Ligand chemistry for surface functionalization in MXenes: A review

Abstract

Surface chemistry of MXenes is of significant interest due to its potential to control their final optoelectronic and physicochemical properties, and address the oxidation and dispersion stabilities of MXenes. Surface chemistry of MXenes can be manipulated by either MXene synthesis via chemical etching or post surface functionalization method. Although numerous reviews have explored MXene synthesis methods, there has been a lack of focus on post surface functionalization. This review aims to fill this gap by summarizing recent advancements in the MXene surface functionalization chemistry, and elucidating mechanisms, properties, and future perspectives of functionalized MXenes. We discuss organic ligand molecules, such as organic salts, catechols, phosphonates, carboxylates, and silanes, which can be employed to surface-functionalize MXene through covalent or non-covalent bond interaction. This comprehensive review offers valuable insights for scientists and engineers in utilizing functionalized MXenes across diverse applications, including EMI shielding, energy storage, electronics, optoelectronics, and sensors.

INTRODUCTION

MXenes, a rapidly expending family of two-dimensional materials comprising transition metal carbides, carbonitrides or nitrides. MXenes have a layered structure with a general formula of M_n+1X_nT_x, where M is an early transition metal, X is nitrogen or carbon, and T_x is a surface functional group such as OH, F, or O.^1,2 In other words, above and below are the layers of metal, between them the layers of carbon or nitrogen are found to be sandwiched, and the suffix (ene) has been used to showcase the 2D nature of graphene. Since Naguib et al. produced titanium carbide (Ti₃C₂T_x) in 2011 utilizing a wet chemical etching method using HF as the etchant, Ti₃C₂T_x MXene has been considered one of the most explored MXenes owing to their excellent conductive nature, abundant surface terminations, enlarged surface area, and good solution processability.³ Moreover, the surface terminal groups allow additional tools to tune the properties of Ti₃C₂T_x MXene via a surface chemistry strategy, enabling the development of MXene-based nanocomposites.⁴ Other two dimensional materials, such as double metal hydroxides (LDH), layered transition metal dichalcogenides (LTMD), and graphene, cannot match these merits.^5–7 Given these unique properties, MXenes have showed great potential in a variety of applications including energy storage,⁸ electronic sensing,⁹ functional coating,¹⁰ plasmonics,¹¹ electrocatalysis,¹² photocatalysis,^13–15 thermal heater,¹⁶ flexible electrode,¹⁷ and electromagnetic interference shielding.^18–20

Generally, MXenes are produced from a precursor MAX phase composed of M_n+1X_n layers interlaced by an A element, which can be aluminum (Al) from IIIA or silicon (Si) from IVA group. The layered MXenes are produced when A layer is selectively etched and removed from the MAX phase (Figure 1A). Furthermore, fabrication of single or few layered nanosheets of MXene is possible through intercalation, exfoliation, and delamination of etched multilayered MXene. Some termination groups, including as F, O, and OH, are terminated during this etching process to balance and stabilize the debonded M atoms on the surface. The delamination of multilayered MXene to single flake have been achieved by using various metal or organic ion intercalants such as Li⁺, Na⁺, K⁺,²¹ TMAOH,²² TBAOH,²³ and cetyltrimethylammonium bromide (CTAB)²⁴ during or after chemical etching, possibly along with physical sonications.

FIGURE 1. (A) Schematic representation of chemical etching synthesis and delamination process of MXene from its MAX precursor (top row), and photographs and SEM images of corresponding experimental results. (bottom row) Reproduced with permission: Copyright 2019, American Chemical Society.103 Reproduced with permission: Copyright 2015, Elsevier.104 Reproduced with permission: Copyright 2016, American Association for the Advancement of Science.19 Reproduced with permission: Copyright 2021, Wiley-VCH.27 (B) Projected density of states and projected band structures for Ti2C and Ti2CF2. Reproduced with permission: Copyright 2017, Royal Society of Chemistry.31 (C) Electrical conductivities for 16 different MXenes. Reproduced with permission: Copyright 2020, American Chemical Society.18 (D) Zeta potential of the surface charge of MXene (Ti3C2Tx) depending on pH with point of zero charge (PZC). Reproduced with permission: Copyright 2019, Royal Society of Chemistry.28 (E) Illustration for catalytic activity of MXenes surface-modified with Pd toward MeOH oxidation. Reproduced with permission: Copyright 2020, American Chemical Society.33 (F) Applications of MXene dispersion in extrusion and ink-jet printing. Reproduced with permission: Copyright 2019, Nature.34 Reproduced with permission: Copyright 2017, American Chemical Society.105

MXene's inherent surface terminations are distinct and not observed in other nanomaterials such as graphene. These terminal groups are qualitatively and quantitatively determined by the etching synthesis route of MXene from its MAX phase.^25–27 Importantly, the surface terminations of MXenes significantly influences their intrinsic physicochemical, optoelectronic, and electrical properties.^28–30 For example, Yunoki and co-workers demonstrated based on DFT calculations that pristine M₂X (X = C or N) without terminations is all metallic as the Fermi level of energy is located at the d bands of the transition metal M, whereas, with terminations like F, OH, or O, additional bands are created under the Fermi level as a consequence of mixing of p-orbitals of fluorine or oxygen with metal's d orbital.(Figure 1B).³¹ This hybridization causes the shifting of the Fermi level to the middle of space present between bands.

Similarly, these characteristics also depend on the type of MXene considered. Gogotsi and co-workers discussed the metallic conductivity of a large number of MXenes, such as M₂CT_x, M₃C₂T_x, and M₄C₃T_x, as illustrated in Figure 1C. Mostly, MXenes based on titanium have higher conductivity. Recently, Ti₃C₂T_x MXene has been reported to have the highest conductivity (up to 24 000 S cm⁻¹) among all available MXenes, which arises from the number of electrons available at the Fermi energy level.³² Another notable property of MXene is its highly negative surface charge, which is mainly generated by the surface terminal groups. Figure 1D presents the zeta potential data, which demonstrates the substantial negative charge on the MXene surface in a broad range of pH (3–13), caused by the existence of hydroxyl and flourine terminal groups. The surface potential became positive after protonation at low pH.²⁸ Similarly, Lang et al. reported the importance of surface terminations on a pristine MXene surface hybrid with Pd nanoparticles for the catalytic methanol oxidation reaction.³³ By applying an electron density difference map (EDDM), they revealed the route of electron transport in the hybrid of MXene with palladium (Figure 1E). The electronic transfer occurred from MXene surface toward Pd through F and OH terminations and facilitated the methanol oxidation reaction. Furthermore, the surface terminations give MXene excellent dispersion stability in water, resulting in remarkable solution processability. The stable MXene aqueous dispersion can be used in spin coating, spray coating, dip coating, extrusion printing, inkjet printing, and painting on various substrates (Figure 1F), which demonstrates the excellent processability of MXene for practical applications.³⁴

Despite their attractive properties and versatility, some problems remain, such as low oxidation stability in water and poor dispersion stability in organic hydrophobic media, which have hindered both the scientific study of MXenes and their practical application. For example, the hydrophilic terminations of MXenes restrict their use in a broad spectrum of surface chemistry applications because MXenes are not dispersible in most organic solvents but in highly polar media, like, propylene carbonate(PC), water, dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) which may not be suitable for many practical applications (Figure 2A).³⁵ In addition, poor adherence to hydrophobic surfaces was observed for the aqueous, pristine MXene dispersion (Figure 2B).³⁵ Therefore, good MXene dispersion in organic solvents is important for achieving good processability for many potential applications, including hydrophobic polymer based MXene composites and organic compound based paints or inks. More importantly, when MXene flakes are held in a diluted aqueous solution for 2 weeks, they oxidize and lose their desirable physical and chemical characteristics such as electrical conductivity (Figure 2C,D).³⁵

FIGURE 2. (A) Pristine Ti3C2Tx suspensions in various organic solvents after being stored for 6 h. (B) Photographic images of various substrates dip-coated with pristine suspensions in aqueous media. (C) Photographic images of suspensions in aqueous media before and after oxidation at a pH of 5.4 and 20°C and the particulate solid obtained from the drying of MXene suspensions after being fully oxidized. Reproduced with permission: Copyright 2023, American Chemical Society.35 (D) High-resolution transmission electron microscopy images of a fully oxidized flake. The fully oxidized Ti3C2Tx flake converts into TiO2 and amorphous carbon. The corresponding selected area electron diffraction (SAED) patterns are shown in the upper insets. (E) Z-contrast scanning TEM (STEM) image indicating the Ti-vacancies (dark contrast regions indicated by yellow arrows) and Ti-rich areas (bright contrast regions indicated by red arrows). Reproduced with permission: Copyright 2019, Royal Society of Chemistry.37 (F) Schematic illustrating the internal electric field caused by the positive side produced around a hole with many Ti-vacancies and the negative side generated by the accumulation of electrons on the convex particle. (G) A plausible mechanism for the oxidation of flakes under acidic, neutral, and basic environments. (H) Proposed oxidation mechanism of dispersion in aqueous media. The letters in italics (a to j) refer to coefficients. Reproduced with permission: Copyright 2021, American Chemical Society.38 (I) Concentration of Ti ions measured from the supernatant of MXene aqueous dispersion before and after oxidation at a pH of 5.4 and 100°C. (J) Mechanism of TiO2 crystallization from Ti3C2Tx suspensions in basic and acidic environments. Reproduced with permission: Copyright 2022, American Chemical Society.41

The oxidation process of pristine MXene must be well comprehended in order to overcome the issues related to stability.³⁶ Xia et al. observed the oxidation process by placing the Ti₃C₂T_x MXene in vials that were wrapped in tin to completely rule out any chances of oxidation induced by light.³⁷ According to the findings, the oxidation of MXene began at the neighbor defective sites at the margins then subsequently spread to the base line of MXene. High-resolution scanning transmission electron microscopy (STEM) picture in Figure 2E indicating the atomic defects by the yellow arrow, whereas the Ti-rich regions appear as bright spots. These defects are supposed to be the sites for nucleation during oxidation, the titanium atoms oxidized to Ti positive ion and oxidation of carbon leads to formation of non-crystalline carbon at titanium vacancy. The electronic passage generates an inner electrical field due to the formation of titanium vacancies. Titanium vacancy represents the area having positive charge, which causes nearby negatively charged C⁴⁺ atoms to lose electrons and oxidize. As TiO₂ nanoparticles develop, the size of the hole and its quantity steadily expands, showing how the continuity of titanium vacancies affecting the subsequent reaction of oxidation of MXene. When the oxidation of carbon is considered the accumulation of electronic holes leads to surplus (+ve) charge, while the excess electrons in atomic flaws lead to negative charges (Figure 2F).

Koo and co-workers looked more deeply into the mechanism underlying the oxidation of titanium based MXene at different pH levels.³⁸ The hydroxyl groups are expected to be the most vulnerable site for oxidation.³⁷ The chemical reaction of the MXene surface's hydroxyl group with proton or hydroxyl ions in the water solution is strongly pH dependent (Figure 2G). The TiO bond's positively charged O atom has more electron localization than the negatively charged titanium atom because the hydroxyl group is protonated by H⁺ ions in an acidic environment. Atoms of titanium with low electron density are more vulnerable to assault by water as well as oxygen. In contrast, the deprotonated enolate (TiO⁻) group in a basic environment becomes more stable with the appearance of sodiated intermediates (TiONa⁺) comprising Na⁺ ions. Further, the coordinated Na⁺ ions allow for water capture, resulting in a bulky solvent cage.^39,40 In addition, an oxidation mechanism of aqueous dispersion suggested based on the results of X-ray diffraction, photoelectron spectroscopy (XPS), and gas chromatography time-of-flight mass spectroscopy (GC-TOF-MS) analysis is provided as equations with coefficients (letters in italics from a to j) in Figure 2H. In addition to producing gases including CH₄, CO₂, and HF, the oxidation also leaves behind nanocrystals of TiO₂ nanocrystals and solid leftovers of non-crystalline carbon. A little amount of fluorine was added to the crystal of TiO₂. In addition, the pH is lowered by the produced gases due to strong solubility in water. When a certain quantity of carbon dioxide and hydrofluoric acid is dissolved in aqueous solution, it causes increment in the concentration of H⁺ ions. Furthermore, the crystallization mechanism for the oxidized TiO₂ was comprehensively studied under different conditions, pH, and temperature (20–100°C).⁴¹ The findings show that titanium ions are initially produced as an intermediary state during disintegration of MXene caused by oxidation. After that, they adhere to the plentiful oxygen present in a medium, nucleates, and forms anatase or rutile crystals of TiO₂ depending on the pH conditions of the aqueous solution. This assertion was substantiated by the detection of titanium ions during the oxidative breakdown of water-based dispersion using inductively coupled plasma mass spectrometry (ICP-MS).⁴¹ 4.38 ppm of [Ti] ions were detected in the fully oxidized water based dispersion, whereas 1.01 ppm of [Ti] ions were observed in pristine aqueous dispersions. As a result, the pH dependence of the crystalline phase in the TiO₂ derived from MXene is strikingly comparable with that of titanium dioxide crystal generated through the traditional solvothermal approach using Ti⁴⁺-ion as a precursor, as described in Figure 2J.⁴¹

Post surface functionalization of MXenes with organic ligand molecules can address both oxidation stability and organic dispersion issues. Replacement of aqueous media with organic solvents is a promising solution because it minimizes contact with oxidation sources, such as water and oxygen molecules, leading to significant retardation of the surface oxidation of pristine MXene.⁴² Due to the ability of organic ligands to regulate the chemistry of surface, physical and chemical characteristics, manufacturing, and uses of nanomaterials, surface chemistry is in this regard the most effective technique for producing an organic dispersions of MXene with improved stability.⁴³ Mainly, two approaches are taken for surface modification via covalent and non-covalent bonds. Weak interatomic or intermolecular interactions are called “non-covalent”, they do not involve any chemical reactions. Non-covalent interactions involve hydrogen bonding (i.e., OH or O of MXene and OH or H of ligand), electrostatic interactions (negative surface or positive edge of MXene and positive cation or native anion of ligands) and van der Waals forces.⁴⁴ Surface functionalization through non-covalent bond interactions is preceded in mild and eco-friendly conditions over a short period of time without causing the oxidative degradation of MXenes.⁴⁴ However, the bond with MXene can be relatively weak, which may lead to lower stability than in the case of covalent bonding with MXene.⁴⁴ In contrast, covalent bonds are strong interactions that involve the equal sharing of electron pairs between two atoms. Examples of covalent bonds in MXenes include TiOP, TiOSi, TiOC(O), TiOC(O)NH, TiOC, TiN, and TiC.⁴⁴ Both approaches have their own merits and demerits. Covalent bonds typically facilitate stronger interactions with MXene, thus allowing for high tolerance in even harsh environments.⁴⁵ However, covalent bonding may require a complex synthetic process or inert or harsh conditions along with a long reaction time to increase the yield of the product.⁴⁵ This may cause oxidative degradation of MXene and increase the cost.⁴⁶

In this article, we aim to review recent advancements in the surface functionalization chemistry of MXenes and understand their mechanisms and the properties of MXene hybrid materials with surface modification. We provide an overview of various organic ligand systems that have been employed to form covalent or non-covalent bonds with the MXene surface (Figure 3), focusing on five key organic ligand molecules: organic salts, catechols, phosphonates, carboxylates, and silanes. The bonding chemistry between MXene surface functional groups and organic ligand molecules, as well as its resulting effect on the physicochemical properties of MXenes, are presented. This will help scientists and engineers understand the functionalization mechanisms and generate interest in the use of functionalized MXene in numerous fields, for examples EMI shielding, storage of energy, electronics, optoelectronics, sensors, and biomedicine.

FIGURE 3. Schematic illustration of MXene surface ligand chemistry using various organic ligand molecules through either covalent or non-covalent bond interaction.

SURFACE FUNCTIONALIZATION WITH ORGANIC LIGAND MOLECULES Organic salts

Organic salts, which are composed of organic cations and either organic or inorganic anions, allow binding with the MXene surface through noncovalent bonds (mainly with electrostatic interactions).⁴⁷ The formed noncovalent bonds allows for simple and easy process, but it could exhibits weak bonding strength and low stability. Meanwhile, organic salts can interact with both the positively charged edge and negatively charged surface of MXenes.⁴⁸ Importantly, the edges of MXene are generally considered active sites for oxidation reactions because they are more susceptible to humidity as well as oxygen.⁴² As a proof of concept, Barsoum and co-workers proposed edge-capped Ti₃C₂T_x MXenes to improve oxidation stability.⁴⁷ Figure 4A illustrates a strategy utilizing edge capping of MXenes with polyanionic salts to protect them from water and oxygen oxidation sources. On the other hand, pristine MXenes are unprotected, thus causing relatively fast oxidative degradation. To confirm capping of the MXene edge, STEM electron energy loss spectroscopy (EELS) was performed on a single Ti₃C₂T_x flake capped by phosphate polyanions (0.1 PTi), as shown in Figure 4B. Note that the suffixes P, B, and Si are used after the salt molar ratios (0.1 M) to indicate the addition of polyphosphate (sodium polyphosphate, Acros Organics), polyborate (sodium tertraborate decahydrate, Alfa Aesar), and polysilicate (sodium metasilicate, Alfa Aesar) salts, respectively. The signals for the Ti, C, and P components were recorded from the vacuum to the MXene flake. The line profiles in Figure 4C clearly indicate that the signal corresponding to the P component is maximum at the edges, while the signals for the Ti and C components are relatively constant from the edges inward. A small signal for the P component was also detectable on the surface of the MXene flake, but it was presumably due to the absorption of polyphosphates at the defects generated during the etching and delamination of the MXene flake. Once the successful edge capping of polyphosphates was confirmed, polysilicates and polyborates were tested to examine the universality of the developed strategy.

FIGURE 4. (A) Schematic illustration of the edge capping mechanism of Ti3C2Tx flakes with organic salts via electrostatic interaction. (B) Scanning TEM-annular dark-field image of Ti3C2Tx for the analysis of electron energy loss spectroscopy (EELS). (C) Normalized intensities of EELS signals for P, Ti, and C components from vacuum to the edge of the Ti3C2Tx flake, as indicated by the arrows. (D) XRD patterns and (E) XPS spectra of 0.1PTi, 0.1SiTi, 0.1BTi, 0Ti, and FTi, respectively. Note that the suffixes P, B, and Si represent the treated organic salts of polyphosphate, polyborate, and polysilicate salts, respectively. The 0.1 in the names indicates that the concentration of each organic salt was 0.1 M. 0Ti (no salt treated) and FTi (fresh MXene) were used as references. Reproduced with permission: Copyright 2019, Wiley-VCH.47 (F) Photographic images of Ti3C2Tx dispersion in water and the NaAsc solution on days 0 and 21. Theoretically simulated molecular configurations for Ti3C2Tx after oxidation (G) in water and (H) in the presence of NaAsc and water. (I) The results for the radial distribution function calculation of TiC bonds obtained by reactive molecular dynamics (ReaxFF) simulations. (J) XRD patterns of dried Ti3C2Tx film after being kept (i) with and (iii) without NaAsc for 21 days: (ii) fresh, pristine Ti3C2Tx, and (iv) Ti3AlC2 MAX phase. (K) Changes in normalized electric conductivity of dried Ti3C2Tx films pretreated by NaAsc and other organic acids as a function of time. Reproduced with permission: Copyright 2019, Cell Press.50

The oxidation state of the Ti₃C₂T_x flakes capped with polyanionic salts was studied using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). First, the XRD results indicated that the 0Ti sample as a control exhibited peaks of the (002), (004), and (006) planes of MXene flakes at 6.5°, 13.0°, and 20.0°, respectively (Figure 4D). In addition, clear peaks corresponding to rutile TiO₂ appear, as indicated by asterisks (JCPDS-# 12-1276). These characteristic peaks indicate the partial oxidation of Ti₃C₂T_x (Ti) to TiO₂. In contrast, these TiO₂ peaks were not observed in any of the other samples after capping with polyanions. Second, the XPS spectra of 0.1PTi, 0.1SiTi, 0.1BTi, 0Ti (no salt treated), and FTi (fresh MXene) samples were plotted along with the gray band around 459 eV corresponding to the binding energies associated with the oxidized Ti⁺⁴ state (TiO₂) (Figure 4E). A signal was observed in the gray band only for the spectrum of 0Ti, demonstrating the positive effect of these salts on the oxidation stability. The XRD and XPS results proved that the polyanions that were selected for edge capping effectively prevented the flakes of Ti₃C₂T_x MXene from getting oxidized while they were being stored.

A similar mechanism was applied for improving the stability of Ti₃C₂T_x MXenes using sodium l-ascorbate (NaAsc).^49,50 For example, Green and co-workers utilized NaAsc as an antioxidant and monitored Ti₃C₂T_x MXene dispersion in NaAsc solution and water for 21 days (Figure 4F).⁵⁰ The Ti₃C₂T_x MXene dispersion exhibited a drastic change in color from day 0 to day 21 due to the formation of TiO₂ and C when stored in water without antioxidants. However, the Ti₃C₂T_x MXene dispersion in aqueous media with NaAsc exhibited no color change or aggregation. Reactive molecular dynamics (ReaxFF) molecular simulations were carried out to explain the interaction between NaAsc and Ti₃C₂T_x nanosheet. The calculation results suggest that the higher stability of the Ti₃C₂T_x nanosheet (Figure 4G,H) can be attributed to the shielding effect by the ascorbates that interact with the Ti atoms. In addition, radial distribution function (RDF) calculations were performed to quantify the structural stability of Ti₃C₂T_x after oxidation. The Ti-C RDF calculations show a peak at 2.1 A° (Figure 4I), which is associated with a crystalline structure of Ti₃C₂T_x MXene. The peak intensity for the NaAsc system is higher than that of the water system, demonstrating a more stable Ti₃C₂T_x crystalline structure with NaAsc. Given the promising simulation results, the extent of oxidation was experimentally studied with XRD and electrical conductivity changes. As XRD patterns shown in Figure 4J, Ti₃C₂T_x capped with sodium l-ascorbate retained (002) peak at a 2θ angle of 6.5°, while the (002) peak of untreated Ti₃C₂T_x completely disappeared owing to oxidation after 21 days. The electrical conductivity of dried neat MXene film dropped below 10⁻⁸ S cm⁻¹ after being stored in water while the film made from NaAsc-treated Ti₃C₂T_x solution retained conductivity (5.7 ± 0.5 × 10² S cm⁻¹) (Figure 4K). The relative conductivity stability for the period of storage strongly supports that NaAsc prevents the oxidation of Ti₃C₂T_x MXene. On the other hand, to some extent decrease in the electrical conductivity after modification is caused by electrically insulting nature of NaAsc.

In contrast to the above edge-capping strategy, the electrostatic interaction between negative MXene surface sites and positive organic ammonium ions was applied to achieve high dispersion stability of Ti₃C₂T_x MXene. For example, Huang and co-workers suggested the utilization of cetyltrimethylammonium bromide (CTAB) to adjust the hydrophilicity of Ti₃C₂T_x MXene.⁵¹ Surface functionalization using electrostatic interactions between the negatively charged MXene surface and the positively charged N(CH₃)₃ moiety of CTAB allowed the facile formation of stable oil-in-water high internal Pickering emulsions (HIPE) (Figure 5A). The as-synthesized Ti₃C₂T_x MXenes have a negative surface potential of −35.4 mV in water due to the existence of terminal groups (F, Cl, O, and OH).⁵² Once CTAB is adsorbed on the surface of MXene, the resulting attachment of the long aliphatic alkyl chain of CTAB increases the hydrophobicity of Ti₃C₂T_x MXene. Interestingly, CTAB-treated MXene aqueous dispersions form an oil-in-water emulsion (Pickering emulsion) when an oil phase is added to the MXene aqueous dispersion. The pH of the water phase is a key factor in determining an emulsion's stability. Figure 5B demonstrates that when the pH of the MXene aqueous solution is acidic (pH < 7), stable emulsions cannot form and eventually, the aggregated MXenes gets settled at the boundary of macroscopic separation between water phase and the dodecane. However, when the pH of the MXene aqueous solution is neutral or even basic, reliable emulsions have been achieved. These findings imply that the presence of H⁺ encourages protonation of the MXene surface, which ultimately leads to displacement of the positive charged N(CH₃)₃ component of CTAB and emulsion instabilities. Figure 5C shows a fluorescent optical image of the emulsion with dodecane droplets dyed with Nile red at a pH of 8.45. Thus, a template for creating porous substances featuring a structure that is cellular may therefore be created using HIPE. The incorporation of a hydrophilic monomer (2-hydroxyethyl methacrylate) and a starting material into the continual water stage of emulsions composed of oil and water, as well as polymerization initiated by temperature, results in the production of materials with pores. (Figure 5D,E). The porous materials produced are relatively resistant to external forces owing to their high-compressional strength (Figure 5F). Similarly, Barsoum and co-workers reported that Ti₃C₂T_x MXenes functionalized with di(hydrogenated tallow)benzyl methyl ammonium chloride (DHT) achieved good dispersion stability in highly nonpolar solvents.⁵³ Briefly, the Li⁺ ions present in the multilayer space after selective etching and washing were replaced by DHT with the rapid occurrence of the cation exchange reaction. For 10 days, the Ti₃C₂T_x MXene with DHT functionalization became organophilic and easily dispersed in solvents that are nonpolar like hexane, toluene, decalin, and chloroform, as shown in Figure 5G. Ti₃C₂T_x MXenes, however, were untreated and hydrophilic, making it impossible for them to disperse in solvents having nonpolar nature. (Figure 5H). In addition, the DHT-treated Ti₃C₂T_x MXene in nonpolar solvents exhibits no sign of oxidation reaction after being dispersed for >10 days, indicating high-oxidation stability, in contrast to the aqueous Ti₃C₂T_x MXene dispersion, which shows severe oxidation within 10 days.

FIGURE 5. (A) Schematic showing the formation of oil-in-water Pickering emulsions and high internal phase Pickering emulsions (HIPEs) generated from the modified Ti3C2Tx-MXene. The blue and yellow colors indicate water and dodecane, respectively. (B) Photograph for emulsion stability as a function of pH at a ratio of 0.5 mg of CTAB to 7.5 mg of Ti3C2Tx. (C) Fluorescent image of Ti3C2Tx dispersion at a pH of 8.45. Note that the oil is labeled Nile red, and the scale bar is 200 μm. (D) Digital photograph and (E) SEM image of porous Ti3C2Tx material. (F) Image showing the resistance to an external force of a 50-g weight. Reproduced with permission: Copyright 2018, Royal Society of Chemistry.51 Photographic images for the dispersion of (G) Ti3C2Tx treated with DHT and (H) untreated Ti3C2Tx after 10 days. (I) Scheme illustrating DHT-treated Ti3C2Tx dispersion in xylene and the solution mixing process for preparing a linear low-density polyethylene (LLDPE) composite. (J) Stress–strain curves of neat LLDPE, LLDPE composite specimen with 1.12 vol% DHT-treated Ti3C2Tx, and untreated Ti3C2Tx specimens. (K) Influence of the solvent and d-spacing on the electrical conductivity of vacuum-filtered MXene films before and after drying. The results of Mo2CTx are also presented here. Reproduced with permission: Copyright 2020, Cell Press.53

To prove the composite fabrication processability with hydrophobic polymers, low-density polyethylene (LLDPE) nanocomposites were fabricated simply by adding DHT-treated Ti₃C₂T_x MXene into a solution of LLDPE pellets dissolved in a hydrophobic p-xylene solvent at 125°C (Figure 5I). Interestingly, the incorporation of 1.12 vol% DHT-functionalized Ti₃C₂T_x into LLDPE resulted in an 11% and 32% improvement in the elastic moduli and tensile strength, respectively (Figure 5J). This is attributed to the fact that DHT-functionalized Ti₃C₂T_x is homogenously distributed in the hydrophobic LLDPE matrix owing to improved hydrophobicity. In contrast, a 2% decrease in the moduli and 9.2% increment in the tensile strength were observed for the untreated MXene composites. However, one problem of this surface functionalization was a large decrease in electrical conductivity because the incorporation of insulating organic salt ligands into the gallery of MXene sheets significantly increased the d-spacing from 1.46 to 3.67 nm (Figure 5K). Additionally, the DHT-functionalized MXenes showed improved electrochemical performance for supercapacitor applications. Freestanding films prepared from DHT-treated Ti₃C₂T_x MXene. When subjected to different levels of current density, rate efficiency was demonstrated by dispersions of MXene, and the resulting electrodes exhibited capacitances of 280, 266, 253, 225, and 185 F g⁻¹ at 0.5, 1, 2, 5, and 10 A g⁻¹, respectively. After being exposed to large power excursions the amount of capacitance that was recovered is 96% of the cycle performed first at 1 A g⁻¹.

In addition, polymeric salts can not only modify the MXene surface but also provide a matrix polymer for MXene composites to improve the mechanical properties and oxidation stability. Sodium alginate (SA), a linear polysaccharide copolymer salt derived from seaweed, has functional groups (OH, COO, and O) that allow for noncovalent interactions of both electrostatic attraction and hydrogen bonding with the hydroxyl termination of MXenes. Given its advantageous properties, Faisal et al. reported a simplest and easy fabrication technique for highly flexible nacre-like MXene-SA composite films with uniform distribution for excellent EMI shielding performance.¹⁹ The schematic for the layer structure of noncovalently bonded MXene-SA composites is shown in Figure 6A. All free-standing films were fabricated by vacuum filtration of colloidal solutions of pristine MXenes or their composites. Cross-sectional transmission electron microscopy (TEM) images of the Ti₃C₂T_x-SA composite films confirmed the intercalation of SA layers between MXene nanosheets through strong hydrogen bonding and electrostatic interactions. Therefore, individual Ti₃C₂T_x flakes were homogeneously distributed in the SA matrix (Figure 6B). Pristine MXene has an electrical conductivity of 4665 S cm⁻¹. As the MXene filler loading increased, the electrical conductivity gradually increased to 3000 S cm⁻¹ at 90 wt% (Figure 6C). Surprisingly, excellent electrical conductance has been recorded for composite (MXene-SA), over 1000 S cm⁻¹ up to an SA loading of 40 wt%, owing to the strong interaction between MXene and SA. As the MXene content increased, the electromagnetic interference shielding effectiveness (EMI SE) of the composite increased to 57 dB at 90 wt% at a fixed thickness of 8 μm (Figure 6D). Pristine MXene and the MXene-SA composite demonstrated the best EMI shielding capabilities among the previously reported synthetic materials in terms of EMI SE at comparable thickness (Figure 6E). Furthermore, pristine MXene and its composite were used to compare with pure aluminum and copper foils at a thickness of 8 and 10 μm, respectively. While the electrical conductivity values of MXene are around two orders of magnitude lower than those of the foils, EMI SE values of MXene are found to be quite similar to those of the foils due to unique layered structure of the MXene films.

FIGURE 6. (A) Schematic for the preparation of sodium alginate (SA)-treated Ti3C2Tx composite. (B) TEM image of the Ti3C2Tx composite treated with 30 wt% of SA. (C) The change in electrical conductivity for Ti3C2Tx-SA composites as a function of MXene content. (D) EMI SEs of Ti3C2Tx-SA composites for different MXene contents at a fixed thickness of approximately 8 μm. (E) Comparison of EMI SEs of MXenes with the values reported in the literature. Each symbol indicates a set of material categories as follows: Ti3C2Tx MXenes (red star), Ti3C2Tx-SA composite (purple star), molybdenum MXenes (green filled circle), copper and aluminum foils (black diamond), metals (blue diamond), graphene (open circle), carbon fibers and nanotubes (open square), graphite (black filled circle), and other materials (blue filled circle). Reproduced with permissions: Copyright 2016, American Association for the Advancement of Science.19 (F) Top-view SEM image of the Ti3C2Tx-SA aerogel mesh. (G) Electrical conductivities and bulk densities of aerogel meshes prepared from Ti3C2Tx (MX), Ti3C2Tx-SA ink (MS), and Ti3C2Tx-SA ink crosslinked calcium ions (MSCa0.2). (H) Comparison of SER, SEA, and SET of MX, MS, and MSCa0.2 samples at a thickness of 700 μm. Reproduced with permission: Copyright 2023, Elsevier.54

Yu and co-workers also reported a modification of MXene with SA to adjust the rheological properties required for developing ink to print aerogel meshes.⁵⁴ The MXene-SA (MS) aerogel meshes and their aerogel meshes crosslinked with calcium ions (MSCa_0.2) resulted in the formation of continuous networks (Figure 6F). Interestingly, the inclusion of SA increased the conductive properties of the printed aerogel meshes while simultaneously making the MXene inks easier to print with. This is because the addition of SA and ions of calcium makes the MXene sheets tightly packed within the printed aerogel frameworks and guarantees the creation of a continued network, both of which are beneficial for increasing the printed aerogel meshes' conductance ability for electricity and shielding against EMI. In contrast, the additives employed to change ink rheology are insulating electricity and, in many cases, have a negative impact on the conductivity of the printed materials. As shown in Figure 6G, the MS and MSCa_0.2 aerogel meshes exhibit conductivities of 2.76 × 10¹ S cm⁻¹ and 2.85 × 10¹ S cm⁻¹, respectively, which are higher than the conductivity of the pristine MXene aerogel mesh (2.05 × 10¹ S cm⁻¹). In comparison to pure MXene, the EMI SE of MS and MSCa_0.2 aerogel meshes is as high as 65.3 dB and 64.2 dB, respectively (Figure 6H). These aerogel meshes also have a significantly higher specific EMI SE value (EMI SE/t, where t is the thickness, in mm). Given the high electrical conductivity, the aerogel meshes was shown to have great Joule heating performance over a range of temperatures from 40 to 135°C along with fast response, accurate temperature regulation, and superior cycling stability. These characteristics suggest that the developed aerogels have great potential for EMI shielding and electronic device application. In addition to the works presented in Figure 6, Zou et al. reviewed additive-intervened intercalants such as cation- and anion-ligand molecules, surface functionalized MXene hybrids, and their applications.⁵⁵ Importantly, the performance of energy and environmental,⁵⁶ biological,⁵⁷ and electronic and optical application⁵⁸ was enhanced through adapting proper molecules followed by MXene surface functionalization.

Catechols

Catechols, or ortho-dihydroxyaryl compounds, are special class of molecules with two OH groups adjacent to each other on the benzene ring.⁵⁹ The mode of surface attachment of these molecules is very strong, as found in adhesive-pad proteins secreted by marine mussels. Owing to their strong binding interactions with various substrates, catechol systems are suitable candidates for the surface functionalization of a wide range of nanomaterials. Moreover, by employing catechol ligands featuring long hydrophobic alkyl tails, the surface properties of the nanomaterials can be easily modified and regulated.

The most commonly known system, DOPA, or 3,4-dihydroxy-l-phenylalanine, has catechol functionality with an amine group and can be a suitable system for binding various inorganic surfaces.⁶⁰ Such molecules generally have a non-covalent interaction through hydrogen bonding between the catechol head and the hydroxyl groups present on the surface of MXene.³⁵ Surface functionalization with catechol molecules is considered as an easy process to handle, as it can be carried out at moderate pH conditions at room temperature and has a very short reaction time (in a minute). Ko et al. reported the utilization of single catechol-based molecules for the functionalization of MXene for improved stability and high processability.³⁵ Hydroxyl terminal groups (OH) have been utilized for surface functionalization with alkylated DOPA ligand molecules (alkylated 3,4-dihydroxy-l-phenylalanine, also called ADOPA). ADOPA1 (or AD1) and ADOPA4 (or AD4) represent the ligand molecules that are synthesized by esterification of 3,4-dihydroxy-l-phenylalanine (DOPA) with a hydrophobic tail of 1H,1H,2H,2H-perfluoro-1-hexanol and 1-decanol, respectively (Figure 7A).

FIGURE 7. (A) Schematic representation of the surface functionalization of Ti3C2Tx MXene with ADOPA. (B) TEM image of an AD1-Ti3C2Tx flake (inset: SAED pattern). (C) F 1s and N 1s high-resolution XPS spectra of ADOPA1-Ti3C2Tx. (D) MD simulation result of the number density of ADOPA1 (center of mass) as a function of the distance from the MXene surface. (E) Comparison of the electrical conductivity of AD1-Ti3C2Tx with the conductivities of other available surface-functionalized MXene systems, including C10PA-Ti3C2Tx and DHT-Ti3C2Tx. (F) Total contribution of projected density of states from carbon pz orbitals of the benzene ring in ADOPA1 adsorbed on Ti3C2(OH)2.Source: Reproduced with permission: Copyright 2022, American Chemical Society.35

Transmission electron microscopy (TEM) analysis showed that the delaminated single-sheet morphology was retained after the surface functionalization of MXene with ADOPA1 (Figure 7B). In addition, hexagonal symmetry can be seen from the selected area electron diffraction (SAED) pattern of AD1-Ti₃C₂T_x (inserted in the TEM image)³⁴ showing a stable interaction between ADOPA1 molecules and the Ti₃C₂T_x MXene surface without the generation of any defects. Moreover, as shown in Figure 7C, the high-resolution N 1s spectra of AD1-Ti₃C₂T_x show the presence of only the primary amine peak at 400.5 eV and no peak corresponding to the secondary amine group, which indicates that no polymerization of ADOPA molecules took place during the functionalization process. Similarly, the presence of two peaks for the TiF bond and CF bond in ADOPA1-Ti₃C₂T_x indicates the successful functionalization of the above molecule on the surface of MXene. This is indicative of strong hydrogen bonding interactions between the OH terminals of the MXene surface and the OH and NH₂ groups of ADOPA and efficient attachment of ADOPA molecules on the MXene surface, which was also confirmed by molecular dynamics (MD) simulations (Figure 7D). In addition, during the surface functionalization of MXene, the ADOPA molecules exhibited a parallel orientation between the MXene sheets, resulting in a minimal increase in the interlayer spacing in the dried film state. Therefore, the functionalization of ADOPA minimized the conductivity reduction. ADOPA-treated MXene (AD1-Ti₃C₂T_x) exhibited a conductivity of 6404 S cm⁻¹ which was very close to that of pristine MXene. Other control organic ligands, such as decylphosphonate (C10PA) and di-(hydrogenated tallow)benzyl methylammonium chloride (DHT), showed a significant reduction in electrical conductivity (Figure 7E). The reason for this high conductivity has been theoretically analyzed using DFT calculations. From the projected density of states, the carbon p_z orbitals of the benzene ring (not the alkylated tail) in the ADOPA molecule are significantly in the conduction band near the Fermi level, indicating that the electronic conductivity of AD1-Ti₃C₂T_x can be attributed to the strong π–π interactions between ADOPA and MXene (Figure 7F).

Owing to the presence of strong hydrophilic termination groups, pristine Ti₃C₂T_x MXenes are not homogeneously dispersed and agglomerated in organic solvents except DMSO, DMF, NMP, and PC, which have high-boiling points and toxicities. However, MXenes, surface-functionalized with ADOPA molecules, are well dispersible in various organic solvents, including ethanol, methanol, isopropyl alcohol, acetone, and acetonitrile, owing to their improved hydrophobicity (Figure 8A).³⁵ Furthermore, ADOPA functionalization chemistry is universally applicable to other types of MXenes in nine different MXene systems, including four M₂X (Ti₂CT_x, Nb₂CT_x, Mo₂CT_x, and V₂CT_x), three M₃X₂ (Ti₃C₂T_x, Ti₃CNT_x, and Mo₂TiC₂T_x), and two M₄X₃ (Mo₂Ti₂C₃T_x and Ti₄N₃T_x). The surface-functionalized MXenes exhibited excellent dispersion stability and formed a liquid crystalline phase at high concentrations in various organic solvents, as confirmed by polarized optical microscopy (POM) images (Figure 8B). Surface functionalization improves hydrophobicity and oxidation stability. The functionalized Ti₃C₂T_x MXene exhibited an increased water contact angle of 106° compared to the 60° of the pristine MXene (Figure 8C). Pristine MXene showed a 58% reduction in electrical conductivity in the 85/85 test (at 85°C and 85% relative humidity), whereas the functionalized MXene showed a conductivity decrease of less than 2% (Figure 8D). This is attributed to the passivation effect of the ADOPA ligands adsorbed on the MXene surface. This prevented the contact of water and oxygen molecules with the functionalized MXene film and enhanced the oxidation stability in the dried state even under harsh conditions. The ADOPA-functionalized Ti₃C₂T_x MXene exhibited a total shielding effectiveness (SE_T) of 80 dB at a thickness of 30 μm, comparable to that of pristine MXene (Figure 8E). The addition of ADOPA ligands to MXene also improved the dielectric relaxation behavior, which in turn increased the absorption shielding effectiveness (SE_A) and energy dissipation of electromagnetic waves. The ADOPA ligand chemistry significantly expands the utility of MXenes in coating and printing electrical circuits on hydrophobic substrates. ADOPA-functionalized MXenes bind to a variety of substrates, including metals, polymers, and particularly the extremely hydrophobic polytetrafluoroethylene (Teflon), well due to the presence of hydrophobic (fluorinated tales) and hydrophilic (DOPA) groups (Figure 8F). This makes it possible to screen print uniformly on very fine-fibered white cotton cloth (Figure 8G). The liquid-crystalline MXene paste can also make it easier to create adaptable, organized electrodes with the excellent electric conductivity required for applications in electronics (Figure 8H). The utility of the ADOPA-Ti₃C₂T_x viscous dispersion has been demonstrated through applications such as the fabrication of conductive electrodes by uniform screen-printing on fine cotton fibers to light an LED.

FIGURE 8. (A) Digital photographs of AD1-functionalized Ti3C2Tx MXene dispersion in different organic solvents (top) and nine different AD1-functionalized MXene dispersions in ethanol (bottom). (B) Polarized optical microscopy images of different AD1-MXenes at a concentration of 50 mg mL−1 in ethanol. (C) Water contact angle of pristine and AD1-Ti3C2Tx MXene films. (D) Change in electrical conductivity of AD1-, AD4-, and pristine Ti3C2Tx MXene films under 85°C and 85% RH test conditions for 20 days. (E) EMI shielding data for AD1- and pristine MXene at 12.4 GHz. (F) Digital photographs of various substrates dip coated with AD1-Ti3C2Tx MXene ethanol dispersion. (G) Digital photograph and optical microscope (red frame), SEM (blue frame), and TEM (green frame) images of fabric screen-printed with AD1-Ti3C2Tx ethanol dispersions. (H) Digital photographs of highly concentrated AD1-Ti3C2Tx paste and the electrically conductive “MXene” letters written with AD1-Ti3C2Tx.Source: Reproduced with permission: Copyright 2022, American Chemical Society.35

Similar to single catechol molecules, polymeric catechol ligand systems are also useful for the surface functionalization of MXenes. Lee et al. demonstrated that the functionalization of polydopamine on the MXene surface improved its thermal stability as well as its mechanical and EMI shielding properties.⁶¹ Figure 9A,B illustrates the overall scheme of in situ polymerization, surface functionalization, and binding mechanisms of polydopamine on the surface of MXene. Here, in situ polymerization is possible through oxidative electron transfer from dopamine to MXene, and the formation of various intermediates can be seen during this process in Figure 9B. Most of these intermediates (leucodopaminechrome and 5,6-dihydroxyindole) form a ligand-to-metal charge transfer complex (LMCT) with Ti of MXene through a coordinate bond. On the other hand, polydopamine constituents that are not involved in coordinate bonding interact with the OH terminal group of MXene (i.e., hydrogen bonds). Owing to the strong interaction (LMCT and H-bonding) between polydopamine molecules and the MXene surface, the functionalized MXene sheets were found to be highly aligned with a uniform morphology, as shown in the SEM images (Figure 9C). To further verify the uniform alignment of the sheets, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed. The resulting scattering patterns of polydopamine-functionalized MXene (PDTM) in Figure 9D indicate narrower (00l) peaks compared to those of the pure MXene film (MX), which supports the well-aligned MXene flakes after surface functionalization with polydopamine. The strong binding also led to improved adhesion force and adhesion energy in the functionalized MXene (Figure 9E). Owing to the increased adhesion energy and parallel assembly via in situ polydopamine coating, the MXenes functionalized with 5% polydopamine loading (PDTM5) exhibited 5.39 times larger tensile strength than pristine MXene (MX) (Figure 9F). Another important study was conducted to investigate the effect of polydopamine on the work function of functionalized MXenes. Figure 9G shows ultraviolet photoelectron spectroscopy (UPS) analysis showing a decrease in the working function of the functionalized MXene, which is another evidence of smooth electron transfer between the molecule and the MXene surface. This leads to polymerization, and the strong interaction of dopamine molecules with the MXene surface, which facilitates in-plane electron transport because of the improved flake alignment (Figure 9H). The surface functionalization of MXene with polydopamine not only enhances its mechanical and electrical properties but also its oxidative stability. The compact, aligned, and densely packed flakes did not allow the interaction of oxygen and water molecules with the MXene surface, and the sheet resistance remained almost the same even at high temperatures (170°C), as shown in Figure 9I. This thermal stability was studied by thermogravimetric analysis-mass spectroscopy (TGA-MS) analysis, which showed the higher stability of functionalized MXene compared to pristine MXene (Figure 9J). This is because of the removal of residual water molecules from the interlayer space of MXene flakes after functionalization, which is also responsible for the increased oxidative stability of polydopamine-functionalized MXene. The utility of polydopamine-functionalized MXene for EMI shielding applications was also studied with different film thicknesses (Figure 9K). The degree of reflection shielding effectiveness (SE_R) was quite similar for pristine and functionalized MXenes, whereas the absorption shielding effectiveness (SE_A) was enhanced in the functionalized MXene. This enhancement is due to the conduction loss from the enhanced electrical conductivity and the polarization loss from the dipoles of the polydopamine molecules.

FIGURE 9. (A) Schematic representation of the surface functionalization of MXenes with polydopamine showing bonding with OH groups of MXene. (B) Detailed process of in situ polymerization of dopamine molecule. (C) SEM cross-sectional image of polydopamine-treated MXene. (D) GIWAXS pattern of polydopamine-treated MXene. (E) Comparison of adhesion force and energy in pristine MXene and polydopamine-MXenes. (F) Mechanical properties of pristine MXene and polydopamine-treated MXene. (G) UPS data for pristine and functionalized MXene. (H) Electrical conductivity of MXene and hybrid films. (I) Comparison of change in electrical resistances of the pristine and functionalized MXenes upon heating. (J) TGA-MS data showing thermal stability after functionalization. (K) EMI shielding property of functionalized MXene with different thicknesses.Source: Reproduced with permission: Copyright 2022, American Chemical Society.61

In practical printing and coating applications, the viscosity of MXene dispersion is a key parameter. Tuning the rheological properties can be effectively achieved through surface functionalization using organic ligands.⁶² Deng et al. reported the functionalization of MXene with polydopamine (PDA) to tune its rheological properties and utilized it in important applications such as EMI shielding, anti-counterfeiting, and security patterns.⁶³ To showcase the rheological property of polydopamine-functionalized MXene, the shear thinning characteristic in Figure 10A shows that the functionalized MXene ink gives a higher steady- shear viscosity (129.8 Pa s) than pristine MXene ink (3.2 Pa s) at the same concentration of 40 mg mL⁻¹, indicating that PDA-functionalized MXene (p-MXene) is highly useful in direct ink writing, screen printing, and other related applications. Another important property and application of this hybrid material is its IR emissivity for creating IR anti-counterfeiting and security patterns. The IR emissivity increased by 21.5%–37.8% for the p-MXene-10, p-MXene-20, p-MXene-30, p-MXene-40, and p-MXene050 samples with the loading of PDA contents of 10, 20, 30, 40, and 50 wt% on MXene (Figure 10B). The increment is due to the increased loading of PDA molecules on MXene, which suggests that the IR emissivity property can be controlled by the degree of surface functionalization. Also, the IR detection temperature was investigated with different PDA loadings for higher temperatures (up to 100°C). The IR detection temperature was found to be quite low under low molecule loading (Figure 10C). This is because of the low emission characteristics of pristine MXene. Figure 10D shows that the temperatures detected by the IR thermal images are lower than that of the graphite heating plate (69.9°C), which depends positively on the infrared emissivity of the coatings. The increased adhesion between the MXene sheets due to surface functionalization is the primary contributor to the high-thermal conductivity.⁶⁴ Utilizing the above advantages of p-MXene, various complex patterns can also be drawn on simple substrates, and MXene ink can be easily used directly in pens for writing conductive patterns, as shown in Figure 10E. High-precision print of intricate and beautiful anti-counterfeiting designs is another use for the superior rheological and printing potential qualities of the inks of p-MXene. (Figure 10F). Thus, features like p-MXene inks' outstanding printing ability and notable tunability in IR emissivity can be controlled by varying the ligand loading content on MXene. This facilitates infrared anti-counterfeiting, security patterns, and many other useful devices for practical applications. In addition, p-MXene conductive ink with tunable viscosity is used to fabricate polarization microgratings for switchable EMI shielding devices (Figure 10G, red dot box). A printed micrograting can be switched to block or allow EM wave absorption or transmission by varying its direction from α = 0° to 90°. When the direction of the grating is parallel to the direction of the electric field of the incident EM waves, they can be absorbed more effectively than in the perpendicular direction. This is because the parallel lines of the coated MXene micrograting can complete the conductive path under EM wave irradiation. The shielded EM power in perpendicular direction depends on the slit width. As the slit width increases, the shielded EM power decreases in the vertical direction (Figure 10H). Utilizing microgratings with a 300 μm slit width allows for the quantitative adjustment of EM wave transmission. By altering α (angle between the orientation direction of the slit and the electric field direction) to 45°, 60°, 75°, and 90°, the degree of EM wave shielding can also be controlled to 59.7%, 40.4%, 19%, and 6.4%, respectively (Figure 10I).

FIGURE 10. (A) Rheological properties of the pristine and functionalized MXene dispersions. (B) IR emissivity properties of pristine and functionalized MXene films with different PDA contents. (C) Infrared-detected temperatures of pure and functionalized MXenes as a function of graphite substrate temperature. (D) IR thermal images of pristine and functionalized MXene with different PDA contents on graphite plate substrates at 70°C. (P1:MXene, P2:p-MXene-10, P3: p-MXene-20, P4: p-MXene-30, P5: p-MXene-40, P6: p-MXene-50.) (E) Application of moderately viscous MXene for drawing complex structures. (F) IR thermal image of a screen-printed complex flower with functionalized MXene for IR anti-counterfeiting. (G) Schematic representation of the switching mechanism of EMI shielding. (H) Effect of micrograting direction on shielding. (I) Shielded EM power as a function of micrograting direction.Source: Reproduced with permission: Copyright 2022, American Chemical Society.63

In addition to the above properties of catechol functionalized MXene, its importance and applications in various fields have been reported. For example, Li et al. demonstrated the synthesis of Ti₃C₂-polydopamine hybrid and its application in high-performance capacitive deionization.⁶⁵ The polydopamine showed redox capacitive behavior which also help to prevent oxidation in MXene and selective toward Na⁺ ions. Similarly, the importance of MXene catechol-based hybrid has been highlighted by Chang and co-workers.⁶⁶ Dopamine and sodium ascorbate were responsible for the improved oxidation stability of MXene in the hybrid. Further, other surface modifications have also been done with enzyme and photosensitizer to obtain photodynamic therapeutic properties to kill cancer cells.

Phosphates

Phosphate-based organic ligands have also been considered as potential candidates for the surface modification of nanomaterials with an oxide layer due to the ability of phosphonic acids (PAs), such as RPO₃H₂ (R = hydrophobic alkyl group), to react with various metallic salts and oxides, forming P-O-metal covalent bonds.⁶⁷ P-O-metal bonding occurs when phosphonate groups displace hydroxide ions coordinated to the metal exposed on the surface, leading to the release of water or alcohol molecules during the condensation reaction.⁶⁸ Phosphonic acids possess three potential binding sites and multiple coordination modes, such as tridentate coordination bridges or chelators.⁶⁹ The coordination modes are influenced by factors such as the susceptible crystallographic planes, type of metal on the oxide surface, and, most importantly, the grafting mechanism and conditions, including pH and temperature. The synthesis and purification methods for phosphonic acid are relatively easy. They form strong covalent bonds and less susceptible to self-condensation as compared to silanes.

Kim et al. described a straightforward and practical method for improving the stability of MXene (Ti₃C₂T_x) dispersion in organic solvents by simultaneously using an interfacial chemical grafting process and a phase transition technique for alkyl phosphonic acid molecules at ambient temperature.⁷⁰ Figure 11A shows a schematic illustrating the mechanism of covalent bond formation between alkyl phosphonic acid molecules and the MXene surface. First, Ti₃C₂T_x MXene flakes were dispersed in an acid-water solution to induce the protonation of their hydroxyl terminal groups. Then, the hydrogenated ones interacted with the alkyl phosphonic acids dispersed in organic solvents such as chloroform and hexanol to produce B as an intermediate at the boundary between the water and organic phases. Subsequently, the nucleophilic addition of phosphonic acid to the surface of titanium (Ti₃C₂T_x) during the condensation reaction leads to the formation of a TiOP linkage in an intermediate labeled C, along with the production of water as a side product. Bidentate (D) and tridentate (E) phosphonate modes are generated by the further addition of phosphonic acid through nucleophilic reactions. After functionalization with phosphonate E, MXene flakes with three binding modes exist in four distinct resonance-stabilized configurations at the boundary between the aqueous and organic phases. MXene flakes with modified surface move to the organic medium simultaneously owing to their enhanced hydrophobicity. External heat activation is not required, as all the addition and successive condensation reactions occur naturally under acid-driven conditions at room temperature.⁶⁹ For effective grafting and then transfer of phase to occur, a low pH is necessary for the activation of the surface hydroxyl group of Ti₃C₂T_x, which results in a successful condensation reaction on the MXene surface in a day at room temperature, ultimately increasing the dispersion stability in the non-polar phase (Figure 11B). Additionally, the functionalized MXene flakes can be redispersed in other desired organic solvents. The disappearance of peak at 8.6 ppm in the ¹H NMR spectrum for Ti₃C₂T_x-C₁₂PA (Figure 11C), the morphology of the uniformly functionalized MXene single sheet along with its clear hexagonal diffraction pattern in selected area electron diffraction (SAED), and the uniform distribution of phosphine (P) atoms in the EDS P atom mapping strongly confirmed that dodecylphosphonic acid (C₁₂PA) was successfully grafted on MXene surface through a condensation reaction that resulted in the development of a TiOP tridentate covalent linkage.

FIGURE 11. (A) Schematic illustrating the mechanism of simultaneous interfacial condensation reaction and phase transfer. (B) Photographs showing the immiscible biphase mixtures of Ti3C2Tx dispersed water phase and alkylphosphonic acid-dissolved chloroform organic phase with variation in pH of the Ti3C2Tx aqueous phase before and after reaction. (C) 1H NMR spectra of C12PA and Ti3C2Tx-C12PA. (D) Electrical conductivity of pristine Ti3C2Tx and Ti3C2Tx-C12PA films. (E) Contact angle measurements of Ti3C2Tx-C12PA films at room temperature. (F) Normalized intensity of the peak at 760 nm for pristine Ti3C2Tx in water, Ti3C2Tx-C12PA in hexanol, and Ti3C2Tx-C12PA in chloroform as a function of storage time. (G) Photographs of the Ti3C2Tx-C12PA/SEBS composite film at pre-strains (ε) of 0%, 20%, and 40%. (H) Relative resistance versus strain for neat Ti3C2Tx and Ti3C2Tx-C12PA/SEBS films with filler content of 80 and 90 wt%.Source: Reproduced with permission: Copyright 2019, American Chemical Society.70

The conductivity of pristine Ti₃C₂T_x MXene film was measured at 5220.0 S cm⁻¹, which was then reduced to a value of 708.2 S cm⁻¹ for Ti₃C₂T_x-C₁₂PA, due to the adsorption of 5 wt% insulating ligands on the conductive MXenes (Figure 11D). The water contact angle increased from 50° to 101.2° after functionalization, demonstrating that the surface became comparatively more hydrophobic (Figure 11E). The results indicate that the hydrophilic Ti₃C₂T_x flakes in aqueous phase were effectively functionalized with C₁₂PA at the interface between water and organic solvent via acid-catalyzed condensation reaction to form tridentate TiOP covalent bonds. At the same time, the hydrophobically modified Ti₃C₂T_x flakes transferred to the organic phase of chloroform, resulting in a more stable dispersion of MXene in the non-polar organic phase. In order to investigate the oxidation stability, the absorbance peak at approximately 760 nm in the ultraviolet–visible (UV–Vis) profile was monitored (Figure 11F). The Ti₃C₂T_x-C₁₂PA dispersions in hexanol and chloroform demonstrated no decay in peak intensity, whereas pristine Ti₃C₂T_x aqueous dispersions showed rapid decay to zero. It indicates that the pristine Ti₃C₂T_x crystals underwent full oxidation and were transformed into rutile TiO₂. The Ti₃C₂T_x-C₁₂PA flakes in hexanol and chloroform maintained a two-dimensional sheet structure without oxidation even after being stored for 3 months. The improved oxidation stability has been attributed to the low solubility of O₂ and H₂O molecules in the organic solutions. The hydrophobic ligand molecules can passivate the surface of Ti₃C₂T_x flakes, which increases oxidation resilience by preventing the oxidation source molecules from accessing the surface.⁷¹

Additionally, to showcase a potential application in flexible electrodes, a MXene polymer composite with a hydrophobic polymer matrix was prepared by the simple solution mixing technique and coated on the substrate via spray coating method. A styrene-ethylene-butylene-styrene/liquid paraffin solution in chloroform was mixed with a Ti₃C₂T_x-C₁₂PA chloroform dispersion to produce Ti₃C₂T_x-C₁₂PA/SEBS composites having 80 and 90 wt% filler contents. The Ti₃C₂T_x-C₁₂PA/SEBS hybrid film was evenly deposited on an elastic acrylate substrate and the composite film exhibited no evidence of fracture or breakage up to 40% strain (Figure 11G). Similarly, the resistance (R/R₀) of the Ti₃C₂T_x-C₁₂PA/SEBS hybrid films did not significantly increase until 45% strain (Figure 11H). In contrast, the pristine MXene showed an abrupt resistance increase at 10% strain, indicating that there could have been an imperfection or break before 10% strain. This suggests that owing to the improved hydrophobicity of the functionalized MXene, flexible electrode with acceptable conductance to a reasonably high strain can be designed using the Ti₃C₂T_x-C₁₂PA/SEBS composites. Phosphoric acid functionalized MXene nonpolar polymer nanocomposites can be utilized in stretchable electronics to create flexible and stretchable electrodes. These electrodes can withstand mechanical deformation and stretching while maintaining their electrical conductivity. They can be integrated into stretchable circuits, sensors, and displays, enabling the development of flexible and wearable electronics.

Phosphoric acid functionalized MXenes improve interfacial adhesion and lubrication properties, reducing friction and minimizing contact forces. They form a boundary layer, enhance wear resistance, and protect surfaces from damage. These lubricating systems are suitable for extreme conditions and can be used as additives in oils or greases. They contribute to sustainable lubrication practices by reducing reliance on harmful additives in traditional lubricants. Qing and co-workers reported that Ti₃C₂ MXenes functionalized with tetradecyl phosphonic acid (TDPA-Ti₃C₂) exhibited outstanding wear resistance and friction reduction capabilities in castor oil for tribological applications (Figure 12A).⁷² When the amount of TDPA-Ti₃C₂T_x in the castor oil was raised from zero to 0.2 weight percent, the coefficient of friction (COF) and wear rate (W_r) initially declined and then raised, showing the nanofiller additives were disseminated in the oil at the optimum concentration for achieving the best lubricating performance (Figure 12B).⁷³ In comparison to unmodified castor oil, castor oil containing 0.1 wt% TDPA-Ti₃C₂T_x exhibited the greatest lubricating effect, with the reduction observed in the values of COF being 27.9% and W_r being 55.1%. The mechanism of lubrication highlighted that the decrease in COF and W_r may be due to two factors: (1) The long alkyl chain of TDPA penetrated the oil molecules, and the phosphonic acid part of TDPA was bound to the hydroxyl group on the MXene surface, resulting in a stable dispersion of Ti₃C₂ in the oil. (2) The consistent distribution of TDPA-Ti₃C₂T_x in castor oil aids in the formation of a homogeneous layer of lubricating film between two friction surfaces, thus preventing the formation of depressions and scraps on the friction surface. In castor oil, the modified Ti₃C₂T_x exhibited great dispersibility as well as good stability. Ti₃C₂T_x is a 2D layered material that may be expanded by adding TDPA to the layers along with mixing it with the OH surface termination. Thus, oil molecules penetrate the layer of nanoparticles, producing a flawless fusion with them and enhancing dispersibility (Figure 12A(i)). As a solid lubricant, the direct addition of Ti₃C₂T_x to castor oil leads to the agglomeration of particles; thus, no lubrication can be achieved (Figure 12A(ii)).

FIGURE 12. (A) Schematic illustrating the mechanism of lubrication for (i) pristine MXene and (ii) TDPA-modified MXene. (B) Average coefficients of friction and wear rates as a function of TDPA-Ti3C2Tx concentration in castor oil. Reproduced with permission: Copyright 2021, Elsevier B.V.72 (C) Wear volumes for four different lubricants. (Normal load: 200 N; temperature: 30°C; stroke: 1 mm; frequency: 25 Hz). Reproduced with permission: Copyright 2022, American Chemical Society.75 (D) Schematic of the fabrication of organic phosphonic acid (OPA)-modified MXene membranes (E) Separation performance of pure and OPA-modified MXene membranes for CR. (F) Comparison between the separation performances of the PA/MXene membrane and various previously reported membranes (marked with the molecular weight of dye used in corresponding studies). (G) The stability of pure and OPA-modified MXene membranes in natural, acidic, and alkaline solutions. Reproduced with permission: Copyright 2021, Royal Society of Chemistry.79 (H) Memory structure of Ti3C2Tx-OP MXene, current–voltage characteristics, and retention time. Reproduced with permission: Copyright 2020, American Chemical Society.80

Guo and co-workers reported similar tribological experimental results using Ti₃C₂T_x MXenes functionalized with octadecylphosphonic acid (ODPA) (Ti₃C₂T_x-ODPA) incorporated in a 500 solvent neutral (SN) supramolecular gel.⁷⁴ The Ti₃C₂T_x-ODPA supramolecular gel with a 500 SN content of 0.1 wt% decreased the COF by 46.32% and W_r by 81.18% when compared to pure 500 SN oil (Figure 12C).⁷⁵ After surface functionalization, the enhanced compatibility of the base oils with Ti₃C₂T_x-ODPA prevented the flakes from clustering. In addition, the mechanical strength and layered structure of Ti₃C₂T_x-ODPA enhanced the tribological performance of the oil. Because of these outstanding properties, they can be used as lubricants in low-maintenance gadgets, including roller bearings and certain gears.⁷⁶

Phosphoric acid functionalized MXenes offer potential applications in membrane technology, particularly for water permeation and filtration processes. These membranes provide enhanced water permeation, selective permeation, antifouling properties, mechanical strength and stability, and compatibility with various membrane fabrication techniques. The hydrophilic nature of MXenes helps mitigate fouling issues, ensuring long-term use in water permeation applications. Yi et al. demonstrated that the surface functionalization of MXenes using organic phosphoric acids (OPAs) can modulate the transport channels of MXene-based membranes and enhance the permeability, rejection, and stability of water filtration membranes (Figure 12D).^77–79 MXenes functionalized with a variety of organic phosphoric acid ligands include iminodi(methylphosphonic acid) (2P), nitrilotri(methylphosphonic acid) (3P), diethylenetriaminepentakis(methylphosphonic acid) (5P), and phytic acid (PA) solutions. A covalent TiOP linkage was developed between the phosphonic acid molecule and MXene flakes through interfacial nucleophilic addition and successive condensation reactions. The OPA-modified MXene membranes are loosely stacked but have greater thicknesses (2.814, 2.957, 3.430, and 3.525 mm for 2P, 3P, 5P, and PA/MXene membranes, respectively), which offers preliminary proof of the effective incorporation and opening of nanochannels. The finding is that the OPA-modified MXene membranes (2P/MXene, 3P/MXene, and PA/MXene) with controlled nanochannels function much better than the pure MXene membrane in terms of both water flux and rejection of dye (Figure 12E). With an outstanding Congo Red (CR) dye refusal of 99.7%, PA/MXene with the largest and most controlled interspace offers an impressive permeability of 510 L m⁻² h⁻¹ bar⁻¹. Given that OPA modification reduces the MXene membranes' water contact angles, the improved hydrophilic surface property of the OPA-modified MXene membrane further leads to rapid transportation of water and high water permeance. The greater effectiveness of the OPA-regulated transportation nanochannels was demonstrated by the PA/MXene membrane's excellent permeability to water and molecular sieving efficiency in comparison to prior-reported membranes (Figure 12F). The modified MXene membranes preserved their color and structure integrity, showing no signs of oxidation even after being submerged in diverse solvents for 24 days. In contrast, the pure MXene membrane exhibited a noticeable color change to yellow, signifying considerable oxidation (Figure 12G). This indicates that the modified MXene membranes exhibit a high level of resistance to harsh environments due to the passivation effect of OPA molecules. The covalent bonding of OPA molecules to the MXene surface effectively shields the MXenes from water- and oxygen-induced oxidation.

Phosphoric acid functionalized MXenes offer potential applications in ternary memory devices, which store three distinct states instead of binary (0 and 1) states. These devices enable multilevel cell operation, enhanced stability and endurance, low-power consumption, and compatibility with existing semiconductor fabrication processes. By tuning resistive switching characteristics, these devices can store more than two bits of information per memory cell, enhancing storage capacity and density. Sun et al. reported that Ti₃C₂T_x MXene functionalized with octyl phosphonic acid (Ti₃C₂T_x-OP) exhibited good oxidation stability in open air and in the organic phase and stable ternary memory behavior (OFF/ON1/ON).⁸⁰ This indicates that Ti₃C₂T_x-OP can be used for memory devices (Figure 12H). Three resistance states can be categorized as ternary memory, with 0, 1, and 2 denoting high, intermediate, and low states of resistance, respectively. The modified device has three resistance states (OFF/ON1/ON2 = 1:10^2.7:10^4.1) that are particularly crucial for precise reading. These states make the modified device distinguishable from the unmodified one, which shows no memory performance. The modified MXene-based memory device exhibited a low threshold voltage, constant retention duration, good on/off rate, and significant ternary device yield (58%). Ti₃C₂T_x-OP MXene behaves like a Write once, read many (WORM) and is unable to go back to ON1 or OFF.

Carboxylates

Carboxylates can serve as valuable ligands for functionalizing the surface of metal oxide nanomaterials.⁸¹ Electrostatic attraction and hydrogen bonding are examples of non-covalent bonding interactions that contribute to the functionalization, while ester-like connection, linking, and chelation are examples of covalent bonding interactions. With benefits like non-toxicity and low cost, numerous studies have demonstrated the effectiveness of carboxylate ligands as an organic ligand for nanomaterials, enabling tuning the physicochemical and surface properties of nanomaterials.⁸² On the other hand, the process might require harsh conditions and complex process with high cost for functionalization.

To date, a few studies have proposed MXene surface modification using carboxylic acids in the form of covalent bonds^83–85 and noncovalent bonds.⁸⁶ Cho and co-workers described the production of hybrid materials (MXene@PCE) via the addition of a brush-type polycarboxylate ether (PCE) to Ti₃C₂T_x MXene (MXene@PCE).⁸⁴ The polyacrylic acid (PAA) segments shown in Figure 13A bind to the MXene surface through covalent TiO connections, while the polyethylene glycol (PEG) segments are flexible enough to allow for controlled steric spacing that protects against intense colloidal interactions. To anchor the carboxylates of PAA on the MXene surface, PCE polymer aqueous solution (molecular weight:50 kg mol⁻¹, 92.6% PEG segmentation) was poured into the MXene dispersion. The resulting mixture was thoroughly mixed for 30 min at room temperature to obtain MXene-polymer hybrid materials. This immersion method can facilitate the formation of surface covalent ester bonds by a condensation reaction between the TiO₂ surface hydroxyl and carboxylic groups. Generally, the strong colloidal interactions lead to the precipitation. To avoid colloidal interactions (i.e., the vdW and electronic double layer (EDL) interactions) between MXene flakes, the Flory radius (RF) of PEG segments ranged from 1.1 nm (poor solvent) to 6.8 nm (good solvent) as an attached semi-dilute brush, allowing for adequate steric spacing. A high-resolution cross-sectional TEM of MXene@PCE (30 wt% Ti₃C₂T_x MXene) reveals that the MXene sheets (black color) were equally mixed by the PCE (white color), and the MXene layer distance was determined to be 1.81 nm (Figure 13B). Sodium alginate (SA), polyethylene oxide (PEO), and polyvinyl alcohol (PVA) were used to investigate the effect of polymeric dispersants on the suppression of colloidal interactions alongside PCE. Figure 13C shows the UV–Vis absorbance of MXene@polymer hybrids in various ionic solutions. The absorbance of the MXene@polymer hybrid in the ionic mixture increased differently after PCE and PVA surface modification compared to untreated MXene and MXene modified with PAA, SA, and PEO. From these results, the extent of dispersion of MXene and the polymer composite can be arranged in the order PEO ≈ SA < PAA < PVA < PCE. Based on these findings, ionic groups like COO on PCE, SA, and PAA, appear to be favorable for the dispersion because the double-layer repulsion is strengthened as the charge density of the surface increases in ionic liquids.

FIGURE 13. (A) Ti3C2Tx/comb-type polycarboxylate ether (PCE) hybrid surfaces and the subsequent interplay potential profiles. (B) High resolution TEM image for MXene@PCE (30 wt%). (C) UV–Vis absorption variations at 750 nm for bare Ti3C2Tx, Ti3C2Tx@polyethylene oxide (PEO), Ti3C2Tx@sodium alginate (SA), Ti3C2Tx@polyacrylic acid (PAA), Ti3C2Tx@polyvinyl alcohol (PVA), and Ti3C2Tx@PCE hybrids in hydrochloric acid (HCl), sodium hydroxide base (NaOH), sodium chloride (NaCl), calcium chloride (CaCl2), and an artificial seawater (SW) [NaOH: NaCl:CaCl2]. (D) Size distribution and integral curves measured via DLS for bare MXene and MXene@PCE (30 wt%) in the SW. (E) Photographic images of bare Ti3C2Tx and Ti3C2Tx with 30 wt% PCE, PEO, and PAA dispersion in various polar and nonpolar solvents (top), and their relative optical absorbance as a function of the solubility parameters of the solvents (bottom). A0.5 and A0 indicate the absorbance at 0.5 and 0 hours after the initial dispersions. DMSO, dimethyl sulfoxide; EA, ethylenediamine; DMF, N,N′-dimethylformamide; NMP, N-methyl-2-pyrrolidone; CB, chlorobenzene; THF, tetrahydrofuran; CF, chloroform; TOL, toluene; HEX, hexane. (F) Humidity stability of bare Ti3C2Tx and Ti3C2Tx@PCE hybrid films under 40% relative humidity (RH) at 20°C as a function of the exposure time. (G) EMI shielding efficiency (SE) of bare Ti3C2Tx and Ti3C2Tx@PCE prepared from HPW and SW.Source: Reproduced with permission: Copyright 2022, American Association for the Advancement of Science.84

In addition, the R_F (2.5 nm) or the grafted semi-dilute brush height (1.1–6.8 nm) of the PEG segment provides effective shielding from precipitation. Figure 13D shows the results of a dynamic light scattering (DLS) analysis of the particle size distribution in colloidal mixtures of bare MXene and MXene@PCE hybrids in seawater (SW). MXene@PCE (30 wt%) exhibited an average size of approximately 445 nm due to no signs of big-sized agglomerations. In contrast, base MXene exhibited a much larger average particle size than the functionalized one because of the presence of a considerable number of agglomerated particles larger than 5 μm. These results indicate that the incorporation of PCE significantly enhances dispersion stability. Dispersions of MXene@polymer in different organic solvents were photographed and their optical absorbance was measured after 30 min (Figure 13E). Among the possible polymers, a PEO with lower polarity is sufficient for dispersing MXene in less polar solvents like chlorobenzene. While a steady rise in affinity for solvents with higher polarity was observed for PVA and PAA. The experimental results presented in the bottom row of Figure 13E are consistent with the theoretical estimate. Although PEG segments made up the majority of PCE's steric spacing groups, the MXene@PCE composites showed significantly better dispersibility in almost water-insoluble organic solvents, with a solubility parameter value of 18 MPa^0.5 (toluene, polar contribution <7%). The segmented comb-type anchor-spacer structures are responsible for excellent dispersibility. Furthermore, interfacial anchoring of MXene with organic polymeric molecules improves oxidation stability. Under 40% relative humidity at 20°C, the conductive nature of the MXene@PCE hybrid film was measured to assess its oxidation stability (Figure 13F). Oxidation stability was evaluated by monitoring the change in electrical conductivity of the MXene@PCE hybrid film under relative humidity of 40% at 20°C, as shown in Figure 13F. The MXene@PCE hybrid did not show any change in resistance, while pristine MXene showed a significant increase in resistance. The results indicate that the PEG segments in the solid state can provide a protection shield against moisture or air, along with the hydrophobicity of the PEG segments. These encouraging findings prompted further investigation into the EMI shielding ability of MXene@PCE hybrid films produced via SW were tested. The pristine MXene film fabricated using high-purity water (HPW) demonstrated a high and stable EMI SE of 47.4 dB as an averaged value in the range of 0.5–18 GHz, while the EMI SE of the MXene film fabricated using SW decreased to 13.1 dB (Figure 13G). The dramatic decrease in EMI SE was due to the non-uniform film thickness caused by the poor dispersion stability of MXenes in ionic solutions. The penetration of electromagnetic radiation occurs through the holes formed in the film. But as the PCE polymer content went up from 0% to 10% to 20% to 30%, the film's EMI SE went up from 13.1 to 24.2, 33.7, and 42.2 dB. This is because the dispersion stability of MXenes and the ensuing film homogeneity are both enhanced by the incorporation of PCE polymeric molecules. Importantly, bare MXene without PCE indicated a high reflection shielding mechanism,⁸⁷ whereas PCE intercalated in MXenes permitted multiple reflections between MXene layers leading to the enhancing a high absorption shielding mechanism.⁸⁸ Besides the above EMI shielding properties, porous MXene/polyimide aerogel developed through covalent interaction between hydroxyl group of MXene and carboxylic acid group of poly(amic acid) followed by thermal annealing demonstrated the feasibility of aerogels as flexible strain sensor and microwave absorption coating materials applications.⁸³

Silanes

Surface functionalization with silane groups is one of the most commonly used techniques for metal oxides with OH surface functional groups, as silane coupling agents provide low cost and good accessibility.⁸⁹ Silanes are hydrolyzed in the presence of water to create the corresponding hydroxysilane, which can also react with the OH surface groups of Ti₃C₂T_x MXene to form a covalent bond (TiOSi) through a silylation reaction (Figure 14A).⁹⁰ Once the alkoxy groups of silanes are converted to hydroxy groups, they can have a strong covalent interaction with the OH groups present on the MXene surface through hydrogen bonding. Water molecules are eliminated by condensation reactions, and covalent bonds are formed.

FIGURE 14. (A) Schematics for surface modification of MXene by (3-aminopropyl)triethoxysilane (APTES-MXene). (B) Zeta potential of Ti3C2Tx MXene and APTES-Ti3C2Tx. (C) Water contact angle measurement of APTES-Ti3C2Tx film. Reproduced with permission: Copyright 2019, Elsevier.90 (D) Oxidation and dispersion stability of n-octyltriethoxysilane-functionalized in the DCM. (E) UTS and Young's modulus of pure PLA and OTES-Ti3C2Tx/PLA nanocomposite membranes. CLSM images of attached MC3T3-E1 cells on (F) pure PLA and (G) OTES-Ti3C2Tx/PLA nanocomposite membranes. Reproduced with permission: Copyright 2018, Elsevier.106 (H) Digital image of the water contact angle of the functionalized MXene, (I) Digital image showing the self-cleaning test on the functionalized MXene-coated filter paper. (J) Photothermal heating curves of various samples under 808 nm. (K) Optical image of light-driven motions of filter paper, paper@PDMS, paper@PDMS@m-F and paper@PDMS@d-F coated with the fluorinated silane modified multilayered MXene (m-F) and delaminated MXene (d-F). Reproduced with permission: Copyright 2019, Royal Society of Chemistry.94

Silanes surface functionalization tunes the surface potential (or charge) of MXenes. Pristine MXene has a strong negative potential because of surface terminations.⁵² The surface functionalization of (3-aminopropyl)triethoxysilane (APTES) ligand molecules changes the surface potential (or charge) of MXenes from negative to positive because the NH₂ groups of the APTES grafted on MXenes are likely to accept protons to convert ammonium groups with positive charges (Figure 14B). The surface hydrophobicity of MXenes functionalized with alkylated silanes is controlled by the length of the hydrophobic alkyl group, which influences the dispersion stability in organic dispersions and the oxidation stability under environmental conditions. For example, functionalization of MXene with 1H,1H,2H,2H-perfluorodecyltriethoxysilane significantly increased the water contact angle from 57.9° (pristine MXene) to 136° (functionalized MXene). The increased hydrophobicity is due to the presence of a long fluorinated alkyl tail in the silane ligand molecule (Figure 14C).⁹¹

Similarly, N-octyltriethoxysilane (OTES)-functionalized MXenes (OTES-Ti₃C₂T_x) showed significantly improved hydrophobicity, which led to improved dispersion stability in hydrophobic organic solvents such as dichloromethane (DCM) (Figure 14D). Owing to the good dispersion stability in DCM, the functionalized MXene dispersions retained their absorbance intensity at 800 nm even after a long storage time. However, the pristine MXene dispersions showed a rapid decrease in the absorbance intensity at 800 nm with storage time because pristine MXenes are not dispersible and rapidly precipitate in DCM because of the strong hydrophilic nature of the MXene surface. The hydrophobic OTES-Ti₃C₂T_x was also homogeneously distributed in a hydrophobic polymer such as PLA. Therefore, homogeneous OTES-Ti₃C₂T_x/PLA nanocomposites could be prepared, which led to an increase in the final mechanical properties such as ultimate tensile strength (UTS) and Young's modulus (Figure 14E). The biocompatibility of MXenes is strongly affected by surface chemistry.⁹² The OTES-Ti₃C₂T_x/PLA nanocomposite membrane was much more compatible with the preosteoblast-like MC3T3-E1 cells than the pure PLA membrane, which was confirmed by confocal laser scanning microscopy (Figure 14F,G). The biocompatibility might be due to the surface functionalization of MXene with OTES ligand molecules. The increased hydrophobicity of the MXene surface led to a decreased cytotoxicity effect in the final nanocomposite.⁹³

Cao et al. reported that the functionalization of MXene with fluorinated alkyl silane (FAS) significantly improved surface hydrophobicity for self-cleaning and light-driven self-propelled machine applications.⁹⁴ The functionalized MXene exhibited superhydrophobic behavior with a large contact angle of 163.1 ± 1.5° (Figure 14H). This superhydrophobicity demonstrates the self-cleaning characteristics of the functionalized MXenes (Figure 14I). In addition to superhydrophobicity, it also exhibited photothermal-conversion behavior under NIR irradiation (Figure 14J). Upon irradiation with an NIR laser at 808 nm for a short period of time, paper@PDMS/m-F and paper@PDMS/d-F coated with the functionalized multilayer and delaminated MXenes, respectively, underwent rapid heating compared to MXene-uncoated papers. The temperature increased up to 116.6°C after just 3 min of laser irradiation, which led to a local temperature gradient and difference in surface tension in the liquid. This resulted in the motion of the MXene-coated paper on the liquid (Figure 14K). The combined effect of the superhydrophobicity of MXene after surface functionalization and the strong localized surface plasmon resonance of MXene are possible reasons behind this light-driven, self-propelled motion of hybrid MXene. However, surface functionalization with silanes has several drawbacks. During the reaction, hydroxyl silanes not only react with the OH groups of MXenes but also experience self-condensation.⁸⁹ This causes non-uniform grafting or coating of silane on the MXene surface, leading to a significant decrease in electrical conductivity.

Zhao et al. reported that the MXenes functionalized with trimethoxy(1H,1H,2H,2H-perfluorodecyl)silane exhibited a solar steam conversion efficiency of 71%, and stability in high salinity conditions under sun over 200 h in solar desalination application.⁹⁵ This outcome using functionalized MXene encourages for the development of highly efficient and stable photothermal transduction for water purification from seawater. Similarly, utilization of ultrathin Ti3C2-MXene nanosheets functionalized with APTES for the detection of cancer biomarker (carcinoembryonic antigen, CEA) has been demonstrated by Salama and co-workers.⁹⁶ The hybrid exhibited a wide range of linear detection range of 0.0001–2000 ng mL⁻¹ with sensitivity of 37.9 μA ng⁻¹ mL cm⁻² per decade.

Other ligand molecules

In addition to the five representative organic ligand systems discussed above, many other organic ligand molecules have also been considered for the surface functionalization of MXenes, which are briefly discussed in this section. For the covalent functionalization of MXene, many coupling reactions and reagents have been utilized, including click chemistry, in-situ polymerization, and cross-linking reactions. Click chemistry is an efficient method for yielding a covalent bond product without creating byproducts. McDaniel et al. covalently functionalized multilayer MXenes with dodecyl isocyanate.⁹⁷ Click reactions between isocyanates and alcohols or thiols lead to the formation of urethane covalent bonds (RNCO-O or RNCO-S). The surface-modified MXene-based nanocomposites showed significantly improved nanofiller dispersion in a thiourethane network when filled to a 5 wt% concentration. Functionalization with isocyanate grafting increased the interlayer distance once the oxygen-based terminations were fully functionalized via the formation of amide linkages (TiOC (O)NH) between MXenes and alkylisocyanate. Thus, a high functionalization yield and better dispersibility were achieved. In situ polymerization reactions are another useful strategy reported for the covalent functionalization of MXene, which addresses their long-term stability issues. Yun et al. demonstrated that the aqueous stability of delaminated MXene was enhanced after functionalization with heterocyclic aromatic amines.⁹⁸ The strong bonding of pyrrole molecules to the exposed edges of MXene provided a passivation effect and enhanced their capacitive efficiency. The pyrrole functionalized-MXene supercapacitor exhibited a specific capacitance of 253.6 F g⁻¹, surpassing MXene and pyridine@MXene values, offering potential applications in energy storage, catalysis, sensors, and electronic devices. In addition, Fan et al. used polypyrrole (PPy) and microemulsions as dual spacers to fabricate functionalized Ti₃C₂-MXene composite films, enabling fast electron/ion transport channels.⁹⁹ To facilitate quick ion diffusion kinetics and eliminate electrolyte imbibition steps, ionic liquid-based microemulsions spontaneously adsorb onto PPy-Ti₃C₂Tx nanosheets. This liquid “spacer” gives the composite films excellent specific capacitance, rate capability, and cycling stability between 4 and 50°C. This was the first report of an IL-based microemulsion being used as a spacer between MXene materials in an IL electrolyte. It presents a novel method for increasing the electrochemical efficiency of MXenes in different ionic liquid solutions for supercapacitor applications. Wang et al, presented a dip-coating method to create highly conductive, water resistant, and flexible MXene-derived textiles with outstanding EMI shielding and Joule heating performance.¹⁰⁰ The researchers modified MXene sheets with in situ polymerized PPy and coated them onto polyethylene terephthalate (PET) textiles. The MXene sheets were functionalized through in situ polymerization of PPy. This covalent bonding between MXene and PPy enhances the interfacial interactions and improves the overall properties of the MXene-decorated textile, such as electrical conductivity, EMI shielding effectiveness, and structural stability. The PPy/MXene-decorated textile exhibited high electrical conductivity, reaching 1000 S m⁻¹. By applying a silicone coating, the textile became hydrophobic, maintaining long-term stability while remaining flexible and air permeable. Moreover, the silicone-coated textile demonstrated excellent Joule heating performance, making it suitable for EMI shielding clothing, wearable intelligence garments, personal heating equipment and flexible electronics. Crosslinking reaction is also very useful for surface modification. Zhang et al. demonstrated that MXenes may also be used as organic crosslinkers for acrylamide reactions. Compared to hydrogels made with the common organic crosslinker N,N-methylene bisacrylamide, the resulting hydrogel had better mechanical characteristics after MXene functionalization.¹⁰¹

For non-covalent surface functionalization, many other organic ligand molecules have also been investigated. The non-covalent functionalization of Ti₃C₂T_x utilizing two unique cationic porphyrins was reported by Shameel Thurakkal and Xiaoyan Zhang, and the two resulting hybrids exhibited high water stability against oxidation.¹⁰² The electrostatic interactions between the porphyrin and MXene nanosheets were used for surface modification and investigation by zeta potential and photophysical studies. The hybrids exhibited a redshifted Soret band with wholly quenched fluorescence emission, which is an indication of efficient energy or electron transmission between the cationic porphyrins and Ti₃C₂T_x. Interestingly, MXene-modified hybrid materials exhibit controlled porphyrin release in acidic environments suitable for biomedical applications. Wu et al. devised a carbon nanoplating technique to effectively stabilize metastable MXene against structural deterioration.⁵ By hydrothermally carbonizing glucose at 160°C, carbon nanoplating on Ti₃C₂-MXene was accomplished, and the glucose molecules were preferentially adsorbed on the surface of MXene through hydrogen bond interactions, leading to a reduction in the surface free energy. Subsequently, they were changed in situ into a more conductive carbon layer on the MXene surface by means of intermolecular polymerization. MXenes could withstand hydrothermal reactions and annealing operations without damage due to carbon nanoplating. Furthermore, the developed MXene-based materials possessed attractive structures and characteristics, such as 2D hierarchical nanohybrids that improve electrochemical processes, reaction kinetics, structural stability, and electrical performance.

SUMMARY AND PERSPECTIVES

In this review, we have thoroughly surveyed existing MXene ligand chemistry for surface functionalization and discussed the rational design and recent advances in various organic ligands, including organic salts, catechols, phosphonates, carboxylates, and silanes, along with their modification mechanisms and property tunability in dispersion, oxidation stability, electrical and mechanical properties, and potential applications, as summarized in Table 1.

TABLE 1 Summary of the characteristics of five organic ligands used for MXene surface modification.

[Table omitted. See PDF]

Abbreviations: ACE, acetone; CB, chlorobenzene; CF, Chloroform; c-HEX, cyclohexane; COF, coefficient of friction; DCM, dichloromethane; decalin, decahydronaphthalene; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; EA, ethyl acetate; EC, electric conductivity; EMI SE, electromagnetic interference shielding effectiveness; EtOH, ethanol; HEX, hexane; IPA, isopropyl alcohol; MeCN, acetonitrile; MeOH, methanol; NMP, N-methyl-2-pyrrolidone; PC, propylene carbonate; RT, retention time; THF, tetrahydrofuran; TOL, toluene; UTS, ultimate tensile strength; WP, water permeation.

Organic salts offer promising benefits such as cost-effectiveness, biocompatibility, and environmental friendliness. However, weak electrostatic secondary bonding and poor stability under harsh conditions are drawbacks. Modification with organic salts improved dispersion and oxidation stability, and the resulting organic-salt-modified MXenes exhibited reasonable electrical conductivity depression, slightly improved oxidation stability, and excellent dispersion stability in nonpolar solvents. These have been applied in EMI shielding, porous materials, 3D printing inks, composites, and supercapacitor electrodes. However, further improvements in electrical conductivity and oxidation stability are required.

Catechol derivatives form strong hydrogen bonds and π–π interactions with MXene surface terminations through a fast and easy modification method. The catechol ligands achieved high electrical conductivity, excellent dispersion stability in polar solvents, enhanced oxidation stability, and improved tensile strength and have been applied in EMI shielding, photodynamic therapy, and conductive inks. However, they require improvement in dispersion stability in nonpolar solvents, and their relatively high material cost is a disadvantage.

Phosphate ligands form strong, stable covalent bonds with MXene surface terminations. However, their high cost, strong acidic reaction conditions, and low electrical conductivity are disadvantages. Despite these disadvantages, they offer good dispersion in various solvents and environments and high oxidation stability. Their applications include energy storage, sensors, lubricants, membranes, and electrocatalysts.

Carboxylate ligands offer covalent bond interactions, property tunability, and the ability to incorporate various end-functional groups. Large conductivity decreases and complex reaction processes are drawbacks. However, they can demonstrate superior dispersion capabilities and high oxidation stability and have been applied in EMI shielding and sensors.

Silane ligands offer strong covalent bond interactions and are inexpensive, accessible, and easy to modify with various functional groups. However, they are not ideal for uniform MXene surface modification owing to the competing side reactions of homo-condensation, poor dispersion stability, and low electrical conductivity. Limited studies on dispersion and oxidation stability exist; however, their applications in biomedicine, seawater desalination, and self-propelled machines have been investigated.

In summary, the surface functionalization of MXenes using organic ligand molecules has shown great potential in tuning their physicochemical properties and enhancing dispersion and oxidation stability. However, additional research is still required to thoroughly understand the chemistry of each ligand, overcome the limitations of each ligand system, and develop appropriate potential applications for modified MXenes.

To address these challenges, perspectives and potential directions for future studies on MXene surface functionalization are proposed in the following aspects:

1. Develop environmentally friendly, cost-effective, high-yield, and scalable surface modification methods using new or existing organic ligands.
2. Conduct comprehensive and comparative studies on MXene surface modification and the resulting effects on the final electronic, optoelectronic, and electrochemical properties, as limited research has been conducted thus far.
3. Examine the retardation mechanism of MXene oxidation using organic ligand molecules to enable the scientific community to devise new strategies for enhancing the physicochemical properties of MXenes in many practical applications.
4. Investigate the surface functionalization of MXenes other than Ti₃C₂T_x using organic ligand molecules to broaden their potential applications. Approximately, 40 different types of MXenes have been synthesized and most of the synthesized MXenes suffered from oxidative degradation. Surprisingly, MXenes other than titanium-based Ti₃C₂T_x MXenes have rarely been investigated for surface functionalization chemistry.
5. Conduct theoretical predictions and computational simulations to elucidate the physicochemical properties of functionalized MXenes, which can offer valuable insights and guidelines for experimental science in MXene surface chemistry.
6. Address the technical challenges associated with mass production and process integration to fulfill the needs of industrial applications, such as low cost and mild, straightforward reaction conditions.

AUTHOR CONTRIBUTIONS

Sungmin Jung, Ujala Zafar and L. Satish Kumar Achary: Conceptualization, visualization, and writing – original draft. Chong Min Koo: Supervision, writing – review & editing, and funding acquisition.

ACKNOWLEDGMENTS

This study was supported by grants from the Basic Science Research Program (2021M3H4A1A03047327 and 2022R1A2C3006227) through the National Research Foundation of Korea, funded by the Ministry of Science, ICT, and Future Planning; the Fundamental R&D Program for Core Technology of Materials and the Industrial Strategic Technology Development Program (20020855), funded by the Ministry of Trade, Industry, and Energy, Republic of Korea. This study was also partially supported by a grant (CRC22031-000) from Korea Institute of Science and Technology and the National Research Council of Science & Technology (NST), funded by the Korean Government (MSIT). Furthermore, this study was partially supported by a start-up fund (S-2022-0096-000) and the Postdoctoral Research Program of Sungkyunkwan University (2022).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.