Ti₃C₂ MXene: recent progress in its fundamentals, synthesis, and applications

Abstract

In this rapidly developing society, it is always crucial to exploit new materials with suitable properties to meet specific application demands. Two-dimensional (2D) transition metal carbon/nitrides (MXenes) are a novel graphene-like material with exciting research potential in recent years. Among them, Ti₃C₂ debuts in a central position due to its relatively longer research history, mature synthetic process, and incredibly rich store of merits, such as good flexibility, large specific surface area, abundant termination groups, excellent electrical conductivity, and light-to-heat conversion ability. In this review, recent research progress on Ti₃C₂ MXene and its composites was updated mainly from three aspects, including their fundamentals, synthesis, and applications. It has been found that diverse applications of Ti₃C₂-based composites are inseparable and correlated with each other, which were linked by their unique physicochemical properties. In the end, a summary and a perspective on future opportunities and challenges of Ti₃C₂ were given to offer theoretical and technical guidelines for further investigation on MXene family.

摘要

在这个快速发展的社会中, 为了满足特定应用需求, 开发具有合适性能的新材料始终是至关重要的。近年来, 二维 (2D) 过渡金属碳/氮化物 (MXenes) 是一种新型的类石墨烯材料, 具有令人兴奋的研究潜力。其中, Ti₃C₂占据引领地位, 这是因为其拥有较长的研究历史、成熟的合成工艺以及极其丰富的优点, 例如良好的柔韧性、较大的比表面积、丰富的端基、优异的导电性和光热转换能力。本篇综述主要从基本性质、合成方法和应用三个方面对Ti₃C₂ MXene及其复合材料的最新研究进展进行了更新总结。经过分析得知, Ti₃C₂基复合材料在不同领域的应用通过其独特的物理化学性质而相互关联, 密不可分。最后, 文章对Ti₃C₂的未来机遇和挑战进行了总结和展望, 为进一步研究MXene家族材料提供了理论和技术指导。

1 Introduction

Since the single-layer graphene was amazedly acquired from graphite by a stripping method in 2004 [1], its unique electrochemical and optoelectronic properties excited a huge research enthusiasm on the fabrication and exfoliation of two-dimensional (2D) layered materials, such as silicone [2], germanene [3], phosphorene [4], MoS₂ [5], and clays [6]. In 2011, Naguib et al. [7] originally extracted the Al layers from Ti₃AlC₂ by hydrofluoric acid (HF) to form a novel laminated Ti₃C₂ nanocrystal, which owned graphene-like properties and morphology and therefore was defined as “MXene.” Thereafter, more transition metal carbides and/or nitrides such as Ti₂C, Ti₃C₂, Ti₃CN, Ta₄C₃, Nb₂C, Nb₄C₃ and V₂C were discovered to extensively expand this burgeoning MXene family [8,9,10]. The general formula of MXene is M_n+1X_nT_x (n = 1–3), in which M means early transition metals (i.e., Sc, Ti, V, Cr, Zr, Hf, Nb, Mo, Ta, and W), X stands for carbon and/or nitrogen, and T_x refers to the surface terminations (such as –OH, –O, and –F) which was formed during the chemical etching process [11]. The originally obtained MXene from its MAX phase usually appears as an accordion-like structure, which is not only conducive to the interpenetration of ions for specific applications (such as batteries and sensors), but also highly flexible and shapeable, because single- or few-layer form of MXenes can be readily obtained by a facile delamination step from their accordion-like multilayer form [12,13,14,15,16], and even more, quantum dot (QD) form of MXenes can also be achieved by further treatment such as hydro-/solvothermal [17, 18], ball milling [19] and micro-explosion [20] strategies.

Although the MXene family was continually expanding with the assistance of both theoretical and experimental technologies, Ti₃C₂T_x still debuts in a central role among the MXenes up to now for the following reasons. (1) The preparation process of Ti₃C₂T_x is simple and low-cost with a wide market prospect [21]. (2) Ti₃C₂T_x is fairly flexible in various structures including three-dimensional (3D) accordion-like multilayers, 2D lamellar nanosheets, and zero-dimensional (0D) ultra-small QDs, which can meet different requirements in diverse situations [22]. (3) The electronic energy band structure and physicochemical properties of Ti₃C₂T_x are feasibly steerable by controlling the abundant functional groups on its surface [23]. (4) Apart from the terminations on the surface, local defects also exist in Ti₃C₂T_x, which can be fully utilized in the preparation of MXene-based composites [24]. (5) Ti₃C₂T_x exhibited outstanding electrical conductivity even with the surface terminations and local defects [25]. (6) In Ti₃C₂T_x, carbon is the basic element for constructing living organism, and transition metal Ti is inert to living beings. Therefore, Ti₃C₂T_x is non-toxic and biodegradable, which has unparalleled research potential in the field of biomedicine [26].

Although Ti₃C₂T_x has multifarious excellent characteristics, there are still some shortcomings that hinder its current development. For example, the structure of Ti₃C₂T_x tends to inevitably collapse in a humid environment, because the exposed Ti atoms on the surface make it easily oxidized. In addition, the synthesis methods to obtain Ti₃C₂T_x were still limited, which usually required the use of hazardous fluorinated reagent [22]. In order to further expand the application areas of Ti₃C₂T_x and accelerate its development pace, researchers have constructed it into hybrids with other materials including metal [25], metal oxide [27], metal sulfide [28], polymer [29], protein [30], and natural organic molecules [31] to combine their merits.

Hitherto, the research on Ti₃C₂T_x and its hybrids is still in its infancy. Thus, it is quite necessary to update the recent research progress and milestones on Ti₃C₂T_x MXene. In this review, we initially introduced the fundamentals of Ti₃C₂T_x MXene, such as their atomic structure, crystal phase, morphologies, and physicochemical properties. Subsequently, we emphasized the synthesis methods of various formed Ti₃C₂T_x and their composites. Next, we summarized the diverse applications of Ti₃C₂T_x in different fields, including energy storage, electromagnetic interference, photothermal conversion, biomedical application, sensor, and photocatalysis. Besides, the synthesis strategies and applications of Ti₃C₂T_x-based composites are classified and summarized in Table 1 [15, 19, 24, 25, 28, 29, 31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58]. Meanwhile, as shown in Fig. 1, linked by their unique physicochemical properties, diverse applications of Ti₃C₂T_x-based composites are inseparable and correlated with each other, just like the pieces of a jigsaw puzzle. Finally, a perspective on future opportunities and challenges of MXene materials was provided. It aims to offer theoretical and technical guidelines for the preparation and application of Ti₃C₂T_x and other similar MXene materials (Fig. 1).

Table 1 Summary of synthetic approaches and applications of Ti₃C₂-based composites

Fig. 1

Schematic diagram of applications of different dimensional Ti₃C₂T_x MXene in diverse and correlative fields

2 Fundamentals

In-depth understanding and research on the fundamentals of Ti₃C₂T_x are not only instructive to promoting its fabrication strategy, but also conducive to discovering and extending its great potential in various fields. Therefore, in this part, we will demonstrate its fundamentals from the aspects of atomic structure, crystal phase, morphology, and various physicochemical properties in detail.

2.1 Atomic structure and crystal phase

To understand the atomic structure of Ti₃C₂ MXene, one must retrospect its precursor, Ti₃AlC₂ MAX. As shown in Fig. 2a, the crystal structure of Ti₃AlC₂ involves the periodic superposition of six atomic layers (Ti–C–Ti–C–Ti–Al) along its cross section [22]. Typically, the Ti layer is closely packed and interleaved with the Al element layer, and C atoms fill the octahedral sites on the Ti layers [11]. Fortunately, Ti₃C₂ MXene can be easily prepared from the Ti₃AlC₂ MAX by selectively removing “Al” layer through etching methods, because Ti–C bound layers with metallic bond energy are more stable than Ti–Al layers [7, 13, 59]. After the etching treatment, the exposed metal sites on Ti₃C₂ surface have higher redox activity than the carbon sites in the framework. Thereby, the bare Ti₃C₂ possesses high surface energy and prefers to quickly react with the anions in the solution to form the functional terminations, such as –O, –OH, and/or –F, which were noted as T_x, where x is the number of the terminal groups [12, 60].

Fig. 2

Atomic structure of Ti₃C₂ MXene. a Scheme of Ti₃C₂T_x MXene formation from Al etching in Ti₃AlC₂ MAX; b comparison of XRD patterns of Ti₃AlC₂ and Ti₃C₂. Reproduced with permission from Ref. [36]. Copyright 2018, Wiley. c TEM image of Ti₃C₂ nanoflakes and (inset) its corresponding SAED pattern. Reproduced with permission from Ref. [16]. Copyright 2016, Wiley

Technically, X-ray diffraction (XRD) analysis could tell the transition of the crystal phases from Ti₃AlC₂ MAX to Ti₃C₂ MXene. As displayed in Fig. 2b, XRD pattern of the initial Ti₃AlC₂ shows a spiculate (104) diffraction peak at 2θ ≈ 39°, as well as other typical diffraction peaks, such as those at the 2θ values of 9.4°and 19.0°, which correspond to (002) and (004) crystal planes, respectively [13]. These characteristic peaks all confirmed the hexagonal crystal phase of Ti₃AlC₂. The etching of Al elements from Ti₃AlC₂ to form Ti₃C₂ was evidenced by the disappearance of the (104) peak, as well as the shift of (002) and (004) peaks to a lower angle [36]. At the same time, the diffraction peaks of (002) and (004) were obviously broadened and weakened, which indicated that the interplanar spacing between the Ti₃C₂ multilayers became larger and thus, the c-lattice parameter (c-LP) increased [12]. Nevertheless, the selected area electron diffraction (SAED) pattern of few-layer Ti₃C₂ shows the typical hexagonal diffraction pattern (inset in Fig. 2c), indicating that the quality of the Ti₃C₂ nanosheets is intact and the hexagonal structure is still maintained after peeling off [38].

2.2 Morphology

Obviously, the macro- and micro-appearance of Ti₃AlC₂ will change dramatically after being etched into Ti₃C₂T_x. From a macro-perspective, the dense powder became fluffy, and its color changed from black to dark purple or bright black (Fig. 3a, b). Microscopically, it can be observed from scanning electron microscopy (SEM) image that the bulk Ti₃AlC₂ exhibited a compact layered structure (Fig. 3c), while Ti₃C₂T_x presented an accordion-like shape (Fig. 3d) [8]. Through some specific treatment (see Sect. 3.2), the accordion Ti₃C₂T_x could be peeled off to form 2D single-layer nanosheets, which are more favored by researchers due to its larger specific surface area and unique quantum size effect. In reality, the tested thickness of Ti₃C₂T_x nanosheets by atomic force microscope (AFM) technique was usually 0.5–2.0 nm with the presence of surface groups and absorbed water molecules [61,62,63,64,65]. For example, Lipatov et al. [16] prepared monolayer Ti₃C₂T_x (Fig. 3e), and the AFM height curve in Fig. 3f shows that it was about 1.5 nm in thickness. Similar results have also been reported in other studies related to Ti₃C₂T_x, where the thicknesses were tested as 0.74, 0.98 and 1.63 nm, respectively [65,66,67,68].

Fig. 3

Reproduced with permission from Ref. [8]. Copyright 2012, American Chemical Society. e TEM and f AFM images of 2D Ti₃C₂T_x monolayers. Reproduced with permission from Ref. [16]. Copyright 2016, Wiley. g TEM and h HRTEM images of 0D Ti₃C₂T_x QDs. Reproduced with permission from Ref. [10]. Copyright 2018, The Royal Society of Chemistry

Morphology of Ti₃C₂ MXene. Photographs of a Ti₃AlC₂ MAX powder and b Ti₃C₂ MXene powder; SEM images of c Ti₃AlC₂ precursor and d 3D Ti₃C₂ multilayers.

For the lamellar size of Ti₃C₂T_x monolayer, it varies along with the utilized preparation methods. For instance, the Ti₃C₂T_x monolayer flakes prepared by a harsh chemical corrosion method in 40% HF aqueous solution were usually broken down to nanometer level in lamellar size (100–400 nm) [69]. In comparison, those prepared in milder conditions such as LiF/HCl and NH₄Cl etching methods often possess much larger size in micrometer level [70]. Generally, the larger the lamellar size of nanosheets is, the more the active sites are exposed. However, sometimes, small flakes are also in demand due to their agility. Especially, Ti₃C₂T_x QDs can be prepared by proper methods such as hydro-/solvothermal [17, 18, 39], ball milling [19], and micro-explosion [20] strategies. As shown in Fig. 3g, h, Ti₃C₂T_x QDs with the lattice spacing of 0.21 nm are relatively uniform in size, and the average size is less than 10 nm [10].

2.3 Physicochemical properties

As a new member of 2D materials, Ti₃C₂ MXene with a regular metal atom skeleton displays some unique properties. With the assistance of abundant surface functional groups on Ti₃C₂, the obtained Ti₃C₂T_x can perfectly combine the metal conductivity and the hydrophilicity [10, 59]. Moreover, specific characteristics of Ti₃C₂T_x can be achieved by controlling and adjusting these terminal functional groups, which is of great research value.

In this part, three typical physicochemical properties of Ti₃C₂T_x including electrical conductivity, hydrophilicity, and mechanical stability will be introduced in detail to further recognize its fundamentals.

2.3.1 Electrical conductivity

Undeniably, one of the most critical factors that motivate researchers to keep enthusiastic in the research of Ti₃C₂T_x is its unparalleled conductivity. It is the conductive carbide core along with transition metal oxide-like surfaces that plays a central role in the electrical conductivity of Ti₃C₂T_x [71]. It has been reported that a pure Ti₃C₂T_x MXene film with a thickness of 940 nm prepared by a scalable blade coating process using large-sized MXene flakes can exhibit the electrical conductivity up to ≈ 15,100 S·cm⁻¹ [72]. Compared with the conductivity of graphene films (1.5 × 10⁴ S·cm⁻¹) [73], the conductivity of Ti₃C₂T_x is in the same order of magnitude. Therefore, it is supposed to be a promising alternative to graphene.

The electrical conductivity of Ti₃C₂T_x is usually influenced by the factors including terminal groups [74], interlamellar spacing between flakes, and ambient temperature. Firstly, the electrical conductivity of Ti₃C₂T_x can be distinctly improved by removing part of the surface functional groups via annealing in vacuum [75]. For example, Wang et al. [23] roasted Ti₃C₂T_x nanosheets at 600 °C for 1 h, and successfully reduced the amount of the functional groups to shorten the conduction path of electrons, which resulted in the enhancement of its conductivity by nearly three times. Secondly, increasing the interlamellar spacing between the flakes influences the electrical properties of Ti₃C₂T_x. For instance, the conductivity of multilayer Ti₃C₂T_x was reported to be one order of magnitude lower than that of single-layer one ((4600 ± 1100) S·cm⁻¹) [16]. Furthermore, the used intercalants to increase the interlayer spacing of the multilayered Ti₃C₂T_x also decreased its resistivity. For instance, Muckley et al. [76] reported that H₂O and Li⁺ intercalation caused the resistance value of Ti₃C₂T_x to decrease from 41 to 10 Ω. Thirdly, temperature is also a vital effect for the electron conductivity of Ti₃C₂T_x MXene. For example, Halim et al. [77] and Bergmann [78] confirmed that the resistivity of the 2D Ti₃C₂T_x nanofilm increases from 4.8 to 6.1 μΩ·m with the decrease in temperature when the temperature is below 100 K, which is attributed to the common electron backscattering phenomenon in 2D metals materials.

The feature of good electro-conductivity has now been used as the most dazzling advantage of Ti₃C₂T_x in various fields including electromagnetic wave absorption and shielding, energy conversion, sensors, catalysis, etc. Thereby, more efforts should be invested in the development and optimization of the MXene's conductivity.

2.3.2 Hydrophilicity and hydrophobicity

Ti₃C₂T_x MXene was usually derived by the acid etching process in a watery environment, thus abundant polar terminal groups were formed on its surface. These terminations always resulted in strong hydrophilicity of Ti₃C₂T_x nanosheets, which has been taken full advantage in some typical application fields, such as photocatalytic pollutant degradation, biomedicine, and seawater desalination. For instance, Cai et al. [79] made full use of the abundant hydrophilic functional groups on the surface of Ti₃C₂T_x to form a strong interface contact with Ag₃PO₄ to promote the separation of electron–hole pairs and achieve high-efficiency photocatalytic degradation of organic pollutants. Furthermore, Singh et al. [80] confirmed that Ti₃C₂T_x featured with good hydrophilicity showed excellent cell adhesion to mouse fibroblasts, which exhibited good biocompatibility. In addition, Zhang et al. [81] designed an amphiphilic MXene aerogel for solar desalination by vertically arranged Ti₃C₂T_x, where the upper layer was hydrophobic, and the bottom layer was hydrophilic. The designed aerogel was featured with adjustable and ordered vertical-array structure, which achieved a daily seawater output of 6 L·m⁻² due to the feasibility of steam escape, moisture transfer, and little heat loss.

On the other hand, a hydrophobic surface of Ti₃C₂T_x suitable for specific environments can also be obtained by adjusting its surface functional terminations or introducing it into other functional materials to construct composites [82]. For example, Zhao et al. [83] prepared a hydrophobic Ti₃C₂T_x MXene film terminated with trimethoxy groups for solar steam conversion. The efficiency reached 71%, ensuring efficient and long-term stable light and heat transmission in seawater with high salinity. In addition, Liu et al. [84] prepared hydrophobic porous Ti₃C₂-based foam by fixing the Ti₃C₂ film between two ceramic wafers, which were coated with 80% hydrazine monohydrate and then calcined in 90 °C. Compared with the hydrophilic Ti₃C₂ films with a contact angle of 59.5°, the Ti₃C₂-based foam exhibited hydrophobic character with a contact angle of 94.0°. The unique hydrophobicity-made Ti₃C₂ foam possessed excellent durability in humid environments and also improved its stability in water.

Although the abundant surface terminal groups endow Ti₃C₂T_x MXene with excellent hydrophilic characteristics, it is easily oxidized by the air or water to form TiO₂, and subsequently results in the structure collapse of Ti₃C₂T_x, which is a non-negligible issue hindering the further development and wide application scope of MXene-based materials. Up to now, many researchers have taken effort to tackle this problem by various methods such as saving in organic antioxidant, isolating from oxygen, and refrigerating in a low temperature. For instance, Zhao et al. [85] utilized sodium L-ascorbate (NaAsc) as an antioxidant to prevent the oxidation of Ti₃C₂T_x nanosheets in wet environments. The L-ascorbate group could associate with the terminations of Ti₃C₂T_x to restricting their further reactions with water in colloidal solution. The as-obtained Ti₃C₂T_x sheets retained its inchoate morphology after even half a year. Analogously, dimethyl sulfoxide (DMSO), ethanol, and n-picoline have also been proven to prolong the lifetime of Ti₃C₂T_x to ca. ~ 30 days [86]. However, the utilized organic additives will inevitably remain in the Ti₃C₂T_x layers and cause the decreased conductivity. Moreover, in order to remove these organic reagents, the exposure time of MXene to water and air will be increased. For long-term storage of Ti₃C₂T_x, the more direct and effective method is to store it in powder form under vacuum [87], or to inject inert gas into the solution to isolate the MXene from water and/or oxygen [88]. Furthermore, Habib et al. [89] recently verified that Ti₃C₂T_x oxidized fastest in liquid media and slowest in solid media (including polymer matrix). Thereafter, Zhang et al. [87] proposed that freezing the MXene aqueous dispersion at − 20 °C could effectively prevent the formation of TiO₂ nanoparticles on the edge of Ti₃C₂T_x in the early stage of oxidation.

2.3.3 Mechanical stability

Mechanical stability of equipment and devices has become a key consideration in their practical applications. So far, due to the excellent mechanical stability, Ti₃C₂T_x nanosheet has been proven to be one of the chosen materials for the next generation of flexible functional devices [90, 91]. The mechanical properties of Ti₃C₂ nanosheets largely depend on the interaction force between individual nanosheets. When the thickness of the Ti₃C₂T_x film is 940 nm, the tensile strength can reach 570 MPa, and the corresponding Young's modulus in the vertical direction is about 20.6 GPa [72]. In order to adapt to the development of intelligent technology, reducing the thickness of the product while ensuring its mechanical stability is extremely important for flexible electronic devices. Therefore, the tensile strength of Ti₃C₂ nanosheets with a thickness of about 40 nm optimized by Firestein et al. [92] can be as high as 670 MPa, which can be proved by in situ transmission electron microscopy (TEM) tensile strength testing. Meanwhile, the Young's modulus perpendicular to the direction of the Ti₃C₂T_x nanosheet was measured to be 120–140 GPa. In addition, the ultimate tensile strength (UTS) can be significantly affected by defects in the structure of Ti₃C₂T_x. It was reported that when the defect concentration in the structure increased from 2% to 8%, the UTS of Ti₃C₂T_x nanosheets decreased from 21.6 to 18.9 GPa [92]. Therefore, it can be concluded that the mechanical properties of Ti₃C₂T_x can be adjusted by controlling the thickness of the nanosheets and concentration of the structural defects.

3 Synthesis methods

Contemporarily, the chief strategy to obtain Ti₃C₂ MXene is selectively etching Al element layers from the Ti₃AlC₂ MAX phase, which could be obtained by the bottom-up synthesis method from atoms and molecules, such as chemical vapor magnetron sputtering [77] and self-propagating high-temperature synthesis (MASHS) method [93]. To improve the work efficiency, most researchers preferred to directly purchase Ti₃AlC₂ powder from companies, such as Beijing Lianli New Technology Co., Ltd, China [36], Jilin 11 Technology Co., Ltd, China [94], Macklin [28], and Sinopharm Chemical Reagent Co. Ltd. [79]. In this section, we firstly present the technologies for preparing Ti₃C₂T_x multilayer flakes from Ti₃AlC₂ MAX precursor, and then illustrate the delamination methods of Ti₃C₂T_x to obtain few-layer or single-MXene nanosheets. Subsequently, the synthetic strategies of 2D-nanosheet and 0D-QD MXenes were expounded. At last, the design and fabrication strategies of Ti₃C₂T_x-based composites were also summarized.

3.1 Synthesis of Ti₃C₂T_x multilayer flakes

Different from other 2D materials, such as graphene and MoS₂, the force between the MAX layers is much stronger due to the presence of M–A metal bonds [13]. In 2011, Gogotsi et al. unprecedentedly reported that Ti₃C₂T_x multilayer could be produced by selective etching Al atoms from Ti₃AlC₂ MAX phase in 50% hydrofluoric acid (HF) aqueous solution (Fig. 4a) [7]. Typically, when Ti₃AlC₂ is immersed in HF aqueous solution, the etching reaction equations are as follows:

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

(1)

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

(2)

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

(3)

In Eq. (1), Al atoms are exfoliated from the Ti₃AlC₂ MAX phase to form Ti₃C₂. After etching, the exposed Ti atoms will become the reaction sites, which is easy to react with water (Eq. (2)) and HF (Eq. (3)) to produce Ti₃C₂(OH)₂ and Ti₃C₂F₂, respectively. In most cases, Eqs. (2, 3) occurred simultaneously [7].

Fig. 4

Reproduced with permission from Ref. [7]. Copyright 2011, Wiley. b Etching Ti₃AlC₂ by XHF₂ bifluoride, where X presents K⁺, Na⁺, and NH₄⁺. Reproduced with permission from Ref. [97]. Copyright 2017, Elsevier. c Etching Ti₃SiC₂ by Lewis acids such as CuCl₂, FeCl₂, and AgCl through a high-temperature molten salt method. Reproduced with permission from Ref. [99]. Copyright 2020, Nature. d Etching Ti₃AlC₂ by alkali (NaOH) solution. Reproduced with permission from Ref. [100]. Copyright 2014, The Royal Society of Chemistry

Synthesis strategies of Ti₃C₂T_x multilayers. a Etching Ti₃AlC₂ by HF. 

However, HF is a dangerous and toxic substance due to that it is strongly corrosive and irritating effect to the skin, eyes, and respiratory tract. Alternatively, a LiF/HCl mixed solution was then developed as a milder etchant. As illustrated in Eq. (4), the HF species with a controllable concentration can be in situ produced from LiF and HCl, and serve as the etchant for Ti₃AlC₂. Interestingly, it has been found that the Ti₃C₂T_x MXene produced by this mild etching method usually possessed a larger planar size and fewer atomic defects than that produced by HF etching [95]. Similarity, Wang et al. [96] employed NH₄F to remove Al element from bulk Ti₃AlC₂ for preparing multilayered Ti₃C₂ by a facial hydrothermal strategy. Followed Eqs. (5, 6), this method also avoids the direct utilization of HF.

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

(4)

$${\text{NH}}_{{4}} {\text{F }} + {\text{ H}}_{{2}} {\text{O }} = {\text{ NH}}_{{3}} \cdot{\text{H}}_{{2}} {\text{O }} + {\text{ HF}}$$

(5)

$${\text{3NH}}_{{4}} {\text{F }} + {\text{ AlF}}_{{3}} = \left( {{\text{NH}}_{{4}} } \right)_{{3}} {\text{AlF}}_{{6}}$$

(6)

Although the optimized LiF/HCl etching method avoids the direct utilization of HF, danger is still not changed in essence. Therefore, Feng et al. [97] attempted to use XHF₂ (where X presents K⁺, Na⁺, NH₄⁺) to replace HF as the etchant for the production of Ti₃C₂T_x based on Eq. (7) (Fig. 4b). Therein, NH₄HF₂ allows extra intercalation of NH⁴⁺ cations and NH₃ molecules in the etching reactions. In this way, Ti₃C₂T_x can be obtained in a single reaction with a large interplanar spacing, so the accordion structure can be better maintained.

$${\text{Ti}}_{{3}} {\text{AlC}}_{{2}} + {\text{ XHF}}_{{2}} = {\text{ X}}_{{{a}}} {\text{AlF}}_{{{b}}} + {\text{ AlF}}_{{3}} + {\text{ H}}_{{2}} + {\text{ Ti}}_{{3}} {\text{C}}_{{2}}$$

(7)

The above-mentioned etching mechanism is still based on utilizing detrimental HF or fluoride solution [98]. To explore a non-hazardous fabrication process, Li et al. [99] proposed a generic strategy to directly etch A-element from the MAX phases by Lewis acids such as CuCl₂, FeCl₂ and AgCl through a high-temperature molten salt method. During the redox process, the exposed Si atoms in the Ti₃SiC₂ were oxidized into Si⁴⁺ by cations of Lewis acid. However, the excess Cu²⁺ in the reaction system will react with the exposed Ti atoms on the surface of Ti₃C₂ to form Cu metal particles, which needs to be removed by ammonium persulfate ((NH₄)₂S₂O₈, APS) solution (Fig. 4c).

Except the Lewis acidic molten salt method, alkali etching is another feasible method, because Al is amphoteric, soluble in both acids and bases. For example, Xie et al. [100] immersed massive Ti₃AlC₂ in the NaOH solution (1 mol·L⁻¹) and continuously stirred at 80 °C for 100 h to remove the Al atomic layer (Fig. 4d). Finally, multilayered Ti₃C₂T_x was obtained by further removing the un-etched Al element and generating hydroxyl terminal functional groups in 1 mol·L⁻¹ H₂SO₄ at 80 °C for 2 h.

Overall, the production of Ti₃C₂T_x MXene multilayer flakes can be achieved by selective etching steps in different ways. However, the multilayer flakes possess small specific surface area and limited amount of available reactive sites, which greatly suppress their potential applications. Therefore, enlarging the interlayer space of Ti₃C₂T_x to gain few-layer ones is necessary. We summarized the delamination methods of Ti₃C₂T_x multilayers in the following section.

3.2 Delamination of Ti₃C₂T_x multilayers

For delamination of Ti₃C₂T_x multilayers, the commonly used strategies of Ti₃C₂T_x are intercalation and ultrasonication methods. Hitherto, the intercalators that have been applied in delamination of Ti₃C₂T_x mainly include some organic reagents such as DMSO [12], tetrabutylammonium hydroxide (TBAOH) [101], tetramethylammonium hydroxide (TMAOH) [13], N-methylpyrrolidone [102] and urea [15]. For instance, Mashtalir et al. [12] reported the formation of Ti₃C₂T_x-based “paper” by the DMSO intercalation, where the DMSO molecules would get into the separated sheets of Ti₃C₂T_x by sonication, and resulted in increasing lattice spacing of Ti₃C₂T_x. Similarly, Chia et al. [101] also took advantage of the large volume of TBAOH molecules to enter the interlayer of Ti₃C₂T_x to promote its delamination. Yang et al. [15] successfully obtained delaminated Ti₃C₂T_x by simultaneous calcination of Ti₃C₂T_x multilayers and urea (Table 1, No. 34). Owing to the abundant functional groups on the surface of Ti₃C₂T_x, urea can be tightly adsorbed on its surface, and NH₃ generated by urea during the calcination process can further be employed as a gas template to increase the layer spacing of Ti₃C₂T_x and promote its delamination.

However, the production yield of 2D monolayer Ti₃C₂T_x sheet by delamination is usually unsatisfactory (≤ 20%), which lies in the remaining Ti–Al bonds and potential Ti–Ti bonds between adjacent Ti₃C₂T_x layers after etching. Innovatively, Han et al. [13] used TMAOH as the intercalator, and the yield of monolayered Ti₃C₂T_x achieved the recorded highest value of 74% by a facile hydrothermal-assisted intercalation method (Fig. 5). Compared with DMSO, TMAOH plays better corrosion to Al, and the spatial structure of TMAOH is relatively larger to weaken the interaction between the adjacent Ti₃C₂T_x sheets. Moreover, the formation of Al(OH)⁴⁻ during the corrosion process will further expand the interlayer space.

Fig. 5

Reproduced with permission from Ref. [13]. Copyright 2019, American Chemical Society

Delamination of Ti₃C₂T_x multilayers by intercalators. a Synthesis process of 2D Ti₃C₂T_x nanosheets by hydrothermal and ultrasonication strategy using tetramethylammonium hydroxide (TMAOH) and ascorbic acid (AA) as intercalators; SEM images of b Ti₃AlC₂ precursor, c 3D Ti₃C₂T_x multilayers and d 2D Ti₃C₂T_x nanosheets.

To date, the difficulty in layering Ti₃C₂ is generally explained by the strong interaction of potential Ti−Ti bonds between adjacent Ti₃C₂ layers. Meanwhile, since Ti₃C₂T_x is easily oxidized in humid conditions, the layered structure inevitably collapses to a certain extent. When choosing an intercalation agent, one should consider whether it will accelerate the oxidation of Ti₃C₂T_x or not. Therefore, it is still a challenging hot topic for researchers to explore more efficient and environmental-friendly intercalations.

3.3 Synthesis of few-/mono-layer Ti₃C₂T_x

Apart from delamination of multilayers, it is feasible to directly prepare uniform and high-quality single-layer Ti₃C₂T_x by adjusting the type and ratio of the etchant and the precursor. Originally, Ghidiu et al. [103] successfully produced single-layer Ti₃C₂T_x flakes from Ti₃AlC₂ via LiF/HCl etching strategy, in which the molar ratio of LiF to Ti₃AlC₂ was 5:1. However, the average diameter of Ti₃C₂T_x flakes obtained after etching was less than 1 μm and had an uneven thickness. In addition, in order to obtain ultra-thin structure and prevent oxidation, the cumbersome steps of sonicating MXene for 1 h under the protection of argon atmosphere cannot be omitted. Thereafter, Lipatov et al. [16] proposed an optimized method to directly etch Ti₃AlC₂ into high-quality monolayer Ti₃C₂T_x flakes, only through changing the molar ratio of LiF to Ti₃AlC₂ without sonication (Fig. 6). When the molar ratio of LiF to Ti₃AlC₂ increased from 5:1 to 7.5:1, the significantly increased Li⁺ amount not only promoted the etching efficiency of Al, but also improved the product quality with a uniform thickness and a smooth surface. In addition, the size of Ti₃C₂T_x flakes was distinctly larger (4–15 μm) than the previous ones (200–500 nm).

Fig. 6

Reproduced with permission from Ref. [16]. Copyright 2016, Wiley

Synthesis of Ti₃C₂T_x monolayers by chemical etching method. a Synthesis process of Ti₃C₂T_x flakes via LiF–HCl etching methods (Route 1: molar ratio of LiF to MAX being 5:1, Route 2: molar ratio of LiF to MAX being 7.5:1.0); b SEM and c TEM images of obtained Ti₃C₂T_x flakes by Route 1; d SEM and e TEM images of obtained Ti₃C₂T_x flakes by Route 2.

In addition to the wet chemical method, direct fabrication of Ti₃C₂ MXene nanosheets can also be achieved by electrochemical methods (Fig. 7). Yang et al. applied Ti₃AlC₂ as the anode in a weak alkaline solution containing ammonium chloride (NH₄Cl) and TMAOH (Fig. 7a) [70]. In this way, Ti₃AlC₂ could be etched in a short time (5 h), and a large number of single or double layered Ti₃C₂T_x with an average size of ca. 2 μm could be obtained. Meanwhile, most of Ti₃C₂ (> 90%) thickness was about 1.2 nm, which is in line with the single layer (Fig. 7b).

Fig. 7

Reproduced with permission from Ref. [70]. Copyright 2018, Wiley. b SEM and AFM images of Ti₃C₂T_x nanosheets with corresponding statistical size distribution and relevant height profiles; c optical photographs of electrochemical etching process from 0 to 120 min

Synthesis of Ti₃C₂T_x monolayers by electrochemical anodic etching method. a Schematic diagram of electrochemical anodic etching process of bulk Ti₃AlC₂ into Ti₃C₂T_x nanosheets.

3.4 Synthesis of Ti₃C₂T_x quantum dots

Beside of 3D and 2D MXenes, 0D MXenes are also worthy of study due to its potential applications in optical and biological fields. It is because the quantum confinement of small-sized QDs can lead to the expansion of the band gap, and 0D Ti₃C₂T_x has tunable photoluminescence characteristics, which are very suitable for biological imaging and functionalization [20, 104, 105]. Therefore, it is of vital significance to summarize and explore the preparation methods of Ti₃C₂T_x MXene QDs.

The methods of preparing Ti₃C₂T_x MXene QDs mainly include hydro-/solvothermal method [17, 18], ball milling method [19], and micro-explosion method [20] from 3D or 2D MXenes. Most of these strategies follow a similar principle that the bulk structure is cut into smaller quantum particles with the assistance of external force through the defects in the material, which are regarded as the reaction sites. Among the reported strategies, hydro-/solvothermal method was regarded as the most acceptable process for synthesizing Ti₃C₂T_x QDs due to its mild operation conditions. Particularly, Xue et al. [17] prepared Ti₃C₂T_x QDs by a facile hydrothermal route and confirmed that the size and thickness of the obtained Ti₃C₂T_x QDs could be controlled by the hydrothermal temperature and reaction time (Fig. 8a). The appropriate reaction temperature for the synthesis of Ti₃C₂T_x QDs should be 100–180 °C, the pH of the solution was 6–9, and the synthesis time was determined by both temperature and pH value [105]. Analogously, Niu et al. used DMSO, dimethylformamide (DMF), and ethanol as the alternative solvents to synthesize Ti₃C₂T_x QDs by a solvothermal process [18]. Compared with the hydrothermal method, the solvothermal method could accurately control the size and crystallinity of the sample. However, during the solvothermal heating process, carbon QDs were inevitably generated from the utilized organic precursors, which are usually unwanted by-products.

Fig. 8

Reproduced with permission from Ref. [17]. Copyright 2017, Wiley. b Formation mechanism of red phosphorus-Ti₃C₂T_x QDs composites via ball milling method. Reproduced with permission from Ref. [19]. Copyright 2018, Wiley. c Preparation process of Ti₃C₂T_x QDs by a micro-explosion strategy. Reproduced with permission from Ref. [20]. Copyright 2020, Wiley

Synthesis of Ti₃C₂T_x quantum dots. a Schematic diagram of effect of hydrothermal temperatures on prepared Ti₃C₂T_x QDs.

In addition, the ball milling method is also applied to prepare Ti₃C₂T_x QDs, which usually has a high yield. The final morphology and physical properties of the sample depend on the milling speed, milling time, and ball-to-powder weight ratio. For instance, Zhang et al. [19] synthesized Ti₃C₂T_x QDs with the thickness of 2–5 nm using red phosphorus (red-P) and multilayered micro-sized Ti₃C₂T_x as precursors under a high ball milling shear force. The Ti–O–P bonds were formed during the ball milling process, which was the key point to result in the delamination and diminution of Ti₃C₂T_x MXene flakes (Fig. 8b). Beside P, other solid materials such as silicon, sulfur, and carbon could also be added in the ball milling process to induce the formation of Ti–O–X (X = P, C, S, Si) bonds, and it has been found that the stronger the –O–X bond energy is, the smaller the size of the obtained Ti₃C₂T_x QDs is. Generally, this strategy is suitable for the preparation of composite materials (Table 1, No. 33) [19].

In order to explore a more environmental-friendly and efficient method, Li et al. [20] tactfully utilized the great temperature difference between liquid nitrogen and hot deionized water to prepare Ti₃C₂T_x QDs by a self-made micro-explosion method (Fig. 8c). In this strategy, the multilayer Ti₃C₂T_x MXene was firstly flooded with liquid nitrogen, and then hot water was poured in. The water seal formed by hot water and the vaporization of liquid nitrogen promoted the micro-explosion of the layers, thereby destroying the layered structure into Ti₃C₂T_x QDs.

3.5 Synthesis of Ti₃C₂T_x-based composites

Owing to the unique structure and surface properties of Ti₃C₂T_x, various kinds of materials including metal [106], metallic oxide [107], metal-salt [36], metallic sulfide [37] and organic cellulose [49, 108] have been successfully combined with Ti₃C₂T_x to construct composites for different applications, and it has been widely recognized that the performance of hybrids could be significantly improved with the help of Ti₃C₂T_x. This section lists some mainly used preparation strategies for the Ti₃C₂T_x-based hybrids in recent years, involving hydro-/solvothermal method, deposition method, self-assembly method, in situ grow method, self-reduction method, and freeze-dried method, which are summarized in Table 1 for convenience of reference.

3.5.1 Hydro-/solvothermal method

Hydro-/solvothermal method has become the most commonly used strategy for preparing Ti₃C₂T_x-based hybrids due to its convenient and simple characteristics. As early as 2017, Ran et al. [37] pioneered the idea of combining CdS with Ti₃C₂ nanoparticles by a hydrothermal strategy, where the Ti₃C₂ nanoparticles served as an efficient co-catalyst for photocatalytic H₂ production (Table 1, No. 6). Since then, articles about employing Ti₃C₂ in the field of photocatalysis have sprung up. For example, Cao et al. [36] prepared a novel 2D/2D Ti₃C₂T_x/Bi₂WO₆ heterojunction by in situ growth of Bi₂WO₆ nanosheets on Ti₃C₂T_x ultra-thin nanosheets through a hydrothermal method (Table 1, No. 4). Theoretically, when Bi³⁺ and Ti₃C₂T_x nanosheets coexisted in an aqueous solution, Bi³⁺ were readily anchored on the surface of Ti₃C₂T_x, because a number of terminal groups (–O or –OH) on the surface of Ti₃C₂T_x resulted in a negative potential. After a simple hydrothermal treatment, with the existence of Na₂WO₆ and cetyltrimethylammonium bromide (CTAB), the intimate contact between Ti₃C₂T_x and Bi₂WO₆ was established. Wang et al. [35] rationally designed Ti₃C₂T_x as a 2D platform to achieve in situ growth of flower-like Zn₂In₂S₅ microsphere under an oxygen-free hydrothermal condition (Table 1, No. 3). Jiao and Liu [109] also proposed a facile hydrothermal method that allowed MoS₂ nanosheets grown on Ti₃C₂T_x vertically. Zuo et al. found that under the solvothermal condition, ZnIn₂S₄ nanosheets could grow uniformly on the surface of a single-layer Ti₃C₂ to fabricate a heterojunction, which was beneficial to photocatalytic hydrogen production [28].

3.5.2 Deposition method

Through the deposition methods, the chemical substances can be uniformly dispersed on the surface of the receptor objects, and the reaction conditions are not harsh. Thereby, the deposition methods including electrochemical deposition and chemical vapor deposition are also widely applied in the preparation of Ti₃C₂T_x-based hybrids. Recently, Yang et al. [38] synthesized a Zn@Ti₃C₂ cathode through a constant-voltage electrochemical deposition technology in a two-electrode system (Table 1, No. 10). Typically, the Ti₃C₂ nanoflakes were used as the work electrode, while a commercial zinc plate was used as the counter electrodes in the reaction system. During the reaction, Zn nanosheets grew vertically on the Ti₃C₂ ultra-thin sheets to form a 3D petal-like structure, which was applied for fabricating biodegradable energy storage systems that deal with increasing amounts of electronic-waste, such as heavy metals. Similarly, Gan et al. [42] prepared a crystalline lithium metal anode with a hair-layer structure by electrochemically depositing lithium on the lithiophilic (LTO)/Ti₃C₂ composite material (Table 1, No. 12). As shown in Fig. 9a, the hair-like LTO grown on the surface of Ti₃C₂ and between the layers could provide a uniformly distributed electric field, which was beneficial to induce the uniform deposition of Li⁺. Furthermore, the page-shaped Ti₃C₂ also provided a favorable foundation for Li⁺ intercalation. Finally, metal Li anchored smoothly and uniformly on the surface and between layers of Ti₃C₂ to form a mechanically stable structure for lithium anode of the battery. In addition, Li et al. [41] proposed a strategy of fabricating Ti₃C₂T_x/CNTs nanocomposite for high-performance electromagnetic wave absorbing material via a facile chemical vapor deposition (Table 1, No. 11). Typically, Ti₃C₂T_x powder was dispersed in an aqueous solution with a certain concentration of nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) as a catalyst, and then freeze-dried. Subsequently, using C₂H₄ as the carbon source, the mixed powder was calcinated at 800 °C, and thus CNTs in situ grew on the surface of Ti₃C₂T_x to form the Ti₃C₂T_x/CNTs nanohybrids (Fig. 9b). In this composite, the proper multilaminate structure of Ti₃C₂T_x could effectively prevent the CNTs from stacking.

Fig. 9

Reproduced with permission from Ref. [42]. Copyright 2020, The Royal Society of Chemistry. b Synthesis process of Ti₃C₂T_x/CNTs hybrids by a chemical vapor deposition method. Reproduced with permission from Ref. [41]. Copyright 2017, The Royal Society of Chemistry

Preparation of Ti₃C₂T_x-based composites by deposition methods. a Schematic illustration showing Li deposition behavior on LTO/Ti₃C₂/Cu electrode.

3.5.3 Self-assembly method

Attributed to the abundant terminal groups (−O, −F, and −OH) on its surface, Ti₃C₂T_x prefers to attract and react with other materials to form strong interfacial contact. To this end, some typical Ti₃C₂-based composites could be rationally fabricated by self-assembly methods. For instance, Xie et al. [43] successfully constructed a 3D porous structured composite of Ti₃C₂T_x and CNTs through an electrostatic self-assembly method (Table 1, No. 13). In detail, Ti₃C₂T_x nanofilms had a hydrophilic surface with a negatively Zeta potential (−63.3 mV), while CNTs were treated by CTAB and thus had a positively Zeta potential (+ 74.8 mV). In the suspension, CNTs were automatically adsorbed on the surface of Ti₃C₂T_x nanofilms through the electrostatic attraction, finally forming a compact structured composite (Fig. 10a). Besides, Guo et al. [46] prepared serine-modified Ti₃C₂T_x by using active sites on serine (such as hydroxyl, amino, and carboxyl) to undergo an esterification reaction with the abundant hydroxyl groups on the surface of Ti₃C₂T_x (Table 1, No. 16). Subsequently, epoxidized natural rubber (ENR) was introduced into the serine-Ti₃C₂T_x in order to overcome the mechanical deformation caused by the inherent rigidity and brittle structure of Ti₃C₂T_x. Since the amino group on the serine molecule and the epoxy group on the ENR chain underwent a ring-opening reaction, a serine-grafted epoxy natural rubber (S-ENR) emulsion was formed. Therefore, the Ti₃C₂T_x nanosheets uniformly dispersed therein were selectively pushed into the gaps of the S-ENR latex microspheres and connected to each other, inducing an ordered 3D conductive network structure (Fig. 10b). Particularly, the natural latex can effectively prevent Ti₃C₂T_x from oxidation during the reaction. Therefore, the mechanical flexibility and sensing performance of the prepared Ti₃C₂/S-ENR sensor can be significantly improved.

Fig. 10

Reproduced with permission from Ref. [43]. Copyright 2016, Elsevier. b Formation diagram of MXene/rubber-based supramolecular elastomer network via esterification and latex self-assembly process. Reproduced with permission from Ref. [46]. Copyright 2020, American Chemical Society

Preparation of Ti₃C₂T_x-based composites by self-assembly methods. a Formation diagram of MXene/CNTs via electrostatic self-assembly of negatively charged MXene and positively charged CNTs.

Developed rapidly from the 1990s, LBL self-assembly is a simple and multi-functional surface modification method, which can achieve finer control of the material structure, and can also apply uniform coating to the sample. Recently, the driving force of LBL has gradually expanded from electrostatic attraction to hydrogen bonds, and even chemical bonds, which makes it extensively applied in preparing composite materials [110,111,112]. For instance, An et al. [29] synthesized a Ti₃C₂/PDAC (poly (diallyldimethylammonium chloride)) humidity sensor through a LBL self-assembly method (Table 1, No. 17), using the characteristics that the surface charge of Ti₃C₂ is negative and that of the polymer electrolyte is positive in an aqueous solution with a pH of about 5. Besides, Ma et al. [55] took advantage of the formed hydrogen bond (HB) between the −OH functional group on bacterial celluloses (BCs) and the F− terminal group of Ti₃C₂T_x to synthesize a transparent BCs/MXene composite film, which exhibited an excellent tensile strength and flexibility. In addition to the dominant HB, the density functional theory (DFT) calculations have also been used to testify that a strong chemical bond (Ti–O covalent bond) was generated between Ti₃C₂T_x MXene and BCs.

3.5.4 In situ grown method

Typically, Ti₃C₂T_x is extremely unstable in a humid environment and is easily oxidized by water or air. It is the exposed Ti atoms on the surface of Ti₃C₂T_x that are regarded as the nucleation sites to form TiO₂. Inspiring by this feature, Low et al. [50] reported a simple calcination method to in situ convert Ti₃C₂T_x into TiO₂ nanoparticles at 350 °C with a ramping rate of 10 °C·min⁻¹. TiO₂ nanoparticles with uniform size in situ grew on the Ti₃C₂T_x surface, which effectively avoided the particle agglomeration happened in the traditional TiO₂ preparation process (Table 1, No. 22). More significantly, there were a large number of voids between the formed TiO₂ nanoparticles to increase the surface area of the hybrid, which is beneficial to the photocatalytic CO₂ reduction reaction. Similarly, but not the same, Peng et al. [58] directly dispersed Ti₃C₂T_x in a hydrochloric acid solution containing NaBF₄ by stirring and ultrasonication, followed by hydrothermal oxidation to easily grow TiO₂ in situ on Ti₃C₂T_x (Table 1, No. 21). In this way, an atomic-level heterojunction was easily formed between TiO₂ and Ti₃C₂T_x, which could effectively reduce the charge recombination caused by defects in the bare TiO₂. In addition, Ti₃C₂-IONPs@PEG (iron oxide nanoparticles @amino-[poly (ethylene–glycol)]) compound can also be obtained by an in situ grown method (Table 1, No. 23) [51]. Firstly, by employing ferrous sulfate as the precursor, the IONPs can be grown in situ on the surface of Ti₃C₂ through a redox reaction under alkaline conditions, because the prepared Ti₃C₂ contains negatively charged −O end groups. Afterward, in order to improve the stability of the complex in vivo, PEG was modified on the prepared Ti₃C₂-IONPs by means of electrostatic interactions between Ti₃C₂ and PEG. The constructed composite system can be applied for specific cancer treatment in the tumor microenvironment.

3.5.5 Self-reduction method

It is worth noting that during the formation of the single-layer/ultra-thin Ti₃C₂T_x MXene flakes through the acid etching of Al from Ti₃AlC₂, even under mild conditions, some adjacent Ti atoms on the surface of Ti₃C₂T_x will also inevitably be corroded away, which results in the formation of Ti vacancies [113]. These Ti-deficit vacancies are unstable and highly reductive, which can serve as reducing agents for some high-valence metal substances [114]. Taking advantage of this feature, Zhao et al. [25] prepared stable Pt single-atom catalyst over ultra-thin Ti_3−xC₂T_y nanofilms with a large number of Ti defects by a synchronous self-reduction stabilization method at room temperature (Fig. 11). In addition, Zou et al. [52] prepared the MXene/Ag composite material by direct self-reduction of AgNO₃ in an aqueous solution containing Ti₃C₂T_x (Table 1, No. 24). Similarly, Zhang et al. [115] dispersed poly(vinylpyrrolidone) (PVP) and Ti₃C₂T_x in deionized water at a ratio of 1:1. When the AgNO₃ solution is added to the above solution, a large amount of dot-shaped Ag nanocrystalline will be formed, and as the reaction time increases, part of the dot-shaped particles will transform into long strips of precipitate. Finally, urchin-like MXene-based composite will be obtained by regulating the reaction time.

Fig. 11

Reproduced with permission from Ref. [25]. Copyright 2019, American Chemical Society

Simultaneous self-reduction and stabilization process for preparation of Pt₁/Ti_3−xC₂T_y catalyst.

3.5.6 Freeze-drying method

Freeze-drying process is based on the sublimation of ice crystals, that is, the moisture in the composites will be directly changed from solid to gas under vacuum conditions. Moreover, the volume of the sample will not change significantly during the freeze-drying process, so its structure can be well preserved. Thereby, the freeze-drying method can be regarded as an efficient solution to prepare the Ti₃C₂-based compounds with a porous structure and sufficient surface area. Recently, Shi et al. [53] prepared the 3D porous MXene/graphene oxide (MGO) aerogels by a freeze-drying method (Table 1, No. 26). Typically, the prepared Ti₃C₂ nanosheets and GO were uniformly dispersed in the aqueous solution, and sonicated for 10 min in argon atmosphere. Subsequently, the 3D MGO aerogels can be facially obtained after freeze-drying the as-prepared homogeneous solution for 3 days. The highly conductive 3D MGO aerogel can be employed as an anode material for ultra-stable and high-capacity lithium metal batteries.

3.5.7 Other methods

Except the methods as mentioned above, other methods were also reported to prepare Ti₃C₂T_x-based composites, such as ultrasound reduction and vacuum filtration. In general, ultrasound will cause the micro-bubbles in the solution to undergo acoustic cavitation and burst, and the rupture of micro-bubbles will generate high temperature. At the same time, water molecules will homogenize at high temperature to produce highly active H· and OH· free radicals. Therefore, Liu et al. [106] slowly added HAuCl₄ into the Ti₃C₂T_x suspension under magnetic stirring, and prepared Au/Ti₃C₂T_x composite by sonicating, centrifuging, and drying under the protection of nitrogen, where Au ions were induced to be loaded on Ti₃C₂T_x nanosheets by the reductive H· free radicals.

In addition, filtration method also occupies an important position in the synthesis of Ti₃C₂-based composites due to its simple operation, wide application, and high efficiency. For example, Wang et al. [31] constructed a Ti₃C₂/NMC (natural microcapsule) flexible pressure sensor with assistance of vacuum filtration (Table 1, No. 27). Typically, monolayer Ti₃C₂ aqueous suspension was slowly added into the NMC ethanol suspension, and intensively stirred to form a uniform solution. Then, the Ti₃C₂/NMC biocomposite film was obtained through vacuum filtration with a fine polypropylene fiber membrane.

4 Applications

After introducing the structure, properties, and synthesis of Ti₃C₂T_x MXene, we now turn to their application prospect. Ascribed to their simple preparation process and unique characteristics, the Ti₃C₂-based composite materials have attracted vast attention in the fields of electromagnetic interference, photothermal conversion, biomedicine, energy storage, sensors, photocatalysis, etc. In this section, we will outline the applications of Ti₃C₂ in these fields in detail.

4.1 Electromagnetic wave absorption and shielding

With the development of portable electronic devices and military technology, the exacerbated pollution caused by electromagnetic (EM) wave negatively affected human health and normal electronic communication equipment. In other words, the development of the high-tech intelligent era has shown a huge demand for protecting electronic instruments and human body from electromagnetic waves.

Recently, Ti₃C₂ MXene has become a hotspot in the microwave absorption (MA) and electromagnetic interference (EMI) fields, because the excellent electrical conductivity and unique layered structure of Ti₃C₂ facilitate some incident electromagnetic waves to be reflected effectively, which improves the conduction loss of the shielding materials to a large extent. Moreover, the presence of terminations on the surface of Ti₃C₂ can cause inherent defects, thus leading to an increase in dipolar polarization, which render Ti₃C₂ a promising candidate for novel MA and EMI absorbing applications. Han et al. [116] designed and fabricated an electromagnetic absorbing material based on Ti₃C₂T_x with an accordion structure. The gained Ti₃C₂T_x films afforded abundant interfaces, which extended the path of electromagnetic wave, and resulted in the attenuation of the electromagnetic wave (Fig. 12a). Moreover, the Ti₃C₂T_x films were annealed and obtained further improved electronic conductivity due to the decrease amount of hydroxyl groups and formation of amorphous carbon on their surface. The reflection coefficient (RC) value of the annealed Ti₃C₂T_x with a loading of 55 wt% on a wax matrix can achieve − 30 dB, and the efficient absorption frequency domain reached up to 2.8 GHz (Fig. 12b). Besides, carbon nanotubes (CNT) were also a good candidate to combine with Ti₃C₂ for electromagnetic interference shielding. For example, Weng et al. [47] fabricated Ti₃C₂/CNT nanocomposite films through a spin spray LBL assembly method (Table 1, No. 18). The obtained thin film shows high conductivity and high electromagnetic shielding efficiency due to the synergistic enhancement effect of Ti₃C₂ and CNT with a layer-by-layer structure. Zhou et al. [57] improved the flexibility and structural stability of Ti₃C₂ nanosheets by repeatedly spraying the composite of Ti₃C₂ and cellulose nanofibers (CNF) on bacterial cellulose (BC) (Table 1, No. 31). Subsequently, CNF/Ti₃C₂/BC films with a compacted layered structure were obtained by hot pressing, which had excellent electrical conductivity and unmatched flexibility. It is worth noting that the CNF/Ti₃C₂/BC composite film was still stable after multiple washings and exhibited good wrinkle resistance, and the shielding effect for EMI was as high as 60 dB. Analogously, Ma et al. [55] took advantage of the chemical bond and hydrogen bond interaction between BC and MXene to synthesize ultra-thin and transparent BCs/Ti₃C₂ films through a simple filtration process (Table 1, No. 29). The composite films exhibited an EMI shielding effectiveness up to ∼ 69,455.2 dB·cm²·g⁻¹.

Fig. 12

Reproduced with permission from Ref. [116]. Copyright 2016, American Chemical Society. c Schematic diagram of water evaporation over Ti₃C₂T_x MXene via its light-to-heat conversion property; d temperature change process of PVDF film and MXene-PVDF film under same condition of illumination and (insets) infrared camera photos of PVDF film and MXene-PVDF film. Reproduced with permission from Ref. [117]. Copyright 2017, American Chemical Society

Applications of Ti₃C₂ in electromagnetic wave absorption and photothermal conversion. a Schematic illustration of electromagnetic wave absorption in Ti₃C₂T_x MXenes. b Comparison of reflection coefficients of 55 wt% Ti₃C₂T_x on a wax matrix versus frequency with different thicknesses.

Overall, the investigation on Ti₃C₂-based hybrids for MA and EMI has obtained a sequence of crucial progress up to date. Nevertheless, compared with the mature graphene-based materials, the MA and EMI properties and mechanisms of Ti₃C₂-based ones are still required to be further researched, which is a long journey for Ti₃C₂ to be practically applied [30].

4.2 Photothermal conversion

As mentioned above, Ti₃C₂ has been testified to be a promising electromagnetic absorbing material. Sunlight, as the optical energy, is also one kind of widespread electromagnetic waves present in our life. Considering that ultimate fate of the absorbed waves is to dissipate in the form of heat within the material demonstrably [30], it can be reasonably conjectured that Ti₃C₂ can act as a promising light-to-heat conversion material by converting the absorbed microwave into heat energy. Actually, Li et al. [117] fabricated a self-floating Ti₃C₂ film by vacuum filtration with polyvinylidene fluoride (PVDF) membrane as the substrate, and applied it for photothermal water evaporation under sun irradiation (Fig. 12c). Under illumination, the equilibrium temperature of Ti₃C₂-PVDF film achieved a much higher value than that of the pure PVDF membrane (Fig. 12d). In addition, the efficiency of water evaporation was enhanced with increasing Ti₃C₂ ratio. This work has demonstrated that Ti₃C₂ can really be served as a promising photothermal conversion material. Thereafter, Wang et al. [32] successfully constructed a 3D nanoflower–nanosheet structure Cu₃BiS₃/Ti₃C₂ for photothermal seawater desalination (Table 1, No. 5), and the light absorption in the visible-light region exceeded 90%. However, in real life, the propagation of light is multi-angled. In order to improve the ability of the photothermal conversion materials to capture multi-angle light, Yang et al. rationally constructed a Co₃O₄/Ti₃C₂ nanocomposite material as a 3D spherical evaporator (Table 1, No. 9) [40]. The 0D Co₃O₄ was uniformly dispersed on the surface of Ti₃C₂ through its abundant functional groups, and synergistically improved the efficiency of Ti₃C₂ for the absorption and utilization of sunlight. Importantly, the uniformly distributed high-density Co₃O₄ nanoparticles on the surface of Ti₃C₂ effectively blocked the contact of the surface terminal groups with water and oxygen, thereby significantly improving the chemical stability of Ti₃C₂ MXene.

4.3 Biomedical applications

Besides water evaporation and seawater desalination, the photothermal conversion property of Ti₃C₂ was also successfully employed in biomedical field. Liu et al. [118] modified the Ti₃C₂ nanosheets with Al(OH)₄⁻ to enhance its optical absorption capacity according to the previous report by Xuan et al. [119], then mixed it with doxorubicin (DOX) and hyaluronic acid (HA) to achieve the purpose of killing cancer cells and destroying tumor tissue. Under laser (808 nm, 0.8 W·cm⁻²) irradiation, the tumor in mice was completely eliminated without reoccurrence after the injection of the functional Ti₃C₂, which exhibited a high photothermal conversion efficiency of 58.3% in the near-infrared (NIR) range (Fig. 13a). Subsequently, Liang et al. [51] combined photothermal conversion effect of Ti₃C₂ with catalysis reactivity of iron oxide nanoparticles (IONPs) to synthesize a consecutive catalytic system for tumor microenvironment (TME)-specific cancer therapy. The system was fabricated by anchoring natural glucose oxidase (GOD) and superparamagnetic IONPs onto ultra-thin Ti₃C₂ nanosheets (Fig. 13b). Glucose was consumed in the presence of GOD to produce hydrogen peroxide and gluconic acid, which were further catalyzed into hydroxyl radicals by IONPs via Fenton reaction. Simultaneously, the NIR light was effectively converted into thermal energy through the light-to-heat ability of Ti₃C₂ nanofilm, which dramatically enhanced the efficiency of catalytic reaction. Finally, the cancer cells were killed by the generated toxic hydroxyl radicals, realizing the high synergistic cancer-therapeutic result.

Fig. 13

Reproduced with permission from Ref. [118]. Copyright 2017, American Chemical Society. b Utilization of glucose oxidase (GOD) and iron oxide nanoparticles (IONPs) modified Ti₃C₂T_x nanoplatforms in synergistic photothermal-catalysis for anti-cancer application under near-infrared light. Reproduced with permission from Ref. [51]. Copyright 2019, American Chemical Society. c Simulated process of urea adsorption on 2D Ti₃C₂T_x MXenes dialysis from patient’s dialysate. Reproduced with permission from Ref. [26]. Copyright 2018, American Chemical Society

Applications of Ti₃C₂ MXene in biomedicine. a Synthetic process of Ti₃C₂T_x-based nanoplatform with a synergistic photothermal effect for tumor chemotherapy, where I.V. injection refers to intravenous injection, and PTT/PDT refers to photothermal/photodynamic therapy.

Interestingly, if the 2D Ti₃C₂ nanosheets are further cut into quantum dots, the unique fluorescence effect and excellent dispersion of Ti₃C₂T_x QDs will provide more suitable services for biomedicine due to the quantum confinement effect. Specifically, Xue et al. [17] have confirmed the development potential of Ti₃C₂T_x QDs as multicolor cell probes in biomedicine and optoelectronic applications by labeling RAW264.7 cells (derived from tumors, induced by Abelson murine leukemia virus). In addition, Li et al. [20] illustrated that Ti³⁺ in the Ti₃C₂T_x QDs could react with H₂O₂ in tumor cells to generate excess toxic hydroxyl radicals, achieving the goal of synergistically killing cancer cells.

Apart from eliminating cancer cells as mentioned above, Ti₃C₂T_x can even serve as a wearable artificial kidney to remove urea, which can provide continuous dialysis for patients with advanced kidney disease (Fig. 13c) [26]. Ti₃C₂T_x has adjustable interlayer spacing and terminal groups, which is advantageous for urea adsorption. The urea adsorption efficiency could reach 99% at room temperature when the content of Ti₃C₂T_x in aqueous solution was 0.83 g·ml⁻¹. More importantly, the cell viability remained stable even in the Ti₃C₂T_x solution with a high concentration (up to 200 μg·ml⁻¹), which demonstrated the biological harmlessness of Ti₃C₂T_x. However, due to the limitations of the 2D structure of Ti₃C₂T_x MXene, it cannot meet the requirements of sustainable drug release, which is of great significance to clinical medicine. Therefore, Xing et al. [49] for the first time synthesized a Ti₃C₂-integrated cellulose hydrogel by using a chemical cross-linking reaction in a low-temperature NaOH−urea−H₂O system (Table 1, No. 32). Replacing terminations on the Ti₃C₂ nanoplatforms by cellulose can make up for the lack of proper biocompatibility of Ti₃C₂. Meanwhile, such cellulose-modified Ti₃C₂ not only improves the carrying capacity of anti-cancer drugs, but the drug release performance of Ti₃C₂ has also been excellently optimized, which can effectively kill tumors and prevent their recurrence.

However, the long-term biosafety of Ti₃C₂ is still a problem to be systematically researched. Further research on controlling the terminal functional groups of Ti₃C₂T_x should be conducted for future biomedical applications. In addition, although Ti₃C₂T_x has excellent dispersibility in pure water, its stability in physiological media needs to be ameliorated [120]. Furthermore, in order to realize the convert of Ti₃C₂T_x from laboratory research to medical clinical application, the researching on safe, facile and controllable preparation method of Ti₃C₂T_x with high quality and large quantity is still crucially needed.

4.4 Energy conversion and storage

Batteries and supercapacitors have been a hot research topic for a long time. Nevertheless, the existence issues of low capacity and unstable structure make their practical applications difficult. Fortunately, Ti₃C₂T_x-based composite materials emerged and were regarded as a promising electrode candidate for batteries, supercapacitors, and power generators, benefiting from their characteristics of mechanical stability, good electron conductivity, unique multilayered structure, and abundant surface terminations. For example, Shi et al. [53] constructed a stabilized 3D Ti₃C₂ MXene/graphene (MG) framework by anchoring Ti₃C₂ nanosheets on rGO as a dendrite-free lithium (Li)-metal anode (Fig. 14a, Table 1, No. 26). Compared to pure rGO electrodes, the 3D MG effectively suppressed the formation of Li dendrites, because the abundant terminations on the surface of Ti₃C₂T_x were beneficial for the adsorption of Li ions (Fig. 14b). It was noted that the 3D MG electrode maintained an excellent stability and reached a high Coulombic efficiency of ∼99% within 2700 h at 5 mAh·cm⁻².

Fig. 14

Applications of Ti₃C₂ MXene in energy conversion and storage. Schematic illustration of a Ti₃C₂ MG anode and b processes of Li⁺ plating and striping into MG and pure rGO electrodes, respectively. Reproduced with permission from Ref. [53]. Copyright 2019, American Chemical Society. c Schematic diagram of charging and discharging process of Ti₃C₂ MXene as cathode in an aqueous zinc-ion battery; d typical galvanostatic charge−discharge curves of Ti₃C₂T_x with different functional groups at current density of 0.5 A·g⁻¹. Reproduced with permission from Ref. [121]. Copyright 2021, American Chemical Society

In addition to Li⁺ batteries, Ti₃C₂T_x is also a potential anode material for sodium ion (Na⁺) batteries. For example, Wang et al. [45] induced the positively charged conductive polyaniline (PANI) into negative-charged Ti₃C₂T_x (Table 1, No. 15). The strong covalent Ti–N bond formed between Ti₃C₂T_x and PANI ensured the stability of the 3D network structure. Moreover, the increased spacing between Ti₃C₂T_x layers and the 3D network structure provided more transmission channels for Na⁺, which improved the diffusion kinetics of Na⁺ to promote the rate performance and prolong the life of the battery. Homoplastically, Li et al. [121] synthesized MXene functionalized with halogen terminal groups as the cathode of water-based zinc-ion (Zn²⁺) batteries with extraordinary electrochemical performance (Fig. 14c). In particular, Ti₃C₂Br₂ and Ti₃C₂I₂ showed distinct electric capacities of 97.6 and 135 mAh·g⁻¹, respectively (Fig. 14d).

In the prospective energy storage and conversion systems, supercapacitors also occupy an important position, in which mechanical strength is one of the most important test criterions for their practical applications. Chang et al. [54] designed a highly stretchable, bendable, and efficient Ti₃C₂/elastomer electrode by Ag NWs for supercapacitor (Table 1, No. 28). This work indicated that the densely stacked nanosheets in the laminated film are very competitive in electrochemical energy storage due to their high flexibility. However, laminated films usually hinder the diffusion and adsorption of electrolyte ions due to the severe self-stacking of nanosheets. Thereby, Dong et al. reported a molecular ligand-assisted assembly strategy to successfully assemble Ti₃C₂T_x and rGO. After the heat treatment, an ultra-high energy density of 42.1 Wh·L⁻¹ can be achieved due to the superlattice formed in the MXene–rGO (Table 1, No. 14) [44]. Moreover, the flexible Ti₃C₂T_x/Nb₂CT_x composite membrane was fabricated through alternately stacked by vacuum-assisted filtration (Table 1, No. 30) [56]. After the introduction of Ti₃C₂T_x into Nb₂CT_x, the interlayer spacing was significantly increased compared with that of pure Nb₂CT_x, and the conductivity of the composite membrane was significantly improved. The obtained Ti₃C₂T_x/Nb₂CT_x composite membrane exhibited a mass specific capacitance of 370 F·g⁻¹ at a scan rate of 2 mV·s⁻¹, and a capacitance retention rate of 56.1% at 200 mV·s⁻¹.

4.5 Sensor

As mentioned above, Ti₃C₂T_x has been applied in the wearable electronic devices such as electromagnetic shielding equipment and artificial kidney, all of which are kinds of sensors. Nowadays, most electronic sensors faced with inevitable fractures and low sensitivity in the process of operation, which immensely limits their application in practice. Fortunately, Ti₃C₂T_x can be combined with traditional sensor materials to improve their electrical and/or humidity sensitivity. Besides, the favorable mechanical strength of Ti₃C₂T_x can significantly improve the mechanical stability and bending resistance of the sensor after introducing into polymer materials. Inspired by the supramolecular interactions of amino acids as the building blocks of a protein, Guo et al. [46] designed a self-healing flexible electric sensor by regularly configuring Ti₃C₂ thin films around the serine-grafted epoxidized natural rubber (S-ENR) latex to form a 3D conductive network via hydrogen bonding interaction (Table 1, No. 16). When people make various expressions such as smiling, astonishing, and frowning, the facial muscle states are totally different, which leads to the discrepancies of electrical signals (Fig. 15a). The obtained flexible electric sensor could detect those different tiny electrical signals due to its high sensitivity and precision rate. Analogously, inspired by sensing mechanisms of human skin, Wang et al. [31] constructed a Ti₃C₂/natural microcapsule (NMC) biocomposite membranes to achieve the human—machine interactions (Fig. 15b, Table 1, No. 27). The stable and flexible sensing device has realized voice recognition via detecting and distinguishing different signals from human motion and pulses. Up to now, the developments of Ti₃C₂-based functional composites in sensor applications are still in initial stage. However, the combination of Ti₃C₂ has effectively overcome the disadvantages of traditional sensors, such as inevitable buckling and fracturing in their working process, which paved the broad road.

Fig. 15

Reproduced with permission from Ref. [46]. Copyright 2020, American Chemical Society. b Relative current change of Ti₃C₂/NMC (natural microcapsule) flexible sensor when finger was bent at different angles. Reproduced with permission from Ref. [31]. Copyright 2019, American Chemical Society

Applications of Ti₃C₂ MXene in sensors. a Respond of Ti₃C₂-based sensors to currents generated by different voices and facial expressions.

4.6 Photocatalysis

Photocatalytic technology based on semiconductor materials is regarded as an effective measure to remit the issues of energy crisis and environmental pollution [122]. Up to now, the semiconductors including TiO₂, CdS, ZnIn₂S₄, and g-C₃N₄ have been widely studied due to their suitable band structure and controllable morphologies [123,124,125,126,127]. However, the traditional single semiconductor catalyst has poor light utilization efficiency and low carrier separation efficiency. Supporting co-catalysts and building heterojunctions are common strategies to solve the above problems. In view of this, the merits of Ti₃C₂T_x including appropriate energy band structure, excellent electron conductivity, popular price, and hydrophilic surface endowed it with incredible potential in the photocatalysis application fields, such as hydrogen (H₂) production from water splitting, CO₂ reduction, nitrogen (N₂) fixation, and pollutant degradation.

4.6.1 Hydrogen production

For H₂ production, Ran et al. [37] for the first time utilized theoretical calculations to systematically evaluate the ability of Ti₃C₂T_x as a co-catalyst in the process of photocatalytic H₂ production from water splitting. Generally, the entire H₂ evolution reaction (HER) pathway can be summarized into three processes: (I) initial state of H⁺  + e⁻, (II) intermediate adsorption of H*, and (III) final production of H₂ [128]. Therein, the Gibbs free energy (|ΔG_H*|) of the second stage is the key factor to evaluate the performance of the catalyst, the most theoretically suitable value of which is zero for H₂ evolution [129]. Ran et al. [37] constructed a 4 × 4 × 1 O-terminated Ti₃C₂T_x supercell (Ti₃C₂O_x) as shown in Fig. 16a, and the Gibbs free energies of the Ti₃C₂O_x with different hydrogen atomic (H*) coverage on the surface were calculated based on the DFT method. As shown in Fig. 16b, the |ΔG_H*| value for Ti₃C₂O_x with an H* coverage of θ = 1/2 was inspiringly 0.00283 eV, extremely close to zero, which indicated that it is beneficial to promote the H₂ production efficiency of semiconductors from the thermodynamic point of view. Therefore, they accordingly modified CdS with Ti₃C₂ QDs for enhanced visible-light photocatalytic H₂ production, which achieved an extremely high H₂ production rate of 14,342 mmol·h⁻¹·g⁻¹ with the corresponding apparent quantum efficiency of 40.1% at 420 nm (Table 1, No. 6). According to the relationship between the band structures of CdS and Ti₃C₂, the charge transfer mechanism was following the schemes in Fig. 16c, d. It was also found that the photocatalytic activity of the composite was enhanced with the decreasing atomic ratio of F to O on the Ti₃C₂T_x surface (Fig. 16e).

Fig. 16

Reproduced with permission from Ref. [37]. Copyright 2017, Nature

DFT calculation results of O-terminated Ti₃C₂T_x. a Side and top views of structure model for a 4 × 4 × 1 O-terminated Ti₃C₂T_x supercell; b calculated ∆G_H* diagram of HER at equilibrium potential (U = 0 V) on surface of a 2 × 2 × 1 Ti₃C₂O_x supercell at different H* coverage (1/8, 1/4, 3/8, 1/2, 5/8 and 3/4) conditions (inset: side and top views of a 2 × 2 × 1 Ti₃C₂O_x supercell at 1/2 H* coverage); c energy band structures of CdS and Ti₃C₂, and charge transfer mechanism of CdS/Ti₃C₂ system under visible-light irradiation; d charge transfer route over interface of CdS/Ti₃C₂ system under visible-light illumination; e influence of Ti₃C₂’s surface F to O atomic ratio on photocatalytic activity.

Homoplastically, taking advantage of suitable Fermi level position, a large number of hydrophilic functional groups, and good electronic conductivity, Ti₃C₂ was also verified to be an effective co-catalyst of the ZnIn₂S₅ [35] and ZnIn₂S₄ [28] photocatalysts for H₂ production (Table 1, Nos. 3 and 7). Furthermore, Su et al. [107] experimentally confirmed that monolayer Ti₃C₂ performed better in improving the photocatalytic H₂ production activity of TiO₂ than multilayer Ti₃C₂, because the monolayer one with abundant hydrophilic functional groups on its surface tended to adsorb water molecule and simultaneously provided more active sites for H₂ production. More importantly, there are abundant Ti vacancy defects with strong reducibility on the monolayer or ultra-thin Ti₃C₂T_x nanosheets prepared by the wet etching method, which can realize the reduction of high-valent metal salts without adding extra reducing agents and energy input [130]. Based on this feature, Li et al. [24] reported a simple reduction and self-assembly strategy to construct a ternary CdS@Au/MXene composite photocatalyst (Table 1, No. 8), which exhibited a photocatalytic H₂ production rate of 5371 μmol·g⁻¹·h⁻¹ under visible-light irradiation, nearly 27 times higher than that of pure CdS.

4.6.2 CO₂ conversion

Similar to the role played in photocatalytic H₂ production, Ti₃C₂ also can serve as a co-catalyst to enhance the performance of semiconductors for photocatalytic CO₂ reduction. For example, Cao et al. [36] demonstrated that the yields of CH₄ and CH₃OH produced from CO₂ reduction on the Ti₃C₂/Bi₂WO₆ nanohybrid (Table 1, No. 4) were, respectively, 4 and 6 times higher than those produced on pure Bi₂WO₆ nanofilms (Fig. 17a). The intimate contact between Ti₃C₂ nanosheets and Bi₂WO₆ ultra-thin nanofilms was the key factor to improve the separation and transfer efficiency of photogenerated charge carriers, which greatly enhanced the performance of photocatalytic CO₂ reduction. Since the Fermi level of Ti₃C₂ with –O termination was more positive than the conduction band edge of Bi₂WO₆, the photoinduced electrons can transfer from Bi₂WO₆ to Ti₃C₂ (Fig. 17b). Similarly, the Ti₃C₂/g-C₃N₄ composite prepared by Yang et al. [15] also facilitated the electron transferring from g-C₃N₄ to Ti₃C₂, which was beneficial to the improvement of the CO₂ reduction performance, and the highest CO yield reached up to 5.19 μmol·h⁻¹·g⁻¹.

Fig. 17

Reproduced with permission from Ref. [36]. Copyright 2018, Wiley

Application of Ti₃C₂ MXene in CO₂ conversion. a Photocatalytic activity of pure Bi₂WO₆ and Ti₃C₂/Bi₂WO₆ (TBx) nanohybrids with different Ti₃C₂ contents; b energy level diagram of Bi₂WO₆ and Ti₃C₂ and photoinduced electron transfer process at interface of Ti₃C₂/Bi₂WO₆ hybrids.

In addition, CO₂ can also be converted into other green energy sources through photocatalytic reactions with the assistance of Ti₃C₂. Zhao et al. [25] used Ti₃C₂ nanosheets rich in Ti vacancies to construct a single-atom catalyst (SAC) Pt₁/Ti_3−xC₂T_y through a self-reduction process at room temperature (Table 1, No. 25). Owing to the high reduction ability of the Ti vacancies, single-atom Pt can be successfully stabilized on the Ti defects through strong metal–carbon bonds. Compared with traditional single-atom catalysts, the utilization of Ti₃C₂ nanosheets as supporting substrates can effectively overcome the shortcomings of reduced catalytic efficiency caused by the aggregation of dispersed single atoms. Meanwhile, the inherent reducing ability of defective Ti₃C₂ can omit the conditions of reducing atmosphere (H₂ or Ar) required in the traditional preparation process of SAC. In this way, CO₂ can contact the evenly distributed Pt atoms on the nanosheets more efficiently, so that the functionalization efficiency of CO₂ can be effectively improved when amines and silanes are present in the system.

It has to be mentioned that Ti₃C₂ MXene can also construct composite catalysts with other semiconductors in the shape of QDs to optimize the capabilities of CO₂ reduction and conversion. Compared with 2D ultra-thin nanosheets, 0D QDs can be more uniformly dispersed in the liquid and have abundant active edge sites. Reported by Zeng et al. [39], the negatively charged Cu₂O NWs/Cu modified with poly(sodium 4-styrenesulfonate) (PSS) and the positively charged Ti₃C₂T_x QDs modified with polyethylenimine (PEI) were combined to fabricate Ti₃C₂T_x QDs/Cu₂O NWs/Cu composites through electrostatic self-assembly strategy (Table 1, No. 19). Under calcination in argon atmosphere, PSS and PEI molecules can be completely removed so that Ti₃C₂T_x QDs and Cu₂O NWs were closely contacted. The efficient conversion of CO₂ to methanol by ternary Ti₃C₂T_x QDs/Cu₂O NWs/Cu was 8.25 times higher than that of the pure Cu₂O NWs/Cu. Similarly, Ti₃C₂T_x QDs were also employed to improve the photocatalytic CO₂ reduction ability of TiO₂/C₃N₄ (Table 1, No. 20) [48]. The conversion rates of CO and CH₄ were 3 and 8 times higher than those of pure TiO₂ and C₃N₄, respectively.

4.6.3 Nitrogen photofixation

For practical applications in industries, the ultimate goal of N₂ fixation is to realize its conversion to ammonia (NH₃), which is of great significance for the modern agricultural production, because NH₃ is the main source of nitrogen element in artificial fertilizers. Generally, the difficulty for the synthesis of NH₃ from N₂ lies in the strong N≡N triple bond energy (945 kJ·mol⁻¹) of N₂ [126], and it is indispensable to accumulate multiple electrons on the surface of the photocatalyst because N₂ reduction is a multi-charge transfer process.

Currently, the outstanding conductivity of Ti₃C₂T_x lays the foundation for its excellent performance in the N₂ reduction reactions. Besides, the exposed Ti vacancies on the surface and edges of the Ti₃C₂T_x and the large specific surface area could provide abundant active sites for the N₂ adsorption and activation. In 2019, Qin et al. [131] reported that 0D/2D AgInS₂/Ti₃C₂ Z-scheme heterojunction prepared by a hydrothermal method was applicable in solar photocatalytic N₂ fixation. The photocatalytic N₂ fixation rate under visible light (< 400 nm) could reach 38.8 μmol·g⁻¹·h⁻¹. The Z-scheme heterostructure formed between Ti₃C₂ and AgInS₂ accelerated the accumulation of multiple electrons on the surface of Ti₃C₂, thereby achieving efficient charge separation and migration (Fig. 18a). In addition, this work used DFT simulations to investigate the adsorption and activation behavior of N₂ on the Ti₃C₂ (001) surface. The result showed that the maximum adsorption energy of NH₃ on Ti₃C₂ was −5.20 eV, and the length of the N≡N bond increased from 0.1098 to 0.1334 nm, indicating a weakening of the N≡N bond. The above results confirmed that N₂ molecules tended to be activated with the aid of Ti₃C₂ (Fig. 18b).

Fig. 18

Reproduced with permission from Ref. [131]. Copyright 2019, Elsevier

Application of Ti₃C₂ in nitrogen photofixation. a Scheme for spatial charge separation and transportation during photocatalytic N₂ reduction over AgInS₂/Ti₃C₂ nanosheets; b adsorption configuration of N₂ molecule on Ti₃C₂ (001) surface with a (2 × 2 × 1) supercell and its corresponding N–N bond length.

Based on the above research, Liao et al. [132] compounded Ti₃C₂ with P25 TiO₂ and measured the performance of photocatalytic NH₃ synthesis. Under full-spectrum light irradiation, the NH₃ generation rate of the optimized Ti₃C₂/P25 sample was 5 times higher than that of bare P25. Similarly, the DFT calculations also confirmed that Ti₃C₂ can significantly improve the efficiency of N₂ chemical adsorption and activation. Furthermore, Sun et al. [133] developed 1D/2D CdS nanorod@Ti₃C₂ MXene composites for photocatalytic NH₃ synthesis with the N₂ fixation rate of 293.06 μmol·g⁻¹·h⁻¹.

Herein, N₂ photofixation on MXene materials is still in its infancy. More in-depth research is needed to reveal the N₂ adsorption model on MXene, as well as the effect and mechanism of MXene in the reaction process of photonitrogen fixation and ammonia synthesis.

4.6.4 Pollutant degradation

Ti₃C₂T_x also plays a central role in photocatalytic pollutant cleanup. For example, Peng et al. [58] synthesized (001) TiO₂/Ti₃C₂T_x composites by a facile hydrothermal process, which exhibited significantly enhanced photocatalytic degradation efficiency of methyl orange (MO) than bare TiO₂. As illustrated in Fig. 19a–c, on the basis of DFT calculation results, the work function values of the Ti₃C₂T_x with −OH terminal groups and TiO₂ with (001) surface exposed were calculated to be 1.800 and 4.924 eV, respectively. Since the “work function” of a material is defined as the difference between its Fermi level (E_f) and the vacuum level (E_vac), it can be inferred that the E_f of the OH-terminated Ti₃C₂T_x is higher than that of the (001) TiO₂. Therefore, after they are tightly contacted, a new balanced Fermi level would be formed between their original ones. Meanwhile, the energy band edge of (001) TiO₂ will bend downward to form a Schottky heterojunction with Ti₃C₂(OH)_x, which will trap the holes from TiO₂ and thus largely reduce the recombination of charge carriers. Under this circumstance, the Ti₃C₂ was regarded to be a reservoir of photogenerated holes. Similar mechanism was also applied to explain the enhanced photocatalytic performance of (111) TiO₂/Ti₃C₂T_x and Cu/TiO₂@Ti₃C₂T_x composites (Table 1, Nos. 1 and 2) [33, 34].

Fig. 19

Reproduced with permission from Ref. [58]. Copyright 2016, American Chemical Society. d SEM image of rice crust-like TiO₂/Ti₃C₂ composite; e schematic illustration for electronic structure of Ti₃C₂ and (101) facets of anatase TiO₂; f energy level diagram of TiO₂/Ti₃C₂ composite before and after contact, and formation mechanism of Schottky junction for electrons trapping. Reproduced with permission from Ref. [50]. Copyright 2018, Elsevier

Comparison of charge transfer mechanisms in TiO₂/Ti₃C₂ composites in different situations. a HRTEM image and (inset) model structure of TiO₂/Ti₃C₂ interface; b initial and final optimized interface structures of (001)TiO₂/OH-terminated Ti₃C₂ simulated by DFT; c energy level diagram of TiO₂/ Ti₃C₂ composite before and after contact, and formation mechanism of Schottky junction for holes trapping.

However, there are some arguments to the contrary. For example, Low et al. [50] reported that the Ti₃C₂ acted as the trap of electrons in the TiO₂/Ti₃C₂ composite for highly efficient photocatalytic CO₂ reduction. The difference might lie in the chosen model structures for the DFT calculations. As shown in Fig. 19d–f, Ti₃C₂ with no terminal groups and TiO₂ with (101) surface exposed were chosen to be the calculating object, whose work functions were 4.46 and 4.25 eV, respectively. Therefore, after their contact, the energy band edge of (101) TiO₂ will bend upward to form a Schottky heterojunction with Ti₃C₂. In this case, the Ti₃C₂ served as a reservoir of photogenerated electrons from TiO₂ instead.

Therefore, in order to determine the actual role of Ti₃C₂ in the composite photocatalyst by DFT calculations, it is necessary to build that structural model as close to reality as possible, paying particular attention on the terminal groups on Ti₃C₂ and surface atomic configuration of TiO₂. Different calculation methods may also influence the final conclusion on the proposed charge transfer mechanism.

Overall, in the field of photocatalysis, (1) Ti₃C₂T_x with different terminations could serve as electrons or holes acceptor to play as an effective co-catalyst in various photocatalysis systems. (2) Owing to appropriate Fermi level, Ti₃C₂ could couple with various semiconductor materials to establish Schottky junction, which could suppress recombination of electron or hole. (3) The intrinsic Ti vacancies in Ti₃C₂T_x could act as reduction sites for cations to form single-atom catalyst and binary/ternary composite catalyst. However, the proposed mechanisms of Ti₃C₂T_x as the co-catalyst in the previous literatures were actually inadequate and worthy further consideration. Meanwhile, the development of new strategies to prepare Ti₃C₂T_x with different end groups to regulate its Gibbs free energy and energy band structure is still a challenge worthy of breakthrough in the field of photocatalysis.

5 Summary and future opportunities

To summarize, considerable achievements and excellent progress have been made in the Ti₃C₂T_x material, which has been recognized as one of the most thoroughly studied materials in the MXene family due to its various excellent properties, such as flexible structures, good conductivity, hydrophilicity, and mechanical stability. Consequently, researchers have taken much effort to combine the advantages of Ti₃C₂T_x with traditional inorganic, organic, and polymer materials to overcome the development obstacles in the conventional functional materials. So far, the explored preparation methods of the Ti₃C₂T_x-based composites include hydro-/solvothermal method, deposition, self-assembly, in situ growing, freeze-drying method, and other strategies. Followed by, the obtained composite has been successfully applied to energy storage, electromagnetic interference, photothermal conversion, biomedicine, sensors, photocatalysis, and other fields, and has made considerable progress. By summarizing the examples mentioned in this review, it can be seen that the discovered properties of Ti₃C₂T_x have been successfully applied in corresponding fields, following some certain disciplines.

• (1) A large number of terminal groups such as –O, –F, and –OH on the surface of Ti₃C₂T_x render it a negative surface, which is particularly attractive to positively charged ions or ligands, providing more active sites and possibilities for the reactions in need. In addition, the abundant groups located on the surface of Ti₃C₂T_x endow it with intrinsic hydrophilicity, which makes it possible to achieve uniform dispersion in aqueous solution for those reactions occurred in aqueous systems. Furthermore, polymers, organic molecules, and inorganic nanomaterials can achieve the functionalization of Ti₃C₂T_x through electrostatic physical adsorption or surface chemistry strategies.

• (2) Adjustable forms of Ti₃C₂T_x MXene make it adaptable to different needs. First, the accordion-like multilayered structure of Ti₃C₂T_x is conducive to multiple reflections of electromagnetic waves, increasing energy consumption and improving electromagnetic wave absorption and shielding capabilities. After delamination, Ti₃C₂T_x can exist in a few-layer/monolayer form, which offers a large specific surface area and thus abundant active sites for reactions. Therefore, it hides huge development potential in energy conversion and storage, wearable smart devices, and biomedicine. After further regulation, Ti₃C₂T_x can be presented in the form of quantum dots, which is characteristic by fluorescence emission due to quantum size effect. The unique optical properties not only make Ti₃C₂T_x show its brilliance in the field of biomedicine, but also have positive influence on the wide applications of Ti₃C₂T_x in optical-related fields.

• (3) Besides, Ti₃C₂T_x possesses the special skill of light-to-heat conversion capability, which makes MXene widely favored in the fields of biomedicine, sensor, and energy conversion.

• (4) Owing to the appropriate band structure and excellent conductivity, Ti₃C₂T_x plays the role of co-catalyst in the field of photocatalysis by improving the carrier transport efficiency. Also, it can serve as an electrode material to enhance the energy storage capacity and optimize the sensitivity of the device in the sensor.

Hitherto, despite the fact that many efforts have been implemented in the research of Ti₃C₂T_x and its composites, there are still several key points to be solved.

• (1) Owing to the limitations of the preparation process through wet chemical etching, bare Ti₃C₂ without terminal surface groups has not yet been obtained. These difficult-to-control end groups will influence the electro-conductivity and surface properties of Ti₃C₂T_x to a certain extent, which in turn affects its application scale. Besides, Ti₃C₂T_x is easily oxidized in a humid environment, which will result in inevitable structural collapse and performance decline or even disappearance. Therefore, exploring a new and smart strategy for preparing bare Ti₃C₂ is still a huge challenge.

• (2) So far, most of the methods for preparing Ti₃C₂T_x still rely on highly corrosive acid solutions. The utilization of these reagents will cause adverse effects on human body as well as environment. To this end, the exploitation of novel, polluting-free and non-toxic preparation strategies still needs a lot of attention. Moreover, except for the preparation of Ti₃C₂T_x by selective etching MAX from up to bottom, the bottom-to-up fabrication technology has not yet been achieved, that is directly synthesizing Ti₃C₂T_x from single atoms.

• (3) As the current researches on the properties of Ti₃C₂T_x are not sufficient, the reaction mechanism of Ti₃C₂T_x in various application fields is still unclear and uncertain. Subsequent research could take advantage of theory calculation to interpret the true role of Ti₃C₂T_x in the composite materials.

• (4) Although Ti₃C₂T_x has been clinically proven to be harmless to living organisms, its application in biomedical fields is still in its infancy. Especially, the long-term life safety of Ti₃C₂T_x needs to be further confirmed.

• (5) Concerning technical issues for mass production and process integration, most of the current research is limited to the laboratory, and has not been able to test in actual conditions. There is still a long way to go to commercialize Ti₃C₂T_x and Ti₃C₂T_x-based composites.

Although the research on Ti₃C₂ and its composites is still in initial stage and many unknowns lie ahead, it is unable to conceal its enormous potential in diverse practical areas. Just keep our passion and faith in scientific knowledge, and it is hoped that this review can give some useful guidance to researchers in the related fields and pave the way for further development of MXene-based composites with bright prospect.