METHOD OF MAKING BLUE EMITTING GRAPHENE QUANTUM
DOTS FROM COAL

TECHNICAL FIELD
[0001] The present disclosure, in general, relates to a high yield simple synthesis procedure for developing blue emitting graphene quantum dots from most abundant natural source coal using the combination of recycling wet chemical oxidation and acid-free hydrothermal techniques with simple purification process. More particularly, the present disclosure relates to a method of making blue emitting graphene quantum dots from coal.
BACKGROUND
[0002] Background description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed subject matter, or that any publication specifically or implicitly referenced is prior art.
[0003] Graphene quantum dots (GQDs) are one of the emerging class of nanomaterials, which is positioned in the crossroad of an electronically conductive highway of graphene, the photoluminescent (PL) street of quantum dots and the green, a widely unexplored lane of carbogenic nanoparticles. [L. Lingling, W. Gehui, Y. Guohai, P. Juan, Z. Jianwei and Z. Jun-Jie, Nanoscale 2013, 5, 4015–4039] & [X. T. Zheng, A. Ananthanarayanan, K. Q. Luo and P. Chen, Small 2015, 11, 1620–1636]. GQDs have a large surface area, low cytotoxicity and excellent solubility. Due to their quantum confinement and edge effects, GQDs display stable photoluminescent (PL) properties. [S. Kim, S. W. Hwang, M. K. Kim, D. Y. Shin, C. O. Kim, S. B. Yang, J. H. Park, E. Hwang, S. H. Choi, G. W. Ko, S. H. Sim, C. S. Sone, H. J. CHoi, S. K. Bae and B. H. Hong, ACS Nano, 2012, 6, 8203–8208], [Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai and L. Qu, J. Am. Chem. Soc., 2012, 134, 15–18], & [L. Wang, S. J. Zhu, H. Y. Wang, S. N. Qu, Y. L. Zhang, Q. D. Chen, H. L. Xu, W. Han, B. Yang and H. B. Sun, ACS Nano, 2014, 8, 2541–2547]. These properties make GQDs spectacular for applications in optoelectronics, photovoltaics, bio-imaging and organic light-emitting diodes. [V. Gupta, N. Chaudhary, R. Srivastava, G. D. Sharma, R. Bhardwaj and S. Chand, J. Am. Chem. Soc., 2011, 133, 9960–9963], [H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C. H. A. Tsang, X. Yang and S. T. Lee, Angew. Chem., Int. Ed., 2010, 49, 4430–4434], [X. Zhai, P. Zhang, C. Liu, T. Bai, W. Li, L. Dai and W. Liu, Chem. Commun., 2012, 48, 7955–7957], & [W. Kwon, Y. H. Kim, C. L. Lee, M. Lee, H. C. Choi, T. W. Lee and S. W. Rhee, Nano Lett., 2014, 14, 1306–1311].
[0004] Semiconductor Quantum Dots (SQDs) with size-dependent physical properties are building blocks for several applications, but their large-scale applications would be jammed by their recognized toxicity and prospective environmental hazard arising from the release of metal ions such as Cd2+, Pb2+. [A. M. Derfus, W. C. W. Chan, S. N. Bhatia; Nano Lett. 2004, 4, 11–18] & [A. M. Derfus, W. C. W. Chan, S. N. Bhatia; Nano Lett. 2004, 4, 11–18]. Therefore, environmentally friendly, biocompatible fluorescent GQDs with tunable emission are considered to be next-generation nanomaterials as a potentially inexpensive and safe alternative to SQDs such as CdX (X=S, Se, Te) and PbS.
[0005] Generally, GQDs are synthesized through solvothermal cutting, chemical or electrochemical oxidation, microwave treatment. However, all these aforesaid techniques suffer to some degree of disadvantages like the requirement of complex, expensive and time-consuming procedures, high temperature and different synthetic conditions which limit their wide application. [X. Yan, X. Cui, L. S. Li, J. Am. Chem. Soc. 2010, 132, 5944], [R. Liu, D. Wu, X. Feng, K. Mullen; J. Am. Chem. Soc. 2011, 133, 15221], [F. Wang, Z. Xie, H. Zhang, C. Y. Liu, Y. Q. Zhang; Adv. Funct. Mater. 2011, 21, 1027], [Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai, L. Qu; J. Am. Chem. Soc. 2012, 134, 15], & [Q. Wang, H. Zheng, Y. Long, L. Zhang, M. Gao, W. Bai; Carbon 2011, 49, 3134]. Moreover, the initial precursor of those methods is graphene oxide which needs to be synthesized from graphite first and those methods take extensive time and multiple stages to get graphene quantum dots with poor yield and hence can be expensive in bulk volumes.
[0006] Furthermore, most of the GQD synthesis routes are not direct and also require complicated procedures wherein the initial precursor is exposed to a strong acid or hazards chemical solvent. [Y. Sun, S. Wang, C. Li, P. Luo, L. Tao, Y. Wei and G. Shi, Phys. Chem. Chem. Phys., 2013, 15, 9907–9913]. GQD purification typically requires also a very long time to ensure removal of acid or solvent. Purification also comprises a neutralization process that involves a strong base, resulting in the formation of a large quantity of salt.
[0007] Therefore, there is a need in the state of the art of a direct method for synthesis of GQDs without the use of strong acid or hazards solvent is essential.

OBJECTS OF THE DISCLOSURE
[0008] Some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0009] It is a general object of the present disclosure to provide a direct method for synthesis of semiconductor quantum dots (GQDs) without the use of strong acid or hazards solvent is essential.
[0010] It is an object of the present disclosure to provide a method of making blue emitting graphene quantum dots from coal.
[0011] It is another object of the present disclosure to provide synthesis and characterization of high quantum yield based blue emitting graphene quantum dots from coal through simplistic technique and find out its potentiality as a fluorescence agent
[0012] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

SUMMARY
[0013] This summary is provided to introduce concepts related to a method of making blue emitting graphene quantum dots from coal. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0014] The present disclosure relates to a method of making blue emitting graphene quantum dots (GQDs) from coal. The method includes chemically oxidizing the coal in an acidic environment followed by distillation of acid; hydrothermal cutting of the oxidized coal at 175-200° C for 6-8 hours to obtain a GQD solution; purifying the GQD solution through a 0.45 mm polytetrafluoroethylene membrane; and dialyzing a filtrate of the GQD solution in 1000 Da dialysis bag, say, for 5 days.
[0015] In an aspect, the coal is bituminous type coal.
[0016] In an aspect, the acidic environment comprises a combination of concentrated hydrochloric acid and concentrated nitric acid at 90° C.
[0017] In an aspect, the hydrothermal cutting is performed in an autoclave using Oxone as an Oxidant.
[0018] In an aspect, the GQDs are having functional groups in combinations of epoxy, alkoxy and hydroxyl groups on their aromatic surfaces.
[0019] In an aspect, the GQDs are having defective graphitic nature.
[0020] In an aspect, the GQDs are having oxygen addends on their edges.
[0021] In an aspect, the GQDs are having diameters ranging from 3 to 6 nm.
[0022] In an aspect, the GQDs are having high intense blue emitting photoluminescence property at emission spectra of 450 nm and ~7.9% quantum yields efficiency.
[0023] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.
[0024] It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.
[0025] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
[0027] FIG. 1 illustrates a method of making blue emitting graphene quantum dots (GQDs) from coal, in accordance with an embodiment of the present disclosure;
[0028] FIG. 2 illustrates UV-vis absorptions spectra of an aqueous solution of GQDs (inset: a photograph taken under UV light);
[0029] FIG. 3 illustrates Fourier-Transform Infrared (FTIR) spectra of the GQDs;
[0030] FIG. 4 illustrates X-ray diffraction (XRD) pattern of the GQDs;
[0031] FIG. 5 illustrates Raman spectra of GQDs;
[0032] FIG. 6A and 6B illustrate High-Resolution Transmission Electron Microscopy (HRTEM) image of GQDs; and
[0033] FIG. 7 illustrates photoluminescence spectra of GQDs at the different excitation wavelength.
[0034] The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
[0035] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0036] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0037] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0038] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0039] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0040] Embodiments explained herein pertain to a method of making blue emitting graphene quantum dots (GQDs) from coal. GQDs have established wonderful attention in nanoscience and nanotechnology because of their large surface area, low cytotoxicity, excellent solubility, and tunable band gap. On contrary, natural carbon resources coal might contain some districts or bunches that are graphite-like in nature. Hence, such graphitic structure exist in coal could be released through facile top-down technique with simple purification step to produce value added high photoluminescent based GQDs. Accordingly, blue emitting graphene quantum dots (GQDs) from coal are required to be developed by inexpensive, environmental friendly and simple synthesis technique in high yield.
Synthesis of GQDs from Bituminous Coal
[0041] In embodiments described in the present disclosure, GQDs derived from bituminous coal is synthesized through inexpensive facile acid-free oxidative cutting followed by neutral oxidant assisted hydrothermal method with simple dialysis tubing-based purification process.
[0042] In a typical procedure, coal of about 1 gm was treated with concentrated hydrochloric acid of about 100 ml at 90° C for 2 hr. The solution was cooled to room temperature and filtered and washed with water-acetone mixture and dried in a hot air oven. Then, the dried coal of about 500 mg was chemically oxidized with concentrated nitric acid of about 100 ml at 90° C for 6 hours (chemical oxidation). Afterward, nitric acid (HNO3) is separated from the sample by normal distillation technique at 200° C and oxidized coal is collected. Then, 100 mg oxidized coal was hydrothermally treated in presence of neutral oxidant Oxone (250 mg) and water (100 mL) solvent in an autoclave at 200°C for 8 hr. The neutral mixture was then filtered through a 0.45 mm polytetrafluoroethylene membrane and the filtrate was dialyzed in 1000 Da dialysis bag for 5 days.
Mechanism of GQDs Synthesis
[0043] FIG. 1 illustrates a method 100 of making blue emitting graphene quantum dots (GQDs) from coal, in accordance with an embodiment of the present disclosure. The order in which the method 100 is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any appropriate order to carry out the method 100 or an alternative method. Additionally, individual blocks may be deleted from the method 100 without departing from the scope of the subject matter described herein.
[0044] At block 102, the method 100 includes chemically oxidizing the coal in an acidic environment.
[0045] At block 104, the method 100 includes hydrothermal cutting of the oxidized coal at 200° C for 8 hours to obtain a GQD solution.
[0046] At block 106, the method 100 includes purifying the GQD solution through a 0.45 mm polytetrafluoroethylene membrane.
[0047] At block 108, the method includes dialyzing a filtrate of the GQD solution in 1000 Da dialysis bag for 5 days.
[0048] In an aspect, the coal is bituminous type coal.
[0049] In an aspect, the acidic environment comprises a combination of concentrated hydrochloric acid and concentrated nitric acid at 90° C.
[0050] In an aspect, the hydrothermal cutting is performed in an autoclave using Oxone as an Oxidant.
[0051] In an aspect, the GQDs are having functional groups in combinations of epoxy, alkoxy and hydroxyl groups on their aromatic surfaces.
[0052] In an aspect, the GQDs are having defective graphitic nature.
[0053] In an aspect, the GQDs are having oxygen addends on their edges.
[0054] In an aspect, the GQDs are having diameters ranging from 3 to 6 nm.
[0055] In an aspect, the GQDs are having high intense blue emitting photoluminescence property at emission spectra of 450 nm and ~7.9% quantum yields efficiency.
[0056] Thus, the mechanism of GQDs synthesis comprises of the combination of chemical oxidation and hydrothermal redox reaction. This reaction is accelerated using Oxone as an oxidant to obtain GQDs by a hydrothermal redox reaction. Graphitic segments of Coal are continually broken during hydrothermal redox reaction using Oxone to afford an acid-free reaction for mass production of GQDs. Because of the chemical functionalization followed by oxidation cleavage (edge defect) through hydrothermal redox cutting, natural carbon materials coal can be turned into very small sized GQDs. This acid-free method, not requiring the neutralization process of strong acids, but also shows a simple, recycling and environmental friendly purification process, which could be effective for scale-up synthesis at high yield.

Characterization of GQDs
UV–vis spectra:
[0057] FIG. 2 illustrates the UV-visible absorption spectrum of aqueous GQDs solution. In FIG. 2, GQDs exhibit an obvious absorption peak at 270 nm with a shoulder peak at 360 nm in the UV–vis absorption spectra. The absorption peak at 270 nm is because of p–p* transition of C=C and the absorption peak at 360 nm corresponds to n–p* transition which is a characteristic feature of GQDs. These characteristic peaks signify a typical absorption of an aromatic sp2 domains and represent the existence of GQDs. The inset of FIG. 2 reflects the optical imaging of GQDs with UV light (360 nm) showing blue emission.
Fourier–Transform Infrared (FTIR) Spectroscopy:
[0058] The FTIR spectrum of GQDs, in FIG. 3, shows the peak related to C–O stretching of alkoxy groups at 1115 cm-1 region. The characteristics peaks at 1380 cm-1 are attributed to C–O (epoxy) stretching vibrations. An aromatic C=C stretching peak is observed at 1630 cm-1. The peak at ~2820 cm-1 is assigned for aliphatic -CH modes and the peak at 3135 cm-1 is due to the aromatic -CH stretching modes. The peak at 3420 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 epoxy, alkoxy and hydroxyl groups on their aromatic surfaces that impart solubility in various solvents.
X-ray diffraction (XRD):
[0059] The XRD pattern shows a broad peak centered at 2? = 23.80 (FIG. 4). 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. In addition, GQDs are oxidized on the edges because of the very small size.
Raman Spectroscopy:
[0060] Raman spectroscopy is also utilized to characterize the GQDs, as shown in FIG. 5. GQDs shows “disorder” D band at 1358 cm-1 and the crystalline G-band at 1592 cm-1 with a relative intensity ratio ID/IG of 0.75. The G-band at 1592 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 groups. The D band at 1358 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 carboxyl, hydroxyl, alkoxy and epoxy groups are introduced to the edges and onto the basal plane, as shown in the FTIR spectrum.

HRTEM Analysis:
[0061] FIGS. 6A and 6B show high-resolution TEM (HRTEM) image of coal-derived GQDs, showing a uniform distribution of particles with a relatively narrow size distribution between 3 to 6 nm diameter. The average diameter of the GQDs is around 4 nm. The high-resolution TEM image (FIG. 6B) with a clear lattice fringe structure indicates high crystallinity of the GQDs.

Fluorescence Characteristics: Photoluminescence Study
[0062] 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 320 to 420 nm; the strongest peak is excited at 350 nm which emitted bright blue photoluminescence at an emission spectra of 450 nm region (FIG. 7). The PL spectrum can be considered as a transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Blue emission was attributed to the zigzag effect with a carbene-like triplet ground state s1p1. The blue luminescence of GQDs is generated from intrinsic states in the highly crystalline structure. It is well known that the photophysical properties of GQDs are influenced by a combination of factors including their size, shape, and functionalization. The quantum confinement effect is a major property of quantum dots that have a size-dependent effect on their PL properties; smaller quantum dots usually lead to a blue-shifted emission. Using Rhodamine B as a reference, the quantum yield (QY) of GQDs is 7.9 %, which is maximum than other reported work based on coal as a raw material as shown in Table 1.

Table 1: Comparison of GQDs Quantum Yield derived from coal
References Quantum Yield (Q.Y)
Methods of producing graphene quantum dots from coal and coke (Patent: WO2014179708A1) 5.35 %
Coal as an abundant source of graphene quantum dots (Nature Communications; DOI: 10.1038/ncomms3943) 1.0 %
Graphene quantum dots, graphene oxide, carbon
quantum dots and graphite nanocrystals in coals (Nanoscale, 2014, 6, 7410–7415) 1.8 %
Present Study 7.9 %

TECHNICAL ADVANTAGES
[0063] The present disclosure utilizes a precursor material (coal) which is cheap and abundantly available.
[0064] The present disclosure proposes a chemical oxidation synthesis route which is acid-free and simple regenerable solvent treatment process is employed.
[0065] The present disclosure proposes an acid/alkali and chemical solvent-free hydrothermal method. Hence, this method does not require the elimination of a large quantity of salt formed from neutralization of a strong acid or the removal of chemical solvent for purification purpose.
[0066] The present disclosure proposes a method of making GQDs which has a moderately high (~40%) product yield.
[0067] The present disclosure proposes making of synthesized GQDs which shows blue photoluminescence property with significantly high quantum yield (~7.9%) efficiency than other reported coal-based work and shows a uniform distribution of particle size in the range of 3-6 nm.
[0068] Furthermore, those skilled in the art can appreciate that the above description does not provide specific details of the manufacture or design of the various components. Those of skill in the art are familiar with such details, and unless departures from those techniques are set out, techniques, known, related art or later developed designs and materials should be employed. Those in the art can choose suitable manufacturing and design details.
[0069] It should be understood, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, as apparent from the discussion herein, it is appreciated that throughout the description, discussions utilizing terms such as “oxidizing,” or “cutting,” or “purifying,” or “dialyzing,” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

[0070] Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0071] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

We claim:

1. A method of making blue emitting graphene quantum dots (GQDs) from coal, the method comprising:
chemically oxidizing the coal in an acidic environment followed by distillation of acid;
hydrothermal cutting of the oxidized coal at 175- 200° C for 6- 8 hours to obtain a GQD solution;
purifying the GQD solution through a 0.45 mm polytetrafluoroethylene membrane; and
dialyzing a filtrate of the GQD solution in 1000 Da dialysis bag.
2. The method as claimed in claim 1, wherein the coal is bituminous type coal.
3. The method as claimed in claim 1, wherein the acidic environment comprises a combination of concentrated hydrochloric acid and concentrated nitric acid at 90° C.
4. The method as claimed in claim 1, wherein the hydrothermal cutting is performed in an autoclave using Oxone as an Oxidant.
5. The method as claimed in claims 1-4, wherein the GQDs are having functional groups in combinations of epoxy, alkoxy and hydroxyl groups on their aromatic surfaces.
6. The method as claimed in claims 1-4, wherein the GQDs are having defective graphitic nature.
7. The method as claimed in claims 1-4, wherein the GQDs are having oxygen addends on their edges.
8. The method as claimed in claims 1-4, wherein the GQDs are having diameters ranging from 3 to 6 nm.
9. The method as claimed in claims 1-4, wherein the GQDs are having high intense blue emitting photoluminescence property at emission spectra of 450 nm and ~7.9% quantum yields efficiency.