Method of fabrication of a suspension of mxene compound in the form of flakes and associated thin film

Abstract

The invention relates to a method of fabrication of a suspension of a compound with the general formula Mn+1Xn in the form of flakes of length L (or lateral size) comprised between 1 and 15 micrometers and of thickness (or height H) comprised between 1 and 20 nanometers, which is essentially characterized in that same is fabricated from a MAX phase compound with the general formula Mn+1AXn, and in that said method comprises at least the following successive steps: - at least two successive chemical attacks of said MAX phase compound, each chemical attack being carried out with an aqueous solution containing hydrochloric acid at a molar concentration strictly greater than 6M mixed with either only lithium fluoride at a molar concentration strictly greater than 1M, or lithium fluoride and hydrofluoric acid, - a first series of wash cycles using deionized and deoxygenated water, each of the wash cycles comprising the addition of a volume V1 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+1Xn which is strictly less than 015M, followed by a centrifugation at more than 1681 rcf and the removal of the supernatant, - a second series of wash cycles using deionized and deoxygenated water, each of the wash cycles comprising the addition of a volume V2 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+1Xn strictly less than 0.05M, followed by centrifugation at more than 1681 rcf and by the removal of the supernatant.

Description

DESCRIPTIONTitle: Method of fabrication of a suspension of MXene compound in the form of flakes and associated thin film.TECHNICAL FIELD

[0001] The invention relates to MXene compounds synthesized from a precursor called MAX phase.

[0002] The invention relates more particularly to a method of fabrication of a suspension of MXene compound in the form of flakes.

[0003] PRIOR ART AND DISADVANTAGES OF THE PRIOR ART

[0004] The MXene compounds were discovered in 2011 (Naguib et al. Adv. Mater.23,4248, 2011). The compound has the general formula Mn+1Xn, where n = 1 , 2 or 3, M is chosen from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, and X is chosen from C or N. Same may alternatively be called Mn+1XnTx where T corresponds to a termination chosen from the groups O, OH, F or any other halogen. The compound is described and characterized in the publication WO2021 177712.

[0005] MXenes are lamellar compounds or 2-dimensional (2D) materials that can be delaminated to form single-flakes of the thickness of the repeating unit on the order of a nanometer. The repeating units have the structure M-X-M, M-X-M-X-M or M-X-M-X-M-X for n=1 , 2 or 3 respectively.

[0006] Such materials have many applications, in particular due to the lamellar structure thereof, the electrical conductivity thereof and / or the property thereof of intercalating and de-intercalating species in the interplanar spaces. The MXene most studied for such purpose is same one with the formula Ti3C2Tx.

[0007] In a known manner, the MXene compounds are synthesized from precursors commonly referred to as "MAX phase-type compounds" or "MAX phase compounds". The MAX phase compounds have the general formula Mn+iAXn, where n=1 , 2 or 3, where M and X are the same as for the MXenes, and A is chosen from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, TI, and Pb. The structure thereof is similar to the structure of MXenes, but additionally comprise the element A which is present in the form of a layer between the repeating units Mn+iXn. Moreover, the MAX phase compounds do not have terminations. The fabrication of MXenes consists in chemically removing the element A from the MAX phaseprecursor. In most cases, a MAX phase precursor where A is aluminum is preferred, the associated method consisting in chemically attacking the MAX precursor with an acidic aqueous solution containing hydrofluoric acid. Aluminum is thereby removed from the space between the repetitive units Mn+iXnleaving Tx terminations, with T which is most often -F, -OH or =0, attached to the element M located outside the repeating unit. The MAX precursor is finely ground beforehand to ensure a good chemical attack yield. After the chemical attack, the product is washed and centrifuged several times in order to remove excess reagent used in the attack as well as soluble by-products such as AIF3. The MXene particles thereby obtained are, in most cases, bulk or three-dimensional particles without a defined geometric shape, as shown in some articles, including the publication Alhabeb et al. Chem. Mater. 29, 7633, 2017 using scanning electron microscopy (SEM) images. Such three-dimensional particles exhibit a high dispersion in both size and shape. The structure of such particles can induce limited electrical conductivity properties and can also be limiting in terms of the rate of intercalation of other species within the crystal. Poor interconnectivity between crystals may also be assumed during the assembly thereof, e.g. for fabricating thin films useful for various applications. Furthermore, the high dispersion in size and shape of the three-dimensional particles does not make it easy to obtain thin flakes after delamination with a homogeneous size distribution (useful for certain applications), and additional size selection operations are thus necessary.

[0008] In order to have better assembly and conductivity for certain applications, many laboratories aim to obtain MXene particles in the form of thin flakes, also called delaminated particles, and not MXene three-dimensional particles or three- dimensional MXene particles. In the literature of the field of two-dimensional materials, the flake can refer both to a particle formed of a single layer of repeating units (single-layer flakes) and to a particle formed of a few layers of repeating units, which can go up to about ten layers (few-layer flakes). The flakes formed by a few layers of repeating units are still quite thin and give a flexibility close to the flexibility of single-flakes, which can also be considered to be two- dimensional particles.

[0009] Additional delamination procedures may be added if the goal is to obtain flakes (2D) and not three-dimensional MXene particles. In most cases, theprocedure for obtaining flakes consists in fabricating a suspension of three- dimensional MXene particles, then adding a chemical compound that can intercalate into the interplanar spaces and then subjecting the suspension to ultrasound treatment. The disadvantage of such procedure is that same breaks the flakes produced into smaller pieces, which is disadvantageous for certain properties such as electrical conductivity or mechanical strength that degrade by decreasing the length L (also called lateral size) of the flakes. In order to avoid any ultrasound treatment, an MXene synthesis method giving rise directly to delaminated flakes or particles has been proposed. Thereof is a variant of the method by attack with a hydrofluoric acid solution and consists in using a mixed solution of fluoride salt, typically Li, and a strong acid, typically HCI, as described in the aforementioned article from 2017. However, in Zhang et al. Adv. Mater. 32, 2001093, 2020, the authors obtained a low yield of delaminated MXene by repeating the variant published in the 2017 publication, yield which was less than 10% (the yield being defined as the mass of delaminated MXene relative to the mass of the starting MAX precursor). On the other hand, in said publication the authors propose a new variant, still based on a single-step chemical attack with a solution of hydrofluoric acid in water, obtained by diluting lithium fluoride with hydrochloric acid, but increasing the concentrations thereof compared to the 2017 publication. With such modified variant, they obtained a delaminated MXene yield of about 60% or less. It should be noted that the initial MAX precursor was subjected to a particle size selection operation by suspension and selective sedimentation, in order to obtain a more homogeneous dispersion of the MAX precursor size. Such modified variant was repeated by other publications in the following years, but the yield of delaminated MXene is not reproducible and appears to be lower than the yield published by the aforementioned 2020 publication. Methods for producing MXene flakes do not result in high yield nor are reproducible, and, furthermore, the flake size dispersion is significant, which is partly due to the fact that the MAX precursor does not have a homogeneous particle size dispersion. Steps of particle size selection for the MAX precursor or for MXene flakes can be added, but thereof reduces the final effective yield of MXene sheets.

[0010] With regard to the synthesis of the MAX phase precursor, the most widespread method, more particularly for fabricating a MAX phase precursor, more particularly with the formula TisAIC2 used to fabricate an MXene with the formula TisC2Tx, consists in feeding in a mixture of carbide or nitride powders of the metal M (such as MC or MN), the metal A (such as Al), and the metal M needed to reach the stoichiometry Mn+iAXn, into a tube furnace under a flow of inert gas. In the particular case of TisAIC2, the reaction mixture is generally composed of powders of TiC, Al and Ti. The aluminum in the starting mixture may be in a slight over-stoichiometry. A mixing of the three elements (e.g. Ti, Al and C) is normally avoided because the reaction is very exothermic and there is a risk of explosion. The use of a tube furnace under a flow of inert gas requires a temperature rise of several hours to temperatures on the order of 1300-1500°C but also plateau of several hours at the maximum temperature. There are other synthesis methods, some using high pressures and / or conditions that are more difficult to control and / or using another reaction mixture.

[0011] In the publication Zhou etal., J. Mater. Scie. 40, 2099, 2005, even before the first synthesis of MXene, the authors synthesized the MAX phase precursor by the spark plasma sintering (SPS) technique using a mixture of TiC, Ti, Al and about 20% Si relative to Al. The mixture is fed into the typical graphite sample holder of an SPS device. A product in the form of a very compact pellet, without porosity, is obtained. Such absence of porosity makes the synthesis of MXene from such a MAX phase precursor very slow and difficult because, according to the prior art, to implement a reasonable synthesis rate of the MXene compound, it is necessary that the MAX precursor is in the form of powders of sub-millimetric sizes, the obtaining of such powders being made difficult from a compact pellet.

[0012] In Patent publication FR3127751 , the MAX phase precursor is obtained by the spark plasma sintering technique by placing the mixed powders in a ceramic container, e.g. containing alumina. The MAX phase precursor obtained is porous and has intrinsic properties ensuring the fabrication of an MXene compound in the form of tablets with a length L comprised between 1 and 15 micrometers and a thickness (also called height H) comprised between 0.2 and 1 micrometers which are free of impurities (such as by-products containing aluminum or unattacked MAX phase), and which have a relatively homogeneous size distribution.

[0013] The fabrication of MXene compound in the form of flakes of length L or lateral size comprised between approximately 1 and 15 micrometers and of thickness or height H comprised between approximately 1 and 20 nanometers from a MAX phase precursor is known, in particular from the publication Zhang et al., Adv. Mater., 32, 2001093, 2020 and the supporting Information thereof which provides for the chemical attack of a MAX phase compound with a solution in water containing 9M hydrochloric acid and approximately 3.1M lithium fluoride (corresponding to 1.6g LiF in 20ml of water), followed by a series of wash cycles using deionized water each comprising a centrifugation at 3500 rpm, equivalent to 1345 ref (relative centrifuge field) for the centrifuge rotor, which has no units and refers to a multiple of the earth’s gravitational acceleration g of 9.80665 m / s2), the number of wash cycles being suitable for obtaining a final pH of about 6. An additional purification step is required to remove the undelaminated MXene compound as well as the unreacted MAX phase compound. Such step is carried out by centrifugation at 1500 rpm (equivalent to 247 ref) for 30 minutes, followed by the removal of sediments and the recovery of the supernatant. The reaction yield calculated by the ratio between the weight of MXene in the form of flakes and the weight of MAX phase compound used is less than or equal to 60%. Such evaluation of the reaction yield can be obtained directly by drying and weighing MXene, or indirectly by keeping MXene in solution and evaluating the weight thereof by measuring the percentage absorption of UV-visible light. To this end, it is necessary to make sure that the material follows the Lambert-Beer law for which the absorption of light is proportional to the concentration of the material to be analyzed.

[0014] An application of interest of MXene flakes is the fabrication of thin films that conduct electricity. Such films can be fabricated by different techniques, such as filtration or tape casting (e.g. by the technique called doctor blade) of aqueous suspensions or inks of MXene flakes, as described in the aforementioned 2020 publication, or in the publication Mathis et al., ACS Nano 15, 6420, 2021. The electrical conductivity of thin films made from such a compound of MXene flakes is less than 2 MS / m, generally comprised between 1 and 2 MS / m, which is insufficient for certain applications.

[0015] PURPOSE OF THE INVENTION

[0016] The aim of the invention is to fabricate a MXene compound in the form of flakes from a MAX phase precursor, making it possible to reproducibly obtain an MXene compound in the form of flakes devoid of undelaminated MXene compound and unreacted MAX phase compound, and exhibiting a reaction yield higher than the yield of the prior art. Furthermore, the invention relates to the fabrication of a MXene compound in the form of flakes from which the thin films produced have an electrical conductivity greater than same of the compounds of the prior art described hereinabove.

[0017] DISCLOSURE OF THE INVENTION

[0018] To this end, the invention relates to a method of fabrication of a suspension of a compound with the general formula Mn+iXn, where n = 1 , 2 or 3, M is chosen from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y, and Ta, and X is chosen from C or N, which compound is in the form of flakes of length L comprised between 1 and 15 micrometers and of height H comprised between 1 and 20 nanometers, which method is characterized in that the compound is fabricated from a MAX phase compound with the general formula Mn+iAXn, where n = 1 , 2 or 3, M is chosen from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y, and Ta, A is chosen from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, TI, and Pb, and X is chosen from C or N, and in that said method comprises at least the following successive steps:- at least two successive chemical attacks of said MAX phase compound, each chemical attack being carried out with an aqueous solution containing hydrochloric acid at a molar concentration strictly greater than 6M mixed with either only lithium fluoride at a molar concentration strictly greater than 1 M, or lithium fluoride and hydrofluoric acid,- a first series of wash cycles using deionized and deoxygenated water, each of the wash cycles comprising the addition of a volume V1 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXnwhich is strictly less than 0.15M, followed by a centrifugation at more than 1681 ref and the discharge of the supernatant,- a second series of wash cycles using deionized and deoxygenated water, each of the wash cycles comprising the addition of a volume V2 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXn strictly less than 0.05M, followed by a centrifugation at more than 1681 refand the discharge of the supernatant, and- obtaining an aqueous suspension of Mn+iXn, the equivalent molar concentration of Mn+iXnduring the second series of cycles being less than the equivalent molar concentration of Mn+iXnduring the first series of cycles, and the last series of wash cycles is carried out until the pH of the solution differs by no more than 0.5 units from the pH of the deionized and deoxygenated water used in the last wash cycle.

[0019] The method of the invention may include the following optional features considered alone or in all possible technical combinations:- in the compound of the general formula Mn+iXn, n = 1 , 2 or 3, M is Ti and X is chosen from C or N, and in the MAX phase compound of general formula Mn+iAXn, n = 1 , 2 or 3, M is Ti, A is Al and X is chosen from C or N,- preferably, in the compound of the general formula Mn+iXn, n = 2, M is Ti and X is C, and in the MAX phase compound of general formula Mn+iAXn, n = 2, M is Ti, A is Al and X is C.- each chemical attack is carried out with an aqueous solution containing hydrochloric acid at a molar concentration strictly greater than 6M, lithium fluoride at a molar concentration of at least 1 M and hydrofluoric acid at a molar concentration of at least 1 M.- each chemical attack is carried out with an aqueous solution containing hydrochloric acid at a molar concentration of 9M, lithium fluoride at a molar concentration of 1 M and hydrofluoric acid at a molar concentration of 2M.- the first series of wash cycles includes at least three wash cycles and the second series of wash cycles includes at least two wash cycles.- the equivalent molar concentration of Mn+iXnduring the first series of wash cycles is comprised between 0.06 and 0.09M.- the equivalent molar concentration of Mn+iXnduring the second series of wash cycles is comprised between 0.005 and 0.03M.- the equivalent molar concentration of Mn+iXnduring the first series of wash cycles is 0.075 M and in that the equivalent molar concentration of Mn+iXnduring the second series of cycles wash cycles is 0.0125M.- the centrifugation during the first and second series of wash cycles is carried outat about 8512 ref.- the aqueous suspension of Mn+iXnin the form of flakes obtained after the last wash cycle is converted into an alcohol-based suspension, preferably containing ethanol.- The MAX phase compound is obtained by a spark plasma sintering method in a spark plasma sintering device (1 ) including a graphite die (2) and two graphite punches (3a, 3b) defining a hollow chamber (4), characterized in that same includes at least the steps of:- mixing powders,- placing the pre-mixed powders in a closed container (5) made of insulating ceramic material and housed in the hollow chamber (4),- performing the spark plasma sintering operation, and- obtaining a pellet made of the MAX phase compound.

[0020] The invention further relates to a thin film produced from the suspension of compound with the general formula Mn+iXnin the form of flakes obtained according to the method defined hereinabove and involving the production of the MAX phase compound by the spark plasma sintering method, wherein the thin film has an electrical conductivity of at least 2 MS / m and a thickness comprised between 0.05 and 2 pm.

[0021] PRESENTATION OF FIGURES

[0022] Other features and advantages of the invention will be clear from the description thereof which is given below as a non-limiting example, with reference to the enclosed figures, among which:

[0023] [Fig. 1] Figure 1 is a schematic representation of the device used to implement the spark plasma sintering (SPS) method for the fabrication of the MAX phase precursor;

[0024] [Fig. 2] Figure 2 is a scanning electron microscopy micrograph taken at a magnification of x600, of a section of the MAX phase precursor pellet obtained according to Example 0 by the spark plasma sintering method, which is roughly fractured;

[0025] [Fig. 3] Figure 3 is a scanning electron microscopy micrograph taken at a magnification of x2500, of a section of the MAX phase precursor pellet obtainedaccording to Example 0 by the spark plasma sintering method, which is roughly fractured;

[0026] [Fig. 4] Figure 4 is an X-ray diffractogram of the MAX phase compound obtained according to Example 0 and illustrated in figures 2 and 3;

[0027] [Fig. 5] Figure 5 is a scanning electron microscopy micrograph taken at a magnification of x10000, of MXene flakes in the suspension of the invention, deposited on a vitreous carbon substrate;

[0028] [Fig. 6] Figure 6 is a scanning electron microscopy micrograph taken at a magnification of x6000, of MXene flakes in the suspension of the invention, diluted and deposited on a porous alumina membrane substrate;

[0029] [Fig. 7] Figure 7 is an X-ray diffractogram and the magnification of a suspension of MXene flakes of the invention on which are illustrated the identification positions of the MAX phase compound (reference A) and lithium fluoride (reference B) when present;

[0030] [Fig. 8] Figure 8 is a scanning electron microscopy micrograph taken at a magnification of x2500, of MXene tablets of the prior art;

[0031] [Fig. 9] Figure 9 is a scanning electron microscopy micrograph taken at a magnification of x4500, of MXene tablets of the prior art;

[0032] [Fig. 10] Figure 10 illustrates the electrical conductivity measurements of films fabricated from the MXene flake suspension of the invention obtained using the MAX phase compound of Example 0, but also fabricated from the suspension of MXene flakes of the invention of Example 3 obtained using a commercial MAX phase compound.

[0033] DETAILED DESCRIPTION OF THE INVENTION

[0034] The invention relates to the fabrication of a suspension of a MXene compound with the general formula Mn+iXn, wherein n = 1 , 2 or 3, M is chosen from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y, and Ta, and X is chosen from C or N. The invention also covers such a compound alternately called Mn+iXnTx where T corresponds to a termination chosen from the groups O, OH, F or any other halogen.

[0035] According to the invention, the compound Mn+iXnis in the form of a flake, a flade being defined in the dimensions thereof by a mean lateral size of the basal plane or length L comprised between 1 and 15 micrometers and of a thickness orheight H comprised between 1 and 20 nanometers. The flake is flexible and can thus be in a corrugated form. A flake is thereby distinguished from the tablets, the subject matter of publication FR3127751 , for which the height of the crystals is comprised between 0.2 and 1 micrometers, which makes the platelets rigid, in contrast to the flexibility of the flakes of the invention.

[0036] The suspension of the compound Mn+iXnin the form of a flake is obtained from a MAX phase precursor, in particular a MAX phase precursor with the formula TisALC2. Such precursor may be obtained commercially and sold under the name TisAIC2 or may advantageously be obtained according to the method described hereinafter.

[0037] The method of fabrication of the MAX phase precursor, particularly but not exclusively a MAX phase precursor with the formula TisAIC2, is based on the known spark plasma sintering (SPS) technique.

[0038] With reference to Figure 1 , the method involves the use of a device 1 including a graphite die 2 and a lower 3a and an upper 3b punches, respectively, delimiting a hollow chamber 4. Only said configuration corresponds to the SPS apparatuses commonly sold and used to carry out the sintering technique.

[0039] The device 1 further includes a closed hollow container 5 housed in the hollow chamber 4 and consisting of a hollow cylinder 6 made of an insulating material resistant at high temperatures and chemically neutral, and covers in the form of a lower disc 7a and an upper disc 7b, respectively, made of the same material and affixed to the two ends of the hollow cylinder 6. For example, the hollow cylinder 6 and the covers 7a, 7b could be made of alumina, or of another insulating ceramic material. The mixture of powders used to synthesize the phase precursor MAX is placed in the hollow alumina container 5. The known operations of applying vacuum and increasing temperature are then applied. After cooling, a pellet of cylindrical shape is obtained.

[0040] To synthesize a MAX phase precursor with composition TisAIC2, commercial powders of titanium carbide, aluminum and titanium are used, which are mixed in an agate crucible. The mixture is passed through a ball mill to mix the three components more finely. The composition of the mixture TiC: Al: Ti can be stoichiometric (molar ratio 2: 1 : 1 respectively), but preferably with a slight excess of aluminum, more particularly a molar ratio of 2: 1.1 : 1 , respectively.

[0041] The powder mixture is then fed into the hollow alumina container 5 of the device of Figure 1 , the hollow cylinder 6 and the lower cover 7a of which are already in place. The powders are compacted in the hollow container 5 and then the upper cover 7b of the container 5 and the upper graphite punch 3b are positioned. A vacuum is applied and then a thermal cycle comprising a rapid rise (from about 15 to 30 minutes) to a temperature between about 1450 and 1600°C, more particularly at 1450°C, for a time comprised between 5 and 15 minutes. A porous pellet of cylindrical shape is obtained.

[0042] Thereby, according to such method, the powder mixture is not in contact with the graphite, is not directly subjected to the applied current and is isolated from the applied pressure. Thereof is what makes it possible to obtain a MAX phase precursor in the form of a porous pellet. Advantageously, the MAX phase precursor is obtained according to Example 0 hereinafter.

[0043] According to the invention, the fabrication of the suspension of the MXene compound of the invention is carried out from the MAX phase compound obtained either as described hereinabove or commercially, and firstly provides for at least two chemical attacks.

[0044] It is necessary to carry out at least two chemical attacks, in particular to make sure that the MAX phase compound has fully reacted but also to optimize the reproducibility of the method. Indeed, a single chemical attack generates in the final product, the presence of the MAX phase precursor which has not reacted, which requires an additional step to remove same.

[0045] In the following description, the values indicated for molar concentrations include a margin of 20% resulting from the uncertainties relating to the measurements.

[0046] Each chemical attack is carried out with an aqueous solution containing hydrochloric acid at a molar concentration strictly greater than 6M mixed with either only lithium fluoride at a molar concentration strictly greater than 1 M, or lithium fluoride and hydrofluoric acid.

[0047] The first chemical attack makes it possible to obtain an intermediate suspension composed predominantly of Mn+iXnand to a lesser extent a MAX phase compound. Nevertheless, it is considered generically in the rest of thedescription that at least two chemical attacks are carried out “of said MAX phase compound”.

[0048] The first chemical attack is carried out for a period comprised between 12 hours and 3 days, preferably 2 days, at a temperature comprised between 30 and 60°C, preferably 35°C. The second chemical attack is carried out for a period comprised between 5 hours and 5 days, preferably 4 days, at a temperature comprised between 30 and 60°C, preferably 35°C.

[0049] When the chemical attacks are carried out with a mixture of hydrochloric acid and lithium fluoride, the molar concentration of hydrochloric acid is preferentially 9M, and the molar concentration of lithium fluoride is preferentially comprised between 2 and 3M.

[0050] If the use of such a mixture to carry out chemical attacks falls within the scope of the invention by serving to produce a suspension of MXene compound in the form of flakes devoid of non-delaminated MXene compound and of unreacted MAX phase compound, and exhibiting a reaction yield higher than the yield of the prior art, the suspension obtained possibly having lithium fluoride impurities in the form of micrometric or submicrometric particles.

[0051] Advantageously, to prevent the presence of lithium fluoride impurities, hydrofluoric acid is added to the mixture of hydrochloric acid and lithium fluoride. Preferably, the concentration of lithium fluoride is decreased. More preferentially, the molar concentration of hydrochloric acid is 9M, the molar concentration of lithium fluoride is 1 M and the molar concentration of hydrofluoric acid is 2M.

[0052] After the chemical attacks, the method of the invention provides at least two series of wash cycles using deionized and deoxygenated water with stirring, each of the wash cycles comprising the addition of a volume V1 (for the first series of wash cycles) and V2 (for the second series of wash cycles), respectively, of deionized and deoxygenated water to obtain a determined molar concentration of Mn+iXn. Instead of referring to the molar concentration (in moles per liter or M) of MXene during washes, reference is made to a concentration of MXene calculated by dividing the weight (in grams) of the MAX phase precursor compound used by the molecular weight (in grams per mole) of the MAX phase compound (the molecular weight of TisAIC2 is 194.64 grams per mol), then dividing again by the volume (in liters) V1 or V2 of the water in the associatedwash cycle series. It is thereby possible to dispense with the concentration errors due to the imprecision of the molecular weight of MXene due to variable T- terminations and to the small losses of MXene at each wash step. In the following description, the MXene concentration thereby evaluated is called "equivalent molar concentration of Mn+iXn".

[0053] For the first series of wash cycles, the equivalent molar concentration of Mn+iXnis strictly less than 0.15M, preferably comprised between 0.06 and 0.09 M and more preferentially 0.0075M.

[0054] For the second series of wash cycles, the equivalent molar concentration of Mn+iXn is strictly less than 0.05M, preferably comprised between 0.005 and 0.03M and more preferentially 0.025M or 0.0125M.

[0055] Such an adjustment of the equivalent molar concentrations of Mn+iXnduring the first and second series of wash cycles serves to obtain an MXene compound in the form of flakes and to prevent the elaboration of a MXene compound in the form of three-dimensional particles (either tablets or other forms of three-dimensional particles) or as a mixture of three-dimensional particles and flakes.

[0056] According to the invention, each wash cycle comprises the addition of deionized and deoxygenated water with stirring, the MXene suspension then being subjected to a centrifugation step at more than 4000 rpm (equivalent to 1681 ref for the rotor used), preferably at 9000 rpm (equivalent to 8512 ref), for a period comprised between 5 and 20 minutes, preferably for 10 minutes. The supernatant is then discharged. A volume of deionized and deoxygenated water V1 or V2 (depending on the series of wash cycles) is then added to the sediment, and the tube containing the water and sediment is manually shaken to detach the sediment from the bottom of the tube, then stirred by magnetic stirring or vortex stirring for about 5 minutes to disperse and wash the sediment in the incorporated water. Each wash cycle may include a plurality of washes thereby carried out.

[0057] According to the invention, the last series of wash cycles is carried out until the pH of the solution differs by no more than 0.5 units from the pH of the deionized and deoxygenated water used in the washes. Commonly and as an example, the last series of wash cycles is performed until the pH of the solution reaches a pH of between 4.5 and 5. Since the pH of the deionized water can vary,the pH to be reached for the last series of wash cycles is chosen to be equal to or lower than the pH of the deionized water used in the wash cycles by a maximum of 0.5.

[0058] Preferentially, the method comprises two series of wash cycles, the first series of wash cycles comprising three identical wash cycles carried out according to the preceding description, and the second series of wash cycles including two identical wash cycles carried out according to the preceding description.

[0059] At the end of the wash cycles, no additional wash step is carried out since the suspension of compound Mn+iXnis devoid of MAX phase compound or nondelaminated MXene compound as confirmed by X-ray diffractogram or scanning electron microscopy analyzes carried out at different places on the sample.

[0060] In order to calculate the yield of the method of the invention, batches were intended to be dried in order to measure the mass of MXene by weighing. A complete batch of MXene suspension obtained according to the invention from a weight ml of the MAX phase compound is placed in a vacuum evaporator and heated to a temperature of about 80°C for about 12 hours, until no further variation in weight is observed. A weight m2 is thereby obtained. The yield is calculated as a percentage of the weight obtained, m2, relative to the weight of the starting MAX phase compound, ml . The yield thereby calculated of the MXene compound in the form of sheets in the aqueous suspension obtained is greater than 85%.

[0061] More precisely, a batch of MXene is synthesized from a weight of the MAX phase, m(MAX); the aqueous, alcohol-based or aqueous-alcohol-based suspension of MXene then being obtained in the form of flakes; the suspension is then dried by applying vacuum and heating at 80°C overnight; the solid obtained is weighed and the operation is repeated for a few hours to be sure that the weight is stabilized; thereof gives the weight of MXene in the form of the flakes and solid, m(MXene). The percentage yield as defined hereinabove is thus m(MXene) divided by m(MAX) multiplied by one hundred. According to the invention, the aqueous suspension of Mn+iXnin the form of flakes obtained after the last wash cycle is converted into an alcohol-based suspension, preferably containing ethanol. Such a suspension is very stable and can be used for several months.

[0062] According to the invention, the suspension of Mn+iXnin the form of flakes is not dried to avoid any instability and / or complicate the subsequent thin film formation method which would require to again put the dry product in suspension. The suspension obtained can be used directly to produce thin film deposits.

[0063] Thin films are thereby produced from the alcohol-based suspension of Mn+iXnin the form of flakes obtained using the doctor blade or tape casting technique. The alcohol-based suspension is first converted into an aqueous or aqueous-alcohol-based ink or slurry with a viscosity suitable for casting. A suitable casting machine or device is then used to cast the film with the doctor blade on a flat substrate, which can be compact or made of a porous membrane. After drying in ambient air, the film is dried under vacuum at about 80°C for 4 to 16 hours. The electrical conductivity is measured by the four-point Hall effect method, corrected with the measurement of the thickness of the film which is measured by scanning electron microscopy in a plurality of places of the film edge in order to obtain an average conductivity and an error interval. The electrical conductivity which is measured is comprised between 2 and 3.4 MS / m.

[0064] Example 0: Synthesis of the MAX phase precursor of composition TisAIC2 according to a modified spark plasma sintering method.

[0065] A commercial spark plasma sintering apparatus marketed under the name of Dr. SINTER Lab. Jr. (SPS-211 Lx model) by Fuji Electronic Industrial Co. Ltd., is used. The device is modified as described with reference to Figure 1 by keeping the graphite die 2 and the two graphite punches 3a and 3b, and by adding the hollow alumina cylinder 6 to the hollow chamber 4, and the lower 7a and upper 7b alumina disks thereby forming a hollow alumina container 5. The mixture of powders which will be described thereafter is fed into the container 5, compacted, and kept electrically insulated from the current flowing through the punches and from the graphite die. The mixture is also kept isolated from the pressure applied by the punches and remains without contact with the graphite throughout the operation.

[0066] Commercial powders of titanium carbide, aluminum, and titanium are mixed in an agate crucible in respective molar proportions of 2: 1.1 : 1 , the mixture of powders having a total weight of about 5 grams. The mixture is passed through a ball mill with a bowl and tungsten carbide balls for a period of 1 hour 15 minutesat a speed of 300 rpm. 2.5 grams of the powder thereby ground are fed into the hollow cylinder 6 and the lower cover 7a already in place in the device. The powders are compacted, then the upper cover 7b is positioned at the end of the hollow cylinder 6, thus forming the hollow container 5. The graphite upper punch 3b is positioned, the whole assembly 1 is positioned in the SPS device and the vacuum is applied. The following thermal cycle is then applied: a rise in temperature to 580°C in 6 minutes, a plateau at 580°C for 5 minutes, a rise in temperature to 1450°C in 12 minutes, and a plateau at 1450°C for 8 minutes. Once the thermal cycle is complete, the temperature decreases rapidly below 580°C in about 5 to 10 minutes. A porous pellet of cylindrical shape is obtained. The porosity of the pellet is greater than 40%, most often about 50% or more. The porosity is evaluated by the difference between the measured volume of the pellet and the volume of the compact material calculated from the theoretical density and the weight of the pellet.

[0067] Figures 2 and 3 illustrate the morphology by scanning electron microscopy of the MAX phase precursor thereby obtained, after coarse crushing of the porous pellet. For the precursor obtained, it is found that the grains are bonded together.

[0068] With reference to Figure 4, the X-ray diffractogram with the radiation source Cu Ka, of the MAX phase compound obtained according to the present example, corresponds to the crystalline phase of the TisAIC2 compound, to which is added a low intensity peak at the angle 29 36.0° assigned to the titanium carbide phase, TiC, which is present in the initial powder. The titanium carbide phase is estimated to be less than 5% by volume of crystalline phases in the MAX phase compound.

[0069] Energy-dispersive X-ray spectroscopy (EDX) spectra of the MAX phase compound in the present example show the presence of the three elements present in the composition of the MAX phase, namely titanium, aluminum and carbon, as well as the absence of impurities such as oxygen or any other element.

[0070] Example 1 : Synthesis of a MXene compound with the formula Ti3C2Tx, in the form of a suspension of flakes, from the roughly crushed MAX phase precursor of Example 0, using a chemical attack solution of 1 M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid.

[0071] Porous pellets of the MAX phase compound of Ti3AIC2 obtained according to Example 0 are roughly crushed with an agate mortar, leaving predominantly pieces of about 1-2 millimeters. A first chemical attack is carried out with a hydrofluoric acid solution. For this purpose, 2 grams of the MAX phase compound with the formula TisAIC2 obtained in Example 0 and roughly crushed are placed in a flask made of fluorinated ethylene propylene copolymer (FEP) plastic with a capacity of 125 ml. 40 ml of an aqueous solution containing 1 M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid prepared beforehand are added, then stirring is carried out with a magnetic stirrer at room temperature for 5 minutes. Stirring is continued by placing the flask in a thermostatically controlled bath at 35°C for approximately 48 hours.

[0072] Following the first attack, the suspension is distributed into fluoroethylene propylene (FEP) centrifuge tubes with a nominal capacity of 50 ml (actual capacity of less than 40 ml), with the same quantity in each tube, then the tubes are centrifuged at the same time at 9000 rpm (equivalent to 8512 ref in the rotor used) for 10 minutes. The supernatant is then removed and the sediments are kept.

[0073] The second chemical attack is then carried out by transferring the sediment into a clean, dry FEP flask, and then 40 ml of the same aqueous solution containing 1M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid are poured, the mixture is stirred in a bath thermostatically controlled at 35°C for about 92 hours.

[0074] At the end of the second chemical attack, two series of wash cycles are carried out with the addition of deionized and deoxygenated water with stirring for about 5 minutes, followed by centrifugation (by distributing the suspension in several FEP centrifugation tubes of nominal volume 50ml) at 9000 rpm (8512 ref) for 10 minutes, removing the supernatants and keeping the sediments for the next cycle.

[0075] In the first series of wash cycles, the total volume used V1 is about 136 ml, which corresponds to an equivalent molar concentration of MXene, as defined hereinabove, of about 0.075M.

[0076] 3 wash cycles are carried out in the first series until the supernatant has a pH of about 2.5.

[0077] In the second series of wash cycles, the total volume used V2 is about 408 ml, which corresponds to a molar equivalent concentration of MXene of about 0.025M.

[0078] 2 wash cycles are carried out in the second series until the supernatant has a pH of about 4.5.

[0079] The sediment is then washed with ethanol during two cycles of rinsing and centrifugation with ethanol, then stored as a suspension in ethanol. Alternatively, the sediment can be dried by heating same under vacuum at a temperature comprised between 40 and 120°C.

[0080] A suspension of MXene compound with the formula Ti3C2Tx, or else more generally TisC2, is thereby obtained, in the form few-layer flakes particles being dispersed in the solvent.

[0081] Figures 5 and 6 illustrate the morphology of the MXene obtained in flake form.

[0082] In Figure 5, thereof is a deposit of one or a plurality of drops of suspension without dilution on a smooth substrate such as vitreous carbon. It is observed that the surface of the vitreous carbon substrate is covered with MXene flakes which are superposed.

[0083] In Figure 6, thereof is a deposit of a drop of diluted suspension on a porous substrate such as an anodized aluminum oxide membrane. Isolated MXene flakes are observed. On scanning electron microscopy images in top view, such as the images shown in Figure 5 or Figure 6, it can be seen that the size distribution of the flakes is relatively homogeneous, namely a length L or lateral size of the sheets, comprised between 1 and 15 microns.

[0084] On scanning electron microscopy images of the edge of films made with the MXene in the form of flakes, it is also found that thereof are flakes and it is possible to estimate the thickness thereof. It can be seen that the thickness or height H of the flakes is comprised between 1 nm and 20 nm.

[0085] The X-ray diffractogram of Figure 7 is very reproducible and characterized by a large peak at an angle of 29 between 6.9° and 7.1 ° and satellite peaks with very low intensities relative to the large peak.

[0086] It was found that the MXene according to the invention does not reveal the small characteristic peak at the angle 26 of 9.5° which would indicate thepresence of the MAX phase compound which has not reacted. It was also found that the MXene suspension does not show a small peak at the 20 angle of 45° which would indicate the presence of LiF.

[0087] The EDX spectrum of the compound in the present example shows the presence of the two main elements present in the composition of the MXene compound, namely titanium and carbon, plus the presence of fluorine, oxygen and chlorine which would correspond to the T-terminations, in particular the groups -F, -Cl, =0 or -OH. The EDX spectrum also shows, as already mentioned hereinabove, the absence of the Al element, which means that the entirety of the MAX phase precursor has been attacked and that no by-product containing aluminum such as the oxide AI2O3, remains.

[0088] Example 2: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension of flakes, from the roughly crushed MAX phase precursor of Example 0, using a chemical attack solution of 1 M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid.

[0089] The procedure is as in Example 1 , but with 0.5 g of MAX phase compound of Example 0, 10 ml of the same solution of 1 M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid in both steps of chemical attack, and volumes V1 and V2 in the two series of wash cycles of about 34ml and 204ml, respectively, representing equivalent molar MXene concentrations of approximately 0.075M and 0.0125M, respectively, the equivalent molar concentration of MXene in the second wash cycle being about half same in the second wash cycle of Example 1 . The product is MXene in the form of flakes, without the presence of MAX phase or impurities such as LiF particles. The method was repeated several times and is reproducible.

[0090] Example 3: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension of flakes, from the commercially obtained MAX phase precursor, using a chemical attack solution of 1 M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid.

[0091] The procedure is as in Example 2, but with 0.5 g of a commercially obtained MAX phase compound, without additional grinding. 10ml of the same 1M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid solution as in Example 2 are used in the two steps of chemical attack, and the same volumesV1 and V2 are used in the two series of wash cycles of approximately 34ml and 204ml respectively, which represents equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product is MXene in the form of flakes, without the presence of MAX phase or impurities such as LiF particles, as in Example 2 where the MAX phase compound was same synthesized according to Example 1 . However, in addition to flakes, the product shows small particles about 500 nm in length that could be three-dimensional MXene particles or smaller thicker flakes that could have appeared because commercial MAX compounds are usually finely ground.

[0092] Example 4: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension of flakes, from the roughly crushed MAX phase precursor of Example 0, using a chemical attack solution of 3M lithium fluoride hydrofluoric acid and 9M hydrochloric acid.

[0093] The procedure is as in Example 2 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 3M lithium fluoride and 9M hydrochloric acid in the two steps of chemical attack. The volumes V1 and V2 in the two series of wash cycles are the same as in Example 2, about 34ml and 204ml, respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is a suspension of MXene in the form of flakes, without the presence of MAX phase which has not reacted, but the significant presence of impurities such as LiF particles detected by scanning electron microscopy and by X-ray diffractogram is noted. Under scanning electron microscopy, the impurities are in the form of micrometric or submicrometric cubic particles. In the X-ray diffractogram, a large peak is observed at an angle 29 of 45° characteristic of a LiF peak, the intensity ratio of the LiF peak to the large MXene peak typically being between 0.1 and 0.3. The method was repeated several times and is reproducible.

[0094] Example 5: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension of flakes, from the roughly crushed MAX phase precursor of Example 0, using a chemical attack solution of 2.5M lithium fluoride hydrofluoric acid and 9M hydrochloric acid.

[0095] The procedure is as in Example 4 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 2.5M lithium fluorideand 9M hydrochloric acid in the two steps of chemical attack. The volumes V1 and V2 in the two series of wash cycles stay the same as in Example 4, about 34ml and 204ml, respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is a suspension of MXene in the form of flakes, without the presence of MAX phase which has not reacted, but with a significant presence of impurities such as LiF particles, the presence thereof being detected by scanning electron microscopy and by X-ray diffractogram.

[0096] Example 6: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension of flakes, from the roughly crushed MAX phase precursor of Example 0, using a chemical attack solution of 2M lithium fluoride hydrofluoric acid and 9M hydrochloric acid.

[0097] The procedure is as in Example 5 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 2M lithium fluoride and 9M hydrochloric acid in the two steps of chemical attack. The volumes V1 and V2 in the two series of wash cycles stay the same as in Example 5, about 34ml and 204ml, respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is a suspension of MXene in the form of flakes, without the presence of MAX phase which has not reacted, but with a significant presence of impurities such as LiF particles, the presence thereof being detected by scanning electron microscopy and by X-ray diffractogram.

[0098] Example 7: Electrical conductivity measurements of the MXene suspension of the invention by thin film deposition.

[0099] A portion of the alcohol-based suspension of MXene sheets obtained in Example 1 (or Example 2) is taken for the case of a suspension obtained from the MAX phase compound of Example 0, or obtained in Example 3 in the case of a suspension obtained from the commercial MAX phase compound, and is placed in an FEP centrifuge tube. The centrifugation is carried out at room 9000 rpm 10 to 20 minutes. The supernatant is removed and the sediment is kept wet. Alternatively, deionized and deoxygenated water can be added, stirred and a second centrifugation can be carried out to reduce the concentration of alcohol.Deionized and deoxygenated water is gradually added to the sediment, mixing with a bar or any other known means until a viscous ink is obtained.

[0100] Film deposition is carried out using the doctor blade technique. To this end, a flat substrate is prepared with a polymeric membrane as the spreading surface, and adhesive strips on both edges along the casting direction, with defined heights. A quantity of ink is placed at one end and the ink is then spread with a glass bar to the other end. It is allowed to dry for a few minutes in ambient air.

[0101] The same operation can be repeated on the already deposited part in order to obtain a film with a greater thickness. The viscosity of the ink, the height of the adhesive strips on the edges and the number of passes will determine the final thickness of the film which will in any case be greater than 0.05 microns and may be up to several tens of microns so that the layer is self-supporting. This thickness makes the final film opaque. The final film is left to dry in a glove box and is then subjected to drying under vacuum at 80°C for 4 to 16 hours. Same can stay on the membrane support thereof or be detached with regard to its self- supporting nature.

[0102] The electrical conductivity is measured by Hall effect with a commercial apparatus such as the Ecopia HMS 5500 including 4 tips arranged to approximately form a square of at least 1 cm2. The assumed film thickness in the calculation program is then corrected by the actual value to obtain the correct electrical conductivity. The actual thickness of the film is obtained by scanning electron microscopy measurement within the cross-section of the film and measured in a plurality of places in order to obtain an average as well as an interval of variability of the thickness and, hence, of the conductivity.

[0103] Table 1 below reports the electrical conductivity values and the thickness of each tested film.Table 1

[0104] With reference to Figure 10 and Table 1 , it can be seen that the electrical conductivity obtained with the suspension of MXene flakes obtained from a MAX phase compound of Example 0 (reference 10 - suspensions of Examples 1 or 2) is greater than 2, whereas the electrical conductivity obtained with the MXene flake suspension obtained from a commercial MAX phase compound (reference 11 - suspension of Example 3) is less than 2.

[0105] It can also be seen that the thickness of the film of the invention according to these examples is comprised between 0.1 and 1.5 pm making it self-supporting and opaque. Counter-Example 1 : Synthesis of a MXene compound with the formula Ti3C2Tx, in the form of a suspension of tablets, from the roughly crushed MAX phase precursor of Example 0.

[0106] The procedure is as in Example 2 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 2M lithium fluoride and 6M hydrochloric acid in the two steps of chemical attack, and with a single series of wash cycles. The volume of deionized and deoxygenated water per wash cycle is approximately 204 ml, which represents an equivalent molar concentration of MXene of about 0.0125M. The product obtained is a suspension of MXene in the form of tablets as seen by scanning electron microscopy. Figures 8 and 9 illustrate the morphology of MXene in the form of a tablet consisting of two planar and parallel faces with a relatively homogeneous crystal size distribution. More precisely, the length L of the tablets is comprised between 1and 15 microns, and the height H thereof is comprised between 0.2 and 1 microns. The X-ray diffractogram of the MXene platelets resembles the diffractogram of MXene flakes but has greater variability in the position of the main peak and in the positions of the satellite peaks. No LiF impurities or unreacted MAX phase are detected. The EDX spectrum of MXene tablets resembles the spectrum of MXene flakes with the presence of the two main elements present in the composition of the MXene compound, namely titanium and carbon, plus the presence of fluorine, oxygen and chlorine which could correspond to the T-terminations. The method was repeated and is reproducible in that the product obtained is systematically in the form of tablets.

[0107] Counter-Example 2: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0108] The procedure is as in Example 2 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 2M lithium fluoride and 6M hydrochloric acid, and with a single chemical attack step lasting 48 h. Two series of wash cycles are carried out with volumes of deionized and deoxygenated water V1 and V2 of about 34ml and 204ml respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is predominantly a suspension of MXene but with the presence of MAX phase compound which has not reacted completely as detected by the X-ray diffractogram. The method is not reproducible with respect to the shape of the MXene particles, since a suspension of MXene in the form of flakes, and a suspension of MXene in the form of tablets, were alternately obtained.

[0109] Counter-Example 3: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0110] The procedure is as in Example 2 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 3M lithium fluoride and 9M hydrochloric acid, and with a single chemical attack step lasting 48 h, as in Counter-Example 2. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of 34ml and 204ml, respectively,representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is predominantly a suspension of MXene, with the presence of MAX phase compound which has not reacted completely as detected by the X-ray diffractogram. The method is not reproducible with respect to the shape of the MXene particles, since a suspension of MXene in the form of flakes, and a suspension of MXene in the form of tablets, were alternately obtained.

[0111] Counter-Example 4: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0112] The procedure is as in Example 2 with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 1 M lithium fluoride (hence less LiF than in Example 2) and 9M of hydrochloric acid for the two steps of chemical attack, lasting 48h and 92h. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of 34ml and 204ml, respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is a suspension of MXene without the presence of any MAX phase compound. LiF particle impurities are not observed either, but MXene suspension contains a mixture of particles in the form of flakes and of tablets.

[0113] Counter-Example 5: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0114] The procedure is as in Example 2, with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 3M hydrofluoric acid and 9M hydrochloric acid in the two steps of chemical attacks of 48 h and 92 h, respectively. The aqueous solution thus does not contain any lithium salt but has the same concentration of fluoride as in Example 2. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of 34ml and 204ml, respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is a suspension of MXene with particles in the form of tablets without the presence ofany MAX phase compound. LiF particles impurities are also not observed since there is no Li cation in the attack solution.

[0115] Counter-Example 6: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0116] The procedure is as in Example 2, with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 2M potassium fluoride and 6M hydrochloric acid in the two steps of chemical attacks of 48 h and 92 h, respectively. The aqueous solution thus does not contain any lithium salt which is replaced by potassium in the form of potassium fluoride which is more soluble than lithium fluoride. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of 34ml and 204ml, respectively, representing equivalent molar concentrations of MXene of about 0.075M and 0.0125M, respectively. The product obtained is a suspension of MXene in the form of tablets, with a small amount of unreacted MAX phase compound. LiF particles impurities are also not observed since there is no Li cation in the attack solution.

[0117] Counter-Example 7: Synthesis of an MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0118] The procedure is as in Example 2, with 0.5 g of MAX phase compound of Example 0, but with 10 ml of an aqueous solution containing 3M lithium chloride and 9M hydrochloric acid in the two steps of chemical attack which of a respective duration of 48 h and 92 h. The aqueous solution thus does not contain any fluoride. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of 34ml and 204ml, respectively. The product obtained is the unreacted MAX compound.

[0119] Counter-Example 8: Synthesis of an MXene compound with the formula TisC2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0120] The procedure is as in Example 2, with 2 g of MAX phase compound of Example 0, and 40 ml of an aqueous solution containing 1 M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid in the two steps of chemical attacks fora respective duration of 48 h and 92 h. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of about 68ml and 136ml, respectively, representing equivalent molar concentrations of MXene of about 0.15M and 0.075M, respectively. The product obtained is a suspension of MXene with particles in the form of tablets, without the presence of any MAX phase compound.

[0121] Counter-Example 9: Synthesis of a MXene compound with the formula Ti3C2Tx, in the form of a suspension, from the roughly crushed MAX phase precursor of Example 0.

[0122] The procedure is as in Example 4, but with 1 g of MAX phase compound of Example 0 and 20 ml of the same aqueous solution 3M of lithium fluoride and 9M of hydrochloric acid as in Example 4 for the two steps of chemical attack of a respective duration of 48 h and 92 h. Two series of wash cycles were carried out with volumes of deionized and deoxygenated water V1 and V2 of about 34ml and 204ml, respectively, representing equivalent molar concentrations MXene of about 0.150M and 0.025M, respectively, i.e. concentrations higher than the concentrations of Example 4 with regard to the doubling of the weight of the MAX phase precursor compound while keeping the same volumes V1 and V2. The product obtained is a suspension of MXene with a mixture of particles in the form of flakes, predominantly, but also with a significant presence of particles in the form of tablets. The presence of a MAX phase compound is not observed. On the other hand, LiF particle impurities are observed.

[0123] [Table 2] below shows the results obtained in terms of the product obtained in the final suspension, lithium fluoride impurities, the presence of a MAX phase compound and the reproducibility of the method according to the operational conditions of the chemical attacks and of the first and second series of wash cycles for the Counter-Examples 1 to 9 explained hereinabove and outside the scope of the invention, and Examples 1 to 6 explained hereinabove and operated according to the method of the invention.

[0124] In all the Examples and Counter-Examples presented in Table 2, the centrifugation operation which were carried out after each wash cycle were at a speed of 9000 rpm (equivalent to 8512 ref for the rotor used).Table 2: Counter-Examples and Examples of the invention according to operational conditions (ND: Not Determined)

Claims

CLAIMS[Revendication 1 ] A method of fabrication of a suspension of a compound of the general formula Mn+iXn, where n = 1 , 2 or 3, M is chosen from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y, and Ta, and X is chosen from C or N, which compound is in the form of flakes of length L comprised between 1 and 15 micrometers and of height H comprised between 1 and 20 nanometers, characterized in that the compound is fabricated from a MAX phase compound with the general formula Mn+iAXn, where n = 1 , 2 or 3, M is chosen from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y, and Ta, A is chosen from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, TI, and Pb, and X is chosen from C or N, and in that said method comprises at least the following successive steps:- at least two successive chemical attacks of said MAX phase compound, each chemical attack being carried out with an aqueous solution containing hydrochloric acid at a molar concentration strictly greater than 6M mixed with either only lithium fluoride at a molar concentration strictly greater than 1 M, or lithium fluoride and hydrofluoric acid,- a first series of wash cycles using deionized and deoxygenated water, each of the wash cycles comprising the addition of a volume V1 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXnwhich is strictly less than 0.15M, followed by centrifugation at more than 1681 ref and removal of the supernatant,- a second series of wash cycles using deionized and deoxygenated water, each of the wash cycles comprising the addition of a volume V2 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXn strictly less than 0.05M, followed by a centrifugation at more than 1681 ref and the removal of the supernatant, and- obtaining an aqueous suspension of Mn+iXn, the equivalent molar concentration of Mn+iXnduring the second series of cycles being less than the equivalent molar concentration of Mn+iXnduring the first series of cycles, and the last series of wash cycles is carried out until the pH of the solution differs by no more than 0.5 units from the pH of the deionized and deoxygenated water used in the last wash cycle.[Revendication 2] The method according to claim 1 , characterized in that in the compound of the general formula Mn+iXn, n = 1 , 2 or 3, M is Ti and X is chosen from C or N, and in the MAX phase compound of general formula Mn+iAXn, n = 1 , 2 or 3, M is Ti, A is Al and X is chosen from C or N.[Revendication 3] The method according to any of the preceding claims, characterized in that each chemical attack is carried out with an aqueous solution containing hydrochloric acid at a molar concentration strictly greater than 6M, lithium fluoride at a molar concentration of at least 1 M and hydrofluoric acid at a molar concentration of at least 1 M.[Revendication 4] The method according any of claims 1 and 2, characterized in that each of the chemical etchings is carried out with an aqueous solution comprising hydrochloric acid in a molar concentration of 9M, lithium fluoride in a molar concentration of 1 M and hydrofluoric acid in a molar concentration of 2 M. [Revendication 5] The method according to any of the preceding claims, characterized in that the first series of wash cycles includes at least three wash cycles and the second series of wash cycles includes at least two wash cycles. [Revendication 6] The method according to any of the preceding claims, characterized in that the equivalent molar concentration of Mn+iXnduring the first series of wash cycles is comprised between 0.06 and 0.09M.[Revendication 7] The method according to any of the preceding claims, characterized in that the equivalent molar concentration of Mn+iXnduring the second series of wash cycles is comprised between 0.005 and 0.03M. [Revendication 8] The method according to claim 3, characterized in that the equivalent molar concentration of Mn+iXnduring the first series of wash cycles is 0.075M and in that the equivalent molar concentration of Mn+iXnduring the second series of wash cycles is 0.0125M.[Revendication 9] The method according to any of the preceding claims, characterized in that the centrifugation during the first and second series of wash cycles is carried out at about 8512 ref.[Revendication 10] The method according to any of the preceding claims, characterized in that the aqueous suspension of Mn+iXnin the form of flakes obtained after the last wash cycle is converted into an alcohol-based suspension, preferably containing ethanol.[Revendication 11] The method according to any of the preceding claims, characterized in that the MAX phase compound is obtained by a spark plasma sintering method in a spark plasma sintering device (1 ) including a graphite die (2) and two graphite punches (3a, 3b) defining a hollow chamber (4), characterized in that same includes at least the steps of:- mixing powders,- placing the pre-mixed powders in a closed container (5) made of insulating ceramic material and housed in the hollow chamber (4),- performing the spark plasma sintering operation, and- obtaining a pellet made of the MAX phase compound.[Revendication 12] A thin film produced from the suspension of compound with the general formula Mn+iXnin the form of flakes obtained according to the method of claim 11 , characterized in that same has an electrical conductivity of at least 2 MS / m and a thickness comprised between 0.05 to 2 pm

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