V2CTx MXene Research Articles

On the rapid in situ oxidation of two-dimensional V2CTz MXene in culture cell media and their cytotoxicity

The plethora of emerging two-dimensional (2D) materials exhibit wide potential application in novel technologies and advanced devices. However, their stability in environmental conditions could be an issue, affecting their application possibilities and posing health risks. Moreover, their decomposed leftovers can also induce a negative influence on human health. In particular, transition metal carbides commonly referred to as MXenes are susceptible to environmental oxidation being decomposed toward transition metal oxides and carbide-derived carbon. In this study we focused on the oxidation-state-related in vitro cytotoxicity of delaminated V2CTz onto immortalized keratinocytes (HaCaT) and malignant melanoma (A375) human cell lines. Due to the fact, that the V2CTx MXenes are least stable from all known obtained MXenes up to date, the vanadium ones were a practical choice to visualize the oxidation-cytotoxic correlation keeping the standards of 24–48 h of cell culturing. We found that the oxidation of V2CTz highly increases their cytotoxicity toward human cells, which is also time and dose dependent. The identified mode of action relates to the cell cycle as well as cellular membrane disintegration through direct physicochemical interactions.

In Situ Electrochemical Synthesis of MXenes without Acid/Alkali Usage in/for an Aqueous Zinc Ion Battery

AbstractThe traditional method to fabricate a MXene based energy storage device starts from etching MAX phase particles with dangerous acid/alkali etchants to MXenes, followed by device assembly. This is a multistep protocol and is not environmentally friendly. Herein, an all‐in‐one protocol is proposed to integrate synthesis and battery fabrication of MXene. By choosing a special F‐rich electrolyte, MAX V2AlC is directly exfoliated inside a battery and the obtained V2CTX MXene is in situ used to achieve an excellent battery performance. This is a one‐step process with all reactions inside the cell, avoiding any contamination to external environments. Through the lifetime, the device experiences three stages of exfoliation, electrode oxidation, and redox of V2O5. While the electrode is changing, the device can always be used as a battery and the performance is continuously enhanced. The resulting aqueous zinc ion battery achieves outstanding cycling stability (4000 cycles) and rate performance (97.5 mAh g−1 at 64 A g−1), distinct from all reported aqueous MXene‐based counterparts with pseudo‐capacitive properties, and outperforming most vanadium‐based zinc ion batteries with high capacity. This work sheds light on the green synthesis of MXenes, provides an all‐in‐one protocol for MXene devices, and extends MXenes’ application in the aqueous energy storage field.

Iontronics Using V2CTx MXene-Derived Metal-Organic Framework Solid Electrolytes.

Electronic applications of porous metal–organic frameworks (MOFs) have recently emerged as an important research area. However, there is still no report on using MOF solid electrolytes in iontronics, which could take advantage of the porous feature of MOFs in the ionic transport. In this article, MXene-derived two-dimensional porphyrinic MOF (MX-MOF) films are demonstrated as an electronic-grade proton-conducting electrolyte. Meanwhile, the MX-MOF film shows high quality, chemical stability, and capability of standard device patterning processes (e.g., dry etching and optical and electron beam lithography). Using the commercialized nanofabrication processes, an electric double-layer (EDL) transistor is demonstrated using the MX-MOF film (derived from V2CTx MXene) as an ionic gate and MoS2 film as a semiconducting channel layer. The EDL transistor, operated by applying an electric field to control the interaction between ions and electrons, is the core device platform in the emerging iontronics field. Therefore, The MX-MOF, confirmed as a solid electrolyte for EDL transistor devices, could have a significant impact on iontronics research and development.

Insight into Adsorption Performance and Mechanism on Efficient Removal of Methylene Blue by Accordion-like V2CTx MXene.

Dye-bearing wastewaters leading to the water pollution and ecological upset is a crucial issue in the textile industry. Herein, we report a facile method using two-dimensional transition metal carbides (MXenes) for the removal of the methylene blue (MB) in the water. The accordion-like V2CTx MXene is originally demonstrated to have high and spontaneous adsorption capacity of MB at 111.11 mg·g-1, thrice over that of Ti3C2Tx as previously reported. The wide lamellar space of V2CTx is certain to have large accommodation for MB. The electrostatic interaction effect and hydrogen bond between V2CTx and MB not only promote the efficient adsorption process but also provide the selectivity between anionic and cationic dyes. Combined with good reusability, we anticipate that the V2CTx MXene is a promising candidate for the removal of cationic dyes from textile-dye-bearing wastewaters.

H2O2 assisted hydrothermal oxidation of partially etched vanadium carbides (MXene) and their electrochemical properties as anode for Li-ion batteries

The novel V2CTx MXenes have gained increasing attention as anode materials in lithium-ion batteries, but to boost their practical to the theoretical values is still challenging. Herein, the partially etched V2CTx was oxidized in the presence of H2O2 through a hydrothermal reaction and the electrochemical properties of as-obtained oxidized V2CTx were assessed. It is illustrated that the oxidized V2CTx possesses a large surface interlayer spacing, and enriched with VO2 (A) and O-terminate groups. Once used as anode materials for Li-ion batteries, the oxidized V2CTx shows enhanced specific lithium ion storage capacities of 318 mAh·g−1 at 100 mA·g−1 after 100 cycles, and 125 mAh·g−1 at 1000 mA·g−1 after 1000 cycles, and better cycling stability and excellent rate performance over the V2CTx.

K+ alkalization promoted Ca2+ intercalation in V2CT MXene for enhanced Li storage

Abstract Although MXenes is highly attractive as anode materials of lithium ion batteries, it sets a bottleneck for higher capacity of the V2CTx MXene due to the limited interlayer space and the derived surface terminations. Herein, the cation intercalation and ion-exchange were well employed to achieve a K+ and Ca2+ intercalated V2CTx MXene. A larger interlayer distance and low F surface terminations were thereof obtained, which accelerates the ion transport and promotes the delicate surface of V2CTx MXene. As a result, a package of enhanced capacity, rate performance and cyclability can be achieved. Furthermore, the ion exchange approach can be extended to other 2D layered materials, and both the interlayer control and the surface modification will be achieved.

Phase Transition Induced Unusual Electrochemical Performance of V2CTX MXene for Aqueous Zinc Hybrid-Ion Battery.

Nonbattery behavior related phase transition of electrodes is usually not favorable for any batteries because it results in performance degradation at all times. Here, we demonstrate a zinc hybrid-ion battery (ZHIB) with an unusual capacity enhancement even within 18 000 cycles by employing V2CTX MXene as the cathode, enormously differing from all reported counterparts with capacity degradation initiated within hundreds of cycles. The dominated mechanisms are determined to be MXene delamination and an unexpected phase transition during cycling. Both the original cathode and secondary derivative contribute to capacity simultaneously, resulting in the unusual capacity enhancement. Consequently, the specific capacity of 508 mAh g-1 (highest for all reported aqueous zinc-ion batteries) and high energy density of 386.2 Wh kg-1 are realized. Also, the quasi-solid-state batteries fabricated can output stably at -20 °C and in bending, twisting, stabbing, and cutting conditions. Our work brings an effective approach, that is, utilizing "unstable" electrode materials, which should usually be avoided, to achieve continuously enhanced performance of a battery. The idea to use both original and secondary materials for energy storage may be developed to be a general method to achieve extraordinary cycling stability of batteries.

MXene Derived Metal-Organic Frameworks.

Synthesis of nanoscale metal-organic frameworks (MOFs) is a highly challenging task because conventional soluble metal salt precursors are not easy to manipulate spatially, thus normally leading to bulk MOFs. In the present work, V2CTx MXene is demonstrated for the first time as a metal precursor to fabricate two-dimensional (2D) MOF nanosheets, whose thickness (6 to 18 nm) can be tuned by varying the reaction temperature. The highly electronegative surface atoms of MXene and sufficient accessible attacking sites for ligands are responsible for the evolution of 2D MOF nanosheets. Moreover, highly oriented and smooth MOF thin films have been grown based on these nanosheets using a convenient spin coating process. With the impregnation of nonvolatile H3PO4, the MOF thin film exhibits a proton-conducting property. This study demonstrates that high-quality 2D MOF sheets and thin films are enabled by 2D MXene precursors. We believe that the high-quality MOF films prepared in this study pave the way for many device applications.

The preparation of V2CTx by facile hydrothermal-assisted etching processing and its performance in lithium-ion battery

In this study, high-purity V2CTx MXene was successfully synthesized by etching V2AlC with fluoride and hydrochloric acid mixed solution using a hydrothermal-assisted method. This method is more concise and effective and has a low level of danger. The morphology and structure of the V2CTx MXene was characterized by X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The electrochemical properties were investigated as an anode material for lithium ion batteries. The results show that the prepared V2CTx had a higher purity and showed excellent electrochemical properties as an anode of lithium-ion batteries. And V2CTx prepared with different etching system can he obtained with high yield and excellent purity by changing the reactive conditions of the system. However, electrochemical performance of V2CTx MXene obtained at different etching system is quite different. V2CTx synthesized in the mixed solution of ammonium fluoride and hydrochloric acid has the best performance, which originates from more accessible active sites for ion in the enlarged interlayer distance, and the smaller impedance of V2CTx.

Two-dimensional vanadium carbide (V2CTx) MXene as supercapacitor electrode in seawater electrolyte

In this study, two-dimensional V2CTx MXene has been prepared by selectively etching Al layers from V2AlC MAX phase by NaF+HCl etching at 90 °C for 72 h and its performance as supercapacitor (SC) electrode were tested using simulating seawater as electrolyte. V2CTx MXene-based electrodes shows a good capacitance of 181.1 F/g, which is in accordance with the volumetric specific capacitance of 317.8 F/cm3, and with 89.1% capacitance retention even after 5000 cycle. Compared with other MXenes, V2CTx have better electrochemical performance as SC electrode. This work provides an innovative strategy to apply V2CTx MXene as SC electrode in safety and effective seawater electrolyte.

Precious‐Metal‐Free Electrocatalysts for Activation of Hydrogen Evolution with Nonmetallic Electron Donor: Chemical Composition Controllable Phosphorous Doped Vanadium Carbide MXene

AbstractThe insufficient strategies to improve electronic transport, the poor intrinsic chemical activities, and limited active site densities are all factors inhibiting MXenes from their electrocatalytic applications in terms of hydrogen production. Herein, these limitations are overcome by tunable interfacial chemical doping with a nonmetallic electron donor, i.e., phosphorization through simple heat‐treatment with triphenyl phosphine (TPP) as a phosphorous source in 2D vanadium carbide MXene. Through this process, substitution, and/or doping of phosphorous occurs at the basal plane with controllable chemical compositions (3.83–4.84 at%). Density functional theory (DFT) calculations demonstrate that the PC bonding shows the lowest surface formation energy (ΔGSurf) of 0.027 eV Å−2 and Gibbs free energy (ΔGH) of –0.02 eV, whereas others such as P‐oxide and PV (phosphide) show highly positive ΔGH. The P3–V2CTx treated at 500 °C shows the highest concentration of PC bonds, and exhibits the lowest onset overpotential of –28 mV, Tafel slope of 74 mV dec−1, and the smallest overpotential of ‐163 mV at 10 mA cm−2 in 0.5 m H2SO4. The first strategy for electrocatalytically accelerating hydrogen evolution activity of V2CTx MXene by simple interfacial doping will open the possibility of manipulating the catalytic performance of various MXenes.

Insights into the thermal and chemical stability of multilayered V2CTx MXene.

We report on the thermal stability of multilayered V2CTx MXenes under different atmospheres by combining in situ Raman spectroscopy with ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) in order to elucidate and monitor the molecular, electronic, and structural changes of both the surface and bulk of the V2CTx MXene which has recently received much attention. The MXene samples were heated up to 600 °C in inert (N2), oxidative (CO2, air), and reductive (H2) environments under similar conditions. In situ Raman showed that the V[double bond, length as m-dash]O vibration for two-dimensional vanadia is preserved up to 600 °C under N2, while its intensity reduces under H2. When heated above 300 °C under either CO2 or air, V2CTx slightly oxidizes or transforms into V2O5, respectively. Furthermore, SEM revealed the presence of an accordion-like layered structure for the MXene under N2 and H2, while under CO2 and air the layered structure collapses and forms VO2 (V4+) and V2O5 (V5+) crystals, respectively. XPS reveals that, regardless of the gas, surface V species oxidize above 300 °C during the dehydration process. Finally, we demonstrated that the partial dehydration of V2CTx results in the partial oxidation of the material, and the total dehydration is achieved once 700 °C is reached. We believe that our methodology is a unique alternative to tune the dehydration, oxidation, and properties of V2CTx, which allows for the expansion of applications of MXenes.

Assembling 2D MXenes into Highly Stable Pseudocapacitive Electrodes with High Power and Energy Densities.

Electrochemical capacitors (ECs) that store charge based on the pseudocapacitive mechanism combine high energy densities with high power densities and rate capabilities. 2D transition metal carbides (MXenes) have been recently introduced as high-rate pseudocapacitive materials with ultrahigh areal and volumetric capacitances. So far, 20 different MXene compositions have been synthesized and many more are theoretically predicted. However, since most MXenes are chemically unstable in their 2D forms, to date only one MXene composition, Ti3 C2 Tx , has shown stable pseudocapacitive charge storage. Here, a cation-driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V2 CTx and Ti2 CTx MXenes from their chemically unstable delaminated single-layer flakes. The electrochemical performance of electrodes fabricated using assembled V2 CTx flakes surpasses Ti3 C2 Tx in various aqueous electrolytes. These electrodes show specific capacitances as high as 1315 F cm-3 and retain ≈77% of their initial capacitance after one million charge/discharge cycles, an unprecedented performance for pseudocapacitive materials. This work opens a new venue for future development of high-performance supercapacitor electrodes using a variety of 2D materials as building blocks.

Conductive transparent V2CTx (MXene) films

To date, the vast majority of research on the emerging 2D material MXene has been conducted on Ti3C2Tx. Herein, we report the first study on the optoelectronic properties of V2CTx MXene, and its processing into flexible, transparent films. Films were prepared from aqueous colloidal suspensions produced using tetrabutylammonium hydroxide, TBAOH, as the dispersant. After annealing, the DC conductivities, σdc, of the thicker filtered and thinner spin cast films were both 3250 ± 100 S/cm. The optical absorption spectrum of 10 nm films was found to be free of distinct transitions throughout the 500 to 2700 nm range. The absorption coefficient at a wavelength of 550 nm was determined to be ≈1.22 ± 0.05 × 105 cm−1, which renders it more than twice as transparent as Ti-based MXenes. As a result, a common figure of merit for transparent conductors—of the ratio of the DC to optical conductivities—of 6.5 is calculated for the V2CTx films.

Two-Dimensional Vanadium Carbide (MXene) as a High-Capacity Cathode Material for Rechargeable Aluminum Batteries.

Rechargeable aluminum batteries (Al batteries) can potentially be safer, cheaper, and deliver higher energy densities than those of commercial Li-ion batteries (LIBs). However, due to the very high charge density of Al3+ cations and their strong interactions with the host lattice, very few cathode materials are known to be able to reversibly intercalate these ions. Herein, a rechargeable Al battery based on a two-dimensional (2D) vanadium carbide (V2CTx) MXene cathode is reported. The reversible intercalation of Al3+ cations between the MXene layers is suggested to be the mechanism for charge storage. It was found that the electrochemical performance could be significantly improved by converting multilayered V2CTx particles to few-layer sheets. With specific capacities of more than 300 mAh g-1 at high discharge rates and relatively high discharge potentials, V2CTx MXene electrodes show one of the best performances among the reported cathode materials for Al batteries. This study can lead to foundations for the development of high-capacity and high energy density rechargeable Al batteries by showcasing the potential of a large family of intercalation-type cathode materials based on MXenes.

Porous Two‐Dimensional Transition Metal Carbide (MXene) Flakes for High‐Performance Li‐Ion Storage

AbstractHerein we develop a chemical etching method to produce porous two‐dimensional (2D) Ti3C2Tx MXenes at room temperature in aqueous solutions. The as‐produced porous Ti3C2Tx (p‐Ti3C2Tx) have larger specific surface areas and more open structures than their pristine counterparts, and can be fabricated into flexible films with, or without, the addition of carbon nanotubes (CNTs). The as‐fabricated p‐Ti3C2Tx/CNT films showed significantly improved lithium ion storage capabilities compared to pristine Ti3C2Tx based films, with a very high capacity of ≈1250 mAh g−1 at 0.1 C, excellent cycling stability, and good rate performance (330 mAh g−1 at 10 C). Using the same chemical etching method, we also made porous Nb2CTx and V2CTx MXenes. Therefore, this study provides a simple, yet effective, procedure to introduce pores into MXenes and possibly other 2D sheets that in turn, can enhance their electrochemical properties.