TITLE:
SIMPLE AND RAPID COST-EFFECTIVE APPROACH FOR SYNTHESIS OF GREEN FLUORESCENT GRAPHENE QUANTUM DOTS FROM COAL
HELD OF INVENTION:
The present invention relates to a simple method of developing green fluorescent based graphene quantum dots using wet chemical oxidation coupled with innovative rapid hydrothermal techniques from most abundant natural source coal. More particularly, the invention relates to the synthesis and characterization of graphene quantum dots and find out its potentiality as a fluorescence agent
BACKGROUND:
Graphene quantum dots (GQDs) are one or few-layered graphene nanosheets with a lateral plane size less than 100 nm [1]. Photoluminescent (PL) 0D graphene quantum dots (GQDs) are emerging as attractive candidate's due to the quantum confinement and edge effects, stable fluorescence, high surface area, high electrical conductivity and low toxicity [2-4]. Graphene quantum dots (GQDs) have various potential applications, like bioimaging, biosensing, light emitting diodes, catatysrs, organic photovoftaic devices and nanodevice fabrication [5-9].
Semiconductor quantum dots (SQDs) with size-dependent physical properties are building blocks for various applications, but their large-scale applications would be impeded by their documented toxicity and potential environmental hazard arising from the release of metal ions such as Cd2+ [10-11]. Compared with conventional semiconductor quantum dots such as CdX (X=S, Se, Te) and PbS, GQDs are more environmentally friendly, biocompatible and photostable.
Till now, two main strategies e.g. top-down and bottom-up have been developed for synthesizing GQDs. The top-down strategy is to cut 2D graphene sheets or other carbon materials into GQDs via hydrothermal cutting, chemical oxidation,

electrochemical oxidation, microwave, or ultrasonication treatment. Although these techniques are feasible to produce GQDs in the laboratory, the inevitably complex and time-consuming operations restrict the large-scale synthesis and wide applications of GQDs. In addition, it is difficult to control the sizes of the obtained GQDs by top-down technique [12-16].
The "bottom-up" methods, based on carbonizing some special organic precursors via thermal treatment, usually allow accurate control over the morphology, size distribution and lattice dimensions of products. However, large scale applications of the colloidal GQDs synthesized by bottom-up technique are hindered by the expensive special equipment or tedious reaction, low yield and multistep purification procedures [17-20].
Furthermore, the starting material of those methods is graphene oxide which needs to be synthesized from graphite first and those methods take long time and multiple steps to get graphene quantum dots with low yield and therefore can be expensive in bulk quantities.
References:
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SUMMARY OF THE INVENTION
Green fluorescent based graphene quantum dots (GQDs) have been synthesized from bituminous coal through facile chemical oxidative treatment coupled with innovative rapid hydrothermal method using autoclave or pressure cooker at high yield (~45 %).
The GQD chemistry and morphology were characterized by means of FTIR, XRD, Raman, UV-vis spectroscopy, Fluorescence spectroscopy and HRTEM analysis.

This technique can produce very small GQDs with an average diameter between 4-8 nm along with a monodisperse size distribution pattern. The graphene quantum dots exhibit a green intense luminescence (Quantum Yield ~ 3.5 %) in the visible range with an excitation wavelength dependent fluorescence, suggesting its potentiality as a fluorescence agent.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 Provides a schematic representation for the synthesis of graphene
quantum dots (GQDs) from Coal
Fig. 2 Represents UV-vis absorptions spectra of aqueous solution of GQDs.
Fig. 3 Provides FTIR spectra of GQDs
Fig. 4 Reflects XRD pattern of GQDs
Fig. 5 Provides Raman spectra of GQDs
Fig. 6 Shows HRTEM Image of GQDs
Fig. 7 Reflects photoluminescence (PL) spectra of GQDs at different excitation
wavelength (inset: photograph taken under UV light)
Fig. 8 PL emission spectrum of GQDs at pH 2, 3, 7 and 10.
DETAILED DESCRIPTION
Coal might contain some regions or clusters that are graphite-like in nature. Therefore, this graphitic structure could be released through oxidative cleavage and separated from coal through simple treatment to produce value added carbon nano material. Hence, the objective of the work is development of graphene quantum dots from coal by direct and rapid synthesis technique in high yield.
Synthesis of GQDs
In present disclosure, GQDs derived from bituminous coal (Fig. 1) is synthesized through inexpensive facile one-step wet chemical oxidation route coupled with

innovative and rapid hydrothermal method. In a typical procedure, coal (2 gm) was suspended at ice cold condition in the mixture of conc, sulphuric add (50 mL) and nitric acid (20 mL) in presence of sodium nitrate (2 gm) at 90° C for 4 hr (chemical oxidation). The mixture of conc sulphuric acid, nitric add in presence of sodium nitrate at 90* C being referred as an acidic environment The ratio of conc, sulphuric acid and conc, nitric acid is 5:2 in the acidic environment
The solution was cooled to room temperature and add 100 mL ice cold water followed by sonicated upto i hr. Then, 100 mL hot water is mixed with solution and the predpitate formed is filtered with the separating funnel. Sodium hydroxide [1 (M) NaOH] solution is poured into the supernatant solution until the pH was 7. The neutral mixture was then filtered through a 0.45-mm polytetrafluoroethylene membrane. Finally, the suspended diluted solution was hydrothermally treated (hydrothermal cutting) in a pressure cooker/autoclave (50 mL)at 120-130°Cfor2-3hrs.
After being cooled down to room temperature, the product was purified witii Whatmann 42 filter paper.
Characterization of GQDs tJV-vis spectra:
The UV-vislble absorption spectrum of aqueous GQDs solution is shown in Rg. 2. In the UV-vislble spectrum, the shoulder absorption peak at 280 nm is assigned to the n-n* transition of aromatic sp2 domains, which is similar to that of chemically reduced GO. However, it also shows a new weak absorption peak at 365 nm due to n-n* transition. Inset is the photograph of GQDs aqueous solution taken under 365 nm UV light which green fluorescence is visible by naked eye.

Fourier-Transform Infrared (FTIR) Spectroscopy:
The FTIR spectrum of GQDs (Fig, 3) shows the peak related to C-O stretching of alkoxy groups at 1105 cm-1. The emerging peaks at 1399 cm-1 are attributed to C-O (epoxy) stretching vibrations. An aromatic C=C stretching peak is observed at 1640 cm-1. The peaks at 1750 cm-1 vibrations is responsible for C=O stretching vibration. The peak at 3125 cm-1 is due to the aromatic -CH stretching modes and the peak at ~2915 cm-1 is assigned for aliphatic -CH modes. The peaks at 3425 cm-1 is associated with the O-H stretching vibration. The spectral results described above reflect that GQDs have various oxygenated functional groups such as carboxylic acid, epoxy, alkoxy and hydroxyl groups on their aromatic surfaces that impart solubility in various solvents.
X-ray diffraction (XRD)
The XRD pattern shows a broad peak centered at 28 = 25.5. The interlayer spacing is 0.354 nm, which is broader than that of graphite. This result could be attributed to the oxygen addend-containing groups introduced in the exfoliation and oxidation of defective graphitic structure in coal, which enhanced the interlayer distance. However, the intertayer distance of GQDs is smaller than the graphene oxide, which could be explained that GQDs are only oxidized on the edges because of the very small size. Raman Spectroscopy:
Raman spectroscopy is also utilized to characterize the GQDs, as shown in Fig. 5. GQDs shows "disorder" D band at 1365 cm"1 and the crystalline G-band at 1605 cm-1 with a relative intensity ratio ID/IG of 0.77, The G-band at 1605 cm"1 is due to E2G, mode at the C-point, arising from the stretching in sp2 hybridized carbon, bonded either with neighboring carbon atoms or with oxygen in the form of carboxyl and carbonyl groups. The D band at 1365 cm-1, which is a prominent

feature in the spectrum, indicates the creation of sp3 domains due to the extensive oxidation. During the oxidation, oxygen-containing groups, including carbonyl, carboxyl, hydroxyl and epoxy groups are introduced to the edges and onto the basal plane, as shown in the FTIR spectrum. The presence of these hydrophilic groups created the GQDs soluble in water. HRTEM Analysis:
Fig. 6 shows HRTEM image of coal derived GQDs, showing uniform distribution of particles with a relatively narrow size distribution between 4 to 8 nm diameter. Fluorescence Characteristics: Photoluntinescence Study The PL spectra are generally broad and dependent on excitation wavelength, the PL peaks shifted to longer wavelengths with a maximum intensity as the excitation wavelength is changed from 300 to 340 nm; the strongest peak is excited at 340 nm which emitted bright green photoluminescence at an emission spectra of 445 nm region (Rg. 7). The PL spectrum can be considered as a transition from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO). Developed GQDs exhibit fluorescent quantum yields of ~3.5 % using quinine sulfate as a reference standard. To analyze the chemical environment dependence PL behavior of GQDs, we performed the pH dependent PL at pH 2, pH 3, pH 7 and pH 10 (Fig. 8). Herein, PL emission is pH dependent and the intensity is highest at pH 7. A red shift of emission spectra from 435 to 475 nm with decreasing intensity is observed as the pH changed from 7 to 2. This result would be attributed to the following explanations. The free zigzag sites of the GQDs are protonated and a complex between the zigzag sites and H+ is formed in acidic solution. Thus, the emissive state becomes inactive in PL The aggregation in acid solution reduces the band gap and consequently a red-shift excitation is observed. The deprotonation of carboxyi groups of GQDs in alkaline solution increases the electrostatic repulsions between them, which overcome the trend of aggregation through layer-layer stacking.

Advantages:
The advantages of our current GQDs synthesis techniques are mentioned below:
> Precursor material (coal) is cheap and abundant available.
> Synthesis route is direct and rapid (~12 hrs.) and separation method is very simple.
> Innovative hydrothermal (pressure cooker/autoclave treatment) route is utilized and occurs at low temperature (130 °C).
> Product yield is moderately high (~45%).
> Developed GQDs shows green fluorescent property of ~3.5% Q.Y. efficiency with uniform distribution of partide size (4-8 nm range).

I / We claim:
1. A method of making green emitting graphene quantum dots from a coal,
the method comprising:
chemically oxidizingthe coal in an acidic environment;
filtering the oxidized coal in neutral pH via polytetrafluoroethylene membrane and retaining the size of 0.45 mm;
hydrothermal cutting the oxidized coal at 120-130 °C for 2-3 hrs.; and
purifying the oxidized coal by Whatmann filter paper technique.
2. The method as claimed in claim 1, wherein the coal is bituminous type coal.
3. The method as claimed in daim 1, wherein the acidic environment comprises a combination of cone, sulphuric acid, conc, nitric acid and sodium nitrate at 90° C.
4. The method as claimed in claim 3, wherein the ratio of conc, sulphuric acid and conc nitric add is 5:2.
5. The method as claimed in daim 1, wherein the hydrothermal cutting is performed in an autoclave.
6. The method as claimed in claim 1, wherein the hydrothermal cutting is performed in a pressure cooker,
7. The method as daimed in claim 1, wherein product yield Is moderately high (~45%).
8. The graphene quantum dots having functional groups in combinations of carboxylic add, epoxy, alkoxy and hydroxyl groups on their aromatic surfaces as claimed in any of the claims 1-7.
9. The graphene quantum dots having defective graphitic nature as claimed in any of the claims 1-7.
10. The graphene quantum dots having oxygen addends on their edges as claimed in any of the claims 1-7.
11. The graphene quantum dots having diameters ranging from 4 to 8 nrn as claimed in any of the claims 1-7.

12. The graphene quantum dots having green emitting fluorescenceat an emission spectra of 445 nm and ~3.5% quantum yields efficiency with pH dependent PL emission property as claimed in any of the claims 1-7.