WO2022015210A1

WO2022015210A1 - Method for preparing a two-dimensional material with the formula mn+1xnts or(m1x,ny)2cts

Info

Publication number: WO2022015210A1
Application number: PCT/SE2020/050729
Authority: WO
Inventors: Ahmed EL GHAZALY; Joseph HALIM; Johanna ROSÉN; Heba Ahmed; Amgad REZK; Leslie Yeo
Original assignee: Royal Melbourne Institute Of Technology
Priority date: 2020-07-13
Filing date: 2020-07-13
Publication date: 2022-01-20

Abstract

A method for preparing a two-dimensional material having the formula M_n+1X_nT_s is provided. The method comprises a step (S101) of providing a dispersion comprising a three-dimensional material having the formula M_n+1AX_n dispersed in a first aqueous solution comprising fluoride ions. The method further comprises a step (S103) of exposing the dispersion to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves, thereby obtaining a second aqueous solution comprising dispersed M_n+1X_nT_s. In the formulas above, n is 1, 2, 3 or higher, M is at least one transition metal, A is at least one A-group element, X is at least one of C, N and O. T_s is a surface termination.

Description

METHOD FOR PREPARING A TWO-DIMENSIONAL MATERIAL WITH THE FORMULA M_h+iC„T₅ OR (Ml_x,n_v)₂CT_s

TECHNICAL FIELD

The present disclosure relates in general to a method for preparing a two-dimensional material with the formula M_n+iXnT_s, wherein n is 1, 2, 3 or higher, M constitutes at least one transition metal, X is at least one of C, N and O, and T_s constitutes a surface termination. The present disclosure further relates in general to a method for preparing a two-dimensional material with the formula (Mlx,i"iv)2CT_s, wherein Ml constitutes a transition metal, h is a vacancy, T_s constitutes a surface termination and wherein the sum of x and y is 1.

BACKGROUND

So called MAX phases, or MAX phase alloys, constitute a class of materials with the formula M_n+iAX_n where n = 1, 2, 3 or higher, M constitutes at least one transition metal, A constitutes at least one A- group element, and X is at least one of C, N and O. MAX phases are in the literature often divided into different classes of MAX phases depending on the relative amounts of the M, A and X elements and the most common classes constitute 211 MAX phases, 312 MAX phases and 413 MAX phases.

In general, MAX phases have a layered hexagonal crystal structure with P63 /mmc symmetry. Each unit cell comprises two formula units. Near-closed packed layers of the M-element(s) are interleaved with pure A-group element(s) layers, with the X-atoms filling the octahedral sites between the former. Therefore, MAX phases form laminated structures. These laminated structures have anisotropic properties as a result of the structure.

MAX phases may be synthesized by bulk synthesis wherein the constituent elements of the intended MAX phase are mixed in the intended amounts of the MAX phase and subjected to high temperature so as to form the MAX phase. Examples of such bulk synthesis methods include hot isostatic pressing (HIP), reactive sintering, self-propagating high temperature synthesis (SHS), and combustion synthesis. MAX phases may also be synthesized using thin film synthesis methods, such as by physical vapor deposition (PVD) or chemical vapor deposition (CVD).

It is previously known to synthesize two-dimensional materials known as MXenes, from MAX phases. MXenes are a class of two-dimensional inorganic compounds which consist of a few atoms thick layers of transition metal carbides, nitrides or carbonitrides. These layers may be stacked on top of each other, if desired. MXenes are often described with the formula M_n+iXn. However, since the surfaces of MXenes are terminated by functional groups, a more correct description is the formula M_n+iX_nT_s, where T_s is a functional group such as O, F and/or OH.

Unlike synthesis of other two-dimensional materials, such as graphene, MXenes cannot be synthesized by simply exfoliating a corresponding three-dimensional material due to the strong bonds within the crystal structure. Instead, the synthesis of MXenes comprises selective etching of a corresponding MAX phase to thereby remove the A-atoms of the MAX phase, followed by washing and delamination of the individual layers. For example, a MAX phase such as M2AIC (M denominating a transition metal) may be etched in hydrofluoric acid (HF), resulting in removal of the A-layer and formation of two dimensional M2C sheets. It is also previously known to for example etch using hydrochloric acid (HCI) combined with lithium fluoride (LiF). Most of the currently known methods for synthesis of MXenes requires addition of acid. Examples of MXenes that have been previously synthesized include T12C, V2C, Nb2C, T13C2, T13CN, Nb^ and Ta^.

Naguib et al., "Two-Dimensional Nanocrystals Produced by Exfoliation of TisAiCf , Advanced Materials, 2011, 23, 4248-4253, reported synthesis of a two-dimensional material starting from the MAX phase T13AIC2. They extracted the Al from T13AIC2 by use of hydrofluoric solution, followed by intercalation of DMSO for delamination, and thereby arrived at isolated layers of T13C2.

WO 2017/204708 Al discloses a method for synthesizing MXenes with ordered vacancies. This may be achieved by etching certain quaternary MAX phases. These MAX phases have the formula (M1_X,M2_V)₂AIC, wherein Ml is transition metal selected from a first group and M2 is a transition metal from another group, the sum of x and y is 1, and x is from 0.60 to 0.75. It was found that by appropriate selection of the Ml and M2 elements, it was possible to obtain quaternary alloys demonstrating chemical ordering in the M-plane of the MAX phase. This may in turn enable etching out the M2 element, in addition to the etching of the Al, thereby arriving at a MXene with ordered vacancies. Such a MXene may thus be described by the formula (Ml_x,q_y)2C wherein h constitutes a vacancy.

MXenes have shown a great potential for applications such as energy storage and electromagnetic shielding due to their properties. Moreover, MXenes are both conductive and hydrophilic, allowing for co-assembly with polar species. However, the synthesis of MXenes constitutes a concern since it is time-consuming (in general requiring at least 24h for the etching step), and also requires tedious and hazardous waste management procedures to take care of the waste products, in particular the acid used. This is an obstacle against industrial scale production of MXenes, in practice requiring batches in the order of at least kilograms. Thus, there is a desire for improved possibilities for synthesis of MXenes such that they can reach their full commercial potential.

SUMMARY

The object of the present invention is to provide an improved method for preparing MXenes.

The object is achieved by the appended independent claim(s).

In accordance with the present disclosure, a method for preparing a two-dimensional material having the formula M_n+iXnT_s is provided. The method comprises a step of providing a dispersion comprising a three-dimensional material having the formula M_n+iAX_n dispersed in a first aqueous solution comprising fluoride ions. The method further comprises a step of exposing the dispersion to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves, thereby obtaining a second aqueous solution comprising dispersed M_n+iXnT_s. In the formulas above, n is 1, 2, 3 or higher, M is at least one transition metal, A is at least one A-group element, X is at least one of C, N and O. T_s is a surface termination.

The method results in a considerable reduction of the time it takes to synthesize the two- dimensional material compared to the conventional synthesis of MXene with similar yield. Moreover, the method is safe and sustainable since it requires a minimum amount of chemicals. Furthermore, the method can easily be upscaled to industrial production by allowing massive parallelization.

The above described method can also be used for preparing MXenes comprising ordered vacancies by utilizing certain MAX phases in the method. Thus, a method for preparing a two-dimensional material having the formula (Ml_x,q_v)2CT_s is also provided. Just like the first method disclosed above, said method for preparing a two-dimensional formula (Ml_x,q_v)2CT_s comprises a step of providing a dispersion comprising a three-dimensional material having the formula M_n+iAX_n dispersed in a first aqueous solution comprising fluoride ions. The method further comprises a step of exposing the dispersion to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves, thereby obtaining a second aqueous solution comprising dispersed (Ml_x,q_v)2CT_s. In the formulas above Ml constitutes a transition metal, h is a vacancy, and T_s is a surface termination. In the method for preparing a two-dimensional material having the formula (Ml_x,q_v)2CT_s, the three- dimensional material having the formula M_n+iAX_n constitutes (Ml_x,M2_y)₂AIC, the sum of x and y is 1, and x is from 0.60 to 0.75. Furthermore,

Ml is selected from a first group of transition metals consisting of Cr, Mo, Nb, Ta, Ti, V and W, and M2 is selected from a second group of transition metals consisting of Sc and Y; or Ml is Ti and M2 is selected from the group consisting of Nb, Ta, V and W; or Ml is Sc and M2 is either Mo or W; or Ml is Cr and M2 is Ta; or

Ml is selected from the group consisting of Cr, Nb, Ta and V, and M2 is Ti.

In the methods described above, the step of providing the dispersion may comprise mixing the three- dimensional material having the formula M_n+iAX_n, with water and a fluorine based chemical.

Thereby, the dispersion can be easily be provided.

The fluorine based chemical may be LiF. Thereby, lithium ions may be intercalated between the M_n+iX_nT_s, in addition to water, which causes an increase in the interlayer spacing. This in turn facilitates the delamination of the M_n+iXnT_s layers.

The dispersion may comprise a concentration of fluoride ions such that the ratio A:F is from 1:3 to 1:5 in the dispersion. Thereby, there will be a sufficient amount for enabling the removal of the A- layers of the three-dimensional material while at the same time not producing an excessive amount of waste. Preferably, the dispersion comprises a concentration of fluoride ions such that the ratio A:F is from 1:3 to 1:4.

The step of exposing the dispersion to substrate subjected to surface acoustic waves (SAWs) and/or surface reflected bulk waves (SRBWs) may comprise dispensing the dispersion onto a piezoelectric substrate subjected to surface acoustic waves and/or surface reflected bulk waves. Thereby, the acoustic waves efficiently split the water in the aqueous solution, which in turn enables formation of localized H F at the three-dimensional material, as well as cause delamination into the two- dimensional material.

The dispersion may be dispensed in the form of drops onto the piezoelectric substrate. The drops will thereby on the substrate be divided into smaller drops which in turn inter alia facilitates the formation of localized H F where needed to etch the three-dimensional material. The frequency of the surface acoustic waves and/or the surface reflected bulk waves may be from 10 MFIz to 1000 MFIz. At higher frequencies than 1000 MFIz, it may be difficult to achieve the desired effect since such high frequencies result in small attenuation lengths. At frequencies below 10 MFIz, there may be a risk for cavitation during the delamination. This is due to the risk of the higher penetration length of the waves at lower frequencies reaching the surface of the solution, causing undulations in said surface and breaking the cohesive forces, which in turn may lead to formation of bubbles.

The amplitude of the surface acoustic waves and/or the surface reflected bulk waves may be from 0.5 nm to 20 nm. FHigher amplitudes than 20 nm may in some cases cause undesired oxidation of the two-dimensional material. Albeit possible, lower amplitudes than 0.5 nm may lead to problems in the control of the method.

The three-dimensional material having the formula the formula M_n+iAX_n may be in a particulate form having a particle size of up to 50 pm. Thereby, the three-dimensional material is sufficiently small to be adequately dispersed in the first aqueous solution. Furthermore, it is sufficiently small to be able to remove the A-layers. Preferably, the three-dimensional material having the formula the formula Mn_+iAXn may be in a particulate form having a particle size of up to 30 pm.

The method may further comprise a step of filtering the M_n+iXnT_s or (Ml_x,q_v) CT_s from the second aqueous solution. During such a filtering step, the individual layers of M_n+iXnT_s or (Ml_x,q_v) CT_s may be automatically stacked to each other. This may in turn inter alia provide an easier handling thereof and enables obtaining free-standing sheets with a thickness of up to at least 5 pm.

BREIF DESCRIPTION OF DRAWINGS

Fig. 1 illustrates a side view of the atomic structure of a conventional 211 MAX phase,

Fig. 2 represents a flowchart schematically illustrating one exemplifying embodiment of a method for preparing a two-dimensional material having the formula M_n+iXnT_s in accordance with the present disclosure,

Fig. 3 illustrates the result of a surface acoustic wave treatment of a dispersion comprising Mn_+iAXn dispersed in the first aqueous solution comprising fluoride ions, exemplified for a case where the M_n+iAX_n constitutes T13AIC2 and the fluoride ions result from addition of LiF,

Fig. 4 shows FIAADF-STEM imaging, at different magnifications, of a T13C2T_S MXene produced by the method according to the present disclosure,

Fig. 5 shows a cross section SEM image of a free-standing film formed of a T13C2T_S MXene produced by the method according to the present disclosure.

DEFINITIONS

A two-dimensional material constitutes a material consisting of a single layer of atoms or crystal cells, and is sometimes referred to as a "single layer material". Thus, in a two dimensional material, the atoms or, where applicable, crystal cells are repeated in two dimensions (x and y direction) but not in the third dimension (z direction), in contrast to a three-dimensional material where the atoms/crystal cells are repeated in all directions. Flowever, as well known to the skilled person, no material constitutes a perfectly two-dimensional material since there will always be normally occurring defects present. Therefore, when the term "two-dimensional material" is used in the present disclosure, it shall be considered to encompass both a perfect two-dimensional material as well as a two-dimensional material comprising normally occurring defects. Furthermore, a two- dimensional material shall not be considered to necessarily be flat but may for example also have a singled-curved, double-curved, undulating, rolled-up, or tube shape without departing from the scope of the present invention.

In the present disclosure, the term M_n+iAX_n is used to describe so called MAX phases, or MAX phase alloys. MAX phases constitute a class of materials with the general formula M_n+iAX_n where n = 1 to 3, M constitutes at least one transition metal, A constitutes at least one A-group element, and X is at least one of C, N and O. MAX phases with compositions diverging from n being an integer are also known. For example, n may diverge from an integer as a result of the synthesis or due to defects in the material. Moreover, MAX phases with n above 3 have been reported in the literature. Thus, MAX phases may be more appropriately described with the formula M_h±i-_d Ai-_a X_n±_P , wherein n= 1, 2, 3 or higher, d <0.2, a <0.2 and p<0.2, M is at least one transition metal, A is at least one A-group element, and X is at least one of C, N and O. Flowever, the formula M_n+iAX_n is used throughout to in order to facilitate the reading of the present disclosure. When M_n+iAX_n is used herein, it shall thus be interpreted as M_h±i-d Ai-_a Xn±_P.

Correspondingly, MXenes are in the present disclosure described by the general formula M_n+iX_nT_s. When M_n+iX_nT_s is used herein, it shall thus be interpreted as M_h±i-_dX_n±T_s, wherein d and p are as given above for the corresponding MAX phase. T_s constitutes the surface termination of the MXene and is formed by one or more functional groups, such as O, F and/or OH. The skilled person is well aware that the amount of functional groups is not given in the general formula M_n+iX_nT_s, but inherently results from the synthesis or the environment to which the MXene is exposed.

In the same way, when MXenes comprising ordered vacancies are described in the present disclosure, they are described by the general formula (M1_c,h_n)₂qT₅. When (M1_c,h_n)₂qT₅ is used herein, it shall thus be interpreted as (Ml_x± ,r\_v±_£) - Ci±_pT_s, wherein d and p are as given above, b is < 0.1, and e is < 0.1. It should also be noted that ordered vacancies are different from potential vacancies resulting as normally occurring defects. Ordered vacancies are repeated throughout the structure in an ordered way.

DETAILED DESCRIPTION

The invention will be described in more detail below with reference to the accompanying drawings, and certain embodiments. The invention is however not limited to the embodiments discussed but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate the invention.

In accordance with the present disclosure, a method for preparing a two-dimensional material having the formula M_n+iX_nT_s is provided. The method comprises a step of providing a dispersion comprising a three-dimensional material having the formula M_n+iAX_n dispersed in a first aqueous solution comprising fluoride ions. The method further comprises a step of exposing the dispersion to a substrate subjected to surface acoustic waves (SAWs) and/or surface reflected bulk waves (SRBWs).

In other words, the dispersion is brough into contact with the substrate subjected to SAW or Thereby, a second aqueous solution comprising dispersed M_n+iX_nT_s is obtained. In the formulas above, n is 1, 2, 3 or higher, M is at least one transition metal, A is at least one A-group element, X is at least one of C, N and O. T_s is a surface termination. According to one alternative M constitutes one transition metal. According to another alternative, M comprises two transition metals.

The temperature of the dispersion, when subjected to the surface acoustic wave treatment, may be between 20°C and 50°C, but is not limited thereto.

The step of providing a dispersion may comprise mixing the three-dimensional material having the formula M_n+iAX_n, with deionized water and a fluorine based chemical. No further additions need to be made to the dispersion. Thus, according to one alternative, the dispersion may consist of deionized water, a fluorine based chemical and the three-dimensional material having the formula M_n+iAX_n. This has the advantage of minimizing the amount of chemicals needed and reducing the amount of waste products. It is however possible to add further solvents and/or other additives, if desired, to the dispersion. Examples of additional solvents include alcohols, such as ethanol.

Examples of other additives include additives for further improving the delamination of the three- dimensional material into the dispersed M_n+iXnT_s. One example of such an additive is tetrabutylammonium hydroxide (TBAOH).

The fluorine based chemical may suitably be lithium fluoride (LiF). However, other fluorine based chemicals may also be used, including mixtures of different fluorine based chemicals. Examples of other fluorine based chemicals that may be used include NaF or KF.

As described in the present disclosure, it has been found that the two-dimensional M_n+iXnT_s material can be synthesized from a corresponding M_n+iAX_n phase through a fast and energy efficient method without the need for additions of acids by exploiting the nonlinear electromechanical coupling afforded by surface-localized vibrations in the form of surface acoustic waves (SAWs) or surface reflected bulk waves (SRBWs). Inducing self-ionization of pure water to facilitate the production of free radicals in the absence of any catalysts, and excitation of an aqueous solution of the dispersed M_n+iAX_n in the presence of low concentration of fluoride ions, leads to the production of localized HF. The localized HF in turn selectively etch the A-layers from the M_n+iAX_n. The result is individual Mn_+iXnT_s sheets dispersed in water. From this, continuous M_n+iXnT_s films may be filtered. The resulting M_n+iXnT_s demonstrate properties which are comparable to M_n+iXnT_s produced by conventional methods. Furthermore, the present method reduces the time for synthesis from the 24 hours typically needed for the conventional methods to well below 1 hour for a corresponding yield. Since the method utilizes a very low amount of chemicals and avoids the need of strong acids, the method also overcomes the obstacle of the waste handling of the conventional method. Moreover, a SAW or SRBW device allows for massive parallelization which in turn leads to excellent potential for upscaling to industrial production.

Surface acoustic waves (SAWs) are acoustic waves traveling along the surface of a material exhibiting elasticity. SAWs can inter alia be used to drive microfluidic actuation. Owing to the mismatch of sound velocities in the SAW substrate, in which surface acoustic wave propagates, and a fluid in contact therewith, the SAWs can be transferred to the fluid. This may in turn create inertial forces in the fluid. Furthermore, it is previously known that SAWs may be used for splitting of water. Surface reflected bulk waves (SRBWs) are a similar type of waves but are, in contrast to SAWs, generated within the bulk of the piezoelectric substrate. The SRBW are exited from the surface of the piezoelectric substrate when the acoustic wavelength is comparable to the piezoelectric substrate thickness. These SRBWs can be excited with the SAWs on the same substrate, if desired.

A surface acoustic wave device generally comprises at least one metal electrode which is the so- called interdigital transducer (IDT). The interdigital transducer is disposed on a piezoelectric elastic substrate (hereinafter piezoelectric substrate). An electric signal is converted by the interdigital transducer into a surface wave by exploiting the piezoelectric effect of the piezoelectric substrate. The piezoelectric substrate may for example be formed of quartz, lithium niobite, lithium tantalite or the like. Surface acoustic wave or surface reflected bulk wave devices are typically fabricated by photolithography. A surface reflected bulk wave device has a configuration similar to a surface acoustic wave device.

One example of a previously known SAW device which may be used in the present method is disclosed in WO 2014/132228 A1 which describes an apparatus for atomizing liquids. Said apparatus comprises a piezoelectric substrate having a working surface, an interdigital transducer located on the working surface for generating surface acoustic waves in the working surface, and a liquid delivery arrangement including a porous member for supplying the liquid to be atomized. It should however be noted that, for use in the method according to the present disclosure, the liquid delivery arrangement of the SAW device described in WO 2014/132228 A1 can be replaced for example by a dispensing device configured to dispense drops of the dispersion onto the piezoelectric substrate, such a dispensing device comprising a nozzle which is not in contact with the piezoelectric substrate..

One example of a previously known SRBW device which may be used in the present method is disclosed in WO 2016/179664 A1 which describes an acoustic wave microfluidic device. Said device comprises an electric acoustic transducer on a substrate and a source of a substance that is movable to the substrate. More specifically, the substance is drawn or pulled from a reservoir onto the substrate. It should however be noted that, for use in the method according to the present disclosure, the device described in WO 2016/179664 A1 can modified such that the dispersion need not be drawn out of a reservoir by the acoustic waves, but may be dispensed onto the piezoelectric substrate by a dispensing device, such a dispensing device not being in contact with the piezoelectric substrate.

In accordance with the present method, the dispersion is exposed to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves in other words, the dispersion is brought into contact with the substrate. This may preferably be made by dispensing the dispersion onto a piezoelectric substrate subjected to surface acoustic waves and/or surface reflected bulk waves, i.e. the substrate of a SAW device or a SRBW device, suitably in the form of drops. The drop/flow rate can for example be between 1 microliter/minute to 2 ml/minute. When the dispersion is subjected to the piezoelectric substrate of the device, water of the dispersion is split and driven to self-ionization to generate hydrogen protons and hydroxyl radicals. The hydrogen protons then combine with the fluorine ions in the dispersion to form localized hydrofluoric acid (HF). The fact that localized HF is formed may be concluded by the fact that the pH of the dispersion is temporarily reduced when subjected to the substrate of the device, despite the fact that no further additions are made to the dispersion after it has been dispensed. The localized H F then reacts with the A-atoms of the three- dimensional material resulting in M_n+iXnT_s and a fluoride hydrate of the A-atoms. The fluoride hydrate is soluble in water.

The strong mechanical vibrations associated with the SAWs results in delamination into single sheets of dispersed M_n+iXnT_s in the resulting aqueous solution. Thus, the SAWs both serve the purpose of splitting water to enable the formation of localized H F and the purpose of delamination.

According to one exemplifying embodiment, the frequency of the surface acoustic waves or surface reflected bulk waves provided to the substrate of the device during the acoustic wave treatment may be from 10 to 1000 Mz. For example, the frequency may be from 20 MFIz to 100 MFIz. Moreover, the amplitude of the surface acoustic waves or surface reflected bulk waves in the substrate of the device may be from 0.5 nm to 20 nm. The acoustic irradiation is generated by applying an input voltage to the piezoelectric substrate, the input voltage can for example be between 1- 100 Vrms.

In case the fluoride ions of the dispersion are achieved by addition of LiF, lithium ions will be intercalated between the M_n+iXnT_s layers in addition to the water. This in turn causes an increase in the interlayer spacing in the same way as previously known to occur in a conventional etching procedure using LiF and HCI. This consequently further facilitates the delamination into single sheets of suspended M_n+iXnT_s in the resulting aqueous solution.

In accordance with the present method, the amount of chemicals needed is significantly reduced compared to the conventional methods for synthesizing M_n+iXnT_s. In fact, only a low concentration of a fluorine based chemical, such as LiF, is needed to form the localized H F sufficient to remove the A- group atoms. By adequately selecting an appropriate concentration of fluoride ions in relation to the added amount of the three-dimensional material M_n+iAX_n in the aqueous solution, the amount of waste can be minimized. Theoretically, the concentration of fluoride ions should be such that the ratio between the A-group elements of the three dimensional M_n+iAX_n to fluoride atoms, i.e. A:F in the dispersion, is 1:3. Thereby, it would in theory be ensured that all of the fluoride ions of the dispersion are consumed which in turn minimizes the waste treatment needed. In practice, it may however be advantageous to use a somewhat higher concentration of fluoride ions to ensure that a sufficient amount of localized H F is formed where desired in the dispersion. Thus, the dispersion may suitably comprise a concentration of fluoride ions such that the ratio A:F is from 1:3 to 1:5, preferably the ratio A:F is from 1:3 to 1:4. It should however also be noted that a lower concentration of fluoride ions may in some situations be used, if desired, if for example seeking to avoid waste management. It is also possible to use higher concentrations of fluoride ions, such as up a ratio A:F of 1:10.

In case there is a residual amount of LiF in the product, this can for example be removed by washing with HCI, possibly followed by washing in LiCI in order to reintercalate Li ions. The concentrations of HCI and LiCI, respectively, can be fairly low, such as 1 M HCI and 1M LiCI.

The method may comprise further steps, if desired. For example, the surface terminations of the M_n+iX_nT_s dispersed in the second aqueous solution may be altered. This may be performed by any previously known method therefore, and will therefore not be further discussed in the present disclosure. Furthermore, the M_n+iXnT_s may be filtered out of the second aqueous solution. This may be performed by a conventional method therefore. Thereby, it is for example possible to obtain free standing sheets of MXene having a desired thickness. Such a thickness could for example be up to at least 5 pm. During filtering, the individual layers of MXene spontaneously form stacked structures whereby the free-standing sheets are formed. These free-standing sheets may for example be used as electrodes or any other previously proposed application for MXenes. In case there is a desire to extract a single layer MXene, this can be extracted from the second aqueous solution by any previously known method therefore, such as spin coating or drop casting.

The three-dimensional material having the formula M_n+iAX_n from which the two-dimensional material is to be produced by means of the present method may be produced in accordance with any conventional method therefore. The three-dimensional M_n+iAX_n material may suitably be in particulate form. The size of the particles should be appropriately selected to ensure that they may be sufficiently dispersed in the first aqueous solution and that the A-group elements may be selectively etched throughout the particles without risking causing undesired defects to the M-layers when subjected to the acoustic wave treatment. Suitably, the particle size of the M_n+iAX_n may be up to 50 pm. Preferably, the particle size is up to 30 pm.

It has further been found that the above described method for preparing a two-dimensional material having the formula M_n+iXnT_s can be used also for preparing MXenes comprising ordered vacancies. This may be achieved by appropriate selection of the three-dimensional material, i.e. the MAX phase, used for preparing the two-dimensional material. In such a case, the MAX phase used constitutes a MAX phase comprising two transition metals purposively selected for achieving in-plane chemical ordering of the transition metals in the M-layer of the MAX-phase. This enables removing one of the transition metals in the same step as the removal of the A-atoms, as described in W02017/204708

Al.

For the purpose of preparing a MXene having ordered vacancies, in present disclosure described with the formula (Ml_x,n_v)2CT_s, the MAX phase used is (Ml_x,M2_y)₂AIC, wherein the sum of x and y is 1 and x is from 0.60 to 0.75. Preferably x is 2/3. Furthermore, the Ml and M2 elements are purposively selected so as to achieve chemical in-plane ordering in the M-plane of the MAX phase. This may be achieved by Ml and M2 being selected in accordance with any one of the following alternatives (a)- (e): a) Ml is selected from a first group of transition metals consisting of Cr, Mo, Nb, Ta, Ti, V and W, and M2 is selected from a second group of transition metals consisting of Sc and Y; or b) Ml is Ti and M2 is selected from the group consisting of Nb, Ta, V and W; or c) Ml is Sc and M2 is either Mo or W; or d) Ml is Cr and M2 is Ta; or e) Ml is selected from the group consisting of Cr, Nb, Ta and V, and M2 is Ti For the purpose of obtaining a MXene comprising ordered vacancies, the MAX phase may preferably be selected from the group consisting of (Mo_x,Sc_y)2AIC, (Mo_x,Y_y)2AIC, (W_x,Sc_v)2AIC, (W_X,Y_V)2AIC, (Mo_x,Er_v)2AIC and (Mo_x,Ho_v)2AIC.

The ordered vacancies obtained in the two-dimensional material results from the etching out of M2 elements of the MAX-phase. This may be obtained in the same step as the etching out of the A atoms (i.e. in this case Al) by means of the localized HF formed.

As previously described, MAX phases typically comprise three elements, M, A and X, forming for example M₂AX in the case of 211 MAX phase. Figure 1 illustrates a side view of the atomic structure of a conventional 211 MAX phase. As can be seen from Figure 1, near-closed packed payers of the M- element are interleaved with pure A-group element layers, with the X atoms filling the octahedral sites between the former. By removing the A-group element layers, a MXene may be prepared. The removal of the A-group element layers is achieved by etching followed by delamination (also known as exfoliation).

Figure 2 represents a flowchart schematically illustrating one exemplifying embodiment of the method for preparing a two-dimensional material having the formula M_n+iXnT_s according to the present disclosure. The method comprises a first step, S101, of providing a dispersion comprising a three-dimensional material having the formula M_n+iAX_n dispersed in a first aqueous solution comprising fluoride ions. The method comprises a second step, S103, of exposing the dispersion (provided in step S101) to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves. Thereby, a second aqueous solution comprising dispersed M_n+iXnT_s is obtained. The method may further comprise a third step, S105, of filtering out the M_n+iXnT_s from the second aqueous solution as obtained in step S103.

Figure 3 schematically illustrates the result of the surface acoustic wave treatment of the dispersion comprising M_n+iAX_n dispersed in the first aqueous solution comprising fluoride ions. It is here exemplified for a case where the M_n+iAX_n constitutes T1₃AIC₂ (MAX phase) and the fluoride based chemical constitutes LiF. The SAWs cause self-ionization of water in the dispersion according to:

H₂O^H⁺+OH^*+e The protons then combine with the fluorine ions to form localized hydrofluoric acid (HF) which in turn reacts with the Al in the T1₃AIC₂ to produce T13C2T_S (MXene) and aluminum fluoride hydrate (AIF3.XH2O), which is soluble in water, according to:

UF + H₂O ^ HF + LiOH

T1₃AIC₂ + HF + H₂0 -> Ti₃C₂T_s + AIF₃.XH₂O + H₂

Lithium ions are intercalated between the T13C2T_S layers, in addition to water, which causes an increase in the interlayer spacing. By means of the SAWs, the individual layers are easily separated so as to be dispersed in the resulting aqueous solution.

Experimental results

A T13AIC2 MAX phase was synthesized by mixing TiC (~2 pm particle size, 99.5% purity), Ti (< 44 pm particle size, 99.9% purity), and Al (200 mesh size, 99% purity) powders, acquired from Alfa Aesar, for 5 mins in an agate mortar and pestle. The TiC:Ti:AI molar ratio was 2:1:1. The mixed powder was loaded in an alumina crucible and inserted in a MTI1700 tube furnace. The furnace was first pumped down and purged with Ar twice and then filled with Ar gas. The powder was heated up with a rate of 5 ^°C min ¹ to 1450^°C, at which it was held for 2 hrs, prior to being cooled to 50^°C at a rate of 5 ^°C min ¹. The slightly sintered MAX phase that was obtained was crushed using an agate mortar and pestle until an average particle size of 35 pm was attained.

To produce the dispersion used to produce MXene T1₃C₂T_S, 100 mg of the T1₃AIC₂ MAX phase was dispersed in an aqueous solution of 0.5 M lithium fluoride (LiF; 98+%, Sigma-Aldrich) in deionized Milli-Q water (18.2 MW cm, Merck Millipore, Bayswater, VIC, Australia, pH 6.8 ) to create a 5 mg ml^-1 MAX phase suspension. In the experiments, the dispersion was dispensed manually using a syringe, onto the surface of a SAW device, at the focal point of the SAW device, as a sessile drop.

The SAW device used consisted of a 128°-rotated Y-cut X-propagating single-crystal lithium niobate piezoelectric substrate (Roditi Ltd, London, UK) on which a focusing-elliptical single-phase unidirectional transducer (SPUDT) comprising 30 electrode finger pairs with an eccentricity of 0.83 was patterned using standard photolithographic processes. A primary function generator (SML01; Rhode & Schwarz, North Ryde, NSW, Australia) was used to generate a sinusoidal electric signal at an applied voltage of 20 Vrms, which was subsequently amplified using a high-frequency 5 W amplifier (LYZ-22+, Mini Circuits, Brooklyn, NY, USA) and applied to the SPUDT to generate a uniform SAW whose frequency f is related to its wavelength l through f = c/l, wherein c = 3995 m s ¹ is the speed at which the SAW propagates in LiNbOs. The input sinusoidal signal to the amplifier was set at a frequency f = 30 MHz to match the resonant frequency of the SPUDT, as determined by the width and spacing of its fingers (equal to l/4 wherein l = 132 pm at 30MHz). The dispensed drops of the dispersion were, when reaching the surface of the activated SAW device, immediately nebulized into aerosol droplets that contained the delaminated T1₃C₂T_S sheets.

The entire setup was enclosed within a collection tube to prevent loss of the aerosols and to facilitate ease of collection.

5 ml of the collected solution was directly used for high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). HAADF-STEM was performed in a double- corrected, monochromated FEI Titan3 60-300 operated at 300 kV. Imaging was performed with a beam convergence semi-angle of 22 mrad and a camera length of 0.185 m. The beam current during imaging was ~20 pA. Samples for HAADF-STEM were prepared by dispersing the ready material on a lacey carbon TEM grid. Fig. 4 shows HAADF-STEM imaging of the MXene obtained. A low magnification image as shown in (A) reveals that the material exhibits a two-dimensional structure, as seen by the extended areas uniform contrast that represent single or multiple stacked layers. Sheet sizes up to one micron were found. In addition, it is also possible to observe local agglomerations of nanoscale particles that appear as bright clouds in the image. The lattice resolved structure from the same area is shown in (B) in which the hexagonally close packed structure of T1₃C₂T_S is clearly visible, though the structure also contains vacancies (dark spots) and vacancy clusters (dark irregular patches). In (B), the same image is shown at higher magnification, revealing the individual atomic columns. Additionally, the sheet is locally identified to exhibit surface terminations, as evident from the apparently sparsely close-packed bright atoms across the sheet given that the column intensity is highly dependent on the local sheet alignment in relation to the e- beam.

Furthermore, 15 ml of the collected solution was sonicated for 30 mins under N₂ flow to improve the dispersion of MXene in the solution, followed by centrifugation for 10 mins at 3000 rpm. The supernatant was used to prepare a free-standing film of 34 mm diameter and 2 pm thickness for further analysis. The film was dried in air. Scanning electron microscopy (SEM) was performed on a Zeiss Supra 50 VP (Carl Zeiss SMT AG, Oberkochen, Germany). Fig. 5 shows a cross section image of the film having a thickness of 2 pm. To evaluate and quantify the MXene surface terminations, XPS measurements were performed on a free-standing film (Kratos AXIS Ultra^DLD, Manchester, UK) under monochromatic A\-K_a (1486.6 eV) radiation. The sample was mounted using carbon double tape and clamped on the sample holder and grounded with a copper strip. The X-ray beam, with a spot size of 300 x 800 pm, irradiated the surface of the sample at an angle of 45° with respect to the surface ray. The electron analyzer received photoelectrons perpendicular to the surface of the sample with an acceptance angle of ±15°.The result from the XPS measurements showed that the chemical formula Ti₃C₂0o_.8(OH)o_.4Fo_.8. 0.2H₂O_ads was obtained. The moles of surface termination groups of the Mxene produced according to the present method showed a ratio (0+OH):F of 60%. This is close to a ratio of 52% obtained for a corresponding Mxene produced by the conventional in-situ LiF+ HCI etching method.

The above results demonstrate that the present method for preparing MXene is able to eliminate the use of harmful acids, and is effective in terms of limited equipment required and minimized waste management, compared to conventional MXene synthesis methods. In addition, the product is safe to handle as the solution only temporally attains a pH value of about 4-5 upon SAW exposure, and then recovers to a value of about pH 6.8 within minutes after relaxation of the SAW. Moreover, the entire procedure is fast, reducing the synthesis time from a minimum of 24 hours with conventional synthesis methods to about 40 minutes in total for an equivalent yield. The etching of the MAX phase of the dispersion to MXene when subjected to the SAW device occurs essentially instantly, likely in the order of milliseconds or even less. Furthermore, it should be noted that the here presented results were attained by one SAW device only. Thus, the method also has a great potential for upscaling through massive parallelization.

Claims

1. A method for preparing a two-dimensional material having the formula M_n+iX_nT_s, the method comprising the steps of: providing a dispersion comprising a three-dimensional material having the formula M_n+iAX_n dispersed in a first aqueous solution comprising fluoride ions, and exposing the dispersion to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves, thereby obtaining a second aqueous solution comprising dispersed M_n+iX_nT_s; wherein n is 1, 2, 3 or higher,

M is at least one transition metal,

A is at least one A-group element,

X is at least one of C, N and O; and T_s is a surface termination.

2. The method according to claim 1, further comprising filtering the M_n+iX_nT_s from the second aqueous solution.

3. A method for preparing a two-dimensional material having the formula (Ml_x,r|_y)₂CT_s, the method comprising the steps of: providing a dispersion comprising a three-dimensional material having the formula M_n+iAX_n dispersed in a first aqueous solution comprising fluoride ions, and exposing the dispersion to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves, thereby obtaining a second aqueous solution comprising dispersed (Ml_x,n_v)₂CT_s; wherein

Ml constitutes a transition metal, h is a vacancy, and

T_s is a surface termination; and wherein the three-dimensional material having the formula M_n+iAX_n constitutes (M1_X,M2_V)₂AIC, the sum of x and y is 1, x is from 0.60 to 0.75, and wherein either Ml is selected from a first group of transition metals consisting of Cr, Mo, Nb, Ta, Ti, V and W, and M2 is selected from a second group of transition metals consisting of Sc and Y; or

Ml is Ti and M2 is selected from the group consisting of Nb, Ta, V and W; or Ml is Sc and M2 is either Mo or W; or Ml is Cr and M2 is Ta; or

Ml is selected from the group consisting of Cr, Nb, Ta and V, and M2 is Ti.

4. The method according to claim 3, further comprising filtering the (M1c,h_n^T₅ from the second aqueous solution.

5. The method according to any one of the preceding claims, wherein the step of providing a dispersion comprises mixing the three-dimensional material having the formula M_n+iAX_n, with water and a fluorine based chemical.

6. The method according to claim 5, wherein the fluorine based chemical is lithium fluoride (LiF).

7. The method according to any one of the preceding claims, wherein the dispersion comprises a concentration of fluoride ions such that the ratio A:F is from 1:3 to 1:5, preferably the ratio A:F is from 1:3 to 1:4.

8. The method according to any one of the preceding claims, wherein the step of exposing the dispersion to a substrate subjected to surface acoustic waves and/or surface reflected bulk waves comprises dispensing the dispersion onto a piezoelectric substrate subjected to surface acoustic waves and/or surface reflected bulk waves.

9. The method according to claim 8, wherein the dispersion is dispensed in the form of drops onto the piezoelectric substrate.

10. The method according to any one of the preceding claims, wherein the frequency of the surface acoustic waves and/or the surface reflected bulk waves is from 10 to 1000 MFIz.

11. The method according to any one of the preceding claims, wherein the amplitude of the surface acoustic waves and/or the surface reflected bulk waves is from 0.5 nm to 20 nm.

12. The method according to any one of the preceding claims, wherein the three-dimensional material having the formula M_n+iAX_n is in a particulate form having a particle size of up to 50 pm, preferably a particle size up to 30 pm.