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.

Upscaled Synthesis Protocol for Phase-Pure, Colloidally Stable MXenes with Long Shelf Lives.

MXenes are electrically conductive 2D transition metal carbides/nitrides obtained by the etching of nanolaminated MAX phase compounds, followed by exfoliation to single- or few-layered nanosheets. The mainstream chemical etching processes have evolved from pure hydrofluoric acid (HF) etching into the innovative "minimally intensive layer delamination" (MILD) route. Despite their current popularity and remarkable application potential, the scalability of MILD-produced MXenes remains unproven, excluding MXenes from industrial applications. This work proposes a "next-generation MILD" (NGMILD) synthesis protocol for phase-pure, colloidally stable MXenes that withstand long periods of dry storage. NGMILD incorporates the synergistic effects of a secondary salt, a richer lithium (Li) environment, and iterative alcohol-based washing to achieve high-purity MXenes, while improving etching efficiency, intercalation, and shelf life. Moreover, NGMILD comprises a sulfuric acid (H2 SO4 ) post-treatment for the selective removal of the Li3 AlF6 impurity that commonly persists in MILD-produced MXenes. This work demonstrates the upscaled NGMILD synthesis of (50 g) phase-pure Ti3 C2 Tz MXene clays with high extraction yields (>22%) of supernatant dispersions. Finally, NGMILD-produced MXene clays dry-stored for six months under ambient conditions experience minimal degradation, while retaining excellent redispersibility. Overall, the NGMILD protocol is a leap forward toward the industrial production of MXenes and their subsequent market deployment.

Effect of terminal groups on the degradation stability of Ti3C2Tz MXenes.

MXenes are 2D nanomaterials which have gained considerable attention from researchers since their discovery in 2011. However, the propensity of these 2D nanomaterials to degrade affects their shelf life. While many studies have focused on the external factors affecting the degradation of MXenes, the effect of internal factors such as terminal groups is not well understood. In this paper, we use -Br and -Cl terminations as model terminal groups to compare the degradation stability of MXenes. From our experiments we observe that -Br terminated ML-Ti3C2Tz degrades faster than -Cl terminated ML-Ti3C2Tz. Our study confirms that terminal groups do affect the degradation rate of Ti3C2Tz. The results suggest that the differences in bond dissociation energy of the M-X bond are responsible for variations in the degradation stability of MXenes. This model study can be generalized to compare the effect of terminal groups on the degradation stability of MXenes.

Advances in the Chemical Stabilization of MXenes.

MXenes are 2D nanomaterials with a wide array of possible compositions; they feature a unique combination of properties such as high electrical conductivity, hydrophilicity, and colloidal stability which makes them attractive for a variety of applications. However, the shelf life and industrial utility of MXenes face challenges due to their tendency to oxidize and disintegrate, particularly in dispersions. Thus, it is crucial to find effective ways to ensure the degradation stability of MXenes. This feature article reviews the key factors affecting the degradation of MXenes such as pH, concentration of the dispersion, humidity, and storage temperature. In addition, we review our group's progress in mitigating the degradation of MXenes such as low-temperature storage, the use of antioxidants, and thermal annealing, particularly for Ti3C2Tz. These simple approaches may allow for applications of MXenes on a commercial scale.

Exfoliation, delamination, and oxidation stability of molten salt etched Nb2CTz MXene nanosheets.

Despite numerous prior reports of molten salt etching of MAX phases, few of these reports achieved water-dispersible MXene nanosheets, and none for Nb-based MXenes. Here we demonstrate the synthesis and aqueous dispersibility of Nb2CTZ nanosheets via molten salt etching and utilizing a KOH wash to add hydroxyl surface groups. However, little is known about the oxidation of molten salt etched MXenes compared to acid-etched MXenes. Our results indicate slower oxidation behavior for MXenes etched by molten salts, which may be due to the decreased amount of oxygen-containing terminal groups.

Interparticle interactions and rheological signatures of Ti3C2Tz MXene dispersions.

HYPOTHESIS: We hypothesize that dispersed Ti3C2Tz MXene particle interactions are reflected in the bulk viscoelastic properties of the dispersions and can be analyzed using classical colloidal theory for anisotropic particles. The relevant kinetic theory for Brownian anisotropic particles is given by the Doi and Edwards (D-E) Model, and the Maxwell Model is used to fit the relaxation times as a function of frequency. Such behavior is relevant to a variety of MXene processing techniques, particularly printing and coating. EXPERIMENTS: Small oscillatory shear tests were performed for dilute Ti3C2Tz MXene aqueous dispersions as a function of their concentration and temperature. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), ζ potential measurements, Dynamic Light Scattering (DLS) were used to characterize the Ti3C2Tz MXene nanoparticles. FINDINGS: Ti3C2Tz dispersions show gel-like and viscous-like behavior at low and high temperatures, respectively. Experimental relaxation times fitted to the Maxwell model are found to be close to the theoretical values. However, at high temperatures, relaxation time values differ due to the inter-particle interactions, even in the dilute concentration regime. For Ti3C2Tz dispersions, aggregation, and clustering can have dramatic consequences for dispersion rheology, including gelation, as the sample transitions from liquid-like to solid-like behavior.

Water-dispersible Ti3C2Tz MXene nanosheets by molten salt etching.

Molten-salt etching of Ti3AlC2 MAX phase offers a promising route to produce 2D Ti3C2Tz (MXene) nanosheets without hazardous HF. However, molten-salt etching results in MXene clays that are not water dispersible, thus preventing further processing. This occurs because molten-salt etching results in a lack of -OH terminal groups rendering the MXene clay hydrophobic. Here, we demonstrate a method that produces water-dispersible Ti3C2Tz nanosheets using molten salt (SnF2) to etch. In molten salt etching, SnF2 diffuses between the layers to form AlF3 and Sn as byproducts, separating the layers. The stable, aqueous Ti3C2Tz dispersion yields a ζ potential of -31.7 mV, because of -OH terminal groups introduced by KOH washing. X-ray diffraction and electron microscopy confirm the formation of Ti3C2Tz etched clay with substantial d-spacing as compared with clay etched with HF. This work is the first to use molten salt etching to successfully prepare colloidally stable aqueous dispersions of Ti3C2Tz nanosheets.

Synthesis and Electronic Applications of Particle-Templated Ti3C2Tz MXene-Polymer Films via Pickering Emulsion Polymerization.

MXene/polymer composites have gained widespread attention due to their high electrical conductivity and extensive applications, including electromagnetic interference (EMI) shielding, energy storage, and catalysis. However, due to the difficulty of dispersing MXenes in common polymers, the fabrication of MXene/polymer composites with high electrical conductivity and satisfactory EMI shielding properties is challenging, especially at low MXene loadings. Here, we report the fabrication of MXene-armored polymer particles using dispersion polymerization in Pickering emulsions and demonstrate that these composite powders can be used as feedstocks for MXene/polymer composite films with excellent EMI shielding performance. Ti3C2Tz nanosheets are used as the representative MXene, and three different monomers are used to prepare the armored particles. The presence of nanosheets on the particle surface was confirmed by X-ray photoelectron spectroscopy and scanning electron microscopy. Hot pressing the armored particles above Tg of the polymer produced Ti3C2Tz/polymer composite films; the films are electrically conductive because of the network of nanosheets templated by the particle feedstocks. For example, the particle-templated Ti3C2Tz/polystyrene film had an electrical conductivity of 0.011 S/cm with 1.2 wt % of Ti3C2Tz, which resulted in a high radio frequency heating rate of 13-15 °C/s in the range of 135-150 MHz and an EMI shielding effectiveness of ∼21 dB within the X band. This work provides a new approach to fabricate MXene/polymer composite films with a templated electrical network at low MXene loadings.

One-step hydrothermal synthesis of porous Ti3C2Tz MXene/rGO gels for supercapacitor applications.

Titanium carbide/reduced graphene oxide (Ti3C2Tz/rGO) gels were prepared by a one-step hydrothermal process. The gels show a highly porous structure with a surface area of ∼224 m2 g-1 and average pore diameter of ∼3.6 nm. The content of GO and Ti3C2Tz nanosheets in the reaction precursor was varied to yield different microstructures. The supercapacitor performance of Ti3C2Tz/rGO gels varied significantly with composition. Specific capacitance initially increased with increasing Ti3C2Tz content, but at high Ti3C2Tz content gels cannot be formed. Also, the retention of capacitance decreased with increasing Ti3C2Tz content. Ti3C2Tz/rGO gel electrodes exhibit enhanced supercapacitor properties with high potential window (1.5 V) and large specific capacitance (920 F g-1) in comparison to pure rGO and Ti3C2Tz. The synergistic effect of EDLC from rGO and redox capacitance from Ti3C2Tz was the reason for the enhanced supercapacitor performance. A symmetric two-electrode supercapacitor cell was constructed with Ti3C2Tz/rGO, which showed very high areal capacitance (158 mF cm-2), large energy density (∼31.5 µW h cm-2 corresponding to a power density of ∼370 µW cm-2), and long stability (∼93% retention) after 10 000 cycles.

Electronic and Optical Property Control of Polycation/MXene Layer-by-Layer Assemblies with Chemically Diverse MXenes.

MXenes, 2D nanomaterials derived from ceramic MAX phases, have drawn considerable interest in a wide variety of fields including energy storage, catalysis, and sensing. There are many possible MXene compositions due to the chemical and structural diversity of parent MAX phases, which can bear different possible metal atoms "M", number of layers, and carbon or nitrogen "X" constituents. Despite the potential variety in MXene types, the bulk of MXene research focuses upon the first MXene discovered, Ti3C2T. With the recent discovery of polymer/MXene multilayer assemblies as thin films and coatings, there is a need to broaden the accessible types of multilayers by including MXenes other than Ti3C2Tz; however, it is not clear how altering the MXene type influences the resulting multilayer growth and properties. Here, we report on the first use of MXenes other than Ti3C2Tz, specifically Ti2CTz and Nb2CTz, for the layer-by-layer (LbL) assembly of polycation/MXene multilayers. By comparing these MXenes, we evaluate both how changing M (Ti vs Nb) and "n" (Ti3C2Tzvs Ti2CTz) affect the growth and properties of the resulting multilayer. Specifically, the aqueous LbL assembly of each MXene with poly(diallyldimethylammonium) into films and coatings is examined. Further, we compare the oxidative stability, optoelectronic properties (refractive index, absorption coefficient, optical conductivity, and direct and indirect optical band gaps), and the radio frequency heating response of each multilayer. We observe that MXene multilayers with higher "n" are more electrically conductive and oxidatively stable. We also demonstrate that Nb2CTz containing films have lower optical band gaps and refractive indices at the cost of lower electrical conductivities as compared to their Ti2CTz counterparts. Our work demonstrates that the properties of MXene/polycation multilayers are highly dependent on the choice of constituent MXene and that the MXene type can be altered to suit specific applications.

Room temperature crack-healing in an atomically layered ternary carbide.

Ceramic materials provide outstanding chemical and structural stability at high temperatures and in hostile environments but are susceptible to catastrophic fracture that severely limits their applicability. Traditional approaches to partially overcome this limitation rely on activating toughening mechanisms during crack growth to postpone fracture. Here, we demonstrate a more potent toughening mechanism that involves an intriguing possibility of healing the cracks as they form, even at room temperature, in an atomically layered ternary carbide. Crystals of this class of ceramic materials readily fracture along weakly bonded crystallographic planes. However, the onset of an abstruse mode of deformation, referred to as kinking in these materials, induces large crystallographic rotations and plastic deformation that physically heal the cracks. This implies that the toughness of numerous other layered ceramic materials, whose broader applications have been limited by their susceptibility to catastrophic fracture, can also be enhanced by microstructural engineering to promote kinking and crack-healing.

Layer-by-Layer Assembly of Reduced Graphene Oxide and MXene Nanosheets for Wire-Shaped Flexible Supercapacitors.

As the demand for wearable electronic devices increases, interest in small, light, and deformable energy storage devices follows suit. Among these devices, wire-shaped supercapacitors (WSCs) are considered key components of wearable technology due to their geometric similarity to woven fiber. One potential method for creating WSC devices is the layer-by-layer (LbL) assembly technique, which is a "bottom-up" method for electrode fabrication. WSCs require conformal and adhesive coatings of the functional material to the wire-shaped substrate, which is difficult to obtain with other processing techniques such as vacuum filtration or spray-coating. However, the LbL assembly technique produces conformal and robust coatings that can be deposited onto a variety of substrates and shapes, including wires. In this study, we report WSCs made using the LbL assembly of alternating layers of positively charged reduced graphene oxide functionalized with poly(diallyldimethylammonium chloride) and negatively charged Ti3C2Tx MXene nanosheets conformally deposited on activated carbon yarns. In this construct, the added LbL film enhances capacitance, energy density, and power density by 240, 227, and 109%, respectively, relative to the uncoated activated carbon yarn, yielding high specific and volumetric capacitances (237 F g-1, 2193 F cm-3). In addition, the WSC possesses good mechanical stability, retaining 90% of its initial capacity after 200 bending cycles. This study demonstrates that LbL coatings on carbon yarns are promising as linear energy storage devices for fibrous electronics.

Flocculation of MXenes and Their Use as 2D Particle Surfactants for Capsule Formation.

MXenes, transition metal carbides or nitrides, have gained great attention in recent years due to their high electrical conductivity and catalytic activity, hydrophilicity, and diverse surface chemistry. However, high hydrophilicity and negative ζ potential of the MXene nanosheets limit their processability and interfacial assembly. Previous examples for modifying the dispersibility and wettability of MXenes have focused on the use of organic ligands, such as alkyl amines, or covalent modification with triethoxysilanes. Here, we report a simple method to access MXene-stabilized oil-in-water emulsions by using common inorganic salts (e.g., NaCl) to flocculate the nanosheets and demonstrate the use of these Pickering emulsions to prepare capsules with shells of MXene and polymer. Ti3C2Tz nanosheets are used as the representative MXene. The salt-flocculated MXene nanosheets produce emulsions that are stable for days, as determined by optical microscopy imaging. The incorporation of a diisocyanate in the discontinuous oil phase and diamine in the continuous water phase led to interfacial polymerization and the formation of capsules. The capsules were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), confirming the presence of both polymer and nanosheets. The addition of ethanol to the capsules led to the removal of the toluene core and retention of the shell structure. The ability to assemble MXene nanosheets at fluid-fluid interfaces without the use of ligands or cosurfactants expands the accessible material constructs relevant for biomedical engineering, water purification, energy storage, electromagnetic electronics, catalysis, and so on.

Minimizing two-dimensional Ti3C2Tx MXene nanosheet loading in carbon-free silicon anodes.

Silicon anodes are promising for high energy batteries because of their excellent theoretical gravimetric capacity (3579 mA h g-1). However, silicon's large volume expansion and poor conductivity hinder its practical application; thus, binders and conductive additives are added, effectively diluting the active silicon material. To address this issue, reports of 2D MXene nanosheets have emerged as additives for silicon anodes, but many of these reports use high MXene compositions of 22-66 wt%, still presenting the issue of diluting the active silicon material. Herein, this report examines the question of what minimal amount of MXene nanosheets is required to act as an effective additive while maximizing total silicon anode capacity. A minimal amount of only 4 wt% MXenes (with 16 wt% sodium alginate and no carbon added) yielded silicon anodes with a capacity of 900 mA h gSi-1 or 720 mA h gtotal-1 at the 200th cycle at 0.5 C-rate. Further, this approach yielded the highest specific energy on a total electrode mass basis (3100 W h kgtotal-1) as comapared to other silicon-MXene constructs (∼115-2000 Wh kgtotal-1) at a corresponding specific power. The stable electrode performance even with a minimal MXene content is attributed to several factors: (1) highly uniform silicon electrodes due to the dispersibility of MXenes in water, (2) the high MXene aspect ratio that enables improved electrical connections, and (3) hydrogen bonding among MXenes, sodium alginate, and silicon particles. All together, a much higher silicon loading (80 wt%) is attained with a lower MXene loading, which then maximizes the capacity of the entire electrode.

Spark Plasma Sintered B4C-Structural, Thermal, Electrical and Mechanical Properties.

The structural, thermal, electrical and mechanical properties of fully dense B4C ceramics, sintered using Spark Plasma Sintering (SPS), were studied and compared to the properties of B4C ceramics previously published in the literature. New results on B4C's mechanical responses were obtained by nanoindentation and ring-on-ring biaxial strength testing. The findings contribute to a more complete knowledge of the properties of B4C ceramics, an important material in many industrial applications.

Rapid Heating of Silicon Carbide Fibers under Radio Frequency Fields and Application in Curing Preceramic Polymer Composites.

Silicon carbide (SiC) fibers are widely used as a reinforcement in ceramic matrix composites due to their high mechanical strength and superior thermal resistance. Here, we investigate the rapid radio frequency (RF) heating response of two types of SiC fibers (Hi-Nicalon and Sylramic) in the 1-200 MHz frequency range. Hi-Nicalon fibers exhibit a surprisingly rapid RF heating response of 240 °C/s in the perpendicular orientation, and this property could be exploited for oven-free and noncontact processing of composites with SiC fibers. The presence of excess carbon on the surface of Hi-Nicalon fibers is most likely responsible for the RF heating response and significantly higher temperatures in the parallel as compared to perpendicular alignment of fibers to the electric field. The RF heating response of Hi-Nicalon SiC fibers was utilized to heat preceramic polymers (polycarbosilanes) infiltrated in SiC fibers and cure them to ceramic matrix composites (CMCs) using RF applicators. A noncontact RF heating setup to pyrolyze the precursor polymers under inert conditions and make SiC/SiC composites is also developed.

Heating of Ti3C2Tx MXene/polymer composites in response to Radio Frequency fields.

Here we report for the first time that Ti3C2Tx/polymer composite films rapidly heat when exposed to low-power radio frequency fields. Ti3C2Tx MXenes possess a high dielectric loss tangent, which is correlated with this rapid heating under electromagnetic fields. Thermal imaging confirms that these structures are capable of extraordinary heating rates (as high as 303 K/s) that are frequency- and concentration-dependent. At high loading (and high conductivity), Ti3C2Tx MXene composites do not heat under RF fields due to reflection of electromagnetic waves, whereas composites with low conductivity do not heat due to the lack of an electrical percolating network. Composites with an intermediate loading and a conductivity between 10-1000 S m-1 rapidly generate heat under RF fields. This finding unlocks a new property of Ti3C2Tx MXenes and a new material for potential RF-based applications.

Layer-by-Layer Assembly of Polyaniline Nanofibers and MXene Thin-Film Electrodes for Electrochemical Energy Storage.

The growing demand for compact energy storage devices may be met through the use of thin-film microbatteries, which generally rely on charge storage in thin or conformal layers. A promising technique for creating thin-film electrodes is layer-by-layer (LbL) assembly, based on the alternating adsorption of oppositely charged species to a surface to form a nanostructured electrode. Thin-film energy storage devices must have a high energy density within a limited space, so new electrode structures, materials, and assembly methods are important. To this end, both two-dimensional MXenes and polyaniline nanofibers (PNFs) have shown promising energy storage properties. Here, we report on the LbL assembly of positively charged PNFs and negatively charged Ti3C2Tx MXenes into hybrid electrodes for thin-film energy storage devices. The successful assembly is demonstrated in which MXenes and PNFs are deposited in films of 49 nm/layer pair thickness. The resulting composition was 77 wt % PNFs and 23 wt % MXenes. The charge storage process was deconvoluted into faradaic/non-faradaic contributions and separated into contributions from PNFs and MXenes. A sandwich cell showed a maximum areal capacity, energy, and power of 17.6 µA h cm-2, 22.1 µW h cm-2, and 1.5 mW cm-2, respectively, for PNF/MXene multilayers of about 2 µm thickness. This work suggests the possibility of using LbL PNF/MXene thin films as electrode materials for thin-film energy storage devices used in next-generation small electronics.

Surface-agnostic highly stretchable and bendable conductive MXene multilayers.

Stretchable, bendable, and foldable conductive coatings are crucial for wearable electronics and biometric sensors. These coatings should maintain functionality while simultaneously interfacing with different types of surfaces undergoing mechanical deformation. MXene sheets as conductive two-dimensional nanomaterials are promising for this purpose, but it is still extremely difficult to form surface-agnostic MXene coatings that can withstand extreme mechanical deformation. We report on conductive and conformal MXene multilayer coatings that can undergo large-scale mechanical deformation while maintaining a conductivity as high as 2000 S/m. MXene multilayers are successfully deposited onto flexible polymer sheets, stretchable poly(dimethylsiloxane), nylon fiber, glass, and silicon. The coating shows a recoverable resistance response to bending (up to 2.5-mm bending radius) and stretching (up to 40% tensile strain), which was leveraged for detecting human motion and topographical scanning. We anticipate that this discovery will allow for the implementation of MXene-based coatings onto mechanically deformable objects.

On the stochastic phase stability of Ti2AlC-Cr2AlC.

The quest towards expansion of the M n+1AX n design space has been accelerated with the recent discovery of several solid solution and ordered phases involving at least two M n+1AX n end members. Going beyond the nominal M n+1AX n compounds enables not only fine tuning of existing properties but also entirely new functionality. This search, however, has been mostly done through painstaking experiments as knowledge of the phase stability of the relevant systems is rather scarce. In this work, we report the first attempt to evaluate the finite-temperature pseudo-binary phase diagram of the Ti2AlC-Cr2AlC via first-principles-guided Bayesian CALPHAD framework that accounts for uncertainties not only in ab initio calculations and thermodynamic models but also in synthesis conditions in reported experiments. The phase stability analyses are shown to have good agreement with previous experiments. The work points towards a promising way of investigating phase stability in other MAX Phase systems providing the knowledge necessary to elucidate possible synthesis routes for M n+1AX n systems with unprecedented properties.