Comparative Study of Five Distinct Etching Strategies to Synthesize 2D (Mo2/3Nd1/3)2CTx i-MXene From (Mo2/3Nd1/3)2AlC i-MAX Phase.

The discovery of in-plane ordered MAX (i-MAX) phases containing lanthanides provides a promising route to novel MXenes. Most MXenes have transition elements occupying the M site. However, the distinctive optical and magnetic properties of lanthanides could yield multifunctional MAX phases and MXenes with applications in aeronautics, radiation shielding, and space technology. Previously, lanthanide-containing i-MAX phases were etched to produce ordered vacancy MXenes via lanthanide removal. Here, we report the first synthesis of (Mo2/3Nd1/3)2CTx i-MXene retaining Nd and compare wet chemical etching, fluorine-free molten salt etching, and hydrothermal etching routes. A key challenge is removing Al without also removing Nd. Etching with concentrated hydrofluoric acid (HF) yielded i-MXene along with neodymium fluoride. Etching using lithium fluoride (LiF) and hydrochloric acid (HCl) produced an accordion-like morphology typical of MXenes. Mixed acids (HF and HCl) yielded more uniform etching compared to other etchants. Both molten salt etching and alkali etching proved less effective. We discuss the potential and challenges of each etching method. Based on the similar chemical behavior of lanthanides, we expect results for the Nd-containing i-MAX phase to be representative. This comparative study reveals significant morphological and structural differences, paving the way for the synthesis of a new family of lanthanide-containing i-MXenes.