Fast and High-Yield Anhydrous Synthesis of Ti3 C2 Tx MXene with High Electrical Conductivity and Exceptional Mechanical Strength.

2D transition metal carbides or nitrides (MXenes) have attracted considerable attention from materials scientists and engineers owing to their physicochemical properties. Currently, MXenes are synthesized from MAX-phase precursors using aqueous HF. Here, in order to enhance the production of MXenes, an anhydrous etching solution is proposed, consisting of dimethylsulfoxide as solvent with its high boiling point, NH4 HF2 as an etchant, CH3 SO3 H as an acid, and NH4 PF6 as an intercalant. The reaction temperature can be increased up to 100 °C to accelerate the etching and delamination of Ti3 AlC2 MAX crystals; in addition, the destructive side reaction of the produced Ti3 C2 Tx MXene is suppressed in the etchant. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high-quality monolayer Ti3 C2 Tx with an electrical conductivity of 8200 S cm-1 and yield of over 70%. The Ti3 C2 Tx MXene fabricated via this modified synthesis exhibits different surface structures and properties arising from more F-terminations than those of Ti3 C2 Tx synthesized in aqueous HF2 T. The atypical surface structure of Ti3 C2 Tx MXene results in an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is five times larger than those of Ti3 C2 Tx MXenes synthesized in aqueous HF solution (31.7 ± 7.8 MPa).

Tunable Ti3C2Tx MXene-Derived TiO2 Nanocrystals at Controlled pH and Temperature.

While two-dimensional (2D) Ti3C2Tx MXene in aqueous dispersions spontaneously oxidizes into titanium dioxide (TiO2) nanocrystals, the crystallization mechanism has not been comprehensively understood and the resultant crystal structures are not controlled among three representative polymorphs: anatase, rutile, and brookite. In this study, such control on the lattice structures and domain sizes of the MXene-derived TiO2 crystallites is demonstrated by means of the oxidation conditions, pH, and temperature (3.0-11.0 and 20-100 °C, respectively). It is observed that the formation of anatase phase is preferred against rutile phase in more basic and hotter oxidizing solutions, and even 100% anatase can be obtained at pH 11.0 and 100 °C. At lower pH and temperature, the portion of rutile phase increases such that it reaches ∼70% at pH 3 and 20 °C. Under certain circumstances, small portion of brookite phase is also observed. Smaller domain sizes of both anatase and rutile phases are observed in more basic oxidizing solutions and at lower temperatures. Based on these experimental results, we propose the crystallization mechanism in which the oxidative dissociation of Ti3C2Tx first produces Ti ions as the intermediate state, and they bind to abundant oxygen in the aqueous dispersions, and nucleate and crystallize into TiO2.

A High Aspect Ratio Serpentine Structure for Use As a Strain-Insensitive, Stretchable Transparent Conductor.

The development of strain-insensitive stretchable transparent conductors (TCs) is essential for manufacturing stretchable electronics. Despite recent progress, achieving a high optoelectronic performance under applied strain of 50% continues to present a significant challenge in this research field. Herein, an ultratall and ultrathin high aspect ratio serpentine metal structure is described that exhibits a remarkable stretching ability (the resistance remains constant under applied strain of 100%) and simultaneously provides an excellent transparent conducting performance (with a sheet resistance of 7.6 Ω î¨-1 and a transmittance of 90.5%). It is demonstrated that the highly stretchable transparent conducting properties can be attributed to the high aspect ratio feature. A high aspect ratio (aspect ratio of 17-367) structure permits facile deformation of the serpentine structure with in-plane motion, leading to a high stretching ability. In addition, this structural feature avoids the classic tradeoff between optical transmittance and electrical conductance, providing a high electrical conductance without decreasing the optical transmittance. The practical utility of these devices is tested by using these TCs as stretchable interconnectors among LEDs or in wearable VOC gas sensors.

Influence of graphene thickness and grain boundaries on MoS2 wrinkle nanostructures.

Controlling wrinkle nanostructures of two-dimensional materials is critical for optimizing the material properties and device performance. In this study, we demonstrated the in situ synthesis of large-area MoS2 wrinkles on graphene by chemical-vapor-deposition-assisted sulfurization, and investigated the influence of graphene thickness and grain structures on the feature dimensions of MoS2 wrinkle nanostructures. The height, width, and overall surface roughness of the MoS2 wrinkles diminish as the number of graphene layers increases, which was further verified by determining the binding energy of graphene layers by density functional theory calculations. Furthermore, the feature dimensions of MoS2 wrinkle nanostructures were also influenced by graphene domain boundaries because of the difference in graphene nucleation density. This may be attributed to the influence of the mechanical properties of graphene substrates on the overall feature dimensions of MoS2 wrinkles, which are directly correlated with the interfacial adhesion energy. We believe that our findings may contribute toward the controllable synthesis of MoS2 wrinkle nanostructures and other two-dimensional materials used for high-performance devices.

Direct observation of molybdenum disulfide, MoS2, domains by using a liquid crystalline texture method.

Because the properties of molybdenum disulfide (MoS2) are strongly influenced by the sizes and boundaries of its domains, the direct visualization of large-area MoS2 domains is one of the most important challenges in MoS2 research. In the current study, we developed a simple and rapid method to observe and determine the boundaries of MoS2 domains. The technique, which depends on observations of nematic liquid crystal textures on the MoS2 surface, does not damage the sample and is not limited by domain size. Thus, this approach should significantly aid not only efforts aimed at gaining an understanding of the relationships between grain boundaries and properties of MoS2 but also those focusing on how domain sizes are controlled during large-area synthesis.

The Influence of Cu Lattices on the Structure and Electrical Properties of Graphene Domains during Low-Pressure Chemical Vapor Deposition.

The influence of various Cu lattices on the texturing of graphene domains during low-pressure chemical vapor deposition was investigated in a large area. The results show that the sizes and shapes of graphene domains grown on Cu(111) substrates match well with those of the underlying Cu(111) domains, which seem to be quasi-single-crystalline. In contrast, on other Cu substrates such as (100) and more intermediate domains, graphene islands with poly-domains (ca. 85 %) are significantly nucleated, eventually merging into polycrystalline graphene. Within the overall channel-length range, graphene from a Cu foil shows a higher resistance compared to graphene from a Cu(111) domain, with the extracted average channel resistances being 34.51 Ω µm(-1) for Cu(111) and 66.17 Ω µm(-1) for the Cu foil.

Functionalized MXene ink enables environmentally stable printed electronics.

Establishing dependable, cost-effective electrical connections is vital for enhancing device performance and shrinking electronic circuits. MXenes, combining excellent electrical conductivity, high breakdown voltage, solution processability, and two-dimensional morphology, are promising candidates for contacts in microelectronics. However, their hydrophilic surfaces, which enable spontaneous environmental degradation and poor dispersion stability in organic solvents, have restricted certain electronic applications. Herein, electrohydrodynamic printing technique is used to fabricate fully solution-processed thin-film transistors with alkylated 3,4-dihydroxy-L-phenylalanine functionalized Ti3C2Tx (AD-MXene) as source, drain, and gate electrodes. The AD-MXene has excellent dispersion stability in ethanol, which is required for electrohydrodynamic printing, and maintains high electrical conductivity. It outperformed conventional vacuum-deposited Au and Al electrodes, providing thin-film transistors with good environmental stability due to its hydrophobicity. Further, thin-film transistors are integrated into logic gates and one-transistor-one-memory cells. This work, unveiling the ligand-functionalized MXenes' potential in printed electrical contacts, promotes environmentally robust MXene-based electronics (MXetronics).

Single stranded 1D-helical Cu coordination polymer for ultra-sensitive ammonia sensing at room temperature.

With the increasing demand for ammonia applications, there is a significant focus on improving NH3 detection performance at room temperature. In this study, we introduce a groundbreaking NH3 gas sensor based on Cu(I)-based coordination polymers, featuring semiconducting, single stranded 1D-helical nanowires constructed from Cu-Cl and N-methylthiourea (MTCP). The MTCP demonstrates an exceptional response to NH3 gas (>900% at 100 ppm) and superior selectivity at room temperature compared to current materials. The interaction mechanism between NH3 and the MTCP sensor is elucidated through a combination of empirical results and computational calculations, leveraging a crystal-determined structure. This reveals the formation of NH3-Cu and NH3-H3C complexes, indicative of a thermodynamically favorable reaction. Additionally, Ag-doped MTCP exhibits higher selectivity and a response over two times greater than the original MTCP, establishing it as a prominent NH3 detection system at room temperature.

Biocompatible and Oxidation-Resistant Ti3 C2 Tx MXene with Halogen-Free Surface Terminations.

Surface chemistry influences not only physicochemical properties but also safety and applications of MXene nanomaterials. Fluorinated Ti3 C2 Tx MXene, synthesized using conventional HF-based etchants, raises concerns regarding harmful effects on electronics and toxicity to living organisms. In this study, well-delaminated halogen-free Ti3 C2 Tx flakes are synthesized using NaOH-based etching solution. The transversal surface plasmon mode of halogen-free Ti3 C2 Tx MXene (833 nm) confirmed red-shift compared to conventional Ti3 C2 Tx (752 nm), and the halogen-free Ti3 C2 Tx MXene has a different density of state by the high proportion of -O and -OH terminations. The synthesized halogen-free Ti3 C2 Tx exhibits a lower water contact angle (34.5°) and work function (3.6 eV) than those of fluorinated Ti3 C2 Tx (49.8° and 4.14 eV, respectively). The synthesized halogen-free Ti3 C2 Tx exhibits high biocompatibility with the living cells, as evidenced by no noticeable cytotoxicity, even at very high concentrations (2000 µg mL⁻1 ), at which fluorinated Ti3 C2 Tx caused ≈50% reduction in cell viability upon its oxidation. Additionally, the oxidation stability of halogen-free Ti3 C2 Tx is enhanced unexpectedly, which cumulatively provides a good rationale for pursuing the halogen-free routes for synthesizing MXene materials for their uses in biomedical and therapeutic applications.

Effect of Substitutional Oxygen on Properties of Ti3 C2 Tx MXene Produced Using Recycled TiO2 Source.

MXenes are an emerging class of 2D materials with unique properties including metallic conductivity, mechanical flexibility, and surface tunability, which ensure their utility for diverse applications. However, the synthesis of MXenes with high crystallinity and atomic stoichiometry in a low-cost process is still challenging because of the difficulty in controlling the oxygen substitute in the precursors and final products of MXenes, which limits their academic understanding and practical applications. Here, a novel cost-effective method is reported to synthesize a highly crystalline and stoichiometric Ti3 C2 Tx MXene with minimum substitutional oxygen impurities by controlling the amount of excess carbon and time of high-energy milling in carbothermal reduction of recycled TiO2 source. The highest used content (2 wt%) of excess-carbon yields TiC with the highest carbon content and minimal oxygen substitutes, which leads to the Ti3 AlC2 MAX phase with improved crystallinity and atomic stoichiometry, and finally Ti3 C2 Tx MXene with the highest electrical conductivity (11738 S cm-1 ) and superior electromagnetic shielding effectiveness. Additionally, the effects of carbon content and substitutional oxygen on the physical properties of TiC and Ti3 AlC2 are elucidated by density-functional-theory calculations. This inexpensive TiO2 -based method of synthesizing high-quality Ti3 C2 Tx MXene can facilitate large-scale production and thus accelerate global research on MXenes.

Simple Approach to Enhance Long-Term Environmental Stability of MXene Using Initiated Chemical Vapor Deposition Surface Coating.

Developing a methodology to enhance long-term stability is one of the most important issues in MXene research, since they are prone to oxidation in the ambient environment. Although various approaches have been suggested to improve the stability of MXene, they have suffered from complicated processes and limited applicability to various types of MXene nanostructures. Herein, we report a simple and versatile technique to enhance the environmental stability of MXenes. Ti3C2Tx MXene films were decorated with a highly hydrophobic polymer, 1H,1H,2H,2H-perfluorodecyl methacrylate (PFDMA), using initiated chemical vapor deposition (iCVD) where iCVD allows the facile postdeposition of polymer films of desired thickness on MXene films. The oxidation resistance was evaluated by fabricating MXene gas sensors and measuring the change in signal-to-noise ratio (SNR) of volatile organic compound (VOC) gases under harsh conditions (RH 100% at 50 °C) for several weeks where the performance in the absence and presence of PFDMA was compared. The results show that while the SNR of PFDMA-Ti3C2Tx sensors was retained, a dramatic increase of the noise level and a decrease in the SNR were observed in pristine Ti3C2Tx. We believe that this simple and nondestructive method will offer great potential to enhance the stability of a wide range of MXenes.

N-p-Conductor Transition of Gas Sensing Behaviors in Mo2CTx MXene.

It is highly important to implement various semiconducting, such as n- or p-type, or conducting types of sensing behaviors to maximize the selectivity of gas sensors. To achieve this, researchers so far have utilized the n-p (or p-n) two-phase transition using doping techniques, where the addition of an extra transition phase provides the potential to greatly increase the sensing performance. Here, we report for the first time on an n-p-conductor three-phase transition of gas sensing behavior using Mo2CTx MXene, where the presence of organic intercalants and film thickness play a critical role. We found that 5-nm-thick Mo2CTx films with a tetramethylammonium hydroxide (TMAOH) intercalant displayed a p-type gas sensing response, while the films without the intercalant displayed a clear n-type response. Additionally, Mo2CTx films with thicknesses over 700 nm exhibited a conductor-type response, unlike the thinner films. It is expected that the three-phase transition was possible due to the unique and simultaneous presence of the intrinsic metallic conductivity and the high-density of surface functional groups of the MXene. We demonstrate that the gas response of Mo2CTx films containing tetramethylammonium (TMA) ions toward volatile organic compounds (VOCs), NH3, and NO2 is ∼30 times higher than that of deintercalated films, further showing the influence of intercalants on sensing performance. Also, DFT calculations show that the adsorption energy of NH3 and NO2 on Mo2CTx shifts from -0.973, -1.838 eV to -1.305, -2.750 eV, respectively, after TMA adsorption, demonstrating the influence of TMA in enhancing sensing performance.

Field-induced orientational switching produces vertically aligned Ti3C2Tx MXene nanosheets.

Controlling the orientation of two-dimensional materials is essential to optimize or tune their functional properties. In particular, aligning MXene, a two-dimensional carbide and/or nitride material, has recently received much attention due to its high conductivity and high-density surface functional group properties that can easily vary based on its arranged directions. However, erecting 2D materials vertically can be challenging, given their thinness of few nanometres. Here, vertical alignment of Ti3C2Tx MXene sheets is achieved by applying an in-plane electric field, which is directly observed using polarised optical microscopy and scanning electron microscopy. The electric field-induced vertical alignment parallel to the applied alternating-current field is demonstrated to be reversible in the absence of a field, back to a random orientation distribution. Interdigitated electrodes with uniaxially aligned MXene nanosheets are demonstrated. These can be further modulated to achieve various patterns using diversified electrode substrates. Anisotropic electrical conductivity is also observed in the uniaxially aligned MXene nanosheet film, which is quite different from the randomly oriented ones. The proposed orientation-controlling technique demonstrates potential for many applications including sensors, membranes, polarisers, and general energy applications.

Universal Ligands for Dispersion of Two-Dimensional MXene in Organic Solvents.

Ligands can control the surface chemistry, physicochemical properties, processing, and applications of nanomaterials. MXenes are the fastest growing family of two-dimensional (2D) nanomaterials, showing promise for energy, electronic, and environmental applications. However, complex oxidation states, surface terminal groups, and interaction with the environment have hindered the development of organic ligands suitable for MXenes. Here, we demonstrate a simple, fast, scalable, and universally applicable ligand chemistry for MXenes using alkylated 3,4-dihydroxy-l-phenylalanine (ADOPA). Due to the strong hydrogen-bonding and π-electron interactions between the catechol head and surface terminal groups of MXenes and the presence of a hydrophobic fluorinated alkyl tail compatible with organic solvents, the ADOPA ligands functionalize MXene surfaces under mild reaction conditions without sacrificing their properties. Stable colloidal solutions and highly concentrated liquid crystals of various MXenes, including Ti2CTx, Nb2CTx, V2CTx, Mo2CTx, Ti3C2Tx, Ti3CNTx, Mo2TiC2Tx, Mo2Ti2C3Tx, and Ti4N3Tx, have been produced in various organic solvents. Such products offer excellent electrical conductivity, improved oxidation stability, and excellent processability, enabling applications in flexible electrodes and electromagnetic interference shielding.

Reduction of Electrochemically Exfoliated Graphene Films for High-Performance Electromagnetic Interference Shielding.

Two-dimensional graphene is of great interest for electromagnetic interference (EMI) shielding owing to its inherent electrical conductivity, lightweight, and excellent mechanical flexibility even at minor thicknesses. However, the complex synthesis and quality-control difficulties limit its application. In this study, we demonstrate that electrochemically exfoliated graphene (EEG) with post-reduction treatment is a promising candidate for lightweight EMI shielding materials. A facile electrochemical exfoliation approach produces a high-quality multilayer graphene with a high electrical conductivity of ∼600 S cm-1, owing to its low degree of oxidation. The reduction of EEG by three different methods, including chemical, thermal, and microwave treatments, causes the removal of surface functional groups as well as significant changes in the microstructure of the final films. The reduced graphene films by microwaves, which are driven by the improved electrical conductivity and large volume expansion, exhibit an EMI shielding effectiveness of 108 dB at a thickness of 125 µm, one of the largest EMI shielding values ever reported for graphene at comparable thicknesses.

Mechanism and Kinetics of Oxidation Reaction of Aqueous Ti3C2Tx Suspensions at Different pHs and Temperatures.

Understanding the oxidation reaction of aqueous Ti3C2Tx MXene suspensions is very important for fostering fundamental academic studies as well as widespread industrial applications. Herein, we investigated the mechanism and kinetics of the oxidation reaction of aqueous Ti3C2Tx suspensions at various pH and temperature conditions. Through comprehensive analysis, the mechanism of the chemical oxidative degradation of aqueous Ti3C2Tx colloids was established. Chemical oxidation produces solid products such as TiO2 and amorphous carbon as well as various gaseous species including CH4, CO, CO2, and HF. Additionally, our comprehensive kinetic study proposes that aqueous Ti3C2Tx dispersions are degraded via an acid-catalyzed oxidation reaction, where, under acidic conditions, the protonation of the hydroxyl terminal groups on the Ti3C2Tx flakes induces electron localization on titanium atoms and accelerates their oxidation reaction. In contrast, under basic conditions, the electrostatically alkali-metalized hydroxyl intermediates forming a bulky solvent cage results in less electron localization on titanium atoms, and thus retards their oxidative degradation.

Large-Area Ti3C2Tx-MXene Coating: Toward Industrial-Scale Fabrication and Molecular Separation.

Large-scale fabrication of MXene films is in high demand for various applications, but it remains difficult to meet industrial requirements. In this study, we develop a slot-die coating method for the preparation of large-area MXene membranes. The technique allows the fabrication of continuous and scalable coatings with a rapid coating speed of 6 mm s-1. The thickness can be readily controlled from the nanometer scale to the micrometer scale, and the alignment of the nanosheet is enhanced by the shear force of the slot-die head. Molecular separation experiments employing a film with a thickness of approximately 100 nm are performed. A nanofiltration performance with water permeance of 190 LMH/bar and molecular weight cutoff of 269 Da is achieved, surpassing previously reported results obtained using MXene-based nanofiltration membranes. The stability of the membrane is highlighted by its nanofiltration performance of 30 days under harsh oxidizing conditions, which is the longest operation ever achieved for a 2D material-based membrane. The extraordinary stability of the film suggests its high potential for industrial and practical applications. The antioxidizing phenomena can be attributed to self-protection of the MXene surface by adsorbed organic molecules, which are particularly stabilized with positively charged molecules via chemisorption.

Electromagnetic Shielding of Monolayer MXene Assemblies.

Miniaturization of electronics demands electromagnetic interference (EMI) shielding of nanoscale dimension. The authors report a systematic exploration of EMI shielding behavior of 2D Ti3 C2 Tx MXene assembled films over a broad range of film thicknesses, monolayer by monolayer. Theoretical models are used to explain the shielding mechanism below skin depth, where multiple reflection becomes significant, along with the surface reflection and bulk absorption of electromagnetic radiation. While a monolayer assembled film offers ≈20% shielding of electromagnetic waves, a 24-layer film of ≈55 nm thickness demonstrates 99% shielding (20 dB), revealing an extraordinarily large absolute shielding effectiveness (3.89 × 106 dB cm2 g-1 ). This remarkable performance of nanometer-thin solution processable MXene proposes a paradigm shift in shielding of lightweight, portable, and compact next-generation electronic devices.

Interfacial Assembly of Ultrathin, Functional MXene Films.

MXenes are a prominent family of two-dimensional materials because of their metallic conductivity and abundant surface functionalities. Although MXenes have been extensively studied as bulk particles or free-standing films, thin and transparent films are needed for optical, optoelectronic, sensing, and other applications. In this study, we demonstrate a facile method to fabricate ultrathin (∼10 nm), Ti3C2Tx MXene films by an interfacial assembly technique. The self-assembling behavior of MXene flakes resulted in films with a high stacking order and strong plane-to-plane adherence, where optimal films of 10 nm thickness displayed a low sheet resistance of 310 Ω/□. By using surface tension, films were transferred onto various types of planar and curved substrates. Moreover, multiple films were consecutively transferred onto substrates from a single batch of solution, showing the efficient use of the material. When the films were utilized as gas sensing channels, a high signal-to-noise ratio, up to 320, was observed, where the gas response of films assembled from small MXene flakes was 10 times larger than that from large flakes. This work provides a facile and efficient method to allow MXenes to be further exploited for thin-film applications.