Chemistry of two-dimensional transition metal carbides (MXenes)

With consumer trends pushing toward smaller, faster, more flexible, multitasking devices, researchers striving to meet these needs have targeted two-dimensional (2D) materials - and graphene in particular - as holding the most promise for use in advanced applications. But in 2011, a significant interest has been triggered by a newly discovered family of novel 2D materials - layered transitional metal carbides and carbonitrides, named MXenes. Those compounds were of general formula Mn+1XnTx, where M stands for metal atom, X is C and/or N, n = 1, 2 or 3, and Tx represents surface groups. Being initially suggested as a material for electrical energy storage systems, MXenes' properties and their potential applications have not been explored. This work is the first complete study of MXenes' chemistry that sheds light on the chemical composition, structure and properties of these novel materials and possible routes of its modification. The research was focused on 2D titanium carbide, Ti₃C₂T_x, chosen as the representative of the MXene family. The kinetic study of Ti₃C₂T_x synthesis discovered the main synthesis parameters, viz. temperature, time and particle size, that affect the etching process and define the quality of final product. MXenes were found to be able to spontaneously accommodate various ions and small organic molecules between the layers leading to preopening of the structure. A major challenge of large scale production of delaminated, atomically thin 2D MXene layers was solved with two delamination techniques involving dimethyl sulfoxide and isopropyl amine pre-intercalation followed by sonication in water. Ti₃C₂T_x was also found to possess adsorptive and photocatalytic properties, revealing its potential for environmental applications. It also showed limited stability in water and in the presence of oxygen, providing important practical information on proper handling and storage of MXene materials. Completion of this work allowed the performance of energy storage devices to be improved significantly, viz. Li-ion batteries and electrochemical capacitors, and gave rise to many other potential applications such as MXene-polymer composites, sorption, and catalysis. More importantly, it opened a path to the large-scale synthesis of thin, single-layer MXene sheets and led to establishing MXenes as fully-fledged members of the growing family of 2D materials.