MXene Films Research Articles

MXene-Based Mixed Conductor Interphase for Dendrite-Free Flexible Al Organic Battery.

Al batteries are promising post-Li battery technologies for large-scale energy storage applications owing to their low cost and high theoretical capacity. However, one of the challenges that hinder their development is the unsatisfactory plating/stripping of the Al metal anode. To circumvent this issue, an ultrathin MXene layer is constructed on the surface of Al by in situ chemical reactions at room temperature. The as-prepared flexible MXene film acts like armor to protect the Al-metal by its high ionic conductivity and high mechanical flexibility. The MXene endow the Al anode with a long cyclic life of more than 5000 h at ultrahigh current density of 50 mA cm-2 for Al//Al batteries and a retention of 100% over 200 cycles for 355 Wh kg-1 PTO//Al batteries. This work provides fresh insights into the formation and regulation of stable electrode-electrolyte interfaces as well as effective strategies for improving Al metal batteries.

Interface Engineering to Drive High-Performance MXene/PbS Quantum Dot NIR Photodiode.

The realization of a controllable transparent conducting system with selective light transparency is crucial for exploring many of the most intriguing effects in top-illuminated optoelectronic devices. However, the performance is limited by insufficient electrical conductivity, low work function, and vulnerable interface of traditional transparent conducting materials, such as tin-doped indium oxide. Here, it is reported that two-dimensional (2D)titanium carbide (Ti3 C2 Tx ) MXene film acts as an efficient transparent conducting electrode for the lead sulfide (PbS) colloidal quantum dots (CQDs) photodiode with controllable near infrared transmittance. The solution-processed interface engineering of MXene and PbS layers remarkably reduces the interface defects of MXene/PbS CQDs and the carrier concentration in the PbS layer. The stable Ti3 C2 Tx /PbS CQDs photodiodes give rise to a high specific detectivity of 5.51×1012 cmW-1 Hz1/2 , a large dynamic response range of 140dB, and a large bandwidth of 0.76MHz at 940nm in the self-powered state, ranking among the most exceptional in terms of comprehensive performance among reported PbS CQDs photodiodes. In contrast with the traditional photodiode technologies, this efficient and stable approach opens a new horizon to construct widely used infrared photodiodes with CQDs and MXenes.

Enhanced memristive effect of laser-reduced graphene and ferroelectric MXene-based flexible trilayer memristors

Ferroelectric memory devices based on 2D materials are gaining popularity in research due to having non-volatile data storage, low power consumption and improved endurance and data retention properties. In this study, we discussed the synthesis of self-standing graphene oxide (GO) and MXene films via chemical oxidation and etching routes. As synthesized MXene (nFE-MXene) was converted into ferroelectric (FE-MXene) by heating the nFE-MXene self-standing film. XRD, SEM, EDS and elemental mapping supported crystal structure as well as chemistry of prepared material. Ferroelectric hysteresis measurements at various frequencies (100Hz–400Hz) confirmed the presence of polarization domains in FE-MXene. Graphene oxide was laser reduced (rGO) to implement as metal electrodes for metal-insulator metal (MIM) memory device architecture. Three trilayer memory systems (rGO/nFE-MXene/rGO, rGO/FE-MXene/rGO and rGO/FE-MXene/nFE-MXene/rGO) were fabricated showing non-volatile bipolar switching. The memory performances were checked for a working window of ±4V. Further, the elevated memory performance of rGO/FE-MXene/nFE-MXene/rGO memristive device at low operating voltage (˂ ±3V) is discussed for efficient working device application with an on/off ratio of 103, higher data retention (4000 s) and endurance up to 1000 cycles.

Charge Carriers Localization Effect Revealed through Terahertz Spectroscopy of MXene: Ti3C2Tx.

The transport properties of charge carriers in MXene, a promising material, have been studied using terahertz time-domain spectroscopy (THz-TDS) to examine its potential applications in optical and electronic devices. However, previous studies have been limited by narrow frequency ranges, which have hindered the understanding of the intrinsic mechanisms of carrier transport in MXenes. To address this issue, ultrabroadband THz-TDS with frequencies of up to 15 THz to investigate the complex photoconductances of MXene (Ti3C2Tx) films with different thicknesses are employed. The findings indicate that the electronic localization is substrate-dependent, and this effect decreases with an increase in the number of layers. This is attributed to the screening effect of the high carrier density in Ti3C2Tx. Additionally, the layer-independent photocarrier relaxations revealed by optical pump THz probe spectroscopy (OPTP) provide evidence of the carrier heating-induced screening effect. These results are significant for practical applications in both scientific research and various industries.

Flexible TiVCTx MXene film for high-performance magnesium-ion storage device

Magnesium-based battery system has emerged as the potential candidate to beyond Li-ion battery system due to the numerous advantageous of magnesium anode, such as natural abundance, high capacity and dendrites free. However, the selection of cathode materials and the intercalation of magnesium-ions in the cathode host material remains a challenge due to the strong interaction of highly polar divalent magnesium ions with the cathode material, making the diffusion of magnesium ions relatively difficult. Herein, the flexible TiVCTx MXene film was developed via a facile and economical approach. As the cathode host material for magnesium-ion storage, the freestanding TiVCTx MXene film displays a high specific capacity of 111 and 135 mAh g−1 at a current density of 0.05 A g−1 for magnesium-ion batteries (MIB) and Mg/Li hybrid batteries (MLHB). Furthermore, a long-term cycling stability over 1000 cycles was demonstrated and a detailed investigation of the unique long activation phenomenon of MXene films during cycling. More importantly, the reaction mechanism of magnesium-ion storage was validated, i.e., the MXene interlayer spacing variation with the reversible Mg2+ diffusion behavior. This work reveals the magnesium storage mechanism of MXene materials and provides a new pathway for high-performance storage of magnesium-ion cathode materials.

Self-standing porous MXene film via in situ formed microgel induced by 2- methylimidazole for high rate supercapacitor

Ti3C2Tx MXene has been considered a potential supercapacitor electrode material on account of its fast electron transfer performance, large volume pseudocapacitance, excellent hydrophilicity and mechanical properties. However, the serious stacking of MXene nanosheets reduced the exposed active sites and hindered diffusion of ions, leading to bad electrochemical property. Here, we report a strategy to transform MXene nanosheets into 3D MXene microgels induced by 2-methylimidazole. Benefitting from the existence of microgels, it facilitates the rapid vacuum filtration to prepare the porous MXene film, thus enhancing the ion transfer efficiency. Furthermore, 2-methylimidazole can also act as an intercalated agent to increase the layer spacing of MXene nanosheets. Therefore, the as-prepared porous MXene film demonstrates a superior rate capacity, and its capacitance reaches 223.4 F g−1 at the scan rates of 1000 mV s−1. Moreover, the assembled MXene//AC asymmetrical supercapacitor displays a 16.88 Wh kg−1 at a power density of 799 W kg−1, and maintains 13.11 Wh kg−1 at a power density of 8 kW kg−1. This work develops a simple way to prepare the porous MXene film for supercapacitors with excellent electrochemical performance.

Ultrathin MXene film interaction with electromagnetic radiation in the microwave range

The quick progress in communication technologies demands superior electromagnetic interference (EMI) shielding materials. However, achieving a high shielding effectiveness (SE) with thin films, which is needed for microscale, flexible, and wearable devices, through absorption of EM radiation remains a challenge. 2D titanium carbide MXene, Ti3C2Tx, has been shown to efficiently reflect electromagnetic waves. In this paper, we investigated the electromagnetic shielding of ultrathin printed Ti3C2Tx films and recorded absorption up to 50% for 4 nm-thick films. This behavior is explained by impedance matching. Analysis of the sheet impedance in the X-band frequency range allows us to correlate the EMI shielding mechanism with the electrical conductivity measured within the same range. The average bulk in-plane conductivity for 4 to 40 nm-thick films reaches 106 S/m, while the average relaxation time is estimated at around 2.3 ps. Our figures of merit are similar to those reported for ultrathin metal films, such as gold, showing that an abundant MXene material can replace noble metals. We demonstrate that the MXene conductivity mechanism does not change from direct current to THz. The conventional method of reporting EMI SE is correlated with absolute values of transmitted, reflected, and absorbed power, which allows us to interpret previous results on MXene EMI shielding. Considering the easy deposition of thin MXenes films from solution onto a variety of surfaces, our findings offer an attractive alternative for shielding microscale devices and personal electronics.

Unraveling the Role of Metal Vacancy Sites and Doped Nitrogen in Enhancing Pseudocapacitance Performance of Defective MXene.

Nitrogen-doped titanium carbides (MXene) films exhibit extraordinary volumetric capacitance when high-concentration sulfuric acid electrolyte is utilized owing to the enhancement of pseudocapacitance. However, the energy storage mechanism of nitrogen-doped MXene is unclear due to the complex electrode structure and electrolyte ions' behavior. Here, based on pristine MXene (Ti3C2O2), three different MXene structures are constructed by introducing metal vacancy sites and doped nitrogen atoms, namely, defective MXene (Ti2.9C2O2), nitrogen-doped MXene (Ti3C2O1.9N0.1), and nitrogen-doped MXene with metal vacancy sites (Ti2.9C2O1.9N0.1). Then, the density functional theory (DFT)-based calculations coupled with the effective screening medium reference interaction site method (ESM-RISM) are applied to reveal the electrochemical behavior at the electrode/electrolyte interfacial area. Through analyzing the electronic structure, electrical double-layer capacitance (EDLC), and equilibrium potential of the pseudocapacitance reaction, the specific effect of structural changes on their performance can be clarified: metal vacancy sites can reduce the potential difference of gap layer (Outer Helmholtz plane) at charged state and increase the electronic capacity of Ti, which can be used to explain the high pseudocapacitance, low charge transfer resistance and high-rate capacity properties of nitrogen-doped MXene observed in experiments.

Multiresponsive MXene Actuators with Asymmetric Quantum‐Confined Superfluidic Structures

AbstractMXene, which is known for its high electrical/thermal conductivity, surface hydrophilicity, excellent mechanical flexibility, and chemical stability, is a versatile and smart material for soft actuators. However, most MXene actuators are fabricated by combining MXene with other inert materials to form a bilayer or multilayer structure. Considering the strain mismatch at multimaterial interfaces under frequent deformation, MXene‐based actuators are generally associated with poor stability, which limits their practical applications. Herein, inspired by the natural quantum‐confined superfluidic (QSF) effect, a multiresponsive MXene actuator that can be driven by moisture, light, and electricity by engineering an asymmetric QSF structure on both sides of the MXene film is reported. The actuation mechanism of the MXene film can be attributed to nonuniform water adsorption, transport, and desorption within the asymmetric QSF channels under moisture, photothermal, and electrothermal stimuli. Interestingly, MXene actuators can be flexibly formed into various shapes under moisture‐assisted mechanical compression, which not only enhances their multiresponsive actuation, but also permits a more complex deformation. As proof‐of‐concept demonstrations, various intriguing applications including a dual‐role robot, a smart shielding curtain, and a dragonfly robot, are fabricated, revealing the potential of MXene actuators for soft robotics.

All-in-one integrated flexible PE@PET/MXene films for high-performance electromagnetic shields with self-reinforced conductivity and mechanical properties

MXene-based composites are deemed as promising electromagnetic interference (EMI) shielding candidates to address the rapidly increasing electromagnetic interference and radiation concerns. However, developing high-performance flexible polymer-MXene composites with excellent EMI shielding efficiency (EMI SE), good mechanical properties and superior serving stability is a significant challenge. Herein, a type of all-in-one designed lightweight, thin, ultra-flexible PE@PET/MXene films are developed using a novel PE@PET nonwoven fabric (NWF) as the supporter. A mussel-inspired polydopamine modification facilitates the uniform loading of MXene, achieving high electrical conductivity of the resultant PE@PET/MXene films. A simple hot-pressing process is then employed to melt the sheath PE component of PE@PET to serve as a “self-glue” in adhering and wrapping the loaded MXene nanosheets, while maintaining the core PET fibers as the internal “steel” skeleton. As such, the resultant PE@PET/MXene films exhibit multiple self-enhancement metrics involving high mechanical strength, good electrical conductivity, robust serving stability, and superior EMI shielding performance. An optimized PE@PET/MXene film can preserve an excellent EMI SE of 50.44 dB and an ultra-high absolute shielding effectiveness (SSE/t) of 10606 dB cm2 g−1. The versatility of PE@PET/MXene EMI shielding films is further indicated by integrating the flame-retardant additives to endow its flame-retardant function, demonstrating excellent versability and expandability.

First Observation of Induced Ferroelectric and Magnetoelectric Properties in Pristine and Ni‐Intercalated Mo2TiC2Tx Double Transition Metal MXene

AbstractIn recent years, multiferroic (MF) materials have attained remarkable attention because of their exceptional ability to demonstrate both ferroelectric (FE) and magnetic properties simultaneously which has lead to their applications in memory, sensors, and energy storage devices. Here, a simple synthesis approach is used to enhance the ferroelectricity and induce the multiferroicity in double transition metal carbide (DTM) MXene film. The successful synthesis is confirmed by X‐ray Diffraction (XRD), Raman Spectroscopy, and Scanning Electron Microscopy (SEM) analysis. In addition, the FE and magnetic hysteresis (MH) loops are conducted to observe the nature of synthesized materials. Furthermore, the magnetoelectric (ME) effect is also examined to explore its significance in data storage and ME signal generation devices. Thus, the findings reveal the first report on the co‐existence of FE and magnetic properties of DTM MXene for potential applications in nano‐electronic devices of future technology.

Implementation of High-Capacity 3D Ti3C2TX MXene Supercapacitors with Terminal Group Modification.

MXene is a highly latent capacity electrode material for supercapacitors, but its capacity limits its development. Herein, we have constructed an independently cross-linked three-dimensional (3D) Ti3C2TX MXene film (Zn-A-MXene) with a hydroxylation surface through a zinc ion (Zn2+) and NaOH. The alkalization of NaOH is used to replace the -F functional group that is not conducive to electrochemical reactions and cross-link the MXene nanosheets through the electrostatic interaction of zinc ions. The synergistic effect can greatly improve the effective area of the electrode, the accessibility of the electrolyte, and the specific capacitance. The 3D Zn-A-MXene films exhibit an extremely high capacity (465.1 F g-1 at 1 A g-1). The all-solid-state flexible supercapacitor assembled using a 3D Zn-A-MXene thin film also has a high energy density of 9.55 Wh kg at a power density of 603.16 W kg. After 5000 cycles, the flexible supercapacitor still has 81.25% of its initial capacity, demonstrating good cycling stability. This work furnishes the innovative idea for constructing high-capacity MXene flexible supercapacitors.

An Ultra-Thin MXene Film for Multimodal Sensing of Neuroelectrical Signals with Artifacts Removal.

Neuroelectrical signals transmitted onto the skin tend to decay to an extremely weak level, making them highly susceptible to interference from the environment and body movement. Meanwhile, for comprehensively understanding cognitive nerve conduction, multimodal sensing of neural signals, such as magnetic resonance imaging (MRI) and functional near-infrared spectroscopy (fNIRS), is highly required. Previous metal or polymer conductors cannot either provide a seamless on-skin feature for accurate sensing of neuroelectrical signals or be compatible with multimodal imaging techniques without opto- and magnet- artifacts. Herein, a ≈20nm thick MXene film that is able to simultaneously detect electrophysiological signals and perform imaging by MRI and fNIRS with high fidelity is reported. The ultrathin film is made of crosslinked Ti3 C2 Tx film via poly (3,4-ethylene dioxythiophene): polystyrene sulfonate (PEDOT:PSS), showing a record high electroconductivity and transparency combination (11000Scm-1 @89%). Among them, PEDOT: PSS not only plays a cross-linking role to stabilize MXene film but also shortens the interlayer distance for effective charge transfer and high transparency. Thus, it can achieve a low interfacial impedance with skin or neural surfaces for accurate recording of electrophysiological signals with low motion artifacts. Besides, the high transparency originating from the ultrathin feature leads to good compatibility with fNIRS and MRI without optical and magnetic artifacts, enabling multimodal cognitive neural monitoring during prolonged use.

NH3-Induced In Situ Etching Strategy Derived 3D-Interconnected Porous MXene/Carbon Dots Films for High Performance Flexible Supercapacitors

HighlightsPorous MXene (p-MC) films with 3D-interconnected ion transmission paths are constructed by NH3-induced in situ etching strategy.The flexible p-MC film electrodes exhibit a superior capacitance of 688.9 F g−1 with a good rate capability.The as-assembled p-MC solid-state flexible supercapacitors reveal superior electrochemical performance (99.7 F g−1 at 1 A g−1) and excellent flexibility.

Heterogeneous Ti3C2Tx MXene-MWCNT@MoS2 Film for Enhanced Long-Term Electromagnetic Interference Shielding in the Moisture Environment.

MXene, as a novel two-dimensional (2D) material, has unique inherent features such as lightweight, flexibility, high electrical conductivity, customizable surface chemistry, and facile solution processability. However, utilizing MXene (Ti3C2Tx) films for long-term electromagnetic interference (EMI) shielding poses challenges, as they are susceptible to chemical deterioration through oxidation into TiO2. In this work, an ultrathin heterogeneous film of Ti3C2Tx MXene integrated with multiwalled carbon nanotubes supporting MoS2 clusters (MXene/MWCNT@MoS2) was developed. The heterogeneous film with 15 wt % of MWCNT@MoS2 clusters exhibited improved EMI shielding performance such as the highest EMI shielding effectiveness of 50 dB and the specific shielding effectiveness of 20,355 dB cm2 g -1, mainly attributed to the excellent electrical conductivity, distinctive porous structure, and multiple interfacial interactions. The heterogeneous films underwent extended exposure to a moisture environment (35 days), and their structural stability and EMI shielding performance were enhanced by the integration of MWCNT@MoS2 clusters. As a result, the engineered heterostructure of multilayered hybrid films holds promise as a viable option for improving the EMI shielding effectiveness and stability of Ti3C2Tx MXene.

Spray deposited Graphene@MXene on a flexible polymer substrate for Terahertz Shielding

The rapid progress in telecommunications over the past decade brought about a societal revolution. However, the proliferation of electromagnetic waves has also presented significant challenges that require our attention. In this regard, we presented a simple spray-coating method for developing a lightweight and flexible graphene-coated MXene film for terahertz shielding. The MXene/graphene layered structure, with a 3:1 wt ratio and a thickness of 1.63 μm, achieved a shielding effectiveness (SE) of 47 dB, which was remarkably similar to that of the pristine MXene with a thickness of 0.8 μm. Notably, even after one month of storage, the SE of the composite consisting of 75 wt % MXene-25 wt % Graphene remained at 94% of its initial value. In contrast, the SE was reduced to 72% of its initial value in the case of 100 wt % MXene. The hydrophobic graphene layer, acting as a barrier against water molecules, emerged as the key factor responsible for maintaining the shielding efficiency. Furthermore, theoretical modeling based on the transfer matrix method was employed to correlate the behavior of electromagnetic waves at different interfaces of thin films.

Label-Free Chemiresistive Sensors Based on Self-Assembled Ti3C2Tx MXene Films for Monitoring of Microcystin-LR in Water Samples.

Herein, we propose a label-free chemiresistive sensor for the highly sensitive and selective detection of microcystin (MC)-LR in water samples. The sensor uses a layer-by-layer (LBL) assembled conductive film consisting of Ti3C2Tx nanosheets as the sensing channel. It is further modified by using an aptamer for the specific recognition of MC-LR. The response signal is based on the change in resistance of the conductive channel upon binding of MC-LR with the aptamer. Our novel strategy is the first concept proposed for immobilizing the aptamer containing -SH on the channel surface through a Ti-S bond under weakly alkaline condition. The resulting sensor is highly sensitive and stable for the detection of MC-LR, with a detection limit of 0.18 ng L-1 and a wide linear range from 1 to 104 ng L-1. We used the sensor to continuously monitor MC-LR released by cultivated Microcystis aeruginosa, showing a strong relationship between MC-LR and cell density. Furthermore, the sensor was successfully used to measure MC-LR in freshwater lakes with moderate algal blooms, and the results agreed well with those obtained by liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. The present study provides a reliable method for highly sensitive and selective detection of MC-LR in environmental waters.

Transport channel engineering between MXene interlayers for Zn‐ion hybrid microsupercapacitor with enhanced energy output and cycle stability

AbstractTwo‐dimensional (2D) transition metal carbonitrides/nitrides (MXene) materials have proven to be promising alternatives as novel capacitor‐type electrodes for aqueous Zn‐ion hybrid microsupercapacitors (ZHMSCs). However, during self‐assembly processes, serious restacking between 2D MXene nanosheets induced by strong van der Waals forces makes ion transport channels narrow within the compact MXene film electrodes, which would result in poor energy output of ZHMSCs. Herein, interlayer transport channel engineering is designed by intercalating bacterial cellulose (BC) between MXene interlayers to develop MXene/BC electrodes with fast ion transport channels in contrast to pure MXene electrodes. Benefiting from fast anion intercalation/deintercalation on MXene/BC capacitor‐type cathode and reversible Zn stripping/plating on Zn foil anode, the fabricated ZHMSCs exhibit wide working potential windows (1.36 V), high areal capacitance (404 mF cm−2), and landmark areal energy density (94 µWh cm−2 at 1 mA cm−2). The areal capacitance and energy density of the developed ZHMSCs are much higher than those of the ZHMSCs based on pure MXene capacitor‐type cathode (239 mF cm−2/57 µWh cm−2 at 1 mA cm−2). Besides, the developed ZHMSCs can perform more than 10,000 cycles, showing outstanding capacity retention. In general, our work provides a novel strategy to break through the performance bottlenecks afflicting MXene‐based ZHMSCs.

Long‐Term Infrared Stealth by Sandwich‐Like Phase‐Change Composites at Elevated Temperatures via Synergistic Emissivity and Thermal Regulation

AbstractThe rapid development of infrared surveillance technologies has attracted great attention for scientists to design advanced functional materials with prominent infrared stealth and thermal camouflage effectiveness. In the current study, a sandwich‐like functional composite based on a crosslinked polyimide aerogel, a meso‐erythritol (mE)‐based phase‐change composite, and an MXene film has been developed to achieve long‐term thermal camouflage at elevated temperatures. In this composite system, the lower aerogel layer can act as a barrier to insulate heat transfer through its layer‐stacking structure under ultralow directional thermal conduction. The introduction of the middle phase‐change composite layer ensures that the composite system obtains a dynamical temperature‐regulation capability through sensible and latent heat absorption of mE as a phase change material, while the upper MXene layer provides a very low emissivity surface for the system. As a result, the developed composite achieves a significant reduction in the thermal radiation temperature of a high‐temperature target. Moreover, the MXene film exhibits good electromagnetic interference shielding effectiveness, making the sandwich‐like composite obtain a thermal camouflage capability in various complicated scenarios. This work provides a promising approach for the design of advanced functional materials to realize long‐term infrared stealth and thermal camouflage of high‐temperature targets in security protection and counter‐surveillance.

Pulsed Force Kelvin Probe Force Microscopy through Integration of Lock-In Detection.

Kelvin probe force microscopy measures surface potential and delivers insights into nanoscale electronic properties, including work function, doping levels, and localized charges. Recently developed pulsed force Kelvin probe force microscopy (PF-KPFM) provides sub-10 nm spatial resolution under ambient conditions, but its original implementation is hampered by instrument complexity and limited operational speed. Here, we introduce a solution for overcoming these two limitations: a lock-in amplifier-based PF-KPFM. Our method involves phase-synchronized switching of a field effect transistor to mediate the Coulombic force between the probe and the sample. We validate its efficacy on two-dimensional material MXene and aged perovskite photovoltaic films. Lock-in-based PF-KPFM successfully identifies the contact potential difference (CPD) of stacked flakes and finds that the CPDs of monoflake MXene are different from those of their multiflake counterparts, which are otherwise similar in value. In perovskite films, we uncover electrical degradation that remains elusive with surface topography.