Influencing Factors on Synthesis and Properties of MXene: A Review

Abstract

MXene has a unique two-dimensional layered structure, a large specific surface area, and good electrical conductivity, stability, magnetic properties, and mechanical properties. Attributed to its structure and properties, MXene has entered various application fields, such as tumors, antibiotics, batteries, hydrogen storage, sensors, electromagnetic shielding, etc. Studies on MXene are implemented in various fields. The preparation methods, surface functional groups, and the preparation technology of composite materials directly affect the performance of MXene and determine its application fields. As significantly important issues, many influencing factors exist in the preparation process of MXene and the process of composite material design, which directly impact the properties and applications of MXene. This paper investigates and analyzes the factors influencing the preparation, properties, and application of MXene, explores the research progress of MXene, and provides critical insights for synthesizing and modifying effective MXene adsorbents. Moreover, this review discusses the influencing factors on applications, i.e., CO₂ reduction, energy storage, heavy metal removal, etc., and compares key factors, including surface modification, pH, reaction time, and adsorption capacity, etc. Furthermore, this review provides an overview on current status and offers recommendations.

1. Introduction

MXene is a type of two-dimensional material consisting of a transition metal carbide, nitride, or carbon nitride. MXene is a kind of nanoelectronic material with preferable electrochemical stability. Owing to its unique optical and electrical properties, MXene and its derivatives have shown potential applications to energy storage, catalysis, electronic/optoelectronic devices, energy storage, and energy conversion applications. Meanwhile, two-dimensional nanosheets of MXene and its derivatives can be prepared in a scalable manner in the liquid phase, which are used for the preparation of self-assembled films and hybrid structures, also suitable for large-area production.

MXenes (M_n+1X_nT_x) are usually synthesized by etching “A” in the precursor MAX phase, where M is the early transition metal, A is the third or fourth main family element, X is the C or N element, and T is the surface chemical group. For “T”, different etching methods can make their chemical groups different, and different atoms can also be modified to them by post-processing methods. To gain a clearer understanding of the two-dimensional structure of MXenes, the atomic structure of the monolayer MXene formed by etching and ultrasonication has a “sandwich” structure, with three layers of Ti atoms arranged alternately with two layers of C atoms, and the functional groups of “T” are connected to the Ti atoms in the outermost layer (Figure 1).

Hydrofluoric acid etching is the most conventional method for preparing MXene. The reaction of the MAX phase and HF is divided into two processes. For instance, Ti₃C₂ is etched by HF corrosion, then Ti₃C₂ is prepared by soaking Ti₃AlC₂ in hydrofluoric acid solution, and, at last, the Al atoms in Ti₃AlC₂ are completely stripped out (Equations (1) and (2)):

Ti₃AlC₂ + 3 HF = AlF₃ + 3/2 H₂ + Ti₃C₂

(1)

Ti₃C₂ + 2 H₂O = Ti₃C₂(OH)₂ + H₂ or Ti₃C₂ + 2HF = Ti₃C₂F₂ + H₂

(2)

MXene has a structure similar to graphene. Owing to the large number of MAX phases containing multiple elements, a large number of special properties can be prepared by etching MAX phases.

(1)

Electrical and optical properties

MXene has good conductivity, which can serve as a photocatalyst and a electrocatalyst. MXene can be used to catalyze the hydrogen evolution reaction, the oxygen evolution and reduction reaction, the carbon dioxide reduction reaction, the nitrogen reduction reaction, and the degradation of pollutants.

(2)

Heat endurance

Based on the density functional theory (DFT)–first principle band structure theory, computational results reveal that the lattice energy of MXene is negative. Lattice energy is an important parameter to judge crystal stability. Thus, MXene is relatively stable [2].

(3)

Mechanical properties

Stretching along the MXene datum, its elastic modulus and bending strength are significantly larger than those of multilayer graphene having the same thickness, and MXene shows better mechanical properties.

(4)

Magnetic performance

Ta_n+1C_n shows good long-range magnetic ordering, and MXene phases such as Cr₂CT_x and Cr₂NT_x also show strong ferromagnetism. The magnetic property of MXene, modified by active functional groups, is reduced or even disappears.

MXene has a unique morphological structure and rich properties. As discussed, MXene can be readily dispersed in water-based liquid and exhibits electrical conductivity between a conductor and semiconductor, which makes it widely used in sensors, composite materials, electrochemical energy-storage materials, and catalysts. However, influencing factors on the synthesis and properties of MXene are significantly important, as it directly impacts on the sufficiency of MXene’s production and performance. This article provides an overview of influencing factors on the synthesis and properties of MXene. A comparative investigation of important factors affecting MXene synthesis, modification, and properties is discussed. Moreover, this review describes the applications that correspond to the key properties of MXenes, including MXenes’ catalytic performance, energy storage, and adsorption mechanisms, among others.

2. Influencing Factors on MXene Synthesis

As mentioned, MXenes are normally generated by the selective etching of its layered MAX precursor, which is usually composed of ternary carbide or nitride synergy. A variety of techniques are available to etch MAX phases, including high temperature (up to 800 °C) reactions using molten salt, reactions with gaseous halides, and etching the A elements within a vacuum. Selective removal of the A element from the MAX phase is a key method in the synthesis of MXenes. The functional groups depend highly on the etching process parameters (etchants, etching temperature, etching time, etc.).

2.1. Selection of Materials

Hydrofluoric acid, used to produce MXene, contains active functional groups such as hydroxyl fluoroions. The occurrence of the end group may produce steric resistance and modify the performance of the MXene. When the layer A atoms in the MAX phase are Al and Si elements, MAX phases are dissolved more easily in HF acid, and the process become more productive. Ti₃C₂T_x (MXene) prepared from Ti₃SiC₂ (MAX) has better antioxidant properties compared to MXene synthesized from Ti₃AlC₂ [3]. Moreover, when the HF concentration is low, the layer A atoms cannot be completely etched; in contrast, strong corrosion by highly concentrated HF destroys the basic structure of the MAX phase, or even completely dissolves the MAX phase. The HF solution used in the experiment is ultratoxic and corrosive, which has a certain danger in practical operation.

Instead of HF, a mixed solution of HCl and LiF can be applied to produce the Ti₃C₂ two-dimensional layered structure material; etching Ti₃AlC₂ at 40 °C for 45 h gives excellent performance [4]. The Ti₃C₂ two-dimensional layered structure materials have superior flexibility and strength, which can be folded repeatedly, and the structure remains intact. The 2D nanosheet has a large transverse size and no significant nanoscale defects, indicating that the etcher composed of HCl and LiF is milder [5]. Similarly, a mixture of NaF, KF, CsF, CaF₂, and HCl or H₂SO₄ can also be used as etchers to replace HF.

Application of NH₄HF₂ to etch MXene provides for stripping and intercalation that can be performed simultaneously (Equations (3) and (4)) [6]:

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

(3)

Ti₃C₂ + a NH₄HF₂ + b H₂O = (NH₃)c(NH₄)dTi₃C₂(OH)xFy

(4)

The NH₃ and NH₄⁺ are intercalated at the same time, making the distribution of the prepared Ti₃C₂T_x atomic layers more even.

To obtain a single or few-layered sheet structure, intercalators are applied to separate the stacked Ti₃C₂ layer. The boiling point of dimethyl sulfoxide (DMSO) is high and DMSO is difficult to remove from solution [7]. The residue may replace the active group on the MXene phase surface and make the lame layer bond. Other intercalators have been tried, but they are generally available only for specific MXene phases, and less complete monolayer sheet structures are obtained [8,9].

Sodium ion intercalates with grafted sulfonacid functional groups, which are capable in achieving even dispersion of the MXene phase (Figure 2), which subsequently enhances its electrochemical activity [10]. It was confirmed that Na⁺ was successfully inserted into the MXene interlayers; the increased layer spacing weakened the binding force between the layers, and the layers were gradually separated, which is beneficial for grafting the sulfonic acid functional groups on the MXene surface and conducting the aryl diazine salt modification. A diazosalt solution is required as a surface modifier for conjugation.

Oxygen-functionalized MXenes provide greater mechanical strength than that of either fluorine-functionalized or hydroxyl-functionalized MXenes.

2.2. Parameters during Process

(1)

Impact factors of the in situ etching method

Direct etching of the Ti₃C₂T_x (MXene) sheet can be accomplished by using a sufficient concentration of HF solvent. More improvements can be observed during in situ etching on the spacing and morphology of a Ti₃C₂T_x (MXene) nanosheet; it is found that there are fewer slice layers, and that Ti₃C₂T_x (MXene) nanosheets with large layer spacing can be prepared, which are also different from the accordion forms and effectively improve the specific surface area utilization rate of Ti₃C₂T_x (MXene). The MXenes prepared by in situ etching show a higher purity. If the concentration, environment, time, and etching mode of HF acid can be effectively controlled, whether directly or indirectly to generate HF acid, the MAX phase material can be more effectively stripped.

• A higher concentration of HF makes a more thorough dissection, which provides thinner MXene two-dimension material [11].
• With short etching time and low temperature, the MXenes prepared are in a wide frequency range with high impedance matching and a large attenuation coefficient [12].

Overall, better stripping effects are associated with a higher concentration of HF acid and suitable etching time. Insufficient etching time with low etching temperature causes incomplete MAX stripping and produces impurities, leading to difficulty in obtaining a complete monolayer or large spacing of MXenes. HF acid concentration, etching temperature, time, etching mode, and mechanism analysis are the important parts of the research.

(2)

Impact factors of the electrochemical-etching method

The electrochemical-etching method is a well-known electrolytic leaching process: according to the principles of electrochemistry, certain electrolytes can selectively remove a certain metal (or semiconductor). The structure of the MXenes phase obtained by electrochemical etching is also laminated. The production rate is ten times higher than the hydrofluoric acid-etching method. The corresponding Ti₂CT_x (MXene) phase material can be prepared by electrochemical etching by placing Ti₂AlC in an aqueous etching solution of HCl [13]:

Ti₂AlC + yCl⁻ + (2 x + z)H₂O → Ti₂C(OH)₂xClyOz + Al₃₊ + (x + z)H₂ + (y + 3)e

(5)

The MXene phase can be successfully prepared, but it is difficult to obtain directly. After etching by the HCl solution, the MAX phase forms three layers: the innermost layer is not an etched MAX phase, the middle layer is MXenes, and the outermost layer is carbide-derived carbon (CDC). If HCl is applied as an etcher in the electrochemical-etching method, further separation is required to complete etching the MAX phase and solves purification problems.

Factors affecting the electrochemical-etching method are complicated:

• Selection of etcher—Use the mixture of NH₄Cl and TMAOH as an electrolyte, apply a voltage of +5 V, use tetramethyloxic ammonium hydroxide for separation, and a few layers Ti₃C₂T_x can be achieved [14];
• The electrode is the crucial factor (Figure 3)—Applications to three-dimensional composite electrodes result in a flower-like MXenes structure, which has a strong ion adsorption force [15];
• Voltage significantly affects Ti₃C₂ formation—Increased voltage and ultrasound can promote the stratification of Ti₃C₂, but the Ti₃C₂ formed still adheres to the working electrode, inducing excessive corrosion and CDC formation [16].

The MXenes phase prepared by electrochemical etching does not contain the F-functional group, which is an environmentally friendly method. The MXenes prepared are still laminated; thus production is faster than HF acid etching. However, it cannot be made in one step, which requires further separation of the MXene from the outer carbide-derived carbon (CDC) and the inner MAX without etching. Hence, additional research is necessary for the electrochemical-etching preparation, especially concerning the separation of sediment, etching of the MAX phase, and impurity treatment.

(3)

Impact factors of the molten fluorinated salt method

The molten salt method refers to the process in which the reactant and the molten salt are configured into a uniform mixture according to a certain proportion. The molten salt then fully melts through heating, the reactant is completely reacts in the dissolved molten salt environment, and the product develops with certain morphological characteristics [17]. Compared to the etching method, the molten salt method requires a short preparation time and results in no pollution. However, it is found that the as-prepared MXene contains fluoride salt impurities, which requires further study of the purification problem.

Reaction temperature is the most important impact factor in the molten fluorinated salt method. The temperate correlates with the type of molten salt system (

Table 1. Molten salt system vs. suitable reaction temperature.

Molten Salt System	Reaction Temperature
LiF–NaF–KF [18]	600 °C
NaF–KF [19]	850 °C

).

To obtain better capacity by using the as-prepared MXene phase, production of the F-functional group is avoided. Compared to the electrochemical-etching method, the preparation process is one-step etching. However, improvement via purification of the MXenes products is necessary and associated with controlling the reaction temperature.

(4)

Impact factors of chemical vapor deposition process

The chemical vapor deposition (CVD) method is a gas phase reaction at high temperature. The substrate is soaked in the metal salt solution according to the difference in the electronic ability of the metal atoms on the substrate surface, thus forming a rough structure [20]. Xu et al. 2015 developed a CVD process to prepare MXenes; they used a very low concentration of methane as the carbon source, Cu foil as the substrate, and a temperature above 1085 °C, thereby preparing a large area of high-quality 2D ultrathin Mo₂C crystal that showed two-dimensional characteristics of the superconducting transition and strong anisotropy in the direction of the magnetic field [21]. The high growth temperature allowed Cu to melt and a Mo–Cu alloy to form at the liquid Cu/Mo interface. By reacting with the carbon atoms from the decomposition of methane molecules, Mo atoms subsequently diffuse from the interface to the surface of the liquid Cu to form Mo₂C crystals.

• Low concentration of methane is crucial to obtaining ultrathin Mo₂C crystals instead of graphene [21];
• Thickness depends on the behavior of the high-quality, ultrathin α-Mo₂C crystals;
• The logarithmic temperature impacts significantly on the thin crystals that are exhibited.

In addition to the CVD methods, NaOH and H₂SO₄ can also be employed instead of HF acid for etching. A Ti₃C₂T_x thin film material etched by NaOH at 270 °C is used to produce MXenes, without the surface functional group F-, and its specific capacitance is three times larger compared to the material prepared by HF acid etching [22]. A simple hydrothermal auxiliary method is also applied to increase the production of MXene sheets by up to 74%. Few-layered or single-layer MXene is prepared using organic molecule intercalation, which shows better optical and electrocatalytic properties than those prepared by conventional HF etching. In addition, there are the bottom-up salt-film and pulsed-laser-deposition methods to prepare MXenes, but a few layered or single layer of MXene cannot be obtained, and the organization and performance also need to be investigated [23].

3. Impacts of MXene Modification on Its Properties

3.1. Effects of Surface Modification on MXene Properties

The poor compatibility of MXenes with hydrophobic polymer leads to low interface binding strength, without excellent mechanical properties. The types of hydrophilic functional groups are relatively single, leading to difficulties in meeting the performance requirements for certain application fields. For instance, it does not work well for electrical energy storage—the existence of negative electric terminal groups (-F, -OH) hinder the transport of electrolyte ions and slow the diffusion dynamics of Li⁺ and Na⁺, thus seriously reducing the energy storage capacity of the battery [24]. Hence, surface modification of MXene is very important for the interface regulation of MXene [25].

Surface modification can change the properties of MXene:

• Characteristics of the MXene surface are associated with friction and adhesion properties. With negative friction factors, MXene has better hydrophilicity and more adhesivity [26]. The interaction of strength and friction may increase with the hydrophilicity of the MXene surface.
• The changes in electrostatic potential around the surface of the material affect the electronic structure and work function of MXene. The surface groups can strongly affect the density of state and the work function [27].
• The influence of N-doping mode and doping process on the electrical properties of MXene structure has been explored [28]. There are three possible routines for N-doping: lattice substitution, functional group substitution, and surface adsorption.
• In the acidic aqueous phase, alkyl phosphoric acid ligands were applied via interfacial nucleophilic addition and sequential condensation reactions to form a Ti-O-P bond, grafting to the MXene surface. Relying on the spatial stability of long alkyl chains and the solvation effects of strong nonpolar effects, the solubility of MXene in nonpolar organic solvents is enhanced [29].

3.2. Impact Factors on the Catalytic Performance of MXene

The properties of MXenes, such as good electrical conductivity and their charge separation and transfer capability, enable MXenes to act as a catalyst in photocatalytic reactions, which can be used for photocatalytic reduction of CO₂, water photodecomposition for hydrogen evolution, and photodegradation of organic pollutants. Since MXenes have been widely used in the catalysis field, a variety of improvement methods were implemented for promoting the functional material catalyst (

Table 2. Studies on the catalytic performance of MXene for CO₂ reduction.

Materials	Synthesis	Production	Improvement	Production Rate (µmol·L−1·h−1)	Reaction Condition	Mechanism
Ti3C2/C3N4 [33]	Sonochemical method	CO/CH4	3.64 times	CO: 10.67 CH4: 4.19	UV–vis irradiation for 4 h	Schottky heterojunction
Ti3C2/Bi2WO6 [34]	In situ grown on Ti3C2	CH4/CH3OH	4.6 times	CH4: 1.78 CH3OH: 0.44	Anaerobic/ solar irradiation	2D/2D heterojunction
Ti3C2/g-C3N4 [35]	Calcining the mixture of multilayered Ti3C2 particles and urea	CO/CH4	8.1 times	CO: 5.19 CH4: 0.044	Visible light irradiation (λ ≥ 420 nm)	Interface contact between 2D g-C3N4 and Ti3C2
Ti3C2/Cu2O [36]		CH3OH	8.25 times 2.15 times	CH3OH: 78.50	300 W Xe lamp irradiation	Ti3C2 QDs promote charge transfer
TiO2/C3N4/Ti3C2 [37]	Interfacial self-assembly	CO/CH4	3 times 8 times	CO: 4.39 CH4: 1.20	Anaerobic/xenon lamp irradiation	Heterojunction (lower carrier recombination rate)

).

• Plasma treatment was performed to enhance the surface functional groups on the Ti₃C₂. The enhanced surface functional groups on MXenes benefit in providing abundant active sites on the surface for photocatalytic reactions [30].
• Oxygen vacancies embedded in the Ti₃C₂O₂ MXenes can favor a highly selective photocatalytic CO₂ reduction. Based on calcination, TiO₂ nanoparticles (NPs) were grown in situ on highly conductive MXenes Ti₃C₂ [31]. A unique rice husk-like structure was obtained by evenly distributing NPs over Ti₃C₂. By producing CH₄, the optimized TiO₂/Ti₃C₂ composite exhibited 3.7-times higher photocatalytic CO₂ reduction performance compared to the commercial TiO₂.
• Double heterojunction (S-type heterojunction at the TiO₂/C₃N₄ interface and Schoteryl heterojunction at the C₃N₄/TCQD interface) play a major role in improving photocatalytic activity and jointly accelerate the electron-hole pair separation, migration, and utilization of photogenerated charge carriers with strong redox capacity [32].
• Adjusted MXene contributes to optimal photoactivity. The addition of MXene improves the catalyst efficiency significantly, as reported in many studies [33,34], while excessive addition of black MXene results in decreased photoactivity due to the light-shielding effect [35].
• Ti₃C₂ serves as a co-catalyst that enhances the performance of semiconductors in reduction of CO₂. By constructing composite catalysts with other semiconductors in the shape of QDs, it effectively optimizes the capabilities of CO₂ reduction and conversion. Compared with 2D ultrathin nanosheets, 0D QDs can be more uniformly dispersed in the liquid and have abundant active edge sites. In the process of CO₂ reduction, based on the photocatalysis by Ti₃C₂ QDs/Cu₂O NWs/Cu, the yield of methanol reached 153.38 ppm·cm⁻², which is an improvement of 8.25 fold from Cu₂O NWs/Cu [34].

3.3. Impact Factors on the Energy Storage and Conversion Properties

Three main categories of MXene applications associated with its energy storage and conversion properties are rechargeable batteries, supercapacitors, and electrocatalysis. Various impact factors are investigated and employed to improve MXene performance.

• Metal oxide can provide a sufficient Li⁺ or Na⁺ ion reservoir. The ion-intercalation sites make conductive MXene serve as effective pathways for electron transfer, which can also enlarge the interlayer spacing of layered MXene.
• Surface-exposed MXene can be used as a high-capacity anode material for non-lithium-ion batteries. In particular, for the Mg²⁺ and Al³⁺ batteries, the capacitance and ion mobility of the exposed MXene are higher than that of the oxygen-sealed MXene. Mg and Al can form a stable metal layer on the surface of the exposed MXene, with a high theoretical capacity [38].
• Metallic conductivity, unique 2D structure, and surface defects are important properties of MXene, which make it a prominent electrocatalyst (introduced in Section 3.1).

3.4. Impact Factors on Microwave Absorbing Properties

MXene has great potential in microwave absorption [39]. MXene has a quantitatively controllable layered structure. The multilayer material layer spacing can be flexibly adjusted according to the different preparation methods. Single-layer and few-layer materials provide the premise for constructing three-dimensional structures. The interlayer structure can also create multiple reflection and scattering of electromagnetic waves between materials. The high conductivity of MXene makes it have strong dielectric and polarization loss [40]. Some species, especially MXenes with “M” of Cr and Mn, have magnetic-loss potential. However, the high conductivity of MXene leads to the interface reflection of high-impedance matching difference, which improve the impedance matching and electromagnetism capacity of the material. MXene is often combined with other materials to improve microwave absorption properties.

Embedding low concentration TiO₂ on Ti₂CT_x can enhance the absorbing performance of MXene.

• Ti₃CNT_x exhibits exceptionally excellent microwave absorption properties after annealing, owing to the significant increase in the electrical conductivity, voids, and dipolar polarization capability of Ti₃CNT_x after annealing [41].
• Building a special morphology, such as three-dimensional porous structure, multilayer wave absorption structure, shell structure, flower structure, etc., can improve the electromagnetic wave absorption and multiple reflection capability.
• Construction of a single material structure, such as designing hollow, porous, or shell structure magnetic particles instead of solid magnetic particles, can meet the requirements for multiple reflections and low weight.

3.5. Impact Factors on Adsorption Properties

MXene has the function of adsorption, which is a certain application prospect due to its particular layer structure and surface properties. Accordingly, MXene is applied to abate heavy metals and organic pollutants. The adsorption of heavy metal Pb²⁺ by Ti₃C₂ occurs at a high rate and in a large amount, and with strong reversibility and high sensitivity, especially associated with alkali metal intercalation [42]. Moreover, the MXenes can be intercalated and exfoliated into nanosheets, and thus various types of organic matter, such as urea and methylene blue, can be intercalated into the MXenes. Therefore, MXene is a promising adsorbent material to remove heavy metals and pollutants from wastewater. Control of the functional groups on the MXene surface can improve the adsorption performance during catalytic applications.

• Modifications of MXene may yield more effective results on heavy metal removal. Alkalization-grafting with sodium ions and a silane coupling agent (APTES) is the most effective modification method so far; it achieved better adsorption capacity (385 mg/g) by using a lower amount of adsorbent (0.1 g/L) and equilibrium time (30 min).
• One of the most important mechanisms of the adsorption is ion exchange, which replaces heavy metal ions by attaching to the surface of MXene functional groups (

  Table 3. Impact factors on Pb²⁺ removal.

  Adsorbent	Applied MXene Modification	N2 Surface Area (A) (m2/g), Point of Zero Charge (PZC)	Adsorbent Dose D (g/L), Optimum pH, Equilibrium Time T (min)	Maximum Adsorption Capacity Qm (mg/g)
  Multilayer Ti3C2Tx − 45 [43]	Drying the hydrofluoric acid-etched Ti3AlC2 powder	A: 76.4	D: 0.5; T: 600	185
  Multilayer Ti3C2Tx − 35 [43]	Drying the hydrofluoric acid-etched Ti3AlC2 powder	A: 65.4	D: 0.5; T: 600	164
  Multilayer Ti3C2Tx − 25 [43]	Drying the hydrofluoric acid-etched Ti3AlC2 powder	A: 19.8	D: 0.5; T: 600	119
  Multilayer Ti3C2Tx [44]	Drying the hydrofluoric acid-etched Ti3AlC2 powder	A: 10.0	D: 0.05; pH: 6 T: 120	~36
  Multilayer Ti3C2Tx/alginate [45]	Mixing of sodium alginate with Ti3C2Tx		D: 1; pH: 6 T: 15	383
  Multilayer Ti3C2Tx − KH570 [46]	Mixing of silane coupling agent (KH570) with Ti3C2Tx at 70C	A: 75.4 PZC: 2.6	D: 3.2; pH: 5 T: 30	147
  Ti2CTx − EHL [47]	Biosurfactant enzymatic hydrolysis lignin mixed with Ti2CTx	A: 22.5 PZC: 3.2	D: 1.6; pH: 5 T: 1440	233
  Ti2CTx − CS [47]	Biosurfactant chitosan mixed with Ti2CTx		-	93.5
  Ti2CTx − LS [47]	Biosurfactant lignosulfonatemixed with Ti2CTx		-	104
  Delaminated alk − Ti3C2Tx [48]	Alkalization intercalation modification with sodium ions	A: 72.0 PZC: 3.8	D: 0.1; pH: 6.3 T: 30	188
  Delaminated alk − Ti3C2Tx − NH2 [48]	Alkalization-grafting with sodium ions and silane coupling agent (APTES)	A: 129.2 PZC: 4.1	D: 0.1; pH: 6.3 T: 30	385
  Delaminated alk − Ti3C2Tx NH2 [48]	Alkalization-grafting with sodium ions and silane coupling agent (APTES)	A: 129.2 PZC: 4.1	D: 0.1; pH: 6.3 T: 30	118
  Multilayer Ti3C2(OH/ONa)xF2 − x [37]	Alkalization–intercalation method		D: 0.5; pH: 6.8 T: 120	~140

  ). In the case of alkylated Ti₃C₂(OH/ONa)_xF_2−x adsorbing Pb²⁺, the Pb²⁺ adsorbed is a function of pH with more preferential sorption at higher levels within the range 5–7 [42]. At low pH, the low affinity of MXenes toward Pb²⁺ may regenerate Pb²⁺ from used flakes in the solutions [43].

Removal of heavy metals was investigated under different pH, adsorbent concentration, initial concentration, adsorption temperature, and contact time. However, more attention should be dedicated to the purpose and applications of MXene design, which may enhance the adsorption capacity [43].

4. Conclusions

Properties of MXene can be affected by various factors in the preparation process, while the influencing factors in the preparation and application of composite materials can also affect the performance of MXene.

Etching temperature, types of etchers, and reaction time are key factors, which not only effect MAX stripping and productivity, but also influence the properties of MXenes in applications. Different etchers can cause different functional groups to graft onto the surface of MXene, which can obviously alter its intrinsic characteristics and thus affect the performance. Clarification and understanding of the surface properties of MXene are important for its performance and application. In situ etching is the classic method, which is relatively mild and most widely used, and the process is simple. It can use ions to intercalate MXenes directly to increase the interlayer moment of MXenes and reduce the interlayer force. Electrochemical etching can create MXenes with surface functional groups without F-, but currently MAX forms MXenes through a two-step process. Electrode and voltage are significant factors. Molten salt is a new preparation method, with fast reaction, safe process, and no pollution or hazard, but effectively controlling fluoride salt impurities is a crucial factor in its preparation mechanism, and reaction temperature is also an important influencing parameter. In the chemical vapor deposition (CVD) method, the substrate is soaked in the metal salt solution, according to the difference in the electronic capability of the metal atoms on the substrate surface. It requires a low concentration of methane, and the thickness depends on the behavior of high-quality ultrathin α-Mo₂C crystals.

MXene is modified according to the demand and the structure optimization. Different structures have different effects on its electrochemical, photothermal, and microwave absorption properties, and its hydrophilic and hydrophobic characteristics. Exploring the properties of MXene, such as friction, adhesion, conductivity, and photothermal conversion, helps to expand its application fields. For surface modification, the interface between different components of most composite materials is connected through physical interactions (hydrogen bond, van der Waals force, etc.) with poor mechanical properties. To improve its comprehensive performance, further exploration of the modification using chemical bonding cooperation is required. The thermal and chemical stability and antioxidant performance of MXene can be improved by surface modification. To expand the comprehensive understanding of MXene properties, the influencing mechanism of MXene composite properties on the existing basis need clarification. Concerning the application for controlling environmental pollutants, compared to drying the hydrofluoric acid-etched Ti₃AlC₃ powder (reaction condition: 0.5 g/L, 600 min), MXene with the modification method of APTES requires lower adsorbent doses (0.1 g/L) and shorter equilibrium time (20 or 30 min) when removing similar amounts of Pb²⁺, and even provides greater adsorption capacity (mg/g).

Advanced methods for MXene synthesis and modification have achieved extraordinary progresses during the last decade. The following points provide further applications and key issues that can be explored:

• Databases and computational models are needed to address the parameters of MXene synthesis, modification, and the positive/negative factors for application in various fields.
• Extend the advantages of MXene in engineering practice, e.g., make significant contributions to carbon reduction programs.
• MXenes as adsorbents are applied to remove heavy metals from synthetic wastewater. Thus, MXene is also expected to be investigated in removing pollutants (e.g., heavy metals, salts, and organic pollutants) in multiple media (air pollution, solid waste, etc.), and applied to the actual complex and complicated environment.
• Chemicals are introduced in the process of MXene synthesis and modification, hence, occupational hygiene, environmental issues, and laboratory safety should be considered.