(30g) Facile Synthesis of Halogen Terminated Mxene from Different MAX Phases for Electromagnetic Interface Shielding

Two-dimensional (2D) layered metal carbides and nitrides are known as MXenes, with the versatile properties (high electrical and thermal conductivity) of 2D MXenes driving interest in various applications (batteries, fuel cells). Generally, MXenes are demonstrated by a general chemical formula of M_n+1X_nT_x (n = 1−4), where M represents an early transition metal such as Ti, V, Mo etc.., where X is carbon/nitrogen, and T_x represents the surface terminations such as Br. Conventionally, MXenes are produced by the selective etching of the A interlayer from their parent three-dimensional (3D) MAX phase (M_n+1AX_n). Thus, the ability to modify the surface chemistry of MXenes is an important topic. However, an experimental investigation of new surface functional group termination on MXenes remains a challenge.

Over the past decade, an exhaustive amount of research has been conducted on the synthesis of MXenes via aqueous HF-based methods that leave an array of hydrophilic termination groups on the surface of the MXene, namely -OH, -O and -F. The serious toxicity issues raised by these compounds hinders the scale up production of MXenes. Moreover, different surface terminations, other than fluorine and hydroxide-based systems, are expected to broaden the applicable chemistries for the functionalization of MXenes. Recently, tuning surface modifications on MXenes were explored by solid-state methods such as molten salt etching. However, these methods suffer from several drawbacks; for example, the experimental conditions of the alkali- based etch are harsh making it hard for scale-up production, the molten salt method require relatively high temperatures (~550 °C), and finally the yields for the electrochemical synthesis approach are fairly low. Besides, regardless of the different etching routes available in the literature, MXene sheets suffer from uncontrollable surface terminations which inhibits the extent of possible functionalization. This has limited not only the production of high-quality MXenes but also the prospect of repeatable, theoretical studies in the literature to predict MXenes properties prior to use in many device applications.

Therefore, the potential of developing non-fluorine based etching procedures is of great interest as it will further development and mass production of MXenes for commercial applications. In this work, a novel non-aqueous, halogenated (bromine) etching from different MAX phases has been developed at room temperature to investigate the effect of surface chemistry on the properties of the MXene. Figure 1 illustrates that as etching proceeds, the color changes from dark red to light yellow over 24 hours at room temperature, indicating the consumption of Br₂, and it is evident that the halogen etching methods produce colloidally stable, multi layered MXene (Ti₃C₂Br) after processing. The XPS spectroscopy confirms the removal of the Al interlayer while preserving the titanium carbide backbone of the initial MAX phase (Fig. 2). Scanning Electron Microscopy is used to confirm the presence of exfoliated MXene flakes (Fig. 3). The reaction (bromine etching) on the MAX phase surface is monitored via wide-angle X-ray scattering by drop-casting aliquots from the reaction slurry onto silicon wafers (Fig. 4). A peak shift is clearly seen in figure 4 from 10 degrees to 7.5 degrees, indicating the presence of brominated MXenes (Ti₃C₂Br). The disappearance of the peak at 40 degrees indicates the removal of aluminum. The significance of this work is that it not only provides a complimentary, non-toxic, fluoride-free route to prepare MXenes, but also increases access to new chemistries for surface derivatization, which expands compositional-tunability of optical, electrical, and chemical properties. Applications of the halogen etch to different MAX phases will also be presented.