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

preprint
submitted on 29.05.2019 and posted on 30.05.2019 by Joseph Halim, Kevin Cook, Per Eklund, johanna Rosen, Michel W. Barsoum

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, Tz 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, Mo2TiC2Tz and Mo2Ti2C3Tz, 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 Mo2TiC2Tz 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 Mo2Ti2C3Tz 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.

Funding

the Swedish Research Council (Project Grant No. 621-2014-4890 and 642-2013-8020)

the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009-00971)

The Swedish Foundation for Strategic Research (SSF) is also acknowledged for Project Funding (EM16-0004)

History

Email Address of Submitting Author

joseph.halim@liu.se

Institution

Linköping University

Country

Sweden

ORCID For Submitting Author

0000-0002-7502-1215

Declaration of Conflict of Interest

No conflict of interest

Version Notes

Submitted to Applied Surface Science, this is a preprint

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