  MXenes are a new, and growing, family of 2D materials with very promising
properties for a wide variety of applications. Obtained from the etching of MAX
phases, numerous properties can be targeted thanks to the chemical richness of
the precursors. Herein, we highlight how etching agents govern surface
chemistries of Ti$_3$C$_2$T$_x$, the most widely studied MXene to date. By
combining characterization tools such as X-ray diffraction, X-ray
photoelectron, Raman and electron energy loss spectroscopies, scanning and
transmission electron microscopies and a surface sensitive electrochemical
reaction-the hydrogen evolution reaction, HER-we clearly demonstrate that the
etching agent (HF, LiF/HCl or FeF$_3$/HCl) strongly modifies the nature of
surface terminal groups (F, OH and/or O), oxidation sensitivity, delamination
ability, nature of the inserted species, interstratification, concentration of
defects and size of flakes. Beyond showing how using these different
characterization tools to analyze MXenes, this work highlights that the MXene
synthesis routes can influence targeted applications.
