2D Carbides and Nitrides (MXenes)

MXenes are a new family of two-dimensional (2D) transition metal carbides, carbonitrides and nitrides that were discovered and developed in collaboration with Prof. Barsoum’s group, that can be used in many applications. These applications include lithium-ion and sodium-ion energy storage systems, electromagnetic interference (EMI) shielding, and water purification. MXenes are highly desirable in EMI shielding due to their good flexibility, easy processing, and high conductivity with minimal thickness, having the highest EMI shielding effectiveness of all synthetic materials of similar thickness. MXenes are also promising antibacterial agents, with higher efficiency than graphene oxide in diminishing bacterial cell viability.

 

These 2D layered materials are called ‘MXenes’ because we produce them by etching A layer from MAX phases and we added the suffix ‘ene’ to emphasize their similarity to graphene. MAX phases are a large family (60+ members) of hexagonal layered ternary transition metal carbides, carbonitrides and nitrides with composition of Mn+1AXn, where M stands for an early transition metal (such as: Ti, V, Cr, Nb, etc.), A stands for a group A element (such as: Al, Si, Sn, In, etc.), X stands for carbon and/or nitrogen, and n=1, 2, or 3. Several etching processes have been developed to synthesize particular MXenes. One of these etching processes is carried out by simply immersing a MAX phase in hydrofluoric acid at room temperature. To produce MXene ‘clay’, the MAX phase Ti3AlC2 is immersed in a solution of lithium fluoride in hydrochloric acid. MXenes are produced with compositions of M2X, M3X2, and M4X3. We recently synthesizedcarbides MXenes  composed of two transition metals. These double transition metal MXenes are formulated as M’2M”C2 or M’2M”2C3, where M’ and M” are different transition metals. The MXene family was limited to carbides and carbonitrides until 2016, when we reported the first MXene nitride, Ti4N3. Synthesis of this MXene involved another synthesis procedure of mixing its corresponding MAX phase in a molten eutectic fluoride salt solution and treating it at a high temperature.

DFT calculations showed that MXenes’ band gap can be tuned by changing the surface termination, for example bare MXenes are metallic conductors, while OH or F terminated are often semiconductors with small band gaps. Multilayer MXenes are electronically conductive with conductivity similar to or exceeding that of multilayer graphene. Unlike graphene, MXenes show hydrophilic behavior that allows for easy dispersion in aqueous solutions.

We have shown that MXenes can be intercalated with a variety of organic molecules and inorganic salts which not only enables synthesis of different intercalation compounds but also leads to new applications for these materials. We continue investigating MXene synthesis by exploring structure and surface termination of MXenes in order to define their chemical formulas and control chemical composition and also by studying the intercalation process, understanding the involved mechanisms and the structure of intercalated MXenes. These materials could be used for a wide range of applications including electronic devices, sensors, reinforcement for composites, and energy storage materials.