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High-Temperature Behavior and Surface Chemistry of Carbide MXenes Studied by Thermal Analysis
journal contribution
posted on 2019-04-03, 00:00 authored by Mykola Seredych, Christopher Eugene Shuck, David Pinto, Mohamed Alhabeb, Eliot Precetti, Grayson Deysher, Babak Anasori, Narendra Kurra, Yury GogotsiTwo-dimensional (2D)
transition-metal carbides and nitrides (MXenes)
have attracted significant attention due to their electronic, electrochemical,
chemical, and optical properties. However, understanding of their
thermal stability is still lacking. To date, MXenes are synthesized
via top-down wet chemical etching, which intrinsically results in
surface terminations. Here, we provide detailed insight into the surface
terminations of three carbide MXenes (Ti3C2Tx, Mo2CTx, and Nb2CTx) by performing
thermal gravimetric analysis with mass spectrometry analysis (TA–MS)
up to 1500 °C under a He atmosphere. This specific technique
enables probing surface terminations including hydroxyl (−OH),
oxy (O), and fluoride (−F) and intercalated species,
such as salts and structural water. The MXene hydrophilicity depends
on the type of etching (hydrofluoric acid concentration and/or mixed
acid composition) and subsequent delamination conditions. We show
that the amount of structural water in Ti3C2Tx increases with decreasing O-containing
surface terminations. The thermal stability of Ti3C2Tx is improved by employing a
low HF concentration or using a mixture of etchant acids, such as
H2SO4/HF or HCl/HF instead of only HF, due to
the reduced defect density. When tetramethylammonium hydroxide (TMAOH)
is used for delamination, new N-containing species appear on the MXene
surface. Moreover, free-standing films produced from Ti3C2Tx etched with different
HF concentrations and delaminated using TMAOH have similar TA–MS
profiles, indicating that the post-treatment of Ti3C2Tx controls its surface chemistry.
The thermal stability of MXenes strongly depends on their chemical
composition and structure; Ti3C2Tx is more thermally stable than the fewer-atomic-layered
Mo2CTx or Nb2CTx, and Mo2CTx is more/less thermally stable than Nb2CTx.