Process for manufacturing a suspension of MXene compound in the form of sheets and associated thin film.

Abstract

The invention relates to a method for manufacturing a suspension of a compound of general formula Mn+1Xn in the form of sheets of length L (or lateral size) of between 1 and 15 micrometers and of a thickness (or height H) of between 1 and 20 nanometers, which is essentially characterized in that it is manufactured from a compound of the MAX phase type of general formula Mn+1AXn, and in that said method comprises at least the following successive steps: at least two successive chemical attacks of said compound of the MAX phase type, each of the chemical attacks being carried out with an aqueous solution comprising hydrochloric acid at a molar concentration strictly greater than 6M in a mixture with either only lithium fluoride at a molar concentration strictly greater than 1M, or lithium fluoride and hydrofluoric acid, a first series of washing cycles with deionized and deoxygenated water,each of the washing cycles comprising the addition of a volume V1 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+1Xn to the MAX phase type compound which is strictly less than 0.15M, followed by centrifugation at more than 1681 fcr and removal of the supernatant, a second series of washing cycles with deionized and deoxygenated water, each of the washing cycles comprising the addition of a volume V2 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+1Xn to the MAX phase type compound which is strictly less than 0.05M, followed by centrifugation at more than 1681 fcr and removal of the supernatant. Figure to be published with the abstract: Figure 5,

Description

Title of the invention: Method for manufacturing a suspension of MXene compound in the form of sheets 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 the process for manufacturing a suspension of MXene compounds in the form of sheets. PRIOR ART AND DISADVANTAGES OF PRIOR ART

[0003] Mxene compounds were discovered in 2011 (Naguib et al. Adv.Master.23,4248, 2011). This compound has the general formula Mn+lXn, where n = 1, 2 or 3, M is selected from Ti, V, Cr, Zr, Nb, Mo, Hf, Sc, Mn, Y and Ta, and X is selected from C or N. It can alternatively be called Mn+lXnTx where T corresponds to a terminal selected from O, OH, F or any other halogen groups. This compound is described and characterized in publication WO2021177712.

[0004] MXenes are lamellar compounds or 2-dimensional (2D) materials that can be delaminated to form monolayers of the repeating unit with a thickness of the order of nanometers. These repeating units have the structure MXM, MXMXM or MXMXMX for n=1, 2 or 3 respectively.

[0005] These materials have numerous applications, notably due to their lamellar structure, their electrical conductivity and / or their property of intercalating and deintercalating species in interplanar spaces. The MXene most studied for this purpose is that of the formula Ti3C2Tx.

[0006] In known manner, these MXene compounds are synthesized from precursors commonly called "MAX phase type compounds" or "MAX phase compounds". These MAX phase compounds have the general formula Mn+iAXn, with n=1, 2 or 3, where M and X are the same as for MXenes, and A is selected from Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb. Their structure is similar to that of MXenes, but additionally comprise the element A which is present in the form of a layer between the repeating units Mn+iXn. The MAX phase compounds do not otherwise carry a termination. The manufacture of MXenes consists of chemically removing the element A from the MAX phase type precursor. In most cases, a MAX phase precursor where A is aluminum is preferred, the associated method consisting of chemically attacking the MAX precursor with an acidic aqueous solution containing hydrofluoric acid.Aluminum is thus removed from the space between the repeating Mn+iXn units leaving Tx endings, with T most often being - . F, -OH or =0, attached to the M element located outside the repeating unit. The MAX precursor is first finely ground to ensure good etching performance. After etching, the product is washed and centrifuged several times to remove excess etching reagent and soluble by-products such as A1F3. The resulting MXene particles are, in most cases, bulk 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. These bulk particles exhibit a high dispersion in both size and shape. The structure of these particles can induce limited electrical conductivity properties and can also be limiting in terms of the intercalation rate of other species within the crystal.Poor interconnectivity between crystals can also be assumed during their assembly, for example, to make thin films, useful for various applications. In addition, the high dispersion in size and shape of the bulk particles does not allow easy production of thin sheets after delamination with a homogeneous size distribution (useful for some applications), and additional size selection operations are thus necessary.

[0007] In order to have a better assembly and a better conductivity for certain applications, many laboratories aim to obtain MXene particles in the form of thin sheets, also called delaminated particles, and not volumetric or three-dimensional MXene particles. In the literature in the field of two-dimensional materials, the sheet can refer as much to a particle formed of a single layer of repeating units or mono-sheet (in English "single-layer flakes"), as to a particle formed of a few layers of repeating units, which can go up to about ten layers (in English "few-layer flakes"). The sheets formed of a few layers of repeating units are still quite thin and retain a flexibility close to that of mono-sheets, which can also be considered as two-dimensional particles.

[0008] Additional delamination procedures may be added if the objective is to obtain (2D) sheets and not three-dimensional MXene particles. In most cases, the procedure for obtaining sheets consists of manufacturing a suspension of three-dimensional MXene particles, then adding a chemical compound that can be intercalated in the interplanar spaces, and subjecting the suspension to an ultrasonic treatment. The disadvantage of this procedure is that it breaks the sheets produced into smaller pieces, which is disadvantageous for certain properties such as electrical conductivity or mechanical strength which deteriorate by reducing the length L (also called lateral size) of the sheets. In order to avoid any ultrasonic treatment, a process for synthesizing MXene directly resulting in delaminated sheets or particles has been proposed. This is a variant of the method by attack with a hydrofluoric acid solution and consists of using a mixed solution of fluoride salt, typically Li, and a strong acid, typically HCl, as described in the aforementioned 2017 article. However, in the publication Zhang et al. Adv. Mater. 32, 2001093, 2020, the authors obtain a low yield of delaminated MXene by using the variant published in the 2017 publication, a yield which is less than 10% (the yield being defined as the mass of delaminated MXene relative to the mass of the starting MAX precursor).In return, in this 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 their concentrations compared to the 2017 publication. With this modified variant they obtain a yield of delaminated MXene of approximately 60% or lower. It should be noted that the starting MAX precursor was subjected to a particle size selection operation by selective suspension and sedimentation, in order to obtain a more homogeneous size dispersion of the MAX precursor. This modified variant was taken up by other publications in subsequent years, but the yield of delaminated MXene is not reproducible and it seems to be lower than that published by the above-mentioned 2020 publication.The processes for obtaining MXene sheets therefore do not give rise to a high or reproducible yield, and, in addition, the dispersion of sheet sizes is important, which is partly due to the fact that the MAX precursor does not have a homogeneous particle size dispersion. Size selection steps for the particles of the MAX precursor or the MXene sheets can be added, but this reduces the final effective yield of the MXene sheets.

[0009] Concerning the synthesis of the MAX phase precursor, the most widespread method, in particular for manufacturing a MAX phase precursor, in particular of formula Ti3 A1C2 used to manufacture an MXene of formula Ti3C2Tx, consists of introducing a mixture of carbide or nitride powders of the metal M (such as MC or MN), the metal A (such as Ai), and the metal M necessary to reach the stoichiometry Mn+iAXn, into a tubular furnace under an inert gas flow. In the particular case of Ti3AlC2, the reaction mixture is generally composed of powders of TiC, Al and Ti. The aluminium in the starting mixture may be in a slight over-stoichiometry. The mixture of the three elements (for example, Ti, Al and C) is normally avoided because the reaction is very exothermic and there is a risk of explosion. The use of a tubular furnace under an inert gas flow requires a temperature rise of several hours until temperatures of the order of 1300-1500°C but also plateaus 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 a different reaction mixture.

[0010] In the publication Zhou et al., 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 the Al. The mixture is introduced into the graphite sample holder typical of an SPS device. A product in the form of a very compact pellet, without porosity, is obtained. This lack of porosity makes the synthesis of MXene from such a MAX phase precursor very slow and difficult because, according to the state of the art, it is necessary, in order to implement a reasonable synthesis rate of the MXene compound, for the MAX precursor to be in the form of powders of sub-millimeter sizes, obtaining such powders being made difficult from a compact pellet.

[0011] 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, for example based on alumina. The MAX phase precursor obtained is porous and has intrinsic properties ensuring the manufacture of an MXene compound in the form of platelets with a length L of between 1 and 15 micrometers and a thickness (also called height H) of between 0.2 and 1 micrometer which are free of impurities (such as by-products containing aluminum or unattacked MAX phase), and which have a relatively homogeneous size distribution.

[0012] The manufacture of MXene compound in the form of sheets of length L or lateral size between approximately 1 and 15 micrometers and thickness or height H 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 its appendix (in English called "Supporting Information") which provides for the performance of a chemical attack of a MAX phase type compound with a solution in water containing 9M hydrochloric acid and approximately 3.1M lithium fluoride (corresponding to 1.6g of LiF in 20mL of water), followed by a series of washing cycles with deionized water each comprising a centrifugation at 3,500 rpm, equivalent to 1,345 iron for the centrifuge rotor (iron meaning "relative centrifugal force", equivalent to ref or "relative centrifugal field" in English, which has no unit and designates a multiple of the Earth's gravitational acceleration g of 9.806 65 m / s2), the number of washing cycles being adapted to obtain a final pH of approximately 6. An additional purification step is necessary to remove the compound. Undelaminated MXene and the unreacted MAX phase compound. This step is performed by centrifugation at 1500 rpm (equivalent to 247 iron) for 30 minutes, followed by removal of sediment and recovery of the supernatant. The reaction yield calculated by the ratio between the mass of MXene in the form of sheets and the mass of MAX phase compound used is less than or equal to 60%. This evaluation of the reaction yield can be obtained directly by drying and weighing the MXene, or indirectly by keeping the MXene in solution, and evaluating its mass by measuring the percentage absorption of UV-visible light. To do this, it is necessary to ensure 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.

[0013] An interesting application of MXene sheets is the manufacture of thin films that conduct electricity. These films can be manufactured by different techniques, such as filtration or tape casting (for example, by the technique called "doctor blade") of aqueous suspensions or inks of MXene sheets, as described in the 2020 publication named above, 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 sheets is less than 2 MS / m, generally between 1 and 2 MS / m, which is insufficient for certain applications. OBJECTIVE OF THE INVENTION

[0014] The invention aims at the manufacture of an MXene compound in the form of sheets from a MAX phase precursor, making it possible to reproducibly obtain an MXene compound in the form of sheets devoid of non-delaminated MXene compound and of MAX phase compound which has not reacted, and having a reaction yield higher than that of the prior art. Furthermore, the invention aims at the manufacture of an MXene compound in the form of sheets from which the thin films produced have an electrical conductivity higher than that of the compounds of the prior art previously exposed. STATEMENT OF THE INVENTION

[0015] For this purpose, the invention relates to a method for manufacturing a suspension of a compound of 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 sheets of length L between 1 and 15 micrometers and height H between 1 and 20 nanometers, which method is characterized in that it is manufactured from a compound of the MAX phase type of 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, Tl 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 type compound, each of the chemical attacks being carried out with an aqueous solution comprising hydrochloric acid at a molar concentration strictly greater than 6M in a mixture with either only lithium fluoride at a molar concentration strictly greater than 1M, or lithium fluoride and hydrofluoric acid, • a first series of washing cycles with deionized and deoxygenated water, each of the washing cycles comprising the addition of a volume VI of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXn to the MAX phase type compound which is strictly less than 0.15M, followed by centrifugation at more than 1681 iron and the evacuation of the supernatant, • a second series of washing cycles with deionized and deoxygenated water, each of the washing cycles comprising the addition of a volume V2 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXn to the MAX phase type compound strictly less than 0.05M, followed by centrifugation at more than 1681 iron and the evacuation of the supernatant, and • obtaining an aqueous suspension of Mn+iXn, the molar concentration equivalent to Mn+iXn in the MAX phase type compound during the second series of cycles being lower than the molar concentration equivalent to Mn+iXn in the MAX phase type compound during the first series of cycles, and the last series of wash cycles being 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.

[0016] The method of the invention may include the following optional characteristics considered in isolation or according to all possible technical combinations: • each of the chemical attacks is carried out with an aqueous solution comprising hydrochloric acid at a molar concentration strictly greater than 6M, lithium fluoride at a molar concentration of at least 1M and hydrofluoric acid at a molar concentration of at least 1M. • each of the chemical attacks is carried out with an aqueous solution containing hydrochloric acid at a molar concentration of 9M, lithium fluoride at a molar concentration of 1M and hydrofluoric acid at a molar concentration of 2M. • the first series of wash cycles comprises at least three wash cycles and in that the second series of wash cycles comprises at least two wash cycles. • the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the first series of washing cycles is between 0.06 and 0.09M. • the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the second series of washing cycles is between 0.005 and 0.03M. • the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the first series of washing cycles is 0.075M and in that the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the second series of washing cycles is 0.0125M. • centrifugation during the first and second series of washing cycles is carried out at approximately 8512 iron. • the aqueous suspension of Mn+iXn in the form of sheets obtained after the last washing cycle is transformed into an alcoholic suspension, preferably based on ethanol. • the MAX phase type compound is obtained by a spark plasma sintering process in a spark plasma sintering device (1) comprising a graphite matrix (2) and two graphite punches (3a, 3b) defining a hollow chamber (4), characterized in that it comprises at least the steps of: • mixture of powders, • placement of the pre-mixed powders in a closed container (5) made of an insulating ceramic material and housed in the hollow chamber (4), • carrying out the spark plasma sintering operation, and • obtaining a pellet made of the MAX phase type compound.

[0017] The invention further relates to a thin film made from the suspension of compound of general formula Mn+iXn in the form of sheets obtained according to the method previously defined and involving the obtaining of the compound of MAX phase type by the spark plasma sintering method, which thin film has an electrical conductivity at least equal to 2 MS / m. PRESENTATION OF FIGURES

[0018] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which:

[0019] [Fig-1] [Fig.l] is a schematic representation of the device used to put implemented the spark plasma sintering (SPS) process for the manufacture of the MAX phase precursor;

[0020] [Fig.2] [Fig.2] is a scanning electron 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 process which is roughly fractured;

[0021] 2] [Fig.3] is a scanning electron micrograph taken at a magnification of x2500 of a section of the MAX phase precursor pellet obtained according to example 0 by the spark plasma sintering process which is roughly fractured;

[0022] [Fig.4] [Fig.4] is an X-ray diffractogram or DRX of the MAX phase compound obtained according to Example 0 and illustrated in Figures 2 and 3;

[0023] ] [Fig.5] [Fig.5] is a scanning electron micrograph taken at a magnification of xlOOOO of MXene sheets in the suspension of the invention deposited on a glassy carbon substrate;

[0024] [Fig.6] [Fig.6] is a scanning electron micrograph taken at x6000 magnification of MXene sheets in the suspension of the invention diluted and deposited on a glassy carbon substrate;

[0025] [Fig.7] [Fig.7] is an X-ray diffractogram and its magnification of a suspension of MXene sheets of the invention on which are illustrated the identification positions of the MAX phase compound (reference A) and lithium fluoride (reference B) when present;

[0026] [Fig.8] [Fig.8] is a scanning electron micrograph taken at x2500 magnification of prior art MXene platelets;

[0027] [Fig.9] [Fig.9] is a scanning electron micrograph taken at x4500 magnification of prior art MXene platelets;

[0028] [Fig. 10] [Fig. 10] illustrates the electrical conductivity measurements of films made from the suspension of MXene sheets of the invention obtained using the MAX phase compound of Example 0, but also made from the suspension of MXene sheets of the invention of Example 3 obtained using a commercial MAX phase compound. DETAILED DESCRIPTION OF THE INVENTION

[0029] The invention relates to the manufacture of a suspension of an MXene compound of 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 selected from C or N. The invention also covers such a compound alternatively titled Mn+iXnTx where T corresponds to a terminal selected from O, OH, F or any other halogen groups.

[0030] According to the invention, the compound Mn+iXn is in the form of a sheet, a sheet being defined in its dimensions by an average lateral size of the basal plane or length L of between 1 and 15 micrometers and a thickness or height H of between 1 and 20 nanometers. The sheet is flexible and can therefore be in a wavy form. A sheet is thus distinguished from the platelets, the subject of publication FR3127751, for which the height of the crystals is between 0.2 and 1 micrometer, which makes these platelets rigid, in contrast to the flexibility of the sheets of the invention.

[0031] The suspension of the Mn+iXn compound in the form of a sheet is obtained from a MAX phase precursor, in particular a MAX phase precursor of formula Ti3AlC2. This precursor can be obtained commercially and sold under the name Ti3AlC2, or advantageously be obtained according to the process described below.

[0032] The manufacturing process of the MAX phase precursor, particularly but not exclusively a MAX phase precursor of formula Ti3AlC2, is based on the known spark plasma sintering technique called spark plasma sintering SPS.

[0033] With reference to [Fig.l], this method involves the use of a device 1 comprising a graphite matrix 2 and punches respectively lower 3a and upper 3b delimiting a hollow chamber 4. This configuration alone corresponds to the SPS devices commonly sold and used to carry out this sintering technique.

[0034] The device 1 further comprises a hollow, closed container 5 housed in the hollow chamber 4 and consisting of a hollow cylinder 6 made of an insulating material, resistant to high temperatures and chemically neutral, and of disc-shaped covers, respectively lower 7a and upper 7b, 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 may be made of alumina, or of another insulating ceramic material. The mixture of powders used to synthesize the MAX phase precursor is placed in the hollow alumina container 5. The known operations of applying a vacuum and a rise in temperature are then applied. After cooling, a cylindrical pellet is obtained.

[0035] To synthesize a MAX phase precursor of composition Ti3AlC2, 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 more finely mix the three components. The composition of the TiC mixture: Al:Ti can be stoichiometric (molar ratio of 2:1:1 respectively), but preferably with a slight excess of aluminum, especially a molar ratio of 2:1,1:1 respectively.

[0036] The powder mixture is then introduced into the hollow alumina container 5 of the device of [Fig.l] whose hollow cylinder 6 and lower cover 7a are already in place. The powders are packed into 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 (of approximately 15 to 30 min) to a temperature between approximately 1450 and 1600°C, more particularly to 1450°C, for a time of between 5 and 15 min. A porous pellet of cylindrical shape is obtained.

[0037] Thus, according to this 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. This 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 below.

[0038] According to the invention, the manufacture of the suspension of the MXene compound of the invention is carried out from the MAX phase type compound obtained either as described above or commercially, and firstly provides for at least two chemical attacks.

[0039] It is necessary to carry out at least two chemical attacks, in particular to ensure that the MAX phase compound has completely reacted but also to optimize the reproducibility of the process. Indeed, a single chemical attack generates in the final product the presence of the MAX phase precursor which has not reacted, which requires carrying out an additional step to eliminate it.

[0040] In the following description, the indicated values ​​of molar concentrations include a margin of 20% resulting from uncertainties relating to the measurements.

[0041] Each chemical attack is carried out with an aqueous solution comprising 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.

[0042] The first chemical attack makes it possible to ensure that an intermediate suspension is obtained, composed mainly of Mn+iXn and a minor part of the MAX phase type compound. Nevertheless, it is considered generically in the remainder of the description that at least two chemical attacks are carried out "on said MAX phase type compound".

[0043] The first chemical attack is carried out for a period of between 12 hours and 3 days, preferably 2 days, at a temperature of between 30 and 60°C, preferably 35°C. The second chemical attack is carried out for a period of between 5 hours and 5 days, preferably 4 days, at a temperature of between 30 and 60°C, preferably 35°C.

[0044] When the chemical attacks are carried out with a mixture of hydrochloric acid and lithium fluoride, the molar concentration of hydrochloric acid is preferably 9M, and the molar concentration of lithium fluoride is preferably between 2 and 3M.

[0045] If the use of such a mixture to carry out chemical attacks falls within the scope of the invention by making it possible to produce a suspension of MXene compound in the form of sheets devoid of non-delaminated MXene compound and of MAX phase compound which has not reacted, and having a reaction yield higher than that of the prior art, the suspension obtained may have lithium fluoride impurities in the form of micrometric or sub-micrometric particles.

[0046] Advantageously, to avoid 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 preferably, the molar concentration of hydrochloric acid is 9M, the molar concentration of lithium fluoride is 1M and the molar concentration of hydrofluoric acid is 2M.

[0047] After the chemical attacks, the method of the invention provides at least two series of washing cycles with deionized and deoxygenated water under agitation, each of the washing cycles comprising the addition of a volume respectively VI (for the first series of washing cycles) and V2 (for the second series of washing cycles) 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 the washings, reference is made to an MXene concentration calculated by dividing the mass (in grams) of the precursor MAX phase compound used by the molecular weight (in grams per mole) of the MAX phase type compound (the molecular weight of Ti3AlC 2 is 194.64 grams per mol), then dividing once again by the volume (in liters) VI or V2 of the water of the associated series of washing cycles.This overcomes the concentration errors due to the imprecision of the molecular weight of MXene due to the variable T-termini and the small losses of MXene at each washing step. In the following description, this MXene concentration thus evaluated is called "equivalent molar concentration of Mn+iXn".

[0048] For the first series of washing cycles, the equivalent molar concentration of Mn+iXn is strictly less than 0.15M, preferably between 0.06 and 0.09M and more preferably 0.0075M.

[0049] For the second series of washing cycles, the equivalent molar concentration of Mn+iXn is strictly less than 0.05M, preferably between 0.005 and 0.03M and more preferably 0.025M or 0.0125M.

[0050] Such an adjustment of the equivalent molar concentrations of Mn+iXn during the first and second series of washing cycles makes it possible to obtain an MXene compound in the form of sheets and to avoid the production of MXene compound in the form of volume particles (either platelets or other forms of three-dimensional particles) or in a mixture of volume particles and sheets.

[0051] According to the invention, each washing cycle comprises the addition of deionized and deoxygenated water with stirring, then the MXene suspension is subjected to a centrifugation step at more than 4000 revolutions per minute (equivalent to 1681 iron for the rotor used), preferably at 9000 revolutions per minute (equivalent to 8512 iron), for a period of between 5 and 20 minutes, preferably for 10 minutes. The supernatant is then removed. Then, a volume of deionized and deoxygenated water VI or V2 (depending on the series of washing cycles) is added to the sediment, and the tube containing the water and the sediment is stirred manually in order to detach the sediment from the bottom of the tube, then stirred by magnetic stirring or vortex stirring for approximately 5 minutes in order to disperse and wash the sediment in the incorporated water. Each washing cycle may comprise several washes thus carried out.

[0052] According to the invention, the last series of washing cycles is carried out until the pH of the solution differs by at most 0.5 units from the pH of the deionized and deoxygenated water used during the washings. Commonly and by way of example, the last series of washing cycles is carried out 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 washing cycles is chosen to be a pH equal to or lower by a maximum of 0.5 than that of the deionized water used during the washing cycles.

[0053] Preferably, the method comprises two series of washing cycles, the first series of washing cycles comprises three identical washing cycles operated according to the preceding description, and the second series of washing cycles comprises two identical washing cycles operated according to the preceding description.

[0054] At the end of the washing cycles, no additional washing step is carried out since the suspension of Mn+iXn compound is devoid of MAX phase compound or non-delaminated MXene compound as confirmed by analyses by X-ray diffractogram or by Scanning Electron Microscopy carried out at different locations of the sample.

[0055] In order to calculate the yield of the process of the invention, batches were dedicated 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 mass ml of the MAX phase compound is placed in a vacuum evaporator and heated at a temperature of about 80°C for about 12 hours, until no further variation in weight is observed. This gives a mass m2. 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 thus calculated of the MXene compound in the form of sheets in the aqueous suspension obtained is greater than 85%.

[0056] More precisely, a batch of MXene is synthesized starting from a mass of the MAX phase, m(MAX); then the aqueous, alcoholic or hydroalcoholic suspension of MXene in the form of sheets is obtained; then this suspension is dried by applying vacuum and heating at 80°C overnight; the solid obtained is weighed and this operation is repeated for a few hours to ensure that the weight is stabilized; this gives the mass of MXene in the form of sheets and solid, m(MXene). The percentage yield as defined above is, therefore, m(MXene) divided by m(MAX) multiplied by one hundred. According to the invention, the aqueous suspension of Mn+iX in the form of sheets obtained after the last washing cycle is transformed into an alcoholic suspension, preferably based on ethanol. Such a suspension is very stable and can be used for several months.

[0057] According to the invention, the suspension of Mn+iXn in the form of sheets is not dried to avoid any instability and / or complicate the subsequent process of forming thin layers which would require resuspending the dry product. The suspension obtained can be directly used to produce thin layer deposits.

[0058] Thin films are thus produced from the alcoholic suspension of Mn+[ Xn in the form of sheets obtained using the tape casting technique (doctor blade or tape casting in English). The alcoholic suspension is first transformed into an aqueous or hydroalcoholic ink or slip of a viscosity suitable for casting. Then, a suitable casting machine or device is used to cast the layer with the doctor blade onto a flat substrate, which may be compact or made of a porous membrane. After drying in ambient air, the layer is dried under vacuum at about 80°C for 4 to 16 hours. The electrical conductivity is measured by the four-point method with Hall effect, corrected with the measurement of the thickness of the layer which is measured by Scanning Electron Microscopy at several locations on the edge of the layer in order to obtain an average conductivity as well as an error interval.The electrical conductivity that is measured is between 2 and 3.4 MS / m.

[0059] Example 0: Synthesis of the MAX phase precursor of composition Ti3AlC2 according to a modified spark plasma sintering process.

[0060] A commercial spark plasma sintering apparatus marketed under the name Dr. SINTER Lab. Jr. (model SPS-21 ILx) by Fuji Electronic Industrial Co. Ltd. is used. The apparatus is modified as described with reference to [Fig.l] by retaining the graphite die 2 and the two graphite punches 3a and 3b, and by adding in the hollow chamber 4 the hollow alumina cylinder 6, and the lower 7a and upper 7b alumina discs thus forming a hollow alumina container 5. The powder mixture which will be described later is introduced into the container 5, packed, and kept electrically isolated 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.

[0061] Commercial powders of titanium carbide, aluminum, and titanium are mixed in an agate crucible, in respective molar proportions of 2: 1.1: 1, the powder mixture having a total mass of approximately 5 grams. The mixture is passed through a ball mill with a bowl and tungsten carbide balls for a period of 1 hour 15 minutes at a speed of 300 revolutions per minute. 2.5 grams of the powder thus ground are introduced inside the hollow cylinder 6 and the lower cover 7a already in place in the device. The powders are packed, then the upper cover 7b is positioned at the end of the hollow cylinder 6 thus forming the hollow container 5. The upper graphite punch 3b is positioned, the entire assembly 1 is positioned in the SPS apparatus and the vacuum is applied.The following thermal cycle is then applied: a temperature rise to 580°C in 6 min, a plateau at 580°C for 5 min, a temperature rise to 1450°C in 12 min, and a plateau at 1450°C for 8 min. Once the thermal cycle is complete, the temperature decreases rapidly below 580°C in about 5 to 10 min. A cylindrical porous pellet is obtained. The porosity of the pellet is greater than 40%, most often around or more than 50%. This 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 mass of the pellet.

[0062] Figures 2 and 3 illustrate the morphology by Scanning Electron Microscopy of the MAX phase precursor thus obtained, after coarse crushing of the porous pellet. For the precursor obtained, it can be seen that the grains are welded together.

[0063] With reference to [Fig.4], the X-ray diffractogram with the Cu Ka radiation source, of the MAX phase compound obtained according to this example corresponds to the crystalline phase of the Ti3AlC2 compound, to which is added a low intensity peak at the angle 20 36.0° assigned to the titanium carbide phase, TiC, which is present in the initial powder. This titanium carbide phase is estimated to be less than 5% by volume of crystalline phases in the MAX phase compound.

[0064] Energy-dispersive X-ray spectroscopy (EDX) spectra of the MAX phase compound of this example show the presence of the three elements present in the composition of this MAX phase, namely titanium, aluminum and carbon, as well as the absence of impurities such as oxygen or any other element.

[0065] Example 1: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension of sheets, from the coarsely crushed MAX phase precursor of Example 0, using a chemical etching solution of 1M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid.

[0066] Porous pellets of the MAX phase compound of Ti3AlC2 obtained according to Example 0 are roughly crushed with an agate mortar, leaving mostly pieces of about 1-2 millimeters. A first chemical attack is carried out with a hydrofluoric acid solution. For this, 2 grams of the MAX phase compound of formula Ti3AlC2 obtained in Example 0 and roughly crushed are placed in a 125 mL fluorinated ethylene propylene copolymer (FEP) plastic bottle. 40 mL of a previously prepared aqueous solution containing 1 M lithium fluoride, 2 M hydrofluoric acid and 9 M hydrochloric acid are added, then stirring is carried out with a magnetic stirrer at room temperature for 5 min. Stirring is continued by placing the bottle in a thermostatically controlled bath at 35°C for about 48 hours.

[0067] Following this first attack, the suspension is distributed into fluoroethylene propylene (FEP) centrifuge tubes with a nominal capacity of 50 mL (actual capacity 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 8,512 iron in the rotor used) for 10 minutes. The supernatant is then removed and the sediment is retained.

[0068] The second chemical attack is then carried out by transferring the sediments into a clean, dry FEP bottle, then 40 mL of the same aqueous solution containing 1 M lithium fluoride, 2 M hydrofluoric acid and 9 M hydrochloric acid is poured in, and the mixture is stirred in a thermostatically controlled bath at 35°C for approximately 92 hours.

[0069] At the end of this second chemical attack, two series of washing cycles are carried out with the addition of deionized and deoxygenated water under agitation for approximately 5 minutes, followed by centrifugation (by distributing the suspension into several FEP centrifuge tubes with a nominal volume of 50 mL) at 9000 rpm (8512 iron) for 10 minutes, removing the supernatants and keeping the sediments for the following cycle.

[0070] In the first series of washing cycles, the total volume used V1 is approximately 136 mL, which corresponds to an equivalent molar concentration of MXene as defined previously of approximately 0.075 M.

[0071] Three washing cycles are carried out in this first series until the supernatant has a pH of approximately 2.5.

[0072] In the second series of washing cycles, the total volume used V2 is approximately 408 mL, which corresponds to an equivalent molar concentration of MXene of approximately 0.025 M.

[0073] Two washing cycles are carried out in this second series until the supernatant has a pH of approximately 4.5.

[0074] The sediment is then washed with ethanol in two ethanol rinsing and centrifugation cycles and then stored as a suspension in ethanol. Alternatively, the sediment can be dried by heating it under vacuum at a temperature between 40 and 120°C.

[0075] A suspension of MXene compound of formula Ti3C2Tx or even more generally Ti3C2 is thus obtained, in the form of particles of the few-layer flake type dispersed in the solvent.

[0076] Figures 5 and 6 illustrate the morphology of MXene obtained in the form of a sheet.

[0077] In [Fig.5], it is a deposit of one or more drops of suspension without dilution on a smooth substrate such as glassy carbon. It is observed that the surface of the glassy carbon substrate is covered with the overlapping MXene layers.

[0078] In [Fig.6], it is a deposit of a drop of diluted suspension on a porous substrate such as an anodized aluminum oxide membrane. Isolated MXene sheets are observed. In top view scanning electron microscopy images, such as those in [Fig.5] or [Fig.6], it can be seen that the size distribution of the sheets is relatively homogeneous, namely a length L or lateral size of the sheets between 1 and 15 microns.

[0079] Scanning electron microscopy images of the edge of the films made with this MXene in the form of sheets also show that they are sheets and it is possible to estimate their thickness. The thickness or height H of the sheets is between 1 nm and 20 nm.

[0080] The X-ray diffractogram of [Fig.7] is very reproducible and characterized by a large peak at an angle 20 between 6.9° and 7.1° and satellite peaks with very low intensities compared to the large peak.

[0081] It is noted that the MXene according to the invention does not show the small characteristic peak at the 20° angle of 9.5° which would indicate the presence of unreacted MAX phase compound. It is also noted that the MXene suspension does not show a small peak at the 20° angle of 45° which would indicate the presence of LiF.

[0082] The EDX spectrum of the compound of this example shows the presence of the two main elements present in the composition of this MXene-type compound, namely titanium and carbon, plus the presence of fluorine, oxygen and chlorine which would correspond at the T-termini, especially at the -F, -Cl, =O or -OH groups. The EDX spectrum also shows, as already mentioned previously, the absence of the element Al, which means that the entire MAX phase precursor has been attacked and no aluminum-containing by-products such as the oxide A12O3 remain.

[0083] Example 2: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension of sheets, from the coarsely crushed MAX phase precursor of Example 0, using a chemical etching solution of 1M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid.

[0084] The procedure is as in Example 1, but with 0.5g of MAX phase compound of Example 0, 10mL of the same solution of 1M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid in the two chemical etching steps, and volumes VI and V2 in the two series of washing cycles of approximately 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075M and 0.0125M respectively, the equivalent molar concentration of MXene of the second washing cycle representing approximately half that of the second washing cycle of Example 1. The product is MXene in the form of sheets, without the presence of MAX phase, nor impurities of the LiF particle type. The process was repeated several times and is reproducible.

[0085] Example 3: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension of sheets, from the MAX phase precursor obtained commercially, using a chemical attack solution of 1M lithium fluoride, 2M hydrofluoric acid and 9M hydrochloric acid.

[0086] The procedure is as in Example 2, but with 0.5 g of a commercially obtained MAX phase compound, without additional grinding. 10 mL of the same 1 M lithium fluoride solution, 2 M hydrofluoric acid and 9 M hydrochloric acid as in Example 3 are used in the two chemical etching steps, as well as the same volumes VI and V2 in the two series of washing cycles of approximately 34 mL and 204 mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075 M and 0.0125 M respectively. The product is MXene in the form of sheets, without the presence of MAX phase, nor impurities such as LiF particles, as for Example 2 where the MAX phase compound was that synthesized according to Example 1.However, in addition to the leaflets, the product shows small particles of about 500 nm in length that could be bulk MXene particles or smaller thicker leaflets that could have appeared because commercial MAX compounds are usually finely ground.

[0087] Example 4: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension of sheets, from the MAX phase precursor of example 0 coarsely crushed using a chemical attack solution of 3M lithium fluoride and 9M hydrochloric acid.

[0088] The procedure is as in Example 2 with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 3M lithium fluoride and 9M hydrochloric acid in the two chemical etching steps. The volumes VI and V2 in the two series of washing cycles are the same as in Example 2, approximately 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075M and 0.0125M respectively. The product obtained is a suspension of MXene in the form of sheets, without the presence of unreacted MAX phase, but we note the significant presence of impurities of the LiF particle type detected by Scanning Electron Microscopy and by X-ray diffractogram. In the Scanning Electron Microscope, these impurities appear in the form of micrometric or sub-micrometric cubic particles.In the X-ray diffractogram, a prominent peak is observed at an angle 20 of 45° characteristic of a LiF peak, the intensity ratio of the LiF peak to the large MXene peak being typically between 0.1 and 0.3. The process has been repeated several times and is reproducible.

[0089] Example 5: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension of sheets, from the MAX phase precursor of example 0 coarsely crushed using a chemical attack solution of 2.5M lithium fluoride and 9M hydrochloric acid.

[0090] The procedure is as in Example 4, with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 2.5M lithium fluoride and 9M hydrochloric acid in the two chemical etching steps. The volumes VI and V2 in the two series of washing cycles remain the same as in Example 4, approximately 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075M and 0.0125M respectively. The product obtained is a suspension of MXene in the form of sheets, without the presence of unreacted MAX phase, but with a significant presence of impurities of the LiF particle type, its presence being detected by Scanning Electron Microscopy and by X-ray diffractogram.

[0091] Example 6: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension of sheets, from the MAX phase precursor of example 0 coarsely crushed using a chemical attack solution of 2M lithium fluoride and 9M hydrochloric acid.

[0092] 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 2 M lithium fluoride and 9 M hydrochloric acid in the two chemical etching steps. The volumes VI and V2 in the two series of washing cycles remain the same as in Example 5, approximately 34 mL and 204 mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075 M and 0.0125 M respectively. The product obtained is a suspension of MXene in the form of sheets, without the presence of unreacted MAX phase, but with a significant presence of impurities of the LiF particle type, its presence being detected by Scanning Electron Microscopy and by X-ray diffractogram.

[0093] Example 7: Electrical conductivity measurements of the MXene suspension of the invention by thin layer deposition.

[0094] A portion of the alcoholic suspension of the MXene sheets obtained in Example 1 (or in Example 2) in 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, is taken and placed in an FEP centrifuge tube. Centrifugation is carried out at 9000 rpm for 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 carried out in order to reduce the alcohol content. 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.

[0095] The layer deposition is carried out using the tape casting technique ("doctor blade" in English). To do this, a flat substrate is prepared with a polymer membrane as a sliding surface, and adhesive strips on both edges in the casting direction with defined heights. A quantity of ink is placed in one end and then the ink is slid with a glass rod to the other end. It is left to dry for a few minutes in ambient air.

[0096] The same operation can be repeated on the part already deposited in order to obtain a layer with 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 layer. The final layer is left to dry in a glove box, then it is subjected to vacuum drying at 80°C for 4 to 16 hours. It can remain on its membrane support or be peeled off.

[0097] The electrical conductivity is measured by Hall effect with a commercial device such as the Ecopia HMS 5500 comprising 4 points arranged approximately forming a square of at least 1 cm2. The assumed thickness of the film in the calculation program is then corrected by the actual value in order to obtain the conductivity correct electrical. The actual film thickness is obtained by Scanning Electron Microscopy measurement in the film slice, and measured in several locations in order to obtain an average as well as a variability range of the thickness and, therefore, the conductivity.

[0098] With reference to [Fig. 10], it can be seen that the electrical conductivity obtained with the suspension of MXene sheets obtained from a MAX phase compound of example 0 (reference 10) is greater than 2, whereas the electrical conductivity obtained with the suspension of MXene sheets obtained from a commercial MAX phase compound (reference 11) is less than 2.

[0099] Counter-Example 1: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a platelet suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0100] The procedure is as in Example 2, with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 2M lithium fluoride and 6M hydrochloric acid in the two chemical etching steps, and with a single series of washing cycles. The volume of deionized and deoxygenated water per washing cycle is approximately 204mL, which represents an equivalent molar concentration of MXene of approximately 0.0125M. The product obtained is a suspension of MXene in the form of platelets as visualized by Scanning Electron Microscopy. Figures 8 and 9 illustrate the morphology of MXene in the form of a platelet consisting of two flat and parallel faces with a relatively homogeneous crystal size distribution. More precisely, the length L of the platelets is between 1 and 15 microns, and their height H is between 0.2 and 1 micron.The X-ray diffractogram of MXene platelets resembles that of MXene sheets but shows greater variability in the position of the main peak and those of the satellite peaks. No LiF impurity or unreacted MAX phase is detected. The EDX spectrum of MXene platelets resembles that of MXene sheets with the presence of the two main elements present in the composition of this MXene-type compound, namely titanium and carbon, plus the presence of fluorine, oxygen and chlorine which could correspond to the T-termini. The process was repeated and is reproducible in that the product obtained is systematically in the form of platelets.

[0101] Counter-Example 2: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0102] The procedure is as in Example 2 with 0.5g of MAX phase compound from Example 0, but with 10mL of an aqueous solution containing 2M lithium fluoride and 6M hydrochloric acid, and with a single chemical attack step. lasting 48 hours. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of approximately 34 mL and 204 mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075 M and 0.0125 M respectively. The product obtained is mainly 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 process is not reproducible with respect to the shape of the MXene particles, since an MXene suspension in the form of sheets and an MXene suspension in the form of platelets were alternately obtained.

[0103] Counter-Example 3: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0104] The procedure is as in Example 2, with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 3M lithium fluoride and 9M hydrochloric acid, and with a single chemical attack step lasting 48h as in Counter-Example 2. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075M and 0.0125M respectively. The product obtained is mainly a suspension of MXene, with the presence of the MAX phase compound which has not reacted completely as detected by the X-ray diffractogram. The process is not reproducible with respect to the shape of the MXene particles, since a suspension of MXene in the form of sheets and a suspension of MXene in the form of platelets were alternately obtained.

[0105] Counter-Example 4: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0106] The procedure is as in Example 2 with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 1M of lithium fluoride (therefore, less LiF than in Example 2) and 9M of hydrochloric acid for the two chemical etching steps lasting 48h and 92h respectively. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of 34mL and 204mL respectively, which represents equivalent molar concentrations of Mxene of approximately 0.075M and 0.0125M respectively. The product obtained is a suspension of MXene without the presence of MAX phase compound. The presence of impurities of the LiF particles is also not observed, but the MXene suspension contains a mixture of particles in the form of sheets and platelets.

[0107] Counter-Example 5: Synthesis of an MXene compound of formula Ti3C2Tx, in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0108] The procedure is as in Example 2, with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 3M hydrofluoric acid and 9M hydrochloric acid in the two chemical etching steps of 48h and 92h respectively. The aqueous solution therefore does not contain lithium salt but has the same fluoride concentration as in Example 2. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075M and 0.0125M respectively. The product obtained is a suspension of MXene with particles in the form of platelets without the presence of MAX phase compound. LiF particle impurities are also not observed since there is no Li cation in the etching solution.

[0109] Counter-Example 6: Synthesis of an MXene compound of formula Ti3C2Tx in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0110] The procedure is as in Example 2 with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 2M potassium fluoride and 6M hydrochloric acid in the two chemical etching steps of 48h and 92h respectively. The aqueous solution therefore does not contain lithium salt which is replaced by potassium in the form of potassium fluoride which is more soluble than lithium fluoride. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.075M and 0.0125M respectively. The product obtained is a suspension of MXene in the form of platelets, with a small amount of MAX phase compound which has not reacted. No LiF particle impurities are observed since there is no Li cation in the etching solution.

[0111] Counter-Example 7: Synthesis of an MXene compound of formula Ti3C2Tx in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0112] The procedure is as in Example 2, with 0.5g of MAX phase compound of Example 0, but with 10mL of an aqueous solution containing 3M lithium chloride and 9M hydrochloric acid in the two chemical attack stages lasting 48h and 92h respectively. The aqueous solution therefore does not contain fluoride. Two series of washing cycles are carried out with volumes of deionized water and deoxygenated VI and V2 of 34mL and 204mL respectively. The product obtained is the unreacted MAX compound.

[0113] Counter-Example 8: Synthesis of an MXene compound of formula Ti3C2Tx in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0114] The procedure is as in Example 2, with 2g of MAX phase compound of Example 0, and 40mL of an aqueous solution containing 1M of lithium fluoride, 2M of hydrofluoric acid and 9M of hydrochloric acid in the two chemical attack steps lasting 48h and 92h respectively. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of approximately 68mL and 136mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.15M and 0.075M respectively. The product obtained is a suspension of MXene with particles in the form of platelets, without the presence of MAX phase compound.

[0115] Counter-Example 9: Synthesis of an MXene compound of formula Ti3C2Tx in the form of a suspension, from the coarsely crushed MAX phase precursor of Example 0.

[0116] The procedure is as in Example 4, but with 1g of MAX phase compound of Example 0 and 20mL of the same 3M aqueous solution of lithium fluoride and 9M hydrochloric acid as in Example 4 for the two chemical etching steps lasting 48h and 92h respectively. Two series of washing cycles are carried out with volumes of deionized and deoxygenated water VI and V2 of approximately 34mL and 204mL respectively, which represents equivalent molar concentrations of MXene of approximately 0.150M and 0.025M respectively, i.e. concentrations higher than those of Example 4 with regard to the doubling of the mass of the precursor MAX phase compound while keeping the same volumes VI and V2. The product obtained is a suspension of MXene with a mixture of particles in the form of predominantly sheets but also with a significant presence of particles in the form of platelets. No presence of MAX phase compound is observed.On the other hand, impurities in the LiF particles are observed.

[0117] [Table 1] below presents the results obtained in terms of product obtained in the final suspension, lithium fluoride impurities, presence of MAX phase type compound and reproducibility of the process according to the operational conditions of the chemical attacks and the first and second series of washing cycles for counter-examples 1 to 9 previously explained and outside the scope of the invention, and examples 1 to 6 previously explained and operated according to the process of the invention.

[0118] In all the examples and counter-examples presented in [Table 1], the centrifugation operation which is carried out after each washing cycle is at a speed of 9000 revolutions per minute (equivalent to 8,512 iron for the rotor used). Chemical attacks Water washes Product obtained Compound phase MAX Number of chemical attacks Composition Equivalent molar concentration in M ​​Xene during the first series of washing cycles Equivalent molar concentration in M ​​Xene during the second series of washing cycles Sheets and / or plates / compound phase eMAX Impurities LiF Presence of compound of type MAX phase Procedure reproducibility Counter example 1 Example 0 2 2ML iF, 6 MH Cl 0.0125M - Plates NO NO Example 20 2ML iF, 6 MH Cl 0.075M 0.0125M Plates or Sheets NO YES NO Counter Example 3 Example 0 1 3ML iF, 9 MH Cl 0.075M 0.0125M Plates or Sheets NO Example 4 I ON 0 2 1ML iF, 9 MH Cl 0.075M 0.0125M Sheets and platelets NON NON ND Counter example 5 Example 0 2 3MH F, 9M HCl 0.075M Platelets NON NON ND Counterexample 6 Example 0 2 2MK F, 6M HCl 0.075M 0.0125M Platelets NO Traces ND Counterexample 7 Example 0 2 3ML iCl, 9 MH Cl 0.075M 0.0125M MA X phase compound at 100% NO- YES ND Counterexample 8 Example 0 2 1ML iF, 2 MHF, 9MH Cl 0.15M 0.075M Platelets YES NO ND Counterexample 9 Example 0 2 2ML iF, 9 MH Cl 0.15M 0.025M Sheets and Platelets YES NO ND Example 1 Example 0 2 1ML iF, 2 MHF, 9MH Cl 0.075M 0.025M Sheets Traces NO YES Example 2 Example 0 2 1ML iF, 2 MHF, 9MH Cl 0.075M 0.0125M Sheets NO NO YES Example 3 Commercial 2 1ML iF, 2 MHF, 9MH Cl 0.075M 0.0125M Sheets NO NO ND Example 4 Example 0 2 3ML iF, 9 MH Cl 0.075M 0.0125M Sheets YES NO YES Example 5 Example 0 2 2.5M LiF, 9MH Cl 0.075M 0.0125M Sheets YES NO ND Example 6 Example 0 2 2ML iF, 9 MH Cl 0.075M 0.0125M Sheets YES NO ND Table 1: Counterexamples and examples of the invention according to operational conditions (ND: Not Determined)

Claims

1. Claims Process for manufacturing a suspension of a compound of general formula Mn+iXn, where n = 1, 2 or 3, M is Ti and X is chosen from C or N, which compound is in the form of sheets of length L between 1 and 15 micrometers and height H between 1 and 20 nanometers, characterized in that it is manufactured from a compound of the MAX phase type of general formula Mn+iAXn, where n = 1, 2 or 3, M is Ti, A is Al, and X is chosen from C or N, and in that said process comprises at least the following successive steps: • at least two successive chemical attacks of said MAX phase type compound, each of the chemical attacks being carried out with an aqueous solution comprising hydrochloric acid at a molar concentration strictly greater than 6M in a mixture with either only lithium fluoride at a molar concentration strictly greater than 1M, or lithium fluoride and hydrofluoric acid, • a first series of washing cycles with deionized and deoxygenated water, each of the washing cycles comprising the addition of a volume V1 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXn to the MAX phase type compound which is strictly less than 0.15M, followed by centrifugation at more than 1681 iron and the evacuation of the supernatant, • a second series of washing cycles with deionized and deoxygenated water, each of the washing cycles comprising the addition of a volume V2 of deionized and deoxygenated water with stirring to obtain an equivalent molar concentration of Mn+iXn to the MAX phase type compound strictly less than 0.05M, followed by centrifugation at more than 1681 iron and the evacuation of the supernatant, and • obtaining an aqueous suspension of Mn+iXn, the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the second series of cycles being lower at the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the first series of cycles, and the last series of washing cycles being 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 during the last washing cycle.

2. Method according to claim 1, characterized in that in the compound of formula Tin+iXn, n=2 and X is C, and in that in the compound of phase MAX type of formula Tin+iAlXn, n=2 and X is C.

3. Method according to any one of the preceding claims, characterized in that each of the chemical attacks is carried out with an aqueous solution comprising hydrochloric acid at a molar concentration strictly greater than 6M, lithium fluoride at a molar concentration of at least 1M and hydrofluoric acid at a molar concentration of at least 1M.

4. Method according to any one of claims 1 and 2, characterized in that each of the chemical attacks is carried out with an aqueous solution comprising hydrochloric acid at a molar concentration of 9M, lithium fluoride at a molar concentration of 1M and hydrofluoric acid at a molar concentration of 2M.

5. A method according to any one of the preceding claims, characterized in that the first series of washing cycles comprises at least three washing cycles and in that the second series of washing cycles comprises at least two washing cycles.

6. Method according to any one of the preceding claims, characterized in that the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the first series of washing cycles is between 0.06 and 0.09M.

7. Method according to any one of the preceding claims, characterized in that the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the second series of washing cycles is between 0.005 and 0.03M.

8. A method according to claim 4, characterized in that the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the first series of washing cycles is 0.075M and in that the equivalent molar concentration of Mn+iXn to the MAX phase type compound during the second series of wash cycles is 0.0125M.

9. A method according to any preceding claim, characterized in that the centrifugation during the first and second series of washing cycles is carried out at approximately 8512 iron.

10. Method according to any one of the preceding claims, characterized in that the aqueous suspension of Mn+iXn in the form of sheets obtained after the last washing cycle is transformed into an alcoholic suspension, preferably based on ethanol.

11. Method according to any one of the preceding claims, characterized in that the MAX phase type compound is obtained by a spark plasma sintering process in a spark plasma sintering device (1) comprising a graphite matrix (2) and two graphite punches (3a, 3b) defining a hollow chamber (4), characterized in that it comprises at least the steps of: - mixing powders, - placing the premixed powders in a closed container (5) made of an insulating ceramic material and housed in the hollow chamber (4), - carrying out the spark plasma sintering operation, and - obtaining a pellet made of the MAX phase type compound.

12. Thin film produced by depositing a thin layer on a support from the suspension of compound of general formula Mn+iXn in the form of sheets obtained according to the method of claim 11, characterized in that it is capable of being detached from the support used for the deposition of the thin layer and has an electrical conductivity at least equal to 2 MS / m.

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