High-throughput printing of combinatorial materials from aerosols.

The development of new materials and their compositional and microstructural optimization are essential in regard to next-generation technologies such as clean energy and environmental sustainability. However, materials discovery and optimization have been a frustratingly slow process. The Edisonian trial-and-error process is time consuming and resource inefficient, particularly when contrasted with vast materials design spaces1. Whereas traditional combinatorial deposition methods can generate material libraries2,3, these suffer from limited material options and inability to leverage major breakthroughs in nanomaterial synthesis. Here we report a high-throughput combinatorial printing method capable of fabricating materials with compositional gradients at microscale spatial resolution. In situ mixing and printing in the aerosol phase allows instantaneous tuning of the mixing ratio of a broad range of materials on the fly, which is an important feature unobtainable in conventional multimaterials printing using feedstocks in liquid-liquid or solid-solid phases4-6. We demonstrate a variety of high-throughput printing strategies and applications in combinatorial doping, functional grading and chemical reaction, enabling materials exploration of doped chalcogenides and compositionally graded materials with gradient properties. The ability to combine the top-down design freedom of additive manufacturing with bottom-up control over local material compositions promises the development of compositionally complex materials inaccessible via conventional manufacturing approaches.

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.

Thickness-independent capacitance of vertically aligned liquid-crystalline MXenes.

The scalable and sustainable manufacture of thick electrode films with high energy and power densities is critical for the large-scale storage of electrochemical energy for application in transportation and stationary electric grids. Two-dimensional nanomaterials have become the predominant choice of electrode material in the pursuit of high energy and power densities owing to their large surface-area-to-volume ratios and lack of solid-state diffusion1,2. However, traditional electrode fabrication methods often lead to restacking of two-dimensional nanomaterials, which limits ion transport in thick films and results in systems in which the electrochemical performance is highly dependent on the thickness of the film1-4. Strategies for facilitating ion transport-such as increasing the interlayer spacing by intercalation5-8 or introducing film porosity by designing nanoarchitectures9,10-result in materials with low volumetric energy storage as well as complex and lengthy ion transport paths that impede performance at high charge-discharge rates. Vertical alignment of two-dimensional flakes enables directional ion transport that can lead to thickness-independent electrochemical performances in thick films11-13. However, so far only limited success11,12 has been reported, and the mitigation of performance losses remains a major challenge when working with films of two-dimensional nanomaterials with thicknesses that are near to or exceed the industrial standard of 100 micrometres. Here we demonstrate electrochemical energy storage that is independent of film thickness for vertically aligned two-dimensional titanium carbide (Ti3C2T x ), a material from the MXene family (two-dimensional carbides and nitrides of transition metals (M), where X stands for carbon or nitrogen). The vertical alignment was achieved by mechanical shearing of a discotic lamellar liquid-crystal phase of Ti3C2T x . The resulting electrode films show excellent performance that is nearly independent of film thickness up to 200 micrometres, which makes them highly attractive for energy storage applications. Furthermore, the self-assembly approach presented here is scalable and can be extended to other systems that involve directional transport, such as catalysis and filtration.

Design of Atomic Ordering in Mo2Nb2C3Tx MXenes for Hydrogen Evolution Electrocatalysis.

The need for novel materials for energy storage and generation calls for chemical control at the atomic scale in nanomaterials. Ordered double-transition-metal MXenes expanded the chemical diversity of the family of atomically layered 2D materials since their discovery in 2015. However, atomistic tunability of ordered MXenes to achieve ideal composition-property relationships has not been yet possible. In this study, we demonstrate the synthesis of Mo2+αNb2-αAlC3 MAX phases (0 ≤ α ≤ 0.3) and confirm the preferential ordering behavior of Mo and Nb in the outer and inner M layers, respectively, using density functional theory, Rietveld refinement, and electron microscopy methods. We also synthesize their 2D derivative Mo2+αNb2-αC3Tx MXenes and exemplify the effect of preferential ordering on their hydrogen evolution reaction electrocatalytic behavior. This study seeks to inspire further exploration of the ordered double-transition-metal MXene family and contribute composition-behavior tools toward application-driven design of 2D materials.

MXene Free Standing Films: Unlocking the Impact of Flake Sizes in Microwave Resonant Structures in Humid Environments.

Microwave communication devices necessitate elements with high electrical conductivity, a property which was traditionally found in metals (e.g., copper). However, in applications such as satellite communications, metals prevent the payload from achieving lightweight and flexible characteristics. Here, we demonstrate the development of MXene film microwave resonators, leveraging MXene's high electrical conductivity and unique mechanical properties. To investigate resonant performance in humid conditions and study the effects of MXene's processing and treatment, MXene films with different flake sizes are prepared and exposed to cyclic humidity. For the large- and small-flake Ti3 C2 MXene films in cyclic humidity, the large-flake film demonstrates higher electrical conductivity, higher resonance quality factor (150 and 35 as unloaded, and loaded), and less fluctuation of performance (≈1.7% total shift in resonance frequency). Further, by implementing MXene films of two different diameters, the correlation between film size and resonant frequency is demonstrated. By introducing an active resonant configuration, the effect of MXene degradation and microwave losses can be compensated. This active feedback loop demonstrates a ≈300 times increase in the quality factor of MXene resonators. As a building block for terrestrial and satellite communication modules, MXene resonators potentiate the replacement of metals in achieving unique electrical and mechanical properties.

MXenes Antibacterial Properties and Applications: A Review and Perspective.

The mutations of bacteria due to the excessive use of antibiotics, and generation of antibiotic-resistant bacteria have made the development of new antibacterial compounds a necessity. MXenes have emerged as biocompatible transition metal carbide structures with extensive biomedical applications. This is related to the MXenes' unique combination of properties, including multifarious elemental compositions, 2D-layered structure, large surface area, abundant surface terminations, and excellent photothermal and photoelectronic properties. The focus of this review is the antibacterial application of MXenes, which has attracted the attention of researchers since 2016. A quick overview of the synthesis strategies of MXenes is provided and then summarizes the effect of various factors (including structural properties, optical properties, surface charges, flake size, and dispersibility) on the biocidal activity of MXenes. The main mechanisms for deactivating bacteria by MXenes are discussed in detail including rupturing of the bacterial membrane by sharp edges of MXenes nanoflakes, generating the reactive oxygen species (ROS), and photothermal deactivating of bacteria. Hybridization of MXenes with other organic and inorganic materials can result in materials with improved biocidal activities for different applications such as wound dressings and water purification. Finally, the challenges and perspectives of MXene nanomaterials as biocidal agents are presented.

Spherical Templating of CoSe2 Nanoparticle-Decorated MXenes for Lithium-Sulfur Batteries.

Two-dimensional MXenes produce competitive performances when incorporated into lithium-sulfur batteries (LSBs), solving key problems such as the poor electronic conductivity of sulfur and dissolution of its polysulfide intermediates. However, MXene nanosheets are known to easily aggregate and restack during electrode fabrication, filtration, or water removal, limiting their practical applicability. Furthermore, in complex electrocatalytic reactions like the multistep sulfur reduction process in LSBs, MXene alone is insufficient to ensure an optimal reaction pathway. In this work, we demonstrate for the first time a loose templating of sulfur spheres using Ti3C2Tx MXene nanosheets decorated with polymorphic CoSe2 nanoparticles. This work shows that the templating of sulfur spheres using nanoparticle-decorated MXene nanosheets can prevent nanosheet aggregation and exert a strong electrocatalytic effect, thereby enabling improved reaction kinetics and battery performance. The S@MXene-CoSe2 cathode demonstrated a long cycle life of 1000 cycles and a low capacity decay rate of 0.06% per cycle in LSBs.

In Situ N-Doped Graphene and Mo Nanoribbon Formation from Mo2 Ti2 C3 MXene Monolayers.

Since the advent of monolayered 2D transition metal carbide and nitrides (MXenes) in 2011, the number of different monolayer systems and the study thereof have been on the rise. Mo2 Ti2 C3 is one of the least studied MXenes and new insights to this material are of value to the field. Here, the stability of Mo2 Ti2 C3 under electron irradiation is investigated. A transmission electron microscope (TEM) is used to study the structural and elemental changes in situ. It is found that Mo2 Ti2 C3 is reasonably stable for the first 2 min of irradiation. However, structural changes occur thereafter, which trigger increasingly rapid and significant rearrangement. This results in the formation of pores and two new nanomaterials, namely, N-doped graphene membranes and Mo nanoribbons. The study provides insight into the stability of Mo2 Ti2 C3 monolayers against electron irradiation, which will allow for reliable future study of the material using TEM. Furthermore, these findings will facilitate further research in the rapidly growing field of electron beam driven chemistry and engineering of nanomaterials.

Temperature-dependent mechanical properties of Tin+1CnO2 (n = 1, 2) MXene monolayers: a first-principles study.

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (named as MXenes) have become of the fastest growing family of 2D materials in terms of compositions and their applications in different areas. One of the least explored properties of MXenes is their mechanical properties. While the basic elastic properties of MXenes have been studied by first-principles, the effects of temperature on the elastic properties have never been explored. In this study, we investigate temperature-dependent structural and mechanical properties of the titanium-containing MXenes (Tin+1CnO2 (n = 1, 2)) based on the first-principles calculations combined with quasi-harmonic approximation. The effective Young's modulus of a single layer of Ti2CO2 and Ti3C2O2 is calculated to be 565 and 482 GPa, respectively, at 0 K. By increasing temperature to 1000 K, Young's moduli of Ti2CO2 and Ti3C2O2 decrease to 469 GPa and 442 GPa, respectively, which indicates a larger reduction in stiffness in thinner MXenes at higher temperatures. Our calculations of the temperature-dependent bond strengths within MXenes showed that titanium and carbon atoms in Ti3C2O2 form stronger bonds than Ti2CO2 and atomic bonds in Ti2CO2 lose their stiffness more than Ti3C2O2 with increasing temperatures. The Debye temperature of these monolayers is also calculated to provide a comparison of the thermal conductivity between these monolayers, in which the results show that the Ti3C2O2 has a higher thermal conductivity than Ti2CO2. Our calculated electronic properties results of the monolayers are also shown that the electrical conductivity of the monolayers would not change with temperature. Our study extends MXenes applications to high-temperature applications, such as structural composite components and aerospace coatings.

A Tungsten-Based Nanolaminated Ternary Carbide: (W,Ti)4C4- x.

Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their two-dimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten-based nanolaminated ternary phase, (W,Ti)4C4- x, synthesized by an Al-catalyzed reaction of W, Ti, and C powders at 1600 °C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure ( P63 /mmc) with lattice parameters, a = 3.00880(7) Å, and c = 19.5633(6) Å and a nominal chemistry of (W,Ti)4C4- x (actual chemistry, W2.1(1)Ti1.6(1)C2.6(1)). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times.

Antimicrobial Properties of 2D MnO2 and MoS2 Nanomaterials Vertically Aligned on Graphene Materials and Ti3C2 MXene.

Two-dimensional (2D) nanomaterials have attracted considerable attention in biomedical and environmental applications due to their antimicrobial activity. In the interest of investigating the primary antimicrobial mode-of-action of 2D nanomaterials, we studied the antimicrobial properties of MnO2 and MoS2, toward Gram-positive and Gram-negative bacteria. Bacillus subtilis and Escherichia coli bacteria were treated individually with 100 µg/mL of randomly oriented and vertically aligned nanomaterials for ∼3 h in the dark. The vertically aligned 2D MnO2 and MoS2 were grown on 2D sheets of graphene oxide, reduced graphene oxide, and Ti3C2 MXene. Measurements to determine the viability of bacteria in the presence of the 2D nanomaterials performed by using two complementary techniques, flow cytometry, and fluorescence imaging showed that, while MnO2 and MoS2 nanosheets show different antibacterial activities, in both cases, Gram-positive bacteria show a higher loss in membrane integrity. Scanning electron microscopy images suggest that the 2D nanomaterials, which have a detrimental effect on bacteria viability, compromise the cell wall, leading to significant morphological changes. We propose that the peptidoglycan mesh (PM) in the bacterial wall is likely the primary target of the 2D nanomaterials. Vertically aligned 2D MnO2 nanosheets showed the highest antimicrobial activity, suggesting that the edges of the nanosheets were likely compromising the cell walls upon contact.

Effect of Synthesis on Performance of MXene/Iron Oxide Anode Material for Lithium-Ion Batteries.

Two-dimensional heterostructures, such as Fe2O3/MXene nanoparticles, can be attractive anode materials for lithium-ion batteries (LIBs) due to the synergy between high lithium-storage capacity of Fe2O3 and stable cyclability and high conductivity provided by MXene. Here, we improved the storage performance of Ti3C2T x (MXene)/Fe2O3 nanocomposite by confining Fe2O3 nanoparticles into Ti3C2T x nanosheets with different mixing ratios using a facile and scalable dry ball-milling process. Composites of Ti3C2T x-25 wt % Fe2O3 and Ti3C2T x-50 wt % Fe2O3 synthesized by ball-milling resulted in uniform distribution of Fe2O3 nanoparticles on Ti3C2T x nanosheets with minimum oxidation of MXene as compared to composites prepared by hydrothermal or wet sonication. Moreover, the composites demonstrated minimum restacking of the nanosheets and higher specific surface area. Among all studied composites, the Ti3C2T x-50 wt % Fe2O3 showed the highest reversible specific capacity of ∼270 mAh g-1 at 1C (∼203 mAh g-1 based on the composite) and rate performance of 100 mAh g-1 at 10C. This can open the door for synthesizing stable and high-performance MXene/transition metal oxide composites with significantly enhanced electrochemical performance for LIB applications.

MoS2 -on-MXene Heterostructures as Highly Reversible Anode Materials for Lithium-Ion Batteries.

Two-dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition-metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS2 -on-MXene heterostructures through in situ sulfidation of Mo2 TiC2 Tx MXene. The computational results show that MoS2 -on-MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li2 S adsorption during the intercalation and conversion reactions. These characteristics render the as-prepared MoS2 -on-MXene heterostructures stable Li-ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications.

Fabrication, biodegradation behavior and cytotoxicity of Mg-nanodiamond composites for implant application.

Mg-based biodegradable implants offer several advantages over their non-degradable or degradable polymeric counterparts used today. However, the low corrosion resistance of Mg in physiologic environment remained as concerns. In this research, nanodiamond (ND) was uniformly dispersed in Mg matrix to induce a protective layer on Mg surface during corrosion. Compared with pure Mg, fabricated Mg-ND nanocomposites had lower corrosion rates, higher corrosion potential, and higher corrosion resistance. Specifically, the corrosion rate of Mg was reduced by 4.5 times by adding 5 wt% of ND particles. Corrosion inhibition effect of ND was thus validated. The chemical interaction and physical adsorption of the ions from simulated body fluid on ND might be the main reason for enhanced corrosion resistance. In vitro biocompatibility test results indicated that Mg-ND nanocomposites were biocompatible since cells growing in contact with corrosion products of Mg-ND maintained high cell viability and healthy morphology. Therefore, Mg-ND nanocomposites with homogenous ND dispersion, enhanced corrosion resistance, and good biocompatibility might be an excellent candidate material for biodegradable implant application.

Accelerating 2D materials discovery.

A large-scale theory-driven approach predicts many new 2D materials.

Treatment of carbon electrodes with Ti3C2Tx MXene coating and thermal method for vanadium redox flow batteries: a comparative study.

One of the significant challenges of vanadium redox flow batteries is connected to the negative electrode where the main reaction of V(ii)/V(iii) and the side reaction of hydrogen evolution compete. To address this issue, we used titanium carbide (Ti3C2Tx) MXene coating via drop-casting to introduce oxygen functional groups and metals on the carbon electrode surface. Characterization through scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the even distribution of Ti3C2Tx MXene on the electrodes and the presence of titanium and termination groups (-O, -Cl, and -F). The cyclic voltammetry analysis of MXene-coated electrodes showed more sharp electrochemical peaks for the V(ii)/V(iii) reaction than thermal-treated electrodes, even at relatively high scan rates. Notably, a relatively high reaction rate of 5.61 × 10-4 cm s-1 was achieved for the V(ii)/V(iii) reaction on MXene-coated electrodes, which shows the competitiveness of the method compared to thermal treatment (4.17 × 10-4 cm s-1). The flow battery tests, at a current density of 130 mA cm-2, using MXene-coated electrodes showed pretty stable discharge capacity for over 100 cycles. In addition, the voltage and energy efficiency were significantly higher than those of the system using untreated electrodes. Overall, this work highlights the potential application of MXene coating in carbon electrode treatment for vanadium redox flow batteries due to remarkable electrocatalytic activity and battery performance, providing a competitive method for thermal treatment.

Strong support effect induced by MXene for the synthesis of metal sulfides nanosheet arrays with sulfur vacancies towards selective CO2-to-CO photoreduction.

Selective CO2-to-CO photoreduction is under intensive research and requires photocatalysts with tuned microstructures to accelerate the reaction kinetics. Here, we report CuInS2 nanosheet arrays with sulfur vacancies (VS) grown on the two-dimensional (2D) support of Ti3C2Tx MXene for CO2-to-CO photoreduction. Our results reveal that the use of Ti3C2Tx induces strong support effect, which causes the hierarchical nanosheet arrays growth of CuInS2 and simultaneously leads to charge transfer from CuInS2 to Ti3C2Tx support, resulting in VS formed in CuInS2. The strong support effect based on Ti3C2Tx is proven to be applicable to prepare a series of different metal indium sulfide arrays with VS. CuInS2 nanosheet arrays with VS supported on Ti3C2Tx benefit the photocatalytic selective reduction of CO2 to CO, manifesting a remarkable over 14.8-fold activity enhancement compared with pure CuInS2. The experimental and computational investigations pinpoint that VS of CuInS2 resulting from the support effect of Ti3C2Tx lowers the barrier of the rate-limiting step of *COOH → *OH + *CO, which is the key to the photoactivity enhancement. This work demonstrates MXene support effects and offers an effective approach to regulate the atomic microstructure of metal sulfides toward enhancing photocatalytic performance.

Alkali cation stabilization of defects in 2D MXenes at ambient and elevated temperatures.

Transition metal carbides have been adopted in energy storage, conversion, and extreme environment applications. Advancements in their 2D counterparts, known as MXenes, enable the design of unique structures at the ~1 nm thickness scale. Alkali cations have been essential in MXenes manufacturing processing, storage, and applications, however, exact interactions of these cations with MXenes are not fully understood. In this study, using Ti3C2Tx, Mo2TiC2Tx, and Mo2Ti2C3Tx MXenes, we present how transition metal vacancy sites are occupied by alkali cations, and their effect on MXene structure stabilization to control MXene's phase transition. We examine this behavior using in situ high-temperature x-ray diffraction and scanning transmission electron microscopy, ex situ techniques such as atomic-layer resolution secondary ion mass spectrometry, and density functional theory simulations. In MXenes, this represents an advance in fundamentals of cation interactions on their 2D basal planes for MXenes stabilization and applications. Broadly, this study demonstrates a potential new tool for ideal phase-property relationships of ceramics at the atomic scale.

Effect of vacancies and edges in promoting water chemisorption on titanium-based MXenes.

The functionality of two-dimensional (2D) transition metal carbides and nitrides (MXenes) in technological applications greatly depends on their wettability. For instance, MXenes' layer stability against degradative oxidation is notably reduced when stored in aqueous solutions, leading to the transformation into oxides. In this work, we study water adsorption on Ti-based MXenes by ab initio calculations. The energy gains for the molecular adsorption on Tin+1XnT2 is evaluated as a function of the termination (T = F, O, OH, mixture), the carbon/nitrogen ratio (X = C, N), the layer thickness (n) and water coverage. MXenes' hydrophilicity tends to increase due to the presence of defects as vacancies and flake edges. We demonstrate that physical adsorption occurs through hydrogen bonding on both defect-free layers and layers containing C/N or Ti atomic vacancies, with -OH terminations providing the strongest interactions (0.40-0.65 eV). In contrast, strong water chemisorption is observed on surfaces with a single termination vacancy (0.60-1.20 eV), edges (0.75-0.85 eV), and clusters of defects (1.00-1.80 eV). We verified that the presence of undercoordinated Ti atoms on the surface is the key factor in promoting H2O chemisorption, i.e., the degradative oxidation.

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.