coordination chemistry reviews...2. synthesis, structure and property of mxenes 2.1. synthesis of...

Coordination Chemistry Reviews 352 (2017) 306–327

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Coordination Chemistry Reviews

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Review

Recent advance in MXenes: A promising 2D material for catalysis, sensorand chemical adsorption

https://doi.org/10.1016/j.ccr.2017.09.0120010-8545/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding authors.E-mail addresses: [email protected] (C. Xu), [email protected] (K.-Y. Wong), [email protected] (D. Astruc), [email protected] (P. Zh

1 These authors contributed equally to this work.

Jing Zhu a,1, Enna Ha b,1, Guoliang Zhao a,1, Yang Zhou a, Deshun Huang a, Guozong Yue a, Liangsheng Hu b,Ning Sun b, Yong Wang b, Lawrence Yoon Suk Lee b, Chen Xu a,⇑, Kwok-Yin Wong b,⇑, Didier Astruc c,⇑,Pengxiang Zhao a,⇑a Institute of Materials, China Academy of Engineering Physics, No. 9, Huafengxincun, Jiangyou City 621908, Sichuan, PR ChinabDepartment of Applied Biology and Chemical Technology and the State Key Laboratory of Chirosciences, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, Chinac ISM, University of Bordeaux, 351 Cours de la Libération, Talence Cedex 33405, France

a r t i c l e i n f o

Article history:Received 3 July 2017Accepted 12 September 2017Available online 5 October 2017

Keywords:MXenesNanomaterials2D materialsCatalysisSensorAdsorption

a b s t r a c t

MXenes, a new, very recently emerging family of two-dimensional (2D) early transition metal carbidesand/or nitrides, has attracted a great influence in the fields of physics, material science, chemistry, andnanotechnology. In this review, their synthesis, compelling physical, chemical, as well as their variouspotential applications in catalysis, sensors and adsorption are highlighted. First, the synthesis, structuralvariety, and chemical and physical properties are summarized. Then the electroactivity, durability, easeof functionalization of MXenes toward electrocatalysis and photocatalysis applications are introduced.The specific properties of metallic conductivity, biocompatibility, hydrophilic surface, and 2D layeredatomic structure that make MXenes promising candidates in sensing of rapid, easy, and label-free detec-tion are consequently discussed. Finally, how MXenes could be considered as ideal adsorbents due to theadvantages of large surface area and abundant active sites is underlined. Promising theoretical calcula-tions and first remarkable performances in these applications are also highlighted.

� 2017 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3072. Synthesis, structure and property of MXenes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

2.1. Synthesis of MXenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309
2.1.1. Multi-layered stacked MXenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3092.1.2. Single/multi-layered MXenes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
2.2. Structure of MXenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
2.2.1. Chemical composition and atomic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3112.2.2. Surface chemical structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
2.3. Properties of MXenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
2.3.1. Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3132.3.2. Mechanical property. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3132.3.3. Electronic property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3142.3.4. Surface chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314
3. MXenes for catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315
3.1. MXenes for CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3163.2. MXenes for dehydrogenation of hydrogen storage materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3163.3. MXenes as catalysts for oxygen reduction reaction (ORR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3163.4. MXenes as catalysts for oxygen evolution reaction (OER) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
ao).

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J. Zhu et al. / Coordination Chemistry Reviews 352 (2017) 306–327 307

3.5. MXenes as catalysts for hydrogen evolution reaction (HER) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
4. MXenes as sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319
4.1. MXenes for electrochemical biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3194.2. MXenes for H2O2 detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3194.3. MXenes for gas sensing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3194.4. MXenes for detection of macromolecules and cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

5. MXenes for chemical adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
5.1. Application of MXenes in adsorption of neutral gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3215.2. Application of MXenes in adsorption of cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3235.3. Application of MXenes in adsorption of anions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
6. Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326

1. Introduction

Since the discovery of graphene in 2004, the two-dimensional(2D) topological materials appear as one of the fast growing familyof recently emerging nanomaterials [1]. Due to their unique chem-ical and physical properties, 2D materials are widely used inenergy storage and material science. Generally speaking, 2D mate-rials refer to the materials with one dimension restricted to single-atom or few-atom layer (typically less than 5 nm); however, theother dimension could be larger than 100 nm, or up to a fewmicrometers [2–6].

Up to now, at least 18 kinds of nanomaterials are included inthe family of 2D materials beyond graphene: hexagonal boron

MXenes that have been reported with three different formulas: M2X, M3X2 and M4

three different forms: mono-M elements (for example, Ti2C and Nb4C3); a solid sdouble-M elements, in which one transition metal occupies the perimeter layers

er M layers are Mo and the central M layers are Ti). Solid solutions on the X site prht 2017 Nature Publishing Group.

nitride (h-BN) [7], graphitic carbon nitride (g-C3N4) [8], transitionmetal dichalcogenides (TMDs) [9], black phosphorus (BP) [10],III–VI layered semiconductors [11], metal phosphorus trichalco-genides [12], layered double hydroxides (LDHs) [13], metal oxides,transition metal oxyhalides [14], metal halides [15], perovskites[16] and niobates [17], silicates and hydroxides (clays) [18], somemetal–organic frameworks (MOFs) [19], covalent–organic frame-works (COFs) [20] and polymers [21], metals [22], non-layer struc-tured metal oxides [23], non-layer structured metal chalcogenides[24], and early transition metal carbides and/or nitrides (MXenes)[25].

It is worth noting that the last member (MXenes) in this largefamily is very young and has grown rapidly in recent years. Over

X3, where M is an early transition metal and X is carbon and/or nitrogen. They can beolution of at least two different M elements (for example, (Ti,V)3C2 and (Cr,V)3C2); orand another fills the central M layers (for example, Mo2TiC2 and Mo2Ti2C3, in whichoduce carbonitrides. NA, not available. Reprinted with the permission from Ref. [25].

Fig. 2. (a) Schematic representation of the exfoliation process for Ti3AlC2. (b) Transmission electron microscope (TEM) images for stacked layers of Ti3C2Tx. (c) Model of the Li-intercalated structure of Ti3C2 (Ti3C2Li2). Reprinted with the permission from Ref. [29]. Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Schematicrepresentation of Mo2TiAlC2 to Mo2TiC2Tx. (e) and (f) High-resolution scanning transmission electron microscopy (HRSTEM) of Mo2TiAlC2 and Mo2TiC2Tx, respectively. (g)TEM image of Mo2TiC2Tx showing the layered structure throughout the sample. Reprinted with the permission from Ref. [35]. Copyright 2015 American Chemical Society.

Table 1Process conditions for MXenes synthesis from MAX phases by HF etching.

Type MXenes Precursor HF Concentration (wt%) Time (h) T (�C) Ref.

M2X Ti2CTx Ti2AlC 10 10 RT [33,38]48 24 RT [39]

V2CTx V2AlC 50 90 RT [30,40,41]Nb2CTx Nb2AlC 50 90 RT [30,42]

50 40 55 [43]Mo2CTx Mo2Ga2C 25 160 55 [31,32](Ti,Nb)2CTx (Ti,Nb)2AlC 50 28 RT [33]

M3X2 Ti3C2Tx Ti3AlC2 50 2 RT [29]10 24 RT [44]40 48 RT [45]49 12 60 [46]50 18 RT [38]

Hf3C2Tx Hf3Al4C6 35 60 RT [37](V0.5Cr0.5)3C2Tx (V0.5Cr0.5)3AlC2 50 69 RT [33]Mo2TiC2Tx Mo2TiAlC2 50 48 RT [35,47]

M4X3 Ta4C3Tx Ta4AlC3 50 72 RT [33]Nb4C3Tx Nb4AlC3 50 90 RT [48]Mo2Ti2C3Tx Mo2Ti2AlC3 50 96 RT [35,47]

308 J. Zhu et al. / Coordination Chemistry Reviews 352 (2017) 306–327

70 kinds of MXenes have been reported up to now (Fig. 1) [25–27].They have been successfully obtained by the selective chemicaletching of ‘‘A” in ‘‘MAX” phases. Generally, the ‘‘MAX” phases havea formula of Mn+1AXn, where ‘‘M” means early d-transition metal,‘‘A” represents the main group sp-element, and ‘‘X” indicates Cand/or N [28]. The structures of M2X, M3X2 and M4X3 are shownin Fig. 1. For instance, the first MXene material, Ti3C2 was synthe-sized in 2011 by selectively etching the Al atoms in layered hexag-onal ternary carbide, Ti3AlC2, with hydrofluoric acid (HF) at roomtemperature [29]. Normally MXenes exist with more than one‘‘M” element and two forms of structures: solid solutions andordered phases. The former represents a random arrangement oftwo different transition metals that is observed in the ‘‘M” layers.

The latter is composed of a single layer or double layers of a singletransition metal that is sandwiched between the layers of a secondtransition metal in a 2D carbide structure.

Although several reviews have been involved in the prepara-tion, property and potential applications (for examples, energystorage, transparent conductors, and environmental remediation)of MXenes, none of them paid close attention to their applicationsin chemistry. To the best of our knowledge, the key process forchemical application of MXenes should be their functionalizationproviding MXenes new properties and potential evaluation. There-fore, in this review, focus is on the chemical functionalization ofMXenes and their applications, including catalysis, sensor, andchemical adsorption.


2. Synthesis, structure and property of MXenes

2.1. Synthesis of MXenes

The synthetic routes of MXenes have attracted extensive inter-est since the discovery of Ti3C2Tx through HF etching of Ti3AlC2 [29]in 2011. Many new methods of synthesis have been developed toprepare these materials.

2.1.1. Multi-layered stacked MXenesThe first synthesis of MXenes was reported by Naguib et al. Ti3-

AlC2 powders were immersed in a 50 wt% concentrated HF solutionat room temperature for 2 h. The resulting suspension was thenwashed several times using deionized water and centrifuged toprecipitate the powders. The reaction mechanism was suggestedto follow a selective etching of Al atomic layers from the layeredprecursors (Ti3AlC2 phases). Such selective etching is possiblebecause the Ti-Al bonds are more easily cleavable than Ti-C bonds(Fig. 2(a)). The preferential etching of the Ti-Al bond in Ti3AlC2

phases with Al is summarized as:

Ti3AlC2 þ 3HF ! AlF3 þ 1:5H2 þ Ti3C2 ð1Þ

Ti3C2 þ 2H2O ! Ti3C2ðOHÞ2 þH2 ð2Þ

Ti3C2 þ 2HF ! Ti3C2F2 þH2 ð3ÞReactions (2) and (3) indicate the generation of surface groups

(OH and F) on MXenes during the process, ending up as the surfaceterminations of 2D Ti3C2 exfoliated layers. It should be noted thatReactions (2) and (3) are simplified with the assumption that theterminations are OH or F, respectively, but in fact they are mostlikely a combination of both (Fig. 2(b and c)). Many new kinds ofMXenes, such as V2CTx [30], Mo2CTx [31,32] and Ta4C3Tx [33], havebeen produced by extending the selective etching method with HFto other MAX phases. The etching conditions for different MXenesare provided in Table 1. Solid solutions, such as (Nb0.8Ti0.2)4C3Tx[34], (Ti0.5,Nb0.5)2CTx [33], and (V0.5,Cr0.5)3C2Tx [33] have also beenreported, in which the two transition elements were believed to berandomly distributed on the M-sites. Anasori et al. [35] synthe-sized the first ordered double-M-element MXenes (Mo2TiC2Txand Mo2Ti2C3Tx) as well as Mo-layers sandwiched TiC2 layers(Fig. 2(d–g)), which further expanded the family of 2D materials,offering more choices of structures, chemistry, and ultimately use-ful properties. Zr-based and Hf-based MXenes such as Zr3C2Tx havenot been produced by etching Zr-based and Hf-based MAX phasesup to now, which limits the extension of the MXene family. Zhouet al. [36] successfully synthesized the first layered Zr3C2Tx byextending this HF-etching protocol to Zr3Al3C5, an alternative lay-ered ternary transition metal carbide beyond the Mn+1AXn phases.Zhou et al. [37] also synthesized Hf3C2Tx by selectively etchingAl4C4 from Hf3Al4C6.

The use of concentrated HF in the synthesis of MXenes limitedthe applications of these materials because of the dangerousness ofHF. Therefore, it is necessary to develop an alternative, safe and fastprocess for exfoliation and delamination. Ghidiu et al. [49] etchedAl from Ti3AlC2 using less harmful hydrochloric acid (HCl) andlithium fluoride salts (LiF), achieving high-yield MXenes with afast, novel, facile and single-step method, in which etching andintercalation were performed simultaneously. The detailed syn-thetic procedure is as follows: firstly, certain amount of LiF wasdissolved in 6M HCl, followed by slow addition of Ti3AlC2 powdersinto the solution, and then the temperature of mixture was raisedto 40 �C and maintained for 45 h. After cooling to room tempera-ture, the resulting products were washed and centrifuged withseveral cycles of water to remove the reaction by-products. The

as-prepared materials were hydrophilic and swelled in volumewith claylike shape after the water hydration. Finally, the resultingMXenes were dried into a solid powder or rolled into films withtens of micrometers in thickness and high conductivity. Comparedto the HF-etched MXenes, the MXene particles did not show theaccordion-like morphology; they appeared larger flakes presum-ably with negligible nanoscale defects. In light of this finding,Shahzad et al. [50] modified the etching route by using a higherLiF:Ti3AlC2 molar ratio (7.5:1 instead of 5:1), and sonication wasnot applied for the delamination step, which enables more Li+ ionsfor intercalation. In this process, the etchant solution was preparedby completely dissolving 1 g LiF in 20 mL of 6 M HCl, followed bygradual addition of 1 g Ti3AlC2 and the reaction was allowed forproceeding at 35 �C for 24 h. The quality of Ti3C2Tx flakes synthe-sized with different molar ratios of LiF to Ti3AlC2 (5:1 and 7.5:1)was evaluated by Lipatov et al. [50]. Ti3C2Tx flakes exfoliated with5:1 (LiF: Ti3AlC2) were 200–500 nm in diameter with uneven edgesdecorated with TiO2 nanoparticles and numerous pin holes,whereas Ti3C2Tx flakes exfoliated with a molar ratio of 7.5:1 were4–15 mm laterally, which showed well-defined and clean edgeswith no visible pin holes. Furthermore, the former was less orderedand had varied thicknesses, while the later was more orderlystacked, with a uniform height of ca. 2.7 nm (single layer with sur-face adsorbates).

So far, many new synthetic routes have been reported. Wanget al. [51] employed a simple hydrothermal method to prepare Ti3-C2Tx by immersing Ti3AlC2 powders in NH4F solution sealed in aTeflon-lined stainless steel autoclave at 150 �C for 24 h. Thismethod avoids the use of HF, thus is easier and safer; however,the method is more complicated. Feng et al. [52] introduced aneasier and less hazardous method by etching MAX phases withmilder etchant, i.e., NH4HF2, NaHF2, and KHF2 solution at 60 �C,which could simultaneously intercalate cations, such as NH4

+, Na+,and K+, and effectively etch the Al layers. The corresponding mech-anism was elaborately demonstrated as follows:

Ti3AlC2 þ XHF2 ! XaAlFb þ AlF3 þH2 þ Ti3C2 ð4Þ

AlF3 þ cH2O ! AlF3 � cH2O ð5Þ

Ti3C2 þ XHF2 þH2O ! Ti3C2FxðOHÞyXz ð6ÞUp to now, all attempts to produce nitride-based MXenes by HF

etching are unsuccessful. Note that the calculated cohesive ener-gies of Tin+1Nn are less than those of Tin+1Cn, whereas the formationenergies of Tin+1Nn from Tin+1AlNn are higher than those of Tin+1Cn

from Tin+1AlCn. The lower cohesion energy implies a lower stabilityof the structure, whereas the higher formation energy of theMXenes from their corresponding Al containing MAX phasesrequires more energy for their extraction. Indeed this implies thatthe Al atoms are bonded more strongly in Tin+1AlNn compared tothe case of Tin+1AlCn. These two factors may explain why nitrideMXenes have not been produced by HF etching to date. Anotherdistinct possibility is that the Tin+1Nn layers dissolve in the HF solu-tion due to their lower stability [25]. Despite the failure of HF etch-ing, Urbankowski et al. [53] utilized a molten salt method toprepare Ti4N3Tx by thermally treating a mixture with equal massesof Ti4AlN3 powders and fluoride salt such as KF, LiF, and NaF at550 �C for 0.5 h under Ar atmosphere (Fig. 3).

All MXenes introduced above were produced by etching Aatoms (A = Al, Si, etc.) from MAX phases and surface groups (OH,F, O). However, the properties of bare Mn+1Xn and surface groupscontaining Mn+1XnTx are different; thus it is necessary to developnew methods to produce bare MXenes. Xu et al. [54] developed achemical vapor deposition (CVD) process to grow high-quality 2DMXene a-Mo2C crystals on a Cu/Mo foil under methane

Fig. 3. Schematic illustration of the synthesis of Ti4N3Tx by molten salt treatment of Ti4AlN3 at 550 �C under Ar, followed by delamination of the multilayered MXene byTBAOH. Reprinted with the permission from Ref. [53]. Copyright 2016 The Royal Society of Chemistry.

Fig. 4. Schematic representation for the MXenes delamination process: MXenes reacted with organic bases that cause multilayered MXene powder (pictured in bottom left)to swell significantly. By simply hand shaking or mild sonication in water, the layers delaminated and formed a stable colloidal solution (right side). A typical ScanningElectron Microscope (SEM) image of the as synthesized Ti3CNTx multi-layer MXene is shown top left. Reprinted with the permission from Ref. [58]. Copyright 2015 The RoyalSociety of Chemistry.


atmosphere, which acts as the carbon source at temperaturesabove 1085 �C. High treating temperature melt Cu and henceMo-Cu alloy formed at the Cu/Mo liquid interface. As Xu proposed,this allowed Mo atoms to diffuse from the interface to the liquid Cusurface to form Mo2C crystals by reacting with methane.

2.1.2. Single/multi-layered MXenesWith strong M–A bonds replaced by weak bonds (such as,

hydrogen bond and van der Waals forces), the intercalation anddelamination of multi-layered stacked MXenes into single or fewlayers have been achieved. Mashtalir et al. [55] reported the inter-calation by hydrazine, N,N-dimethylformamide and urea, whichresulted in an increase of the c-lattice parameters of surface func-tionalized f-Ti3C2. Lukatskaya et al. [56] successfully demonstratedthe spontaneous intercalation of cations (including Na+, K+, NH4+,Mg2+, and Al3+) from aqueous salt solutions between 2D Ti3C2

MXene layers. The intercalation of organic materials and cationsincreases the interlayer spacings of MXenes, which allows theintercalation and de-intercalation of large metal ions and particles.

However, the interlayer spacings are uncontrollable, which limitsthe selective intercalation of various metal ions and particles. Luoet al. [57] prepared 2D Ti3C2 MXene with controllable interlayerspacing between 1 and 2.708 nm. The synthesis was achieved asfollows: Ti3C2 MXene was produced by intercalating cationic sur-factants with various lengths of hydrophobic alkyl chains. Uponthe immerse of Ti3C2 the cationic surfactant solution, the cationicsurfactant was spontaneously assembled and intercalated betweenthe negatively charged layers of Ti3C2 by electrostatic interactions,causing an increase in the interlayer spacing between Ti3C2 layers.

The weak bonding between Mn+1Xn layers allows sonication toseparate the layers from each other. However, the low yields ofdelaminated flakes by such method restrict the applications ofMXenes where fully delaminated layers are required. Mashtaliret al. [55] introduced the first high-yield delamination of Ti3C2Txinto a mixture of narrow-sized single and multi-layered Ti3C2Txby intercalation of dimethyl sulfoxide (DMSO) between themulti-layered Ti3C2Tx at room temperature for 18 h under constantstirring, followed by sonication in water for 6 h. However, this

Fig. 5. Scheme summarizing the synthetic procedure of Ti2C with various morphologies. The materials were derived from HF etched MXenes, mediated by ultrasonication inthe presence of p-phosphonic acid calix[n]arenes (PCXn). Note the Tyndall effect images for corresponding samples as a confirmation of generating colloidal solution.Reprinted with the permission from Ref. [59]. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


might be caused by the co-intercalation or capillary condensationof water from ambient air into the interlayer space. This approachhas only been successful with Ti3C2Tx, since DMSO was not able tointercalate into in the case of other MXenes. Another approach todelaminate Ti3C2Tx was achieved by treating Ti3AlC2 with a mix-ture of HCl and LiF, instead of using HF directly, as discussed above.Again, this approach has been successful only with Ti3C2Tx up todate.

Many efforts have been done to produce other single/few-layered MXenes. Naguib et al. [58] introduced another method toproduce large scale delaminated Ti3CNTx, V2CTx, and Nb2CTx byintercalating large organic bases (such as tetrabutylammoniumhydroxide (TBAOH) and choline hydroxide) into multi-layeredstacked MXenes (Fig. 4). The delaminated MXenes showed largerd-spacing and reduced F content, which is due to the replacementof O and OH. Urbankowski et al. [53] also produced delaminatedTi4N3Tx by using TBAOH as an intercalating agent, with the aid ofsonication.

Excellent electronic and mechanical properties of 2D laminarmaterials require effective means of both exfoliation and facilitat-ing control over morphology beyond the layered structure. To solvethis problem, Vaughn et al. [59] used a new method to develop Ti2-CTx morphology of plates, crumpled sheets, spheres, and scrolls byselective the ring size of intercalating p-phosphonic calix[n]arenes(n = 4, 5, 6, or 8) (Fig. 5). Following ultrasonication in aqueous ofMXene in the presence of PCX4 resulted in thin sheets displayingmicron sized lateral dimensions, while sonication with PCX5 wasproduced crumpled-up sheet morphology. This crumpling is most

likely a consequence of supramolecular and/or covalent interplaybetween the phosphonic acid and/or phenolic moieties of PCX5with the MXene sheets coupled with the fivefold symmetry cal-ixarenes avoiding self-assembly into close packed flat 2D sheetson the surface of the MXene, which would have symmetry con-straints. The delamination of MXene by PCX6 gained unprece-dented spherical particles. The greater conformational flexibilityof PCX6 compared with PCX4 and PCX5 may be the crucial pointin controlling the formation of spherical particles. The largestphosphonic acid calixarene PCX8 assisted delamination results inscrolls morphology probably because of its greatest conformationalflexibility and dexterity. The authors conjecture the M-PCX8 scrollsarise from the flexible calixarene being able to effectively interplaywith an MXene sheet from either side of the plane of themacrocycle.

2.2. Structure of MXenes

2.2.1. Chemical composition and atomic structureMXenes originate from ternary nitrides and carbides with a for-

mula of Mn+1AXn (n = 1–3), namely MAX phase [29]. In MAX phase,‘M’ refers to transition metal elements, ‘A’ mostly refers to group13 and 14 elements, and X is usually C and/or N. MAX phase is alarge family of materials with more than 70 members [25,60]. Asone of groups in MAX phase, 19 kinds of MXenes have been syn-thesized, and there are still dozens of potential MXenes have beenpredicted by theoretical simulation [25,35,61] since the first dis-covery of Ti3C2 type in 2011 (Fig. 1).

Fig. 6. Three types of MAX phase unit cells and crystal structures: (a) 211, (b) 312, and (c) 413 phases. Reprinted with the permission from Ref. [62]. Copyright 2011 AnnualReviews.


The structure of MXene derives from the MAX phase. The MAXphases have three types of unit cells with hexagonal close struc-ture (space group P63/mmc) as shown in Fig. 6. The pure A layerinterleaves in close packed M layers, and the octahedral sites areoccupied by X atoms; the bonds of the MAX phase are metallic,covalent, and ionic [62,63]. The M–X bonds are exceptionallystrong with mixed covalent/metallic/ionic characteristics [64],but the binding of the A-group layer is relatively weak and morereactive. So it is possible to remove A-group atoms to obtainMXenes [25,29]. In fact, the bonding mode of MXene is differentfrom those of other layered materials like graphite in which weakvan der Waals interactions hold similar structures [65], consider-ing that the M–A bond is metallic.

Similar to MAX phases, MXenes have a hexagonal close-packedstructure with X atoms filling the octahedral interstitial sites andmainly in three types of packing ways: BcA–AcB (M2X–M2X), BcAbC–CbAcB (M3X2-M3X2), and BaCbAcB–BcAbCaB (M4X3–M4X3)[61]. The atomic orders of M could follow ABABAB (in M2X) andABCABC (in M3X2, M4X3), which is vital for the structural stability[31,35]. If more than one element occupies the M sites, MXenesexist in two atomic arrangement forms: solid solution and orderedphases [35,47]. The former is in a random arrangement of differenttransition metals in the same M layer, and the ordered MXeneshave a pure M element with single or double layers, and is interca-lated in a sandwich structure by second transition metal. Theordered MXenes are energetically more stable than MXenes in

solid solution forms, as suggested by density functional theory(DFT) calculations. In addition, 2D transition metal carbonitrideswere also reported, such as Ti3CN [33].

With a layered structure similar to that of graphene, MXenesare widely used as 2D materials [66]. The traditional methodslike mechanical cleavage or ultrasonication are not easilyemployed to separate MX layer from MAX precursor due to rel-atively strong bonding of A-containing layer [67]. A selectiveetching method was reported by Gogotsi’s group, and the lay-ered structure MXenes was obtained experimentally [29]. Theirresults showed the exfoliated Ti3C2 nanosheets with a thicknessranging from single, double to multi-layers. Having a similarhexagonal structure, the electronic properties of MXenes areconsidered comparable to those of graphene. In fact, as-prepared MXene flakes have some intrinsic defects like atomicvacancies because of etching and delamination process [68,69].Monolayer Ti3C2Tx (Tx-functional group) flakes were synthesizedvia a minimally intensive layer delamination method and differ-ent point defects were characterized by using aberration cor-rected atomic-resolution STEM. Their results showed that Tivacancies are easily formed in the outer Ti sub-layer, and theconcentration of defects was tuned by adjusting HF dosages(Fig. 7). At the same time, the defect structure was supportedby DFT-based calculation, and the defects caused the change ofsurface morphology and termination groups but not stronglyalter the metallic conductivity [68].

Fig. 7. The atomic defects found in MXenes. (a) Comparison between experimental and simulated high-angle annular dark-field imaging-STEM (HAADF-STEM) images ofTi3C2Tx. (b), (c) and (d) the influencing factors of the formation of Ti vacancies. Reprinted with the permission from Ref. [68]. Copyright 2016 American Chemical Society.


2.2.2. Surface chemical structureHF is usually used as the etching agent with a selective etching

reaction, due to the fact that A atoms are relatively weakly bonded.The exposed Ti is very active after the removal of A; therefore, it iseasy to bond extremely electronegative charged F, O or OH asligands, which is the Tx in the formula Mn+1XnTx [29]. Consideringthe effects of synthetic conditions, the surface functionalizedMXenes are often noted as Mn+1Xn(OH)xOyFz (Mn+1XnTx, in short)and non-terminated MXenes are yet being synthesized [61]. Inmost cases, there is a random distribution and mixture of termina-tions on MXene surfaces [55,69–71]. A few of reports have con-firmed that it is possible to produce MXenes with specificterminations via post-synthesis [72–74].

Experimentally, the surface terminations of MXenes depend onthe synthetic conditions, such as the enchant, etching and delam-ination conditions, the types of M element, post-synthetic proce-dure, and storage [61]. In addition, as suggested by theoreticalcalculations, MXenes should have many specific properties[26,75–77]. Variation on the surface functional group significantlyaffects the properties of MXenes. For example, the surface termina-tion of oxygen tends to promote the highest capacity comparedwith H and/or OH-terminated MXenes [78]. The researchersexpand the surface terminations like P, Si, and methoxy groups.Besides, Xie et al. predicts that O-terminated MXenes can decom-pose into bare MXenes, suggesting a route to prepare bare MXenenanosheets [79,80].

2.3. Properties of MXenes

2.3.1. StabilityThe stability of MXenes varies with the compositions, solutions,

and reaction conditions [81–86]. A systematic ab initio simulationwas reported by Ivanovskii’s group. Analysis based cohesive

energies and formation energies indicated that MXenes werehighly stable [87]. Moreover, self-consistent charge density-functional tight-binding (DFTB) calculation studies showed thatthe hydroxylated MXene derivatives with random distribution ofC and N atoms are the most stable phases [77]. However, it stillremains a question whether MXenes and their surface structuresare stable under realistic reaction conditions. Some studies onthe thermal stability of MXenes have been reported, showing thecompositional and structural changes during heating. Li et al.[82] showed that anatase TiO2 formed in the Ti2C nanosheets uponheating to 500 K, then transforms to rutile TiO2, suggesting the sur-face groups in MXenes are eliminated with increasing temperature.Therefore, it is possible to tune the properties of MXenes by properheat treatment. Naguib et al. [86] also showed that Ti3C2Tx wasrapidly oxidized in air at 1150 �C for 30 s, and a hybrid structureof thin sheets with disordered graphitic carbon decorated withnanocrystalline anatase was obtained. Nevertheless, the as-prepared 2D Ti3C2 was still stable even upon heating to 1200 �Cunder argon [85]. In addition, a study of the 2D Zr3C2Tz showedthat there was no apparent mass loss in the thermogravimetricanalysis (TGA) tests of Zr3C2Tz from 500 to 1200 �C, and Zr3C2Tzexhibited relatively better ability to maintain 2D nature and struc-tural integrity than Ti-based MXenes (Fig. 8) [36].

On the other hand, Ti3C2Tx is not stable in aqueous solution,especially in the presence of dissolved oxygen. It can be oxidizedand transformed into titanium hydroxide [88,89]. The oxidationmight occur with O2 adsorption by under-coordinated Ti atomsin Ti3C2Tx [90]. Thus water is not the best medium for MXene stor-age after synthesis.

2.3.2. Mechanical propertyThe first study of 2D early transition metal carbides (Ti2C, Ti3C2,

Ti4C3, V2C, Cr2C, Zr2C, Hf2C, Ta2C, Ta3C2, and Ta4C3) indicated that

Fig. 8. (a) TGA and differential scanning calorimetry (DSC) curves of Ti3C2 and 2DZr3C2Tz from RT to 1200 �C in argon atmosphere, (b) X-ray diffraction patterns ofTi3C2 MXene and 2D Zr3C2Tz before and after annealing in vacuo at 1000 �C for 2 h.These figures show the stability of MXenes. Reprinted with the permission from Ref.[36]. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Table 2the band gap of MXenes with various surface groups.

MXenes Band gap/eV Type Ref.

Ti2CO2 0.24 Indirect band gap [26]Ti3C2F2 0.10 – [29]Ti3C2(OH)2 0.05 – [29]Sc2CO2 1.80 Indirect band gap [26]Sc2C2F2 1.03 Indirect band gap [26]Sc2C(OH)2 0.45 Direct band gap [26]Zr2CO2 0.88 Indirect band gap [26]Hf2CO2 1.00 Indirect band gap [26]Mo2CF2 0.25 Indirect band gap [93]

Fig. 9. Scheme of the relative energy position of the lowest NFE state for MXenes,graphene, BN, and MoS2. The solid line represents the energy position of thevacuum level. Reprinted with the permission from Ref. [92]. Copyright 2016American Physical Society.


MXenes might possess high elastic moduli when stretched alongthe basal planes, and their mechanical properties should be signif-icantly better than those of multilayer graphene of comparablethickness [66]. Later, it was experimentally proved by Gogotsi’sgroup that T3C2Tx MXene, incorporated with an electrically neutralpolyvinyl alcohol (PVA) to produce T3C2Tx/polymer composites,holds high mechanical property which is about 15,000 times ofits own weight [91].

2.3.3. Electronic propertyAll the MXenes are metallic in the absence of surface function-

alization. Due to the functionalization, some MXenes becomesemiconductors with energy gaps about 0.25–2.0 eV [26] (Table 2),such as Sc2CF2, Sc2C(OH)2, Sc2CO2, Ti2CO2, and Zr2CO2. Khazaei

et al. [92] revealed the existence of nearly free electron (NFE) statesin a variety of MXenes (terminated with F, O, OH) and OH-terminated MXene with positively charged surfaces located nearthe Fermi level (Fig. 9). This generates the dipole moments allow-ing electron transfer which causes semiconducting properties.

According to the first-principle calculations, together with thelow dimensionality of MXenes, their electronic and magnetic prop-erties differ greatly from those of their corresponding MAX phasesolids upon an appropriate chemical surface treatment by differenttermination (F, OH, and O), such as ferromagnetic, magnetic, andSeebeck coefficient [26]. For example, Ti2CO2 acquires a large See-beck coefficient of about 1140 lV/K at very low temperatures of100 K [26].

Furthermore, MXenes have excellent electromagnetic waveabsorption [94] and can be used as light-to-heat conversion mate-rial generating heat to reach certain applications, like photother-mal ablation of tumor [95] and light-to-water–vapor generation[96].

2.3.4. Surface chemical propertiesThe chemical properties are often affected by the presence of

surface groups that are introduced in the preparation. MXenematerials exhibit hydrophilic surface, good electronic conductivity,and chemical stability [25,33,55]. Considering the etching process,the chemistry of exfoliated MXenes are much closer to Ti3C2(OH)x-OyFz rather than the idealized structure of pure carbide layer Ti3C2.These hydrophilic functionalities (OH and O) enable MXenes toform strong connections with various semiconductors and watermolecules in aqueous solutions [97]. It can be also observed thatvarious MXene materials have small contact angles [33]. Theexposed terminal metal sites (for example, Ti, Nb, or V) on MXenesmight lead to much stronger redox reactivity, and the MXenes arestable in 1 M H2SO4 solution [49].

In particular, the properties of 2DMXenes are tuned by tailoringsurface functional groups in order to obtain special electrical (e.g.,band gap and conductivity) and electrochemical (e.g., pseudoca-pacitance) properties [29,72,98]. For example, MXenes are poten-tially usable as a promising, stable, and active non-precioushydrogen evolution reaction (HER) catalysts as suggested bystate-of-the-art density functional calculations [99]. Their resultsshow that the Gibbs free energy for adsorption of atomic hydrogenon the terminated O atoms (e.g., Ti2CO2) is close to the ideal value(0 eV), and the O/OH-terminated MXenes are the most stable

Fig. 10. (a) Scheme of a volcano plot between the numbers of electron surface O atoms gains (Ne) and the absolute value of the free energy of hydrogen adsorption (DGH). (b)The volcano curve of Gibbs free energies of hydrogen adsorption of MXene materials. (c) HER activity and stability of MXenes. Reprinted with the permission from Ref.[32,99,100]. Copyright 2016 American Chemical Society.

Fig. 11. SEM characterization of pristine Ti3AlC2 (a) and Ti3C2X2 (b); TEM images of Ru/Ti3C2X2 (c) and Ru/Ti3C2X2 (d) recycle five times. Reprinted with the permission fromRef. [115]. Copyright 2017 Elsevier. (e) Effect of catalyst loadings on the catalytic activity of Ru/Ti3C2X2. Reprinted with the permission from Ref. [105]. Copyright 2014Elsevier.


states. At the same time, all of them are metallic, thus favoringexcellent charge transfer. Another study also shows that Ti2CO2

and W2CO2 are highly active catalysts for HER [100]. In addition,Mo2CTx and Ti2CTx are experimentally shown to be a promisingHER catalyst [32] (Fig. 10).

3. MXenes for catalysis

As newly developed 2D materials, MXenes show attractivechemical properties in electrocatalysis and photocatalysis, becauseof their excellent electroactivity, durability, and ease of functional-


ization [101,102]. MXenes are however still not extensively stud-ied in these fields yet. Only a few catalytic reactions have beenrecently reported either theoretical or experimental studies withMXenes, including CO oxidation [28,103], dehydrogenation ofhydrogen storage materials [104–106], oxygen reduction reaction(ORR) [107,108], oxygen evolution reaction (OER) [102,109], andthe HER [32,99,110–112]. In these reactions, MXenes could act asthe catalyst, co-catalyst, or supporting substrate.

3.1. MXenes for CO oxidation

For CO oxidation, Zhou et al. introduced the single Ti atom-anchored Ti2CO2 (a typical MXene monolayer). Disclosed by first-principles computations, the high diffusion barriers indicate thatTi2CO2 substrate prevents Ti atoms from agglomeration. Theenergy barriers are even comparable to those of many noble metalcatalysts, which demonstrate that Ti2CO2 without any noble metalcould exhibit very high activity for CO oxidation. In addition, theCO oxidation process prefers the Eley–Rideal (ER) mechanism thanthe Langmuir–Hinshelwood (LH) mechanism [28,103].

3.2. MXenes for dehydrogenation of hydrogen storage materials

The high cycle stability, good reversibility, and low dehydro-genation temperature are most significant properties to evaluatethe catalysts for hydrogen storage materials [113]. Basically, theTi-based catalysts, particularly, those with Ti-O, Ti-F, and Ti-Cbonds are beneficial to a new dehydrogenation process [104–106,114]. Therefore, the MXenes with large amounts of such bondsare remarkably efficient for the dehydrogenation of solid-statehydrogen storage materials. For instance, Zhou et al., synthesizedthe nanoflower-shaped MXene Ti3C2(OHxF1�x)2 and their deriva-tive A0.9R0.1–TiO2/C (containing 90% anatase TiO2 and 10% rutileTiO2). These materials exhibit an unexpected catalytic activity forthe dehydrogenation of NaAlH4 with capacity of �3.08 wt% within85 min at 100 �C and 240 min at 90 �C at a stable rate, and keepstable even after ten cycles, which is the best Ti-based catalystfor the dehydrogenation of NaAlH4 reported so far [104]. In addi-tion, the MXenes was also used as supports for metal nanoparticlesin the catalytic dehydrogenation. Fan’s group loaded Ru nanoparti-cles with a mean particle size of 2.4 nm on Ti3C2X2 (Fig. 11) andthese composites easily catalyzed the hydrolysis of NaBH4 at roomtemperature without any additives, the best amount of catalystbeing 6.99 wt% with a high hydrogen generation rate of 59.04 L/gRu�min in a quantified study (Fig. 11(e)) [105].

3.3. MXenes as catalysts for oxygen reduction reaction (ORR)

ORR, OER, and HER are three fundamental reactions for renew-able energy conversion and storage. They are widely investigatedin water splitting, fuel cells, and metal–air batteries [116]. Recentadvance in these reactions involves the investigations of thereplacement of noble metal catalyst by earth-abundant elementsthat are low-cost and highly active. Hence, the MXenes that arecomposed of earth-abundant elements are potentially one of themost promising materials in this field.

ORR is an important cathodic reaction for fuel cells and themost widely used cathode catalysts are highly dispersed noblemetals or noble metal nanoparticles (NPs). Use of the MXene mate-rials, Ti3C2X2 (X = OH, F), as a support, has two advantages for thisreaction. Firstly, as a titanium-rich material, it possesses gooddurability under mechanical, acidic, and oxidative duress; sec-ondly, it is not just as good as carbon support that has effectiveconductive channels for electrons, but also it efficiently avoidsthe corrosion of the support which always happens in noblemetal/carbon catalyst [107,108]. For instance, Ag has relatively

high ORR activity and stability in alkaline electrolyte. Nonetheless,their performances reported are not satisfactory. One of promisingstrategies for further increasing catalytic performance is to preparea more conductive supporter coupled with some effective bimetal-lic catalysts. Peng’s group synthesized a series of MXene-Ag com-posites by directly mixing AgNO3 and alkalization intercalatedMXene (alk-MXene, Ti3C2(OH/ONa)2) solution containingpolyvinylpyrrolidone (PVP) at room temperature. This uniquealk-MXene-Ag0.9Ti0.1 nanowire with width of �42 ± 5 nm compos-ite shows good electrochemical activity toward ORR by enhancingthe conductivity, increasing abundant active sites and providing asynergistic effect. It possesses the best ORR activity with the onsetpotential (EORR) 0.921 V vs. RHE, the half-wave potential (E1/2)0.782 V vs. RHE at 1600 rpm, best structure stability and reversibil-ity after 1000 cycles, which is better than commercial Ag/C catalystand pure Ag nanowires reported previously (Fig. 12).

3.4. MXenes as catalysts for oxygen evolution reaction (OER)

OER is another important reaction related to the energy systemslike electrolytic/solar water splitting and rechargeable metal-airbatteries. OER may be considered as the opposite reaction ofORR. Thus, developing efficient electrocatalysts for OER is of greatsignificance [102,109]. MXenes are highly promising as electrocat-alysts because of their hydrophilic surface with remarkable electri-cal conductivity and stability. A recent report of the preparation ofhybrid film of overlapped g-C3N4 and Ti3C2 nanosheets (TCCN) isillustrated in Fig. 13. In this film, Ti3C2 is coupled with g-C3N4

through Ti-N interaction, forming a porous free-standing film,and exhibiting excellent catalytic performance in OER by deliver-ing a Ej=10 (10.0 mA cm2 current density) value of 1.65 V, whichis lower than that of IrO2/C (Ej=10 = 1.70 V), comparable or even bet-ter than those of the state-of the-art precious-metal catalysts. Thecatalytic nanosheets further show high stability with only a slightanodic current decrease after 10 h reaction, accompanying no mor-phology change.

3.5. MXenes as catalysts for hydrogen evolution reaction (HER)

HER is the last but not the least important reaction in energyconversion, because the hydrogen generation using suitable photo-catalysts or electrocatalysts would allow solving global energyproblem [32,99,110–112]. For instance, the MXene material Ti3C2

has an outstanding electrical conductivity that provides its excep-tional capability to transport electrons. Thus, it could be an idealco-catalyst for HER. Indeed, DFT calculations suggested that theTi3C2 retains its outstanding electrical conductivity, even withnumerous functionalities such as F, O groups that terminateTi3C2. Therefore, Ti3C2 were incorporated with CdS to give a highvisible light photocatalytic hydrogen production activity. Theseproperties are attributed to the favorable Fermi level position, elec-trical conductivity, and hydrogen evolution capacity of the Ti3C2

nanoparticles (Fig. 14). Moreover, Ti3C2 nanoparticles is also anefficient co-catalyst on ZnS or ZnxCd1�xS [110].

In 1972, Fujishima and Honda found that hydrogen was photo-electrochemically generated with n-type TiO2 and Pt electrodes.Since this pioneering work, photocatalyzed HER has widelyexpanded, and the MXenes are also recently involved in this area[117]. The interaction between the TiO2 and certain MXene mate-rials was shown to enhance photocatalytic HER compared to thatof pure rutile TiO2. For instance, Ti3C2Tx (Tx: surface terminationsO, OH, and F) in TiO2/Ti3C2Tx composites (Fig. 15) provides a 2Dplatform for intimate interactions with uniformly grown TiO2

nanoparticles and facilitates the migration and separation of pho-togenerated charge carriers. These features improve the photocat-alytic behavior of the TiO2/Ti3C2Tx (5 wt%) composites in the

Fig. 12. The preparation and electronic properties of MXene-Ag composites. Reprinted with the permission from Ref. [108]. Copyright 2016 American Chemical Society.

Fig. 13. Fabrication scheme of porous TCCN film is as shown. Reprinted with the permission from Ref. [102]. Copyright 2016Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


photocatalytic hydrogen evolution with a 400% enhancement com-pared with that of pure rutile TiO2. [118]. In addition, otherMXenes (Nb2CTx and Ti2CTx) were also incorporated with TiO2 asco-catalysts for enhanced the hydrogen production.

Additionally, theoretical calculations also predicted and guidedthe investigations of MXenes for photocatalysis. For instance,based on DFT and deformation potential theory, 2D Zr2CO2 andHf2CO2 are considered as promising photocatalysts for water

Fig. 14. Photocatalytic performance and spectroscopy/(photo)electrochemical characterization. (a) A comparison of the photocatalytic H2-production activities (Ti3C2 to CdSwith various mass ratios, Pt–CdS, NiS–CdS, Ni–CdS and MoS2–CdS). (b) Ultraviolet–visible diffuse reflectance spectra. (c) Time-resolved photoluminescence (PL) spectra. (d)Electrochemical impedance spectra (EIS) Nyquist plots of pristine CdS and Ti3C2 to CdS (2.5 wt%: 97.5 wt%,) electrodes measured under the open-circle potential and visiblelight irradiation. Reprinted with the permission from Ref. [110]. Copyright 2017 Nature Publishing Group.


splitting because i) they have a remarkable anisotropic carriermobility that effectively promotes the migration and separationof photogenerated electron-hole pairs; ii) they have a strong opti-cal absorption in both visible light and ultraviolet wavelengths; iii)they are stable in water based on ab initiomolecular dynamics sim-ulations [111]. Importantly, a recent report indicated that the O⁄

and OH⁄ terminated MXenes are most stable states and favorablefor the HER, since the oxygen atoms are the active sites for HER.DFT calculations suggested that the Gibbs free energies for theadsorption of atomic hydrogen (DGH⁄0) are close to 0 with oxygenterminated MXenes, which indicates the weak interaction betweenhydrogen atoms and the surface of MXenes and subsequent pro-motion of hydrogen release [99]. Similarly, Wang’s group con-ducted DFT studies on MXenes with a fully O-terminated surface,including 10 mono-metal in carbides and 7 bimetallic carbides.In their work, Ti2CO2 and W2CO2 are found to be highly active cat-alysts for HER. Furthermore, they set up a volcano plot between the

absolute value of the free energy of hydrogen adsorption |DGH| andthe number of electron surface O atom gains (Ne), which is usefulfor description of the relationship between the HER performanceand O-terminated MXenes (Fig. 6(a)). With this simple descriptor,TiVCO2 is extracted with improves the HER performances of Ti2CO2

and W2CO2 among 7 bimetallic carbides [100].In addition to the photocatalytic applications, MXene materials

are also good candidates as active and stable electrocatalysts forthe HER. Combined the computational methods after screening ofdifferent 2D layered M2XTx (M = metal; X = (C, N); and Tx = surfacefunctional groups) and experimental results, Vojvodic’s groupfound that Mo2CTx exhibits far higher HER activity with an initialoverpotential of (10 mA cm2 current density, Ej=10) 283 mV vsRHE than that of Ti2CTx (Ej=10 = 609 mV vs RHE). In addition, com-pared with MoS2, the basal plane of Mo2CTx is catalytically activefor the HER; however, only the edge sites of the 2 H phase inMoS2 are active [32].

Fig. 15. SEM images of (a) TiO2, (b) TiO2/Ti3C2Tx (5 wt%), (c) TiO2/Ti3C2Tx (50 wt%), and (d) Ti3C2Tx. Reprinted with the permission from Ref. [118]. Copyright 2016 Wiley-VCHVerlag GmbH & Co. KGaA, Weinheim.

Fig. 16. Schematic representation summarizing the organ-like structure of MXene-Ti3C2 nanomaterial encapsulating hemoglobin. Reprinted with the permission fromRef. [122]. Copyright 2015 The Electrochemical Society.


4. MXenes as sensors

As mentioned above, MXenes have the unique properties ofmetallic conductivity, hydrophilic surface, and 2D layered atomicstructure. These properties make MXenes promising candidatesin carriers, novel electronic materials, and parts of sensing materi-als for rapid, easy, and label-free detection.

4.1. MXenes for electrochemical biosensors

Direct electron transfer (DET) between an enzyme and an elec-trode is essential for the fabrication of mediator-free electrochem-ical biosensors. Due to the fact that the electroactive center isdeeply embedded inside the protein structure, however, DET pro-cess is difficult to distinguish between the protein and the elec-trode surface. Furthermore, the protective microenvironments forretaining the activity of protein are of prime importance. Therefore,

introducing MXene as an efficient pathway to facilitate the DETand retain the bioactivity of immobilized enzymes [119–121] isof great interest.

Zhu et al. [122] demonstrated that the new graphene-likeMXene Ti3C2 has the paralleled flake-like structure that showsgreat tendency to be organ-like curve nanolayers with one endclosed and the other opened, accessible for enzyme adsorption(Fig. 16). In brief, the enzyme could be easily entrapped by the sur-face functional groups of nanolayers and funnelled toward theinterior of the nanomaterials. These materials exhibit excellentmobility of charge carriers and make them superb media for poten-tial electrical communication between the protein and the elec-trode that is composed of MXene-Ti3C2/hemoglobin/nafion andglassy carbon electrode (GCE). The fabricated mediator-freebiosensors showed good performance for the detection of H2O2

with a wide linear range from 0.1 to 260 lM and a remarkablylow detection limit of 20 nM.

4.2. MXenes for H2O2 detection

Loading the TiO2 nanoparticles onto these materials offers addi-tional advantages for H2O2 detection [123]. On one hand, the depo-sition of TiO2 with great biocompatibility may provide shieldingmicroenvironments for the enzymes to retain their stability andactivity. On the other hand, TiO2-Ti3C2 notably enhances the activesurface area available for enzyme adsorption compared to Ti3C2.Some further study also disclosed that the efficiency of MXenesmodified GCE for H2O2 reduction and nitrite detection, and glucosedetection is at a low detection limits [124–126].

4.3. MXenes for gas sensing

MXenes are also involved in the fields of gas sensing, which is acritical area nowadays. Nonetheless, selective adsorption andreversible capture and release is an important burning issue forfuel cell application. A recent research indicated that the MXene-Ti2C nanolayers are good sensors and capturers for NH3. Since

Fig. 17. A schematic illustration of (a) side, and (b) top view of the adsorption of NH3, H2, CH4, CO, CO2, N2, NO2, or O2 molecule on monolayer Ti2CO2. Reprinted with thepermission from Ref. [127]. Copyright 2015 American Chemical Society.

Fig. 18. The adsorption energies and Mulliken/Hirshfeld charge transfer for theadsorption of gas molecules on monolayer Zr2CO2. Reprinted with the permissionfrom Ref. [128]. Copyright 2016 Elsevier.


the N atom of NH3 located directly above the Ti atom in Ti2CO2

exhibited the strongest binding energy compared to H2, CH4, CO,CO2, N2, NO2 (Fig. 17). The adsorbed NH3 on monolayer Ti2CO2

can be easily released by decreasing the applied biaxial strains,which is applicable in the fuel cell application. Meanwhile, theelectronic conductivity of Ti2CO2 was greatly increased after theadsorption of NH3 [127].

Subsequently, the mechanism of the reversible NH3 adsorptionon O-terminated MXene (M2CO2, M = Sc, Ti, Zr, and Hf) was alsoinvestigated [128]. First of all, NH3 was chemisorbed on M2CO2

with obvious charge transfer compared with other gases(Fig. 18). By utilizing electrons injected into M2CO2, the behaviorof NH3 adsorption could be varied from chemisorption tophysisorption, because the adsorption energy increased dramati-cally; thus, NH3 adsorption could be highly reversible. The results

highlight the fact that O-terminated semiconducting MXenes arepromising candidates as NH3 sensor and capturer.

Moreover, Chen et al. [40] studied the effect of CO2 and temper-ature as a smart factor to adjust the dispersion state through thechange of transmittance and conductive of V2C (Fig. 19). Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) was grafted on2D MXene vanadium carbide (V2C) through self-initiated pho-tografting and photopolymerization (SIPGP). Firstly, PDMAEMAchains are hydrophilic and expand when the temperature is belowthe lower critical solution temperature (LCST). Upon the increase oftemperature above the LCST, PDMAEMA chains become hydropho-bic and shrink, resulting in the improvement of the transmittance.Additionally, when CO2 was bubbled into the water to formcharged ammonium bicarbonate, the conductivity increasedapparently due to the CO2 responsive behavior. The analysis indi-cated the temperature and CO2 are stimuli to the V2C@PDMAEMAsystem by altering the transmittance and conductive activity.

4.4. MXenes for detection of macromolecules and cells

Besides small molecules and gases, the MXene materials arealso important for macromolecules and cells detection. Forinstance, Zhi’s group [129] prepared three different Ti3C2 MXeneQuantum dots (Ti3C2 MQD) through a facile hydrothermal methodat 100 �C (MQD-100), 120 �C (MQD-120), and 150 �C (MQD-150).These MQDs exhibited photoluminescence with quantum yieldsaround 10%. MQD-100 and MQD-120 were used as a cellular probein vitro bio-imaging for RAW264.7 cells. Shi’s group pioneered thedetection of neutral activity upon MXenes by fabricating ultrathinconductive MXene-micropattern-based Field-Effect Transistor(FET) device. The FET devices were used in detecting neurotrans-mitters dopamine through doping effect, provoked by the p–pinteraction between dopamine and the electrons from the terminalgroups (e.g., OH or F). Additionally, the device could also be used as

Fig. 19. Schematic representation of the preparation of smart V2C@PDMAEMA hybrid systems. The C atoms are denoted by off-white color, the V atoms are denoted by purplecolor, occupying every other elemental atomic plane, the Al atoms are denoted by blue color and occupy every third plane. For simplicity, only two layers of the V2C are shownhere. Reprinted with the permission from Ref. [40]. Copyright 2015 The Royal Society of Chemistry.


a real-time probe for cultured primary hippocampal neurons withexcellent biocompatibility even in long term culturing [130].

In summary, MXenes have been shown to be promising materi-als for the fabrication of sensors with wide potential applicationsin environmental analysis and biomedical detection for its conduc-tivity, hydrophilic surface, and biocompatibility.

5. MXenes for chemical adsorption

As 2D materials, MXenes have the advantages of large surfacearea and abundant active sites. They could thus be considered asideal adsorbents. Progress has been made in the application ofMXenes on adsorption through both theoretical and experimentalresearches.

5.1. Application of MXenes in adsorption of neutral gases

A seminal work on the adsorption of neutral gas by MXeneswas the investigation of MXene phases as hydrogen storagemedia by first-principles calculations [131]. Generally, threebonding modes of hydrogen storage are presented: chemisorp-tion of dissociated H atoms, physisorption of H2 molecules, andKubas-type binding of the H2 molecules with transition-metal(TM) atoms [132]. For chemisorption, the bonding betweenhydrogen atoms and host materials is too strong to releasehydrogen at moderate temperatures. On the other hand, thebinding strength is so weak for physisorption that hydrogenstorage can only operate around liquid nitrogen temperature.However, for Kubas-type interaction, the TM d orbitals hybridizewith the H2 r and r⁄ orbitals to form TM-H2 complexes, in

which the adsorption energy of H2 molecules is as low as theenergy of reversible hydrogen storage at room temperature andambient pressure. Thus, the Kubas-type hydrogen storage modeis a promising direction in this area. In order to meet the gravi-metric storage capacity target (5.5 wt% by 2015) set by Depart-ment of Energy of the United States (U.S. DOE), a material isrecalled with high specific surface area, light weight, no metal-clustering behavior, and Kubas-type hydrogen storage modebased on the above Ti2C. According to the first-principle totalenergy pseudo potential calculations, it was shown that H2

was adsorbed on both sides of the Ti2C layered structure. Themaximum hydrogen storage capacity was up to 8.6 wt%, whichwas contributed to three modes: chemisorption (1.7 wt%),physisorption (3.4 wt%), and Kubas-type interaction (3.4 wt%).Progressively, 2D MXene Sc2C have shown the highest gravimet-ric hydrogen storage capacities (9.0 wt%) using first principlestotal energy pseudo potential calculations, because of its highestsurface area per weight among all possible MXene phases [133].

Very recently, the hydrogen storage capacity of 2D MXene Cr2Cwas also investigated using DFT [134]. In this research, weak elec-trostatic interaction [135] was taken into account, which is usefulin the reversible hydrogen storage capacity at near ambient condi-tions such as Kubas interaction. The binding energy of weak elec-trostatic interaction was 0.26 eV with significant charge transferof 0.09 e from Cr to H atom, which indicated a reversible hydrogenstorage capacity at ambient conditions. Therefore, the total rever-sible hydrogen storage capacity under ambient conditions(through Kubas and weak electrostatic interactions) was up to6.4 wt%, which is better than that of Sc, Ti, and V-based MXenematerials.

Fig. 20. (i) Methane adsorption isotherms from 0 to 60 bar of Ti3C2 MXene prepared using various fluoride salts with HCl at 25 �C: (A1) LiF; (A2) NaF; (A3) KF; (A4) NH4F,respectively. (ii) Methane adsorption isotherms from 0 to 60 bar of Ti2C MXene prepared by different fluoride salts with HCl at 25 �C: (B1) LiF; (B2) NaF; (B3) KF; (B4) NH4F,respectively. Reprinted with the permission from Ref. [137]. Copyright 2017 Elsevier.


Additionally, the adsorption ability of various gases includingNH3, H2, CH4, CO, CO2, N2, NO2, and O2 on monolayer Ti2CO2 byfirst-principles simulations was reported [127]. Several typical

and possible adsorption sites for each gas molecule were consid-ered on the monolayer Ti2CO2, including the top sites over Ti, C,or O atom, and bridge sites between neighboring O and O, or Ti

Fig. 21. Removal mechanism of Cr(VI) by the Ti3C2Tx nanosheets. Reprinted withthe permission from Ref. [89]. Copyright 2015 American Chemical Society.


and O atoms. Besides, as mentioned above in Fig. 17, several typicalorientations of gas molecules with respect to the monolayer Ti2CO2

surface were also considered.The above gas adsorption performances on MXenes were

mainly based on theoretical studies. Recently, an experimentalstudy on CH4 adsorption on MXenes was reported by Liu et al.[136,137]. Two kinds of MXenes (Ti3C2 and Ti2C) have been pre-pared by etching Ti3AlC2 and Ti2AlC with various fluoride salts inHCl, including LiF, NaF, KF, and NH4F. The CH4 adsorption iso-therms of these as-prepared MXenes are shown in Fig. 20. TheCH4 adsorption capacity of all samples was enhanced with theincrease of pressure. However, among them only the MXene exfo-liated by NH4F and LiF could preserve large amount of methaneunder the normal pressure (A1, A4 and B1 and B4), whereasMXenes exfoliated by NaF and KF adsorb methane under high pres-sure and release it under low pressure (A2, A3 and B2 and B3). Fur-thermore, Ti2C-MXene had higher CH4 adsorption capacity thanTi3C2-MXene due to their larger theoretical specific surface area.Thus, the MXene prepared by NH4F and LiF may have an importantapplication in capturing CH4, while the others may be used in stor-age of CH4.

5.2. Application of MXenes in adsorption of cations

Since the discovery of MXenes, one of the most important appli-cation perspectives is in the area of energy storage, particularly inthe field of lithium ion batteries (LIBs). The remarkable propertiesof MXenes in the LIBs were largely provided by their excellentadsorption capacity of Li ions. As for Ti3C2, Li ions intercalate intothe space vacated by the Al atoms to form Ti3C2Li2, with Li ionscapacity of 320 mAh/g, which is comparable to the 372 mAh/g ofgraphite for LiC6 [29]. Additionally, F and/or OH groups are alwaysterminated on MXene layers in the etching process. Therefore,their influence on Li adsorption capacity was also investigated[138]. Briefly, the bare Ti3C2 monolayer exhibits a higher Li storagecapacity and a lower barrier for Li diffusion. On the other hand, thefunctional groups of F and OH on the surface of MXenes block Litransport and decrease Li storage capacity. External strain and Liconcentration on the adsorption of Li on Ti2C layer were also stud-ied by first-principles calculations [139]. The results demonstratedthat the binding energy of Li atoms decreases monotonically withexternal strains, whereas it is weakly dependent on the Liconcentration.

Another important application of MXenes lies on the adsorptionof heavy metal ions for environmental remediation because of theunique properties of MXenes. Firstly, titanium has high sorptionaffinity toward metallic ions, especially heavy metals. In addition,some ion-change sites can be obtained by cation substitution withhydroxyl groups on the exposed Ti surfaces of MXenes, whichmight provide preferential sorption of toxic metals. Finally, the lay-ered structure of the MXenes with large surface areas enhancessequestration of target metal-pollutants. Representatively, a 2Dalk-MXene (Ti3C2(OH/ONa)xF2�x) material, prepared by chemicalexfoliation followed by alkalization intercalation, exhibits excel-lent sorption behavior for toxic Pb(II) with the applied sorptioncapacities of 4500 kg water per alk-MXene, and produces treatedwater with Pb(II) contents below the drinking water standard rec-ommended by theWorld Health Organization (10 lg/L) [140]. Typ-ically, the alk-MXene shows very good selectivity of Pb(II) over thecoexistence of common cations such as Ca(II) and Mg(II), becauseof the strong affinities between Ti–O and Pb(II) ions that mightresult from inner-sphere complex formation of Pb onto the surfaceof alk-MXene. First-principles calculations disclosed that the func-tional groups on alk-MXene surfaces have great impacts on theadsorption property [141]. Basically, the ion adsorption efficiencyof alk-MXene decreases by the occupation of the F atom, while

increases with the intercalation of Li, Na, and K atoms. The abun-dance of hydroxyl sites in MXenes that are vertical to the titaniumatoms has stronger ability to adsorb the metal ion than other posi-tions. In addition, MXenes M2X(OH)2 in the series M = Sc, Ti, V, Cr,Zr, Nb, Mo, Hf, Ta, and X = C or N for Pb adsorption have been inves-tigated by first-principle calculations [142]. The results demon-strated that the presence of N is more effective for Pb(II)adsorption compared to C due to the lower formation energies ofM2N(O2H2�2xPbx) than those of M2C(O2H2�2xPbx). Hexavalent chro-mium Cr(VI), is another highly toxic metal that is widely used inindustrial processes. Research showed that the 2D Ti3C2Tx(T = OH and F) nanosheets exhibited unique reductive removal per-formance for toxic Cr(VI) from water [89]. The maximum removalcapacity was 250 mg/g by Ti3C2Tx nanosheets delaminated by 10%HF solution, and the residual concentration of Cr(VI) in treatedwater was less than 5 ppb, which is far below the drinking waterstandard recommended by the World Health Organization. Mech-anistic investigation revealed that Cr(VI) was firstly reduced to Cr(III) and subsequently adsorbed on the Ti3C2Tx surfaces (Fig. 21).The negatively charged Cr2O7

2� is firstly electrostatically attractedto the positively charged Ti3C2Tx surfaces due to the presence ofabundant protonated hydroxyl groups. Subsequently, the electronsare transferred from Ti3C2Tx to Cr(VI), and reduction reactions areinitiated. Finally, the produced Cr(III) are partly anchored on thesurface of the nanosheets via covalent bonds with [Ti–O] bond toform Ti–O–Cr(III). Very recently, the adsorption of barium (Ba)using Ti3C2 nanomaterials has also been reported [143]. The Ti3C2

nanosheet showed an outstanding removal capacity for Ba, withthe maximum adsorption capacity of 9.3 mg/g for an initial bariumconcentration of 55 ppm. This performance is superior to those ofother adsorbents such as activated carbons and carbon nanotubes.Additionally, the Ti3C2 MXene showed a good selectivity towardbarium removal over other coexisting metal ions such as As(III),Pb(II), Cr(IV), Ca(II), Sr(II), and Ba(II).

The modification of the raw MXene is sometimes necessary inorder to improve the capacities of adsorption. For instance, Liu’sgroup prepared a new urchin-like rutile TiO2-C/TiC 2D materialwith a high amount of (1 1 0) facets by in situ solvothermal alco-holysis of MXene in FeCl3 solution [144]. The Fe(III) ions in MXeneplayed an important role in transforming the intermediate productof anatase TiO2-C (1-ATC) into rutile TiO2-C (u-RTC), and the u-RTCdisplayed a higher Cr(VI) adsorption capacity than that of rawMXene. The improved adsorption capacity is possibly due to theinhibition of H2O adsorption by bridging oxo-groups according tothe first-principle calculation.

Nuclear waste pollution is another challenge for environmentand human health. Adsorption is considered to be a promisingway to remedy contamination. Based on their excellent perfor-mance on heavy metal adsorbing, 2D MXenes were applied for

Fig. 22. Snapshots of uranyl inner- and outer-sphere adsorption configurations on Ti3C2(OH)2 nanosheet. (a) TiO2-UO2(H2O)3, (b) TiO(OH)-UO2(H2O)3 and (c) Ti(OH)2-UO2(H2O)3 are uranyl bidentate adsorption configurations; (d) TiO-UO2(H2O)4 and (e) TiOH-UO2(H2O)4 are uranyl monodentate adsorption configurations; (f) [TiOH. . .UO2

(H2O)5]p and (g) [TiOH. . .UO2(H2O)5]v are uranyl outer-sphere adsorption configurations. Reprinted with the permission from Ref. [146]. Copyright 2016 Elsevier.

Fig. 23. (a) Time dependence of MB (C0 = 0.05 mg mL�1) and AB80 (C0 = 0.06 mg mL�1) concentrations in aqueous solutions with suspended Ti3C2Tx particles in the dark.Chemical structures of corresponding dyes are shown as insets. (b) Adsorption isotherms of MB on Ti3C2Tx. Reprinted with the permission from Ref. [88]. Copyright 2014 TheRoyal Society of Chemistry.


uranyl species adsorption from both theoretical and experimen-tal aspects [145]. One of the advantages lies in the fact that it ispossible to make radionuclide permanently fixed in the carbideceramics after high-temperature solid-state sintering of the 2DMXene ceramic materials. The first theoretical study on theadsorption of uranyl ions by hydroxylated titanium carbide Ti3-C2(OH)2 was carried out using DFT simulations [146]. Two inter-action mechanisms were considered, including inner-sphere andouter-sphere adsorption modes, in which chemical bondsbetween the U atom and the surface O or OH group were formedfor the first mode, whereas electrostatic interaction and hydro-gen bonding occurred for the second mode (Fig. 22). Fig. 22(a)–(c) represent the uranyl (UO2

2+) bidentate inner-sphereadsorption configurations with U binding two, one, or none ofthe deprotonated Os groups, respectively. Fig. 22(d) and (e) rep-resent the uranyl mono-dentate inner-sphere adsorption config-urations with U binding deprotonated Os groups and

protonated Os(H) groups, respectively. According to the resultsof the binding energy, the stability of the inner-sphere adsorp-tion configurations follows the order of Fig. 22(a) > 22(d) > 22(b) > 22(e) > 22(c). It is concluded that uranyl preferred to bindthe deprotonated Os adsorption site rather than the protonatedOs(H) adsorption site. Bidentate adsorption configuration prefersto form under high pH with surface OH groups that are alldeprotonated, whereas monodentate adsorption configurationprefers to form under low pH conditions with all surface OHgroups that remain protonated. For the most stable inner-sphere bidentate adsorption configuration (Fig. 22(a)), theadsorption capacity of the nanosheet for uranyl ions reaches595.3 mg/g. Fig. 22(f) and (g) represent the uranyl outer-sphereadsorption configurations with the axis of uranyl parallel andvertical to the Ti3C2(OH)2 nanosheet, respectively. The bindingenergy calculation indicates that the species of Fig. 22(g) isslightly more stable than that of Fig. 22(f).

Fig. 24. Simple synthesis steps and application for phosphate removal. Reprinted with the permission from Ref. [147]. Copyright 2016 The Royal Society of Chemistry.


Later, the same group reported the first experimental researchon using the multilayered V2CTx MXene as a potential and efficientadsorbent for uranium (U(VI)) capture from aqueous solutions.This material has the high U(VI) uptake capacity of 174 mg/g atpH 5.0, comparable with, or even higher than, conventional inor-ganic nanomaterials. DFT calculations suggested that the interac-tion between U(VI) and V2CTx follows an ion-exchangemechanism, which is a typical for a chemisorption process. Similarto Ti3C2(OH)2 nanosheet aforementioned, the most energeticallyfavorable adsorption configuration is the bidentate inner-spherecoordinated with U binding the deprotonated hydroxyl functional-ized V2C surface. Accordingly, the calculated maximum uptakecapacity of V2C(OH)2 nanosheets for uranium is 536 mg/g.

In accordance with the adsorption of metal cations, multilay-ered Ti3C2Tx was found to disclose efficient adsorption ability ofthe cationic dye methylene blue (MB) [88]. Interestingly, the Ti3C2-Tx nanosheets showed preferential adsorption of the cationic dyeMB over the anionic dye Acid Blue 80 (AB80) in the dark (Fig. 23(a)). This is primarily due to favorable electrostatic interactionsbetween the positive MB molecules and the negatively chargedMXene surfaces. The adsorption capacity of Ti3C2Tx for MB wasabout 39 mg/g, calculated from Langmuir isotherm (Fig. 23(b)).

5.3. Application of MXenes in adsorption of anions

By surface modification, 2D MXenes can also be applied to theadsorption of anions such as phosphate. For instance, a novel sand-wiched structural 2D MXene-iron oxide (MXI) material has beenprepared by selectively exfoliating the Al layer followed by mag-netic ferric oxide intercalation (Fig. 24) [147]. The as-preparedMXI nanocomposite consists of layered MXene, magnetite-Fe3O4,

and maghemite-Fe2O3. Among them, the ultrafine maghemite-Fe2O3 phase plays an important role in widening the layer distanceand increasing the number of available activated Ti-OH layers byintercalation into the interior layers of MXene. Indeed, themagnetite-Fe3O4 phase distributed on the surface of the MXeneand the gap between the multiple layers of MXene provides a highmagnetic separation force. The resultant MXI nanocomposite exhi-bits superior treatment capacities of 2400 kg/kg in real phosphatewastewater applications. Additionally, the sorption selectivity ofphosphate from common competitive anions (such as SO4

2�, Cl�,and NO3

�) were evaluated. Compared with the commercialArsenXnp (Purolite UK) for highly selective capture of phosphate/arsenate and the typical anion-exchange resin D201, the obtainedMXI displays a higher sorption capacity and remarkable selectivitywith high ionic strengths. Fourier transform infrared spectroscopy(FT-IR) analysis suggested the presence of strong specific affinitiesbetween the Fe-Ti hydroxides and phosphates. X-ray photoelec-tron spectroscopy (XPS) results demonstrated the synergeticadsorption enhancement toward phosphate in water. Thus, theefficient sequestration of phosphates was ascribed to the formationof unique nano-ferric oxide morphology. The factors include thewide layer distance and abundant activated layers by the interca-lation of ultrafine nano-Fe2O3, the strong complexation of phos-phate onto the embedded magnetic nano-Fe3O4 with a uniquesandwich-structure, as well as the stimulated Ti–O terminal withinMXene.

6. Conclusion and outlook

Since the first synthesis by Naguib et al. of Ti3C2 in 2011 byselectively etching the Al atoms in layered hexagonal ternary


carbide Ti3AlC2 with hydrofluoric acid, over 60 kinds of MXeneshave been reported up to now by the selective chemical etchingof ‘‘A” in ‘‘MAX” phases. Generally the ‘‘MAX” phases have a for-mula of Mn+1AXn, where ‘‘M” is an early d transition metal, ‘‘A” isthe main group sp-element, and ‘‘X” is C and/or N. The challengeherewith is the chemical functionalization of the MXene surfacein order to be able to achieve chemical applications including forinstance catalysis, sensing, and adsorption. Meanwhile it couldbe expanding this research field in the future, although the floodof results in these few years is remarkably promising. In catalysis,the environmentally and energetically most important reactions(ORR, OER, and HER) have already been successfully addressed inthe first reports, and the demand is still exceptionally high in thisarea. Other very crucial features include the surface ionization forsensing and adsorption of substrates by ion pairing toward an ele-gant and useful surface supramolecular chemistry.

Acknowledgements

Financial support from the National Natural Science Foundationof China (21507117, 21601166), the China Academy of EngineeringPhysics (PY201479, PY2014710), Discipline Development Founda-tion of Science and Technology on Surface Physics and ChemistryLaboratory (XKFZ201505, XKFZ201506), and foundation of Insti-tute of Materials, China Academy of Engineering Physics(TP20160208) are gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.ccr.2017.09.012.

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