Guidelines for Synthesis and Processing of Chemically Stable Two-Dimensional V₂CT_<i>x</i> MXene

Vanadium carbide MXenes, specifically V2CTx, have shown promise for applications ranging from energy storage and sensing to electronics and optics. In the past decade, however, research involving V2CTx has been mostly limited to its multilayered form due to instability of delaminated V2CTx in its colloidal state. In this paper, we report on mild synthesis conditions which result in high-quality V2CTx and an ion exchange process coupled with flocculation that increases the shelf life of this MXene in aqueous suspension by about 3 orders of magnitude, from a few hours to several months. We discuss the etching and delamination mechanisms and provide a guideline for researchers working with this MXene composition. We explain the effect of etchant formulation, delamination chemicals, and postprocessing on the quality, chemical stability, and optoelectronic properties of the synthesized V2CTx. We also demonstrate that during ion-exchange and flocculation tetrabutylammonium or tetramethylammonium ions are replaced with lithium cations. The produced precipitates from delaminated V2CTx can not only be stored in suspension for a few months without degradation but can also be redispersed and processed into films. Those MXene films show distinct improvements in the optical and electronic properties. Their electrical conductivities in the dry state can exceed 1000 S cm–1, a value not previously achievable for V2CTx. The major improvements in shelf life and properties of V2CTx demonstrated in this work are expected to allow fundamental studies of properties of this MXene and greatly expand its range of potential applications. The proposed approach may be applicable to other MXenes that require the use of quaternary amines for delamination.