Challenges and opportunities in tailoring MAX phases as a starting materials for MXenes development

ABSTRACT

There is plethora of parameters to take into consideration during designing of the 2D MXene. Some of them are crucial for obtaining particular morphology others are important for purity and surface chemistry. While the latter mainly corresponds to Mene etching and delamination, the first and second ones are critical from material design point of view. In other words, how we synthesise and prepare the MAX phase truly determines the efficiency and feasibility of the process of MXene synthesis. Therefore, these two (MAX and MXene) stages are considered in this study as a step-wise aporoach in which changing of each single parameter may highly influence the final product. As can be seen, preparation of the MXene materials with repeteable features and properties is a great challenge that needs special attention in the upcoming future.

KEYWORDS:

• MXene
• max phases
• synthesis

Introduction

Since the discovery of the unique properties of graphene, the interest in two-dimensional (2D) materials has greatly increased [1]. There are already recognised basic characteristics of materials characterised by a two-dimensional (2D) crystal structure. The family of 2D materials is still expanding and can be divided into several smaller groups i.e.: transition metal dichalcogenides (e.g. MoS₂, WS₂, MoSe₂, WSe₂, MoTe₂, WTe₂) [2], nitrides (e.g. GaN, h-BN, h-AlN, h-GaN, Ca₂N) [3], di-transition metal sulphides, selenides and phosphides (e.g. Zr₂S, Hf₂S, Ti₂S, Ti₂Se, Zr₂Se, Hf₂Se, Ta₂P, Ta₂P, W₂P, Nb₂P, etc.) [3,4], organic materials (e.g. organic and metal-organic covalent frameworks) [5], metal oxides and hydroxides [6,7], Xenes (e.g. graphene, phosphorene, borophene, silicene, stanene, germanene) [8], as well as MXenes (e.g. Ti₂C, Ti₃C₂, Ti₄C₃, Ti₂N, etc.) [9].

The youngest member of this group, as well as one of the most exciting from those mentioned above, is the MXenes. They can also be named as early transition metal carbides, nitrides, carbonitrides and their hybrids. They were first discovered in 2011 by Michael Naguib, Michel Barsoum and Yuri Gogotsi from Drexel University, USA [10]. They are obtained from the so-called MAX phases [11] and are characterised by many intriguing properties [9]. The individual stages of MXene preparation are summarised in Figure 1.

Figure 1. MXene phase synthesis diagram.

In this work, we discuss the challenges in the management of MAX phase properties for future applications in MXenes technology. This journey begins with the parent MAX phases that are the starting materials for the preparation of MXenes. Methods of their synthesis and their properties can have a huge impact on the bioactive properties of their derivatives – MXenes. Also, the purity of the reactants, the phase composition of the product of synthesis, the course of the grinding process, etching as well as the delamination processes are considered to influence properties of the final 2D flakes.

The parent MAX phases

MAX phases are also named ‘Nowotny-phases’ due to the work of Hans Nowotny in the 1960s [12,13] or ternary ‘T-phases’, ‘H-phases’ or ‘Hagg-phases’ [13–15]. The group of Nowotny described over 100 carbides and nitrides. The research on Nowotny phases was boosted around the mid-1990s, when a group of Michel W. Barsoum synthesised almost single-phase Ti₃SiC₂. The innovative material was called the ‘MAX phase’ [11,16]. It should be noted that ‘MAX phase’ reflects the composition of the material i.e. M_n+1AX_n, where M is an early transition metal (e.g. Ti); A is a metal of group 13 or 14 (e.g. Al or Si); X is carbon or/and nitrogen atoms; while; n = 1, 2, and 3 or even higher. The n = 4 allows preparation of ‘514ʹ phases such as e.g. (Ti_0.5, Nb_0.5)₅AlC₄ [17]. In fact, the obtained (Ti_0.5, Nb_0.5)₅AlC₄ MAX phase is the mixture of other numerous phases, mostly of a nonlayered structure.

Structures of the MAX phases can be systematised into six individual groups of the M₂AX (phase 211), M₃AX₂ (phase 312), M₄AX₃ (phase 413), M₅AX₄ (phase 514), M₆AX₅ (phase 615), and M₇AX₆ (phase 716) [18] (the crystal structure of phases 211, 312 and 413 is shown in Figure 2). All of them can be described as hexagonal unit cells of the M₆X octahedral of the P63/mmc space group, interleaved with single 2D layers composed of the A element. It should also be noted that the M6X octahedra are twinned with respect to each other. It has been shown that the 211 phases are characterised by the lattice parameters of a ~ 3 Å and c ~ 13 Å. Whereas the 312 phases c ~ 18 Å; and c ~ 23–24 Å for the 413 phases [17]. Thus, it can be seen that these materials are generally anisotropic. Due to the presence of covalent-metal-ion bonds, MAX phases are, like ceramics, characterised by good oxidation stability, thermal stability, a relatively high melting point and high strength properties. Like metals, they show good electrical and thermal conductivity, good resistance to thermal shocks, resistance to abrasive wear and are relatively easily machined [18–21]. Such a diverse set of MAX phase properties predisposes them to many applications, i.e.: protective barriers, materials for work at elevated temperatures, abrasion-resistant materials and many others [22–26].

Figure 2. Crystal structure of phases a) 211, b) 312 and c) 413 [29.

The family of MAX phases has developed vigorously since boosting the research by Professor Barsoum and co-workers. Since then many other MAX phases have been explored e.g.: Ti₂CdC, Ti₂GaC, Ti₂GaN, Ti₂InN, Zr₂InN, or Nb₂GaC [24]. Processes for the MAX phase preparation are carried out using various techniques [25,26]. These methods can be divided into those which incorporate stoichiometric mixtures of elements (powders) [27] as well as techniques for thin films preparation [28,29]. In this study, the most interesting are the methods that allow the obtaining of MAX phases in the form of powders because the powders are the starting material for the production of MXene phases.

MAX phase preparation of the powders and bulk materials includes such methods as Hot pressing (HP) [30,31], Hot isostatic pressing (HIP) [32], Slip casting (SC) [33], Self-propagating high-temperature synthesis (SHS) [34–36], Pressure-assisted self-propagating high-temperature synthesis (PSHS) [14,37], as well as Spark Plasma Sintering (SPS) [38] and Mechanical alloying (MA) [39,40]. For all of these methods, the content of component A is extremely important. This component is reactive at high temperature and undergoes partial oxidation and evaporation, which usually forces it to be added in some excess. In Ti/Al/C systems this component is Al. The appropriate proportion of Al and the ratio of Al/C allows obtaining MAX phases with high purity. The most viable problem with their synthesis mentioned above is purity. It is, however, widely known that every single batch of MAX material is a de facto mixture of different phases of unique mass composition that is usually difficult to repeat. The mentioned methods usually involve mixing precursor powders in an appropriate molar ratio, which is then processed with the chosen method. The most widely used method for the preparation of MAX phases is the SHS method. This method allows the production of large amounts of material; however, the synthesis product is usually porous, which predisposes the SHS method to the production of powders rather than dense materials. For the preparation of the Ti₃AlC₂ MAX phase, it involves mixing elemental powders (Ti, Al and graphite) or Ti₃Al with graphite powder and subsequent pressing to enable better control over the SHS process.

However still, after ignition, the development of the highly exothermal synthesis is practically uncontrollable. The parameters of temperature and soaking time cannot be controlled and are unrepeatable [11]. It should be noted that the purity of the powders used is also a matter of concern due to the fact that every additional element will also be transferred to the final MXene [41]. Then, the equipment used needs to enable the local ignition so that the reaction can self-propagate until the substrates are fully consumed. Moreover, the final composition of the MAX material obtained after a particular process relates to many factors and also strictly depends on the applied method of synthesis (Table 1) [42]. For example, in the case of Ti₃AlC₂ and Ti₂AlC, the SHS process is the kind of equilibrium in which the thermodynamic stability of Ti₃AlC₂ and Ti₂AlC is different depending on process conditions. Bai et al. [43] revealed the reaction mechanism in which Ti₃AlC₂ is formed together with the presence of Ti₂AlC. It should be noted that the energy for TiC formation is rather low so that it forms easily. After the formation of octahedral Ti₆C two different reaction paths occur that compete thermodynamically with each other. The first one leads to the Ti₂AlC which is also thermodynamically more stable in relation to Ti₃AlC₂. Then, the Ti₂AlC directly transforms into Ti₃AlC₂ which is indeed difficult especially since this is an additional step in the reaction pathway. The second path leads from Ti₆C octahedral to TiC, which in turn transforms into Ti₃AlC₂.

Table 1. Synthesis conditions and phase composition of MAX phase. Adopted from [42].

Experiment	Conditions	Phase composition [vol. %]
	powder from SHS used in SHS-HIP	60% Ti3SiC2 40% TiC
HIP	1200°C 1 h 100 MPa	50% Ti3SiC2 50% TiC
HIP	1 800°C lh 100 MPa	20% Ti3SiC2 80% TiC
HIP	1400°C 1 h 25 MPa	75% Ti3SiC2 25% TiC
HIP	1400°C 3 h 25 MPa	82% Ti3SiC2 18% TiC
SHS-HIP	1800°C 15 min 100 MPa	30% Ti3SiC2 10% TiC
GPCS* from raw mixture	maxt. temp. 18OO ^oC, next keeping at 1400°C for 1 h 25 MPa	45% Ti3SiC2 55% TiC
GPCS* from raw mixture	max. temp. 1800°C 100 MPa	10% Ti3SiC2 70% TiC silicides

*Gas-pressure combustion synthesis

In addition to Ti₃AlC₂, a large amount of Ti₂AlC is present in the final product as well as cubic TiC. When we consider a mixture of elemental powders (i.e. Ti, Al and C), in this Ti-Al-C system the first action that occurs is the exothermic reaction between Ti and Al. Subsequently, the exothermic reaction in the Ti–C system is induced and thus generates a high temperature. Due to the presence of high temperatures, the Ti-Al liquid is formed. The previously formed TiC is dissolved and the Ti-Al liquid is further formed. Finally, the Ti₃AlC₂ is formed between the TiC and the molten Ti-Al. As can be seen, the final product consists of Ti₃AlC₂, Al₃Ti as well as TiC and the wt% of the Ti₃Al₂ is usually far below 80% [38]. Ti₃SiC₂ is the second most frequently studied system. In this case, the Ti₃SiC₂ reaction path runs through the intermediate phases. Carbon and silicon diffuse into titanium and form TiC_x and Ti₅Si₃C_x, these phases, in turn, react to form Ti₃SiC₂ [44].

The best method to control MAX phase synthesis parameters is the Spark Plasma Sintering (SPS) technique. The SPS method is usually used for reactive sintering of dense MAX phases. However, applying modifications to the die allows the obtaining of porous MAX phases that are later easier to granulate to powder. This solution makes the synthesis process very similar to the SHS method with the difference that it is possible to control the parameters of the process.

As can be seen from the literature data, summarised in Table 2, the SPS method allows the synthesis of almost pure MAX phases (of course with strict control of the contributions of individual components of the synthesis reaction). The purity of the MAX phases after the synthesis process is extremely important due to the quality of the final product, which are the MXene phases. As can be seen, all of the unwanted reaction by-products can be present in the final MAX phase. They can also be present in the MXene (if not all Al₃Ti is dissolved in hydrofluoric acid) as well as probably – in 2D flakes e.g. small amounts of cubic TiC are attached to edges of 2D flakes. Therefore, it is not only necessary to select and optimise the MAX phase synthesis parameters, but also subsequent processes. Usually, the grinding process is carried out after the synthesis step. This conducted in a rotary-vibratory or planetary mill in a chosen solvent (e.g. isopropanol) [30]. WC balls are used as a milling medium. Due to the fact that the obtained MAX phase is a hard type of material, the problem of surface abrasion of the used milling balls should be minimised. It should be, however, noted that both milling balls, as well as a container in which the product is placed, can be subjected to surface-abrasion during the milling process. Accordingly, optimisation of the grinding parameters is very important. Nevertheless, the final product is usually contaminated with materials used for grinding. In the milling process, it is also extremely important to obtain particles of the right size. In paper [45] the influence of Ti₃AlC₂ particle size on the production process of MXene phases was proved. The reduction in particle size from 53 to 38 µm allowed to reduce etching (50% HF) time from 19 to 2 hours. The same nature of changes was also observed for other MAX phases [46].

Table 2. Synthesis of Ti₃AlC₂ MAX phase using Spark Plasma Sintering (SPS) process with different conditions and starting materials. Adopted from [33].

Starting powders	Molar ratio of starting powders	Preparation before SPS	SPS parameters (temp., pressure, heating rate, dwell time)	Final product purity (in wt%)
ONE STEP SYNTHESIS
Ti, Al, C	3:1.1:1.8	Mixed in tubula mixer for 24 h	800–1450°C, 50 MPa, 50°C/min, 0–60 min	100%
Ti, Al, C	3:(1/1.1/1.2/1.3):2	High energy ball milled for 9 h	600–250°C, 35 MPa, 80°C/min, 5–10 min	99.4%
Ti, Al4C3, TiC	7:1:3	Mixed in tubula shaker for 24 h	1250–1450°C, 50 MPa, 50°C/min, 0–15 min	100%
Ti, Al, TiC	1:1:2	Mixed in tubula shaker for 24 h	800–1450°C, 50 MPa, 50°C/min, 0–15 min	99%
Ti, Al, TiC	2:2:3	Mixed in tubula shaker for 24 h	800–1450°C, 50 MPa, 50°C/min, 0–15 min	100%
TiH2, Al, TiC	1:1:1.8	Mixed in tubula shaker for 24 h	1200–1400°C, 50 MPa, 50°C/min, 0–60 min	100%
TiH2, Al, C	3:1.1:1.8	Mixed in tubula shaker for 24 h	900–1500°C, 50 MPa, 50°C/min, 0–60 min	80%
TWO STEP SYNTHESIS
Ti, Al, C	2:1:1 (SHS-SPS)	Ball milled with alcohol for 24 h	1250°C, 22 MPa, 600°C/min, 5 min	100%
Ti, Al, C	3:1.2:2 (in situ SPS)	Al4C3 sintered from mixture of Al and C, then the mixture was again sintered	1250°C, 22 MPa, 600°C/min, 5 min	Nearly 80%
TiC, Al, Si, Ti	2:1.2-x):x:1:1 where x = 0.05, 0.1, 0.2, 0.3	Mixed in ethanol for 24 h	1150–1300°C, 50 MPa, 80°C/min, 8 min	100%

The synthesis of the MAX phase’s thin films can also be carried out using several different techniques. These include physical vapour deposition (PVD) [20,47,48], chemical vapour deposition (CVD) [49,50], and Solid-state reactions (SR) [51,52]. There is also a lot of room for research on Atomic Layer Deposition (ALD) which should enable the control to the MAX phases structure on the level of layer-by-layer deposition [53,54]. The first and most commonly used method of synthesising MAX thin film is the CVD method. Already in 1972, Nickl et al. studied the possibility of the synthesis of Ti₃SiC₂ by CVD using TiCl₄-SiCl₄-CCl₄-H₂ as precursors and introduced a deposition ternary diagram at 1200°C and 100,000 Pa. The second most commonly used method for the synthesis of MAX film is the PVD method. This method involves sputtering material from a target and then depositing it on the substrate. So far, such phases as Ti₃SiC₂, Ti₃GeC₂, Ti₂AlN and V₂AlC have been obtained by PVD method [50,51,55,56].

As can be seen, the optimisation of the MAX phase’s synthesis for high repeatability is an additional challenge for technologists. This also includes the synthesis of MAX phases of thin films [57,58]. However, when taking into consideration the aspect of purity, there is no doubt that the MAX thin films prepared using e.g. PVD processes are characterised with much higher purity in comparison to bulk MAX phases. The reason is the number of processing steps to which MAX phases’ precursors and final products are subjected. In the case of the PVD process, the only critical step in which impurities can be incorporated into the final material involves the process of vapour deposition that is also connected with the purity of the starting targets.

Parent non-MAX phases

Several studies also revealed the possibility of synthesising MXenes from types of starting materials other than MAX phases. This allows a significant expansion of the number of obtainable materials with better properties than, e.g. Ti₃C₂. One of such materials is Zr₃C₂T_x which shows much higher thermal stability, up to 1200 °C, compared to Ti₃C₂T_x (800 °C) [59]. Others also analysed other MXene obtained from non-MAX phase precursors. Meshikian et al. [60] reported the preparation of the Mo₂C MXene by the etching of gallium layers from a parent Mo₂Ga₂C ternary metal carbide. It should be noted that the Mo₂Ga₂C cannot be considered as the MAX phase due to the composition of Mn+1An+1Xn instead of M_n+1AX_n. It is presumed that the Mo₂C MXene is a promising thermoelectric material [61]. These materials were also studied by other authors [62,63]. Subsequently, Zhou et al. [33] synthesised Zr₃C₂ MXene from Zr₃Al₃C₅ by etching energetically-favourable Al₃C₃ layers contrary to only aluminium. In this case, the non-MAX phase Zr₃Al₃C₅ material was characterised with a different composition i.e. M_n+2A_n+2X_n. Exciting research was presented by Zha et al. [64] They attempted to obtain Hf₃C₂ from Hf₃Al₄C₆. However, as a result of etching with HF, they obtained cubic HfC. This is due to the presence of strong Hf-C bonds. To obtain MXene, they added a small amount of Si to HF-Al-C to form Hf₃[Al(Si)]₄C₆. As a result of selective etching of [Al(Si)]₄C₄, they obtained Hf₃C₂-MXene.

Synthesis of MXenes

The synthesis of MXene phases was developed by Naguib et al. [65]. In the MAX phases, the element A is more reactive than MX; therefore, it can be etched using aqueous solution of HF acid. In the case of Ti₃AlC₂ phases, the reactions during etching occur as follows:

(1) $2T2i_{\hspace{0.17em}}232Al2C222+3HF=Al2F232+3/22H222+T2i232C22$(1)

(2) $2T2i232C_{\hspace{0.17em}}222+22H222O=T2i232C_{\hspace{0.17em}}22{\left(2OH\right)}_{\hspace{0.17em}}222+2H_{\hspace{0.17em}}22$(2)

(3) $2T2i_{\hspace{0.17em}}232C222+2HF=Ti32C_{\hspace{0.17em}}222F222+2H22$(3)

According to reaction 1, the HF reacts with Al to form AlF₃, which is then removed with distilled water during the powder washing. The Al place is occupied by functional groups -OH, = O and/or -F (reaction 2 and 3). The process of MXene phases synthesis is schematically shown in Figure 3.

Figure 3. Scheme of MXene synthesis from MAX phases [65].

The factors affecting the synthesis of MXene from MAX phases are particle size, etching time and acid concentration. All of these factors affect the efficiency of the process and the structure of the finished product. In addition, the type of MAX phases also have a great impact on the synthesis process. The number of layers n and the type of element M affect the conditions of the etching process. Higher M-A binding energy and more layers require a higher acid concentration and/or longer etching time. In the final step, two methods are used to separate the exfoliated MXene into single flakes. The first is to sonicate the exfoliated MXene in isopropanol or methanol. Due to the relatively strong bonds between the individual layers, the sonication process gives low efficiency. In addition, the obtained layers are significantly fragmented, which makes it impossible to obtain large flakes. The second method is intercalation that breaks the bonds between the layers and separates them from each other. This method has been successfully applied to the delamination of Ti₃C₂Tx by Mashtalir et al. [66]. As mentioned above, the parameters of the individual synthesis steps affect the structure and properties of the obtained MXene phases. Shortening the etching time translates into a reduction of surface oxidation and a decrease in the formation of defects. This in turn has a significant influence on the electrical conductivity of the MXene phases. In general, the smaller the defect concentration and the larger the flake size, the higher the electrical conductivity. This can be achieved through mild etching conditions and the elimination of the sonication step from the delamination process [9].

The resulting family of MXenes

MAX phases are considered as the primary starting material for MXenes. The unique feature of these materials applicable for the 2D-world is that the M-X bonds are strong of a covalent-metallic-ionic character and the M-A bonds are much weaker. According to this, these materials are described as 2D layers (having the composition of M_n+1X_n) chemically linked via layers of the A element [30]. The already identified MXenes were presented in Figure 4. Several new types of MXenes phases with not only single metal but also with several metals or with carbon and nitrogen (carbonitrides) can now be distinguished. More specifically, the original division of the MXenes has been expanded into new types of structures.

Figure 4. The family of M_n+1X_n phases characterised with the formulas of M₂X, M₃X₂, and M₄X₃ (i.e. M = early transition metal; X = C and/or N, NA = not available) were categorised as following: mono-M elements; solid solution of M elements, as well as ordered double-M elements. Also, the solid solutions of the X element result in obtaining carbonitrides (i.e. C,N) [67,68].

The family was primarily divided into three different formulas of M₂X, M₃X₂, and M₄X₃ (i.e. M = early transition metal; X = C and/or N). These correspond to the aforementioned groups of the 211, 312, and 413 stoichiometries. Subsequently, the MXenes phases can be further categorised into mono-M elements, a solid solution of two different M elements as well as ordered double-M elements. The mono-M elements correspond to the simple compositions of e.g. Ti₃C₂, Ti₂C, and Nb₄C. More complicated systems include a solid solution of a minimum of two different M elements. These include (Ti,V)₃C₂ or (Cr,V)₃C₂. The situation in which one transition metal occupies the perimeter layers and another fills the central M layers was recognised and connected with MAX phases characterised by the presence of ordered double-M elements. The examples are related to Mo₂TiC₂ or Mo₂Ti₂C₃ [9,33,68–70]. In these MAX phases, molybdenum atoms occupy the outer M layers whereas titanium atoms occupy central M layers. It should be also remembered that the additional variations of the possible compositions come from the X-element. The presence of both carbon and nitrogen in interleaving X layers results in obtaining carbonitrides e.g. Ti₃(C,N)₂.

Modification of MXenes surface

Recently, there has been a growing interest in surface modification of MXene phases. Such solutions have already been used in the case of other materials. Li and co-authors modified the surface of carbon nanotubes by electroless plating to make them magnetic [71]. In addition, Wozniak and co-authors researched surface modifications of multilayer graphene and graphene oxide by Ni-P, and Al₂O₃ coating, respectively [72–74]. The work aimed to improve composites’ mechanical and tribological properties reinforced with modified phases through a better bonding at the matrix–reinforcement interface. Similar assumptions were used by others authors for the MXene phases. Petrus et al. applied a coating containing 3% Y₂O₃ and 3% Al₂O₃ using the sol-gel method on the Ti₃C₂ surface [[75]]. The microstructure of the powder before and after the modification process is shown in Figure 5. As a result of the research, they found that the application of surface modification affects the stability of Ti₃C₂ particles in a solution of water and isopropanol (Figure 6). Coated particles had greater stability in water and slightly lower in isopropanol compared to uncoated particles. Moreover, they did not observe significant differences between the water and isopropanol stability for the coated particles.

Figure 5. Microstructure of powders a) Ti₃C₂-MXene unmodified, b) Ti₃C₂-MXene modified with Y₂O₃ and Al₂O₃ [75].

Figure 6. Zeta potential od Ti₃C₂T_x and modified Ti₃C₂T_x in different solutions [ [75]].

The use of particles with Al₂O₃/Y₂O₃ coating in composites based on SiC improved the mechanical properties compared to composites with unmodified MXene. Moreover, a change in the phase composition of the obtained composites was observed. In the case of sinters containing modified MXene, a greater content of the α-SiC phase was observed. The authors explain the increase in the content of the α-SiC phase by the presence of a liquid phase resulting from the reaction of Al₂O₃-TiO₂ and Y₂O₃-TiO₂. The mechanism of MXene degradation during the composite sintering process has also changed. Both composites with the addition of modified and unmodified MXene exhibit carbon structures resulting from the degradation of MXene. However, they are less defective in composites reinforced with modified MXene. The authors explain the change in the mechanism of MXene decomposition by the presence of oxides deposited on the MXene surfaces. Their presence is expected to increase the amount of the liquid phase and accelerate the diffusion of Ti from inside the MXene. As a result, a faster ordering of the remaining carbon structure can take place.

Similar research results were presented by Cygan at all [76]. The authors produced Al₂O₃ composites reinforced with Ti₃C₂ unmodified and covered with Mo and Ti coatings. As a result of the sintering process of Al₂O₃ with the addition of two-dimensional Ti₃C₂ phases at the temperature of 1300 °C, MXene particles were completely degraded. In addition, despite the high density of sinters, it was not possible to perform hardness and K_IC measurements using the indenter method due to the high brittleness of the material. To minimise the oxidation effect of MXene, the particles were covered with metallic, Ti, and Mo coatings. The coatings were applied with the PVD method. In the case of sinters with surface-modified MXene phases, graphite-like structures were observed apart from the TiC phase (Figure 7). This proves that the oxidation mechanism of Ti₃C₂ particles has changed. The authors explain that the formation of such structures occurs during MXene degradation without oxygen. In addition, they observed an increase in hardness of about 10% and 15% in the case of KIC for sinters with the addition of surface-modified MnSe compared to pure Al₂O₃. The improvement of mechanical properties is related to carbon structures that act as a reinforcing phase (similar to MLG).

Figure 7. Microstructure of the Al₂O₃ sinter with the addition of Ti₃C₂-Mo a-b) 1 wt.%, C) 2 wt.% and EDS analysis [76].

The basis of the presented studies can be concluded that the surface modifications of MXene phases contribute to improving their stability at elevated temperatures and allow the shape of the properties of composites with their participation.

Applications of MXenes

Due to its unique 2D structure and excellent properties, MXene is used in energy storage, biomedicine, electronics, photonics, and catalysis. There are also reports of the use of MXene as a reinforcement phase in composites based on polymer, metal and ceramic matrix [76–84]. However, a major problem in the production of composites is the low thermal stability of MXene. Despite the low thermal stability and the difficulty of preserving MXene in consolidated composites. It is possible to shape the microstructure and strength properties of composites by adding MXene.

One of the biggest and most promising areas of MXene application is energy storage. Shortly after their discovery, Naguib et al. [85] observed that Ti₂C had a reversible capacity five times higher than the MAX Ti₂AlC phase and showed a stable capacity of 225 mAhg⁻¹. Zhu et al. [86] researched structural and electronic properties of Li decorated Nb₂C and Nb₂CX₂. Obtained research results, specific capacities of 305 mAh/g, and low diffusion barrier for Li on Nb₂C, make Nb₂C a promising electrode material for Li-ion batteries. Wang et al. [87] investigated Ti₃C₂ as the S cathode host for Li-S batteries. They showed that unmodified Ti₃C₂ could not be used in Li-S batteries because of too strong interactions between the decomposition of Li₂S_n/S₈ and Ti₃C. Functional terminated Ti₃C can achieve high performance in Li-S batteries. S and O were selected as the best functional groups for Ti₃C₂ surface modification.

Recently, T₃C₂ has been extensively studied as potential material for gas and biosensors. Studies have shown that T₃C₂ can successfully detect gases such as ethanol, methanol, acetone, and ammonia at room temperature [88]. In addition, it has been shown that the functionalization of the MXene surface allows increasing the selectivity of gas sensors. Mo₂CO₂ and V₂CO₂ have been shown to have selectivity towards NO, while Nb₂CO₂ and Ti₂CO₂ towards NH₃ [89]. Liu et al. [90] investigated the use of Ti₃C₂ to produce biosensors for the detection of NO₂⁻ in water samples. Another use of MXene was proposed by Rakhi et al. [91]. They designed a biosensor to detect glucose in the presence of other electroactive substances using Au/MXene nanocomposites.

Research on MXena is also being conducted concerning the possible storage of hydrogen. Hydrogen molecules are predicted to adsorb to the MXene surface in molecular form and then dissociate to hydrides. In contrast, hydrogen molecules in the second and third surface layers are adsorbed in the molecular form. The research showed that the possible hydrogen storage capacity of Ti₂C, Sc₂C, Cr₂C, and V₂C [92–94] is about 7% by weight.

Since some MXene (Ti₂CO₂, Zr₂CO₂, Hf₂CO₂, Sc₂CO₂, and Sc₂CF₂) are semiconductors with light absorption in the visible light region and good catalytic properties, they are potential candidates for photocatalytic applications. An example of such a reaction is the production of hydrogen by splitting water into semiconductor substrates with the help of sunlight, which consists of generating an electron-hole pair (by absorbing sunlight) in a photocatalytic material [93]. Research also indicates that the use of MXen heterostructures with other semiconductors, e.g. TiO₂, can be used in various photocatalytic reactions, can significantly improve the photocatalytic activity of MXene [92].

As mentioned before, the large number of MXene that can be produced, their unique properties, and 2D structures allow them to be used in many important industries. Moreover, because these materials are relatively poorly tested. There remain a lot of possibilities for new MXene applications.

Conclusions

The MXene phases are of great interest because of their properties. However, their synthesis is still very difficult. As described above, the procedure for producing MXene is a multi-step process. Starting from the synthesis of the MAX phases to the delamination process and a number of intermediate steps. Each of these steps has a significant impact on the efficiency of the synthesis process but also on the quality of the final product. Therefore, it is necessary to optimise each of the processes and care for the purity of the product at every technological stage. The multi-stage process requires an interdisciplinary approach to the synthesis of MXene phases. Knowledge and skills in the synthesis of MAX phases and further processing processes such as delamination or exfoliation are necessary. Only careful attention to all these steps will allow the development of a high-performance process to obtain high-quality MXene phases.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The study was accomplished thanks to the funds allotted be the National Science Centre within the framework of the research project ‘OPUS 13ʹ no. UMO-2017/25/B/ST8/01205.