MXene/TiO2 Heterostructure-Decorated Hard Carbon with Stable Ti-O-C Bonding for Enhanced Sodium-Ion Storage

Hard carbon (HC) has attracted considerable attention in the application of sodium-ion battery (SIB) anodes, but the poor realistic capacity and low rate performance severely hinder their practical application. Herein we report a solvent mechanochemical protocol for the in situ fabrication of the HC-MXene/TiO₂ electrode by functionalizing MXene to improve the electrochemical performance of the batteries. MXene (Ti₃C₂T_x) with abundant oxygen-containing functional groups reacts with HC particles in the ball milling process to form a Ti-O-C covalent cross-linked HC-MXene composite, in which the edge of the MXene nanosheets is in situ oxidized by air to form TiO₂ nanorods, forming a regular 1D/2D MXene/TiO₂ heterojunction structure. Ti-O-C covalent bonding can protect the heterojunction structures from pulverization and detachment from the current collector during charge/discharge cycles due to sodium-ion intercalation/detachment, thus improving the stability of the electrode structure. Meanwhile, the MXene/TiO₂ heterojunction can form a 3D conductive network and provide more active sites. The resulting HC-MXene/TiO₂ electrode exhibits superior electrode capacity (660 mAh g^-1), making it a promising anode material for SIBs. This simple and efficient method for preparing MXene/TiO₂ heterojunction-decorated HC provides a new perspective on the structural design of MXene and carbon material composites for SIBs.

Host-Guest Intercalation Chemistry in MXenes and Its Implications for Practical Applications

The ever-increasing demand on developing layered materials for practical applications, such as electrochemical energy storage, responsive materials, nanofluidics, and environmental remediation, requires the profound understanding and artful exploitation of interlayer engineering or intercalation chemistry. The past decade has witnessed the massive exploration of a recently discovered 2D material-transition metal carbides, carbonitrides, and nitrides (referred to as MXenes), which began to take hold of a myriad of applications owing to the abundant possibilities on their compositions and intercalation states. However, application-targeted manipulation of the material performance of MXenes is constrained by the dearth of deep comprehension on fundamental intercalation chemistry/physics. To this end, the aim of this review is to provide a holistic discussion on the intercalation chemistry in MXenes and the physical properties of MXene intercalation compounds. On the basis of this, potential solutions for the challenges confronted in the synthesis, tuning of material properties, and practical applications are proposed, which are also expected to reinvigorate the exploration of layered materials that are similar to MXenes.

High specific capacitance cotton fiber electrode enhanced with PPy and MXene by in situ hybrid polymerization

Fiber electrodes are the main functional elements of flexible and textile-based storage devices. This study proposes a Polypyrrole (PPy) and MXene composite, grown on cotton fiber, as a high capacitance electrode. Pyrrole (Py) and MXene are processed and deposited along with an in-situ polymerization. The mass and areal capacitance of the assembled (PPy/MXene)@Cotton electrode respectively reach to 506.6 F g^-1, at current density of 1 A g^-1 and 455.9 mF cm^-2 at scan rate of 0.9 mA cm^-2. These values outperform the PPy@Cotton fiber electrode, around 45.8% and 119% respectively. As-prepared fiber electrodes with mechanical strength of 107.3 MPa and conductivity of 60.8 S/m, offer intriguing application prospects in the field of weaving and flexible fibrous supercapacitors.

Double transition metal MXene (TixTa4-xC3) 2D materials as anodes for Li-ion batteries

A bi-metallic titanium-tantalum carbide MXene, TixTa(4-x)C3 is successfully prepared via etching of Al atoms from parent TixTa(4-x)AlC3 MAX phase for the first time. X-ray diffractometer and Raman spectroscopic analysis proved the crystalline phase evolution from the MAX phase to the lamellar MXene arrangements. Also, the X-ray photoelectron spectroscopy (XPS) study confirmed that the synthesized MXene is free from Al after hydro fluoric acid (HF) etching process as well as partial oxidation of Ti and Ta. Moreover, the FE-SEM and TEM characterizations demonstrate the exfoliation process tailored by the TixTa(4-x)C3 MXene after the Al atoms from its corresponding MAX TixTa(4-x)AlC3 phase, promoting its structural delamination with an expanded interlayer d-spacing, which can allow an effective reversible Li-ion storage. The lamellar TixTa(4-x)C3 MXene demonstrated a reversible specific discharge capacity of 459 mAhg-1 at an applied C-rate of 0.5 °C with a capacity retention of 97% over 200 cycles. An excellent electrochemical redox performance is attributed to the formation of a stable, promising bi-metallic MXene material, which stores Li-ions on the surface of its layers. Furthermore, the TixTa(4-x)C3 MXene anode demonstrate a high rate capability as a result of its good electron and Li-ion transport, suggesting that it is a promising candidate as Li-ion anode material.

Filtration Materials Modified with 2D Nanocomposites-A New Perspective for Point-of-Use Water Treatment

Point-of-use (POU) water treatment systems and devices play an essential role in limited access to sanitary safe water resources. The filtering materials applied in POU systems must effectively eliminate contaminants, be readily produced and stable, and avoid secondary contamination of the treated water. We report an innovative, 2D Ti3C2/Al2O3/Ag/Cu nanocomposite-modified filtration material with the application potential for POU water treatment. The material is characterized by improved filtration velocity relative to an unmodified reference material, effective elimination of microorganisms, and self-disinfecting potential, which afforded the collection of 99.6% of bacteria in the filter. The effect was obtained with nanocomposite levels as low as 1%. Surface oxidation of the modified material increased its antimicrobial efficiency. No secondary release of the nanocomposites into the filtrate was observed and confirmed the stability of the material and its suitability for practical application in water treatment.

MXenes for Rechargeable Batteries Beyond the Lithium-Ion

Research on next-generation battery technologies (beyond Li-ion batteries, or LIBs) has been accelerating over the past few years. A key challenge for these emerging batteries has been the lack of suitable electrode materials, which severely limits their further developments. MXenes, a new class of 2D transition metal carbides, carbonitrides, and nitrides, are proposed as electrode materials for these emerging batteries due to several desirable attributes. These attributes include large and tunable interlayer spaces, excellent hydrophilicity, extraordinary conductivity, compositional diversity, and abundant surface chemistries, making MXenes promising not only as electrode materials but also as other components in the cells of emerging batteries. Herein, an overview and assessment of the utilization of MXenes in rechargeable batteries beyond LIBs, including alkali-ion (e.g., Na⁺ , K⁺ ) storage, multivalent-ion (e.g., Mg²⁺ , Zn²⁺ , and Al³⁺ ) storage, and metal batteries are presented. In particular, the synthetic strategies and properties of MXenes that enable MXenes to play various roles as electrodes, metal anode protective layers, sulfur hosts, separator modification layers, and conductive additives in these emerging batteries are discussed. Moreover, a perspective on promising future research directions on MXenes and MXene-based materials, ranging from material design and processing, fundamental understanding of the reaction mechanisms, to device performance optimization strategies is provided.

Toward Nanotechnology-Enabled Approaches against the COVID-19 Pandemic

The COVID-19 outbreak has fueled a global demand for effective diagnosis and treatment as well as mitigation of the spread of infection, all through large-scale approaches such as specific alternative antiviral methods and classical disinfection protocols. Based on an abundance of engineered materials identifiable by their useful physicochemical properties through versatile chemical functionalization, nanotechnology offers a number of approaches to cope with this emergency. Here, through a multidisciplinary Perspective encompassing diverse fields such as virology, biology, medicine, engineering, chemistry, materials science, and computational science, we outline how nanotechnology-based strategies can support the fight against COVID-19, as well as infectious diseases in general, including future pandemics. Considering what we know so far about the life cycle of the virus, we envision key steps where nanotechnology could counter the disease. First, nanoparticles (NPs) can offer alternative methods to classical disinfection protocols used in healthcare settings, thanks to their intrinsic antipathogenic properties and/or their ability to inactivate viruses, bacteria, fungi, or yeasts either photothermally or via photocatalysis-induced reactive oxygen species (ROS) generation. Nanotechnology tools to inactivate SARS-CoV-2 in patients could also be explored. In this case, nanomaterials could be used to deliver drugs to the pulmonary system to inhibit interaction between angiotensin-converting enzyme 2 (ACE2) receptors and viral S protein. Moreover, the concept of "nanoimmunity by design" can help us to design materials for immune modulation, either stimulating or suppressing the immune response, which would find applications in the context of vaccine development for SARS-CoV-2 or in counteracting the cytokine storm, respectively. In addition to disease prevention and therapeutic potential, nanotechnology has important roles in diagnostics, with potential to support the development of simple, fast, and cost-effective nanotechnology-based assays to monitor the presence of SARS-CoV-2 and related biomarkers. In summary, nanotechnology is critical in counteracting COVID-19 and will be vital when preparing for future pandemics.

Nb-Doped MXene With Enhanced Energy Storage Capacity and Stability

MXenes present unique features as materials for energy storage; however, limited interlayer distance, and structural stability with ongoing cycling limit their applications. Here, we have developed a unique method involving incorporating Nb atoms into MXene (Ti₃C₂) to enhance its ability to achieve higher ionic storage and longer stability. Computational analysis using density functional theory was performed that explained the material structure, electronic structure, band structure, and density of states in atomistic detail. Nb-doped MXene showed a good charge storage capacity of 442.7 F/g, which makes it applicable in a supercapacitor. X-ray diffraction (XRD) indicated c-lattice parameter enhancement after Nb-doping in MXene (from 19.2A° to 23.4A°), which showed the effect of the introduction of an element with a larger ionic radius (Nb). Also, the bandgap changes from 0.9 eV for pristine MXene to 0.1 eV for Nb-doped MXene, which indicates that the latter has the signature of increased conductivity due to more metallic nature, in support of the experimental results. This work presents not only the effect of doping in MXene but also helps to explain the phenomena involved in changes in physical parameters, advancing the field of energy storage based on 2D materials.

Condition optimization for exfoliation of two dimensional titanium carbide (Ti3C2T x )

The new class of 2D MXene material exfoliated from transition metal carbides receives increasing research interest due to its extraordinary properties and high potential in energy and environmental applications. However, the exfoliation of Ti₃C₂T _x , a widely studied MXene, from its precursor Ti₃AlC₂ by chemical etching in HF solution remains to be optimized. This study investigated the optimum exfoliation condition through systematic evaluating potential effects of reaction parameters, including the weight ratio of Ti₃AlC₂ in HF solution, etching time, reaction temperature, repeating etching, and sonication, on the yield, purity, and structure of produced Ti₃C₂T _x . Results show that a high weight percentage (5 wt%) of Ti₃AlC₂ etching at 50 °C for 36 h produced highly exfoliated MXene material. Etching at lower weight percentages (0.6-2.5 wt%) of Ti₃AlC₂ resulted in observable byproduct (AlF₃). Degradation of MXene layers with AlF₃ enrichment was observed under prolonged etching or higher temperatures. Room temperature etching failed to exfoliate Ti₃AlC₂ and the repeated etching denatured the MXene material. Introduction of controlled sonication during the etching produced highly exfoliated MXene with minimum etching time, which can be a promising alternative for high quality MXene production.

Synthesis of two-dimensional titanium nitride Ti4N3 (MXene)

We report on the synthesis of the first two-dimensional transition metal nitride, Ti4N3-based MXene. In contrast to the previously reported MXene synthesis methods - in which selective etching of a MAX phase precursor occurred in aqueous acidic solutions - here a molten fluoride salt is used to etch Al from a Ti4AlN3 powder precursor at 550 °C under an argon atmosphere. We further delaminated the resulting MXene to produce few-layered nanosheets and monolayers of Ti4N3Tx, where T is a surface termination (F, O, or OH). Density functional theory calculations of bare, non-terminated Ti4N3 and terminated Ti4N3Tx were performed to determine the most energetically stable form of this MXene. Bare and functionalized Ti4N3 are predicted to be metallic. Bare Ti4N3 is expected to show magnetism, which is significantly reduced in the presence of functional groups.

Recent Advances in Two-Dimensional Materials beyond Graphene

The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here, we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.

25th anniversary article: MXenes: a new family of two-dimensional materials

Recently a new, large family of two-dimensional (2D) early transition metal carbides and carbonitrides, called MXenes, was discovered. MXenes are produced by selective etching of the A element from the MAX phases, which are metallically conductive, layered solids connected by strong metallic, ionic, and covalent bonds, such as Ti2 AlC, Ti3 AlC2 , and Ta4 AlC3 . MXenes -combine the metallic conductivity of transition metal carbides with the hydrophilic nature of their hydroxyl or oxygen terminated surfaces. In essence, they behave as "conductive clays". This article reviews progress-both -experimental and theoretical-on their synthesis, structure, properties, intercalation, delamination, and potential applications. MXenes are expected to be good candidates for a host of applications. They have already shown promising performance in electrochemical energy storage systems. A detailed outlook for future research on MXenes is also presented.

Two-dimensional transition metal carbides

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti(2)AlC, Ta(4)AlC(3), (Ti(0.5),Nb(0.5))(2)AlC, (V(0.5),Cr(0.5))(3)AlC(2), and Ti(3)AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.