(155c) Tuning 3D Assembly of Ti3C2 Mxene Nanosheets through Mxene/Polyelectrolyte Complexation

MXene (Ti₃C₂T_x, T = OH, O, F), a new hydrophilic member of the 2D materials family, is a metal carbide with excellent prospects in the fields of electrochemistry and environmental protection. However, for each specific application, taking full advantage of the inherent properties of MXene nanosheets relies highly on the components’ assemblies. MXene nanosheets are produced as a highly stable colloidal dispersion in water (at neutral pH), thanks to their abundant ionized surface groups, i.e., long-range double layer repulsion. Besides, the presence of hydrated intercalant molecules such as Lithium ions on the nanosheets’ surfaces results in short-range hydrophobic repulsion. Therefore, to engineer the 3D assembly of MXene nanosheets, their self-assembly behavior with respect to the surrounding water should be triggered and controlled by external stimuli. In this regard, various methods have been applied in the literature to control the assembly of MXene nanosheets and prevent their stacking upon removing the water. For example, directional freezing, layer-by-layer assembly, in-situ polymerization of monomers in the presence of MXene, and chemical crosslinking of MXene/polymer systems are the major studied methods. However, in most of these methods, there is no flexibility in tuning the 3D morphology MXene assembly, i.e., the final morphology is either fixed or not controllable at all. In this work, the goal was to take the advantage of the negatively charged MXene surface and their strong interactions with positively charged polyelectrolytes to tune the assembly of nanosheets. Ti₃C₂T_x MXene was synthesized from its MAX precursor, Ti₃AlC₂, through a minimally intensive layer delamination method to reach single-layer nanosheets with high aspect ratio and minimal in-plane defects. We show that through the diffusion of a polyelectrolyte into a liquid crystalline MXene suspension, the 3D porous assembly of MXene nanosheets with tunable morphology is triggered. By adjusting the mass flux of polymer chains into the MXene phase, we could control the kinetics of the assembly process, and the average pore size in the final structures from 10 to 40 µm. Besides, we show that at a constant mass flux of polymer, the morphology of the porous structures i.e., pore shape and pore size, is governed by the strength of MXene/polyelectrolyte interactions, controlled by polymer molecular weight, ionic strength, and MXene surface charge density. This automatic and flexible assembly process, which is purely thermodynamic based, can open a new way toward designing MXene structures with the tunable accessible surface area for electrochemical and biomedical applications.