WO2020009627A1

WO2020009627A1 - Process for production of a substantially two-dimensional sheet of transition metal carbide, nitride or carbonitride

Info

Publication number: WO2020009627A1
Application number: PCT/SE2018/050727
Authority: WO
Inventors: Per Persson; Lars-Åke NÄSLUND
Original assignee: Per Persson; Naeslund Lars Aake
Priority date: 2018-07-03
Filing date: 2018-07-03
Publication date: 2020-01-09

Abstract

The present disclosure relates to a process for production of a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride have a reduced amount of oxygen in the surface termination thereof and which may be used for providing a substantially two-dimensional sheet with a tailored surface termination or for capturing, removing and/or storing CO_x. The present disclosure also relates to a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination and use thereof.

Description

PROCESS FOR PRODUCTION OF A SUBSTANTIALLY TWO-DIMENSIONAL SHEET OF TRANSITION

METAL CARBIDE, NITRIDE OR CARBONITRIDE

TECHNICAL FIELD

The present disclosure relates in general to a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride, and a process for production of such a substantially two-dimensional sheet. The present disclosure also relates to a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination, and a method for manufacturing such a substantially two-dimensional sheet.

BACKGROUND

MXenes are a class of substantially two-dimensional inorganic compounds which consists of a few atoms thick layers of transition metal carbides, nitrides, or carbonitrides. They may be available as free-standing structurally stable single sheets, or as stacked assemblies of such sheets. MXenes are often described with the formula M_h+iC_h, wherein M is a transition metal, X is C and/or N and n is 1, 2 or 3. However, since the surfaces of MXenes are inevitably terminated by functional groups resulting from the synthesis of the MXene, a more accurate description of a MXene may be given by the formula M_n+iXnT_s, where T_s denominates the surface terminations and comprises functional groups such as -O, -OH, -Cl, and/or -F. Furthermore, n need not necessarily be an integer as will be discussed below. MXenes are synthesised from so called MAX phases as will be described further below.

So called MAX phases, or MAX phase alloys, constitute a class of materials with the 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. The most commonly known and investigated MAX phase is Ti3AIC2. MAX phases with compositions diverging from n being an integer are also known, and 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-_dAi-_aCh±r, 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. MAX phases have a layered hexagonal crystal structure with P6s/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.

The synthesis of MXenes comprises etching of various MAX phases to thereby remove the A-atoms of the MAX phase resulting in a stacked assembly of sheets of M_h+i-_d Xn±_PT_s. The individual sheets of the stacked assembly may then, if desired, easily be separated from each other. The separation step is sometimes referred to as exfoliation or delamination. As a specific example of the synthesis of MXene, the MAX phase M2AIC (M denominating a transition metal) may be etched in hydrofluoric acid (HF), resulting in removal of the Al-layer of the MAX phase and formation of substantially two- dimensional M2C sheets which are surface terminated with -F, -O, and -OFI groups, the sheets present as a stacked assembly. The individual sheets may thereafter be separated from each other to free-standing individual two-dimensional M2C sheets. Specific examples of MXenes that have been previously synthesized include T12C, T12N, V2C, Nb2C, T13C2, T13CN, Nb^, and Ta^, each which comprises at least one surface termination resulting from the synthesis.

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 immersing in 50% concentrated

hydrofluoric solution and thereby arrived at layers of T13C2. Naguib et al concluded that the exposed Ti atoms located on the bottom and top of the T13C2 layers were saturated by -OFI or -F groups. They explained that the surface termination is likely a result of the following simplified reactions that occurs when T13AIC2 is immersed in FIF:

T1₃AIC₂ + 3HF = AIFs + 3/2H₂ + T1₃C₂ (Eq. 1)

T13C₂ + 2H₂0 = Ti₃C₂(OH)₂ + H₂ (Eq. 2)

Ti₃C₂ + 2H F = T13C₂F₂ + H₂ (Eq. 3)

WO 2014/088995 also discloses synthesis of various MXenes from corresponding MAX phases, wherein the MXenes comprises surface terminations as a result of the synthesis.

As described above, the surface termination of MXenes is a result of the chemical etching process during synthesis. The surface terminations cover the surface of the MXene sheet by being bound to the transition metal atoms. The surface termination is dependent of the etching solution used during synthesis and comprises in most cases made of -O, -OFI, and/or -F. The most frequently used etching solution comprises FIF, but etching solutions comprising for example KF may also be used in some cases. In both cases, surface terminations comprising -F groups will be present on the surface of the MXene. In some cases, the etching solution may also comprise HCI, in which case there is a high likelihood also for a surface termination comprising -Cl groups. Other functional groups may also be present as a result of the etching solution used during synthesis. The ratio of the various groups of the surface termination may be somewhat controlled by appropriate selection of the etching solution used during synthesis of the MXene. It has furthermore been proposed to modify the surface termination by subjecting the MXene to a different solution, for example by intercalation with lithium ions. Such a modification does however not lead to a removal of the already present surface terminations but merely adds additional surface terminations. Thus, even when seeking to modify the surface termination, a significant amount of oxygen containing groups will remain on the surface of the MXene sheet.

Ingmar Persson et al, "On the organization and thermal behaviour of functional groups on T C MXene surfaces in vacuum", 2D Mater. 5 (2018) 015002, reported that F can be desorbed by a thermal treatment. However, oxygen remains on the surface of the MXene sheet as a surface termination despite such a thermal treatment. Therefore, there is still a considerable amount of remaining surface terminations on the sheet of MXenes.

Despite the efforts described above, it has not been possible to completely remove the surface terminations. It can be envisaged that removing or at least significantly reducing the amount of the surface terminations of MXenes could enable development of new commercial applications for MXenes. Moreover, the presence of the surface terminations makes investigation of the true properties of the actual MXene difficult. In case the surface terminations could be removed or at least significantly reduced, investigation of the MXenes would be facilitated which in turn would increase the understanding of their potential.

Angel Morales-Garcia et al., "CO2 abatement by two-dimensional MXene carbides", Journal of Materials Chemistry A, 30 January 2018, reports CO2 storage on carbide M2C MXenes (M= Ti, Zr, Hf,

V, Nb, Ta, Mo, W) (0001) surfaces by using DFT PBE calculations including D3 Grimme correction dispersion. Based on these theoretical considerations, they predicted that the M2C MXenes would be more effective that their three dimensional bulk counterparts for CO2 capture, storage and activation. These are however only theoretical considerations, and assume a MXene surface free of surface terminations. SUMMARY

The object of the present invention is to provide a substantially two-dimensional free-standing sheet of a transition metal carbide, nitride, or carbonitride, or a stacked assembly of such sheets, which has a considerably lower amount of oxygen containing surface terminations compared to previously known substantially two-dimensional sheets of transition metal carbides, nitrides, or carbonitrides.

The object is achieved by the process according to the independent claim 1.

In accordance with the present invention, a process for production of a substantially two- dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride is provided. The process comprises the steps of:

a) synthesising a substantially two-dimensional sheet with the general formula M_n+iXnT_s by chemically etching so as to remove A atoms from a three-dimensional material with the formula M_n+iAX_n, wherein M constitutes at least one transition metal, A constitutes at least one A-group element, X constitutes C and/or N, n= 1, 2, 3 or higher, and T_s constitutes a surface termination of the substantially two-dimensional sheet;

b) placing the substantially two-dimensional sheet in an enclosed chamber, preferably in a vacuum chamber;

c) introducing a first gas into the chamber, the first gas consisting of hydrogen gas and optionally one or more inert gases, thereby desorbing oxygen from the surface of the substantially two-dimensional sheet by reaction with hydrogen so as to form H O molecules; and

d) removing H O molecules from the chamber,

Thereby, a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride, having a reduced amount of oxygen in the surface termination T_s, is obtained.

The oxygen that is desorbed from the surface in step c) above may be present in the surface termination T_s in the form of oxygen atoms, in the form of hydroxide groups or any other oxygen containing functional group on the surface.

The above-described process enables obtaining a substantially two-dimensional sheet, or a stacked assembly thereof, of a transition metal carbide, transition metal nitride, or transition metal carbonitride with a considerably lower amount of surface terminations. More specifically, a substantially two-dimensional sheet with a considerably lower amount of oxygen on its surfaces may be obtained. This enables usage of the substantially two-dimensional sheet in new applications. For example, the substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride can be provided with a tailored surface termination enabling new possible commercial applications. In other words, the process provides the possibility of obtaining a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride which can be functionalized as desired. Furthermore, the surface of the substantially two-dimensional sheet has a higher reactivity, which means that it may more efficiently adsorb atoms from for example surrounding gases in a controllable manner.

Step c) of the process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride may be performed at a temperature of 400°C - 750 °C and a hydrogen pressure of at least 2 mbar. Thereby, oxygen is efficiently desorb from the surface of the substantially two-dimensional sheet.

Step c) of the process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride may comprise continuously flowing the first gas into the chamber. Thereby, it can be ensured that a sufficient hydrogen pressure is maintained during said step and that a sufficient amount of hydrogen atoms are available for the reaction. This leads in turn to a process of higher efficiency.

The step of removing FhO molecules from the chamber may comprise continuously pumping out gaseous species from the chamber. Thereby, the risk of the surface absorbing the desorbed oxygen is minimised, which in turn leads to a more efficient process.

The process may further comprise a step of subjecting the substantially two-dimensional sheet with the general formula M_n+iXnT_s to a thermal heat treatment prior to step c) so as to desorb F atoms from the surface of the substantially two-dimensional sheet. Thereby, the amount of surface terminations is further reduced.

The present disclosure also relates to a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride, the substantially two-dimensional sheet comprising a first surface and a second surface, wherein each of the first surface and the second surface comprises less than 30 % oxygen atoms per number of surface transition metal atoms of the corresponding surface; preferably wherein each of the first surface and the second surface comprises equal to or less than about 25 % oxygen atoms per number of surface transition metal atoms of the corresponding surface. Such a substantially two-dimensional sheet is obtainable by the process as described above. This leads to the possibility for new applications. For example, such a substantially two-dimensional sheet may be used for capturing and/or storing carbon dioxide or for removing carbon dioxide from a carbon dioxide containing gas efficiently.

The present disclosure also relates to a stacked assembly comprising a plurality of the substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride as described above. Such a stacked assembly may for example be used for capturing and/or storing carbon dioxide or for removing carbon dioxide from a carbon dioxide containing gas efficiently.

Moreover, a method for manufacturing a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination T_s is provided. The method comprises:

a) performing the process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride as described above, so as to obtain a substantially two-dimensional sheet having a reduced amount of oxygen in the surface termination T_s;

b) introducing a second gas into the chamber, and allowing atoms from the second gas to react with the surface of the substantially two-dimensional sheet having a reduced amount of oxygen in the surface termination T_s, so as to provide the desired tailored surface termination on a surface of the substantially two-dimensional sheet.

Thereby, a substantially two-dimensional sheet with a purposively selected surface termination is achieved. This may for example enable new applications. Furthermore, it may in some cases improve the properties of such a two-dimensional sheet in already available or suggested applications because it enables controlling the surface termination achieved.

The second gas may for example comprise or consist of carbon dioxide. In such a case, the tailored surface termination will comprise carbon dioxide, dissociation products of carbon dioxide, or reaction products of carbon dioxide and other molecules that may be included in the second gas. Alternatively, the second gas may for example comprise or consist of nitrogen, ammonia, hydrazine, amidogen, hydrogen cyanide, or methylamine. In such a case, the tailored surface termination will comprise nitrogen.

The present disclosure further relates to a method of capturing, removing and/or storing CO_x from a CO* containing gas, wherein the method comprises:

providing an enclosure comprising the substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride disclosed above, the stacked assembly disclosed above, or a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride obtained by the process disclosed above or a stacked assembly thereof; and

introducing the CO_x containing gas into the enclosure, thereby causing CO_x to adhere to the surface of the substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride.

Thereby, capture, removal and/or storage of CO_x, such as carbon monoxide and/or carbon dioxide, can efficiently be achieved. The CO_x containing gas could for example be a biogas.

The method for capture, removal and/or storage of CO_x from a CO_x containing gas may further comprises a step of desorbing the CO_x from the surface of the substantially two-dimensional sheet of transition metal carbide, transition metal nitride or transition metal carbonitride by subjecting the two-dimensional sheet of transition metal carbide, transition metal nitride or transition metal carbonitride to a hydrogen atmosphere, preferably at a temperature of 400-750 °C and a hydrogen pressure of at least 2 mbar. Thereby, the method can for example be cycled by absorbing and desorbing CO_x multiple times as desired.

The present disclosure also relates to a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride or transition metal carbonitride having a tailored surface termination comprising nitrogen. Such a substantially two-dimensional sheet is obtainable by the method described above. This enables new applications for the substantially two-dimensional sheet. For example, the substantially two-dimensional transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination comprising nitrogen may efficiently be used in a supercapacitor or in a topological insulator. BREIF DESCRIPTION OF DRAWINGS

Fig. 1 schematically illustrates a side view of a three-dimensional 312 MAX phase.

Fig. 2 schematically illustrates a side view of a plurality of substantially two-dimensional sheets of a transition metal carbide, transition metal nitride, or transition metal carbonitride obtainable from the three-dimensional MAX phase of Fig. 1.

Fig. 3a schematically illustrates a side view of a substantially two-dimensional sheet with the

formula M_n+iXnT_s in the state obtained after chemical etching.

Fig. 3b schematically illustrates the substantially two-dimensional sheet of Fig. 3a when subjected to H2 leading to desorption of oxygen from the surface termination and formation of FI2O in the atmosphere surrounding the substantially two-dimensional sheet.

Fig. 3c schematically illustrates the resulting substantially two-dimensional sheet being essentially free from surface terminations after having being subjected to FI2.

Fig. 4 schematically illustrates a flow chart of a process, in accordance with the present invention, for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride having a reduced amount of oxygen in the surface termination.

Fig. 5 schematically illustrates a flow chart of a method, in accordance with the present invention, for manufacturing a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination.

Fig. 6a schematically illustrates a side view of a substantially two-dimensional sheet having a

reduced amount of surface terminations or being essentially free from surface terminations.

Fig. 6b schematically illustrates the substantially two-dimensional sheet of Figure 6a when subjected to ammonia leading to adsorption of nitrogen on the surface of the substantially two- dimensional sheet. Fig. 6c schematically illustrates the resulting substantially two-dimensional sheet obtained after having been subjected to ammonia thereby containing a tailored surface termination comprising or consisting of nitrogen.

Fig. 7 schematically illustrates a flow chart of a method of capturing, removing, or storing CO_x from a COx containing gas according to the present invention.

Fig. 8 schematically illustrates a flow chart of the method for capturing, removing, and/or storing COx according to one exemplifying embodiment comprising multiple steps of adsorbing and desorbing CO_x on the surface of the substantially two-dimensional sheet.

Fig. 9 illustrates results from experimental tests in terms of reduction of inherent oxygen adatoms on a MXene surface in a H atmosphere and the corresponding H O evolution.

DETAILED DESCRIPTION

The invention will in the following be described in more detail 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.

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". One example of a two-dimensional material consisting of a single layer of atoms is graphene. MXenes constitutes an example of a material consisting of a single layer of crystal cells. 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. However, 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, in the present disclosure, the term "substantially two-dimensional material" is used, which 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 or a substantially two-dimensional material shall not be considered to necessarily be flat but may for example also have a single-curved, double-curved, undulating, rolled-up, or tube shape without departing from the scope of the present invention.

Moreover, in the present disclosure, the expression "essentially free from oxygen containing surface terminations", "essentially free from any surface termination" and similar expressions are used for the purpose of defining that the respective surface terminations have been removed. It will be understood by the skilled person that said expressions are intended to clarify that the intention is that there should be no such surface terminations. In reality however, there is always a risk of a presence of an unintended small amount of surface terminations remaining on the surface of the substantially two-dimensional material, and therefore it is not accurate to say that it is necessarily completely free of surface terminations. However, when the "essentially free from ... surface termination(s)"-expressions are used in the present disclosure, it shall be considered to mean that the amount of such surface terminations shall be neglectable. In particular, this would mean less than 5 % of such surface terminations based on the amount of transition metal atoms at the corresponding surface.

In the present disclosure, the substantially two-dimensional sheet is described to have a general formula M_n+iXnT_s or M_n+iXn, wherein M constitutes at least one transition metal, A is at least one A- group element, X is at least one of C and N, and n is 1, 2, 3 or higher. It is however common general knowledge that n does not necessarily have to be an integer. The substantially two-dimensional sheet is produced from a corresponding three-dimensional with the formula M_n+iAX_n by removal of the A atoms (constituting at least one A-group element). During synthesis of a M_n+iAX_n, deviations from n being an integer may occur; for example due to unintended loss of X atoms or an excess addition of X atoms provided to compensate for such an unintended loss of X atoms, partial sublimation of A atoms, etc. Therefore, the three dimensional material is more accurately described by the formula M_h+i-_dAi-_aCh±r, 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 and N. Since the above described deviations remain when synthesising the two-dimensional material from the three- dimensional material, the two-dimensional material will in reality have the formula M_h+i-_dCh±_rT₅. Therefore, whenever the formula M_n+iXnT_s is used in the present disclosure, it shall be considered to actually mean M_h+i-_dCh±rT₅. Correspondingly, whenever the formula M_n+iXn is used in the present disclosure, it shall be considered to actually mean M_h+i-_dCh±_R. This applies mutatis mutandis in case of specific compositions being given wherein M and X are specified, for example T1₃C₂ is in reality considered to mean Tΐ3-_dq2±_R. The formulas M_n+iAX_n, M_n+iXnT_s, and M_n+iXn are simply used for facilitating the reading of the present disclosure (including the appended claims).

In accordance with the present invention, a process is provided for producing a substantially two- dimensional sheet of a transition metal carbide, a transition metal nitride, or transition metal carbonitride, said two-dimensional sheet having a reduced amount of oxygen in the surface termination. In the present disclosure, a reduced amount of oxygen shall be considered in relation to the amount of oxygen in a surface termination of a substantially two-dimensional sheet as obtained by the conventional synthesis by chemical etching of a three-dimensional material so as to remove the A-elements. The process may also be used for producing a stacked assembly of such two- dimensional sheets, wherein each sheet of the stacked assembly comprises a reduced amount of oxygen in the surface termination.

The process comprises a first step of synthesising a substantially two-dimensional sheet with the general formula M_n+iXnT_s, or a stacked assembly comprising a plurality of such two-dimensional sheets. The synthesis is conducted by chemically etching a three-dimensional material having the formula M_n+iAX_n (a so-called MAX phase) so as to remove substantially all of the A atoms. The chemical etching can be performed by means of any previously known etching solution for this purpose, such as an HF containing aqueous etching solution optionally with an addition of HCI. The chemical etching removes the A-layers of the three dimensional M_n+iAX_n, leaving a stacked assembly of two-dimensional sheets of M_n+iXnT_s wherein T_s constitutes a surface termination. A single two- dimensional sheet of the stacked assembly can easily be separated from the other two-dimensional sheets in accordance with previously known methods, thereby obtaining a single free-standing two- dimensional sheet with the general formula M_n+iXnT_s.

It should however be noted that in many practical uses, it is advantageous not to separate the substantially two-dimensional sheets of the stacked assembly from each other, but to use the stacked assembly as obtained. The subsequent steps of the present process, as will be described below, may be performed on a substantially two-dimensional sheet isolated from such a stacked assembly, or on the entire stacked assembly with its plurality of substantially two-dimensional sheet.

In the formulas given above, M constitutes at least one transition metal, A constitutes at least one A- group element, X constitutes C and/or N, and n is 1, 2, 3 or higher integer. The MAX phase may, if desired be a quartenary MAX phase in which case it may comprise two different M elements. Such a quaternary MAX phase may be described with the formula (Ml, M2)_n+iAX_n wherein Ml and M2 represents the two different transition metal elements. The MAX phase may for example also comprise more than one A-group element. Such a MAX phase may have the formula M_n+i(Al, A2)X_n wherein A1 and A2 represents the two different A-group elements. Any previously known MAX phase may be used in accordance with the present invention, as long as it can be chemically etched to remove the A-atoms so as to receive a MXene.

The two-dimensional sheet, i.e. the MXene, will inevitably comprise the surface termination after the chemical etching since the surface of the two-dimensional sheet is highly reactive. The precise composition of the surface termination depends on the etching solution used, as previously disclosed.

The three dimensional M_n+iAX_n phase material may be syntheses in accordance with any previously known method. For example, M_n+iAX_n may be synthesised by bulk synthesis wherein the constituent elements M, A and X are mixed in the intended amounts of the M_n+iAX_n phase and subjected to high temperature so as to form the M_n+iAX_n phase. Examples of such bulk synthesis methods include hot isostatic pressing (HIP), reactive sintering, self-propagating high temperature synthesis (SHS), and combustion synthesis. The M_n+iAX_n phase may also be synthesised using thin film synthesis methods, such as by physical vapour deposition (PVD) or chemical vapour deposition (CVD), if desired.

In the second step of the process according to the present invention, the substantially two- dimensional sheet with the formula M_n+iXnT_s, or a stacked assembly comprising a plurality of such substantially two-dimensional sheets, is placed in an enclosed chamber with a controlled atmosphere. The second step can be performed on a single isolated substantially two-dimensional sheet, a plurality of the substantially two-dimensional sheet being randomly oriented in relation to each other, or a plurality of the substantially two-dimensional sheet being at least partly oriented in relation to each other. A stacked assembly is one example of a plurality of the substantially two- dimensional sheet wherein the plurality of substantially two-dimensional sheets are essentially oriented in relation to each other. Thus, the second step is performed on at least one such substantially two-dimensional sheet with the formula M_n+iXnT_s. The enclosed chamber can suitably be a vacuum chamber. The controlled atmosphere could for example be obtained by evacuating the chamber in accordance with conventional processes. This placement of the at least one of the substantially two-dimensional sheet obtained from the chemical etching in an enclosed chamber is performed in order to ensure the intended reactions in the subsequent steps. Before the substantially two-dimensional sheet (or a plurality thereof) is placed in the enclosed chamber the substantially two-dimensional sheet is preferably subjected to a drying step for the purpose of easy handling and minimising the amount of species to be removed from the enclosed chamber during subsequent steps. How the drying step is performed is not critical, and drying can for example be performed in ambient air and room temperature if desired.

It should also be noted that the steps of synthesising the substantially two-dimensional sheet by chemical etching the three-dimensional material and the subsequent step of desorbing oxygen from the surface of the substantially two-dimensional sheet (as will be described further below) need not follow each other directly in time. In other words, the synthesis of the substantially two-dimensional sheet can be made and the resulting substantially two-dimensional sheet, comprising the surface terminations resulting from the etching, stored in any previously known manner, including in ambient air and room temperature, for any period of time before the subsequent steps leading to alteration of the composition of the surface termination. It may also be possible to perform other previously known steps for modification of the surface termination, such as subjecting the substantially two-dimensional sheet to various solutions, before the steps of desorbing oxygen from the surface of the substantially two-dimensional sheet, if desired.

In case the surface termination comprises -F groups, the process may also advantageously comprise a step of subjecting the two-dimensional sheet obtained from the chemical etching step (or any subsequent previously known process for alteration of the composition of the surface termination) to a heat treatment so as to remove the -F groups before the subsequent steps of desorbing oxygen containing functional groups. Said heat treatment may suitably be performed under vacuum for the purpose of avoiding any unintended reaction between the surface of the substantially two- dimensional sheet and potential surrounding gaseous species.

The heat treatment to desorb -F can be performed before placing the substantially two-dimensional sheet in the enclosed chamber, or can be performed in the above disclosed enclosed chamber in which the substantially two-dimensional sheet is placed. In other words, the heat treatment to desorb fluorine atoms from the surface is performed as a step preceding the step of desorbing oxygen from the surface and may optionally be performed in the same enclosed chamber as the following step of desorbing oxygen. Performing the desorption of -F in the same chamber as desorption of oxygen provides an easy and efficient process, and is therefore desirous for example from a cost perspective. It is however not critical that the heat treatment to desorb -F from the surface is performed in the same enclosed chamber as the step of desorbing oxygen from the surface of the substantially two-dimensional sheet. The step of removing the -F groups from the surface of the substantially two dimensional sheet may suitably be performed by heating to a temperature of 600-800 °C, preferably 650-750 °C. Such a step may also lead to a spontaneous reorganisation of oxygen atoms by oxygen atoms filling the sites where the F-atoms have been.

In a third step of the process according to the present invention, a first gas is introduced into the enclosed chamber. The first gas constitutes hydrogen gas, or a mixture of hydrogen gas and one or more inert gases, for example argon. This will lead to a presence of hydrogen atoms in the environment around the substantially two-dimensional sheet inside the enclosed chamber. The hydrogen atoms will combine with oxygen atoms of the surface terminations, irrespective of the oxygen being present at -O or -OFI, thereby forming FhO molecules. The FhO molecules will spontaneously leave the surface of the two-dimensional sheet. Thus, the introduction of hydrogen gas (alone or together with one or more inert gases) into the vacuum chamber will result in desorption of -O and possible -OFI groups from the surface of the substantially two-dimensional sheet. Thereby, the surface termination of the substantially two-dimensional sheet will have a considerably lower amount of oxygen therein as a result of being subjected to a hydrogen atmosphere as described above. Therefore, the third step efficiently removes oxygen containing surface terminations from the substantially two-dimensional sheet without in itself replacing the surface terminations with other surface terminations.

It should be noted that an auxiliary gas may also be introduced into the chamber in addition to the first gas, as long as the auxiliary gas will not react with the surface of the substantially two- dimensional sheet. By way of example, in case the first gas consists solely of hydrogen gas, one or more inert gases may be separately introduced into the chamber as auxiliary gas prior to, during and/or after the introduction of the first gas into the chamber.

The third step is suitably performed at a temperature of 400-750 °C, preferably 400-700 °C, and at a hydrogen pressure inside the enclosed chamber of at least 2 mbar. At a temperature above 750 °C, there could be a risk of damaging the substantially two-dimensional sheet due to the high temperature. Furthermore, at temperatures above 750 °C, there is a risk of risk of redistribution of elements between adjacent two-dimensional sheets (such as in a stacked assembly) increasing the risk of formation of a three dimensional material. In other words, at too high temperatures it may be more difficult to maintain the two-dimensional material property and therefore also the surfaces of the sheet. At a temperature below 400 °C, the reaction may be inferior leaving too much oxygen remaining on the surface of the substantially two-dimensional sheet. Furthermore, the amount of oxygen being removed from the surface termination increases with increased hydrogen pressure inside the enclosed chamber. Therefore, the hydrogen pressure inside the enclosed chamber may suitably be 4 mbar of higher. Hydrogen pressures up to 10 mbar, or even higher, may be used.

The third step may be performed for a duration of time sufficient to obtain the desired result, and is dependent of the temperature and the hydrogen pressure. A suitable time for the third step can however be determined by a person skilled in the art by trial and error. As a practical example, the duration of the third step could be at least a few minutes. There is no practical upper limit for the duration of the third step, but there is no reason for performing the third step for longer periods of times than a few hours as the amount of desorbed oxygen from the surface is not significant thereafter.

The introduction of the first gas into the vacuum chamber could advantageously be performed by continuously flowing the first gas into the enclosed chamber. This ensures an appropriate hydrogen pressure to assure sufficient amount of free hydrogen atoms available for reaction with surface terminations of the substantially two-dimensional sheet.

The process further comprises, in a fourth step, removing H O molecules from the chamber to ensure that said molecules will not react again with the surface of the substantially two-dimensional sheet. Removing the H O molecules could be performed continuously or intermittently.

Advantageously, the removal of H O molecules from the chamber is performed by continuously pumping out gaseous species from the chamber at the same time as the first gas is pumped into the chamber. The rate at which the gaseous species are pumped out may be selected such as to ensure that there is a remaining suitable hydrogen pressure inside the chamber for continued desorption of remaining oxygen atoms on the surface of the substantially two-dimensional sheet, in case the gaseous species are pumped out during the step of desorption of oxygen from the surface of the sheet. Alternatively, the third and the fourth step may be performed one after another and repeated multiple times.

Figure 1 schematically illustrates a side view of a three-dimensional material with the formula M_n+iAX_n, wherein M constitutes at least one transition metal, A constitutes at least one A-group element, X constitutes C and/or N. In other terms, Figure 1 schematically illustrates a side view of a MAX phase. The MAX-phase shown is a so called 312 MAX phase, which means that n=2. The most common example of such a MAX phase is Ti AIC . As shown in Figure 1, the MAX phase forms a laminated structure. Near-closed packed layers of the M element(s) are interleaved with layers of A-group element(s). The X-atoms fills octahedral sites between the M element(s).

A MXene can be produced by chemically etching a MAX phase so as to remove essentially all of the A-elements. The structure resulting from such a chemical etching of the three-dimensional material according to Figure 1 is shown in Figure 2. Figure 2 schematically illustrates a side view of three substantially two-dimensional sheets of a transition metal carbide, nitride, or carbonitride, in a stacked assembly. In Figure 2, the surface terminations have been omitted in order to more clearly show the configuration of the stacked assembly of individual two-dimensional sheets. The step of chemically etching the three-dimensional MAX phase according to the present invention can be performed in accordance with any previously known process for this purpose, as long as it removes the A-atoms.

Figures 3a-3c schematically illustrate a side view of a substantially two-dimensional sheet and the principle of a part of the process according to the present invention. In Figure 3a, the substantially two-dimensional sheet in the state obtained after the chemical etching is shown. The substantially two-dimensional sheet has the formula M_n+iXnT_s as described above. In Figure 3a, the surface termination T_s is demonstrated as oxygen atoms O only. Figure 3b illustrates the step of subjecting the substantially two-dimensional sheet shown in Figure 3a to a first gas, here illustrated as consisting solely of hydrogen gas F . Flydrogen will combine with the oxygen present on the surface and form FbO molecules. The FbO molecules will inevitably leave the surface of the substantially two- dimensional sheet. Thus, the oxygen containing surface terminations of the substantially two- dimensional sheet are desorbed from the surface of the substantially two-dimensional sheet. Figure 3c illustrates the resulting substantially two-dimensional sheet being essentially free from surface terminations. The surface of the substantially two-dimensional sheet may be described as a bare surface. It should in this context be noted that the process does not need be performed to the extent that the substantially two-dimensional sheet is essentially free from surface terminations. This is simply used in Figure 3c for ease of illustration. The substantially two-dimensional sheet may thus still comprise a reduced amount of oxygen in the surface termination, for example up to 30 % oxygen based on the number of transition metal atoms at the surface of the substantially two-dimensional sheet.

Figure 4 illustrates a flow chart of the process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride having a reduced amount of oxygen in the surface termination. The process is also applicable for producing a two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride being essentially free from oxygen containing surface terminations.

As shown in Figure 4, the process comprises the steps of: a) synthesising a substantially two-dimensional sheet with the general formula M_n+iXnT_s by chemically etching so as to remove A atoms from a three-dimensional material with the formula M_n+iAX_n, wherein M constitutes at least one transition metal, A constitutes at least one A-group element, X constitutes C and/or N, n= 1, 2, 3 or higher, and T_s constitutes a surface termination of the substantially two-dimensional sheet; (S100)

b) placing the substantially two-dimensional sheet in an enclosed chamber, preferably in a vacuum chamber; (S110)

c) introducing a first gas into the chamber, the first gas consisting of hydrogen gas and optionally one or more inert gases, thereby desorbing oxygen from the surface of the substantially two-dimensional sheet by reaction with hydrogen so as to form H O molecules; (S120) and

d) removing H O molecules from the chamber (S130).

The resulting substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride will thereby have a reduced amount of oxygen in the surface termination, or even be essentially free from oxygen in the surface termination. Therefore, the resulting substantially two-dimensional sheet will has a highly reactive surface, even more reactive than prior to the step of desorbing oxygen. If being exposed to for example ambient conditions, the surface will immediately adsorb oxygen again. Thus, in order to maintain the intended surface conditions, the substantially two-dimensional sheet should remain in a controlled environment until used as intended in order to avoid such unintended surface reactions. Such a controlled environment could for example be the controlled hydrogen environment previously disclosed, an inert gas environment, such as an argon environment, or a high vacuum environment.

The above described process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride leads to a considerable reduction of the amount of oxygen containing surface terminations. Thus, by means of the above described process, it is possible to obtain a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride, the substantially two-dimensional sheet comprising a first surface and a second surface, wherein each of the first surface and the second surface comprises less than 30 % oxygen atoms per number of surface transition metal atoms of the corresponding surface. In particular, the present process enables a substantially two- dimensional sheet wherein each of the first and second surface comprises 25 % oxygen atoms per number of surface transition metal atom of the corresponding surface or less. A substantially two- dimensional sheet having a reduced amount of oxygen containing surface terminations is particularly suitable for serving as an intermediate product in the manufacture of a substantially two- dimensional sheet a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination T_s. It may also be suitable to use for example in applications such as capture, removal, and/or storage of carbon oxides (CO_x), such as carbon dioxide.

The ability of the surface of the substantially two-dimensional sheet to adsorb other, purposively selected, atoms increases with reduced amount of surface terminations. Thus, according to one exemplifying embodiment, a two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride that is essentially free from oxygen containing surface terminations, preferably essentially free from any surface terminations, is provided.

It should be noted that in many potential commercial applications, it will be advantageous to use the above described substantially two-dimensional sheet in the form of a stacked assembly thereof. This has the advantage of providing large area surfaces. This is for example advantageous when intended to be used for CO_x capture, removal, and/or storage. For other applications, it would be

advantageous to use the resulting above described substantially two-dimensional sheet having a reduced amount of surface terminations as a single free-standing substantially two-dimensional sheet. One such example could be when the substantially two-dimensional sheet with a tailored surface termination, such as nitrogen, is to be used in a supercapacitor. Both a free-standing substantially two-dimensional sheet and a stacked assembly of such a sheet are encompassed by the scope of the present invention.

The present disclosure also relates to a method for manufacturing a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination T_s. A tailored surface termination is in the present disclosure considered to mean a surface termination that is purposively selected dependent of the intended use of the substantially two-dimensional sheet, and is thus different from an inherent surface termination resulting from the chemical etching process for synthesising the substantially two-dimensional sheet from a corresponding three-dimensional material.

The method for manufacturing a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination T_s, comprises the following steps. Firstly, the process for production of a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride as described above, so as to obtain a substantially two-dimensional sheet having a reduced amount of oxygen in the surface termination T_s, is performed. The substantially two-dimensional sheet having a reduced amount of oxygen containing surface terminations is maintained in the chamber under a controlled atmosphere since the surface of the substantially two-dimensional sheet is highly reactive and thus prone to adsorb atoms from the surrounding atmosphere. Thereafter, a second pre selected gas is introduced into the chamber. This will lead to atoms from said second gas reacting with the surface of the substantially two-dimensional sheet resulting in a tailored surface termination on the surface of the substantially two-dimensional sheet.

The second gas may for example be carbon dioxide or a gas comprising carbon dioxide. This will lead to an adsorption of carbon dioxide on the surface of the substantially two-dimensional sheet. In other words, the tailored surface termination will comprise, or consist of, carbon dioxide or dissociation products of carbon dioxide.

The second gas may alternatively comprise or consist of carbon monoxide. This will lead to an adsorption of carbon monoxide on the surface of the substantially two-dimensional sheet. As an example only, the second gas may be a gas mixture comprising both carbon dioxide and carbon monoxide if desired.

Alternatively, the second gas may comprise or consist of nitrogen gas (N₂), ammonia (NH₃), hydrazine (N₂H₄), amidogen (NH₂), hydrogen cyanide (HCN) or methylamine (CH₃N H₂). Mixtures of two or more of the aforementioned gases are also plausible. This will lead to adsorption of nitrogen atoms on the surface of the substantially two-dimensional sheet. In other words, the tailored surface termination will comprise, or even consist of, nitrogen atoms. Nitrogen gas, ammonia, hydrazine, amidogen or mixtures thereof may be preferred in case a substantially pure nitrogen containing surface termination is desired since these gases only comprise nitrogen and hydrogen. In the case of the carbon containing gases hydrogen cyanide or methylamine, there may be a possibility of obtaining a surface termination that also comprises carbon. Figure 5 illustrates a flow chart of the method for manufacturing a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination T_s. As shown in Figure 5, the method comprises the steps of: a) performing the process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride as described above, so as to obtain a substantially two- dimensional sheet having a reduced amount of oxygen in the surface termination T_s (S200);

b) introducing a second gas into the chamber, and allowing atoms from the second gas to react with the surface of the substantially two-dimensional sheet having a reduced amount of oxygen in the surface termination T_s, so as to provide the desired tailored surface termination on a surface of the substantially two-dimensional sheet (S210).

The pressure inside the chamber during the step of introducing the second gas so as to provide the tailored surface termination can be selected in dependence of the second gas used as well as the temperature inside the chamber. Flowever, since the surface of the substantially two-dimensional sheet with reduced amount of oxygen containing surface terminations is highly reactive as described above, quite moderate pressures and temperatures are sufficient. In many cases, a temperature above room temperature and pressures above 1 mbar may be sufficient. Suitably, the temperature inside the chamber when introducing the second gas could be from 80 °C up to 750 °C. The duration of said step may depend on the second gas used, the temperature, and the pressure. Flowever, a few minutes may be sufficient to obtain the desired result. In most instances, a duration of approximately 30 minutes is sufficient to provide a surface termination which is stable and which will not be altered when the substantially two-dimensional sheet with the tailored surface termination is used in the intended application. The upper limit for the duration of this step is only limited by practical and economical reasons.

Figures 6a-6c illustrate the principles of providing a tailored surface termination to the substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride. In Figure 6a, the substantially two-dimensional sheet as also shown in Figure 3c is illustrated, i.e. the substantially two-dimensional sheet with a reduced amount of surface terminations or even being essentially free from surface terminations. Figure 6b illustrates the substantially two-dimensional sheet when subjected to the second gas, in the figure illustrated by ammonia NH3. Nitrogen N from the ammonia will attach to the surface of the substantially two- dimensional sheet leaving free hydrogen gas H2 in the atmosphere surrounding the sheet. Figure 6c illustrates the resulting substantially two-dimensional sheet comprising a surface termination comprising or even consisting of nitrogen.

The above described process may therefore be used to obtain a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination comprising or even consisting of nitrogen. Such a substantially two- dimensional sheet having a tailored surface termination comprising nitrogen may suitably be used in a supercapacitor or in a topological insulator. The nitrogen containing surface termination will enable electrons to move along the surface of the substantially two-dimensional sheet while the interior of the substantially two-dimensional sheet acts as an insulator.

The present disclosure also relates to a method of capturing, removing and/or storing CO_x (such as carbon monoxide CO and/or carbon dioxide CO2) from a CO_x containing gas. A flow chart

representing this method is illustrated in Figure 7. The method comprises a step, S300, of providing an enclosure comprising a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a reduced amount of oxygen in the surface termination or being essentially free from oxygen containing surface terminations.

Preferably, a stacked assembly of a plurality of said two-dimensional sheet is provided in the enclosure. The substantially two-dimensional sheet may be obtained as described above. In such a case, the enclosure may suitably be the same enclosed chamber as the substantially two-dimensional sheet has been produced in. The substantially two-dimensional sheet may suitably be the substantially two-dimensional sheet having a first surface and a second surface, wherein each of the first surface and the second surface comprises less than 30% oxygen atoms per number of surface transition metal atom as described above.

In a second step, S310, of the method of capturing, removing and/or storing CO_xfrom a CO_x containing gas, the CO_x containing gas is introduced into the enclosure. This causes CO_x from the CO_x containing gas to adhere to the surfaces of the substantially two-dimensional sheet of the transition metal carbide, transition metal nitride, or transition metal carbonitride. Thereby, the CO_x is captured by the substantially two-dimensional sheet. This removes CO_x from the CO_x containing gas. The CO_x captured from the CO_x contain gas can thus be stored by means of the substantially two-dimensional sheet. The CO* containing gas may for example be biogas. Biogas is a gas mixture resulting from breakdown of organic matter. Biogas primarily comprises methane (CH4) and carbon dioxide (CO2) but may also usually comprise additional gaseous species dependent of the organic matter from which it is derived.

The method for capturing, removing and/or storing CO_x from a CO_x containing gas may also be used on other forms of CO_x containing gases. Examples of such other CO_x containing gases includes, but is not limited to, various flue gases.

According to an embodiment of the method for capturing, removing and/or storing CO_x, the method may optionally further comprise a step, S320, of desorbing CO_x from the surfaces of the substantially two-dimensional sheet if desired. This step may for example be a step of releasing the stored CO_x. The released CO_x may thereafter be used as desired for various industrial purposes, and the substantially two-dimensional sheet or stacked assembly thereof may be reused for capturing, removing and/or storing CO_x. The method may therefore be a cycling method, as illustrated in Figure 8, in case the captured CO_x is desorbed and new CO_x groups are captured on the surfaces of the substantially two-dimensional sheet. Alternatively, the desorption of CO_x from the surface of the substantially two-dimensional sheet may be performed only once, if desired.

The step of desorbing CO_x from the surface of the substantially two-dimensional sheet may advantageously be performed by subjecting the substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride, having CO_x captured on the surface thereof, to a hydrogen gas atmosphere. This may suitably be performed at a temperature of 400-750 °C and a hydrogen pressure of at least 2 mbar. Better results are achieved with higher hydrogen pressures. Therefore, the hydrogen pressure is preferably at least 4 mbar. This step is suitably performed in the same enclosure as the capture of the CO_x on the surface of the substantially two-dimensional sheet.

Experimental results

Experimental result 1 A substantially two-dimensional sheet of a transition metal carbide (a so called MXene) was synthesised by subjecting a bulk-synthesised T13AIC2 MAX phase to a chemical etching step in aqueous HF etching solution so as to remove the Al atoms. A single two-dimensional sheet was separated from the stacked assembly resulting from the chemical etching. The obtained MXene had a composition of Ti3C20i_.gFo_.i, wherein the oxygen and fluorine atoms constitutes surface

terminations of the sheet.

The obtained MXene was thereafter subjected to a vacuum annealing procedure at about 700 ° in order to desorb the F-groups from the surface.

Thereafter, the MXene was pre-heated and subjected to a H2 atmosphere at different pressures, ranging from 1-10 mbar, at different elevated temperatures. During this experiment, the structure was observed by high-resolution transmission electron microscopy (FIRTEM) and diffraction, and the MXene composition was followed by electron energy loss spectroscopy (EELS). Furthermore, the residual gas was monitored using a residual gas analyser (RGA). The MXene sheet remained intact throughout the experiment.

Figure 9 illustrates parts of the result. The figure illustrates the reduction of the inherent O adatoms on the MXene surface in a H2 atmosphere and the corresponding H2O evolution. It can be seen that while the Ti:0 ratio increases because of the O adatoms desorption, H2O is formed in the O+H2 ->Fl₂0 conversion and detected by the RGA.

The results obtained also for example showed that the original composition of Ti₃C₂0i_.gFo_.i was reduced to T13C2O0.3 after 2 h of 8 mbar H2 flow at 500 °C.

The above-disclosed results demonstrate that elevated temperatures and H2 environments are efficient in removing the inherent O adatoms from the MXene surface.

Experimental result 2

The MXene T13C2O0.3 obtained in Experimental result 1 by exposure to 8 mbar H2 flow at 500 °C during 2 h was subsequently subjected to CO2 immediately following the H2 atmosphere. The MXene was subsequently cooled to ambient temperatures (no applied heating). Successful adsorption of CO2 on the MXene surface was achieved at ambient temperature conditions and at 2 mbar CO2.

Presumably, CO2 would adsorb on the surfaces at both higher and lower temperatures and at higher and lower pressures. After 30 minutes exposure to CO2, the composition of the structure was measured and found to be approximately Tii:Ci:Oi, which suggests complete saturation of the MXene surfaces by CO2. The saturation was presumably achieved more or less immediately after the exposure onset, although the total exposure was 30 min.

Claims

1. Process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride, the process comprising the steps of a) synthesising a substantially two-dimensional sheet with the general formula M_n+iXnT_s by chemically etching so as to remove A atoms from a three-dimensional material with the formula M_n+iAX_n, wherein M constitutes at least one transition metal, A constitutes at least one A-group element, X constitutes C and/or N, n= 1, 2, 3 or higher, and T_s constitutes a surface termination of the substantially two-dimensional sheet;

d) removing H O molecules from the chamber,

thereby obtaining a substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride, the substantially two- dimensional sheet having a reduced amount of oxygen in the surface termination T_s.

2. The process according to claim 1, wherein step c) is performed at a temperature of 400°C - 750 °C and a hydrogen pressure of at least 2 mbar.

3. The process according to any one of claims 1 and 2, wherein step c) comprises continuously flowing the first gas into the chamber.

4. The process according to any one of the preceding claims, wherein the step of removing H O molecules from the chamber comprises continuously pumping out gaseous species from the chamber.

5. The process according to any one of the preceding claims, further comprising subjecting the substantially two-dimensional sheet with the general formula M_n+iXnT_s to a thermal heat treatment prior to step c) so as to desorb F atoms from the surface of the substantially two- dimensional sheet.

6. A substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride, the substantially two-dimensional sheet comprising a first surface and a second surface, wherein each of the first surface and the second surface comprises less than 30 % oxygen atoms per number of transition metal atoms of the corresponding surface; preferably wherein each of the first surface and the second surface comprises equal to or less than about 25 % oxygen atoms per number of transition metal atoms of the corresponding surface.

7. A stacked assembly comprising a plurality of the substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride according to claim 6.

8. Method for manufacturing a substantially two-dimensional sheet of a transition metal

carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination T_s, the method comprising:

a) performing the process for production of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride according to any one of claims 1-5, so as to obtain a

substantially two-dimensional sheet having a reduced amount of oxygen in the surface termination T_s;

9. Method according to claim 8, wherein the second gas comprises or consists of carbon

dioxide.

10. Method according to claim 8, wherein the second gas comprises or consists of nitrogen, ammonia, hydrazine, amidogen, hydrogen cyanide, or methylamine.

11. Method of capturing, removing, and/or storing CO_x from a CO_x containing gas, the method comprising: providing an enclosure comprising the substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride according to claim 6, the stacked assembly according to claim 7, or a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride obtained by the process according to any one of claims 1-5 or a stacked assembly thereof; and

12. Method according to claim 11, wherein the CO_x containing gas comprises biogas.

13. Method according to any one of claims 11 or 12, further comprising a step of desorbing the CO_x from the surface of the substantially two-dimensional sheet of transition metal carbide, transition metal nitride or transition metal carbonitride by subjecting the two-dimensional sheet of transition metal carbide, transition metal nitride or transition metal carbonitride to a hydrogen atmosphere, preferably at a temperature of 400-750 °C and a hydrogen pressure of at least 2 mbar.

14. A substantially two-dimensional sheet of a transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination comprising nitrogen.

15. Use of a substantially two-dimensional transition metal carbide, transition metal nitride, or transition metal carbonitride having a tailored surface termination comprising nitrogen in a supercapacitor or in a topological insulator.

16. Use of a substantially two-dimensional sheet of transition metal carbide, transition metal nitride, or transition metal carbonitride according to claim 6, or stacked assembly according to claim 7, for capturing and/or storing carbon dioxide or for removing carbon dioxide from a carbon dioxide containing gas.