Perspectives of 2D MXene Tribology

Functionalized MXene composites for protection on metals in electric power

Metals used in electric power suffer from icing, wear, and corrosion problems, resulting in high energy consumption, economic losses, security risks, and increased CO2 emission. To address these problems, researchers have turned to two-dimensional (2D) transition metal carbide or nitride (MXene) materials, which possess strong near-infrared adsorption, photothermal conversion, shear ability, low friction coefficient, and impermeability. These properties make MXene a promising candidate for surface protection on metals in electric power, including anti-icing, anti-wear, and anti-corrosion applications. However, the comprehensively protective ability and the promising application of MXene in electric power have not yet been reported. In this review, recent progress in MXene-based composites for anti-icing, anti-wear, and anti-corrosion in electric power is summarized to understand the protective mechanisms and the promising applications. First, the chemical and structure of MXene are briefly introduced, followed by a summary of its intrinsic properties. Next, the latest research on deicing MXene composite coatings, anti-wear MXene-based composites and coatings, and anti-corrosive MXene coatings, along with the corresponding mechanisms, is discussed. Finally, the challenges and opportunities of MXene-based composites in electric power are highlighted. This review provides guidance for understanding the comprehensively protective abilities of MXene and rationally designing MXene-based materials used in electric power.

Recent advancements on 2D nanomaterials as emerging paradigm for the adsorptive removal of microcontaminants

Water reservoirs are facing increasing prevalence of microcontaminants originating from agricultural runoff, industrial effluents, and domestic wastewater. The persistence of microcontaminants leads to disruptions in aquatic ecosystems and poses potential long-term health risks to humans, even at minimal concentrations. However, traditional wastewater treatment methods are inefficient to eliminate the microcontaminants because of their intricate chemical structures and low concentration. In this regard, nano-adsorption employing nanomaterials as adsorbents presents a viable alternative, offering enhanced efficiency and specificity towards the removal of microcontaminants. Amongst all, two-dimensional (2D) nanomaterials, including graphene oxide (GO), layered double hydroxides (LDHs), MXenes, and boron nitrides (BNs), exhibit distinctive characteristics such as a high surface area, remarkable chemical stability, and tendency of diverse surface functionalization, rendering them particularly effective in adsorbing pollutants from water. Therefore, the present review provides an exhaustive literature and comparative analysis of the aforementioned 2D nanomaterials-based adsorbents concerning their efficacy in adsorbing microcontaminants of pharmaceuticals and personal care products origin such as antibiotics, steroids, bisphenols, phthalates, parabens, and benzophenones. The different aspects of 2D adsorbents including adsorption capacity, mechanisms involved, kinetic and isotherm models followed for removal of a variety of microcontaminants have been congregated. Also, the information on recyclability, reusability, and stability of the adsorbents has been summarized to highlight their viability. Further, the limitations and future aspects related to the use of 2D nanomaterials-based adsorbents towards pollutant removal have been discussed. Overall, 2D nanomaterials holds great promise as efficient adsorbents for environmental remediation and can also be explored for industrial adsorption applications.

Advanced Solid Lubrication with COK-47: Mechanistic Insights on the Role of Water and Performance Evaluation

Metal-organic framework (MOF) nanoparticles have attracted widespread attention as lubrication additives due to their tunable structures and surface effects. However, their solid lubrication properties have been rarely explored. This work introduces the positive role of moisture in solid lubrication in the case of a newly described Ti-based MOF (COK-47) powder. COK-47 achieves an 8.5-fold friction reduction compared to AISI 304 steel-on-steel sliding under room air. In addition, COK-47 maintains a similarly low coefficient of friction (0.1-0.2) on various counterbodies, including Al2O3, ZrO2, SiC, and Si3N4. Notably, compared to other widely studied MOFs (ZIF-8, ZIF-67) and 2D materials powder (MXene, TMD, rGO), COK-47 exhibits the lowest friction (≈0.1) under the same experimental settings. Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscope, and transmission electron microscopy indicate that the tribofilm is an amorphous film obtained by hydrolysis of COK-47 in the air with moisture. Density functional theory further confirms that water catalyzes the decomposition of COK-47, a crucial step in forming the tribofilm. This study demonstrates the idea of utilizing MOF and water-assisted lubrication mechanisms. It provides new insights into MOF applications in tribology and highlights interdisciplinary contributions of mechanical engineering and chemistry.

Formation of hydrocarbons and carbon oxides in MXene reactions with water under varying oxidative conditions

Titanium carbide/carbonitride MXenes have garnered significant attention due to their remarkable properties, versatile solution processability, and broad range of potential applications. However, when exposed to the environment, MXenes are susceptible to degradation, which ultimately leads to the formation of metal oxides, a process that may be regarded as either disadvantageous or beneficial, depending on the application of a MXene and our knowledge about the underlying mechanisms. Therefore, it is very important to understand the reactivity of MXenes in different environments and conditions. Although researchers have made efforts to understand MXene degradation in air and water, our knowledge of the involved processes and even products of degradation remains incomplete. Here, we study the degradation of MXenes (Ti2CTx, Ti3C2Tx, and Ti3CNTx) under various oxidative conditions, in the presence of hydrogen peroxide, oxygen, ambient air, and argon. Gaseous products of MXene degradation in an aqueous environment were examined using gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) working in series. In addition to methane and carbon dioxide, gaseous products including higher hydrocarbons were identified and analyzed. This research further deepens our understanding of the fundamental chemistry of MXenes.

Surface Functionalization of Ti3C2Tx MXenes in Epoxy Nanocomposites: Enhancing Conductivity, EMI Shielding, Thermal Conductivity, and Mechanical Strength

MXenes have gained significant attention as multifunctional fillers in MXene-polymer nanocomposites. However, their inherently hydrophilic surfaces pose challenges in compatibility with hydrophobic polymers such as epoxy, potentially limiting composite performance. In this study, high-crystalline Ti3C2Tx MXenes were functionalized with alkylated 3,4-dihydroxy-l-phenylalanine ligands, transforming the hydrophilic MXene flakes into a more hydrophobic form, thus significantly enhancing compatibility with the epoxy matrix. This surface functionalization enabled uniform dispersion and supported the formation of a percolation network within the epoxy matrix at a low filler loading of just 0.12 vol %. Consequently, the functionalized MXene-epoxy nanocomposites exhibited remarkable performance, including an electrical conductivity of 8200 S m-1, outstanding electromagnetic interference (EMI) shielding effectiveness (SE) of 100 dB at 110 GHz (61 dB at 8.2 GHz), improved thermal conductivity of 1.37 W m-1 K-1, and a 300% increase in tensile toughness (271 KJ m-3). These properties substantially outperformed those of their nonfunctionalized counterparts and surpassed previously reported MXene-polymer nanocomposites. This study underscores the critical role of surface functionalization in unlocking the full potential of two-dimensional (2D) MXenes in polymer composites, providing a pathway to advanced multifunctional nanocomposite materials.

Self-Lubricating Tribo-Catalytic Activity of 2D High Entropy Alloy Nanoflakes

High Entropy Alloys (HEAs) have garnered attention due to their remarkable tribological attributes. Predominantly, failure mechanisms in HEAs emanate from stress-induced dislocations, culminating in crack propagation and film delamination. In this study, we report on the synthesis of 2D HEA of (MoWNbTaV)0.2S2 which facilitates shear-induced energy dissipation at sliding interfaces. The ball-on-disk tribological investigations demonstrate unprecedentedly low average coefficients of friction (0.076) and wear rates (10-9 mm3 (N∙m)-1) under high contact pressures (0.936 GPa) within ambient conditions. Employing multi-scale characterizations alongside molecular dynamic simulations, we elucidate that the presence of the HEA triggers tribocatalytic activity under high contact pressures emerging as a pivotal factor in extending lubricant lifespan during tribological tests. The resilient lubriciousness coupled with the facile spray coating methodology of (MoWNbTaV)0.2S2 in ambient environments paves the way for the development of a new class of solid lubricants based on 2D HEA.

Ti3C2TXderived layered MXenes as friction and wear reducing additives in lubricating oils: a detailed review

Friction and wear are critical aspects that significantly impact the efficiency and durability of mechanical systems. The demand for improved lubricating oils capable of reducing friction and wear has spurred the exploration of advanced additives. Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides (MXene), a new class of materials, have emerged as promising additives with exceptional tribological properties. This review paper aims to understand the usability of MXene, specifically the ones derived from Ti3C2TXas anti-friction and antiwear additives in lubricating oils. An elaborate discussion is presented about the synthesis and characterization techniques employed in the synthesis of Ti3C2TX(MXene), emphasizing their unique structural and surface properties that could contribute to their tribological performance, followed by their influence on the lubricant's tribological properties is thoroughly discussed. The underlying anti-friction and anti-wear mechanisms, their ability to form tribofilms on sliding surfaces, reduce direct metal-to-metal contact, and minimize wear are also highlighted. Additionally, the role of MXene in modifying the lubricant's chemical and physical interactions with sliding surfaces is analyzed. This review also attempts to identify and address the roadblocks hindering the use of Ti3C2TXMXene in lubricating oils, such as their aggregation tendencies, stability under extreme conditions, and potential side effects on lubricant properties along with the tentative strategies to overcome these hurdles. Relevant experimental findings in which Ti3C2TXderived 2D nano-sheets have been explored as friction and wear-reducing additives in different lubricating oils are critically assessed. Although these MXene are claimed to be highly effective as lubricant additives in lubricating oils owing to their unique properties and versatile chemistry, further research is urgently needed to address the challenges and optimize the formulation and integration of MXene into lubricating oils for practical implementation. This article comprehensively discusses Ti3C2TXMXene as friction and wear-reducing additives in lubricating oils, highlighting the pressing need for further research and the potential for future developments in this field.

Engineering the next generation of MXenes: challenges and strategies for scalable production and enhanced performance

Two-dimensional nanomaterials, such as MXenes, have garnered significant attention due to their excellent properties, including electrical conductivity, mechanical strength, and thermal stability. These properties make them promising candidates for energy storage and catalysis applications. However, several challenges impede their large-scale production and industrial application. Issues such as high production costs, safety concerns related to toxic etching agents, instability in oxidative environments, and the complex synthesis process must be addressed. In this review, we systematically analyze current methodologies for scaling up MXene production, focusing on the synthesis and etching of MAX phases, delamination strategies, and the production of MXene derivatives. We explore strategies for overcoming challenges like aggregation, oxidation, and cost, presenting optimization techniques for enhancing electrochemical performance and stability. The review also discusses the applications of MXenes in batteries and supercapacitors, emphasizing their potential for large-scale use. Finally, we provide an outlook on future research directions for MXene to develop safer and more cost-effective production methods to improve the performance of MXene in order to realize its commercial potential in energy technologies.

Emerging two dimensional MXene for corrosion protection in new energy systems: Design and mechanisms

With the development of new and clean energy (offshore wind power, fuel cells, aqueous zinc ion batteries, lithium-ion batteries, etc.), the corrosion and security problems in special environments of the new energy system have attracted much attention. Corrosion protection on the metals applied in new energy system can reduce the economic loss, security risk, and energy consumption, as well as guarantee the efficiency of energy system. Traditional coatings face challenges in agglomeration of nano fillers, structural control, environmental issues, and poor conductivity, which limits the applications. With features in controllable surface chemistry and composition, rich surface terminations, better conductivity than graphene oxide, high aspect-ratio, strong impermeability, and low friction coefficient, the two-dimensional (2D) MXene presents potential for applications in corrosion protection in new energy systems. Despite progress has been made in the MXene for corrosion protection, there is still a lack of comprehensive review regarding the design and mechanisms of anti-corrosive MXene-based materials for corrosion protection in new energy system. In this review, a brief induction of MXene and the specially four corrosive environments (offshore wind power at deep sea, bipolar plates in PEMFC environments, zinc anode in AZIBs, and current collectors in Li-ion battery) are presented. Importantly, the design strategies and mechanisms of the MXene-based anti-corrosive coatings on metals used in the special environments are discussed in detail. Finally, the challenges and research trends in the MXene-based coatings for new energy systems are prospected. This review provides further understanding of corrosion in new energy and would expand the application prospects of MXene.

Effect of the Ti2CT x (T x = O, OH, and H) Functionalization on the Formation of (TiO2)5/Ti2CT x Composites

First-principles density functional theory calculations are carried out on the (TiO2)5 cluster supported on the Ti2CT x (0001) surface with different chemical terminations, i.e., -H, -O, and -OH, to study the interaction and understand the Ti2CT x functionalization effect on the formation of (TiO2)5/Ti2CT x composites. Results show an exothermic interaction for all cases, whose strength is driven by the surface termination, promoting weaker bonds when the MXene is functionalized with H atoms. For Ti2CH2 and Ti2C(OH)2 MXenes, the interaction is accompanied by a charge transfer towards the titania cluster. All adsorptions are accompanied by a significant structural deformation of the titania nanocluster. The analysis of the density of states of (TiO2)5/Ti2CH2 and (TiO2)5/Ti2C(OH)2 composites shows a clear almost metallic character with titania-related states close to the Fermi level. However, for (TiO2)5/Ti2CO2, the band positions are similar to those of a Type-I heterojunction. Overall, the MXene surface termination influence on the TiO2/MXene interaction is unveiled, providing more stable composite formations when the MXene surface is functionalized with -H and -OH groups, where the adsorption process is accompanied by significant charge transfer.

Ti3C2T x -UHMWPE Nanocomposites-Towards an Enhanced Wear-Resistance of Biomedical Implants

There is an urgent need to enhance the mechanical and biotribological performance of polymeric materials utilized in biomedical devices such as load-bearing artificial joints, notably ultrahigh molecular weight polyethylene (UHMWPE). While two-dimensional (2D) materials like graphene, graphene oxide (GO), reduced GO, or hexagonal boron nitride (h-BN) have shown promise as reinforcement phases in polymer matrix composites (PMCs), the potential of MXenes, known for their chemical inertness, mechanical robustness, and wear-resistance, remains largely unexplored in biotribology. This study aims to address this gap by fabricating Ti3C2T x -UHMWPE nanocomposites using compression molding. Primary objectives include enhancements in mechanical properties, biocompatibility, and biotribological performance, particularly in terms of friction and wear resistance in cobalt chromium alloy pin-on-UHMWPE disk experiments lubricated by artificial synovial fluid. Thereby, no substantial changes in the indentation hardness or the elastic modulus are observed, while the analysis of the resulting wettability and surface tension as well as indirect and direct in vitro evaluation do not point towards cytotoxicity. Most importantly, Ti3C2T x -reinforced PMCs substantially reduce friction and wear by up to 19% and 44%, respectively, which was attributed to the formation of an easy-to-shear transfer film.

Macro-Superlubricity Induced by Tribocatalysis of High-Entropy Ceramics

Macroscale superlubricity has attracted considerable attention as a promising strategy to minimize frictional energy dissipation and achieve near-zero wear. However, realizing macroscale superlubricity with prolonged durability remains an immense challenge, particularly on engineering steels. Current superlubricants render steel surfaces susceptible to corrosion, causing severe wear and superlubrication failure. Herein, high-entropy ceramics (HEC) with catalytic properties are innovatively introduced to prevent corrosion of engineering steels and achieve macro-superlubricity through tribo-catalytic effect. Furthermore, this catalytically induced superlubricity system exhibits an ultra-low friction coefficient of 0.0037 under contact pressure up to 1.47 GPa, an ultra-long cycle lifetime of 1.25 × 106 cycles (corresponding sliding distance up to 5 km), and an extremely low wear rate of 3.032 × 10-10 mm3·N-1·m-1 on the HEC surface. Based on the experimental analysis and theoretical simulation, the in situ formed HEC nanocrystals reduce the Gibbs free energy of hydrolysis of PA molecules into inositol and phosphoric acid molecules in the lubricant. Notably, the hydrolysis products favorably contributed to the reduction of shear force in the lubrication system, which is essential for achieving macroscale superlubricity over a long time. This study provides a new perspective for designing robust superlubricity systems by harnessing the tribocatalytic effect of high-entropy materials.

Effects of Intercalation on ML-Ti3C2Tz MXene Properties and Friction Performance

Intercalation in two-dimensional (2D) materials can modify their physical, chemical, and electronic properties. This modification enables the tailoring of 2D material characteristics, enhancing their performance and expanding their applications in various fields. The friction performance of 2D materials such as MoS2 and graphite has a strong dependence on their interlayer spacing, and they exhibit an increase in d-spacing associated with a reduction in friction performance. The ability to control the interlayer spacing of Ti3C2Tz MXene has proven beneficial for energy storage applications such as batteries and supercapacitors, but no one has utilized this control of interlayer spacing for lubrication. In this study, we demonstrate that interlayer spacing of multilayer (ML) Ti3C2Tz MXene can be controlled through chemical intercalation and its direct effects on the electrical conductivity and friction performance. We observed a notable decrease in electrical conductivity in vacuum-filtered ML-Ti3C2Tz MXene films, which was attributed to an increased internal resistance resulting from the expansion of the interlayer gap. We also found a significant reduction in the coefficient of friction for ML-Ti3C2Tz MXene with an increased d-spacing. This reduction is attributed to a weakened attraction of individual ML-Ti3C2Tz MXene layers (intercalated). Under a tangential force, it becomes easier to slide within the larger interlayer gap with weakened van der Waals forces. This work provides insights into the tunability of MXene properties through interlayer spacing, offering potential applications requiring materials with specific electrical and friction characteristics.

In Situ Transition of Amorphous Carbon to Graphite-like Structures Using MXene as a Template for Fast and Long-Lasting Macrosuperlubricity

Achieving fast and long-lasting superlubricity in two-dimensional (2D) materials under high-stress conditions is challenging due to their susceptibility to structural deformations, limited load-bearing capacity, oxidation, and thermal degradation. This study introduces an innovative strategy by utilizing a composite of MXene and H-DLC, where, under high-stress conditions, H-DLC acts as a preferential energy-absorbing phase. MXene serves as a template to rapidly and continuously transform the absorbed energy into graphene-like structures, forming an in situ heterogeneous MXene/graphene-like interface. This process achieves long-lasting macroscopic superlubricity. Friction tests indicate that, under high-stress conditions (∼1.5 GPa Hertz pressure), the coefficient of friction (CoF) of the composite films rapidly decreases to macroscopic superluberic regimes of ∼0.003, with a friction lifespan more than ten times that of the original H-DLC films. In-depth experimental research and tribology-focused molecular dynamics simulations have shown that carbon atoms diffusing from decomposed H-DLC form graphene-like structures under high contact stress, which then evolve into MXene/graphene-like heterostructures. Molecular dynamics simulations reveal that the formation of this heterostructure involves a transition from sp3 to sp2 carbon structures, accompanied by significant energy absorption. Our research presents the lowest CoF achieved by MXene or MXene/H-DLC nanocomposite so far.

Improved tribological properties of MXene nanosheet filler-modified PPO composites

MXenes, a class of two-dimensional (2D) nanomaterials, exhibit exceptional properties such as outstanding mechanical and thermal stability, along with diverse surface characteristics, making them highly promising in the tribology. However, their tendency to aggregate within the polymeric matrix adversely affects the tribological performance of the polymer. In this study, glass fiber (GF) surfaces were modified with polydopamine (PDA), allowing smaller MXene nanosheets to adhere to the GF surface, whereas the larger MXene nanosheets were dispersed throughout the matrix. This approach effectively enhanced the dispersion of MXene nanosheets in the polymeric matrix, facilitating the preparation of polyphenylene oxide (PPO)/MXene composite materials. Compared with the pure PPO sample, the results showed that the average friction coefficient and wear rate of the PPO/MXene composites were reduced by 46.25% and 98.34%, respectively, due to the distinct roles of different MXene nanosheet sizes in the polymeric matrix. Furthermore, a uniform lubricating film was formed during the friction of the polymer composite, enhancing its tribological performance. This study proposes a novel design strategy to enhance MXene nanosheet dispersion and optimize their lubricating properties.

Illuminating Pathways to Dynamic Nanotribology: Light-Mediated Active Control of Interfacial Friction with Nanosuspensions

Active control of nanotribological properties is a challenge. Materials responsive to external stimuli may catalyze this paradigm shift. Recently, the nanofriction of a thin film is modulated by light, ushering in phototribology. This frontier is expanded here, by investigating photoactive nanoparticles in lubricants to confer similar functionality to passive surfaces. Quartz-crystal microbalance (QCM) is employed to assess the phototribological behavior of aqueous suspensions of titanium dioxide nanoparticles. A comparison of dark and illuminated conditions provides the first demonstration of tuning the interfacial friction in solid-nanosuspension interfaces by light. Cyclic tests reveal reversible transitions between higher (dark) and lower friction (illuminated) regimes. These transitions are underpinned by transient states with surface charge variations, as confirmed by Zeta potential measurements. The accumulated surface charge increases repulsion within the system and favors sliding. Upon cessation of illumination, the system returns to its prior equilibrium state. These findings impact not only nanotribology but nanofluidics and nanorheology. Furthermore, the results underscore the need to consider light-induced effects in other scenarios, including the calculation of activity coefficients of photoactive suspensions. This multifaceted study introduces a new dimension to in operando frictional tuning, beckoning a myriad of applications and fundamental insights at the nanoscale.

Solid-Solution MXenes: Synthesis, Properties, and Applications

ConspectusMXenes, among other two-dimensional (2D) materials such as graphene, hexagonal BN, transition metal dichalcogenides (TMDs), 2D metal-organic frameworks (MOFs), and covalent organic frameworks (COFs), are the fastest growing class discovered thus far. The general formula of MXenes is Mn+1XnTx, where M, X, and Tx represent an early transition metal (Ti, V, Nb, Mo, etc.), C and/or N, and the surface functional groups (typically, O, OH, F, Cl), respectively, and n can be between 1 and 4. MXenes as a class of materials have extraordinary properties, such as high electrical conductivity, nonlinear optical properties, solution processability, scalability and ease of synthesis, redox capability, and tunable surface properties, among others; the specific properties, however, depend on their chemistry. Since the initial report of the first MXene in 2011, the research community has primarily focused on Ti3C2Tx, and the amount of research work to investigate its synthesis and properties has increased exponentially over the years. In materials science, alloying is a useful way of synthesizing new materials to improve the properties of a class of materials. Advancement of steel and synthesis of inorganic semiconductors can be regarded as some of the major historical advancements in the concept of alloying. Thus, just one year after the initial report of MXenes, the first solid-solution MXene, (TiNb)2CTx, was reported, which demonstrates the inherent chemical tunability of this class of materials.MXenes have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites, presenting promising routes for tailoring their properties. X-site solid-solutions include carbonitride MXenes and are the least studied class of MXenes to date. Comparatively, multi-M MXenes have acquired significant attention, leading to the extreme example of high-entropy solid-solution MXenes. By using multiple M elements, a significant expansion of the structural and chemical diversity is possible, giving rise to novel chemical, magnetic, electronic, and optical properties that cannot be accessed by single-M MXenes. Solid-solution MXenes represent the largest and most tunable class of MXenes; solid-solution MXenes are those that have multiple metals that are randomly distributed on their M sites with no distinct chemical ordering. Using multiple M elements in MXenes, it is possible to synthesize novel MXene structures that cannot be produced otherwise, such as M5X4Tx MXenes. Based on their chemistry, it is possible to rationally control the electronic, optical, mechanical, and chemical properties in a way that no other class of MXenes can. In some cases, the resultant property is linearly related to the chemistry, such as the electrical conductivity, while in other cases the properties are nonlinear or emergent: optical properties, enabling these MXenes to fulfill roles that no other MXene, or 2D material, can.In this Account, we discuss the recent progress in the synthesis, properties, applications, and outlook of solid-solution MXenes. Importantly, we demonstrate how multi-M solid-solutions can be used to tailor properties for specific applications easily.

Recent developments of artificial intelligence in MXene-based devices: from synthesis to applications

Two-dimensional transition metal carbides, nitrides, or carbonitrides (MXenes) have garnered remarkable attention in various energy and environmental applications due to their high electrical conductivity, good thermal properties, large surface area, high mechanical strength, rapid charge transport mechanism, and tunable surface properties. Recently, artificial intelligence has been considered an emerging technology, and has been widely used in materials science, engineering, and biomedical applications due to its high efficiency and precision. In this review, we focus on the role of artificial intelligence-based technology in MXene-based devices and discuss the latest research directions of artificial intelligence in MXene-based devices, especially the use of artificial intelligence-based modeling tools for energy storage devices, sensors, and memristors. In addition, emphasis is given to recent progress made in synthesis methods for various MXenes and their advantages and disadvantages. Finally, the review ends with several recommendations and suggestions regarding the role of artificial intelligence in fabricating MXene-based devices. We anticipate that this review will provide guidelines on future research directions suitable for practical applications.

Maintaining the 2D Structure of MXene via Self-Assembled Monolayers for Efficient Lubrication in High Humidity

MXene is considered as a promising solid lubricant due to facile shearing ability and tuneable surface chemistry. However, it faces challenges in high-humidity environments where excessive water molecules can significantly impact its 2D structure, thus deteriorating its lubricating properties. In this work, the self-assembled monolayers are formed on MXene by surface chlorination (MXene-Cl) and fluorination (MXene-F), and their friction behaviors in high/low humidity are investigated. The results indicate that MXene-F and MXene-Cl can maintain a relatively constant friction coefficient (CoF) (MXene-F ∼0.76, MXene-Cl ∼0.48) under both high (75%) and low (25%)-relative humidity (RH) environments. Meanwhile, the MXene-F and MXene-Cl display a lower CoF than the pristine MXene (MXene CoF∼1.18) in high humidity. The above phenomena are mainly attributed to the preservation of its 2D layered structure, the increased layer spacing, and superficial partial oxidation for SAMs-functionalized MXene under high humidity during friction. Interestingly, MXene-Cl with moderate water resistance has a lower CoF than that of MXene-F with complete water resistance. The nanostructured water adsorption capacity and larger interlayer spacing of MXene-Cl make it exhibit a lower CoF compared to MXene-F. The findings of this study offer valuable guidance for tailoring MXene by surface chemical functionalization as an efficient solid lubricant in high humidity.

Accurate Simulation for 2D Lubricating Materials in Realistic Environments: From Classical to Quantum Mechanical Methods

2D materials such as graphene, MoS2, and hexagonal BN are the most advanced solid lubricating materials with superior friction and anti-wear performance. However, as a typical surface phenomenon, the lubricating properties of 2D materials are largely dependent on the surrounding environment, such as temperature, stress, humidity, oxygen, and other environmental substances. Given the technical challenges in experiment for real-time and in situ detection of microscopic environment-material interaction, recent years have witnessed the acceleration of computational research on the lubrication behavior of 2D materials in realistic environments. This study reviews the up-to-date computational studies for the effect of environmental factors on the lubrication performance of 2D materials, summarizes the theoretical methods in lubrication from classical to quantum-mechanics ones, and emphasizes the importance of quantum method in revealing the lubrication mechanism at atomic and electronic level. An effective simulation method based on ab initio molecular dynamics is also proposed to try to provide more ways to accurately reveal the friction mechanisms and reliably guide the lubricating material design. On the basis of current development, future prospects, and challenges for the simulation and modeling in lubrication with realistic environment are outlined.

Unveiling the power of MXenes: Solid lubrication perspectives and future directions

The interaction between two surfaces leads to the generation of friction and wear of material. Friction and wear are some of the major challenges that may readily be overcome by the third part of tribology called lubrication. Utilizing solid lubricants including polymers, carbon-based materials, soft metals, transition metal dichalcogenides, along with their potential benefits and drawbacks in dry environments can reduce friction. Recently, an emerging class of two-dimensional (2D) transition metal nitrides, carbides or carbonitrides commonly known as MXenes have emerged as an attractive alternative for solid lubrication because of their ability to establish wear-resistant tribo layers and well as low friction and shear strength. Furthermore, the inherent hydrophilic nature of these substances has led to limited dispersion stability and phase compatibility when combined with pure base oils. As a result, their potential use as solid lubricants and lubricant additives has been impeded. To address this issue and enhance the applicability of MXenes as solid lubricants, their surface modification can be an attractive tool. Therefore, this review provides a succinct summary of the current state-of-the-art in surface functionalization of MXenes, a subject that has not yet been thoroughly addressed. Further, the mechanical behavior of MXenes and composites has been discussed, followed by the potential of MXenes as a solid lubricant at micro- and macro-scale. Finally, the existing opportunities and challenges of the research area have been discussed with possible future research directions. We believe, this article will be a valuable resource for MXenes and opens the door to improve the chemical, physical and mechanical properties of MXenes in various applications, such as solid lubrication.

2D material-based surface plasmon resonance biosensors for applications in different domains: an insight

The design of surface plasmon resonance (SPR) sensors has been greatly enhanced in recent years by the advancements in the production and integration of nanostructures, leading to more compact and efficient devices. There have been reports of novel SPR sensors having distinct nanostructures, either as signal amplification tags like gold nanoparticles (AuNPs) or as sensing substrate-like two-dimensional (2D) materials including graphene, transition metal dichalcogenides (TMDCs), MXene, black phosphorus (BP), metal-organic frameworks (MOFs), and antimonene. Such 2D-based SPR biosensors offer advantages over conventional sensors due to significant increases in their sensitivity with a good figure of merit and limit of detection (LOD). Due to their atomically thin structure, improved sensitivity, and sophisticated functionalization capabilities, 2D materials can open up new possibilities in the field of healthcare, particularly in point-of-care diagnostics, environmental and food monitoring, homeland security protection, clinical diagnosis and treatment, and flexible or transient bioelectronics. The present study articulates an in-depth analysis of the most recent developments in 2D material-based SPR sensor technology. Moreover, in-depth research of 2D materials, their integration with optoelectronic technology for a new sensing platform, and the predicted and experimental outcomes of various excitation approaches are highlighted, along with the principles of SPR biosensors. Furthermore, the review projects the potential prospects and future trends of these emerging materials-based SPR biosensors to advance in clinical diagnosis, healthcare biochemical, and biological applications.

Comprehensive synthesis of Ti3C2Tx from MAX phase to MXene

MXenes are a large family of two-dimensional materials that have attracted attention across many fields due to their desirable optoelectronic, biological, mechanical and chemical properties. There currently exist many synthesis procedures that lead to differences in flake size, defects and surface chemistry, which in turn affect their properties. Herein, we describe the steps to synthesize Ti3C2Tx-the most important and widely used MXene, from a Ti3AlC2 MAX phase precursor. The procedure contains three main sections: synthesis of Ti3AlC2 MAX, wet chemical etching of the MAX in hydrofluoric acid/HCl solution to yield multilayer Ti3C2Tx and its delamination into single-layer flakes. Three delamination options are described; these use LiCl, tertiary amines (tetramethyl ammonium hydroxide/ tetrabutyl ammonium hydroxide) and dimethylsulfoxide respectively. These procedures can be adapted for the synthesis of MXenes beyond Ti3C2Tx. The MAX phase synthesis takes about 1 week, with the etching and delamination each requiring 2 d. This protocol requires users to have experience working with hydrofluoric acid, and it is recommended that users have experience with wet chemistry and centrifugation; characterization techniques such as X-ray diffraction and particle size analysis are also essential for the success of the protocol. While alternative synthesis methods, such as minimally intensive layer delamination, are desirable for certain MXenes (such as Ti2CTx) or specific applications, this protocol aims to standardize the more commonly used hydrofluoric acid/HCl etching method, which produces Ti3C2Tx with minimal concentration of defects and the highest conductivity and serves as a guideline for those working with MXenes for the first time.

Ion pair sites for efficient electrochemical extraction of uranium in real nuclear wastewater

Electrochemical uranium extraction from nuclear wastewater represents an emerging strategy for recycling uranium resources. However, in nuclear fuel production which generates the majority of uranium-containing nuclear wastewater, fluoride ion (F-) co-exists with uranyl (UO22+), resulting in the complex species of UO2Fx and thus decreasing extraction efficiency. Herein, we construct Tiδ+-PO43- ion pair extraction sites in Ti(OH)PO4 for efficient electrochemical uranium extraction in wastewater from nuclear fuel production. These sites selectively bind with UO2Fx through the combined Ti-F and multiple O-U-O bonds. In the uranium extraction, the uranium species undergo a crystalline transition from U3O7 to K3UO2F5. In real nuclear wastewater, the uranium is electrochemically extracted with a high efficiency of 99.6% and finally purified as uranium oxide powder, corresponding to an extraction capacity of 6829 mg g-1 without saturation. This work paves an efficient way for electrochemical uranium recycling in real wastewater of nuclear production.

Wear Mechanisms, Composition and Thickness of Antiwear Tribofilms Formed from Multi-Component Lubricants

The antiwear properties of tribofilms formed on steel surfaces lubricated with various multi-component lubricants were investigated at an elevated temperature and under load-speed conditions conducive to sliding in the boundary lubrication regime. The lubricants contained base oil, reduced-level (secondary) zinc dialkyl dithiophosphate (ZDDP), and nitrogenous dispersant. The wear resistance of the tribofilms produced from different oil blends was evaluated in the context of the rate of change in the sliding track volume (wear rate for material loss) and the load-bearing capacity, chemical composition, and thickness of the tribofilms. Surface profilometry and scanning electron microscopy were used to quantify the wear performance and detect the prevailing wear mechanisms, whereas X-ray photoelectron spectroscopy elucidated the chemical composition and thickness of the tribofilms. The oil blends without ZDDP did not produce tribofilms with adequate antiwear properties, whereas the oil blends containing ZDDP and dispersant generated tribofilms with antiwear characteristics comparable to those of tribofilms produced from blends with a higher ZDDP content. Although dispersants can suspend oil contaminants and preserve the cleanness of the sliding surfaces, it was found that they can also reduce the antiwear efficacy of ZDDP. This was attributed to an additive-dispersant antagonistic behavior for surface adsorption sites affecting tribofilm chemistry and mechanical properties. Among the blends containing a mixture of ZDDP and dispersant, the best antiwear properties were demonstrated by the tribofilm produced from the blend consisting of base oil, 0.05 wt% ZDDP, and a bis-succinimide dispersant treated with ethylene carbonate. The findings of this investigation demonstrate the potential of multi-component lubricants with reduced-content ZDDP and nitrogen-based dispersant to form effective antiwear tribofilms.

Staphylococcus aureus epidemiology, pathophysiology, clinical manifestations and application of nano-therapeutics as a promising approach to combat methicillin resistant Staphylococcus aureus

Staphylococcus aureus is a Gram-positive bacterium and one of the most prevalent infectious disease-related causes of morbidity and mortality in adults. This pathogen can trigger a broad spectrum of diseases, from sepsis and pneumonia to severe skin infections that can be fatal. In this review, we will provide an overview of S. aureus and discuss the extensive literature on epidemiology, transmission, genetic diversity, evolution and antibiotic resistance strains, particularly methicillin resistant S. aureus (MRSA). While many different virulence factors that S. aureus produces have been investigated as therapeutic targets, this review examines recent nanotechnology approaches, which employ materials with atomic or molecular dimensions and are being used to diagnose, treat, or eliminate the activity of S. aureus. Finally, having a deeper understanding and clearer grasp of the roles and contributions of S. aureus determinants, antibiotic resistance, and nanotechnology will aid us in developing anti-virulence strategies to combat the growing scarcity of effective antibiotics against S. aureus.

Solar-driven surface-heating membrane distillation using Ti3C2Tx MXene-coated spacers

The emergence of two-dimensional (2D) MXenes as efficient light-to-heat conversion materials offers significant potential for solar-based desalination, particularly in photothermal interfacial evaporation, enabling cost-effective solar-powered membrane distillation (MD). This study investigates solar-powered MD afforded by a photothermally functionalized spacer, which is built by spray-coating Ti3C2Tx MXene sheets on metallic spacers. 2D Ti3C2Tx MXene gives an ultrahigh photothermal conversion efficiency; thereby, by Ti3C2Tx MXene-coated metallic spacer, this rationally designed spacer allows for a localized photothermal conversion and interfacial feed heating effect on the membrane surface, especially for MD operation. As a feed spacer and a photothermal element, Ti3C2Tx MXene-coated metallic spacer exhibited stable enhanced water flux of up to 0.36 kg·m-2h-1 under one sun illumination for a feed salinity of 35 g·L-1, corresponding energy conversion efficiency of 28.3 %. Overall, the developed photothermal Ti3C2Tx MXene-coated spacers displayed great potential in enhancing the performance, scalability, and feasibility of solar-driven MD process, paving the way for further development of photothermal elements that can be implemented in solar MD applications.

Annealing Ti3C2Tz MXenes to Control Surface Chemistry and Friction

Although surface terminations (such as ═O, -Cl, -F, and -OH) on MXene nanosheets strongly influence their functional properties, synthesis of MXenes with desired types and distribution of those terminations is still challenging. Here, it is demonstrated that thermal annealing helps in removing much of the terminal groups of molten salt-etched multilayered (ML) Ti3C2Tz. In this study, the chloride terminations of molten salt-etched ML-Ti3C2Tz were removed via thermal annealing at increased temperatures under an inert (argon) atmosphere. This thermal annealing created some bare sites available for further functionalization of Ti3C2Tz. XRD, EDS, and XPS measurements confirm the removal of much of the terminal groups of ML-Ti3C2Tz. Here, the annealed ML-Ti3C2Tz was refunctionalized by -OH groups and 3-aminopropyl triethoxysilane (APTES), which was confirmed by FTIR. The -OH and APTES surface-modified ML-Ti3C2Tz are evaluated as a solid lubricant, exhibiting ∼70.1 and 66.7% reduction in friction compared to a steel substrate, respectively. This enhanced performance is attributed to the improved interaction or adhesion of functionalized ML-Ti3C2Tz with the substrate material. This approach allows for the effective surface modification of MXenes and control of their functional properties.

Functional MXenes: Progress and Perspectives on Synthetic Strategies and Structure-Property Interplay for Next-Generation Technologies

MXenes are a class of 2D materials that include layered transition metal carbides, nitrides, and carbonitrides. Since their inception in 2011, they have garnered significant attention due to their diverse compositions, unique structures, and extraordinary properties, such as high specific surface areas and excellent electrical conductivity. This versatility has opened up immense potential in various fields, catalyzing a surge in MXene research and leading to note worthy advancements. This review offers an in-depth overview of the evolution of MXenes over the past 5 years, with an emphasis on synthetic strategies, structure-property relationships, and technological prospects. A classification scheme for MXene structures based on entropy is presented and an updated summary of the elemental constituents of the MXene family is provided, as documented in recent literature. Delving into the microscopic structure and synthesis routes, the intricate structure-property relationships are explored at the nano/micro level that dictate the macroscopic applications of MXenes. Through an extensive review of the latest representative works, the utilization of MXenes in energy, environmental, electronic, and biomedical fields is showcased, offering a glimpse into the current technological bottlenecks, such asstability, scalability, and device integration. Moreover, potential pathways for advancing MXenes toward next-generation technologies are highlighted.

Review of two-dimensional nanomaterials in tribology: Recent developments, challenges and prospects

From our ordinary lives to various mechanical systems, friction and wear are often unavoidable phenomena that are heavily responsible for excessive expenditures of nonrenewable energy, the damages and failures of system movement components, as well as immense economic losses. Thus, achieving low friction and high anti-wear performance is critical for minimization of these adverse factors. Two-dimensional (2D) nanomaterials, including transition metal dichalcogenides, single elements, transition metal carbides, nitrides and carbonitrides, hexagonal boron nitride, and metal-organic frameworks have attracted remarkable interests in friction and wear reduction of various applications, owing to their atomic-thin planar morphologies and tribological potential. In this paper, we systematically review the current tribological progress on 2D nanomaterials when used as lubricant additives, reinforcement phases in the coatings and bulk materials, or a major component of superlubricity system. Additionally, the conclusions and prospects on 2D nanomaterials with the existing drawbacks, challenges and future direction in such tribological fields are briefly provided. Finally, we sincerely hope such a review will offer valuable lights for 2D nanomaterial-related researches dedicated on tribology in the future.

MXene Lubricated Tribovoltaic Nanogenerator with High Current Output and Long Lifetime

Tribovoltaic nanogenerators (TVNGs) have the characteristics of high current density, low matched impedance and continuous output, which is expected to solve the problem of power supply for small electronic devices. However, wear occurrence in friction interface will seriously reduce the performance of TVNGs as well as lifetime. Here, we employ MXene solution as lubricate to improve output current density and lifetime of TVNG simultaneously, where a high value of 754 mA m-2 accompanied with a record durability of 90,000 cycles were achieved. By comparing multiple liquid lubricates with different polarity, we show that conductive polar liquid with MXene as additive plays a crucial role in enhancing the electrical output performance and durability of TVNG. Moreover, the universality of MXene solution is well demonstrated in various TVNGs with Cu and P-type Si, and Cu and N-GaAs as material pairs. This work may guide and accelerates the practical application of TVNG in future.

Engineering the surface of Nbn+1CnTx MXenes to versatile bio-activity towards microorganisms

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) are potential antibacterial agents. However, their activity against microorganisms is not fully understood. It could relate to MXenes' surface which further influences their biocidal action. Herein, we report no continuous biocidal activity for delaminated 2D niobium-based MXenes (Nbn+1XnTx) such as Nb2CTx and Nb4C3Tx prepared with HF/TMAOH protocol. Biocidal activity towards Bacillus subtilis and Staphylococcus aureus microorganisms was achieved by surface-functionalization with lysozyme macromolecule. MXenes' engineering with lysozyme changed MXene's surface charge from negative into positive thus enabling the elimination of bacteria cells during 48 h of incubation. In contrast, Nb4C3Tx functionalized with collagen stimulated the growth of Bacillus subtilis by 225 %, showing MXene's biocompatibility towards this particular strain. Altogether, our results show that MXenes are incredibly bio-tunable. Opposing bio-effects such as antimicrobial or growth-stimulating can be achieved towards various microorganisms with rational surface engineering.

In-situ synthesized V2CTx MXene-based immune tag for the electrochemical detection of Interleukin 6 (IL-6) from breast cancer cells

Interleukin-6 (IL-6) is a proinflammatory cytokine with a critical role in immune regulation and treatment of many diseases, including breast cancer. Herein, we developed a novel V2CTx MXene-based immunosensor for rapid and accurate IL-6 detection. The chosen substrate was V2CTx, a 2-dimensional (2D) MXene nanomaterial with excellent electronic properties. Prussian blue (Fe4[Fe(CN)6]3), used for its electrochemical properties, and spindle-shaped gold nanoparticles (Au SSNPs), used to combine with antibodies, were in-situ synthesized on the surface of the MXene. The in-situ synthesis ensures a firm chemical connection compared to other tags formed by a less stable physical absorption. Inspired by a sandwich ELISA test, the modified V2CTx tag was captured by the electrode surface with cysteamine to detect the analyte, IL-6, after being attached with a capture antibody (cAb). Benefiting from an increased surface area, an enhanced charge transfer rate, and a firm connection of the tag, this biosensor exhibited excellent analytical performance. The high sensitivity, high selectivity, and wide detection range covering the IL-6 level of both healthy individuals and breast cancer patients were obtained to meet clinical demands. Herein, this V2CTx MXene-based immunosensor is a potential therapeutic and diagnostic point-of-care alternative to routine ELISA IL-6 detection methods.

d-Band Center Optimization of Ti3C2Tx MXene Nanosheets for Ultrahigh NO2 Gas Sensitivity at Room Temperature

MXene exhibits numerous advantageous properties such as high electronic conductivity, high surface area, and ease of surface modification via tailoring of functional groups. However, the mechanism by which MXene functionalization enhances gas sensing performance has not yet been well understood, let alone the development of a rational sensor design optimization strategy. This work presents a functionalization methodology for MXene based on d-band center modulation, which can be implemented by introducing Fe onto the surface of Ti3C2Tx nanosheets, for significantly improved gas sensing response and selectivity. The strategy is demonstrated in the design of gas sensors. The optimized gas sensor shows a response of 50% toward 10 ppm of NO2 at room temperature, which is over 6-fold improvement from its pristine counterpart, an unprecedented performance level among all reported MXene gas sensors. XPS characterizations, valence band analyses, and density functional theory (DFT) calculations all indicate that the underlying enhancement mechanism can be attributed to the tuning of the d-band center energy toward the Fermi level. This work provides a new design strategy based on the optimization of the d-band center energy and adds a much needed systematic and quantitative method to the design of two-dimensional materials based semiconducting gas sensors.

Advanced progress on the significant influences of multi-dimensional nanofillers on the tribological performance of coatings

Over the past two decades, nanofillers have attracted significant interest due to their proven chemical, mechanical, and tribological performances. However, despite the significant progress realized in the application of nanofiller-reinforced coatings in various prominent fields, such as aerospace, automobiles and biomedicine, the fundamental effects of nanofillers on the tribological properties of coatings and their underlying mechanisms have rarely been explored by subdividing them into different sizes ranging from zero-dimensional (0D) to three-dimensional (3D) architectures. Herein, we present a systematic review of the latest advances on multi-dimensional nanofillers for enhancing the friction reduction and wear resistance of metal/ceramic/polymer matrix composite coatings. Finally, we conclude with an outlook for future investigations on multi-dimensional nanofillers in tribology, providing possible solutions for the key challenges in their commercial applications.

Recent Progress in MXene Hydrogel for Wearable Electronics

Recently, hydrogels have attracted great attention because of their unique properties, including stretchability, self-adhesion, transparency, and biocompatibility. They can transmit electrical signals for potential applications in flexible electronics, human-machine interfaces, sensors, actuators, et al. MXene, a newly emerged two-dimensional (2D) nanomaterial, is an ideal candidate for wearable sensors, benefitting from its surface's negatively charged hydrophilic nature, biocompatibility, high specific surface area, facile functionalization, and high metallic conductivity. However, stability has been a limiting factor for MXene-based applications, and fabricating MXene into hydrogels has been proven to significantly improve their stability. The unique and complex gel structure and gelation mechanism of MXene hydrogels require intensive research and engineering at nanoscale. Although the application of MXene-based composites in sensors has been widely studied, the preparation methods and applications of MXene-based hydrogels in wearable electronics is relatively rare. Thus, in order to facilitate the effective evolution of MXene hydrogel sensors, the design strategies, preparation methods, and applications of MXene hydrogels for flexible and wearable electronics are comprehensively discussed and summarized in this work.

Ultrahigh Lubricity between Two-Dimensional Ice and Two-Dimensional Atomic Layers

Low temperature and high humidity conditions significantly degrade the performance of solid-state lubricants consisting of van der Waals (vdW) atomic layers, owing to the liquid water layer attached/intercalated to the vdW layers, which greatly enhances the interlayer friction. However, using low temperature in situ atomic force microscopy (AFM) and friction force microscopy (FFM), we unveil the unexpected ultralow friction between two-dimensional (2D) ice, a solid phase of water confined to the 2D space, and the 2D molybdenum disulfides (MoS₂). The friction of MoS₂ and 2D ice is reduced by more than 30% as compared to bare MoS₂ and the rigid surface. The phase transition of liquid water into 2D ice under mechanical compression has also been observed. These new findings can be applied as novel frictionless water/ice transport technology in nanofluidic systems and promising high performance lubricants for operating in low temperature and high humidity environments.

Layer-Dependent Nanowear of Graphene Oxide

The mechanical performance and surface friction of graphene oxide (GO) were found to inversely depend on the number of layers. Here, we demonstrate the non-monotonic layer-dependence of the nanowear resistance of GO nanosheets deposited on a native silicon oxide substrate. As the thickness of GO increases from ∼0.9 nm to ∼14.5 nm, the nanowear resistance initially demonstrated a decreasing and then an increasing tendency with a critical number of layers of 4 (∼3.6 nm in thickness). This experimental tendency corresponds to a change of the underlying wear mode from the overall removal to progressive layer-by-layer removal. The phenomenon of overall removal disappeared as GO was deposited on an H-DLC substrate with a low surface energy, while the nanowear resistance of thicker GO layers was always higher. Combined with density functional theory calculations, the wear resistance of few-layer GO was found to correlate with the substrate's surface energy. This can be traced back to substrate-dependent adhesive strengths of GO, which correlated with the GO thickness originating from differences in the interfacial charge transfer. Our study proposes a strategy to improve the antiwear properties of 2D layered materials by tuning their own thickness and/or the interfacial interaction with the underlying substrate.