High-Temperature Behavior and Surface Chemistry of Carbide MXenes Studied by Thermal Analysis

Two-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 degrees 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 Ti(3)C(2)Tx 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 Mo(2)CTx or Nb2CTx, and Mo2CTx is more/less thermally stable than Nb2CTx.