XPS of Cold Pressed Multilayered and Freestanding Delaminated 2D Thin Films of Mo2TiC2Tz and Mo2Ti2C3Tz (MXenes)

MXenes, transition metal carbides that are synthesized from the top down by etching of their 3D parent layered solids, the MAX phases, are the latest family of the two-dimensional solids discovered. When the A layers – mostly Al – are etched they are replaced by surface terminations, T_z mainly comprised of O-, OH- and F-terminations. One of the few techniques capable of quantifying these surface terminations is X-ray photoelectron spectroscopy, XPS. Herein, we undertook an XPS study of the out-of-plane ordered MXenes, Mo₂TiC₂T_z and Mo₂Ti₂C₃T_z, in both multilayered, ML, cold pressed and delaminated thin film forms. The harsh conditions needed to etch these MAX phases into MLs, results in their partial oxidation. The hydroxide used to delaminate the MLs results in further oxidation and a reduction in the F-content. In all cases, etching resulted in a decrease in the Ti to Mo ratio implying that the former atoms are selectively etched. In all but the ML Mo₂TiC₂T_z case, the Mo to C ratio was also reduced. It follows that the loss of Ti also results in the loss of C atoms. With the exception of the ML Mo₂Ti₂C₃T_z case, the number of termination moles per formula unit, z, was > 2, which is energetically unfavorable and thus unlikely. However, if one assumes that not all of the O signal is coming from terminations but rather from O atoms that replace C atoms in the MX blocks, then z ≈ 2. This work is an important step in quantifying and understanding the effects of etching on terminations and structure in these Mo and Ti-based MXenes.