WO2017081688A1

WO2017081688A1 - A method of producing graphene quantum dots and a product thereof

Info

Publication number: WO2017081688A1
Application number: PCT/IL2016/051217
Authority: WO
Inventors: Matat BUZAGLO; Michael SHTEIN; Oren Regev
Original assignee: B. G. Negev Technologies And Applications Ltd., At Ben-Gurion University
Priority date: 2015-11-10
Filing date: 2016-11-10
Publication date: 2017-05-18

Abstract

It is provided a method for producing graphene quantum dots (GQD) by manipulating and controlling flow of an aqueous suspension of graphite flakes through micro-channels wherein manipulating and controlling the flow comprises cycling the aqueous suspension of graphite flakes through the micro-channels that may be embedded within a micro fluidizer.

Description

A METHOD OF PRODUCING GRAPHENE QUANTUM DOTS

AND A PRODUCT THEREOF

TECHNICAL FIELD

The present disclosed subject matter relates to methods of producing graphene quantum dots, and more particularly, the present disclosure relates to affordable techniques for producing graphene quantum dots (GQD) and the GQD produced by the disclosed techniques.

BACKGROUND

Graphene quantum dots (GQD) are nano-particles with lateral dimensions smaller than 100 nanometer, possessing unique optical, electronic, spin, and photoelectric properties induced by the quantum confinement effect and edge effect. These induced unique physical properties lead to potential applications in various fields, such as photovoltaic, bio-imaging, light-emitting diodes, and sensors.

Presently, several commercially available techniques are employed for GQD preparations. These techniques mainly include electron beam lithography, chemical synthesis, electrochemical preparation, microwave assisted hydrothermal method, and the ultrasonic exfoliation method. However, these techniques come with a price tag of approximately two million USD per kilogram, which limits GQD commercialization solely to biological applications, such as cellular imaging, molecular tracking in live cells, bio-sensing, drug delivery, or the like, in which small quantities of these low toxicity quantum dots are required.

BRIEF SUMMARY

It is an object of the present disclosure to teach a new method of producing GQD through which the produced GQD are affordable, a fact that can open the product to additional and vast applications.

According to a first aspect of the present disclosed subject matter, a method is provided for producing graphene quantum dots (GQD) by manipulating and controlling flow of an aqueous suspension of graphite flakes through micro-channels.

According to another embodiment of the subject matter, manipulating and controlling the flow comprising cycling the aqueous suspension of graphite flakes through said micro-channels and wherein said micro-channels are embedded within a micro fluidizer.

According to another embodiment of the subject matter, the producing method further comprises preparing the aqueous suspension with graphite flakes.

According to another embodiment of the subject matter, the producing method further comprises injecting the aqueous suspension into the micro fluidizer with substantially high pressure pump.

According to another embodiment of the subject matter, the producing method further comprises stabilizing the aqueous suspension while cycling the aqueous suspension through the micro fluidizer.

According to another embodiment of the subject matter, a dispersion resulting from the aqueous suspension that is obtained from the micro fluidizer is filtered in order to extract the GQD.

According to another embodiment of the subject matter, the preparing of the aqueous suspension further comprises utilizing high shear mixer for reducing the graphite flakes to an average size of about 400 micrometer.

According to another embodiment of the subject matter, injecting the aqueous suspension into the micro fluidizer is performed by a substantially high-pressure pump at about 400 meter per second subsequently applying shear forces of about 10 reciprocal second on the graphite flakes. According to another embodiment of the subject matter, the micro channels comprises at least one Z-shaped micro-channel.

According to another embodiment of the subject matter, the aqueous suspension is cycled through the at least one Z-shaped micro-channel at about 100 mL per minute.

According to another embodiment of the subject matter, said at least one Z- shaped micro-channel has an inner diameter that is gradually reduced.

According to another embodiment of the subject matter, the inner diameter starts at 400 micrometer (having inlet pressure of 5000 psi), further reduced to 200 micrometer (having inlet pressure of 15000 psi), and further reduced to 87 micrometer (having inlet pressure of 27000 psi) at the end of at the least one Z-shaped micro-channel.

According to another embodiment of the subject matter, stabilizing the aqueous suspension during the cycling is performed by introduction of a surfactant.

According to another embodiment of the subject matter, the surfactant is polyethylene glycol p - (1, 1, 3, 3 - tetramethylbutyl) - phenyl ether.

According to another embodiment of the subject matter, the solution is filtered by stirred cell under pressure of about 3bar through tracked-etch membranes with approximately 15 nm pore size.

According to a second aspect of the present disclosed subject matter, graphene quantum dots (GQD) are produced in accordance with the embodiments described herein before.

According to a third aspect of the present disclosed subject matter, an aqueous solution of GQD is produced in accordance with the embodiments described herein before.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosed subject matter belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosed subject matter, suitable methods and materials are described below. In case of conflict, the specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the disclosed subject matter described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present disclosed subject matter only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the disclosed subject matter. In this regard, no attempt is made to show structural details of the disclosed subject matter in more detail than is necessary for a fundamental understanding of the disclosed subject matter, the description taken with the drawings making apparent to those skilled in the art how the several forms of the disclosed subject matter may be embodied in practice.

In the drawings:

Fig. 1A schematically illustrates a cross sectional view along a Z-shaped channel for producing GQD, in accordance with some exemplary embodiments of the disclosed subject matter;

Fig. IB schematically outlines flow profile within the Z-shaped channel shown in Figure 1A and microstructure presentation along the channel, in accordance with some exemplary embodiments of the disclosed subject matter;

Fig. 2 shows a flowchart diagram depicting GQD production process, in accordance with some exemplary embodiments of the disclosed subject matter;

Fig. 3 shows a graph depicting a spatial distribution of GQD produced in a microfluidization process, in accordance with the exemplary embodiments of the disclosed subject matter;

Fig. 4 shows transmission electron microscopy (TEM) micrograph of GQD produced in a microfluidization process, in accordance with an exemplary embodiments of the disclosed subject matter; Fig. 5 shows high resolution TEM micrograph of the GQD produced in the microfluidization process, in accordance with some exemplary embodiments of the disclosed subject matter;

Fig. 6 shows an atomic-force microscopy (AFM) image and analysis (inset) depicting the GQD thickness, in accordance with some exemplary embodiments of the disclosed subject matter;

Fig. 7 shows photoluminescence spectrum (PL spectra) of the GQD produced in the microfluidization process, in accordance with the exemplary embodiment of the disclosed subject matter;

Fig. 8 shows a contour map for excitation and emission of the GQD produced in the microfluidization process, in accordance with some exemplary embodiments of the disclosed subject matter; and

Fig. 9 shows Raman spectra of graphite, few layers graphene (FLG), and graphene quantum dots (GQDs,) as produced in the microfluidization process, in accordance with some exemplary embodiments of the disclosed subject matter;

DETAILED DESCRIPTION

Before explaining at least one embodiment of the disclosed subject matter in detail, it is to be understood that the disclosed subject matter is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The disclosed subject matter is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. The drawings are generally not to scale. For clarity, non-essential elements were omitted from some of the drawings.

The terms "comprises", "comprising", "includes", "including", and "having" together with their conjugates mean "including but not limited to". The term "consisting of" has the same meaning as "including and limited to". The term "consisting essentially of" means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.

As used herein, the singular form "a", "an" and "the" include plural references unless the context clearly dictates otherwise. For example, the term "a compound" or "at least one compound" may include a plurality of compounds, including mixtures thereof.

Throughout this application, various embodiments of this disclosed subject matter may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed subject matter. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range.

It is appreciated that certain features of the disclosed subject matter, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosed subject matter, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the disclosed subject matter. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

In some exemplary embodiments, utilization of the present disclosure, exhibits a simple, inexpensive and environmentally friendly microfluidization-based method for fabricating non-functionalized graphene quantum dots (GQD). It should be noted that the term "microfluidization" in the present disclosure refers to the technology of manipulating and controlling flow of fluids, ranging from micro-liters to few liters, through micro- channels, wherein the diameter of the channels ranges between tens and hundreds of micrometers. Microfluidization fabrication may be easily scalable through alignment of micro-channels in parallel, which ensures the entire product stream experiences identical shear, and resulting in consistent product quality ranging from 1 milliliter to 60 liters per- minute. In some exemplary embodiments, the method of the present disclosure allows utilizing various organic solvents and carbon-based raw materials so as to provide an ideal framework to large-scale production of GQD to be used for various applications and usability.

Referring now to Fig. 1A, schematically illustrating a cross sectional view along a Z-shaped channel for producing GQD, in accordance with some exemplary embodiments of the disclosed subject matter. A Z-shaped channel 100, also known as micro-channel, may have an inner path 111 with a diameter ranging from tens of micrometers to hundreds of micrometer. Additionally or alternatively, the inner path 111 may zigzag along the Z- shaped channel 100. Furthermore, the inner path 111 diameter may vary along the channel. In some exemplary embodiments, at least one Z-shaped channel 100 may be utilized within a microfluidization apparatus. Each Z-shaped channel 100 comprises inlet 102 for collecting aqueous suspension and outlet 105 for emitting aqueous solution. It should be noted that there are microfluidizations with channels having other profiles than the Z profile, such as Y profile. Other profiles of microfluidizations in which the diameter and/or the direction of the tube change in order to vary the fluid regimes of the fluid that flows within the channels can be utilized for production of GQD in accordance with the present disclosure without limiting the scope of protection.

Referring now to Fig. IB schematically outlines flow profile within the Z-shaped channel and microstructures presentations along the channel, in accordance with some exemplary embodiments of the disclosed subject matter. In some exemplary embodiments, the flow rate within the main portions of the channel as depicted in the left hand-side of Fig. IB, is an exemplary flow rate of 400 meter per second in the center of the channel. The flow brakes when the direction of flow is changed so that turbulence is formed in the transition zones. Within these flow regimes, a suspension, from which GQD are produced, flows. Microfluidization may be a dynamic high-pressure homogenization process, in which high shear rates greater than 10 /sec. are applied on the suspension of particles pressurized through the micro-sized channels, such as the Z-shaped channel 100. In some exemplary embodiments, this technique may generate shear that is few orders of magnitude stronger than conventional rotor-based or other homogenization techniques. As mentioned herein before and as shown in Figure 2B - the graphite suspension that enters the inlet of the channel 100 flows within the channel while during cycling within the channel and enduring the intense flow and shear, graphite flakes 103 are exfoliated into graphene sheets 104 and further fragmented into nano sized GQD 105, that are non- functionalized.

Reference is now made to Fig. 2, showing a flowchart diagram depicting the GQD production process according to some exemplary embodiments of the present disclosure. It should be mentioned that this process was used in order to experimentally produce GQD according to the method disclosed. The experimental details are provided herein:

In Step 210, aqueous suspension of graphite flakes is prepared. In some exemplary embodiments, the aqueous graphite suspension 122 as shown in Fig. IB, is prepared by mixing graphite flakes with water. According to the performed experiment, graphite flakes having lateral dimensions of a few millimeters (CAS 7782-42-5, 1%) were pre- mixed with water using high shear mixer ( 15000 rpm for 30 min) until the average size of the flakes is reduced to 400μπι. This size was determined in accordance with other parameters of the experiment; however, other sizes can be prepared according to the specific parameters of the system.

In Step 215, the suspension comprising graphite and water may be injected by high-pressure pump (up to 30 kpsi) into the micro-channels at velocity of 400 meter per second. In some exemplary embodiments, the high-pressure injection may apply shear forces of 10 per second on the solid particles, such as the graphite flakes or the like.

In Step 220, the aqueous suspension is cycled in a micro fluidizer. In the exemplary embodiment, the aqueous suspension was cycled at 100 mL per minute through a commercially available micro fluidizer, lab Homogenizer M-110P, Micro- fluidics. Other fluidizers or homogenizers can be used without limiting the scope of the protection of this disclosure. It will be noted that such micro fluidizer may be equipped with at least one micro-channel, such as a Z-shaped channel 100 of Fig. 1A, wherein the diameter of its inner path 111 is gradually reduced. In this case, the inner path diameter started from 400μπι (having inlet pressure of 5000 psi), further reduced to 200μπι (having inlet pressure of 15000 psi), and further reduced to 87μπι (having inlet pressure of 27000 psi) at the last cycling stage. It should also be noted that, other dimensions or parameters may be utilized without limiting the scope of the present disclosure.

In Step 230, adding a surfactant to the aqueous suspension in the cycling process is performed. In some exemplary embodiments, suspension such as polyethylene glycol p- (l,l,3,3-tetramethylbutyl)-phenyl ether is added. In this exemplary embodiment, Triton X-100 (CAS 9002-93-1, 0.5wt%) was added to the aqueous suspension during the last microfluidizing cycle in order to stabilize the GQD in a resulted aqueous solution.

In Step 240, the resulted aqueous solution that exits the microfluidizer though the outlet may be filtered. In this exemplary embodiment, the resulted aqueous solution was filtered by stirred cell under pressure of 3bar, through tracked-etch membranes with 15 nm pore size (Whatman^® Nuclepore™ Track-Etched Membranes, Sigma- Aldrich).

The resulting aqueous solution was examined and the properties of the GQD that were produced in the above mentioned apparatus and according to the steps mentioned herein were evaluated. Figure 3 showing a graph depicting a spatial distribution of GQD produced in a microfluidization process, in accordance with the exemplary embodiment of the disclosed subject matter and to Figures 4 and 5 which are showing transmission electron microscopy (TEM) micrograph and high resolution transmission electron microscopy (HRTEM) micrograph, respectively, of GQD produced in the microfluidization process. The produced nano-particles that are established to be GQD are of 2.7±0.7 nanometer in diameter as shown in Figure 3 with hexagonal symmetry in the real and reciprocal spaces, as shown Figures 4 and 5 with its inset, with graphitic in-plane lattice spacing of 0.2 nanometer. It should be noted, that the distribution in figure 3 is based on over 200 (HRTEM)-imaged particles.

Reference is now made to Figure 6 showing an atomic-force microscopy (AFM) image and analysis (in an inset) depicting the GQD thickness, in accordance with the exemplary embodiments of the disclosed subject matter. The AFM micrograph reveals that the GQD produced in the experiment are of thickness of 2-4 nanometers, i.e. two to four layers of graphene sheets.

It will be noted that the AFM images were taken with Veeco Dimension 3100 AFM in tapping mode using VEECO and RTESP models silicon probes. The dispersion was spin coated on Si0₂ wafers dried by evaporation at ambient temperature for 24 hours before measuring.

The GQD fluorescence activity is evident also in Raman Scattering (RS) as opposed to graphite and few-layered graphene as shown in Figure 9. It is known in the art that the GQD is characterized by G and D bands at 1589cm^"1 and 1331cm^"1, respectively, similar to that of blue luminescent hydrothermally synthesized GQD. It also should be noted that the low D-to-G peak intensity ratio smaller than 0.3 may indicate low GQD defect densities.

RS of the GQD were measured by a Jobin-Yvon HR Labram micro-Raman spectroscope at 514 nanometer on quartz slide. The liquid samples dried out on the slide from a 200 micro liter drop before the measurement.

Reference is now made to Figures 7 and 8 that shows photoluminescence spectrum (PL spectra) of the GQD produced in the microfluidization process and a contour map for excitation and emission of the GQD produced in the microfluidization process in accordance with the exemplary embodiment of the disclosed subject matter, respectively. Further study of the optical properties of the GQD, prepared according to a method of the present disclosure, comprise a detailed UV-Vis and PL studies that were carried out. The excitation-independent PL behavior, as shown in Figure 7, indicates comparable size uniformity with PL features of ultrasonically prepared GQD that are known in the art. Test results shown in Figure 7 depict that changing the excitation wavelength from 280 to 310 nanometers, yields invariable PL spectra characterized by a strong emission peak at approximately 400 nanometers. It should be noted that this behavior is in contrast with PL spectra of most luminescent carbon nano particles such as carbon quantum dots CQD. It will be noted that the UV-Vis spectra was measured by Jasco V-530 spectrometer using quartz cuvettes while the dispersant absorption was subtracted from the spectrum of the sample. Additionally or alternatively, the PL spectra of GQD dispersions in quartz cuvettes were measured by FLS920P Spectrofluorimeter and the PL spectrum of the pure dispersant were subtracted by TX-100 with maximum excitation wavelength at 365nm.

It will be understood that CQD are typically quasi-spherical nano particles comprised of amorphous to nano crystalline cores with predominantly graphitic carbon

(sp 2 carbon) fused by diamond-like sp 3 hybridized carbon insertions. It will be noted that, in the present disclosed subject matter, the term sp refer to hybridized atoms in sigma and pi bonding. CQDs are dependent of excitation wavelength, demonstrating a bath chromic shift due to optical selection of differently sized nano particles (quantum effect) and/or different emissive traps on their surfaces. The excitation-emission contour of the GQD of the present disclosure possesses an excitation wavelength maximizes at approximately 310nanometer, as depicted by black dotted lines of Figure 8 and emission maximizes at approximately 400nanometer due to their small sizes and lack of functionality.

Reference is now made again to Figure 5 showing a HRTEM micrograph of GQD produced in a microfluidization process depicted in Figure 2. Figure 5 inset is resulting from fast Fourier transform of the HRTEM image, and reveals a crystalline hexagonal structure. The TEM micrographs were obtained by FEI Tecnai 12 G2 TWIN TEM operated at 120 kV. The dashed circles indicate lattice imaging obtained by HRTEM operated at 200 kV.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

Claims

1. A method for producing graphene quantum dots (GQD) by manipulating and controlling flow of an aqueous suspension of graphite flakes through micro- channels.

2. The method of Claim 1, wherein manipulating and controlling the flow comprising cycling the aqueous suspension of graphite flakes through said micro-channels and wherein said micro-channels are embedded within a micro fluidizer.

3. The method of Claims 1-2, wherein the producing method further comprises preparing the aqueous suspension with graphite flakes.

4. The method of Claims 1-3, wherein the producing method further comprises injecting the aqueous suspension into the micro fluidizer with substantially high pressure pump.

5. The method of Claims 1- 4, wherein the producing method further comprises stabilizing the aqueous suspension while cycling the aqueous suspension through the micro fluidizer.

6. The method of Claims 1- 5, wherein a dispersion resulting from the aqueous suspension that is obtained from the micro fluidizer is filtered in order to extract the GQD.

7. The method of Claim 3, wherein the preparing of the aqueous suspension further comprises utilizing high shear mixer for reducing the graphite flakes to an average size of about 400 micrometer.

8. The method of Claim 4, wherein injecting the aqueous suspension into the micro fluidizer is performed by a substantially high-pressure pump at about 400 meter per second subsequently applying shear forces of about 10 reciprocal second on the graphite flakes.

9. The method of Claim 1, wherein the micro channels comprises at least one Z- shaped micro-channel.

10. The method of Claim 9, wherein the aqueous suspension is cycled through the at least one Z-shaped micro-channel at about 100 mL per minute.

11. The method of Claim 9, wherein said at least one Z-shaped micro-channel has an inner diameter that is gradually reduced.

12. The method of Claim 11, wherein the inner diameter starts at 400 micrometer (having inlet pressure of 5000 psi), further reduced to 200 micrometer (having inlet pressure of 15000 psi), and further reduced to 87 micrometer (having inlet pressure of 27000 psi) at the end of at the least one Z-shaped micro- channel.

13. The method of Claim 5, wherein stabilizing the aqueous suspension during the cycling is performed by introduction of a surfactant.

14. The method of Claim 13, wherein the surfactant is polyethylene glycol p - (1, 1, 3, 3 - tetramethylbutyl) - phenyl ether.

15. The method of Claim 5, wherein the solution is filtered by stirred cell under pressure of about 3bar through tracked-etch membranes with approximately 15 nm pore size.

16. Graphene quantum dots (GQD) produced in accordance with the method of Claims 1 to 15.

17. An aqueous solution of GQD produced in accordance with the method of Claims 1 to 15.