Low-temperature molten-salt synthesis of Mo2Ti2C3Tx, Ta4C3Tx, and Cr2TiC2Tx MXenes within one hour

MXenes are a rapidly expanding family of two-dimensional transition metal carbides and nitrides that hold promise for applications in energy storage, catalysis, and electronics. However, conventional solution-based fluoride etching of their layered MAX phase precursors is slow, hazardous, and often incomplete. Here we demonstrate a low-temperature molten salt (LTMS) approach that enables rapid and efficient synthesis of three MXene materials, Mo 2 Ti 2 C 3 T x , Ta 4 C 3 T x , and Cr 2 TiC 2 T x , from their respective MAX phase precursors using molten NH 4 HF 2 at 180 °C. Complete conversion to MXenes is achieved within 40-45 min, representing more than a 100-fold reduction in reaction time compared to conventional HF etching of the corresponding MAX phases. The LTMS products exhibit the characteristic accordion-like morphology, high crystallinity, and uniform elemental distribution, with no residual Al detected by elemental analysis methods. Notably, Mo 2 Ti 2 C 3 T x prepared by the LTMS method shows minimal oxidation relative to HF-etched samples. These results establish the LTMS approach as a rapid, scalable, and safer route for synthesizing high-quality MXenes, especially those derived from MAX phases that are poorly etchable in HF. The LTMS-synthesized MXenes may find many applications, including those in gas sensing, as demonstrated in this study by fabrication of Mo 2 Ti 2 C 3 T x sensors and their detection of water vapors. • Low-temperature molten-salt (LTMS) synthesis of Mo 2 Ti 2 C 3 T x , Ta 4 C 3 T x , and Cr 2 TiC 2 T x MXenes • Complete MAX-to-MXene conversion achieved within 40–45 min at 180 °C using NH 2 HF 2 • LTMS products show high crystallinity, uniform morphology, and minimal oxidation • LTMS outperforms conventional HF etching (>100× faster, safer, and more efficient) • LTMS-synthesized Mo 2 Ti 2 C 3 T x exhibits chemiresistive gas sensing toward water vapor