Effect of Ammonium and Sodium Fluorides Etchants on the Synthesis of Double Transition Metal (Mo₂TiC₂Tx) MXenes

Abstract Recent advancements in MXene research have extended beyond mono-metal compositions to the more complex double transition metal (DTM) MXenes, which exhibit remarkable structural versatility. However, etching DTM MAX phases to produce these MXenes remains a challenging process. Although lithium fluoride (LiF) combined with hydrochloric acid (HCl) has been the standard approach for this purpose, alternative fluoride salts have not been thoroughly investigated. In this study, Mo2TiC2Tx MXene was successfully extracted from Mo2TiAlC2 MAX phase using a conventional hydrothermal method to etch aluminum (Al). Two non-LiF etchant solutions, ammonium fluoride (NH4F) combined with HCl and sodium fluoride (NaF) combined with HCl were tested to extract Mo2TiC2Tx MXene, their Al-etching efficiencies were also compared. The impact of varying reaction times (72, 48, 24, and 12 h) on the Al-etching process was also examined at a constant temperature. Results showed that the NH4F and HCl mixture removed Al in a shorter period of 24 h at 180°C. Additionally, this combination achieved the highest Al removal efficiency, producing a stacked lamellar MXene structure with relatively wider galleries compared to the other samples.