Two-dimensional Metal Carbides Research Articles

Potassium Ion-Assisted Self-Assembled MXene-K-CNT Composite as High-Quality Sulfur-Loaded Hosts for Lithium-Sulfur Batteries.

We successfully synthesized hybrid MXene-K-CNT composites composed of alkalized two-dimensional (2D) metal carbide and carbon nanotubes (CNTs), which were employed as host materials for lithium-sulfur (Li-S) battery cathodes. The unique three-dimensional (3D) intercalated structure through electrostatic interactions by K+ ions in conjunction with the scaffolding effect provided by CNTs effectively inhibited the self-stacking of MXene nanosheets, resulting in an enhanced specific surface area (SSA) and ion transport capability. Moreover, the addition of CNTs and in situ-grown TiO2 considerably improved the conductivity of the cathode material. K+ ion etching created a more hierarchical porous structure in MXene, which further enhanced the SSA. The 3D framework effectively confined S embedded between nanosheet layers and suppressed volume changes of the cathode composite during charging/discharging processes. This combination of CNTs and alkalized nanosheets functioned as a physical and chemical dual adsorption system for lithium polysulfides (LiPSs). When subjected to a high current at 1.0C, S@MXene-K-0.5CNT with S-loaded of 1.2 mg cm-2 had an initial capacity of 919.6 mAh g-1 and capacity decay rate of merely 0.052% per cycle after 1000 cycles. Moreover, S@MXene-K-0.5CNT maintained good cycling stability even at a high current of up to 5.0C. These impressive results highlight the potential of alkalized 2D MXene nanosheets intercalated with CNTs as highly promising cathode materials for Li-S batteries. The study findings also have prospects for the development of next-generation Li-S batteries with high energy density and prolonged lifespans.

Synergistically coupling of ternary hydrotalcite and Ti3C2Tx-MXene nanosheets boosting electronic transmission in energy storage

Layered double hydroxides (LDH) and two-dimensional metal carbides (Ti3C2Tx) garner significant attention in recent years. LDH/Ti3C2Tx nanohybrids combine the unique characteristics of individual component, making them a promising option for high-power energy storage applications. Here, we present a composite electrode constructed by synergistic coupling of Ti3C2Tx and ternary hydrotalcite (NiCoMn-LDH). Strong interfacial interactions between the NiCoMn-LDH array and Ti3C2Tx nanoflakes, along with effective electron coupling, contribute to enhanced structural stability, conductivity, and electrolyte accessibility, thereby significantly promoting the kinetics of redox reactions. The intriguing “lace-like” structure fully utilizes the high activity of NiCoMn-LDH and the excellent conductivity of Ti3C2Tx, allowing LDH/Ti3C2Tx to exhibit an impressive output capacitance of 1102.9 F g−1 at 1 A g−1, coupled with notable rate capability (66.87 % at 20 A g−1). The constructed LDH/Ti3C2Tx//AC hybrid supercapacitor (HSC) achieves a remarkable energy density of 47.3 Wh kg−1 at 800 W kg−1. Following 5000 cycles, the specific capacity of HSC decreases by merely 8.3 %, while achieving a Coulombic efficiency of 99 %, showcasing remarkable durability and reversible reaction characteristics. This study provides a streamlined and effective methodology for crafting LDH/Ti3C2Tx nanohybrids, showcasing robust coupled interfaces and exceptional electrochemical energy storage application.

Antibacterial MXene composite with excellent U/V selectivity for uranium extraction

Uranium, as a radioactive element, is the source of utilizable energy for mankind in the future. Large quantities of uranium resources are contained in seawater, so uranium extraction from seawater is undoubtedly the most feasible method. However, the presence of competitive ions such as vanadium hinders the efficient extraction of uranium, which is currently a challenge that needs to be solved. Here, silane coupling agent (KH550) was grafted onto two-dimensional metal carbide (MXene). Subsequently, salicylaldoxime (SA) was deposited onto MXene particles using self-polymerization of dopamine (DA) to prepare PDA/SA deposited MXene composite grafted with KH550 (PDA/SA/MXene-KH550). The adsorbent exhibited excellent selectivity for uranium ions with a uranium/vanadium selectivity ratio of 4.8. In antibacterial experiments, the PDA/SA/MXene-KH550 composite showed excellent antibacterial performance against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), demonstrating the ability to adapt to complex marine microbial contamination. In addition, the maximum experimental uranium adsorption capacity of the composite was 332.3 mg/g, following the pseudo-second-order kinetic model and the Langmuir isotherm model. After five adsorption-desorption cycles, the elution efficiency of PDA/SA/MXene-KH550 for uranium can still reach 74.7 %, which demonstrated that the adsorbent had superior stability and regeneration ability, providing the possibility for actual enrichment of uranium.

NIR-II-amplify high-entropy MXene-based sonosensitizer as sonodynamic therapy promotes methicillin-resistant Staphylococcus aureus–Infected wound healing

The growing pace of two-dimensional (2D) transitional metal carbides and nitrides (MXenes) has become a spotlight for recent research. Similarly, the need to close the gap left by conventional antibiotics, fighting off bacterial infections, and drug resistance has become eminent. Especially, MXenes are widely used as excellent photothermal agents. However, there are limited reports on MXenes as sonosensitizer. Herein, a high-entropy MXene (HEM), TVNMC was fabricated for the generation of reactive oxide species (ROS) against Methicillin-resistance Staphylococcus aureus (MRSA). Interestingly the energy storage and conversion from the diverse composition of the TVNMC MXenes allowed the generation of ROS under US irradiation and even enhanced under Photothermal therapy. This combinatorial therapy undertook a near-total kill and eradication of MRSA with its biofilm. Moreover, TVNMC MXene simultaneously eliminated MRSA abscesses while promoting wound healing with negligible anomaly to murine tissues under sonodynamic and photothermal therapy, making the TVNMC MXene a promising sonosensitizer for possible biomedical usage. Overall, this system provides great potential for the use of high-entropy MXenes as a new type of sonosensitizer in anti-infection therapy.

Flashlight treatment for instantaneous structuring of dense MXene film into porous MXene/TiO2 nanocomposite for lithium-ion battery anodes

MXenes, the two-dimensional metal carbides and nitrides, have been considered a new class of electrode materials with their noticeable performance in various energy storage systems. Furthermore, the unique property and morphology allow MXenes to be assembled into freestanding films, which can be applied in energy storage devices with lightweight and high energy density. However, their tendency to restack and aggregate remains a challenge to enhance the electrochemical performance of MXenes. In this study, we propose a facile method to fabricate a porous MXene-TiO2 freestanding anode with the help of a flashlight. Simply heating the MXene films with a flashlight in milliseconds increases the interlayer spacing of the MXenes and simultaneously forms TiO2 on the MXene surface. Along with the widened interlayer and formation of TiO2, improved wettability, easy electrolyte penetration, and reduced electrode resistance were achieved. As a result, this freestanding anode exhibits nearly 50 times higher Li+ storage capacity (148 mAh/g) than pristine MXene film (3 mAh/g) at 0.05 A/g and excellent cycle stability up to 1500 cycles without degradation.

Exploring the oxidation behaviors of the Ti[sbnd]V[sbnd]Cr[sbnd]Mo high-entropy MAX at 800 °C for its self-lubricity

Given the unique nanolaminate structure, the MAX phase ceramics which are composed of alternatively stacked two-dimensional (2D) metal (M) carbides (X) and main group (A) atomic layers have shown promising properties at elevated temperatures. Recently, an excellent self-lubricating performance of a high-entropy (HE) (TiVCrMo)3AlC2 MAX is demonstrated at 800 °C in the air. However, the formation mechanism of a self-lubricative tribofilm is still waiting for clarified. In this work, the HE (TiVCrMo)3AlC2 MAX is synthesized and its oxidation behaviors are systematically evaluated at 800 °C in the air. A two-stage parabolic oxidation behavior is detected, which can be mainly ascribed to the volatilization of MoO3 species coupled with the thickening of the compound oxide scale. Meanwhile, the HE composition results in chemically complex phases including Rutile and vacancy-ordered TiO species in the oxide scale, which contribute to a higher ion diffusion rate for mass transportation. Consequently, the continued volatilization of MoO3 is beneficial for the enhanced lubricity and anti-wear performance of the (TiVCrMo)3AlC2 MAX at 800 °C.

Asymmetric Supercapacitors based on 1,10-phenanthroline-5,6-dione Molecular Electrodes Paired with MXene.

An efficient approach to increase the energy density of supercapacitors is to prepare electrode materials with larger specific capacitance and increase the potential difference between the positive and negative electrodes in the device. Herein, an organic molecular electrode (OME) is prepared by anchoring 1,10-phenanthroline-5,6-dione (PD), which possesses two pyridine rings and an electron-deficient conjugated system, onto reduced graphene oxide (rGO). Because of the electron-deficient conjugated structure of PD molecule, PD/rGOs exhibit a more positive redox peak potential along with the advantages of high capacitance-controlled behaviour and fast reaction kinetics. Additionally, the small energy gap between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) leads to increased conductivity in PD/rGO. To assemble the asymmetric supercapacitor (ASC), a two-dimensional metal carbide, as known as MXene, with a chemical composition of Ti3C2Tx is selected as the negative electrode due to its exceptional performance, and PD/rGO-0.5 is employed as the positive electrode. Consequently, the working voltage is expanded up to 1.8 V. Through further electrochemical measurements, the assembled ASC (PD/rGO-0.5//Ti3C2Tx) achieves a remarkable energy density of 36.8 Wh kg-1. Remarkably, connecting two ASCs in series can power 73 LEDs, showcasing its promising potential for energy storage applications.

Graphenization of graphene oxide films for strongly anisotropic thermal conduction and high electromagnetic interference shielding

A macroscopic carbon film that has a “graphenic” structure (defect-free in-plane graphene lattice yet absence of Bernal type graphitic stacking order), may inherit the exotic properties of rotated/slipped few-layer graphene (e.g., high flexibility and anisotropic transport properties). However, the fabrication of such a film remains challenging and unreported. Here we show that the annealing of reduced electrochemical graphene oxide (REGO), which has fewer in-plane defects than the conventional graphene oxide from Hummers’ method, leads to a nearly intact graphene lattice (Raman ID/IG down to 0.03) without restoring the graphitic stacking order (I2D/IG > 1 and a single Lorentzian 2D peak). Atomic resolution transmission electron microscopy verifies the unique graphenic structure with random interlayer rotations. Density functional theory calculations imply the low defect density of REGO can suppress the interplanar interaction between undercoordinated carbon atoms, therefore inhibiting cross-plane migration of vacancies and the consequent establishment of stacking order. The annealed, graphenic REGO film is highly flexible and has an in-plane to cross-plane thermal conductivity ratio of 835 (among the highest reported for anisotropic thermal conductors). This high anisotropic ratio is attributed to the inhibited through-plane heat transport (0.26 W m−1 K−1) by the interlayer rotations and the high in-plane thermal conductivity (218 W m−1 K−1) enabled by intact graphene lattice. The graphenic films show high electromagnetic interference shielding effectiveness of 80.3 dB at a thickness of 53 μm and 42.4 dB at 5.7 μm, with an absolute effectiveness (>70000 dB cm2 g−1) superior to two-dimensional metal carbides (MXenes) and Al foil.

Two-dimensional graphitic metal carbides: structure, stability and electronic properties

Via first-principles computational modeling and calculations, we propose a new class of two-dimensional (2D) atomically thin crystals that contain metal-C3 (MC3) moieties periodically distributed in a graphenic lattice, which we refer to as 2D graphitic metal carbides (g-MCs). Most g-MCs are dynamically stable as verified by the calculated phonon spectra. Our detailed chemical bonding analyzes reveal that the high stability of g-MCs can be attributed to a unique bonding feature, which manifests as the carbon-backbone-mediated metal–metal interactions. These analyzes provide new insights for understanding the stability of 2D materials. It is found that the calculated electronic band gaps and magnetic moments (per unit cell) of g-MCs can range from 0 to 1.30 eV and 0 to 4.40 μ B, respectively. Highly tunable electronic properties imply great potential of 2D g-MCs in various applications. As an example, we show that 2D g-MnC can be an excellent electrocatalyst towards CO2 reductive reaction for the formation of formic acid with an exceptionally high loading of Mn atoms (∼43 wt%). We expect this work to simulate new experiments for fabrication and applications of g-MCs.

Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, which provides an ideal platform for both fundamental and applied studies of interfaces. Good progress has been achieved in the functionalization of MXenes with small inorganic ligands, but relatively little work has been reported on the covalent bonding of various organic groups to MXene surfaces. Here we synthesize a family of hybrid MXenes (h-MXenes) that incorporate amido- and imido-bonding between organic and inorganic parts by reacting halogen-terminated MXenes with deprotonated organic amines. The resulting hybrid structures unite tailorability of organic molecules with electronic connectivity and other properties of inorganic 2D materials. Describing the structure of h-MXene necessitates the integration of concepts from coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic constituents. h-MXenes also exhibit superior stability against hydrolysis.

Electrochemical characterization of Ti3C2Tx MXene prepared via a molten salt etching route in an acetonitrile-based electrolyte

Two-dimensional metal carbide and nitride materials (MXenes) with controlled Cl- and O-surface terminations (MS-Ti3C2Tx) were prepared by a molten salt etching route and their electrochemical properties were investigated in nitrile-based electrolytes. The results show that the power performance of MS-Ti3C2Tx was greatly improved in acetonitrile (ACN)-based non-aqueous organic electrolytes, which are conventional electrolytes for electrochemical capacitors. The addition of a lithium salt to produce 2 M LiTFSI in ACN results in an increase in the specific capacitance. Moreover, replacing LiTFSI by LiFSI drastically improves the power performance of MXenes at a high scan rate (95 mAh g−1 at 900C). These results highlight the importance of properly matching the electrolyte composition to the electrode material and shed light on MXenes as high-rate electrode materials for energy storage applications.

Ag decorated 3D honeycomb-like MXene architecture as an advanced lithium-ion anode material towards high capacity and long-term cycle capability

MXenes, a distinct class of two-dimensional metal carbides, are pursued to be promising potential for driving applications of lithium-ion storage with remarkable flexibility and conductivity. However, the electrochemical behavior of MXene-based anodes is far from reaching a satisfactory level due to severe interlayer stacking in MXenes, which immensely deteriorates the intrinsic conductivity in bulk and hampers the kinetic transport of Li+ resulting in significant capacity fading as well as inferior rate capability. Hence, we proposed an effective tactic to significantly boost the electrochemical ability by constructing a hollow MXene/Ag composite comprised of 3D honeycomb-like MXene with Ag nanoparticles. Melamine formaldehyde (MF), the sacrificial template followed by thermal decomposition, can effectively transform 2D MXene nanosheets into a 3D hollow structure. Accordingly, rational 3D honeycomb-like MXene/Ag composite exhibits impressive rate performance and enhanced cycling stability of 310 mAh/g after 2000 cycles at 3 A/g with availably suppressing anode swelling, compared to pristine 2D MXene nanosheets, evincing the potentiality as a desirable anode for excellent electrochemical behavior lithium-ion batteries (LIBs).

Synergistic adsorption of U(VI) from seawater by MXene and amidoxime mixed matrix membrane with high efficiency

Two-dimensional metal carbides (MXene) and amidoxime-based materials have attracted more and more attention in heavy metal adsorption. Herein, we prepared a mixed matrix membrane of amidoximized polyacrylonitrile with MXene particles by a simple phase separation method (M/PAO mixed matrix membrane). The M/PAO mixed matrix membrane had excellent U(VI) ion selectivity compared to vanadium. Under the synergistic adsorption of the two functional adsorption units, the maximum adsorption capacity can reach 10.31 g/m2 (364.46 mg/g) at pH = 6, while the adsorption capacity of U(VI) can reach 37.49 mg/m2 (1.3225 mg/g) in a seven-day natural seawater experiment, which was far more than that of single amidoxime-based adsorbent. Pseudo-second-order kinetics and Langmuir adsorption model were used to study the adsorption behavior. The results of various characterization methods showed the complexation mechanism between the M/PAO mixed matrix membrane and U(VI) ions. Considering its excellent adsorption capacity and facile preparation process, we believe that the M/PAO mixed matrix membrane is one of the most promising candidates for large-scale U(VI) extraction from seawater.

A realistic take on MXenes for electrochemical reduction of carbon dioxide

As part of concerted efforts in carbon capture and utilisation, electrochemical carbon dioxide reduction reaction (CO2RR) is a promising approach to realise a circular carbon economy. Two-dimensional metal carbides and nitrides (MXenes) have been touted as an attractive class of materials for CO2RR due to their tuneable electronic and surface properties, which provide a possible avenue to break linear scaling relations of intermediate binding energies that plague traditional transition metal catalysts. Despite the overwhelming number of theoretical studies with optimistic prospects for MXenes as CO2RR electrocatalysts, there remains a myriad of unresolved problems, alongside unexplored design opportunities, that require further experimental optimization to realise the promised potential of MXenes. Herein, we discuss how MXenes may break the above-mentioned scaling relations, as well as methods of MXenes modification that may improve their catalytic performance, including defect engineering and MXenes heterostructures. Lastly, we conclude by summarizing the current challenges and possible strategies to bring the potential of MXenes to fruition.

Comparative performance analysis of HTLs in MXene-assisted perovskite solar cells

To improve the efficiency of the perovskite solar cells (PSCs) and enhance their optoelectronic properties, modification of the structure of these devices may help. In this regard, two-dimensional (2D) metal carbides, nitrides, and carbonitrides called MXenes have shown desirable potential and electrochemical properties. We model the incorporation of Ti3C2TX MXene, where TX is the surface-terminating functional group, into the PSC structure using the SCAPS simulator to investigate the role of the hole transport layers in improving the efficiency and understanding the physics beyond. Moreover, variations of key parameters such as the thickness of the involved layers, defect density of the absorber layer, and doping concentrations are considered. Meanwhile, the influence of the interface layer defects on the PSC performance is discussed. Intending to optimized the structure, we proposed several substances for the hole transport layer (HTL). For an optimized device with CuI, CuSCN, Spiro-OMeTAD, and Cu2O HTLs, a power conversion efficiency (PCE) of about 20.60% is achieved.

Two-Dimensional Titanium Carbides (Ti3C2) Mxene-Based Patterned-Electrodes for High Capacity Micro-Supercapacitors

With the fast growth of wearable & miniaturized electronic devices, internet of things (IoT), and 5G telecommunications, the need for ultrathin, lightweight, and flexible energy storage devices has expanded significantly. Micro-supercapacitors (MSCs) with electrically isolated interdigitated electrodes, have attracted great interest as major power sources for micro-electronic devices, offering high power efficiency, superior rate capability, and excellent cycle durability. MXenes, two-dimensional (2D) metal carbides and nitrides, have been considered as a promising candidate for state-of-the-art energy storage applications, due to their intriguing combinational properties such as excellent electrical conductivity, large surface area, tunable surface chemistry, and pseudo-capacity. 1 In this work, we report titanium carbides (Ti3C2) MXene-based MSCs with sub-micron gap between electrodes exhibiting high areal capacitance and superior rate capability, realized by microfabrication method based on focused ion beam (FIB) and photolithography.2 We also demonstrate a facile and reliable process for scalable manufacturing of on-chip and flexible MSCs for wearable/portable and miniaturized microelectronics. The MXene MSCs fabricated utilizing the cutting-edge nanofabrication technology in an 8-inch wafer have shown an outstanding volumetric capacitance and stable cyclic performances, suggesting the developed manufacturing process can accelerate commercialization of MXene-based energy storage devices as power sources for next-generation wearable and miniaturized microelectronics.3 [References] 1 Energy Environ. Sci. 9 2847 (2016) 2 Nano Energy 81 (2021), 105616 3 Chem. Eng. J., 450, 138456 (2022)

Bioinspired Ti3 C2 Tx MXene-Based Ionic Diode Membrane for High-Efficient Osmotic Energy Conversion.

Bioinspired asymmetric nanofluidic ion channels with ionic diode behavior that can boost the osmotic energy (so-called blue energy) conversion are highly desirable, especially if they can be easily constructed and modified. Two-dimensional (2D) metal carbides and nitrides, known as MXenes, combine hydrophilic surfaces and tunable surface charge properties, providing a shortcut to prepare asymmetric nanofluidic ion channels. Here, we report a mechanically robust, flexible, and scale-up-friendly asymmetric Ti3 C2 Tx MXene-based ionic diode membrane with a highly rectified current and demonstrate its potential use in reverse electrodialysis osmotic energy conversion. Under the salinity gradient of synthetic seawater and river water, our ionic diode membrane-based generator's power density is 8.6 W m-2 and up to 17.8 W m-2 at a 500-fold salinity gradient, outperforming the state-of-the-art membranes. The design of MXene-based ionic diode-type membrane provides a facile and general strategy in developing large-scale 2D nanofluidics and selective ion transport.

Studying Plasmon Dispersion of MXene for Enhanced Electromagnetic Absorption.

2D metal carbides and nitrides (MXene) are promising candidates for electromagnetic (EM) shielding, saturable absorption, thermal therapy, and photocatalysis owing to their excellent EM absorption. The plasmon resonances in metallic MXene micro/nanostructures may play an important role in enhancing the EM absorption; however, their contribution has not been determined due to the lack of a precise understanding of its plasmon behavior. Here, the use of high-spatial-resolution electron energy-loss spectroscopy to measure the plasmon dispersion of MXene films with different thicknesses is reported, enabling accurate analysis of the EM absorption of complex MXene structures in a wide frequency range via a theoretical model. The EM absorption of MXene can be excited at the desired frequency by controlling the momentum (e.g., the sizes of the nanoflakes for EM excitation) as the strength can be enhanced by increasing the layer number and the interlayer distance in MXene. For example, a 3nm interlayer distance can nearly double the plasmon-enhanced EM absorption in MXene nanostructures. These findings can guide the design of advanced ultrathin EM absorption materials for a broad range of applications.

(Digital Presentation) Deconvoluting and Quantifying Real-Time Fluxes and Ionic Currents of Various Species Using in Situ Electrochemical Quartz Crystal Microbalance Measurements

Electrochemical quartz crystal microbalance (EQCM) is a powerful technique to screen the gravimetric response of electrochemical electrodes. In this study, a straightforward mathematical method is proposed for extracting and deconvoluting the real-time fluxes and ionic currents of two species based on the EQCM measurement results. We creatively propose the concept of Flux cyclic voltammograms (CVs) and ionic current CVs of various species and apply them to the real-time analyses of molecules/ions dynamics. For proof of concept, Ti3C2T x MXene, a most studied two-dimensional metal carbide, is investigated as a supercapacitor electrode in a 1M H2SO4 electrolyte. The H2O and H+ flux CV plots are highly symmetrical, indicating reversible inserting/deserting species fluxes. The highest fluxes along with maximum hydration numbers are obtained at the peak current potential. This suggests the significant contribution of double-layer capacitance originates from the insertion of hydrated H+. The H+ CV with the ionic current induced by H+ flux overlaps the real CV, confirming that H+ is the only interactive ion for screening the electrode charge. Lastly, we also validate the proposed strategy using Ti3C2T x MXene electrode in 1M KCl electrolyte and YP80 porous carbon electrode in 1 M LiCl electrolyte.

Insights into MXenes-based electrocatalysts for oxygen reduction

Due to the urgent need for a sustainable energy economy, fuel cells, which use clean and renewable fuels (hydrogen and methanol) for efficient power generation, have attracted more and more attention in recent years. The oxygen reduction reaction(ORR) on the cell's cathode is the core of its electrochemical conversion process, and the performance of the ORR electrocatalyst determines the overall performance and efficiency of related devices. Developing efficient, durable, and low-cost electrocatalysts for ORR has always been the goal of people's exploration. In recent years, MXenes, a new family of two-dimensional (2D) metal carbides, nitrides, and carbonitrides, have emerged and attracted great interest due to their excellent electrical conductivity and high surface area. The MXene-based materials inspire the design and preparation of electrocatalysts with increased ORR activity, high selectivity, and long load life. This review has provided an overview of MXenes, then described their synthesis methods, and finally explained the progress of MXene (mainly Ti3C2Tx) in the field of oxygen reduction.