MXenes are a unique class of two-dimensional (2D) materials that have gained considerable attention in recent years due to their remarkable properties, including high electrical conductivity, mechanical strength, and hydrophilicity. These materials are typically synthesized by selectively etching the "A" layer from a parent MAX phase, where "M" is a transition metal, "A" is an element such as aluminum, and "X" is carbon or nitrogen. The resulting MXenes have a layered structure and are ideally suited for applications in energy storage, catalysis, sensors and environmental remediation. The continued exploration of new synthesis techniques has significantly advanced the understanding of MXenes, paving the way for their widespread use in various technological fields.
Figure 1. Structure and applications of 2D MXenes [1].
Conventional Etching Techniques
The traditional synthesis of MXenes involves etching the "A" layer from MAX phases using hydrofluoric acid (HF). This process selectively removes the metallic bonds between the M and A layers, yielding multilayered MXene flakes. While effective, HF etching poses significant safety risks due to the acid's toxicity and corrosiveness. Additionally, the resulting MXenes often require further processing to achieve monolayer dispersion, limiting their practical application. Despite these challenges, HF remains a foundational method due to its reliability in producing MXenes like Ti3C2Tx.
Advances in Safer Etching Methods
Recent innovations focus on reducing reliance on HF. For instance, minimally intensive layer delamination (MILD) methods use milder etchants like hydrochloric acid (HCl) combined with lithium fluoride (LiF), generating in situ HF at lower concentrations. This approach improves safety while maintaining etching efficiency. Additionally, molten salt etching has enabled fluoride-free synthesis, producing MXenes with fewer surface defects. These advancements not only mitigate environmental and health risks but also enhance control over surface terminations (e.g., -O, -OH, -F), which dictate MXene properties.
Intercalation and Delamination Strategies
Post-etching, intercalation with organic molecules (e.g., tetraalkylammonium ions) or solvents (e.g., DMSO) is often employed to weaken interlayer van der Waals forces, facilitating delamination into single-layer flakes. Recent work emphasizes optimizing intercalants to prevent restacking and improve colloidal stability. Moreover, sonication-assisted delamination and mechanical shearing methods have also gained traction, enabling scalable production of high-quality MXene dispersions. These strategies are crucial for applications requiring high surface area, such as supercapacitors and electromagnetic interference shielding.
Emerging Synthesis Techniques
Beyond liquid-phase etching, novel approaches like chemical vapor deposition (CVD) and template-assisted growth are being explored. CVD allows direct growth of MXene films on substrates, bypassing the need for etching and delamination. Template methods using layered precursors or sacrificial materials offer precise control over MXene morphology and thickness. Such techniques aim to address limitations of conventional synthesis, including defects and inhomogeneity, while expanding the library of MXene compositions.
The synthesis of MXenes has evolved significantly, transitioning from hazardous HF-based methods to safer, more controllable techniques. Innovations in etching, delamination, and alternative synthesis routes have expanded their applicability and performance.
Alfa Chemistry believes that with the continuous advancement of technology, MXenes will play an increasingly important role in next-generation materials and devices. If you are interested in MXenes and related materials, feel free to contact us.
Mxene Related Materials
Reference
- VahidMohammadi, A.; Rosen, J.; et al. The world of two-dimensional carbides and nitrides (MXenes). Science. 2021, 372(6547): eabf1581.
