Strategies to Prepare 2D MXenes

Abstract

MXenes have rapidly become a key focus in contemporary materials research, particularly because of their unique structural and electrical properties since their discovery. Consequently, they find application across various domains such as supercapacitors, batteries, and hydrogen storage. However, a significant challenge lies in optimizing existing methodologies or devising novel ones to enable high-quality, cost-effective, and scalable production, thereby broadening their potential applications. This section delves into the diverse range of top-down and bottom-up approaches employed for the production of 2D MXenes today. In addition to conventional wet-chemical synthesis methods, it elucidates innovative techniques like halogen etching, thermal reduction, UV-induced selective etching, and algae extraction, which have surfaced in recent years. Furthermore, it provides intricate insights into the optimization of process parameters within the chemical vapor deposition method utilized for synthesizing 2D MXenes.

2.1 Introduction

Since the synthesis of the first MXene in 2011, its demand has been increasing. MXenes have been used in various applications such as batteries, supercapacitors, hydrogen storage, and biosensors due to their high electrical conductivity, high volumetric electrochemical capacitance, adjustable band gap, high thermal conductivity, and high strength resistance [1, 2]. MXenes are characterized by the chemical formula M_n+1X_nT_x (M = Ti, V, Mo, Hf, Cr, etc.; X = C, N; T = –OH, –O, –F, etc.) and can be synthesized using a variety of top-down and bottom-up methods. Presently, top-down methods are favored due to their low cost, ease of use, and scalability for large-scale 2D MXene synthesis. However, the quality of products synthesized using top-down methods is often lower, leading to the preference for bottom-up methods in producing high-quality 2D MXenes, albeit at a higher complexity, cost, and smaller-scale production [3]. In this chapter, 2D MXene preparation strategies are described under two sections as top-down methods and bottom-up methods.

2.2 Top-Down Methods

Top-down methods stand out particularly in terms of being used in the industrial field, as they allow products to be synthesized on a larger scale and at lower cost. For the synthesis of 2D M_n+1X_nT_x MXenes with these methods, the layers of A atoms are removed from the M_n+1AX_n MAX phase (A = Al, Si, Zn, etc.) by selective etching. Top-down methods are presented here under the headings of wet-chemical synthesis of 2D MXene and other advanced top-down methods as well as fabrication of 2D MXene films, respectively. The wet-chemical synthesis part was examined under four subtitles, respectively, as fluorine-based acid etchants, fluorine-free base etchants, molten-salt etchants, and electrochemical etching.

2.2.1 Wet-Chemical Synthesis of 2D MXene

2.2.1.1 Fluorine-Based Acid Etchants

One of the frequently used methods for the synthesis of MXene from the MAX phase is the selective etching of A atoms using HF solvent as described in Fig. 2.1a. In 2011, Naguib et al. synthesized nanometer-thick Ti₃C₂ MXene crystal layers for the first time by removing Al from the Ti₃AlC₂ MAX phase [4]. The Al atoms in the MAX phase, which act as a bridge between the Ti₃C₂ layers and connect the layers with metallic bonds, were etched using HF solvent. Accordingly, it was observed that dangling bonds terminated with –OH or –F surface groups were formed in the synthesized 2D Ti₃C₂ layers. The chemical reactions occurring in the process are shown in Eqs. 2.1–2.3. After centrifugation, 2D layers were obtained by replacing stronger metallic bonds in the layered structure with weak van der Waals bonds.

$$\text{Ti}_{3}\text{AlC}_{2}+3\text{HF}\to \text{AlF}_{3}+(3/2)\text{H}_{2}+\text{Ti}_{3}\text{C}_{2}$$

(2.1)

$$\text{Ti}_{3}\text{C}_{2}+2\text{H}_{2}\text{O}\to \text{Ti}_{3}\text{C}_{2}{(\text{OH})}_{2}+\text{H}_{2}$$

(2.2)

$$\text{Ti}_{3}\text{C}_{2}+2\text{HF}\to \text{Ti}_{3}\text{C}_{2}\text{F}_{2}+\text{H}_{2}$$

(2.3)

As a further study to optimize the standard HF-based etching method and examine the underlying mechanism, Chang et al. observed the effect of HF treatment time on the etching process [5]. Accordingly, after 20 h of HF treatment, it was observed that the Ti₃AlC₂ MAX phase transformed into Ti₃C₂ MXene phase with a layered structure as a result of the selective etching of Al atoms, as shown in Fig. 2.1b–d. In addition, the synthesis of the MXene phase was proven with the mostly disappearance of the peaks of the Ti₃AlC₂ MAX phase after 20 h of treatment in the XRD pattern given in Fig. 2.1e. In another study, Liu et al. observed that the performance of the etching process increased due to the higher solubility of the HF solvent as a result of increasing the etching temperature [6]. In addition, it was observed that the number of bonds terminated with –OH and –F groups is higher in etching processes at higher temperatures, due to greater ion mobility.

Fig. 2.1

a Schematic representation of MXene synthesis from the MAX phase using HF [7]. SEM images showing unetched and etched MAX phases; b before etching process, c after 20 h, and d high-resolution image of the etched sample, respectively. e XRD pattern of unetched and etched MAX phases. Adapted with permission from [5]. Copyright (2013), Copyright The Authors, some rights reserved; exclusive licensee [Elsevier]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/

As a variant of 2D MXene, Hf₃C₂T_x (T_x; –OH, –O, –F groups), few-layered flakes with the 2–5 μm lateral dimensions were synthesized by Zhou et al. using HF from the Hf₃(Al-Si)₄C₆ MAX phase having the A layers doped with Si [8]. With this method different from the standard HF-based etching, the length of the bonds between the Hf-C atoms increased and weakened due to the doping of Si atoms, resulting in stronger bonds between the C-Al atoms with the adhesive energy decreasing at the etching interface which increased the efficiency of the etching process. As a result, Hf₃C₂T_x MXene flakes were obtained by removing Al₄C₄ molecules from the structure.

In addition to using HF acid etchant directly, it can be formed as a product by using a mixture of HCl and LiF acid as shown in Eq. (2.4). In 2015, Anasori et al. examined the MXene structures of about 20 double transition metals carbides using density functional theory (DFT) calculations and synthesized Mo₂TiC₂Tx, Mo₂Ti₂C₃T_x, and Cr₂TiC_xT_x double metal carbide 2D MXene flakes by etching with a mixture of HCl and LiF solvents followed by delamination including DMSO intercalation and sonication steps [9]. Cr₂TiC₂T_x and Mo₂Ti₂C₃T_x MXene flakes synthesized with this method were found to contain more contamination compared to Mo₂TiC₂T_x MXene flakes.

$$\text{LiF}+\text{HCl}\to \text{HF}+\text{LiCl}$$

(2.4)

To investigate the electrochemical and heterogeneous electron transfer properties of MXenes, Nayak et al. synthesized few layer and multilayer 2D Ti₃C₂T_x (T_x; –OH, –O, –F groups) MXene crystals using two methods named Clay and minimum intensive layer delamination (MILD), as depicted in Fig. 2.2a–b [10]. In the Clay method, the Ti₃AlC₂ MAX phase was etched using a mixture of LiF and HCl solvent to obtain the bulk Ti₃C₂T_x phase, and then the ultrasonication method was used to break the weak bonds between the layers. The MILD method was carried out to synthesize few-layered MXene phases with simple shaking instead of sonification, using a mixture of LiF and HCl solvent with increased LiF ratio in the etching process. As a result, multilayer (10–13 layered) Ti₃C₂T_x MXene flakes were obtained in the clay method, while few layer (1–2 layered) Ti₃C₂T_x MXene flakes were obtained in the MILD method (Fig. 2.2c–f).

Fig. 2.2

SEM images of a few-layered and b multilayered MXene flakes of varying thickness, shown in c, d AFM images and e, f thickness distributions [10]. g Raman spectra of Ti₃AlC₂ and Ti₃C₂ after etching [13]. h Diagrams illustrating a plasma treatment-assisted DBD plasma system method used to obtain nitrogen-doped Ti₃C₂T_x, adapted with permission from [14]. Copyright (2021), Copyright: The Authors, some rights reserved; exclusive licensee [Elsevier]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/, and i an etching process assisted by SAWs to synthesize Ti₃C₂T_x MXene flakes [15]

Furthermore, Shayesteh et al. developed the evaporated-nitrogen MILD (EN-MILD) process by adding dry nitrogen to the system to improve the MILD method and increase the stability, capacitance, and electrical conductivity of MXene flakes [11]. By using nitrogen environment in EN-MILD method, the acid concentration was increased by providing partial evaporation of the LiF and HCl solvent mixture used for etching, and hence, etching and delamination processes could be performed with higher efficiency. Moreover, by increasing the etching time, high-efficiency etching was achieved even using a lower concentration solvent mixture. In addition, it was observed that the number of defects in the synthesized multilayer MXene flakes was lower while the sizes of the flakes were higher, and the number of TiO₂ intermediates formed during the synthesis was significantly reduced in the EN-MILD method compared to the MILD method.

Due to the low solubility of the LiF solvent in water and therefore the increased etching time, a high temperature is required to dissolve in the etching solution. Accordingly, when synthesizing 2D Mo₂CT_x (T_x; –OH, –O, –F groups) MXene flakes, Guo et al. decreased the etching time from several days to 24 h by using NaF, KF, and NH₄F solvents at lower temperatures than the solubility temperature of LiF [12]. Further, Wang et al. synthesized Ti₃C₂T_x MXene flakes from Ti₃AlC₂ MAX phase via etching using NH₄F solvent in aqueous solution [13]. The water in the aqueous solution reacted with the NH₄F solvent and formed NH₃ and HF. It has been reported that etching is carried out when the HF compound reacts with Al atoms in the MAX phase, which is proved with Raman spectra as shown in Fig. 2.2g. Etching reactions were also observed to be more effective by increasing the NH₄F concentration or the reaction temperature or the reaction time for the synthesized few-layered Ti₃C₂T_x flakes.

In another study, Chen et al. obtained nitrogen-doped Ti₃C₂T_x MXene flakes using a plasma treatment-assisted dielectric barrier discharge (DBD) plasma system at low temperatures [14] (Fig. 2.2h). Multilayer Ti₃C₂T_x MXene flakes were synthesized from Ti₃AlC₂ MAX phase by etching using a solvent mixture of LiF and HCl, and then NH₄HCO₃/Ti₃C₂T_x MXene flakes were obtained by immersing the MXene flakes in the saturated NH₄HCO₃ solution and allowing NH₄HCO₃ to enter between Ti₃C₂T_x layers. After that, nitrogen-doped Ti₃C₂T_x MXene flakes were obtained by using NH₄HCO₃/Ti₃C₂T_x MXene flakes in DBD plasma system with a mixture of NH₃ and Ar gas as plasma gas. Accordingly, the NH₄HCO₃ group, which is connected by weak bonds between the layers, decomposes into NH₃, CO_2, and H₂O with the effect of heat during the DBD plasma treatment. As a result, the layers are easily and quickly separated from each other and 2D MXene flakes can be obtained. At the same time, nitrogen atoms join the dangling bonds on the surfaces, forming bonds that terminate with nitrogen. Also, the number of active nucleation sites increases as a result of Ar ions bombarding the surface of the MXene flakes, and the nitrogen doping is controlled by adjusting the concentration of the NH₃/Ar gas mixture.

A reduced etching time from a few days to 15 min was obtained by a microwave-assisted method involving HCl/LiF etching while obtaining Ti₃C₂T_x MXene flakes from the Ti₃AlC₂ MAX phase by Zhu et al. [2]. In the process, 15 min after etching with a LiF/HCl solvent mixture, the solution is rapidly heated using microwaves, resulting in a rapid and violent release of H₂ gas and H₂O vapor into the air environment. Thus, the weak bonds between the Ti₃C₂T_x MXene flakes are easily broken and the layers are separated, eliminating the need for an additional delamination step and significantly shortening the synthesis time. In addition, Ghazaly et al. were able to synthesize monolayer Ti₃C₂T_x MXene flakes in milliseconds using a very low concentration (0.05 M) LiF solution for etching with the assistance of surface acoustic waves (SAWs) [15] (Fig. 2.2i). Accordingly, the H₂O compound in the solution is decomposed into H⁺ and OH⁻ ions with SAWs applied to the 0.05 M LiF aqueous solution containing the Ti₃AlC₂ MAX phase dispersed on the substrate surface. The H⁺ protons lower the pH of the solution and allow the LiF solvent to more efficiently etch the Al element in the MAX structure. The resulting Li⁺ and OH⁻ ions are weakly bound between the Ti₃C₂T_x layers. Following the etching, the weak bonds between the layers are broken by local mechanical vibrations applied to the substrate surface, and the single-layer Ti₃C₂T_x MXene flakes are suspended above the water and then collected.

2.2.1.2 Fluorine-Free Base Etchants

Solvents used in fluorine-based etching methods are very harmful to the environment and human health, as well as limiting their usage areas due to the presence of fluorine-terminated bonds on the surface of the material [13, 16, 17]. Furthermore, intermediates such as AlF₃ formed as a result of the reaction are quite difficult to be dissolved in any solvent [16]. As a result of such reasons, fluorine-free processes have been developed to synthesize 2D MXene structures. In 2014, Xie et al. conducted one of the early studies of fluorine-free processes using NaOH solvent to synthesize Pt nanoparticle-doped Ti₃C₂T_x MXene layers [18]. Ti₃AlC₂ MAX phase was kept in NaOH solvent heated to 80 °C for 100 h and immersed in H₂SO₄ solution at 80 °C to remove the impurities formed by etching. Due to the inability of NaOH to be a good enough solvent, very limited etching of the Al atoms was observed. After that, Xuan et al. used tetramethylammonium hydroxide (TMAOH) solvent to synthesize hundred nanometer thick Ti₃C₂T_x plate-like crystals [19] (Fig. 2.3a). Accordingly, while TMA⁺ cations bond between T₃C₂T_x layers, OH⁻ anions combine with Al atoms to form bonds terminated with Al(OH)₄⁻ groups. Moreover, Li et al. synthesized high-quality Ti₃C₂T_x nanosheets with large lateral size for the first time by fluorine-free process using a KOH solvent [16]. Water was used to easily remove the intermediate KAlO₂ formed in the reaction.

Fig. 2.3

a Schematic illustration showing the TMAOH intercalation and delamination processes to obtain MXene flakes from the Ti₃AlC₂ MAX phase. Adapted with permission from [19]. Copyright (2016), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/. b Schematic representation of etching processes in NaOH/water solution at different NaOH concentrations and temperatures. Adapted with permission from [17]. Copyright (2018), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/. MXene flakes were synthesized in the intercalation process using c TMAOH, d DMSO, e DMF, and f ethanol solvents, respectively [20]

In another study, Li et al. utilized NaOH in water to obtain a high-quality multilayer Ti₃C₂T_x (T_x; –OH, –O groups) MXene phase from Ti₃AlC₂ MAX phase [17]. In the etching process, while Al atoms in the MAX phase form aluminum hydroxide compounds with OH⁻ ions, Na atoms bond with weak bonds between layers, and bonds terminated with –O and –OH groups are formed after the delamination process. It was observed that the efficiency of the etching process decreased when the reaction temperature was reduced from 270 to 250 °C, and the impurities increased as the oxidation rate of Ti atoms increased due to the decrease in NaOH concentration at 270 °C (Fig. 2.3b). In addition, Li et al. used NaOH etching solvent to synthesize Ti₃C₂T_x nanolayers with a high specific surface area in order to increase the electrocatalytic effect, and also used TMAOH intercalation reagent in the delamination process [20]. The effect of TMAOH intercalation reagent on the size of MXene flakes was compared using other intercalation reagents such as N, N-dimethylformamide (DMF), DMSO, and ethanol in the same process as shown in Fig. 2.3c–f, respectively. Accordingly, it was observed that, due to the presence of positively charged TMA⁺ ions in the TMAOH intercalation compound, it exfoliated more effectively than electroneutral DMSO, DMF, and ethanol, yet eventually smaller (50–100 nm) multilayer/few-layer MXene flakes yielded.

2.2.1.3 Molten-Salt Etchants

Particularly in the synthesis of 2D nitride MXenes, the use of acidic solution is not sufficient for an effective etching process due to the high formation energy of Ti_n+1N_n and the strong binding of Al atoms to nitride atoms in Ti_n+1AlN_n MAX phase [21, 22]. Therefore, another advanced method, the molten salt method, is used by applying a very high temperature for etching. In 2016, Urbankowski et al. synthesized the first Ti₄N₃T_x nitride MXene flakes from the Ti₄AlN₃ MAX phase using the molten salt method [22]. They applied the method by heating a mixture of LiF, NaF, and KF fluorinated salts containing the MAX phase to 550 °C in an Ar atmosphere. As a result of selective etching, it was observed that bonds terminated with F groups were formed and Al atoms were removed from the structure. In order to obtain single-layered flakes from the multi-layered Ti₄N₃T_x MXene structure and to eliminate the contamination caused by unetched MAX phases, the MXene structure was immersed in tetrabutylammonium hydroxide (TBAOH) solution and then 2D MXene flakes were obtained by breaking the weak bonds between Ti₄N₃ layers via sonication. Following that, washing process with deionized water was used to remove TBAOH residues and the synthesized 2D MXene flakes were collected by centrifugation. In addition, according to the DFT calculations, the probability of formation of bonds terminated with the –O group in Ti₄N₃T_x structure was found to be considerably high compared to the –F and –OH groups. Further, it was observed that bare Ti₄N₃ layers that do not terminate with any group have the highest density of states.

Li et al. investigated in detail the mechanism of the molten salt method and succeeded in obtaining different multilayer MXenes using various Cl-based molten salts from many different MAX structures [23]. Accordingly, to synthesize Ti₃C₂Cl₂ multilayer MXene, the Ti₃SiC₂ MAX precursor was immersed in CuCl₂ molten salt at 750 °C (Fig. 2.4a–d). High temperature molten salt provides more active etching of the MAX phase. During the etching process, Si cations that are weakly bonded to Ti atoms combine with Cl⁻ anions to form SiCl₄ compounds, while Cu²⁺ cations and some Cl⁻ anions bind to Ti atoms in Ti₃C₂ layers to form the Ti₃C₂Cl₂/Cu multilayer MXene. The multilayer MXene is then dipped in ammonium persulfate (APS) solution to remove Cu atoms and other impurities. However, during the APS process, the –O functional group is attached to Ti atoms instead of the Cu atoms removed from the multilayer MXene. Also, multilayer structures of Ti₂CT_x, Ti₃C₂T_x, Ti₃CNT_x, Nb₂CT_x, Ta₂CT_x, Ti₂CT_x, Ti₃C₂T_x MXene were synthesized using Ti₂AlC, Ti₃AlC₂, Ti₃AlCN, Nb₂AlC, Ta₂AlC, Ti₂ZnC, Ti₃ZnC₂ MAX phases and CdCl₂, FeCl₂, CoCl₂, CuCl₂, AgCl, NiCl₂ molten salts (Fig. 2.4e–j). In addition, the fact that it is very difficult to synthesize Ta₂CT_x and Ti₃C₂T_x multilayer MXenes using Ta₂AlC and Ti₃SiC₂ MAX phases show how effective the molten salt etching method is. They also succeeded in synthesizing Ti₃C₂T_x multilayer MXene using CuBr₂ and CuI molten salts and functionalized the surface of MXene structures as a result of different anion groups attached to the surface of the MXene layers. In another study, Li et al. synthesized 2D Ti₃C₂Cl₂ and Ti₂CCl₂ MXene flakes using ZnCl₂ molten salt solution and novel Ti₃ZnC₂, Ti₂ZnC, Ti₂ZnN, and V₂ZnC MAX phases [24]. The etching process was carried out by heating the prepared MAX/ZnCl₂ mixture to 550 °C in Ar gas environment. During etching, the weakly bound Zn atoms in the MAX phase dissolve in the ZnCl₂ molten salt solution, while the Cl atoms bond between the Ti₃C₂ layers. Then, ZnCl₂ residues were removed by washing with deionized water.

Fig. 2.4

a–d Schematic illustration of obtaining Ti₃C₂T_x MXene flakes as a result of the reactions of Ti₃SiC₂ MAX phase immersed in CuCl₂ molten salt solution heated to 750 °C and washed with ammonium persulfate (APS) solution. e–j SEM images of various MXene flakes synthesized using different molten salts or MAX phases (Scale Bar: 2 μm) [23]. k Schematic representation of the effect of the MSE method on the MAX phase shows the surface terminations of the synthesized MXene flakes. l SEM image of MXene flakes synthesized using the MSE method. Adapted with permission from [25]. Copyright (2021), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/

In the molten salt method, an extra cleaning process is required to remove the remaining contaminations from the structure after the reaction. To prevent this and control surface modification, Shen et al. developed the molten salt-assisted electrochemical etching method (MSE) [25] (Fig. 2.4k–l). Accordingly, there is a nickel cathode in the cuvette containing the LiCl/KCl molten salt mixture, and the Ti₃AlC₂ (or Ti₃SiC₂) MAX phase is used as the anode and immersed in the solution. In the process carried out in the Ar gas environment, the solution was heated to 450 °C and a voltage of 2 V was applied. Due to the thermochemical effect, Al atoms in the MAX phase were oxidized and formed the AlCl₃ compound, which evaporated at high temperature. At the same time, some –Cl anion groups were attached to the site of the etched A atoms, resulting in the formation of bonds terminating with the –Cl group. In addition, Li/K cations are kept away from the MXene structure by being coated on the cathode due to the electrochemical effect. As a result, Ti₃C₂Cl₂ MXene flakes were synthesized without any contamination. To control the surface modification, the reaction was carried out under the same conditions by adding LiO₂ (or Li₂S) salt additive to the molten salt solution containing the synthesized Ti₃C₂Cl₂ MXene flakes.

Thus, depending on the type of additive salt used, bonds terminating with –O or –S group instead of –Cl group could be obtained, and it was observed that the surface modification property could be controlled. In addition, it has been observed that the molten salt solution used during the process can be used repeatedly, and it has been reported that the MSE process is an environmentally friendly and sustainable method.

2.2.1.4 Electrochemical Etching

Electrochemical etching is another method that does not use highly toxic chemicals and is used to synthesize 2D MXenes in a shorter time and quite simply [3]. In 2013, Sun et al. synthesized layered 2D Ti₂CT_x MXene structures from Ti₂AlC MAX phase for the first time by electrochemical etching using an HCl electrolyte solution [26] (Fig. 2.5a). Accordingly, while Ti₂AlC was used as the working (anode) electrode in the HCl electrolyte solution, Pt foil was used as the counter (cathode) electrode. Due to the electrochemical effect created by the applied electrical force, the bonds between Ti and Al atoms are broken and –Cl, –O, –OH groups are bonded instead of Al atoms. It has been observed that phase differences occur on the surfaces of the MAX structures during the reaction. Since the surface of the MAX structures was etched stronger than the inner regions, it was observed that not only the A atoms were eliminated but also the Ti atoms were etched and the carbon-derived carbide (CDC) phase was formed. It was also observed that increasing the etching time, applied voltage, or the concentration of the HCl solvent in the electrolyte solution increased the CDC ratio. A sonication bath with deionized water was used to remove the CDC layer after etching and layered 2D MXene structures were obtained.

Fig. 2.5

a Schematic representation of the electrochemical etching process used to synthesize the 2D Ti₂C_xT_x MXene flakes and the products formed according to the steps [26]. b–k Analysis of the electrochemical etching mechanism of the Ti₃AlC₂ MAX phase using DFT calculations. Adapted with permission from [27]. Copyright (2018), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/

Yang et al. succeeded in synthesizing single-layer TiC₂ MXene flakes by electrochemical etching method using a solvent mixture of NH₄Cl and TMAOH as an electrolyte solution [27]. The electrochemical etching mechanism was investigated in more detail by DFT calculations. When the Ti₃AlC₂ MAX phase used as the electrode is loaded with a positive charge, the Cl ions in the solution move towards the surface of the MAX phase (Fig. 2.5b–c). The number of Cl ions bound to Ti atoms on the surface of the MAX phase increases with time and the distance between the Ti₃C₂ layers increases (Fig. 2.5d–g). The resulting Al atoms form an AlCl₃ compound with Cl ions and move away from the MAX structure (Fig. 2.5h–i). Instead of Al atoms, OH anions with an ionic radius smaller than the distance between the layers are bonded between the Ti₃C₂ layers (Fig. 2.5j–k). At the same time, NH⁴⁺ and TMA⁺ cations are collected on the other electrode. Furthermore, electrolyte solutions such as H₂SO₄, HNO₃, NaOH, FeCl₃ were used as well as NH₄Cl for comparison in the electrochemical etching process. Accordingly, it has been observed that electrolyte solutions containing Cl⁻ anion groups are more effective for electrochemical etching due to the effective bonding of Al atoms and Cl⁻ ions. Delamination with TMAOH solution is used to obtain single-layer TiC₂ MXene flakes from the multi-layered Ti₃C₂T_x MXene structure obtained after etching. TMA⁺ cations are weakly bonded between Ti₃C₂ layers, while the OH anion group is attached to dangling bonds on the surfaces of T₃C₂ layers. Single-layer Ti₃C₂ MXene flakes were obtained by mechanically breaking the weak bonds between the layers as a result of continuous stirring. The dimensions of the flakes collected by centrifugation go up to about 18 μm and their average size is 2.4 μm.

Additionally, Pang et al. were able to synthesize Ti₃CT_x, Cr₂CT_x, and V₂CT_x MXene plates using the thermal-assisted electrochemical etching method and develop a universal method for the synthesis of many different MXenes [28]. They used composite Ti₂AlC-CB/CFC electrode consisting of carbon black additive (CB) and carbon fiber cloth (CFC) to increase the efficiency of the electrochemical etching method. The composite electrode shows low Warburg resistance, causes increased ion diffusion to the electrode surface, and increases the efficiency of reactions in the electrochemical etching process. In addition, it was observed that the efficiency of the delamination process increased and Ti₃CT_x structures with rougher surfaces were obtained when the solution temperature was increased from 25 to 50 °C during the electrochemical etching process and with the applied voltage of 0.3 V. Furthermore, laminar Ti₃CT_x MXene structures were observed when the etching time was increased from 3 to 9 h. In the case of the more difficultly synthesized Cr₂CT_x and V₂CT_x MXenes, higher voltages of 1 V and 0.5 V were applied, respectively, for 9 h etching at 50 °C. The lateral size of the synthesized multilayer Cr₂CT_x plates is up to 25 μm, while the lateral sizes of the multilayer Ti₂CT_x and V₂CT_x MXene plates are greater than 1 μm.

2.2.2 Other Advanced Top-Down Methods

Besides the most commonly used methods described above, interesting methods such as halogen etching, thermal reduction, UV-induced selective etching, algae extraction, and ball milling are also used to synthesize high-quality 2D MXenes. Jawaid et al. succeeded in obtaining the MXene phase by using halogen (I₂, Br₂) and interhalogen compounds (IBr, ICl) to etch A atoms from the MAX phase at room temperature [29] (Fig. 2.6a). Accordingly, as a result of the significant increase in the activity of I₂, Br₂ halogens in high dielectric solvents (such as acetonitrile), TiBr_x structures are formed as a result of the loss of selectivity due to the etching of the X layer as well as the A layer in the MAX phase. Therefore, nonpolar solvents such as cyclohexane (CH) are used to increase selectivity. In addition, the formation of more reactive by-products on the surface with the increase of Br₂ concentration in the solution reduces the efficiency of the etching process, while lowering the concentration value decreases the etching rate since it cannot be adsorbed on the MAX surface sufficiently. Tetrabutylammonium halides (TBAX⁻; X; F⁻, Cl⁻, Br⁻) are used to disperse the synthesized Ti₃C₂T_x MXene flakes. In interhalogen compounds, the halogen atom with higher electronegativity participates in the etching process. Since interhalogens are stable in low dielectric organic solutions, MXene flakes can be synthesized using solvents such as tetrahydrofuran (THF), acetonitrile, chloroform. In addition, it has been stated that halogen etching method can be used to obtain MXene structures from Cr₂AlC and V₂AlC MAX phases as well as Ti₂AlC MAX phase by using halogen etching method.

Fig. 2.6

a Schematic view and experimental images of the halogen etching process [29]. b AFM images show dissolution and reduction in the thickness of MXene flakes exposed to water over time; initial, 1 week, and 2 weeks, respectively. Adapted with permission from [30]. Copyright (2021), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/. c SEM images show the transformation in the structure of the MAX phase at different temperatures (Scale bar: 1 μm). Adapted with permission from [31]. Copyright (2020), Copyright: The Authors, some rights reserved; exclusive licensee [Elsevier]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/. d Schematic illustration showing the synthesis of few-layered MXene flakes using DMSO solvent by the HEBM method [34]

Furthermore, Shi et al. synthesized Ti₃C₂T_x (Tx = O⁻, OH⁻) MXene sheets from the MAX phase by etching with halogen (I₂) in anhydrous acetonitrile and delamination with HCl solvent [30]. Since some AlI₃ by-product remained as impurity between Ti₃C₂T_x layers after etching, it was immersed into HCl solution to remove the impurity, and delamination was performed at the same time. In addition, it was observed that the performance increased proportionally with the increase in temperature, and the Al concentration decreased from 16.7 to 0.9 wt% when the temperature was increased from room temperature to 100 °C. The thickness of the oxygen-rich Ti₃C₂T_x sheets synthesized by the I₂ halogen etching process is mostly a few layers and their lateral sizes are 1.8 μm. It has also been observed that MXene sheets have a high stability value for up to 2 weeks without decomposition in water due to the non-aqueous etching process (Fig. 2.6b).

In another study, Mei et al. synthesized multilayer Ti₂C MXene flakes from Ti₂SC MAX phase using a very simple, environmentally friendly, and easy-to-control thermal reduction method [31]. Accordingly, the Ti₂SC MAX phase is placed in a sealed tube furnace with Ar/H gas flow by a quartz boat, and the temperature is increased from 400 to 800 °C with a heating rate of 10 °C/min, and multi-layered 2D Ti₂C MXene flakes are synthesized in 2–4 h. The most important part of the thermal reduction method is to find the appropriate temperature value (Fig. 2.6c). It was observed that, when the temperature exceeded 500 °C, the S atoms in the MAX phase began to etch, and Ti₂C, MXene flakes and TiO₂ particles were formed at 800 °C. It has also been reported that MXene flakes return to the MAX phase when the temperature reaches 900 °C. However, it was noted that more S atoms were etched than MXene flakes left at 800 °C for 30 min and then at 900 °C for 30 min.

In another approach, Mei et al. succeeded in synthesizing mesoporous 2D Mo₂C plate-like structures using UV-induced selective etching from the ultraviolet (UV) wavelength-sensitive Mo₂Ga₂C MAX phase [32]. With this method, it has been shown that 2D MXene structures can be easily synthesized with UV-sensitive MAX phases in a very short time, within a few hours, without the use of any dangerous acidic chemicals. Accordingly, Ga atoms in the Mo₂Ga₂C MAX phase are etched under UV irradiation due to strong UV absorption. Etched Mo₂C MXene plates were delaminated using H₃PO₄ solvent. The synthesized few layered Mo₂C MXene plates have a graphene-like structure, and interestingly, the plates have mesoporous structures with a pore size in the 2–16 nm range. In addition, due to the solvent used in the delamination process, they have bonds that terminate with the –O group.

In addition to these studies, Zada et al. succeeded in synthesizing V₂C MXene nanoflakes for use as photothermal agents in photothermal therapy applications from the V₂AlC MAX phase using the algae extraction method [33]. Al atoms in the V₂AlC MAX phase are etched with an algae extraction mixture containing organic acids. Moreover, many biological compounds intercalate between the V₂C layers during the etching process and also accelerate the delamination process. The few-layer V₂C MXene nanoplates, synthesized after 48 h, have lateral sizes in the range of 50–100 nm. In addition, it was observed that MXene nanoplates with smaller lateral size and thinner thickness were synthesized with increasing etching and delamination times. Thus, a very low cost, completely environmentally friendly, simple and easy-to-control method has been developed.

In addition, Wu et al. used high energetic ball milling (HEBM) method in the exfloation process to obtain few-layer 2D Ti₃C₂T_x nanoplates from multilayered Ti₃C₂T_x MXene particles [34]. Accordingly, DMSO solvent was used in the HEBM method to prevent oxidation of MXene particles during the process and for an effective intercalation (Fig. 2.6d). MXene nanoplates were obtained by breaking the weak bonds between Ti₃C₂T_x layers due to the mechanical force applied by ball milling method. In addition, when the ball milling method is applied without using DMSO solvent, it was observed that the MXene structure turned into TiO₂ and graphitic carbon structures. Further, it has been reported that multilayer MXene structures exist when different solvents such as DMF, ethanol, HF are used and the delamination process is not as efficient as when DMSO is used. In another study, He et al. used PDDA solution-assisted ball milling (P-BM) method in the delamination process to obtain functionalized few-layered MXene nanoplates from multilayer Ti₃C₂T_x MXene particles [35]. The high viscosity of the PDDA solution prevents the steel balls from hitting the MXene particles violently and facilitates the exfoliation process by preventing the agglomeration of the newly exfoliated nanoplates. It was observed that several-layered Ti₃C₂T_x MXene nanoplates thus synthesized had a surface functionalized with the PDDA group intercalated between Ti₃C₂ layers and had larger lateral size (hundreds of nm).

2.2.3 Fabrication of 2D MXene Films

Although a wide variety of advanced methods have been developed for the synthesis of 2D MXene flakes, the development of large-scale, low-cost, and low-toxicity production methods for high-quality 2D MXene thin films is of great importance. In 2014, Michael et al. showed that Ti₃C₂T_x MXene 2D film can be produced via the rolling method using etched multilayer MXene particles called clay-like MXene paste [36] (Fig. 2.7a). In addition, it is stated that printing can be made on the desired surface by using clay-like MXene paste as ink. It has been also reported that clay-like MXene paste can be formed into the desired shape when wet and MXene structures in inch sizes can be obtained after drying. Thus, it has been shown that MXene structures can be produced easily, quickly, and at large scales (Fig. 2.7b–c). This opened a new page for producing 2D MXene films via many methods.

Fig. 2.7

a Schematic representation of clay-like Ti₃C₂T_x MXene paste making and various uses. b Plate produced using clay-like MXene paste (left) swells and shrinks (left) during the drying process (top cross-section views). c The inch-size MXene film obtained after the rolling process [36]. SEM images of d lateral area (Scale bar: 500 nm) and e top surface area (Scale Bar: 1 μm) of MXene thin films produced by spin coating. f Image of MXene thin film coated on a flexible substrate (polyetherimide polymer). Adapted with permission from [40]. Copyright (2016), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/. g Schematic representation of the blade coating method. h Image of Ti₃C₂T_x MXene thin film with a length of 1 m, a width of 10 cm, and a thickness of 940 nm, coated on Celgrad membrane by blade coating method. i Digital photograph showing thin film carrying 40 g in durability test. j Cross-sectional SEM image showing MXene flakes lined up in a highly coherent array. k SEM image showing that a 62.5 μm radius fold was formed by 180° bending of the thin film. l Low-magnification and m high-magnification SEM images show the formation of folds with extremely small radius down to 1.3 μm. Adapted with permission from [42]. Copyright (2020), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/

Vacuum filtration method was developed by Ling et al. to produce flexible, high electrical conductive 2D MXene films [37]. Accordingly, the suspended solution containing 2D MXene flakes is filtered with the help of vacuum pump, and a 2D film with a thickness of 13 μm is obtained, consisting of thousands of MXene flakes stacked on the filter. Moreover, the film thickness could be easily controlled by adjusting the volume of solution used, and 2D MXene films with thicknesses less than 1 μm could be produced. Thus, it has been shown that MXene 2D films with high electrical conductivity, high flexibility, and low density can be produced with a facile and cost-effective method. In another study, Kim et al. synthesized inch-size Mo-based MXene 2D films such as Mo₂CT_x, Mo₂TiC₂T_x, and Mo₂TiC₃T_x by vacuum filtration method [38]. Moreover, the electrical properties of the 2D MXene film at different temperatures were investigated, and it was observed that the electrical properties of the thermally treated (800 K) films were improved. It has been stated that the reason for this is that the MXene flakes become very close to each other as a result of the evaporation of the intercalation compounds between the MXene layers and the functional groups on the surfaces at temperatures above 500 K. Thus, it was observed that Mo₂TiCT_x MXene 2D films thermally treated at 800 K have high electrical conductivity (1380 S/cm), large Seebeck coefficient (–47.3 μV/K) and high thermoelectric power (3.09 × 10^–4 W m⁻¹ K⁻²). Furthermore, Shuck et al. carried out large-scale production using a specially designed vacuum filtration-assisted reactor to synthesize 2D Ti₃C₂T_x MXene structures in different batch sizes (1 and 50 g), showing that large-scale production did not alter the properties of MXene [39].

In addition, Dillon et al. produced inch-sized MXene 2D films of different thicknesses on the substrate surface by spin-coating method using a colloidal solution containing single/few layer Ti₃C₂T_x MXene plates [40] (Fig. 2.7d–f). The thickness of Ti₃C₂T_x MXene 2D film was changed between 2 and 81 nm to examine the relationship between thickness and electrical conductivity. It was observed that the electrical properties did not change with the increase in thickness, since few-layered film with a thickness of 2 nm showed bulk conductivity. It has also been reported that the inch size Ti₃C₂T_x MXene 2D film has very high electrical conductivity (6500 ± 800 S/cm). Further, Hantanasirisakul et al. produced a 70 nm thick transparent Ti₃C₂T_x MXene 2D film on a glass substrate by spray coating method using a suspended solution [41]. Also, Zhang et al. used blade coating method with few-layered Ti₃C₂T_x MXene flakes to obtain high-quality 2D MXene film with high electrical conductivity and high mechanical strength [42]. Accordingly, since MXene flakes with a large area and thin thickness have a high aspect ratio, large flakes are dispersed in the solution. Large flakes dispersed in solution have been reported to self-assemble and form agglomerates. Also, the viscosity of the solution decreased when shear stress was applied to the gel-like solution. Thus, it was observed that the large flakes dispersed in the solution aligned very easily in a certain pattern in the direction of the shear stress applied by the blade (Fig. 2.7g). Therefore, several meters long and 940 nm thick 2D Ti₃C₂T_x MXene film was synthesized with excellent electrical conductivity (15,100 S/cm), outstanding tensile strength (570 MPa), a record among thin films, and high Young modulus (20.6 GPa) (Fig. 2.7h–j). It has also been observed that the produced 2D MXene thin film has very high flexibility and high stability as a result of many repeated bending processes. The produced 2D MXene thin film was reported to retain its stability even after repeated 5000 times bending at 180° angle, a 62.5 μm radius curl is formed as a result of bending, and the large flakes are able to be compressed to form fold with an extremely small radius of 1.3 μm (Fig. 2.7k–m).

2.3 Bottom-Up Methods

The bottom-up synthesis method is applied to fabricate precise MXene structures by increasing the controllability of the growth parameters. Also, this approach provides a clean and acid reaction-free situation without using any functional group. One of the most important members of this approach is the well-known chemical vapor deposition (CVD) process.

2.3.1 Chemical Vapor Deposition Method

The CVD process has proven to be a reliable, inexpensive, and flexible method for growing two-dimensional structures such as nanotubes, nanofibers, graphene, and MXene. This method offers efficient uniformity due to more controllable parameters compared to other alternatives [43]. Compared to the top-down approaches, CVD-MXene offers fewer structural impurities and lower levels of defects and atomic disorder. Furthermore, the instability of MXenes synthesized via top-down techniques under atmospheric conditions limits their widespread application. The bottom-up approach described here involves exposing a reactive substrate to volatile precursors via the heated reaction chamber to affect chemical reactions and deposition on the surface. As a result, this approach has been used extensively in the preparation of carbide, nitride, and silicon-based compounds [44]. The synthesis strategies influence the controlled growth process of the MXene structure and its inherent features. With the emergence of novel MXene structures and specifications such as Mo₂C, W₂C, WC, TaC, or NbC, the emergence of a new approach towards bottom-up synthesis extends MXene capability to more sensitive applications. CVD is a well-established key process in the bottom-up approach. As a result, the significance of CVD in MXene synthesis is well understood [45] but still needs to be improved and optimized. A single-step CVD process with controllable parameters allows precise fine-tuning of each feature to achieve the required thickness, morphology, grain size, and 2D structure quality [46]. In addition, the CVD technique exhibits superior controllability over the thickness and size of MXene structures, facilitating the synthesis of extended lateral heterostructures of graphene and MXene [47].

The synthesis method of 2D MXene crystals influences grain boundaries, imperfections, crystal structure, and thickness. Furthermore, the shape formation regime is a major consequence of chemical stability, and the electrical conductivity of the MXene family is thickness-dependent [48]. Consequently, tight control of the growth process is essential to obtain the relevant properties that CVD offers for future MXene-based applications. Chemical reactions at high temperatures also generate vertical van der Waals band heterostructures, which are crucial for the specific properties of MXene. The CVD approach could also produce an MXene/graphene heterostructure in multiple steps using different gases at different thermal conditions. Cooling duration can also be adjusted as a critical parameter to control flake growth rate. Some CVD furnaces with an upstream heating zone provide some fixed chemical combinations for precursors [49]. Numerous complex factors need to be considered, including effective optimization of the synthesis process and consequent mass production of the MXene structure, which is still debated by researchers. In general, the MXene growth compound consists of stacked metal foils positioned in a quartz tube. Because of their higher potential for atomic surface mobility and reaction within the molten metal, the dominant transition metals used to control MXene production are preferably selected from the third through sixth periodic table element groups. The tube is closed on both sides and coupled with a gas control system and a mechanical vacuum pump as a pressure optimizer. In order to prevent the air pollution in the early stage, the vacuum pump is linked with the air exhaust system. The tube is preheated by the furnace after being filled with the appropriate carrier gas ratio and then the temperature rises to the set-point. In general, the temperature reported is the melting point of the catalyst or a temperature in the range of the needed values for MXene formation. The temperature stabilization performance also has an impact on the quality of the final product. Mo₂C is one of the most studied MXene structures, specifically by CVD where stacked Mo/Cu foils are used as a precursor and a catalyzer, respectively. Above 1085 °C, copper foil becomes liquid and covers the Mo foil completely. The temperature limits of reaction activation are typically near the melting point of the catalyst and the recrystallization temperature of the transition metal. The process ends with the deactivation of the furnace, which moves the sample out of the hot zone and cools it down in the accompanying gas flow. According to the latest study results, at least 800 ℃ is required for the development of carbon-based MXene structures [50]. Figure 2.8a–b explains the schemes of system configuration and growth mechanism of Mo₂C in different shapes and single or multilayer forms under different growth conditions. Figure 2.8c shows the possibility of a structural change with a multi-step CVD operation from Mo₂C to MoN, and Fig. 2.8d illustrates the Mo₂C growth inhibition by initial growth of a graphene layer. In conclusion, since the synthesis of 2D MXene structures with desired and high optoelectronic properties is of great importance, the important parameters that can be controlled in the CVD method will be discussed in this section.

Fig. 2.8

The principles of bottom-up method CVD: a the system configuration [51], b the schematic of nucleation mechanism, adapted with permission from [52]. Copyright (2021), Copyright: The Authors, some rights reserved; exclusive licensee [Elsevier]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/, c Multi-step MXene flakes growing by Mo₂C to MoN transformation, adapted with permission from [49]. Copyright (2022), Copyright: The Authors, some rights reserved; exclusive licensee [Elsevier]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/, d is the MXene vertical growth rate blocking by initial growth graphene layer, adapted with permission from [53]. Copyright (2017), Copyright: The Authors, some rights reserved; exclusive licensee [John Wiley and Sons]. Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/

2.3.1.1 Catalyzer Type and Thickness

Because transition metals such as Mo have high recrystallization temperatures, surface atoms require much activation energy for atomic transition [54]. The molten metal catalyst facilitates the diffusion of metal atoms to the gas/liquid interface. The atoms meet decomposed gas atoms at the liquid interface, resulting in a chemical reaction. The molten metal is a reactive environment that provides an atomic motion environment through the applied kinetic energy. Catalytic metals generally have little ability to perform chemical reactions by purging precursor gases. The kinetic energy of MXene formation is provided by the high temperature and atomic transition in molten metal. In addition, it is possible to dissolve gas atoms in molten metal to achieve deeper penetration and higher vertical growth rates. The early melting temperatures of the catalyst provide the earliest nucleation points and a general guideline for the controllable growth of large-scale, high-quality samples. Catalytic metals are typically copper, gold, indium, and their alloys. The lower melting point and hydrogen solubility increase the potential of MXene formation [55,56,57]. With a precisely engineered thickness, the catalyst thickness can be a reliable parameter to enable an affordable method of growing ultrathin 2D materials. So it can demonstrate the importance of the process by varying the thickness of the catalyst foil on the transition metal film, which readily diffuses to react with the hydrocarbon gas [58]. In addition, the applied catalyst thickness causes MXene flakes with a few nanometers of thickness to grow on the superlattice. The thicker the catalyst, the more crystallographic layers stack up and the more carbon vacancies there are. By modifying the thickness of the Cu foil, the Mo₂C vertical growth range can be varied from the superlattice to hundreds of nanometers thick. Due to their lower melting points, alloyed catalysts outperform pure catalytic metal catalysts. Temperature control is crucial in this process as there are few options for all-metal catalysts. Lowering the melting point of the catalyst allows for more diffusion of the supported transition metal towards the liquid interface. The alloyed catalysts are obtained to lower the growth temperature of MXene crystals. Several alloyed catalysts such as Ag-Cu and In-Cu alloys could be used for Mo₂C synthesis at temperatures below 1000 °C [55, 56]. By increasing the Sn content, Sn-Cu alloyed catalysts show higher carbon absorption than pure copper. The viscosity of the liquid Sn-Cu alloy is an important factor in reducing the resistance of the liquid to atomic movement and enabling faster diffusion of Mo. The density of MXene nucleation inversely increases with growth temperature, which has a non-linear effect on the viscosity of the alloyed catalyst. Accordingly, an optimal growth temperature is required to provide growth kinetics. As aforementioned, the growth temperatures can be reduced to 880 °C, with similar growth rates for pure Cu above 1085 °C [55].

2.3.1.2 Ratio of the Gases

The crystal morphology and flake thickness could be controlled by changing growth parameters such as the CH₄:H₂ gas combination ratio [56]. Injected gases are classified into two types: carriers and precursors. Carbides and nitrides are the two most important groups of transition metal compounds. According to research, the formation of carbide MXenes by catalytic metals is easier than that of the nitride group due to undesirable nitride compositions. Methane (CH₄) or ammonia (NH₃) flows into the quartz chamber as carbon and nitrogen sources during CVD operation [59]. As previously stated, the most researched CVD-grown MXenes are transition metal carbides such as Mo₂C, W₂C, WC, TaC, and NbC, but binary nitrides such as Co₃N, WN, Fe₃N, CrN, and VN have less relevance in CVD products. Because exothermic reactions of nitride composition demand less energy for catalytic activity, N₂ is a superior candidate for Ta₂C, Ta₂N, Ti₂C, and Ti₂N production than Mo₂C and W₂C [59]. M_n+1C_n is the most prevalent chemical composition for carbides, while M_n+1N_n is the chemical composition for nitrides. The functional groups (precursor gases) breakdown by interaction with the mass flow of hydrogen and/or nitrogen. The gas integration ratio is an important parameter in controlling lateral or vertical flake development as well as surface coverage. In the following, the carbide phase will be discussed more than the nitride phase.

2.3.1.3 CH₄ Flow Rate

The major process of crystal development on the crystal face is 2D nucleation due to carbon supersaturation. As a result, increasing the flow rate of carbon allows crystallographic faces with distinct crystal morphologies to vary, with the resultant flake forms shifting from triangle to polyhedron as the carbon precursor is increased [58]. Due to the extended rapid extension of graphene structure at low methane flow rates, graphene domains appear to dominate on the surface of molten metal. Graphene prevents the interfacial diffusion of Mo atoms and passivates the Mo interaction with the gaseous hydrocarbon. When the flow rate of CH₄ increases significantly to reach the surface saturation limits, Mo atoms diffuse along the liquid-metal interface and react with the hydrocarbon molecules, resulting in the formation of Mo₂C crystals based on the first nucleated graphene. Researchers demonstrate a wide range of methane flow rate utilization ranging from 0.3 to 25 sccm. Different flow rates support the formation of flakes ranging from triangular to polyhedral as the methane flow rate increases. In addition, methane decomposition increases with operating temperature, leading to carbon saturation [48, 53]. The methane flow rate is a critical factor in regulating the formation of the graphene/MXene heterostructure. The data supported the idea that crystal morphologies and crystallographic structures may be reformed by modifying the flow rate to the supersaturation of the precursors. Due to limited carbon diffusion, the initial Mo₂C nucleus gradually expands into a variety of different shaped flakes with low carbon content [58, 60, 61] Fig. 2.9a–c shows the diversity of MXene flakes formation at different CH₄ fluxes and carbon concentrations. Lower than 1 sccm methane ratio results in cubic-shaped flake, and with increasing flow rate the shapes begin to expand in star shape, and finally changing to hexagonal type above 5 sccm methane.

Fig. 2.9

MXene flake by changing the precursor flow rate; a under 1 sccm CH₄, b between 1 and 5 sccm CH₄, c above 5 sccm CH₄ (Scale Bar: 5 µm) [51], and d 80 sccm H₂, e 100 sccm H₂ f 120 sccm H₂ (Scale Bar: 10 µm). g Real state micro-Raman peaks of Mo₂C and h graphene micro-Raman peaks in context and i the calculated Mo₂C peaks [51]

2.3.1.4 Carrier Gas Flow Rate

Hydrogen flow as a carrier gas causes lateral growth by decreasing nucleation density and increasing MXene flake domains. Hydrogen atoms interact preferentially with methane and dehydrogenate at high temperatures, which increases the proportion of carbon in the reaction tube. Due to the rapid deposition of graphene on the metal catalyst surface, the low fraction of H₂ flow rate inhibits growth of MXene single crystals. As a result, graphene structure predominates in the MXene/graphene heterostructure [48]. Meanwhile, in a hot atmosphere, hydrogen acts as a corrosive agent, resulting in negative consequences on the obtained compounds. Therefore, an optimal H₂ flow rate is required to achieve the desired structure [62]. Figure 2.9d–f shows the changes in flake growth and morphologies at a constant methane flow rate of 5 sccm by increasing the H₂ from 80 to 120 sccm at 1200 ℃. The images show the direct effect of hydrogen flux on the lateral growth of flakes and eroded copper exposed to high hydrogen flows.

2.3.1.5 Operation Temperature

The dissolution of precursor atoms within the molten catalyst plays an important role in atomic diffusion and reaction with accumulated transition metal atoms. In addition, increasing the growth temperature promotes gas dissolution in the liquid metal. Melting point and viscosity in the molten state are the most important factors in varying metal catalyst type [56]. The increased wettability of the metal catalyst is beneficial for increasing flake coverage, but the high viscosity is disadvantageous for the MXene growth process. The lower melting point of some metallic catalysts, such as Au and In, affects their catalytic ability and determines the growth temperature of MXenes. It is evident that increasing the process temperature causes more metal diffusion due to increased atomic mobility. In addition, higher temperature (above the melting point of the catalyst metal) promotes the diffusion of transition metal atoms into the solid/liquid interface, increasing the probability of forming a carbide/nitride heterostructure [56, 57].

Additionally, the surface energy of the molten metal increases directly with temperature, which results in an increase in atomic dispersion and excessive precursor activity. By applying higher temperatures to the flakes, the vertical growth rate changed, resulting in large pyramidal growth regions with clearly defined terraces [61].

2.3.1.6 Growth Time

MXene structures, which are grown at different process durations, form in isolated flake structures and they are transformed into continuous films when the duration is increased. However, the prolonged deposition process allows more Mo atoms to enter the first-grown graphene domains and combine through interfacial diffusion to Mo₂C with hydrocarbon interaction. Typically, hexagonal Mo₂C flakes with a lateral range of about 50 µm appear after 30 min and expand to 100 µm after 60 min [53]. Figure 2.9g–i provides that the formation of graphene in context is almost inevitable due to the gradual temperature change.

CVD offers simplicity and diverse development opportunities as a bottom-up approach by tuning multiple parameters for the synthesis of 2D MXene structures with high potential for use in a wide variety of future electronic and optoelectronic applications. However, it is still challenging to obtain large-area and high-quality 2D MXene crystals with controlled thicknesses. The combination of multiple purified precursors and temperature ranges results in a scalable MXene synthesis with tunable thicknesses and excellent physical and chemical properties. Temperature and gas flow rate are among the most studied process parameters for CVD growth of 2D MXene crystals. But catalytic metals formed in shots (In, Ga) or as foils (Cu, Au) are placed on the surface of source transition metals, where such studies show the importance of the catalyzers for controllable MXene formation. In all situations, the thickness of the 2D MXene flakes decreases with increasing catalyst thickness. In addition, increasing the reaction time by choosing a lower melting point catalyst reduces the vertical growth rate.

2.3.2 Other Bottom-Up Methods to Produce 2D MXene

Bottom-up approaches are not limited to CVD where self-assembly, plasma-enhanced pulsed laser deposition (PEPLD), and template-assisted growth are also possible strategies to obtain 2D MXene structures [3]. Transition metal carbides (TMCs) and nitrides (TMNs) mainly consist of the crystalline structures of their bulk counterparts and the bottom-up ultrathin synthesis strategies are able to control their superior properties, in general avoiding functional groups. For example, as fabricated 2D MXene structures, the Mo₂C crystals show three crystalline structures including hexagonal (β), orthorhombic (α), and cubic phase allotropes. Occupying carbon atoms within the Mo interstitial octahedrons is a factor responsible for the phase variation. The PEPLD method is highly reliable to control the crystal structure and obtain multiphase allotropes [63].

A further advantage of interfacial self-assembly is that it allows high-performance composites to be organized into hierarchically ordered structures, for example, drug delivery, magnetic materials, or energy storage—without the risk of oxidation of MXene layers. The MXene sandwich composite structures such as Co-NiS/MXene nanosheets and nanorods constructed by self-assembly include hydrogen bonds within the structure and show good electrochemical performance for lithium and sodium ion storage [64].

The template-assisted growth approach essentially uses a metallic framework that could be removed by chemical etching to obtain a free-standing 3D MXene architecture. This method offers the possibility to generate macroscopic 3D MXene structures with high gravimetric capacity. This also allows the hydrophilicity of MXene to be tuned by exchanging H⁺ with a cationic surfactant. In addition, the template-assisted growth MXene/binder polymerization leads to the formation of a porous monolithic cell structure [65]. Obviously, the main purpose of all bottom-up approaches is precise growth control of structure formation, and the resulting composition is a direct function of the synchronization of several growth parameters.