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.

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.

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.

Microplastic ingestion perturbs the microbiome of Aedes albopictus (Diptera: Culicidae) and Aedes aegypti.

Microplastics (MPs) are common environmental pollutants; however, little is known about their effects after ingestion by insects. Here we fed Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) mosquito larvae 1 µm polystyrene MPs and examined the impacts of ingestion on adult emergence rates, gut damage, and fungal and bacterial microbiota. Results show that MPs accumulate in the larval guts, resulting in gut damage. However, little impact on adult emergence rates was observed. MPs are also found in adult guts postemergence from the pupal stage, and adults expel MPs in their frass after obtaining sugar meals. Moreover, MPs effects on insect microbiomes need to be better defined. To address this knowledge gap, we investigated the relationship between MP ingestion and the microbial communities in Ae. albopictus and Ae. aegypti. The microbiota composition was altered by the ingestion of increasing concentrations of MPs. Amplicon sequence variants (ASVs) that contributed to differences in the bacterial and fungal microbiota composition between MP treatments were from the genera Elizabethkingia and Aspergillus, respectively. Furthermore, a decrease in the alpha diversity of the fungal and bacterial microbiota was observed in treatments where larvae ingested MPs. These results highlight the potential for the bacterial and fungal constituents in the mosquito microbiome to respond differently to the ingestion of MPs. Based on our findings and the effects of MP ingestion on the mosquito host micro- and mycobiome, MP pollution could impact the vector competence of important mosquito-transmitted viruses and parasites that cause human and animal diseases.

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.

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.

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.

Recycle and Reuse of Continuous Carbon Fibers from Thermoset Composites Using Joule Heating.

This study demonstrates a new and sustainable methodology for recycling continuous carbon fibers from end-of-life thermoset composite parts using Joule heating. This process addresses the longstanding challenge of efficiently recovering carbon fibers from composite scrap and reusing them to make fresh composites. The conductive carbon fibers volumetrically heat up when an electric current is passed through them, which in turn rapidly heats up the surrounding matrix sufficiently to degrade it. Fibers can be easily separated from the degraded matrix after the direct current (DC) heating process. Fibers reclaimed using this method were characterized to determine their tensile properties and surface chemistry, and compared against both as-received fibers and fibers recycled using conventional oven pyrolysis. The DC- and oven-recycled fibers yielded similar elastic modulus when compared against as-received fibers; however, an around 10-15 % drop was observed in the tensile strength of fibers recycled using either method. Surface characterization showed that DC-recycled fibers and as-received fibers had similar types of functional groups. To demonstrate the reusability of recycled fibers, composites were fabricated by impregnation with epoxy resin and curing. The mechanical properties of these recycled carbon fiber composites (rCFRCs) were compared against conventional recycling methods, and similar modulus and tensile strength values were obtained. This study establishes DC heating as a scalable out-of-oven approach for recycling carbon fibers.

Radio Frequency Heating of Washable Conductive Textiles for Bacteria and Virus Inactivation.

The ongoing COVID-19 pandemic has increased the use of single-use medical fabrics such as surgical masks, respirators, and other personal protective equipment (PPE), which have faced worldwide supply chain shortages. Reusable PPE is desirable in light of such shortages; however, the use of reusable PPE is largely restricted by the difficulty of rapid sterilization. In this work, we demonstrate successful bacterial and viral inactivation through remote and rapid radio frequency (RF) heating of conductive textiles. The RF heating behavior of conductive polymer-coated fabrics was measured for several different fabrics and coating compositions. Next, to determine the robustness and repeatability of this heating response, we investigated the textile's RF heating response after multiple detergent washes. Finally, we show a rapid reduction of bacteria and virus by RF heating our conductive fabric. 99.9% of methicillin-resistant Staphylococcus aureus (MRSA) was removed from our conductive fabrics after only 10 min of RF heating; human cytomegalovirus (HCMV) was completely sterilized after 5 min of RF heating. These results demonstrate that RF heating conductive polymer-coated fabrics offer new opportunities for applications of conductive textiles in the medical and/or electronic fields.

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.

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.

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.