Review of MXenes as new nanomaterials for energy storage/delivery and selected environmental applications Byung-Moon Jun1,§, Sewoon Kim1,§, Jiyong Heo2, Chang Min Park3, Namguk Her2, Min Jang4, Yi Huang5, Jonghun Han2 (), and Yeomin Yoon1 ()
1 Department of Civil and Environmental Engineering, University of South Carolina, Columbia, 300 Main Street, SC 29208, USA 2 Department of Civil and Environmental Engineering, Korea Army Academy at Young-Cheon, 495 Hogook-ro, Kokyungmeon, Young-Cheon, Gyeongbuk
ABSTRACT Energy and environmental issues presently attract a great deal of scientific attention. Recently, two-dimensional MXenes and MXene-based nanomaterials have attracted increasing interest because of their unique properties (e.g., remarkable safety, a very large interlayer spacing, environmental flexibility, a large surface area, and thermal conductivity). In 2011, multilayered MXenes (Ti3C2Tx, a new family of two-dimensional (2D) materials) produced by etching an A layer from a MAX phase of Ti3AlC2, were first described by researchers at Drexel University. The term “MXene” was coined to distinguish this new family of 2D materials from graphene, and applies to both the original MAX phases and MXenes fabricated from them. We present a comprehensive review of recent studies on energy and environmental applications of MXene and MXene-based nanomaterials, including energy conversion and storage, adsorption, membrane, photocatalysis, and antimicrobial. Future research needs are discussed briefly with current challenges that must be overcome before we completely understand the extraordinary properties of MXene and MXene-based nanomaterials.
KEYWORDS MXenes, MXene-based nanomaterials, energy storage, environment, applications
1 Introduction Energy and environmental issues are presently at the forefront of global attention [1, 2]. Heavy-duty energy conversion/storage devices are required both for electric vehicles [3] and stationary batteries [4]. While current energy conversion/storage methods/devices including water-splitting, batteries, and supercapacitors have been well developed, further development is still essential because: (i) it is difficult to achieve contemporaneous high energy conversion and power density and (ii) rechargeable devices are expensive and restricted in terms of use [5]. Two-dimensional (2D) materials including graphene/graphene-based nanocomposites [6, 7], metal- organic frameworks [8, 9], and MXenes [10, 11] have attracted increasing attention because of their unique physicochemical properties. Graphene/graphene-based nanocomposites may possibly serve as next-generation energy devices [12], but MXenes are also very attractive in this context, being very safe, and exhibiting large interlayer spacing, environmental flexibility, and outstanding biocompatibility [13]. In addition, such electrodes are able not only to store large quantities of charge per unit volume but also to discharge rather rapidly [14]. Various nanomaterials have found different applications in water/wastewater treatment [15–18], catalysts/ energy conversion [19–24], and environmental sensing [25, 26]. For
example, graphenes adsorb heavy metals [27–29] and organic contaminants [30–36] from water and wastewater. Table 1 summarizes the removal of selected heavy metals and organic contaminants by graphene-based nanoadsorbents. Recently, metal-organic frameworks (i.e., inorganic-organic hybrid composites) have been widely used as adsorbents removing both inorganic and organic contaminants [37, 38]. However, MXenes may be better in these contexts, because of their exceptional physicochemical properties (a large surface area, excellent stability, a high melting point, outstanding oxidation resistance, extraordinary electrical/thermal conductivity, and hydrophilicity) [10, 39]. MXenes and MXene-based materials have been used as innovative adsorbents and as next-generation membranes removing environmental contaminants [40–43].
A few recent reviews on the use of MXenes used for energy conversion and storage have appeared [5, 44, 45]; these differ from our present review in that we include all recent developments. In addition, while there is a review study on environmental applications of MXenes, that study is still limited to membrane only [46]. To the best of our knowledge, no comprehensive review of the environmental applications of MXenes has appeared. Thus, it is useful to explore how MXenes and MXene-based materials technologically enhance environmental applications. Our review is structured as follows: (i) fabrication and properties of MXenes; (ii) the use of MXenes for
energy conversion and storage; and (iii) the adsorptive, membrane, photocatalytic, and antimicrobial properties of MXenes. We also herald future research on MXenes for energy and environmental applications.
2 Fabrication and properties of MXenes
2.1 Fabrication of MXenes
Several recent studies have comprehensively reviewed the current fabrication status of MXenes [47–49]. In brief, in 2011, multilayered MXenes (Ti3C2Tx, a new family of 2D materials) produced by etching an A layer from a MAX phase of Ti3AlC2, were first described by researchers at Drexel University [50]. MAX phases consist of hexagonally layered, ternary transition metal carbides, carbonitrides, and nitrides of the general formula Mn+1AXn, where M stands for an early transition metal (e.g., Cr, Nb, Ti, V, and etc.), A stands for an element from groups (e.g., Al, As, Cd, P, and etc.), X stands for carbon and/or nitrogen, and n = 1, 2, or 3 [51, 52]. The term “MXene” was coined to distinguish this new family of 2D materials from graphene, and applies to both the original MAX phases and MXenes fabricated from them [53]. Multilayered MXene flakes were created via wet etching with hydrofluoric acid (HF) [54–56], HCl-LiF [57–59], or HCl-NaF [60] yielding various MXenes including Cr2TiC2 [61], Mo2Ti2C3 [61], Mo2TiC2 [61], Mo2ScC2 [62], Mo2C [63], (Nb0.8Zr0.2)4C3 [64], (Nb0.8Ti0.2)4C3 [64], Nb2C [65], Nb4C3 [64], Ti3CN [61], TiNbC [61], Ta4C3 [61], Ti4N3 [66], Ti2C [61], Ti3C2 [50], (Ti0.5Nb0.5)2C [61], (V0.5Cr0.5)3C2 [61], and V2C,25 Zr3C2 [63].
Figure 1(a) shows the historical timeline of MXene fabrication:
(i) in 2011, the first multilayered MXene (Ti3C2Tx) was developed [50]; (ii) in 2012, various MXenes including Ta4C3Tx, Ti2CTx, Ti3CNTx, (TiNb)2CTx, and (V,Cr)3C2Tx appeared [67]; (iii) in 2013, single-layer MXenes were isolated via intercalation and delamination with organic compounds [61]; (iv) in 2014, the use of in situ HF etchants including HCl-LiF [11] or NH4HF2 [68] to produce MXenes was introduced; (v) in 2015, large-scale delamination of various MXenes employing an amine-assisted method or tetrabutylammonium hydroxide was presented [69]; (vi) in 2015, delamination of large single flakes of Ti3C2Tx (< 2 μm in thickness without sonication) by modifying the HCl-LiF-etching method was reported [70]; and (vii) in 2017, synthesis of an MXene with ordered divacancies (Mo1.3CTx) from (Mo2/3Sc1/3)2Al was described [71]. Of the various MXenes (over 70 members) produced by selective etching of atomic layers from different MAX phases, Ti3C2Tx is one of the most commonly studied. Figure 1(b) shows the synthesis of Ti3C2Tx using various etching methods employing HF or in situ HF-etching [53]. HF at three different concentrations (5%, 10%, and 30% by weight) was applied for various times (5, 18, and 24 h); all conditions adequately fabricated Ti3C2Tx. However, in the in situ method, hazardous HF was used indirectly (as HCl-LiF or NH4HF2) over an etching time of 24 h. Employing either HF or NH4HF2, delamination is achieved by addition of large organic molecules dissolved in dimethyl sulfoxide or tetramethylammonium hydroxide, followed by sonication. With HCl-LiF, a clay-like Ti3C2Tx intercalated with Li+ can be delaminated with or without sonication (thus, via minimally intensive layer delamination), depending on the etchant concentrations (HCl and LiF) [53].
Table 1 Summary of removal of selected heavy metals and organic species by graphene-based nanoadsorbentsa
Adsorbent Contaminant Co (mg/L) Experimental condition qm (mg/g) Main finding Ref.
Graphenes Sb 3 pH 3–11 temp. = 303 K synthetic water
8.1 The pseudo-second-order kinetic shows the best correlation for the adsorption, suggesting that the process was governed by the chemical process
[29]
GOs Pb 1–60 pH 5.6 single/mixed solution synthetic water
227.2 Once used as filter media in sand columns, GO efficiently removed Pb from water. In the mixed solution system, while Pb showed good sorption performance, the GOs-sand columns still effectively reduced both contaminants
[213]
rGOs Cd 0.2–10 pH 2–12 temp. = 298 K synthetic water
0.499–6.32 The oxygen-containing groups might be first occupied by Cd over 1-naphthol and the straight H-bonding could be replaced by a moderately strong cation– interaction between Cd and 1-naphthol
[214]
rGOs–iron oxide
Pb 10–15 pH 5 temp. = 303 K synthetic water
455 Adsorption ability of Pb(II) on rGOs-iron oxides is considerably lower than that of Pb(II) on GOs-iron oxides, owing to the difference between GOs-iron oxides and rGOs-iron oxides
[215]
Graphenes Oils toluene chloroform
0–100 Spongy graphene synthetic seawater
86,000 The results exhibited some excellent features of spongy graphene: quick and efficient absorption of organic solvents and oils, high restoration of absorbates, recyclability, and long-life of the sorbent material
[31]
GOs 17-estradiol 0.05–4 pH 3–12 temp. = 298–338 K various background ions
129–149 The large adsorption affinity of GOs for 17-estradiol could be mostly owing to hydrogen bonds and– interactions. The adsorption capacity was insignificantly influenced by the solution pH
[216]
rGOs Sulfapyridine sulfathiazole
0–75 pH 1–12 temp. = 298 K synthetic WW
35–191 (sulfapyridine)34.9–245 (sulfathiazole)
The uncharged species showed strong adsorption affinity and high contributions to the general adsorption of sulfonamides, which attributed to the great hydrophobicity and -electron accepting capability
[217]
GOs–Fe3O4 Tetracycline antibiotics
50 pH 3–10, various ionic strength room temp. synthetic water
35.5–45.0 It is extraordinary that the ionic strength and the pH of solution nearly had no effect on the adsorption process, which makes GOs–Fe3O4 to be the possible adsorbents in the application of removing TCs from environment water samples
[218]
GOs–Fe3O4–SiO2
p-nitrophenol 100–800 pH 3–9, various ionic strength temp. = 298–318 K
1,142–1,549 The Fe3O4@mSiO2/GO hybrid composites might be readily separated from solutions through an external magnetic force
[219]
a Co = initial concentration; qm = maximum adsorption capacity; GOs = graphene oxideds; rGOs = reduced graphene oxides; temp. = temperature.
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The structural, mechanical, electronic, surface, optical, magnetic, and transport properties of MXenes are significantly influenced by the MXene precursor used [10], the etchant [49], the etching protocol, the intercalation method [72], and sonication frequency [5, 53]. Figure 2 shows top and side views of the pristine MXene M2X (Fig. 2(a)), Model M2X-1 with two functional groups on top of hollow site A (Fig. 2(b)), and Model M2X-2 with two functional groups on top of hollow site B (Fig. 2(c)) [45]. As transition metal ions have co-ordination numbers of six, it is assumed that transition metals in MXenes form six chemical bonds to X atoms and surface chemical groups, causing production of M2XO2, M2XF2, and M2X(OH)2 [73]. In addition to Models M2X-1 and M2X-2, two other configurations are possible; in Model M2X-3, two functional groups are on top of a single hollow site and, in Model M2X-4, one functional group is on top of hollow site A and the other on top of hollow site B [73]. When sufficient transition metal electrons are donated to both X and attached functional groups, Model M2X-2 is the most solid configuration [73].
In theory, mechanical tensile stress is influenced by F, OH, and O functionalization in Tin+1Cn; Ti2CO2 tolerates biaxial and uniaxial tensions much higher than are acceptable to graphene, because surface functionalization reduces atomic layer breakdown, enhancing mechanical elasticity [74]. For example, the exceptional solidification afforded by oxygen functionalization is presumably attributable to substantial charge transfer between inner Ti–C and outer Ti–C bonds [75]. Chemical exfoliation of MAX phases is very commonly used when synthesizing MXenes, but mechanical exfoliation is also possible [45]. General bonding in the ab plane is stronger than in the c direction, presumably because C33 < C11 in terms of the elastic constants of some MAX phases [76, 77]. Both MAX phases and pristine MXenes are metallic, but some MXenes exhibit semicon-ductive properties via surface functionalization [73]. Several theoretical studies found that the MXenes were both topologically trivial metals/semiconductors and topologically non-trivial semimetals/
semiconductors, depending on the flexibility of relativistic spin-orbit coupling [78, 79].
Only a few experimental studies have evaluated MXene optical properties using Ti3C2Tx thin films [67, 80]. A thin film based on a MAX phase (i.e., Ti3AlC2) exhibited relatively low transmittance (30%) [67], but visible light (550 nm) transmittance rose to approximately 80% for Ti3C2Tx [80] and to 90% for intercalated Ti3C2Tx/NH4HF2 (Ti3C2.3O1.2F0.7N0.2) of various Ti3C2Tx thicknesses [80]. In general, both pristine and most functionalized MXenes would be expected to be nonmagnetic because of solid covalent bonding between the transition metal, the X element, and attached functional groups [81]. However, even strong covalent bonds can be modified by external strain, triggering magnetism based on the discharge of d electrons [82]. In fact, most pristine MXenes become naturally magnetic. For example, near-semi-metallic ferromagnetism was evident in pristine Ti2N and Ti2C of formula units 1.9 and 1.0 mB, respectively [83]. A few theoretical studies have explored electronic transport in MXenes [79, 84]. When comparing pristine and functionalized Ti3C2, the type of surface functionalization greatly affected electronic transport. For example, the current for any given bias voltage of Ti3C2F2 was fourfold that of pristine Ti3C2 [84].
3 MXenes for energy applications
3.1 Energy conversion
Chemical energy is directly converted into electrical power by fuel cells [85]. In general, the common electrochemical processes are oxygen reduction, oxygen evolution, and hydrogen evolution, used in water-splitting, regenerative fuel cells, and metal-air batteries, respectively [86]. During oxygen reduction, H2O or H2O2 is produced by reaction of oxygen with electrons and protons at the cathodes of metal-air batteries and fuel cells [85]. The lack of effective oxygen reduction catalysts has limited commercialization of metal-air batteries [87]. Recently, Xue et al. reported superb oxygen reduction by Mn3O4 nanocomposites combined with Ti3C2 MXenes produced by the simple method shown in Fig. 3(a); this is the first example of a non-noble metal-MXene composite catalyst mediating oxygen reduction [88]. The Mn3O4 nanoparticles are evenly distributed on MXenes in the nanocomposite structure (Fig. 3(b)). This is very important; highly conductive MXene layers permit direct charge transfer, greatly improving electrode performance [89]. Rotating disk
evaluation showed that the Mn3O4-MXene nanocomposites had the same initial potential (0.89 V) as Pt/C, but a greater positive potential than Mn3O4-acetylene black (0.80 V) (Fig. 3(c)) [88]. Both the positive initial potential and high current density of Mn3O4-MXene nanocomposites contribute to the outstanding oxygen reduction capacity. Also, the high-level electron transfer of Mn3O4-MXene nanocomposites reflects a superior catalytic capacity. The composites were also more stable than Mn3O4-acetylene black (Fig. 3(d)), presumably because highly conductive MXene enhanced both the conductivity and stability of Mn3O4-MXene nanocomposites [88]. In particular, wide-open, terminal metal sites (Ti) afforded considerably higher redox reactivity than carbon nanomaterials [90].
Electrochemical oxygen evolution is critical in terms of renewable energy applications (metal-air batteries and water-splitting [91, 92]), but it is difficult to overcome the significant overpotential and sluggish kinetics of proton-coupled charge transfer; effective electrocatalysts are lacking [93]. Yu et al. synthesized a novel, non-precious metal electrocatalyst exhibiting outstanding structural strength, and excellent interfacial junctions and electrical properties, by layering double hydroxides synergistically combined with a highly conductive MXene (FeNi-layered double hydroxides/Ti3C2-Mxene) [92]. This catalyst exhibited a lower overpotential (240 mV) than FeNi-layered double hydroxides/2Ti3C2-Mxene (270 mV) and FeNi-layered double hydroxides/0.5Ti3C2-Mxene (298 mV). The poor catalytic behavior of both of the latter catalysts is probably attributable to the relatively small content of the active FeNi/double hydroxide phase in the former catalyst and the relatively large content of the poorly conductive FeNi/double hydroxide phase in the latter catalyst; MXene support was inadequate [92]. In another study, a new catalyst (a 2D metal-organic framework combined with the MXene cobalt 1,4-benzenedicarboxylate-Ti3C2Tx) afforded good electrocatalytic oxygen evolution, attributable to the porous structure, fast mass/ion transport, and the high proportion of exposed, active metal centers [91]. The nanocomposite afforded a current density of 10 mA/cm2 at a potential of 1.64 V, similar to that of a common IrO2-based catalyst [94], and superior to those of earlier transition metal-based catalysts (oxides and sulfides) [95, 96]. In general, although both fuel cells and metal-air batteries are commonly used for energy storage, fuel cells require optimization of oxygen reduction and metal-air batteries require both efficient oxygen reduction and oxygen evolution during discharging and charging, respectively [97].
Hydrogen has been considered an ideal clean, recyclable energy source [98, 99]. In general, most electrocatalysts for hydrogen evolution are based on Pt, but, recently many alternatives such as borides, metal sulfides, carbides, nitrides, and phosphides have been discovered [100]. However, the electrocatalytic capacities require improvement
in terms of electrical conductivity, active site reactivity, and numbers of active sites [101, 102]. Li et al. fabricated ultrathin MXene nano-sheets rich in fluorine groups, which synergistically improved electrocatalytic performance by enhancing the reactivities of active sites and the numbers of such sites [103]. Ti2CTx-rich fluorine nanocomposites exhibit very effective catalytic activity, attributable to the small onset overpotential (75 mV) and the high exchange current density (0.41 mA/cm2). Experimental findings combined with density functional theory indicate that fluorine groups not only enhance proton adsorption kinetics but also reduce charge transfer resistance. Therefore, both active site reactivity and electrode kinetics improve increasing electrocatalytic capacity [103].
Hydrogen production from water via different types of photocatalysis has attracted a great deal of research attention [104]. Of the many semiconductor-based photocatalysts, including C3N4, TiO2, CoO, and MoS2 [105], Nb2O5 exhibits exceptional chemical and photochemical properties [106]. Su et al. showed that Nb2O5-C-Nb2C (an MXene) split water to yield a very high hydrogen production rate of 7.81 μmol/(h·gcat) (four-fold that of pristine Nb2O5) [107], presumably because of increased photoinduced carrier transport and enhancement of photogenerated electron-hole pair departure. The Nb2C surface accepts and transports electrons from Nb2O5. An et al. explored the synergistic effects of Ti3C2 MXene and Pt co-catalysts combined with g-C3N4; photocatalytic hydrogen evolution was most effective [108]. Ti3C2/Pt/g-C3N4 nanocomposites exhibited high-level hydrogen production (5.1 mmol/(h·g)) attributable to an increase in reactive sites and enhanced electrical conductivity during photoreduction, which was 3–4-fold better than afforded by Ti3C2/g-C3N4 and Pt/g-C3N4. The greater activity of the Ti3C2/Pt/g-C3N4 photocatalyst was attributed to enhanced separation of photogenerated charge carriers and effective electron transfer from the g-C3N4 conduction band to Ti3C2 and Pt [108].
Several previous studies have shown extensive theoretical efforts on energy conversion and storage devices for MXenes and MXene- based nanomaterials [56, 109–111]. Tang et al. investigated the electronic properties and Li storage capability of Ti3C2 by using density functional theory (DFT) computations [56]. The findings showed that Li adsorption occurs due to a strong Coulomb interaction with Ti3C2-based hosts but well preserves its structural integrity. The original Ti3C2 monolayer shows a relatively small barrier for Li diffusion and high Li storage capacity. A separate study employed DFT theory to evaluate MXenes (M3C2) as CO2 conversion catalysts, predicting the formation of OCHO• and HOCO• radicals in the initial hydrogenation phases through impulsive reactions [110]. The findings provide atomic level understandings into the computer- aided screening for high-performance catalysts and the insights of electrochemical mechanisms for CO2 reduction to energy-rich hydrocarbon fuels. Pandey et al. also employed DFT calculations to evaluate diverse polymorphs (M2X, M3X2, and M4X3) and their performance as electrocatalysts for the hydrogen evolution reaction using the free energy of hydrogen adsorption at equilibrium coverage as an activity descriptor [111]. The results showed that the hydrogen adsorption energy could vary depending on the number of metal layers in the structure, which indicate that the catalytic activity degree of MXenes can be adjusted by controlling the layer thickness.
In addition, the mechanisms of the enhanced electrochemical performance for various MXenes/MBenes and MXene-based nanomaterials were well elucidated from theoretical point of view [112–115]. Zhou et al. theoretically evaluate various heterostructures of N-doped graphene supported by MXene monolayers as bifunctional electrocatalysts for both oxygen reduction reaction and hydrogen evolution reaction [115]. The primary results showed that the graphitic sheet on V2C and Mo2C MXenes are very active with an oxygen reduction reaction overpotential and reaction free energies for the hydrogen evolution reaction becoming zero. Those exceptional
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catalytic behaviors presumably occur due to the electronic coupling between the graphitic sheet and the MXene, which also empathize a common mechanism for controlling the catalytic properties of 2D hybrid nanomaterials. In a separate study, theoretical analyses showed that the edges of the MXene nanoribbons could readily adsorb H species and act as the reaction sites for hydrogen evolution [114]. The binding power of the MXene edge depends on the d band center of metal atoms in MXenes. Particularly, the nanoribbons of Ti3C2 and solid solution exhibit high activity for the hydrogen evolution reaction with the sorption free energy becoming zero. These theoretical findings highlight the principle for fabricating MXene nanostructures for electrocatalysts with fast kinetics, and shed light on the application of MXenes with more than one metal element for a wide range of electrochemical reactions. Guo et al. investigated new 2D crystals (Mo2B2 and Fe2B2; MBenes = extended 2D MXene family) for Li ion batteries and electrocatalysis [112]. Those 2D MBenes showed greater catalytic activity for the hydrogen evolution reaction with hydrogen adsorption Gibbs free energy approaching the optimal value (0 eV), suggesting their promising application as an electrocatalyst for hydrogen evolution. A separate study exhibited that the 2D MBene sheet and its functionalized products show metallic ferromagnetic performance with the Curie temperature above room temperature ranging from 345 to 600 K [113]. The ferromagnetism and metallicity primarily arise from the p–d hybridization mechanism and crystal field splitting.
3.2 Energy storage
3.2.1 Supercapacitors
A supercapacitor is an energy storage device intermediate between batteries and conventional capacitors; the power density is greater than that of a battery and the energy density greater than that of a conventional capacitor [85]. Supercapacitor energy storage is of three types: (i) a rapidly reversible redox reaction (e.g., using hydroxides or transition metal oxides); (ii) reversible chemical adsorption/desorption (e.g., oxygen adsorption onto Au or Pt); and, (iii) reversible electrochemical doping/de-doping (e.g., using a polypyrrole or polyaniline) [116]. One conventional supercapacitor is a double-layer capacitor used for energy storage; a double electric interface lies between the electrolyte used for charge storage and the electrode-active material [6]. Such capacitors exhibit outstanding strengths because both charge and discharge processes are principally physical in nature, but the capacities remain relatively small [85]. Pseudo-capacitors (i.e., asymmetric supercapacitors) use oxidation- reduction or reversible electrochemical adsorption/desorption for energy storage [117], allowing deposition of materials on various surfaces and sub-surfaces [118].
2D titanium carbide has often been used to form supercapacitor electrodes, affording good electrical conductivity, a high melting point, good hydrophilicity, and high-level electrochemical surface action [119]. Ti3C2Tx-reduced graphene oxides (rGOs) were used to fabricate asymmetric flexible micro-supercapacitors, greatly improving ion transport in devices with interdigitated electrodes [58]. A Ti3C2Tx-rGO asymmetric micro-supercapacitor exhibited an energy density of 8.6 mWh/cm3 at a power density of 0.2 W/cm3 under a 1-V voltage window; 97% of the initial capacitance remained after 10,000 cycles. Figure 4(a) shows the preparation of asymmetrical, flexible Ti3C2Tx-rGO nanocomposites [58]. Fu et al. reported a large areal capacitance (314 F/cm3) of Ti3C2Tx-single walled carbon nanotubes self-assembled into nanocomposite electrodes; 95% of the initial capacitance remained after 10,000 cycles at a mass loading of 1.7 mg/cm2 [59]. A high areal capacitance (610 F/cm3) was evident even at this high mass-loading rate. Figure 4(b) shows the preparation of a Ti3C2Tx-single walled carbon nanotubes self-assembling composite film [59]. Xia et al. synthesized BiOCl nanosheets immobilized on
the surfaces of Ti3C2Tx MXene nanocomposite flakes via simple and cost-effective chemical bath deposition [120]. Figure 4(c) shows nanocomposite preparation. A nanocomposite electrode exhibited excellent volumetric specific capacitances of 397 F/cm3 at 1 A/g and 228 F/cm3 at 15 A/g. In addition, a very high energy density (15.2 Wh/kg) at a power density of 567 W/kg was evidenced by a symmetrical supercapacitor assembled using BiOCl nanosheets/ immobilized Ti3C2Tx MXene nanocomposites; the performance was significantly better than that of an earlier symmetric supercapacitor created using Ti3C2Tx MXene nanocomposites [121]. The former supercapacitor exhibited cycling capability; the volumetric capacitance fell by only 5% after 5,000 cycles at 10 A/g [120].
3.2.2 Batteries
Batteries are essential energy-storage devices; improvements are critical because of demands posed by consumer electronics and electric vehicles [39]. Supercapacitors afford large power densities, short recharging times, and long operation times, but remain expensive and of low efficiency [122]. Unlike lead-acid batteries, Ni–Cd, Ni–metal–hydride, and Li batteries are inexpensive, exhibit high energy densities, are extremely stable, have high cycle lives, exhibit good electronic conductivities, and do not self-discharge [123]. In the time since Li batteries were developed in the 1960s, graphite has been the most widely employed anodic material, with a specific capacity of 372 mAh/g [124]. Although all of Ni, K, Ca, Mg, and Al batteries have been studied [125], the development of new materials for Li batteries is essential. In this context, MXenes are valuable candidates, exhibiting high electrical conductivity, rapid molecule and ion transport, low operating voltages, and high storage capacities [44]. Naguib et al. were the first to explore the potential of MXenes as new anodic materials for Li batteries; an exfoliated 2D Ti2C MXene was fabricated by etching Al from Ti2AlC [54]. The new material exhibited a significantly greater surface area (approximately 10-fold) compared to that of the MAX phase (Ti2AlC), and also a five-fold greater specific capacity (225 mAh/g) than Ti2AlC, attributable to the open structure, larger surface area, relatively weak bonds between MX layers on the Ti2C surface, and larger interlayer gaps between exfoliated Ti2C sheets [54].
2D heterostructures combining MoS2 with conductive supports have been used for energy storage [126]. Chen et al. fabricated 2D MoS2-MXene heterostructures with metallic properties, as revealed
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by computational analyses [127]; the structures exhibited large, reversible capacities of approximately 550 mAh/g at a current density of 50 mA/g, presumably because the MXene (Ti3C2Tx) improved Li and Li2S adsorption by stabilizing heterostructures during intercalation and conversion [128]. Organic molecules are often used to prevent layer restacking, thus retaining the interlayer spaces [129]. Glycine- MXene hybrids have been used to enhance such spacing and to form possible Ti–N bonds, improving charge storage and cycling [57]. Glycine-Ti3C2Tx nanocomposite electrodes exhibited a capacitance (325 F/g) at 10 mV/s about twice that of pristine Ti3C2Tx electrodes, presumably attributable to the lower ion diffusion-resistance of
the former nanocomposite, associated with enhanced interlayer spacing. The electrodes were extremely stable and indeed, capacitance improved somewhat after 20,000 cycles as the interlayer spacing in-creased further [57]. Huang et al. fabricated sandwich-like Na0.23TiO2 nanobelt/Ti3C2 MXene nanocomposites for high-drain Li/Na batteries; these exhibited a significant increase in capacity (from 47 to 138 mAh/g) when the current density decreased from 3 to 0.1 A/g
[130]. Good charge retention (approximately 90%) was evident after 180 cycles at 0.1 A/g; after 4,000 cycles, a discharge capacity of 56 mAh/g was achieved at a high current density (2 A/g). Table 2 summarizes recent MAX phase/MXene materials; their synthetic
Table 2 MXenes: Summary of material composition, synthetic method, morphology, application, and performance in energy storage and conversion modified from [44]a
Mo2Ga2C Mo2CTx HF etching/DFT, SEM, XRD Nanosheets Hydrogen evolution reaction 609 mV at 10 mA/cm2 [220]Ti3AlC2 Ti3C2-C3N4 HF etching/XRD, SEM, TEM, AFM Nanosheets Oxygen evolution reaction 210 mV at 10 mA/cm2 [221]Ti3AlC2 Ti3C2Tx HCl–LiF/XRD, TEM, SEM Clay Supercapacitors 900 F/cm3 at 2 mV/s [11] Ti3AlC2 Ti3C2Tx-EG HF etching/XRD, TEM, SEM Nanosheets Supercapacitors 33 F/cm2 at 2 mV/s [222]Ti3AlC2 Ti3C2Tx HF etching/EIS, XRD Paper Supercapacitors 340 F/cm3 at 1 A/g [10] Ti3AlC2 Ti3C2Tx-CNT HF etching/XRD, SEM, TEM Film Li-ion batteries 1,250 mAh/g at 0.1 C [55] Ti2AlC Ti2COx HF etching/SEM Particles Li-ion batteries 225 mAh/g at 1 C [54] Ti3AlC2 Ti3C2Tx HF etching/NA Film Supercapacitors 528 F/cm3 at 2 mV/s [56] Ti3AlC2 Ti3C2Tx HF etching/TEM, SEM, CA, XRD, EIS Nanosheets Supercapacitors 517 F/g at 1 A/g [223]Hf3Al4C6 Hf3C2Tz HF etching/XRD, SEM, TEM, AFM Nanosheets Li-ion batteries
Na-ion batteries 1,567 mAh/cm3 at 200 mA/g 504 mAh/cm3 at 200 mA/g
[125]
V2AlC V2C HCl–NaF/XRD, SEM, XPS, TEM Nanosheets Li-ion batteries 467 mAh/g at 50 mA/g [60] Mo2TiAlC2 Mo2TiC2 HF etching/DFT, SEM, TEM Paper Li-ion batteries 176 mAh/g at 1 C [70] Nb2AlC3 Nb2C3Tx HF etching/XRD, SEM, XPS Nanosheets Li-ion batteries 222 mAh/g at 4 C [64] Ti3AlC2 Ti3C2 HF etching/XRD, SEM, TEM, PSD Paper Li-ion batteries 410 mAh/g at 1 C [150]Ti3AlC2 Ti3C2Tx NH4F-hydrothermal/XRD, SEM, Raman, XPS Nanosheets Supercapacitors 141 F/g at 2 A/g [224]Mo2Ga2C Mo2TiCx HCl–LiF/TEM, XPS, SEM Paper Supercapacitors
Li-ion batteries 196 F/g at 2 mV/s 560 mAh/g at 0.4 A/g
[225]
Nb2AlC Nb2CTx-CNT HF etching/XRD, TEM, SEM Paper Li-ion batteries 400 mAh/g at 0.5 C [69] Ti2AlC Ti2COx HF etching/XRD NA Li-ion batteries 65 mAh/g at 10 C [226]V2AlC V2CTx HF etching/SEM, TEM Nanosheets Li-ion batteries 260 mAh/g at 1 C [65] V2AlC NaV2CTx-HC HF etching/EIS, XRD Nanosheets Na-ion batteries 100 F/g at 0.2 mV/s [227]Ti2AlC Ti2CTx HF etching/TEM Nanosheets Na-ion hybrid capacitor 90 mAh/g at 1 C [228]Ti3AlC2 Ti3C2Tx HF etching/NA Nanosheets Na-ion batteries
K-ion batteries 370 mAh/g at 0.2 mV/s 260 mAh/g at 0.2 mV/s
[229]
NA Sulphur-Ti2C NA/SEM, XPS Nanosheets Li–S batteries 1,200 mAh/g at 5 C [230]Ti3AlC2 Ti3C2Tx-polypyrrole HCl–LiF/SEM, TEM, FTIR Paper Supercapacitors 416 F/g at 5 mV/s [231]Ti3AlC2 Ti3C2Tx HF etching/XRD, SEM, TEM, NMR Nanosheets Na-ion batteries 270 mAh/g at 20 mA/g [232]Ti3AlC2 d-Ti3C2Tx-glycine HCl–LiF/SEM, Raman, FTIR, XRD Film Li-ion batteries 324 F/g at 1 V/s [57] Ti3AlC2 Ti3C2Tx-rGO HCl–LiF/XRD, SEM Nanosheets Supercapacitors 8.6 mWh/cm3 at 0.2 W/cm3 [58] Ti3AlC2 Ti3C2Tx-CNT HCl–LiF/XRD, SEM, TEM Nanosheets Supercapacitors 314 F/cm3 at 1.7 mg/cm2 [59] Ti3AlC2 Na0.23TiO2-Ti3C2 HF etching/SEM, TEM, XRD, XPS Sandwich Li/Na-ion batteries 673 mAh/g at 3 C [130]Ti3AlC2 Ti3C2Tx HF etching/NA Nanosheets Supercapacitors 2.8 mWh/cm3 at 0.225 W/cm3 [233]Ti3AlC2 Ti3C2-rGO HF etching/XRD, Raman, XPS, SEM, TEM Film Li-ion batteries 336 mAh/g at 0.05 A/g [234]Ti3SiC2 Ti3C2Tx-CMK5 HF etching/XRD, SEM, TEM Nanosheets Li-ion batteries 749 mAh/g at 1 C [235]Ti3AlC2 Li4Ti5O12-Ti3C2Tx HF etching/SEM, XRD, TEM Nanosheets Li-ion batteries 116 mAh/g at 10 A/g [236]Ti3AlC2 Ti3C2Tx-SnS2 HF etching/SEM, TEM, XRD Sandwich Na-ion batteries 882 mAh/g at 0.1 A/g [237]Ti3AlC2 BiOCl-Ti3C2Tx HF etching/XRD, SEM, TEM, XPS Nanosheets Supercapacitors 397 F/cm3 at 1 A/g [120]Ti3AlC2 Ti3C2Tx-CNT HF etching/XRD, SEM Nanosheets Li-ion batteries 489 mAh/g at 0.05 A/g [238]Ti3AlCN Ti3C2Tx HCl–LiF/TEM, AFM, SEM Nanosheets Supercapacitors 61 mF/cm2 at 5 μA/cm2 [239]Ti3AlC2 SnS-Ti3C2Tx HF etching/XRD, Raman, XPS, SEM, TEM Nanosheets Na-ion batteries 413 mAh/g at 0.1 A/g [240]
aAFM = atomic force microscopy; CA = contact angle; CMK5 = mesoporous carbon; CNT = carbon nanotube; DFT = density functional theory; EG = exploited graphene; EIS = electrochemical impedance spectroscopy; FTIR = Fourier-transform infrared spectroscopy; HC = hard carbon; HF = hydrofluoric acid; NA = not available; NMR = nuclear magnetic resonance; PSD = particle size distribution; rGO = reduced graphene oxide; SEM = scanning electron microscopy; TEM = transmission electron microscopy; XPS =X-ray photon spectroscopy; XRD = X-ray diffraction.
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methods, characterization techniques, morphologies, energy con-version and storage applications, and performances.
4 MXenes for environmental applications
4.1 Adsorption
Many waterborne inorganic and organic contaminants occur worldwide [131–136]; effective water and wastewater treatments are essential. Existing methods include coagulation-flocculation- sedimentation, softening, addition of granular/powdered activated carbon or carbon nanomaterials, oxidation (e.g., chlorination, ozonation, ultraviolet (UV) light oxidation, and sonodegradation), biological degradation, and membrane filtration [16, 17, 137–148]. Adsorption is favored, being feasible, extensively applicable, and relatively low-cost [149]. As is true of graphene-based nanomaterials, MXenes can be intercalated with, and delaminated into, nanosheets; various organics (e.g., urea [150] and methylene blue [42, 151]) and heavy metals (e.g., Ba [152], Cr [43], Hg [153], Cu [154], and U [155]) naturally intercalate various MXenes, suggesting that MXenes may be valuable adsorbents; it is thus necessary to understand the interactions between MXenes and contaminants. Aqueous removal of contaminants by MXenes is affected by the nature of the contaminants (e.g., inorganic/heavy metals or organics, size/shape, functional group, hydrophilicity, and charge), the properties of the adsorbent (e.g., surface area, functional group, hydrophilicity, and charge), and water quality (e.g., adsorbate concentration, inorganic/ organic issues, pH, and temperature).
The MXene Ti3C2Tx adsorbed the cationic dye methylene blue from water to the extent of approximately 40 mg/g [42], thus lower than that of commercial activated carbons (up to 1,000 mg/g) [156], but comparable to that of materials having similar surface areas (pristine kaolinite (approximately 15 mg/g) [157] and graphenes-Fe3O4 (approximately 45 mg/g) [158]). Thus, MXenes of various structures with diverse surface chemistries may adsorb as well as do other 2D materials [42]. Methylene blue intercalation with MXenes is possible, given the behaviors of dyes with other layered materials such as graphite oxide [159] and clay nanoadsorbents [160]. X-ray diffraction revealed that the c-lattice constant of the MXene interlayer space (0.7–2 nm) [152] was larger than the methylene blue molecule (1.7 × 0.76 × 0.325 nm3 (l × w × h) [161]). The interlayer spacing of Ti3C2Tx was expanded by tuning the surface functional groups (from –F to –OH) [151]. The adsorption capacity for methylene blue of different functionalized MXenes decreased in the order NaOH-Ti3C2Tx (184 mg/g) > LiOH-Ti3C2Tx (119 mg/g) > Ti3C2Tx (100 mg/g) > KOH-Ti3C2Tx (74.2 mg/g). The adsorption behavior of alk-Ti3C2Tx followed the Langmuir model, indicating that alk-Ti3C2Tx exhibited homogeneous adsorption surfaces and engaged in single-layer adsorption. X-ray diffraction indicated that the c-lattice constants increased in alk-Ti3C2Tx MXenes, presumably via a “pillar effect” imparted by surrounding alkali metal ions. Although exfoliation or splitting of alk-Ti3C2Tx MXenes caused by increments in the c-lattice constants (30% for NaOH-Ti3C2Tx and LiOH-Ti3C2Tx, and 20% for KOH-Ti3C2Tx) remain of concern, both NaOH and LiOH enlarge MXene interlayer spacing, allowing facile intercalation of methylene blue molecules into the interlayer between alk-Ti3C2Tx sheets, parallel to their surfaces. However, dye adsorption is not entirely attributable to intercalation of KOH-Ti3C2Tx and the increment in the c-lattice constant is relatively small [151].
MXenes adsorptively removed various heavy metals (Ba [152], Cr [43], Hg [153], Cu [154], and U [155]) from waters of various qualities. 2D Ti3C2Tx nanosheets removed > 99% of Ba2+ under optimized conditions; the adsorption capacity was 9.3 mg/g [152]. The pH was important during adsorption, because it affects adsorbent surface charge and both speciation and ionization [162]. In general, maximum Ba2+ adsorption onto Ti3C2Tx nanosheets was evident at
pH 6–7 because: (i) at acidic pH values, MXenes with less-negative surfaces (the pH point of zero charge = pH 2.7 [154]) exhibited lower affinities for heavy metal actions; (ii) at neutral pH, the MXenes were negatively charged, modulated principally by electrostatic attraction; and (iii) at basic pH values, Ba(OH)2 precipitated, and was thus removed from the bulk solution [163]. Figure 5 shows adsorption of Ba2+ onto MXene surfaces: (i) adsorption occurred in intercalated and unstacked MXene interlayers [164]; and (ii) adsorption was facilitated by –OH, –F, and –O groups on MXene surfaces [152]. Delaminated Ti3C2Tx afforded significantly more Cu2+ removal (40.9 mg/g) than commercial activated carbon (15.1 mg/g) [154]. The Cu2+ adsorption capacity of nanosheets increased from 15 to 40 mg/g as the pH rose from 2 to 5, but rapidly decreased at pH 6 (to 20 mg/g), because: (i) at low pH, delaminated Ti3C2Tx nanosheets were less negatively charged than at higher pH values, reducing electrostatic attraction between nanosheets and Cu2+ and creating high-level competition between H+ and Cu2+ for adsorption sites [165]; and (ii) at pH 5–6, adsorption was minimal because of Cu2+ precipitation as Cu(OH)2, depending on the solution Cu2+
concentration [166]. Although TiO2-based materials have been widely used as
photocatalysts, their adsorption capacities for both organic and inorganic compounds are rather low [167, 168]. Urchin-like rutile TiO2-C combined with a Ti3C2(OH)0.8F1.2 MXene nanocomposite exhibited a high Cr(VI) adsorption capacity (approximately 225 mg/g), which was significantly greater than those of pristine Ti3C2(OH)0.8F1.2 MXene (62 mg/g) and TiO2-C (11 mg/g) [43]. Adsorption of Cr(VI) varied by the solution pH: (i) at pH 1–6, adsorption increased slightly with increasing pH, presumably because of mutual attraction between Cr(VI) ions and TiO2 in the rutile/MXene nanocomposite; under acidic conditions, monovalent HCrO4
− predominated, and sequestered protonated Ti-OH2
+ rather weakly [169]; but (ii) under basic conditions, strong electrostatic repulsion was in play between Cr2O7
2− and deprotonated Ti-O–, significantly reducing Cr(VI) adsorption [170]. In addition, high-level-OH competition reduced Cr(VI) adsorption [43].
An MXene completely removed Ba2+ within 60 min; this is one of the fastest rates reported [152], while Perlite required > 60 min to remove only 80% of Ba2+ [171]. Ti3C2Tx nanosheets exhibited fast Ba2+ adsorption kinetics (approximately 35% Ba2+ removal within a few seconds), because many Ba2+-binding sites were available [171] given the multilayer nature of the MXene and complete saturation of all available adsorption sites via chemical bonding with Ba2+. A pseudo-second-order model rather accurately described Cu2+ adsorption kinetics onto delaminated Ti3C2Tx nanosheets [154], possibly via capture by surface functional groups such as –F, –O, and –OH [172], the hydrophilic nature of the MXene, and the fact that surface functional groups (C-Ti-Fx, C-Ti-(OH)x, and C-Ti-Ox) engaged in strong electrostatic reactions via inner-sphere complex formation between positively charged Cu2+ and negatively charged
terminal groups (–O and –OH) on the surfaces of delaminated nanosheets [173].
Thermodynamic parameters such as the standard entropy change (ΔS0), the standard enthalpy change (ΔH0), and the standard free-energy change (ΔG0) yield in-depth information on internal energy changes associated with adsorption. The Cu2+ adsorption capacity of delaminated Ti3C2Tx nanosheets was examined at 298, 308, and 318 K (Cu0 = 25 mg/L at pH 5) [154]. A slight increase in maximum adsorption capacity (39.2 to 43.3 mg/g) with increasing temperature suggested that Cu2+ adsorption onto MXene nanosheets was endothermic. In addition, the positive ΔS0 (0.152 J/(mol·K)) and the increasingly negative ΔG0 (–22.0 to –25.1 kJ/mol) recorded as temperature increased indicated that adsorption was both spontaneous and occurred at high affinity; at higher temperatures, adsorption was rapid after simple dehydration of Cu2+ [174]. Shahzad et al. observed similar thermodynamic trends when Hg(II) was adsorbed by magnetic Ti3C2Tx nanocomposites [153]. Relatively high adsorption levels of 54.2, 54.8, 55.4, and 56.6 mg/g were evident at 288, 298, 308, and 318 K, respectively; the ΔG0 values became increasingly negative (–27.0 to –34.9 kJ/mol) with increasing tem-perature, indicating strong and spontaneous adsorbent-adsorbate interactions.
It is assumed that ion adsorption affinities vary by the selectivities and chemical potentials of adsorbents. Ba2+ removal by Ti3C2Tx nanosheets occurred in the presence of several other metal ions including As, Ca, Cr, Pb, and Sr, added to evaluate their effects on Ba2+ adsorption [152]. Ba2+ removal was highest (98%), followed by Ca/Pb/Sr (94%) and As (approximately 70%), indicating that Ba2+ was most effectively removed by the MXene, possibly because (i) adsorption was affected by ion electronegativity (As (2.2) > Cr (1.56) ≥ Pb (1.55) > Ca (1.04) > Sr (0.99) ≥ Ba (0.97)) and material affinity [175]; and (ii) surface hydroxyl groups of the MXene bound Ba2+ more strongly than other metal ions [176]. Table 3 shows the adsorptive removals of selected dyes and heavy metals by MXenes.
4.2 Membrane
Membrane processes such as forward osmosis, reverse osmosis, nanofiltration, ultrafiltration, and microfiltration have been commonly used to treat water and wastewater [15, 138, 140, 177–181]. Membrane filtration affords several advantages (low-level/no chemical requirement,
high removal efficiency, and a small carbon footprint), but trade-offs are apparent between membrane selectivity and permeability [182]. In recent years, many efforts have been made to overcome this limitation by developing membranes made of ultrathin 2D materials including graphene-based nanomaterials (e.g., graphenes, graphene oxides (GOs), and rGOs) [183, 184], metal-organic frameworks [185–187], covalent organic nanosheets [188–190], and MXenes [40, 41, 191]. Compared to conventional materials, the 2D materials allow fabrication of ultrathin coating/separating layers enabling rapid passage of selected solutes, affording both excellent permeation and remarkable selectivity [46]. In particular, MXene-based membranes on different support materials have been used to separate waterborne organic and inorganic components (Fig. 6).
Ding et al. fabricated a 2D lamellar MXene membrane on an anodic aluminum oxide support; the membrane exhibited extremely high water permeability (> 1,000 L/(m2·h·bar)) and rapid removal (> 90%) of Evans blue, cytochrome, and gold nanoparticles [40]. The extent of bovine serum albumin removal (> 99%) and that of rhodamine B (85%) allowed estimation of membrane pore size (2–5 nm). Therefore, size exclusion explained the removal of large solutes/particles (Evans blue (1.2 nm × 3.1 nm), cytochrome (2.5 nm × 3.7 nm), and gold nanoparticles (5 nm)) from water; a small molecule (< 1 nm) such as K3[Fe(CN)6] (0.9 nm × 0.9 nm) was not readily removed (32% only). The water permeability of the membrane (1,120 L/(m2·h·bar)) was approximately 10-fold higher than that of commercial polyethersulfone membranes with nominal molecular weight cut-offs of 30 kDa (111 L/(m2·h·bar)) and 50 kDa (130 L/ (m2·h·bar)), and the MXene membrane afforded much better removal (32%–> 99%) than commercial membranes (0.15%–34%) of all solutes/particles tested [40]. The enhanced water permeability is attributable to the nanochannels and large interspaces associated with the loosely lamellar structure of the MXene membrane. Also, a new 2D MXene-polyethersulfone composite membrane exhibited relatively small water fluxes (57.5 and 58.8 L/(m2·h·bar)) attributable to fouling with high-level removal of Congo red (93%) and gentian violet (80%); larger water fluxes (230, 316, and 218 L/(m2·h·bar)) were evident during low-level inorganic salt removal (23% of MgCl2, 13.2% of Na2SO4, and 13.8% of NaCl) [41]. The pristine polyethersulfone ultrafiltration membrane (nominal molecular weight cut-off 10 kDa) did not effectively remove these inorganic salts (< 5% of the totals).
Table 3 Summary of adsorptive removal of selected species by MXenesa
MXene Species Co (mg/L) Experimental condition qm (mg/g) Main finding Ref.Ti3C2Tx Methylene blue 50 Kinetic = 8 h
temp. = 293 K synthetic water
38.9 The adsorption ability of Ti3C2Tx for methylene blue is somewhat lower than commonly used commercial activated carbons, but similar with other materials
[42]
Ti3C2Tx
Ti3C2Tx-LiOH Ti3C2Tx-NaOH Ti3C2Tx-KOH
Methylene blue 50 pH 6–9 temp. = 298 K synthetic water
99.9 119 184 74.2
Adsorption degree alk-Ti3C2Tx follows the Langmuir model, which indicates that alk-Ti3C2Tx have homogeneous adsorption surfaces and only can have one adsorbate layer
[151]
Ti3C2Tx Ba 1–55 pH 2–13 temp. = 298 K synthetic water
9.3 The maximum adsorption capacity (9.3 mg/g at Ba = 55 mg/L) is higher than the adsorption capacity of other adsorbents such as activated carbons and carbon nanotubes
[152]
Ti3C2Tx Cu 25 pH 2–6 temp. = 298–318 K synthetic water
86.5 The adsorption capacity of delaminated-Ti3C2Tx was approximately 3 times higher than that of a commercially available activated carbon
[154]
Ti3C2Tx U 100 pH 5 temp. = 293 K synthetic water
214 The adsorption of U(VI) in hydrated versus dry Ti3C2Tx was significantly enhanced, primarily due to the more flexible nature and much larger interlayer space of hydrated Ti3C2Tx
[155]
Magnetic Ti3C2Tx Hg 25–1,000 pH 2–9 temp. = 288–318 K synthetic water
1,128 The maximum experimental adsorption capacity for Hg(II) by magnetic Ti3C2Tx was significantly greater than that previously reported for 2D nano-materials and nanocomposites
[153]
Ti3C2(OH)0.8F1.2 Cr 10 pH 3–6 temp. = 298K synthetic water
62 The chemical exfoliation of MXene generally takes place in solutions containing F ions, and then the surface of MXene is covered by F groups
[43]
a Co = initial concentration; qm = maximum adsorption capacity; temp. = temperature.
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In addition, the flux of the pristine membrane declined in the presence of Congo red and gentian violet, indicating that the introduction of MXene nanoparticles into the membrane surface reduced hydrophobicity and thus, membrane fouling [41].
Compared to a 2D MXene-polyethersulfone composite membrane [41], a MXene-Ag membrane of average pore size 2.1 nm exhibited slightly better removal of inorganic salts (26%, 41%, and 50% of NaCl, MgCl2, and AlCl3, respectively) at similar water fluxes (approximately 420 L/(m2·h·bar)) [191]. In addition, rhodamine B (79.9%), methyl green (92.3%), and bovine serum albumin (> 99%) were effectively removed; however, both the extent of removal and flux were significantly affected by membrane thickness. The optimum membrane thickness for rhodamine B removal (79.9%) was 470 nm at a water flux of 387 L/(m2·h·bar); the flux fell significantly to 234 and 125 L/(m2·h·bar) at membrane thicknesses of 969 and 1,420 nm with a similar extent of rhodamine B removal (81.6% and 82.8%). An MXene-Ag membrane fouled with methyl green and bovine serum presented at 50 mg/L exhibited remarkable flux recovery (91%–97%) compared to that of the pristine MXene membrane (81%–86%) [191], attributable to the enhanced hydrophilicity of the MXene-Ag membrane with additional nanopores. The water contact angle decreased from 40° to 35° as the Ag nanoparticle level rose from 0 to 21%, confirming that Ag improved membrane surface hydrophilicity [192]. Figure 7(a) shows a schematic of, and the mechanism of dye removal by, the MXene-Ag composite membrane.
Graphene is impermeable [193], and many studies have used graphene-based membranes for water treatment [138, 139, 183, 184, 194, 195]. Kang et al. fabricated a 90-nm-thick Ti3C2Tx-GO membrane on a porous support (polycarbonate/nylon) that inhibited target solute movement through inter-edge defects or poorly packed spaces [196]. A membrane with an effective interlayer spacing of approximately 0.5 nm exhibited high-level removal rates (68% for methyl red (neutral, 0.49 nm), 99% for methylene blue (positive, 0.51 nm), 94% for Rose Bengal (negative, 0.59 nm), and 100% for Coomassie brilliant blue (negative, 0.8 nm)). However, the pristine Ti3C2Tx membrane exhibited lower removal rates (40% (methyl red), 95% (methylene blue), 66% (Rose Bengal), and 95.4% (Coomassie brilliant blue)). The poor removal of methyl red is attributable to its relatively small size and hydrodynamic diameter, and its neutral charge. However, Rose Bengal removal was less effective than that of methylene blue although the former molecule is larger, presumably because the negative charge of the former molecule triggered low-level electrostatic attraction to the MXene surface [196]. Ionic salts (NaCl, Na2SO4, MgSO4, and MgCl2) were very poorly removed (< 10%). Thus, the interlayer spaces of the swollen Ti3C2Tx-GO membrane were too large to allow of complete salt ion removal, but the negatively charged membrane surface removed some ions via electrostatic repulsion. Figure 7(b) shows schematics of the steric exclusion of hydrated ions and dyes by the Ti3C2Tx-GO membrane. Table 4 summarizes the removal of selected species by MXene membranes.
4.3 Photocatalytic and antimicrobial applications
When two cationic dyes (methylene blue and acid blue 80) were subjected to UV for 5 h in the presence of Ti3C2Tx, degradation was very rapid (81% and 62%, respectively) [42], but acid blue 80 was not degraded at all and methylene blue to only 18% over 20 h in the dark, presumably because UV triggered formation of TiH4O4 and/or TiO2 on the Ti3C2Tx surface, enhancing photocatalysis [197, 198]. Although the photocatalytic mechanisms in play require further study, it is possible that, as is true for graphene-titania hybrids, MXene-supported TiO2 composites may find applications in catalysis, environmental remediation, energy conversion, and energy storage [42]. Li et al. showed that photocatalytic degradation of methylene blue was enhanced under UV in the presence of titania-carbon nanocomposites derived from 2D Ti2CTx [199]. Figure 8(a) shows
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a schematic of high-energy ball-milling of Ti2CTx to form titania- carbon. Such nanocomposites have valuable photocatalytic applications; carbon efficiently improved TiO2 photocatalytic capacity by inhibiting recombination of photoexcited e− and h+ pairs [200, 201]. Furthermore, such recombination was inhibited by O–Ti–C bond formation in the nanosheets; these bonds transported photo-excited electrons from the conduction band of TiO2 to the nanosheets. In particular, disordered, uniform, graphitic carbon nanosheets bearing immobilized TiO2 nanoparticles serve as highly conductive electron acceptors [199]. In another study, Ti3C2-TiO2-CuO nanocomposites were fabricated via decomposition of mixtures of 2D MXene (Ti3C2) (Fig. 8(b)) [202]. The nanocomposites exhibited excellent photocatalytic degradation of methyl orange (> 99% in 80 min); the photoelectron separation vacancies of CuO and Ti3C2 were enhanced by UV, explaining photocatalysis by these nanocomposites (Fig. 8(c)).
The control of microbial activity is often very challenging; surfaces are exposed for long periods to complex media rich in bacteria and nutrients [203]. In particular, biofouling of membranes used in wastewater treatment plants is a major issue in that membrane efficiency is seriously compromised [204]. Surface nanomaterials inhibit bacterial growth via nano-bio interaction [205]. Pandey et al. found that an MXene-Ag (21%) membrane inhibited E. coli much more than did pristine MXene and control polyvinylidene difluoride membranes [191]. In addition, the Ag nanoparticle levels in MXenes correlated positively with growth inhibition, presumably because both Ag and TiO2 particles formed on the membrane surfaces. TiO2 on the MXene surface enhanced antimicrobial behavior, particularly under UV [206]. Ag nanoparticles are commonly used as antimicrobial agents [207]. Rasool et al. showed that a Ti3C2Tx membrane on a polyvinylidene difluoride support inhibited Bacillus subtilis and E. coli growth (> 99%) much more effectively than did the control polyvinylidene difluoride membrane (73% for Bacillus subtilis and 67% for E. coli) [208]. In another study, the antibacterial activity of a Ti3C2Tx MXene against Gram-negative E. coli and Gram-positive Bacillus subtilis [209] was explored. A MAX dispersion exhibited very low antimicrobial rates (14% for E. coli and 18% for Bacillus subtilis). However, when bacteria were exposed to a colloidal solution
of delaminated Ti3C2Tx MXene, the growth inhibition rates were 98% and 97%, respectively. Similar trends were evident in previous studies; several graphene- and cellulose-based nanomaterials exhibited stronger antimicrobial activities against Gram-positive than against Gram-negative bacteria, presumably reflecting bacterial cell-wall differences [210, 211]. Gram-negative and Gram-positive membranes are both negatively charged but differ in terms of the isoelectric point (approximately pH 4–5 and pH 7, respectively) [212]. Thus, at pH 7, E. coli with a relatively more negative charge could resist the Ti3C2Tx MXene more so than Bacillus subtilis [209].
a Co = organic initial concentration; NA = not available.
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5 Conclusions and outlook While there are various nanomaterials for broad applications in energy capture and storage, MXenes have quickly developed over the previous decade into a various group of types, hybrids, and composites for numerous applications. We have reviewed progress in the use of MXenes and MXene-based materials for energy and environmental applications. Over the last few years, substantial progress has been made. The materials show great promise in terms of energy conversion and storage, being very safe, and exhibiting very large interlayer spaces, environmental flexibility, and outstanding biocompatibility. The electrochemical performance of MXene-based materials is enhanced by increasing the specific surface area, electrical conductivity, hydrophilicity, and stability. Numerous findings have shown that MXene-based nanomaterials have achieved great capacitance performances in neutral, aqueous, and sulphuric acid electrolytes, indicating that these are very promising materials as supercapacitor electrodes. In addition, various MXene-based nano-materials have shown great capacity and stability throughout the charge/discharge processes of Li batteries. In addition, Na and other larger ions were intercalated into the interlayers of various MXene-based nanomaterials, suggesting the favorable potential of the nanomaterials as well for battery–capacitor hybrid devices.
The MXene-based nanomaterials may also play useful roles in environmental processes. MXene-based nanomaterials have shown great potential adsorption abilities in the removal of a diverse range of inorganic and organic contaminants, presumably due to their large surface areas having different functional groups. The sorption process for inorganic and organic contaminants appears to be affected mainly by the properties of compounds, solution chemistry, and surface chemistry/properties of MXene-based nanomaterials. MXene-based nanomaterials are promising alternatives to traditional adsorbents (e.g., granular/powdered activated carbon) and other nanomaterials (e.g., graphene-based nanomaterials) widely used to treat water and wastewater. The recent studies have shown that MXene-based membranes on different support materials have very high potentials to effectively remove organic and inorganic contaminants, while the water permeability of their membranes is also much higher than that of commercial membranes. In particular, controlling or combining these emerging 2D materials successfully into a membrane structure has appeared to considerably improve membrane performance based on water flux, membrane fouling, which could ultimately result in longer lifetime, lower energy consumption, and lower maintenance cost.
While current energy conversion and storage methods/devices have been developed, further development of high-performance MXene-based nanomaterials is critical for the following reasons: (i) it is difficult to contemporaneously achieve high-level energy conversion with adequate energy storage; and (ii) existing power supplies for rechargeable devices are expensive and suboptimal. In particular, appropriate spacing of MXene-based layers is essential, obviating the need for restacking, maintaining a high surface area, and allowing target molecules and ions to intercalate. MXene-based nanomaterials must be cost-competitive. Also, in addition to MXene- based nanomaterials both energy and environmental implications of MBenes and MBene-based nanomaterials may need to be considered. While these emerging MXene-based nanomaterials are promising in the design of great performance membranes, numerous challenges have to still be overcome to develop accurate nanochannels with long term stability. Comprehensive ecotoxicological assessments and life-cycle analyses of MXene-based nanomaterials are imperative, but MXene-based nanomaterials remain very promising.
Acknowledgements This research was supported by the Korea Ministry of Environment,
‘GAIA Project, 2018002470005’ (Republic of Korea). This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B04033506), and a grant (code 19IFIP-B088091-06) from Industrial Facilities & Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government (Republic of Korea).
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