condition, with the concentration of sodium cholate ranging between 0.1 g and 0.4 g; shear mixing of exfoliated graphene worms at room temperature, with the ratio of exfoliated graphene worms to sodium cholate being in the ratio of 1:12.5 to 1:50, at an rpm rate of 5,000 to 6,000, for one hour to four hours; allowing the suspension to stay at room temperature overnight; collecting 400 mL of upper solution through a pipette; centrifuging the collected upper solution, with the centrifugation rpm being between 1,000 and 4,000, for 10 minutes to 40 minutes; collecting the supernatant; filtering the collected solution through 0.45 µm Polyvinylidene fluoride membranes, and washing with distilled water; and drying the obtained graphene in air.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates a Table which depicts the variation in the process parameters, in accordance with the present disclosure;
Figure 2 illustrates a general flowchart of an eco-friendly process of producing defect-free, high specific surface area graphene, by aqueous-based exfoliation, in accordance with the present disclosure;
Figure 2a illustrates a flowchart of an embodiment of an eco-friendly process of producing defect-free, high specific surface area graphene, by aqueous-based exfoliation, in accordance with the present disclosure;
Figure 2b illustrates a flowchart of another embodiment of an eco-friendly process of producing defect-free, high specific surface area graphene, by aqueous-based exfoliation, in accordance with the present disclosure; and
Figure 2c illustrates a flowchart of yet another embodiment of an eco-friendly process of producing defect-free, high specific surface area graphene, by aqueous-based exfoliation, in accordance with the present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
Throughout this specification, the use of the word "comprise" and “include”, and variations such as "comprises" "comprising", “includes”, and “including” may imply the inclusion of an element or elements not specifically recited.
Throughout this specification, the disclosure of any range is to be construed as being inclusive of the lower limit of the range and the upper limit of the range.
Exfoliated Graphene Worms (EGW) exfoliated into few layered graphene (FLG), Multi layered graphene (MLG), and Graphene Nanosheets (GNS), by high shear mixing in aqueous surfactant solution, is disclosed.
To prepare Exfoliated Graphite Worms (EGW) exfoliated into few layered graphene (FLG), Multi layered graphene (MLG) and Graphene Nanosheets (GNS) initially, calculated amount of sodium cholate was dissolved in 500 mL of distilled water in 1 litre beaker under stirring condition. Then, calculated amount of EGW above surfactant solution was shear mixed at room temperature at different process parameters (as illustrated in Figure 1).
As illustrated in Figure 2, after shear mixing under ambient conditions, the obtained suspension was allowed to stay at room temperature overnight. Then, 400 mL (80%) of upper solution was collected with the help of pipette. Subsequently, the collected upper solution was centrifuged, with the supernatant being collected. Finally, the collected solution is filtered through Polyvinylidene fluoride (PVDF, 0.45 µm) membranes and washed with distilled water.
The obtained graphene was dried in air until there was no water surplus. As illustrated in Figure 2a, Figure 2b, and Figure 2c, all the samples were processed by similar shear mixing by varying the mainly 3 process parameters, like reaction time, rpm and the ratio of EGW and surfactant.
Preparation of FLG Synthesis
Shear mixing of EGW was performed, with the parameters of EGW and NaC being 1:12.5, at an rpm rate at 5,000, for 2 hours. The centrifugation rpm was at 4,000 for 40 minutes.
(A) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing time (up to 3 hours);
(B) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing time (up to 4 hours); and
(C) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing rate (up to 6,000 rpm).
Preparation of MLG Synthesis
EGW and NaC in 1:25 ratio, at an rpm rate at 5,000, for 1 hour. The centrifugation rpm was at 2,000 for 20 minutes.
(A) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing time (up to 2 hours);
(B) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing time (up to 3 hours);
(C) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing rate (up to 6,000 rpm).
Preparation of GNS Synthesis
EGW and NaC in 1:50 ratio, at an rpm rate of 5,000, for 1 hour. The centrifugation rpm was at 1,000 for 10 minutes.
(A) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing time (up to 2 hours);
(B) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing time (up to 3 hours);
(C) All the parameters were the same as above (previous paragraph), with the exception of an increase in the shear mixing rate (up to 6,000 rpm).
All the prepared samples were characterized by XRD, Raman, SEM, and BET analysis. In XRD patterns of the EGW and graphene samples, two diffraction peaks (0 0 2) and (0 0 4) were observed on both the XRD patterns at the same positions, which suggested that the graphite lattice was retained after the shear exfoliation from EGW to graphene. The relative intensity ratio of graphene and EGW samples reflections for (0 0 2) and (0 0 4) were 0.044 and 0.017, respectively, confirming a significant decrease in the intensity ratio for the as-prepared graphene. The relative intensity ratio of (0 0 2) and (0 0 4) reflections can be considered as an indication of positional disorder in the graphene nanosheet caused by the exfoliation of the graphite layers.
As per Raman spectra, it was clear that, distinctly different from that of EGW, the graphene sample exhibited a single and almost symmetrical 2D peak at about 2692 cm-1, also demonstrating its few-layer feature. Based on the intensity ratio (I2D/IG) of 2D (2716 cm-1) and G (1581 cm-1) bands, which was calculated as 0.9, we deduced that the average layer number for the graphene sheets should be less than 5.
In addition, it is well-known that the signal intensity of D band is a sensitive indicator for the graphene with defects. For the graphene powder obtained in the present work, the calculated average intensity ratio (ID/IG) of D and G bands for the graphene was 0.45, which is significantly lower than those reported previously, suggesting that the produced graphene has fewer defects and was of high quality.
The above processes were repeated for 25 times using the sediments of the first cycle. The yield of each cycle decreased up to the 25th cycle. All the graphene samples showed similar kind of Raman pattern, which indicated the good quality of all the samples.
Huge specific surface area is an important feature of graphene that enhances the specific properties like conductivity, catalytic activity, and interface interaction between graphene and matrix in composite materials. The specific surface area of processed graphene samples were analyzed by Brunauer-Emmett-Teller (BET) method. The BET surface areas of graphene samples were observed to be in the range of 320 m2/g – 550 m2/g.
The disclosed process offers the following advantages: FLG, MLG and GNS of high quality were produced by simple shear mixing of EGW in aqueous surfactant solutions; high specific surface area graphene was produced by controlled process parameters; eco-friendly alternative to produce defect-free graphene over already existing methods, such as chemical vapour deposition (CVD), microwave synthesis, and electrochemical process (this process requires readily available distilled water and sodium cholate (NaC) which are environmentally non-harmful rather than toxic organic solvents); requires less power relatively when compared to other liquid exfoliation methods like sonication, thereby enabling the process to be scalable and economically viable; and large amounts of defect-free few layered graphene can be produced by scaling up the process to hundreds of liters.
It will be apparent to a person skilled in the art that the above description is for illustrative purposes only and should not be considered as limiting. Various modifications, additions, alterations and improvements without deviating from the spirit and the scope of the disclosure may be made by a person skilled in the art. Such modifications, additions, alterations and improvements should be construed as being within the scope of this disclosure.
,CLAIMS:1. An eco-friendly process of producing defect-free, high specific surface area graphene by aqueous-based exfoliation, comprising the steps of:

dissolving sodium cholate in 500 mL of distilled water under stirring condition, with the concentration of sodium cholate ranging between 0.1 g and 0.4 g;

shear mixing of exfoliated graphene worms at room temperature, with the ratio of exfoliated graphene worms to sodium cholate being in the ratio of 1:12.5 to 1:50, at an rpm rate of 5,000 to 6,000, for one hour to four hours;

allowing the suspension to stay at room temperature overnight;

collecting 400 mL or 80% of upper solution through a pipette;

centrifuging the collected upper solution, with the centrifugation rpm being between 1,000 and 4,000, for 10 minutes to 40 minutes;

collecting the supernatant;

filtering the collected solution through 0.45 µm Polyvinylidene fluoride membranes, and washing with distilled water; and

drying the obtained graphene in air.

2. The process of producing graphene by aqueous-based exfoliation as claimed in claim 1, wherein the ratio of exfoliated graphene worms to sodium cholate is 1:12.5.

3. The process of producing graphene by aqueous-based exfoliation as claimed in claim 1, wherein the shear mixing rpm rate is 5,000 for four hours.

4. The process of producing graphene by aqueous-based exfoliation as claimed in claim 1, wherein the centrifugation rpm is 4,000 for 40 minutes.