Two-Dimensional Nanostructured Ti₃C₂T_x MXene for Ceramic Materials: Preparation and Applications

Abstract

Ti₃C₂T_x MXene, a novel two-dimensional transition metal carbide with nanoscale dimensions, has attracted significant attention due to its exceptional structural and performance characteristics. This review comprehensively examines various preparation methods for Ti₃C₂T_x MXene, including acid etching, acid–salt composite etching, alkali etching, and molten salt etching. It further discusses several strategies for interlayer exfoliation, highlighting the advantages and limitations of each method. The effects of these techniques on the nanostructure, surface functional groups, interlayer spacing, and overall performance of Ti₃C₂T_x MXene are evaluated. Additionally, this paper explores the diverse applications of Ti₃C₂T_x MXene in ceramic materials, particularly its role in enhancing mechanical properties, electrical and thermal conductivity, as well as oxidation and corrosion resistance. The primary objective of the review is to provide scientific insights and theoretical guidance for the preparation of Ti₃C₂T_x MXene and its further research and innovative applications in ceramic materials, advancing the development of high-performance, multifunctional ceramics.

1. Introduction

With the continuous advancement of materials science, two-dimensional (2D) materials have emerged as highly promising candidates for a wide range of applications in fields such as energy storage, sensors, catalysis, and biomedicine, owing to their unique structures and remarkable properties [1,2,3]. Among these, 2D transition metal carbides, nitrides, or carbonitrides, collectively known as MXene, have garnered significant attention due to their excellent electrical conductivity, thermal stability, mechanical properties, and the ability to modulate surface functionality [4,5]. Since the discovery of MXene in 2011, over 30 distinct compounds have been synthesized, including Ti₃C₂, Nb₂C, V₄C₃, and Ti₃N₄. The general chemical formula for MXene is M_n+1X_nT_x, where M represents a transition metal, X denotes carbon or nitrogen, A is typically an element from group III or IV, and T represents various surface functional groups. By selectively etching the A-layer elements from the MAX phase, MXene exhibits a unique, layered structure while displaying diverse physicochemical properties.

Among various MXene materials, Ti₃C₂T_x was the first to be discovered. Its unique structure and excellent performance have made it a focal point of research. Ti₃C₂T_x has a layered structure, consisting of alternating layers of Ti and C atoms. This structure not only preserves the high strength and superior mechanical properties of the MAX phase but also imparts notable conductivity, increased interlayer spacing, and a higher specific surface area [6,7]. These outstanding characteristics make Ti₃C₂T_x highly promising in the fields of electronics, thermodynamics, and mechanics. Moreover, Ti₃C₂T_x MXene offers flexible surface tunability, enabling the introduction of various functional groups such as -F, -OH, and -O. These groups provide it with high surface reactivity and adjustable physicochemical properties. The diversity in surface functionalization further enhances its compatibility with different matrix materials, expanding its potential for use in composite materials [8,9,10]. As one of the most extensively studied materials in the MXene family, Ti₃C₂T_x MXene has attracted significant attention, particularly regarding its preparation methods and applications in composite materials.

The synthesis process of Ti₃C₂T_x MXene plays a crucial role in determining its structure and performance. Therefore, developing efficient fabrication methods to produce Ti₃C₂T_x MXene with optimal properties has become a key focus of current research. Traditional synthesis methods typically rely on hydrofluoric acid (HF) etching, which effectively removes the A-layer elements from MAX phases, yielding high-quality Ti₃C₂T_x MXene. However, HF etching has drawbacks, including strong corrosiveness, significant environmental pollution, and low etching efficiency [11,12]. Consequently, in recent years, alternative etching methods have been proposed, such as acid–salt composite etching, alkaline etching, and molten salt etching, to improve the preparation efficiency and quality of MXene while reducing environmental harm [13,14,15,16]. After etching, additional delamination processes are often required to achieve single-layer or few-layer MXene [17].

Currently, the applications of Ti₃C₂T_x MXene primarily focus on supercapacitors, batteries, sensors, and electromagnetic shielding [18,19,20,21]. For example, Wan et al. [22] synthesized GaOOH/Ti₃C₂T_x composites via a hydrothermal method, using gallium nitrate (Ga(NO₃)₃) as the gallium source. As a supercapacitor electrode material, the GaOOH/Ti₃C₂T_x composite prepared with a 0.06 M gallium source concentration achieved a specific capacitance of 542.1 F·g⁻¹. After 5000 charge–discharge cycles, the capacitance decreased by only 3.4%. Dai et al. [23] synthesized CoFe₂O₄/Ti₃C₂T_x MXene/CNFs composites by combining Ti₃C₂T_x with porous CoFe₂O₄ derived from metal–organic frameworks (MOFs) and integrating it with carbon nanofibers (CNFs). This composite effectively mitigated the volume expansion issue during battery charge–discharge cycles. It exhibited excellent rate performance, maintaining 332 mAh·g⁻¹ at 20 A·g⁻¹. After 1000 cycles, it retained a remarkable capacity of 972 mAh·g⁻¹ at 2 A·g⁻¹. Zhang et al. [24] synthesized 2D a-Fe₂O₃/Ti₃C₂T_x MXene composites through solvothermal and annealing methods. In this process, a-Fe₂O₃ was in situ grown on both the surface and interlayers of Ti₃C₂T_x MXene. The composite exhibited exceptional gas sensitivity (20.27 at 100 ppm) and ultra-low detection limits for H₂S gas (10.19 ppb) at room temperature (25 °C). Although the application of Ti₃C₂T_x MXene in ceramic materials is still in its early stages, its unique 2D structure, superior mechanical properties, outstanding electrochemical performance, and tunable surface functionality render it highly promising for ceramic applications. As a reinforcing phase in ceramic materials, Ti₃C₂T_x MXene can significantly improve the strength and toughness of ceramics, while also imparting excellent oxidation and corrosion resistance. However, Ti₃C₂T_x MXene faces challenges in practical applications, particularly concerning thermal stability. Existing studies show that Ti₃C₂T_x MXene is prone to oxidation and degradation in high-temperature environments, limiting its use in ceramics, especially for applications requiring high-temperature sintering or long-term high-temperature stability. Specifically, under high-temperature conditions, surface oxidation of Ti₃C₂T_x MXene can lead to the destruction of its layered structure, weakening its adhesion to the ceramic matrix and reducing the overall material performance. Therefore, maintaining the integrity and functionality of Ti₃C₂T_x MXene during ceramic material fabrication is essential not only for ensuring its reinforcing effect in ceramics but also for enhancing the reliability and service life of ceramic-based composites in high-temperature environments. Finding ways to improve the thermal stability and high-temperature resistance of Ti₃C₂T_x MXene in ceramic material preparation is a critical challenge to address.

This review focuses on the preparation methods of Ti₃C₂T_x MXene and its applications in ceramic materials. Firstly, it explores various etching techniques, including acid etching, acid–salt composite etching, alkaline etching, and molten salt etching, while analyzing their working principles and effects on the structure and performance of MXene. Additionally, several emerging etching methods are introduced. Next, the review examines the delamination strategies for Ti₃C₂T_x MXene, with a focus on how different treatment methods effectively achieve its delamination. Finally, the paper summarizes the applications of Ti₃C₂T_x MXene in ceramic materials, particularly its potential to enhance the mechanical properties of ceramics. Through this review, we aim to provide scientific insights for further research and applications of Ti₃C₂T_x MXene, to lay the groundwork for its innovative applications in ceramics.

2. Etching Methods for Ti₃C₂T_x MXene

2.1. Acid Etching

In 2011, Naguib et al. [25] pioneered the use of hydrofluoric acid (HF) to etch Ti₃AlC₂, a member of the MAX phase family. Ti₃AlC₂ exhibits a typical M_n+1AX_n structure, where M (Ti) elements are bonded by metallic bonds, X (C) elements are covalently bonded to the M (Ti) elements, and the A (Al) layer is connected to the M (Ti) layer by weaker metallic bonds. During the etching process, HF breaks the Ti–Al metallic bonds in Ti₃AlC₂, leading to the removal of the Al layer and the formation of 2D Ti₃C₂T_x MXene. The specific etching process is illustrated in Figure 1 [26]. The chemical reactions involved are as follows [27]:

Ti₃AlC₂ + 3HF → AlF₃ + Ti₃C₂ + 2/3H₂

(1)

Ti₃C₂ + 2H₂O → Ti₃C₂(OH)₂ + H₂

(2)

Ti₃C₂ + 2HF → Ti₃C₂F₂ + H₂

(3)

The Al-based MAX phase, Ti₃AlC₂, is the most commonly used precursor for etching Ti₃C₂T_x MXene. However, Si-based MAX phases, such as Ti₃SiC₂, can also be used to synthesize Ti₃C₂T_x MXene, although their application is less common due to the stronger Ti–Si bonds. In 2020, Noor et al. [28] successfully etched Ti₃SiC₂ to obtain Ti₃C₂T_x MXene using a mixed solution of HF and hydrogen peroxide (H₂O₂). In their study, single-layer MXene was obtained by ultrasonic cooling in an ice bath, without the need for intercalation agents. X-ray diffraction (XRD) analysis (Figure 2a) showed that the peak at 2θ = 39°, corresponding to the Ti₃SiC₂ MAX phase, disappeared, indicating the successful removal of the silicon layer and the formation of Ti₃C₂T_x MXene. Furthermore, the (002) peak of the delaminated Ti₃C₂T_x MXene shifted from 8.9° to 5.9°, with the interlayer spacing increasing from 9.87 Å to 15.22 Å, further confirming the delamination process. Energy-dispersive X-ray spectroscopy (EDX) in Figure 2b,c demonstrated a reduction in silicon content following the HF/H₂O₂ treatment. Raman spectroscopy (Figure 2e) revealed that after the removal of the silicon layer, the single-layer Ti₃C₂T_x MXene surface adsorbed terminal groups such as -O, -OH, and -F. These surface modifications increased the number of atoms within the unit cell and influenced the lattice structure of Ti₃C₂T_x MXene. The morphology of the etched Ti₃C₂T_x MXene, shown in Figure 2d,f, exhibited an accordion-like structure, similar to that obtained from etching Al-based MAX phases.

In 2022, Kiran et al. [29] introduced a novel electric-field-assisted flash sintering technique to rapidly synthesize the Ti₃SiC₂ MAX phase. They then used HF to etch the Ti₃SiC₂ MAX phase to obtain Ti₃C₂T_x MXene. The etching process began with HF treatment to remove surface oxides from the MAX phase, followed by high-energy ultrasonic treatment of the pre-treated solution using a high-power ultrasonic cell disruptor. Assisted by high-energy ultrasound, HF acid effectively disrupted the Ti–Si bonds in the MAX phase, resulting in the formation of layered Ti₃C₂T_x MXene with an interlayer spacing of 14.56 Å.

A substantial body of research indicates that the quality of the precursor material plays a crucial role in determining the properties of Ti₃C₂T_x MXene after etching. Scheibe et al. [30] used a TiC–Ti₂AlC powder mixture as the starting material and prepared three different Ti₃AlC₂ MAX phase precursors under argon, air, and oxygen atmospheres, named Ti₃AlC₂–Ar, Ti₃AlC₂–Air, and Ti₃AlC₂–O₂, respectively. These precursors were then etched with HF to investigate how the structure of the MAX phase, influenced by different oxygen concentrations, affects the resulting Ti₃C₂T_x MXene. XRD (Figure 3a–c) and scanning electron microscopy (SEM) (Figure 3d–f) analyses revealed that the oxygen concentration significantly impacted the formation of α-Al₂O₃ inclusions between the MAX phase layers. As the oxygen concentration increased, the amount of a-Al₂O₃ inclusions also increased. During subsequent grinding and MXene preparation, these α-Al₂O₃ impurities were partially released, resulting in the formation of a porous structure in the MXene. However, some α-Al₂O₃ particles remained within the MXene and showed a tendency to grow, which reduced the specific surface area of the MXene. The specific surface areas of Ti₃C₂T_x–Ar, Ti₃C₂T_x–Air, and Ti₃C₂T_x–O₂ were measured as 13.7, 13.64, and 5.96 m²/g, respectively.

Wang et al. [31] demonstrated that Ti₃AlC₂ typically contains trace amounts of TiC, which can influence the lateral area and interlayer spacing of Ti₃C₂T_x after etching. These factors affect the mobility of charge carriers and the formation of a conductive network in Ti₃C₂T_x, ultimately impacting its electromagnetic absorption properties. Therefore, controlling the TiC content in the Ti₃AlC₂ MAX to adjust the structure and properties of the resulting MXene represents a feasible strategy for tailoring its performance. In addition, Ahmadian et al. [32] investigated the effects of three different ball-milling parameters (milling time, interval time, and rotation speed) on the synthesis of Ti₃AlC₂ MAX phase and the resulting Ti₃C₂T_x MXene. The study revealed that the intermediate Ti and Al compounds generated during the milling process are critical for the formation of Ti₃AlC₂. If these intermediate compounds do not form, the subsequent formation of the MAX phase during the spark plasma sintering (SPS) process is hindered. Furthermore, the temperature increase during milling plays a critical role in facilitating the formation of Ti and Al compounds. The optimal ball milling process involved shorter rest periods. HF etching of Ti₃AlC₂ MAX phase, synthesized under different milling parameters, resulted in Ti₃AlC₂ MXene particles with sizes ranging from 0.1 μm to 0.5 μm. Experimental results indicated that the highest quality MAX and MXene were achieved using a milling speed of 400 rpm, a milling time of 18 h, and a 5 min interval time.

Additionally, the structure and properties of Ti₃C₂T_x MXene are significantly influenced by the etching time. Benchakara et al. [33] demonstrated that high-concentration HF etching induces expansion of the Ti₃C₂T_x MXene due to the generation of H₂, resulting in a characteristic accordion-like structure. In contrast, low-concentration HF etching leads to insufficient etching, causing a more tightly stacked structure of Ti₃C₂T_x MXene. Furthermore, HF concentration plays a crucial role in regulating the content and distribution of surface functional groups on MXene, which in turn affects its surface properties. A higher HF concentration increases the -F content while decreasing the -O content in Ti₃C₂T_x, thereby raising the overpotential for the hydrogen evolution reaction (HER). Density functional theory (DFT) calculations further indicate that as the ratio of fluorine substituting oxygen on the MXene surface increases, the free energy change of Ti₃C₂T_x increases monotonically, which reduces its HER activity and simultaneously diminishes its oxidation stability. Specifically, the introduction of -F functional groups reduces the surface energy of Ti₃C₂T_x, decreasing its hydrophilicity and causing uneven electron cloud distribution, which in turn impacts its electronic conductivity and chemical stability. This alteration can lead to instability in the electrochemical performance of Ti₃C₂T_x under certain conditions.

Tran et al. [34] employed 49% HF to etch the Ti₃AlC₂ MAX phase at 5 °C and synthesize Ti₃C₂T_x MXene, exploring the influence of varying etching times on the quality of the resulting MXene. Figure 4 presents the XRD patterns and SEM images of the original Ti₃AlC₂ powder and the MXene prepared after various etching durations. From the intensity of the main peaks and the appearance of secondary peaks, it was found that Ti₃C₂T_x prepared after 24 h of etching exhibited superior quality compared to the other samples, a conclusion consistent with the SEM observations. Notably, after 12 h of etching, a clear TiC phase appeared as a secondary phase. After 36 h of etching, several characteristic peaks of Ti₃C₂T_x weakened, indicating structure degradation, which was also reflected in the SEM images. The Ti₃C₂T_x prepared displayed exceptional adsorption capabilities, achieving a removal efficiency of 91.9% within 5 min when 40 mg of Ti₃C₂T_x was added to a 20 μM methylene blue (MB) solution. This remarkable performance is closely associated with the presence of -F functional groups on the Ti₃C₂T_x surface. The -F groups confer a negative charge to the MXene surface, while the positively charged MB molecules interact electrostatically, facilitating the efficient removal of MB. This phenomenon highlights the critical role of surface functional groups in determining surface charge distribution and adsorption properties, with fluorine groups enhancing the adsorption capacity of MXene through polar interactions and electrostatic adsorption.

Similarly, Sumbe et al. [35] used 48% HF to etch Ti₃AlC₂ and examined the effect of etching time on the morphology of the resulting MXene. SEM images of Ti₃AlC₂ and Ti₃C₂T_x MXene prepared with varying etching times are shown in Figure 5. The results show that the MAX phase initially exhibits a tightly stacked layered structure. After 24 h of HF etching, some layers began to delaminate (Figure 5c,d), which was attributed to the selective removal of the aluminum layer. As the etching time increased to 36 h, more layers underwent delamination (Figure 5e,f). After 48 h of etching, over 90% of the layers had delaminated (Figure 5g,h). These findings suggest that the etching time should be carefully optimized, considering the structure of the original MAX phase, the concentration of the etching agent, and the desired properties of the MXene.

In addition to HF, researchers have explored alternative acidic solutions for the preparation of Ti₃C₂T_x MXene. In 2019, Scheibe et al. [36] investigated three distinct etching methods: (1) HF alone, (2) chlorosulfonic acid (ClHSO₃), and (3) a combination of HF and chlorosulfonic acid to etch Ti₃AlC₂, yielding Ti₃C₂T_x MXene with varying surface functional groups. The study found that MXene etched with either HF or chlorosulfonic acid alone exhibited paramagnetic behavior. However, when a mixture of HF and chlorosulfonic acid was used, the MXene displayed two distinct types of free radicals and/or defects in their electron paramagnetic resonance spectra, as well as a mixed antiferromagnetic/paramagnetic behavior. This magnetic ordering was attributed to the sulfur-based functional groups (e.g., -S₂ and -SO₃H) introduced by the chlorosulfonic acid. The results highlighted a strong correlation between the surface functional groups and the properties of Ti₃C₂T_x MXene, particularly the significant alteration in its electronic structure and magnetic properties upon the introduction of sulfur-based groups. This modification opens new avenues for the potential use of Ti₃C₂T_x MXene in magnetism and electronics.

In 2011, Naguib et al. [25] conducted geometric optimization of hydroxylated and fluorinated Ti₃C₂T_x MXene layers using DFT. The results revealed that the monolayer Ti₃C₂ structure exhibited typical metallic behavior, whereas the hydroxylated and fluorinated Ti₃C₂T_x demonstrated semiconducting properties, with energy gaps of 0.05 eV and 0.1 eV between the valence and conduction bands, respectively. This suggests that by modulating the types of surface functional groups on Ti₃C₂T_x, it is possible to effectively regulate its electronic cloud distribution and band gap, thereby tuning its performance. The -OH functional group enhances the hydrophilicity of MXene by forming hydrogen bonds with water molecules, thereby improving its performance in applications such as batteries and sensors. In contrast, the -F functional group significantly alters the electronic conductivity and stability of MXene, reducing its activity in oxidation reactions. Moreover, studies have shown that the presence of -F functional groups significantly lowers the surface energy, hydrophilicity, and electrical conductivity of Ti₃C₂T_x MXene, thus impacting its overall performance. Consequently, reducing the -F content on the surface of Ti₃C₂T_x MXene has become a key research focus [37]. In 2020, Xu et al. [38] successfully synthesized Ti₃C₂T_x MXene with -F functional groups on its surface by etching Ti₃AlC₂ with HF. The MXene was subsequently used as a co-catalyst for graphite-like carbon nitride (g-C₃N₄) and subjected to plasma treatment in a N₂ and H₂ atmosphere at 500 °C. This treatment effectively transformed the Ti–C functional groups on the Ti₃C₂T_x surface into Ti–O groups, particularly Ti⁴⁺, while removing some of the -F functional groups. This modification notably enhanced the photocatalytic activity of the Ti₃C₂T_x MXene, mainly due to Ti⁴⁺’s ability to capture photo-induced electrons from g-C₃N₄, improving the separation of electrons and holes under light irradiation, and ultimately enhancing photocatalytic performance. In the same year, Yang et al. [39] prepared Ti₃C₂T_x MXene from Ti₃AlC₂ via HF etching and further treated it with sodium hydroxide (NaOH) solution to form alkali Ti₃C₂T_x. This alkali treatment converted some Ti₃C₂F_x into Ti₃C₂O_x, effectively reducing the fluorination caused by the HF etching and significantly improving the performance of MXene in NH₃ and humidity sensors. For the NH₃ sensor, the response signal of the alkali-treated Ti₃C₂T_x to 100 ppm NH₃ at 25 °C was twice that of the non-alkalized sample. This enhancement was primarily attributed to the adsorption of NH₃ molecules on the Ti atoms of Ti₃C₂T_x, where they form chemical bonds. Additionally, the presence of -O functional groups on the surface of alkali-treated Ti₃C₂T_x further strengthened the N–Ti bonding sites, increasing the adsorption capacity for NH₃. In the humidity sensor, residual sodium ions (Na⁺) in the interlayer of the alkali-treated Ti₃C₂T_x reacted with water molecules to form [Na(H₂O)m]⁺, promoting the adsorption of water molecules.

The acid etching method, particularly HF etching, remains one of the most widely used and extensively studied methods for preparing Ti₃C₂T_x MXene. This method offers the advantages of simplicity and operational convenience, effectively removing the Al or Si layers from the MAX phase to produce pure Ti₃C₂T_x MXene. Additionally, HF etching allows for the optimization of MXene’s interlayer spacing and surface functional groups by adjusting the etching conditions. However, the environmental and safety concerns associated with HF etching cannot be overlooked. HF solutions are highly corrosive and pose significant safety risks to personnel and the environment, especially in large-scale production where waste disposal becomes challenging. Moreover, the fluorine-rich functional groups produced by HF etching can reduce the chemical reactivity or electrochemical stability of MXene in certain applications. While HF etching remains a cornerstone of MXene research and applications, its environmental and safety drawbacks have driven researchers to explore more sustainable alternatives.

2.2. Acid–Salt Composite Etching

To overcome the drawbacks associated with directly using HF etching agents, researchers have explored alternative methods that generate HF in situ. In 2014, Ghidiu et al. [40] pioneered the use of a mixture of lithium fluoride (LiF) and hydrochloric acid (HCl) to generate HF in situ, successfully etching the Ti₃AlC₂ MAX phase and obtaining Ti₃C₂T_x MXene. The Ti₃C₂T_x produced via this method exhibited volumetric expansion upon hydration, allowing it to be molded like clay. After drying, it transformed into a highly conductive solid or could be rolled into thin films several tens of micrometers thick. These films exhibited a capacitance of up to 900 F/cm³ and demonstrated excellent cycling stability and rate performance. Compared to traditional HF etching methods, the LiF + HCl etching process facilitates the preparation of single-layer MXene sheets with large lateral dimensions. Studies showed that over 70% of the Ti₃C₂T_x MXene sheets had lateral dimensions ranging from 0.5 to 1.5 μm, with approximately 70% of the sheets consisting of 1 to 2 layers.

In 2016, Ghidiu et al. [41] further investigated the impact of LiCl on the etching of Ti₃AlC₂ MAX phase into Ti₃C₂T_x MXene. Their study revealed the presence of Li⁺ ions between the MXene layers, which eliminated the need for subsequent intercalation steps and facilitated direct delamination. Additionally, as shown in Figure 6, they demonstrated successful ion exchange, replacing Li⁺ with cations such as K⁺, Na⁺, Rb⁺, Mg²⁺, and Ca²⁺. The study revealed that the intercalated cations induced changes in the material’s behavior, such as expansion at the unit lattice level in response to humidity, mimicking the swelling properties of clay minerals. In the same year, Sharma et al. [42] employed oxide molten liquid calorimetry to study the energy changes during the transformation of the MAX phase to MXene when etched with the HCl–LiF solution. Their findings showed that the MXene produced via this method exhibited exceptionally high stability. Compared to MXene etched with high-concentration HF, those treated with the HCl–LiF mixture displayed significantly larger interlayer spacing, which was primarily attributed to the intercalation of Li⁺ ions. Further work by Kumar et al. [43] in 2020 examined the effect of etching temperature (25 °C, 50 °C, and 80 °C) on the quality of Ti₃C₂T_x MXene synthesized from Ti₃AlC₂ using a concentrated hydrochloric acid (12 M) and LiF mixture solution. The study found that all MXene layers produced by etching had a thickness of less than 50 nm. As the etching temperature increased, both the etching of the MAX phase and the delamination of the MXene layers were enhanced. However, excessively high temperatures led to deterioration in the quality of the MXene, resulting in surface defects that reduced the active sites for pseudo-capacitive redox reactions and, consequently, a decrease in specific capacitance. Elevated temperatures also led to the formation of additional oxidized groups on the MXene surface, which hindered the formation of the electric double layer and further reduced specific capacitance. Electrochemical impedance spectroscopy analysis revealed that MXene etched at 50 °C exhibited superior performance in supercapacitors, with lower series resistance, smaller charge transfer resistance, and reduced Warburg impedance.

During the in situ etching process, where fluoride salts react with strong acids to generate HF, an undesirable byproduct that often forms is AlF₃·3H₂O. This byproduct can negatively affect the purity and properties of the Ti₃C₂T_x MXene. The typical accordion-like structure of pure Ti₃C₂T_x MXene is shown in Figure 7a, while Figure 7b–d illustrate the MXene structure containing AlF₃·3H₂O impurities [44]. In 2019, Cockreham et al. [44] proposed a method using cobalt fluorides (CoF₂/CoF₃) with HCl to in situ generate HF, successfully etching Ti₃AlC₂. They also investigated the factors influencing the formation of AlF₃·3H₂O impurities in the MXene, focusing on the etching environment. Their study revealed that the ionic strength of the etching solution is a critical factor driving the formation of AlF₃·3H₂O impurities. Specifically, the ionic strength directly impacts the thermodynamic stability of the AlF₃ complex. When the ionic strength of the solution is between approximately 8.5 and 10 M, the AlF₃ complex remains stable, promoting the formation of AlF₃·3H₂O impurities in the Ti₃C₂T_x MXene. Conversely, when the ionic strength is either below 8.5 M or above 10 M, the formation of AlF₃·3H₂O impurities is minimized, resulting in a product closer to pure MXene. This finding underscores the importance of carefully controlling the ionic strength during the etching process to minimize undesirable impurities and achieve higher-quality Ti₃C₂T_x MXene.

In acid–salt composite etching methods, the choice of fluoride salts significantly influences the structure and properties of the resulting MXene. In 2020, Benchakar et al. [33] examined the effects of different fluoride salts, including LiF and iron fluoride (FeF₃), on the surface chemistry of Ti₃C₂T_x MXene. The study demonstrated that etching with a LiF/HCl mixed solution produced conductive, clay-like MXene material, which facilitates further processing [45]. When LiF was replaced with FeF₃, the conversion rate from MAX phase to MXene was accelerated, primarily due to the oxidative properties of Fe³⁺, which is beneficial for reducing the production costs of MXene. This occurs because, during Ti₃C₂T_x synthesis, Fe³⁺ can oxidize the surface Ti atoms, forming Ti₃C₂T_x@TiO₂-xF_2x composites. This oxidation process aids in the exfoliation of MXene, thus reducing the production cost. However, in contrast to the LiF/HCl method, the FeF₃-based etching does not spontaneously exfoliate MXene in water. Furthermore, increasing the etching temperature and prolonging the etching time led to MXene dissolution, reducing the overall yield. In 2024, Yao et al. [46] employed high-intensity ultrasonic-assisted technology to successfully synthesize a series of MAX–MXene heterostructures (MAM) by adjusting the composition of etching agents (HCl and sodium fluoride (NaF)). Figure 8a,b show the microstructures of the original MAX phase and MAM. The original MAX phase (Figure 8a) displays an uneven size distribution and a compact, block-like structure. After etching with NaF+HCl, the MAX phase begins to exfoliate and break down, partially forming MAM nanostructures, as shown in the low-magnification SEM image in Figure 8b. Figure 8c–e show MAM products prepared with varying amounts of etching agents. It is evident that as the etching agent concentration increases, the MAM layer thickness decreases, and surface impurities, such as aluminum fluoride (AlF₃) compounds containing O and C, are reduced [47]. Furthermore, the study found that the MXene generated on the MAM surface significantly enhanced electron transport efficiency, resulting in improved conductivity (with a minimum resistance of just 7 kΩ). The material’s surface also featured abundant adsorption sites, which greatly enhanced its gas sensing performance.

In recent years, researchers have expanded beyond traditional HCl etching methods to explore composite etchants that combine other acids with fluoride salts. In 2024, Huang et al. [48] pioneered the use of Lewis acids and boron trifluoride (BF₃) as electron-deficient etchants in sulfuric acid (H₂SO₄) solution to etch aluminum from Ti₃AlC₂ MAX phase, resulting in the formation of B-doped Ti₃C₂T_x MXene. The specific etching process is shown in Figure 9a. As seen in the XRD results in Figure 9b, the incorporation of B led to broader diffraction peaks and an increase in the interlayer spacing of the material. Further analysis of the specific surface area of Ti₃AlC₂ MAX phase and Ti₃C₂T_x MXene (Figure 9c) revealed that after etching, the specific surface area of the B-doped Ti₃C₂T_x MXene (2.9125 m²/g) was approximately four times greater than that of the original MAX phase (0.7331 m²/g). This increase in surface area is attributed to the bonding of exposed Ti with elements from the etching solution, leading to functionalization with -F, -O, and -B groups, which in turn enhances the surface area. Additionally, a comparison of Figure 9d,e reveals that the etched Ti₃C₂T_x MXene transitioned from the dense, stacked structure of Ti₃AlC₂ to a more distinctly delaminated structure. The B-doped Ti₃C₂T_x MXene exhibited a larger interlayer spacing, with B inserted between the MXene layers. This enhancement contributed to a greater pseudocapacitive effect, improving the charge transfer and storage capabilities of the MXene electrode. At a current density of 1 A/g, the material demonstrated an energy density of 3.97 Wh/kg, a power density of 383.90 W/kg, and a maximum capacitance of 396 F/g. Additionally, the MXene showed excellent capacitance retention, highlighting its promising supercapacitor characteristics.

In 2024, Jee et al. [49] developed a more environmentally friendly method for synthesizing Ti₃C₂T_x MXene by employing a weak acid, phosphoric acid (H₃PO₄), in combination with LiF, zinc fluoride (ZnF₂), and copper fluoride (CuF₂) as etchants. The specific etching reaction pathway is as follows:

H₃PO₄ → 3H⁺ + PO4³⁻/CuF₂ → Cu²⁺ +2F⁻

(4)

Al³⁺ + 3F⁻ → AlF₃/Al³⁺ + PO4³⁻ → AlPO₄

(5)

Ti₃AlC₂ → Al³⁺ +3e⁻ + Ti₃C₂/Cu²⁺ +2e⁻ → Cu

(6)

In this method, Al in the Ti₃AlC₂ MAX phase is removed through two etching mechanisms. First, Al reacts with phosphate ions (PO₄³⁻) and fluoride ion (F⁻) to form aluminum phosphate (AlPO₄) and aluminum fluoride (AlF₃). Second, lithium ions (Li⁺), zinc ions (Zn²⁺), and copper ions (Cu²⁺) facilitate the remove of Al from Ti₃AlC₂ via an electrochemical displacement reaction. The combination of H₃PO₄ and CuF₂ demonstrates significantly higher Al removal efficiency compared to LiF and ZnF₂. This enhanced efficiency is attributed to the superior ionization tendency of Cu²⁺. The H₃PO₄/CuF₂ combination allows for rapid etching of Ti₃AlC₂ MAX at low temperatures (35 °C) within just 4 h, achieving substantial etching effects. In contrast, traditional HF etching methods typically require more than 24 h to achieve similar results.

Compared to traditional HF etching, the acid–salt composite etching method mitigates some of the limitations associated with HF etching. By introducing fluoride salts into an acidic solution, HF is generated in situ, thereby avoiding the direct use of HF and reducing both safety and environmental risks. This method also lowers the fluorine content on the MXene surface, improving its chemical stability and electrochemical performance. Furthermore, the presence of salts helps regulate ion concentration and diffusion rates in the reaction system, enhancing etching efficiency and ensuring more uniform product quality. However, despite these advantages, the method still operates within an acidic environment, meaning its environmental impact is not entirely mitigated. Additionally, in comparison to single-acid etching methods, the acid–salt composite etching approach introduces greater process complexity, requiring precise control over the acid-to-salt ratio and other reaction conditions. While this method addresses some of the shortcomings of traditional HF etching, further refinement is necessary to enhance its efficiency and sustainability.

2.3. Alkaline Etching

Compared to traditional MAX phase etching methods, it is theoretically possible to etch the A-layer using an alkaline solution. In 2018, Li et al. [50] successfully etched MAX phases using a NaOH-assisted hydrothermal process, resulting in Ti₃C₂T_x MXene with a purity of up to 92% (NaOH–Ti₃C₂T_x). Since this etching process does not involve fluoride compounds, the resulting MXene surface functional groups are primarily -OH and -O, rather than -F. The hydroxyl and oxygen functional groups contribute to the formation of pseudo-capacitive effects associated with hydrogen ion (H⁺), thereby enhancing the capacitive performance of the MXene. Testing results indicate that in 1 M H₂SO₄, the volumetric capacitance of NaOH–Ti₃C₂T_x (511 F/cm³, 1.63 mV/s) is 43.9% higher than that of Ti₃C₂T_x (355 F/cm³, 2.2 g/cm³). Additionally, after 10,000 cycles, it retains 89.1% of its capacitance, demonstrating excellent stability and durability. However, it is important to note that the formation of aluminum hydroxide and hydrated alumina is inevitable in the alkaline etching environment, which may hinder MXene synthesis. Despite this, under high-temperature and high-pressure conditions, concentrated alkaline solutions can effectively dissolve aluminum oxides and hydroxides, thus promoting MXene synthesis. As shown in Figure 10a–d, the content of Ti₃C₂T_x increases with rising temperature. This can be attributed to the endothermic nature of the dissolution reactions of Al oxides and hydroxides, where higher temperatures facilitate these reactions, thereby promoting MXene synthesis [51,52]. In contrast, lowering the temperature halts the reaction, indicating that temperature is a critical factor influencing the progress of the etching process. Additionally, at a constant reaction temperature, decreasing the alkaline concentration leads to a significant increase in the water content of the obtained MXene samples. Further experimental results reveal that when Ti₃C₂T_x is treated with 27.5 M NaOH at 270 °C, the {002} interlayer spacing increases to 24 Å, which is considerably larger than the {002} interlayer spacing of Ti₃C₂T_x prepared by the HF method (20 Å), as reported by Naguib et al. [25] and Mashtalir et al. [53]. This suggests that sodium ion (Na⁺) from the high-concentration NaOH solution is incorporated into the interlayer structure of the MXene. This hypothesis was supported by X-ray photoelectron spectroscopy (XPS) analysis (Figure 10e), where a decrease in the Al signal of Ti₃C₂T_x was accompanied by the appearance of a Na signal. SEM images in Figure 10f–h show that the NaOH–Ti₃C₂T_x exhibits a closely stacked layered structure, distinct from the typical accordion-like structure obtained using high-concentration (50%) HF etching, and more similar to the MXene structure formed by the slow H₂ generation during low-concentration (5%) HF treatment. The resulting Ti₃C₂T_x has lateral dimensions of approximately 80–200 nm, a specific surface area of 16 m²/g (higher than the 9 m²/g of the original Ti₃AlC₂), and an interlayer spacing of approximately 1.6 nm (larger than the 0.93 nm of the original Ti₃AlC₂). These results suggest that the NaOH-assisted hydrothermal method can effectively tune the structure and performance of Ti₃C₂T_x, providing new insights for MXene synthesis and application.

In 2024, Khan et al. [54] successfully synthesized Ti₃C₂T_x@Al–NaOH composites using an alkaline-assisted hydrothermal method at various NaOH concentrations (22.5 M, 25 M, 30 M, 35 M, and 40 M) at 280 °C. As shown in the XRD patterns in Figure 11a–c, most of the diffraction peaks in the etched products disappeared compared to pure Ti₃AlC₂, and the peaks between 5° and 80° were significantly weakened and broadened. These results suggest that the reaction between Ti₃AlC₂ and NaOH at high temperature led to the formation of a new structure. Furthermore, the Ti₃C₂T_x@Al–NaOH (30 M) sample exhibited a slight shift in the (002) peak position, along with the appearance of a characteristic (104) peak, further confirming the surface functionalization of the MXene with -OH and -O groups. The presence of these functional groups enhances the pseudocapacitive effect of Ti₃C₂T_x. Further investigations revealed that the Ti₃C₂T_x@Al–NaOH sample synthesized at a NaOH concentration of 30 M with 15 h of etching demonstrated a capacitance increase of approximately 465% compared to Ti₃C₂T_x MXene synthesized using the traditional HF etching method. This significant enhancement in capacitance indicates that Ti₃C₂T_x@Al–NaOH samples have considerable potential for use in supercapacitors and other energy storage devices.

In 2024, Colkesen et al. [55] utilized both potassium hydroxide (KOH) alkaline solution and HF as etchants to etch Ti₃AlC₂, thereby obtaining Ti₃C₂T_x with different surface functional groups. The etching equations for both methods are as follows:

Ti₃AlC₂ + 3HF → Ti₃C₂T_x + 3/2H₂ + AlF₃

(7)

Ti₃AlC₂ + KOH + H₂O → Ti₃C₂T_x + KAlO₂ + 3/2H₂

(8)

Figure 12a,c show the SEM images of Ti₃C₂T_x MXene synthesized by HF and KOH etching (i.e., HF–MXene and KOH–MXene). Both exhibit the characteristic accordion-like structure, confirming the successful synthesis of 2D MXene nanosheets. Figure 12b,d present the energy dispersive spectroscopy (EDS) results for HF–MXene and KOH–MXene. Compared to HF–MXene (3.42 at.%), the Al content in KOH–MXene (1.77 at.%) is significantly reduced, indicating that KOH is more effective than HF in removing metallic Al from the MAX phase. Additionally, the study reveals that the O content in MXene primarily originates from deionized water washing. The oxygen content in KOH–MXene (31.52 at.%) is notably higher than in HF–MXene (18.33 at.%). This can be attributed to the fact that during HF etching, the -F groups occupy part of the exposed Ti surface, leaving only the remaining free surface for -O groups. As a result, the oxygen content in HF–MXene is considerably lower than in KOH–MXene. Further investigation indicates a significant difference in the ability of HF–MXene and KOH–MXene to remove Sr²⁺. HF–MXene achieves a maximum Sr²⁺ removal rate of 82.0%, while KOH–MXene removes 99.3% of Sr²⁺. The Sr²⁺ adsorption capacity of HF–MXene is 180.48 mg/g, while KOH–MXene’s is 218.40 mg/g. These data suggest that KOH–MXene demonstrates superior adsorption capacity for Sr²⁺ compared to HF–MXene, which is closely related to the differences in surface functional groups. During the adsorption process, Sr²⁺ is effectively adsorbed onto the KOH–MXene surface via electrostatic interactions with -OH and -O functional groups. In contrast, the -F functional groups on the surface of HF–MXene reduce its ability to adsorb Sr²⁺. Additionally, KOH–MXene outperforms HF–MXene in terms of reusability. KOH–MXene retains over 97% removal efficiency after five consecutive adsorption cycles, whereas HF–MXene stabilizes around 80%. These results demonstrate that KOH–MXene has superior adsorption performance and recyclability, highlighting its potential for environmental pollution remediation.

Alkaline etching has emerged as a novel method for preparing MXene, using high-concentrated alkaline solutions to etch the MAX phase and produce MXene materials with fluoride-free terminations. This fluoride-free structure significantly enhances MXene’s hydrophilicity and environmental compatibility, broadening its potential applications in specialized fields such as biomedicine. Additionally, alkaline etching typically offers high reaction efficiency, enabling rapid etching within a relatively short time frame. However, several challenges remain. First, alkaline etching generally requires high concentrations of alkaline solutions, elevated temperatures, and extended processing times, leading to high energy consumption and increased safety risks in large-scale production. Second, MXene synthesized using alkaline etching often exhibits a characteristic accordion-like morphology, necessitating further intercalation and delamination steps to obtain single- or few-layer MXene. This adds to the complexity and costs of the process. Additionally, alkaline waste generated during etching can pose environmental risks if not properly managed. Future research should focus on reducing energy consumption and waste treatment costs while optimizing the etching process to enhance the quality and safety of MXene.

2.4. Molten Salt Etching

In 2020, Li et al. [56] introduced a novel method for synthesizing MXene by controlling the redox reaction of A-site elements in MAX phases using Lewis acidic molten salts. This innovative method not only expanded the synthesis routes for MXene but also significantly improved its electrochemical performance, particularly in non-aqueous electrolytes. Furthermore, this method allows for the selective tuning of MXene’s surface functional groups by choosing specific Lewis acid etchants. The etching steps are illustrated in Figure 13, with the corresponding reaction given as follows:

Ti₃SiC₂ + 2CuCl₂ → Ti₃C₂ + SiCl₄↑ + 2Cu

(9)

Ti₃C₂ + CuCl₂ → Ti₃C₂Cl₂ + Cu

(10)

Traditional MXene synthesis methods typically require extended processing times and impose stringent etching conditions, resulting in low efficiency and limited scalability. These factors hinder the commercialization and broader applications of MXene. Therefore, there is a pressing need for more efficient methods that allow for the rapid synthesis of MXene. In 2024, Wang et al. [57] developed a rapid, large-scale synthesis method for various MXenes, including V₄C₃T_x, Nb₄C₃T_x, Mo₂TiC₂T_x, Mo₂CT_x, and Ti₃C₂T_x, using ammonium bifluoride (NH₄HF₂) as the etchant in combination with ultra-fast low-temperature molten salt (LTMS) etching. In a single reaction, a yield of Ti₃C₂T_x can exceed 100 g, with the resulting Ti₃C₂T_x exhibiting a multi-layer structure with an interlayer spacing of 1.27 nm. The etching process is depicted in Figure 14a, with the reaction equation as follows:

Ti₃AlC₂ + 3NH₄HF₂ → Ti₃C₂ + (NH₄)₃AlF₆ + 3/2H₂

(11)

Figure 14b highlights three key advantages of the LTMS method over conventional methods. First, molten NH₄HF₂ exhibits good fluidity, enabling it to quickly penetrate the Al atomic layers within the MAX phase. This accelerates the removal of by-products such as (NH₄)₃(AlF)₆, preventing interlayer clogging in the MXene. Second, the H₂ produced during the etching process increases the interlayer spacing of the MXene, which enhances the accessibility of Al atomic reaction sites, further promoting the etching reaction. Finally, the LTMS etching process is exothermic, and the heat generated during the reaction rapidly raises the temperature of the etching system, accelerating the etching kinetics and completing the reaction in just 5 min. In contrast, traditional high-temperature molten salt methods typically require temperatures above 700 °C, whereas the LTMS method achieves the etching of Ti₃AlC₂ at a relatively low temperature of 130 °C, offering a significant advantage in terms of energy efficiency. These characteristics make the LTMS method highly promising for cost-effective, large-scale industrial production of MXene.

Molten salt etching has gained attention as a novel, environmentally friendly method for MXene preparation. Unlike traditional acid or alkali etching, this method employs high-temperature molten salts to etch the MAX phase, eliminating the need for hazardous acid or alkaline solutions and significantly reducing risks to both personnel and the environment. The method also offers high etching efficiency and excellent process controllability, making it an attractive option for the large-scale, environmentally friendly production of MXene. However, molten salt etching has some limitations. The process requires high temperatures, leading to substantial energy consumption, and the selection and disposal of molten salts must be carefully managed to minimize environmental impacts. Despite its potential, further research is needed to address challenges related to energy efficiency, salt recycling, and waste management, paving the way for a more sustainable and cost-effective production process.

2.5. Other Etching Methods

In recent years, researchers have explored a variety of etching methods for synthesizing Ti₃C₂T_x MXene, proposing several novel techniques that differ significantly from traditional methods. In 2021, Shi et al. [58] introduced an innovative method for synthesizing Ti₃C₂T_x MXene using iodine, resulting in MXene with oxygen-rich terminal groups. The etching mechanism of this method is illustrated in Figure 15a–c, and the corresponding chemical reactions are as follows:

Ti₃AlC₂ + (x + 3)/2I₂ → Ti₃C₂I_x + AlI₃

(12)

Ti₃C₂I_x + x/2O₂ → Ti₃C₂O_x + x/2I₂

(13)

Ti₃C₂I_x + xH₂O → Ti₃C₂(OH)_x + xHI

(14)

AlI₃ + 3H₂O → Al(OH)₃ + 3HI

(15)

Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O

(16)

Characterization of the iodine-etched Ti₃C₂T_x MXene (IE–MXene) was performed using TEM and AFM, with the results presented in Figure 15d–i. These characterizations revealed that the crystal structure of IE–MXene remained intact, with no obvious lattice defects, owing to the mild etching treatment in anhydrous solvents. Furthermore, as shown in Figure 15h,i, more than 71% of the layers in IE–MXene exhibited a thickness of less than 5 nm, with an average lateral size of 1.8 μm, significantly larger than the MXene produced by traditional HF etching (with a lateral size typically under 500 nm) [59,60]. The larger flake size and thinner structure of IE–MXene facilitate the preparation of MXene films through dispersion filtration. Compared to HF-etched MXene, IE–MXene exhibited superior environmental stability due to the absence of fluoride ions and the presence of oxygen-rich functional groups on its surface. These oxygen-containing groups not only enhanced the environmental stability of IE–MXene but also provided active sites, thereby facilitating the adsorption of H⁺ and significantly improving its specific capacitance.

In 2022, Oh et al. [61] proposed a rapid and efficient method for synthesizing Ti₃C₂T_x MXene using anhydrous dimethyl sulfoxide (DMSO) solution. This method employs DMSO as a solvent, combined with ammonium bifluoride (NH₄HF₂) and methanesulfonic acid (CH₃SO₃H) as etchants, to achieve high-quality Ti₃C₂T_x etching at 100 °C within just 4 h. The resulting DMSO–Ti₃C₂T_x exhibits a flake size of 4.2 ± 2.2 µm, with the etching process detailed in Figure 16a. The XRD pattern in Figure 16b confirms the complete etching of Ti₃AlC₂. SEM images in Figure 16c,d reveal the typical morphology of exfoliated single-layer Ti₃C₂T_x nanosheets. Further analysis using TEM and SAED (Figure 16e,f) demonstrates the hexagonal crystal structure of the DMSO–Ti₃C₂T_x flakes. AFM measurements (Figure 16g) indicate that the thickness of single-layer DMSO–Ti₃C₂T_x is approximately 1.6 nm, consistent with the single-layer thickness reported by Lipatov et al. [62]. Notably, in contrast to Ti₃C₂T_x synthesized by traditional HF etching, DMSO–Ti₃C₂T_x does not disperse in pure water [63]. However, it achieves excellent dispersion in a DMSO–water mixed solution through simple stirring. As shown in Figure 16h, the etching rate of this method significantly outperforms other conventional aqueous or anhydrous synthesis methods, highlighting its exceptional efficiency [64,65,66,67,68,69]. Additionally, DMSO–Ti₃C₂T_x exhibits outstanding mechanical properties, with films fabricated from it achieving an ultimate tensile strength of 167 ± 8 MPa, attributed to the larger flake size.

Wang et al. [70] demonstrated that tetramethylammonium hydroxide (TMAOH) solution can be used as an effective etchant for Ti₃AlC₂, successfully synthesizing fluorine-free Ti₃C₂T_x. The Ti₃C₂T_x produced by this method exhibits abundant active sites and excellent hydrophilicity. The etching process is depicted in Figure 17a, and the corresponding chemical reaction is as follows:

Ti₃AlC₂ + OH⁻ + H₂O → Ti₃C₂T_x + Al(OH)₃ + H₂

(17)

Figure 17b–d show the microscopic morphology of Ti₃AlC₂, HF-etched Ti₃C₂T_x (F-Ti₃C₂T_x), and TMAOH-etched Ti₃C₂T_x (Ff–Ti₃C₂T_x). The Ff–Ti₃C₂T_x exhibits a typical few-layer nanosheet morphology with a lateral size of approximately 15 μm, a layer thickness of ~0.6 nm, and a specific surface area of 17.37 m²/g. TEM analysis (Figure 17e,f) reveals lattice spacings of 0.227 nm and 0.270 nm for Ff–Ti₃C₂T_x, corresponding to the (103) and (110) planes of Ti₃C₂, respectively, indicating a well-ordered crystal structure. Elemental mapping via TEM (Figure 17h–k) demonstrates that the O, C, and Ti are uniformly distributed across the Ff–Ti₃C₂T_x nanosheets, further confirming the structural consistency. Fourier transform infrared spectroscopy (FTIR) (Figure 17g) was employed to analyze the surface functional groups. The results indicate that compared to F–Ti₃C₂T_x, Ff–Ti₃C₂T_x contains a greater abundance of oxygen-containing functional groups, such as C=O, C-O, O-H, and Ti-O. The presence of these groups greatly improves the hydrophilicity of Ff–Ti₃C₂T_x, underscoring its versatility for various applications.

In addition to the previously discussed etching methods, Chen et al. [71] introduced a novel approach in 2023 using Bis(trifluoromethanesulfonyl)imide (TFSI) as an etchant to selectively dissolve the Al layers in Ti₃AlC₂, thereby synthesizing Ti₃C₂T_x MXene nanosheets. The etching process is illustrated in Figure 18a. SEM images in Figure 18b,d show the morphology of Ti₃AlC₂ MAX before and after TFSI treatment, respectively. Prior to treatment, Ti₃AlC₂ MAX exhibits a distinct micron-scale layered structure, whereas the TFSI-treated Ti₃C₂T_x MXene displays a characteristic accordion-like morphology. TEM analysis (Figure 18c) further confirms that Ti₃C₂T_x MXene maintains a well-ordered crystal structure, with a lattice spacing of 0.25 nm corresponding to the (006) plane of Ti₃C₂. A key advantage of this method is that TFSI serves both as an etchant and as an intercalating agent, simplifying the experimental procedure while enhancing efficiency. This dual functionality streamlines the synthesis process, offering an effective and rational approach to producing high-quality Ti₃C₂T_x MXene.

The above content provides a comprehensive overview of various etching methods for Ti₃C₂T_x MXene, each offering unique advantages depending on specific requirements and experimental conditions.

Table 1. The representative synthesis methods for Ti₃C₂T_x MXene.

Etching Methods	Etchant	Etching Environment	Ref.	Advantages	Disadvantages
Acid etching	50% HF	Room temperature, 24 h	[72]	①Simple process, easy to operate; ②Effectively removes Al or Si layers from MAX phase, resulting in high-purity Ti3C2Tx MXene.	①HF solution is highly corrosive, posing safety risks; ②Waste liquid treatment is challenging, with significant environmental impact; ③Surface rich in fluorine functional groups, which may reduce chemical activity or lead to instability in performance.
	49% HF	50 °C, 36 h	[73]
	48% HF	Room temperature, 24 h	[74]
	40% HF	40 °C, 30 h	[75]
	50% HF	25 °C, 24 h	[76]
	40% HF	25 °C, 24 h	[77]
Acid–salt composite etching	50 mL 6 M HCl + NH4F	50 °C, 24 h	[78]	①HF is generated in situ, reducing the safety and environmental risks of directly using HF; ②Reduces the content of fluorine functional groups, enhancing chemical stability and electrochemical performance; ③The introduction of salt improves etching efficiency and uniformity of the product quality.	①Still an acidic system, environmental impact is not fully eliminated; ②High process complexity, requiring precise control of the acid and salt ratio and reaction conditions.
	4 g LiF + 50 mL 9 M HCl	Room temperature, 200 rpm, 72 h	[79]
	12 M LiF + 9 M HCl	Room temperature, 24 h	[80]
	6.0/7.5/9.0/12.0 M LiF + 9.0 M HCl	Room temperature, 300 rpm, 24 h	[81]
	2 g LiF + 40 mL HCl (9.0 mol·L−1)	35 °C, 30 h	[82]
	2 g NaF + 40 mL HCl (12 M)	60 °C, 48 h	[83]
	0.8 g LiF + 10 mL 9 M HCl	Room temperature, 48 h	[84]
Alkaline etching	30 mL 10 M NaOH	Room temperature, 1–5 day	[85]	①Generates fluorine-free terminated MXene, improving hydrophilicity and environmental friendliness; ②High etching efficiency, short reaction time.	①Requires high-concentration alkaline solution, high temperature, and long reaction times, leading to high energy consumption and increased safety risks; ②May generate alkaline waste liquid, requiring proper disposal to prevent environmental pollution
	50 mL 30%NaOH	100 °C, 24 h	[86]
	50 mL 7 mol/LKOH	180 °C, Hydrothermal treatment, 24 h	[55]
	50 mL 22.5/25/30/35/40 M NaOH	280 °C, 15 h	[54]
Molten salt etching	NH4HF2 molten salt	130 °C, 5 min	[57]	①Avoids the use of strong acids or bases, offering better chemical safety and environmental friendliness; ②Suitable for large-scale green production.	①Requires high-temperature operation, leading to higher energy consumption.
Other etching methods	NH4HF2 (6 g), CH3SO3H (6 mL) and NH4PF6 (6 g) dissolved in anhydrous DMSO (34 mL)	100 °C, 4 h	[61]	/	/
	40mL TMAOH solution	40 °C, Oil bath, 1 week	[70]
	400 mg TFSI + 50 mL Acetic acid solution	150 W, Ultrasonic homogenizer stirring, 2 h	[71]

summarizes the key etching parameters of different etching methods, facilitating the selection of the most suitable approach for different technical needs.

It is important to note that due to the abundance of active sites on the MXene surface, it is highly reactive with environmental substances like water and oxygen, resulting in alterations to the surface functional groups and, as a consequence, influencing the material’s properties. This issue is particularly prominent under humid or high-temperature conditions, where MXene is susceptible to oxidation or degradation, which can result in performance degradation. Therefore, Ti₃C₂T_x MXene typically cannot remain stable in solution for extended periods and is often required to be freshly prepared for immediate use, limiting its long-term storage and practical applications. To address this issue, researchers are exploring various methods to improve the stability of MXene, thereby enhancing its durability and extending its lifetime.

In 2022, Ashok et al. [87] improved the stability of Ti₃C₂T_x by treating it with NH₄OH following etching Ti₃AlC₂ using an HCl and LiF solution. During this process, NH₄OH reacted with the in situ generated HF, forming the buffer compound NH₄F, which played a crucial role in regulating the oxidation behavior and exfoliation properties of Ti₃C₂T_x. This modification enhanced the structural integrity and morphology of Ti₃C₂T_x, leading to a superior antioxidant performance in air. Moreover, Ashok et al. observed that the Ti₃C₂T_x produced via in situ HF etching exhibited strong pH dependence in its colloidal solution. Under neutral pH conditions, the edges of Ti₃C₂T_x carried negative charges, while the surface exhibited positive charges, thus promoting stability. In acidic conditions, Ti₃C₂T_x underwent sedimentation, whereas in alkaline pH, flocculation occurred only when the pH exceeded 12. After NH₄OH modification, the pH of the solution stabilized around 6, enabling Ti₃C₂T_x to remain stable for over 40 days without aggregation.

In 2023, Li et al. [88] treated Ti₃C₂T_x, etched with LiF and HCl, using tetramethylammonium hydroxide (TMAOH). This treatment successfully removed etching by-products and increased the number of oxygen-containing functional groups on the Ti₃C₂T_x surface, significantly enhancing its durability and lifetime. After TMAOH treatment, the relative atomic ratio of fluorine in Ti₃C₂T_x decreased from 41.44% to 11.34%, while the oxygen content increased from 13.36% to 36.77%. When comparing the colloidal solution of Ti₃C₂T_x treated with TMAOH (O–Ti₃C₂T_x) and traditional MXene colloid solutions (Figure 19a–d), it was found that traditional MXene solutions degraded through oxidation within 10 days, whereas O–Ti₃C₂T_x exhibited no significant degradation even after two months. Additionally, the microstructure of O–Ti₃C₂T_x remained largely unchanged after two months (Figure 19e), demonstrating superior stability compared to traditional MXene. Li et al. also found that O–Ti₃C₂T_x MXene films showed significantly better durability and lifetime in a controlled temperature and humidity environment compared to traditional Ti₃C₂T_x films. As shown in the XRD (Figure 19f), the traditional Ti₃C₂T_x film exhibited peaks corresponding to TiO₂ after two months, indicating significant oxidation, while no such phenomenon was observed in the O–Ti₃C₂T_x film.

In 2024, Tan et al. [89] employed Tris(hydroxymethyl)aminomethane (Tris)-HCl buffer solutions to stabilize Ti₃C₂T_x MXene dispersions. The Tris in the buffer formed Ti–N bonds with the Ti atoms on the surface of Ti₃C₂T_x, effectively preventing oxidation and degradation. By adjusting the pH of the Tris-HCl buffer, they achieved enhanced durability and extended lifetime for Ti₃C₂T_x MXene dispersions. Experimental results (Figure 20a) showed that untreated Ti₃C₂T_x liquid exhibited noticeable color changes and oxidation after 14 days, whereas the Ti₃C₂T_x dispersion treated with Tris-HCl buffer exhibited significantly improved stability. Specifically, the Ti₃C₂T_x dispersion treated with 0.25 mg/mL Tris-HCl remained stable for 150 days. Moreover, oxidation degradation substantially affected the lateral dimensions of Ti₃C₂T_x. Figure 20b–g show the surface state of Ti₃C₂T_x treated with 0.25 mg/mL Tris-HCl buffer and untreated Ti₃C₂T_x. The untreated Ti₃C₂T_x displayed visible white boundaries after 14 days, indicating the onset of oxidation, which worsened by day 35, resulting in the fragmentation of the Ti₃C₂T_x sheets. In contrast, Ti₃C₂T_x treated with 0.25 mg/mL Tris-HCl maintained its surface morphology without significant changes. The Tris-HCl buffer not only prevented oxidation but also acted as an interlayer spacer, facilitating ion diffusion and improving the material’s electrochemical performance. After treatment, Ti₃C₂T_x films exhibited a capacitance of 212.4 F/g at a current density of 500 A/g, which remained at 156 F/g after 35 days, significantly higher than the 33.4 F/g observed for untreated Ti₃C₂T_x films.

3. Delamination

The accordion-like structure of MXene obtained through etching is not the final product and requires further delamination. Although the etching process effectively disrupts the interlayer chemical bonds in the MAX phases, relaxing the layered structure of MXene, van der Waals forces and strong bonds formed during etching may still exist in certain cases. These mechanical resistances hinder further delamination. As a result, additional measures are necessary to overcome these forces, expand the interlayer spacing, and facilitate successful delamination. Typically, delamination is achieved by introducing intercalants that penetrate and expand the interlayer structure of MXene, reducing interlayer interactions and enabling the separation of single-layer MXene sheets. Following the introduction of intercalants, ultrasonic or vibration treatment is commonly employed to accelerate the delamination process. Conventional delamination methods often use high-speed centrifugation, where centrifugal forces are applied to separate MXene layers. Centrifugation speeds are typically set above 3500 rpm to generate sufficient force for layer separation. However, excessively high speeds can lead to the breakage of MXene nanosheets, reducing their lateral size and compromising their structural integrity [25]. Therefore, the development of novel delamination techniques to improve efficiency has become a key area of research.

For Ti₃C₂T_x MXene, chloride salts are among the most commonly used intercalants. In 2016, Ghidiu et al. [41] employed a mixed solution of HF and LiCl to etch the Ti₃AlC₂ MAX phase. After etching, the sample was washed with 6 M HCl to remove the LiF precipitate. Subsequently, chloride salt solutions (LiCl, NaCl, KCl, RbCl, MgCl₂ and CaCl₂) were introduced for intercalation, resulting in a Ti₃C₂T_x MXene with an interlayer spacing of approximately 25 Å. Studies have shown that different cations exhibit varying intercalation effects on MXene. Moreover, due to its clay-like properties, MXene responds differently to humidity changes depending on the type of intercalant used.

In 2022, Shekhirev et al. [90] proposed a soft delamination method using Li⁺ as an intercalant. In this approach, etched Ti₃C₂T_x MXene is immersed in a LiCl solution and stirred at 350 rpm for 18 h at 35 °C, yielding single-layer MXene nanosheets with a large surface area. This method not only prevents nanosheet fragmentation but also preserves the size of the original MAX phase particles. By employing large MAX particles as precursors, the resulting MXene exhibited lateral dimensions of up to 40 μm and layer thicknesses ranging from 2.5 to 3 nm. Figure 21 illustrates the microstructure of both the etched MAX phase and the delaminated MXene, demonstrating that the soft delamination method effectively preserved the size of the MAX phase flakes while minimizing structural damage. This method provides a promising foundation for the industrial application of MXene.

While traditional chemical intercalation methods can achieve the delamination of multilayer MXene, these processes are time-consuming and generate substantial chemical waste. Additionally, intercalants can influence the properties of MXene, and some may be toxic, limiting their applications in various fields. To address these challenges, Inman et al. [91] proposed a physical delamination method in 2022, using a three-roll mill to shear multilayer MXene. This approach enables the delamination of single-layer or few-layer Ti₃C₂T_x MXene without the need for chemical intercalants. The process is illustrated in Figure 22a. By applying a shear rate exceeding 15,000 s⁻¹, the van der Waals forces between layers are effectively weakened, allowing for successful delamination. This method streamlines the delamination process while eliminating residual chemical intercalants. The resulting Ti₃C₂T_x exhibited a capacitance of 337 F·g⁻¹ in a 3 M H₂SO₄ electrolyte, comparable to that produced via LiCl intercalation. However, a small fraction of undelaminated layers remained in the product. Notably, this method is not limited to Ti₃C₂T_x and can be extended to V₂CT_x MXene. The 3RM-V₂CT_x prepared using this method demonstrated electrochemical performance in a 5 M LiCl electrolyte comparable to that of Li-V₂CT_x prepared by ion exchange, highlighting the scalability and potential of the three-roll milling method for industrial MXene production.

In 2023, Inman et al. [92] introduced a high-pressure homogenization (HPH) method for delamination, which uses a combination of high shear, cavitation, and impact forces to effectively separate MXene layers without requiring post-treatment or chemical intercalants. This approach significantly enhances delamination efficiency compared to conventional methods. The schematic of the delamination process is shown in Figure 22b. As the MXene solution passes through the HPH unit, the reduced pore sizes subject the solution to intense shear forces. As the solution moves from a smaller nozzle to a larger one, sudden pressure changes induce bubble formation, which aids in the delamination of the accordion-like MXene structure. The resulting solution forms a colloidal suspension similar to that obtained through LiCl intercalation. Since MXene produced by HPH (HPH-MXene) lacks Li⁺ to neutralize surface charges, its Zeta potential is expected to be more negative than that of LiCl-etched MXene (Li-MXene). However, the measured Zeta potential of HPH-MXene is −27.4 ± 0.3 mV, slightly higher than the −31.3 ± 0.8 mV of Li-MXene. This discrepancy may be related to the particle size and solution stability, as MXene solutions with more single-layer nanoflakes typically exhibit higher Zeta potentials. Furthermore, Raman spectroscopy analysis (Figure 22c–h) revealed that HPH-MXene has lower oxidation levels and fewer defects than Li-MXene, further demonstrating that the HPH method produces higher-quality MXene compared to traditional chemical intercalation methods.

As MXene materials continue to demonstrate significant potential across various applications, evaluating the environmental impact of their synthesis processes has become increasingly important. To support the sustainable development of MXene, conducting a Life Cycle Assessment (LCA) is crucial. The synthesis of Ti₃C₂T_x MXene involves various etching methods, each with distinct environmental impacts, making LCA a crucial tool for systematically comparing these differences. Through LCA, a comprehensive comparison of the environmental impacts of commonly used synthesis methods can be made, providing valuable data to guide the optimization and environmentally friendly development of MXene synthesis processes.

In 2024, Ungureanu et al. [93] conducted the first systematic comparison of the life cycle environmental impacts associated with seven different methods for synthesizing Ti₃C₂T_x MXene. The results, presented in

Table 2. Midpoint environmental impacts for producing 1 g of Ti₃C₂T_x [93].

Impact Category	Unit	Path A [25,94]	Path B [95]	Path C [96]	Path D [97]	Path E [98]	Path F [99]	Path G [100]
Global warming	kg CO2 eq	1.44 × 1000	4.91 × 1000	7.08 × 1000	8.42 × 1000	7.54 × 1000	1.58 × 1001	1.40 × 1001
Stratospheric ozone depletion	kg CFC11 eq	5.05 × 10−07	1.61 × 10−06	2.57 × 10−06	3.00 × 10−06	2.82 × 10−06	5.44 × 10−06	5.33 × 10−06
Ionizing radiation	kBq Co-60 eq	2.27 × 10−01	7.52 × 10−01	1.50 × 1000	1.77 × 1000	1.82 × 1000	1.96 × 1000	3.27 × 1000
Ozone formation, human health	kg NOx eq	2.11 × 10−03	7.95 × 10−03	9.98 × 10−03	1.24 × 10−02	9.72 × 10−03	2.28 × 10−02	1.94 × 10−02
Fine particulate matter formation	kg PM2.5 eq	2.88 × 10−03	1.04 × 10−02	1.63 × 10−02	1.96 × 10−02	1.76 × 10−02	2.79 × 10−02	3.30 × 10−02
Ozone formation, terrestrial ecosystems	kg NOx eq	2.16 × 10−03	8.13 × 10−03	1.02 × 10−02	1.26 × 10−02	9.94 × 10−03	2.32 × 10−02	2.00 × 10−02
Terrestrial acidification	kg SO2 eq	3.99 × 10−03	1.57 × 10−02	2.04 × 10−02	2.48 × 10−02	1.99 × 10−02	4.18 × 10−02	3.80 × 10−02
Freshwater eutrophication	kg P eq	8.01 × 10−04	2.83 × 10−03	4.65 × 10−03	5.53 × 10−03	5.21 × 10−03	8.24 × 10−03	9.75 × 10−03
Marine eutrophication	kg N eq	6.09 × 10−05	2.49 × 10−04	3.41 × 10−04	4.07 × 10−04	4.25 × 10−04	9.86 × 10−04	7.59 × 10−04
Terrestrial ecotoxicity	kg 1,4-DCB	4.05 × 1000	1.72 × 1001	2.76 × 1001	2.54 × 1001	1.86 × 1001	4.29 × 1001	4.38 × 1001
Freshwater ecotoxicity	kg 1,4-DCB	1.05 × 10−01	3.50 × 10−01	5.75 × 10−01	6.39 × 10−01	5.50 × 10−01	1.24 × 1000	1.05 × 1000
Marine ecotoxicity	kg 1,4-DCB	1.34 × 10−01	4.51 × 10−01	7.35 × 10−01	8.16 × 10−01	6.99 × 10−01	1.59 × 1000	1.34 × 1000
Human carcinogenic toxicity	kg 1,4-DCB	1.61 × 10−01	7.90 × 10−01	8.17 × 10−01	1.09 × 1000	7.09 × 10−01	1.73 × 1000	1.51 × 1000
Human noncarcinogenic toxicity	kg 1,4-DCB	1.50 × 1000	5.46 × 1000	8.71 × 1000	9.69 × 1000	8.31 × 1000	1.75 × 1001	1.65 × 1001
Land use	M2a crop eq	2.88 × 10−02	9.82 × 10−02	1.56 × 10−01	1.84 × 10−01	1.71 × 10−01	2.80 × 10−01	3.28 × 10−01
Mineral resource scarcity	kg Cu eq	8.39 × 10−03	3.40 × 10−02	3.49 × 10−02	4.16 × 10−02	3.09 × 10−02	1.23 × 10−01	5.89 × 10−02
Fossil resource scarcity	kg oil eq	3.42 × 10−01	1.17 × 1000	1.77 × 1000	2.15 × 1000	1.92 × 1000	3.08 × 1000	3.61 × 1000
Water consumption	m3	1.23 × 10−02	3.62 × 10−02	5.10 × 10−02	5.93 × 10−02	4.95 × 10−02	1.34 × 10−01	2.53 × 10−01

and Figure 23a, highlight the variations in environmental impact across these methods. To enable a more accurate comparison, the researchers applied normalization and weighted scoring methods. Based on the scoring results (Figure 23b), the environmental impact follows this order: pathway A < B < C < E < D < F < G, with human health being the most affected category, followed by ecosystem damage and resource consumption. Figure 23c illustrates the environmental impacts at various steps of Ti₃C₂T_x MXene synthesis, including MAX phase preparation, etching, and delamination. It is clear that the etching step has the most significant environmental impact, primarily due to its extended duration and high energy consumption. To further validate the reliability of their findings, Ungureanu et al. performed uncertainty analysis using Monte Carlo (MC) simulations, with the results shown in Figure 23d. The simulations revealed the variability within the 95% confidence interval. The analysis confirmed that pathway A, compared to the other six methods, exhibits the least environmental impact, consistent with the trend observed in Figure 23b, thus supporting the robustness of their conclusions. This comprehensive LCA not only identifies the environmental impacts of various synthesis methods but also provides strong data support for the environmental optimization of future MXene synthesis processes.

4. Applications of Ti₃C₂T_x MXene in Ceramics

Ceramic materials are highly valued for their exceptional strength, hardness, high-temperature resistance, and corrosion resistance, making them essential in applications such as aerospace, military protection, electronics, and energy systems [101,102,103]. However, the highly ordered atomic structure that contributes to these beneficial properties also renders ceramics prone to poor crack resistance and brittle fracture when subjected to external forces. This brittleness remains a significant challenge for the broader application of ceramics. To mitigate this limitation, researchers have explored various strategies, with the incorporation of second-phase materials emerging as one of the most effective solutions. In recent years, nanoscale second-phase materials such as carbon nanotubes (CNTs), graphene, nano-silicon nitride (Si₃N₄), nano-silicon carbide (SiC), and MXene have attracted considerable attention due to their excellent mechanical properties and unique nanoscale effects [104,105,106]. Among these, Ti₃C₂T_x MXene stands out due to its distinctive physicochemical properties, particularly its 2D nanostructure, high electrical conductivity, and exceptional mechanical performance. These advantages make Ti₃C₂T_x MXene a promising candidate for reinforcing ceramic matrix composites in applications like aerospace, military defense, and energy storage.

In 2017, Fei et al. [107] conducted pioneering research on incorporating Ti₃C₂T_x MXene as a reinforcing phase in bauxite, leading to the development of Ti₃C₂T_x–Al₂O₃ composite ceramics through high-temperature sintering. Their study demonstrated that increasing the weight fraction of Ti₃C₂T_x in the Al₂O₃ matrix significantly enhanced several mechanical properties, including volume density, hardness, bending strength, and fracture toughness. Specifically, at a sintering temperature of 1500 °C, the Al₂O₃ composite ceramics containing 2 wt.% Ti₃C₂T_x exhibited approximately 300%, 150%, and 300% improvements in hardness, bending strength, and fracture toughness, respectively, compared to pure Al₂O₃ ceramics. Microstructural analysis, as shown in Figure 24, revealed that during high-temperature sintering, part of the Ti₃C₂T_x underwent melting (Figure 24b), while the majority retained its characteristic accordion-like structure [108]. Figure 24c illustrates how the melted MXene filled the pores within the Al₂O₃ matrix and was firmly anchored at the Al₂O₃ grain boundaries through interfacial interactions, significantly improving the relative density of the composite ceramics. Furthermore, the incorporation of Ti₃C₂T_x reduced the sintering temperature of Al₂O₃ ceramics and inhibited excessive grain growth, which is crucial for enhancing the mechanical properties of ceramic materials. Notably, Ti₃C₂T_x effectively hindered crack propagation, inducing cracks to follow more tortuous paths via the crack deflection mechanism, thereby relieving stress and markedly improving the fracture toughness of the composite ceramics.

In 2021, Petrus et al. [109] explored the incorporation of Ti₃C₂T_x MXene into SiC powders and successfully fabricated Ti₃C₂T_x–SiC composites through powder metallurgy at 1900 °C for 30 min. The effects of Ti₃C₂T_x incorporation on the microstructure and mechanical properties of the composites were systematically investigated. SEM images (Figure 25a–c) reveal that, at low Ti₃C₂T_x content, the interface between the Ti₃C₂T_x layers and the SiC matrix exhibited strong bonding with a well-formed, tight connection, showing no visible gaps or discontinuities. However, as the Ti₃C₂T_x content increased, noticeable voids appeared between the Ti₃C₂T_x layers and the SiC matrix, indicating a reduction in interface bonding strength. Further elemental analysis via EDS identified two distinct layered structures within the composite: one composed of thicker graphite layers (Figure 25d), and the other consisting of thinner C-Ti layers (Figure 25e). These layered structures effectively suppressed the growth of SiC grains, contributing to an improvement in the composite’s hardness. Compared to pure SiC ceramics (20.7  ±  0.4 GPa), the hardness of the SiC composite ceramic containing 1.5 wt.% Ti₃C₂T_x increased by over 10%, reaching 23.2  ±  0.4 GPa. Additionally, these layered structures hinder crack propagation through mechanisms such as crack deflection and bridging, significantly enhancing the fracture toughness of the composite material. Specifically, the fracture toughness of the SiC composite with 0.2 wt.% Ti₃C₂T_x increased by nearly 50%, from 3.1  ±  0.2 MPa·m^1/2 in pure SiC ceramics to 4.5  ±  0.2 MPa·m^1/2 in the composite.

In the same year, Petrus et al. [110] employed powder metallurgy to fabricate SiC composite ceramics reinforced with Ti₃C₂T_x and surface-modified Ti₃C₂T_x (Ti₃C₂T_xM). The surface modification was achieved using a sol–gel method with a Y₂O₃/Al₂O₃ mixture. This study aimed to further explore the impact of surface modification on the microstructure and mechanical properties of composite ceramics. The fracture surface morphologies, shown in Figure 26a–c, reveal the presence of flaky structures at the grain boundaries in the MXene-reinforced SiC composites. Notably, compared to Ti₃C₂T_x, the layered structure formed by Ti₃C₂T_xM is considerably thinner, indicating that surface modification improves the dispersion and interface bonding of MXene within the composite. XRD analysis in Figure 26d shows that the addition of Ti₃C₂T_x alters the phase composition of the SiC composite ceramics, with Ti₃C₂T_xM proving more effective in promoting the transformation from β-SiC to α-SiC. Further TEM analysis (Figure 26g–n) demonstrates that SiC composite ceramics containing Ti₃C₂T_xM exhibit a more compact microstructure and fewer defects compared to those reinforced with Ti₃C₂T_x. This improvement is attributed to the presence of Al₂O₃ and Y₂O₃ oxide layers on the surface of Ti₃C₂T_xM, which react with the residual TiO₂ on Ti₃C₂T_x surfaces to form a liquid phase during sintering. This liquid phase promotes the sintering process around the reinforcing phase, enhancing the interface quality and reducing the formation of internal defects, thus significantly improving the mechanical properties of the composite ceramics. The addition of MXene reinforcement significantly enhanced the fracture toughness of the composite ceramics compared to pure SiC. Specifically, the SiC composite containing 1 wt.% Ti₃C₂T_x exhibited the highest fracture toughness, reaching 5 MPa·m^1/2, more than 50% higher than that of pure SiC ceramics and 15% higher than the SiC composite with 2.5 wt.% Ti₃C₂T_x. These results suggest that surface-modified Ti₃C₂T_x reinforcement effectively enhances the mechanical properties of SiC composite ceramics, particularly in terms of fracture toughness.

As a reinforcing phase, Ti₃C₂T_x can significantly influence the residual stress levels and stress distribution within the matrix material. In 2022, the Petrus team [111] employed finite element analysis to explore the impact of Ti₃C₂T_x on the stress state in SiC composite ceramics. Figure 27 shows the distribution of equivalent stress and principal stresses (S1, S2, and S3) in regions near the Ti₃C₂T_x layers within the SiC composite. The results show that the edge regions of the Ti₃C₂T_x layers experience high compressive stress, with values approaching 700 MPa. The compressive stress fields play a crucial role in enhancing the material’s fracture toughness, as compressive stress helps close cracks and induces crack deflection, thereby preventing crack propagation.

As research into MXene as a reinforcing phase in ceramic materials continues to advance, a deeper understanding of this 2D nanomaterial has emerged. Studies have shown that MXene is highly susceptible to decomposition in air at temperatures as low as 200 °C, though its stability improves in inert environments such as argon. However, when exposed to temperatures exceeding 800 °C, MXene can undergo phase transformations, forming anatase. At temperatures above 1000 °C, MXene will completely oxidize, transitioning into rutile [112,113]. This thermal instability presents a challenge in the preparation of ceramic materials, which often requires high-temperature sintering processes. During sintering, MXene sheets are particularly vulnerable to oxidation and degradation, a phenomenon that is even more pronounced in single-layer MXene. Consequently, maintaining the thermal stability of MXene in high-temperature environments has become an urgent issue to address [114].

To address this challenge, in 2018, Guo et al. [115] employed the cold sintering process (CSP) to successfully co-sinter Ti₃C₂T_x MXene with ZnO ceramics, producing ZnO–Ti₃C₂T_x nanocomposites at low temperatures and effectively preventing the oxidation of Ti₃C₂T_x. As shown in Figure 28a, the relative density of the ZnO–Ti₃C₂T_x composites prepared by this method exceeded 90%. Furthermore, SEM images comparing the original powders and the sintered bodies (Figure 28b–e) clearly reveal that the addition of Ti₃C₂T_x significantly suppressed grain growth in ZnO ceramics. TEM characterization results (Figure 28f–l) further demonstrate that the CSP technique effectively prevents diffusion between Ti₃C₂T_x and the ZnO ceramic matrix, allowing Ti₃C₂T_x to be uniformly distributed along the ZnO grain boundaries, thus preventing grain coarsening. Additionally, the study revealed that as the Ti₃C₂T_x content increased, the electrical conductivity of the ZnO–Ti₃C₂T_x nanocomposites increased by 1–2 orders of magnitude. Nanocomposites containing 0.5 wt.% Ti₃C₂T_x exhibited a 40–50% improvement in hardness and elastic modulus compared to pure ZnO ceramics. When the Ti₃C₂T_x content reached 5 wt.%, the hardness and elastic modulus increased by over 150%. These results demonstrate that the CSP technique offers an effective route for the preparation of 2D MXene–ceramic composites.

In 2020, Wozniak et al. [10] successfully employed SPS technology to prepare Si₃N₄ ceramics reinforced with Ti₃C₂T_x MXene. SPS technology offers numerous advantages, including rapid heating rates, short sintering times, low sintering temperatures, quick cooling, and minimal holding times, all of which help reduce MXene decomposition during the sintering process. The lower sintering temperatures and rapid sintering process also play a crucial role in controlling the grain growth of Si₃N₄ ceramics, thereby further enhancing their mechanical properties. The study found that the incorporation of MXene significantly improved the hardness and fracture toughness of the Si₃N₄ ceramics. Moreover, XRD analysis presented in Figure 29 showed that the addition of MXene modified the phase transformation of Si₃N₄, facilitating the transition from α-Si₃N₄ to β-Si₃N₄ and introducing a new Si₂N₂O phase. These phase changes not only altered the ceramic’s phase composition but also had a significant impact on its mechanical properties.

Similarly, Wozniak et al. [116] utilized the SPS process to prepare SiC–x/Ti₂C composite ceramics (where x = 0.2, 0.5, 0.7, 1, 1.5, 2, 2.5, and 3 wt%) reinforced with Ti₂C MXene. The results showed that the inclusion of Ti₂C significantly enhanced the relative density, hardness, and fracture toughness of the SiC ceramics compared to pure SiC. Among the composites, those containing 1.5 wt.% Ti₂C exhibited the best overall performance. The study demonstrated that the presence of MXene facilitated material transport during the sintering process, improving the relative density of the composite ceramics. As the MXene content increased, the SiC grain size gradually decreased, further boosting the mechanical properties. Crack propagation behavior, shown in Figure 30, revealed that delaminated 2D MXene as a reinforcement phase was particularly effective at hindering crack propagation. This mechanism forced the composite ceramics to absorb more energy during fracture. Moreover, crack bridging was observed in SiC composites containing delaminated MXene, which significantly contributed to the improvement in fracture toughness.

In 2021, Cygan et al. [117] proposed a structural modification strategy to prevent the oxidation and degradation of Ti₃C₂T_x MXene at high temperatures. They achieved this by sputtering thin Ti or Mo metal layers onto the surface of Ti₃C₂T_x. In the Ti₃C₂T_x–Al₂O₃ composite ceramics, the Ti/Mo metal layer served as a protective barrier, effectively preventing atomic migration and reorganization, thus limiting the diffusion of external environments and significantly altering the degradation process. The SEM images, shown in Figure 31, reveal that the Ti₃C₂Tx–Al₂O₃ composite ceramics exhibited a dense microstructure. Compared to Al₂O₃ composites with unmodified Ti₃C₂T_x, the composites with modified Ti₃C₂T_x displayed a noticeable shift in fracture mode, transitioning from intergranular fracture to a mixed fracture mode, where both intergranular and transgranular fractures coexisted. Regarding mechanical properties, the composite ceramics with modified Ti₃C₂T_x demonstrated significant improvements. Specifically, the hardness and fracture toughness of the Al₂O₃ composites containing 0.5 wt.% modified Ti₃C₂T_x increased by 10% and 15%, respectively, compared to those containing unmodified Ti₃C₂T_x.

In addition to enhancing the mechanical properties of ceramics, the incorporation of Ti₃C₂T_x MXene also significantly influences other critical aspects, including electromagnetic shielding, antioxidant properties, corrosion resistance, and fluorescence characteristics.

In 2021, Ding et al. [118] introduced Ti₃C₂T_x MXene into polyborosilazane (SiBCN) to prepare TiC/SiBCN nanocomposite ceramics with exceptional electromagnetic absorption properties through pyrolysis treatment. TEM images shown in Figure 32 indicate that a substantial amount of TiC nanoparticles formed during the pyrolysis process. The formation of TiC nanoparticles not only enhanced the interface polarization but also provided a medium for the multiple reflections of electromagnetic waves. Additionally, the inherent defects in the TiC particles further boosted the dipole polarization effect. Together, these mechanisms collectively contributed to a significant enhancement in the electromagnetic absorption performance of composite ceramics. Remarkably, the TiC/SiBCN composites containing 5 wt.% Ti₃C₂T_x MXene achieved a minimum reflection coefficient of −45.44 dB at 10.93 GHz and demonstrated an effective absorption bandwidth spanning nearly the entire X-band, from 8.35 to 12.40 GHz. Furthermore, the study revealed that these ceramics exhibited excellent thermal stability, maintaining consistent electromagnetic absorption performance at temperatures up to 600 °C. This makes Ti₃C₂T_x-infused SiBCN ceramics a promising material for advanced applications in radar and communication systems, where high-frequency absorption is crucial.

In 2022, Lyu et al. [119] successfully developed Ti₃C₂T_x/diatom frustule-derived porous silica (DFPS) composites (DM), which exhibited exceptional mechanical properties and outstanding electromagnetic interference (EMI) shielding performance. This was achieved by immersing DFPS ceramics in a Ti₃C₂T_x solution followed by annealing in an argon atmosphere. Figure 33a,b show the surface morphology of the Ti₃C₂T_x /DFPS composites, illustrating that Ti₃C₂T_x uniformly coated the DFPS framework, forming an efficient conductive network that enhances the material’s ability to absorb electromagnetic waves. The electromagnetic shielding mechanism of the Ti₃C₂T_x/DFPS composite, depicted in Figure 33d, is primarily driven by absorption shielding. Specifically, the porous, layered Ti₃C₂T_x coating on the DFPS framework significantly improves the reabsorption and reflection efficiency of electromagnetic waves. Moreover, the impedance mismatch between Ti₃C₂T_x and the DFPS surface promotes the reflection and scattering of electromagnetic waves, simultaneously improving the material’s overall absorption capacity. Figure 33c compares the compressive strength and EMI shielding performance of various materials, demonstrating that the Ti₃C₂T_x/DFPS composite achieves an optimal balance between mechanical strength and shielding effectiveness.

In 2022, Liang et al. [120] incorporated Ti₃C₂T_x MXene into quaternary Si–B–C–N ceramics to investigate its impact on the material’s oxidation resistance. As shown in Figure 34a–f, a dense oxide layer formed on the surface of the Si-B-C-N ceramics at 1100 °C. With increasing Ti₃C₂T_x content, from 1.0 wt.% to 3.0 wt.%, the thickness of the oxide layer decreased from 1.4 μm to 0.8 μm, indicating that Ti₃C₂T_x enhanced the oxidation resistance of the Si-B-C-N ceramics. However, when the temperature increased to 1600 °C, the oxidation of the Si-B-C-N ceramics significantly accelerated (Figure 34a–g). Specifically, Si–B–C–N ceramics with 1.0 wt.% and 1.5 wt.% Ti₃C₂T_x exhibited severe blistering on their surface, whereas those with 3.0 wt.% Ti₃C₂T_x maintained a dense and intact oxide layer. The results in Figure 34h,i show that at elevated temperature, TiO₂ particles embedded within the SiO₂ oxide layer created a pinning effect that effectively hindered the diffusion of SiO₂, preventing further oxidation of the internal material. This antioxidant behavior is critical for extending the lifespan and performance of ceramic materials in high-temperature and oxidative environments, such as in aerospace and defense applications.

In 2020, Du et al. [121] synthesized Ti₃C₂T_x particles coated with TiO₂ and MoS₂ (Ti₃C₂T_x@TiO₂/MoS₂) using a hydrothermal method. These particles were incorporated into Ni–P coatings via electroplating, significantly improving the corrosion resistance of the coatings. The incorporation of Ti₃C₂T_x@TiO₂/MoS₂ particles modified the microstructure of the composite coating. Figure 35a–e show the surface morphology of Ni–P coatings with varying concentrations of Ti₃C₂T_x@TiO₂/MoS₂ particles. It was observed that the surface of the Ni–P coating without Ti₃C₂T_x@TiO₂/MoS₂ particles was relatively smooth. However, as the concentration of Ti₃C₂T_x@TiO₂/MoS₂ particles increased, the surface became rougher, primarily due to the excellent conductivity of the Ti₃C₂T_x@TiO₂/MoS₂ particles, which facilitated the formation of larger crystals within the coating. The inset in the top-right corner indicates that the addition of Ti₃C₂T_x@TiO₂/MoS₂ particles significantly altered the wettability of the coating, making it more hydrophilic, with a maximum contact angle of 120°. This increase in contact angle contributed to enhanced corrosion resistance. Moreover, the Ti₃C₂T_x@TiO₂/MoS₂ particles acted as cathodes within the coatings, enhancing the polarization of the anode. As shown in Figure 35f, when the concentration of Ti₃C₂T_x@TiO₂/MoS₂ particles reached 6 g/L, the composite coating exhibited the lowest corrosion current and the highest polarization resistance, demonstrating optimal corrosion resistance.

In 2023, Guan et al. [122] examined the impact of Ti₃C₂T_x MXene on the corrosion resistance of silicate-bonded ceramic coatings (SBCC). Pure SBCC exhibits poor physical barrier properties, allowing corrosive agents like water and oxygen to penetrate directly, limiting their corrosion resistance. In contrast, the addition of Ti₃C₂T_x significantly enhanced the structural integrity of SBCC, effectively preventing the diffusion of corrosive agents and substantially improving its corrosion resistance. The experimental results showed that SBCC with 1.6 wt.% Ti₃C₂T_x exhibited exceptional corrosion resistance in 3.5 wt.% NaCl, 5 wt.% NaOH, and 5 wt.% H₂SO₄ solutions, as shown in Figure 36. The most remarkable corrosion resistance was observed in 5 wt.% H₂SO₄ solution, where the impedance modulus of 3.45 × 10⁶ Ω·cm² was approximately 79% higher than that of pure SBCC. Furthermore, the corrosion current density was 1.18 × 10⁻⁸ A/cm², approximately one order of magnitude lower than that of pure SBCC.

In the same year, Dong et al. [123] explored the impact of Ti₃C₂T_x MXene on the fluorescence performance of Eu³⁺-doped 8 wt.% yttria-stabilized zirconia (8YSZ: Eu³⁺) powder. Microstructural analysis of the powder, as shown in Figure 37, reveals that the addition of Ti₃C₂T_x does not significantly alter the crystal structure of the 8YSZ: Eu³⁺ powder. However, some 8YSZ: Eu³⁺ particles are encapsulated by Ti₃C₂T_x flakes, while others are embedded between them. When compared to pure 8YSZ: Eu³⁺ powder, the luminescent intensity of the MXene/8YSZ: Eu³⁺ composites is markedly higher. This enhancement suggests that Ti₃C₂T_x MXene plays a crucial role in improving the optical properties of ceramic materials, which is particularly advantageous for applications requiring enhanced fluorescence or luminescence, such as in sensors and lighting technologies.

These studies further emphasize the multifunctional advantages of Ti₃C₂T_x MXene in enhancing the properties of ceramic materials. By incorporating Ti₃C₂T_x, ceramics can exhibit not only improved mechanical properties but also enhanced electromagnetic absorption, oxidation resistance, corrosion resistance, and optical properties such as fluorescence. These improvements significantly expand the potential applications of ceramic materials in high-performance fields, including aerospace, military protection, electronics, and advanced coatings. The unique properties of Ti₃C₂T_x MXene make it a promising candidate for further research and development in advanced ceramic composites.

5. Summary and Outlook

This review summarizes the preparation methods of Ti₃C₂T_x MXene and its research progress in ceramic materials. As a novel 2D nanomaterial, Ti₃C₂T_x MXene has become a significant research focus in the field of material science, owing to its unique layered structure and excellent electrical, thermal, mechanical, and chemical properties. Ti₃C₂T_x MXene shows great promise, particularly in enhancing and functionalizing ceramic materials, offering remarkable potential.

In the preparation of Ti₃C₂T_x MXene, HF etching remains the most common and well-established method due to its simplicity and high yield. However, HF etching poses potential environmental and health hazards, and the -F functional groups on the surface of the Ti₃C₂T_x can negatively impact its subsequent applications. As a result, F-free or low-toxicity etching alternatives have become a research hotspot in recent years. Techniques such as acid–salt composite etching, alkaline etching, and molten salt etching have gained attention, as these methods can reduce environmental pollution while improving the quality and yield of Ti₃C₂T_x. Despite progress in laboratory settings, challenges remain for large-scale production, particularly in terms of process stability, cost control, and scalability. In the future, composite etching methods that combine multiple chemical approaches may offer the most promising solution, effectively balancing environmental sustainability with the quality of Ti₃C₂T_x.

The delamination process of Ti₃C₂T_x is crucial for producing high-quality single-layer or few-layer Ti₃C₂T_x nanosheets. Traditional chemical delamination methods typically rely on intercalants, but the use of such intercalants can not only negatively impact the performance of Ti₃C₂T_x but also generate substantial chemical waste. In recent years, advances in novel physical delamination technologies have made it possible to achieve effective delamination of Ti₃C₂T_x and produce high-quality nanosheets without the need for intercalants. By optimizing delamination equipment, controlling delamination conditions, and integrating advanced auxiliary delamination techniques, the delamination process for Ti₃C₂T_x can become more efficient and environmentally friendly, thus promoting its widespread application.

In the application of ceramic materials, Ti₃C₂T_x MXene, as a reinforcement phase, has demonstrated the ability to significantly enhance the mechanical properties, electromagnetic absorption, oxidation resistance, and corrosion resistance of ceramic materials. To address the instability of Ti₃C₂T_x at high temperatures, researchers have proposed using novel sintering techniques, such as CPS and SPS, to prepare Ti₃C₂T_x–ceramic matrix composites with favorable outcomes. However, Ti₃C₂T_x MXene still faces several challenges in its application to ceramics. First, the large-scale synthesis of high-quality Ti₃C₂T_x remains a significant hurdle. Second, issues related to the dispersion of Ti₃C₂T_x and its compatibility with the ceramic matrix have not been fully resolved, which limits its widespread application. To address these challenges, future research should focus on the following areas: enhancing the surface functionalization of Ti₃C₂T_x to improve its compatibility with ceramic matrices, developing new delamination methods to prevent aggregation of Ti₃C₂T_x within the ceramic matrix, and further optimizing sintering processes, particularly to enhance the stability of Ti₃C₂T_x under high-temperature conditions.

Looking toward future research directions, the challenges in applying Ti₃C₂T_x MXene in ceramic materials will mainly be focused on large-scale synthesis, surface functionalization, and optimization of sintering processes. Future research could make breakthroughs in the following practical aspects: (1) developing more economical, environmentally friendly, and efficient synthesis processes, particularly focusing on the optimization and commercialization of F-free etching methods; (2) investigating surface modification and functionalization techniques to improve the dispersion and interfacial bonding of Ti₃C₂T_x with ceramic matrices, thereby enhancing its reinforcement effects; (3) exploring the impact of different sintering techniques on the properties of Ti₃C₂T_x-ceramic composites, especially in terms of their stability and mechanical performance at high temperatures.

In conclusion, Ti₃C₂T_x MXene, as a highly promising 2D material, holds great potential for applications in ceramic materials. With continuous innovations in synthesis technologies, improvements in functionalization methods, and optimizations in sintering processes, Ti₃C₂T_x MXene is expected to become an ideal choice for enhancing the performance of ceramic materials. This will drive the development of ceramics toward higher performance and multifunctionality, promoting their applications in various practical fields.