FIELD OF THE INVENTION

The present invention relates in general to MXenes and in particular to a method of producing the same.

BACKGROUND OF THE INVENTION

MXenes (pronounced maxenes) are a relatively new class of two-dimensional inorganic material having a layered or sheet-like morphology made of from a few atom thick layers of transition metal carbides, nitrites or carbonitrides.

MXenes combine unique conductivity and surface chemistry properties making them eminently suitable for use in numerous applications such as energy storage, photocatalysis, water purification, sensors, electronic device s/components, biomedical, coatings and composites.

Applications for MXenes are only just now being fully realised and there is an increasing demand for their production.

There are two main methods currently used for producing MXenes, the so-called top-down approach and the bottom-up approach.

The top-down approach generally involves subjecting a precursor material commonly referred to as MAX or MAX-phase to chemical etching in a stirred reactor using aqueous hydrofluoric acid (HF) to form etched MAX-phase that can be subsequently exfoliated, typically in a separate sonication and/or solvent treatment step, to afford MXenes. While such top-down approaches can successfully produce MXenes, they are often timeconsuming, require significant energy input and afford rather poor yields (c.a. less than 20%). It is not uncommon for the etching process to be performed at high temperature for over 24+ hours, followed by the exfoliation step. Furthermore, the use of HF in the stirred reactors typically employed presents significant OH&S concerns to operators. Such OH&S concerns also make scaling up such production of MXenes problematic.

The bottom-up approach involves the formation of the MXenes from an atomic/molecular level, for example using chemical vapour deposition (CVD), template methods and plasma-enhanced pulsed laser deposition (PEPLD). While high-quality MXenes can be produced, the bottom-up approach requires the use of high temperature, high pressure and expensive equipment. The approach is also not particularly well suited for scale up.

There remains an opportunity to develop a simpler, safer and/or scalable process for producing MXenes.

SUMMARY OF THE INVENTION

The present invention provides a process for producing MXenes, the process comprising subjecting to ball milling in an inert atmosphere a reaction mixture comprising a combination of MAX-phase and a liquid comprising hydrogen fluoride, wherein the ball milling is performed using ball milling equipment that presents to the reaction mixture a surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate.

In one embodiment, the liquid comprising hydrogen fluoride is introduced into the ball mill.

In another embodiment, the hydrogen fluoride is generated in situ within the ball mill.

In a further embodiment, the hydrogen fluoride is generated in situ from a fluoride salt.

In one embodiment, the fluoride salt is selected from lithium fluoride, ammonium bifluoride and ammonium fluoride.

In a further embodiment, the hydrogen fluoride is generated in situ within the ball mill from a fluoride salt used in combination with water and/or a mineral acid.

The present invention may also be described as providing a process for producing MXenes, the process comprising subjecting to ball milling in an inert atmosphere a reaction mixture comprising a combination of MAX-phase and a liquid comprising a mixture of a fluoride salt and water, wherein the hydrogen fluoride is generated in situ within the ball mill from the fluoride salt, and wherein the ball milling is performed using ball milling equipment that presents to the reaction mixture a surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate.

The present invention may also be described as providing a process for producing MXenes, the process comprising subjecting to ball milling in an inert atmosphere a reaction mixture comprising a combination of MAX-phase and a liquid comprising a mixture of a fluoride salt and a mineral acid, wherein the hydrogen fluoride is generated in situ within the ball mill from the fluoride salt, and wherein the ball milling is performed using ball milling equipment that presents to the reaction mixture a surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate.

In one embodiment, the mineral acid is hydrochloric acid.

Surprisingly, it has now been found MXenes can be produced with excellent purity at high yield (e.g. >60%) using a unique top down approach where MAX-phase is ball milled using specific ball milling equipment under an inert atmosphere in the presence of a liquid comprising hydrogen fluoride (HF). Without wishing to be limited by theory, it is believed the ball milling process provides a unique environment in the presence of HF that promotes in situ etching of the MAX-phase to afford etched MAX-phase that is subsequently exfoliated also in situ to afford the MXenes. After ball milling, the pristine MXene product can be obtained simply by washing the reaction product, for example with water, via centrifugation.

The process in accordance with the invention can advantageously be performed in a safe and efficient manner. The ball milling apparatus can be readily sealed so as to reduce/avoid operator exposure to hazardous HF and, if desired, the HF can be generated in situ. Furthermore, the process can be readily scaled and produce MXenes in both an economic and time effective manner.

MXene production in accordance with the process of the invention can be undertaken in as little as a few hours. Without wishing to be limited by theory, it is believed the intense forces exerted on the MAX-phase by the milling balls in the unique milling environment during ball milling not only accelerates HF etching of the MAX-phase, but also exfoliation of the so formed etched MAX-phase, to produce the MXenes. The process in accordance with the invention therefore affords the MXene in a one-step ball milling action.

In other words, the hydrogen fluoride promotes formation of an etched MAX-phase within the ball mill and the so formed etched MAX-phase is exfoliated during ball milling to form the MXenes.

Accordingly, the present invention may be described as a process for producing MXenes, the process comprising subjecting to ball milling in an inert atmosphere a reaction mixture comprising a combination of MAX-phase and a liquid comprising hydrogen fluoride, wherein the ball milling is performed using ball milling equipment that presents to the reaction mixture a surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate, and wherein the hydrogen fluoride promotes formation of an etched MAX-phase within the ball mill and the so formed etched MAX-phase is exfoliated during ball milling to form the MXenes.

Further aspects and embodiments of the invention are discussed in more detail below.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will herein be described with reference to the following non-limiting drawings in which: Figure 1 illustrates an XRD pattern of ball milled MXene (Ti3C2) prepared according to Example 1. The XRD results exhibit typical characteristic peaks of the sample fabricated by the ball milling process of the invention, which is consistent with the characteristic peaks of MXene in the literature. After ball milling, the MAX phase's characteristic peaks disappeared, and the (002) peak that belonged to the MXene was shifted from 9.49° to 6.87°. In addition to the position change of the (002) peak, it also became weaker and broaden because of the decrease of particle size and thickness, which indicate that the MXene nanosheets were obtained;

Figure 2 illustrates an SEM image of ball milled MXene (TisCA) nanosheets prepared according to Example 1;

Figure 3 illustrate a TEM image of ball milled MXene (Ti sCA) nanosheets prepared according to Example 1.

Figure 4 illustrates an XRD pattern of samples after reducing the per-cycle centrifugation time in the post-treatment (Example 2: 30 min per cycle; Example 3: 10 min per cycle); and

Figure 5 illustrates an SEM image of ball milled MXene (T CS) nanosheets prepared according to Example 2 (A - left) and Example 3 (B - right); and

Figure 6 illustrates cyclic voltammetry data obtained using the MXene sample produced in Example 1.

DETAILED DESCRIPTION OF THE INVNETION

Described herein is a process for producing MXenes, the process comprising subjecting to ball milling a combination of MAX-phase and a liquid comprising hydrogen fluoride. The MXenes

As used herein, the term "MXene(s)" is intended to define a group of transition metal carbides, nitrites or carbonitrides.

The MXenes may comprise one or two transition metal(s), providing for so called “single transitional metal MXene” or a “double transition metal MXene”, respectively.

Single transition metal MXenes may be represented by the general formula M _n +iX _n , where M is at least one Group IIIB, IVB, VB, or VIB metal (i.e. groups 3-6 of the periodic table); each X is C or N (i.e. stoichiometrically X=C _x N _y , including where x+y=l); and n=l, 2, or 3. MXenes may therefore be represented by the general formula M2X, M3X2 and M4X3.

Double transition metal MXenes may be represented by the general formula M'zM' nXn+i, where M' and M" each comprise different Group IIIB, IVB, VB, or VIB metals (i.e. groups 3-6 of the periodic table); each X is C orN (i.e. stoichiometrically X=C _x N _y , including where x+y=n+l); and n=l or 2. MXenes may therefore be represented by the general formula M'2M''X ₂ and MW'^.

The present invention advantageously enables a diverse range of MXenes to be produced.

As described herein, M, M', and M" refer to one or more members of the Groups IIIB, IVB, VB, or VIB (i.e. groups 3-6 of the periodic table), either alone or in combination, said members including Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W.

Single transitional metal MXenes having the empirical formula M _n +iX _n , wherein X is C, N, or a combination thereof, and n=l, 2, or 3 gives rise to a number of possible compositions. Exemplary compositions when n=l include, without limitation, those wherein the empirical formula of the crystalline phase is SC2C, SC2N, Ti2C, Ti2N, M02C, V2C, V2N, CnC. CnN, ZnC, ZnN, Nb2C, Nb2N, H zC. and H iN. Exemplary compositions when n=2 include, without limitation, those wherein the empirical formula of the crystalline phase is Ti3C2, Ti3N2, V3C2, V3C2, Ta3C2, and Ta3N2. Exemplary compositions when n=3 include, without limitation, those wherein the empirical formula is Ti k V4C3, V4N3, Ta4C3 and Ta4N3.

Double transition metal MXenes having the empirical formula M'2M"nXn+i wherein X is X, N, or a combination thereof, and n=l or 2 gives rise to a number of possible compositions. Exemplary compositions when n=l include, without limitation, those wherein the empirical formula of the crystalline phase is Mo2TiC2, M02VC2, Mo2TaC2, Mo2NbC2, Cr ₂ TiC ₂ , Cr ₂ VC ₂ , Cr ₂ TaC ₂ , Cr ₂ NbC ₂ , Ti ₂ NbC ₂ , Ti ₂ TaC ₂ , V ₂ TaC ₂ , or V ₂ TiC ₂ . Exemplary compositions when n=2 include, without limitation, those wherein the empirical formula of the crystalline phase is Mo2Ta2C3, Cr2Ti2C3, Cr2V2C3, Cr2Nb2C3, Cr2Ta2C3, Nb2Ta2C3, Ti ₂ Nb ₂ C3, Ti ₂ Ta ₂ C3, V ₂ Ta ₂ C3, V ₂ Nb ₂ C3, or V ₂ Ti ₂ C3.

The MXenes produced may be described as comprising one or more layers of a substantially two-dimensional array of crystal cells having an empirical formula as described herein.

The expression comprising "a substantially two-dimensional array of cells” refers to a characteristic of MXene materials. The two-dimensional array of crystal cells may be viewed as an array of cells extending in an x-y plane, with the z-axis defining the thickness of the material. It may be desired that the z-dimension is defined by the dimension of approximately one crystal cell, but it should be appreciated in practice the material may have more than single crystal cell thicknesses. The top and bottom surfaces extending in the x-y plane of the array are available for chemical modification/fiinctionalisation.

Those skilled in the art will appreciate that the surface of MXenes may not be M- terminated, but rather present various functional groups depending upon their method of production. For example, MXenes produced via HF etching may present a surface replete with functional groups such as -O, -OH and -F. However, since such surface functionality is typically not known with any degree of certainty it is commonplace to define MXenes by way of empirical formulas such as M _n +iX _n and M'2M"nXn+i as herein described. Nevertheless, if relevant, it can sometimes be helpful to define MXenes by way of an empirical formula that helps designate such surface functionality. For example, the MXenes can be further defined by way of empirical formulas such as M _n +iX _n T _x and M'2M"nX _n +iT _x , where M, X, n are as herein described and T is a terminal group, for example selected from as -O, -OH and -F, and x is the number of terminal groups.

MXenes produced in accordance with the invention may present as a monolayer or a stacked monolayer assembly with two or more of such layers (i.e. multilayer). The number of layers forming a multilayer assembly is not necessarily limited to any particular value, but such a multilayer assembly may comprise between 2 to about 50 layers. In certain embodiments, MXenes produced according to the invention have a lower range of at least 2 layers, 3 layers, 4 layers, 5 layers, 10 layers, 15 layers, or 20 layers and an upper range of not more than 50 layers, 45 layers, 40 layers, 35 layers, 30 layers, 25 layers, 20 layers, 15 layers, 10 layers or 5 layers, including, without limitation, any range between a lower range and an upper range.

In one embodiment, MXenes produced in accordance with the invention present as a monolayer or a stacked monolayer assembly of no more than , 40 layers, 30 layers, 25 layers, 20 layers, 15 layers, 10 layers or 5 layers.

While there is no particular limitation on the shape of MXenes produced in accordance with the invention, it can be convenient to describe such shapes as having a major and minor planar dimension (or x-axis and y-axis dimensions, using the envisioned x-y plane as described above). For example, if a quadrilateral or pseudo-quadrilateral shape, the major and minor dimension is the length and width dimensions. In some embodiments, the ratio of the lengths of the major and minor axes is in the range of about 1 to about 10 (1: 10) to about 10 to about 1 (10: 1), about 1 to about 5 (1:5) to about 5 to about 1 (5: 1), about 1 to about 3 (1:3) to about 3 to about 1 (3: 1), or about 1 to about 2 (1:2) to about 2 to about 1 (2: 1).

In some embodiments, the thickness of the MXenes is in the range of the thickness of one monolayer to several of such layers, for example in the range of the thickness of a monolayer to 30 layers, 25 layers, 20 layers, 15 layers, 10 layers or 5 layers. In further embodiments, the thickness of the MXenes is in the range of 1 nm to 50 nm, 1 nm to 45 nm, 1 nm to 40 nm, 1 nm to 35 nm, 1 nm to 30 nm, 1 nm to 25 nm, 1 nm to 20 nm, 1 nm to 15 nm, 1 nm to 10 nm, or 1 nm to 5 nm.

Advantageously, the thickness of MXenes produced in accordance with the invention can be adjusted by variation of the ball milling parameters used, as will be discussed in more detail below.

The thickness of the MXenes can be readily determined using atomic force microscopy (AFM), scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

The MXene product produced in accordance with the invention may be provided in the form of a colloidal suspension, for example as an aqueous colloidal suspension.

As used herein, the expression MXene "colloidal suspension" is intended to mean MXene in the form of particulate matter is distributed and suspended throughout a liquid phase, for example an aqueous liquid.

The MAX Phase

The present invention provides a process for producing MXenes from MAX-phase.

The process in accordance with the invention may be described as a top-down selective etching of MAX-phase for producing MXenes.

Those skilled in the art will be familiar with conventional principles of top-down selective etching of MAX-phase for producing MXenes. For example, the MAX-phase can be viewed as a precursor material to the MXenes. A given MAX-phase itself has a layered structure with a similar composition to its related MXene, except for an additional component, typically generically known as the "A-group element", which serves to strongly bind the layers within the structure together. Subjecting MAX-phase to the HF etching removes component A to provide for a so called etched MAX-phase that has a layered structure with significantly reduced binding between the layers. The resulting etched MAX-phase can then be exfoliated to provide for the MXenes.

Without wishing to be limited by theory, the conventional mechanisms of etching MAX- phase and subsequent exfoliation of the etched MAX-phase are believed to operate in the process of the present invention. However, the application of ball milling according to the present invention has surprisingly been found to accelerate and enhance the overall production of MXenes. For example, the ball milling is believed to enhance the rate of etching of the MAX-phase and also the rate of exfoliation of the so formed etched MAX- phase to produce the MXenes with an improved yield. It is also believed ball milling enhances removal of any oxide protective layer on the MAX-phase (for example the aluminium oxide protective layer on Ti3AlC2) to enable more efficient etching of the MAX-phase. Once formed, the etched MAX-phase is then subjected to the high-speed relative motion between the ball-milling balls and the ball-milling jar that imparts shear forces to promote exfoliation of the etched MAX-phase to afford the MXene.

There is no particular limitation on the MAX-phase that may be used in accordance with the invention, provided it is susceptible to etching by HF.

Suitable MAX-phase compositions are generally recognized as comprising layered, hexagonal carbides, nitrites or carbonitrides.

Suitable MAX-phase compositions will generally have an empirical formula of Mn+iAXnor M' ₂ M"AX _n+ i, where M, M ¹ , M", X and n are as herein described. A is commonly known as the "A-group element" that is removed from the MAX-phase during etching. Suitable A-group elements include, but not limited to, Al, Si or a combination thereof.

MAX-phase compositions having an empirical formula of M _n +iAX _n or M'2M"AXn+i allow for the preparation of MXene M _n +iX _n or M zM'nXn+i, respectively.

As described above for MXene materials, M, M', and M" are transition metals, X is C or N, or a combination thereof, and n may be 1 or 2 or 1, 2, or 3 depending on the MXene to be formed.

Examples of suitable M _n +2AX _n MAX-phase compositions when n=l include, but are not limited to: SC ₂ A1C, Ti ₂ AlC, V ₂ A1C, Ti ₂ AlN, V ₂ A1N, Cr ₂ AlC, Cr ₂ AlN, Nb ₂ AlC, Ta ₂ AlC.

Examples of suitable M _n +iAX _n MAX phase compositions when n=2 include, but are not limited to: Ti3AlC ₂ , V3A1C ₂ and Ta3AlC ₂ .

Examples of suitable M _n +iAX _n MAX phase compositions when n=3 include, but are not limited to: Ti4AlN3, V4AIC3, Nb4AlC3, and Ta4AlC3.

Examples of suitable M' ₂ M" _n AX _n +i MAX phase compositions when n=l include, but are not limited to: MO ₂ T;A1C ₂ , MO ₂ VA1C ₂ , MO ₂ TaAlC ₂ , Mo ₂ NbAlC ₂ , Cr ₂ TiAlC ₂ , Cr ₂ VAlC ₂ , Cr ₂ TaAlC ₂ , Cr ₂ NbAlC ₂ , Ti ₂ NbAlC ₂ , Ti ₂ TaAlC ₂ , V ₂ TaAlC ₂ , or V ₂ TiAlC ₂ .

Examples of suitable M' ₂ M" _n AX _n +i MAX phase compositions when n=2 include, but are not limited to: Mo ₂ Ta ₂ AlC3, Cr ₂ Ti ₂ AlC3, Cr ₂ V ₂ AlC3, Cr ₂ Nb ₂ AlC3, Cr ₂ Ta ₂ AlC3, Nb ₂ Ta ₂ AlC3, Ti ₂ Nb ₂ AlC ₃ , Ti ₂ Ta ₂ AlC ₃ , V ₂ Ta ₂ AlC3, V ₂ Nb ₂ AlC3, or V ₂ Ti ₂ AlC ₃ .

MAX-phase used in accordance with the invention may be obtained by any suitable conventional means. For example, constituent components of a given MAX phase may be subjected to ball milling to promote intimate mixing and then resulting mixture can be heated to high-temperature under an inert atmosphere to produce the MAX-phase.

For avoidance of any doubt, MAX-phase used in accordance with the invention is preformed MAX-phase. In other words, the process according to the invention comprises subjecting to ball milling a combination of preformed MAX-phase and a liquid comprising hydrogen fluoride. The scope of the invention is not intended to embrace using ball milling only as step in the production of MAX-phase. The liquid comprising HF

The process in accordance with the invention comprises ball milling MAX-phase in combination with a liquid comprising HF.

There is no particular limitation on the type of liquid that may be used provided the HF is soluble in the liquid.

In one embodiment, the liquid comprising HF is an aqueous liquid comprising HF.

Such an aqueous HF composition may comprise one or more other aqueous soluble liquids, for example an aqueous soluble alcohol, ether, ketone, nitrile or amine. Should the aqueous liquid comprise one or more other aqueous soluble liquids, they will typically be present at less than 50 vol %, 40 vol %, 30 vol %, 20 vol %, 10 vol %, or 5 vol %.

The concentration of HF in the liquid used will generally range from about 0.5 wt. % to about 50 wt. %, or about 0.5 wt. % to about 20 wt. %, or about 0.5 wt. % to about 5 wt. %.

The liquid comprising the HF may be prepared prior to undertaking the ball milling and introduced into the ball mill so as to perform the invention. According to such an embodiment, the liquid comprising HF is introduced into the ball mill.

Alternatively, one or more reagents that can react so as to form HF may be introduced into the ball mill so as to produce the liquid comprising HF in situ (i.e. within the ball mill itself). According to such an embodiment, the liquid comprising HF is generated in situ within the ball mill.

Those skilled in the answer will know of suitable reagents that can be used to form HF. For example, a fluoride salt may be used to generate the HF in situ.

In one embodiment, the liquid comprising HF is generated in situ using a fluoride salt. In such an embodiment, the fluoride salt may be introduced into the ball mill in solid or liquid form. For example, an aqueous liquid comprising the fluoride salt may be introduced into the ball mill or the fluoride salt may be introduced into the ball mill that already comprises an aqueous liquid.

Where the liquid comprising HF is generated in situ using a fluoride salt, the ball mill will of course also comprise any other reagents required to promote reaction of the fluoride salt to produce HF in situ. Such reactive combination of reagents are well known to those skilled in the art.

For example, the liquid comprising HF may be generated in situ using a combination of a fluoride salt (e.g. lithium fluoride) and a mineral acid. Alternatively, the liquid comprising HF may be generated in situ using a combination of a fluoride salt (e.g. ammonium bifluoride) and water.

In one embodiment, the fluoride salt is selected from lithium fluoride, ammonium bifluoride and ammonium fluoride.

In another embodiment, the mineral acid is hydrochloric acid.

When used, and the mineral acid will generally be present in the solution at a concentration ranging from about 1 mol/L to about 12 mol/L, or about 5 mol/L to about 10 mol/L, or about 7 mol/L to about 9 mol/L.

The ball mill and milling

An important feature of the present invention is the use of ball milling to produce the MXenes.

Provided a number or parameters (discussed below) are met, the present invention can advantageously be performed using conventional ball milling equipment. Suitable ball milling equipment includes, but is not limited to, planetary ball mills, horizontal ball mills and vertical ball mills.

Importantly, ball milling according to the present invention is conducted under an inert atmosphere. Suitable inert atmospheres include, but are not limited to, nitrogen, argon, helium and combinations thereof.

In one embodiment, ball milling is conducted under an inert vacuum (i.e. where any remaining atmosphere in the vacuum is an inert atmosphere as herein described). Suitable vacuum pressures include, but are not limited to, from about -0.05 MPa to about -0.5 MPa, or -0.08 MPa to about -0.1 MPa.

Ball milling and may also be conducted at atmospheric pressure or pressures greater than atmospheric pressure under an inert atmosphere as herein described.

Without wishing to be limited by theory, it is believed ball milling of the reaction mixture under an inert atmosphere facilitates the rapid production of high-purity MXene.

The time for which ball milling is conducted may vary depending upon the ball milling equipment used, the speed at which ball milling is conducted and the amount of Max- phase/liquid comprising HF being ball milled. However, ball milling will typically be conducted for a period of time ranging from about 1 hour to about 6 hours.

In one embodiment, ball milling is conducted for a period of time ranging from about 1 hour to about 6 hours, or from about 1 hour to about 5 hours, or from about 1 hour to about 4 hours, or from about 1 hour to about 3 hours, or from about 1 hour to about 2 hours.

Notably, MXene production in accordance with the present invention may be undertaken significantly faster relative to conventional modes of production.

Ball milling may be conducted continuously or intermittently over the ball milling period.

For example, ball milling may be conducted continuously for 30 minutes, followed by a rest period of 10 minutes, with that cycle repeated over the entire ball milling period.

Where the ball milling process generates undesirable high temperatures the process may be performed by introducing a rest period of non-ball milling. That rest period of non-ball milling can assist with reducing temperature in the ball mill throughout the process.

In one embodiment, the ball milling process includes an intermittent period of non-ball milling.

As those skilled in the art will appreciate, ball milling equipment suitable for use in accordance with the invention will include a milling jar and milling balls. There is no particular limitation on the material from which the milling jar and balls are made provided they present to the reaction mixture (i.e. MAX-phase and liquid comprising HF) a surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate.

For example, the milling jar and balls may be entirely made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate. Alternatively, only the surface of the milling jar and ball that comes into contact with the reaction mixture may be made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate.

As a practical example, the milling jar and milling balls used in accordance with the invention may be made entirely from polytetrafluoroethylene. Alternatively, the milling jar may be made from steel and have an inner lining (that makes contact with the reaction mixture) made of polytetrafluoroethylene and the milling balls may be made from silicon carbide. In both cases, the milling jar and milling balls will only present to the reaction mixture a surface made of polytetrafluoroethylene/silicon carbide.

In one embodiment, the milling balls are made from silicon carbide.

Without wishing to be limited by theory, it is believed performing the ball milling under an inert atmosphere in combination with using a milling jar and balls having a reaction mixture contact surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate greatly enhances the reaction environment so as to afford MXenes in not only high-purity but also high yield.

The size of the milling jars and balls used can be readily adjusted by those skilled in the art to suit the scale and amount of reagents being used.

In one embodiment, the milling jar has a volume ranging from about 40 ml to about 450 ml, or about 50 ml to about 150 ml, or about 50 ml to about 100 ml.

Those skilled in the art will also be able to readily select the size of the milling balls to suit the task at hand, including taking into account the volume of the milling jar.

In one embodiment, the milling balls have a diameter ranging from about 2 mm to about 20 mm, or about 5 mm to about 15 mm, or about 8 mm to about 10 mm.

Those skilled in the art will also be able to readily select the milling speed to suit the task at hand, including taking into account the diameter of the milling balls.

In one embodiment, ball milling is conducted at a speed ranging from about 100 rpm to about 1000 rpm, or about 200 rpm to about 800 rpm, or about 300 rpm to about 600 rpm.

There is no particular limitation on the temperature at which the ball milling is conducted provided it suitably processes the MAX-phase and liquid comprising HF to produce the MXenes. For example, ball milling may be conducted at a temperature ranging from about 10° C to 50° C, or from about 15° C to about 45° C, or from about 20° C to about 45° C, or from about 25° C to about 45° C.

The thickness of MXene produced can advantageously be tailored by adjusting milling parameters such as the milling time and milling speed. For example, the thickness of MXene can be decreased with the increase of ball milling time while keeping the ball milling speed constant. The thickness of MXene will also be changed by a change in the ball milling speed. For example, under a condition of constant ball milling time, an increase of ball milling speed can reduce the area and thickness of MXene.

The process of producing the MXenes

The process of producing MXenes in accordance with the invention comprises subjecting to ball milling in an inert atmosphere a combination of MAX-phase and a liquid comprising HF.

As discussed herein, ball milling equipment suitable for use in accordance with the invention will typically comprise a milling jar and milling balls.

Suitable milling jars and milling balls are those that will present to the reaction mixture a surface made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate. In other words, the present invention is performed such that during the reaction mixture only comes into contact with a surface of the ball milling equipment that is made from one or more of silicon carbide, polytetrafluoroethylene, silicon nitride, zirconium oxide and agate. In combination with the inert atmosphere, the surface characteristics of those materials have surprisingly been found to not only be suitable for undertaking ball milling, but also enhance the production of high-purity MXene in high yield.

The reaction mixture comprising the MAX-phase and the liquid comprising HF will be contained within the milling jar, together with the milling balls, during the ball milling process. Liquid comprising HF may be introduced into the milling jar and/or reagents that react to produce HF may be introduced into the milling jar, with the HF being produced in situ.

As an example of HF being produced in situ, MAX-phase, fluoride salt and mineral acid can be introduced separately or together into the milling jar. The required inert atmosphere can be provided by means well known to those skilled in the art.

During ball milling and the milling jar will generally be sealed. Sealing of the jar is particularly beneficial given the potential hazardous nature of HF. Sealing of the jar also assists with maintaining the inert atmosphere.

In one embodiment, ball milling is conducted using a sealed milling jar.

In a further embodiment, ball milling is conducted using a hermetically sealed milling jar.

The amount of HF introduced into the milling jar or generated in situ relative to the amount of MAX-phase used in accordance with the invention will be sufficient to promote the required degree of etching that enables MXene to be formed. Those skilled in the art will be able to readily determine the required amount HF for a given amount of MAX- phase to use in accordance with the invention in order to form the required etched MAX- phase. Generally, the HF provided in the liquid will be in an amount that is in excess of that required to complete etching of the MAX-phase.

The MAX-phase and the liquid comprising HF are subjected to ball milling using the ball milling parameters described herein.

Not only can the process of the invention rapidly produce MXenes, the yield of MXenes is also enhanced relative to conventional top-down techniques for producing MXenes. For example, yields of at least 30 wt. %, or at least 40 wt. %, or at least 50 wt. %, or at least 60 wt. %, or at least 70 wt. %, or at least 80 wt. % can be obtained, relative to the mass of the MAX-phase processed.

The process itself can advantageously be performed in a safe and efficient manner. The ball milling apparatus can be readily sealed so as to reduce/avoid operator exposure to hazardous HF and, if desired, the HF can be generated in situ. Furthermore, the process can be readily scaled to produce large quantities of high quality MXenes in both an economic and time effective manner.

After ball milling, the MXenes produced are present in the resulting ball milled product. In other words, ball milling process represents one step procedure for producing MXenes.

Accordingly, the process may be described as producing a ball milled product comprising the MXenes.

In one embodiment, the process affords a ball milled product comprising the MXenes.

The present invention therefore also provides a process for producing MXenes, the process comprising subjecting to ball milling a combination of MAX-phase and a liquid comprising HF to afford a ball milled product comprising the MXenes.

The MXenes per se can be obtained simply by washing the ball milled product, for example with water, such as de-ionised water, via centrifugation.

The ball milled product produced in accordance with the invention will typically have quite a low pH, for example a pH<l and may contain reaction by product ions such as Li+, Cl- etc. It may therefore be desirable to treat the ball milled product in order to remove such residual impurities from the MXene rich composition. Centrifugal cleaning has been found to be a particularly useful method to remove the residual impurities and neutralize acidity in the product.

Adopting such a treatment approach, the ball milled product can be subjected to centrifugation (e.g. 8000 rpm to 15000 rpm or about 10000 rpm) until the pH approaches neutrality, for example a pH 6-7.

The resulting sample can then be centrifuged at a lower speed (e.g. 1500 rpm to 3000 rpm) to remove any un-exfoliated MAX phase or etched MAX phase. In one embodiment, the MXenes produced from the ball milling are subjected to washing process.

In a further embodiment, the washing process is conducted by centrifugation.

In another embodiment, the washing process is conducted for a period of less than one hour, or less than 30 minutes, or less than 20 minutes, or less than 15 minutes, or about 10 minutes.

The MXenes produced are of high-purity. For example, the purity of MXenes produced in accordance with the invention is at least 95 %, or at least 99 %, or at least 99%, or at least 99.99%.

The resulting MXene can be readily characterized by conventional analytical techniques such as XRD, XPS, SEM, and TEM. XRD data can be used to show that characteristic peaks belonging to MAX phase in the final product are not present and the characteristic peak (002) belonging to MXene has also shifted to a low angle, which is consistent with the data for the successful preparation of MXene in the literature. XPS testing can be used to show removal of the A layer in the final product, which is accordance with MAX phase transition to MXene. SEM and TEM data can be used to show that the prepared MXene has a clear two-dimensional microstructure, with large lateral dimensions and thin dimensions in the Z-axis. That is also consistent with the transition from bulk MAX phase to 2D MXene.

MXenes produced in accordance with the invention exhibit unique conductivity and surface chemistry properties making them eminently suitable for use in numerous applications such as energy storage, photo-catalysis, water purification, sensors, electronic devices/components, biomedical, coatings and composites.

EXAMPLES

Equipment: Planetary Ball Mill, Centrifuge Reagents: Hydrochloric acid (HC1), Lithium fluoride (LiF), Ti3AlC2, Ti2AlC Operating procedures: Directly add 0.8 g of LiF, 10 mL of 9 M HC1, and 0.5 g of Ti3AlC2 powder into a ball mill jar, then sealed the jars after adding mill balls. The sealed jar was purged with nitrogen 3 times. The milling time was controlled with 2 h 30 mins (every 30 mins stop 10 mins). The milling speed was optimized at 400 rpm. After milling, the mixture was washed with deionized H2O via centrifugation (1 h per cycle at 10000 rpm) for 5-6 cycles, until pH 6-7 was achieved.

Details of the operating parameters

Example 1:

Milling time: 2 h 30 mins (every 30 mins stop 10 mins)

Milling speed: 400 rpm

Milling atmosphere: Nitrogen

Milling jar materials: Outer material - Stainless steel

Inner lining material - Polytetrafluoroethylene

Milling ball materials: Silicon carbide (SiC)

Post-treatment parameters

Centrifugal speed: 10000 rpm

Centrifugal time: 1 h per cycle

Cycles: 5-6 time until pH = 6-7

Example 2:

Milling time: 2 h 30 mins (every 30 mins stop 10 mins)

Milling speed: 400 rpm

Milling atmosphere: Nitrogen Milling jar materials: Outer material - Stainless steel

Inner material - Polytetrafluoroethylene

Milling ball materials: Silicon carbide (SiC)

Post-treatment parameters

Centrifugal speed: 10000 rpm

Centrifugal time: 30 min per cycle

Cycles: 5-6 time until pH = 6-7

Example 3:

Milling time: 2 h 30 mins (every 30 mins stop 10 mins)

Milling speed: 400 rpm

Milling atmosphere: Nitrogen

Milling jar materials: Outer material - Stainless steel

Inner material - Polytetrafluoroethylene

Milling ball materials: Silicon carbide (SiC)

Post-treatment parameters

Centrifugal speed: 10000 rpm

Centrifugal time: 10 min per cycle

Cycles: 5-6 time until pH = 6-7

Results

XRD results (Figure 1) on the sample produced from Example 1 exhibit the typical characteristic peaks of the sample fabricated by the ball milling method, which is consistent with the characteristic peaks of MXene in the literature. After ball milling the MAX phase's characteristic peaks vanished, and the (002) peak that belonged to the MXene was shifted from 9.49° to 6.87°. In addition to the position change of the (002) peak, it also became weaker and broaden because of particle size and thickness reduction, which indicate the MXene nanosheets were obtained.

XPS analysis (Table 1) was performed to study the chemical composition and analyse the chemical state of the sample surface from Example 1. The XPS survey spectra showed the presence of titanium, carbon, oxygen, fluorine, and aluminium in the ball milled samples. After ball milling using this technique, the aluminium element has been successfully removed, thus confirming the successful preparation of MXene.

Tablel illustrates the element contents of ball milled MXene (Ti3C2).

SEM images (Figure 2) show that the ball-milled MXene nanosheets from Example 1 have larger lateral dimensions and thinner thickness, exhibiting a distinct 2D microstructure. TEM images (Figure 3) also reveal that the ball-milled MXene nanosheets have relatively thin thickness and large lateral dimensions.

XRD tests were carried out to ensure that a reduced centrifuge time process would not affect the product quality. As shown in the XRD pattern of Figure 4, after reducing the per- cycle centrifugation time in the post-treatment (Example 2: 30 min per cycle; Example 3: 10 min per cycle), the characteristic peaks belonging to the MAX phase precursors have disappeared. While the (002) peaks were shifted to the left. In addition, the SEM images (A) (Example 2) and (B) (Example 3) shown in Figure 5 demonstrate that both samples present typical two-dimensional microstructural features after reducing the post-processing time. The prepared samples both showed a large specific surface area and a thin thickness and reducing the post-treatment time did not affect the size distribution of the samples. In summary, the post-processing time has been reduced from 1 h per cycle to 30 minutes per cycle and further to 10 minutes per cycle, but it didn’t affect the metrics of the prepared MXene and significantly reduce the overall preparation time of Mxene. Cyclic voltammetry results (Figure 6) on the MXene sample produced in Example 1 show the typical characteristic redox peaks of MXene, which are similar to the peaks of MXene in the literature. The sample show a stable potential window of -0.6 to 0.2 V (vs. Ag/AgCl) in acidic electrolyte (1 M sulfuric acid). The pair of peaks at -0.4 to -0.5 V are the results of Ti surface redox reaction and the proton intercalation/de-intercalation. The results confirm that the sample has a capacitance of -226 F/g at slow scan (2mV/s) rate and -73 even at high scan rate (0.5 V/s). For electrochemical measurements including cyclic voltammetry, typically, MXene aqueous dispersion were adjusted to a concentration of ~2mg/mL. About 5 pL of the dispersion was dropped on a clean glassy carbon electrode and dried naturally, 1 pL of Nafion (3% in ethanol) was drop casted on top and dried overnight to ensure sufficient adhesion. The MXene coated electrode was cycled in IM sulfuric acid electrolyte as various scan rate (from 2 to 500 mV/s) with an Ag/AgCl reference electrode and a graphite counter electrode.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.