MXenes

MXenes are potentially the largest class of 2D materials discovered so far. With a general formula of M_n+1X_nT_x, M is an early transition metal (Ti, V, Nb, Ta, etc.), X is C and/or N, T_x represents the surface groups (-O, -OH, -F, -Cl), and n = 1–4, over 30 stoichiometric phases have already been discovered, with many more predicted computationally. This class of materials has been widely studied owing to their exceptional properties, including hydrophilicity, scalability, mechanical strength, thermal stability, redox capability, and ease of processing. Because MXenes inherit their structure from M_n+1AX_n­ (MAX) phase precursors, understanding MAX phase synthesis leads to control over flake size, defect density, and chemical composition of the resultant MXene. One understudied, yet important class of MXenes are solid-solution MXenes, where multiple elements are randomly distributed within the M or X layers. These MXenes exhibit tunable properties that are directly related to their chemistry, and act as a way to study why they have the properties they do. By understanding this relationship, it then becomes possible to rationally design new MXenes targeting specific properties and applications.\

My work on MXenes include synthesis of the precursor MAX phases, studying their etching kinetics, and synthesis of resultant MXenes. I am very interested in synthesizing novel MXenes, such as the M₅X₄T_x family as well as determining fundamental relationships between MXene chemistry and their properties. The use of multiple M or X elements is a great way to do this, allowing us to get deep insight into their optical, chemical, electronic, and other properties. Moreover, because all aspects of MXene synthesis affect their properties (such as defect density, flake size, surface groups), it is important that we rationally understand how each synthesis step affects their properties.

 

Examples of projects in MXenes:

1. Synthesis of (Ti_2-yNb_y)CT_x or Ti₂C_1-yN_yT_x MXenes. How does changing y affect their optical, electronic, and chemical properties. A project like this involves synthesis of the precursor MAX phases, determining conditions to topochemically convert them into MXenes, delaminating them, then using multiple characterization techniques to understand their properties.
2. Etching kinetics of MXenes. We know that HF concentration, temperature, and time affect the topochemical etching kinetics of MXenes, as they affect all other reactions. If we are producing Ti₃C₂T_x for example, if we target 90% conversion, how will these variables affect the resultant MXene defect density, flake size, and surface functional groups? Can we generalize these results to other MXenes?
3. As of now, there exist only three M₅X₄T_x MXenes. Theoretical calculations predict that there are many others across other chemistries, but where are they? Synthesis of novel M₅AX₄ MAX phases with specific chemistries plays into deep thermodynamics of complex quaternary or quinary systems. Can we do cyclical synthesis using DFT as a roadmap to producing others?