Recent progress and advances in the environmental applications of MXene related materials

Junyu Chen a, Qiang Huang a, Hongye Huang a, Liucheng Mao a, Meiying Liu *a, Xiaoyong Zhang *a and Yen Wei *bc
aCollege of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China. E-mail: lmy5305@iccas.ac.cn; zhangxiaoyong@ncu.edu.cn
bDepartment of Chemistry and the Tsinghua Center for Frontier Polymer Research, Tsinghua University, Beijing, 100084, P. R. China. E-mail: weiyen@tsinghua.edu.cn
cDepartment of Chemistry and Center for Nanotechnology and Institute of Biomedical Technology, Chung-Yuan Christian University, Chung-Li 32023, Taiwan

Received 5th October 2019 , Accepted 19th December 2019

First published on 4th February 2020


MXenes are a new type of two-dimensional (2D) transition metal carbide or carbonitride material with a 2D structure similar to graphene. The general formula of MXenes is Mn+1XnTx, in which M is an early transition metal element, X represents carbon, nitrogen and boron, and T is a surface oxygen-containing or fluorine-containing group. These novel 2D materials possess a unique 2D layered structure, large specific surface area, good conductivity, stability, and mechanical properties. Benefitting from these properties, MXenes have received increasing attention and emerged as new substrate materials for exploration of various applications including, energy storage and conversion, photothermal treatment, drug delivery, environmental adsorption and catalytic degradation. The progress on various applications of MXene-based materials has been reviewed; while only a few of them covered environmental remediation, surface modification of MXenes has never been highlighted. In this review, we highlight recent advances and achievements in surface modification and environmental applications (such as environmental adsorption and catalytic degradation) of MXene-based materials. The current studies on the biocompatibility and toxicity of MXenes and related materials are summarized in the following sections. The challenges and future directions of the environmental applications of MXene-based materials are also discussed and highlighted.


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Junyu Chen

Junyu Chen was born in Jiangxi, China, in 1994. In 2015, he joined the group of Professor Xiaoyong Zhang and obtained his B.S. (2017) from the Department of Chemistry, Nanchang University. He is now a master's student under the supervision of Professor Xiaoyong Zhang. At present, he is a joint student in the group of Professor Yen Wei, Tsinghua University. His research is focused on surface modification by photo-initiated CRP and mussel-inspired chemistry and their biomedical applications.

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Hongye Huang

Hongye Huang was born in Jiangxi, China, in 1995. He received his B.S. (2017) in the School of Materials Science and Engineering from Nanchang University. He is now studying at the same institute under the supervision of Professor Naigen Zhou and Professor Yen Wei. His research interests mainly focus on carbon nanomaterials and AIE-based materials and their biomedical applications.

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Meiying Liu

Dr Meiying Liu was born in Jiangxi, China, in 1981. She received her B.S. (2004) from the Medical College of Nanchang University and her PhD (2011) from Shanghai Institute of Micro-system and Information Technology (SIMIT), Chinese Academy of Sciences (CAS). After a postdoc at the Institute of Chemistry, CAS, with Professor Lei Jiang, she took a position at Nanchang University as an associate professor. She has published over 200 papers with total citations over 7100 and an h-index of 47. Her research is focused on nanobiosensors, bio-inspired smart nanochannels, mussel-inspired chemistry, and AIE-based materials and their biomedical applications.

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Xiaoyong Zhang

Dr Xiaoyong Zhang was born in Jiangxi, China, in 1980. He received his B.S. (2004) from Wuhan University of Science and Technology and his Ph.D. (2011) from Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences (CAS). After postdoctoral research at the Department of Chemistry, Tsinghua University with Professor Yen Wei, he took a position at Nanchang University as an associate professor. He has published more than 290 papers, with over 12[thin space (1/6-em)]000 total citations, and has an h-index of 57. His research is focused on AIE-based materials and their biomedical applications, mussel-inspired chemistry, and carbon nanomaterials.

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Yen Wei

Dr Yen Wei is a Chair Professor of Chemistry and Director of the Tsinghua Center for Frontier Polymer Research at Tsinghua University in China. He received his undergraduate diploma (1979) and MS (1981) from Peking University, and he obtained his PhD from the City University of New York (1986). After postdoctoral work at MIT, he joined Drexel University in 1987, where he became Full Professor in 1995. He has coauthored more than 1070 articles, with over 37[thin space (1/6-em)]000 citations, and has an h-index of 91. He joined Tsinghua University in November 2009, and his current research focuses on polymers and nano-materials for bioscience, biomedicine and energy technology.


1. Introduction

With the continuous development of the economy and industry, the ecological environment has been seriously damaged and there is serious water pollution. The protection of water resources and the treatment of water pollution have become the biggest concerns of modern society.1,2 Pollutants in environmental water could be divided into inorganic and organic compounds. Among them, toxic heavy metals and organic dyestuffs have become the main sources of wastewater as a result of industrial production and household waste.3,4 Besides, chemicals like radioactive elements, antibiotic residues and waste gas also pose a threat to human health.5,6 All these pollutants may cause irreversible damage to organisms even at a trace concentration. For overcoming severe environmental issues, many techniques have been developed for the removal of these pollutants, mainly including the following: adsorption, ion exchange, filtration, sedimentation, electrochemical approaches, catalytic degradation, chemical oxidation, etc.7–21 Adsorption is now considered to be the most economical and effective method in wastewater treatment for its low cost, high availability and low operating requirements.22,23 Besides, in order to achieve a better processing effect, adsorption also offers diversification in adsorbent design. In the process of adsorption, electrostatic force, ion exchange and chemical binding are the main interaction forces between absorbents and pollutant molecules. Due to its versatile properties, activated carbon is the most widely studied adsorbent in water pretreatment, while its high cost and complicated preparation process limit its application in further research.24 Thus, searching for a more suitable material for environmental remediation is still an urgent task for all researchers.

Since the discovery of graphene in 2004, a new research field of 2D-nanomaterials has opened.25 Advances in graphene applications have opened a new avenue of research of 2D-materials. Graphene-like 2D-materials such as transition metal dichalcogenides, BN nano-sheets, silicene, and layered transition metal oxides or hydroxides were discovered in succession and attracted tremendous attention due to their unique physicochemical properties like facile tailored surface chemistry and high specific surface area.26–29 Following these graphene-like nanomaterials, a new family of 2D-materials named MXenes were discovered by selective etching of the interlayer Al atoms of a transition metal carbide Ti3AlC2 in 2011.30,31 Generally, MXenes are transition metal carbides, nitrides or borides with a formula of Mn+1XnTx, where M represents a transition metal, X represents carbon, nitrogen or boron, and T is a surface functionality terminated group (including –F, –OH, –O, etc.).32,33 MXenes have been used in many fields such as high-efficiency catalysts, separation membranes, conductive films, electromagnetic adsorption, biological sensors and labels, high capacity battery solar cells and photo-luminescent LED devices.18,34–47 Analysis shows that publications of MXenes have been showing an increasing trend year by year (Fig. 1A). In addition to the advantages of other traditional 2D-materials, MXenes, with the inherent properties of high surface activity, hydrophilicity, chemical stability and biological compatibility, are promising support materials to satisfy the above-mentioned requirements in environmental remediation.48 Most recently, effort has been devoted to exploring the applications of MXenes in environmental adsorption and environmental degradation, as witnessed by the rapidly increased publications and citations on this topic (Fig. 1B). It has been used as an efficient absorbent and separation membrane for removing environmental pollutants, including dyes, metals ions, and even gases.49,50


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Fig. 1 (A) Summary of publications of MXenes and MXenes in environmental applications from 2012 to 2019, (B) summary of citations of MXenes and MXenes in environmental applications from 2012 to 2019. Results are obtained via Web Of Science, Nov., 2019.

It is noteworthy that MXenes are very young but extensively studied 2D-materials, and their applications in environmental remediation are being further investigated. On the basis of a literature investigation, some reviews have summarized the progress in the synthesis and applications of MXenes, and most of them mainly covered energy storage and biomedical applications, and only a few of them refer to environmental remediation.51–53 There are also many reports on the structure and surface properties of MXenes, but there are relatively few studies on surface modification. In this review, we first summarized and discussed some common surface modification methods of MXenes and their utilization for the fabrication of MXene-based materials. Recent advances in the environmental applications of MXenes and MXene-based composites including organic, inorganic compounds, waste gas adsorption, and the catalytic degradation of chemicals will be discussed in detail. The toxicity of MXenes and their further investigation in environmental remediation will be summarized and discussed.

2. Surface properties and modification of MXenes

The MAX phase is a kind of ternary ceramic material. Its chemical composition can be expressed as Mn+1AXn, where M represents a transition metal element (such as Ti, V, Nb, Ta, Sc, Zr, Hf) and A represents a third group or a fourth group element such as Al, Ga, Si or Ge, etc.54,55 X represents C, N and B, and n = 1, 2, 3 (M2AX, M3AX2, M4AX3, etc.) (Fig. 2A).56 Depending on the value of n, MAX can be divided into 211, 312, and 413 phases. At present, more than 70 kinds of MAX phases have been reported, and more than 20 kinds of derived MXenes have been synthesized and studied. The MAX phase has a layered hexagonal structure in which the X atoms are filled in the octahedron formed by the metal M atoms, and the MX layer is alternately arranged with the A atomic layer.57 With a covalent bond between M and X and a metallic bond between M and A, the MAX phase exhibits very stable chemical properties and high mechanical strength. However, the bond between M and A is relatively weak compared with the M–X bond. When treating MAX with HF, the bond between M and A was cleaved more easily than that between M and X, thus the layer A compound can be selectively etched, as presented in Fig. 2B.58 The surface of the obtained Mn+1Xn was subsequently substituted by T groups (oxygen or fluorine-containing groups, including –H, –O, –OH, and –F.).59 After the process of etching, the interaction between MXene (Mn+1XnTx) layers was greatly weakened, which allowed them to be easily exfoliated. Usually, the surface properties of MXenes could be changed and enhanced by controlling the ratio and type of T group in the synthesis process, which could be artificially manipulated to –Cl, –Br, –OCl, –OBr, –CN, etc.60 With all possible combinations of three components (M, X, and T), more than 23[thin space (1/6-em)]000 results can be enumerated, as illustrated in Fig. 2C. However, limitations of MXene functionalization exist by merely changing the chemical environment, and it is difficult to impart new properties to them without post-modification by using various functional molecules. Further investigation on the surface modification of MXenes based on post-modification should be conducted to meet the performance requirements of their applications in various fields.
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Fig. 2 (A) Atomistic structures of six different crystalline MAX phases. Reprinted with permission.56 Copyright 2018, The Royal Society of Chemistry. (B) Structure of MAX phases and the corresponding MXenes. Reprinted with permission.58 Copyright 2014, WILEY-VCH. (C) MXene composition. With 11 early transition metals M (blue) of IIIB to VIB groups; X (B, C, N) (colored in yellow, boron was highlighted with red artificially) and 14 surface functional groups T/T′ (pink); a pool of 23[thin space (1/6-em)]870 MXenes are generated. Reprinted with permission.60 Copyright 2018, American Chemical Society.

Similar to 2D materials, the post-modification of graphene-like materials with organics has gained great achievement.27,61–64 Effective strategies have been employed including non-covalent and covalent modifications, wherein non-covalent modification is realized by a combination of electrostatic attraction, hydrogen bonding, and van der Waals forces.65,66 Covalent surface modification could be divided into three main chemical methods:67–69 (1) small molecules such as organic amines, epoxy compounds, acid halide or acid anhydride were used for bonding with the surface oxygen-containing groups; (2) polymer surface grafting via a “grafting onto” method; polymers with functional groups terminated are covalently attached to the surface of 2D materials to prepare multifunctional composites; (3) surface-initiated polymerization based on “grafting from” strategies; these methods with highly controllable polymerization procedures have become the well-received ways for the surface modification of 2D materials, and the stability of MXenes in aqueous solution may be greatly enhanced via polymer modification. Abundant oxygen containing groups on the surface of MXenes could be used as active sites for covalent modification using small molecules, surface-active initiators, and even polymers.70–84 As mentioned above, taking advantage of surface chemical modifications, the intrinsic properties of 2D MXene materials can be engineered to a large extent.

Generally, MXenes are hydrophilic due to their abundant oxygen terminal groups, hence surface modification with appropriate functional groups is easy to enrich their performance for pollutant remediation. Based on the literature, many studies were conducted on the surface chemical modification of MXenes using functional small molecules and polymers, including silane coupling agents, epoxy compounds, biological macromolecules and various polymers.85 Abundant hydroxyl groups could serve as active sites for covalent binding, for instance, as demonstrated in Fig. 3A, Kumar et al. have successfully introduced amino groups on the surface of an MXene layer via chemical coupling.43 The resultant amino-modified MXenes could serve as an excellent platform for facile immobilization of various biomolecules, including bio-receptors, enzymes, amino acids, aptamers, DNA, etc. Thus bio-detection, bio-catalysis and bio-sensing based on multifunctional MXene composites would be easily realized. Besides, Wang et al. conducted an effective reaction for large-scale delamination of MXenes by introducing diazonium surface chemistry.86 Typically, sulfanilic acid diazonium salts form strong aryl-surface linkages, resulting in expanded interlayer space and weakening the bonds between MX layers. Therefore, the multilayer of MXenes was easily delaminated via mild sonication. The same group performed surface modification via in situ functionalization of Ti3C2 with diazonium ions as shown in Fig. 3B.87 Covalently grafted Ti3C2 with –C6H4SO3H groups was proved to have enhanced electrochemical properties, which indicated that surface chemical modification is of great value in improving the performance of MXenes. Besides silane agents, epichlorohydrin and trimesoyl chloride were also used to conduct etherification and esterification reactions with hydroxyl groups on the surface of MXenes.39,88Fig. 3C shows a process to form MXene/cellulose hydrogels; epichlorohydrin was first covalently bonded with the hydroxyl groups of MXenes, serving as the cross-linking agent between cellulose and MXenes. This indicates that the hydroxyl groups on the MX surface could perform various chemical reactions like other materials with active hydroxyl sites.


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Fig. 3 (A) Schematic illustration of Ti3C2–MXene functionalization. Reprinted with permission.43 Copyright 2018, Elsevier. (B) Schematic representation of diazonium ion functionalization for the MXene Ti3C2. Reprinted with permission.87 Copyright 2018, Elsevier. (C) Schematic illustrating the fabrication process of MXene/cellulose aerogel composites. Reprinted with permission.39 Copyright 2018, The Royal Society of Chemistry.

The modification of MXenes using polymers has proven to be a versatile method to expand their application fields. By introducing functional polymers, the performance of the obtained MXene-based composites would be diversified and could be preconceived. As mentioned above, the surface modification of MXenes using polymers has been focused on two methods: non-covalent and covalent modifications. Currently, the surface modification of MXenes using polymers usually involves electrostatic attraction, hydrogen-bond interaction, physical absorption, etc. For example, pyrrole and EDOT are monomers of typical conductive polymers, which have been employed for in situ polymerization on the surface of Ti3C2Tx.89,90 MXenes in this process served both as catalysts and substrates with a large surface area for polymerization and alignment of the polymer chain. In the PPy/Ti3C2Tx composite, the interaction between the polymer chain and MXenes originated from the hydrogen bond formed between the N and H of PPy and oxygen groups on the surface of Ti3C2Tx. Furthermore, biomacromolecules are also introduced for the surface modification of MXenes owing to their biocompatible properties. Among these, a soybean phospholipid (SP) is a widely used economical and efficient molecule for surface modification, which has been reported to be suitable for the decoration of MXenes by Lin et al.91 The schematic of the surface modification of Ti3C2 is shown in Fig. 4A. SP modified Ti3C2 was found to have enhanced biodistribution and circulation in physiological environments. Additionally, poly(ethylene glycol) (PEG) also showed admirable performance in strengthening the stability of MXenes in physiological solutions.92


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Fig. 4 (A) Schematics of the surface modification of exfoliated Ti3C2 nanosheets modified with the soybean phospholipid (Ti3C2–SP). Reprinted with permission.91 Copyright 2016, American Chemical Society. (B) Schematic representation of preparing V2C@PDMAEMA smart hybrid systems. Reprinted with permission.95 Copyright 2015, The Royal Society of Chemistry. (C) Schematic illustration of p-Ti3C2Tx-initiated polymerization and subsequent gelation. Reprinted with permission.96 Copyright 2019, The Royal Society of Chemistry.

Compared with non-covalent modification, covalent modification using polymers could endow MXenes with more stable properties and multiple functions. Although there are relatively few studies on this topic, some significant progress has been made. Polydopamine (PDA) is rich in catechol and amino groups, and this structure allows it to interact covalently and non-covalently with organic and inorganic surfaces; much progress has been made in PDA modified materials.67 Recently, PDA was chosen as a surface modification agent for improving the structural stability of MXenes.93 After a layer of PDA was formed on the surface of Ti3C2Tx, the cobalt catalyst was absorbed on the layers and carbon nanotubes were formed to obtain Ti3C2Tx@CNT 3D composites. In addition, Wang et al. also modified Ti3C2Tx with polydopamine to enhance its capacitive performance.94 Moreover, benefiting from the abundant surface active sites on their surface, PDA-modified MXenes are promising to play a role in many other fields, including environmental adsorption and biomedical application. Surface-initiated free radical polymerization has also been studied as an effective way for the modification of MXenes. For example, self-initiated photo-grafting and photo-polymerization (SIPGP) was employed for the preparation of “smart” MXene polymeric composites with CO2 and temperature response properties by Chen et al.95 Typically, as demonstrated in Fig. 4B, the hydroxyl groups on the surface of V2C were used as photoactive sites for the surface-initiated polymerization of PDMAEMA. Most recently, Tao et al. synthesized a novel MXene material with peroxide decoration, which served as a hydroxyl radical provider for initiating the surface polymerization of various monomers.96 This process is very effective with strong universality for acrylic monomers, as shown in Fig. 4C. By simply mixing the monomer with an MXene suspension, polymerization occurs after nitrogen was added for minutes without adding any co-initiator or external energy support. Characterization results were as expected, indicating that surface-initiated polymer modification is a promising method for the preparation of functional MXene-based composites. Inspired by this work, other controllable radical polymerization methods such as ATRP, RAFT, and SET-LRP could also be applied in the preparation of MXene-based composites. Moreover, the polymer surface modification of MXenes has a very broad prospect in environmental remediation. Using the method of surface modification mentioned above, MXenes can be facilely functionalized and endowed with non-native properties, paving the bright way to applications.

3. Application in chemical adsorption

3.1. Adsorption of organic compounds

Dyes, as one of the most threatening organic pollutants in water, have caused enormous harm to human society and the natural environment. Effective removal of dyes from wastewater is an urgent task for all researchers. Owing to their high specific surface area, layered structure and negatively charged surface, MXenes have already been used as novel adsorbents for the removal of cationic dyes from aqueous solutions.97–100 For example, the cationic and anionic dye adsorption performance of the synthesized h-Ti3C2 was evaluated.98 The results show that cationic dyes (MB) could be adsorbed effectively, while the anionic dye (MO) could hardly be adsorbed by MXenes, indicating that MXenes are prone to adsorb cationic dyes for their negatively charged surface. The MB adsorption behavior of alkali-treated MXenes was reported for the first time by Wei et al.100 In their work, the adsorption performance of pristine-Ti3C2Tx and three types of alkali-treated Ti3C2Tx was demonstrated. Results revealed that alkali-treated Ti3C2Tx shows a better adsorption performance than pristine-Ti3C2Tx. The adsorption mechanism was also concluded as monolayer coverage based on the Langmuir isotherm study. Typically, NaOH-treated Ti3C2Tx shows a comparable adsorption capacity (189 mg g−1) to other 2D-materials, which indicated that alkali-treated Ti3C2Tx is a competitive material for MB adsorption. Surface modification by polymers always enhances the surface chemical properties of materials and even endows them with novel properties. Polymer functionalized MXenes for dye adsorption have also been reported, and carboxyl functionalized MXene-COOH@(PEI/PAA) nanocomposites were fabricated by a layer-by-layer method.97 The negatively charged surface of MXenes has been enhanced by introducing abundant carboxyl groups on their surface. In order to make a comparison between pristine MXenes and MXene-COOH@(PEI/PAA) nanocomposites, the removal of safranine T (ST), methylene blue (MB), and neutral red (NR) dyes was evaluated. Adsorption results show that the efficient removal of these dyes has significantly increased after the modification. Notably, this polymer modification should be a facile method for improving the adsorption properties of pristine MXenes, suggesting that surface modification is of great importance in MXene adsorption application.

Similar to other 2D-materials, the delaminated structure makes MXenes intercalated with organic molecules, including urea, dimethyl sulfoxide, isopropylamine, tetrabutylammonium hydroxide (TBAOH), choline hydroxide, n-butylamine and cationic dyes.101–104 This indicated that MXenes could be used as efficient adsorbents for these organics; adsorption of urea in aqueous solution has also been investigated.104 This work evaluated the effect of the MXene structure on the adsorption of urea; three types of MXenes were studied, including Ti3C2Tx, Ti2CTx, and Mo2TiC2Tx. Fig. 5C shows the schematic of urea molecules intercalated with MXenes. The macroscopic and microscopic morphologies of the as-synthesized MXenes are shown and layered MXene sheets could be clearly identified through the SEM image. Experimentally, urea removal was quantitatively analyzed, and the efficiency of urea adsorption significantly increased with the increase in the loading mass of MXenes. Besides, Ti3C2Tx removed over 98% of urea from aqueous solution within 4 minutes, showing the highest urea adsorption capacity of 9.7 mg g−1 (Fig. 5E). Analysis results revealed that the formation of an H-bond between the interlayer oxygen-containing groups and urea molecules was the main driving force for the adsorption process to take place. Furthermore, no obvious cytotoxicity was observed through the assessment of Ti3C2Tx biocompatibility by using 3T3 fibroblast cells, suggesting that MXenes could be used as biocompatible adsorbents for efficient removal of urea from aqueous solution.


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Fig. 5 (A) Digital photograph of ∼7 g of Ti3C2Tx powder in a 20 mL glass vial. (B) Scanning electron microscopy image of the Ti3C2Tx powder. (C) Schematic of Ti3C2Tx MXene powder intercalated with urea molecules. (D) Schematic of MXene nanosheets used as the adsorbent. (E) Experimental data of the adsorption of urea by MXenes. Reprinted with permission.104 Copyright 2018, American Chemical Society.

3.2. Adsorption of inorganic ions/compounds

Unlike most organic pollutants, heavy metal ions do not degrade in living organisms, but accumulate over time, which causes greater damage to organisms. Therefore, it is very urgent to remove heavy metal ions from natural water.105–107 MXenes and their derivatives with abundant oxygen-containing groups and large surface areas have attracted tremendous attention as desirable adsorbents for heavy metal cation removal.108,109 Significantly, research indicates that transition metal elements contained in MXenes have a strong adsorption affinity for various heavy metal ions.110–112 Recently, Peng et al. reported that an alk-MXene phase (Ti3C2(OH/ONa)xF2−x) with alkalization intercalation shows unique adsorption performance toward Pb(II).111 A series of batch adsorption experiments revealed that alk-MXenes possess remarkable lead adsorption capacity (140 mg g−1), typically, the maximum theoretical adsorption capacity is nearly 2800 mg g−1. Besides, the desired selective adsorption properties of Pb(II) were confirmed. The results show that the removal efficiency of Pb(II) was barely affected by its coexistence with common competing cations, including Mg(II) and Ca(II). These results demonstrated that MXenes have great potential for practical use in Pb(II) removal from aqueous solution. Inspired by this work, continuous theoretical research studies on the adsorption mechanism of Pb(II) were reported based on the study of the first-principles calculations.113 Besides Pb(II), other metal ions like Cu(II), Zn(II), and Cd(II) also show low reaction formation energies with MXenes, ranging from −1.0 to −3.3 eV, indicating their excellent heavy metal removal ability. Based on the study of density functional theory (DFT), adsorption of metal ions is closely related to the formation of a potential hydroxyl trap in the adsorption process, and the hydroxyls and oxygen atoms on the surface of MXenes provide enough electron pairs to bond with heavy metal atoms. Therefore, a stable bond between MXenes and Pb including Pb–O was formed. In addition, further theoretical investigation revealed that nearly all MXenes except for Sc2C(OH)2 and Zr2C(OH)2 have the ability for Pb adsorption. Besides, both the kinetics study and adsorption efficiency suggested that Ti2C(OH)2 is the most promising MXene material for heavy metal removal.

Apart from physisorption and chemical bonding, the reducibility of MXenes also plays an important role in the adsorption of heavy metal ions, especially for high valent oxidizing metal ions, i.e., Cr(VI). Recently, Ying et al. found that the typical MXene materials such as Ti3C2Tx nanosheets demonstrated excellent adsorption performance for Cr(VI).114 At pH 2.0, Cr(VI) anions were firstly adsorbed on the surface of protonated nanosheets, and as pH increased, due to the strong reducing properties of nanosheets, the adsorbed Cr(VI) began to reduce to Cr(III) through electron transfer. Theoretically, the generated Cr(OH)3 would precipitate completely at pH 5.6, while Cr(III) is almost completely eliminated at pH 5.0, suggesting that a part of Cr(III) was absorbed on Ti3C2Tx nanosheets to form a Ti–O–Cr(III) bond. Compared with other adsorbing materials, the competitive adsorption capacity of Ti3C2Tx was achieved (250 mg g−1) and the content of Cr(VI) in treated water was found to reach the standard of the WHO. Furthermore, the desired effect was achieved by adsorption evaluation of K3[Fe(CN)6], KMnO4 and NaAuCl4, and the results showed that MXene-based materials could potentially be used for the removal of high valent oxidizing metal ions. MXene derivatives were also synthesized for the adsorption of Cr(VI). For example, by controlling the prior growth of some lattice planes, urchin-like rutile TiO2–C/TiC nanocomposites with a high amount of (110) facets were fabricated.115 The size and urchin-like morphology of the nanocomposites were confirmed by TEM and FESEM. Compared with the low adsorption ability of pristine MXenes (about 62 mg g−1), these urchin-like rutile TiO2–C/TiC nanocomposites display a high Cr(VI) removal performance (over 225 mg g−1). Experiments show that the excellent Cr(VI) selective adsorption ability of these nanocomposites was maintained even when general anions coexisted in wastewater. All results indicated that the derivatives of MXenes may also show great potential in the removal of heavy metal ions.

High-valent halogen ions such as BrO3 are potential carcinogens that are widely spread in natural water and pose a great threat to the health of organisms. In the common water treatment process, BrO3 could reduce to a low-valent and less toxic bromide ion (Br) by reduction treatment. The reducibility of MXenes may satisfy this need. Recently, Ti3C2Tx was firstly reported for the efficient reduction of BrO3 to Br in aqueous solution.116 The reducing properties of MXenes towards BrO3 were evaluated; Fig. 6(A) shows that the reduction capacity increases with the increase in the Ti3C2Tx concentration until it reaches up to 15 mg L−1 and the removal capacity finally reaches 321.8 mg g−1 in 50 min. Fig. 6(D) reveals the XRD pattern before and after reduction; the formation of TiO2 and amorphous carbon was confirmed in the reduction process, indicating that the reduction plays a leading role after the adsorption on the surface of Ti3C2Tx.


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Fig. 6 (A) Effect of the concentration of Ti3C2Tx on the reduction of BrO3. (B) Reduction (%) of different concentrations of bromate using 15 mg L−1 Ti3C2Tx at pH 7 and 25 °C. (C) Effect of coexisting anions on the reduction of bromate by Ti3C2Tx. (D) XRD pattern of Ti3C2Tx before and after the reduction of BrO3 to Br. Reprinted with permission.116 Copyright 2018, American Chemical Society.

To date, Pb(II) and Cr(VI) are the most deeply investigated heavy metal ions in the environmental applications of MXenes. Due to the remarkable adsorption performance reported earlier, the detection and adsorption studies of other heavy metals such as Hg(II), Ba(II), Cd(II) and Cu(II) have also been reported in succession.110,117,118 For example, Fe2O3 nanoparticle-deposited magnetic Ti3C2Tx MXene-based nanocomposites (MGMX) were prepared through a facile hydrothermal process, and their adsorption performance towards Hg(II) for water purification was evaluated.119 SEM and TEM analyses revealed that the 2D layered structure was not damaged after Fe2O3 nanoparticles were deposited, and even after the adsorption of Hg(II), their morphology was preserved with no obvious changes. A series of adsorption experiments at different pH values and temperatures was conducted to explore the effect of adsorption of Hg(II). Under the optimized conditions, highly selective adsorption of Hg(II) was achieved with the maximum adsorption capacity of 1128.41 mg g−1. Adsorption mechanism analysis revealed that the oxygenated terminal groups play an active role in the process of Hg(II) hydrolysis. Through the adsorption reference experiment by using primitive non-magnetic Ti3C2Tx, Fe2O3 also plays an important role in the adsorption of Hg(II) for its small size and large surface area. Besides, due to their magnetic properties, excellent stability and outstanding recyclability of MGMX were also obtained. After four continuous recovery cycles, their removal efficiencies could still be maintained over 90%. Results indicated that MXenes could be a great promising candidate for Hg(II) or other heavy metal adsorption. Also, Ti3C2Tx was proved to be highly efficient at removing Ba(II), the removal efficiency could reach 100% under optimal conditions.118 The authors found that MXenes have a strong affinity for divalent metal ions, so several divalent ions were selected for adsorption experiments and the results indicated that the order of affinity towards MXenes is Ba > Sr > Ca > Pb > Cr > As. Besides heavy metal ions, MXenes could even be used for highly toxic metallic vapor adsorption in coal gas, i.e., mercury.120 Oxygen vacancy (Ov)-Ti2CO2 was involved in the adsorption of Hg0 and achieved better performance than the original Ti2CO2, and the oxidation mechanism shows that the chemisorption of Hg0 involves three steps: Hg0 → Hg (ads) → HgO (ads) → HgO. The produced HgO could be strongly absorbed by Ov-Ti2CO2, this research provides a novel solution for mercury removal, which could be of widespread interest in the treatment of non-ionic metals.

Radioactive contamination has increased manifold with the development of the nuclear industry; effective removal of high-level radioactive waste is a research topic of great concern in the environmental fields.121–124 Uranium is one of the most widely used radioactive elements and is considered to be one of the major radioactive contaminants due to its high radioactivity and biotoxicity. With a half-life of hundreds of millions of years, if it is not properly removed from nuclear waste, it will pose a huge threat to environmental safety and human health.125 However, the strong acidity, strong radiation and a large amount of coexisting ions of radioactive waste liquids are the main challenges for an efficient recovery of uranyl ions from radioactive wastewater.126–128 Based on the density functional theory (DFT) computational simulation in 2016, Zhang et al. have predicted that MXenes could be promising adsorbents for the removal of uranyl.122 Results have shown that there may be two adsorption mechanisms in this process, outer-sphere and inner-sphere adsorption configuration. Typically, strong hydrogen bonds were formed between uranyl and hydroxylated Ti3C2(OH)2 nanosheets, which play the main role in the chemical adsorption process. Following this theoretical simulation, it was confirmed that MXenes have remarkable adsorption properties towards uranium (U(VI)) with a high removal capacity of 174 mg g−1 by using V2CTx.128 In addition, more efficient adsorption properties for U(VI) by using Ti2CTx have been reported.129 Through a batch of experiments, the mechanism of sorption–reduction in the removal process of U(VI) was revealed. As shown in Fig. 7A, UO22+ was first absorbed on the surface of Ti2CTx, and when electrons were transferred from Ti2CTx to UO22+ ions, part of Ti2CTx was converted into unformed TiO2. The kinetic data (Fig. 7E) show that adsorption equilibrium could be achieved within 48 hours, and the maximum adsorption capacity was up to 470 mg g−1. Most recently, the same group reported stable MXene-based hierarchical titanate nanostructures (HTNs) for effective removal of Eu(III) from aqueous solution.130 Characterization based on DFT revealed that the excellent absorption properties towards Eu(III) mainly contribute to the formation of inner-sphere surface complexes in interlayer confined space. Significantly, these studies provide some new ideas for this direction and show impressive results, indicating that MXenes are promising candidates for efficient removal of radioactive species from wastewater. Based on the above conclusions, further investigations should focus on improving the surface adsorption properties of MXenes via surface modification, including non-covalent and covalent strategies, which would enhance their adsorption performance towards various metal anions.


image file: c9nr08542d-f7.tif
Fig. 7 (A) Illustration of the mechanism of UO22+ adsorption. (B) SEM image. U(VI) removal from aqueous solution by multilayered Ti2CTx as a function of (C) pH, (D and E) contact time, and (F) initial U(VI) concentration. Reprinted with permission.129 Copyright 2018, American Chemical Society.

3.3. Adsorption of gases

Gaseous pollutants are one of the greatest concerns in environmental remediation, especially with the development of industry and the increase of population.131 These gaseous pollutants can cause serious respiratory infections in the human body and lead to serious health problems. The accurate detection of air pollutants and their effective removal are of great significance to environmental remediation. Investigation has been conducted for gaseous pollutant removal using MXene-based materials. In recent years, based on the density functional theory (DFT) calculation, He et al. have proved that more than 20 kinds of reported MXene-based materials could be promising candidates for H2 adsorption and storage.132,133 A series of applications on the adsorption and detection of gaseous pollutants and storage of gaseous fuel has been reported recently.134–142 For instance, monolayer Ti2CO2 was synthesized for highly selective NH3 sensing and adsorption.139 Due to the appropriate adsorption energy (−0.37 eV) of NH3 on Ti2CO2, the adsorbed NH3 could be easily adsorbed onto the solid surface. Also, Persson et al. reported highly-efficient adsorption performance of Ti3C2Tx for CO2, and its adsorption performance reached 12 mol kg−1, which even exceeded the maximum value of CO2 adsorption reported in current studies.135 Based on the DFT calculation, Li et al. predicted that MXenes could even convert CO2 into fuels such as methane, which provided a guiding significance for MXenes in the field of renewable energy research.143 SO2 adsorption has also been systematically researched using several monolayer M2CO2, where M is Sc, Hf, Zr, and Ti.138 Results indicated that Sc2CO2 is the most effective substrate, and its adsorption capacity could be enhanced via biaxial strains and an external electric field (E-field). Besides, the recovery of SO2 from Sc2CO2 could be facilely realized by changing the E-field.

MXene-based materials have been reported to detect and adsorb volatile organic compounds (VOCs), including formaldehyde, methane, acetone, ethanol and propanal.137,141,144 For example, four kinds of etched MXenes have been studied for methane adsorption, and they were obtained by etching MAX with fluoride salts in HCl, including LiF, NaF, KF, and NH4F, respectively.141 The results revealed that the highest absorption capacity is 8.5 cm3 g−1 for Ti3C2 and 11.6 cm3 g−1 Ti2C. After methane was absorbed under high pressure, MXenes made from LiF and NH4F could keep methane under normal pressure while MXenes made from NaF and KF would release methane under low pressure (Fig. 8). The results show that MXenes have great potential for VOC adsorption and detection, and further exploration in this field is necessary. In addition to VOCs, MXenes are also used to prepare high-efficiency nanofiber filters for the removal of PM2.5.145 Enhanced removal performance towards PM2.5 was achieved by modifying polyacrylonitrile (PAN) nanofibers with Ti3C2Tx; the experiment shows that nearly 99.7% of PM2.5 could be removed even under low pressure. Antibacterial activity tests revealed that this MXene-modified membrane has antibacterial activity, thus it is suitable for filtering bacteria-containing air, and is a promising candidate for designing air purification equipment, especially in the field of biomedicine.


image file: c9nr08542d-f8.tif
Fig. 8 Methane adsorption isotherms from 0 to 60 bar of the Ti3C2 MXene prepared by different fluoride salts with HCl at 25 °C: (A) LiF; (B) NaF; (C) KF; (D) NH4F, respectively. Reprinted with permission.141 Copyright 2017, Elsevier.

4. Progress in catalytic degradation

Semiconductor heterojunction-based catalysts attract wide attention because of their use in fuel production and for degradation of various pollutants in aqueous solution.146 MXenes have been proved to have a wide absorption wavelength ranging from ultraviolet to infrared, and their advantages have been gradually highlighted in the field of photothermal conversion. Therefore, their potential for photocatalytic degradation is also worth exploring.147 Recently, various MXene-based heterojunction materials were investigated for effective photocatalytic degradation of organic compounds and production of hydrocarbon fuels, including g-C3N4/Ti3C2, TiO2/Ti3C2Tx, Ti3C2/Bi2WO6, a-Fe2O3/Ti3C2, etc.148–152 Based on the theoretical calculation, MXene-based materials were reported to have a narrow bandgap that can change with altering surface properties and structures.30,60,153,154 The alterability of the bandgap indicates that they can be converted into heterostructure materials with potential for catalytic degradation applications.

Due to the excessive use of pharmaceuticals, the incomplete absorption of drug molecules by the human body causes a large number of pharmaceuticals to be excreted through feces and urine, resulting in the accumulation of a large number of pharmaceuticals in natural water.155–157 Efficient removal of these molecules from water has become a hotspot in environmental science research. Lately, the photocatalytic decomposition of an antibiotic ciprofloxacin (CIP) was reported by using a formed heterojunction g-C3N4/Ti3C2 composite (CNTC).148 The photocatalytic performance investigation of CNTC towards CIP reveals that the degradation rate was greatly enhanced after the addition of Ti3C2 to form a heterostructure (Fig. 9A and B). The mechanism is illustrated in Fig. 9C; under the irradiation of visible light, the generated electrons (e) could rapidly transfer from g-C3N4 to Ti3C2, and the abundant photogenerated holes (h+) on g-C3N4 could effectively degrade CIP. Simultaneously, the dissolved oxygen in the solution can be reduced to superoxide compounds (˙O2) by the acquired electrons on the surface of Ti3C2 and the generated ˙O2 ions can attack the CIP molecules, thereby enhancing the degradation rate of CIP. Besides, a TiO2/Ti3C2Tx heterojunction composite was also synthesized via a simple hydrothermal method for photo-degradation of carbamazepine (CBZ, an antiepileptic drug).149 The adsorption/photocatalytic degradation process of CBZ was investigated, and the results revealed that the adsorption of CBZ molecules on the surface of composites was the necessary first step for the following photo-degradation procedure to take place. Subsequently, the generated hydroxyl radical on the surface of the composite reacts with CBZ to eventually oxidize it to water and carbon dioxide through two possible pathways.


image file: c9nr08542d-f9.tif
Fig. 9 Plot of Ct/C0versus time (A) and the corresponding kinetic rates (B) for the degradation of CIP during the photocatalytic processes. (C) Schematic illustration of the charge transfer and separation in the CNTC nanocomposite for organic pollutant oxidation with h+ and ˙O2 to oxidation products under visible light irradiation. Reprinted with permission.148 Copyright 2019, Elsevier.

Bisphenol A as a typical toxic EDC (endocrine-disrupting chemical) with high yield and wide application has made a serious impact on human health. The effective removal of this EDC has become a crucial task in environmental remediation. A heterogeneous catalyst sandwich-like Co3O4/MXene composite was used to activate peroxymonosulfate (PMS) for the degradation of Bisphenol A.158 A batch of experiments revealed that the catalytic efficiency reached the highest when the loading percentage of Co3O4 was 20%. The proposed mechanism suggested that both the generated SO4˙and ˙OH radicals were the key factor in the process of degradation. Very recently, photo-degradation of VOCs (HCHO and CH3COCH3) was also evaluated by using MXene-deposited Bi2WO6 nanoplates.159 Under light irradiation, HCHO and CH3COCH3 were effectively degraded by the prepared Bi2WO6/Ti3C2 composites with the maximum v(CO2) of 72.8 and 85.3 μmol g−1 h−1 respectively. Combined with DTF simulations, characterization revealed that the photo-oxidation generated active species (˙O2 and ˙OH) promoted the photo-degradation of VOCs. In addition, the effective catalytic degradation of various dyes by MXenes and their derivatives has also been reported, including methylene blue (MB), acid blue 80 (AB80), Rhodamine B (RhB), etc.151,160 The recent advances in dye degradation and its mechanism and discovery are summarized in Table 1. These studies provide an ideal strategy for the removal of organic pollutants from aqueous solution, which consolidated the position of MXenes in the field of environmental remediation.

Table 1 Summary of the degradation of dye pollutants by MXenes and their derivatives
Catalyst Dyes Concentration and volume Experimental conditions Degradation efficiency Mechanism Ref.
mp-MXene/TiO2−x NDs RhB 30 mg L−1, 15 mL 500 W Xe lamp with a cooling system, room temperature 96% of RhB was degraded in 10 min The Fenton reaction and photocatalysis process work synergistically to accelerate the degradation of RhB 167
Layered BiOBr/Ti3C2 (BTC) RhB 20 mg L−1, 100 mL 300 W Xe lamp (λ > 420 nm), room temperature 89.3% of RhB in 50 min Synergetic effects of excellent light capture and electron transfer performance of composites have been proved, the photogenerated ˙O2 and ˙OH were activated as the key effect in degradation 168
CeO2/Ti3C2 nanocomposites RhB 20 mg L−1, 50 mL 500 W Hg lamp, 283.6 K 75% of RhB in 90 min The CeO2/Ti3C2 nanocomposites have narrower bandgap energies than pure CeO2, which allows them to make better use of photon energy 169
TiO2/C nanocomposites RhB 2.0 × 10−5 M, 100 mL 175 W Hg lamp, 283 K 90% of RhB in 5 min The synergistic effect between TiO2 and carbon makes it possible to generate more oxygen vacancies and photogenerated electron–hole pairs, which will accelerate the photodegradation process 170
a-Fe2O3/Ti3C2 MXene composites RhB 10 mg L−1, 100 mL 500 W Xe lamp with a UV-cutoff filter (λ > 420 nm), room temperature 98% of RhB in 120 min Photogenerated electrons could facilely transfer from Fe2O3 to Ti3C2; high electron–hole separation efficiency is the key effect 151
BiOCl/Ti3C2 nanocomposites MO 20 mg L−1, 100 mL 300 W Xe lamp, room temperature 96% of MO in 40 min Nano-scale contact between BiOCl and Ti3C2 promotes charge transfer; the photogenerated holes and ˙OH radicals significantly improved the photocatalytic activity 171
TiO2/Ti3C2 nanocomposites MO 20 mg L−1, 100 mL 175 W Hg lamp, 283 K 98% of MO in 30 min The formation of a heterojunction structure between TiO2 and Ti3C2 may generate effective electron–hole separation 172
TiO2@C nanosheets MB 20 mg L−1, 50 mL 500 W Hg lamp (λ < 400 nm), 298 K 85.7% of MB in 30 min The intimate structure between TiO2 and C effectively suppresses the recombination of photo-induced electron–hole pairs, and the presence of Ti3+ and oxygen vacancy defects also promotes the photocatalysis process 173
MXene–Co3O4 nanocomposites MB, RhB 12.5 mg L−1 (MB), 5 mg L−1 (RhB), 100 mL 15 mL of H2O2 (30%), room temperature 128.91 mg g−1 of MB in 300 min and 47.076 mg g−1 of RhB in 100 min H2O2 was then effectively catalyzed to produce free ˙OH radical species and ultimately promote the degradation of the MB molecules 174
Ti3C2Tx MB, AB80 0.012 mg L−1 (MB), 0.06 mg L−1 (AB80), 40 mL UV-C lamp (254 nm), room temperature 81% of MB and 62% of AB80 in 5 h The formation of titanium hydroxide or TiO2 on the surface of MXenes may contribute to the photocatalytic degradation process, which is very complex and needs further research 160
BGFO-20Sn/MXene CR 300 W Xe lamp, room temperature Nearly 100% of CR in 2 h Catalyst absorbed light energy and charge generated. MXene sheets act as acceptors as they trap electrons and enhance the separation of charge carriers; the CR was effectively degraded by the produced O2 and OH active species 175
CdS@Ti3C2@TiO2 nanocomposites RhB, MB, SCP, phenol 20 mg L−1, 200 mL Visible light with 300 mW cm−2, wavelength between 400 and 1050 nm, room temperature 100% of RhB, MB, SCP and phenol in 60, 88, 88 and 150 min, respectively Photo-generated superoxide radicals (˙O2) and hydroxyl radicals (˙OH) play a key role in the efficient photo-degradation of organic pollutants 162


Besides forming semiconductor heterojunctions with the above-mentioned materials to enhance their photocatalytic performance, MXenes also play a suitable role as co-catalysts and in the formation of a cascading junction (Z-scheme) structure. Ran et al. reported for the first time that MXenes act as co-catalysts with CdS for highly efficient photocatalytic H2 production.161 Cauliflower-structured CdS/Ti3C2 nanoparticles were obtained by a hydrothermal strategy, and the maximum hydrogen production was recorded (14[thin space (1/6-em)]342 μmol h−1 g−1) when the mass ratios of Ti3C2 to CdS was 2.5. Based on the experimental and theoretical calculations, the excellent hydrogen production performance contributed to a fast photoinduced electron transfer from CdS to Ti3C2, leading to an efficient proton reduction on the surface of Ti3C2. In addition, fabrication of an MXene-based Z-scheme (cascading junction structure) photocatalyst was also reported very recently, which was also proved to be a good alternative to achieve enhanced photocatalytic activity.162 Complete degradation of RhB, MB, sulfachloropyridazine (SCP) and phenol compounds was achieved by using the as-synthesized CdS@Ti3C2@TiO2 nanocomposites under visible light irritation. This enhanced degradation performance could be attributed to the Z-scheme formation, as characterized by the transient photocurrent response tests and the electron spin resonance (EPR) technique. Such an excellent photocatalytic degradation efficiency even surpasses that of many widely accepted g-C3N4-based photocatalysts.163 The catalytic hydrogen evolution reaction (HER) is an important direction in the field of catalysis, which is of great significance for providing sustainable energy and environmental protection. Recently, the performance of MXenes (Mo2CTx) in the HER was demonstrated for the first time; both theory and experiment revealed that Mo2CTx is a promising candidate for catalytic hydrogen production.164 Most recently, Li's group has systematically investigated the performance of non-metallic heteroatom doped MXene materials in the HER based on well-defined DFT calculations, which provided guidance for the fabrication of renewable energy and environmental protection devices.165,166

5. Toxicity evaluation

The increased use of MXenes in environmental remediation and biomedical applications has attracted a lot of research interest. The study of MXene biotoxicity is of great significance in both environmental and biomedical applications. In this process, it is necessary that MXene materials do not produce new impurities and maintain their chemical stability upon coming into contact with water. Lately, the investigation of an in vitro cytotoxicity test of a typical MXene by using cancer cells and normal cells has been reported.176 For the first time, the biotoxicity of Ti3C2 was studied by incubation of cell lines including A549, MRC-5, A375 and HaCaT at different concentrations varying from 0 to 500 mg L−1. Both MTT and calcein-AM assays were performed for the evaluation of cell viability, as shown in Fig. 10A and B; the results indicated that the number of living cells was inversely proportional to the concentration of MXenes, and the damage to cancer cells was higher than that to normal cells. Typically, the MTT assay showed that even at the highest concentration (500 mg L−1) of MXenes, the cell viability of HaCaT cells still reached 80%, indicating their excellent biocompatibility and low environmental toxicity. Significant results were presented with the goal of enhancing material-based cell therapy and tissue engineering by using Ti3C2 based quantum dots (MQDs).177 In this investigation, their biocompatibility for stem cells was evaluated, as shown in Fig. 10C–F. Since there were almost no differences in the number or morphology of cells, culturing with MQDs showed no obvious toxicity towards mesenchymal stem cells (MSCs) and induced pluripotent stem cell (iPSC)-derived fibroblasts (iPS-Fibs).
image file: c9nr08542d-f10.tif
Fig. 10 (A) MTT assay and (B) calcein-AM assay results of the cell viability of A549, MRC-5, A375, and HaCaT cells after 24 h cultivation at different concentrations of MXenes. Reprinted with permission.176 Copyright 2017, Elsevier. Biocompatibility of Ti3C2 MQDs with stem cell populations. (C) Ti3C2 MQDs were co-cultured with rat bone-marrow-derived mesenchymal stem cells for 24 h and subsequently stained for live (calcein, green) and dead (7-AAD, red) cells. (D) No difference was observed in the percentage of live cells between the control and MQD group. (E) MQDs were subsequently co-cultured with human iPSC-derived fibroblasts for 24 h and subsequently stained for live and dead cells. (F) No difference was observed in viability between MQD and control groups. Reprinted with permission.177 Copyright 2019, WILEY-VCH.

Cytotoxicity evaluation from microscopic cells to living organisms or even at the eco-toxicological level is necessary. For the first time, zebrafish was used as the model for evaluating the potential toxicity of MXenes. The experiment revealed that the median lethal concentration (LC50) of Ti3C2Tx was found to be 257.46 μm mL−1 and the no observed effect concentration (NOEC) was recorded to be 50 μm mL−1. The results suggested that Ti3C2Tx is non-toxic at a certain concentration (below 100 μm mL−1), and it can be safely used because it does not pose a serious threat to zebrafish embryos or other animals. Moreover, MXenes have been reported in recent years for biosensing, diagnosis, cell imaging, and drug delivery, which further indicates that their excellent biocompatibility is recognized.44,178–181 Although these current research studies are not perfect, they are enough to provide evidence that MXenes are relatively non-toxic absorbents to the environment and organisms, and further investigations on the environmental toxicity of MXenes are needed in the future.

6. Summary and prospects

The efficient removal of various pollutants from the environment is still a challenge, and the adsorption and catalytic degradation techniques are usually thought to be the most facile and effective strategies. The rapidly growing concern in using MXene-related materials for environmental applications has been stimulated by the attractive discoveries of exceptional properties and performance of graphene-based nanomaterials, transition metal dichalcogenides (TMDs, typically MoS2), layered double hydroxides (LDHs), etc. However, their inherent hurdles are also exposed, such as the high cost and hydrophobicity of graphene, the hydrophobicity of MoS2 and the difficulty in the large-scale production of monolayer MoS2 with high quality, and the low stability of LDHs in complex solutions. Although these difficulties have been alleviated through the efforts of researchers, it is still a challenge to solve them fundamentally.

Based on their excellent performance reported in the literature, MXenes and related composites have proven to be alternative substance materials with great potential in environmental remediation. The unique morphology of MXene-based materials endows them with a large specific surface area, high hydrophilicity, layer thickness controllability, structural adjustability and compositional diversity, which make them great candidates for environmental applications. Benefitting from these properties, the applications of MXene-based materials in environmental remediation, including adsorption and catalytic degradation of pollutants, including dyestuff, organic compounds, heavy metal ions, oxidative metal ions, radioactive contaminants, antibiotics and even waste gases, have been widely investigated in recent years. We have reviewed recent advances in the applications of MXenes and MXene-based hybrids for environmental adsorption and catalytic degradation. However, compared with the extensive research on other two-dimensional materials, the study on MXenes is still in the preliminary stage, and the following unsolved problems in their properties and applications have been exposed, which need to be further evaluated in the future: (1) the methods of etching using HF need to be improved, and the large amounts of acidic gas and waste liquid generated in the preparation process need to be controlled. Although much work has been done, there are still many possibilities to be explored. More environmentally friendly preparation approaches need to be investigated. (2) The surface properties of MXene-based materials are not well studied. For example, the influence of surface properties on the stability of MXenes in aqueous solution and the surface modification of MXenes also needs further systematic investigation, which will bring great convenience to expand the applications of MXenes. In particular, in the adsorption of pollutants, the adsorption performance could be greatly enhanced after appropriate surface engineering. (3) MXenes have been extensively investigated in biomedical applications and these results preliminarily indicate their low toxicity, while the toxicity and effects of MXenes and their composites on the environment and human beings have not been systematically studied and the toxicity mechanism is not clear yet. However, we have a reason to believe that the performance of MXenes in environment applications will be more impressive after solving these difficulties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Science Foundation of China (No. 21788102, 21865016, 21474057, 21564006, 21561022, 21644014, and 51673107).

References

  1. I. Ali, Chem. Rev., 2012, 112, 5073–5091 CrossRef CAS PubMed.
  2. C. Santhosh, V. Velmurugan, G. Jacob, S. K. Jeong, A. N. Grace and A. Bhatnagar, Chem. Eng. J., 2016, 306, 1116–1137 CrossRef CAS.
  3. E. A. Laws, Aquatic pollution: an introductory text, John Wiley & Sons, 2017 Search PubMed.
  4. X. Jin, C. Yu, Y. Li, Y. Qi, L. Yang, G. Zhao and H. Hu, J. Hazard. Mater., 2011, 186, 1672–1680 CrossRef CAS PubMed.
  5. R. Gothwal and T. Shashidhar, Clean: Soil, Air, Water, 2015, 43, 479–489 CAS.
  6. J. E. Burgess, S. A. Parsons and R. M. Stuetz, Biotechnol. Adv., 2001, 19, 35–63 CrossRef CAS PubMed.
  7. G. Crini, Prog. Polym. Sci., 2005, 30, 38–70 CrossRef CAS.
  8. S. Wang and Y. Peng, Chem. Eng. J., 2010, 156, 11–24 CrossRef CAS.
  9. K.-M. Yao, M. T. Habibian and C. R. O'Melia, Environ. Sci. Technol., 1971, 5, 1105–1112 CrossRef CAS.
  10. A. Matilainen, M. Vepsäläinen and M. Sillanpää, Adv. Colloid Interface Sci., 2010, 159, 189–197 CrossRef CAS PubMed.
  11. C. Comninellis, Electrochim. Acta, 1994, 39, 1857–1862 CrossRef CAS.
  12. D. Bahnemann, Sol. Energy, 2004, 77, 445–459 CrossRef CAS.
  13. C. Comninellis, A. Kapalka, S. Malato, S. A. Parsons, I. Poulios and D. Mantzavinos, J. Chem. Technol. Biotechnol., 2008, 83, 769–776 CrossRef CAS.
  14. X. Ren, C. Chen, M. Nagatsu and X. Wang, Chem. Eng. J., 2011, 170, 395–410 CrossRef CAS.
  15. X. Zhang, Q. Huang, F. Deng, H. Huang, Q. Wan, M. Liu and Y. Wei, Appl. Mater. Today, 2017, 7, 222–238 CrossRef.
  16. Q. Huang, M. Liu, J. Chen, K. Wang, D. Xu, F. Deng, H. Huang, X. Zhang and Y. Wei, Appl. Surf. Sci., 2016, 387, 285–293 CrossRef CAS.
  17. Q. Huang, M. Liu, J. Zhao, J. Chen, G. Zeng, H. Huang, J. Tian, Y. Wen, X. Zhang and Y. Wei, Appl. Surf. Sci., 2018, 427, 535–544 CrossRef CAS.
  18. G. Zeng, L. Huang, Q. Huang, M. Liu, D. Xu, H. Huang, Z. Yang, F. Deng, X. Zhang and Y. Wei, Appl. Surf. Sci., 2018, 459, 588–595 CrossRef CAS.
  19. Q. Huang, M. Liu, L. Mao, D. Xu, G. Zeng, H. Huang, R. Jiang, F. Deng, X. Zhang and Y. Wei, J. Colloid Interface Sci., 2017, 499, 170–179 CrossRef CAS PubMed.
  20. Q. Huang, J. Zhao, M. Liu, J. Chen, X. Zhu, T. Wu, J. Tian, Y. Wen, X. Zhang and Y. Wei, J. Taiwan Inst. Chem. Eng., 2018, 82, 92–101 CrossRef CAS.
  21. Q. Huang, J. Zhao, M. Liu, Y. Li, J. Ruan, Q. Li, J. Tian, X. Zhu, X. Zhang and Y. Wei, J. Taiwan Inst. Chem. Eng., 2018, 86, 174–184 CrossRef CAS.
  22. I. Ali and V. Gupta, Nat. Protoc., 2006, 1, 2661 CrossRef CAS PubMed.
  23. Y. Liu, H. Huang, D. Gan, L. Guo, M. Liu, J. Chen, F. Deng, N. Zhou, X. Zhang and Y. Wei, Ceram. Int., 2018, 44, 18571–18577 CrossRef CAS.
  24. A. Demirbas, J. Hazard. Mater., 2009, 167, 1–9 CrossRef CAS PubMed.
  25. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669 CrossRef CAS PubMed.
  26. A. Gupta, T. Sakthivel and S. Seal, Prog. Mater. Sci., 2015, 73, 44–126 CrossRef CAS.
  27. M. Xu, T. Liang, M. Shi and H. Chen, Chem. Rev., 2013, 113, 3766–3798 CrossRef CAS PubMed.
  28. Q. Tang and Z. Zhou, Prog. Mater. Sci., 2013, 58, 1244–1315 CrossRef CAS.
  29. G. R. Bhimanapati, Z. Lin, V. Meunier, Y. Jung, J. Cha, S. Das, D. Xiao, Y. Son, M. S. Strano and V. R. Cooper, ACS Nano, 2015, 9, 11509–11539 CrossRef CAS PubMed.
  30. M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi and M. W. Barsoum, Adv. Mater., 2011, 23, 4248–4253 CrossRef CAS PubMed.
  31. B. Anasori, Y. Xie, M. Beidaghi, J. Lu, B. C. Hosler, L. Hultman, P. R. Kent, Y. Gogotsi and M. W. Barsoum, ACS Nano, 2015, 9, 9507–9516 CrossRef CAS PubMed.
  32. M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi and M. W. Barsoum, ACS Nano, 2012, 6, 1322 CrossRef CAS PubMed.
  33. J. Wang, T.-N. Ye, Y. Gong, J. Wu, N. Miao, T. Tada and H. Hosono, Nat. Commun., 2019, 10, 2284 CrossRef PubMed.
  34. J. Zheng, B. Wang, A. Ding, B. Weng and J. Chen, J. Electroanal. Chem., 2018, 816, 189–194 CrossRef CAS.
  35. C. Ke, Y. Chen, Q. Deng, S. H. Jeong, T. S. Jang, S. Du, H. E. Kim, Q. Huang and C. M. Han, Mater. Lett., 2018, 229, 114–117 CrossRef.
  36. Z. Chen, Y. Han, T. Li, X. Zhang, T. Wang and Z. Zhang, Mater. Lett., 2018, 220, 305–308 CrossRef CAS.
  37. X. Li, X. Yin, C. Song, M. Han, H. Xu, W. Duan, L. Cheng and L. Zhang, Adv. Funct. Mater., 2018, 28, 1803938 CrossRef.
  38. Y. Yue, N. Liu, Y. Ma, S. Wang, W. Liu, C. Luo, H. Zhang, F. Cheng, J. Rao and X. Hu, ACS Nano, 2018, 12, 4224–4232 CrossRef CAS PubMed.
  39. J. Yue, X. Xi, C. Yu, Y. Liu, Y. Rui and G. X. Sui, J. Mater. Chem. C, 2018, 6, 8679–8687 RSC.
  40. X. Li, C. Wang, Y. Cao and G. Wang, Chem. – Asian J., 2018, 13, 2742–2757 CrossRef CAS PubMed.
  41. L. Zhuang, M. L. Zhao, L. Han, C. Dai and C. Yu, J. Mater. Chem. B, 2018, 6, 3541–3548 RSC.
  42. Q. Pan, Y. Zheng, S. Kota, W. Huang, S. Wang, H. Qi, S. Kim, Y. Tu, M. W. Barsoum and C. Y. Li, Nanoscale Adv., 2019, 1, 395–402 RSC.
  43. S. Kumar, Y. Lei, N. H. Alshareef, M. Quevedo-Lopez and K. N. Salama, Biosens. Bioelectron., 2018, 121, 243–249 CrossRef CAS PubMed.
  44. X. Han, J. Huang, H. Lin, Z. Wang, P. Li and Y. Chen, Adv. Healthcare Mater., 2018, 7, 1701394 CrossRef PubMed.
  45. M. Liu, P. A. Gurr, Q. Fu, P. A. Webley and G. G. Qiao, J. Mater. Chem. A, 2018, 6, 23169–23196 RSC.
  46. Q. Xu, L. Ding, Y. Wen, W. Yang, H. Zhou, X. Chen, J. Street, A. Zhou, W.-J. Ong and N. Li, J. Mater. Chem. C, 2018, 6, 6360–6369 RSC.
  47. Q. Xu, W. Yang, Y. Wen, S. Liu, Z. Liu, W.-J. Ong and N. Li, Appl. Mater. Today, 2019, 16, 90–101 CrossRef.
  48. V. M. H. Ng, H. Huang, K. Zhou, P. S. Lee, W. Que, J. Z. Xu and L. B. Kong, J. Mater. Chem. A, 2017, 5, 3039–3068 RSC.
  49. J. Zhu, E. Ha, G. Zhao, Y. Zhou, D. Huang, G. Yue, L. Hu, N. Sun, Y. Wang and L. Y. S. Lee, Coord. Chem. Rev., 2017, 352, 306–327 CrossRef CAS.
  50. B.-M. Jun, S. Kim, J. Heo, C. M. Park, N. Her, M. Jang, Y. Huang, J. Han and Y. Yoon, Nano Res., 2018, 1–17 Search PubMed.
  51. B. Anasori, M. R. Lukatskaya and Y. Gogotsi, Nat. Rev. Mater., 2017, 2, 16098 CrossRef CAS.
  52. A. Sinha, H. Zhao, Y. Huang, X. Lu, J. Chen and R. Jain, TrAC, Trends Anal. Chem., 2018, 105, 424–435 CrossRef CAS.
  53. Y. Lei, Y. Cui, Q. Huang, J. Dou, D. Gan, F. Deng, M. Liu, X. Li, X. Zhang and Y. Wei, Ceram. Int., 2019, 45, 17653–17661 CrossRef CAS.
  54. M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin and Y. Gogotsi, Chem. Mater., 2017, 29, 7633–7644 CrossRef CAS.
  55. J. Peng, X. Chen, W.-J. Ong, X. Zhao and N. Li, Chem, 2019, 5, 18–50 CAS.
  56. M. Khazaei, A. Ranjbar, K. Esfarjani, D. Bogdanovski, R. Dronskowski and S. Yunoki, Phys. Chem. Chem. Phys., 2018, 20, 8579–8592 RSC.
  57. M. Magnuson, J. Halim and L.-Å. Näslund, J. Electron Spectrosc. Relat. Phenom., 2018, 224, 27–32 CrossRef CAS.
  58. M. Naguib, V. N. Mochalin, M. W. Barsoum and Y. Gogotsi, Adv. Mater., 2014, 26, 992–1005 CrossRef CAS PubMed.
  59. S. Lai, J. Jeon, S. K. Jang, J. Xu, Y. J. Choi, J.-H. Park, E. Hwang and S. Lee, Nanoscale, 2015, 7, 19390–19396 RSC.
  60. A. C. Rajan, A. Mishra, S. Satsangi, R. Vaish, H. Mizuseki, K.-R. Lee and A. K. Singh, Chem. Mater., 2018, 30, 4031–4038 CrossRef CAS.
  61. D. Wu, F. Zhang, H. Liang and X. Feng, Chem. Soc. Rev., 2012, 41, 6160–6177 RSC.
  62. Y. Guo, K. Xu, C. Wu, J. Zhao and Y. Xie, Chem. Soc. Rev., 2015, 44, 637–646 RSC.
  63. Q. Huang, M. Liu, J. Chen, Q. Wan, J. Tian, L. Huang, R. Jiang, Y. Wen, X. Zhang and Y. Wei, Appl. Surf. Sci., 2017, 419, 35–44 CrossRef CAS.
  64. J. Zhao, Q. Huang, M. Liu, Y. Dai, J. Chen, H. Huang, Y. Wen, X. Zhu, X. Zhang and Y. Wei, J. Colloid Interface Sci., 2017, 505, 168–177 CrossRef CAS PubMed.
  65. M. Singh, M. Holzinger, M. Tabrizian, S. A. Winters, N. C. Berner, S. Cosnier and G. S. Duesberg, J. Am. Chem. Soc., 2015, 137, 2800–2803 CrossRef CAS PubMed.
  66. T. Kuila, P. Khanra, S. Bose, N. H. Kim, B.-C. Ku, B. Moon and J. H. Lee, Nanotechnology, 2011, 22, 305710 CrossRef PubMed.
  67. M. Liu, G. Zeng, K. Wang, Q. Wan, L. Tao, X. Zhang and Y. Wei, Nanoscale, 2016, 8, 16819–16840 RSC.
  68. X. Zhang, K. Wang, M. Liu, X. Zhang, L. Tao, Y. Chen and Y. Wei, Nanoscale, 2015, 7, 11486–11508 RSC.
  69. J. Chen, M. Liu, H. Huang, F. Deng, L. Mao, Y. Wen, L. Huang, J. Tian, X. Zhang and Y. Wei, J. Mol. Liq., 2018, 259, 179–185 CrossRef CAS.
  70. Q.-Y. Cao, R. Jiang, M. Liu, Q. Wan, D. Xu, J. Tian, H. Huang, Y. Wen, X. Zhang and Y. Wei, Mater. Sci. Eng., C, 2017, 80, 578–583 CrossRef CAS PubMed.
  71. Q.-Y. Cao, R. Jiang, M. Liu, Q. Wan, D. Xu, J. Tian, H. Huang, Y. Wen, X. Zhang and Y. Wei, Mater. Sci. Eng., C, 2017, 80, 411–416 CrossRef CAS PubMed.
  72. H. Huang, D. Xu, M. Liu, R. Jiang, L. Mao, Q. Huang, Q. Wan, Y. Wen, X. Zhang and Y. Wei, Mater. Sci. Eng., C, 2017, 78, 862–867 CrossRef CAS.
  73. L. Huang, S. Yang, J. Chen, J. Tian, Q. Huang, H. Huang, Y. Wen, F. Deng, X. Zhang and Y. Wei, Mater. Sci. Eng., C, 2019, 94, 270–278 CrossRef CAS PubMed.
  74. R. Jiang, H. Liu, M. Liu, J. Tian, Q. Huang, H. Huang, Y. Wen, Q.-Y. Cao, X. Zhang and Y. Wei, Mater. Sci. Eng., C, 2017, 81, 416–421 CrossRef CAS PubMed.
  75. R. Jiang, M. Liu, C. Li, Q. Huang, H. Huang, Q. Wan, Y. Wen, Q.-Y. Cao, X. Zhang and Y. Wei, Mater. Sci. Eng., C, 2017, 80, 708–714 CrossRef CAS PubMed.
  76. X. Zhang, X. Zhang, B. Yang, M. Liu, W. Liu, Y. Chen and Y. Wei, Polym. Chem., 2014, 5, 356–360 RSC.
  77. X. Zhang, X. Zhang, B. Yang, M. Liu, W. Liu, Y. Chen and Y. Wei, Polym. Chem., 2014, 5, 399–404 RSC.
  78. Q. Wan, Q. Huang, M. Liu, D. Xu, H. Huang, X. Zhang and Y. Wei, Appl. Mater. Today, 2017, 9, 145–160 CrossRef.
  79. J. Chen, M. Liu, Q. Huang, L. Huang, H. Huang, F. Deng, Y. Wen, J. Tian, X. Zhang and Y. Wei, Chem. Eng. J., 2018, 337, 82–90 CrossRef CAS.
  80. Z. Long, M. Liu, R. Jiang, Q. Wan, L. Mao, Y. Wan, F. Deng, X. Zhang and Y. Wei, Chem. Eng. J., 2017, 308, 527–534 CrossRef CAS.
  81. R. Jiang, M. Liu, H. Huang, L. Mao, Q. Huang, Y. Wen, Q. Y. Cao, J. Tian, X. Zhang and Y. Wei, Dyes Pigm., 2018, 153, 99–105 CrossRef CAS.
  82. H. Ouyang, M. Zhou, Y. Guo, M. He, H. Huang, X. Ye, Y. Feng, X. Zhou and S. Yang, Fitoterapia, 2014, 96, 152–158 CrossRef CAS PubMed.
  83. X. Zhang, J. Li, B. Xie, B. Wu, S. Lei, Y. Yao, M. He, H. Ouyang, Y. Feng, W. Xu and S. Yang, Front. Pharmacol., 2018, 9, 282 CrossRef PubMed.
  84. X. Zhang, M. He, S. Lei, B. Wu, T. Tan, H. Ouyang, W. Xu and Y. Feng, J. Pharm. Biomed. Anal., 2018, 156, 221–231 CrossRef CAS PubMed.
  85. C. Ding, J. Liang, Z. Zhou, Y. Li, W. Peng, G. Zhang, F. Zhang and X. Fan, Chem. Eng. J., 2019, 378, 122205 CrossRef CAS.
  86. H. Wang, J. Zhang, Y. Wu, H. Huang, G. Li, X. Zhang and Z. Wang, Appl. Surf. Sci., 2016, 384, 287–293 CrossRef CAS.
  87. H. Wang, J. Zhang, Y. Wu, H. Huang and Q. Jiang, J. Phys. Chem. Solids, 2018, 115, 172–179 CrossRef CAS.
  88. X. Wu, L. Hao, J. Zhang, X. Zhang, J. Wang and J. Liu, J. Membr. Sci., 2016, 515, 175–188 CrossRef CAS.
  89. M. Boota, B. Anasori, C. Voigt, M. Q. Zhao, M. W. Barsoum and Y. Gogotsi, Adv. Mater., 2016, 28, 1517–1522 CrossRef CAS PubMed.
  90. C. Chen, M. Boota, X. Xie, M. Zhao, B. Anasori, C. E. Ren, L. Miao, J. Jiang and Y. Gogotsi, J. Mater. Chem. A, 2017, 5, 5260–5265 RSC.
  91. H. Lin, X. Wang, L. Yu, Y. Chen and J. Shi, Nano Lett., 2016, 17, 384–391 CrossRef PubMed.
  92. J. Xuan, Z. Wang, Y. Chen, D. Liang, L. Cheng, X. Yang, Z. Liu, R. Ma, T. Sasaki and F. Geng, Angew. Chem., Int. Ed., 2016, 55, 14569–14574 CrossRef CAS PubMed.
  93. X. Li, J. Zhu, L. Wang, W. Wu and Y. Fang, Electrochim. Acta, 2017, 258, 291–301 CrossRef CAS.
  94. H. Wang, L. Li, C. Zhu, S. Lin, J. Wen, Q. Jin and X. Zhang, J. Alloys Compd., 2019, 778, 858–865 CrossRef CAS.
  95. J. Chen, K. Chen, D. Tong, Y. Huang, J. Zhang, J. Xue, Q. Huang and T. Chen, Chem. Commun., 2015, 51, 314–317 RSC.
  96. N. Tao, D. Zhang, X. Li, D. Lou, X. Sun, C.-W. Wei, J. Li, J. Yang and Y.-N. Liu, Chem. Sci., 2019, 10, 10765–10771 RSC.
  97. K. Li, G. Zou, T. Jiao, R. Xing, L. Zhang, J. Zhou, Q. Zhang and Q. Peng, Colloids Surf., A, 2018, 553, 105–113 CrossRef CAS.
  98. P. Chao, W. Ping, C. Xin, Z. Yongli, Z. Feng, C. Yonghai, W. Hongjuan, Y. Hao and P. Feng, Ceram. Int., 2018, 44, 18886–18893 CrossRef.
  99. W. Gan, X. Shang, X. H. Li, J. Zhang and X. Fu, Colloids Surf., A, 2018, 561, 218–225 CrossRef.
  100. Z. Wei, Z. Peigen, T. Wubian, Q. Xia, Z. Yamei and S. Zhengming, Mater. Chem. Phys., 2018, 206, 270–276 CrossRef CAS.
  101. O. Mashtalir, M. Naguib, V. N. Mochalin, Y. Dall'Agnese, M. Heon, M. W. Barsoum and Y. Gogotsi, Nat. Commun., 2013, 4, 1716 CrossRef PubMed.
  102. M. Naguib, R. R. Unocic, B. L. Armstrong and J. Nanda, Dalton Trans., 2015, 44, 9353–9358 RSC.
  103. O. Mashtalir, M. R. Lukatskaya, M. Q. Zhao, M. W. Barsoum and Y. Gogotsi, Adv. Mater., 2015, 27, 3501–3506 CrossRef CAS PubMed.
  104. F. Meng, M. Seredych, C. Chen, V. Gura, S. Mikhalovsky, S. Sandeman, G. Ingavle, T. Ozulumba, L. Miao and B. Anasori, ACS Nano, 2018, 12, 10518–10528 CrossRef CAS PubMed.
  105. L. Cui, Y. Wang, L. Gao, L. Hu, L. Yan, Q. Wei and B. Du, Chem. Eng. J., 2015, 281, 1–10 CrossRef CAS.
  106. M.-Q. Jiang, X.-Y. Jin, X.-Q. Lu and Z.-L. Chen, Desalination, 2010, 252, 33–39 CrossRef CAS.
  107. P. Gu, J. Xing, T. Wen, R. Zhang, J. Wang, G. Zhao, T. Hayat, Y. Ai, Z. Lin and X. Wang, Environ. Sci.: Nano, 2018, 5, 946–955 RSC.
  108. L. Fu, Z. Yan, Q. Zhao and H. Yang, Adv. Mater. Interfaces, 2018, 5, 1801094 CrossRef.
  109. Y. Wu, H. Pang, Y. Liu, X. Wang, S. Yu, D. Fu, J. Chen and X. Wang, Environ. Pollut., 2018, 246, 608–620 CrossRef PubMed.
  110. X. Guo, X. Zhang, S. Zhao, Q. Huang and J. Xue, Phys. Chem. Chem. Phys., 2016, 18, 228–233 RSC.
  111. Q. Peng, J. Guo, Q. Zhang, J. Xiang, B. Liu, A. Zhou, R. Liu and Y. Tian, J. Am. Chem. Soc., 2014, 136, 4113–4116 CrossRef CAS PubMed.
  112. M. Hua, S. Zhang, B. Pan, W. Zhang, L. Lv and Q. Zhang, J. Hazard. Mater., 2012, 211–212, 317–331 CrossRef CAS.
  113. J. Guo, H. Fu, G. Zou, Q. Zhang, Z. Zhang and Q. Peng, J. Alloys Compd., 2016, 684, 504–509 CrossRef CAS.
  114. Y. Ying, Y. Liu, X. Wang, Y. Mao, W. Cao, P. Hu and X. Peng, ACS Appl. Mater. Interfaces, 2015, 7, 1795–1803 CrossRef CAS PubMed.
  115. G. Zou, J. Guo, Q. Peng, A. Zhou, Q. Zhang and B. Liu, J. Mater. Chem. A, 2016, 4, 489–499 RSC.
  116. R. P. Pandey, K. Rasool, P. Abdul Rasheed and K. A. Mahmoud, ACS Sustainable Chem. Eng., 2018, 6, 7910–7917 CrossRef CAS.
  117. X. Zhu, B. Liu, H. Hou, Z. Huang, K. M. Zeinu, L. Huang, X. Yuan, D. Guo, J. Hu and J. Yang, Electrochim. Acta, 2017, 248, 46–57 CrossRef CAS.
  118. A. K. Fard, G. Mckay, R. Chamoun, T. Rhadfi, H. Preud'Homme and M. A. Atieh, Chem. Eng. J., 2017, 317, 331–342 CrossRef CAS.
  119. A. Shahzad, K. Rasool, W. Miran, M. Nawaz, J. Jang, K. A. Mahmoud and D. S. Lee, J. Hazard. Mater., 2018, 344, 811–818 CrossRef CAS PubMed.
  120. X. Gao, Y. Zhou, Y. Tan, Z. Cheng, B. Yang, Y. Ma, Z. Shen and J. Jia, Appl. Surf. Sci., 2019, 464, 53–60 CrossRef CAS.
  121. W. Mu, S. Du, X. Li, Q. Yu, H. Wei, Y. Yang and S. Peng, Chem. Eng. J., 2019, 358, 283–290 CrossRef CAS.
  122. Y.-J. Zhang, J.-H. Lan, L. Wang, Q.-Y. Wu, C.-Z. Wang, T. Bo, Z.-F. Chai and W.-Q. Shi, J. Hazard. Mater., 2016, 308, 402–410 CrossRef CAS PubMed.
  123. S. Li, L. Wang, J. Peng, M. Zhai and W. Shi, Chem. Eng. J., 2019, 366, 192–199 CrossRef CAS.
  124. L. Wang, H. Song, L. Yuan, Z. Li, P. Zhang, J. K. Gibson, L. Zheng, H. Wang, Z. Chai and W. Shi, Environ. Sci. Technol., 2019, 53, 3739–3747 CrossRef CAS PubMed.
  125. Y. J. Zhang, Z. J. Zhou, J. H. Lan, C. C. Ge, Z. F. Chai, P. Zhang and W. Q. Shi, Appl. Surf. Sci., 2017, 426, 572–578 CrossRef CAS.
  126. L. Wang, W. Tao, L. Yuan, Z. Liu, Q. Huang, Z. Chai, J. K. Gibson and W. Shi, Chem. Commun., 2017, 53, 12084–12087 RSC.
  127. D. Humelnicu, C. Blegescu and D. Ganju, J. Radioanal. Nucl. Chem., 2014, 299, 1183–1190 CrossRef CAS.
  128. L. Wang, L. Yuan, K. Chen, Y. Zhang, Q. Deng, S. Du, Q. Huang, L. Zheng, J. Zhang and Z. Chai, ACS Appl. Mater. Interfaces, 2016, 8, 16396–16403 CrossRef CAS PubMed.
  129. L. Wang, H. Song, L. Yuan, Z. Li, Y. Zhang, J. K. Gibson, L. Zheng, Z. Chai and W. Shi, Environ. Sci. Technol., 2018, 52, 10748–10756 CrossRef CAS PubMed.
  130. P. Zhang, L. Wang, L.-Y. Yuan, J.-H. Lan, Z.-F. Chai and W.-Q. Shi, Chem. Eng. J., 2019, 370, 1200–1209 CrossRef CAS.
  131. S. Wang, H. Sun, H. M. Ang and M. O. Tade, Chem. Eng. J., 2013, 226, 336–347 CrossRef CAS.
  132. Q. Hu, D. Sun, Q. Wu, H. Wang, L. Wang, B. Liu, A. Zhou and J. He, J. Phys. Chem. C, 2013, 117, 14253–14260 CrossRef CAS PubMed.
  133. Q. Hu, H. Wang, Q. Wu, X. Ye, A. Zhou, D. Sun, L. Wang, B. Liu and J. He, Int. J. Hydrogen Energy, 2014, 39, 10606–10612 CrossRef CAS.
  134. Á. Morales-García, A. Fernández-Fernández, F. Viñes and F. Illas, J. Mater. Chem. A, 2018, 6, 3381–3385 RSC.
  135. I. Persson, J. Halim, H. Lind, T. W. Hansen, J. B. Wagner, L. Å. Näslund, V. Darakchieva, J. Palisaitis, J. Rosen and P. O. Persson, Adv. Mater., 2019, 31, 1805472 CrossRef PubMed.
  136. J. Low, L. Zhang, T. Tong, B. Shen and J. Yu, J. Catal., 2018, 361, 255–266 CrossRef CAS.
  137. Y.-Y. Chen, Q. Lin, Y.-M. Zhang, H. Yao, T.-B. Wei, Y.-Q. Fan, X.-W. Guan, G.-F. Gong and Q. Zhou, Spectrochim. Acta, Part A, 2019, 218, 263–270 CrossRef CAS PubMed.
  138. S. Ma, D. Yuan, Z. Jiao, T. Wang and X. Dai, J. Phys. Chem. C, 2017, 121, 24077–24084 CrossRef CAS.
  139. X.-F. Yu, Y.-C. Li, J.-B. Cheng, Z.-B. Liu, Q.-Z. Li, W.-Z. Li, X. Yang and B. Xiao, ACS Appl. Mater. Interfaces, 2015, 7, 13707–13713 CrossRef CAS PubMed.
  140. A. Yadav, A. Dashora, N. Patel, A. Miotello, M. Press and D. Kothari, Appl. Surf. Sci., 2016, 389, 88–95 CrossRef CAS.
  141. F. Liu, A. Zhou, J. Chen, J. Jia, W. Zhou, L. Wang and Q. Hu, Appl. Surf. Sci., 2017, 416, 781–789 CrossRef CAS.
  142. S. J. Kim, H.-J. Koh, C. E. Ren, O. Kwon, K. Maleski, S.-Y. Cho, B. Anasori, C.-K. Kim, Y.-K. Choi and J. Kim, ACS Nano, 2018, 12, 986–993 CrossRef CAS PubMed.
  143. N. Li, X. Chen, W.-J. Ong, D. R. MacFarlane, X. Zhao, A. K. Cheetham and C. Sun, ACS Nano, 2017, 11, 10825–10833 CrossRef CAS PubMed.
  144. F. Liu, A. Zhou, J. Chen, H. Zhang, J. Cao, L. Wang and Q. Hu, Adsorption, 2016, 22, 915–922 CrossRef CAS.
  145. X. Gao, Z.-K. Li, J. Xue, Y. Qian, L.-Z. Zhang, J. Caro and H. Wang, J. Membr. Sci., 2019, 586, 162–169 CrossRef CAS.
  146. H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and X. Wang, Chem. Soc. Rev., 2014, 43, 5234–5244 RSC.
  147. R. Li, L. Zhang, L. Shi and P. Wang, ACS Nano, 2017, 11, 3752–3759 CrossRef CAS PubMed.
  148. N. Liu, N. Lu, Y. Su, P. Wang and X. Quan, Sep. Purif. Technol., 2019, 211, 782–789 CrossRef CAS.
  149. A. Shahzad, K. Rasool, M. Nawaz, W. Miran, J. Jang, M. Moztahida, K. A. Mahmoud and D. S. Lee, Chem. Eng. J., 2018, 349, 748–755 CrossRef CAS.
  150. S. Cao, B. Shen, T. Tong, J. Fu and J. Yu, Adv. Funct. Mater., 2018, 28, 1800136 CrossRef.
  151. H. Zhang, M. Li, J. Cao, Q. Tang, P. Kang, C. Zhu and M. Ma, Ceram. Int., 2018, 44, 19958–19962 CrossRef CAS.
  152. Z. Zeng, Y. Yan, J. Chen, P. Zan, Q. Tian and P. Chen, Adv. Funct. Mater., 2019, 29, 1806500 CrossRef.
  153. Q. Tang, Z. Zhou and P. Shen, J. Am. Chem. Soc., 2012, 134, 16909–16916 CrossRef CAS PubMed.
  154. A. Enyashin and A. Ivanovskii, Comput. Theor. Chem., 2012, 989, 27–32 CrossRef CAS.
  155. F. Méndez-Arriaga, S. Esplugas and J. Giménez, Water Res., 2008, 42, 585–594 CrossRef PubMed.
  156. H. Yang, G. Li, T. An, Y. Gao and J. Fu, Catal. Today, 2010, 153, 200–207 CrossRef CAS.
  157. J. Hartmann, P. Bartels, U. Mau, M. Witter, W. Tümpling, J. Hofmann and E. Nietzschmann, Chemosphere, 2008, 70, 453–461 CrossRef CAS.
  158. Y. Liu, R. Luo, Y. Li, J. Qi, C. Wang, J. Li, X. Sun and L. Wang, Chem. Eng. J., 2018, 347, 731–740 CrossRef CAS.
  159. G. Huang, S. Li, L. Liu, L. Zhu and Q. Wang, Appl. Surf. Sci., 2020, 503, 144183 CrossRef CAS.
  160. O. Mashtalir, K. M. Cook, V. Mochalin, M. Crowe, M. W. Barsoum and Y. Gogotsi, J. Mater. Chem. A, 2014, 2, 14334–14338 RSC.
  161. J. Ran, G. Gao, F.-T. Li, T.-Y. Ma, A. Du and S.-Z. Qiao, Nat. Commun., 2017, 8, 13907 CrossRef CAS PubMed.
  162. Q. Liu, X. Tan, S. Wang, F. Ma, H. Znad, Z. Shen, L. Liu and S. Liu, Environ. Sci.: Nano, 2019, 6, 3158–3169 RSC.
  163. U. Ghosh and A. Pal, J. Ind. Eng. Chem., 2019, 79, 383–408 CrossRef CAS.
  164. Z. W. Seh, K. D. Fredrickson, B. Anasori, J. Kibsgaard, A. L. Strickler, M. R. Lukatskaya, Y. Gogotsi, T. F. Jaramillo and A. Vojvodic, ACS Energy Lett., 2016, 1, 589–594 CrossRef CAS.
  165. B. Ding, W.-J. Ong, J. Jiang, X. Chen and N. Li, Appl. Surf. Sci., 2020, 500, 143987 CrossRef CAS.
  166. B. Huang, N. Zhou, X. Chen, W.-J. Ong and N. Li, Chem. – Eur. J., 2018, 24, 18479–18486 CrossRef CAS PubMed.
  167. X. Cheng, L. Zu, Y. Jiang, D. Shi, X. Cai, Y. Ni, S. Lin and Y. Qin, Chem. Commun., 2018, 54, 11622–11625 RSC.
  168. C. Liu, Q. Xu, Q. Zhang, Y. Zhu, M. Ji, Z. Tong, W. Hou, Y. Zhang and J. Xu, J. Mater. Sci., 2019, 54, 2458–2471 CrossRef CAS.
  169. W. Zhou, J. Zhu, F. Wang, M. Cao and T. Zhao, Mater. Lett., 2017, 206, 237–240 CrossRef CAS.
  170. Y. Gao, H. Chen, A. Zhou, Z. Li, F. Liu, Q. Hu and L. Wang, Nano, 2015, 10, 1550064 CrossRef CAS.
  171. W. Lian, L. Wang, X. Wang, C. Shen, A. Zhou and Q. Hu, Funct. Mater. Lett., 2019, 12, 1850100 CrossRef CAS.
  172. Y. Gao, L. Wang, A. Zhou, Z. Li, J. Chen, H. Bala, Q. Hu and X. Cao, Mater. Lett., 2015, 150, 62–64 CrossRef CAS.
  173. J. Li, S. Wang, Y. Du and W. Liao, Ceram. Int., 2018, 44, 7042–7046 CrossRef CAS.
  174. S. Luo, R. Wang, J. Yin, T. Jiao, K. Chen, G. Zou, L. Zhang, J. Zhou, L. Zhang and Q. Peng, ACS Omega, 2019, 4, 3946–3953 CrossRef CAS PubMed.
  175. A. Tariq, S. I. Ali, D. Akinwande and S. Rizwan, ACS Omega, 2018, 3, 13828–13836 CrossRef CAS PubMed.
  176. A. Jastrzębska, A. Szuplewska, T. Wojciechowski, M. Chudy, W. Ziemkowska, L. Chlubny, A. Rozmysłowska and A. Olszyna, J. Hazard. Mater., 2017, 339, 1–8 CrossRef PubMed.
  177. A. Rafieerad, W. Yan, G. L. Sequiera, N. Sareen, E. Abu-El-Rub, M. Moudgil and S. Dhingra, Adv. Healthcare Mater., 2019, 1900569 CrossRef PubMed.
  178. H. Lin, Y. Chen and J. Shi, Adv. Sci., 2018, 5, 1800518 CrossRef PubMed.
  179. C. Dai, H. Lin, G. Xu, Z. Liu, R. Wu and Y. Chen, Chem. Mater., 2017, 29, 8637–8652 CrossRef CAS.
  180. Q. Xue, H. Zhang, M. Zhu, Z. Pei, H. Li, Z. Wang, Y. Huang, Y. Huang, Q. Deng and J. Zhou, Adv. Mater., 2017, 29, 1604847 CrossRef PubMed.
  181. A. Szuplewska, D. Kulpińska, A. Dybko, M. Chudy, A. M. Jastrzębska, A. Olszyna and Z. Brzózka, Trends Biotechnol., 2019 DOI:10.1016/j.tibtech.2019.09.001.

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