Nitrogen-Doped Graphene-Like Carbon Intercalated MXene Heterostructure Electrodes for Enhanced Sodium- and Lithium-Ion Storage.

MXene is investigated as an electrode material for different energy storage systems due to layered structures and metal-like electrical conductivity. Experimental results show MXenes possess excellent cycling performance as anode materials, especially at large current densities. However, the reversible capacity is relatively low, which is a significant barrier to meeting the demands of industrial applications. This work synthesizes N-doped graphene-like carbon (NGC) intercalated Ti3C2Tx (NGC-Ti3C2Tx) van der Waals heterostructure by an in situ method. The as-prepared NGC-Ti3C2Tx van der Waals heterostructure is employed as sodium-ion and lithium-ion battery electrodes. For sodium-ion batteries, a reversible specific capacity of 305 mAh g-1 is achieved at a specific current of 20 mA g-1, 2.3 times higher than that of Ti3C2Tx. For lithium-ion batteries, a reversible capacity of 400 mAh g-1 at a specific current of 20 mA g-1 is 1.5 times higher than that of Ti3C2Tx. Both sodium-ion and lithium-ion batteries made from NGC-Ti3C2Tx shows high cycling stability. The theoretical calculations also verify the remarkable improvement in battery capacity within the NGC-Ti3C2O2 system, attributed to the additional adsorption of working ions at the edge states of NGC. This work offers an innovative way to synthesize a new van der Waals heterostructure and provides a new route to improve the electrochemical performance significantly.

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.

Understanding the mechanism of Nb-MXene bioremediation with green microalgae.

Rapidly developing nanotechnologies and their integration in daily applications may threaten the natural environment. While green methods of decomposing organic pollutants have reached maturity, remediation of inorganic crystalline contaminants is major problem due to their low biotransformation susceptibility and the lack of understanding of material surface-organism interactions. Herein, we have used model inorganic 2D Nb-based MXenes coupled with a facile shape parameters analysis approach to track the mechanism of bioremediating 2D ceramic nanomaterials with green microalgae Raphidocelis subcapitata. We have found that microalgae decomposed the Nb-based MXenes due to surface-related physicochemical interactions. Initially, single and few-layered MXene nanoflakes attached to microalgae surfaces, which slightly reduced algal growth. But with prolonged surface interaction, the microalgae oxidized MXene nanoflakes and further decomposed them into NbO and Nb2O5. Since these oxides were nontoxic to microalgal cells, they consumed Nb-oxide nanoparticles by an uptake mechanism thus enabling further microalgae recovery after 72 h of water treatment. The uptake-associated nutritional effects were also reflected by cells' increased size, smoothed shape and changed growth rates. Based on these findings, we conclude that short- and long-term presence of Nb-based MXenes in freshwater ecosystems might cause only negligible environmental effects. Notably, by using 2D nanomaterials as a model system, we show evidence of the possibility of tracking even fine material shape transformations. In general, this study answers an important fundamental question about the surface interaction-associated processes that drive the mechanism of 2D nanomaterials' bioremediation as well as provides the fundamental basis for further short- and long-term investigations on the environmental effects of inorganic crystalline nanomaterials.

Transition Metal Carbo-Chalcogenide "TMCC:" A New Family of 2D Materials.

Here, a new family of 2D transition metal carbo-chalcogenides (TMCCs) is reported, which can be considered a combination of two well-known families, TM carbides (MXenes) and TM dichalcogenides (TMDCs), at the atomic level. Single sheets are successfully obtained from multilayered Nb2 S2 C and Ta2 S2 C using electrochemical lithiation followed by sonication in water. The parent multilayered TMCCs are synthesized using a simple, scalable solid-state synthesis followed by a topochemical reaction. Superconductivity transition is observed at 7.55 K for Nb2 S2 C. The delaminated Nb2 S2 C outperforms both multilayered Nb2 S2 C and delaminated NbS2 as an electrode material for Li-ion batteries. Ab initio calculations predict the elastic constant of TMCC to be over 50% higher than that of TMDC.

Pre-Sodiated Ti3C2Tx MXene Structure and Behavior as Electrode for Sodium-Ion Capacitors.

Layered titanium carbide (Ti3C2Tx) MXene is a promising electrode material for use in next-generation electrochemical capacitors. However, the atomic-level information needed to correlate the distribution of intercalated cations with surface redox reactions, has not been investigated in detail. Herein we report on sodium preintercalated MXene with high sodium content (up to 2Na per Ti3C2Tx formula) using a solution of Na-biphenyl radical anion complex (E0 ≈ -2.6 SHE). Multiple sodiation sites and formation of a two-dimensional sodium domain structure at interfaces/surfaces is identified through combined computational simulations with neutron pair distribution function analysis. The induced layer charges and the redox process characterized by the density-functional tight-binding method on a local scale are found to greatly depend on the location of sodium ions. Electrochemical testing of the pre-sodiated MXene as an electrode material in a sodium-ion capacitor shows excellent reversibility and promising performance, indicating the feasibility of chemical preintercalation as an approach to prepare MXene electrodes for ion capacitors.

Structure and Dynamics of Aqueous Electrolytes Confined in 2D-TiO2/Ti3C2T2 MXene Heterostructures.

The synthesis of heterostructures of different two-dimensional (2D) materials offers an approach to combine advantages of different materials constituting the heterostructure and ultimately enhance their performance for applications such as electrochemical energy storage, achieving high energy, and high-power densities. Understanding the behavior of ions and solvents in confinement between these dissimilar layers is critical to understand their performance and control. Considering aqueous electrolytes, we explore the heterostructure of 2D lepidocrocite-type TiO2 (2D-TiO2) and hydroxylated or O-terminated Ti3C2 MXene using ReaxFF molecular dynamics simulations and elastic/quasielastic neutron scattering techniques. Simulating a bilayer water intercalation, we find that the extent of interlayer hydration is impacted most by the surface terminations on the MXene and is marginally affected by 2D-TiO2. However, the introduction of 2D-TiO2 decreases the water self-diffusion due to the notch sites (i.e., surface oxygen ridges) entrapping water molecules. Intercalating alkali cations into the heterostructures, we find that Li+ is predominantly adsorbed at the 2D-TiO2 surface instead of the MXenes with the preferential occupation of the notch sites. In contrast, Na+ forms a planar solvation with water, while K+ is adsorbed both at the O-terminated MXene and 2D-TiO2. This behavior is altered when OH-terminated MXene is involved-the repulsion from the protons on the MXene surface forces the K+ ions to be adsorbed exclusively to 2D-TiO2, while Na+ retains some of its solvation in the water layer due to its smaller size. In OH-terminated MXenes, we see a consistent transfer of protons from the MXene surface toward 2D-TiO2, implying a greater capacity to store protons in the heterostructures. Of the three cations simulated, Na+ hinders the proton migration the least and Li+ the most because of its position near the 2D-TiO2 surface. Therefore, 2D-TiO2/MXene heterostructures are likely to exhibit a higher energy density but lower power density, especially with Na+ intercalation.

Impact of Cation Intercalation on the Electronic Structure of Ti3C2Tx MXenes in Sulfuric Acid.

Intercalation in Ti3C2Tx MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the Ti3C2Tx nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the Ti3C2Tx MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li-, Na-, K-, and Mg-intercalated Ti3C2Tx MXenes dispersed initially in water and 10 mM sulfuric acid (H2SO4) using X-ray absorption spectroscopy (XAS). The cation intercalation is found to dramatically influence the chemical environment of Ti atoms. The Ti oxidation of the MXene increases progressively upon intercalation of cations of larger sizes after drying in air, while interestingly a low Ti oxidation is observed for all intercalated MXenes after dispersion in diluted H2SO4. In situ XAS at the Ti L-edge was conducted during electrochemical oxidation to probe the changes in the Ti oxidation state in the presence of different cations in H2SO4 aqueous electrolyte. By applying the sensitivity of the Ti L-edge to probe the oxidation state of Ti atoms, we demonstrate that cation-intercalation and H2SO4 environment significantly alter the Ti3C2Tx surface chemistry.