Introduction:
A new family of two dimensional (2D) carbides, carbonitrides and nitrides – labeled MXenes – was discovered. Since then the number of papers on these materials has increased exponentially for several reasons amongst them: their hydrophilic nature, excellent electronic conductivities and ease of synthesizing large quantities in water. This unique combination of properties and ease of processing has positioned them as enabling materials for a large, and quite varied, host of applications from energy storage to electromagnetic shielding, transparent conductive electrodes, electrocatalysis, to name a few. Since the initial synthesis of Ti3C2 in hydrofluoric acid, many more compositions were discovered, and different synthesis pathways were explored. Most of the work done so far has been conducted on top-down synthesis where a layered parent compound is etched and then exfoliated. Three bottom-up synthesis methods, chemical vapor deposition, a template method and plasma enhanced pulsed laser deposition have been reported. The latter methods enable the synthesis of not only high-quality ultrathin 2D transition metal carbide and nitride films, but also those that could not be synthesized by selective etching. This article reviews and summarizes the most important breakthroughs in the synthesis of MXenes and high-quality ultrathin 2D transition metal carbide and nitride films.
In particular, it is easy for MXenes to form composites with other materials such as polymers, oxides, and carbon nanotubes, which further provides an effective way to tune the properties of MXenes for various applications. Not only have MXenes and MXene-based composites come into prominence as electrode materials in the energy storage field as is widely known, but they have also shown great potential in environment-related applications including electro/photocatalytic water splitting, photocatalytic reduction of carbon dioxide, water purification and sensors, thanks to their high conductivity, reducibility and biocompatibility. In this review, we summarize the synthesis and properties of MXenes and MXene-based composites and highlight their recent advances in environment-related applications. Challenges and perspectives for future research are also outlined.
Properties:
While MAX phases are stiff, they will be machined as easily as some metals. They can all be machined manually employing a hacksaw, despite the very fact that a number of them are 3 times as stiff as titanium metal, with an equivalent density as titanium.
Applications:
Ti3SiC2, Ti3AlC2, Ti2AlN, and Ti4AlN3 are well-known members of this family. To examine the feasibility of these compounds in high-temperature applications, it is important to test their high temperature stability.
Certain compositions possess good irradiation and thermal shock resistance, along with good machinability, and are being considered as structural and functional materials for Gen IV nuclear
reactor component
MXenes, as conductive layered materials with tunable surface terminations, have been shown to be promising for energy storage applications (Li-ion batteries and supercapacitors), composites, photocatalysis,water purification,gas sensors,transparent conducting electrodes,neural electrodes,as a metamaterial,SERS substrate,photonic diode,electrochromic device,and triboelectric nanogenerator (TENGs),to name a few.
Lithium-ion batteries (LIBs)
Some MXenes have been investigated experimentally thus far in LIBs (e.g. V2CTx, Nb2CTx, Ti2CTx, and Ti3C2Tx. V2CTx has demonstrated the highest reversible charge storage capacity among MXenes in multi-layer form (280 mAhg−1 at 1C rate and 125 mAhg−1 at 10C rate). Nb2CTx in multi-layer form showed a stable, reversible capacity of 170 mAhg−1 at 1C rate and 110 mAhg−1 at a 10C rate. Although Ti3C2Tx shows the lowest capacity among the four MXenes in multi-layer form, it can be easily delaminated via sonication of the multi-layer powder. By virtue of higher electrochemically active and accessible surface area, delaminated Ti3C2Tx paper demonstrates a reversible capacity of 410 mAhg−1 at 1C and 110 mAhg−1 at 36C rate. As a general trend, M2X MXenes can be expected to have greater capacity than their M3X2 or M4X3 counterparts at the same applied current, since M2X MXenes have the fewest atomic layers per sheet.
In addition to the high power capabilities of MXenes, each MXene has a different active voltage window, which could allow their use as cathodes or anodes in batteries. Moreover, the experimentally measured capacity for Ti3C2Tx paper is higher than predicted from computer simulations, indicating that further investigation is required to ascertain the charge storage mechanism on MXene surfaces.
Sodium-ion batteries
MXenes also exhibit promising performances for sodium-based energy storage devices. Na+ should diffuse rapidly on MXene surfaces, which is favourable for fast charging/discharging. Two layers of Na+ can be intercalated in between MXene layers. As a typical example, multi-layered Ti2CTx MXene as a negative electrode material showed a capacity of 175 mA h g−1 and good rate capability for electrochemical sodium-ion storage. It is possible to tune the Na-ion insertion potentials of MXenes by changing the transition metal and surface functional groups. V2CTx MXene has been successfully applied as a positive electrode material for sodium-ion storage. Porous MXene-based paper electrodes have also been reported, which exhibited high volumetric capacities and stable cycling performance, demonstrating that MXenes are promising for sodium-based energy storage devices where size matters.
Super capacitors
Super capacitor electrodes based on Ti3C2 MXene paper in aqueous solutions demonstrate excellent cyclability and the ability to store 300-400 F/cm3, which translates to three times as much energy as for activated carbon and graphene-based capacitors. Ti3C2 MXene clay shows a volumetric capacitance of 900 F/cm3, a higher capacitance per unit of volume than most other materials, and does not lose any of its capacitance through more than 10,000 charge/discharge cycles.
Composites
FL-Ti3C2 (the most studied MXene) nanosheets can mix intimately with polymers such as polyvinyl alcohol (PVA), forming alternating MXene-PVA layered structures. The electrical conductivities of the composites can be controlled from 4×10−4 to 220 S/cm (MXene weight content from 40% to 90%). The composites have tensile strength up to 400% stronger than pure MXene films and show better capacitance up to 500 F/cm3. A method of alternative filtration for forming MXene-carbon nanomaterials composite films is also devised. These composites show better rate performance at high scan rates in super capacitors. The insertion of polymers or carbon nanomaterials between the MXene layers enables electrolyte ions to diffuse more easily through the MXene’s, which is the key for their applications in flexible energy storage devices.
Porous MXenes
Porous MXenes (Ti3C2, Nb2C and V2C) have been produced via a facile chemical etching method at room temperature. Porous Ti3C2 has a larger specific surface area and more open structure, and can be filtered as flexible films with, or without, the addition of carbon nanotubes (CNTs). The as-fabricated p-Ti3C2/CNT films showed significantly improved lithium ion storage capabilities, with a capacity as high as 1250 mA·h·g−1 at 0.1 C, excellent cycling stability, and good rate performance.
Antennas
Scientists at Drexel University in the US have created spray on antennas that perform as well as current antennas found in phones, routers and other gadgets by painting MXene’s onto everyday objects, widening the scope of the Internet of things considerably.
Optoelectronic devices
MXene SERS substrates have been manufactured by spray-coating and were used to detect several common dyes, with calculated enhancement factors reaching ~106. Titanium carbide MXene demonstrates the SERS effect in aqueous colloidal solutions, suggesting the potential for biomedical or environmental applications, where MXene can selectively enhance positively charged molecules. Transparent conducting electrodes have been fabricated with titanium carbide MXene showing the ability to transmit approximately 97% of visible light per nanometer thickness. The performance of MXene transparent conducting electrodes depends on the MXene composition as well as synthesis and processing parameters.
