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 (Ti₃C₂T<i>_x</i>, Mo₂CT<i>_x</i>, and Nb₂CT<i>_x</i>) 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 Ti₃C₂T<i>_x</i> increases with decreasing O-containing surface terminations. The thermal stability of Ti₃C₂T<i>_x</i> 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 Ti₃C₂T<i>_x</i> etched with different HF concentrations and delaminated using TMAOH have similar TA–MS profiles, indicating that the post-treatment of Ti₃C₂T<i>_x</i> controls its surface chemistry. The thermal stability of MXenes strongly depends on their chemical composition and structure; Ti₃C₂T<i>_x</i> is more thermally stable than the fewer-atomic-layered Mo₂CT<i>_x</i> or Nb₂CT<i>_x</i>, and Mo₂CT<i>_x</i> is more/less thermally stable than Nb₂CT<i>_x</i>.