Guidelines for Synthesis and Processing of Chemically Stable Two-Dimensional V₂CT_x MXene

Abstract

Vanadium carbide MXenes, specifically V 2 CT x , have shown promise for applications ranging from energy storage and sensing to electronics and optics. In the past decade, however, research involving V 2 CT x has been mostly limited to its multilayered form due to instability of delaminated V 2 CT x in its colloidal state. In this paper, we report on mild synthesis conditions which result in high-quality V 2 CT x 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 V 2 CT x . We also demonstrate that during ionexchange and flocculation tetrabutylammonium or tetramethylammonium ions are replaced with lithium cations. The produced precipitates from delaminated V 2 CT x 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 V 2 CT x . The major improvements in shelf life and properties of V 2 CT x 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.

Removing Fluoride-Terminations from Multilayered V2CTx MXene by Gas Hydrolyzation

“…At higher temperatures (∼320–480 °C), the removal of chemisorbed water due to OH-terminations is suggested. 30 , 35 However, the XRD data show that this temperature also results in a degradation of the MXene structure, as the (002) reflections of V 2 C T x are significantly reduced after annealing up to 600 °C. Similar to hydrolyzation at 400 °C ( Figure 2 ), the formation of broad reflections from oxycarbide (VC x O y ) appear at this temperature.…”

“… 48 Based on the results presented here, the application of hydrolysis using a continuous flow of Ar(g) with a high partial pressure of H 2 O(g) opens up for new possibilities for post etching treatments of MXenes beyond only V 2 C T x . 35 , 56 …”

Thermal Properties of MXenes and Relevant Applications

“…In particular, top‐down methods, based on the selective removal of A layers from MAX phases (which are the precursors for MXenes) with a subsequent delamination procedure by certain intercalants in most cases, are widely adopted by most research groups due to their simplicity and economic nature [30] . Though the stronger bonding strength of M−X (mixed ionic and covalent bonds) compared to that of M−A (metallic bonds) allows the selective etching, the produced MXene sheets (typically about a few micrometers in size, as shown in Figure 1(a)) [31] usually contain various surface terminations (i.e., oxygen and/or fluorine containing functional groups) which are affected by the etchants and the synthesis conditions. On the contrary, preparing MXenes from atomic building blocks (by bottom‐up approaches) can dramatically eliminate the structural defects and the lateral size of the fabricated sheets is up to 100 μm (as shown in Figure 1(b)), [32] providing the opportunity to evaluate the intrinsic properties of monolayer MXenes.…”

“…Figure 1. (a) Scanning electron microscope (SEM) image of a V 2 CTx MXene sheet (prepared by acid etching) on a substrate of anodized aluminum oxide [31]. Reproduced with permission from Ref[31].…”

“…(a) Scanning electron microscope (SEM) image of a V 2 CTx MXene sheet (prepared by acid etching) on a substrate of anodized aluminum oxide [31]. Reproduced with permission from Ref[31]. Copyright 2022 American Chemical Society.…”