Adhesion Between MXenes and Other 2D Materials

MXene-Based Nanocomposites for Supercapacitors: Fundamentals and Applications

MXene-based nanocomposite materials with other 2D materials have made a large impact in the field of energy storage, particularly in the area of supercapacitors. Combining conductive 2D MXene with other 2D materials, such as transition metal oxide, transition metal dichalcogenides, and layered double hydroxide, improves the electrochemical energy storage properties of resulting MXene-based nanocomposites. The interface of MXene and 2D nanocomposite materials allows an improved electrochemical performance for energy storage applications. In this review, state-of-the-art research progress in 2D/2D MXene-based nanocomposite synthesis, structural and morphological properties, and electrochemical performance for supercapacitors is explored. 2D MXene nanocomposites electrochemical properties in terms of specific capacitance, energy, power densities, and stability are discussed. This study shows that this rapidly developing field has an important impact on the next-generation supercapacitor.

MXene-Coated Liquid Metal Nanodroplet Aggregates

Combining droplets of liquid metal (LM) with nanomaterials often introduces synergistic thermal or electrical properties that are not found in the constituent materials alone. However, in these existing systems, LM droplets maintain a statistically uniform dispersion and are not capable of self-assembly or aggregation. These composites are limited by their need for high volume fractions of LM (>60 vol %) to achieve high thermal properties, introducing LM leaking as a drawback for thermal management and wearable electronic applications. In this work, we show that coating nanoscale droplets of eutectic gallium-indium (EGaIn) LM with small volume fractions of Ti3C2Tx MXenes (0.25 vol %) results in a unique LM morphology in which droplets self-assemble to form semisolid aggregates. This is accomplished by wrapping MXene sheets around individual LM droplets to create "sticky" particles that form self-assembled aggregates when mixed with a silicone oil. By introducing aggregation as a design parameter in soft LM composites, the thermal and electric resistance of the composite is shown to change dramatically. In contrast to silicone-based composites containing LM droplets or MXene nanosheets alone, these MXene-LM-silicone-based composites exhibit an exponential increase in thermal and electrical conductivity with decreasing interfacial thickness with significantly lower LM volume fractions (25 vol %) while avoiding LM rupture and bleed-out. This could enable more effective composites, reducing the amount of filler material required for thermal interface materials (TIM) and printed electronics.

A first-principles study of multilayer Ti3C2Tx MXene model

We proposed a more realistic albeit slightly complicated multilayer Ti3C2Tx model and performed a comprehensive theoretical study of its structural and electronic properties. In this work, we constructed various multilayer Ti3C2Tx structures considering different concentrations of hydrofluoric acid (HF; 5, 10, and 48 wt%) as the etchant. The validity of our ternary mixed O/OH/F-terminated Ti3C2Tx multilayer models is confirmed by the consistency of the calculated d-spacing (9.60 ± 0.07 Å), simulated X-ray diffraction (XRD) spectra and the predicted adhesion energy (0.77 ± 0.15 J m-2) with the reported experimental measurements. The uniform terminated and mixed terminated multilayer Ti3C2Tx exhibit metallic characteristics, similar to those of monolayer Ti3C2Tx. We found a stronger interaction between the interlayers with OH-rich ternary mixed terminated Ti3C2Tx surfaces, due to the formation of hydrogen bonds between the hydroxyl groups and adjacent layers of F/O terminal groups as supported by the crystal orbital Hamilton population (COHP) calculation. From this finding, we propose that multilayer Ti3C2Tx etched with a strong HF acid could be easily exfoliated into monolayer sheets due to smaller adhesion energy. Based on this work, we believe that the current findings will offer a fundamental understanding and a useful baseline multilayer model for the future investigation of the hydrogen and ion storage and diffusion properties in the MXene multilayer application.

Revealing the Interlayer Interaction Forces in 2D Graphene Materials by Graphene-Wrapped Nanoprobe

Understanding the interlayer interaction between 2D layered structures is critical for the construction of various micro- and nanoscale functional devices. However, both the normal and the tangential interlayer interactions between 2D layered materials have rarely been studied simultaneously. In this work, an immersion and lift-up method is proposed to wrap a layer of graphene flakes onto a plasma-pretreated atomic force microscopy (AFM) nanoprobe for the measurements of interaction forces by AFM. The normal interactions (adhesion force and adhesion energy) and tangential interactions (friction force) between two different probes (Pt-coated probe and graphene-wrapped probe) and two different 2D graphene materials [graphene and graphene oxide (GO)] were systematically measured, respectively. The adhesion energies of Pt-GO, Pt-graphene, graphene-GO, and graphene-graphene were measured to be 0.72 ± 0.05, 0.41 ± 0.03, 0.19 ± 0.02, and 0.10 ± 0.02 J m-2, respectively. The graphene-graphene contact pair showed the lowest adhesion force (5.57 ± 1.03 nN) and adhesion energy (0.10 ± 0.02 J m-2), which was attributed to the strong covalent bonds and charge density distribution. The friction coefficients of Pt-GO, graphene-GO, Pt-graphene, and graphene-graphene were determined to be 0.38, 0.14, 0.054, and 0.013. The graphene-graphene tribo-pair exhibited a superlow friction state for a long time, which was attributed to incommensurate contact and weak van der Waals interactions. These findings provide a technical route to reveal the interlayer interactions of various 2D layered materials, which can be widely applied in microelectromechanical systems.

Stabilizing Ti3C2Tx MXene flakes in air by removing confined water

MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti[Formula: see text]C[Formula: see text]T[Formula: see text] MXene monoflakes have exceptional thermal stability at temperatures up to 600[Formula: see text]C in air, while multiflakes readily oxidize in air at 300[Formula: see text]C. Density functional theory calculations indicate that confined water between Ti[Formula: see text]C[Formula: see text]T[Formula: see text] flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti[Formula: see text]C[Formula: see text]T[Formula: see text] films at 600[Formula: see text]C, resulting in substantial stability improvement in multiflake films (can withstand 600[Formula: see text]C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti[Formula: see text]C[Formula: see text]T[Formula: see text] oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability.

Construction of Monolayer Ti3C2Tx MXene on Nickel Foam under High Electrostatic Fields for High-Performance Supercapacitors

Ti3C2Tx MXene, as a common two-dimensional material, has a wide range of applications in electrochemical energy storage. However, the surface forces of few-layer or monolayer Ti3C2Tx MXene lead to easy agglomeration, which hinders the demonstration of its performance due to the characteristics of layered materials. Herein, we report a facile method for preparing monolayer Ti3C2Tx MXene on nickel foam to achieve a self-supporting structure for supercapacitor electrodes under high electrostatic fields. Moreover, the specific capacitance varies with the deposition of different-concentration monolayer Ti3C2Tx MXene on nickel foam. As a result, Ti3C2Tx/NF has a high specific capacitance of 319 mF cm-2 at 2 mA cm-2 and an excellent long-term cycling stability of 94.4% after 7000 cycles. It was observed that the areal specific capacitance increases, whereas the mass specific capacitance decreases with the increasing loading mass. Attributable to the effect of the high electrostatic field, the self-supporting structure of the Ti3C2Tx/NF becomes denser as the concentration of the monolayer Ti3C2Tx MXene ink increases, ultimately affecting its electrochemical performance. This work provides a simple way to overcome the agglomeration problem of few-layer or monolayer MXene, then form a self-supporting electrode exhibiting excellent electrochemical performance.

Atomic Force Microscopy beyond Topography: Chemical Sensing of 2D Material Surfaces through Adhesion Measurements

Developing new functionalities of two-dimensional materials (2Dms) can be achieved by their chemical modification with a broad spectrum of molecules. This functionalization is commonly studied by using spectroscopies such as Raman, IR, or XPS, but the detection limit is a common problem. In addition, these methods lack detailed spatial resolution and cannot provide information about the homogeneity of the coating. Atomic force microscopy (AFM), on the other hand, allows the study of 2Dms on the nanoscale with excellent lateral resolution. AFM has been extensively used for topographic analysis; however, it is also a powerful tool for evaluating other properties far beyond topography such as mechanical ones. Therefore, herein, we show how AFM adhesion mapping of transition metal chalcogenide 2Dms (i.e., MnPS3 and MoS2) permits a close inspection of the surface chemical properties. Moreover, the analysis of adhesion as relative values allows a simple and robust strategy to distinguish between bare and functionalized layers and significantly improves the reproducibility between measurements. Remarkably, it is also confirmed by statistical analysis that adhesion values do not depend on the thickness of the layers, proving that they are related only to the most superficial part of the materials. In addition, we have implemented an unsupervised classification method using k-means clustering, an artificial intelligence-based algorithm, to automatically classify samples based on adhesion values. These results demonstrate the potential of simple adhesion AFM measurements to inspect the chemical nature of 2Dms and may have implications for the broad scientific community working in the field.

Button shear testing for adhesion measurements of 2D materials

Two-dimensional (2D) materials are considered for numerous applications in microelectronics, although several challenges remain when integrating them into functional devices. Weak adhesion is one of them, caused by their chemical inertness. Quantifying the adhesion of 2D materials on three-dimensional surfaces is, therefore, an essential step toward reliable 2D device integration. To this end, button shear testing is proposed and demonstrated as a method for evaluating the adhesion of 2D materials with the examples of graphene, hexagonal boron nitride (hBN), molybdenum disulfide, and tungsten diselenide on silicon dioxide and silicon nitride substrates. We propose a fabrication process flow for polymer buttons on the 2D materials and establish suitable button dimensions and testing shear speeds. We show with our quantitative data that low substrate roughness and oxygen plasma treatments on the substrates before 2D material transfer result in higher shear strengths. Thermal annealing increases the adhesion of hBN on silicon dioxide and correlates with the thermal interface resistance between these materials. This establishes button shear testing as a reliable and repeatable method for quantifying the adhesion of 2D materials.

Advancements in MXene-based composites for electronic skins

MXenes are a class of two-dimensional (2D) materials that have gained significant attention in the field of electronic skins (E-skins). MXene-based composites offer several advantages for E-skins, including high electrical conductivity, mechanical flexibility, transparency, and chemical stability. Their mechanical flexibility allows for conformal integration onto various surfaces, enabling the creation of E-skins that can closely mimic human skin. In addition, their high surface area facilitates enhanced sensitivity and responsiveness to external stimuli, making them ideal for sensing applications. Notably, MXene-based composites can be integrated into E-skins to create sensors that can detect various stimuli, such as temperature, pressure, strain, and humidity. These sensors can be used for a wide range of applications, including health monitoring, robotics, and human-machine interfaces. However, challenges related to scalability, integration, and biocompatibility need to be addressed. One important challenge is achieving long-term stability under harsh conditions such as high humidity. MXenes are susceptible to oxidation, which can degrade their electrical and mechanical properties over time. Another crucial challenge is the scalability of MXene synthesis, as large-scale production methods need to be developed to meet the demand for commercial applications. Notably, the integration of MXenes with other components, such as energy storage devices or flexible electronics, requires further developments to ensure compatibility and optimize overall performance. By addressing issues related to material stability, mechanical flexibility, scalability, sensing performance, and power supply, MXene-based E-skins can develop the fields of healthcare monitoring/diagnostics, prosthetics, motion monitoring, wearable electronics, and human-robot interactions. The integration of MXenes with emerging technologies, such as artificial intelligence or internet of things, can unlock new functionalities and applications for E-skins, ranging from healthcare monitoring to virtual reality interfaces. This review aims to examine the challenges, advantages, and limitations of MXenes and their composites in E-skins, while also exploring the future prospects and potential advancements in this field.

Fast and high-resolution mapping of van der Waals forces of 2D materials interfaces with bimodal AFM

High-spatial resolution mapping of van der Waals forces is relevant in several fields ranging from nanotechnology to colloidal science. The emergence of two-dimensional heterostructures assembled by van der Waals interactions has enhanced the interest of those measurements. Several AFM methods have been developed to measure the adhesion force between an AFM probe and the material of interest. However, a reliable and high-resolution method to measure the Hamaker constant remains elusive. We demonstrate that an atomic force microscope operated in a bimodal configuration enables fast, quantitative, and high-resolution mapping of the Hamaker constant of interfaces. The method is applied to map the Hamaker constant of monolayer, bilayer and multilayer MoS2 surfaces. Those interfaces are characterized with Hamaker constant and spatial resolutions of, respectively, 0.1 eV and 50 nm.

3D Assembly of MXene Networks using a Ceramic Backbone with Controlled Porosity

Transition metal carbides (MXenes) are novel 2D nanomaterials with exceptional properties, promising significant impact in applications such as energy storage, catalysis, and energy conversion. A major barrier preventing the widespread use of MXenes is the lack of methods for assembling MXene in 3D space without significant restacking, which degrades their performance. Here, this challenge is successfully overcome by introducing a novel material system: a 3D network of MXene formed on a porous ceramic backbone. The backbone dictates the network's 3D architecture while providing mechanical strength, gas/liquid permeability, and other beneficial properties. Freeze casting is used to fabricate a silica backbone with open pores and controlled porosity. Next, capilary flow is used to infiltrate MXene into the backbone from a dispersion. The system is then dried to conformally coat the pore walls with MXene, creating an interconnected 3D-MXene network. The fabrication approach is reproducible, and the MXene-infiltrated porous silica (MX-PS) system is highly conductive (e.g., 340 S m-1 ). The electrical conductivity of MX-PS is controlled by the porosity distribution, MXene concentration, and the number of infiltration cycles. Sandwich-type supercapacitors with MX-PS electrodes are shown to produce excellent areal capacitance (7.24 F cm-2 ) and energy density (0.32 mWh cm-2 ) with only 6% added MXene mass. This approach of creating 3D architectures of 2D nanomaterials will significantly impact many engineering applications.

Controllable interface-tailored strategy to reduce the nanotribological properties of Ti3C2Txby depositing MoS2using atomic layer deposition

Ti3C2TxMXene has attracted widespread attention in lubrication owing to its unique structure and surface properties. However, the inferior nanotribological properties of Ti3C2Txstill limit its applications in nano lubricants. Herein, we propose a controllable interface-tailored strategy to reduce the nanotribological properties of Ti3C2Txby depositing MoS2nano-sheet on its surface using atomic layer deposition (ALD). The nanotribological properties of the MoS2/Ti3C2Txnanocomposites synthesized by ALD are studied by atomic force microscope for the first time. At the optimal 20 ALD MoS2cycles, the nanofriction of MoS2/Ti3C2Txhas been reduced by 57%, 46%, and 44% (at 5, 10, and 15 nN load, respectively), while the adhesion has been reduced by 59%, compared to the original Ti3C2Tx. The results can contribute to understanding of the nanotribological mechanisms of Ti3C2Txcomposites and provide the potential prospects for Ti3C2Txas a nanoscale adjustable lubricant.

Unveiling the tribological potential of MXenes-current understanding and future perspectives

Reducing energy consumption and CO2 emissions by improving the tribological performance of mechanical systems relies on the development of new lubrication concepts. Two-dimensional (2D) materials have been the subject of extensive tribological research due to their unique physical and chemical properties. 2D transition metal carbides, nitrides, and carbonitrides (MXenes), with their tuneable chemistry and structure, are a relatively new addition to the family of 2D materials. MXenes' good strength and stiffness, easy-to-shear ability, capability to form wear-resistant tribofilms, and the possibility to control their surface chemistry make them appealing candidates to be explored for tribological purposes. This review provides a comprehensive overview of MXenes' tribology, covering their structure-property relationship, synthesis approaches, deposition methods to generate MXene coatings for tribological purposes, and their fundamental tribological mechanisms. Furthermore, detailed insights into studies exploring MXenes' tribological performance from the nano- to the macro-scale are presented with special emphasis on their use as self-lubricating solid lubricants, lubricant additives, and reinforcement phases in composites.

Optically Active MXenes in Van der Waals Heterostructures

The vertical integration of distinct 2D materials in van der Waals (vdW) heterostructures provides the opportunity for interface engineering and modulation of electronic as well as optical properties. However, scarce experimental studies reveal many challenges for vdW heterostructures, hampering the fine-tuning of their electronic and optical functionalities. Optically active MXenes, the most recent member of the 2D family, with excellent hydrophilicity, rich surface chemistry, and intriguing optical properties, are a novel 2D platform for optoelectronics applications. Coupling MXenes with various 2D materials into vdW heterostructures can open new avenues for the exploration of physical phenomena of novel quantum-confined nanostructures and devices. Therefore, the fundamental basis and recent findings in vertical vdW heterostructures composed of MXenes as a primary component and other 2D materials as secondary components are examined. Their robust designs and synthesis approaches that can push the boundaries of light-harvesting, transition, and utilization are discussed, since MXenes provide a unique playground for pursuing an extraordinary optical response or unusual light conversion features/functionalities. The recent findings are finally summarized, and a perspective for the future development of next-generation vdW multifunctional materials enriched by MXenes is provided.

Can MXene be the Effective Nanomaterial Family for the Membrane and Adsorption Technologies to Reach a Sustainable Green World?

Environmental pollution has intensified and accelerated due to a steady increase in the number of industries, and exploring methods to remove hazardous contaminants, which can be typically divided into inorganic and organic compounds, have become inevitable. Therefore, the development of efficacious technology for the separation processes is of paramount importance to ensure the environmental remediation. Membrane and adsorption technologies garnered attention, especially with the use of novel and high performing nanomaterials, which provide a target-specific solution. Specifically, widespread use of MXene nanomaterials in membrane and adsorption technologies has emerged due to their intriguing characteristics, combined with outstanding separation performance. In this review, we demonstrated the intrinsic properties of the MXene family for several separation applications, namely, gas separation, solvent dehydration, dye removal, separation of oil-in-water emulsions, heavy metal ion removal, removal of radionuclides, desalination, and other prominent separation applications. We highlighted the recent advancements used to tune separation potential of the MXene family such as the manipulation of surface chemistry, delamination or intercalation methods, and fabrication of composite or nanocomposite materials. Moreover, we focused on the aspects of stability, fouling, regenerability, and swelling, which deserve special attention when the MXene family is implemented in membrane and adsorption-based separation applications.

Dissipation Mechanisms and Superlubricity in Solid Lubrication by Wet-Transferred Solution-Processed Graphene Flakes: Implications for Micro Electromechanical Devices

Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS₂ and WS₂. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.

Perspectives of 2D MXene Tribology

The large and rapidly growing family of 2D early transition metal carbides, nitrides, and carbonitrides (MXenes) raises significant interest in the materials science and chemistry of materials communities. Discovered a little more than a decade ago, MXenes have already demonstrated outstanding potential in various applications ranging from energy storage to biology and medicine. The past two years have witnessed increased experimental and theoretical efforts toward studying MXenes' mechanical and tribological properties when used as lubricant additives, reinforcement phases in composites, or solid lubricant coatings. Although research on the understanding of the friction and wear performance of MXenes under dry and lubricated conditions is still in its early stages, it has experienced rapid growth due to the excellent mechanical properties and chemical reactivities offered by MXenes that make them adaptable to being combined with other materials, thus boosting their tribological performance. In this perspective, the most promising results in the area of MXene tribology are summarized, future important problems to be pursued further are outlined, and methodological recommendations that could be useful for experts as well as newcomers to MXenes research, in particular, to the emerging area of MXene tribology, are provided.

Nanoscale MXene Interlayer and Substrate Adhesion for Lubrication: A Density Functional Theory Study

Understanding the interlayer interaction at the nanoscale in two-dimensional (2D) transition metal carbides and nitrides (MXenes) is important to improve their exfoliation/delamination process and application in (nano)-tribology. The layer-substrate interaction is also essential in (nano)-tribology as effective solid lubricants should be resistant against peeling-off during rubbing. Previous computational studies considered MXenes' interlayer coupling with oversimplified, homogeneous terminations while neglecting the interaction with underlying substrates. In our study, Ti-based MXenes with both homogeneous and mixed terminations are modeled using density functional theory (DFT). An ad hoc modified dispersion correction scheme is used, capable of reproducing the results obtained from a higher level of theory. The nature of the interlayer interactions, comprising van der Waals, dipole-dipole, and hydrogen bonding, is discussed along with the effects of MXene sheet's thickness and C/N ratio. Our results demonstrate that terminations play a major role in regulating MXenes' interlayer and substrate adhesion to iron and iron oxide and, therefore, lubrication, which is also affected by an external load. Using graphene and MoS₂ as established references, we verify that MXenes' tribological performance as solid lubricants can be significantly improved by avoiding -OH and -F terminations, which can be done by controlling terminations via post-synthesis processing.

High-Performance NixCo3-xO4/Ti3C2Tx-HT Interfacial Nanohybrid for Electrochemical Overall Water Splitting

This study highlights the facet structure control of regular Ni_xCo_3-xO₄ nanoplates and interfacial modulation through elemental doping and morphologically fitted assembly of Ti₃C₂T_x nanosheets for high performances in OER/HER and overall water splitting. Over the resulting Ni_0.09Co_2.91O₄/Ti₃C₂T_x-HT in a solution of 1 M KOH, the OER and HER overpotentials of 262 and 210 mV, respectively, are achievable at a current density of 10 mA cm^-2. In the case of the overall water splitting by using Ni_0.09Co_2.91O₄/Ti₃C₂T_x-HT as anode and cathode catalysts, only a potential of 1.66 V is needed to obtain a current density of 10 mA cm^-2, and the catalysts can stand for a period of 70 h, remarkably outperforming the RuO₂-Pt/C-based catalyst and benefiting from the intensive association and interfacial function between the Ti₃C₂T_x and Ni_xCo_3-xO₄ nanosheets. Interestingly, a surface reconstruction from the (112) to (111) facet structure occurred upon the fine-tuned Ni doping of regular Ni_xCo_3-xO₄ hexagonal nanoplates and led to a highly active catalyst surface. At x = 0.09, the amount of Ni³⁺ becomes the highest, which is favorable for the generation of the critical OH intermediates on Ni_xCo_3-xO₄/Ti₃C₂T_x-HT. The current study documented the significance of the well-controlled interfacial assembly of transition-metal oxide/MXenes as an effective electrocatalyst in the OER/HER and overall water splitting processes and provided the insights into the structure-performance correlation over such kinds of precious metal-free catalysts.