The world of two-dimensional carbides and nitrides (MXenes)
A family of fine materials
Two-dimensional (2D) materials have aroused interest due to the unusual properties that emerge from these confined structures. There is a growing family of 2D metal carbides and nitrides known as MXenes which contain an odd number of layers in which the metals (M) sandwich layers of carbon or nitrogen (X). VahidMohammadi et al. reviewed the progress made in synthesizing this growing library of materials. Combinations of mixed metals can be used, as well as a range of surface terminations, allowing the properties to be adjusted. However, there are still challenges to improve synthesis methods and develop techniques that can be scaled up.
Science, abf1581, this issue p. eabf1581
The appreciation that the synthesis of two-dimensional (2D) materials does not necessarily require laminated precursors linked by van der Waals has led to the discovery of many new materials, including MXenes – 2D transition metal carbides and nitrides, products by selective etching of strongly bonded materials. layered solids. Ti3VS2 was first reported in 2011 and paved the way for the synthesis of Ti2C, Ta4VS3, and other MXenes of their MAX phase precursors, demonstrating three possible types of structures (M2X, M3X2, and M4X3). M5VS4 was produced later, further increasing structural diversity and bringing the number of theoretically possible compositions to over 100, including those with in-plane and out-of-plane order of metal atoms. Considering various surface terminations of MXenes, the number of distinct compositions increases by another order of magnitude. MXenes’ ability to form carbonitrides and solid solutions suggests a potentially infinite number of compositions and opens a new era of computational-based 2D atomistic design.
MXenes adds a large number of building blocks, mostly metallic conductors, to the 2D family of materials, most of which are dielectrics, semiconductors, or semi-metals. Using the tunable properties of MXenes, devices ranging from transistors to supercapacitors, batteries, antennas and sensors can be constructed from 2D nanosheets using additive manufacturing or other coating and processing techniques. MXenes have already shown various electronic, optical, chemical and mechanical properties, and the concept of MXetronics (fully MXene optoelectronics) has been proposed. High electronic conductivity allows their use in current collectors, interconnects and conductive inks. MXenes possess electrochemically and chemically tunable plasmonic properties, with interband transitions and plasmon resonance peaks spanning the entire ultraviolet, visible and near infrared range, allowing their applications in electrochromic and photothermal therapy. Their strong interaction with electromagnetic waves ranging from terahertz to gigahertz is used in shielding and communication against electromagnetic interference. The redox activity of transition metal atoms on the surface of MXene allows electrochemical storage of energy in batteries and supercapacitors as well as electrocatalysis. Controlled spacing between 2D sheets is used for gas separation, water purification, and dialysis. The surface charge of MXenes allows aqueous treatment without surfactants or binders as well as the formation of liquid crystals. Organic molecules, polymers and ions can be interposed between the layers of MXene, allowing tuning of properties and multilayer assemblies. The non-toxic and environmentally friendly titanium-based MXenes made of abundant elements, and their hybrids and composites with polymers, ceramics and metals are particularly eye-catching.
While progress has been made in the preparation of MXene carbides, the synthesis of nitrides is lagging behind. Vapor phase synthesis is required for the integration of MXenes on chips using current microfabrication device technology. Large-scale and environmentally friendly synthesis methods are the key to widespread use of MXenes in future additive manufacturing technologies. Precise control of structure and surface chemistry, including fault and strain engineering, should pave the way for theoretically predicted intrinsically semiconducting, topologically insulating and ferromagnetic MXenes, as well as other discoveries in physics and chemistry of MXenes. Mechanically tough, environmentally stable, and highly conductive MXenes can have a major impact on flexible, printable and portable self-powered electronics. However, using MXenes in combination with other 2D materials to build heterostructures and devices by self-assembly from a solution is perhaps the most exciting prospect.
A decade after the first report, the family of two-dimensional (2D) carbides and nitrides (MXenes) comprises structures with three, five, seven or nine layers of atoms in the form of an ordered or solid solution. Dozens of MXene compositions have been produced, resulting in MXene with mixed surface terminations. MXenes have shown useful and tunable electronic, optical, mechanical, and electrochemical properties, leading to applications ranging from optoelectronics, electromagnetic interference shielding, and wireless antennas to energy storage, catalysis, detection and medicine. We present here a prospective review of the MXenes domain. We discuss the challenges to be overcome and describe lines of research that will deepen the fundamental understanding of the properties of MXenes and allow their hybridization with other 2D materials in various emerging technologies.