Solid-state fluorescent graphene-based quantum dots

Abstract

The invention relates to a method for preparing graphene quantum dots (GQDs), the method comprising the steps of: i) suspending a carbon source in a mixture of water and sulphuric acid; ii) microwaving the obtained suspension to a temperature of between 20°C and 200°C, in particular between 100°C and 180°C; iii) recovering and optionally washing and / or drying the obtained carbonaceous residue; iv) suspending the obtained residue in nitric acid; v) microwaving the obtained suspension to a temperature of between 20°C and 200°C, in particular between 100°C and 180°C; and vi) recovering the obtained graphene-based quantum dots (GQDs).

Description

[0001] Solid-state fluorescent graphene-based quantum dots

[0002] Field of invention

[0003] The present invention relates to a method for preparing graphene quantum dots (GQDs), the graphene quantum dots that can be obtained according to this method, and their use, in particular in energy production devices such as light-emitting diodes, batteries, or solar cells.

[0004] Technical background

[0005] Graphene is an infinite 2D monolayer of sp-bonded hexagonal carbon lattice 2 which exhibits interesting zero band gap properties due to electron delocalization.

[0006] As a result, this gives graphene a semi-metallic behavior. Thus, electrons move through graphene with virtually no resistance, leading to high electrical conductivity. Thanks to other remarkable properties such as strength, lightness, thermal conductivity, and transparency, graphene holds great promise in the following fields: semiconductors, electronics, and energy storage applications.

[0007] However, large-scale production of high-quality graphene and its low processability still represent technical obstacles that must be overcome to enable graphene to reach industrial scale. In addition, the development of stable and homogeneous graphene dispersions, without agglomeration, remains a major challenge. Furthermore, the zero band gap of graphene hinders their implementation in optoelectronic devices as well as in field-effect transistors (FETs).

[0008] In this context, the recent discovery of graphene quantum dots (GQDs) and rapid progress in their synthetic preparation offer an opportunity to improve the efficiency of optoelectronic devices.

[0009] The major interest of these nanoparticles lies in their photoluminescence, which is the origin of various applications, notably in the field of energy (heterogeneous photocatalysis, photovoltaic devices, LEDs, etc.) and also at the interface of biology for imaging. These carbon quantum dots (GQDs) combine several favorable attributes of traditional semiconductor-based quantum dots (namely, nanometric size, size- and wavelength-dependent luminescence emission, resistance to photobleaching, ease of bioconjugation) without implying intrinsic toxicity or rarity of the elements, or rigorous, expensive or inefficient preparation steps.

[0010] GQDs are functionalized graphene nanofragments with a lateral size generally less than 10 nm. Their anisotropic morphology comes from the lateral dimension being larger than their height. GQDs systematically possess graphitic networks within their structures, as evidenced by high-resolution microscopy (HR-TEM) images of their structures. Their height is generally between 0.4 and 4 nm, as shown by statistical AFM analysis, which corresponds to a few GQDs (from 1 to 10) stacked on top of each other.

[0011] GQDs can be considered as polycyclic aromatic hydrocarbons (PAHs) larger than 1 nm.

[0012] The multi-step organic synthesis of GQDs is very challenging, typically involving many successive chemical transformations starting from a commercially available chemical. As GQDs are composed of many fused aromatic rings, their synthesis typically involves one or more key intramolecular cyclization steps. Although difficult to scale up to industrial scale, this stepwise approach has the notable advantage of producing atomically precise nanographene structures, in terms of heteroatom size and doping (nature, position, and concentration of the heteroatom).On the other hand, chemists have also developed approaches to GQD synthesis (bottom-up or top-down), in most cases in a single step, which result in the preparation of a mixture of GQDs with a statistical distribution in terms of size and chemistry. As a result, the macroscopic properties are derived from those of the total population of GQDs.

[0013] Top-down synthesis involves the chemical decomposition of large carbon-based materials (carbon fibers, graphene oxide GO, coal, fullerenes, graphite, etc.) into small fragments with concentrated acids. Graphene oxide is typically the ideal starting material due to the presence of numerous oxygen-containing functional groups that facilitate chemical cleavage to nano-sized GQDs. However, graphene oxide does not occur naturally and must therefore be prepared from various materials such as coal or anthracite by the complex chemical Hummers approach. In this context, the use of graphite as a natural source has been evaluated as a substitute for graphene oxide, with less success to date due to lower synthetic yields.Top-down syntheses are possible through hydrothermal or solvothermal splitting, microwave-assisted exfoliation, electrochemical methods, and oxidation.

[0014] On the other hand, one-step bottom-up synthesis involves carbonizing organic precursors (citric acid, glucose, glutamic acid, hexa-perihexabenzocoronene, etc.) by microwave-assisted pyrolysis, solvothermal heating, or pulsed laser irradiation. However, bottom-up approaches generally suffer from lower yields associated with purification problems in removing unreacted small-sized organic materials.

[0015] Current syntheses described in the literature are mainly based on hydrothermal reaction techniques that require time intervals of several hours. Thus, manufacturing techniques (laser ablation, electron beam lithography, electrochemical syntheses) remain at low yield levels for high costs, meaning that production for commercial applications remains difficult to envisage.

[0016] There is therefore a real need today to have a rapid GQD synthesis process, easy to implement and allowing GQDs to be isolated with good yields.

[0017] Summary of the invention

[0018] A simple, rapid and efficient process has now been developed to provide access to GQDs with good yields.

[0019] The invention relates firstly to a method for preparing graphene-based quantum dots (GQDs) comprising the steps of: i) Suspending a carbon source in a mixture of water and sulfuric acid; ii) Heating by microwave the suspension obtained at a temperature between 20°C and 200°C, in particular between 100°C and 180°C, iii) Recovering and optionally washing and / or drying the carbon residue obtained; iv) Suspending the residue obtained in nitric acid; v) Heating by microwave the suspension obtained at a temperature between 20°C and 200°C, in particular between 100°C and 180°C, and vi) Recovering the graphene-based quantum dots (GQDs) obtained.

[0020] More specifically, the inventors were able to show that by carrying out, in a first phase, a treatment of a carbon source in sulfuric acid, followed by a second phase of treatment in nitric acid, the two stages being carried out by microwave heating, it was possible to obtain GQDs in a very rapid time, in particular in around ten minutes, at low cost, and in a very reproducible manner.

[0021] Advantageously, the GQDs obtained at the end of these two phases are relatively homogeneous and very easy to purify by simple extraction with an organic solvent. Thus, it is not necessary to resort to heavy and costly extraction methods to implement such as dialysis or chromatography methods commonly used in the field.

[0022] According to another advantage, the carbon source used in this process can be a source of organic carbon, in particular renewable, in particular organic waste such as plant waste, which reduces the ecological impact of the process.

[0023] According to yet another advantage, this method makes it possible to access GQDs having very advantageous characteristics, in particular a very homogeneous size distribution, in particular monomodal, and very good solubility in aqueous or organic solvents. Furthermore, unexpectedly, the inventors were able to demonstrate that these GQDs exhibited white fluorescence in the solid state, that is to say that when the material is irradiated in the solid state, the spectrum of white light is obtained, without it being necessary to use a polymer matrix such as PMMA, as a diluent. This property is particularly advantageous for certain applications such as OLED devices, white light emitters, organic transistors (OFET), fluorescent sensors, optical sensors and the field of anti-counterfeiting.

[0024] According to embodiments, the method according to the invention further comprises one or more of the following characteristics:

[0025] - the carbon source is a renewable organic material, particularly plant-based;

[0026] - plant organic matter is made up of plant waste;

[0027] - the carbon source is in powder form;

[0028] - the proportion of water by volume in the water / sulfuric acid mixture used in step i) varies between 0 and 50% of the total volume;

[0029] - the concentration by weight of the carbon source relative to the volume of the water / sulfuric acid mixture used in step i) is between 0.1 and 100 g / L, in particular between 1 and 30 g / L;

[0030] - microwave heating in step ii) and / or v) is carried out at a pressure of between 5 and 50 bars;

[0031] - step iii) of recovery includes the steps of:

[0032] - dilution of the reaction mixture obtained in step ii) in water;

[0033] - recovery of the carbon residue obtained, in particular by centrifugation, and elimination of the supernatant; and possibly

[0034] - repeating the previous dilution and recovery steps until the supernatant has a neutral pH, in particular between 6.5 and 7.5;

[0035] - the concentration of the carbon residue in the nitric acid in step iv) is between 1 and 40 g / L, in particular between 5 and 25 g / L;

[0036] - the recovery of GQDs includes an extraction step with an organic solvent;

[0037] - the recovery stage includes the steps of:

[0038] - dilution of the reaction mixture obtained in step v) in water;

[0039] - extraction of the aqueous phase with an organic solvent;

[0040] - washing the organic phase with water and / or an acid solution;

[0041] - recovery of the organic phase and elimination of the extraction solvent, whereby GQDs are obtained.

[0042] The inventors were able to characterize the GQDs obtained according to the method of the invention using different methods, in particular X-ray induced photoelectron spectrophotometry (XPS) and infrared (IR). Thus, according to a second aspect, the invention relates to graphene quantum dots capable of being obtained according to the method of the invention.

[0043] It also concerns graphene quantum dots characterized in that their spectrum obtained by X-ray induced photoelectron spectrometry (XPS) N1s includes the following energy bands:

[0044] These quantum dots are notably obtained by the process according to the invention.

[0045] According to embodiments, the graphene quantum dots according to the invention comprise one or more of the following characteristics:

[0046] - their spectrum obtained by X-ray induced photoelectron spectrometry (XPS) of C1s further includes 4 energy bands between 282 and 290 eV, in particular the following bands (in eV): 284.6; 286; 287.4; and 288.8;

[0047] - their 01s spectrum obtained by X-ray induced photoelectron spectrometry (XPS) also includes 3 energy bands between 530 and 536 eV;

[0048] - their infrared spectrum also includes the following absorption bands (in cm -1 ): 3000, 1695, 1574, 1123 and 1021;

[0049] - They present a monomodal particle size distribution in number;

[0050] - the number mode is between 1 and 10 nm, in particular between 1 and 5 nm;

[0051] - the median D50 is between 1 and 5 nm, notably between 2 and 4 nm;

[0052] - the standard deviation is between 1 and 20%, in particular between 6 and 12%;

[0053] - they exhibit white luminescence in the solid state;

[0054] According to a final aspect, the invention relates to the use of graphene quantum dots according to the invention, in imaging, detection, optoelectronic, energy production, energy storage (batteries and supercapacitors), photocatalytic devices, as well as within conductive or composite materials.

[0055] According to embodiments, the graphene quantum dots according to the invention are used in energy production devices such as light-emitting diodes, batteries, or solar cells.

[0056] Brief description of the figures

[0057] Figure 1 represents the XPS 01s spectrum of the graphene quantum dots according to the invention.

[0058] Figure 2 represents the XPS C1s spectrum of the graphene quantum dots according to the invention.

[0059] Figure 3 represents the XPS N1s spectrum of the graphene quantum dots according to the invention.

[0060] Figure 4 represents the FT-IR spectrum of the graphene quantum dots according to the invention.

[0061] Figure 5 represents the fluorescence spectrum of the graphene quantum dots according to the invention in ultrapure water, as a function of the excitation wavelength (values ​​indicated by an arrow)

[0062] Figure 6 represents the fluorescence spectrum of the graphene quantum dots according to the invention in the solid state.

[0063] Figure 7 represents an image of the GQDs according to the invention obtained by transmission electron microscopy (TEM). detailed

[0064] The invention is now described in more detail and in a non-limiting manner in the following description.

[0065] Method (A) for preparing GQDs

[0066] According to a first aspect, the invention relates to a method for preparing graphene-based quantum dots (GQDs) comprising the steps of: i) Suspending a carbon source in a mixture of water and sulfuric acid; ii) Heating by microwave the suspension obtained at a temperature between 20°C and 200°C, in particular between 100°C and 180°C, iii) Recovering and optionally drying the carbon residue obtained; iv) Suspending the residue obtained in nitric acid; v) Heating by microwave the suspension obtained at a temperature between 20°C and 200°C, in particular between 100°C and 180°C, and vi) Recovering the graphene-based quantum dots (GQDs) obtained.

[0067] For the purposes of this description, the term "graphene-based quantum dots", also hereinafter referred to as "GQDs", refers to any nanoparticle composed mainly of carbon atoms, in particular carbon, fused and aromatic rings. It mainly designates anisotropic nanoobjects, i.e. planar (wider than tall) and crystallized, in particular exclusively crystallized. By extension, for the purposes of this application, this term also includes "carbon nanodots" (also called "carbon dots") which are spherical carbon particles which may have crystallized parts. Preferably, the term "graphene-based quantum dots", or GQDs, designates anisotropic and crystallized nanoobjects.

[0068] [Step i)]

[0069] Step i) consists of suspending a carbon source, more particularly a renewable organic matter, in a mixture of water and sulfuric acid.

[0070] Renewable organic matter refers to material derived from living nature and includes animal and plant matter, which is generally produced by agriculture, forestry or fishing. It regenerates continuously in relatively short cycles (from a few days to a few decades).

[0071] Examples of animal matter include wool, animal fats, etc.

[0072] Examples of plant materials include cereals, wood, rubber, cotton, algae, vegetable fats, fruits and vegetables.

[0073] For the purposes of the invention, the plant material used in step i) may be dehydrated or dried plant material. However, it does not include vegetable coals, i.e. coals obtained in particular by pyrolysis of plant material.

[0074] Preferably, the carbon source is a plant material. According to a preferred embodiment, the plant organic material consists of plant waste. This embodiment is particularly advantageous since it allows the waste to be recycled and the graphene quantum dots to be prepared from an abundant, renewable and inexpensive raw material.

[0075] The carbon source can be used in the form of a powder. This advantageously makes it possible to obtain a homogeneous dispersion in the water / sulfuric acid mixture, and therefore a homogeneous reactivity of all the material used in steps i) and ii) of the process. The plant waste powder can, for example, be obtained by drying in an oven at a temperature between 60°C and 100°C, then grinding.

[0076] This carbon source is then suspended in a water / sulfuric acid (H2SO4) mixture. Sulfuric acid is, in particular, concentrated sulfuric acid.

[0077] The proportion of water by volume in the water / sulfuric acid mixture used in step i) can vary widely. In particular, it can be between 0 and 50% of the total volume of the mixture.

[0078] The concentration by weight of the carbon source relative to the volume of the water / sulfuric acid mixture used in step i) may vary. It may in particular be between 0.1 and 100 g / L, in particular between 1 and 30 g / L.

[0079] [Step ii)]

[0080] Step ii) consists of heating by microwave the suspension obtained in step i) to a temperature between 20°C and 200°C, in particular between 100°C and 180°C, in particular at approximately 150°C. This heating is generally carried out at a pressure between 1 and 10 bars, in particular between 2 and 5 bars.

[0081] Advantageously, at a temperature of 150°C and a pressure of 4 bars, step iii) can be carried out in less than 10 minutes, in particular in 5 minutes.

[0082] Without wishing to be limited to a particular theory, steps i) and ii) would promote dehydration reactions of the plant material to form an oxygenated carbonaceous material.

[0083] Preferably, this step is not carried out under an inert atmosphere, so as to promote the incorporation of oxygen into the carbon residue.

[0084] [Step iii)]

[0085] Step iii) consists of recovering and possibly washing and / or drying the carbon residue obtained in step ii). This step iii) may in particular include the recovery and washing of the carbon residue according to the steps of:

[0086] - dilution of the reaction mixture obtained in step ii) in water;

[0087] - recovery of the carbon residue obtained, in particular by centrifugation, and elimination of the supernatant; and possibly

[0088] - repeating the previous dilution and recovery steps until the supernatant has a neutral pH, in particular between 6.5 and 7.5.

[0089] This step makes it possible in particular to reduce, or even substantially eliminate, the presence of sulfuric acid in the carbon residue.

[0090] The recovered and washed carbon residue can then be dried, for example in an oven at a temperature between 60°C and 90°C for several hours.

[0091] [Step iv)]

[0092] Step iv) consists of treating the carbon residue in nitric acid. The concentration of the carbon residue in nitric acid in step iv) may be between 1 and 40 g / L, in particular between 5 and 25 g / L.

[0093] [Step v)]

[0094] Step v) consists of heating the suspension obtained by microwave to a temperature between 20°C and 200°C, in particular between 100°C and 180°C, and in particular to approximately 150°C. This step is preferably carried out with stirring.

[0095] The microwave heating in step v) can be carried out at a pressure of between 5 and 60 bars, in particular between 10 and 40 bars.

[0096] Advantageously, at a temperature of 150°C and a pressure of 35 bars, step v) can be carried out in less than 10 minutes, in particular in 5 minutes.

[0097] The conversion rate of the carbon residue used in step v) into graphene quantum dots at the end of step v) is generally 100%. The progress of the conversion reaction can be easily followed: at the beginning of the reaction in step v), the carbon residue is not soluble in the reaction medium and becomes soluble as it is converted. The reaction is generally complete when the initial carbon residue has completely disappeared, i.e. is completely solubilized in the form of GQDs.

[0098] Without wanting to be limited to a particular theory, steps iv) and v) would allow the chemical cutting of fragile chemical bonds in order to lead to the formation of functionalized fragments, called GQDs.

[0099] [Step vi)] Step vi) consists of the recovery of the graphene-based quantum dots (GQDs) obtained in step iv).

[0100] This step can be carried out using conventional methods such as chromatography, extraction, dialysis and / or ultracentrifugation.

[0101] Preferably, this step is carried out by extraction of the reaction medium with an organic solvent, in particular a polar organic solvent, in particular a protic polar solvent, such as an alcohol, more particularly an aliphatic alcohol.

[0102] Advantageously, the graphene quantum dots are recovered in the organic phase which can be washed and concentrated under reduced pressure. The resulting residue can be dried under vacuum, at a temperature of around 80°C, typically for 2 hours.

[0103] Advantageously, the method of the invention makes it possible to obtain graphene quantum dots with good yields, in particular between 15 and 70%, in particular between 20 and 60%, this yield being calculated by taking the ratio between the mass of GQDs recovered and the mass of hydrocarbon material used in step i).

[0104] GQDs

[0105] According to a second aspect, the invention relates to graphene quantum dots capable of being obtained according to the method as defined above.

[0106] According to a third aspect, the invention relates to graphene quantum dots characterized in that their spectrum obtained by X-ray induced photoelectron spectrometry (XPS) N1 s comprises the following energy bands:

[0107] In the context of the present invention, relative intensity means the value of the area under the curve of one of the bands relative to the sum of the area under the curve of the two bands. More particularly, the N1 s spectrum of the quantum dots according to the invention, obtained by X-ray induced photoelectron spectrometry (XPS), comprises two energy bands:

[0108] - one being characterized by a binding energy between 400 eV and 403 eV, and a relative intensity between 30% and 60%, and

[0109] - the other being characterized by a binding energy between 404 eV and 406 eV, and a relative intensity between 40% and 70%.

[0110] Preferably, the sum of the relative intensities of these two energy bands is equal to 100%.

[0111] X-ray induced photoelectron spectrometry (XPS) can be performed with a Kratos Axis Ultra spectrometer (Kratos Analytical, UK) equipped with a monochromatic Al Ka ​​source (1486.6 eV). Spectra are recorded at a 90° elevation angle over an area of ​​0.7 x 0.3 mm. High-resolution spectra are measured with a step size of 0.1 eV.

[0112] The XPS N1 s spectrum of the graphene quantum dots according to the invention can comprise the following energy bands (eV): 401.5 and 405.8.

[0113] More particularly, the XPS N1 s spectrum of the graphene quantum dots according to the invention can comprise the following two energy bands:

[0114] Thus, the N1 s spectrum of the quantum dots according to the invention, obtained by X-ray induced photoelectron spectrometry (XPS), comprises two energy bands:

[0115] - one being characterized by a binding energy of 401.5 eV, and a relative intensity of 55.4%, and

[0116] - the other being characterized by a binding energy of 405.8 eV, and a relative intensity of 44.6%.

[0117] The graphene quantum dots according to the invention may be further characterized in that their spectrum obtained by X-ray induced photoelectron spectrometry (XPS) of C1 s comprises 4 energy bands whose binding energies are between 282 and 290 eV. In particular, the graphene quantum dots according to the invention may be further characterized by an XPS spectrum of C1 s comprising the energy bands characterized by the following binding energies (in eV): 284.6; 286; 287.4; and 288.8.

[0118] This spectrum is measured with a Kratos Axis Ultra spectrometer (Kratos Analytical, UK), equipped with a monochromatic Al Ka ​​source (1486.6 eV). The spectra are recorded at a 90° elevation angle over an area of ​​0.7 x 0.3 mm. High-resolution spectra are measured with a step size of 0.1 eV.

[0119] More specifically, the graphene quantum dots according to the invention can be further characterized by an XPS spectrum of the C1 s comprising the following energy bands:

[0120] These bands can be measured with a Kratos Axis Ultra spectrometer (Kratos Analytical, UK), notably equipped with a monochromatic Al Ka ​​source (1486.6 eV). The spectra are notably recorded at a 90° elevation angle over an area of ​​0.7 x 0.3 mm. The high-resolution spectra are notably measured with a step of 0.1 eV.

[0121] The graphene quantum dots according to the invention can be further characterized in that their O1 s spectrum obtained by X-ray induced photoelectron spectrometry (XPS) comprises 3 energy bands whose binding energies are between 530 and 536 eV.

[0122] More particularly, the graphene quantum dots according to the invention can be further characterized in that their XPS O1 s spectrum comprises the following energy bands (in eV):

[0123] These bands can be measured with a Kratos Axis Ultra spectrometer (Kratos Analytical, UK), notably equipped with a monochromatic Al Ka ​​source (1486.6 eV). The spectra are notably recorded at a 90° elevation angle over an area of ​​0.7 x 0.3 mm. The high-resolution spectra are notably measured with a step of 0.1 eV.

[0124] The quantum dots according to the invention can be further characterized in that their infrared spectrum comprises the following absorption bands (in cm' 1 ): 3000 (broad COOH band), 1695 (C=O band), 1574, in particular the bands 3000, 1695, 1574, 1123 and 1021.

[0125] The graphene quantum dots according to the invention can be characterized in that they have:

[0126] - An XPS N1 s spectrum including the following energy bands (in eV): 401.5 and 405.8; and

[0127] - An infrared spectrum comprising the following absorption bands (in cm' 1 ): 3000; 1695; 1574; 1123 and 1021.

[0128] Furthermore, the quantum dots according to the invention can be characterized in that they have a monomodal particle size distribution in number.

[0129] The particle size distribution of graphene quantum dots can be measured in number using the dynamic light scattering (DLS) method on a VASCO KIN apparatus (Cordouan Technologies, Pessac, France). The measurements were performed on freshly prepared suspensions in ultrapure water at 25°C and in triplicate. The data were analyzed by Data Nano Kin® software in multimodal number determination mode.

[0130] In particular, the number mode can be between 1 and 10 nm, especially between 1 and 5 nm.

[0131] The D50 median in number can be between 1 and 5 nm, notably between 2 and 4 nm.

[0132] The standard deviation can be between 1 and 20%, especially between 6 and 12%.

[0133] Very advantageously, the graphene quantum dots according to the invention can be characterized in that they exhibit white luminescence in the solid state.

[0134] The graphene quantum dots according to the invention can be further characterized in that they exhibit luminescence in water. More particularly, this luminescence is characterized in that the emission wavelength varies as a function of the excitation wavelength. More particularly, for an excitation wavelength varying from 300 nm to 420 nm, the emission wavelength of the GQDs according to the invention varies from 430 nm to 510 nm.

[0135] Furthermore, GQDs are characterized by a high solubility, of more than 500 g / L in aliphatic alcohols, including ethanol. This solubility is particularly advantageous in the field of ink formulation, in particular for inkjet printing, which are generally composed of an aliphatic alcohol solvent. The GQDs according to the invention can therefore be advantageously incorporated into inks, used in the field of electronics in particular.

[0136] In addition, GQDs are characterized by an interesting solubility of 10g / L in water, which allows their use in the field of biology, particularly in the field of imaging or diagnosis.

[0137] Use of GQDs

[0138] According to a fourth aspect, the invention relates to the use of graphene quantum dots according to the invention, in imaging, detection, optoelectronic, energy production, energy storage devices, in particular batteries and / or supercapacitors, photocatalytics, as well as within conductive or composite materials.

[0139] The graphene quantum dots according to the invention can be used in energy production devices such as light-emitting diodes, batteries, or solar cells.

[0140] Process (B) for the preparation of GQDs

[0141] According to yet another aspect, the invention relates to a method for preparing graphene quantum dots (GQDs) comprising the steps of: a. Suspending a carbon source in nitric acid, the carbon source being selected from coal, coal coke, biochar, asphalt and mixtures thereof; b. Heating by microwave the suspension obtained at a temperature between 20°C and 200°C, in particular between 100°C and 180°C, at a pressure between 5 and 50 bars; and c. Recovering the graphene-based quantum dots (GQDs) obtained. The carbon source used in step a) can be prepared according to steps i) to 111) of method (A), as defined above.

[0142] Furthermore, steps a), b) and c) are identical to steps iv), v) and vi) respectively, defined above.

[0143] Examples

[0144] Materials and methods

[0145] All chemicals used (reagents and solvents) in the method are of reagent grade and are used directly. The water used is ultrapure water, i.e. with a resistivity of 18.2 mQ.cm at 25°C. Step 1 of the synthesis is carried out in a Masterwave BTR microwave reactor from Anton Paar. Step 2 of the synthesis is carried out in a Multiwave 5000 microwave reactor from Anton Paar. FT-IR spectra are carried out on a Nicolet iS5 instrument. XPS spectra are carried out on a Kratos Axis Ultra instrument (Kratos Analytical, UK). The size of the GQD particles was determined by dynamic light scattering on a VASCO KIN instrument from Cordouan Technologies. All measurements were carried out on freshly prepared suspensions in ultrapure water, at 25°C and in triplicate.Data were analyzed using Nano Kin software using a multimodal number distribution and expressed as a mean (± standard deviation). Transmission electron microscopy analyses were performed on a JEM - ARM 200F Cold FEG TEM / STEM apparatus operating at 200 kV. Grids were prepared by depositing a drop of GQDs solution (1 mg / mL in water) on a carbon-coated copper grid for 4 minutes. The drop was then removed with blotting paper, the grid was then dried at 80°C for 20 minutes. UV-vis spectra were measured on a Varian Cary® spectrophotometer. Fluorescence emission spectra were measured on a Horiba Jobin Yvon FluoroMax-3 spectrofluorimeter at 25°C. XPS analyses were performed with a Ka-monochromatized Al source.

[0146] Experimental protocol

[0147] Step 0: The banana peels are dried in an oven at 75°C for

[0148] 48h then ground. Step i) to iii): 5g of dry organic matter is treated in the monowave BTR microwave with 160 mL H2SO4 and 40 mL of FW in a 1 L Teflon tube at 150°C for 5'. Dilution in water (700 mL), centrifuged and washed with water. Drying of the residue overnight at 75°C. Obtaining 4.6g (92%)

[0149] Step iv) to vi): 3.2 g of carbon (obtained in step 1) are distributed in 8 tubes and are treated with conc. HNO3 (40 mL / per tube) in the Multiwave microwave at 150°C for 5'. The reaction medium is diluted in water (1.5 L). The aqueous medium is extracted with an aliphatic alcohol (2 x 800 mL). The organic phases are washed with water (2 x 400 mL) then with 1 M HCl (3 x 600 mL). The organic phase is then concentrated under reduced pressure. The residue is dried under vacuum at 80°C for 2 h. The final yield is 22%. 700 mg of GQDs (22%) are obtained.

[0150] The obtained GQDs were characterized by N1s, O1s and C1s X-ray induced photoelectron spectrometry (XPS), FT-IR and spectrofluorometry.

[0151] The same protocol was implemented using olive pits, orange peels and oak acorns. The obtained GQDs were characterized by N1s, O1s and C1s X-ray induced photoelectron spectrometry (XPS), FT-IR and spectrofluorometry. The spectra obtained are similar to those obtained with banana peels.

Claims

Demands 1. A process for preparing graphene quantum dots (GQDs) comprising the steps of: i) Suspending a carbon source in a mixture of water and sulfuric acid, said carbon source being a renewable organic material; ii) Microwave heating the resulting suspension to a temperature between 20°C and 200°C, in particular between 100°C and 180°C; iii) Recovering and optionally washing and / or drying the carbon residue obtained; iv) Suspending the residue obtained in nitric acid; v) Microwave heating the resulting suspension to a temperature between 20°C and 200°C, in particular between 100°C and 180°C; and vi) Recovering the graphene-based quantum dots (GQDs) obtained.

2. A process according to claim 1, wherein the carbon source is a renewable plant organic matter.

3. A method according to any one of the preceding claims, wherein the microwave heating in step ii) and / or v) is carried out at a pressure between 5 and 50 bar.

4. A process according to any one of the preceding claims, wherein the concentration of the carbon residue in nitric acid at step iv) is between 1 and 40 g / L, in particular between 5 and 25 g / L.

5. Quantum graphene dots that can be obtained according to the process as defined in any one of claims 1 to 4.

6. Quantum dots of graphene characterized in that their spectrum obtained by X-ray photoelectron spectrometry (XPS) N1 s includes the following energy bands:

7. Quantum dots according to claim 6, further characterized in that their spectrum obtained by X-ray induced photoelectron spectrometry (XPS) of C1 s comprises 4 energy bands between 282 and 290 eV, in particular the following bands (in eV): 284.6; 286; 287.4; and 288.

8.

8. Quantum dots according to claim 6 or 7, further characterized in that their O1 s spectrum obtained by X-ray induced photoelectron spectrometry (XPS) comprises 3 energy bands between 530 and 536 eV.

9. Quantum dots according to claims 6 to 8, characterized in that their infrared spectrum comprises the following absorption bands (in cm⁻¹) -1 ): 3000, 1695, 1574, 1123 and 1021.

10. Quantum dots according to any one of claims 6 to 9, characterized in that they exhibit a unimodal number particle size distribution.

11. Quantum dots according to claim 10, characterized in that the number mode is between 1 and 10 nm, in particular between 1 and 5 nm.

12. Quantum dots according to any one of claims 10 and 11, characterized in that the median D50 is between 1 and 5 nm, in particular between 2 and 4 nm.

13. Quantum dots according to any one of claims 6 to 12, characterized in that they exhibit white luminescence in the solid state 14. Use of graphene quantum dots according to any one of claims 6 to 13, in imaging, detection, optoelectronic, energy production, and energy storage devices, including batteries and / or supercapacitors, photocatalytic devices, as well as in conductive or composite materials.

15. Use according to claim 14, in energy production devices such as light-emitting diodes, batteries, or solar cells.

Images