Symposium NM03—2D MXenes—Synthesis, Properties and Applications

Two-dimensional (2D) materials offer advantages that their 3D counterparts do not. The conventional method for the bulk synthesis of 2D materials has predominantly been through etching layered solids. Herein, we convert - through a bottom-up approach – 10 binary and ternary titanium carbides, nitrides, borides, phosphides, and silicides into 2D flakes by immersing them in a tetramethylammonium hydroxide solution at temperatures in the 25 to 85 °C range. Based on X-ray diffraction, density functional theory, X-ray photoelectron, electron energy loss, Raman, X-ray absorption near edge structure spectroscopies, transmission and scanning electron microscope images and selected area diffraction, we conclude that the resulting flakes are C-containing, anatase-based 2D layers that are in turn comprised of ≈ 5x10 Ų nanofilaments in cross-section some of which are few microns long. Electrodes made from some of these films performed well in lithium-ion and lithium-sulphur systems. These materials also reduce the viability of cancer cells thus showing potential in biomedical applications. Synthesizing 2D materials, at near ambient conditions, with non-layered, inexpensive, precursors (e.g., TiC) is paradigm shifting and will undoubtedly open new and exciting avenues of research and applications.

MXenes are potentially the largest class of 2D materials discovered so far. With a general formula of M_n+1X_nT_x, M is an early transition metal (Ti, V, Nb, Ta, etc.), X is C and/or N, T_x represents the surface groups (-O, -OH, -F, -Cl), and n = 1–4, over 30 stoichiometric phases have already been discovered, with many more predicted computationally. This class of materials has been widely studied owing to their exceptional properties, including hydrophilicity, scalability, mechanical strength, thermal stability, redox capability, and ease of processing. Because MXenes inherit their structure from M_n+1AX_n (MAX) phase precursors, understanding MAX phase synthesis leads to control over flake size, defect density, and chemical composition of the resultant MXene. One understudied, yet important class of MXenes are solid-solution MXenes, where multiple elements are randomly distributed within the M layers. Herein, three interrelated solid solution systems, (Ti_2-yV_y)CT_x, (Ti_2-yNb_y)CT_x, (Nb_2-yV_y)CT_x are used as models to study the optical, electronic, and electrochemical properties. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. These MXenes exhibit tunable properties that are directly related to their chemistry. By understanding this relationship, it then becomes possible to rationally design new MXenes targeting specific properties and applications.

Since the pioneering studies with graphene, several 2D layered materials have emerged, with a wide range of new properties and applications. MAX phases consist of layered carbides and nitrides. Their general formula is M_n+1AX_n, where M is an early transition metal, n=1 to 3, A is an element from IIIA and IVA, and X is carbon and/or nitrogen. When an A-site atom is removed from the structure, it produces a large interlayer distance in the remaining MX, forming the phase called MXene, M_n+1X_nT_z, where T stands for z surface functionalities.¹ Due to the 2D nanosized layered morphology, and composition, MXenes have various applications, such as membranes, water purification, metal ion-based batteries and supercapacitors, (photo)catalysis, sensors, and optoelectronics. The removal of A, however, is a challenging process. After a decade of research on MXene preparation, the standard etching process still relies on using aqueous HF, either directly or by in situ formation. Though much progress has been made, with new syntheses based on molten salts, hydrothermal or organic halogen etching, fine-tuning of MXene is hardly achieved.² Etching methods and conditions have a determinant influence on surface functionalities and defects. Thus, developing alternative etching methods is needed to control and tailor the MXene surfaces, properties, and applications.
Ionic liquids compose a wide variety of organic-inorganic salts with rich chemistry, compositions, and physical-chemical properties. Here we present novel approaches to perform MAX etching using ionic liquids (IL).³ Different strategies are used to achieve MXene structures. Hydrolysis of F-containing IL under mild conditions (T<100°C, ambient pressure) leads to selective etching of A while intercalating large organic cations like ethyl methyl imidazolium, leading to MXenes of large interlayer spacing and lower degree of fluorine content compared to HF-based methods. IL also proved to work as an etchant in water- and solvent-free media, acting as an etchant, synthesis media, and intercalant in an inert atmosphere (glovebox). This is possible due to the wide range of reaction temperatures that IL can handle. Different MAX phases and structures (M = Ti, V, n = 1, 2) are successfully etched with IL, demonstrating that IL-based etching can be a generic route for MXene synthesis.

MXenes, a class of two-dimensional (2D) materials, are synthesized by etching MAX phase precursors to produce multilayer MXenes, where individual MXene sheets are held together by van der Waals forces. Typically, single 2D flakes of MXene are produced by chemical intercalation of multilayer MXenes. MXene delamination by chemical intercalation is a time-intensive process that produces excess waste. In addition, a limited number of the intercalants can be used and along with their toxicity these intercalants can limit the synthesis of certain MXene chemistries as well as their achievable electronic and electrochemical properties. This work demonstrates an alternative approach to the delamination of multilayer MXenes by applying high shear mixing to multilayer MXenes to produce single- and few-layer thick flakes. While this approach does not strictly require chemical intercalants it can be used in concert with chemical intercalants to allow the delamination of MXenes that are hard to delaminate.. Three-roll milling is an industrially mature technology with high throughput that is capable of producing shear rates over 10,000 s^-1. With the use of a three-roll mill, single-layer and few-layer Ti₃C₂T_x flakes were produced without chemical intercalants. The high shear produced Ti₃C₂T_x flakes showed a capacitance of 337 F g^-1, comparable to flakes made with LiCl intercalation in 3 M H₂SO₄.While V₂CT_x was unable to be shear delaminated purely by high shear, single-layer and few-layer flakes of V₂CT_x were produced by high shear with the aid of LiCl, which does not delaminate V₂CT_x MXene itself, as an intercalant. The capacitance of the LiCl shear delaminated V₂CT_x demonstrated a high capacitance of 187 F g^-1 at 2 mV s^-1 in 5 M LiCl.

Surface-enhanced Raman scattering (SERS) is a valuable analytical technique that provides ultra-low detection limits and highly selective molecular fingerprint sensing. However, the practical use is still limited by the cost of disposable substrates and their stability as the standard materials for SERS substrates use noble metals (Ag, Au, Pt). Two-dimensional transition metal carbides and carbonitrides (MXenes) are promising plasmonic materials for SERS, allowing sensitive detection of analyte molecules. Their surface plasmon resonances, metallic characters, and rich surface chemistries enable both SERS electromagnetic and chemical enhancements. We have synthesized seven MXenes – Nb₂C, Mo₂C, Ti₂C, V₂C, Ti₃C₂, Mo₂TiC₂, and Ti₃CN, and tested them as SERS substrates, detecting the 10^-7 M concentrations of Rhodamine 6G dye. Ti₃C₂ and Ti₂C show the highest enhancement and were compared to the commercially available substrates, where Ti₃C₂ outperformed gold nanoparticles. The best performing MXene was tested with a variety of dyes, suggesting the versatility of MXene sensing. This study elucidates the capability of MXenes beyond Ti₃C₂ to perform as SERS substrates, showing the potential for MXene-based SERS sensors that are inexpensive, easy to fabricate, and optimize for specific applications.

Since the discovery of the first MXene, the number of different MXenes has increased substantially. Thanks to the diverse structural and chemical composition of MXenes, denoted as M_n+1X_nT_x, a plethora of MXene structures and chemistries with record-breaking electronic, optical, electrochemical, and mechanical properties has been predicted and synthesized, with many more yet to be demonstrated. By varying the structure and composition of MXenes, it is possible to tailor properties of MXenes towards specific applications for optimal performance. The key to realizing these new MXene structures and realizing improved properties of known MXenes is by further developing the synthetic process. In fact, many of the MXene research challenges for the next decade identified by MXene researchers during the 3^rd International Conference of MXenes in late 2020,¹ were directly or indirectly related to the synthesis of MXenes.
While the number of synthetic strategies to fabricate MXenes has also expanded, these new techniques are still in their infancy; most synthetic efforts still rely heavily on hazardous hydrofluoric acid (HF). In order to translate future synthetic efforts to successful synthetic protocols and fabricate new MXenes with the desired composition and properties, there is a clear need to deepen our understanding of the synthesis process. Every step in the MXene fabrication procedure, beginning from precursor synthesis to the etching and final delamination steps, requires a conscientious effort to expand our fundamental knowledge of synthesis-structure-properties relations to guide further progress in MXenesynthesis.
This talk will provide an overview of MXene synthesis beyond a description of the known synthetic procedures. Instead, the focus of this talk is to identify trends and gaps in knowledge through a critical examination of the synthesis process to guide future synthetic efforts. Starting with the MAX and non-MAX precursors, the effect of precursors on the structure, composition and properties of the resulting MXenes produced will be described. The subsequent steps of etching of the MAX phase material to fabricate multilayer MXene, and the delamination of the multilayer MXene into individual MXene flakes, will be evaluated with a focus on the fundamental studies undergirding the etching and delamination processes. In particular, we highlight theoretical work, molecular simulations and ex situ experiments to deepen our understanding of the relevant mechanisms to guide future successful theory-guided synthetic efforts. The talk will conclude with a discussion of existing gaps in MXene synthesis and how to close them.
References:
Y. Gogotsi and Q. Huang. ACS Nano 2021, 15, 4, 5775–5780

Transition metal carbides and nitrides are a particularly valuable group of materials because they combine useful properties found in metals and ceramics.^1-2 These materials have shown high melting temperature, corrosion resistance, high strength and hardness, and good catalytic activity.^1-2 Recently, synthesizing 2D transition metal carbides and nitrides has led to interesting new materials with the same excellent properties, as well as the high surface area to volume ratio of a 2D material. The most common way to synthesize 2D transition metal carbides and nitrides is by a top-down etching method using bulk MAX phases. The transition metal (“M”) is arranged in a nearly hexagonally close packed structure, with C or N (“X”) in the octahedral interstitials.^1-2 These “M-X” layers are held together by a group 13-16 element (“A”), which can be preferentially etched to form 2D “M-X” layers, known as MXenes.¹ However, this etching process causes defects, as well as surface terminations, which can be difficult to control.¹

Recently, ultrathin transition metal carbides have been synthesized by chemical vapor deposition (CVD).² Ultrathin Mo₂C has been synthesized via CVD using a Cu and Mo foil stack as the substrate.² The Cu foil is melted, and the stack is exposed to hydrocarbons at high temperature. It has been proposed that the Mo diffuses through the liquid Cu and reacts with the hydrocarbons, forming ultrathin Mo₂C crystals.^1-2 Unlike MXenes synthesized by MAX phase etching, these crystals are very high quality with low number of defects and no surface terminations.² These high-quality crystals have already been used for applications such as Josephson junctions, hydrogen evolution reaction, photosensors, etc., however, the synthesis mechanism is not completely understood.^1-2

This work systematically analyzes the effects of using different metal alloys (Ag-Cu, In-Cu) as substrates to determine the mechanism of Mo₂C synthesis by CVD. Parameters such as composition of the alloy substrate, synthesis time, cooling rate, CH₄ partial pressure, temperature, and surface energy of the substrate are varied to determine the effects of each parameter on the Mo₂C synthesis mechanism. Ag alone is not suitable for Mo₂C synthesis under these conditions, despite having similar Mo solubility to liquid Cu. However, Mo₂C flakes can be synthesized using Ag-Cu alloys, and the flake size increases with the percentage of Cu in the Ag-Cu alloy substrate. It is determined that the mechanism of Mo₂C synthesis on liquid metals is by Mo diffusion through the liquid alloy, where it eventually reaches the surface and reacts with surface C. This suggests that successful Mo₂C synthesis requires the substrate to dehydrogenate methane and form surface C. Substrate choice can also play an important role in Mo₂C coalescence. Alloys that do not have solid solubility and that segregate at high temperatures prevent Mo₂C coalescence, such as Ag-Cu alloys. However, Mo₂C coalescence occurs on substrates where the alloys remain mixed during cooling, such as In-Cu alloys. Also, graphene and Mo₂C heterostructures can be synthesized by increasing the CH₄ partial pressure. In-Cu alloys reduce the melting temperature, making lower temperature synthesis at 800 °C possible. However, these Mo₂C flakes are much smaller, and there is lower surface coverage. These results may be due to an increased viscosity of the liquid alloy, as well as less efficient pyrolysis of the CH₄ at lower temperatures. This indicates that there is a low-temperature limit for Mo₂C synthesis using CH₄ as a precursor, similar to graphene. Thus, the bottom-up synthesis of high quality, uniform films of transition metal carbides and nitrides is highly dependent on substrate choice, which can be optimized using a comprehensive understanding of the synthesis mechanism.

(1) Anasori, B.; Gogotsi, Y., 2D Metal Carbides and Nitrides (MXenes). Springer: 2019.

(2) Xu, C.; et al.; Nature materials 2015, 14 (11), 1135

MXenes (M_n+1X_nT_x) as two-dimensional materials with tunable surface functional groups are promising for a variety of applications including energy storage, water purification, and photocatalysis. Since their discovery a decade ago, many compositional and structural variations have been proposed and synthesized, revealing a broad family of materials with distinct structural, electronic, and electrochemical properties. Here, instead of varying the “M” metal atom site as has mostly commonly been done, we study varying the “X” site atoms via high throughput density functional theory (DFT) calculations enabled by an efficient and reproducible workflow. We substitute carbons for combinations of boron, carbon, and nitrogen. i.e. M₃B₂T₂, M₃BCT₂, M₃C₂T₂, M₃CNT₂, M₃N₂T₂, M₃BNT₂ (where M=Ti, T=unterminated, F, O, or OH). We obtain and analyze the table structures and their resultant structural and electronic properties for all these combinations. Results are contrasted with the established properties of Ti₃C₂T_x. Based on their distinct properties, synthesis efforts for several of the novel materials are encouraged.
This research was sponsored by the Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences.

In the first decade of research detailing 2D MXenes, methods to produce MXenes with one or two transition metals (M) were reported. Inspired by the two fast-growing fields of high-entropy compounds and 2D MXenes, We recently synthesized two multi-principal-element high-entropy M₄C₃T_x MXenes, TiVNbMoC₃T_x and TiVCrMoC₃T_x as well as their precursor TiVNbMoAlC₃ and TiVCrMoAlC₃ high-entropy MAX phases. We used X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) to study the phase composition and establish equimolar stoichiometry 1:1:1:1 for the four principal transition metal elements Ti:V:Nb:Mo and Ti:V:Cr:Mo in these MXenes. Scanning transmission electron microscopy (STEM) studies were used to determine structure and the distribution of the M elements in these M₄C₃T_x MXenes. First-principles calculations were used to compute the formation energies and to further explore synthesizability of these high-entropy phases. To further evaluate the effect of entropic stabilization in these systems, we evaluated the phase formation of the high-entropy MAX phases using non-equimolar transition metals, such as Ti:V:Nb:Mo 1.3:1.3:1.3:0.1. In this talk we describe the formation of high-entropy MXenes and their precursors and our recent understanding of these phases. The synthesis of high-entropy MXenes significantly expands the compositional variety of the MXene family to further tune their properties, including electronic, magnetic, electrochemical, catalytic, high temperature stability, and mechanical behavior.

MXenes are a widely studied class of two-dimensional materials due to their promising applications in fields like energy storage, EMI shielding, catalysis, sensing, etc. Although various synthesis routes have been proposed for MXene synthesis, the top-down etching of MAX phases with hydrofluoric acid (HF) remains the most common MXene synthesis method. However, the toxicity and severe health hazards associated with concentrated HF remain a major challenge in the large-scale synthesis of MXenes. While other etchants like a combination of alkali fluorides and hydrochloric acid (HCl) have been proposed in the literature, their relative safety is seldom taken into consideration. On comparing the NFPA hazard classification of various etchants proposed in the literature, it was found that most of the non-HF etchant alternatives themselves rank high in their hazard classification. Moreover, most of the non-HF etching routes also require elevated temperatures or an additional intercalation step which increases process time and the number of chemicals required. To address this challenge, we propose the use of a relatively safer family of quaternary ammonium salts in combination with a suitable solvent for MXene synthesis at room temperature and without the need for an additional intercalation step. We propose that the large size of quaternary ammonium ions would promote intercalation during the process of etching, thereby eliminating the need for a separate intercalation step and additional chemicals. To test this hypothesis, Ti₃C₂T_z MXene synthesis was chosen as a model process and a selected quaternary ammonium salt in combination with a suitable aqueous solvent was used as an etchant. On etching the parent Ti₃AlC₂ MAX phase for different amounts of time in different molarities of etchant, it was found that in each case, Ti₃C₂T_z MXenes were successfully synthesized. Moreover, the synthesized Ti₃C₂T_z MXenes also had a tunable functional group distribution, which varied with the concentration of etchant and the etching duration. Moreover, the MXene synthesis process was also optimized for a selected sample to obtain high yields of >30%. Furthermore, Li⁺ de/intercalation properties were also observed in the Ti₃C₂T_z MXene clay etched with the proposed etchant, which makes it a suitable candidate for battery applications. Thus, using the proposed novel MXene synthesis process, Ti₃C₂T_z MXenes were successfully synthesized at room temperature without the need for an additional intercalation step with functional group tunability and high yields. These results open the possibility of using a range of quaternary ammonium salts in aqueous solutions for safer and scalable MXene synthesis.

MXene is expected to be used in a wide range of material applications because of its diverse material properties derived from its various compositions and surface groups. Single layer MXene is necessary to obtain material quality that meets the demanding of these MXene applications. However, the problem is that the high-quality single layer MXene reported so far has chlorine groups on the surface. In general, materials containing chlorine cause corrosion and other failures in electrical components. Furthermore, halogen-free materials are required due to the increasing importance of halogen regulations as a result of the recent rise in environmental awareness. However, it is known that to obtain single layered MXene without chlorine groups requires TMAOH(tetramethylammonium hydroxide), which is difficult to remove, or exfoliation processes such as ultrasonication, which involves reducing the particle size, resulting in a significant decrease in conductivity [1].
In this study, we have succeeded in obtaining highly conductive halogen-free MXene by our original synthesis method. In addition, halogen-free MXene is less susceptible to moisture absorption than MXene with chlorine groups, and thus achieves high material reliability.
References
[1] Mohamed Alhabeb, Kathleen Maleski, Babak Anasori, Pavel Lelyukh, Leah Clark, Saleesha Sin and Yury Gogotsi, Chem. Mater. 2017, 29, 7633−7644.

Recently, 2D materials MXene gained significant attention in various research fields due to their unique properties like chemical diversity, hydrophilicity, 2D morphology, metallic conductivity, etc. Inspite of the promising properties of MXenes, these materials are not gaining momentum in various applications because of their poor chemical or oxidation stability. A few reported literatures stated that organic solvents with low temperature storage could prevent the oxidation of MXene flakes due to the presence of less number of dissolved oxygen molecules in the solvents. Till now, mostly investigated material regarding oxidation stability is Ti₃C₂Tx MXene only due to its high capacitance and excellent high metallic conductivity. In this regard, Ti₂CT_x MXene also delivers comparable electrical and electrochemical properties like the previous one, but it is getting less attention due to a lack of knowledge about their suitable storage medium for a prolonged period without oxidation/ degradation. To address this issue, this paper reports the best suitable medium for preventing or delaying the degradation or oxidation of Ti₂CT_x MXene. Herein, Ti₂CT_x MXene was dispersed in different solvents for a duration of 3 months at ambient and freezing conditions and characterized thereafter using XRD, Raman spectroscopy, SEM, TEM and XPS. The results suggest that the best suitable medium to store Ti₂CT_x MXene for 3 months is IPA or DMSO at -20°C or Water at -80°C.

MXenes are an emerging family of two-dimensional materials. They are synthesized by etching the A element -typically from periodic groups 13 or 14- from a MAX phase crystal (M denoting an early transition metal and X either is carbon or nitrogen). In MAX crystals, the A element is located between MX layers and forms metallic bonds with the M element. Upon the removal of the A-element, functional surface groups, T_x, can be introduced producing ordered MX stacked nanosheets where consecutive layers are held together by van der Waals (vdW) forces. MXenes (M_n+1X_nT_x with n=1,2,3) owe their large design space, and consequently their broad variety of properties, to a wide range of chemical modifications possible in M and functional T_x surface groups ^[1].
In this contribution, we will present a study of MXenes functionalized with organic surface groups. As part of our study, we will utilize atomic resolution scanning transmission electron microscopy (STEM) to determine the structure of the material as well as the nature of local defects. Additionally, we will use electron energy loss spectroscopy (EELS) and x-ray energy dispersive spectroscopy (XEDS) to gain insights into the local chemical composition and bonding environment with high spatial resolution.
Organic-metallic hybrid MXenes pose a unique challenge to high-resolution STEM imaging since the organic material in the sample is highly electron-beam sensitive. At typical doses required for high resolution imaging, organic material will readily degrade. In this study, we borrow elements of transmission electron cryomicroscopy (Cryo-TEM), namely cooling our samples in-situ to liquid nitrogen temperatures. This will increase the robustness of the hybrid MXenes allowing us to conduct high-resolution imaging of the organic-to-metallic interface, as well as to gain insights into the arrangement of organics between MX layers.
To cool the MXene sheets, we will use a Gatan cold stage capable of reaching liquid nitrogen temperatures. The primary instrument we will utilize is an aberration-corrected cold field emission JEOL ARM200CF operating at 200kV primary electron energy. With the emission current set to 12A, the electron probe will be operated at 24mrad convergence semi-angle. High angle annular dark field, low angle annular dark field and annular bright field detectors will be set to inner angles of 75 mrad, 30 mrad, 11mrad respectively. Atomic scale chemical identification will be conducted using a large solid-angle Oxford XMAX100TLE XEDS detector. The ARM200CF is equipped with a post-column Gatan Continuum GIF spectrometer which allows will allow us to probe unoccupied density of states (DOS) with high spatial resolution. Using this approach, we will demonstrate that metal-organic hybrid MXenes form highly ordered, flexible structures with well defined elemental layers.^[2]
[1] Gogotsi, Yury, and Babak Anasori. "The rise of MXenes." (2019): 8491-8494.
^[2] This project is supported by a supported by the National Science Foundation (DMREF CBET-1729420) and made use of instruments in the Electron Microscopy Service at the UIC Research Resources Center. The acquisition of UIC JEOL JEM ARM200CF is supported by an MRI-R² grant from the National Science Foundation (Grant No. DMR-0959470) and the upgraded Gatan Continuum spectrometer was supported by a grant from the NSF (DMR-1626065).

Chemical vapor deposition is a promising technique to produce Mo₂C crystals with large area, controlled thickness, and reduced defect density. We have studied the effects of CVD processing parameters (effect of catalyst, catalyst type and thickness, reaction temperature and duration) on the growth of 2D Mo₂C crystals. SEM, EDS, Raman spectroscopy, XPS, TEM and XRD studies show that hexagonal Mo₂C crystals, which are orthorhombic, grow along the [100] direction together with graphene an amorphous carbon thin film on Cu and In, respectively [1, 2, 3]. The activation energy for the growth of Mo₂C on graphene was calculated to be 3.76 eV, and the diffusion of Mo to the Cu surface through uncovered Cu or graphene vacancies/defects was determined to be the rate-limiting step [2]. The growth mechanism is examined and discussed in detail, and a model is proposed. AFM studies agree well with the proposed model, showing that the vertical thickness of the Mo₂C crystals decreases inversely with the thickness of catalyst (Cu or In) for a given reaction time [3]. This study is based on work supported by the Air Force Office of Scientific Research under Award Number FA9550-19-1-7048.

Since the discovery of MXene in 2011 number of two-dimensional transition metal carbides or nitrides increasing steadily. To date, more than 40 MXene members have been developed and in time may develop the largest family in 2D materials. Conventionally, MXene materials are synthesized from the MAX phase (M_n+1AX_n) using selective chemical etching method where a hydrofluoric-based solution is used for selective etching of metal layers (element A) in the M_n+1AX_n. As hydrogen fluoride is toxic in nature, it creates major health and environmental issues. In that context, the research community looking for a green synthesis method for the preparation of MXene. Present work demonstrates a simple, easy and green synthesis of Ti₃C₂T_x MXene. The synthesized material is characterized with XRD and SEM for crystallography and morphological study. The XRD results confirm the formation of Ti₃C₂T_x along with various functional groups. The obtained SEM results showed that the multilayered structure of MXene was clearly observed at low resolution itself also EDS confirm the presence of Ti and C as major element percentages. The synthesized material can be used for further applications like energy storage devices, sensors, etc.

Since the first report by Naguib et al. in 2011, Ti₃C₂T_x MXene prepared by selective etching of monoatomic Al layers in the Ti₃AlC₂ MAX phase, in particular, has attracted tremendous attention. This originates from its high electrical conductivity, hydrophilicity, and ability to intercalate cations and store charge, which are favored for electrodes, supercapacitors and energy storage applications. Many studies have been focusing on the deposition process, compositing technology, and optimization of physical shape through improvement of etching and intercalation for improving fundamental performance of the MXene nanosheets. In many studies to improve the fundamental performance of Ti₃C₂T_x MXene, commercial MAX phase was used as a starting material and focused on optimizing the selective etching process of the Al layer. In this study, the synthesizing strategy of MAX phase was revisited to lower the oxygen concentration inside the MAX and MXene nanosheets. In the well-known MAX synthesis, Ti powder is used as a Ti source, in which case the high oxygen affinity of Ti increases the oxygen content in MAX. Such oxygen remains in MXene even after Al etching, and most of it constitutes surface termination, but excessive oxygen formed oxides (TiO₂ and Al₂O₃, etc.) inside the MXene and act as defects. To improve this, we propose a 2-step synthesis process consisting of the first step of synthesizing TiC from TiO₂ and the immediately following second step of synthesizing Ti₃AlC₂ MAX from the first step output. Interestingly, MAX synthesized through this two-step process contained significantly less oxygen than commercial MAX phases and those synthesized from Ti powders prepared as a comparison group. We prepared Ti₃C₂T_x MXene nanosheets using this MAX, and compared its properties with those of the MXene nanosheets prepared from the MAX synthesized with Ti as a starting material.

MXenes are 2D transition metal carbides or nitrides, and many studies are being conducted due to the fascinating properties of various MXene phase. Nevertheless, the problem of oxidation stability and narrow solvent compatibility due to the presence of hydrophilic terminal groups (O or OH) should be urgently overcome. Herein, universal ligand system for the preparation of organic-dispersible MXene was designed through bio-inspired chemistry. The resulting organic MXene inks or pastes have been utilized in various application including electromagnetic interference shielding, electrodes for supercapacitors, etc. This work was supported by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Govermment (MOTIE) (P00008500, The Competency Development Program for Inudstry Specialist).

Two-dimensional (2D) transition metal carbide and nitrides, MXenes, exhibit promising properties in different applications such as energy storage, sensing, and optoelectronics, to name a few.(1) These materials have a general formula of M_n+1X_nT_x where M is a transition metal, X is carbon and/or nitrogen, and T_x represents surface functional groups on the outer layers. Since the discovery of MXenes in 2011, research has mainly focused on Ti₃C₂T_x because of its established synthesis protocols and high chemical stability, while investigation of other experimentally made, stoichiometric MXenes has been trailing behind. Among other MXenes, M₂XT_x MXenes such as Vanadium carbide (V₂CT_x) promise exceptional theoretical properties due to the increased active surface area in M₂XT_x MXenes and the multiple available oxidation states in Vanadium systems. Despite its attractive properties, however, research related to V₂CT_x has mostly been limited to its multilayered state due to rapid oxidation after delamination. Herein, we show that through post-processing protocols the shelf life of delaminated V₂CT_x dispersions can be extended from only a few hours to multiple months with almost no degradation. Lithium-ion exchange is performed to remove the amine intercalants that are a byproduct of MXene delamination. Through this process, the excess of cations cause the MXene to crash out of solution. In this flocculated state, the MXene exhibits greatly enhanced stability and can be redispersed into water by washing with DI water. Various characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis, and thermogravimetric analysis were used to investigate the materials. This work shows the instability of certain MXenes can be overcome using proper solution processing techniques.

1. A. VahidMohammadi, J. Rosen, Y. Gogotsi, The world of two-dimensional carbides and nitrides (MXenes). Science. 372, eabf1581 (2021).

Although the family of well over 50 experimentally realized MXenes has grown to include one or more transition metals from groups 3-6 of the 3d-5d blocks of the periodic table, W has remained a difficult transition metal to include in the typical MXene structure. Although MXenes are typically synthesized from their respective parent MAX phase precursors, further exploration of alternative precursors can provide more effective ways of creating MXene possible with non-typical transition metals. Here, we discuss the synthesis of W₂TiC₂T_x, an out-of-plane ordered MXene where the ordering defines the structure as containing a layer of titanium with two carbon layers bonded to the top and bottom respectively which are further sandwiched between two tungsten layers completing the M₃X₂ formula. To synthesize this MXene, the nano-laminated ternary transition metal carbide non-MAX precursor (W,Ti)₄C_4-x was chemically immersed in hydrofluoric acid followed by tetramethylammonium-hydroxide treatment to yield single-to-few layer flakes that can then be vacuum filtered to form W₂TiC₂T_x free-standing films. The successful synthesis of new W-containing out-of-plane ordered double transition metal MXene derived from a non-MAX precursor exemplifies the complexity of MXene that is possible via synthesis involving non-MAX precursors. With the addition of W₂TiC₂T_x to the out-of-plane ordered double transition metal sub-family of MXenes, there is increased potential for the tunability of electronic, magnetic, optical, and electrochemical properties due to chemical diversity.

To improve the properties of MXenes and explore new applications, it is of paramount importance to study the synthetic mechanism, in addition to obtaining direct insight on the impact of different etchants on the structure, such as defects, and the properties, such as electrical conductivity. Despite its importance, this has been very challenging due to the atomic layer of A-elements being etched and the elaborateness required in investigating the inner structural changes of individual MAX particles. Here, we try to observe the etching behavior of Ti₃AlC₂ MAX phase at an atomic-scale and correlate the structural changes of Ti₃AlC₂ MAX phase and we suggest the etching mechanism of atomic Al layers in the MAX phase during MXene synthesis. For this purpose, we tried to observe the etching state of atomic Al layers in etched MAX phases depending on Al etchants and etching times. At first, we confirmed the etching state of Al layers after etching with LiF and HCl according to specific etching times (3, 6, and 24 hours). For all etchants, we show that etching starts from the edges and propagates along the basal planes, leading to the stepwise etching of Al layers. However, the etching mechanism is very different for the grain boundary depending on the types of etchants. HF and NH₄HF₂ etchants can break poly-crystals at their grain boundaries into single-crystals, while poly-crystal particles remain after LiF/HCl etching. Meanwhile, in an earlier stage, the etching of Al atoms occurred at relatively weak edge layers, which are exposed to etchant including inside of MAX phase like voids, in MAX particle. In these weak edge layers, one side boned Ti₃C₂-Al layers exist dominantly, and they can have relatively lower bonding energy than both sides bonded Al-Ti₃C₂-Al layers, which are inside of MAX phase particle. Then, an etching process proceeds to the inner layers and next layers sequentially from starting points. Additionally, we revealed the role of Cl^- ion for the MXene delamination. It can induce the easier intercalation of Li⁺ ions between Ti₃C₂ layers, resulting in effective delamination of MXene sheets. We expect that our results can open the ways to develop MXene synthesis methods for high quality and mass production and give us the possibility for tuning the properties of synthesized MXene to their proper application.

Surface-Enhanced Raman Spectroscopy (SERS) has emerged as a powerful spectral detection technology. At this moment in time, noble metal films/nanostructures are readily used as effective SERS substrates for various chemo/biosensing, however, lower sensitivity and cost is limiting factor associated with them. Lately, with the advent of MXenes, they have gained significant interest in the SERS field due to their ease of preparation, scalability, excellent optical and chemical properties, high spectral stability, and strong anti-interference ability. They can also bind probe molecules stably and firmly on their surface that’s conducive to obtaining stronger Raman signals. In the present work, we report for the first time Mo₂CT_xMXene/Au hybrid as a sensitive SERS substrate utilizing methylene blue as a model chemical. The Mo₂C MXene is synthesized from the Mo₂Ga₂C MAX phase, where the Ga layer was etched to form Mo₂CT_x layers. The synthesized Mo2C SEM-EDX analysis shows the elemental mapping of Mo (63.33%) and C (36.67%) indicating the uniform distribution of Mo and C and no Ga. Also, the AFM results of the delaminated MXene display the average height of 27 nm of the layered structure. Further, the in-situ hybridization of the MXene was carried out with Au for the formation of MXene/Au hybrids. The as-prepared noble Mo₂C MXene/Au hybrid demonstrates an excellent SERS characteristic in terms of enhancement factor, sensitivity, and detection limit. This could be assigned to synergetic plasmon enhancement by Au nanoparticles and chemical transfer by Mxenes as well as excellent chemisorption of dye molecules due to high specific surface area. These initial findings can be significantly implemented in the designing and development of MXene hybrids-based SERS substrates for the chemical and biological sensing of targeted molecules.

MXene is one of the attractive 2-dimensional materials because of its low bandgap, relatively low resistance, and composition diversity to manipulate its electrical/physical property. This MXene can be an active branch of solid-state physics with considerable potential for spintronics, magnetoelectronics, and the long-term frontier of information processing technology. For that, magneto-conductance (same as magnetoresistance) study (of material) is an important candidate to understand transport behavior and/or dynamics of charge carriers for the future opportunity of applications. Also, the study on a practical material system such as inkjet printed MXene mesh is actually required. Herein, we report an experimental case of extrinsic negative magneto-conductance and observation of weak anti-localization (WAL)/weak localization (WL) crossover at inkjet printed Ti₃C₂T_x MXene mesh system. The observation of WAL corresponds that this Ti₃C₂T_x MXene mesh system has strong spin-orbit coupling at low temperature which opens possible application for spin-orbitronics. From WAL/WL magneto-conductance analysis, we estimate that the inkjet-printed MXene mesh has a strong spin-orbit field up to Hso ~ 0.84 T at 1.9 K temperature. This work will not only lead to a deep understanding of the microscopic charge carrier transport behavior but will also lead to a broad range of applications for spintronics (especially spin-orbitronics) and information processing technology.

Since the discovery of MXene in 2011 number of two-dimensional transition metal carbides or nitrides increasing steadily. To date, more than 40 MXene members have been developed and in time may develop the largest family in 2D materials. Conventionally, MXene materials are synthesized from the MAX phase (M_n+1AX_n) using selective chemical etching method where a hydrofluoric-based solution is used for selective etching of metal layers (element A) in the M_n+1AX_n. As hydrogen fluoride is toxic in nature, it creates major health and environmental issues. In that context, the research community looking for a green synthesis method for the preparation of MXene. Present work demonstrates a simple, easy and green synthesis of Ti₃C₂T_x MXene. The synthesized material is characterized with XRD and SEM for crystallography and morphological study. The XRD results confirm the formation of Ti₃C₂T_x along with various functional groups. The obtained SEM results showed that the multilayered structure of MXene was clearly observed at low resolution itself also EDS confirm the presence of Ti and C as major element percentages. The synthesized material can be used for further applications like energy storage devices, sensors, etc.

Since the first report by Naguib et al. in 2011, Ti₃C₂T_x MXene prepared by selective etching of monoatomic Al layers in the Ti₃AlC₂ MAX phase, in particular, has attracted tremendous attention. This originates from its high electrical conductivity, hydrophilicity, and ability to intercalate cations and store charge, which are favored for electrodes, supercapacitors and energy storage applications. Many studies have been focusing on the deposition process, compositing technology, and optimization of physical shape through improvement of etching and intercalation for improving fundamental performance of the MXene nanosheets. In many studies to improve the fundamental performance of Ti₃C₂T_x MXene, commercial MAX phase was used as a starting material and focused on optimizing the selective etching process of the Al layer. In this study, the synthesizing strategy of MAX phase was revisited to lower the oxygen concentration inside the MAX and MXene nanosheets. In the well-known MAX synthesis, Ti powder is used as a Ti source, in which case the high oxygen affinity of Ti increases the oxygen content in MAX. Such oxygen remains in MXene even after Al etching, and most of it constitutes surface termination, but excessive oxygen formed oxides (TiO₂ and Al₂O₃, etc.) inside the MXene and act as defects. To improve this, we propose a 2-step synthesis process consisting of the first step of synthesizing TiC from TiO₂ and the immediately following second step of synthesizing Ti₃AlC₂ MAX from the first step output. Interestingly, MAX synthesized through this two-step process contained significantly less oxygen than commercial MAX phases and those synthesized from Ti powders prepared as a comparison group. We prepared Ti₃C₂T_x MXene nanosheets using this MAX, and compared its properties with those of the MXene nanosheets prepared from the MAX synthesized with Ti as a starting material.

MXenes are 2D transition metal carbides or nitrides, and many studies are being conducted due to the fascinating properties of various MXene phase. Nevertheless, the problem of oxidation stability and narrow solvent compatibility due to the presence of hydrophilic terminal groups (O or OH) should be urgently overcome. Herein, universal ligand system for the preparation of organic-dispersible MXene was designed through bio-inspired chemistry. The resulting organic MXene inks or pastes have been utilized in various application including electromagnetic interference shielding, electrodes for supercapacitors, etc. This work was supported by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Govermment (MOTIE) (P00008500, The Competency Development Program for Inudstry Specialist).

Two-dimensional (2D) transition metal carbide and nitrides, MXenes, exhibit promising properties in different applications such as energy storage, sensing, and optoelectronics, to name a few.(1) These materials have a general formula of M_n+1X_nT_x where M is a transition metal, X is carbon and/or nitrogen, and T_x represents surface functional groups on the outer layers. Since the discovery of MXenes in 2011, research has mainly focused on Ti₃C₂T_x because of its established synthesis protocols and high chemical stability, while investigation of other experimentally made, stoichiometric MXenes has been trailing behind. Among other MXenes, M₂XT_x MXenes such as Vanadium carbide (V₂CT_x) promise exceptional theoretical properties due to the increased active surface area in M₂XT_x MXenes and the multiple available oxidation states in Vanadium systems. Despite its attractive properties, however, research related to V₂CT_x has mostly been limited to its multilayered state due to rapid oxidation after delamination. Herein, we show that through post-processing protocols the shelf life of delaminated V₂CT_x dispersions can be extended from only a few hours to multiple months with almost no degradation. Lithium-ion exchange is performed to remove the amine intercalants that are a byproduct of MXene delamination. Through this process, the excess of cations cause the MXene to crash out of solution. In this flocculated state, the MXene exhibits greatly enhanced stability and can be redispersed into water by washing with DI water. Various characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis, and thermogravimetric analysis were used to investigate the materials. This work shows the instability of certain MXenes can be overcome using proper solution processing techniques.

1. A. VahidMohammadi, J. Rosen, Y. Gogotsi, The world of two-dimensional carbides and nitrides (MXenes). Science. 372, eabf1581 (2021).

Although the family of well over 50 experimentally realized MXenes has grown to include one or more transition metals from groups 3-6 of the 3d-5d blocks of the periodic table, W has remained a difficult transition metal to include in the typical MXene structure. Although MXenes are typically synthesized from their respective parent MAX phase precursors, further exploration of alternative precursors can provide more effective ways of creating MXene possible with non-typical transition metals. Here, we discuss the synthesis of W₂TiC₂T_x, an out-of-plane ordered MXene where the ordering defines the structure as containing a layer of titanium with two carbon layers bonded to the top and bottom respectively which are further sandwiched between two tungsten layers completing the M₃X₂ formula. To synthesize this MXene, the nano-laminated ternary transition metal carbide non-MAX precursor (W,Ti)₄C_4-x was chemically immersed in hydrofluoric acid followed by tetramethylammonium-hydroxide treatment to yield single-to-few layer flakes that can then be vacuum filtered to form W₂TiC₂T_x free-standing films. The successful synthesis of new W-containing out-of-plane ordered double transition metal MXene derived from a non-MAX precursor exemplifies the complexity of MXene that is possible via synthesis involving non-MAX precursors. With the addition of W₂TiC₂T_x to the out-of-plane ordered double transition metal sub-family of MXenes, there is increased potential for the tunability of electronic, magnetic, optical, and electrochemical properties due to chemical diversity.

To improve the properties of MXenes and explore new applications, it is of paramount importance to study the synthetic mechanism, in addition to obtaining direct insight on the impact of different etchants on the structure, such as defects, and the properties, such as electrical conductivity. Despite its importance, this has been very challenging due to the atomic layer of A-elements being etched and the elaborateness required in investigating the inner structural changes of individual MAX particles. Here, we try to observe the etching behavior of Ti₃AlC₂ MAX phase at an atomic-scale and correlate the structural changes of Ti₃AlC₂ MAX phase and we suggest the etching mechanism of atomic Al layers in the MAX phase during MXene synthesis. For this purpose, we tried to observe the etching state of atomic Al layers in etched MAX phases depending on Al etchants and etching times. At first, we confirmed the etching state of Al layers after etching with LiF and HCl according to specific etching times (3, 6, and 24 hours). For all etchants, we show that etching starts from the edges and propagates along the basal planes, leading to the stepwise etching of Al layers. However, the etching mechanism is very different for the grain boundary depending on the types of etchants. HF and NH₄HF₂ etchants can break poly-crystals at their grain boundaries into single-crystals, while poly-crystal particles remain after LiF/HCl etching. Meanwhile, in an earlier stage, the etching of Al atoms occurred at relatively weak edge layers, which are exposed to etchant including inside of MAX phase like voids, in MAX particle. In these weak edge layers, one side boned Ti₃C₂-Al layers exist dominantly, and they can have relatively lower bonding energy than both sides bonded Al-Ti₃C₂-Al layers, which are inside of MAX phase particle. Then, an etching process proceeds to the inner layers and next layers sequentially from starting points. Additionally, we revealed the role of Cl^- ion for the MXene delamination. It can induce the easier intercalation of Li⁺ ions between Ti₃C₂ layers, resulting in effective delamination of MXene sheets. We expect that our results can open the ways to develop MXene synthesis methods for high quality and mass production and give us the possibility for tuning the properties of synthesized MXene to their proper application.

Surface-Enhanced Raman Spectroscopy (SERS) has emerged as a powerful spectral detection technology. At this moment in time, noble metal films/nanostructures are readily used as effective SERS substrates for various chemo/biosensing, however, lower sensitivity and cost is limiting factor associated with them. Lately, with the advent of MXenes, they have gained significant interest in the SERS field due to their ease of preparation, scalability, excellent optical and chemical properties, high spectral stability, and strong anti-interference ability. They can also bind probe molecules stably and firmly on their surface that’s conducive to obtaining stronger Raman signals. In the present work, we report for the first time Mo₂CT_xMXene/Au hybrid as a sensitive SERS substrate utilizing methylene blue as a model chemical. The Mo₂C MXene is synthesized from the Mo₂Ga₂C MAX phase, where the Ga layer was etched to form Mo₂CT_x layers. The synthesized Mo2C SEM-EDX analysis shows the elemental mapping of Mo (63.33%) and C (36.67%) indicating the uniform distribution of Mo and C and no Ga. Also, the AFM results of the delaminated MXene display the average height of 27 nm of the layered structure. Further, the in-situ hybridization of the MXene was carried out with Au for the formation of MXene/Au hybrids. The as-prepared noble Mo₂C MXene/Au hybrid demonstrates an excellent SERS characteristic in terms of enhancement factor, sensitivity, and detection limit. This could be assigned to synergetic plasmon enhancement by Au nanoparticles and chemical transfer by Mxenes as well as excellent chemisorption of dye molecules due to high specific surface area. These initial findings can be significantly implemented in the designing and development of MXene hybrids-based SERS substrates for the chemical and biological sensing of targeted molecules.

MXene is one of the attractive 2-dimensional materials because of its low bandgap, relatively low resistance, and composition diversity to manipulate its electrical/physical property. This MXene can be an active branch of solid-state physics with considerable potential for spintronics, magnetoelectronics, and the long-term frontier of information processing technology. For that, magneto-conductance (same as magnetoresistance) study (of material) is an important candidate to understand transport behavior and/or dynamics of charge carriers for the future opportunity of applications. Also, the study on a practical material system such as inkjet printed MXene mesh is actually required. Herein, we report an experimental case of extrinsic negative magneto-conductance and observation of weak anti-localization (WAL)/weak localization (WL) crossover at inkjet printed Ti₃C₂T_x MXene mesh system. The observation of WAL corresponds that this Ti₃C₂T_x MXene mesh system has strong spin-orbit coupling at low temperature which opens possible application for spin-orbitronics. From WAL/WL magneto-conductance analysis, we estimate that the inkjet-printed MXene mesh has a strong spin-orbit field up to Hso ~ 0.84 T at 1.9 K temperature. This work will not only lead to a deep understanding of the microscopic charge carrier transport behavior but will also lead to a broad range of applications for spintronics (especially spin-orbitronics) and information processing technology.

A large family of two-dimensional transition metal carbides and nitrides (MXenes) raises interest for many applications due to their high electrical conductivity, mechanical properties [1], potentially tunable electronic structure [2], nonlinear optical properties [3], and the ability to be manufactured in the thin film state [4]. However, their chemistry that is key to development of these applications, still remains largely terra incognita [5,6]. In this presentation we will discuss recent progress in understanding fundamental MXene chemistry and harnessing it for development of applications.
For example, during delamination and storage in ambient air environment, spontaneous oxidation of MXene flakes leads to formation of titanium oxide, a process that can be harnessed for simple, inexpensive, and environmentally benign manufacturing MXene–titania composites for optoelectronics, sensing, and other applications [7]. We show that partially oxidized MXene thin films containing the in situ formed phase of titanium oxide have a significant photoresponse in the UV region of the spectrum. The relaxation process of photoexcited charge carriers, which takes a long time (∼24 h), can be accelerated in the presence of oxygen and water vapor in the atmosphere. These properties of spontaneously formed MXene-titania thin films make them attractive materials for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications.
Other selected examples illustrating connections between understanding MXene chemistry and development of their applications will also be considered.



References
Y. Li, S. Huang, C. Wei, C. Wu, V. N. Mochalin, Nature Communications, 10, 3014 (2019)
M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, Advanced Materials, 26(7), 992-1005 (2014)
J. Yi, L. Du, J. Li, L. Yang, L. Hu, S. Huang, Y. Dong, L. Miao, S. Wen, V. N. Mochalin, et al., 2D Materials, 6, 045038 (2019)
Y. Dong, S. Chertopalov, K. Maleski, B. Anasori, L. Hu, S. Bhattacharya, A. M. Rao, Y. Gogotsi, V. N. Mochalin, R. Podila, Advanced Materials, 30(10), 1705714 (2018)
S. Huang, V. N. Mochalin, Inorganic Chemistry, 58(3), 1958 (2019)
S. Huang, V. N. Mochalin, ACS Nano 14(8), 10251-10257 (2020)
S. Chertopalov, V. N. Mochalin, ACS Nano, 12(6), 6109-6116 (2018)

MXenes are a class of two-dimensional transition metal carbides that show great promise for a range of applications, from energy storage to catalysis. The functional properties of MXenes depend critically on the surface functionalisation of the MXene layers (e.g. -OH, -F, -O); a greater understanding of the surface terminations, and the influence of synthesis on the surface chemistry, will enable the properties of MXenes to be optimised for a given application. However, as an aperiodic assembly of light atoms, the surface groups remain challenging to study via conventional techniques.
Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for the characterisation of MXenes that is sensitive to the local environment and electronic properties of the chosen nucleus, with no requirement for long-range periodic order. In this work, we report an exhaustive study of the surface and bulk chemistry of Ti₃C₂T_x, Nb₂CT_x, and Nb₄C₃T_x MXenes [1,2].
¹H and ¹⁹F NMR are used to quantify -OH and -F terminations and demonstrate that they are intimately mixed on the MXene surfaces. Furthermore, the effect of different etching procedures on the relative proportions of the terminations is determined. Strongly hydrogen bonded interlayer water is also identified and found to be present even after vacuum drying at 200 °C. ¹³C, ²⁷Al, ^47/49Ti and ⁹³Nb NMR are sensitive to the transformation from MAX to MXene due to the changes in the electronic structure. They also revealed a number of by-products and impurities in the MXenes, including aluminium oxides and fluorides, NbC, Nb metal, and unreacted MAX phase. Several of these are amorphous or nanocrystalline and therefore diffraction-silent, indicating that diffraction alone is insufficient to characterise MXene phase composition. NMR of the quadrupolar transition metal nuclei is challenging, but also highly sensitive to the local symmetry; in combination with density functional theory calculations of the electric field gradients at the nucleus, this afforded a detailed view of the local coordination environment. Overall, NMR provides a comprehensive picture of the phase composition, surface chemistry, and bulk structure of MXenes which can be used to correlate structure–property relations and tailor MXene synthesis for next-generation devices.
1. Hope, M. A.; Forse, A. C.; Griffith, K. J.; Lukatskaya, M. R.; Ghidiu, M.; Gogotsi, Y.; Grey, C. P. Phys. Chem. Chem. Phys. 2016, 18 (7), 5099–5102.
2. Griffith, K. J.; Hope, M. A.; Reeves, P. J.; Anayee, M.; Gotosi, Y; Grey, C. P. J. Am. Chem. Soc. 2020, 142 (44), 18924–18935.

Two-dimensional material, MXene and its applications have gained tremendous attention since its discovery in 2011. ¹ There have been enormous efforts to tailor MXene properties making it a viable prospect in various applications such as energy storage (rechargeable batteries, supercapacitors), energy generation (photocatalysis and electrocatalysis, solar cells), environmental remediation (water treatment etc.) and biomedical applications. ^{2 3 4 5} The sensitivity of two-dimensional materials could be achieved by surface modification through engineering defects through surface functionalisation.
Plasma functionalisation is one of the most common modification methods in defect engineering ⁶ with the advantages of low cost, environmentally benign, scalability. Plasma modification generates an efficient production of reactive ions with Ti₂C MXene, that can effectively produce defects such as oxygen vacancies and change the structure of Ti₂C MXenes. In this work, atmospheric pressure plasma printing, with helium as a carrier gas and oxygen as a reducing gas, was used to (i)ionise and print simultaneously which resulted (ii) in the in situ formation TiO₂ from prepared Ti₂C MXene which could form a heterostructure with improved bandgap and (iii) and simultaneously generate oxygen vacancies (via reduction of Ti⁴⁺ forming Ti³⁺ ).
Various characterisation techniques were employed to establish the correlation between the surface groups and termination groups ofTi₂C MXene before and after helium-oxygen plasma functionalisation and printing. SEM and TEM investigation showed that controlled oxygen plasma treatment broke Ti-C bonds and generated abundant Ti-O bonds on Ti₂C MXene. Raman imaging was also used to examine plasma-induced damage in Ti₂C MXene after oxygen plasma treatment and not only changes in D/G ratio but also active anatase TiO₂ after the plasma oxidization process. These measurements showed not only conversion of Ti₂C to TiO₂ but also defect generation in Ti₂C MXenes after 3 minutes of plasma treatment. XPS results showed the varying oxidation states on the surface of Ti₂C MXene after different plasma exposure times. In addition, the surface tension of Ti₂C MXene was thoroughly investigated by the contact angle and topography measurements. The nanocomposites were applied for photodegradation of organic pollutants.
References
1. Halim, A., Qu, K. Y., Zhang, X. F., & Huang, N. P. (2021). Recent Advances in the Application of Two-Dimensional Nanomaterials for Neural Tissue Engineering and Regeneration. ACS Biomaterials Science & Engineering, 7(8), 3503-3529.
2. Pang, J., Mendes, R. G., Bachmatiuk, A., Zhao, L., Ta, H. Q., Gemming, T.,& Rummeli, M. H. (2019). Applications of 2D MXenes in energy conversion and storage systems. Chemical Society Reviews, 48(1), 72-133.
3. Liu, A., Liang, X., Ren, X., Guan, W., Gao, M., Yang, Y., & Ma, T. (2020). Recent Progress in MXene Based Materials: Potential High Performance Electrocatalysts. Advanced Functional Materials, 30(38), 2003437.
4. Nguyen, T. P., Nguyen, D. M. T., Le, H. K., Vo, D. V. N., Lam, S. S., Varma, R. S., & Van Le, Q. (2020). MXenes: Applications in electrocatalytic, photocatalytic hydrogen evolution reaction and CO2 reduction. Molecular Catalysis, 486, 110850.
5. Jeon, M., Jun, B. M., Kim, S., Jang, M., Park, C. M., Snyder, S. A., & Yoon, Y. (2020). A review on MXene-basednanomaterials as adsorbents in aqueous solution. Chemosphere, 127781.
6. Felten, A., Eckmann, A., Pireaux, J. J., Krupke, R., & Casiraghi, C. (2013). Controlled modification of mono-and bilayer graphene in O2, H2 and CF4 plasmas. Nanotechnology, 24(35), 355705.

Ti₃C₂T_x MXene has drawn immense attention over the last decade for its high performance in broadly distributed application areas, resulting from its unique electronic and physicochemical properties. As the applications of Ti₃C₂T_x have broadened, its degradation mechanisms have been thoroughly studied, and it is now well-understood that Ti₃C₂T_x converts to TiO₂ over time via hydrolysis while dispersed in water.¹ While this has been shown to negatively impact the electrical conductivity of prepared films, there are now a few works recognizing potential to take advantage of nanostructured blends of Ti₃C₂T_x with TiO₂ for wide-ranging optoelectronic and photocatalytic applications.^2,3 In this work, we outline a method for tunable conversion of Ti₃C₂T_x to TiO₂ by treatment of colloidal Ti₃C₂T_x dispersions with gaseous ozone. By adjusting the time of ozone treatment, we precisely control the degree of conversion of the Ti₃C₂T_x/TiO₂ blend while retaining much of the 2D structure of the MXene flake. Further, through very short ozone treatment times, we show that it is possible to solely modify the surface terminations of the MXene flake, with minimal TiO₂ formation. The results presented here outline an alternative method for assembly of unique nanostructured Ti₃C₂T_x/TiO₂ composites with powerful implications in photocatalysis and optoelectronic devices.
(1) Huang, S.; Mochalin, V. N. Hydrolysis of 2D Transition-Metal Carbides (MXenes) in Colloidal Solutions. Inorg. Chem. 2019, 58 (3), 1958–1966.
(2) Yang, L.; et al. Performance Improvement of MXene-Based Perovskite Solar Cells upon Property Transition from Metallic to Semiconductive by Oxidation of Ti₃C₂T_x in Air. J. Mater. Chem. A 2021, 9 (8), 5016–5025.
(3) Agresti, A.; et al. Titanium-Carbide MXenes for Work Function and Interface Engineering in Perovskite Solar Cells. Nat. Mater. 2019, 18 (11), 1228–1234.

MXenes, a new class of early transition metal carbide, nitrides and carbonitrides, have gained tremendous attention in the scientific community due to their interesting property combination. Applied as solid lubricant coatings, MXenes have shown an outstanding performance resulting in an excellent durability and longevity (ultra-high wear resistance), which even outperformed state-of-the art solid lubricant including graphene and MoS₂. This contribution presents the latest findings regarding MXenes’ ability to form beneficial tribolayers depending on MXenes’ number of layers (few- versus multi-layer systems), coatings thickness (amount of lubricious material), applied load (thermomechanical) and sliding velocity (kinetics).
Introduction
MXenes nano-sheets have experienced tremendous attention in the scientific community since their discovery in 2011. In the last 5 years, the tribological research community has started to explore their friction and wear performance when used as lubricant additives, solid lubricant coatings and reinforcement phase in composites. Especially when using MXenes for solid lubrication, promising results have been verified. MXene coatings tend to demonstrate an ultra-high wear resistance being particularly beneficial for the durability and longevity of these coatings. These beneficial properties are traced back to the formation of a thin MXene-rich tribolayer. Little is known about the structural and compositional properties of these tribolayers. The underlying kinetics and driving forces are yet to be explored. More knowledge about the involved mechanisms and kinetics is urgently needed, which is expected to significantly boost this entire research topic.
Therefore, we have designed tribological ball-on-disk experiments to understand the influence of the number of layers (few- versus multi-layers), the coatings thickness and the tribological testing conditions (normal load, sliding velocity and relative humidity) on the tribofilm formation. Combined with advanced materials characterization, these tests allow us to draw some important conclusions about the involved thermomechanical aspects and underlying kinetics of the layer formation.
Methods
We fabricate solid lubricant coatings using few- and multi-layer Ti₃C₂T_X nano-sheets on stainless steel substrates. The nano-sheets are deposited in different concentrations by spray-coating thus enabling a range of different coating thicknesses. Afterwards, we perform tribological ball-on-disk experiments with varying normal load (Hertzian contact pressure) and sliding speed to assess the involved thermomechanical and kinetic aspects of the underlying tribolayer formation.
Discussion
Based upon the experiments conducted, we verify thermomechanical and kinetic aspects of the involved tribolayer formation, which align well with the respective temporal evolution of the coefficient of friction. When exceeding a critical value of the applied normal load (Hertzian contact pressure), the formation of a stable tribolayer with beneficial friction and wear properties is not possible. More importantly, the same conclusion can be drawn when exceeding a critical sliding velocity, which clearly shows the kinetic aspect of the involved layer formation. We also verify that increasing the respective thickness of the MXene coatings does not necessarily result in more beneficial effects (low friction, low wear, and long-lasting effects). Concerning energy application, the material of choice tends to go towards mono-layer MXenes. Regarding tribological research, no scientific study has systemically addressed whether it is more beneficial to use few- or multi-layer MXenes. This contribution also sheds some light on this open question, thus giving some important guidelines and recommendation for future tribological experiments using MXenes.

MXenes have become an important class of 2D materials with functional applications ranging from electrochemical energy storage, catalysis, nanoelectronics, optoelectronics and sensors. Their unique physical and chemical properties are derived from their surfaced functional group chemistry and transition metal carbide, nitride and carbonitride atomic structure. New insight into MXene synthesis and measurement of functional properties can be elucidated at the atomic and nanoscale using in situ scanning transmission electron microscopy (STEM) based characterization methods. Specialized heating and electrical biasing platforms have been used to investigate the thermally induce growth of new MXene stoichiometric structures and ionic transport mechanisms and kinetics. During thermal exposure of Ti₃C₂T_x MXenes at temperatures above 500C, it was found that through atomic resolution STEM imaging that new hexagonal TiC layers from on one or both sides of single layer Ti₃C₂ following a Frank-van der Merwe growth mechanisms. ReaxFF and DFT calculations reveals the step-edge barrier that needs to be overcome for Ti and C atoms to migrate and diffuse on the surface to form the new layers. Similar behavior is observed for other MXenes as will be discussed. To better understand ionic transport behavior, probe based in situ electrical biasing TEM holders were used to understand the dynamics response of ion intercalation and corresponding kinetics of lattice plane expansion at nanoscale spatial resolution. Ion transport kinetics measurements revealed a decrease in lattice expansion as a result of the formation of SEI like inorganic phases. The in situ S/TEM research presented here aide to better understand materials transformations barriers and pathways in 2D MXenes.

The oxidation of 2D MXenes jeopardizes their shelf life, both in colloidal dispersions and in functional devices. Our group has previously shown that the addition of antioxidants is an effective approach to enhance the oxidation stability of MXenes. However, the nature of the antioxidant-MXene interactions remains unknown. This work systematically examines the interactions between three classes of organic antioxidant candidates, α-hydroxy acids, polycarboxylic acids, and phenols, with Ti_n+1C_nT_x, specifically Ti₃C₂T_x and Ti₂CT_x, nanosheets. There antioxidants (e.g., citric acid, tartaric acid, and oxalic acid) were proven to protect MXenes exceptionally well. Analyzing each antioxidant’s molecular structure and efficacy indicates that chelation interactions with the transition metal layers are the key to protecting MXenes.

MXenes have been extensively investigated for a range of applications from energy storage to catalysis. However, few studies take advantage of the inherent stability of the interior M-X transition metal carbide core for use in extreme environments. This interior M-X core gives MXenes potential to be used as highly stable transition metal carbides in inert conditions such as ultra-high temperature (> 3,000 C) or highly radioactive environments. In this study, we investigated the high-temperature behavior of two M₃C₂T_x MXenes of Ti₃C₂T_x and Mo₂TiC₂T_x from room temperature to 2,000 C using in-situ annealing up to 1,100 C using two-dimensional x-ray diffraction (XRD²) and ex-situ XRD² sintered MXenes up to 2,000 C annealed via spark plasma sintering. In the case of Ti₃C₂T_x, we identified the formation of nanolamellar cubic disordered carbon vacancy TiC_y with strong preferential (111) plane orientation formed along the original basal plane of Ti₃C₂T_x MXene. Our cross-sectional scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) results further confirmed the preferentially ordered phases that keep their lamellar morphology up to 2,000 C in inert environments. In the case of Mo₂TiC₂T_x, which is an ordered double-transition metal MXenes, our results indicated a combination of micro- and nano-scale structures of molybdenum and titanium carbides. These findings confirm that MXenes remain as carbides under inert environments at high temperatures of at least 2000 C and identify a unique capability of MXenes to synthesize MXene-derived carbides with unique morphologies that are not possible to make via the traditional carbide synthesis routes.

MXenes are a relatively new class of two-dimensional (2D) transition metal carbides, nitrides and carbonitrides. Due to their hydrophilicity, high surface area, activated metallic hydroxide sites, and environmentally friendly characteristics, MXenes have aroused great interest for a variety of applications—including environmental remediation and water treatment. Given the relative newness of this class of materials, many uncertainties exist about their environmental behavior, fate, and transport; and, their stability under conditions relevant to natural and engineered environments has not been explored. Here, we investigate the colloidal and chemical stabilities of Ti₃C₂T_x MXenes in aqueous suspensions in the presence of O₂, free chlorine, and/or UV light as a function of pH (5-9) and free chlorine concentration (20-200 mg L^-1). We find that pH in the range of 5-9 and UV irradiation (λ_max = 365 nm, fluence = 6.36 x 10^-2 mE cm^-1, up to 240 min) do not affect the stabilities of aqueous Ti₃C₂T_x MXene colloidal solutions over the experimental timeframe as assessed by UV–visible absorption spectroscopy. In contrast, exposure to HOCl/OCl^– induced aggregation of Ti₃C₂T_x MXenes. At low chlorine concentrations, solutions with dark green color were observed, indicating good colloidal stability of Ti₃C₂T_x MXenes. When the chlorine concentration exceeded a threshold, Ti₃C₂T_x solutions became unstable, forming visible agglomerates. For all conditions evaluated, Ti₃C₂T_x reacted with free chlorine. The rates of reaction with free chlorine were enhanced under UV irradiation and were highest at pH 6, where HOCl is the dominant species in the solution. We also find that the rates of reaction in the presence of free chlorine were lower for MXene solutions in phosphate buffer than those in deionized water. We attribute this to the adsorption of phosphate ions to the edges of Ti₃C₂T_x MXene flakes reducing their propensity for oxidation. We show that oxidation starts at the edges and its kinetics follow a single exponential decay. Observed reactivity has direct implications for MXene life cycle assessments and for material performance in water treatment technologies, such as membrane and disinfection processes, which utilize chlorine and/or UV irradiation.

Due to their versatile chemistry and two-dimensional morphology, MXenes can serve as unique precursors for the synthesis of other classes of materials. The presence of transition metals in the layer structure facilitates redox activity of the produced phases in intercalation reactions. Tunability of the surface terminations via selecting reagents in the MAX phase etching process enables additional degree of control of MXene-derived materials. In this presentation, the focus will be put on the lattice reconstruction approaches achieved through wet chemistry methods. The reorganization of the V₂CT_x framework via anionic sublattice modification leading to the formation of chemically preintercalated bilayered vanadium oxides (BVOs) will be demonstrated. Modification of the MAX phase etching protocols will be shown to have an effect on the morphology of the produced oxides. The electrochemical charge storage capability in cells with non-aqueous electrolytes will be correlated with morphology, structure and composition of the V₂CT_x-derived BVOs. While this study focused on BVOs, this synthesis route can be used with other MXenes enabling the production of oxides with different chemistries or mixed oxides. Furthermore, it is possible for new metastable phases with enhanced functionalities that cannot be synthesized using traditional routes to be produced by utilizing MXenes as precursors.

Nanomaterials, particularly two-dimensional (2D) materials, may provide new opportunities to develop future technologies. Two-dimensional metal carbides and nitrides (MXenes) are the largest and yet growing family of 2D materials. Their unique and diverse electronic, optical, and electrochemical properties have already distinguished them from other widely studied 2D materials. Unlike other 2D materials, MXenes have rich chemistry with more than 30 different available compositions. The structure of MXenes can comprise of one or two different types of transition metals in a random solid solution (such as (Ti,V)₂CT_x) or ordered (in-plane or out-of-plane, such as Mo₂TiC₂T_x) form.¹ The surface functional groups of MXenes can be modified, enabling MXenes with metallic behavior having conductivity values over 15,000 S cm^-1 and semiconducting or superconducting electronic properties with a range of work functions predicted to be from ~2.6 eV to ~6 eV. The tunable optoelectronic properties of MXenes along with their high mechanical stiffness (Young’s modulus reaching ~0.4 TPa for Nb₄C₃T_x MXene), rapid ion-transport properties, redox-active surfaces, and their non-toxicity have rendered these materials as promising candidates for various emerging technologies such as transparent conductive electrodes, smart and conductive textiles for sensing and energy-storing wearable devices, biomedical and biosensing, photovoltaics, and high energy and power density supercapacitors. However, similar to any new material and technology, scalability, killer applications, and usability of these materials are yet to be shown in order to unleash their practical potential for future devices and technologies. In this talk, we aim to discuss MXenes and their unique properties for emerging technologies, focusing on their promise for next-generation energy storage systems. We discuss the optimism about these materials and why we believe they may provide a path to the promised land of fast and reliable charge storage devices.
1. VahidMohammadi, A.; Rosen, J.; Gogotsi, Y. The World of Two-Dimensional Carbides and Nitrides (MXenes). Science. 2021, 372 (6547), eabf1581.

Two-dimensional (2D) MXenes, thanks to their high conductivity and electrochemically active surface, are important candidates for electrochemical energy storage devices such as batteries and supercapacitors, however, their energy density is limited by a narrow voltage window in aqueous electrolytes. Recently, an anomalous electrochemical behaviour of Ti₃C₂T_x MXene in water-in-salt electrolytes has been demonstrated, allowing insertion/extraction of partially solvated cations, leading to an abrupt change of interlayer spacing between MXene flakes. This unusual phenomenon enhances charge storage at positive potentials, which allows a wider working potential window, and, in this way, high energy density can be realized. In the present work, we prepared a MoO₃/Ti₃C₂ hybrid, which was electrochemically studied in saturated LiCl electrolyte and a stable potential window of 1.8V was achieved. Cyclic voltammograms reveal multiple redox peaks of MoO₃ in addition to the well-separated peaks of Ti₃C₂, which lead to a high specific charge (>400 C/g at 0.2 mV/s) and higher rate capability (>70% retention up to 10 mV/s) compared to carbon containing MoO₃, which retains <5% of its capacity up to 10 mV/s. These results demonstrate that not only a large amount of charge can be stored by adding highly conducting MXenes to less conductive oxides, but redox capacity at high rates can also be achieved in absence of any other conductive additives or binders. Moreover, this approach can be extended to other oxides and less conductive materials.

MXenes constitute a large family (40+ phases) of two-dimensional transition metal carbides/nitrides with a composition of M_n+1X_nT_x (M is an early transition metal, X is carbon or nitrogen, n =1-4, and T_x stands for surface terminations). Their excellent electrical conductivity and capability to host ions make them ideal candidates as electrode materials for electrochemical energy storage. While intercalation plays an important role in the applications of MXenes since it is the mechanism for storing ions in it, less is known about the effect of pre-intercalation on the electrochemical performance of MXenes. We will discuss few examples where pre-intercalation introduced a significant change in the electrochemical performance of MXenes. First example, Na⁺, K⁺, and Mg²⁺ pre-intercalated multilayer Ti₃C₂T_x were explored as aqueous supercapacitor electrode with adjustable areal loadings (5.2 to 20.1 mg/cm²). K-Ti₃C₂T_x exhibited the highest capacitances at different scan rates, achieving a gravimetric capacitance of up to 300 F/g, comparable to delaminated MXene but with an ultra-high areal capacitance of up to 5.7 F/cm², which is manyfold higher than the 0.5 F/cm² which can be achieved for delaminated MXene. The other example is using pre-intercalation of alkylammonium (AA) cations with different chain lengths to study the effect of interlayer spacing in MXene on their performance as electrodes for supercapacitor in room-temperature ionic liquids (RTIL) electrolytes that offer larger potential windows, leading to higher energy density. We found that pre-intercalated MXene with an interlayer spacing of ≈2.2 nm can operate within a voltage window of > 3V in neat EMIMTFSI electrolyte and deliver a large specific capacitance of 257 F/g (1428 mF/cm² and 492 F/cm³) -an order of magnitude higher than pristine MXene- leading to high-energy density achieving those of batteries without compromising their high-power density. We will also report on using pre-intercalation to achieve new compositions with high alkali metal content (e.g., Ti₃C₂OFNa₂) that can be used as electrode materials for ion capacitor directly without the need for extra steps of electrochemical ion insertion in half-cell configuration before assembling full cells.

MXenes have shown promise for a range of applications due to their unique physical and chemical properties. The high electrical conductivity, 2D and layered structure, and rich surface chemistry of MXenes have sparked great interest in their properties as electrode materials for energy storage devices. However, similar to other 2D materials, the applications of MXenes in batteries and supercapacitors is dependent on their assembly into electrode structures with high electrical and ionic conductivities. The main focus of this talk is our group’s recent research on the assembly of MXene flakes into electrode structures with high energy and power densities. The effects of synthesis conditions and controlling the flake size on the specific capacitance and rate capability of MXene electrodes will be discussed. Furthermore, our recent work on the assembly of MXene heterostructures to improve pseudocapacitive properties will be presented. The heterostructured MXenes show superior chemical and electrochemical stabilities as well as energy storage capability. Finally, some of our recent studies on 3D printing of MXene electrodes will be presented and discussed.

Two-dimensional (2D) materials of different compositions can be stacked like building blocks to form unique heterostructures combining the properties of dissimilar materials to achieve new paradigms that can’t be achieved by the individual materials alone. One interesting family of 2D materials are transition metal oxides (TMO), which are relatively inexpensive and have promise in the field of electrochemical energy storage. Because 2D TMO are either insulators or semiconductors with wide bandgap, conductive additives must be used for electrochemical energy storage applications. Electrically conductive 2D transition metal carbides, MXenes, have already shown enormous potential in the field of electrochemical energy storage, due to their capabilities to host ions and protons in addition to their high electrical conductivity. The wide variety of available MXenes and TMOs with different properties allows the tuning of heterostructure architecture to optimize performance.

We have achieved novel supercapacitor architectures which show properties which combine and exceed the properties of either material alone. Ti₃C₂T_x-Nb₂O₅ heterostructures demonstrate capacitance of over 200 F/g in neutral aqueous electrolytes Li₂SO₄ and Nb₂SO₄. In the same electrolytes, Nb₄C₃T_x-Nb₂O₅ demonstrates a capacitance up to 100 F/g. These capacitances exceed what can be achieved each material alone. In case of Ti₃C₂T_x-Nb₂O₅, we observed multiple redox peaks, assigned mostly to Ti₃C₂T_x. which have the effect of expanding the high-current area of the cyclic voltammagram curve. Interestingly, such redox peaks were not observed in neutral aqueous electrolyte for MXene alone. The use of high-resolution TEM, as well as allows atomic-scale resolution of the heterostructure interface, showing that MXene and TMO nanosheets are stacked to form the heterostructure interface. STEM/EELS and X-ray absorption spectroscopy shows the effect of the interface between MXene and TMO on the oxidation states of titanium and niobium, which is supported by density functional theory (DFT) calculations.

Smart fabrics are attracting tremendous interest at both academic and industrial levels. In the next future, smart textile will likely integrate different kind of microdevices as energy harvesting, luminescent actuators and sensors for human health monitoring, control motion and others. In this framework, the development of textile energy storage plays a key role as power source for low-consumption microdevices. The design of fibrous-shaped electrodes have been considered as a suitable strategy but their compatibility with conventional textile machining technique has been rarely demonstrated.[1] On the contrary, microbatteries and microcapacitors can be easily coated on fabric surface by printing technologies.[2] Despite graphene and carbon nanostructures still represent the most common printed materials, MXenes, a new class of 2D materials, are revolutioning materials science and their applications.[3] Their metallic conductivity, outstanding water processability and electrochemical energy storage properties make MXenes the ideal candidate materials for ink-based printing of energy storage device. [4] In this work, inkjet and aerosol printing were proposed as suitable techniques for fast and easy Ti₃C₂ MXenes printing as symmetric microcapacitors on textile. The Ti₃C₂ MXenes were produced with the MILD etching route and ink preparation was optimized to achieve printable solvent-free inks. The electrical conductivity was investigated as well as the substrate effect. The electrochemical properties of the all-printed devices, coupled with both solvent and aqueous based solid-state electrolyte, were investigated at rest and under mechanical deformation. Finally, a comparison between the two techniques for micro-scaled printing device was provided.
Bibliography
1. Zhou Y, Wang C-H, Lu W, Dai L (2020) Recent Advances in Fiber-Shaped Supercapacitors and Lithium-Ion Batteries. Advanced Materials 32:1902779. https://doi.org/10.1002/adma.201902779
2. Choi K-H, Ahn DB, Lee S-Y (2018) Current Status and Challenges in Printed Batteries: Toward Form Factor-Free, Monolithic Integrated Power Sources. ACS Energy Lett 3:220–236. https://doi.org/10.1021/acsenergylett.7b01086
3. Gogotsi Y, Anasori B (2019) The Rise of MXenes. ACS Nano 13:8491–8494. https://doi.org/10.1021/acsnano.9b06394
4. Uzun S, Schelling M, Hantanasirisakul K, et al (2021) Additive-Free Aqueous MXene Inks for Thermal Inkjet Printing on Textiles. Small 17:2006376. https://doi.org/10.1002/smll.202006376

Supercapacitors are one of the most promising electrochemical energy-storage (EES) devices, and bridge the gap between batteries and capacitors in terms of power and energy density. 2D layered transition metal carbides (MXenes) like Ti₃C₂T_x are of great interest due to the presence of fast surface redox storage mechanisms (pseudocapacitance) allowing much higher energy and power densities compared to commonly used electrostatic double-layer capacitors.
However, in all EES systems the fundamental physical interactions and/or chemical reactions depend on the accessible surface- or interface area. In a recent work, a very thin (90 nm) Ti₃C₂T_x electrode showed exceptional capacitance up to 450 F/g and rate performance up to 100 V/s.[1] Nonetheless, practically useful electrodes with much higher thickness and therefore mass prepared, for example, by rolled MXene clay or vacuum filtration suffer from long diffusion pathways and reduced accessible surface area.[1][2] Therefore, highly porous 3D structures such as foams and aerogels composed of 2D materials are highly beneficial due to their large specific surface area, high electrochemical activity, and reduced ion transport distance. Hence, to enable efficient use of their properties, 2D materials need to be arranged into a sophisticated and well-defined macroscopic 3D structure.
In this work we introduce a novel concept that enables the creation of a hierarchical network structure that can be tailored in both micro- and nanoscale in a controlled manner. [3][4] The concept is based on a wet-chemical approach [3] using an aqueous MXene dispersion to coat the surface of a highly porous 3D network structures based on tetrapodal zinc oxide. By thorough controlling of the coating process the layer thickness and therefore the nanostructure can be precisely adjusted (from few nm to ~300 nm). After zinc oxide removal, a freestanding, conductive and mechanically stable 3D scaffold out of MXenes remains that can be subsequently compressed to a certain thickness (from 2000 µm to 50 µm) to control the microstructure and final density.
We demonstrate that by optimizing the nano- and microstructure of these MXene foams in combination with the versatile processing route, practically useful supercapacitor electrodes with areal loadings of more than 5 mg/cm² and densities of up to 1200 mg/cm³ can be fabricated. These electrodes show superior rate capability as well as high gravimetric and volumetric capacitance.
References:
[1] M. R. Lukatskaya et al., “Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides,” Nat. Energy, vol. 6, no. July, pp. 1–6, 2017, doi: 10.1038/nenergy.2017.105.
[2] M. Ghidiu, M. R. Lukatskaya, M. Q. Zhao, Y. Gogotsi, and M. W. Barsoum, “Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance,” Nature, vol. 516, no. 7529, pp. 78–81, 2015, doi: 10.1038/nature13970.
[3] F. Rasch et al., “Wet-Chemical Assembly of 2D Nanomaterials into Lightweight, Microtube-Shaped, and Macroscopic 3D Networks,” ACS Appl. Mater. Interfaces, vol. 11, no. 47, pp. 44652–44663, 2019, doi: 10.1021/acsami.9b16565.
[4] F. Schütt et al., “Hierarchical self-entangled carbon nanotube tube networks,” Nat. Commun., vol. 8, no. 1, pp. 1–10, 2017, doi: 10.1038/s41467-017-01324-7.

Recently, MXenes have emerged as a promising family of materials for a variety of energy storage devices, with much of the work surrounding supercapacitors (SCs) based on Ti₃C₂T_x. Nanosheets of this material combine a metallically conductive titanium carbide core and a surface functionalised by a range of polar and pseudocapacitive surface groups such as F, O and OH – denoted T_x. Conductivity of up to 20000 S/cm,¹ and specific capacitance of up to 450 F/g ² makes it an ideal material for SCs. However, control of the electrode architecture is needed, as overly dense and/or thick electrodes compromise both rate performance and specific capacitance, due to long ion diffusion pathways inhibiting fast charge transfer and restriction of the available surface area.³ Therefore, we describe herein two methods to create hierarchically structured 3D Ti₃C₂T_x electrodes for distinct scales and applications.
At smaller scales, such as IoT and portable electronics, micro-supercapacitors (MSCs) may present a solution to the requirement for compact energy storage, and directly incorporating MSCs into the product through printing is an appealing concept. To address the ion transport and capacitance issues of more conventional dense, 2D printed films,⁴ recent studies have demonstrated 3D printing of MXene-based SCs, though these have been achieved using composites ⁵ or post-processing steps such as freeze-drying.⁶ We show that aerosol-jet printing (AJP) of aqueous Ti₃C₂T_x inks can be used to fabricate high aspect ratio 3D structures up to 1 mm in height and with feature sizes as small as 20 μm, without the aid of supports, binders, or post-processing and in a wide range of geometries such as walls, pillars, and pyramids. Electrodes with 3D pillars show capacitance up to 300 F/g at 5 mV/s in three-electrode configuration, retaining 51% of this capacitance at 500 mV/s. When assembled into symmetric, interdigitated MSCs with gel electrolyte, the 3D structured MSCs exhibit up to 97% greater areal capacitance than mass equivalent planar MSCs, up to 138 mF/cm² at 5 mV/s.
For larger, macroscopic electrodes, we propose a method to create hierarchical 3D Ti₃C₂T_x “aeromaterial” with tuneable nano- and micro-structure. Using sacrificial templates of sintered ZnO tetrapods,⁷ and careful control of the coating process, highly porous MXene structures with tailored layer thickness are obtained. These structures may be compressed to a desired thickness, from 2 mm to 0.05 mm to tailor their microstructure. Optimising the above properties, we have demonstrated MXene aeromaterial electrodes with gravimetric, volumetric, and areal capacitance up to 350 F/g, 175 F/cm³ and 920 mF/cm², respectively, at 20 mV/s. The electrodes retain approximately 67% capacitance at 200 mV/s and over 90% capacitance after 10000 cycles. The combination of density up to 1.2 g/cm³, stability, and excellent electrochemical performance make these electrodes promising candidates for future SCs.

1. Mathis, T. S. et al. Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti₃C₂ MXene. ACS Nano 15, 6420–6429 (2021).
2. Lukatskaya, M. R. et al. Ultra-high-rate pseudocapacitive energy storage in two-dimensional transition metal carbides. Nat. Energy 6, 1–6 (2017).
3. Ghidiu, M., Lukatskaya, M. R., Zhao, M. Q., Gogotsi, Y. & Barsoum, M. W. Conductive two-dimensional titanium carbide “clay” with high volumetric capacitance. Nature 516, 78–81 (2015).
4. Zhang, C. (John) et al. Additive-free MXene inks and direct printing of micro-supercapacitors. Nat. Commun. 10, 1795 (2019).
5. Wang, Y. et al. Three-dimensional porous MXene/layered double hydroxide composite for high performance supercapacitors. J. Power Sources 327, 221–228 (2016).
6. Yang, W. et al. 3D Printing of Freestanding MXene Architectures for Current-Collector-Free Supercapacitors. Adv. Mater. 31, 1902725 (2019).
7. Schütt, F. et al. Hierarchical self-entangled carbon nanotube tube networks. Nat. Commun. 8, 1–10 (2017).

The last few years witnessed an enormous growth of research interest in developing miniaturised flexible and wearable electronics to substitute conventional rigid and bulky configurations. Due to their intrinsic properties and unique ability to combine features like breathability and flexibility with strength retention due to stress/strain and environmental conditions, textiles have become very attractive for the invisible technological integration, currently known as "e-textiles", and steadily are turning into a new generation of wearable electronics. Such technologies require sophisticated electronics, which set high requirements for the energy source. These are usually met by attaching a conventional battery to the garment, limiting the design freedom and being inconvenient for the user due to its bulkiness and weight.

Accordingly, there is a significant demand for thin, flat and flexible energy storage sources, which survive washing, drying, ironing and/or dry-cleaning and can be fully embedded into the textile. This demand for miniaturised energy storage in wearable electronics stimulates rapid research in micro-supercapacitors (MSCs), which provide long lifetimes and high power density. Even more effort is placed into synthesising novel nanomaterials to ensure that these MSCs exhibit high areal and volumetric capacitances. There is a broad range of manufacturing methods of MSCs; however, when textiles are used as substrates, the fabric must sustain its breathability and flexibility. Direct printing is a solution since nanomaterials can often be formulated into active material functional inks.

With our work, we aim to develop the methodology for fabricating flexible current collector-free MSCs with aqueous Ti₃C₂T_x MXene inks (no additives/binders/surfactants) directly embedded into the textiles via aerosol-jet printing (AJP). AJP is a relatively new additive manufacturing method and has not been used in the production of MXene based fabric MSCs. This technique shows excellent potential due to its superior resolution, high precision, cost-effectiveness for fast prototyping and bespoke uses, digitalised maskless nature, and compatibility with a wide range of nanomaterial inks and their viscosities. Ti₃C₂T_x MXene inks are of the main interest for this work due to their superior electrical conductivity, superlative performance in supercapacitors, mechanical flexibility, and easy processability into stable, viscous, aqueous, colloidal suspensions without additives/binders/surfactants, even in solvents like water.

This research work introduces a new methodology in developing e-textiles with AJP using additive-free aqueous Ti₃C₂T_x Mxene inks. Our preliminary studies determined the factors impacting the fabrics selection and their compatibility with the functional inks. We also optimised AJ printing parameters by depositing Ti₃C₂T_x MXene films on fabrics and analysing them with four-probe resistance measurements and SEM.

Then, the systematic study was performed by fabricating the MX@Fabric-#L electrodes via printing 10, 20, 30, 50, and 100 layers on cotton, polycotton, and polyester fabrics with optimised AJ printing parameters. These electrodes were then characterised electrochemically in the three-electrode study analysing CVs, GCD, EIS and cycling stabilities.

Based on the results of the three-electrode study, we developed and optimised the current collector-free symmetrical MSC prototypes (MX@C-15L IDDs) on cotton fabric via AJP with aqueous Ti₃C₂T_x MXene ink. Furthermore, fortunately, because of the versatility of AJP, such devices could also be manufactured in tandem, i.e. printed in series and parallel.

This work is an essential step in developing the textile-based supercapacitor electrodes and fully integrated MSCs into the fabric networks via the aerosol-jet printing technology, versatility and universality of which may allow exploiting a wider variety of fabrics to meet the requirements of the broad range of applications.

Wearable electronics and the internet of things (IoT) promise a new era of interconnectivity with the implementation of electronic applications that will improve the productivity of industries, safety of workers, and effectiveness of health monitoring, to name a few. Many of these applications will require devices that are flexible in their final use case, increasing demand for flexible energy storage solutions. Two-dimensional (2D) transition metal carbides and nitrides called MXenes, offer unique charge storage properties, metallic conductivity, high volumetric capacitance, hydrophilicity, and solution processibility, as well as mechanical flexibility and robustness; properties which render these materials promising for flexible wearable energy storage technologies. In this work, Ti₃C₂T_x MXene based textile supercapacitors have been fabricated with woven cotton textiles, solid-state LiCl/PVA gel electrolyte, and additive-free Ti₃C₂T_x paint. The supercapacitor device is assembled by stacking Ti₃C₂T_x coated electrodes between LiCl/PVA gel-soaked porous cotton separators in series. The stacked design maximizes the active Ti₃C₂T_x loading while minimizing the internal resistance of the device and the gap between electrodes. The combination of high areal loading of Ti₃C₂T_x and the ability to easily integrate devices in series led to the development of 6 V textile-based supercapacitors with high capacitance. This work demonstrates the capability of such MXene-based devices to power microcontrollers and introduces a platform technology that can be implemented across a wide range of wearable electronic and IoT applications.

There has been significant demand for wearable, miniaturized, portable and integrated microelectronic devices that are lightweight, ultrathin, wearable and compatible with the on-chip design concept microsupercapacitors (MSCs). Two-dimensional transition-metal carbides and nitrides (MXenes) are promising candidates for state-of-the-art energy storage applications because of their excellent combination of properties, which include high conductivity, high packing density, and pseudocapacitive behavior which leads to the improvement in volumetric and areal capacitances. In our work, MXene and MXene hybrids are utilized to fabricate coplanar interdigitated on-chip MSCs via nanofabrication process. The resultant devices feature a gap of sub-micrometers between fingers, achieving excellent capacitances and superior rate capability, and their high-power performance benefits from the hybridization of materials with different dimensionalities (i.e., 2D MXene and 1D CNTs). Furthermore, the focused ion beam (FIB) fabrication process is compared with the conventional lithography patterning techniques (i.e., photolithography and electron beam lithography methods).

To design an effective photocatalyst, we demonstrate the use of nano-sized, delaminated Ti₃C₂T_x (N-Ti₃C₂T_x) that is characterized by high transparency and tailored surface functional groups as an effective cocatalyst. Coupling TiO₂ and N-Ti₃C₂T_x produced a hybrid photocatalyst (TiO₂-N-Ti₃C₂T_x) with superior activity in pollutant degradation using UV light. The superior photocatalytic activity of TiO₂-N-Ti₃C₂T_x is correlated to the small size of N-Ti₃C₂T_x that facilitates a strong interfacial contact with TiO₂, high transparency of N-Ti₃C₂T_x that enables good interaction between photons and TiO₂, and presence of abundant =O and OH terminals on the surface of N-Ti₃C₂T_x that promotes adsorption of the pollutant on the surface of the hybrid photocatalyst. The study provides a rational approach for effectively coupling TiO₂ and Ti₃C₂T_x to induce beneficial charge transfer properties and synergistic effects that have far-reaching applications in photocatalysis.

MXenes are a growing family of two-dimensional transition metal carbides with an ever-widening variety of compositional possibilities with impressive material behaviors. One sub-group of MXenes that has grown in recent years in the search for more exotic material properties is double-transition metal (DTM) MXenes. Here, we present the synthesis of an out-of-plane partially ordered (Mo_2+αNb_2-α)AlC₃T_x DTM MAX phase and their derivative out-of-plane ordered (Mo_2+αNb_2-α)C₃T_x DTM MXenes. We investigated the synthesis of the full range of Mo-Nb compositions from Mo-rich to Nb-rich (Mo_n, Nb_4-n)AlC₃ MAX phases (0.3 ≤ n ≤ 3.7). Our x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy results indicated that only Mo-rich (n > 2) leads to the formation of a MAX phase which suggests the out-of-plane ordered nature of this phase. Using these methods, we further analyze changes within the a and c-lattice parameters as well as the atomic ratios across an as-mixed molar range of Mo and Nb. After verification of these partially ordered phases, we exfoliated the (Mo_2+αNb_2-α)C₃T_x MXenes in an aqueous hydrofluoric acid solution and then delaminated the MXene to single-to-few layer flakes using tetramethylammonium hydroxide. These newly reported partially ordered (Mo_2+αNb_2-α)C₃T_x MXenes represent further expansion of the possible compositions of DTM MXenes by adding Nb as an interior transition metal with Mo as an exterior transition metal in ordered MXene phases. While further characterization is still needed, we speculate that (Mo_2+αNb_2-α)C₃T_x MXenes may provide a new composition to further tune their electrochemical, mechanical and electronic properties and provide another option toward application-driven design of MXene compositions.

Electrocatalyst with low price, electrocatalytic activity, and durability in oxygen reaction such as oxygen evolution and reduction reactions is highly required in fuel cell and metal–oxygen battery to replace noble metal–based electrocatalysts. Bimetallic organic frameworks (BMOFs) have two different metals in the inorganic nodes with an organic linker, thereby shows a synergistic effect and enhanced properties. In this study, bimetallic (cobalt and manganese) MOFs epitaxially grown on Ti₃C₂T_x MXene sheet by solvothermal synthesis. Porphyrin structure of TCPP utilized as organic linker and unpaired electrons between MOF and Ti₃C₂T_x MXene contributes to enhance their electrocatalytic activity and cycle stability. As-prepared CMT–MXene is hired as electrocatalyst in lithium-oxygen batteris and exhibits a high stability for 312 cycles at current density of 500 mA g^-1_CMTM+SuperP.

MXenes are a new family of two-dimensional (2D) nanomaterials exhibiting extraordinary physical properties and attracting much interest from researchers with respect to their possible applications. In this work, Ti3C2-MXene materials obtained from Ti3AlC2 MAX-phase precursors (based on different preparation techniques) have been studied. They have been thoroughly characterized with XRD, EDS-SEM/STEM and their electrical and thermal properties have been investigated in a broad temperature range from 300 K down to 3 K. Measurements of the electrical resistivity, Seebeck coefficient, thermal conductivity, and specific heat have been carried out. The experimental results were supported with theoretical calculations of the electronic structure and related physical properties.

A relatively low value of the electrical resistivity is observed with some decrease with lowering the temperature and an upturn at low temperatures. The low-temperature specific heat data were analyzed with respect to possible metallicity and lattice properties of the materials. A large variation of the Seebeck coefficient with temperature was observed, depending on the preparation. A comparison of the experimental data with theoretical calculations has been made. The results obtained on the properties of the materials studied are analyzed and considered with respect to low-temperature sensing and possible space applications.

The last few years witnessed an enormous growth of research interest in developing miniaturised flexible and wearable electronics to substitute conventional rigid and bulky configurations. Due to their intrinsic properties and unique ability to combine features like breathability and flexibility with strength retention due to stress/strain and environmental conditions, textiles have become very attractive for the invisible technological integration, currently known as "e-textiles", and steadily are turning into a new generation of wearable electronics. Such technologies require sophisticated electronics, which set high requirements for the energy source. These are usually met by attaching a conventional battery to the garment, limiting the design freedom and being inconvenient for the user due to its bulkiness and weight.

Accordingly, there is a significant demand for thin, flat and flexible energy storage sources, which survive washing, drying, ironing and/or dry-cleaning and can be fully embedded into the textile. This demand for miniaturised energy storage in wearable electronics stimulates rapid research in micro-supercapacitors (MSCs), which provide long lifetimes and high power density. Even more effort is placed into synthesising novel nanomaterials to ensure that these MSCs exhibit high areal and volumetric capacitances. There is a broad range of manufacturing methods of MSCs; however, when textiles are used as substrates, the fabric must sustain its breathability and flexibility. Direct printing is a solution since nanomaterials can often be formulated into active material functional inks.

With our work, we aim to develop the methodology for fabricating flexible current collector-free MSCs with aqueous Ti₃C₂T_x MXene inks (no additives/binders/surfactants) directly embedded into the textiles via aerosol-jet printing (AJP). AJP is a relatively new additive manufacturing method and has not been used in the production of MXene based fabric MSCs. This technique shows excellent potential due to its superior resolution, high precision, cost-effectiveness for fast prototyping and bespoke uses, digitalised maskless nature, and compatibility with a wide range of nanomaterial inks and their viscosities. Ti₃C₂T_x MXene inks are of the main interest for this work due to their superior electrical conductivity, superlative performance in supercapacitors, mechanical flexibility, and easy processability into stable, viscous, aqueous, colloidal suspensions without additives/binders/surfactants, even in solvents like water.

This research work introduces a new methodology in developing e-textiles with AJP using additive-free aqueous Ti₃C₂T_x Mxene inks. Our preliminary studies determined the factors impacting the fabrics selection and their compatibility with the functional inks. We also optimised AJ printing parameters by depositing Ti₃C₂T_x MXene films on fabrics and analysing them with four-probe resistance measurements and SEM.

Then, the systematic study was performed by fabricating the MX@Fabric-#L electrodes via printing 10, 20, 30, 50, and 100 layers on cotton, polycotton, and polyester fabrics with optimised AJ printing parameters. These electrodes were then characterised electrochemically in the three-electrode study analysing CVs, GCD, EIS and cycling stabilities.

Based on the results of the three-electrode study, we developed and optimised the current collector-free symmetrical MSC prototypes (MX@C-15L IDDs) on cotton fabric via AJP with aqueous Ti₃C₂T_x MXene ink. Furthermore, fortunately, because of the versatility of AJP, such devices could also be manufactured in tandem, i.e. printed in series and parallel.

This work is an essential step in developing the textile-based supercapacitor electrodes and fully integrated MSCs into the fabric networks via the aerosol-jet printing technology, versatility and universality of which may allow exploiting a wider variety of fabrics to meet the requirements of the broad range of applications.

Wearable electronics and the internet of things (IoT) promise a new era of interconnectivity with the implementation of electronic applications that will improve the productivity of industries, safety of workers, and effectiveness of health monitoring, to name a few. Many of these applications will require devices that are flexible in their final use case, increasing demand for flexible energy storage solutions. Two-dimensional (2D) transition metal carbides and nitrides called MXenes, offer unique charge storage properties, metallic conductivity, high volumetric capacitance, hydrophilicity, and solution processibility, as well as mechanical flexibility and robustness; properties which render these materials promising for flexible wearable energy storage technologies. In this work, Ti₃C₂T_x MXene based textile supercapacitors have been fabricated with woven cotton textiles, solid-state LiCl/PVA gel electrolyte, and additive-free Ti₃C₂T_x paint. The supercapacitor device is assembled by stacking Ti₃C₂T_x coated electrodes between LiCl/PVA gel-soaked porous cotton separators in series. The stacked design maximizes the active Ti₃C₂T_x loading while minimizing the internal resistance of the device and the gap between electrodes. The combination of high areal loading of Ti₃C₂T_x and the ability to easily integrate devices in series led to the development of 6 V textile-based supercapacitors with high capacitance. This work demonstrates the capability of such MXene-based devices to power microcontrollers and introduces a platform technology that can be implemented across a wide range of wearable electronic and IoT applications.

There has been significant demand for wearable, miniaturized, portable and integrated microelectronic devices that are lightweight, ultrathin, wearable and compatible with the on-chip design concept microsupercapacitors (MSCs). Two-dimensional transition-metal carbides and nitrides (MXenes) are promising candidates for state-of-the-art energy storage applications because of their excellent combination of properties, which include high conductivity, high packing density, and pseudocapacitive behavior which leads to the improvement in volumetric and areal capacitances. In our work, MXene and MXene hybrids are utilized to fabricate coplanar interdigitated on-chip MSCs via nanofabrication process. The resultant devices feature a gap of sub-micrometers between fingers, achieving excellent capacitances and superior rate capability, and their high-power performance benefits from the hybridization of materials with different dimensionalities (i.e., 2D MXene and 1D CNTs). Furthermore, the focused ion beam (FIB) fabrication process is compared with the conventional lithography patterning techniques (i.e., photolithography and electron beam lithography methods).

To design an effective photocatalyst, we demonstrate the use of nano-sized, delaminated Ti₃C₂T_x (N-Ti₃C₂T_x) that is characterized by high transparency and tailored surface functional groups as an effective cocatalyst. Coupling TiO₂ and N-Ti₃C₂T_x produced a hybrid photocatalyst (TiO₂-N-Ti₃C₂T_x) with superior activity in pollutant degradation using UV light. The superior photocatalytic activity of TiO₂-N-Ti₃C₂T_x is correlated to the small size of N-Ti₃C₂T_x that facilitates a strong interfacial contact with TiO₂, high transparency of N-Ti₃C₂T_x that enables good interaction between photons and TiO₂, and presence of abundant =O and OH terminals on the surface of N-Ti₃C₂T_x that promotes adsorption of the pollutant on the surface of the hybrid photocatalyst. The study provides a rational approach for effectively coupling TiO₂ and Ti₃C₂T_x to induce beneficial charge transfer properties and synergistic effects that have far-reaching applications in photocatalysis.

MXenes are a growing family of two-dimensional transition metal carbides with an ever-widening variety of compositional possibilities with impressive material behaviors. One sub-group of MXenes that has grown in recent years in the search for more exotic material properties is double-transition metal (DTM) MXenes. Here, we present the synthesis of an out-of-plane partially ordered (Mo_2+αNb_2-α)AlC₃T_x DTM MAX phase and their derivative out-of-plane ordered (Mo_2+αNb_2-α)C₃T_x DTM MXenes. We investigated the synthesis of the full range of Mo-Nb compositions from Mo-rich to Nb-rich (Mo_n, Nb_4-n)AlC₃ MAX phases (0.3 ≤ n ≤ 3.7). Our x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy results indicated that only Mo-rich (n > 2) leads to the formation of a MAX phase which suggests the out-of-plane ordered nature of this phase. Using these methods, we further analyze changes within the a and c-lattice parameters as well as the atomic ratios across an as-mixed molar range of Mo and Nb. After verification of these partially ordered phases, we exfoliated the (Mo_2+αNb_2-α)C₃T_x MXenes in an aqueous hydrofluoric acid solution and then delaminated the MXene to single-to-few layer flakes using tetramethylammonium hydroxide. These newly reported partially ordered (Mo_2+αNb_2-α)C₃T_x MXenes represent further expansion of the possible compositions of DTM MXenes by adding Nb as an interior transition metal with Mo as an exterior transition metal in ordered MXene phases. While further characterization is still needed, we speculate that (Mo_2+αNb_2-α)C₃T_x MXenes may provide a new composition to further tune their electrochemical, mechanical and electronic properties and provide another option toward application-driven design of MXene compositions.

Electrocatalyst with low price, electrocatalytic activity, and durability in oxygen reaction such as oxygen evolution and reduction reactions is highly required in fuel cell and metal–oxygen battery to replace noble metal–based electrocatalysts. Bimetallic organic frameworks (BMOFs) have two different metals in the inorganic nodes with an organic linker, thereby shows a synergistic effect and enhanced properties. In this study, bimetallic (cobalt and manganese) MOFs epitaxially grown on Ti₃C₂T_x MXene sheet by solvothermal synthesis. Porphyrin structure of TCPP utilized as organic linker and unpaired electrons between MOF and Ti₃C₂T_x MXene contributes to enhance their electrocatalytic activity and cycle stability. As-prepared CMT–MXene is hired as electrocatalyst in lithium-oxygen batteris and exhibits a high stability for 312 cycles at current density of 500 mA g^-1_CMTM+SuperP.

MXenes are a new family of two-dimensional (2D) nanomaterials exhibiting extraordinary physical properties and attracting much interest from researchers with respect to their possible applications. In this work, Ti3C2-MXene materials obtained from Ti3AlC2 MAX-phase precursors (based on different preparation techniques) have been studied. They have been thoroughly characterized with XRD, EDS-SEM/STEM and their electrical and thermal properties have been investigated in a broad temperature range from 300 K down to 3 K. Measurements of the electrical resistivity, Seebeck coefficient, thermal conductivity, and specific heat have been carried out. The experimental results were supported with theoretical calculations of the electronic structure and related physical properties.

A relatively low value of the electrical resistivity is observed with some decrease with lowering the temperature and an upturn at low temperatures. The low-temperature specific heat data were analyzed with respect to possible metallicity and lattice properties of the materials. A large variation of the Seebeck coefficient with temperature was observed, depending on the preparation. A comparison of the experimental data with theoretical calculations has been made. The results obtained on the properties of the materials studied are analyzed and considered with respect to low-temperature sensing and possible space applications.

MXenes, a large family of 2D transition metal carbides/nitrides, having unique surface termination with excellent electrical conductivity shows great performance in energy storage, electrocatalysis, sensing, and many more applications. Among over 40 MXenes which have been reported to date, only one carbonitride (Ti₃CNT_x) and a few nitrides have been introduced so far. In previous studies, Ti₃CNT_x has showed distinct and properties and better performance in many applications in comparison to Ti₃C₂T_x. Such distinct behavior of Ti₃CNT_x is a strong motivation to develop other carbonitrides based MXenes. Herein, we report on a new 2D carbonitride MXene, viz. Ti₂C_0.5N_0.5T_x (T_x stands for surface terminations), and the only second carbonitride after Ti₃CNT_x until now. A new type of in situ HF using HCl+KF was employed to etch Al from Ti₂AlC_0.5N_0.5 to synthesize multilayer Ti₂C_0.5N_0.5T_x powders from Ti₂AlC_0.5N_0.5. The use of water-soluble KF instead of LiF allowed us to achieve a salt-free multilayer MXene. Spontaneous intercalation of tetramethylammonium followed by sonication in water resulted in a large-scale delamination of this new titanium carbonitride into 2D sheets. This new family member, multilayer Ti₂C_0.5N_0.5T_x MXene showed higher specific capacities and larger electroactive surface area than those of Ti₂CT_x powders. Multilayer Ti₂C_0.5N_0.5T_x powders exhibited a specific capacity of 182 mAh g^-1 at 20 mA g^-1, the highest among all reported multilayer MXene electrodes in Na-ion batteries with excellent cycling stability.

Two-dimensional (2D) materials, as transition-metal carbides and/or nitrides (MXenes), have recently gained great attention due to their combination of electrochemical, electrical and mechanical properties, which can be tailored for the specific application by controlling the chemical composition. As a consequence, MXenes find applications in many fields as energy storage, sensing, photocatalysis and many more ^[1].
MXenes are obtained by selective etching of a precursor layered material, known as MAX phase, and characterized by the general formula M_n+1X_nT_x, where M is a transition metal with n =1-4, X represents carbon and/or nitrogen atoms and T_x represents surface terminations of different nature (-O, -OH, -F and -Cl). The latter aspect is the one responsible for one important property related to the hydrophilicity of MXenes, which allows easy processing in water and/or solutions ^[2]. This aspect is particularly attractive for inkjet processing, as an easy water-based ink can be formulated. In addition, in aqueous solutions Ti₃C₂X flakes are negatively charged allowing the development of additive-free conductive inks.
Eventually, the MXenes extremely high conductivity is particularly attractive for final application as flexible current collector for lithium-ion batteries. The current collector can be considered as a bottleneck, especially when considering substrates selection, i.e. flexible/wearable substrates. Different silver or copper nanoparticles inks are commercially available, but they do require high curing temperatures, which are incompatible with plastic-based substrates, in order to reach the desired electrical conductivity. Another option is carbon-based inks, which despite being more cost-effective, they have limitations in terms of moderate electrical conductivity, which affects the Ohmic losses, and so the overall performance, of the battery device ^[3].
Inkjet printing fabrication requires specific ink properties, in order to ensure a clogging-free condition, in terms of viscosity and surface tension of the ink. Generally, a clogging-free jetting is also forecast when particle size dimension is between 1/100 < d < 1/50 of the nozzle diameter ^[4]. This is a particularly meaningful aspect when considering MXene-based inks, as flake size should be finely controlled in order to avoid large or multilayered flakes, which can favor nozzle clogging. However, too small flakes can compromise the final electric conductivity.
In this study, we provide three methods to control the flake size of Ti₃C₂X MXene specifically for inkjet printing application, i.e. ball miller, probe sonicator and bath sonicator. The effect on the flake size for the three methods is compared for different number of hours, i.e. 1, 3 and 5 hours. The best processing method will be the one providing a good flake delamination and sufficiently small flakes in order to make it compatible with IJP, within the lowest possible time, as to avoid Ti₃C₂X oxidation, which inevitably lowers its electrical conductivity. Once chosen the optimal method, the Ti₃C₂X aqueous-based ink will be used as current collector for lithium-ion batteries. Preliminary studies on inkjet printed full cell LTO/LMO and LTO/LFP were already obtained. Inkjet printing of the full cells on flexible substrates will be performed, using Ti₃C₂X as current collector. Morphological and electrical analysis on the printed substrates, combined with an in-depth electrochemical characterization in order to understand the battery performance, will be discussed.
[1] M. Shekhirev, C. E. Shuck, A. Sarycheva, Y. Gogotsi, Progress in Materials Science 2021, 120, 100757.
[2] K. Maleski, C. E. Ren, M.-Q. Zhao, B. Anasori, Y. Gogotsi, ACS Appl. Mater. Interfaces 2018, 10, 24491.
[3] S. Uzun, M. Schelling, K. Hantanasirisakul, T. S. Mathis, R. Askeland, G. Dion, Y. Gogotsi, Small 2021, 17, 2006376.
[4] Z. Tang, K. Fang, M. N. Bukhari, Y. Song, K. Zhang, Langmuir 2020, 36, 9481.

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have attracted attention due to their unique electrochemical performance in a variety of energy storage systems. MXenes have the general formula of M_n+1X_nT_x, where M is the transition metal, X is carbon, and T represents the surface terminations (F, O, OH).¹ Since their discovery, the primary candidate of study has been Ti₃C₂T_x, but many MXenes with different compositions and M_n+1X_nT_x structures have been synthesized. Vanadium containing MXenes are particularly attractive for redox energy storage because of the presence of V transition metal with multiple oxidation states. Herein, we study V₂CT_x and V₄C₃T_x, as electrodes in supercapacitors with various aqueous electrolytes. The vanadium containing MXenes, specifically V₂CT_x, show high charge storage capability in 6M KOH electrolyte (>250 F g^-1) with distinct redox couples in their cyclic voltammograms. In the acidic electrolytes, both MXenes show pseudocapacitive behavior with V₄C₃T_x showing multiple redox couples in a wide potential window (~ 1V). Through comparing these two MXenes we demonstrate that change in the number of layers of MXenes affects charge storage mechanism and electrochemical behavior of these materials. Our results, provide new insights into structure- chemistry-electrochemical performance relations in MXenes and, therefore, may guide design of advanced electrodes based on these materials for pseudocapacitive energy storage.

Rising demands to achieve higher energy density (> 500 Wh/Kg) and reduce in cost per kWh for complete electrification of Vehicles by 2030, lithium-sulfur batteries are a potential solution. However, the major challenges such as the polysulfide shuttle effect and the insulating natures of sulfur and the final discharge products, hinder lithium-sulfur batteries from their practical applicability. From recent reports, MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides are shown to be efficient in trapping the polysulfides thus increasing the cycle life and performance of the battery. Due to MXenes’ metal like conductivity, the versatility in terms of composition, structure, and tunable surface chemistry makes it more interesting to develop integrated MXene-sulfur cathodes. With almost 30 synthesized MXenes and more than 100 predicted structures, this study aims to screen and select the best performing MXenes for use as cathodes and coatings on separators in Li-S batteries. We expect this work to provide a future pathway in developing MXene based high energy density Li-S batteries

Triboelectric charging comprises the frictional contact of two materials, and their contact electrification usually depends on the difference in polarity in the triboelectric series. This limits the choices of materials for triboelectric contact pairs and hinders research and further development of TENGs. Two-dimensional (2D) transition metal carbides (Ti₃C₂T_x MXene) have received great attention for potential applications in triboelectric nanogenerators (TENGs) among 2D materials owing to their rich surface chemistry and metallic conductivity. However, the poor stability of MXene-based TENGs to oxidation and humidity has been overlooked, which affects device performance for its prolonged use. Here, we report a facile covalent functionalization approach through a silylation reaction to efficiently stabilize the MXene against oxidation, humidity and to improve its surface properties. The functionalization of MXene with Aminopropyyl-triethoxy silanes (APTES) and Flourooctyl silanes (FOTS) not only stabilizes MXene against humidity but also expands the material selection for MXene-based TENGs. We also conducted simulation studies to prove the presented idea.

The current pandemic, Covid-19 is not only a global pandemic and public health crisis; it also has severely affected the global economy and financial markets. The consequences of COVID-19 pandemic teach us a lesson: the needs for epidemic preparedness and prediction. Like this COVID-19 pandemic, the situation would be very different if we can contain and control the outbreak in time, e.g. employing an effective screening and monitoring method which has the capabilities of fast and real-time monitoring the vital signs from infected patients even when they are asymptomatic and at very early stages. The similar challenge is also existing for chronic diseases such as diabetes and asthma real-time health monitoring and in-treatment in order to reduce the mortality and complicates.
This presentation focuses on a novel, rapid, smart sensing system using new functionalized 1D/2D NMOS/MXene nanocomposites as the key sensing components which can instantly “smell” the diseases through detection of volatile organic compounds (VOCs) in human breath. Here, the 1-Dimensional Nanostructured metal oxide semiconductors (1D NMOS) include ZnO, WO₃, TiO₂, etc., while 2-Dimensional nanosheets mainly refer to multi-layered Ti₃C₂T_x. If succeeded, this device could be an effective tool to intervent the chain of transmission and address the challenges which we have experienced during COVID-19 and provide in-time self-management for patients with chronic diseases. Also, this research will guide to develop an innovative point-of-care diagnostic technique which can accurately and rapidly measure the concentrations of trace metabolites in exhaled breath and identify diseases and infections non-invasively and free of risk. Due to the miniaturized size, low-cost fabrication, AI integrated functionality, this smart device can not only be widely used at central facilities (airports, shopping centers, schools, train stations, and other public places) to rapid screen and early detect infectious diseases but also be conveniently used as a wearable device for non-invasive but rapid disease monitoring. This will be able to significantly reduce the burden of health-care system and improve life quality for patients.

MXenes, two-dimensional (2D) transition metal carbides or nitrides, have recently shown huge potential for gas sensing applications [1,2]. We have successfully synthesized 2D Ti₃C₂T_x and Mo₂CT_x flakes via selective chemical etching of Al and Ga from the Ti₃AlC₂ and Mo₂Ga₂C precursors correspondingly. The comprehensive characterization of the MXene by XRD, SEM, TEM, AFM, and XPS shows high quality flakes with lateral dimensions of 0.5-1 µm. Suspension of well-delaminated MXenes were deposited on a multi-contact array to study their chemiresistive performance for organic vapors (methanol, ethanol, n-butanol, acetone, ammonia) and humidity at 10¹-10⁴ ppm concentrations in dry air, which was used to emulate practical sensing environments. We demonstrate that Mo₂CT_x MXenes are highly sensitive towards H₂O vapor in comparison to the organic analytes showing a dc chemiresistive response at a 10 ppm detection limit. While partially oxidation of Ti₃C₂T_x MXenes considerably improves their chemiresistive response to organic analytes at low-ppm concentrations showing faster and a qualitatively different sensor response to volatile analytes compared to pristine Ti₃C₂T_x. In addition to their high sensitivity, developed MXenes enable a selective recognition of analytes, such as low molecular weight alcohols, as shown by linear discriminant analysis. We suggest that the observed performance of partially oxidized Ti₃C₂T_x and Mo₂CT_x MXenes in dry air meets many sensing applications and opens avenues for implementation of new 2D concepts in practice.
1. K. Deshmukh, et al. Coord. Chem. Rev. 2020, 424, 213514.
2. H. Pazniak, et al. ACS Appl. Nano Mater. 2020, 3, 3195-3204.

Microwave devices have proven to function as reliable, low-cost, and real-time sensing devices in various industrial, biomedical, and environmental sectors. These devices are fabricated using metal traces as the conductive element of the sensor structure. Metal-based sensors are not useful for specific sensing applications such as strain sensing and gas sensing, due to their rigid structure and negligible intrinsic sensitivity to gas, respectively. This poses a significant limitation in gas detection applications such as air quality evaluation. To overcome this limitation, an additional interface material is required for the metal-based microwave sensors. The adsorption of the gas molecule by the interface material changes its electrical properties where it would be interpreted by the microwave structure. The addition of interface materials affects the sensor's form factor and increases implementation complexity. Another disadvantage of metal-based microwave sensors is their rigidity, impeding their development for a range of applications such as strain sensing and wearable devices. Hence, an approach for addressing the shortcomings of metallic microwave sensors is to replace metal elements with highly conductive, flexible, lightweight, and sustainable nanomaterials.
As an alternative to metal, titanium carbide (Ti₃C₂T_x), MXene, has been recently investigated for the development of microwave devices due to its high conductivity (up to 10,000 S/cm) and superior mechanical features, including excellent flexibility. Recently, MXene-based microwave devices, such as antennas and resonators, have been developed using spray coating technologies or film structures. However, to the best of our knowledge, no previous work has studied MXene membrane-based microwave structures for microwave sensing applications, in which the membrane can simultaneously function as the sensing and microwave element, eliminating the need for additional interface materials.

Here, a MXene membrane is developed for realizing microwave components (e.g., antennas and resonators), with a concurrent operation for gas sensing. A novel MXene resonator/sensor integrated with an active feedback loop is proposed for humidity sensing application. The developed MXene membrane serves as both the resonating and the sensing element, where the addition of an active feedback loop constructively improves the sensing resolution. Adsorption of water molecules changed the conductivity of the MXene membrane, resulting in an increase in the membrane's resistance. As a result, the MXene resonator dissipated more electromagnetic power, altering the sensor's electrical parameters, including resonant frequency and resonant amplitude. Employing this active MXene-based resonator, we were able to sense humidity level variations between 0% to 70% RH, where increasing the humidity level resulted in a 4.46 MHz frequency shift in the sensor's response. We further report developing a Ti₃C₂T_x MXene membrane to realize an antenna sensor for detecting VOC gases (e.g., Acetone, IPA, Ethanol, and Methanol) at room temperature. Wirelessly monitoring the transmission coefficient between the MXene and a secondary antenna showed that the proposed membrane can detect four VOC types. For Acetone, the detection range was found to be between 8 to 80 ppt. The developed MXene membranes demonstrated promising performance as microwave components with potential in gas and humidity sensing applications in the environmental and industrial sectors.

Lightweight, ultrathin and flexible electromagnetic interference (EMI) shielding materials are highly needed to protect electronic circuits and portable telecommunication devices and eliminate cross-talk between devices and device components. Here, we show that a 2D transition metal carbonitride, Ti₃CNT_x MXene, with a moderate electrical conductivity provides a higher shielding effectiveness compared to more conductive Ti₃C₂T_x or metal foils of the same thickness. This exceptional shielding performance of Ti₃CNT_x was achieved by film annealing and is attributed to an anomalously high absorption of electromagnetic waves in the layered metamaterial-like structure. These results provide guidance for designing advanced EMI shielding materials, but also highlight the need for exploring fundamental mechanisms behind interaction of electromagnetic waves with 2D materials.

Surface chemistry controls not only the physicochemical, electrochemical, and optoelectronic properties, but also environment stability and processability of nanomaterials. MXenes, transition metal carbides/carbonitrides/nitrides, are a very large family of 2D materials with the general formula M_n+1X_nT_x, where M, X, T_x, and n represent transition metal(s), carbon or nitrogen, surface terminal groups such as –OH, -O and -F, and integers ranging from 1 to 4, respectively. MXenes have been attracted in many electronic, electrochemical, and optoelectronic applications, due to their high electronic conductivity (~15000 S/cm), hydrophilicity, and solution processability. Unfortunately, however, how to control surface chemistry of MXenes is not well established yet, which hinders the use of MXene in real applications.
In this presentation, the presenter will briefly demonstrate two ways to control the surface chemistry of MXenes, including anhydrous synthesis and ligand chemistry. The developed surface chemistry not only control the surface functionality and physicochemical properties of MXenes, but also enables preparation of stable MXene dispersions with good oxidation stability. The stable MXene dispersions provide an opportunity to prepare printable flexible MXene electrodes for various applications including EMI shielding, flexible joule heater, LED display and energy storage.

Synaptic devices with linear high-speed switching can accelerate learning in artificial neural networks (ANNs). Electrochemical random-access memories (ECRAMs), where resistive switching occurs by ion insertion into a redox-active channel, has emerged as a most promising component for ANNs due to their linear switching and low noise. ECRAMs using 2D materials and metal oxides however suffer from slow ion kinetics, whereas organic ECRAMs enable high-speed operation but face challenges toward on-chip integration due to poor temperature stability of polymers. Here, ECRAMs using 2D titanium carbide (Ti3C2Tx) MXene that combine the high speed of organics and the integration compatibility of inorganic materials in a single high-performance device are demonstrated. These ECRAMs combine the speed, linearity, write noise, switching energy, and endurance metrics essential for parallel acceleration of ANNs, and importantly, they are stable after heat treatment needed for back-end-of-line integration with Si electronics. The high speed and performance of these ECRAMs introduces MXenes as promising candidates for devices operating at the nexus of electrochemistry and electronics.

Bioelectronic technologies are powerful tools to study, diagnose, and treat a wide variety of disorders. Examples of these technologies include implantable neuromodulation devices for neurological and mental disorders, peripheral nerve stimulators for pain and incontinence, wearable muscle sensors for rehabilitation and prosthesis control, and brain-computer interfaces. The vast majority of bioelectronic interfaces, however, still rely on noble metals and silicon, which are expensive to source and process and are intrinsically inadequate to address the mechanical, chemical, and electrical properties of excitable circuits in the body. Furthermore, manufacturing bioelectronic interfaces from these materials require fabrication schemes that are time-consuming, do not scale, and critically limit the maximum attainable resolution and coverage. Solution processing is a cost-effective manufacturing alternative, but biocompatible conductive inks matching the performance of conventional metals are lacking.
Thanks to the remarkable combination of high electronic conductivity, high capacitance, biocompatibility, and processability from aqueous dispersions, Ti₃C₂ is emerging as an ideal candidate for replacing traditional materials and establish low-cost, additive-free, solution-based manufacturing of bioelectronic interfaces.
Recently, we have developed a novel manufacturing approach to fabricate high-resolution electrodes based on Ti₃C₂ MXene for recording and stimulation at high-resolution and multiscale, ranging from implantable neural electrodes for small rodents, all the way up to dry (i.e., gel-free) wearable sensors for brain (EEG), muscle (EMG), and cardiac (ECG) monitoring in humans.¹
These novel electrode structures match – and in some cases exceed – the electrochemical impedance and stimulating charge injection performance of conventional bioelectronic materials such as Au and Pt. Furthermore, the low-density and magnetic susceptibility mismatch between Ti₃C₂ and biological tissues enable artifact-free imaging with magnetic resonance (MRI) and computed tomography (CT), which is precluded when using metals such as Pt.
To demonstrate the feasibility and versatility of this electrodes for multiscale bioelectronics, we show examples of a number of applications ranging from dry EEG for studying attention and cognition, to EMG for rehabilitation and prosthesis control, to invasive brain recording and stimulation in rodents and pigs.
1. Driscoll, N., Erickson, B., Murphy, B. B., Richardson, A. G., Robbins, G., Apollo, N. V., ... & Gogotsi, Y. Medaglia, J.M, Vitale, F. (2021). MXene-infused bioelectronic interfaces for multiscale electrophysiology and stimulation. Science Translational Medicine, 13(612), eabf8629.

The use of dyes in many chemical industries, such as in the production of plastics, paints, papers, textile, has been increasing in recent years. Issues with environmental pollution related to dyes still exist if these chemicals are allowed to flow into wastewater bodies, since they are non-biodegradable, toxic, mutagenic, and carcinogenic. In this work, nanocomposites of GO-AgMOF and MXene-AgMOF were fabricated for the first-time using silver-based metal-organic framework (AgMOF). AgMOF has been functionalized by its heterogeneous bondage with graphene-oxide (GO) and Ti₃C₂T_x MXene and investigated for the adsorption of cationic methylene blue (MB) and anionic orange G (OG) dyes. The nanocomposites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and energy dispersive spectroscopy, before and after the adsorption. Batch adsorption tests suggested pseudo first-order kinetics of adsorption, suggesting a physical mechanism of adsorption. The effects of initial dye concentration and of pH were also studied. Very high adsorption efficiencies were observed for MB, with GO-AgMOF, MXene-AgMOF, and AgMOF able to remove 99.9%, 99%, and 98% of this dye from water at an initial concentration of 200 mg/L and an adsorbent mass of 0.01 g. In case of OG, adsorption efficiency did not exceed 20%, suggesting the electrostatic nature of the driving force for adsorption. The results suggest that these nanocomposites can be promising adsorbents for cationic contaminants, since 399.9 mg/g adsorption capacity was observed for GO-AgMOF. Incorporation of MXene into AgMOF enabled enhanced functionality to AgMOF, such as improving the adsorption capacity toward anionic and cationic dyes, suggesting that MXene-AgMOF can be further investigated for environmental remediation applications.

Characteristic properties of two-dimensional (2D) transition metal carbides and nitrides (MXenes), such as high conductivity, hydrophilicity, and catalytic activity have led to a growing research interest for their use in environmental remediation and water treatment applications. The ability to process MXenes into flexible films with negative surface charge and hydrophilicity adds a possibility to control ion flux and biofouling by applying a small potential to the membrane. MXene shows a much higher antibacterial efficiency toward both Gram-negative and Gram-positive bacteria as compared with other 2D nanomaterials. Consequently, MXene membranes demonstrated outstanding water flux, selective rejection to salts and organic molecules, which makes it ideal UF/NF membrane materials. Moreover, MXene has been successfully used for the efficient adsorption and removal of heavy metals such as Hg, and Cu. This talk summarizes the recent advances in the applications of MXenes as adsorbents, desalination membranes, electrodes for electrochemical deionization, and catalytic or antibacterial agents for water purification and other environmental remediation processes. The overview also features discussions on the computational attempts, biocompatibility, and environmental impact in the exploration of MXenes for water applications, highlighting the challenges and opportunities of these advanced 2D materials. The biocompatibility and cytotoxicity assessment of MXene and their impact on the environment will be highlighted.

Two-dimensional materials based on transition metal carbides have been intensively studied due to their unique properties including hydrophilicity and structural diversity and have shown a great potential in several applications, such as energy storage, sensing and optoelectronics. Two-dimensional (2D) titanium carbide MXene (Ti₃C₂T_x) was prepared by exfoliating MAX phase (Ti₃AlC₂) powders using solutions of concentrated hydrofluoric acid (HF). The effect of the mean particle size of the prepared MXene on the specific surface area (SSA) and carbon dioxide adsorption capacity was investigated using N₂ and CO₂ gas physisorption. The Ti₃C₂T_x MXene specific surface area was increased from 14.4 m²/g to 20.0 m²/g when the MAX phase was reduced from 8 μm to 1 μm. However, the CO₂ adsorption capacity decreased for MXene produced from 1μm MAX phase, indicating the need for mesopores. Furthermore, amine intercalation reduced the measured surface area and carbon capture capacity by binding to the MXene surface. An observed specificity of CO₂ over N₂ adsorption suggest that MXenes have the potential for carbon capture applications.

Having a proven alternative to fossil fuel, sustainable, affordable, and efficient green hydrogen generation is one of the burning issues of the current research. A lot of materials (including 2D materials) and their heterostructures have been unitized to get the desired efficiency along with the stability, however with limited success. Most of the reported work on the 2D materials ignore electron SPIN to address the existing issue in the photoelectrochemical water splitting, such as high overpotential, side product formation, high recombination, inefficient charge separation & transport, stability, and efficiency. We will present our recent work on the asymmetric Janus MoSSe heterostructured with Mo₂C MXenes grown nanostructured Si and GaN substrate as photocathode for spin manipulated charge transport in a photoelectrochemical system. First, we probe Janus MoSSe and Mo2C for their structure (XRD, Raman), Morphology (SEM, HRTEM, AFM), electronic (XPS), optical (PL, UV-Vis, TCSPC), and magnetic (PPMS, MCD). We compare the performance with the symmetric 2D materials MoS₂ and MoSe₂ grow with the same methodology. Results indicate superior (1.5 times higher) PEC performance for Janus MoSSe, which has been attributed to the spin to charge conversion in the presence of an external magnetic field. Further, the density functional theory (DFT) simulation validates the claim and supports the existence of Rashba and Zeeman type spin splitting of the Janu MoSSe/Mo₂C photocathode. These findings will offer a fertile ground to cultivate asymmetric 2D material for a wide range of applications.

Electrochemical nitrogen reduction reaction (NRR) technology is a viable alternative to reducing the energy and greenhouse gas footprint from the current ammonia (NH₃) production technology. Most NRR catalysts operate by using an associative or dissociative mechanism, during which the NRR competes with the hydrogen evolution reaction (HER), resulting in low selectivity. In this presentation, we report on a new active catalyst for NRR that evokes the Mars-van Krevelen (MvK) mechanism to increase the selectivity of NRR towards NH₃. The MvK mechanism reduces the competition between NRR and HER by eliminating the adsorption and dissociation processes of nitrogen gas (N₂) at the active sites for NH₃ synthesis. This new catalyst, two-dimensional (2D) Ti₂N nitride MXene, was synthesized via an oxygen-assisted molten salt fluoride etching technique. We confirmed its phase purity and stability in aqueous electrolytes using various characterization techniques. The Ti₂N nitride MXene catalyst achieved a high Faradaic efficiency (FE) of 19.85% towards NH₃ at an applied potential of –250 mV vs. RHE with a yield of 11.33 μg/cm²/hr in a 0.1M hydrochloric acid (HCl) N₂-saturated electrolyte. Electrocatalytic activity and selectivity obtained in an Ar-saturated electrolyte support an MvK mechanism. These results can be expanded to a broad class of systems evoking the MvK mechanism and constitute the foundation of NRR technology based on MXenes.

Ti₃C₂T_X nano-sheets (MXenes) are important biocompatible materials in treating infectious diseases thanks to their excellent light-conversion property. In this study, we exploited this feature to design photothermal antibacterial therapy using few- (FX) and multi-layer (MX) Ti₃C₂T_X nano-sheets. The conversion of light to heat resulted higher in FX, but MX depicted a better efficacy in inhibiting growth of S. aureus and E. coli due to MXenes’ reversible bacteria trapping mechanism. For FX, 72% of E. coli and 46% of S. aureus viable cells survived, while the effect was more pronounced after the treatment with MX (25 µg/mL), showing 37% of E. coli and 23% of S. aureus viable cells. The Near-Infrared (NIR) laser treatment increased the antibacterial efficacy of both materials. NIR LASER treatment for 5 min at 5.7 W cm^-2 combined with 100 µg/mL of MX completely killed both species indeed. Photothermal effect of FX (100 µg/mL) with a lower power density (3 W cm^-2) showed an antibacterial efficiency similar to that of MX (87% on E. coli, 95% on S.aureus). The combined NIR LASER-MXene treatment induced an irreversible cell death linked to the loss of cell integrity (DNA release quantification and bacteria debris observation). Taken together, our results show that MXene demonstrated a high antibacterial efficiency, which increased when combined with NIR LASER at relatively-low power densities resulting in complete cell death and consequent DNA release. This outlines the importance for future studies of antibacterial effects of MXenes in wound healing as well as possible studies in MXenes embedding in textiles to inhibit microbial growth.

Two-dimensional (2D) transition metal carbides/carbonitrides (MXenes) are rapidly growing as multimodal nanoplatforms in biomedicine.[1-3]
Envisaging future MXene-based biomedical nanotools and drug nanoformulations and considering the central importance of the immune response for any clinical translation of nanomaterials, we evaluated the immune impact of MXenes ex vivo on human primary immune cells, exploiting our expertise on the biological effects of 2D materials.[4-6]
To this end, we selected three highly stable and well-characterized MXenes: Mo₂Ti₂C_3, Nb₄C_3, and Ta₄C₃. Following material synthesis and characterization, we demonstrated their excellent immunocompatibility on human peripheral blood mononuclear cells (PBMCs), by providing a high-dimensional immune and functional profiling.
We assessed immune cell functionality trough the measurement of 40 secreted cytokines by Luminex technology. Overall, the 2D materials selected in this work exhibited only a slight modulation on the inflammatory mediators analyzed. Moreover, by RNA-Seq, we investigated in depth the impact on the whole gene expression. All materials showed an overall slight impact in human PBMCs, modulating the expression of a limited number of genes compared to the positive controls concanavalin A (ConA) and lipopolysaccharides (LPS). In particular, even if all materials had a similar effect on gene expression modulation, Ta₄C₃ had a slightly higher impact compared to Mo₂Ti₂C₃ and Nb₄C₃. In our analysis, a total of more than 18000 genes were modulated. In detail, the expression of 879, 981, and 1353 genes was modulated by Mo₂Ti₂C_3, Nb₄C_3, and Ta₄C₃, respectively, while the positive controls ConA and LPS modulated the expression of 18340 and 4740 genes, respectively.
Taken together, our results provide a compendium of knowledge on the biocompatibility of MXenes for the safe development of multi-functioning nanosystems in biomedicine.
References:
1. Gogotsi, Y. & Anasori, B. The Rise of MXenes. ACS Nano (2019).
2. Fusco, L. et al. Graphene and other 2D materials: a multidisciplinary analysis to uncover the hidden potential as cancer theranostics. Theranostics (2020).
3. Unal M.A. et al. 2D MXenes with antiviral and immunomodulatory properties: a pilot study against SARS-CoV-2. Nano Today (2021).
4. Gazzi A. et al. Photodynamic Therapy Based on Graphene and MXene in Cancer Theranostics. Front Bioeng Biotechnol (2019).
5. Orecchioni M et al. Single cell mass cytometry and transcriptome profiling reveal the impact of graphene on human primary immune cells. Nature Communications (2017).
6. Gazzi A. et al. Graphene, other carbon nanomaterials and the immune system: toward nanoimmunity-by-design. J Phys materials (2020).

MXenes, a class of two-dimensional (2D) transition metal carbides, carbonitrides or nitrides, have been widely used and studied for functional electronics and optoelectronic devices due to their excellent properties such as metallic conductivity, tunable work function, water dispersibility. Here, we report an atomic-switch-type artificial synapse based on Ti3C2Tx MXene nanosheets with surface functional groups, which successfully mimics the dynamics of biological synapses such as excitatory/inhibitory postsynaptic current (EPSC/IPSC), paired-pulse facilitation (PPF), and long-term potentiation/depression (LTP/LTD) characteristics. Through in-depth analyses using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS), we found the synaptic dynamics in which the conductive metallic filaments gradually formed or annihilated on the surface of MXene with distributed functional groups by diffusion and redox reaction of Cu²⁺ ions. Finally, to demonstrate the applicability of the artificial MXene synapse for hardware neural networks, we investigated the synaptic characteristics in terms of the cycle-to-cycle variation (CCV), nonlinearity (NL), asymmetricity (AS), dynamic range (DR) in LTP/LTD curves and performed training and inference tasks using a convolutional neural network (CNN) and the Canadian-Institute-For-Advanced-Research-10 (CIFAR-10) dataset.

Cadmium Telluride (CdTe) is a high-efficiency thin-film photovoltaic technology that has seen tremendous commercial success over the past decade; however, the fabrication of low-cost, ohmic hole contacts remains a challenge due to the high ionization potential of CdTe, limiting the external potential that can be feasibly reached even as other characteristics of the device improve.^1,2 MXenes, a family of 2D materials with rapidly growing scientific and commercial interest, offer a promising route to forming low-cost, low-barrier contacts due to their demonstrated high work function, metallic conductivity, and facile solution processing from benign solvents.³ In this work, Ti₃C₂T_x MXene processed from aqueous solution is used as a highly efficient back contact material for CdTe solar cells, resulting in high power-conversion efficiencies and excellent stability. The role of defect passivation, Schottky barrier formation, and light trapping in Ti₃C₂T_x-contacted devices is probed, elucidating pathways for the future development of this potent combination of materials. The modularity of the expansive MXene family of materials presents a promising strategy for developing next-generation hole contacts for CdTe.

References:
1. Duenow, J. N. & Metzger, W. K. Back-surface recombination, electron reflectors, and paths to 28% efficiency for thin-film photovoltaics: A CdTe case study. Journal of Applied Physics 125, 053101 (2019).
2. Liyanage, G. K., Phillips, A. B., Alfadhili, F. K., Ellingson, R. J. & Heben, M. J. The Role of Back Buffer Layers and Absorber Properties for >25% Efficient CdTe Solar Cells. ACS Appl. Energy Mater. 2, 5419–5426 (2019).
3. Yin, L. et al. MXenes for Solar Cells. Nano-Micro Lett. 13, 78 (2021).

Heat transfer fluids are often made out of conventional fluids such as water, motor oil, ethylene glycol, silicone oil, among others. Given the limited heat transfer performance of these fluids, the incorporation of solid particles as an additive suspended in the base fluid is a basic heat transfer enhancement approach. Nevertheless, several issues are known to hinder the application of this strategy, such as sedimentation, erosion, fouling, and higher pressure drop in the flow channel. To overcome these issues, a nanotechnology-based heat transfer fluid known as “nanofluids” have been explored. They are created by dispersing and stably suspending nanoparticles with typical lengths of 1–50 nm in standard heat transfer fluids. A relatively little amount (~1 vol%) of nanoparticles can make huge enhancements in the thermal properties of host fluids. Beyond the numerous nanosized materials that are available and well-studied (e.g., carbon nanotubes, metal and oxide nanoparticles, graphene oxide, and boron nitride). MXenes, a new class of 2D layered materials, are been explored for a wide range of applications, and more recently has been applied as a promising nanomaterial for the preparation of efficient heat transfer nanofluids.
In the present work, MXene (Ti₃C₂T_x) nanoplatelets are used to prepare silicone-based nanofluids with enhanced thermo-physical properties. The study focuses on preparation, characterization, thermal properties, thermal stability and performance investigation of silicone oil nanofluids containing various amounts of MXene additives. The MXenes were synthesized using a wet chemistry method based on mixture of HCl and LiF. The base fluid, a mixture of silicone oil and the toluene, are kept in the ratio of 1:1 (60 mL). The mixture is stirred with 400 rpm at 50 °C for 30 min. Adding MXene nanoplatelets to silicone oil is carried out with the dilution method by Toluene. Commercially available nano-sized alumina is also incorporated in the liquid medium to verify possible synergetic effects when combined with MXenes, and also for comparison purposes. Therefore, a face centered central composite design (CCF) is applied to statistically evaluate the optimum concentratios of the nanomaterials for maximum thermal conductivity, in which the design levels varies from 0.0 to 0.10 wt% of Ti₃C₂T_x and/or Alumina. The thermal conductivity of the nanofluids is accessed using a C-Therm TCi device (25−100 °C). Viscosity is measured at various temperatures ranging from 25 to 100 °C. Optical properties and chemical structure are evaluated using UV–Vis and FTIR. Morphology and composition of the synthesized MXene are studied using SEM-EDS and XRD, respectively.

Detecting neurotransmitters is critical in the prognosis and diagnosis of neurodegenerative diseases, including Parkinson’s disease. Recently, MXene, a newly discovered 2D material, has drawn significant attention due to its superior electrical properties to the thin metal films, electrochemical property, and unique surface functionalities. However, the proper binding of biomolecules on MXene remains challenging, hindering MXene from being utilized for wide biosensing applications. Here, we present the development of conformal surface modification of MXene with aptamers (Ti₃C₂T_x) by depositing an ultrathin (< 10 nm) polymer and its utility for a biosensor to detect neurotransmitters. We coated a thin layer of poly(hexavinyldisiloxane) (pHVDS) directly on MXene via initiated chemical vapor deposition (iCVD), which resulted in abundant vinyl (C=C) functional groups on MXene without damaging its electrical property. Then, we used the C=C functional group further for the covalent bonding of thiolated aptamers via the thiol-ene reaction, confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and water contact angle (WCA) measurements. With dopamine- or serotonin-specific aptamers, we can selectively monitor changes in electrical resistance of MXene according to minute variations of the neurotransmitters’ concentration. We confirmed that the current-voltage (I-V) curves of MXene changed as we applied different concentrations of neurotransmitters in an aqueous condition. We emphasize that the iCVD enabled almost eliminating compromise of the electrical property of MXene due to the several nanometer-scaled deposition of pHVDS.

MXenes are an emerging class of 2D layered nanomaterials that provide large surface area, hydrophilicity, high ion transport properties, low diffusion barrier, biocompatibility, and ease of surface functionalization. Due to their unique features, MXenes have gained substantial attention in fields such as batteries and super capacitors and their application in chemical and biological sensors is growing. This presentation is focused on the synthesis and characterization of a MXene based nano-hybrid for sensing applications. The developed MXene-nanoparticles hybrid was then used as a transducer surface and supporting material for the immobilization of the biomolecule in an electrochemical biosensor. The incorporation of MXene hybrid into the sensor design enhanced the efficiency of the developed sensor in terms of charge transfer kinetics, biomolecule loading capacity, and amplification of the biomolecular recognition phenomenon. Opportunities for developing wearable health sensors and sensors for detection of environmental pollutants will be highlighted

By excellent properties of 2D MXene Ti₃C₂T_x such as surface functional groups and wonderful conductivity, MXene was widely applied in sensing detection, specifically gas sensors. The sensing mechanism of Ti₃C₂T_x was proposed based on charge interaction between gas molecules and the surface functional groups of Ti₃C₂T_x, especially with oxygen and hydroxyl group. Herein, our study focusses on improving the hydroxyl groups onto MXene surface and transfer as-synthesized MXene onto sensing device for inorganic gas detection such NH₃, NO₂, CO₂. MXene Ti₃C₂T_x was co-etching with NaOH to convert fluoride groups into oxygen and hydroxyl groups which was proved by XPS measurement. MXene then was formed into a thin MXene film around few hundred nanometers onto flatted PDMS stamp. This is a simple technique to obtain precise sizes of MXene sensing layer by using customized PDMS molds. The MXene sensing layer is homogeneity and high coverage which can be adaptable on various thickness of electrode layer. The sheet resistance of MXene was measured around 4.88 Ω/sq and the average conductivity approximately 208.84 S/cm. The first pre-assessment of sensing gases onto NH₃ at 100 ppm achieved 11.4 % sensitivity of the response which is a positive result of our further gas sensing application. In addition, gases desorption by light-to-heat conversion of MXene also will be investigated to regenerate MXene based sensors.

Ti₃C₂T_X nano-sheets (MXenes) are important biocompatible materials in treating infectious diseases thanks to their excellent light-conversion property. In this study, we exploited this feature to design photothermal antibacterial therapy using few- (FX) and multi-layer (MX) Ti₃C₂T_X nano-sheets. The conversion of light to heat resulted higher in FX, but MX depicted a better efficacy in inhibiting growth of S. aureus and E. coli due to MXenes’ reversible bacteria trapping mechanism. For FX, 72% of E. coli and 46% of S. aureus viable cells survived, while the effect was more pronounced after the treatment with MX (25 µg/mL), showing 37% of E. coli and 23% of S. aureus viable cells. The Near-Infrared (NIR) laser treatment increased the antibacterial efficacy of both materials. NIR LASER treatment for 5 min at 5.7 W cm^-2 combined with 100 µg/mL of MX completely killed both species indeed. Photothermal effect of FX (100 µg/mL) with a lower power density (3 W cm^-2) showed an antibacterial efficiency similar to that of MX (87% on E. coli, 95% on S.aureus). The combined NIR LASER-MXene treatment induced an irreversible cell death linked to the loss of cell integrity (DNA release quantification and bacteria debris observation). Taken together, our results show that MXene demonstrated a high antibacterial efficiency, which increased when combined with NIR LASER at relatively-low power densities resulting in complete cell death and consequent DNA release. This outlines the importance for future studies of antibacterial effects of MXenes in wound healing as well as possible studies in MXenes embedding in textiles to inhibit microbial growth.

Two-dimensional (2D) transition metal carbides/carbonitrides (MXenes) are rapidly growing as multimodal nanoplatforms in biomedicine.[1-3]
Envisaging future MXene-based biomedical nanotools and drug nanoformulations and considering the central importance of the immune response for any clinical translation of nanomaterials, we evaluated the immune impact of MXenes ex vivo on human primary immune cells, exploiting our expertise on the biological effects of 2D materials.[4-6]
To this end, we selected three highly stable and well-characterized MXenes: Mo₂Ti₂C_3, Nb₄C_3, and Ta₄C₃. Following material synthesis and characterization, we demonstrated their excellent immunocompatibility on human peripheral blood mononuclear cells (PBMCs), by providing a high-dimensional immune and functional profiling.
We assessed immune cell functionality trough the measurement of 40 secreted cytokines by Luminex technology. Overall, the 2D materials selected in this work exhibited only a slight modulation on the inflammatory mediators analyzed. Moreover, by RNA-Seq, we investigated in depth the impact on the whole gene expression. All materials showed an overall slight impact in human PBMCs, modulating the expression of a limited number of genes compared to the positive controls concanavalin A (ConA) and lipopolysaccharides (LPS). In particular, even if all materials had a similar effect on gene expression modulation, Ta₄C₃ had a slightly higher impact compared to Mo₂Ti₂C₃ and Nb₄C₃. In our analysis, a total of more than 18000 genes were modulated. In detail, the expression of 879, 981, and 1353 genes was modulated by Mo₂Ti₂C_3, Nb₄C_3, and Ta₄C₃, respectively, while the positive controls ConA and LPS modulated the expression of 18340 and 4740 genes, respectively.
Taken together, our results provide a compendium of knowledge on the biocompatibility of MXenes for the safe development of multi-functioning nanosystems in biomedicine.
References:
1. Gogotsi, Y. & Anasori, B. The Rise of MXenes. ACS Nano (2019).
2. Fusco, L. et al. Graphene and other 2D materials: a multidisciplinary analysis to uncover the hidden potential as cancer theranostics. Theranostics (2020).
3. Unal M.A. et al. 2D MXenes with antiviral and immunomodulatory properties: a pilot study against SARS-CoV-2. Nano Today (2021).
4. Gazzi A. et al. Photodynamic Therapy Based on Graphene and MXene in Cancer Theranostics. Front Bioeng Biotechnol (2019).
5. Orecchioni M et al. Single cell mass cytometry and transcriptome profiling reveal the impact of graphene on human primary immune cells. Nature Communications (2017).
6. Gazzi A. et al. Graphene, other carbon nanomaterials and the immune system: toward nanoimmunity-by-design. J Phys materials (2020).

MXenes, a class of two-dimensional (2D) transition metal carbides, carbonitrides or nitrides, have been widely used and studied for functional electronics and optoelectronic devices due to their excellent properties such as metallic conductivity, tunable work function, water dispersibility. Here, we report an atomic-switch-type artificial synapse based on Ti3C2Tx MXene nanosheets with surface functional groups, which successfully mimics the dynamics of biological synapses such as excitatory/inhibitory postsynaptic current (EPSC/IPSC), paired-pulse facilitation (PPF), and long-term potentiation/depression (LTP/LTD) characteristics. Through in-depth analyses using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS), we found the synaptic dynamics in which the conductive metallic filaments gradually formed or annihilated on the surface of MXene with distributed functional groups by diffusion and redox reaction of Cu²⁺ ions. Finally, to demonstrate the applicability of the artificial MXene synapse for hardware neural networks, we investigated the synaptic characteristics in terms of the cycle-to-cycle variation (CCV), nonlinearity (NL), asymmetricity (AS), dynamic range (DR) in LTP/LTD curves and performed training and inference tasks using a convolutional neural network (CNN) and the Canadian-Institute-For-Advanced-Research-10 (CIFAR-10) dataset.

Cadmium Telluride (CdTe) is a high-efficiency thin-film photovoltaic technology that has seen tremendous commercial success over the past decade; however, the fabrication of low-cost, ohmic hole contacts remains a challenge due to the high ionization potential of CdTe, limiting the external potential that can be feasibly reached even as other characteristics of the device improve.^1,2 MXenes, a family of 2D materials with rapidly growing scientific and commercial interest, offer a promising route to forming low-cost, low-barrier contacts due to their demonstrated high work function, metallic conductivity, and facile solution processing from benign solvents.³ In this work, Ti₃C₂T_x MXene processed from aqueous solution is used as a highly efficient back contact material for CdTe solar cells, resulting in high power-conversion efficiencies and excellent stability. The role of defect passivation, Schottky barrier formation, and light trapping in Ti₃C₂T_x-contacted devices is probed, elucidating pathways for the future development of this potent combination of materials. The modularity of the expansive MXene family of materials presents a promising strategy for developing next-generation hole contacts for CdTe.

References:
1. Duenow, J. N. & Metzger, W. K. Back-surface recombination, electron reflectors, and paths to 28% efficiency for thin-film photovoltaics: A CdTe case study. Journal of Applied Physics 125, 053101 (2019).
2. Liyanage, G. K., Phillips, A. B., Alfadhili, F. K., Ellingson, R. J. & Heben, M. J. The Role of Back Buffer Layers and Absorber Properties for >25% Efficient CdTe Solar Cells. ACS Appl. Energy Mater. 2, 5419–5426 (2019).
3. Yin, L. et al. MXenes for Solar Cells. Nano-Micro Lett. 13, 78 (2021).

Heat transfer fluids are often made out of conventional fluids such as water, motor oil, ethylene glycol, silicone oil, among others. Given the limited heat transfer performance of these fluids, the incorporation of solid particles as an additive suspended in the base fluid is a basic heat transfer enhancement approach. Nevertheless, several issues are known to hinder the application of this strategy, such as sedimentation, erosion, fouling, and higher pressure drop in the flow channel. To overcome these issues, a nanotechnology-based heat transfer fluid known as “nanofluids” have been explored. They are created by dispersing and stably suspending nanoparticles with typical lengths of 1–50 nm in standard heat transfer fluids. A relatively little amount (~1 vol%) of nanoparticles can make huge enhancements in the thermal properties of host fluids. Beyond the numerous nanosized materials that are available and well-studied (e.g., carbon nanotubes, metal and oxide nanoparticles, graphene oxide, and boron nitride). MXenes, a new class of 2D layered materials, are been explored for a wide range of applications, and more recently has been applied as a promising nanomaterial for the preparation of efficient heat transfer nanofluids.
In the present work, MXene (Ti₃C₂T_x) nanoplatelets are used to prepare silicone-based nanofluids with enhanced thermo-physical properties. The study focuses on preparation, characterization, thermal properties, thermal stability and performance investigation of silicone oil nanofluids containing various amounts of MXene additives. The MXenes were synthesized using a wet chemistry method based on mixture of HCl and LiF. The base fluid, a mixture of silicone oil and the toluene, are kept in the ratio of 1:1 (60 mL). The mixture is stirred with 400 rpm at 50 °C for 30 min. Adding MXene nanoplatelets to silicone oil is carried out with the dilution method by Toluene. Commercially available nano-sized alumina is also incorporated in the liquid medium to verify possible synergetic effects when combined with MXenes, and also for comparison purposes. Therefore, a face centered central composite design (CCF) is applied to statistically evaluate the optimum concentratios of the nanomaterials for maximum thermal conductivity, in which the design levels varies from 0.0 to 0.10 wt% of Ti₃C₂T_x and/or Alumina. The thermal conductivity of the nanofluids is accessed using a C-Therm TCi device (25−100 °C). Viscosity is measured at various temperatures ranging from 25 to 100 °C. Optical properties and chemical structure are evaluated using UV–Vis and FTIR. Morphology and composition of the synthesized MXene are studied using SEM-EDS and XRD, respectively.

Detecting neurotransmitters is critical in the prognosis and diagnosis of neurodegenerative diseases, including Parkinson’s disease. Recently, MXene, a newly discovered 2D material, has drawn significant attention due to its superior electrical properties to the thin metal films, electrochemical property, and unique surface functionalities. However, the proper binding of biomolecules on MXene remains challenging, hindering MXene from being utilized for wide biosensing applications. Here, we present the development of conformal surface modification of MXene with aptamers (Ti₃C₂T_x) by depositing an ultrathin (< 10 nm) polymer and its utility for a biosensor to detect neurotransmitters. We coated a thin layer of poly(hexavinyldisiloxane) (pHVDS) directly on MXene via initiated chemical vapor deposition (iCVD), which resulted in abundant vinyl (C=C) functional groups on MXene without damaging its electrical property. Then, we used the C=C functional group further for the covalent bonding of thiolated aptamers via the thiol-ene reaction, confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and water contact angle (WCA) measurements. With dopamine- or serotonin-specific aptamers, we can selectively monitor changes in electrical resistance of MXene according to minute variations of the neurotransmitters’ concentration. We confirmed that the current-voltage (I-V) curves of MXene changed as we applied different concentrations of neurotransmitters in an aqueous condition. We emphasize that the iCVD enabled almost eliminating compromise of the electrical property of MXene due to the several nanometer-scaled deposition of pHVDS.

MXenes are an emerging class of 2D layered nanomaterials that provide large surface area, hydrophilicity, high ion transport properties, low diffusion barrier, biocompatibility, and ease of surface functionalization. Due to their unique features, MXenes have gained substantial attention in fields such as batteries and super capacitors and their application in chemical and biological sensors is growing. This presentation is focused on the synthesis and characterization of a MXene based nano-hybrid for sensing applications. The developed MXene-nanoparticles hybrid was then used as a transducer surface and supporting material for the immobilization of the biomolecule in an electrochemical biosensor. The incorporation of MXene hybrid into the sensor design enhanced the efficiency of the developed sensor in terms of charge transfer kinetics, biomolecule loading capacity, and amplification of the biomolecular recognition phenomenon. Opportunities for developing wearable health sensors and sensors for detection of environmental pollutants will be highlighted

By excellent properties of 2D MXene Ti₃C₂T_x such as surface functional groups and wonderful conductivity, MXene was widely applied in sensing detection, specifically gas sensors. The sensing mechanism of Ti₃C₂T_x was proposed based on charge interaction between gas molecules and the surface functional groups of Ti₃C₂T_x, especially with oxygen and hydroxyl group. Herein, our study focusses on improving the hydroxyl groups onto MXene surface and transfer as-synthesized MXene onto sensing device for inorganic gas detection such NH₃, NO₂, CO₂. MXene Ti₃C₂T_x was co-etching with NaOH to convert fluoride groups into oxygen and hydroxyl groups which was proved by XPS measurement. MXene then was formed into a thin MXene film around few hundred nanometers onto flatted PDMS stamp. This is a simple technique to obtain precise sizes of MXene sensing layer by using customized PDMS molds. The MXene sensing layer is homogeneity and high coverage which can be adaptable on various thickness of electrode layer. The sheet resistance of MXene was measured around 4.88 Ω/sq and the average conductivity approximately 208.84 S/cm. The first pre-assessment of sensing gases onto NH₃ at 100 ppm achieved 11.4 % sensitivity of the response which is a positive result of our further gas sensing application. In addition, gases desorption by light-to-heat conversion of MXene also will be investigated to regenerate MXene based sensors.

This talk will focus on the device applications of MXenes and MXene-derived functional materials. Our group has been developing device concepts that capitalize on the rich and promising properties of MXenes. These include using MXenes as electrical contacts, active materials, and precursors to prepare various types of functional materials.
For example, the excellent electrical conductivity of MXenes makes them good candidates as contact materials electronics both as local and global contacts. We have demonstrated that MXenes are promising as electrical contacts in thin-film electronics, 2D electronics, printed electronics, quantum dot electronics, CMOS devices, and solar cells. Further, due to the high carrier concentration of MXenes, they exhibit plasmonic properties which we used to develop broad-band plasmonic photodetectors working in the visible range. Capitalizing on the abundant surface charges of MXenes, we have developed conducting MXene-polymer hydrogels with unique (skin-like) sensing capabilities that outperform existing hydrogel sensors. These hydrogels could detect magnitude and signs of stress, speed, facial expression, touch, and sound. The hydrogel could also harvest energy from ultrasound, which can be stored or coupled to power other devices making them potentially useful for implanted powered sources. Finally, we have developed a new direction in MXene transformations, where the 2D nature of MXenes is leveraged to make high-performance functional materials. For example, highly-textured ferroelectric crystals, piezoelectric crystals, and piezoluminescent crystals were fabricated. Besides, 2D metal-organic frameworks and their thin films were made using MXene as a metal source resulting in highly-texture MOF thin films at the wafer scale. These MXene-derived functional materials are shown to be useful for many promising applications.

MXenes are unique two-dimensional (2D) nanomaterials that combine high electrical conductivity with the excellent hydrophilic property. They exhibit interesting energy transition properties that make them promising candidates for various energy harvesting applications. In their general M_n+1X_nT_x formula, M is an early transition metal, X is a carbon or/and nitrogen, T_x stands for surface terminations (e.g., -OH, -F, -Cl, etc.), and n=1-4. Such unique structure and chemistry is an additional asset which makes MXenes promising in terms of tunability.
While many transition features for MXenes can now be easily controlled, their light-to-catalytical conversion is not fully recognized. Consequently, understanding their ability to convert sunlight into chemically active species remains a challenge. Our study aimed at analyzing the photocatalytic activity of 2D Ti₃C₂T_x MXene stepwise after etching into a multilayered structure (ML) and further delamination into single flakes (SL). To elucidate ML/SL MXenes’ efficiency in utilizing the solar light to photo-active dyes decomposition, we used various light ranges as well as simulated and natural sunlight. Obtained results were further correlated with MXenes’ bandgap function and physicochemical features.
Our results have shown excellent photocatalytic activity of 2D Ti₃C₂T_x without the need for any post-processing or supporting compounds. MXene’s efficiency in converting the solar light energy into a catalytical function was related to the energy source and specific to the decomposed dye. In addition, the MXene photocatalyst showed high adsorption capacity, which additionally boosted the process parameters. Furthermore, it was regenerated and successfully reused, thus proving MXenes robustness in photocatalytic processes. Overall, our findings support MXenes’ practical environmental applications toward water cleaning technologies.

Exploratory theoretical predictions in uncharted structural and compositional space are integral to materials discoveries. A more recent addition to the family of MAB phases is new types of chemically ordered quaternary borides, i-MAB (M’4/3M”2/3AlB2) and o-MAB (M’4M”SiB2), in which the M-atoms are in-plane and out-of-plane chemically ordered, respectively. Both types of phases can be chemically exfoliated into 2D sheets, and selectively prepared in multilayer form, as delaminated single-layer sheets in colloidal suspension, or as additive-free filtered films. For example, Ti4MoSiB2 o-MAB can be used to derive 2D TiOxCly of high yield. (Mo2/3Y1/3)2AlB2 and (Mo2/3Sc1/3)2AlB2 i-MAB can be used for realization of so called boridene in the form of single-layer 2D sheets with ordered metal vacancies, Mo4/3B2-xTz (Tz = -F, -O, -OH). The present talk will summarise the results to date of 2D materials synthesis from laminated borides, the mechanisms behind the realization of these 2D materials, and evaluation of selected properties.

Two-dimensional (2D) nanomaterials such as MXenes, graphene, layered transition metal dichalcogenides (TMDs), etc., are emerging nanomaterials that are getting closer to commercialization. Specifically, MXenes, a family of ultrathin layered 2D transition metal carbides, nitrides, and carbonitrides, are steadily advancing as novel inorganic nanosystems for various electronic applications. While the metallic conductivity, solution processability, hydrophilic nature, and presence of various surface functional groups enable the use of 2D MXenes in aqueous systems possible, studies aiming to use MXenes in aqueous and especially biological systems are limited. Also, being new materials, they have not been exposed to the environment. The fundamental to understanding the interactions of MXenes with the environment and biological systems is to evaluate their behavior in various biological buffers. We investigate the structural and functional properties of the most explored Ti₃C₂T_x (T = F, OH) MXenes in N-substituted biological buffers like N-(2-Hydroxyethyl)-piperazine-N’-ethanesulfonic acid (HEPES) and 3-(N-morpholino)-propane sulfonic acid (MOPS). UV-vis measurements of MXenes when dissolved in solution reveal a striking impact on their absorption spectra in the presence of all buffer molecules. XRD and electrical conductivity measurements of MXene films exposed to buffer molecules show that these molecules intercalate between MXene nanosheets and significantly affect their electrical conductivity. The interactions of Ti₃C₂T_x MXenes with N-substituted zwitterionic buffer molecules are structure-dependent and are partially reversible. This suggests the interactions are driven by weak bonding interactions like hydrogen bonding and/or electrostatic interactions. This study is an important step toward understanding the stability of MXenes in aqueous media and the ability to predict the interactions of MXenes and nitrogen-containing molecules and later larger biomolecules. The nature of the interactions between MXenes and buffer molecules, revealed in our study, opens new possibilities for developing MXenes containing chemical sensing thin films for use in aqueous media.

2D MXenes have attracted tremendous interest due to their great potential in plenty of applications, especially for electrochemical energy storage, but still suffering from the serious restacking-caused low specific area and long ion diffusion pathways. Here we report a self-assembly method, which enables the construction of MXene hydrogels with porous structure and thus excellent electrochemical performance for supercapacitors, including a mass loading/thickness-independent rate capability and a high areal capacitance up to 3 F cm^-2, surpassing most state-of-the-art supercapacitor electrodes.

Spinal cord injury (SCI) is a devastating condition that disrupts both sensory and motor function, with very limited prospects of functional recovery. Electrical stimulation (ES) has become a common clinical remedy to lessen the impact of SCI-induced pain after injury. However, regeneration-focused lesion site electrostimulation has not had clinical translation yet, despite promising evidence of directing axonal growth and encouraging cord repair. Furthermore, neural interfaces still face challenges over long implantation times due to delamination, insufficient water barrier properties and inflammatory responses. MXenes possesses unique properties for developing ES systems that can wrap around the injured cord to deliver charge safely and efficiently.
Here we show the aerosol jet 3D printing (AJP) of a neural interface cuff with highly conductive MXene (Ti₃C₂T_X) electrodes, protected and insulated by a polytetrafluoroethylene (PTFE) structure.
MXene films were produced using doctor blade, vacuum-assisted and AJP-printing to assess the effect of fabrication methods on their physical properties and biocompatibility. Conductivity, hydrophilicity and mechanical properties from the films were evaluated, with AJP-printed films expressing enhanced conductivity (∼12000 S/cm) and hydrophilicity (35°). To assess biocompatibility, NSC-34 mouse motor neurons were seeded on the MXene films to study the morphology influence onto the cells over 3 days. Results showed that all MXene films were biocompatible, supporting neuron cell viability via similar proliferation and metabolism rate to tissue culture polystyrene controls. Also, neurons grown on AJP-printed films displayed enhanced neuronal neurite outgrowth and cell morphology. Top and cross-section SEM images of the device showed conformal deposition of MXenes onto a printed PTFE substrate, facilitated by the surfactant-induced hydrophilicity of the PTFE commercial ink. After sintering at 360°C, the PTFE nanoparticles coalesced, effectively bounding the printed MXenes onto its hydrophobic surface. During the thermal treatment, the surfactant and remaining moisture from the PTFE ink evaporated, switching the behaviour of the PTFE surface from hydrophilic (41°) to hydrophobic (125°) (p<0.0001).
In conclusion, this research proposes the direct adhesion of 3D printed MXenes after PTFE sintering, as a facile method to construct bespoke neural electrode implants for stimulation of the injured spinal cord, while limiting abiotic and biotic faults.

As electronic technology advances, the need in safe and long-lasting energy storage devices that occupy minimum volume arises. Short charging times of several seconds to minutes, with energy densities comparable to those of batteries, can be achieved in pseudocapacitors. These are sub-class of supercapacitors, where capacitance is mediated by fast redox reactions and can enable at least an order of magnitude more energy to be stored than in typical electrical double layer capacitors. Transition metal oxides (e.g. RuO₂, MnO₂) and conducting polymers (e.g. polyaniline) serve as typical examples. However, these materials are often high in cost and/or suffer from low cycling stability. As a result, the search for new pseudocapacitive materials constitutes an important direction today.
In my talk, I will discuss how the key performance metrics of pseudocapacitors – capacitance and charging rates – can be pushed to the limits in the materials that combine good electrical and ionic conductivities (ensuring fast charge transfer and hence charging rates) with high density of redox-active and ionically accessible sites (enabling high capacitance and charging rates). In particular, I will discuss the electrochemistry of 2D transition metal carbides (MXenes), with an emphasis on the mechanism of charge storage and factors affecting the overall electrochemical responce in these materials.

Transition metal carbides and nitrides (MXenes) exhibit the unique combination of excellent solution processability and outstanding metallic electrical conductivity, showing great potential for electromagnetic interference (EMI) shielding. To enable practical EMI shielding applications, the nanoscale 2D MXene sheets should be assembled into macroscopic architectures with satisfactory structural and functional properties. Here, we will present our recent efforts in developing functional composite materials based on 2D Ti₃C₂T_x MXene for EMI shielding applications. We fabricated a composite hydrogel incorporating MXene and poly(acrylic acid) through a biomineralization-inspired assembly route. This multifunctional hydrogel-type shielding material exhibits excellent stretchability and recyclability, fast self-healing capability, unique absorption-dominated shielding property, and sensing ability. Furthermore, to regulate the dispersion of MXene in the polymer composites and boost the EMI shielding performance, we will demonstrate a fabrication approach of highly conductive MXene composites by a monomer induced gelation method followed by in-situ polymerization to allow the formation of highly continuous and robust MXene network in the nanocomposites, leading to the facile creation of multifunctional highly efficient EMI shielding materials.

The very few reports published on the electrochemical properties of Double Transition Metal MXenes (DTMs) suggests that they perform much better than their mono metal counterparts. This is true for both ordered and solid solution DTMs[1-3]. Despite the theoretical predictions on the stability of a number of double transition metal MAX phases, only a few have been synthesized and etched into their corresponding MXenes. In an attempt to scale up the synthesis and processing of DTMs, we present the synthesis and structural studies of ordered Cr₂TiAlC₂, Cr₂Ti₂AlC₃ and solid solution Ti₂NbAlC₂ MAX phases and their etching into corresponding MXenes. In-situ high temperature XRD data collected on the admix of activated metal powders was used to optimize and model the MAX phase synthesis in high temperature tube furnace. Electrochemical studies such as CV, EIS and CCD of the synthesized DTMs compared with Ti₃C₂T_x will be presented and discussed.


[1] David Pinto et al., “Synthesis and Electrochemical Properties of 2D Molybdenum Vanadium Carbides – Solid Solution MXenes,” J. Mater. Chem. A 8, no. 18 (2020): 8957–68, https://doi.org/10.1039/D0TA01798A.

[2] Likui Wang et al., “Adjustable Electrochemical Properties of Solid-Solution MXenes,” Nano Energy 88 (2021): 106308, https://doi.org/https://doi.org/10.1016/j.nanoen.2021.106308.

[3] Babak Anasori et al., “Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes),” ACS Nano 9, no. 10 (October 27, 2015): 9507–16, https://doi.org/10.1021/acsnano.5b03591.

Electrochromic (EC) materials show great promise as one of the key functional components in future internet-of-things (IoT) applications such as flexible/3D displays, optical camouflage, smart windows, and smart sensor systems. However, realizing flexible EC devices with high energy-efficiency and fast switching speed is fundamentally challenging. This research is to investigate the in-situ oxidation of MXenes for two-dimensional (2D) transition metal oxides (TMOs) and develop the TMO/MXene heterostructures as building blocks for next generation 2D materials based electronic devices. Though the TMOs thin films have been fabricated for a long time using various methods such as sol-gel coating, pulsed laser deposition, and radio-frequency (RF) sputtering, the films are composed of bulky agglomerates which are rigid and usually over hundreds of nanometers thick. Such form of the TMOs is not suitable for the desired electronic devices which are flexible and could be integrated with other 2D materials. Here we demonstrate fast, and high-coloration-efficiency EC devices based on self-assembled 2D V₂O₅/MXene heterostructure, while the liquid/liquid interfacial self-assembly (LLIA) technique was used to fabricate the heterostructure. With the expectation of low contact resistance between the 2D TMO and MXenes, we further demonstrate the high-performance EC devices based on such heterostructures.

Among two-dimensional nanomaterials, transition metal carbides/carbonitrides (MXenes)[1] have gained remarkable attention for their potential as biomedical nanotools [2, 3]. Due to their unique combination of physiochemical properties, MXenes enable a wide range of biomedical applications. Among these, we have recently explored the use of MXenes to fight against SARS-CoV-2, demonstrating the immune modulatory properties of Ti₃C₂ MXene [4, 5]. The comprehension of biomolecular effects of MXenes on immune cells is a prerequisite for their exploitation in future translational applications. To characterize the complex interactions between MXenes and immune cells, we applied the Labelling Immune Partnerships by SorTagging Intercellular Contacts (LIPSTIC)[6] approach to nanomaterials (LIPSTIC). The intercellular communication between T cells and antigen-presenting dendritic cells (DCs), a key phenomenon in the immune response, was investigated after exposure to two highly stable and well-characterized MXenes: V₄C₃ and Ti₃C₂. Cell-specific intercellular communication between DCs and T cells was drastically decreased by the former which induced immunosuppression. Moreover, an anti-inflammatory activity of V₄C₃ was revealed by functional analyses and cytokine quantification. Our results open the way for i) new investigations on the promising immunomodulatory properties of novel MXenes in the context of autoimmune diseases and ii) a novel methodological approach in nanotoxicology and nanomedicine.
References
1. Gogotsi, Y et al. Front. Bioeng. Biotechnol. 2019; 7, 295.
2. Fusco, L; Gazzi, A; Peng, G et al. Theranostics 2020, 10(12):5435-5488.
3. Gazzi, A et al. J. Phys. Mater. 2020; 3 034009
4. Weiss, C et al. ACS Nano. 2020;14(6):6383-6406.
5. Unal, MA et al. Nano Today. 2021; 38:101136.
6. Pasqual, G et al. Nature. 2018; 553 (7689), 496–500.

Ever since the discovery of two-dimensional Ti₃C₂ MXenes, there has been a tremendous interest in the study of their structure, properties, and applications. This class of 2D transition metal carbides and nitrides are produced by the exfoliation of bulk MAX phases and can have a wide variety of stoichiometry and composition, which allows control over their properties. Since they are easily dispersed in water, they can easily be processed into a variety of forms such as powders, free-standing films, aerogels, etc. which also allows for tunability of their properties. MXenes are well-known for having a very high metallic conductivity, hydrophilic nature due to a number of surface terminations, high mechanical stiffness and strength, and tunable plasmonic, electrochromic, and optoelectronic properties. These properties combined with the high surface area of 2D MXenes have led to their utilization in battery anodes due to their exceptional capacitive charge storage. They are shown to have use in composites, as transparent conducting electrodes in LEDs and photovoltaics, EM wave shielding, and in gas and electrochemical sensors among other applications. However, a detailed understanding of their optical and electronic properties at the nanoscale level remains a key open question which is relevant to their utilization for a wide range of applications.

Tip-enhance Raman Spectroscopy (TERS) is a technique which uses a nanoscale plasmonic scanning probe to generate Raman signal from a nanoscale focal volume, thereby providing deep sub-diffraction limited optical spatial resolution proportional to that of Atomic Force Microscopy. TERS has been used to study nanoscale heterogeneities in other 2D materials such as graphene, transition metal dichalcogenides as well as their van der Waals heterostructures. Here, we report for the first time TERS imaging of mono- and few-layer crystals of 2D Ti₃C₂T_x MXenes (Tx is surface groups such as =O, -OH and -F) deposited on gold substrate with 785 nm and 830 nm excitation wavelengths, matching the transverse plasmon resonance of 2D Ti₃C₂T_x located at 780-800 nm. TERS spectra collected from the monolayers are strongly dominated by the intense peak A1g (Ti, C, T_x) at 203 cm^-1. As the number of layers increase, relative intensity of the resonant 126 cm^-1 and 725 cm^-1 peaks as compared to that of the 203 cm^-1 peak also increase, though the absolute intensity of the peaks decrease. In addition, we observed peculiar TERS response from wrinkles in MXene sheets, which commonly appear in crystals of Ti3C2Tx (and other 2D materials) deposited from colloidal suspensions. TERS spectra of the wrinkles featured a strongly enhanced absolute intensity of the 126 cm^-1 and 725 cm^-1 peaks, even in wrinkles that were up to 20 nm high. Using TERS for nanoscale spectroscopic characterization of Ti₃C₂T_x allows collecting fast Raman maps with deep sub-diffraction resolution with a laser power density on the sample about an order of magnitude lower compared to previously conducted confocal microRaman measurements. In addition, we show that the intensity of the TERS response from the mono- to few-layer crystals of Ti₃C₂T_x can be used to track early stages of degradation in ambient conditions, well before noticeable morphological changes start to appear in these crystals. In summary, TERS imaging of 2D Ti₃C₂T_x MXenes has enabled detailed nanoscale spectroscopic characterization which is essential to the understanding of their crystal structure and properties.

Since their discovery one decade ago, the MXenes have developed into the largest family of two dimensional materials with a broad array of predicted and reported properties. Theory and computation have been useful to explore many variations from the standard (M_n+1X_nT_x) formula, chiefly by varying the M atom to consider alloys and their possible ordering. Some of the variations include ordered multi-metal atom MXenes, where the different species of metal atoms are ordered in different layers, and more recently high-entropy MXenes where many metal atoms form a solid solution. These all result in different structural and electronic properties. In this presentation I will first review the range of structures that have been predicted in the MXene family. Second, I will consider some of the surprising opportunities remaining for new materials, based on the results from recent high throughput density functional calculations looking at variations on the “X” site, where we have studied many seemingly overlooked borides, nitrides, and their alloys with carbon. Results are contrasted with the established properties of Ti₃C₂. Finally, looking forward to the next few years of MXene research, I will outline some of the properties that have remained out-of-reach and the challenges posed to theory by recent experiments.

This research was sponsored by the Fluid Interface Reactions, Structures, and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences.

Due to the increasing specialization of space and earth-bound mechanical systems, the use, study, and synthesis of solid lubricants has gained attention over the last decade. The discovery of two-dimensional (2D) and hexagonal layered solid materials such as graphene, and members of the metal dichalcogenides family revolutionized the solid lubricants’ field with their ability to reduce the friction and applicability to extreme conditions of space. Very recently, it was shown that early transition metal carbides and nitrides (MXenes) as a new class of 2D layered material also offer promising solid lubrication properties at the macro- and nanoscales. Nevertheless, since the first titanium carbide MXene, Ti₃C₂T_x, was synthesized in 2011, friction property of Ti₃C₂T_x has not been fully understood. Lateral force microscopy (LFM) is an ideal technique to investigate the friction of Ti₃C₂T_x at the nanoscale under controllable environmental conditions. Here, LFM was employed to scrutinize the effect of layer thickness on the friction property of Ti₃C₂T_x MXene flakes on silicon substrates at the nanoscale in dry nitrogen environment. We have observed that a reduction of the friction coefficient to the superlubric regime could be achieved at the nanoscale, and the decrease became more pronounced with the increase of the thickness of Ti₃C₂T_x MXene flakes from single- to trilayers. Even though the further increase of the layer thickness slightly increased the friction coefficient, the superlubricity was preserved for bulk Ti₃C₂T_x MXene flakes. Moreover, the friction behaviors of Ti₃C₂T_x MXene flakes with different thicknesses were investigated at different time intervals, and a drastic deterioration from superlubricity to solid lubricity regime was observed as a consequence of oxidative degradation on Ti₃C₂T_x MXene flakes.

Three-dimensional (3D) porous structures based on MXene nanosheets have recently emerged as high-performance materials for electronic and biomedical applications. However, the high colloidal stability of single-layered MXene nanosheets in water complicates their assembly into a multiscale structure. In this work, we demonstrate that the controlled diffusion of a positively charged polyelectrolyte (PE) into the single-layer MXene suspension allows for a range of free-standing 3D structures with tunable porosity. The MXene/PE assemblies are developed by the complexation of PE chains and MXene nanosheets at the interface and the subsequent diffusion of PE chains into the bulk of MXene suspension. The shape and orientation of the pores made between MXene nanosheets are engineered by the colloidal behavior of the MXene suspension and the MXene/PE molecular interactions. Such interactions, in turn, are controlled by tuning the ionization degrees of the components and the polyelectrolyte chain length. Also, we show that the same molecular approach can be applied to design MXene-based hollow fibers. The final 3D porous hybrids are electroconductive (10 to 20 S/cm) and provide a basis for the rational design of MXene-based macro assemblies as promising candidates for electromagnetic interference shielding, tissue engineering, and environmental remediation applications.

MXene’s are a new intriguing class of 2D materials. They have numerous theoretical applications from ion transport, to battery electrode, to a potential Pt replacement in catalysis. One major hurdle to MXene’s being used in these theoretical applications is how to produce sufficient quantities of this 2D materials in bulk for these applications. The original way that Yury Gogotsi made this class of material involves a HF etch of MAX group alloys such as Ti3AlC2 to make Ti3C2. That method has difficulty with scaling due to the dangers associated with working with large quantities of HF and is also limited by what a practical M group can be due to some of the known M groups such as Mo and W being refractory metals. One workaround is to do a bottom up nanoscale growth of a MXene on an epitaxial surface. MXene’s being hexagonal layered crystals can epitaxially grow on and be stable on hexagonal crystals at far lower temperatures than free standing metal carbides. While people in the past have demonstrated the growth of MXene’s on Copper foil a hexagonal close packed material and graphene another hexagonal layered nanomaterial, these methods have very limited yield per surface area only on the order a couple of nanograms per square cm. Herein we demonstrate a way to a patterned growth of various types of MXenes on milled graphite powder by a sol-gel method that makes a MXene-Graphite composite. We also demonstrate that this sol-gel patterning can produce multiple grams on the small batch scale and that it can be fairly easily scaled up.

Fresh-water scarcity in arid and semi-arid regions is a global problem that is increasing due to population growth, climate change, and environmental degradation. The scientific community is always looking for new materials to achieve effective sea and brackish water desalination to reduce water scarcity. Commonly, theoretical and experimental methods make a synergy to better understand and explain the chemical and physical processes in water desalination electrodes. In this way, experimental evidence pointed to Mo1.33C MXene as an efficient ion intercalation material, in which both Na cations and Cl anions are removed. However, the atomic-scale understanding of the Physico-chemical processes due to the cation and anion interaction with the MXene is still unknown. We report the Na cation and Cl anion interaction with OH functionalized Mo1.33C monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through multiple van der Waals interactions with the MXene, which increases its adsorption energy. Electrostatic potential isosurface calculations evidence reactive sites. Oxygen atoms have an affinity for the electropositive Na atoms, whereas hydrogen atoms -of the hydroxyl groups- interact with the electronegative Cl atoms. Bader change analysis and projected densities of states help clarify the way cations and anions attach to the MXene layer. Our findings explain why OH-functionalized Mo1.33C can efficiently remove both anions and cations based on their affinities with the functional groups present in the MXene.
We thank DGAPA-UNAM projects IA100822, IN110820, and IN105722 for partial financial support. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, projects LANCAD-UNAM-DGTIC-051, LANCAD-UNAM-DGTIC-150 and LANCADUNAM-DGTIC-368. J.G.S. acknowledges THUBAT-KAAL IPICYT supercomputing center for computational resources. The authors thankfully acknowledge the computer resources, technical expertise, and support provided by the Laboratorio Nacional de Supercomputo del Sureste de Mexico, LNS-CONACYT member of the network of national laboratories. DMMP thanks CONACyT for the postdoctoral fellowship. We thank Aldo Rodriguez-Guerrero for his technical support.

The discovery of MXenes expands the 2D family by adding desirable highly conductive and stable members, providing more building blocks for ubiquitous electronics. In addition to the widely used selective etching method to synthesize MXenes in liquid phase, several other methods such as chemical vapor deposition (CVD) and atomic substitution method are also developed recently to synthesize MXenes on solid state substrates. In the first part of my talk, I will first discuss our recent progress on the synthesis of nitride MXenes and related ultrathin metal nitrides through the atomic substitution of ultrathin layered metal chalcogenides.¹ This approach allows us to prepare the self-aligned 2D lateral heterostructures between nitride MXenes and the metal chalcogenides such as MoN_x and MoS₂, which is a metal-semiconductor junction. We further apply this structure to field-effect transistors with MoN_x as metallic contacts and MoS₂ as the semiconducting channel material. In the second part of my talk, I will introduce the synthesis of 2D transition metal oxides (TMOs, e.g. TiO₂) through the oxidation of MXenes (e.g. Ti₃C₂T_x) and the liquid-liquid self-assembly method to fabricate large-area films of both 2D TMOs and MXenes.² We further demonstrate flexible, fast, and high-coloration-efficiency EC devices based on self-assembled 2D TiO₂/Ti₃C₂T_x heterostructures, with the Ti₃C₂T_x layer as the transparent electrode, and the 2D TiO₂ layer as the EC layer. Benefiting from the well-balanced porosity and connectivity of these assembled nanometer-thick heterostructures, they present fast and efficient ion and electron transport, as well as superior mechanical and electrochemical stability. We further demonstrate large-area flexible devices which could potentially be integrated onto curved and flexible surfaces for future ubiquitous electronics.
References:
(1) Cao, J.; Li, T.; Gao, H.; Lin, Y.; Wang, X.; Wang, H.; Palacios, T.; Ling, X. Realization of 2D Crystalline Metal Nitrides via Selective Atomic Substitution. Sci. Adv. 2020, 6 (2), eaax8784.
(2) Li, R.; Ma, X.; Li, J.; Cao, J.; Gao, H.; Li, T.; Zhang, X.; Wang, L.; Zhang, Q.; Wang, G.; Hou, C.; Li, Y.; Palacios, T.; Lin, Y.; Wang, H.; Ling, X. Flexible and High-Performance Electrochromic Devices Enabled by Self-Assembled 2D TiO2/MXene Heterostructures. Nat. Commun. 2021, 12 (1), 1587.

MXenes have a wide range of applications in energy devices, sensors, lithium-ion batteries, sodium-ion batteries, and supercapacitors due to their novel two-dimensional (2D) layered carbides or carbonitride structure, high electrical conductivity, large specific surface area, and excellent electrochemical properties. Several MXene composites with carbon nanotubes and graphene, have been studied for energy devices over the past years, but these composites sometimes lack various structural and electrochemical properties, directing research toward more flexible solutions, literally and metaphorically. Polymers are an excellent choice for synthesizing MXene composites due to their versatility, compatibility, and low cost. Polypyrrole (PPy) has been widely used in energy devices as an outstanding conducting polymer due to its high theoretical specific capacity, reversible electrochemical redox characteristics, and high conductivity. However, pristine PPy tends to agglomerate together, which may hinder the diffusion of electrolyte ions. Many researchers have concentrated their efforts in recent years on the combination of conducting polymers and carbon materials to improve the electrochemical performance. In the present work, we present the synthesis of Ti₃C₂ MXene/PPy composite via in-situ polymerization of pyrrole monomer. Comprehensive morphological, structural and electrochemical studies were carried out to develop the understanding of Ti₃C₂ MXene/PPy composite with the fundamentals of electrochemistry. Inherent electrochemical behaviours were studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry, and linear sweep voltammetry (LSV). The studies revealed that the current density of Ti₃C₂ MXene/PPy composite is several folds higher than PPy_, which attributes to high electrocatalytic activity. Further, electrochemical surface area (ECSA) of Ti₃C₂ MXene/PPy composite has been determined to investigate the electron transfer kinetics and specific catalytic activity. We investigated the electrocatalytic water splitting activity of Ti₃C₂ MXene/PPy composite_, for electrocatalytic green hydrogen fuel production (>90% faradaic efficiency) by water splitting. This observed enhancement we attributed to the improved electrocatalytic behaviors of Ti₃C₂ MXene/PPy composite, which stimulates the chemical reaction, as supported with DFT calculations. This study makes a big splash by demonstrating MXene/polymer composites for clean and green hydrogen energy production.

Pioneering discovery of graphene paved a way for various interesting 2D materials, such as graphene oxides, TMDs, MXene and h-BN, while attracted enormous research attentions. How to design the rational architectures of those 2D materials is crucial for the realization of their profound implications in real-world applications, including electrical, energy, environmental and biomedical application fields. In this regard, nanoscale assembly, precise control over the orientational/positional ordering and complex interface among 2D layers, is essential for the future progress of 2D materials especially for energy storage/conversion and environmental remediation. This presentation will highlight the recent progress, status, future prospects, and challenges associated to the nanoscopic assembly of 2D materials, particularly targeting at energy and environmental applications. Geometric dimensional diversity of 2D material assembly is focused, based on the novel assembly mechanisms, including 1D fiber spinning from colloidal liquid crystalline phase, 2D film formation by interfacial tension and 3D nanoporous architecture assembly by electrochemical process. Relevant critical advantageous of the 2D material assembly will be introduced for the application fields, including wearable device, secondary batteries/supercapacitors, single atom catalysts, sensors, and water decontamination.

The class of two-dimensional metal carbides and nitrides known as MXenes attract significant attention owing to their unparalleled properties and wide range of applications. The controllable surface chemistry provide room to tune the properties and expand the application of MXenes, although experimental exploration of new surface terminals remains a challenge. Herein, we shown a MXene surface chemistry tuning strategy based on Lewis acidic molten salt etching, which can be used to tune the constituent of termination exactly. MXene with the termninations of -P, -As, Sb, -S, -Se and –Te were successfully synthesized through the aforemention route. The influence of the termninations on the properties of MXene was investigated. The results indicate the effectiveness of the Lewis acidic molten salt etching route for tuning the surface chemistry of MXenes, and also show promise for expanding the MXene family towards various applications.

Two-dimensional (2D) materials exhibit unique and unusual properties that are unimaginable in bulk materials, resulting in many promising and novel applications. In particular, MXenes, a large family of 2-D transition metal carbides, nitrides and carbonitrides have outperformed other 2D materials and rapidly positioning in numerous promising applications, including energy storage, electronics, medicine, sensors etc. due to their excellent mechanical, electrical and tunable optical properties. However, for the development of these 2D MXenes, synthesis of MAX phase is very pertinent as a precursor material. Conventional techniques available are highly expensive, very complex, power-consuming involves high temperature and pressure and longer duration, which makes the MAX phase synthesis difficult. Therefore, the current quest for MAX phase synthesis has been to circumvent the expensive, complex, time and power-consuming conventional synthesis procedures. Here, we have introduced a novel flash sintering technique, generally used as a rapid sintering and densification process for ceramic materials, for fast synthesis of Ti₃SiC₂ MAX phase. Unlike the conventional method, this method is aided by joule heating due to which the preparation of MAX phase from the elemental Ti/Si/C powders is completed within seconds, both in air as well as in vacuum. Application of DC voltage in the range of 30-50 V/cm, leads to a sudden rise in current flow to 200-300 mA/mm² within the compact Ti/Si/C mixture resulting in the heating of the sample. We successfully synthesized two-dimensional Ti₃C₂T_X MXene nanosheets from the flash synthesized Ti₃SiC₂ MAX phase by oxidant-assisted HF etching. The MXene thus obtained exhibits comparable properties to that previously reported from conventionally synthesized Ti₃SiC₂ MAX phase. The use of flash sintering technique presented here for synthesis of MAX phase is an innovative and scalable approach capable of producing MAX phases in large quantities in a short time. Thus, it could be a breakthrough for rapid synthesis of wide range of MAX phases on an industrial scale.

MXenes are synthesized by selective etching of A-group element from bulk MAX phases via wet etching process. Because the A-group elements between M layers has atomic-scale layer thickness and wide lateral dimension (~ 15 μm) with strong chemical bonds between A and M elements in MAX phases, selective removal process of A layer should be very complicate. Thus, understanding where the etching of A-element layer starts and how it proceeds depending on types of etchants is the first step, and essential to obtaining high quality of MXenes. In spite of intensive efforts, exploring the etching mechanism of the atomic Al layer is still uncertain. Herein, we report for the first time the etching mechanism of Al atomic layers from Ti₃AlC₂ MAX phases by observing the structural and morphological changes of Ti₃C₂T_x MXenes as a function of etching time via high-resolution electron microscopy. We demonstrate that the atomic Al layers are etched by two distinct mechanisms depending on the types of wet-etchants: For LiF/HCl etchant, the etching begins from either defect edges or/and edges of Al atomic layers exposed at side surface of Ti₃AlC₂ MAX top (or bottom) layers, which are relatively weak Ti-Al bonding sites. The etching proceeds from the edges into inner Al atoms in a single Al layer, and at the same time etches Al atoms at under layers by attacking weak Ti-Al bonding sites generated during the etching. For HF and NH₄HF₂ etchants, however, excess amount of HF starts to attack not only edges of atomic Al layers exposed at side surface of Ti₃AlC₂ MAX top (or bottom) layers, but also grain boundary of poly-crystalline Ti₃AlC₂ MAX multi-layer. Hence, more atomic Al layers are exposed to etching solutions for further etching, leading to fully etched multi-layer MXene particles.

MXenes are receiving intense interests in energy storage field due to their unique characteristics. Here, we will show our recent work on the application of MXene-based materials and electrodes for energy storage. A series of 3D structured MXene electrodes, including MXene foam, porous MXene film, 3D carbon-coated MXene were constructed, which present high capacity, excellent cycle and rate performances for batteries and supercapacitors. Moreover, some MXene-based nanohybrids were prepared, in which MXene nanosheets can not only provide efficient pathways for fast transport of electrons and ions, but also buffer the volume change of the active materials during charge/discharge, making the nanocomposites exhibit much enhanced performance as anodes for LIBs. In addition, we proposed a novel strategy to employ 2D MXene nanosheets as a multifunctional conductive binder for electrode fabrication of batteries and supercapacitors, through a simple vacuum-assisted co-filtration of MXene sheets and various active material (such as activated carbon, hard carbon, graphite carbon, silicon, sulfur, etc.). Different from the conventional insulate binder such as PVDF and PTFE, the MXene nanosheets act as conductive binder, active material as well as flexible backbone, and can also buffer the volume change of the active materials. As a result, the MXene-bonded electrodes present much superior electrochemical performances to the conventional PVDF/PTFE-bonded electrodes, indicating a promising electrode fabrication strategy for energy storage.

MXene is one of the most exciting 2D materials currently, which has incredible potential in various applications. In this work, we report Polyethyleneimine (PEI) conjugated MXene nanosheets, which are cationic in nature. PEI_Ti₃C₂T_x (PEI_MX) was produced by etching Ti₃AlC₂, ultrasonicated along with PEI, and then hydrothermally treated at 200 ⁰C for 24 hours. PEI is a positively charged polymer containing repeating amine groups. The as-obtained nanosheets were characterized through XRD, AFM, TEM, FIR, UV-vis, and PL spectroscopy. The thickness of PEI conjugated nanosheet was found to be 2.63±0.85 nm compared to the thickness of non-conjugated control Ti₃C₂ which was found to be 6.82±1.66 nm, proving its utilization both as a surfactant as well as a functionalizing agent. The use of polymer also assists in forming a stable aqueous dispersion. The small-sized nanosheets were found to be highly biocompatible and exhibited blue fluorescence. The photoluminescence of the 2D nanosheets enables them to be also utilized as cell labeling probes. The PEI_MX are highly scalable and possess intrinsic NIR activity, fluorescence, small size, and cationic character which can be applied for a variety of applications like cell imaging and photothermal therapy.

Due to their ultrathin layered structure and rich elemental variety, MXenes are emerging as one of promising electrode candidates in energy generation and storage. MXenes are generally synthesized via hazardous fluoride-containing reagents from robust MAX materials, unfortunately resulting in plenty of inert fluoride functional groups on the surface that noticeably decline their performance. We used density-functional theory (DFT) calculations to show the etching feasibility of hydrochloric acid (HCl) on various MAX phases. Based on this theoretical guidance, we have experimentally demonstrated fluoride-free Mo2C MXenes with high efficiency about 98%. The Mo2C electrodes produced by this process exhibit high electrochemical performance in supercapacitors and sodium ion batteries. Moreover, in situ hydrothermal strategy is also implemented to successfully synthesize unique endogenous hetero-MXenes of amorphous MoS2 coupling with fluoride-free Mo2CTx directly from Mo2Ga2C MAX. The distinctive morphology and heterojunction structure caused by the introduction of MoS2 endow the hetero-MXenes with extraordinary structural stability and optimized Li+ storage mechanism with improved charge transport and lithium ion adsorption capabilities. This strategy enables the development of fluoride-free MXenes and opens a new window to explore their potential in energy storage applications.

Detecting low concentrations of gaseous explosive formulations based on NOx derivatives has been challenging, especially when the hazard is extremely severe. MXene (e.g. Ti₃C₂T_x,) family of 2D layered compounds have shown promise for sensing gases due to their unique properties such as high surface area, strong metallic conductivity, high hydrophilicity, good mechanical properties and active surface chemistry. Ti₃C₂T_x is the most extensively researched MXene and in this work we have used a composite based on the same for NOx detection. Indeed, we have observed that selectivity for the NO₂ gas in Ti₃C₂T_X can be considerably improved by loading the same with Fe₃O₄ nanoparticles. Fe₃O₄ gives the electron for the transition from NO₂ to NO₃^-, whilst Ti₃C₂T_X provides a large surface area for this reaction to take place and also aids in the provision of extra electrons. In this context, a metamaterial-inspired sensor is used to detect the NOx gas,wherein the sensor is made to interact with Fe₃O₄ NPloaded MXene. The properties are studied as a function of variation of dielectric constant of Fe₃O₄-loaded -MXene as a result of the toxic gas interaction. The sensor is a complementary slit-ring resonator (CSRR) operating at 430 MHz, and is initially simulated for its resonant frequency and power using COMSOL software. The unit cell sensor is fabricated on a copper-clad with FR-4 substrate. Separately, Fe₃O₄-doped-MXene is exposed to the NOx gas (varying from 0 to 150 ppm) and the NOx-purged Fe₃O₄-doped-MXene is obtained; which is subjected to the CSRR device. A systematic shift, both in frequency and power is obtained as the purged gas concentration changes from 0 to 150 ppm. The sensor is studied for its sensitivity, accuracy, and recovery properties. The Fe₃O₄-doped-MXene is carefully evaluated for its structure, chemical composition and interaction with NOx molecules. A two-stage device is hence illustrated, for hazardous gas sensing. The details are discussed in this presentation.

The demand for room-temperature chemical sensing has been increasing due to the emergence of flexible, portable, and low-power devices. To achieve this, two-dimensional (2D) nanomaterials have drawn attention due to their high surface area, and high tunability, and ability to detect molecules at room temperature. MXenes, a large class of 2D transition metal carbides/nitrides with interesting properties, are one of the most recent group of nanomaterials to be utilized as sensing materials. By exploiting the simultaneous existence of metallic-like electrical conductivity and high-density surface functional groups, we were able to previously demonstrate that Ti₃C₂ MXene sensors show an exceptionally high single-to-noise ratio. Here, the fabrication of laminated MXene thin films and our strategies to apply them as tunable chemical sensors will be suggested, with special focuses directed at methods to utilize the intercalation and surface chemistry of MXene laminates. We demonstrate that by deliberately intercalating cations into the interlayers of MXene laminates, the gas response and selectivity were greatly enhanced compared to gas sensors based on pristine MXene films. Also, we show that modifying the surface functionalities of MXene can be utilized to enhance molecule detection.

The bonding and debonding of surface -O terminals of Ti₃C₂T_x MXene with hydronium in acid electrolyte may provide pseudocapacitive charge storage. Alkali, n-butyllithium, and ammonium persulfate etc. have been proved to be efficient media to engineer more surface oxygen functional groups on Ti₃C₂T_x for enhanced charge storage. Heteroatom doping is also a feasible method to modulate Ti₃C₂T_x MXene surface to boost charge storage capability. Thus far, nitrogen and sulfur have been successfully doped into MXene, which exhibits ameliorative electrochemical performance (EC). These hybrid elements tune the electronic band structure of MXene and increase its conductivity. The doping of other elements with lower electronegativity into MXene as electron donors to improve its electrochemical behavior remains a domain waiting for exploration.

In this work, phosphorus with inferior electronegativity to nitrogen and sulfur is successfully grafted on Ti₃C₂T_x MXene via heating in argon and phosphine atmosphere. X-ray diffraction (XRD) results show that the incorporated phosphorus serves as pillar to resist the shrinkage of interlayer path caused by the elimination of surface functional groups (-OH, -F, -Cl) of MXene during heating process, which benefits its rate performance. X-ray photoelectron spectroscopy (XPS) results confirm the increased valence state of titanium after doping and the existence of incorporated phosphorus in the form of Ti-O-P on MXene surface. EC characterization indicates that the phosphorus-doped (1.9 at.%) Ti₃C₂T_x exhibits a capacitance of 276.9 F g^-1 at 2mV s^-1 in 1M H₂SO₄, much higher than that of the pristine counterpart (177.7 F g^-1). It is concluded that this enhanced capacitance is mainly attributed to newly formed surface Ti-O-P groups, which serve as new active sites for charge storage, providing extra capacitance. Moreover, the modified Ti₃C₂T_x with phosphorus delivers a capacitance retention of ~90% after cycling at 10A g^-1 for 10000 times, indicating its good cycle stability. In summary, our work proved that the introduction of phosphorus is an efficient way to maintain the interlayer path open and promote the pseudocapacitive behavior of Ti₃C₂T_x MXene.

Expanding the MXene design space from double-transition-metal (DTM) MXenes to include high-entropy MXenes with four or more principal elements provides a powerful approach for tailoring MXene properties. While DTM MXenes have been characterized to reveal microstructures that strongly influence material properties, high-entropy MXenes have been synthesized only recently, with little known about their microstructures and properties. If the development of the analogous bulk 3D high-entropy alloys is any indication, understanding the behaviors of high-entropy MXenes across the huge design space would be challenging. Therefore, we present a timely high-throughput first-principles study of the microstructures of two high-entropy MXenes: (Ti-V-Nb-Mo)₄C₃ and (Ti-V-Cr-Mo)₄C₃. Using Monte Carlo simulations powered by cluster expansion, we predict that, across a wide range of compositions, the predominant behavior in both high-entropy MXenes is the emergence of interlayer ordering at low temperatures. The degree of ordering highly depends on the composition and temperature, with Cr having the strongest preference for the outer layers, followed by Mo, V, Nb, and Ti. Regardless of the interlayer ordering, the atoms within each layer are randomly dispersed, with one notable exception: Nb and V tend to segregate from each other in the inner layers. Our results provide a set of design principles that guides experiments in the search of MXenes with enhanced properties within the huge compositional space.

In this talk, a manufacturing method for additive/binder- or composite-free, entirely pure 1D MXene fibers with high electrical conductivity is introduced. Aa promising building block for manufacturing multifunctional macroscopic structures, MXenes (Ti₃C₂T_x) is successfully assembled into fibers via a wet-spinning method. Without using binders and composites, pure MXene fibers are fabricated from a relatively large Ti₃C₂T_x MXene sheet (5.11 μm²) at a high concentration (25 mg mL^-1), demonstrating highly stable colloidal properties with a liquid–crystalline phase. The wet-spinning of the liquid crystal MXene dispersion produces flexible, meter-long, continuous MXene fibers with ultrahigh electrical conductivity. In addition, the wet-spinning method of MXene is further developed via using a deformable MXene gel system through strong electrostatic interactions. Due to its defect-free structure, MXene fiber has dense lamella, as well as a highly aligned multiscale structure. As an electrical wire, the prepared MXene fiber exhibits outstanding electrical conductivity and a high mechanical modulus compared with other MXene-based assemblies. The MXene fibers are expected to have wide-spread applications, such as electrical wires and signal transmission.